Methods for refining molten iron

Abstract

This invention provides a technique for preventing splashing during the refining of high-Si-concentration molten pig iron without increasing the number of converters used. One method is a refining process comprising: a first step of charging molten pig iron with a Si concentration of 0.7% by mass or higher into a converter; a second step of blowing oxygen-containing gas through a top-blown lance to remove a portion of the Si from the molten metal; a third step of discharging slag from the converter; a fourth step of performing desiliconization, dephosphorization, and decarburization treatments by blowing oxygen through the top-blown lance to remove the remaining Si from the molten metal; and a fifth step of tapping the steel that has undergone desiliconization, dephosphorization, and decarburization treatments. In this method, in the fifth step, the slag generated in the fourth step remains in the converter. In the first step of the next processing, molten pig iron is charged while the slag from the preceding processing remains in the converter, and the remaining slag is reused in the second step of the next processing.

Description

[Technical Field]

[0001] This invention relates to a method for refining molten iron in a converter. In this specification, the unit of mass "t" represents 10³ kg. The "N" preceding the unit of gas volume indicates standard conditions, which are a temperature of 0°C and a pressure of 101325 Pa. The numerical range "x~y" means "above x and below y," and includes boundary values. Furthermore, "molten iron" refers to molten metal primarily composed of Fe, and includes "molten pig iron" and "molten steel" containing C. The C content of the "pig iron" is 3% to 5% by mass, and the C content of the "steel" is 2.1% by mass or less. [Previous Technology]

[0002] In the steelmaking process, the composition of the molten pig iron transported to the converter varies widely, and the Si concentration can sometimes reach 2% by mass. This phenomenon occurs particularly frequently when the blast furnace tapping rate decreases. When the Si concentration of the molten pig iron is high, it can be mixed with molten pig iron with a low Si concentration and charged into the converter. On the other hand, if molten pig iron with a high Si concentration is continuously transported, it will exceed the converter's processing capacity, leading to problems such as the molten pig iron solidifying in the blast furnace pot due to prolonged standby. Therefore, increasing the upper limit of the Si concentration in the molten pig iron that can be processed in the converter is an issue.

[0003] Methods that have been consistently implemented for processing molten pig iron with high Si concentration include the following: For example, there is a method in which desiliconization is performed in a blast furnace or converter, followed by processing in a decarburization converter. In this case, specialized equipment is required for performing desiliconization in the blast furnace. Furthermore, when performing desiliconization in a converter, issues arise regarding heat loss due to the increased number of loadings, or a decrease in production capacity due to the use of two converters.

[0004] In recent years, as a refining method in converters, the process of removing slag from the converter after desiliconization and dephosphorization treatment and then performing decarburization blowing has gradually become mainstream (for example, see Patent Document 1 or Patent Document 2). This method allows for the design of slag composition or molten steel temperature control ranges during desiliconization and dephosphorization treatments and decarburization treatments, thus enabling efficient processing. Compared to continuously performing desiliconization, dephosphorization, and decarburization treatments without slag removal during the process, this method requires less slag and has the advantage of being able to handle even situations with high Si concentrations in molten pig iron. Furthermore, in Patent Document 2, splashing is suppressed by adjusting the top-blown oxygen or bottom-blown gas volume. [Prior Art Documents] [Patent Documents]

[0005] Patent Document 1: Japanese Patent Application Publication No. 2022-105879; Patent Document 2: Japanese Patent Application Publication No. 2020-180335 [Summary of the Invention]

[0006] [Problem to be Solved by the Invention] However, the aforementioned prior art has the following problem that must be solved. That is, the technology described in Patent Document 1 has an upper limit on the Si concentration, and in order to suppress splashing caused by slag bubbling, the upper limit of the Si concentration in the molten pig iron is limited to 0.6% by mass. In addition, even the technology described in Patent Document 2 can only process molten pig iron with a Si concentration of 0.80% by mass or less.

[0007] This invention was made to solve the aforementioned problem, and its object is to provide a technique for preventing splashing during the refining of molten pig iron with high Si concentration without increasing the number of converters used. [Means for Solving the Problem]

[0008] The refining method for molten iron of the present invention, which advantageously solves the aforementioned problem, is as follows, wherein the refining method is performed sequentially: a first step, charging molten pig iron with a Si concentration of 0.7% by mass or more into a converter; a second step, blowing oxygen-containing gas from a top-blown lance to remove a portion of the Si in the molten liquid; a third step, discharging the slag in the converter to the outside of the furnace; a fourth step, performing desiliconization, dephosphorization, and decarburization treatments by blowing oxygen from the top-blown lance to remove the remaining Si in the molten liquid; and a fifth step, tapping the molten steel that has undergone desiliconization, dephosphorization, and decarburization treatments, wherein the method is characterized in that, in the fifth step, the slag generated in the fourth step remains in the converter, and in the first step, molten pig iron is charged while the slag remaining in the fifth step, which is the immediate preceding treatment, is still in the converter, and in the second step, the remaining slag is reused.

