Production method of molten steel

Abstract

To provide a production method of molten steel capable of efficiently dissolving solid reduced iron in an electric furnace.SOLUTION: Disclosed is a production method of molten steel by heating and melting solid reduced iron in an electric furnace. The solid reduced iron is supplied such that the value (F / D2) exceeds 0.01 and is less than 0.20, where F (t / min) is the supply rate of the solid reduced iron and D (m) is the diameter of the surface of the molten steel when a specified amount of solid reduced iron is melted in the electric furnace.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] Compared to scrap, solid reduced iron contains more iron oxide and gangue, which are components other than metallic iron, and further contains voids that reduce thermal conductivity, so it is necessary to melt it more efficiently. Therefore, Patent Documents 1 to 4 each disclose a technique for heating and melting solid reduced iron in an electric furnace. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2021-102798 A [Patent Document 2] JP 2001-316715 A [Patent Document 3] JP 2017-57431 A [Patent Document 4] JP 2018-119693 A Summary of the Invention [Problem to be solved by the invention]

[0004] In general, it is known that in solid-liquid melting, the heat and mass transfer is promoted by stirring the liquid, and the melting rate of solid reduced iron is improved. On the other hand, in converter-type equipment, bottom blowing is often used as a method of stirring the molten iron, but in the case of electric furnaces, the bath depth is shallow, and it is difficult to obtain sufficient stirring power when bottom blowing is used because the floating distance of the gas bubbles is short. In addition, excessively increasing the flow rate of bottom blown gas can cause the molten iron to scatter, making the arc unstable, or the molten iron to adhere to the electrodes, accelerating the wear of the electrodes and accelerating the wear of the refractories.

[0005] In view of the above problems, an object of the present invention is to provide a method for producing molten steel that can efficiently melt solid reduced iron in an electric furnace. [Means for solving the problem]

[0006] Generally, most of the commercially available solid reduced iron contains about 0.5 to 2.0 mass % of carbon, and it is known that carbon in the solid reduced iron reacts with unreduced iron oxide or oxygen in the molten iron at high temperatures to generate CO gas from the solid reduced iron. Therefore, the present inventors have intensively studied conditions for stirring the molten iron using the gas generated from the solid reduced iron and efficiently melting the solid reduced iron. As a result, they have found that the molten iron can be efficiently stirred and the melting rate of the solid reduced iron can be improved by appropriately setting the supply rate of the solid reduced iron per cross-sectional area in the electric furnace and controlling the amount of gas generated.

[0007] The present invention is as follows. [1] A method for producing molten steel by heating and melting solid reduced iron in an electric furnace to produce molten steel, the method comprising the steps of: supplying solid reduced iron, which contains a component that dissolves in molten iron to generate gas, so as to satisfy the following formula (1): 0.01 <F / D 2 <0.20 ···(1) In formula (1), F represents the supply rate (t / min) of the solid reduced iron, and D represents the diameter (m) of the molten steel surface when a specified amount of solid reduced iron is melted in the electric furnace. [2] The method for producing molten steel according to the above item [1], wherein the solid reduced iron is supplied so as to further satisfy the following formula (2): 0.1<(F / D 2 ) / Σ(W i / M i )<19.0 ···(2) (2) In the formula, W i represents the concentration (mass%) of gas component i in the solid reduced iron, M i represents the molecular weight of gas component i. [3] The method for producing molten steel according to the above [1] or [2], wherein the gas generated from the solid reduced iron is at least one selected from the group consisting of CO, H2O, CO2, hydrocarbons, and metal vapor. Effect of the Invention

[0008] According to the present invention, solid reduced iron can be efficiently melted in an electric furnace. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a diagram for explaining the internal structure of an electric furnace. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a diagram for explaining the internal structure of an electric furnace. The electric furnace 1 includes an electrode 2 for heating solid reduced iron and molten iron by an arc, a charging section 3 for charging the solid reduced iron, carbonaceous material, quicklime, and the like, and an exhaust section (not shown) for discharging gas and dust generated by reducing and melting the solid reduced iron. Although one electrode 2 is shown in Fig. 1, the number of electrodes may be two or more, and the electric furnace may be of either a direct current type or an alternating current type. There may be two or more charging sections 3.

