Refining method for molten iron

Abstract

Provided is a technique for preventing slopping without increasing the number of converters used when refining molten iron that has a high Si concentration. A refining method according to the present invention includes a first step for charging molten iron that has an Si concentration of at least 0.7 mass% into a converter, a second step for blowing an oxygen-containing gas from a top-blowing lance to remove a portion of the Si in the molten metal, a third step for discharging slag from inside the converter to the outside of the furnace, a fourth step for blowing oxygen gas from the top-blowing lance to perform desiliconization that removes the remainder of the Si in the molten metal, dephosphorization, and decarbonization, and a fifth step for tapping the molten steel obtained by the desiliconization, dephosphorization, and decarbonization. Slag generated at the fourth step is left inside the converter at the fifth step, molten iron is charged into the converter on top of the slag left from the previous process at the first step of the next process, and the slag is reused at the second step of the next process.

Description

Iron smelting method

[0001] This invention relates to a method for refining molten iron in a converter. In this specification, the unit of mass "t" is 10 ３ The unit is expressed in kilograms. The "N" preceding the unit of gas volume indicates standard conditions, which mean a temperature of 0°C and a pressure of 101325 Pa. The range "x to y" indicates a range of values, meaning "more than or equal to x and less than or equal to y," and includes boundary values. Furthermore, "molten iron" refers to molten metal mainly composed of Fe, and includes "molten pig iron" and "molten steel" which contain carbon. "Pig iron" has a carbon content of 3 to 5% by mass, and "steel" has a carbon content of 2.1% by mass or less.

[0002] In the steelmaking process, the composition of molten iron transported to the converter varies, and the Si concentration can reach 2% by mass. This phenomenon occurs frequently, especially when the amount of blast furnace iron produced decreases. When the Si concentration of molten iron is high, it is sometimes mixed with molten iron with a lower Si concentration before being charged into the converter. On the other hand, if molten iron with a high Si concentration is transported continuously, it will exceed the converter's processing capacity, leading to problems such as the molten iron solidifying in the ladle due to prolonged waiting. Therefore, raising the upper limit of the Si concentration in molten iron that can be processed by the converter has been a challenge.

[0003] The following methods have been conventionally used to process molten iron with a high Si concentration. For example, one method involves performing desiliconization in the molten iron ladle or in a converter, and then charging the iron into a converter for decarburization. In this case, dedicated equipment is required to perform desiliconization in the molten iron ladle. Furthermore, performing desiliconization in a converter presents challenges such as heat loss due to the increased number of charging cycles and a decrease in production capacity due to the use of two converters.

[0004] In recent years, a process in which slag is removed from the converter after desiliconization and dephosphorization, followed by decarburization and blowing, has become the mainstream refining method in converters (see, for example, Patent Documents 1 and 2). In this method, the control range of the slag composition and molten steel temperature can be designed separately for the desiliconization and dephosphorization process and the decarburization process, allowing for efficient processing. This method also has the advantage of requiring less slag compared to the case where desiliconization, dephosphorization, and decarburization are performed continuously without removing slag during the process, and can therefore be used even when the Si concentration in the molten iron is relatively high. Furthermore, Patent Document 2 suppresses slopping by adjusting the amount of top-blown oxygen and bottom-blown gas.

[0005] Japanese Patent Publication No. 2022-105879 Japanese Patent Publication No. 2020-180335

[0006] However, each of the above-mentioned conventional technologies has the following problems that need to be solved. Specifically, the technology described in Patent Document 1 has an upper limit on the Si concentration, and in order to suppress slopping due to slag forming, the upper limit of the Si concentration in molten iron was limited to 0.6 mass%. Furthermore, even in the technology described in Patent Document 2, the Si concentration in molten iron that can be processed is limited to 0.80 mass% or less.

[0007] The present invention was made to solve the aforementioned problems, and aims to provide a technology for preventing slopping when refining molten iron with a high Si concentration without increasing the number of converters used.

