Ultra-low-temperature steel for hydrogen energy storage and transportation equipment and preparation process thereof

Abstract

An ultra-low-temperature steel for hydrogen energy storage and transportation equipment and a preparation process thereof, including chemical elements with mass percentages of C: 0.06-0.1%, Ni: 6.5-10.5%, Mn: 1.0-1.5%, Cr: 0.8-1.5%, Si: 0.3-0.5%, Al: 0.3-0.5%, and the rest being Fe elements; on the basis of conventional Ni-based ultra-low-temperature steel components, the ratio is adjusted, the components are optimized, the grain refinement and dynamic recrystallization are promoted, and the low-temperature toughness of the steel is improved; the 'austenite + forging process' is applied to densify and refine the parent phase austenite (fcc phase) grains, and then through solid phase transformation, the bcc phase (ferrite or martensite) is formed from the refined parent phase; the grain ultra-refinement can significantly improve the low-temperature toughness of the steel, the grain ultra-refinement treatment increases the grain boundary, effectively hinders the crack propagation, reduces the ductile-brittle transition temperature, and significantly improves the low-temperature toughness.

Description

Technical Field

[0001] This invention belongs to the field of ultra-low temperature steel technology, and specifically relates to an ultra-low temperature steel for hydrogen energy storage and transportation equipment and its preparation process. Background Technology

[0002] Cryogenic steel is mainly used for the storage and transportation of liquid hydrogen, liquefied natural gas, liquid nitrogen, and liquid oxygen, and has wide applications in the energy and chemical industries. Key components of hydrogen energy storage and transportation equipment, such as containers, pipelines, and valves, operate for extended periods in cryogenic environments below -200°C, making them critical equipment in cryogenic industries. As operating temperatures decrease, steel materials transition from a ductile to a brittle state. Defects or load fluctuations at this temperature can trigger sudden brittle fracture, leading to serious safety accidents. To improve the safety of hydrogen energy storage and transportation equipment, containers, pipelines, and valves used in cryogenic environments are typically made of Ni-based cryogenic steel. Adding Ni increases the retained austenite content, promoting austenite reversion and improving low-temperature toughness. Generally, as the Ni content increases, the low-temperature toughness of Ni-based steel gradually improves while maintaining sufficient strength to ensure good low-temperature performance of hydrogen energy storage and transportation equipment.

[0003] In recent years, numerous patent applications have focused on regulating the strength and toughness of Ni-based cryogenic steels. For example, patent application "A heat treatment process for improving the low-temperature impact toughness of 9Ni steel" (CN104745770A) improves the low-temperature impact toughness and optimizes the mechanical properties of 9Ni steel through processes such as two-phase quenching, secondary quenching, and secondary normalizing. Patent application "A strengthening and toughening treatment process for high-nickel and low-carbon series steels" (CN106893816B) combines multi-pass heat treatment with cryogenic treatment to improve strength and low-temperature toughness through the carbide precipitation strengthening effect.

[0004] With the continuous development of hydrogen energy storage and transportation equipment, the requirements for low-temperature toughness, strength, and safety are constantly increasing. Under the principle of controllable cost (without increasing Ni content), developing Ni-based ultra-low temperature steel with good processability, excellent low-temperature toughness, and high strength is extremely crucial. Generally, the strength and toughness of steel materials are contradictory; higher strength results in lower toughness, and vice versa. To solve this problem, this invention, based on the theory of ultra-fine grains, develops an ultra-low temperature steel for hydrogen energy storage and transportation equipment and its preparation process without increasing the Ni content. This process can reduce the ductile-brittle transition temperature, significantly improve low-temperature toughness, and maintain good strength. Summary of the Invention

[0005] To overcome the problems existing in the prior art, the present invention aims to provide an ultra-low temperature steel for hydrogen energy storage and transportation equipment and its preparation process. Based on the theory of grain refinement, without increasing the Ni content, grain refinement is achieved at the microscale by implementing multi-stage hot deformation and forging processes at different temperatures. This improves grain uniformity, increases grain boundaries, effectively hinders crack propagation, and reduces the ductile-brittle transition temperature. It can significantly improve the low-temperature toughness of the ultra-low temperature steel for hydrogen energy storage and transportation equipment while maintaining good strength.

