Decarburization refining method

Abstract

To provide a decarburization refining method capable of reducing the use amount of a charcoal material.SOLUTION: In a decarburization refining method, molten iron (M) is stored in a refining furnace (100), a top blowing lance (2) is arranged above the molten iron (M), and oxygen is blown toward the molten iron (M) from a nozzle (21) provided at the tip of the top blowing lance (2) to decarburize and refine the molten iron (M). In the decarburization refining method, the nozzle (21) having a main hole (211) and a plurality of sub-holes (212) arranged around the main hole (211) is used, and the vertical distance (H) from the surface (Ma) of the molten iron (M) to the tip (21a) of the nozzle (21) is set to 3700-5400 mm, and the blowing amount of oxygen from the nozzle (21) is set to 20-75 Nm3 / hr / ton.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a decarburization refining method. [Background technology]

[0002] Refining is essential for the production of steel. In particular, refining is essential when producing steel containing chromium (Cr), such as stainless steel. In refining, for example, decarburization is performed. Refining including decarburization uses refining furnaces such as converters and AOD furnaces. By blowing oxygen (O2) into the molten pig iron contained in the refining furnace, excess carbon (C) in the molten pig iron is burned and removed from the molten pig iron. The molten pig iron contained in the refining furnace includes molten steel tapped from an electric furnace. In this specification, the molten pig iron contained in the refining furnace before oxygen blowing is collectively referred to as molten pig iron. By removing oxygen from the molten pig iron in the refining furnace, steel with a desired C content can be obtained.

[0003] For example, Japanese Patent Laid-Open Publication No. 9-41018 (Patent Document 1) describes a method for decarburizing and refining Cr-containing molten steel using a refining furnace equipped with a top-blowing lance. The decarburization and refining method of Patent Document 1 aims to suppress dust generation and oxidation loss of Cr while maintaining high productivity. To achieve this aim, a multi-hole nozzle is provided at the tip of the top-blowing lance. In the multi-hole nozzle of Patent Document 1, multiple main holes are arranged near the periphery of the nozzle, and sub-holes are arranged at or near the center of the nozzle.

[0004] In the decarburization refining method of Patent Document 1, oxygen is blown onto the molten pig iron from a multi-hole nozzle of a top blowing lance. In this case, the oxygen blown mainly from the main hole burns C in the molten pig iron primarily to generate CO gas, and the oxygen blown mainly from the sub-hole burns the CO gas secondary to generate CO2 gas. At this time, since the main hole is arranged around the sub-hole arranged near the center of the nozzle, the speed at which the oxygen jet blown from the main hole collides with the molten pig iron is relatively small. In addition, since the hole arranged near the center of the nozzle is the sub-hole, the speed of the oxygen blown onto the molten pig iron on the central axis of the lance is attenuated. As a result, the decarburization by the primary combustion proceeds while the generation of dust is suppressed. Furthermore, since the outward movement of heat generated by the secondary combustion is blocked by the oxygen jet blown from the main hole, the secondary combustion heat is imparted to the molten pig iron by the oxygen jet blown from the sub-hole, raising the temperature of the molten pig iron. This suppresses the oxidation loss of Cr. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-41018 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, when producing Cr-based stainless steel, oxygen is blown onto the molten pig iron in a refining furnace to perform decarburization and refining, and alloy raw materials such as Cr and nickel (Ni) are added to the molten pig iron to adjust the composition. However, the temperature of the molten pig iron drops as the alloy raw materials are added, and there is a risk that the heat provided by the combustion of the C in the molten pig iron alone will be insufficient, which may promote the oxidation of Cr. To compensate for this heat deficiency, carbonaceous materials (e.g., graphite, coal, pitch coke, etc.) are usually added to the molten pig iron during refining, and the combustion heat of the carbonaceous materials is used to raise the temperature of the molten pig iron.

