High-toughness steel plate for liquid hydrogen storage and transportation and method for manufacturing the same

Abstract

The application belongs to the field of steel material and its preparation technology, and discloses a high strength and toughness steel plate for liquid hydrogen storage and transportation and a preparation method thereof. The mass percentage of each chemical component is controlled and the film-like reversed austenite formation tendency index is satisfied. Through reasonable component proportioning, high cleanliness smelting, continuous casting, rolling and heat treatment synergic control, a dual-phase structure of tempered martensite and film-like reversed austenite is obtained, the volume fraction of the film-like reversed austenite is 8% to 20%, and the volume fraction of the block or island-like reversed austenite is not more than 3%. The steel plate has the room temperature yield strength of greater than or equal to 600 MPa, the tensile strength of greater than or equal to 750 MPa and the impact energy of greater than or equal to 70 J at-253 DEG C in the thickness range of 6 to 50 mm, the structure and performance in the thickness direction are uniform, the steel plate has high strength, super-low temperature high toughness and excellent hydrogen damage resistance, and is suitable for liquid hydrogen storage tanks and related low-temperature pressure vessels.

Description

Technical Field

[0001] This invention relates to the field of steel materials and their preparation technology, and in particular to a high-strength and high-toughness steel plate for liquid hydrogen storage and transportation and its preparation method. Background Technology

[0002] Liquid hydrogen, as a high-energy-density carrier suitable for large-scale and long-distance transportation in the hydrogen energy system, has broad application prospects in transportation, aerospace, storage and transportation terminals, and new energy power plants. Liquid hydrogen has a boiling point of approximately 20K (about -253℃) at atmospheric pressure. Key equipment such as liquid hydrogen storage tanks, pipelines, valves, and transport containers operate in extremely low-temperature environments for extended periods during service, and simultaneously endure the combined effects of multiple factors, including internal pressure fluctuations, start-up and shutdown cycles, loading and unloading, localized stress concentration at welded joints, and the long-term effects of the hydrogen medium. Therefore, extremely high requirements are placed on the strength, toughness, and hydrogen damage resistance of structural materials.

[0003] Currently, engineering materials used in cryogenic and ultracryogenic environments mainly fall into two categories: one is austenitic stainless steel, represented by 304L, 316L, and 304LN; the other is high-Ni cryogenic steel, represented by 9%Ni steel. Austenitic stainless steel exhibits good toughness and weldability at low temperatures and has been widely used in LNG storage and transportation equipment and some cryogenic media environments. However, the yield strength of this type of steel is typically low, mostly only 200MPa~400MPa. As liquid hydrogen storage and transportation equipment develops towards higher pressure resistance, larger capacity, and lighter weight, it is often necessary to increase wall thickness to meet load-bearing requirements, leading to increased structural weight, material consumption, and manufacturing costs. High-Ni cryogenic steel, such as 9%Ni steel, through quenching, tempering, and partial inverse austenite modification, can achieve both high strength and good toughness at -196℃ and has been applied in fields such as LNG storage tanks. However, the design of this type of steel is mainly geared towards operating conditions of -196℃. In the lower liquid hydrogen temperature range (close to -253℃), there is still room for improvement in its low-temperature impact toughness, crack propagation resistance, and resistance to hydrogen damage. It is still difficult to fully meet the requirements for long-term safe service of liquid hydrogen equipment.

[0004] As liquid hydrogen storage and transportation equipment develops towards specialization, high strength, thin walls, and long lifespan, existing technologies mainly suffer from the following shortcomings: (1) It is difficult to balance strength and toughness in the liquid hydrogen temperature range. Although austenitic stainless steel has good low-temperature toughness, its strength is relatively low, which is not conducive to the thinning and weight reduction of high pressure equipment. Although traditional high-Ni low-temperature steels such as 9%Ni steel have a certain strength base, they may still experience problems such as decreased toughness and insufficient crack propagation resistance in the liquid hydrogen environment close to -253℃, making it difficult to meet the requirements of high strength and ultra-low temperature high toughness at the same time.

[0005] (2) Insufficient resistance to hydrogen damage. In liquid hydrogen service environments, hydrogen tends to accumulate in inclusions, segregation zones, grain boundaries, martensitic lath interfaces, and stress concentration areas, thereby inducing hydrogen-induced damage or promoting crack propagation. Existing material systems are mostly based on the design concepts of conventional cryogenic steel or stainless steel, and the system design for hydrogen diffusion, hydrogen trap distribution, and cryogenic fracture behavior under liquid hydrogen media conditions is insufficient, making it difficult to achieve stable resistance to hydrogen damage at a high strength level.

[0006] (3) Insufficient precision in controlling alloy composition and inverted austenite microstructure. Existing technologies usually improve strength and toughness by increasing Ni content or adding alloying elements such as Mo and Cu. However, most of these technologies are still based on empirical design of element content ranges. There is insufficient research on the synergistic effect between stabilizing austenite elements and their correspondence with the volume fraction, morphology distribution and stability of inverted austenite. This can easily lead to improper control of inverted austenite morphology, which in turn affects the strength and toughness matching and service stability of the material in the liquid hydrogen temperature range.

[0007] (4) Insufficient uniformity of microstructure and properties in the thickness direction. Liquid hydrogen storage tanks and pressure vessels typically use steel plates with a thickness of 6mm to 50mm or even greater. If the rolling and heat treatment processes are not properly controlled, uneven microstructure, property fluctuations, central segregation zones, or localized brittle areas can easily occur in the thickness direction of the plate, thereby reducing the overall service reliability of the steel plate.

