METHOD FOR REFINING MOLTEN IRON

Abstract

A method for refining molten iron that can stably produce low-nitrogen steel is proposed. In this method for refining molten iron, untreated molten iron with a carbon concentration [C].sub.i between 0.5 mass % and 3.0 mass %, both inclusive, is placed into a vessel, and oxygen is blown onto the untreated molten iron under atmospheric pressure while a hydrogen gas, a hydrocarbon gas, or a mixture gas of these gases is blown in to perform a decarburization and denitrification treatment of the untreated molten iron. It is preferable, for example, that a nitrogen concentration [N].sub.f in treated molten iron after being subjected to the decarburization and denitrification treatment be 30 mass ppm or lower; that treated molten iron after being subjected to the decarburization and denitrification treatment be further subjected to a vacuum degassing treatment; that the untreated molten iron include molten iron obtained by melting a cold iron source; that the untreated molten iron be a mixture of primary molten iron obtained by melting a cold iron source in a melting furnace and molten pig iron having a carbon concentration of 2.0 mass % or higher; that the cold iron source include reduced iron; and that the vessel be a converter.

Claims

1. A method for refining molten iron, wherein untreated molten iron with a carbon concentration [C].sub.i between 0.5 mass % and 3.0 mass %, both inclusive, is placed into a vessel, and oxygen is blown onto the untreated molten iron under atmospheric pressure while a hydrogen gas, a hydrocarbon gas, or a mixture gas of these gases is blown in to perform a decarburization and denitrification treatment of the untreated molten iron.

2. The method for refining molten iron according to claim 1, wherein a nitrogen concentration [N].sub.f in treated molten iron after being subjected to the decarburization and denitrification treatment is 30 mass ppm or lower.

3. The method for refining molten iron according to claim 1, wherein treated molten iron after being subjected to the decarburization and denitrification treatment is further subjected to a vacuum degassing treatment.

4. The method for refining molten iron according to claim 1, wherein the untreated molten iron includes molten iron obtained by melting a cold iron source.

5. The method for refining molten iron according to claim 1, wherein the untreated molten iron is a mixture of primary molten iron obtained by melting a cold iron source in a melting furnace and molten pig iron having a carbon concentration of 2.0 mass % or higher.

6. The method for refining molten iron according to claim 4, wherein the cold iron source includes reduced iron.

7. The method for refining molten iron according to claim 1, wherein the vessel is a converter.

8. The method for refining molten iron according to claim 2, wherein treated molten iron after being subjected to the decarburization and denitrification treatment is further subjected to a vacuum degassing treatment.

9. The method for refining molten iron according to claim 2, wherein the untreated molten iron includes molten iron obtained by melting a cold iron source.

10. The method for refining molten iron according to claim 3, wherein the untreated molten iron includes molten iron obtained by melting a cold iron source.

11. The method for refining molten iron according to claim 8, wherein the untreated molten iron includes molten iron obtained by melting a cold iron source.

12. The method for refining molten iron according to claim 2, wherein the untreated molten iron is a mixture of primary molten iron obtained by melting a cold iron source in a melting furnace and molten pig iron having a carbon concentration of 2.0 mass % or higher.

13. The method for refining molten iron according to claim 3, wherein the untreated molten iron is a mixture of primary molten iron obtained by melting a cold iron source in a melting furnace and molten pig iron having a carbon concentration of 2.0 mass % or higher.

14. The method for refining molten iron according to claim 8, wherein the untreated molten iron is a mixture of primary molten iron obtained by melting a cold iron source in a melting furnace and molten pig iron having a carbon concentration of 2.0 mass % or higher.

15. The method for refining molten iron according to claim 5, wherein the cold iron source includes reduced iron.

Description

DESCRIPTION OF EMBODIMENTS

[0026] An embodiment of the present invention will be specifically described below.

[0027] As a first step, in a steelmaking melting furnace, an iron source is melted and heated using electric energy. Here, as the steelmaking melting furnace, an electric furnace, such as an arc furnace or an induction furnace, can be used. In this case, as the iron source, not only a solid iron source, such as scrap or reduced iron, but also molten iron that has been melted by another process may be used. As the heat energy supplied to melt the solid iron source and heat the iron source, not only electric energy but also combustion heat of metal etc. may be supplementarily used. It is preferable that these energies be renewable energies from the viewpoint of cutting down on CO.sub.2 emissions.

