Refining methods for molten steel and methods for manufacturing steel

Abstract

This invention proposes a refining method for low-sulfur molten steel that reduces nitrogen concentration and the formation of alumina inclusions. The method is characterized by the following steps: when refining the molten steel to reduce sulfur concentration, at least one of the following is added as a deoxidizer: a metallic silicon source; at least one of the following is added as a slag-forming agent: an alumina source and a calcium fluoride source for promoting slag formation. Furthermore, the slag composition is adjusted to suppress the reduction of alumina in the slag by the molten steel. Simultaneously, relative to the silicon concentration [Si] (mass%) in the molten steel, the thickness HS (m) of the molten slag layer satisfies HS > 0.02 × [Si]¹ / ⁶; the sulfur concentration is set to 0.020% by mass or less, and the total aluminum concentration is set to 0.005% by mass or less.

Description

Technical Field

[0001] This invention relates to a refining process for molten steel, and more particularly to a desulfurization process for molten steel. It also relates to a method for manufacturing steel that reduces nitrogen concentration and the formation of alumina inclusions. In this specification, "t" (ton) refers to metric tons, or 1000 kg. "N" preceding the volume unit for gases indicates the gas volume at standard conditions of 0°C and 101325 Pa. "l" (l) refers to 10⁻³ m³. 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 the slag. "x~y" indicates a numerical range, meaning x above and y below, including boundary values. Prior Technology

[0002] Sulfur (S) in steel can worsen its hot brittleness or corrosion resistance, and reduce its toughness or workability. Therefore, it is required to reduce the S concentration in steel. Similarly, nitrogen (N) can reduce the ductility of steel or the toughness of welded joints. Therefore, it is required to reduce the N concentration in steel.

[0003] In manufacturing methods using molten steel produced in a blast furnace, desulfurization is typically performed during the molten steel tapping stage to ensure the desired sulfur concentration in the finished product. However, when a strict sulfur concentration of less than 20 ppm by mass is required, a secondary refining process after tapping the steel from the converter is necessary. In this case, processes using ladle refining equipment such as the LF (Ladle Furnace) or vacuum degassing equipment such as the RH (Ruhrstahl-Heraeus) are widely employed.

[0004] In the method of manufacturing molten steel by melting a chill in an electric furnace, the sulfur concentration of the molten steel tapped from the electric furnace is at a high level. Therefore, even products that do not require strictly regulated sulfur concentrations almost always require a secondary refining process for desulfurization.

[0005] Regarding methods for efficiently desulfurizing molten steel using LF, for example, Patent Document 1 proposes a method for determining the slag composition in the ladle. Adding metallic aluminum, a deoxidizer, to the molten steel before ladle refining increases the concentration of calcium oxide and magnesium oxide, which are alkaline oxides. [Previous Technical Documents] [Patent Literature]

[0006] Patent Document 1: Japanese Patent Application Publication No. 2012-12648 Summary of the Invention

[0007] (The problem the invention aims to solve) However, the aforementioned conventional techniques have the following unresolved problems. Due to the addition of a metallic aluminum source, the concentration of molten aluminum in the steel increases, leading to an increase in the formation of alumina-based inclusions with re-oxidation. The presence of alumina-based inclusions causes problems such as wire breakage. Furthermore, damage to rail materials also becomes a problem. Therefore, the upper limit of the total aluminum concentration is strictly controlled. Adding silicon, which has a weaker deoxidizing power than aluminum, is also considered to suppress the formation of alumina-based inclusions. However, to promote the desulfurization reaction, sufficient deoxidation is necessary. Therefore, when using silicon for deoxidation, compared to using aluminum, there is a problem of reduced desulfurization efficiency during the desulfurization process.

[0008] To reduce molten oxygen concentration and promote desulfurization through the deoxidation of silicon, it is effective to reduce the silicon oxide concentration and activity in the slag during ladle refining. Therefore, it is effective to increase the amount of calcium oxide, a basic oxide, added. Adding calcium oxide, a basic oxide, also enhances the desulfurization capacity of the slag.

[0009] When the calcium oxide concentration in the slag increases, the slag becomes less prone to impurity. Therefore, substances to promote impurity must be added. Alumina or calcium fluoride are generally used. However, when the alumina concentration in the slag increases, the alumina concentration is reduced by the molten silicon in the steel due to the enhanced deoxidation caused by silicon. Furthermore, the increased concentration of molten aluminum in the steel increases the risk of alumina inclusion formation. Therefore, the slag composition must be appropriately controlled to balance ensuring desulfurization capacity with preventing an increase in molten aluminum concentration.

[0010] Furthermore, LF tank refining processes are mostly carried out at atmospheric pressure. In this case, the molten steel is exposed to air during processing due to gas agitation, leading to an increase in nitrogen concentration. In other words, it is difficult to simultaneously achieve gas agitation and low-NOx treatment.

