Electric furnace operation method, sponge-like reduced iron production method, and molten steel production method

Abstract

This sponge-like reduced iron has a nitrogen content of less than 20 ppm.

Description

[Technical Field]

[0001] The present invention provides Electric furnace operation method This application claims priority to Japanese Patent Application No. 2024-023433, filed on February 20, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] For example, techniques for producing sponge-like reduced iron (porous reduced iron), such as a reduced iron production method using a shaft furnace, are known (Patent Documents 1 and 2). In these techniques, instead of producing liquid molten pig iron (pig iron) as in the blast furnace method, iron ore or its agglomerates are reduced in solid form to produce porous, highly porosity sponge-like reduced iron (hereinafter also referred to as sponge iron). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japan Utility Model Publication No. 63-167162 [Patent Document 2] Japanese Patent Publication No. 2000-204419 Summary of the Invention [Problem to be solved by the invention]

[0004] High-quality steel, especially steel used for producing thin sheets for deep drawing, is required to have an extremely low nitrogen content of less than 20 ppm by mass. In the following, ppm by mass will be simply referred to as ppm.

[0005] In the blast furnace / converter process, the following process is known to produce steel with such low nitrogen content. Specifically, a carbon source such as a carbonaceous material is added to the molten pig iron in the converter, and then an oxygen-containing gas is blown into the converter to promote the decarburization reaction, causing N to adsorb onto the surface of the CO bubbles. This process aims to denitrify the molten pig iron. The nitrogen content of the molten pig iron tapped from the converter is approximately 30 ppm. This process is possible because the molten pig iron contains a high carbon content. In other words, coke is used to produce molten pig iron in a blast furnace, which increases the carbon content of the molten pig iron. Furthermore, when this molten steel is treated in an RH degasser, the molten steel is denitrified by reducing the pressure, further reducing the nitrogen content of the molten steel. As a result, the nitrogen content of the molten steel is reduced to less than 20 ppm.

[0006] On the other hand, the nitrogen content of sponge-like reduced iron produced in a shaft furnace is 20 ppm or higher. The sponge-like reduced iron is transported to an electric furnace to become molten steel, where impurities are removed, but nitrogen is not sufficiently removed. Even if a carbon source is added to the molten steel in the electric furnace, as in the blast furnace / converter process, the denitrification reaction does not proceed sufficiently. Because shaft furnaces do not use large amounts of carbon sources such as coke, the carbon content of the sponge-like reduced iron transported to the electric furnace is low, and the CO bubbles necessary for denitrification are not sufficiently generated. Furthermore, because electric furnaces are less airtight than converters, atmospheric nitrogen may be mixed into the electric furnace and, ultimately, the molten steel. Therefore, it has been difficult to produce steel with low nitrogen content using shaft furnaces and electric furnaces. For example, the nitrogen content of molten steel tapped from an electric furnace is approximately 50–100 ppm.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide sponge reduced iron having a nitrogen content of less than 20 ppm, a method for producing sponge reduced iron, and a method for producing molten steel. [Means for solving the problem]

