Molten steel denitrification method

Abstract

Provided is a technique whereby the nitrogen concentration in molten steel in a melting furnace or a refining furnace can be reduced to a low nitrogen concentration region inexpensively and stably. In a molten steel denitrification method in which an oxygen-containing gas and a carbon source to be conveyed by a conveying gas are supplied to molten steel filled into a melting furnace or a refining furnace, the oxygen-containing gas and the carbon source are simultaneously blown under the following conditions: in a plan view, the oxygen-containing gas and the carbon source are introduced into the melting furnace or the refining furnace; with respect to a triangle [Delta] XYZ formed by connecting the tip (point X) of a blow-in tube discharge hole of the carbon source, the intersection point (point Y) of the central axis extension line of the carbon source in the blow-in direction and the inner wall of the furnace, and the tip (point Z) of an oxygen lance discharge hole, a point W is a position on a side XY which is 2 / 3 of the length of the side XY from the point X, and an angle XZW is an angle [theta] 1 (DEG). In a plan view, the blowing-in direction [theta] 2 (DEG) of the oxygen-containing gas, which opens from a side ZX passing through the tip (point Z) of the oxygen lance discharge hole to the Y side, is a positive value and is within the range of [theta] 1 or less.

Description

Technical Field

[0001] The present invention relates to a method for removing nitrogen from molten steel by supplying an oxygen-containing gas and a carbon source to molten steel in a process of manufacturing steel by treating iron source materials such as scrap, reduced iron and pig iron in a melting furnace or a refining furnace. Background Art

[0002] Nitrogen is a harmful component for metal materials. In the conventional steelmaking process, nitrogen [N] in molten iron is mainly removed by adsorbing it on the bubble surface of carbon monoxide (CO) generated during the decarburization of molten iron. Therefore, for molten steel with low carbon concentration, since the amount of carbon monoxide generated is limited, nitrogen cannot be removed to a low concentration by the same method. Here, molten iron refers to molten metal mainly composed of iron, that is, containing more than 50%, and includes molten metal in which a cold iron source is melted, and molten metals mixed with these, in addition to molten iron and molten steel.

[0003] On the other hand, for the purpose of protecting the global environment, it is required to reduce CO2 emissions. Therefore, it is necessary to change the steelmaking process from the conventional method using blast furnaces and converters to the method of melting scrap and reducing iron. In this case, the carbon concentration of the obtained molten iron becomes low, and low-nitrogen steel may not be smelted for the above reasons.

[0004] Therefore, several methods have been proposed for effectively denitrifying molten steel by reacting carbon [C] with an oxygen-containing gas in molten steel in an electric furnace and utilizing carbon monoxide bubbles generated in the reaction. For example, Patent Document 1 discloses conditions for the carbon injection rate and oxygen supply rate for injecting carbon into molten steel, and a nitrogen concentration limit (≤ 3 vol.%) in a carrier gas when injecting carbon into molten steel. Furthermore, by simultaneously promoting denitrification and suppressing nitrogen absorption, the nitrogen concentration [N] in molten steel at the end of refining can be reduced to 25 mass ppm or less.

[0005] Patent Document 2 discloses the use of a hydrocarbon-containing gas as a carrier gas for blowing a carbon source into molten steel. Compared with the case of using air as a carrier gas, this can promote carbon addition and reduce the nitrogen concentration [N] in the molten steel to less than about 20 mass ppm.

[0006] In addition, Patent Document 3 discloses that carbon and oxygen are injected simultaneously and separately into the same region in the furnace. In addition, denitrification of molten steel is accelerated by about 5 mass ppm compared to the case where only oxygen is injected.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 11-012634

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 03-028312

[0011] Patent Document 3: Japanese Patent Application No. 2002-533566 Summary of the invention

[0012] Problems to be solved by the invention

[0013] However, the above-mentioned prior art has the following problems. Air or nitrogen is usually used as a carrier gas when blowing a carbon source into molten steel in an electric furnace. In contrast, in the methods described in Patent Documents 1 and 2, argon or a hydrocarbon-containing gas is used as a carrier gas. Pipes for introducing these gases different from the usual ones, gas storage tanks, and huge equipment investments are required.

