A method for controlling nitrogen content in S460NL steel

Abstract

This invention belongs to the field of iron and steel smelting technology, specifically relating to a method for controlling the nitrogen content in S460NL steel. The method includes: primary refining in an electric furnace or converter → refining in an LF furnace → refining in a VD furnace → casting. After vacuuming in the VD furnace, nitrogen gas is blown into the ladle through two permeable bricks at the bottom to achieve the target nitrogen content. After nitrogen blowing, argon gas is switched for soft stirring. This invention, by simultaneously blowing nitrogen gas through two permeable bricks at the bottom of the ladle to increase the nitrogen content of high-strength S460NL steel used in wind power flanges after vacuum refining in the VD furnace, offers advantages such as lower cost, higher steel purity, higher composition hit rate, and simpler operation compared to adding manganese nitride ferroalloys.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, and specifically relates to a method for controlling the nitrogen content in S460NL steel. Background Technology

[0002] With the rapid increase in wind turbine power and the increasing demand for weight reduction in wind turbine towers, there is an urgent need for higher-strength wind turbine flanges to replace low-strength ones. Currently, most wind turbine flanges domestically and internationally are 355MPa grade, with a small number at 420MPa grade. To promote the high-quality and rapid development of my country's wind power industry, S460NL steel for 460MPa grade high-strength wind turbine flanges has been developed. To improve strength without affecting the weldability and corrosion resistance of the wind turbine flange, the carbon content of S460NL steel should not be too high; the carbon mass fraction needs to be controlled at ≤0.2%. Therefore, a certain amount of nitrogen needs to be added to increase strength. The production process of S460NL steel is: primary refining in an electric furnace or converter → refining in an LF furnace → vacuum refining in a VD furnace → casting (continuous casting or ingot casting), requiring an N content ≤0.025%.

[0003] Currently, S460NL steel is produced by adding manganese nitride alloy in an LF furnace to increase nitrogen content. This method has a low nitrogen recovery rate; 7.5 kg of manganese nitride is required to smelt 1 ton of steel, increasing the cost by approximately 60 yuan per ton. Furthermore, the nitrogen content in the molten steel fluctuates significantly during the subsequent VD vacuum smelting process, making it difficult to precisely control the final nitrogen content (currently controlled within the range of 35-180 ppm). This results in significant fluctuations in the mechanical properties of the product, particularly its low-temperature impact toughness. Data analysis of the relationship between nitrogen content and mechanical properties shows that a stable nitrogen content of 120 ± 10 ppm yields optimal mechanical properties. Therefore, to obtain stable nitrogen content and excellent mechanical properties, there is an urgent need in this field to develop a method for stably controlling the nitrogen content in S460NL steel. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for controlling the nitrogen content in S460NL steel.

[0005] Specifically, the present invention is achieved through the following technical solutions:

[0006] A method for controlling the nitrogen content in S460NL steel includes: primary refining in an electric furnace or converter → refining in an LF furnace → refining in a VD furnace → casting. After the VD furnace vacuuming is completed, nitrogen gas is blown into the ladle through two permeable bricks at the bottom of the ladle to make the nitrogen content in the steel reach the target content. After the nitrogen blowing is completed, argon gas is switched for soft stirring.

[0007] The method described above for controlling the nitrogen content in S460NL steel involves a nitrogen flow rate of 3.8-4.2 NL / min / t for each of the permeable bricks.

[0008] In the above-described method for controlling the nitrogen content in S460NL steel, the nitrogen blowing time is controlled to be 13-17 minutes.

[0009] The above-mentioned method for controlling the nitrogen content in S460NL steel requires that when the nitrogen content in the steel is 35 < [N] ≤ 45 ppm, the nitrogen blowing time should be controlled at 15-17 min; when the nitrogen content in the steel is 45 < [N] ≤ 55 ppm, the nitrogen blowing time should be controlled at 13-15 min.

[0010] In the above method for controlling the nitrogen content in S460NL steel, the duration of soft stirring is 2-2.5 min.

[0011] The above-mentioned method for controlling the nitrogen content in S460NL steel involves stirring with argon gas during the refining process in the LF furnace without introducing nitrogen gas. After alloying treatment, the steel leaves the station and the nitrogen content is initially measured.

