JPH07157820A - Method for promoting decarburization of molten steel - Google Patents

Abstract

(57) [Summary] [Objective] To provide a method for promoting decarburization of molten steel under reduced pressure in an extremely low carbon region. [Composition] (1) In refining steel under reduced pressure, Fe, Mn, Cr
And a method of decarburizing molten steel, in which a powder consisting of one or more of Ni oxides is sprayed onto a molten steel surface together with a carrier gas, the alkaline earth metal oxide (MgO,
A method for promoting decarburization of molten steel, in which 5 to 20% by weight of powder comprising one or more of CaO, SrO ₂ , BaO), Al ₂ O ₃ and SiO ₂ is mixed and sprayed. (2) The method for promoting decarburization according to the above (1), in which powder is sprayed in a region where [C] is 0.005% or less. (3) As a carrier gas, Ar gas, N ₂ gas, O ₂ gas or O
_The method for promoting decarburization according to (1) or (2) above, which uses a refining gas containing ₂ gases. [Effect] It is possible to improve the decarburization rate from the low carbon region, reduce the reached carbon concentration and shorten the treatment time, and it is possible to stably obtain an ultra low carbon steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for promoting decarburization of molten steel under a reduced pressure, particularly, decarburization from a low carbon concentration region (20 to 50 ppm by weight).

[0002]

2. Description of the Related Art Generally, reduction of carbon in steel is carried out by utilizing a vacuum treatment to accelerate a CO reaction (reaction between carbon [C] and oxygen in molten steel) under reduced pressure. However, in the case of such an ordinary treatment, in the low carbon concentration range where [C] is 20 to 50 ppm by weight (hereinafter, simply referred to as ppm), even if the transfer of carbon at the reaction interface is sufficient, decarburization is performed. The reaction is stagnant and it is difficult to achieve extremely low carbonization ([C] <20ppm). The reason for this is that, although the above decarburization reaction is the rate-limiting chemical reaction at the reaction interface in molten steel, the reaction nuclei necessary for promoting the reaction are insufficient in molten steel in the low carbon concentration region, This is because the interfacial area is decreasing.

Currently, mass-produced steel is generally decarburized by a VOD device or an RH vacuum processing device.
Further ultra low carbonization of steel is desired, and carbon concentration in molten steel is 20
In order to meet the requirement of less than ppm, there are problems such as extension of treatment time, increase of treatment cost, lowering of molten steel temperature and complication of process.

In the past, various methods have been developed to solve these problems. As a method of promoting the decarburization reaction by increasing the reaction boundary area, a method of injecting gas into molten steel in a vacuum chamber to increase stirring power, or a metal oxide powder serving as an oxygen supply source as an oxidizing agent There is a method of spraying molten steel onto the surface of molten steel (see Japanese Patent Publication No. 1-25370).

However, in the method utilizing gas agitation, it is necessary to flow gas in order to prevent molten steel from entering the gas blowing tuyere even during decarburization in a high carbon concentration region where stirring power is not required. Therefore, it is disadvantageous in terms of the processing cost related to maintaining the degree of vacuum and the gas intensity,
Even with such a method, the decarburization reaction is delayed from the low carbon concentration region within the above range.

On the other hand, in the method of spraying an oxidizing agent, a metal oxide powder acting as an oxidizing agent is introduced into molten steel to supply an oxygen source into the molten steel, and the powder acts as a reaction nucleus in the molten steel. Therefore, it is effective for decarburization.

However, since all the oxidizer powder is used for the reaction, it loses its sustainability to act as a reaction nucleus.
The melting point of such an oxidizer powder is also close to the temperature of molten steel, and once dissolved in molten steel, it no longer forms a reaction nucleus.Therefore, deoxidization to the extremely low carbon concentration region where the carbon concentration is less than 20 ppm, which is required in recent years, is achieved. This method is still insufficient for efficient charcoal.

[0008]

Thus, in the decarburization treatment of molten steel in the case of using vacuum refining, the decarburization limit practically occurs due to the stagnation of the decarburization reaction occurring from the low carbon concentration region at the final stage of decarburization. In addition, there are problems such as an increase in treatment cost due to a longer treatment time and a complicated process for preventing a decrease in molten steel temperature.

An object of the present invention is to provide a decarburization accelerating method capable of improving the decarburization limit and shortening the treatment time in an extremely low carbon concentration region in refining molten steel under reduced pressure. is there.

