A high-strength and high-plasticity metastable austenitic stainless steel and its preparation method

Abstract

The present invention discloses a kind of high-strength and high-plasticity metastable austenitic stainless steel and preparation method thereof, belong to metal material composition design and cold working field.A kind of high-strength and high-plasticity metastable austenitic stainless steel, by weight percentage, its chemical composition includes C:0.05%-0.3%, Si:0.2%-2%, Mn:0.2%-2%, Cr:12%-20%, Ni:5%-10%, Cu:0.2%-3%, remainder is Fe, the metastable austenitic stainless steel has the multiphase structure of stable layered martensite-austenite interphase distribution, produces the Lüdes band of continuous expansion and is accompanied by the transformation of austenite to martensite during stretching.The addition of Cu element can enhance the stacking fault energy of material, makes the austenite in matrix more stable, compared to traditional 301 stainless steel, retains more retained austenite, so as to realize ultra-high strength while also retaining considerable elongation, has both high strength and high plasticity.

Description

Technical Field

[0001] The present invention belongs to the field of metal material composition design and cold processing, and in particular relates to a high-strength and high-plasticity metastable austenitic stainless steel and a preparation method thereof. Background Art

[0002] Austenitic stainless steel, as a steel grade with excellent corrosion resistance, is widely used in various industries. Components made of austenitic stainless steel can be seen in food processing, transportation, low-temperature storage, and aerospace. Although it has good plasticity, its low strength limits its further application.

[0003] There are currently many methods to improve the strength of austenitic stainless steel. Among them, grain refinement has a significant improvement on the yield strength of the material, but its tensile strength improvement is limited, about 200MPa-300MPa. Chinese patent CN202410617970.X discloses a high-strength ultrafine-grained stainless steel. Although its elongation is increased to 25% through ultrafine grain strengthening, its maximum tensile strength is only 900MPa and its maximum yield strength is only 770MPa, which is difficult to reach the level of ultra-high strength stainless steel; Chinese patent CN201510717109.1 discloses a high-hardness, high-strength, pitting-resistant ultrafine-grained stainless steel and its preparation method. The tensile strength of its stainless steel sintered block is also only about 900MPa; grain refinement technology generally requires the use of cold deformation technology and heat treatment technology in combination to be realized, and compared with the coarse-grained state, it will also produce a certain loss in elongation. Another strengthening method involves combining cold deformation with aging to introduce fine nanoscale precipitates, significantly increasing the tensile strength and yield strength of the material. However, previous studies have shown that this strengthening method requires the addition of components that can form fine precipitates and a long aging period during the heat treatment phase, which poses significant challenges in terms of cost and processing cycle time. During the plastic deformation of mild steel or stainless steel, Lüders bands form, which can significantly increase the elongation of the stainless steel. However, as Gao S, Bai Y, Zheng R et al. ("Mechanism of huge Lüders-type deformation in ultrafine grained austenitic stainless steel"), published in Scripta Materialia (2019, 159:28-32), describe, while combining ultrafine grains with Lüders bands to improve the performance of stainless steel, the resulting tensile strength is limited to 1000-1200 MPa, making it difficult to break through the ultra-high strength level of 1500 MPa. Summary of the Invention

[0004] The purpose of the present invention is to provide a high-strength and high-plasticity metastable austenitic stainless steel and a preparation method thereof. Through the process of smelting-heat treatment-hot deformation-solution treatment-cold deformation, the stability of austenite in the phase structure is adjusted by adding Cu element during the smelting process. During the stretching process, the high-strength and high-plasticity metastable austenitic stainless steel can form an extended Lüders band accompanied by the transformation of austenite to martensite, taking into account both high strength and good plasticity.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] The present invention provides a high-strength and high-plasticity metastable austenitic stainless steel. The chemical composition thereof comprises, by weight percentage, C: 0.05%-0.3%, Si: 0.2%-2%, Mn: 0.2%-2%, Cr: 12%-20%, Ni: 5%-10%, Cu: 0.2%-3%, and the balance is Fe. The high-strength and high-plasticity metastable austenitic stainless steel has a stable multiphase structure with layered martensite and austenite interphase distribution, generates a continuously expanding Lüders band during the stretching process, and is accompanied by the transformation of austenite to martensite, thereby having both high strength and high toughness.

