Method for producing molten iron

Abstract

To provide a method for producing molten iron with which reduced iron having low carbon concentration can efficiently be melted.SOLUTION: In the method for producing molten iron by melting reduced iron having a carbon concentration of less than 1 mass% in an electric furnace, the reduced iron is charged into the molten iron under the condition that the difference Δ T (°C) between the temperature of the molten iron before charging the reduced iron and the liquidus temperature of the molten iron satisfies Δ T> 15-5C when the carbon concentration in the reduced iron is C (mass%).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing molten iron for melting reduced iron having a low carbon concentration. [Background technology]

[0002] In recent years, in order to achieve carbon neutrality, a method of producing molten steel by melting reduced iron in an electric furnace has been widely adopted. Reduced iron contains a few percent of carbon, which contributes to promoting the melting of reduced iron. Specifically, when reduced iron is melted, the carbon in the reduced iron dissolves in the molten iron, lowering the melting point of the molten iron and promoting the melting of reduced iron. Furthermore, when reduced iron is melted, the carbon in the reduced iron reacts with oxygen in the iron oxide in the reduced iron to generate CO gas, which stirs the molten iron around the reduced iron, promoting the melting of the reduced iron.

[0003] On the other hand, reduced iron is characterized by being more difficult to melt than scrap because it contains iron oxide and gangue components other than metallic iron and voids that reduce thermal conductivity. Therefore, various technologies have been proposed for efficiently melting reduced iron. Patent Document 1 discloses a method in which a cold iron source is preheated in a preheating chamber and then introduced into a melting chamber for melting. Patent Document 2 discloses a method in which reduced iron is charged so that the specific charging rate of reduced iron is less than a predetermined value. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-316715 [Patent Document 2] Japanese Patent Application Publication No. 2023-93079 [Non-patent literature]

[0005] [Non-Patent Document 1] Hirai, M., Kanamaru, K., Mori, H.: Materials for the 5th Solidification Phenomena Conference of the 19th Committee on Steelmaking, Japan Society for the Promotion of Science (Solidification 46), (1968). Summary of the Invention [Problem to be solved by the invention]

[0006] From the viewpoint of achieving carbon neutrality, the carbon concentration in reduced iron is also tending to decrease, and there is an increasing trend in the use of reduced iron produced using hydrogen or the like as a reducing agent rather than carbon-derived reduced iron for melting in electric furnaces. The methods described in Patent Documents 1 and 2 do not anticipate the use of reduced iron with such low carbon concentrations, and therefore are unable to melt reduced iron efficiently.

[0007] In view of the above-mentioned problems, an object of the present invention is to provide a method for producing molten iron that can efficiently melt reduced iron having a low carbon concentration. [Means for solving the problem]

[0008] The present inventors conducted extensive research into optimal conditions for melting reduced iron based on the findings that the higher the temperature of the molten iron, the easier it is to melt reduced iron, and that the liquidus temperature of the molten iron varies depending on the carbon concentration in the molten iron. As a result, they found that, under conditions where the carbon concentration in the reduced iron is low, the melting rate of the reduced iron can be improved by appropriately controlling the difference between the temperature of the molten iron before the introduction of the reduced iron and the liquidus temperature of the molten iron, relative to the carbon concentration in the reduced iron.

[0009] The present invention is as follows. [1] A method for producing molten iron by melting reduced iron having a carbon concentration of less than 1.00% by mass in an electric furnace, comprising: a temperature difference ΔT (°C) between the temperature of the molten iron before the reduced iron is added and the liquidus temperature of the molten iron satisfies the following formula (1): ΔT>15-5C (1) In the formula, C represents the carbon concentration (mass %) in the reduced iron. [Effects of the Invention]

[0010] According to the present invention, reduced iron with a low carbon concentration can be efficiently melted. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram illustrating the internal structure of an electric furnace. [Figure 2] FIG. 1 is a graph showing the relationship between the carbon concentration in reduced iron and the difference ΔT (° C.) between the temperature of molten iron before the reduced iron was charged and the liquidus temperature. DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Hereinafter, the term "reduced iron" refers to solid reduced iron. FIG. 1 is a diagram illustrating the internal structure of an electric furnace. The electric furnace 1 includes an electrode 2 that heats solid reduced iron and molten iron by arc, a charging section 3 into which reduced iron, carbonaceous material, quicklime, etc. are charged, and an exhaust section (not shown) for discharging gas and dust generated by melting the reduced iron. Although FIG. 1 shows one electrode 2, the number of electrodes may be two or more, and the electric furnace may be either a direct current or an alternating current type. There may be two or more charging sections 3.