[0008] The gist of the present invention is as follows. (1) According to one aspect of the present invention Electric furnace operation method has a nitrogen content of less than 20 ppm Using sponge-like reduced iron . (2) A method for producing sponge reduced iron according to another aspect of the present invention includes the steps of: a step of determining the relationship between the nitrogen content of the sponge-like reduced iron and the denitrification temperature and retention time; a step of determining the denitrification temperature and retention time of the sponge-like reduced iron from the relationship; and a step of retaining the sponge-like reduced iron at the denitrification temperature and for the retention time, Based on the relationship between the nitrogen content of the sponge-like reduced iron and the retention time, the sponge-like reduced iron having a nitrogen content of 20 ppm or more is subjected to denitrification treatment. (3) In the method for producing sponge-like reduced iron according to (2) above, the denitrified sponge-like reduced iron may be cooled in a nitrogen-free atmosphere. (4) In a method for producing molten steel according to yet another aspect of the present invention, the sponge-like reduced iron produced by the method for producing sponge-like reduced iron described in (2) above is transported to an electric furnace in a nitrogen-free atmosphere to produce molten steel. (5) In the method for producing molten steel according to (4) above, the sponge reduced iron having a temperature of 650° C. or higher and a nitrogen content of less than 20 ppm may be transported to the electric furnace. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide sponge reduced iron having a nitrogen content of less than 20 ppm, a method for producing sponge reduced iron, and a method for producing molten steel. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing an apparatus for producing sponge reduced iron. [Figure 2] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 100% by volume, 1.013×10 Pa, and 700° C. [Figure 3] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 90% by volume, N2 10% by volume, 1.013 × 105 Pa, and 700°C. [Figure 4]10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 80 vol %, N2 20 vol %, 1.013 × 105 Pa, and 700°C. [Figure 5] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 70 vol %, N2 30 vol %, 1.013 × 105 Pa, and 700°C. [Figure 6] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 50 vol %, N2 50 vol %, 1.013 × 105 Pa, and 700°C. [Figure 7] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was set to 36% by volume of H2 and 64% by volume of N2, 1.013×105 Pa, and 700°C. [Figure 8] FIG. 1 is a schematic diagram showing an apparatus for producing molten steel. DETAILED DESCRIPTION OF THE INVENTION

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference

[0012] <1. Sponge-like reduced iron manufacturing equipment> 1 is a schematic diagram showing an apparatus for producing sponge-like reduced iron 1. The apparatus for producing sponge-like reduced iron 1 according to this embodiment includes a shaft furnace 1a, a reducing gas heating device 2, a dust removing device 3, a dehydration device 4, and a denitrification device 5.

[0013] The shaft furnace 1a is an apparatus for producing sponge-like reduced iron by reducing iron oxide raw materials. The shaft furnace 1a, for example, operates as follows: First, iron oxide raw materials (e.g., iron oxide pellets) are charged into the shaft furnace 1a from above, and reducing gas is blown into the shaft furnace 1a from below. The reducing gas is heated to a predetermined temperature (e.g., about 900 to 950°C) by a reducing gas heater 2 and then blown into the shaft furnace 1a. The reducing gas blown into the shaft furnace 1a reduces the iron oxide raw materials in the shaft furnace 1a. This direct reduction process produces sponge-like reduced iron (DRI). The sponge-like reduced iron is discharged from the bottom of the shaft furnace 1a.

[0014] The sponge-like reduced iron immediately after being discharged from the shaft furnace 1a has a nitrogen content of 20 ppm or more. The nitrogen content of the sponge-like reduced iron is measured in accordance with JIS G1228:1997 inert gas dissolution-thermal conductivity method (ISO / DIS 10720).

[0015] The sponge-like reduced iron may be discharged in a heated state or in a cooled state. The sponge-like reduced iron discharged in a heated state is also called HDRI (Hot Direct Reduced Iron), and the sponge-like reduced iron discharged in a cooled state is also called CDRI (Cold Direct Reduced Iron). The temperature of HDRI is, for example, 650°C or higher, and the temperature of CDRI is, for example, lower than 650°C. The temperatures of HDRI and CDRI are measured, for example, by a radiation thermometer or a thermocouple (the thermocouple is inserted into the DRI).

[0016] The sponge-like reduced iron is porous reduced iron, and preferably has a porosity of 40% or more. The porosity is measured by the method specified in JIS M8716:1990.

[0017] Furnace top gas (exhaust gas) is discharged from the top of the shaft furnace 1a. The furnace top gas is, for example, hydrogen gas and steam gas, or carbon monoxide gas and carbon dioxide gas, or a mixture of these. The furnace top gas may be discharged to the outside of the system, or may be circulated as in this embodiment.

[0018] The furnace for producing sponge reduced iron is not limited to a shaft furnace, and may be, for example, a kiln furnace.

[0019] The dust remover 3 removes dust from the furnace gas using a dust filter, etc. Thereafter, a part of the furnace gas is used as fuel gas for the reducing gas heating device 2, and the remainder is introduced into the dehydration device 4 as circulation gas.