[0014] In addition, the method described in Patent Document 3 is to inject carbon near the area where oxygen is injected. The area where carbon is injected locally obtains a carbon concentration that is higher than the carbon concentration of the area where carbon is not injected (i.e., the average carbon concentration of the molten steel). Therefore, it is meaningful to promote the production of carbon monoxide gas in this area. However, there is no definition of this local area, that is, there is no record of the extent to which the distance between the carbon injection site and the oxygen injection site is allowed to differ, and only a slightly "about 20 cm" embodiment is shown. The carbon source injection pipe is mostly embedded in the furnace wall. In this case, in order to be able to inject carbon near the oxygen injection site, it is necessary to set up a spray gun for the carbon source, a spray gun position adjustment device and other equipment modifications, which is not realistic.

[0015] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technology capable of stably reducing the nitrogen concentration in molten steel in a melting furnace or a refining furnace to a low nitrogen concentration region without requiring large-scale investment.

[0016] Means for solving problems

[0017] The method for denitrifying molten steel of the present invention, which advantageously solves the above-mentioned problems, supplies an oxygen-containing gas and a carbon source transported by a transport gas to the molten steel filled in a melting furnace or a refining furnace, wherein the oxygen-containing gas and the carbon source are simultaneously blown under the following conditions: in a top plan view, with respect to a triangle ΔXYZ formed by connecting the front end (point X) of the ejection hole of the blowing pipe of the carbon source, the intersection point (point Y) of the extension line of the central axis of the blowing direction of the carbon source and the furnace inner wall, and the front end (point Z) of the ejection hole of the oxygen supply lance, a position on the side XY that is 2 / 3 of the length of the side XY from the point X is set as point W, an angle ∠XZW is set as angle θ1 (°), and in a top plan view, an oxygen-containing gas blowing direction θ2 (°) that opens to the Y side from the side ZX passing through the front end (point Z) of the ejection hole of the oxygen supply lance is a positive value and is within a range below θ1.

[0018] It should be noted that, in the method for denitrifying molten steel of the present invention, the following solutions may be more preferable solving means.

[0019] (a) When there are a plurality of carbon source injection pipes, the above-mentioned conditions are satisfied in relation to at least one carbon source injection pipe.

[0020] (b) Ensure that the time t1 (min) for simultaneously supplying the carbon source and the oxygen-containing gas from the carbon source blowing pipe and the oxygen supply lance is at least 30% of the time t2 (min) required from the start of melting of the cold iron source to the start of tapping of the molten steel in the furnace.

[0021] (c) The point W is located at the midpoint of the side XY.

[0022] (d) The melting furnace is an electric furnace.

[0023] Effects of the Invention

[0024] According to the method for denitrifying molten steel of the present invention, the nitrogen concentration in molten steel in a melting furnace or a refining furnace can be stably reduced to a low nitrogen concentration region without requiring large-scale investment. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is schematically shown as an example of an apparatus for implementing the present invention, wherein (a) is a longitudinal sectional view and (b) is a top plan view.

[0026] Figure 2 The schematic diagram shows an outline of an experiment to investigate the effect of the direction of simultaneous blowing of oxygen and carbon on denitrification, wherein (a) shows the case where oxygen-containing gas is blown from the oxygen lance on the slag discharge side, and (b) shows the case where oxygen-containing gas is blown from the oxygen lance on the steel tapping side.

[0027] Figure 3 It is a schematic diagram showing the blowing direction of the oxygen-containing gas suitable for denitrification when the oxygen-containing gas and the carbon source are blown simultaneously.

[0028] Figure 4 This is a graph showing the effect of the simultaneous supply time rate R (%) of the oxygen-containing gas and the carbon source according to the present invention on the nitrogen concentration in molten steel. DETAILED DESCRIPTION

[0029] The following is a detailed description of the embodiments of the present invention. It should be noted that the drawings are schematic and sometimes different from reality. In addition, the following embodiments illustrate the devices and methods used to concretize the technical concept of the present invention, and are not intended to limit the configuration to the following configuration. That is, the technical concept of the present invention can be subjected to various changes within the technical scope described in the claims.