[0008] (5) The hot working process window for Cu-containing systems is unclear. To improve strength and tempering stability, Cu is introduced into some high-Ni steels. However, under higher heating temperatures and longer holding times, Cu may segregate, increasing the risk of hot working cracks. Existing technologies lack clear control over the matching relationship between Cu content and high-temperature holding regimes, making it difficult to balance the strengthening effect with the safety of hot working.

[0009] Therefore, while existing austenitic stainless steels possess good low-temperature toughness, their strength is relatively insufficient. Existing 9% Ni steel and related high-Ni low-temperature steels, although possessing a certain strength base, still have shortcomings in toughness, resistance to hydrogen damage, and thickness-direction uniformity in the liquid hydrogen temperature range. Thus, it is necessary to develop a new type of high-strength, high-toughness steel plate suitable for liquid hydrogen storage and transportation equipment to meet the comprehensive requirements of high strength, high toughness, good microstructural stability, and resistance to hydrogen damage under the extreme service conditions of liquid hydrogen. Summary of the Invention

[0010] The purpose of this invention is to address the shortcomings of existing steel plate materials for the storage and transportation of liquid hydrogen (approximately -253°C) by providing a high-strength and tough steel plate for liquid hydrogen storage and transportation, and its preparation method, focusing on solving the following technical problems: (1) The problem of balancing high strength and high toughness in the liquid hydrogen temperature range; Commonly used low-temperature steels mainly include austenitic stainless steel and 9% Ni steel. While austenitic stainless steel possesses good low-temperature toughness, its yield strength is generally low, making it difficult to meet the requirements of liquid hydrogen storage tanks and pressure vessels under high pressure, large volume, thinning, and lightweight conditions. Traditional high-Ni low-temperature steels such as 9% Ni steel are primarily designed for -196℃ operating conditions. In liquid hydrogen environments approaching -253℃, their low-temperature impact toughness and crack propagation resistance remain insufficient, making it difficult to simultaneously meet the high strength and ultra-low temperature high toughness requirements of liquid hydrogen storage and transportation equipment. Therefore, this invention aims to provide a special steel plate that can balance high strength and high toughness in the liquid hydrogen temperature range within a thickness range of 6mm to 50mm.

[0011] (2) The problem of hydrogen-induced cracking and high sensitivity to low-temperature embrittlement in liquid hydrogen environment; Under the influence of liquid hydrogen, hydrogen tends to accumulate in inclusions, grain boundaries, martensitic lath interfaces, and stress concentration areas, thereby inducing hydrogen-induced cracking, delaying fracture, or promoting crack propagation. Existing materials do not adequately address the specific needs of hydrogen damage resistance under liquid hydrogen conditions in terms of cleanliness control, inclusion morphology regulation, and low-temperature microstructure design, making it difficult to maintain stable hydrogen damage resistance at high strength levels. Therefore, this invention aims to reduce the impact of harmful impurities and inclusions in steel on the safety of service in liquid hydrogen and improve the resistance of steel plates to crack initiation and propagation in a liquid hydrogen environment.

[0012] (3) The problem of uneven microstructure and properties in the thickness direction; For liquid hydrogen storage steel plates with a thickness range of 6mm to 50mm, improper control of rolling and heat treatment processes can easily lead to differences between the surface and core layers in terms of microstructure, grain size, inverted austenite content, hardness, and residual stress. This can result in localized areas of uneven hardness or brittleness, affecting overall service reliability. Therefore, this invention aims to improve the uniformity of microstructure and properties along the thickness direction of the steel plate, and enhance the overall mechanical property stability of the steel plate across its entire thickness range.

[0013] (4) The problem of lack of quantitative design of alloy ratio and process window for Cu-containing high-Ni steel; In existing technologies, improving the strength and toughness of materials is often achieved by increasing the Ni content or adding elements such as Cu and Mo. However, a systematic design is lacking regarding the synergistic ratio of key alloying elements and their relationship with microstructure control. Furthermore, insufficient research has been conducted on the matching relationship between the heat treatment regime and hot working safety during high-temperature heating of Cu-containing high-Ni steel, which can easily lead to increased risk of hot working cracks, difficulty in microstructure control, and performance fluctuations. Therefore, this invention aims to establish a method for matching composition design and hot working processes suitable for liquid hydrogen storage and transportation steel plates, so as to balance microstructure control effects and hot working safety.

[0014] In summary, the present invention aims to provide a high-strength and high-toughness steel plate for liquid hydrogen storage and transportation and its preparation method, so as to solve the problems in the prior art that it is difficult to achieve both high strength and high toughness in the liquid hydrogen temperature range, insufficient resistance to hydrogen damage, poor uniformity of microstructure and properties in the thickness direction, and unclear hot working process window of Cu-containing high-Ni steel, thereby meeting the requirements of liquid hydrogen storage and transportation equipment for high strength, high toughness, good microstructure stability and service reliability.

[0015] The technical solution of the present invention is as follows: A high-strength and high-toughness steel plate for liquid hydrogen storage and transportation, the chemical composition of which, by mass percentage, is: C: 0.03%~0.08%, Si: 0.10%~0.50%, Mn: 0.30%~0.80%, P≤0.005%, S≤0.003%, Ni: 12.5%~16.0%, Cu: 0.10%~1.0%, Cr: 0.10%~0.40%, Mo: 0.10%~0.40%, Ti: 0.01%~0.040%, Al: 0.020%~0.060%, N≤0.0050%, O≤15ppm, with the balance being Fe and unavoidable impurity elements; The chemical composition, by mass percentage, satisfies the film-like inversion austenite formation tendency index Aγ: Aγ=Ni+Mn+Cu, and 14%≤Aγ≤17%.