[0010]

Means for Solving the Problems The gist of the present invention is as follows (1)
It is in the decarburization acceleration method of (3).

(1) In refining steel under reduced pressure, powder of one or more of iron oxide, manganese oxide, chromium oxide and nickel oxide is sprayed on the surface of molten steel together with carrier gas. A decarburizing method, wherein the powder contains an alkaline earth metal oxide (MgO, CaO,
A method for promoting decarburization of molten steel, characterized by mixing 5 to 20% by weight of powder comprising one or more of SrO ₂ , BaO), Al ₂ O ₃ and SiO ₂ and spraying.

(2) The above-mentioned is characterized in that powder is sprayed in a region where [C] in molten steel is 0.005% by weight or less.
(1) The method for promoting decarburization of molten steel as described above.

(3) Argon gas, nitrogen gas, oxygen gas or a refining gas containing oxygen gas is used as a carrier gas for spraying the powder.
(1) or the method for promoting decarburization of molten steel according to (2) above.

[0014]

In the method of the present invention, in addition to the powder of one or more of the oxides of Fe, Mn, Cr, and Ni, alkaline earth metal oxides (MgO, CaO 2, SrO 2, BaO) are added. ), Al ₂ O ₃ and SiO ₂ and one or more kinds of powder selected from them are mixed and sprayed onto the surface of the molten steel under reduced pressure by a carrier gas.

When the mixed powder is sprayed onto the surface of the molten steel together with the carrier gas, the powder actually penetrates into the molten steel, so that the same effect as when the powder is blown below the surface of the molten steel can be obtained. it can.

The oxide powders of Fe, Mn, Cr and Ni are not only a source of oxygen but also an oxidant which can be a nucleus of the decarburization reaction. The oxidant is composed of one kind or a combination of two or more kinds of Fe oxide, Mn oxide, Cr oxide and Ni oxide. All of these are oxides that can supply oxygen that dissociates in molten steel and acts on the decarburization reaction, and the reduced metal element dissolves quickly in molten steel and is useful depending on the steel type. It is selected from oxides that include elements that can be utilized as various elements and that are relatively inexpensive and easily available.

Since all of these oxides are easily decomposed at the molten steel temperature, the oxygen supply action is the same even if they are used alone or in combination of two or more kinds, and the combination is selected depending on the composition of the molten steel type. Good.

By the way, a vacuum decarburization process (for example, RH
In a process using a vacuum degasser, for example, in the case of ordinary steel, the oxygen concentration in the molten steel is about 300 to 500 ppm in the low carbon region ([C]: 20 to 50 ppm), and this value slows the decarburization reaction. However, in order to further accelerate the decarburization reaction, it is necessary to increase the reaction interface area in some way.

The above-mentioned oxidizing agent causes dissolution of the oxidizing agent itself or decomposition due to decarburization reaction at the molten steel temperature. Therefore, when only the oxidizing agent is sprayed, the oxide powder which becomes a reaction nucleus in such a low carbon concentration region is generated. It is not possible to maintain the state of the body and increase the reaction interface area, and the effect of greatly promoting decarburization is poor.

Therefore, the oxidizer is mixed with one or more of the weakly oxidizable alkaline earth metal oxides, Al ₂ O ₃ and SiO ₂ , which are thermodynamically stable in molten steel. Then, by increasing the reaction nuclei, the reaction boundary area is increased and the reaction is promoted. The thermodynamically stable oxide referred to here refers to those that do not easily react with molten steel and may exist by invading molten steel as reaction nuclei for decarburization,
Hereinafter, these are referred to as reaction accelerators.

The oxide acting as a reaction accelerator is thermodynamically stable with respect to the molten steel at the refining temperature for decarburization, and at the same time, the vapor pressure of the metal element is high and the molten steel is reduced in pressure. Those that are difficult to disassemble are suitable.

Alkaline earth metal oxides having such properties include MgO, CaO, SrO and Ba.
O is suitable.

As the other reaction accelerator, Al ₂ O ₃ or SiO ₂ is used for the same reason.

The above-mentioned oxides all have the same action as a reaction accelerator, and the same effect can be obtained by using one kind or a combination of two or more kinds.

By mixing these in an amount of 5 to 20% by weight (hereinafter simply referred to as "%") with respect to the oxidizing agent, reaction nuclei that are not extinguished by the decarburization reaction are supplied to increase the reaction interface area,
Thereby, decarburization of molten steel can be effectively promoted even in a low carbon concentration region.

