Method for dephosphorizing molten iron

Abstract

To provide a method for dephosphorizing molten iron with which it is possible to effectively improve phosphorus distribution and promote dephosphorization reaction of molten iron by imparting electric energy by an electrochemical method. This method for dephosphorizing molten iron, in which an electrode in contact with molten iron is used as a positive electrode and an electrode in contact with only molten slag is used as a negative electrode, and a current is applied between the molten slag and the molten iron through the electrodes, is characterized in that the applied current density I of the current satisfies the following relational expression (1) with respect to 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. [Expression 1]: I ≥ α × In (Lp' / Lp) × T In the relational expression (1), I is the applied current density (A / m2), α is a constant, Lp is the phosphorus distribution (-) of the molten slag, Lp' is the required phosphorus distribution (-) of the molten slag, and T is the molten steel temperature (K) of the molten iron.

Description

Method for dephosphorizing molten iron

[0001] The present invention relates to a method for dephosphorizing molten iron, and more particularly to a method for dephosphorizing molten iron that can accelerate the dephosphorization reaction.

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

[0003] In recent years, the trend toward the production of high-quality steel has increased, and there is a demand for further reducing the phosphorus content in molten steel. However, due to factors such as the deterioration of the quality of iron ore, which is the raw material for molten steel, the load imposed by the dephosphorization process on molten steel has become increasingly severe. From this technical perspective, it is essential to develop a technology that can reduce the phosphorus concentration in molten steel after processing.

[0004] In conventional dephosphorization techniques for molten steel, since phosphorus oxide contained in the molten steel exhibits acidity, 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 results in insufficient slag formation of CaO contained in the lime-based flux absorbed into the molten steel, resulting in low dephosphorization efficiency. To improve this low dephosphorization efficiency and compensate for the reaction efficiency of the dephosphorization reaction, it is necessary to use an excess amount of refining material in the molten steel. Using an excess amount of refining material in the molten steel increases the cost of refining the molten steel. Furthermore, when attempting to dephosphorize the molten steel to an extremely low phosphorus concentration, an excessive amount of molten slag is generated. As a result, problems arise, such as insufficient capacity of the molten steel refining equipment and the slag removal equipment.

[0006] Therefore, in the dephosphorization process of molten steel aiming at ultra-low phosphorus, fluorite (CaF ２ A method has been adopted in which halides such as fluorine-containing nitrate are added to achieve both dephosphorization ability and a reduction in the amount of molten slag, thereby maintaining a high dephosphorization rate down to the extremely low phosphorus range.

[0007] However, this fluorite (CaF ２ Adding halides such as fluorine (F) 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 being restricted because of the problem of elution. From this perspective, adding fluorine (CaF) to molten steel is becoming increasingly common. ２ Therefore, there is a need for a technology for efficiently removing phosphorus from molten steel without increasing the basicity of the slag.

[0008] In light of the above situation, research has focused on electrical energy and has been conducted on technologies for efficiently dephosphorizing phosphorus from molten steel without increasing the basicity of the molten slag. For example, Non-Patent Document 1 discloses a method for promoting dephosphorization reactions that organizes the reaction that occurs at the interface between slag and molten iron (hereinafter referred to as the slag-metal reaction) based on electrochemical concepts. Non-Patent Document 1 demonstrates the concept of promoting reactions using electrical energy, using the example of how polarizing the potential of molten iron to the noble side oxidizes and removes iron in the slag, while polarizing it to the noble side reduces iron ions in the slag and returns them to the molten iron. Since then, various studies related to slag-metal reactions 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 for each charge by passing a 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 recycling impurities such as phosphorus and sulfur in steelmaking slag by transferring them to and absorbing them into molten scrap iron that has sunk to the bottom of the furnace. The method for recycling steelmaking slag described in Patent Document 2 involves 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, thereby reducing the phosphorus and sulfur in the slag and transferring them into the molten scrap iron.

