DC electric furnace and metal melting method

Abstract

To provide a DC electric furnace and a metal melting method that can efficiently melt a melting raw material in a short time.SOLUTION: A DC electric furnace is characterized in that a third virtual region has an area which is 0.1 to 1.0 times as large as the area of a second virtual region, where a first virtual region is a virtual region where two or more virtual circles on a surface of molten metal in pouring which have their centers at intersections of perpendiculars to the molten metal surface which pass respective center axes of a plurality of upper electrodes and the surface of the molten metal in pouring and also have radii equal to the sum of doubles of the radii of the upper electrodes and a maximum voltage overlap with each other, the second virtual region is defined with a virtual circle on the surface of the molten metal which has its center at the intersection of the center axis of a raw material feed pipe and the surface of molten iron in pouring and also has a radius 0.5 time as large as the vertical distance from a lower end of the raw material feed pipe to the surface of the molten iron, and the third virtual region is a virtual region of the part where the first virtual region and the second virtual region overlap with each other.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In an electric arc furnace, there is an upper limit to the current value per upper electrode. In a DC system, the upper limit per electrode is relaxed compared to an AC system that has a skin effect, but when large amounts of power are required, using multiple upper electrodes is an option. When charging a cold iron source, as in the technology disclosed in Patent Document 1, preheated scrap is charged between two electrodes, and by utilizing the high-temperature region of the molten iron, a configuration is shown that makes it possible to melt the raw materials more quickly.

[0003] In addition, in DC electric furnaces, current flows between the upper and bottom electrodes, creating a current density distribution that penetrates the molten iron. Specifically, the current density is high directly below the top electrode, and decreases with increasing distance from the top electrode. Also, because the electrode surface area directly above the bottom electrode is generally larger than that of the top electrode, the current density is lower than that directly below the top electrode. As a result, electromagnetic forces cause a flow in both directions. It is known that in DC electric furnaces, this flow causes the temperature of molten steel at the bottom of the furnace to be closer to the temperature at the top than in AC electric furnaces. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-300449 Summary of the Invention [Problem to be solved by the invention]

[0005] The main raw materials used for melting include iron oxides such as iron ore and dust, which are reduced using natural gas, carbonaceous materials, or, more recently, hydrogen gas. These materials are often used to produce sponge-like direct reduced iron (DRI), or hot briquetted iron (HBI), which is hot-formed from DRI. Furthermore, to enable efficient melting and the production of low-nitrogen steel, flat-bath operation is commonly used, in which a pre-charge of molten steel is left in the furnace and a cold iron source is continuously charged therein. For reduced iron, a conceivable method involves drilling a hole in the furnace lid, connecting a raw material charging pipe located on the lid to the hole, and continuously charging the raw material into the furnace from above through the raw material charging pipe. A conceivable charging position is to aim for the high-temperature area between the electrodes. However, there is a spread in the location where the reduced iron falls on the molten iron surface. Patent Document 1 does not disclose clear guidelines for the specific installation position or angle of the raw material charging pipe, taking into account this spread.

[0006] Furthermore, since scrap generally sinks in the molten iron, the vertical flow of molten steel mentioned above also tends to promote the melting of scrap accumulated at the bottom of the furnace, making it reasonable to charge it between the electrodes. On the other hand, the apparent density of reduced iron in HBI is 5000 kg / m 3 The average diameter of HBI is about 100-250 mm, while that of DRI is even lower, floating on the molten iron. It has been confirmed that HBI sinks in the slag, but DRI floats in the slag for a period between charging and melting. However, horizontal flow is also generated in the molten iron and slag by the arc jet and oxygen jet, causing the reduced iron to move from its charging position. While apparent density has the greatest influence on floating on the molten iron, the size and shape of the reduced iron also have a significant effect. In other words, HBI is only a maximum of 100-250 mm in the longitudinal direction and a maximum of 50-100 mm in the transverse direction, while DRI is even smaller.