[0006] To achieve the above objectives, the specific technical solution adopted by the present invention is as follows:

[0007] An ultra-low temperature steel for hydrogen energy storage and transportation equipment comprises the following chemical elements by mass percentage: C: 0.06-0.1%, Ni: 6.5-10.5%, Mn: 1.0-1.5%, Cr: 0.8-1.5%, Si: 0.3-0.5%, Al: 0.3-0.5%, with the remainder being Fe.

[0008] A process for preparing ultra-low temperature steel for hydrogen energy storage and transportation equipment, the specific steps of which are as follows:

[0009] Step 1, Homogenization treatment: The sample with the following mass percentages: C: 0.06-0.1%, Ni: 6.5-10.5%, Mn: 1.0-1.5%, Cr: 0.8-1.5%, Si: 0.3-0.5%, Al: 0.3-0.5%, and the remainder being Fe, is heated to 1100-1160℃ and held for 120-180 minutes until all alloying elements and carbides are dissolved, resulting in a uniform single-phase austenitic structure.

[0010] Step 2, primary forging: The obtained single-phase austenite is forged in a forging temperature range of 850-1050℃, a forging ratio of 50-80%, and a final forging temperature of 850-950℃, followed by water cooling to room temperature;

[0011] Step 3, austenitization: Reheat the sample after the first forging to 800-900℃ and hold for 20-40 minutes;

[0012] Step 4, Secondary forging: Spray cooling is performed on the austenitized sample. When the sample is cooled to 50-100°C below the A1 temperature line, secondary forging is performed: the forging ratio is 50-70%, the forging direction is perpendicular to the "first forging" direction, the final forging temperature is 100°C below the A1 temperature line, and the sample is immediately water-cooled to room temperature after forging.

[0013] Step 5, Heating the two-phase region: Reheat the sample after the second forging to the two-phase region 50°C above the A1 temperature line, and hold for 15 to 30 minutes to obtain a uniform and fine ferrite + austenite two-phase structure.

[0014] Step 6, Three-stage forging: The sample heated in the two-phase region is spray-cooled to a temperature range of 250-300°C below the A1 temperature line, and then forged three times with a forging ratio of 20-30% and a final forging temperature of 300°C below the A1 temperature line.

[0015] Step 7, Four-time forging: Immediately after the three-time forging, the sample is reheated to 50-100°C below the A1 temperature line and held for 10-20 minutes. After the temperature is uniform, the sample is forged four times: the forging temperature range is 100-150°C below the A1 temperature line, the forging ratio is 50-70%, the forging direction is perpendicular to the "three-time forging" direction, and the final forging temperature is 300°C below the A1 temperature line. After forging is completed, the sample is immediately water-cooled to room temperature.

[0016] The A1 temperature line is approximately 620–670℃.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1. Alloy Composition Optimization: Based on the conventional Ni-based ultra-low temperature steel composition, the proportions were adjusted and the composition optimized to promote grain refinement and dynamic recrystallization. By strictly controlling C (0.065–0.1%), the ductile-brittle transition temperature was lowered, improving low-temperature toughness; the addition of Ni (6.5–10.5%) lowered the Al temperature line of the alloy (the eutectoid transformation temperature, which varies with alloy composition; the Al temperature line is approximately 620–670℃). Figure 1 As shown), A3 (is the temperature line for the interconversion of ferrite and austenite, which varies with the alloy composition; in this invention, A3 ≈ 680~730℃, as...) Figure 1 (As shown) Critical point, refines the ferrite phase in the alloy, promotes austenite reversal transformation, and significantly reduces the ductile-brittle transition temperature; Adding Cr: 0.8-1.5% improves austenite stability, lowers the martensite Ms point, and facilitates dynamic recrystallization; Adding Mn: 1-1.5% expands the austenite region in the steel, thereby improving toughness; Adding Si: 0.3-0.5% can refine the grains in the steel, avoid coarse grains, and improve the low-temperature toughness of the steel; Adding Al: 0.3-0.5% can form AlN with N, refine the grains, and improve the low-temperature toughness of the steel.

[0019] 2. Forging and Dynamic Recrystallization: Implementing "secondary forging" and "quadruple forging" can refine grains. Combined with dynamic recrystallization, fine and uniform equiaxed grains can be formed. The combination of these two processes can obtain fine and uniform equiaxed grains. The forging ratio must be greater than 50% to increase the driving force for dynamic recrystallization. The selection of the forging temperature is extremely critical. It should be 100-150℃ below the A1 temperature line and above the nose temperature of the C curve to facilitate dynamic recrystallization. Dynamic recrystallization is affected by the phase transformation driving force, nucleation rate, and diffusion rate. If the temperature is too high, the diffusion rate is fast, but the phase transformation driving force and nucleation rate are low, making it difficult to complete dynamic recrystallization; conversely, if the temperature is too low, the diffusion rate is low, which is also not conducive to dynamic recrystallization.

