Design method for DC electric furnaces, DC electric furnaces, and operating method for DC electric furnaces.

Abstract

To provide a DC electric furnace designing method capable of restraining wear of an electrode, a ceiling member and a furnace wall due to collision of arcs, a DC electric furnace, and a DC electric furnace operating method.SOLUTION: A DC electric furnace designing method includes: a step of arranging three electrodes into a regular triangle, and calculating coefficients α, β and γ in relation expressions (i), (ii) of a maximum current value I per electrode, a distance L between a center of the regular triangle and a center of the electrode, and a shortest distance D between the center of the electrode and a furnace wall, based on simulation for evaluating a thermal flow when the current value per electrode and the distance between the center of the regular triangle and the center of the electrode are varied; and a step of optimizing at least any of the maximum current value I or the distance L. L≥αI+β...(i) D≥γ...(ii)SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Iron produced using the blast furnace method involves reducing iron ore with coke, resulting in high CO2 emissions. One way to reduce CO2 emissions is to produce molten iron by melting iron scrap or hydrogen-reduced iron (DRI) in an electric furnace, and then produce molten steel using existing steelmaking processes centered on converters.

[0003] During steelmaking, cylindrical electrodes of a certain radius and length, which are gripped and suspended from above the ceiling of the electric furnace by a gripping device and inserted into the furnace through an opening in the ceiling, are exposed to a high-temperature atmosphere, resulting in a large proportion of oxidation wear on their sides. To address this, a technology has been disclosed that applies an oxidation-preventive treatment to the electrodes to suppress lateral oxidation and extend the lifespan of the electrodes (Patent Document 1). Furthermore, with the increase in scrap, the main raw material, furnace sizes and power levels are being increased to improve melting capacity. On the other hand, such increases in size and power levels lead to greater wear on the refractory materials of the furnace body, making furnace body cooling essential. Therefore, a technology has been disclosed that cools the ceiling of the electric furnace to extend its lifespan and improve its durability (Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2000-169845 [Patent Document 2] Japanese Patent Application Publication No. 9-196567 [Overview of the project] [Problems that the invention aims to solve]

[0005] Electric furnaces are broadly classified into AC electric furnaces, which use alternating current (AC) as the power source for arc melting, and DC electric furnaces, which use direct current (DC). Since AC electric furnaces use three-phase AC, when increasing the number of electrodes, the total number of electrodes must be a multiple of three. Therefore, AC electric furnaces have limited flexibility in terms of dimensions when scaling up. In contrast, DC electric furnaces allow for increasing the number of electrodes one at a time, thus offering greater flexibility in furnace dimensions when scaling up.

[0006] However, when two or more electrodes are used in a DC electric furnace, the arcs will attract each other due to the electromagnetic force acting between them. Therefore, if the distance between the electrodes is too small relative to the dimensions of the DC electric furnace, or if the current applied to the electrodes is too large, the arcs will collide in the center between the electrodes. The colliding arcs can cause a high-temperature region to extend towards the ceiling material, further heating and damaging the electrodes, or even melting the ceiling material or furnace walls. This problem can become even more pronounced when three electrodes are used in a DC electric furnace.

[0007] Therefore, the present invention aims to provide a DC electric furnace design method, a DC electric furnace, and a DC electric furnace operation method that can suppress wear and tear on electrodes, ceiling members, and furnace walls due to collisions between arcs when three electrodes are used. [Means for solving the problem]

