A precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel

Abstract

The present invention discloses a precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel, including converter smelting, LF furnace refining, RH furnace refining, nitrogen content detection, continuous casting and other steps. First, the converter and LF furnace are used in combination for preliminary nitrogen addition, and then the RH furnace is used for refining. The molten steel is nitrogen-added by a vacuum treatment process, and the nitrogen content in the molten steel is further increased, so that the nitrogen content of the molten steel can be stably reached 300ppm~450ppm, or even higher. The molten steel is then sampled and tested to obtain the specific nitrogen content in the molten steel. According to the measured nitrogen content, the type of protective gas in the subsequent continuous casting process is determined. If the nitrogen content is lower than the lower limit, nitrogen sealing protection casting is used to increase nitrogen. If the nitrogen content is middle limit or above, argon sealing protection casting is used to further accurately control the nitrogen content. The present invention has the advantages that the molten steel is not easy to be polluted, the smelting cost can be reduced, and the nitrogen addition is stable.

Description

Technical Field

[0001] The present invention belongs to the technical field of nitrogen-containing high-strength steel smelting, and specifically relates to a precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel. Background Art

[0002] As one of the important elements in steel, nitrogen is a strong austenite-forming element. It has the ability to expand and stabilize austenite in duplex stainless steel. In terms of its contribution to austenite formation, the nitrogen equivalent is 30 times that of nickel. With the addition of nitrogen, the corrosion resistance of stainless steel can be significantly improved. Nitrogen can also improve the stability of austenite and reduce the formation rate of harmful intermetallic phases. The solid solution strengthening and grain refinement of nitrogen can improve the strength, wear resistance and corrosion resistance of steel. Some stainless steel, oriented silicon steel, high manganese steel, microalloy steel and some quenched and tempered steel all need to add some nitrogen. Therefore, precise control of the nitrogen content in steel is conducive to improving the comprehensive mechanical properties of nitrogen-containing steel. In the current smelting process of nitrogen-containing steel, nitrogen addition is a difficult problem for the steelmaking process, especially for steels with a particularly narrow range of nitrogen content requirements. Different smelting equipment and conditions require a matching nitrogen addition process to meet production needs.

[0003] Most steel companies use the method of using converters to smelt pre-treated molten iron. After stirring, slag washing, heating, vacuum, and blowing in the refining furnace, nitrogen-containing alloys or nitrogen-containing alloy wire feeding and nitrogen blowing in the refining process are used to achieve nitrogen addition. This method can basically meet the current nitrogen content control of nitrogen-containing steel grades and is a relatively effective nitrogen addition method. However, there are still the following problems: First, the purity of the molten steel after vacuum treatment is relatively high, and the addition of nitrogen-increasing materials is prone to secondary oxidation pollution of the molten steel, resulting in fluctuations in product quality; second, the use of nitrogen-containing alloys to increase nitrogen is relatively expensive, which increases smelting costs and reduces market competitiveness; third, it is difficult to achieve stable control of nitrogen content by blowing nitrogen at the bottom of the refining process, and the processing cycle is long, which affects steelmaking efficiency and is not conducive to stable and smooth production. Therefore, it is an objective need to develop a precise nitrogen control smelting method for nitrogen-containing high-strength steel that is not easy to pollute molten steel, can reduce smelting costs, and has stable nitrogen addition. Summary of the invention

[0004] In order to solve the problems existing in the background technology, the purpose of the present invention is to provide a precise nitrogen control smelting method for nitrogen-containing high-strength steel which is not easy to be polluted by molten steel, can reduce smelting costs, and increase nitrogen stably.

