Secondary refining method of molten steel and method for manufacturing steel

Abstract

This invention provides a secondary refining method for molten steel that can stably achieve low nitrogen concentration ranges at high speeds without causing operational problems, and a method for manufacturing steel. The secondary refining method for molten steel is as follows: An Al addition step is combined with adding a metallic Al substance to the molten steel to produce Al-containing steel, and a CaO addition step is combined with adding a CaO substance to the molten steel to form a slag containing CaO and Al₂O₃. Then, oxygen-containing gas is injected through the slag into the Al-containing steel to perform an oxygen-feeding treatment, including denitrification. This method includes: adjusting the Al concentration [Al] in the molten steel before the oxygen-feeding treatment. i (mass%) is set as the value calculated by the following formula (A) based on the stirring dynamic density ε (W / t) during slag formation [A1]. e The above represents the Al concentration [Al] at the end of the oxygen delivery treatment. f The content is set to 0.03% by mass or higher. The above-mentioned steel manufacturing method includes: adjusting the composition of the obtained molten steel before casting. [Al] e =‑0.072×ln(ε)+0.5822···(A).

Description

Technical Field

[0001] This invention relates to a method for secondary refining of molten steel by means of a reaction based on molten steel filling a reaction vessel such as a ladle, slag added to / formed on the molten steel, and oxygen-containing gas injected into the slag, and a method for manufacturing steel. Background Technology

[0002] Nitrogen is a harmful component for metallic materials. In existing steelmaking processes, nitrogen [N] in molten iron is adsorbed onto the surface of carbon monoxide bubbles generated primarily during the decarburization process of molten pig iron and removed. Therefore, for molten steel with low carbon concentration, since the amount of carbon monoxide produced is limited, nitrogen cannot be removed to a low concentration using the same method.

[0003] On the other hand, in order to reduce CO2 emissions, steelmaking processes need to be changed from using existing blast furnaces and converters to methods that melt scrap iron and reduced iron. In this case, the carbon concentration of the resulting molten iron is reduced, and for the reasons mentioned above, there is a risk that low-nitrogen steel cannot be smelted.

[0004] Therefore, methods for denitrification from molten steel that has used slag have been proposed. For example, Patent Document 1 proposes the following method: In an electric furnace, scrap iron is used as the main iron source to melt molten steel. After the steel is tapped to another refining vessel, a substance containing metallic Al and CaO is added to the surface of the molten steel, so that the CaO / Al2O3 (hereinafter referred to as C / A) reaches a mass ratio of 0.8 to 1.2. Oxygen-containing gas is supplied to the molten steel, thereby carrying out a denitrification reaction utilizing the AlN formation reaction. Even molten steel with low carbon concentration can be smelted into low-nitrogen steel at low cost.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2007-211298

[0008] Non-patent literature

[0009] Non-patent literature 1: Ueno et al., Iron and Steel, 101(2015), 74 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] However, the above-mentioned prior art has the following problems.

[0012] That is, the technology described in Patent Document 1 involves setting the C concentration in the molten steel before treatment to 0.01 to 0.05% by mass, without adding carbon to the molten steel. In the pre-denitrification process, decarburization (primary refining) is carried out from the molten metal in a converter or electric furnace by introducing oxygen. In this case, if the C concentration in the molten steel is below 0.05% by mass, the following problems occur: the decarburization efficiency decreases sharply, FeO is generated, the yield decreases, and the processing time in the converter or electric furnace is prolonged.

[0013] Furthermore, the method in Patent Document 1 suffers from the problem of refractory loss during casting. This can be presumed to be due to the fact that if the C / A ratio decreases, the slag deteriorates, leading to increased refractory loss.

[0014] Furthermore, in the method described in Patent Document 1, when oxygen is supplied to the molten steel, the carbon in the molten steel reacts with the oxygen to produce carbon monoxide gas. The slag present on the molten steel expands, also causing overflow. It can be assumed that the slag overflow is due to the rapid generation of CO gas.

[0015] In addition to nitrogen removal, sulfur removal from molten steel is also a secondary refining process, but Patent Document 1 does not describe simultaneous denitrification and desulfurization. Therefore, for sulfur removal from molten steel, a separate process must be set up, such as simultaneously heating the slag in an LF (ladle furnace) with electrodes while slag is formed with CaO and Al2O3 as the main components, and then bringing the slag into contact with the molten steel to remove sulfur, which increases production costs.