[0016] The high-strength and tough steel plate used for liquid hydrogen storage and transportation has a dual-phase structure of tempered martensite and film-like inverted austenite, with a volume fraction of 8% to 20% for film-like inverted austenite and a volume fraction of no more than 3% for blocky or island-like inverted austenite.

[0017] The high-strength and tough steel plate used for liquid hydrogen storage and transportation has a thickness of 6mm to 50mm, a room temperature yield strength ≥600MPa, a tensile strength ≥750MPa, and a Charpy V-notch impact energy ≥70J at -253℃.

[0018] A method for preparing a high-strength and tough steel plate for liquid hydrogen storage and transportation, comprising the following specific steps: Step S1: Clean steelmaking and continuous casting to obtain a continuously cast billet; Step S2: Heating the continuously cast billet to obtain a high-temperature continuously cast billet; Step S3: High-temperature continuous casting billet is rolled to obtain precision rolled steel plate; Step S4: The precision-rolled steel plate is quenched in the two-phase region to obtain a quenched steel plate; Step S5: The quenched steel plate is tempered to obtain the final steel plate, which is used as a high-strength and tough steel plate for liquid hydrogen storage and transportation.

[0019] In step S1, an integrated high-cleanliness metallurgical process of converter, ladle refining and continuous casting is adopted to control the P in the molten steel to ≤0.005%, S to ≤0.003%, N to ≤0.0050% and O to ≤15ppm, and Ca treatment is carried out in the ladle refining stage to obtain the continuously cast billet.

[0020] In step S2, the continuously cast billet obtained in step S1 is heated to 1100℃~1150℃, and hot charging and hot delivery are used to utilize the residual heat of the continuously cast billet, ensuring that the total holding time t of the continuously cast billet above 1100℃ in the furnace satisfies the following relationship with the Cu content: The total holding time t is in min, and the Cu content is in wt.%; high-pressure descaling is performed after the furnace is removed from the furnace.

[0021] Step S3 includes two stages: roughing and finishing rolling. Rough rolling stage: The continuously cast billet after high-pressure descaling in step S2 is subjected to multi-pass rolling. The initial rolling temperature is 1050℃~1070℃, and the single-pass reduction rate of rough rolling is ≥10%. Finishing stage: The cumulative reduction rate of finishing rolling is ≥60%. Finishing rolling adopts slightly asymmetric rolling. The speed of the upper and lower work rolls is interchanged in adjacent passes of the reversible rolling mill, and the speed ratio is 1.03~1.08. The starting temperature of finishing rolling is ≤880℃ and the finishing temperature is ≥830℃. After rolling, the steel plate is cooled to room temperature with water at ≥15℃ / s to obtain the finished steel plate.

[0022] In step S4, the precision-rolled steel plate obtained in step S3 is heated to 630℃~660℃, and the holding time per millimeter of precision-rolled steel plate is 2 minutes. Then, it is cooled to room temperature at a cooling rate of ≥15℃ / s to obtain a quenched steel plate.

[0023] In step S5, the quenched steel plate obtained in step S4 is heated to 520℃~550℃, and the holding time for each millimeter of quenched steel plate is 2 minutes. Then it is air-cooled to room temperature to obtain the final steel plate.

[0024] This invention relates to a high-strength and high-toughness steel plate for liquid hydrogen storage and transportation. Based on a 12.5%–16.0% high-Ni matrix, it utilizes the synergistic design of elements such as C, Si, Mn, Ni, Cr, Mo, Cu, Ti, and Al, combined with the quantitative constraint of the film-like inversion austenite formation tendency index Aγ, to establish a relatively clear correspondence between chemical composition, post-heat-treated microstructure, and liquid hydrogen service performance. The roles of each element and related parameters in this invention are as follows: C (0.03%~0.08%): Used to ensure the strength of the tempered martensitic matrix and to form fine precipitates with Cr, Mo and other elements, thereby improving yield strength and tempering stability; however, excessive content can easily form coarse carbides, which is not conducive to low-temperature toughness and microstructure stability, so it is controlled within a low range.

[0025] Si (0.10%~0.50%): mainly used as a deoxidizing element. An appropriate amount of Si is beneficial to inhibit the coarsening of carbides released from water. However, too much Si is not conducive to toughness and weldability. Therefore, it should be controlled at a low to medium level to balance cleanliness and toughness.

[0026] Mn (0.30%~0.80%): provides moderate solid solution strengthening and hardenability, and participates in the formation and stabilization of inverted austenite. Mn, together with Ni and Cu, constitutes the Aγ parameter, which is beneficial for controlling the volume fraction and stability of film-like inverted austenite; the Mn content is controlled to avoid the adverse effects of excessive segregation and the formation of long strips of MnS with S.

[0027] Ni (12.5%~16.0%): This is the core alloying element of the present invention. It significantly improves hardenability and low-temperature toughness, reduces the martensite initiation temperature, ensures that stable inverted austenite is retained after quenching and tempering in the two-phase region, promotes the formation and stabilization of inverted austenite, and is the dominant element in Aγ. It plays a decisive role in the strength and toughness matching in the liquid hydrogen temperature region.

[0028] Cr (0.10%~0.40%) and Mo (0.10%~0.40%): Both improve hardenability, tempering stability and resistance to tempering softening, and promote the formation of dispersed carbides; however, excessive amounts can lead to coarse carbides, segregation bands and embrittlement tendency; they are not included in the Aγ calculation, but are used as auxiliary strengthening elements to control their range separately.

