Production method of molten steel

Abstract

To provide a production method of molten steel capable of efficiently carburizing by using diverse carbonaceous material.SOLUTION: The present invention relates to a production method of molten steel for producing molten steel by feeding a carbonaceous material to a molten iron surface from a lance while dissolving an iron source in an electric furnace. The production method includes arranging the lance so that an angle of the lance from the molten iron surface in a static state becomes equal to or more than 20° and a ratio x / d between a direct distance x in a lance center axis direction from a tip of the lance to the molten iron surface in the static state and an outlet diameter d of a nozzle at the tip of the lance becomes equal to or less than 140, and top-blowing the carbonaceous material from the lance to the molten iron surface in the condition that a center flowing speed of carrier gas in the lance tip is equal to or more than 50 m / s, and a feed rate of the carbonaceous material per sum of a specified dissolved amount of the iron source and seed molten iron is equal to or less than 0.6 kg / (min*t-steel).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing molten steel using an electric furnace. [Background technology]

[0002] Traditionally, carbonaceous materials have been used in electric arc furnaces to add carbur to molten iron and as a heat source and CO2 source. However, because carbonaceous materials have a low apparent density, when solid carbonaceous materials are added by free fall, they may remain on the slag and not reach the molten iron. Therefore, operations generally involve transporting granular carbonaceous materials from a lance using a carrier gas to supply them to the molten iron. However, because the inertial force of the carbonaceous materials supplied from the lance is weak, it is difficult for the granular carbonaceous materials to reach the molten iron even when transported using a carrier gas.

[0003] Therefore, many techniques have been proposed for efficiently injecting carbonaceous material into molten iron. Patent Document 1 discloses a method in which carbonaceous material is injected from the center of a lance using a carrier gas, and fuel and combustion-supporting gas are injected from the outer periphery and combusted, thereby penetrating the carbonaceous material into the molten iron.

[0004] Meanwhile, biomass charcoal derived from wood and other sources has recently been attracting attention as a carbon-neutral carbonaceous material. Patent Document 2 discloses a method of using biomass charcoal derived from coconut shells, which has a bulk density equivalent to that of coke breeze and a fixed carbon content of 75% by mass or more, as a carbonaceous material that is less likely to scatter and can be efficiently carbonized. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2021 / 090654 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-46724 Summary of the Invention [Problem to be solved by the invention]

[0006] Biomass charcoal, which has been attracting attention in recent years as a carbon-neutral carbon material, generally has an apparent density of approximately 0.50 g / cm 3 Many of them have a density less than 100% and, in addition, they are prone to scattering due to gas generation caused by the moisture and volatile components they contain, making it even more difficult for them to reach the molten iron. The method described in Patent Document 1 uses one or more of coke powder, coal, plastic, etc. as the carbonaceous material, but using biomass charcoal reduces the efficiency of penetration into the molten iron. Another issue is the cost of fuel and combustion-supporting gas. Furthermore, the method described in Patent Document 2 uses biomass charcoal made from coconut palm or oil palm, but the plants used as raw materials are limited, and biomass charcoal with a lower apparent density cannot be used.

[0007] In view of the above-mentioned problems, an object of the present invention is to provide a method for producing molten steel that can efficiently carburize using a variety of carbonaceous materials. [Means for solving the problem]

[0008] The present invention has been made to solve the above-mentioned problems, and is as follows. [1] A method for producing molten steel in an electric furnace by supplying a carbonaceous material from a lance to a surface of molten iron while melting an iron source, comprising: The lance is installed so that the angle of the lance from the surface of the molten iron in a stationary state is 20° or more, and the ratio x / d of the linear distance x from the tip of the lance to the surface of the molten iron in a stationary state in the direction of the central axis of the lance to the outlet diameter d of the nozzle at the tip of the lance is 140 or less, a method for producing molten steel, characterized in that the carbonaceous material is top-blown from the lance onto a surface of molten iron under conditions of a central flow velocity of a carrier gas at a tip of the lance being 50 m / s or more and a supply rate of the carbonaceous material per total amount of a specified amount of iron source to be dissolved and seed molten metal being 0.6 kg / (min t-steel) or less. [2] The method for producing molten steel according to [1] above, wherein biomass charcoal obtained by carbonizing plants is used as the carbonaceous material. [3] The apparent density of the carbonaceous material is 0.50 g / cm 3 The method for producing molten steel according to the above [1], characterized in that the temperature is less than [Effects of the Invention]