[0016] This invention was made in view of the following circumstances, and its object is to provide a secondary refining method for molten steel. This method, while simultaneously denitrifying the molten steel by contacting slag with Al-containing molten steel and injecting oxygen-containing gas for secondary refining, can stably achieve a low nitrogen concentration range at high speed without melting or damaging the refractory materials of the ladle or other containers, and without causing slag overflow. Furthermore, this invention provides a secondary refining method for molten steel in which the denitrification and desulfurization treatments are performed in the same process, enabling efficient denitrification or both denitrification and desulfurization. Additionally, this invention provides a method for manufacturing steel using molten steel refined through this secondary refining method.

[0017] Problem Solving Methods

[0018] The inventors conducted in-depth research on these issues and found that by properly managing the Al concentration in the molten steel when injecting oxygen-containing gas through the slag to reach the molten steel, even slag compositions with low C / A ratios can promote slagging by heating Al, suppress decarburization reactions, and reduce the rate of CO gas generation.

[0019] The secondary refining method for the first molten steel of the present invention advantageously solves the above-mentioned problems. The method comprises: combining an Al addition step (adding a metallic Al substance to the molten steel to produce Al-containing molten steel) and a CaO addition step (adding a CaO substance to the molten steel to produce CaO-containing molten steel) to form a slag containing CaO and Al₂O₃; then, injecting oxygen-containing gas through the slag into the Al-containing molten steel to perform an oxygen-feeding treatment including denitrification; the method further comprises: adjusting the Al concentration [Al] in the molten steel before the oxygen-feeding treatment. i (mass%) is set as the value calculated by the following formula (A) based on the stirring dynamic density ε (W / t) during slag formation described above [A1]. e The above represents the Al concentration [Al] at the end of the above oxygen delivery treatment. f Set to 0.03% by mass or higher.

[0020] [Al] e =-0.072×ln(ε)+0.5822···(A).

[0021] Furthermore, it can be considered that, in the secondary refining method for the first molten steel of the present invention, the following (a) to (e) are more preferred solutions:

[0022] (a) The above-mentioned Al addition step includes the process of adding a metallic Al substance to the above-mentioned molten steel to produce deoxidized molten steel;

[0023] (b) In the above oxygen delivery treatment middle, When the above-mentioned oxygen-containing gas is supplied, the indentation depth L caused by the injection of oxygen-containing gas will be reduced. S With respect to the thickness L of the aforementioned slag S0 The ratio of L S / L S0 Set to 1.0 or higher;

[0024] (c) Set C / A(-) to 0.4 or more and 1.8 or less, wherein C / A(-) is the mass ratio of CaO concentration (CaO) (mass%) to Al2O3 concentration (Al2O3) (mass%) in the above-mentioned slag;

[0025] (d) The MgO concentration (MgO) (mass%) in the above slag is set to be 0.25 or less based on its mass ratio to the CaO concentration (CaO) (mass%).

[0026] (e) The above-mentioned oxygen supply treatment brings the surface temperature of the Al-containing molten steel and slag to 9.3 × 10⁻⁶. 4 Reduced pressure gas atmosphere below Pa; etc.

[0027] The second refining method for molten steel according to the present invention advantageously solves the above-mentioned problems. The method is as follows: a slag containing CaO and Al2O3 is formed on Al-containing molten steel filled in a container; oxygen-containing gas is injected through the slag into the Al-containing molten steel; and the slag is brought into contact with the molten steel to remove nitrogen and sulfur from the molten steel. The method includes: in the oxygen supply treatment carried out in any of the above-mentioned second refining methods for molten steel, maintaining the Al concentration in the molten steel at 0.05% by mass or more, and controlling the ratio of CaO concentration (by mass) to Al2O3 concentration (by mass) in the slag, C / A(-), to be 1.8 or more and 2.2 or less.

[0028] The steel manufacturing method of the present invention, which advantageously solves the above-mentioned problems, is characterized in that the composition of the molten steel melted by any of the above-mentioned secondary refining methods of the first molten steel or the secondary refining method of the second molten steel is arbitrarily adjusted, and then cast.

