Method for dephosphorizing molten iron

Abstract

To provide a method for dephosphorizing molten iron, which can effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron by applying electric energy by an electrochemical method, comprising applying a current between the molten slag and the molten iron through an electrode in contact with the molten iron as an anode and an electrode in contact only with the molten slag as a cathode, the current density I applied by the current satisfies the following relational expression (1) between the molten steel temperature T of the molten iron, the phosphorus distribution Lp of the molten slag, and the required phosphorus distribution Lp' of the molten slag: [0010] TIFF0007541655000020.tif13147In equation (1), I is the applied current density (A / m 2 ), α is a constant, Lp is the phosphorus distribution in the molten slag (-), Lp' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K).

Description

[Technical field]

[0001] The present invention relates to a method for dephosphorization of molten iron, and more particularly to a method for dephosphorization of molten iron, capable of accelerating a dephosphorization reaction. [Background technology]

[0002] Phosphorus contained in molten steel is likely to segregate at grain boundaries in materials formed from the molten steel, and reduces the strength and toughness of the steel formed from the molten steel. For this reason, the phosphorus content in the molten steel is generally controlled to be low when producing steel from molten steel.

[0003] In recent years, the trend towards producing high-grade steel has increased, and there is a demand to further reduce the phosphorus content in molten steel. However, due to reasons such as the deterioration of the quality of iron ore, which is the raw material for molten steel, the load caused by the dephosphorization process in molten steel is becoming increasingly severe. From such a technical point of view, it is essential to develop a technology to reduce the phosphorus concentration in molten steel after the molten steel is treated.

[0004] In the conventional dephosphorization technology of molten steel, since the phosphorus oxide contained in the molten steel is acidic, a lime-based flux such as CaO is added to the molten steel to increase the basicity of the molten slag formed during the production of molten steel. By adding a lime-based flux such as CaO to the molten steel, the dephosphorization ability of the molten slag can be improved and the equilibrium phosphorus concentration of the molten steel can be reduced.

[0005] However, molten slag with high basicity has a high melting temperature, and when the molten steel temperature drops during hot metal pretreatment, the viscosity of the molten slag increases. This causes insufficient slag formation of CaO contained in the lime-based flux absorbed into the molten steel, which tends to reduce the dephosphorization efficiency. In order to increase the low dephosphorization efficiency and compensate for the reaction efficiency of the dephosphorization reaction, it is necessary to use an excess of refining material in the molten steel. When an excess of refining material is used in the molten steel, the cost of refining the molten steel increases. Furthermore, when dephosphorization is performed to reduce the phosphorus concentration in the molten steel to an extremely low phosphorus concentration, the amount of molten slag becomes excessive. As a result, problems such as insufficient capacity of the molten steel refining equipment and the slag removal equipment occur.

[0006] Therefore, in the dephosphorization process of molten steel aiming at extremely low phosphorus, a method has been adopted in which halides such as fluorite (CaF2) are added to promote the formation of slag from CaO contained in the lime-based flux and reduce the amount of flux added, thereby achieving both dephosphorization capacity and a reduction in the amount of molten slag, thereby maintaining a high dephosphorization rate up to the extremely low phosphorus range.

[0007] However, adding halides such as fluorite (CaF2) to molten steel increases the content of fluorine (F) in the molten slag that is formed. In recent years, with increasing social interest in environmental issues, the use of fluorine (F) is restricted due to the problem of elution. From this perspective, it has become difficult to use halides such as fluorite (CaF2) in molten steel. Therefore, there is a demand for technology that can efficiently dephosphorize phosphorus in molten steel without increasing the basicity of the slag.

[0008] In view of the above situation, research has been conducted on technology to efficiently dephosphorize phosphorus in molten steel without increasing the basicity of the molten slag, focusing on electrical energy. For example, Non-Patent Document 1 discloses a method for promoting dephosphorization reactions, in which the reaction occurring at the interface between slag and molten iron (hereinafter referred to as slag-metal reaction) is organized based on the concept of electrochemistry. According to Non-Patent Document 1, the concept of promoting reactions by electrical energy is presented, using as a concrete example the fact that iron is oxidized and removed in the slag by polarizing the potential of molten iron to the noble side, and conversely, when the potential is polarized to the noble side, the iron ions in the slag are reduced and returned to the molten iron. Since then, various studies related to the slag-metal reaction have been reported based on Non-Patent Document 1.

[0009] Patent Document 1 discloses a method for reducing the content of granular iron in the molten slag and the variation in the metallic iron content in the slag from charge to charge by passing an electric current through an electrode in contact with the molten slag and an electrode in contact with the iron bath.

[0010] Patent Document 2 discloses a method for transferring impurities such as phosphorus and sulfur in steelmaking slag to molten scrap iron that has sunk to the bottom of the hearth and absorbing them to be recycled as steelmaking slag. The method for recycling steelmaking slag described in Patent Document 2 reduces the phosphorus and sulfur in the slag by applying an electric current to an electrode on the slag side as the anode and an electrode on the molten scrap iron side as the cathode, and transfers the phosphorus and sulfur in the slag to the molten scrap iron. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 7158570 specification [Patent Document 2] Japanese Patent Application Publication No. 11-302719 [Non-patent literature]

[0012] [Non-Patent Document 1] Masanori Tokuda, "Coupling Phenomena in Slag-Metal Reactions," Journal of the Japan Institute of Metals, Vol. 15, No. 6 (1976), accepted February 17, 1976 Summary of the Invention [Problem to be solved by the invention]

