Method for melting ultra-low phosphorus steel

Abstract

The present invention provides a method for melting extremely low phosphorus steel in a refining process using two converters (first converter and second converter), which does not require the addition of a slag accelerator during decarburization blowing, does not require a long decarburization blowing time for slag slag formation, and reduces the amount of slag generated throughout the entire refining process. [Solution] Partial desilicate blowing of molten iron 11 is performed in the second converter 1B (step (b-2)), the partially desilicate molten iron 12 produced is charged into the first converter 1A for desilicate dephosphorization blowing (step (a-2)), and with all or part of the desilicate slag 21 generated in the partial desilicate blowing in the second converter 1B remaining in the converter, the desilicate dephosphorization molten iron 13 is charged into the second converter 1B for decarburization blowing (step (b-7)).

Description

Technical Field

[0001] The present invention relates to a method for melting extra-low phosphorus steel using a converter.

Background Art

[0002] In an integrated steelmaking process for pig iron, hot metal discharged from a blast furnace contains high concentrations of impurity elements such as carbon, silicon, and phosphorus. As a device for removing impurities in hot metal and producing molten steel containing predetermined components, a converter is often used, and particularly a top-bottom blown converter is suitable. In a top-bottom blown converter, hot metal is refined by blowing oxygen from an upper blowing lance at the upper part of the converter and blowing bottom blowing gas from the bottom of the converter.

[0003] In recent years, as a method of performing dephosphorization blowing and decarburization blowing for removing impurities in hot metal using a single converter, a method of performing dephosphorization blowing - intermediate slag removal - decarburization blowing is known. Since silicon in hot metal is the most easily oxidized, silicon is first oxidized at the initial stage of dephosphorization blowing, and then dephosphorization proceeds. For example, the method described in claim 1 of Patent Document 2 and the method described in refining method 2 of Patent Document 3 correspond to the method of performing dephosphorization blowing - intermediate slag removal - decarburization blowing using a single converter.

[0004] In the above method of performing dephosphorization blowing - intermediate slag removal - decarburization blowing using a single converter, there is a large amount of slag remaining in the converter after intermediate slag removal, and as a result, a large amount of slag remains after dephosphorization blowing. Therefore, phosphoric acid in the dephosphorization slag is carried over to the decarburization blowing. Therefore, when trying to melt extra-low phosphorus steel, it may be difficult with this method.

[0005] A known method for melting ultra-low phosphorus steel involves using two converters (first converter and second converter). In the first converter, dephosphorization blowing is performed, and the dephosphorized molten iron is tapped and charged into the second converter, where decarburization blowing is performed. When the dephosphorized molten iron is tapped from the first converter, the dephosphorized slag remains in the first converter and can be separated from the dephosphorized molten iron. Therefore, phosphoric acid in the slag is not carried over to the decarburization blowing in the second converter, making it possible to melt ultra-low phosphorus steel. Regarding the desiliconization of the molten iron, the reaction proceeds in the initial stages of dephosphorization blowing, so the silicic acid produced by desiliconization is contained in the dephosphorized slag. Claim 1 of Patent Document 1 and Claim 4 of Patent Document 2 are relevant. Patent Document 3 describes a refining method 1 in which desilicate, intermediate slag removal, and dephosphorization are performed in one converter, the molten iron is tapped and charged into another converter, and decarburization is performed.

[0006] The above method is suitable for the production of ultra-low phosphorus steel because it separates the refining process into a dedicated dephosphorization converter and a dedicated decarburization converter, thus avoiding the need to carry over the dephosphorized slag generated by dephosphorization blowing in the dedicated dephosphorization converter to the decarburization blowing in the dedicated decarburization converter.

