Patent Application
20240271235
The present invention relates to a method of removing nitrogen in molten steel charged in a reaction vessel, such as a ladle, through reactions among the molten steel, slag added and formed on top of the molten steel, and an oxygen-containing gas blown onto the slag, and to a production method of steel smelted by this method.
Nitrogen is a harmful component for metal materials. In a conventional steelmaking process, nitrogen [N] in molten iron is removed mainly by having it adsorbed onto the surfaces of air bubbles of carbon monoxide that is generated during a decarburization treatment of molten pig iron. Therefore, when it comes to molten steel with a low carbon concentration, due to the limited amount of carbon monoxide to be generated, a similar technique cannot remove nitrogen to a low concentration.
Meanwhile, to reduce CO2 emissions, the steelmaking process needs to shift from a conventional method of using a blast furnace or a converter to a method of melting scrap or reduced iron. In that case, molten iron obtained has a low carbon concentration, which may make it impossible to smelt low-nitrogen steel for the above-described reason.
in this context, some methods of removing nitrogen from molten steel using slag have been proposed. For example, Patent Literature 1 shows a method in which an Al concentration in molten steel is held at a concentration of 0.7 mass % or higher in a VOD furnace for at least five minutes to form aluminum nitride (hereinafter “AlN”) and thereby remove nitrogen.
Patent Literature 2 shows a method in which molten steel is smelted in an electric furnace using iron scrap as a main iron source, and after the molten steel is discharged into another refining vessel and held therein, a denitrification flux including an Al-containing substance is added to make the AlN transition to slag, and then an oxygen-containing gas is blown onto the molten steel to remove nitrogen.
Patent Literature 3 shows a method in which molten metal is charged into a refining vessel having a gas top-blowing function, and after the surface of this molten metal is covered with slag composed mainly of CaO and Al2O3, an oxidizing gas is blown onto the surface of this covering slag to such an extent that this gas does not directly contact the molten metal to thereby remove nitrogen.
However, these conventional technologies have the following problems.
The technologies described in Patent Literatures 1 and 2, which use the formation of AlN for denitrification, have a problem in that part of the AlN formed remains in the molten steel and constitutes a starting point of cracking during casting in a later step.
Moreover, smelting low-nitrogen steel with a nitrogen content in the order of a few tens of mass ppm by a denitrification method using the formation of AlN requires an Al concentration of at least about a few mass % to 10 mass %, with the solubility products of Al and N taken into account. Or effectively utilizing the denitrification reaction requires an initial nitrogen concentration in the order of a few hundred mass ppm. The problem is that the technologies described in Patent Literatures 1 and 2 are extremely costly for smelting low-nitrogen steel in terms of process and therefore applicable only to those types of steel that have large amounts of dissolved nitrogen, such as stainless steel.
As conditions for shielding the molten steel from the oxidizing gas, the technology described in Patent Literature 3 presents the following:
As for condition (1), the amount of slag increases according to the size of the vessel into which molten steel is charged. As for condition (2), specific control means and control ranges are not described, and a method for checking whether the molten steel is shielded from the gas is not clear. Thus, compatible conditions are ambiguous. The present inventors have confirmed that when a test is conducted using the same ranges as those in the compatible example described in Patent Literature 3, the denitrification speed becomes actually slow as the movement of nitrogen between the slag and the metal is restricted as a result of an increase in apparent oxygen partial pressure in the slag-metal interface due to the oxidizing gas, which makes this technology not practical for operation.
The present invention has been contrived in view of these circumstances, and an object thereof is to propose a molten steel denitrification method by which an extremely low nitrogen concentration range can be stably reached in a short time when performing a denitrification treatment of molten steel using slag. The present invention further proposes a steel production method that uses molten steel smelted by this molten steel denitrification method.
As a result of vigorously conducting studies in view of the above-described problems, the present inventors have found that, to achieve a high denitrification speed in a denitrification treatment of blowing an oxygen-containing gas onto slag and removing nitrogen in molten steel through the slag, it is necessary to reduce the T·Fe concentration in the slag after the treatment to or below a certain value.
A molten steel denitrification method according to the present invention that advantageously solves the above-described problems is a molten steel denitrification method in which CaO-and-Al2O3-containing slag is 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, and then an oxygen-containing gas is blown from above the slag to perform a denitrification treatment, characterized in that T·Fe in the slag after the denitrification treatment is set to 3.0 mass % or lower.
Molten steel denitrification methods according to the present invention in which the following apply could be more preferable solutions:
A steel production method according to the present invention that advantageously solves the above-described problems is characterized in that molten steel smelted by any one of the above-described molten steel denitrification methods is cast after its components are arbitrarily adjusted.
The present invention makes it possible to stably remove nitrogen to an extremely low nitrogen concentration range in a short time when performing a denitrification treatment of molten steel using slag.
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.
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 12 or may be performed in a step before entering the vacuum vessel 13. The step of deoxidizing the molten steel 3 (deoxidation 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 deoxidation step is preferable, because then the temperature rise of the molten steel due to the deoxidation reaction can be used to flux the slag. Performing the CaO addition step after the Al addition step is further preferable, because this can reduce deoxidation failure or variation in the slag composition due to the added Al-containing substance being hindered by the thick slag from reaching the molten steel.
To form the CaO-and-Al2O3-containing slag 4, Al2O3 resulting from adding the CaO-containing substance and deoxidizing the molten steel is used. This may be done using, as the CaO-containing substance, for example, calcium aluminate that is a pre-melted or pre-mixed product. As for the slag composition, a higher melting ratio (fluxing ratio) of the slag is more advantageous for the denitrification reaction, and a mass ratio C/A between CaO and Al2O3 is preferably within a range of 0.4 to 1.8 and more preferably within a range of 0.7 to 1.7.
