High nitrogen austenitic stainless steel for ultra-low temperature environment and preparation method thereof

Abstract

This invention belongs to the field of metallic materials technology, and discloses a high-nitrogen austenitic stainless steel for ultra-low temperature environments and its preparation method. The composition is: C 0.01%, Si: 0.1%~0.3%, Mn: 1.5%~2.0%, Cr: 16.5%~18.5%, Ni: 12.5%~14.5%, Mo: 2.0%~3.0%, N: 0.30%~0.40%, with the balance being Fe and unavoidable impurity elements. The preparation method includes high-pressure nitrogen injection, electric arc furnace roughing, vacuum induction furnace refining, temperature-controlled cooling, rolling, and heat treatment. During the roughing process, aluminum powder is added in stages to suppress nitride formation, and a "nitrogen solubility index" is proposed. N S "Used to characterize the efficient dissolution behavior of nitrogen. The resulting material has a yield strength > 430 MPa, tensile strength > 740 MPa, impact energy > 420 J at room temperature, and impact energy > 260 J in a liquid hydrogen environment."

Description

Technical Field

[0001] This invention relates to the field of metal materials technology, and in particular to a high-nitrogen austenitic stainless steel for ultra-low temperature environments and its preparation method. Background Technology

[0002] With the rapid development of cutting-edge fields such as aerospace, cryogenic storage and transportation, polar exploration, nuclear fusion devices, and high-energy physics experiments, more stringent technical requirements have been placed on the comprehensive performance of metallic materials under extreme low-temperature environments. This is especially true in liquid nitrogen. Even liquid hydrogen Under ultra-low temperature service conditions, materials must simultaneously possess excellent strength, plasticity, impact toughness, and microstructure stability, as well as good corrosion resistance, in order to effectively cope with the challenges of complex working conditions such as low-temperature brittle fracture, microcrack propagation, thermal stress fatigue, and low-temperature medium corrosion.

[0003] Austenitic stainless steel, due to its face-centered cubic (FCC) crystal structure, inherently possesses excellent low-temperature toughness and corrosion resistance, making it a key structural material in the manufacturing of cryogenic engineering equipment. The 304 and 316 series austenitic stainless steels, based on this design, are widely used in the liquid nitrogen temperature range. However, traditional austenitic stainless steels (such as 316L and 316LN) face several performance bottlenecks in the more severe liquid hydrogen environment, mainly in the following aspects:

[0004] 1) The low-temperature toughness is severely insufficient, making it difficult to meet the service requirements of liquid hydrogen;

[0005] Although conventional austenitic stainless steel is in the liquid nitrogen temperature range It retains a certain degree of ductility and toughness, but in the more demanding liquid hydrogen environment... Under these conditions, its impact toughness decreases significantly, making it prone to low-temperature brittle fracture, especially in areas with defects or stress concentrations, which seriously threatens the safe service of hydrogen storage containers, cryogenic pipelines, and aerospace cryogenic structural components.

[0006] 2) The material strength is too low, making it impossible to balance lightweight and high load-bearing capacity;

[0007] While traditional austenitic stainless steels such as 304 and 316L possess a certain degree of ductility at ultra-low temperatures, their yield strength is generally below 400 MPa, which cannot meet the design requirements of "high strength and high toughness" in liquid hydrogen systems. This limits their further application in critical load-bearing components, and there is an urgent need to introduce strengthening mechanisms to improve their strength.

[0008] 3) Nitrogen is difficult to stably dissolve in steel, and the smelting process faces multiple technical bottlenecks;

[0009] Nitrogen, as an austenite stabilizer and a strong solid solution strengthening element, has outstanding advantages in improving strength, refining grains, and enhancing corrosion resistance. However, due to the thermodynamic solubility limit of nitrogen in molten steel and the tendency of nitrogen to escape at high temperatures, traditional smelting methods often cannot achieve an efficient and controllable nitrogen solid solution process. This process is often accompanied by problems such as nitride precipitation, porosity formation, and uneven composition, resulting in large fluctuations in the final material properties and poor microstructural stability, which restricts the reliability of its engineering applications.