Considering the supply of oxygen to the reaction nuclei,
The practical mixing ratio of the reaction accelerator is preferably 5 to 20%. If it is less than 5%, only the decarburization reaction promoting effect in the low carbon concentration region, which is similar to the usual blowing of only oxide powder, can be obtained. On the other hand, if it exceeds 20%, the ratio of the oxidizing agent becomes small, the amount of oxygen required for decarburization cannot be sufficiently supplied, and the molten steel temperature is lowered, which is a disadvantageous condition for promoting the decarburization reaction.

The powder particle size of each of the above oxidizing agents or reaction accelerators is preferably in the range of 0.002 to 1 mm on average.
The finer the powder, the better in terms of supplying reaction nuclei, but 0.00
If it is less than 2 mm, it is practically difficult to penetrate into the molten steel even if it is sprayed on the surface of the molten steel. On the other hand, if it exceeds 1 mm, the reaction rate will decrease. More practically desirable is 0.05-0.
The range is 3 mm.

The powders may be mixed in advance, or may be mixed in the middle of the spraying line or at the outlet. Hereinafter, the mixed powder used in the method of the present invention is referred to as a decarburizing agent.

Since the decarburization reaction is rate-determined by the chemical reaction at the reaction interface as described above, even if oxygen is sufficiently supplied under reduced pressure and carbon migration at the reaction interface is sufficient, the decarburization reaction is low. Stagnation from the carbon concentration range. Therefore, if the above-mentioned deoxidizing agent and decarburizing agent acting as a reaction accelerator are blown in, the reaction nuclei can be increased and the reaction interfacial area can be increased. It is possible to accelerate the carbon reaction and efficiently obtain ultra-low carbon steel.

The oxidizer finally reacts and decomposes in some form with the molten steel, and the metal component elements remain in the molten steel, but the reaction accelerator floats and agglomerates through suspension and convection in the molten steel, and finally Will be excluded from the system.

In order to obtain the above-mentioned effect of the decarburizing agent, it is best to spray the decarburizing agent powder on the surface of the molten steel with a carrier gas under reduced pressure so that the decarburizing agent powder can efficiently penetrate into the molten steel.

In the method of the present invention, the decarburizing reaction is carried out by spraying the decarburizing agent powder onto the surface of the molten steel using argon gas, nitrogen gas, oxygen gas or a refining gas containing oxygen gas as a carrier gas. The decarburizing agent powder sprayed on the surface of the molten steel penetrates into the molten steel, the oxidizing agent serves as an oxygen supply source for the decarburizing reaction, and the reaction accelerator serves as a reaction nucleus to accelerate the decarburizing reaction.

When oxygen gas or a refining gas containing oxygen gas is used as a carrier gas, the carrier gas also serves as a supply source of oxygen, which is more effective in promoting the decarburization reaction. Furthermore, at this time, the effect of supplementing heat by raising the temperature at the fire point can be expected.

The oxygen gas concentration in the carrier gas can be changed, and in the case of steel types containing a large amount of Mn that easily evaporates and Cr that easily oxidizes, a mixture of an inert gas such as argon gas and oxygen gas is used. It is desirable to use gas.
Further, the oxygen gas to be sprayed need not be pure oxygen gas, and in the region where the decarburization rate decreases, the use of a mixed gas of oxygen and argon gas, etc., makes it possible to use an oxide that does not contribute to decarburization. Can be suppressed.

The method of the present invention comprises a RH vacuum degasser, D
It can be carried out using a steel refining device under reduced pressure such as an H vacuum degassing device, a VOD device, and a tank degassing device.

An example of an apparatus for carrying out the method of the present invention will be described with reference to FIG.

FIG. 1 is a vertical sectional view of an RH vacuum degassing apparatus. The vacuum tank 1 is installed above the ladle 3, and the molten steel 6 melted in a converter or the like is housed in the ladle 3. Vacuum tank 1
Two dip pipes, that is, an ascending pipe 2a and a descending pipe 2b, are installed in the lower part of the soaker. While depressurizing the vacuum chamber 1, the two dipping pipes are immersed in the molten steel 6 in the ladle 3, The molten steel 6 is pulled up into the vacuum tank 1, an inert gas is introduced from the circulating gas blowing tuyere 4 to reflux the molten steel 6 based on the gas lift principle, and vacuum decarburization treatment is performed. Inside the vacuum chamber 1, a powder top blowing lance 5
The decarburizing agent powder 7 is provided with a refining gas containing argon gas, nitrogen gas, oxygen gas or oxygen gas as a carrier gas from a nozzle at its tip during decarburization treatment.
Is sprayed on the surface of molten steel to accelerate the decarburization reaction.