[0005] The precise nitrogen-controlled smelting method of nitrogen-containing high-strength steel of the present invention comprises the following steps: ① Converter smelting: Converter smelting includes the early stage of smelting, the middle stage of smelting and the late stage of smelting. In the early stage of smelting and the middle stage of smelting, argon is blown at the bottom and oxygen is blown at the top. In the middle stage of smelting, slag foaming agent is added for denitrification treatment. In the late stage of smelting, top oxygen blowing is stopped and deoxidizer is added to deoxidize slag and molten steel. The supply intensity of bottom blowing argon is 0.04-0.05m / (min·t steel). After the molten steel is deoxidized, the oxygen content in the molten steel is controlled below 30ppm, and the nitrogen content in the molten steel is controlled below 15ppm. The converter tapping temperature is controlled at 1615℃-1635℃. ② LF furnace refining: the molten steel after the converter smelting in step ① is transferred to the LF furnace for refining, and nitrogen is added by top blowing nitrogen and bottom blowing nitrogen at the same time. The supply intensity of the top blowing nitrogen is 3-4m / (min·t steel), and the supply intensity of the bottom blowing nitrogen is 0.04-0.05m / (min·t steel), so that the nitrogen content in the molten steel is 160ppm-200ppm, and the outlet temperature of the molten steel is controlled at 1540℃-1560℃; 3. RH furnace refining: the molten steel after the LF furnace refining in step ② is transferred to the RH furnace refining for vacuum degassing, the vacuum chamber is evacuated to make the vacuum degree of the vacuum chamber less than 100Pa, and after maintaining for 15min to 25min, part of the vacuum pump group is turned off, and the vacuum chamber is filled with nitrogen to stabilize the vacuum degree of the vacuum chamber between 15kPa and 25kP, and maintained for 20min to 30min, so that the nitrogen content in the molten steel is increased to 300ppm to 450ppm; ④ Nitrogen content detection: Sampling and testing the molten steel after RH furnace refining in step ③ to obtain the nitrogen content in the molten steel; ⑤ Continuous casting: During the continuous casting process, the long nozzle, the submerged nozzle and the top of the ladle are all cast with gas protection.

[0006] Furthermore, in step ①, the slag foaming agent is slag steel obtained by hot stuffing and magnetic separation of steel-making slag, which contains 40-50wt% of iron oxide, and the addition amount is 4-5Kg / t steel.

[0007] Furthermore, in step ①, the deoxidizer is an aluminum deoxidizer.

[0008] Furthermore, in step ②, the gun position of the top spray gun is controlled between 1.2m and 1.6m, and the nitrogen blowing time is 50s to 100s.

[0009] Furthermore, in step ②, the purity of nitrogen is 99.99%.

[0010] Furthermore, in step ③, when the vacuum chamber is filled with nitrogen, the flow rate of nitrogen is less than 50m 3 / h. After the vacuum degree of the vacuum chamber is stable, increase the flow rate of nitrogen to 60m 3 / h~80m 3 / h.

[0011] Furthermore, in step ⑤, the type of protective gas is switched according to the nitrogen content of the molten steel. If the nitrogen content is at the lower limit, nitrogen sealing protection casting is used to increase nitrogen; if the nitrogen content is at the middle limit or above, argon sealing protection casting is used.

[0012] Further, in step ①, the early stage of smelting is the stage from the start of oxygen blowing to the stage when the oxygen blowing amount of the converter top blowing reaches 70% of the total oxygen blowing amount in the whole converter smelting process, the middle stage of smelting is the stage when the oxygen blowing amount of the converter top blowing reaches 70% to 90% of the total oxygen blowing amount in the whole converter smelting process, and the late stage of smelting is the stage when the oxygen blowing amount of the converter top blowing reaches 90% of the total oxygen blowing amount in the whole converter smelting process to the end of blowing.

[0013] The beneficial effects of the present invention are: First, the present invention overturns the traditional nitrogen-increasing thinking of steelmaking, and adopts a nitrogen-increasing process that is completely different from the prior art, that is, in the converter smelting process, a slag foaming agent is used for deep denitrification treatment, and a deoxidizer is used to deeply deoxidize the slag and molten steel. Denitrification and deoxidation are first carried out in the converter smelting process, and then in the LF furnace refining process, the molten steel is subjected to a violent nitrogen-increasing treatment, and the nitrogen content of the molten steel is quickly and efficiently controlled between 160ppm and 200ppm. The main principle used is: when the nitrogen content in the molten steel is low, it has a large nitrogen absorption driving force, and a large amount of nitrogen can be supplied in a short time to achieve the purpose of violent and rapid nitrogen increase of the molten steel. When the nitrogen content of the molten steel is already high, the nitrogen absorption efficiency will be relatively slow and difficult. The principle of first decreasing and then increasing can quickly and efficiently increase the nitrogen content in the molten steel during the converter smelting and LF furnace refining process.