[0007] Traditionally, when melting ultra-low phosphorus steel while suppressing an increase in slag generation, it was common practice to perform refining using a dedicated dephosphorization converter and a dedicated decarburization converter. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2009-256727 [Patent Document 2] Japanese Patent Publication No. 2005-60740 [Patent Document 3] Japanese Patent Publication No. 2019-14958 [Overview of the project] [Problems that the invention aims to solve]

[0009] The demand for high-grade steel continues to rise, necessitating advancements in melting technology for ultra-low phosphorus steel, which has a lower phosphorus concentration than conventional steels. Meanwhile, the quality of iron ore used as raw material for blast furnace molten iron is declining, and the phosphorus concentration in the molten iron is expected to increase. Under these circumstances, there is a need to develop refining methods that can melt ultra-low phosphorus steel with a small amount of slag.

[0010] Generally, the melting of ultra-low phosphorus steel requires the use of large amounts of dephosphorization flux, which in turn generates a large amount of slag. As a result, the cost of dephosphorization flux, including quicklime, increases, and the amount of dephosphorization slag produced also increases, creating a need to expand the demand for reuse of this slag. However, the likelihood of increased demand for roadbed materials and other uses for reusing dephosphorization slag is small, making it difficult to solve the problem of expanding demand.

[0011] It is predicted that obtaining large quantities of low-phosphorus iron ore at low cost will become increasingly difficult in the future. As a result, the phosphorus concentration of molten iron produced in blast furnaces from low-grade iron ore is expected to increase. Consequently, in order to melt extremely low-phosphorus steel using converters, it will be necessary to use more dephosphorization flux and generate more dephosphorization slag.

[0012] For the reasons stated above, it is necessary to develop a refining method that can melt down ultra-low phosphorus steel with a smaller amount of dephosphorization flux.

[0013] In the aforementioned process, which uses two converters to carry out refining separately in a converter dedicated to dephosphorization and a converter dedicated to decarburization, in the converter dedicated to dephosphorization, the desiliconization reaction proceeds before the dephosphorization reaction occurs, generating high-temperature SiO2. This high-temperature SiO2 can cause the dephosphorization flux, such as quicklime, to slag. On the other hand, in the converter dedicated to decarburization, the molten iron has already been completely desiliconized, so high-temperature SiO2 is not generated, making it difficult to cause the dephosphorization flux, such as quicklime, to slag.

[0014] Patent Document 1 proposes adding premelt flux and a TiO2 source to accelerate the slag formation of dephosphorization fluxes such as quicklime in a dedicated decarburization converter. However, this method requires the use of expensive premelt flux and TiO2 source, and the addition of these also increases the amount of slag discharged.

[0015] In a converter dedicated to decarburization, if flux slag formation is slow and takes time, it becomes difficult to suppress spitting that occurs when the oxygen jet sprayed onto the molten iron from the top-blowing lance collides with the molten iron using the cover slag. This necessitates lowering the acid supply rate until the cover slag is formed, reducing the converter's productivity.

[0016] Furthermore, in dephosphorization blowing using a dedicated dephosphorization converter, all the silicic acid formed by the oxidation of silicon in molten iron is included in the dephosphorized slag. When forming dephosphorized slag by adding flux such as CaO, it is necessary to add flux in proportion to the amount of silicic acid in the slag in order to ensure the basicity required for dephosphorization blowing.

[0017] The present invention aims to provide a method for melting extremely low-phosphorus steel in a refining process using two converters, which does not require the addition of a slag accelerator such as a premelt flux to promote slag formation of the dephosphorus flux formed during decarburization blowing, does not require extending the decarburization blowing time for slag formation, and can reduce the amount of slag generated throughout the entire refining process. [Means for solving the problem]

[0018] This invention focuses on the above points and aims to accelerate the slag formation of dephosphorization fluxes such as quicklime in both desiliconization-dephosphorization and decarburization-dephosphorization, without adding slag formation accelerators such as premelt fluxes. This is achieved by leaving the high-temperature SiO2 produced by partially desiliconizing the molten iron in the furnace and utilizing it in decarburization-desiliconization-dephosphorization, and by desiliconizing the remaining Si in the molten iron in desiliconization-dephosphor