[0007] This type of shape is significantly different from scrap, and is significantly affected by the flow of molten iron and slag, not only in terms of density but also in terms of size and shape. Therefore, the injection position must be determined taking into account the flow of reduced iron, and it has not been shown that aiming for the space between the upper electrodes is necessarily advantageous for high-speed melting. On the other hand, due to the size and shape described above, continuous charging using bucket or horizontal shaft methods poses problems of fusion and heat exchange during melting, making continuous injection from above necessary. Therefore, the policy for formulating equipment specifications related to this injection position is extremely important.

[0008] SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a DC electric furnace and a metal melting method that can solve the above problems and melt raw materials more efficiently in a shorter time. [Means for solving the problem]

[0009] [1] A DC electric furnace includes a plurality of upper electrodes and a cylindrical raw material charging tube inserted into the upper surface of the furnace body and through which molten raw materials including reduced iron are charged into the furnace, wherein a first virtual region is defined as a virtual region where two or more imaginary circles on the molten metal surface at the time of tapping overlap, the first virtual region being centered at the intersection of a perpendicular to the molten metal surface passing through the central axes of the plurality of upper electrodes and the molten metal surface at the time of tapping, and the radius being the sum of twice the radius (mm) of the upper electrodes and the maximum voltage (V); a second virtual region is defined as a virtual circle on the molten metal surface that is centered at the intersection of the central axis of the raw material charging tube and the molten metal surface at the time of tapping, and the radius being 0.5 times the vertical distance from the lower end of the raw material charging tube to the molten metal surface at the time of tapping; and a third virtual region is defined as a virtual region where the first virtual region and the second virtual region overlap, wherein the ratio of the area of ​​the third virtual region to the area of ​​the second virtual region is 0.1 or more and 1.0 or less. [2] The DC electric furnace according to [1], wherein the raw material charging pipe has an inner diameter of 150 mm or more and 1000 mm or less. [3] The DC electric furnace according to [1], wherein the raw material charging pipe is inserted into the upper surface of the furnace body so that the angle between the central axis and a plane horizontal to the molten metal surface during tapping is 20° or more and 80° or less. [4] A method for melting metals in a DC electric furnace according to any one of [1] to [3], wherein the amount of reduced iron charged from the raw material charging pipe is 10% by mass or more of the total amount of melted raw materials charged into the charge, and the amount of remaining molten iron in the previous charge is 20% by mass or more and 60% by mass or less of the total amount of tapped steel. [Effects of the Invention]

[0010] According to the above-mentioned configuration, by optimizing the arrangement of the raw material introduction pipes and the like, the raw material can be melted more efficiently in a short time. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing the structure of a DC electric furnace according to one embodiment of the present invention. [Figure 2] 2 is a cross-sectional view of the DC electric furnace shown in FIG. 1 taken along line II-II. [Figure 3] FIG. 2 is a schematic diagram showing the arrangement of an upper electrode and a raw material introduction tube in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted.

[0013] (Structure of a DC electric furnace according to one embodiment) Fig. 1 is a cross-sectional view showing the structure of a DC electric furnace according to one embodiment of the present invention. In the drawings and in the following description, three mutually orthogonal directions are referred to as the +x direction, the +y direction, and the +z direction. Although not shown, the direction opposite the +x direction is referred to as the -x direction, the direction opposite the +y direction is referred to as the -y direction, and the direction opposite the +z direction is referred to as the -z direction. The plane including the surface of the molten iron 21 is referred to as the xy plane, but the molten iron surface does not need to be strictly flat and may have irregularities.

[0014] The DC electric furnace 1 is a large electric furnace, and can be a tilting type in which the furnace body tilts, or a stationary type in which the furnace body does not tilt. As shown in the figure, the furnace body of the DC electric furnace 1 includes a furnace cover 2, furnace wall copper panels 3, furnace wall refractories 4, and hearth bottom refractories 5. In addition to reduced iron, iron-containing scrap, iron-containing dust, and pig iron and ingots generated in steelworks may be fed into the DC electric furnace 1 as iron sources. Converter or electric furnace dust granulated product can be used as the iron-containing dust.

