TW202246528A - Method for refining molten iron - Google Patents

Abstract

The present invention proposes a method for refining molten iron, the method enabling the stable production of a low nitrogen steel. A method for refining molten iron, wherein molten iron before treatment having a carbon concentration (C)i of 0.5% by mass to 3.0% by mass is held in a container, and decarburization and denitrification of the molten iron before treatment are performed by blowing oxygen into the molten iron before treatment at the atmospheric pressure, while blowing a hydrogen gas, a hydrocarbon gas, or a mixed gas thereof into the molten iron before treatment. It is preferable that the nitrogen concentration (N)f of the molten iron after treatment, namely after decarburization and denitrification, is 30 ppm by mass or less; it is preferable that the molten iron after treatment, namely after decarburization and denitrification, is further subjected to a vacuum degassing treatment; it is preferable that the molten iron before treatment contains molten iron that is obtained by melting a cold iron source; it is preferable that the molten iron before treatment is obtained by mixing primary molten iron that is obtained by melting a cold iron source in a melting furnace and hot metal having a carbon concentration of 2.0% by mass or more; it is preferable that the cold iron source contains reduced iron; and it is preferable that the container is a converter furnace.

Description

Refining method of molten iron

The present invention relates to a method for obtaining low-nitrogen steel by reducing the nitrogen concentration in the decarburized molten iron when decarburizing molten iron having a carbon concentration of 3.0% by mass or less before treatment to obtain molten steel. In particular, it is a preferable method when a part or all of the molten iron is obtained by melting a cold iron source before processing.

In recent years, from the viewpoint of preventing global warming, the development of technologies for reducing the consumption of fossil fuels and reducing the generation of CO ₂ gas has been promoted in the iron and steel industry. In conventional iron and steel plants, iron ore was reduced using carbon to produce molten pig iron. For the production of this molten pig iron, about 500 kg of carbon source is required per 1 ton of molten pig iron for the reduction of iron ore and the like. On the other hand, in the case of producing molten steel from cold iron sources such as iron scrap as a raw material, a carbon source required for reduction of iron ore is not required, and only sufficient heat energy for melting the cold iron source is required. Therefore, the amount of CO ₂ emitted can be significantly reduced.

When molten steel is obtained by melting a chilled iron source in a melting furnace such as an electric furnace, the nitrogen concentration at the time of tapping may be higher than in the case of tapping blast furnace pig iron with a converter. In the process of refining blast furnace pig iron by converter, nitrogen is mainly removed by adsorbing on carbon monoxide bubbles generated by decarburization, and the nitrogen concentration during tapping is generally low. Specifically, blast furnace pig iron contains about 4% by mass of carbon, and the amount of carbon monoxide generated by decarburization refining is sufficiently large, so that low-nitrogen steel with a nitrogen concentration of about 20 mass ppm can be smelted. However, in the case of using cold iron source, the concentration of carbon in molten iron after melting the cold iron source is low, and the amount of carbon monoxide produced is limited, so it is difficult to remove nitrogen to a low concentration. If vacuum degassing treatment or the like is performed on the molten iron after melting the cold iron source, denitrification can be performed to a certain extent. However, the region where the denitrification reaction occurs is limited to the surface of the molten steel in contact with the vacuum atmosphere in the vacuum tank, so the upper limit of the nitrogen concentration that can be stably melted is about 40 mass ppm.

In addition, in general, since reduced iron is produced by reduction with natural gas or the like, it contains 0.5% by mass to 2.0% by mass of carbon. Therefore, decarburization refining is required in molten iron obtained by melting the reduced iron, and denitrification can be performed to some extent in this case. In addition, from the viewpoint of increasing the amount of denitrification, it is conceivable to combine the molten iron obtained by melting reduced iron in an electric furnace and blast furnace pig iron to increase the carbon concentration in the molten iron, and then denitrify in a converter. carbon refining. However, in order to reduce _CO2 production in the future, it is considered to reduce the production of blast furnace pig iron and increase the use of cold iron sources. In this way, it is presumed that the carbon concentration at the time of loading the converter is lowered and it is difficult to sufficiently reduce the tapping nitrogen concentration.