[0019] That is, the gist of the present invention is as follows. [1] In a method for refining hot metal for melting extra-low phosphorus steel using two converters (a first converter and a second converter), hot metal with a Si concentration in the hot metal of 0.40% by mass or more is charged into the second converter, and desiliconization blowing is performed until the Si concentration in the hot metal becomes 0.15% by mass or more and less than 0.30% by mass to obtain partially desiliconized hot metal. The partially desiliconized hot metal is discharged from the tapping hole of the second converter into a hot metal ladle. The partially desiliconized hot metal is charged into the first converter, and desiliconization and dephosphorization blowing with the addition of a CaO-containing substance are performed in the first converter to obtain desiliconized and dephosphorized hot metal. Then, the desiliconized and dephosphorized hot metal is discharged from the tapping hole of the first converter into a hot metal ladle. The desiliconized and dephosphorized hot metal is charged into the second converter again, and decarburization blowing with the addition of a CaO-containing substance is performed in the second converter. At this time, all or part of the desiliconization slag generated by the desiliconization blowing in the second converter is left in the furnace and used for the decarburization blowing in the second converter. A method for melting extra-low phosphorus steel, characterized in that. [2] Among the desiliconization slags generated by the desiliconization blowing in the second converter, 4.5 kg / t or more and 9.0 kg / t or less are left in the furnace and used for the decarburization blowing in the second converter. The method for melting extra-low phosphorus steel according to [1], characterized in that.

Advantages of the Invention

[0020] In the process of refining using two converters, the present invention leaves all or part of the desiliconization slag generated by the partial desiliconization blowing in the converter and uses it for the decarburization blowing, thereby promoting the slagging of the dephosphorization flux formed during the decarburization blowing without the need to add a slagging promoter such as a premelt flux, without the need to lengthen the decarburization blowing time for slagging of the slag, and reducing the amount of slag generated in the entire refining process. A method for melting extra-low phosphorus steel can be provided.

Brief Description of the Drawings

[0021] [Figure 1]It is a diagram showing the refining process of the present invention. [Figure 2] It is a diagram showing the refining process of the comparative example. [Figure 3] It is a diagram showing the relationship between the Si concentration of hot metal and the slag discharge amount for each refining process.

Mode for Carrying Out the Invention

[0022] In the present invention, ultra-low phosphorus steel means steel having a phosphorus concentration of 0.005% by mass or less.

[0023] First, a conventional process of performing refining separately using two converters, a dephosphorization dedicated converter and a decarburization dedicated converter, will be described based on FIG. 2. In FIG. 2, the first converter 1A is a dephosphorization dedicated converter, and the second converter 1B is a decarburization dedicated converter.

[0024] In step (a-11) in the first converter 1A, hot metal 11 is charged into the first converter 1A from the hot metal ladle 2 via the furnace mouth 6, and in step (a-12), desiliconization and dephosphorization blowing of the hot metal 11 are performed. In the desiliconization and dephosphorization blowing, since the entire amount of silicon in the hot metal is oxidized and contained in the desiliconization and dephosphorization slag, in order to ensure the basicity required for dephosphorization, a flux such as CaO is added in an amount corresponding to the amount of silicic acid in the slag. In step (a-13), the desiliconized and dephosphorized hot metal 13 is discharged into the hot metal ladle 4. All of the silicic acid generated by the desiliconization reaction remains in the first converter 1A as a component of the desiliconization and dephosphorization slag 22 and is discharged into the slag pan 5 in step (a-14).

[0025] In step (b-11) in the second converter 1B, the desiliconized and dephosphorized hot metal 13 is charged into the second converter 1B from the hot metal ladle 4, and in step (b-12), decarburization blowing is performed. Since no silicic acid source is contained in the converter, in order to form slag during decarburization blowing, as described in Patent Document 1, it is necessary to add a premelt flux or extend the decarburization blowing time while preventing the occurrence of spitting to slag the slag. In step (b-13), the molten steel 14 is discharged into the molten steel ladle 9, and the decarburization slag 23 remaining in the converter is discharged into the slag pan 5 in step (b-14).

