1000MPa-grade high-strength plastic stainless steel plate and preparation method thereof

Abstract

The invention discloses a 1000MPa-grade high-strength plastic stainless steel plate and a preparation method thereof, and belongs to the field of metal materials. The stainless steel comprises the following chemical components: less than or equal to 0.031% of C, 17.39-18.21% of Cr, 7.39-8.21% of Ni, less than or equal to 1.81% of Mn, less than or equal to 0.71% of Si, 0.29-0.51% of Mo, 0.19-0.61% of Cu, 0.029-0.081% of N and the balance of Fe and inevitable impurities. The room-temperature yield strength of the formed stainless steel is greater than or equal to 887 MPa, the room-temperature tensile strength is greater than or equal to 980 MPa, the room-temperature ductility is greater than or equal to 14.5%, and the spot corrosion resistance PREN is greater than or equal to 20. The low-temperature (-196 DEG C) yield strength is larger than or equal to 985 MPa, the low-temperature (-196 DEG C) tensile strength is larger than or equal to 1740 MPa, and the low-temperature (-196 DEG C) ductility is larger than or equal to 26.5%. The mechanical property, the corrosion resistance and the cold forming performance of the stainless steel plate are well balanced, and a low-cost solution can be provided for application of the stainless steel plate in the fields of ocean structures, space flight and aviation, energy, chemical industry and the like.

Description

Technical Field

[0001] This invention relates to the field of high-performance metal materials technology, and in particular to a 1000MPa-grade high-strength and ductile stainless steel sheet and its preparation method. Background Technology

[0002] With the development of high-end equipment in aerospace, energy and chemical industries, including cryogenic storage and transportation, high-strength structural safety and low cost, stringent requirements have been placed on stainless steel materials for room temperature, low temperature (liquid nitrogen) and industrial cost reduction.

[0003] For large-scale industrial production of stainless steel plates, cold deformation of 300 series stainless steel is an effective way to achieve low cost and high strength. Currently, commonly available 301 and 304 series stainless steels in their hardened state can achieve different levels of room temperature strength. Compared to 304 series stainless steel, 301 series stainless steel, due to its lower Cr and Ni equivalents and poorer austenitic stability, can undergo significant work hardening during room temperature deformation, resulting in high strength and high plasticity. However, its lower Cr and Ni content also leads to poorer corrosion resistance, especially in the sensitization effect of the heat-affected zone (HAZ) after welding, which exacerbates corrosion in the HAZ. 304L stainless steel has excellent corrosion resistance, and its lower carbon content effectively improves the sensitization characteristics of the HAZ after welding. However, 304L stainless steel exhibits low strength in the solution-treated state and insufficient elongation after cold rolling for high strength. Adding excessive Cr and Ni elements can improve the strength of 304L stainless steel to some extent, but this is accompanied by insufficient low-temperature strength and elongation.

[0004] Therefore, existing technologies cannot provide a stainless steel sheet with a room temperature tensile strength of 1000 MPa, good low-temperature mechanical properties, and high corrosion resistance, which severely restricts the widespread use of high-performance, low-cost stainless steel in the energy, chemical, and aerospace fields. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a 1000MPa grade high-strength and ductile stainless steel sheet and its preparation method, at least to solve one of the following problems: traditional 304 series cold-rolled austenitic stainless steel has insufficient strength and low room temperature elongation, making it difficult to meet the requirements for high strength and ductility at room temperature; traditional 301 series cold-rolled metastable austenitic stainless steel has insufficient corrosion resistance and low low temperature elongation, making it difficult to meet corrosion resistance requirements; and the current high cost of martensitic stainless steel in terms of adjusting the corrosion resistance and room / low temperature performance of the alloy through higher levels of elements such as Mo and Co.

[0006] The objective of this invention is mainly achieved through the following technical solutions:

[0007] This invention provides a 1000MPa grade high-strength and ductile stainless steel sheet, the chemical composition of which, by mass percentage, includes: C≤0.031%, Cr 17.39~18.21%, Ni 7.39~8.21%, Mn≤1.81%, Si≤0.71%, Mo 0.29~0.51%, Cu 0.19~0.61%, N 0.029~0.081%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, the chemical composition, by mass percentage, includes: C≤0.03%, Cr 17.4~18.2%, Ni 7.4~8.2%, Mn≤1.8%, Si≤0.7%, Mo 0.3~0.5%, Cu 0.2~0.6%, N0.03~0.08%, with the balance being Fe and unavoidable impurities.

[0009] Furthermore, the chemical composition, by mass percentage, is 26.6% ≤ Cr + Ni + Mn ≤ 27.6%.

[0010] Furthermore, the microstructure of the hot-rolled high-strength and high-ductility stainless steel sheet is austenite, with a ferrite volume fraction not exceeding 2%.

[0011] Furthermore, the high-strength, high-ductility stainless steel sheet has a room temperature yield strength ≥887MPa, a room temperature tensile strength ≥985MPa, and a room temperature elongation ≥14.5%.