[0011] Patent No. 7158570 Specification JP 11-302719 Publication

[0012] Masanori Tokuda, "Coupling Phenomena in Slag-Metal Reactions," Journal of the Japan Institute of Metals, Vol. 15, No. 6 (1976), accepted February 17, 1976

[0013] However, such conventional techniques still have the following problems to be solved. Specifically, Non-Patent Document 1 discloses a method for accelerating the dephosphorization reaction by electrochemically characterizing the slag-metal reaction. However, the methods for accelerating the dephosphorization reaction of molten iron described in Non-Patent Document 1 all involve changing the mixed potential of the dephosphorization reaction system of molten iron or the equilibrium potential of the phosphorus reaction using additives or the like, and do not accelerate the dephosphorization reaction by applying electrical energy from an external source. On the other hand, the technology disclosed in Patent Document 1 applies an electric current between the slag and metal. In the molten slag treatment method described in Patent Document 1, the purpose of applying an electric current between the slag and metal is to reduce the content and variation of iron nuggets present in the slag. In other words, the effect of applying an electric current between the slag and metal in the molten slag treatment method described in Patent Document 1 is to promote the aggregation and coarsening of iron nuggets, without taking into account the impact on the electrochemical reaction, and it does not accelerate the dephosphorization reaction from molten iron.

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

[0015] The present invention has been made in view 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 accelerate the dephosphorization reaction of molten iron by applying electrical energy by an electrochemical method.

[0016] Therefore, the present inventors conducted various experiments to solve the above-mentioned problems, and as a result, 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 the molten slag, the required phosphorus distribution in the molten slag, and the molten steel temperature of the molten iron. The present invention was made based on the above-mentioned findings, and its gist is as follows.

[0017] That is, the present invention provides a method for dephosphorizing molten iron that advantageously solves the above-mentioned problems, in which 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, wherein the applied current density I satisfies the following relational expression (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': 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 of the molten iron (K).

[0018] The method for dephosphorizing molten iron according to the present invention is characterized in that: (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): In the relational expression (2), I is the applied current density (A / m ２), LP is the phosphorus distribution (-) of the molten slag, LP' is the required phosphorus distribution (-) of the molten slag, and T is the molten steel temperature (K) of the molten iron. (b) No arc discharge occurs when the current is applied between the molten slag and the molten iron, (c) the current value is 5000 (A) or less, and (d) the liquid phase ratio of the molten slag is 60 vol.% or more. These are considered to be more preferable solutions.

[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.

[0020] 1 is a schematic diagram showing an outline of a molten iron dephosphorization apparatus used to implement the molten iron dephosphorization method according to the present embodiment; 2 is a graph showing the relationship between the dephosphorization treatment time (min) of molten iron and the phosphorus concentration (mass%) when the molten iron dephosphorization method according to the present embodiment is implemented; 3 is a graph showing the relationship between the applied current density when a current is applied to an electrode using the molten iron dephosphorization apparatus and the phosphorus distribution of phosphorus contained in the molten iron;

[0021] [First Embodiment] A method for dephosphorizing molten iron according to the first embodiment will be described. In this method, 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 an electric current is applied between the molten slag and the molten iron through these two electrodes. A molten iron dephosphorization apparatus used to implement the method for dephosphorizing molten iron according to this embodiment will be described.

[0022] <Outline of the Molten Iron Dephosphorization Apparatus> Fig. 1 is a schematic diagram showing an outline of the molten iron dephosphorization apparatus used to carry 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 pig iron, etc. It is preferable that the MgO crucible 101 is made of a material that has low solubility in molten iron and is thermodynamically stable. In addition to the MgO crucible 101, other crucibles that can be used include CaO crucibles, Al crucibles, and the like. ２ O ３The 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 m. ２ For example, 0.018 m ２ may be.

[0023] The ramming material 102 is a refractory material. The ramming material 102 covers the sidewall and 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 from the viewpoint 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 sidewall of the induction melting furnace 103.

[0024] The molten iron dephosphorization apparatus 100 further 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, not the molten iron 300 present below the molten slag 200. The cathode 104 is preferably made of a heat-resistant material because the temperature of the molten slag 200 charged in the MgO crucible 101 is extremely high. The cathode 104 is preferably made of graphite or artificial graphite. The cathode 104 may be in the shape 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 formed from 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 anode 105 may be formed from a composite material such as carbon or C-MgO. The anode 105 may be in the form of, for example, the 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] A cathode 104 attached to the induction melting furnace 103 is connected to the negative pole of a DC power supply 106 installed outside the induction melting furnace 103 via a conductor 107. Anode 105 attached to the induction melting furnace 103 is connected to the positive pole of the DC power supply 106 installed outside the induction melting furnace 103 via a conductor 107. In this way, an electric circuit is formed by connecting both electrodes consisting of the cathode 104 and the anode 105 to the DC power supply 106 installed outside the induction melting furnace 103.

