CN1596316A

CN1596316A - Method of manufacturing low phosphorous hot metal

Info

Publication number: CN1596316A
Application number: CN02823559.2A
Authority: CN
Inventors: 松野英寿; 村井刚; 菊地良辉; 樱井荣司; 竹滨良平; 川畑凉; 小平悟史; 菊地一郎; 田野学; 清水宏
Original assignee: NKK Corp
Current assignee: JFE Steel Corp; JFE Engineering Corp
Priority date: 2000-12-21
Filing date: 2002-08-27
Publication date: 2005-03-16
Also published as: TW550295B; JP2002249814A

Abstract

The invention is to implement an efficient dephosphorization treatment without adding a large amount of CaF2 and with a small addition amount of a refining agent. The invention is made based on the finding that a very efficient dephosphorization refinement utilizing an inhomogeneous molten state of slag can be implemented by feeding a gaseous oxygen and a refining agent onto a bath surface of a molten iron in a specific mode under conditions wherein a post-treatment slag is reduced appreciably in comparison to a conventional case. The dephosphorization treatment is performed by blowing the gaseous oxygen and at least a part of the refining agent onto the bath surface of the molten iron through a top blowing lance. Concurrently, the post-treatment slag rate is controlled to 30 kg/ton of molten iron or less, preferably to 20 kg/ton of molten iron or less, and more preferably 10 kg/ton of molten iron or less. In addition, the dephosphorization treatment be performed for an molten iron having a Si content of 0.15 mass% or less, more preferably 0.07 mass% or less, and particularly preferably 0.03 mass% or less.

Description

Method for manufacturing low-phosphorus molten iron

Technical Field

The present invention relates to a method for efficiently manufacturing low-phosphorous molten iron by dephosphorization for pretreating molten iron.

Background

In the past, a molten iron pretreatment method in which dephosphorization is performed at an iron-making stage has been widely used instead of the converter method. This is because the lower the refining temperature of the dephosphorization reaction, the easier the dephosphorization reaction proceeds thermodynamically, and the less refining agent is used for the dephosphorization treatment.

In general, in the pretreatment of molten iron, a solid oxygen source such as iron oxide is added to molten iron to conduct desiliconization, and after slag generated in the desiliconization is removed, a refining agent is added to conduct dephosphorization. Generally, a CaO-based refining agent such as lime is used as a refining agent for dephosphorization, and a solid oxygen source (iron oxide or the like) and gaseous oxygen are used as an oxygen source. As the treatment vessel, a mixer ladle car, a ladle (ladle), a converter type vessel, or the like is used. CaF for promoting slagging of CaO-based refining agent is widely used₂(fluorite).

As the dephosphorization conditions of the prior art, for example, Japanese patent laid-open publication No. 7-70626 discloses that the basicity of the slag is 0.6 to 2.5, the treatment completion temperature is 1250 to 1400 ℃, the bottom-blowing stirring power is 1.0 kg/ton or more, and the oxygen feed rate is 2.5Nm³And/ton molten iron or more. In this technique, the reason why the basicity of the slag is set to 2.5 or less is that the basicity above this deteriorates the fluidity of the slag, so that it is necessary to perform the treatment at a high temperature which is unfavorable for dephosphorization. Further, when the basicity of slag is 2.5 or less, dephosphorization is more likely to proceed as the basicity of slag is higher.

Furthermore, Japanese patent application laid-open No. 8-311523 discloses a method of blowing CaO powder and 0.7 to 2.0 Nm/N into molten iron in a converter-type container by a top-blowing lance³Oxygen per ton of molten iron, and blowing 0.05-0.30 Nm from the bottom or side wall of the converter-type vessel³Method for stirring gas for/min/ton of molten ironIn this method, by controlling the amount of oxygen supplied by top-blowing and bottom-blowing appropriately, the slag is rapidly formed (CaO is formed) and the concentration of FeO in the slag is controlled appropriately, whereby dephosphorization can be efficiently performed.

The conventional dephosphorization refining technique for molten iron mainly disclosed in Japanese unexamined patent publication Hei 7-70626 and Japanese unexamined patent publication Hei 8-311523 is based on the problem that the slag after treatment is uniformly melted and the slag-metal is nearly balanced, as judged by the analysis of the dephosphorization equilibrium equation. Therefore, the dephosphorization ability of the slag (phosphorus distribution Lp is mass% (P)/mass% [ P], mass% (P): the concentration of P in the slag, mass% [ P]: the concentration of P in the metal) and the amount of the slag were also determined by the above-described subjects and then operated.

The phosphorus distribution Lp of the slag is related to the basicity of the slag, and the higher the basicity of the slag, the higher the phosphorus distribution Lp. However, it is considered that the conventional high basicity of slag deteriorates the fluidity of slag, and is unfavorable for dephosphorization. On the other hand, since the phosphorus distribution Lp becomes low when the slag basicity is low, it is necessary to add lime in large amounts (SiO is also added as required)₂Source) to increase the amount of slag.

As can be seen from the above, in the prior art, in order to ensure a predetermined phosphorus distribution Lp, the necessary basicity is set, and here the slag basicity is setNext, the basicity required to achieve the target P content is determined, and a refining agent is added, but the basicity of the slag cannot be increased so much in view of the fluidity of the slag, so that the amount of the slag (the amount of the slag after the treatment) of about 40 to 50 kg/ton of molten iron is operated in a general dephosphorization treatment for the molten iron containing about 0.2 mass% of Si. For example, in Japanese unexamined patent publication Hei 8-311523, the amount of CaO (refining agent) charged is determined according to the P content in molten iron to be dephosphorized, and when the P content before dephosphorization is about 0.10 mass% of the ordinary level, about 20 kg/ton of molten iron is charged, and the slag in the refining vessel in dephosphorization is the portion of the slag formed by charging CaO and the SiO formed by the desiliconization of molten iron₂Partial, dephosphorizing reaction of formed P₂O₅A portion, a slag portion (FeO, MnO, etc.) derived from other molten iron components, a slag portion derived from the above-mentioned steps, and a slag portion derived from the melting loss of the furnace body (Al)₂O₃MgO, etc.), a slag portion originally adhering to the furnace body, a slag portion carried by charged scrap, a slag portion generated from the addition of ore, etc., and the like, and the total amount thereof (the amount of slag after the treatment) is generally about 2 to 2.5 times the amount of CaO charged, so that in the case of charging about 20 kg/ton of molten iron of CaO as described above, the amount of slag after the treatment inevitably reaches the amount of CaOAbout 40-50 kg/ton molten iron.

In recent years, from the viewpoint of environmental protection and the like, it has been demanded to reduce the amount of slag generated as much as possible in a refining step mainly including a dephosphorization step, but there is a limit to reducing the amount of slag by the above-mentioned conventional techniques, and therefore, it has not been possible to sufficiently meet the demand for reducing the amount of slag generated.

In addition, CaF added for promoting slagging of refining agent₂In recent years, in consideration of the influence of F on the environment, it has been demanded to reduce CaF as much as possible even in refining of steel₂Amount of CaF used, therefore CaF addition is attempted₂There is also a limit to improve the dephosphorization efficiency.

Disclosure of Invention

It is an object of the present invention to provide a process for the preparation of a catalyst which does not involve the addition of CaF in large amounts₂And with a small amountThe amount of the refining agent (2) can effectively conduct dephosphorization, and the amount of slag produced can be minimized.

In dephosphorization refining of molten iron, a method of blowing oxygen gas from a top-blowing lance onto the molten iron surface is preferably used as a method of supplying oxygen gas by adding an oxygen source and a refining agent as a CaO source to the molten iron, in view of suppressing a temperature drop and effectively promoting the production of FeO. In such a method of supplying oxygen, the slag is extruded by the energy of oxygen in the refining vessel and divided into a portion exposed to the liquid surface (molten iron surface) and a portion covered with the slag, and the existence state of the slag in the refining vessel is not uniform. Therefore, the present inventors have conducted studies on a dephosphorization method capable of stably achieving high dephosphorization efficiency in a state where a small amount of a refining agent is added without following the conventional method of considering the problem of keeping a slag in a uniformly molten state in a refining vessel, and as a result, have found that dephosphorization refining can be performed very efficiently by supplying oxygen and a refining agent to the molten ironsurface in a specific manner under the condition where the amount of the slag after the treatment is made much lower than that of the conventional technique, and further preferably under the condition where the Si content in the molten iron before the treatment is made a predetermined level or lower, contrary to the conventional method of considering the uniform melting of the slag, by utilizing the non-uniform molten state of the slag.

The method for producing a low-phosphorus hot metal of the present invention is based on the finding that a refining agent as a CaO source and an oxygen source are added to a vessel containing a hot metal to produce ironA process for producing a low-phosphorus molten iron by dephosphorization in a preliminary treatment of water, characterized in that the dephosphorization is carried out by blowing oxygen and at least a part of a refining agent into the molten iron surface by means of a top-blowing lance, and the amount of slag after the treatment is controlled to be 30 kg/ton or less of the molten iron. Further, it is more preferable that the amount of the slag after the treatment is 20 kg/ton or less, preferably 10 kg/ton or less. The method of the present invention utilizes the direct dephosphorization in the molten iron surface area of the oxygen-blown molten iron and the P fixation in the outer area thereof to fix the slag mainly in the solid phase without adding a large amount of CaF₂And a small amount of a refining agent is added to effectively conduct dephosphorization.

In order to make the effects of the present invention more effective, it is desirable to subject the low-Si molten iron to dephosphorization. That is, it is desirable to provide the molten iron having an Si content of 0.15 mass% or less, preferably 0.07 mass% or less, more preferably 0.03 mass% or less, with the optimum conditions for stably generating the dephosphorization reaction by the above mechanism byconducting the dephosphorization treatment.

As a method of adding the refining agent from the top-blowing lance to the molten iron surface, it is preferable that at least a part of the refining agent supplied from the top-blowing lance is blown to the molten iron surface to which oxygen is blown, and it is more preferable that at least a part of the refining agent supplied from the top-blowing lance is blown to a fire point generated in the molten iron surface by blowing oxygen. Further, it is preferable that at least a part of the refining agent is blown to the molten iron surface by using oxygen as a carrier gas. Thus, the oxygen is supplied to effectively slag the refining agent in the molten iron surface region where a large amount of FeO is produced, and the dephosphorization reaction can be effectively promoted.

In the present invention, CaF is used₂The amount of (A) is not more than 2 kg/ton of molten iron or substantially no CaF is added₂Under the conditions of (3), dephosphorization can be effectively carried out.

In the present invention, it is desirable that the molten iron having a P content of 0.10 mass% or more is dephosphorized and refined to a P content (standard value of steel component) required for the crude steel or less, and it is more desirable that the P content in the molten iron after the dephosphorization is 0.01 mass% or less. In this way, in the continuous converter blowing, substantially no slag former is used, and only decarburization refining can be performed substantially.

In the present invention, under the above basic conditions, various desirable embodiments described below can be adopted.

In embodiment 1, the dephosphorization is performed at a feed rate B (kg/min/ton of molten iron) in terms of CaO of the refining agent to be blown onto the molten iron surface and at a feed rate A (Nm) of oxygen to be blown onto the molten iron surface³/min/ton of molten iron) satisfies the following expression (1), and desirably satisfies the following expression (2). Thus, the amount of FeO produced by the oxygen supply and the amount of CaO supplied are properly balanced, and higher dephosphorization efficiency can be obtained.

0.3≤A/B≤7……(1)

1.2≤A/B≤2.5……(2)

In embodiment 2, a ladle type or a torpedo type hot metal ladle type vessel is used as a vessel for charging molten iron, and oxygen and at least a part of a refining agent are blown to the molten iron surface by a top-blowing lance, and a gas containing powder is blown to the molten iron by an immersion lance and/or a blowing nozzle to conduct dephosphorization. Thus, in the dephosphorization using a ladle type or a hot metal mixer type vessel, molten iron can be appropriately stirred and higher dephosphorization efficiency can be obtained.

In embodiment 2, the powder to be blown into the molten iron by the dipping lance and/or the blowing nozzle is preferably a part of the refining agent, and the amount of oxygen to be blown into the molten iron surface by the top-blowing lance is preferably 0.7Nm³Less than/min/ton molten iron. Further, it is preferable to effectively perform the dephosphorization by blowing 80 mass% or more of the amount of refining agent added in the dephosphorization to the molten iron surface by a top-blowing lance. In addition, in the case where substantially the total amount of the refining agent is injected to the molten iron surface by the top-blowing lance and added by injecting the refining agent into the molten iron by the dipping lance and/or the injection nozzle, it is desirable that the amount of the refining agent to be added by the top-blowing lance is 20 to 80 mass% based on the total amount of the refining agent tobe added, so that the effect of injecting the refining agent into the molten iron surface and the effect of stirring the molten iron by injecting the refining agent into the molten iron can be well balanced.

In embodiment 3, the dephosphorization is performed under the condition that the supply rate of the refining agent and the supply rate of the oxygen gas to be blown to the molten iron surface satisfy the following expressions (3) and (4). Thus, since the refining agent can be added in a minimum necessary amount without adding an unnecessary amount of the refining agent at the latter stage of the dephosphorization, the dephosphorization can be efficiently performed with a small amount of the refining agent.

(C1/D1)＞(C2/D2)……(3)

C1＞C2……(4)

Wherein C1: average value of CaO-converted supply rates of refining agents (kg/min/ton molten iron) in the early stage of dephosphorization

C2: average value of CaO-converted supply rates of refining agents (kg/min/ton molten iron) at the latter stage of dephosphorization

D1: average value (Nm) of oxygen supply rate in the early stage of dephosphorization³/min/ton molten iron)

D2: average value (Nm) of oxygen gas supply rate at the latter stage of dephosphorization³/min/ton molten iron)

In this embodiment 3, the refining agent supply rate and the oxygen gas supply rate in terms of CaO may be continuously and/or stepwise changed during the dephosphorization.

In embodiment 4, molten iron having an Si content of 0.15 mass% or less is subjected to dephosphorization by blowing oxygen and at least a part of a refining agent to the molten iron surface by a top-blowing lance, and lime in an amount of the sum of the lime amount Wcao-P (kg/ton of molten iron) obtained by the following equation (5) and the lime amount Wcao-Si (kg/ton of molten iron) obtained by the following equation (6) is added as the refining agent in the dephosphorization. This allows efficient dephosphorization to be carried out with a minimum amount of refining agent added.

Wcao-P ═ (molten iron [ P]-target [ P]) × (10/62) × 56 × 3/η cao … (5)

Wherein, the molten iron [ P]: p concentration (mass%) in molten iron before dephosphorization

Target [ P]: p concentration (mass%) in molten iron after targeted dephosphorization

η cao (lime efficiency) is 0.5-1

Wcao-Si ═ hot metal [ Si]× (10/28) × 56 × 2 … (6)

Wherein, the molten iron [ Si]: si concentration (mass%) in molten iron before dephosphorization

In embodiment 4, it is desirable that lime of 80 mass% or more of the lime amount Wcao-P (herein, Wcao-P obtained by using η CaO ═ 1) is blown to the molten iron surface by a top-blowing lance to perform effective treatment, and 1 or more kinds of lime can be selected from among iron-making slags containing lime powder, lump quicklime, lump limestone and unreacted CaO and used as a refining agent corresponding to the lime amount Wcao-Si.

In embodiment 5, the depth L of the depression formed in the molten iron surface by blowing oxygen or blowing a refining agent using oxygen as a carrier gas, which is defined by the following equation (7), is controlled to be 200 to 500 mm. This makes it possible to provide oxygen to the fire point of the reaction zone in a suitable form, and to add a small amount of a refining agent for more effective dephosphorization.

L＝L₀×exp{(-0.78×L_H)/L₀}……(7)

L₀＝63×{(F₀₂/n)/d_t}^2/3

Wherein L is_H: height of the top-blowing gun (mm)

F₀₂: velocity of oxygen supply (Nm) by top-blowing lance³/hr)

n: number of nozzle holes of top-blowing gun

d_t: nozzle aperture (mm) of top-blowing gun (average aperture of all nozzle holes in the case where nozzle apertures of a plurality of nozzle holes are different)

In embodiment 6, CaF is added to molten iron having an Si content of 0.15 mass% or less₂The amount of (A) is 1 kg/ton or less of molten iron or substantially no CaF is added₂Under the condition of (1), blowing oxygen and at least a part of refining agent to the molten iron liquid surface by a top blowing gun to carry out dephosphorization treatment, and simultaneously enabling the molten iron temperature to be 1360-1450 ℃ when the dephosphorization treatment is finished. Thus, the dephosphorization can be efficiently performed even in the high-temperature treatment, and the residual heat in the subsequent step can be sufficiently ensured.

In embodiment 7, a substance that absorbs heat of molten iron due to a chemical reaction and/or a thermal decomposition reaction is supplied to a molten iron surface region where oxygen is supplied. Thus, the temperature rise in the molten iron surface area to which oxygen is supplied can be suppressed without inhibiting the slagging of the refining agent, and therefore higher dephosphorization efficiency can be obtained.

In the 7 th embodiment, it is desirable that a substance which absorbs heat of molten iron at least partially by a chemical reaction and/or a thermal decomposition reaction is supplied to the molten iron surface by blowing oxygenThe resulting fire point. As the substance which absorbs the heat of molten iron by the chemical reaction and/or thermal decomposition reaction, it is desirable to select 1 or more species from carbon dioxide, water vapor, nitrogen oxide, metal carbonate and metal hydroxide, and it is particularly desirable to select the substance from the group consisting of carbon dioxide, water vapor, nitrogen oxide, metal carbonate and metal hydroxideDecomposition to CO₂Or H₂Metal carbonate of O, CO produced by thermal decomposition₂Or H₂More than 1 kind of metal hydroxide of O is selected. Among them, CaCO is particularly preferable₃、Ca(OH)₂、CaMg(CO₃)₂At least 1 selected from the above.

In embodiment 7, instead of a part or all of the refining agent as the CaO source, CaCO, which is a product of the refining agent and absorbs the heat of molten iron by a chemical reaction and/or a thermal decomposition reaction, may be used as the refining agent₃、Ca(OH)₂、CaMg(CO₃)₂More than 1 kind of the oxygen is selected and supplied to the molten iron liquid level area to which the oxygen is supplied. In this embodiment, it is desirable to remove the CaCO₃、Ca(OH)₂、CaMg(CO₃)₂At least a part of the 1 or more substances selected in (b) is supplied to the fire point generated on the molten iron surface by injecting oxygen.

The above-described 1 st to 7 th embodiments of the method of the present invention may be carried out individually, or 2 or more of the embodiments may be carried out in any combination, and the effect of the method of the present invention is more excellent as the combination conditions are increased.

Drawings

FIG. 1 is a graph showing the relationship between the amount of slag after dephosphorization and the P content in molten iron.

FIG. 2 is a graph showing the relationship between the Si content in molten iron before dephosphorization and the amount of slag after dephosphorization.

FIG. 3 is an explanatory view showing an embodiment of the method of the present invention using a converter type vessel.

