Method for producing molten iron

Abstract

This invention provides a method for producing molten iron that can further suppress nitrogen absorption. [Solution] A method for producing molten iron by melting a solid iron source with an arc in an electric furnace, wherein, when the diameter of the electric furnace is D (m), a carbon material is added to the molten iron within a range of D / 5 (m) from the inner wall of the furnace at the center of the lower end of the slag outlet when viewed from above, and the supply rate of volatile matter and water in the carbon material is V (kg / min), and the area of ​​the slag outlet is S (m 2 In this case, the carbon material is added under the condition that the ratio V / S exceeds 0.10.

Description

Technical Field

[0001] The present invention relates to a method for producing molten iron with suppressed nitrogen absorption.

Background Art

[0002] In recent years, there has been an increasing demand for steel grades with lower nitrogen concentrations, and operations for suppressing nitrogen absorption are generally carried out in an electric furnace. When nitrogen enters the electric furnace, nitrogen is entrained in the arc and nitrogen absorption occurs at the high-temperature arc spot. Therefore, generally, nitrogen absorption is suppressed by forming slag to cover the arc spot with the forming slag. On the other hand, Patent Document 1 discloses a technique for preventing nitrogen absorption into molten steel by introducing a secondary raw material containing volatile components and supplying oxygen to form a gas shield on the surface of the molten steel with the gas generated by the reaction between the volatile components of the secondary raw material and oxygen.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In order to form slag or cover the entire surface of the molten metal with a gas shield, control for maintaining that state is a problem. If the control is not sufficient, nitrogen absorption cannot be sufficiently suppressed.

[0005] In view of the above problems, an object of the present invention is to provide a method for producing molten iron that can more effectively suppress nitrogen absorption.

Means for Solving the Problems

[0006] The inventors focused on the fact that even when the slag door is closed, a gap can be created due to deformation of the slag door or adhesion of slag to the slag outlet, allowing atmospheric nitrogen to enter through the slag outlet. They discovered that nitrogen absorption inside the furnace can be prevented by generating gas near the slag outlet to prevent nitrogen from entering through the slag outlet.

[0007] The present invention is as follows: [1] A method for producing molten iron by melting a solid iron source with an arc in an electric furnace, When the diameter of the electric furnace is D (m), and the electric furnace is viewed from above, carbon material is added to the molten iron within a range of D / 5 (m) from the inner wall of the furnace at the center of the lower end of the slag outlet, and the supply rate of volatile matter and water in the carbon material is V (kg / min), and the area of ​​the slag outlet is S (m²). 2 A method for producing molten iron, characterized by adding the carbon material under the condition that the following equation (1) is satisfied when ) is assumed. 0.10 <V / S ···(1) [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for producing molten iron that can further suppress nitrogen absorption. [Brief explanation of the drawing]

[0009] [Figure 1] This is a diagram illustrating the operation of an electric furnace. [Figure 2] This is a diagram illustrating the location where the charcoal material is added. [Figure 3] This is a schematic diagram of an electric furnace viewed from above, with a protruding section near the hot water outlet. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a diagram illustrating the operation of the electric furnace 20 in this embodiment. The electric furnace 20 includes electrodes 23 for heating solid reduced iron, molten iron 21 and slag 22 by arc, an input section 24 for introducing solid reduced iron, carbon material, quicklime, etc., and an exhaust section 25 for discharging gas and dust generated by melting solid reduced iron or scrap. The electric furnace 20 may be DC or AC, and there may be two or more input sections 24. In this case, it is desirable that one section be for adding only carbon material. Although Figure 1 shows two electrodes 23, in the case of an AC electric furnace, there is another electrode (not shown) behind these, for a total of three electrodes 23. If the electric furnace 20 is DC, there may be one or two electrodes in addition to the three.