[0013] However, such conventional techniques have the following problems to be solved. That is, Non-Patent Document 1 discloses a method for accelerating the dephosphorization reaction after electrochemically sorting out the slag-metal reaction. However, all of the methods for accelerating the dephosphorization reaction of molten iron described in Non-Patent Document 1 involve changing the mixed potential of the dephosphorization reaction system of molten iron or the equilibrium potential of the phosphorus reaction by using additives or the like, and do not involve applying electrical energy from the outside to accelerate the dephosphorization reaction. On the other hand, the technique disclosed in Patent Document 1 applies an electric current between the slag and metal. In the method for treating molten slag described in Patent Document 1, the purpose of applying an electric current between the slag and metal is to reduce the content of the iron particles present in the slag and the variation of the iron particles. In other words, in the method for treating molten slag described in Patent Document 1, the effect of passing an electric current between the slag and metal is to promote the coarsening and aggregation of the iron particles, so the effect on the electrochemical reaction is not taken into consideration, and the dephosphorization reaction from the molten iron is not promoted.

[0014] In the technology disclosed in Patent Document 2, electrical energy is used for current heating to melt the molten slag and scrap iron, and does not contribute to electrochemical reactions. In addition, the technology disclosed in Patent Document 2 promotes so-called rephosphorization, which removes phosphorus from molten slag and transfers it to molten iron, but does not promote dephosphorization reaction from molten iron.

[0015] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for dephosphorizing molten iron, which can effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron by applying electrical energy by an electrochemical technique. [Means for solving the problem]

[0016] In order to solve the above problems, the inventors conducted various experiments and discovered that in a method for dephosphorizing molten iron by applying an electric current between molten slag and molten iron, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron by controlling the applied electric current based on the relationship between phosphorus distribution in molten slag, required phosphorus distribution in molten slag, and the molten steel temperature of molten iron. The present invention was made based on the above findings and is summarized as follows.

[0017] That is, the method for dephosphorization of molten iron according to the present invention, which advantageously solves the above-mentioned problems, is a method for dephosphorization of molten iron, in which an electrode in contact with the molten iron is an anode and an electrode in contact only with the molten slag is a cathode, and a current is applied between the molten slag and the molten iron through both electrodes, wherein the applied current density I satisfies the following relationship (1) between the molten steel temperature T of the molten iron, the phosphorus distribution in the molten slag LP, and the required phosphorus distribution in the molten slag LP'.

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[0018] The method for dephosphorizing molten iron according to the present invention includes the steps of: (a) the carbon concentration [C] of the carbon contained in the molten iron is 4.0 mass% or less, and the applied current density I (A / m 2 ) satisfies the following relational expression (2):

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[0019] According to the present invention, by applying electrical energy to molten iron by an electrochemical method, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron without modifying the slag. [Brief description of the drawings]

[0020] [Figure 1] 1 is a schematic diagram showing an overview of a molten iron dephosphorization apparatus used to carry out a molten iron dephosphorization method according to an embodiment of the present invention. [Diagram 2] 1 is a graph showing the relationship between the dephosphorization treatment time (min) of molten iron and the phosphorus concentration (mass%) when the dephosphorization method of molten iron according to the present embodiment is carried out. [Diagram 3] 1 is a graph showing the relationship between the applied current density and phosphorus distribution of phosphorus contained in molten iron when a current is applied to electrodes using a molten iron dephosphorization apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] [First embodiment] A method for dephosphorizing molten iron according to the first embodiment will be described. The method for dephosphorizing molten iron according to this embodiment is a method for dephosphorizing molten iron in which an electrode in contact with the molten iron is an anode and an electrode in contact only with the molten slag is a cathode, and a current is applied between the molten slag and the molten iron through both electrodes. A molten iron dephosphorization apparatus for implementing the method for dephosphorizing molten iron according to this embodiment will be described.

[0022] <Outline of molten iron dephosphorization equipment> Fig. 1 is a schematic diagram showing an overview of a molten iron dephosphorization apparatus used in carrying out the molten iron dephosphorization method according to this embodiment. As shown in Fig. 1, the molten iron dephosphorization apparatus 100 includes an MgO crucible 101, a ramming material 102, and an induction melting furnace 103. The MgO crucible 101 is charged with molten iron formed by melting scrap, molten iron, etc. The MgO crucible 101 is preferably made of a material that has low solubility in molten iron and is thermodynamically stable. In addition to the MgO crucible 101, a CaO crucible or an Al2O3 crucible can be used as a crucible. The shape of the MgO crucible 101 is not particularly limited, but may be cylindrical. When the MgO crucible 101 is cylindrical, its cross-sectional area is 0.005 to 0.030 m2. 2 For example, 0.018 m 2 may be also possible.

[0023] The ramming material 102 is a refractory material. The ramming material 102 covers the side wall and the bottom surface of the MgO crucible 101, and is attached to the inner wall of the induction melting furnace 103. The thickness of the ramming material 102 is preferably 100 to 150 mm in terms of heat resistance and durability. The induction melting furnace 103 is a crucible-type low-frequency induction furnace. The induction melting furnace 103 can hold molten iron while maintaining a high temperature. An induction heater may be provided on the side wall of the induction melting furnace 103.

[0024] Furthermore, the molten iron dephosphorization apparatus 100 includes a cathode 104 and an anode 105 for applying a current between the molten slag 200 and the molten iron 300 charged in the MgO crucible 101. The cathode 104 contacts only the molten slag 200, and does not contact the molten iron 300 present on the lower surface of the molten slag 200. The cathode 104 is preferably made of a heat-resistant material because the molten slag temperature of the molten slag 200 charged in the MgO crucible 101 becomes extremely high. The material for forming the cathode 104 is preferably graphite or artificial graphite. The cathode 104 may be in the form of a rod or a plate.