[0029] Ti (0.01%~0.040%): mainly used to refine the microstructure and fix some N and C, and reduce the adverse effects of interstitial elements; however, excessive content can easily form coarse TiN, which is not conducive to low-temperature toughness, so it is controlled in a low range.

[0030] Al (0.020%~0.060%): mainly used as a deoxidizing element, combined with Ca treatment to improve the morphology of inclusions and reduce the adverse effects of sharp oxides on low-temperature toughness and crack initiation; excessive content can easily form coarse Al2O3 inclusions, so it must be strictly controlled.

[0031] N (≤0.0050%): N is an interstitial element that needs to be strictly controlled. If it is too high, it will easily form coarse nitrides and complex inclusions, reducing low-temperature toughness and microstructure uniformity. Therefore, it is controlled at a low level.

[0032] Harmful elements such as P, S and O: They are controlled at low levels through clean metallurgy and Ca treatment to reduce segregation embrittlement, inclusion-induced crack initiation and adverse fracture risks in liquid hydrogen environment.

[0033] The film-like inverted austenite formation tendency index Aγ: This invention specifies Aγ = Ni + Mn + Cu, with 14% ≤ Aγ ≤ 17%, used to characterize the synergistic effect of Ni, Mn, and Cu on the formation and stabilization of film-like inverted austenite. Ni is the dominant term, while Mn and Cu are auxiliary terms. The film-like inverted austenite formation tendency index, combined with two-phase quenching and tempering processes, is beneficial for obtaining a two-phase microstructure of tempered martensite and film-like inverted austenite, and for controlling the volume fraction of film-like inverted austenite to be 8%–20%, and the volume fraction of blocky or island-like inverted austenite to be no more than 3%.

[0034] Through the synergistic design of the above elements, this invention achieves a quantitative and controllable correlation between "composition-structure-performance", providing a clear alloying design basis for special steel plates for liquid hydrogen environments.

[0035] Compared with the prior art, the beneficial effects of the present invention include: (1) A compositional design method for the film-like reverse austenite formation tendency index Aγ was established; This invention introduces a film-like inverted austenite formation tendency index Aγ and limits the range of Aγ. It synergistically controls three elements—Ni, Mn, and Cu—that are closely related to the formation and stability of inverted austenite, establishing a clear correspondence between chemical composition and the volume fraction and morphological distribution of film-like inverted austenite. Through this parameter design, it can promote the formation of a duplex microstructure in steel plates after subsequent heat treatment, with tempered martensite as the matrix and film-like inverted austenite as the main component. The volume fraction of film-like inverted austenite is controlled at 8%–20%, and the volume fraction of blocky or island-like inverted austenite is controlled to no more than 3%, thereby improving the strength-toughness balance and microstructural stability in the liquid hydrogen temperature range.

[0036] (2) Parametric control of the hot working process window for Cu-containing high-Ni steel was achieved; This invention addresses the problem of Cu segregation and increased risk of hot working cracking in high-Ni Cu steel during high-temperature heating. It proposes a method where the total holding time t (min) of continuously cast billets above 1100℃ is related to the Cu content (wt.%) in the steel. The control relationship ensures the safe operation of hot working while maximizing the beneficial effects of Cu on strength and tempering stability. Compared with existing technologies, this invention couples the Cu content with the high-temperature holding regime, improving the operability and stability of process control for Cu-containing high-Ni steel.

[0037] (3) A two-phase organization control route suitable for liquid hydrogen operating conditions has been formed; This invention employs a process route combining clean metallurgy, heating and alloy solution treatment, controlled rolling, two-phase quenching, and tempering to achieve a refined dual-phase microstructure of tempered martensite and film-like inverted austenite in the steel plate. This microstructure design improves crack passivation capability, impact energy absorption capability, and low-temperature service stability, while reducing the formation of unfavorable structures such as blocky or island-like inverted austenite. Therefore, it is more suitable for the comprehensive requirements of high strength, ultra-low temperature toughness, and microstructure stability under liquid hydrogen storage and transportation conditions.

[0038] (4) It takes into account high strength, high toughness in the liquid hydrogen temperature range, and feasibility of engineering implementation; Through the coordinated control of composition design and process regulations, the steel plate of this invention achieves stable chemical composition, microstructure, and mechanical properties within a thickness range of 6mm to 50mm, realizing a room temperature yield strength ≥600MPa, tensile strength ≥750MPa, and Charpy V-notch impact energy ≥70J at -253℃, while also exhibiting good thickness-direction microstructure and property uniformity. Compared with existing technologies such as the relatively low strength of austenitic stainless steel and the insufficient performance reserves of traditional 9% Ni steel in the liquid hydrogen temperature range, this invention is more suitable for engineering applications of steel plates for liquid hydrogen storage tanks and related cryogenic pressure vessels. Attached Figure Description

[0039] Figure 1 The impact fracture morphology of Example 3; Figure 2 The impact fracture morphology is shown in Comparative Example 6. Detailed Implementation

[0040] The chemical composition of the steel plate used in Example 1, by mass percentage, is as follows: C: 0.03%, Si: 0.10%, Mn: 0.80%, P: 0.003%, S: 0.001%, Ni: 12.50%, Cu: 1.0%, Cr: 0.10%, Mo: 0.10%, Ti: 0.010%, Al: 0.02%, N: 0.0030%, O ≤ 12 pm, with the balance being Fe and unavoidable impurities. Aγ = 14.3%, and the steel plate thickness is 6 mm. The specific preparation process is as follows: S1: Clean steelmaking and continuous casting. Converter smelting, LF and VD ladle refining, Ca treatment at the end of refining to transform inclusions into fine and rounded Ca-modified inclusions, and continuous casting to obtain a continuous casting billet with a thickness of 200mm.

