Low-nitrogen production method for low-temperature steels

Abstract

The application discloses a low-nitrogen production method of low-temperature steel. The method comprises the following steps: when the converter tapping is 10-20%, silicon iron, metal manganese and nickel are added into the molten steel to perform weak deoxidization and alloying; during LF refining, 0.15-0.35 kg / t of calcium carbide and low-carbon steel slag surface deoxidizer are added in the slagging stage to adjust the slag composition to contain 50-55% of CaO, 30-35% of Al2O3, 3-6% of SiO2, 4-7% of MgO, 2-5% of (T.Fe+MnO) and other inevitable impurity components in percentage by mass; during RH vacuum refining, metal aluminum is added into the molten steel after the vacuum degree is reduced to 1.5 mbar, and 2-4 kg / t of low-carbon steel slag surface deoxidizer is added into the ladle slag surface; and then continuous casting is performed to obtain a continuous casting billet with N content of less than or equal to 0.002%.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation technology, and relates to a method for producing low-temperature steel, particularly a low-nitrogen method for producing low-temperature steel. Background Technology

[0002] Natural gas is a high-quality energy source with advantages such as high calorific value and low pollution, and is widely used in power generation, vehicle fuel, residential use, and industry. Natural gas generally undergoes liquefaction, and liquefied natural gas (LNG) has unparalleled advantages in storage and transportation compared to other methods. Therefore, the construction of numerous LNG relay stations and receiving terminals is an inevitable trend.

[0003] The main storage device for LNG is a large LNG storage tank, whose inner shell is welded from cryogenic steel. Because it operates at a low temperature of -162°C, it has special performance requirements for cryogenic steel, including excellent low-temperature toughness, high strength, high structural stability and weldability.

[0004] In most cases, the presence of nitrogen in steel significantly impacts its properties, reducing toughness, weldability, and hot stress zone toughness, and to some extent increasing brittleness. Furthermore, nitrogen can cause cracking in continuously cast billets during the continuous casting process. Therefore, effectively controlling the nitrogen content in steel is a hot topic in the research of low-temperature steels. Summary of the Invention

[0005] The purpose of this invention is to provide a method for producing low-temperature steel, and more particularly a method for producing low-nitrogen low-temperature steel.

[0006] To achieve the above-mentioned objective, one embodiment of the present invention provides a low-nitrogen production method for low-temperature steel. The method employs a process route including KR desulfurization, converter smelting, LF refining, RH vacuum refining, and continuous casting to prepare low-temperature steel continuously cast billets.

[0007] The KR desulfurization process: the temperature of the molten iron leaving the station after desulfurization is 1300-1350℃, and the S content is ≤0.0015% by mass percentage;

[0008] The converter smelting process is as follows: the tapping temperature of molten steel is 1590-1630℃, and the tapping steel has a C content of 0.02-0.05%, an O content of 0.045-0.085%, a P content of ≤0.005%, and a S content of ≤0.003% by mass percentage.

[0009] The LF refining process includes a sequential heating stage, an alloying stage, and a slag-forming stage. During the slag-forming stage, 0.15–0.35 kg / t of calcium carbide and low-carbon steel slag surface deoxidizer are added to adjust the slag composition to contain 50–55% CaO, 30–35% Al2O3, 3–6% SiO2, 4–7% MgO, 2–5% (T.Fe+MnO), and other unavoidable impurities by mass percentage. The tapping temperature of the molten steel is 1610–1630℃.

[0010] The RH vacuum refining process employs an RH vacuum furnace with a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump, and two-stage water circulation pumps sequentially arranged in the vacuum exhaust pipeline of the vacuum chamber. Within 3 minutes of the arrival of molten steel, the two-stage water circulation pump, the fourth-stage steam pump, the third-stage steam pump, the second-stage steam pump, and the first-stage steam pump are turned on in sequence, and the vacuum level is reduced to below 1.5 mbar within 4 minutes of the arrival of molten steel. The boosting gas flow rate within 4 minutes of the arrival of molten steel is 100–120 Nm³. 3 / h, the lifting gas flow rate after 4 minutes is 230-250 Nm 3 / h; After the vacuum level drops to 1.5 mbar, add metallic aluminum to the molten steel and add 2-4 kg / t of low-carbon steel slag surface deoxidizer to the slag surface of the ladle. Then continue vacuum treatment for 15-20 min; then turn off the two-stage vacuum pump of the vacuum chamber. After the vacuum chamber rises to above 5 mbar, reduce the lifting gas flow rate to 180-200 Nm. 3 / h; then continue processing for 10-15 minutes, then break the air and unload the steel;

[0011] The continuous casting process involves hoisting the molten steel from the RH vacuum refining process to the continuous casting platform and letting it stand for more than 10 minutes before casting to obtain a continuously cast billet.

