Method for removing nitrogen from molten steel, method for simultaneously removing nitrogen and sulfur, and method for manufacturing steel

Abstract

This invention provides a method for efficiently denitrifying or desulfurizing molten steel by performing a denitrification process that can stably reach extremely low nitrogen concentration ranges at high speeds, and by performing the denitrification and desulfurization processes in the same process. A method for denitrifying molten steel involves forming a slag (4) containing CaO and Al2O3 on molten steel (3) filled in a container (1), spraying oxygen-containing gas from above the slag, and bringing the slag into contact with the molten steel to remove nitrogen from the molten steel. The method includes: ensuring that the Al concentration in the molten steel is above a value determined by formula (1) based on the stirring power density, and spraying oxygen-containing gas such that the indentation depth L of the slag generated by the spraying of oxygen-containing gas is... S With respect to the thickness L of the aforementioned slag S0 The ratio is L S / L S0 ≥1. Al concentration in molten steel (mass%) = -0.072 × ln(stirring power density (W / t)) + 0.5822···(1).

Description

Technical Field

[0001] This invention relates to a method for removing nitrogen from molten steel by contacting molten steel filled in a ladle or other reaction vessel with slag added to or formed on the molten steel, or a method for removing nitrogen and sulfur simultaneously. This invention also relates to a method for manufacturing steel smelted by these methods. Background Technology

[0002] Nitrogen is a harmful component for metallic materials. In existing steelmaking processes, nitrogen [N] in molten iron is removed by adsorbing onto the surface of carbon monoxide bubbles generated primarily during the decarburization process of molten pig iron. 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 must be changed from the existing methods using blast furnaces and converters to methods that melt scrap iron and reduced iron. In this case, the carbon concentration of the resulting molten iron becomes lower, and for the reasons mentioned above, there is a risk that low-nitrogen steel cannot be smelted.

[0004] Therefore, some methods for denitrification from molten steel that has used slag have been proposed. For example, Patent Document 1 shows a method in which the Al concentration in the molten steel is maintained at a concentration of 0.7% by mass or more for at least 5 minutes in a VOD furnace, and denitrification is carried out by the formation of aluminum nitride (hereinafter AlN).

[0005] In addition, Patent Document 2 shows the following method: in an electric furnace, molten steel is smelted using scrap iron as the main iron source. After the steel is tapped to another refining container and held, a denitrification flux containing Al-containing substances is added. Oxygen-containing gas is injected into the molten steel to form slag, while AlN is transferred to the slag to perform denitrification.

[0006] In addition, Patent Document 3 shows the following method: molten metal is loaded into a refining container with a gas top blowing function, and the surface of the molten metal is covered with slag with CaO and Al2O3 as the main components. Then, the oxidizing gas is sprayed onto the slag-covered surface to a degree that the oxidizing gas will not come into direct contact with the molten metal, thereby performing denitrification.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 5-320733

[0010] Patent Document 2: Japanese Patent Application Publication No. 2007-211298

[0011] Patent Document 3: Japanese Patent Application Publication No. 8-246024

[0012] Non-patent literature

[0013] Non-Patent Literature 1: Iron and Steel, 101(2015), p. 74 Summary of the Invention

[0014] The problem that the invention aims to solve

[0015] However, the aforementioned prior art has the following problems.

[0016] That is, the technology described in Patent Documents 1 and 2 utilizes the generation of AlN for denitrification. A portion of the generated AlN remains in the molten steel, which may become the starting point for cracking during subsequent casting processes.

[0017] Furthermore, in order to smelt low-nitrogen steel with a nitrogen content of around tens of ppm by mass using a denitrification method that utilizes AlN generation, an initial nitrogen concentration of at least several % to 10% by mass or several hundred ppm by mass is required, considering the solubility product of Al and N. For the technologies described in Patent Documents 1 and 2, the process cost for smelting low-nitrogen steel becomes very high, and there is a problem that it can only be applied to steel grades with high dissolved nitrogen content, such as stainless steel.