[0027] The top 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 top surfaces of the MgO crucible 101, the ramming material 102, and the induction melting furnace 103, and serves to maintain the temperature of the molten iron 300 charged into 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 dephosphorization method of 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 an electric current is applied between the molten slag and the molten iron through these two electrodes. Hereinafter, the dephosphorization of molten iron by applying electric current in the dephosphorization method of molten iron according to this embodiment will be described.

[0029] Commercially 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 industrially pure iron is heated and melted using an induction melting furnace 103 to produce molten iron 300. The phosphorus concentration of the phosphorus contained in the molten iron 300 is adjusted to fall within a predetermined range, thereby producing the molten iron 300. Here, the phosphorus concentration of the phosphorus contained in the molten iron 300 may be 0.01 to 0.20% by mass, preferably 0.05 to 0.15% by mass, and more preferably 0.08% by mass. The total amount of the molten iron 300 produced by melting the industrially pure iron can be set to 5 to 30 kg, preferably 10 to 20 kg, and more preferably 15 kg.

[0030] Furthermore, within the molten iron dephosphorization apparatus 100, a flux is introduced onto the upper surface of the molten iron 300 to form a molten slag 200 on the upper surface of the molten iron 300. The amount of flux introduced can be determined appropriately depending on the internal volume of the MgO crucible 101, the total amount of the molten iron 300, and the like. The amount of flux introduced 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 composition of the flux is not particularly limited as long as it contains components that can form the molten slag 200. The composition of the flux may include, for example, CaO, SiO ２ , FeO, MgO, etc. The flux may contain CaO, SiO, etc. ２ When FeO and MgO are contained, the content thereof is 22.5 (% CaO), 28.0 (% SiO ２ ), 42.5 (% FeO), 7.0 (% MgO).

[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 in 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, a 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 a cathode 104 and an anode 105, which are used to apply an electric current between the molten slag 200 and the molten iron, to be inserted into the molten slag 200.

[0033] A 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, an 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 formed from C-MgO bricks, which are carbon-containing refractories, and the molten iron 300. A DC power supply 106 is used between the two 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 of the current applied between the electrodes is preferably determined taking into account the dephosphorization treatment time, the required phosphorus distribution taking into account the target phosphorus concentration, and the power cost. Specifically, by setting the current density of the current applied between the electrodes to increase the current density, the attainable phosphorus concentration can be reduced and the dephosphorization rate can be increased. Therefore, by applying a large current between the two electrodes, the treatment time required for the dephosphorization treatment of molten iron can be shortened and molten iron (molten metal) with a phosphorus concentration equal to or lower than the target concentration can be obtained, but the power cost becomes excessive. From this technical viewpoint, the current density can be set so as to satisfy the following relational expression (1), and a current can be applied between the electrodes. 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 of the molten iron (K).

[0035] In the dephosphorization method for molten iron according to this embodiment, the value of the applied current density I calculated using the relational expression (1) is the current density that can achieve the minimum dephosphorization effect for 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 relational expression (5) described below, increasing the reaction rate of the dephosphorization reaction of molten iron improves the productivity of molten steel. Second, 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 cost increase. Therefore, it is desirable to determine the upper limit of the applied current density used to dephosphorize molten iron, taking into account the allowable operation time, cost, and allowable current of the power source. From this technical perspective, being able to calculate the value of the applied current density I using the relational expression (1) is extremely significant because it not only ensures the dephosphorization effect of molten iron, but also provides suitable conditions for the dephosphorization method taking into account the allowable operation time, cost, and allowable current of the power source.

[0037] Specifically, in the method for dephosphorizing molten iron according to this 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, 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, which is preferable. ２ ) or less, it is possible to reduce the power cost when dephosphorizing the molten iron 300. In the dephosphorization method for 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 current between the two electrodes.

