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

Abstract

The invention belongs to the technical field of metal materials, and discloses high-nitrogen austenitic stainless steel for an ultralow-temperature environment and a preparation method thereof. The steel comprises the following components in percentage by weight: 0.01% of C # imgabs0 #, 0.1%-0.3% of Si, 1.5%-2.0% of Mn, 16.5%-18.5% of Cr, 12.5%-14.5% of Ni, 2.0%-3.0% of Mo, 0.30%-0.40% of N and the balance of Fe and inevitable impurity elements. The preparation method comprises the steps of high-pressure nitrogen injection, electric arc furnace crude smelting, vacuum induction furnace refining, temperature control cooling, rolling and heat treatment, aluminum powder is added in stages in the crude smelting process to inhibit nitride generation, and the nitrogen dissolution index NS is proposed to be used for representing the effective dissolution behavior of nitrogen. The yield strength of the obtained material at room temperature is greater than 430 MPa, the tensile strength is greater than 740 MPa, the impact energy is greater than 420 J, and the impact energy in a liquid hydrogen environment is greater than 260 J.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and in particular to high-nitrogen austenitic stainless steel for ultra-low temperature environments and a preparation method thereof. Background Art

[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, the comprehensive performance of metal materials in extreme low-temperature environments has put forward more stringent technical requirements. Even liquid hydrogen Under ultra-low temperature service conditions, the material must have excellent strength, plasticity, impact toughness and microstructural stability, as well as good corrosion resistance, in order to effectively cope with complex working conditions such as low-temperature brittle fracture, microcrack propagation, thermal stress fatigue and low-temperature medium corrosion.

[0003] Austenitic stainless steel has excellent low-temperature toughness and corrosion resistance due to its face-centered cubic (FCC) crystal structure, and is a key structural material in the current manufacture of low-temperature engineering equipment. The 304 and 316 series austenitic stainless steels designed as the basis for liquid nitrogen are widely used in the liquid nitrogen temperature range. However, traditional austenitic stainless steels (such as 316L and 316LN) face multiple performance bottlenecks in the harsher liquid hydrogen environment, mainly reflected in the following aspects:

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

[0005] Although conventional austenitic stainless steel is in the liquid nitrogen temperature range It still maintains a certain degree of plasticity and toughness under the harsher liquid hydrogen environment. Under these conditions, its impact toughness is significantly reduced, and it is prone to low-temperature brittle fracture, especially in areas with defects or stress concentrations, which is more sensitive and seriously threatens the safe service of hydrogen storage containers, low-temperature transmission pipelines and aviation cryogenic structural parts.

[0006] 2) The material strength is relatively low, and it is impossible to achieve both lightweight and high load-bearing capacity;

[0007] Although material systems represented by traditional austenitic stainless steels such as 304 and 316L have certain ductility at ultra-low temperatures, their yield strength is generally lower than 400 MPa, which cannot meet the design requirements of "high strength and high toughness" in liquid hydrogen systems. This limits their further application in key load-bearing components, and there is an urgent need to introduce a strengthening mechanism to improve strength.

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

[0009] As an austenite stabilizer and strong solid-solution strengthening element, nitrogen offers significant advantages in improving strength, grain refinement, and corrosion resistance. However, due to the thermodynamic solubility limit of nitrogen in molten steel and its tendency to escape at high temperatures, traditional smelting methods often fail to achieve an efficient and controllable nitrogen solid solution process. This process is often accompanied by nitride precipitation, porosity formation, and compositional inhomogeneities, resulting in significant fluctuations in the final material's properties and poor structural stability, limiting its reliability in engineering applications.