[0009] Furthermore, regarding the iron refining method of the present invention, (a) during the second step, the basicity of the slag is in the range of 0.9 or more and 1.1 or less, and the Si concentration at the end of the second step is 0.3% by mass or less, where the basicity of the slag refers to the ratio of CaO to SiO2 in the slag on a mass basis; (b) the Si concentration at the end of the second step is 0.1% by mass or more and 0.2% by mass or less; and (c) the slag discharge rate in the third step is 30% to 80%, etc., which are more desirable solutions to the problem. [Effects of the Invention]

[0010] According to the present invention, even when the Si concentration is 0.7% by mass or more in molten pig iron, splashing can be prevented without increasing the number of converters used. Therefore, it is possible to suppress heat loss to the outside of the system and prevent a decrease in the converter's production capacity.

Implementation Method

[0012] Hereinafter, embodiments of the present invention will be specifically described. Furthermore, the following embodiments are illustrative of methods for embodying the technical concept of the present invention, and do not necessarily define the structure as described below. 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.

[0013] (Converter blowing method) Figures 1(A) to 1(E) are schematic diagrams illustrating a method for refining molten iron according to one embodiment. The method for refining molten iron in this embodiment is, for example, a converter blowing method. The method for refining molten iron in this embodiment is a method of repeatedly performing steps one through five in the same converter.

[0014] In the first step shown in Figure 1(A), molten pig iron 9a with a Si concentration of 0.7% by mass or more is charged into the converter 1 via a self-loading pot 14. At this time, a chill source 11 or residual slag 17 from the previous treatment can also be pre-placed into the converter 1. If the Si concentration of the charged molten pig iron 9a is less than 0.7% by mass, the previous refining method for molten iron can be applied. The upper limit of the Si concentration of the charged molten pig iron 9a is preferably 1.5% by mass or less. At Si concentrations exceeding the upper limit, excessive splashing may interrupt the blowing process.

[0015] In the second step shown in Figure 1 (B), a desiliconization process is performed by blowing oxygen-containing gas 12, which serves as the oxygen source, onto the molten iron through a top-blown lance 2 to remove a portion of the Si. During this desiliconization process, bottom-blown gas 13 is supplied from the bottom-blown tuyeres 3 to agitate the molten iron 9. The Si concentration in the molten iron at the end of the second step is preferably 0.3% by mass or less. More preferably, the Si concentration in the molten iron at the end of the second step is 0.1% by mass or more and 0.2% by mass or less. Furthermore, during the second step, the basicity of the slag is preferably in the range of 0.9 or more and 1.1 or less. Here, the basicity of the slag refers to the ratio of CaO to SiO2 in the slag on a mass basis, and the same applies below. The oxygen-containing gas used as the oxygen source can be pure oxygen, a mixture of oxygen and an inert gas, or CO2 gas.

[0016] In the third step shown in Figure 1 (C), the converter 1 is tilted so that the slag 10 generated by the desiliconization process is discharged from the furnace mouth to the outside of the converter 1.

[0017] In the fourth step shown in Figure 1 (D), oxygen-containing gas 12, which serves as an oxygen source, is blown in by a top-blown lance 2 to remove the remaining Si from the molten iron 9, thereby producing molten steel 9b. In this process, bottom-blown gas 13 is blown in from the bottom-blown tuyeres 3 to stir the molten metal.

[0018] In the fifth step shown in Figure 1 (E), the refined molten steel 9b is tapped from the melt discharge port 4 into the ladle. After tapping, slag 17 generated in the fourth step remains in the converter 1. The slag 17 is carried to the next processing while remaining, and the processing is repeated from the first step.

