Molten steel refining method and steel manufacturing method

Abstract

We propose a method for accurately estimating the S concentration in molten steel, enabling desulfurization of molten steel without causing desulfurization defects. This method for refining molten steel contained in a ladle involves inputting refining conditions, including the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel, into a mathematical model before the start of the refining process or at any time during the refining process, to estimate the concentrations of two or more elements in the molten steel, including S, during the refining process, and determining the conditions for the subsequent refining process based on the estimated concentrations of elements in the molten steel and the target concentrations of elements for the refining process.

Description

[Technical Field]

[0001] The present invention relates to a method for refining molten steel contained in a ladle, and more particularly to a method for desulfurizing molten steel, and also to a method for producing steel including said method. [Background technology]

[0002] Sulfur (S) contained in steel leads to hot brittleness, deterioration of corrosion resistance, and reduction of toughness and workability. Therefore, there is a need to reduce the S concentration in steel. Furthermore, in recent years, there has been an increasing demand for higher purity and cleanliness in steel, and efforts are needed to further reduce the sulfur content of steel, that is, to promote desulfurization and suppress resulfurization during the refining process of molten iron.

[0003] Desulfurization of molten iron is carried out in the preliminary treatment process of molten iron tapped from a blast furnace and in the secondary refining process of molten steel. For desulfurization in the secondary refining process of molten steel, processes using ladle refining equipment such as a ladle furnace (LF) or vacuum degassing equipment such as a refining furnace (RH) are widely used. As a method for efficiently desulfurizing molten steel in an LF, for example, Patent Document 1 proposes a method for determining the amount of flux input required for desulfurization based on information such as the sulfur concentration in the molten steel before LF treatment, slag components, and molten steel temperature. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2005-517807 [Non-patent literature]

[0005] [Non-Patent Document 1] S.Ohguchi, DGCRobertson, B.Deo, P.Grieveson and JHEJeffes: Ironmaking Steelmaking, 11(1984) p202. [Non-patent document 2] R. Hundermark: The electrical conductivity of melter type slags, University of Cape Town (2003). Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above-mentioned conventional technology has the following problems that must be solved. Specifically, the method disclosed in Patent Document 1 does not take into account the effects of oxidation-reduction reactions of components in molten steel other than S during the refining process, and fluctuations in the molten steel components or slag components during the refining process may reduce desulfurization capacity, resulting in poor desulfurization. Furthermore, the method only calculates the S concentration in molten steel and the amount of flux required for desulfurization of molten steel using information before the start of the refining process, and does not disclose a method for adjusting the amount of flux (desulfurization material) added using information on the molten steel components, etc., during the refining process.

[0007] Therefore, in order to solve the above-mentioned problems of the conventional techniques, the present invention aims to propose a method for accurately estimating the S concentration in molten steel when refining the molten steel contained in a ladle, thereby enabling the desulfurization of the molten steel without causing desulfurization defects, and also to propose a steel manufacturing method including the method. [Means for solving the problem]

[0008] While studying the above-mentioned problems of the conventional techniques, the inventors discovered that in the refining of molten steel using LF equipment, it is effective to determine the refining treatment conditions based on the component concentrations of slag present in the ladle, and this led to the development of the present invention.

[0009] That is, the method for refining molten steel according to the present invention, which advantageously solves the above-mentioned problems, is a method for refining molten steel contained in a ladle, and is characterized in that refining conditions, including the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel before the start of the refining process or at any time during the refining process, are input into a mathematical model, the concentrations of two or more elements in the molten steel, including S, during the refining process are estimated, and the conditions for the subsequent refining process are determined based on the estimated concentrations of elements in the molten steel and the target concentrations of elements in the refining process.

[0010] The method for refining molten steel according to the present invention comprises the steps of: a. The refining treatment conditions to be determined include at least one of the amount of desulfurization material added, the amount of alloy added, the flow rate of stirring gas supplied into the molten steel, and the amount of electric power for arc heating; b. The mathematical model is a model created using refining conditions including information about the ladle; c. The electrical conductivity of the slag is 10 S / m or more and 300 S / m or less; This would be a more preferable solution to the problem.

