Lower electrode structure of a DC electric furnace and operating method of a DC electric furnace

Abstract

The present invention provides a lower electrode structure for a DC electric furnace and an operating method for a DC electric furnace that can suppress steel leakage due to the melting of the electrode rods of the lower electrode in a DC electric furnace with two or more upper electrodes. [Solution] A DC electric furnace 1 for manufacturing molten steel, characterized in that two or more upper electrodes 3 are installed on the upper electrode 2, the lower electrode 4 has numerous electrode rods 5 embedded in the refractory material at the bottom of the furnace, these electrode rods 5 are connected to one electrode support plate 6 installed below them, power supply cables 7 are connected to the electrode support plate 6 at three or more locations, and no electrode rods are installed in an area of ​​at least 500 mm x 500 mm directly above the connection points of the power supply cables 7. A method for operating a DC electric furnace having the lower electrode structure, characterized in that a current of 300 kA to 450 kA is applied.

Description

[Technical Field]

[0001] The present invention relates to a lower electrode structure for a DC electric furnace and a method for operating a DC electric furnace. [Background technology]

[0002] Traditionally, the blast furnace process has been used as the primary steelmaking process. The blast furnace process uses iron ore as its main raw material and reduces it with coke, thus emitting CO2. On the other hand, the electric arc furnace process uses scrap as its main raw material, resulting in lower CO2 emissions. In order to reduce CO2 emissions in the steelmaking process, it is necessary to switch from the blast furnace process, which emits a large amount of CO2, to the electric arc furnace process, which emits less. In order to replace the amount of molten steel produced by the blast furnace process with electric arc furnaces, it is necessary to increase the capacity of electric arc furnaces compared to conventional ones. While the average size of conventional electric arc furnaces is less than 100 tons, electric arc furnaces intended to replace blast furnaces have a capacity of about 500 tons.

[0003] In electric furnaces, an AC or DC power source is used to apply an electric current between the upper electrode and the molten iron, generating an arc from the upper electrode and heating it. In conventional electric furnaces, a maximum current of about 100kA was applied. AC furnaces use three upper electrodes, so the current per electrode is about 30-40kA. DC electric furnaces generally have a single upper electrode, and a current of nearly 100kA is applied to that electrode. DC electric furnaces using two electrodes have been reported, but the practical application of DC electric furnaces using three electrodes has not been reported.

[0004] In a large electric furnace with a capacity of approximately 500 tons and an output of approximately 250 kW, it is considered necessary to install three electrodes even in a DC electric furnace in order to achieve the target heating amount. In that case, the total current would be approximately 300 kA, assuming 100 kA per upper electrode.

[0005] In a DC electric furnace, current flows from the lower electrode installed at the bottom of the furnace through the molten steel and via an arc generated from the upper electrode of the upper electrode. The lower electrode is typically constructed by embedding numerous metal rods, each about 40 mm in diameter, as electrode rods in the bottom of the furnace and converging them with an electrode support plate at the bottom of the furnace body. When the current is increased, there is a risk that the electrode rods of the lower electrode will melt, causing steel leakage from the bottom of the furnace. This invention provides a method for suppressing the amount of melting of the electrode rods of the lower electrode and suppressing steel leakage in a large DC electric furnace using two or more upper electrodes.

[0006] In DC electric furnaces, lower electrodes are often used, which are electrode rods with a diameter of about 40 mm embedded in the bottom of the furnace. A typical lower electrode structure is shown in Figure 15 (Patent Document 1). The upper end of the electrode rod 31 is in contact with the molten steel, and the temperature rises due to Joule heating by the electric current, so the temperature rises above the melting point, and a certain range from the upper end becomes melted. The area enclosed by the dashed line in Figure 15(A) is melted when the electric furnace is in operation, and solidifies again when the temperature drops. While melting of the electrode rod in such a lower electrode is not a problem if the amount of melting is small, if the amount of melting increases, there is a risk of molten steel leaking through the gap between the electrode rod and the refractory material (steel leakage). If steel leakage occurs, the electric furnace becomes unusable, and it may be impossible to operate for several months until it is repaired.