[0020] 3. Forging and Solid-State Phase Transformation: An "austenite + forging process" is applied to densify and refine the parent austenite (fcc phase) grains. Then, through a solid-state phase transformation, the refined parent phase forms an even finer bcc phase (ferrite or martensite), resulting in significant grain refinement. This process is repeated, with the newly formed parent phase grains becoming even finer, further refining the subsequent bcc phase grains, ultimately achieving fine, uniform equiaxed grains and ultra-fine grains. To avoid coarse parent austenite, the austenitizing temperature and heating time should be progressively reduced in each cycle.

[0021] 4. Grain Refinement Strengthening: Grain refinement can simultaneously improve the strength, plasticity, and toughness of metallic materials. The finer and more uniform the grains, the more grains per unit volume, the fewer impurities at grain boundaries, and the smaller the deformation dispersed across individual grains during material deformation. This results in more uniform deformation, effectively hindering crack propagation and leading to higher material strength, better toughness, and better plasticity. Ultra-fine grain refinement can significantly improve the low-temperature toughness of steel. The increased grain boundaries from ultra-fine grain treatment effectively hinder crack propagation, lower the ductile-brittle transition temperature, and significantly improve low-temperature toughness. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the preparation process of the present invention.

[0023] Figure 2 This is the grain orientation diagram (EBSD) of the ferrite bcc structure of the present invention.

[0024] Figure 3 This is a SEM image of the parent phase austenite grains. Detailed Implementation

[0025] The present invention will now be described in a clearer and more complete manner with reference to the accompanying drawings.

[0026] Example 1:

[0027] A cryogenic steel for hydrogen energy storage and transportation equipment has the following chemical element mass percentages: C: 0.08%, Ni: 9.8%, Mn: 1.1%, Cr: 1.2%, Si: 0.5%, Al: 0.35%, with the balance being Fe.

[0028] A manufacturing process for ultra-low temperature steel for hydrogen energy storage and transportation equipment specifically includes the following steps:

[0029] Step 1, Homogenization treatment: The sample with the following mass percentages: C: 0.08%, Ni: 9.8%, Mn: 1.1%, Cr: 1.2%, Si: 0.5%, Al: 0.35%, and the remainder being Fe, was placed in an argon-protected furnace and heated to 1120℃, held for 150 minutes, until all alloying elements and carbides were dissolved to obtain a uniform single-phase austenitic structure;

[0030] Step 2, First forging: The obtained single-phase austenitic sample is taken out of the furnace and forged at a forging temperature of 1000℃, a forging ratio of 60%, and a final forging temperature of 850℃. The austenitic grains are initially refined and densified. Then, it is water-cooled to room temperature to obtain a microstructure mainly composed of martensite.

[0031] Step 3, austenitization: The sample after one forging is reheated to 850℃ and held for 30 minutes to perform austenitization treatment, so as to obtain a fine austenitic microstructure and avoid the formation of coarse grains.

[0032] Step 4, Secondary Forging: After austenitization, the sample is spray-cooled to approximately 600℃ (50-100℃ below the A1 temperature line) before forging begins. The forging deformation direction is perpendicular to the "first forging" direction, the forging ratio is 60%, and the final forging temperature is 500℃. At this temperature, the grains undergo dynamic recrystallization, resulting in refined and uniform equiaxed grains. After forging, the sample is immediately water-cooled to room temperature to obtain a microstructure dominated by fine lath martensite and supplemented by retained austenite.

[0033] Step 5, Heating in the two-phase region: Reheat the sample after the second forging to 700℃ (about 50℃ above the A1 temperature line) in the two-phase region and hold for 20 minutes to avoid grain growth and obtain a uniform and fine ferrite + austenite two-phase structure.

[0034] Step 6, Three-stage forging: The sample heated in the two-phase region is spray-cooled to 400℃ (250-300℃ below the A1 temperature line), and then forged three times. The final forging temperature is 300℃, and the forging ratio is 30%. At this temperature, a solid-state phase transformation occurs to form ferrite, and the forging process can significantly refine the grain structure.

