Molten steel denitrification method and steel production method

Abstract

A molten steel denitrification method, wherein an extremely low nitrogen concentration range is stably reached in a short time without use of a top-blown gas, is a denitrification process wherein CaOandAl.sub.2O.sub.3-containing slag formed by a combination of an Al addition step of adding a metalAl-containing substance to molten steel to deoxidize and turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the molten steel is brought into contact with the Al-containing molten steel to remove nitrogen in the molten steel, in which the molten steel is stirred at a stirring power density of 60 W/t or higher. In the denitrification process, a surface of the molten steel or the slag is subjected to an atmosphere of 1.010.sup.5 Pa or lower. In a steel production method, the obtained molten steel is cast after the components are adjusted.

Claims

1. A molten steel denitrification method that is a denitrification process in which CaOandAl.sub.2O.sub.3-containing slag formed by a combination of an Al addition step of adding a metalAl-containing substance to molten steel to deoxidize and turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the Al-containing molten steel to remove nitrogen in the molten steel without use of a top-blown gas, wherein the molten steel is stirred at a stirring power density of 60 W/t or higher, a temperature of the molten steel, an Al concentration in the molten steel, an atmospheric pressure in a furnace and a slag composition are adjusted, and either condition (A) or (B) is selected so as to achieve a N concentration [N].sub.f at 35 mass ppm or lower: (A) wherein in the denitrification process, an MgO concentration in the slag is set to 5.0 mass % or lower, except for 0, and (B) wherein in the denitrification process, a temperature T.sub.f of the molten steel undergoing the denitrification process is increased by 5 C. or more each time the MgO concentration in the slag increases by 1.0 mass % beyond 5.0 mass %, compared to a molten steel temperature during said denitrification process required to reduce the N concentration [N].sub.f in the molten steel to a predetermined value when the MgO concentration is 5.0 mass % in the slag.

2. The molten steel denitrification method according to claim 1, wherein, in the denitrification process, a surface of the molten steel or the slag is subjected to an atmosphere of 1.010.sup.5 Pa or lower.

3. The molten steel denitrification method according to claim 2, wherein, in the Al addition step, an Al concentration [Al], in mass %, in the molten steel before the denitrification process is set to be equal to or higher than a value [Al].sub.e calculated by Formula (A) based on the stirring power density , in W/t, during the denitrification process,
[Al].sub.e=0.072ln()+0.5822(A).

4. The molten steel denitrification method according to claim 1, wherein, in the Al addition step, an Al concentration [Al], in mass %, in the molten steel before the denitrification process is set to be equal to or higher than a value [Al].sub.e calculated by Formula (A) based on the stirring power density , in W/t, during the denitrification process,
[Al].sub.e=0.072ln()+0.5822(A).

5. A steel production method wherein molten steel produced by the molten steel denitrification method according to claim 1 is cast after a composition of the molten steel is adjusted.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view showing one example of a device suitable for a molten steel denitrification method according to one embodiment of the present invention.

(2) FIG. 2 is a graph showing an influence of a molten steel stirring power density on a relationship between a furnace internal pressure P and an upper limit Max[N].sub.f of variations in an achieved nitrogen concentration.

(3) FIG. 3 is a graph showing a relationship between the stirring power density and an achieved nitrogen concentration [N].sub.f.

(4) FIG. 4 is a graph showing an influence of an MgO concentration (MgO) in slag on the achieved nitrogen concentration [N].sub.f.

(5) FIG. 5 is a graph showing molten steel temperatures T.sub.f for obtaining the same achieved nitrogen concentration [N].sub.f when the MgO concentration (MgO) in slag is changed.

(6) FIG. 6 is a graph showing a relationship between an Al concentration [Al].sub.e in molten steel and the stirring power density for obtaining an achieved nitrogen concentration [N].sub.f of 25 mass ppm.

DESCRIPTION OF EMBODIMENTS

(7) Embodiments of the present invention will be specifically described below. The drawings are schematic and may differ from the reality. The following embodiments illustrate a device and a method for embodying the technical idea 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 idea of the present invention within the technical scope described in the claims.