[0011] When the solid reduced iron is charged, first, while the seed liquor is left, the electrode 2 is lowered to the vicinity of the molten iron surface, and an arc is generated from the lower end of the electrode 2. The dotted line 4 in FIG. 1 indicates the molten iron surface with only the seed liquor. Then, the solid reduced iron is charged from the charging section 3, and the solid reduced iron is heated. Note that the arc may be generated after the initial amount of solid reduced iron is charged. Also, when the solid reduced iron is initially charged, auxiliary materials such as a lime source and a carbonaceous material may be charged together with the solid reduced iron for the purpose of adjusting the basicity and improving the reduction efficiency. As a result, the iron oxide (FeO) in the solid reduced iron is reduced by the carbon in the solid reduced iron, generating CO gas, which agitates the molten iron and the solid reduced iron. Note that the solid reduced iron is continuously charged from the charging section 3 while the arc is being generated. Then, the target amount of solid reduced iron is charged and all of it is heated and melted. Then, molten steel, excluding the amount of seed molten steel to be used in the next charge, is tapped from a tapping hole (not shown) provided at the bottom of the electric furnace, and then slag is discharged from a slag discharge port (not shown).

[0012] As described above, if the solid reduced iron and molten iron are efficiently stirred by the gas generated by the melting of the solid reduced iron, the melting rate of the solid reduced iron can be improved. Therefore, the solid reduced iron is charged from the charging section 3 under conditions that allow efficient generation of gas and sufficient stirring of the solid reduced iron. Specifically, the solid reduced iron is charged so as to satisfy the condition of the following formula (1). 0.01 <F / D 2 <0.20 ···(1)

[0013] Here, F represents the feed rate (t / min) of the solid reduced iron, and D represents the diameter (m) of the molten steel surface at the stage where a specified amount of solid reduced iron is melted. The solid line 5 in FIG. 1 indicates the molten steel surface at the stage where a specified amount of solid reduced iron is melted, and represents the sum of the specified amount of melted solid reduced iron and the seed molten iron. Here, in the example of FIG. 1, the diameter D represents the diameter of the iron bath in the circular furnace body excluding the part corresponding to the tapping hole. In this way, when the molten steel surface at the stage where a specified amount of solid reduced iron is melted has a part that is not circular when viewed from above, such as the tapping hole, the diameter D is the diameter (m) of a circle with an arc on the opposite side to the part such as the tapping hole. The specified amount of melted solid reduced iron is the amount of tapping steel that is predetermined for the electric furnace.

[0014] Value F / D 2 When F / D is less than 0.01, the solid reduced iron particles are too far apart, making it difficult for the gas generated from each solid reduced iron to reach each other, and the mixing efficiency decreases. 2 When the ratio is 0.20 or more, the solid reduced iron particles come too close to each other and tend to pile up on the molten iron surface, preventing contact with the molten iron. This makes it difficult for the reaction to generate gases such as CO to occur, and also prevents the solid reduced iron from dissolving into the molten iron.

[0015] The type of solid reduced iron is not particularly limited, and may be HBI (Hot Briquetted Iron) or DRI (Direct Reduced Iron). The components of the solid reduced iron are not particularly limited as long as they are components that generate gas by reactions such as evaporation, reduction, and thermal decomposition. In the above example, an example in which CO gas is generated has been described, but different gases or multiple types of gases may be generated depending on the components of the solid reduced iron. For example, when the solid reduced iron contains moisture or hydroxide, water vapor (H2O) is generated, and when the solid reduced iron contains carbonate, CO2 gas is generated. When the solid reduced iron contains volatile components in coal, hydrocarbons are generated. Furthermore, when the solid reduced iron is mixed with Zn or Na derived from dust, these metal vapors are generated.

[0016] As described above, the types and combinations of gases generated may vary depending on the components of the solid reduced iron, so it is preferable to further satisfy the condition of the following formula (2) in consideration of the types of gases. Note that when an auxiliary material such as carbonaceous material is charged together with the solid reduced iron, the gas generated from the auxiliary material is excluded. 0.1<(F / D 2 ) / Σ(W i / M i )<19.0 ···(2)

[0017] Here, W i represents the concentration (mass%) of gas component i in the solid reduced iron, and M i represents the molecular weight of gas component i. For example, when the solid reduced iron contains graphite or cementite, it is assumed to contain CO as a gas component, and the calculation is performed using the concentration of the gas component CO in the solid reduced iron. When the solid reduced iron contains carbonate, it is assumed to contain CO2 as a gas component, and the calculation is performed using the concentration of the gas component CO2 in the solid reduced iron. Σ(W i / M i A low value of Σ(W) in equation (2) means that the gas agitation per solid reduced iron particle is weak. In other words, the dissolution rate of the solid reduced iron is slow and it becomes easy for the solid reduced iron to accumulate, so it is necessary to reduce the supply rate F of the solid reduced iron. On the other hand, i / M i ) means that the amount of gas generated is large, so that the dissolution rate of the solid reduced iron is high and it is difficult for the solid reduced iron to accumulate. Therefore, the supply rate F of the solid reduced iron can be increased.