[0026] In the conventional process described above, which uses two converters—one dedicated to dephosphorization and the other to decarburization—the high-temperature SiO2 generated by the desiliconization of Si in the molten iron could only be used for dephosphorization blowing in the dedicated dephosphorization converter (first converter 1A). Therefore, in the decarburization blowing performed in the dedicated decarburization converter (second converter 1B) in step (b-12), the slag formation of the dephosphorization flux is accelerated by adding a slag formation accelerator such as a premelt flux. Furthermore, the decarburization blowing time needs to be extended to facilitate slag formation. In addition, there is a need to reduce the amount of slag generated throughout the entire refining process.

[0027] Next, the entire process of the present invention's method for refining molten iron to produce ultra-low phosphorus steel using two converters 1 (first converter 1A, second converter 1B) will be explained based on Figure 1.

[0028] In the process of the present invention, as shown in Figure 1, partial desiliconization of molten iron 11 is performed in steps (b-1) to (b-3) of the second converter 1B, desiliconization and dephosphorization of the partially desiliconized molten iron 12 is performed in steps (a-1) to (a-3) of the first converter 1A, and decarburization is performed in steps (b-6) to (b-8) of the second converter 1B to produce molten steel 14. In the blowing in the first converter 1A in step (a-2) and the blowing in the second converter 1B in steps (b-2) and (b-7) shown in Figure 1, blowing is performed on the molten iron 31 and slag 32 in the converter using oxygen supply from the top blowing lance 8 and bottom blowing gas injection (not shown). The process will be explained in order below.

[0029] In each of the blowing processes described below (partial desiliconization blowing, desiliconization dephosphorus blowing, decarburization blowing), the combined mass ratio of CaO and SiO2 contained in the slag after blowing (CaO / SiO2) is called the basicity of the blend. In calculating the basicity of the blend, for the CaO content, the CaO content unit value in the added CaO-containing material is used, and for the SiO2 content, the SiO2 content generated by the combustion of silicon in the charged molten iron (molten iron 11, partially desiliconized molten iron 12, desiliconization dephosphorus molten iron 13) during blowing, the desiliconization slag 21 remaining in the converter, and the total SiO2 unit value in the added silica and other blending materials are used.

[0030] First, in step (b-1), molten iron 11 with a Si concentration of 0.40 mass% or more is charged into the second converter 1B. The molten iron is charged into the second converter 1B from the ladle 2 via the furnace opening 6. The reason for limiting the use of molten iron with a Si concentration of 0.40 mass% or more is that the effects of the present invention can be fully obtained when molten iron with a Si concentration of 0.40 mass% or more is used.

[0031] In step (b-2) of the second converter 1B, partial desiliconization blowing is performed to oxidize only a portion of the silicon in the molten iron. In this step, partial desiliconization blowing is performed until the Si concentration in the molten iron is 0.15% by mass or more and less than 0.30% by mass, resulting in partially desiliconized molten iron 12. The reason for keeping the Si concentration in the molten iron after partial desiliconization blowing at 0.15% by mass or more is to ensure good slag formation for dephosphorization blowing in the subsequent desiliconization and dephosphorization blowing in the first converter 1A. Furthermore, the reason for keeping the Si concentration in the molten iron after partial desiliconization blowing at 0.30% by mass or less is that if the Si concentration in the molten iron is excessive, the amount of CaO used in slag formation in the subsequent desiliconization and dephosphorization blowing will be excessive. In the partial desiliconization blowing of step (b-2), a CaO-containing substance may be added as a slag-forming material so that the aforementioned basicity of the blend is approximately 0.3 to 0.7. If the basicity of the mixture is 0.3 or higher, the activity of SiO2, the desiliconization product, is moderately reduced, making the desiliconization reaction easier to proceed. If it is 0.7 or lower, it is possible to avoid excessive addition of CaO for desiliconization.