[0008] [1] A DC electric furnace equipped with three electrodes, wherein the three electrodes are arranged such that the shape formed by connecting the centers of the electrodes is an equilateral triangle, and the coefficients α, β, and γ in relational equations (i) and (ii) for the maximum current value per electrode I, the distance L between the center of the equilateral triangle and the center of the electrode, and the shortest distance D between the center of the electrode and the furnace wall are calculated based on a simulation that evaluates the heat flow in the DC electric furnace due to arc discharge generated by the three electrodes when the current value per electrode and the distance between the center of the equilateral triangle and the center of the electrode are varied; and a method for designing a DC electric furnace, comprising the steps of: optimizing at least one of the maximum current value per electrode I or the distance L between the center of the equilateral triangle and the center of the electrode using relational equations (i) and (ii) and the calculated coefficients α, β, and γ. L≧αI+β ···(i) D≧γ ···(ii) [2] A DC electric furnace equipped with three electrodes, wherein the three electrodes are arranged such that the shape formed by connecting the centers of the electrodes is an equilateral triangle, and the coefficients α, β, and γ in relational equations (iii) and (iv) for the maximum current value I per electrode, the distance L between the center of the equilateral triangle and the center of the electrode, the shortest distance D between the center of the electrode and the furnace wall, and the cross-sectional radius k of the electrode are calculated based on a simulation that evaluates the heat flow in the DC electric furnace due to arc discharge generated by the three electrodes when the current value per electrode and the distance between the center of the equilateral triangle and the center of the electrode are varied; and a method for designing a DC electric furnace, comprising the steps of optimizing at least one of the maximum current value I per electrode or the distance L between the center of the equilateral triangle and the center of the electrode using relational equations (iii) and (iv) and the calculated coefficients α, β, and γ. L≧αI+β+k ···(iii) D≧γ+k ···(iv) [3] A DC electric furnace having three electrodes, wherein the three electrodes are arranged such that the shape formed by connecting the centers of the electrodes is an equilateral triangle, the maximum current value per electrode is I (kA), the distance L (mm) between the center of the equilateral triangle and the center of the electrode, and the shortest distance D (mm) between the center of the electrode and the furnace wall satisfy relations (v) and (vi). L≧15.3I+443 ···(v) D≧1870 ···(vi) [4] A DC electric furnace having three electrodes, wherein the three electrodes are arranged such that the shape formed by connecting the centers of each electrode is an equilateral triangle, and the maximum current value per electrode is I (kA), the distance L (mm) between the center of the equilateral triangle and the center of the electrode, the shortest distance D (mm) between the center of the electrode and the furnace wall, and the cross-sectional radius k of the electrode satisfy the relationships (vii) and (viii). L≧15.3I+443+k ···(vii) D≧1870+k ···(viii) [5] The diameter d (m) of the molten steel surface in the DC electric furnace, the bath depth h (m), the radius R (m) of the sphere whose cross-section is the molten steel surface, and the volume V (m³) of the molten steel. 3 A DC electric furnace as described in [3] or [4], satisfying the relationships (ix) to (xi).

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[0009] According to the above configuration, by optimizing the current value per upper electrode and the distance between the center of the furnace and the center of the upper electrode, it is possible to suppress the spread of the high-temperature region due to the collision of arcs to the ceiling member or the like, and it is possible to suppress the wear of the electrode, the ceiling member, and the furnace wall.

Brief Description of the Drawings

[0010] [Figure 1] It is a cross-sectional view of a DC electric furnace according to an embodiment of the present invention. [Figure 2] It is a cross-sectional view taken along line II-II of the DC electric furnace shown in FIG. [Figure 3] It is a diagram for explaining the gravitational force acting on the arc emitted from the upper electrode in the DC electric furnace according to an embodiment of the present invention. [Figure 4] It is a diagram showing the temperature distribution of the arc emitted from the upper electrode, simulated in the DC electric furnace shown in FIG. [Figure 5] It is a diagram showing the flow velocity distribution of the arc emitted from the upper electrode, simulated in the DC electric furnace shown in FIG. [Figure 6] It is a graph showing the relationship between the current per upper electrode and the arc power derived from the simulation results. [Figure 7] It is a graph showing the relationship between the distance between the center of the furnace and the center of the upper electrode and the maximum current value derived from the simulation results. [Figure 8] It is a graph showing the relationship between the distance between the center of the upper electrode and the furnace wall and the temperature of the furnace wall derived from the simulation results. [Figure 9] It is a diagram showing the relationship between the product of the current flowing through the upper electrode and the arc length and the arc power derived from the simulation results. [Figure 10] It is a diagram for explaining the relationship between the diameter of the molten steel surface and the bath depth. [Figure 11] It is a graph showing the relationship between the diameter of the molten steel surface and h / d derived from the simulation results.