[0011] In view of this situation, the present invention aims to provide a refining method for molten steel that reduces nitrogen concentration and the formation of alumina-based inclusions. Furthermore, it aims to provide a method for manufacturing steel incorporating this method. (Technical means to solve the problem)

[0012] The present invention provides a refining method for molten steel that is beneficial in solving the above-mentioned problems. The method is characterized in that, when refining the molten steel to reduce sulfur concentration, at least one of the following is added: a metallic silicon source as a deoxidizer, at least a calcium oxide source as a slag-forming agent, an alumina source for promoting slag formation, and a calcium fluoride source. Furthermore, by adjusting the slag composition, the reduction of alumina in the slag by the molten steel is suppressed. Simultaneously, relative to the silicon concentration [Si] (mass%) in the molten steel, the thickness HS (m) of the molten slag layer satisfies the following relationship: The sulfur concentration is set to 0.020% by mass or less, and the total aluminum concentration is set to 0.005% by mass or less. [Relation 1] HS>0.02×[Si]1 / 6 Wherein, HS: thickness of molten slag layer (m); [Si]: silicon concentration in molten steel (mass %).

[0013] Furthermore, the following are preferred solutions in the refining method for molten steel of the present invention: (a) Add a calcium oxide source in such a way that the ratio of calcium oxide concentration to silicon oxide concentration in the slag is in the range of 1.5 to 3.5 based on 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 alumina 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 alumina concentration in the slag is in the range of 2.5 to 5.0. (b) For every 1 ton of molten steel, the amount of aluminum source added as a deoxidizer shall be set at less than 0.1 kg in terms of aluminum content. (c) Part or all of the molten steel for which the above refining process is performed is molten steel produced by an electric furnace. (d) The above-mentioned refining treatment to reduce sulfur concentration is carried out under reduced pressure, or the above-mentioned refining treatment is followed by degassing under reduced pressure. (e) In the above refining process to reduce sulfur concentration, the silicon concentration in the molten steel is set to 0.15% by mass or less, metallic silicon is added to make the silicon concentration of the product specifications after the refining process, and the stirring time of the molten steel is set to within 5 minutes after the addition.

[0014] The steel manufacturing method of the present invention, which is beneficial to solving the above-mentioned problems, is characterized in that, in the process of manufacturing steel, molten steel refined by any of the above-mentioned steel refining methods is solidified to obtain a casting, wherein the concentration of aluminum insoluble in acid in the casting is set to be 0.004% by mass or less. (Compared to the effectiveness of previous technologies)

[0015] According to the present invention, when refining molten steel contained in a ladle, even if the sulfur concentration in the molten steel is at a high level at the beginning of the ladle treatment, desulfurization can still be carried out stably until the sulfur concentration reaches the product specifications. In particular, it can suppress the rise in the molten aluminum concentration in the molten steel and suppress the formation of nitrogen concentration and alumina inclusions. Implementation

[0016] The following describes specific embodiments of the present invention. These embodiments are illustrative of apparatus or methods for embodying the technical concept of the present invention, and are not intended to be specific to the description. That is, the technical concept of the present invention can be modified in various ways within the scope of the technology described in the claims.

[0017] In the development of this invention, the inventors used an LF (Ladle Refining Furnace) to refine the molten steel after it had been melted in a converter or electric furnace and placed in a ladle. The target molten steel had a sulfur concentration of 0.020% by mass or less and a total aluminum concentration of 0.005% by mass or less.

[0018] In this processing step, the changes in sulfur and total aluminum concentrations in the molten steel were investigated under various changes in the composition concentrations of the molten steel and slag, as well as other operating conditions. When the waste was dissolved in an electric furnace and the molten steel was drained, the sulfur concentration in the molten steel was approximately 0.030–0.040% by mass, and the total aluminum concentration was below 0.001% by mass. Silicon was added to deoxidize the molten steel before ladle refining using LF. The silicon concentration in the deoxidized molten steel was 0.15–0.5% by mass.

[0019] Subsequently, a calcium oxide source was added and the molten steel was stirred. The compositional changes in the molten steel were investigated when the mass ratio of calcium oxide concentration to silicon oxide concentration in the slag varied. The results showed that when the mass ratio of calcium oxide concentration to silicon oxide concentration was less than 1.5, no increase in the total aluminum concentration in the molten steel was observed. Furthermore, the desulfurization reaction did not proceed.

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

[0021] Under conditions of adding an alumina source, and with the mass composition ratio (calcium oxide concentration) / (alumina concentration) in the slag set to 5.0 or less, and the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) set to 1.6 or more, slag sludge formation was confirmed. At this point, it became possible to desulfurize to 0.020% by mass of the target sulfur concentration.

