Induction melting furnace for refining

Abstract

To improve the durability of refractory materials in crucible-type induction melting furnaces for refining. [Solution] In melting in a crucible-type induction melting furnace for refining, the presence of refining slag reduces the durability of refractories. By configuring a solenoid coil in a set of three stages and applying a three-phase alternating current, a moving magnetic field is formed. The stirring flow induced by the single-phase coil is further induced by the moving magnetic field, causing the radiating flow on the molten steel surface to reverse into a centripetal flow. The flow of floating molten slag is dispersed from the furnace wall by the reversal, suppressing chemical reactions with refractories. A frequency converter is installed to optimize the distribution of heating and stirring by setting the frequency according to the furnace conditions.

Description

Technical Field

[0001] The present invention relates to an induction melting furnace for refining suitable for mass melting of scrap iron.

Background Art

[0002] Scrap iron is an extremely useful resource generated in the country at over 20 million tons per year. Part of it is used as an auxiliary raw material for converters, and most of it is recycled into high-grade steel products and general-purpose steel products by arc furnaces. Melting by an arc furnace also has a refining effect. Today, including subsequent secondary refining, it can be said that it is close to the mature stage in terms of equipment, operation, quality, and energy efficiency as a mass production steelmaking method.

[0003] On the other hand, induction furnaces, especially crucible types, are small in capacity and are applied to melting of special metals and small-lot, multi-variety production of high-grade products due to their convenience, but they have problems in energy cost and refractory cost. Recently, large crucible-type induction furnaces (capacity 50 - 100 tons) for melting scrap iron have been put into practical use overseas and are becoming popular in the production of low-grade mass-produced steel.

[0004] The advantages of induction furnaces compared to arc furnaces include that the load is stable and the adverse impact on the power supply is small, the dust is overwhelmingly less and the countermeasures against air pollution are inexpensive and easy, the equipment cost is low, and although the energy efficiency is slightly inferior, there is no significant difference because electrode rods are not required. The disadvantages include that it is specialized in melting and has a weak refining function, there are limitations on the thickness of the refractory between the molten steel and the coil due to induction efficiency and the durability of the furnace wall is short, when melting a large amount of scrap iron, the generation of slag mainly composed of FeO increases, and this slag easily corrodes the non-sintered refractory essential for the induction furnace and further reduces the durability. When slag is formed for refining, the durability becomes an even more serious problem.

[0005] Another feature of the induction furnace is that stirring acts by electromagnetic induction in parallel with heating. Although stirring promotes melting, when slag is present, the reaction with the refractory also progresses. In a crucible-type induction furnace, as shown in Figure 1B of this application, a pinch action is induced by the solenoid coil, causing a radiating flow to appear on the molten steel surface. A close observation of the melting process reveals that molten slag rises from the center of the liquid surface, collides with the walls on the radiating flow, and penetrates them. This accelerates the corrosion of the refractory material. While it is expected that the slag will solidify and the reaction will be suppressed just above the liquid surface on the furnace wall, the reaction products in the solidified areas will crack during repeated heating and cooling, a problem inherent to refractory materials, resulting in reduced durability. It can be said that induction furnaces and slag do not mix well.

[0006] Patent Document 1 discloses a method for improving the circuit by dividing a solenoid coil into two parts, thereby enabling timely adjustment of dissolving capacity and stirring force. The phase difference between the two coils is applied to enhance stirring. In this method as well, the flow pattern within the crucible is, in principle, identical to that of conventional flows, as shown in Figure 10 in the cited document. If molten slag and molten steel flow coexist, the progression of refractory corrosion is unavoidable. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Published Patent Application 2006-38351 [Overview of the project] [Problems that the invention aims to solve]

[0008] In a crucible-type induction melting furnace, electromagnetic induction causes a central bubbling and diverting flow along the furnace walls at the molten metal surface. When refined metal is used as raw material, very little molten slag is generated, but when a large amount of scrap iron is melted, a large amount of low-melting-point slag is produced. As suspended slag collides with the furnace wall and is drawn into the molten steel, a chemical reaction occurs between the molten slag and the refractory material, causing the refractory material to corrode. Directly above the reaction surface, the slag solidifies, forming reaction products between the slag and the refractory material. Cracks develop during heating and cooling, and in both cases, the molten slag consumes the refractory material.