The form of supplying the stirring gas 10 into the molten steel may be, other than the above-described method, for example, a form of injecting it into the molten steel through an injection lance for blowing in an inert gas. Next, preferred embodiments of the present invention will be described in detail along with how they were developed.
A first embodiment was devised out of necessity to clearly and quantitatively present conditions advantageous for denitrification, for, even when a test was conducted within the range of the compatible example in Patent Literature 3, denitrification was not stable and the reached nitrogen concentration was not reduced, either. In a small-sized high-frequency vacuum induction melting furnace satisfying the configuration requirements of
Of the tests described above, in a test in which the T·Fe concentration (T·Fe) in the slag was 15 mass % or higher, it was clearly recognizable with the naked eye that the oxygen gas had penetrated through the slag layer and exposed the surface of the molten steel. By contrast, in a test in which the T·Fe concentration was lower than 15%, clear exposure of the surface of the molten steel was not recognized at any locations including the surface onto which the oxygen gas was blown. Thus, removing nitrogen to a low nitrogen concentration range in accordance with what was described in Patent Literature 3 proved difficult. The result of the study as just described led to the development of the first embodiment, i.e., a molten steel denitrification method in which CaO-and-Al2O3-containing slag is 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, and then an oxygen-containing gas is blown from above the slag to perform a denitrification treatment, wherein T·Fe in the slag after the denitrification treatment is set to 3.0 mass % or lower. The lower limit of T·Fe in the slag may be 0 mass %. 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.
A second embodiment was found in the course of conducting tests in the aforementioned small-sized high-frequency vacuum induction melting furnace to address the challenge of how to control the T·Fe concentration (T·Fe) in the slag after the denitrification treatment to 3.0 mass % or lower. First, an X-ray diffraction (XRD) analysis was performed on each of slags after the denitrification treatment, respectively obtained in a test in which the oxygen gas clearly penetrated through the slag layer and the nitrogen concentration [N] in the molten steel after the treatment was higher than 35 mass ppm, and a test in which the surface of the molten steel was not exposed during the test and the concentration decreased to 35 mass ppm or lower. As a result, as shown in
To control the ratio Ls/Ls0 of the slag depression depth, a method of increasing or decreasing the lance height or the gas flow rate, appropriately shaping the nozzle tip of the gas top-blowing lance, and various other methods can be adopted. The present inventors have confirmed that if, for example, the value of L/Ls0 when the lance height is changed and the value of Ls/Ls0 when the gas flow rate is changed are the same, the T·Fe concentrations (T·Fe)f in the slag are equivalent, and that no difference due to the difference in control means occurs. Depending on the scale of the device, the thickness of the slag being treated can decrease for reasons such as part of the slag infiltrating into the refractory or the slag getting involved into the molten steel as the molten steel is stirred. However, the upper limit value of the ratio Ls/Ls0 of the slag depression depth should be adjusted to be lower than 0.9 as appropriate based on this technical idea.
A third embodiment was found in the course of conducting studies to make the present invention applicable also to a facility in which it is difficult to control the T·Fe concentration in the slag through the ratio L/Ls0 of the slag depression depth for some reason, such as ascending and descending of the top-blowing lance being controlled stepwise. Specifically, this embodiment involves reducing the oxygen gas concentration in the oxygen-containing gas. In a test using the aforementioned small-sized high-frequency vacuum induction furnace, a denitrification treatment was performed while an inert gas was supplied through the gas pipe 9 to reduce the oxygen concentration in the gas blown onto the slag from 1.5 mass % (industrial crude Ar level) to 0.1 mass ppm (industrial Ar level). Here, as the inert gas, a gas that does not include nitrogen is used. As a result, as shown in
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 smelt 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 an even lower concentration to solve this problem. In the aforementioned small-sized high-frequency vacuum induction melting furnace, a minimum required Al concentration [Al], for reducing the nitrogen [N]f in molten steel to 25 mass ppm was studied. As a result, as shown in
A fifth embodiment was found in the course of studying an influence that a reached degree of vacuum P inside the vacuum vessel exerted on the reached nitrogen concentration [N]f. In the aforementioned small-sized high-frequency vacuum induction melting furnace, the reached nitrogen concentration [N]f was studied by performing a denitrification treatment several times at different timings, with the ratio Ls/Ls0 of the slag depression depth set to 0.9 in the case where the gas blown onto the CaO-and-Al2O3-containing slag was an oxygen gas and with the ratio Ls/Ls0 of the slag depression depth set to 1.2 in the case of a diluted gas (with an oxygen concentration in the gas 0.1 ppm to 1.5 mass %). As a result, as shown in
A sixth embodiment was found in the course of studying an influence of the MgO concentration (MgO) in the CaO-and-Al2O3-containing slag. Using the aforementioned small-sized high-frequency vacuum induction melting furnace, a study was conducted on a molten steel temperature Tf that was required to reduce the nitrogen [N]f in the molten steel to 25 mass ppm when the MgO concentration (MgO) in the CaO-and-Al2O3-containing slag was changed over a range of 0 mass % to a saturated concentration. As a result, as shown in
It is preferable that molten steel smelted by the above-described molten steel denitrification method be 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.
In the following, examples of the present invention will be described in detail. Using the device having the configuration of
Table 1 shows the test conditions and the results. Treatments No. 1 to 7 in which the T·Fe concentration (T·Fe) in the slag is sufficiently low produced good results with the N concentration [N]f after the treatment being 35 mass ppm or lower. By contrast, in treatment No. 8 in which T·Fe concentration (T·Fe) in the slag is high, denitrification in same treatment time was insufficient.
When applied to a steelmaking 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 CO2 and is industrially useful.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-098118 | Jun 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/020007 | 5/12/2022 | WO |
