CN103088187A - Method of producing steel - Google Patents

Abstract

The present invention provides a method for producing steel capable of producing a slab with less inclusions and excellent cleanliness even under high-speed casting. In the undeoxidized state, steel is directly tapped from the converter to the ladle. After tapping, metal Al or Al scum is added to the slag in the ladle to reduce the low-level oxides in the slag, and the slag is added to the slag containing The bulk composition of hydroxide and carbon oxide is the MgO source so that the MgO concentration in the slag becomes 6 to 15% by mass, and then, in the vacuum degassing device, the carbon in molten steel is reacted with dissolved oxygen to form The dissolved oxygen concentration is reduced to below 0.050% by mass. After the dissolved oxygen concentration reaches below 0.050% by mass, the molten steel is deoxidized with metal Al, and Mn is not added to the undeoxidized molten steel. When the Mn concentration of molten steel needs to be adjusted, the Al After deoxidation, Mn-containing metal is added to adjust the Mn concentration, and then the molten steel is cast into a slab by a continuous casting machine.

Description

steel manufacturing method

technical field

The present invention relates to a method for producing high-clean steel with less oxide-based non-metallic inclusions.

Background technique

In the case of Al-killed steel, metal Al is added as a deoxidizer when tapping the steel from the converter or after tapping to remove the oxygen in the molten steel that has increased due to oxidation refining (decarburization refining) in the converter, and then, by continuous casting Machine casting into cast slabs as raw material for rolling. By adding this metal Al, Al ₂ O ₃ is produced as a deoxidation product in molten steel _, and when this Al ₂ O ₃ cannot float _out of molten steel and is separated and remains in the cast sheet, steel products will generate cracks and surface defects.

Therefore, in order to reduce Al ₂ O ₃ remaining in the steel, the method of adding metal Al or Al dross to the ladle containing the molten steel after tapping is conventionally used to reduce the low-order oxidation of the slag existing on the surface of the hot molten steel. (easily-reductive oxide) (FeO, MnO) to reduce the lower oxides in the slag, and then carry out Al deoxidation (for example, refer to Patent Document 1). In addition, Al dross is generated at the stage of remelting for recycling aluminum metal used as beverage cans, building materials, or aluminum wheels for automobiles, and contains about 30% by mass to about 40% by mass of metal Al. A mixture of aluminum oxides and nitrides.

In addition, in order to reduce deoxidation products, a technique of restricting the type and order of addition of deoxidizers has also been proposed. For example, Patent Document 2 proposes a method of adding deoxidizers in the order of increasing deoxidizing ability, specifically, a method of adding in the order of Mn→Si→Al. According to Patent Document 2, by adding deoxidizing agents in the order of increasing deoxidizing ability, the aggregation of deoxidized products proceeds to form deoxidized products composed of various compounds, thereby promoting flotation and separation from molten steel. However, this method has a problem in that a large amount of MnO is formed to increase the concentration of MnO in the slag. MnO is a low-level oxide, and reacts with Al in molten steel having a strong deoxidizing ability added later to continuously form Al ₂ O ₃ in molten steel, making it difficult to obtain steel with high cleanliness.

In contrast, Patent Document 3 proposes a method of first adding Al scum by adjusting the amount of addition so that dissolved oxygen remains, then sequentially adding Mn and Si, and finally adding metal Al to perform deoxidation. In this deoxidation method, although the amount of MnO produced can be reduced, there is a problem in that Mn is added before deoxidation by Al, as in Patent Document 2, and the production of MnO cannot be completely suppressed.

In addition, a technique of using Ti and Ca as main deoxidizers without using metal Al has also been proposed (for example, refer to Patent Document 4). This technique is aimed at steels containing Ti, but Ti is more expensive than Al and cannot be applied to ordinary steels. kind.

On the other hand, Patent Document 5 and Patent Document 6 propose a technique of adding an MgO source to slag on a molten steel bath surface as a technique for making deoxidized products harmless. According to Patent Document 5 and Patent Document 6, after adding the MgO source, when the slag and molten steel are stirred, the Al ₂ O ₃ of the deoxidation product reacts with the MgO source to form a spinel of MgO-Al ₂ O ₃ , MgO- Al ₂ O ₃ spinel has low aggregation and bonding, and can keep the deoxidized product at a fine level, thereby enabling detoxification.