[0007] The present disclosure has been made in consideration of the above-mentioned circumstances. An object of the present disclosure is to provide a decarburization and refining method capable of reducing the amount of carbonaceous material used. [Means for solving the problem]

[0008] In the decarburization and refining method according to the present disclosure, molten pig iron is placed in a refining furnace, a top-blowing lance is arranged above the molten pig iron, and oxygen is blown toward the molten pig iron from a nozzle provided at the tip of the top-blowing lance to decarburize and refine the molten pig iron. In the decarburization and refining method, a nozzle having a main hole and a plurality of sub-holes arranged around the main hole is used, the vertical distance from the surface of the molten pig iron to the tip of the nozzle is set to 3700 to 5400 mm, and the amount of oxygen blown from the nozzle is set to 20 to 75 Nm 3 / hr / ton. Effect of the Invention

[0009] According to the decarburization refining method of the present disclosure, the amount of carbonaceous material used can be reduced. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a refining furnace used in the decarburization refining method according to this embodiment. [Diagram 2] FIG. 2 is a plan view of the tip of the nozzle as viewed from below. [Diagram 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a diagram showing test results in the examples. [Diagram 5] FIG. 5 shows test results in the examples. [Figure 6] FIG. 6 shows test results in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present disclosure will be described. In the following description, the embodiment of the present disclosure will be described by way of examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and specific materials may be exemplified, but the present disclosure is not limited to these examples.

[0012] In a decarburization and refining method according to an embodiment of the present disclosure, molten pig iron is accommodated in a refining furnace, a top-blowing lance is disposed above the molten pig iron, and oxygen is blown toward the molten pig iron from a nozzle provided at the tip of the top-blowing lance to decarburize and refine the molten pig iron. In this decarburization and refining method, a nozzle having a main hole and a plurality of sub-holes arranged around the main hole is used, the vertical distance from the surface of the molten pig iron to the tip of the nozzle is set to 3700 to 5400 mm, and the amount of oxygen blown from the nozzle is set to 20 to 75 Nm 3 / hr / ton (first configuration).

[0013] In general, in a decarburization refining method using a refining furnace, a top blowing lance is arranged so that the vertical distance from the surface of the molten pig iron to the tip of the nozzle is 100 to 2000 mm. That is, the nozzle of the top blowing lance is arranged at a position relatively close to the surface of the molten pig iron. Furthermore, from the viewpoint of shortening the time required for decarburization, the amount of oxygen blown from the nozzle is relatively large.

[0014] In contrast, in the decarburization refining method according to the first configuration, the top blowing lance is positioned so that the vertical distance from the surface of the molten pig iron to the nozzle tip is 3700 to 5400 mm. In other words, compared to the general method, the nozzle of the top blowing lance is positioned at a position significantly farther away from the surface of the molten pig iron. Furthermore, the amount of oxygen blown from the nozzle is 20 to 75 Nm 3 / hr / ton, which is significantly smaller than that of conventional methods.

[0015] In the decarburization refining method according to the first configuration, a multi-hole nozzle having a main hole and a plurality of sub-holes is used as the nozzle of the top blowing lance. In this nozzle, a plurality of sub-holes are arranged around the main hole. In this case, the oxygen blown from the main hole burns C in the molten pig iron primarily to generate CO gas, and decarburization progresses. The oxygen blown from the sub-hole burns the CO gas secondary to generate CO2 gas. The oxygen jet blown from the sub-hole flows along the oxygen jet blown from the main hole and entrains the CO gas floating above the molten pig iron. This causes the CO gas to burn secondary to generate CO2 gas. The heat generated by the secondary combustion heats the atmosphere in the furnace and raises the temperature of the molten pig iron. At that time, as described above, the nozzle is arranged at a position away from the surface of the molten pig iron, and the amount of oxygen blown from the nozzle is small. Therefore, the oxygen jet blown from the sub-hole can entrain CO gas one after another over a wide range from the nozzle to the molten pig iron, and as a result, secondary combustion occurs over a wide range and a large amount of heat is generated. That is, the secondary combustion is significantly promoted. As a result, even if the combustion heat of the carbonaceous material is reduced, the temperature of the molten pig iron can be sufficiently increased. Therefore, according to the decarburization refining method of the first configuration, the amount of carbonaceous material used can be reduced.