[0009] According to the present invention, when producing molten steel, it is possible to efficiently carburize using a variety of carbonaceous materials. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram for explaining how carbonaceous material is blown from above in an electric furnace. [Figure 2] FIG. 10 is a diagram for explaining the state in which carbonaceous material is blown from the lance upward. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this embodiment, an example in which carbonaceous material is top-blown in an electric furnace will be described, but the electric furnace may be of either a DC type or an AC type.

[0012] FIG. 1 is a diagram illustrating the top blowing of carbonaceous material in an electric furnace. FIG. 1(a) is a cross-sectional side view of the electric furnace, and FIG. 1(b) is a schematic top view of the electric furnace. The electric furnace shown in FIG. 1 has one or more electrodes 3 installed at the top. Current is supplied to the electrodes 3 from a power source (not shown). The molten iron 2 in the furnace body 1 is heated at the arc spot by an arc formed by the electrodes 3. A slag discharge port 5 is provided in the side wall of the furnace body 1, and a lance 4a for supplying granular carbonaceous material and a lance 4b for supplying oxygen gas are inserted through the slag discharge port 5. The tips of both lances 4a and 4b are single-hole nozzles, and the height of the lance tip from the molten iron surface and the angle of the lance relative to the molten iron surface can be changed using a manipulator (not shown).

[0013] Next, a specific method for producing molten steel will be described. First, a portion of the molten steel produced in the pre-charge is left in the furnace body 1 as a seed molten metal. Scrap is added to the seed molten metal, and the electrode 3 is lowered to near the molten iron surface. An arc is generated from the bottom of the electrode 3, and the scrap is melted. At this time, powdered carbonaceous material is blown upward from the lance 4a along with carrier gas toward the molten iron surface. Oxygen gas is also supplied from the lance 4b to generate CO gas and form slag. Low-cost nitrogen gas, for example, is used as the carrier gas blown upward along with the powdered carbonaceous material. Once all the scrap has been melted and molten steel has been produced, the molten steel is tapped from the tap hole 6 located at the bottom of the furnace body 1.

[0014] Next, various conditions for efficiently allowing carbonaceous material with a low apparent density, such as biomass charcoal, to reach the molten iron will be described. First, regarding the angle of the lance 4a for top-blowing the carbonaceous material, it is necessary to ensure a vertical velocity of the molten iron surface sufficient to reach the molten iron surface when the carbonaceous material is blown from above at the center velocity of the carrier gas described below. Therefore, the angle of the lance from the stationary molten iron surface is set to 20° or more. If the angle is less than 20°, the vertical velocity of the molten iron surface will be insufficient. Since the larger the angle of the lance 4a from the stationary molten iron surface, the more efficiently the carbonaceous material can reach the molten iron surface, the upper limit of the angle of the lance 4a is preferably 90°, at which the vertical velocity is maximized.

[0015] Furthermore, the height of the lance 4a is subject to the following conditions. First, the longer the linear distance from the tip of the lance to the stationary molten iron surface in the lance central axis direction, the more the velocity of the carbonaceous material decreases, making it more difficult for the carbonaceous material to reach the molten iron surface. Furthermore, the smaller the outlet diameter of the nozzle at the tip of the lance, the more the amount of carbonaceous material passing through per unit area of the nozzle. This causes the carbonaceous material to concentrate at a specific spot on the molten iron surface, reducing the efficiency of the carbonaceous material reaching the molten iron surface. Based on these points, the inventors conducted experiments and other verifications. As shown in Figure 2, when the linear distance from the tip of the lance to the stationary molten iron surface in the lance central axis direction is x (m) and the outlet diameter of the nozzle at the tip of the lance is d (m), the velocity of the carbonaceous material does not excessively decrease and the carbonaceous material efficiently reaches the molten iron surface as long as the ratio x / d is 140 or less. The carbonaceous material may be blown from above while the tip of the lance 4a is immersed in the slag. Alternatively, the height of the tip of the lance 4a and the stationary molten iron surface may be the same (ratio x / d = 0). On the other hand, if the height of the tip of the lance 4a is lower than the surface of the molten iron in a stationary state (ratio x / d<0), even if the tip of the lance is exposed by the spraying of the carrier gas, the tip of the lance will be damaged by the splashing of molten iron, etc., so it is preferable that the ratio x / d be 0 or greater.