[0010] In summary, to meet the performance requirements of key basic materials in future extreme low-temperature environments (especially in the liquid hydrogen temperature range), it is urgent to develop a new type of high-nitrogen austenitic stainless steel for ultra-low temperature environments and its preparation method. This new high-nitrogen austenitic stainless steel should possess high strength, high toughness, high purity, and stable nitrogen solid solution, and a smelting-forming integrated preparation system that can be industrially implemented, has strong controllability of nitrogen element, uniform microstructure, and extremely low impurity content. This system should be able to overcome nitrogen dissolution limitations, suppress the formation of inclusions and precipitated phases, and achieve a comprehensive improvement in the reliability, safety, and long service life of the material under liquid hydrogen temperature conditions. Summary of the Invention

[0011] This invention aims to solve key technical challenges faced by existing austenitic stainless steels in ultra-low temperature environments (especially the liquid hydrogen temperature range, approximately -252.87°C). First, traditional smelting processes struggle to achieve stable solid solution of nitrogen at high concentrations. Nitrogen easily escapes at high temperatures or reacts with impurities to form nitrides, leading to large fluctuations in composition and unstable microstructure, severely impacting the low-temperature performance of the steel. Second, existing processes have limited control over impurities such as oxygen, sulfur, and hydrogen in molten steel. These impurities easily induce grain boundary embrittlement, inclusion-induced fracture, or hydrogen embrittlement failure under ultra-low temperature conditions, becoming significant factors restricting material reliability. Furthermore, traditional high-nitrogen stainless steels are mostly designed based on liquid nitrogen environments, which cannot meet the higher requirements for impact toughness and fracture resistance under liquid hydrogen environments. Simultaneously, they lack corresponding systematic smelting and heat treatment processes such as atmosphere control, nitrogen injection regulation, vacuum impurity removal, and temperature-controlled cooling, making it difficult to achieve a good balance between nitrogen content control, microstructure regulation, and performance stability. Therefore, there is an urgent need to develop a high-nitrogen austenitic stainless steel for ultra-low temperature environments and its preparation method to meet the requirements of key equipment for extreme service performance in liquid hydrogen environments.

[0012] To address the aforementioned problems, this invention proposes a high-nitrogen austenitic stainless steel for ultra-low temperature environments and its preparation method. By optimizing the alloy composition and controlling the nitrogen content to 0.30%~0.40%, a nitrogen and argon mixed atmosphere is employed for protection. This is combined with an integrated metallurgical process including two-stage high-pressure nitrogen injection, vacuum induction refining, staged aluminum powder reduction, and pressure-controlled cooling. This achieves efficient nitrogen solidification and effective removal of impurity elements, resulting in a stainless steel material with uniform microstructure, high purity, and excellent strength and toughness. The yield strength of the final steel at room temperature is... ,tensile strength Impact power ,exist Maintains its properties even in liquid hydrogen environment It is suitable for extreme low-temperature service scenarios such as liquid hydrogen storage and transportation, cryogenic equipment, and polar structures.

[0013] The technical solution of this invention is as follows: A high-nitrogen austenitic stainless steel for ultra-low temperature environments, having the following chemical composition by mass percentage: C 0.01%, Si: 0.1%~0.3%, Mn: 1.5%~2.0%, Cr: 16.5%~18.5%, Ni: 12.5%~14.5%, Mo: 2.0%~3.0%, N: 0.30%~0.40%, balance being Fe and unavoidable impurity elements;

[0014] A method for preparing high-nitrogen austenitic stainless steel for ultra-low temperature environments includes the following steps:

[0015] S1. Add alloy materials according to the composition requirements, and carry out rough refining in an electric arc furnace with a pressure control system. The smelting temperature is controlled between 1600℃ and 1650℃. A mixture of nitrogen and argon is introduced to form a protective atmosphere, and the partial pressure of nitrogen in the mixture is maintained between 0.6MPa and 0.8MPa.