[0011] Furthermore, a slag outlet 26 for removing slag 22 is provided on the side of the main body of the electric furnace 20, and a molten iron outlet 27 for discharging molten iron 21 is provided on the bottom of the main body of the electric furnace 20. The slag outlet 26 and the molten iron outlet 27 are configured to be openable and closable as needed. The electric furnace 20 can also be tilted toward the slag outlet 26 by a tilting device (not shown). In the example of Figure 1, the molten iron outlet 27 is provided on the bottom of the furnace, but it may also be provided on the side opposite to the slag outlet 26. In this case, the electric furnace 20 can be tilted toward the slag outlet 26 side and the molten iron outlet 27 side by a tilting device (not shown). Figure 1(a) shows the removal of slag 22, and Figure 1(b) shows the discharging of molten iron.

[0012] Next, a method for producing molten iron by melting solid reduced iron as a solid iron source in an electric furnace 20 will be described in detail. First, with the electric furnace 20 not tilted and with seed molten metal remaining, the input of solid reduced iron and carbon material is started from the input section 24. There are no particular limitations on the type of solid reduced iron; it may be HBI (Hot Briquetted Iron) or DRI (Direct Reduced Iron). In addition, when the solid reduced iron is initially input, auxiliary materials such as a lime source may be added at the same time to adjust the basicity.

[0013] Alternatively, the charcoal material may be added not from the input section 24, but by blowing powdered charcoal material onto the molten iron surface along with a carrier gas (Ar gas) from a wall lance (not shown) installed inside the furnace. The lance may also be inserted through the slag outlet 26 to blow powdered charcoal material; however, since atmospheric nitrogen is easily drawn into the area around the lance inserted into the slag outlet 26, it is preferable to either add the charcoal material from the input section 24 or blow powdered charcoal material from the wall lance when adding charcoal material. In this embodiment, the location for adding charcoal material is within a range close to the slag outlet 26; detailed conditions will be described later.

[0014] Then, the electrode 23 is lowered to near the molten iron surface and an arc is generated from the lower end of the electrode 23. When the solid reduced iron is heated by the arc, the solid reduced iron is reduced and melted to become molten iron, and the gangue components in the solid reduced iron become slag. As described above, the amount of molten iron 21 and slag 22 increases as the reduction and melting of the solid reduced iron progresses. Then, the addition of solid reduced iron and carbon material and heating by arc are continued until the amount of molten iron 21 and slag 22 reaches a predetermined amount. Note that before adding the solid reduced iron and carbon material, the electrode 23 may be lowered to near the molten iron surface and an arc may be generated from the lower end of the electrode 23.

[0015] Next, a method for producing molten iron by melting scrap as a solid iron source in an electric furnace 20 will be described in detail. First, with the electric furnace 20 not tilted, the furnace lid is opened and a predetermined amount of scrap is put in from a scrap bucket (not shown) installed above the furnace lid. Then, the furnace lid is closed and the electrode 23 is lowered to near the molten iron surface, and an arc is generated from the lower end of the electrode 23 to add carbon material. The method of adding carbon material is the same as when dissolving solid reduced iron. When the scrap is heated by the arc, the scrap melts and becomes molten iron. As described above, the amount of molten iron 21 and slag 22 increases as the melting of the scrap progresses. Then, the addition of carbon material and heating by the arc are continued until all of the predetermined amount of scrap has melted.

[0016] Next, a specific method for suppressing nitrogen absorption in this embodiment will be described. FIG. 2 is a diagram for explaining the position where the carbonaceous material is added. FIG. 2(a) is a view of the inside of the electric furnace 20 seen from above, and FIG. 2(b) is an enlarged view of the vicinity of the slag discharge port 26. In this embodiment, whether the carbonaceous material is charged from the charging section 24 or sprayed from the wall lance, when looking at the inside of the furnace from above, the carbonaceous material containing volatile matter (VM) is added onto the molten iron within a range of radius D / 5 (m) or less from the inner side wall 32 of the furnace at the center of the lower end of the slag discharge port 26 (the shaded portion within the dotted line 33).

