How molten iron is refined

Abstract

A method for refining molten iron is provided that uses the heat of high-temperature slag remaining in a reaction vessel to preheat a cold iron source, preventing slag bumping and enabling stable operation in a safe state without loss of production. The method utilizes the heat of the slag generated after the refining of molten iron to preheat the cold iron source. The method includes a first step of pouring the molten iron refined with the residual slag remaining in the reaction vessel from the reaction vessel, a second step of charging the cold iron source from a scrap chute and a third step of charging the next charge of molten iron into the reaction vessel and refining the molten iron, wherein the mass Wsl of the residual slag, the mass Wsc of the cold iron source charged from the scrap chute, and the mass Who of the cold iron source charged from the furnace hopper in the second step satisfy predetermined conditions.

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 that can avoid the bumping phenomenon of molten iron even when the amount of cold iron material including a cold iron source used is significantly increased and the next charge of molten iron is charged while leaving slag in the converter. That is, the present invention relates to a method for refining molten iron that reduces greenhouse gas emissions by increasing the amount of cold iron used in a converter, and that, when refining molten iron, makes it possible to effectively reuse the slag that has already been generated after refining as slag.

[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 increased use of cold iron in the steelmaking process. By charging cold iron into a reactor in addition to the molten iron tapped from a blast furnace, 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) can be reduced. As a result, it is possible to reduce the amount of molten iron used per unit of crude steel produced.

[0004] In other words, in recent years, technological development to reduce CO2 gas emissions has been required to prevent global warming. Integrated steelworks extract iron from iron ore and use the extracted iron as a raw material to manufacture final products such as steel plates, steel pipes, steel sections, steel bars, and galvanized steel sheets. In conventional integrated steelworks, iron ore is reduced with a carbon source to produce molten iron. Producing this molten iron requires 500 kg of carbon source per ton of molten iron for the reduction of iron ore.

[0005] On the other hand, when molten steel is produced using a cold iron source such as iron scrap or solid reduced iron as a raw material, a carbon source, which is required for reducing iron ore, is not required. Therefore, when producing molten steel using a cold iron source as a raw material, only a sufficient amount of heat energy is required to melt the cold iron source. From this technical perspective, producing molten steel using a cold iron source as a raw material can significantly reduce CO2 gas emissions.

[0006] 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.

[0007] 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.

[0008] In the reactor, slag is added to the molten iron, and impurities are removed by the slag that forms. This slag reaches high temperatures of 1300-1400°C for dephosphorization and 1600-1700°C for decarburization, and contains a large amount of heat. Typically, the slag after these processes is solidified by adding a coolant such as dolomite and left in the reactor. The unsolidified 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.

[0009] Specifically, if the next charge of molten pig iron is charged while the molten decarburization slag remains in a reaction vessel such as a converter, the iron oxide in the decarburization slag will react rapidly with the carbon in the molten pig iron, causing the slag to bump. To solve this problem, various methods have been adopted, including waiting until the decarburization slag has solidified before charging the next charge of molten pig iron, or adding a coolant such as raw dolomite, CaCO3, iron ore, or scale to the decarburization slag to forcibly solidify it.

[0010] However, if an excessive amount of slag former is charged into the converter, the amount of slag generated in the converter increases. On the other hand, if the amount of coolant required for refining the next charge of molten pig iron is charged into the converter, the amount of decarburization slag that can be solidified is limited, and the decarburization slag that cannot be solidified must be discharged from the converter.

[0011] 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. The sensible heat of the slag generated during the refining of molten pig iron in a converter has not been effectively utilized until now. Therefore, by utilizing the sensible heat of the slag generated during the refining of molten pig iron to preheat the cold iron source, it is possible to increase the amount of cold iron source used in the converter without requiring a new heat source. Generally, molten pig iron produced in a blast furnace is decarburized and refined in a converter before being turned into steel. The decarburized slag generated during the refining of molten pig iron in this converter is not discharged from the converter when the molten steel is tapped, but is instead partially or entirely left in the converter and reused for refining the next charge of molten pig iron.

[0012] Theoretically, the phosphorus concentration of the decarburization slag after decarburization blowing, which finishes refining molten pig iron at a high temperature, is lower than the phosphorus concentration of the slag after pretreatment blowing, which finishes refining molten pig iron at a low temperature in a converter. Therefore, by using the decarburization slag after decarburization blowing in the dephosphorization blowing of molten pig iron and allowing the decarburization slag after decarburization blowing to contribute to the dephosphorization reaction of the phosphorus contained in the molten pig iron, the amount of slag former to be newly added to the converter can be reduced. That is, by leaving the decarburization slag generated after the decarburization blowing of the hot metal used in the immediately preceding charge in the converter in a hot state and performing pretreatment blowing of the hot metal to be used in the next charge, it is possible to expect a significant reduction in the amount of added slag formers and a significant increase in the amount of cold iron source used.

[0013] From this technical viewpoint, a method of adding a certain amount of deoxidizer (C, Si, Al) to decarburization slag has been proposed as a converter operation method that can avoid the occurrence of CO boiling when charging the next charge of molten pig iron while leaving the slag and molten steel of the previous charge (for example, Patent Document 1). That is, Patent Document 1 discloses a converter operation method in which two or more charges of molten pig iron after pre-treatment blowing are continuously blown without discharging slag, in which, after the tapping of the previous charge is completed, a predetermined amount or more of deoxidizer is added to and mixed with the slag in the converter, and then the main raw materials and slag former of the next charge are added and operation is continued.

[0014] Furthermore, several methods have been disclosed that can preheat a cold iron source using the residual slag left in the reaction vessel while preventing the slag from bumping. For example, Patent Document 2 discloses a technique in which steel is tapped while leaving the decarburized slag produced 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.

[0015] Furthermore, Patent Document 3 discloses a method in which a solid iron source is added to the residue (slag and molten iron) in a converter after steel is tapped, thereby solidifying the remaining molten iron.

[0016] The technology disclosed in Patent Document 2 recycles decarburization slag, while the technology disclosed in Patent Document 3 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]

[0017] [Patent Document 1] Japanese Patent Application Publication No. 4-52207 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-256839 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-228102 Summary of the Invention [Problem to be solved by the invention]

[0018] However, the above-mentioned conventional techniques have the following problems to be solved. The converter operation method described in Patent Document 1 has a major problem with the cost of deoxidizers, and is currently not widely used in practice. In the molten pig iron refining method described in Patent Document 2, excessive tilting of the refining vessel accelerates deterioration of the tilting mechanism of the refining vessel, leading to increased equipment maintenance costs. Furthermore, even if the decarburization slag generated in the molten pig iron refining method is reused in the decarburization blowing of the molten pig iron, it cannot be expected to contribute to the dephosphorization reaction, and therefore the effect of reducing the use of new slag formers cannot be expected.

[0019] Furthermore, in the molten iron refining method described in Patent Document 2, when scrap containing moisture is used as the cold iron source, there is a risk of a steam explosion occurring when the scrap comes into contact with high-temperature molten slag. Therefore, measures have been taken, such as removing the moisture contained in the scrap by, for example, heating the scrap containing moisture in advance with a burner. However, there is a problem in that the equipment investment and fuel costs for heating, which are required to remove moisture from the scrap containing moisture by heating it in advance with a burner, increase. That is, the techniques described in Patent Documents 2 and 3 have a problem in that they are unable to deal with the bumping of slag that can occur when a cold iron source containing moisture gets into the high-temperature slag in the reaction vessel due to the difference in specific gravity. Possible examples of a cold iron source containing moisture include water that gets into depressions in plate-shaped scrap, which can unavoidably occur during rainy weather, and wet powdery scrap.

[0020] 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.

[0021] Furthermore, the above-mentioned prior art also has problems in terms of preventing reactions with molten iron. The solid iron source charge amount Wsc in the formula shown in Patent Document 2 is a value determined based on the molten steel production plan, assuming that the production volume of molten steel is ensured, and is technically an uncontrolled parameter. This requires either reducing the amount of decarburized slag remaining Wsl or increasing the number of reciprocating tilts N. The former does not achieve the purpose of preheating the cold iron source because the total heat content of the slag is reduced, while the latter increases non-steelmaking time, reducing molten iron productivity.

[0022] In Patent Document 3, the solid iron source is charged to solidify the remaining molten iron in the reaction vessel, and the slag is discharged from the furnace in a molten state. Therefore, the method described in Patent Document 3 does not achieve the purpose of preheating the cold iron source with the remaining slag. Furthermore, Patent Document 3 describes that the solid iron source remains 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.

[0023] In light of these problems, we conducted extensive research and discovered that the steam explosion that causes slag bumping is 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 this rapid volume expansion of water. Here, when the moisture contained in the cold iron source evaporates upon contact with the high-temperature materials remaining in the furnace, the amount of moisture evaporated per unit amount of moisture depends mainly on the temperature of the high-temperature materials remaining in the furnace.

[0024] 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).

[0025] 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 from being brought into the reaction vessel due to the cold iron source when the surface temperature of the high-temperature materials remaining in the furnace, at which the temperature of the high-temperature slag is about 140°C to about 300°C, shortens the time it takes for moisture to evaporate, has been studied.

