Molten iron refining method

Abstract

Provided is a molten iron refining method suitable for performing refining using a solid iron raw material.　This molten iron refining method comprises: a charging step for charging a converter with a solid iron raw material, a CaO fluxing agent, and molten pig iron to convert the same into molten iron; a first blowing step for supplying oxygen to the molten iron in the converter; a slag discharge step for discharging slag generated in the first blowing step from the converter; a second blowing step for supplying oxygen to the molten iron remaining in the converter after the slag discharge step is performed; and a discharge step for discharging the molten iron subjected to the second blowing step. In either the first blowing step or the second blowing step, a temperature-raising material containing Si is added, and in the first blowing step and the second blowing step in which the temperature-raising material is added, the temperature-raising material is added such that the Si content in the molten iron becomes 0.8-1.9 mass%. In at least one of the first blowing step and the second blowing step, the converter is charged with the solid iron raw material.

Description

Method for refining molten iron

[0001] The present invention relates to a refining method using a converter or a steelmaking furnace of converter type.

[0002] In a converter or the like, by using iron scrap as a solid iron raw material, the blending ratio of hot metal is reduced. In order to melt a large amount of iron scrap, a thermal margin is required. For example, in Patent Document 1, after charging hot metal and scrap into a converter, it is disclosed that a treatment for promoting the melting of scrap is performed by supplying a heat source.

[0003] Japanese Patent Application Laid-Open No. 2019-031725

[0004] In Patent Document 1, two converters are used, and there are problems of large heat loss and long production time. In addition, development of a method for refining molten iron that can use more solid iron raw materials is desired.

[0005] The present invention has been made in view of the above problems, and an object thereof is to provide a method for refining molten iron suitable for refining using a solid iron raw material.

[0006] In order to solve the above problems, the present invention has the following features.

[0007] [1] A charging step of charging a converter with solid iron raw material, CaO solvent, and molten iron to make molten iron; a first blowing step of supplying oxygen to the molten iron in the converter; a slag discharge step of discharging the slag generated in the first blowing step from the converter; a second blowing step of supplying oxygen to the molten iron remaining in the converter after the slag discharge step; and a discharge step of discharging the molten iron after the second blowing step, wherein a heat-raising material containing Si is added in either the first blowing step or the second blowing step, and the heat-raising material is added in the first blowing step and the second blowing step such that the Si content of the molten iron is 0.8 mass% or more and 1.9 mass% or less. A method for refining molten iron, wherein the solid iron raw material is charged into the converter in at least one of the first blowing step and the second blowing step. [2] The method for refining molten iron according to [1], wherein the solid iron raw material includes a specific solid iron raw material having a sulfur content of 0.015 mass% or less, and 85% or more of the total amount of the solid iron raw material charged into the converter is the specific solid iron raw material. [3] The method for refining molten iron according to [1] or [2], wherein the solid iron raw material is charged when the converter is open upwards. [4] The method for refining molten iron according to any one of [1] to [3], wherein the solid iron raw material is charged into the converter after the discharge step has been performed while slag remains in the converter.

[0008] In the molten iron refining method of the present invention, solid iron raw material is charged into the converter in at least one of the first blowing step, which supplies oxygen to the molten iron charged into the converter, and the second blowing step, which supplies oxygen to the molten iron remaining in the converter. This makes it possible to maintain the temperature of the molten iron in a temperature range suitable for refining. Therefore, even when refining is performed using solid iron raw material in a single converter, refining can be carried out with minimal heat loss. As a result, it becomes possible to use a large amount of solid iron raw material in refining. Furthermore, it becomes possible to shorten the steelmaking production time.

[0009] This is a flow chart of a molten iron refining method. This is an explanatory diagram showing the charging process in step S01 of Figure 1. This is an explanatory diagram showing the first blowing process in step S02 of Figure 1. This is an explanatory diagram showing the slag removal process in step S03 of Figure 1. This is an explanatory diagram showing the second blowing process in step S04 of Figure 1. This is an explanatory diagram showing the discharge process in step S05 of Figure 1.