[0020] The dehydration device 4 dehydrates the circulating gas. Thereafter, the circulating gas is mixed with a reducing gas introduced from the outside and introduced into the reducing gas heating device 2. Although not shown, carbon dioxide gas may be removed from the circulating gas.

[0021] The reducing gas heating device 2 heats the reducing gas with heat generated by burning the fuel gas. The heated reducing gas is blown into the shaft furnace 1a. The combusted fuel gas is discharged outside the system as exhaust gas.

[0022] The sponge-like reduced iron discharged from the shaft furnace 1a is introduced into the denitrification unit 5. The denitrification unit 5 denitrifies the sponge-like reduced iron using hydrogen gas. The denitrification proceeds according to the following reaction formula. The gas used for denitrification may be mixed with other types of gases as long as the denitrification is not hindered. Examples of such gases include argon gas, carbon monoxide gas, and carbon dioxide gas. N (in sponge-like reduced iron) + (3 / 2)H2 → NH3 The conditions required for denitrification (e.g., the temperature of the hydrogen gas) may be determined, for example, according to the research of Mori et al. (Iron and Steel, 58 (1972), 1264). The denitrification unit 5 performs denitrification until the nitrogen content of the sponge-like reduced iron becomes less than 20 ppm. The nitrogen content of the sponge-like reduced iron can be adjusted to any value by adjusting the operating conditions of the denitrification unit 5. The denitrification treatment is performed, for example, at 600°C or higher. The gas generated from the denitrification unit 5, i.e., the denitrification treatment exhaust gas, contains hydrogen gas and ammonia gas, and therefore may be mixed with a reducing gas introduced from an external source. By providing the denitrification unit 5, it is possible to denitrify sponge-like reduced iron in any form.

[0023] The denitrification conditions will now be described in detail. The inventors performed a simulation calculation of the denitrification behavior of sponge-like reduced iron using hydrogen. The denitrification reaction shown in the above chemical reaction formula was assumed to be rate-limited by the surface of sponge-like reduced iron, and the reaction rate was calculated using the value presented by Grabke (Ber. Bunsenges. Physik. Chem., (1968), 533, 541).

[0024] Figure 2 shows the denitrification atmosphere with 100% H2 by volume and 1.013 x 10 5 10 is a graph showing simulation results of the holding time and nitrogen content of sponge-like reduced iron for each initial nitrogen content at 100 Pa and 700° C. The denitrification atmosphere refers to the atmosphere in which the sponge-like reduced iron is denitrified. H2100 vol%, 1.013 x 10 5 In a denitrification atmosphere at 700°C and 200 Pa, it was found that the nitrogen content of sponge-type reduced iron with an initial nitrogen content of 30 ppm was reduced to 20 ppm in 3.6 minutes. The nitrogen content of sponge-type reduced iron with an initial nitrogen content of 50 ppm was reduced to 20 ppm in 8 minutes, and the nitrogen content of sponge-type reduced iron with an initial nitrogen content of 100 ppm was reduced to 20 ppm in 13.8 minutes.

[0025] Figure 3 shows the denitrification atmosphere: H2 90% by volume, N2 10% by volume, 1.013 × 10 510 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 4 shows the denitrification atmosphere with 80% H2 by volume, 20% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 5 shows the denitrification atmosphere with 70% H2 by volume, 30% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 6 shows the denitrification atmosphere with 50% H2 by volume, 50% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 7 shows the denitrification atmosphere with 36% H2 by volume, 64% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C.

[0026] As the N2 concentration in the denitrification atmosphere increases, the time required to reduce the nitrogen content of the sponge-like reduced iron to 20 ppm or less increases. The time required to reduce the nitrogen content of the sponge-like reduced iron from 30 ppm to 20 ppm is 6.1 minutes when the N2 concentration in the denitrification atmosphere is 10% by volume, as shown in Figure 3; 9.1 minutes when the N2 concentration is 20% by volume, as shown in Figure 4; 13.8 minutes when the N2 concentration is 30% by volume, as shown in Figure 5; and 38.6 minutes when the N2 concentration is 50% by volume, as shown in Figure 6. When the N2 concentration is 64% by volume, the N2 concentration (equilibrium concentration) at which the above reaction equation reaches equilibrium is 20 ppm, as shown in Figure 7, so it is not possible to reduce the nitrogen content below 20 ppm.

