Method for refining molten steel and method for producing steel

Abstract

This paper proposes a method for refining low-sulfur molten steel that reduces the nitrogen concentration and the generation of alumina-based inclusions. In performing refining treatment to reduce the sulfur concentration in molten steel, at least a metallic silicon source is added as a deoxidizer, at least a calcium oxide source is added as a slag former, and at least one of an aluminum oxide source and a calcium fluoride source is added to promote slag formation. The slag composition is adjusted to suppress the reduction of aluminum oxide in the slag by molten steel, and the thickness of the molten slag layer, HS (m), is set to satisfy the following: HS > 0.02 × [Si] for the silicon concentration [Si] (mass%) in the molten steel. 1 / 6 The method is characterized by satisfying the above, setting the sulfur concentration to 0.020 mass % or less, and setting the total aluminum concentration to 0.005 mass % or less.

Description

[Technical Field]

[0001] The present invention relates to a method for refining molten steel, and in particular to a method for desulfurizing molten steel. It also relates to a method for producing steel with reduced nitrogen concentration and reduced generation of alumina-based inclusions. In this specification, the unit of mass "t" means metric ton, i.e., 1000 kg. The letter "N" before the unit of gas volume means the gas volume at standard conditions of a temperature of 0°C and a pressure of 101325 Pa. The letter "l" before the unit of volume means 10 -3 m 3 The symbol [M] indicates that element M is dissolved in molten steel, and the symbol (R) indicates that a substance with chemical formula R is contained in slag. "x~y," which indicates a range of values, means greater than or equal to x and less than or equal to y, and includes the boundary value. [Background technology]

[0002] S (sulfur) contained in steel leads to deterioration of hot brittleness and corrosion resistance of steel, as well as reduced toughness and workability. Therefore, there is a need to reduce the S concentration in steel. Furthermore, N (nitrogen) leads to reduced ductility of steel and reduced weld toughness. Therefore, there is a need to reduce the N concentration in steel.

[0003] In molten steel manufacturing methods using molten pig iron produced in a blast furnace, desulfurization is often performed on the molten pig iron tapped from the blast furnace to ensure the sulfur concentration meets product specifications. However, when stricter specifications require desulfurization to a sulfur concentration of 20 mass ppm or less, desulfurization must be performed in the secondary refining process of the molten steel after it is tapped from the converter. In such cases, processes using ladle refining equipment such as a ladle furnace (LF) or vacuum degassing equipment such as a Ruhrstahl-Heraeus (RH) are widely used.

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

[0005] Regarding a method 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, a deoxidizer, to molten steel before ladle refining to increase the concentrations of calcium oxide and magnesium oxide, which are basic oxides. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-12648 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques still have the following problems to be solved. Adding a metallic aluminum source increases the dissolved aluminum concentration in molten steel, which increases the amount of alumina-based inclusions generated due to reoxidation. The presence of alumina-based inclusions can cause breakage problems in wire materials, for example, and rail materials, for example. Therefore, the upper limit of the total aluminum concentration is strictly controlled. It is also possible to suppress the generation of alumina-based inclusions by adding silicon, which has a weaker deoxidizing power than aluminum. However, sufficient deoxidation is required to promote the desulfurization reaction. Therefore, there is a problem that the desulfurization efficiency during desulfurization is lower when deoxidizing using silicon than when deoxidizing using aluminum.

[0008] In order to reduce the dissolved oxygen concentration and promote desulfurization by deoxidizing with silicon, it is effective to reduce the silicon oxide concentration in the slag during ladle refining, thereby reducing the silicon oxide activity. To achieve this, it is effective to increase the amount of calcium oxide, a basic oxide. Adding calcium oxide, a basic oxide, also has the effect of increasing the desulfurization ability of the slag.

[0009] Increasing the calcium oxide concentration in the slag makes it difficult for the slag to form slag. Therefore, it is necessary to add a substance to promote slag formation. Aluminum oxide or calcium fluoride is generally 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 increases the risk of forming alumina-based inclusions. Therefore, it is necessary to appropriately control the slag composition so that desulfurization ability can be maintained while avoiding an increase in the dissolved aluminum concentration.