[0007] In conventional electric furnaces with a capacity of 100 tons or less, lower electrode structures have been developed to suppress the risk of steel leakage. Specifically, as shown in Figure 15(B), the current flowing through the electrode rods 31 is distributed to prevent concentration, and measures are taken to prevent the current density of the electrode rods 31 from becoming too high. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Utility Model Publication No. 7-35998 [Patent Document 2] Japanese Patent Application Publication No. 4-214180 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Conventional electric furnaces typically had a capacity of around 100 tons. However, in order to achieve the same steel tapping volume using electric furnaces as in blast furnaces in steel mills that have traditionally used blast furnaces, the tapping volume per cycle needs to be around 300 tons. In recent years, "molten steel retention operation," where a certain amount of molten steel is left in the furnace during tapping to continue the next melting process, has become the mainstream method for electric furnaces. Taking this retained molten steel into account, a large electric furnace with a tapping volume of 300 tons per cycle would need a capacity of around 500 tons, which is significantly larger than conventional electric furnaces.

[0010] The current used in such large-scale electric furnaces is several times greater than that in conventionally sized electric furnaces. In conventional DC electric furnaces, a single upper electrode is used to apply a current of about 80kA, achieving 60kW of heating. In large DC electric furnaces with a capacity of about 500t, a heating capacity of 200kW or more is required, necessitating a total current of 300kA or more. The current value that can be applied to each upper electrode depends on the cross-sectional area of ​​the electrode. The maximum current that can be stably applied with graphite electrodes used in recent years is about 120-150kA, so at least two upper electrodes are needed to apply a current of 300kA, and three are needed to apply a current of more than that. The structure of conventional lower electrodes is specialized for cases where there is only one upper electrode.

[0011] The present invention aims to provide a lower electrode structure for a DC electric furnace and an operating method for a DC electric furnace that can suppress steel leakage due to melting of the electrode rods of the lower electrode when there are two or more upper electrodes. [Means for solving the problem]

[0012] In other words, the gist of this invention is as follows: [1] In a direct current electric furnace for producing molten steel, two or more upper electrodes are installed on the upper electrode, and a large number of electrode rods are embedded in the refractory material at the furnace bottom for the lower electrode. These electrode rods are connected to one electrode support plate installed below them, and power supply cables are connected to the electrode support plate at three or more positions. The electrode rods are not installed in at least a 500 mm × 500 mm range directly above the connection part of the power supply cable. A lower electrode structure of a direct current electric furnace is characterized by this. [2] The lower electrode structure of the direct current electric furnace according to [1], wherein the length of the electrode rod of the lower electrode is in the range of 1080 mm to 1760 mm. [3] In a direct current electric furnace having the lower electrode structure according to [1] or [2], a method of operating a direct current electric furnace, characterized in that a current of 300 kA or more and 450 kA or less is applied.

Effect of the Invention

[0013] According to the present invention, in a large direct current electric furnace, it is possible to suppress the risk of molten steel leakage from the lower electrode.

Brief Explanation of Drawings

[0014] [Figure 1] It is a schematic diagram of a direct current electric furnace having three upper electrodes. (A) is a front sectional view, and (B) is a plan view. [Figure 2] (A) is a front schematic view showing the current path of a direct current electric furnace, and (B) and (C) are calculation mesh diagrams of the lower electrode. (B) is a top view, and (C) is a perspective view. [Figure 3] It is a diagram showing the numerical analysis results. In a vertical section passing through the center of the electric furnace, (A) shows the current density distribution of the entire electric furnace, and (B) shows the current density distribution of the electrode rods. [Figure 4] It is a diagram showing the numerical analysis results when there is one power supply cable. (A) is a plan view of the electrode rod arrangement, and (B) shows the temperature distribution of the electrode rod located at viewpoint 25. [Figure 5] It is a diagram showing the numerical analysis results when there are three power supply cables. (A) is a plan view of the electrode rod arrangement, and (B) shows the temperature distribution of the electrode rod located at viewpoint 25. [Figure 6]This is a schematic diagram showing the location of the power supply cable connection points. [Figure 7] This diagram shows the relationship between the number of connection points in the power supply cable and the maximum melting length of the electrode rod. [Figure 8] This figure shows the relationship between the number of connection points in the power supply cable and the difference between the maximum and minimum melting lengths of the electrode rods. [Figure 9] This figure shows the relationship between the number of connection points in the power supply cable and the minimum unmelted length of the electrode rod. [Figure 10] This figure shows the results of the numerical analysis; (A) is a schematic diagram of the melting of the electrode rod, and (B) shows the temperature distribution of the electrode rod. [Figure 11] This diagram shows the relationship between electrode rod length and minimum unmelted length when there is one power supply cable. [Figure 12] This diagram shows the relationship between electrode rod length and minimum unmelted length when there are three power supply cables. [Figure 13] This diagram shows the current density distribution of the electrode support plate when there is only one power supply cable. [Figure 14] This diagram shows the current density distribution of the electrode support plate when there are three power supply cables. [Figure 15] This diagram shows the structure of a conventional lower electrode, with (A) being a partial front cross-sectional view and (B) being a partial plan view. [Figure 16] This diagram shows the structure of a conventional lower electrode, with (A) being a partial front cross-sectional view and (B) being a partial plan view. [Modes for carrying out the invention]