[0012] In the above-mentioned method for controlling the nitrogen content in S460NL steel, after the VD furnace refining is completed, the ladle is vacuumed and degassed in the VD furnace. During the vacuuming process, only argon gas is blown for stirring. The vacuum is maintained at a vacuum degree of ≤67pa for ≥15 minutes before the vacuum is broken.

[0013] In the above-described method for controlling the nitrogen content in S460NL steel, the casting is either continuous casting or ingot casting.

[0014] On the other hand, the present invention also provides an S460NL steel, which is prepared by the above-described method.

[0015] In another aspect, the present invention also provides the application of the above-mentioned S460NL steel in the preparation of high-strength wind power flanges.

[0016] The technical solution of the present invention has the following beneficial effects:

[0017] This invention, after the vacuum refining process in VD, simultaneously blows nitrogen through two permeable bricks at the bottom of the ladle to nitrogen-enrich the S460NL steel used in high-strength wind power flanges. Compared to nitrogen enrichment by adding manganese nitride ferroalloy, this method has advantages such as lower cost, higher steel purity, higher composition hit rate, and simpler operation.

[0018] Cost Advantage: Compared to adding manganese ferronitride (Mn nitride) for nitrogen enrichment, using nitrogen blowing significantly reduces costs. Taking S460NL steel for high-strength wind turbine flanges produced by a steel mill as an example, adding Mn ferronitride requires approximately 7.5 kg / t, with an Mn ferronitride unit price of 13,550 yuan / ton, resulting in a cost of 13,550 / 1000 × 7.5 = 101.6 yuan / t steel. Using nitrogen blowing, approximately 7.5 kg / t of ordinary manganese ferronitride is sufficient to meet the corresponding Mn content requirements, with an ordinary Mn ferronitride unit price of 5,500 yuan / ton, resulting in a cost of 5,500 / 1000 × 7.5 = 41.3 yuan / ton steel. Therefore, compared to adding Mn ferronitride, using nitrogen blowing saves 101.6 - 41.3 = 60.3 yuan / ton in costs.

[0019] Higher purity: Purity directly affects the mechanical properties of the finished wind turbine flange. Using alloy nitrogen enhancement involves adding manganese ferronitride alloy after the VD void is broken, and the next step is direct casting. It is difficult to guarantee the purity of the added alloy material, which will inevitably increase the content of non-metallic inclusions and P, S, O, and N gases in the steel, contaminating the molten steel. However, the method of using nitrogen enhancement by blowing nitrogen through two holes at the bottom of the ladle does not have the problem of contaminating the molten steel. At the same time, it can also make the inclusions float more fully, making the molten steel purer.

[0020] High component hit rate and simpler operation: To save costs, adding manganese ferromanganese alloy for nitrogen enhancement requires adding high-carbon ferromanganese and manganese ferromanganese in two or more steps to ensure the Mn and N content in the molten steel. Therefore, there is mutual interference between Mn and N components, making the operation complicated and affecting the production rhythm. However, by using nitrogen blowing for nitrogen enhancement, high-carbon / medium-carbon ferromanganese is added directly, and the manganese and nitrogen content are controlled separately without interference. The operation is simple and the manganese and nitrogen content can be precisely controlled. Attached Figure Description

[0021] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention.

[0022] Figure 1 A schematic diagram of the arrangement of double-hole nitrogen blowing permeable bricks at the bottom of the steel ladle;

[0023] Wherein, 1 is the first diameter, 2 is the second diameter, 3 is the first breathable brick, 4 is the second breathable brick, and 5 is the trunnion. Detailed Implementation

[0024] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0025] When a range of values ​​is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0026] To precisely control the nitrogen content (target requirement 120±10ppm) in S460NL steel used for high-strength wind power flanges, this invention provides a method that not only increases nitrogen content by blowing nitrogen through two holes at the bottom of the ladle, but also precisely controls the nitrogen content in the steel, effectively reduces costs, and is simple to operate.

[0027] Specifically, the method for controlling the nitrogen content in S460NL steel according to the present invention includes: primary refining in an electric furnace or converter → refining in an LF furnace → refining in a VD furnace → casting. After the VD furnace vacuuming is completed, nitrogen gas is blown into the ladle through two permeable bricks at the bottom of the ladle to make the nitrogen content in the steel reach the target content. After the nitrogen blowing is completed, argon gas is switched for soft stirring.