FIG. 4 is a graph showing the relationship between the amount of the refining agent added by the top-blowing lance relative to the total amount of the refining agent and the required amount of lime in embodiment 2 of the method of the present invention.

FIG. 5 is a graph showing the relationship between the ratio of the amount of refining agent added by the top-blowing lance to the total amount of refining agent added and the dephosphorization ratio in the case where all the refining agent is blown to the molten iron surface by the top-blowing lance and is added by the immersion lance and/or the blowing nozzle in embodiment 2 of the method of the present invention.

Fig. 6 is an explanatory view showing an example of an implementation of embodiment 2 of the method of the present invention.

FIG. 7 is a graph showing the relationship between the unit consumption of CaO and the dephosphorization efficiency required for the dephosphorization method of embodiment 3 of the present invention and the conventional method so that the P content in the molten iron after the dephosphorization is 0.012 mass%.

FIG. 8 is a graph showing the relationship between the Si content in molten iron and the required lime amount in the method according to embodiment 4 of the present invention and the conventional method.

FIG. 9 is a graph showing the relationship between the amount of lime required for dephosphorization and the lime efficiency η cao and the P content in molten iron after dephosphorization in the method according to embodiment 4 of the present invention and the conventional method.

FIG. 10 is a graph showing the relationship between the P content in molten iron after dephosphorization and the ratio X/Wcao-P between the amount of lime X blown from a top-blowing lance onto the molten iron surface and the amount of lime Wcao-P for dephosphorization in the 4 th embodiment of the process of the present invention.

FIG. 11 is a graph showing the relationship between the depth L of a depression formed in the molten iron surface by injecting oxygen or injecting a refining agent using oxygen as a carrier gas, the dephosphorization efficiency, and the P content in the molten iron after the dephosphorization in the method 5 according to the embodiment of the present invention.

FIG. 12 shows CaF not added in embodiment 6 of the method of the present invention₂The relationship between the Si content in the molten iron during dephosphorization, the temperature of the molten iron after dephosphorization and the efficiency of dephosphorization lime.

FIG. 13 shows a CaF of dephosphorization at 1360 to 1450 ℃ after the dephosphorization in the 6 th embodiment of the process of the present invention₂Graph of the relationship between the addition amount and the efficiency of dephosphorized lime.

FIG. 14 is an explanatory view showing an example of a mode of supplying oxygen, a refining agent and a heat absorbing substance to the molten iron surface by a top-blowing lance in embodiment 7 of the method of the present invention.

FIG. 15 is an explanatory view schematically showing the state of slag/metal at the start of tapping in the conventional method using a converter type vessel and the method of the present invention.

FIG. 16 is an explanatory view schematically showing the state of slag/metal in the vicinity of a taphole at the end of tapping in a conventional method using a converter type vessel and the method of the present invention.

FIG. 17 is a graph showing the relationship between the P content in molten iron after dephosphorization and the ratio A/B of the oxygen gas supply rate A and the CaO refining agent supply rate B in the example of embodiment 1 of the method of the present invention.

Detailed Description

The dephosphorization mechanism of molten iron has been recognized in the past, in which CaO and SiO generated by supplying oxygen are added to a refining vessel₂FeO reacts to form a molten mass to generate CaO-SiO₂FeO is a uniform slag having a high dephosphorization ability, and dephosphorization of molten iron is carried out by the reaction of this slag with P in molten iron. To address such a dephosphorization mechanism, the basicity of the slag is determined in consideration of the fluidity and dephosphorization ability of the slag as described above, and the amount of the slag required to achieve the target P at the basicity of the slag is determined. On the contrary, the present inventors have found that a very effective dephosphorization and refining can be carried out by a completely different mechanism from the conventional technique by a treatment method in which oxygen and a refining agent are blown onto the molten iron surface by a top-blowing lance under a condition that the amount of slag after the treatment is considerably reduced as compared with the conventional technique and further under a condition that the Si content in the molten iron before the treatment is desired to be a predetermined level or less.

The details and preferred embodiments of the present invention based on such knowledge will be described below.

In the method of the present invention, when dephosphorizing as a pretreatment of molten iron is performed by adding an oxygen source and a refining agent as a CaO source to a vessel (refining vessel) in which molten iron is charged to perform dephosphorization as a pretreatment of molten iron, oxygen and at least a part of the refining agent are blown to the molten iron liquid surface by a top-blowing lance to perform dephosphorization. After oxygen is blown to the molten iron liquid surface through the top-blowing lance, a large amount of FeO is generated by the oxygen impacting the liquid surface, so that the condition which is very favorable for promoting the slagging of the refining agent is formed.

Further, oxygen and a refining agent may be blown to the molten iron surface by a top-blowing lance, and a carrier gas other than oxygen (e.g., N) may be used₂And an inert gas such as Ar) is blown to the molten iron surface, and in this case, it is also desirable to blow a part or the whole of the refining agent to the molten iron surface region to which the oxygen is supplied (blown). This is because the molten iron surface region to which the oxygen gas is supplied is a region where FeO is produced by the supplied oxygen, and by directly adding CaO to such a surface region, the slagging of CaO can be effectively promoted, and the contact efficiency between CaO and FeO also becomes high. This is achieved byIn addition, in the region of the molten iron surface where oxygen is supplied, it is preferable to supply the refining agent to a region called "fire" resulting from top-blowing oxygen. This ignition point is a molten iron surface region of the highest temperature at which the oxygen gas jet impinges, and is a region in whichthe oxidation reaction by the oxygen gas is concentrated and the oxygen gas jet agitates, so that it can be said that the effect of supplying CaO is most remarkable. In this sense, it is desirable to use oxygen as the carrier gas for blowing the refining agent onto the molten iron surface, and in this case, the oxygen is blown onto the molten iron surface together with the refining agent to directly supply the refining agent to the ignition point, and as a result, the efficiency of contact between CaO and FeO on the molten iron surface is greatly improved.

In the method of the present invention, the mode of adding oxygen and the refining agent is aimed at efficiently proceeding the dephosphorization reaction by the following basic principle.

That is, when a refining agent (CaO) is blown by a top-blowing lance to a molten iron surface region (preferably, an ignition point) to which oxygen is supplied in an optimum state, the CaO reacts rapidly with FeO generated at the ignition point to be melted (slagged) and forms a CaO-FeO-based melt. The molten CaO-FeO system is formed such that the kinetic energy of oxygen is pushed from the molten iron surface region to which oxygen is supplied around the fire to a region where the potential energy of oxygen around the molten iron is low, and the molten CaO-FeO reacts with Si in the molten iron to reduce FeO and thereby form a molten iron of CaO-FeO system2 CaO. SiO is formed in accordance with the Si content in the molten iron before treatment₂And the like, a stable solid phase. Further, when the Si content in the molten iron is lowered to some extent by the above reaction, the CaO-FeO-based melt starts to react with phosphorus to form 3 CaO. P₂O₅The same stable solid phase of (a). As a result, as the dephosphorization proceeds, the amount of slag (or most of the slag) which is produced and pushed out of the molten iron surface region to which oxygen is supplied from the fire center to the outer region thereof in order becomes 2 CaO. SiO₂、3CaO·P₂O₅Is present in a stable solid phase. Since the solid-phase slag thus produced is very stable, it does not remelt even if the basicity of the slag is low. By virtue of the fact that the dephosphorization reaction proceeds directly in the region centered on the fire and the slag pushed to the outside is present mainly in the solid phase, effective dephosphorization can be carried out by adding a small amount of refining agent.

As described above, in the method of the present invention, focusing on the mechanism of utilizing the direct dephosphorization reaction in the molten iron surface region centered on the fire and the P fixation of the slag mainly in the solid phase in the outer region thereof, the dephosphorization reaction proceeds efficiently, and the dephosphorization reaction utilizing the above mechanism cannot be stably realized only by injecting oxygen and a refining agent into the molten iron surface alone. That is, in order to stably realize the dephosphorization reaction by the above mechanism, the above specific supply method of oxygen and refining agent is used, and the treatment is carried out with a sufficiently small amount of slag, specifically, the amount of slag after the treatment is 30 kg/ton or less of molten iron, preferably 20 kg/ton or less of molten iron, more preferably 10 kg/ton or less of molten iron. From the same viewpoint, the molten iron to be dephosphorized is preferably a low-Si molten iron, more specifically, a molten iron having an Si content of 0.15 mass% or less, still more preferably 0.07 mass% or less, and most preferably 0.03 mass% or less.

The reason why the treatment is carried out with a small amount of slag in the present invention is as follows. In order to efficiently cause the dephosphorization reaction of the above-mentioned specific mechanism to occur, oxygen gas passing through the top-blowing lance must be supplied to the molten iron surface in so-called soft blowing (low dynamic pressure). That is, the above mechanism is used to control the ignition point in the dephosphorization reactionThe molten iron surface area of the core to which oxygen is supplied becomes a main generation site of FeO, CaO supplied to this area and slagging reacts with FeO to generate a CaO-FeO system melt, and this CaO-FeO system melt directly reacts with P in molten iron to form 3 CaO.P₂O₅A stable solid phase of (a). On the other hand, in the case where oxygen is supplied by soft blowing in a state where the amount of slag produced is large and the slag layer is thick as in the conventional technique, the oxygen jet flow cannot penetrate the slag layer and the oxygen cannot be supplied properly to the molten iron surface, so that FeO produced on the molten iron surface is insufficient, and therefore the amount of CaO — FeO-based melt produced is small. On the other hand, if oxygen is supplied by hard blowing (high dynamic pressure) so that the oxygen jet can penetrate the thick slag layer formed, the supply region is in a strongly stirred state, and even if FeO is produced, it is reduced by C in the molten iron, and in this case, the required amount of FeO cannot be secured, so that the amount of CaO — FeO system melt produced is also reduced. When the amount of slag is large, the amounts of FeO and CaO-FeO-based molten metal produced cannot be stably secured in both soft blowing and hard blowing, and it is difficult to stably produce the dephosphorization reaction by the above-described mechanism. Therefore, in order to appropriately supply oxygen to the molten iron surface by soft blowing, the dephosphorization reaction by the above mechanism is efficiently performed, and it is essential to limit the amount of slag and to sufficiently thin the slag layer. Therefore, in the present invention, the amount of the slag after the treatment is 30 kg/ton or less. For theabove reasons, it is desirable that the amount of slag after the treatment is as small as possible, particularly preferably 20 kg/ton or less, more preferably 10 kg/ton.

Further, the reason why the dephosphorization treatment of the low-Si molten iron is desired in the present invention is as follows. Such asIn the above-described specific dephosphorization process, CaO, which is supplied to the molten iron surface region to which oxygen is supplied around the fire point (i.e., the main FeO generation region), is slagged to react with FeO to generate a CaO — FeO system melt, and this CaO — FeO system melt directly reacts with P in the molten iron to conduct dephosphorization, but if the Si content in the molten iron is high, the generated CaO — FeO system melt is consumed in the reaction with Si, and the direct dephosphorization cannot be sufficiently facilitated. Therefore, the dephosphorization by the above mechanism is to be stably carried outThe most suitable conditions for the reaction are conditions satisfying the amount of the slag after the above-mentioned treatment, and the Si content in the molten iron subjected to the dephosphorization is sufficiently low. In addition, when the Si content in the molten iron is small, SiO is contained₂The generated amount is small, which is beneficial to reducing the amount of the slag after treatment. Therefore, in the present invention, it is desirable to dephosphorize the molten iron having an Si content of 0.15 mass% or less, more preferably 0.07 mass% or less, and most preferably 0.03 mass% or less.

The post-treatment slag amount in the present invention means the amount of slag in the refining vessel (vessel in which molten iron is charged) at the time of completion of the dephosphorization treatment. The amount of slag after the treatment can be determined by a method of calculating the mass balance between the amount of lime added and the CaO concentration in the slag (slag analysis value), a method of adding an isotope indicator such as yttrium oxide or strontium oxide to the slag and analyzing the isotope indicator concentration in the slag after the treatment, a method of directly measuring the slag thickness, or the like.

FIG. 1 is a graph showing the relationship between the amount of slag after dephosphorization and the P content in molten iron, and shows the average value of the P content in molten iron after dephosphorization and the magnitude of the deviation, according to the results of experiments conducted by the present inventors. FIG. 1 is a graph showing the accumulation of P content in molten iron after 6 to 72ch dephosphorization in the post-treatment slag amount ranges of 5 to 10 kg/ton molten iron, more than 10 to 20 kg/ton molten iron, more than 20 to 30 kg/ton molten iron, more than 30 to 40 kg/ton molten iron, and more than 40 to 50 kg/ton molten iron.

In this test, molten iron tapped in a blast furnace was desiliconized in a tapping yard of the blast furnace or in a ladle as needed, and then desulfurized in a ladle with mechanical stirring, and thereafter dephosphorized in a converter type vessel (300 tons). The molten iron before dephosphorization comprises the following components: 4.5 to 4.7 mass%, Si: 0.01 to 0.28 mass%, Mn: 0.15 to 0.25 mass%, P: 0.10 to 0.11 mass%, S: 0.001 to 0.003 mass%. Refining agent for dephosphorizationLime powder with the particle size of less than 1mm is blown to the liquid level of molten iron by an oxygen lance by taking oxygen as carrier gas. No CaF is added in refining agent₂. The blowing time was fixed at 10 minutes for stirringMixing molten iron, providing 0.05-0.15 Nm from the furnace bottom³Min/ton molten iron. The unit consumption of lime and oxygen varies according to the Si content in the molten iron, and the desiliconized portion of lime and oxygen together (2 calcium silicate: formationof 2 CaO. SiO₂Stoichiometric portion of) were determined as fixed 3.5 kg/ton molten iron, 9 Nm/ton molten iron, respectively³Per ton of molten iron. The temperature of molten iron before and after dephosphorization is 1250-1350 ℃. The amount of the processed slag was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

The higher the amount of slag after the dephosphorization in FIG. 1, the higher the P content, and the larger the deviation on the upper limit side. On the contrary, when the amount of slag after the treatment is 30 kg/ton or less, the variation of the upper limit of the P content is remarkably reduced, and the P content is 0.020% by mass at the maximum. When the amount of the slag after the dephosphorization treatment is less than 20 kg/ton of molten iron, the P content in the molten iron after the dephosphorization treatment is 0.015 mass% at most, and when the amount of the slag after the dephosphorization treatment is less than 10 kg/ton of molten iron, the P content in the molten iron after the dephosphorization treatment is 0.010 mass% at most. For the above reasons, the amount of slag after the treatment is preferably 30 kg/ton or less, more preferably 20 kg/ton or less, and most preferably 10 kg/ton or less.

FIG. 2 is a graph showing the relationship between the Si content in molten iron before dephosphorization and the amount of slag after dephosphorization in the test of FIG. 1. According to this figure, when the Si content in the molten iron before treatment is high, the amount of lime added increases and the amount of slag also increases, so the amount of slag and the Si content in the molten iron before treatment are closely related. Therefore, it was determined that the Si content in the molten iron before dephosphorization should be 0.15 mass% or less in order to reduce the amount of slag after dephosphorization to 30 kg/tonor less. Similarly, it was determined that the Si content in the molten iron before dephosphorization should be 0.07 mass% or less in order to reduce the amount of slag after the dephosphorization to 20 kg/ton or less, and that the Si content in the molten iron before dephosphorization should be 0.03 mass% or less in order to reduce the amount of slag after the dephosphorization to 10 kg/ton or less. From the above reasons, in the present invention, it is preferable to conduct dephosphorization of molten iron having an Si content of 0.15 mass% or less, more preferably 0.07 mass% or less, and most preferably 0.03 mass% or less.

As described above, when the Si content in the molten iron is low, the reaction consumption ratio of the produced CaO-FeO-based melt to Si is reduced, and the effect of promoting the dephosphorization reaction using the CaO-FeO-based melt as it is obtained, and it is considered that the results shown in FIG. 1 reflect the effect.

The Si content in the molten iron before dephosphorization can be adjusted as follows.

Molten iron is supplied from an apparatus for manufacturing molten iron such as a blast furnace, and as a method for reducing the Si content of the manufactured molten iron, a method of reducing the total amount of charged silicic acid portions by pretreating raw materials for manufacturing molten iron, or a method of performing low-temperature operation and offset charging of coke for suppressing a reduction reaction of silicic acid in a furnace such as a blast furnace, is known. Therefore, when the Si content of the molten iron produced in a blast furnace or the like is 0.15 mass% or less, the molten iron can be dephosphorized without being subjected to desiliconization.

On the other hand, when the Si content of molten iron produced in a blast furnace or the like exceeds 0.15 mass%, desiliconization is performed in a tapping field, a ladle or the like of the blast furnace before dephosphorization, and dephosphorization is performed when the Si content of molten iron before dephosphorization is 0.15 mass% or less.

In general, the desiliconization of molten iron is carried out by adding a solid oxygen source and oxygen to molten iron, and for example, a method of adding a solid oxygen source such as sintered powder or rolled iron oxide scale by charging it from above onto the molten iron surface and blowing it into the molten iron, or a method of blowing oxygen into the molten iron surface or into the molten iron is employed.

The hot metal desiliconization may be performed by adding an oxygen source to a hot metal stream flowing from a tapping site of a blast furnace to a transfer container such as a ladle, for example, in addition to the tapping site and the ladle of the blast furnace. In order to improve the desiliconization efficiency, it is also possible to blow stirring gas into the molten iron in the vessel, or to add a CaO source such as quicklime to adjust the basicity of the slag, thereby reducing iron oxide in the desiliconized slag as much as possible and improving the reduction efficiency.

When the dephosphorization is performed after the desiliconization of the molten iron, slag such as desiliconized slag is discharged in advance, and it is desired to effectively perform the dephosphorization by suppressing the incorporation of silicic acid portions as much as possible. Therefore, before the dephosphorization, slag is separated from the molten iron by a mechanical slag discharge device or by manual operation, and then the dephosphorization is performed.

In the method of the present invention, the method of blowing oxygen and the refining agent to the molten iron surface by the top-blowing lance is not particularly limited, and for example, among a plurality of lance holes of the top-blowing lance, oxygen may be supplied only to the molten iron surface from a part of the lance holes, and the refining agent may be supplied to the molten iron surface from the other lance holes by using oxygen or a gas other than oxygen (for example, an inert gas such as nitrogen or Ar) as a carrier gas. This allows the addition of a refining agent to the molten iron surface to which oxygen is supplied. In this case, it is preferable to use a top-blowing lance having a main lance hole in the center of the lance and a plurality of sub lance holes around the main lance hole, and to supply oxygen from the sub lance holes to the molten iron surface and supply a refining agent from the main lance hole to the molten iron surface using oxygen or a gas other than oxygen as a carrier gas. Alternatively, different top-blowing lances may be used for blowing oxygen and for blowing a refining agent using oxygen or a gas other than oxygen as a carrier gas. However, in all cases, it is particularly desirable that the carrier gas of the refining agent is oxygen in order to slag the refining agent as described above most efficiently.