[0012] As a further improvement to one embodiment, the LF refining process involves adjusting the slag composition during the slag-forming stage to contain 3-5% (T,Fe+MnO) by mass percentage.

[0013] As a further improvement to one embodiment, the chemical composition of the continuously cast billet includes N ≤ 0.002% by mass percentage.

[0014] As a further improvement of one embodiment, the chemical composition of the continuously cast billet, in mass percentage, includes: C: 0.03-0.10%, Si: 0.15-0.35%, Mn: 0.5-1.6%, Ni: 0.4-10.0%, Al: 0.015-0.055%, Cu≤0.015%, Mo≤0.50%, Cr≤0.70%, Nb≤0.035%, TO≤10ppm, P≤0.005%, S≤0.002%, N≤0.002%, H≤1.5ppm.

[0015] As a further improvement to one embodiment, the LF refining process is as follows: argon is blown into the bottom throughout the process; the flow rate of argon during the power-on heating stage is 400-500 NL / min; the flow rate of argon during the alloying stage is 300-400 NL / min; the flow rate of argon during the slag-forming stage is 500-600 NL / min; and the flow rate of argon during the remaining time is 150-250 NL / min.

[0016] As a further improvement to one embodiment, the converter process involves: before tapping, the composition of the molten steel is tested, and the total amounts M1, M2, and M3 of ferrosilicon, metallic manganese, and nickel to be added are determined based on the test results and the target chemical composition; when the steel reaches 10-20% of its mass, ferrosilicon, metallic manganese, and nickel are added to the molten steel for weak deoxidation and alloying, until the steel reaches 60-70% of its mass, at which point the weights of ferrosilicon, metallic manganese, and nickel are k×M1, k×M2, and M3 respectively, where k is 60-80%, and then the addition is stopped; then 5-8 kg / t of lime and 10-15 kg / t of calcium aluminate slag are added for slag formation, and all of these are added when the steel reaches 80-90% of its mass; then the mixture is stirred for 2-5 minutes, and then all the molten steel is transported to the LF refining furnace for the LF refining process.

[0017] As a further improvement to one embodiment, the LF refining process involves adding the remaining (1-k)×M1 ferrosilicon and (1-k)×M2 metallic manganese during the alloying stage.

[0018] As a further improvement to one embodiment, in the converter process: during the tapping process, the flow rate of bottom-blown argon gas in the ladle is 400-600 NL / min, and after tapping is completed, the flow rate of bottom-blown argon gas in the ladle is increased to 800-1000 NL / min.

[0019] As a further improvement to one embodiment, the converter smelting process uses molten iron, nickel plates, and scrap steel from the KR desulfurization process for smelting, wherein the total weight of nickel plates and scrap steel accounts for 20-25% of the total weight of molten iron, nickel plates, and scrap steel.

[0020] The chemical composition of the nickel plate, by mass percentage, includes Ni ≥ 99%, P ≤ 0.025%, S ≤ 0.008%, with the balance being Fe and other unavoidable impurities; the chemical composition of the scrap steel, by mass percentage, includes Si ≤ 0.6%, Mn ≤ 1.8%, Al ≤ 0.08%, P ≤ 0.02%, S ≤ 0.01%, with the balance being Fe and other unavoidable impurities.

[0021] As a further improvement to one embodiment, the continuous casting process has the following characteristics: casting speed of 0.8 to 1.1 m / min; and full-process protective casting is carried out, with argon blowing flow rate of 150 to 250 L / min for the long nozzle and argon blowing flow rate of 3 to 5 L / min for the stopper rod and immersion nozzle. Argon blowing begins in the tundish 5 minutes before casting begins and stops after the first round of tundish covering agent addition.