[0016] On the other hand, the angle and height of the lance 4b for supplying oxygen gas are not particularly limited, but it is preferable to set the conditions so that the range of blowing oxygen is at least wider than the range of blowing the carbonaceous material from the top from the lance 4a. Moreover, since the lances 4a and 4b are inserted from positions close to each other, it is preferable to set the angle and height of the lance 4b to conditions similar to those of the lance 4a for blowing the carbonaceous material from the top.

[0017] Next, the conditions for top blowing of the carbonaceous material will be described. In this embodiment, the apparent density is 0.50 g / cm 3The carbonaceous material is blown from the top under the condition that the central flow velocity of the carrier gas at the lance tip of the lance 4a is 50 m / s or more so that even carbonaceous material of less than 50 m / s can reach the molten iron. If the central flow velocity of the carrier gas at the lance tip is less than 50 m / s, the initial velocity of the carbonaceous material is insufficient, and an increased amount of the carbonaceous material does not reach the molten iron. On the other hand, the flow velocity of the oxygen gas supplied from the lance 4b is not particularly limited, but is preferably about 100 to 400 m / s from the viewpoint of increasing the thermal efficiency from the electrode 3 by foaming the slag.

[0018] The supply rate of the carbonaceous material blown from the top of the lance 4a must be 0.6 kg / (min·t-steel) or less. Here, the carbonaceous material supply rate represents the amount per ton of molten iron, including unmelted scrap. In other words, the carbonaceous material supply rate per total amount of the specified melting iron source and the seed molten iron must be 0.6 kg / (min·t-steel) or less. The specified melting amount refers to the predetermined tapping rate for the electric furnace. If the carbonaceous material supply rate exceeds 0.6 kg / (min·t-steel), unmelted carbonaceous material tends to accumulate on the slag surface, forming clumps that prevent the material from reaching the molten iron surface. As a result, the molten iron cannot be efficiently recarburized. Although the slower the carbonaceous material supply rate, the less recarburized the molten iron becomes, theoretically, a rate greater than 0 kg / (min·t-steel) ensures efficient recarburization of the molten iron. For the carburizing effect to be significant, the carbonaceous material supply rate should be 0.1 kg / (min·t-steel) or more.

[0019] Next, the carbonaceous material to be blown from the lance 4a will be described. As mentioned above, various kinds of carbonaceous material can be used. 3Biomass charcoal of less than 10 ...

[0020] Under the above conditions, molten steel with a target carbon concentration can be produced by top-blowing carbon material from a lance to carburize the scrap while melting it. Note that, although the present embodiment has been described as a method for producing molten steel by melting scrap as an iron source, the present invention can also be applied to the melting of reduced iron. [Example]

[0021] Next, an example of the present invention will be described, but the conditions are merely examples for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of this example. The present invention can be implemented in various ways to achieve the object of the present invention without departing from the gist of the present invention.

[0022] In the electric furnace shown in Figure 1, scrap was charged with 60 t of seed molten metal remaining, and an arc was generated from electrode 3 to melt the scrap. Nitrogen gas was supplied as a carrier gas from lance 4a, and a carbonaceous material with the composition shown in Table 1 was blown upward toward the molten iron surface. At the same time, oxygen gas was also supplied from lance 4b, which was on a separate system from the carbonaceous material. The particle size of the carbonaceous material used was 0.3 to 2.0 mm, and the nozzle outlet diameter d of both lances 4a and 4b was 28 mm.