[0012] Furthermore, the high-strength and high-ductility stainless steel sheet has a liquid nitrogen temperature yield strength ≥985MPa, a liquid nitrogen temperature tensile strength ≥1740MPa, and a liquid nitrogen temperature elongation ≥26.5%.

[0013] Furthermore, after the high-strength and high-ductility stainless steel sheet is kept at 300-500℃ for 1-3 hours, its yield strength is ≥1016MPa and its tensile strength is ≥1128MPa at room temperature.

[0014] Furthermore, the high-strength, high-ductility stainless steel sheet has a pitting corrosion resistance of ≥20 (PREN, calculated as Cr+3.3Mo+16N).

[0015] Furthermore, the average austenite grain size of the high-strength, high-ductility stainless steel sheet before final rolling is 30-50 μm.

[0016] On the other hand, the present invention provides a method for preparing 1000MPa grade high-strength and ductile stainless steel sheet, comprising the following steps:

[0017] (1) Smelting and casting are carried out using an electric furnace and refining method to obtain billets;

[0018] (2) Hot rolling the billet;

[0019] (3) The slab is hot rolled, and then rolled and pickled to obtain white cold-rolled raw material;

[0020] (4) The cold-rolled raw material is subjected to cold rolling for no less than two rolling passes to obtain stainless steel sheet.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] 1. This invention designs a specific alloy composition system that inherently possesses the potential to become "metastable austenite" in metallurgical terms. Through multi-element microalloying synergy: strictly controlling C ≤ 0.031% (anti-sensitization), Mo 0.29-0.51% (improving corrosion resistance and strength), N 0.029-0.081% (stabilizing microstructure, improving strength, and reducing cost), and Cu 0.19-0.61% (improving formability), a low-cost, high-performance alloy formula is formed. The final product is a high-strength, high-ductility stainless steel sheet: achieving an excellent balance between room temperature strength and ductility: room temperature yield strength ≥ 887 MPa, tensile strength ≥ 980 MPa, and elongation ≥ 14.5%; low-temperature (-196℃) toughness: yield strength ≥ 985 MPa, tensile strength ≥ 1740 MPa, and elongation ≥ 26.5%. This combination achieves a simultaneous leap in strength and plasticity at low temperatures, far exceeding that of traditional austenitic stainless steel. Its core mechanism lies in the precise control of metastable austenite, which undergoes a toughening martensitic phase transformation (TRIP effect) during low-temperature deformation. Excellent corrosion resistance: pitting corrosion equivalent PREN≥20, improving intergranular corrosion sensitivity and overcoming the inherent defect of poor corrosion resistance in 301 series stainless steel. Good thermal stability: after being held at 300-500℃ for 1-3 hours, the yield strength is ≥1016MPa, the tensile strength is ≥1128MPa, and the elongation is ≥14% at room temperature, ensuring stable performance even after service in high-temperature zones below 500℃.

[0023] 2. This invention employs an alloy range of 17.39–18.21% Cr and 7.39–8.21% Ni, which effectively controls the metastable characteristics of the austenite phase. This avoids the poor austenite stability and low corrosion resistance of 301 series stainless steel, while also avoiding the high austenite stability and insufficient low-temperature strengthening effect of 304 series stainless steel. Mo is added to further improve the corrosion resistance and strength of the designed alloy. N is added to control the potential for excessive ferrite phase after adding Mo, while also improving material strength and corrosion resistance. The added Cu further improves the corrosion resistance of stainless steel while effectively improving its ductility and cold formability.

[0024] 3. Compared to traditional austenitic stainless steels, which suffer from limited strengthening at low temperatures due to the excessive stability of austenite or premature embrittlement due to its instability, the high-strength, high-ductility stainless steel system of this invention achieves a highly stable austenitic structure through precise control of the alloy composition. This austenite remains stable even under low-temperature (liquid nitrogen) conditions. This austenite exhibits significant low-temperature instability during deformation, with substantial martensitic transformation observed in the early stages of strain. It achieves a good balance of yield strength ≥985MPa, tensile strength ≥1740MPa, and elongation ≥26.5% at -196℃, effectively ensuring the safety of the structure during low-temperature service and providing more material options for equipment design in extreme low-temperature environments.

[0025] 4. By controlling the Cr, Mo, and N elements within an optimized range, this invention enables the material to have a pitting corrosion resistance equivalent PREN≥20, fundamentally avoiding intergranular corrosion sensitivity and achieving a balance between excellent mechanical properties and high corrosion resistance.

[0026] 5. Compared with improving performance by adding high amounts of expensive elements such as Mo and Co, this invention controls the Mo content at a low level while ensuring overall performance, and uses N to partially replace Ni, which significantly reduces the cost of the alloy.