[0016] In the first configuration of the decarburization refining method, for example, the molten pig iron contains chromium (second configuration). When the molten pig iron contains chromium, at least a Cr raw material is charged into the molten pig iron in a refining furnace. Although the temperature of the molten pig iron appears to drop due to the charging of the Cr raw material, the temperature of the molten pig iron can be sufficiently increased because the secondary combustion is significantly promoted as described above.

[0017] In the first or second configuration of the decarburization refining method, it is preferable that the main hole is provided on the central axis of the top lance, and the sub-holes are provided at equal intervals around the central axis of the top lance (third configuration). In this case, primary combustion by oxygen blown from the main hole is promoted, and decarburization progresses effectively. Furthermore, secondary combustion by oxygen blown from the sub-hole occurs evenly around the oxygen jet blown from the main hole. This effectively promotes secondary combustion.

[0018] In the decarburization refining method of the third configuration, each of the sub-holes is preferably inclined away from the main hole from the inlet to the outlet, and the extension line of the central axis of each of the sub-holes intersects with the surface of the molten pig iron before intersecting with the wall surface of the refining furnace (fourth configuration). In this case, secondary combustion is effectively promoted and damage to the furnace wall refractory can be suppressed.

[0019] In any one of the first to fourth configurations of the decarburization refining method, it is preferable that the main hole has a converging-diverging shape and each of the sub-holes has a straight shape (fifth configuration). In this case, even if the nozzle is located at a position away from the surface of the molten iron, the oxygen jet blown out from the main hole can reach the molten iron sufficiently. On the other hand, the oxygen jet blown out from the sub-hole has a moderate speed and can sufficiently entrain CO gas.

[0020] In any one of the first to fifth configurations of the decarburization refining method, the sum of the minimum cross-sectional areas of the sub-holes is preferably 3% to 30% of the sum of the minimum cross-sectional areas of the main hole and the sub-holes (sixth configuration). For example, when the hole shape is tapered and divergent, the minimum cross-sectional area of ​​the hole is the cross-sectional area of ​​the throat. When the hole shape is straight, the minimum cross-sectional area of ​​the hole is the cross-sectional area of ​​an arbitrary position, for example, the cross-sectional area of ​​the inlet of the hole or the cross-sectional area of ​​the outlet of the hole. The minimum cross-sectional area of ​​the hole corresponds to the amount of oxygen blown out from the hole. If the sum of the minimum cross-sectional areas of the sub-holes is 3% to 30% of the sum of the minimum cross-sectional areas of all the holes, the promotion of primary combustion and the promotion of secondary combustion can be achieved at the same time.

[0021] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In each drawing, the same or corresponding components are denoted by the same reference numerals, and redundant description will not be repeated.

[0022] The configuration of a refining furnace used in a decarburization refining method according to an embodiment of the present disclosure will be described with reference to FIGS. 1 to 3. FIG.

[0023] [Refining furnace] Fig. 1 is a schematic diagram showing the configuration of a refining furnace 100 used in the decarburization refining method according to this embodiment. Fig. 1 shows a cross section of a furnace body 1 of the refining furnace 100 during refining, cut along a plane along the up-down direction. In this specification, the up-down direction corresponds to the vertical direction, and the direction perpendicular to the up-down direction corresponds to the horizontal direction.

[0024] The refining furnace 100 shown in FIG. 1 is an AOD furnace. The refining furnace 100 includes a furnace body 1, a top blowing lance 2, and a tuyere 3. The furnace body 1 has a roughly barrel shape and is configured to be tiltable. The furnace body 1 has an opening 1a at the top. The opening 1a is used for introducing molten iron, inserting the top blowing lance 2, charging alloy raw materials, discharging molten steel, and discharging slag. The furnace body 1 is made of a refractory material 11. The entire refractory material 11 is covered with a steel shell 12. When refining is performed, molten iron M is accommodated in the furnace body 1. The molten iron M is accumulated at the bottom of the furnace body 1.