[0026] Specifically, when a cold iron source is added to a reaction vessel containing hot 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 of the hot slag depends on the ratio of the amount of cold iron source added to the amount of residual slag. Therefore, we believe that it is possible to specify conditions under which slag bumping due to a reaction between the residual slag and the water in the cold iron source does not occur, based on the value calculated from the ratio of the amount of residual slag in the reaction vessel (Wsl (kg)) to the amount of cold iron source added to the reaction vessel for preheating (Wsc (kg)), Wsc / Wsl (-), and the final temperature of the molten iron in the process with residual slag (Tf (°C)). We have found these conditions.

[0027] Furthermore, the inventors have found a condition for charging a predetermined amount of cold iron source into a reaction vessel using a scrap chute, and then charging a water-free furnace cold iron source, in accordance with a value calculated from a predetermined relational expression showing the relationship between the amount of slag remaining in the reaction vessel Wsl (kg), the amount of cold iron source Wsc (kg) to be charged into the reaction vessel using a scrap chute, and the final temperature Tf (°C) of the molten iron in the treatment with the slag remaining.

[0028] The present invention has been made to solve the above problems, and its purpose is to propose conditions under which the heat of the high-temperature slag remaining in the reaction vessel can be used to preheat the cold iron source in a safe and stable manner without bumping of the slag, without loss of production time. Another object of the present invention is to provide a method for refining molten iron, which can avoid the phenomenon of bumping of molten iron by rapidly reacting oxygen in the slag with carbon in the molten iron even when the next charge of molten iron is charged while slag is left in a reaction vessel such as a converter. Another object of the present invention is to provide a method for refining molten iron that utilizes the sensible heat of slag generated in the molten iron refining process to preheat the cold iron material, thereby achieving a significant reduction in the amount of heating materials and slag formers used, while also significantly increasing the amount of cold iron material, including the cold iron source. Another object of the present invention is to provide a method for refining molten iron that can avoid steam explosions even when the amount of cold iron material, including the cold iron source, used is significantly increased while leaving slag in the converter and the next charge of molten pig iron is charged. [Means for solving the problem]

[0029] In view of these problems, the inventors have conducted extensive research and have found that slag bumping does not occur during the refining of molten iron by setting the following conditions: the ratio of the mass of slag remaining in the reaction vessel to the total amount of cold iron source charged into the reaction vessel, the temperature of the hot material remaining in the furnace in the treatment in which slag remains, the mass of the cold iron source charged from the scrap chute, and the mass of the cold iron source charged from the top of the furnace.

[0030] 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 pouring molten iron, which has been refined while leaving some or all of the slag as residual slag in a reaction vessel, from the reaction vessel; a second step of charging a cold iron source from a scrap chute and a cold iron source to be added from above the furnace as coolants for cooling the residual slag to solidify it; and a third step of charging a next charge of molten iron into the reaction vessel and refining it, wherein in the second step, the mass Wsl of the residual slag, the mass Wsc of the cold iron source to be added from the scrap chute, and the mass Who of the cold iron source to be added from above the furnace satisfy any one of the following conditions (A) to (C): Condition (A): The total amount W of the cold iron source, which is the sum of the mass Wsc of the cold iron source charged from the scrap chute and the mass Who of the cold iron source charged from the furnace, is charged within the ranges of the following relational expressions (1) and (2): Condition (B): The mass Wsc of the cold iron source charged from the scrap chute is within the range of the following relational expression (2), the remainder required as a coolant is the cold iron source charged from the furnace afterwards or simultaneously, and the total mass W of the cold iron source, which is the sum of the mass Wsc of the cold iron source charged from the scrap chute and the mass Who of the cold iron source charged from the furnace, satisfies the following relational expression (1), Condition (C): The mass Who of the cold iron source charged from the furnace is set within the range of the following relational expression (3), and after the cold iron source is charged from the furnace, the remainder required as a coolant is charged as the cold iron source from the scrap chute. Relation (1): W / Wsl>5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321 Equation (2): W / Wsl≦8.64×10 -7 ×Tf 1.947 Equation (3): W / Wsl≧6.591×10 -6 ×Tf 1.695 In the above relational expressions (1) to (3), Wsl: mass of residual slag (t), W: Mass of cold iron source charged before hot metal charging (t), Tf: represents the temperature (°C) of the hot material remaining in the furnace in the first step.

[0031] The method for refining molten iron according to the present invention includes the steps of: (a) charging a cold iron source that is likely to contain moisture, including a powdered cold iron source and a cold iron source having voids, through a scrap chute; (b) the cold iron source charged in the second step contains reduced iron; (c) when the temperature Tf of the residual high-temperature material in the furnace is higher than the melting point of the reduced iron to be charged, the reduced iron is charged into the reactor in multiple batches from the furnace, and the amount of the reduced iron initially charged from the furnace into the reaction vessel, Who1(t), is 6.6 mass% or less with respect to the mass Wsl(t) of the residual slag in the reaction vessel; (d) the refining of the molten iron includes a pre-treatment blowing process for desiliconizing and dephosphorizing the molten iron of the next charge, an intermediate slag removal process for removing the slag remaining in the reaction vessel by tilting the reaction vessel, and a decarburization process for decarburizing and final dephosphorizing by adding auxiliary materials to the reaction vessel, (e) A more preferable means for solving the problem is that the refining of the molten iron includes a desiliconization blowing process for desiliconizing the molten iron of the next charge, an intermediate slag removal process for removing the slag remaining in the reaction vessel by tilting the reaction vessel, and a dephosphorization blowing process for dephosphorizing the molten iron by adding auxiliary materials to the reaction vessel. [Effects of the Invention]

[0032] 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 safely and stably without any loss of production time, without bumping of the slag. Even if the next charge of molten pig iron is charged while the decarburization slag remains in the converter, the iron oxide in the decarburization slag reacts rapidly with the carbon in the molten pig iron, preventing the bumping of the molten pig iron. Furthermore, the present invention overcomes the problems and disadvantages of the prior art. When the decarburization slag generated in a previous charge after refining is reused as a slag former in the next charge, the decarburization slag can be efficiently cooled and solidified without increasing the burden on the operation and equipment, thereby increasing the amount of recycled slag in the next charge, and enabling an increase in the amount of cold iron source used and a reduction in the amount of new slag former used. [Brief explanation of the drawings]

[0033] [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 bumping confirmation test in which a cold iron source wetted with water is added to slag. [Figure 3] 1 is a graph showing the results of a molten iron bumping confirmation test in which a cold iron source wetted with water is poured into molten iron. DETAILED DESCRIPTION OF THE INVENTION

[0034] 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 idea of ​​the present invention, and are not intended to limit the configuration to the following. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims. Next, a preferred embodiment of the present invention will be described in detail, including the background.

[0035] [First embodiment] The method for refining molten iron according to this embodiment utilizes the heat of slag generated after the completion of refining of a previous charge of molten iron to preheat a cold iron source. The method includes the following steps: a first step of pouring the refined molten iron from a reaction vessel while leaving some or all of the slag as residual slag in the reaction vessel; a second step of charging a cold iron source from a scrap chute and a cold iron source from above the furnace as coolants for cooling the residual slag to solidify it; and a third step of charging a next charge of molten iron into the reaction vessel and refining it. In the second step, the mass Wsl of the residual slag, the mass Wsc of the cold iron source charged from the scrap chute, and the mass Who of the cold iron source charged from above the furnace satisfy any one of the following conditions (A) to (C). Hereinafter, each step included in the method for refining molten iron according to this embodiment will be described.

[0036] Here, in the second step included in the method for refining molten iron according to this embodiment, the conditions (A) to (C) are: Condition (A): The total amount of cold iron sources W, which is the sum of the mass Wsc of the cold iron sources charged from the scrap chute and the mass Who of the cold iron sources charged from the furnace, is charged within the ranges of the following relational expressions (1) and (2). Condition (B): The mass Wsc of the cold iron source charged from the scrap chute is within the range of the following relational expression (2), the remainder of the cold iron source required as a coolant is the cold iron source charged from above the furnace after the cold iron source is charged from the scrap chute or at the same time as the cold iron source is charged from the scrap chute, and the total amount W of the cold iron source, which is the sum of the mass Wsc of the cold iron source charged from the scrap chute and the mass Who of the cold iron source charged from above the furnace, satisfies the following relational expression (1), Condition (C): The mass Who of the cold iron source charged from the furnace is set within the range of the following relational expression (3), and after the cold iron source is charged from the scrap chute, the remainder of the cold iron source required as a coolant is charged from the scrap chute. Furthermore, the relational expressions (1) to (3) employed in the conditions (A) to (C) included in the method for refining molten iron according to this embodiment are: Relationship (1): W / Wsl>5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321 Relational formula (2): W / Wsl≦8.64×10 -7 ×Tf 1.947 Relational formula (3): W / Wsl≧6.591×10 -6 ×Tf 1.695 In the above relational expressions (1), (2), and (3), Wsl is the mass of the residual slag (t), W is the mass of the cold iron source charged before the hot metal is charged (t), and Tf is the temperature of the residual hot material in the furnace in the first process (°C).

[0037] The method for refining molten iron according to this embodiment is carried out in a reaction vessel such as a converter-type refining 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.

[0038] (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.

[0039] 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.

[0040] 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. 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 so-called residual molten iron 8 remaining in the reaction vessel 1, which is refined molten iron produced after the refining of the pre-charge molten iron, which serves as pre-treatment, is completed.

