METHOD FOR SECONDARILY REFINING MOLTEN STEEL AND METHOD OF PRODUCING STEEL

Abstract

A method for secondarily refining molten steel s by which a low nitrogen concentration range can be stably reached at high speed without causing operational problems, and a method of producing steel are proposed. In the method for secondarily refining molten steel, CaO-and-Al.sub.2O.sub.3-containing slag is formed by a combination of an Al addition step of adding a metal-Al-containing substance to molten steel to turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the molten steel, and then an oxygen blowing process including a denitrification process is performed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel, in which an Al concentration [Al].sub.i (mass %) in the molten steel immediately before the oxygen blowing process is equal to or higher than a value [Al].sub.e calculated by Formula (A) based on a stirring power density ? (W/t) during formation of the slag, and an Al concentration [Al].sub.f upon completion of the oxygen blowing process is 0.03 mass % or higher. In the method of producing steel, the obtained molten steel is cast after adjusting the composition.

[Al].sub.e=?0.072?In(?)+0.5822 . . . (A)

Claims

1. A method for secondarily refining molten steel wherein CaO-and-Al.sub.2O.sub.3-containing slag is formed by a combination of an Al addition step of adding a metal-Al-containing substance to molten steel to turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the molten steel, and then an oxygen blowing process including a denitrification process is performed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel, characterized in that: an Al concentration [Al].sub.i (mass %) in the molten steel immediately before the oxygen blowing process is equal to or higher than a value [Al].sub.e calculated by Formula (A) based on a stirring power density ? (W/t) during formation of the slag; and an Al concentration [Al].sub.f upon completion of the oxygen blowing process is 0.03 mass % or higher,
[Al].sub.e=?0.072?In(?)+0.5822 (A).

2. The method for secondarily refining molten steel according to claim 1, wherein the Al addition step includes a step of adding a metal-Al-containing substance to the molten steel to turn the molten steel into deoxidized molten steel.

3. The method for secondarily refining molten steel according to claim 1, wherein, in the oxygen blowing process, the oxygen-containing gas is supplied such that a ratio L.sub.s/L.sub.s0 between a thickness L.sub.s0 of the slag and a depth L.sub.s of a depression resulting from the blowing of the oxygen-containing gas is 1.0 or higher.

4. The method for secondarily refining molten steel according to claim 1, wherein a mass ratio C/A (?) between a CaO concentration (CaO) (mass %) and an Al.sub.2O.sub.3 concentration (Al.sub.2O.sub.3) (mass %) in the slag is between 0.4 and 2.2, both inclusive.

5. The method for secondarily refining molten steel according to claim 1, wherein a mass ratio of an MgO concentration (MgO) (mass %) to a CaO concentration (CaO) (mass %) in the slag is 0.25 or lower.

6. The method for secondarily refining molten steel according to claim 1, wherein, in the oxygen blowing process, surfaces of the Al-containing molten steel and the slag are subjected to a depressurized atmosphere of 9.3?10.sup.4 Pa or lower.

7. A method for secondarily refining molten steel in which CaO-and-Al.sub.2O.sub.3-containing slag is formed on top of Al-containing molten steel charged in a vessel, and nitrogen and sulfur in the molten steel are removed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel and bringing the slag and the molten steel into contact with each other, characterized in that, an oxygen blowing process according to the method for secondarily refining molten steel according to claim 1 is performed, and during the oxygen blowing process, an Al concentration in the molten steel is kept at 0.05 mass % or higher, and a ratio C/A (?) between a CaO concentration (mass %) and an Al.sub.2O.sub.3 concentration (mass %) in the slag is controlled between 1.8 and 2.2, both inclusive.

8. A method of producing steel characterized in that molten steel produced by the method for secondarily refining molten steel according to claim 1 is cast after optionally adjusting the composition.

9. A method for secondarily refining molten steel in which CaO-and-Al.sub.2O.sub.3-containing slag is formed on top of Al-containing molten steel charged in a vessel, and nitrogen and sulfur in the molten steel are removed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel and bringing the slag and the molten steel into contact with each other, characterized in that, an oxygen blowing process according to the method for secondarily refining molten steel according to claim 2 is performed, and during the oxygen blowing process, an Al concentration in the molten steel is kept at 0.05 mass % or higher, and a ratio C/A (?) between a CaO concentration (mass %) and an Al.sub.2O.sub.3 concentration (mass %) in the slag is controlled between 1.8 and 2.2, inclusive.