[0013] When adding reduced iron, first, with the seed molten iron 4 remaining, the electrode 2 is lowered to near the molten iron surface, and an arc is generated from the lower end of the electrode 2. Then, the reduced iron is added through the charging section 3 and heated to melt. Note that the arc may be generated after the initial amount of reduced iron is charged. Furthermore, when the reduced iron is initially charged, auxiliary materials such as lime source and carbonaceous material may also be added to adjust the basicity and improve the reduction efficiency. This reduces the iron oxide (FeO) in the reduced iron by carbon in the reduced iron or molten iron, generating CO gas, which agitates the molten iron and reduced iron. Note that the reduced iron does not necessarily undergo reduction after melting; reduction may also occur during the melting of the reduced iron. Reduced iron is continuously added through the charging section 3 while the arc is being generated.

[0014] Furthermore, when reduced iron is charged from the charging section 3, the temperature of the seed molten iron drops, so an arc is generated at a specified power to charge the reduced iron at a rate that keeps the temperature of the seed molten iron constant. Once the target amount of reduced iron has been charged and all of it has melted, the molten iron, excluding the amount of seed molten iron to be used in the next charge, is tapped from a tapping hole (not shown) located at the bottom of the electric furnace, and then the slag is discharged from a slag discharge port (not shown).

[0015] Next, more detailed conditions for charging reduced iron will be described. In this embodiment, since the purpose is to efficiently melt reduced iron with a low carbon concentration, it is assumed that reduced iron with a carbon concentration of less than 1.00 mass % is used, including reduced iron produced by hydrogen reduction. The type of reduced iron is not particularly limited, and may be hot briquetted iron (HBI) or direct reduced iron (DRI).

[0016] Generally, the greater the difference between the temperature of molten iron and its liquidus temperature, which is the temperature at which the molten iron starts to solidify, the greater the melting rate of reduced iron, but a low carbon concentration in the reduced iron reduces the amount of CO gas generated, making it difficult to obtain the stirring effect of CO gas. Therefore, the inventors conducted experiments to investigate the relationship between the carbon concentration in the reduced iron, the temperature of the molten iron, and the liquidus temperature, which is the temperature at which the molten iron starts to solidify, and investigated the optimal range for the melting rate when using reduced iron with a carbon concentration of less than 1.00 mass%.

[0017] In the experiment, 150 t of molten iron was prepared in an electric furnace as a seed molten metal. After measuring the temperature, an arc was quickly generated from the electrode, and reduced iron was continuously charged into the charging section at a constant rate, with 2 t of reduced iron being charged in one minute. One minute after the reduced iron charging was completed, the interior of the furnace was observed to determine whether any unmelted reduced iron remained. In the experiment, reduced iron with various carbon concentrations was used, and the carbon concentration of the molten iron was adjusted by charging a carbonaceous material or the like according to the carbon concentration of the reduced iron before charging. The liquidus temperature of the molten iron was calculated using Hirai's formula (the following formula (2) or (3)) described in Non-Patent Document 1, and the difference ΔT (°C) between the temperature of the molten iron before charging and the liquidus temperature was calculated. In formulas (2) and (3), [C] represents the carbon concentration (mass%) in the molten iron. Liquidus temperature (℃)=1538-55[C]-80[C] 2 ([C]≦0.5) ···(2) Liquidus temperature (℃)=1494+21[C]-52[C] 2 (0.5<[C]<1.0) (3)

[0018] Figure 2 shows the relationship between the carbon concentration in reduced iron and the difference ΔT (°C) between the temperature of the molten iron before the reduced iron was added and the liquidus temperature. As can be seen from the results shown in Figure 2, a boundary line was found between the case where all the reduced iron is melted and the case where some unmelted reduced iron is present. Specifically, when the carbon concentration in the reduced iron is C (mass%), if the following formula (1) is satisfied, the reduced iron can be melted efficiently even when a low-concentration reduced iron is used. ΔT>15-5C (1)

[0019] In this embodiment, the temperature difference from the temperature of the molten iron before the reduced iron was charged is used, but because the charging rate of the reduced iron is controlled so as to keep the molten iron temperature constant according to the arc voltage, the temperature of the molten iron while the reduced iron is being melted is substantially the same as the temperature of the molten iron before the reduced iron was charged. Another reason for using the temperature of the molten iron before the reduced iron was charged is that measuring the temperature while the reduced iron is being charged would result in unstable measurements.

