Method for solidifying residual decarburized slag

Abstract

This invention provides a method for solidifying residual decarburized slag in a converter without using a solidification agent with a high CaO content. [Solution] After tapping the molten steel following decarburization and blowing, a first step is performed to remove the slag generated during decarburization and blowing so that a predetermined amount of slag remains in the converter as residual slag. A second step involves adding a solidifying agent to the residual slag in the converter, A method for solidifying residual decarburized slag, comprising a third step of mixing the residual slag and the solidifying agent by tilting the converter, The method for solidifying residual decarburized slag is characterized in that the solidifying material contains magnetic separation scrap and contains less than 30% by mass of CaO.

Description

Technical Field

[0001] The present invention relates to a method for solidifying decarburization slag generated during decarburization blowing. In this specification, the unit of mass "t" means 1000 kg. In addition, total iron (T.Fe) is the sum of Fe (iron content) in metallic iron and iron oxide contained in magnetic separation chips described later. Further, "x to y" representing a range of numerical values means x or more and y or less, including the boundary values.

Background Art

[0002] In the operation of a converter, a part of the decarburization slag generated in the previous blowing may be used as a slag-forming material, a fluxing material, and a converter lining material in the next blowing. At that time, in order to prevent the molten iron from splashing when the molten iron is charged for the next blowing, it is necessary to solidify the residual slag of the previous blowing. Examples of general solidifying materials used to solidify the residual slag of the previous blowing include those described in Patent Document 1. Patent Document 1 discloses a hot slag recycling method. In the step of charging a solidifying material into the slag remaining in the converter (solidifying material charging step) in this method, a solidifying material containing 30% by mass or more of CaO (hereinafter referred to as "high CaO-containing solidifying material") is used. Specifically, the solidifying material is charged into the hot slag remaining in the converter, and the charging amount is 250 kg or more per 1 t of hot slag.

[0003] Since the molten slag remaining in the converter has a low melting point of about 1100 to 1250°C, it takes time to completely solidify. Therefore, as described in Patent Document 1, it is generally practiced to charge a solidifying material to rapidly solidify the molten slag. By charging the solidifying material, the molten slag is deprived of its heat and cooled. Further, since the molten slag has a CaO-SiO2-FeO(-MgO) composition, it has been considered effective that the CaO component of the solidifying material mixes into the molten slag to obtain a more CaO-enriched slag component. Specifically, the basicity (CaO / SiO2) increases, and the melting point of the molten slag rapidly becomes a high melting point, thereby promoting the solidification of the molten slag. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-105586 [Overview of the project] [Problems that the invention aims to solve]

[0005] In recent years, from the perspective of reducing CO2 emissions, there has been a desire to expand the use of cold iron sources such as magnetic separation scrap and other scrap materials. This inevitably leads to a decrease in the proportion of molten iron and a reduction in the Si source derived from molten iron. In the method using the solidification material described in Patent Document 1, in the subsequent blowing stage, it becomes necessary to add silica in an amount corresponding to the excess CaO content in order to maintain the basicity of the slag (the mass ratio of CaO to SiO2 contained in the slag (CaO / SiO2)). This leads to an increase in slag volume, and there is a risk that molten iron may erupt from the converter nozzle. Furthermore, adding solidification material with a high CaO content leads to increased costs. Moreover, as the source of SiO2 derived from raw materials decreases in the future, from the perspective of operational load, there will be no need to use solidification material containing an excess amount of CaO.

[0006] This invention was made to solve the above problems and aims to provide a technology for solidifying residual decarburized slag in a converter without using a solidifying agent with a high CaO content. [Means for solving the problem]

[0007] The inventors investigated a method for solidifying molten slag remaining in the converter after decarburization and blowing in the pre-charge converter. As a result, while conventional methods generally involved using a solidifying agent with a high CaO content to solidify residual slag, they discovered a method that uses a low CaO-containing solidifying agent containing magnetic separation scrap to solidify residual slag as quickly as, or even faster than, conventional methods.