[0041] 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.

[0042] 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 FeO concentration 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.

[0043] (Second process: The cold iron source is fed into the reaction vessel from the scrap chute and furnace top) 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 or from above the furnace, with some or all of the slag generated by refining the pre-charge molten iron remaining in the reaction vessel. In the second step, the cold iron source 5 may be charged into the reaction vessel 1 through the scrap chute 6 or 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.

[0044] The cold iron source 5 may be carbon steel scrap, pig iron scrap, or solid reduced iron 15A such as granular pig iron or reduced iron that can be wound up into the furnace hopper 14, and loaded into the scrap chute 6. 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. Here, the cold iron source 5 is a cold iron source that is prone to contain moisture and often contains a small amount of moisture. For example, the cold iron source 5 is powdery scrap or scrap with voids, such as steel turning scrap, pipes, motors, and press scrap, and these cold iron sources 5 are prone to contain moisture.

[0045] 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.

[0046] 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 charged from the scrap chute 6 or the cold iron source charged from the furnace, and becomes solidified slag.

[0047] 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 may be a cold iron source that is likely to contain moisture and that includes a powdery cold iron source and a cold iron source having voids. 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 is adopted as the cold iron source 5, it is desirable to charge the solid reduced iron 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.

[0048] 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, solid reduced iron with a large specific surface area may be used when charging the cold iron source 5 into the converter. 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, and the solidification of the residual slag, which is molten slag, can be rapidly promoted.

[0049] Furthermore, by using the cold iron source 5 used to cool the residual slag and solidify it into solidified slag, and by making the length of the solid reduced iron in the longitudinal direction 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 iron oxide in the slag 4 reacts rapidly with the carbon in the molten pig iron, thereby preventing the bumping of the molten pig iron.

[0050] 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 was measured using a reaction vessel 1 such as a heating furnace in the refining of general molten iron, the results showed that if the slag temperature of the slag was 1100°C or less, the residual slag would solidify and become solidified slag under any conditions. Furthermore, in the temperature range considered during decarburization refining of molten iron in general, it was found that even if the slag has a composition with variously changed basicity, iron oxide concentration, etc., it is possible to solidify all of the residual slag by setting the slag temperature to 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.

[0051] From this perspective, in the method for refining molten iron according to this embodiment, it has been found that 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 is sufficient to add a cold iron source 5 in an amount that ensures 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.

[0052] Therefore, in the second step, the cold iron source 5 is charged or added to the residual slag so that the mass Wsl of the residual slag, the mass Wsc(t) of the cold iron source charged from the scrap chute, the mass Who(t) of the cold iron source charged from the furnace top, and the temperature Tf (°C) of the residual hot material in the furnace satisfy any one of the following conditions (A) to (C).

[0053] That is, in the method for refining molten iron according to this embodiment, the cold iron source 5 is charged or added to the residual slag present in the reaction vessel 1 so that the mass Wsl(t) of the residual slag, the mass Wsc(t) of the cold iron source charged from the scrap chute, and the mass Who(t) of the cold iron source charged from the furnace satisfy any one of the following conditions (A) to (C): Here, in the second step, the sum of the mass Wsc of the cold iron source charged from the scrap chute 6 to the remaining slag in the reaction vessel 1 and the mass Who of the cold iron source charged from above the furnace is the total amount of the cold iron source charged before charging the molten iron, and is the mass W(t) of the cold iron source. Conditions (A) to (C) that must be met when charging or adding the cold iron source 5 into the reaction vessel 1 will be explained below.

[0054] <Cold iron source charging conditions (A)> Figure 2 is a graph showing the results of a slag bumping confirmation test in which a cold iron source was charged into slag. The slag bumping confirmation test uses slag and a cold iron source wet with water to check whether slag bumping occurs when the cold iron source is charged into the slag. Details of the slag bumping confirmation test will be described later.

[0055] As shown in Figure 2, it is clear that there are regions A and C where slag bumping occurs, and regions B1 and B2 where slag bumping does not occur. The symbols "●" in Figure 2 represent plots of conditions under which slag bumping did not occur, and the symbols "x" represent plots of conditions under which slag bumping did occur. That is, as shown in Figure 2, regions A and C where slag bumping occurs and regions B1 and B2 where slag bumping does not occur can be organized by the temperature of the remaining hot material in the furnace, Tf (°C), and W' / Wsl' (-), which is the ratio of the amount of cold iron source W' (kg) added to the amount of remaining slag Wsl' (kg).

[0056] Condition (A), defined based on the results of the slag bumping confirmation test shown in Figure 2, stipulates that the amount of cold iron source W, which is the sum of the mass of the cold iron source Wsc charged from the scrap chute and the mass of the cold iron source Who charged from the furnace, relative to the mass Wsl of the residual slag, be charged within the ranges defined by the following relational expressions (1) and (2). Here, relational expressions (1) and (2) are as follows: The introduction of relational expressions (1) and (2) will be described later. Relation (1): W / Wsl>5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321 Relational formula (2): W / Wsl≦8.64×10 -7 ×Tf 1.947

[0057] That is, the condition (A) defines the condition for charging the cold iron source 5 into the remaining slag present in the reaction vessel 1 within the range of the region B1 where slag bumping does not occur. According to condition (A), by charging the mass W of the cold iron source 5, which is the sum of the mass Wsc of the cold iron source 5 charged from the scrap chute 6 and the mass Who of the cold iron source 5 charged from the furnace, relative to the mass Wsl of the remaining slag in the reaction vessel 1, within the range of the following relational expressions (1) and (2), it is possible to avoid exceeding the region A where slag bumping occurs and entering the region C where slag bumping occurs.

[0058] Here, region A where slag bumping occurs is the range where slag bumping occurs due to gas generated by a reaction between iron oxide in the unsolidified slag and carbon in the molten iron when the next charge of molten iron 10 is charged. Region C where slag bumping occurs is the range where slag bumping occurs due to a steam explosion caused by the moisture contained in the cold iron source 5 immediately after or some time after the cold iron source 5 is charged through the scrap chute 6.

[0059] In this way, according to the condition (A) 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 the following from occurring: bumping of slag due to gas generated by a sudden reaction between iron oxide in the residual slag and carbon in the molten iron when the next charge of molten iron 10 is charged; and bumping of slag due to steam explosion caused by the moisture contained in the cold iron source 5.

[0060] In the method for refining molten iron according to this embodiment, the mass Wsl(t) 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] In this way, in the method for refining molten iron according to this embodiment, by selecting condition (A) in the second step and setting the weight of the coolant, such as the cold iron source 5 and solid reduced iron, used to completely solidify the slag 4 remaining in the reaction vessel 1, such as a converter-type refining furnace, the remaining slag can be completely solidified into solid slag.

[0062] As a result, the iron oxide contained in the residual slag, which is molten slag, does not react with the carbon contained in the next charge of molten pig iron charged into the converter-type refining furnace because the decarbonized slag solidifies to form solidified slag. That is, in the method for refining molten iron according to this embodiment, by selecting the condition (A) in the second step, the sensible heat energy of the residual slag remaining in the reaction vessel 1 can be absorbed into a coolant such as scrap or a cold iron source 5, thereby making effective use of the energy, without wasting it by discharging it.

[0063] In other words, the method for refining molten iron according to this embodiment solves the problems and disadvantages of the conventional technology by selecting condition (A) in the second step. When the decarburization slag generated in the previous charge after refining is reused as a slag former in the next charge, the decarburization slag can be efficiently cooled and solidified without increasing the operational load or the load on the equipment, thereby increasing the amount of recycled slag for the next charge, and also enabling an increase in the amount of cold iron source 5 used and a reduction in the amount of new slag former used. In the condition (A), the cold iron source 5 may be charged into the reaction vessel 1 through the scrap chute 6 or may be added from the furnace hopper 14. From the viewpoint of securing the total amount of cold iron source required for charging the next charge of molten iron 10 into the reaction vessel 1 and performing refining, it is preferable that the cold iron source 5 be charged into the reaction vessel 1 through the scrap chute 6.

[0064] <Cold iron source charging conditions (B)> Condition (B), defined based on the results of a slag bumping confirmation test in which the cold iron source shown in Figure 2 was charged into slag, specifies that the mass Wsc of the cold iron source 5 charged through the scrap chute 6 must be within the range of the following relational expression (2), the remainder of the required cold iron source 5 must be cold iron source charged from the furnace after the cold iron source 5 has been charged through the scrap chute 6 or simultaneously with the charging of the cold iron source 5 through the scrap chute 6, and the total mass W of the cold iron source, which is the sum of the mass Wsc of the cold iron source charged through the scrap chute and the mass Who of the cold iron source charged from the furnace, must satisfy the above relational expression (1). Here, relational expression (2) is as follows. The introduction of relational expression (2) will be described later. Relational formula (2): W / Wsl≦8.64×10 -7 ×Tf 1.947

[0065] That is, condition (B) specifies the condition that the cold iron source 5 is temporarily stopped from being charged from the scrap chute 6 so as to remain in an area that avoids the range of area C where slag bumping occurs, and then the cold iron source 5 is charged from the furnace hopper 14 to avoid area A where slag bumping occurs.