(8) FIG. 1 shows a device 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 10 having an exhaust system 8 and an alloy addition system 9, a stirring inert gas 7 is blown in through a bottom-blowing nozzle 5 connected to a gas pipe 6 to give a stir. As the stirring inert gas 7, for example, an Ar gas not including a nitrogen gas is preferable.

(9) A step of adding a metal-Al-containing substance to the molten steel 3 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 9 or may be performed in a step before entering the vacuum vessel 10. The step of deoxidizing the molten steel 3 (deoxidization step) may be performed separately from the Al addition step. The CaO addition step can be performed at an arbitrary timing. Performing the CaO addition step after the deoxidization step is preferable, because then the temperature rise of the molten steel due to the deoxidation reaction can be used for formation of slag. 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.

(10) To form the CaO-and-Al.sub.2O.sub.3-containing slag 4, Al.sub.2O.sub.3 resulting from adding the CaO-containing substance and 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 slag formation rate is more advantageous for the denitrification reaction, and a mass ratio C/A between CaO and Al.sub.2O.sub.3 is preferably within a range of 0.4 to 1.8 and more preferably within a range of 0.7 to 1.7.

(11) The form of supplying the stirring inert gas 7 into the molten steel may be, other than the above-described method, for example, a form of injecting into the molten steel through an injection lance for blowing in an inert gas.

(12) Next, preferred embodiments of the present invention will be described in detail along with how they were developed.

First Embodiment

(13) A first embodiment was found in the course of exploring a method for stably removing nitrogen to a low nitrogen concentration in a facility not having a gas top-blowing device. In a small-sized high-frequency vacuum induction melting furnace satisfying the configuration requirements of FIG. 1, the CaOandAl.sub.2O.sub.3-containing slag 4 including MgO at a concentration of 0 to 17 mass % was formed at a 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 inside the furnace was adjusted, a molten steel denitrification process was performed while the molten steel was given a stir at a stirring power density of 200 W/t to 2000 W/t. First, in a denitrification test in which the degree of vacuum in the furnace atmosphere (atmospheric pressure) P(Pa) was varied, as shown in FIG. 2, an upper limit value Max[N].sub.f (mass ppm) of variations in the post-process nitrogen concentration changed according to the stirring power density (W/t). In this case, an initial nitrogen concentration [N].sub.i in the molten steel was 50 mass ppm; the Al concentration [Al] 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. At a low stirring power density ( up to 500 W/t), the upper limit value Max[N].sub.f of variations in the achieved nitrogen concentration remained stable up to an atmospheric pressure P of 1.010.sup.5 Pa. In the case of the facility configuration of FIG. 1, the atmospheric pressure P becomes higher than outside air by a few percent due to the influence of a temperature rise inside the enclosed space and the bottom-blown gas. By contrast, at a high stirring power density (>500 W/t), the upper limit value Max[N].sub.f of variations in the achieved nitrogen concentration started to increase when the atmospheric pressure P exceeded 0.710.sup.5 Pa, and the upper limit value Max[N].sub.f of variations in the post-process achieved nitrogen concentration was found to become larger as the stirring power density became higher. In this embodiment, therefore, a preferable atmospheric pressure P is specified as 1.010.sup.5 Pa or lower, and it is further preferably 0.710.sup.5 Pa or lower. A possible explanation is that, as the molten steel bath is stirred by the bottom-blown gas, the surface of the molten steel bulges and part of the surface becomes exposed, and through that part nitrogen is absorbed into the molten steel.