[0030] Figure 1An example of a device structure preferably suitable for implementing the present invention is shown. Figure 1 In the example, an arc-type electric furnace is used as a refining container of the melting furnace. A pusher 3 is used to push the scrap 2 that is thrown into the furnace body 1 of the arc-type electric furnace from a dump truck or a crane bucket. Then, an arc is generated from a graphite electrode 4 to melt the scrap 2 as a cold iron source to obtain molten steel 5. At the same time, a flux 7 is added to the molten steel 5 from the hopper equipment 6 on the furnace to obtain slag 8. In order to promote the melting of the scrap 2, an oxygen-containing gas 11 is blown from the oxygen piping system 9 through an oxygen spray gun 10 on the slag discharge side to the molten steel 5 on the scrap input side. At the same time, in order to isolate the molten steel 5 from the atmosphere by increasing the carbon concentration in the molten steel 5 or forming the slag 8, a carbon source and a carrier gas 14 are blown onto the molten steel 5 from the carrier gas piping system 12 through a carbon source blowing pipe 13. In the Figure 1 In the example of the embodiment, the carbon source blowing pipe 13 is horizontally buried in the furnace wall 19. As the melting of the scrap proceeds, the oxygen-containing gas 11 is also blown onto the molten steel 5 from the oxygen lance 15 on the steel tapping side. After a predetermined time, the supply of the oxygen-containing gas 11, the carbon source and the carrier gas 14 is stopped. At the same time, the arc energization from the graphite electrode 4 is stopped, and a part of the slag 8 is discharged to the outside of the furnace from the slag discharge port 16. Then, the molten steel 5 is discharged to the outside of the furnace from the steel tapping port 17.

[0031] As a melting furnace and a refining furnace, in addition to an arc-type electric furnace, an induction melting furnace and a ladle refining furnace can also be exemplified. In addition to scrap, the cold iron source can also use reduced iron, preferably an iron source produced using renewable energy. It should be noted that the reduced iron is a substance that can be obtained normally. The oxygen-containing gas 11 can use pure oxygen or a gas diluted with oxygen. In order to promote denitrification, the diluent gas is preferably free of nitrogen, and argon gas, carbon dioxide gas, etc. can be exemplified. Air, etc. can be used as a carrier gas for the carbon source.

[0032] For such processing, the inventors used Figure 1 In a similar arc-type electric furnace, oxygen-containing gas 11, carbon source, and carrier gas 14 are simultaneously and separately sprayed. The influence of the spraying direction and spraying conditions on denitrification at this time was investigated in detail.

[0033] In the investigation, according to Figure 2 In the cases described in (a) and (b), air as a carrier gas and powdered carbon as a carbon source are blown onto the molten steel 5 in a fixed blowing direction. Figure 2 As shown in (a), the direction of the spray hole of the slag discharge side oxygen lance 10 is changed, and oxygen as the oxygen-containing gas is sprayed to the molten steel in any direction of O1 to O3. Figure 2 As shown in (b), the direction of the spray hole of the oxygen lance 15 on the steel-outlet side is changed, and oxygen is sprayed toward the molten steel in any direction of O4 to O6.

[0034] The front ends of the ejection holes of the carbon source blowing pipes 13a and 13b on the slag discharge side and the steel discharge side are set as points X and x, respectively. The intersection points of the extended lines of the central axes of the air and powdered carbon 14 blown from each carbon source blowing pipe in the top view and the furnace wall 19 are set as points Y and y, respectively. The front ends of the oxygen ejection holes on the slag discharge side and the steel discharge side are set as points Z and z, respectively. In addition, the position on the side XY that is 2 / 3 of the length of the side XY from point X is set as point W, and the position on the side xy that is 2 / 3 of the length of the side xy from point x is set as point w. First, as Figure 2 As shown in (a), when the oxygen-containing gas is blown from the oxygen lance 10 on the slag discharge side, the triangles with the front end (point Z) of the oxygen ejection hole on the slag discharge side as vertices are set to ΔXYZ, ΔXWZ, ΔxyZ and ΔxwZ respectively. At this time, the blowing direction O1 of oxygen is an oxygen blowing method in which the extension line of the central axis of the oxygen jet in the top plan view is located outside ΔXWZ and ΔxwZ. The blowing directions O2 and O3 of oxygen are oxygen blowing methods in which the oxygen blowing direction O2 is also located in ΔxwZ, which is the extension line of the central axis of the oxygen jet in the top plan view.

[0035] In addition, if Figure 2 As shown in (b), when the oxygen-containing gas is blown in from the gas supply lance 15 on the steel-out side, the triangles with the front end (point z) of the oxygen ejection hole on the steel-out side as vertices are set to Δxyz, Δxwz, ΔXYz and ΔXWz respectively. At this time, the blowing direction O4 of oxygen is an oxygen blowing method in which the extension line of the central axis of the oxygen jet in the top view is located outside any one of Δxwz and ΔXWz. The blowing directions O5 and O6 of oxygen are oxygen blowing methods in which the extension line of the central axis of the oxygen jet in the top view is located in Δxwz, and the blowing direction O5 of oxygen is also located in ΔXWz.