[0029] The effects of the invention

[0030] According to the present invention, during the secondary refining process involving denitrification of molten steel by injecting oxygen-containing gas into the slag, nitrogen in the molten steel can be stably removed to a low nitrogen concentration range at high speed without melting or damaging the refractory materials of vessels such as ladles, and without causing slag overflow. Furthermore, the removal of sulfur from the molten steel can also be promoted. By casting such low-nitrogen molten steel or low-nitrogen, low-sulfur molten steel with other components adjusted as needed, high-grade steel with excellent economic efficiency can be produced, which is industrially useful. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating an example of an apparatus for a secondary refining method of molten steel suitable for one embodiment of the present invention.

[0032] Figure 2 This shows the Al concentration [Al] in the molten steel adjusted through the Al addition step. i With respect to the reached nitrogen concentration [N] f A diagram showing the relationship between the two.

[0033] Figure 3 This illustrates the methods used to obtain the nitrogen concentration [N]. f =25 ppm (by mass) Al concentration in molten steel before oxygen treatment [Al] e A graph showing the relationship between the stirring dynamic density ε during slag formation and the slag.

[0034] Figure 4 This indicates the Al concentration [Al] at the end of the treatment. f A chart showing the relationship between the slag forming index and the slag forming index.

[0035] Figure 5It shows L S / L S0 The Al concentration [Al] in the molten steel reaches a certain level. f A diagram of the relationship, the L S / L S0 The indentation depth L of the slag is caused by oxygen-containing gas. S With initial slag thickness L S0 The ratio.

[0036] Figure 6 This shows the C / A(-) ratio and the nitrogen concentration [N] reached in the molten steel. f The graph shows the relationship between C / A(-), which is the ratio of CaO concentration (C) to Al2O3 concentration (A) in the slag on a mass basis.

[0037] Figure 7 This shows the ratio of MgO concentration (MgO) to CaO concentration (CaO) in the slag and the nitrogen concentration [N] reaching the molten steel. f A diagram showing the relationship between the two.

[0038] Figure 8 This is a graph showing the relationship between the ratio of MgO concentration (MgO) to CaO concentration (CaO) in slag and the melting loss index of the refractory.

[0039] Figure 9 This indicates the upper limit of the deviation between the furnace pressure P and the nitrogen concentration in the molten steel, denoted as Max[N]. f A diagram showing the relationship between the two.

[0040] Symbol Explanation

[0041] 1 Container

[0042] 2 Refractory materials

[0043] 3. Molten Steel

[0044] 4. Slag containing CaO and Al2O3

[0045] 5. Gas piping (oxygen)

[0046] 6. Gas Top-Blow Spray Gun

[0047] 7 O2-containing gas

[0048] 8 Bottom-blowing nozzles

[0049] 9. Gas piping (non-reactive gases)

[0050] 10. Inactive gas for stirring steel bath

[0051] 11 Exhaust System

[0052] 12 Alloy Addition System

[0053] 13 Vacuum Container

[0054] 14 Middle Cover Detailed Implementation

[0055] The embodiments of the present invention will now be described in detail. It should be noted that the accompanying drawings are schematic diagrams and may sometimes differ from reality. Furthermore, the following embodiments exemplify apparatus and methods for embodying the technical concept of the present invention, and do not limit the configuration to the aforementioned configurations. That is, various modifications can be made to the technical concept of the present invention within the scope of the technology described in the claims.

[0056] exist Figure 1 The preferred apparatus configuration for carrying out the present invention is shown. Molten steel 3 is filled into a container 1, such as a ladle lined with refractory material 2, and slag 4 containing CaO and Al2O3 is formed on it. In a vacuum container 13 equipped with an exhaust system 11 and an alloying system 12, the surfaces of the molten steel 3 and slag 4 are kept under a reduced-pressure gas atmosphere, and O2-containing gas is sprayed onto the slag 4 from a top-blowing gas nozzle 6 connected to a gas pipe 5. The molten steel 3 is stirred by blowing an inactive stirring gas 10 into the slag 4 through a bottom-blowing nozzle 8 connected to a gas pipe 9. The inactive stirring gas 10 is preferably Ar gas, which does not contain nitrogen. The top-blowing gas nozzle 6 is inserted through a middle cover 14 covering the upper part of the container 1.