[0008] The method for solidifying residual decarburized slag according to the present invention, which advantageously solves the above problems, is: The first step involves removing slag after tapping the molten steel following decarburization and blowing, so that a predetermined amount of slag generated during decarburization remains in the converter as residual slag. A second step involves adding a solidifying agent to the residual slag in the converter, A method for solidifying residual decarburized slag, comprising a third step of mixing the residual slag and the solidifying agent by tilting the converter, The solidification material is characterized by containing magnetic separation scrap and containing less than 30% by mass of CaO.

[0009] Furthermore, the method for solidifying residual decarburized slag according to the present invention is (a) In the first step, the residual slag shall be 0.7% by mass or more and 2.6% by mass or less of the total amount charged to the converter, and (b) In the second step, the magnetic separation scrap contained in the solidification material contains 50% by mass or more of total iron, and the solidification material contains 1,000 kg to 1,500 kg of magnetic separation scrap per ton of residual slag, and 0 kg to 200 kg of dolomite per ton of residual slag. These would be more preferable solutions. [Effects of the Invention]

[0010] According to the present invention, after leaving a predetermined amount of slag generated by decarburization blowing in the converter, it is possible to solidify the residual slag remaining in the converter simply by adding a predetermined amount of low-CaO-containing solidifying agent containing magnetic separation scrap, without adding a high-CaO-containing solidifying agent. Therefore, by using the method according to the present invention, the risk of molten iron ejection and other issues that may arise from using solidifying agents with a high CaO content can be eliminated. Furthermore, since a solidifying agent containing magnetic separation scrap is used, it leads to an expansion of the use of cold iron sources and eliminates the increase in costs. Among cold iron sources, magnetic separation scrap and other scrap materials have a higher specific heat and absorb more heat from molten slag compared to solidification materials made of oxides, thus possessing high cooling capacity. In other words, by using solidification materials containing magnetic separation scrap and other scrap materials, thermal energy can be effectively utilized, and the solidification of molten slag can be promoted without raising the melting point of the molten slag itself. In other words, by using the present invention, the solidification of residual slag can be easily achieved, which not only avoids the scattering of molten material and the generation of black smoke during subsequent addition of molten iron, but also reduces costs by eliminating the need for high-CaO-containing solidification materials and reduces the operational load on the steelmaking process. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram showing an overview of a method for solidifying residual decarburized slag according to one embodiment of the present invention. [Modes for carrying out the invention]

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

[0013] The method for solidifying residual decarburized slag according to this embodiment includes the following first to third steps. The first step is to remove slag after tapping the molten steel after decarburization and blowing, so that a predetermined amount of slag generated in decarburization and blowing remains in the converter as residual slag. The second step is to add a solidifying agent to the residual slag in the converter. The third step is to mix the residual slag and the solidifying agent by tilting the converter. Figure 1 is a schematic diagram showing an overview of the method for solidifying residual decarburized slag according to this embodiment. In Figure 1, the symbol "1" represents the converter, the symbol "2" represents the decarburized slag, the symbol "2a" represents the residual slag, the symbol "3" represents the slag pan, the symbol "4" represents the trolley, and the symbol "5" represents the solidifying agent. Each step will be described in detail below.

[0014] <1st process> In the first step of the solidification method for residual decarburized slag according to this embodiment, after the molten steel after decarburization blowing in the previous charge is tapped from the converter, slag is discharged, and a predetermined amount of slag (decarburized slag) generated in decarburization blowing is left in the converter as residual slag. Specifically, referring to Figure 1, the decarburized slag 2 in the converter 1 is discharged into a slag pot 3 placed on a trolley 4 so that a predetermined amount of decarburized slag 2 remains in the converter 1 (Figure 1(a)). After the discharge is completed, the converter 1 is set to an upright position (Figure 1(b)). Here, the inventors investigated the relationship between the amount of residual decarburized slag 2 generated in decarburization blowing and the amount of magnetic separation scrap used in the solidification material 5 described later, through trial and error. As a result, it was found that the residual amount of decarburized slag 2 is preferably 0.5% by mass or more and 3.5% by mass or less relative to the total charge of the converter 1, and more preferably 0.7% by mass or more and 2.6% by mass or less. That is, for example, in the case of a converter with a total charge of 300 tons, it is more preferable to leave behind 2.1 tons or more and 7.8 tons or less of decarburized slag 2. When the residual amount of decarburized slag 2 is within the above range, a thermal margin is secured, allowing heat to be effectively supplied to the magnetic separation scrap contained in the solidification material 5 introduced in the second step described later, and in addition, the refractory material on the inner surface of the converter 1 can be coated. Here, if the residual amount of decarburized slag 2, i.e., the amount of residual slag 2a, exceeds 3.5% by mass, the amount of slag remaining in the converter 1 becomes excessive, which causes delays in the solidification of residual slag 2a. On the other hand, if the amount of residual slag 2a is less than 0.5 mass%, not only will the thermal energy not be effectively utilized, but the coating of the refractory lining with slag will be insufficient, leading to problems such as increased wear of the refractory during subsequent blowing. Here, the time required for one furnace shake, as described later, is about 45 seconds to 1 minute, and if the residual slag 2a can be solidified within that time, there will be no delay in converter operation.