[0027] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0028] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0029] Figure 1 This is an EBSD diagram of Embodiment 1 of the present invention;

[0030] Figure 2 This is a stress-strain curve diagram of Embodiment 1 of the present invention. Detailed Implementation

[0031] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0032] For large-scale industrial production of stainless steel plates, cold deformation of 300 series stainless steel is an effective way to achieve low cost and high strength. Currently, commonly available 301 and 304 series stainless steels in their hardened state can achieve different levels of room temperature strength. Compared to 304 series stainless steel, 301 series stainless steel, due to its lower Cr and Ni equivalents and poorer austenitic stability, can undergo significant work hardening during room temperature deformation, resulting in high strength and high plasticity. However, its lower Cr and Ni content also leads to poorer corrosion resistance, especially in the sensitization effect of the heat-affected zone (HAZ) after welding, which exacerbates corrosion in the HAZ. 304L stainless steel has excellent corrosion resistance, and its lower carbon content effectively improves the sensitization characteristics of the HAZ after welding. However, 304L stainless steel exhibits low strength in the solution-treated state and insufficient elongation after cold rolling for high strength. Adding excessive Cr and Ni elements can improve the strength of 304L stainless steel to some extent, but this is accompanied by insufficient low-temperature strength and elongation.

[0033] To address the aforementioned problems, this invention provides a 1000MPa grade high-strength, high-ductility stainless steel sheet, the chemical composition of which, by mass percentage, comprises: C≤0.031%, Cr17.39~18.21%, Ni7.39~8.21%, Mn≤1.81%, Si≤0.71%, Mo0.29~0.51%, Cu0.19~0.6%, N0.029~0.081%, with the balance being Fe and unavoidable impurities.

[0034] Furthermore, the chemical composition by weight includes: C≤0.03%, Cr 17.4~18.2%, Ni 7.4~8.2%, Mn≤1.8%, Si≤0.7%, Mo 0.3~0.5%, Cu 0.2~0.6%, N 0.03~0.08%, with the balance being Fe and unavoidable impurities.

[0035] Furthermore, 26.6% ≤ Cr + Ni + Mn ≤ 27.6%.

[0036] The functions of each element and the reasons for choosing their dosage are as follows:

[0037] Carbon (C): Carbon is an important interstitial solid solution strengthening element that can give stainless steel sufficient strength. However, it can sensitize the heat-affected zone during the welding process. Therefore, in order to ensure that stainless steel materials have high corrosion resistance, the carbon content needs to be controlled within the range of ≤0.031%.

[0038] Chromium (Cr): The core element that gives stainless steel its "stainless" property. When its content is too high, it can form an extremely thin and dense chromium-rich oxide film (passivation film) on the surface of the steel. After carbon forms partial carbides, the chromium content in the matrix should still be maintained above the critical value for producing the passivation effect, thus providing reliable basic corrosion resistance. However, excessive chromium content will lead to excessive formation of high-temperature ferrite, affecting the material strength and low-temperature performance. Excessively low Cr content will affect the corrosion resistance of stainless steel. Therefore, the chromium content in this invention is controlled at 17.39% to 18.21%. For example, the chromium content is 17.39%, 17.47%, 17.55%, 17.63%, 17.71%, 17.79%, 17.87%, 17.95%, 18.03%, 18.11%, and 18.21%.

[0039] Nickel (Ni): Nickel is a major austenite stabilizing element, endowing stainless steel with excellent toughness, plasticity, and cold working properties. Nickel also improves the stress corrosion resistance of stainless steel in reducing acid and chloride environments. Based on the effective control of the metastable austenitic matrix in this invention, the nickel content is controlled between 7.39% and 8.21%. Exemplary examples include nickel contents of 7.39%, 7.47%, 7.55%, 7.63%, 7.71%, 7.79%, 7.87%, 7.95%, 8.03%, 8.11%, and 8.21%.

[0040] Manganese (Mn) and silicon (Si): Manganese has a certain austenite-stabilizing effect, but its ability is about half that of nickel, and its role is more important in nickel-saving stainless steels. It is also a good deoxidizer and desulfurizer, which can improve the hot working properties of steel. Silicon is mainly added as a deoxidizer, which can improve the casting properties of molten steel and also improve the high-temperature oxidation resistance of steel. The addition of excessive manganese can easily lead to component segregation in stainless steel. Therefore, the manganese content in this invention is controlled at ≤1.81% and the silicon content is controlled at ≤0.71%.

[0041] Molybdenum (Mo): Molybdenum significantly improves the resistance of stainless steel to pitting and intergranular corrosion, especially in media containing chloride ions. It makes the passivation film more stable and less susceptible to damage even in harsh environments. However, excessive Mo content significantly increases alloy costs and introduces an excessive amount of ferrite phase. Therefore, in this invention, the Mo content is controlled at 0.29%–0.51%. Exemplary Mo contents are 0.29%, 0.32%, 0.35%, 0.38%, 0.41%, 0.44%, 0.47%, 0.50%, and 0.51%.