[0015] A plurality of upper electrodes 6 are provided on the furnace lid 2, which is the top surface of the furnace body, in order to supply a large amount of power into the furnace. In the illustrated example, three upper electrodes 6 are provided, but this is not limited to this. The plurality of upper electrodes 6 preferably have the same polarity and are negative. The current per upper electrode 6 is, for example, about 100 kA to 150 kA, but even if it is less than 100 kA or exceeds 150 kA, there is no theoretical effect on the effects of the present invention, which will be described later.

[0016] At the bottom of the furnace body, a bottom electrode 7, which is an anode, and a bottom blowing tuyeres 8 are provided. The bottom electrode 7 is preferably of the multi-pin type, but may also be of the billet type. The hearth refractory 5 is provided with a tapping hole 10 for discharging molten iron 21. The furnace wall refractory 4 is provided with a slag discharge hole 11 for discharging electric furnace slag 22.

[0017] An arc 20 is emitted from the upper electrode 6 to melt the raw materials, producing molten iron 21. Gas is blown into the bottom tuyeres 8, creating a circulating flow through the molten iron 21 and electric furnace slag 22, promoting heat transfer and melting of the surface of the molten iron 21 and the electric furnace slag 22. In the figure, r represents the radius of the upper electrode 6, and C1 represents the intersection of the molten iron surface perpendicular to the central axis of each of the upper electrodes 6 and the molten iron surface at the time of tapping. The molten iron surface at the time of tapping refers to the height of the molten iron 21 immediately before tapping, after all the raw materials have been charged, melted, heated, and refined. The molten iron surface at the time of tapping reflects factors such as the refractory lining plan, the height of the floor through which the slag flows from the slag discharge port, and the tilting angle. The upper electrode 6 is fixed to an electrode arm and holder and is installed so that its central axis is approximately perpendicular to the molten iron surface. The central axis of the upper electrode 6 is considered to move roughly on the same line even when the upper electrode 6 is raised or lowered, so in terms of equipment design, the perpendicular line to the molten metal surface that passes through the central axis of the upper electrode 6 is uniquely determined.

[0018] The raw material charging pipe 9 is cylindrical and is inserted through the furnace lid 2, which is the top surface of the furnace body, and charges molten raw materials containing reduced iron, which has a lower density than molten iron 21, into the furnace. The raw material charging pipe 9 may be inserted through the furnace wall, but is preferably inserted through the furnace lid 2, as described below. The inner diameter of the raw material charging pipe 9 is preferably 150 mm or more and 1000 mm or less. By making the inner diameter of the raw material charging pipe 9 150 mm or more, the molten raw materials can be charged more smoothly into the furnace. Furthermore, by making the inner diameter of the raw material charging pipe 9 1000 mm or less, it is possible to reduce equipment costs while ensuring freedom in the design of the layout of each piece of equipment on the furnace lid 2.

[0019] Furthermore, it is more preferable that the inner diameter of the raw material charging pipe 9 be 150 mm or more and 500 mm or less. By making the inner diameter of the raw material charging pipe 9 500 mm or less, the degree of freedom in the layout design of each piece of equipment in the furnace hood 2 is further increased, and it becomes possible to suppress heat dissipation inside the furnace. It is preferable that the inner diameter of the raw material charging pipe 9 be 1.5 times or more and 5 times or less the maximum size of the melted raw materials to be charged.

[0020] In the figure, C indicates the central axis of the raw material charging pipe 9, and C2 indicates the intersection of the central axis C and the molten iron surface at the time of tapping. Furthermore, P1 indicates the intersection of the molten iron surface at the time of tapping and a perpendicular line passing through the lower end P2 of the raw material charging pipe 9, and H indicates the vertical distance from the lower end P2 of the raw material charging pipe 9 to the molten iron surface at the time of tapping, i.e., the distance between P1 and P2.

[0021] The raw material charging pipe 9 is preferably inserted into the furnace hood 2 so that the angle θ between the central axis C and a plane horizontal to the surface of the molten iron 21 is 20° or more and 80° or less. If the angle θ is 20° or less, the horizontal speed of the charged molten raw materials will be low, which may cause clogging inside the pipe. On the other hand, if the angle θ is 80° or more, it will be necessary to install the raw material charging pipe 9 near the upper electrode 6, which will reduce the flexibility of equipment layout.

