Patent Application
20240271234
The present invention relates to a method for secondarily refining molten steel charged in a reaction vessel, such as a ladle, by reactions among the molten steel, slag added and formed on top of the molten steel, and oxygen-containing gas blown onto the slag, and to a steel production method.
Nitrogen is a harmful element for metal materials. In a conventional steel production process, nitrogen [N] in molten iron is removed mainly by having it adsorbed onto the surfaces of gas bubbles of carbon monoxide that is generated during a decarburization process 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 steel production process needs to shift from a conventional method using a blast furnace and a converter to a method including melting scrap or reduced iron. In that case, molten iron obtained has a low carbon concentration, which may make it impossible to produce low-nitrogen steel for the above-described reason.
In this context, methods of removing nitrogen from molten steel using slag have been proposed. For example, Patent Literature 1 proposes a method for inexpensively producing low-nitrogen steel even when the molten steel has a low carbon concentration. In this method, after molten steel is melted by an electric furnace using scrap as a main iron source and tapped to another refining vessel, a metal-Al-containing substance and CaO are added onto a bath surface of the molten steel such that CaO/Al2O3 (hereinafter, “C/A”) in the mass ratio falls within a range of 0.8 to 1.2, and oxygen-containing gas is supplied to the molten steel to thereby promote a denitrification reaction using an AlN formation reaction.
Patent Literature 1: JP-2007-211298A
Non Patent Literature 1: Ueno et al .: Tetsu-to-Hagane, 101 (2015), 74
However, the above-described conventional technology has the following problems.
It is mentioned in the technology described in Patent Literature 1 that the C concentration in the molten steel before processing is set to 0.01 to 0.05 mass % while no carbon is added to the molten steel. In a step preceding a denitrification process, decarburization (primary refining) of molten metal is performed in a converter or an electric furnace by blowing oxygen. In this case, when the C concentration in the molten steel becomes 0.05 mass % or lower, the decarburization efficiency decreases rapidly, leading to problems such as generation of FeO, a reduction in the iron yield, and prolongation of the processing time in the converter or the electric furnace.
Another problem is that the method of Patent Literature 1 causes erosion of the refractory of the ladle. This is presumably because when the value of C/A becomes low, slag is formed and adds to erosion of the refractory.
Further, the method described in Patent Literature 1 is faced with a challenge that when the oxygen gas is supplied to the molten steel, carbon in the molten steel reacts with the oxygen gas to generate carbon monoxide gas, which causes the slag present on top of the molten steel to expand and overflow. Slag overflow is believed to occur due to rapid generation of the CO gas.
While removing sulfur, other than nitrogen, in molten steel is also a role of secondary refining, Patent Literature 1 does not mention simultaneous processing of denitrification and desulfurization. Thus, removing sulfur in molten steel incurs an additional production cost, as it requires separately providing a process such as forming slag composed mainly of CaO and Al2O3 while performing electrode heating by a ladle furnace (LF), for example, and removing sulfur while bringing the slag and the molten steel into contact with each other.
The present invention has been devised in view of these circumstances, and an object thereof is to propose a molten steel secondary refining method by which, when secondarily refining molten steel by blowing oxygen-containing gas while performing a molten steel denitrification process by bringing slag and Al-containing molten steel into contact with each other, a low nitrogen concentration range can be stably reached at high speed without causing erosion of the refractory of the vessel, such as a ladle, or overflow of the slag. Another object is to propose a molten steel secondary refining method by which this denitrification process and a desulfurization process can be performed within the same process to efficiently remove nitrogen, or nitrogen and sulfur, from molten steel. Further, a steel producing method that uses molten steel produced by this molten steel secondary refining method is proposed.
As a result of vigorously conducting studies in view of the above-described problems, the present inventors have found that appropriately managing the Al concentration in molten steel when blowing oxygen-containing gas so as to pierce slag to reach the molten steel can promote slag formation by heating of Al even when the slag composition has a low C/A ratio, and can thereby restrict the decarburization reaction and reduce the CO gas generation speed.
A first method for secondarily refining molten steel according to the present invention that advantageously solves the above-described problems is a method for secondarily refining molten steel wherein 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 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]i (mass %) in the molten steel immediately before the oxygen blowing process is equal to or higher than a value [Al]e calculated by Formula (A) based on a stirring power density ϵ (W/t) during formation of the slag, and an Al concentration [Al]f upon completion of the oxygen blowing process is 0.03 mass % or higher,
The following characteristics would make the first method for secondarily refining molten steel according to the present invention a more preferable solution:
A second method for secondarily refining molten steel according to the present invention that advantageously solves the above-described problems is a molten steel secondary refining method in which CaO-and-Al2O3-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 one of the first methods for secondarily refining molten steel described above 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 Al2O3 concentration (mass %) in the slag is controlled between 1.8 and 2.2, inclusive.
A method of producing steel according to the present invention that advantageously solves the above-described problems is characterized in that molten steel produced by any of the first methods for secondarily refining molten steel or molten steel produced by the second method for secondarily refining molten steel is cast, after optionally adjusting the composition.
According to the present invention, when performing a secondary refining process including a denitrification process of molten steel by blowing oxygen-containing gas onto slag, it is possible to stably remove nitrogen in the molten steel to a low nitrogen concentration range at high speed without causing erosion of the refractory of the vessel, such as a ladle, and overflow of the slag. It is further possible to promote removal of sulfur in the molten steel. By casting such low-nitrogen molten steel or low-nitrogen, low-sulfur molten steel after adjusting other compositions as necessary, economically excellent high-grade steel can be produced, which makes the present invention industrially useful.
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.
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. 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.
To form the CaO-and-Al2O3-containing slag 4, CaO resulting from adding the CaO-containing substance and Al2O3 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.
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.
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.
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
First, while the amount of Al-containing substance added in the Al addition step was varied, a relationship between an Al concentration [Al]i in the molten steel immediately before the oxygen blowing process involving blowing O2 gas and an achieved N concentration [N]f was studied. As shown in
Next, using the aforementioned small-sized high-frequency vacuum induction melting furnace, the minimum Al concentration [Al]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
Further, using the aforementioned small-sized high-frequency vacuum induction melting furnace, a relationship between the Al concentration [Al]f in molten steel after the oxygen blowing process and a slag forming index was studied while the Al concentration [Al]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
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
The present inventors studied a relationship between the Al concentration [Al]f in the molten steel after the oxygen blowing process and the ratio Ls/Ls0 (−) between a measurement result of the slag thickness Ls0 (m) at the stage where the CaO-and-Al2O3-containing slag has melted before the oxygen blowing process and the depth Ls (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
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 Al2O3 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
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
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
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/SiO2)/Al2O3) 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)/(Al2O3) becomes 1.8 to 2.2 relative to the amount of Al2O3 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 SiO2 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.
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 Ls/Ls0 between the slag thickness Ls0 and the depth Ls of the depression resulting from blowing of the oxygen-containing gas higher than 1.
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.
In the following, examples of the present invention will be described in detail. Using an apparatus having the configuration of
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]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.
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 CO2 emissions and is industrially useful.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-098139 | Jun 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/020010 | 5/12/2022 | WO |