Based on such assumptions, the following techniques have been disclosed as techniques for obtaining low-nitrogen steel. For example, Patent Document 1 proposes a method in which carbon is added to molten steel discharged from a converter, and after Al deoxidation is carried out, oxygen is supplied during vacuum degassing to carry out decarburization and refining, thereby reducing N in molten steel. Concentration [N]≦25 mass ppm.

In addition, Patent Document 2 proposes a denitrification method for molten steel: CaO is added to the bath surface of molten steel without adding carbon, and then a substance containing Al is added to remove nitrogen in molten steel as nitrides in the slag. , and further send oxygen, thereby removing it as nitrogen gas in the gas phase, and reducing the nitrogen concentration to 20 mass ppm or less.

In addition, Patent Document 3 proposes a vacuum refining method in which the nitrogen concentration is reduced to 20 mass ppm by supplying hydrocarbon gas as the reflux gas supplied from the immersion tube to the RH vacuum degassing device to make the bubbles finer. the following. [Prior art literature] [Patent Document]

Patent Document 1: Japanese Patent Laid-Open No. 2004-211120 Patent Document 2: Japanese Patent Laid-Open No. 2007-211298 Patent Document 3: Japanese Patent Laid-Open No. 2000-45013

[Problems to be Solved by the Invention]

However, there still exist the following problems to be solved in the said prior art. In the method described in Patent Document 1, since a carbon source is additionally added to generate carbon monoxide bubbles, there is a problem that the amount of CO ₂ generated increases, and the processing time is prolonged due to the decarburization again in the vacuum degassing process, and the productivity is reduced. topic.

In addition, in the method described in Patent Document 2, it is described that metal Al needs to be added at least 3 kg/t-molten steel, and the cost rises significantly. In addition, after adding metal Al, it is necessary to oxidize and remove Al in molten steel again. Therefore, a reduction in productivity due to an increase in processing time and an increase in the discharge amount of slag become a problem.

In addition, in the method described in Patent Document 3, since the hydrogen concentration in molten iron increases after the hydrocarbon gas is supplied, dehydrogenation treatment is required. Therefore, there is a problem that processing time increases and productivity decreases.

The present invention is made in view of this situation, and its purpose is to provide a method for refining molten iron: under the condition that the usage of cold iron source increases, it will not be accompanied by obvious reduction in productivity or increase in cost, and will not cause slag The amount of production or CO ₂ generation increases, and low-nitrogen steel is stably produced. [Means to solve the problem]

In view of these problems, the inventors have diligently studied a method for promoting denitrification in a process of decarburization and refining in a converter or the like, and have completed the present invention as a result.

In the method for refining molten iron of the present invention that advantageously solves the above-mentioned problems, the untreated molten iron having a carbon concentration [C] _i of 0.5 mass % or more and 3.0 mass % or less is stored in a container, and the The molten iron is blown with oxygen before treatment, and hydrogen or hydrocarbon gas or their mixed gas is blown in to carry out the decarburization and denitrogenation treatment of the molten iron before the treatment.

In addition, regarding the refining method of molten iron of the present invention, it is considered that (a) the nitrogen concentration [N] _f of the treated molten iron subjected to the decarburization and denitrification treatment is set to be 30 mass ppm or less, and (b) The treated molten iron after the decarburization and denitrification treatment is further subjected to vacuum degassing treatment, (c) the molten iron before the treatment includes those obtained by melting cold iron sources, (d) the hot metal before the treatment The molten iron is mixed with primary molten iron obtained by melting the chilled iron from a melting furnace, and molten iron having a carbon concentration of 2.0% by mass or more, (e) the chilled iron source includes reduced iron, (f ) The container is a converter, etc. can become a better solution. [Effect of the invention]

According to the present invention, under the condition that the amount of cold iron source used increases, it is not accompanied by a significant decrease in productivity or an increase in cost, and does not increase the amount of slag generated or CO ₂ generated, and it is possible to stably manufacture processed The nitrogen concentration [N] _f in molten steel is a low-nitrogen steel of 30 mass ppm or less.