(14) Next, a denitrification test was conducted in which the stirring power density was varied from 20 to 1500 W/t while the MgO concentration (MgO) was held constant at 5%, with the furnace atmospheric pressure P being 0.710.sup.5 Pa, the Al concentration [Al] being 1.0 mass %, and the initial nitrogen concentration [N].sub.i, the C/A in the slag composition, the molten steel temperature T.sub.f, and the processing time t being the same as those mentioned above. As a result, as shown in FIG. 3, with the stirring power density at a level of 60 W/t or higher, a low nitrogen concentration range (where the nitrogen concentration [N].sub.f is 35 mass ppm or lower) could be reached. An extremely low nitrogen concentration range (where the nitrogen concentration [N].sub.f is 25 mass ppm or lower) was achieved with the stirring power density at a level of 200 W/t or higher. The result of the study as just described led to the development of the first embodiment, i.e., a molten steel denitrification method that is a denitrification process in which CaO-and-Al.sub.2O.sub.3-containing slag formed by a combination of an Al addition step of adding a metal-Al-containing substance to molten steel to deoxidize and turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the molten steel is brought into contact with the Al-containing molten steel to remove nitrogen in the molten steel, in which the molten steel is stirred at a stirring power density of 60 W/t or higher, or a molten steel denitrification method in which, further, in the denitrification process, the surface of the molten steel or the slag is subjected to an atmosphere of 1.010.sup.5 Pa or lower. While the upper limit of the stirring power density is not particularly limited, the bottom-blown gas, when blown in in a large amount, will be blown through without being effectively used; therefore, with about 5000 W/t as the upper limit, the stirring power density should be appropriately set within such a range that possible troubles accompanying its rise (e.g., sculls adhering to a furnace lid) do not occur. In addition, 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.

Second Embodiment

(15) A second embodiment was found in the course of studying the influence of the MgO concentration (MgO) in the CaOandAl.sub.2O.sub.3-containing slag. A denitrification test was conducted in which the MgO concentration (MgO) in the slag was varied from 0 to 17 mass % while the stirring power density was held constant at 500 W/t, with the furnace atmospheric pressure P being 0.710.sup.5 Pa and the initial nitrogen concentration [N].sub.i, the Al concentration [Al], the C/A in the slag composition, the molten steel temperature T.sub.f, and the processing time t being the same as those mentioned above. As a result, as shown in FIG. 4, when the MgO concentration (MgO) in the slag was at a level of 5 mass % or lower, a low nitrogen concentration range (where the nitrogen concentration [N].sub.f is 35 mass ppm or lower) was reached, but at a concentration higher than that, the achieved nitrogen concentration [N].sub.f did not decrease and remained high. The result of the study as just described led to the development of the second embodiment, i.e., a molten steel denitrification method in which, in addition to the above-described first embodiment, further the MgO concentration (MgO) in the slag is limited to 5 mass % or lower. It is preferable that a molten steel temperature after the denitrification process is used as the molten steel temperature T.sub.f, and that the denitrification process is completed at 1600 C. or higher, although it depends on a casting step that is a later step and a transfer time. While the lower limit of the MgO concentration (MgO) in the slag is not particularly limited, it may be 0 mass %.

Third Embodiment

(16) A third embodiment was found in the course of exploring improvement measures for a decrease in the denitrification speed in the case where it is unavoidable to increase the MgO concentration from the viewpoint of protecting the refractory of the vessel into which the molten steel is charged. Using the aforementioned small-sized high-frequency vacuum induction melting furnace, a study was conducted on the molten steel temperature T.sub.f that was required to reduce the nitrogen [N].sub.f in the molten steel to 25 mass ppm when the MgO concentration (MgO) in the CaOandAl.sub.2O.sub.3-containing slag was changed from 0 mass % to a saturated concentration. As a result, as shown in FIG. 5, at each time when the MgO concentration (MgO) in the slag was increased by 1.0 mass %, the molten steel temperature T.sub.f needed to be raised by about 5 C. As preconditions for the study, the furnace atmospheric pressure P was 410.sup.3 Pa; the Al concentration [Al] was 0.7 mass %; the initial nitrogen concentration [N].sub.i was 50 mass ppm; the C/A in the slag composition was 1.2; the stirring power density was 60 W/t; and the processing time t was 30 minutes. This study has quantitatively revealed the amount of increase in the molten steel temperature that can compensate for a decrease in the denitrification reaction due to an increase in the MgO concentration. The result of the study as just described led to the development of the third embodiment, i.e., a molten steel denitrification method in which, in addition to the first embodiment, the temperature of the molten steel is increased by 5 C. or more at each time when the MgO concentration (MgO) in the slag increases by 1.0 mass % beyond 5.0 mass %.