[0015] <Second process> In the second step of the solidification method for residual decarburized slag according to this embodiment, a solidification material 5 containing magnetic separation scrap and less than 30% by mass of CaO is added to the residual slag 2a in the converter 1 (Figure 1(c)). As magnetic separation scrap, for example, slag discharged from the converter can be magnetically separated and used. Regarding the amount of magnetic separation scrap to be included in the solidification material 5, the inventors diligently investigated through trial and error the amount that could be stably added while balancing the complete solidification of the residual slag 2a within a specified time with the operating time. As a result, it became clear that the amount of magnetic separation scrap to be used is preferably 800 kg to 2500 kg per ton of residual slag 2a, and more preferably 1000 kg to 1500 kg. This makes it possible to completely solidify the residual slag 2a. Furthermore, the use of magnetic separation scrap enables effective utilization of heat in the converter 1 and also provides a CO2 reduction effect by expanding the use of cold iron sources. Here, if the amount of magnetic separation scrap used exceeds 2500 kg, the ratio of the amount of magnetic separation scrap to the amount of residual slag 2a increases, which is considered advantageous for the solidification of residual slag 2a. However, in this case, there is a risk that the blowing time will be prolonged due to the longer time the solidification material containing magnetic separation scrap is added. On the other hand, if the amount of magnetic separation scrap used is less than 800 kg, the cooling capacity for the molten slag residual slag 2a will be insufficient, and the solidification progress of residual slag 2a will be stunted, which is undesirable.

[0016] The solidifying agent 5 according to this embodiment contains less than 30% by mass of CaO. In the solidification material 5, the CaO content derived from each component is as follows: magnetic separation scrap 0.45% to 7% by mass, limestone 95% to 98% by mass, and dolomite used for furnace protection 29.5% to 31.5% by mass. Of these, for example, the CaO content derived from limestone and dolomite relative to the total amount of solidification material 5 is 0% to 17.5% by mass. Here, examples of CaO sources such as limestone include slaked lime, quicklime, and calcium carbonate. Furthermore, the component composition of the magnetic separation scrap is, for example, 70% to 99% by mass of metallic iron, 0.5% to 17% by mass of iron oxide, 0.45% to 6% by mass of CaO, and 0.1% to 1.5% by mass of SiO2. In addition, as magnetic separation scrap, it is preferable that the total iron content (T.Fe), which is the sum of Fe (iron content) in metallic iron and iron oxide, is 50% by mass or more. Here, the CaO content derived from magnetic separation scrap relative to the total amount of solidification material 5 is 0.45% by mass or more and 5.5% by mass or less. While scrap other than magnetic separation scrap is fed in from the scrap chute, the magnetic separation scrap contained in the solidification material 5 is fed in from, for example, a hopper (not shown) located above the converter 1 and used by being cut out from the same hopper. Similarly, limestone and dolomite may also be fed in from their respective dedicated hoppers (not shown). Alternatively, the solidification material 5, which is a pre-mixed mixture of magnetic separation scrap, limestone, and dolomite, may be fed in from the same hopper. Here, the cooling capacity for residual slag 2a improves in proportion to the amount of magnetic separation scrap, but the cutting speed of the magnetic separation scrap depends on the volume of the magnetic separation scrap. For this reason, even in the case of magnetic separation scrap with small particle size, it takes a certain amount of time to cut it out from the hopper. In this embodiment, while the furnace shaking itself, which will be described later, takes a maximum of about 1 minute, for example, it takes about 2.5 minutes to cut out 5 tons of magnetic separation scrap. In other words, cutting becomes possible at a speed of approximately 2 ton / min, allowing for the cutting of a large amount of magnetic separation waste within a limited operating time. Furthermore, regarding the amount of dolomite used, it is preferable to set the upper limit to about 200 kg per ton of residual slag 2a.