[0041] S2: Billet heating and alloy solution treatment. The continuously cast billet obtained in S1 is heated to 1130℃, and the total holding time above 1100℃ is 20 minutes. After being taken out of the furnace, it is descaled by high pressure water.

[0042] S3: Rolling. The continuously cast billet obtained in S2 is rolled. The roughing rolling start temperature is 1060℃, the single-pass reduction rate of roughing rolling is ≥12%, the cumulative reduction rate of finishing rolling is 85%, a slight asymmetric rolling is adopted, the speed ratio is 1.05, the finishing rolling start temperature is 880℃, the finishing rolling temperature is 830℃, and after rolling, it is cooled to room temperature at a cooling rate of ≥15℃ / s to obtain the finished steel plate.

[0043] S4: Two-phase quenching. The fine-rolled steel plate obtained in S3 is heated to 660℃ and held for 12 min, then cooled to room temperature at a rate of ≥15℃ / s to obtain the quenched steel plate.

[0044] S5: Tempering. The quenched steel plate obtained in S4 is heated to 550℃ and held for 12 minutes, then air-cooled to room temperature to obtain the final steel plate.

[0045] Testing revealed that the microstructure of the steel plate in Example 1 consisted of refined tempered martensite and film-like inverted austenite, with a volume fraction of 8% for film-like inverted austenite and a volume fraction of ≤1% for blocky or island-like inverted austenite. The yield strength was 760 MPa, the tensile strength was 890 MPa, and the Charpy V-notch impact energy at -253℃ was 105 J.

[0046] The chemical composition of the steel plate used in Example 2, by mass percentage, is as follows: C: 0.08%, Si: 0.50%, Mn: 0.50%, P: 0.003%, S: 0.001%, Ni: 16.0%, Cu: 0.50%, Cr: 0.40%, Mo: 0.40%, Ti: 0.040%, Al: 0.06%, N: 0.0040%, O ≤ 14 ppm, with the balance being Fe and unavoidable impurities. Aγ = 17.0%, and the steel plate thickness is 50 mm. The specific preparation process is as follows: S1: Clean steelmaking and continuous casting. Converter smelting, LF and VD ladle refining, Ca treatment at the end of refining to transform inclusions into fine and rounded Ca-modified inclusions, and continuous casting to obtain a continuous casting billet with a thickness of 200mm.

[0047] S2: Billet heating and alloy solution treatment. The continuously cast billet obtained in S1 is heated to 1140℃, and the total holding time above 1100℃ is 30 minutes. After being taken out of the furnace, it is descaled by high pressure water.

[0048] S3: Rolling. The continuously cast billet obtained in S2 is rolled. The roughing rolling start temperature is 1050℃, the single-pass reduction rate of roughing rolling is ≥14%, the cumulative reduction rate of finishing rolling is 60%, a slightly asymmetric rolling is adopted, the speed ratio is 1.05, the finishing rolling start temperature is 870℃, the finishing rolling temperature is 840℃, and after rolling, it is cooled to room temperature at a cooling rate of ≥15℃ / s to obtain the finished steel plate.

[0049] S4: Two-phase quenching. The fine-rolled steel plate obtained in S3 is heated to 630℃ and held for 100 min, then cooled to room temperature at a rate of ≥15℃ / s to obtain the quenched steel plate.

[0050] S5: Tempering. The quenched steel plate obtained in S4 is heated to 520℃ and held for 100 minutes, then air-cooled to room temperature to obtain the final steel plate.

[0051] Testing revealed that the microstructure of the steel plate in Example 2 consisted of refined tempered martensite and film-like inverted austenite, with a volume fraction of 20% for film-like inverted austenite and ≤3% for blocky or island-like inverted austenite. The yield strength was 670 MPa, the tensile strength was 790 MPa, and the Charpy V-notch impact energy at -253℃ was 133 J.

[0052] The chemical composition of the steel plate used in Example 3, by mass percentage, is as follows: C: 0.05%, Si: 0.30%, Mn: 0.50%, P: 0.004%, S: 0.002%, Ni: 14.70%, Cu: 0.60%, Cr: 0.20%, Mo: 0.30%, Ti: 0.020%, Al: 0.04%, N: 0.0030%, O ≤ 13ppm, with the balance being Fe and unavoidable impurities. Aγ = 15.8%, and the steel plate thickness is 30mm. The specific preparation process is as follows: S1: Clean steelmaking and continuous casting. Converter smelting, LF and VD ladle refining, Ca treatment at the end of refining to transform inclusions into fine and rounded Ca-modified inclusions, and continuous casting to obtain a continuous casting billet with a thickness of 200mm.

[0053] S2: Billet heating and alloy solution treatment. The continuously cast billet obtained in S1 is heated to 1130℃, and the total holding time above 1100℃ is 25 minutes. After being taken out of the furnace, it is descaled by high pressure water.

[0054] S3: Rolling. The continuously cast billet obtained in S2 is rolled. The roughing rolling start temperature is 1060℃, the single-pass reduction rate of roughing rolling is ≥15%, the cumulative reduction rate of finishing rolling is 80%, a slightly asymmetric rolling is adopted, the speed ratio is 1.05, the finishing rolling start temperature is 870℃, the finishing rolling temperature is 830℃, and after rolling, it is cooled to room temperature at a cooling rate of ≥15℃ / s to obtain the finished steel plate.