[0022] Compared with existing technologies, the beneficial effects of this invention are as follows: On the one hand, by using low-carbon, high-oxygen tapping, and by not adding aluminum during the tapping process, and by using a portion (e.g., coefficient k, i.e., 60-80%) rather than all of ferrosilicon and metallic manganese for weak deoxidation, the nitrogen absorption of molten steel can be greatly reduced, thus lowering the final nitrogen content. On the other hand, by adjusting the composition of the slag, especially by increasing it from the traditional low proportion of T.Fe+MnO to 2-5%, the nitrogen absorption of molten steel can be greatly reduced. In addition, by rapidly drawing a deep vacuum and adding low-carbon steel slag surface deoxidation under vacuum conditions, the microbubbles formed by the C-O reaction in molten steel, the argon bubbles blown in by a large flow rate of gas under deep vacuum, and the deep vacuum steel-molten steel interface reaction can be used to comprehensively and significantly degas the steel, reducing the O and N content. Then, metallic aluminum is added under vacuum conditions to avoid the oxidation and alloying of metallic aluminum, thereby achieving overall low-nitrogen control of low-temperature steel. Attached Figure Description

[0023] Figure 1 This is a process flow diagram of a method for producing low-temperature steel according to an embodiment of the present invention;

[0024] Figure 2 This is a partial structural schematic diagram of an RH vacuum furnace according to one embodiment of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0026] An embodiment of the present invention provides a method for producing low-temperature steel, which can obtain low-temperature steel with a low nitrogen content, for example, the nitrogen content does not exceed 0.002% by mass percentage.

[0027] Specifically, refer Figure 1 , the method adopts a process route including KR desulfurization, converter smelting, LF refining, RH vacuum refining, and continuous casting to prepare a low-temperature steel continuous casting billet. Next, each process of a preferred embodiment will be introduced in detail.

[0028] <KR Desulfurization Process>

[0029] The hot metal from the blast furnace is desulfurized by the KR desulfurization technology.

[0030] In the present invention, the temperature of the hot metal leaving the station after desulfurization is 1300 - 1350 °C, and S ≤ 0.0015% by mass percentage.

[0031] <Converter Smelting Process>

[0032] The hot metal leaving the station of the KR desulfurization process is smelted by a converter.

[0033] In the present invention, the temperature of the molten steel leaving the converter in the converter smelting process is 1590 - 1630 °C, and the C content in the molten steel at tapping is 0.02 - 0.05% by mass percentage, the O content is 0.045 - 0.085%, the P content ≤ 0.005%, the S content ≤ 0.003%, and the N content ≤ 0.001%. And, before tapping, the composition of the molten steel is detected, and the respective total amounts M1, M2, and M3 of ferrosilicon, ferromanganese, and nickel to be added are determined according to the detection results and the target chemical composition; when the tapping reaches 10 - 20%, ferrosilicon, ferromanganese, and nickel are started to be added to the molten steel for weak deoxidation and alloying until the tapping reaches 60 - 70%, and the weights of ferrosilicon, ferromanganese, and nickel added are k×M1, k×M2, and M3 respectively, where k ranges from 60 - 80%, and then the feeding ends. The inventors have found that in this way, by tapping with low carbon and high oxygen, and without adding aluminum first during the tapping process and using partial (for example, the coefficient k, that is, 60 - 80%) rather than all of the ferrosilicon and ferromanganese for weak deoxidation, the absorption of N by the molten steel can be greatly reduced, and the final N content can be reduced.

[0034] In a preferred embodiment, the converter smelting process can be carried out using the hot metal discharged from the KR desulfurization process, nickel plates, and scrap steel for smelting. The total weight of the nickel plates and scrap steel mentioned here accounts for 20 - 25% of the total weight of the hot metal, nickel plates, and scrap steel. Moreover, the chemical composition of the nickel plates, by mass percentage, includes Ni≥99%, P≤0.025%, S≤0.008%, with the balance being Fe and other inevitable impurities; the chemical composition of the scrap steel, by mass percentage, includes Si≤0.6%, Mn≤1.8%, Al≤0.08%, P≤0.02%, S≤0.01%, with the balance being Fe and other inevitable impurities.

[0035] Furthermore, in the converter smelting process, after the addition of ferrosilicon, ferromanganese, and nickel is completed, 5 - 8 kg / t of lime and 10 - 15 kg / t of calcium aluminate synthetic slag are added for slag formation, and all are added when the steel is being tapped to 80 - 90%; then, it is stirred for 2 - 5 min, and then all the molten steel is transported to the LF refining furnace to carry out the LF refining process. In this way, a large slag amount can be achieved to protect the molten steel, which is beneficial for further reducing the nitrogen absorption of the molten steel during subsequent LF refining.