[0032] In step (b-3), some desiliconized molten iron 12 is tapped into the molten iron ladle 3 via the tapping hole 7 of the second converter 1B. Desiliconized slag 21 formed by partial desiliconization blowing remains inside the second converter 1B.

[0033] In step (b-3), the partially desiliconized molten iron 12, which has been tapped from the second converter 1B to the molten iron ladle 3, is poured into the first converter 1A in step (a-1). In step (a-2), desiliconization and dephosphorization blowing with the addition of a CaO-containing substance is performed in the first converter 1A to obtain desiliconized and dephosphorized molten iron 13. As the CaO-containing substance to be added, quicklime, limestone, decarburized slag, ingot-forming slag, etc., can be used. In desiliconization and dephosphorization blowing, CaO-containing substances, silica, and other additives may be added so that the basicity of the blended mixture is approximately 1.1 to 2.0. This is because if the basicity of the blended mixture is 1.1 or higher, the activity of P2O5, which is a dephosphorization product, decreases and the dephosphorization reaction proceeds more easily, and if it is 2.0 or lower, it is possible to avoid excessive addition of CaO for dephosphorization.

[0034] In step (a-3), the desiliconized and dephosphorized molten iron 13 is tapped from the tapping hole 7 of the first converter 1A into the molten iron ladle 4. After tapping, in step (a-4), the entire amount of desiliconized and dephosphorized slag 22 is discharged from the first converter 1A into the slag pan 5.

[0035] In the second converter 1B, after partially removing the desilicate molten iron 12 in step (b-3), desilicate slag 21 remains in the converter (step (b-4)). With all or part of the desilicate slag 21 remaining in the converter, in step (b-6), the desilicate molten iron 13 removed from the first converter 1A in step (a-3) is poured into the second converter 1B. Furthermore, in step (b-7), decarburization blowing is performed in the second converter 1B with the addition of a CaO-containing substance. When the molten desilicate slag 21 (which consists mostly of SiO2) and the added CaO-containing substance are mixed, the slag formation of the CaO-containing substance is accelerated and it melts rapidly, forming molten slag, thus eliminating the need to add slag accelerators such as premelt flux. Furthermore, since molten cover slag is generated in a short time, the time required to reduce the acid supply rate to prevent spitting can be shortened, and the decarburization blowing process (b-7) can be performed while maintaining the high productivity of the converter without extending the decarburization blowing time.

[0036] Regarding the desilicate slag 21 to be left in the converter before the desilicate and dephosphorize molten iron charging in step (b-6), one can choose between two methods: a first method in which the entire amount of desilicate slag 21 remaining in the converter in step (b-4) after tapping in step (b-3) is left in place and the process proceeds to the next step (b-6); and a second method in which a portion of the desilicate slag 21 in the converter is discharged into the slag pan 5 in step (b-5) and the process proceeds to the next step (b-6). In the second method, it is preferable that in the second converter 1B, 4.5 kg / t to 9.0 kg / t of the desilicate slag 21 generated in the desilicate blowing in step (b-2) is left in the furnace in step (b-5) and used for decarburization blowing in step (b-7) of the second converter 1B. If the unit consumption of desilicate slag left in the furnace is 4.5 kg / t or more, the slag formation of CaO-containing materials such as quicklime used in the decarburization blowing process (b-7) can be stably promoted, and if it is 9.0 kg / t or less, it is possible to avoid an excess of desilicate slag necessary for that slag formation.

[0037] After decarburization and smelting are completed in step (b-7) of the second converter 1B, molten steel 14 is tapped from the tapping hole 7 of the second converter 1B into the molten steel ladle 9 in step (b-8). Furthermore, in step (b-9), decarburized slag 23 is discharged from the furnace opening 6 of the second converter 1B into the slag pan 5. [Examples]

[0038] The effects of the present invention were evaluated using two 300-ton converters (Converter 1A and Converter 2B).

