Method for producing low-phosphorus molten iron

Abstract

To provide a method for producing low-phosphorus molten iron with high efficiency when using a specified solid iron source containing SiO2, Al2O3 or the like as a raw material in an electric furnace.SOLUTION: A method for producing low-phosphorus molten iron includes: a first step in which an iron source is charged into a furnace and melted, and slag in the furnace is discharged; a second step in which a secondary raw material containing CaO is charged into the furnace and the remaining solid iron source is additionally charged into the furnace; and a third step in which the secondary raw material containing CaO is charged into the furnace and a part or all of molten iron is discharged after melting, refining, and heating. In the first step, an operation to increase the average temperature of the iron bath is not carried out in a specified 30% period within at least 30% from the end point of the specified section in the specified sections starting at the start of charging of the first solid iron source and ending at the end of discharging slag, or in the first step, discharging slag is started before melting the solid iron source is completed.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing low-phosphorus molten iron using an electric furnace.

Background Art

[0002] Conventionally, electric furnaces mainly use scrap as the raw material iron source. On the other hand, scrap available in the market contains components (tramp elements) such as copper that are difficult to refine and remove. Therefore, depending on the target steel grade, the content of tramp elements is regulated, and thus the amount of scrap used as the iron source is limited. As a result, iron sources derived from iron ore such as reduced iron and pig iron are often used as part of the iron source. However, among the iron sources derived from iron ore, reduced iron contains a large amount of gangue components such as SiO2 and Al2O3, and pig iron contains Si components. When these are used in an electric furnace as the iron source, the oxide components directly transfer to the slag, and Si oxidizes and transfers to the slag. Therefore, the generation of slag increases compared to the case of using only scrap. In addition, iron sources derived from iron ore often have a higher phosphorus concentration than scrap. In electric furnace refining, phosphorus needs to be removed by reacting and transferring from the metal to the slag through slag / metal refining. As a component in the slag, it is desirable to increase the mass concentration ratio of CaO / SiO2 to give the slag the ability to dephosphorize.

[0003] Thus, when using a solid iron source containing gangue components or metallic Si, a large amount of slag is generated just by melting the solid iron source. In addition, in order to remove phosphorus from the metal, it is necessary to charge CaO corresponding to the SiO2 content in the furnace. This causes problems such as an increase in the further amount of slag generated and a deterioration in the unit consumption of auxiliary raw materials. Also, when the amount of slag is large, the amount of iron contained in the slag also increases. If the iron in the slag cannot be separated, the iron is discharged out of the furnace together with the slag during slag removal, which also causes a deterioration in yield.

[0004] Furthermore, in electric furnaces, it is desirable to foam the slag with carbon material or oxygen (slag forming) to improve thermal efficiency and prevent an increase in nitrogen concentration. The foaming state of the slag is greatly affected by the viscosity of the slag, and since viscosity is strongly affected not only by the slag composition but especially by the slag temperature, it is necessary to control these appropriately. For the purposes mentioned above, it is necessary to control the foaming state, that is, the composition and temperature, within a certain range not only during slag removal but also during melting.

[0005] To address these problems, a method has been proposed for converters, as described in Patent Document 1, in which slag with a high SiO2 concentration is discharged in an intermediate stage before the entire amount of CaO is added. However, in converters, the temperature changes in the process before slag discharge and the absolute value of the temperature immediately before slag discharge differ significantly from those of electric furnaces. In the case of converters, the molten iron being charged has a high C concentration, so the liquidus temperature is low, and correspondingly, the molten iron temperature at the time of converter charging is lower than the molten steel temperature at the time of tapping. During converter blowing, the temperature of the molten iron increases monotonically due to the exothermic oxidation of the molten iron components. Therefore, in converters, the iron bath temperature before the start of slag discharge and during slag discharge is significantly higher than at the time of molten iron charging.

[0006] On the other hand, in the case of an electric furnace, the carbon concentration in the solid iron source is lower than that of molten iron charged into a converter, and the iron bath temperature during the melting of the solid iron source is also near the liquidus temperature of the solid iron source.

[0007] Patent Document 2 discloses a method for producing low-phosphorus molten iron using an electric furnace for steelmaking, comprising: a first step of charging a solid iron source and optionally a molten iron source, and using electrical energy to melt and heat these raw materials; a second step of removing some or all of the slag generated during melting; a third step of adding a dephosphorization flux to perform dephosphorization treatment; and a fourth step of tapping the purified low-phosphorus molten iron. The method also discloses adjusting the mass ratio of the slag composition CaO / (SiO2+Al2O3) removed in the second step to within the range of 0.25 to 0.70. This reduces the amount of lime required for the phosphorus reduction of molten iron, and enables the efficient production of low-phosphorus molten iron in an electric furnace for steelmaking. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 10-152714 [Patent Document 2] International Publication No. WO2022 / 054555 [Non-patent literature]

[0009] [Non-Patent Document 1] "The Iron and Steel Handbook, 5th Edition," edited by The Iron and Steel Institute of Japan, 2014. [Overview of the project] [Problems that the invention aims to solve]

[0010] In this invention, a solid iron source containing 1% by mass or more of Si oxide, Al oxide, metallic Si, or metallic Al in total on an oxide basis is referred to as a "specific solid iron source." Reduced iron containing gangue, pig iron containing a large amount of metallic Si, and scrap with soil and other materials attached are considered "specific solid iron sources."

[0011] When a specific solid iron source containing gangue and other materials is charged into an electric furnace and melted by arc, the amount of molten iron and slag increases as the melting progresses. For example, when the melting has progressed to the point where a temperature measuring probe can be inserted into the iron bath through the slag door, measuring the temperature of the iron bath multiple times at the same location reveals that it does not change significantly. Similarly, measuring the temperature of the slag bath using the same method also shows little change, similar to the iron bath. Exceptionally, if a solid iron source is charged into a furnace where some molten iron from the previous charge remains, or if additional solid iron sources or auxiliary materials are charged intermittently, the average temperature of the iron bath may temporarily drop and then rise. However, once the melting stabilizes, the temperature does not change significantly.

[0012] Electric furnaces are characterized by localized heating and weaker agitation compared to converters. Therefore, it can be assumed that there will be temperature differences at the same time depending on the impact position of the arc spot or acid jet, the distance from the furnace wall, the depth, and the distance from the undissolved solid iron source. While it is possible to determine the average temperature distribution of the entire iron bath with a certain degree of accuracy using numerical calculations, it is easy to measure the temperature at the same location each time, and it is common practice to use this temperature as a representative temperature for determining the required power input and deciding whether to dispense the molten iron. Therefore, there is no problem in using the temperature measurement result from one location as the average temperature of the iron bath.

