Direct current electric furnace, and direct current electric furnace designing method

Abstract

To provide a direct current electric furnace 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 designing method.SOLUTION: A direct current electric furnace according to the present invention comprises: three upper electrodes; a furnace bottom electrode provided at a bottom of a furnace body; a first furnace bottom cable connected to the furnace bottom electrode and including a first parallel part extending in a first direction; a second furnace bottom cable including a second parallel part extending in a second direction opposite to the first direction; and a raw material input pipe provided on a side face or an upper face of the furnace body and inputting a raw material including at least DRI or HBI. Length of the first and second parallel parts is 2.0 m or more and 3.5 m or less, and the raw material input pipe is provided so that the HBI or DRI is inputted to a position on a surface of molten iron within 3.2 m from a furnace center.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Iron produced by the blast furnace method is produced by reducing iron ore with coke, so CO 2 A large amount of CO is generated. 2 One method to reduce the amount of waste generated is to produce molten iron by melting scrap iron and DRI (direct reduced iron) in an electric furnace, and then produce molten steel using the existing steelmaking process centered on converters. 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, in DC arc furnaces, a magnetic field generating coil is provided around the electrode at the bottom of the furnace so that the windings go around, generating a DC magnetic field with components that are rotationally symmetrical about the axis of the arc generating electrode. As a result, a technology has been disclosed that can equalize the heat load in any direction and achieve uniform melting because the arc rotates around the core (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 DC electric furnace and a method for designing 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 short period of time in the case of a large electric furnace using three electrodes, and [Means for solving the problem]