Hereinafter, embodiments of the present invention will be specifically described. As a first step, in a melting furnace for steelmaking, the iron source is melted and heated using electric energy. Here, as the melting furnace for steelmaking, an electric furnace such as an electric arc furnace or an induction furnace can be used. At this time, the so-called iron source is not only a solid iron source such as scrap or reduced iron, but molten iron melted in other processes can also be used. In addition, the heat energy supplied for the melting of the solid iron source and heating up of the iron source is not only electric energy, but also the combustion heat of metal, etc. may be used supplementarily. These energy sources are preferably renewable energy sources from the viewpoint of reducing CO ₂ emissions.

As a second step, the melt is discharged into a container such as a ladle. In the case of using reduced iron as a cold iron source, since a large amount of slag due to gangue contained in reduced iron is generated, it is preferable to remove slag as necessary. Slag removal can also be carried out by using a slag dragger (slag dragger). If the freeboard height of the ladle (the height from the top of the ladle to the surface of the molten iron) is insufficient, the furnace body can be tilted before the melt is discharged from the electric furnace, and the melt can be discharged after the slag flow. In addition, the furnace body can also be tilted before discharging the melt from the electric furnace, and the melt can be discharged after the slag flow, and the slag flowing out into the ladle and other containers together with the molten iron can be further deslagged.

As a third step, if necessary, molten iron such as blast furnace pig iron is melt-merged to adjust the carbon concentration [C] _i in the molten iron to 0.5% by mass or more and 3.0% by mass or less, and put it into a reaction vessel, and from the top Oxygen is supplied to the lance, etc. for decarburization and refining. When the carbon concentration [C] _i of the molten iron before treatment is less than 0.5% by mass, the amount of CO gas generated during decarburization is small, and thus denitrification may be insufficient. On the other hand, when the carbon concentration exceeds 3.0% by mass, the effect of reducing the amount of CO ₂ generated becomes small. Furthermore, in the case of combining the melts, the molten pig iron used as the combined melt is preferably molten iron with a carbon concentration of 2.0% by mass or more, and may be molten pig iron in the state of discharging pig iron from a blast furnace, or One or a combination of two or more of desiliconization, dephosphorization, and desulfurization after the pig iron is discharged from the blast furnace. As the reaction vessel, a converter is preferable in terms of the height of the free space (the height from the upper end of the reaction vessel to the surface of the molten iron). As long as it is a container capable of oxygen blowing, it may be a ladle or the like. In addition, oxygen blowing is not only a method of supplying oxygen from the top blowing lance, but also supplying oxygen from the bottom tuyeres. The supply of oxygen from the top blowing lance and the supply of oxygen from the bottom tuyeres may also be used together.

Next, oxygen gas for decarburization is started to be supplied, and hydrogen atom-containing gas including hydrogen gas, hydrocarbon gas, or a mixed gas thereof is supplied from a porous plug or the like provided on the bottom of the furnace. It is considered that when a gas containing hydrogen atoms is supplied to the molten iron, the dissociation reaction of the gas molecules is caused, and the hydrogen atoms are temporarily dissolved in the molten iron and are generated again as fine hydrogen bubbles. It is considered that the denitrification reaction proceeds between the fine air bubbles generated here and the molten iron interface. Therefore, when decarburization refining is carried out using molten iron in which a chilled iron source is melted, even if the amount of bubble generation of carbon monoxide is not sufficient, the nitrogen concentration after decarburization refining can be reduced. Therefore, decarburization and denitrogenation can be performed simultaneously. As a result of intensive studies by the inventors, it was found that the appropriate supply rate of the gas containing hydrogen atoms is a flow rate of about 0.1 Nm ³ /min to 0.3 Nm ³ /min per ton of molten iron. Here, "Nm ³ " means the volume of gas in a standard state. In this manual, the standard state of gas is 0°C, 1 atm (101325 Pa). When the decarburization refining is completed, the supply of oxygen gas is stopped, and the supply of the gas containing hydrogen atoms is stopped. After the gas containing hydrogen atoms is stopped, it is preferable to switch to the supply of an inert gas such as argon in order to suppress clogging of the bottom blowing plug. The supply of the gas containing hydrogen atoms is not limited to the porous plug, and it may be supplied using an injection lance (dipping lance), or a single tube or a double tube.