[0025] The anode 105 is in contact with the molten iron 300. The anode 105 is in contact with the molten slag 200 and the molten iron 300. The anode 105 is preferably made of a heat-resistant material because the molten steel temperature of the molten slag 200 and the molten iron 300 charged in the MgO crucible 101 becomes extremely high. The material for forming the anode 105 may be a composite material such as carbon or C-MgO. The form of the anode 105 may be, for example, a core metal part of a stirring lance that is immersed in the molten iron (molten metal) to stir the molten iron (molten metal) by blowing in an inert gas such as argon gas or nitrogen gas, or a graphite-containing refractory brick installed below the surface of the molten iron (molten metal).

[0026] Cathode 104 attached to induction melting furnace 103 is connected to the negative electrode of DC power supply 106 installed outside induction melting furnace 103 via conductor 107. Anode 105 attached to induction melting furnace 103 is connected to the positive electrode of DC power supply 106 installed outside induction melting furnace 103 via conductor 107. In this way, an electric circuit is formed by connecting both electrodes consisting of cathode 104 and anode 105 to DC power supply 106 installed outside induction melting furnace 103.

[0027] The upper surfaces of the MgO crucible 101, the ramming material 102, and the induction melting furnace 103 included in the molten iron dephosphorization apparatus 100 are covered with a heat insulating board 108. The heat insulating board 108 covers the upper surfaces of the MgO crucible 101, the ramming material 102, and the induction melting furnace 103, and has a role of maintaining the temperature of the molten iron 300 charged in the MgO crucible 101. The heat insulating board 108 is not particularly limited as long as it is made of a material having heat insulating properties.

[0028] <Dephosphorization of molten iron by applying electric current> In the method for dephosphorizing molten iron according to this embodiment, an electrode in contact with the molten iron is used as an anode, and an electrode in contact only with the molten slag is used as a cathode, and a current is applied between the molten slag and the molten iron through both electrodes. Hereinafter, the dephosphorization of molten iron by applying an electric current in the method for dephosphorization of molten iron according to this embodiment will be described.

[0029] Industrial pure iron is charged into an MgO crucible 101. A ramming material 102 is embedded in the outer wall of the MgO crucible 101, and the industrial pure iron is heated and melted using an induction melting furnace 103 to obtain molten iron 300. The molten iron 300 is produced by adjusting the phosphorus concentration of phosphorus contained in the molten iron 300 to fall within a predetermined range. Here, the phosphorus concentration of phosphorus contained in the molten iron 300 may be 0.01 to 0.20 mass%, preferably 0.05 to 0.15 mass%, and more preferably 0.08 mass%. The total amount of the molten iron 300 produced by melting the industrial pure iron can be set to 5 to 30 kg, preferably 10 to 20 kg, and more preferably 15 kg.

[0030] Furthermore, in the molten iron dephosphorization apparatus 100 system, flux is added to the upper surface of the molten iron 300 to form molten slag 200 on the upper surface of the molten iron 300. The amount of flux added can be appropriately determined depending on the inner volume of the MgO crucible 101, the total amount of the molten iron 300, and the like. The amount of flux added may be, for example, 10 to 30 kg / t of molten iron, preferably 15 to 25 kg / t of molten iron, and more preferably 20 kg / t of molten iron.

[0031] The compositional components of the flux are not particularly limited as long as they contain components capable of forming the molten slag 200. The compositional components of the flux may contain, for example, CaO, SiO2, FeO, MgO, etc. When the flux contains CaO, SiO2, FeO, and MgO as its compositional components, the content thereof may be 22.5 (% CaO), 28.0 (% SiO2), 42.5 (% FeO), and 7.0 (% MgO) on a mass basis.

[0032] After the flux is poured onto the top surface of the molten iron 300, the molten steel temperature of the molten iron 300 present inside the MgO crucible 101 is maintained in the range of 1300 to 1700°C, preferably 1585 to 1615°C. By pouring the flux into the molten iron 300 and maintaining that temperature, molten slag 200 and molten iron 300 are formed within the system of the molten iron dephosphorization apparatus 100. The molten slag 200 is formed on the surface of the molten iron 300. A molten slag 200 (slag)-molten iron 300 (metal) interface is formed between the molten slag 200 and the molten iron 300. The molten slag 200 thus formed has a molten slag composition that allows the insertion of a cathode 104 and an anode 105 into the molten slag 200 for use in applying an electric current between the molten slag 200 and the molten iron.

[0033] The cathode 104 is immersed in the molten slag 200 formed in the system of the molten iron dephosphorization apparatus 100. The cathode 104 is immersed only in the molten slag 200. Meanwhile, the anode 105 is immersed in the molten slag 200 and the molten iron 300 formed in the system of the molten iron dephosphorization apparatus 100. That is, the anode 105 may be immersed in the molten slag 200 and the molten iron 300 formed from C-MgO bricks, which are carbon-containing refractories. A DC power source 106 is used between both electrodes consisting of the cathode 104 and the anode 105 installed in the molten iron dephosphorization apparatus 100 in this way, and a DC current is applied to the molten slag 200 and the molten iron 300.