[0057] The process of adding metallic Al to molten steel 3 and deoxidizing it to produce Al-containing molten steel (Al addition step), and the process of adding CaO to molten steel 3 (CaO addition step) can be performed using alloy addition system 12, or can be performed before placing it into vacuum container 13. The deoxidation process of molten steel 3 (deoxidation step) can be performed separately from the Al addition step, or deoxidation can be performed within the Al addition step. If there is a deoxidation step preceding the treatment, the addition of metallic Al can be performed before, after, or separately before or after the deoxidation step. If Al is added before the deoxidation step, it is expected that the temperature of the molten steel will be maintained at a high level through Al combustion; if added after the deoxidation step, it is expected that denitrification will be achieved. Furthermore, both of these effects can be expected when added separately before and after the deoxidation step. The CaO addition step can be performed at any time. If the CaO addition step is performed after the deoxidation step, the increase in molten steel temperature caused by the deoxidation reaction can be used for slag formation, which is therefore preferred. If the CaO addition step is performed after the Al addition step, it can suppress poor deoxidation or deviations in slag composition caused by the added Al-containing substances being blocked from reaching the molten steel by the thick slag. Therefore, it is a further preferred step.

[0058] The formation of slag 4 containing CaO and Al2O3 utilizes the addition of CaO-containing substances and the Al2O3 produced by deoxidation of molten steel. Pre-melted or pre-mixed calcium aluminate can also be used as the CaO-containing substance. Regarding the slag composition, a higher slag melting ratio (hereinafter referred to as slag formation rate) is more conducive to the denitrification reaction.

[0059] Furthermore, from the perspective of promoting slag formation, it is preferable to agitate the molten steel, for example, through bottom blowing. In addition to the methods described above, the agitation gas 10 can also be supplied to the molten steel by, for example, by being injected into the molten steel through a jet lance for blowing in inactive gas.

[0060] Next, the preferred embodiments of the present invention will be described in detail with reference to the development process. It should be noted that in this specification, [M] represents the state in which element M is dissolved and contained in molten steel, and (R) represents the state in which chemical substance R is dissolved and contained in slag, with units indicating their respective composition ratios.

[0061] (First Embodiment)

[0062] The first embodiment was developed to achieve denitrification by contacting Al-containing molten steel with slag, and to remove excess Al from the molten steel by supplying oxygen. This was done while satisfying the requirements... Figure 1 In a small high-frequency vacuum induction melting furnace, relative to 15 kg of molten steel 3, slag 4 containing CaO and Al2O3 is formed in an amount so small that it cannot be visually confirmed to reach the surface of the molten steel. This slag is stirred by bottom-blowing gas, and then O2 gas is injected into the slag. It is visually confirmed that the O2 gas, through its ejection pressure, pushes the slag aside and reaches the surface of the molten steel. For example, at the position on the molten metal bath surface corresponding to the buoyancy point of the bottom-blowing gas, the slag thickness becomes thinner due to the bulging of the bath surface. If O2 gas is injected towards this buoyancy point of the bottom-blowing gas, it can easily penetrate the slag and be directly injected into the molten steel.

[0063] First, by varying the amount of Al-containing material added in the Al addition step, the Al concentration in the molten steel prior to the oxygen-feeding treatment based on O2 gas injection was investigated. i With respect to reaching N concentration [N] f The relationship. For example... Figure 2 The figure shows the Al concentration [Al] in the molten steel before oxygen treatment. i The nitrogen concentration [N] is reached when the concentration is less than 0.03% by mass. f Unstable denitrification to below 35 ppm by mass was not achieved. At this point, the furnace atmosphere pressure P was 5.3 × 10⁻⁶. 3 Pa, the initial nitrogen concentration [N] in molten steel. iThe concentration of MgO in the slag is 50 ppm by mass, the slag composition (calculated as the mass ratio of CaO to Al2O3, C / A) is 1.2, the MgO concentration in the slag is 5% by mass, the stirring power density ε is 396 W / t, and the molten steel temperature T is... f The temperature was 1660℃, and the oxygen treatment time (t) was 25 minutes. This can be attributed to the fact that, through the oxygen-containing gas flowing through the slag, the Al in the molten steel was oxidized and reduced, preventing the formation of aluminum nitride (AlN). It should be noted that if the Al concentration in the molten steel before oxygen treatment [Al] is... i Setting the concentration to 0.1% by mass or higher will allow the nitrogen concentration [N] in the molten steel to reach a certain level. f A concentration of less than 30 ppm by mass is preferred. Furthermore, if the Al concentration [Al] in the molten steel before oxygen treatment is... i Setting it to 1.0% by mass or higher will allow the molten steel to reach a nitrogen concentration [N]. f It is preferred because its concentration is below 25 ppm by mass.