[0018] The technology described in Patent Document 3 includes the following conditions for isolating molten steel from oxidizing gases:

[0019] (1) Ensure that the amount of slag per ton of molten steel is at least 15 kg;

[0020] (2) Control the amount of slag, the amount of bottom-blown gas, the composition of top-blown gas, its flow rate, the height of the spray gun, and the gas atmosphere pressure to an appropriate range.

[0021] However, condition (1) increases the amount of slag as the size of the container filled with molten steel increases, and condition (2) does not describe a specific control method or control range. The method for confirming the barrier between the gas and the molten steel is unclear, so the suitable conditions are unclear. The inventors have confirmed that even when the experiment is conducted within the same suitable range as described in Patent Document 3, in reality, the nitrogen movement between the slag and metal is suppressed because the oxygen partial pressure applied only to the slag-metal interface by the oxidizing gas is increased. As a result, the denitrification rate is slowed down, making it impractical in operation.

[0022] Furthermore, when oxygen-containing gas is supplied without slag penetration, the Al concentration does not decrease or decreases very slowly during the oxygen delivery process. Therefore, in order to reduce the Al concentration in the steel, which is increased due to the presence of metallic Al, to the product standard level, for example, around 0.025–0.040% by mass, an additional Al removal process is required, whereby the oxygen-containing gas reacts with the Al in the steel to remove it. However, performing this Al removal process increases the processing time, becoming a contributing factor to reduced productivity due to the time mismatch with the casting time of subsequent processes.

[0023] In addition to nitrogen, the removal of sulfur from molten steel is also a secondary refining process, but there is no specific record of desulfurization in Patent Documents 1-3. Therefore, to remove sulfur from molten steel, it is necessary to separately set up a process, such as slag formation with CaO and Al2O3 as the main components in an LF (ladle furnace) while electrode heating, and to contact the slag with the molten steel to remove sulfur, which increases production costs.

[0024] This invention was made in view of the following circumstances, and its object is to provide a method for efficiently performing denitrification, or denitrification and desulfurization, of molten steel in the process of denitrification refining of molten steel using slag, by removing high-concentration Al from the steel and performing a denitrification treatment that can stably reach an extremely low nitrogen concentration range at high speed, and then performing this denitrification treatment and desulfurization treatment in the same process. Furthermore, a method for manufacturing steel using molten steel smelted through these denitrification methods is provided.

[0025] Problem Solving Methods

[0026] To address the aforementioned problems, the inventors conducted numerous experiments and discovered that in denitrification processes involving the injection of oxygen-containing gas into the slag and the removal of nitrogen from the molten steel via the slag, when top-blown oxygen reaches the molten steel to eliminate Al, under conditions where the oxygen-containing gas permeates the slag, depending on the composition of the slag outside the surface covering the injected oxygen-containing gas (hereinafter referred to as the ignition point) and the temperature of the molten steel, there exists a region where rapid denitrification, and consequently, rapid denitrification and desulfurization occur simultaneously. This invention is based on the above insights, and its main points are as follows.

[0027] The denitrification method for molten steel of the present invention advantageously solves the above-mentioned problems. This method involves forming a slag containing CaO and Al2O3 on top of molten steel filled in a container, injecting oxygen-containing gas from above the slag, and bringing the slag into contact with the molten steel to remove nitrogen from the molten steel. The method includes: ensuring that the Al concentration in the molten steel is above the value determined by formula (1) based on the stirring dynamic density, and injecting oxygen-containing gas such that the indentation depth L of the slag generated by the injection of oxygen-containing gas... S With respect to the thickness L of the aforementioned slag S0The ratio is L S / L S0 ≥1.