[0015] Figure 1 shows a schematic diagram of a DC electric furnace using three upper electrodes 3. In a DC electric furnace, a common power supply method is to use the lower electrode 4 as positive and the upper electrode 3 as negative. A power supply cable 7 is connected to the electrode support plate 6 of the lower electrode 4, and current flows to the upper electrode 3 via molten steel 21 and arc 23, passing through a number of electrode rods 5. The arc 23 is generated directly below each upper electrode 3, heating the inside of the furnace and melting and heating the raw material. The raw material scrap 24 is fed continuously or intermittently through an input hole (not shown) in the furnace wall or ceiling.

[0016] Patent Document 1 discloses a technique in which the lower electrode is made into a small electrode unit 37, and multiple electrode units 37 are arranged in a distributed manner, as shown in Figure 16. It is thought that this technique can distribute the current flowing through the electrode rod 31, but as shown in Figure 16(B), there is a large area where the electrode rod 31 is not installed, so when applying a current of 300kA to 450kA in a large furnace, it is not possible to install the necessary electrode rods.

[0017] Patent Document 2 states that the theoretical current density that can be applied to the electrode rod is 70 A / cm². 2 Therefore, in reality, 40 A / cm 2 The following is indicated as desirable: The horizontal cross-sectional area of ​​a typical electrode rod is 14 cm². 2 Therefore, the maximum current value of an electrode rod is 560A. To apply a current of 450kA, 450kA ÷ 560A = 804 electrode rods are needed. Considering the unevenness of current density and the safety factor, it is necessary to install approximately 1200 to 1600 electrode rods. The spacing between electrode rods is generally about 80mm (center-to-center distance 120mm), and assuming a dense grid arrangement, each rod requires an area of ​​120 x 120mm. Therefore, to install approximately 1200 to 1600 electrode rods, 17.28 to 23.04m is needed. 2 A certain area is required, which in the case of a circle would be 4.7 to 5.4 meters in diameter. Even in large electric furnaces, the area of ​​the furnace bottom where electrode rods can be installed is only about 6 meters in diameter, so there is no room to install multiple unitized lower electrodes as shown in Figure 16.

[0018] To address the above needs, a lower electrode structure in which electrode rods are densely arranged on a single electrode support plate can be considered to suppress the melting of the electrode rods. Therefore, in this invention, the lower electrode 4 is considered to be a case in which a large number of electrode rods 5 are embedded in the furnace bottom refractory material 14, and these electrode rods 5 are connected to a single electrode support plate 6 installed below it.

[0019] The melting amount of the electrode rods 5 of the lower electrode 4 was evaluated by numerical analysis simulation. Figure 2 shows a schematic diagram of a large 500t DC electric furnace with three upper electrodes 3, a single electrode support plate 6 as the lower electrode 4 with numerous electrode rods 5 arranged on it, and a single power supply cable to supply current to the electrode support plate 6. Figure 2(A) is a schematic front view showing the current path of the DC electric furnace, (B) and (C) are calculation mesh diagrams of the lower electrode 4, with (B) being a top view and (C) being a perspective view.