[0064] Next, in the aforementioned small-scale high-frequency vacuum induction melting furnace, the Al concentration [Al] before oxygen supply was adjusted to the minimum level required to reduce nitrogen in the molten steel to 25 ppm. e An investigation was conducted, and the results show that... Figure 3 The figure shows the required Al concentration [Al] before oxygen delivery. e (mass%) varies depending on the stirring dynamic density ε (W / t) during slag formation. Here, the MgO concentration (MgO) in the slag is set to 0% by mass, and the molten steel temperature T is... f Set the temperature to 1600℃ and the initial nitrogen concentration [N]... i The C / A ratio of the slag composition is set to the same as above. The furnace gas atmosphere pressure P, which is a premise for the investigation, is set to 0.7 × 10⁻⁶. 5 Pa, the stirring power density ε is controlled within the range of 200-2000 W / t to be constant during slag formation and oxygen delivery, and the oxygen delivery time is set to 30 minutes. It is inferred that denitrification based on slag is carried out by contacting slag and molten steel outside the slag formation before oxygen delivery and the fire point (the part of the molten steel surface exposed by oxygen-containing gas) during the oxygen delivery process.

[0065] Furthermore, in the aforementioned small-scale high-frequency vacuum induction melting furnace, the Al concentration [Al] in the molten steel before oxygen treatment is... i The Al concentration [Al] in the oxygen-treated molten steel was varied by 0.02–0.5% by mass. f The relationship between the slag forming index and free space was investigated. Here, the slag forming index is the ratio of slag height calculated based on slag volume to free space. The results are presented below. Figure 4The symbol "×" indicates the presence of slag overflow, and "○" indicates the absence of slag overflow. At this time, the gas atmosphere pressure P inside the furnace is 1×10⁻⁶. 5 Pa, stirring kinetic density ε is 60 W / t, initial nitrogen concentration [N] in molten steel. i The initial carbon concentration (C) was 0.10% by mass, with a slag composition of 1.2 (C / A based on the mass ratio of CaO to Al2O3). The MgO concentration in the slag was 5% by mass, and the molten steel temperature was T. f The temperature was 1660℃, and the treatment time t was 18 minutes. Additionally, the free space in the ladle was 1.5m. Therefore, the Al concentration in the molten steel after oxygen treatment is [Al]. f Slag overflow will occur if the slag concentration is below 0.03% by mass and the slag forming index exceeds 1. This can be attributed to the competition between the oxidation reactions of Al and C based on the injected O2 gas, resulting in CO gas production as the Al concentration decreases, leading to an increase in slag volume. Based on the results of the investigation described above, a first embodiment was obtained, namely a secondary refining method for molten steel, which involves combining an Al addition step (adding a metallic Al substance to the molten steel to produce Al-containing steel) with a CaO addition step (adding a CaO substance to the molten steel) to form a slag containing CaO and Al2O3. Then, oxygen-containing gas is injected through the slag into the Al-containing molten steel to perform an oxygen delivery treatment including denitrification. This method includes adjusting the Al concentration [Al] in the molten steel before the oxygen delivery treatment. i (mass%) is set as the value calculated by the following formula (A) based on the stirring dynamic density ε (W / t) during slag formation described above [A1]. e The above represents the Al concentration [Al] at the end of the above oxygen delivery treatment. f Set to 0.03% by mass or higher. It should be noted that this refers to the Al concentration [Al] after the oxygen delivery treatment is completed. f Regarding the Al concentration [Al] specified in the product standard... d The value is greater than the value of the following formula (A) [Al]. e In this case, it is preferable to use this value as the target value. In the target value [Al] d The value is less than the value of the following formula (A) [Al]. e In such cases, it is preferable to perform the Al oxidation removal process during oxygen delivery.

[0066] [Al] e =-0.072×ln(ε)+0.5822···(A)

[0067] (Second Implementation)