[0010] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 shows a flow chart of a molten iron refining method. As shown in Figure 1, the molten iron refining method involves a charging step in which molten iron is charged into a converter (step S01).

[0011] In step S01, the charging process, solid iron raw materials, a CaO solvent, and molten iron are charged into the converter. Molten iron is produced when these materials are charged into the converter. The molten iron used is, for example, iron tapped from a blast furnace. The solid iron raw materials are not particularly limited, but examples include iron scrap, reduced iron, solid pig iron, pellets, and sponge iron. One or more of these solid iron raw materials may be used. In this embodiment, an example in which iron scrap is used as the solid iron raw material will be described.

[0012] Furthermore, while there are no particular limitations on what constitutes iron scrap, slabs generated at steel mills and crop scraps from steel plates can be used. Specifically, sheet metal scraps generated during cropping or trimming of the front and rear ends of thin or thick steel plates, as well as block-shaped slab scraps such as bottom and top crops from continuously cast slabs, can be used as iron scrap.

[0013] The CaO-based solvent is not particularly limited, but examples include quicklime, calcium carbonate, dolomite, and converter slag. For example, converter slag produced during decarburization refining in a converter can be used. The CaO-based solvent may be one or a mixture of two or more of these.

[0014] Next, a first blowing process is carried out to supply oxygen to the molten iron in the converter (step S02). Then, a slag discharge process is carried out to discharge the slag generated in the first blowing process of step S02 from the converter (step S03).

[0015] Next, a CaO-based solvent addition step is performed (step S04) in which a CaO-based solvent is added to the molten iron remaining in the converter after the slag removal step of step S03.

[0016] Next, a second blowing process is carried out in which oxygen is supplied to the molten iron to which a CaO solvent has been added (step S05). The second blowing process in step S05 performs a dephosphorization and decarburization treatment in which phosphorus and carbon are separated from the molten iron as slag.

[0017] Next, a discharge process is carried out to discharge the molten iron that has undergone the second blowing process in step S05 (step S06).

[0018] Figure 2 shows the charging process in step S01 of Figure 1. As shown in Figure 2, in the charging process of step S01, iron scrap 20 is first charged into the converter 10.

[0019] Next, molten iron 40 is charged from the charging ladle 30 into the converter 10. When the molten iron 40 is charged into the converter 10, it is charged while the converter 10 is opening diagonally upward. More specifically, the molten iron 40 is charged into the converter 10 while the opening axis AX of the converter 10 is tilted at an angle with respect to the vertical direction VD. After that, a CaO medium solvent is charged into the converter 10.

[0020] Figure 3 shows the configuration of the first blowing process in step S02 of Figure 1. As shown in Figure 3, the first blowing process in step S02 is carried out with the opening axis AX of the converter 10 aligned with the vertical direction VD. That is, the first blowing process in step S02 is carried out with the converter 10 opening upwards.

[0021] In the first blowing process of step S02, an oxygen-containing gas is supplied from an upper blowing lance 50 inserted into the converter 10. Through the first blowing process of step S02, silicon and phosphorus are separated from the molten iron 41 as slag 60. The first blowing process is also known as desiliconization and dephosphorization.

[0022] During the first blowing process in step S02 and at least one of the second blowing processes in step S05, a heat-raising agent (not shown) containing Si is charged into the converter 10. At these timings when the heat-raising agent is added, the heat-raising agent is added so that the Si content of the molten iron 41 is 0.8 mass% or more and 1.9 mass% or less. The heat-raising agent containing Si is not particularly limited, but examples include FeSi, SiC, etc. Of these, FeSi is preferably used as the heat-raising agent from the viewpoint of securing an iron source and obtaining a good yield.

[0023] The Si content of the molten iron 41 can be determined, for example, by taking a sample from the molten iron 41 and performing a component analysis. When adding a heat-raising agent, it is advisable to determine the amount of the heat-raising agent to add based on the values ​​obtained from the component analysis.