Embodiments for Carrying Out the Invention

[0011] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0012] Figure 1 is a cross-sectional view of a DC electric furnace according to one embodiment of the present invention. As shown in the figure, 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. Three upper electrodes 6 are provided on the ceiling 2, and a furnace bottom electrode 7 and a bottom blow nozzle 8 are provided on the furnace bottom. The furnace wall copper panel 3 is provided with a furnace top raw material input pipe 9, a tapping hole 10 for tapping molten steel 21, and a slag hole 11 for discharging electric furnace slag 22. In the following description, the diameter of the molten steel surface 21 in the DC electric furnace 1 is denoted as d, and the bath depth as h.

[0013] As the DC electric furnace 1, a tilting type with a tilting furnace body can be used, or a stationary type with a non-tilting furnace body can be used. In addition, one or more of the following three raw materials are fed into the DC electric furnace 1 as the iron source: iron-containing scrap, reduced iron, and iron-containing dust. As the reduced iron, DRI (Direct Reduction Iron), HBI (Hot Briquette Iron), high-phosphorus reduced iron, etc. can be used. As the iron-containing dust, converter dust granules can be used.

[0014] An arc 20 is emitted from the upper electrode 6, melting raw materials such as reduced iron to produce molten steel 21. The raw materials such as reduced iron are added to the DC electric furnace 1 using the upper raw material input pipe 9. In addition, by blowing gas from the bottom blow nozzle 8 and creating a circulating flow in the molten steel 21 and electric furnace slag 22, heat transfer and melting of the surface of the molten steel 21 and the electric furnace slag 22 can be promoted. Normally, the temperature of the molten steel 21 in the DC electric furnace 1 is at most about 1700°C, but the arc 20 generated from the upper electrode 6 to the surface of the molten steel 21 has an internal temperature of 5000°C or more, and the arc spot on the surface of the molten steel 21 is about 2000°C.

[0015] Figure 2 is a cross-sectional view of the DC electric furnace shown in Figure 1, taken along line II-II. As shown, the DC electric furnace 1 has a circular cross-section. The three upper electrodes 6 are arranged such that the shape formed by connecting the centers of each upper electrode 6 is an equilateral triangle. In the following description, the distance between the furnace center C and the center of the upper electrode 6 will be denoted as L, and the distance between the center of the upper electrode 6 and the furnace wall copper panel 3 will be denoted as D.

[0016] Here, using Figure 3, we will explain the attractive force acting on the arc 20 emitted from the two upper electrodes 6 in the DC electric furnace 1 shown in Figure 1. In the illustrated example, the arc 20 is emitted from the upper electrodes 6 toward the molten steel 21. Therefore, the current flowing through the upper electrodes 6 flows in the directions j1 and j2, which are opposite to the direction in which electrons are emitted. As a result of the current flowing through the upper electrodes 6, magnetic flux densities are generated in the directions b1 and b2 shown in the illustration. As shown in the illustration, the magnetic flux densities b1 and b2 act in opposite directions between the two upper electrodes 6. Therefore, since the magnetic flux densities b1 and b2 cancel each other out, the magnetic flux densities b1 and b2 between the two upper electrodes 6 become low. At this time, an electromagnetic force acts on the arc 20 in the direction represented by the cross product of the current density vector and the magnetic flux density vector (f1 and f2 in the figure), causing the arcs 20 to attract each other.

