Direct current electric furnace designing method and direct current electric furnace

Abstract

To provide a direct current electric furnace designing method capable of dissolving a reduction ferrous material such as DRI and HBI having a smaller specific gravity than molten iron and easy to float on a molten metal surface efficiently in a short time, and a direct current electric furnace.SOLUTION: A direct current electric furnace designing method according to the present invention includes: a step of calculating coefficients a, b, c, d, and e in relational expressions (i), (ii) of a distance L from a furnace bottom cable to the furnace bottom, a winding diameter r of the furnace bottom cable and a winding number T of the furnace bottom cable based on simulation for evaluating fluidization of the molten iron when the distance from the furnace bottom cable to the furnace bottom, the winding diameter of the furnace bottom cable and the winding number of the furnace bottom cable are changed; and a step of optimizing at least the winding number T of the furnace bottom cable using the relational expressions (i), (ii) and the calculated coefficients a, b, c, d, and e. T≥(a / V0)b...(i), V0=cL+dr+e...(ii)SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for designing a DC electric furnace and a DC electric furnace. [Background technology]

[0002] The iron source produced by the blast furnace process is produced by reducing iron ore with coke, which generates a large amount of CO2. One method to reduce CO2 emissions is to melt scrap iron and DRI (direct reduced iron) in an electric furnace to produce molten iron, and then use the existing steelmaking process centered on converters to produce molten steel. Melting is performed by an arc generated from the upper electrode installed in the electric furnace to the surface of the molten iron.

[0003] It is known that in DC electric furnaces, the arc is deflected due to the magnetic field caused by the current flowing through the cable from the bottom electrode. As a countermeasure, a magnetic field generating coil is provided around the bottom electrode with a winding so as to generate a DC magnetic field having a rotationally symmetric component about the axis of the arc generating electrode. As a result, the arc rotates around the core, and a technology has been disclosed that can equalize the heat load in any direction and achieve uniform melting (Patent Document 1). In addition, a technology has been disclosed that suppresses the deflection of the arc by setting the vertical component of the magnetic flux density to 90 Gauss or more and the horizontal component to 30 Gauss or less in the arc generating region using a coil wound around the bottom of the furnace (Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 3-140791 [Patent Document 2] Japanese Patent Application Publication No. 6-34279 Summary of the Invention [Problem to be solved by the invention]

[0005] Direct reduced iron is produced by directly reducing iron ore or iron ore pellets with reducing gas such as hydrogen, and is called DRI. Hot briquetted iron (HBI) is produced by hot compression molding of DRI after reduction. The apparent density of DRI is 3500 kg / m 3 The apparent density of HBI is about 5000 to 5500 kg / m 3 The specific gravity of molten iron is about 7000 kg / m 3 Since both DRI and HBI are lighter than molten iron, they float to the surface.

[0006] In the techniques disclosed in Patent Documents 1 and 2, scrap that has settled in the molten iron is efficiently heated, which makes it easier to stir the molten iron near the scrap. However, when reduced iron is used, DRI and HBI, which have a smaller specific gravity than molten iron and float to the surface of the molten iron, need to be dissolved more efficiently and in a shorter time.

[0007] Therefore, an object of the present invention is to provide a design method for a DC electric furnace and a DC electric furnace that are capable of more efficiently melting reduced iron materials such as DRI and HBI, which have a smaller specific gravity than molten iron and tend to float to the surface of the molten iron, in a shorter time when three electrodes are used. [Means for solving the problem]

[0008] [1] In a DC electric furnace comprising three upper electrodes, a bottom electrode provided at the bottom of the furnace body, a bottom cable connected to the bottom electrode and surrounding the bottom electrode, and a raw material charging pipe provided on the side or top of the furnace body for charging raw materials containing at least DRI or HBI into the furnace, the three upper electrodes are provided so that the shape of the center of each electrode is an equilateral triangle, and the distance L from the bottom cable to the bottom of the DC electric furnace, the winding diameter r of the bottom cable, A method for designing a DC electric furnace, comprising: a step of calculating the coefficients a, b, c, d and e in the relational expressions (i) and (ii) for the number of turns T of the bottom cable based on a simulation evaluating the flow of molten iron when the distance from the bottom cable to the bottom of the DC electric furnace, the winding diameter of the bottom cable and the number of turns of the bottom cable are changed; and a step of optimizing at least the number of turns T of the bottom cable using the relational expressions (i) and (ii) and the calculated coefficients a, b, c, d and e. T≧(a / V0) b (i) V0 = cL + dr + e (ii) [2] A DC electric furnace comprising three upper electrodes, a bottom electrode provided at the bottom of the furnace body, a bottom cable connected to the bottom electrode and surrounding the bottom electrode, and a raw material charging pipe provided on the side or top of the furnace body for charging raw materials containing at least DRI or HBI into the furnace, wherein the three upper electrodes are provided so that the shape of the center of each electrode connected to each other forms an equilateral triangle, and the distance L (m) from the bottom cable to the bottom of the DC electric furnace, the winding diameter r (m) of the bottom cable, and the number of turns T of the bottom cable satisfy the relational expressions (iii) and (iv). T≧(0.7 / 1.39V0) 2.08 (iii) V0=-0.605L-0.032r+2.0 (iv) However, 1.25≦L≦2.5 and 4.5≦r≦7.5. [3] The DC electric furnace according to [2], wherein the raw material charging pipe is arranged so that the HBI or the DRI is charged at a position within 3.89 m from the center of the furnace on the surface of the molten iron. Effect of the Invention