The oxygen used in the present invention may be pure oxygen or a gas containing oxygen. As the oxygen source to be added to the refining vessel, a solid oxygen source such as iron oxide (for example, sintered powder or rolled iron oxide scale) may be used in addition to the oxygen gas, and any method such as charging and injecting into molten iron from above may be employed. However, in order to efficiently dephosphorize molten iron by supplying (blowing) oxygen to the molten iron surface as described above, it is desirable that 50% or more of the oxygen source added to the refining vessel is supplied to the molten iron surface through the top-blowing lance, and more desirable that 70% or more of the oxygen source added to the refining vessel is supplied to the molten iron surface through the top-blowing lance.

In addition to the method of blowing a part of the oxygen gas to the molten iron surface, for example, a method of injecting the oxygen gas into the molten iron through an immersion lance or a blowing nozzle provided on a side wall and a bottom of the molten iron container may be used to supply the oxygen gas to the molten iron.

In general, a CaO-based refining agent (refining agent mainly containing CaO) such as lime is used as the refining agent. In addition, powder was used as a refining agent to be blown onto the molten iron surface by a top-blowing lance.

In addition to blowing the refining agent to the molten iron surface by the top-blowing lance, it may be added by charging and injecting a part of the refining agent into the molten iron from above, and in this case, it is desirable that the amount of the refining agent added by these methods is 20 mass% or less of the total amount of the refining agent. If the proportion of the refining agent added by a method other than the method of blowing the refining agent to the molten iron surface by the top-blowing lance exceeds 20 mass% of the total amount, the effect of promoting the dephosphorization reaction by blowing the refining agent together with oxygen to the molten iron surface tends to be reduced.

To improve the dephosphorization efficiency, it is desirable to perform gas stirring on the molten iron. This gas stirring can be performed, for example, by a method of blowing an inert gas such as nitrogen or Ar into the molten iron through an immersion gun and blowing nozzles provided on the side wall and the bottom of the molten iron container. In order to obtain sufficient molten iron stirring performance, it is desirable that the amount of the stirring gas supplied is 0.02Nm³At least/min/ton of molten iron, and if the stirring of molten iron is too strong, the rate of reduction of FeO produced by Cin the molten iron becomes too high, so that the rate is preferably 0.3Nm³Less than/min/ton molten iron.

The vessel (refining vessel) for charging molten iron for dephosphorization is preferably a converter type vessel from the viewpoint of ensuring a sufficient free space, and any vessel such as a ladle and a torpedo type ladle car can be used.

FIG. 3 shows an embodiment of the method of the present invention using a converter type vessel, wherein 1 is a converter type vessel, 2 is a top-blowing lance, and 3 is a bottom-blowing nozzle provided at the bottom of the vessel, and in this example, oxygen is used as a carrier gas, and a refining agent is blown from the top-blowing lance 2 to the surface of molten metal while a stirring gas is blown from the bottom-blowing nozzle 3 into the molten iron.

In the conventional dephosphorization, CaF is added to promote the slagging of CaO₂(Fluoritum)Stone), in recent years, in consideration of the influence of F on the environment, CaF control is required even in refining of steel₂The amount of (2) used. In this regard, the process of the invention is substantially free of added CaF₂(that is, CaF is not added except for the inevitable inclusion in the refining agent₂) Or only adding a small amount of CaF₂In the case of (2), a high dephosphorization efficiency is obtained. Therefore, CaF is added to promote the slagging of CaO₂In the case of (3), the amount of addition is also 2 kg/ton or less, preferably 1 kg/ton or less. Further, as described later, the present invention is similar to the conventional onesCompared with the method, the method can obtain the effect of greatly reducing the loss of the processed slag, and can utilize the method without adding CaF₂Or, the addition amount thereof is controlled to be very small, whereby the fluidity of the slag can be made lower, and the above-mentioned effects can be further improved.

The P content of molten iron before dephosphorization is usually 0.10 mass% or more, but in the present invention, it is desired to conduct dephosphorization refining to the P content required for the crude steel, that is, to the P content required for the steel, that is, to the value not more than the standard value of the steel components (usually 0.020 mass% or less), more preferably 0.010 mass% or less, so that in the converter blowing which is continued, decarburization refining is conducted substantially without charging slag formers, ① can be obtained which can extremely simplify decarburization refining and shorten the refining time, ② can effectively reduce the amount of slag produced in decarburization refining, and ③ can obtain a very high Mn recovery rate in the case where manganese ore is added as a manganese source because slag formers are not substantially used in decarburization refining.

Several preferred embodiments of the process of the present invention are described below. The efficiency of the dephosphorization reaction can be further improved by carrying out the process of the present invention in the embodiment described below.

In embodiment 1 of the present invention, the supply rate B (kg/min/ton of molten iron) of the refining agent blown onto the molten iron surface and the supply rate a (Nm) of the oxygen blown onto the molten iron surface are calculated in terms of CaO³Min/ton of molten iron) to satisfy the following dephosphorization treatment of the formula (1).

0.3≤A/B≤7……(1)

Further, in order to obtain higher dephosphorization reaction efficiency, it is desirable to perform a supply rate B (kg/min/ton of molten iron) of the refining agent to be blown onto the molten iron surface in terms of CaO and a supply rate A(Nm/min/ton of molten iron) of the oxygen to be blown onto the molten iron surface³Min/ton of molten iron) to satisfy the following dephosphorization treatment of the formula (2).

1.2≤A/B≤2.5……(2)

According to the results of the studies conducted by the present inventors, it has been found that in the method of blowing oxygen and a refining agent to the molten iron surface, the dephosphorization reaction is changed depending on the supply rate of oxygen and the supply rate of CaO (refining agent), and specifically FeO is generated in the molten iron surface region to which oxygen is supplied, and there is an ideal supply rate of CaO corresponding to the amount of generated FeO. If the oxygen gas supply rate is too low in the ratio of the oxygen gas and CaO supply rate, FeO corresponding to the amount of CaO supplied cannot be produced in the molten iron surface area to which the oxygen gas is supplied, and CaO cannot be slagging (CaO-FeO system molten mass is produced), and CaO exists in a state where it is not slagging, and thus the effect of efficient dephosphorization cannot be achieved. On the other hand, if the oxygen gas supply rate is too high, the amount of CaO required for dephosphorization with respect to the oxygen supply amount is insufficient, and in this case, a sufficient CaO-FeO system melt cannot be formed. Therefore, the above-mentioned mechanism of dephosphorization is disadvantageous in all cases, and a high dephosphorization rate tends not to be obtained. Further, if the oxygen gas supply rate is too high, the amount of ineffective oxygen other than the oxygen required for dephosphorization becomes large, and since it is consumed by decarburization or the like, the heat source in the subsequent step becomes insufficient, resulting in a significant increase in the operation cost in the decarburization treatment.

When the A/B ratio is less than 0.3,since the amount of CaO supplied is excessive with respect to the amount of oxygen supplied, FeO corresponding to the amount of CaO supplied cannot be generated in the molten iron surface area to which oxygen is supplied. Therefore, the supplied CaO is not sufficiently slag-formed (formed into a CaO-FeO-based molten mass), and CaO is present in a state of not being slag-formed, and thus the dephosphorization is not effectively performed, and the dephosphorization rate tends to be low. On the other hand, if the A/B ratio exceeds 7, the amount of CaO required for dephosphorization based on the oxygen supply amount is insufficient, and in this case, a sufficient CaO-FeO system melt cannot be formed, so that the dephosphorization rate tends to be low. Further, by setting A/B to 1.2 to 2.5, the balance between the amount of FeO produced by supplying oxygen and the amount of CaO supplied is more suitable, and particularly high dephosphorization efficiency can be obtained.

The 2 nd embodiment of the present invention is a dephosphorization method using a ladle type or a torpedo type hot-metal ladle car type vessel, in which in a dephosphorization process using a ladle or a torpedo type hot-metal ladle car type refining vessel, oxygen and at least a part of a refining agent are blown to the molten iron surface by a top-blowing lance, and a powder-containing gas is blown to the molten iron by an immersion lance and/or a nozzle.

The present inventors have conducted studies on a method of more efficiently dephosphorizing molten iron using a ladle type or a mixer type ladle car type refining vessel, and as a result, have found that a method of blowing oxygen and a refining agent to the molten iron surface by a top-blowing lance while blowing a powder-containing gas into the molten iron by an immersion lance or the like is very effective.

In embodiment 2, it is desirable that the amount of oxygen (oxygen feed amount) blown from the top-blowing lance to the molten iron surface be 0.7Nm³Less than/min/ton molten iron. If the amount of oxygen supplied from the top-blowing lance is excessive, slag may be blown out of the refining vessel due to slag foaming. By setting the amount of oxygen supplied from the top-blowing lance to 0.7Nm³Below/min/ton molten iron, slag foaming can be suppressed, and stable operation can be achieved.

In embodiment 2, in addition to the refining agent being blown onto the molten iron surface by the top-blowing lance, a part of the refining agent may be added by being charged from above or by being injected into the molten iron, and in this case, it is also desirable that the amount of the refining agent blown onto the molten iron surface by the top-blowing lance be 80 mass% or more of the total amount of the refining agent. If the proportion of the refining agent added to the molten iron surface by the top-blowing lance is less than 80 mass% of the total amount, the effect of promoting the dephosphorization reaction by blowing the refining agent together with oxygen to the molten iron surface tends to be reduced.

FIG. 4 is a graph showing the amount of refining agent added by a top-blowing lance relative to the total amount of refining agent based on the results of tests conducted by the present inventorsGraph of the ratio of the addition amount to the required lime amount, in this test, for the P content charged into a ladle type vessel (150 tons): 0.10 to 0.11 mass%, Si content: less than 0.07 mass% of molten iron, as oxygen (4.5-5.0 Nm)³One ton of molten iron) as a refining agent of a carrier gas, and lime powder (0 to 6 kg/ton of molten iron) having a particle diameter of 1mm was blown from a top-blowing lance to the molten iron surface, and simultaneously, powder was blown into the molten iron surface through an immersion lance to conduct dephosphorization (treatment time: 15 minutes). The amount of powder blown by the immersion gun was fixed at 90 kg/min. Part or all of the powder used the required residual lime fraction, and the insufficient part used dust (Fe content 40 mass%) or coke powder. In the dephosphorization, no CaF is added into the refining agent₂The amount of the processed slag is below 20 kg/ton molten iron. A lime supply rate B (kg/min/ton molten iron) and an oxygen supply rate A (Nm) to be blown to the molten iron surface³Min/ton of molten iron) was 2.0. The amount of lime added is within the range of the total of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the following expressions (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the following expression (7)) is controlled to be in the range of 200 to 500 mm. The temperature of the molten iron before and after dephosphorization is 1300-1320 ℃. The amount of the slag after treatment is determined by the amount of the added lime and the furnaceAnd (4) calculating the mass balance of the CaO concentration (slag analysis value) in the slag. FIG. 4 shows the amount of lime required to reduce the P content in the treated molten iron to 0.02 mass% or less.

According to fig. 4, as the ratio of the refining agent supplied by the top-blowing lance to the total amount of the refining agent increases, the amount of lime required decreases, and particularly, the amount of lime required decreases to the minimum when the ratio is 80 mass% or more.

The kind of the powder to be blown into the molten iron together with the gas is not particularly limited, and for example, 1 kind or 2 or more kinds of part of a refining agent such as lime powder, dust generated in an iron works such as converter dust, powder mainly containing a carbon source such as coke powder, ironoxide such as sinter ore powder and mill scale, CaCO, etc. may be used₃、Ca(OH)₂、CaMg(CO₃)₂And the like.

When a refining agent such as lime powder is used as the powder, the blown refining agent is heated while floating in molten iron, and the melting of slag is promoted when the slag floats on the molten iron surface.

In addition, the dust generated in iron works can be used to effectively utilize the waste. That is, since the dust is powdery, in order to reuse it, it has been necessary to perform a treatment such as pelletization from the viewpoint of easy use, and in the present embodiment, the powder can be directly reused without requiring labor and cost for pelletization. In addition, the powder mainly containing the carbon source is added carbon to the molten iron, and becomes an effective heat source in the next step. In addition, CaCO₃、Ca(OH)₂、CaMg(CO₃)₂Etc. are thermally decomposed in molten iron to generate gas (CO)₂、H₂O), which contributes to enhancing molten iron stirring, and CaO generated by thermal decomposition has a function as a refining agent. In addition, iron oxide powder becomes a part of the oxygen source in the molten iron.

The type of gas (carrier gas) to be blown into the molten iron together with the powder is not particularly limited, and oxygen (pure oxygen or oxygen-containing gas) or N may be used₂Or inert gas such as Ar. Among them, when the refining agent is blown with oxygen, it is expected that the reaction is accelerated by a so-called transient reaction when floating up in molten iron. However, since oxygen is supplied from the immersion gun or the nozzle, FeO is generated at the tip of the gun or the nozzle, and the life of the gun or the nozzle becomes a problem. In contrast, in useN₂Or Ar, the effect on the reaction cannot be expected, but the life of the gun and nozzle is longer than that of the case of using oxygen. The type of gas used may therefore be selected taking into account the overall cost including gun and nozzle life.

As the apparatus for blowing the refining agent into the molten iron, an immersion lance, a blowing nozzle provided in the refining vessel, or both of them can be used. Any type of nozzle such as a bottom blowing nozzle or a lateral blowing nozzle may be used as the blowing nozzle.

In embodiment 2, when substantially all of the refining agent is added by blowing the refining agent into the molten iron liquid surface by the top-blowing lance and blowing the refining agent into the molten iron by the immersion lance and/or the nozzle, the amount of the refining agent added by the top-blowing lance is preferably 20 to 80 mass% of the total amount of the refining agent. If the proportion of the refining agent to be blown to the molten iron surface by the top-blowing lance exceeds 80 mass% of the total amount of the refining agent, the effect of stirring the molten iron by blowing the refining agent to the molten iron is small, and therefore, it is difficult to obtain stirring power required for the dephosphorization reaction, and if the proportion of the refining agent to be blown to the molten iron surface is less than 20 mass%, the effect of promoting slagging by blowing the refining agent to the molten iron surface cannot be sufficiently obtained.

FIG. 5 is a graph showing the relationship between the ratio of the amount of refining agent added by the top-blowing lance to the total amount of refining agent and the dephosphorization efficiency in the case where all the refining agent is added by blowing the refining agent into the molten iron surface by the top-blowing lance and by blowing the refining agent into the molten iron by the immersion lance and/or the blowing nozzle, based on the results of the test conducted by the present inventors, in which the P content in the charged ladle type vessel (150 tons): 0.10 to 0.11 mass%, Si content: less than 0.07 mass% of molten iron, as oxygen (4.5-5.0 Nm)³One ton of molten iron) as a refining agent of a carrier gas, and a lime powder (0 to 6 kg/ton of molten iron) having a particle diameter of 1mm was blown from a top-blowing lance to the molten iron surface, and a required remaining lime portion (0 to 6 kg/ton of molten iron) was blown by an immersion lance to conduct dephosphorization (treatment time: 15 minutes). In the dephosphorization treatment, CaF is not added into the refining agent₂The amount of the processed slag is below 20 kg/ton molten iron. A lime supply rate B (kg/min/ton molten iron) and an oxygen supply rate A (Nm) to be blown to the molten iron surface³Min/ton of molten iron) was 2.0. The amount of lime added is within the range of the total of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the following expressions (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the following expression (7)) is controlled to be200-500 mm. The temperature of the molten iron before and after dephosphorization is 1300-1320 ℃. The amount of the slag after the treatment was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

According to FIG. 5, the ratio of the refining agent injected into the molten iron surface by the top-blowing lance is greatly decreased in the region of less than 20 mass% and more than 80 mass%.

Fig. 6 is an example of the dephosphorization method applied to the present embodiment when the molten iron dephosphorization is performed in the blast furnace ladle type dephosphorization apparatus. Depending on the Si content of the molten iron tapped from the blast furnace, a desiliconization treatment such as desiliconization in a tapping field of the blast furnace is performed before the dephosphorization treatment if necessary. The dephosphorization is carried out by charging molten iron into a blast furnace ladle 4, injecting lime powder (refining agent) from an immersion lance 5, and blowing the lime powder (refining agent) from a top-blowing lance 2 to the molten iron surface together with oxygen. The feeding speed of the lime powder injected at this time can sufficiently stir the molten iron.

In embodiment 3 of the present invention, the dephosphorization is performed under the condition that the supply rate of the refining agent and the supply rate of the oxygen gas to be blown to the molten iron surface satisfy the following expressions (3) and (4).

(C1/D1)＞(C2/D2)……(3)

C1＞C2……(4)

Wherein, C1: average value of refining agent supply speed (kg/min/ton molten iron) converted into CaO at the early stage of dephosphorization

C2: average value of refining agent supply speed (kg/min/ton molten iron) converted into CaO at the early stage of dephosphorization

D1: average value of oxygen supply rate (Nm) in the early stage of dephosphorization³/min/ton molten iron)

In contrast to the region where the P content in the molten iron is high and the refining agent supply rate is high and the shift of the refining agent into (P) in the slag in which the dephosphorization rate is high dominates the total reaction rate, the P content in the molten iron is low and the shift of[ P]in the metal into the reaction region dominates the total reaction rate in the latter stage of the dephosphorization, so that the proportion of the refining agent effective for the dephosphorization is smaller than in the former stage of the dephosphorization. Therefore, in the above-described specific embodiment, by reducing the ratio of the supply rates of the refining agent and the oxygen gas to be supplied to the molten iron surface (the supply rate of the refining agent/the supply rate of the oxygen gas) and the supply rate of the refining agent in the latter stage of the dephosphorization relative to the former stage of the dephosphorization, the dephosphorization can be efficiently performed with a smaller amount of the refining agent added.

In the method of the present invention, for the above reasons, in order to effectively improve the reactivity of the refining agent, the dephosphorization can be efficiently performed by adding the minimum necessary refining agent at the later stage of the dephosphorization.

FIG. 7 shows that CaF was not added to a converter type dephosphorization refining furnace (300 ton) under the following conditions of ① and ②₂The dephosphorization was conducted, and a graph showing the relationship between the unit consumption of CaO and the dephosphorization efficiency, which is required when the P content in the molten iron after the dephosphorization was 0.012 mass%, was examined.

① the supply rate C of the refining agent blown onto the molten iron surface in terms of CaO (kg/min/ton of molten iron) is fixed over the entire treatment period, and the supply rate C and the oxygen supply rate D (Nm) of the refining agent are set to be constant³/min/ton of molten iron) is fixed throughout the treatment period to conduct dephosphorization treatment.

② average value of refining agent supply rates (kg/min/tonmolten iron) in the former stage of dephosphorization in C1, average value of refining agent supply rates (kg/min/ton molten iron) in the latter stage of dephosphorization in kg/min/ton molten iron in C2, and average value of oxygen supply rates (Nm) in the former stage of dephosphorization in D1³Min/ton molten iron), D2: average value (Nm) of oxygen gas supply rate at the latter stage of dephosphorization³Min/ton molten iron), dephosphorization is performed under the condition of (C1/D1)>(C2/D2).