[0010] Furthermore, ladle refining using LF is often carried out under atmospheric pressure. In this case, the molten steel is exposed to air during processing due to gas agitation, which increases the nitrogen concentration. In other words, it has been difficult to achieve both gas agitation and low nitrogen.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to propose a method for refining molten steel that reduces the nitrogen concentration and the generation of alumina-based inclusions, and a method for producing steel that includes the method. [Means for solving the problem]

[0012] The method for refining molten steel according to the present invention, which advantageously solves the above-mentioned problems, is characterized in that, in performing a refining treatment to reduce the sulfur concentration in molten steel, at least a metallic silicon source is added as a deoxidizer, at least a calcium oxide source is added as a slag former, and at least one of an aluminum oxide source and a calcium fluoride source is added to promote slag formation, the slag composition is adjusted to suppress the reduction of aluminum oxide in the slag by molten steel, and the molten slag layer thickness HS (m) satisfies the following relational expression 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. [Equation 1] HS>0.02×[Si] 1 / 6 Where, HS: molten slag layer thickness (m), [Si]: Silicon concentration in molten steel (mass%) is.

[0013] Further, the method for refining molten steel according to the present invention comprises the steps of: (a) adding a calcium oxide source 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 by mass, (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 5.0; (b) The amount of metallic aluminum source added as a deoxidizer is 0.1 kg or less as metallic aluminum per ton of molten steel; (c) Part or all of the molten steel to be refined is molten steel tapped from an electric furnace; (d) performing the refining treatment for reducing the sulfur concentration under a reduced pressure atmosphere, or performing a degassing treatment under a reduced pressure atmosphere after the refining treatment; (e) during the refining treatment to reduce the sulfur concentration, the silicon concentration in the molten steel is set to 0.15 mass% or less, and after the refining treatment, metallic silicon is added so that the silicon concentration meets the product standard, and the stirring time of the molten steel after the addition is set to 5 minutes or less; This may be a more preferable solution.

[0014] The method for producing steel according to the present invention, which advantageously solves the above-mentioned problems, is characterized in that, in producing steel by solidifying molten steel refined using any of the above-mentioned methods for refining molten steel to obtain a slab, the slab has an acid-insoluble aluminum concentration of 0.004 mass% or less. [Effects of the Invention]

[0015] According to the present invention, when refining molten steel contained in a ladle, it is possible to stably desulfurize the molten steel to a sulfur concentration meeting product specifications, even when the sulfur concentration in the molten steel is high at the start of ladle processing. In particular, it is possible to suppress an increase in the dissolved aluminum concentration in the molten steel and to suppress the nitrogen concentration and the formation of alumina-based inclusions. DETAILED DESCRIPTION OF THE INVENTION

[0016] The following describes in detail the embodiments of the present invention. The following embodiments are intended to exemplify equipment and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0017] In developing the present invention, the inventors poured molten steel produced in a converter or electric furnace into a ladle and then refined the molten steel using LF (ladle refining furnace) equipment. The target chemical composition of the molten steel was a sulfur concentration of 0.020 mass% or less and a total aluminum concentration of 0.005 mass% or less.

[0018] In this treatment process, the changes in the sulfur concentration and total aluminum concentration in the molten steel during treatment were investigated by changing the component concentrations of the molten steel and slag, as well as other operating conditions. When scrap was melted in an electric furnace and tapped, the sulfur concentration in the molten steel was approximately 0.030 to 0.040 mass%, and the total aluminum concentration was 0.001 mass% or less. Before the ladle refining process using the LF, metallic silicon was added to the molten steel to deoxidize it. The silicon concentration in the molten steel after deoxidation was 0.15 to 0.5 mass%.

[0019] After that, a calcium oxide source was added and the molten steel was stirred. The mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the slag was varied to investigate the changes in the components in the molten steel. As a result, when 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 progress either.

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

[0021] When an aluminum oxide source was added, the slag was confirmed to turn into slag 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 this case, it was possible to desulfurize the slag 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, the total aluminum concentration in molten steel during refining using LF tended to increase. As the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) increased, the silicon oxide activity in the slag decreased, and deoxidation using silicon decreased the dissolved oxygen concentration. Furthermore, as the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) decreased, the aluminum oxide activity in the slag increased. This resulted in the reduction of aluminum oxide in the slag by the dissolved silicon in the molten steel. By keeping the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) below 3.5 and the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) above 2.5, the reduction of aluminum oxide in the slag was suppressed. Therefore, it was possible to suppress the total aluminum concentration after refining using LF to below 0.005 mass%.