[0066] According to condition (B), by charging the mass W of the cold iron source 5, which is the sum of the mass Wsc of the cold iron source 5 charged from the scrap chute 6 and the mass Who of the cold iron source 5 charged from the furnace, relative to the mass Wsl of the remaining slag in the reaction vessel 1, within the range of the above-mentioned relational expression (2), it is possible to avoid region C, where slag bumping occurs due to steam explosion of the moisture contained in the cold iron source 5.

[0067] Here, the region C where slag bumping occurs is a range in which slag bumping occurs due to steam explosion caused by the moisture contained in the cold iron source 5 immediately after the cold iron source 5 is charged through the scrap chute 6 or after a while.

[0068] Thus, the condition (B) of the second step included in the method for refining molten iron according to this embodiment is a condition for preventing a phreatic explosion of the moisture contained in the cold iron source 5 after the cold iron source 5 is charged into the reaction vessel 1, and ultimately for avoiding a region of bumping due to unsolidified slag. The method for refining molten iron according to this embodiment focuses on the weight 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 left in the reaction vessel 1 as residual slag, the amount W of the cold iron source 5 charged into the reaction vessel 1, and the temperature Tf (°C) of the high-temperature residue in the furnace, including the residual molten iron 8 after the process for leaving the slag, and finds a condition for preventing a phreatic explosion of the moisture contained in the cold iron source 5 based on a value calculated by a predetermined relational expression using these values.

[0069] The condition for preventing a steam explosion of the moisture contained in the cold iron source 5, which is specified in the condition (B) of the second step, is based on the results of a slag bumping confirmation test described later and a molten iron bumping confirmation test in which a water-soaked cold iron source 5 is poured into molten iron 10. That is, the condition (B) of the second step included in the method for refining molten iron according to this embodiment focuses on the Leidenfrost effect, which occurs when the moisture contained in the cold iron source 5 comes into contact with high-temperature materials remaining in the furnace, such as slag, and is set based on detailed experiments described later to prevent moisture from being brought into the reaction vessel 1 from the cold iron source 5 when the surface temperature of the high-temperature materials remaining in the furnace is around 140 to 300°C, which shortens the evaporation time of the moisture from the high-temperature slag.

[0070] As described above, according to the condition (B) that can be selected in the second step included in the method for refining molten iron according to this embodiment, the cold iron sources 5 that are insufficient with respect to the mass W(t) of the cold iron sources 5, which is the total amount of the cold iron sources required before the next charge of molten iron is charged, can be supplemented by the cold iron sources 5 that are charged from the furnace, thereby making it possible to prevent bumping of slag that would otherwise occur due to a steam explosion caused by the moisture contained in the cold iron sources 5.

[0071] <Cold iron source charging conditions (C)> Condition (C), which is defined based on the results of a slag bumping confirmation test in which the cold iron source shown in Figure 2 was charged into the slag, defines that the mass of the cold iron source Who charged from the furnace must be within the range of the following relational expression (3), and that after the cold iron source is charged from the scrap chute, the remaining cold iron source required as a coolant must be charged from the scrap chute. Here, relational expression (3) is as follows. The introduction of relational expression (3) will be described later. Relational formula (3): W / Wsl≧6.591×10 -6 ×Tf 1.695

[0072] That is, condition (C) specifies the condition under which, when charging the cold iron source 5 into the reaction vessel 1, the cold iron source 5 is first charged from above the furnace to avoid region C where slag bumping occurs, and then the remainder required as a coolant is charged as the cold iron source 5 from the scrap chute 6. According to condition (C), by adding the mass Who of the cold iron source 5 from the furnace within the range of the above-mentioned relational expression (3) relative to the mass Wsl of the residual slag in the reaction vessel 1, it is possible to go beyond the region C where slag bumping occurs and reach the region B2 where slag bumping does not occur. In the second step, the cold iron source 5 charged into the reaction vessel 1 may contain reduced iron, and the cold iron source 5 charged from the furnace preferably contains reduced iron. The reduced iron may include solid reduced iron or powder reduced iron.

[0073] Here, the region B2 where slag bumping does not occur exceeds the region C where slag bumping occurs when a sufficient amount of cold iron source 5 is charged into the reaction vessel 1, and is a range where slag bumping does not occur due to steam explosion caused by the moisture contained in the cold iron source 5 immediately after the cold iron source 5 is charged from the scrap chute 6 or after a while.

[0074] Thus, according to the condition (C) that can be selected in the second step included in the method for refining molten iron according to this embodiment, after the cold iron sources 5 are charged from the furnace hopper 14, any shortage of the cold iron sources 5 relative to the mass W of the cold iron sources 5, which is the total amount of cold iron sources required before the next charge of molten iron is charged, can be made up for by the cold iron sources 5 charged from the scrap chute 6, thereby making it possible to prevent the bumping of slag caused by a steam explosion due to the moisture contained in the cold iron sources 5 from occurring. In the method for refining molten iron according to this embodiment, the conditions (A) to (C) that define the relationship between the mass Wsl of the residual slag, the mass Wsc of the cold iron source 5 charged from the scrap chute 6, and the mass Who of the cold iron source 5 introduced from the furnace can be selected as appropriate depending on the operating conditions of the converter in which the next charge of molten iron 10 is charged and refined.

[0075] In the second step, in order to solidify the slag 4 remaining in the reaction vessel 1 such as a converter-type refining furnace, it is preferable to use a cold iron source 5 from the viewpoint of significantly increasing the amount of cold iron source 5 used. Furthermore, in the second step, a coolant may be used in addition to the cold iron source 5 used to solidify the slag 4 remaining in the reaction vessel 1.

[0076] The coolant is not particularly limited as long as it can solidify the molten slag 4. Examples of coolants include dolomite, calcium carbonate, natural stone, and scale. Scrap may be iron scrap or the like. Dolomite may be raw dolomite or burnt dolomite, and may be a mineral or rock. Natural stone may be iron ore, chromium ore, manganese ore, or the like. Scale may be any oxide that forms on the surface of steel when the steel is exposed to air at high temperatures, and its composition is not particularly limited.

[0077] In the second step included in the method for refining molten iron according to this embodiment, it is preferable to use a large amount of cold iron source 5 to solidify the slag 4 remaining in the reaction vessel 1, from the viewpoint of reducing CO2 gas emissions. The amount of cold iron source 5 used to solidify the slag 4, which is the residual slag charged into the reaction vessel 1, is set in consideration of the relationship with the mass of the remaining slag, which is the slag 4 remaining in the reaction vessel 1 in the first step described above. The temperature of the cold iron source 5 used to solidify the slag 4 remaining in the converter-type refining furnace may be room temperature, and is usually 15 to 25°C.

[0078] (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 10. The third step included in the method for refining the molten iron 10 according to this embodiment includes desiliconization, dephosphorization, decarburization, and the like of the molten iron in the next charge.

[0079] Specifically, after the molten iron 10 is charged, 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 the molten iron 10 is stirred with stirring gas blown from a bottom blowing gas system 13. The process then proceeds to a refining step of the molten iron 10, in which impurities are oxidized and removed. Before or during the refining step of the molten iron 10, a material 15 to be charged onto the furnace may be charged from an upper hopper 14. The material 15 to be charged onto the furnace may be a slag-forming material such as lime or a coolant such as ore.

[0080] In this way, the method for refining molten iron according to this embodiment takes into consideration the relationship between the amount Wsc of the cold iron source 5 charged into the reaction vessel 1 from the scrap chute 6, the amount Wsl (kg) of the slag 4 left inside the reaction vessel 1, and the temperature Tf (°C) of the residual high-temperature material in the furnace in the treatment in which the slag 4 is left in the first step, and by using the value calculated from these data, it is possible to avoid bumping caused by steam explosion of the water contained in the cold iron source 5.

[0081] Therefore, in the method for refining molten iron according to this embodiment, an allowable amount of cold iron source 5 is charged into reaction vessel 1 in order to avoid steam explosion due to moisture contained in the cold iron source 5 charged into reaction vessel 1 from scrap chute 6. For this reason, the amount of cold iron source 5 charged from scrap chute 6 may be less than the amount of cold iron source W, which is the total amount of cold iron source required to refine the next charge of molten iron 10.

[0082] In this case, solid reduced iron 15A is charged into the reaction vessel 1 from the furnace hopper 14 at a timing after cold iron source 5 that is insufficient with respect to the total amount of cold iron source W is charged from the scrap chute 6. That is, the reaction vessel 1 is placed upright, and solid reduced iron 15A is charged from the furnace hopper 14 as the remainder of the cold iron source 5. The solid reduced iron 15A refers to a cold iron source, such as granulated pig iron or reduced iron, that is of a size that can be wound up by the furnace hopper 14.

[0083] 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.

[0084] 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 that constitutes 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.

[0085] 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, in the second step included in the method for refining molten iron according to this embodiment, in order to explain the technical matters used to define the conditions (A) to (C) that define the relationship between the mass Wsl of the residual slag, the mass Wsc of the cold iron source charged from the scrap chute, and the mass Who of the cold iron source charged from the furnace hopper, a slag bumping confirmation test and a molten iron bumping confirmation test that are the basis for these conditions will be described in detail.