[0028] Al concentration in molten steel (mass%) = -0.072 × ln(stirring dynamic density (W / t)) + 0.5822···(1)

[0029] It should be noted that, in the denitrification method for molten steel of the present invention configured as described above, the following (1) to (4) are considered to be more preferred solutions:

[0030] (1) Set C / A(-) to 0.4 or more and 1.8 or less, wherein C / A(-) is the ratio of CaO concentration (mass%) to Al2O3 concentration (mass%) in the above slag;

[0031] (2) Set the MgO concentration in the above slag to below 5.0% by mass;

[0032] (3) When the MgO concentration in the above slag exceeds 5.0% by mass and increases by 1%, the temperature of the above molten steel increases by more than 5°C.

[0033] (4) Reduce the surface pressure of the above-mentioned slag or molten steel to 1.0 × 10⁻⁶. 5 Below Pa.

[0034] Furthermore, the simultaneous denitrification and desulfurization treatment method for molten steel of the present invention advantageously solves the above-mentioned problems. This method involves forming a slag containing CaO and Al2O3 on molten steel filled in a container, injecting oxygen-containing gas from above the slag, and bringing the slag into contact with the molten steel to remove nitrogen and sulfur from the molten steel. The method includes maintaining the Al concentration in the molten steel at 0.05% by mass or more, and controlling the C / A(-) ratio to be 0.7 or more and 1.7 or less, where C / A(-) is the ratio of the CaO concentration (by mass%) to the Al2O3 concentration (by mass%) in the slag.

[0035] Furthermore, the steel manufacturing method of the present invention is characterized in that the composition of the molten steel obtained by the above-described denitrification method or the above-described simultaneous denitrification and desulfurization treatment method is arbitrarily adjusted, and then cast.

[0036] The effects of the invention

[0037] According to the present invention configured as described above, when refining molten steel using slag, high concentrations of Al in the steel can be removed, and in the same process, in addition to performing denitrification treatment that can stably reach an extremely low nitrogen concentration range at high speed, desulfurization treatment can also be performed, resulting in highly efficient denitrification treatment of molten steel. Attached Figure Description

[0038] Figure 1 This is a schematic diagram illustrating an example of an apparatus suitable for the method of the present invention.

[0039] Figure 2 This is a graph showing the relationship between the Al concentration in steel required to achieve a nitrogen concentration of 25 ppm by mass and the stirring kinetic density.

[0040] Figure 3 This is a graph showing the relationship between the CaO concentration to Al2O3 concentration ratio (C / A(-)) in the slag and the nitrogen concentration reached.

[0041] Figure 4 This is a graph showing the effect of MgO concentration in slag on the final nitrogen concentration.

[0042] Figure 5 This is a graph showing the effect of MgO concentration in the slag on the molten steel temperature required to achieve the same nitrogen content.

[0043] Figure 6 This is a graph showing the effect of furnace atmosphere pressure on the upper limit of nitrogen deviation after treatment.

[0044] Figure 7 This is a graph showing the relationship between equilibrium S concentration and C / A ratio.

[0045] Symbol Explanation

[0046] 1 Container

[0047] 2 Refractory materials

[0048] 3. Molten Steel

[0049] 4. Slag

[0050] 5. Gas piping (oxygen)

[0051] 6. Gas top-blowing spray gun

[0052] 7. O2-containing gas

[0053] 8 Bottom-blowing nozzles

[0054] 9. Gas piping (non-reactive gases)

[0055] 10. Inactive gas for stirring steel bath

[0056] 11 Exhaust System

[0057] 12 Alloy Addition System

[0058] 13 Vacuum Container Detailed Implementation

[0059] 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.

[0060] <Regarding the configuration of the apparatus for implementing the present invention>

[0061] exist Figure 1 The preferred apparatus configuration for carrying out the present invention is shown in the diagram. Figure 1 In this process, 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 surface of the molten steel 3 or slag 4 is kept under a reduced-pressure gas atmosphere. O2-containing gas 7 is injected into the slag 4 from a top-blowing lance 6 connected to an oxygen supply pipe 5 and an inactive gas supply pipe 9. Inactive gas 10 for stirring the steel bath is blown into the molten steel 3 through a bottom-blowing nozzle 8 connected to the gas pipe 9, thereby agitating the molten steel 3.