In addition, as a technique of adding an MgO source to molten slag or molten steel, Patent Document 7 proposes a technique of adding an MgO source to molten steel in a vacuum tank immediately after the refining in the RH vacuum degasser is started, A MgO-enriched layer is formed between the molten steel and the slag in the ladle, and the reaction between the molten steel and the slag is suppressed by the MgO-enriched layer. In addition, Patent Document 8 discloses a technique of modifying the slag in the ladle by adding a slag modifier to the ladle, and then adding an MgO source to the slag in the ladle to make the slag in the ladle The slag solidifies, thereby inhibiting the slag in the ladle from flowing out from the ladle to the tundish.

Patent Document 1: Japanese Patent Application Laid-Open No. 5-230516

Patent Document 2: Japanese Patent Application Laid-Open No. 54-94422

Patent Document 3: Japanese Patent Laid-Open No. 2009-120930

Patent Document 4: Japanese Patent Laid-Open No. 2000-144330

Patent Document 5: Japanese Patent Laid-Open No. 2004-169147

Patent Document 6: Japanese Patent Laid-Open No. 2003-171714

Patent Document 7: Japanese Patent Application Laid-Open No. 6-116623

Patent Document 8: Japanese Patent Laid-Open No. 2008-240136

In recent years, from the viewpoint of productivity improvement, the casting speed of continuous casting machines has been increased, and even for steel products with sufficiently high cleanliness that have been obtained only by using the slag modification method proposed in Patent Document 1, problems occur. Defects caused by oxide-based non-metallic inclusions (hereinafter simply referred to as "inclusions"). In particular, defects caused by inclusions were observed in slabs at the time of ladle exchange corresponding to multi-heat continuous casting (referred to as "full continuous casting").

This is because, in the operation where the casting speed has been increased, the slag in the ladle and the inclusions in the molten steel existing near the slag (deoxidized products and CaO in the converter slag or added as a slagging agent (formed by agglomeration of CaO in quicklime in the ladle) is involved in the eddy current formed in the ladle at the end of the injection of molten steel from the ladle into the tundish, and the frequency of outflow to the tundish increases. In addition, the molten steel in the tundish The residence time of the mold is also relatively shortened, so the frequency of direct injection into the mold without flotation and separation increases. It should be noted that when the injection amount (per unit time) of molten steel injected from the ladle into the tundish is increased, the time at which eddy currents are generated in the ladle is advanced. That is, eddy currents start to be generated when the amount of residual molten steel in the ladle is large.

In order to improve the cleanliness of molten steel, it is effective to reduce the lower oxides in the slag. However, in Patent Document 2 and Patent Document 3, MnO is formed as described above, so the effect cannot be said to be sufficient.

In addition, in order to prevent the slag in the ladle from being involved in the eddy current, it is effective to solidify the slag in the ladle, and the technique of adding an MgO source to the slag proposed in Patent Documents 5 to 8 is effective. However, When excessively added, although the slag solidifies, the absorption capacity of the deoxidized product of the slag will be impaired, and the cleanliness may worsen on the contrary. In addition, in order to solidify the slag, it is necessary to react the added MgO source with the slag. In the references 5 to 8, MgO clinker (clinker) is used as the MgO source, and the added MgO clinker and slag need to be reacted. Processing such as stirring. However, when the molten steel is not deoxidized, when the slag is stirred by bubbling gas into the molten steel as is usually done, in order to approach the equilibrium relationship with the oxygen concentration in the molten steel, the low-level oxidation of FeO, MnO, etc. in the slag Therefore, there is a problem that the deoxidized molten steel is re-oxidized due to slag, thereby reducing the cleanliness. In addition, the surfacing area of the gas blown into the molten steel is local, so there is a problem that if the gas blowing rate is insufficient, the slag cannot be uniformly stirred, and the reduction of lower oxides or the slag remains. Inadequately cured areas.

Contents of the invention

The present invention has been made in view of the above circumstances, and its object is to provide a method for producing high-clean steel, which can sufficiently solidify the molten slag in the ladle without being involved in molten steel injected from the ladle even without stirring or the like. In the eddy current formed at the end of the tundish, and compared with the conventional method, the cleanliness of molten steel is further improved, and even under high-speed casting, a slab with less inclusions and excellent cleanliness can be obtained.