[0042] Copper (Cu): Improves the corrosion resistance of stainless steel to certain media, especially its resistance to reducing acids such as sulfuric acid. It also improves the cold workability of stainless steel; therefore, the copper content is controlled between 0.19% and 0.61%. Examples of copper content control include 0.19%, 0.22%, 0.25%, 0.28%, 0.31%, 0.34%, 0.37%, 0.4%, 0.43%, 0.46%, 0.49%, 0.52%, 0.55%, 0.58%, and 0.61%.

[0043] Nitrogen (N): A strong austenite-forming and stabilizing element, its ability far surpasses that of nickel. Therefore, it is often used to partially replace expensive nickel to stabilize the austenitic structure and compensate for the ferrite phase formation tendency brought about by the addition of Cr and Mo. It also has a solid solution strengthening effect, significantly improving the strength of steel with minimal negative impact on corrosion resistance. Excessive addition of N may lead to excessively high austenite stability, which is detrimental to the phase transformation behavior of the alloy under low-temperature conditions. Therefore, the nitrogen content in this invention is controlled at 0.029%–0.081%. Exemplarily, the nitrogen content in this invention is controlled at 0.029%, 0.0329%, 0.0429%, 0.0529%, 0.0629%, 0.0729%, and 0.081%.

[0044] Cr, Ni, and Mn are the three most important austenite-forming and stabilizing elements. To ensure sufficient stability of austenite at room temperature, stable austenite leads to excellent plasticity in the material, and while ensuring corrosion resistance, the metastable characteristics of austenite are determined. Therefore, this invention controls the range of Cr+Ni+Mn to be 26.6% ≤ Cr+Ni+Mn ≤ 27.6%.

[0045] On the other hand, the present invention also provides a method for preparing 1000MPa grade high-strength and ductile stainless steel sheet, comprising the following steps:

[0046] (1) Smelting and casting are carried out using an electric furnace and refining method to obtain billets;

[0047] (2) Hot rolling the billet;

[0048] (3) The slab is hot rolled, and then rolled and pickled to obtain white cold-rolled raw material;

[0049] (4) The cold-rolled raw material is subjected to cold rolling for no less than two rolling passes to obtain stainless steel sheet.

[0050] Furthermore, in step (1), smelting and continuous casting include converter or electric furnace smelting, LF refining, RH / VD vacuum treatment and continuous casting processes, which ensure metallurgical quality and internal cleanliness.

[0051] Further, in step (2), the billet is hot-rolled and opened at a furnace temperature of 1200℃-1260℃. For example, the furnace temperatures are 1200℃, 1210℃, 1220℃, 1230℃, 1240℃, 1250℃, and 1260℃.

[0052] Specifically, the furnace exit temperature is 1200℃-1260℃. This temperature range allows for sufficient and uniform solid solution of alloying elements, aiming to fully dissolve carbide and nitride-forming elements such as Cr, Ni, and Mo, as well as copper (Cu), into the austenite matrix, forming a uniform supersaturated solid solution. This is the material basis for subsequent solid solution strengthening through hot and cold rolling processes, ultimately achieving high strength. If the furnace exit temperature is below 1200℃, segregation may not be completely eliminated, leading to uneven microstructure and impaired toughness and corrosion resistance. If the furnace exit temperature is above 1260℃, the austenite grains will rapidly coarsen. Coarse initial austenite grains directly lead to a decrease in the toughness and plasticity of the final product, especially making it difficult to guarantee low-temperature elongation.

[0053] Further, in step (3), the slab is hot rolled with an initial rolling temperature of ≥1150℃ and a final rolling temperature of ≥900℃. The total reduction during the hot rolling process is controlled to be no less than 50%. After that, the slab is rolled and pickled to obtain white cold-rolled raw material.

[0054] Specifically, in step (3), the slab is hot-rolled at an initial rolling temperature of ≥1150℃. This temperature is closely linked to the furnace exit temperature of 1200℃-1260℃, preventing the slab temperature from dropping excessively to a range unfavorable for rolling during the rolling process and ensuring the stability of the process window. If the initial hot rolling temperature is below 1150℃, the material's deformation resistance increases sharply, which may lead to overload of the rolling load or even edge cracks or surface cracks. At the same time, the microstructure refinement effect is poor, and the grains are coarse or uneven after hot rolling, which poses hidden dangers for subsequent processes and ultimately leads to a decrease in product strength and toughness.