[0036] Further, in the converter process, the components of the calcium aluminate synthetic slag added during the tapping process, by mass percentage, include CaO: 50 - 55%, Al2O3: 30 - 35%, CaF2: 5 - 10%, SiO2≤3%, MgO: 2 - 5%, and other inevitable impurity components, where the phase 12CaO·7Al2O3 accounts for more than 60%, the Al2O3 phase≤5%, and the rest are single-phase or composite phases of CaO, CaF2, SiO2, and MgO.

[0037] Further, in the converter process, the flow rate of argon bottom blowing in the ladle during the tapping process is 400 - 600 NL / min, and after the tapping is completed, the flow rate of argon bottom blowing in the ladle is increased to 800 - 1000 NL / min.

[0038] <LF Refining Process>

[0039] The molten steel tapped from the converter smelting process is refined using an LF furnace.

[0040] Specifically, the LF refining process includes a power-on heating-up stage, an alloying stage, and a slag-making stage in sequence. In the slag-making stage, 0.15 - 0.35 kg / t of calcium carbide and a deoxidizer for the low-carbon steel slag surface are added to adjust the slag composition to contain 50 - 55% of CaO, 30 - 35% of Al2O3, 3 - 6% of SiO2, 4 - 7% of MgO, 2 - 5% of (T.Fe + MnO), and other inevitable impurity components by mass percentage; the molten steel temperature at tapping is 1610 - 1630 °C. Thus, compared with the conventional technology, in the present invention, the slag composition is adjusted by calcium carbide and the deoxidizer for the low-carbon steel slag surface. Especially, the proportion of traditional low-content T.Fe + MnO is increased to 2 - 5%. Through the research of the inventor, it can greatly reduce the nitrogen absorption of molten steel.

[0041] Furthermore, in the slag-making stage, the slag composition is adjusted to contain 50 - 55% of CaO, 30 - 35% of Al2O3, 3 - 6% of SiO2, 4 - 7% of MgO, 3 - 5% of (T.Fe + MnO), and other inevitable impurity components by mass percentage.

[0042] In this application, the specific composition of the deoxidizer for the low-carbon steel slag surface contains 25 - 35% of CaO, 10 - 20% of Al2O3, 5 - 10% of CaF2, 45 - 55% of metallic aluminum, and other inevitable components by mass percentage.

[0043] Furthermore, in the alloying stage, the remaining ferrosilicon of (1 - k) × M1 and metallic manganese of (1 - k) × M2 are added. That is,至此, through the tapping process of the converter smelting process and the alloying stage of this LF refining process, the addition of all ferrosilicon with a weight of M1 and metallic manganese with a weight of M2 is completed.

[0044] In a preferred embodiment, in the LF refining process, argon is blown from the bottom throughout the process. The flow rate of argon blown from the bottom in the power-on heating-up stage is 400 - 500 NL / min, the flow rate of argon blown from the bottom in the alloying stage is 300 - 400 NL / min, the flow rate of argon blown from the bottom in the slag-making stage is 500 - 600 NL / min, and the flow rate of argon blown from the bottom at other times is 150 - 250 NL / min. Thus, by mainly blowing argon from the bottom with a medium and small flow rate to avoid violent tumbling of molten steel, and combined with the weak deoxidation technology in the previous converter smelting process and the slag composition design in the LF refining process, the nitrogen absorption can be further reduced on the basis of reducing nitrogen absorption.

[0045] <RH Vacuum Refining Process>

[0046] An RH vacuum furnace employs a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump, and two-stage water circulation pumps sequentially arranged in the vacuum exhaust pipeline of the vacuum chamber. This type of RH vacuum furnace is known in the art, and for ease of explanation, it is described in the appendix. Figure 2 The simplified diagram illustrates the positional relationship of the first-stage steam pump E1, the second-stage steam pump E2, the third-stage steam pump E3, the fourth-stage steam pump E4, the two-stage water circulation pumps W1 & W2, and the vacuum chamber 10, in order to facilitate understanding of the technical content of the RH vacuum refining process.