The gist of the present invention for solving the above-mentioned problems is as follows.

[1] A method of manufacturing steel, characterized in that,

have:

The process of tapping the molten steel obtained by decarburizing and refining the molten iron directly from the converter to the ladle without deoxidation;

After tapping, add metal Al or Al scum to the slag existing on the molten steel in the ladle, and then add MgO source, while reducing the low-level oxides in the slag, adjust the MgO concentration in the slag to 6 ~15% by mass process;

Next, in a vacuum degasser, react carbon in molten steel with dissolved oxygen in molten steel under reduced pressure to reduce the dissolved oxygen concentration to 0.050% by mass or less, and after the dissolved oxygen concentration in molten steel reaches 0.050% by mass or less , adding metal Al to the molten steel under reduced pressure to deoxidize the molten steel; and

Then use the continuous casting machine to cast molten steel into billets,

Wherein, the above-mentioned MgO source contains hydroxide and carbon oxide, and the gas generated by heating the MgO source to 1000°C and thermally decomposing it is 5 moles or more per 1 kg of the above-mentioned MgO source,

The addition of Mn is not carried out from the time of tapping from the converter to the addition of metal Al to the molten steel in the vacuum degassing device. Mn concentration is adjusted by adding Mn-containing metals to water.

[2] The method for producing steel according to the above [1], wherein a carbon material is added to the molten steel at the beginning of a stage in which carbon in the molten steel and dissolved oxygen in the molten steel are reacted under reduced pressure.

[3] The method for producing steel according to the above [1] or [2], wherein the single-strand molten steel casting rate in the stable casting zone of the continuous casting machine is 4.5 tons/minute or more.

[4] The method for producing steel according to any one of [1] to [3] above, wherein the concentration of dissolved oxygen in molten steel during tapping from the converter is controlled to 0.075% by mass or less.

Invention effect

According to the present invention, metallic Al or Al dross is added to the molten slag present on the bath surface of molten steel tapped from a converter, and then a source of MgO containing hydroxides and carbon oxides is added to the molten slag in the ladle , while reducing the low-level oxides such as iron oxides and manganese oxides in the slag, the melting point of the slag is raised to solidify at least a part of the slag. Therefore, the low-level oxides of the slag in the ladle The content of slag is reduced, and the outflow of slag from the ladle to the tundish is reduced by solidification with the increase of the melting point of the slag caused by the addition of the MgO source containing hydroxides and carbon oxides. In addition, the slag is stirred by the H ₂ O gas and CO ₂ gas generated by the thermal decomposition reaction of the MgO source containing hydroxide and carbon oxide added to the slag, so even without forced stirring, it is possible to The reaction between the metal Al or Al dross and the slag that is put in first can be efficiently carried out, and the dispersion and melting of MgO in the MgO source in the slag can also be promoted. At this time, H ₂ O gas is generated when heated to a relatively low temperature of about 400°C, and CO ₂ gas is generated when heated to a relatively high temperature of about 700°C. The gas effectively stirs the slag. In addition, when a bulk MgO source is used, the injected MgO source is particularly likely to intrude into and disperse into the slag layer. Through these functions, the composition of the slag can be stably controlled even when there is a slight deviation in the distribution of the input materials.

In addition, in the vacuum degasser, carbon in molten steel is reacted with dissolved oxygen under reduced pressure to reduce the concentration of dissolved oxygen to 0.050% by mass or less, and then metal Al is added to deoxidize molten steel. Therefore, deoxidation products can be suppressed. Furthermore, Mn is not added to non-deoxidized molten steel, and when it is necessary to adjust the Mn concentration of molten steel, the Mn concentration is adjusted by adding Mn-containing metals to molten steel after deoxidizing the molten steel with metal Al. Therefore, adding The concentration of dissolved oxygen in the case of Mn is sufficiently low, and the addition of Mn does not cause the formation of MnO, so that the increase in the concentration of MnO in the slag can be prevented.

That is, in the present invention, the cleanliness improvement effect based on the reduction of lower oxides in the slag, the cleanliness improvement effect based on the reduction of the outflow of the molten slag from the ladle, and the cleanliness improvement effect based on the reduced amount of deoxidized products Overlapping, it is possible to produce high-clean steel with very few inclusions.