[0086] <Slag bumping confirmation test> Figure 2 is a graph showing the results of a slag bumping confirmation test in which a water-soaked cold iron source was added to slag. That is, the slag bumping confirmation test shown in Figure 2 was conducted because the test conducted within the scope of the conforming example described in Patent Document 2 did not lead to stable suppression of slag bumping, and because there was a need to clearly and quantitatively show the conditions under which slag bumping occurs.

[0087] The slag bumping confirmation test was performed 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 slag bumping. Next, a ladle made from a processed graphite crucible was used to scoop up C-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.

[0088] 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 W' / Wsl' (-), the ratio of the amount of cold iron source charged W' (kg) to the amount of melted slag Wsl' (kg), was in the range of 0.1–50. The cold iron source was iron flakes with lengths and widths ranging from 3–15 mm, thicknesses ranging from 3–7 mm, and a carbon content of 30 mass ppm or less and an oxygen content of 150 mass ppm or less. The cold iron source was immersed in room-temperature water for approximately 1 minute. The cold iron source was then lightly drained immediately before being introduced into the melting furnace and subjected to a slag bumping test.

[0089] The results of a slag bumping confirmation test using slag and a water-wetted cold iron source are shown in Figure 2. The symbols "●" in Figure 2 represent plots of conditions under which slag bumping did not occur, and the symbols "x" represent plots of conditions under which slag bumping did occur. As shown in Figure 2, the inventors discovered that regions A and C where slag bumping occurred and regions B1 and B2 where slag bumping did not occur can be organized using the slag temperature Tf' and W' / Wsl'(-), which is the ratio of the amount of cold iron source W' (kg) added to the amount of remaining slag Wsl' (kg).

[0090] In region A (hereinafter referred to as "slag bumping region A") where slag bumping occurs on the side where W' / Wsl', the mass ratio of the amount of cold iron source W' (kg) added 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 C-saturated molten iron is charged.

[0091] Furthermore, as W' / Wsl', the mass ratio of the amount of cold iron source W' (kg) added to the amount of residual slag Wsl' (kg), increases, the slag bumping region A reaches region B1 where slag bumping no longer occurs (hereinafter referred to as "slag bumping 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 avoidance region B1 can be approximated by the following relational expression (1), for example. In the molten iron refining method according to this embodiment, the relational expression (1) may vary slightly depending on the molten iron refining conditions, and is not limited to a range that can indicate the boundary between the slag bumping region A and the slag bumping avoidance region B1. W' / Wsl'= 5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321···(1)

[0092] Furthermore, if the mass ratio W' / Wsl' of the added cold iron source W' (kg) to the residual slag Wsl' (kg) is increased beyond the value that can be calculated using relational expression (1), region C (hereinafter referred to as "slag bumping region C") will be reached immediately after the cold iron source is added, or shortly thereafter, where slag bumping occurs and the slag is blown out onto the melting furnace. Based on the graph shown in Figure 2, it has been found that the boundary between slag bumping avoidance region B1 and slag bumping region C can be approximated, for example, by the following relational expression (2). In the molten iron refining method according to this embodiment, the relational expression (2) may vary slightly depending on the molten iron refining conditions, and is not limited to a range that can indicate the boundary between the slag bumping avoidance region B1 and the slag bumping region C. W' / Wsl'=8.64×10 -7 ×Tf 1.947 ···(2)

[0093] Furthermore, as Wsc' / Wsl', the mass ratio of the amount of cold iron source W' (kg) added to the amount of residual slag Wsl' (kg), increases, a region B2 (hereinafter referred to as "slag bumping avoidance region B2") is reached where slag bumping no longer occurs. Based on the graph shown in Figure 2, it was found that the boundary between slag bumping region C and slag bumping avoidance region B2 can be approximated by the following relational expression (3), for example. In the molten iron refining method according to this embodiment, the relational expression (3) may vary slightly depending on the molten iron refining conditions, and is not limited to a range that can indicate the boundary between the slag bumping region C and the slag bumping avoidance region B2. Wsc' / Wsl'≧6.591×10 -6 ×Tf 1.695 ···(3)

[0094] Furthermore, it was found that the division into slag bumping regions A and C and slag bumping 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 regions A and C and slag bumping avoidance regions B1 and B2.

[0095] The inventors believe that the slag bumping region A, where W' / 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. 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 slag bumping region C, where the mass ratio of the added cold iron source Wsc' (kg) to the residual slag Wsl' (kg) is higher than W' / Wsl', the slag appeared to be bursting upwards with force. From this, it is estimated that the moisture in the scrap was rapidly vaporized by the high-temperature slag, causing a steam explosion.

[0096] <Molten iron bumping confirmation test> Fig. 3 is a graph showing the results of a molten iron bumping confirmation test in which a water-wetted cold iron source was poured into molten iron. That is, the molten iron bumping confirmation test shown in Fig. 3 was conducted to confirm whether or not molten iron bumping occurred when a water-wetted cold iron source was poured into molten iron remaining in a reaction vessel. In other words, not only residual slag but also residual molten iron, which is leftover steel from pretreatment, may be present in the reaction vessel. Therefore, in a molten iron bumping confirmation test, we checked whether or not molten iron bumping occurred when a water-wetted cold iron source was poured into the molten iron remaining in the reaction vessel. The results of the molten iron bumping confirmation test are shown in Figure 3. The symbol "●" in Figure 3 represents plots for conditions under which molten iron bumping did not occur, and the symbol "×" represents plots for conditions under which molten iron bumping occurred.

[0097] The molten iron bumping confirmation test was carried out according to the following procedure. First, molten iron was melted in the small-scale high-frequency melting furnace shown in Figure 1, and the temperature of the molten iron was adjusted. Then, the melting furnace was turned off, and a cold iron source immersed in water was added to the molten iron. A test was then carried out to confirm whether or not the molten iron had bumped. Here, the molten iron temperature Tf'' was 1200°C to 1700°C, and the ratio W'' / Wrm'' (-) of the amount of cold iron source added W'' (kg) 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 bumping confirmation test was the same as that used in the slag bumping confirmation test described above, and was subjected to the molten iron bumping confirmation test in the same manner.

[0098] As shown in FIG. 3, the inventors have found that, similar to slag, the region D where molten iron bumps and the region E where molten iron does not bump can be organized by the molten iron temperature Tf'' and the ratio W'' / Wrm''(-) of the amount of cold iron source W'' (kg) added to the amount of molten iron Wrm'' (kg).

[0099] In region D where molten iron bumps (hereinafter referred to as "molten iron bumping region D"), bumping of molten iron was observed immediately after the cold iron source was added or shortly thereafter. As is clear from Figure 3, the range of W'' / Wrm'' - the ratio of the cold iron source mass W'' (kg) to the remaining molten iron mass Wrm'' (kg) - is significantly narrower than in regions A and C of slag bumping, which were revealed in the slag bumping confirmation test.

[0100] Furthermore, in actual operation of a converter, 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 the relational expression (1) in Figure 2, the mass ratio W'' / Wrm'' of the amount of cold iron source to the amount of residual molten iron will automatically be such that the molten iron bumping 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)' that defines the boundary line between the slag bumping avoidance regions B1 and B2 is satisfied.

[0101] 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 Relation (2)': Wsc' / Wsl'≧6.591×10 -6 ×Tf' 1.695

[0102] That is, in the second step of the molten iron refining method according to this embodiment, the relationship between the mass of the residual slag Wsl, the mass of the cold iron source Wsc charged through the scrap chute, and the mass of the cold iron source Who charged through the furnace hopper, which is the total amount of the cold iron source, is defined by the relational expressions (1) to (3) based on the slag bumping confirmation test and the molten iron bumping confirmation test. The relational expressions (1) to (3) used to define the conditions (A) to (C) define the boundary between the slag bumping region A and the slag bumping avoidance region B1, the boundary between the slag bumping avoidance region B1 and the slag bumping avoidance region C, and the boundary between the slag bumping region C and the slag bumping avoidance region B2, which were revealed in the slag bumping confirmation test and the molten iron bumping confirmation test.

[0103] From such a technical viewpoint, the method for refining molten iron according to this embodiment avoids the slag bumping region A and the slag bumping region C based on the relational expressions (1) to (3) when charging the cold iron source 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 for preheating the cold iron source. 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.

[0104] In this way, the method for refining molten iron according to this embodiment allows the slag generated by the refining of the previous charge of molten iron to remain in the reaction vessel, and the weight of the cold iron source used to solidify the remaining slag can be optimized.

[0105] Furthermore, in the method for refining molten iron according to this embodiment, the slag generated by the blowing of the previous charge of molten pig iron is left behind and solidified to form solidified slag. Therefore, even when the next charge of molten pig iron is charged, the oxygen in the decarburized slag does not react rapidly with the carbon in the molten pig iron, preventing the molten pig iron from bumping.

[0106] Furthermore, in the method for refining molten iron according to this embodiment, the cold iron source can be charged into the residual slag generated in the reaction vessel by refining the previous charge of molten iron, and even if the moisture contained in the cold iron source reacts with the residual slag, no bumping of the slag occurs due to a steam explosion of the moisture. In other words, the method for refining molten iron according to this embodiment can significantly increase the amount of cold iron source used, and can greatly contribute to reducing CO2 gas emissions.