[0007] Furthermore, the high-strength and high-plasticity metastable austenitic stainless steel has a tensile strength of ≥1600 MPa, a yield strength of ≥1400 MPa, and an elongation of ≥15%.

[0008] Furthermore, in the phase structure of the high-strength and high-plasticity metastable austenitic stainless steel, the austenite content is 50%-80%, and the martensite content is 20%-50%.

[0009] The present invention provides a method for preparing high-strength and high-plasticity metastable austenitic stainless steel, comprising the following steps:

[0010] (1) Smelting: Smelt according to the design composition of high-strength and high-plasticity metastable austenitic stainless steel and cast into ingots after meeting the design composition requirements;

[0011] (2) Forging: heating and keeping the steel ingot warm, and then forging it into a billet;

[0012] (3) Homogenization: Under the protection of inert atmosphere, heating and heat preservation are carried out to homogenize the composition of metastable austenitic stainless steel;

[0013] (4) Hot rolling: The metastable austenitic stainless steel billet after homogenization is thinned by multiple hot rolling passes and cooled to room temperature to form a metastable austenitic stainless steel plate; the metastable austenitic stainless steel is processed into a plate shape for subsequent further processing;

[0014] (5) Solution treatment: Under the protection of inert atmosphere, the metastable austenitic stainless steel plate is heated, kept warm and cooled for solution treatment; the carbides produced during the hot working process are eliminated, thereby increasing the stability of austenite;

[0015] (6) Cold rolling: At room temperature, the material thickness is reduced after multiple passes of rolling with a constant downward pressure, and the austenite transforms into martensite, with austenite transforming from 92% to 50%-80% and martensite transforming from 8% to 20%-50%. A certain amount of strain is introduced to induce martensite, increasing the yield and tensile strength of the material.

[0016] Furthermore, the equipment used for smelting in step (1) is an argon oxygen decarburization furnace and a ladle refining furnace; the Cu element in the designed composition can improve the stability of austenite to a limited extent, and introduce appropriate strain-induced martensite in the subsequent cold rolling process, and the austenite can be continuously transformed into strain-induced martensite during the stretching process.

[0017] Furthermore, in step (2), the heating temperature is 1100°C-1300°C, the holding time is 1h-2h, the initial forging temperature is 1180°C, and the final forging temperature is 1000°C.

[0018] Furthermore, the heating temperature of the homogenization treatment in step (3) is 1100° C.-1300° C., and the holding time is 1 h-3 h.

[0019] Furthermore, in step (4), the hot rolling temperature is 950°C-1300°C, the thickness reduced by hot rolling is 70%-85%, and the cooling method is water cooling.

[0020] Furthermore, in step (5), the heating temperature is 1050°C-1200°C, the holding time is 1h-3h, and the cooling method is water cooling.

[0021] Furthermore, in step (6), the cold rolling is performed by pressing down 0.1 mm in a single pass and rolling multiple passes until the thinning thickness is 30%-70%.

[0022] The advantages and beneficial effects of the present invention are:

[0023] The present invention designs a high-strength and high-toughness metastable austenitic stainless steel by adding 0.2wt.%-3wt.% of Cu to the traditional austenitic stainless steel, and undergoes smelting-heat treatment-hot deformation-solutionization-cold deformation. The addition of Cu element can improve the stacking fault energy of the material, thereby making the austenite in the material matrix more stable. Through the design of the composition, the stacking fault energy of the austenite in the material is increased but less than or equal to 12mJ / m 2, thus ensuring that a certain amount of strain martensite can still be introduced into the material during the cold deformation process. After cold deformation, the content of retained austenite is higher, which can significantly improve the plasticity of the material. After cold deformation, the material presents a stable multiphase structure of layered martensite-austenite interphase distribution, with an austenite content of 50%-80% and a martensite content of 20%-50% in the phase composition, and a smooth surface.

[0024] Unlike existing stainless steel, the high-strength and high-plasticity metastable austenitic stainless steel of the present invention exhibits a huge Lüders deformation after tensile testing, significantly improving the material's ductility. This is accompanied by the transformation of austenite to martensite, thus enabling the metastable austenitic stainless steel of the present invention to have both high strength and high plasticity.