10. A method for secondarily refining molten steel in which CaO-and-Al.sub.2O.sub.3-containing slag is formed on top of Al-containing molten steel charged in a vessel, and nitrogen and sulfur in the molten steel are removed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel and bringing the slag and the molten steel into contact with each other, characterized in that, an oxygen blowing process according to the method for secondarily refining molten steel according to claim 3 is performed, and during the oxygen blowing process, an Al concentration in the molten steel is kept at 0.05 mass % or higher, and a ratio C/A (?) between a CaO concentration (mass %) and an Al.sub.2O.sub.3 concentration (mass %) in the slag is controlled between 1.8 and 2.2, inclusive.

11. A method for secondarily refining molten steel in which CaO-and-Al.sub.2O.sub.3-containing slag is formed on top of Al-containing molten steel charged in a vessel, and nitrogen and sulfur in the molten steel are removed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel and bringing the slag and the molten steel into contact with each other, characterized in that, an oxygen blowing process according to the method for secondarily refining molten steel according to claim 4 is performed, and during the oxygen blowing process, an Al concentration in the molten steel is kept at 0.05 mass % or higher, and a ratio C/A (?) between a CaO concentration (mass %) and an Al.sub.2O.sub.3 concentration (mass %) in the slag is controlled between 1.8 and 2.2, inclusive.

12. A method for secondarily refining molten steel in which CaO-and-Al.sub.2O.sub.3-containing slag is formed on top of Al-containing molten steel charged in a vessel, and nitrogen and sulfur in the molten steel are removed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel and bringing the slag and the molten steel into contact with each other, characterized in that, an oxygen blowing process according to the method for secondarily refining molten steel according to claim 5 is performed, and during the oxygen blowing process, an Al concentration in the molten steel is kept at 0.05 mass % or higher, and a ratio C/A (?) between a CaO concentration (mass %) and an Al.sub.2O.sub.3 concentration (mass %) in the slag is controlled between 1.8 and 2.2, inclusive.

13. A method for secondarily refining molten steel in which CaO-and-Al.sub.2O.sub.3-containing slag is formed on top of Al-containing molten steel charged in a vessel, and nitrogen and sulfur in the molten steel are removed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel and bringing the slag and the molten steel into contact with each other, characterized in that, an oxygen blowing process according to the method for secondarily refining molten steel according to claim 6 is performed, and during the oxygen blowing process, an Al concentration in the molten steel is kept at 0.05 mass % or higher, and a ratio C/A (?) between a CaO concentration (mass %) and an Al.sub.2O.sub.3 concentration (mass %) in the slag is controlled between 1.8 and 2.2, inclusive.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 is a schematic view showing one example of an apparatus suitable for a molten steel secondary refining method according to one embodiment of the present invention.

[0025] FIG. 2 is a graph showing a relationship between an Al concentration [Al].sub.i in molten steel adjusted in an Al addition step and an achieved nitrogen concentration [N].sub.f.

[0026] FIG. 3 is a graph showing a relationship between an Al concentration [Al].sub.e in molten steel immediately before an oxygen blowing process and a stirring power density ? during slag formation for obtaining an achieved nitrogen concentration [N].sub.f of 25 mass ppm.

[0027] FIG. 4 is a graph showing a relationship between an Al concentration [Al].sub.f upon completion of processing and a slag forming index.

[0028] FIG. 5 is a graph showing a relationship between a ratio L.sub.s/L.sub.s0 between an initial slag thickness L.sub.s0 and a depth L.sub.s of a depression in the slag due to oxygen-containing gas and an achieved Al concentration [Al].sub.f in molten steel.

[0029] FIG. 6 is a graph showing a relationship between a ratio C/A (?) between a CaO concentration (C) and an Al.sub.2O.sub.3 concentration (A) in slag on a mass basis and the achieved nitrogen concentration [N].sub.f in molten steel.

[0030] FIG. 7 is a graph showing a relationship between a ratio of an MgO concentration (MgO) to a CaO concentration (CaO) in slag and the achieved nitrogen concentration [N].sub.f in molten steel.

[0031] FIG. 8 is a graph showing a relationship between a ratio of an MgO concentration (MgO) to a CaO concentration (CaO) in slag and a refractory erosion index.

[0032] FIG. 9 is a graph showing a relationship between a furnace internal pressure P and an upper limit Max [N].sub.f of variations in the achieved nitrogen concentration in molten steel.