[0009] According to the above configuration, the reduced iron materials such as HBI and DRI are likely to move to the center of the furnace, which is a high-temperature region in the furnace, by rotating and stirring the molten iron. Therefore, the reduced iron materials such as DRI and HBI, which have a smaller specific gravity than molten iron and tend to float to the surface of the molten iron, can be melted more efficiently in a short time. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram showing the structure of a DC electric furnace according to one embodiment of the present invention; [Diagram 2] 2 is a cross-sectional view taken along line II-II of the DC electric furnace shown in FIG. 1. [Diagram 3] FIG. 2 is a diagram showing a temperature distribution of an arc emitted from an upper electrode, which is simulated in the DC electric furnace shown in FIG. [Figure 4] 1 is a diagram for explaining an attractive force acting on an arc emitted from an upper electrode in a DC electric furnace according to an embodiment of the present invention. FIG. [Diagram 5] FIG. 10 is a diagram showing the distribution of Lorentz force density by electromagnetic field analysis in a DC electric furnace according to one embodiment of the present invention. [Figure 6] FIG. 6 is a diagram showing the results of an analysis of the flow of molten iron based on the distribution of Lorentz force density shown in FIG. 5. [Figure 7] FIG. 2 is a diagram for explaining an electromagnetic force generated in molten iron by a current flowing through a bottom cable connected to a bottom electrode in a DC electric furnace according to one embodiment of the present invention. [Figure 8] FIG. 1 is a diagram showing the results of an electromagnetic field analysis when a bottom cable is connected to a bottom electrode in a DC electric furnace according to an embodiment of the present invention. [Figure 9]FIG. 9 is a diagram showing the results of an analysis of the flow of molten iron based on the Lorentz force density distribution shown in FIG. 8. [Figure 10] FIG. 13 is a diagram showing the calculation results of the trajectory of HBI in a DC electric furnace in which a bottom cable is not connected to the bottom electrode. [Figure 11] FIG. 13 is a diagram showing the calculation results of the HBI trajectory when the number of turns of the bottom cable connected to the bottom electrode is 0.25 in a DC electric furnace according to one embodiment of the present invention. [Figure 12] FIG. 13 is a diagram showing the calculation results of the HBI trajectory when the number of turns of the bottom cable connected to the bottom electrode is 0.5 in a DC electric furnace according to one embodiment of the present invention. [Figure 13] 1 is a graph showing the results of evaluating the time it takes for an HBI to reach directly below an upper electrode after being turned on in a DC electric furnace according to one embodiment of the present invention. [Figure 14] 1 is a graph showing the results of evaluating the time it takes for an HBI to reach directly below an upper electrode after being turned on in a DC electric furnace according to one embodiment of the present invention. [Figure 15] 1 is a graph showing the relationship between the number of turns of a hearth cable and the rotational flow speed of molten iron in a DC electric furnace according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted.

[0012] (Structure of a DC electric furnace according to one embodiment) Fig. 1 is a diagram showing the structure of a DC electric furnace 1 according to one embodiment of the present invention. In Fig. 1, (a) is a cross-sectional view of the DC electric furnace 1, and (b) is a bottom view of the DC electric furnace 1 as viewed from the furnace bottom side. In the figure and in the following description, three mutually orthogonal directions are defined as +x direction, +y direction, and +z direction. Although not shown in the figure, the direction opposite to the +x direction is defined as -x direction, the direction opposite to the +y direction is defined as -y direction, and the direction opposite to the +z direction is defined as -z direction.

[0013] As shown in the figure, the furnace body of the DC electric furnace 1 includes a ceiling 2, a furnace wall copper panel 3, a furnace wall refractory 4, and a furnace bottom refractory 5. In addition, three upper electrodes 6 are provided on the ceiling 2, and a bottom electrode 7 and a bottom blowing tuyeres 8 are provided at the bottom of the furnace body. An upper electrode cable 6a is connected to the upper electrode 6. In addition, a bottom cable 7a is connected to the bottom electrode 7, and an electric current I flows through the upper electrode cable 6a. The raw material charging pipe 9 is provided on the side or top of the furnace body, and charges raw materials containing at least DRI or HBI into the furnace. In the following description, it is assumed that HBI is charged into the furnace from the raw material charging pipe 9, but DRI may be charged from the raw material charging pipe 9. The furnace wall copper panel 3 is provided with a tapping hole 10 for discharging molten iron 21 and a slag discharge hole 11 for discharging electric furnace slag 22.

[0014] As shown in Fig. 1(b), the bottom cable 7a is connected to the bottom electrode 7 and winds around the bottom electrode 7. Here, the number of turns of the bottom cable 7a is defined as 1.0 when it winds around the bottom electrode 7 in the circumferential direction one turn (360°). In the illustrated example, the bottom cable 7a winds around the bottom electrode 7 half a turn (180°), so the number of turns is 0.5.

[0015] The DC electric furnace 1 may be a tilting type in which the furnace body is tilted, or a stationary type in which the furnace body is not tilted. In addition, one or two of iron-containing scrap and reduced iron are charged into the DC electric furnace 1 as the iron source. HBI, DRI, etc. can be used as the reduced iron. Iron-containing dust may be charged into the furnace as the iron source. Converter dust granulated products can be used as the iron-containing dust. Pr in the figure indicates the position where HBI charged from the raw material charging pipe 9 reaches the surface of the molten iron 21. R in the figure indicates the distance between the furnace center C and the charging position Pr. L indicates the distance between the hearth cable 7a and the hearth, and the range of values ​​that L (m) can take is 1.25≦L≦2.5. r indicates the winding diameter of the hearth cable 7a, and the range of values ​​that r (m) can take is 4.5≦r≦7.5.

