Sponge reduced iron, method for producing sponge reduced iron, and method for producing molten steel

Abstract

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

Description

Sponge-like reduced iron, method for producing sponge-like reduced iron, and method for producing molten steel

[0001] The present invention relates to a sponge-like reduced iron, a method for producing sponge-like reduced iron, and a method for producing molten steel. This application claims priority based on Japanese Patent Application No. 2024-023433, filed on February 20, 2024, the contents of which are incorporated herein by reference.

[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 (see 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).

[0003] Japanese Utility Model Application Publication No. 63-167162 Japanese Patent Application Publication No. 2000-204419

[0004] High-quality steel, particularly 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 a 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 steel tapped from the converter is approximately 30 ppm. This process is possible because the molten pig iron contains a high carbon content. Since coke is used to produce molten pig iron in a blast furnace, the carbon content of the molten pig iron is high. 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 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 more. 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 as containers than converters, there is a possibility that nitrogen from the atmosphere may be mixed into the electric furnace and, ultimately, the molten steel. Therefore, it has been difficult to produce steel with a low nitrogen content using molten steel production methods using shaft furnaces and electric furnaces. For example, the nitrogen content of molten steel tapped from an electric furnace is approximately 50 to 100 ppm.

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

[0008] The gist of the present invention is as follows. (1) According to one aspect of the present invention, sponge-like reduced iron has a nitrogen content of less than 20 ppm. (2) According to another aspect of the present invention, a method for producing sponge-like reduced iron denitrifies 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 a holding time. (3) According to the method for producing sponge-like reduced iron described in (2) above, the denitrified sponge-like reduced iron may be cooled in a nitrogen-free atmosphere. (4) According to yet another aspect of the present invention, a method for producing molten steel comprises transporting the sponge-like reduced iron produced by the method for producing sponge-like reduced iron described in (2) above to an electric furnace in a nitrogen-free atmosphere to produce molten steel. (5) According to the method for producing molten steel described in (4) above, the sponge-like reduced iron having a nitrogen content of less than 20 ppm may be transported to the electric furnace at a temperature of 650°C or higher.

[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.

[0010] 1 is a schematic diagram showing an apparatus for producing sponge-like reduced iron. ２ 100% by volume, 1.013 x 10 ５ 1 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 H ２ 90% by volume, N ２ 10% by volume, 1.013 x 10 ５ 1 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 H ２ 80% by volume, N ２ 20% by volume, 1.013 x 10 ５ 1 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 H ２ 70% by volume, N ２ 30% by volume, 1.013 x 10 ５ 1 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 H ２ 50% by volume, N ２ 50% by volume, 1.013 x 10 ５ 1 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 H ２ 36% by volume, N ２ 64% by volume, 1.013 x 10 ５ 10 is a graph showing simulation calculation results of the holding time and nitrogen content for each initial nitrogen content of sponge-like reduced iron at a temperature of 700° C. and 700 Pa. FIG. 11 is a schematic diagram showing an apparatus for producing molten steel.

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

[0012] 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 removal 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 an iron oxide raw material. The shaft furnace 1a, for example, operates as follows: 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 heating device 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. Through this direct reduction process, sponge-like reduced iron (DRI) is produced. 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 HDRI).

[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] A furnace top gas (exhaust gas) is discharged from the furnace top of the shaft furnace 1a. The furnace top gas may be, 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 circulating gas.

[0020] The dehydration device 4 dehydrates the circulating gas, which is then 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 to the outside of 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)H ２ →NH ３The conditions required for denitrification (e.g., the temperature of the hydrogen gas) may be determined, for example, in accordance with the research of Mori et al. (Iron and Steel, 58 (1972), 1264). The denitrification device 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 device 5. The denitrification treatment is performed, for example, at 600°C or higher. The gas generated from the denitrification device 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 outside. By providing the denitrification device 5, it is possible to denitrify sponge-like reduced iron in any form.

[0023] The denitrification conditions will be described in detail. The present 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 determined using the value presented by Grabke (Ber. Bunsenges. Physik. Chem., (1968), 533, 541).

[0024] FIG. 2 shows the denitrification atmosphere. ２ 100% by volume, 1.013 x 10 ５ 1 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge-like reduced iron at H 2 O 3 and 700° C. The denitrification atmosphere refers to the atmosphere in which the sponge-like reduced iron is denitrified. ２ 100% by volume, 1.013 x 10 ５ It was found that in a denitrification atmosphere at 700°C and 200 Pa, the nitrogen content of sponge-like reduced iron with an initial nitrogen content of 30 ppm was reduced to 20 ppm in 3.6 minutes. It was also found that the nitrogen content of sponge-like reduced iron with an initial nitrogen content of 50 ppm was reduced to 20 ppm in 8 minutes, and that the nitrogen content of sponge-like reduced iron with an initial nitrogen content of 100 ppm was reduced to 20 ppm in 13.8 minutes.