[0008] [1] A DC electric furnace comprising three upper electrodes, a bottom electrode provided at the bottom of the furnace body, a first bottom cable connected to the bottom electrode and including a first parallel portion extending in a first direction which is a radial direction of the bottom electrode, a second bottom cable connected to the bottom electrode and including a second parallel portion extending in a second direction which is a direction opposite to the first direction, and a raw material introduction pipe provided on the side or top of the furnace body for introducing raw materials including at least DRI or HBI into the furnace, wherein the three upper electrodes are provided so that a shape connecting the centers of the respective electrodes forms an equilateral triangle, the length of the first and second parallel portions is 2.0 m or more and 3.5 m or less, and the raw material introduction pipe is provided so that the HBI or the DRI is introduced at a position within 3.2 m from the center of the furnace on the surface of the molten iron. [2] A DC electric furnace comprising three upper electrodes, a bottom electrode provided at the bottom of the furnace body, a first bottom cable connected to the bottom electrode and including a first parallel portion extending in a first direction which is a radial direction of the bottom electrode, a second bottom cable connected to the bottom electrode and including a second parallel portion extending in a second direction which is a direction opposite to the first direction, and a raw material introduction pipe provided on the side or top of the furnace body for introducing raw materials including at least DRI or HBI into the furnace, wherein the three upper electrodes are provided so that the shape of a line connecting the centers of the respective electrodes forms an equilateral triangle, and at least one of the first and second bottom cables includes a circumferential portion which encircles the bottom electrode by 90 degrees to 180 degrees, and the raw material introduction pipe is provided so that the HBI or DRI is introduced at a position within 4.0 m from the center of the furnace on the surface of the molten iron. [3] A DC electric furnace according to [1] or [2], wherein the length X (m) of the first and second parallel portions and the distance L (m) between the first and second bottom cables and the bottom of the furnace satisfy the following formula (i): X≧1.85+0.079L (i) [4] A method for designing a DC electric furnace according to the invention described in [1] or [2], comprising the steps of: calculating coefficients a and b in relational equation (ii) between the length X of the first and second parallel portions and the distance L between the first and second hearth cables and the hearth based on a simulation for evaluating the flow of molten iron when the lengths of the first and second parallel portions are varied; and optimizing at least the length X of the first and second parallel portions using relational equation (ii) and the calculated coefficients a and b. X≧a+bL (ii) 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. 2 is a diagram showing the structure of the bottom of a DC electric furnace according to a comparative example. [Figure 9] FIG. 13 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 a comparative example. [Figure 10] 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 11] FIG. 2 is a diagram showing the results of an analysis of the flow of molten iron in the DC electric furnace shown in FIG. [Figure 12] FIG. 2 is a diagram showing the structure of a DC electric furnace 1 according to another embodiment of the present invention. [Figure 13] FIG. 13 is a diagram showing the results of an analysis of the flow of molten iron in the DC electric furnace shown in FIG. 12. [Figure 14] FIG. 13 is a diagram showing the calculation results of the trajectory of the HBI in a DC electric furnace in which a bottom cable is not connected to the bottom electrode. [Figure 15] FIG. 13 is a diagram showing the calculation results of the trajectory of the HBI when the length of the parallel portion is changed in the DC electric furnace shown in FIG. [Figure 16] 13 is a graph showing the results of evaluating the time it takes for HBI to reach just below the upper electrode after being introduced into the DC electric furnace shown in FIG. 12. [Figure 17] FIG. 13 is a diagram showing the calculation results of the trajectory of the HBI when the length of the parallel portion is changed in the DC electric furnace shown in FIG. [Figure 18] FIG. 13 is a diagram showing the calculation results of the trajectory of the HBI when the length of the circulating part is changed in the DC electric furnace shown in FIG. [Figure 19] FIG. 11 is a diagram showing the structure of a DC electric furnace according to yet another embodiment of the present invention. [Figure 20] 4 is a graph showing the relationship between the length of the parallel part of the hearth cable and the rotational flow speed of molten iron in a DC electric furnace according to one embodiment of the present invention. [Figure 21] 13 is a graph showing the relationship between the length of the parallel part and the distance between the bottom cable and the hearth, derived from the results of a simulation. 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, (b) is a bottom view of the DC electric furnace 1 as viewed from the furnace bottom side, and (c) is a perspective view of the DC electric furnace 1 as viewed from the furnace bottom side. In the drawings and in the following description, three mutually orthogonal directions are defined as +x direction, +y direction, and +z direction. Although not shown in the drawings, 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. A 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 also 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] In addition, two bottom cables 71 and 72 are connected to the bottom electrode 7, and a current I flows through each of them. In the following, the bottom cable 71 will be described as the first bottom cable, and the bottom cable 72 will be described as the second bottom cable. The bottom cable 71 is connected to the bottom electrode 7 and includes a parallel portion 71a, which is a first parallel portion extending in a first direction (+x direction) that is the radial direction of the bottom electrode 7. The bottom cable 72 is connected to the bottom electrode 7 and includes a parallel portion 72a, which is a second parallel portion extending in a second direction (-x direction) that is the opposite direction to the first direction (+x direction). The parallel portion 71a and the parallel portion 72a have the same length, and are indicated by X in the figure. By providing the two bottom cables 71 and 72, electromagnetic forces are generated in the molten iron 21 in opposite directions across the furnace center C. The effect of the present invention is not only obtained by installing the bottom cables 71 and 72 in parallel, but also by installing them in the opposite directions. Therefore, the bottom cables 71 and 72 do not need to be strictly parallel at the parallel parts 71a and 72a, and a deviation of about ±15° or deflection of the bottom cables 71 and 72 is acceptable.

[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 cables 71, 72 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 cables 71, 72, and the range of values ​​that r (m) can take is 4.5≦r≦7.5. Furthermore, the depth from the surface of the molten iron 21 to the bottom of the hearth is defined as H.

[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 connected forms 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). 0 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 RC 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 I 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 and b2 at the generation position of the arc 20. In addition, in a DC electric furnace 1 having 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 cables 71 and 72 to the bottom electrode 7. When a magnetic field generating coil is provided around the bottom electrode 7 so that the winding goes around it as in the technology shown in Patent Document 1, 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, in the case of 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, i.e., in the +y direction. In a large DC electric furnace 1, a current of about 100 kA to 140 kA flows per upper electrode, resulting in a large current of 300 kA to 420 kA in total for the three upper electrodes 6. The bottom electrode 7 may be made of a conductive refractory material or may be made by embedding a large number of metal rods.

[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 cables 71, 72 are 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 depth of the furnace bottom of 1.85 m, and a weight of the molten iron 21 in the furnace 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] The total current flowing through the three upper electrodes 6 is 300 kA. In the case of a large DC electric furnace 1 with a total current value of 300 kA or more, if a current of 300 kA or more is passed through one bottom cable, an extremely large cable must be used. Therefore, it is preferable to use two or more cables as the bottom cables 71, 72. Therefore, in the DC electric furnace 1 according to this embodiment, the two bottom cables 71, 72 are connected to the bottom electrode 7 for explanation.