[0009] To produce general-purpose, mass-produced steel using a crucible-type induction melting furnace, it must compete with arc furnaces, and furnace wall life is a major problem for induction furnaces. The present invention aims to solve the problem of suppressing the reaction of suspended slag with the refractory material of the furnace wall in a crucible-type induction melting furnace, thereby extending the lifespan of the furnace wall. [Means for solving the problem]

[0010] The first aspect of the present invention is an induction melting furnace for scrap iron refining, characterized in that, in a crucible-type induction melting furnace, the solenoid coil wound around the outer circumference of the crucible is divided axially into three or multiples of three, and three-phase AC terminals ij, jk, and ki are connected to each, and by energizing, an axially moving magnetic field is generated in addition to the magnetic field circulating inside and outside the solenoid coil, thereby inducing an upward flow near the inner wall surface of the crucible, reversing the liquid surface divergence flow inherent to the solenoid coil, and separating the suspended slag from the wall surface. The reason it's labeled "for refining" is because it's intended to be used with the active application of slag refining.

[0011] The second invention is an induction melting furnace for scrap iron refining described in the first invention, characterized in that the frequency of the applied three-phase alternating current is variable. The frequency of induction melting furnaces is typically 50 to 1000 Hz, but in recent years, frequency converters have been added to improve melting efficiency, and this invention also utilizes this technology. [Effects of the Invention]

[0012] In this invention, an upward flow is formed along the inner wall surface of the crucible by an induction coil that generates a moving magnetic field. At the molten steel surface, the conventional radiating flow from the center toward the inner wall of the crucible is reversed, causing the suspended slag to separate from the furnace wall and suppressing the corrosion reaction of refractories by the slag. This extends the lifespan of the furnace body.

[0013] By setting the appropriate frequency according to the progress of dissolution, the dissolution efficiency is improved, and by strengthening timely reverse stirring, it is also beneficial for the durability of refractory materials.

[0014] An induction furnace usually uses single-phase alternating current, but in the present invention, three-phase is applied, so the efficiency of the power supply equipment cost and the power transmission equipment cost including the thyristor for frequency conversion is improved.

Brief Description of the Drawings

[0015] [Figure 1] It is a conceptual diagram of the structure of a crucible-type induction melting furnace and the flow of molten metal. A is the present invention, and B is a conventional melting furnace. [Figure 2] It is a conceptual diagram of the flow of molten steel and molten slag in the crucible. A is the present invention, and B is a conventional melting furnace. [Figure 3] It is the simplest structural example of a three-phase traveling magnetic field coil. [Figure 4] It is an example in which a three-phase traveling magnetic field is configured in multiple stages. [Figure 5] It is an example in which the traveling magnetic field is divided into upper and lower parts, and the moving direction is reversed between the upper and lower parts. [Figure 6] It shows the direction of the molten steel flow by the traveling magnetic field with the up-and-down reversal.

Modes for Carrying Out the Invention

[0016] The structure and operation of the crucible-type induction melting furnace for steelmaking according to the present invention will be described with reference to FIG. 1. The right side of the figure is the present invention of the application, and the left side is a conventional melting furnace for comparison. 1 is a crucible, 2 is molten steel, 3 is a three-phase traveling magnetic field coil, 4 is an electromagnetic force vector to which a traveling magnetic field is added, and 5 indicates the flow of molten steel in the crucible. Except for the coil, it is the same as a conventional induction melting furnace. The three-phase traveling magnetic field coil 3 is composed of at least three stages of single-phase solenoids, and they are configured in multiple stages as necessary.

[0017] When an alternating current is applied to the single-phase solenoid coil 6 shown in B, an induced current opposite to the energized current is generated near the wall surface of the molten steel, and an electromagnetic force vector 7 acts, and a vertically symmetric molten steel flow 8 appears. It is observed that it becomes a diffusion flow on the molten steel surface.

[0018] When the i-phase, j-phase, and k-phase of an AC current are appropriately connected to three stages of single-phase solenoid coils, as is well known, a vertically parallel moving magnetic field is formed, which is combined with the intrinsic magnetic flux of the solenoid coils. An electromagnetic force vector 4 is induced. The molten steel rises along the furnace wall, forming a centripetal flow at the surface, a downward flow in the center, and a radiating flow at the bottom.