[0028] In some preferred embodiments, such as Figure 1 As shown, the first breathable brick 3 and the second breathable brick 4 are respectively located at the bottom of the ladle and on the same side of the first diameter 1 of the bottom of the ladle where the trunnion 5 is located. The distances of the first breathable brick 3 and the second breathable brick 4 from the first diameter 1 and the second diameter 2 of the bottom of the ladle, which is perpendicular to the first diameter 1, are respectively half the radius R of the bottom of the ladle.

[0029] Through practice, breathable bricks have been found to be effective. Figure 1 This arrangement can effectively improve the nitrogen absorption rate, and at the same time, when the nitrogen flow rate fluctuates, it will not cause the top slag to be pushed out and the molten steel to be exposed, thus reducing the risk of secondary oxidation of the molten steel.

[0030] In some other implementations, the number of breathable bricks is not limited to two.

[0031] Among them, the main chemical component of the permeable brick is Al2O3 (≥87%) and MgO+Al2O 3+ The Cr2O3 content is ≥92%, and the bulk density is ≥3.05 g / cm³. 3Apparent porosity ≤18%, compressive strength ≥100MPa.

[0032] In some preferred embodiments, the nitrogen flow rate of both permeable bricks is 3.8-4.2 NL / min / t. When the nitrogen content in the steel is 35 < [N] ≤ 45 ppm, the nitrogen blowing time is controlled at 15-17 min; when the nitrogen content in the steel is 45 < [N] ≤ 55 ppm, the nitrogen blowing time is controlled at 13-15 min.

[0033] This invention controls the nitrogen flow rate at 3.8-4.2 NL / min / t, which ensures both the nitrogen enrichment effect and the nitrogen enrichment duration, while also preventing secondary oxidation of the molten steel.

[0034] When the nitrogen flow rate is less than 3.8 NL / min / t, the nitrogen enrichment efficiency is too low, the temperature drop of the molten steel is too large, and it affects the subsequent production rhythm. When the nitrogen flow rate is greater than 4.2 NL / min / t, the nitrogen absorption rate decreases, and there is a risk that the flow rate will fluctuate greatly, causing the top slag to be displaced and resulting in secondary oxidation of the molten steel.

[0035] In some preferred embodiments, after nitrogen blowing is completed, argon gas is switched for soft stirring for 2-2.5 minutes. The nitrogen content in the steel is analyzed. Based on the difference between the measured value and the target value of nitrogen content, the time for supplementary nitrogen blowing is determined according to the nitrogen blowing flow rate (3.8-4.2 NL / min / t steel) and the nitrogen blowing rate. After nitrogen blowing is completed, argon gas is switched for soft stirring. When the specified molten steel temperature requirement is met, the steel is tapped and sent for casting.

[0036] The primary smelting in the electric furnace or converter is carried out using conventional methods of existing technology, and this invention does not impose specific limitations on this.

[0037] In some preferred embodiments, the LF refining process involves stirring with argon gas but not with nitrogen gas. Ferromanganese, ferrosilicon, ferroniobium, and ferrovanadium are added for alloying treatment, and the content of alloying elements is controlled within the target range. The product leaves the station after the target temperature is reached, and the nitrogen content is initially measured.

[0038] In some preferred embodiments, after VD furnace refining, the ladle is evacuated and degassed in the VD furnace. During the evacuation process, only argon gas is blown for stirring, and the vacuum is maintained at ≤67 Pa for ≥15 minutes before being broken. Ensuring the evacuation time in the VD furnace effectively degasses and removes inclusions, while also maintaining the N content range between furnace cycles. Nitrogen replenishment after breaking the vacuum further facilitates nitrogen absorption.

[0039] In this invention, the casting process is either continuous casting or ingot casting, and this invention does not specifically limit the method.