Detailed ways

Hereinafter, although this invention is demonstrated concretely, it is not limited to this.

Put the molten iron into the converter, supply oxygen to the molten iron from the top blowing lance or the bottom blowing tuyere, and decarburize the molten iron to smelt the molten steel. In this converter decarburization refining, quicklime is added to adjust the basicity (mass %CaO/mass %SiO ₂ ) of the slag generated in the furnace to within the range of 3 to 5, and if necessary, to prevent the Dolomite is added due to the melting loss of the refractory body. In addition, manganese ore can also be added into the furnace as a Mn source for adjusting the molten steel composition. In short, it suffices to decarburize and refine molten iron using a conventional converter decarburization refining method.

However, the purpose of the present invention is to produce clean steel with fewer inclusions, and the less the amount of deoxidized products generated, the more beneficial it is for cleaning. Therefore, it is preferable to control the dissolved oxygen concentration in molten steel at the end of decarburization refining to 0.075% by mass the following. Here, dissolved oxygen is not oxygen suspended in molten steel in the form of oxides, but oxygen dissolved in molten steel.

The dissolved oxygen concentration at the end of decarburization and refining is inversely proportional to the carbon concentration in molten steel. Therefore, if the decarburization and refining is completed with a carbon concentration of 0.035% by mass or more, preferably 0.040% by mass or more, the dissolved oxygen concentration can be controlled. 0.075% by mass or less. On the other hand, the present invention targets low carbon steel (carbon concentration: about 0.02% by mass to about 0.07% by mass) and extremely low carbon steel (carbon concentration: 0.0030% by mass or less), and the molten steel at the end of decarburization refining When the carbon concentration exceeds 0.10% by mass, the decarburization and refining under reduced pressure in the vacuum degasser in the subsequent process is prolonged and the productivity is reduced. Therefore, the carbon concentration in molten steel at the end of the decarburization and refining is preferably 0.10% by mass or less. .

Without adding any deoxidizers such as Si, Al, Ti, Ca, etc. into the ladle, the obtained molten steel is directly tapped from the converter into the ladle without being deoxidized. At the end of tapping from the converter to the ladle, when the amount of molten steel in the converter decreases, a vortex is formed in the molten steel in the furnace, and the slag in the converter is involved in the vortex, and flows out into the ladle together with the molten steel, and Stay on the molten steel in the ladle. As mentioned above, at the time of tapping, a deoxidizer cannot be added, but quicklime (CaO purity of about 95% by mass) can be added. By adding this quicklime, the slag flowing out into the ladle is diluted, and the concentration of low-order oxides such as iron oxides and manganese oxides in the slag can be reduced.

First, metal Al or Al dross for reducing lower oxides such as iron oxide and manganese oxide in the slag is added to the slag in the ladle. Make sure that the amount of metal Al or Al scum added is enough to reduce all the iron oxides and manganese oxides in the slag with Al or to reduce a part of them to a harmless level. Specifically, the amount of iron oxides and manganese oxides in the slag is estimated from the amount of dissolved oxygen in molten steel at the end of converter blowing or the carbon concentration in molten steel after tapping, thereby determining the amount of metal Al or Al dross. Add amount. Generally, when the amount of dissolved oxygen in molten steel is small (the concentration of carbon in molten steel after tapping is high), there is a tendency for the amount of lower oxides in the slag to decrease. On the other hand, the amount of dissolved oxygen in molten steel When there is much (the carbon concentration in molten steel after tapping is low), the amount of lower oxides in the slag tends to increase. However, the concentration of lower oxides in the slag varies depending on the type of operation, so it is preferable to regularly perform slag analysis in the ladle to maintain accuracy.