[0016] S2. After the alloy material is melted into molten steel, nitrogen is injected in two stages. In the initial coarse injection stage, the nitrogen injection flow rate is controlled between 15 L / min·ton and 20 L / min·ton, and the nitrogen injection pressure is controlled between 0.6 MPa and 0.8 MPa. In the middle and later refining injection stage, the nitrogen injection flow rate is controlled between 8 L / min·ton and 13 L / min·ton, and the nitrogen injection pressure is controlled between 0.3 MPa and 0.5 MPa. Throughout the process, a mixed gas of nitrogen and argon is used for protection, and the nitrogen partial pressure is maintained between 0.6 MPa and 0.8 MPa. Combined with bottom-blown argon stirring, the roughing process is completed.

[0017] S3. After the roughing process is completed, the molten steel is transferred to a vacuum induction furnace for refining, with the pressure controlled at 0.03MPa~0.05MPa.

[0018] S4. After refining, the molten steel is gradually reduced from 1600℃ to 900℃, with the cooling rate controlled between 3℃ / min and 5℃ / min. Natural cooling is used in the stage below 900℃, with nitrogen atmosphere protection throughout, and finally a steel billet is formed.

[0019] S5. The steel billet is subjected to rolling and heat treatment processes to obtain the final steel plate.

[0020] Controlling the nitrogen solubility index during the roughing process of molten steel Between 0.08 and 0.12, where, Calculate using the following formula:

[0021]

[0022] in, This represents the partial pressure of nitrogen during the crude refining process, expressed in Pa. This refers to the temperature of the molten steel, expressed in °C. For temperature The activity coefficient of nitrogen.

[0023] The temperature The empirical formula for the activity coefficient of nitrogen is:

[0024] .

[0025] In step S2, a trace amount of aluminum powder is added before the initial coarse injection, and the trace amount of aluminum powder accounts for 0.01% to 0.03% of the mass of the molten steel. After the nitrogen injection for the middle and later stages of refinement is completed, a major amount of aluminum powder is added, and the major amount of aluminum powder accounts for 0.05% to 0.15% of the mass of the molten steel.

[0026] The rolling and heat treatment process is specifically as follows:

[0027] S51. Forge the steel billet into a 150mm thick steel ingot. The forging temperature range is 900℃~1100℃. After forging, air cool to room temperature to obtain the steel ingot.

[0028] S52. Heat the steel ingot to 1200℃-1250℃ and hold for 2 minutes per millimeter, then control the rolling process to obtain the steel plate; single-pass reduction rate of rough rolling. 30%, roughing and finishing rolling temperature 1100℃, single-pass reduction rate in finishing rolling 25%, finishing rolling temperature 950℃, then water-cooled to room temperature;

[0029] S53. The steel plate obtained in S52 is subjected to solution heat treatment, held at 1000℃~1100℃ for 1.5 min per millimeter, and then water-cooled to room temperature to obtain the final steel plate.

[0030] The yield strength of the high-nitrogen austenitic stainless steel prepared for ultra-low temperature environments at room temperature ,tensile strength Impact power Impact energy in a liquid hydrogen environment .

[0031] This invention significantly improves the solid solution efficiency, microstructure stability, and ultra-low temperature mechanical properties of nitrogen by systematically constructing a complete smelting and heat treatment process suitable for high-nitrogen austenitic stainless steel.

[0032] First, in the smelting stage, this invention employs an electric arc furnace (EAF) with a pressure control system for preliminary roughing. After the molten steel is formed, a high-pressure nitrogen injection process is introduced, divided into two stages: rough injection and fine injection. Rough injection involves rapidly injecting high-flow-rate, high-pressure nitrogen before the molten steel reaches saturation, increasing the initial nitrogen dissolution rate. Fine injection, on the other hand, involves finely replenishing nitrogen at a lower flow rate after the molten steel gradually approaches saturation, maintaining gas-liquid phase balance and preventing "oversaturation precipitation" or "denitrification reaction." Combined with bottom-blown argon stirring, this effectively improves the nitrogen dissolution efficiency in the molten steel, preventing nitrogen escape and uneven solid solution. To further suppress the formation of nitride inclusions, aluminum powder is added in stages during the smelting process. A trace amount of aluminum powder is added before nitrogen injection for pre-deoxidation, and a large amount of aluminum powder is added after nitrogen injection to strengthen deoxidation and stabilize the oxygen activity of the molten steel, thereby inhibiting the precipitation of AlN and other nitrides and ensuring that nitrogen exists in a solid solution state within the austenitic matrix.