When the treatment is performed so that the nitrogen concentration [N] _f of the molten iron after treatment becomes 30 mass ppm or less, it is possible to produce low-nitrogen steel with a product nitrogen concentration N of 30 mass ppm or less in the rough steel stage such as steel sheet, so it is preferable. Furthermore, when increasing the hydrogen flow rate or using hydrocarbon gas with a large hydrogen content per unit gas volume, etc., adjust the treatment conditions so that the supply of hydrogen atoms increases, and perform treatment so that the nitrogen concentration [N] _f of the molten iron after treatment is When it is 20 mass ppm or less, it becomes an ultra-low nitrogen steel, which is more preferable.

As the fourth step, it is preferable to carry out vacuum degassing treatment after the completion of the decarburization refining, and to carry out casting after adjusting to other predetermined components. Dehydrogenation can be performed by performing vacuum degassing treatment after decarburization refining. Also in this embodiment, compared with the technique described in the patent document 3 which supplies the gas containing a hydrogen atom in a vacuum degassing process, productivity fall can be suppressed. For the vacuum degassing treatment, an RH-type vacuum processing device or a DH-type vacuum processing device, a device with a ladle installed in a vacuum chamber, or the like can be used. [Example]

A 150-ton-scale electric furnace is charged with scrap or reduced iron as a source of cold iron and melted, and the molten slag is removed after being discharged into a ladle. The reduced iron used in the test was produced by reduction with natural gas, and when the carbon concentration was analyzed, it was 1.0% by mass. The molten iron in the pouring bucket after the melt is discharged is put into the pot in the converter and the blast furnace pig iron is melted, and the amount of molten iron is adjusted to 300 t. After the component analysis of the molten iron is carried out, it is loaded into a converter for decarburization blowing. The amount of carbon contained in the blast furnace pig iron used in the combined melt was 4.3% by mass. Various changes were made to the blending ratio of the molten iron in which the cold iron source was melted and the blast furnace pig iron, and the carbon concentration [C] _i (mass %) at the time of charging the converter was also changed in various ways. The oxygen required for decarburization is supplied from the top blowing lance, and the oxygen supply amount is determined based on the carbon and other analytical values (expressed by subscript i) in the molten iron before the converter is charged. Oxygen supply was started, and hydrogen gas, propane gas, or 50 vol% hydrogen-50 vol% propane mixed gas was supplied from a porous plug provided on the bottom of the converter.

After the specified oxygen supply is completed, the supply of hydrogen, propane gas or hydrogen-propane mixed gas is stopped, the bottom blowing gas is converted into argon gas and the steel is tapped into the ladle, and the composition analysis of the molten steel is carried out (using the following marked f expression). Afterwards, the ladle is vacuum-processed in a vacuum degasser, and the casting is performed after adjusting to a predetermined composition.

As comparative conditions, a test was performed under the conditions of supplying argon gas as a bottom blowing gas during decarburization refining in a converter. In addition, on the basis of supplying only argon gas by bottom blowing during decarburization refining in the converter, after tapping the steel into the ladle, supply hydrogen or hydrocarbon gas as the reflux gas condition during the vacuum degassing process Test below.

(Experiments 1 to 3) The molten iron obtained by melting the scrap in the electric furnace and the blast furnace pig iron were put into a pot in a converter, and the amount of the molten iron was adjusted to 300 t. The carbon concentration [C] _e when the electric furnace discharges the melt is 0.2% by mass to 0.3% by mass. As a result of changing the mixing ratio of blast furnace pig iron and electric furnace molten iron, the combined carbon concentration [C] _i was 2.5% by mass to 3.5% by mass. The molten iron thus combined is charged into a converter for decarburization refining. While supplying oxygen for decarburization, 40 Nm ³ /min of argon gas was supplied from a porous plug provided on the bottom of the converter. Composition analysis is carried out after tapping from the converter, followed by vacuum degassing treatment. Argon gas was used as the reflux gas at this time. After the degassing treatment is completed, casting is carried out using a continuous casting machine. As a result, the nitrogen concentration [N] _f (ppm by mass) of the tapped steel from the converter and the nitrogen concentration N (ppm by mass) of the crude steel were both low when the carbon concentration [C] _i charged into the converter exceeded 3.0% by mass. However, when the carbon concentration [C] _i charged into the converter is lower than 3.0% by mass, the nitrogen concentration [N] _f of the tapped steel from the converter and the nitrogen concentration N of the crude steel are both high.