If adding metallic Al or Al dross, immediately add a source of MgO containing hydroxides and carbon oxides to the slag in the ladle. By adding the MgO source containing the hydroxide and the carbon oxide, the MgO concentration of the slag is raised within the range of 6 to 15% by mass. When the MgO concentration of the slag is less than 6% by mass, the solidification of the slag in the ladle becomes insufficient, and the increase of inclusion defects in molten steel cannot be suppressed. In addition, when the MgO concentration of the slag exceeds 15% by mass, the slag solidifies, but the deoxidation product absorption capacity of the slag is impaired, and the cleanliness is degraded on the contrary. The MgO concentration of the slag shown here refers to the value calculated when MgO in the added MgO source is uniformly melted in the slag in the ladle, and some MgO is not melted, or the concentration is locally exceeded The concentration of MgO is included in this range. The standard of the specific addition amount of the MgO source containing hydroxide and carbon oxide is empirically confirmed as about 0.15kg/molten steel-ton~about 0.4kg/molten steel-ton in terms of MgO purity, but flows out with the slag to Since the outflow rate in the ladle and the addition amount of quicklime in the ladle vary, it is necessary to increase or decrease according to these amounts.

The composition of the slag in the ladle after adding metal Al or Al dross is CaO-Al ₂ O ₃ system or CaO-Al ₂ O ₃ -SiO ₂ system, and the melting point of the slag rises due to the MgO brought in from the MgO source. By raising the melting point of the slag, the solidification of the slag starts, thereby reducing the region in the molten state. In addition, when the MgO source containing hydroxide and carbon oxide generates gas through thermal decomposition, a large amount of heat is absorbed, so the average temperature of the slag is compared with the case of using MgO clinker by utilizing the cooling effect generated by the MgO source. Lowers faster and, affected by this, proceeds with solidification of the slag in the ladle.

In addition, the MgO source containing hydroxide and carbon oxides charged into the ladle absorbs heat from the slag to generate H ₂ O gas and CO ₂ gas. The slag in the ladle is stirred by the H ₂ O gas and CO ₂ gas generated by this decomposition reaction, so the metal Al or Al dross that was charged earlier can be efficiently removed without forced stirring of the molten steel or slag. Reactions with slag. In order to efficiently stir the molten slag, it is preferable to use a MgO source in which the gas generated by heating to 1000° C. and thermally decomposed is 5 mol or more per 1 kg of the above-mentioned MgO source. Here, the test temperature of 1000° C. is adopted with reference to the conditions of ordinary ignition loss (ig. loss) measurement. The gas generated by the above-mentioned thermal decomposition does not need to set an upper limit in particular, but when it is necessary to suppress the foaming of slag (foaming), etc., the upper limit can be set appropriately (for example, 40 mol or less per 1 kg of the above-mentioned MgO source, etc.) .

In addition, it is preferable to use 3 moles or more of _H2O generated by thermal decomposition of hydroxide per 1 kg of MgO source, and _{2 moles or} more of CO2 generated by thermal decomposition of contained carbon oxides per 1 kg of MgO source. source of MgO. At this time, H ₂ O gas is generated when heated to a relatively low temperature of about 400°C, and CO ₂ gas is generated when heated to a relatively high temperature of about 700°C. The gas effectively stirs the slag. In the case of using a MgO source that does not contain hydroxide, it takes time to generate and activate the gas after it is charged onto the slag, and therefore, the slag may solidify in a state where the MgO source floats on the slag. Cannot stir effectively.

In addition, when a bulk MgO source is used, the injected MgO source is particularly likely to intrude into and disperse into the slag layer.

Through these functions, the composition of the slag can be stably controlled even when there is a slight deviation in the distribution of the input materials.

The size of the lumps is not limited as long as the intrusion into the slag layer proceeds quickly, but it is preferably about 2 cm or more in terms of a median particle size. From the viewpoint of promoting the melting of the MgO source, about 5 cm is preferable as the upper limit.

As the MgO source to be used, a MgO source in which the concentration of MgO in the MgO source after loss on ignition heated to 1000° C. is specified to be 50% by mass or more is preferable. The reason for this is that substances with a large content such as CaO, Al ₂ O ₃ , and SiO ₂ have little effect of raising the melting point, and in order to raise the melting point, it is necessary to increase the amount of the MgO source added.