[0025] The present invention changes the composition of conventional austenitic stainless steel and adds the Cu element, which changes the mechanical properties of the material. Since Cu exists in the material in the form of replacement atoms, the yield strength of the stainless steel plate in the solid solution state of the present invention can be increased by 50MPa, and the total elongation can be increased to more than 40%;

[0026] The metastable austenitic stainless steel prepared by the present invention retains more retained austenite after cold rolling compared to conventional 301 stainless steel, thereby achieving ultra-high strength while also retaining considerable elongation.

[0027] The high-strength and high-plasticity metastable austenitic stainless steel prepared by the present invention has a yield strength of ≥1400 MPa, a tensile strength of ≥1600 MPa, and a total elongation of ≥15%, achieving a good match between strength and plasticity. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a diagram of the mechanical properties of the high-strength and high-plasticity metastable austenitic stainless steel of Example 1;

[0029] Figure 2 The tensile strain distribution diagram of the high-strength and high-ductility metastable austenitic stainless steel of Example 1, where: (a) is the beginning of the Lüders band expansion during the strain process, and (b) is the end of the Lüders band expansion during the strain process;

[0030] Figure 3 This is a mechanical properties diagram of the high-strength metastable austenitic stainless steel of comparative example 1;

[0031] Figure 4 This is the tensile strain distribution diagram of the high-strength metastable austenitic stainless steel of comparative example 1;

[0032] Figure 5 This is a mechanical properties diagram of high-strength metastable austenitic stainless steel of comparative example 2;

[0033] Figure 6 This is the tensile strain distribution diagram of the high-strength metastable austenitic stainless steel in comparative example 2. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0035] Example 1

[0036] The present invention discloses a high-strength and high-plasticity metastable austenitic stainless steel, whose chemical composition, by weight percentage, includes: C: 0.11%, Cr: 16.48%, Mn: 0.73%, Si: 0.69%, Ni: 6.68%, Cu: 1%, and the balance is Fe. The metastable austenitic stainless steel has a multiphase structure with layered martensite and austenite interphase distribution, has an extended Lüders band during the tensile process, and has both high strength and high toughness. The austenite content in the phase structure is 62%, the martensite content is 38%, the yield strength is 1587 MPa, the tensile strength is 1655 MPa, and the total elongation is 22%.

[0037] A method for preparing high-strength and high-plasticity metastable austenitic stainless steel comprises the following steps:

[0038] (1) Smelting: According to the designed composition: C: 0.11wt.%, Cr: 16.48wt.%, Mn: 0.73wt.%, Si: 0.69wt.%, Ni: 6.68wt.%, Cu: 1wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0039] (2) Forging: The steel ingot is kept at 1200℃ for 2h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0040] (3) Homogenization: The forged sample was heated to 1200 °C under an argon protective atmosphere and kept at this temperature for 2 h;

[0041] (4) Hot rolling: The homogenized steel billet is rolled at 1200℃, and the final rolling temperature is maintained at ≥950℃. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0042] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1100 °C for 1 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 260 MPa, the tensile strength was 966 MPa, the total elongation was 46%, and the untransformed austenite in the fractured sample was 12%, indicating the presence of stable austenite.

[0043] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 50%, the austenite is transformed from 92% to 62%, and the martensite is transformed from 8% to 38%, thereby obtaining high-strength and high-plasticity metastable austenitic stainless steel.

[0044] like Figure 1 As shown, the high-strength and high-plasticity metastable austenitic stainless steel of this embodiment was subjected to mechanical property tests, and the yield strength was 1587 MPa, the tensile strength was 1655 MPa, and the total elongation was 22%; Figure 2 As shown, during the strain process, the high-strength and high-ductility metastable austenitic stainless steel of this embodiment produces an extended Lüders band.

[0045] Example 2

[0046] The present invention discloses a high-strength and high-plasticity metastable austenitic stainless steel, whose chemical composition, measured by weight percentage, includes: C: 0.3%, Cr: 20%, Mn: 2%, Si: 2%, Ni: 10%, Cu: 3%, and the balance is Fe. The metastable austenitic stainless steel has a multiphase structure with layered martensite and austenite interphase distribution, has an extended Lüders band during the tensile process, and has both high strength and high toughness. The austenite content in the phase structure is 80%, the martensite content is 20%, the yield strength is 1400 MPa, the tensile strength is 1600 MPa, and the total elongation is 20%.