[0022] However, as the mass ratio of calcium oxide concentration to alumina concentration in the slag decreases, or as the mass ratio of calcium oxide concentration to silicon oxide concentration increases, there is a tendency for the total aluminum concentration in the molten steel to increase during refining using LF. Since the mass ratio of calcium oxide concentration to silicon oxide concentration increases, the amount of active silicon oxide in the slag decreases, and deoxidation using silicon reduces the molten oxygen concentration. Conversely, as the mass ratio of calcium oxide concentration to alumina concentration decreases, the amount of active alumina in the slag increases. Therefore, the molten silicon in the steel reduces alumina in the slag. By setting the mass ratio of calcium oxide concentration to silicon oxide concentration to 3.5 or less and the mass ratio of calcium oxide concentration to alumina concentration to 2.5 or more, the reduction of alumina in the slag can be suppressed. Therefore, the total aluminum concentration after refining using LF can be suppressed to below 0.005% by mass.

[0023] Under conditions where a calcium fluoride source is added, slag accretion is confirmed when the mass ratio of calcium fluoride concentration to calcium oxide concentration in the slag is set to 0.05 or higher, and the mass ratio of calcium oxide concentration to silicon oxide concentration is set to 1.5 or higher. At this point, desulfurization can be performed to reduce the target sulfur concentration to below 0.020% by mass. Similar to the case using an alumina source, the total aluminum concentration after refining with LF increases as the mass ratio of calcium oxide concentration to silicon oxide concentration increases. By setting the mass ratio of calcium oxide concentration to silicon oxide concentration to 3.5 or lower, the total aluminum concentration after refining with LF can be reduced to below 0.005% by mass. This is because even without the addition of an alumina source, the inevitable reduction of alumina mixed into the slag still occurs, thus increasing the total aluminum concentration in the molten steel.

[0024] When the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) in the slag increases to more than 0.25, it is confirmed that there is abnormal melting loss in the refractory lining.

[0025] Next, the amount of metallic silicon added before refining using LF was reduced to achieve a silicon concentration in the molten steel of 0.15% by mass or less, and the compositional shift of the molten steel during refining was investigated. As a result, the total aluminum concentration in all slag components was reduced to 0.005% by mass or less. This is because the reduction in silicon concentration in the molten steel during refining using LF leads to an increase in molten oxygen concentration and inhibits the reduction of alumina in the slag.

[0026] Furthermore, various variations were made to the thickness of the molten slag layer during the refining process using LF, and the changes in nitrogen concentration during the refining process were investigated. The results showed that increasing the thickness of the molten slag layer could suppress nitrogen absorption by the molten steel. This can be attributed to the effect of the slag blocking the interaction between ambient air and the molten steel. It was also clarified that the higher the silicon concentration in the molten steel, the greater the molten slag layer thickness required to suppress nitrogen absorption. A higher silicon concentration in the molten steel leads to a lower oxygen concentration. Since oxygen is an interfacially active element, a lower oxygen concentration makes nitrogen absorption from the molten steel surface more likely to occur.

[0027] The inventors found that, relative to the silicon concentration [Si] (mass %) in the molten steel, the absorption of nitrogen by the molten steel can be suppressed by making the thickness of the molten slag layer HS (m) satisfy the following relationship 1. [Relation 1] HS>0.02×[Si]1 / 6 Wherein, HS: thickness of molten slag layer (m); [Si]: silicon concentration in molten steel (mass %).

[0028] As a countermeasure to reduce nitrogen concentration, an effective method is to perform tank refining using LF under reduced pressure to reduce the nitrogen partial pressure in the environment, or to perform degassing treatment of molten steel under reduced pressure after the refining treatment.

[0029] This embodiment was completed based on the above review results. The following describes the specific refining method for molten steel and the steel manufacturing method.

[0030] The molten steel, refined or melted once in a converter or electric furnace, is tapped into a ladle. Inevitably, some slag from the converter or electric furnace flows into the ladle. If this amount is excessive, the flowing slag can be removed from the ladle. However, since the silica in the flowing slag acts as a desulfurizing agent, it helps to degrade the impurities contained in the subsequently added calcium oxide, and therefore may not be removed. The ladle is then transported to the LF equipment, where inert gas is supplied through a plug at the bottom of the ladle or an injection lance to agitate the molten steel. Furthermore, electric arc heating is implemented to adjust the temperature of the molten steel.

[0031] Metallic silicon for deoxidation can be added at the beginning of the refining process using LF, or it can be placed in a container before tapping from the converter or electric furnace, or it can be added during tapping. Appropriately sample the composition of the molten steel, and adjust the amount of metallic silicon added while referring to the analytical values ​​to bring the silicon concentration in the molten steel into a predetermined range.