[0025] FIG. 3 shows the denitrification atmosphere. ２ 90% by volume, N ２ 10% by volume, 1.013 x 10 ５4 is a graph showing the results of simulation calculations of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H ２ 80% by volume, N ２ 20% by volume, 1.013 x 10 ５ 5 is a graph showing the results of simulation calculations of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H ２ 70% by volume, N ２ 30% by volume, 1.013 x 10 ５ 6 is a graph showing the results of simulation calculations of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H ２ 50% by volume, N ２ 50% by volume, 1.013 x 10 ５ 7 is a graph showing the results of simulation calculations of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H ２ 36% by volume, N ２ 64% by volume, 1.013 x 10 ５ 10 is a graph showing simulation calculation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content at 700 Pa and 700° C.

[0026] Denitrification atmosphere N ２ As the concentration 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 depends on the N concentration of the denitrification atmosphere. ２ When the concentration is 10% by volume, as shown in FIG. ２ When the concentration is 20% by volume, as shown in FIG. ２ When the concentration is 30% by volume, as shown in FIG. ２ When the concentration is 50% by volume, it is 38.6 minutes, as shown in FIG. ２ When the concentration is 64% by volume, as shown in FIG. 7, the N ２The concentration (equilibrium concentration) is 20 ppm, so it cannot be made below 20 ppm.

[0027] Also, H ２ The lower the concentration, the longer the treatment time. The time required to reduce the nitrogen content of the sponge reduced iron from 30 ppm to 20 ppm is, for example, ２ 50% by volume and Ar 50% by volume, it was 9.8 minutes. ２ 10% by volume and 90% by volume of Ar, it is 71 minutes.

[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 the sponge-like reduced iron 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 nitrogen-free atmosphere containing only a small amount of nitrogen. A nitrogen-free atmosphere means that nitrogen is contained as an impurity, and specifically, a nitrogen-free atmosphere containing only 10% nitrogen. －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 causes 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 reduced iron becomes HDRI, the following treatment may be performed instead of the above. That is, the sponge reduced iron may be cooled in a nitrogen-free atmosphere. For example, such treatment may be performed when the electric furnace 7 is located far from the denitrification device 5 in the sponge reduced iron production apparatus 1 and transportation takes a long time. The temperature of the sponge 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 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 by 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 and discharge in the shaft furnace 1a, and nitridation will progress, resulting in the discharge of 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. Therefore, 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 Spongy Reduced Iron, Method for Denitrifying Spongy Reduced Iron, Method for Producing Molten Steel, and Method for Cooling Spongy Reduced Iron> Next, the methods for producing sponge reduced iron, the method for denitrifying sponge reduced iron, the method for producing molten steel, and the method for cooling sponge 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. Here, the reducing gas is heated to a predetermined temperature (e.g., about 900 to 950°C) by a reducing gas heating device 2, and then blown into the shaft furnace 1a. The reducing gas blown into the shaft furnace 1a then reduces the iron oxide raw material in the shaft furnace 1a. By this direct reduction process, sponge reduced iron (DRI) is produced. 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 circulation gas.

[0041] Next, 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. 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 device 5. The denitrification device 5 denitrifies the sponge-like reduced iron using hydrogen gas. The denitrification device 5 performs denitrification until the nitrogen content of the sponge-like reduced iron becomes less than 20 ppm. As described above, 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 example, referring to FIG. 2, ２ 100% by volume, 1.013 x 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 a 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 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] Note that, 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 device 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 device 5 or in a separately provided cooling device (not shown).

[0046] Furthermore, when the sponge-like reduced iron is placed in a vessel (sealed container) serving 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, since denitrification is carried out within the vessel, the denitrification device 5 may be omitted.

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

[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.

[0050] REFERENCE SIGNS LIST 1 Spongy reduced iron manufacturing apparatus 1a Shaft furnace 2 Reducing gas heating apparatus 3 Dust removal apparatus 4 Dehydration apparatus 5 Denitrification apparatus 6 Transportation means 7 Electric furnace 10 Molten steel manufacturing apparatus

Claims

1. Sponge reduced iron with a nitrogen content of less than 20 ppm.

2. A method for producing sponge-like reduced iron, which comprises denitrifying 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 retention time.

3. The method for producing sponge-like reduced iron according to claim 2, wherein the denitrified sponge-like 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. 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.

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