[0062] The deoxidation of molten steel 3, the addition of metallic Al-containing substances to increase the Al concentration in molten steel 3, and the addition of CaO-containing substances can be carried out from the alloy addition system 12 or in a process before entering the vacuum container 13. The formation of slag 4 containing CaO and Al2O3 is achieved using CaO-containing substances and Al2O3 generated through the deoxidation of molten steel 3, but calcium aluminate, such as pre-melted or premixed products, can also be used as the substance containing CaO and Al2O3. In addition to the above methods, the supply of inactive gas 10 for stirring to molten steel 3 can also be, for example, by injecting it into molten steel 3 through a jet lance for blowing inactive gas.

[0063] Next, the denitrification method for molten steel of the present invention, preferred examples 1 to 4 of the denitrification method for molten steel of the present invention, and the method for simultaneous denitrification and desulfurization of molten steel of the present invention will be described in detail in conjunction with the development process.

[0064] <Features of the denitrification method for molten steel according to the present invention>

[0065] The denitrification method for molten steel of the present invention was characterized by the discovery of the required Al content when performing denitrification treatment on slag outside the flash point under conditions where top-blown oxygen-containing gas permeates the slag phase. In a small high-frequency vacuum induction melting furnace, 15 kg of molten steel was melted to form slag containing 15 kg / t or more of CaO and Al2O3. When oxygen-containing gas was injected into the slag, the minimum Al concentration required to reduce the nitrogen content in the molten steel to 25 ppm by mass was investigated, and the results are as follows: Figure 2 As shown, the required Al concentration changes accordingly with the stirring kinetic density. It can also be seen that at this point, if L... S / L S0 If (-) is 1 or higher, the Al concentration in the molten steel decreases through reaction with oxygen, and the L S / L S0 (-) represents the thickness L of the slag containing CaO and Al2O3 in the molten stage before denitrification treatment. S0 (m) The measured depth L of the slag 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). This invention is based on such a survey.

[0066] It should be noted that, in the above-described denitrification method for molten steel of the present invention, the thickness L of the slag... S0 (m) and the depression L of the slag S The ratio of (m) to L S / L S0 The upper limit of (-) varies depending on the denitrification conditions, equipment configuration, etc., so no specific upper limit is set here.

[0067] A preferred example 1 of the denitrification method for molten steel of the present invention was discovered during an investigation into the effect of slag composition, primarily the ratio of CaO concentration (mass%) to Al2O3 concentration (mass%) in the slag, C / A(-), on denitrification. In the aforementioned small-scale melting furnace, experiments were conducted where the C / A ratio varied from 0.4 to 2.0 with the MgO concentration in the slag set to 0%. Figure 3 As shown, the denitrification performance remained stable within the C / A range of 0.4 to 1.2. From a C / A exceeding 1.2, the denitrification performance began to decline, reaching a point where nitrogen concentration increased sharply above 1.6, and failing to reach a low nitrogen concentration range (nitrogen concentration below 35 ppm by mass) above 1.8. The aforementioned preferred example 1 was proposed based on this investigation.

[0068] Preferred Example 2 of the denitrification method for molten steel of the present invention was discovered during an investigation into the effect of MgO concentration in slag. In the aforementioned small-scale melting furnace, the stirring power density was kept constant at 500 W / t, and the MgO concentration was controlled within the range of 0% to 17% for denitrification treatment. The results are as follows: Figure 4 As shown, a MgO concentration of less than 5% by mass in the slag reaches the low nitrogen concentration range (nitrogen concentration less than 35 ppm by mass), but at higher concentrations, the denitrification performance deteriorates significantly. The above-mentioned preferred example 2 is based on such investigation.

