Method for refining molten steel and steel production method

Abstract

Proposed is a method for refining low sulfur molten steel in which the concentration of nitrogen is low and generation of alumina-based inclusions is reduced. The method is characterized by, when performing a refining treatment for reducing the concentration of sulfur in molten steel, adding at least a metal silicon source as a deoxidizing material, adding at least a calcium oxide source as a slag-making material, adding at least one of an aluminum oxide source and a calcium fluoride source for promoting slag formation, adjusting the composition of a slag to inhibit reduction of aluminum oxide in the slag caused by the molten steel, and setting a molten slag layer thickness HS (m) so as to satisfy HS>0.02×[Si]1 / 6 with respect to the silicon concentration [Si] (mass%) in the molten steel, the concentration of sulfur to at most 0.020 mass%, and the total concentration of aluminum to at most 0.005 mass%.

Description

Method for refining molten steel and method for producing steel

[0001] The present invention relates to a method for refining molten steel, and more particularly to a method for desulfurizing molten steel. It also relates to a method for producing steel with reduced nitrogen concentration and the generation of alumina-based inclusions. In this specification, "t" representing the unit of mass means metric ton, or 1000 kg. "N" preceding the unit of gas volume means the gas volume at standard conditions, a temperature of 0°C and a pressure of 101325 Pa. "l" representing the unit of volume means 10 －３ I understand ３ This means that the element [M] is dissolved in the molten steel, and the symbol (R) is contained in the slag. The range of values ​​"x to y" means x or greater and y or less, including boundary values.

[0002] Sulfur (S) contained in steel leads to hot brittleness, deterioration of corrosion resistance, and a decrease in toughness and workability. Therefore, a reduction in the S concentration in steel is required. Similarly, nitrogen (N) leads to a decrease in ductility and weld toughness of steel. Therefore, a reduction in the N concentration in steel is required.

[0003] In methods for producing molten steel using molten iron produced in a blast furnace, desulfurization is often performed at the stage of tapping the molten iron from the blast furnace to ensure the sulfur content meets product specifications. On the other hand, when desulfurization is required to achieve a strict sulfur content of 20 ppm by mass or less, desulfurization must be performed in the secondary refining process of the molten steel after tapping from the converter. In such cases, processes using ladle refining equipment such as LF (Ladle Furnace) or vacuum degassing equipment such as RH (Ruhrstahl-Heraeus) are widely applied.

[0004] In the method of producing molten steel by melting a cold iron source using an electric furnace, the sulfur concentration of the molten steel is high when tapped from the electric furnace. Therefore, even for products that do not require a strict sulfur concentration standard, desulfurization treatment is almost always necessary in the secondary refining process of the molten steel.

[0005] Regarding methods for efficiently desulfurizing molten steel using LF, for example, Patent Document 1 proposes a method for determining the slag composition in a ladle. This method involves adding metallic aluminum, which is a deoxidizing agent, to the molten steel before ladle refining to increase the concentration of basic oxides such as calcium oxide and magnesium oxide.

[0006] Japanese Patent Publication No. 2012-12648

[0007] However, the aforementioned conventional technology still had the following problems that needed to be solved. Adding a metallic aluminum source increases the dissolved aluminum concentration in the molten steel, which leads to an increase in the amount of alumina-based inclusions generated during reoxidation. The presence of alumina-based inclusions can cause problems such as breakage in wire materials and fracture in rail materials. For this reason, the upper limit of the total aluminum concentration is strictly controlled. Adding silicon, which has weaker deoxidizing power than aluminum, can be considered to suppress the generation of alumina-based inclusions. However, sufficient deoxidation is necessary to promote the desulfurization reaction. Therefore, when using silicon for deoxidation, there is a problem that the desulfurization efficiency is lower during the desulfurization process compared to when using aluminum.

[0008] To reduce the dissolved oxygen concentration and promote desulfurization through silicon-based deoxidation, it is effective to reduce the silicon oxide concentration in the slag during ladle refining and thereby decrease the silicon oxide activity. To achieve this, increasing the amount of calcium oxide, a basic oxide, added is effective. Adding calcium oxide also has the effect of increasing the desulfurization capacity of the slag.