[0107] As described above, according to the method for refining molten iron in the first embodiment, even if a cold iron source is charged in a reaction vessel such as a converter-type refining furnace while slag generated by the blowing of a previous charge of molten pig iron remains as residual slag, the gas generated when the molten iron is charged does not cause slag bumping, and the phenomenon of slag bumping due to a steam explosion can be avoided.

[0108] [Second embodiment] The method for refining molten iron according to the second embodiment is characterized in that, in the above-described embodiment, when the temperature Tf (°C) of the residual high-temperature material in the furnace is higher than the melting point of the reduced iron to be charged in the second step, the reduced iron is charged into the reactor in multiple batches from above the furnace, and the amount Who1(t) of the reduced iron initially charged from the furnace to the reaction vessel is 6.6 mass % or less with respect to the amount Wsl(t) of residual slag remaining in the reaction vessel.

[0109] That is, in the method for refining molten iron according to the present embodiment, when a cold iron source containing reduced iron is adopted as the cold iron source to be charged into a reaction vessel, the reduced iron is charged from above the furnace, from the viewpoint of facilitating adjustment of the amount of the reduced iron to be charged into the reaction vessel. Furthermore, in the method for refining molten iron according to the present embodiment, the reduced iron is charged from above the furnace into the reaction vessel in multiple batches, and the amount of reduced iron to be initially charged from above the furnace is specified.

[0110] Here, the method for refining molten iron according to this embodiment is premised on the condition that the temperature Tf (°C) of the residual high-temperature materials in the furnace is higher than the melting point of the reduced iron charged in the second step. That is, the method for refining molten iron according to this embodiment is premised on the condition that in the first step of discharging molten iron, which has been refined with residual slag remaining in the reaction vessel, from the reaction vessel, the temperature Tf (°C) of the residual high-temperature materials in the furnace, including the residual slag and residual molten iron present in the reaction vessel, is higher than the melting point of the reduced iron from the furnace hopper, and the charged reduced iron melts. The temperature Tf (°C) of the high-temperature materials remaining in the furnace is in the range of 1300 to 1700°C, and the melting point of reduced iron is 1450°C.

[0111] That is, in the method for refining molten iron according to the present embodiment, if the molten iron refined in a state in which some or all of the slag generated by the refining of the molten iron of the previous charge remains in the reaction vessel as residual slag is tapped from the reaction vessel, and then a large amount of solid reduced iron is charged from above the furnace, the solid reduced iron may partially melt, causing the molten reduced iron to adhere to the bottom-blowing plug of the converter, which is the reaction vessel. If the molten reduced iron adheres to the bottom blowing plug of the converter in this way, the bottom blowing plug of the converter will close, causing a serious problem in the operation of the converter.

[0112] Therefore, the inventors have found that, in the method for refining molten iron according to the present embodiment, when the residual slag is solidified to form solidified slag, a cold iron source, which is large scrap, is first charged through the scrap chute, and then reduced iron is charged in multiple portions from above the furnace.

[0113] That is, in the method for refining molten iron according to this embodiment, solid reduced iron is charged into a reaction vessel from above the furnace in multiple batches, and the amount of solid reduced iron charged in the first (initial) batch, at which solid reduced iron is first charged, is specified to a predetermined amount so that the molten solid reduced iron does not adhere to the bottom-blowing plug of the converter in the early stage of converter operation. In the method for refining molten iron according to this embodiment, the amount of reduced iron Who1(t) initially charged into the reaction vessel from the furnace is 6.6 mass% or less with respect to the amount of residual slag Wsl(t) remaining in the reaction vessel.

[0114] It is preferable that the amount of reduced iron Who1(t) initially charged into the reaction vessel is 0.5 mass % or more and 6.6 mass % or less of the amount of residual slag Wsl(t) remaining in the reaction vessel, because the solid reduced iron itself, which is the reduced iron, partially melts and the melted solid reduced iron does not adhere to the bottom-blowing plug of the converter, which is the reaction vessel.

[0115] As described above, in the method for refining molten iron according to the present embodiment, solid reduced iron is charged from above the furnace, and the amount of reduced iron charged Who1(t) at the first (initial) charge of solid reduced iron, at which the charging of solid reduced iron is started, is limited to a predetermined amount, thereby making it possible to prevent the molten solid reduced iron from adhering to the bottom-blowing plug of the converter and causing the bottom-blowing plug to close.

[0116] As described above, according to the method for refining molten iron of the second embodiment, the slag that is generated during the refining of the pre-charge molten iron and remains in the reaction vessel is solidified, and bumping of the slag does not occur. Furthermore, the molten solid reduced iron is prevented from adhering to the bottom-blowing plug of the converter. This makes it possible to refining molten iron safely and stably without loss of production time by utilizing the preheating of the cold iron source.

[0117] [Third embodiment] The method for refining molten iron according to the third embodiment is characterized in that in the above-mentioned embodiment, the refining of the molten iron includes a preliminary blowing process for desiliconizing and dephosphorizing the molten iron of the next charge, an intermediate slag removal process for tilting the reaction vessel to remove the slag remaining in the reaction vessel, and a decarburization process for decarburizing and final dephosphorization by adding auxiliary materials to the reaction vessel.

[0118] That is, the method for refining molten iron according to this embodiment is an application of the "MURC method," which can continuously perform desiliconization, dephosphorization, slag removal, and decarburization in the same converter, to the method for refining molten iron according to the above-mentioned embodiment. The method for refining molten iron according to this embodiment, which continuously performs desiliconization, dephosphorization, slag removal, and decarburization in the same converter, can be applied even when the temperature Tf (°C) of the residual high-temperature materials in the reactor vessel, such as a converter-type refining furnace, is raised to around 1700°C.

[0119] Specifically, in the method for refining molten iron according to this embodiment, after dephosphorization of molten iron in a converter, the slag with a high phosphorus concentration is intermediately discarded, leaving only the molten iron that has been completely desiliconized and dephosphorized, and then a small amount of new lime is added to the slag to remove the remaining phosphorus and decarburize it. The molten iron refined by refining the previous charge of molten iron is then tapped out of the converter, and the remaining slag with a low phosphorus concentration can be reused to dephosphorize the next charge of molten pig iron.

[0120] In other words, the method for refining molten iron according to this embodiment can be said to be a converter operating method that can produce molten iron necessary to increase the purity of steel, while at the same time reducing the amount of slag discharged and realizing effective utilization of resources such as slag and cold iron source. Furthermore, the method for refining molten iron according to this embodiment involves treatment in a single reaction vessel such as a converter, and heat recovery is also possible by recycling the slag in a high-temperature state. This allows the amount of cold iron source that can be used to be increased to the same amount as in the refining of molten iron using a normal converter without preliminary treatment, thereby increasing productivity.

[0121] As described above, the method for refining molten iron according to this embodiment includes a pre-treatment blowing step for desiliconization and dephosphorization, an intermediate slag removal step for tilting the furnace to remove slag from the converter-type refining furnace, and a decarburization step for decarburization and finish dephosphorization by adding a CaO-based auxiliary material to the converter-type refining furnace and blowing it. Therefore, if the slag at the end of the decarburization refining is carried over to the pre-treatment blowing, it will contribute to the dephosphorization reaction, and it will also be possible to reduce the amount of new slag former added.

[0122] As described above, the method for refining molten iron according to this embodiment can meet the needs for producing high-grade steel and the needs for stable production of low-phosphorus, low-sulfur steel. Moreover, the method for refining molten iron according to this embodiment can prevent a decrease in the temperature at which molten iron is charged into a converter, and is not subject to restrictions on the amount of cold iron source used.

[0123] As described above, according to the method for refining molten iron of the third embodiment, the slag generated during the refining of the pre-charge molten iron and remaining in the reaction vessel is solidified, and the cold iron source charged from the scrap chute is charged into the reaction vessel without bumping of the slag, thereby enabling a series of converter operations. The method for refining molten iron of the third embodiment can meet the needs for producing high-grade steel and the needs for stable production of low-phosphorus, low-sulfur steel, and enables stable refining of molten iron utilizing preheating of the cold iron source without loss of production time.

[0124] [Fourth embodiment] The method for refining molten iron according to the fourth embodiment is characterized in that in the above-mentioned embodiments, the refining of the molten iron includes a desiliconization blowing process in which the molten iron of the next charge is desiliconized, an intermediate slag removal process in which the slag remaining in the reaction vessel is removed by tilting the reaction vessel, and a dephosphorization blowing process in which the molten iron is dephosphorized by adding auxiliary materials to the reaction vessel. That is, the method for refining molten iron according to this embodiment is an application of the "DRP method" to the method for refining molten iron according to the above embodiment, which makes maximum use of silicon contained in the molten iron as a heat source and can increase the amount of cold iron source input into the converter.

[0125] Specifically, in the method for refining molten iron according to this embodiment, molten iron is charged into a reaction vessel such as a converter without prior desiliconization treatment of the molten iron, and the blowing is interrupted after the molten iron is desiliconized by blowing the molten iron. After intermediate discharge of the slag with a high SiO2 content present in the furnace, the dephosphorization blowing is resumed. The method for refining molten iron according to this embodiment can be applied when the residual high temperature material temperature Tf (°C) in the reaction vessel such as a converter-type refining furnace is around 1400°C.