Fourth Embodiment

(17) Patent Literature 3 requires an Al concentration [Al] in molten steel of 0.3 mass % to 2 mass % as a concentration needed to increase the ratio of nitrogen distribution between slag and metal, which makes it costly to produce ordinary steel. A fourth embodiment was found in the course of exploring the possibilities of removing nitrogen with the Al concentration [Al] in the molten steel reduced to a lower concentration to solve this problem. As a result of studying the minimum required Al concentration [Al].sub.e for reducing nitrogen in molten steel to 25 mass ppm using the aforementioned small-sized high-frequency vacuum induction melting furnace, it was found that, as shown in FIG. 6, the required Al concentration [Al].sub.e (mass %) varied according to the stirring power density (W/t). 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 0.710.sup.5 Pa; the stirring power density was controlled so as to remain constant within a range of 200 to 2000 W/t during the process; and the processing time t was 30 minutes. The result of the study as just described led to the development of the third embodiment, i.e., a molten steel denitrification method in which, in addition to any one of the first to third embodiments, in the Al addition step, the Al concentration [Al] (mass %) in the molten steel is set to be equal to or higher than a value [Al].sub.e calculated by the following Formula (A) based on the stirring power density (W/t) during the denitrification process.

(18) $\begin{array}{cc}{\left[\mathrm{Al}\right]}_e=-0\text{.072}\ln \left(\right)+0.5822& \left(A\right)\end{array}$
(Steel Production Method)

(19) It is preferable that molten steel produced by the above-described molten steel denitrification method is cast 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 produce high-grade steel which is low-nitrogen steel and of which various components have been adjusted.

EXAMPLES

(20) In the following, examples of the present invention will be described in detail. Using the device having the configuration of FIG. 1, metal Al was added to molten steel at 1600 C. or higher inside a ladle to set the Al concentration in the molten steel to 0.085 to 0.1 mass %. CaO and refractory-protecting MgO were added to form CaOAl.sub.2O.sub.3 binary slag or CaOAl.sub.2O.sub.3MgO ternary slag. Then, a bottom-blown stirring gas was supplied at a stirring power density of 60 to 1000 W/t. The test was conducted using an amount of molten steel of 160 t. The mass ratio C/A between CaO and Al.sub.2O.sub.3 in the slag composition was within a range of 0.4 to 1.8.

(21) Table 1 shows the test conditions and the results. In processes No. 1 to 4 in which the stirring power density is sufficient, the results were favorable with the post-process N concentration [N].sub.f at 35 mass ppm or lower. By contrast, in process No. 5 in which the stirring power density is low, nitrogen was not sufficiently removed in the same processing time t.

(22) TABLE-US-00001 TABLE 1 Slag Molten Steel (MgO) [Al] T.sub.f P [N]i [N]f t No. mass % mass % C. W/t 10.sup.5 Pa massppm massppm min Remarks 1 10 0.1 1600 60 0.67 50 35 30 Invention Example 2 5 0.1 1600 60 0.67 50 30 30 Invention Example 3 10 0.1 1625 200 0.67 50 25 30 Invention Example 4 10 0.085 1625 1000 0.67 50 13 30 Invention Example 5 10 0.1 1600 30 1.0 50 49 30 Comparative Example

INDUSTRIAL APPLICABILITY

(23) 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 molten steel denitrification method according to the present invention can stably mass-produce low-nitrogen steel. Thus, this method contributes to reducing CO.sub.2 and is industrially useful.

REFERENCE SIGNS LIST

(24) 1 Vessel 2 Refractory 3 Molten steel 4 CaOandAl.sub.2O.sub.3-containing slag 5 Bottom-blowing nozzle 6 Gas pipe 7 Stirring inert gas 8 Exhaust system 9 Alloy addition system 10 Vacuum vessel