[0034] The current density applied between the electrodes is preferably determined taking into consideration the dephosphorization treatment time, the required phosphorus distribution considering the target phosphorus concentration, and the power cost. Specifically, by setting the current density applied between the electrodes in order to increase the current density applied between the electrodes, the attainable phosphorus concentration can be reduced and the dephosphorization rate can be increased. For this reason, by applying a large current between the two electrodes, the processing time required for dephosphorization of molten iron can be shortened and molten iron (molten metal) with a phosphorus concentration below the target level can be obtained, but the power cost is excessive. From this technical point of view, the applied current density can be set so as to satisfy the following relational expression (1), and a current can be applied between the electrodes.

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[0035] In the method for dephosphorizing molten iron according to the present embodiment, the value of the applied current density I calculated using the relational expression (1) is the current density that can obtain the minimum dephosphorization effect of the molten iron. Here, there are two main effects when a current density equal to or greater than the value of the applied current density I calculated using the relational expression (1) is applied between the molten slag 200 and the molten iron 300.

[0036] First, as can be seen from the later-described relational equation (5), the productivity of molten steel can be improved by increasing the reaction rate of the dephosphorization reaction of molten iron. Secondly, the cost of electricity required to dephosphorize molten iron increases. In other words, if the applied current density used to dephosphorize molten iron is increased more than necessary, both the productivity and production costs will increase. Therefore, it is desirable to determine the upper limit of the applied current density used to dephosphorize molten iron by taking into account the allowable operation time, cost, allowable current of the power source, etc. From such a technical viewpoint, it is extremely important to be able to calculate the value of the applied current density I using the relational expression (1) since it can ensure the dephosphorization effect of molten iron and provide the optimum conditions for the dephosphorization method of molten iron taking into account the allowable operation time, cost, and allowable current of the power source for dephosphorization of molten iron.

[0037] Specifically, in the method for dephosphorizing molten iron according to the present embodiment, the value of the applied current density I that can be set using the relational expression (1) is 150 to 600 (A / m 2 ), and preferably 200 to 500 (A / m 2 The applied current density I is 150 (A / m 2) or more, it is preferable because the phosphorus concentration of phosphorus contained in the molten iron 300 that can be reached can be reduced and the dephosphorization rate can be increased. 2 ) or less, the power cost for dephosphorizing the molten iron 300 can be suppressed, which is preferable. In the dephosphorization method of molten iron according to this embodiment, the phosphorus concentration of phosphorus contained in the molten iron 300 can be measured after a certain time has elapsed since the start of application of a current between the two electrodes.

[0038] Figure 2 is a graph showing the change in phosphorus concentration over time in molten iron when dephosphorization is performed by applying a current between the electrodes using a molten iron dephosphorization apparatus. That is, Figure 2 shows the relationship between the dephosphorization time (min) and the phosphorus concentration (mass%) of molten iron. As shown in Figure 2, the larger the applied current value, the higher the dephosphorization rate becomes and the lower the achieved phosphorus concentration becomes.

[0039] 3 is a graph showing the relationship between the applied current density and the phosphorus distribution of phosphorus contained in the molten slag when the molten iron is dephosphorized by applying a current to the electrodes using a molten iron dephosphorization apparatus. As shown in FIG. 3, it can be seen that the logarithm of the phosphorus distribution Lp contained in the molten slag 200 tends to increase linearly with the applied current density I applied to both electrodes. This tendency is the same even if the composition of the molten slag 200 is changed. Furthermore, this tendency is maintained even if the molten iron 300 is dephosphorized by combining stirring with a bubbling lance. Furthermore, the present inventors have confirmed that the logarithm of the phosphorus distribution of the molten slag 200 increases linearly with the applied current density I even if the molten steel temperature T when the dephosphorization method for the molten iron 300 is applied is changed.

[0040] The principle by which the dephosphorization method of molten iron according to this embodiment can accelerate the dephosphorization reaction of molten iron is believed to be as follows. That is, by applying a current to the molten iron 300 and the molten slag 200, the potential on the molten iron 300 side is polarized to a more noble potential and the potential on the molten slag 200 side is polarized to a more noble potential through both electrodes. The potential change at this time is called overvoltage. Here, due to the change in Gibbs energy corresponding to the overvoltage, the equilibrium reaction formula and the equilibrium constant K of the dephosphorization reaction of phosphorus contained in the molten iron 300 are as follows:

[0041] [ka]

[0042]

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[0043] By applying a current to both electrodes using the molten iron dephosphorization apparatus 100, the dephosphorization reaction of phosphorus contained in the molten iron 300 progresses, and phosphorus (P) contained in the molten iron 300 is converted into phosphorus ions (P 5+ ) and phosphorus ions (P 5+ ) concentration [P 5+ ] increases. Therefore, the equilibrium constant K of the dephosphorization reaction of phosphorus contained in molten iron 300 increases. As a result, it is considered that the concentration [P] of phosphorus (P) contained in molten iron 300 decreases. In this case, if the reaction order of the dephosphorization reaction of phosphorus contained in molten iron 300 is set to first order and the reaction rate v of the dephosphorization reaction is expressed as a linear equation of the phosphorus concentration [P], the following relational equation (5) is obtained. In relational equation (5), t is the dephosphorization treatment time t (s) of phosphorus contained in molten iron, k is the apparent reaction rate constant of the dephosphorization reaction of phosphorus contained in molten iron, and [P]e is the equilibrium phosphorus concentration when the dephosphorization reaction of phosphorus contained in molten iron reaches dissolution equilibrium.