[0010] In summary, to meet the performance requirements of key base materials in future extreme low-temperature environments (especially in the liquid hydrogen temperature range), it is urgent to develop a high-nitrogen austenitic stainless steel for ultra-low-temperature environments and its preparation method. This new high-nitrogen austenitic stainless steel, which combines high strength, high toughness, high purity, and stable nitrogen solid solution, must also be developed. Furthermore, an industrially scalable, integrated smelting-forming system must be established, characterized by strong nitrogen controllability, uniform microstructure, and extremely low impurity content. This system should overcome nitrogen solubility limitations, inhibit the formation of inclusions and precipitates, and achieve comprehensive improvements in the material's reliability, safety, and longevity under liquid hydrogen temperature conditions. Summary of the Invention

[0011] This invention aims to address key technical challenges facing existing austenitic stainless steel during service in ultra-low temperature environments (particularly in 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, resulting in large composition fluctuations and unstable microstructures, seriously impacting the steel's low-temperature performance. Second, existing processes have limited control over impurities such as oxygen, sulfur, and hydrogen in molten steel. These impurities can easily cause grain boundary embrittlement, inclusion-induced fracture, or hydrogen embrittlement failure at ultra-low temperatures, significantly limiting material reliability. Furthermore, traditional high-nitrogen stainless steels are often designed for liquid nitrogen environments and cannot meet the higher requirements for impact toughness and fracture resistance in liquid hydrogen environments. Furthermore, they lack supporting systematic smelting heat treatment processes, including atmosphere control, nitrogen injection regulation, vacuum impurity removal, and temperature-controlled cooling. This makes 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 extreme service performance requirements of key equipment in liquid hydrogen environments.

[0012] To solve the above problems, the present invention proposes a high-nitrogen austenitic stainless steel for ultra-low temperature environments and a preparation method thereof. By optimizing the alloy composition, controlling the nitrogen content to 0.30% to 0.40%, adopting a nitrogen and argon mixed atmosphere protection, and coordinating an integrated metallurgical process such as two-stage high-pressure nitrogen injection, vacuum induction refining, staged aluminum powder reduction and pressure-controlled cooling, efficient solid solution of nitrogen and effective removal of impurity elements are achieved, thereby obtaining a stainless steel material with uniform structure, high purity, and excellent strength and toughness. The yield strength of the final steel at room temperature is ,tensile strength , impact energy ,exist Still maintain in liquid hydrogen environment , suitable for extreme low-temperature service scenarios such as liquid hydrogen storage and transportation, cryogenic equipment and polar structures.

[0013] The technical solution of the present invention is as follows: A high nitrogen austenitic stainless steel for ultra-low temperature environment has the following chemical composition in 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%, the balance is Fe and unavoidable impurity elements;

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

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

[0016] S2. After the alloy material is melted into molten steel, two-stage nitrogen injection is carried out. In the initial rough injection stage, the nitrogen injection flow rate is controlled between 15L / min·ton and 20L / min·ton, and the nitrogen injection pressure is controlled between 0.6MPa and 0.8MPa. In the middle and late refinement injection stages, the nitrogen injection flow rate is controlled between 8L / min·ton and 13L / min·ton, and the nitrogen injection pressure is controlled between 0.3MPa and 0.5MPa. The whole process is protected by a mixed gas of nitrogen and argon, and the nitrogen partial pressure is maintained between 0.6MPa and 0.8MPa. Bottom blowing of argon is combined with stirring to complete the rough refining.

[0017] After the rough refining is completed, the molten steel is transferred to a vacuum induction furnace for refining, and the pressure is controlled at 0.03MPa~0.05MPa;

[0018] S4. After refining, the molten steel is gradually cooled from 1600°C to 900°C, with the cooling rate controlled between 3°C / min and 5°C / min. Natural cooling is adopted in the stage below 900°C, with nitrogen atmosphere protection throughout the process, to finally form a steel billet.

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

[0020] During the roughing process of molten steel, control the nitrogen solubility index Between 0.08 and 0.12, among which, Calculated using the following formula:

[0021]

[0022] in, is the nitrogen partial pressure during the crude refining process, in Pa; is the temperature of molten steel, in °C; 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 rough injection, and the mass percentage of the trace aluminum powder in the molten steel is 0.01%~0.03%. After the mid-to-late refinement and nitrogen injection is completed, the main amount of aluminum powder is added, and the mass percentage of the main amount of aluminum powder in the molten steel is 0.05%~0.15%.