[0025] The top blowing lance 2 has a tubular shape extending in the vertical direction. Oxygen flows through the top blowing lance 2. The top blowing lance 2 has a double-tube structure to allow cooling water to flow through it. A nozzle 21 is provided at the tip (lower end) of the top blowing lance 2. An oxygen supply system (not shown) is connected to the upper end (not shown) of the top blowing lance 2. Specifically, piping is connected to the upper end of the top blowing lance 2. An oxygen tank is connected to the piping. A pump, a flow rate control valve, a flow meter, etc. are provided in the piping path.

[0026] When refining, the top blowing lance 2 is inserted from the opening 1a of the furnace body 1 and positioned above the molten pig iron M contained in the furnace body 1. Specifically, the top blowing lance 2 is positioned so that the vertical distance H from the surface Ma of the molten pig iron M to the tip 21a of the nozzle 21 is 3700 to 5400 mm. In general, the vertical distance H from the surface Ma of the molten pig iron M to the tip 21a of the nozzle 21 is called the lance height. Therefore, in this embodiment, the lance height is set to 3700 to 5400 mm during refining. In addition, the tip of the top blowing lance 2 is located almost directly above the center of the surface Ma of the molten pig iron M.

[0027] The nozzle 21 has a main hole 211 and a plurality of sub-holes 212. The configuration of the nozzle 21 will be specifically described with reference to Figures 2 and 3. Figure 2 is a plan view of the tip 21a of the nozzle 21 as viewed from below. Figure 3 is a cross-sectional view taken along line III-III in Figure 2.

[0028] In this embodiment, as shown in FIG. 2, the main hole 211 is provided on the central axis C of the top blowing lance 2. That is, in the nozzle 21, the main hole 211 is provided at the center of the nozzle 21. In this case, the central axis of the main hole 211 coincides with the central axis C of the top blowing lance 2. The multiple sub-holes 212 are provided at equal intervals around the central axis C of the top blowing lance 2. In the example shown in FIG. 2, three sub-holes 212 are provided at intervals of 120° around the central axis C of the top blowing lance 2. In this case, the sub-holes 212 are arranged around the main hole 211. The size of the sub-holes 212 is smaller than the size of the main hole 211.

[0029] Referring to FIG. 3, the main hole 211 and each of the sub-holes 212 are continuous with the inner hole 2a of the top blowing lance 2. The main hole 211 has a tapered shape. In this case, the main hole 211 forms a so-called Laval nozzle, and can blow out oxygen at a supersonic flow. The main hole 211 has a throat portion 211a. The diameter of the main hole 211 decreases from the inlet (upper end) toward the throat portion 211a, and increases from the throat portion 211a toward the outlet (lower end). For this reason, among the diameters of the main hole 211, the diameter d1 of the throat portion 211a is the smallest. In this case, among the cross-sectional areas of the main hole 211, the cross-sectional area of ​​the throat portion 211a is the smallest. In other words, the smallest cross-sectional area of ​​the main hole 211 is the cross-sectional area of ​​the throat portion 211a. A diameter d 1 of a throat portion 211 a of the main hole 211 is smaller than a diameter d 0 of an inner hole 2 a of the top blowing lance 2 .

[0030] Each sub-hole 212 has a straight shape. The diameter d2 of the sub-hole 212 is constant over the entire area from the inlet (upper end) to the outlet (lower end). In this case, the cross-sectional area of ​​the sub-hole 212 does not change over the entire area from the inlet to the outlet. Therefore, the minimum cross-sectional area of ​​the sub-hole 212 is the cross-sectional area at any position of the sub-hole 212. For example, the minimum cross-sectional area of ​​the sub-hole 212 may be the cross-sectional area of ​​the inlet of the sub-hole 212 or the cross-sectional area of ​​the outlet of the sub-hole 212. The diameter d2 of each sub-hole 212 is smaller than the diameter d0 of the inner hole 2a of the top blowing lance 2 and smaller than the diameter d1 of the throat portion 211a of the main hole 211.