[0017] Figure 4 shows the temperature distribution of the arc emitted from the upper electrode, simulated in the DC electric furnace 1 shown in Figure 1. Figure 5 shows the velocity distribution of the arc emitted from the upper electrode, simulated in the DC electric furnace 1 shown in Figure 1. In Figures 4 and 5, (a) shows the temperature distribution or velocity distribution inside the DC electric furnace 1 in a perspective view, and (b) shows the cross-section of the arc 20 emitted from the upper electrode 6, respectively.

[0018] Furthermore, as shown in the figure, a region of high temperature extends towards the ceiling 2 due to the collision of arcs 20 at the center between adjacent upper electrodes 6. Also, at the furnace center C, the collision of arcs 20 generated by the three upper electrodes 6 concentrates, causing a region of high temperature to extend towards the ceiling 2. If this region expands further, there is a risk that the arcs 20 will further heat and damage the upper electrodes 6 or melt the ceiling 2. Moreover, as shown in the figure, the temperature tends to be higher in the vicinity of the straight line or extension of the line connecting the furnace center C and the centers of adjacent upper electrodes 6 compared to other parts. This is thought to be due to the collision of arcs 20 emitted from the upper electrodes 6 at the centers of adjacent upper electrodes 6.

[0019] Here, the dimensions of the DC electric furnace 1 are determined, for example, based on the target production volume of molten steel 21. Therefore, in designing the DC electric furnace 1, it is necessary to determine, based on the determined dimensions of the DC electric furnace 1, the distance between the furnace center C and the center of the upper electrode 6, which is sufficient to suppress collisions between arcs 20 and prevent damage to the upper electrode 6 and ceiling 2, and the current value to be applied to the upper electrode 6.

[0020] (Simulation outline and conditions) Since the arc 20 emitted from the upper electrode 6 is hot and fast, it is difficult to directly measure the temperature and flow velocity of the arc 20. Therefore, in this embodiment, a simulation is performed to evaluate the thermal flow in the DC electric furnace 1 due to arc discharge when the current value per upper electrode 6 and the distance between the furnace center C and the center of the upper electrode 6 are varied. Based on the simulation results, the maximum current value I per upper electrode 6, the distance L between the furnace center C and the center of the upper electrode 6, and the distance D between the center of the upper electrode 6 and the furnace wall copper panel 3 are calculated.

[0021] The simulation will be conducted 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 its outline will be explained below. In the simulation, the arc 20 emitted from the upper electrode 6 is considered to be an electromagnetic fluid with conductivity, and coupled calculations of electromagnetic field analysis, fluid analysis, and heat transfer analysis will be performed. First, the method of electromagnetic field analysis will be explained. The current density flowing through the arc 20 is defined as i(A / m 2 Assuming this, the relationship between the magnetic vector potential A and the current density i is expressed as shown in equation (1), where μ0 is the permeability of vacuum.

[0022]

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[0023] The current density i is calculated using equation (2), which is the conservation law of current density. Here, φ is the potential at the inlet and outlet of the current, set as boundary conditions, and σ is the conductivity of the arc (S / m), which is a value that depends on the arc temperature.

[0024]

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[0025] The magnetic vector potential A is calculated from equation (1) based on 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.

[0026]

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[0027] Next, we will explain the methods for fluid analysis and heat transfer analysis. A conductor carrying an electric current in a magnetic field is subjected to the Lorentz force F(N / m 3 ) = i × B is generated. Also, the amount of Joule heating generated in the conductor is q (W / m). 3 )=i 2 / σ is generated. Fluid analysis and heat transfer analysis are performed using the calculated Lorentz force F and Joule heating amount q. In the fluid analysis, the arc flow is analyzed by the law of conservation of mass shown in equation (4) and the Navier-Stokes equations shown in equation (5). Here, u is the velocity, ρ is the density, t is time, p is the pressure, μ is the viscosity, and g is the acceleration due to gravity.

[0028]

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[0029] The heat transfer analysis is performed using equation (6), which is the governing equation for heat transfer analysis. Here C p is specific heat, T is temperature, λ is thermal conductivity, S r Q is the radiant energy. t This is energy loss due to the Thomson effect.

