How molten iron is refined

Abstract

A method for refining molten iron is provided that can be carried out safely and stably without steam explosions or machine losses when using the heat of high-temperature slag remaining in a reaction vessel to preheat a cold iron source. The method uses the heat of slag generated after the completion of refining a previous charge of molten iron to preheat the cold iron source. The method includes a first step of discharging the molten iron refined while it remains in the reaction vessel as residual slag from the reaction vessel, a second step of charging the cold iron source through a scrap chute as a coolant for cooling the residual slag to solidify it, and a third step of charging the next charge of molten iron into the reaction vessel and refining it. In the second step, the K value calculated using a predetermined relational expression using the mass Wsl of the residual slag, the mass Wsc of the cold iron source charged through the scrap chute, and the temperature Tf of the hot material remaining in the furnace in the first step is less than 0, and no waiting time T is required after charging the cold iron source.

Description

[Technical Field]

[0001] The present invention relates to a method for refining molten iron, and more particularly to a method for refining molten iron in which a cold iron source is introduced into slag remaining in a reaction vessel such as a converter, and the heat of the slag is utilized to preheat the cold iron source.

[0002] In this specification, the expression "x~y" means "greater than or equal to x and less than or equal to y" and includes boundary values. Furthermore, chemical formulas such as "SiO2" represent compounds of the specified composition, and notations such as "iron" and "phosphorus" indicate that the element is included regardless of its form. T.Fe means total iron, and represents the total amount of iron regardless of its form. M.Fe means metallic iron. The notation [M] indicates that element M is contained in molten iron. (R) indicates that a compound with chemical formula R is contained in the slag. "Molten iron" refers to a molten metal containing iron as the main component, and includes "molten pig iron" containing approximately 3-4 mass% C and "molten steel" containing approximately 2 mass% or less C. The unit of mass, "t," represents 1000 kg. [Background technology]

[0003] In response to the recent growing need to reduce CO2 emissions, there is a demand for an increase in the amount of cold iron used in the steelmaking process. By charging the cold iron into the reactor in addition to the molten iron tapped from the blast furnace, it is possible to reduce the ratio of molten iron to the molten iron being filled and refined in the reactor (hereinafter referred to as the molten iron blending ratio). As a result, it is possible to reduce the amount of molten iron used per unit of crude steel produced.

[0004] There are various types of cold iron sources. For example, iron scrap is stored in a yard, and reduced iron to be fed into the furnace is stored in an underground bunker and is generally kept at room temperature. These cold iron sources are charged into the reactor vessel through a scrap chute or a hopper above the furnace. To completely dissolve the charged cold iron source in the molten iron, the cold iron source must be heated and the heat required to melt it must be supplied from another source.

[0005] Therefore, if more cold iron is used, the heat that cannot be compensated for by the heat of the molten iron itself or the heat of combustion of impurities must be compensated for by adding a heat raising material or by a forced heat application means from outside. Examples of heat raising materials include graphite, ferrosilicon, and silicon carbide. Examples of heat application means include a burner or arc discharge.

[0006] In the reactor, slag is added to the molten iron, and impurities are removed by the slag that forms. This slag contains a large amount of heat, reaching high temperatures of 1300-1400°C for dephosphorization and 1600-1700°C for decarburization. Typically, the slag after these processes is solidified by adding a coolant such as dolomite and left in the reactor. The remaining slag is discharged from the reactor. This is because if the slag is not solidified, the iron oxide in the slag will react rapidly with the carbon in the molten iron, causing the slag to bump.

[0007] However, if the heat of the slag itself, which is usually not effectively utilized because it is discharged outside the reaction vessel, can be used to preheat the scrap, the amount of heat required to heat and melt the cold iron source can be reduced, and the molten iron blending ratio can be reduced even more than when the cold iron source is at room temperature.

[0008] Therefore, several methods have been disclosed that can preheat the cold iron source using the residual slag remaining in the reaction vessel while preventing the slag from bumping. For example, Patent Document 1 discloses a technique in which steel is tapped while leaving the decarburized slag generated by the decarburization treatment in the reaction vessel, and a solid iron source is charged into the remaining slag. When charging molten iron after charging the solid iron source, Wsl / Wsc·(2-N -2 A method for charging molten iron under conditions that satisfy the equation Wsc < 1 is shown. In the equation, Wsc is the amount of solid iron charged (t / ch), Wsl is the amount of decarburized slag remaining (t / ch), and N is the number of times the refining vessel is tilted back and forth.

[0009] Furthermore, Patent Document 2 discloses a method in which a solid iron source is added to the residue (slag and molten iron) in the converter after steel is tapped, thereby solidifying the remaining molten iron.

[0010] The technology disclosed in Patent Document 1 recycles decarburization slag, while the technology disclosed in Patent Document 2 prevents the outflow of molten iron remaining in the converter during slag removal. Although their objectives are somewhat different, they can both be applied to preheating a cold iron source using slag in a reaction vessel. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-256839 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-228102 Summary of the Invention [Problem to be solved by the invention]

[0012] However, the above-mentioned conventional technologies have the following problems to be solved. That is, the technologies described in Patent Documents 1 and 2 have the problem that they are unable to deal with steam explosions that can occur when a cold iron source containing moisture gets into the high-temperature slag in the reaction vessel due to differences in specific gravity. Possible examples of cold iron sources containing moisture include water that gets into depressions in plate-shaped scrap, which can unavoidably occur during rainy weather, and wet powdery scrap.

[0013] For ease of transportation, cold steel sources are often stored in buildings that are usually covered but not airtight. In such storage areas, it is impossible to prevent the cold steel sources from getting wet due to wind and rain, so to remove moisture, the cold steel sources must be preheated in the storage area. This preheating requires the introduction of equipment and goes against the purpose of utilizing unused heat.

[0014] Furthermore, the above-mentioned prior art also has problems in terms of preventing reactions with molten iron. The amount of solid iron source charge Wsc in the formula shown in Patent Document 1 is a value determined based on a production plan, assuming guaranteed production volume, and is therefore an uncontrolled parameter. This requires either reducing the amount of decarburized slag remaining Wsl or increasing the number of reciprocating tilts N. The former approach fails to achieve the purpose of preheating the cold iron source because the total heat content of the slag is reduced, while the latter approach increases non-steelmaking time, reducing molten iron productivity.

[0015] In Patent Document 2, the solid iron source is added to the reactor for the purpose of solidifying the remaining molten iron in the reactor, and the slag is discharged from the furnace in a molten state. Therefore, the method described in Patent Document 2 does not achieve the purpose of preheating the cold iron source with the remaining slag. Furthermore, Patent Document 2 describes that the solid iron source can be retained in the molten slag by reducing the size of the solid iron source to less than 1 mm, but it is difficult to secure a sufficient amount of such a solid iron source.

[0016] In light of these problems, we conducted extensive research and found that steam explosions are caused by the rapid volume expansion of water contained in the cold iron source that occurs when the water comes into contact with slag or other high-temperature materials remaining in the furnace. We then focused on the conditions for avoiding such rapid volume expansion of water. When water contained in the cold iron source evaporates upon contact with the high-temperature materials remaining in the furnace, the amount of water evaporated per unit amount of water depends mainly on the temperature of the high-temperature materials remaining in the furnace.

[0017] When the surface temperature of the hot materials remaining in the furnace is between 90°C and around 140°C, the higher the surface temperature, the faster the moisture evaporates. Furthermore, when the surface temperature of the hot materials remaining in the furnace is between around 140°C and around 300°C, the higher the surface temperature, the longer it takes for the moisture to evaporate. It is known that once the surface temperature of the hot materials remaining in the furnace reaches around 300°C and the time it takes for the moisture to evaporate reaches its peak, the higher the surface temperature of the hot materials remaining in the furnace becomes, the faster the moisture will evaporate (Leidenfrost effect).

[0018] Therefore, in the present invention, attention is focused on the Leidenfrost effect that occurs when moisture contained in the cold iron source comes into contact with high-temperature materials remaining in the furnace, such as slag, and a method for preventing moisture contained in the cold iron source from being brought into the reaction vessel when the temperature of the high-temperature slag is in the range of about 140°C to about 300°C, at which point the surface temperature of the high-temperature materials remaining in the furnace, at which time the evaporation time of the moisture becomes short, is studied.

[0019] Specifically, when a cold iron source is added to a reactor containing residual high-temperature slag, the temperature of the hot slag drops due to contact between the cold iron source and the hot slag. The amount of temperature drop in the hot slag depends on the ratio of the amount of cold iron source added to the amount of residual slag. Therefore, we hypothesized and discovered that conditions for preventing steam explosions due to the reaction between the residual slag and the moisture in the cold iron source can be determined by providing a time for moisture evaporation based on the ratio of the amount of residual slag in the reactor, Wsl, to the amount of cold iron source added to the reactor for preheating, Wsc, Wsc / Wsl(-), and the temperature Tf of the remaining hot material in the furnace.