[0009] Increasing the calcium oxide concentration in slag makes it less likely to slag. Therefore, it is necessary to add a substance to promote slag formation. Generally, aluminum oxide or calcium fluoride is used. However, as the alumina concentration in the slag increases, the deoxidation by silicon is strengthened, and the aluminum oxide in the slag is reduced by the dissolved silicon in the molten steel. This increases the dissolved aluminum concentration in the molten steel, which raises the risk of alumina-based inclusion formation. Therefore, it is necessary to appropriately control the slag composition to achieve both sufficient desulfurization and avoid an increase in dissolved aluminum concentration.

[0010] Furthermore, ladle refining using LF (Luminous Gas) is often performed under atmospheric pressure. In this process, the molten steel is exposed to the air during processing due to the effects of gas agitation, which leads to an increase in nitrogen concentration. In other words, it has been difficult to achieve both gas agitation and low nitrogen levels simultaneously.

[0011] This invention has been made in view of these circumstances, and its objective is to propose a method for refining molten steel that reduces nitrogen concentration and the generation of alumina-based inclusions. It also aims to propose a method for manufacturing steel that includes this method.

[0012] The present invention provides a method for refining molten steel that advantageously solves the above problems. In refining the molten steel to reduce the sulfur concentration, the method involves adding at least a metallic silicon source as a deoxidizing agent, at least a calcium oxide source as a slag-forming agent, and at least one of an aluminum oxide source and a calcium fluoride source to promote slag slag formation, thereby adjusting the slag composition to suppress the reduction of aluminum oxide in the slag by the molten steel. Furthermore, the method is characterized in that, with respect to the silicon concentration [Si] (mass%) in the molten steel, the molten slag layer thickness HS (m) satisfies the following relational expression 1, the sulfur concentration is 0.020 mass% or less, and the total aluminum concentration is 0.005 mass% or less. [Relational expression 1] HS > 0.02 × [Si] １／６ Here, HS: molten slag layer thickness (m), and [Si]: silicon concentration in molten steel (mass%).

[0013] Furthermore, the molten steel refining method according to the present invention is characterized by: (a) adding a calcium oxide source such that the ratio of calcium oxide concentration to silicon oxide concentration in the slag is in the range of 1.5 to 3.5 by mass; (i) when a calcium fluoride source is added, the ratio of calcium fluoride concentration to calcium oxide concentration in the slag is in the range of 0.05 to 0.25; (ii) when an aluminum oxide source is added, the lower limit of the ratio of calcium oxide concentration to silicon oxide concentration in the slag is set to 1.6, and the ratio of calcium oxide concentration to aluminum oxide concentration in the slag is in the range of 2.5 to 5.0; (b) the amount of metallic aluminum source added as a deoxidizing agent is 0.1 kg or less of metallic aluminum per ton of molten steel; (c) part or all of the molten steel subjected to the refining treatment is molten metal tapped from an electric furnace; and (d) the refining treatment to reduce the sulfur concentration is carried out under a reduced pressure atmosphere, or a degassing treatment is carried out under a reduced pressure atmosphere after the refining treatment. (e) A more preferable solution may be to reduce the silicon concentration in the molten steel to 0.15% by mass or less during the refining process that reduces the sulfur concentration, add metallic silicon after the refining process to achieve the silicon concentration of the product standard, and keep the stirring time of the molten steel within 5 minutes after the addition.

[0014] The present invention provides a method for manufacturing steel that advantageously solves the above problems, characterized in that, when manufacturing steel by solidifying molten steel refined using any of the above molten steel refining methods to obtain a cast slab, the cast slab has an aluminum concentration that does not dissolve in acid of 0.004% by mass or less.

[0015] According to the present invention, when refining molten steel contained in a ladle, it is possible to stably perform desulfurization to the sulfur concentration of the product standard even when the sulfur concentration in the molten steel is high at the start of the ladle process. In particular, it is possible to suppress the increase in dissolved aluminum concentration in the molten steel and to suppress the formation of nitrogen concentration and alumina-based inclusions.

[0016] The embodiments of the present invention will be described in detail below. The following embodiments are illustrative of equipment and methods for realizing the technical idea of ​​the present invention, and do not limit the configuration to those described below. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0017] In developing this invention, the inventors placed molten steel, which had been melted in a converter or electric furnace, into a ladle and then refined the molten steel using an LF (ladle refining furnace) facility. The target composition of the molten steel is a sulfur concentration of 0.020% by mass or less and an upper limit of 0.005% by mass or less for the total aluminum concentration.