[0011] Furthermore, a method for producing steel according to the present invention, which advantageously solves the above-mentioned problems, is characterized in that it includes any one of the above-mentioned methods for refining molten steel as a ladle refining step. [Effects of the Invention]

[0012] According to the present invention, when refining molten steel contained in a ladle, the S concentration in the molten steel can be accurately estimated by taking into account the effects of oxidation-reduction reactions of components in the molten steel other than S, and the molten steel can be desulfurized without causing desulfurization defects. Furthermore, the addition of excessive desulfurization material can be avoided, thereby reducing the refining cost. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a flow chart illustrating a method for determining refining treatment conditions before refining treatment in a molten steel refining treatment method according to an embodiment of the present invention. [Figure 2]FIG. 2 is a flowchart showing a method for estimating component concentrations of molten steel according to the embodiment. [Figure 3] FIG. 2 is a flow chart illustrating a method for determining refining treatment conditions during refining treatment in the molten steel refining treatment method according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the following embodiments are intended to exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to that described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0015] In developing the present invention, the inventors poured molten steel produced in a converter or electric furnace into a ladle and then refined the molten steel using a ladle refining furnace (LF) facility. This process is called the ladle refining process. In this process, molten steel with a target S concentration of 0.0015% by mass or less or 0.0030% by mass or less was produced.

[0016] In the ladle refining process, we investigated the relationship between operational conditions, such as the element concentrations of molten steel before processing, the element concentrations of slag, and the molten steel temperature, and the change in the sulfur concentration in molten steel during processing. As a result, we discovered that the sulfur concentration in molten steel can be accurately estimated, enabling molten steel desulfurization without causing desulfurization defects. The method involves first inputting refining conditions, including the element concentrations of molten steel, slag element concentrations, and molten steel temperature, before the start of the refining process or at any time during the refining process, into a mathematical model, and estimating the concentrations of two or more elements in the molten steel, including S, during the refining process. Then, based on the estimated element concentrations in the molten steel and the target element concentrations for the refining process, the conditions for the subsequent refining process are determined and the refining process is carried out. Specifically, the method is as follows.

[0017] In the molten steel refining method according to this embodiment, first, refining conditions, including the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel, before the start of the refining process or at any time during the refining process, are input into a mathematical model. Then, the concentrations of two or more elements in the molten steel, including S, during the refining process are estimated. Next, it is necessary to determine the conditions for the subsequent refining process based on the estimated concentrations of elements in the molten steel and the target concentrations of elements in the refining process. In the refining process of molten steel, an electrochemical desulfurization reaction, shown in Equation (1) below, proceeds between the molten steel and the slag. It is also known that oxidation-reduction reactions of elements other than S, shown in Equations (2) to (9) below, also proceed competitively. [S]+(O 2- )=(S 2- )+[O] (1) [Si] + 2[O] = (SiO2) (2) [Mn] + [O] = (MnO) (3) 2[P]+5[O]=(P2O5) (4) Fe + [O] = (FeO) (5) [C]+[O]=CO↑ (6) [Ca] + [O] = (CaO) (7) 2[Al]+3[O]=(Al2O3) (8) [Mg] + [O] = (MgO) (9) In the above chemical reaction formula, the notation [M] indicates that element M is contained in the molten steel. Also, the notation (R) indicates that a substance with chemical formula R is contained in the slag. The same applies below.

[0018] The reaction rate equation for the desulfurization reaction of molten steel shown in formula (1) is expressed by the following formula (10). d[S] / dt=-K([S]-[S] e ) (10) Where, [S] is the S concentration in the molten steel (mass%), [S] e : Equilibrium S concentration (mass%) in molten steel in the equilibrium reaction between molten steel and slag, K: reaction rate constant (s -1 ), t: Desulfurization treatment time (s) is.