As a massive MgO source containing hydroxide and carbon oxides, a massive composition (composition of matter), for example, magnesium carbonate and magnesium hydroxide precipitated by adding sodium carbonate to an aqueous solution of a magnesium salt can be used. A MgO source obtained by molding the powder of the composite compound into a briquet, or the like. In this way, when using a MgO source obtained by molding fine primary particles of a magnesium compound into a block using a binder such as cement, the specific surface area is larger than when using sintered particles with a larger particle size such as magnesium oxide clinker. Large, the binding force between the primary particles also disappears during the temperature rise, so the melting and dispersion in the slag are promoted, so that the solidification of the slag proceeds more uniformly. In addition, the hydroxide or carbon oxide contained in the MgO source does not have to be a magnesium compound, as long as it is one or more hydroxides or carbon oxides selected from Mg, Ca, and Al, gas can be generated in the same way. They are used to stir the slag, so they can be mixed with magnesium oxide powder and agglomerated as a source of MgO.

When the MgO source containing hydroxide and carbon oxide is added to the molten slag in the ladle, the ladle containing the molten steel is transported to a vacuum degasser such as an RH vacuum degasser or a DH vacuum degasser.

In the vacuum degasser, at least a part of molten steel is exposed to an atmosphere under reduced pressure. In the present invention, the molten steel is in an undeoxidized state. Therefore, by exposing the molten steel to an atmosphere under reduced pressure, the partial pressure of CO gas in the atmosphere under reduced pressure is low, so that the carbon in the molten steel reacts with the dissolved oxygen in the molten steel, Causes a reaction to generate CO gas (C+O→CO). Through this reaction (referred to as "boiling reaction"), the carbon concentration and dissolved oxygen concentration in molten steel decrease, and the molten steel reaches a decarburized and deoxidized state. In the present invention, the boiling reaction is continued until the dissolved oxygen concentration in molten steel becomes 0.050% by mass or less, preferably 0.030% by mass or less. Carbon and oxygen decrease in proportion to their respective atomic weights.

When the carbon concentration in the molten steel decreases due to the boiling reaction, it is difficult to cause the boiling reaction. Therefore, it is preferable to add a carbon material (coke, graphite, etc.) The carbon concentration rises, thereby promoting the boiling reaction. However, when the amount of carbon material added becomes too much and exceeds the target carbon concentration of the molten steel component, it is necessary to carry out a decarburization treatment accompanied by supply of an oxygen source to the molten steel, which will cause an increase in inclusions in the molten steel. Therefore, it is necessary to add The amount of the carbon material added at the time of the boiling reaction is set within a range in which the carbon concentration of the molten steel after the boiling reaction does not exceed the target carbon concentration of the molten steel component.

In this way, the boiling reaction is continued, the dissolved oxygen concentration in the molten steel is 0.050% by mass or less, and the carbon concentration in the molten steel is at any time within the range of the target steel composition, and the metal Al is added to the molten steel under reduced pressure. to deoxidize molten steel. The dissolved oxygen concentration can be measured by, for example, a measuring device using an oxygen concentration cell as a sensor. Adding metal Al causes a reaction between the added Al and dissolved oxygen (2Al+3O→Al ₂ O ₃ ), the dissolved oxygen suddenly decreases to a concentration of several ppm, and the boiling reaction stops. The dissolved oxygen concentration at the time of adding metal Al does not need to be specified as long as it is 0.050% by mass or less. However, the lower the dissolved oxygen concentration, the smaller the amount of deoxidized products produced. Therefore, it is preferable to reduce the dissolved oxygen concentration as much as possible. The addition amount of metal Al is set so that 0.01-0.07 mass % of Al melt|dissolves in molten steel after removing dissolved oxygen.

Addition of Mn was not performed after tapping from the converter until metal Al was added to molten steel in a vacuum degasser. Si is preferably not added until it is deoxidized from metal Al, similarly to Mn.

However, if it is necessary to adjust Mn or Si based on the steel type composition standard of the melted steel, the composition adjustment is carried out by adding a Mn source or a Si source to the molten steel after deoxidation treatment with metal Al. In this case, Mn-containing metals such as high-carbon ferromanganese (FMnH) or metallic manganese are used for adjustment as the Mn source, but high-carbon ferromanganese, which is the cheapest among Mn-containing metals, contains about 7% by mass of carbon. Carbon ferromanganese, the carbon concentration in molten steel rises. Therefore, the carbon concentration in molten steel at the end of the boiling reaction is adjusted in consideration of the amount of increase in carbon concentration caused by the addition of Mn-containing metals such as high-carbon ferromanganese. Since metallic manganese does not contain carbon, in the case of using metallic manganese as the Mn source, there is no need to consider the amount of increase in carbon concentration. In addition, when it is necessary to adjust trace addition elements such as Nb, V, B, Ca, Ti, etc., it is also carried out after the deoxidation treatment by metal Al.