[0069] Preferred Example 3 of the denitrification method for molten steel of the present invention was discovered during an investigation into improvement strategies for denitrification degradation when the MgO concentration had to be increased due to the need for refractory protection of the container filling the molten steel. Using the aforementioned small-scale high-frequency vacuum induction melting furnace, the MgO concentration in the slag containing CaO and Al2O3 was varied from 0% by mass to a saturation concentration, and the required molten steel temperature for reducing nitrogen in the molten steel to 25 ppm by mass was investigated. The results are as follows... Figure 5 As shown, for every 1.0% by mass increase in MgO concentration in the slag, the molten steel temperature needs to be increased by approximately 5°C. This investigation quantitatively determined the amount of molten steel temperature increase necessary to recover from the denitrification reduction caused by the increased MgO concentration. The aforementioned preferred example 3 is based on this investigation.

[0070] Preferred Example 4 of the denitrification method for molten steel of the present invention was discovered during an investigation into the influence of gas atmosphere pressure on denitrification behavior. Using the aforementioned small high-frequency vacuum melting furnace, after adjusting the gas atmosphere pressure inside the furnace, denitrification treatment of the molten steel was performed while applying stirring with a stirring power density of 200 W / t to 2000 W / t. First, in an experiment where the vacuum degree was controlled, such as... Figure 6 As shown, the upper limit of the deviation of the treated nitrogen concentration changes accordingly with the stirring kinetic density. It can be seen that under low stirring kinetic density, 1.0 × 10⁻⁶ 5 At a certain Pa, the nitrogen concentration stabilizes, but under high stirring dynamic density, it exceeds 0.66 × 10⁻⁶. 5 At a pressure of Pa, the nitrogen concentration begins to increase, and the higher the stirring power density, the greater the upper limit of the deviation from the final nitrogen concentration. Therefore, in this invention, the preferred gas atmosphere pressure is determined to be atmospheric pressure (1.0 × 10⁻⁶). 5 Below Pa), it is further preferably set to 0.66 × 10⁻⁶. 5 The pressure is below Pa. This can be attributed to the fact that a portion of the molten steel surface is exposed due to the agitation of the steel bath, thus leading to nitrogen absorption. The above-mentioned preferred example 4 is based on this investigation.

[0071] <Features of the simultaneous denitrification and desulfurization treatment method for molten steel according to the present invention>

[0072] In investigating the impact of slag C / A ratio on denitrification, starting from cases where desulfurization was also performed under certain conditions, the conditions that could trigger simultaneous denitrification and desulfurization were investigated. The simultaneous denitrification and desulfurization method for molten steel of this invention was discovered during such a process. Using the aforementioned small high-frequency vacuum melting furnace, the pressure was reduced to 0.66 × 10⁻⁶. 5 In the above-mentioned melting furnace of Pa, L S / L S0 Setting the value as 1, oxygen was injected into the slag, and the stirring power density applied to the molten steel was kept constant at 200 W / t. During the experiment, the oxygen activity was measured, and the amount of Al2O3 produced was calculated based on the equilibrium Al concentration. Powdered lime was added without changing the C / A ratio, and the C / A ratio was maintained between 0.4 and 2.0. Based on this, the denitrification and desulfurization behaviors were investigated. Before this treatment, the Al concentration in the molten steel was 0.3%, the steel temperature was kept constant at 1650℃, the nitrogen and sulfur concentrations in the molten steel were 50 ppm and 0.03% by mass, respectively, the MgO concentration in the slag was 0%, and the slag unit consumption was 15 kg / t.