Further, it is considered that the desiliconization component is 2 CaO. SiO₂Removing desiliconized components, dephosphorizing efficiency η_CaOIs defined by the following formula.

η_CaO＝[{([％P]_i-[％P]_f)/(31×2)}×56×3×10]/[W_Cao-{([％Si]_i-[％Si]_f)/28}×56×2×10]

Wherein, W_Cao: unit consumption of CaO (kg/ton molten iron)

[％P]_i: p content in molten iron before dephosphorizationAmount (mass%)

[％P]_f: p content (mass%) in molten iron after dephosphorization

[％Si]_i: si content (mass%) in molten iron before dephosphorization

[％Si]_f: si content (mass%) in molten iron after dephosphorization

In this test, after the molten iron of the blast furnace was desiliconized in a blast furnace casting house and a ladle as required, the molten iron was desulfurized in the ladle, and the molten iron was transferred to a converter type vessel to be dephosphorized. The P content in the molten iron before dephosphorization is 0.10-0.11 mass%, and the Si content is less than 0.07 mass%. Using only CaF-free refining agents₂The quick lime mainly contains CaO. Oxygen gas is mainly used as the oxygen source, and is added by blowing it from a top-blowing lance to the molten iron liquid surface, and a part of the oxygen source is added together with a solid oxygen source (iron ore). The amount of oxygen supplied to the refiner is 4.6 to 9.0 kg/ton of molten iron and 8.6 to 13.6Nm³Further, regarding the dephosphorization treatment of ①, the C/D is set to 0.50 to 0.69kg/Nm ³② dephosphorization is carried out by adjusting C1 to 0.88 to 1.00 kg/min/ton of molten iron, C2 to 0.30 to 0.39 kg/min/ton of molten iron, and C1/D1 to 0.60 to 0.83kg/Nm³The ratio of C2/D2 is 0.38-0.48 kg/Nm³And (C1/D1) × 56-72% ═ C2/D2. The amount of the processed slag is below 20 kg/ton molten iron. The amount of lime added is within the range of the sum of the lime amount Wcao-P (kg/ton molten iron) and the lime amount Wcao-Si (kg/ton molten iron) defined by the following expressions (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent with oxygen as a carrier gas (L value defined by the following expression (7)) is controlled to be in the range of 200 to 500 mm. The temperature of the molten iron before and after dephosphorization is 1300-1320 ℃. The amount of the slag after the treatment was calculated from the mass balance between the amount of lime added and the CaO concentration in the slag (slag analysis value).

From FIG. 7, it was found that the dephosphorization efficiency was high in ② since CaO consumption was less and dephosphorization efficiency was high in ② compared with ①, since sufficient dephosphorization was possible without adding an extra refining agent in the latter stage of refining, high dephosphorization efficiency was obtained.

In embodiment 3, the desired effects can be obtained by setting the refining agent supply rate and the oxygen supply rate to (C1/D1)>(C2/D2) and C1>C2, and it is particularly preferable to set the refining agent supply rate and the oxygen supply rate to (C1/D1) × 30 to 80% ((C2/D2) and C1 × 30 to 80% ((C2). In the case of (C1/D1) × 30%>(C2/D2) and C1 × 30%>C2, the dephosphorization efficiency tends to be decreased due to insufficient supply of the refining agent, while in the case of (C1/D1) × 80%<(C2/D2) and C1 × 80%<C2, the supply of the extra refining agent in the latter stage of the dephosphorization treatment tends to be increased, and the dephosphorization efficiency tends to be decreased.

In embodiment 3, the refining agent and the oxygen gas are supplied under the above conditions during the dephosphorization (before and after the dephosphorization), and the supply rate of the refining agent and the supply rate of the oxygen gas are changed arbitrarily, and may be changed continuously or in stages, or both.

In embodiment 4 of the present invention, molten iron having an Si content of 0.15 mass% or less is subjected to dephosphorization by blowing oxygen and at least a part of a refining agent to the molten iron surface by a top-blowing lance, and in this dephosphorization, lime is added as the refining agent in an amount of the sum of the lime amount Wcao-P (kg/ton of molten iron) obtained by the following expression (5) and the lime amount Wcao-Si (kg/ton of molten iron) obtained by the following expression (6).

Wcao-P ═ (molten iron [ P]-target [ P]) × (10/62) × 56 × 3/η cao … (5)

Wherein, the molten iron [ P]: p content (mass%) in molten iron before dephosphorization

η cao (lime efficiency) is 0.5-1

Wcao-Si ═ hot metal [ Si]× (10/28) × 56 × 2 …(6)

Wherein, the molten iron [ Si]: si content (mass%) in molten iron before dephosphorization

As described above, in the conventional dephosphorization technique, the amount of the slag is determined in advance in accordance with the phosphorus distribution Lp in order to maintain the slag in a uniform liquid phase, and therefore, the amount of the refining agent is substantially fixed to P, Si in an amount equal to or larger than the necessary refining amount. On the contrary, in the present invention, since the mechanism of direct dephosphorization reaction in the molten iron surface region centered on the fire and the mechanism of P fixation by slag mainly in the solid phase in the outer region are utilized, the dephosphorization reaction can be efficiently performed with the minimum refining amount required as described above.

In practice, the amount of lime consumed for fixing P and Si can be calculated by the following equation. In the following formula, Wcao-Po is the amount of lime (kg/ton of molten iron) consumed for fixing P, and Wcao-Sio is the amount of lime (kg/ton of molten iron) consumed for fixing Si.

Wcao-Po ═ (molten iron [ P]-target [ P]) × (10/62) × 56 × 3

Target [ P]: p content (mass%) in molten iron after targeted dephosphorization

Wcao-Sio ═ hot metal [ Si]× (10/28) × 56 × 2

Assuming Total CaO (kg/ton of molten iron) as the Total lime added, the lime efficiency η CaO contributing to dephosphorization can be calculated by the following equation.

η CaO Wcao-P/(Total CaO-Wcao-Sio)

In the present embodiment, the lime efficiency η CaO is first defined to be 0.5 to 1, and the lower limit of η CaO is defined from the viewpoint that the unnecessary lime is not added and the dephosphorization reaction aimed at in the present invention is suitably caused, that is, when η CaO is less than 0.5, the unnecessary lime is substantially added, and not only the effect of the present invention of effecting the dephosphorization treatment by the refining agent added in a small amount is lost, but also the amount of lime is excessively added with respect to FeO produced at a predetermined oxygen consumption, and a large amount of CaO which is not slagging is present, and such CaO which is not slagging inhibits the progress of the dephosphorization reaction.

Therefore, in this embodiment, lime is added in an amount corresponding to the sum of the lime amount Wcao-P (kg/ton of molten iron) determined by the following equation (5) and the lime amount Wcao-Si (kg/ton of molten iron) determined by the following equation (6).

Wcao-P ═ (molten iron [ P]-target [ P]) × (10/62) × 56 × 3/η cao … (5)

Target [ P]: p content (mass%) in molten iron after targeted dephosphorization

η cao (lime efficiency) is 0.5-1

Wcao-Si ═ hot metal [ Si]× (10/28) × 56 × 2 … (6)

The Wcao-P is obtained by treating P in molten iron as 3 CaO. P with η CaO being 0.5-1₂O₅The amount of lime required for fixation,and the Wcao-Si mentioned above is obtained by making Si in molten iron 2 CaO. SiO₂The required amount of lime is fixed.

FIG. 8 is a graph showing, as an example, the comparison of the amount of lime added in the present embodiment corresponding to the Si content in molten iron in the case where molten iron having a P content of 0.11 mass% is dephosphorized so that the P content becomes 0.015 mass%, with the amount of lime added in the dephosphorization in the conventional method, Wcao-Si being the amount of lime required for Si fixation, Wcao-P₁Amount of lime required to fix P (dephosphorize) at η cao ═ 1, Wcao-P_0.5As shown in this figure, the amount of lime required in the conventional method is determined by the phosphorus distribution Lp and the corresponding required amount of slag, so that the amount of lime of W is required regardless of the Si concentration in the molten iron, whereas the amount of lime added in the present embodiment is [ Wcao-Si + Wcao-P]instead₁]～[Wcao-Si+Wcao-P_0.5]It suffices to significantly reduce the amount of lime added as compared with conventional methods.

FIG. 9 is a graph showing the relationship between the amount of lime required for dephosphorization and the lime efficiency η cao in the present embodiment and the conventional method and the P content in molten iron after dephosphorization, wherein the amount of lime required for dephosphorization in the conventional method is [ W-Wcao-Si]in FIG. 8, and it can be judged from FIG. 9 that dephosphorization can be performed at high lime efficiency by using very small amount of lime for dephosphorization in the present embodiment as compared with the conventional method.

In the present embodiment of the 4 th aspect, it is desired that 80 mass% or more of lime of the lime amount Wcao-P (Wcao-P obtained by η cao ═ 1, the same applies hereinafter) is blown to the molten iron surface by the top-blowing lance, and fig. 10 is a graph showing the relationship between the ratio X/Wcao-P of the lime amount X and the lime amount Wcao-P blown from the top-blowing lance to the molten iron surface and the P content in the molten iron after dephosphorization, based on the test results of the present inventors, in which the P content charged into the converter type vessel (340 tons) is 0.095 to 0.135 mass%, and the Si content is 0.02 to 0.10 mass%, respectively% of molten iron, and oxygen (10-15 Nm)³One ton of molten iron) is used as carrier gas, lime powder (4-10 kg/ton of molten iron) with the particle size of 1mm or less is sprayed to the liquid level of the molten iron through a top blowing gun, and dephosphorization treatment is carried out (treatment time: 10 to 14 minutes), and then charging molten iron into a decarburization converter and conducting decarburization blowing. In the dephosphorization treatment, CaF₂The addition amount is below 1 kg/ton molten iron, and the slag amount after treatment is below 30 kg/ton molten iron. A lime supply rate B (kg/min/ton molten iron) and an oxygen supply rate A (Nm) to be blown to the molten iron surface³Min/ton of molten iron) was 1.7. A refining agent comprising oxygen as a carrier gas is blown to the molten iron to control the depth L of a depression formed in the molten iron surface (L value defined by the following expression (7)) to be within a range of 200 to 500 mm. The temperature of the molten iron before and after dephosphorization is 1300-1320 ℃. The amount of the slag after the treatment was calculated from the mass balance between the amount of lime added and the CaO concentration in the slag (slag analysis value).

According to FIG. 10, when the amount of lime X is less than 80% by mass based on the amount of lime Wcao-P, the dephosphorization efficiency tends to be slightly lowered. This is because it is relatively difficultto obtain the high reaction efficiency as described above by directly charging the refining agent into the oxygen supply region at or near the ignition point of the reaction zone.

Since Si is more easily burned than C and Fe, SiO is formed in molten iron during blowing₂May be present stably and it is therefore not necessary to react it with lime at the fire point. Therefore, it is equivalent toFixed formation of SiO₂The Lime source of the Lime amount Wcao-Si is not limited to quicklime, and may be a material containing unreacted Lime (Free Lime). Therefore, the refining agent corresponding to the lime amount Wcao-Si can be selected from 1 or more of lime powder, lump quicklime, lump limestone, and iron-making slag containing unreacted CaO. For example, converter slag (basicity of about 3 to 4) and ladle slag generated in the decarburization step may be used as the iron-making slag.

In embodiment 4, for the above reasons, the Si content of the molten iron subjected to the dephosphorization is 0.15 mass% or less, preferably 0.07 mass% or less, more preferably 0.03 mass% or less, in order to obtain high dephosphorization efficiency when a small amount of refining agent is added. When the Si content in the molten iron exceeds 0.15 mass%, the effect of reducing the amount of the refining agent added in the present embodiment is reduced.

In embodiment 5 of the present invention, the depth L of the depression formed in the molten iron surface by blowing oxygen or blowing a refining agent using oxygen as a carrier gas, which is defined by the following formula (7), is controlled to be 200 to 500 mm.

L＝L₀×exp{(-0.78×L_H)/L₀}……(7)

L₀＝63×{(F₀₂/n)d_t}^2/3

Wherein L is_H: gun height of top blowing gun (mm)

F₀₂: velocity of oxygen supply (Nm) by top-blowing lance³/hr)

n: number of nozzle holes of top-blowing gun

d_t: nozzle aperture (mm) of top-blowing gun (wherein the average aperture of all nozzle holes in the case where the nozzle apertures of the plurality of nozzle holes are different)

In order to obtain high dephosphorization efficiency with a small amount of added refining agent by focusing attention on the mechanism of dephosphorization reaction aimed at in the present invention, it is particularly preferable to use a method of supplying oxygen to the hot spot as the reaction zone, and specifically, to control the depth of the cavity (theoretical cavity depth calculated from the oxygen supply rate, the top-blowing lance structure and the conditions of use) generated in the molten iron surface to an optimum range by blowing oxygen or blowing oxygen and refining agent.

In this case, if the depth of the depression formed in the molten iron surface by the injected oxygen or the injected oxygen and the refining agent is too small, that is, if the injection of the oxygen or the oxygen and the refining agent is too weak, slag foaming occurs outside the ignition point, and the foamed slag interferes with the flow of the oxygen jet flow, so that the supply of oxygen to the ignition point is reduced, which is a condition that is not favorable for improving the dephosphorization efficiency. Further, since the supply of oxygen to the fire becomes unstable, oxygen required for dephosphorization cannot be stably supplied, variation in dephosphorization efficiency becomes large, and 3 CaO. P₂O₅Phosphorus return occurs by decomposition.

On the other hand, if the depth of the depression formed in the molten iron surface by the blown oxygen or the blown oxygen and the refining agent is too large, that is, if the blowing of the oxygen or the blown oxygen and the refining agent is too strong, the oxygen density in the fire is too high, and P corresponding to the FeO produced cannot be sufficiently supplied from the metal. As a result, the extra FeO is decarburized, which also becomes a disadvantageous condition for improving the dephosphorization efficiency.

The depth L of the cavity formed in the molten iron surface by blowing oxygen or blowing a refining agent using oxygen as a carrier gas can be defined by the following equation (7).

L＝L₀×exp{(-0.78×L_H)/L₀}……(7)

L₀＝63×{(F₀₂/n)/d_t}^2/3

Wherein L is_H: gun height of top blowing gun (mm)

F₀₂: velocity of oxygen supply (Nm) by top-blowing lance³/hr)

n: number of nozzle holes of top-blowing gun

d_t: is the nozzle aperture (mm) of a top-blowing gun (wherein the average aperture of all nozzle holes in the case where the nozzle apertures of a plurality of nozzle holes are different)

In the present embodiment, the recess depth of the molten iron surface is controlled to be 200 to 500mmAnd (4) dephosphorization treatment. Fig. 11 is a graph showing the relationship between the depression depth L of the molten iron surface and the dephosphorization efficiency and the P content in molten iron after dephosphorization on the basis of the test results by the present inventors, in which for the P content charged into the converter type vessel (340 tons): 0.095 to 0.135 mass%, Si content: 0.02 to 0.15 mass% of molten iron, and oxygen (10 to 15 Nm)³One ton ofmolten iron) as a carrier gas, and lime powder (4-10 kg/ton of molten iron) having a particle size of 1mm or less is injected into the molten iron liquid surface by a top blowing lance to perform dephosphorization treatment (treatment time: 10 to 14 minutes), and then charging molten iron into a decarburization converter and conducting decarburization blowing. In the dephosphorization treatment, CaF₂The addition amount is below 1 kg/ton molten iron, and the slag amount after treatment is below 30 kg/ton molten iron. A lime supply rate B (kg/min/ton molten iron) and an oxygen supply rate A (Nm) to be blown to the molten iron surface³Min/ton of molten iron) was 1.7. The lime addition amount is within the total range of the lime amount Wcao-P (kg/ton molten iron) and the lime amount Wcao-Si (kg/ton molten iron) defined by the above equations (5) and (6). In addition, the temperature of the molten iron before and after dephosphorization is 1300-1320 ℃. The amount of the slag after the treatment was calculated from the mass balance between the amount of lime added and the CaO concentration in the slag (slag analysis value).

According to FIGS. 11(a) and (b), compared with the case where the depth L of the recess is in the range of 200 to 500mm, the dephosphorization efficiency is lowered by the above-mentioned reasons in the range of less than 200mm and more than 500mm, and the P content in the treated molten iron tends to be high.

In embodiment 6 of the present invention, CaF is added to molten iron having an Si content of 0.15 mass% or less, preferably 0.07 mass% or less, more preferably 0.03 mass% or less₂The addition amount is below 1 kg/ton molten iron or CaF is not substantially added₂(that is, CaF is not added except for inclusion unavoidably contained in the refining agent₂) Under the condition of (1), blowing oxygen and at least a part of refining agent to the molten iron liquid surface by a top blowing gun to carry out dephosphorization treatment, and simultaneously enabling the molten iron temperature to reach 1360-1450 ℃ when the dephosphorization treatment is finished.

The dephosphorization reaction is caused by the oxidation reaction of P, and the conventional ironThe fact that the water temperature is advantageous at low temperatures is common knowledge, and the problem of phosphorus return from slag to metal has been considered in the conventional treatment with high molten iron temperatures. Therefore, it has been considered difficult to reduce the P content in the molten iron to a low level by dephosphorizing at a high temperature of 1360 ℃ or higher. In contrast, the inventors confirmed that the Si content of the molten iron subjected to dephosphorization in the process of the present invention was sufficiently reduced and that CaF was added₂In small or no addition of CaF₂The dephosphorization is performed under the conditions of (1) and, even if the high-temperature treatment is performed, the return of phosphorus from the slag to the metal hardly occurs, and a high dephosphorization reaction efficiency can be obtained. The reason why the rate of phosphorus return is small even when the high-temperature treatment is carried out is considered to be that the refining agent is supplied to the molten iron surface region where a large amount of FeO is generated by the oxygen gas in the method of the present invention, and the contact area between CaO (refining agent) and FeO is greatly increased as compared with the method of charging the lump lime from above, and thus P oxidized by FeO is formed₂O₅The efficiency and rate of reaction with CaO become higher, and the time for melting the CaO-FeO melt can be shortened. That is, it is considered that the reason is that the dephosphorization reaction is instantaneously completed, and the rate of returning phosphorus may becomesmall because the slag melting time thereafter is short.

FIG. 12 shows a converter type vessel (300 ton) without CaF₂The influence of the temperature of molten iron (molten iron temperature at the end of dephosphorization) and the Si content in molten iron before dephosphorization on the dephosphorization efficiency (dephosphorization lime efficiency) was investigated. The dephosphorizing lime efficiency shown in FIG. 12 is a ratio of lime useful for dephosphorization to the total lime (quicklime) added as a refining agent, and is a ratio of phosphorus oxide to 3 CaO. P₂O₅Fixed as the precursor, derived from the stoichiometric ratio.