[0038]

EXAMPLE A decarburization treatment test was conducted using a 170 t scale RH vacuum degassing apparatus shown in FIG.

The molten steel was blown out in the converter to an end point carbon content of about 400 ppm and an end point oxygen content of about 400 ppm, and then tapped in a non-deoxidized state. The basic conditions are as follows.

Undeoxidized molten steel in the ladle (temperature 1640 to 1660 ° C)
After immersing the dipping pipe in, the pressure inside the vacuum chamber was reduced and the molten steel was sucked up into the vacuum chamber. After that, Ar gas was blown from the circulating gas blowing tuyere provided inside the rising pipe to raise the molten steel in the rising pipe so that the molten steel was refluxed, and decarburization treatment was started.

The molten steel components other than [C] before the decarburization treatment are as follows.

Si: 0.2%, Mn: 0.15%, P: 0.010%,
S: 0.005%, Cr: 0.025%, Ti: 0.025%, balance Fe
And inevitable impurities During the treatment, the molten steel is repeatedly sampled periodically to predict the time when [C] = 50ppm, and at that time, a carrier gas having a flow rate of 10 Nl / min is used to perform various demixing in advance. A carbon powder (average particle size of about 0.15 mm) was sprayed. The degree of vacuum was 1 to 2 Torr throughout the decarburizing process, the hole diameter of the spray nozzle was 25 mmφ, and the distance between the lance and the molten steel surface was 2.0 m.

[Test 1] The decarburization treatments were compared under the following conditions according to the above method.

Carrier gas type for powder spraying: Argon gas Inventive example: Decarburizing agent (iron ore powder 90%, MgO powder 10%) Comparative example: Oxidizing agent (iron ore powder only, average particle size) About 0.15m
m) Comparative Example: Ordinary RH decarburization treatment (without powder spraying) The chemical compositions of iron ore and MgO used are as follows.

Iron ore: T.Fe = 63%, Al ₂ O ₃ = 1%, SiO ₂
= 5%, balance = unavoidable impurities MgO: MgO = 98%, CaO = 1%, SiO 2 = 0.3%, Al 2 O 3
= 0.08%, balance = unavoidable impurities FIG. 2 is a diagram showing the time change of [C] at this time. As is clear from FIG. 2, in the example of the present invention and the comparative example, [C] is
The decarburization rate up to the decarburization period of about 50 ppm is within the margin of error due to the timing of analysis and sampling, and there is virtually no difference. However, from the low carbon concentration region in the latter stage of decarburization where the decarburizing agent or the oxidant has begun to be sprayed, the decarburization reaction begins to show a difference, and in the present invention example, the decarburization rate is large in the low carbon concentration region
And the lowest carbon concentration was reached in a short time.

[Test 2] Decarburization was carried out according to the basic conditions and the carrier gas conditions of Test 1 by changing the mixing ratio of the iron ore powder and the MgO powder in the decarburizing agent of Test 1. FIG. 3 is a diagram showing the mixing ratio and [C] with time in this case.

As shown in the figure, in all the examples satisfying the conditions defined by the present invention, the difference in decarburization behavior is apparent from the low carbon concentration region where the blowing of the decarburizing agent was started. In other words, the decarburization rate and the ultimate carbon concentration in the low carbon concentration region are substantially the same within the analytical error range in the three cases where the MgO powder ratio is 5, 10 and 20%, whereas the iron ore 1% for powder
It is worse in the case of adding MgO powder. For iron ore powder
In the example in which 80% MgO powder was mixed, both the decarburization rate and the reached oxygen concentration were even worse.

[Test 3] The mixing ratio of the decarburizing agent was changed to iron ore powder.
It was fixed to 90% and 10% MgO powder, and the carrier gas was changed to argon gas, oxygen gas, and 50% argon-50% oxygen gas for decarburization treatment. Other conditions are basic conditions and test 1
It complied with the conditions.