[0019] In the desulfurization method of molten steel disclosed in Patent Document 1, the equilibrium sulfur concentration [S] is calculated using the component concentrations of slag, the temperature of molten steel, and the activity of free oxygen in the molten steel. e However, the equilibrium S concentration [S] e is also dependent on the concentrations of components other than S in the molten steel. As a result, there is a risk that the estimated S concentration in the molten steel will differ greatly from the actual S concentration in the molten steel during the refining process, resulting in poor desulfurization. Therefore, it was thought that it would be possible to improve the accuracy of estimating the S concentration in molten steel by accurately estimating not only S but also the concentrations of components other than S.

[0020] As a method for estimating the concentrations of two or more components in molten steel taking into consideration the oxidation-reduction reactions of the formulas (1) to (9) and other components in molten steel, for example, the competitive reaction model described in Non-Patent Document 1 can be applied.

[0021] Furthermore, the refining conditions determined in the ladle refining process preferably include at least one of the amount of desulfurization material added, the amount of alloy added, the flow rate of the stirring gas supplied to the molten steel, and the amount of electric power used for arc heating. Regarding the amount of desulfurization material added, the greater the amount added, the more accelerated the desulfurization reaction of the molten steel. On the other hand, increasing the amount added increases the cost of the refining process. Therefore, by appropriately adjusting the amount added, it is possible to promote the desulfurization process while suppressing increases in refining process costs. Regarding the amount of alloy added, it affects the changes in the concentrations of elements in the molten steel. Furthermore, when alloy elements in the molten steel react with oxygen in the air or oxygen in the slag and migrate to the slag, it also affects the changes in the concentrations of elements in the slag. Therefore, by appropriately adjusting the amount of alloy added, it is possible to promote the desulfurization process while maintaining the concentrations of elements in the molten steel other than S within the target range. Regarding the flow rate of the stirring gas supplied to the molten steel, the greater the flow rate of the stirring gas, the more strongly the molten steel is stirred. Therefore, the amount of molten steel supplied to the interface between the molten steel and the slag increases, accelerating the desulfurization reaction. On the other hand, increasing the flow rate of the stirring gas promotes the oxidation reaction of alloying elements in the molten steel by oxygen in the atmosphere. Therefore, by appropriately adjusting the flow rate of the stirring gas, it is possible to promote desulfurization while maintaining the concentrations of elements in the molten steel other than S within the target range. Regarding the electric power consumption of the arc heating, the higher the electric power consumption, the faster the temperature rise rate of the molten steel and slag, which is effective in promoting the desulfurization reaction. On the other hand, a high electric power consumption of the arc heating increases the amount of carbon picked up from the carbon electrode into the molten steel. Therefore, by appropriately adjusting the electric power consumption of the arc heating, it is possible to promote desulfurization while maintaining the carbon concentration in the molten steel and the molten steel temperature within the target range.

[0022] Furthermore, it is preferable that the mathematical model used in the refining method be a model created using refining conditions including information about the ladle. Even when a mathematical model based on thermodynamic or chemical knowledge is used, the accuracy of estimating the S concentration in molten steel may be reduced depending on disturbances such as the state of the refining equipment and the surrounding environment. Therefore, by applying a model created using refining conditions including information about the ladle, it is possible to accurately estimate the component concentrations of molten steel while taking into account the disturbances. Note that examples of information about the ladle include the number of times the ladle has been used, the thickness of the refractory inside the ladle, the weight of the metal and slag adhering to the ladle, the surface temperature of the refractory inside the ladle before pouring the molten steel, and the surface temperature of the ladle shell before or after pouring the molten steel.