[0032] Next, slag-forming agents are added to adjust the slag composition. Taking into account the amount of silicon oxide generated during deoxidation, silicon oxide and calcium oxide sources are added to achieve a slag amount sufficient to ensure the thickness of the molten slag layer to prevent nitrogen pickup. Furthermore, alumina or calcium fluoride sources are added to promote impurities. These slag-forming agents can be added not only during LF treatment but also during tapping in converters or electric furnaces.

[0033] In the refining process using LF, sampling and analysis of molten steel and slag can be appropriately carried out, and alloys and slag-forming agents can be added for composition adjustment to confirm that the composition of the molten steel reaches the predetermined value. The refining process using LF can also be divided into desulfurization treatment and subsequent silicon concentration adjustment. Afterwards, degassing and refining are carried out using RH vacuum degassing equipment or the like as needed, and casting is carried out using a continuous casting machine or the like to produce castings. Molten steel can also be agglomerated and divided into pieces to make castings.

[0034] In the resulting castings, the concentration of aluminum insoluble in acid can be reduced to below 0.004% by mass. Aluminum insoluble in acid is almost entirely composed of non-metallic inclusions such as aluminum oxide or aluminum nitride. This steel, with its reduced non-metallic inclusions, can be appropriately used in steel cables or rails.

[0035] Furthermore, the above description is based on an LF device as an example of an implementation. In addition, the present invention can also be applied to ASEA-SKF devices, VAD (Vacuum Arc Degassing) devices, and VOD (Vacuum Oxygen Decarburization) devices, etc., as described above. [Example]

[0036] (Example 1) In a practical machine with a single processing capacity of approximately 200 tons of molten steel, the molten steel tapped from the electric arc furnace is collected in a container and transported to the LF equipment for secondary refining. Here, the sulfur concentration of the molten steel tapped from the electric arc furnace is 0.030~0.040% by mass, and the total aluminum concentration is less than 0.001% by mass.

[0037] After the refining process using LF begins, Ar gas is supplied at a flow rate of 1000 Nl / min through the bottom blowing plug to stir the molten steel, causing the electrode to descend for arc heating.

[0038] Subsequently, metallic silicon, silicon oxide source, calcium oxide source, and aluminum oxide source for deoxidation were added. At this time, the addition was carried out in a manner that ensured the thickness of the molten slag layer after refining using LF was 0.10 μm. Then, a desulfurization treatment was performed for approximately 60 min. Table 1 shows the composition of the molten steel before and after LF treatment under various experimental conditions, in the columns for silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N]. Furthermore, the slag composition is expressed as the mass ratio (calcium oxide concentration) / (silicon oxide concentration) in the (CaO) / (SiO2) column and the mass ratio (calcium oxide concentration) / (alumina concentration) in the (CaO) / (Al2O3) column. Subsequently, samples were taken from the solidified casting in a continuous casting machine, and the concentration of aluminum insoluble in acid was expressed as insol.Al.

[0039] [Table 1] No. Composition of molten steel before LF treatment Composition of molten steel after LF treatment Composition of slag after LF treatment insol.Al in casting Remark [Si] [S] [T.Al] [N] [Si] [S] [T.Al] [N] (CaO) / (SiO2) (CaO) / (Al2O3) quality% quality% quality% mass ppm quality% quality% quality% mass ppm mass ratio mass ratio quality% 1 0.26 0.035 <0.001 84 0.26 0.0340 <0.001 84 1.1 7.5 <0.001 Comparative example 2 0.25 0.039 <0.001 76 0.24 0.0390 <0.001 76 1.3 9.2 <0.001 Comparative example 3 0.24 0.036 <0.001 69 0.28 0.0350 <0.001 69 1.5 8.3 <0.001 Comparative example 4 0.25 0.031 <0.001 90 0.27 0.0195 0.001 90 1.6 5.0 <0.001 Invention Examples 5 0.25 0.037 <0.001 82 0.24 0.0176 0.001 82 1.8 5.0 <0.001 Invention Examples 6 0.28 0.036 <0.001 75 0.26 0.0118 0.002 75 2.3 4.5 0.001 Invention Examples 7 0.26 0.035 <0.001 76 0.28 0.0056 0.003 76 3.2 4.0 0.002 Invention Examples 8 0.24 0.034 <0.001 94 0.30 0.0030 0.005 94 3.5 3.2 0.004 Invention Examples 9 0.25 0.038 <0.001 81 0.24 0.0023 0.006 81 3.8 3.3 0.005 Comparative example 10 0.26 0.040 <0.001 76 0.29 0.0057 0.005 76 3.4 2.5 0.004 Invention Examples 11 0.28 0.034 <0.001 84 0.24 0.0071 0.008 84 3.3 2.3 0.005 Comparative example 12 0.24 0.036 <0.001 64 0.27 0.0094 0.009 64 3.3 2.0 0.005 Comparative example

[0040] Series No. 1 to 3 underwent LF treatment without adding alumina as a slagging agent. In this case, the mass composition ratio (calcium oxide concentration) / (alumina concentration) of the slag after LF refining, based on the alumina contained in the flowing slag, was 7.5 to 9.2. The total aluminum concentration in the molten steel after LF refining was less than 0.001% by mass, remaining unchanged. Furthermore, the desulfurization reaction was almost non-existent.