[0017] <3rd process> In the third step of the method for solidifying residual decarburized slag according to this embodiment, the converter 1 is tilted to mix the residual slag 2a in the converter 1 with the solidifying agent 5 added in the second step (diagram (d) in Fig. 1). Specifically, it is preferable to perform tilting (rocking) of the converter 0.5 to 1 time. By doing so, the residual slag 2a and the solidifying agent 5 can be surely mixed, and the cooling and solidification of the residual slag 2a proceed. In addition, coating proceeds as the residual slag 2a comes into contact with the entire refractory of the converter 1, and the effect of improving the life of the refractory can also be obtained. When the state where the solidification of the residual slag 2a is completed (diagram (e) in Fig. 1) is confirmed, the next blowing operation is carried out.

[0018] As described above, in the solidification method for residual decarburized slag according to this embodiment, after decarburization and blowing in the previous charge, the steel is tapped and the decarburized slag 2 is discharged from the furnace opening of the converter 1. At that time, a portion of the decarburized slag 2 (residual slag 2a) is left in the converter 1 for use as a slag-forming material, a fluxing material, and a coating material inside the converter. Subsequently, a solidification material 5 containing a predetermined amount of magnetic separation scrap is added to the residual slag 2a left in the converter 1. By solidifying the residual slag 2a, which is molten slag, using a solidification material 5 that contains magnetic separation scrap, which is a cold iron source, and has a CaO content of less than 30% by mass, the use of conventional high-CaO-containing solidification materials can be avoided. This prevents the generation of excess CaO components and avoids the need to add SiO2 components (silica) to adjust the basicity of the slag. Therefore, the risk of abnormal reactions (molten material scattering, black smoke generation) when charging molten iron in the next charge can be reduced. Next, the converter 1 is tilted to mix the residual slag 2a and the solidifying agent 5 in the converter 1, thereby solidifying the residual slag 2a. If solidification of the residual slag 2a is confirmed, scrap and molten iron are charged into the converter 1 in that order, and the next blowing process is started. If solidification of the residual slag 2a cannot be confirmed, the number of tilting (furnace shaking) cycles of the converter 1 can be increased until solidification is confirmed, and / or the amount of magnetic separation scrap contained in the solidifying agent 5 can be increased. In other words, the method for solidifying residual decarburized slag according to this embodiment differs from conventional methods that promote the solidification of molten slag solely by adding a high-CaO-containing solidifying agent, in that it promotes solidification by actively using magnetic separation scrap in proportion to the amount of residual slag. CaO has a high melting point of approximately 2600°C, and conventional methods have utilized this to raise the melting point of residual slag, whereas in this embodiment, the residual slag is cooled by the sensible heat due to the heat capacity of the magnetic separation scrap, which is the main component of the solidifying agent.

[0019] In addition, in the method for solidifying residual decarburized slag according to this embodiment, as described above, the solidifying material 5 includes, in addition to magnetic separation chips, solidifying materials other than magnetic separation chips, such as limestone and dolomite. The addition of the solidifying material other than magnetic separation chips may be performed, for example, simultaneously with the addition of magnetic separation chips or before or after it. Here, by actively using magnetic separation chips, the CaO content of the entire solidifying material 5 can be suppressed to less than 30% by mass. Thus, it becomes possible to freely select, in accordance with an optimal slag composition that is difficult to splash or form, a solidifying material containing an optimal amount of MgO for the refractory coating, etc. As a result, various functions can be imparted to the slag more than in the past.

Example

[0020] Hereinafter, the present invention will be specifically described based on examples. Note that the present invention is not limited to the examples shown below.