[0018] In this processing step, the sulfur concentration and total aluminum concentration in the molten steel were investigated by changing various operating conditions, including the component concentrations of the molten steel and slag. When scrap was melted and tapped in an electric furnace, the sulfur concentration in the molten steel was approximately 0.030–0.040 mass%, and the total aluminum concentration was 0.001 mass% or less. Deoxidation of the molten steel was performed by adding metallic silicon before ladle refining with LF. The silicon concentration in the molten steel after deoxidation was 0.15–0.5 mass%.

[0019] Subsequently, a calcium oxide source was added and the molten steel was stirred, and the changes in the composition of the molten steel were investigated by varying the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the slag. As a result, under conditions where the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was less than 1.5, no increase in the total aluminum concentration in the molten steel was observed. On the other hand, the desulfurization reaction did not proceed either.

[0020] Under conditions where the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was 1.5 or higher, slag slag formation did not progress. Therefore, an aluminum oxide source or a calcium fluoride source was added to induce slag slag formation. Subsequently, the relationship between the sulfur concentration and the total aluminum concentration in the molten steel was investigated.

[0021] Under conditions where an aluminum oxide source was added, slag slag formation was confirmed when the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) in the slag was 5.0 or less, and the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was 1.6 or more. In that case, it was possible to desulfurize to the target sulfur concentration of 0.020 mass% or less.

[0022] However, as the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) in the slag decreased, and as the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) increased, a tendency was observed for the total aluminum concentration in the molten steel to increase during the refining process using LF. An increase in the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) reduces the silicon oxide activity in the slag, and the dissolved oxygen concentration decreases during deoxidation using silicon. Furthermore, as the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) decreases, the aluminum oxide activity in the slag increases. This leads to the reduction of aluminum oxide in the slag by dissolved silicon in the molten steel. By setting the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) to 3.5 or less and the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) to 2.5 or more, the reduction of aluminum oxide in the slag is suppressed. Therefore, it was possible to suppress the total aluminum concentration after the refining process using LF to 0.005 mass% or less.

[0023] Under conditions where a calcium fluoride source was added, slag slag formation was confirmed when the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) in the slag was 0.05 or higher, and the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was 1.5 or higher. In that case, desulfurization to the target sulfur concentration of 0.020 mass% or less was possible. Similar to the case where an aluminum oxide source was used, the total aluminum concentration after refining with LF increased as the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) increased. By setting the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) to 3.5 or less, it was possible to reduce the total aluminum concentration after refining with LF to 0.005 mass% or less. This is because even without the addition of an aluminum oxide source, the reduction of aluminum oxide in the slag inevitably occurs, causing the total aluminum concentration in the molten steel to increase.

[0024] When the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) in the slag was increased to more than 0.25, abnormal melting and erosion of the ladle refractory material was observed.

[0025] Next, we investigated the changes in molten steel components during the refining process at a level where the amount of metallic silicon added before the smelting process using LF was reduced, resulting in a silicon concentration in the molten steel of 0.15% by mass or less. As a result, it was possible to reduce the total aluminum concentration to 0.005% by mass or less in all slag components. This is because lowering the silicon concentration in the molten steel during the smelting process using LF increases the dissolved oxygen concentration, thereby suppressing the reduction of aluminum oxide in the slag.

[0026] Furthermore, the change in nitrogen concentration during the refining process using LF was investigated by varying the thickness of the molten slag layer. As a result, it was found that increasing the thickness of the molten slag layer suppressed nitrogen absorption into the molten steel. This is thought to be due to the effect of the slag acting as a barrier between the atmospheric air and the molten steel. It was also revealed that the higher the silicon concentration in the molten steel, the greater the thickness of the molten slag layer required to suppress nitrogen absorption into the molten steel. The dissolved oxygen concentration decreases as the silicon concentration in the molten steel increases. This is because oxygen is a surfactant element, and nitrogen absorption from the molten steel surface is more likely to occur as the oxygen concentration decreases.

[0027] The inventors' investigation revealed that nitrogen absorption into molten steel is suppressed when the molten slag layer thickness HS (m) satisfies the following relational equation 1, relative to the silicon concentration [Si] (mass%) in the molten steel. [Relational Equation 1] HS > 0.02 × [Si] １／６ Here, HS is the thickness of the molten slag layer (m), and [Si] is the silicon concentration in the molten steel (mass%).