[0017] Here, D represents the diameter (m) of the furnace body. However, the shape of the electric furnace varies. As shown in FIG. 1(b), when there is a protruding portion 29 near the tapping hole 27, as shown in FIG. 3, when the electric furnace 20 is viewed from above, the diameter D is the diameter of the circular furnace body shown by the dotted line excluding the protruding portion 29. Generally, since the tapping hole is provided on the opposite side of the slag discharge port, the diameter D may be calculated as the diameter of the circular furnace body in the direction parallel to the slag discharge port.

[0018] When the carbonaceous material is added, the volatile matter contained in the carbonaceous material decomposes in the molten iron and generates evaporation gas together with the moisture in the carbonaceous material. By adding the carbonaceous material containing volatile matter (VM) onto the molten iron within a range of D / 5 (m) or less from the inner side wall of the furnace at the center of the lower end of the slag discharge port, the intrusion of air from the slag discharge port can be prevented, and nitrogen absorption can be suppressed. Therefore, when the carbonaceous material is added in a range more than D / 5 (m) away from the inner side wall of the furnace at the center of the lower end of the slag discharge port, the intrusion of air from the slag discharge port cannot be prevented, and nitrogen absorption cannot be sufficiently suppressed. Preferably, it is added onto the molten iron within a range of D / 8 (m) or less from the inner side wall of the furnace at the center of the lower end of the slag discharge port.

[0019] In addition, if the slag discharge port is rectangular, the product of the long side and the short side is the area of the slag discharge port. As the area of this slag discharge port increases, the amount of air intrusion increases. Therefore, it is necessary to adjust the amount of carbon material added according to the area of the slag discharge port. Also, the amount of volatile matter (VM) and moisture in the carbon material varies depending on the type of carbon material. Therefore, in the present embodiment, when adding the carbon material onto the molten iron within the above-described range, the supply rate V (kg / min) of (VM + moisture) in the carbon material and the slag discharge port area S (m 2 ) are made to satisfy the following formula (1). 0.10 < V / S ···(1)

[0020] When the condition of formula (1) is not satisfied, the amount of evaporation gas generated is small, and sufficient prevention of air intrusion from the slag discharge port cannot be achieved. Preferably, the ratio V / S is more than 0.15 (kg / min·m 2 ). Regarding the upper limit of the ratio V / S, there is no particular limitation. However, from the perspective of the raw material cost of the carbon material, the ratio V / S is preferably 0.25 (kg / min·m 2 ) or less. Also, the area of the slag discharge port is the area of the slag discharge port 26 on the furnace inner wall 32 side.

[0021] Also, as described above, as a method of adding the carbon material containing volatile matter (VM), either the method of adding from above the charging section or the method of blowing the powdered carbon material from the wall lance may be adopted. When blowing the powdered carbon material from the wall lance, although it depends on the flow rate of the carrier gas and the area S of the slag discharge port, the carrier gas may also contribute to preventing air intrusion from the slag discharge port together with the evaporation gas composed of VM decomposition gas and moisture. However, costs are incurred because it is necessary to install a wall lance as equipment or to blow expensive Ar gas as the carrier gas for low nitrogenization. Also, there may be cases where the air intruded by the carrier gas is involved. In addition, if the powdered carbon material can be blown onto the molten iron within the above-described range, the position where the wall lance is installed may be any position.

[0022] Furthermore, although this embodiment describes an electric furnace that tilts the furnace body when removing slag, the same method can be applied to electric furnaces that do not tilt the furnace body. In the case of an electric furnace that does not tilt the furnace body, it is possible to generate an arc and add charcoal material even while removing slag, but since some of the charcoal material will flow out from the slag outlet and reduce the yield of charcoal material, it is preferable to stop the arc and interrupt the addition of charcoal material when removing slag. [Examples]