[0023] When 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 this case, desulfurization to the target sulfur concentration of 0.020 mass% or less was possible. As with the aluminum oxide source, the total aluminum concentration after LF refining 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, the total aluminum concentration after LF refining could be reduced to 0.005 mass% or less. This is because the total aluminum concentration in molten steel increases even without the addition of an aluminum oxide source due to the reduction of aluminum oxide inevitably present in the slag.

[0024] When the mass composition ratio of calcium fluoride concentration to calcium oxide concentration in the slag was increased to more than 0.25, abnormal corrosion of the ladle refractory was confirmed.

[0025] Next, the amount of metallic silicon added before the LF refining process was reduced, and the change in the molten steel composition during the refining process was investigated at a level where the silicon concentration in the molten steel was 0.15 mass% or less. As a result, it was possible to keep the total aluminum concentration at 0.005 mass% or less for all slag components. This is because reducing the silicon concentration in the molten steel during the LF refining process increases the dissolved oxygen concentration, suppressing the reduction of aluminum oxide in the slag.

[0026] In addition, the change in nitrogen concentration during refining using LF was investigated by varying the thickness of the molten slag layer. The results showed that increasing the thickness of the molten slag layer suppressed nitrogen absorption into molten steel. This is thought to be due to the effect of the slag acting as a barrier between the ambient air and the molten steel. It was also revealed that the higher the silicon concentration in the molten steel, the thicker the molten slag layer required to suppress nitrogen absorption into the molten steel. The higher the silicon concentration in the molten steel, the lower the dissolved oxygen concentration. This is because oxygen is an interfacially active element, and as the oxygen concentration decreases, nitrogen absorption from the molten steel surface becomes more likely.

[0027] As a result of research by the inventors, it was found that nitrogen absorption into molten steel can be suppressed when the molten slag layer thickness HS (m) satisfies the following relational expression 1 with respect to the silicon concentration [Si] (mass%) in molten steel. [Equation 1] HS>0.02×[Si] 1 / 6 where 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 the nitrogen concentration, it is effective to carry out the ladle refining process using LF under a reduced pressure atmosphere to reduce the nitrogen partial pressure in the atmosphere, or to carry out a degassing process of the molten steel under a reduced pressure atmosphere after the refining process.

[0029] This embodiment has been made based on the results of the above investigations, and a specific method for refining molten steel and a method for producing steel will be described below.

[0030] Molten steel that has been primarily 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 is excessive, the slag that flows in can be removed from the ladle. However, silicon oxide in the slag does not need to be removed, as it contributes to the formation of slag in the calcium oxide-containing material that is added later as a desulfurization agent. The ladle is then transported to the LF equipment, where inert gas is supplied from a plug or injection lance installed at the bottom of the ladle to stir the molten steel in the ladle. Arc heating is also used to adjust the temperature of the molten steel.

[0031] The metallic silicon for deoxidization may be added at the start of refining using an LF, placed in a ladle before tapping from a converter or electric furnace, or added during tapping. The molten steel components are sampled as needed, and the amount of metallic silicon added is adjusted while referring to the analytical values ​​so that the silicon concentration in the molten steel falls within a predetermined range.

[0032] Next, slag formers are added to adjust the slag composition. While taking into account the amount of silicon oxide produced by deoxidation, silicon oxide and calcium oxide sources are added to obtain a slag amount that ensures a molten slag layer thickness to prevent nitrogen pickup. Furthermore, aluminum oxide and calcium fluoride sources are added to promote slag formation. These slag formers can be added not only during LF processing, but also during tapping from a converter or electric furnace.

[0033] During the refining process using LF, sampling and analysis of the molten steel and slag may be carried out as appropriate, and alloys and slag formers may be added to adjust the composition and confirm that the molten steel composition has reached the specified values. The refining process using LF may be carried out in two steps: desulfurization and subsequent silicon concentration adjustment. After that, degassing and refining are carried out as needed using an RH vacuum degasser, etc., and then the slab is cast using a continuous casting machine or other machine. The molten steel may also be ingotted and broken down to produce the slab.