[0018] In addition, the value (F / D 2 ) / Σ(W i / M i When the value (F / D) is 0.1 or less, the amount of gas generated is large, but the feed rate F of the solid reduced iron tends to be small, so it takes a long time to dissolve the required amount of solid reduced iron. 2 ) / Σ(W i / M i) is 19 or more, the feed rate F of the solid reduced iron tends to be high while the amount of gas generated is small, so that the solid reduced iron is likely to accumulate on the molten iron when it is added.

[0019] As described above, the types of gases generated were CO, H2O, CO2, hydrocarbons, and metal vapor. As can be seen from formula (2), the molecular weight M i The smaller Σ(W i / M i ) and the feed rate F of the solid reduced iron can be increased. Therefore, gases with smaller molecular weights are preferable from the viewpoint of the productivity of the electric furnace because a larger gas volume can be obtained with a smaller weight and it is not necessary to increase the amount of gas components other than iron in the solid reduced iron. In addition, from the viewpoint of operation, it is preferable that the generated gas is a gas that does not adhere to the dust collection pipe after being cooled in the exhaust system. For the above reasons, it is preferable to use solid reduced iron that generates CO, HO, or CO2 as the main component of the generated gas.

[0020] The gas components in the solid reduced iron can be measured before it is charged into the electric furnace. For example, the gas components in the solid reduced iron can be measured by thermogravimetry-mass spectrometry (TG-MS). EXAMPLES

[0021] Next, an embodiment of the present invention will be described, but the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0022] In an electric furnace with a molten steel surface diameter D of 6.8 m when a specified amount of solid reduced iron is melted, 150 t of molten iron with a C concentration of 0.1-0.3 mass% and a molten iron temperature of 1550-1570°C was held as a seed molten iron, and solid reduced iron was charged for 1 minute at a rate shown in Table 1 while an arc was generated from the electrode. Then, the inside of the furnace was observed 1 minute after the end of charging the solid reduced iron, and the presence or absence of unmelted solid reduced iron was investigated. Specifically, the presence or absence of solid reduced iron was determined by checking whether bubbles were generated in the molten iron or slag due to gas generation.

[0023] HBI was used as the solid reduced iron, and its composition was T.Fe = 85-92%, M.Fe = 83-87%, with other components including gangue components and gas components shown in Table 1. The size of the solid reduced iron was approximately 110 mm in length, approximately 50 mm in width, and approximately 35 mm in thickness. The content of gas components in the solid reduced iron shown in Table 1 was measured by thermogravimetry-mass spectrometry (TG-MS). The test results are shown in Table 1.

[0024] [Table 1]

[0025] In Comparative Examples 1 to 4, the value F / D 2 In contrast, in Examples 5 to 11, since the condition of formula (1) was satisfied, it was confirmed that the unmelted solid reduced iron was not observed and the reduced iron was rapidly dissolved. [Explanation of symbols]

[0026] 1 Electric furnace 2 electrodes 3 Input section

Claims

1. 1. A method for producing molten steel by heating and melting solid reduced iron in an electric furnace to produce molten steel, comprising the steps of: supplying solid reduced iron containing a component that dissolves in molten iron to generate gas so as to satisfy the following formula (1): 0.01<F / D 2 <0.20 ・・・(1) In formula (1), F represents the supply rate (t / min) of the solid reduced iron, and D represents the diameter (m) of the molten steel surface when a specified amount of solid reduced iron is melted in the electric furnace.

2. The method for producing molten steel according to claim 1, characterized in that the solid reduced iron is supplied so as to further satisfy the following formula (2): 0.1<(F / D 2 ) / Σ(W i / M i )<19.0 ・・・(2) (2) In the formula, W i represents the concentration (mass%) of gas component i in the solid reduced iron, M i represents the molecular weight of gas component i.

3. The gas generated from the solid reduced iron is CO, H 2 O, CO 2 3. The method for producing molten steel according to claim 1, wherein the gas is at least one selected from the group consisting of a gaseous hydrocarbon and a metal vapor.

Images