DESCRIPTION OF EMBODIMENTS

[0033] Embodiments of the present invention will be specifically described below. The drawings are schematic and may differ from the reality. The following embodiments illustrate an apparatus and a method for embodying the technical concept of the present invention, and are not intended to restrict the configuration to the one described below. Thus, various changes can be made to the technical concept of the present invention within the technical scope described in the claims.

[0034] FIG. 1 shows an apparatus configuration suitable to implement the present invention. Molten steel 3 is charged into a vessel 1, such as a ladle, that is lined with a refractory 2, and slag 4 containing CaO and Al.sub.2O.sub.3 is formed on top of this molten steel 3. In a state where surfaces of the molten steel 3 and the slag 4 are subjected to a depressurized atmosphere inside a vacuum vessel 13 having an exhaust system 11 and an alloy addition system 12, O.sub.2-containing gas is blown onto the slag 4 through a gas top-blowing lance 6 connected to a gas pipe 5. The molten steel 3 is stirred as a stirring inert gas 10 is blown in through a bottom-blowing nozzle 8 connected to the gas pipe 9. As the stirring inert gas 10, for example, Ar gas not including nitrogen gas is preferable. The top-blowing lance 6 is inserted so as to penetrate an inner lid 14 that covers an upper part of the vessel 1.

[0035] A step of adding a metal-Al-containing substance to the molten steel 3

[0036] to deoxidize the molten steel 3 and turn it into Al-containing molten steel (Al addition step) and a step of adding a CaO-containing substance to the molten steel 3 (CaO addition step) may be performed using the alloy addition system 12 or may be performed in a step before entering the vacuum vessel 13. A step of deoxidizing the molten steel 3 (deoxidization step) may be performed separately from the Al addition step, or a deoxidation process may be performed within the Al addition step. In the case where there is a deoxidation step at an early stage of the process, addition of a metal-Al-containing substance may be performed before the deoxidation step, or may be performed after the deoxidation step, or may be divided and performed before and after the deoxidation step. Adding Al before the deoxidation step can be expected to also have an effect of keeping the temperature of the molten steel high owing to combustion of Al, while adding Al after the deoxidation step can be expected to have a denitrifying effect. Further, when Al is divided and added before and after the deoxidation step, both of these effects can be expected. The CaO addition step can be performed at an arbitrary timing. Performing the CaO addition step after the deoxidization step is preferable, because then a temperature rise of the molten steel due to the deoxidation reaction can be used to slag formation. Performing the CaO addition step after the Al addition step is further preferable, because this can reduce deoxidization failure or variations in the slag composition due to the added Al-containing substance being hindered by the thick slag from reaching the molten steel.

[0037] To form the CaO-and-Al.sub.2O.sub.3-containing slag 4, CaO resulting from adding the CaO-containing substance and Al.sub.2O.sub.3 resulting from deoxidizing the molten steel is used. As the CaO-containing substance, for example, calcium aluminate that is a pre-melted or pre-mixed product may be used. As for the slag composition, a higher ratio of melting of slag (hereinafter referred to as a slag formation rate) is more advantageous for the denitrification reaction.

[0038] To promote slag melting for forming the slag, it is preferable that stirring, for example, bottom-blowing stirring of the molten steel is performed. The method of supplying the stirring gas 10 into the molten steel may be, other than the above-described method, for example, a method of injecting it into the molten steel through an injection lance for blowing in an inert gas.

[0039] Next, preferred embodiments of the present invention will be described in detail along with how they were developed. In this Description, [M] represents a state of element M being dissolved and contained in molten steel, and (R) represents a state of a chemical substance R being dissolved and contained in slag. Units are added to express their respective composition ratios.

FIRST EMBODIMENT

[0040] A first embodiment was developed in an attempt to remove nitrogen by bringing Al-containing molten steel into contact with slag as well as to remove Al excessively contained in the molten steel by blowing oxygen. In a small-sized high-frequency vacuum induction melting furnace satisfying the configuration requirements of FIG. 1, the CaO-and-Al.sub.2O.sub.3-containing slag 4 was formed at the ratio of 15 kg/t or higher relative to 15 kg of the molten steel 3, in such an amount that the surface of the molten steel was not recognizable to the naked eye, and after the molten steel 3 was stirred by bottom-blown gas, O.sub.2 gas was blown onto the slag. It was confirmed with the naked eye that the O.sub.2 gas had pushed its way through the slag by the jet pressure and reached the surface of the molten steel. For example, at a position in the molten metal bath surface corresponding to an emerging point of the bottom-blown gas, the slag thickness is reduced due to bulging of the bath surface. When the O.sub.2 gas is blown toward this emerging point of the bottom-blown gas, the O.sub.2 gas can easily pierce the slag and be directly blown onto the molten steel.