[0039] 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.

[0040] 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.

[0041] (Flow analysis 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.

[0042] (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 the bottom cables 71, 72 connected to the 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.

[0043] In Fig. 7, (a) shows the electromagnetic force when there is no effect of the current I flowing through the bottom cables 71, 72 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 passing 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 (-y direction).

[0044] In Fig. 7, (b) shows the electromagnetic force when there is an effect of the current I flowing through the bottom cables 71, 72 connected to the bottom electrode 7. The direction of the current j4 flowing through the bottom cables 71, 72 is perpendicular to the paper (-z direction). The magnetic flux density b4 caused by the current j4 flowing through the bottom cables 71, 72 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.

[0045] 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 cables 71, 72. Therefore, by installing the bottom cables 71, 72 so as to circle the bottom electrode 7, the electromagnetic force f4 acts to rotate the molten iron 21 horizontally in the furnace.

[0046] (Analysis of the flow of molten iron in a DC electric furnace according to a comparative example) Here, a DC electric furnace 90 according to a comparative example will be described. FIG. 8 is a diagram showing the structure of a DC electric furnace 90 according to a comparative example. In FIG. 8, (a) is a cross-sectional view of the furnace bottom, and (b) is a bottom view of the DC electric furnace 90 seen from the furnace bottom side. As shown in the figure, unlike the DC electric furnace 1 according to this embodiment, only one bottom cable 97a is connected to the bottom electrode 97. The bottom cable 97a extends in the +x direction, which is the radial direction of the bottom electrode 97, and turns 180 degrees around the circumferential direction of the bottom electrode 97. The structure other than the illustrated parts is the same as that of the DC electric furnace 1 shown in FIG. 1, and illustration and description of these are omitted.

[0047] FIG. 9 is a diagram showing the results of an electromagnetic field analysis when a bottom cable 97a is connected to a bottom electrode 97 in a DC electric furnace 90 according to a comparative example. As analysis conditions, the bottom cable 97a is installed at a position 1.85 m from the bottom of the furnace, taking into consideration the bottom refractory 95 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 (2×√3=3.4 m). The bottom cable 97a is drawn out 2.25 m in the radial direction (−x direction) from the furnace center C and is wound 1 / 2 around in the circumferential direction. The current I flowing through the bottom electrode 97 is set to 300 kA.

[0048] In Fig. 9, (a) shows the Lorentz force density distribution when the bottom cable 97a is not connected to the bottom electrode 97, and (b) shows the Lorentz force density distribution when the bottom cable 97a is connected to the bottom electrode 7. Comparing (a) and (b), it can be seen that when the bottom cable 97a is connected, the Lorentz force is inclined in the direction of the current I flowing along the bottom cable 97a (f5, f6 in the figure). From this, it can be seen that by providing the bottom cable 97a in the circumferential direction of the bottom electrode 97, a rotating flow field is formed in which the molten iron 21 rotates in the circumferential direction inside the furnace.

[0049] Fig. 10 is a diagram showing the results of analyzing the flow of molten iron 21 based on the Lorentz force density distribution shown in Fig. 9. The molten iron 21 has a specific gravity of 7000 kg / m 3The analysis was performed with a viscosity coefficient of 0.005 Pa s. In Fig. 10, (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.

[0050] 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 97a. 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.

[0051] (Analysis of the flow of molten iron in a DC electric furnace according to this embodiment) Fig. 11 is a diagram showing the results of an analysis of the flow of molten iron 21 in the DC electric furnace 1 according to the present embodiment shown in Fig. 1. In Fig. 11, (a) is a diagram showing the flow velocity vector of the molten iron 21 on the molten iron surface as viewed from the ceiling 2 side. Also, (b) is a diagram showing the contour lines of the flow velocity of the molten iron 21 on the molten iron 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 furnace bottom as viewed from the furnace bottom side.

[0052] 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 cables 71 and 72. 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.

[0053] (DC electric furnace according to another example of this embodiment) FIG. 12 is a diagram showing the structure of a DC electric furnace 1 according to another embodiment of the present invention. In FIG. 12, (a) is a cross-sectional view of the DC electric furnace 1, (b) is a bottom view of the DC electric furnace 1 as viewed from the furnace bottom side, and (c) is a perspective view of the DC electric furnace 1 as viewed from the furnace bottom side. 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. A raw material input pipe 9 is provided on the side or top of the furnace body, and raw materials containing at least DRI or HBI are input into the furnace. 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.