[0034] In the resulting cast slab, the acid-insoluble aluminum concentration was reduced to 0.004 mass% or less. Most of the acid-insoluble aluminum consists of non-metallic inclusions such as aluminum oxide and aluminum nitride. Steel with reduced non-metallic inclusions is suitable for use in wires and rails.

[0035] Although the above description has been given using an LF facility as an example of an embodiment, the present invention can also be applied to ASEA-SKF facilities, VAD (Vacuum Arc Degassing) facilities, VOD (Vacuum Oxygen Decarburization) facilities, etc. in a similar manner. [Example]

[0036] Example 1 In a commercial furnace with a capacity of approximately 200 t per process, molten steel tapped from an electric furnace was placed in a ladle and transported to the LF facility for secondary refining. 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.

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

[0038] Next, metallic silicon, silicon oxide source, calcium oxide source, and aluminum oxide source for deoxidation were added. The addition was performed so that the molten slag layer thickness after LF refining was 0.10 m. Desulfurization treatment was then performed for approximately 60 min. Table 1 shows the composition of the molten steel before and after LF treatment under each test condition in the silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N] columns. The composition of the slag is also shown in the column for the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the (CaO) / (SiO2) column and the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) in the (CaO) / (Al2O3) column. Furthermore, the acid-insoluble aluminum concentration, obtained by sampling and analyzing the slab after solidification in the continuous caster, is shown as insol.Al.

[0039] [Table 1]

[0040] In Nos. 1 to 3, LF treatment was performed without adding an aluminum oxide source as a slag-forming agent. In these cases, the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) in the slag after refining using LF was 7.5 to 9.2, depending on the aluminum oxide contained in the inflow slag. The total aluminum concentration in the molten steel after refining using LF remained unchanged at 0.001 mass% or less. On the other hand, the desulfurization reaction hardly progressed.

[0041] For Nos. 4 to 12, an aluminum oxide source was added as a slag-forming agent, and the mass composition ratios (calcium oxide concentration) / (silicon oxide concentration) and (calcium oxide concentration) / (aluminum oxide concentration) were varied during refining using LF. For Nos. 4 to 8 and 10, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was between 1.6 and 3.5, and the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) was between 2.5 and 5.0. After refining using LF, these samples achieved sulfur concentrations of 0.020 mass% or less and total aluminum concentrations of 0.005 mass% or less in the molten steel. For No. 9, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) exceeded 3.5, and for Nos. 11 and 12, the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) was less than 2.5. In these studies, the desulfurization reaction progressed during the refining process using LF, but the total aluminum concentration in the molten steel after refining exceeded 0.005 mass%. If the total aluminum concentration in the molten steel after refining was 0.005 mass% or less, the concentration of aluminum insoluble in acid in the 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 refining using LF satisfied the above-mentioned relational expression 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 commercial furnace with a capacity of approximately 200 t per process, molten steel tapped from an electric furnace was placed in a ladle and transported to the LF facility for secondary refining. 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.

[0043] After the start of refining using the LF, Ar gas was supplied from the bottom injection plug at a flow rate of 1000 NL / min to stir the molten steel, and the electrode was lowered to perform arc heating.

[0044] Next, metallic silicon, silicon oxide source, calcium oxide source, and calcium fluoride source for deoxidation were added. The additions were made so that the molten slag layer thickness after LF refining was 0.10 m. Desulfurization treatment was then performed for approximately 60 min. Table 2 shows the composition of the molten steel before and after LF treatment under each test condition in the silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N] columns. The composition of the slag is also shown in the column for the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the column for (CaO) / (SiO2) and the column for the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) in the column for (CaF2) / (CaO). Furthermore, the acid-insoluble aluminum concentration, measured by sampling and analyzing the slab after solidification in the continuous caster, is shown as insol.Al.