[0038] Fig. 2 is a graph showing the change in the phosphorus concentration of molten iron over time when dephosphorization of molten iron is performed by applying a current between electrodes using a molten iron dephosphorization apparatus. That is, Fig. 2 shows the relationship between the dephosphorization treatment time (min) and the phosphorus concentration (mass %) of molten iron. As shown in Fig. 2, the dephosphorization rate increases and the attained phosphorus concentration decreases as the applied current value increases.

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

[0040] The principle by which the dephosphorization method for molten iron according to this embodiment can accelerate the dephosphorization reaction of molten iron is believed to be as follows: When a current is applied to the molten iron 300 and the molten slag 200, the potential on the molten iron 300 side becomes noble and the potential on the molten slag 200 side becomes base, via both electrodes. This potential change is called overpotential. Here, due to the change in Gibbs energy corresponding to the overpotential, 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]

[0042]

[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 the molten iron 300 increases. As a result, it is considered that the concentration [P] of phosphorus (P) contained in the molten iron 300 decreases. In this case, if the reaction order of the dephosphorization reaction of phosphorus contained in the 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 expression (5) is obtained. In relational expression (5), t is the dephosphorization treatment time t (s) of phosphorus contained in the molten iron, k is the apparent reaction rate constant of the dephosphorization reaction of phosphorus contained in the molten iron, and [P]e is the equilibrium phosphorus concentration when the dephosphorization reaction of phosphorus contained in the molten iron reaches dissolution equilibrium.

[0044]

[0045] According to relational expression (5), the equilibrium phosphorus concentration [P]e decreases when the dephosphorization reaction of phosphorus contained in the molten iron 300 reaches dissolution equilibrium, thereby increasing the reaction rate v of the dephosphorization reaction of phosphorus contained in the molten iron 300. In this case, if the molten steel temperature when the dephosphorization reaction of phosphorus contained in the molten iron 300 reaches dissolution equilibrium is T (K), the phosphorus distribution of 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 phosphorus contained in the molten slag 200 after current is applied to the molten iron 300 and the molten slag 200 is Lp', the applied current density I of the current required to obtain the required phosphorus distribution Lp' is expressed by the following relational expression (1):

[0046] 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 of the molten iron (K).

[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 predetermined time has elapsed slag and the concentration of phosphorus in the molten iron C ironThe phosphorus distribution Lp(-) of the molten slag 200 is calculated by measuring the current density I (A / m) corresponding to the required phosphorus distribution Lp'(-) of the molten slag 200. Next, the molten iron dephosphorization apparatus 100 is used to apply a current to both electrodes 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. The phosphorus distribution Lp(-) of the molten slag 200 at the molten steel temperature T (K) is then calculated, and the required phosphorus distribution Lp'(-) of the molten slag 200 is determined by using the relational expression (1). 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 The relationship between the applied current density I (A / m) and 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 calculated. ２ The applied current density I (A / m) and the overvoltage η are proportional to each other and can be expressed by the following relational expression (6). ２ The slope α can be calculated from a graph showing the relationship between the overvoltage η and the temperature.

[0049]

[0050] On the other hand, when a current is applied to the molten iron 300 and the 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 dissolution equilibrium after applying the current. equilibrium are expressed by the following relational expressions (7) and (8), respectively.

[0051] In the relational expression (7), R is the gas constant, T is the molten steel temperature (K), Lp(-) is the phosphorus distribution in the molten slag, and a slag is the activity of phosphorus in the molten slag, a iron represents the activity of phosphorus in molten iron.

[0052]

[0053] Furthermore, based on the relational expressions (7) and (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 expression (9), η represents the overvoltage, R represents the gas constant, F represents the Faraday constant, Z represents the valence, Lp represents the phosphorus distribution (-) in the molten slag at the molten steel temperature T (K), and Lp' represents the required phosphorus distribution (-).

[0054]

[0055] In this way, the dephosphorization method of 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 by using the relational expression (1) in which the constant α is determined, it is possible 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, the phosphorus distribution can be effectively improved and the dephosphorization reaction of molten iron can be promoted 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.

[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] 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 (K). The technical features of the method for dephosphorizing molten iron according to this embodiment will be described below.