[0016] An arc 20 is emitted from the upper electrode 6, and raw materials such as reduced iron are melted to produce molten iron 21. Gas is blown from the bottom blowing tuyeres 8 to generate a circulating flow in the molten iron 21 and electric furnace slag 22, thereby promoting heat transfer and melting of the surface of the molten iron 21 and the electric furnace slag 22. Normally, the temperature of the molten iron 21 in a DC electric furnace 1 is at most about 1700°C, but the internal temperature of the arc 20 generated from the upper electrode 6 to the surface of the molten iron 21 is 5000°C or higher, and even the arc spot on the surface of the molten iron 21 is about 2000°C.

[0017] 2 is a cross-sectional view of the DC electric furnace 1 shown in FIG. 1 taken along line II-II. As shown in the figure, the DC electric furnace 1 has a circular cross section. The three upper electrodes 6 are arranged so that the shape of the center of each upper electrode 6 is an equilateral triangle. Although it is ideal to install the upper electrodes 6 so that they form an equilateral triangle, there are cases where a deviation of about the radius of the upper electrodes 6 occurs due to equipment restrictions, and this deviation does not significantly change the effect of the present invention. In the following description, the distance between the furnace center C and the center of the upper electrodes 6 is defined as RC, and the radius of the surface of the molten iron 21 is defined as d.

[0018] (Outline and conditions of simulation to evaluate thermal flow in DC electric furnace) Since the arc 20 emitted from the upper electrode 6 has a high temperature and a high velocity, it is difficult to directly measure the temperature and flow velocity of the arc 20. For this reason, in this embodiment, a simulation is performed to evaluate the heat flow in the DC electric furnace 1 due to the arc discharge. The simulation is performed based on the method described in Jonas ALEXIS, Marco RAMIREZ, Gerardo TRAPAGA and Par JONSSON, "Modeling of a DC Electric Arc Furnace-Heat Transfer from the Arc" ISIJ International, Vol. 40 (2000), pp. 1089-1097, and an outline of the simulation will be described below.

[0019] In the simulation, the arc 20 emitted from the upper electrode 6 is regarded as an electromagnetic fluid having electrical conductivity, and a coupled calculation of electromagnetic field analysis, fluid analysis, and heat transfer analysis is performed. First, the method of electromagnetic field analysis will be described. The current density flowing through the arc 20 is expressed as i (A / m 2 ), the relationship between the magnetic vector potential A and the current density i is expressed as in equation (1), where μ0 is the magnetic permeability of a vacuum.

[0020]

number

[0021] The current density i is calculated using equation (2), which is the law of conservation of current density, where φ is the potential at the inlet and outlet of the current set as the boundary condition, and σ is the electrical conductivity (S / m) of the arc 20, which is a value that depends on the temperature of the arc 20.

[0022]

number

[0023] The magnetic vector potential A is calculated from equation (1) using the current density distribution calculated from equation (2). Then, the distribution of magnetic flux density B(T) is calculated from equation (3) using the calculated magnetic vector potential A.

[0024]

number

[0025] Next, we will explain the methods of fluid analysis and heat transfer analysis. A conductor with a current flowing in a magnetic field is subjected to the Lorentz force F (N / m 3 ) = i × B occurs. In addition, the amount of Joule heat generated in the conductor is q (W / m 3 )=i 2 / σ occurs. Using the calculated Lorentz force F and Joule heating amount q, fluid analysis and heat transfer analysis are performed. In the fluid analysis, the flow of the arc 20 is analyzed using the law of conservation of mass shown in equation (4) and the Navier-Stokes equation shown in equation (5). Here, u is the flow velocity, ρ is the density, t is time, p is pressure, μ is viscosity, and g is the gravitational acceleration.

[0026]

number

[0027] The heat transfer analysis is performed using the governing equation for heat transfer analysis, equation (6), where C p is the specific heat, T is the temperature, λ is the thermal conductivity, S r is the radiant energy, Q t is the energy loss due to the Thomson effect.

[0028]

number

[0029] By performing the above-mentioned electromagnetic field analysis, fluid analysis, and heat transfer analysis, the temperature and flow rate of the arc 20 can be obtained. As an example, the calculation conditions in the simulation are a diameter of the upper electrode 6 of 750 mm, a distance between the furnace center C and the center of the upper electrode 6 of 2300 mm, and a distance between the upper electrode 6 and the surface of the molten iron 21 of 500 mm. The atmospheric gas in the furnace is Ar, the surface of the molten iron 21 is flat, and the potential of the surface of the molten iron is 0. Furthermore, the current value flowing per upper electrode 6 is 133 kA, and the starting point of the arc 20 is the center of the lower end surface of the upper electrode 6. The electric furnace slag 22 is not taken into consideration.