[0068] The second embodiment requires performing a so-called Al removal process during oxygen delivery, which was developed for the purpose of simplifying the process. In order to meet... Figure 1 In a small high-frequency vacuum induction melting furnace, relative to 15 kg of molten steel 3, slag 4 containing CaO and Al2O3 at a rate of 15 kg / t is formed to a degree that cannot be visually confirmed by the naked eye. O2 gas is injected onto the slag surface at a location other than the buoyancy point of the bottom-blown gas. The inventors et al. [made a study on L...] S / L S0 (-) and the Al concentration [Al] in the molten steel after oxygen treatment. f The relationship was investigated, and the L S / L S0 (-) represents the thickness L of the slag containing CaO and Al2O3 in the molten stage before oxygen delivery treatment. S0 The measured result of (m) is compared with the slag depression depth L obtained by changing the parameters in the formula described in Non-Patent Document 1, specifically the liquid density, gas density, and injection velocity, to values ​​suitable for experimental conditions. S The ratio of (m). The result was the discovery of... Figure 5 As shown, if L S / L S0 If the value is set to 1.0 or higher, the Al concentration in the molten steel can be reduced simultaneously with denitrification during oxygen treatment. At this time, the furnace atmosphere pressure P is 5.3 × 10⁻⁶. 3 Pa, the initial nitrogen concentration [N] in molten steel. i The Al concentration [Al] in the molten steel before oxygen treatment is 50 ppm by mass. i The slag composition, calculated as a CaO / Al2O3 mass ratio (C / A), is 1.2, with a MgO concentration of 10% by mass. The molten steel temperature is T. f The temperature was 1650℃, and the processing time t was 30 minutes. Based on the investigation results described above, a second embodiment was obtained, namely, a secondary refining method for molten steel. This method, in addition to the first embodiment, also involves increasing the thickness L of the slag during the oxygen supply process. S0 The depth L of the depression caused by the blowing of oxygen-containing gas S The ratio of L S / L S0 Set it to 1.0 or higher. It should be noted that if the ratio L... S / L S0 If the ratio is too large, it will cause operational obstacles due to splashing of molten steel, etc. Therefore, it is preferable to keep the ratio L... S / L S0 The upper limit is set at around 1.5, and further optimization is set at around 1.3.

[0069] (Third Implementation)

[0070] The third implementation method was discovered during an investigation into the impact of slag composition, primarily the ratio of CaO concentration (mass%) to Al2O3 concentration (mass%) (C / A) on denitrification. This was achieved while meeting the following requirements. Figure 1 In a small high-frequency vacuum induction melting furnace, where the MgO concentration in the slag is set to 0%, and the C / A ratio is varied from 0.4 to 2.5, as shown in the experiment... Figure 6 As shown, when the C / A ratio is in the range of 0.4 to 2.0, the nitrogen concentration [N] is reached. f Maintain a concentration below 20 ppm by mass. If the C / A ratio exceeds 1.8, the nitrogen concentration [N] is reached. f It begins to rise; if it exceeds 2.0, it reaches the nitrogen concentration [N]. f A sharp increase occurs, and if it exceeds 2.2, it becomes impossible to reach the low nitrogen concentration range (nitrogen concentration [N]). f (35 ppm or less). Based on the survey results described above, a third embodiment was obtained, namely a secondary refining method for molten steel. In addition to the first or second embodiment, the method further sets the mass ratio C / A(-) of the CaO concentration (CaO) (mass%) to the Al2O3 concentration (Al2O3) (mass%) in the slag to be 0.4 or more and 2.2 or less.

[0071] (Fourth implementation)

[0072] The fourth embodiment was discovered during an investigation into the effect of the ratio (MgO) / (CaO)(-) of the MgO concentration (mass%) to the CaO concentration (mass%) in the slag, which is mainly increased due to refractory melting loss, on denitrification. Other parameters besides the MgO concentration in the slag are set as described above. Figure 6 Under the same conditions, with C / A fixed at 1.7, the effect of varying the mass ratio (MgO) / (CaO) on the reached nitrogen concentration [N] was investigated. f The resulting impact will be presented in the following ways. Figure 7 It can be seen that by setting (MgO) / (CaO) to below 0.25, the nitrogen concentration [N] can be achieved. f The concentration is below 35 ppm by mass. Furthermore, by setting the (MgO) / (CaO) ratio to 0.2 or less, the nitrogen concentration [N] can be further reduced. f Similarly, in Figure 8The effect of the ratio (MgO) / (CaO)(-) of the MgO concentration (mass%) to the CaO concentration (mass%) in the slag on the refractory melting loss is shown. If (MgO) / (CaO) is 0.14 or higher, the effect on refractory melting loss is small, and therefore preferred. Based on the investigation results described above, a fourth embodiment was obtained, namely a secondary refining method for molten steel, in which, except for any of the embodiments in the first to third embodiments, the MgO concentration (MgO) (mass%) in the slag is set to a mass ratio of 0.25 or less relative to the CaO concentration (CaO) (mass%). Preferably, the MgO concentration (MgO) (mass%) in the slag is set to a mass ratio of 0.14 to 0.25 relative to the CaO concentration (CaO) (mass%).