[0023] In addition, the electrical conductivity of the slag in the above-mentioned refining process is preferably 10 S / m or more and 300 S / m or less. As mentioned above, increasing the heating rate of the molten steel and slag by arc heating is expected to accelerate the desulfurization reaction. Since the electrical conductivity of the slag is a factor that affects the temperature changes of the molten steel and slag due to arc heating, by appropriately adjusting this value, desulfurization can be performed while maintaining the temperatures of the molten steel and slag within the desired range without excessive consumption of electric power for arc heating. If the electrical conductivity of the slag is less than 10 S / m, the slag's electrical resistance is high, resulting in increased heat generation by arc heating. This causes the slag temperature to become excessively high relative to the molten steel temperature, which can lead to concerns about increased carbon pickup from the carbon electrode into the molten steel and increased wear rate of refractories in the ladle that come into contact with the slag. On the other hand, if the electrical conductivity of the slag is greater than 300 S / m, the slag's electrical resistance is low, so the heat generated by the slag due to arc heating is small. This causes the melting of the slag and desulfurization material in the ladle to stagnate, reducing the slag's desulfurization ability, and there is concern that this may result in poor desulfurization of the molten steel.

[0024] The electrical conductivity of the slag may be measured before the start of the refining process or at any time during the refining process, or calculated using a theoretical formula. The method for measuring electrical conductivity is not limited to a specific method, and examples include the AC two-electrode method, i.e., the AC two-terminal method, and the electromagnetic induction method. The method for calculating electrical conductivity is not limited to a specific method, and examples include a method for calculating electrical conductivity from the slag components and temperature using a theoretical formula, as described in Non-Patent Document 2.

[0025] Next, an example of desulfurization of molten steel using LF equipment based on the molten steel refining method according to this embodiment will be specifically described.

[0026] Molten steel produced using a converter or electric furnace is poured into a ladle, which is then transported to the LF facility. The refining process conditions for the molten steel are then determined according to the flowchart shown in Figure 1. First, the molten steel and slag in the ladle are analyzed to measure the concentrations of the molten steel and slag elements (S1, S2). Next, the temperature of the molten steel in the ladle is measured (S3). The obtained concentrations of the molten steel elements, the slag elements, and the molten steel temperature are input into a control terminal (S4). Next, the assumed refining process conditions are input into the control terminal (S5). The assumed refining process conditions include, for example, the amount of desulfurization material added, the amount of alloy added, the flow rate of stirring gas supplied to the molten steel, and the amount of electric power used for arc heating. Using the input assumed refining process conditions, i.e., the operational data, the concentrations of two or more elements in the molten steel, including S, after the refining process are estimated (S6). It is then determined whether the estimated S concentration in the molten steel is within a predetermined range (S7). If the estimated S concentration in the molten steel is within a predetermined range, the prerequisite refining process conditions are adopted (S8), and the determination of the refining process conditions is terminated. On the other hand, if the estimated S concentration in the molten steel is not within the predetermined range, the prerequisite refining process conditions are changed (S9), and the component concentrations in the molten steel are estimated again (S6). At this time, the same process is repeated until the estimated S concentration in the molten steel falls within the predetermined range. When the estimated S concentration in the molten steel falls within the predetermined range, the changed refining process conditions are adopted (S8), and the determination of the refining process conditions is terminated.

[0027] The means for estimating the component concentrations of molten steel is not limited. As described above, the preferred means for estimating the component concentrations of molten steel is a method of estimation based on a mathematical model created using information about the ladle. For example, according to the flowchart shown in FIG. 2, a mathematical model can be created using past operational data including information about the ladle, and current operational data including information about the ladle can be input to estimate the component concentrations of molten steel. For example, the past operational data including information about the ladle is input to a control terminal (S11). The input data is used to create a mathematical model for estimating the component composition of molten steel after processing (S12). The current operational data including information about the ladle is input (S13). This step corresponds to steps S4 and S5 described above. Then, the component concentrations of molten steel after processing are estimated (S14). This step corresponds to step S6 described above.