[0055] Step (3) involves hot rolling the slab, controlling the final rolling temperature to above 900℃ in the single-phase austenite region to prevent ferrite precipitation or carbide (such as M) formation. 23C6) Sensitive temperature range for significant precipitation (e.g., 750-850℃). This prevents harmful phases from precipitating at grain boundaries, thus preserving the initial plasticity and corrosion resistance of the material, especially avoiding the "sensitization" tendency caused by carbide precipitation. Simultaneously, the higher final rolling temperature provides sufficient temperature margin for subsequent coiling and cooling, reducing internal stress. If the final rolling temperature is below 900℃, chromium carbides may precipitate along the austenite grain boundaries, resulting in chromium depletion near the grain boundaries and severely impairing the material's corrosion resistance, particularly its resistance to intergranular corrosion and pitting. Residual deformation structure: Complete recrystallization is difficult to occur at low temperatures, forming a mixed structure containing work hardening, which is detrimental to the uniform deformation and control of subsequent cold rolling and may also impair the plasticity of the steel.

[0056] Step (3) involves hot rolling the slab. Controlling the total hot rolling reduction to ≥50% is a decisive factor in achieving microstructure modification.

[0057] The functions of step (3) above include: (1) Large cumulative deformation can fully break the casting structure and completely eliminate dendritic segregation and coarse structure in the as-cast state. (2) Significantly refine the grains: Through repeated deformation and recrystallization, a fine and uniform austenitic grain structure is obtained, which improves the density of the structure. (3) Provide an ideal pre-structure for subsequent cold rolling: The uniform and fine hot-rolled structure is a prerequisite for subsequent precise control of cold rolling (controlling grain size and reduction), which directly affects the "meta-stability" of austenite and the uniformity of mechanical properties in the final product.

[0058] If the total hot-rolling reduction is less than 50%, the microstructure may not be sufficiently broken down and refined, potentially retaining dendritic segregation, coarse grains, or casting defects. This uneven microstructure will be directly transferred to the cold-rolling process, resulting in uneven final properties, such as large fluctuations in mechanical properties (especially yield strength and elongation). It will also lead to substandard low-temperature performance; for example, coarse or uneven microstructure will worsen low-temperature toughness and plasticity, making it difficult to achieve the high target of "elongation at liquid nitrogen temperature ≥26.5%". Pickling is used to remove the dense iron oxide scale formed during hot rolling.

[0059] Specifically, step (4) includes cold rolling the cold-rolled raw material to ensure at least two rolling passes, with intermediate annealing between each two rolling passes at a temperature of 1000℃-1200℃.

[0060] Specifically, in step (4), the single-pass cold rolling reduction must be ≥30%, and the cumulative cold rolling deformation in the final rolling pass must be 30%-40%.

[0061] Furthermore, in step (4), the average austenite grain size before final rolling is 30-50 μm.

[0062] Specifically, hot-rolled plates undergo cold rolling, ensuring at least two rolling passes. These two passes strictly control the austenite grain size before the final cold rolling pass, avoiding differences in thermal and mechanical stability caused by austenite grain size, and ensuring the stability of the high-strength, high-ductility stainless steel's room temperature and low-temperature properties. Furthermore, the multi-pass structure incorporates "intermediate annealing" after each large deformation, which is crucial for resetting the material's state and restoring its plasticity. This allows subsequent rolling passes to restart on a "soft" matrix. The single-pass cold rolling reduction must be ≥30% to provide sufficient and uniform deformation energy storage for the recrystallization process during intermediate annealing. This ensures sufficient recrystallization, uniform grain refinement, and process stability. Insufficient single-pass cold rolling reduction will result in incomplete recrystallization, coarse and uneven grains, substandard strength, and decreased low-temperature plasticity. Furthermore, this invention strictly controls the final cold rolling reduction (the cumulative cold rolling deformation in the final rolling stroke is 30%-40%). Within this deformation range, cold-rolled stainless steel can achieve high strength while still exhibiting a certain degree of work hardening during deformation, which is beneficial for the material's forming in application. This specific deformation window allows the material to achieve ultra-high room temperature strength of 1000 MPa while retaining the necessary work hardening capacity and austenitic metastability. This metastable state is precisely 'unlocked' at liquid nitrogen temperature, achieving a simultaneous leap in strength and plasticity through a significant TRIP effect. If the final cold rolling reduction is less than 30%, work hardening is insufficient, and the yield strength and tensile strength at room temperature and low temperature decrease. If the final rolling deformation is greater than 40%, excessive deformation increases energy consumption and rolling losses, but the marginal effect of the resulting strength gain diminishes, the work hardening effect of the material weakens, and the difficulty of subsequent forming processes increases.

[0063] Furthermore, this invention involves intermediate annealing between every two cold rolling passes at a temperature of 1000℃-1200℃. This achieves complete recrystallization and microstructure homogenization, completely softening the deformed microstructure from the previous rolling pass and restoring the material's extremely high plasticity, thus enabling large deformation in the next rolling pass. It also yields fine and uniform equiaxed austenite grains: through the recrystallization process, the elongated, dislocation-filled deformed grains are transformed into new, distortion-free equiaxed grains. This directly controls the microstructure, resulting in an average austenite grain size of 30-50 μm before final rolling. For example, the annealing temperatures are 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, and 1200℃.