[0035] Step 7, Four-time forging: Immediately reheat the sample after three forgings to 600℃ (50-100℃ below the A1 temperature line), hold for 20 minutes, and then perform four forgings after the temperature is uniform: the forging direction is perpendicular to the three forging directions, the initial forging temperature is 600℃, the final forging temperature is 500℃, the forging ratio is 60%, and the sample is immediately water-cooled to room temperature after forging. The final sample microstructure is significantly refined.

[0036] After the above process, the sample from Example 1 was tested. Backscattered electron diffraction (EBSD) was used to observe the grain orientation and size, and the results are as follows: Figure 2 As shown, the average grain size of the bcc structure is approximately 0.45 μm. To measure the grain size of the parent austenite phase, Example 1 underwent short-term tempering at 350°C, followed by polishing and etching. The microstructure was then observed using a scanning electron microscope (SEM). Figure 3 As shown, the average size of the parent austenite grains is approximately 6.5 μm. This indicates that both the parent austenite and the bcc structure grains have been significantly refined.

[0037] Mechanical property testing: The ultrafine processing is a "pretreatment" process. To measure the final "tempered state" mechanical properties, the ultrafine sample was tempered at 560℃ for 2 hours. Measurement results of the "tempered state" sample: Charpy impact absorption energy A was measured at -196℃. k =201J; its tensile strength R was determined at room temperature. m =857±15MPa, with low-temperature toughness increased by 40% and tensile strength increased by 20% compared to traditional processes.

[0038] Example 2:

[0039] A cryogenic steel for hydrogen energy storage and transportation equipment has the following chemical element mass percentages: C: 0.1%, Ni: 10.5%, Mn: 1.5%, Cr: 1.5%, Si: 0.5%, Al: 0.5%, with the balance being Fe.

[0040] A manufacturing process for ultra-low temperature steel for hydrogen energy storage and transportation equipment specifically includes the following steps:

[0041] Step 1, Homogenization treatment: The sample with the following mass percentages: C: 0.1%, Ni: 10.5%, Mn: 1.5%, Cr: 1.5%, Si: 0.5%, Al: 0.5%, and the balance being Fe, is placed in an argon-protected furnace and heated to 1160℃, held for 120 minutes, until all alloying elements and carbides are dissolved to obtain a uniform single-phase austenitic structure;

[0042] Step 2, First forging: The obtained single-phase austenitic sample is taken out of the furnace and forged at a forging temperature of 1100℃, a forging ratio of 80%, and a final forging temperature of 900℃. The austenitic grains are initially refined and densified. Then, it is water-cooled to room temperature to obtain a microstructure mainly composed of martensite.

[0043] Step 3, austenitization: The sample after one forging is reheated to 800℃ and held for 40 minutes to perform austenitization treatment, so as to obtain a fine austenitic microstructure and avoid the formation of coarse grains.

[0044] Step 4, Secondary Forging: After austenitization, the sample is spray-cooled to 600℃ (50-100℃ below the A1 temperature line) before forging begins. The forging deformation direction is perpendicular to the "first forging" direction, the forging ratio is 70%, and the final forging temperature is 500℃. At this temperature, the grains undergo dynamic recrystallization, resulting in refined and uniform equiaxed grains. After forging, the sample is immediately water-cooled to room temperature to obtain a microstructure dominated by fine lath martensite and supplemented by retained austenite.

[0045] Step 5, Heating in the two-phase region: Reheat the sample after the second forging to 675℃ (50℃ above the A1 temperature line) in the two-phase region and hold for 30 minutes to avoid grain growth and obtain a uniform and fine ferrite + austenite two-phase structure.

[0046] Step 6, Three-stage forging: The sample heated in the two-phase region is spray-cooled to about 400℃ (250-300℃ below the A1 temperature line), and then forged three times. The final forging temperature is 300℃ and the forging ratio is 25%. At this temperature, a solid-state phase transformation occurs to form ferrite, and the forging process can significantly refine the grain structure.

[0047] Step 7, Four-time forging: Immediately reheat the sample after three forgings to 600℃ (50-100℃ below the A1 temperature line), hold for 20 minutes, and then perform four forgings after the temperature is uniform: the forging direction is perpendicular to the three forging directions, the initial forging temperature is 600℃, the final forging temperature is 500℃, the forging ratio is 50%, and the sample is immediately water-cooled to room temperature after forging is completed. The final sample microstructure is significantly refined.

[0048] Mechanical property testing: Measurement results of "tempered state" sample: Charpy shock absorption energy A determined at -196℃ k =158J; its tensile strength R was determined at room temperature. m =935±15MPa.