[0047] A method for preparing high-strength and high-plasticity metastable austenitic stainless steel comprises the following steps:

[0048] (1) Smelting: According to the designed composition: C: 0.3wt.%, Cr: 20wt.%, Mn: 2wt.%, Si: 2wt.%, Ni: 10wt.%, Cu: 3wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0049] (2) Forging: The steel ingot is kept at 1100℃ for 2h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0050] (3) Homogenization: The forged sample was heated to 1100 °C under an argon protective atmosphere and kept at this temperature for 3 h;

[0051] (4) Hot rolling: The homogenized steel billet is rolled at 1100°C, and the final rolling temperature is maintained at ≥950°C. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0052] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1200 °C for 2 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 260 MPa, the tensile strength was 996 MPa, the total elongation was 48%, and the untransformed austenite in the fractured sample was 18%, indicating the presence of stable austenite.

[0053] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 70%, the austenite is transformed from 92% to 80%, and the martensite is transformed from 8% to 20%, thereby obtaining high-strength and high-plasticity metastable austenitic stainless steel.

[0054] The high-strength and high-plasticity metastable austenitic stainless steel of this embodiment was subjected to mechanical property testing, and the yield strength was 1400 MPa, the tensile strength was 1600 MPa, and the total elongation was 20%; during the strain process, the high-strength and high-plasticity metastable austenitic stainless steel of this embodiment produced an extended Lüders band.

[0055] Example 3

[0056] The present invention discloses a high-strength and high-plasticity metastable austenitic stainless steel, whose chemical composition, measured by weight percentage, includes: C: 0.05%, Cr: 12%, Mn: 0.2%, Si: 0.2%, Ni: 5%, Cu: 0.2%, and the balance is Fe. The metastable austenitic stainless steel has a multiphase structure in which layered martensite and austenite are distributed interphase, has an extended Lüders band during the tensile process, and has both high strength and high toughness. The austenite content in the phase structure is 50%, the martensite content is 50%, the yield strength is 1530 MPa, the tensile strength is 1700 MPa, and the total elongation is 15%.

[0057] A method for preparing high-strength and high-plasticity metastable austenitic stainless steel comprises the following steps:

[0058] (1) Smelting: According to the designed composition: C: 0.05wt.%, Cr: 12wt.%, Mn: 0.2wt.%, Si: 0.2wt.%, Ni: 5wt.%, Cu: 0.2wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0059] (2) Forging: The steel ingot is kept at 1300℃ for 2h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0060] (3) Homogenization: The forged sample was heated to 1300 °C under an argon protective atmosphere and kept at this temperature for 2 h;

[0061] (4) Hot rolling: The homogenized steel billet is rolled at 1300℃, and the final rolling temperature is maintained at ≥950℃. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0062] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1100 °C for 3 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 240 MPa, the tensile strength was 989 MPa, the total elongation was 48%, and the untransformed austenite in the fractured sample was 7%, indicating the presence of stable austenite.

[0063] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 70%, the austenite is transformed from 92% to 50%, and the martensite is transformed from 8% to 50%, thereby obtaining high-strength and high-plasticity metastable austenitic stainless steel.

[0064] The high-strength and high-plasticity metastable austenitic stainless steel of this embodiment was subjected to mechanical property testing, and the yield strength was 1530 MPa, the tensile strength was 1700 MPa, and the total elongation was 15%; during the strain process, the high-strength and high-plasticity metastable austenitic stainless steel of this embodiment produced an extended Lüders band.

[0065] Example 4

[0066] The present invention discloses a high-strength and high-plasticity metastable austenitic stainless steel, whose chemical composition, by weight percentage, includes: C: 0.15%, Cr: 17%, Mn: 1.1%, Si: 1.3%, Ni: 8.0%, Cu: 1.7%, and the balance is Fe. The metastable austenitic stainless steel has a multiphase structure with layered martensite and austenite interphase distribution, has an extended Lüders band during the tensile process, and has both high strength and high toughness. The austenite content in the phase structure is 69%, the martensite content is 31%, the yield strength is 1470 MPa, the tensile strength is 1620 MPa, and the total elongation is 17%.