[0013] The molten state of the solid iron source is crucial for controlling the average temperature of the iron bath. In the case of an electric furnace, when the solid iron source is melting, the average temperature of the iron bath is affected by the liquidus temperature of the solid iron source. It is assumed that the temperature of the iron bath near heat sources such as the arc, carbon material, and oxidation of iron bath components will be higher than the average temperature. On the other hand, the temperature of the part of the iron bath in contact with the solid iron source is dominated by the heat of fusion of the solid iron source and will be near the liquidus temperature. To promote the melting of the solid iron source, it is necessary to continuously supply heat to compensate for the heat of fusion, as well as sensible heat. If the heat input is increased from a state where the progress of melting and heat input are balanced, the melting rate will also increase, so not all of the heat input will contribute to the temperature rise of the iron bath. Therefore, it is difficult to control the average temperature of the iron bath independently of the liquidus temperature of the solid iron source.

[0014] Figure 1 is a conceptual diagram showing the trend with the processing time in the electric furnace on the horizontal axis and the iron bath temperature on the vertical axis. (A) is the case when the solid iron source is batch charged without hot heel (seed molten metal), (B) is the case when the solid iron source is batch charged with hot heel, and (C) is the case when the solid iron source is continuously charged with hot heel. As shown in Figure 1, regardless of the presence or absence of hot heel (A) and (B), and whether the solid iron source is charged in batch (B) or continuously (C), there is a period during which the dissolution of the solid iron source progresses and the iron bath temperature becomes constant.

[0015] While it is possible to raise the temperature of the iron bath and slag bath to a certain extent by increasing the heat input sufficiently relative to the melting rate of the solid iron source during the melting process, this increases the proportion of heat lost to refractories and cooling water, leading to other problems such as a deterioration in thermal efficiency and refractory material consumption. For these reasons, the operation to raise the iron bath temperature is generally carried out when the melting of the solid iron source is nearing completion or has been completed.

[0016] Therefore, the temperature progression of the iron bath in an electric furnace differs significantly from that of a converter. For this reason, Patent Document 1 does not clarify whether, when removing slag from an electric furnace, the iron bath or slag bath should be kept at the same temperature as during molten metal, or whether it is necessary to heat it to a higher temperature than during molten metal.

[0017] Similar methods have been proposed for electric furnaces, as described in Patent Document 2. This document emphasizes the importance of controlling the viscosity of the slag from the viewpoint of separating slag from molten iron and controlling forming. In addition, the importance of temperature is also described, and the first step before the second step of slag removal includes the dissolution of the raw materials and heating. As described on pages 319-320 of Non-Patent Document 1, dissolution in an electric furnace refers to the process of obtaining a predetermined melt-down component and temperature, and heating refers to the operation of raising the temperature of the iron bath. It is thought that the melted portion of the raw materials is at a temperature above the liquidus temperature once it becomes a liquid phase, and it can be interpreted that a separate heating operation is necessary. Furthermore, as mentioned above, the temperature of the iron bath does not rise significantly during the dissolution of the solid iron source, and it is only after the dissolution is nearing completion or has been completed that the temperature can be substantially increased. Therefore, according to Patent Document 2, since a heating operation is included, it is suggested that the iron bath temperature before slag removal should be higher than the iron bath temperature during dissolution in order to maintain forming and separate the iron.

[0018] Our investigations have revealed that, according to the method described in Patent Document 2, when the iron bath temperature is increased while foaming is being maintained during dissolution, the viscosity rapidly decreases and foaming stops. This is because the iron bath temperature, and consequently the slag bath temperature, rises, causing the foaming to subside. When foaming subsides, slag cannot be discharged, making it difficult to adequately discharge low-basicity slag, which in turn worsens the lime production rate. Furthermore, attempting to discharge the same amount of slag as in an undisturbed state results in longer discharge times and reduced yield due to the outflow of iron along with the slag. In addition, the process time is extended because time is required for dissolving the solid iron source, heating, and slag discharge.

[0019] As described above, conventional technology has the problem that there are no clear guidelines for controlling the iron bath temperature during melting and slag discharge in electric furnaces. The present invention aims to solve this problem and provide an efficient method for producing low-phosphorus molten iron. [Means for solving the problem]

[0020] The inventors conducted experimental and analytical studies on the iron bath temperature during the melting of a solid iron source and the iron bath temperature during slag removal. As a result, they found that if slag forming can be maintained while melting is progressing, a sufficient amount of slag can be removed by performing the slag removal operation without intentionally changing the average temperature of the iron bath. Furthermore, it was found that the impact on yield deterioration due to viscosity reduction is limited, as the amount of molten iron in the electric furnace slag is less than that of the intermediate slag slag from a converter. It is presumed that the low amount of molten iron in the electric furnace slag is due to the pinch effect of arc current. On the contrary, when the average temperature of the iron bath was intentionally increased, slag forming slowed down, resulting in problems such as insufficient slag removal or a decrease in yield if time was taken to ensure sufficient slag removal.

[0021] That is, in a batch-type electric furnace, when using a solid iron source (specific solid iron source) containing 1% by mass or more in total of oxides of SiO2, Al2O3, metallic Si, and metallic Al, such as low-grade reduced iron, as a raw material, by discharging a low basicity slag without heating up, while ensuring the time required for refining, it is possible to suppress the deterioration of the lime unit and the yield, and to suppress the extension of the overall occupation time of the electric furnace.

[0022] The present invention has been made based on the above findings, and its gist is as follows. [1] First Invention When producing molten iron in an electric furnace using a solid iron source (hereinafter referred to as "specific solid iron source") containing 1% by mass or more in total of oxides of SiO2, Al2O3, metallic Si, and metallic Al as part or all of the raw materials, a first step of charging into the furnace an iron source containing 10% by mass or more and 100% by mass or less of the specific solid iron source and allowing melting to proceed, and discharging part or all of the slag in the furnace; a second step of charging a subraw material containing CaO and either additionally charging all the remaining solid iron source into the furnace or not charging it; a third step of either additionally charging a subraw material containing CaO or not charging it, and after melting, refining, and heating up, tapping part or all of the molten iron, the slag discharged in the first step has a mass concentration ratio (CaO / SiO2) of CaO and SiO2 in the range of 0.3 or more and 1.5 or less; at least the mass concentration ratio (CaO / SiO2) of CaO and SiO2 of the slag discharged in the third step or discharged after the third step is 1.2 or more and 4.5 or less; and in the first step, in the section starting from the start of charging the first solid iron source and ending at the end of discharging the slag within the above composition range (hereinafter referred to as "specific section"), an operation of raising the average temperature of the iron bath is not performed for at least 30% or less of the period counted backward from the end point of the specific section. A method for producing low-phosphorus molten iron, characterized by this.