[0008] The present invention provides a method for refining molten iron that advantageously solves the above problems, comprising the steps of: a first step of charging molten iron with a Si concentration of 0.7% by mass or more into a converter; a second step of blowing oxygen-containing gas from an upper blowing lance to remove a portion of the Si in the molten metal; a third step of removing slag from the converter to the outside of the furnace; a fourth step of performing desiliconization, dephosphorization, and decarburization treatments by blowing oxygen gas from the upper blowing lance to remove the remaining Si in the molten metal; and a fifth step of tapping molten steel that has undergone desiliconization, dephosphorization, and decarburization treatments, wherein in the fifth step, the slag generated in the fourth step is left in the converter; in the first step, molten iron is charged while retaining the slag left in the immediately preceding fifth step in the converter; and in the second step, the left slag is reused.

[0009] Furthermore, the molten iron refining method according to the present invention is characterized in that (a) during the second step the basicity of the slag is in the range of 0.9 to 1.1 and the Si concentration at the end of the blowing in the second step is 0.3% by mass or less, where the basicity of the slag is based on the mass of the slag, and SiO ２ (b) the ratio of CaO to (c) 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 removal rate in the third step is 30% to 80%, which are more preferable means of solving the problem.

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

[0011] This is a schematic diagram showing the flow of a molten iron refining method according to one embodiment of the present invention. This is a graph showing the relationship between the estimated Si concentration in the molten metal at the end of the second step and the unit consumption of the foaming inhibitor required in the fourth step, under the condition that the Si concentration in the molten iron charged in the first step is 0.7 mass% or more. This is a graph showing the relationship between the amount of desilicate ΔSi (mass%) in the second step and the unit consumption of the foaming inhibitor required in the second step, under the condition that the Si concentration in the molten iron charged in the first step is 0.7 mass% or more. This is a graph showing the relationship between the estimated Si concentration in the molten metal at the end of the second step and the unit consumption of the foaming inhibitor required in the second and fourth steps. This is a graph showing the relationship between the initial Si concentration in the molten iron in the second step and the desilicate oxygen efficiency α. This is a graph illustrating the blowing patterns of the second and fourth steps in a conventional example and an inventive example. This graph shows the concentration changes of P and Si in molten iron processed with the above blowing pattern, where (a1) and (a2) represent P and Si in the conventional example, and (b1) and (b2) represent P and Si in the inventive example, respectively. This graph shows the influence of the conventional example and the inventive example on the relationship between the initial Si concentration in the molten iron in the second step and the foaming inhibitor unit cost.

[0012] The embodiments of the present invention will be described in detail below. The following embodiments are illustrative examples of methods for realizing the technical concept of the present invention and do not limit the configuration to those described below. That is, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

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

[0014] In the first step shown in Figure 1(A), molten iron 9a with a Si concentration of 0.7 mass% or more is charged into the converter 1 from the charging pot 14. At this time, the converter 1 may already contain a cold iron source 11 or slag 17 remaining from the previous treatment. If the Si concentration of the molten iron 9a to be charged is less than 0.7 mass%, the conventional molten iron refining method can be applied. The upper limit of the Si concentration of the molten iron 9a to be charged is preferably 1.5 mass% or less. If the Si concentration exceeds the upper limit, there is a risk of interruption of the smelting due to excessive slopping.

[0015] In the second step shown in Figure 1(B), a desiliconization treatment is performed by blowing oxygen-containing gas 12 as a gaseous oxygen source from the top blowing lance 2 to remove a portion of the Si in the molten metal. During this desiliconization treatment, bottom blowing gas 13 is supplied from the bottom blowing tuyere 3 to stir the molten iron 9. The Si concentration in the molten metal at the end of the second blowing step is preferably 0.3 mass% or less. The Si concentration in the molten metal at the end of the second blowing step is more preferably 0.1 mass% or more and 0.2 mass% or less. Also, 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 is based on the mass of the slag, where SiO ２ This refers to the ratio of CaO to , and the same applies hereafter. Oxygen-containing gases used as gaseous oxygen sources include pure oxygen, oxygen and inert gases, and CO2. ２ A mixed gas with gas can be used.