[0030]

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[0031] By performing electromagnetic field analysis, fluid analysis, and heat transfer analysis as described above, the temperature and velocity of the arc can be determined. As an example, the calculation conditions in the simulation were as follows: the diameter of the upper electrode 6 was 750 mm, the distance between the furnace center C and the center of the upper electrode 6 was 2300 mm, and the distance between the upper electrode 6 and the surface of the molten steel 21 was 500 mm. In addition, the atmospheric gas in the furnace was Ar, the surface of the molten steel 21 was flat, and the potential of the molten steel surface was set to 0. Furthermore, the current flowing through each upper electrode 6 was set to 133 kA, and the starting point from which the arc 20 is emitted was set to the center of the lower end surface of the upper electrode 6. Electric furnace slag 22 is not considered.

[0032] Generally, in order to achieve the same production volume in an electric arc furnace process as in a large blast furnace process, the amount of molten steel tapped in one batch needs to be around 300 tons. Electric furnaces do not tap all of the molten steel in each batch; in many cases, about one-third of the molten steel is left aside to secure the initial heat source for the next process. Therefore, to tap 300 tons, an electric furnace capacity of about 450 tons is required, and considering the yield, a capacity of about 500 tons is necessary. In order to melt about 300 tons of raw material in an electric furnace in about 40 minutes, it is preferable that the electric furnace has a heat output of about 200 MW or more. For even faster melting, it is preferable that the electric furnace has a heat output of 250 MW or more.

[0033] Figure 6 is a graph showing the relationship between the current per upper electrode and the arc power, derived from the simulation results. Here, the arc power was evaluated for the cases of one upper electrode 6, two upper electrodes 6, and three upper electrodes 6 in the DC electric furnace 1. The current value that can be applied to the upper electrode 6 is determined by the cross-sectional area of ​​the upper electrode 6, and is generally limited to about 160kA.

[0034] As shown in the figure, when there is one upper electrode 6, it is impossible for the arc power to exceed 200 MW. When there are two upper electrodes 6, the arc power will be 200 kW or less when the current is 150 kA, and the arc power can be increased by lengthening the arc. From this, it is possible for the arc power to be 200 MW when there are two upper electrodes 6, but it is considered difficult to reach 250 MW. Therefore, in order for the arc power to be between 200 MW and 250 MW, it is necessary to use three upper electrodes 6. Considering the uniformity of the temperature inside the furnace, it is preferable to arrange the upper electrodes 6 so that the shape formed by connecting the centers of each upper electrode 6 is an equilateral triangle. Here, an equilateral triangle means a triangle in design, and in the DC electric furnace 1, due to distortion of the furnace lid, deformation of the electrodes, etc., a deviation in shape from a perfect equilateral triangle by about the cross-sectional radius of the upper electrode 6 may occur.

[0035] The temperature evaluation points in the simulation are indicated by P1 to P3 in Figures 1 and 2. Evaluation point P1 is the height of the lower surface of the ceiling 2 at the furnace center C, which is 1.8 m above the molten steel 21 surface. Evaluation points P2 and P3 are the locations of the furnace wall refractory material 4. As shown in Figure 2, evaluation point P2 is the location of the furnace wall refractory material 4 on the extension of the line connecting the furnace center C and the center of the upper electrode 6, and evaluation point P3 is the location of the furnace wall refractory material 4 on the extension of the line connecting the furnace center C and the centers of the adjacent upper electrodes 6.

[0036] The heat resistance temperature of the stainless steel used in ceiling 2 is generally around 700°C (973K), and considering the need to suppress deformation due to thermal expansion, it is preferable that the temperature at evaluation point P1 be 800K or lower. Furthermore, since the heat resistance temperature of the refractory covering the furnace wall refractory 4 is around 1500 to 1800°C (1773 to 2073K), it is preferable that the temperatures at evaluation points P2 and P3 be 1900K or lower.