[0023] [2] Second Invention When producing molten iron in an electric furnace using a solid iron source (hereinafter referred to as "specified solid iron source") containing 1% by mass or more of Si oxide, Al oxide, metallic Si, or metallic Al in terms of oxide equivalent total, as part or all of the raw materials, A first step involves charging an iron source containing 10% to 100% by mass of the specified solid iron source into the furnace and proceeding with the melting process, and then removing some or all of the slag from inside the furnace. A second step involves adding auxiliary materials containing CaO, and either adding or not adding all of the remaining solid iron source to the furnace. The process includes a third step in which, after adding or not adding auxiliary materials containing CaO, the molten iron is melted, refined, and heated, and then some or all of the molten iron is poured out. The furnace slag discharged in the first step has a mass concentration ratio of CaO to SiO2 (CaO / SiO2) in the range of 0.3 to 1.5. At a minimum, the mass concentration ratio (CaO / SiO2) of the slag discharged in the third process or after the third process is 1.2 or more and 4.5 or less. Furthermore, the method for producing low-phosphorus molten iron is characterized in that, in the first step, the discharge of slag is started before the melting of the solid iron source is completed.

[0024] [3] Third invention (1) The characteristic feature is that, within the aforementioned specific section, the difference between the minimum and maximum temperatures of the iron bath is kept within 30°C for a period of at least 30% preceding the end of the aforementioned specific section. The method for producing low-phosphorus molten iron described in [1]. [4] Third invention (2) The method for producing low-phosphorus molten iron according to [2], characterized in that, in the first step, the difference between the minimum temperature and maximum temperature of the iron bath is kept within 30°C for a period of at least 30% preceding the end of the specified section (hereinafter referred to as the "specified section"), starting from the time when the initial solid iron source is charged and ending at the time when the slag of the composition range is discharged.

[0025] [5] Fourth invention Of the total CaO content in the furnace and the slag discharged during the first process, A method for producing low-phosphorus molten iron according to any one of [1] to [4], characterized in that the sum of the CaO content in the slag remaining in the furnace after the previous third step and the CaO content contained in the solid iron source is 40% or more. [6] Fifth Invention An efficient method for producing low-phosphorus molten iron according to any one of [1] to [5], characterized by continuously charging at least 10% or more of a solid iron source. [7] Sixth Invention An efficient low-phosphorus molten iron production method according to any one of [1] to [6], characterized in that at least 50% or more of the specified solid iron source is charged in the first step. [Effects of the Invention]

[0026] In an electric furnace, when producing molten iron using a specific solid iron source containing SiO2, Al2O3, etc., as part or all of the raw material, a low-phosphorus molten iron production method is provided, comprising: a first step of charging the iron source into the furnace and proceeding with melting, and removing part or all of the slag from the furnace; a second step of adding auxiliary raw materials containing CaO, and adding or not adding all of the remaining solid iron source to the furnace; and a third step of adding or not adding auxiliary raw materials containing CaO, melting, refining, raising the temperature, and then discharging part or all of the molten iron. In the first step, an efficient low-phosphorus molten iron production method can be provided by not performing an operation to raise the average temperature of the iron bath during a specific 30% period, which is at least 30% backward from the end of a specific section defined as a specific section starting from the start of charging the first solid iron source and ending at the end of slag discharge, or by starting the discharge of slag before the melting of the solid iron source is completed in the first step. [Brief explanation of the drawing]

[0027] [Figure 1] This diagram shows the temperature changes of molten iron in an electric furnace. (A) shows the case where the iron source is charged in batches and there is no hot heel, (B) shows the case where the iron source is charged in batches and there is a hot heel, and (C) shows the case where the iron source is charged continuously and there is a hot heel. [Modes for carrying out the invention]

[0028] 《Matters common to the first and second inventions》 The matters common to the first and second inventions will be explained below.

[0029] The present invention relates to a low-phosphorus molten iron production method, in which molten iron is produced in an electric furnace using a solid iron source (specific solid iron source) containing 1% by mass or more of Si oxide, Al oxide, metallic Si, and metallic Al in total on an oxide basis as part or all of the raw materials, comprising: a first step of charging an iron source containing 10% by mass or more and 100% by mass or less of the specific solid iron source into the furnace and proceeding with melting, and then removing part or all of the slag from the furnace; a second step of adding auxiliary raw materials containing CaO and adding or not adding all of the remaining solid iron source into the furnace; and a third step of adding or not adding auxiliary raw materials containing CaO, melting, refining, and heating, and then tapping out part or all of the molten iron. The furnace slag discharged in the first process has a CaO to SiO2 mass concentration ratio (CaO / SiO2) in the range of 0.3 to 1.5, and at a minimum, the CaO to SiO2 mass concentration ratio (CaO / SiO2) of the slag discharged in the third process, or discharged after the third process, is 1.2 to 4.5.

[0030] Slag is generated when dissolving iron sources that contain a large amount of gangue. The main problem in the generation of low-basicity slag is the Si and Al content. Therefore, the present invention applies to cases where a solid iron source (specific solid iron source) containing 1% by mass or more of Si oxide, Al oxide, metallic Si, or metallic Al in terms of oxide equivalent, in whole or in part, is used. In this invention, concentration is expressed as mass concentration. As a specific solid iron source, for example, directly reduced iron derived from iron ore is assumed. In addition, scrap containing slag generated within the business site, or scrap containing SiO2 or Al2O3 due to soil contamination, or Si and Al dissolved as components in iron are also oxidized to become slag components, so if the total mass ratio of these to the iron source after oxidation is 1% or more, then the relevant scrap and reduced iron are also included. Preheating of these solid iron sources is not required. Hereafter, in this specification, a solid iron source containing 1% or more of Si oxide, Al oxide, metallic Si, and some or all of metallic Al in oxide terms (specific solid iron source) will be referred to as a high-ganele solid iron source, and other solid iron sources will be referred to as low-ganele solid iron sources. There are no particular restrictions on the use of low-ganele solid iron sources, but the influence on the iron bath temperature must be considered. The effect is further enhanced when the total of Si oxide, Al oxide, and some or all of metallic Si and metallic Al in oxide terms contained in the specific solid iron source reaches 2% or more, and the effect is particularly high when the total of Si oxide and the oxide equivalent amount of metallic Si alone reaches 2% or more. As this concentration increases, the effect of the present invention becomes more pronounced, so there is no problem in using reduced iron containing 5% or more Si oxide, for example, produced from pellets widely used in blast furnaces. Since it is not affected by the carbon concentration in the reduced iron, it can also be applied to reduced iron produced using hydrogen.