[0073] (Fifth Embodiment)

[0074] The fifth implementation method was discovered during a study on the effect of achieving vacuum levels on the denitrification reaction. When the required vacuum level is met... Figure 1 In a small high-frequency vacuum induction melting furnace, a slag 4 containing CaO and Al2O3 was formed in an amount so high that the surface of the molten steel could not be determined by visual observation. This slag 4 had a content of 15 kg / t or more relative to 15 kg of molten steel 3, and an MgO concentration of 0–17% by mass. Then, after adjusting the gas atmosphere pressure P inside the furnace, oxygen was introduced into the molten steel by simultaneously applying stirring power of 2500 W / t and blowing O2 gas through the slag. In an oxygen introduction test that changed the vacuum degree (gas atmosphere pressure) P (Pa) of the furnace gas atmosphere, such as… Figure 9 As shown, the upper limit of the deviation of the treated nitrogen concentration is Max[N]. f (mass ppm) The furnace gas atmosphere pressure P is 9.3 × 10⁻⁶. 4 Below Pa, it can be stably maintained in the low nitrogen range of 35 ppm by mass. At this point, the initial nitrogen concentration [N] in the molten steel is... i The Al concentration [Al] is 50 ppm by mass. i The slag composition, calculated as a CaO / Al2O3 mass ratio (C / A), is 1.2, with a MgO concentration of 5% by mass. The molten steel temperature is T. f The temperature was 1600℃, and the processing time t was 30 minutes. Based on the investigation results described above, a fifth embodiment was obtained, namely, a secondary refining method for molten steel, which, in addition to any one of the embodiments 1 to 4, further involves bringing the surface of the aforementioned Al-containing molten steel and slag to a temperature of 9.3 × 10⁻⁶. 4 A reduced pressure gas atmosphere below Pa. Preferably at 6.7 × 10 Pa. 4A reduced pressure gas atmosphere below Pa. It should be noted that excessive pressure reduction will increase equipment costs such as the exhaust system; therefore, the lower limit of the furnace gas atmosphere pressure P is preferably set to 10. 3 Approximately Pa.

[0075] (Sixth Embodiment)

[0076] The sixth embodiment was discovered during research into whether denitrification and desulfurization could be performed simultaneously. The conditions for oxygen supply are preferably selected from any of the embodiments described above. It is known that to promote the desulfurization reaction, the Mannesmann Slag Index (MSI) = ((CaO / SiO2) / Al2O3) is preferably set to a range of 0.25 to 0.45. CaO-containing substances are preferably added in such a way that the mass ratio (CaO) / (Al2O3) is 1.8 to 2.2 relative to the amount of Al2O3 generated during deoxidation and slag conditioning. MgO clinker is added as needed to achieve an MgO concentration of approximately 0.2 based on the mass ratio of the added CaO portion. Regarding SiO2, concentration control is not actively performed. Since a high C / A ratio in the slag composition is beneficial for desulfurization, it becomes unfavorable from the viewpoint of denitrification. Therefore, to promote denitrification, it is preferable to increase the thickness L of the slag when supplying oxygen-containing gas. S0 The depth L of the depression caused by the blowing of oxygen-containing gas S The ratio of L S / L S0 Set it to be greater than 1 and process it with a high vacuum.

[0077] Preferably, for molten steel refined through the aforementioned secondary refining method, other given components are adjusted as needed, and casting is performed after inclusion morphology control and flotation separation. Based on the production of low-nitrogen steel or low-nitrogen and low-sulfur steel, high-grade steels obtained by adjusting various components can be manufactured.

[0078] Example

[0079] The embodiments of the invention will now be described in detail. Figure 1 The apparatus consists of a ladle containing molten steel at 1600°C–1750°C, into which metallic Al is added. To adjust the temperature, the molten steel is heated using an LF (Leakage-Free) device. The addition of Al for deoxidation is performed either before the LF treatment or during Al addition using a VOD (Vacuum-Oxygen Deoxidation) device. In the VOD device, the Al concentration in the molten steel before oxygenation is adjusted by the presence or absence of added Al, thus controlling the [Al] content. iThe concentration was set at 0.02–0.48% by mass. Then, CaO and MgO for refractory protection were added, forming a CaO-Al₂O₃ binary slag or a CaO-Al₂O₃-MgO ternary slag. Oxygen was then injected into the slag. For the molten steel, Ar gas was supplied from a bottom-blowing plug installed at the bottom of the ladle at a stirring power density of 60–600 kW / t. Tests were conducted with a steel volume of 160 t.