[0014] Secondly, the present invention adopts the mode of nitrogen enrichment in the entire smelting process of molten steel, thereby avoiding the disadvantages of low yield, high price, high smelting cost, etc. caused by adding nitrogen-containing alloys to the molten steel, reducing smelting costs, and improving market competitiveness. At the same time, it avoids the nitrogen-containing alloy from bringing in inclusions to increase the inclusions in the steel, ensuring the purity of the molten steel, and at the same time avoids the problem of secondary oxidation pollution of the molten steel that is easily caused by the addition of nitrogen-containing alloys, ensuring the stability of the quality of the steel plate products, reducing quality fluctuations, and preventing the occurrence of steel plate flaw detection failures.

[0015] In summary, the present invention sequentially performs converter smelting, LF furnace refining, RH furnace refining and continuous casting processes on the molten steel, first uses the converter and LF furnace to cooperate for preliminary nitrogen addition, and then uses the RH furnace for refining. After ensuring the removal of the hydrogen content in the molten steel, the molten steel is nitrogenized by using a vacuum treatment process, and the nitrogen content in the molten steel is further increased, so that the nitrogen content of the molten steel can be stably reached 300ppm~450ppm, or even higher, and then the molten steel is sampled and tested to obtain the specific nitrogen content in the molten steel. According to the measured nitrogen content, the type of protective gas in the subsequent continuous casting process is determined. If the nitrogen content is lower than the lower limit, nitrogen sealing protection casting is used for nitrogen addition. If the nitrogen content is middle limit or above, argon sealing protection casting is used to further accurately control the nitrogen content. The present invention has the advantages that the molten steel is not easy to be polluted, the smelting cost can be reduced, and the nitrogen addition is stable. DETAILED DESCRIPTION

[0016] The present invention is further described below in conjunction with the embodiments, but the present invention is not limited in any way. Any changes or substitutions made based on the teachings of the present invention belong to the protection scope of the present invention. Example 1

[0017] The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel described in Example 1 comprises the following steps: ① Converter smelting: Converter smelting includes the early stage, middle stage and late stage of smelting. The early stage of smelting is the stage from the start of oxygen blowing to the oxygen blowing amount of the converter top blowing reaching 70% of the total oxygen blowing amount in the whole converter smelting process. The middle stage of smelting is the stage from the oxygen blowing amount of the converter top blowing reaching 70% to 90% of the total oxygen blowing amount in the whole converter smelting process. The late stage of smelting is the stage from the oxygen blowing amount of the converter top blowing reaching 90% of the total oxygen blowing amount in the whole converter smelting process to the end of blowing. In the early and middle stages of smelting, argon is blown at the bottom while oxygen is blown at the top. In the middle stage of smelting, a slag foaming agent is added to perform denitrification treatment. The slag foaming agent is slag steel obtained by hot stuffing and magnetic separation of steelmaking slag, which contains 40-50wt% of iron oxide, and the addition amount is 4Kg / t steel. In the late stage of smelting, the top blowing of oxygen is stopped, and a deoxidizer is added to deoxidize the slag and molten steel. The deoxidizer is an aluminum deoxidizer, and the gas supply intensity of the bottom blowing argon is 0.04m / (min·t steel). After the molten steel is deoxidized, the oxygen content in the molten steel is controlled to be 21ppm, and the nitrogen content in the molten steel is controlled to be 10ppm. The converter tapping temperature is controlled to be 1615°C. The purpose of adding the slag foaming agent in the present invention is to improve the denitrification capacity of the converter. In the later stage of oxygen blowing, the carbon-oxygen reaction in the furnace slows down, the amount of CO produced is small, and the denitrification capacity is weak. After adding the slag foaming agent, the later stage slag can be activated, a large amount of CO is produced to promote denitrification, and the emulsified slag can effectively cover the surface of the molten steel to prevent the molten steel from contacting with the air, thereby reducing the nitrogen content of the molten steel. Because oxygen is an active element on the surface of the molten steel, the presence of oxygen will play a role in isolating the molten steel from the air. When the oxygen content of the molten steel reaches a certain value, even if nitrogen is blown into the molten steel continuously, the molten steel will hardly increase nitrogen. Therefore, in order to make the molten steel increase nitrogen violently, deoxidation must be carried out first. In addition to the deoxidation effect, aluminum is also an element with a strong ability to combine with nitrogen. After adding aluminum deoxidation, it is more conducive to nitrogen absorption, and the remaining aluminum is left in the steel to play a very strong nitrogen fixation effect, thereby reducing the partial pressure of nitrogen in the molten steel, further promoting the subsequent nitrogen absorption of the molten steel.