[0126] That is, the method for refining molten iron according to this embodiment can reduce the amount of lime required for dephosphorization while making the most of the heat generated by the oxidation of silicon in the molten iron, after intermediately discharging the slag containing high SiO2 from the furnace during the blowing of the molten iron containing high SiO2.

[0127] As described above, according to the method for refining molten iron in the fourth embodiment, the slag that is generated during the refining of the molten iron in the previous charge and remains in the reaction vessel is solidified, and the amount of lime required for dephosphorization can be reduced while making maximum use of the heat of oxidation of silicon in the molten iron without bumping of the slag, and this makes it possible to stably refining molten iron by utilizing the preheating of the cold iron source without loss of production time.

[0128] [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]

[0129] <Invention Example 1 (Example No. 6)> Using molten pig iron, pretreatment blowing, intermediate slag removal, and decarburization blowing were performed in a converter, and the resulting molten steel was tapped from the converter. The composition of the molten pig iron used in refining the pre-charge molten pig iron (component wt%) was C: 3.9-4.2, Si: 0.2-0.5, Mn: 0.20-0.30, P: 0.080-0.120, and S: 0.001-0.003. After decarburization blowing of this molten pig iron, the molten steel temperature before tapping was set to 1650°C. The weight of the decarburization slag remaining in the converter was adjusted to 10-20 t. In Example 1, the weight of the decarburization slag remaining in the converter was set to 15 t.

[0130] Thereafter, scrap was charged into the converter as a coolant through the scrap chute. In Example 1, 8 tons of scrap was charged into the converter through the scrap chute. Furthermore, with the converter body in an upright position, solid reduced iron was charged from the upper hopper. At this time, the amount of solid reduced iron charged from the upper hopper was varied. In Example 1, 3 tons of solid reduced iron was charged from the upper hopper.

[0131] The same molten iron as the previous charge was charged into a converter as the next charge of hot metal, and pre-blowing was performed. The hot metal blending ratio was set within a predetermined range. In Example 1, 289 tons of molten iron was charged into the converter, and the hot metal blending ratio was set to 96.3%. During pre-blowing, consideration was given to silicon oxide produced by the oxidation of silicon contained in the hot metal, calcium oxide contained in the auxiliary materials added during the pre-blowing, and silicon oxide and calcium oxide contained in the slag generated by refining the hot metal of the previous charge and remaining in the next charge. Then, new burnt lime was added from the hopper above the furnace so that the basicity (CaO / SiO2) of the slag after pretreatment blowing was 1.2 to 2.0.

[0132] The temperature of the hot metal at the end of pretreatment blowing was adjusted to 1300°C to 1400°C. Furthermore, the supply rate of oxygen gas and the amount of iron ore charged were adjusted so that the carbon concentration of the hot metal at the end of pretreatment blowing was 2.5 wt% to 3.5 wt%. Here, if there was a concern that the molten pig iron temperature at the end of the pretreatment blowing would not be maintained at 1300°C to 1400°C, carbonaceous material was added to the converter as a heating agent. After the pretreatment blowing was completed, the converter body was tilted with the molten pig iron remaining in the converter, and the slag generated by the pretreatment blowing was discharged outside the converter. The weight of the discharged slag was measured using a weighing scale installed in the slag pan.

[0133] In addition, a portion of the molten pig iron obtained after the pretreatment blowing was taken as a metal sample and analyzed using an analyzer installed on the machine. After intermediate slag removal, the furnace was then turned upright and the molten pig iron obtained after the pretreatment blowing was subjected to decarburization blowing. Taking into account the amount of phosphorus contained in the slag remaining in the converter after intermediate slag removal and the phosphorus concentration in the molten pig iron remaining in the converter after the pretreatment blowing, the amount of newly added burnt lime and other slag formers was adjusted so that the phosphorus concentration in the molten steel at the end of the decarburization blowing would be the set target value. After the decarburization blowing, the molten steel in the converter was tapped into a ladle. Furthermore, when the next charge of molten pig iron was charged into the converter, the presence or absence of bumping and the presence or absence of bumping due to steam explosion were determined by visual inspection. The results of Example 1 are shown in Tables 1 and 2.

[0134] [Table 1]

[0135] [Table 2]

[0136] <Invention Examples 2 to 8 (Examples Nos. 7 to 13)> The next charge of hot metal was refined in the same manner as in Example 1, except that the weight of the decarburization slag left in the converter, the weight of the scrap charged from the scrap chute, the weight of the solid reduced iron charged from the hopper above the converter, the weight of the molten iron charged into the converter, and the hot metal blending ratio were changed. The results of Examples 2 to 8 are shown in Tables 1 and 2.

[0137] <Comparative Examples 1 to 8 (Examples Nos. 1 to 5, 14 to 16)> In Comparative Example 1 (Example No. 1), the cold iron source and hot metal were charged without leaving any pre-charge slag, and then pre-treatment blowing, intermediate slag removal, and decarburization blowing were performed, followed by tapping. Naturally, bumping did not occur during hot metal charging. To achieve the target in-furnace hot metal temperature after pre-treatment blowing, carbonaceous material was added during pre-treatment blowing to heat the hot metal. Furthermore, burnt lime was added from the furnace hopper to adjust the basicity to the target value.

[0138] In Comparative Examples 2 to 3 (Examples Nos. 2 to 3), with the pre-charge slag remaining in the furnace, a cold iron source was introduced through the scrap chute, and then molten pig iron was charged into the converter, followed by pre-treatment blowing, intermediate slag removal, and decarburization blowing. In addition, in Comparative Examples 4 to 8 (Examples Nos. 4 to 5 and 14 to 16), with the pre-charge slag remaining in the furnace, a cold iron source was introduced into the converter through the scrap chute, solid reduced iron was introduced from the furnace upper hopper, and then molten pig iron was charged into the converter, and preliminary treatment blowing, intermediate slag removal, and decarburization blowing were performed.

[0139] As a result, as the amount of residual slag in the pre-charge increased, the amount of new lime added in the pre-treatment blowing became lower than in Comparative Example 1 (Example No. 1), and the phosphorus concentration after the pre-treatment blowing became equivalent (Examples Nos. 2 to 16).

[0140] In Comparative Examples 2 to 5, when the hot metal blending ratio was as high as about 97%, that is, when the amount of cold iron source charged was low compared to the amount of slag remaining in the pre-charge, bumping occurred during hot metal charging, posing a safety problem. This is thought to be because the slag remaining in the furnace did not completely solidify and reacted with the carbon in the hot metal, resulting in the sudden generation of CO gas (Examples Nos. 2 to 5).

[0141] When a sufficient amount of cold iron source was charged relative to the amount of remaining slag in the pre-charge, i.e., when the relational expression (1) was satisfied, bumping was not observed during molten iron charging. This is thought to be because the slag remaining in the furnace was completely solidified by charging a sufficient amount of cold iron source (Examples Nos. 6 to 13 corresponding to Invention Examples 1 to 8). As the hot metal blending ratio decreases, it is necessary to compensate for the heat of dissolution of the cold iron source, so it is necessary to charge carbonaceous material for heating in the pre-treatment blowing. However, at the level where pre-charge slag was left, the amount of carbonaceous material needed for heating was lower than that in Comparative Example 1 (Example No. 1). This is thought to be because the charged cold iron source was heated by the slag remaining in the furnace (Examples Nos. 7 to 13 corresponding to Invention Examples 2 to 8).

[0142] On the other hand, in Comparative Examples 6 to 8, when the hot metal blending ratio was low at approximately 82%, that is, when the amount of cold iron source charged was high relative to the amount of remaining slag in the pre-charge furnace, a steam explosion due to the moisture contained in the cold iron source was observed when the cold iron source was charged into the furnace, posing a safety problem. This is thought to have occurred when a large amount of cold iron source was charged into the furnace through the scrap chute and reached the area where a steam explosion would occur (Examples Nos. 14 to 16).

[0143] <Invention Example 9 (Example No. 20)> The hot metal was subjected to desiliconization blowing, intermediate slag removal, and dephosphorization blowing in a converter, and the resulting molten steel was tapped from the converter. The composition of the hot metal used in refining the pre-charge hot metal (component wt%) was C: 3.9-4.2, Si: 0.2-0.5, Mn: 0.20-0.30, P: 0.080-0.120, and S: 0.001-0.003. After dephosphorization blowing, the temperature of the resulting molten steel before tapping was set to 1380°C. The weight of the dephosphorization slag remaining in the converter was adjusted to 20 t.

[0144] Thereafter, scrap was charged into the converter as a coolant through the scrap chute. In Example 9, 4 tons of scrap was charged into the converter through the scrap chute. Furthermore, with the converter body in an upright position, solid reduced iron was charged from the upper hopper. At this time, the amount of solid reduced iron charged from the upper hopper was varied. In Example 9, 4 tons of solid reduced iron was charged from the upper hopper.

[0145] The same molten iron as the previous charge was charged into a converter as the next charge of molten iron, and desiliconization blowing was performed. The molten iron blending ratio was set within a predetermined range. In Example 9, 292 tons of molten iron was charged into the converter, and the molten iron blending ratio was set to 97.3%. During desiliconization blowing, consideration was given to silicon oxide produced by the oxidation of silicon contained in the molten iron, calcium oxide contained in the auxiliary materials added during the desiliconization blowing, and silicon oxide and calcium oxide contained in the slag generated during the refining of the previous charge of molten iron and left in the next charge. Then, new burnt lime was added from the hopper above the furnace so that the basicity (CaO / SiO2) of the slag after desiliconization blowing was 1.2 to 2.0.