[0039] In a process where molten iron 11 with a Si concentration of 0.40-0.80 mass% and a P concentration of 0.12 mass% is used as molten iron charged into a converter, and molten steel 14 with a P concentration of 0.002 mass% or less is produced as molten steel after the completion of converter refining, the effects of three processes, including the present invention (the present invention process, comparative process 1, and comparative process 2), were compared and evaluated.

[0040] (The process of the present invention) As part of the present invention, the process shown in Figure 1 was carried out. Molten iron 11 was charged into the second converter 1B (step (b-1)), quicklime was added so that the basicity of the blended metal as defined above became 0.5, and partial desiliconization blowing was performed until the Si concentration in the molten iron became 0.2 mass% (step (b-2)). After that, the partially desiliconized molten iron 12 was tapped from the tapping hole 7 into the molten iron ladle 3, and the partially desiliconized molten iron 12 was transferred to the first converter 1A (step (b-3)(a-1)). In step (a-2), quicklime was added so that the basicity of the blended metal became 1.3, and desiliconization and dephosphorization blowing was performed. Subsequently, desiliconized and dephosphorized molten iron 13 was tapped from the tapping hole 7 of the first converter 1A into the ladle 4 (step (a-3)), the desiliconized and dephosphorized molten iron 13 was again poured into the second converter 1B (step (b-6)), and after adding an amount of quicklime expected to yield a P concentration of 0.0018 mass% in the molten steel after decarburization and blowing, and MgO granules for refractory protection, decarburization and blowing were carried out (step (b-7)).

[0041] At that time, in the partial slag discharge process of step (b-5) of the second converter 1B, 4.5 to 9.0 kg / t of the desilicate slag 21 (which remained in the converter in step (b-4)) generated in the partial desilicate blowing of step (b-2) was left in the converter, and the remaining desilicate slag was discharged into the slag pan 5 without being reused. The desilicate slag 21 left in the converter was reused in the decarburization blowing (step (b-7)) of the second converter 1B.

[0042] (Comparison Process 1) In comparative process 1, molten iron 11 was refined using a dedicated dephosphorization converter (first converter 1A) and a dedicated decarburization converter (second converter 1B) according to the process shown in Figure 2.

[0043] First, molten iron 11 is charged into a dedicated dephosphorization converter (first converter 1A) (step (a-11)), quicklime is added to achieve a basicity of 1.3, and desiliconization and dephosphorization blowing is performed (step (a-12)). When the phosphorus concentration in the molten iron reaches approximately 0.02 mass%, the desiliconization and dephosphorization molten iron 13 is tapped out of the tapping hole 7 into the molten iron ladle 4. In step (b-11), the desiliconization and dephosphorization molten iron 13 is transferred to a dedicated decarburization converter (second converter 1B), and quicklime, silica, and MgO granules for refractory protection are added as a dephosphorization flux at a basicity of 3.5 in an amount that is expected to yield a P concentration of 0.0018 mass% in the molten steel after treatment, and decarburization blowing is performed (step (b-12)).

[0044] (Comparison Process 2) In comparative process 2, unlike the method of the present invention, the desilicate slag generated by desilicate blowing in the second converter 1B was not left in the furnace, but was completely discharged in step (b-5), and the slag was refined in a process in which silica equivalent to the amount of discharged desilicate slag recycled in the process of the present invention was added during decarburization blowing in step (b-7).

[0045] The evaluation results of the three processes described above are shown below.

[0046] First, in all three processes—the present invention process, comparative process 1, and comparative process 2—the P concentration of the resulting molten steel achieved the target of 0.002% by mass or less.