[0031] In this embodiment, the sum of the minimum cross-sectional areas of the sub-holes 212 is 3% to 30% of the sum of the minimum cross-sectional areas of the main hole 211 and the sub-holes 212. In this specification, the sum of the minimum cross-sectional areas of the sub-holes 212 may be referred to as the sub-hole cross-sectional area, and the sum of the minimum cross-sectional areas of the main hole 211 and the sub-holes 212 may be referred to as the total hole cross-sectional area. In this embodiment, the dimensions of the main hole 211 and the sub-holes 212 are set so that the sub-hole cross-sectional area is 3% to 30% of the total hole cross-sectional area. For example, the diameter d1 of the throat portion 211a of the main hole 211 is 29 mm, and the diameter d2 of each of the three sub-holes 212 is 6.5 mm. In this case, the sub-hole cross-sectional area is about 13% of the total hole cross-sectional area. The diameter d0 of the inner hole 2a of the top blowing lance 2 is, for example, 167 mm.

[0032] In this embodiment, each of the sub-holes 212 is inclined away from the main hole 211 as it moves from the inlet to the outlet. Furthermore, when the central axis C2 of each of the sub-holes 212 is extended toward the molten iron M, it intersects with the surface Ma of the molten iron M before it intersects with the side wall of the furnace body 1. In this case, for example, the angle θ between the central axis C2 of each of the sub-holes 212 and the central axis C of the top blowing lance 2 is 7 to 12°. In other words, the angle θ between the central axis C2 of each of the sub-holes 212 and the central axis of the main hole 211 is 7 to 12°. In short, the angle θ between the central axis C2 of each of the sub-holes 212 and the central axis of the main hole 211 is the inclination angle of the sub-hole 212 with respect to the main hole 211.

[0033] The sub-holes 212 may have the same or different sizes. The inclination angles (angle θ) of the sub-holes 212 with respect to the main hole 211 may be the same or different.

[0034] Returning to FIG. 1, the tuyere 3 is provided at the bottom of the furnace body 1. For example, the tuyere 3 has a tubular shape and penetrates the side wall of the furnace body 1. The tuyere 3 has a double-tube structure. In the tuyere 3 of the double-tube structure, oxygen and argon gas flow through the inner tube, and argon gas for cooling flows through the outer tube. When refining is performed, oxygen and argon gas are blown into the molten iron M from the tuyere 3.

[0035] The actual refining furnace 100 is equipped with a combustion tower that recovers gases (CO gas and CO2 gas) generated in the furnace body 1. The combustion tower is connected to the opening 1a of the furnace body 1. The combustion tower draws in the CO2 gas that could not be completely combusted in the furnace body 1 together with the CO2 gas and outside air, and generates CO2 gas by subjecting the drawn-in CO gas to secondary combustion. The CO2 gas is discharged from the combustion tower.

[0036] [Decarburization refining method] With reference to Fig. 1, the decarburization refining method according to this embodiment will be described. In a refining furnace 100, molten pig iron M is accommodated in a furnace body 1. Then, using the above-mentioned top blowing lance 2, oxygen is blown toward the molten pig iron M from the nozzle 21 (main hole 211 and sub-hole 212) of the top blowing lance to decarburize and refine the molten pig iron M. In the example of this embodiment, the molten pig iron M contains chromium. In this case, a Cr raw material is charged into the molten pig iron M. Other alloy raw materials are also charged.

[0037] In this case, as described above, the top blowing lance 2 is disposed above the molten iron M so that the vertical distance H from the surface Ma of the molten iron M to the tip 21a of the nozzle 21 is 3700 to 5400 mm. In other words, the lance height is set to 3700 to 5400 mm. This lance height is significantly higher than the lance height used in general methods. Preferably, the lance height is set to 4000 to 5000 mm. In addition, the tip of the top blowing lance 2 is positioned almost directly above the center of the surface of the molten iron M. Furthermore, the amount of oxygen blown from the nozzle 21 is set to 20 to 75 Nm 3 The oxygen blowing rate is set to 1 / hr / ton. This oxygen blowing rate indicates the flow rate of oxygen blown from the nozzle 21 per 1 ton of molten pig iron M. This oxygen blowing rate is significantly smaller than that of general methods. The oxygen blowing rate is adjusted, for example, by a flow rate control valve. Note that oxygen is blown from the nozzle 21 of the top blowing lance 2 onto the molten pig iron M, and oxygen and argon gas are blown from the tuyere 3.