[0018] ② LF furnace refining: the molten steel after the converter smelting in step ① is transferred to the LF furnace for refining, and nitrogen is increased by top blowing nitrogen and bottom blowing nitrogen at the same time. The gun position of the top spray gun is controlled at 1.2m, the nitrogen blowing time is 50s, the purity of nitrogen is 99.99%, the gas supply intensity of the top blowing nitrogen is 3m / (min·t steel), and the gas supply intensity of the bottom blowing nitrogen is 0.04m / (min·t steel), so that the nitrogen content in the molten steel is 160ppm, and the outlet temperature of the molten steel is controlled at 1540℃; the purpose of controlling the gun position at 1.2m is mainly to enable the nitrogen jet to better penetrate the molten pool, so that nitrogen is integrated into the molten steel and the nitrogen content of the molten steel is increased. If the gun position is too high, the impact depth of the nitrogen jet is insufficient, and the nitrogen increase effect will be unsatisfactory. However, if the gun position is too low, the nitrogen jet may impact the bottom of the furnace, resulting in damage to the refractory at the bottom of the furnace and affecting the life of the converter.

[0019] ③ RH furnace refining: transfer the molten steel after LF furnace refining in step ② to RH furnace refining for vacuum degassing, evacuate the vacuum chamber to make the vacuum degree of the vacuum chamber 60Pa, keep it for 15min, turn off part of the vacuum pump group, fill the vacuum chamber with nitrogen, make the vacuum degree of the vacuum chamber stable within 15kPa, keep it for 20min, the nitrogen content in the molten steel increases to 360ppm, when the vacuum chamber is filled with nitrogen, the flow rate of nitrogen is 40m 3 / h. After the vacuum degree of the vacuum chamber is stable, increase the flow rate of nitrogen to 60m 3 / h; ④ Nitrogen content detection: Sampling and testing the molten steel after RH furnace refining in step ③ to obtain the nitrogen content in the molten steel; ⑤ Continuous casting: In the continuous casting process, the long nozzle, the submerged nozzle and the upper part of the ladle are all cast with gas protection. The type of protective gas is switched according to the nitrogen content of the molten steel. If the nitrogen content is lower than the lower limit, nitrogen sealing protection casting is used to increase nitrogen; if the nitrogen content is in the middle limit or above, argon sealing protection casting is used.

[0020] In this embodiment, the molten steel is subjected to converter smelting, LF furnace refining, RH furnace refining and continuous casting processes in sequence, and the nitrogen content of the molten steel entering the LF furnace is controlled to 10ppm, which can ensure that there is sufficient driving force for nitrogen increase in the LF furnace. If the nitrogen content of the molten steel is too high, the nitrogen absorption effect of the drastic nitrogen increase step is poor. Ultimately, the nitrogen content of the molten steel can be stably reached 360ppm, and appropriate nitrogen increase can be performed in the subsequent continuous casting process. After testing, the nitrogen content of the final steel is 365.5ppm, which meets the nitrogen content requirement in nitrogen-containing high-strength steel. Example 2