[0037] Table 1 shows the temperature (K) at each evaluation point when an arc discharge occurs, by varying the current value per upper electrode 6 and the distance between the furnace center C and the center of the upper electrode 6. In Table 1, "Current Value" is the current value per upper electrode 6. "Furnace Center to Electrode Distance" is the distance L between the furnace center C and the center of the upper electrode 6, and "Electrode to Furnace Wall Distance" is the distance D between the center of the upper electrode 6 and the furnace wall copper panel 3. "Arc Length" is the length of the arc 20 emitted from the upper electrode 6 and is equal to the distance between the upper electrode 6 and the molten steel surface 21. "Arc Power" is the power of the arc 20. "Evaluation Point 1" is the temperature at evaluation point P1, and "Furnace Wall Maximum Temperature" is the maximum temperature at the evaluation point where the temperature was highest among evaluation points P2 and P3. In "Equation (7) Judgment," we indicate "○" if Equation (7), which will be explained later, is true, and "×" if it is false. In "Equation (7) Right Side," we show the value of the right side of Equation (7).

[0038] [Table 1]

[0039] Figure 7 is a graph showing the relationship between the distance between the reactor center and the center of the upper electrode and the maximum current value, derived from the simulation results shown in Table 1. The dashed line graph in the figure indicates that the greater the distance L between the reactor center C and the center of the upper electrode 6, the greater the current that can be applied to the upper electrode 6. Furthermore, the relationship between the maximum current value I per upper electrode 6 and the distance L between the reactor center C and the center of the upper electrode 6 is given by equation (7). Rearranging for L, equation (7) becomes equation (8). These equations were derived from the simulation results using the least squares method. I(kA)≦0.0653L(mm)-28.9 (7) L(mm)≧15.3I+443 ···(8)

[0040] In the above simulation, the starting point for the emission of arc 20 was assumed to be the center of the lower end surface of the upper electrode 6. However, in an actual DC electric furnace 1, the starting point for the emission of arc 20 is not necessarily the center of the lower end surface of the upper electrode 6, depending on the wear condition of the upper electrode 6. Equations (7) and (8) can be expressed as equations (9) and (10) below, where k is the cross-sectional radius of the upper electrode 6. I(kA)≦0.0653L(mm)-28.9+k (9) L(mm)≧15.3I+443+k (10)

[0041] Furthermore, equations (8) and (10) relating the maximum current value 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 (11) and (12) using coefficients α and β. L≧αI+β ···(11) L≧αI+β+k ···(12)

[0042] Figure 8 is a graph showing the relationship between the distance between the center of the upper electrode and the furnace wall and the furnace wall temperature, derived from the simulation results shown in Table 1. Graph (a) plots all the data from Table 1, while graph (b) plots the data from Table 1 where "Equation (7) Judgment" is ○.

[0043] In graph (a), as the distance D between the center of the upper electrode 6 and the furnace wall increases, both a tendency for the furnace wall temperature to decrease and a tendency for it to increase can be observed. The inventors found that when the temperature at evaluation point P3 is at its maximum, it is when the collision between the arcs 20 is strong, and this is the result when the relationship between the maximum current value I per upper electrode 6 and the distance L between the furnace center C and the center of the upper electrode 6 does not satisfy equation (7).

[0044] In contrast, graph (b) shows that as the distance D between the center of the upper electrode 6 and the furnace wall increases, the temperature of the furnace wall tends to decrease. From this, it can be said that if the relationship between the maximum current value I per upper electrode 6 and the distance L between the furnace center C and the center of the upper electrode 6 satisfies equation (7), then as the distance D between the center of the upper electrode 6 and the furnace wall increases, the temperature of the furnace wall also decreases. Here, distance D is the length from the center of the upper electrode 6 to the furnace wall on the line extended from the straight line from the furnace center C to the center of the upper electrode 6 to the furnace wall.