[0038] During refining, oxygen is blown toward the molten pig iron M from the main hole 211 and the sub-hole 212 by the top blowing lance 2. At this time, the lance height is significantly higher than in the general method, and the nozzle 21 (main hole 211 and sub-hole 212) of the top blowing lance 2 is disposed at a position significantly distant from the surface Ma of the molten pig iron M. Furthermore, the amount of oxygen blown from the nozzle 21 is significantly smaller than in the general method. Thus, in the refining method according to this embodiment, the nozzle 21 of the top blowing lance 2 is disposed at a position distant from the surface Ma of the molten pig iron M, and the amount of oxygen blown from the nozzle 21 is small, so that the oxygen is softly blown toward the molten pig iron M. The oxygen blown from the main hole 211 primarily combusts C in the molten pig iron M to generate CO gas, and the oxygen blown from the sub-hole 212 secondary combusts the CO gas to generate CO2 gas.

[0039] Specifically, the oxygen jet blown out from the main hole 211 collides with the molten pig iron M, and the C in the molten pig iron M undergoes primary combustion. This primary combustion generates CO gas, and decarburization progresses. The CO gas leaves the molten pig iron M and floats in the space between the molten pig iron M and the nozzle 21. Meanwhile, the oxygen jet blown out from the sub-hole 212 flows along the oxygen jet blown out from the main hole 211 and entrains the CO gas floating above the molten pig iron M. This causes secondary combustion of the CO gas, and generates CO2 gas. The heat generated by the secondary combustion heats the atmosphere in the furnace and raises the temperature of the molten pig iron M. At that time, as described above, the lance height is high, and the amount of oxygen blown out from the nozzle 21 is small. Therefore, the oxygen jet blown out from the sub-hole 212 can entrain CO gas one after another over a wide range from the nozzle 21 to the molten pig iron M. As a result, secondary combustion occurs over a wide range, and a large amount of heat is generated. In other words, the secondary combustion is significantly promoted. As a result, the temperature of the molten iron M can be sufficiently increased even if the combustion heat of the carbonaceous material is reduced.

[0040] [effect] In the decarburization refining method according to the present embodiment, a refining furnace 100 is used that is equipped with a top blowing lance 2 with a nozzle 21 at its tip. In this decarburization refining method, a nozzle 21 having a main hole 211 and a plurality of sub-holes 212 arranged around the main hole 211 is used, the vertical distance H from the surface Ma of the molten pig iron M to the tip 21a of the nozzle 21 is set to 3700 to 5400 mm, and the amount of oxygen blown from the nozzle 21 is set to 20 to 75 Nm 3 / hr / ton. This significantly promotes secondary combustion, and even if the combustion heat of the carbonaceous material is reduced, the temperature of the molten pig iron M can be sufficiently increased. Therefore, according to the decarburization refining method of this embodiment, the amount of carbonaceous material used can be reduced.

[0041] In this embodiment, the molten pig iron M contains chromium. When the molten pig iron M contains about 2 mass % or more of chromium, at least a Cr raw material is charged into the molten pig iron M in the refining furnace 100. The temperature of the molten pig iron M appears to drop due to the charging of the Cr raw material, but according to the decarburization refining method of this embodiment, the secondary combustion is significantly promoted, so that the temperature of the molten pig iron M can be sufficiently increased. Therefore, even when producing, for example, a Cr-based stainless steel, the oxidation loss of Cr can be suppressed.

[0042] In this embodiment, the main hole 211 is provided on the central axis C of the top blowing lance 2, and the multiple sub-holes 212 are provided at equal intervals around the central axis C of the top blowing lance 2. In this case, primary combustion by oxygen blown from the main hole 211 is promoted, and decarburization progresses effectively. Furthermore, secondary combustion by oxygen blown from the sub-hole 212 occurs evenly around the oxygen jet blown from the main hole 211. This effectively promotes secondary combustion, and the temperature of the molten iron M can be raised more sufficiently.