[0041] First, while the amount of Al-containing substance added in the Al addition step was varied, a relationship between an Al concentration [Al].sub.i in the molten steel immediately before the oxygen blowing process involving blowing O.sub.2 gas and an achieved N concentration [N].sub.f was studied. As shown in FIG. 2, when the Al concentration [Al].sub.i in the molten steel immediately before the oxygen blowing process was lower than 0.03 mass %, denitrification could not be stably performed to an achieved N concentration [N].sub.f of 35 mass ppm or lower. In this case, the furnace internal atmospheric pressure P was 5.3?10.sup.3 Pa; the initial nitrogen concentration [N].sub.i in the molten steel was 50 mass ppm; the slag composition had the mass ratio C/A between CaO and Al.sub.2O.sub.3 of 1.2; the MgO concentration (MgO) in the slag was 5 mass %; the stirring power density ? was 396 W/t; the molten steel temperature T.sub.f was 1660? C.; and the oxygen blowing process time t was 25 minutes. A possible explanation for this result is that, due to the oxygen-containing gas going through the slag, Al in the molten steel is oxidized and decreased and is unable to form aluminum nitride (AlN). It is preferable that the Al concentration [Al].sub.i in the molten steel before the oxygen blowing process is 0.1 mass % or higher, because then the achieved nitrogen concentration [N].sub.f in the molten steel can be made equal to or lower than 30 mass ppm. Further, it is preferable that the Al concentration [Al].sub.i in the molten steel before the oxygen blowing process is 1.0 mass % or higher, because then the achieved nitrogen concentration [N].sub.f in the molten steel can be made equal to or lower than 25 mass ppm.

[0042] Next, using the aforementioned small-sized high-frequency vacuum induction melting furnace, the minimum Al concentration [Al].sub.e before oxygen blowing that was required for reducing nitrogen in molten steel to 25 ppm was studied. It turned out that, as shown in FIG. 3, the Al concentration [Al].sub.e (mass %) required before oxygen blowing varied according to the stirring power density ? (W/t) during slag formation. Here, the MgO concentration (MgO) in the slag was 0 mass % and the molten steel temperature T.sub.f was 1600? C., and the initial nitrogen concentration [N].sub.i and the C/A in the slag composition were the same as those mentioned above. As preconditions for the study, the furnace atmospheric pressure P was 0.7?10.sup.5 Pa; the stirring power density ? was controlled so as to remain constant within the range of 200 to 2000 W/t during slag formation and the oxygen blowing process; and the oxygen blowing process time was 30 minutes. It was deduced from this result that contact between the slag and the molten steel at positions other than a hot spot (which refers to a portion where the molten steel surface is exposed due to the oxygen-containing gas) during oxygen blowing or the slag during slag formation before oxygen blowing promoted denitrification by the slag.

[0043] Further, using the aforementioned small-sized high-frequency vacuum induction melting furnace, a relationship between the Al concentration [Al].sub.f in molten steel after the oxygen blowing process and a slag forming index was studied while the Al concentration [Al].sub.i in the molten steel before the oxygen blowing process was varied from 0.02 to 0.5 mass %. Here, the slag forming index was defined as the ratio between the slag height calculated from the slag volume and a freeboard. The result is shown in FIG. 4. The cross mark represents the case where there was slag overflow, and the circle mark represents the case where there was no slag overflow. Here, the furnace internal atmospheric pressure P was 1?10.sup.5 Pa; the stirring power density ? was 60 W/t; the initial nitrogen concentration [N].sub.i in the molten steel was 50 mass ppm; the initial C concentration [C] was 0.10 mass %; the slag composition had the mass ratio C/A between CaO and Al.sub.2O.sub.3 of 1.2; the MgO concentration (MgO) in the slag was 5 mass %; the molten steel temperature T.sub.f was 1660? C.; and the processing time t was 18 minutes. The freeboard of the ladle was 1.5 m. It was found that slag overflow occurred when the Al concentration [Al].sub.f in the molten steel after the oxygen blowing process fell below 0.03 mass % and the slag forming index exceeded 1. A possible explanation is that, as the oxidation reactions of Al and C with the O.sub.2 gas blown in competed against each other, the Al concentration decreased and at the same time CO gas was generated, resulting in an increase in the slag volume. The result of the study as just described led to the development of the first embodiment, i.e., a method for secondarily refining molten steel in which CaO-and-Al.sub.2O.sub.3-containing slag is formed by a combination of an Al addition step of adding a metal-Al-containing substance to molten steel to turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the molten steel, and then an oxygen blowing process including a denitrification process is performed by blowing oxygen-containing gas so as to pierce the slag to reach the Al-containing molten steel, in which an Al concentration [Al].sub.i (mass %) in the molten steel immediately before the oxygen blowing process is equal to or higher than a value [Al].sub.e calculated by Formula (A) based on a stirring power density ? (W/t) during formation of the slag, and an Al concentration [Al].sub.f upon completion of the oxygen blowing process is 0.03 mass % or higher. When an Al concentration [Al].sub.d given in product specifications is higher than the value [Al].sub.e of Formula (A), the value of the Al concentration [Al].sub.d is preferably used as the target value for the Al concentration [Al].sub.f upon completion of the oxygen blowing process. When the target value [Al].sub.d is lower than the value [Al].sub.e of Formula (A), a process of oxidizing and removing Al is preferably performed in the oxygen blowing process.