[0033] Secondly, in the steel refining stage, this invention transfers the roughed steel to a vacuum induction furnace for low-pressure refining, effectively removing harmful impurities such as oxygen, sulfur, and hydrogen. Simultaneously, leveraging the kinetic advantages of the vacuum environment, it further enhances the solid solution stability of nitrogen, laying the foundation for subsequent microstructure control and performance stability. After refining, a temperature-controlled cooling process under a nitrogen protective atmosphere precisely controls the cooling rate, allowing the steel to... Slowly cool to This ensures uniform tissue transformation, stable nitrogen dissolution, and inhibits segregation and the formation of coarse precipitates.

[0034] Furthermore, this invention is the first to propose the concept of "nitrogen solubility index". "As a key characterization parameter reflecting the effective dissolution behavior of nitrogen, a correlation was established between the nitrogen content and the temperature of molten steel, the partial pressure of nitrogen, and the nitrogen activity coefficient." The quantitative relationship between them. This index can not only be used for real-time monitoring of nitrogen smelting behavior, but also serve as an important basis for process design and composition optimization, enabling precise control of the nitrogen dissolution process from a thermodynamic and kinetic perspective.

[0035] Compared with the prior art, the beneficial effects of the present invention include:

[0036] Through the synergistic effect of the aforementioned multiple mechanisms, this invention yields a high-nitrogen austenitic stainless steel for cryogenic environments with a nitrogen content of 0.30%~0.40%, extremely low impurity content, and a fine and uniform microstructure. The final yield strength of the high-nitrogen austenitic stainless steel for cryogenic environments at room temperature is also discussed. ,tensile strength Impact power ,exist Still capable of operating in a liquid hydrogen environment Its impact absorption capacity is significantly better than that of traditional austenitic stainless steels such as 316L and 316LN, and it can be widely used in extreme low-temperature service fields such as liquid hydrogen storage and transportation, aerospace, cryogenic containers, and polar exploration, and has good industrial application prospects. Attached Figure Description

[0037] Figure 1 The image shows the microstructure of the finished steel plate prepared in Example 1 of this invention.

[0038] Figure 2 These are microstructure images of the finished steel plate prepared in Comparative Example 6 of this invention.

[0039] Figure 3 The image shown is an EBSD image of the finished steel plate prepared in Example 3 of this invention.

[0040] Figure 4 This is an EBSD image of the finished steel plate prepared in Comparative Example 1 of the present invention. Detailed Implementation

[0041] Example 1

[0042] In Example 1, high-nitrogen austenitic stainless steel was prepared using the lower limit of the composition. The steel plate thickness was 10 mm, and the specific chemical composition by mass percentage was as follows: C: 0.005%, Si: 0.1%, Mn: 1.5%, Cr: 16.5%, Ni: 12.5%, Mo: 2.0%, N: 0.30%, with the balance being... And unavoidable impurity elements, the specific preparation process is as follows:

[0043] Smelting Process: Alloy materials are added according to the composition requirements. Roughing is carried out in an electric arc furnace with a pressure control system. The smelting temperature is controlled at 1600℃. A protective atmosphere is formed by introducing a mixture of nitrogen and argon, with the nitrogen partial pressure maintained at 0.6 MPa. After the alloy materials melt into molten steel, a two-stage nitrogen injection is performed. Before the initial roughing injection, 0.01% aluminum powder by mass is added. After the mid-to-late refining injection, 0.05% aluminum powder by mass is added. During the initial roughing injection stage, the nitrogen injection flow rate is controlled at 15 L / min·ton, and the nitrogen injection pressure is controlled at 0.6 MPa. During the mid-to-late refining injection stage, the nitrogen injection flow rate is controlled at 8 L / min·ton, and the nitrogen injection pressure is controlled at 0.3 MPa. Bottom-blown argon is used for stirring throughout the process to complete the roughing. The value is 0.097. After roughing, the molten steel is transferred to a vacuum induction furnace for refining, with the pressure controlled at 0.03 MPa. After refining, the molten steel is gradually cooled from 1600℃ to 900℃ at a rate of 5℃ / min. Natural cooling is used below 900℃, with nitrogen atmosphere protection throughout the process, ultimately forming a steel billet.