[0045] Furthermore, if the distance D between the center of the upper electrode 6 and the furnace wall is 1870 mm or more, and equations (7) and (8) are satisfied, then the temperature of the furnace wall is 1900 K or less, which is the heat resistance temperature. In this case, D can be expressed as shown in equation (13) below. Also, if the cross-sectional radius of the upper electrode 6 is k, then equation (13) can be expressed as equation (14), and using the coefficient γ, it can be expressed as equations (15) and (16). D≧1870 ···(13) D≧1870+k ···(14) D≧γ ···(15) D≧γ+k ···(16)

[0046] From Equation (8) for the distance L between the center C of the furnace and the center of the upper electrode 6 and Equation (13) for the distance D between the center of the upper electrode 6 and the furnace wall, the diameter d of the molten steel surface in the DC electric furnace 1 is expressed by the following Equation (17). d≧0.002(15.3I + 443 + 1870) ···(17)

[0047] In addition, in Fig. 8(b), the data of No. 8 in Table 1 has the maximum furnace wall temperature under the condition of satisfying Equation (7). Since the current value at this time is 140 kA, considering the heat resistance of the furnace wall, the current value of the upper electrode 6 is preferably 140 kA or less.

[0048] Fig. 9 is a diagram showing the relationship between the product of the current flowing through the upper electrode and the arc length and the arc power, derived from the simulation results shown in Table 1. From Fig. 9, the relationship between the target arc power Q (MW), the maximum current value I (kA) of the upper electrode 6, and the arc length E (m) is shown as Equation (18). Equation (18) was created using the least squares method from the simulation results. Q≦2.7IE + 23.6 ···(18)

[0049] Fig. 10 is a diagram for explaining the relationship between the diameter of the molten steel surface and the bath depth. As the shape of the DC electric furnace 1, assuming the diameter of the molten steel surface in the DC electric furnace 1 is d and the bath depth is h, the value of h / d affects the refining performance and the like. Although h / d is generally in the range of 0.2 to 0.3, the higher the value, the better the refining performance. As shown in Fig. 10 for the shape of the molten steel part in the DC electric furnace 1, assuming the molten steel surface is a sphere that becomes the cutting surface, and the radius of the sphere is R (m), V (m 3 ) then it is shown as the following relational expressions (19) and (20).

[0050]

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[0051] Figure 11 is a graph showing the relationship between the diameter of the molten steel surface and h / d, derived from the simulation results in Table 1. In DC electric furnace 1, for example, when the amount of molten steel is 76 m³ 3 Assuming (532t), h / d can be shown in Figure 11 from equations (19) and (20). For example, if the current value of the upper electrode 6 is 140kA, then from equation (8), the distance L between the furnace center C and the center of the upper electrode 6 should be 2585mm or more. Also, from equation (13), the distance D between the center of the upper electrode 6 and the furnace wall should be 1870mm or more, so the minimum diameter d of the DC electric furnace 1 is 2 × (2585 + 1870) = 8914mm. In this case, from Figure 11, h / d is 0.25.

[0052] In a DC electric furnace 1 designed according to a design method of one embodiment of the present invention, by satisfying equation (8) for the current value applied to the upper electrode 6 and the distance L between the furnace center C and the center of the upper electrode 6, the expansion of the high-temperature region due to collisions between arcs can be suppressed, and the temperatures of the upper electrode 6 and the ceiling 2 can be kept within an appropriate range. Furthermore, by satisfying equation (13), the temperature of the furnace wall refractory 4 can also be kept within an appropriate range. In addition, the value of h / d can be calculated from equations (17), (19), and (20) for the diameter d of the molten steel surface 21. Furthermore, by optimizing the current value per upper electrode 6 based on equation (7) and equation (18) for the arc power Q, wear and tear of the upper electrode 6, ceiling 2, and furnace wall refractory 4 of the DC electric furnace 1 can be suppressed.