[0019] Figure 2 compares the behavior of molten steel and suspended slag using the conventional method (left side B) and the method of the present invention (right side). In this method, the upward flow 23 of molten steel 22 along the furnace wall 21 forms a centripetal flow 24 at the surface, causing the molten slag 25 to separate from the furnace wall 21. This suppresses the chemical reaction between the slag and the refractory material, contributing to the durability of the refractory material. In the conventional method, the downward flow 26 along the furnace wall draws in the slag 27 below the liquid surface, which then travels in the circulating flow to become the emergent slag 28 at the center of the liquid surface. This slag then travels in the radiating flow 29, colliding with the furnace wall and causing corrosion. Some of it becomes solidified slag 30 just above the liquid surface, while other parts are drawn directly below.

[0020] In induction melting furnaces, molten steel flow occurs in parallel with induction heating, and both are strongly influenced by frequency. Typically, frequencies between 50 Hz and 1000 Hz are used, but generally, higher frequencies have a stronger effect on heating, while lower frequencies have a stronger effect on flow. Although equipment costs will increase, making the frequency easily adjustable with a frequency converter allows for operation under appropriate conditions depending on the situation. [Examples]

[0021] A preferred embodiment of the induction coil will be described. Figure 3 shows the simplest method for forming a coil that generates a moving magnetic field. Solenoid coils 31, 32, and 33 are stacked in three stages, and terminals ij, jk, and ki of a three-phase AC are connected to each stage. Corresponding to a 120° phase difference, three stages of vertically moving magnetic fields are formed in one cycle. The direction of movement of the magnetic field depends on the order of the connections. This is similar to how an induction motor reverses direction by reversing part of its connections.

[0022] Figure 4 shows the same three-stage coil as in Figure 3, but stacked in three stages. It consists of three (42, 43) three-phase, three-stage solenoid coils 41 connected in parallel, which increases the allowable current value and allows for output enhancement without setting the voltage excessively high.

[0023] Figure 5 shows a moving magnetic field coil divided into upper and lower sections, forming a circulating flow with reversed direction. The upper coil 63 and lower coil 64 of the moving magnetic field are connected in opposite directions relative to the crucible 61 and molten steel 62. Two circulating flows are formed, one above the other. The upper circulating flow is slightly strengthened.

[0024] In actual operation, CO bubbles are constantly generated during the melting of scrap iron due to the oxidation of molten steel, although not to the same extent as in arc furnaces. The source of these bubbles is the hearth and furnace walls, and an upward flow of bubbles is formed. The flow conditions are complex, involving induced flow, upward flow, and obstacles caused by undissolved material, and are not as simple as previously described. This invention is effective in suppressing entrapment at the furnace wall.

[0025] Dissolution volume (t) · Dissolution efficiency (t / h) · Output (kW) · Allowable voltage (V) · Allowable current (A) · Conductor cross-sectional area (cm²) 2 ) Optimizing settings such as frequency, number of coil turns, and number of coil stages is not particularly difficult for someone skilled in the art. [Industrial applicability]

[0026] This technology can be effectively used to enlarge induction melting furnaces and reduce costs. [Explanation of Symbols]

[0027] 1; Crucible 2; Molten steel 3; Three-phase moving magnetic field coil 4; Electromagnetic force vector 5; Molten steel flow 6; Single-phase solenoid coil 7; Single-phase electromagnetic force vector 8; Molten steel flow 21; Furnace wall 22; Molten steel 23; Upward flow 24; Centripetal flow 25; Molten slag 26; Downward flow 27; Intake slag 28; Outflow slag 29; Molten slag 30; Solidified slag 31,32,33; Single-phase solenoid coil 34; Power terminals i,j,k 35; Moving magnetic field 41,42,43; Three-phase three-stage solenoid coil 44; Power terminal 51; Upper moving magnetic field three-phase coil 52; Lower moving magnetic field three-phase coil 61; Crucible 62; Molten steel 63; Upper moving magnetic field coil 64; Lower moving magnetic field coil 65,66; Electromagnetic force vector 67,68; Reverse flow

Claims

1. An induction melting furnace for scrap iron refining, characterized in that a solenoid coil wound around the outer circumference of the crucible is divided axially into three or multiples of three, and each is connected to three-phase AC terminals ij, jk, and ki, and when energized, an axial moving magnetic field is generated in addition to the magnetic field circulating inside and outside the solenoid coil, inducing an upward flow near the inner wall surface of the crucible, reversing the solenoid coil's original liquid surface divergence flow, and separating the suspended slag from the wall surface.

2. An induction melting furnace for scrap iron refining according to claim 1, characterized in that the frequency of the applied three-phase alternating current is variable.

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