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

[0027] S51, forging the steel billet into a 150 mm thick ingot at a forging temperature ranging from 900° C. to 1100° C., and air-cooling the steel billet to room temperature after forging to obtain a steel ingot;

[0028] S52, heat the steel ingot to 1200℃-1250℃ and keep it warm for 2min per millimeter, then control rolling to obtain steel plate; the single pass reduction rate of rough rolling is 30%, rough rolling and finishing temperature 1100℃, finishing rolling single pass reduction rate 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, kept at 1000°C to 1100°C for 1.5 minutes per millimeter, and then water-cooled to room temperature to obtain a final steel plate.

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

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

[0032] First, during the smelting stage, the present invention uses an electric arc furnace (EAF) equipped with a pressure control system for preliminary rough refining. After the molten steel is formed, a high-pressure nitrogen injection process is introduced. This process is divided into two stages: coarse injection and fine injection. Coarse injection is used to rapidly inject high-flow, high-pressure nitrogen before the molten steel reaches saturation, thereby increasing the initial dissolved nitrogen content in the molten steel. Fine injection is used to finely replenish nitrogen at a lower flow rate after the molten steel gradually reaches nitrogen saturation, maintaining gas-liquid phase equilibrium and avoiding "supersaturation precipitation" or "denitrification reaction." Combined with bottom-blown argon agitation, this effectively improves the dissolution efficiency of nitrogen 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 to achieve pre-deoxidation, and a major amount of aluminum powder is added after nitrogen injection to enhance deoxidation and stabilize the oxygen activity of the molten steel, thereby suppressing the precipitation of AlN and other nitrides and ensuring that nitrogen remains in solid solution in the austenite matrix.

[0033] Secondly, in the steel liquid refining stage, the present invention transfers the crude steel liquid into a vacuum induction furnace for low-pressure refining treatment, effectively removing harmful impurities such as oxygen, sulfur, and hydrogen. At the same time, by taking advantage of the kinetic advantages of the vacuum environment, the solid solution stability of nitrogen is further improved, laying the foundation for subsequent organizational control and performance stability. After refining, the cooling rate is precisely controlled under a nitrogen protective atmosphere to ensure that the steel liquid is self-cooled. Slowly cool to To ensure uniform structural transformation, stable solid solution of nitrogen and inhibition of segregation and coarse precipitate formation.

[0034] In addition, the present invention proposes for the first time the "nitrogen solubility index As the key characterization parameter reflecting the effective dissolution behavior of nitrogen, the relationship between molten steel temperature, nitrogen partial pressure and nitrogen activity coefficient was established. This index can not only be used to monitor the smelting behavior of nitrogen in real time, but also serve as an important basis for process design and composition optimization, achieving precise control of the nitrogen dissolution process from the perspective of thermodynamics and kinetics.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] Through the synergy of the above multiple mechanisms, the present invention obtains high nitrogen austenitic stainless steel for ultra-low temperature environments with a nitrogen content of 0.30% to 0.40%, extremely low impurity content, and fine and uniform structure. The yield strength of high nitrogen austenitic stainless steel for ultra-low temperature environments at room temperature is ,tensile strength , impact energy ,exist Liquid hydrogen environment still has Its impact absorption capacity is significantly better than that of traditional austenitic stainless steels such as 316L and 316LN. It can be widely used in extreme low-temperature service fields such as liquid hydrogen storage and transportation, aerospace, cryogenic containers, polar exploration, etc., and has good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a microstructure picture of the finished steel plate prepared in Example 1 of the present invention.

[0038] Figure 2 This is a microstructure picture of the finished steel plate prepared in Comparative Example 6 of the present invention.

[0039] Figure 3 This is the EBSD image of the finished steel plate prepared in Example 3 of the present invention.