[0020] Furthermore, conditions were found for charging a predetermined amount of cold iron source into the reaction vessel using a scrap chute, and then setting a waiting time, and then adding the remaining cold iron source, according to a K value calculated from a predetermined relational expression showing the relationship between the amount of slag remaining in the reaction vessel Wsl, the amount of cold iron source Wsc to be charged into the reaction vessel using a scrap chute, and the temperature Tf of the residual high-temperature material in the furnace, which is the temperature of the residual high-temperature material in the furnace including the molten iron and residual slag remaining in the reaction vessel in the treatment in which slag remains.

[0021] The present invention has been made to solve the above problems, and its purpose is to propose conditions under which the heat energy of the high-temperature slag remaining in a reaction vessel can be used to preheat a cold iron source in a safe and stable manner without loss of production time, without causing a steam explosion due to a reaction between the residual slag and the moisture in the cold iron source. [Means for solving the problem]

[0022] In view of these problems, the inventors have conducted extensive research and have found that it is possible to determine the conditions under which a steam explosion does not occur by adjusting the ratio of the amount of slag remaining in the reaction vessel to the amount of cold iron source charged into the reaction vessel, and the temperature of the hot material remaining in the furnace in a process in which slag remains.

[0023] The method for refining molten iron according to the present invention, which advantageously solves the above-mentioned problems, is a method for refining molten iron in which heat contained in slag generated after the completion of refining of a previous charge of molten iron is utilized to preheat a cold iron source, and includes the following steps: a first step of discharging the refined molten iron from the reaction vessel while leaving some or all of the slag as residual slag in the reaction vessel; a second step of charging the cold iron source through a scrap chute as a coolant for cooling the residual slag to solidify it; and a third step of charging the next charge of molten iron into the reaction vessel and refining it; wherein in the second step, the value of K calculated by the following relational expression (1) using the mass Wsl of the residual slag, the mass Wsc of the cold iron source charged through the scrap chute, and the temperature Tf of the residual hot materials in the furnace in the first step is less than 0, and no waiting time T is provided after charging the cold iron source. K = Wsc-8.64 x 10 -7 ×Tf 1.947 ×Wsl···(1) In the relational expression (1), Wsc (kg) represents the mass of the cold iron source charged through the scrap chute, Wsl (kg) represents the mass of the residual slag, and Tf (°C) represents the temperature of the residual hot material in the furnace in the first step.

[0024] The method for refining molten iron according to the present invention, which advantageously solves the above-mentioned problems, is a method for refining molten iron in which the heat of slag generated after the refining of a previous charge of molten iron is used to preheat a cold iron source, and includes a first step of pouring the refined molten iron from a reaction vessel while leaving part or all of the slag as residual slag in the reaction vessel; and a first step of charging a cold iron source from a scrap chute or a furnace top as a coolant for cooling the residual slag to form solidified slag. a second step and a third step of charging a next charge of molten iron into the reaction vessel and refining the molten iron, wherein in the second step, the K value calculated by the following relational expression (1) using the mass Wsl of the residual slag, the mass Wsc of the cold iron source charged from the scrap chute, and the temperature Tf of the hot material remaining in the furnace in the first step is less than 0, a waiting time T after charging the cold iron source is set to 60 seconds or more, and after the waiting time T has elapsed, the remaining cold iron source is charged from the scrap chute or from the furnace. K = Wsc-8.64 x 10 -7 ×Tf 1.947 ×Wsl···(1) In the relational expression (1), Wsc (kg) represents the mass of the cold iron source charged through the scrap chute, Wsl (kg) represents the mass of the residual slag, and Tf (°C) represents the temperature of the residual hot material in the furnace in the first step. The method for refining molten iron according to the present invention includes the steps of: (a) In the third step, a more preferable means for solving the problem is to provide a waiting time T1 immediately before charging the next charge of molten iron into the reaction vessel. [Effects of the Invention]

[0025] According to the present invention, the heat of the high-temperature slag remaining in the reaction vessel can be utilized to preheat the cold iron source, and molten iron can be refining by preheating the cold iron source safely and stably without steam explosions and without loss of production time. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic conceptual diagram showing a facility configuration suitable for application to a method for refining molten iron according to the present invention. [Figure 2] 1 is a graph showing the results of a slag steam explosion confirmation test in which a cold iron source wetted with water was poured into slag. [Figure 3] 1 is a graph showing the results of a molten iron steam explosion confirmation test in which a cold iron source wetted with water was poured into molten iron. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, embodiments of the present invention will be described in detail. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims. Below, preferred embodiments of the present invention will be described in detail, including the background.

[0028] [First embodiment] The method for refining molten iron according to this embodiment is a method for refining molten iron in which the heat contained in the slag generated after the refining of a previous charge of molten iron is completed is used to preheat a cold iron source, and is characterized by including the following steps: a first step of pouring the refined molten iron from the reaction vessel while leaving some or all of the slag in the reaction vessel as residual slag; a second step of charging a coolant from a scrap chute as the cold iron source for cooling the residual slag to solidify it; and a third step of charging the next charge of molten iron into the reaction vessel and refining it. Hereinafter, each step included in the method for refining molten iron according to this embodiment will be described.

[0029] The method for refining molten iron according to this embodiment is carried out in a reaction vessel such as a converter-type refining furnace or an induction melting furnace. Therefore, the method for refining molten iron according to this embodiment can be applied to a multi-refining coverter process (hereinafter referred to as the "MURC process"), which can continuously perform desiliconization, dephosphorization, slag removal, and decarburization in the same converter. Furthermore, the method for refining molten iron according to this embodiment can also be applied to a double-slag refining process (hereinafter referred to as the "DRP process"), which can maximize the use of silicon in molten iron as a heat source and increase the amount of scrap input to the converter.

[0030] (First step: A step of pouring refined molten iron from a reactor vessel while leaving residual slag in the reactor vessel) The method for refining molten iron according to this embodiment is a method for refining molten iron in which the heat of slag generated after the refining of the molten iron in the previous charge is completed is used to preheat the cold iron source. The first step included in the method for refining molten iron according to this embodiment is a step of tapping the refined molten iron from a reaction vessel while leaving some or all of the slag generated by refining the molten iron of the previous charge in the reaction vessel as residual slag. The residual slag is molten slag in a molten state. The temperature of the slag remaining in the reaction vessel is 1700 to 1300°C.

[0031] The amount of slag left in the reactor vessel, such as a converter-type refining furnace, may be a portion of the slag generated by refining the hot metal in the previous charge, or may be the entire slag. The amount of slag left in the reactor vessel is determined taking into account the amount of coolant charged into the reactor vessel in the second step described below to solidify the remaining slag. The residual slag left in the reaction vessel in the first step is solidified by being cooled by a coolant in the second step described below, and becomes solidified slag.

[0032] 1(a) to 1(c) show an apparatus configuration suitable for carrying out the method for refining molten iron according to this embodiment. A pre-charge of molten iron is refined using a reaction vessel 1. By refining the pre-charge of molten iron, the molten iron becomes refined molten iron 10. The reaction vessel 1 is a vessel such as a converter-type refining furnace lined with a refractory material 2. The reaction vessel 1, such as a converter-type refining furnace lined with the refractory material 2, is tilted, and the refined molten iron is tapped out of the reaction vessel 1 through a tapping hole 3. In addition, there may be about 1 ton of refined molten iron 10 remaining in the reaction vessel 1, which is so-called residual molten iron 8, generated after the refining of the pre-charge molten iron, which serves as pre-treatment, is completed.

[0033] The refractory material 2 lining the reaction vessel 1 is made of a material that has sufficient corrosion resistance against the slag 4 generated during the refining of the previous charge of molten iron. MgO-C bricks are generally used. The treatment temperature may be adjusted to prevent the refractory material 2 from dissolving into the slag 4, and an MgO source such as lightly burned dolomite may be added as a furnace charge material 15 from an upper hopper 14 depending on the composition of the slag 4.

[0034] Slag 4 is generated in the reaction vessel 1 by refining the pre-charge molten iron. Although the composition of slag 4 is not particularly specified, it is generally a steelmaking slag whose main components are CaO, SiO2, and FeO, and also contains Al2O3, MgO, P2O5, MnO, S, M.Fe (metallic iron), etc. Here, FeO represents iron oxide, including, for example, FeO and Fe2O3. The basicity C / S, which is expressed as the mass ratio of CaO to SiO2 in the slag, is often 0.5 to 4.5, and the concentration of FeO in the slag is often 3 to 40 mass%. A slag formation promoter such as TiO2 may be charged as the furnace charge material 15 to the slag 4 to lower the melting point of the slag 4.

[0035] (Second step: A step of charging a cold iron source into a reaction vessel through a scrap chute) The second step included in the method for refining molten iron according to this embodiment is a step of charging a cold iron source through a scrap chute while leaving some or all of the slag generated by refining the pre-charge molten iron in the reaction vessel. In the second step, the cold iron source 5 may be charged into the reaction vessel 1 through a scrap chute 6 or may be charged from above the furnace. The cold iron source 5 may be charged from above the furnace using, for example, an above-furnace hopper 14. Specifically, as shown in FIG. 1(a), while leaving some or all of the slag 4 remaining in the reaction vessel 1 as residual slag, the reaction vessel 1 is tilted toward the scrap chute 6 loaded with the cold iron source 5. The tip of the scrap chute 6 is inserted into the reaction vessel 1. Then, the scrap chute 6 is tilted using a crane 7, and the cold iron source 5 is charged into the reaction vessel 1.