[0146] The temperature of the hot metal at the end of the desiliconization blowing was adjusted to 1300-1400°C. If there was concern that the hot metal temperature at the end of the desiliconization blowing would not be maintained at 1300-1400°C, carbonaceous material was added to the converter as a heating agent. After the desiliconization blowing was completed, the converter body was tilted with the hot metal remaining in the converter, and the slag generated by the desiliconization blowing was discharged outside the converter. The weight of the discharged slag was measured using a weighing scale installed in the slag pan.

[0147] After intermediate slag removal, the converter was turned upright and the hot metal obtained after desiliconization was blown to remove phosphorus. Taking into account the amount of phosphorus contained in the slag remaining in the converter after intermediate slag removal and the concentration of phosphorus contained in the hot metal remaining in the converter after desiliconization, the amount of newly added burnt lime and other slag formers was adjusted so that the phosphorus concentration in the molten steel at the end of desiliconization would reach the target value. After desiliconization, the molten steel in the converter was tapped into a ladle. Furthermore, when the next charge of molten pig iron was charged into the converter, the presence or absence of bumping and the presence or absence of bumping due to steam explosion were determined by visual inspection. The results of Example 9 are shown in Tables 3 and 4. In Table 3, solid reduced iron I (above furnace) represents the weight of solid reduced iron initially charged from the furnace hopper before the cold iron source is charged from the scrap chute, and solid reduced iron II (above furnace) represents the weight of solid reduced iron charged from the furnace hopper after the scrap is charged from the scrap chute.

[0148] [Table 3]

[0149] [Table 4]

[0150] <Invention Examples 10 to 14 (Examples Nos. 21 to 22, 25 to 27)> The next charge of hot metal was refined in the same manner as in Example 9, except that the weight of the dephosphorization slag to be left in the converter, the weight of the scrap charged from the scrap chute, the weight of the solid reduced iron charged from the furnace top hopper, the weight of the molten iron charged into the converter, and the hot metal blending ratio were changed. In Examples 12 to 14 (Examples 25 to 27), a large amount of solid reduced iron was first charged from the furnace hopper, and then scrap was charged from the scrap chute to refine molten iron. The results of Examples 10 to 14 are shown in Tables 3 and 4.

[0151] <Comparative Examples 9 to 13 (Examples Nos. 17 to 19, and 23 to 24)> In Comparative Example 9 (Example No. 17), the cold iron source and hot metal were charged without leaving any pre-charge slag, and desiliconization blowing, intermediate slag removal, and dephosphorization blowing were performed, followed by tapping. Naturally, bumping did not occur during hot metal charging. To achieve the target in-furnace hot metal temperature after desiliconization blowing, a carbonaceous material was added during desiliconization blowing. Furthermore, burnt lime was added from the furnace hopper to adjust the hot metal basicity to the target value.

[0152] In Comparative Example 10 (Example No. 18), with 20 (t) of pre-charge slag remaining in the furnace, a cold iron source was introduced through the scrap chute, and then molten pig iron was charged into the converter, followed by desiliconization blowing, intermediate slag removal, and dephosphorization blowing. In Comparative Examples 11 to 13 (Examples Nos. 19, 23, and 24), the cold iron source was introduced into the converter through the scrap chute while the pre-charge slag was left in the furnace. After introducing solid reduced iron from the furnace hopper, the molten pig iron was charged into the converter, and desiliconization blowing, intermediate slag removal, and dephosphorization blowing were performed.

[0153] As a result, as the amount of residual slag in the previous charge increased, the amount of new lime added in desiliconization blowing became lower than in Comparative Example 9 (Example No. 17), and the phosphorus concentration after dephosphorization blowing became equivalent (Examples Nos. 18 to 27).

[0154] In Comparative Examples 10 and 11, when the hot metal blending ratio was as high as about 99%, that is, when the amount of cold iron source charged was low relative to the amount of slag remaining in the pre-charge, bumping occurred during hot metal charging, posing a safety problem. This is thought to be because the slag remaining in the furnace did not completely solidify and reacted with the carbon in the hot metal, resulting in the sudden generation of CO gas (Examples 18 and 19).

[0155] When a sufficient amount of cold iron source was charged relative to the amount of remaining slag in the pre-charge, the relational expression (1) was satisfied, and no bumping phenomenon was observed during the charging of molten iron. This is thought to be because the slag remaining in the furnace was completely solidified by charging a sufficient amount of cold iron source (Examples Nos. 20-22 and 25-27, corresponding to Examples 9-14).

[0156] As the hot metal blending ratio decreases, it is necessary to compensate for the heat of dissolution of the cold iron source, so it is necessary to charge carbonaceous material for heating during desiliconization blowing. However, at the level where pre-charge slag was left, the amount of carbonaceous material needed for heating was lower than in Comparative Example 9 (Example No. 17). This is thought to be because the charged cold iron source was heated by the slag remaining in the furnace (Examples Nos. 20 to 22 and 25 to 27 corresponding to Inventive Examples 9 to 14).

[0157] On the other hand, in Comparative Examples 12 and 13 (Examples Nos. 23 and 24), when the hot metal blending ratio was low at approximately 87%, that is, when the amount of cold iron source charged was high relative to the amount of remaining slag in the pre-charge furnace, a steam explosion due to the moisture contained in the cold iron source was observed when the cold iron source was charged into the furnace, posing a safety problem. This is thought to have occurred when a large amount of cold iron source was charged into the furnace through the scrap chute and reached the area where a steam explosion would occur (Examples Nos. 23 and 24).

[0158] As described above, according to Examples 1 to 14 of the invention and Comparative Examples 1 to 13, the heat of the high-temperature slag remaining in the converter can be utilized to preheat the cold iron source, and molten iron can be refining safely by preheating the cold iron source without the occurrence of slag bumping or steam explosion. [Industrial Applicability]

[0159] 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]

[0160] 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 completion of refining of a pre-charge of molten iron is utilized to preheat a cold iron source, the method comprising: a first step of pouring the refined molten iron from a reaction vessel while leaving a part or all of the slag as residual slag in the reaction vessel; 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 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 mass Wsl of the residual slag, the mass Wsc of the cold iron source charged from the scrap chute, and the mass Who of the cold iron source charged from above the furnace satisfy any one of the following conditions (A) to (C): Condition (A): The total amount W of the cold iron source, which is the sum of the mass Wsc of the cold iron source charged from the scrap chute and the mass Who of the cold iron source charged from above the furnace, is charged within the ranges of the following relational expressions (1) and (2): Condition (B): The mass Wsc of the cold iron source charged from the scrap chute is within the range of the following relational expression (2), the remainder required as a coolant is the cold iron source charged from the furnace afterwards or simultaneously, and the total amount W of the cold iron source, which is the sum of the mass Wsc of the cold iron source charged from the scrap chute and the mass Who of the cold iron source charged from the furnace, satisfies the following relational expression (1), Condition (C): The mass Who of the cold iron source charged from above the furnace is set within the range of the following relational expression (3), and after the cold iron source is charged from above the furnace, the remainder required as a coolant is charged as the cold iron source from a scrap chute. Relational formula (1): W / Wsl>5.224×10 －７ ×Tf ２ -1.779×10 －４ ×Tf-0.4321 Relational formula (2): W / Wsl≦8.64×10 －７ ×Tf １．９４７ Relational formula (3): W / Wsl≧6.591×10 －６ ×Tf １．６９５ In the above relational expressions (1) to (3), Wsl: mass of residual slag (t), W: Mass (t) of cold iron source charged before molten iron charging, Tf: represents the temperature (°C) of the hot material remaining in the furnace in the first step.

2. 2. The method for refining molten iron according to claim 1, wherein the cold iron source likely to contain moisture, which includes a powdery cold iron source and a cold iron source having voids, is charged through a scrap chute.

3. 2. The method for refining molten iron according to claim 1, wherein the cold iron source charged in the second step includes reduced iron.

4. 4. The method for refining molten iron according to claim 3, wherein, when the temperature Tf of the residual high-temperature materials in the furnace is higher than the melting point of the reduced iron to be charged, the reduced iron is charged into the reaction vessel in multiple batches from above the furnace, and an amount Who1(t) of the reduced iron initially charged into the reaction vessel from above the furnace is 6.6 mass% or less with respect to a mass Wsl(t) of the residual slag in the reaction vessel.

5. 5. The method for refining molten iron according to claim 1, wherein the refining of the molten iron comprises a pre-treatment blowing process for desiliconizing and dephosphorizing the molten iron of the next charge, an intermediate slag removal process for tilting the reaction vessel to remove the slag remaining in the reaction vessel, and a decarburization treatment process for decarburizing and final dephosphorizing by adding auxiliary materials to the reaction vessel.

6. 5. The method for refining molten iron according to claim 1, wherein the refining of the molten iron comprises a desiliconization blowing process for desiliconizing the molten iron of the next charge, an intermediate slag removal process for tilting the reaction vessel to remove the slag remaining in the reaction vessel, and a dephosphorization blowing process for dephosphorizing the molten iron by adding auxiliary materials to the reaction vessel.

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