In this way, even in the case of adjusting the Mn concentration of molten steel, the addition timing of the Mn-containing metal is the time when the dissolved oxygen concentration after Al deoxidation is extremely low, so the reaction between Mn and dissolved oxygen in the Mn-containing metal does not occur, When Mn-containing metal is added, the formation of MnO, which is a lower oxide, is prevented.

When the degassing refining in the vacuum degasser is completed, the ladle containing the molten steel is transported to a billet continuous casting machine, and cast by the continuous casting machine to manufacture a billet. In a continuous casting machine, casting is preferably performed by high-speed casting in which a single-strand molten steel casting amount is 4.5 tons/minute or more in a stable casting zone from the viewpoint of improving productivity.

As the casting in the continuous casting machine proceeds, the molten steel in the ladle decreases, and the height of the molten steel in the ladle is extremely low just before the ladle exchange of full continuous casting, and the flow of molten steel from the ladle to the tundish outlet hole (ladle nozzle) The molten steel in the nearby ladle forms a vortex. The slag in the ladle and inclusions in the molten steel near the slag are involved in the eddy current and flow out to the tundish, and a part of the tundish that has flowed out does not completely float out and flows out into the mold to become Cast sheet inclusions. Under high-speed casting, the injection flow rate of molten steel injected from the ladle into the tundish is large, therefore, from the time when the residual molten steel in the ladle is large, an eddy current is formed in the ladle, and the slag in the ladle and the slag existing in the vicinity of the slag The frequency of inclusions in molten steel being drawn into eddy currents increases.

However, in the present invention, the slag in the ladle is solidified by adding the MgO source containing hydroxide and carbon oxide, so the slag in the ladle is not easily involved in the eddy current, and the addition of metal Al or Al floating slag to reduce the low-level oxides in the slag, and adjust the Mn of the molten steel composition after Al deoxidation, so it can prevent the new generation of MnO, reduce the oxygen potential of the slag, and the inclusions in the molten steel near the slag In addition, since Al deoxidation is carried out when the dissolved oxygen concentration is 0.050% by mass or less, the amount of deoxidized products itself decreases, and the cleanliness of molten steel is ensured by overlapping the above effects. As a result, even single-flow molten steel High-speed casting with a casting rate of 4.5 tons/minute or more can also produce a slab with less inclusions and excellent cleanliness.

Example

The present invention will be described in more detail based on examples. The molten steel melted in a top-blown bottom-blown converter with a furnace capacity of 300 tons is refined in an RH vacuum degasser, and then cast in a twin-strand billet continuous caster for casting a billet with a thickness of 235 mm and a width of 1100 mm. The present invention is applied to the process of manufacturing a billet for thin steel plates. In addition, as a comparison, operations outside the scope of the present invention were also performed. Melting was carried out under the same conditions using four furnaces as a unit, and the full continuous casting of the above-mentioned four furnaces was carried out in the continuous casting, and tests were performed using the four furnaces as a unit.

The produced cast sheet is hot-rolled without surface modification to form a thin steel sheet, the obtained thin steel sheet is pickled, then cold-rolled, and the cold-rolled steel sheet is tin-plated. Surface defects caused by oxide-based inclusions in the tin-plated steel sheet were examined.

Table 1 shows the operating conditions in the examples of the present invention and the comparative examples, and the index of surface defects caused by oxide-based inclusions in the steel sheets. Here, the surface defect index shown in Table 1 is based on the degradation rate due to defects in Comparative Example 1 in which the MgO source, which is a bulk composition containing hydroxide and carbon oxide, was not added (1.00). and indexed to the values shown. The MgO ball used as the MgO source which is a bulk composition containing hydroxide and carbon oxide contains magnesium hydroxide, magnesium carbonate and magnesium oxide, and _the Mg(OH) content is about 20% by mass or more, MgCO ₃ content of about 20% by mass or more, loss on ignition (1000°C treatment) of about 20% by mass, and MgO concentration in the residue after loss on ignition of 80% by mass or more is molded into a diameter Lumpy flux obtained from a briquette of about 30mm. When heated to 1000°C, 3.4 mol or more of _H2O is generated by thermal decomposition per 1 kg of MgO source, and 2.4 or more mol of _CO2 is generated by thermal decomposition per 1 kg of MgO source.