[0028] The refining process of molten steel is started based on the refining process conditions determined by the above method. Thereafter, at any point during the refining process, subsequent refining process conditions for the molten steel may be determined and changed according to the flowchart shown in FIG. 3. As with the method shown in FIG. 1, the component concentrations of the molten steel (S21), the component concentrations of the slag (S22), and the molten steel temperature (S23) during the refining process are measured to estimate the concentrations of two or more components in the molten steel, including S, after the refining process (S26). Then, the refining process conditions are determined (S28) so that the estimated S concentration in the molten steel falls within a predetermined range. The refining process conditions are changed to the determined refining process conditions (S30), and the refining process continues. Furthermore, when the estimated S concentration in the molten steel is expected to fall within the predetermined range, the component concentrations of the molten steel may be measured, and the obtained component concentrations of the molten steel may be input into a control terminal to determine whether the measured S concentration in the molten steel falls within the predetermined range. If the measured S concentration in the molten steel falls within the predetermined range, the refining process of the molten steel is terminated. On the other hand, if the measured S concentration in the molten steel is not within the predetermined range, the component concentrations of the slag and the temperature of the molten steel may be additionally measured. Then, similar to the above method, the concentrations of two or more components in the molten steel, including S, after the refining process may be estimated, refining process conditions may be determined so that the estimated S concentration in the molten steel falls within the predetermined range, and the refining process may be continued under the determined refining process conditions.

[0029] The above-mentioned molten steel refining process is included as a ladle refining process, and if necessary, a secondary refining process such as an RH vacuum degasser can be carried out to produce steel materials such as slabs by continuous casting or ingot making-blooming rolling. The steel materials can be made into steel products as they are, or by hot rolling, cold rolling, heat treatment, or other processes. [Example]

[0030] Example 1 In an actual plant with a molten steel capacity of approximately 200 tons per charge, molten iron tapped from a blast furnace was subjected to primary refining in a converter to produce molten steel, and the molten steel was then poured into a ladle and transported to the LF equipment for secondary refining.

[0031] The sulfur concentration in the molten steel after primary refining was between 0.0040% and 0.0050% by mass. Next, the molten steel was desulfurized using LF equipment to achieve a target sulfur concentration of 0.0015% by mass after refining. The refining conditions, such as the amount of desulfurization material and alloy added, the flow rate of the stirring gas supplied to the molten steel, and the electric power consumption for arc heating, were varied. Burnt lime with a particle size of 30 mm or less was used as the desulfurization material. Aluminum shot with a particle size of 30 mm or less was used as the alloy. Ar gas was used as the stirring gas, and it was supplied into the molten steel using an injection lance. The refining conditions and the measurement results of the sulfur concentration in the molten steel before and after refining are shown in Table 1. Table 1 lists the amount of desulfurization material and alloy added, as well as the consumption unit of the desulfurization material and aluminum shot per unit mass of molten steel. The flow rate of the stirring gas supplied to the molten steel is recorded as the cumulative Ar gas consumption per unit of molten steel. The amount of electric power used for arc heating is recorded as the cumulative electric power consumption per unit mass of molten steel. The electrical conductivity of the slag is recorded as a value calculated using the component concentrations and temperature of the slag measured 5 minutes after the start of the refining process, based on the following equations (11) and (12) described in Non-Patent Document 2. κ = 100 × exp(A) (11) A=(19.9-47348 / T)×X Al2O3 +(15.4-24087 / T)×X CaO +(9.2-14151 / T)×X MgO +(-0.5-7478 / T)×X SiO2 +(10.0-9140 / T)×X FeO ×Fe 2+ +(65.4-82447 / T)×X FeO 2 ×Fe 2+ ×Fe 3+ +(-2.6+6642 / T)×X FeO ×Fe 3+ (12) where κ is the electrical conductivity of the slag (S / m), T: slag temperature (K), X i : The ratio of the molar concentration of component i in the slag to the sum of the molar concentrations of all components in the slag (-), Fe 2+ : Fe in slag 2+ ions and Fe 3+ Fe in slag versus total molar concentration of ions 2+ Ratio of molar concentrations of ions (-), Fe 3+ : Fe in slag 2+ ions and Fe 3+ Fe in slag versus total molar concentration of ions 3+ Ratio of ion molar concentrations (-) is.