[0028] As a second step, the molten iron is discharged into a vessel, such as a ladle. When reduced iron is used as the cold iron source, a large amount of slag attributable to gangue contained in the reduced iron is generated. Therefore, performing slag removal as necessary is desirable. Slag removal may be performed using a slag dragger or the like. When the height of the freeboard in the ladle (the height from the upper end of the ladle to the surface of the molten iron) is insufficient, before the molten iron is discharged from the electric furnace, the furnace body may be tilted to flow out the slag. Alternatively, before the molten iron is discharged from the electric furnace, the furnace body may be tilted to flow out the slag, and then the slag flown into a vessel, such as a ladle, along with the molten iron may be further removed.

[0029] As a third step, the molten iron is mixed with molten pig iron, such as blast-furnace molten pig iron, as necessary to adjust the carbon concentration [C].sub.i in the molten iron to between 0.5 mass % and 3.0 mass %, both inclusive. Then, this molten iron is charged into a reaction vessel, and decarburization refining is performed by supplying an oxygen gas through a top-blowing lance etc. When the carbon concentration [C].sub.i in the untreated molten iron is lower than 0.5 mass %, denitrification may become insufficient due to the small amount of CO gas generated during decarburization. On the other hand, when the carbon concentration exceeds 3.0 mass %, the reducing effect on CO.sub.2 generation lessens. In the case where molten metals are mixed, the molten pig iron to be mixed is preferable to have a carbon concentration of 2.0 mass % or higher, and may be molten pig iron as discharged from a blast furnace, or may be molten pig iron that has undergone one of desiliconization, dephosphorization, and desulfurization or a combination of two or more of these treatments after being discharged from a blast furnace. As the reaction vessel, a converter is preferable in terms of the height of the freeboard (the height from the upper end of the reaction vessel to the surface of the molten iron). The reaction vessel should be a vessel in which oxygen blowing is possible, and may also be a ladle or the like. Oxygen blowing is not limited to a method of supplying oxygen through a top-blowing lance, and oxygen may instead be supplied through a bottom-blowing tuyere. A combination of supplying oxygen through a top-blowing lance and supplying oxygen through a bottom-blowing tuyere may be adopted.

[0030] Next, at the same time as supply of an oxygen gas for decarburization is started, a gas containing hydrogen atoms formed by a hydrogen gas or a hydrocarbon gas or a mixture gas of these gases is supplied through a porous plug etc. installed at the bottom of the furnace. It is believed that when a gas containing hydrogen atoms is supplied into molten iron, after a dissociation reaction of gas molecules occurs, hydrogen atoms dissolve temporarily into the molten iron and are then generated as fine hydrogen gas bubbles again. The denitrification reaction is believed to progress between the fine bubbles generated here and the molten iron interface. Therefore, when performing decarburization refining using molten iron obtained by melting a cold iron source, even when the amount of bubbles of carbon monoxide generated is insufficient, the nitrogen concentration after decarburization refining can be reduced. Thus, decarburization and denitrification can be simultaneously performed. As a result of vigorously conducting studies, the present inventors have found that, for the amount to be supplied of the gas containing hydrogen atoms, a flow rate of about 0.1 to 0.3 Nm.sup.3/min per ton of molten iron is appropriate. Here, Nm.sup.3 means a volume of a gas in a normal state. In this Description, the normal state of a gas is 0? C. and 1 atm (101325 Pa). Upon completion of decarburization refining, supply of the oxygen gas is stopped and, at the same time, supply of the gas containing hydrogen atoms is stopped. To prevent clogging of the bottom-blowing plug, it is preferable that supply of the gas containing hydrogen atoms, after it is stopped, be switched to supply of an inert gas, such as an argon gas. The gas containing hydrogen atoms is not limited to being supplied through a porous plug and may instead be supplied using an injection lance (immersion lance), a single pipe, or a double pipe.

[0031] Performing a treatment such that the nitrogen concentration [N] in the treated molten iron becomes 30 mass ppm or lower is preferable, because then low-nitrogen steel with a nitrogen concentration N in the product at the stage of crude steel, such as a steel slab, of 30 mass ppm or lower can be produced. Performing a treatment such that the nitrogen concentration [N]f in the treated molten iron becomes 20 mass ppm or lower through adjustment of the treatment conditions so as to increase the amount of hydrogen atoms supplied, for example, by increasing the flow rate of the hydrogen gas or by using a hydrocarbon-based gas containing a large amount of hydrogen per gas volume is further preferable, because this results in extremely low-nitrogen steel.