[0020] As described above, even if the solubility of reduced iron deteriorates due to a low carbon concentration, solubility can be ensured by ensuring that the difference ΔT between the temperature of the molten iron before the reduced iron is added and its liquidus temperature satisfies equation (1). From the perspective of melting rate, there is no upper limit to the difference ΔT between the temperature of the molten iron before the reduced iron is added and its liquidus temperature. However, if the temperature of the molten iron is excessively high, the life of the refractory material in the furnace may be shortened and the power consumption rate may decrease. Therefore, regardless of the carbon concentration of the reduced iron to be added, it is preferable to keep the temperature of the molten iron before the reduced iron is added below 1650°C.

[0021] On the other hand, there is also a method of increasing the carbon concentration in the molten iron by adding a carbonaceous material or the like to lower the liquidus temperature of the molten iron so that the difference ΔT between the temperature of the molten iron before the reduced iron is added and the liquidus temperature of the molten iron satisfies equation (1). However, if the carbon concentration in the molten iron is excessively increased, the time required for the decarburization process after melting the reduced iron becomes longer, thereby reducing productivity, and the amount of CO gas generated increases, increasing the load on the suction equipment. Therefore, the carbon concentration in the molten iron is preferably less than 2.00 mass%, more preferably less than 1.00 mass%, and even more preferably less than 0.50 mass%.

[0022] If the carbon concentration in the molten iron and the carbon concentration in the reduced iron are both low, the reduction of iron oxide in the reduced iron may not proceed sufficiently. Therefore, if necessary, the carbon concentration in the molten iron may be increased within the above-mentioned range by, for example, spraying a carbonaceous material from a top-blowing lance (not shown). Furthermore, if the amount of reduced iron charged is greater than the amount of seed molten iron before the reduced iron is charged, the carbon concentration of the molten iron may change significantly during the melting of the reduced iron. For example, if the amount of reduced iron produced by hydrogen reduction is greater than the amount of seed molten iron, the carbon concentration in the molten iron may decrease, preventing the reduction of iron oxide in the reduced iron from proceeding sufficiently. In such cases, a carbonaceous material may be continuously charged to maintain the carbon concentration in the molten iron. [Example]

[0023] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. 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.

[0024] 150 t of molten iron with the carbon concentration shown in Table 1 was placed in an electric furnace, and a temperature probe using a consumable thermocouple was immersed in the molten iron to measure the temperature. An arc was then quickly generated from the electrode, and reduced iron was charged at a constant rate for 1 minute. The amount of reduced iron charged was 2 t. One minute after the reduced iron was charged, the interior of the furnace was observed to check for the presence of unmelted reduced iron. If reduced iron remained on the slag, it was considered to be unmelted reduced iron. However, if bubbles were generated in the molten iron or slag due to CO gas generation, it was also considered to be unmelted reduced iron hidden in the slag.

[0025] The reduced iron used in the experiment was an HBI measuring approximately 110 mm in length, 50 mm in width, and 35 mm in thickness. The reduced iron had a total iron content of 85-92% and a mass fraction of 83-87%. The reduced iron contained other gangue components. In this experiment, the amount of reduced iron added (2 t) was small compared to the amount of molten iron before the reduced iron addition (150 t). Therefore, no carbonaceous material was added, and the carbon concentration in the molten iron remained nearly constant. The liquidus temperature of the molten iron was calculated using equations (2) and (3), and the difference ΔT (°C) between the temperature of the molten iron before the reduced iron addition and the liquidus temperature was calculated. Table 1 shows the test results. Note that % in the table indicates mass %.

[0026] [Table 1]

[0027] As shown in Table 1, in all of Examples No. 8 to No. 26, reduced iron was charged under conditions that satisfied formula (1), and therefore no unmelted reduced iron was present. On the other hand, in all of Comparative Examples No. 1 to No. 7, reduced iron was charged under conditions that did not satisfy formula (1), and therefore unmelted reduced iron was present. [Explanation of symbols]

[0028] 1. Electric furnace 2 electrodes 3 Input section 4 types of hot spring

Claims

[Claim 1] A method for producing molten iron by melting reduced iron having a carbon concentration of less than 1.00 mass% in an electric furnace, comprising: a temperature difference ΔT (°C) between the temperature of the molten iron before the reduced iron is added and the liquidus temperature of the molten iron satisfies the following formula (1): ΔT>15-5C...(1) In the formula, C represents the carbon concentration (mass %) in the reduced iron.

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