[0024] By setting the Si content of the molten iron 41 to 0.8 mass% or more, the heat necessary for melting the iron scrap 20, which is the solid iron raw material, in the converter 10 can be secured. As a result, for example, 30 mass% or more of the total amount of material charged into the converter 10 can be made from iron scrap 20, which is the solid iron raw material. In addition, the dephosphorization reaction that occurs in the second blowing process of step S04 can be promoted.

[0025] Furthermore, by limiting the Si content of the molten iron 41 to 1.9 mass% or less, it becomes possible to suppress the amount of slag generated in the converter 10. As a result, it is possible to suppress the occurrence of problems such as slag being ejected outside the converter 10.

[0026] Furthermore, during the first smelting process in step S02 and at least one of the second smelting processes in step S05, iron scrap 20, which is a solid iron raw material, is charged into the converter 10. For example, the iron scrap 20 is charged into the converter 10 in two stages: the charging process in step S01 and the first smelting process in step S02. In the first smelting process, the temperature of the molten iron 41 rises. That is, in the first smelting process in step S02, the amount of Si in the molten iron 41 is sufficient, so the temperature of the molten iron 41 can be maintained in a temperature range suitable for smelting. At that time, the charging of iron scrap 20 helps to maintain the temperature of the molten iron 41 in a temperature range suitable for smelting.

[0027] In the first smelting step of step S02 and the second smelting step of step S05, the amount of iron scrap 20 charged into the converter 10 is preferably 3.0 mass% or less of the total amount of material charged into the converter 10. Furthermore, it is preferable that the amount of iron scrap 20 be 2.5 mass% or less of the total amount of material charged into the converter 10, and more preferably 2.0 mass% or less. By charging the iron scrap 20 in this manner, the temperature of the molten iron 41 can be set to a temperature suitable for smelting.

[0028] Furthermore, it is preferable that 85% or more, preferably 90% or more, of the total amount of solid iron raw materials charged into the converter 10 has a sulfur content of 0.015 mass% or less. Hereinafter, solid iron raw materials with a sulfur content of 0.015 mass% or less will also be referred to as specified solid iron raw materials.

[0029] By using solid iron raw materials with a sulfur content of 0.015 mass% or less for 85% or more of the total amount, the amount of so-called miscellaneous iron scrap containing CaO, FeO, and S is reduced. This reduces the slag volume and suppresses the occurrence of problems. Preferably, the solid iron raw materials used have an FeO purity of 1 mass% or more (maximum 100 mass%).

[0030] Furthermore, using solid iron raw materials with a high sulfur content will increase the sulfur content of the molten steel. In addition, carbon, SiC, and FeSi, which are used as heat-raising materials, contain trace amounts of sulfur. Therefore, if solid iron raw materials with a high sulfur content are used, the sulfur content will exceed a predetermined level, making it impossible to use a large amount of heat-raising material, which may lower the temperature of the molten steel. Therefore, by using specific solid iron raw materials, it becomes possible to raise the temperature of the molten steel above the predetermined level using heat-raising material, and thus enable the use of more solid iron raw materials.

[0031] Furthermore, when charging the iron scrap 20 in the first blowing process of step S02, it is preferable to charge the iron scrap 20 into the converter 10 before the oxygen-containing gas is supplied from the upper blowing lance 50.

[0032] Figure 4 shows the configuration of the slag removal process in step S03 of Figure 1. As shown in Figure 3, the slag removal process in step S03 is performed with the opening axis AX of the converter 10 at approximately 90° with respect to the vertical direction VD. That is, the slag removal process in step S03 is performed with the converter 10 open to the side. By performing the slag removal process in step S03, P can be incorporated into the slag and discharged to the outside, thereby reducing the concentration of P in the molten iron 41. Note that the slag removal process in step S03 is an optional process.