[0022] The reduced iron having a density lower than that of the molten iron 21, which is charged from the raw material charging pipe 9, is, for example, the above-mentioned DRI and HBI, but is not limited to these. In the following description, it is assumed that the reduced iron having a density lower than that of the molten iron 21 is HBI.

[0023] It is desirable to charge the entire amount of HBI into the furnace through the raw material charging pipe 9, but for example, less than 30 mass% of the HBI to be charged may be charged into the furnace by other means (e.g., horizontal conveyor, shaft furnace, bucket charging, etc.). Furthermore, melting raw materials other than HBI may be charged from a source other than the raw material charging pipe 9, and may be charged from the raw material charging pipe 9 simultaneously with the HBI or at a different timing.

[0024] In the metal melting method in the DC electric furnace 1 according to this embodiment, hot heel operation may be performed in which the entire amount of molten iron 21 melted in the furnace is not poured out, but a portion of the molten iron 21 is left in the furnace and carried over to a subsequent charge. In this case, HBI or the like, which is reduced iron having a lower density than the molten iron 21 charged from the raw material charging pipe 9, preferably accounts for 10% by mass or more of the total raw materials for melting charged in the charge. If HBI or the like accounts for 10% by mass or less of the total raw materials for melting charged in the charge, the amount or rate of charging of HBI or the like becomes limited, and the effect of improving melting efficiency according to the present invention is not significantly achieved. Alternatively, the entire amount of raw materials for melting charged may be HBI or the like.

[0025] Furthermore, the amount of remaining molten iron in the pre-charge is preferably 20% by mass or more and 60% by mass or less of the total tapping amount. If the amount of remaining molten iron in the pre-charge is less than 20% by mass of the total tapping amount, the heat capacity of the molten iron 21 may be insufficient when the HBI is charged from the raw material charging pipe 9, particularly at the initial stage of raw material charging, and the temperature of the molten iron 21 may drop excessively, resulting in a decrease in the melting efficiency of the HBI. If the amount of remaining molten iron in the pre-charge is more than 60% by mass of the total tapping amount, the effect of the present invention remains unchanged, but the furnace will ultimately contain 1.6 times or more of the molten iron 21 as the tapping amount. This may require an increase in the furnace capacity, which may increase equipment costs.

[0026] Figure 2 is a cross-sectional view of the DC electric furnace shown in Figure 1 taken along line II-II. In the figure, the DC electric furnace 1 is shown with a circular horizontal cross section, omitting the tapping hole. However, the shape is not limited to a circle, and it may be elliptical. Reference numeral 20a in the figure denotes a virtual circle on the molten metal surface at the time of tapping, with its center at the intersection C1 of the molten metal surface and a perpendicular line passing through the central axis of each upper electrode 6 with the molten metal surface at the time of tapping and its radius R1. The radius R1 of the virtual circle 20a is equal to the sum of twice the radius r (mm) of the upper electrode 6 and the maximum voltage (V) of the upper electrode 6. The virtual region where two or more virtual circles 20a overlap is referred to as the first virtual region. The first virtual region is the range where the molten metal surface of the molten metal 21 is stably heated to a high temperature by the arc 20 emitted from the upper electrode 6, and is hereinafter referred to as the high-temperature range S1.

[0027] In the figure, S2 is an imaginary circle on the molten iron surface at the time of tapping, with its center at the intersection C2 between the central axis C of the raw material charging pipe 9 and the molten iron surface at the time of tapping and its radius R2. The radius R2 of the imaginary circle is equal to 0.5 times the vertical distance H from the lower end P2 of the raw material charging pipe 9 to the molten iron surface at the time of tapping. The imaginary circle shown in S2 is a second imaginary region, which is the region on the molten iron surface where 80% or more of the HBI charged into the furnace from the raw material charging pipe 9 reaches. Therefore, it is hereinafter referred to as the reduced iron drop center of gravity range S2. The imaginary region where the high temperature range S1, which is the first imaginary region, and the reduced iron drop center of gravity range S2, which is the second imaginary region, overlap is a third imaginary region, hereinafter referred to as region S3.