[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 this embodiment employs the relational expression (2) to apply a current having an applied current density I to the molten slag 200 and the molten iron 300, and then measures the phosphorus concentration, thereby obtaining the molten slag distribution Lp′.

[0060] Furthermore, in the dephosphorization method of molten iron according to this embodiment, the current applied to the molten slag and the molten iron is controlled by the current value. Therefore, the dephosphorization method of molten iron according to this embodiment can reduce the effect of changes in the electrical properties of the molten iron on the change in overpotential. By applying a current having an applied current density that satisfies the relational expression (2) employed in the dephosphorization method of molten steel according to this embodiment to the molten slag and the molten iron, phosphorus distribution can be effectively improved without modifying the molten slag, and the dephosphorization method of molten iron according to this embodiment can be applied to molten iron with various compositional components.

[0061] In the dephosphorization method for molten iron according to this 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. A carbon concentration [C] of 4.0 mass% or less of the carbon contained in the molten iron to which a current is applied is preferable because it ensures the carbon content that can be contained in the molten iron. The carbon concentration [C] of the carbon contained in the molten iron to which a current is applied may be 0.1 mass% or more. Thus, the dephosphorization method for 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 the carbon concentration that can be contained in the 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 overvoltage 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 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-described embodiments, no arc discharge occurs when the current is applied between the molten slag and the molten iron. The following describes technical features of the method for dephosphorizing molten iron according to this embodiment.

[0064] The dephosphorization method of molten iron according to this embodiment is carried out under conditions that prevent arc discharge from occurring due to the current applied between the molten slag and the molten iron. That is, the dephosphorization method of molten iron according to this embodiment promotes the reaction between the phosphorus contained in the molten iron and iron oxide by preventing arc discharge from occurring due to the current applied between the molten slag and the molten iron, thereby enabling effective dephosphorization of the molten iron.

[0065] The generation of arc discharge within the system of the molten iron dephosphorization apparatus due to the current applied between the molten slag and the molten iron is undesirable from the viewpoint of promoting the dephosphorization reaction of the molten iron. The reason for adopting conditions that prevent the generation of arc discharge due to the current applied between the molten slag and the molten iron in the method for dephosphorizing molten iron according to this embodiment will be explained below.

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

[0067]

[0068] Here, in the relational expression (11) showing the equilibrium constant Kp, a ｐ２Ｏ５ is the diphosphorus pentoxide (P) contained in molten iron. ２ O ５ ) activity, a Ｆｅ is the activity of iron (Fe) contained in molten iron, a ｐ is the activity of phosphorus (P) contained in molten iron, a ＦｅＯand represent the activity of iron oxide (FeO) contained in the molten iron. ｐ can be expressed by the following relational expression (12) using the phosphorus concentration [P] of phosphorus contained in molten iron and its activity coefficient fp.

[0069]

[0070] Furthermore, in the relational expression (11) showing the equilibrium constant Kp, the logarithm of both sides is taken to obtain the diphosphorus pentoxide (P ２ O ５ The relationship between the activity of iron (Fe), the activity of phosphorus (P), and the activity of iron oxide (FeO) is expressed by the following relational expression (13).

[0071]

[0072] Therefore, 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 accelerated by lowering the reaction temperature T. Generally, in dephosphorization methods of molten iron, arc discharge occurs due to the current applied between the molten slag and the molten iron, which increases the reaction temperature T of the dephosphorization reaction of molten iron. From this technical viewpoint, the dephosphorization method of molten iron according to this embodiment can accelerate the dephosphorization reaction of molten iron by lowering the reaction temperature T while preventing the arc discharge from occurring due to the current applied between the molten slag and the molten iron.

[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 (FeO) contained in molten iron promotes the reaction between iron oxide contained in molten slag and phosphorus, thereby promoting the dephosphorization reaction of molten iron. ２ O ５ ) contained in the molten slag by reducing the activity of diphosphorus pentoxide (P ２ O ５ ) decomposition reaction proceeds, the reverse reaction of the dephosphorization reaction of molten iron can be suppressed.