[0036] The scrap is loaded into the furnace, and the moment when the graphite electrode 4 starts to be energized is regarded as the start of melting (the start of treatment). Powdered carbon is immediately blown into the furnace from the carbon source blowing pipes 13a and 13b on the slag discharge side and the steel discharge side using air as a carrier. At the same time, as shown in Table 1, oxygen is blown from the oxygen lance 10 on the slag discharge side in the direction of O1 to O3 for 15 to 45 minutes, and then the oxygen supply is stopped. Immediately, oxygen is blown from the oxygen lance 15 on the steel discharge side in the direction of O4 to O6 for 15 to 45 minutes. After stopping the power supply from the graphite electrode 4, a sample of the molten steel 5 in the furnace is taken out. The sample is analyzed in order to investigate the carbon concentration and nitrogen concentration.

[0037] It should be noted that all the scraps 2 put into the furnace are new chips with a nitrogen concentration of about 22 mass ppm. The carbon source put into the furnace uses powdered carbon sprayed from the carbon source blowing pipe 13 and block carbon added from the furnace hopper equipment 6. Then, the carbon concentration [C] in the molten steel at the end of the treatment is adjusted to about 0.05 mass%. The results are recorded in Table 1 together with the conditions.

[0038] [Table 1]

[0039]

[0040] As shown in Table 1, in all of Levels 1 to 12, the carbon concentration [C] of the sample before tapping is approximately 0.05 mass ppm. On the other hand, the nitrogen concentration [N] fluctuates greatly from 12 mass ppm to 136 mass ppm. At a level where the oxygen injection direction includes either O1 or O4 or both, the shorter the oxygen injection time in the direction, the lower the nitrogen concentration. It can be seen that at a level where neither O1 nor O4 is included, the nitrogen concentration [N] can be stably reduced to less than 20 mass ppm.

[0041] From this result, it is clear that it is not necessary to inject oxygen and carbon into the same region as in Patent Document 3. Figure 3 As shown, when the front end of the carbon source injection pipe ejection hole is set as point X, the intersection of the extension line of the central axis of the carbon source injection direction in the top view and the furnace inner wall is set as point Y, the position on the side XY that is 2 / 3 of the length of the side XY from point X is set as point W, the front end of the oxygen supply lance ejection hole is set as point Z, and the angle ∠XWZ of the triangle ΔXWZ is set as angle θ1 (°), it can be seen that if the blowing direction θ2 (°) of the oxygen-containing gas opened to the Y side from the side ZX passing through the front end of the oxygen supply lance ejection hole (point Z) in the top view is positive and less than θ1, a high denitrification effect can be obtained by simultaneously blowing carbon and oxygen. This is because, if the angle θ2 indicating the blowing direction of oxygen is 0 or a negative value, most of the oxygen jet is directed toward the furnace wall, which may damage the furnace wall 19. In addition, in the case of having multiple carbon source injection pipes, it is preferred that the above conditions are satisfied between at least one carbon source injection pipe. It should be noted that since the linear velocity of the carbon source into which oxygen is also blown decays as it moves away from the nozzle, it is more preferable to set point W to move toward point X from a position 2 / 3 of the length of side XY away from point X. Specifically, point W is the midpoint of side XY.

[0042] The angle θ1 (°) can be calculated from the equipment drawing, or the length L of the sides XY, YZ, and XZ can be used. XY (m), L YZ (m) and L XZ (m) is calculated based on the cosine theorem of a triangle with reference to the following mathematical formulas 1 to 3.

[0043] [Mathematical formula 1]

[0044]

[0045] [Mathematical formula 2]

[0046]

[0047] [Mathematical formula 3]

[0048]

[0049] Next, the experimental results recorded in Table 1 are sorted by the effective simultaneous supply time of carbon and oxygen. The time required from the start of scrap melting to the start of tapping of molten steel in the furnace is set to t2 (min). The time for simultaneously supplying carbon and oxygen from the oxygen-containing gas supply hole and the carbon source supply hole in the oxygen injection direction between at least one carbon source injection pipe to meet the above conditions is set to t1 (min). When the effective simultaneous supply time rate of carbon and oxygen is sorted by R = t1 / t2 × 100 (%), as shown in FIG. Figure 4 As shown, the longer the simultaneous supply time rate R is, the lower the nitrogen concentration [N] analyzed in the sample before steel tapping is.