[0036] The cold iron source 5 may be carbon steel scrap, pig iron scrap, or solid reduced iron 15A such as granulated pig iron or reduced iron that can be wound up into the furnace hopper 14, and loaded into the scrap chute 6 as reduced iron. Examples of carbon steel scrap include heavy scrap, press scrap, shredder scrap, new scrap, and steel turnings. Examples of pig iron scrap include old pig iron and pig iron turnings. The cold iron source 5 is a cold iron source that is prone to moisture and often contains a small amount of moisture. For example, the cold iron source 5 may be powdery scrap or scrap with voids, such as steel turnings, pipes, motors, or press scrap, and these scraps are prone to moisture content.

[0037] In the second step, after the cold iron source 5 is charged into the reaction vessel 1, the reaction vessel 1 may be tilted back and forth to mix the cold iron source 5 with the slag 4. While the reciprocal tilting of the reaction vessel 1 is not necessarily required, it is more preferable to perform the reciprocal tilting of the reaction vessel 1 from the viewpoint of promoting dispersion of the cold iron source 5 and solidification of the slag 4. As described above, there may be about one ton of residual molten iron 8, which is refined molten iron generated after the refining of the pre-charge molten iron, which is a pre-treatment, is completed, in the reaction vessel 1. Therefore, the residual high-temperature material in the furnace, which is formed by mixing the slag 4, the cold iron source 5, and the residual molten iron 8, is present in the reaction vessel 1.

[0038] In the second step, the residual slag present in the reaction vessel 1 is generated after the refining of the previous charge of molten iron is completed, and even if residual molten iron 8, which is the refined molten iron, is coexistent, it is cooled and solidified by the cold iron source 5 charged from the scrap chute 6 or the cold iron source 5 charged from the furnace, and becomes solidified slag.

[0039] Here, in the second step included in the method for refining molten iron according to this embodiment, the cold iron source 5 charged into the reaction vessel 1 includes a powdery cold iron source and a cold iron source having voids, and may be a cold iron source that is likely to contain moisture. The cold iron source that is likely to contain moisture is preferably charged through the scrap chute 6. Note that, in the method for refining molten iron according to this embodiment, when reduced iron such as solid reduced iron 15A is adopted as the cold iron source 5, it is preferable to charge the solid reduced iron 15A into the reaction vessel 1 from above the furnace using an upper furnace hopper 14 or the like, from the viewpoint of facilitating adjustment of the amount of the reduced iron charged into the reaction vessel 1.

[0040] Furthermore, the cold iron source 5 may include reduced iron, and the length of the reduced iron in the longitudinal direction may be 300 mm or less. That is, in the method for refining molten iron according to this embodiment, when the cold iron source 5 is charged into the reaction vessel 1 such as a converter-type refining furnace, solid reduced iron 15A having a large specific surface area may be used. That is, in the method for refining molten iron according to this embodiment, by using solid reduced iron as the cold iron source 5, the residual slag can easily become entangled on the surface of the solid reduced iron 15A, and the solidification of the residual slag, which is molten slag, can be rapidly promoted.

[0041] Furthermore, by using the cold iron source 5 used to cool the residual slag and turn it into solidified slag, and by making the longitudinal length of the solid reduced iron 15A 300 mm or less, the effect of rapidly progressing the solidification of the residual slag was remarkable. In this way, in the method for refining molten iron according to this embodiment, the cold iron source 5 is charged into the reaction vessel 1 and the residual slag is solidified. Therefore, even when the next charge of molten pig iron is charged, the oxygen in the slag 4 reacts rapidly with the carbon in the molten pig iron, thereby preventing the bumping of the molten pig iron.

[0042] In the second step, the residual slag and a small amount of residual molten iron 8 present in the reaction vessel 1 constitute the residual high-temperature material in the furnace. The temperature of the residual high-temperature material in the furnace is defined as the residual high-temperature material temperature in the furnace Tf (°C). Here, when the solidification temperature of the slag 4 was measured using a reaction vessel 1 such as a heating furnace in a typical smelting of molten iron, the results showed that if the slag temperature of the slag 4 was 1100°C or less, the residual slag would solidify and become solidified slag under any conditions.

[0043] Furthermore, in the temperature range considered during decarburization refining of molten iron in general, even if the slag 4 has a composition with variously changed basicity, iron oxide concentration, etc., it was found that all residual slag can be solidified by setting the temperature of the slag 4 at 1100°C or less. On the other hand, in general decarburization refining to which the refining method of molten iron according to this embodiment is applied, the temperature at the end of refining of molten iron is 1750°C at most.

[0044] From this perspective, in the method for refining molten iron according to this embodiment, in order to completely solidify the residual slag, which is part or all of the slag 4 generated by refining the molten iron of the previous charge, it has been found that it is sufficient to charge into the reaction vessel 1 an amount of cold iron source 4 that can ensure the amount of heat removal required to lower the temperature Tf (°C) of the residual high-temperature materials in the furnace, which is the temperature of the residual high-temperature materials in the furnace including the residual slag and the residual molten iron 8, from 1750°C to 1100°C.

[0045] Therefore, in the second step, the relationship between the mass Wsl of the residual slag, the mass Wsc of the cold iron source 5 charged through the scrap chute 6, and the temperature Tf of the residual hot materials in the furnace is taken into consideration, and using these parameters, for example, when charging the cold iron source 5 into the reaction vessel 1 using the scrap chute 6, the standby time can be determined by subtracting the product of the mass Wsl (kg) of the residual slag and the temperature Tf (°C) of the residual hot materials in the reaction vessel 1 from the mass Wsc (kg) of the cold iron source 5. Specifically, the relationship between the mass Wsl of the residual slag, the mass Wsc of the cold iron source 5 charged through the scrap chute, and the temperature Tf of the residual hot materials in the furnace is taken into consideration, and the K value is calculated, for example, using the following relational expression (1): K = Wsc-8.64 x 10 -7 ×Tf 1.947 ×Wsl···(1) In the relational expression (1), Wsc represents the mass of the cold iron source 5 charged through the scrap chute 6, Wsl represents the mass of the residual slag, and Tf (°C) represents the temperature of the residual hot material in the furnace in the reaction vessel 1 in the process in which the slag 4 remains. The introduction of the relational expression (1) will be described later.

[0046] In the method for refining molten iron according to this embodiment, the mass Wsc of the cold iron source 5 charged into the reaction vessel 1 from the scrap chute 6, the mass Wsl of the residual slag, and the temperature Tf (°C) of the residual high-temperature material in the furnace of the reaction vessel 1 are selected as parameters for determining the waiting time T, and the K value calculated by the relational expression (1) is calculated taking into account the relationship between these parameters.

[0047] Here, the K value calculated by the relational expression (1) is used to determine whether or not it is necessary to set a waiting time T for evaporating the moisture contained in the cold iron source 5. That is, in the second step, it is possible to determine whether or not to set a waiting time T after charging the cold iron source 5 based on the K value calculated using the relational expression (1).

[0048] That is, in the method for refining molten iron according to this embodiment, the waiting time T for evaporating the moisture contained in the cold iron source 5 can be determined according to the K value calculated by the relational expression (1) using the amount Wsc of the cold iron source 5 charged from the scrap chute 6, the amount Wsl of the residual slag, and the temperature Tf (°C) of the residual high-temperature material in the furnace in the reaction vessel 1 after the first step of the treatment in which the slag 4 remains after blowing. Alternatively, the waiting time T can be set as well. That is, in the method for refining molten iron according to this embodiment, if the K value calculated by the relational expression (1) is less than 0, it is not necessary to provide the waiting time T after charging the cold iron source 5 into the reaction vessel 1.

[0049] Figure 2 is a graph showing the results of a slag steam explosion confirmation test in which a cold iron source was charged into slag. The slag steam explosion confirmation test uses slag and a cold iron source wet with water to check whether a steam explosion occurs when the cold iron source is charged into the slag. Details of the slag steam explosion confirmation test will be described later.

[0050] As shown in Figure 2, it is clearly understood that there are three regions: region A where slag bumping occurs (hereinafter referred to as "slag bumping region A"), region C where steam explosion occurs (hereinafter referred to as "steam explosion region C"), and regions B1 and B2 where slag bumping and steam explosion do not occur. Here, the symbol "●" in Figure 2 indicates plots under conditions where slag bumping and steam explosions did not occur, and the symbol "×" indicates plots under conditions where steam explosions did occur. That is, as shown in Figure 2, slag bumping region A and steam explosion region C, as well as regions B1 and B2 where slag bumping and steam explosions did not occur, can be organized by the temperature of the remaining hot materials in the furnace Tf (°C) and W' / Wsl'(-), which is the ratio of the amount of cold iron source charged W' (kg) to the amount of remaining slag Wsl' (kg).