[0021] The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel described in Example 2 includes the following steps: ① Converter smelting: Converter smelting includes the early stage, middle stage and late stage of smelting. The early stage of smelting is the stage from the start of oxygen blowing to the oxygen blowing amount of the converter top blowing reaching 70% of the total oxygen blowing amount in the whole converter smelting process. The middle stage of smelting is the stage from the oxygen blowing amount of the converter top blowing reaching 70% to 90% of the total oxygen blowing amount in the whole converter smelting process. The late stage of smelting is the stage from the oxygen blowing amount of the converter top blowing reaching 90% of the total oxygen blowing amount in the whole converter smelting process to the end of blowing. In the early and middle stages of smelting, argon is blown at the bottom while oxygen is blown at the top. In the middle stage of smelting, a slag foaming agent is added to perform denitrification treatment. The slag foaming agent is slag steel obtained by hot stuffing and magnetic separation of steelmaking slag, which contains 40-50wt% of iron oxide, and the addition amount is 4.5Kg / t steel. In the late stage of smelting, the top blowing of oxygen is stopped, and a deoxidizer is added to deoxidize the slag and molten steel. The deoxidizer is an aluminum deoxidizer, and the gas supply intensity of the bottom blowing argon is 0.05m / (min·t steel). After the molten steel is deoxidized, the oxygen content in the molten steel is controlled to be 26ppm, and the nitrogen content in the molten steel is controlled to be 13ppm. The converter tapping temperature is controlled to be 1625°C. The purpose of adding the slag foaming agent in the present invention is to improve the denitrification capacity of the converter. In the later stage of oxygen blowing, the carbon-oxygen reaction in the furnace slows down, the amount of CO produced is small, and the denitrification capacity is weak. After adding the slag foaming agent, the later stage slag can be activated, a large amount of CO is produced to promote denitrification, and the emulsified slag can effectively cover the surface of the molten steel to prevent the molten steel from contacting with the air, thereby reducing the nitrogen content of the molten steel. Because oxygen is an active element on the surface of the molten steel, the presence of oxygen will play a role in isolating the molten steel from the air. When the oxygen content of the molten steel reaches a certain value, even if nitrogen is blown into the molten steel continuously, the molten steel will hardly increase nitrogen. Therefore, in order to make the molten steel increase nitrogen violently, deoxidation must be carried out first. In addition to the deoxidation effect, aluminum is also an element with a strong ability to combine with nitrogen. After adding aluminum deoxidation, it is more conducive to nitrogen absorption, and the remaining aluminum is left in the steel to play a very strong nitrogen fixation effect, thereby reducing the partial pressure of nitrogen in the molten steel, further promoting the subsequent nitrogen absorption of the molten steel.

[0022] ② LF furnace refining: the molten steel after the converter smelting in step ① is transferred to the LF furnace for refining, and nitrogen is increased by top blowing nitrogen and bottom blowing nitrogen at the same time. The gun position of the top spray gun is controlled at 1.4m, the nitrogen blowing time is 80s, the purity of nitrogen is 99.99%, the gas supply intensity of the top blowing nitrogen is 3.5m / (min·t steel), and the gas supply intensity of the bottom blowing nitrogen is 0.05m / (min·t steel), so that the nitrogen content in the molten steel is 180ppm, and the outlet temperature of the molten steel is controlled at 1550℃; the purpose of controlling the gun position at 1.4m is mainly to enable the nitrogen jet to better penetrate the molten pool, so that nitrogen is integrated into the molten steel and the nitrogen content of the molten steel is increased. If the gun position is too high, the impact depth of the nitrogen jet is insufficient, and the nitrogen increase effect will be unsatisfactory. However, if the gun position is too low, the nitrogen jet may impact the bottom of the furnace, resulting in damage to the refractory at the bottom of the furnace and affecting the life of the converter.