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

[0075] In the method for dephosphorizing molten iron according to this embodiment, the current value required to prevent arc discharge by applying a current between the molten slag and the molten iron is preferably 5,000 (A) or less. In the method for dephosphorizing molten iron according to this embodiment, if the current value applied between the molten slag and the molten iron is 500 to 5,000 (A) or less, arc discharge will not occur, energy balance in the dephosphorization reaction of molten iron can be achieved, and thermal efficiency in the dephosphorization treatment of molten iron can be ensured. From this technical viewpoint, the molten iron dephosphorization apparatus to which the method for dephosphorizing molten iron according to this 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 a DC electric furnace, high-temperature exhaust gas can be used for preheating, and heat radiation caused by opening the furnace lid when charging scrap can be prevented, thereby reducing energy consumption.

[0077] In addition, DC electric furnaces are increasingly being equipped with scrap preheating and continuous charging equipment, as well as the eccentric bottom tapping method. The eccentric bottom tapping method for DC electric furnaces improves efficiency by allowing for rapid tapping without tilting the furnace body, and also makes it difficult for slag to flow into the ladle during tapping, which is advantageous for maintaining the cleanliness of the molten steel.

[0078] As described above, according to the third embodiment of the invention, by preventing arc discharge, lowering the reaction temperature T, and setting 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 dephosphorization method of the third embodiment of the invention 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 methods 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 this 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 a cathode 104 to be immersed only in the molten slag 200. An electrode made of a conductive material serves as the cathode 104, immersed only in the molten slag 200.

[0081] In the dephosphorization method of molten iron according to this embodiment, the components of the molten slag 200 used in the dephosphorization treatment of phosphorus contained in the molten iron 300 are CaO, SiO, which are generally used in dephosphorization refining. ２ The molten slag 200 preferably contains FeO, MgO, etc. As is clear from the relational expressions (1) and (2), the method for dephosphorizing molten iron according to this embodiment can increase the required phosphorus distribution in the molten slag 200 without any particular limitations on the composition of the molten slag 200.

[0082] In the molten iron dephosphorization method according to this embodiment, the cathode 104 of the molten iron dephosphorization apparatus 100 must be immersed only in the molten slag 200. To improve the efficiency of the dephosphorization reaction, the liquid phase ratio of the molten slag 200 is preferably 60 vol.% or more. The liquid phase ratio of the molten slag 200 may be any value that allows the cathode 104 and anode 105 used to apply current between the molten slag 200 and the molten iron to be inserted into the molten slag 200. A liquid phase ratio of 60 vol.% or more is preferable because it allows the cathode 104 to be fully immersed in the molten slag 200, thereby accelerating the dephosphorization reaction of the molten iron. A liquid phase ratio of 95 vol.% or less is preferable because it facilitates the operation of the molten iron dephosphorization apparatus 100. The liquid phase ratio of the molten slag 200 refers to the proportion of the liquid phase in the molten slag 200.

[0083] As described above, according to the fourth embodiment of the present 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, which effectively improves phosphorus distribution and accelerates the dephosphorization reaction of molten iron.

[0084] [Other Embodiments] The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various modifications can be made to the configuration and details of the present invention that are understandable to those skilled in the art within the technical scope of the present invention. Furthermore, programs, systems, or devices that combine the separate features included in each embodiment in any manner are also included in the technical scope of the present invention.

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

[0086] (Example 1) The dephosphorization method for molten iron according to the present embodiment was adopted to dephosphorize molten iron using an electric furnace. Scrap, iron phosphide (FeP), carbonaceous material, and CaO—SiO ２The molten steel raw materials were melted by an AC arc, and 300 t of molten steel and 30 kg / t of molten slag were obtained in the electric furnace.

[0087] Next, the furnace graphite electrode used for the arc AC 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. Furthermore, argon gas (Ar) was introduced at 2.0 Nm using the stirring lance. ３ / min, and an average current density of 300 (A / m ２ A direct current of 2100 A was applied for 30 minutes to dephosphorize the molten steel (Level 1). During the dephosphorization of the molten steel, samples were taken before the dephosphorization (0 min), 10 minutes after the start of the dephosphorization, 20 minutes after the start of the dephosphorization, and 30 minutes after the start of the dephosphorization (completion of the dephosphorization). The phosphorus concentration in the molten steel was measured, and the actual phosphorus distribution was calculated. Table 1 shows the phosphorus distribution and slag composition ratio over the dephosphorization time of the molten steel. The liquid phase ratio of the slag and whether or not the relationship shown in Equation (2) above was satisfied are also shown. In Example 1, the required phosphorus distribution of the molten slag was set to 100. In addition, in the method for dephosphorizing molten iron of Example 1, the presence or absence of arc generation was confirmed, and the current value applied between the molten slag and the molten steel was set to a predetermined current value.