[0047] In the RH vacuum refining process, within 3 minutes of the arrival of molten steel, the two-stage water circulation pumps W1 & W2, the fourth-stage steam pump E4, the third-stage steam pump E3, the second-stage steam pump E2, and the first-stage steam pump E1 are turned on in sequence, and the vacuum degree is reduced to below 1.5 mbar within 4 minutes of the arrival of molten steel; and the flow rate of the lifting gas within 4 minutes of the arrival of molten steel is 100-120 Nm³. 3 / h, the lifting gas flow rate after 4 minutes is 230-250 Nm 3 / h; After the vacuum level drops to 1.5 mbar, add metallic aluminum to the molten steel and add 2-4 kg / t of low-carbon steel slag surface deoxidizer to the slag surface of the ladle. Then continue vacuum treatment for 15-20 min; then turn off the two-stage vacuum pump of the vacuum chamber. After the vacuum chamber rises to above 5 mbar, reduce the lifting gas flow rate to 180-200 Nm. 3 / h; then continue processing for 10-15 minutes, then break the vacuum and tap the steel. In this way, on the one hand, by rapidly drawing a deep vacuum, and on the other hand, by adding metallic aluminum to the molten steel and adding low-carbon steel slag surface deoxidizer to the slag surface under vacuum, deoxidation can be achieved by utilizing the micro bubbles formed by the C-O reaction of the molten steel, the argon bubbles blown in by the high flow rate of gas under deep vacuum, and the deep vacuum steel-molten steel interface reaction, which can comprehensively and significantly degas the steel and reduce the O and N content. Then, by adding metallic aluminum and low-carbon steel slag surface deoxidizer under vacuum conditions, the oxidation and alloying of metallic aluminum and the absorption of gas during slag formation are avoided. In addition, under the slag composition design of the LF refining process, the treatment of this RH vacuum refining process can also avoid the problem of high oxidation caused by the slag composition design of the LF refining process.

[0048] <Continuous casting process>

[0049] The molten steel from the RH vacuum refining process is hoisted to the continuous casting platform and allowed to stand for at least 10 minutes before casting begins, resulting in a continuously cast billet. This billet is a low-temperature steel continuously cast billet, and its chemical composition, by mass percentage, includes N ≤ 0.002%.

[0050] In one specific embodiment, the chemical composition of the continuously cast billet, by mass percentage, includes: C: 0.03–0.10%, Si: 0.15–0.35%, Mn: 0.5–1.6%, Ni: 0.4–10.0%, Al: 0.015–0.055%, Cu≤0.015%, Mo≤0.50%, Cr≤0.70%, Nb≤0.035%, TO≤10ppm, P≤0.005%, S≤0.002%, N≤0.002%, and H≤1.5ppm. Wherein, Cu is an impurity element rather than an alloying element. Mo, Cr, and Nb may be impurities rather than alloying elements in some specific embodiments, while they may be alloying elements in others. For example, in one embodiment where Mo, Cr, and Nb are impurity elements, the chemical composition of the continuously cast billet, by mass percentage, is: C: 0.03–0.10%, Si: 0.15–0.35%, Mn: 0.5–0.9%, Ni: 0.4–10.0%, Al: 0.015–0.055%, Cu≤0.015%, Mo≤0.010%, Cr≤0.015%, Nb≤0.006%, TO≤10ppm, P≤0.005%, S≤0.002%, N≤0.002%, H≤1.5ppm, with the remainder being iron and unavoidable impurities.

[0051] Preferably, in the continuous casting process, the casting speed is 0.8 to 1.1 m / min; and full-process protective casting is carried out, with the argon blowing flow rate of the long nozzle being 150 to 250 L / min, and the argon blowing flow rate of the stopper rod and the submerged nozzle being 3 to 5 L / min. Argon blowing begins in the tundish 5 minutes before casting begins and stops after the first round of tundish covering agent addition is completed.

[0052] Furthermore, the thickness of the continuously cast billet is 220mm or 320mm, and the width is 1500-2300mm.

[0053] Thus, compared with the prior art, the beneficial effects of one embodiment of the present invention are as follows: On the one hand, by tapping steel with low carbon and high oxygen, and by not adding aluminum during the tapping process, and by using a portion (e.g., coefficient k, i.e., 60-80%) rather than all of ferrosilicon and metallic manganese for weak deoxidation, the nitrogen absorption of molten steel can be greatly reduced, thus reducing the final nitrogen content; on the other hand, by adjusting the composition of the slag, especially by increasing it from the traditional low proportion of T.Fe+MnO to 2-5%, the nitrogen absorption of molten steel can be greatly reduced; in addition, by rapidly drawing a deep vacuum and adding low carbon steel slag surface deoxidation under vacuum, the microbubbles formed by the C-O reaction of molten steel, the argon bubbles blown in by a large flow of gas under deep vacuum, and the deep vacuum steel-molten steel interface reaction can be used to comprehensively and significantly degas the steel, reducing the O and N content; then, metallic aluminum is added under vacuum conditions to avoid the oxidation and alloying of metallic aluminum, thereby achieving overall low nitrogen control of low-temperature steel.