[0032] As a fourth step, upon completion of the decarburization refining, a vacuum degassing treatment is performed, and casting is performed preferably after being otherwise adjusted to a predetermined composition. By performing a vacuum degassing treatment after decarburization refining, hydrogen can be removed. This embodiment can prevent a decrease in productivity compared with the technology described in Patent Literature 3 in which a gas containing hydrogen atoms is supplied in a vacuum degassing treatment. For the vacuum degassing treatment, an RH vacuum treatment device, a DH vacuum treatment device, a facility with a ladle installed inside a vacuum chamber, etc. can be used.

EXAMPLES

[0033] Scrap or reduced iron as a cold iron source was charged into a 150 t-scale electric furnace and melted. After the molten iron was discharged into a ladle, slag removal was performed. The reduced iron used in the test was reduced iron produced through reduction with a natural gas, and the carbon concentration was analyzed to be 1.0 mass %. The discharged molten iron inside the ladle and blast-furnace molten pig iron were mixed in a converter-charging pot so as to adjust the amount of molten iron to 300 t. After the components of the molten iron were analyzed, the molten iron was charged into a converter and subjected to decarburization blowing. The amount of carbon contained in the blast-furnace molten pig iron used as the molten metal to be mixed was 4.3 mass %. The mixing ratio between the molten iron obtained by melting a cold iron source and the blast-furnace molten pig iron were changed to various ratios, and the carbon concentration [C].sub.i (mass %) at the time of charging into the converter was also changed to various concentrations. An oxygen gas needed for decarburization was supplied through a top-blowing lance, and the amount of oxygen gas to be supplied was determined based on analytical values (represented with a suffix i) of carbon and others in the molten iron before being charged into the converter. At the same time as supply of the oxygen gas was started, a hydrogen gas, a propane gas, or a mixture gas of 50 vol % hydrogen and 50 vol % propane was supplied through a porous plug installed at the bottom of the converter.

[0034] After the predetermined amount of oxygen was supplied, supply of a hydrogen gas, a propane gas, or a mixture gas of hydrogen and propane was stopped, and the bottom-blown gas was switched to an argon gas. The molten steel was discharged into a ladle, and the components of the molten steel were analyzed (represented with a suffix f). Thereafter, the ladle was subjected to a vacuum treatment in a vacuum degassing device, and the molten steel was cast after being adjusted to a predetermined composition.

[0035] A test was conducted under, as comparative conditions, conditions where an argon gas was supplied as a bottom-blown gas during decarburization refining in a converter. Further, a test was conducted under conditions where only an argon gas was supplied by being bottom-blown during decarburization refining in a converter, and after the molten steel was discharged into a ladle, a hydrogen gas or a hydrocarbon gas was supplied as a circulating gas during a vacuum degassing treatment.

(Tests 1 to 3)

[0036] Molten iron obtained by melting scrap in an electric furnace and blast-furnace molten pig iron were mixed in a converter-charging pot so as to adjust the amount of the mixed molten iron to 300 t. The carbon concentration [C].sub.e of the molten iron at the time of discharging from the electric furnace was 0.2 to 0.3 mass %. When the mixing ratio between the blast-furnace molten pig iron and the electric-furnace molten iron was changed, the carbon concentration [C].sub.i after the mixing was 2.5 to 3.5 mass %. The molten iron thus mixed was charged into a converter and subjected to decarburization refining. While an oxygen gas for decarburization was being supplied, an argon gas was supplied at 40 Nm.sup.3/min through a porous plug installed at the bottom of the converter. After the molten steel was discharged from the converter, the components were analyzed, and further a vacuum degassing treatment was performed. As the circulating gas in this case, an argon gas was used. Upon completion of the degassing treatment, casting was performed using a continuous casting machine.

[0037] As a result, under a condition where the carbon concentration [C].sub.i at the time of charging into the converter exceeded 3.0 mass %, both the nitrogen concentration [N].sub.f (mass ppm) at the time of discharging from the converter and the crude steel nitrogen concentration N (mass ppm) were low. However, when the carbon concentration [C].sub.i at the time of charging into the converter is at a level below 3.0 mass %, both the nitrogen concentration [N].sub.f at the time of discharging from the converter and the crude steel nitrogen concentration N were high.