[0050] Generally, low nitrogen steel refers to steel with a product nitrogen concentration of 40 ppm or less. Therefore, the nitrogen concentration [N] in the molten steel is set to be 40 mass ppm or less as a low nitrogen concentration region. First, in relation to the injection pipe of at least one carbon source, it is necessary to satisfy the condition that the angle θ2 (°) of the oxygen injection direction from the oxygen supply lance opening from the side XZ to the Y side is a positive value and is within the range of θ1 or less. Furthermore, in order to form a low nitrogen concentration region, it is preferred to ensure that the effective carbon and oxygen simultaneous supply time rate R that meets the above conditions relative to the operating time is 30% or more. In addition, ultra-low nitrogen steel refers to steel with a product nitrogen concentration of 25 ppm or less. Therefore, the nitrogen concentration [N] in the molten steel is set to be 25 ppm or less as an extremely low nitrogen concentration region. Therefore, in order to become an extremely low nitrogen concentration region, it is more preferred to ensure that the effective carbon and oxygen simultaneous supply time rate R that meets the above conditions relative to the operating time is 45% or more.

[0051] The present invention has been proposed based on such investigations, and can stably reduce the nitrogen concentration in molten steel in a melting furnace or a refining furnace to a low nitrogen concentration region without requiring large-scale investment.

[0052] [Example]

[0053] In an AC arc type electric furnace containing about 60 to 70 tons of molten steel 5 with a residual carbon concentration of 0.03 to 0.06 mass%, new chips are loaded as 10 to 15 tons of scrap 2 (nitrogen content of about 22 mass ppm) per time using a pusher 3. At the same time, the scrap 2 is melted using an arc from a graphite electrode 4. 1.8 tons of coke is added to the obtained molten steel 5, and powdered carbon is blown into the furnace together with an air carrier at a rate of 10 to 20 kg / min from the carbon source blowing pipe 13a on the slag discharge side and at a rate of 20 to 30 kg / min from the carbon source blowing pipe 13b on the steel discharge side to add carbon to the molten steel 5. Oxygen is supplied from the slag discharge side by a lance 10 at a rate of 60 to 70 Nm 3 / min oxygen is sprayed into the molten steel, and after the oxygen supply from the oxygen spray lance 10 on the slag discharge side stops, the oxygen spray lance 15 on the steel discharge side is immediately sprayed at 40-50 Nm 3 / min oxygen is blown into the molten steel to perform decarburization. The time t2 (min) required from the start of scrap melting at the start of power-on to the start of steel tapping in the furnace is 50 to 60 minutes, and the carbon concentration [C] in the molten steel tapped from the electric furnace is 0.03 to 0.06% by mass. In the relationship between the oxygen lance 10 on the slag discharge side and the carbon source blowing pipe 13a on the slag discharge side and the carbon source blowing pipe 13b on the steel tapping side, θ1 is 95° and 105°, respectively. In the relationship between the oxygen lance 15 on the steel tapping side and the carbon source blowing pipe 13a on the slag discharge side and the carbon source blowing pipe 13b on the steel tapping side, θ1 is 40° and 85°, respectively. The oxygen blowing direction θ2 and the effective simultaneous supply time rate R of carbon and oxygen are arbitrarily adjusted to confirm the denitrification behavior. The temperature of the obtained molten steel was 1590-1600°C, the melting amount was 210-220 t per treatment, and the slag volume was 100-150 kg / t-molten steel. The operating conditions and results are shown in Table 2.

[0054] [Table 2]

[0055]

[0056] Test No. 1 to 5, in which the relationship between the oxygen injection direction θ2 on the slag discharge side or the steel tapping side and any carbon source supply hole satisfies the condition of being positive and less than θ1, are invention examples. Test No. 6, in which the oxygen injection direction θ2 on the slag discharge side and the steel tapping side is 0 or negative or greater than θ1 in terms of the relationship between the carbon source supply on the slag discharge side and the steel tapping side, is a comparative example. Compared with the comparative example, the nitrogen concentration [N] at the time of steel tapping in the invention example is less than 1 / 2.