[0027] In addition, the lower the H2 concentration, the longer the treatment time. longThe time required to reduce the nitrogen content of the sponge-like reduced iron from 30 ppm to 20 ppm is, for example, 9.8 minutes in the case of 50 vol% H2 and 50 vol% Ar, and 71 minutes in the case of 10 vol% H2 and 90 vol% Ar.

[0028] As described above, the holding time for the denitrification treatment can be calculated by simulation. Therefore, the holding time for holding the sponge-like reduced iron at the denitrification temperature can be determined based on the relationship between the nitrogen content of the sponge-like reduced iron and the holding time for each condition, such as the denitrification temperature and the denitrification atmosphere.

[0029] The nitrogen content of the sponge-like reduced iron to be denitrified is 20 ppm or more. The nitrogen content of the sponge-like reduced iron to be denitrified may be, for example, 50 ppm or more, or 100 ppm or more. The nitrogen content of the sponge-like reduced iron to be denitrified may be, for example, 60,000 ppm (6 mass%) or less. In consideration of productivity, the nitrogen content of the sponge-like reduced iron to be denitrified is preferably 2,000 ppm or less.

[0030] FIG. 8 is a schematic diagram showing an apparatus 10 for producing molten steel. As shown in FIG. 8, the sponge-like reduced iron discharged from the denitrification unit 5 is transported to an electric furnace 7 by, for example, a transport means 6. That is, the sponge-like reduced iron having a nitrogen content of less than 20 ppm is transported to the electric furnace 7 in a nitrogen-free atmosphere. The transported sponge-like reduced iron may be HDRI or CDRI. In this embodiment, "transport" includes charging into the electric furnace 7. Here, the transport to the electric furnace 7 is performed by the transport means 6 in a nitrogen-free atmosphere. The sponge-like reduced iron is then melted in the electric furnace 7 to produce molten steel. Here, the nitrogen-free atmosphere includes not only a nitrogen-free atmosphere but also a case where nitrogen is contained in a small amount. "Containing a small amount of nitrogen" means a case where nitrogen is contained as an impurity, and specifically, 10 -3This means that the nitrogen-free atmosphere contains approximately 100% nitrogen. A nitrogen-free atmosphere is, for example, an atmosphere that does not contain nitrogen, does not react with the sponge-like reduced iron, or hardly induces any harmful reactions to the sponge-like reduced iron, such as reoxidation. Examples of nitrogen-free atmospheres include hydrogen gas, carbon dioxide gas, carbon monoxide gas, argon, and mixtures thereof. Strictly speaking, argon does not react with the sponge-like reduced iron. While hydrogen gas may reduce the sponge-like reduced iron, this reaction is not harmful to the sponge-like reduced iron. Carbon monoxide gas may reduce and carburize the sponge-like reduced iron, but these reactions are not harmful to the sponge-like reduced iron. Carbon dioxide gas may reoxidize the sponge-like reduced iron, but the rate of reoxidation is sufficiently low. If the interior of the transportation means 6 contains nitrogen, the nitrogen-containing atmosphere is replaced with the nitrogen-free atmosphere described above.

[0031] When the sponge-like reduced iron becomes HDRI, the following treatment may be performed instead of the above. That is, the sponge-like reduced iron may be cooled in a nitrogen-free atmosphere. For example, when the electric furnace 7 is located far from the denitrification device 5 in the sponge-like reduced iron production apparatus 1 and transportation takes a long time, such treatment may be performed. The temperature of the sponge-like reduced iron after the denitrification treatment and cooling may be, for example, less than 650°C.