[0047] Figure 3 shows the actual slag discharge figures, with the molten iron Si concentration on the horizontal axis. This slag discharge figure represents the total amount of desilicate slag 21, desilicate and dephosphorize slag 22, and decarburize slag 23 discharged outside the furnace. The present invention process is represented by white circles, comparative process 1 by black circles, and comparative process 2 by white squares. In Figure 3, in the present invention process, the amount of desilicate slag 21 left in the converter during the partial slag removal process of step (b-5) is set to 6.0 kg / t.

[0048] In comparative process 1, since desiliconization is not performed before desiliconization and dephosphorization blowing (step (a-12) in Figure 2), the amount of quicklime added in step (a-12) increases in proportion to the molten iron Si concentration, resulting in a large amount of slag discharged in molten iron with a high Si concentration. In contrast, comparative process 2, in which desiliconization is performed before desiliconization and dephosphorization blowing (step (a-2) in Figure 1), and the present invention process, the amount of slag discharged was significantly reduced. In comparative process 2, the amount of slag discharged was kept to a level only slightly higher than that of the present invention process.

[0049] Furthermore, in comparative processes 1 and 2, high-temperature molten SiO2 could not be used in decarburization blowing, which resulted in a longer slag formation time and a longer period of time during the initial blowing stage when slag formation was insufficient, requiring the acid supply rate to be kept at a low level.

[0050] On the other hand, as shown in Figure 3, the process of the present invention resulted in the lowest slag discharge and the ability to utilize high-temperature molten SiO2 in the decarburization furnace, leading to rapid slag formation and a total blowing time approximately 10% shorter compared to comparative processes 1 and 2.

[0051] Furthermore, in the process of the present invention, under the condition that the Si concentration of the molten iron 11 to be charged is 0.60 mass%, the amount of desilicate slag 21 left in the converter during the partial slag removal process of step (b-5) was evaluated for cases of 4.5 kg / t and 9.0 kg / t. As a result, it was found that the amount of slag discharged was 40 kg / t when the amount of desilicate slag 21 left in the converter was 4.5 kg / t, and the amount of slag discharged was 45 kg / t when the amount of desilicate slag 21 left was 9.0 kg / t. [Explanation of symbols]

[0052] 1 Converter 1A First Converter 1B 2nd converter 2. Molten iron pot 3. Molten iron pot 4. Molten iron pot 5 Slagpan 6 Hearth 7 Steel tapping hole 8. Upward-blowing lance 9. Molten steel pot 11 Molten iron 12 Partially desiliconized hot metal 13 Desiliconization and dephosphorization of molten iron 14 Molten steel 21 Desiliconized slag 22 Desiliconized and dephosphorized slag 23 Decarburized slag 31 Molten iron 32 slag

Claims

1. In a method for refining molten iron to produce extremely low phosphorus steel using two converters (first converter, second converter), Molten iron with a Si concentration of 0.40% by mass or more is charged into the second converter, and desiliconization blowing is performed until the Si concentration in the molten iron becomes 0.15% by mass or more and less than 0.30% by mass to obtain partially desiliconized molten iron, and the partially desiliconized molten iron is tapped into the molten iron ladle from the tapping hole of the second converter. The partially desiliconized molten iron is poured into the first converter, and desiliconization and dephosphorization blowing is performed in the first converter with the addition of a CaO-containing substance to obtain desiliconized and dephosphorized molten iron. Then, the desiliconized and dephosphorized molten iron is poured into the molten iron ladle from the tapping hole of the first converter. The desiliconized and dephosphorized molten iron is poured back into the second converter, and decarburization blowing is performed in the second converter with the addition of a CaO-containing substance. A method for melting extremely low phosphorus steel, characterized in that, in the second converter, all or part of the desilicate slag generated in the desilicate blowing is left in the furnace and used in the decarburization blowing of the second converter.

2. A method for melting extremely low phosphorus steel according to claim 1, characterized in that, of the desilicate slag generated in the desilicate blowing in the second converter, 4.5 kg / t or more and 9.0 kg / t or less is left in the furnace and used for the decarburization blowing in the second converter.

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