[0051] Based on the results of a slag steam explosion confirmation test in which a cold iron source was charged into slag, as shown in FIG. 2, the condition adopted in the method for refining molten iron according to this embodiment specifies that the mass Wsc of the cold iron source 5 charged through the scrap chute 6 satisfies the K value calculated by relational expression (1) below 0.

[0052] That is, the condition adopted in the method for refining molten iron according to this embodiment specifies that the K value calculated by relational expression (1) is less than 0. The K value calculated by relational expression (1) being less than 0 specifies the condition under which the charging of the cold iron source 5 from the scrap chute 6 is stopped to temporarily remain within the region B1 where a steam explosion does not occur, and then the steam explosion region C is avoided.

[0053] In accordance with the condition adopted in the method for refining molten iron according to this embodiment, that the value of K calculated by the relational expression (1) is less than 0, the mass Wsc of the cold iron source 5 charged into the reaction vessel 1 from the scrap chute 6 is set so that the value of K calculated from the relational expression (1) is less than 0 relative to the mass Wsl of the residual slag present in the reaction vessel 1. This makes it possible to avoid the water contained in the cold iron source 5 from entering the steam explosion region C.

[0054] Here, the steam explosion region C is a range in which a steam explosion occurs when the moisture contained in the cold iron source 5 comes into contact with the slag 4 immediately after the cold iron source 5 is introduced into the reaction vessel 1 from the scrap chute 6, or after a short time has passed.

[0055] As described above, the condition for the second step included in the method for refining molten iron according to this embodiment is a condition for making the K value calculated from the relational expression (1) less than 0, and for preventing steam explosion of the water contained in the cold iron source 5 after the cold iron source 5 is charged into the reaction vessel 1.

[0056] The method for refining molten iron according to this embodiment focuses on the mass Wsl of the residual slag, which is a part or all of the slag 4 generated after the refining of the previous charge of molten iron is completed and remains in the reaction vessel 1 as residual slag, the amount Wsc of the cold iron source 5 charged into the reaction vessel 1, and the temperature Tf (°C) of the residual high-temperature material in the furnace, including the residual molten iron 8 after the treatment that left the slag 4, and uses these values ​​to set the K value calculated by relational expression (1) to less than 0, thereby finding the conditions for preventing steam explosion of the moisture contained in the cold iron source 5. The condition for preventing a steam explosion of the moisture contained in the cold iron source 5, which is specified as a condition for the second step, is that the K value calculated by the relational expression (1) is less than 0. This condition is based on the results of a slag steam explosion confirmation test described later and a molten iron steam explosion confirmation test in which a water-wet cold iron source 5 is poured into molten iron 10.

[0057] That is, the conditions for the second step included in the method for refining molten iron according to this embodiment focus on the Leidenfrost effect that occurs when moisture contained in the cold iron source 5 comes into contact with the high-temperature materials remaining in the furnace, such as the slag 4, and are set based on detailed experiments described below to prevent moisture from being brought into the reaction vessel 1 due to the cold iron source 5 when the surface temperature of the high-temperature materials remaining in the furnace is around 140 to 300°C, at which point the temperature of the high-temperature slag shortens the evaporation time of moisture.

[0058] In this way, according to the conditions that can be selected in the second step included in the method for refining molten iron according to this embodiment, it is possible to prevent a steam explosion caused by the moisture contained in the cold iron source 5.

[0059] In other words, the method for refining molten iron according to this embodiment can prevent the following problems from occurring: bumping of the slag 4 caused by gas generated by a sudden reaction between oxygen in the residual slag and carbon in the molten iron 10 when the next charge of molten iron 10 is charged; and steam explosion caused by moisture contained in the cold iron source 5.

[0060] In the method for refining molten iron according to this embodiment, the mass Wsl of the residual slag, which is part or all of the slag 4 generated by refining the molten iron in the previous charge, can be calculated from the amount of slag former added in the charge, the amount of gangue contained in the other added auxiliary materials, and estimated values ​​of the weight of iron oxide and the weight of manganese oxide generated by oxidation of the molten iron.

[0061] As described above, the method for refining molten iron according to this embodiment makes it possible to avoid a steam explosion by setting the K value calculated by the relational expression (1) to less than 0 and evaporating the water contained in the cold iron source 5 charged into the reaction vessel 1 from the scrap chute 6 without setting a waiting time. In other words, the method for refining molten iron according to this embodiment uses the relational expression (1) to calculate the K value, which is an index for determining whether or not to set the waiting time T, and can avoid the steam explosion region C based on the calculated K value.

[0062] In the method for refining molten iron according to this embodiment, the value K calculated by the relational expression (1) is set to less than 0, so that there is no need to provide a waiting time T for evaporating the moisture contained in the cold iron source 5. This makes it possible to refining molten iron safely and stably, without loss of time, by utilizing the preheating of the cold iron source 5.

[0063] (Third process: The next charge of molten iron is charged into the reactor and refined.) The method for refining molten iron according to this embodiment includes a third step of charging the next charge of molten iron into the reaction vessel and refining it. That is, as shown in Fig. 1(b), the third step is a step of tilting the reaction vessel 1 again toward the crane 7, tilting the molten iron ladle 9 to charge the next charge of molten iron 10 into the reaction vessel 1, and then refining the next charge of molten iron. The third step included in the method for refining molten iron according to this embodiment includes desiliconization, dephosphorization, and decarburization of the next charge of molten iron 10. The third step included in the method for refining molten iron according to this embodiment includes a preliminary treatment for desiliconization and dephosphorization, an intermediate slag removal treatment, and a decarbonization treatment for decarburization and final dephosphorization. That is, refining such as desiliconization, dephosphorization, and decarburization is performed continuously in the reaction vessel 1.

[0064] When the molten iron 10 is charged into the reactor 1, if the moisture brought into the reactor 1 (furnace) by the cold iron source 5, such as scrap, does not evaporate and remains, there is a concern of a steam explosion due to a reaction between the molten iron 10, such as molten pig iron, and the moisture. Therefore, when charging scrap containing moisture into the reactor 1, it is desirable to set a separate waiting time T1 between the completion of the scrap charging and the start of the charging of the molten iron 10. The waiting time T1 may be set uniformly or may be determined based on Wsc / Wsl. Furthermore, to shorten the waiting time T1 required for dispersing and evaporating the moisture, it is desirable to perform furnace shaking after the completion of the charging of the scrap into the reactor 1. Furthermore, in the second step, the waiting time T1, which is set separately after the completion of the charging of the cold iron source 5 into the reactor 1 from the scrap chute 6 and before the charging of the next charge of molten iron 10, will be described later.

[0065] Specifically, after the molten iron 10 is charged into the reaction vessel 1, the reaction vessel 1 is placed upright as shown in FIG. 1(c). Then, oxygen gas 12 is blown from an oxygen supply lance 11 onto the molten iron 10 or slag 4, while stirring the molten iron 10 using stirring gas blown from a bottom-blowing gas system 13. The molten iron 10 then undergoes a refining process in which impurities are oxidized and removed. Before or during the refining process of the molten iron 10, a furnace-loaded material 15 may be added from an upper hopper 14. The furnace-loaded material 15 may be a slag-forming material such as lime or a coolant such as ore. The furnace-loaded material 15 may be a slag-forming material such as lime or a coolant such as ore, or it may also be solid reduced iron 15A of a size that can be lifted up into the upper hopper 14.

[0066] In this regard, the method for refining molten iron according to this embodiment charges the reaction vessel 1 with an allowable amount of cold iron source 5 to avoid steam explosion due to moisture contained in the cold iron source 5 charged into the reaction vessel 1 from the scrap chute 6. For this reason, the amount of cold iron source 5 charged from the scrap chute 6 may be less than the necessary and sufficient total amount Wt of cold iron source charge.

[0067] In this case, solid reduced iron 15A can be charged into the reaction vessel 1 from above the furnace at a timing after the cold iron source 5, which is insufficient for the total amount Wt of cold iron source input, is charged from the scrap chute 6 as the remaining cold iron source. That is, the reaction vessel 1 is placed upright, and the solid reduced iron 15A is charged from the furnace top hopper 14. The solid reduced iron 15A refers to the cold iron source 5, such as granulated pig iron or reduced iron, which has a size that can be lifted up by the furnace top hopper 14.

[0068] In the method for refining molten iron according to this embodiment, the solid reduced iron 15A can be introduced after the cold iron source 5 is charged into the reaction vessel 1 through the scrap chute 6, regardless of whether the process of refining the next charge of molten iron 10 is started or after the process has started. However, when the molten iron temperature is low, such as before the start of the refining process of the next charge of molten iron 10, the drop in the molten iron temperature in the early stage of the refining process is large due to the endothermic heat that accompanies the melting of the cold iron source 5. Therefore, when the temperature of the molten iron drops to a temperature obtained by adding approximately 60°C to the solidification temperature of the molten iron calculated from the C-Fe binary phase diagram, the gas outlet of the bottom-blown gas system 13 is blocked due to the solidification of the molten iron 10 or the growth of the solidification phase of the cold iron source 5.