[0040] The present invention discloses a method for controlling the nitrogen content in S460NL steel for high-strength wind turbine flanges. After vacuum diversion (VD) is completed, nitrogen gas is blown into the steel through two permeable bricks at the bottom of the ladle to increase the nitrogen content. This method can stably control the nitrogen content at 120±10ppm, ensuring the mechanical properties and stability of the finished flange. Compared with the existing method of adding manganese ferronitride alloy to increase nitrogen, this method has advantages such as lower cost, higher steel purity, higher component hit rate, and simpler operation.

[0041] Example

[0042] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments, unless otherwise specified, are performed according to conventional methods and conditions.

[0043] Example 1

[0044] The high-strength wind turbine flange steel S460NL is produced using the process of "electric furnace smelting → LF furnace refining → VD vacuum smelting → φ800mm continuous casting large round billet". The production process is as follows:

[0045] Electric furnace smelting: Smelting is carried out according to the smelting process requirements, with the final C being 0.071%, the final P being 0.005%, and the final temperature of the electric furnace being 1650℃. The auxiliary materials added at tapping include lime and synthetic slag. The alloys added with the steel stream include aluminum shot, ferromanganese, and silicon-manganese alloy. The steel output is 82.7 tons.

[0046] LF Refining: The entire LF refining process is purged with argon gas. Medium-carbon ferromanganese, high-carbon ferromanganese, ferrosilicon, ferroniobium, ferrovanadium, aluminum shot, and aluminum wire are used to adjust the elements such as C, Mn, Si, Al, P, S, Nb, and V to the target requirements. The weight percentage of the chemical composition at the LF endpoint is as follows (%): C: 0.167, Si: 0.31, Mn: 1.567, P: 0.008, S: 0.001, Nb: 0.03, V: 0.06, Ti: 0.0028, Cr: 0.05, Ni: 0.02, Cu: 0.01, Mo: 0.0036, Al: 0.064; balance is Fe. The LF endpoint temperature is 1672℃, and the endpoint [N] is 85ppm.

[0047] VD Vacuum Treatment: After LF refining, VD vacuum treatment is performed. After the ladle is hoisted into the VD station, argon gas is blown on, and the temperature is measured at 1660℃. After the vacuum reaches 67Pa and is maintained for 15 minutes, the vacuum is broken, and the temperature is measured at 1605℃. The composition and gas content mass percentage (%) are as follows: C: 0.168, Si: 0.34, Mn: 1.576, P: 0.007, S: 0.002, Nb: 0.03, V: 0.06, Ti: 0.0025, Cr: 0.05, Ni: 0.02, Cu: 0.01, Mo: 0.0033, Al: 0.036. At this point, the N content in the steel was 36 ppm. Argon blowing was stopped, and nitrogen blowing was switched. The nitrogen blowing flow rates of No. 1 and No. 2 permeable bricks were 340 NL / min (approximately 4.11 NL / min / t steel) and 335 NL / min (approximately 4.05 NL / min / t steel), respectively. The total flow rate of the two permeable bricks was 675 NL / min. After blowing nitrogen for 17 min, argon blowing was switched to 2 min. A gas sample was taken for analysis, and the [N] content was 124 ppm, which met the target requirements.

[0048] Example 2

[0049] A company uses the process of "electric furnace smelting → LF furnace refining → VD vacuum smelting → φ800mm continuous casting large round billet" to produce high-strength wind turbine flange steel S460NL. The production process is as follows:

[0050] Electric furnace smelting: Smelting is carried out according to the smelting process requirements, with the final C being 0.074%, the final P being 0.004%, and the final temperature being 1642℃. The auxiliary materials added during tapping are lime and synthetic slag. The alloys added with the steel stream are aluminum shot, ferromanganese, and silicon-manganese alloy. The steel output is 82.5 tons.

[0051] LF Refining: The entire LF refining process is purged with argon gas. Medium-carbon ferromanganese, high-carbon ferromanganese, ferrosilicon, ferroniobium, ferrovanadium, aluminum shot, and aluminum wire are used to adjust the elements such as C, Mn, Si, Al, P, S, Nb, and V to the target requirements. The weight percentage of the chemical composition at the LF endpoint is as follows (%): C: 0.165, Si: 0.40, Mn: 1.614, P: 0.006, S: 0.001, Nb: 0.03, V: 0.06, Ti: 0.0034, Cr: 0.05, Ni: 0.02, Cu: 0.01, Mo: 0.0037, Al: 0.062; balance is Fe. The LF endpoint temperature is 1668℃, and the endpoint [N] is 90ppm.