[0064] If the annealing temperature for intermediate annealing between two cold rolling passes is not within the range of this invention, it will cause mixed crystals and precipitation (too low temperature) or coarse grains (too high temperature), ultimately affecting the matching of the material's room temperature / low temperature performance.

[0065] In summary, this invention controls the alloy composition range in the stainless steel system, effectively regulating the metastable characteristics of the austenite phase. This avoids the poor austenite stability and low corrosion resistance of 301 series stainless steel, while also avoiding the high austenite stability and insufficient low-temperature strengthening effect of 304 series stainless steel. This invention adds Mo to further improve the corrosion resistance and strength of the designed alloy. It adds N to control the potential for excessive ferrite phase after adding Mo, while also improving material strength and corrosion resistance. Adding Cu further improves the corrosion resistance of stainless steel while effectively improving its ductility and cold formability. In the high-strength stainless steel system, this invention strictly controls the austenite grain size before the final cold rolling, avoiding differences in thermal and mechanical stability caused by austenite grain size, and ensuring the stability of the high-strength, ductile stainless steel's room temperature and low-temperature properties. In the high-strength stainless steel system, this invention, in conjunction with the alloy composition and austenite grain size, strictly controls the final cold rolling reduction (30%–40%). Within this deformation range, cold-rolled stainless steel achieves high strength while still exhibiting a certain degree of work hardening during deformation, which is beneficial for material forming. As the deformation amount increases, the material strength further improves, but the work hardening process is significantly accelerated, hindering material forming. In the high-strength, high-ductility stainless steel system of this invention, through alloy composition and deformation amount control, a highly stable austenitic structure is obtained, which remains stable under low-temperature (liquid nitrogen) conditions. This austenite exhibits significant low-temperature stability during low-temperature deformation, effectively ensuring the safety of the structure during low-temperature service, and has broad application prospects in high-end fields such as aerospace and deep-sea equipment.

[0066] The advantages of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0067] Example 1

[0068] This embodiment provides a 1000MPa grade high-strength and ductile stainless steel sheet and its preparation method.

[0069] The chemical composition of the high-strength, high-ductility stainless steel sheet in this embodiment is shown in Table 1.

[0070] The preparation methods for 1000MPa grade high-strength and ductile stainless steel plates include:

[0071] Step (1): Smelting and continuous casting include converter or electric furnace smelting, LF refining, RH / VD vacuum treatment and continuous casting processes, which ensure metallurgical quality and internal cleanliness.

[0072] Step (2): The billet is hot rolled and opened, and the billet exit temperature is 1260℃.

[0073] Step (3): The hot-rolled slab after the initial rolling is hot-rolled at an initial rolling temperature of 1230℃ and a final rolling temperature of 950℃, with the total reduction controlled at 60% during the hot rolling process. The hot-rolled slab is then pickled to become a white-skinned cold-rolled raw material.

[0074] Step (4): Cold rolling, with two rolling passes and intermediate annealing between the two passes at a temperature of 1000℃. The cold rolling reduction in the first pass is 36%, and the cumulative cold rolling deformation in the second pass is 36%.

[0075] Example 2

[0076] This embodiment provides a 1000MPa grade high-strength and ductile stainless steel sheet and its preparation method.

[0077] The chemical composition of the high-strength, high-ductility stainless steel sheet in this embodiment is the same as that in Example 1.

[0078] The preparation methods for 1000MPa grade high-strength and ductile stainless steel plates include:

[0079] Step (1): Smelting and continuous casting include converter or electric furnace smelting, LF refining, RH / VD vacuum treatment and continuous casting processes, which ensure metallurgical quality and internal cleanliness.

[0080] Step (2): The billet is hot rolled and opened, and the billet exit temperature is 1200℃.

[0081] Step (3): The billet is hot rolled at an initial rolling temperature of 1150℃ and a final rolling temperature of 900℃. The total reduction during the hot rolling process is controlled to be no less than 50%. The hot-rolled plate is then pickled to make it a white cold-rolled raw material.

[0082] Step (4): Cold rolling, using two rolling passes, with intermediate annealing between the two passes at a temperature of 1200℃. The cold rolling reduction in the first pass is 30%, and the cumulative cold rolling deformation in the second pass is 35.5%.

[0083] Example 3

[0084] This embodiment provides a 1000MPa grade high-strength and ductile stainless steel sheet and its preparation method.

[0085] The chemical composition of the high-strength, high-ductility stainless steel sheet in this embodiment is shown in Table 1.

[0086] The preparation methods for 1000MPa grade high-strength and ductile stainless steel plates include:

[0087] Step (1): Smelting and continuous casting include converter or electric furnace smelting, LF refining, RH / VD vacuum treatment and continuous casting processes, which ensure metallurgical quality and internal cleanliness.

[0088] Step (2): The billet is hot rolled and opened, and the billet exit temperature is 1230℃.