[0020] The arc of a DC electrode, due to the balance between its characteristic of shortening the current path and the electromagnetic force attracting each other, exhibits a distribution tilted towards the center of the three upper electrodes. Numerical analysis simulations of the arc show that this tilt angle is approximately 4°. Assuming the distance from the upper electrode to the molten metal surface (arc length) is 500 mm, the arc position at the molten metal surface is estimated to be approximately 34 mm from the center of the upper electrode. Therefore, even if the arc forms directly below the upper electrode, it does not result in a significant error.

[0021] As a numerical analysis simulation to predict the amount of melting in the electrode rod 5 of the lower electrode 4, the shape of the arc 23 was fixed, and it was assumed that the current reached the surface of the molten steel 21 vertically from the upper electrode 3. The distribution of the DC current from the molten steel 21 to the electrode rod 5, electrode support plate 6, and power supply cable 7 was calculated. From the obtained current density distribution, Joule heat was calculated, and the region above the melting point of the electrode rod 5 was evaluated as the melting region through heat transfer analysis.

[0022] Molten steel 21 has a surface diameter of 7.38m, a furnace bottom diameter of 7.0m, a bath depth of 1.9m, and a capacity of 79m³. 3 The density of the molten steel was set to 7 t / m³. 3Then, the molten steel mass is 553 t. The upper electrode 3 has a diameter of 800 mm and is arranged as an equilateral triangle with the distance from the center of the furnace to the center of the upper electrode 3 being 1.6 m. The arc diameter is 12 cm. The electrode support plate 6 of the lower electrode 4 has a diameter of 5 m, and the electrode rod 5 is a square with a size of 40 mm × 40 mm. In reality, the electrode rod 5 often has a circular cross-section with a diameter of about 45 mm, and the cross-sectional area of □40 mm × 40 mm is 16 cm 2 Therefore, it corresponds to φ46 mm (area 16.61 cm 2 ). The power supply cable is arranged as one, and the power supply cable 7 is connected to the center of the electrode support plate 6. If the electrode rod is installed directly above the position of the power supply cable at the center of the electrode support plate, the current will concentrate at this position and the current density will become very high. Therefore, as shown in the calculation mesh diagrams of the lower electrode 4 in Figures 2(B) and (C), the electrode rod is not installed in the area of 1.16 × 1.16 m directly above the power supply cable (the electrode rod non-installed area 26). The distance between the electrode rods 5 is such that the distance between the centers of the electrode rods 5 is 120 mm. The thickness of the electrode support plate 6 is 100 mm, and the length of the electrode rod 5 is 1160 mm. The total number of electrode rods 5 is 1280.

[0023] The current analysis of the electric furnace was performed using the finite element method based on Ohm's law and the law of conservation of current shown in Equations (1) and (2). In Equation (1), the boldface i is the current density vector (A / m 2 ), σ is the conductivity (S / m), and φ is the potential (V), indicating that the current flows in proportion to the potential gradient and conductivity. Equation (2) represents the law of conservation of current, meaning that the sum of the current flowing into and out of the infinitesimal volume is 0.

Equation

Equation

[0024] By taking the divergence of Equation (1) and substituting Equation (2), Equation (3) is obtained. Equation (3) is the Poisson equation, and the solution can be obtained by the general finite element method.

Equation

[0025] To solve equation (3), the conductivity of each phase is required. Assuming the molten steel is at a constant temperature of 1600°C, the arc at 12000K, and the power supply cable at 30°C, the conductivity (S / m) of each phase is calculated as follows: 5 , arc 1.0 × 10 4 The power supply cable is 3.79 x 10 6 The electrode rod 5 and electrode support plate 6 were made of low-carbon steel, and the temperature dependence of their conductivity was taken into consideration. These conductivity values ​​can be obtained from research materials on arcs and metal data books.

[0026] The Joule heat was calculated from the obtained current density, and a heat transfer analysis was performed using the finite element method. The target of the heat transfer analysis was the electrode rod 5 and the electrode support plate 6. The surface of the electrode rod 5 in contact with the molten steel 21 was kept at a constant temperature of 1600°C, and the lower surface of the electrode support plate 6 was water-cooled. The heat transfer coefficient of the electrode support plate 6 in contact with the water-cooled section was set to 1000 W / K / m. 2 The steady-state temperature calculation was performed with a cooling water temperature of 30°C. Other interfaces were assumed to be adiabatic.