[0031] The first stage aims to discharge as much low-basicity slag as possible. Therefore, this stage is designated for charging 10% or more of the high-gane solid iron source (specific solid iron source) that causes slag generation. If the slag is discharged at a lower charging amount and the process moves to the second stage, increasing the slag's basicity, the effect of discharging low-basicity slag cannot be fully achieved. Furthermore, for this purpose, the higher the proportion of high-gane solid iron source charged in the first stage, the better, with an upper limit of 100%.

[0032] The optimal range for slag composition was determined by the mass concentration ratio of CaO and SiO2, which greatly affects dephosphorization. For dephosphorization to proceed efficiently from the second process onward, the removal of SiO2, which inhibits dephosphorization, is important. Furthermore, to increase the utilization efficiency of CaO, it is necessary to remove it with the lowest possible CaO concentration. Therefore, the ratio of CaO to SiO2 is the optimal control guideline. In addition to SiO2, Al2O3 was listed as a component of the high-gangstone solid iron source because it is thought to have the adverse effect of diluting the CaO concentration.

[0033] If the mass concentration ratio of CaO to SiO2 in the furnace slag discharged in the first step is lower than 0.3, the viscosity becomes too high, leading to unstable forming, difficulty in rapid discharge, and a deterioration of iron yield in terms of iron oxide reduction and granular iron content. Also, if the mass concentration ratio of CaO to SiO2 is higher than 1.5, it leads to a deterioration in CaO unit consumption and slag generation. Since the dephosphorization reaction can occur in the first step as well, there are no particular restrictions on refining. In order to adjust the mass concentration ratio of CaO to SiO2 in the furnace slag discharged in the first step to the above range, auxiliary materials containing CaO may be added in the first step.

[0034] The second step involves adding auxiliary materials containing CaO to increase the basicity of the slag (CaO / SiO2), and completing the charging of any remaining solid iron source that was not charged in the first step. Dissolution and refining are expected to proceed in this step, but there are no particular restrictions. Heating and slag removal are not required, but there are no particular restrictions. If there is no remaining solid iron source to charge, the process proceeds seamlessly to the third step.

[0035] The third step completes the dissolution of the solid iron source charged in the first and second steps, controls the slag composition to the final target, and refines and raises the temperature of the iron bath phosphorus to meet the tapping conditions. Since refinement continues during dissolution, there is no specific order between dissolution and refinement. Similarly, the refinement reaction may also proceed during heating, and there is no specific order between refinement and heating. Because predetermined temperature conditions are set before tapping, the heating operation must be completed before tapping.

[0036] The end of the third stage is defined as the completion of molten iron tapping. After the iron bath composition reaches the target, slag removal and molten iron tapping are performed. There are no restrictions on the composition of the molten iron at this tapping stage; for example, in terms of carbon concentration in the iron, it can range from molten pig iron to molten steel. Tapping can be done in one go, in two or more stages, or some of the molten iron can be left in the furnace without any problem.

[0037] If the molten metal is tapped from the bottom of the furnace, slag removal and tapping are separate operations, except for the slag that inevitably flows out. Similarly, if the tapping is from the side wall and the outlets for slag and molten iron are separate, slag removal and tapping are separate operations. On the other hand, if there is only one opening in the side wall, slag removal and tapping are the same operation. If slag removal comes first, slag removal is included in the third process. If tapping comes first, slag removal is an operation that comes after the third process. In either case, it is possible to remove as much molten iron and slag as possible, or to leave some to be used as hot heal or hot recycled slag in the next first process. If it is desirable to remove as much slag as possible, it is best to remove the slag before raising the temperature, as in the first process.

[0038] In the third step, it is necessary to complete the dephosphorization and refining process. Therefore, CaO is added as needed in the third step, but this is unnecessary if sufficient CaO has been charged in the second step. There are no particular restrictions on the slag composition in the second step or its discharge, but from the standpoint of dephosphorization and refining, it is effective for the mass concentration ratio of CaO to SiO2 in the slag discharged in the third step, or after the third step, to be between 1.2 and 4.5. If it is lower than 1.2, dephosphorization will not be performed sufficiently, so the lower limit was set at 1.2. As for the upper limit, a higher value indicates higher refining capacity, but considering the drawbacks such as CaO dissolution and foaming and deterioration of slag discharge due to an increase in the solid phase ratio in the slag, the upper limit was set at 4.5. Although the upper limit of the concentration ratio for the slag composition range in the first process was set at 1.5, depending on the actual slag composition in the first process and the amount of SiO2 contained in the gangue solid iron source dissolved in the second and subsequent processes, the mass concentration ratio of CaO to SiO2 in the slag discharged in the third process or after the third process may be lower than 1.5.

[0039] In any process, it is acceptable to discharge the slag in two or more stages. The composition of the slag in each process is expected to change continuously. These will be controlled to stay within the specified conditions based on mass balance, experience, etc. From the objective, it is most desirable that the average composition of the discharged slag falls within the specified conditions. This includes cases where control is intentional, as well as cases where the composition falls within the specified range as a result. The average composition of the discharged slag may be managed by analyzing the slag in the furnace, during discharge, or recovered after discharge, and by using the discharge amount, or by using a value calculated from the mass balance.

[0040] When adding CaO in any of the processes, it may be done all at once, in multiple steps, or continuously in powder form. CaO-containing auxiliary materials can include quicklime, limestone, dolomite, and slag generated within the process or facility.

[0041] Features of the First Invention The first invention is characterized in that, in the first step, within a section (hereinafter referred to as the "specific section") that starts at the time of charging the first solid iron source and ends at the time of discharge of slag within the specified composition range, no operation is performed to increase the average temperature of the iron bath during a period of at least 30% prior to the end of the specific section (hereinafter referred to as the "specific 30% period").

[0042] As mentioned above, we found that if slag forming can be maintained while dissolution is progressing, a sufficient amount of slag can be removed by performing the slag removal operation without raising the average temperature of the iron bath, that is, without intentionally changing the average temperature of the iron bath.