[0023] 3. RH furnace refining: transfer the molten steel after LF furnace refining in step ② to RH furnace refining for vacuum degassing, evacuate the vacuum chamber to make the vacuum degree of the vacuum chamber 80Pa, keep it for 20min, turn off part of the vacuum pump group, fill the vacuum chamber with nitrogen, make the vacuum degree of the vacuum chamber stable within 20kPa, keep it for 25min, increase the nitrogen content in the molten steel to 420ppm, and when filling the vacuum chamber with nitrogen, the flow rate of nitrogen is 45m 3 / h. After the vacuum degree of the vacuum chamber is stable, increase the flow rate of nitrogen to 70m 3 / h; ④ Nitrogen content detection: Sampling and testing the molten steel after RH furnace refining in step ③ to obtain the nitrogen content in the molten steel; ⑤ Continuous casting: In the continuous casting process, the long nozzle, the submerged nozzle and the upper part of the ladle are all cast with gas protection. The type of protective gas is switched according to the nitrogen content of the molten steel. If the nitrogen content is lower than the lower limit, nitrogen sealing protection casting is used to increase nitrogen; if the nitrogen content is in the middle limit or above, argon sealing protection casting is used.

[0024] In this embodiment, the molten steel is subjected to converter smelting, LF furnace refining, RH furnace refining and continuous casting processes in sequence, and the nitrogen content of the molten steel entering the LF furnace is controlled to 13ppm, which can ensure that there is sufficient driving force for nitrogen increase in the LF furnace. If the nitrogen content of the molten steel is too high, the nitrogen absorption effect of the drastic nitrogen increase step will be poor. Ultimately, the nitrogen content of the molten steel can be stably reached at 420ppm, and appropriate nitrogen increase can be performed in the subsequent continuous casting process. After testing, the nitrogen content of the final steel is 423.5ppm, which meets the nitrogen content requirement in nitrogen-containing high-strength steel. Example 3

[0025] The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel described in Example 3 includes the following steps: ① Converter smelting: Converter smelting includes the early stage, middle stage and late stage of smelting. The early stage of smelting is the stage from the start of oxygen blowing to the oxygen blowing amount of the converter top blowing reaching 70% of the total oxygen blowing amount in the whole converter smelting process. The middle stage of smelting is the stage from the oxygen blowing amount of the converter top blowing reaching 70% to 90% of the total oxygen blowing amount in the whole converter smelting process. The late stage of smelting is the stage from the oxygen blowing amount of the converter top blowing reaching 90% of the total oxygen blowing amount in the whole converter smelting process to the end of blowing. In the early and middle stages of smelting, argon is blown at the bottom while oxygen is blown at the top. In the middle stage of smelting, a slag foaming agent is added to perform denitrification treatment. The slag foaming agent is slag steel obtained by hot stuffing and magnetic separation of steelmaking slag, which contains 40-50wt% of iron oxide, and the addition amount is 4-5Kg / t steel. In the late stage of smelting, the top blowing of oxygen is stopped, and a deoxidizer is added to deoxidize the slag and molten steel. The deoxidizer is an aluminum deoxidizer, and the gas supply intensity of the bottom blowing argon is 0.04m / (min·t steel). After the molten steel is deoxidized, the oxygen content in the molten steel is controlled to be 30ppm, and the nitrogen content in the molten steel is controlled to be 15ppm. The converter tapping temperature is controlled to be 1635°C. The purpose of adding the slag foaming agent in the present invention is to improve the denitrification capacity of the converter. In the later stage of oxygen blowing, the carbon-oxygen reaction in the furnace slows down, the amount of CO produced is small, and the denitrification capacity is weak. After adding the slag foaming agent, the later stage slag can be activated, a large amount of CO is produced to promote denitrification, and the emulsified slag can effectively cover the surface of the molten steel to prevent the molten steel from contacting with the air, thereby reducing the nitrogen content of the molten steel. Because oxygen is an active element on the surface of the molten steel, the presence of oxygen will play a role in isolating the molten steel from the air. When the oxygen content of the molten steel reaches a certain value, even if nitrogen is blown into the molten steel continuously, the molten steel will hardly increase nitrogen. Therefore, in order to make the molten steel increase nitrogen violently, deoxidation must be carried out first. In addition to the deoxidation effect, aluminum is also an element with a strong ability to combine with nitrogen. After adding aluminum deoxidation, it is more conducive to nitrogen absorption, and the remaining aluminum is left in the steel to play a very strong nitrogen fixation effect, thereby reducing the partial pressure of nitrogen in the molten steel, further promoting the subsequent nitrogen absorption of the molten steel.