[0067] A method for preparing high-strength and high-plasticity metastable austenitic stainless steel comprises the following steps:

[0068] (1) Smelting: According to the designed composition: C: 0.15wt.%, Cr: 17wt.%, Mn: 1.1wt.%, Si: 1.3wt.%, Ni: 8.0wt.%, Cu: 1.7wt.%, the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0069] (2) Forging: The steel ingot is kept at 1300℃ for 1.5h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0070] (3) Homogenization: The forged sample was heated to 1300 °C under an argon protective atmosphere and kept at this temperature for 1 h;

[0071] (4) Hot rolling: The homogenized steel billet is rolled at 1300℃, and the final rolling temperature is maintained at ≥950℃. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0072] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1050 °C for 3 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 267 MPa, the tensile strength was 980 MPa, the total elongation was 40%, and the untransformed austenite in the fractured sample was 20%, indicating the presence of stable austenite.

[0073] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 30%, the austenite is transformed from 92% to 69%, and the martensite is transformed from 8% to 31%, thereby obtaining high-strength and high-plasticity metastable austenitic stainless steel.

[0074] The high-strength, high-plasticity metastable austenitic stainless steel of this embodiment was subjected to mechanical property testing, and the yield strength was 1470 MPa, the tensile strength was 1620 MPa, and the total elongation was 17%. During the strain process, the high-strength, high-plasticity metastable austenitic stainless steel of this embodiment produced an extended Lüders band.

[0075] Example 5

[0076] The present invention discloses a high-strength and high-plasticity metastable austenitic stainless steel, whose chemical composition, by weight percentage, includes: C: 0.2%, Cr: 19%, Mn: 1.7%, Si: 1.5%, Ni: 6.0%, Cu: 2%, and the balance is Fe. The metastable austenitic stainless steel has a multiphase structure with layered martensite and austenite interphase distribution, has an extended Lüders band during the tensile process, and has both high strength and high toughness. The austenite content in the phase structure is 70%, the martensite content is 30%, the yield strength is 1480 MPa, the tensile strength is 1630 MPa, and the total elongation is 16%.

[0077] A method for preparing high-strength and high-plasticity metastable austenitic stainless steel comprises the following steps:

[0078] (1) Smelting: According to the designed composition: C: 0.2wt.%, Cr: 19wt.%, Mn: 1.7wt.%, Si: 1.5wt.%, Ni: 6.0wt.%, Cu: 2wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0079] (2) Forging: The steel ingot is kept at 1300℃ for 1h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0080] (3) Homogenization: The forged sample was heated to 1300 °C under an argon protective atmosphere and kept at this temperature for 1 h;

[0081] (4) Hot rolling: The homogenized steel billet is rolled at 1300℃, and the final rolling temperature is maintained at ≥950℃. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0082] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1100 °C for 3 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 270 MPa, the tensile strength was 970 MPa, the total elongation was 41%, and the untransformed austenite in the fractured sample was 14%, indicating the presence of stable austenite.

[0083] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 30%, the austenite is transformed from 92% to 70%, and the martensite is transformed from 8% to 30%, thereby obtaining high-strength and high-plasticity metastable austenitic stainless steel.

[0084] The high-strength and high-plasticity metastable austenitic stainless steel of this embodiment was subjected to mechanical property testing, and the yield strength was 1480 MPa, the tensile strength was 1630 MPa, and the total elongation was 16%; during the strain process, the high-strength and high-plasticity metastable austenitic stainless steel of this embodiment produced an extended Lüders band.

[0085] Example 6

[0086] The present invention discloses a high-strength and high-plasticity metastable austenitic stainless steel, whose chemical composition, by weight percentage, includes: C: 0.23%, Cr: 18.5%, Mn: 0.5%, Si: 0.6%, Ni: 6.3%, Cu: 1%, and the balance is Fe. The metastable austenitic stainless steel has a multiphase structure with layered martensite and austenite interphase distribution, has an extended Lüders band during the tensile process, and has both high strength and high toughness. The austenite content in the phase structure is 65%, the martensite content is 35%, the yield strength is 1560 MPa, the tensile strength is 1670 MPa, and the total elongation is 18%.