[0043] In this embodiment, each of the sub-holes 212 is inclined so as to move away from the main hole 211 as it moves from the inlet to the outlet. In addition, when the central axis C2 of each of the sub-holes 212 is extended toward the molten iron M, it intersects with the surface Ma of the molten iron M before the side wall of the furnace body 1. In this case, for example, the angle θ between the central axis C2 of each of the sub-holes 212 and the central axis C of the top blowing lance 2 is 7 to 12°. If the angle θ between the central axis C2 of the sub-hole 212 and the central axis C of the top blowing lance 2 is too small, the oxygen jet blown from the sub-hole 212 may interfere with the oxygen jet blown from the main hole 211, and the promotion of secondary combustion may be suppressed. On the other hand, if the angle θ is too large, the oxygen jet blown from the sub-hole 212 may head toward the side wall of the furnace body 1, and the refractory 11 may be damaged. According to the decarburization refining method of the fourth configuration, the secondary combustion is effectively promoted, and damage to the furnace wall refractories 11 can also be suppressed.

[0044] In this embodiment, the main hole 211 has a tapered-divergent shape, and each of the sub-holes 212 has a straight shape. In this case, even if the nozzle 21 is disposed at a position away from the surface Ma of the molten iron M, the oxygen jet blown out from the main hole 211 can sufficiently reach the molten iron M. On the other hand, since the shape of the sub-hole 212 is straight, the oxygen jet blown out from the sub-hole 212 spreads at an appropriate speed and can sufficiently entrain CO gas.

[0045] In this embodiment, the sum of the minimum cross-sectional areas of the sub-holes 212 is 3% or more and 30% or less of the sum of the minimum cross-sectional areas of the main hole 211 and the sub-holes 212. The minimum cross-sectional area of ​​a hole corresponds to the amount of oxygen blown out from that hole. If the sum of the minimum cross-sectional areas of the sub-holes 212 (sub-hole cross-sectional area) is 3% or more and 30% or less of the sum of the minimum cross-sectional areas of all holes (total hole cross-sectional area), it is possible to achieve both promotion of primary combustion and promotion of secondary combustion. EXAMPLES

[0046] In order to verify the effect of the decarburization refining method according to this embodiment, the following actual test was carried out using the refining furnace shown in FIG. 1. In the test, blowing, i.e., decarburization refining, was carried out on molten pig iron of various steel types (compositions) using multiple charges. In the test, the nozzle of the top blowing lance was changed in various ways. Specifically, as an example of the present invention, a nozzle 21 having a main hole 211 and a sub-hole 212 as shown in FIG. 2 and FIG. 3 was used. As a comparative example, a nozzle having a main hole 211 but no sub-hole 212 was used. In both the example of the present invention and the comparative example, the lance height was set to 3700 to 5400 mm, and the amount of oxygen blown from the nozzle was set to 20 to 75 Nm 3 The blowing rate was set at / hr / ton. The blowing was performed while intermittently feeding the carbonaceous material into the smelting furnace.

[0047] FIG. 4 is a diagram showing an example of the relationship between the blowing progress rate and the secondary combustion rate. FIG. 4 shows the results when molten pig iron of a certain steel type (composition) is decarburized and refined. The blowing progress rate shown on the horizontal axis of FIG. 4 means the time lapse from the start of oxygen blowing from the nozzle to the end of blowing. In the example shown in FIG. 4, the time from the start of oxygen blowing to the end of blowing was just under one hour, which was the same for the present invention example and the comparative example. The vertical axis of FIG. 4 shows the secondary combustion rate. The secondary combustion rate means the ratio of the amount of CO2 gas to the total amount of CO gas and CO2 gas in the furnace body. The amount of CO gas, i.e., the CO concentration, was estimated from the O2 (oxygen) concentration and CO2 concentration measured at the exhaust port of the combustion tower. The secondary combustion rate shown on the vertical axis of FIG. 4 is expressed as a ratio when the peak value of the secondary combustion rate of the comparative example is set to 1. The supply amount of carbonaceous material was almost the same in the present invention example and the comparative example. As shown in FIG. 4, when comparing the average value of the secondary combustion rate between the comparative example and the present invention example, the present invention example was about twice as large as the comparative example.