[0044] Rolling and heat treatment process: The steel billet is forged into a 150mm thick ingot at 1000℃, and then air-cooled to room temperature. The ingot is then heated to 1230℃, held for 300 minutes, and then rolled under controlled conditions. The single-pass reduction rate in the roughing mill is... 30%, roughing rolling finishing temperature is 1110℃, and finishing rolling single-pass reduction rate The steel plate was rolled at 25% at a finishing temperature of 960℃ to a final thickness of 10mm, and then water-cooled to room temperature. Subsequently, the steel plate was heated to 1000℃ and held for 15 minutes before being water-cooled to room temperature to obtain the final steel plate.

[0045] Upon testing, the steel plate of Example 1... The final content was 0.30%, the yield strength at room temperature was 439 MPa, and the tensile strength was 752 MPa. The impact energy of the impact is liquid hydrogen environment The impact energy of the impact is .

[0046] Example 2

[0047] In Example 2, high-nitrogen austenitic stainless steel was prepared using the upper limit of the composition. The steel plate thickness was 30 mm, and the specific chemical composition by mass percentage was as follows: C: 0.005%, Si: 0.3%, Mn: 2.0%, Cr: 18.5%, Ni: 14.5%, Mo: 3.0%, N: 0.40%, with the balance being... And unavoidable impurity elements, the specific preparation process is as follows:

[0048] Smelting Process: Alloy materials are added according to the composition requirements. Roughing is carried out in an electric arc furnace with a pressure control system. The smelting temperature is controlled at 1650℃. A protective atmosphere is formed by introducing a mixture of nitrogen and argon, with the nitrogen partial pressure maintained at 0.8 MPa. After the alloy materials melt into molten steel, a two-stage nitrogen injection is performed. Before the initial roughing injection, 0.03% aluminum powder by mass is added. After the mid-to-late refining injection, 0.15% aluminum powder by mass is added. During the initial roughing injection stage, the nitrogen injection flow rate is controlled at 20 L / min·ton, and the nitrogen injection pressure is controlled at 0.8 MPa. During the mid-to-late refining injection stage, the nitrogen injection flow rate is controlled at 13 L / min·ton, and the nitrogen injection pressure is controlled at 0.5 MPa. Bottom-blown argon is used for stirring throughout the process to complete the roughing. The value is 0.096. After roughing, the molten steel is transferred to a vacuum induction furnace for refining, with the pressure controlled at 0.05 MPa. After refining, the molten steel is gradually cooled from 1600℃ to 900℃ at a rate of 3℃ / min. Natural cooling is used below 900℃, with nitrogen atmosphere protection throughout the process, ultimately forming a steel billet.

[0049] Rolling and heat treatment process: The steel billet is forged into a 150mm thick ingot at 1000℃, and then air-cooled to room temperature. The ingot is then heated to 1230℃, held for 300 minutes, and then rolled under controlled conditions. The single-pass reduction rate in the roughing mill is... 30%, roughing rolling finishing temperature is 1100℃, and finishing rolling single-pass reduction rate The steel plate was rolled at 25% at a finishing temperature of 950℃ to a final thickness of 30mm, and then water-cooled to room temperature. Subsequently, the steel plate was heated to 1000℃ and held for 45 minutes before being water-cooled to room temperature to obtain the final steel plate.

[0050] Upon testing, the steel plate of Example 2... The final content was 0.40%, the yield strength at room temperature was 477 MPa, and the tensile strength was 784 MPa. The impact energy of the impact is liquid hydrogen environment The impact energy of the impact is .