[0028] As a measure to reduce nitrogen concentration, it is effective to perform ladle refining using LF under a reduced pressure atmosphere to lower the partial pressure of nitrogen in the atmosphere, or to perform degassing treatment of molten steel under a reduced pressure atmosphere after the refining treatment.

[0029] This embodiment was made based on the above-described findings, and the specific method for refining molten steel and the method for manufacturing steel will be explained below.

[0030] Molten steel, which has been primary refined or melted in a converter or electric furnace, is tapped into a ladle. Some slag from the converter or electric furnace inevitably flows into the ladle. If the amount of slag is excessive, it may be removed from the ladle. However, since silicon oxide in the incoming slag contributes to the slag formation of calcium oxide-containing substances that are subsequently added as desulfurizing agents, it does not need to be removed. The ladle is then transported to the LF (Reflux) equipment, and inert gas is supplied from a plug or injection lance installed at the bottom of the ladle to agitate the molten steel inside. Arc heating is also performed to adjust the temperature of the molten steel.

[0031] Deoxidizing metallic silicon may be added at the start of the refining process using LF, placed in the ladle before tapping in a converter or electric furnace, or added during tapping. The amount of metallic silicon added is adjusted so that the silicon concentration in the molten steel is within the specified range, while referring to the analysis results of the molten steel components that are sampled as appropriate.

[0032] Next, slag-forming agents are added to adjust the slag composition. While taking into account the amount of silicon oxide generated by deoxidation, silicon oxide sources and calcium oxide sources are added to ensure a sufficient amount of slag to maintain a molten slag layer thickness to prevent nitrogen pickup. In addition, aluminum oxide sources or calcium fluoride sources are added to promote slag formation. These slag-forming agents can be added not only during the LF treatment but also during tapping in the converter or electric furnace.

[0033] During the refining process using LF, sampling and analysis of molten steel and slag may be performed as appropriate, and alloys and slag-forming agents for adjusting the composition may be added to confirm that the composition of the molten steel has reached a predetermined value. The refining process using LF may also be divided into desulfurization and subsequent silicon concentration adjustment. After that, if necessary, degassing refining may be performed using an RH vacuum degassing device, etc., and then casting may be performed using a continuous casting machine, etc. to produce cast slabs. Alternatively, the molten steel may be formed into ingots and divided into ingots to produce cast slabs.

[0034] In the resulting cast slabs, the concentration of aluminum that does not dissolve in acid could be reduced to 0.004% by mass or less. Most of the aluminum that does not dissolve in acid consists of non-metallic inclusions such as aluminum oxide and aluminum nitride. Steel with reduced non-metallic inclusions in this way is suitable for application to wires and rails.

[0035] The above description illustrates the embodiment using an LF (Luminous Flue) facility as an example. However, the present invention can also be applied to other facilities such as ASEA-SKF (Airflow-Based Fuel) facilities, VAD (Vacuum Arc Degassing) facilities, and VOD (Vacuum Oxygen Decarburization) facilities in accordance with the above.

[0036] (Example 1) In a full-scale machine with a molten steel amount of about 200 t per treatment, the molten steel discharged from the electric furnace was accommodated in a ladle and transported to the LF facility for secondary refining treatment. Here, the sulfur concentration of the molten steel discharged from the electric furnace was 0.030 to 0.040 mass%, and the total aluminum concentration was 0.001 mass% or less.

[0037] After the start of the refining treatment using LF, Ar gas was supplied from the bottom blowing plug at a flow rate of 1000 Nl / min to stir the molten steel, and the electrode was lowered for arc heating.

[0038] Then, a metal silicon for deoxidation, a silicon oxide source, a calcium oxide source, and an aluminum oxide source were added. At that time, they were added so that the molten slag layer thickness after the refining treatment using LF became 0. (Here, the component compositions of the molten steel before and after the LF treatment under each test condition are shown in the columns of silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N] in Table 1. Also, the component composition of the slag is the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the column of (CaO) / (SiO ２ ), and the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) in the column of (CaO) / (Al ２ O ３ ). Furthermore, the aluminum concentration that does not dissolve in the acid obtained by sampling and analyzing the slab after solidification by the continuous casting machine is shown as insol. Al.