[0049] Example 3:

[0050] A cryogenic steel for hydrogen energy storage and transportation equipment has the following chemical element mass percentages: C: 0.06%, Ni: 6.5%, Mn: 1.1%, Cr: 0.8%, Si: 0.3%, aluminum (Al): 0.3%, with the balance being Fe and unavoidable impurities.

[0051] A manufacturing process for ultra-low temperature steel for hydrogen energy storage and transportation equipment specifically includes the following steps:

[0052] Step 1, Homogenization treatment: The sample with the following mass percentages: C: 0.06%, Ni: 6.5%, Mn: 1.1%, Cr: 0.8%, Si: 0.3%, Al: 0.3%, and the balance being Fe, is placed in an argon-protected furnace and heated to 1100℃, held for 180 minutes, until all alloying elements and carbides are dissolved to obtain a uniform single-phase austenitic structure;

[0053] Step 2, First forging: The obtained single-phase austenitic sample is taken out of the furnace and forged at a forging temperature of 1050℃, a forging ratio of 50%, and a final forging temperature of 950℃. The austenitic grains are initially refined and densified. Then, it is water-cooled to room temperature to obtain a microstructure mainly composed of martensite.

[0054] Step 3, austenitization: The sample after one forging is reheated to 900℃ and held for 20 minutes to perform austenitization treatment, so as to obtain a fine austenitic microstructure and avoid the formation of coarse grains.

[0055] Step 4, Secondary Forging: After austenitization, the sample is spray-cooled to approximately 600℃ (50-100℃ below the A1 temperature line) before forging begins. The forging deformation direction is perpendicular to the "first forging" direction, the forging ratio is 50%, and the final forging temperature is 500℃. At this temperature, the grains undergo dynamic recrystallization, resulting in refined and uniform equiaxed grains. After forging, the sample is immediately water-cooled to room temperature to obtain a microstructure dominated by fine lath martensite and supplemented by retained austenite.

[0056] Step 5, Heating in the two-phase region: Reheat the sample after the second forging to 720℃ (50℃ above the A1 temperature line) in the two-phase region and hold for 15 minutes to avoid grain growth and obtain a uniform and fine ferrite + austenite two-phase structure.

[0057] Step 6, Three-stage forging: The sample heated in the two-phase region is spray-cooled to 400℃ (250-300℃ below the A1 temperature line), and then forged three times. The final forging temperature is 300℃, and the forging ratio is 20%. At this temperature, a solid-state phase transformation occurs to form ferrite, and the forging process can significantly refine the grain structure.

[0058] Step 7, Four-time forging: Immediately reheat the sample after three forgings to 600℃ (50-100℃ below the A1 temperature line), hold for 20 minutes, and then perform four forgings after the temperature is uniform: the forging direction is perpendicular to the three forging directions, the initial forging temperature is 600℃, the final forging temperature is 500℃, the forging ratio is 70%, and the sample is immediately water-cooled to room temperature after forging is completed. The final sample microstructure is significantly refined.

[0059] Mechanical property testing: Measurement results of "tempered state" sample: Charpy shock absorption energy A determined at -196℃ k =195J; its tensile strength R was determined at room temperature. m =778±15MPa.

[0060] Example 4:

[0061] A cryogenic steel for hydrogen energy storage and transportation equipment has the following chemical element mass percentages: C: 0.09%, Ni: 8.5%, Mn: 1.0%, Cr: 1.3%, Si: 0.4%, Al: 0.5%, with the balance being Fe and unavoidable impurities.

[0062] A manufacturing process for ultra-low temperature steel for hydrogen energy storage and transportation equipment specifically includes the following steps:

[0063] Step 1, Homogenization treatment: The sample with the following mass percentages: C: 0.09%, Ni: 8.5%, Mn: 1.0%, Cr: 1.3%, Si: 0.4%, Al: 0.5%, and the balance being Fe, is placed in an argon-protected furnace and heated to 1130℃, held for 120 minutes, until all alloying elements and carbides are dissolved to obtain a uniform single-phase austenitic structure;

[0064] Step 2, First forging: The obtained single-phase austenitic sample is taken out of the furnace and forged at a forging temperature of 1050℃, a forging ratio of 50%, and a final forging temperature of 900℃. The austenitic grains are initially refined and densified. Then, it is water-cooled to room temperature to obtain a microstructure mainly composed of martensite.