[0032] [Table 1]

[0033] In the above operation, in levels No. 1 to 4, before the start of the refining process, the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel were measured, and the concentrations of two or more molten steel elements, including S, were estimated based on a mathematical model created without using information about the ladle. Then, the refining process conditions were determined and the refining process was carried out. In levels No. 5 to 8, five minutes after the start of the refining process, the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel were measured, and the concentrations of two or more molten steel elements, including S, were estimated based on a mathematical model created without using information about the ladle. Then, the refining process conditions were determined and the refining process was carried out. Here, the refining process conditions determined were any of the amount of desulfurization material charged, the amount of alloy charged, the flow rate of the stirring gas supplied into the molten steel, and the amount of electric power for arc heating. In the case of No. 9, the concentrations of two or more molten steel components, including S, were estimated based on a mathematical model created without using information about the ladle, both before the start of the refining process and five minutes after the start of the refining process. Then, the refining process conditions were determined and the refining process was carried out. In the case of No. 10, the concentrations of two or more molten steel components, including S, were estimated based on a mathematical model created using information about the ladle, both before the start of the refining process and five minutes after the start of the refining process. Then, the refining process conditions were determined and the refining process was carried out. On the other hand, in the cases of No. 11 and 12 as comparative examples, the refining process conditions were determined based only on past operational performance and the operator's experience, without using a mathematical model. In the cases of No. 13 and 14, the S concentration in the molten steel was estimated based on a mathematical model created without using information about the ladle, and which only estimated the S concentration in the molten steel, both before the start of the refining process and five minutes after the start of the refining process. Then, the refining process conditions were determined and the refining process was carried out.

[0034] From the results shown in Table 1, it was confirmed that, at the levels of Nos. 1 to 10, which are compatible with the present invention, the refining process could be carried out without causing insufficient or excessive desulfurization. It was also confirmed that the S concentration in the molten steel after the refining process was within the range of 0.0013 to 0.0015 mass%. On the other hand, it was confirmed that, at the level of No. 11, the amount of desulfurization material charged was insufficient, resulting in insufficient desulfurization. At the level of No. 12, the amount of desulfurization material charged was excessive, reducing the S concentration in the molten steel to 0.0010 mass%, resulting in excessive desulfurization. At the levels of Nos. 13 and 14, the S concentration in the molten steel after the refining process was 0.0011 mass% or 0.0017 mass%, confirming that, compared to the levels of Nos. 1 to 10, insufficient or excessive desulfurization occurred.

[0035] From the results shown in Table 1, it was confirmed that at the level of No. 9, the S concentration in the molten steel after refining was 0.0014 mass%, and desulfurization treatment was possible with an accuracy of -0.0001 mass% compared to the target of 0.0015 mass%.

[0036] From the results shown in Table 1, it was confirmed that at the level of No. 10, the S concentration in the molten steel after refining was 0.0015 mass%, and desulfurization was possible without excess or deficiency relative to the target of 0.0015 mass%.

[0037] <Example 2> In an actual plant with a molten steel capacity of approximately 200 tons per charge, molten iron tapped from a blast furnace was subjected to primary refining in a converter to produce molten steel, and the molten steel was then poured into a ladle and transported to the LF equipment for secondary refining.