[0044]

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[0045] According to the relational expression (5), the equilibrium phosphorus concentration [P]e at the time when the dephosphorization reaction of the phosphorus contained in the molten iron 300 reaches the dissolution equilibrium decreases, and thus the reaction rate v of the dephosphorization reaction of the phosphorus contained in the molten iron 300 increases. In this case, if the molten steel temperature at the time when the dephosphorization reaction of the phosphorus contained in the molten iron 300 reaches the dissolution equilibrium is T (K), the phosphorus distribution of the phosphorus contained in the molten slag 200 when no current is applied to the molten iron 300 and the molten slag 200 is Lp, and the required phosphorus distribution of the phosphorus contained in the molten slag 200 after the current is applied to the molten iron 300 and the molten slag 200 is Lp', the current application density I of the current required to obtain the required phosphorus distribution Lp' is expressed by the following relational expression (1).

[0046]

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[0047] That is, in the relational expression (1), the phosphorus concentration C of phosphorus contained in the molten slag at the molten steel temperature T (K) after a certain time has elapsed slag and the concentration of phosphorus in the molten iron, C iron and calculates the phosphorus distribution Lp(-) of the molten slag 200. Next, a current is applied to both electrodes using the molten iron dephosphorization apparatus 100 to determine the required phosphorus distribution Lp'(-) of the molten slag 200 when the dephosphorization reaction of the phosphorus contained in the molten iron 300 progresses and reaches dissolution equilibrium. Then, the phosphorus distribution Lp(-) of the molten slag 200 at the molten steel temperature T (K) is calculated, the required phosphorus distribution Lp'(-) of the molten slag 200 is set, and the applied current density I (A / m 2 ) can be calculated.

[0048] Here, the constant α in the relational expression (1) is calculated as follows: First, the applied current density I (A / m 2) and the overvoltage η that occurs between the electrodes formed by the cathode 104 and the anode 105 when a current is applied to the molten iron 300 and the molten slag 200. 2 ) and the overvoltage η are proportional to each other and can be expressed by the following equation (6). Therefore, the applied current density I (A / m 2 The slope α can be calculated from a graph showing the relationship between the overvoltage η and the temperature

[0049]

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[0050] On the other hand, when a current is applied to molten iron 300 and molten slag 200, the change in Gibbs energy ΔG initial and the change in Gibbs energy ΔG when the phosphorus in molten iron reaches the solution equilibrium after the application of electric current. equilibrium are expressed by the following relations (7) and (8), respectively.

[0051]

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[0052]

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[0053] Furthermore, from the relational expressions (7) to (8), the overvoltage η generated between the electrodes formed by the cathode 104 and the anode 105 when a current is applied to the molten iron 300 and the molten slag 200 is expressed by the following relational expression (9). The constant α in the relational expression (1) can be calculated by comparing the relational expressions (6) and (9). In the relational equation (9), η is the overpotential, R is the gas constant, F is the Faraday constant, Z is the valence, Lp is the phosphorus distribution in the molten slag at the molten steel temperature T (K) (-), and Lp' is the required phosphorus distribution (-).

[0054]

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[0055] In this manner, the method for dephosphorizing molten iron according to this embodiment determines the phosphorus distribution Lp(-) in the molten slag and the required phosphorus distribution Lp'(-) in the molten slag at the molten steel temperature T (K), and uses the relational expression (1) in which the constant α is determined to determine the applied current density I of the current required to obtain the required phosphorus distribution Lp'.

[0056] As described above, according to the first embodiment of the present invention, by controlling the current density of the applied current based on the relationship between the phosphorus distribution in the molten slag, the required phosphorus distribution in the molten slag, and the molten steel temperature of the molten iron, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of the molten iron.

[0057] [Second embodiment] A method for dephosphorizing molten iron according to a second embodiment will be described. The method for dephosphorizing molten iron according to this embodiment is the same as the method for dephosphorizing molten iron according to the above embodiment, except that the carbon concentration [C] of carbon contained in the molten iron is 4.0 mass% or less and the applied current density I (A / m 2 ) satisfies the following relational expression (2).

[0058]

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[0059] In the dephosphorization method of molten iron according to the present embodiment, the constant α in the relational expression (1) employed in the dephosphorization method of molten iron according to the above embodiment is calculated as follows: α=5.264×10 -2 In other words, the method for dephosphorizing molten steel according to the present embodiment employs the relational expression (2) to measure the phosphorus concentration after applying a current having an applied current density I to the molten slag 200 and the molten iron 300, thereby obtaining the molten slag distribution Lp′.

[0060] Furthermore, in the method for dephosphorizing molten iron according to the present embodiment, the current applied to the molten slag and the molten iron is controlled by the current value, so that the method for dephosphorizing molten iron according to the present embodiment can reduce the effect on the above-mentioned change in overvoltage even if the electrical properties of the molten iron change. Furthermore, by applying a current having an applied current density that satisfies the relation (2) employed in the method for dephosphorizing molten steel according to this embodiment to the molten slag and molten iron, it is possible to effectively improve phosphorus distribution without modifying the molten slag, and the method for dephosphorizing molten iron according to this embodiment can be applied to molten iron with various compositional components.

[0061] In the dephosphorization method of molten iron according to the present embodiment, the carbon concentration [C] of carbon contained in the molten iron to which a current is applied is preferably 4.0 mass% or less. If the carbon concentration [C] of carbon contained in the molten iron to which a current is applied is 4.0 mass% or less, it is preferable because the carbon content that can be contained in the molten iron can be secured. Note that the carbon concentration [C] of carbon contained in the molten iron to which a current is applied may be 0.1 mass% or more. As described above, the method for dephosphorization of molten iron according to this embodiment is advantageous in that it can be applied without any particular restriction on the carbon concentration within the range of carbon concentrations that can be contained in molten iron.