[0041] For series No. 4 to 12, an alumina source is added as a slagging agent to achieve various variations in the mass composition ratios (calcium oxide concentration) / (silicon oxide concentration) and (calcium oxide concentration) / (alumina concentration) for refining using LF. For series No. 4 to 8 and 10, the mass composition ratios (calcium oxide concentration) / (silicon oxide concentration) are between 1.6 and 3.5, and between 2.5 and 5.0. These ratios achieve a sulfur concentration of 0.020% by mass and a total aluminum concentration of 0.005% by mass in the molten steel after LF refining. For series No. 9, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) exceeds 3.5; for series No. 11 and 12, the mass composition ratio (calcium oxide concentration) / (alumina concentration) is less than 2.5. Although desulfurization was carried out during the refining process using LF, the total aluminum concentration in the molten steel after refining exceeded 0.005% by mass. If the total aluminum concentration in the molten steel after refining was less than 0.005% by mass, the concentration of aluminum insoluble in acid, i.e., the concentration of aluminum existing in the form of inclusions, in the cast steel after continuous casting also stabilized at less than 0.004% by mass. Furthermore, the relationship between the thickness of the molten slag layer after refining using LF and the silicon concentration in the molten steel during desulfurization met the above-mentioned Equation 1, and no nitrogen absorption by the molten steel occurred.

[0042] (Example 2) In a practical machine with a single processing capacity of approximately 200 tons of molten steel, the molten steel tapped from the electric furnace is collected in a container and transported to the LF equipment for secondary refining. Here, the sulfur concentration of the molten steel tapped from the electric furnace is 0.030~0.040% by mass, and the total aluminum concentration is less than 0.001% by mass.

[0043] After the refining process using LF begins, Ar gas is supplied at a flow rate of 1000 Nl / min through a bottom-blown plug to stir the molten steel, thereby lowering the electrode and performing arc heating.

[0044] Subsequently, metallic silicon, silicon oxide source, calcium oxide source, and calcium fluoride source for deoxidation were added. At this time, the addition was carried out in a manner that ensured the thickness of the molten slag layer after refining using LF was 0.10 μm. Then, a desulfurization treatment was performed for approximately 60 min. Table 2 shows the composition of the molten steel before and after LF treatment under various experimental conditions, with columns for silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N]. Furthermore, the slag composition is expressed as the mass ratio (calcium oxide concentration) / (silicon oxide concentration) in the (CaO) / (SiO2) column and the mass ratio (calcium fluoride concentration) / (calcium oxide concentration) in the (CaF2) / (CaO) column. Furthermore, solidified castings were sampled on a continuous casting machine, and the concentration of aluminum insoluble in acid was expressed as insol.Al.

[0045] [Table 2] No. Composition of molten steel before LF treatment Composition of molten steel after LF treatment Composition of slag after LF treatment insol.Al in casting Remark [Si] [S] [T.Al] [N] [Si] [S] [T.Al] [N] (CaO) / (SiO2) (CaF2) / (CaO) quality% quality% quality% mass ppm quality% quality% quality% mass ppm mass ratio mass ratio quality% 13 0.24 0.036 <0.001 94 0.25 0.0350 <0.001 94 1.5 0.03 <0.001 Comparative example 14 0.26 0.034 <0.001 86 0.24 0.0192 <0.001 86 1.5 0.05 <0.001 Invention Examples 15 0.27 0.036 <0.001 75 0.27 0.0164 <0.001 75 1.5 0.09 <0.001 Invention Examples 16 0.23 0.038 <0.001 64 0.26 0.0123 0.001 64 1.8 0.10 <0.001 Invention Examples 17 0.29 0.039 <0.001 62 0.25 0.0081 0.002 62 2.0 0.08 0.001 Invention Examples 18 0.24 0.040 <0.001 91 0.24 0.0054 0.003 91 2.5 0.15 0.002 Invention Examples 19 0.25 0.034 <0.001 92 0.25 0.0043 0.004 92 2.9 0.20 0.003 Invention Examples 20 0.29 0.036 <0.001 94 0.28 0.0035 0.005 94 3.5 0.25 0.003 Invention Examples twenty one 0.26 0.031 <0.001 73 0.27 0.0031 0.006 73 3.7 0.25 0.005 Comparative Example twenty two 0.24 0.035 <0.001 81 0.29 0.0023 0.008 81 3.8 0.25 0.005 Comparative Example twenty three 0.28 0.039 <0.001 64 0.26 0.0027 0.005 64 3.5 0.27 0.003 Example of development twenty four 0.26 0.030 <0.001 91 0.25 0.0019 0.009 91 3.8 0.28 0.006 Comparative Example