[0073] The result is as follows Figure 7 As shown, it can be seen that each increase in C / A from 0.4 leads to a decrease in the equilibrium sulfur concentration that can be confirmed from the desulfurization behavior. The minimum value is reached when C / A is 1.7, and the equilibrium sulfur concentration increases when C / A is greater than 1.7. The desulfurization rate is determined by the following equation (2), which uses the difference between the current sulfur concentration and the equilibrium sulfur concentration as the driving force. Therefore, an increase in the equilibrium sulfur concentration means a decrease in the desulfurization rate.

[0074] -d[%S] / dt=k·([%S]-[%S] e (2)

[0075] In the formula, [%S] represents the sulfur concentration in the steel. e The equilibrium sulfur concentration is represented by k, the proportionality constant is represented by t, and time is represented by t.

[0076] like Figure 3 As shown, within the range of C / A below 1.8, the nitrogen concentration has reached the low nitrogen concentration range (nitrogen concentration in steel below 35 ppm by mass). From the perspective of simultaneous denitrification and desulfurization, it is not very meaningful to increase the C / A to the maximum C / A of 1.7 or above within the range of C / A during denitrification.

[0077] On the other hand, within the C / A range of 0.4 to 1.2, according to Figure 7The equilibrium reaches a point where the sulfur concentration decreases monotonically. Therefore, from a desulfurization perspective, it is preferable to increase the C / A ratio. However, according to... Figure 3 When the nitrogen concentration reaches a C / A ratio of 0.7 or higher, the increase becomes slight (from 15 to 16 ppm by mass). From a denitrification perspective, it is preferable to lower the C / A ratio. Therefore, when performing simultaneous denitrification and desulfurization treatment, the lower limit of the C / A ratio can be set to 0.7 or higher. The method for simultaneous denitrification and desulfurization of molten steel according to the present invention is based on such investigation.

[0078] <Features of the steel manufacturing method of the present invention>

[0079] Preferably, for molten steel smelted using the aforementioned denitrification method or the simultaneous denitrification and desulfurization method, the composition can be adjusted to other given specifications as needed, and casting can be performed after inclusion morphology control and floating separation. Based on the production of low-nitrogen steel, high-grade steels can be manufactured by adjusting various compositions.

[0080] Example

[0081] The embodiments of the invention will now be described in detail. Figure 1 The apparatus consisted of a ladle containing molten steel at temperatures above 1650°C. Metallic Al was added to the molten steel, with an Al concentration of 0.12–0.15% by mass. CaO and MgO (for refractory protection) were also added, forming either a CaO-Al₂O₃ binary slag or a CaO-Al₂O₃-MgO ternary slag. The slag was then supplied with a stirring power density of 500 W / t. Tests were conducted with a steel volume of 160 t. The nitrogen concentration before denitrification or simultaneous denitrification and desulfurization was adjusted to 50 ppm by mass, and the sulfur concentration was adjusted to 300 ppm by mass. The test results are shown in Table 1 below.

[0082]

[0083] According to the results in Table 1, the stirring dynamic density and Al concentration in the steel satisfy equation (1), and Ls / L S0 Examples 1 to 10 of the present invention with ≥1 and Ls / L S0 Compared to Comparative Example 1, which has a nitrogen content of less than 1, the treated nitrogen content is below 35 ppm by mass, indicating that effective denitrification can be performed.

[0084] Furthermore, according to the results in Table 1, Examples 2-10, with a C / A ratio of 0.4 or higher and 1.8 or lower, are preferred (Preferred Example 1) because they show better post-treatment nitrogen (N) compared to Example 1, which has a C / A ratio exceeding 1.8. Additionally, according to the results in Table 1, Examples 4, 6, 8, and 10, with an MgO concentration of 5.0% by mass or lower in the slag, are preferred (Preferred Example 2) because they show better post-treatment nitrogen (N) compared to Examples 1-3, which have an MgO concentration exceeding 5.0% by mass. Furthermore, according to the results in Table 1, Examples 5, 7, and 9, where the MgO concentration in the slag exceeds 5.0% by mass but each 1% increase in MgO concentration raises the molten steel temperature by 5°C or more, show the same post-treatment nitrogen (N) as Examples 4, 6, and 8, which have an MgO concentration of 5% by mass, and are preferred (Preferred Example 3). Additionally, according to the results in Table 1, reducing the surface pressure of the slag or molten steel to 1.0 × 10⁻⁶ is also preferred. 5 Examples of inventions with a strength of less than Pa (6-10) and exceeding 1.0 × 10⁻⁶ Pa 5 Compared with Invention Examples 1 to 5 of Pa, the treated N is better and is preferred (Preferred Example 4).