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

[0041] Example 1

[0042] In this embodiment 1, the lower limit of the composition is used to prepare high nitrogen austenitic stainless steel. The thickness of the steel plate is 10 mm. The specific chemical composition is as follows by mass percentage: C: 0.005%, Si: 0.1%, Mn: 1.5%, Cr: 16.5%, Ni: 12.5%, Mo: 2.0%, N: 0.30%, and the balance is and inevitable impurity elements, the specific preparation process is as follows:

[0043] Smelting process: Alloy materials are added according to the composition requirements, and rough smelting is carried out in an electric arc furnace with a pressure control system. The smelting temperature is controlled at 1600℃, and a mixed gas of nitrogen and argon is introduced to form a protective atmosphere. The nitrogen partial pressure in the mixed gas is maintained at 0.6MPa. After the alloy material is melted into molten steel, two-stage nitrogen injection is carried out. Before the initial rough injection, 0.01% by mass of aluminum powder is added. After the mid-to-late refinement nitrogen injection is completed, 0.05% by mass of aluminum powder is added. In the initial rough injection stage, the nitrogen injection flow rate is controlled at 15L / min·ton, and the nitrogen injection pressure is controlled at 0.6MPa. In the mid-to-late refinement injection stage, the nitrogen injection flow rate is controlled at 8L / min·ton, and the nitrogen injection pressure is controlled at 0.3MPa. The whole process is coordinated with bottom blowing argon stirring to complete the rough refining; during the rough refining process, The molten steel is then transferred to a vacuum induction furnace for refining at a pressure of 0.03 MPa. After refining, the temperature of the molten steel is gradually lowered from 1600°C to 900°C at a cooling rate of 5°C / min. Natural cooling is used below 900°C, with nitrogen atmosphere protection throughout the entire process, ultimately forming a 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℃ and kept at this temperature for 300min before controlled rolling. The single pass reduction rate of rough rolling is 30%, rough rolling and finishing temperature is 1110℃, finishing rolling single pass reduction rate The steel plate was heated to 1000℃ and kept at that temperature for 15 minutes before being cooled to room temperature to obtain the final steel plate.

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

[0046] Example 2

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

[0048] Smelting process: Alloy materials are added according to the composition requirements, and rough smelting is carried out in an electric arc furnace with a pressure control system. The smelting temperature is controlled at 1650℃, and a mixed gas of nitrogen and argon is introduced to form a protective atmosphere. The nitrogen partial pressure in the mixed gas is maintained at 0.8MPa. After the alloy material is melted into molten steel, two-stage nitrogen injection is carried out. Aluminum powder with a mass percentage of 0.03% is added before the initial rough injection, and aluminum powder with a mass percentage of 0.15% is added after the mid-to-late refinement nitrogen injection is completed. In the initial rough injection stage, the nitrogen injection flow rate is controlled at 20L / min·ton, and the nitrogen injection pressure is controlled at 0.8MPa. In the mid-to-late refinement injection stage, the nitrogen injection flow rate is controlled at 13L / min·ton, and the nitrogen injection pressure is controlled at 0.5MPa. The whole process is coordinated with bottom blowing argon stirring to complete the rough refining. The molten steel is then transferred to a vacuum induction furnace for refining at a pressure of 0.05 MPa. After refining, the temperature of the molten steel is gradually lowered from 1600°C to 900°C at a cooling rate of 3°C / min. Natural cooling is used below 900°C, with nitrogen atmosphere protection throughout the entire process, ultimately forming a 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℃ and kept at this temperature for 300min before controlled rolling. The single pass reduction rate of rough rolling is 30%, rough rolling and finishing temperature is 1100℃, finishing rolling single pass reduction rate The steel plate was heated to 1000℃ and kept at that temperature for 45 minutes before being cooled to room temperature to obtain the final steel plate.