[0043] To avoid intentionally altering the average temperature of the iron bath, this can be achieved by not intentionally changing the heat balance estimated based on the operating conditions and furnace conditions exemplified below (hereinafter referred to as "specific operating and furnace conditions"). Other conditions may be added or used in part, or the conditions may be simplified, for example, by keeping the ratio of input power to charged main raw materials constant. The operating conditions should preferably include: (1) changes in temperature, quantity, and composition (components that change the liquidus temperature, such as carbon concentration) of the iron bath and slag; (2) means, changes, and cumulative heat input; (3) timing, location, quantity, composition, density, and temperature of the main and auxiliary raw materials; (4) elapsed time since the last solid iron source was added; (5) the state of electrode coating (submerging) by slag; (6) conditions for stirring and acid supply by gas or electromagnetic force; and (7) the amount of exhaust gas drawn in and the influence of the atmosphere drawn into the furnace that is affected by this. Furthermore, it is preferable to consider furnace body conditions such as furnace shape, size, refractory wear condition, cooling structure, and cooling strength.

[0044] Even without intentionally changing the operating conditions or furnace conditions described above, the observed temperature will change due to factors such as changes in the electrode coating state by slag, unavoidable fluctuations in the composition and feeding rate of the raw materials, and the accuracy of temperature measurement. However, unless intentional changes are made, or unless easily observable and predictable state changes are ignored, the change will not be significant enough to affect the slag forming state. For example, in the case of batch charging where the heat input is constant, and assuming that no temperature rise was predicted in the heat balance described above, the temperature of the iron bath may actually rise as the solid iron source melts. In this case, since the intended mismatch in the heat balance has not been introduced, the temperature rise will be gradual, and no significant change will occur in the slag forming state.

[0045] Furthermore, if the electrode coating by slag remains more stable than expected, the temperature may rise higher than anticipated. In this case, the calculated molten iron temperature will fluctuate, but since the temperature changes are repeated within a certain range and the forming state also fluctuates within a certain range, it does not significantly affect the slag removal performance.

[0046] Furthermore, the average temperature of the iron bath may change due to rapid changes in the contact area between the undissolved iron source and the iron bath caused by the collapse of accumulated raw materials, or due to the falling of solids adhering to the furnace walls and lid. If these occur regularly, they should be included in the scope of the aforementioned operating conditions and taken into consideration.

[0047] In any case, maintaining a constant heat balance is a necessary control for operational management and is possible for those skilled in the art; in fact, it is also possible to intentionally prevent the average temperature of the iron bath from rising.

[0048] Here, the important period for intentionally not raising the average temperature of the iron bath is the period within 30% of the time preceding the end of the specific section (specific 30% period), which starts from the beginning of charging the first solid iron source and ends at the end of discharging slag within the aforementioned composition range. Immediately after charging the solid iron source, depending on the quantitative relationship between the solid iron source and molten iron, the temperature of the iron bath will initially decrease before rising towards a state where melting proceeds stably. During this period, even considering the heat balance based on the aforementioned operating conditions and furnace conditions, operations intended to raise the temperature may be performed. Furthermore, slag forming is unstable during this period, and there is little direct relationship with the slag properties at the time of slag discharge. On the other hand, we have found that by not performing operations to raise the average temperature of the iron bath for at least 30% of the total time of the specific section (specific 30% period) prior to the end of slag discharge, it is possible to discharge slag while maintaining the forming state.

[0049] The most destabilizing factor for the forming state and the most detrimental to slag removal is when the temperature is intentionally increased during the specific 30% period by increasing heat input or decreasing heat removal compared to the heat balance prior to that period. Even if the temperature rises as a result due to factors such as low accuracy of the heat balance prior to the specific 30% period or differences in furnace conditions from predictions, if the temperature is intentionally increased by altering the heat balance during the specific 30% period, the forming state becomes unstable due to changes in the rate of temperature increase, and as a result, slag removal deteriorates.

[0050] Before or during slag removal, operations such as stopping arc current or acid supply may be performed, which can lead to a decrease in the average temperature of the iron bath. However, since this does not lead to changes in the foaming state or deterioration of slag removal performance, there are no particular restrictions on this.

[0051] Features of the second invention The second invention is characterized in that, in the first step, the discharge of slag is started before the dissolution of the solid iron source is completed.

[0052] Before the solid iron source has completely melted, that is, while melting is in progress, the heat input from the arc is consumed as heat of melting, and the iron bath does not rise in temperature. As mentioned above, the rise in the average temperature of the iron bath is closely related to the completion of the melting of the solid iron source. Therefore, the objective of removing slag without raising the average temperature of the iron bath can also be achieved by removing the slag while melting is in progress, that is, when melting is nearing completion or before it is completed.

[0053] The timing at which the melting of the solid iron source added up to a certain point in time is completed can be estimated from the heat balance based on some or all of the specific operation and furnace conditions described above, or from an analysis that also takes into account the heat transfer rate. Alternatively, it can be derived using empirical formulas that take into account the heat balance and heat transfer, or by using data analysis. The timing at which the average temperature of the iron bath rises can also be empirically determined and this can be set as the timing at which the melting of the solid iron source is completed. Here, the solid iron sources that should be considered as being in the process of melting include specific solid iron sources containing a total of 1% or more of SiO2, Al2O3, etc., and other solid iron sources, because it is necessary to consider the effect on the iron bath temperature rather than the generation of slag. The inventors have found that in order for a significant rise in the average temperature of the iron bath to not occur, at least 5% or more of solid iron equivalent in weight to the molten iron must remain unmelted.

[0054] Even when slag removal is started before dissolution is complete, operations that lead to a decrease in the dissolution rate of the solid iron source, such as stopping arc current or acid supply, may be performed before or during slag removal. However, since this does not lead to a change in the foaming state or a deterioration in slag removal performance, there are no particular restrictions.

[0055] Furthermore, if the slag is discharged in several stages during the first stage, it is desirable to start the final discharge of low-basicity slag before the dissolution of the solid iron source is complete, in order to discharge as much low-basicity slag as possible. Moreover, it is desirable that the dissolution is not complete until the final discharge of low-basicity slag is finished.

[0056] Features of the third invention The third invention is characterized in that, in the first step of the first and second inventions, the difference between the minimum and maximum temperatures of the iron bath is kept within 30°C during a period of at least 30% preceding the end of the specified section (specified 30% period), within a section (specified section) that starts at the time of charging the first solid iron source and ends at the time of discharge of slag within the specified composition range.

[0057] As described in the descriptions of the first and second inventions, by removing slag without intentionally raising the average temperature of the iron bath, it is possible to secure a sufficient amount of slag while maintaining the foaming state. However, as mentioned above, due to issues with the accuracy of the heat balance and fluctuations in the furnace conditions such as slag coating, the average temperature of the iron bath may rise even when no intended temperature-raising operation is performed in terms of the heat balance.