[0039]

[0040] For Nos. 1 to 3, the LF treatment was performed without adding an aluminum oxide source as a slag-forming agent. In that case, due to the aluminum oxide contained in the inflowing slag, the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) in the slag after the refining treatment using LF was 7.5 to 9.2. The total aluminum concentration in the molten steel after the refining treatment using LF did not change and was 0.001 mass% or less. On the other hand, the desulfurization reaction hardly proceeded.

[0041] For samples No. 4 to 12, an aluminum oxide source was added as a slag-forming agent, and the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) and (calcium oxide concentration) / (aluminum oxide concentration) were varied while refining was performed using LF. For samples No. 4 to 8 and 10, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was in the range of 1.6 to 3.5 and the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) was in the range of 2.5 to 5.0. These samples were able to achieve a sulfur concentration of 0.020 mass% or less and a total aluminum concentration of 0.005 mass% or less in the molten steel after refining with LF. For sample No. 9, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was greater than 3.5, and for samples No. 11 and 12, the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) was less than 2.5. In these cases, the desulfurization reaction proceeded during the smelting process using LF, but the total aluminum concentration in the molten steel after smelting exceeded 0.005 mass%. If the total aluminum concentration in the molten steel after smelting was 0.005 mass% or less, the concentration of aluminum that does not dissolve in acid in the cast slab after continuous casting, i.e., the concentration of aluminum present as inclusions, remained stable at 0.004 mass% or less. Furthermore, the thickness of the molten slag after the smelting process using LF satisfied the above relational equation 1 in relation to the silicon concentration in the molten steel during the desulfurization process, and no nitrogen absorption into the molten steel was observed.

[0042] (Example 2) In a real-world machine with a processing capacity of approximately 200 tons per batch, the molten steel tapped from the electric furnace was placed in a ladle and transported to the LF (Refining Foam) equipment for secondary refining. Here, the sulfur concentration of the molten steel tapped from the electric furnace was 0.030 to 0.040% by mass, and the total aluminum concentration was 0.001% by mass or less.

[0043] After initiating the refining process using LF, Ar gas was supplied from the bottom-blowing plug at a flow rate of 1000 Nl / min to agitate the molten steel, and the electrodes were lowered to perform arc heating.

[0044] Thereafter, metallic silicon for deoxidation, a silicon oxide source, a calcium oxide source, and a calcium fluoride source were added. At that time, the addition was made so that the thickness of the molten slag layer after the refining treatment using LF became 0.10 m. Then, a desulfurization treatment was performed for about 60 min. Table 2 shows the component compositions of the molten steel before and after the LF treatment under each test condition in the columns of silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N], respectively. Also, the component composition of the slag is the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the column of (CaO) / (SiO ２ ), and the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) in the column of (CaF ２ ) / (CaO). Furthermore, the aluminum concentration that does not dissolve in acid, which was analyzed by sampling the slab after solidification in the continuous casting machine, is shown as insol. Al.

[0045]

[0046] For samples No. 13-24, a calcium fluoride source was added as a slag-forming agent to vary the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) and mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) in the slag, and then refining was carried out using LF. For samples No. 14-20, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was between 1.5 and 3.5, and the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) was between 0.05 and 0.25. These samples achieved a sulfur concentration of 0.020% by mass or less and a total aluminum concentration of 0.005% by mass or less in the molten steel after refining with LF. In sample 13, the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) was less than 0.05, so slag formation did not progress, desulfurization did not proceed during the refining process using LF, and there was no change in the total aluminum concentration after the refining process. In samples 21, 22, and 24, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) exceeded 3.5. In these samples, the desulfurization reaction progressed during the refining process using LF, but the total aluminum concentration after the refining process exceeded 0.005 mass%. If the total aluminum concentration in the molten steel after refining is 0.005 mass% or less, the concentration of aluminum that does not dissolve in acid in the cast slab after continuous casting, i.e., the concentration of aluminum present as inclusions, remains stable at 0.004 mass% or less. In samples 23 and 24, the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) exceeded 0.25, resulting in significant erosion of the ladle refractory. Furthermore, the thickness of the molten slag after refining using LF satisfied the above relational equation 1 in relation to the silicon concentration in the molten steel during the desulfurization process, and no nitrogen absorption into the molten steel was observed.

[0047] (Example 3) In a real machine with a processing capacity of approximately 200 tons per batch, the molten steel tapped from the electric furnace was placed in a ladle and transported to the LF equipment for secondary refining. Here, the sulfur concentration of the molten steel tapped from the electric furnace was 0.030 to 0.040 mass%, and the total aluminum concentration was 0.001 mass% or less.