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

[0051] Example 3

[0052] In this embodiment 3, the intermediate composition value is used to prepare high nitrogen austenitic stainless steel. The thickness of the steel plate is 20 mm. The specific chemical composition is as follows in percentage by mass: C: 0.005%, Si: 0.2%, Mn: 1.8%, Cr: 17.5%, Ni: 13.5%, Mo: 2.5%, N: 0.30%, and the balance is and inevitable impurity elements, the specific preparation process is as follows:

[0053] Smelting process: Alloy materials are added according to the composition requirements, and rough smelting is carried out in an electric arc furnace with a pressure control system. The smelting temperature is controlled at 1630℃, and a mixed gas of nitrogen and argon is introduced to form a protective atmosphere. The nitrogen partial pressure in the mixed gas is maintained at 0.7MPa. After the alloy material is melted into molten steel, two-stage nitrogen injection is carried out. Before the initial rough injection, 0.02% by mass of aluminum powder is added. After the mid-to-late refinement nitrogen injection is completed, 0.10% by mass of aluminum powder is added. In the initial rough injection stage, the nitrogen injection flow rate is controlled at 18L / min·ton, and the nitrogen injection pressure is controlled at 0.7MPa. In the mid-to-late refinement injection stage, the nitrogen injection flow rate is controlled at 10L / min·ton, and the nitrogen injection pressure is controlled at 0.4MPa. The whole process is coordinated with bottom blowing argon stirring to complete the rough refining; during the rough refining process, The molten steel is then transferred to a vacuum induction furnace for refining at a pressure of 0.04 MPa. After refining, the temperature of the molten steel is gradually lowered from 1600°C to 900°C at a cooling rate of 4°C / min. Natural cooling is used below 900°C, with nitrogen atmosphere protection throughout the entire process, ultimately forming a 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℃ and kept at this temperature for 300min before controlled rolling. The single pass reduction rate of rough rolling is 30%, rough rolling and finishing temperature is 1110℃, finishing rolling single pass reduction rate The steel plate was heated to 1000℃ and kept at that temperature for 30 minutes before being cooled to room temperature to obtain the final steel plate.

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

[0056] Comparative Example 1 (Uncontrolled Nitrogen Partial Pressure)

[0057] Comparative Example 1 adopts the same chemical composition and process flow as Example 3. The only difference is that the pressure of the smelting atmosphere is not controlled during the rough refining stage. The nitrogen and argon mixed gas is introduced under normal pressure (about 0.1 MPa), which fails to reach the nitrogen partial pressure range of 0.6 MPa to 0.8 MPa specified in the technical solution. The final nitrogen content of Comparative Example 1 was 0.22%, and the yield strength at room temperature was , tensile strength is , the impact energy at room temperature is , liquid hydrogen temperature zone The impact energy is only In Comparative Example 1, due to insufficient nitrogen partial pressure, the solubility of nitrogen in the molten steel is significantly reduced, resulting in insufficient and uneven distribution of solid-solution nitrogen, thereby affecting the uniformity of the structure and the low-temperature toughness.

[0058] Comparative Example 2 (canceling the two-stage nitrogen injection)

[0059] Comparative Example 2 adopts the same chemical composition and process flow as Example 3, but does not adopt the "two-stage nitrogen injection" strategy in the rough refining stage. Instead, a single-stage nitrogen injection operation with a fixed flow rate of 12 L / min·ton is used. Although the nitrogen partial pressure is kept at The range is within this range, but it does not meet the smelting path of “initial coarse injection and mid-to-late refined injection” combined with bottom blowing argon stirring, and the progressive control of the nitrogen dissolution process cannot be achieved.

[0060] After testing, the nitrogen content of the steel plate of Comparative Example 2 is 0.27%, and the room temperature yield strength is ,tensile strength , room temperature impact energy , Liquid hydrogen environment impact energy In Comparative Example 2, due to the lack of dynamic control of the nitrogen injection process, nitrogen is locally oversaturated or escapes in the molten steel, and the nitrogen dissolution efficiency and uniformity are reduced, so stable solid solution cannot be achieved.