[0019] (Reasons for limiting the basicity of the slag in the second step) The lower limit of the basicity of the slag is set to 0.9. The reason is to prevent phosphorus from the slag 17 left over from the previous treatment from being distributed into the molten iron 9, and to prevent excessive foaming due to increased viscosity caused by low basicity. In addition, by carrying slag 17 from the previous treatment, the amount of lime added to the furnace in the second step can be reduced, thereby reducing costs and reducing the heat lost in the furnace due to the addition of lime at room temperature. On the other hand, the upper limit of the basicity of the slag is set to 1.1. For dephosphorization, a basicity of 1.2 to 2.0 is appropriate. However, in this invention, by sacrificing the dephosphorization effect, for high-Si molten pig iron with a Si content of 0.7% by mass or more, the blowing process can be completed smoothly while suppressing splashing. In the blowing process of high-Si molten pig iron, if the target basicity of the slag before slag discharge is set at 1.2 to 2.0, it cannot be achieved without excessive addition of CaO. This results in an increased slag volume, increased slag foaming, and an increase in the basic CaO concentration. In this embodiment, since the second step is not for dephosphorization, it is not necessary to excessively increase the basicity to prevent cost increases caused by the use of lime. Therefore, during the second step, the basicity of the slag is set in the range of 0.9 or higher and 1.1 or lower.

[0020] (The Si concentration in the melt at the end of the second blowing step is 0.3% by mass or less) If the Si concentration in the melt decreases, the oxygen-based desiliconization rate decreases, while the decarburization rate increases. Therefore, when the Si concentration in the melt is high and the amount of slag is large, slag foaming based on CO gas is promoted, making splashing more likely. For example, Figure 2 shows the relationship between the estimated Si concentration in the melt at the end of the second step and the basic unit of the foaming inhibitor required in the fourth step, under the condition that the Si concentration in the molten pig iron 9a charged in the first step is 0.7% by mass or more. If desiliconization is not sufficiently carried out in the second step, the tendency to splashing will be aggravated in the fourth step. According to the results in Figure 2, from the viewpoint of suppressing splashing in the fourth step, the Si concentration in the melt at the end of the second step is preferably set to 0.3% by mass or less.

[0021] Figure 3 shows the relationship between the amount of desiliconization in the second step, i.e., the difference ΔSi (mass%) between the Si concentration before and after treatment and the amount of Si concentration in the molten pig iron 9a charged in the first step being 0.7% by mass or more, and the basic unit of the foaming inhibitor required in the second step. If excessive desiliconization is performed in the second step, the possibility of slag foaming caused by the decarburization reaction increases, and the tendency to splash in the second step will be aggravated.

[0022] Figure 4 schematically illustrates the relationship between the estimated Si concentration in the melt at the end of the second step and the basic units of the foaming inhibitor required in the second and fourth steps, summarizing the results. Based on these results, in order to ensure stable processing without splashing in either the second or fourth step, the Si concentration in the melt at the end of the second step is preferably in the range of 0.1% by mass or more and 0.2% by mass or less.

[0023] If the results are summarized, the target Si concentration at the end of the second step is preferably designed for blowing in the range of 0.1% by mass or more and 0.2% by mass or less. Furthermore, if no tendency to sputter is observed in the second step, the Si concentration in the melt can also be set to more than 0.0% by mass in order to reduce the Si source carried in the fourth step. On the other hand, if the sputtering tendency is so high that it is difficult to reduce the Si concentration in the melt to 0.2% by mass in the second step, blowing is temporarily interrupted and the process is moved to the next step. Even in the case described above, the Si concentration in the melt is preferably reduced to 0.3% by mass or less in order to reduce the Si source carried in the fourth step.

[0024] (Method for determining the basicity of slag in the second step) The basicity of slag is calculated by dividing the mass of CaO in the auxiliary raw materials fed into converter 1 before the end of the second step by the sum of the mass of SiO2 generated by the desiliconization treatment of Si in the molten pig iron and the mass of SiO2 in the auxiliary raw materials.

[0025] (Method for determining the target Si concentration in the molten metal at the end of the second step) The amount of oxygen required per 1 t of molten iron in the oxygen-containing gas blown in the second step (the basic unit of oxygen blowing in the second step) is expressed by the following equation (1). Equation (1) (the basic unit of oxygen blowing in the second step) [Nm3 / t] ={ΔSi[kg / t-Fe] / M(Si)×22.4[Nm3 / kmol]} / (desiliconization oxygen efficiency α) Here, M(Si) is the atomic weight of Si and is 28 kg / kmol. The basic unit of desiliconization in the second step, ΔSi[kg / t-Fe], that is, the amount of reduction in the mass of Si in the molten iron per 1 t of molten iron, is expressed by the following equation (2). (2) Equation (Desiliconization amount ΔSi) [kg / t-Fe] = {(Si concentration in molten pig iron) [mass%] - (target Si concentration in melt discharge) [mass%]} × 10 Furthermore, the desiliconization oxygen efficiency α is determined based on past performance as shown in Figure 5, as shown in Table 1. If the above relationship is used, and the Si concentration in the molten iron at the beginning of the second step is set to 1.2% by mass, and the target Si concentration at the end of the second step is set to 0.2% by mass, then the basic unit of oxygen blowing in the second step is {(1.2-0.2) × 10 / 28} × 22.4 / 0.8 = 10 [Nm3 / t].