[0027] Current flow analysis and heat transfer analysis were performed repeatedly until the solution became constant. As an example of the numerical analysis results, Figure 3 shows the current density distribution from molten steel to electrode rod to electrode support plate when a current of 300 kA is applied.

[0028] Figure 3(A) shows the current density distribution in a vertical cross-section of the center of the electric furnace. The current density is shown by varying shades of gray, as shown on the far right of Figure 3(A). The electrode rods 5 from the power supply cable 7 show a high current density, but the current density in the molten steel 21 decreases because of its large volume. Near the surface of the molten steel, the position of the upper electrode 3 (arc 23) affects the current density distribution, but the structure of the electrode rods 5 of the lower electrode 4 is dominant in determining the current density of the molten steel 21 near the bottom of the furnace. Therefore, it can be considered that the position of the upper electrode 3 has little effect on the current density distribution of the lower electrode 4. In other words, there is no significant difference whether the upper electrode 3 of the upper electrode 2 has two or three upper electrodes.

[0029] Figure 3(B) shows the current density distribution of electrode rods 5 located on the x-axis including the center of the electric furnace. No electrode rods 5 are installed in the central part where the power supply cable 7 is connected (the position where the x-coordinate is 0 in Figure 3(B)). In the area surrounding this point, the current density is higher as the x-coordinate approaches 0, and decreases towards the outer periphery.

[0030] Figure 4 shows the temperature distribution of the electrode rod 5 and electrode support plate 6 at a side view of the lower electrode 4, which is divided in the center (position 25 in Figure 4(A)). Since no electrode rods are installed in the central part, the electrode rod 27 at the back is visible. The melting point of electrode rod 5 was set to 1500°C, and the thermal conductivity in the region above 1500°C was set to 200 W / m / K, taking into account the flow effect. The temperature distribution of electrode rod 5 and electrode support plate 6 is shown in shades of gray as shown on the far right of Figure 4(B). The region above 1500°C where the electrode rod 5 melted is shown in white. It can be confirmed that the melted length of electrode rod 5 is large in the region close to the power supply cable 7 in the center and where the current density of electrode rod 5 is high as shown in Figure 3(B). The electrode rod that was most melted (the electrode rod with the longest white length) was at point A, which is closest to the center, and the maximum melted length of the electrode rod was 490 mm.

[0031] Figure 5 shows the calculation results when the power supply cable 7 is connected to the electrode support plate 6 at three locations. In the plan view of the electrode rod 5 shown in Figure 5(A), the three locations where the electrode rod 5 is not placed are the connection points for the power supply cable 7. Three areas (electrode rod unplaced area 26 in Figure 5(A)) are placed directly above the power supply cable to avoid placing the electrode rod 5 when connecting the power supply cable, and their shape is 0.68m × 0.56mm. The temperature distribution of the electrode rod 5 and electrode support plate 6 is shown when the device is cut along the dashed line in Figure 5(A) and viewed from the side (position 25 in Figure 4(A)). Since no electrode rod is installed in the electrode rod unplaced area 26, the electrode rod 27 at the back is visible. The temperature distribution of the electrode rod 5 and electrode support plate 6 is shown in shades of gray as shown on the far right of Figure 4(B). The area above the electrode rod 5 that melted at 1500℃ or higher is shown in white. The total number of electrode rods 5 was 1301, which is almost the same as the 1280 rods connected at a single point. The total current value was also the same at 300kA. The electrode rod that melted the most was electrode rod 27 at the back of the area 26 where no electrode rods were installed, at point B or C, which is closest to the space of the connection part of the power supply cable 7. The maximum melted length was 350mm. In this way, by distributing the connection positions between the electrode support plate 6 and the power supply cable 7 to three locations, it is possible to suppress the maximum melted length of the electrode rods 5 and ensure the minimum unmelted length.