[0054] In addition, two bottom cables 71 and 72 are connected to the bottom electrode 7, and a current I flows through each of them. In the following, the bottom cable 71 will be described as the first bottom cable, and the bottom cable 72 as the second bottom cable. The bottom cable 71 is connected to the bottom electrode 7 and includes a parallel part 71a, which is a first parallel part extending in a first direction (+x direction) that is the radial direction of the bottom electrode 7, and a winding part 71b that winds around the bottom electrode 180 degrees in the circumferential direction. The bottom cable 72 is connected to the bottom electrode 7 and includes a parallel part 72a, which is a second parallel part extending in a second direction (-x direction) that is the opposite direction to the first direction (+x direction), and a winding part 72b that winds around the bottom electrode 180 degrees in the circumferential direction.

[0055] Fig. 13 is a diagram showing the results of analyzing the flow of molten iron 21 in the DC electric furnace 1 shown in Fig. 12. In Fig. 13, (a) is a diagram showing the flow velocity vector of the molten iron 21 on the molten iron surface as viewed from the ceiling 2 side. Also, (b) is a diagram showing the contour lines of the flow velocity of the molten iron 21 on the molten iron 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.

[0056] As shown in the figure, the molten iron 21 forms a rotating flow field rotating in the circumferential direction, and the maximum value of the flow velocity is about 1.0 m / s. The flow velocity of the molten iron 21 is maximum at the portion indicated by Rv in FIG. 13(b) and (c). 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 cables 71 and 72. 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. Unlike the case of the DC electric furnace 90 according to the comparative example shown in FIG. 8, in the DC electric furnace 1 shown in FIG. 12, the bottom cables 71 and 72 include the circumferential parts 71b and 72b, so that the electromagnetic force generated in the molten iron 21 is large. Therefore, the flow of the molten iron 21 is larger than that of the DC electric furnace 90 according to the comparative example shown in FIG. 8, and the maximum flow velocity of the molten iron 21 is about 1.0 m / s.

[0057] 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.

[0058]

number

[0059] 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 coefficient C 1, C 2 and C 3 , drag F D is expressed by the following equations (8) and (9).

[0060]

number

[0061] 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.

[0062]

number

[0063] 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.

[0064] FIG. 14 is a diagram showing the calculation result of the trajectory of the HBI in a DC electric furnace in which the bottom cables 71, 72 are not connected to the bottom electrode. The diameter of the furnace is 9.6 m, the bath depth is 1.8 m, and the capacity of the molten iron 21 is about 500 t. In FIG. 14, (a) shows the trajectory of the HBI when the distance R of the charging position Pr from the furnace center C is 1 m, and (b) shows the trajectory of the HBI when the distance R is 3.2 m. When the bottom cables 71, 72 are not connected to the bottom electrode 7, as explained using FIG. 6, the flow of the molten iron 21 is not a flow that rotates in the furnace, but a flow that springs up from the vicinity of the furnace center C. Therefore, as shown in the figure, the HBI on the molten iron 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.

[0065] Fig. 15 is a diagram showing the calculation results of the trajectory of the HBI when the length X of the parallel portion is changed in the DC electric furnace 1 shown in Fig. 12. In Fig. 15, (a) shows the trajectory of the HBI when the length X of the parallel portion is 1.5 m, (b) shows the trajectory when the length X of the parallel portion is 2.0 m, and (c) shows the trajectory when the length X of the parallel portion is 2.5 m. The trajectory of the HBI was measured for 120 seconds from the time of being thrown onto the molten metal surface. The throwing position Pr of the HBI was set at a distance R of 1.3 m from the center C of the furnace.

[0066] As shown in (a), when the length X of the parallel portion is 1.5 m, the rotational flow velocity of the molten iron 21 is not generated sufficiently, so that the HBI moves away from the high-temperature region directly below the upper electrode 6 after being poured in. Therefore, it is difficult to say that the HBI is melted efficiently. Also, as shown in (b) and (c), when the length X of the parallel portion is 2.0 m or more, the rotational flow field of the molten iron 21 acts on the HBI, which has a smaller specific gravity than the molten iron 21, so that the HBI moves to the high-temperature region directly below the upper electrode 6 after being poured in. Therefore, it is understood that the HBI is melted efficiently.