[0030] FIG. 3 is a diagram showing the temperature distribution of the arc 20 discharged from the upper electrode 6, which is simulated in the DC electric furnace 1 shown in FIG. 1. In FIG. 3, (a) shows the temperature distribution or flow velocity distribution in the DC electric furnace 1 in a perspective view, and (b) shows the cross section of the arc 20 discharged from the upper electrode 6. In the figure, the temperature distribution of the arc 20 is shown in a three-dimensional contour surface, (a) shows a temperature surface of 3000K, and (b) shows the temperature distribution in the cross section of the arc 20 discharged from the two upper electrodes 6. As shown in the figure, in the DC electric furnace 1, the vicinity of the furnace center C, which is an area surrounded by the three upper electrodes 6, becomes a high-temperature area. Therefore, if the HBI, which is the raw material for melting, gathers near the furnace center C due to the flow of the molten iron 21, the HBI will melt efficiently.

[0031] (Background of the study leading to the idea of ​​the DC electric furnace according to the embodiment) Here, the attractive force acting on the arc 20 emitted from the two upper electrodes 6 in the DC electric furnace 1 shown in Fig. 1 will be described with reference to Fig. 4. As described above, there are three upper electrodes 6 in this embodiment, but for simplicity, the description will be given assuming that there are two upper electrodes 6. In the illustrated example, the arc 20 is emitted from the upper electrode 6 toward the molten iron 21. Therefore, the current flowing through the upper electrode 6 flows in the direction indicated by j1 and j2 in the figure, which is the opposite direction to the direction in which electrons are emitted.

[0032] Then, due to the current flowing through the upper electrode 6, magnetic flux density is generated in the directions indicated by b1 and b2 in the figure. Between the two upper electrodes 6, the magnetic flux densities b1 and b2 act in opposite directions, and are cancelled out. As a result, the magnetic flux densities b1 and b2 between the two upper electrodes 6 become low. An electromagnetic force acts on the arc 20 in a direction indicated by the cross product of the current density vector and the magnetic flux density vector, that is, in the directions indicated by f1 and f2 in the figure, so the arcs 20 attract each other.

[0033] Thus, the degree of deflection of the arc 20 is greatly influenced by the distribution of the magnetic flux densities b1, b2 at the generation position of the arc 20. In addition, in a DC electric furnace 1 with one upper electrode 6, it is known that the arc 20 is deflected by the magnetic field caused by the current I flowing from the bottom cable 7a to the bottom electrode 7. As in the technology shown in Patent Document 1, when a magnetic field generating coil is provided around the bottom electrode 7 so that the winding goes around, if the DC electric furnace 1 is enlarged, the distance between the generation position of the arc 20 and the magnetic field generating coil becomes farther, making it difficult to suppress the deflection of the arc 20. Furthermore, when there are three upper electrodes 6, the distance between the upper electrodes 6 becomes smaller than the distance between the generation position of the arc 20 and the magnetic field generating coil, and it becomes more difficult to suppress the deflection of the arc 20 due to the influence of the electromagnetic force in the direction in which the arcs 20 attract each other.

[0034] Therefore, the inventors discovered that it is possible to improve the melting efficiency of the HBI by moving the HBI floating on the molten iron surface to the high temperature region near the furnace center C by rotating and stirring the molten iron 21, rather than controlling the deflection of the arc 20 with a coil. The present invention is particularly effective in large DC electric furnaces 1 where differences between high temperature and low temperature regions are likely to occur.

[0035] (Electromagnetic field analysis of molten iron) In the DC electric furnace 1 according to this embodiment, the upper electrode 6 is a negative electrode, and the bottom electrode 7 is a positive electrode. Therefore, the current flows from the bottom to the top of the furnace, that is, in the +y direction. In a large DC electric furnace 1, a current of about 100 kA to 140 kA flows per upper electrode, so that a total current of 300 kA to 420 kA flows for three upper electrodes 6. The bottom electrode 7 may be made of a conductive refractory material or may be made of many embedded metal rods. Regardless of the configuration of the bottom electrode 7, one bottom cable 7a is connected to the bottom electrode 7.

[0036] Below, we will explain the results of an electromagnetic field analysis of the Lorentz force density and the direction and magnitude of the electromagnetic force in the molten iron 21 when the bottom cable 7a is installed vertically below the bottom electrode 7 (-y direction) and an arc 20 is released from the upper electrode 6 to the molten iron surface.

[0037] As conditions for the electromagnetic field analysis, the shape of the DC electric furnace 1 was assumed to have a radius of 4.77 m, a bath depth of 1.85 m, and a weight of about 500 t. The upper electrodes 6 were arranged at a distance RC of 2 m from the furnace center C, so that the shape of the center of each electrode formed an equilateral triangle. The length of the arc 20, which is the distance between the lower end of the upper electrode 6 and the surface of the molten iron 21, was 600 mm, and the diameter of the upper electrode 6 was 750 mm. The structure of the bottom electrode 7 was assumed to be a conductor with a diameter of 4 m, in which multiple metal rods were installed within a range of a diameter of 4 m. The area of ​​the arc 20 was assumed to be equal to the diameter cross section of the upper electrode 6. The total value of the current flowing through the three upper electrodes 6 was assumed to be 300 kA.

[0038] Fig. 5 is a diagram showing the distribution of Lorentz force density by electromagnetic field analysis in a DC electric furnace 1 according to one embodiment of the present invention. In Fig. 5, (a) is a diagram showing the distribution of Lorentz force density on the surface of molten iron 21 as viewed from the ceiling 2 side, and (b) is a diagram showing the distribution of Lorentz force density at the bottom of the furnace as viewed from the bottom side. In addition, (c) is a diagram showing the distribution of Lorentz force density in the molten iron 21 in the cross section taken along line AA directly below the upper electrode 6 in (a), and (d) is a diagram showing the direction of the electromagnetic force in the molten iron 21 in the cross section taken along line AA.