[0048] After initiating the refining process using LF, Ar gas was supplied from the bottom-blowing plug at a flow rate of 1000 Nl / min to agitate the molten steel, and the electrodes were lowered to perform arc heating.

[0049] Subsequently, metallic silicon, silicon oxide source, calcium oxide source, and aluminum oxide source for deoxidation were added. The addition was done so that the molten slag layer thickness after the LF refining treatment was 0.10 m. Then, a desulfurization treatment was performed for approximately 60 min. The silicon concentration in the molten steel at the start of the LF refining treatment was varied, and after the desulfurization treatment was completed, metallic silicon was added to achieve the silicon concentration of the product standard. After stirring for a certain period, the LF refining treatment was terminated. Table 3 shows the component composition of the molten steel before LF treatment, after desulfurization treatment, and after LF treatment under each test condition, in the columns for silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N], respectively. The component composition of the slag is given by the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) as (CaO) / (SiO ２ In the column, enter the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) as (CaO) / (Al ２ O ３ This is shown in the column. Furthermore, the concentration of aluminum that does not dissolve in the acid, obtained by sampling and analyzing the cast slab after solidification in a continuous casting machine, is shown as insol. Al. In addition, the thickness of the molten slag after refining using LF satisfies the above relation equation 1 in relation to the silicon concentration in the molten steel during the desulfurization process, and no nitrogen absorption into the molten steel was observed.

[0050]

[0051] For samples No. 25-29, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the slag after desulfurization was set to over 3.5, and the silicon concentration in the molten steel during desulfurization was set to 0.15 mass% or less. This made it possible to desulfurize the sulfur concentration in the molten steel after desulfurization to 0.020 mass% or less, and to set the total aluminum concentration to 0.005 mass% or less. This is because keeping the silicon concentration in the molten steel low during desulfurization leads to a high dissolved oxygen concentration, which suppresses the reduction of aluminum oxide in the slag. On the other hand, for samples No. 30 and 31, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the slag after desulfurization was set to over 3.5, and the silicon concentration in the molten steel during desulfurization was set to over 0.15 mass%. In these cases, although the desulfurization process progressed, the excessive dissolved silicon in the molten steel reduced the aluminum oxide in the slag, resulting in a total aluminum concentration in the molten steel exceeding 0.005% by mass after desulfurization. For samples No. 25-28, the stirring time after adding metallic silicon to achieve the silicon concentration of the product standard was set to 5 minutes or less. Within this range, the aluminum oxide in the slag was not excessively reduced by the dissolved silicon in the molten steel, and the total aluminum concentration in the molten steel after refining with LF was kept below 0.005% by mass. On the other hand, in sample No. 29, the stirring time after adding metallic silicon exceeded 5 minutes, resulting in an excessive increase in the total aluminum concentration during stirring. Consequently, the total aluminum concentration in the molten steel after refining with LF exceeded 0.005% by mass. If the total aluminum concentration in the molten steel after refining is 0.005% by mass or less, the concentration of aluminum that does not dissolve in acid in the cast slab after continuous casting, i.e., the concentration of aluminum present as inclusions, also remains stable at 0.004% by mass or less. Furthermore, the thickness of the molten slag after refining using LF satisfies the above relation equation 1 in relation to the silicon concentration in the molten steel during desulfurization, and no nitrogen absorption into the molten steel was observed.

[0052] (Example 4) In a real-world machine with a processing capacity of approximately 200 tons per batch, the molten steel tapped from the electric furnace was placed in a ladle and transported to the LF (Refining Foam) equipment for secondary refining. Here, the sulfur concentration of the molten steel tapped from the electric furnace was 0.030 to 0.040% by mass, and the total aluminum concentration was 0.001% by mass or less.

[0053] After initiating the refining process using LF, Ar gas was supplied from the bottom-blowing plug at a flow rate of 1000 Nl / min to agitate the molten steel, and the electrodes were lowered to perform arc heating.