[0028] Below, we will explain the optimal installation conditions for the raw material charging pipe 9 based on the results of experimental and calculation studies of the trajectory of the HBI charged from the raw material charging pipe 9. When assuming the high-temperature portion of the molten metal surface, it is assumed that the power and voltage supplied to the DC electric furnace 1 are at their maximum. In this case, since the potential gradient in the air is 1 V / mm, the distance (mm) between the upper electrode 6 and the molten metal surface is considered to be approximately equal to the maximum voltage (V). For example, when the maximum voltage supplied to the DC electric furnace 1 is 500 V, the distance between the upper electrode 6 and the molten metal surface can be considered to be 500 mm in the practice of the present invention.

[0029] Because the behavior of the arc 20 is strongly affected by the current value, the range in which the high temperature occurs is also affected by the current value. Although the arc 20 fluctuates, the range in which the high temperature is stable is the high temperature range S1 in the figure, where two or more imaginary circles 20a overlap.

[0030] When there are two upper electrodes 6, the high-temperature range S1 is uniform, but when there are three or more upper electrodes 6, a sufficient effect can be obtained by selecting a virtual region where two or more virtual circles 20a overlap as the high-temperature range S1. Furthermore, when there are three or more upper electrodes 6, the high-temperature range S1 may be a virtual region where three or more virtual circles 20a overlap. In this case, the high-temperature range S1 becomes hotter than when two virtual circles 20a overlap, and the melting efficiency is further improved.

[0031] Next, we investigated the range of fall of the HBI introduced from the raw material introduction tube 9 onto the molten metal surface. The cylindrical raw material introduction tube 9 inserted into the furnace lid 2 was set to 1 m in length in the material axis direction, which was approximately 10 times the longitudinal length of the HBI, so that the HBI would flow at a constant speed. The inner diameter was 300 mm, and the angle θ between the central axis C and the plane horizontal to the molten metal surface was set to 30° and 45°.

[0032] Under these conditions, as described above, 80% or more of the HBI introduced into the furnace from the raw material introduction pipe 9 reaches the range S2 of the center of gravity of the reduced iron falling. It is desirable that the raw material introduction pipe 9 be straight near its lower end. This is because, compared to when the raw material introduction pipe 9 has a bend, the frequency with which the HBI bounces inside the pipe is reduced, thereby concentrating the falling range of the HBI.

[0033] The influence of the molten iron surface position on the proportion of HBI that falls into the reduced iron fall center of gravity range S2 will be discussed below. As described above, in the DC electric furnace 1 according to this embodiment, not all of the molten iron 21 melted in the furnace is tapped; instead, a portion of the molten iron 21 is left in the furnace and carried over to the subsequent charge, and raw materials for melting are continuously charged into the furnace. The distance from the raw material charging pipe 9 to the molten iron surface is longest at the start of the charge, i.e., when only the remaining molten iron (hot heel) from the previous charge remains in the furnace as raw materials for melting, and before the introduction of other raw materials for melting begins. Furthermore, the distance from the raw material charging pipe 9 to the molten iron surface decreases as the molten iron surface rises as the raw materials are charged and melted.

[0034] As described above, the center of the range S2 of the center of gravity of the reduced iron drop is the intersection C2 between the central axis C of the raw material charging pipe 9 and the molten iron 21 surface at the time of tapping. In reality, the intersection C2 changes with fluctuations in the molten iron surface. The vertical distance H from the lower end P2 of the raw material charging pipe 9 to the molten iron 21 surface at the time of tapping also fluctuates. Comparing the initial stage of the raw material charging with the hot heel stage, it was found that the ratio of HBI falling into the range S2 of the center of gravity of the reduced iron drop, which was 80% at the time of tapping, only decreased to about 50% at most.