[0021] <Example> To confirm the effects of the present invention, converter operations according to the following inventive examples and comparative examples were carried out. Here, an example using the method for solidifying residual decarburized slag according to the present invention was taken as an inventive example, and an example using other methods was taken as a comparative example. Specifically, Examples 1 to 6 are comparative examples, and Examples 7 to 11 are inventive examples. The results of the inventive examples and comparative examples are shown in Table 1 below. In Table 1, regarding the abnormal reaction during the charging of hot metal at the second blowing, when no flare was observed, it was evaluated as "○", when a slight flare was observed, it was evaluated as "△", and when a significant flare was observed and the operation was hindered, it was evaluated as "×". Furthermore, in both the inventive examples and comparative examples, when the solidification of the residual slag was confirmed, scrap and hot metal were charged for the second blowing. On the other hand, when the residual slag did not solidify, the solidification of the residual slag was promoted by increasing the number of tilts (shaking) of the converter. Here, the determination of whether the residual slag solidified was made by visually confirming the movement of the residual slag by tilting the converter by an operator of the crane or the like.

[0022] (Inventive Example 1) In Invention Example 1, the total charge volume of the converter was set to 381.0 t, and during the slag discharge in the first step, the operation was carried out so that 3 t (0.787 mass%) of decarburized slag remained in the converter as residual slag. In addition, 3 t of magnetically separated slag obtained by magnetically separating the slag discharged from the converter and 0.5 t of dolomite were used as solidifying agents. The calculated CaO content of the solidifying agent was 9 mass%. After adding the solidifying agent containing the magnetically separated slag, solidification of the residual slag was confirmed after tilting the converter back and forth just once. Furthermore, no flames were observed during the charging of molten iron in the subsequent blowing. In this invention example, the magnetically separated slag and dolomite were each fed from hoppers dedicated to each solidifying agent, which were located above the converter. Here, it took 1.5 minutes to cut out 3 t of magnetically separated slag, and the magnetically separated slag was added at a cutting speed of 2 t / min.

[0023] (Example of Invention 2) In Invention Example 2, the total charge volume of the converter was set to 369.6 t, and the operation was carried out so that 5 t (1.353 mass %) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 5 t of magnetically separated slag obtained by magnetically separating the slag discharged from the converter and 0.5 t of dolomite were used as solidifying agents. The calculated CaO content of the solidifying agent was 8 mass %. After adding the solidifying agent containing the magnetically separated slag, solidification of the residual slag was confirmed after tilting the converter back and forth just once. Furthermore, no flames were observed during the charging of molten iron in the subsequent blowing. In this invention example, the magnetically separated slag and dolomite were each fed from hoppers dedicated to each solidifying agent, which were located above the converter. Here, it took 2.5 minutes to cut out 5 t of magnetically separated slag, and the magnetically separated slag was added at a cutting speed of 2 t / min.

[0024] (Example of Invention 3) In Invention Example 3, the total charge volume of the converter was set to 377.5 t, and the operation was carried out so that 5 t (1.325 mass %) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 5 t of magnetically separated slag obtained by magnetically separating the slag discharged from the converter was used as the solidification material. Here, since only magnetically separated slag was used as the solidification material, the CaO content in the solidification material was 6 mass %. After the addition of the magnetically separated slag, solidification of the residual slag was confirmed by tilting the converter back and forth 0.5 times. Furthermore, no flame was observed when charging molten iron in the subsequent blowing. In this invention example, the magnetically separated slag was fed in from a dedicated hopper located above the converter. Here, it took 2.5 minutes to cut out 5 t of magnetically separated slag, and the magnetically separated slag was added at a cutting speed of 2 t / min.

[0025] (Example of Invention 4) In Invention Example 4, the total charge volume of the converter was set to 355.0 t, and the operation was carried out so that 5 t (1.408 mass%) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 5 t of magnetically separated slag obtained by magnetically separating the slag discharged from the converter was used as the solidification material. Here, since only magnetically separated slag was used as the solidification material, the CaO content in the solidification material was 6 mass%. After the addition of the magnetically separated slag, solidification of the residual slag was confirmed after tilting the converter back and forth just once. Furthermore, no flame was observed when charging molten iron in the subsequent blowing. In this invention example, the magnetically separated slag was fed in from a dedicated hopper located above the converter. Here, it took 2.5 minutes to cut out 5 t of magnetically separated slag, and the magnetically separated slag was added at a cutting speed of 2 t / min.