[0061] Comparative Example 3 (without vacuum refining)

[0062] Comparative Example 3 adopts the same chemical composition and process flow as Example 3, but eliminates the step of transferring to a vacuum induction furnace for refining after rough refining, i.e., the prescribed refining operation is not performed, and the key processes of deoxidation, desulfurization and dehydrogenation in the molten steel are omitted.

[0063] After testing, the nitrogen content of the steel plate of Comparative Example 3 is 0.33%, but due to the high impurity level, the yield strength is ,tensile strength , the impact energy at room temperature is , the liquid hydrogen impact energy is Since impurities are not effectively removed, non-metallic inclusions and brittle phases are formed, which significantly weaken the low-temperature toughness and stability.

[0064] Comparative Example 4 (canceling controlled cooling after refining)

[0065] In this comparative example 4, the roughing and refining steps were carried out strictly in accordance with Example 3, but after the refining was completed, the molten steel to No controlled cooling rate was used in this stage , but directly adopt natural cooling method.

[0066] After testing, the nitrogen content of the steel plate of Comparative Example 4 is 0.34%, and the mechanical strength changes little (yield strength ,tensile strength ), but the impact performance is significantly reduced, and the impact energy at room temperature is , the impact energy in the liquid hydrogen temperature zone is Due to the lack of uniform thermal history control during the natural cooling process, the microstructure becomes coarse, nitrogen segregation and nitride precipitation increase, which seriously affects the ultra-low temperature impact performance.

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

[0068] In this comparative example 5, aluminum powder was not added in stages before and after nitrogen injection in crude refining, and pre-deoxidation and deep deoxidation control were not achieved. Aluminum powder was added before nitrogen injection.

[0069] After testing, the nitrogen content of the steel plate of Comparative Example 5 is 0.32%, and the yield strength is ,tensile strength , room temperature impact energy 、Liquid hydrogen impact energy is Due to the imperfect aluminum deoxidation process, the oxygen activity is high and nitrogen reacts with oxygen to produce , AlN inclusions inhibit the stable solid solution behavior of nitrogen, causing organizational embrittlement.

[0070] Comparative Example 6 (no controlled rolling)

[0071] The comparative example 6 omits the provisions on controlling the rolling reduction rate and the final rolling temperature. The single-pass rolling reduction rate and the final rolling temperature are not strictly controlled in the rough rolling and finishing rolling. The one-time rolling process is used to directly form the Steel plate, other processes are the same as in Example 3.

[0072] After testing, the steel plate of comparative example 6 The content remains at 0.35%, and the yield strength is ,tensile strength , but the structure is coarse and the stress concentration is serious. The impact energy at room temperature is only , the liquid hydrogen impact energy is reduced to The lack of process control in thermal deformation leads to grain coarsening and significant enhancement of texture directionality, which reduces toughness and uniformity.

[0073] Example 1 ( Figure 1) shows an equiaxed structure with fine and uniform grains and clear boundaries, indicating that nitrogen partial pressure control, two-stage nitrogen injection and reasonable heat treatment effectively promote the homogenization of the structure and strengthen the phase stability; Comparative Example 6 ( Figure 2 ) shows coarse grains and strong texture directionality, which verifies that the failure to implement controlled rolling leads to grain growth, stress concentration, and reduced low-temperature toughness. The EBSD map of Example 3 ( Figure 3 ) shows that the grain orientation distribution is discrete and the proportion of high-angle grain boundaries is high, which is beneficial to improving the comprehensive mechanical properties; while Comparative Example 1 ( Figure 4 ) The grain outline is blurred, the orientation is aggregated, and the low-angle grain boundaries increase, indicating that failure to control the nitrogen partial pressure will cause insufficient nitrogen solid solution and structural segregation, thereby weakening the low-temperature performance of the material.

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

Claims

1. A high nitrogen austenitic stainless steel for ultra-low temperature environment, characterized in that: The high nitrogen austenitic stainless steel for ultra-low temperature environment 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%, and the balance is Fe and inevitable impurity elements.