[00002]$\begin{array}{cc}{\left[\mathrm{Al}\right]}_e=-0.072?\ln \left(?\right)+0.5822& \left(A\right)\end{array}$

SECOND EMBODIMENT

[0044] A second embodiment was developed with an intention to simplify the step of the oxygen blowing process, which requires performing a so-called Al elimination process. In a small-sized high-frequency vacuum induction melting furnace satisfying the configuration requirements of FIG. 1, the CaO-and-Al.sub.2O.sub.3-containing slag 4 was formed at the ratio of 15 kg/t or higher relative to 15 kg of the molten steel 3, in such an amount that the surface of the molten steel was not recognizable to the naked eye, and O.sub.2 gas was blown onto the slag surface at a position other than an emerging point of bottom-blown gas.

[0045] The present inventors studied a relationship between the Al concentration [Al].sub.f in the molten steel after the oxygen blowing process and the ratio L.sub.s/L.sub.s0 (?) between a measurement result of the slag thickness L.sub.s0 (m) at the stage where the CaO-and-Al.sub.2O.sub.3-containing slag has melted before the oxygen blowing process and the depth L.sub.s (m) of a depression in the slag resulting when parameters in the formula described in Non Patent Literature 1, namely the liquid density, the gas density, the jet speed, etc., are changed to values complying with experimental conditions. As a result, as shown in FIG. 5, it was found that L.sub.s/L.sub.s0 being 1.0 or higher could reduce the Al concentration in the molten steel at the same time as removing nitrogen during the oxygen blowing process. In this case, the furnace internal atmospheric pressure P was 5.3?10.sup.3 Pa; the initial nitrogen concentration [N].sub.i in the molten steel was 50 mass ppm; the Al concentration [Al].sub.i in the molten steel before the oxygen blowing process was 0.7 mass %; the slag composition had the mass ratio C/A between CaO and Al.sub.2O.sub.3 of 1.2; the MgO concentration (MgO) in the slag was 10 mass %; the molten steel temperature T.sub.f was 1650? C.; and the processing time t was 30 minutes. The result of the study as just described led to the development of the second embodiment, i.e., a method for secondarily refining molten steel in which, in addition to the first embodiment, the oxygen-containing gas is supplied in the oxygen blowing process such that the ratio L.sub.s/L.sub.s0 between the thickness L.sub.s0 of the slag and the depth L.sub.s of the depression resulting from the blowing of the oxygen-containing gas is 1.0 or higher. The ratio L.sub.s/L.sub.s0 being too high causes interference with operation due to spitting of molten steel etc.; therefore, the upper limit of the ratio L.sub.s/L.sub.s0 is preferably about 1.5 and further preferably about 1.3.