(Experiments 4 to 7) The molten iron obtained by melting the scrap in the electric furnace and the blast furnace pig iron were put into a pot in a converter, and the amount of the molten iron was adjusted to 300 t. The carbon concentration [C] _e of the melt discharged from the electric furnace is 0.2% by mass to 0.3% by mass. As a result of changing the mixing ratio of blast furnace pig iron and electric furnace molten iron, the combined carbon concentration [C] _i was 2.5% by mass to 2.8% by mass. The molten iron thus combined is charged into a converter for decarburization refining. While supplying oxygen for decarburization, 40 Nm ³ /min of argon gas was supplied from a porous plug provided on the bottom of the converter. Composition analysis is carried out after tapping from the converter, followed by vacuum degassing treatment. As the reflux gas at this time, hydrogen gas or propane gas was used. After the degassing treatment is completed, casting is carried out using a continuous casting machine. As a result, the nitrogen concentration [N] _f in the steel from the converter was higher, but the denitrification reaction was promoted during the vacuum degassing process, and the nitrogen concentration N in the crude steel was lower. However, the crude steel hydrogen concentration H (ppm by mass) is relatively high.

(Experiments 8 to 11) The molten iron obtained by melting the scrap in the electric furnace and the blast furnace pig iron were put into a pot in the converter, and the amount of the molten iron was adjusted to 300 t. The carbon concentration [C] _e of the melt discharged from the electric furnace is 0.2% by mass to 0.3% by mass. As a result of changing the mixing ratio of blast furnace pig iron and electric furnace molten iron, the combined carbon concentration [C] _i was 2.5% by mass to 2.8% by mass. The molten iron thus combined is charged into a converter for decarburization refining. While supplying oxygen for decarburization, 40 Nm ³ /min of argon gas was supplied from a porous plug provided on the bottom of the converter. Composition analysis is carried out after tapping from the converter, followed by vacuum degassing treatment. As the reflux gas at this time, hydrogen gas or propane gas was used. Component analysis is performed during the vacuum degassing process, and the vacuum process is continued until the hydrogen concentration becomes below the specified concentration. After the degassing treatment is completed, casting is carried out using a continuous casting machine. As a result, the nitrogen concentration [N] _f in the steel from the converter was higher, but the denitrification reaction was promoted during the vacuum degassing process, and the nitrogen concentration N in the crude steel was lower. Furthermore, the crude steel hydrogen concentration H is also low. However, the vacuum degassing process time is greatly increased.

(Experiments 12 to 26) The molten iron obtained by melting the scrap in the electric furnace and the blast furnace pig iron were put into a pot in a converter, and the amount of the molten iron was adjusted to 300 t. The carbon concentration [C] _e of the melt discharged from the electric furnace is 0.2% by mass to 0.3% by mass. As a result of changing the mixing ratio of blast furnace pig iron and electric furnace molten iron, the combined carbon concentration [C] _i was 0.6% by mass to 2.8% by mass. The molten iron thus combined is charged into a converter for decarburization refining. While supplying oxygen for decarburization, 40 Nm ³ /min of hydrogen gas or propane gas or a mixed gas thereof was supplied from a porous plug provided on the bottom of the converter. Composition analysis is carried out after tapping from the converter, followed by vacuum degassing treatment. Argon gas was used as the reflux gas at this time. After the degassing treatment is completed, casting is carried out using a continuous casting machine. As a result, both the converter tapped nitrogen concentration [N] _f and the crude steel nitrogen concentration N were low. The hydrogen concentration [H] _f of the steel from the converter is higher, but the hydrogen concentration H of the crude steel is lower by vacuum degassing. In addition, extension of the vacuum degassing treatment time was not observed.