[0026] ② LF furnace refining: the molten steel after the converter smelting in step ① is transferred to the LF furnace for refining, and nitrogen is increased by top blowing nitrogen and bottom blowing nitrogen at the same time. The gun position of the top spray gun is controlled at 1.6m, the nitrogen blowing time is 100s, the purity of nitrogen is 99.99%, the gas supply intensity of the top blowing nitrogen is 4m / (min·t steel), and the gas supply intensity of the bottom blowing nitrogen is 0.04m / (min·t steel), so that the nitrogen content in the molten steel is 200ppm, and the outlet temperature of the molten steel is controlled at 1560℃; the purpose of controlling the gun position at 1.6m is mainly to enable the nitrogen jet to better penetrate the molten pool, so that nitrogen is integrated into the molten steel and the nitrogen content of the molten steel is increased. If the gun position is too high, the impact depth of the nitrogen jet is insufficient, and the nitrogen increase effect will be unsatisfactory. However, if the gun position is too low, the nitrogen jet may impact the bottom of the furnace, resulting in damage to the refractory at the bottom of the furnace and affecting the life of the converter.

[0027] 3. RH furnace refining: The molten steel after LF furnace refining in step ② is transferred to RH furnace refining for vacuum degassing. The vacuum chamber is evacuated to make the vacuum degree of the vacuum chamber 100Pa. After maintaining for 25 minutes, part of the vacuum pump group is turned off and the vacuum chamber is filled with nitrogen to stabilize the vacuum degree of the vacuum chamber at 25kP. This is maintained for 30 minutes to increase the nitrogen content in the molten steel to 450ppm. When the vacuum chamber is filled with nitrogen, the flow rate of nitrogen is 50m 3 / h. After the vacuum degree of the vacuum chamber is stable, increase the flow rate of nitrogen to 80m 3 / h; ④ Nitrogen content detection: Sampling and testing the molten steel after RH furnace refining in step ③ to obtain the nitrogen content in the molten steel; ⑤ Continuous casting: In the continuous casting process, the long nozzle, the submerged nozzle and the upper part of the ladle are all cast with gas protection. The type of protective gas is switched according to the nitrogen content of the molten steel. If the nitrogen content is lower than the lower limit, nitrogen sealing protection casting is used to increase nitrogen; if the nitrogen content is in the middle limit or above, argon sealing protection casting is used.

[0028] In this embodiment, the molten steel is subjected to converter smelting, LF furnace refining, RH furnace refining and continuous casting processes in sequence, and the nitrogen content of the molten steel entering the LF furnace is controlled to 15ppm, which can ensure that there is sufficient driving force for nitrogen increase in the LF furnace. If the nitrogen content of the molten steel is too high, the nitrogen absorption effect of the drastic nitrogen increase step is poor. Ultimately, the nitrogen content of the molten steel can be stably reached 450ppm, and the nitrogen increase is stopped in the subsequent continuous casting process. After testing, the nitrogen content of the final steel is 450ppm, which meets the nitrogen content requirement in nitrogen-containing high-strength steel.

[0029] In the present invention, the molten steel is subjected to converter smelting, LF furnace refining, RH furnace refining and continuous casting processes in sequence, the converter and LF furnace are first used in combination to carry out preliminary nitrogen addition, and then the RH furnace is used for refining, after ensuring that the hydrogen content in the molten steel is removed, the molten steel is nitrogen added by a vacuum treatment process, and the nitrogen content in the molten steel is further increased, so that the nitrogen content of the molten steel can be stably reached to 300ppm-450ppm, and low-cost and precise control of the nitrogen content of the molten steel in the range of 300ppm-450ppm is achieved, which can replace the current mainstream domestic alloy nitrogen addition process, and greatly reduce the smelting cost of nitrogen-containing high-strength steel on the premise of further improving stability and achieving a higher nitrogen content.