[0035] In addition, we also investigated the following cases: the angle θ that the central axis C makes with respect to the surface of the molten iron 21 and the horizontal plane was changed from 20° to 90° (vertical); the case where the raw material charging pipe 9 was inserted into the furnace wall (the side of the furnace body); and the case where the inner diameter of the raw material charging pipe 9 was changed. As a result of the investigation, we found that the lower limit of the falling ratio of HBI into the range S2 of the center of gravity of the reduced iron falling was about 50% of that in the case of the molten iron surface at the time of tapping.

[0036] To further improve the melting efficiency of the HBI, it is preferable to drop the HBI onto the high-temperature part of the molten metal surface. That is, in the region S3 where the high-temperature region S1 and the center-of-gravity region S2 of the reduced iron fall overlap, the ratio of the area of ​​S3 to the area of ​​S2 (hereinafter referred to as "S3 / S2"), which indicates how much of the HBI has dropped onto the high-temperature region, is preferably large.

[0037] However, because the HBI floats on the molten iron 21, when S3 / S2 is large (for example, when 1), if the HBI are concentrated and dropped in a small area, the molten iron 21 that gets trapped between the HBI particles may solidify due to heat loss by the HBI particles, and multiple HBI particles may form a single lump. This significantly reduces the melting efficiency of the HBI. It has also been found that in such a case, a vicious cycle may occur in which the HBI loses heat to the molten iron 21, making it difficult to maintain the temperature of the molten iron 21. Note that if large lumps of HBI are not formed, the HBI will not remain at the position where it reaches the molten iron surface, but will disperse and dissolve due to the influence of the flow of the molten iron 21. Therefore, it is preferable to make the region S3 as large as possible within the range in which large lumps of HBI are not formed.

[0038] Based on the above, the optimum range for region S3 was investigated. As a result, it was found that the arrangement of the upper electrode 6, the arrangement of the raw material charging pipe 9, and the power supply conditions should be determined so that the ratio of the area of ​​S3 to the area of ​​S2, S3 / S2, falls within the range shown in the following formula (1). If S3 / S2 is less than 0.1, not only will the heat from the high-temperature section not be used effectively to melt the HBI, but the molten iron 21 may be overheated, which may accelerate wear of the upper electrode 6, the furnace wall refractory 4, and the furnace bottom refractory 5. Furthermore, if S3 / S2 is 1.0, the reduced iron drop center of gravity range S2 can be made smaller than the high-temperature range S1. 0.1≦S3 / S2≦1.0 (1)

[0039] Furthermore, it is more preferable that S3 / S2 be in the range shown in the following formula (2). When S3 / S2 is 0.25 or more, HBI receives sufficient heat from the high-temperature part, further promoting melting. Furthermore, in order to ensure the productivity of molten iron 21, the generation of large HBI agglomerates can be suppressed by setting the range S2 of the center of gravity of the reduced iron drop to a certain extent. In this case, by setting the upper limit of S3 / S2 to 0.9, it is possible to achieve both high-speed HBI charging and promotion of melting. 0.25≦S3 / S2≦0.9 (2)

[0040] In order to concentrate the HBI's arrival position on the molten metal surface in the high-temperature range S1, it is necessary to take measures such as limiting the trajectory of the falling material at the outlet of the material inlet pipe 9. Such measures may cause clogging of the melting materials and increase the frequency of removing the metal and electric furnace slag 22 deposits. However, the DC electric furnace 1 according to this embodiment is less likely to cause such problems, and it is possible to improve the efficiency of stable melting of the melting materials.

[0041] Furthermore, by inserting the raw material charging pipe 9 through the furnace lid 2, the distance from the range S2 of the center of gravity of the reduced iron falling can be made shorter than when the raw material charging pipe 9 is inserted through the furnace wall, thereby ensuring flexibility in facility layout. Therefore, to ensure that S3 / S2 is 0.1 or more, it is preferable to insert the raw material charging pipe 9 through the furnace lid 2.