[0065] Step 3, austenitization: The sample after one forging is reheated to 860℃ and held for 30 minutes to perform austenitization treatment, so as to obtain a fine austenitic microstructure and avoid the formation of coarse grains.

[0066] Step 4, Secondary Forging: After austenitization, the sample is spray-cooled to 600℃ (50-100℃ below the A1 temperature line) before forging begins. The forging deformation direction is perpendicular to the "first forging" direction, the forging ratio is 70%, and the final forging temperature is 500℃. At this temperature, the grains undergo dynamic recrystallization, resulting in refined and uniform equiaxed grains. After forging, the sample is immediately water-cooled to room temperature to obtain a microstructure dominated by fine lath martensite and supplemented by retained austenite.

[0067] Step 5, Heating in the two-phase region: Reheat the sample after the second forging to 710℃ (50℃ above the A1 temperature line) in the two-phase region and hold for 15 minutes to avoid grain growth and obtain a uniform and fine ferrite + austenite two-phase structure.

[0068] Step 6, Three-stage forging: The sample heated in the two-phase region is spray-cooled to 400℃ (250-300℃ below the A1 temperature line), and then forged three times. The final forging temperature is 300℃, and the forging ratio is 30%. At this temperature, a solid-state phase transformation occurs to form ferrite, and the forging process can significantly refine the grain structure.

[0069] Step 7, Four-time forging: Immediately reheat the sample after three forgings to 600℃ (50-100℃ below the A1 temperature line), hold for 15 minutes, and then perform four forgings after the temperature is uniform: the forging direction is perpendicular to the three forging directions, the initial forging temperature is 600℃, the final forging temperature is 500℃, the forging ratio is 60%, and the sample is immediately water-cooled to room temperature after forging is completed. The final sample microstructure is significantly refined.

[0070] Mechanical property testing: Measurement results of "tempered state" sample: Charpy shock absorption energy A determined at -196℃ k =166J; its tensile strength R was measured at room temperature. m =873±15MPa.

[0071] Example 5:

[0072] A cryogenic steel for hydrogen energy storage and transportation equipment has the following chemical element mass percentages: C: 0.07%, Ni: 7.8%, Mn: 1.4%, Cr: 1.2%, Si: 0.5%, Al: 0.4%, with the balance being Fe.

[0073] A manufacturing process for ultra-low temperature steel for hydrogen energy storage and transportation equipment specifically includes the following steps:

[0074] Step 1, Homogenization treatment: The sample with the following mass percentages: C: 0.08%, Ni: 9.8%, Mn: 1.1%, Cr: 1.2%, Si: 0.5%, Al: 0.35%, and the remainder being Fe, was placed in an argon-protected furnace and heated to 1120℃, held for 150 minutes, until all alloying elements and carbides were dissolved to obtain a uniform single-phase austenitic structure;

[0075] Step 2, First forging: The obtained single-phase austenitic sample is taken out of the furnace and forged at a forging temperature of 1000℃, a forging ratio of 60%, and a final forging temperature of 850℃. The austenitic grains are initially refined and densified. Then, it is water-cooled to room temperature to obtain a microstructure mainly composed of martensite.

[0076] Step 3, austenitization: The sample after one forging is reheated to 890℃ and held for 30 minutes to perform austenitization treatment, so as to obtain a fine austenitic microstructure and avoid the formation of coarse grains.

[0077] Step 4, Secondary Forging: After austenitization, the sample is spray-cooled to 600℃ (50-100℃ below the A1 temperature line) before forging begins. The forging deformation direction is perpendicular to the "first forging" direction, the forging ratio is 60%, and the final forging temperature is 500℃. At this temperature, the grains undergo dynamic recrystallization, resulting in refined and uniform equiaxed grains. After forging, the sample is immediately water-cooled to room temperature to obtain a microstructure dominated by fine lath martensite and supplemented by retained austenite.

[0078] Step 5, Heating the two-phase region: Reheat the sample after the second forging to 700℃ (about 50℃ above the A1 temperature line) in the two-phase region and hold for 25 minutes to avoid grain growth and obtain a uniform and fine ferrite + austenite two-phase structure.

[0079] Step 6, Three-stage forging: The sample heated in the two-phase region is spray-cooled to 400℃ (250-300℃ below the A1 temperature line), and then forged three times. The final forging temperature is 300℃, and the forging ratio is 25%. At this temperature, a solid-state phase transformation occurs to form ferrite, and the forging process can significantly refine the grain structure.