[0062] As described above, according to the invention of the second embodiment, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron without being affected by changes in overpotential due to changes in the electrical properties of molten iron or by the concentrations of compositional components contained in the molten iron, such as the carbon concentration [C] and phosphorus concentration [P].

[0063] [Third embodiment] A method for dephosphorizing molten iron according to a third embodiment will now be described. The method for dephosphorizing molten iron according to this embodiment is characterized in that, in the method for dephosphorizing molten iron according to the above-mentioned embodiments, no arc discharge occurs when the current is applied between the molten slag and the molten iron. Hereinafter, technical features included in the method for dephosphorizing molten iron according to this embodiment will be described.

[0064] The method for dephosphorizing molten iron according to this embodiment performs dephosphorization of molten iron under conditions that prevent arc discharge from occurring due to the current applied between the molten slag and the molten iron. That is, the method for dephosphorizing molten iron according to this embodiment prevents arc discharge from occurring due to the current applied between the molten slag and the molten iron, thereby accelerating the reaction between phosphorus contained in the molten iron and iron oxide, and effectively performing dephosphorization of the molten iron.

[0065] In view of promoting the dephosphorization reaction of molten iron, it is undesirable for an arc discharge to occur within the system of the molten iron dephosphorization apparatus due to the current applied between the molten slag and the molten iron. Hereinafter, the reason for adopting the conditions for preventing the arc discharge from occurring due to the current applied between the molten slag and the molten iron in the dephosphorization method of molten iron according to this embodiment will be explained.

[0066] In the method for dephosphorization of molten iron according to this embodiment, as shown in the following equilibrium reaction formula (10), phosphorus (P) contained in the molten iron before dephosphorization reacts with iron oxide (FeO) contained in the molten iron to become diphosphorus pentoxide, and the iron oxide (FeO) contained in the molten iron before dephosphorization is reduced to iron (Fe). The equilibrium constant Kp in the chemical equilibrium reaction formula (10) is as shown in the following relational formula (11).

[0067] [ka]

[0068]

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[0069]

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[0070] Furthermore, taking the logarithm of both sides of equation (11), which shows the equilibrium constant Kp, and rearranging the relationship between the activity of diphosphorus pentoxide (P2O5), iron (Fe), phosphorus (P), and iron oxide (FeO) contained in the molten iron, gives the following equation (13).

[0071]

number

[0072] Then, as is clear from relational equation (13), it can be understood that the dephosphorization reaction of molten iron shown by chemical equilibrium reaction equation (10) can be promoted by lowering the reaction temperature T. Generally, in methods for dephosphorization of molten iron, an arc discharge is generated by applying a current between the molten slag and the molten iron, thereby increasing the reaction temperature T of the dephosphorization reaction of molten iron. From this technical viewpoint, the method for dephosphorization of molten iron according to the present embodiment can promote the dephosphorization reaction of molten iron by preventing the generation of an arc discharge due to the current applied between the molten slag and the molten iron and by lowering the reaction temperature T.

[0073] Furthermore, from the chemical equilibrium reaction equation (10) and the relational equation (11) showing the equilibrium constant Kp, it is clear that increasing the activity of iron oxide (FetO) in molten iron promotes the reaction between the iron oxide and phosphorus in the molten slag, and promotes the dephosphorization reaction of molten iron. In addition, decreasing the activity of diphosphorus pentoxide (P2O5) can suppress the reverse reaction of the dephosphorization reaction of molten iron caused by the progress of the decomposition reaction of diphosphorus pentoxide (P2O5) in the molten slag.

[0074] On the other hand, the activity of phosphorus (P) in molten iron is p According to the equation (12), the activity of phosphorus (P) a p is the product of the phosphorus concentration [P] of phosphorus in molten iron and its activity coefficient fp. Here, the activity coefficient fp is larger as the carbon concentration [%C] in the molten steel is higher. Therefore, by increasing the carbon concentration [%C] in the molten steel and increasing the activity coefficient fp, the activity a of phosphorus (P) in the molten iron p can be made larger.

[0075] In the method for dephosphorizing molten iron according to the present embodiment, the current value of the current required to prevent arc discharge from occurring when applying a current between the molten slag and the molten iron is preferably 5000 (A) or less. In the method for dephosphorizing molten iron according to the present embodiment, if the current value applied between the molten slag and the molten iron is 500 to 5000 (A) or less, an arc discharge does not occur, and energy balance in the dephosphorization reaction of the molten iron can be achieved, thereby ensuring thermal efficiency in the dephosphorization treatment of the molten iron. From such a technical viewpoint, the molten iron dephosphorization apparatus to which the method for dephosphorizing molten iron according to the present embodiment can be applied is preferably a DC electric furnace.

[0076] DC electric furnaces require less electricity, electrodes and refractories per unit of production, and produce less noise and flicker. Furthermore, by installing scrap preheating and continuous charging equipment in DC electric furnaces, high-temperature exhaust gas can be used for preheating, and heat radiation caused by opening the furnace lid when charging scrap can be prevented, reducing energy consumption.

[0077] In addition, DC electric furnaces are increasingly being equipped with scrap preheating and continuous charging equipment, and the eccentric bottom tapping method is being adopted. If the eccentric bottom tapping method is adopted for DC electric furnaces, it is efficient because steel can be tapped quickly without tilting the furnace body, and it is also preferable in terms of maintaining the cleanliness of the molten steel because slag is less likely to flow into the ladle during tapping.