[0032] The transport means 6 may be a transport pipe connecting the denitrification apparatus 5 and the electric furnace 7, or may be a sealed container. When the sponge-like reduced iron is placed in a vessel (sealed container) and transported to the electric furnace, the vessel may be filled with a hydrogen gas atmosphere. In this case, since denitrification is carried out within the vessel, the denitrification apparatus 5 may be omitted.

[0033] When the sponge-like reduced iron is transported to the electric furnace as a gas, the transport gas may be hydrogen gas. In this case, denitrification is carried out in the piping, so the denitrification device 5 may be omitted.

[0034] As described above, according to this embodiment, sponge-like reduced iron having a nitrogen content of less than 20 ppm can be produced. Furthermore, such sponge-like reduced iron can be transported to an electric furnace to produce molten steel while ensuring that the nitrogen content does not exceed 20 ppm. This facilitates the production of high-grade steel. Furthermore, the quality of high-grade steel can be improved.

[0035] Even if denitrification of the sponge-like reduced iron progresses in the shaft furnace 1a and the nitrogen content becomes less than 20 ppm, the sponge-like reduced iron will come into contact with a nitrogen-containing atmosphere during cooling in the shaft furnace 1a or discharge, and nitridation will progress, resulting in discharged sponge-like reduced iron with a nitrogen content of 20 ppm or more.

[0036] According to this embodiment, after denitrification by the denitrification device 5, (1) the sponge-like reduced iron can be transported at a high temperature in a nitrogen-free atmosphere, and (2) it can be cooled in a nitrogen-free atmosphere, so that the sponge-like reduced iron can be used in an electric furnace while maintaining a low nitrogen content (less than 20 ppm).

[0037] <2. Method for producing sponge-like reduced iron, method for denitrifying sponge-like reduced iron, method for producing molten steel, and method for cooling sponge-like reduced iron> Next, a method for producing sponge-like reduced iron, a method for denitrifying sponge-like reduced iron, a method for producing molten steel, and a method for cooling sponge-like reduced iron will be described.

[0038] First, sponge reduced iron is produced using a shaft furnace 1a. That is, first, an iron oxide raw material (e.g., iron oxide pellets) is charged into the shaft furnace 1a from above, and a reducing gas is blown into the shaft furnace 1a from below. The reducing gas is heated to a predetermined temperature (e.g., about 900 to 950°C) by a reducing gas heater 2 and then blown into the shaft furnace 1a. The reducing gas blown into the shaft furnace 1a reduces the iron oxide raw material in the shaft furnace 1a. This direct reduction process produces sponge reduced iron (DRI). The sponge reduced iron is discharged from the bottom of the shaft furnace 1a.

[0039] The sponge-like reduced iron immediately after being discharged from the shaft furnace 1a has a nitrogen content of 20 ppm or more. The sponge-like reduced iron may be discharged in a heated state (HDRI) or in a cooled state (CDRI).

[0040] On the other hand, furnace top gas (exhaust gas) containing hydrogen gas, carbon monoxide gas, water vapor, and carbon dioxide gas is discharged from the furnace top of the shaft furnace 1a. Dust is removed from the furnace top gas by a dust remover 3. Thereafter, part of the furnace top gas is used as fuel gas for the reducing gas heating device 2, and the remainder is introduced into a dehydration device 4 as a circulating gas.

[0041] Next, the dehydration device 4 dehydrates the circulating gas. After that, the circulating gas is mixed with reducing gas introduced from the outside and introduced into the reducing gas heating device 2. Although not shown, carbon dioxide gas may be removed from the circulating gas. The reducing gas heating device 2 heats the reducing gas with heat generated by burning fuel gas. The heated reducing gas is blown into the shaft furnace 1a.

[0042] The sponge-like reduced iron discharged from the shaft furnace 1a is introduced into the denitrification unit 5. The denitrification unit 5 denitrifies the sponge-like reduced iron using hydrogen gas. The denitrification unit 5 performs denitrification until the nitrogen content of the sponge-like reduced iron becomes less than 20 ppm. As described above, the retention time for maintaining the sponge-like reduced iron at the denitrification temperature can be determined based on the relationship between the nitrogen content of the sponge-like reduced iron and the retention time. For example, referring to FIG. 2, when 100% by volume of H2, 1.013×10 5 In order to denitrify sponge-like reduced iron having a nitrogen content of 50 ppm in an atmosphere of Pa to reduce the nitrogen content of the sponge-like reduced iron to 20 ppm or less, the sponge-like reduced iron should be held at the denitrification temperature of 700°C for 8 minutes or more.