[0055] S4: Two-phase quenching. The fine-rolled steel plate obtained in S3 is heated to 640℃ and held for 60 min, then cooled to room temperature at a rate of ≥15℃ / s to obtain the quenched steel plate.

[0056] S5: Tempering. The quenched steel plate obtained in S4 is heated to 530℃ and held for 60 minutes, then air-cooled to room temperature to obtain the final steel plate.

[0057] Testing revealed that the microstructure of the steel plate in Example 3 consisted of refined tempered martensite and film-like inverted austenite, with a volume fraction of 12% for film-like inverted austenite and ≤2% for blocky or island-like inverted austenite. The yield strength was 730 MPa, the tensile strength was 820 MPa, and the Charpy V-notch impact energy at -253°C was 114 J.

[0058] Comparative Example 1 (Aγ value exceeds the range defined in this invention); Comparative Example 1 is a comparative experiment based on Example 3, with only the contents of Mn, Ni, and Cu adjusted. The specific chemical composition by mass percentage is as follows: C: 0.05%, Si: 0.30%, Mn: 0.80%, P: 0.004%, S: 0.002%, Ni: 16.0%, Cu: 0.8%, Cr: 0.20%, Mo: 0.30%, Ti: 0.020%, Al: 0.04%, N: 0.0030%, O ≤ 13 ppm, with the balance being Fe and unavoidable impurity elements. The chemical composition of this steel is within the scope of this invention, but the value of Aγ is 17.6%, which is outside the scope of this invention. The steel plate thickness is 30 mm, and the specific preparation process is the same as in Example 3.

[0059] The test results show that, because the Aγ value of Comparative Example 1 reached 17.6%, exceeding the 14-17 range defined in this invention, although the content of each individual alloying element was within the range defined in this invention, the total amount of stable austenite elements was too high, leading to an excessively strong tendency for inverted austenite formation during the two-phase region treatment. Testing revealed that the total fraction of inverted austenite in this steel plate increased to 26%, with the volume fraction of blocky or island-shaped inverted austenite exceeding 5%. The room temperature yield strength of this steel plate was 540 MPa, the tensile strength was 680 MPa, and the Charpy V-notch impact energy at -253℃ was 88 J. The results indicate that when Aγ exceeds the range defined in this invention, even if the content of each individual alloying element is still within the range defined in this invention, the steel plate is still prone to problems such as excessively high inverted austenite content and uncontrolled morphology, resulting in a disruption of the overall balance between microstructure and properties, making it difficult to meet the technical requirements of this invention.

[0060] Comparative Example 2 (Aγ value did not reach the range defined in this invention); Comparative Example 2 is a comparative experiment based on Example 3, with only the contents of Mn, Ni, and Cu adjusted. The specific chemical composition by mass percentage is as follows: C: 0.05%, Si: 0.30%, Mn: 0.30%, P: 0.004%, S: 0.002%, Ni: 12.5%, Cu: 0.1%, Cr: 0.20%, Mo: 0.30%, Ti: 0.020%, Al: 0.04%, N: 0.0030%, O ≤ 13 ppm, with the balance being Fe and unavoidable impurity elements. The chemical composition of this steel is within the scope of this invention, but the value of Aγ is 12.9%, which is outside the scope of this invention. The steel plate thickness is 30 mm, and the specific preparation process is the same as in Example 3.

[0061] The test results show that, because the Aγ value of Comparative Example 2 is only 12.9%, which is lower than the 14-17 range specified in this invention, although the content of each individual alloying element is still within the range specified in this invention, the total amount of stable austenite elements is low, resulting in insufficient formation of inverted austenite during the two-phase region treatment. The test results show that the total fraction of inverted austenite in this steel plate is 6%, mainly manifested as a small amount of discontinuous distribution at the martensite lath / bulb interface. The room temperature yield strength of this steel plate is 785 MPa, the tensile strength is 860 MPa, and the Charpy V-notch impact energy at -253℃ is 52 J. The results indicate that when Aγ is lower than the range specified in this invention, although the steel plate can maintain high strength, the insufficient volume fraction of film-like inverted austenite significantly reduces crack passivation and impact energy absorption capacity, making it difficult to meet the technical requirements of this invention for microstructure and ultra-low temperature toughness.

[0062] Comparative Example 3 (Aγ value is within the range defined by this invention, but individual alloying elements exceed the range defined by this invention). Comparative Example 3 is a comparative experiment based on Example 3, with only the contents of Mn, Ni, and Cu adjusted. The specific chemical composition by mass percentage is as follows: C: 0.05%, Si: 0.30%, Mn: 1.0%, P: 0.004%, S: 0.002%, Ni: 13.0%, Cu: 1.5%, Cr: 0.20%, Mo: 0.30%, Ti: 0.020%, Al: 0.04%, N: 0.0030%, O ≤ 13 ppm, with the balance being Fe and unavoidable impurity elements. The chemical composition of this steel is not within the scope of this invention, but the value of Aγ is 15.5%, which is within the range defined by this invention, and the steel plate thickness is 30 mm. Except for adjusting the total holding time of the continuously cast billet above 1100°C in step S2 to 13 min, the rest of the preparation process is the same as in Example 3.