[0088] (Invention Examples 2 to 6) In Invention 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 The dephosphorization treatment of molten steel was carried out in the same manner as in Example 1, except that the applied current value was changed to 5000 (A) or less (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 changed to 3000 (A / m ２ ) and the applied current 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. Table 1 also shows the slag liquid phase ratio and whether or not the relationship shown in the above-mentioned relational expression (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 and 2) Dephosphorization of 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 did 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. Table 1 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 of 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 8). Table 1 shows the phosphorus distribution (actual phosphorus distribution) and slag composition ratio over the dephosphorization time of the molten steel. It also shows the liquid phase ratio of the slag and whether or not the relationship shown in the above formula (2) was satisfied.

[0091]

[0092] Table 1 shows the test conditions and results for each level employed in Examples 1 to 6 and Comparative Examples 1 to 3. Table 1 also shows the required phosphorus distribution, and whether the calculated actual phosphorus distribution reached this required phosphorus distribution was evaluated. As is clear from Table 1, under the condition of 100% slag liquid phase (Examples 1 to 2, 4 to 6), when the above-mentioned relational expression (2) was not satisfied, the calculated actual phosphorus distribution (the phosphorus distribution obtained from the experimental results) did not reach the required phosphorus distribution (Comparative Example 2). On the other hand, under the condition of 100% slag liquid phase (Examples 1 to 6), when a DC current with a current density satisfying the above-mentioned relational expression (2) was applied, the required phosphorus distribution was achieved (Examples 1 to 6). Furthermore, as shown in Examples 1 to 5, the required phosphorus distribution was also achieved extremely well even under the condition of applying a current of 5000 A or less to prevent arc discharge. From this technical viewpoint, it has become clear that the method for dephosphorizing molten iron according to this embodiment can be suitably carried out in a DC electric furnace that can prevent arc discharge from occurring.

[0093] Furthermore, it can be seen that the higher the average current density of the DC current applied to the molten steel, the faster the required phosphorus distribution is reached, thereby shortening the dephosphorization treatment time. Furthermore, when the slag liquid fraction is 30%, the solidified slag becomes an obstacle and prevents the electrode from being immersed, making the dephosphorization method of molten iron according to the present invention unapplicable (Comparative Example 3). However, when the slag liquid fraction is 60 vol.% or more, it is possible to immerse the electrode in the molten slag. However, from the viewpoint of the reaction efficiency of the dephosphorization reaction, a slag liquid fraction of 60 vol.% or more is preferable. These trends are independent of the carbon concentration [C] or the phosphorus concentration [P] of the molten iron.

[0094] As described above, the dephosphorization method 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 DC current having a current density that satisfies the above-mentioned relational expression (2) is applied. In other words, it has become clear that by using the dephosphorization method of molten iron according to the present invention and setting predetermined conditions, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron.

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

[0096] REFERENCE SIGNS LIST 100 Molten iron dephosphorization apparatus 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 Heat insulating board 200 Molten slag 300 Molten iron

Claims

1. A method for dephosphorizing molten iron, comprising applying a 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, wherein the applied current density I of the current 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': 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 the carbon contained in the molten iron is 4.0 mass% or less, and the applied current density I (A / m ２ 2. The method for dephosphorizing molten iron according to claim 1, characterized in that: 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. A method for dephosphorizing molten iron as claimed in claim 1 or 2, characterized in that no arc discharge occurs by applying the current between the molten slag and the molten iron.

4. A method for dephosphorizing molten iron according to claim 3, characterized in that the current value is 5000 (A) or less.

5. A method for dephosphorizing molten iron as claimed in claim 1 or 2, characterized in that the liquid phase rate of the molten slag is 60 vol. % or more.

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