[0087] A method for preparing high-strength and high-plasticity metastable austenitic stainless steel comprises the following steps:

[0088] (1) Smelting: According to the designed composition: C: 0.23wt.%, Cr: 18.5wt.%, Mn: 0.5wt.%, Si: 0.6wt.%, Ni: 6.3wt.%, Cu: 1wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0089] (2) Forging: The steel ingot is kept at 1300℃ for 1h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0090] (3) Homogenization: The forged sample was heated to 1300 °C under an argon protective atmosphere and kept at this temperature for 1 h;

[0091] (4) Hot rolling: The homogenized steel billet is rolled at 1200℃, and the final rolling temperature is maintained at ≥950℃. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0092] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1100 °C for 1 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 267 MPa, the tensile strength was 1015 MPa, the total elongation was 45%, and the untransformed austenite in the fractured sample was 17%, indicating the presence of stable austenite.

[0093] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 45%, the austenite is transformed from 92% to 65%, and the martensite is transformed from 8% to 35%, thereby obtaining high-strength and high-plasticity metastable austenitic stainless steel.

[0094] The high-strength, high-plasticity metastable austenitic stainless steel of this embodiment was subjected to mechanical property testing, and the yield strength was 1560 MPa, the tensile strength was 1670 MPa, and the total elongation was 18%. During the strain process, the high-strength, high-plasticity metastable austenitic stainless steel of this embodiment produced an extended Lüders band.

[0095] Example 7

[0096] The present invention discloses a high-strength and high-plasticity metastable austenitic stainless steel, whose chemical composition, by weight percentage, includes: C: 0.25%, Cr: 18.5%, Mn: 1%, Si: 1%, Ni: 6.5%, Cu: 1%, and the balance is Fe. The metastable austenitic stainless steel has a multiphase structure with layered martensite and austenite interphase distribution, has an extended Lüders band during the tensile process, and has both high strength and high toughness. The austenite content in the phase structure is 66%, the martensite content is 34%, the yield strength is 1530 MPa, the tensile strength is 1610 MPa, and the total elongation is 16.3%.

[0097] A method for preparing high-strength and high-plasticity metastable austenitic stainless steel comprises the following steps:

[0098] (1) Smelting: According to the designed composition: C: 0.25wt.%, Cr: 18.5wt.%, Mn: 1wt.%, Si: 1wt.%, Ni: 6.5wt.%, Cu: 1wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0099] (2) Forging: The steel ingot is kept at 1300℃ for 1h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0100] (3) Homogenization: The forged sample was heated to 1250 °C under an argon protective atmosphere and kept at this temperature for 1 h;

[0101] (4) Hot rolling: The homogenized steel billet is rolled at 1200°C, and the final rolling temperature is maintained at ≥950°C. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0102] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1100 °C for 1 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 253 MPa, the tensile strength was 1010 MPa, the total elongation was 45%, and the untransformed austenite in the fractured sample was 13%, indicating the presence of stable austenite.

[0103] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 45%, the austenite is transformed from 92% to 66%, and the martensite is transformed from 8% to 34%, thereby obtaining high-strength and high-plasticity metastable austenitic stainless steel.

[0104] The high-strength, high-plasticity metastable austenitic stainless steel of this embodiment was subjected to mechanical property testing, and the yield strength was 1530 MPa, the tensile strength was 1610 MPa, and the total elongation was 16.3%. During the strain process, the high-strength, high-plasticity metastable austenitic stainless steel of this embodiment produced an extended Lüders band.

[0105] Comparative Example 1

[0106] A high-strength metastable austenitic stainless steel, which differs from Example 1 in that no Cu element is added, and has the following chemical composition: C: 0.11 wt.%, Cr: 16.48 wt.%, Mn: 0.73 wt.%, Si: 0.69 wt.%, Ni: 6.68 wt.%, and the balance is Fe. The metastable austenitic stainless steel does not produce a multiphase structure with layered martensite and austenite interphase distribution, and no Lüders band expands during the tensile process. The strength is extremely high but the plasticity is extremely poor. The austenite content in the phase structure is 8%, the martensite content is 92%, the yield strength is 1939 MPa, the tensile strength is 2028 MPa, and the total elongation is 2.76%.