[0045] [Table 2]

[0046] For Nos. 13–24, a calcium fluoride source was added as a slag-forming agent, and the mass composition ratios (calcium oxide concentration) / (silicon oxide concentration) and (calcium fluoride concentration) / (calcium oxide concentration) in the slag were varied. For Nos. 14–20, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) was 1.5–3.5, and the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) was 0.05–0.25. After refining using the LF, these samples achieved sulfur concentrations of 0.020 mass% or less and total aluminum concentrations of 0.005 mass% or less in the molten steel. For No. 13, the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) was less than 0.05, and slag formation did not progress. Desulfurization did not progress during refining using the LF, and the total aluminum concentration remained unchanged after refining. In samples Nos. 21, 22, and 24, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) exceeded 3.5. Although the desulfurization reaction progressed during the refining process using the LF, the total aluminum concentration after refining exceeded 0.005 mass%. If the total aluminum concentration in the molten steel after refining was 0.005 mass% or less, the acid-insoluble aluminum concentration in the slab after continuous casting, i.e., the aluminum concentration present as inclusions, remained stable at 0.004 mass% or less. In samples Nos. 23 and 24, the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) exceeded 0.25, resulting in significant corrosion of the ladle refractory. Furthermore, the thickness of the molten slag after refining using the LF satisfied the above-mentioned relationship with 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 commercial furnace with a capacity of approximately 200 t per process, molten steel tapped from an electric furnace was placed in a ladle and transported to the LF facility for secondary refining. 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 the start of refining using the LF, Ar gas was supplied from the bottom injection plug at a flow rate of 1000 NL / min to stir the molten steel, and the electrode was lowered to perform arc heating.

[0049] Next, metallic silicon, silicon oxide source, calcium oxide source, and aluminum oxide source for deoxidation were added. The additions were made so that the molten slag layer thickness after LF refining was 0.10 m. Desulfurization treatment was then performed for approximately 60 min. The silicon concentration in the molten steel at the start of LF refining was varied. After desulfurization, additional metallic silicon was added to achieve the product-specific silicon concentration. After stirring for a set period, the LF refining treatment was terminated. Table 3 shows the composition of the molten steel before, after, and after LF treatment under each test condition in the silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N] columns. The mass composition ratio of the slag (calcium oxide concentration) / (silicon oxide concentration) is shown in the (CaO) / (SiO2) column, and the mass composition ratio (calcium oxide concentration) / (aluminum oxide concentration) is shown in the (CaO) / (Al2O3) column. Furthermore, the concentration of aluminum insoluble in acid, which was obtained by sampling and analyzing the slab after solidification in the continuous casting machine, is shown as insol.Al. Furthermore, the thickness of the molten slag after refining using LF satisfied the above relational expression 1 in relation to the silicon concentration in the molten steel during desulfurization, and no absorption of nitrogen into the molten steel was observed.

[0050] [Table 3]

[0051] In Nos. 25 to 29, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the slag after desulfurization exceeded 3.5, and the silicon concentration in the molten steel during desulfurization was 0.15 mass% or less. This enabled the sulfur concentration in the molten steel after desulfurization to be reduced to 0.020 mass% or less, while the total aluminum concentration was reduced to 0.005 mass% or less. This was because reducing the silicon concentration in the molten steel during desulfurization increased the dissolved oxygen concentration, suppressing the reduction of aluminum oxide in the slag. On the other hand, in Nos. 30 and 31, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the slag after desulfurization exceeded 3.5, and the silicon concentration in the molten steel during desulfurization exceeded 0.15 mass%. Although desulfurization progressed in these cases, the excess dissolved silicon in the molten steel reduced the aluminum oxide in the slag, resulting in a total aluminum concentration in the molten steel after desulfurization exceeding 0.005 mass%. In Nos. 25 to 28, the stirring time after adding metallic silicon after desulfurization was set to 5 min or less to achieve the silicon concentration required for the product. 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 using an LF was 0.005 mass% or less. In contrast, in No. 29, the stirring time after adding metallic silicon exceeded 5 min, resulting in an excessive increase in the total aluminum concentration during stirring. Consequently, the total aluminum concentration in the molten steel after refining using an LF exceeded 0.005 mass%. As long as the total aluminum concentration in the molten steel after refining was 0.005 mass% or less, the concentration of aluminum insoluble in acid in the slab after continuous casting, i.e., the concentration of aluminum present as inclusions, was also consistently 0.004 mass% or less. Furthermore, the thickness of the molten slag after refining using an LF satisfied the above-mentioned relational expression 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 commercial furnace with a capacity of approximately 200 t per process, molten steel tapped from an electric furnace was placed in a ladle and transported to the LF facility for secondary refining. 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.