[0054] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

[0055] Several embodiments are provided below, in which continuously cast billets are prepared using the method described in this invention. The chemical composition and thickness of the continuously cast billets in these embodiments are shown in Table 1 below.

[0056] [Table 1]

[0057]

[0058]

[0059] [Table 1 continued]

[0060]

[0061] It can be seen that by using the method provided by the present invention, a continuously cast billet with N content ≤0.002% by mass percentage can be obtained, and the contents of elements such as TO, P, S, and H are also low.

[0062] The continuous casting billets in each embodiment are rolled into steel plates, for example: the continuous casting billets are fed into a heating furnace for heating, with the temperature in the preheating section rising to 750-850℃, the heating section to 1100-1200℃, and the soaking section to 1150-1200℃; then rolled, with an initial rolling temperature of 1030-1130℃ and a final rolling temperature of 800-850℃, a rolling amount of 10-15% per pass, and a plate thickness of 5-60mm; the rolled plate is then subjected to a second quenching, with the first quenching at 800-900℃ and the second quenching at 700-800℃; tempering is then performed at a tempering temperature of 560-620℃ to obtain the finished low-temperature steel plate.

[0063] The mechanical properties and low-temperature performance of the finished low-temperature steel plates from each embodiment were tested, and the results are shown in Table 2.

[0064] [Table 2]

[0065]

[0066]

[0067] It is evident that the continuously cast billet prepared by the method of the present invention exhibits excellent low-temperature toughness after being rolled into a sheet. It should be noted that the continuously cast billet prepared by the method of the present invention, when rolled into a sheet using other techniques (such as existing known techniques), also possesses excellent low-temperature toughness. Thus, the method of the present invention can obtain continuously cast billets with low nitrogen content and high purity, which can be used to prepare low-temperature steel sheet products with excellent low-temperature toughness, meeting market performance requirements for low-temperature steel.

Claims

1. A method for producing low-nitrogen steel at low temperatures, characterized in that, The method employs a process route including KR desulfurization, converter smelting, LF refining, RH vacuum refining, and continuous casting to prepare low-temperature steel continuously cast billets; the chemical composition of the continuously cast billets, in mass percentage, includes: C: 0.03~0.10%, Si: 0.15~0.35%, Mn: 0.5~1.6%, Ni: 0.4~10.0%, N≤0.002%; The KR desulfurization process: the temperature of the molten iron leaving the station after desulfurization is 1300~1350℃, and the sulfur content (S) is ≤0.0015% by mass. The converter smelting process is as follows: the tapping temperature of the molten steel is 1590~1630℃, and the tapping steel has a C content of 0.02~0.05%, an O content of 0.045~0.085%, a P content of ≤0.005%, a S content of ≤0.003%, and a N content of ≤0.001% by mass percentage. Before tapping, the composition of the molten steel is tested, and the total amounts of ferrosilicon, metallic manganese, and nickel plates to be added, M1, M2, and M3 respectively, are determined based on the test results and the target chemical composition. When the steel reaches 10~20% tapping, ferrosilicon, metallic manganese, and nickel are added to the molten steel for weak deoxidation and alloying. When the steel reaches 60~70% tapping, the weights of ferrosilicon, metallic manganese, and nickel added are k×M1, k×M2, and M3 respectively, where k is 60~80%, and then the addition is stopped. The LF refining process includes a sequential heating stage, an alloying stage, and a slag-forming stage. During the slag-forming stage, 0.15~0.35 kg / t of calcium carbide and low-carbon steel slag surface deoxidizer are added to adjust the slag composition to contain 50~55% CaO, 30~35% Al2O3, 3~6% SiO2, 4~7% MgO, 2~5% (T.Fe+MnO), and other unavoidable impurities by mass percentage. The tapping temperature of the molten steel is 1610~1630℃. The RH vacuum refining process employs an RH vacuum furnace with a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump, and two-stage water circulation pumps sequentially arranged in the vacuum exhaust pipeline of the vacuum chamber. Within 3 minutes of the arrival of molten steel, the two-stage water circulation pump, the fourth-stage steam pump, the third-stage steam pump, the second-stage steam pump, and the first-stage steam pump are turned on in sequence, and the vacuum level is reduced to below 1.5 mbar within 4 minutes of the arrival of molten steel. The boosting gas flow rate within 4 minutes of the arrival of molten steel is 100~120 Nm³. 3 / h, the lifting gas flow rate after 4 minutes is 230~250Nm 3 / h; After the vacuum level drops to 1.5 mbar, add metallic aluminum to the molten steel and add 2~4 kg / t of low-carbon steel slag surface deoxidizer to the slag surface of the ladle. Then continue vacuum treatment for 15~20 min; then turn off the two-stage vacuum pump of the vacuum chamber. After the vacuum chamber rises to above 5 mbar, reduce the lifting gas flow rate to 180~200 Nm. 3 / h; then continue processing for 10~15min, then break the air and unload the steel; The continuous casting process involves hoisting the molten steel from the RH vacuum refining process to the continuous casting platform and letting it stand for more than 10 minutes before casting to obtain a continuously cast billet.