[0067] FIG. 16 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 the DC electric furnace 1 shown in FIG. 12. 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 of 2 m from the center C of the furnace. In the graph, the length X of the parallel part is changed in a range of 2.0 m to 3.5 m, and the relationship between the charging position Pr of the HBI and the time it takes for the HBI to reach just below the upper electrode 6 is shown. As shown in the figure, the longer the length X of the parallel part, the greater the electromagnetic force acting on the molten iron 21, and therefore the shorter the time it takes to reach just below the upper electrode 6. In other words, for efficient melting of the HBI, it can be said that a longer length X of the parallel part is preferable.

[0068] Here, the size of a typical HBI is about 30×50×130 mm. It is known that with this size, after the HBI is charged into the molten iron 21, one HBI melts in about 40 to 60 seconds. In the DC electric furnace 1, the temperature of the molten steel at the charging position of the HBI decreases, so if the stirring of the molten iron 21 is insufficient or the transportation of the HBI is inappropriate, the melting rate drops more than that of a single HBI. Therefore, efficient melting in a large DC electric furnace 1 means that the melting rate is about the same as when one HBI is charged. Therefore, when multiple HBIs are charged intermittently, the temperature of the molten iron 21 near the charging position Pr drops, so it is important to move the HBI to a high temperature region quickly. From the graph, it can be seen that in order for the HBI to reach directly under the upper electrode 6 within 40 seconds, which is the melting time after the HBI is charged, the charging position Pr needs to be at a distance R≦3.2 m from the furnace center C.

[0069] Figure 17 is a diagram showing the calculation results of the trajectory of the HBI when the length X of the parallel part is 2.5 m and the winding angle of the winding parts 71b and 72b is changed in the DC electric furnace 1 shown in Figure 12. The length X of the parallel part is 2.5 m. In Figure 17, (a) shows the trajectory of the HBI when the bottom cables 71 and 72 are wound around the bottom electrode 7 by 180 degrees in the winding parts 71b and 72b, respectively, and (b) shows the trajectory of the HBI when they are wound around the bottom electrode 7 by 90 degrees. The HBI insertion position Pr was set to a position at a distance R = 1 m from the furnace center C.

[0070] As shown in the figure, when the length X of the parallel portion is 2.5 m, the HBI moves to the vicinity of the furnace center C regardless of whether the orbiting portions 71b, 72b rotate 180 degrees or 90 degrees in the circumferential direction of the bottom electrode 7, and therefore it can be seen that the HBI is melted efficiently. In addition, the maximum flow speeds of the molten iron 21 at the surface are 0.96 m / s and 1.226 m / s when the orbiting portions 71b, 72b rotate 90 degrees and 180 degrees in the circumferential direction of the bottom electrode 7, respectively.

[0071] Fig. 18 is a diagram showing the calculation results of the trajectory of the HBI when the length of the winding parts 71b, 72b is changed in the DC electric furnace 1 shown in Fig. 12. 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. The graph shows the relationship between the HBI input position Pr and the time when the HBI reaches just below the upper electrode 6 when the bottom cables 71, 72 are wound 90 degrees and 180 degrees in the winding parts 71b, 72b.

[0072] As shown in the figure, in both cases where the bottom cables 71, 72 are rotated by 90 degrees and 180 degrees in the winding sections 71b, 72b, the flow speed of the molten iron 21 is different as described above, but the time for the HBI to reach the high temperature area directly below the upper electrode 6 is approximately the same. Therefore, in both cases where the bottom cables 71, 72 are rotated by 90 degrees or more and 180 degrees or less in the winding sections 71b, 72b, the time for the HBI to reach the high temperature area directly below the upper electrode 6 is approximately the same. In addition, it can be seen that in order for the HBI to reach the high temperature area directly below the upper electrode 6 within 40 seconds after being introduced, the introduction position Pr needs to be set to a position of R≦4.0 m.