[0051] Example 3

[0052] In Example 3, high-nitrogen austenitic stainless steel was prepared using intermediate composition values. The steel plate thickness was 20 mm, and the specific chemical composition by mass percentage was as follows: C: 0.005%, Si: 0.2%, Mn: 1.8%, Cr: 17.5%, Ni: 13.5%, Mo: 2.5%, N: 0.30%, with the balance being... And unavoidable impurity elements, the specific preparation process is as follows:

[0053] Smelting Process: Alloy materials are added according to the composition requirements. Roughing is carried out in an electric arc furnace with a pressure control system. The smelting temperature is controlled at 1630℃. A protective atmosphere is formed by introducing a mixture of nitrogen and argon, with the nitrogen partial pressure maintained at 0.7 MPa. After the alloy materials melt into molten steel, a two-stage nitrogen injection is performed. Before the initial roughing injection, 0.02% aluminum powder by mass is added. After the mid-to-late refining injection, 0.10% aluminum powder by mass is added. During the initial roughing injection stage, the nitrogen injection flow rate is controlled at 18 L / min·ton, and the nitrogen injection pressure is controlled at 0.7 MPa. During the mid-to-late refining injection stage, the nitrogen injection flow rate is controlled at 10 L / min·ton, and the nitrogen injection pressure is controlled at 0.4 MPa. Bottom-blown argon is used for stirring throughout the process to complete the roughing. The value is 0.095. After roughing, the molten steel is transferred to a vacuum induction furnace for refining, with the pressure controlled at 0.04 MPa. After refining, the molten steel is gradually cooled from 1600℃ to 900℃ at a rate of 4℃ / min. Natural cooling is used below 900℃, with nitrogen atmosphere protection throughout the process, ultimately forming a steel billet.

[0054] Rolling and heat treatment process: The steel billet is forged into a 150mm thick ingot at 1000℃, and then air-cooled to room temperature. The ingot is then heated to 1230℃, held for 300 minutes, and then rolled under controlled conditions. The single-pass reduction rate in the roughing mill is... 30%, roughing rolling finishing temperature is 1110℃, and finishing rolling single-pass reduction rate The steel plate was rolled at 25% at a finishing temperature of 960℃ to a final thickness of 20mm, and then water-cooled to room temperature. Subsequently, the steel plate was heated to 1000℃ and held for 30 minutes before being water-cooled to room temperature to obtain the final steel plate.

[0055] Upon testing, the steel plate of Example 3... The final content is 0.35%, the yield strength at room temperature is 451 MPa, and the tensile strength is 769 MPa. The impact energy of the impact is liquid hydrogen environment The impact energy of the impact is .

[0056] Comparative Example 1 (Nitrogen partial pressure not controlled)

[0057] Comparative Example 1 used the same chemical composition and process flow as Example 3. The only difference was that the smelting atmosphere pressure was not controlled during the roughing stage. The nitrogen and argon mixture was introduced under normal pressure (approximately 0.1 MPa), failing to reach the nitrogen partial pressure range of 0.6 MPa to 0.8 MPa specified in the technical solution. During the roughing process... The value was 0.014. Testing showed that the final nitrogen content of Comparative Example 1 was 0.22%, and the room temperature yield strength was... Tensile strength is Room temperature impact energy is Liquid hydrogen temperature zone Impact energy is only In Comparative Example 1, due to insufficient nitrogen partial pressure, the solubility of nitrogen in molten steel was significantly reduced, resulting in insufficient and unevenly distributed dissolved nitrogen, which affected the uniformity of the microstructure and low-temperature toughness.

[0058] Comparative Example 2 (without two-stage nitrogen injection)

[0059] Comparative Example 2 used the same chemical composition and process flow as Example 3, but instead of the "two-stage nitrogen injection" strategy in the roughing stage, it used a single-stage nitrogen injection operation with a fixed flow rate of 12 L / min·ton, although the nitrogen partial pressure was maintained at... Within the range, but not meeting the smelting path of "initial coarse injection and mid-to-late stage fine injection" combined with bottom blowing argon stirring, it is impossible to achieve gradual control of the nitrogen dissolution process.

[0060] Testing revealed that the nitrogen content of the steel plate in Comparative Example 2 was 0.27%, and its room temperature yield strength was... ,tensile strength Room temperature shock energy Impact energy of liquid hydrogen environment In Comparative Example 2, the lack of dynamic control over the nitrogen injection process led to localized supersaturation or escape of nitrogen in the molten steel, resulting in reduced nitrogen dissolution efficiency and uniformity, thus preventing the achievement of stable solid solution.