[0069] In such a case, ineffective stirring of the bath of molten iron 10 can have adverse effects, such as overoxidation of the slag 4, a decrease in iron yield, and a decrease in the refining characteristics of the molten iron 10. In addition, gas can escape through the gaps in the furnace wall through the joints of the refractories 2, leading to equipment problems such as the refractories 2 falling off. Therefore, it is preferable to charge the solid reduced iron 15A from the furnace hopper 14 after the start of the refining process of the molten iron 10, when the temperature of the molten iron can be increased by the heat of impurity combustion using oxygen gas.

[0070] When the stirring gas is supplied from the bottom-blowing gas system 13, a certain amount of gas continues to flow as a standby flow even before the refining process of the next charge of molten iron 10, in order to prevent clogging of the gas outlet. The flow rate of the gas constituting the standby flow is determined based on the amount of residual molten iron 8 and slag 4 and the diameter of the gas outlet of the bottom-blowing gas system 13, so that the gas discharge pressure does not fall below the static pressure. The type of gas blown from the bottom-blowing gas system 13 is generally an inert gas, such as N2 or Ar.

[0071] Next, a method for refining molten iron according to a preferred embodiment of the present invention will be described in detail, including the background. That is, before explaining the technical matters included in the second step included in the method for refining molten iron according to this embodiment, a slag steam explosion confirmation test and a molten iron steam explosion confirmation test, which are the prerequisites for the second step, will be described in detail.

[0072] <Slag steam explosion confirmation test> Figure 2 is a graph showing the results of a slag steam explosion confirmation test in which a water-soaked cold iron source was poured into slag. That is, the slag steam explosion confirmation test shown in Figure 2 was conducted because conducting the test within the scope of the conforming example described in Patent Document 1 did not lead to stable suppression of steam explosions, and because there was a need to clearly and quantitatively indicate the conditions under which a steam explosion phenomenon occurs.

[0073] The slag steam explosion confirmation test was conducted using the following procedure. First, a CaO-SiO2-FetO ternary slag was melted in a small high-frequency melting furnace meeting the structural requirements shown in Figure 1, and the slag temperature was adjusted. Then, the power to the melting furnace was turned off, and a cold iron source immersed in water was simultaneously dropped into the molten slag to check for the presence or absence of a steam explosion. Next, a ladle made from a processed graphite crucible was used to scoop up carbon-saturated molten iron at 1200-1300°C, which had been melted in a separate furnace, and this was charged into the melting furnace on top of the slag.

[0074] The slag composition was 20-50 mass% CaO, 10-40 mass% SiO2, and 10-40 mass% FeO. The slag temperature Tf' was 1200-1700°C, and the ratio Wsc' / Wsl'(-) of the amount of cold iron charged Wsc' (kg) to the amount of melted slag Wsl' (kg) was in the range of 0.1-50. The cold iron source used was iron flakes with a length and width ranging from 3 to 15 mm, a thickness ranging from 3 to 7 mm, a carbon concentration of 30 mass ppm or less, and an oxygen concentration of 150 mass ppm or less. The iron flakes used as the cold iron source were immersed in room-temperature water for about 1 minute. The cold iron source was then lightly drained immediately before being placed in the melting furnace and subjected to a slag steam explosion confirmation test.

[0075] The results of a slag steam explosion confirmation test using slag and a water-wet cold iron source are shown in Figure 2. The symbols "●" in Figure 2 represent plots of conditions under which slag bumping and steam explosions did not occur, while the symbols "x" represent plots of conditions under which steam explosions did occur. As shown in Figure 2, the inventors discovered that the slag bumping region A, steam explosion region C, and regions B1 and B2 where slag bumping and steam explosions did not occur can be organized using the slag temperature Tf' and Wsc' / Wsl'(-), which is the ratio of the amount of cold iron source added Wsc' (kg) to the amount of remaining slag Wsl' (kg).

[0076] In the slag bumping region A where slag bumping occurs on the side where Wsc' / Wsl', which is the mass ratio of the amount of added cold iron source Wsc' (kg) to the amount of residual slag Wsl' (kg), is low, the slag does not bump immediately after the cold iron source is added or even after a while has passed since the cold iron source was added. The slag bumping region A is characterized by the occurrence of slag bumping due to the gas generated when carbon-saturated molten iron is charged. From this technical viewpoint, in the method for refining molten iron according to this embodiment, in order to avoid slag bumping region A shown in FIG. 2 where slag bumping occurs due to gas generated when carbon-saturated molten iron is charged into the reaction vessel, it is necessary to charge into the reaction vessel an amount of cold iron source sufficient to solidify all of the residual slag remaining in the reaction vessel after blowing.

[0077] Furthermore, as Wsc' / Wsl', the mass ratio of the amount of cold iron source Wsc' (kg) added to the amount of residual slag Wsl' (kg), increases, the slag bumping region A reaches region B1 where slag bumping and steam explosions no longer occur (hereinafter referred to as "slag bumping and steam explosion avoidance region B1"). Based on the graph shown in Figure 2, it was found that the boundary between slag bumping region A and slag bumping and steam explosion avoidance region B1 can be approximated, for example, by the following relational expression (1A): In the method for refining molten iron according to this embodiment, the relational expression (1A) may vary slightly depending on the refining conditions of the molten iron, and is not limited to a range that can indicate the boundary between the slag bumping region A and the slag bumping and steam explosion avoidance region B1. Wsc' / Wsl'= 5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321···(1A)

[0078] Furthermore, if the mass ratio Wsc' / Wsl' of the added cold iron source Wsc' (kg) to the residual slag Wsl' (kg) is increased beyond the value that can be calculated using relational expression (1A), immediately after the cold iron source is added, or shortly thereafter, a steam explosion region C is reached, in which a steam explosion occurs and slag is ejected above the melting furnace. Based on the graph shown in Figure 2, it was found that the boundary between slag bumping and steam explosion avoidance region B1 and steam explosion region C can be approximated, for example, by the following relational expression (1B): In the method for refining molten iron according to this embodiment, the relational expression (1B) may vary slightly depending on the refining conditions of the molten iron, and is not limited to a range that can indicate the boundary between the slag bumping and steam explosion avoidance region B1 and the steam explosion region C. Wsc' / Wsl'=8.64×10 -7 ×Tf 1.947 (1B)

[0079] Furthermore, as Wsc' / Wsl', the mass ratio of the amount of cold iron source Wsc' (kg) added to the amount of residual slag Wsl' (kg), increases, a region B2 is reached where steam explosions no longer occur (hereinafter referred to as "slag bumping and steam explosion avoidance region B2"). Based on the graph shown in Figure 2, it was found that the boundary between steam explosion region C and slag bumping and steam explosion avoidance region B2 can be approximated, for example, by the following relational expression (2). In the method for refining molten iron according to this embodiment, the relational expression (2) may vary slightly depending on the refining conditions of the molten iron, and is not limited to a range that can indicate the boundary between the steam explosion region C and the slag bumping and steam explosion avoidance region B2.

[0080] Wsc' / Wsl'≧6.591×10 -6 ×Tf 1.695 ···(2)

[0081] Furthermore, it was also found that the division into slag bumping region A, steam explosion region C, and slag bumping and steam explosion avoidance regions B1 and B2 does not depend on the slag composition, at least within the range of the tests conducted this time. Furthermore, it is not clear what theory determines the thresholds for slag bumping region A, steam explosion region C, and slag bumping and steam explosion avoidance regions B1 and B2.

[0082] The inventors believe that the slag bumping region A, where Wsc' / Wsl' is lower, is a region where the amount of cold iron source added is small compared to the amount of residual slag, so the molten slag does not solidify sufficiently, and the reaction between the C contained in the molten iron being charged and the FeO in the slag progresses rapidly.

[0083] In other words, it is estimated that the reaction between the C contained in the charged molten iron and the FeO in the slag, [C] + (FeO) = CO↑ + Fe, became a liquid-liquid reaction. Furthermore, in the steam explosion region C, where the mass ratio of the added cold iron source Wsc' (kg) to the residual slag Wsl' (kg) was higher (Wsc' / Wsl'), the slag appeared to be flying upwards with force. Based on this, it is estimated that the steam explosion occurred when the moisture in the scrap was suddenly vaporized by the high-temperature slag.

[0084] <Confirmation test for molten iron steam explosion> Figure 3 is a graph showing the results of a molten iron steam explosion confirmation test in which a water-wetted cold iron source was poured into molten iron. That is, the molten iron steam explosion confirmation test shown in Figure 3 was conducted to confirm whether a molten iron steam explosion occurred when a water-wetted cold iron source was poured into molten iron remaining in a reaction vessel. Here, not only residual slag but also molten iron remaining from pretreatment may be present in the reaction vessel. Therefore, in a molten iron steam explosion confirmation test, we confirmed whether a molten iron steam explosion would occur when a water-wet cold iron source was poured into the molten iron remaining in the reaction vessel. The results of the molten iron steam explosion confirmation test are shown in Figure 3. In Figure 3, the symbol "●" indicates plots under conditions where there was no steam explosion of molten iron, and the symbol "×" indicates plots under conditions where a steam explosion of molten iron occurred.