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

[0017] In the fourth step shown in Figure 1(D), oxygen-containing gas 12 is blown from the top blowing lance 2 as a gaseous oxygen source, and desiliconization, dephosphorization, and decarburization treatments are performed to remove the remaining Si in the molten iron 9, resulting in molten steel 9b. During this process, bottom blowing gas 13 is blown in from the bottom blowing tuyere 3 to agitate the molten metal.

[0018] In the fifth step shown in Figure 1(E), the molten steel 9b after refining is tapped from the tapping port 4 into a ladle. After tapping, the slag 17 generated in the fourth step remains in the converter 1. This slag 17 is left in place and carried over to the next processing step, and the process 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 reasons for this are to prevent phosphorus in the slag 17 carried 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 over the 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 phosphorus removal, a basicity of 1.2 to 2.0 is appropriate. However, in this invention, by sacrificing the phosphorus removal effect, it is possible to complete the blowing of high-Si molten iron with a Si content of 0.7 mass% or more without problems while suppressing slopping. In the smelting of high-silicon iron, if the target basicity of the slag before slag discharge is set to 1.2 to 2.0, this cannot be achieved without adding an excess of CaO, which in turn increases the amount of slag and makes slag forming more likely, as well as increasing the CaO unit consumption. In this embodiment, the second step is not for the purpose of dephosphorization treatment, so in order to prevent cost deterioration due to the use of lime, it is not necessary to raise the basicity excessively. Therefore, during the period of the second step, the basicity of the slag is kept in the range of 0.9 to 1.1.

[0020] (The Si concentration in the molten metal at the end of the second blowing process is 0.3 mass% or less.) As the Si concentration in the molten metal decreases, the desilicate rate by oxygen gas decreases, and the decarburization rate increases. Therefore, when the Si concentration in the molten metal is high and the amount of slag is large, slag forming by CO gas is promoted, making slopping more likely to occur. For example, Figure 2 shows the relationship between the estimated Si concentration in the molten metal at the end of the second process and the unit amount of foaming inhibitor required in the fourth process, under the condition that the Si concentration in the molten iron 9a charged in the first process is 0.7 mass% or more. If desilicate has not progressed sufficiently in the second process, the tendency for slopping will worsen in the fourth process. From the results in Figure 2, it is preferable to keep the Si concentration in the molten metal at the end of the second process at 0.3 mass% or less from the viewpoint of suppressing slopping in the fourth process.

[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 unit consumption of the foaming inhibitor required in the second step, under the condition that the Si concentration in the molten iron 9a charged in the first step is 0.7 mass% or higher. If excessive desiliconization is performed in the second step, the risk of slag forming due to the decarburization reaction increases, and the tendency for slopping in the second step worsens.

[0022] Figure 4 schematically shows the relationship between the estimated Si concentration in the molten metal at the end of the second step and the unit consumption of the foaming inhibitor required in the second and fourth steps, summarizing the above results. From these results, it is preferable that the Si concentration in the molten metal at the end of the second step is in the range of 0.1% by mass or more and 0.2% by mass or less in order to stably process without causing slopping in either the second or fourth step.

[0023] In summary, the results above indicate that it is preferable to design the blowing process so that the target Si concentration at the end of the second process is in the range of 0.1% by mass or more and 0.2% by mass or less. If there is no tendency for slopping to occur in the second process, the Si concentration in the molten metal may be set to more than 0.0% by mass in order to reduce the Si source carried over to the fourth process. On the other hand, if the tendency for slopping is so high that it is difficult to reduce the Si concentration in the molten metal to 0.2% by mass in the second process, the blowing process should be temporarily suspended and the process should be moved to the next process. Even in that case, it is preferable to reduce the Si concentration in the molten metal to 0.3% by mass or less in order to reduce the Si source carried over to the fourth process.

[0024] (Method for determining the basicity of slag in the second process) The mass of CaO in the auxiliary raw materials introduced into converter 1 by the end of the second process is the amount of SiO produced by the desiliconization treatment of Si in molten iron. ２ and SiO in auxiliary materials ２ The basicity of the slag is calculated by dividing by the sum of its masses.