[0078] As described above, according to the invention of the third embodiment, by reducing the reaction temperature T without generating an arc discharge and setting the current value of the current applied between the molten slag and the molten iron to 5000 (A) or less, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of the molten iron. The method for dephosphorization of molten iron according to the third embodiment can be carried out using a DC electric furnace.

[0079] [Fourth embodiment] A method for dephosphorizing molten iron according to a fourth embodiment will be described. The method for dephosphorizing molten iron according to this embodiment is characterized in that the liquid phase ratio of the molten slag is 60 vol.% or more in the method for dephosphorizing molten iron according to the above embodiments. The technical features of the method for dephosphorizing molten iron according to this embodiment will be described below.

[0080] In the method for dephosphorizing molten iron according to the present embodiment, molten slag 200 is formed in a molten iron dephosphorization apparatus 100, such as a refining reaction vessel, into which molten iron 300 (molten metal) is charged. At this time, the molten slag 200 is added to the upper surface of the molten iron 300 so as to have a thickness sufficient to allow the cathode 104 to be immersed only in the molten slag 200. An electrode made of a conductive material is immersed only in the molten slag 200 to form the cathode 104.

[0081] In the dephosphorization method of molten iron according to the present embodiment, the components of the molten slag 200 used in the dephosphorization treatment of phosphorus contained in the molten iron 300 are preferably molten slag 200 containing CaO, SiO2, FeO, MgO, etc., which are generally used in dephosphorization refining. According to the dephosphorization method of molten iron according to the present embodiment, as is clear from the relational expressions (1) and (2), the component composition of the molten slag 200 is not particularly limited, and the required phosphorus distribution of the molten slag 200 can be increased.

[0082] In the method for dephosphorization of molten iron according to the present embodiment, the cathode 104 of the molten iron dephosphorization apparatus 100 must be immersed only in the molten slag 200, and the liquid phase ratio of the molten slag 200 is preferably 60 vol.% or more in order to improve the reaction efficiency of the dephosphorization reaction. The liquid phase ratio of the molten slag 200 may be a value that allows the cathode 104 and the anode 105 used to apply a current between the molten slag 200 and the molten iron to be inserted into the molten slag 200. If the liquid phase ratio of the molten slag 200 is 60 vol.% or more, the cathode 104 can be sufficiently immersed in the molten slag 200, and the dephosphorization reaction of the molten iron can be promoted. If the liquid phase ratio of the molten slag 200 is 95 vol.% or less, the operation of the molten iron dephosphorization apparatus 100 is easy, and this is preferable. The liquid phase ratio of the molten slag 200 refers to the ratio of the liquid phase in the molten slag 200.

[0083] As described above, according to the fourth embodiment of the invention, by setting the liquid phase ratio of the molten slag to 60 vol.% or more, the cathode of the molten iron dephosphorization apparatus can be sufficiently immersed in the molten slag, effectively improving phosphorus distribution and accelerating the dephosphorization reaction of molten iron.

[0084] [Other embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical scope of the present invention. In addition, programs, systems, or devices that combine the separate features included in each embodiment in any way are also included in the technical scope of the present invention. EXAMPLES

[0085] The effects of the present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0086] (Example 1) The molten iron dephosphorization method according to the present embodiment was adopted to carry out dephosphorization of molten iron using an electric furnace. Scrap, iron phosphide (FeP), carbonaceous material, and CaO-SiO2-FeO-MgO slag were charged into the electric furnace of the electric furnace, and these molten steel raw materials were melted by an AC arc. 300 t of molten steel and 30 kg / t of molten slag were obtained inside the electric furnace.

[0087] Next, the graphite electrode used for the arc alternating current was immersed in the molten slag to serve as the cathode. The core metal part of the stirring lance was immersed in the molten steel to serve as the anode. In addition, argon gas (Ar) was introduced at 2.0 Nm 3 / min, while blowing current between the molten slag and molten steel at an average current density of 300 (A / m 2 ), and a direct current of 2100 (A) was applied for 30 minutes to carry out dephosphorization of the molten steel (Level 1). In the dephosphorization of molten steel, samples were taken before the dephosphorization (0 min), 10 min after the start of the dephosphorization, 20 min after the start of the dephosphorization, and 30 min after the start of the dephosphorization (completion of the dephosphorization), and the phosphorus concentration in the molten steel was measured to determine the phosphorus distribution (actual phosphorus distribution). Table 1 shows the phosphorus distribution and slag composition ratio over the dephosphorization time of the molten steel. Also shown are the slag liquid phase rate and whether or not the relationship shown in the above relational formula (2) is satisfied. In Example 1, the required phosphorus distribution of the molten slag was set to 100. Additionally, in the method for dephosphorizing molten iron of Example 1, the presence or absence of arc generation was confirmed, and the current value of the current applied between the molten slag and molten steel was set to a predetermined current value.

[0088] (Examples 2 to 6) In Examples 2 to 5, the average current density of the direct current applied between the molten slag and the molten steel was set to 300 to 350 (A / m 2 ), and the applied current value was changed to 5000 (A) or less, and the same procedure as in Example 1 was repeated to carry out dephosphorization of molten steel (Levels 2 to 5). On the other hand, in Example 6, the average current density of the direct current applied between the molten slag and the molten steel was set to 3000 (A / m 2) and the applied current value was set to 21,000 (A) to generate an arc discharge (Level 9). Table 1 shows the phosphorus distribution (actual phosphorus distribution) and slag composition ratio over the dephosphorization time of molten steel. It also shows the slag liquid phase rate and whether or not the relationship shown in the above relational formula (2) is satisfied. In addition, in the dephosphorization methods of molten iron in Examples 2 to 6, the presence or absence of arc generation was confirmed, and the current value applied between the molten slag and molten steel was set to a predetermined current value.