[0046] Series No. 13-24 involve adding calcium fluoride as a slag-forming agent to vary the mass ratios of calcium oxide concentration / silicon oxide concentration and calcium fluoride concentration / calcium oxide concentration in the slag, thus enabling refining treatment using LF. Series No. 14-20 have mass ratios of calcium oxide concentration / silicon oxide concentration between 1.5 and 3.5, and mass ratios of calcium fluoride concentration / calcium oxide concentration between 0.05 and 0.25. These ratios achieve a sulfur concentration of less than 0.020% by mass and a total aluminum concentration of less than 0.005% by mass in the molten steel after refining using LF. Series No. 13 has a mass ratio of calcium fluoride concentration / calcium oxide concentration less than 0.05, so slag impurity treatment is not performed, and desulfurization is not carried out during refining using LF; therefore, the total aluminum concentration remains unchanged after refining. For series No. 21, 22, and 24, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) exceeds 3.5. Although desulfurization was carried out during the refining process using LF, the total aluminum concentration after refining exceeds 0.005% by mass. If the total aluminum concentration in the molten steel after refining is below 0.005% by mass, the concentration of aluminum insoluble in acid in the castings after continuous casting, i.e., the concentration of aluminum existing in the form of inclusions, also stabilizes to below 0.004% by mass. For series No. 23 and 24, the mass composition ratio (calcium fluoride concentration) / (calcium oxide concentration) exceeds 0.25, and the melting loss of the refractory in the refractory trough is significant. Furthermore, the relationship between the thickness of the molten slag layer after refining using LF and the silicon concentration of the molten steel during desulfurization satisfies the above-mentioned Equation 1, and no nitrogen absorption by the molten steel is observed.

[0047] (Example 3) In a practical machine with a single processing capacity of approximately 200 tons of molten steel, the molten steel tapped from the electric arc furnace is collected in a container and transported to the LF equipment for secondary refining. Here, the sulfur concentration of the molten steel tapped from the electric arc furnace is 0.030~0.040% by mass, and the total aluminum concentration is less than 0.001% by mass.

[0048] After the refining process using LF begins, Ar gas is supplied at a flow rate of 1000 Nl / min through a bottom-blown plug to stir the molten steel, and the electrode is lowered to perform arc heating.

[0049] Subsequently, metallic silicon, silicon oxide source, calcium oxide source, and aluminum oxide source for deoxidation were added. At this time, the addition was carried out to achieve a molten slag layer thickness of 0.10 μm after refining using LF. Then, a desulfurization treatment was performed for approximately 60 minutes. The silicon concentration in the molten steel at the start of the LF refining treatment was varied, and metallic silicon was added again after the desulfurization treatment to achieve the silicon concentration required for the finished product. The LF refining treatment was then terminated after stirring for a certain period. Table 3 shows the composition of the molten steel before, after desulfurization, and after LF treatment under various experimental conditions, with columns for silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N]. Furthermore, the slag composition is expressed in the (CaO) / (SiO2) column as the mass ratio of calcium oxide concentration to silicon oxide concentration, and in the (CaO) / (Al2O3) column as the mass ratio of calcium oxide concentration to aluminum oxide concentration. Furthermore, samples of solidified castings were taken from the continuous casting machine, and the concentration of aluminum insoluble in acid was expressed as insol.Al. Moreover, the relationship between the thickness of the molten slag layer after LF refining and the silicon concentration in the molten steel during desulfurization satisfies Equation 1 above, and no nitrogen absorption by the molten steel is observed.