Claims

1. A precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel, characterized in that: The following steps are involved: ① Converter smelting: The converter smelting includes an early stage of smelting, a middle stage of smelting and a late stage of smelting. In the early stage of smelting and the middle stage of smelting, argon is blown at the bottom and oxygen is blown at the top. In the middle stage of smelting, a slag foaming agent is added to carry out denitrification treatment. In the late stage of smelting, the top blowing of oxygen is stopped, and a deoxidizer is added to deoxidize the slag and molten steel. The supply intensity of the bottom blowing argon is 0.04-0.05m / (min·t steel). After the molten steel is deoxidized, the oxygen content in the molten steel is controlled to be below 30ppm, and the nitrogen content in the molten steel is controlled to be below 15ppm. The converter tapping temperature is controlled to be 1615℃-1635℃. ② LF furnace refining: the molten steel after the converter smelting in step ① is transferred to the LF furnace for refining, and nitrogen is added by top blowing nitrogen and bottom blowing nitrogen at the same time. The supply intensity of the top blowing nitrogen is 3-4m / (min·t steel), and the supply intensity of the bottom blowing nitrogen is 0.04-0.05m / (min·t steel), so that the nitrogen content in the molten steel is 160ppm-200ppm, and the outlet temperature of the molten steel is controlled at 1540℃-1560℃; 3. RH furnace refining: the molten steel after the LF furnace refining in step ② is transferred to the RH furnace refining for vacuum degassing, and the vacuum chamber is evacuated to make the vacuum degree of the vacuum chamber less than 100Pa. After maintaining for 15min to 25min, part of the vacuum pump group is turned off, and the vacuum chamber is filled with nitrogen to stabilize the vacuum degree of the vacuum chamber between 15kPa and 25kP. It is maintained for 20min to 30min, so that the nitrogen content in the molten steel is increased to 350ppm to 450ppm; ④ Nitrogen content detection: Sampling and testing the molten steel after RH furnace refining in step ③ to obtain the nitrogen content in the molten steel; ⑤ Continuous casting: During the continuous casting process, the long nozzle, the submerged nozzle and the top of the ladle are all cast with gas protection.

2. The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel according to claim 1, characterized in that: In step ①, the slag foaming agent is slag steel obtained by hot stuffing and magnetic separation of steel-making slag, which contains 40-50wt% of iron oxide, and the addition amount is 4-5Kg / t steel.

3. The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel according to claim 1, characterized in that: In step ①, the deoxidizer is an aluminum deoxidizer.

4. The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel according to claim 1, characterized in that: In step ②, the gun position of the top spray gun is controlled between 1.2m and 1.6m, and the nitrogen blowing time is 50s to 100s.

5. The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel according to claim 1, characterized in that: In step ②, the purity of nitrogen is 99.99%.

6. The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel according to claim 1, characterized in that: In step ③, when the vacuum chamber is filled with nitrogen, the flow rate of nitrogen is less than 50m 3 / h. After the vacuum degree of the vacuum chamber is stable, increase the flow rate of nitrogen to 60m 3 / h~80m 3 / h.

7. The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel according to claim 1, characterized in that: In step ⑤, the type of protective gas is switched according to the nitrogen content of the molten steel. If the nitrogen content is at the lower limit, nitrogen sealing protection casting is used to increase nitrogen. If the nitrogen content is at the middle limit or above, argon sealing protection casting is used.

8. The precise nitrogen-controlled smelting method for nitrogen-containing high-strength steel according to claim 1, characterized in that: In step ①, the early stage of smelting is the stage from the start of oxygen blowing to the stage when the oxygen blowing amount of the converter top blowing reaches 70% of the total oxygen blowing amount in the whole converter smelting process, the middle stage of smelting is the stage when the oxygen blowing amount of the converter top blowing reaches 70% to 90% of the total oxygen blowing amount in the whole converter smelting process, and the late stage of smelting is the stage when the oxygen blowing amount of the converter top blowing reaches 90% of the total oxygen blowing amount in the whole converter smelting process to the end of blowing.