2. The method for producing low-nitrogen steel at low temperatures according to claim 1, characterized in that, The LF refining process involves adjusting the slag composition during the slag-making stage to contain 3-5% (T, Fe + MnO) by mass percentage.

3. The low-nitrogen production method of low-temperature steel according to claim 1, wherein the chemical composition of the continuously cast billet, in mass percentage, is: C: 0.03~0.10%, Si: 0.15~0.35%, Mn: 0.5~1.6%, Ni: 0.4~10.0%, Al: 0.015~0.055%, Cu≤0.015%, Mo≤0.50%, Cr≤0.70%, Nb≤0.035%, TO≤10ppm, P≤0.005%, S≤0.002%, N≤0.002%, H≤1.5ppm, with the balance being iron and unavoidable impurities.

4. The method for producing low-nitrogen steel at low temperatures according to claim 1, characterized in that, The LF refining process is as follows: argon is blown into the bottom throughout the process. During the power-on heating stage, the flow rate of argon is 400~500 NL / min; during the alloying stage, the flow rate of argon is 300~400 NL / min; during the slag-forming stage, the flow rate of argon is 500~600 NL / min; and during the remaining time, the flow rate of argon is 150~250 NL / min.

5. The method for producing low-nitrogen steel at low temperatures according to claim 1, characterized in that, The converter process involves adding 5-8 kg / t of lime and 10-15 kg / t of calcium aluminate slag to form a slag, which is completed when the steel reaches 80-90% concentration. After stirring for 2-5 minutes, all the molten steel is transported to the LF refining furnace for the LF refining process.

6. The method for producing low-nitrogen steel at low temperatures according to claim 1, characterized in that, The LF refining process involves adding the remaining (1-k)×M1 ferrosilicon and (1-k)×M2 metallic manganese during the alloying stage.

7. The method for producing low-nitrogen steel at low temperatures according to claim 6, characterized in that, The converter process is as follows: during the tapping process, the flow rate of bottom-blown argon gas in the ladle is 400~600 NL / min, and after tapping is completed, the flow rate of bottom-blown argon gas in the ladle is increased to 800~1000 NL / min.

8. The method for producing low-nitrogen steel at low temperatures according to claim 1, characterized in that, The converter smelting process involves smelting molten iron, nickel plates, and scrap steel from the KR desulfurization process, wherein the total weight of nickel plates and scrap steel accounts for 20-25% of the total weight of the molten iron, nickel plates, and scrap steel. The chemical composition of the nickel plate, by mass percentage, includes Ni ≥ 99%, P ≤ 0.025%, S ≤ 0.008%, with the balance being Fe and other unavoidable impurities; the chemical composition of the scrap steel, by mass percentage, includes Si ≤ 0.6%, Mn ≤ 1.8%, Al ≤ 0.08%, P ≤ 0.02%, S ≤ 0.01%, with the balance being Fe and other unavoidable impurities.

9. The method for producing low-nitrogen steel at low temperatures according to claim 1, characterized in that, The continuous casting process involves a casting speed of 0.8~1.1 m / min and full-process protective casting. The argon blowing flow rate at the long nozzle is 150~250 L / min, and the argon blowing flow rate at the stopper rod and submerged nozzle is 3~5 L / min. Argon blowing begins in the tundish 5 minutes before casting begins and stops after the first round of tundish covering agent addition.

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