2. The method for preparing high nitrogen austenitic stainless steel for ultra-low temperature environment according to claim 1, characterized in that: The steps are as follows: S1. Add alloy materials according to the composition requirements and use an electric arc furnace with a pressure control system for rough refining. The smelting temperature is controlled between 1600℃ and 1650℃. A mixed gas of nitrogen and argon is introduced to form a protective atmosphere. The nitrogen partial pressure in the mixed gas is maintained between 0.6MPa and 0.8MPa. S2. After the alloy material is melted into molten steel, two-stage nitrogen injection is carried out. In the initial rough injection stage, the nitrogen injection flow rate is controlled between 15L / min·ton and 20L / min·ton, and the nitrogen injection pressure is controlled between 0.6MPa and 0.8MPa. In the middle and late refinement injection stages, the nitrogen injection flow rate is controlled between 8L / min·ton and 13L / min·ton, and the nitrogen injection pressure is controlled between 0.3MPa and 0.5MPa. The whole process is protected by a mixed gas of nitrogen and argon, and the nitrogen partial pressure is maintained between 0.6MPa and 0.8MPa. Bottom blowing of argon is combined with stirring to complete the rough refining. After the rough refining is completed, the molten steel is transferred to a vacuum induction furnace for refining, and the pressure is controlled between 0.03MPa and 0.05MPa. S4. After refining, the molten steel is gradually cooled from 1600°C to 900°C, with the cooling rate controlled between 3°C / min and 5°C / min. Natural cooling is adopted in the stage below 900°C, with nitrogen atmosphere protection throughout the process, to finally form a steel billet. S5. The steel billet is subjected to rolling and heat treatment processes to obtain a final steel plate.

3. The method for preparing high nitrogen austenitic stainless steel for ultra-low temperature environment according to claim 2, characterized in that: During the roughing process of molten steel, control the nitrogen solubility index Between 0.08 and 0.12, among which, Calculated using the following formula: in, is the nitrogen partial pressure during the crude refining process, in Pa; is the temperature of molten steel, in °C; Temperature The activity coefficient of nitrogen.

4. The method for preparing high nitrogen austenitic stainless steel for ultra-low temperature environment according to claim 3, characterized in that: The temperature The empirical formula for the activity coefficient of nitrogen is: 。 5. The method for preparing high nitrogen austenitic stainless steel for ultra-low temperature environment according to claim 2, characterized in that: In step S2, a trace amount of aluminum powder is added before the initial rough injection, and the mass percentage of the trace aluminum powder in the molten steel is 0.01%~0.03%. After the mid-to-late refinement and nitrogen injection is completed, the main amount of aluminum powder is added, and the mass percentage of the main amount of aluminum powder in the molten steel is 0.05%~0.15%.

6. The method for preparing high nitrogen austenitic stainless steel for ultra-low temperature environment according to claim 2, characterized in that: The rolling and heat treatment process is specifically as follows: S51, forging the steel billet into a 150 mm thick ingot at a forging temperature ranging from 900° C. to 1100° C., and air-cooling the steel billet to room temperature after forging to obtain a steel ingot; S52, heat the steel ingot to 1200℃-1250℃ and keep it warm for 2min per millimeter, then control rolling to obtain steel plate; the single pass reduction rate of rough rolling is 30%, rough rolling and finishing temperature 1100℃, finishing rolling single pass reduction rate 25%, finishing rolling temperature 950℃, then water-cooled to room temperature; S53. The steel plate obtained in S52 is subjected to solution heat treatment, kept at 1000°C to 1100°C for 1.5 minutes per millimeter, and then water-cooled to room temperature to obtain a final steel plate.

7. The method for preparing high nitrogen austenitic stainless steel for ultra-low temperature environment according to any one of claims 2 to 6, characterized in that: The prepared high-nitrogen austenitic stainless steel for ultra-low temperature environments has a yield strength of >430MPa, a tensile strength of >740MPa, an impact energy of >420J at room temperature, and an impact energy of >260J in a liquid hydrogen environment.

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