[0050] [Table 3] No. Composition of molten steel before LF treatment Composition of molten steel after desulfurization treatment Composition of slag after desulfurization treatment Composition of molten steel after LF treatment Stir time In the casting insol.Al Remark [Si] [S] [T.Al] [N] [Si] [S] [T.Al] [N] (CaO) / (SiO2) (CaO) / (Al2O3) [Si] [S] [T.Al] quality% quality% quality% mass ppm quality% quality% quality% mass ppm mass ratio mass ratio quality% quality% quality% min quality% 25 0.1 0.038 <0.001 75 0.11 0.0106 0.003 75 3.8 3.0 0.25 0.0064 0.004 4 0.003 Invention Examples 26 0.13 0.034 <0.001 64 0.12 0.0085 0.003 64 3.6 3.0 0.26 0.0054 0.005 5 0.003 Invention Examples 27 0.14 0.037 <0.001 62 0.13 0.0072 0.004 62 3.8 2.9 0.24 0.0040 0.004 3 0.003 Invention Examples 28 0.15 0.031 <0.001 91 0.15 0.0054 0.005 91 3.7 3.0 0.25 0.0041 0.005 5 0.004 Invention Examples 29 0.15 0.030 <0.001 92 0.14 0.0065 0.005 92 3.6 3.0 0.24 0.0038 0.007 7 0.005 Comparative example 30 0.17 0.039 <0.001 84 0.16 0.0046 0.006 84 3.6 2.9 0.23 0.0038 0.006 5 0.005 Comparative Example 31 0.2 0.034 <0.001 76 0.2 0.0035 0.007 76 3.7 2.9 0.24 0.0030 0.007 3 0.006 Comparative Example

[0051] The mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) of the slag after desulfurization treatment in series No. 25-29 exceeds 3.5, and the silicon concentration in the molten steel during desulfurization treatment is below 0.15% by mass. This allows for desulfurization of the sulfur concentration in the molten steel to below 0.020% by mass, and reduces the total aluminum concentration to below 0.005% by mass. This is because the silicon concentration in the molten steel becomes low and the oxygen concentration becomes high during desulfurization treatment, thus inhibiting the reduction of alumina in the slag. On the other hand, the mass composition ratio (calcium oxide concentration) / (silicon oxide concentration) of the slag after desulfurization treatment in series No. 30 and 31 exceeds 3.5, and the silicon concentration in the molten steel during desulfurization treatment exceeds 0.15% by mass. This is because, although desulfurization is performed, the excess silicon in the molten steel leads to the reduction of alumina in the slag, resulting in a total aluminum concentration in the molten steel exceeding 0.005% by mass after desulfurization. Nos. 25-28 involve adding metallic silicon after desulfurization to achieve the desired silicon concentration for the finished product, with a stirring time of 5 minutes or less. Within this range, the alumina in the slag is not excessively reduced by the silicon in the molten steel, allowing the total aluminum concentration in the molten steel after refining using LF to be below 0.005% by mass. On the other hand, in No. 29, the stirring time after adding metallic silicon exceeds 5 minutes, resulting in an excessive increase in the total aluminum concentration during stirring. Furthermore, the total aluminum concentration in the molten steel after refining using LF exceeds 0.005% by mass. If the total aluminum concentration in the refined molten steel is below 0.005% by mass, then the concentration of aluminum insoluble in acid in the continuously cast steel, i.e., the concentration of aluminum existing in the form of inclusions, also remains stable at below 0.004% by mass. Furthermore, the relationship between the thickness of the molten slag layer after refining using LF and the silicon concentration in the desulfurized steel satisfies Equation 1 above, and no nitrogen absorption by the molten steel is observed.

[0052] (Example 4) In a practical machine with a single processing capacity of approximately 200 tons of molten steel, the molten steel tapped from the electric arc furnace is collected in a container and transported to the LF equipment for secondary refining. Here, the sulfur concentration of the molten steel tapped from the electric arc furnace is 0.030~0.040% by mass, and the total aluminum concentration is less than 0.001% by mass.

[0053] After the refining process using LF begins, Ar gas is supplied at a flow rate of 1000 Nl / min through a bottom-blown plug to stir the molten steel, and the electrode is lowered to perform arc heating.

[0054] Subsequently, metallic silicon, silicon oxide source, calcium oxide source, and calcium fluoride source for deoxidation are added. Then, a desulfurization treatment of approximately 60 minutes is performed. After refining using LF, the container is transported to an RH-type degassing device, where the molten steel is refluxed in a vacuum tank for vacuum degassing. Table 4 shows the composition of the molten steel before, after, and after vacuum degassing under various test conditions, in the columns for silicon concentration [Si], sulfur concentration [S], total aluminum concentration [T.Al], and nitrogen concentration [N]. Furthermore, the slag composition is shown in the (CaO) / (SiO2) column as the mass ratio (calcium oxide concentration) / (silicon oxide concentration), and in the (CaF2) / (CaO) column as the mass ratio (calcium fluoride concentration) / (calcium oxide concentration). Also, the thickness HS of the molten slag after refining using LF is shown. Furthermore, samples were taken from the solidified castings on a continuous casting machine, and the concentration of aluminum insoluble in acid was expressed as insol.Al.