[0032] The greater the melting length of the electrode rod 5, the greater the risk of steel leakage. It is thought that the more connections there are to the power supply cable 7 and the more dispersed they are, the less concentrated the current density on the electrode rod 5 will be, and the smaller the maximum melting length will be. Table 1 shows the results of examining the number of connections to the power supply cable 7, the maximum melting length, the difference between the maximum and minimum melting lengths, and the minimum unmelted length. The area where electrode rods 5 should not be installed directly above the connection points of the power supply cable 7 (electrode rod uninstalled area 26) was set to 1.16m x 1.16m when there was one connection point for the power supply cable 7, and to 0.68m x 0.56m when there were two or more connection points. The number of electrode rods 5 was adjusted so that there were approximately 1300 on the outer perimeter. The current values ​​were set to 150kA, 300kA, and 450kA, and the current values ​​of the power supply cables were made uniform. Figure 6 shows the connection points of the power supply cable 7 and the areas on the electrode support plate 6 where the electrode rods 5 are not installed (electrode rod uninstalled areas 26). Figures 6(A) to (F) show that the number of electrode rod uninstalled areas 26 (number of power supply cable installation points) ranges from 1 to 6. The numbers in Figure 6 represent the distance (m) between connection points. The greater the distance between connection points, the greater the effect of distributing the current. In this study, the minimum distance between connection points is 1.0m and the maximum is 2.8m.

[0033] [Table 1]

[0034] Table 1 confirms that the maximum melting length decreases as the number of connection points increases. The difference between the maximum and minimum melting lengths also decreases as the number of connection points increases. The maximum melting length does not decrease significantly even when the number of connection points exceeds 5, and the difference between the maximum and minimum lengths does not decrease further. Therefore, it is clear that the effective number of connection points is 3 or 4.

[0035] Figures 7 to 9 show the results of graphing the values ​​from Table 1, with the horizontal axis representing the number of connection points and the vertical axis representing the maximum melted length, the difference between the maximum and minimum, and the minimum unmelted length, respectively. Figure 9 shows the minimum unmelted length, which is the length of the electrode rod 5 that remains unmelted. To suppress steel leakage, it is empirically desirable to ensure a minimum unmelted length of 600 mm. In the case of a maximum current of 450 kA, it is necessary to have three or more connection points in order to ensure a minimum unmelted length of 600 mm or more. Therefore, in this invention, the electrode rod 5 is connected to one electrode support plate 6 installed below it, and power supply cables 7 are connected to the electrode support plate 6 at three or more locations.

[0036] In the analysis described above, the electrode rod length was set to 1160 mm (one type). In the following analysis, several different electrode rod lengths will be used, and the analysis will be performed for each case.

[0037] Figure 10(A) shows a schematic diagram of electrode rod melting. If the length of electrode rod 5 is D and the melted length is d, then the unmelted length L is Dd. Although the electrode rod 5 and the furnace bottom refractory 14 are constructed to prevent gaps, it is difficult to completely prevent the formation of minute gaps due to deformation caused by heat and weight. The shorter the unmelted length L, the greater the risk of molten steel leaking through this gap.

[0038] Figure 10(B) shows the temperature distribution of the electrode rod with the longest melting length when the power supply cable 7 is connected at one point and 300kA is applied (100kA per upper electrode). Figure 10 shows the temperature distribution when the electrode rod length D from the support plate is 860mm and 1160mm. The region with a temperature higher than the assumed melting point of 1500℃ is melted, and the melting lengths are 307mm for the electrode rod length D of 860mm and 490mm for the electrode rod length D of 1160mm. The reason why the melting length is shorter for the electrode rod length D is that the distance from the electrode support plate 6, which is the cooling surface, is shorter, and the thermal conductivity is higher because the molten region becomes a fluid.

[0039] Figure 11 shows the results of evaluating the relationship between the minimum unmelted length and the electrode rod length D when a current of 300kA or 450kA (100kA and 150kA per rod) is applied, with only one connection point for the power supply cable 7. It was confirmed that the minimum unmelted length becomes shorter when the electrode rod length is 1200mm or more. In the case of a high current value of 150kA / rod, the minimum unmelted length becomes shorter, increasing the risk of steel leakage when a large current is applied. It is not possible to secure an unmelted length of 600mm, which is necessary to suppress the risk of steel leakage.