[0063] The test results show that although the Aγ value of Comparative Example 3 is 15.5%, which is within the range defined by this invention, the steel plate still exhibits unfavorable microstructure evolution characteristics during rolling and heat treatment because the contents of Mn and Cu are both higher than the limits defined by this invention. The test results show that the total fraction of inverted austenite in this steel plate is 18%, with some of the inverted austenite transforming from a film-like structure to a thicker, shorter rod-like, blocky, or island-like morphology. The volume fraction of blocky or island-like inverted austenite exceeds 5%, and the microstructure uniformity decreases, with significant component segregation in local areas. The room temperature yield strength of this steel plate is 645 MPa, the tensile strength is 770 MPa, and the Charpy V-notch impact energy at -253℃ is 64 J. The results indicate that even if Aγ is within the range defined by this invention, if the content of any single alloying element exceeds the limits defined by this invention, it will still lead to unstable control of the inverted austenite morphology and decreased microstructure uniformity, making it difficult to stably obtain the dual-phase microstructure dominated by film-like inverted austenite required by this invention, thus hindering the stable matching of microstructure and properties.

[0064] Comparative Example 4 (continuous casting billet heating time is relatively long); Comparative Example 3 is a comparative experiment based on Example 3, but with a change in the heating regime of step S2. The continuously cast billet is heated to 1200°C, and the total holding time above 1100°C is extended to 60 minutes. All other conditions remain unchanged.

[0065] The test results show that although the chemical composition and Aγ value of Comparative Example 4 are within the limits defined by this invention, the high heating temperature and prolonged holding time of the continuously cast billet led to poor microstructure uniformity, increasing the tendency for local component agglomeration and unfavorable microstructure formation. The test revealed that the total fraction of inverted austenite in the steel plate was 14%, with some inverted austenite unevenly distributed, and the volume fraction of blocky or island-shaped inverted austenite exceeding 4%. The room temperature yield strength of the steel plate was 700 MPa, the tensile strength was 805 MPa, and the Charpy V-notch impact energy at -253℃ was 43 J. The results indicate that when the continuously cast billet is held at high temperatures for an excessively long time, resulting in t > 20 / Cu, even if the steel plate composition and Aγ parameters meet the requirements defined by this invention, it will still lead to a decrease in microstructure control stability and a significant deterioration in extremely low-temperature toughness, making it difficult to meet the technical requirements of this invention.

[0066] Comparative Example 5 (without asymmetric rolling); Comparative Example 5 is a comparative experiment based on Example 3, except that the slight asymmetric rolling in the finishing stage is changed to symmetrical rolling with equal speeds of the upper and lower work rolls, while the other conditions remain the same.

[0067] The test results show that in this comparative example, there are still obvious segregation bands and layered structures in the center of the plate thickness. The film-like inverted austenite is unevenly distributed in the thickness direction, forming localized banded regions with alternating hard and soft phases. The yield strength is 640 MPa, the tensile strength is 760 MPa, and the KV2 at -253℃ is 62 J. This indicates that asymmetric rolling has a positive effect on breaking up center segregation, promoting microstructure homogenization, and ensuring continuous distribution of film-like inverted austenite, which is one of the preferred process features of this invention.

[0068] Comparative Example 6 (without quenching in the two-phase region); Comparative Example 6 is a comparative experiment based on Example 3, keeping the composition and rolling process unchanged, but omitting the S4 two-phase region quenching step, with the other conditions remaining the same.

[0069] The test results show that the microstructure of this comparative example is mainly composed of tempered martensite and a small amount of dispersed retained austenite, with a volume fraction of 3% for inverted austenite. It lacks film-like inverted austenite continuously distributed along the martensite lath / block interface. The yield strength is 800 MPa, the tensile strength is 890 MPa, and the KV2 at -253℃ is only 33 J, exhibiting obvious low-temperature brittle fracture characteristics. Figure 2 As shown, it is clear that it cannot meet the requirements of this invention for 8~20% film-like inverted austenite and high toughness at extremely low temperatures, indicating that the two-phase quenching step is crucial for controlling the inverted austenite.

[0070] Comparative Example 7 (quenching temperature in the two-phase region is too high). Comparative Example 7 is a comparative experiment based on Example 3, except that the quenching temperature of the S4 two-phase region is increased from 660℃ to 700℃, while the other conditions remain the same.

[0071] The test results show that when the temperature in the two-phase region is too high, the equilibrium volume fraction of the γ phase increases significantly and Ni and Cu are significantly enriched. After quenching, the volume fraction of inverted austenite is 28%, and it is predominantly blocky and island-like in shape, while the proportion of film-like inverted austenite decreases significantly. The yield strength is 650 MPa, the tensile strength is 740 MPa, and the KV2 at -253℃ is 65 J. This does not meet the overall requirements of this invention for an inverted austenite volume fraction of 8-20%, predominantly film-like, and a balance between strength and toughness, thus verifying the necessity of controlling the quenching temperature window in the two-phase region.

[0072] Comparative Example 8 (without tempering); Comparative Example 8 is a comparative experiment based on Example 3, omitting the S5 tempering step, while keeping all other conditions the same.

[0073] The test results show that the microstructure of this comparative example consists of high-dislocation quenched martensite and a certain amount of inverted austenite, with high residual stress, unstable local carbide distribution, and obvious hydrogen-sensitive brittle regions. The yield strength is 780 MPa, the tensile strength is 900 MPa, and the KV2 at -253℃ is 44 J. This comparative example demonstrates that medium-temperature tempering is crucial for eliminating quenching brittleness and stabilizing the duplex microstructure.