[0043] The gas generated from the denitrification device 5, i.e., the denitrification treated exhaust gas, contains hydrogen gas and ammonia gas, and therefore may be used as a heat source and fuel for the reducing gas heating device 2, or may be mixed with the reducing gas introduced from the outside while controlling the nitrogen concentration.

[0044] The sponge-like reduced iron discharged from the denitrification unit 5 is transported to an electric furnace 7 by, for example, a transport means 6. The transport to the electric furnace 7 is performed in a nitrogen-free atmosphere, which prevents the sponge-like reduced iron from being nitrided. The sponge-like reduced iron is then melted in the electric furnace 7 to produce molten steel.

[0045] When the sponge reduced iron becomes HDRI, the following treatment may be performed instead of the above. That is, the denitrified sponge reduced iron may be cooled in a nitrogen-free atmosphere. This allows the sponge reduced iron to be cooled while suppressing nitridation of the sponge reduced iron. For example, when the electric furnace 7 is located far from the denitrification unit 5 in the sponge reduced iron production apparatus 1 and transportation takes a long time, such treatment may be performed. The temperature of the sponge reduced iron after cooling may be less than 650°C, for example. The sponge reduced iron may be cooled in the denitrification unit 5 or in a separately provided cooling unit (not shown).

[0046] Furthermore, when the sponge-like reduced iron is placed in a vessel (sealed container) as the transportation means 6 and transported to the electric furnace 7, the inside of the vessel may be filled with a hydrogen gas atmosphere. In this case, denitrification is carried out inside the vessel, so the denitrification device 5 may be omitted.

[0047] Furthermore, when the transport means 6 is a transport pipe and the sponge reduced iron is transported as a gas to the electric furnace 7, the transport gas may be hydrogen gas. In this case, denitrification is performed in the pipe, so the denitrification device 5 may be omitted. [Example]

[0048] CDRI with a nitrogen content of 30 ppm was placed in a denitrification apparatus and held at 700°C in a hydrogen gas atmosphere for 5 minutes. The nitrogen content was measured using the method described above. The nitrogen content of the CDRI was then measured and found to have been reduced to 19 ppm. This demonstrates that the denitrification apparatus 5 described above can denitrify sponge-like reduced iron to less than 20 ppm.

[0049] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0050] 1. Sponge-like reduced iron manufacturing equipment 1a Shaft furnace 2. Reducing gas heating device 3 Dust removal equipment 4 Dehydration equipment 5 Denitrification equipment 6 Means of transportation 7. Electric furnace 10 Molten steel manufacturing equipment

Claims

1. A method for operating an electric furnace using sponge-like reduced iron having a nitrogen content of less than 20 ppm.

2. A step of determining the relationship between the nitrogen content of sponge reduced iron and the denitrification temperature and retention time; determining a denitrification temperature and a retention time of the sponge reduced iron from the relationship; The method for producing sponge-like reduced iron includes holding the sponge-like reduced iron at the denitrification temperature for the holding time, and denitrifying the sponge-like reduced iron having a nitrogen content of 20 ppm or more based on the relationship between the nitrogen content of the sponge-like reduced iron and the holding time.

3. The method for producing sponge reduced iron according to claim 2 , wherein the denitrified sponge reduced iron is cooled in a nitrogen-free atmosphere.

4. A method for producing molten steel, comprising transporting the sponge-like reduced iron produced by the method for producing sponge-like reduced iron according to claim 2 to an electric furnace in a nitrogen-free atmosphere to produce molten steel.

5. 5. The method for producing molten steel according to claim 4, wherein the sponge reduced iron having a temperature of 650°C or higher and a nitrogen content of less than 20 ppm is transported to the electric furnace.

Images