[0058] In addition, if the average temperature of the iron bath can be quantitatively controlled in the aforementioned specific section, even more stable slag discharge will be possible. The inventors continued their research and investigated the range of change in the average temperature of the iron bath. As a result, they found that by keeping the range of change in the average temperature of the iron bath to 30°C or less during a period of at least 30% preceding the end point (specific 30% period) in a section (specific section) that starts from the start of charging the first solid iron source and ends at the end of discharge of slag in the low basicity composition range, the foaming state can be made more stable while slag is discharged, and when the range of change is 10°C or less, extremely stable slag discharge can be achieved.

[0059] While controlling the range of variation in the average temperature of the iron bath is possible based on the knowledge and experience of electric furnace operators, for example, it could be done by estimating and observing the heat balance based on the aforementioned specific operating and furnace conditions, heat transfer, changes in the contact area between the solid iron source and the iron bath, the state of slag coating, or by making predictions based on accumulated performance data and experience, or by using methods based on actual temperature measurements.

[0060] If a decrease in the average temperature of the iron bath is predicted or observed through observation of the furnace conditions, such as slag coating, or through actual temperature measurements, it may be possible to intentionally increase the average temperature of the iron bath for a certain period. However, the intended increase should only be within the range corresponding to the predicted or observed temperature decrease. It is important not to continue the average temperature increase.

[0061] In particular, when slag removal is initiated without raising the temperature while dissolution is in progress, it is possible to avoid the average temperature of the iron bath rising by more than 5°C before and after the start of slag removal, and in this case, the most stable slag removal is possible.

[0062] Features of the fourth invention The fourth invention is characterized in that, in the first to third inventions, the total amount of CaO in the furnace and the slag discharged during the first step is 40% or more of the sum of CaO in the slag remaining in the furnace after the previous third step and the CaO contained in the solid iron source.

[0063] For the CaO content in the first step, some of the slag from the third step of the previous heat cycle in the electric furnace can be reused. This allows for the introduction of a new solid containing CaO in the current heat cycle, reducing the energy and time required to dissolve it. In this case, it is beneficial that the basicity of the slag at the end of the third step of the previous heat cycle is higher than that of the slag at the end of the first step of the current heat cycle.

[0064] Furthermore, high-gane solid iron sources may contain CaO due to binders, and this should also be taken into consideration when managing the basicity of the slag. This CaO is expected to dissolve into the slag when the solid iron source dissolves, and as mentioned above, this also leads to a reduction in the energy and time required for dissolution.

[0065] After considering the cost impact of the new CaO content in addition to these energy and time reductions, it was found that a sufficient effect is achieved when the total amount of CaO content in the furnace and the slag discharged in the first process, including the CaO content in the slag remaining in the furnace after the previous third process discharge and the CaO content in the solid iron source charged in the first process, is 40% or more.

[0066] There are no particular restrictions on the ratio of new CaO content from the second process onward to other CaO content.

[0067] Features of the Fifth Invention The fifth invention is characterized in that, in the first to fourth inventions, at least 10% or more of the solid iron source is continuously charged.

[0068] Continuously charging a solid iron source improves the reliability of stable control of the average temperature of the iron bath. This is because, when a hot heel is present or when a sufficient amount of molten iron is available, continuously charging the solid iron source makes it easier to suppress abrupt changes in the quantitative relationship between molten iron and the solid iron source, and to maintain a constant contact area. To achieve this effect, it is desirable to continuously charge at least 10% of the total solid iron source, which is the sum of high-ganeille solid iron source and low-ganeille solid iron source. There is no upper limit, as the temperature becomes more stable as the ratio increases. Furthermore, this is a requirement from the perspective of temperature control, and distinction based on gangue content is unnecessary. Stability will be further improved if preheating is performed.

[0069] Continuous charging of solid iron sources is carried out using, for example, horizontal conveyors, shafts, and input chutes (input pipes). Since the solid iron source itself is not a fluid, there may be moments when it is not being charged into the furnace depending on, for example, the vibration period of the horizontal conveyor or the conveying speed to the input chute, and it is also permissible to intentionally stop charging at certain times. However, compared to batch charging, in which solid iron sources contained in containers of a fixed volume, such as scrap buckets, are charged into the electric furnace, continuous charging is sufficient, and therefore these are called continuous charging.

[0070] In the case of continuous charging, leaving hot water in the furnace during the third stage of the previous heat cycle (hot heal) and using it as seed water for the first stage of the current heat cycle is also effective in stabilizing the iron bath temperature.

[0071] Features of the sixth invention The sixth invention is characterized in that, in the first to fifth inventions, at least 50% or more of the specified solid iron source is charged in the first step.

[0072] Since the first step is the process of discharging low-basicity slag, it is desirable to charge a large amount of specific solid iron sources (gange solid iron sources) containing SiO2, Al2O3, etc., which inhibit dephosphorization, in the first step. In order to reduce the amount of slag generated and the CaO consumption per unit, it is desirable to charge at least 50% of the gange solid iron source by weight in the first step, and the more the better, so there is no upper limit.

[0073] If there are any constraints on the division ratio of the total amount of solid iron source input into each process, it is desirable to use a larger proportion of high-ganeite solid iron source in the first process and a larger proportion of low-ganeite solid iron source in the second process. In other words, it is desirable to make the proportion of high-ganeite solid iron source to the total amount of solid iron source charged in the first process higher than that of the second process.

[0074] Solid iron sources containing gangue components often also contain relatively high levels of phosphorus. From this perspective, it is desirable to charge as much of the high-gangue solid iron source as possible in the first step in order to complete the dissolution of the high-gangue solid iron source as early as possible. [Examples]

[0075] The present invention was implemented using a 175-ton electric furnace. During the previous heat cycle, 60 tons of iron bath were left in the furnace as a hot heel to serve as part of the iron source for the current heat cycle. Approximately 130 tons of solid iron source were then charged in for melting and refining. After refining was complete, 110-120 tons of molten metal were discharged, and 60 tons of iron bath were left in the furnace as a hot heel for the next heat cycle.

[0076] In all comparative examples and examples except for Example 4-2, 70% of the solid iron source to be charged was direct reduced iron (SiO2=4%, Al2O3=1%, CaO=1%), which is a specific solid iron source (high-gane solid iron source). Only in Example 4-2 was the CaO content in the direct reduced iron, which is a specific solid iron source (high-gane solid iron source), set to 2% (SiO2=4%, Al2O3=1% was the same as in the other cases). In all cases, scrap was used as the low-gane solid iron source (30% of the solid iron source) in addition to 90-100 tons of high-gane solid iron source.

[0077] The process involved the following steps in this order: firstly, charging a solid iron source into a furnace with a hot heel and proceeding with the melting process, followed by the removal of some or all of the slag from the furnace; secondly, adding auxiliary materials containing CaO, and either adding or not adding all of the remaining solid iron source to the furnace; and thirdly, adding or not adding auxiliary materials containing CaO, followed by melting, refining, and heating, and then tapping out some or all of the molten iron.