[0089] Step (3): The hot-rolled slab after the billet is opened is hot-rolled at an initial rolling temperature of 1210℃ and a final rolling temperature of 920℃. The total reduction during the hot rolling process is controlled to be no less than 50%. The hot-rolled plate is pickled and then made into a white cold-rolled raw material.

[0090] Step (4): Cold rolling, using two rolling passes, with intermediate annealing performed between the second and third passes at a temperature of 1080℃. The single-pass cold rolling reduction is 34%, and the cumulative cold rolling deformation in the final rolling pass is 34%.

[0091] Example 4

[0092] This embodiment provides a 1000MPa grade high-strength and ductile stainless steel sheet and its preparation method.

[0093] The chemical composition of the high-strength, high-ductility stainless steel sheet in this embodiment is the same as that in Example 3.

[0094] The preparation methods for 1000MPa grade high-strength and ductile stainless steel plates include:

[0095] Step (1): Smelting and continuous casting include converter or electric furnace smelting, LF refining, RH / VD vacuum treatment and continuous casting processes, which ensure metallurgical quality and internal cleanliness.

[0096] Step (2): The billet is hot rolled and opened, and the billet exit temperature is 1240℃.

[0097] Step (3): The hot-rolled slab after the initial rolling is hot-rolled at an initial rolling temperature of 1160℃ and a final rolling temperature of 910℃. The total reduction during the hot rolling process is controlled to be no less than 50%. The hot-rolled plate is pickled to make it a white cold-rolled raw material.

[0098] Step (4): Cold rolling, using three rolling passes. The first pass involves a cold rolling reduction of 30%, followed by intermediate annealing at 1160℃. The second pass involves a cold rolling reduction of 30%, followed by intermediate annealing at 1100℃. The third pass results in a cumulative cold rolling deformation of 37%.

[0099] Example 5

[0100] This embodiment provides a 1000MPa grade high-strength and ductile stainless steel sheet and its preparation method.

[0101] The chemical composition of the high-strength, high-ductility stainless steel sheet in this embodiment is shown in Table 1.

[0102] The preparation methods for 1000MPa grade high-strength and ductile stainless steel plates include:

[0103] Step (1): Smelting and continuous casting include converter or electric furnace smelting, LF refining, RH / VD vacuum treatment and continuous casting processes, which ensure metallurgical quality and internal cleanliness.

[0104] Step (2): The billet is hot rolled and opened, and the billet exit temperature is 1230℃.

[0105] Step (3): The hot-rolled slab after the initial rolling is hot-rolled at an initial rolling temperature of 1185℃ and a final rolling temperature of 915℃. The total reduction during the hot rolling process is controlled to be no less than 50%. The hot-rolled slab is then pickled to make it a white cold-rolled raw material.

[0106] Step (4): Cold rolling, using two rolling passes, with intermediate annealing between the two passes at a temperature of 1060℃. The cold rolling reduction in the first pass is 35%, and the cumulative cold rolling deformation in the second pass is 34%.

[0107] Comparative Example 1

[0108] The elemental composition of the steel does not meet the requirements of this invention by mass percentage. The differences in its elemental composition are shown in Table 1. The rest is the same as in Example 1.

[0109] Comparative Example 2

[0110] The elemental composition of the steel does not meet the requirements of this invention by mass percentage. The differences in its elemental composition are shown in Table 1. The rest is the same as in Example 1.

[0111] Comparative Example 3

[0112] The elemental composition of the steel does not meet the requirements of this invention by mass percentage. The differences in its elemental composition are shown in Table 1. The rest is the same as in Example 1.

[0113] Performance testing

[0114] The mechanical properties of the above embodiments and comparative examples were tested, and the tensile tests were performed in accordance with GB / T 228.

[0115] Table 1 Chemical composition of Examples 1-5 and Comparative Examples 1-3 (wt.%, balance: Fe and unavoidable impurities)

[0116] element C Cr Ni Mn Si Mo Cu N Example 1 0.018 17.77 8.11 1 0.4 0.39 0.34 0.07 Example 2 0.028 17.87 8.3 1.1 0.43 0.47 0.37 0.053 Example 3 0.02 18.21 7.95 1.4 0.21 0.42 0.46 0.029 Example 4 0.021 18.21 7.71 1.3 0.31 0.32 0.58 0.081 Example 5 0.025 17.5 7.9 1.2 0.28 0.44 0.28 0.07 Comparative Example 1 0.05 16.5 6.5 0.90 0.20 1.0 0.06 0.070 Comparative Example 2 0.02 16.6 10.3 1.5 0.20 2 / 0.01 Comparative Example 3 0.02 18.2 8.2 1.5 0.20 / / 0.02

[0117] Table 2 Summary of performance test results for Examples 1-5 and Comparative Examples 1-3

[0118]