[0057] In the test No. 3, in which the effective simultaneous supply time rate R of carbon and oxygen was ensured to be 30% or more, the nitrogen concentration [N] at the time of steel tapping was about 1 / 4 of the low nitrogen concentration region (≤40 mass ppm) compared with the comparative example. In the tests No. 4 and No. 5, in which the effective simultaneous supply time rate R of carbon and oxygen was ensured to be 45% or more, the nitrogen concentration [N] at the time of steel tapping reached the extremely low nitrogen concentration region (≤25 mass ppm).

[0058] The units used in this manual are converted as follows.

[0059] As a unit of mass, 1t=1000kg.

[0060] The volume unit [N] of the gas volume indicates the standard state, that is, the state at 0°C and 101325 Pa.

[0061] [Industrial Applicability]

[0062] According to the method for denitrifying molten steel of the present invention, the nitrogen concentration in molten steel in a melting furnace or a refining furnace can be stably reduced to a low nitrogen concentration region without requiring large-scale investment, and is therefore industrially useful.

[0063] Description of Reference Numerals

[0064] 1 Furnace body

[0065] 2 Waste

[0066] 3 propulsion machine

[0067] 4 Graphite Electrode

[0068] 5 Molten Steel

[0069] 6. Furnace hopper equipment

[0070] 7. Flux

[0071] 8 Slag

[0072] 9. Oxygen piping system

[0073] 10 Oxygen spray gun for slag discharge side

[0074] 11 Oxygen-containing gas (oxygen)

[0075] 12Carbon source and carrier gas piping system

[0076] 13 Carbon source blowing tube

[0077] 13a (slag discharge side) carbon source blowing pipe

[0078] 13b (steel outlet side) carbon source blowing pipe

[0079] 14 Carbon source and carrier gas (powdered carbon and air)

[0080] 15 Oxygen spray gun on the steel tapping side

[0081] 16 Slag discharge port

[0082] 17 tap hole

[0083] 18 Exhaust

[0084] 19 Furnace Wall

[0085] X, x The front end of the carbon source blowing pipe ejection hole

[0086] Y, y The intersection of the central axis of the carbon source blowing direction and the furnace inner wall in the top view

[0087] Z, z oxygen spray gun oxygen outlet front end

[0088] Oxygen injection direction (central axis) in the top view of O1~O6 and On

Claims

1. A method for denitrifying molten steel, comprising supplying an oxygen-containing gas and a carbon source transported by a transport gas to molten steel filled in a melting furnace or a refining furnace, wherein: The oxygen-containing gas and the carbon source are blown simultaneously under the following conditions: in a top view, relative to a triangle ΔXYZ formed by connecting the front end (point X) of the ejection hole of the blowing pipe of the carbon source, the intersection (point Y) of the extension line of the central axis of the blowing direction of the carbon source and the inner wall of the furnace, and the front end (point Z) of the ejection hole of the oxygen supply lance, the position on the side XY at a distance of 2 / 3 of the length of the side XY from the point X is set as point W, the angle ∠XZW is set as angle θ1 (°), and the blowing direction θ2 (°) of the oxygen-containing gas opening from the side ZX passing through the front end (point Z) of the ejection hole of the oxygen supply lance to the Y side in the top view is a positive value and within a range below θ1.

2. The method for denitrification of molten steel according to claim 1, wherein: When there are a plurality of carbon source injection pipes, the above conditions are satisfied in relation to at least one carbon source injection pipe.

3. The method for denitrification of molten steel according to claim 1, wherein: Ensure that the time t1 (min) for simultaneously supplying the carbon source and the oxygen-containing gas from the carbon source blowing pipe and the oxygen supply lance is at least 30% of the time t2 (min) required from the melting of the cold iron source to the tapping of the molten steel in the furnace.

4. The method for denitrification of molten steel according to claim 2, wherein: Ensure that the time t1 (min) for simultaneously supplying the carbon source and the oxygen-containing gas from the carbon source blowing pipe and the oxygen supply lance is at least 30% of the time t2 (min) required from the melting of the cold iron source to the tapping of the molten steel in the furnace.

5. The method for denitrifying molten steel according to any one of claims 1 to 4, wherein: The point W is located at the midpoint of the side XY.

6. The method for denitrifying molten steel according to any one of claims 1 to 4, wherein: The melting furnace is an electric furnace.

7. The method for denitrification of molten steel according to claim 5, wherein: The melting furnace is an electric furnace.

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