[0078] In the first process, the section starting from the start of charging the first solid iron source and ending at the end of discharge of slag within the aforementioned composition range is designated as the "specified section," and the period within the specified section that extends at least 30% backward from the end of the specified section is designated as the "specified 30% period."

[0079] Table 1 shows the manufacturing conditions and results for the examples. Values ​​and items that fall outside the scope of the present invention are underlined. In Table 1, "C / S" for the first and third processes represents the mass concentration ratio (CaO / SiO2) of CaO to SiO2 in the furnace slag discharged in each process. The composition of the slag discharged between processes was determined by mass balance, and the amount of discharged slag was measured using a weighing machine on the slag receiving cart. "Introduced CaO" (%) for the first process represents the ratio of the total weight of CaO in the furnace and the slag discharged in the first process to the sum of the weight of CaO in the slag remaining in the furnace after the previous heat and the weight of CaO contained in the solid iron source charged in the first process of the current heat. In other words, the remainder of this ratio is CaO added in the first process of the current heat, separate from these CaO. The amount of slag remaining after the previous heat was calculated by considering the history of the charging auxiliary materials, products, and discharged slag for each heat, after complete molten metal extraction and complete slag removal (a state in which as much slag and molten iron as possible is removed at the end of a heat). Furthermore, the amount of CaO was calculated by multiplying this amount of slag by the analyzed CaO concentration. The amount of CaO charged in the first process was calculated from the weight of each charging material and their respective CaO concentrations.

[0080] Table 1 shows the proportion of the specific solid iron source charged in the first step out of the total specific solid iron source charged throughout the entire process. Only Example 6 charged the entire specific solid iron source in the first step; in all other examples (excluding Comparative Example 1), 47% of the total specific solid iron source was charged in the first step, and the remainder was charged in the second step.

[0081] Table 1 shows whether or not an operation to increase the average temperature of the iron bath was performed during the specified 30% period within the specified section, and the difference between the maximum and minimum temperatures of the iron bath during the specified 30% period. Whether or not an operation to increase the average temperature of the iron bath was performed was determined by whether or not the heat balance estimated based on the "specified operation and furnace conditions" was intentionally altered.

[0082] Table 1 describes whether the melting of the charged solid iron source was completed or not before the slag discharge in the first process, or whether a determination of melting completion was made. The slag discharge rate in the first process is shown in Table 1. The slag discharge rate (%) was evaluated as the total weight of slag discharged in the first process, relative to the total weight of slag remaining in the furnace after the previous heat and the total weight of slag produced in the first process. Each weight was calculated using the same method as described above.

[0083] The CaO content in the CaO-containing auxiliary materials added throughout the entire process is shown in Table 1 as "New T.CaO (kg / t)". Table 1 also shows whether or not the charged iron source was continuously charged. "Slag generation amount" was evaluated by weighing the amount of slag discarded throughout the entire process, and the total was recorded as kg / t. "Molten iron [P]" refers to the concentration (mass%) of [P] contained in the molten iron dispensed in the third step. "Yield loss" (%) was evaluated as a percentage of the total weight of iron contained in the generated slag as iron oxide and metallic iron, and iron in the dust collected by the exhaust gas dust collector, divided by the total weight of charged iron, throughout the entire heat process. "Tap to Tap (min)" is the elapsed time from the start time of the first step of the current heat cycle to the start time of the first step of the next heat cycle.

[0084] For the overall evaluation, three items were evaluated: slag generation amount, yield loss, and Tap to Tap. The thresholds for each were set at 120 kg / t, 4.0%, and 65 min. A score of ○ was given if all three items were below the threshold, and a score of × was given if even one item was not satisfied. In addition, a score of ◎ was given if the slag amount and yield loss were 100 kg / t and 3.0% or less, respectively.

[0085] [Table 1]

[0086] Comparative Examples 1 to 2-3 in Table 1 are comparative examples.

[0087] Comparative Example 1 is a comparative example in which intermediate slag removal was not performed. High basicity slag was incorporated from the beginning. Although the process time was shorter due to the absence of intermediate slag removal, both the amount of slag generated and the amount of new T.CaO were excessive, resulting in a low yield.

[0088] In Comparative Example 2-1, no operations intended to increase the temperature were performed during the dissolution process in the first step. Furthermore, within the specific 30% period, the unavoidable temperature increase was suppressed to 8°C in the period before dissolution was complete. On the other hand, after dissolution was complete and before intermediate slag removal (during the specific 30% period), an operation was performed to increase the average temperature of the iron bath, resulting in a temperature increase of 25°C. That is, the temperature increase during the specific 30% period was 33°C. The overall time increased due to waiting for dissolution to complete in the first step and the heating time. There was a slight re-phosphorusting during the heating. Because foaming subsided during slag removal in the first step, the slag removal time was extended to ensure the slag removal rate, iron also leaked out, and the yield decreased.

[0089] Comparative Example 2-2 performed the same temperature control as Comparative Example 2-1 (an operation to raise the average temperature of the iron bath during a specific 30% period), but the temperature increase before slag removal was reduced to 10°C. The temperature increase during the specific 30% period was 18°C. The overall time was shortened by the amount of the temperature increase compared to Comparative Example 2-1. Although the foaming during the first step of slag removal was not as severe as in Comparative Example 2-1, it subsided, and the slag removal time was extended to ensure a sufficient slag removal rate, resulting in a longer Tap to Tap time of 68 mins. Iron leakage occurred, and the yield decreased.

[0090] Comparative Examples 2-3 involved increasing the average temperature of the iron bath during a specific 30% period, and slightly raising the temperature before intermediate slag removal. Furthermore, ensuring a high slag removal rate was not required, and a time limit was imposed on the slag removal process. The foaming process became unstable due to the waiting period for dissolution and the temperature increase operation. The slag removal rate decreased due to the time limit. The amount of slag generated and new T.CaO increased, resulting in a low yield.

[0091] Examples 1-1 to 6 in Table 1 are examples of the present invention.

[0092] In Example 1-1, no heating operation was performed during the specific 30% period before intermediate slag removal. Temperature changes during the dissolution process were not measured or evaluated. As a result, it was possible to maintain low levels of slag generation and new T.CaO consumption while improving yield and keeping the overall time extension within an acceptable range.