[0025] (Method for determining the target Si concentration in the molten metal at the end of the second process) The amount of oxygen required per ton of molten iron as oxygen in the oxygen-containing gas blown in during the second process (oxygen blowing rate per unit in the second process) is expressed by the following equation (1). (1) Equation (Oxygen blowing rate per unit in the second process) [Nm³ ３ / t] = {ΔSi[kg / t - Fe] / M(Si)×22.4[Nm ３ / kmol]} / (desiliconization efficiency α) Here, M(Si) is the atomic weight of Si, which is 28 kg / kmol. The desiliconization amount per unit ΔSi[kg / t - Fe] in the second step, that is, the reduction amount per ton of hot metal in the mass of Si in the hot metal, is given by the following formula (2). Formula (2) (Desiliconization amount ΔSi)[kg / t - Fe] = { (hot metal Si concentration)[mass%] - (target Si concentration at tapping)[mass%]} × 10 Also, the desiliconization efficiency α was determined as shown in Table 1 from past performance shown in Fig. 5. Using the above relational expressions, when the Si concentration in the hot metal at the start of the second step is 1.2 mass% and the target Si concentration at the end of the second step is 0.2 mass%, the oxygen injection amount per unit in the second step is { (1.2 - 0.2) × 10 / 28} × 22.4 / 0.8 = 10[Nm ３ / t].

[0026]

[0027] In this embodiment, in one converter, the desiliconization treatment may be completed before the decarburization rate increases, the bottom blowing gas may be flowed for 2 to 3 minutes to promote the separation of hot metal slag, intermediate slag removal may be performed, and the remaining desiliconization treatment, dephosphorization treatment, and decarburization treatment may be carried out.

[0028] The slag removal rate in the intermediate slag removal is preferably 30% to 80%. More preferably, it is 60% to 80%. If the slag removal rate is less than 30%, a large amount of viscous slag with a basicity near 1 remains, and slopping occurs due to the CO gas generated during decarburization in the fourth step. On the other hand, although the higher the slag removal rate is, the better, if slag removal exceeding 80% is attempted, there is a risk of flowing out to the hot metal.

[0029] The oxygen supply rate when decarburizing and dephosphorizing the hot metal thus obtained in the fourth step can be 45000 Nm ３ / h. Conventionally, when the hot metal containing 0.7 mass% or more of Si is used as the starting material, slopping occurred when supplying oxygen at such a high oxygen supply rate, but according to the method of the present invention, slopping does not occur. Thereby, it is possible to shorten the blowing time.

[0030] (Example 1) Figure 7 shows the changes in P and Si concentrations in molten iron when high-Si molten iron is refined under the smelting pattern and design shown in Figure 6. In the inventive example (Figures 7(b1) and (b2)), the amount of molten iron charged was 320 tons, and a cold iron source of 60 tons was used. The Si concentration in the molten iron was 1.2% by mass. There were 3 tons of residual slag carried over from the previous treatment, and 5 tons of lime and 6 tons of recycled slag were used as auxiliary raw materials. The target basicity of the slag in the second step was 1.0. The smelting time in the second step was 7 min, and the smelting time in the fourth step was 15 min. Pure oxygen gas was used as the top blowing gas, and Ar gas, an inert gas, was used as the bottom blowing gas. In the conventional example (Figures 7(a1) and (a2)), the Si concentration in the molten iron was set to 0.4% by mass, the target basicity of the slag in the second step was set to 1.6, and the blowing pattern was different, but otherwise it was the same as the inventive example.

[0031] In the initial stage of the second-stage mixing pattern, the inventive example has a lower upward blowing flow rate than the conventional example. In the final stage of the second-stage mixing pattern, the inventive example has a lower bottom blowing flow rate than the conventional example. This is because the inventive example does not aim to remove phosphorus in the second stage, and therefore does not require FeO generation necessary for phosphorus removal, nor the final stirring. The lance height is designed to match the upward blowing flow rate so that the dynamic pressure at the bath surface remains constant.