[0073] (DC electric furnace according to yet another example of this embodiment) FIG. 19 is a diagram showing the structure of a DC electric furnace 1 according to yet another embodiment of the present invention. In FIG. 19, (a) is a cross-sectional view of the DC electric furnace 1, (b) is a bottom view of the DC electric furnace 1 as viewed from the furnace bottom side, and (c) is a perspective view of the DC electric furnace 1 as viewed from the furnace bottom side. 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. The raw material input pipe 9 is provided on the side or top of the furnace body, and inputs raw materials containing at least DRI or HBI into the furnace. 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.

[0074] In addition, two bottom cables 71, 72 are connected to the bottom electrode 7, and a current I flows through each of them. In the following, the bottom cable 71 will be described as the first bottom cable, and the bottom cable 72 as the second bottom cable. The bottom cable 71 is connected to the bottom electrode 7 and includes a parallel part 71a, which is a first parallel part extending in a first direction (+x direction) which is the radial direction of the bottom electrode 7, and a winding part 71b which winds around the bottom electrode 180 degrees. The bottom cable 72 is connected to the bottom electrode 7 and includes a parallel part 72a, which is a second parallel part extending in a second direction (-x direction) which is the opposite direction to the first direction (+x direction). Here, the bottom cable 72 does not include a winding part. Such a structure is effective in that the bottom cables 71, 72 can be drawn out only on one side of the bottom of the furnace (the -x direction side in the illustrated example) when there are restrictions on the installation conditions of the DC electric furnace 1.

[0075] In the illustrated structure, analysis of the flow of the molten iron 21 based on the Lorentz force density distribution in the molten iron 21 showed that the molten iron 21 formed a rotating flow field rotating in the circumferential direction, and the maximum flow velocity of the molten iron 21 was about 1.05 m / s. As described with reference to Fig. 13, the maximum flow velocity of the molten iron 21 when it forms a flow rotating in the circumferential direction is about 1.0 m / s, so if the flow velocity of the molten iron 21 is 1.0 m / s or more, the HBI can move to the vicinity of the furnace center C, which is a high-temperature region. Therefore, it can be said that the illustrated structure can achieve a flow velocity sufficient to move the HBI to directly below the upper electrode 6, which is a high-temperature region.

[0076] (Optimization of the length of the parallel section) FIG. 20 is a graph showing the relationship between the length X of the parallel part of the bottom cables 71, 72 and the rotational flow speed of the molten iron 21 in the DC electric furnace 1 according to one embodiment of the present invention. The results in FIG. 20 are obtained by changing the length X of the parallel part of the bottom cables 71, 72 and calculating the flow speed of the molten iron 21 in each of the cases of the "parallel type", "parallel + round" and "mixed type" structures of the DC electric furnace 1. The "parallel type" is a structure in which the bottom cables 71, 72 have only the parallel parts 71a, 72a as shown in FIG. 1. The "parallel + round" is a structure in which the bottom cables 71, 72 have the parallel parts 71a, 72a and the round parts 71b, 72b as shown in FIG. 14. The "mixed type" is a structure in which the bottom cables 71, 72 have the parallel parts 71a, 72a as shown in FIG. 19, and only one of the bottom cables 71 has the round part 71b.

[0077] As explained using Figures 15(b) and (c), when the length X of the parallel portion is 2.0 m or more, the HBI moves due to centripetal force to the vicinity of directly below the upper electrode 6, which is a high-temperature region. Therefore, from the graph in Figure 20, it can be seen that the rotational flow velocity of the molten iron 21 needs to be 0.6 m / s or more. Also, from the graph, the flow velocity of the molten iron 21 is faster in the "parallel + circulating" and "mixed types" than in the "parallel type". Therefore, it can be seen that the HBI will melt efficiently under conditions where the minimum flow velocity of the molten iron 21 in the "parallel type" can be ensured.