[0026] (Example of Invention 5) In Invention Example 5, the total charge volume of the converter was set to 376.4 t, and the operation was carried out so that 5 t (1.328 mass %) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 5 t of magnetically separated slag obtained by magnetically separating the slag discharged from the converter was used as the solidification material. Here, since only magnetically separated slag was used as the solidification material, the CaO content in the solidification material was 6 mass %. After the addition of the magnetically separated slag, solidification of the residual slag was confirmed by tilting the converter back and forth 0.5 times. Furthermore, no flame was observed when charging molten iron in the subsequent blowing. In this invention example, the magnetically separated slag was fed in from a dedicated hopper located above the converter. Here, it took 2.5 minutes to cut out 5 t of magnetically separated slag, and the magnetically separated slag was added at a cutting speed of 2 t / min.

[0027] (Comparative Example 1) In Comparative Example 1, the total charge volume of the converter was set to 373.1 t, and the operation was carried out so that 3 t (0.804 mass %) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, instead of using magnetic separation scrap as a solidifying agent, 1.1 t of limestone and 1 t of dolomite were used. The calculated CaO content of the solidifying agent was 64 mass %. After the addition of the solidifying agent, solidification of the residual slag was confirmed after tilting the converter back and forth just once. Furthermore, no flames were observed during the charging of molten iron in the subsequent blowing. In this comparative example, the limestone and dolomite were each added from hoppers dedicated to each solidifying agent, which were located above the converter.

[0028] (Comparative Example 2) In Comparative Example 2, the total charge volume of the converter was set to 383.9 t, and the operation was carried out so that 3 t (0.781 mass%) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 2 t of magnetically separated slag obtained by magnetically separating the slag discharged from the converter was used as the solidification material. Here, since only magnetically separated slag was used as the solidification material, the CaO content in the solidification material was 6 mass%. After the addition of the magnetically separated slag, solidification of the residual slag was not confirmed after tilting the converter back and forth once, and solidification of the residual slag was finally confirmed after tilting it back and forth twice. Furthermore, no flame was observed when charging molten iron in the next blowing. In this comparative example, the magnetically separated slag was fed in from a dedicated hopper located above the converter. Here, it took 1 minute to cut out 2 t of magnetically separated slag, and the magnetically separated slag was added at a cutting speed of 2 t / min.

[0029] (Comparative Example 3) In Comparative Example 3, the total charge volume of the converter was set to 372.6 t, and the operation was carried out so that 3 t (0.805 mass%) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 1 t of magnetically separated slag obtained by magnetically separating the slag discharged from the converter, 1 t of limestone, and 0.5 t of dolomite were used as solidifying agents. The calculated CaO content of the solidifying agent was 46 mass%. After adding the solidifying agent containing the magnetically separated slag, solidification of the residual slag was confirmed after tilting the converter back and forth just once. Furthermore, no flames were observed during the charging of molten iron in the subsequent blowing. In this comparative example, the magnetically separated slag, limestone, and dolomite were each fed into dedicated hoppers for each solidifying agent located above the converter. Here, it took 0.5 minutes to cut out 1 ton of magnetic separation waste, and the magnetic separation waste was added at a cutting speed of 2 tons / minute.

[0030] (Comparative Example 4) In Comparative Example 4, the total charge volume of the converter was set to 378.1 t, and the operation was carried out so that 5 t (1.322 mass %) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 3 t (outside the range) of magnetically separated slag obtained by magnetically separating the slag discharged from the converter, 1 t of limestone, and 0.5 t of dolomite were used as solidifying agents. The calculated CaO content of the solidifying agent was 28 mass %. After adding the solidifying agent containing the magnetically separated slag, solidification of the residual slag was confirmed after tilting the converter back and forth just once. At this point, a slight flame was observed when charging molten iron in the next blowing stage. In this comparative example, the magnetically separated slag, limestone, and dolomite were each fed into dedicated hoppers for each solidifying agent located above the converter. Here, it took 1.5 minutes to cut out 3t of magnetic separation waste, and the magnetic separation waste was added at a cutting speed of 2t / min.