THIRD EMBODIMENT

[0046] A third embodiment was found in the course of studying an influence on denitrification exerted by the slag composition, mainly the ratio C/A (?) between the CaO concentration (mass %) and the Al.sub.2O.sub.3 concentration (mass %) in the slag. In a test in which, using a small-sized high-frequency vacuum induction melting furnace satisfying the configuration requirements of FIG. 1, the MgO concentration in slag was 0% while C/A was varied from 0.4 to 2.5, the achieved nitrogen concentration [N].sub.f remained at 20 mass ppm or lower when C/A was within the range of 0.4 to 2.0 as shown in FIG. 6. When C/A exceeded 1.8, the achieved nitrogen concentration [N].sub.f started to increase. When 2.0 was exceeded, the achieved nitrogen concentration [N].sub.f rose rapidly, and when 2.2 was exceeded, a low nitrogen concentration range (where the nitrogen concentration [N].sub.f is 35 ppm or lower) was no longer reached. The result of the study as just described led to the development of the third embodiment, i.e., a method for secondarily refining molten steel in which, in addition to the first embodiment or the second embodiment, the mass ratio C/A (?) between the CaO concentration (CaO) (mass %) and the Al.sub.2O.sub.3 concentration (Al.sub.2O.sub.3) (mass %) in the slag is between 0.4 and 2.2, inclusive.

FOURTH EMBODIMENT

[0047] A fourth embodiment was found in the course of studying an influence on denitrification exerted by the slag composition, mainly the ratio (MgO)/(CaO) (?) of the MgO concentration (mass %) to the CaO concentration (mass %) in the slag that increases as a refractory erodes. Using the same conditions as in FIG. 6 except for the MgO concentration (MgO) in the slag, the mass ratio (MgO)/(CaO) was varied with C/A fixed at 1.7 to study the influence on the achieved nitrogen concentration [N].sub.f. The result is shown in FIG. 7. It can be seen that (MgO)/(CaO) being 0.25 or lower can make the achieved nitrogen concentration [N].sub.f equal to or lower than 35 mass ppm. Further, (MgO)/(CaO) being 0.2 or lower can make the achieved nitrogen concentration [N].sub.f even lower. Similarly, FIG. 8 shows an influence on erosion of the refractory exerted by the ratio (MgO)/(CaO) (?) of the MgO concentration (mass %) to the CaO concentration (mass %) in the slag. (MgO)/(CaO) being 0.14 or higher is preferable because it has little influence on erosion of the refractory. The result of the study as just described led to the development of the fourth embodiment, i.e., a method for secondarily refining molten steel in which, in addition to one of the first to third embodiments, the mass ratio of the MgO concentration (MgO) (mass %) to the CaO concentration (CaO) (mass %) in the slag is 0.25 or lower. The mass ratio of the MgO concentration (MgO) (mass %) to the CaO concentration (CaO) (mass %) in the slag is preferably within the range of 0.14 to 0.25.

FIFTH EMBODIMENT

[0048] A fifth embodiment was found in the course of studying an influence on a denitrification reaction exerted by a degree of vacuum reached. In a small-sized high-frequency vacuum induction melting furnace satisfying the configuration requirements of FIG. 1, the CaO-and-Al.sub.2O.sub.3-containing slag 4 including MgO at the concentration of 0 to 17 mass % was formed at the ratio of 15 kg/t or higher relative to 15 kg of the molten steel 3, in such an amount that the surface of the molten steel was not recognizable to the naked eye. After the atmospheric pressure P inside the furnace was adjusted, a process of blowing oxygen to the molten steel was performed by blowing O.sub.2 gas so as to pierce the slag to reach the slag, while giving the molten steel a stir at the stirring power density of 2500 W/t. In an oxygen blowing test in which the degree of vacuum in the furnace atmosphere (atmospheric pressure) P (Pa) was varied, as shown in FIG. 9, an upper limit value Max [N].sub.f (mass ppm) of variations in the nitrogen concentration after processing could be stably maintained in a low nitrogen region of 35 mass ppm at a furnace atmospheric pressure P of 9.3?10.sup.4 Pa or lower. In this case, the initial nitrogen concentration [N].sub.i in the molten steel was 50 mass ppm; the Al concentration [Al].sub.i was 0.7 mass %; the slag composition had a mass ratio C/A between CaO and Al.sub.2O.sub.3 of 1.2; the MgO concentration (MgO) in the slag was 5 mass %; the molten steel temperature T.sub.f was 1600? C.; and the processing time t was 30 minutes. The result of the study as just described led to the development of the fifth embodiment, i.e., a method for secondarily refining molten steel in which, in addition to one of the first to fourth embodiments, surfaces of the Al-containing molten steel and the slag are subjected to a depressurized atmosphere of 9.3?10.sup.4 Pa or lower. A depressurized atmosphere of 6.7?10.sup.4 Pa or lower is preferable. Since excessive depressurization causes an increase in facility costs of the exhaust system etc., the lower limit of the furnace atmospheric pressure P is preferably about 10.sup.3 Pa.