In all the examples and comparative examples, after tapping the undeoxidized molten steel into the ladle, Al dross was added to the molten slag in the ladle, and in all the examples and some comparative examples, MgO balls were further added for melting. Slag control. In addition, in Comparative Example 8, after throwing Al dross and MgO clinker on the slag in the ladle, the lance covered with the refractory was immersed in molten steel for about 2 m, and the blasting rate was about 1 Nm ³ /min. Ar gas was injected for 3 minutes for stirring. The MgO clinker used in Comparative Example 8 is obtained by calcining magnesium hydroxide extracted from seawater, contains 90% by mass or more of MgO, and contains SiO ₂ , CaO, Al ₂ O ₃ , Fe ₂ O as impurities. ₃ and a small amount of oxides.

In Examples 1 to 5 of the present invention, compared with Comparative Example 1, inclusions could be reduced to 1/3 or less. On the other hand, in Comparative Example 2 in which the addition amount of MgO balls containing hydroxide and carbon oxide was large, and the MgO content in the slag exceeded the range of the present invention, and the addition amount of MgO balls was small, and the MgO content in the slag was In Comparative Example 4 in which the content did not fall within the range of the present invention, a sufficient effect of reducing inclusions was not obtained. In addition, in Comparative Example 3 in which no Mn source was added but MgO balls were not added to the boiling reaction by RH vacuum degassing, a sufficient effect of reducing inclusions was not obtained. In Comparative Example 6, although MgO balls were used to control the slag composition, the dissolved oxygen concentration before Al addition was higher than 0.050% by mass, and therefore, a sufficient effect of reducing inclusions was not obtained. In addition, in Comparative Example 7, although MgO balls were used to control the composition of the slag, but the Mn source was added during the boiling reaction, the concentration of MnO in the slag increased, and a sufficient effect of reducing inclusions was not obtained. Also, in Comparative Example 8 in which slag was controlled by adding MgO clinker instead of adding MgO balls and performing bubbling and stirring, a sufficient effect of reducing inclusions was not obtained.

As described above, it has been confirmed that by applying the present invention, steel with less inclusions and excellent cleanliness can be produced even under high-speed casting with a single-strand molten steel casting amount of 4.5 tons/min or more.

Claims (4)

1. A method of manufacturing steel, characterized in that, have: The process of tapping the molten steel obtained by decarburizing and refining the molten iron directly from the converter to the ladle without deoxidation; After tapping, add metal Al or Al scum to the slag existing on the molten steel in the ladle, and then add MgO source, while reducing the low-level oxides in the slag, adjust the MgO concentration in the slag to 6 ~15% by mass process; Next, in a vacuum degasser, react carbon in molten steel with dissolved oxygen in molten steel under reduced pressure to reduce the dissolved oxygen concentration to 0.050% by mass or less, and after the dissolved oxygen concentration in molten steel reaches 0.050% by mass or less , adding metal Al to the molten steel under reduced pressure to deoxidize the molten steel; and Then use the continuous casting machine to cast molten steel into billets, Wherein, the MgO source contains hydroxide and carbon oxide, and the gas generated by heating the MgO source to 1000° C. and thermally decomposing it is 5 moles or more per 1 kg of the MgO source, The addition of Mn is not carried out from the time of tapping from the converter to the addition of metal Al to the molten steel in the vacuum degassing device. Mn concentration is adjusted by adding Mn-containing metals to water. 2. The method for producing steel according to claim 1, wherein the carbon material is added to the molten steel at the beginning of the stage of reacting the carbon in the molten steel with the dissolved oxygen in the molten steel under reduced pressure. 3. The manufacturing method of steel according to claim 1 or 2, characterized in that the single-strand molten steel casting rate in the stable casting zone of the continuous casting machine is 4.5 tons/minute or more. 4. The method for producing steel according to any one of claims 1 to 3, wherein the dissolved oxygen concentration in the molten steel during tapping from the converter is controlled to be 0.075% by mass or less.