[0080] Step 7, Four-time forging: Immediately reheat the sample after three forgings to 600℃ (50-100℃ below the A1 temperature line), hold for 20 minutes, and then perform four forgings after the temperature is uniform: the forging direction is perpendicular to the three forging directions, the initial forging temperature is 600℃, the final forging temperature is 500℃, the forging ratio is 70%, and the sample is immediately water-cooled to room temperature after forging is completed. The final sample microstructure is significantly refined.

[0081] Mechanical property testing: Measurement results of "tempered state" sample: Charpy shock absorption energy A determined at -196℃ k=172J; its tensile strength R was determined at room temperature. m =822±15MPa.

[0082] Comparative Example 1:

[0083] A cryogenic steel for hydrogen energy storage and transportation equipment and its preparation process were disclosed, using a sample with the same composition as in Example 1. The sample was heat-treated according to conventional methods, without ultrafine refining. The key steps of the conventional process were: (1) The sample was heated to 900°C in an argon-protected furnace and held for 60 minutes, then water-quenched to room temperature; (2) After quenching, the sample was tempered at 560°C for 120 minutes, then air-cooled to room temperature. Mechanical property testing: Charpy impact absorption energy A was measured at -196°C. k =106J; its tensile strength R was determined at room temperature. m =718±15MPa, the mechanical properties are significantly reduced.

[0084] Comparative Example 2:

[0085] A cryogenic steel for hydrogen energy storage and transportation equipment and its preparation process were disclosed, using a sample with the same composition as in Example 1. The sample was heat-treated according to conventional methods, without ultrafine refining. The key steps of the conventional process were: (1) The sample was heated to 800°C in an argon-protected furnace and held for 60 minutes, then quenched in water to room temperature; (2) The sample was reheated to 690°C and held for 40 minutes, then quenched in water to room temperature; (3) After quenching, the sample was tempered at 560°C for 120 minutes, then air-cooled to room temperature. Mechanical property testing: Charpy impact absorption energy A was measured at -196°C. k =134J; its tensile strength R was determined at room temperature. m =752±15MPa, the mechanical properties are significantly reduced.

[0086] Comparative Example 3:

[0087] An ultra-low temperature steel for hydrogen energy storage and transportation equipment and its preparation process are disclosed, using a sample with the same composition as in Example 1. The main steps employ an ultra-fine refining process, but all forging processes are eliminated. Mechanical property testing results for the tempered sample: Charpy impact absorption energy A measured at -196℃. k =146J; its tensile strength R was determined at room temperature. m =753±15MPa, the mechanical properties are significantly reduced.

[0088] This invention utilizes the principle of grain refinement, combined with alloy composition optimization, forging and dynamic recrystallization, and forging and solid-state phase transformation processes, to develop an ultra-low temperature steel for hydrogen energy storage and transportation equipment and its preparation process. The beneficial effects obtained are as follows:

[0089] (1) Microstructure grain refinement:

[0090] After ultrafine refining, a microstructure dominated by martensite with residual austenite and ferrite as secondary components was obtained, exhibiting fine and uniform grains. EBSD observation of the sample revealed that the bcc structure grain orientation was as follows... Figure 2 As shown, the average bcc grain size is 0.3–0.8 μm.

[0091] After short-term tempering at 350℃ and etching with a CuCl2 saturated picric acid solution, the austenite grains of the parent phase were observed by SEM. Figure 3 As shown, the parent phase fcc austenite grains are refined to 5–8 μm. This indicates that after grain refinement treatment, both the final bcc grains and the parent phase fcc austenite grains are significantly refined, and the ultra-refinement of the parent phase fcc grains is the basis for further grain refinement.

[0092] (2) Improve low-temperature toughness and strength:

[0093] The ultra-fine grain processing technology for ultra-low temperature steel used in hydrogen energy storage and transportation equipment developed in this invention is a "pretreatment" process. After subsequent conventional high-temperature tempering, the steel reaches a "tempered state" before mechanical property testing and comparison. The ultra-fine treated samples are tempered at 580℃ for 2 hours, and then their mechanical properties are measured. The low-temperature impact toughness of the "tempered state" samples is measured at -196℃, with the highest Charpy impact absorption energy reaching over 200J, an improvement of 45% compared to the traditional process. The tensile strength of the "tempered state" samples is measured at room temperature, reaching a maximum of over 950MPa, an improvement of 30% compared to the traditional process.