[0039] As shown in the figure, a large Lorentz force density is generated in the molten iron 21 directly below the upper electrode 6, and an electromagnetic force is generated in the direction (-y direction) pushing the molten iron 21. Also, on the side of the bottom electrode 7, an electromagnetic force is generated in the direction (+y direction) toward the surface of the molten iron 21, but since the current density is lower than directly below the upper electrode 6, it can be seen that the Lorentz force density is smaller.

[0040] (Evaluation of the flow of molten iron) FIG. 6 is a diagram showing the results of analyzing the flow of the molten iron 21 based on the distribution of the Lorentz force density shown in FIG. 5. In FIG. 6, (a) is a diagram showing the flow of the molten iron 21 on the molten iron surface as viewed from the ceiling 2 side, and (b) is a diagram showing the flow of the molten iron 21 in the cross section of line BB directly below the upper electrode 6. As shown in the figure, the force pushing the molten iron 21 is strong directly below the upper electrode 6, so the molten iron 21 flows toward the bottom of the furnace (-y direction) and forms a flow that rises toward the molten iron surface after colliding with the bottom of the furnace. Near the molten iron surface, the molten iron 21 rises from the bottom of the furnace to the molten iron surface near the center C of the furnace and forms a flow toward the furnace wall (+x direction). HBI charged in a situation where such a flow of molten iron 21 is formed flows near the furnace wall, which is not a high-temperature region, and the melting efficiency of the HBI decreases.

[0041] (Flow of molten iron caused by electromagnetic force due to current in the bottom cable) 7 is a diagram for explaining an electromagnetic force generated in the molten iron 21 due to a current I flowing through a bottom cable 7a connected to a bottom electrode 7 in a DC electric furnace 1 according to an embodiment of the present invention. For the sake of simplicity, the explanation will be given assuming that there is one upper electrode 6, but the mechanism by which the electromagnetic force is generated is the same even when there are three upper electrodes 6.

[0042] In FIG. 7, (a) shows the electromagnetic force when there is no effect of the current I flowing through the bottom cable 7a connected to the bottom electrode 7. The magnetic flux density b3 caused by the arc current j3 flowing from the bottom electrode 7 to the upper electrode 6 is generated in the circumferential direction based on the right-hand rule. Therefore, the electromagnetic force f3 is generated in the direction toward the furnace center C based on Fleming's left-hand rule. At this time, the cross-sectional area of ​​the bottom electrode 7 is larger than the current-carrying area between the upper arc 20 and the molten iron 21, so the electromagnetic force f3 near the molten iron surface is strong. Therefore, the molten iron 21 flows from the molten iron surface toward the hearth bottom (from the +y direction to the -y direction).

[0043] In Fig. 7, (b) shows the electromagnetic force when there is an effect of the current I flowing through the bottom cable 7a connected to the bottom electrode 7. The direction of the current j4 flowing through the bottom cable 7a is perpendicular to the paper (-z direction). The magnetic flux density b4 caused by the current j4 flowing through the bottom cable 7a acts in the clockwise direction in the figure based on the right-hand screw rule. Due to the action of the generated magnetic flux density b4 and the current j3 flowing through the molten iron 21, an electromagnetic force is generated in the direction of f4 shown in the figure.

[0044] That is, as shown in the figure, an electromagnetic force f4 is generated in the molten iron 21 in the same direction as the current j4 in the bottom cable 7a. Therefore, by installing the bottom cable 7a so as to circle around the bottom electrode 7, the electromagnetic force f4 acts to rotate the molten iron 21 horizontally in the furnace.

[0045] FIG. 8 is a diagram showing the results of an electromagnetic field analysis when a bottom cable 7a is connected to the bottom electrode 7 in a DC electric furnace 1 according to an embodiment of the present invention. As analysis conditions, the bottom cable 7a is installed at a position 1.85 m from the bottom of the furnace, taking into consideration the bottom refractory 5 and the like, and the bath depth is also set to 1.85 m. In this case, the distance to the position where the arc 20 is released is 3.7 m, which is farther than the distance between the upper electrodes 6 (2 × √3 = 3.4 m). One bottom cable 7a is connected from the center of the bottom electrode 7. The bottom cable 7a is pulled out 2.25 m in the radial direction (+x direction) from the furnace center C and wound 1 / 2 around in the circumferential direction (number of turns 0.5). The current I flowing through the bottom electrode 7 is set to 300 kA.

[0046] In Fig. 8, (a) shows the Lorentz force density distribution when the bottom cable 7a is not connected to the bottom electrode 7, and (b) shows the Lorentz force density distribution when the bottom cable 7a is connected to the bottom electrode 7. Comparing (a) and (b), it can be seen that when the bottom cable 7a is connected, the Lorentz force is inclined in the direction of the current I along the bottom cable 7a (f5, f6 in the figure). From this, it can be seen that by installing the bottom cable 7a in the circumferential direction of the bottom electrode 7, a rotating flow field is formed in which the molten iron 21 rotates in the circumferential direction inside the furnace.