[0026] [Table 1] Si concentration in molten pig iron [quality%] Desiliconization efficiency α[%] 0.70 55 0.80 60 0.90 65 1.00 70 1.10 75 ≥1.20 80

[0027] Furthermore, in this embodiment, the desiliconization process can also be completed in a converter before the decarburization rate increases. After the bottom-blown gas flows for 2 to 3 minutes to promote the separation of pig iron slag, intermediate slag discharge is carried out, and the remaining material is subjected to desiliconization, dephosphorization and decarburization processes.

[0028] The slag removal rate in the intermediate slag discharge should preferably be 30% to 80%, preferably 60% to 80%. When the slag removal rate is less than 30%, a large amount of highly viscous slag with a basicity of around 1 remains, which will cause splashing due to the CO gas generated during the decarburization process in the fourth step. On the other hand, the higher the slag removal rate, the better, but if a slag removal rate of more than 80% is to be implemented, it is possible that even molten pig iron may flow out.

[0029] The oxygen supply rate during the dephosphorization and decarburization of the molten pig iron thus obtained in the fourth step can be set to 45,000 Nm³ / h. Previously, when using molten pig iron containing 0.7% by mass or more of Si as the starting material, such a high oxygen supply rate would cause splashing, but according to the method of the present invention, splashing does not occur. This shortens the blowing time. [Example]

[0030] (Example 1) Figure 7 shows the concentration shift of P and Si in molten iron during refining of high-Si molten pig iron based on the blowing mode or design shown in Figure 6. In the inventive example (Figure 7(b1) and Figure 7(b2)), the amount of molten pig iron charged is set to 320 t, and a 60 t chill source is used. The Si concentration in the molten pig iron is 1.2% by mass. The residual slag left over from the previous treatment is 3 t, and 5 t of lime and 6 t of recycled slag are used as by-products. The basicity of the slag in the second step is targeted at 1.0. The blowing time in the second step is 7 min, and the blowing time in the fourth step is 15 min. Pure oxygen gas is used as the top blowing gas, and Ar gas is used as an inert gas as the bottom blowing gas. In the existing examples (Figure 7(a1) and Figure 7(a2)), the Si concentration in the molten pig iron is set to 0.4% by mass, and the basicity target of the slag in the second step is set to 1.6. Except for the different blowing mode, the other settings are the same as those in the invention example.

[0031] In the initial stage of the second-step refining process, the top-blowing flow rate of the invention is lower than that of the conventional example. In the final stage of the second-step refining process, the bottom-blowing flow rate of the invention is lower than that of the conventional example. In the invention, the second step is not for P removal, and therefore does not require FeO generation or final stirring for P removal. The spray gun height is designed based on the top-blowing flow rate to ensure a constant bath surface dynamic pressure.

[0032] In the fourth step of the blowing mode, in the invention example where the amount of slag increases, the maximum value of the top-blowing flow rate is lower than that of the existing example in order to suppress splashing. The lance height is designed according to the top-blowing flow rate to ensure that the dynamic pressure of the bath surface is constant.

[0033] In the conventional example, the basicity of the slag in the second step is in the range of 1.5 to 1.7, while in the inventive example it is in the range of 0.9 to 1.1. According to Figure 7, in the inventive example, for Si, which is oxidized and removed before P or C, since the oxygen supply ends before the oxidation and removal of Si is completed, P can hardly be oxidized and removed at the end of the second step. On the other hand, in the conventional example, Si is completely removed in the second step. However, the absolute amount of Si that can be removed in the inventive example is greater.

[0034] Figure 8 shows the influence of the existing example and the inventive example on the relationship between the initial Si concentration and the basic unit of the foaming inhibitor in the molten iron during the second step. The melting pig iron temperature is in the range of 1300℃ to 1350℃, and the molten pig iron blending rate is below 90%. According to Figure 8, there is a tendency for the basic unit of the foaming inhibitor to increase as the Si concentration increases. It is found that in the inventive example, even if the initial Si concentration in the molten iron is above 0.7% by mass, a smaller amount of foaming inhibitor can be used for blowing compared to the existing example.