[0085] The molten iron steam explosion confirmation test was conducted according to the following procedure. First, molten iron was melted in a small-scale high-frequency melting furnace meeting the structural requirements shown in Figure 1, and its temperature was adjusted. Then, the power to the melting furnace was turned off, and a cold iron source immersed in water was simultaneously poured into the molten iron. A test was then conducted to confirm whether or not a steam explosion occurred in the molten iron. Here, the molten iron temperature Tf'' was 1200°C to 1700°C, and the ratio Wsc'' / Wrm''(-), which is the ratio of the amount of cold iron source Wsc'' (kg) poured in to the amount of molten iron Wrm'' (kg), was in the range of 0.1 to 3.0. The cold iron source used in the slag steam explosion confirmation test was the same as that used in the slag steam explosion confirmation test described above, and was subjected to the molten iron steam explosion confirmation test in the same manner.

[0086] As shown in Figure 3, the inventors discovered that, similar to slag, region D where molten iron will undergo a phreatic explosion and region E where molten iron will not undergo a phreatic explosion can be organized using the molten iron temperature Tf'' and Wsc'' / Wrm''(-), which is the ratio of the amount of cold iron source Wsc'' (kg) added to the amount of molten iron Wrm'' (kg).

[0087] In region D where molten iron undergoes a steam explosion (hereinafter referred to as "molten iron steam explosion region D"), a steam explosion in which molten iron shoots up was confirmed immediately after the cold iron source was added, or shortly thereafter. As is clear from Figure 3, the range of Wsc'' / Wrm'', which is the ratio of the amount of cold iron source Wsc'' (kg) to the amount of remaining molten iron Wrm'' (kg), is significantly narrower than those of slag bumping region A and steam explosion region C, which were revealed in the slag steam explosion confirmation test.

[0088] Furthermore, in actual converter operation, the amount of residual molten iron in the reaction vessel is about 1 ton, while the amount of residual slag exceeds 10 to 20 tonnes. Therefore, if the amount of cold iron source is equal to or greater than the lower limit that satisfies relational expression (1A) in Figure 2, the mass ratio Wsc'' / Wrm'' of the amount of cold iron source Wsc'' to the amount of residual molten iron Wrm'' will automatically be such that the molten iron steam explosion region D in Figure 3 can be avoided. Therefore, even if both residual slag and residual molten iron are present in the reaction vessel, there is no problem as long as the following relational expression (1)' or relational expression (2)', which defines the boundary line between the slag bumping and steam explosion avoidance regions B1 and B2, is satisfied.

[0089] Relational formula (1)' 5.224×10 -7 ×Tf' 2 -1.779×10 -4 ×Tf'-0.4321 <Wsc’ / Wsl’≦8.64×10 -7 ×Tf' 1.947 (1)'

[0090] Relational formula (2)' Wsc' / Wsl'≧6.591×10 -6 ×Tf' 1.695 (2)'

[0091] That is, in the second step included in the method for refining molten iron according to this embodiment, the relational expression (1) used to calculate the K value, which is an index for determining whether or not to set the waiting time T, from the relationship between the mass Wsl of the residual slag and the mass Wsc of the cold iron source charged through the scrap chute, is based on the slag steam explosion confirmation test and the molten iron steam explosion confirmation test. The relational expression (1) used to calculate the K value, which is an index for determining whether or not to set the waiting time T, defines the boundary line between the slag boiling and steam explosion avoidance region B1 and the steam explosion region C, which was revealed from the slag steam explosion confirmation test and the molten iron steam explosion confirmation test.

[0092] From such a technical viewpoint, the method for refining molten iron according to this embodiment uses the relational expression (1) to calculate the K value, which is an index for determining whether or not to set the waiting time T, when the cold iron source is charged into the reaction vessel in order to utilize the heat of the residual slag generated after the refining of the previous charge of molten iron is completed, and can avoid the slag bumping region A and the steam explosion region C.

[0093] As a result, the method for refining molten iron according to this embodiment can avoid slag bumping caused by gas generated when molten iron is charged into a reaction vessel, and can avoid steam explosions that occur when a cold iron source is charged into a reaction vessel.

[0094] In this way, the method for refining molten iron according to this embodiment allows the slag generated by refining the molten iron of the previous charge to remain in the reaction vessel, and optimizes the weight of the coolant used to solidify the slag.

[0095] Furthermore, in the method for refining molten iron according to this embodiment, the slag generated by the blowing of the previous charge of molten iron is left behind and solidified to form solidified slag. Therefore, even when the next charge of molten iron is charged, the oxygen in the decarburized slag does not react rapidly with the carbon in the molten iron, preventing the bumping of the molten iron.

[0096] Therefore, in the method for refining molten iron according to this embodiment, the K value is set to be less than 0 based on the K value calculated by the relational expression (1). Therefore, there is no need to set a predetermined waiting time T for evaporating the moisture contained in the cold iron source, and a steam explosion caused by the evaporation of the moisture due to a reaction between the moisture contained in the cold iron source and the slag does not occur.

[0097] In the method for refining molten iron according to the present embodiment, when the mass Wsc (kg) of the cold iron source introduced using the scrap chute is a part of the total amount Wt (kg) of introduced cold iron source, the remaining cold iron source may be introduced from an above-furnace hopper or the like after the cold iron source from the scrap chute is charged into the reaction vessel.

[0098] As described above, according to the method for refining molten iron in the first embodiment, even if a cold iron source is charged while slag generated by refining the previous charge of molten iron remains in the reaction vessel, no steam explosion occurs. Moreover, the method for refining molten iron in the first embodiment uses relational expression (1) to calculate the K value, which is an index for determining whether or not to set the waiting time T, and based on the calculated K value, it is possible to avoid the steam explosion region C without setting a waiting time. Furthermore, the method for refining molten iron according to the first embodiment does not require a waiting time for evaporating the moisture contained in the cold iron source, and therefore, it is possible to refining molten iron safely and stably without loss of production time by utilizing the preheating of the cold iron source.

[0099] Furthermore, in the method for refining molten iron according to this embodiment, the slag generated by refining the previous charge of molten iron is left in the reaction vessel and solidified, so that even when the next charge of molten iron is charged and the slag reacts with the moisture contained in the cold iron source, no phreatic explosion of moisture occurs. In other words, the method for refining molten iron according to this embodiment can significantly increase the amount of cold iron source used and greatly contribute to reducing CO2 gas emissions.

[0100] [Second embodiment] The method for refining molten iron according to the second embodiment is characterized in that, in the above-described embodiment, when the K value is less than 0, the waiting time is set to 60 seconds or more, and after the waiting time has elapsed, the remaining cold iron source is introduced from the scrap chute or the furnace. That is, the method for refining molten iron according to this embodiment has a technical feature in that, when it is assumed that a steam explosion will occur due to evaporation of the moisture contained in the cold iron source caused by a reaction between the moisture contained in the cold iron source and the residual slag based on the K value calculated by the above-mentioned relational expression (1), the charging of the cold iron source from the scrap chute into the reaction vessel is temporarily stopped, and a predetermined waiting time T is set for evaporation of the moisture contained in the cold iron source.

[0101] That is, the method for refining molten iron according to this embodiment is based on the premise that the cold iron source required to solidify the entire amount of residual slag is charged into the reaction vessel, and after the cold iron source is charged so that the K value calculated by the above relational expression (1) is less than 0, a predetermined waiting time T is set. In the method for refining molten iron according to this embodiment, after a predetermined waiting time T has elapsed, the moisture contained in the cold iron source is evaporated. Thereafter, in the method for refining molten iron according to the present embodiment, when the mass Wsc of the cold iron source charged using the scrap chute is a part of the total amount Wt of charged cold iron sources, the amount of cold iron source that is insufficient from the total amount Wt of charged cold iron sources can be charged from the scrap chute or from above the furnace as the remaining cold iron source.

[0102] Here, in the method for refining molten iron according to this embodiment, the total amount Wt of cold iron sources required to be charged is the sum of the mass Wsc of the cold iron sources charged using the scrap chute and the mass of the remaining cold iron sources charged from the scrap chute or furnace top hopper after a predetermined waiting time T has elapsed. The remaining cold iron sources are calculated by subtracting the mass Wsc of the cold iron sources charged from the scrap chute from the total amount Wt of cold iron sources charged or charged into the reaction vessel in the refining of the next charge of molten iron.

[0103] In this way, the method for refining molten iron according to this embodiment can avoid the occurrence of a steam explosion caused by the evaporation of moisture due to a reaction between the moisture contained in the cold iron source and the residual slag, and by charging the mass of the remaining cold iron source charged from the scrap chute or the furnace into the reaction vessel after the predetermined waiting time T has elapsed, the total amount Wt of cold iron source charge required for charging and refining the next charge of molten iron can be secured and charged into the reaction vessel. After the predetermined waiting time T has elapsed, the remaining cold iron source that can be charged from the scrap chute or the furnace hopper may be a cold iron source containing moisture, or may be a cold iron source that does not contain moisture, such as solid reduced iron.