[0055] [Table 4] No. Composition of molten steel before LF treatment Composition of molten steel after LF treatment Composition of slag after LF treatment Molten slag Thickness HS After degassing In the casting insol.Al Remark [Si] [S] [T.Al] [N] [Si] [S] [T.Al] [N] (CaO) / (SiO2) (CaF2) / (CaO) [N] quality% quality% quality% mass ppm quality% quality% quality% mass ppm mass ratio mass ratio m mass ppm quality% 32 0.16 360 <0.001 94 0.16 176 0.003 126 2.2 0.14 0.0080 98 0.001 Comparative example 33 0.18 340 <0.001 86 0.18 182 0.003 87 2.3 0.16 0.0168 75 0.001 Invention Examples 34 0.26 360 <0.001 75 0.26 152 0.002 116 2.2 0.15 0.0124 96 0.001 Comparative example 35 0.23 380 <0.001 64 0.23 192 0.003 66 2.3 0.14 0.0224 58 0.002 Invention Examples 36 0.27 390 <0.001 62 0.27 143 0.002 108 2.1 0.15 0.0152 94 0.001 Comparative example 37 0.35 400 <0.001 91 0.35 172 0.003 91 2.3 0.18 0.0236 84 0.001 Invention Examples 38 0.38 340 <0.001 92 0.38 164 0.002 134 2.2 0.14 0.0160 96 0.001 Comparative example 39 0.46 360 <0.001 94 0.46 143 0.003 93 2.2 0.16 0.0196 85 0.002 Invention Examples 40 0.41 310 <0.001 73 0.41 152 0.002 139 2.1 0.13 0.0156 98 0.001 Comparative example

[0056] The relationship between the thickness of the molten slag after refining with LF and the silicon concentration in the molten steel during desulfurization satisfies Equation 1 above, indicating no absorption of excess nitrogen in the molten steel. On the other hand, the relationship between the thickness of the molten slag after refining with LF and the silicon concentration in the molten steel during desulfurization does not satisfy Equation 1, and the absorption of nitrogen in the molten steel is significant. By implementing vacuum degassing after refining with LF, the nitrogen concentration in the molten steel is reduced. This has been confirmed as an effective means to achieve low-NOx levels. (Industrial applicability)

[0057] The refining method for molten steel of the present invention allows for stable desulfurization of molten steel contained in a ladle, even when the sulfur concentration in the molten steel is high at the beginning of the ladle treatment, until the sulfur concentration reaches the product specifications. In particular, it can suppress the increase in the concentration of molten aluminum in the molten steel and inhibit the formation of nitrogen concentration and alumina inclusions. Therefore, it can easily produce steel with reduced nitrogen concentration and alumina inclusions, which is extremely useful in industry.

Claims

1. A refining method for molten steel, wherein during the refining treatment of molten steel to reduce sulfur concentration, at least one of the following is added as a deoxidizer: a metallic silicon source; at least one of the following is added as a slag-forming agent: an alumina source and a calcium fluoride source to promote slag formation; and the composition of the slag is adjusted to inhibit the reduction of alumina in the slag by the molten steel. Simultaneously, relative to the silicon concentration [Si] (mass%) in the molten steel, the thickness HS (m) of the molten slag layer satisfies the following relationship: The sulfur concentration is set to 0.020% by mass or less, and the total aluminum concentration is set to 0.005% by mass or less; [Relationship 1] HS>0.02×[Si]¹ / ⁶ Wherein, HS: Thickness of molten slag layer (m); [Si]: Silicon concentration in molten steel (mass %).

2. The refining method for molten steel as described in Request 1, wherein, Calcium oxide source is added in a manner that, based on mass, makes the ratio of calcium oxide concentration to silicon oxide concentration in the slag range from 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 in the range of 0.05 to 0.25; (ii) When an alumina 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 alumina concentration in the slag is in the range of 2.5 to 5.

0.

3. The refining method for molten steel as described in Request 1, wherein, For every 1 ton of molten steel, the amount of aluminum source added as a deoxidizer is set to less than 0.1 kg (calculated as aluminum).

4. The refining method for molten steel as described in Request 1, wherein, Part or all of the molten steel that has undergone the above refining process is molten steel produced from an electric furnace.

5. The refining method for molten steel as described in Request 1, wherein, The refining process described above, which reduces the sulfur concentration, is carried out under reduced pressure, or the degassing process is carried out under reduced pressure after the refining process described above.

6. The refining method for molten steel as described in Request 1, wherein, In the refining process described above to reduce sulfur concentration, the silicon concentration in the molten steel is set to 0.15% by mass or less. Add metallic silicon to achieve the silicon concentration required for the finished product after refining. After adding the silicon, set the stirring time of the molten steel to within 5 minutes.

7. A method for manufacturing steel, wherein the steel is produced by solidifying molten steel refined by any one of the refining methods of claims 1 to 6 to obtain a casting, wherein the concentration of aluminum insoluble in acid in the casting is set to 0.004% by mass or less.