In this test, after the molten iron of the blast furnace was desiliconized in a blast furnace casting house and a ladle as necessary, the molten iron was desulfurized in the ladle, and the molten iron was transferred to a converter type vessel to be dephosphorized, and at this time, the Si content of the molten iron to be dephosphorized and the temperature of the molten iron after the treatment were variously changed. The P content in the molten iron before dephosphorization is 0.10-0.11 mass%, the Si content is less than 0.15 mass%, and the P content in the molten iron is less than 0.02 mass% by dephosphorization.

Use of only CaF-free refining agents₂The quick lime mainly contains CaO. In addition, oxygen gas is mainly used as the oxygen source, and is added by blowing it from a top-blowing lance to the molten iron liquid surface, and a part of the oxygen source is added together with a solid oxygen source (iron ore). The amount of oxygen other than desiliconization is controlled to 10 to 11Nm³Range per ton of molten iron. In addition, the time of dephosphorization is set to be 10-11 minutes, the temperature of molten iron before dephosphorization and the addition amount of waste materials are adjusted,and the temperature of molten iron after dephosphorization is controlled. The amount of the processed slag is below 30 kg/ton molten iron.

In FIG. 12, ○ represents a test example (a) in which lime is charged from above and added while the molten iron temperature after dephosphorization is 1260 to 1350 ℃, ▲ represents a test example (B) in which lime (lime powder having a particle size of 1mm or less) is blown to the molten iron surface while the oxygen is used as a carrier gas and the molten iron temperature after dephosphorization is 1360 to 1450 ℃, the amount of lime added is varied in the range of 5 to 10 kg/ton of molten iron in accordance with the Si content in the molten iron, and in the test example (B), the lime supply rate B (kg/min/ton of molten iron) blown to the molten iron surface and the oxygen supply rate A (Nm/min/ton of molten iron) blown to the molten iron surface are set to the respective supply rates³Min/ton of molten iron) was 1.7. The lime addition amount is within the total range of the lime amount Wcao-P (kg/ton molten iron) and the lime amount Wcao-Si (kg/ton molten iron) defined by the above-mentioned expressions (5) and (6). Further, the depth L of the depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. The amount of the processed slag is calculated by the mass balance of the amount of the added lime and the concentration of CaO in the slag.

According to FIG. 12, the lower the Si content in the molten iron, the more CaO is consumed at 2 CaO. SiO irrespective of the method of supplying lime and the temperature of the molten iron at the time of completion of the dephosphorization₂The lower the above ratio, the higher the dephosphorization efficiency, but the more the dephosphorization effect was improved by injecting lime together with oxygen into the molten iron surface, as compared with the case where the molten iron temperature at the end of dephosphorization was 1260 to 1350 ℃ in the method of charging lime from above and adding it (test example (a))Temperature of waterThe dephosphorization and liming efficiency is improved under the condition of 1360-1450 ℃ (test example (b)). Further, such an effect is more remarkable as the Si content in the molten iron is lower. The reason why the dephosphorization reaction is advantageous in view of the average temperature is considered to be that the rate of phosphorus return is decreased in the test example (b) due to the meltability of the slag and the fixation of the dephosphorization product in the results of FIG. 12.

FIG. 13 is a view showing CaF in a method of blowing a refining agent together with oxygen gas to the molten iron surface₂The influence of the amount of the refining agent on the dephosphorization efficiency (dephosphorization lime efficiency) was shown in the same manner as in the test example (b) of FIG. 12, except that the converter type vessel used in the test of FIG. 12 was used, and the manner of addition of the refining agent and the oxygen source, the amount of the refining agent, the treatment time, and the like were also shown in the same manner as in the test example (b) of FIG. 12. In addition, the temperature of the molten iron is 1360-1450 ℃ after the dephosphorization treatment is finished. The CaF is added in its entirety by charging from above at the beginning of the blowing₂. The amount of the processed slag was determined to be 30 kg/ton of molten iron.

According to FIG. 13, CaF₂When the addition amount is less than 1 kg/ton of molten iron, the dephosphorization and lime efficiency is improved. CaF₂Has CaO melting promoting effect, and is prepared by adding CaF₂The liquid phase ratio of the slag is increased. However, whenthe treatment temperature (molten iron temperature) is 1360 ℃ or higher, CaF is added₂If the slag liquid phase ratio is increased, the phosphorus return rate from the slag to the metal is likely to be increased and approach the equilibrium value, and therefore, the dephosphorization efficiency is deteriorated. Therefore, in order to increase the dephosphorization efficiency by setting the treatment temperature (molten iron temperature) to 1360 ℃ or higher, it is necessary to use CaF₂The amount of the additive is kept to a minimum (not more than 1 kg/ton of molten iron or substantially not added).

When the molten iron temperature at the end of the dephosphorization treatment exceeds 1450 ℃, the effect of increasing the P concentration value in the molten iron in equilibrium with the slag is greater than the effect of melting CaO in the molten iron at a high temperature. Therefore, the temperature of the molten iron at the end of the dephosphorization should be 1450 ℃ or lower.

From the above results, it is apparent that CaF is added to molten iron having an Si content of 0.15 mass% or less, preferably 0.07 mass% or less, more preferably 0.03 mass% or less₂The addition amount is below 1 kg/ton molten iron or substantially not addedThe dephosphorization is performed under the condition of (1), and even if the temperature of molten iron at the end of the dephosphorization is at a high temperature of 1360-1450 ℃, the dephosphorization can be performed with high dephosphorization efficiency.

In the present embodiment, since a high molten iron temperature can be ensured at the end of the dephosphorization, the residual heat in the subsequent step can be sufficiently ensured. In addition, because the temperature of the treated molten iron is high, the T.Fe in the slag can be suppressed to be low, and the yield of the dephosphorized iron can be improved.

In general, the temperature of the molten iron before the dephosphorization is about 1250 to 1350 ℃, and the temperature of the molten iron at the time of completion of the dephosphorization is adjusted, and in the case of the dephosphorization using a converter type dephosphorization refining furnace for dissolving the scrap, a method of controlling the amount of the scrap to be charged may be mentioned. In addition, in the case of dephosphorization using a ladle type vessel such as a ladle or a hot metal mixer type vessel, a method of adjusting the amount of solid oxygen source to be charged into sintered powder or the like may be mentioned. Therefore, the temperature of the molten iron at the end of the treatment can be adjusted to be 1360-1450 ℃ by the method.

Further, as a specific method for controlling the temperature of molten iron at the end of dephosphorization, a method of calculating the temperature of molten iron during dephosphorization from the gas component analysis value of the exhaust gas generated during dephosphorization and the temperature of the exhaust gas is most easily performed based on this. That is, in this method, the exhaust gas is subjected to gas composition analysis to determine CO and CO₂The concentration and the amount of gas produced are calculated from the temperature of the exhaust gas. Then, the calorific value in the furnace is calculated, and the molten iron temperature can be calculated on the basis of the calorific value.

In embodiment 7 of the present invention, a substance that absorbs heat of molten iron due to a chemical reaction and/or a thermal decomposition reaction is supplied to the molten iron surface region to which oxygen is supplied.

In the molten iron surface area where oxygen is blown, a large amount of iron oxide is generated by oxygen striking the molten iron surface, which is a very favorable condition for promoting slagging of the refining agent. On the other hand, however, high temperature regions are formed in the region of the liquid surface (in particular the fire point) on which the oxygen impinges as a result of oxidation reactions, which are advantageous for melting the lime, but have an adverse effect in terms of dephosphorization balance.

In view of the above problem, the present inventors have studied a method for making the liquid surface area of the molten iron to which oxygen is supplied a temperature condition favorable for the dephosphorization reaction, and as a result, have found that by supplying a substance that absorbs the heat of the molten iron by a chemical reaction and/or a thermal decomposition reaction to the liquid surface area of the molten iron to which oxygen is supplied, the effect of promoting the slagging of the refining agent by oxygen is not inhibited, and the temperature rise of the liquid surface area of the molten iron to which oxygen is supplied is appropriately suppressed, and a higher dephosphorization reaction efficiency can be obtained.

Since a substance (hereinafter referred to as "endothermic substance") that absorbs heat of molten iron due to a chemical reaction and/or a thermal decomposition reaction is added (supplied) to the molten iron surface, it is possible to suppress excessive rise in the molten iron temperature due to heat generation caused by supply of oxygen to the molten iron surface, and therefore it is necessary to supply the endothermic substance to the molten iron surface region to which oxygen is supplied. It is also desirable to provide oxygen to a region of molten iron surface where oxygen is supplied, particularly a region called "fire point" which is generated in molten iron by blowing oxygen with a top-blowing lance. This ignition point is a region of the molten iron liquid surface where the highest temperature is formed by the impact ofthe oxygen jet, and is a region where the oxidation reaction (reaction to generate FeO) by the oxygen is concentrated and stirring by the oxygen jet is caused, and it can be said that the most significant effect of adding the endothermic substance is obtained.

The endothermic substance is not particularly limited as long as it can extract heat from molten iron by chemical reaction, thermal decomposition reaction or both reactions when added to molten iron. Therefore, the heat absorbing substance can be gas or solid.

Examples of the gas usable as the endothermic substance include carbon dioxide, water vapor, and Nitrogen Oxide (NO)_X) Etc., 1 or more of them may be used. These gaseous endothermic substances are supplied to the molten iron surface to react mainly with Fe (for example, in the case of 、 ) At this time, the molten iron absorbs heat. As a result, Fe is oxidized by oxygen ( ) The resulting heat generation, on the contrary, becomes an overall heat absorption or a substantial reduction in the amount of heat generation. Among the above-mentioned gas heat-absorbing substances, carbon dioxide and water vapor which are generated in large amounts in steel works are easily available, and are particularly suitable for use because of their high heat effect. Further, even if nitrogen is mixed into these gases, the purity is lowered to a greater or lesser extent, but since the dephosphorization is not performed at the final stage of the steel making, there is no particular problem. CO and H formed by reduction of carbon dioxide and steam supplied thereto₂The heat quantity of the exhaust gas can be increased by recovering a part of the exhaust gas during the dephosphorization.

Examples of the solid usable as the endothermic substance include metal carbonates and metal hydroxides, and particularly, carbonates and hydroxides of alkali metals and alkaline earth metals are preferable, and 1 or more kinds thereof can be selected and used. The solid endothermic substances are supplied to the molten iron surface to mainly cause thermal decomposition reaction, and at this time, the molten iron absorbs heat and CO is generated by thermal decomposition₂Or H₂O, this CO₂Or H₂O also functions as the heat absorbing substance, and therefore, a particularly high heat absorbing effect can be obtained. As the carbonate of such a metal, CaCO is exemplified₃、CaMg(CO₃)₂、MgCO₃、Na₂CO₃、FeCO₃、MnCO₃、NaHCO₃(sodium hydrogencarbonate), and the like, and Ca (OH) may be mentioned as a metal hydroxide₂、Mg(OH)₂、Ba(OH)₂、Al(OH)₃、Fe(OH)₂、Mn(OH)_n、Ni(OH)_nEtc., from which 1 or more kinds can be selected for use.

In addition, CaCO is contained in these solid heat-absorbing substances₃、Ca(OH)₂、CaMg(CO₃)₂The above-mentioned CaO is not only easily available, but also generated by thermal decomposition, and is particularly preferably used because it has an advantage of functioning as a refining agent. Typically, these solid heat-absorbing materials are not firedOr semi-burned limestone and dolomite.

Further, since thermal decomposition does not proceed rapidly when the particle size of the solid endothermic substance is too large, it is preferable that the particle size is a powder having an average particle size of 5mm or less.

The gas endothermic substance and the solid endothermic substance may be used together, or the gas endothermic substance may be used as a part or all of the carrier gas when the solid endothermic substance is supplied to the molten iron surface.

The method of adding the endothermic substance (gas and/or solid) is not particularly limited, and the endothermic substance may be added by a top-blowing lance or other lance by blowing the endothermic substance onto the molten iron surface, or by charging the endothermic substance from the upper surface (charging the endothermic substance by a discharger or the like in the case of a solid endothermic substance), but in order to reliably supply the endothermic substance to the molten iron surface region (particularly, the "ignition point") where oxygen is supplied, it is desirable to supply the endothermic substance to the molten iron surface by a lance, and it is particularly desirable to supply the endothermic substance to the molten iron surface by a top-blowing lance.

When the endothermic material is supplied to the molten iron surface by the top-blowing lance, ① may be used to mix the endothermic material with oxygen (in the case of a solid endothermic material, oxygen is used as a carrier gas) and supply the mixture to the molten iron surface from the same lance hole, ② may be used to supply the endothermic material and oxygen to the lance through different gas supply lines and supply the mixture to the molten iron surface from separate lance holes (in the case of a solid endothermic material, a carrier gas other than oxygen may be used for supplying the endothermic material).

The method ① is more preferable from the viewpoint of reliably supplying the endothermic substance to the molten iron surface area where oxygen is supplied, but the method ② may be a method in which the endothermic substance supplied through a predetermined lance hole is supplied to the molten iron surface area where oxygen is supplied through another lance hole₂Or an inert gas such as Ar, and as described laterThe gas can be heated to absorb heat (such as CO)₂) Used as carrier gas.

In the method ①, among the plurality of lance holes, only oxygen may be supplied from a part of the lance holes, and oxygen mixed with the endothermic substance (and optionally a refining agent) may be supplied to the molten iron surface from another lance hole.

Further, in any of the

methods

① and ②, the carrier gas other than oxygen or oxygen, or the refining agent alone or in combination with the endothermic substance (gas and/or solid) may be mixed with the gaseous endothermic substance and supplied to the molten iron surface.

The endothermic substance (gas and/or solid) or the endothermic substance and the refining agent are supplied to the molten iron surface by the top-blowing lance in a state where the endothermic substance and the oxygen are mixed, and for example, the endothermic substance may be supplied to a part or all of an oxygen supply line (a manifold, a pipe, an oxygen passage in the lance, etc.) of the top-blowing lance and mixed with the oxygen.

Other supply means (e.g., other lances) other than the top-blowing lance may be used to supply the endothermic substance (gas and/or solid) or the endothermic substance and the refining agent to the molten iron surface. The top-blowing lance other than the top-blowing lance may be a lance capable of supplying the powdery material at a fixed position in the furnace as in the top-blowing lance, and a sampling lance or the like for sampling, temperature measurement or the like may be used as long as the cooling capacity in the furnace is not a problem. Further, the durability of the device such as a discharger and an inflow device which are charged from above at high temperature and the accuracy of the charging position are not problematic.

The dephosphorization reaction can be most effectively promoted by blowing (charging) oxygen or another carrier gas into the molten iron surface region to which oxygen is supplied (particularly, the "hot spot" region) and directly supplying a heat-absorbing substance into this region. In this case, a method of blowing oxygen gas, a refining agent, and a heat absorbing substance (gas and/or solid) discharged from the top-blowing lance to the molten iron surface in a mixed state may be employed, and in addition, for example, oxygen gas may be supplied to the molten iron surface from some of the plurality of lance holes of the top-blowing lance, and the refining agent and the heat absorbing substance (gas and/or solid) may be supplied to the molten iron surface from other lance holes as necessary by using oxygen gas or a gas other than oxygen gas (for example, inert gas such as nitrogen gas and Ar) as a carrier gas. In this case, it is particularly preferable to use a top-blowing lance having a main lance hole at the center of the lance and a plurality of sub lance holes around the main lance hole, and to supply oxygen from the sub lance holes to the molten iron surface, and to supply a refining agent and a heat absorbing material (gas and/or solid) from the main lance hole to the molten iron surface by using oxygen or a gas other than the oxygen as a carrier gas as required. In addition, different top blowing guns can be used for blowing oxygen and blowing the refining agent and the heat-absorbing substance. However, in order to slag the refining agent most efficiently in each case, it is particularly desirable to blow the refining agent and endothermic substances (gas and/or solid) together with oxygen to the molten iron surface.

FIGS. 14(a) to (e) show some examples of a method of supplying oxygen, a refining agent and a heat absorbing material to the molten iron surface using a top-blowing lance. Wherein FIG. 14(a) is a view showing a manner in which oxygen, a refining agent and a heat absorbing substance (gas and/or solid) are supplied (blown to the molten iron level) from a lance hole after being mixed; FIG. 14(b) is a view showing a manner in which oxygen and a refining agent are supplied from a part of the lance holes and oxygen and a heat absorbing substance (gas and/or solid) are supplied (blown to the molten iron surface) from the other lance holes, respectively; FIG. 14(c) is a view showing a manner in which a carrier gas and a refining agent other than oxygen are supplied from a part of the lance holes and oxygen and a heat absorbing substance (gas and/or solid) are supplied (blown to the molten iron surface) from the other lance holes, respectively; FIG. 14(d) is a view showing a manner in which a gas endothermic substance and a refining agent are supplied from a part of the lance holes, and oxygen and an endothermic substance (gas and/or solid) are supplied from the other lance holes (blown to the molten iron surface), respectively; fig. 14(e) shows a mode in which oxygen and a refining agent are supplied from a part of the lance holes, and a gas endothermic substance or a gas endothermic substance and a solid endothermic substance are supplied from the other lance holes (blown to the molten iron surface). The manner of supplying the oxygen, the refining agent and the endothermic substance to the molten iron level is not limited thereto.

CaCO in the solid heat absorbing material₃、Ca(OH)₂、CaMg(CO₃)₂CaO is generated by thermal decomposition, and the CaO functions as a refining agent, so that the CaO is obtained in the present embodimentIn an embodiment, the solid endothermic substance may be provided in place of a part or all of a CaO-based refining agent (mainly quicklime), and dephosphorization may be performed by using CaO generated from the substance as a substantially refining agent. That is, in this case, instead of a part or all of the CaO series refining agent, a refining agent-forming substance is used, and the substance is removed from CaCO as an endothermic substance that absorbs the heat of molten iron by a chemical reaction and/or a thermal decomposition reaction₃、Ca(OH)₂、CaMg(CO₃)₂At least 1 kind of substances (hereinafter referred to as "refining agent producing/endothermic substances") are selected and supplied to the molten iron surface area to which oxygen is supplied.

In this method, the refining agent supplied to the molten iron surface is thermally decomposed to generate and absorb the heat of molten iron, and CaO and CO are generated as the refining agent and the endothermic material by thermal decomposition₂Or H₂O, this CO₂Or H₂The reaction of O with Fe has an advantage of absorbing heat of molten iron, and has an effect similar to the effect of supplying both a CaO refining agent and a heat absorbing substance to the molten iron surface area to which oxygen is supplied, and as a result, a high dephosphorization efficiency can be obtained.

In this case, the refining agent-forming/endothermic substance is preferably supplied to a region called "ignition point" formed by oxygen supply using a top-blowing lance in the molten iron surface region to which oxygen is supplied, for the same reason as described above.

The refining agent-forming/endothermic substance is generally added as unbaked or semi-baked limestone or dolomite. If the particle size of the endothermic substance generated by the refining agent is too large, thermal decomposition or the like cannot proceed rapidly, and therefore, it is preferable that the particles be in the form of powder having an average particle size of 5mm or less.