[0107] A method for preparing high-strength metastable austenitic stainless steel comprises the following steps:

[0108] (1) Smelting: According to the designed composition: C: 0.11wt.%, Cr: 16.48wt.%, Mn: 0.73wt.%, Si: 0.69wt.%, Ni: 6.68wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0109] (2) Forging: The steel ingot is kept at 1200℃ for 2h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0110] (3) Homogenization: The forged sample was heated to 1200 °C under an argon protective atmosphere and kept at this temperature for 2 h;

[0111] (4) Hot rolling: The homogenized steel billet is rolled at 1200℃, and the final rolling temperature is maintained at ≥950℃. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0112] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1100 °C for 1 h and then cooled to room temperature. The mechanical properties of the solution treated plate were tested. The yield strength was 210 MPa, the tensile strength was 1300 MPa, and the total elongation was 37%. There was no residual austenite in the sample after fracture, and the austenite was completely converted into martensite.

[0113] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 50%. The austenite content in the phase structure is 8% and the martensite content is 92%, thereby obtaining high-strength metastable austenitic stainless steel.

[0114] like Figure 3 As shown in FIG, the mechanical properties of the comparative high-strength metastable austenitic stainless steel were tested, and the yield strength was 1939 MPa, the tensile strength was 2028 MPa, and the total elongation was 2.67%; Figure 4 As shown in the figure, during the strain process, the high-strength metastable austenitic stainless steel in this comparative example did not show the expansion of the Lüders band. This indicates that the lack of Cu element will lead to extremely poor austenite stability and easy martensitic transformation, thus only high-strength and low-plasticity stainless steel products can be obtained, and high strength and high plasticity cannot be taken into account at the same time.

[0115] Comparative Example 2

[0116] A high-strength metastable austenitic stainless steel, which differs from Example 1 in that the Cu element is replaced by the N element, and has the following chemical composition: C: 0.11wt.%, Cr: 16.48wt.%, Mn: 0.73wt.%, Si: 0.69wt.%, Ni: 6.68wt.%, N: 0.15wt.%, and the balance is Fe. Although the metastable austenitic stainless steel produces a multiphase structure with layered martensite and austenite phase distribution, there is no extended Lüders band during the tensile process, and the strength-plasticity matching is poor. The austenite content in the phase structure is 68%, the martensite content is 32%, the yield strength is 1537MPa, the tensile strength is 1655MPa, and the total elongation is 7.3%.

[0117] A method for preparing high-strength metastable austenitic stainless steel comprises the following steps:

[0118] (1) Smelting: According to the designed composition: C: 0.11wt.%, Cr: 16.48wt.%, Mn: 0.73wt.%, Si: 0.69wt.%, Ni: 6.68wt.%, N: 0.15wt.%, and the balance is Fe, smelting is carried out in an argon oxygen decarburization furnace + a ladle refining furnace, and casting into steel ingots after meeting the above composition requirements;

[0119] (2) Forging: The steel ingot is kept at 1200℃ for 2h, the initial forging temperature is 1180℃, and the final forging temperature is 1000℃ to obtain a metastable austenitic stainless steel billet;

[0120] (3) Homogenization: The forged sample was heated to 1200 °C under an argon protective atmosphere and kept at this temperature for 2 h;

[0121] (4) Hot rolling: The homogenized steel billet is rolled at 1200℃, and the final rolling temperature is maintained at ≥950℃. After multiple rolling, the thickness is reduced by 75%, and then water-cooled to room temperature to obtain a metastable austenitic stainless steel plate;

[0122] (5) Solution treatment: The metastable austenitic stainless steel plate was solution treated at 1100 °C for 1 h and then cooled to room temperature. The mechanical properties of the plate after solution treatment were tested. The yield strength was 252 MPa, the tensile strength was 966 MPa, the total elongation was 77%, and the untransformed austenite in the fractured sample was 23%, indicating that the austenite was more stable.

[0123] (6) Cold rolling: The solution treated plate is rolled multiple times with a single pass pressing down 0.1 mm until the thickness is reduced by 50%. The austenite content in the phase structure is 68% and the martensite content is 32%, thereby obtaining high-strength metastable austenitic stainless steel.

[0124] like Figure 5 As shown in FIG, the mechanical properties of the comparative high-strength metastable austenitic stainless steel were tested, and the yield strength was 1537 MPa, the tensile strength was 1655 MPa, and the total elongation was 7.3%; Figure 6 As shown in the figure, during the strain process, the high-strength metastable austenitic stainless steel in this comparative example did not show any expansion of the Lüders band. This indicates that although the addition of nitrogen greatly improves the stability of austenite, the extremely high austenite stability will make it difficult for the Lüders band to propagate and expand during the tensile process of the material, and necking will occur in the middle of the Lüders band, leading to fracture. The strength and plasticity are poorly matched, and high strength and high plasticity cannot be taken into account at the same time.