[0023] The top blowing of carbonaceous material and oxygen gas was stopped when the temperature of the molten iron after scrap melting reached 1650-1670°C and the carbon concentration in the molten iron reached 0.08-0.10 mass%, and 175 tons of molten steel was obtained. After the experiment was completed, the carbonaceous material consumption rate for each test was compared.

[0024] [Table 1]

[0025] [Table 2]

[0026] The apparent densities in Table 1 were measured using the method described in JIS Z 8807:2012, and the components were measured using the method described in JIS M 8812:2006. The power consumption rates after scrap melting due to arc generation were all within the range of 0.42 to 0.44 MWh / t-steel. The "moisture" in Table 1 is included in the "volatile components," and the remainder is ash.

[0027] The carbonaceous material supply conditions and test results are shown in Table 2. Compared to the conditions of the present invention, in Comparative Example No. 2, the central flow velocity of the nitrogen gas, which is the carrier gas for the carbonaceous material, was too small, resulting in an insufficient initial velocity of the carbonaceous material, and an increased amount of the carbonaceous material did not reach the molten iron. This resulted in a high carbonaceous material consumption rate. In Comparative Example No. 3, the lance angle from the molten iron surface was too small, resulting in an insufficient velocity in the vertical direction of the molten iron surface, and an increased amount of the carbonaceous material did not reach the molten iron. This resulted in a high carbonaceous material consumption rate.

[0028] In Comparative Example No. 4, the linear distance x from the tip of the lance to the stationary molten iron surface in the lance central axis direction was long, resulting in a large ratio x / d. This resulted in a decrease in the speed of the carbonaceous material, and an increase in the amount of carbonaceous material that could not reach the molten iron. This resulted in a high carbonaceous material consumption rate. In Comparative Example No. 5, the carbonaceous material supply rate was too high, resulting in undissolved carbonaceous material accumulating on the slag surface and preventing it from reaching the molten iron surface, resulting in an increase in the amount of carbonaceous material that could not reach the molten iron. This resulted in a high carbonaceous material consumption rate. Furthermore, in Comparative Example No. 1, the central flow velocity of the nitrogen gas and the lance angle were too small, and the ratio x / d and the carbonaceous material supply rate were too high, resulting in an increase in the amount of carbonaceous material that could not reach the molten iron. This resulted in a high carbonaceous material consumption rate.

[0029] In the comparative examples No. 6 and No. 7, biomass charcoal was used as the carbonaceous material, but the central flow velocity of the nitrogen gas and the lance angle were too small, and the ratio x / d and the carbonaceous material supply rate were too large, so the amount that did not reach the molten iron increased. As a result, the carbonaceous material consumption rate was high.

[0030] In contrast, the carbonaceous material consumption rate was low at 11.6 to 13.5 kg / t-steel in Examples No. 8 to No. 12. In particular, No. 11 and No. 12 were examples in which biomass charcoal was used, and these examples also succeeded in suppressing the carbonaceous material consumption rate. [Explanation of symbols]

[0031] 1 Furnace body 2. Molten iron 3 electrodes 4a,4b Lance

Claims

1. A method for producing molten steel in an electric furnace by supplying a carbonaceous material from a lance to a surface of molten iron while melting an iron source, comprising: The lance is installed so that the angle of the lance from the surface of the molten iron in a stationary state is 20° or more, and the ratio x / d of the linear distance x from the tip of the lance to the surface of the molten iron in a stationary state in the direction of the central axis of the lance to the outlet diameter d of the nozzle at the tip of the lance is 140 or less, A method for producing molten steel, characterized in that the carbonaceous material is top-blown from the lance onto a surface of molten iron under conditions of a central flow velocity of a carrier gas at a tip of the lance being 50 m / s or more and a supply rate of the carbonaceous material per total of a specified amount of dissolved iron source and seed molten metal being 0.6 kg / (min t-steel) or less.

2. 2. The method for producing molten steel according to claim 1, wherein the carbonaceous material is biomass charcoal obtained by carbonizing plants.

3. The apparent density of the carbonaceous material is 0.50 g / cm 3 2. The method for producing molten steel according to claim 1, wherein the temperature is less than 100°C.

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