(Tests 4 to 7)

[0038] Molten iron obtained by melting scrap in an electric furnace and blast-furnace molten pig iron were mixed in a converter-charging pot so as to adjust the amount of the mixed molten iron to 300 t. The carbon concentration [C].sub.e of the molten iron at the time of discharging from the electric furnace was 0.2 to 0.3 mass %. When the mixing ratio between the blast-furnace molten pig iron and the electric-furnace molten iron was changed, the carbon concentration [C].sub.i after the mixing was 2.5 to 2.8 mass %. The molten iron thus mixed was charged into a converter and subjected to decarburization refining. While an oxygen gas for decarburization was being supplied, an argon gas was supplied at 40 Nm.sup.3/min through a porous plug installed at the bottom of the converter. After the molten steel was discharged from the converter, the components were analyzed, and further a vacuum degassing treatment was performed. As the circulating gas in this case, a hydrogen gas or a propane gas was used. Upon completion of the degassing treatment, casting was performed using a continuous casting machine.

[0039] As a result, while the nitrogen concentration [N].sub.f of the molten steel at the time of discharging from the converter was high, the crude steel nitrogen concentration N was low owing to the denitrification reaction being promoted during the vacuum degassing treatment. However, the crude steel hydrogen concentration H (mass ppm) was high.

(Tests 8 to 11)

[0040] Molten iron obtained by melting scrap in an electric furnace and blast-furnace molten pig iron were mixed in a converter-charging pot so as to adjust the amount of the mixed molten iron to 300 t. The carbon concentration [C].sub.e of the molten iron at the time of discharging from the electric furnace was 0.2 to 0.3 mass %. When the mixing ratio between the blast-furnace molten pig iron and the electric-furnace molten iron was changed, the carbon concentration [C].sub.i after the mixing was 2.5 to 2.8 mass %. The molten iron thus mixed was charged into a converter and subjected to decarburization refining. While an oxygen gas for decarburization was being supplied, an argon gas was supplied at 40 Nm.sup.3/min through a porous plug installed at the bottom of the converter. After the molten steel was discharged from the converter, the components were analyzed, and further a vacuum degassing treatment was performed. As the circulating gas in this case, a hydrogen gas or a propane gas was used. The components were analyzed during the vacuum degassing treatment, and the vacuum treatment was continued until the hydrogen concentration became equal to or lower than a predetermined concentration. Upon completion of the degassing treatment, casting was performed using a continuous casting machine.

[0041] As a result, while the nitrogen concentration [N].sub.f of the molten steel at the time of discharging from the converter was high, the crude steel nitrogen concentration N was low owing to the denitrification reaction being promoted during the vacuum degassing treatment. Further, the crude steel hydrogen concentration H was also low. However, the vacuum degassing treatment time increased significantly.

(Tests 12 to 26)

[0042] Molten iron obtained by melting scrap in an electric furnace and blast-furnace molten pig iron were mixed in a converter-charging pot so as to adjust the amount of the mixed molten iron to 300 t. The carbon concentration [C].sub.e of the molten iron at the time of discharging from the electric furnace was 0.2 to 0.3 mass %. When the mixing ratio between the blast-furnace molten pig iron and the electric-furnace molten iron was changed, the carbon concentration [C].sub.i after the mixing was 0.6 to 2.8 mass %. The molten iron thus mixed was charged into a converter and subjected to decarburization refining. While an oxygen gas for decarburization was being supplied, a hydrogen gas or a propane gas or a mixture gas of these gases was supplied at 40 Nm.sup.3/min through a porous plug installed at the bottom of the converter. After the molten steel was discharged from the converter, the components were analyzed, and further a vacuum degassing treatment was performed. As the circulating gas in this case, an argon gas was used. Upon completion of the degassing treatment, casting was performed using a continuous casting machine.

[0043] As a result, both the nitrogen concentration [N].sub.f of the molten steel at the time of discharging from the converter and the crude steel nitrogen concentration N were low. While the hydrogen concentration [H].sub.f of the molten steel at the time of discharging from the converter was high, the crude steel hydrogen concentration H was low as a result of performing the vacuum degassing treatment. The vacuum degassing treatment time was found not to be prolonged.