FIG. 4 is a diagram showing the conditions of the carrier gas species and the time change of [C] at this time. As shown in the figure, the decarburization behavior is almost as good as the examples of the present invention shown in FIGS. Further, no difference due to the carrier gas species is observed from the low carbon concentration range where the spraying of the decarburizing agent was started. That is, the decarburization rate and the ultimate carbon concentration in the low carbon concentration region are as follows even if the oxygen concentration in the carrier gas is 100%.
There is no difference within the analytical error. Therefore, since a method of using oxygen gas as a carrier gas is effective in the sense of supplying oxygen gas for decarburization, a method of using only oxygen gas as a carrier gas has a particularly low end point oxygen amount in a converter, It is considered to be effective when the oxygen source supplied to the decarburization reaction is small.

[Test 4] Iron oxide + 10% in decarburizing agent powder
SiO ₂ , iron oxide + 10% Al ₂ O ₃ , iron oxide + 10% MgO,
Decarburization was performed using argon gas as the carrier gas, respectively. Other conditions were in accordance with the basic conditions and the conditions of Test 1.

FIG. 5 is a diagram showing the kind of decarburizing agent powder and the time change of [C] at this time.

As shown in the drawing, since the composition of any powder is within the range of the present invention, the decarburization rate in the low carbon concentration range is sufficiently high and the ultimate carbon concentration is 10 ppm or less. From this result, it is understood that the method of the present invention is excellent as a method for melting ultra-low carbon steel having a carbon concentration of 10 ppm or less.

[Test 5] The composition of the molten steel was Si: 0.45%, Mn:
1.3%, P: 0.08%, manganese oxide + 10% SiO ₂ , manganese oxide + 10% MgO, iron oxide + 10% MgO, and argon gas as carrier gas were used as the decarburizing agent powder. A decarburization process was performed. Other conditions were in accordance with the basic conditions and the conditions of Test 1.

FIG. 6 is a diagram showing the kind of decarburizing agent powder and the time change of [C] at this time.

As shown in the drawing, since the composition of each powder is within the range of the present invention example, the decarburization rate in the low carbon concentration range is sufficiently high and the reached carbon concentration is 10 ppm or less.
In addition, it is understood that manganese oxide can also be used for decarburization as an oxidizing agent in steel types in which an increase in Mn content is allowed.

[0056]

EFFECTS OF THE INVENTION According to the method of the present invention, it is possible to improve the decarburization rate from a low carbon concentration region, reduce the ultimate carbon concentration and shorten the treatment time when performing decarburization treatment of molten steel under reduced pressure. Therefore, ultra low carbon steel can be stably obtained.

[Brief description of drawings]

1 is a schematic vertical sectional view of an RH vacuum degassing apparatus for carrying out the method of the present invention.

FIG. 2 is a diagram showing a decarburization treatment method and a temporal change in carbon concentration in molten steel.

FIG. 3 is a diagram showing a time change of carbon concentration in molten steel when the mixing ratio of iron ore powder and MgO powder in the decarburizing agent is changed.

FIG. 4 is a diagram showing an example of changes over time in carbon concentration in molten steel according to the method of the present invention.

FIG. 5 is a diagram showing an example of changes over time in carbon concentration in molten steel according to the method of the present invention.

FIG. 6 is a diagram showing an example of changes over time in carbon concentration in molten steel according to the method of the present invention.

[Explanation of symbols]

1: Vacuum tank, 2a: Immersion pipe (upward pipe), 2b: Immersion pipe (downward pipe), 3: Ladle, 4: Circulating gas blowing tuyere, 5: Powder top blowing lance, 6: Molten steel, 7 : Carrier gas and decarburizing powder

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kaoru Mame Sumitomo Metal Industries, Ltd. 4-53-3 Kitahama, Chuo-ku, Osaka-shi, Osaka

Claims (3)

[Claims] 1. In refining steel under reduced pressure, powder of at least one of iron oxide, manganese oxide, chromium oxide and nickel oxide is sprayed onto the surface of molten steel together with a carrier gas to remove molten steel. A method of treating charcoal, wherein the powder contains alkaline earth metal oxides (MgO, CaO, Sr).
A method for promoting decarburization of molten steel, characterized by mixing 5 to 20% by weight of powder consisting of one or more of O ₂ , BaO), Al ₂ O ₃ and SiO ₂ and spraying. 2. The method for accelerating decarburization of molten steel according to claim 1, wherein the powder is sprayed in a region where [C] in the molten steel is 0.005% by weight or less. 3. The molten steel according to claim 1, wherein argon gas, nitrogen gas, oxygen gas or a refining gas containing oxygen gas is used as a carrier gas for spraying the powder. Decarburization promotion method.