[0035] (Example 2) Under the same blowing conditions as in Example 1, the effect of the estimated Si concentration at the end of the second step on the amount of foaming inhibitor used in the second and fourth steps was investigated. The Si concentration in the molten pig iron was 1.0% by mass. The results are summarized in Table 2. It was found that for No. 2 to No. 4, where the Si concentration at the end of the second step was 0.1% by mass or more and 0.3% by mass or less, the basic unit of foaming inhibitor used in the second and fourth steps was 1.5 kg or less per ton of molten iron. In particular, it was found that for No. 2 and No. 3, the basic unit of foaming inhibitor used in the second and fourth steps was 1.3 kg or less per ton of molten iron. On the other hand, when the Si concentration at the end of the second step was less than the lower limit (No. 1), due to concerns about splashing caused by slag foaming, the basic unit of foaming inhibitor used in the second step was 1.7 kg per ton of molten iron, which became excessive. Furthermore, when the Si concentration exceeds the upper limit (No. 5, No. 6) at the end of the second step of blowing, due to concerns about splashing caused by slag bubbling, the basic unit of the foaming inhibitor used in the fourth step becomes excessive, which is more than 1.8 kg per 1 t of molten iron.

[0036] [Table 2] No. When the second step ends Estimated Si concentration Foaming inhibitor dosage Second step Fourth step quality% kg / t kg / t 1 0 1.7 0.6 2 0.1 1.3 0.9 3 0.2 1.0 1.2 4 0.3 0.7 1.5 5 0.4 0.3 1.8 6 0.5 0.0 2.1 [Simplified Explanation of the Diagram]

[0011] Figure 1 is a schematic diagram showing the flow chart of a method for refining molten iron according to an embodiment of the present invention. Figure 2 is a graph showing the relationship between the estimated Si concentration in the molten iron charged in the first step and the basic unit of the foaming inhibitor required in the fourth step, under the condition that the Si concentration in the molten pig iron charged in the first step is 0.7% by mass or more. Figure 3 is a graph showing the relationship between the amount of desiliconization ΔSi (mass%) in the second step and the basic unit of the foaming inhibitor required in the second step, under the condition that the Si concentration in the molten pig iron charged in the first step is 0.7% by mass or more. Figure 4 is a graph showing the relationship between the estimated Si concentration in the molten iron at the end of the second step and the basic unit of the foaming inhibitor required in the second and fourth steps. Figure 5 is a graph showing the relationship between the initial Si concentration in the molten iron in the second step and the desiliconization efficiency α. Figure 6 is a graph illustrating the blowing modes of the second and fourth steps in a conventional example and an inventive example. Figure 7 is a graph showing the concentration shift of P and Si, expressed as the concentration of iron processed in the blowing mode. Figures 7(a1) and 7(a2) represent P and Si in the conventional example, respectively, and Figures 7(b1) and 7(b2) represent P and Si in the inventive example, respectively. Figure 8 is a graph showing the influence of the conventional example and the inventive example on the relationship between the initial Si concentration and the basic unit of the foaming inhibitor in the molten pig iron in the second step.

Claims

1. A method for refining molten iron, comprising the following refining method, wherein the refining method is performed sequentially: a first step, charging molten pig iron with a Si concentration of 0.7% by mass or more into a converter; a second step, blowing oxygen-containing gas from a top-blown lance to remove a portion of the Si in the molten iron; a third step, discharging the slag from the converter to the outside of the furnace; a fourth step, performing desiliconization, dephosphorization, and decarburization treatments by blowing oxygen from the top-blown lance to remove the remaining Si in the molten iron; and a fifth step, tapping the molten steel that has undergone desiliconization, dephosphorization, and decarburization treatments, wherein, in the fifth step, the slag generated in the fourth step remains in the converter; in the first step, molten pig iron is charged while the slag remaining in the fifth step, which is the immediate preceding treatment, is still in the converter; and in the second step, the remaining slag is reused.

2. The method for refining molten iron as described in claim 1, wherein, During the second step, the basicity of the slag is in the range of 0.9 or higher and 1.1 or lower, and the Si concentration at the end of the second step is 0.3% by mass or lower. Here, the basicity of the slag refers to the ratio of CaO to SiO2 in the slag on a mass basis.

3. The method for refining molten iron as described in claim 2, wherein, The Si concentration at the end of the second step of blowing is above 0.1% by mass and below 0.2% by mass.

4. The method for refining molten iron as described in claim 2 or 3, wherein, The slag removal rate in the third step is 30% to 80%.

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