The refining agent generating/heat-absorbing substance may be used togetherwith the gas heat-absorbing substance described above, or a gas heat-absorbing substance may be used as part or all of the carrier gas when the refining agent generating/heat-absorbing substance is supplied to the molten iron surface.

The method of adding the refining agent-forming/endothermic substance is not particularly limited, and the refining agent-forming/endothermic substance may be added by blowing it to the molten iron surface with a top-blowing lance or another lance, by loading it from the top (by a tripper or the like), or the like.

When the refining agent producing/endothermic substance is supplied to the molten iron surface by the top-blowing lance, ① may be used to mix the refining agent producing/endothermic substance with oxygen (using oxygen as a carrier gas) and supply the mixture to the molten iron surface from the same lance hole, ② may be used to supply the refining agent producing/endothermic substance and oxygen into the lance through separate gas supply lines and supply the mixture to the molten iron surface from the respective lance holes (using a carrier gas other than oxygen for supplying the refining agent producing/endothermic substance).

The method ① is preferably employed from the viewpoint of surely supplying the refining agent generating/endothermic substance to the molten iron surface area to which oxygen is supplied, but the method ② may be adapted such that the refining agent generating/endothermic substance supplied through a predetermined lance hole is supplied to the molten iron surface area to which oxygen is supplied through another lance hole₂Or an inert gas such as Ar, or a gas heat-absorbing substance (e.g., CO)₂) Used as carrier gas.

In the method ①, among the plurality of lance holes of the top-blowing lance, oxygen may be supplied only to the molten iron surface from some of the lance holes, oroxygen mixed with the refining agent producing/endothermic substance may be supplied to the molten iron surface from other lance holes.

The refining agent producing/endothermic substance may be supplied to the molten iron surface by the top-blowing lance in a state where the refining agent producing/endothermic substance is mixed with oxygen, for example, by supplying the refining agent producing/endothermic substance to a part or all of an oxygen supply line (a header, a pipe, an oxygen passage in the lance, etc.) of the top-blowing lance and mixing the refining agent producing/endothermic substance with oxygen.

Further, the refining agent producing/endothermic substance may be supplied to the molten iron surface by a supply method other than the top-blowing lance (for example, other lance). The lance other than the top-blowing lance may be a lance capable of supplying the powdery material at a fixed position in the furnace as in the top-blowing lance, and a sampling lance or the like for sampling, temperature measurement or the like may be used as long as the cooling capacity in the furnace is not a problem. Further, the durability of the device such as a discharger and an inflow device which are charged from above at high temperature and the accuracy of the charging position are not problematic.

The oxygen gas to be supplied to the refining agent-forming/endothermic substance may be pure oxygen or an oxygen-containing gas.

The above-described 1 st to 7 th embodiments of the method of the present invention may be used alone, or 2 or more embodiments (however, the 2 nd embodiment is limited to the case of using a ladle type or a mixer type ladle car type vessel as the refining vessel) may be combined and carried out, and it goes without saying that the effect of the method of the present invention is better as the combination conditions are more increased.

As described above, the method of the present invention can effectively conduct dephosphorization treatment with a minimum amount of refining agent to be added, and has an advantage that the properties and state of the produced slag are mainly solid phase, so that the loss of slag can be appropriately prevented at the time of tapping after the treatment.

In order to improve the efficiency of dephosphorization reaction after dephosphorization, it is important to increase the phosphorus concentration in the slag so that the slag does not flow out together with the metal during tapping after dephosphorization (particularly during tapping from a refining vessel having a tap hole such as a converter-type vessel). That is, when the P content in the molten iron after the dephosphorization is performed with the phosphorus allocation Lp of about 200 and the P content in the treated molten iron is about 0.015 mass% (predetermined value: 0.020 mass%), when about 5 kg/ton of molten iron is tapped, phosphorus is carried to the decarburization blowing converter as much as 0.015 mass%, and therefore lime for dephosphorization is required in the decarburization blowing converter. However, this does not achieve the original purpose of the molten iron pretreatment. It is therefore important to prevent the slag from flowing out to bring the dephosphorized slag to the next process.

As methods for minimizing the outflow of slag into the following steps after the dephosphorization treatment in the converter type vessel, there are (1) a slag cutting technique in tapping from the converter type vessel, (2) a method for reducing the fluidity of slag by controlling the slag components after the treatment, and (3) a method for removing slag (deslagging) from a ladle after tapping.

However, these conventional methods have problems that the loss of slag cannot be stably prevented, the cost of using consumables is increased, the temperature of molten iron is lowered by the time for operation, and the yield of iron is lowered by removing slag.

On the contrary, according to the method of the present invention, since the surface region of the molten iron, which forms the fire center, is formed as described above and is formed mainly in the solid phase by being pushed out in sequence to the outer region thereof, the fluidity of the slag at the end of the dephosphorization is extremely small as compared with the slag produced by the conventional dephosphorization, and as a result, the slag can be effectively prevented from flowing out at the tapping after the end of the dephosphorization (particularly at the tapping from the refining vessel having the tap hole such as the converter type vessel). Furthermore, as described above, in the absence of CaF₂Or let CaF₂The amount of (B) is 1 kg/ton or less, and the effect of suppressing the increase in slag fluidity can be further improved.

The mechanism of preventing the slag from flowing out when the slag produced by the method of the present invention is tapped will be described below by comparing it with the slag produced by the conventional method. Fig. 15 shows the state of slag/metal at the start of tapping in a converter-type vessel. In the case of the conventional method shown in FIG. 15(a), the method utilizes the reduction of the slag basicity or the addition of a large amount of CaF₂The slag is fully melted, the slag foams and the slag thickness is increased. Therefore, after tilting the furnace during tapping, slag initially passes through the tap hole, and slag outflow inevitably occurs. In contrast, in the case of the method of the present invention shown in FIG. 15(b), since the slag is present mainly in a solid phase, the slag thickness is very thin, and the slag outflow occurring at the start of tapping is a negligiblelevel.

FIG. 16 shows the state of slag/metal in the vicinity of the taphole at the end of tapping. In the conventional method shown in fig. 16(a), the molten slag on the metal is entrained into the eddy current and flows out. In contrast, in the case of the method of the present invention shown in fig. 16(b), since the slag is mainly in a solid phase, the slag is hardly entrained in the eddy current of the metal because the slag interferes and is integrated with the eddy current of the metal.

Examples

[ example 1]

Molten iron tapped in a blast furnace was subjected to desiliconization in a tapping yard or a ladle as needed, followed by desulfurization in the ladle with mechanical stirring, and thereafter dephosphorization was carried out in a converter type vessel (300 tons). The molten iron before dephosphorization comprises the following components: 4.5 to 4.7 mass%, Si: 0.01 to 0.28 mass%, Mn: 0.15 to 0.25 mass%, P: 0.10 to 0.11 mass%, S: 0.001 to 0.003 mass%. As a refining agent for dephosphorization, lime powder having a particle size of 1mm or less was used, and this was blown to the molten iron surface by a lance with oxygen as a carrier gas. No CaF is added in refining agent₂. The blowing time is fixed at 10 minutes, and 0.05-0.15 Nm is provided from the furnace bottom for stirring molten iron³Min/ton molten iron. The unit consumption of lime and oxygen varies according to the Si content in the molten iron, and the desiliconized portion of lime and oxygen together (2 calcium silicate: formation of 2 CaO. SiO₂Stoichiometric portion of) of molten iron, respectively, set to a fixed value of 3.5 kg/ton of molten iron, 9Nm³Per ton of molten iron. A lime supply rate B (kg/min/ton molten iron) and an oxygen supply rate A (Nm) to be blown to the molten iron surface³Min/ton of molten iron) was 1.7. The amount of lime added is within the range of the total of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. The temperature of molten iron before and after dephosphorization is 1250-1350 ℃. The amount of the processed slag was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

The results and dephosphorization conditions of the examples are shown in Table 1. The average values shown in table 1 are values obtained by averaging 6 to 72ch dephosphorization treatment performed on each of the ranges of slag amounts after the treatments of 5 to 10 kg/ton of molten iron, more than 10 to 20 kg/ton of molten iron, more than 20 to 30 kg/ton of molten iron, more than 30 to 40 kg/ton of molten iron, and more than 40 to 50 kg/ton of molten iron.

TABLE 1

No.	After treatment Amount of slag (kg/T)	Number of tests (ch)	Average Amount of lime (kg/T)	Average Amount of oxygen (Nm³/T)	Molten iron composition (% by mass)					Average Dephosphorization rate (％)	Categories
					Molten iron composition (% by mass)							Before dephosphorization		After dephosphorization
					Average [Si]	Average [P]	Average [P]	Maximum of [P]	Minimum size [P]			Before dephosphorization		After dephosphorization
					Average [Si]	Average [P]	Average [P]	Maximum of [P]	Minimum size [P]			1	5～10	16	5.1	9.3	0.04	0.113	0.008	0.010	0.007	93	Examples of the invention
2	More than 10 to 20	72	6.2	9.5	0.07	0.111	0.011	0.014	0.009	91	Examples of the invention	1	5～10	16	5.1	9.3	0.04	0.113	0.008	0.010	0.007	93	Examples of the invention
2	More than 10 to 20	72	6.2	9.5	0.07	0.111	0.011	0.014	0.009	91	Examples of the invention	3	More than 20 to 30	13	8.1	9.9	0.12	0.112	0.012	0.019	0.009	89	Examples of the invention
4	More than 30 to 40	6	11.7	10.6	0.21	0.110	0.014	0.023	0.010	87	Comparative example	3	More than 20 to 30	13	8.1	9.9	0.12	0.112	0.012	0.019	0.009	89	Examples of the invention
4	More than 30 to 40	6	11.7	10.6	0.21	0.110	0.014	0.023	0.010	87	Comparative example	5	More than 40 to 50	9	13.4	11.0	0.25	0.111	0.015	0.027	0.010	86	Comparative example

[ example 2]

Molten iron tapped in a blast furnace was subjected to desiliconization in a tapping yard or a ladle as needed, followed by desulfurization in the ladle with mechanical stirring, and thereafter dephosphorization was carried out in a converter type vessel (300 tons). The P content before dephosphorization is 0.10-0.11 mass%, and the Si content is less than 0.15 mass%. The temperature of molten iron before and after dephosphorization is 1250-1350 ℃, quicklime mainly containing CaO is used as a refining agent for dephosphorization, powder sieved below 200 meshes is used, and the unit consumption of CaO is 5-15 kg/ton of molten iron according to the content of Si in the molten iron.

In this dephosphorization, the refining agent was blown to the liquid surface by a top-blowing lance using oxygen as a carrier gas, and the refining agent and the oxygen source were supplied (blowing time: 10 minutes) at the oxygen supply rate A (Nm/min)³/min/ton of molten iron) and the feeding speed B of the refining agent (kg/min/ton of molten iron)The operation was carried out under different conditions. For stirring molten iron, 0.05 to 0.15Nm is supplied from a bottom-blowing nozzle at the bottom of the furnace³And blowing nitrogen gas at the flow rate of/min/ton molten iron into the molten iron. No CaF is added in refining agent₂The amount of the slag after the treatment is less than 30 kg/ton of molten iron. The amount of lime added is within the range of the total of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. The amount of the processed slag was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

FIG. 17 shows the oxygen gas supply rate A (Nm)³/min/ton of molten iron) and the feeding speed B of a refining agent (kg/min/ton of molten iron), and the content of P after dephosphorization. As can be seen from the figure, the example of the invention in which A/B is in the region of 0.3 to 7 has the P content in the molten iron after dephosphorization as the target [ P []]The concentration of the silicon compound is 0.015 mass% or less, and particularly, when the Si content in the molten iron before dephosphorization is 0.10 mass% or less, [ P]which is a low P standard can be stably achieved]Less than or equal to 0.010 percent by mass. Furthermore, the example of A/B in the region of 1.2-2.5 can be obtained with particularly low [ P]]The highest dephosphorization efficiency can be obtained in this region.

In contrast, the examples in which the a/B is in the region less than 0.3 and more than 7 do not reach the P content of 0.015 mass% or less which is the target [ P]concentration in the molten iron after the dephosphorization.

[ example 3]

The molten iron discharged from a blast furnace is desiliconized in a tapping site, then the molten iron is put into a ladle and is desiliconized in the ladle, and the ladle is moved to a dephosphorization site after slag is discharged and is dephosphorized.

In dephosphorization, lime powder (refining agent) is blown to the molten iron liquid level by a top-blowing lance with oxygen as a carrier gas, and simultaneously, lime powder is blown into the molten iron by an immersion lance. In addition, in the comparative example, the lime powder was blown into the molten iron by an immersion lance without using a top-blowing lance. The treatment time was set to 20 minutes. The amount of the slag after the treatment is below 20 kg/ton of molten iron. In the present invention example, the amount of lime added is within the range of the total of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. The temperature of molten iron before and after dephosphorization is 1300-1320 ℃. The amount of the processed slag was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

Table 2 shows the results and dephosphorization conditions of the examples.

TABLE 2

No.	Molten iron composition before dephosphorization		Temperature of molten iron		Dephosphorization conditions				After dephosphorization Iron melt [ P ]] (mass%)	Dephosphorization rate (％)	Categories
	Molten iron composition before dephosphorization		Temperature of molten iron		Dephosphorization conditions							[Si] (mass%)	[P] (mass%)	Before dephosphorization (℃)	After dephosphorization (℃)	Amount of refining agent added (kg/T)^*1		CaF₂ Adding amount of (kg/T)	Amount of oxygen (Nm³/T)
	Top blowing gun	Immersion gun														Amount of refining agent added (kg/T)^*1
	Top blowing gun	Immersion gun		1	0.05	0.112	1312	1325								3.9	1.0			-	13	0.005	96	Examples of the invention
2	0.04	0.120	1342	1	0.05	0.112	1312	1325	1330	3.3	1.5	-	10	0.004	97	3.9	1.0	Examples of the invention		-	13	0.005	96	Examples of the invention
2	0.04	0.120	1342	3	0.02	0.115	1338	1320	1330	3.3	1.5	-	10	0.004	97	2.3	1.5	Examples of the invention	-	6	0.007	94	Examples of the invention
4	0.06	0.118	1310	3	0.02	0.115	1338	1320	1328	2.8	2.7	-	7	0.006	95	2.3	1.5	Examples of the invention	-	6	0.007	94	Examples of the invention
4	0.06	0.118	1310	5	0.07	0.116	1348	1332	1328	2.8	2.7	-	7	0.006	95	1.8	4.0	Examples of the invention	0.6	6	0.005	96	Examples of the invention
6	0.02	0.116	1325	5	0.07	0.116	1348	1332	1324	-	3.8	-	15	0.019	84	1.8	4.0	Comparative example	0.6	6	0.005	96	Examples of the invention
6	0.02	0.116	1325	7	0.07	0.113	1343	1325	1324	-	3.8	-	15	0.019	84	-	5.8	Comparative example	-	5	0.026	77	Comparative example
8	0.03	0.120	1330	7	0.07	0.113	1343	1325	1326	-	4.4	0.4	4	0.022	82	-	5.8	Comparative example	-	5	0.026	77	Comparative example
8	0.03	0.120	1330	9	0.04	0.110	1329	1328	1326	-	4.4	0.4	4	0.022	82	-	4.5	Comparative example	1.5	3	0.031	72	Comparative example

^*1 top blowing gun: blowing to the molten iron level

Immersing in a gun: blowing into molten iron

[ example 4]

The molten iron tapped from a blast furnace is desiliconized in a tapping house or a ladle as required, desulfurized in the ladle by mechanical stirring, and then dephosphorized in a converter-type vessel (300 tons). in the dephosphorization, the temperature of the molten iron before and after the dephosphorization is set to 1250 to 1350 ℃³Stirring the molten iron with nitrogen gas for a period of time of one minute/ton of molten iron, and carrying out dephosphorization for 9 minutes. The amount of the processed slag is less than 30 kg/ton molten iron. In the present invention, the lime addition amount is in the range of the total amount of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. From the amount of lime added and the CaO concentration in the slag (slag analysis value)) The mass balance of (2) calculates the amount of slag after treatment.

Table 3 shows the results and dephosphorization conditions of the examples.

TABLE 3

No.	Before dephosphorization Composition of molten iron (mass%)		Conditions of dephosphorization										After dephosphorization Iron melt [ P ]] (mass%)	Dephosphorization rate (％)	Categories
			Conditions of dephosphorization													Refining agent Adding amount of (kg/T)	CaF₂ Adding amount of (kg/T)	Amount of oxygen (Nm³/T)	C1 ^*1	C2 ^*1	D1 ^*1	D2 ^*1	C1/D1 (kg/Nm³)	C2/D2 (kg/Nm³)	Refining agent Addition method^*2
			[Si]	[P]
	1	0.07	[Si]	[P]	0.111	5.8	-	8.6	1.00	0.36	1.21	0.76														0.83	0.47	Blowing of	0.009	92	Examples of the invention
2	1	0.07	0.06	0.105	0.111	5.8	-	8.6	1.00	0.36	1.21	0.76	4.4	-	9.3	0.85	0.30	1.42	0.85	0.60	0.35	Blowing of	0.011	90	Examples of the invention	0.83	0.47	Blowing of	0.009	92	Examples of the invention
2	3	0.03	0.06	0.105	0.103	4.6	-	9.6	0.88	0.33	1.45	0.88	4.4	-	9.3	0.85	0.30	1.42	0.85	0.60	0.35	Blowing of	0.011	90	Examples of the invention	0.61	0.38	Blowing of	0.012	88	Examples of the invention
4	3	0.03	0.04	0.108	0.103	4.6	-	9.6	0.88	0.33	1.45	0.88	5.5	-	9.6	0.91	0.36	1.33	0.85	0.68	0.42	Blowing of	0.008	93	Examples of the invention	0.61	0.38	Blowing of	0.012	88	Examples of the invention
4	5	0.05	0.04	0.108	0.111	5.0	0.2	9.2	0.88	0.39	1.36	0.85	5.5	-	9.6	0.91	0.36	1.33	0.85	0.68	0.42	Blowing of	0.008	93	Examples of the invention	0.65	0.46	Blowing of	0.010	91	Examples of the invention
6	5	0.05	0.01	0.108	0.111	5.0	0.2	9.2	0.88	0.39	1.36	0.85	5.0	0.7	9.1	0.88	0.30	1.28	0.79	0.69	0.38	Blowing of	0.009	92	Examples of the invention	0.65	0.46	Blowing of	0.010	91	Examples of the invention
6	7	0.03	0.01	0.108	0.114	7.9	-	10.2	0.88	0.88	1.42	0.91	5.0	0.7	9.1	0.88	0.30	1.28	0.79	0.69	0.38	Blowing of	0.009	92	Examples of the invention	0.62	0.97	Blowing of	0.015	87	Examples of the invention
8	7	0.03	0.07	0.112	0.114	7.9	-	10.2	0.88	0.88	1.42	0.91	6.7	1.5	10.4	0.91	0.61	1.45	0.91	0.63	0.67	Blowing of	0.011	90	Examples of the invention	0.62	0.97	Blowing of	0.015	87	Examples of the invention
8	9	0.06	0.07	0.112	0.105	10.0	-	13.1	-	-	1.45	-	6.7	1.5	10.4	0.91	0.61	1.45	0.91	0.63	0.67	Blowing of	0.011	90	Examples of the invention	-	-	Jacket	0.015	86	Comparative example
10	9	0.06	0.06	0.108	0.105	10.0	-	13.1	-	-	1.45	-	12.0	0.3	13.6	-	-	1.52	-	-	-	Jacket	0.014	87	Comparative example	-	-	Jacket	0.015	86	Comparative example
10	11	0.02	0.06	0.108	0.113	5.8	1.3	12.8	-	-	1.42	-	12.0	0.3	13.6	-	-	1.52	-	-	-	Jacket	0.014	87	Comparative example	-	-	Jacket	0.039	65	Comparative example

^*1C 1: average value of lime feeding rate (kg/min/T) at preliminary stage of dephosphorization ^*2, blowing: blowing lime powder (refining agent) to the liquid level of molten iron by using top-blowing gun

C2: and (3) loading the average value (kg/min/T) of lime supply speed in the later period of dephosphorization: from above, lime (refining agent) is loaded

D1: average value (Nm)of oxygen gas supply rate in the early stage of dephosphorization³The first is (C1/D1) less than or equal to (C2/D2)

D2: average value (Nm) of oxygen supply rate in the latter stage of dephosphorization³/min/T)

[ example 5]

Molten iron tapped from a blast furnace was desiliconized in a tapping site, and then charged into a ladle, and desiliconized in the ladle, and after slag discharge, charged into a converter type vessel (300 tons).