[0061] Comparative Example 3 (without vacuum refining)

[0062] Comparative Example 3 uses the same chemical composition and process flow as Example 3, but omits the step of refining in a vacuum induction furnace after roughing, i.e., the prescribed refining operation is not implemented, and the key processes of deoxidation, desulfurization and dehydrogenation in the molten steel are omitted.

[0063] Testing revealed that the nitrogen content of the steel plate in Comparative Example 3 was 0.33%, but due to the high level of impurities, its yield strength was [missing value]. ,tensile strength Room temperature impact energy is The impact work of liquid hydrogen is Because impurities were not effectively removed, non-metallic inclusions and brittle phase precipitation occurred, significantly weakening low-temperature toughness and stability.

[0064] Comparative Example 4 (speed control cooling after refining is removed)

[0065] In Comparative Example 4, both the roughing and refining steps were strictly performed according to Example 3, but after refining, the molten steel spontaneously... to The stage did not employ controlled cooling rate. Instead, it adopts a natural cooling method.

[0066] Testing revealed that the nitrogen content of the steel plate in Comparative Example 4 was 0.34%, with little change in mechanical strength (yield strength). ,tensile strength However, its impact performance is significantly reduced, with a room temperature impact energy of [missing value]. The impact work in the liquid hydrogen temperature range is The lack of uniform thermal process control during natural cooling leads to coarsening of the microstructure, increased nitrogen segregation and nitride precipitation, which severely affects the cryogenic impact performance.

[0067] Comparative Example 5 (Aluminum powder not added in stages)

[0068] In Comparative Example 5, aluminum powder was not added in stages before and after nitrogen injection in the roughing process, and pre-deoxidation and deep deoxidation control were not achieved. Aluminum powder was added before nitrogen injection in both cases.

[0069] Testing revealed that the nitrogen content of the steel plate in Comparative Example 5 was 0.32%, and its yield strength was [missing value]. ,tensile strength Room temperature shock energy The impact work of liquid hydrogen is Due to an imperfect aluminum deoxidation process, high oxygen activity and the reaction of nitrogen and oxygen to form... AlN inclusions inhibit the stable solid solution behavior of nitrogen, causing tissue embrittlement.

[0070] Comparative Example 6 (Uncontrolled rolling)

[0071] Comparative Example 6 omits the regulations regarding the control of rolling reduction rate and final rolling temperature. The single-pass reduction rate and final rolling temperature are not strictly controlled during roughing and finishing rolling; a one-pass reduction process is used for direct forming. The steel plate and other processes are the same as in Example 3.

[0072] Tests showed that, although the steel plate of Comparative Example 6... The content remains at 0.35%, and the yield strength is... ,tensile strength However, the microstructure is coarse and stress concentration is severe, resulting in a room temperature impact energy of only [missing information]. The impact energy of liquid hydrogen decreased to The lack of process control in thermal deformation leads to grain coarsening and a significant increase in texture orientation, which reduces toughness and uniformity.

[0073] Example 1 ( Figure 1The comparative example 6 () exhibits a fine, uniform, and clearly defined equiaxed microstructure, indicating that nitrogen partial pressure control, two-stage nitrogen injection, and appropriate heat treatment effectively promoted microstructure homogenization and enhanced phase stability; Figure 2 The results showed coarse grains and strong texture directionality, verifying that the lack of controlled rolling led to grain growth, stress concentration, and reduced low-temperature toughness. (EBSD diagram of Example 3) Figure 3 The comparison example 1 shows a discrete grain orientation distribution and a high proportion of high-angle grain boundaries, which is beneficial to improving the overall mechanical properties; while the comparison example 1 ( Figure 4 The blurred grain outlines, oriented aggregation, and increased low-angle grain boundaries indicate that uncontrolled nitrogen partial pressure will cause insufficient nitrogen solid solution and microstructure segregation, thus weakening the low-temperature performance of the material.