[0089] (Comparative Examples 1 to 2) The dephosphorization treatment of the molten steel was carried out in the same manner as in Example 1, except that no direct current was applied between the molten slag and the molten steel (Level 6). In addition, the average current density of the direct current applied between the molten slag and the molten steel was set to 200 (A / m 2 ), and dephosphorization of molten steel was carried out under conditions that do not satisfy the above-mentioned relational expression (2) (Level 7). Table 1 shows the phosphorus distribution (actual phosphorus distribution) and slag composition ratio over the dephosphorization time of molten steel. It also shows the slag liquid phase ratio and whether or not the relationship shown in the above-mentioned relational expression (2) is satisfied.

[0090] Comparative Example 3 The dephosphorization treatment of molten steel was carried out in the same manner as in Example 1, without applying a direct current between the molten slag and the molten steel (Level 8). Table 1 shows the phosphorus distribution (actual phosphorus distribution) and the slag composition ratio over the dephosphorization time of the molten steel. It also shows the slag liquid phase ratio and whether or not the relationship shown in the above formula (2) is satisfied.

[0091] [Table 1]

[0092] Table 1 shows the test conditions and results at each level adopted in Examples 1 to 6 and Comparative Examples 1 to 3. Table 1 also shows the required phosphorus distribution, and evaluates whether the calculated actual phosphorus distribution reaches this. As is clear from Table 1, under the condition where the slag liquid phase rate is 100% (Examples 1 to 2, 4 to 6), it was found that the calculated actual phosphorus distribution (phosphorus distribution obtained from the experimental results) does not reach the required phosphorus distribution under the condition that does not satisfy the above-mentioned relational formula (2) (Comparative Example 2). On the other hand, under the condition that the slag liquid phase rate is 100% (Invention Examples 1 to 2, 4 to 6), when a direct current with a current density satisfying the above relational formula (2) is applied, it is found that the required phosphorus distribution is achieved (Invention Examples 1 to 6). Furthermore, as shown in Examples 1 to 5, it was found that the required phosphorus distribution can be achieved extremely well even under conditions in which the applied current value is set to 5000 (A) or less and arc discharge is not generated. From such a technical viewpoint, it has become clear that the method for dephosphorizing molten iron according to the present embodiment can be suitably carried out by a DC electric furnace that can prevent arc discharge from occurring.

[0093] Furthermore, it is found that the larger the average current density of the direct current applied to the molten steel, the sooner the required phosphorus distribution is reached, and the dephosphorization treatment time can be shortened. Also, when the slag liquid phase rate is 30%, the solidified slag becomes an obstacle and the electrode cannot be immersed, so the dephosphorization method of molten iron according to the present invention cannot be applied (Comparative Example 3). However, if the slag liquid fraction is 60 vol.% or more, it is possible to immerse the electrode in the molten slag, but from the viewpoint of the reaction efficiency of the dephosphorization reaction, it is preferable that the slag liquid fraction is 60 vol.% or more. Note that these trends do not depend on the carbon concentration [C] of carbon or the phosphorus concentration [P] of phosphorus contained in the molten iron.

[0094] As described above, the method for dephosphorization of molten iron according to the present invention can achieve the required phosphorus distribution under the conditions that the slag liquid phase ratio is set to 60 vol.% or more and a direct current with a current density that satisfies the above-mentioned relational expression (2) is applied. In other words, it has become clear that by using the method for dephosphorization of molten iron according to the present invention and setting specific conditions, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron. [Industrial Applicability]

[0095] INDUSTRIAL APPLICABILITY The method for dephosphorization of molten iron according to the present invention can effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron without modifying slag, and is therefore extremely useful industrially, contributing to the development of related industries such as the steelmaking industry. [Explanation of symbols]

[0096] 100 Molten iron dephosphorization equipment 101 MgO crucible 102 Ramming material 103 Induction melting furnace 104 Cathode (graphite electrode) 105 Anode (MgO-C electrode) 106 DC power supply 107 Conductor 108 Insulation Board 200 Molten Slag 300 Molten Iron

Claims

1. A method for dephosphorizing molten iron, comprising the steps of: applying an electric current between the molten slag and the molten iron through an electrode that is in contact with the molten iron as an anode and an electrode that is in contact only with the molten slag as a cathode, the method comprising the steps of: a current density I of the current satisfying the following relationship (1) between a molten steel temperature T of the molten iron, a phosphorus distribution LP of the molten slag, and a required phosphorus distribution LP' of the molten slag: [0010] In the relational expression (1), I is the applied current density (A / m ２ ), α is a constant, LP is the phosphorus distribution in the molten slag (-), LP' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature T (K).

2. The carbon concentration [C] of carbon contained in the molten iron is 4.0 mass% or less, The applied current density I (A / m ２ 2. The method for dephosphorizing molten iron according to claim 1, characterized in that: [0025] In the relational expression (2), I is the applied current density (A / m ２ ), LP is the phosphorus distribution in the molten slag (-), LP' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K).

3. 3. The method for dephosphorizing molten iron according to claim 1, wherein no arc discharge occurs by applying the current between the molten slag and the molten iron.

4. 4. The method for dephosphorizing molten iron according to claim 3, wherein the current value of the electric current is 5000 (A) or less.

5. 3. The method for dephosphorizing molten iron according to claim 1, wherein the liquid phase rate of the molten slag is 60 vol. % or more.