[0104] In the second step of the method for refining molten iron according to this embodiment, the predetermined waiting time T for evaporating the water contained in the cold iron source must be long enough to prevent a steam explosion caused by the water vaporizing due to a reaction between the water contained in the cold iron source and the slag. The waiting time T is 60 seconds or longer. By setting the waiting time T to 60 seconds or more, it is possible to prevent the occurrence of a steam explosion even if the water contained in the cold iron source evaporates. The waiting time T may be 60 seconds or more, but from the viewpoint of stably refining molten iron utilizing the preheating of the cold iron source without loss of time, it is preferable that the waiting time T be 120 seconds or less. In the method for refining molten iron according to this embodiment, the waiting time T depends on the amount of moisture contained in the cold iron source and the temperature of the hot material remaining in the furnace after the slag is left behind, but does not depend on the compositional components of the slag.

[0105] As described above, according to the method for refining molten iron according to the second embodiment, the waiting time T is set so that a steam explosion can be prevented, which would otherwise occur if the moisture contained in the cold iron source reacts with the slag and evaporates. Therefore, even if the cold iron source is charged, no steam explosion occurs. Moreover, the method for refining molten iron according to the second embodiment makes it possible to refining molten iron safely and stably without loss of production time by preheating the cold iron source, by setting an appropriate waiting time. Furthermore, according to the method for refining molten iron according to the second embodiment, when the mass Wsc of the cold iron source charged using the scrap chute becomes part of the total amount Wt of the cold iron source after a predetermined waiting time T has elapsed, the amount of the cold iron source that is insufficient from the total amount Wt of the cold iron source can be charged or charged from the scrap chute or from the furnace as the remaining cold iron source.

[0106] [Third embodiment] The method for refining molten iron according to the third embodiment is characterized in that, in the method for refining molten iron according to the first or second embodiment, an additional waiting time T1 is provided immediately before the next charge of molten iron is charged into the reaction vessel when the third step is carried out. That is, the method for refining molten iron according to this embodiment can avoid a steam explosion caused by the remaining moisture even if the moisture brought into the reaction vessel by the cold iron source charged from a scrap chute, such as scrap, remains without evaporating when the next charge of molten iron is charged in the third step.

[0107] In the method for refining molten iron according to the above embodiment, even if the cold iron source is charged in the second step so as to avoid a steam explosion due to the moisture contained in the cold iron source, the moisture contained in the cold iron source may not evaporate sufficiently. If the moisture contained in the cold iron source charged from the scrap chute does not evaporate sufficiently in the second step, a steam explosion may occur due to the remaining moisture when the next charge of molten iron is charged into the reaction vessel in the third step.

[0108] From this technical viewpoint, the method for refining molten iron according to this embodiment includes an additional waiting time T1 immediately before charging the next charge of molten iron into the reaction vessel, in addition to the waiting time T. The waiting time T1 is preferably 30 to 150 seconds, depending on the type of cold iron source charged through the scrap chute. Setting the waiting time T1 to 30 seconds or more is preferable because it allows sufficient evaporation of the moisture contained in the cold iron source charged through the scrap chute. Setting the waiting time T1 to 150 seconds or less is preferable because it allows for safe and stable refining of molten iron without loss of production. Furthermore, the waiting time T1 may be set uniformly or may be determined based on the Wsc / Wsl ratio depending on the operating mode of the converter. Furthermore, in order to shorten the waiting time T1 required for dispersing and evaporating the moisture, it is desirable to perform furnace shaking after the charging of the cold iron source in the second step is completed.

[0109] [Other embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical scope of the present invention. [Example]

[0110] <Example 1> The mass of the slag remaining in the induction melting furnace after blowing was adjusted to 10 kg, with some or all of the slag remaining as residual slag. The composition of the slag remaining in the induction melting furnace was adjusted to a CaO concentration of 50 mass%, an SiO2 concentration of 22 mass%, and an FeO concentration of 28 mass%. The residual high-temperature material temperature in the furnace, Tf (°C), was defined as the temperature of the slag remaining in the induction melting furnace, and this slag temperature was set to 1400°C, taking into account the temperature of the residual molten iron remaining in the induction melting furnace after blowing the previous charge of molten iron.

[0111] Next, 10 kg of cold iron was prepared for charging into the induction melting furnace. The cold iron was made of iron flakes with a length and width of 3 to 15 mm, a thickness of 3 to 7 mm, a carbon concentration of 30 mass ppm or less, and an oxygen concentration of 150 ppm or less.

[0112] The K value was calculated using the relationship (1) using the amount of cold iron source Wsc (kg) of iron flakes after lightly draining the water, the amount of residual slag Wsl (kg), and the temperature of the residual hot material in the furnace Tf (°C). As a result, the K value calculated using relational expression (1) was -1.5. As the K value calculated in Example 1 was less than 0 (K<0), no particular waiting time T was set. It was also confirmed that in Example 1, even when drained iron flakes were charged into the residual slag in the induction melting furnace, no steam explosion occurred at all.

[0113] Furthermore, in Example 1, of the iron flakes that were the cold iron source to be charged into the induction melting furnace, dry iron flakes that had not been immersed in water were charged into the converter from a hopper above the furnace. That is, it was found that in Example 1, the heat of the slag remaining in the induction melting furnace can be used to preheat the cold iron source, and molten iron can be safely refined without steam explosions. Table 1 shows the results of refining molten iron in Example 1.

[0114] <Examples 2 to 8> Molten iron was refining in the same manner as in Example 1, except that the component composition of the slag remaining in the induction melting furnace, the slag temperature of the slag remaining in the induction melting furnace, which is the temperature of the hot material remaining in the furnace, and the amount of the cold iron source of iron flakes were changed. In Examples 2 to 8, the K value calculated by relational expression (1) was set to be less than 0 (K<0), so no particular waiting time T was set. It was also confirmed that in Examples 2 to 8, no steam explosion occurred when drained iron flakes, which were used as a cold iron source, were charged into the residual slag in the induction melting furnace. Table 1 shows the refining results of molten iron in Examples 2 to 8.

[0115] <Comparative Examples 1 to 4> Except for changing the chemical composition of the slag remaining in the induction melting furnace, the slag temperature of the slag remaining in the induction melting furnace, which is the temperature of the hot material remaining in the furnace, and the amount of cold iron source iron flakes immersed in water, molten iron was refining in the same manner as in Example 1. Here, in Comparative Examples 1 to 4, the K value calculated by the relational expression (1) was set to be 0 or more (K≧0). In Comparative Examples 1 to 4, it was confirmed that a steam explosion occurred when drained iron flakes, which were used as a cold iron source, were charged into the residual slag in the induction melting furnace. Table 1 shows the refining results of molten iron in Comparative Examples 1 to 4.

[0116] [Table 1]

[0117] According to Table 1, even if drained iron flakes are charged into the residual slag in an induction melting furnace, it is clear that steam explosions can be avoided by adjusting the amount of cold iron source Wsc (kg) of the drained iron flakes, the amount of residual slag Wsl (kg), and the temperature of the residual hot material in the furnace Tf (°C) so that the K value is less than 0 (K<0) according to the relational expression (1). On the other hand, it was revealed that even if drained iron flakes are charged into the residual slag in an induction melting furnace, if the amount of cold iron source Wsc (kg) of the drained iron flakes, the amount of residual slag Wsl (kg) of the residual slag, and the temperature of the residual hot material in the furnace Tf (°C) are adjusted so that the K value is 0 or greater (K ​​≥ 0) according to the relationship (1), steam explosions will occur.

[0118] <Examples 9 to 14> Molten iron was refining in the same manner as in Example 1, except that the composition of the slag remaining in the induction melting furnace, the slag temperature of the slag remaining in the induction melting furnace, which is the residual hot material temperature in the furnace Tf (°C), and the amount of cold iron source Wsc (kg) of iron flakes immersed in water were changed so that the K value calculated by relational formula (1) was 0 or more (K≧0). That is, in Examples 9 to 14, the K value calculated by relational formula (1) was set so that it was 0 or more (K≧0).

[0119] Here, from the results of refining molten iron in Examples 1 to 8 and Comparative Examples 1 to 4, it is clear that steam explosions occurred in Examples 9 to 14. Therefore, in Examples 9 to 14, the iron flakes immersed in water were divided into two groups, designated as cold iron source I and cold iron source II. These cold iron sources I and II were then separately charged into the slag. A waiting time T was set to allow the moisture contained in cold iron source I, the iron flakes to be charged first, to evaporate. After the set predetermined waiting time T had elapsed, cold iron source I, the iron flakes to be charged second, was charged. In Examples 9 to 14, the waiting time T was set to 60 seconds or more in order to thoroughly evaporate the water contained in the iron flakes and completely prevent the occurrence of steam explosions. Table 2 shows the refining results of molten iron in Examples 9 to 14.

[0120] <Comparative Examples 5 to 7> Molten iron was refining in the same manner as in Examples 9 to 14, except that the waiting time T for evaporating the moisture contained in the cold iron source I, which is the iron flakes charged first, was set to less than 60 seconds. Table 2 shows the refining results of molten iron in Comparative Examples 5 to 7.