[0032] In the fourth step, the blowing pattern, as demonstrated in this invention where the slag volume is large, has a lower maximum upward blowing flow rate than conventional examples to suppress slopping. The lance height is designed to match the upward blowing flow rate so that the bath surface dynamic pressure remains constant.

[0033] In the conventional example, the basicity of the slag in the second step was in the range of 1.5 to 1.7, whereas in the inventive example, it was in the range of 0.9 to 1.1. As shown in Figure 7, in the inventive example, since the acid supply is completed before the oxidation and removal of Si, which is removed before P and C, almost no P is removed by 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 is greater in the inventive example.

[0034] Figure 8 shows the influence of the conventional example and the inventive example on the relationship between the initial Si concentration in molten iron and the foaming inhibitor unit consumption in the second step. In both cases, the molten iron temperature was in the range of 1300 to 1350°C, and the molten iron blending ratio was 90% or less. From Figure 8, it can be seen that in both cases, the foaming inhibitor unit consumption tends to increase as the Si concentration increases. In the inventive example, it can be seen that even when the initial Si concentration in molten iron is 0.7 mass% or higher, blowing can be performed with a smaller amount of foaming inhibitor than in the conventional example.

[0035] (Example 2) Under the same blowing conditions as in the inventive example of Example 1, the effect of the estimated Si concentration at the end of the second process on the amount of foaming inhibitor used in the second and fourth processes was investigated. The Si concentration in the molten iron was 1.0 mass%. The results are summarized in Table 2. For Nos. 2 to 4, where the Si concentration at the end of the second process blowing was between 0.1 mass% and 0.3 mass%, the unit consumption of foaming inhibitor used in the second and fourth processes was excellent, at 1.5 kg or less per ton of molten iron. In particular, for Nos. 2 and 3, the unit consumption of foaming inhibitor used in the second and fourth processes was excellent, at 1.3 kg or less per ton of molten iron. On the other hand, when the Si concentration at the end of the second process blowing was below the lower limit (No. 1), the unit consumption of foaming inhibitor used in the second process was excessive at 1.7 kg per ton of molten iron due to concerns about slopping caused by slag forming. Furthermore, when the Si concentration at the end of the second blowing process exceeds the upper limit (No. 5, 6), the amount of forming inhibitor used in the fourth process is excessive, at 1.8 kg or more per ton of molten iron, due to concerns about slopping caused by slag forming.

[0036]

[0037] 1 Converter 2 Acid supply lance (top blowing lance) 3 Bottom blowing tuyeres 4 Hot spout 9 Molten iron 9a Molten pig iron 9b Molten steel 10 Slag (after desiliconization and the second process) 11 Cold iron source 12 Oxygen-containing gas (gas oxygen source) 13 Bottom blowing gas 14 Charging pan 17 Slag (after the fourth process and after dephosphorization and decarburization)

Claims

1. A refining method for molten iron, comprising the steps of:

1. A first step of charging molten iron with a Si concentration of 0.7 mass% or more into a converter; 2. A second step of blowing oxygen-containing gas from an upper blowing lance to remove a portion of the Si in the molten metal; 3. A third step of removing slag from the converter to the outside of the furnace; 4. A fourth step of performing desiliconization, dephosphorization, and decarburization treatments by blowing oxygen gas from the upper blowing lance to remove the remaining Si in the molten metal; and 5. Tapping molten steel that has undergone desiliconization, dephosphorization, and decarburization treatments, wherein in the fifth step, the slag generated in the fourth step is left in the converter; in the first step, molten iron is charged while retaining the slag left in the immediately preceding fifth step in the converter; and in the second step, the leftover slag is reused.

2. During the second step, the basicity of the slag is in the range of 0.9 to 1.1, and the Si concentration at the end of the second step is 0.3% by mass or less. Here, the basicity of the slag is based on the mass of the slag, where SiO ２ The method for refining molten iron according to claim 1, wherein the ratio of CaO to is the ratio of to .

3. The method for refining molten iron according to claim 2, wherein the Si concentration at the end of the second step of blowing is 0.1% by mass or more and 0.2% by mass or less.

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

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