[0074] Comparative Example 9 (tempering temperature too low); Comparative Example 9 is a comparative experiment based on Example 3, except that the tempering temperature was reduced from 560°C to 450°C, while all other conditions remained the same.

[0075] The test results show that due to the low tempering temperature, the high-dislocation martensite and some metastable inverted austenite were not sufficiently stabilized, resulting in high residual stress and dislocation density, and insufficient and uneven precipitation and distribution of alloy carbides. The yield strength was 730 MPa, the tensile strength was 810 MPa, and the KV2 at -253℃ was 45 J. This indicates that an excessively low tempering temperature cannot achieve the strength-toughness balance required by this invention, and the tempering regime of 520~560℃ set in S5 is necessary.

[0076] It should be understood that the embodiments and comparative examples described in this specification are for illustrative purposes only and not for limiting the present invention. Any equivalent substitutions or combinations made to the chemical composition of the steel plate, the value of the Aγ parameter, the heating regime, the rolling process, the two-phase quenching and tempering process, etc., within the spirit and scope of the claims of the present invention, shall fall within the protection scope of the present invention.

[0077] Table 1 Comparison of mechanical properties between the embodiments of the invention and the comparative examples

Claims

1. A high-strength and high-toughness steel plate for liquid hydrogen storage and transportation, characterized in that, The chemical composition, by mass percentage, is as follows: C: 0.03%~0.08%, Si: 0.10%~0.50%, Mn: 0.30%~0.80%, P≤0.005%, S≤0.003%, Ni: 12.5%~16.0%, Cu: 0.10%~1.0%, Cr: 0.10%~0.40%, Mo: 0.10%~0.40%, Ti: 0.01%~0.040%, Al: 0.020%~0.060%, N≤0.0050%, O≤15ppm, with the balance being Fe and unavoidable impurity elements; The aforementioned chemical composition, by mass percentage, satisfies the tendency index Aγ for film-like inverted austenite formation: Aγ = Ni + Mn + Cu, and 14% ≤ Aγ ≤ 17%; the high-strength and tough steel plate used for liquid hydrogen storage and transportation has a dual-phase structure of tempered martensite and film-like inverted austenite, with the volume fraction of film-like inverted austenite being 8%~20%, and the volume fraction of blocky or island-like inverted austenite not exceeding 3%; The specific steps of the preparation method are as follows: Step S1: Clean steelmaking and continuous casting to obtain a continuously cast billet; Step S2: Heating the continuously cast billet to obtain a high-temperature continuously cast billet; Step S3: High-temperature continuous casting billet is rolled to obtain precision rolled steel plate; Step S4: The precision-rolled steel plate is quenched in the two-phase region to obtain a quenched steel plate; Step S5: The quenched steel plate is tempered to obtain the final steel plate, which is used as a high-strength and tough steel plate for liquid hydrogen storage and transportation. Step S3 includes two stages: roughing and finishing rolling. Rough rolling stage: The continuously cast billet after high-pressure descaling in step S2 is subjected to multi-pass rolling. The initial rolling temperature is 1050℃~1070℃, and the single-pass reduction rate of rough rolling is ≥10%. Finishing stage: The cumulative reduction rate of finishing rolling is ≥60%. Finishing rolling adopts slightly asymmetric rolling. The speed of the upper and lower work rolls is interchanged in adjacent passes of the reversible rolling mill, and the speed ratio is 1.03~1.

08. The starting temperature of finishing rolling is ≤880℃ and the finishing temperature is ≥830℃. After rolling, the steel plate is cooled to room temperature with water at ≥15℃ / s to obtain the finished steel plate.

2. The high-strength and high-toughness steel plate for liquid hydrogen storage and transportation according to claim 1, characterized in that, The high-strength and tough steel plate used for liquid hydrogen storage and transportation has a thickness of 6mm to 50mm, a room temperature yield strength ≥600MPa, a tensile strength ≥750MPa, and a Charpy V-notch impact energy ≥70J at -253℃.

3. The high-strength and high-toughness steel plate for liquid hydrogen storage and transportation according to claim 1, characterized in that, In step S1, an integrated high-cleanliness metallurgical process of converter, ladle refining and continuous casting is adopted to control the P in the molten steel to ≤0.005%, S to ≤0.003%, N to ≤0.0050% and O to ≤15ppm, and Ca treatment is carried out in the ladle refining stage to obtain the continuously cast billet.

4. The high-strength and high-toughness steel plate for liquid hydrogen storage and transportation according to claim 1, characterized in that, In step S2, the continuously cast billet obtained in step S1 is heated to 1100℃~1150℃, and hot charging and hot delivery are used to utilize the residual heat of the continuously cast billet, ensuring that the total holding time t of the continuously cast billet above 1100℃ in the furnace satisfies the following relationship with the Cu content: The total holding time t is in min, and the Cu content is in wt.%; high-pressure descaling is performed after the furnace is removed from the furnace.

5. The high-strength and high-toughness steel plate for liquid hydrogen storage and transportation according to claim 1, characterized in that, In step S4, the precision-rolled steel plate obtained in step S3 is heated to 630℃~660℃, and the holding time per millimeter of precision-rolled steel plate is 2 minutes. Then, it is cooled to room temperature at a cooling rate of ≥15℃ / s to obtain a quenched steel plate.

6. The high-strength and high-toughness steel plate for liquid hydrogen storage and transportation according to claim 1, characterized in that, In step S5, the quenched steel plate obtained in step S4 is heated to 520℃~550℃, and the holding time for each millimeter of quenched steel plate is 2 minutes. Then it is air-cooled to room temperature to obtain the final steel plate.

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