[0023] Next, the present invention will be further described based on examples. However, the conditions in the examples are merely one example of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0024] Similar to the electric furnace shown in Figure 1, a rectangular slag outlet was installed on the side wall of the furnace body, and a molten metal outlet was installed at the bottom of the furnace. In a tiltable DC electric furnace (with one electrode) with a furnace body diameter D of 7m and a molten iron melting capacity of 120t, 10t of molten iron was left as seed molten metal. Then, HBI with the composition shown in Table 1 as solid reduced iron, general coal or anthracite as carbon material, and quicklime for basicity adjustment were added. Before adding the carbon material, the ash content, volatile matter, fixed carbon, and moisture content were analyzed according to the method specified in JIS M8812. The results of these analyses are shown in Table 2. The area S of the slag outlet in the electric furnace was 1.4m². 2 (1.0m x 1.4m) was used, but in some tests, the refractory lining of the electric furnace was changed, and the area S of the slag outlet was reduced to 1.0m 2 We also conducted a test with dimensions of (0.8m x 1.3m).

[0025] [Table 1]

[0026] [Table 2]

[0027] Then, the electrode was lowered to the molten iron surface to generate an arc and melt the HBI, and while the arc was being generated, HBI, carbon material, and quicklime were added. Since some iron was present in the slag, a total of 150 tons of HBI was added to produce molten iron so that the amount of molten iron dissolved would be 120 tons. At that time, the type of carbon material, the method and location of addition of the carbon material, and the supply rate V of (VM + water) in the carbon material were changed. total Experiments were conducted with varying parameters. In the experiment where carbon material was sprayed from a wall lance, Ar gas was used as the carrier gas, and the solid-gas ratio (carbon material supply rate (kg / min) / carrier gas supply rate (kg / min)) was set to 10. The carbon material supply time was 40 minutes in all cases. After all HBI was dissolved, a temperature-measuring sampling probe was inserted into the molten iron through the slag door, and temperature measurement and sampling were performed. The nitrogen analysis of the sample was then performed. Each of the above experiments was performed for 5 channels, and the average nitrogen concentration was calculated. The invention was evaluated as having been effective if the average nitrogen concentration was 30 ppm or less. The experimental results are shown in Table 3.

[0028] [Table 3]

[0029] As shown in Table 3, in all of Examples 1 to 5, the carbon material was added to the molten iron within 1.4 m of the inner wall of the furnace at the center of the lower end of the slag outlet, and the carbon material was added under conditions that satisfied equation (1), resulting in a low nitrogen concentration in the molten iron.

[0030] On the other hand, in Comparative Example 1, carbon material was added from above from the input section onto the molten iron within 1.4 m of the inner furnace wall at the center of the lower end of the slag outlet. However, the conditions did not satisfy equation (1), resulting in insufficient evaporation and failure to adequately suppress nitrogen absorption. In Comparative Example 2, carbon material was added under conditions that satisfied equation (1), but because the carbon material was added from above from the input section to an area more than 1.4 m away from the inner furnace wall at the center of the lower end of the slag outlet, it was not possible to adequately prevent the intrusion of air from the slag outlet. Furthermore, in Comparative Example 3, similar to Comparative Example 1, carbon material was added from above from the input section onto the molten iron within 1.4 m of the inner furnace wall at the center of the lower end of the slag outlet. However, the conditions did not satisfy equation (1), resulting in insufficient evaporation and failure to adequately suppress nitrogen absorption. [Explanation of Symbols]

[0031] 20 Electric furnace 21 Molten iron 22 slag 23 electrodes 24 Input section 25 Exhaust section 26. Sludge outlet 27 Hot water outlet 28 Slugdoor 32 Furnace inner wall

Claims

[Claim 1] A method for producing molten iron by melting a solid iron source with an arc in an electric furnace, When the diameter of the electric furnace is D (m), and the electric furnace is viewed from above, carbon material is added to the molten iron within a range of D / 5 (m) from the inner wall of the furnace at the center of the lower end of the slag outlet, and the supply rate of volatile matter and water in the carbon material is V (kg / min), and the area of ​​the slag outlet is S (m). 2 A method for producing molten iron, characterized by adding the carbon material under the condition that the following equation (1) is satisfied when the above is the case. 0.10<V / S...(1)

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