[0047] Fig. 9 is a diagram showing the results of analyzing the flow of molten iron 21 based on the Lorentz force density distribution shown in Fig. 8. The molten iron 21 has a specific gravity of 7000 kg / m 3 The analysis was performed with a viscosity coefficient of 0.005 Pa s. In Fig. 9, (a) is a diagram showing the flow velocity vector of the molten iron 21 on the molten metal surface as viewed from the ceiling 2 side. (b) is a diagram showing the contour lines of the flow velocity of the molten iron 21 on the molten metal surface as viewed from the ceiling 2 side, and (c) is a diagram showing the contour lines of the flow velocity of the molten iron 21 at the hearth as viewed from the hearth side.

[0048] As shown in the figure, the molten iron 21 forms a flow that rotates in the circumferential direction, and the maximum flow speed is about 0.7 m / s. At the bottom of the furnace, a rotating flow field is formed in which the molten iron 21 rotates in the circumferential direction inside the furnace due to the electromagnetic force caused by the current I flowing through the bottom cable 7a. On the other hand, at the molten iron surface, since the electromagnetic force caused by the current flowing from the arc 20 to the molten iron surface is large, a circumferential flow is formed outside the triangle connecting the centers of the upper electrodes 6.

[0049] (Outline and conditions of simulation for evaluation of HBI movement) Since the temperature inside the furnace is extremely high, it is difficult to directly measure the trajectory of the HBI moving inside the furnace. Therefore, in this embodiment, a simulation is performed to evaluate the trajectory of the HBI. The following analysis was performed to evaluate the trajectory of the HBI when it is charged from the raw material charging pipe 9 of the DC electric furnace 1. The simulation was performed based on the method described in Turbulence Second Edition, HINZE, McGRAW-HILL, New York, 1975, pp. 460-462, and an outline of the simulation will be described below. The motion of a sphere in a fluid can be expressed by the BBOT equation shown in the following formula (7). Note that v is the particle velocity, F D is the drag force, v f is the velocity of the fluid and g is the acceleration due to gravity.

[0050]

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[0051] Here, the particle and fluid densities are ρ and ρ f Let d be the particle diameter and C be the drag coefficient. D Then, the coefficients C1, C2 and C3, and the drag force F D is expressed by the following equations (8) and (9).

[0052]

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[0053] The second term on the right hand side of equation (9) is the total differential of the flow velocity. This can be expressed as equation (10) below.

[0054]

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[0055] The viscosity of molten iron 21 is 0.005 Pa s and the density is 7000 kg / m 3 If the particle diameter is 5 cm and the fluid velocity range is 0.05 to 1 m / s, the Reynolds number Re is about 420 to 84,000. The drag coefficient C in this range is D is about 0.4 to 0.5. Therefore, the drag coefficient C D is 0.4, diameter 5cm, specific gravity 5500kg / m 3 The fluid has a specific gravity of 7000kg / m 3 The calculation was performed assuming that the HBI rises to the surface of the molten iron 21 and does not move toward the hearth bottom.

[0056] (Evaluation of HBI's trajectory) FIG. 10 is a diagram showing the calculation result of the trajectory of the HBI in the DC electric furnace 1 without the bottom cable 7a connected to the bottom electrode 7. In FIG. 10, (a) shows the trajectory of the HBI when the charging position is R=1m, and (b) shows the trajectory of the HBI when the charging position is R=3.2m. The trajectory of the HBI was taken for 120 seconds from the charging to the molten metal surface. When the bottom cable 7a is not connected to the bottom electrode 7, as shown in FIG. 6, the flow of the molten iron 21 is not a flow that rotates in the furnace, but a flow that springs from the vicinity of the furnace center C. Therefore, as shown in the figure, the HBI on the molten metal surface flows in the direction of the furnace wall. As described above, the high-temperature region of the molten iron 21 in the DC electric furnace 1 is the vicinity of the furnace center C surrounded by the upper electrode 6. Therefore, it is difficult to say that the trajectory of the HBI shown in the figure is a condition for efficiently melting the HBI.

[0057] FIG. 11 is a diagram showing the calculation result of the trajectory of the HBI when the number of turns of the bottom cable 7a connected to the bottom electrode 7 in the DC electric furnace 1 according to one embodiment of the present invention is 0.25. In FIG. 11, (a) shows the flow velocity distribution of the molten iron 21 on the molten iron surface as seen from the ceiling 2 side. (b) shows the trajectory of the HBI as seen from the ceiling 2 side when the distance R between the furnace center C and the charging position Pr is 1 m, and (c) shows the trajectory of the HBI as seen from the ceiling 2 side when the distance R is 3.2 m. The trajectory of the HBI was taken for 120 seconds from the time of charging to the molten iron surface. When the distance R is 1 m, the charging position Pr of the HBI is on the perpendicular bisector of the two upper electrodes 6.

[0058] As shown in the figure, when the number of turns of the bottom cable 7a is 0.25, the electromagnetic force in the rotational direction is small and no rotational flow field is formed. Therefore, as shown in (b), the HBI charged at a distance R = 1 m flows in the direction just below the upper electrode 6, where the flow toward the bottom of the furnace is strong. This is effective for melting the HBI. However, as shown by the dashed line in the figure, the HBI sinks into the molten iron 21 just below the upper electrode 6 and then moves toward the furnace wall, so it is difficult to say that it is melted efficiently.

[0059] On the other hand, as shown in (c), when the HBI is charged at a distance R = 3.2 m, the HBI moves near the furnace wall and then reaches just below the upper electrode 6. Here, the size of a typical HBI is about 30 × 50 × 130 mm. It is known that with this size, after being charged into the molten iron 21, one HBI melts in about 40 to 60 seconds. In the illustrated case, it takes about 120 seconds for the HBI to reach just below the upper electrode 6, and it is difficult to say that the melting of the HBI is efficient.