In the dephosphorization, lime powder (refining agent) is blown to the molten iron surface by means of a top-blowing lance with oxygen as carrier gas, while in some embodiments the loading of lump lime from above is used simultaneously. In the comparative example, lime powder was not blown by a top-blowing gun, but lump lime was added from above. In each embodiment, 0.07 to 0.12Nm of blowing gas is blown from the bottom of a converter-type vessel³And (4) dephosphorizing for 8-14 minutes by min/ton of nitrogen of molten iron. In the dephosphorization, the temperature of the molten iron before and after the dephosphorization is 1250-1350 ℃, and the amount of the slag after the dephosphorization is below 30 kg/ton. In the present example, the lime supply rate B (kg/min/ton molten iron) and the oxygen supply rate A (Nm) were measured for the molten iron surface³Min/ton of molten iron) was 1.7. The temperature of the molten iron before and after dephosphorization is 1250-1350 ℃. The amount of the processed slag was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

Tables 4 to 7 show the results and dephosphorization conditions of the examples.

TABLE 4

No.	Molten iron composition (% by mass)				Calculated value of lime amount^*1			Lime addition^*2		Slag addition of iron-making furnace^*3		Categories
	Molten iron composition (% by mass)				Calculated value of lime amount^*1			Lime addition^*2		Slag addition of iron-making furnace^*3			Before dephosphorization		Target [P]	After dephosphorization [P]	Wcao-Si (kg/T)	Wcao-P (kg/T)	Total up to	Blowing of (kg/T)	Jacket (kg/T)	Adding amount of (kg/T)	CaO part^*4 (kg/T)
	[Si]	[P]											Before dephosphorization
	[Si]	[P]	1	0.15	0.115	0.013	0.013	6.0	2.76～5.53	8.76～11.53	4.23		4.88	-										-	Examples of the invention
2	0.13	0.112	1	0.15	0.115	0.013	0.013	6.0	2.76～5.53	8.76～11.53	4.23	0.013	4.88	-	0.016	5.2	2.68～5.37	7.88～10.57	4.38	3.85	-	-	Examples of the invention	-	Examples of the invention
2	0.13	0.112	3	0.12	0.108	0.013	0.016	4.8	2.57～5.15	7.37～9.95	5.18	0.013	2.55	-	0.016	5.2	2.68～5.37	7.88～10.57	4.38	3.85	-	-	Examples of the invention	-	Examples of the invention
4	0.08	0.132	3	0.12	0.108	0.013	0.016	4.8	2.57～5.15	7.37～9.95	5.18	0.013	2.55	-	0.013	3.2	3.22～6.45	6.42～9.65	5.14	1.64	-	-	Examples of the invention	-	Examples of the invention
4	0.08	0.132	5	0.02	0.125	0.013	0.016	0.8	3.03～6.07	3.83～6.87	3.93	0.013	0.26	-	0.013	3.2	3.22～6.45	6.42～9.65	5.14	1.64	-	-	Examples of the invention	-	Examples of the invention
6	0.03	0.107	5	0.02	0.125	0.013	0.016	0.8	3.03～6.07	3.83～6.87	3.93	0.013	0.26	-	0.017	1.2	2.55～5.09	3.75～6.29	2.91	1.19	-	-	Examples of the invention	-	Examples of the invention
6	0.03	0.107	7	0.07	0.099	0.013	0.011	2.8	2.33～4.66	5.13～7.46	3.81	0.013	1.68	-	0.017	1.2	2.55～5.09	3.75～6.29	2.91	1.19	-	-	Examples of the invention	-	Examples of the invention
8	0.04	0.118	7	0.07	0.099	0.013	0.011	2.8	2.33～4.66	5.13～7.46	3.81	0.013	1.68	-	0.011	1.6	2.85～5.69	4.45～7.29	4.15	0.65	-	-	Examples of the invention	-	Examples of the invention
8	0.04	0.118	9	0.09	0.113	0.013	0.013	3.6	2.71～5.42	6.31～9.02	6.00	0.013	0.66	-	0.011	1.6	2.85～5.69	4.45～7.29	4.15	0.65	-	-	Examples of the invention	-	Examples of the invention
10	0.05	0.108	9	0.09	0.113	0.013	0.013	3.6	2.71～5.42	6.31～9.02	6.00	0.013	0.66	-	0.018	2.0	2.57～5.15	4.57～7.15	4.06	0.87	-	-	Examples of the invention	-	Examples of the invention
10	0.05	0.108	11	0.08	0.116	0.013	0.009	3.2	2.79～5.58	5.99～8.78	5.26	0.013	1.08	-	0.018	2.0	2.57～5.15	4.57～7.15	4.06	0.87	-	-	Examples of the invention	-	Examples of the invention
12	0.03	0.117	11	0.08	0.116	0.013	0.009	3.2	2.79～5.58	5.99～8.78	5.26	0.013	1.08	-	0.014	1.2	2.82～5.64	4.02～6.84	2.97	0.25	3.20	0.80	Examples of the invention	-	Examples of the invention
12	0.03	0.117	13	0.08	0.099	0.013	0.008	3.2	2.33～4.66	5.53～7.86	2.93	0.013	1.80	4.00	0.014	1.2	2.82～5.64	4.02～6.84	2.97	0.25	3.20	0.80	Examples of the invention	1.00	Examples of the invention

^*1 Wcao-Si: (1) calculated value of formula ^*2, blowing: blowing lime powder (refining agent) to the liquid level of molten iron by using top-blowing gun

Wcao-P: (2) the calculated value of formula is loaded: charging blocky lime (refining agent) from above

Totaling: Wcao-Si + Wcao-P ^*3 loading from above

^*4 amount of free lime in iron-making slag

TABLE 5

No	Level of molten iron Depth of depression (mm)	Dephosphorization rate (％)	Lime efficiency (-)	Amount of lime injected /Wcao-P ^*1	Categories
No	Level of molten iron Depth of depression (mm)	Dephosphorization rate (％)	Lime efficiency (-)	Amount of lime injected /Wcao-P ^*1	Categories	1	300	88.7	0.887	1.53	Examples of the invention
2	325	85.7	0.857	1.63	Examples of the invention	1	300	88.7	0.887	1.53	Examples of the invention
2	325	85.7	0.857	1.63	Examples of the invention	3	256	85.2	0.852	2.01	Examples of the invention
4	268	90.2	0.902	1.59	Examples of the invention	3	256	85.2	0.852	2.01	Examples of the invention
4	268	90.2	0.902	1.59	Examples of the invention	5	289	87.2	0.872	1.29	Examples of the invention
6	295	84.1	0.841	1.14	Examples of the invention	5	289	87.2	0.872	1.29	Examples of the invention
6	295	84.1	0.841	1.14	Examples of the invention	7	312	88.9	0.889	1.63	Examples of the invention
8	415	90.7	0.907	1.46	Examples of the invention	7	312	88.9	0.889	1.63	Examples of the invention
8	415	90.7	0.907	1.46	Examples of the invention	9	210	88.5	0.885	2.21	Examples of the invention
10	450	83.3	0.833	1.58	Examples of the invention	9	210	88.5	0.885	2.21	Examples of the invention
10	450	83.3	0.833	1.58	Examples of the invention	11	480	92.2	0.922	1.88	Examples of the invention
12	290	88.0	0.990	1.05	Examples of the invention	11	480	92.2	0.922	1.88	Examples of the invention
12	290	88.0	0.990	1.05	Examples of the invention	13	360	91.9	0.973	1.26	Examples of the invention

^*1 Wcao-P is the amount of lime calculated from η cao ═ 1

TABLE 6

No.	Molten iron composition (% by mass)				Calculated value of lime amount (kg/T) ^*1	Lime addition ^*2		Categories
	Molten iron composition (% by mass)					Lime addition ^*2			Before dephosphorization		Target [P]	After dephosphorization [P]	Blowing of (kg/T)	Jacket (kg/T)
	[Si]	[P]							Before dephosphorization
	[Si]	[P]	14	0.10		0.122	0.013		0.015	13.97					-	13.97	Comparative example
15	0.07	0.108	14	0.10	0.013	0.122	0.013	0.011	0.015	13.97	12.18	-	12.18	Comparative example	-	13.97	Comparative example
15	0.07	0.108	16	0.02	0.013	0.113	0.013	0.011	0.009	12.82	12.18	-	12.18	Comparative example	-	12.82	Comparative example

^*1 slag basicity: 3, Lp: the amount of lime required is calculated from 240

^*2, blowing: blowing lime powder (refining agent) from top-blowing gun to molten iron liquid level

Loading: charging blocky lime (refining agent) from above

TABLE 7

No.	Level of molten iron Depth of depression (mm)	Dephosphorization rate (％)	Lime efficiency (-)	Categories
No.	Level of molten iron Depth of depression (mm)	Dephosphorization rate (％)	Lime efficiency (-)	Categories	14	450	87.7	0.330	Comparative example
15	450	89.8	0.299	Comparative example	14	450	87.7	0.330	Comparative example
15	450	89.8	0.299	Comparative example	16	330	92.0	0.320	Comparative example

[ example 6]

In the dephosphorization, oxygen was blown to the molten iron surface by a top-blowing lance, at the same time, ① was used as a carrier gas to blow lime powder (refining agent) having a particle size of 3mm or less to the molten iron surface, ② was added as a refining agent by a method in which lime cake (refining agent) having a particle size of 3mm or less was charged from above, ② was blown to the molten iron from the bottom of the converter type vessel at 0.1Nm³Stirring the molten iron with nitrogen gas at a rate of/min/ton of molten iron supply, and simultaneously carrying out dephosphorization for 10-11 minutes. And adjusting the temperature of the molten iron before dephosphorization and the addition amount of the waste, and controlling the temperature of the molten iron after the dephosphorization. The amount of the processed slag is less than 30 kg/ton molten iron. In the present invention, the lime addition amount is in the range of the total amount of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of the cavity formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above-mentioned formula (7)) is controlled to be200-500 mm. The amount of the processed slag was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

Table 8 shows the results and dephosphorization conditions of the examples.

TABLE 8

No.	Molten iron composition before dephosphorization		Temperature of molten iron		Dephosphorization conditions				Dephosphorized iron Water [ P ]](Mass) Volume%)	Dephosphorization ratio (%)	Categories
	Molten iron composition before dephosphorization		Temperature of molten iron		Dephosphorization conditions							[Si](quality) ％)	[P](quality) ％)	Before dephosphorization (. degree.C.)	After dephosphorization (℃)	Refining agent additive Dosage (kg/T)	CaF₂Adding Volume (kg/T)	Refining agent additive Adding method^*1	Amount of oxygen (Nm³/T)
	1	0.05	0.106	1320	1375	6.5	-	Blowing of				[Si](quality) ％)	[P](quality) ％)	Before dephosphorization (. degree.C.)	After dephosphorization (℃)	Refining agent additive Dosage (kg/T)	CaF₂Adding Volume (kg/T)	Refining agent additive Adding method^*1	Amount of oxygen (Nm³/T)	10.2	0.012	89	Examples of the invention
2	1	0.05	0.106	1320	1375	6.5	-	Blowing of	0.04	0.110	1315	1400	6.1	-	Blowing of	9.5	0.014	87	Examples of the invention	10.2	0.012	89	Examples of the invention
2	3	0.08	0.102	1285	1446	7.7	-	Blowing of	0.04	0.110	1315	1400	6.1	-	Blowing of	9.5	0.014	87	Examples of the invention	10.3	0.015	85	Examples of the invention
4	3	0.08	0.102	1285	1446	7.7	-	Blowing of	0.02	0.115	1267	1440	5.3	-	Blowing of	9.6	0.014	88	Examples of the invention	10.3	0.015	85	Examples of the invention
4	5	0.09	0.106	1354	1421	8.1	-	Blowing of	0.02	0.115	1267	1440	5.3	-	Blowing of	9.6	0.014	88	Examples of the invention	10.2	0.012	89	Examples of the invention
6	5	0.09	0.106	1354	1421	8.1	-	Blowing of	0.06	0.108	1302	1367	6.9	-	Blowing of	10.1	0.008	93	Examples of the invention	10.2	0.012	89	Examples of the invention
6	7	0.07	0.110	1314	1385	7.3	0.2	Blowing of	0.06	0.108	1302	1367	6.9	-	Blowing of	10.1	0.008	93	Examples of the invention	9.7	0.011	90	Examples of the invention
8	7	0.07	0.110	1314	1385	7.3	0.2	Blowing of	0.03	0.107	1298	1375	5.7	0.7	Blowing of	10.2	0.014	87	Examples of the invention	9.7	0.011	90	Examples of the invention
8	9	0.06	0.108	1315	1410	6.9	1.6	Jacket	0.03	0.107	1298	1375	5.7	0.7	Blowing of	10.2	0.014	87	Examples of the invention	10.4	0.032	70	Comparative example
10	9	0.06	0.108	1315	1410	6.9	1.6	Jacket	0.15	0.106	1320	1403	10.5	-	Jacket	10.2	0.034	68	Comparative example	10.4	0.032	70	Comparative example
10	11	0.05	0.106	1343	1465	6.5	-	Jacket	0.15	0.106	1320	1403	10.5	-	Jacket	10.2	0.034	68	Comparative example	10.1	0.039	63	Comparative example
12	11	0.05	0.106	1343	1465	6.5	-	Jacket	0.06	0.112	1298	1321	6.9	-	Jacket	9.9	0.033	71	Comparative example	10.1	0.039	63	Comparative example

^*1, blowing: blowing lime powder (refining agent) to the liquid level of molten iron by using top-blowing gun

Loading: charging blocky lime (refining agent) from above

[ example 7]

The molten iron tapped from a blast furnace was desiliconized in a tapping site, and then charged into a ladle, and then desiliconized in the ladle, and after slag discharge, charged into a converter type vessel (300 tons) for dephosphorization. In this dephosphorization, lime powder (refining agent) having a particle size of 1mm or less and a heat absorbing substance are blown to the molten iron surface by a top-blowing lance using oxygen as a carrier gas. Using CaCO as heat-absorbing substance₃Or Ca (OH)₂(particle diameter is below 1 mm), and mixing with lime powder at a predetermined ratio. Blowing 0.1Nm into molten iron from the bottom of a converter-type vessel³Stirring the molten iron with nitrogen gas at a rate of/min/ton of molten iron supply, and simultaneously carrying out dephosphorization for 10-11 minutes. And adjusting the temperature of the molten iron before dephosphorization and the addition amount of the waste, and controlling the temperature of the molten iron after the dephosphorization. The amount of the processed slag is less than 30 kg/ton molten iron. In the present invention, the lime addition amount is in the range of the total amount of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. From the amount of lime added and the CaO concentration in the slag (slag analysis)Value) the amount of slag after treatment was calculated.

Table 9 shows the results and dephosphorization conditions of the examples.

TABLE 9

No.	Molten iron composition before dephosphorization		Temperature of molten iron		Dephosphorization conditions						After dephosphorization Iron melt [ P ]] (mass%)	Dephosphorization rate (％)	Categories
	Molten iron composition before dephosphorization		Temperature of molten iron		Dephosphorization conditions									[Si](quality) ％)	[P](mass%)	Before dephosphorization (℃)	After dephosphorization (℃)	Refining agent Adding amount of (kg/T)	CaF₂ Adding amount of (kg/T)	Refining agent Addition method Method of^*1	Amount of oxygen (Nm³/T)	Heat absorbing material
	Species of	Adding amount of (kg/T)																				Heat absorbing material
	Species of	Adding amount of (kg/T)	1	0.03	0.108	1319	1423	5.7	-	Blowing of												10.6	CaCO₃	3.1	0.009	92	Examples of the invention
2	0.05	0.111	1	0.03	0.108	1319	1423	5.7	-	Blowing of	1347	1426	6.5	-	Blowing of	10.8	CaCO₃	2.3	0.010	91	Examples of the invention	10.6	CaCO₃	3.1	0.009	92	Examples of the invention
2	0.05	0.111	3	0.08	0.105	1299	1447	7.7	-	Blowing of	1347	1426	6.5	-	Blowing of	10.8	CaCO₃	2.3	0.010	91	Examples of the invention	9.9	CaCO₃	6.5	0.011	90	Examples of the invention
4	0.03	0.106	3	0.08	0.105	1299	1447	7.7	-	Blowing of	1275	1412	5.7	-	Blowing of	10.5	Ca(OH)₂	2.1	0.012	89	Examples of the invention	9.9	CaCO₃	6.5	0.011	90	Examples of the invention
4	0.03	0.106	5	0.04	0.111	1297	1385	6.1	0.3	Blowing of	1275	1412	5.7	-	Blowing of	10.5	Ca(OH)₂	2.1	0.012	89	Examples of the invention	10.2	CaCO₃	3.4	0.008	93	Examples of the invention
6	0.06	0.104	5	0.04	0.111	1297	1385	6.1	0.3	Blowing of	1311	1374	6.9	0.8	Blowing of	10.5	Ca(OH)₂	2.3	0.009	91	Examples of the invention	10.2	CaCO₃	3.4	0.008	93	Examples of the invention

[ example 8]

Molten iron discharged from a blast furnace was subjected to desiliconization in a ladle, and after slag discharge, the molten iron was charged into a converter type vessel (300 tons) to be subjected to dephosphorization. In this dephosphorization, about 0.1Nm is blown from the bottom of the converter type vessel³The molten iron is stirred by stirring gas (nitrogen) per ton of molten iron, and oxygen, lime powder (CaO series refining agent) and gas heat-absorbing substance are supplied from the upper part of the molten iron liquid surface to the molten iron liquid surface through a top blowing lance. CaF is not added in the refining agent₂. The amount of the processed slag is below 30 kg/ton molten iron. A lime supply rate B (kg/min/ton molten iron) and an oxygen supply rate A (Nm) to be blown to the molten iron surface³Min/ton of molten iron) was 1.7. The amount of lime added is within the range of the total of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. The temperature of molten iron before and after dephosphorization is 1250-1350 ℃. The amount of the slag after the treatment was calculated from the mass balance of the amount of lime added and the CaO concentration in the slag (slag analysis value).