Claims

1. A process for preparing ultra-low temperature steel for hydrogen energy storage and transportation equipment, characterized in that, Includes the following steps: Step 1, Homogenization treatment: Heat the sample containing the following mass percentages to 1100~1160℃ and hold for 120~180 minutes until all alloying elements and carbides are dissolved to obtain a uniform single-phase austenitic structure; the mass percentages are: C: 0.06~0.1%, Ni: 6.5~10.5%, Mn: 1.0~1.5%, Cr: 0.8~1.5%, Si: 0.3~0.5%, Al: 0.3~0.5%, with the remainder being Fe. Step 2, primary forging: The obtained single-phase austenite is forged in a forging temperature range of 850~1050℃, a forging ratio of 50~80%, and a final forging temperature of 850~950℃, followed by water cooling to room temperature; Step 3, austenitization: Reheat the sample after the first forging to 800~900℃ and hold for 20~40 minutes; Step 4, Secondary forging: The austenitized sample is spray-cooled and then forged again when it is cooled to 50-100°C below the A1 temperature line. The forging ratio is 50-70%, the forging direction is perpendicular to the direction of the first forging, and the final forging temperature is 100°C below the A1 temperature line. After forging, the sample is immediately water-cooled to room temperature. Step 5, Heating the two-phase region: Reheat the sample after the second forging to the two-phase region 50°C above the A1 temperature line, and hold for 15-30 minutes to obtain a uniform and fine ferrite + austenite two-phase structure. Step 6, Three-stage forging: The sample heated in the two-phase region is spray-cooled and cooled to 250~300℃ below the A1 temperature line for three-stage forging. The forging ratio is 20~30%, and the final forging temperature is 300℃ below the A1 temperature line. Step 7, Four-time forging: Immediately after the three-time forging, the sample is reheated to 50-100°C below the A1 temperature line and held for 10-20 minutes. After the temperature is uniform, the sample is forged four times. The forging temperature range is 100-150°C below the A1 temperature line, the forging ratio is 50-70%, the forging direction is perpendicular to the direction of the three-time forging, and the final forging temperature is 300°C below the A1 temperature line. After forging, the sample is immediately water-cooled to room temperature.

2. The preparation process according to claim 1, characterized in that, The A1 temperature line is 620~670℃.

3. The preparation process according to claim 1, characterized in that, In step one, the sample is placed in an argon-protected furnace and heated.

4. The preparation process according to claim 1, characterized in that, In step four, the initial forging temperature of the secondary forging is 600℃; in step seven, the initial forging temperature of the fourth forging is 600℃.

5. A cryogenic steel for hydrogen energy storage and transportation equipment, characterized in that, The preparation process described in any one of claims 1 to 4 comprises the following chemical elements in mass percentage: C: 0.06~0.1%, Ni: 6.5~10.5%, Mn: 1.0~1.5%, Cr: 0.8~1.5%, Si: 0.3~0.5%, Al: 0.3~0.5%, with the remainder being Fe.

6. The ultra-low temperature steel for hydrogen energy storage and transportation equipment according to claim 5, characterized in that, The chemical elements by mass percentage are: C: 0.08%, Ni: 9.8%, Mn: 1.1%, Cr: 1.2%, Si: 0.5%, Al: 0.35%, with the balance being Fe.

7. The ultra-low temperature steel for hydrogen energy storage and transportation equipment according to claim 5, characterized in that, The chemical elements are: C: 0.1%, Ni: 10.5%, Mn: 1.5%, Cr: 1.5%, Si: 0.5%, Al: 0.5%, with the balance being Fe.

8. The ultra-low temperature steel for hydrogen energy storage and transportation equipment according to claim 5, characterized in that, The chemical elements by mass percentage are: C: 0.06%, Ni: 6.5%, Mn: 1.1%, Cr: 0.8%, Si: 0.3%, Al: 0.3%, with the balance being Fe.

9. The ultra-low temperature steel for hydrogen energy storage and transportation equipment according to claim 5, characterized in that, The chemical elements by mass percentage are: C: 0.09%, Ni: 8.5%, Mn: 1.0%, Cr: 1.3%, Si: 0.4%, Al: 0.5%, with the balance being Fe.

10. The ultra-low temperature steel for hydrogen energy storage and transportation equipment according to claim 5, characterized in that, The average size of the bcc structure grains in its microstructure is 0.3~0.8μm, and the average size of the fcc grains in the parent austenite phase is 5~8μm.

Images