[0033] Figure 5 shows the configuration of the second blowing process in step S05 of Figure 1. As shown in Figure 5, the second blowing process in step S05 is carried out with the opening axis AX of the converter 10 aligned with the vertical direction VD. That is, the second blowing process in step S05 is carried out with the converter 10 opening upwards.

[0034] In the second blowing process of step S05, oxygen-containing gas is supplied from an upper blowing lance 50 inserted into the converter 10. The second blowing process of step S05 separates silicon and carbon from the molten iron 41 as slag 60. The second blowing process is also known as desiliconization or decarburization.

[0035] Furthermore, when iron scrap 20 is charged in the second smelting process in step S05, for example, the iron scrap 20 is charged into the converter 10 in two stages: the charging process in step S01 and the second smelting process in step S05. Alternatively, for example, the iron scrap 20 is charged into the converter 10 in three stages: the charging process in step S01, the first smelting process in step S02, and the second smelting process in step S05. In this case, the iron scrap 20 can be charged in the same way as in the first smelting process in step S02. In the second smelting process, the temperature of the molten iron 41 rises. At this time, the charging of iron scrap 20 helps maintain the temperature of the molten iron 41 in a temperature range suitable for smelting. Note that the decrease in the temperature of the molten iron 41 due to the addition of iron scrap 20 in the second smelting process is often compensated for by the heat rise due to decarburization. If the temperature of the molten iron 41 drops significantly due to the addition of iron scrap 20, it is advisable to add a heat-raising agent.

[0036] Furthermore, a CaO-based solvent may be added to the molten iron 41 remaining in the converter 10 after the slag removal process in step S03. The CaO-based solvent can be the same as that described in the charging process in step S01.

[0037] The CaO-based solvent may be added before the iron scrap 20 is charged, or it may be added after the iron scrap 20 is charged. Alternatively, the CaO-based solvent may be added at the same time as the iron scrap 20 is charged.

[0038] Figure 6 shows the discharge process of step S06 in Figure 1. As shown in Figure 6, the discharge process of step S06 is performed with the opening axis AX of the converter 10 at approximately 90° with respect to the vertical direction VD. That is, the discharge process of step S06 is performed with the converter 10 opening to the side.

[0039] Furthermore, after the discharge process in step S06 is performed, slag 60 remains in the converter 10. The charging process in step S01 in Figure 1 should ideally be carried out under these conditions. That is, the iron scrap 20 should be charged into the converter 10 with slag 60 remaining after the discharge process in step S06. By carrying out the charging process under these conditions, the heat generated during the refining process can be effectively utilized in the next refining process.

[0040] As described above, according to the molten iron refining method of the present invention, in at least one of the first blowing step in step S02 and the second blowing step in step S05, the iron scrap 20 is charged into the converter 10. This makes it possible to maintain the temperature of the molten iron 41 in a temperature range suitable for refining. In other words, if all the iron scrap 20 that is to be charged into the converter 10 is charged in the charging step in step S01, the thermal margin will be small, and it will not be possible to charge a large amount of iron scrap 20.

[0041] In the molten iron refining method of the present invention, a heat-raising agent containing Si is added in either the first blowing step in step S02 or the second blowing step in step S05. Furthermore, in the first blowing step in step S02 and the second blowing step in step S05 in which the heat-raising agent is added, the heat-raising agent is added such that the Si content of the molten iron 41 is 0.8 mass% or more and 1.9 mass% or less. In addition, by charging iron scrap 20 in at least one of the first blowing step in step S02 and the second blowing step in step S05, the thermal margin of the converter 10 can be increased. As a result, it becomes possible to use a large amount of iron scrap 20 in refining. Moreover, since refining can be performed using iron scrap 20 in one converter 10, refining can be performed with less heat loss than when refining is performed in two converters 10, making it possible to shorten the steelmaking production time.

[0042] The amount of usable solid iron raw material was investigated through steelmaking and refining. Iron scrap was used as the solid iron raw material in the experiment. FeSi was used as the heat-raising material.