(Tests 27 to 41)

[0044] Molten iron obtained by melting reduced iron in an electric furnace and blast-furnace molten pig iron were mixed in a converter-charging pot so as to adjust the amount of the mixed molten iron to 300 t. The carbon concentration [C].sub.e of the molten iron at the time of discharging from the electric furnace was 1.0 to 1.1 mass %. When the mixing ratio between the blast-furnace molten pig iron and the electric-furnace molten iron was changed, the carbon concentrations [C].sub.i in tests No. 31, 36, and 41 in which unmixed molten iron was used were 0.9 mass %, while the carbon concentrations [C].sub.i of other mixed molten irons were 1.4 to 2.9 mass %. Thus, unmixed molten iron or mixed molten iron was charged into a converter and subjected to decarburization refining. While an oxygen gas for decarburization was being supplied, a hydrogen gas or a propane gas or a mixture gas of these gases was supplied at 40 Nm.sup.3/min through a porous plug installed at the bottom of the converter. After the molten steel was discharged from the converter, the components were analyzed, and further a vacuum degassing treatment was performed. As the circulating gas in this case, an argon gas was used. Upon completion of the degassing treatment, casting was performed using a continuous casting machine.

[0045] As a result, both the nitrogen concentration [N].sub.f of the molten steel at the time of discharging from the converter and the crude steel nitrogen concentration N were low. While the hydrogen concentration [H].sub.f of the molten steel at the time of discharging from the converter was high, the crude steel hydrogen concentration H was low as a result of performing the vacuum degassing treatment. The vacuum degassing treatment time was found not to be prolonged.

[0046] The test conditions and results having been described above are collectively shown in Tables 1-1 to 1-3. The product components in these tables represent values obtained by analyzing the components sampled from a cast steel slab as crude steel components.

TABLE-US-00001 TABLE 1-1 When discharged Mixed When from with charged When discharged Vacuum Product Type of electric blast- into from converter degassing treatment components cold furnace furnace converter Type of bottom- [N]f [H]f Type of N H iron [C]e molten [C]i blown gas during mass mass circulating Treatment mass mass No. source mass % pig iron mass % decarburization ppm ppm gas time ppm ppm Remarks 1 Scrap 0.2 Mixed 3.5 Argon 23 4 Argon 25 25 2 Comparative Example 2 Scrap 0.2 Mixed 2.8 Argon 35 3 Argon 25 38 2 Comparative Example 3 Scrap 0.3 Mixed 2.5 Argon 39 4 Argon 25 43 2 Comparative Example 4 Scrap 0.2 Mixed 2.8 Argon 34 3 Hydrogen 25 26 9 Comparative Example 5 Scrap 0.3 Mixed 2.5 Argon 37 4 Hydrogen 25 24 9 Comparative Example 6 Scrap 0.2 Mixed 2.8 Argon 34 3 Propane 25 17 9 Comparative Example 7 Scrap 0.3 Mixed 2.5 Argon 37 4 Propane 25 18 9 Comparative Example 8 Scrap 0.2 Mixed 2.8 Argon 34 3 Hydrogen 40 27 1 Comparative Example 9 Scrap 0.3 Mixed 2.5 Argon 37 4 Hydrogen 40 23 2 Comparative Example 10 Scrap 0.2 Mixed 2.8 Argon 34 3 Propane 40 19 1 Comparative Example 11 Scrap 0.3 Mixed 2.5 Argon 37 4 Propane 40 18 1 Comparative Example 12 Scrap 0.2 Mixed 2.8 Hydrogen 22 8 Argon 25 24 1 Invention Example 13 Scrap 0.3 Mixed 2.5 Hydrogen 21 8 Argon 25 23 2 Invention Example 14 Scrap 0.3 Mixed 1.7 Hydrogen 24 8 Argon 25 26 2 Invention Example 15 Scrap 0.2 Mixed 1.2 Hydrogen 21 9 Argon 25 23 1 Invention Example