[0121] [Table 2]

[0122] According to Table 2, even if drained iron flakes are charged into the residual slag in an induction melting furnace, by adjusting the amount of cold iron source Wsc (kg) of the drained iron flakes, the amount of residual slag Wsl (kg) of the residual slag, and the temperature of the residual hot material in the furnace Tf (°C) so that the K value is 0 or more (K ≥ 0) according to the relational expression (2), it has become clear that a steam explosion can be avoided by setting the waiting time T to 60 seconds or more.

[0123] On the other hand, it was revealed that even if the waiting time T was set to less than 60 seconds, the water contained in the iron flakes immersed in water could not be completely evaporated, and therefore steam explosions could not be avoided.

[0124] Furthermore, from the results of refining molten iron in Examples 9 to 14, it was found that the waiting time T was 60 seconds or more, regardless of the amount of residual slag or the amount of cold iron source iron flakes immersed in water.

[0125] <Examples 15-20> Next, we investigated steam explosions caused by contact between molten iron and the moisture contained in iron flakes (scrap), which are the cold iron source. In Examples 15 to 20, the composition of the slag remaining in the induction melting furnace, the slag temperature of the slag remaining in the induction melting furnace (Tf (°C)), which is the residual hot material temperature in the furnace, and the amount of cold iron source iron flakes immersed in water (Wsc (kg)) were changed, and the K value calculated using relational formula (1) was set to be 0 or greater (K≧0), and molten iron was refining in the same manner as in Example 1. That is, in Examples 15 to 20, as in Examples 9 to 14, the K value calculated using relational formula (1) was set to be 0 or greater (K≧0).

[0126] In Example 15, to prevent a steam explosion between the slag and the cold iron source, 13.0 kg of cold iron source I, which was iron flakes with a moisture content of 10.0 kg and sufficient to satisfy the relational expression (2), was charged into the induction melting furnace. Sixty seconds after charging the cold iron source I, 2.0 kg of cold iron source II, which was iron flakes that had been drained, was charged into the induction melting furnace. After charging the cold iron source II into the induction melting furnace, a waiting time T1 of 10 seconds elapsed. Then, 100 kg of 1400°C molten iron was charged into the induction melting furnace, and the presence or absence of a steam explosion was confirmed. Furthermore, in Example 15, the above-mentioned molten iron refining conditions were adopted, and the molten iron refining test was repeated 10 times, and the presence or absence of a steam explosion in each test was confirmed. After completing the 10 molten iron refining tests, the steam explosion probability was calculated from the number of confirmed steam explosions. The probability of a steam explosion occurring between the slag and the cold iron source and between the molten iron and the cold iron source was calculated by confirming the steam explosion occurring between the slag and the cold iron source and the steam explosion occurring between the molten iron and the cold iron source. In addition, in Examples 16 to 20, molten iron was refining in the same manner as in Example 15, except that the slag temperature of the slag remaining in the induction melting furnace, which was the residual high-temperature material temperature in the furnace Tf (°C), the amount of cold iron source I, which was iron flakes not containing moisture, and cold iron source II, which was iron flakes that had been drained, charged into the induction melting furnace, and the waiting time T1 set after the cold iron source II was charged into the induction melting furnace were changed. The probability of a steam explosion occurring between the slag and the cold iron source and the probability of a steam explosion occurring between the molten iron and the cold iron source were calculated. Table 3 shows the refining results of molten iron in Examples 15 to 20.

[0127] <Examples 21 to 26> Molten iron was refining in the same manner as in Examples 15 to 20, except that the waiting time T1 for evaporating the water contained in the cold iron source II, which is the iron flakes added second time, was set to 30 seconds or more. In Examples 21 to 26, the probability of a steam explosion occurring between the slag and the cold iron source and the probability of a steam explosion occurring between the molten iron and the cold iron source were also calculated. Table 3 shows the results of refining molten iron in Examples 21 to 26.

[0128] [Table 3]

[0129] According to Examples 15 to 20 in Table 3, the waiting time T1 immediately before charging the next charge of molten iron into the induction melting furnace was short, at 10 to 20 seconds, so the probability of a steam explosion occurring between the slag and the cold iron source was 0%, but it was found that the probability of a steam explosion occurring between the molten iron and the cold iron source was not 0%.

[0130] Furthermore, as is clear from Examples 21 to 26 in Table 3, when the waiting time T1 was set to 30 to 40 seconds, the probability of a steam explosion occurring between the slag and the cold iron source and the probability of a steam explosion occurring between the molten iron and the cold iron source could both be reduced to 0%.

[0131] From the above results, it was confirmed that by setting the waiting time T1 to 30 seconds or more, it is possible to avoid not only steam explosions caused by contact between the slag and the moisture contained in the iron flakes (scrap), but also steam explosions caused by contact between the molten iron in the next charge and the moisture contained in the iron flakes (scrap). In other words, it has become clear that according to the present invention, by setting the waiting time T1 to 30 seconds or more, a stable molten iron refining method can be provided in which no hydrogen explosions occur at all.

[0132] As described above, according to Examples 1 to 26 and Comparative Examples 1 to 7, the heat of the high-temperature slag remaining in the induction melting furnace can be utilized to preheat the cold iron source, and molten iron can be refining by utilizing the preheating of the cold iron source in a safe manner without causing a steam explosion. [Industrial Applicability]

[0133] INDUSTRIAL APPLICABILITY The method for refining molten iron according to the present invention is industrially useful because it can be applied to a blast furnace-converter process to reduce the molten iron blending ratio and contribute to reducing CO2 emissions. [Explanation of symbols]

[0134] 1 reaction vessel 2 Refractories 3 Steel tapping hole 4. Slug 5 cold iron source 6. Scrap Chute 7 Crane 8 Residual molten iron 9. Hot Metal Ladle 10 Molten Iron 11 Oxygen supply lance 12 Oxygen gas 13 Bottom blown gas system 14 Furnace hopper 15 Materials charged onto the reactor 15A Solid reduced iron

Claims

1. A method for refining molten iron, in which heat of slag generated after refining of a pre-charge of molten iron is completed is utilized to preheat a cold iron source, a first step of pouring the refined molten iron from the reaction vessel while leaving a part or all of the slag in the reaction vessel as residual slag; A second step in which the coolant for cooling the residual slag to solidify it is a cold iron source charged from a scrap chute and a remaining cold iron source charged from above the furnace; and a third step of charging a next charge of molten iron into the reaction vessel and refining the molten iron. In the second step, the K value calculated by the following relational expression (1) using the mass Wsl of the residual slag, the mass Wsc of the cold iron source charged from the scrap chute, and the temperature Tf of the residual hot material in the furnace in the first step is less than 0, A method for refining molten iron, characterized in that no waiting time T is provided after the cold iron source is charged. K = Wsc-8.64×10 －７ ×Tf １．９４７ ×Wsl・・・(1) In the relational expression (1), Wsc (kg) represents the mass of the cold iron source charged through the scrap chute, Wsl (kg) represents the mass of the residual slag, and Tf (°C) represents the temperature of the residual hot material in the furnace in the first step.

2. A method for refining molten iron, in which heat of slag generated after refining of a pre-charge of molten iron is completed is utilized to preheat a cold iron source, a first step of pouring the refined molten iron from the reaction vessel while leaving a part or all of the slag in the reaction vessel as residual slag; A second step in which a coolant for cooling the residual slag to solidify it is introduced from a scrap chute or from the furnace as a cold iron source I and a cold iron source II; and a third step of charging a next charge of molten iron into the reaction vessel and refining the molten iron. In the second step, the K1 value calculated by the following relational expression (1) using the mass Wsl of the residual slag, the mass Wsc1 of the cold iron source I charged from the scrap chute, and the temperature Tf of the residual high-temperature material in the furnace in the first step is less than 0, After the cold iron source I is charged, a waiting time T is set to 60 seconds or more, and after the waiting time T has elapsed, a cold iron source II is charged from the scrap chute or from above the furnace, and the K2 value calculated by the following relational expression (1a) using a mass Wsl of the residual slag, a mass Wsc1 of the cold iron source I charged from the scrap chute, a mass Wsc2 of the cold iron source II charged from the scrap chute or from above the furnace, and a temperature Tf of the residual hot material in the furnace in the first step is set to 0 or more. K1 = W｡c1-8.64×10 －７ ×Tf １．９４７ ×Wsl・・・(1) K2 = Wsc1+Wsc2-8.64×10-7×Tf 1.947×Wsl...(1a) In the relational expressions (1) and (1a), Wsc1 (kg) represents the mass of the cold iron source I, Wsc2 (kg) represents the mass of the cold iron source II, Wsl (kg) represents the mass of the residual slag, and Tf (°C) represents the temperature of the residual hot material in the furnace in the first step.

3. 3. The method for refining molten iron according to claim 1, wherein in the third step, a waiting time T1 is provided immediately before the next charge of molten iron is charged into the reaction vessel.

Images