(Experiments 27 to 41) The molten iron obtained by melting the reduced iron in the electric furnace and the blast furnace pig iron were put into a pot in a converter, and the amount of the molten iron was adjusted to 300 t. The carbon concentration [C] _e of the melt discharged from the electric furnace is 1.0% by mass to 1.1% by mass. The carbon concentration [C] _i of test No.31, No.36, and No.41 in which the blending ratio of blast furnace pig iron and electric furnace molten iron was changed to maintain a molten state was 0.9% by mass, and the carbon concentration [C] of other combined _i is 1.4% by mass to 2.9% by mass. In this way, the molten iron is kept in a molten state, or the combined molten iron is charged into the converter for decarburization and refining. While supplying oxygen for decarburization, 40 Nm ³ /min of hydrogen gas or propane gas or a mixed gas thereof was supplied from a porous plug provided on the bottom of the converter. Composition analysis is carried out after tapping from the converter, followed by vacuum degassing treatment. Argon gas was used as the reflux gas at this time. After the degassing treatment is completed, casting is carried out using a continuous casting machine. As a result, both the converter tapped nitrogen concentration [N] _f and the crude steel nitrogen concentration N were low. The hydrogen concentration [H] _f of the steel from the converter is higher, but the hydrogen concentration H of the crude steel is lower by vacuum degassing. Prolongation of the vacuum degassing treatment time was also not observed.

The above test conditions and results are collectively shown in Tables 1-1 to 1-3. The composition of the product in the table is the value collected from the cast slab as the crude steel composition and subjected to composition analysis.

[Table 1-1] No. cold iron source When the electric furnace discharges the melt Combined melt of blast furnace pig iron Converter loading Bottom-blown gas species during decarburization When the converter is tapping Vacuum degassing Product composition exam preparation [C] _e [C] _i [N] _f [H] _f Reflux gas species processing time N h quality% quality% massppm massppm massppm massppm 1 scrap 0.2 have 3.5 argon twenty three 4 argon 25 25 2 comparative example 2 scrap 0.2 have 2.8 argon 35 3 argon 25 38 2 comparative example 3 scrap 0.3 have 2.5 argon 39 4 argon 25 43 2 comparative example 4 scrap 0.2 have 2.8 argon 34 3 hydrogen 25 26 9 comparative example 5 scrap 0.3 have 2.5 argon 37 4 hydrogen 25 twenty four 9 comparative example 6 scrap 0.2 have 2.8 argon 34 3 propane 25 17 9 comparative example 7 scrap 0.3 have 2.5 argon 37 4 propane 25 18 9 comparative example 8 scrap 0.2 have 2.8 argon 34 3 hydrogen 40 27 1 comparative example 9 scrap 0.3 have 2.5 argon 37 4 hydrogen 40 twenty three 2 comparative example 10 scrap 0.2 have 2.8 argon 34 3 propane 40 19 1 comparative example 11 scrap 0.3 have 2.5 argon 37 4 propane 40 18 1 comparative example 12 scrap 0.2 have 2.8 hydrogen twenty two 8 argon 25 twenty four 1 Invention example 13 scrap 0.3 have 2.5 hydrogen twenty one 8 argon 25 twenty three 2 Invention example 14 scrap 0.3 have 1.7 hydrogen twenty four 8 argon 25 26 2 Invention example 15 scrap 0.2 have 1.2 hydrogen twenty one 9 argon 25 twenty three 1 Invention example

[Table 1-2] No. cold iron source When the electric furnace discharges the melt Combined melt of blast furnace pig iron Converter loading Bottom-blown gas species during decarburization When the converter is tapping Vacuum degassing Product composition exam preparation [C] _e [C] _i [N] _f [H] _f Reflux gas species processing time N h quality% quality% massppm massppm massppm massppm 16 scrap 0.3 have 0.6 hydrogen twenty three 7 argon 25 25 2 Invention example 17 scrap 0.2 have 2.8 propane 16 8 argon 25 18 1 Invention example 18 scrap 0.3 have 2.5 propane 17 8 argon 25 18 2 Invention example 19 scrap 0.3 have 1.7 propane 18 8 argon 25 19 2 Invention example 20 scrap 0.2 have 1.2 propane 18 9 argon 25 20 1 Invention example twenty one scrap 0.3 have 0.6 propane 17 7 argon 25 18 2 Invention example twenty two scrap 0.2 have 2.8 50% hydrogen-50% propane mix 17 8 argon 25 18 1 Invention example twenty three scrap 0.3 have 2.5 50% hydrogen-50% propane mix 18 7 argon 25 19 1 Invention example twenty four scrap 0.3 have 1.7 50% hydrogen-50% propane mix 18 8 argon 25 19 2 Invention example 25 scrap 0.2 have 1.2 50% hydrogen-50% propane mix 19 9 argon 25 20 1 Invention example 26 scrap 0.3 have 0.6 50% hydrogen-50% propane mix 19 8 argon 25 20 2 Invention example 27 Reduced iron 1.1 have 2.9 hydrogen twenty three 8 argon 25 twenty four 1 Invention example 28 Reduced iron 1.0 have 2.4 hydrogen 25 8 argon 25 27 2 Invention example 29 Reduced iron 1.1 have 1.7 hydrogen 25 8 argon 25 28 2 Invention example 30 Reduced iron 1.1 have 1.4 hydrogen twenty four 9 argon 25 25 1 Invention example