[0074] It should be understood that the above embodiments are only used to illustrate the technical principles and preferred solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Any equivalent substitutions or functional improvements made to the process flow, proportioning range, metallurgical parameters, etc., in accordance with the claims of the present invention should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for preparing high-nitrogen austenitic stainless steel for ultra-low temperature environments, characterized in that, This high-nitrogen austenitic stainless steel for ultra-low temperature environments has the following chemical composition by mass percentage: C≤0.01%, Si: 0.1%~0.3%, Mn: 1.5%~2.0%, Cr: 16.5%~18.5%, Ni: 12.5%~14.5%, Mo: 2.0%~3.0%, N: 0.30%~0.40%, with the balance being Fe and unavoidable impurity elements; The preparation method of the high-nitrogen austenitic stainless steel for ultra-low temperature environments includes the following steps: S1. Add alloy materials according to the composition requirements, and carry out rough refining in an electric arc furnace with a pressure control system. The smelting temperature is controlled between 1600℃ and 1650℃. A mixture of nitrogen and argon is introduced to form a protective atmosphere, and the partial pressure of nitrogen in the mixture is maintained between 0.6MPa and 0.8MPa. S2. After the alloy material is melted into molten steel, nitrogen injection is carried out in two stages. In the initial roughing stage, the nitrogen injection flow rate is controlled between 15 L / min·ton and 20 L / min·ton, and the nitrogen injection pressure is controlled between 0.6 MPa and 0.8 MPa. In the middle and later refining injection stage, the nitrogen injection flow rate is controlled between 8 L / min·ton and 13 L / min·ton, and the nitrogen injection pressure is controlled between 0.3 MPa and 0.5 MPa. The entire process is protected by a mixed gas of nitrogen and argon, with the nitrogen partial pressure maintained between 0.6 MPa and 0.8 MPa, and bottom-blown argon is used for stirring to complete the roughing process. S3. After the roughing process is completed, the molten steel is transferred to a vacuum induction furnace for refining, with the pressure controlled between 0.03MPa and 0.05MPa. S4. After refining, the molten steel is gradually reduced from 1600℃ to 900℃, with the cooling rate controlled between 3℃ / min and 5℃ / min. Natural cooling is used in the stage below 900℃, with nitrogen atmosphere protection throughout, and finally a steel billet is formed. S5. The steel billet is subjected to rolling and heat treatment processes to obtain the final steel plate; Its characteristic is that the nitrogen solubility index is controlled during the roughing process of molten steel. Between 0.08 and 0.12, where, Calculate using the following formula: ； in, This represents the partial pressure of nitrogen during the crude refining process, expressed in Pa. This refers to the temperature of the molten steel, expressed in °C. This represents the activity coefficient of nitrogen at temperature T. The characteristic is that the empirical formula for the activity coefficient of nitrogen at the temperature T is: 。 2. The method for preparing high-nitrogen austenitic stainless steel for ultra-low temperature environments according to claim 1, characterized in that, In step S2, a trace amount of aluminum powder is added before the initial coarse injection, and the trace amount of aluminum powder accounts for 0.01% to 0.03% of the mass of the molten steel. After the nitrogen injection for the middle and later stages of refinement is completed, a major amount of aluminum powder is added, and the major amount of aluminum powder accounts for 0.05% to 0.15% of the mass of the molten steel.

3. The method for preparing high-nitrogen austenitic stainless steel for ultra-low temperature environments according to claim 1, characterized in that, The rolling and heat treatment process is specifically as follows: S51. Forge the steel billet into a 150mm thick steel ingot. The forging temperature range is 900℃~1100℃. After forging, air cool to room temperature to obtain the steel ingot. S52. Heat the steel ingot to 1200℃-1250℃ and hold for 2 minutes per millimeter. Then control the rolling to obtain the steel plate. The single-pass reduction rate of roughing rolling is ≥30%, the finishing rolling temperature is ≥1100℃, the single-pass reduction rate of finishing rolling is ≥25%, the finishing rolling temperature is ≥950℃, and then water-cool to room temperature. S53. The steel plate obtained in S52 is subjected to solution heat treatment, held at 1000℃~1100℃ for 1.5 min per millimeter, and then water-cooled to room temperature to obtain the final steel plate.

4. The method for preparing high-nitrogen austenitic stainless steel for ultra-low temperature environments according to any one of claims 1-3, characterized in that, The prepared high-nitrogen austenitic stainless steel for ultra-low temperature environments has a yield strength >430MPa, tensile strength >740MPa, impact energy >420J at room temperature, and impact energy >260J in liquid hydrogen environment.

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