[0048] FIG. 5 is a diagram showing the relationship between the oxygen consumption rate and the amount of decarburization. In FIG. 5, each plot shows the oxygen consumption rate and the amount of decarburization in each charge used in the test. The oxygen consumption rate indicates the amount of oxygen blown out from the nozzle per ton of molten iron, and the amount of decarburization indicates the amount of decarburization per ton of molten iron. In FIG. 5, the oxygen consumption rate shown on the horizontal axis and the amount of decarburization shown on the vertical axis are each expressed as a ratio when the average value of the conventional example is set to 1. When the oxygen consumption rate is the same, the larger the amount of decarburization, the more oxygen is consumed for decarburization. In other words, when the oxygen consumption rate is the same, the smaller the amount of decarburization, the more oxygen is consumed for secondary combustion. As shown in FIG. 5, the amount of decarburization in the present invention example clearly tends to be lower than that in the comparative example. This result is consistent with the result shown in FIG. 4.

[0049] FIG. 6 is a diagram showing the unit consumption of carbonaceous material for each of the examples of the present invention and the comparative example. The unit consumption of carbonaceous material means the weight of carbonaceous material supplied per unit mass of molten iron. In FIG. 6, the unit consumption of carbonaceous material is expressed as a ratio to the unit consumption of carbonaceous material in the comparative example. As shown in FIG. 6, the unit consumption of carbonaceous material in the example of the present invention was reduced by 7.4% in average supply amount from the unit consumption of carbonaceous material in the comparative example.

[0050] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit of the present disclosure.

[0051] For example, the refining furnace 100 is not limited to an AOD furnace, and may be another refining furnace such as a converter. In the nozzle 21, the main hole 211 may be provided substantially on the central axis C of the top blowing lance 2. In other words, the main hole 211 may be provided near the central axis C of the top blowing lance 2.

[0052] The shapes of the main hole 211 and the sub-holes 212 are not particularly limited. For example, the main hole 211 may have a straight shape. The sub-holes 212 may have a tapered shape. The number of the sub-holes 212 is not particularly limited as long as there is more than one. For example, the number of the sub-holes 212 may be two, four, or five. [Explanation of symbols]

[0053] 100: Smelting furnace 1: Furnace body 2: Top blowing lance 21: Nozzle 21a: Tip 211: Main hole 212: Secondary hole H: distance M: Molten iron Ma: Surface C: Central axis C2: Central axis

Claims

1. A decarburization and refining method comprising: storing molten pig iron in a refining furnace; arranging a top blowing lance above the molten pig iron; and blowing oxygen toward the molten pig iron from a nozzle provided at a tip of the top blowing lance to decarburize and refine the molten pig iron, Using the nozzle having a main hole and a plurality of sub-holes arranged around the main hole, The vertical distance from the surface of the molten iron to the tip of the nozzle is 3700 to 5400 mm, The amount of oxygen blown from the nozzle is 20 to 75 Nm ３ / hr / ton.

2. The decarburization refining method according to claim 1, The decarburization and refining method, wherein the molten iron contains chromium.

3. The decarburization refining method according to claim 1, The main hole is provided on a central axis of the top blowing lance, and the plurality of sub-holes are provided at equal intervals around the central axis of the top blowing lance.

4. The decarburization refining method according to claim 3, each of the plurality of sub-holes is inclined so as to move away from the main hole as it moves from the inlet to the outlet, and an extension line of a central axis of each of the plurality of sub-holes intersects with the surface of the molten iron before a side wall of a furnace body of the refining furnace.

5. The decarburization refining method according to claim 1, The main hole has a converging-diverging shape, and each of the plurality of sub-holes has a straight shape.

6. The decarburization refining method according to claim 1, a sum of the minimum cross-sectional areas of the plurality of sub-holes is 3% to 30% of a sum of the minimum cross-sectional areas of the main hole and the plurality of sub-holes.

Images