[Table 1-3] No. cold iron source When the electric furnace discharges the melt Combined melt of blast furnace pig iron Converter loading Bottom-blown gas species during decarburization When the converter is tapping Vacuum degassing Product composition exam preparation [C] _e [C] _i [N] _f [H] _f Reflux gas species processing time N h quality% quality% massppm massppm massppm massppm 31 Reduced iron 1.0 none 0.9 hydrogen 25 7 argon 25 27 2 Invention example 32 Reduced iron 1.1 have 2.9 propane 16 8 argon 25 18 1 Invention example 33 Reduced iron 1.0 have 2.4 propane 17 8 argon 25 19 2 Invention example 34 Reduced iron 1.1 have 1.7 propane 17 8 argon 25 19 2 Invention example 35 Reduced iron 1.1 have 1.4 propane 18 9 argon 25 19 1 Invention example 36 Reduced iron 1.0 none 0.9 propane 18 9 argon 25 19 2 Invention example 37 Reduced iron 1.1 have 2.9 50% hydrogen-50% propane mix 18 8 argon 25 19 1 Invention example 38 Reduced iron 1.0 have 2.4 50% hydrogen-50% propane mix 18 9 argon 25 19 1 Invention example 39 Reduced iron 1.1 have 1.7 50% hydrogen-50% propane mix 18 8 argon 25 19 1 Invention example 40 Reduced iron 1.1 have 1.4 50% hydrogen-50% propane mix 17 9 argon 25 19 2 Invention example 41 Reduced iron 1.0 none 0.9 50% hydrogen-50% propane mix 19 9 argon 25 20 1 Invention example [Industrial availability]

According to the refining method of molten iron of the present invention, under the condition that the amount of cold iron source used increases, there is no obvious reduction in productivity or increase in cost, and it does not increase the amount of slag produced or the amount of _CO2 produced, and can Low-nitrogen steel with a nitrogen concentration of 30 mass ppm or less can be stably produced. It is industrially useful because blast furnace pig iron and chilled iron sources can be used together in existing steel mills with consistent operations, and it can reduce CO ₂ emissions and produce high-grade steel at the same time.

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Claims (7)

A method for refining molten iron, comprising accommodating untreated molten iron having a carbon concentration [C] _i of 0.5% by mass to 3.0% by mass in a container, blowing oxygen into the untreated molten iron under atmospheric pressure, and The decarburization and denitrogenation treatment of the molten iron before the treatment is carried out by blowing hydrogen gas or hydrocarbon gas or these mixed gases. The method for refining molten iron according to claim 1, wherein the nitrogen concentration [N] _f of the treated molten iron after the decarburization and denitrification treatment is set to be 30 mass ppm or less. The method for refining molten iron according to Claim 1 or Claim 2, wherein the treated molten iron after the decarburization and denitrogenation treatments are further subjected to vacuum degassing treatment. The method for refining molten iron according to any one of claim 1 to claim 3, wherein the pre-treatment molten iron includes one obtained by melting cold iron sources. The method for refining molten iron as described in any one of claim 1 to claim 3, wherein the molten iron before treatment is mixed with primary molten iron obtained by melting chilled iron from a melting furnace, and the carbon concentration is 2.0% by mass or more of molten iron. The method for refining molten iron according to Claim 4 or Claim 5, wherein the cold iron source contains reduced iron. The method for refining molten iron according to any one of claim 1 to claim 6, wherein the container is a converter.