[0085] Furthermore, according to the results in Table 1, the Invention Examples 8 to 10, which have an Al concentration of 0.05% by mass or more and a C / A ratio of 0.7 or more and 1.7 or less, compared with the Invention Examples 1 to 7 and Comparative Example 1, which have a C / A ratio outside this range, show that the sulfur concentration after treatment is 35 ppm by mass or less and the nitrogen concentration after treatment is 25 ppm by mass or less. Both are good, and simultaneous denitrification and desulfurization can be appropriately achieved.

[0086] Industrial applicability

[0087] The denitrification method for molten steel of the present invention can be applied to steelmaking processes that use electric furnaces or similar methods to melt low-carbon scrap iron and reduced iron to produce molten steel, thus enabling stable mass production of low-nitrogen steel. Therefore, it helps to reduce CO2 and is useful in industry.

Claims

1. A method for denitrifying molten steel, comprising forming a slag containing CaO and Al2O3 on top of molten steel filling a container, injecting oxygen-containing gas from above the slag, and bringing the slag into contact with the molten steel to remove nitrogen from the molten steel. The method includes: Based on the stirring dynamic density, ensure that the Al concentration in the molten steel is above the value determined by equation (1), and inject oxygen-containing gas so that the slag depression depth L caused by the injection of oxygen-containing gas is increased. S With respect to the thickness L of the slag S0 The ratio is L S / L S0 ≥1, Al concentration in molten steel = -0.072 × ln(stirring kinetic density) + 0.5822···(1) In the formula, the unit of Al concentration in molten steel is mass%, and the unit of stirring power density is W / t.

2. The denitrification method for molten steel according to claim 1, wherein, The C / A ratio is set to 0.4 or higher and 1.8 or lower, where C / A is the ratio of CaO concentration to Al2O3 concentration in the slag, with CaO concentration expressed as mass% and Al2O3 concentration expressed as mass.

3. The denitrification method for molten steel according to claim 1 or 2, wherein, The MgO concentration in the slag is set to below 5.0% by mass.

4. The denitrification method for molten steel according to claim 1 or 2, wherein, The MgO concentration in the slag exceeds 5.0% by mass, and for every 1% increase, the temperature of the molten steel increases by more than 5°C.

5. The method for denitrifying molten steel according to any one of claims 1 to 4, wherein, The surface pressure of the slag or molten steel is reduced to 1.0 × 10⁻⁶. 5 Below Pa.

6. A method for simultaneously denitrifying and desulfurizing molten steel, comprising forming slag containing CaO and Al2O3 on top of molten steel filling a container, injecting oxygen-containing gas from above the slag, and bringing the slag into contact with the molten steel to remove nitrogen and sulfur from the molten steel. The method includes: In the denitrification method according to any one of claims 1 to 5, the Al concentration in the molten steel is maintained at 0.05% by mass or more, and the C / A ratio is controlled at 0.7 or more and 1.7 or less, wherein C / A is the ratio of CaO concentration to Al2O3 concentration in the slag, and the unit of CaO concentration is by mass% and the unit of Al2O3 concentration is by mass.

7. A method for manufacturing steel, the method comprising: The composition of the molten steel produced by the denitrification method of any one of claims 1 to 5 or the simultaneous denitrification and desulfurization treatment method of the molten steel described in claim 6 is arbitrarily adjusted, and then cast.

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