TABLE-US-00002 TABLE 1-2 When discharged Mixed When from with charged When discharged Vacuum Product Type of electric blast- into from converter degassing treatment components cold furnace furnace converter Type of bottom- [N]f [H]f Type of N H iron [C]e molten [C]i blown gas during mass mass circulating Treatment mass mass No. source mass % pig iron mass % decarburization ppm ppm gas time ppm ppm Remarks 16 Scrap 0.3 Mixed 0.6 Hydrogen 23 7 Argon 25 25 2 Invention Example 17 Scrap 0.2 Mixed 2.8 Propane 16 8 Argon 25 18 1 Invention Example 18 Scrap 0.3 Mixed 2.5 Propane 17 8 Argon 25 18 2 Invention Example 19 Scrap 0.3 Mixed 1.7 Propane 18 8 Argon 25 19 2 Invention Example 20 Scrap 0.2 Mixed 1.2 Propane 18 9 Argon 25 20 1 Invention Example 21 Scrap 0.3 Mixed 0.6 Propane 17 7 Argon 25 18 2 Invention Example 22 Scrap 0.2 Mixed 2.8 Mixture of 50% 17 8 Argon 25 18 1 Invention Example hydrogen and 50% propane 23 Scrap 0.3 Mixed 2.5 Mixture of 50% 18 7 Argon 25 19 1 Invention Example hydrogen and 50% propane 24 Scrap 0.3 Mixed 1.7 Mixture of 50% 18 8 Argon 25 19 2 Invention Example hydrogen and 50% propane 25 Scrap 0.2 Mixed 1.2 Mixture of 50% 19 9 Argon 25 20 1 Invention Example hydrogen and 50% propane 26 Scrap 0.3 Mixed 0.6 Mixture of 50% 19 8 Argon 25 20 2 Invention Example hydrogen and 50% propane 27 Reduced 1.1 Mixed 2.9 Hydrogen 23 8 Argon 25 24 1 Invention Example iron 28 Reduced 1.0 Mixed 2.4 Hydrogen 25 8 Argon 25 27 2 Invention Example iron 29 Reduced 1.1 Mixed 1.7 Hydrogen 25 8 Argon 25 28 2 Invention Example iron 30 Reduced 1.1 Mixed 1.4 Hydrogen 24 9 Argon 25 25 1 Invention Example iron

TABLE-US-00003 TABLE 1-3 When discharged Mixed When from with charged When discharged Vacuum Product Type of electric blast- into from converter degassing treatment components cold furnace furnace converter Type of bottom- [N]f [H]f Type of N H iron [C]e molten [C]i blown gas during mass mass circulating Treatment mass mass No. source mass % pig iron mass % decarburization ppm ppm gas time ppm ppm Remarks 31 Reduced 1.0 Not 0.9 Hydrogen 25 7 Argon 25 27 2 Invention Example iron mixed 32 Reduced 1.1 Mixed 2.9 Propane 16 8 Argon 25 18 1 Invention Example iron 33 Reduced 1.0 Mixed 2.4 Propane 17 8 Argon 25 19 2 Invention Example iron 34 Reduced 1.1 Mixed 1.7 Propane 17 8 Argon 25 19 2 Invention Example iron 35 Reduced 1.1 Mixed 1.4 Propane 18 9 Argon 25 19 1 Invention Example iron 36 Reduced 1.0 Not 0.9 Propane 18 9 Argon 25 19 2 Invention Example iron mixed 37 Reduced 1.1 Mixed 2.9 Mixture of 50% 18 8 Argon 25 19 1 Invention Example iron hydrogen and 50% propane 38 Reduced 1.0 Mixed 2.4 Mixture of 50% 18 9 Argon 25 19 1 Invention Example iron hydrogen and 50% propane 39 Reduced 1.1 Mixed 1.7 Mixture of 50% 18 8 Argon 25 19 1 Invention Example iron hydrogen and 50% propane 40 Reduced 1.1 Mixed 1.4 Mixture of 50% 17 9 Argon 25 19 2 Invention Example iron hydrogen and 50% propane 41 Reduced 1.0 Not 0.9 Mixture of 50% 19 9 Argon 25 20 1 Invention Example iron mixed hydrogen and 50% propane

INDUSTRIAL APPLICABILITY

[0047] The method for refining molten iron according to the present invention can stably produce low-nitrogen steel with a nitrogen concentration of 30 mass ppm or lower under the condition of an increased amount of cold iron source used, without a significant decrease in productivity or cost increase, and without adding to the amount of slag generated or the amount of CO.sub.2 generated. This method is industrially useful in that it allows existing integrated ironworks to reduce CO.sub.2 emissions and produce high-grade steels at the same time while using blast-furnace molten pig iron and a cold iron source in combination.