[0053] After the start of refining using the LF, Ar gas was supplied from the bottom injection plug at a flow rate of 1000 NL / min to stir the molten steel, and the electrode was lowered to perform arc heating.

[0054] Next, metallic silicon, silicon oxide source, calcium oxide source, and calcium fluoride source were added for deoxidation. Desulfurization treatment was then performed for approximately 60 min. After refining using the LF, the ladle was transported to an RH-type degassing system, and the molten steel was returned to a vacuum vessel for vacuum degassing. Table 4 shows the composition of the molten steel before, after, and after LF treatment under each test condition in the silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N] columns. The composition of the slag is also shown in the columns for the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) in the (CaO) / (SiO2) column and the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) in the (CaF2) / (CaO) column. The thickness of the molten slag after refining using the LF, HS, is also shown. Furthermore, the acid-insoluble aluminum concentration, measured by sampling and analyzing the cast slab after solidification in the continuous caster, is shown as insol.Al.

[0055] [Table 4]

[0056] In Nos. 33, 35, 37, and 39, the molten slag thickness after refining using an LF satisfied the above-mentioned relationship with the silicon concentration in molten steel during desulfurization, and no excessive nitrogen absorption into the molten steel was observed. On the other hand, in Nos. 32, 34, 36, 38, and 40, the molten slag thickness after refining using an LF did not satisfy the above-mentioned relationship with the silicon concentration in molten steel during desulfurization, and significant nitrogen absorption into the molten steel was observed. Vacuum degassing after refining using an LF reduced the nitrogen concentration in the molten steel. This confirmed that vacuum degassing is an effective method for achieving low nitrogen. [Industrial Applicability]

[0057] The method for refining molten steel according to the present invention enables stable desulfurization of molten steel stored in a ladle to a sulfur concentration meeting product specifications, even when the sulfur concentration in the molten steel is high at the start of ladle processing. In particular, it is possible to suppress an increase in the dissolved aluminum concentration in the molten steel and to suppress the formation of nitrogen and alumina-based inclusions. Therefore, it becomes easier to produce steel with reduced nitrogen and alumina-based inclusions, and is therefore industrially useful.

Claims

1. When carrying out refining treatment to reduce the sulfur concentration in molten steel, At least a metal silicon source is added as a deoxidizer, at least a calcium oxide source is added as a slag former, and at least one of an aluminum oxide source and a calcium fluoride source is added to promote slag formation, thereby adjusting the slag composition to suppress the reduction of aluminum oxide in the slag by molten steel, The thickness of the molten slag layer HS (m) is set to satisfy the following relational expression 1 with respect to the silicon concentration [Si] (mass%) in the molten steel: A method for refining molten steel, comprising adjusting the sulfur concentration to 0.020% by mass or less and the total aluminum concentration to 0.005% by mass or less. [Relationship 1] HS>0.02×[Si] １／６ Where HS: molten slag layer thickness (m), [Si]: silicon concentration in molten steel (mass%) is.

2. By mass, adding a calcium oxide source so that the ratio of calcium oxide concentration to 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 calcium fluoride concentration to calcium oxide concentration in the slag is set to a range of 0.05 to 0.25, (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, 2. The method for refining molten steel according to claim 1, wherein the ratio of calcium oxide concentration to aluminum oxide concentration in the slag is set to a range of 2.5 to 5.

0.

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

4. 2. The method for refining molten steel according to claim 1, wherein a part or all of the molten steel to be refined is molten steel tapped from an electric furnace.

5. 2. The method for refining molten steel according to claim 1, wherein the refining treatment for reducing the sulfur concentration is carried out in a reduced pressure atmosphere, or the refining treatment is followed by a degassing treatment in a reduced pressure atmosphere.

6. During the refining process for reducing the sulfur concentration, the silicon concentration in the molten steel is set to 0.15 mass% or less, After the refining treatment, metallic silicon is added so that the silicon concentration meets the product standard.

2. The method for refining molten steel according to claim 1, wherein the stirring time of the molten steel after the addition is set to 5 minutes or less.

7. When producing steel by solidifying molten steel refined using the method for refining molten steel according to any one of claims 1 to 6 to obtain a cast piece, In the slab, the concentration of acid-insoluble aluminum is 0.004 mass% or less.

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