[0060] FIG. 12 is a diagram showing the calculation result of the trajectory of the HBI when the number of turns of the bottom cable 7a connected to the bottom electrode 7 is 0.5 in the DC electric furnace 1 according to one embodiment of the present invention. In FIG. 12, (a) shows the flow velocity distribution of the molten iron 21 on the molten iron surface as seen from the ceiling 2 side. (b) to (f) show the trajectory of the HBI as seen from the ceiling 2 side when the charging position Pr is changed, where (b) is the case where the distance R between the furnace center C and the charging position Pr is 1 m. In addition, (c) is the case where the distance R is 2.4 m, (d) is the case where the distance R is 3.7 m, (e) is the case where the distance R is 4.1 m, and (f) is the case where the distance R is 4.5 m. The trajectory of the HBI was taken for 120 seconds from the charging to the molten iron surface.

[0061] As shown in (a), when the number of turns of the bottom cable 7a is 0.5, a rotating flow field of the molten iron 21 is formed. Therefore, as shown in (b) to (f), a centripetal force is generated in the injected HBI, and the HBI moves toward the furnace center C. In addition, it can be said that the farther the injection position Pr is from the furnace center C, the longer it takes for the HBI to reach directly below the upper electrode 6.

[0062] (Evaluation of the time it takes for the HBI to reach the high temperature region) 13 is a graph showing the results of evaluating the time it takes for the HBI to reach just below the upper electrode 6 after being charged in a DC electric furnace 1 according to one embodiment of the present invention. As described above, the high temperature region is an area surrounded by the upper electrode 6, and the upper electrode 6 is provided at a distance R = 2 m from the furnace center C. In addition, the number of turns of the bottom cable 7a connected to the bottom electrode 7 is 0.5. As shown in the figure, it can be seen that the farther the HBI charging position Pr is from the furnace center C, the longer it takes for the HBI to reach just below the upper electrode 6. In addition, when the HBI charging position Pr exceeds the distance R = 4 m, the time it takes for the HBI to reach the high temperature region from the charging position Pr increases rapidly.

[0063] As described above, it is known that the HBI melts in about 40 to 60 seconds after being added to the molten iron 21. Efficient melting in a large DC electric furnace 1 means that the melting speed is about the same as when one HBI is added. Therefore, when multiple HBIs are added intermittently, it is necessary to move the HBI to the high temperature zone as soon as possible, considering that the temperature of the molten iron 21 near the introduction position Pr decreases. From the graph shown in the figure, it can be seen that in order for the HBI to reach the high temperature zone within 40 seconds, which is the melting time after the HBI is added, the distance R needs to be ≦3.89 m.

[0064] Fig. 14 is a graph showing the results of evaluating the time it takes for the HBI to reach just below the upper electrode 6 after being turned on in a DC electric furnace 1 according to one embodiment of the present invention. As in the case of Fig. 13, the area just below the upper electrode 6 is a high temperature region, and the distance between the upper electrode 6 and the furnace center C is 2 m. Also, unlike the case of Fig. 13, the number of turns of the bottom cable 7a connected to the bottom electrode 7 is 1.0.

[0065] When the number of turns of the bottom cable 7a is 1.0, the electromagnetic force generated by the current I flowing through the bottom cable 7a is larger than when the number of turns is 0.5. Therefore, the rotational flow speed of the molten iron 21 increases, and the time when the HBI reaches directly below the upper electrode 6 is shortened. As shown in the figure, in order for the HBI to reach the high temperature zone within 40 seconds, which is the melting time after the HBI is charged, it is necessary to set the charging position Pr to a distance R≦4.32 m.

[0066] From the results of evaluating the time it takes for the HBI to reach just below the upper electrode 6 shown in Figures 13 and 14, it can be said that it is preferable to set the number of turns of the bottom cable 7a to 0.5 to 1.0 and to set the distance R of the HBI injection position Pr to 3.89 m or less. In this case, it is preferable to set the material injection pipe 9 so that the HBI is injected at a position within 3.89 m from the center C of the furnace on the surface of the molten iron. The same effect can be obtained even if the number of turns of the bottom cable 7a is 1.0 or more.

[0067] (Optimization of bottom cable installation conditions) Fig. 15 is a graph showing the relationship between the number of turns of the bottom cable 7a and the maximum rotational flow speed of the molten iron 21 surface in a DC electric furnace 1 according to one embodiment of the present invention. In Fig. 15, (a) is a graph showing the rotational flow speed of the molten iron 21 when the number of turns of the bottom cable 7a and the distance between the bottom cable 7a and the hearth are changed. Also, (b) is a graph showing the rotational flow speed of the molten iron 21 when the number of turns of the bottom cable 7a and the winding diameter r of the bottom cable 7a are changed.

[0068] In (a), as described above, the range of values ​​that the distance L (m) between the hearth cable 7a and the hearth can take is 1.25≦L≦2.5, so the number of turns of the hearth cable 7a was changed and the flow velocity of the molten iron 21 was calculated for each of the cases of L=1.25m, 1.55m, 1.85m, and 2.15m. It is considered that the distance L between the hearth cable 7a and the hearth has a large effect on the flow velocity of the molten iron 21. It can be seen that the closer the distance L between the hearth cable 7a and the hearth, the greater the electromagnetic force generated in the molten iron 21, and therefore the flow velocity of the molten iron 21 increases.