[0042] As described above, in the DC electric furnace 1 according to this embodiment, it is preferable to drop the HBI onto the high-temperature portion of the molten metal surface to improve melting efficiency. Specifically, it is preferable to arrange the material charging pipe 9 so as to optimize S3 / S2, which is the ratio of the area of ​​S3 to the area of ​​S2, for the region S3 where the high-temperature region S1 and the region S2 where the center of gravity of the reduced iron falls overlap. This ratio indicates how much of the HBI has fallen onto the high-temperature region. More specifically, by setting S3 / S2 to be 0.1 or more and 1.0 or less, it is possible to suppress a decrease in the melting efficiency of the HBI and wear on the upper electrode 6, the furnace wall refractory 4, and the furnace bottom refractory 5, and to melt the molten material containing the HBI more efficiently in a shorter time.

[0043] (Example) The following describes the results of experiments and analyses conducted to verify the effects of the above-described embodiment. In this example, a DC electric furnace with a capacity of approximately 300 tons was used, with a hot heel volume of 100 tons and 400 tons of molten iron held in the furnace during tapping. The main raw materials for melting were reduced iron and scrap, and all of the reduced iron was introduced through two raw material introduction pipes inserted through the top of the furnace. The inner diameter of the raw material introduction pipes was 500 mm. The reduced iron introduced into the furnace was HBI, accounting for 60 mass% of the total raw materials introduced. Scrap was introduced into the furnace by a horizontal conveyor.

[0044] The thermal flow in a DC electric furnace due to arc discharge was evaluated by numerical analysis simulation. The simulation was 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. It was confirmed that the high-temperature range S1 calculated by this method was a range with a higher temperature than the regions other than the high-temperature range S1, even in the results obtained by the above-mentioned numerical analysis.

[0045] In addition, in No. 1 to No. 5 described below, HBI was charged offline into the raw material charging section, and the falling range of the HBI at a position corresponding to the molten iron surface during tapping was observed to calculate the falling center of gravity range S2 of the reduced iron. The trajectory of the HBI reaching the molten iron surface was evaluated by numerical analysis based on the method described in Turbulence Second Edition, HINZE, McGRAW-HILL, New York, 1975, pp. 460-462. Based on the above thermal flow simulation, the input energy required for the HBI temperature to reach 1550°C was calculated. The input energy, converted into melting energy per weight of reduced iron based on the operational results obtained in advance, was used as the "electricity consumption rate."

[0046] Table 1 shows design examples in which the number of upper electrodes was varied from 2 to 4 and the maximum voltage supplied to the DC electric furnace was varied from 500 V to 800 V, and the results of evaluating the melting efficiency of the raw materials in each case are shown. In Table 1, Nos. 1 to 4 are examples in which S3 / S2 was 0.1 or more and 1.0 or less, and Nos. 5 and 6 are comparative examples in which S3 / S2 was outside this range.

[0047] In Examples No. 1 to No. 4, the maximum voltage is reduced by increasing the number of upper electrodes from two to three or four, and the overall height of the equipment is reduced by shortening the distance from the bottom of the raw material inlet tube to the molten metal surface during tapping. Considering the reduction in the maximum voltage and arc length, which reduces the impact on furnace wall wear, and the localization of the heat source per upper electrode, the pitch circle diameter (PCD) is increased when the number of upper electrodes is three or four compared to when the number of upper electrodes is two. Furthermore, by selecting an upper electrode with a smaller radius when the number of upper electrodes is three or four compared to when the number of upper electrodes is two, equipment costs can be reduced.

[0048] Figure 3 is a schematic diagram showing the arrangement of the upper electrode and the raw material introduction pipe in an embodiment of the present invention. In Figure 3, (a), (b), (c), and (d) show the arrangements of the upper electrode and the raw material introduction pipe in Examples 1 to 4 of Table 1, respectively. Note that only one raw material introduction pipe and one reduced iron drop center of gravity range S2 are shown in the figure. In reality, there is another raw material introduction pipe and one reduced iron drop center of gravity range S2 under similar conditions at a rotationally symmetrical position of 180° in (a), (c), and (d) and 120° in (b) with respect to the furnace center; however, this is not shown for simplicity.