[0093] Examples 1-2 did not involve heating during the specific 30% period before intermediate slag removal. During the specific 30% period, the slag coverage rate was higher than expected, resulting in better heat transfer efficiency. Although the iron bath temperature rose by 35°C during the melting process, this was not due to short-term heating after melting was complete, as shown in the comparative example. The forming state remained stable, and slag removal was carried out smoothly. As a result, the amount of slag generated and the new T.CaO unit consumption were kept low, the yield was improved, and the overall time extension was kept within an acceptable range.

[0094] In Example 2-1, the melting timing was determined based on heat balance and heat transfer analysis, and slag removal was started reliably earlier than the completion of melting. As a result, no heating operation was required during the specific 30% period before intermediate slag removal. Consequently, slag removal was performed smoothly. As a result, the amount of slag generated and the amount of new T.CaO per unit were kept low, the yield was improved, and the extension of the overall time was kept within an acceptable range.

[0095] In Example 2-2, no heating operation was performed before intermediate slag removal. During a specific 30% period, the dissolution of the solid iron source was slower than expected, and the temperature of the iron bath rose by 32°C during the dissolution process. However, after confirming the presence of undissolved solid iron source in the furnace using a measuring rod, slag was removed without further heating, and the slag was removed smoothly. As a result, the amount of slag generated and the new T.CaO unit consumption were kept low, the yield was improved, and the overall time extension was kept within an acceptable range.

[0096] In Example 3, instead of performing a temperature increase operation before intermediate slag removal, the temperature rise during a specific 30% period was controlled while measuring it, and the range of change was suppressed to 15°C. As a result, slag removal was smoother compared to Examples 1 and 2. As a result, the amount of slag generated and the amount of new T.CaO per unit were kept low, the yield was improved, and the extension of the overall time was kept within an acceptable range.

[0097] In Example 4-1, the amount of recycled slag from the preheating process was increased, resulting in nearly 70% of the CaO content being excluding newly charged T.CaO. As a result, the unit consumption of newly charged T.CaO was reduced, the amount of slag generated was also reduced, and the yield was further improved.

[0098] Example 4-2 recycled a similar amount of slag from the preheating process as in Example 4-1, and further used directly reduced iron containing 2% CaO as a specific solid iron source (high-gane solid iron source). In this case, the target phosphorus output level was higher than in the other examples, so the basicity of the first and third processes was set lower. In the first process, in addition to the CaO contained in the slag and reduced iron from the preheating process (CaO brought into the first process), no new CaO was added, resulting in 100% CaO being brought into the first process. As a result, the amount of T.CaO added per unit and the amount of slag generated were reduced, and the yield was further improved due to the smaller amount of slag.

[0099] In Example 5, by continuously adding a solid iron source, the unavoidable temperature changes during dissolution were controlled within a very narrow range. As a result, the foaming state was very stable, and the intermediate sludge discharge was stable.

[0100] In Example 6, all of the high-gangstone solid iron source (specific solid iron source) was charged in the first step. As a result, a large amount of SiO2 was discharged in the intermediate waste, which significantly reduced the amount of slag generated and the amount of new T.CaO produced, while also improving the yield.

Claims

1. When producing molten iron in an electric furnace using a solid iron source (hereinafter referred to as "specified solid iron source") containing 1% by mass or more of Si oxide, Al oxide, metallic Si, or metallic Al in terms of oxide equivalent total, as part or all of the raw materials, A first step involves charging an iron source containing 10% to 100% by mass of the specified solid iron source into the furnace and proceeding with the melting process, and then removing some or all of the slag from inside the furnace. A second step involves adding auxiliary materials containing CaO, and either adding or not adding all of the remaining solid iron source to the furnace. The process includes a third step in which, after adding or not adding auxiliary materials containing CaO, the molten iron is melted, refined, and heated, and then some or all of the molten iron is poured out. The furnace slag removed in the first step consists of CaO and SiO ２ The mass concentration ratio (CaO / SiO ２ ) is in the range of 0.3 or more and 1.5 or less, At a minimum, CaO and SiO in the slag discharged in the third step or after the third step. ２ The mass concentration ratio (CaO / SiO ２ ) is between 1.2 and 4.5, A method for producing low-phosphorus molten iron, characterized in that, in the first step, within a section (hereinafter referred to as the "specified section") that starts at the time of charging the first solid iron source and ends at the time of discharge of the furnace slag to be removed in the first step, no operation is performed to increase the average temperature of the iron bath during a period of at least 30% prior to the end of the specified section.

2. When producing molten iron in an electric furnace using a solid iron source (hereinafter referred to as "specified solid iron source") containing 1% by mass or more of Si oxide, Al oxide, metallic Si, or metallic Al in terms of oxide equivalent total, as part or all of the raw materials, A first step involves charging an iron source containing 10% to 100% by mass of the specified solid iron source into the furnace and proceeding with the melting process, and then removing some or all of the slag from inside the furnace. A second step involves adding auxiliary materials containing CaO, and either adding or not adding all of the remaining solid iron source to the furnace. The process includes a third step in which, after melting, refining, and heating, some or all of the molten iron is poured out, with or without the addition of auxiliary materials containing CaO. The furnace slag removed in the first step consists of CaO and SiO ２ The mass concentration ratio (CaO / SiO ２ ) is in the range of 0.3 or more and 1.5 or less, At a minimum, CaO and SiO in the slag discharged in the third step or after the third step. ２ The mass concentration ratio (CaO / SiO ２ ) is between 1.2 and 4.5, Furthermore, the method for producing low-phosphorus molten iron is characterized in that, in the first step, the discharge of slag is started before the melting of the solid iron source is completed.

3. The method for producing low-phosphorus molten iron according to claim 1, characterized in that, within the specified section, the difference between the minimum temperature and the maximum temperature of the iron bath is 30°C or less during a period of at least 30% preceding the end of the specified section.

4. The method for producing low-phosphorus molten iron according to claim 2, characterized in that, in the first step, the difference between the minimum temperature and maximum temperature of the iron bath is kept within 30°C for a period of at least 30% preceding the end of the specified section (hereinafter referred to as the "specified section"), starting from the time when the initial solid iron source is charged and ending at the time when the furnace slag to be discharged in the first step is finished.

5. Of the total amount of CaO in the furnace and the slag discharged during the first process, A method for producing low-phosphorus molten iron according to any one of claims 1 to 4, characterized in that the sum of the CaO content in the slag remaining in the furnace after the previous third step and the CaO content contained in the solid iron source is 40% or more.

6. A method for producing low-phosphorus molten iron according to any one of claims 1 to 4, characterized by continuously charging at least 10% or more of a solid iron source.

7. A method for producing low-phosphorus molten iron according to any one of claims 1 to 4, characterized in that at least 50% or more of the specified solid iron source is charged in the first step.

Images