[0054] Subsequently, metallic silicon, silicon oxide source, calcium oxide source, and calcium fluoride source for deoxidation were added. Then, a desulfurization treatment was performed for approximately 60 minutes. After the refining treatment using LF, the ladle was transported to an RH-type degassing facility, and the molten steel was refluxed into a vacuum chamber for vacuum degassing treatment. Table 4 shows the component composition of the molten steel before LF treatment, after treatment, and after vacuum degassing under each test condition, in the columns for silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N], respectively. The component composition of the slag is given by the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) as (CaO) / (SiO ２ In the column, enter the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) (CaF ２ This is shown in the column for ) / (CaO). The thickness HS of the molten slag after refining using LF is also shown. Furthermore, the concentration of aluminum that does not dissolve in acid, obtained by sampling and analyzing the cast slab after solidification in a continuous casting machine, is shown as insol. Al.

[0055]

[0056] For samples No. 33, 35, 37, and 39, the molten slag thickness after refining with LF satisfied the above relational equation 1 in relation to the silicon concentration in the molten steel during desulfurization, and no excessive nitrogen absorption into the molten steel was observed. On the other hand, for samples No. 32, 34, 36, 38, and 40, the molten slag thickness after refining with LF did not satisfy the above relational equation 1 in relation to the silicon concentration in the molten steel during desulfurization, and significant nitrogen absorption into the molten steel was observed. By performing vacuum degassing after refining with LF, the nitrogen concentration in the molten steel decreased. It was confirmed that this is an effective means of achieving low nitrogen content.

[0057] The molten steel refining method according to the present invention makes it possible to stably perform desulfurization to the product standard sulfur concentration even when the sulfur concentration in the molten steel is high at the start of the ladle refining process. In particular, it is possible to suppress the increase in dissolved aluminum concentration in the molten steel and to suppress the formation of nitrogen concentration and alumina-based inclusions. Therefore, it becomes easier to manufacture steel with reduced nitrogen concentration and alumina-based inclusions, which is industrially useful.

Claims

1. A method for refining molten steel, wherein, in order to reduce the sulfur concentration in molten steel, at least a metallic silicon source is added as a deoxidizing agent, at least a calcium oxide source is added as a slag-forming agent, and at least one of an aluminum oxide source and a calcium fluoride source is added to promote slag slag formation, thereby adjusting the slag composition to suppress the reduction of aluminum oxide in the slag by the molten steel, and the molten slag layer thickness HS (m) satisfies the following relational equation 1 with respect to the silicon concentration [Si] (mass%) in the molten steel, the sulfur concentration is 0.020 mass% or less, and the total aluminum concentration is 0.005 mass% or less. [Relational Equation 1] HS > 0.02 × [Si] １／６ Here, HS: molten slag layer thickness (m), and [Si]: silicon concentration in molten steel (mass%).

2. A method for refining molten steel according to claim 1, wherein, on a mass basis, a calcium oxide source is added so that the ratio of the calcium oxide concentration to the silicon oxide concentration in the slag is in the range of 1.5 to 3.5, (i) when a calcium fluoride source is added, the ratio of the calcium fluoride concentration to the calcium oxide concentration in the slag is in the range of 0.05 to 0.25, and (ii) when an aluminum oxide source is added, the lower limit of the ratio of the calcium oxide concentration to the silicon oxide concentration in the slag is set to 1.6, and the ratio of the calcium oxide concentration to the aluminum oxide concentration in the slag is in the range of 2.5 to 5.

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3. The method for refining molten steel according to claim 1 or 2, wherein the amount of metallic aluminum source added as a deoxidizing agent is 0.1 kg or less of metallic aluminum per ton of molten steel.

4. The method for refining molten steel according to any one of claims 1 to 3, wherein part or all of the molten steel subjected to the refining process is molten metal drawn from an electric furnace.

5. A method for refining molten steel according to any one of claims 1 to 4, wherein the refining treatment to reduce the sulfur concentration is performed under a reduced pressure atmosphere, or a degassing treatment is performed under a reduced pressure atmosphere after the refining treatment.

6. A method for refining molten steel according to any one of claims 1 to 5, wherein during the refining process to reduce the sulfur concentration, the silicon concentration in the molten steel is kept to 0.15% by mass or less, metallic silicon is added after the refining process to achieve the silicon concentration of the product standard, and the stirring time of the molten steel after the addition is kept to within 5 minutes.

7. A method for producing steel, wherein the molten steel refined using the molten steel refining method described in any one of claims 1 to 6 is solidified to obtain a cast slab, and the cast slab has an aluminum concentration that does not dissolve in acid of 0.004% by mass or less.

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