[0043] The tests were conducted in the manner shown in Table 1. In the inventive examples, heating materials were charged until the Si content of the molten iron reached 0.8 mass% or more. In the inventive examples, 85 to 100% of the total amount of the solid iron raw materials was the specific solid iron raw material with an S content of 0.015 mass% or less. The charging of the solid iron raw materials and the heating materials was carried out in the manner shown in Table 1.

[0044] In Table 1, ▲ means "+", and ▽ means "-".

[0045] In Inventive Examples 1 to 7, among the materials charged into the converter, the solid iron raw materials could be charged at 25 mass% or more. That is, it was found that in Inventive Examples 1 to 7, even when using one converter, more solid iron raw materials could be used than in Comparative Example 6 which is a conventional example. As a result, heat loss can be suppressed more than when using a plurality of converters.

[0046] Also, in Inventive Examples 1 to 7, slag ejection was not confirmed in any case. On the other hand, as shown in Comparative Examples 8 and 9, when the Si concentration in the hot metal is higher than in other examples, if the charging amount of the solid iron raw materials is 25 mass% or more, a large amount of heating materials are required for heating. Therefore, in Comparative Examples 8 and 9, the slag volume becomes extremely higher than in Comparative Example 6 which is a conventional example. As a result, slag ejection occurred in Comparative Examples 8 and 9.

[0047] As described above, in Inventive Examples 1 to 7, 85% or more of the total amount of the solid iron raw materials has an S content of 0.015 mass% or less. It can be seen that the amount of heating materials in Inventive Examples 1 to 7 is equal to or lower than that in Comparative Example 6 which is a conventional example. Therefore, in Inventive Examples 1 to 7, the slag volume can be reduced more than in Comparative Examples 8 and 9. As a result, the occurrence of slag ejection can be suppressed. Also, in Inventive Examples 1 to 7 of the present invention, the production time became shorter than in Comparative Example 6 which is a conventional example.

[0048] In this example, the amount of solid iron raw material charged is 32 mass% in Invention Examples 6 and 7 and Comparative Example 9. In Invention Examples 6 and 7, a slag removal process is performed, whereas in Comparative Example 9, a slag removal process is not performed. Since the slag volume in Invention Examples 6 and 7 is significantly lower than that in Comparative Example 9, it was found that the slag volume is reduced by the slag removal process.

[0049] Comparative Examples 2, 8, and 10-12 had higher concentrations of [P] in the molten iron compared to the conventional method because intermediate slag removal was not performed. Furthermore, Comparative Examples 8 and 9 could not be produced due to problems such as slag ejection.

[0050] 10 Converter 20 Iron scrap 30 Charging ladle 40 Molten pig iron 41 Molten iron 50 Top blow lance 60 Slag

Claims

1. A process comprising: a charging step of charging a converter with solid iron raw material, a CaO solvent, and molten iron to form molten iron; a first smelting step of supplying oxygen to the molten iron in the converter; a slag discharge step of discharging the slag generated in the first smelting step from the converter; a second smelting step of supplying oxygen to the molten iron remaining in the converter after the slag discharge step; and a discharge step of discharging the molten iron after the second smelting step, wherein a heat-raising material containing Si is added in either the first smelting step or the second smelting step, and the heat-raising material is added in the first smelting step and the second smelting step such that the Si content of the molten iron is 0.8 mass% or more and 1.9 mass% or less. A method for refining molten iron, wherein in at least one of the first and second refining steps, the solid iron raw material is charged into the converter.

2. The method for refining molten iron according to claim 1, wherein the solid iron raw material includes a specified solid iron raw material having a sulfur content of 0.015 mass% or less, and 85% or more of the total amount of the solid iron raw material charged into the converter is the specified solid iron raw material.

3. The method for smelting molten iron according to claim 1 or 2, wherein the solid iron raw material is charged when the converter is open upwards.

4. The method for refining molten iron according to any one of claims 1 to 3, wherein the solid iron raw material is charged into the converter with slag remaining after the discharge step has been performed.

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