[0049] The arrangement of the upper electrode and the raw material charging pipe in No. 5, a comparative example, was the same as in (a), but the molten metal level at the time of tapping (not shown) was different from that in No. 1. In addition, in No. 6, the same electric furnace as in No. 5 was used, but the hot heel amount was set to 30 tons, and 300 tons of melted raw materials were charged, resulting in a melt volume of 330 tons in the furnace.

[0050] [Table 1]

[0051] When furnace No. 5 (comparison example) was used, the HBI was added in 40 minutes, and then power supply was continued. It took 55 minutes to reach the specified temperature. The power consumption rate was evaluated as 615 kWh / t. When furnace No. 6 (also a comparison example) was used, the HBI did not spread sufficiently on the molten metal surface after the HBI was added, resulting in the formation of large unmelted lumps. As a result, it took 65 minutes to melt the lumps and bring the entire molten metal surface temperature to the specified temperature, resulting in a power consumption rate of 620 kWh / t.

[0052] When furnace No. 1 (Example) was used, the HBI charge amount, charge rate, and power supply were the same as those of furnace No. 5 (Comparative Example), and it took 54 minutes to reach the specified temperature, 1 minute earlier. The power consumption rate was evaluated as 606 kWh / t. The evaluation results for furnaces No. 2 to No. 4 are shown in Table 1.

[0053] In terms of operation, the lower the power consumption rate, the better. Based on the composition of the HBI used in this study and the heat transfer efficiency gained from experience when scrap is used as the main raw material, a power consumption rate of approximately 610 kWh / t was assumed. Therefore, in the "Evaluation" column in Table 1, cases where the power consumption rate was 610 kWh / t or less were marked with a "Yes," and cases where the power consumption rate was higher than 610 kWh / t were marked with an "X." The results shown in Table 1 indicate that the Examples in which the S3 / S2 ratio was 0.1 or more and 1.0 or less had better melting efficiency than the Comparative Examples.

[0054] 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 to which the present invention pertains can conceive of various modifications or alterations 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]

[0055] 1...DC electric furnace, 2...furnace cover, 3...furnace wall copper panel, 4...furnace wall refractory, 5...furnace bottom refractory, 6...upper electrode, 7...furnace bottom electrode, 9...raw material inlet pipe, 21...molten iron, S1...high temperature range, S2...reduced iron drop center of gravity range, S3...area.

Claims

1. a plurality of upper electrodes; a cylindrical raw material charging pipe that is inserted into the upper surface of the furnace body and that charges molten raw materials containing reduced iron into the furnace; In a DC electric furnace comprising: a first imaginary region is defined as a region where two or more imaginary circles on the molten metal surface at the time of tapping overlap, the imaginary circles having a center at the intersection of a perpendicular line to the molten metal surface passing through the central axis of each of the plurality of upper electrodes and the molten metal surface at the time of tapping, and a radius equal to the sum of twice the radius (mm) of the upper electrode and the maximum voltage (V); a second imaginary region is an imaginary circle on the molten iron surface, the second imaginary region having a center at the intersection of the central axis of the raw material introduction pipe and the molten iron surface at the time of tapping, and a radius of 0.5 times the vertical distance from the lower end of the raw material introduction pipe to the molten iron surface at the time of tapping; When a virtual area where the first virtual area and the second virtual area overlap is defined as a third virtual area, a ratio of an area of ​​the third virtual region to an area of ​​the second virtual region is 0.1 or more and 1.0 or less; DC electric furnace.

2. The raw material charging pipe has an inner diameter of 150 mm or more and 1000 mm or less.

2. The DC electric furnace according to claim 1.

3. The raw material charging pipe is inserted into the upper surface of the furnace body so that the angle between the central axis and a plane horizontal to the molten metal surface during tapping is 20° or more and 80° or less.

2. The DC electric furnace according to claim 1.

4. 4. A method for melting metal in a DC electric furnace according to claim 1, the amount of reduced iron charged from the raw material charging pipe is 10 mass% or more of the total amount of molten raw materials charged into the charge, A metal melting method in which the amount of remaining molten metal in the previous charge is 20% by mass or more and 60% by mass or less of the total amount of tapped steel.

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