[0038] The sulfur concentration in the molten steel after primary refining was between 0.0040% and 0.0050% by mass. Next, the molten steel was desulfurized using LF equipment to achieve a target sulfur concentration of 0.0030% by mass. The refining conditions, such as the amount of desulfurization material and alloy added, the flow rate of the stirring gas supplied to the molten steel, and the electric power used for arc heating, were varied. The desulfurization material used was burnt lime with a particle size of 30 mm or less. The alloys used were ferrosilicon and ferromanganese with a particle size of 30 mm or less. Ar gas was used as the stirring gas, and the Ar gas was supplied into the molten steel using an injection lance. Table 2 shows the refining conditions and the measurement results of the sulfur concentration in the molten steel before and after refining. Table 2 lists the amount of desulfurization material and alloy added, as well as the consumption unit of the desulfurization material, ferrosilicon, and ferromanganese per unit mass of molten steel. The flow rate of the stirring gas supplied to the molten steel is shown as the cumulative Ar gas consumption per unit of molten steel. The amount of electric power used for arc heating is shown as the cumulative electric power consumption per unit mass of molten steel. As in Example 1, the electrical conductivity of the slag was calculated based on the above formulas (11) and (12) described in Non-Patent Document 2 using the component concentrations and temperature of the slag measured 5 minutes after the start of the refining process.

[0039] [Table 2]

[0040] In the above operation, in the cases of Nos. 15 to 18, the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel were measured before the start of the refining process, and the concentrations of two or more molten steel elements, including S, were estimated based on a mathematical model created without using any information about the ladle. Then, the refining conditions were determined and the refining process was carried out. In the cases of Nos. 19 to 22, the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel were measured 5 minutes after the start of the refining process, and the concentrations of two or more molten steel elements, including S, were estimated based on a mathematical model created without using any information about the ladle. Then, the refining conditions were determined and the refining process was carried out. Here, the refining conditions determined were any of the amount of desulfurization material charged, the amount of alloy charged, the flow rate of the stirring gas supplied to the molten steel, and the amount of electric power for arc heating. In the case of No. 23, the concentrations of two or more molten steel components, including S, were estimated based on a mathematical model created without using information about the ladle, both before the start of the refining process and five minutes after the start of the refining process. Then, the refining process conditions were determined and the refining process was carried out. In the case of No. 24, the concentrations of two or more molten steel components, including S, were estimated based on a mathematical model created using information about the ladle, both before the start of the refining process and five minutes after the start of the refining process. Then, the refining process conditions were determined and the refining process was carried out. On the other hand, in the cases of Nos. 25 and 26, as comparative examples, the refining process conditions were determined based only on past operational performance and the operator's experience, without using a mathematical model. In the cases of Nos. 27 and 28, the S concentration in the molten steel was estimated based on a mathematical model created without using information about the ladle, and which only estimated the S concentration in the molten steel, both before the start of the refining process and five minutes after the start of the refining process. Then, the refining process conditions were determined and the refining process was carried out.

[0041] From the results shown in Table 2, it was confirmed that, in the cases of Nos. 15 to 24, which are compatible with the present invention, the refining process was able to be carried out without causing insufficient or excessive desulfurization. It was also confirmed that the S concentration in the molten steel after the refining process was within the range of 0.0028 to 0.0030 mass%. On the other hand, in the case of No. 25, it was confirmed that the amount of desulfurization material charged was insufficient, resulting in insufficient desulfurization. In the case of No. 26, it was confirmed that the amount of desulfurization material charged was excessive, reducing the S concentration in the molten steel to 0.0023 mass%, resulting in excessive desulfurization. In the cases of Nos. 27 and 28, the S concentration in the molten steel after the refining process was 0.0025 mass% or 0.0032 mass%, which confirmed that, compared to the cases of Nos. 15 to 24, insufficient or excessive desulfurization occurred.

[0042] From the results shown in Table 2, it was confirmed that at the level of No. 23, the S concentration in the molten steel after refining was 0.0029 mass%, and desulfurization treatment was possible with an accuracy of -0.0001 mass% compared to the target of 0.0030 mass%.

[0043] From the results shown in Table 2, it was confirmed that at the level of No. 24, the S concentration in the molten steel after refining was 0.0030 mass%, and desulfurization was possible without excess or deficiency relative to the target of 0.0030 mass%.