[0119] Table 3 Summary of Thermal History Test Results

[0120] serial number Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Comparative Example 3 Insulation time (h) 2 3 1 3 3 1 1 / Insulation temperature ℃ 300 400 500 500 500 300 300 / room temperature yield strength after insulation (MPa) 1016 1039 1016 1046 1028 942 968 / Tensile strength at room temperature after insulation (MPa) 1151 1165 1131 1153 1128 1085 866 / Elongation at room temperature after insulation (%) 15 14 14 16 15 10.5 20 /

[0121] As can be seen from Tables 1 and 2, the high-strength and ductile stainless steel sheet of the present invention exhibits excellent mechanical properties, with a room temperature yield strength ≥887MPa, a room temperature tensile strength ≥980MPa, a room temperature elongation ≥14.5%, and a pitting corrosion resistance (PREN, calculated as Cr+3.3Mo+16N) ≥20. At -196℃, the yield strength ≥985MPa, the tensile strength ≥1740MPa, and the elongation ≥26.5%. It achieves a good match between strength and ductility at both room temperature and low temperature, and also has excellent corrosion resistance, ensuring the stability of the high-strength and ductile stainless steel's performance at both room temperature and low temperature.

[0122] Combining Table 1 and Figure 1 Its microstructure (represented by Example 1, see...) Figure 1 The material is predominantly cold-deformed austenite, with an average original austenite grain size not exceeding 50 μm. Table 3 shows that the high-strength, high-ductility stainless steel sheet of this invention, after being held at 300-500℃ for 1-3 hours, exhibits a yield strength ≥1128 MPa, tensile strength ≥1016 MPa, and elongation ≥14% at room temperature. This ensures that even after service in high-temperature zones below 500℃, repeated service will not affect its performance. Deviating from the composition and process design of this invention, even with the same heat treatment, will not yield the comprehensive properties of high strength, high ductility, and good stability simultaneously.

[0123] In contrast, all comparative examples have obvious defects because the mass percentage of elemental composition does not meet the requirements of this invention and deviates from the technical solution of this invention. Specifically:

[0124] Comparative Example 1 shows that the austenite has insufficient stability, and the martensitic phase transformation is significant at room temperature and low temperature, resulting in a low elongation of the material.

[0125] Comparative Example 2 shows that the austenite has high stability and is difficult to effectively trigger the martensitic phase transformation at low temperatures, resulting in lower strength.

[0126] Comparative Example 3 has alloy main components similar to those required by this invention, but the overall PREN value is lower. The lack of alloying elements such as Mo, Cu, and N also results in lower material strength.

[0127] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A 1000MPa grade high-strength, high-ductility stainless steel sheet, characterized in that, The chemical composition, by mass percentage, includes: C≤0.031%, Cr 17.39~18.21%, Ni 7.39~8.21%, Mn≤1.81%, Si≤0.71%, Mo 0.29~0.51%, Cu 0.19~0.61%, N 0.029~0.081%, with the balance being Fe and unavoidable impurities.

2. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The chemical composition, by mass percentage, includes: C≤0.03%, Cr 17.4~18.2%, Ni 7.4~8.2%, Mn≤1.8%, Si≤0.7%, Mo 0.3~0.5%, Cu 0.2~0.6%, N 0.03~0.08%, with the balance being Fe and unavoidable impurities.

3. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The chemical composition, by mass percentage, is 26.6% ≤ Cr + Ni + Mn ≤ 27.6%.

4. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The microstructure of the hot-rolled high-strength, high-plasticity stainless steel sheet is austenite, with a ferrite volume fraction not exceeding 2%.

5. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The high-strength, high-ductility stainless steel sheet has a room temperature yield strength ≥887MPa, a room temperature tensile strength ≥980MPa, and a room temperature elongation ≥14.5%.

6. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The high-strength, high-ductility stainless steel sheet has a yield strength ≥985MPa, a tensile strength ≥1740MPa, and an elongation ≥26.5% at -196℃.

7. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The high-strength, high-plasticity stainless steel sheet, after being kept at 300-500℃ for 1-3 hours, has a yield strength ≥1128MPa and a tensile strength ≥1016MPa at room temperature.

8. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The high-strength, high-ductility stainless steel sheet has a pitting corrosion resistance PREN value of = Cr + 3.3Mo + 16N ≥ 20, where Cr, Mo, and N are the mass percentages of each element.

9. The high-strength, high-ductility stainless steel sheet according to claim 1, characterized in that, The high-strength, high-plasticity stainless steel sheet has an average austenite grain size of 30-50 μm before final rolling.

10. A method for preparing a 1000MPa grade high-strength and ductile stainless steel sheet, used to prepare the high-strength and ductile stainless steel sheet according to any one of claims 1-9, characterized in that, Includes the following steps: (1) Smelting and casting are carried out using an electric furnace and refining method to obtain billets; (2) Hot rolling the billet; (3) The slab is hot rolled, and then rolled and pickled to obtain white cold-rolled raw material; (4) The cold-rolled raw material is subjected to cold rolling for no less than two rolling passes to obtain stainless steel sheet.

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