[0052] VD Treatment: After LF refining, VD vacuum treatment is performed. After the ladle is placed in the VD station, argon gas is purged and the temperature is measured at 1653℃. After the vacuum reaches 67Pa and is maintained for 15 minutes, the vacuum is broken and the temperature is measured at 1604℃. The composition and gas content are measured, and the weight percentage of the components is as follows (%): C: 0.176, Si: 0.38, Mn: 1.608, P: 0.007, S: 0.002, Nb: 0.03, V: 0.06, Ti: 0.0027, Cr: 0.05, Ni: 0.02, Cu: 0. 01. Mo: 0.0035; Al: 0.033; At this time, the N content in the steel is 44 ppm. Argon blowing is stopped and nitrogen blowing is switched. The nitrogen blowing flow rates of No. 1 and No. 2 permeable bricks are 323 NL / min (about 3.92 NL / min / t steel) and 340 NL / min (about 4.12 NL / min / t steel) respectively. The total flow rate of the two permeable bricks is 663 NL / min. After blowing nitrogen for 15 min, argon blowing is switched for 2 min. A gas sample is taken for analysis. The [N] content is 123 ppm, which meets the target requirements.

[0053] Example 3

[0054] A company uses the process of "electric furnace smelting → LF furnace refining → VD vacuum smelting → φ800mm continuous casting large round billet" to produce high-strength wind turbine flange steel S460NL. The production process is as follows:

[0055] Electric furnace smelting: Smelting is carried out according to the smelting process requirements, with the final C being 0.056%, the final P being 0.005%, and the final temperature being 1642℃. The auxiliary materials added during tapping are lime and synthetic slag. The alloys added with the steel stream are aluminum shot, ferromanganese, and silicon-manganese alloy. The steel output is 82.2 tons.

[0056] LF Refining: The entire LF refining process is purged with argon gas. Medium-carbon ferromanganese, high-carbon ferromanganese, ferrosilicon, ferroniobium, ferrovanadium, aluminum shot, and aluminum wire are used to adjust the elements such as C, Mn, Si, Al, P, S, Nb, and V to the target requirements. The weight percentage of the chemical composition at the LF endpoint is as follows (%): C: 0.167, Si: 0.41, Mn: 1.623, P: 0.006, S: 0.001, Nb: 0.03, V: 0.06, Ti: 0.0038, Cr: 0.05, Ni: 0.02, Cu: 0.01, Mo: 0.0033, Al: 0.061; the balance is Fe. The LF endpoint temperature is 1675℃, and the endpoint [N] is 96ppm.

[0057] VD treatment: After LF refining, VD vacuum treatment is performed. After the ladle is placed in the VD station, argon gas is blown on and the temperature is measured to be 1654℃. After the vacuum reaches 67Pa and is maintained for 15 minutes, the vacuum is broken and the temperature is measured to be 1605℃. The composition and gas content are measured. The weight percentage of the composition is as follows (%): C: 0.175, Si: 0.39, Mn: 1.603, P: 0.0071, S: 0.001, Nb: 0.03, V: 0.06, Ti: 0.0028, Cr: 0.05, Ni: 0.03, Cu: 0.01, Mo: 0.0034, Al: 0.032. At this point, the N content in the steel was 47 ppm. Argon blowing was stopped, and nitrogen blowing was switched. The nitrogen blowing flow rates of No. 1 and No. 2 permeable bricks were 342 NL / min (approximately 4.16 NL / min / t steel) and 339 NL / min (approximately 4.12 NL / min / t steel), respectively, with a total flow rate of 681 NL / min for the two permeable bricks. After blowing nitrogen for 14 min, argon blowing was switched to 2.5 min. A gas sample was taken for analysis, and the [N] content was 121 ppm, which met the target requirements.

[0058] Example 4

[0059] A company uses the process of "electric furnace smelting → LF furnace refining → VD vacuum smelting → φ800mm continuous casting large round billet" to produce high-strength wind turbine flange steel S460NL. The production process is as follows:

[0060] Electric furnace smelting: Smelting is carried out according to the smelting process requirements, with the final C content being 0.055%, P content being 0.005%, and the final temperature being 1643℃. Auxiliary materials added during tapping include lime and synthetic slag. Alloys added with the steel stream include aluminum shot, ferromanganese, and silicon-manganese alloy. The steel output is 82.9 tons.