[0044] Example 3 In an actual plant with a molten steel capacity of approximately 200 tons per charge, molten iron tapped from a blast furnace was subjected to primary refining in a converter to produce molten steel, and the molten steel was then poured into a ladle and transported to the LF equipment for secondary refining.

[0045] The sulfur concentration in the molten steel after primary refining was between 0.0040% and 0.0050% by mass. Next, LF equipment was used to desulfurize the molten steel to a target sulfur concentration of 0.0015% by mass. The refining conditions, including the amount of desulfurization material and alloy added, the flow rate of the stirring gas supplied to the molten steel, and the power consumption for arc heating, as well as the electrical conductivity of the slag, were varied. Burnt lime with a particle size of 30 mm or less was used as the desulfurization material. Aluminum shot with a particle size of 30 mm or less was used as the alloy. Ar gas was used as the stirring gas, and the Ar gas was supplied into the molten steel using an injection lance. The refining conditions and the measurement results of the sulfur concentration in the molten steel before and after refining are shown in Table 3. Table 3 lists the amount of desulfurization material and alloy added, as well as the consumption unit of the desulfurization material and aluminum shot per unit mass of molten steel. The flow rate of the stirring gas supplied to the molten steel is shown as the cumulative Ar gas consumption per unit of molten steel. The amount of electric power used for arc heating is shown as the cumulative electric power consumption per unit mass of molten steel. As in Example 1, the electrical conductivity of the slag was calculated based on the above formulas (11) and (12) described in Non-Patent Document 2 using the component concentrations and temperature of the slag measured 5 minutes after the start of the refining process.

[0046] [Table 3]

[0047] In the above operation, for Nos. 29 to 36, before the start of the refining process, the concentrations of elements in the molten steel, the concentrations of elements in the slag, and the temperature of the molten steel were measured, and the concentrations of two or more molten steel elements, including S, were estimated based on a mathematical model created without using information about the ladle. Then, the refining process conditions were determined and the refining process was carried out. Here, the determined refining process condition was the amount of desulfurization material charged.

[0048] The results shown in Table 3 indicate that the refining process was performed without insufficient or excessive desulfurization in the samples Nos. 29 to 36, which are compatible with the present invention. Furthermore, it was confirmed that the sulfur concentration in the molten steel after refining was within the range of 0.0013 to 0.0015 mass%. The samples Nos. 31 to 36, which had slag electrical conductivities of 10 S / m or higher, were able to keep the amount of carbon picked up in the molten steel lower than the samples Nos. 29 and 30, which had slag electrical conductivities of less than 10 S / m. Furthermore, the samples Nos. 29 to 34, which had slag electrical conductivities of 300 S / m or lower, were able to reduce the sulfur concentration in the molten steel after refining more than the samples Nos. 35 and 36, which had slag electrical conductivities greater than 300 S / m.

Claims

1. A method for refining molten steel contained in a ladle, comprising: The refining conditions, including the component concentrations of the molten steel, the component concentrations of the slag, and the molten steel temperature, at any time before the start of the refining process or during the refining process are input into a mathematical model, and the concentrations of two or more components in the molten steel, including S, during the refining process are estimated, and the conditions for the subsequent refining process are determined based on the estimated component concentrations of the molten steel and the component concentrations targeted for the refining process. A method for refining molten steel, wherein the refining conditions to be determined include at least one of the amount of desulfurization material charged, the amount of alloy charged, the flow rate of stirring gas supplied into the molten steel, and the amount of electric power for arc heating.

2. 2. The method for refining molten steel according to claim 1, wherein the mathematical model is a model created using refining conditions including information about the ladle.

3. 2. The method for refining molten steel according to claim 1, wherein the slag has an electrical conductivity of 10 S / m or more and 300 S / m or less.

4. A method for producing steel, comprising the molten steel refining method according to any one of claims 1 to 3 as a ladle refining step.

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