[0031] (Comparative Example 5) In Comparative Example 5, the total charge volume of the converter was set to 374.3 t, and the operation was carried out so that 5 t (1.336 mass %) of decarburized slag remained in the converter as residual slag during the slag discharge in the first step described above. In addition, 4 t (outside the range) of magnetically separated slag obtained by magnetically separating the slag discharged from the converter was used as a solidifying agent. Here, since only magnetically separated slag was used as the solidifying agent, the CaO content in the solidifying agent was 6 mass %. After the addition of the magnetically separated slag, solidification of the residual slag was confirmed by tilting the converter back and forth once. Here, a slight flame was observed when charging molten iron in the next blowing. In this comparative example, the magnetically separated slag was fed in from a dedicated hopper located above the converter. Here, it took 2 minutes to cut out 4 t of magnetically separated slag, and the magnetically separated slag was added at a cutting speed of 2 t / min.

[0032] (Comparative Example 6) In Comparative Example 6, the total charge volume of the converter was set to 382.8 t, and the operation was carried out so that 5 t (1.306 mass %) of decarburized slag remained in the converter as residual slag during the slag removal in the first step described above. In addition, 1 t of limestone and 0.5 t of dolomite were used as solidifying agents. The calculated CaO content of the solidifying agents was 73 mass %. After the addition of the solidifying agents, solidification of the residual slag was confirmed after tilting the converter back and forth just once. At this point, a slight flame was observed when charging molten iron in the next blowing stage. In this comparative example, the limestone and dolomite were each added from hoppers dedicated to the respective solidifying agents, which were located above the converter.

[0033] [Table 1]

[0034] As is clear from the results shown in Table 1, despite using a large quantity of magnetically separated scrap (3-5 tons) in the inventive examples, the extraction speed after being fed from the hopper was generally high, at approximately 1.9 t / min to 2.1 t / min, allowing for addition without affecting the operating time. Furthermore, in the inventive examples, solidification of residual slag was observed with only 0.5 furnace shakes. This is thought to be due to the amount of magnetically separated scrap contained in the solidification material and the cooling capacity it provides. In addition, regarding the presence and extent of flames during molten iron charging in the subsequent blowing, no flames were observed in any of the inventive examples, resulting in a "○" rating. [Industrial applicability]

[0035] According to the present invention, by using a solidification material containing a predetermined amount of magnetic separation scrap instead of a high-CaO-containing solidification material, decarburized slag after decarburization and blowing can be rapidly solidified. In other words, the method for solidifying residual decarburized slag according to the present invention not only suppresses abnormal reactions when molten iron is charged into the converter, but also enables the effective use of thermal energy in the converter by expanding the use of magnetic separation scrap, which is a cold iron source. This makes it possible to reduce the environmental burden (CO2 reduction) in the steelmaking process. Therefore, the method for solidifying residual decarburized slag according to the present invention has high industrial applicability to pig iron refining using a converter. [Explanation of symbols]

[0036] 1 Converter 2. Decarburized slag 2a Residual slug 3 Slag pot 4 carts 5. Solidifying agent

Claims

1. The first step involves removing slag after tapping the molten steel following decarburization and blowing, so that a predetermined amount of slag generated during decarburization remains in the converter as residual slag. A second step involves adding a solidifying agent to the residual slag in the converter, A method for solidifying residual decarburized slag, comprising a third step of mixing the residual slag and the solidifying agent by tilting the converter, A method for solidifying residual decarburized slag, characterized in that the solidifying material contains magnetic separation scrap and contains less than 30% by mass of CaO.

2. The method for solidifying residual decarburized slag according to claim 1, characterized in that, in the first step, the residual slag is 0.7% by mass or more and 2.6% by mass or less of the total amount charged to the converter.

3. The method for solidifying residual decarburized slag according to claim 1 or 2, characterized in that, in the second step, the magnetic separation scrap contained in the solidification material contains 50% by mass or more of total iron, and the solidification material comprises 1,000 kg to 1,500 kg of magnetic separation scrap per ton of residual slag, and 0 kg to 200 kg of dolomite per ton of residual slag.

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