[0061] LF Refining: The entire LF refining process is purged with argon gas. Medium-carbon ferromanganese, high-carbon ferromanganese, ferrosilicon, ferroniobium, ferrovanadium, aluminum shot, and aluminum wire are used to adjust the elements such as C, Mn, Si, Al, P, S, Nb, and V to the target requirements. The weight percentage of the chemical composition at the LF endpoint is as follows (%): C: 0.163, Si: 0.43, Mn: 1.618, P: 0.007, S: 0.001, Nb: 0.03, V: 0.06, Ti: 0.0038, Cr: 0.05, Ni: 0.02, Cu: 0.01, Mo: 0.0040, Al: 0.060; the balance is Fe. The LF endpoint temperature is 1677℃, and the endpoint [N] is 103ppm.

[0062] VD Treatment: After LF refining, VD vacuum treatment is performed. After the ladle is placed in the VD station, argon gas is blown on, and the temperature is measured at 1655℃. After the vacuum reaches 67Pa and is maintained for 15 minutes, the vacuum is broken, and the temperature is measured at 1606℃. The composition and gas content are measured. The weight percentage of the composition is as follows (%): C: 0.172, Si: 0.40, Mn: 1.61, P: 0.007, S: 0.001, Nb: 0.03, V: 0.06, Ti: 0.0030, Cr: 0.05, Ni: 0.02, Cu: 0.01, Mo: 0.0039, Al: 0.032. At this point, the N content in the steel was 54 ppm. Argon blowing was stopped, and nitrogen blowing was switched. The nitrogen blowing flow rates for permeable bricks #1 and #2 were 320 NL / min (approximately 3.86 NL / min / t steel) and 340 NL / min (approximately 4.1 NL / min / t steel), respectively, with a total flow rate of 660 NL / min for the two permeable bricks. After blowing nitrogen for 13 min, argon blowing was switched to 2.5 min. A gas sample was taken for analysis, and the [N] content was 118 ppm, which met the target requirements.

[0063] The present invention has been disclosed above with reference to preferred embodiments. However, those skilled in the art should understand that these embodiments are merely illustrative of the invention and should not be construed as limiting its scope. It should be noted that any variations and substitutions equivalent to these embodiments should be considered to be covered within the scope of the claims. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for controlling the nitrogen content in S460NL steel, comprising: The process is as follows: primary refining in an electric furnace or converter → refining in an LF furnace → refining in a VD furnace → casting. The feature is that after the VD vacuuming is completed, nitrogen gas is blown into the ladle through the first and second permeable bricks at the bottom of the ladle to make the nitrogen content in the steel reach the target content. After the nitrogen blowing is completed, argon gas is switched to perform soft stirring for 2-2.5 minutes. The nitrogen flow rate of the first and second permeable bricks is 3.8-4.2 NL / min / t; when the nitrogen content in the steel is 35 < [N] ≤ 45 ppm, the nitrogen blowing time is controlled at 15-17 min; when the nitrogen content in the steel is 45 < [N] ≤ 55 ppm, the nitrogen blowing time is controlled at 13-15 min. After the VD furnace refining is completed, the ladle is degassed under vacuum in the VD furnace. During the vacuuming process, only argon gas is blown and stirred. The vacuum is maintained at a vacuum degree of ≤67pa for ≥15 minutes before the vacuum is broken. The first and second breathable bricks are located at the bottom of the ladle and on the same side of the first diameter of the bottom of the ladle where the trunnion is located. The distances of the first and second breathable bricks from the first diameter and the second diameter of the bottom of the ladle (which is perpendicular to the first diameter) are respectively half the radius R of the bottom of the ladle.

2. The method according to claim 1, characterized in that, Argon gas is blown for stirring during the refining process in the LF furnace, but nitrogen gas is not blown in. After alloying treatment, the product leaves the station and the nitrogen content is initially measured.

3. The method according to claim 1, characterized in that, The casting is either continuous casting or die casting.

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