The top-blown oxygen lance uses a lance having 1 central hole and 3 peripheral holes as a lance hole.

Lime powder having a particle size of 3mm or less is used, and oxygen gas is fed from a feeding device as a carrier gas, and the lime powder is fed into a top-blowing lance through a pipe and supplied to the molten iron surface from a center hole together with the oxygen gas. On the other hand, oxygen gas is supplied to the top-blowing lance through another piping line and supplied to the molten iron surface from the outer peripheral hole. The total oxygen supply is 1.5Nm³Min/ton molten iron.

Gas endothermic substances of predetermined concentrations are added to the two oxygen gas lines, respectively. Carbon dioxide and steam are used as the heat absorbing material, and the mixing ratio of the carbon dioxide and the steam to oxygen is 10-40 vol% (based on 100% of oxygen)

In the comparative example, oxygen gas was supplied from a top-blowing lance to the molten iron surface, and lump lime (CaO-based refining agent) was charged from above.

Table 10 shows the results and dephosphorization conditions of the examples.

Watch 10

No.	Molten iron composition (% by mass)			Dephosphorization conditions				Categories
	Molten iron composition (% by mass)			Dephosphorization conditions					Before dephosphorization [Si]	[P]		Heat absorbing material		Lime making Dosage of (kg/T)	Dephosphorization Time of day (minute)
	Before dephosphorization	After dephosphorization	Species of	Adding amount of (％)^*1						[P]		Heat absorbing material
	Before dephosphorization	After dephosphorization	Species of	Adding amount of (％)^*1	1	0.09	0.110			0.011	CO ₂	5	5.6			6.4	Examples of the invention
2	0.08	0.108	0.010	CO ₂	1	0.09	0.110	10	5.2	0.011	CO ₂	5	5.6	6.1	Examples of the invention	6.4	Examples of the invention
2	0.08	0.108	0.010	CO ₂	3	0.07	0.109	10	5.2	0.008	CO ₂	25	4.6	6.1	Examples of the invention	5.2	Examples of the invention
4	0.07	0.109	0.007	CO ₂	3	0.07	0.109	40	4.3	0.008	CO ₂	25	4.6	5.0	Examples of the invention	5.2	Examples of the invention
4	0.07	0.109	0.007	CO ₂	5	0.13	0.109	40	4.3	0.010	CO ₂	25	5.8	5.0	Examples of the invention	6.6	Hair-like deviceExample of the invention
6	0.07	0.107	0.010	H₂O	5	0.13	0.109	10	5.1	0.010	CO ₂	25	5.8	5.9	Examples of the invention	6.6	Hair-like deviceExample of the invention
6	0.07	0.107	0.010	H₂O	7	0.07	0.110	10	5.1	0.008	H₂O	25	4.8	5.9	Examples of the invention	5.5	Examples of the invention
8	0.07	0.105	0.015	-	7	0.07	0.110	-	8.0	0.008	H₂O	25	4.8	11.0	Comparative example	5.5	Examples of the invention
8	0.07	0.105	0.015	-	9	0.08	0.108	-	8.0	0.014	-	-	8.5	11.0	Comparative example	11.5	Comparative example

^*1 relative to 100% by weight of oxygen

[ example 9]

The molten iron discharged from the blast furnace is desiliconized in a ladle, and after slag is discharged, dephosphorization is continued in the ladle. In this dephosphorization, 3 Nm/min was blown into the molten iron from 1 dipping gun³While stirring molten iron with nitrogen, oxygen, lime powder (CaO-based refining agent) and a heat absorbing substance were supplied from above the liquid surface by a top-blowing lance in a manner of ① to ④, and CaF was not added to the refining agent₂. The amount of the processed slag is below 30 kg/ton molten iron. A lime supply rate B (kg/min/ton molten iron) and an oxygen supply rate A (Nm) to be blown to the molten iron surface³Min/ton of molten iron) was 1.7. The amount of lime added is within the range of the total of Wcao-P (kg/ton of molten iron) and Wcao-Si (kg/ton of molten iron) defined by the above-mentioned formulas (5) and (6). Further, the depth L of a depression formed in the molten iron surface by blowing a refining agent using oxygen as a carrier gas (L value defined by the above expression (7)) is controlled within the range of 200 to 500 mm. The temperature of molten iron before and after dephosphorization is 1250-1350 ℃. The amount of the slag after treatment is controlled byAnd calculating the mass balance of the added lime amount and the CaO concentration in the slag (slag analysis value).

① in the embodiment of the invention, a top-blowing gun is used for supplying oxygen, lime powder and CO₂。

② example of the invention, a top-blowing gun is used to supply oxygen, lime powder, CaCO₃(limestone) or Ca (OH)₂(slaked lime).

③ in the embodiment of the invention, a top-blowing gun is used for supplying oxygen, lime powder and CO₂、CaCO₃(limestone).

④ example of the invention, the top-blowing gun is used to supply oxygen and CaCO₃(limestone) or Ca (OH)₂(slaked lime).

Lime powder, CaCO₃(limestone) or Ca (OH)₂Powders having a particle diameter of 1mm or less are used, and fed from a feeding device using oxygen as a carrier gas, transported in a pipe, and supplied to the liquid surface together with the oxygen jet from 3 gun holes at the tip of the top-blowing gun while being merged with oxygen supplied through another pipe at the inlet of the top-blowing gun. The total oxygen supply is 6000Nm per hour³。

With respect to CO₂The mixing ratio to oxygen was set to 25% by volume (a number based on 100% by volume of oxygen). Lime powder, CaCO₃(limestone) or Ca (OH)₂The amount of CaO is 70-80 kg/min.

In the comparative example, oxygen was supplied to the molten iron surface by a top-blowing lance, while lime powder was sprayed into the molten iron by an immersion lance.

Table 11 shows the results and dephosphorization conditions of the examples.

TABLE 11

No.	Molten iron composition (% by mass)			Dephosphorization conditions					Categories
	Molten iron composition (% by mass)			Dephosphorization conditions						Before dephosphorization [Si]	[P]		Heat absorbing material			Lime making Dosage of (kg/T)	Dephosphorization time (minute)
	Before dephosphorization	After dephosphorization	Gas (es)	Solid body							[P]		Heat absorbing material
				Solid body		Species of	Addition amount (kg/T)	Examples of the invention
				1	0.08	Species of	Addition amount (kg/T)	Examples of the invention	0.115		0.008	CO₂	-	-	5.0			9.8	Examples of the invention
2	0.07	0.114	0.008	1	0.08	CO₂	CaCO₃	1.1	0.115	4.4	0.008	CO₂	-	-	5.0	9.7	Examples of the invention	9.8	Examples of the invention
2	0.07	0.114	0.008	3	0.12	CO₂	CaCO₃	1.1	0.116	4.4	0.011	CO₂	CaCO₃	1.2	5.9	9.7	Examples of the invention	13.0	Examples of the invention
4	0.08	0.115	0.009	3	0.12	-	CaCO₃	3.6	0.116	3.3	0.011	CO₂	CaCO₃	1.2	5.9	13.2	Examples of the invention	13.0	Examples of the invention
4	0.08	0.115	0.009	5	0.08	-	CaCO₃	3.6	0.114	3.3	0.010	-	CaCO₃	8.5	-	13.2	Examples of the invention	14.2	Examples of the invention
6	0.07	0.113	0.011	5	0.08	-	Ca(OH)₂	3.5	0.114	2.2	0.010	-	CaCO₃	8.5	-	11.5	Examples of the invention	14.2	Examples of the invention
6	0.07	0.113	0.011	7	0.07	-	Ca(OH)₂	3.5	0.113	2.2	0.010	-	Ca(OH)₂	6.0	-	11.5	Examples of the invention	11.6	Examples of the invention
8	0.06	0.114	0.017	7	0.07	-	-	-	0.113	7.9	0.010	-	Ca(OH)₂	6.0	-	16.5	Comparative example	11.6	Examples of the invention
8	0.06	0.114	0.017	9	0.15	-	-	-	0.114	7.9	0.021	-	-	-	10.5	16.5	Comparative example	20.1	Comparative example

Possibility of industrial utilization

The invention is used for manufacturing molten iron with low phosphorus content in the intermediate process of steel making.

Claims

1. A process for producing a low-phosphorus molten iron by adding a refining agent comprising an oxygen source and a CaO source to a vessel containing a molten iron and conducting dephosphorization by preliminary treatment of the molten iron, characterized in that the dephosphorization is conducted by blowing oxygen and at least a part of the refining agent onto the molten iron surface by means of a top-blowing lance, and the amount of slag after the treatment is 30 kg/ton or less of the molten iron.

2. The method of manufacturing molten iron having low phosphorus content according to claim 1, wherein at least a part of the refining agent supplied from the top-blowing lance is blown to a molten iron surface area where oxygen is blown.

3. The method of manufacturing molten iron having low phosphorus content according to claim 2, wherein at least a part of the refining agent supplied from the top-blowing lance is blown onto a fire point formed on the molten iron surface by the blown oxygen.

4. The method of manufacturing molten iron having low phosphorus content according to claim 2 or 3, wherein at least a part of the refining agent is blown onto the molten iron surface by using oxygen as a carrier gas.

5. The method of manufacturing a molten iron having low phosphorus content according to claim 1, 2, 3 or 4, wherein the molten iron having an Si content of 0.15 mass% or less is subjected to dephosphorization.

6. The method of manufacturing a molten iron having low phosphorus content according to claim 1, 2, 3 or 4, wherein the molten iron having an Si content of 0.07 mass% or less is subjected to dephosphorization.

7. The method of manufacturing a molten iron having low phosphorus content according to claim 1, 2, 3 or 4, wherein the molten iron having an Si content of 0.03 mass% or less is subjected to dephosphorization.

8. The method of manufacturing a molten iron having low phosphorus content according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the amount of the slag after the treatment is 20 kg/ton or less.

9. The method of manufacturing a molten iron having low phosphorus content according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the amount of the slag after the treatment is 10 kg/ton or less.

10. The method of manufacturing a molten iron having low phosphorus content according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein a supply rate B (kg/min/ton of molten iron) in terms of CaO of the refining agent to be blown onto the molten iron surface and a supply rate A (Nm/m) of oxygen to be blown onto the molten iron surface are set to be equal to each other³Min/ton molten iron) satisfies the following formula (1),

0.3≤A/B≤7……(1)。

11. the method of manufacturing a molten iron having low phosphorus content according to claim 10, wherein the supply rate B (kg/min/ton of molten iron) in terms of CaO of the refining agent to be blown onto the molten iron surface and the supply rate B to the molten iron are controlled so as to be adjusted in accordance with the amount of CaOThe supply rate A (Nm) of oxygen gas blown onto the liquid surface³Min/ton molten iron) satisfies the following formula (2),

1.2≤A/B≤2.5……(2)。

12. the method for manufacturing a low-phosphorous molten iron according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11, wherein a vessel of a ladle type or a torpedo type is used as the vessel in which the molten iron is charged, and oxygen and at least a part of the refining agent are blown to the molten iron surface by a top-blowing lance while a gas containing powder is blown to the molten iron by an immersion lance and/or a blowing nozzle.

13. The method of manufacturing molten iron having low phosphorus content according to claim 12, wherein the powder to be blown into the molten iron through the immersion lance and/or the blowing nozzle is a part of the refining agent.

14. The method of manufacturing molten iron having low phosphorus content according to claim 12 or 13, wherein the amount of oxygen blown into the molten iron surface by the top-blowing lance is 0.7Nm³Less than/min/ton molten iron.

15. The method of manufacturing molten iron having low phosphorus content according to claim 12, 13 or 14, wherein 80 mass% or more of the amount of refining agent added in the dephosphorization is blown to the molten iron surface by the top-blowing lance.

16. The method of manufacturing a low-phosphorous hot metal as claimed in claim 12, 13, 14 or 15, wherein the amount of the refining agent added by the top-blowing lance is 20 to 80 mass% of the total amount of the refining agent added while substantially the total amount of the refining agent is blown to the molten iron liquid surface by the top-blowing lance and is added by blowing into the molten iron by the immersion lance and/or the blowing nozzle.

17. The method of manufacturing a low-phosphorous hot metal as claimed in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16, wherein the dephosphorization is performed under the condition that the supply rate of the refining agent and the supply rate of the oxygen gas to be blown to the molten metal surface satisfy the following expressions (3) and (4),

(C1/D1)＞(C2/D2)……(3)

C1＞C2……(4)

wherein, C1: average value of CaO-converted supply rates of refining agents (kg/min/ton molten iron) in the early stage of dephosphorization

D2: average value (Nm) of oxygen gas supply rate at the latter stage of dephosphorization³/min/ton molten iron).

18. The method of manufacturing molten iron having low phosphorus content according to claim 17, wherein the refining agent supply rate and the oxygen gas supply rate in terms of CaO can be continuously and/or stepwise changed during the dephosphorization.

19. The process for producing a low-phosphorus molten iron according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 or 18, wherein the molten iron having an Si content of 0.15 mass% or less is dephosphorized by blowing oxygen gas and at least a part of the refining agent to the molten iron surface by means of a top-blowing lance, and in the dephosphorizing treatment, lime is added as the refining agent in an amount corresponding to the sum of Wcao-P (kg/ton molten iron) in the amount determined by the following expression (5) and Wcao-Si (kg/ton molten iron) in the amount determined by the following expression (6),

Wcao-P ═ (molten iron [ P]-target [ P]) × (10/62) × 56 × 3/η cao … (5)

Wherein, the molten iron [ P]: the P concentration (mass%) in the molten iron before dephosphorization,

target [ P]: the P concentration (mass%) in the molten iron after the targeted dephosphorization treatment,

η cao (lime efficiency) is 0.5-1,

Wcao-Si ═ hot metal [ Si]× (10/28) × 56 × 2 … (6)

Wherein, the molten iron [ Si]: si concentration (mass%) in molten iron before dephosphorization.

20. The method of manufacturing a low-phosphorous hot metal as claimed in claim 19, wherein 80 mass% or more of the lime content Wcao-P (Wcao-P determined as η cao ═ 1, among others) is blown to the molten metal surface by a top-blowing lance.

21. The method of manufacturing low-phosphorous hot metal according to claim 19 or 20, wherein 1 or more kinds selected from the group consisting of lime powder, lump quicklime, lump limestone and iron-making slag containing unreacted CaO are used as the refining agent corresponding to the lime amount Wcao-Si.

22. The method of producing a low-phosphorus molten iron according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or 21, wherein a depth L of a depression formed in a molten iron surface by blowing oxygen or blowing a refining agent using oxygen as a carrier gas defined by the following formula (7) is controlled to 200 to 500mm,

L＝L₀×exp{(-0.78×L_H)/L₀}……(7)

L₀＝63×{(F₀₂/n)/d_t}^2/3

wherein L is_H: gun height of top blowing gun (mm)

F₀₂: velocity of oxygen supply (Nm) by top-blowing lance³/hr)

n: number of nozzle holes of top-blowing gun

d_t: a nozzle aperture (mm) of a top-blowing gun (wherein the average aperture of all nozzle holes in the case where the nozzle apertures of a plurality of nozzle holes are different).

23. The method of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,14. 15, 16, 17, 18, 19, 20, 21 or 22, which is a method for producing a low-phosphorous molten ironCharacterized in that CaF is added₂The amount of (A) is not more than 2 kg/ton of molten iron or substantially no CaF is added₂The dephosphorization is carried out under the condition of (1).

24. The method of manufacturing low-phosphorous hot metal as claimed in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 or 22, wherein CaF is added to the hot metal having an Si content of 0.15 mass% or less₂The amount of (A) is 1 kg/ton or less of molten iron or substantially no CaF is added₂Under the condition (1), oxygen and at least a part of refining agent are blown to the molten iron liquid surface by a top blowing gun to carry out dephosphorization treatment, and the molten iron temperature at the end of dephosphorization treatment is 1360-1450 ℃.

25. The method of manufacturing molten iron having low phosphorus content according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 or 24, wherein a substance that absorbs heat of molten iron through a chemical reaction and/or a thermal decomposition reaction is supplied to a molten iron surface region to which oxygen is supplied.

26. The method of manufacturing molten iron having low phosphorus content according to claim 25, wherein at least a part of the substance which absorbs heat of molten iron by a chemical reaction and/or a thermal decomposition reaction is supplied to the ignition point generated on the molten iron surface by the blowing of oxygen.

27. The method of manufacturing molten iron having low phosphorus content according to claim 25 or 26, wherein the substance that absorbs heat of molten iron by a chemical reaction and/or a thermal decomposition reaction is at least 1 selected from carbon dioxide, water vapor, nitrogen oxide, metal carbonate, and metal hydroxide.

28. The method of manufacturing molten iron having low phosphorus content according to claim 27, wherein the substance which absorbs heat of molten iron by a chemical reaction and/or a thermal decomposition reaction is a substance which generates CO by thermal decomposition₂Or H₂Of OMetal carbonates, CO production by thermal decomposition₂Or H₂At least 1 selected from metal hydroxides of O.

29. The method of manufacturing molten iron having low phosphorus content according to claim 28, wherein the substance for absorbing heat of molten iron by a chemical reaction and/or a thermal decomposition reaction is derived from CaCO₃、Ca(OH)₂、CaMg(CO₃)₂At least 1 selected from the above.

30. The method of manufacturing molten iron having low phosphorus content according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 or 24, wherein instead of a part or all of the refining agent as the CaO source, a substance which is a product of the refining agent and absorbs heat of molten iron by a chemical reaction and/or a thermal decomposition reaction is used as the CaCO source₃、Ca(OH)₂、CaMg(CO₃)₂More than 1 kind of the oxygen is selected and supplied to the molten iron liquid level area to which the oxygen is supplied.

31. The method of claim 30, wherein the molten iron containing phosphorus is selected from CaCO₃、Ca(OH)₂、CaMg(CO₃)₂At least a part of the 1 or more selected from the above is supplied to the fire point generated on the molten iron surface by injecting oxygen.

32. The method of manufacturing a low-phosphorous hot metal as claimed in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or 31, wherein a hot metal having a P content of 0.10 mass% or more is dephosphorized and refined to a P content (standard value of steel component) required for a raw steel or less.

33. The method of manufacturing molten iron having low phosphorus content according to claim 32, wherein the content of P in the molten iron after the dephosphorization is 0.010 mass% or less.