[0078] 20, the flow velocity V (m / s) of the molten iron 21 in the case of the “parallel type” can be expressed by the following approximate formula (11): where X (m) is the length of the parallel parts 71a, 72a of the hearth cables 71, 72. V = 0.376X - 0.156 (11)

[0079] It is considered that the distance L between the hearth cables 71, 72 and the hearth has a large effect on the flow velocity of the molten iron 21. The shorter the distance L between the hearth cables 71, 72 and the hearth, the larger the electromagnetic force generated in the molten iron 21, and the flow velocity of the molten iron 21 increases. As a result of evaluating the influence of the change in the distance L on the flow velocity of the molten iron 21 by numerical analysis, the change in the flow velocity with respect to the change in the distance L was 0.03. From this, an equation considering the influence of 0.03 on the arc 20 for the distance L (m) between the hearth cables 71, 72 and the hearth was evaluated by numerical analysis, and the electromagnetic force generated in the molten iron 21 was expressed as the following equation (12). Note that the numerical analysis was performed in the range of L = 1.25m to 2.15m, with L = 1.85 (m) as the standard. V=0.376X-0.156-0.03(L-1.85) ···(12)

[0080] In addition, the length X (m) of the parallel part required to set the rotational flow velocity V of the molten iron 21 to 0.6 m / s for the HBI to move to the vicinity immediately below the upper electrode 6, which is a high-temperature region, is expressed by the following formula (13) from formula (12). Figure 21 shows a graph showing the relationship between the length X of the parallel part of the bottom cables 71, 72 and the distance L between the bottom cables 71, 72 and the bottom of the hearth, which is derived from the result of formula (13). X≧1.85+0.079L (13)

[0081] Furthermore, the relational expression (13) between the length X of the parallel part of the bottom cables 71, 72 and the distance L between the bottom cables 71, 72 and the hearth can be expressed as the expression (14) using the coefficients a and b. As described with reference to Fig. 20, the coefficients a and b can be calculated based on a simulation that evaluates the flow of the molten iron 21 when the length X of the parallel part of the bottom cables 71, 72 is changed. X≧a+bL (14)

[0082] In a DC electric furnace 1 designed according to a design method according to an embodiment of the present invention, at least the length X of the parallel portion is optimized using formula (14) and the calculated coefficients a and b. This allows the molten iron 21 to be rotationally stirred, so that reduced iron materials such as HBI and DRI, which have a smaller specific gravity than the molten iron 21, tend to move to the vicinity of the furnace center C, which is a high-temperature region in the furnace. This makes it possible to efficiently melt reduced iron materials such as HBI and DRI.

[0083] 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]

[0084] 1...DC electric furnace, 2...ceiling, 3...furnace wall copper panel, 4...furnace wall refractory, 5...furnace bottom refractory, 6...upper electrode, 7...furnace bottom electrode, 9...raw material input tube, 20...arc, 21...molten iron, 22...electric furnace slag, 71,71...furnace bottom cable, 71a,72a...parallel portion, 71b,72b...circulating portion, X...length of parallel portion.

Claims

1. Three upper electrodes; A bottom electrode provided at the bottom of the furnace body; a first bottom cable connected to the bottom electrode and including a first parallel portion extending in a first direction, which is a radial direction of the bottom electrode; a second bottom cable connected to the bottom electrode and including a second parallel portion extending in a second direction opposite to the first direction; 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, The length of the first and second parallel portions is 2.0 m or more and 3.5 m or less, The raw material injection pipe is provided so that the HBI or the DRI is injected into a position within 3.2 m from the center of the furnace on the surface of the molten iron. DC electric furnace.

2. Three upper electrodes; A bottom electrode provided at the bottom of the furnace body; a first bottom cable connected to the bottom electrode and including a first parallel portion extending in a first direction, which is a radial direction of the bottom electrode; a second bottom cable connected to the bottom electrode and including a second parallel portion extending in a second direction opposite to the first direction; 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, At least one of the first and second bottom cables includes a winding portion that winds around the bottom electrode by 90 degrees or more and 180 degrees or less in the circumferential direction, The raw material injection pipe is provided so that the HBI or the DRI is injected into a position within 4.0 m from the center of the furnace on the surface of the molten iron. DC electric furnace.

3. 3. The DC electric furnace according to claim 1, wherein the length X (m) of the first and second parallel parts and the distance L (m) between the first and second bottom cables and the bottom of the furnace satisfy the following formula (i): X≧1.85+0.079L...(i)

4. A method for designing a DC electric furnace according to claim 1 or 2, comprising the steps of: Calculating coefficients a and b in the relational expression (ii) between the length X of the first and second parallel parts and the distance L between the first and second bottom cables and the hearth based on a simulation for evaluating the flow of molten iron when the lengths of the first and second parallel parts are changed; optimizing the length X of at least the first and second parallel portions using the relation (ii) and the calculated coefficients a and b. X≧a+bL...(ii)

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