[0069] In (b), the rotational flow velocity of the molten iron 21 was calculated for each case where the number of turns of the bottom cable 7a and the winding diameter r of the bottom cable 7a were changed. As described above, the range of values ​​that the winding diameter r (m) of the bottom cable 7a can take is 4.5≦r≦7.5, so the number of turns of the bottom cable 7a was changed and the flow velocity of the molten iron 21 was calculated for each case where r=4.5m, 5.5m, 6.5m, and 7.5m. It is considered that the winding diameter r of the bottom cable 7a affects the flow velocity of the molten iron 21. From the comparison of the graphs (a) and (b), it can be said that the influence of the distance L between the bottom cable 7a and the hearth on the flow velocity of the molten iron 21 is greater than the winding diameter r of the bottom cable 7a.

[0070] From the graphs (a) and (b), if the distance from the bottom cable 7a to the hearth is L (m), the number of turns of the bottom cable 7a is T, and the winding diameter of the bottom cable 7a is r (m), the following approximate equations (11) and (12) can be obtained for the flow velocity V (m / s) of the molten iron 21. V=1.39V0T 0.48 (11) V0=-0.605L-0.032r+2.0 (12)

[0071] As explained with reference to Fig. 9, the maximum flow velocity when the molten iron 21 forms a rotational flow field rotating in the circumferential direction is about 0.7 m / s. Therefore, if the flow velocity V of the molten iron 21 is 0.7 m / s or more, the HBI moves to the vicinity of the furnace center C. Therefore, from equation (11), the number of turns T required to make the flow velocity V of the molten iron 21 0.7 m / s or more is expressed by the following equation (13). T≧(0.7 / 1.39V0) 2.08 (13)

[0072] Furthermore, the equations (13) and (12) regarding the relationship between the current I per upper electrode 6 and the distance L between the furnace center C and the center of the upper electrode 6 can be expressed as equations (14) and (15) using the coefficients a, b, c, d, and e, respectively. As described with reference to Fig. 15, the coefficients a, b, c, d, and e can be calculated based on a simulation evaluating the flow of the molten iron 21 when the distance L from the bottom cable 7a to the bottom of the DC electric furnace 1, the winding diameter r of the bottom cable 7a, and the number of turns T of the bottom cable 7a are changed. T≧(a / V0) b (14) V0 = cL + dr + e (15)

[0073] In the DC electric furnace 1 designed according to the design method of one embodiment of the present invention, at least the number of turns T of the hearth cable 71a is optimized using the formulas (14) and (15) and the calculated coefficients a, b, c, d, and e. This makes it easier for reduced iron materials such as HBI and DRI, which have a smaller specific gravity than the molten iron 21, to move to the vicinity of the furnace center C, which is a high-temperature region in the furnace, by rotating and stirring the molten iron 21. This makes it possible to efficiently melt reduced iron materials such as HBI and DRI.

[0074] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art in the technical field to which the present invention pertains can come up with various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0075] Reference Signs List 1: DC electric furnace, 2: ceiling, 3: furnace wall copper panel, 4: furnace wall refractory material, 5: furnace bottom refractory material, 6: upper electrode, 7: furnace bottom electrode, 7a: furnace bottom cable, 9: raw material injection tube, 20: arc, 21: molten iron, 22: electric furnace slag.

Claims

1. Three upper electrodes; A bottom electrode provided at the bottom of the furnace body; a bottom cable connected to the bottom electrode and surrounding the bottom electrode; A raw material introduction pipe provided on a side or top surface of the furnace body for introducing raw materials containing at least DRI or HBI into the furnace; In a DC electric furnace comprising: The three upper electrodes are provided such that a shape obtained by connecting the centers of the respective electrodes forms an equilateral triangle, Calculating the coefficients a, b, c, d and e in the relational expressions (i) and (ii) of the distance L from the bottom cable to the bottom of the DC electric furnace, the winding diameter r of the bottom cable and the number of turns T of the bottom cable based on a simulation to evaluate the flow of molten iron when the distance from the bottom cable to the bottom of the DC electric furnace, the winding diameter of the bottom cable and the number of turns of the bottom cable are changed; optimizing at least the number of turns T of the bottom cable using the relationship (i) and (ii) and the calculated coefficients a, b, c, d and e. T≧(a / V ０ ) ｂ ・・・(i) V ０ =cL+dr+e ・・・(ii)

2. Three upper electrodes; A bottom electrode provided at the bottom of the furnace body; a bottom cable connected to the bottom electrode and surrounding the bottom electrode; A raw material introduction pipe provided on a side or top surface of the furnace body for introducing raw materials containing at least DRI or HBI into the furnace; In a DC electric furnace comprising: The three upper electrodes are provided such that a shape obtained by connecting the centers of the respective electrodes forms an equilateral triangle, A DC electric furnace, wherein the distance L (m) from the bottom cable to the bottom of the DC electric furnace, the winding diameter r (m) of the bottom cable, and the number of turns T of the bottom cable satisfy the relational expressions (iii) and (iv). T≧(0.7 / 1.39V ０ ) ２．０８ ・・・(iii) V ０ =-0.605L-0.032r+2.0 ·····(iv) However, 1.25≦L≦2.5 and 4.5≦r≦7.

5.

3. The raw material injection pipe is provided so that the HBI or the DRI is injected into a position within 3.89 m from the center of the furnace on the surface of the molten iron.

3. The DC electric furnace according to claim 2.

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