SIXTH EMBODIMENT

[0049] A sixth embodiment was found in the course of exploring the possibilities of simultaneously processing denitrification and desulfurization. It is preferable that conditions for the oxygen blowing process is selected from those of the above-described embodiments. It is known that, to promote the desulfurization reaction, Mannesmann Slag Index (MSI)=((CaO/SiO.sub.2)/Al.sub.2O.sub.3) is preferably within the range of 0.25 to 0.45. It is preferable that a CaO containing substance is added such that the mass ratio (CaO)/(Al.sub.2O.sub.3) becomes 1.8 to 2.2 relative to the amount of Al.sub.2O.sub.3 generated during the deoxidation process and slag adjustment, and that MgO clinker is added as necessary such that the mass ratio of the MgO concentration relative to the amount of CaO added becomes about 0.2. The concentration of SiO.sub.2 is not actively controlled. A slag composition advantageous for desulfurization has a high C/A, which is, on the other hand, disadvantageous from the viewpoint of denitrification.

[0050] Therefore, when supplying oxygen-containing gas to promote denitrification, it is preferable that the process is performed at a high degree of vacuum, with the ratio L.sub.s/L.sub.s0 between the slag thickness L.sub.s0 and the depth L.sub.s of the depression resulting from blowing of the oxygen-containing gas higher than 1.

[0051] Molten steel produced by the above-described method for secondarily refining molten steel is cast preferably after additionally it is adjusted to a predetermined composition and form control and floating separation of inclusions are performed as necessary. It is possible to manufacture high-grade steel which is low-nitrogen steel or low-nitrogen, low-sulfur steel and of which various compositions have been adjusted.

EXAMPLES

[0052] In the following, examples of the present invention will be described in detail. Using an apparatus having the configuration of FIG. 1, metal Al was added to molten steel having the temperature of 1600? C. to 1750? C. inside a ladle. To adjust the temperature, the molten steel was heated in an LF apparatus. Al for deoxidation was added all at once before the LF processing or at the time of adding Al in a VOD apparatus. In the VOD apparatus, the Al concentration in the molten steel before oxygen blowing was adjusted according to whether Al was added, and [Al].sub.i was 0.02 to 0.48 mass %. After CaO and refractory-protecting MgO were added to form CaO-Al.sub.2O.sub.3 binary slag or CaO-Al.sub.2O.sub.3-MgO ternary slag, oxygen gas was blown onto the slag. Ar gas was supplied to the molten steel through a bottom-blowing plug that was mounted at the lower part of the ladle at the stirring power density of 60 to 600 KW/t. The test was conducted using an amount of molten steel of 160 t.

[0053] Table 1-1 and Table 1-2 show the test conditions and the results. The examples of the present invention could make the achieved nitrogen concentration [N].sub.f in the molten steel after the oxygen blowing process equal to or lower than 35 mass ppm. By contrast, comparative examples failed to achieve that. Further, in tests No. 7 and 8, the processing was forced to be discontinued due to slag overflow. Under test conditions No. 5, 14, and 16 in which C/A in the slag was within the range of 1.8 to 2.2, the desulfurization efficiency was also found to be excellent. In processes No. 11 and 15, partial solidification without slag formation was observed in the slag.