[0080] The test conditions and results are shown in Tables 1-1 and 1-2. This invention enables the molten steel after oxygen treatment to achieve a nitrogen concentration [N]. f The concentration reached below 35 ppm by mass. On the other hand, the comparative example did not reach this level. Furthermore, tests No. 7 and 8 had to be interrupted due to slag overflow. It was found that tests No. 5, 14, and 16, with a C / A ratio in the slag ranging from 1.8 to 2.2, also exhibited excellent desulfurization performance. In treatments No. 11 and 15, the slag did not slagify, and partial solidification was observed.

[0081]

[0082]

[0083] Industrial applicability

[0084] The secondary refining of molten steel according to the present invention is suitable for steelmaking processes in which low-carbon scrap iron and reduced iron are melted in electric furnaces or the like to produce molten steel. It can stably produce low-nitrogen steel or low-nitrogen and low-sulfur steel in large quantities, and therefore helps to reduce CO2, which is useful in industry.

Claims

1. A secondary refining method for molten steel, comprising: combining an Al addition step (adding a metallic Al substance to the molten steel to produce Al-containing molten steel) and a CaO addition step (adding a CaO substance to the molten steel) to form a slag containing CaO and Al2O3; then, injecting oxygen-containing gas through the slag into the Al-containing molten steel to perform an oxygen supply treatment including denitrification. The method includes: The Al concentration [Al] in the molten steel before it undergoes oxygen treatment. i Let ε be the value calculated using the following formula (A) based on the stirring dynamic density ε during slag formation [A1]. e The above refers to the Al concentration [Al]. i The unit is mass%, and the unit of stirring dynamic density ε is W / t. The Al concentration [Al] at the end of the oxygen delivery treatment. f Set to 0.03% by mass or higher. [To the] e =-0.072×ln(ε)+0.5822···(A)。 2. The secondary refining method for molten steel according to claim 1, wherein, The Al addition step includes the process of adding a metallic Al substance to the molten steel to produce deoxidized molten steel.

3. The secondary refining method for molten steel according to claim 1 or 2, wherein, In the oxygen delivery process, when the oxygen-containing gas is supplied, the indentation depth L caused by the blowing of the oxygen-containing gas is reduced. S With respect to the thickness L of the slag S0 The ratio of L S / L S0 Set it to 1.0 or higher.

4. The secondary refining method for molten steel according to any one of claims 1 to 3, wherein, The C / A(-) ratio is set to 0.4 or higher and 2.2 or lower. The C / A(-) ratio is the mass ratio of CaO concentration to Al2O3 concentration in the slag. The CaO concentration is expressed in mass% and the Al2O3 concentration is expressed in mass.

5. The secondary refining method for molten steel according to any one of claims 1 to 4, wherein, The MgO concentration in the slag is set to be below 0.25 based on its mass ratio to CaO concentration, where MgO concentration is expressed as mass% and CaO concentration is expressed as mass.

6. The secondary refining method for molten steel according to any one of claims 1 to 5, wherein, The oxygen supply treatment brings the surface temperature of the Al-containing molten steel and slag to 9.3 × 10⁻⁶. 4 A depressurized gas atmosphere below Pa.

7. A secondary refining method for molten steel, comprising: forming a slag containing CaO and Al2O3 on top of Al-containing molten steel filled in a container; injecting an oxygen-containing gas through the slag into the Al-containing molten steel, thereby bringing the slag into contact with the molten steel to remove nitrogen and sulfur from the molten steel. The method includes: In the oxygen supply treatment of the secondary refining method of molten steel according to any one of claims 1 to 6, the Al concentration in the molten steel is maintained at 0.05% by mass or more, and the ratio of CaO concentration to Al2O3 concentration in the slag, C / A(-), is controlled to be 1.8 or more and 2.2 or less, wherein the unit of CaO concentration is by mass% and the unit of Al2O3 concentration is by mass.

8. A method for manufacturing steel, the method comprising: The composition of the molten steel obtained by the secondary refining method of any one of claims 1 to 7 is arbitrarily adjusted, and then cast.

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