[0053] Preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, but the present invention is not limited to these examples. It will be obvious to those skilled in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these will naturally also be understood to fall within the technical scope of the present invention. [Explanation of Symbols]

[0054] 1... DC electric furnace, 2... Ceiling, 3... Copper panels on the furnace wall, 4... Refractory material on the furnace wall, 5... Refractory material at the bottom of the furnace, 6... Upper electrode, 7... Bottom electrode, 20... Arc, 21... Molten steel, 22... Electric furnace slag.

Claims

1. In a DC electric furnace equipped with three electrodes, The three electrodes are arranged such that the shape formed by connecting the centers of each electrode is an equilateral triangle. The steps include: calculating the coefficients α, β, and γ in relational equations (i) and (ii) for the maximum current value I per electrode, the distance L between the center of the equilateral triangle and the center of the electrode, and the shortest distance D between the center of the electrode and the furnace wall, based on a simulation that evaluates the heat flow in the DC electric furnace due to arc discharge generated by the three electrodes when the current value per electrode and the distance between the center of the equilateral triangle and the center of the electrode are varied; A method for designing a DC electric furnace, comprising the step of optimizing at least one of the maximum current value I per electrode or the distance L between the center of the equilateral triangle and the center of the electrode, using relational equations (i), (ii) and the calculated coefficients α, β, and γ. L≧αI+β ...(i) D ≥ γ ... (ii)

2. In a DC electric furnace equipped with three electrodes, The three electrodes are arranged such that the shape formed by connecting the centers of each electrode is an equilateral triangle. The steps include: calculating the coefficients α, β, and γ in relational equations (iii) and (iv) concerning the maximum current value I per electrode, the distance L between the center of the equilateral triangle and the center of the electrode, the shortest distance D between the center of the electrode and the furnace wall, and the cross-sectional radius k of the electrode, based on a simulation that evaluates the heat flow in the DC electric furnace due to arc discharge generated by the three electrodes when the current value per electrode and the distance between the center of the equilateral triangle and the center of the electrode are varied; A method for designing a DC electric furnace, comprising the step of optimizing at least one of the maximum current value I per electrode or the distance L between the center of the equilateral triangle and the center of the electrode, using relational equations (iii), (iv) and the calculated coefficients α, β, and γ. L≧αI+β+k...(iii) D≧γ+k...(iv)

3. In a DC electric furnace equipped with three electrodes, The three electrodes are arranged such that the shape formed by connecting the centers of each electrode is an equilateral triangle. A DC electric furnace in which the maximum current value per electrode is I (kA), the distance L (mm) between the center of the equilateral triangle and the center of the electrode, and the shortest distance D (mm) between the center of the electrode and the furnace wall satisfy relations (v) and (vi). L≧15.3I+443...(v) D≧1870...(vi)

4. In a DC electric furnace equipped with three electrodes, The three electrodes are arranged such that the shape formed by connecting the centers of each electrode is an equilateral triangle. A DC electric furnace in which the maximum current value per electrode I (kA), the distance L (mm) between the center of the equilateral triangle and the center of the electrode, the shortest distance D (mm) between the center of the electrode and the furnace wall, and the cross-sectional radius k of the electrode satisfy relations (vii) and (viii). L≧15.3I+443+k...(vii) D≧1870+k...(viii)

5. The diameter d (m) of the molten steel surface in the DC electric furnace, the bath depth h (m), the radius R (m) of the sphere whose cross-section is the molten steel surface, and the volume V (m³) of the molten steel. ３ The DC electric furnace according to claim 3 or 4, wherein the given expression satisfies relational equations (ix) to (xi). [Math 1]

6. In the method for operating a DC electric furnace according to claim 3 or 4, A method for operating a DC electric furnace, wherein the arc power Q (MW) of the arc discharge generated by the three electrodes is given by E (m) and satisfies relations (xii) and (xiii). I≦0.0653L-28.9...(xii) Q≦2.7IE+23.6...(xiii)

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