TABLE-US-00001 TABLE 1-1 Before LF VOD Amount of Amount of Slag Al added Al added (C/A)* (MgO) (MgO/CaO) Slag ? L.sub.S/L.sub.S0 P kg/t-molten kg/t-molten Tf t No. mass % pierced? W/t Pa steel steel ? C. min Remarks 1 1.20 9.2 0.2 Yes 396 1.1 13300 0.85 2.78 1630 13 Invention Example 2 1.20 9.2 0.2 Yes 582 1.1 5320 0.85 2.78 1635 5 Invention Example 3 1.50 10.1 0.2 Yes 288 1.3 1330 0.85 2.78 1660 19 Invention Example 4 1.50 10.1 0.2 Yes 288 1.3 1330 0.00 3.63 1660 19 Invention Example 5 2.00 11.1 0.2 Yes 288 1.4 1330 0.85 2.78 1660 25 Invention Example 6 0.80 7.7 0.2 Yes 288 1.1 13300 0.85 2.78 1630 13 Invention Example 7 0.80 2.0 0.05 No 288 0.8 13300 0.85 0.00 1630 Comparative Example 8 0.80 7.7 0.2 Yes 288 1.1 13300 0.85 0.50 1660 Comparative Example 9 0.80 7.7 0.2 Yes 288 1.1 13300 0.85 2.78 1660 13 Invention Example 10 0.80 7.7 0.2 No 288 0.9 13300 0.85 2.78 1660 13 Comparative Example 11 0.30 4.1 0.2 Yes 288 1.1 13300 0.85 2.78 1660 13 Comparative Example 12 0.40 5.1 0.2 Yes 288 1.1 13300 0.85 2.78 1660 13 Invention Example 13 1.80 10.7 0.2 Yes 288 1.1 13300 0.85 2.78 1660 13 Invention Example 14 1.90 10.9 0.2 Yes 288 1.1 13300 0.85 2.78 1660 13 Invention Example 15 2.30 11.5 0.2 Yes 288 1.1 13300 0.85 2.78 1660 13 Comparative Example 16 2.20 11.4 0.2 Yes 288 1.1 13300 0.85 2.78 1660 13 Invention Example 17 1.00 8.5 0.2 Yes 61 1.1 101323 0.85 2.78 1660 19 Comparative Example 18 1.00 8.5 0.2 Yes 70 1.1 93000 0.85 4.78 1660 18 Invention Example *(C/A) represents the mass ratio between CaO (C) and Al.sub.2O.sub.3 (A) in the slag.

TABLE-US-00002 TABLE 1-2 Before process Element composition of VOD molten steel Slag Refractory [C] [Al]e [Al]i [Al]f [N]i [N]f [S]i [S]f No. overflowed? eroded? mass % mass % mass % mass % massppm massppm mass % mass % Remarks 1 No No 0.03 0.15 0.28 0.04 40 25 0.04 0.007 Invention Example 2 No No 0.01 0.12 0.28 0.05 60 30 0.05 0.006 Invention Example 3 No No 0.06 0.17 0.28 0.04 45 20 0.05 0.003 Invention Example 4 No No 0.06 0.17 0.28 0.04 45 20 0.05 0.003 Invention Example 5 No No 0.10 0.17 0.28 0.05 60 25 0.05 0.001 Invention Example 6 No No 0.03 0.17 0.28 0.03 40 25 0.05 0.006 Invention Example 7 Yes No 0.05 0.17 0.02 0.02 45 0.05 Comparative Example 8 Yes No 0.03 0.17 0.05 0.02 55 0.05 Comparative Example 9 No No 0.03 0.17 0.28 0.04 55 28 0.05 0.006 Invention Example 10 No No 0.03 0.17 0.28 0.04 55 56 0.05 0.006 Comparative Example 11 No Yes 0.03 0.17 0.28 0.04 55 57 0.05 0.04 Comparative Example 12 No Yes 0.03 0.17 0.28 0.04 55 20 0.05 0.04 Invention Example 13 No No 0.03 0.17 0.28 0.04 55 35 0.05 0.003 Invention Example 14 No No 0.03 0.17 0.28 0.05 55 30 0.05 0.001 Invention Example 15 No No 0.03 0.17 0.28 0.05 55 56 0.05 0.001 Comparative Example 16 No No 0.03 0.17 0.28 0.05 55 33 0.05 0.001 Invention Example 17 No Yes 0.06 0.29 0.28 0.05 53 55 0.05 0.05 Comparative Example 18 No Yes 0.06 0.28 0.48 0.06 55 35 0.05 0.006 Invention Example *In tests No. 7 and 8, processing was discontinued due to slag overflow.

INDUSTRIAL APPLICABILITY

[0054] When applied to a steel production process of producing molten steel by melting low-carbon scrap or reduced iron in an electric furnace etc., the secondarily refining molten steel according to the present invention can stably mass-produce low-nitrogen steel or low-nitrogen, low-sulfur steel. Thus, this method contributes to reducing CO.sub.2 emissions and is industrially useful.

REFERENCE SIGNS LIST

[0055] 1 Vessel [0056] 2 Refractory [0057] 3 Molten steel [0058] 4 CaO-and-Al.sub.2O.sub.3-containing slag [0059] 5 Gas pipe (oxygen gas) [0060] 6 Gas top-blowing lance [0061] 7 O.sub.2-containing gas [0062] 8 Bottom-blowing nozzle [0063] 9 Gas pipe (inert gas) [0064] 10 Inert gas for stirring molten steel bath [0065] 11 Exhaust system [0066] 12 Alloy addition system [0067] 13 Vacuum vessel [0068] 14 Inner lid