[0040] Figure 12 shows the evaluation results of the relationship between the minimum unmelted length and the electrode rod length D when there are three connections to the power supply cable and currents of 300kA and 450kA (100kA / rod, 150kA / rod) are applied. In the case of 100kA / rod, the minimum unmelted length was 600mm or more when the electrode rod length was 840mm or more. In the case of 150kA, in order to make the minimum unmelted length 600mm or more, the electrode rod length D should be 1080mm or more, and even with a length of 1760mm, it was 600mm or more. Therefore, in the present invention, it is preferable to set the electrode rod length of the lower electrode in the range of 1080mm to 1760mm. It is also preferable to apply a current of 300kA to 450kA in a DC electric furnace.

[0041] Figure 13 shows the current density distribution of the electrode support plate 6 when the power supply cable is connected at one point and 450kA is applied. Figure 13 illustrates the placement of the electrode rods 5, with the area without electrode rods being 1.16m × 1.16m. In the case of a single connection shown in Figure 13, the current density of the electrode support plate 6 at the electrode rod placement location near the connection point is a maximum of 1.64 × 10⁻¹⁰. 6 (A / m 2 The current density is as follows.

[0042] Figure 14 shows the current density distribution of the electrode support plate 6 when connected at three points. Figure 14(A) shows the current density distribution of the electrode support plate 6 when the power supply cable is connected at three points and 450kA is applied. Figure 14(A) shows the placement of the electrode rods 5, with the area without electrode rods being 0.68m × 0.56m. Figure 14(B) shows the current density of the electrode support plate at 0.7~1.1 × 10⁻¹⁰. 6 A / m 2 This is a diagram showing an extracted portion, and Figure 14(C) is an enlarged view of that extracted portion. The maximum current density of the electrode support plate 6 when connected in three places is 1.10 × 10 6 (A / m 2 ) was. Patent Document 2 states that the theoretical current density that can be applied to the electrode rod is 70 A / cm². 2 (=0.7 × 10 6 (A / m 2 It has been shown that the current density is approximately 0.7 × 10⁻¹⁰. When an electrode rod is placed in an area where no electrode rods are installed, a current of approximately the same magnitude as the current density in the electrode support plate flows through that electrode rod. Therefore, the current density of the electrode support plate 6 shown in Figure 13 is approximately 0.7 × 10⁻¹⁰. 6 (A / m 2 It is considered that electrode rods should not be installed in the range above this point.

[0043] Figures 14(B) and (C) show that the current density of the electrode support plate 6 is 0.7 × 10⁻⁶. 6 from 1.10 × 10 6 (A / m 2 The range of ) is shown in shades of gray, and 0.7 × 10 6 (A / m 2 The area where the value was greater than or equal to 500mm x 500mm was the area. Therefore, if the maximum applied current is 450kA and there are three connection points, it is sufficient to avoid installing electrode rods in the 500mm x 500mm area directly above the connection points. [Explanation of Symbols]

[0044] 1 DC electric furnace 2 Upper electrode 3 Upper electrode 4 Lower electrode 5 Electrode rod 6 Electrode support plate 7 Power supply cable 11 Ceiling 12 Furnace wall copper panel 13 Furnace wall refractories 14 Hearth refractories 21 Molten steel 22 slag 23 Arc 24 Scrap 25 perspectives 26 Area where no electrode rod is installed 27. Electrode rod on the far side 31 Electrode rod 32 Stamped Refractories 33 Firebricks 35 Electrode support plate 36 Power supply cable 37 Electrode Unit

Claims

1. A lower electrode structure for a DC electric furnace used for manufacturing molten steel, characterized in that two or more upper electrodes are installed on the upper electrode, a number of electrode rods are embedded in the refractory material at the bottom of the furnace for the lower electrode, these electrode rods are connected to a single electrode support plate installed below them, power supply cables are connected to the electrode support plate at three or more locations, and the electrode rods are not installed in an area of ​​at least 500 mm x 500 mm directly above the connection points of the power supply cables.

2. The lower electrode structure of a DC electric furnace according to claim 1, characterized in that the length of the electrode rod of the lower electrode is in the range of 1080 mm to 1760 mm.

3. A method for operating a DC electric furnace having the lower electrode structure described in claim 1 or claim 2, characterized by applying a current of 300 kA or more and 450 kA or less.

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