[0125] Performance Analysis:

[0126] The high-strength and high-plasticity metastable austenitic stainless steel of the present invention has a stable multiphase structure with layered martensite and austenite interphase distribution. During the stretching process, a continuously expanding Lüders band is generated accompanied by the transformation of austenite to martensite. Therefore, it has both high strength and high toughness. The strength and plasticity comparison of the high-strength and high-plasticity metastable austenitic stainless steel of Example 1 and the existing metastable austenitic stainless steel are shown in the following table:

[0127] type Tensile strength (MPa) Elongation (%) Example 1 High-strength and high-plasticity metastable austenitic stainless steel 1655 22 301-60CR 1760 3 16Cr6Mn-70%CR 1428 5.1 301-80%CR 1600 8 304-CR 1480 10 301-CR 1500 12 301-UFG 1008 37 316-HT 910 30 301-30%CR 1305 33 304-UFG 1056 34 304-FG 1148 40 301-ECAP 1200 41

[0128] As can be seen from the above table, the existing metastable austenitic stainless steel has a very poor match between strength and plasticity. Combined with the comparative example, it can be seen that the high-strength and high-plasticity metastable austenitic stainless steel of the present invention contains copper elements and can form an extended Luders band during the stretching process. Therefore, it has a good match between strength and plasticity, and has both high strength and high plasticity.

Claims

1. A high-strength and high-plasticity metastable austenitic stainless steel, characterized in that: The chemical composition, by weight percentage, includes C: 0.05%-0.3%, Si: 0.2%-2%, Mn: 0.2%-2%, Cr: 12%-20%, Ni: 5%-10%, Cu: 0.2%-3%, and the balance is Fe. The high-strength and high-plasticity metastable austenitic stainless steel has a stable multiphase structure with layered martensite and austenite interphase distribution, and produces a continuously expanding Lüders band during the stretching process, accompanied by the transformation of austenite to martensite. The high-strength and high-plasticity metastable austenitic stainless steel has a tensile strength of ≥1600 MPa, a yield strength of ≥1400 MPa, and an elongation of ≥15%; In the phase structure of the high-strength and high-plasticity metastable austenitic stainless steel, the austenite content is 50%-80%, and the martensite content is 20%-50%.

2. A method for preparing high-strength and high-plasticity metastable austenitic stainless steel according to claim 1, characterized in that: The following steps are involved: (1) Smelting: Smelt according to the design composition of high-strength and high-plasticity metastable austenitic stainless steel and cast into ingots after meeting the design composition requirements; (2) Forging: heating and keeping the steel ingot warm, and then forging it into a billet; (3) Homogenization: Heating and heat preservation for homogenization under inert atmosphere protection; (4) Hot rolling: The metastable austenitic stainless steel billet after homogenization is thinned by multiple hot rolling passes and cooled to room temperature to form a metastable austenitic stainless steel sheet; (5) Solution treatment: Under the protection of inert atmosphere, the metastable austenitic stainless steel plate is subjected to solution treatment by heating, holding and cooling; (6) Cold rolling: At room temperature, with a constant downward pressure, a single pass is pressed down by 0.1 mm and rolled multiple times until the thickness is thinned to 30%-70%. Austenite transforms to martensite, austenite transforms from 92% to 50%-80%, and martensite transforms from 8% to 20%-50%.

3. The method for preparing high-strength and high-plasticity metastable austenitic stainless steel according to claim 2, characterized in that: The equipment used for smelting in step (1) is an argon oxygen decarburization furnace and a ladle refining furnace.

4. The method for preparing high-strength and high-plasticity metastable austenitic stainless steel according to claim 2, wherein: In step (2), the heating temperature is 1100°C-1300°C, the holding time is 1h-2h, the initial forging temperature is 1180°C, and the final forging temperature is 1000°C.

5. The method for preparing high-strength and high-plasticity metastable austenitic stainless steel according to claim 2, characterized in that: The heating temperature of the homogenization treatment in step (3) is 1100°C-1300°C, and the holding time is 1h-3h.

6. The method for preparing high-strength and high-plasticity metastable austenitic stainless steel according to claim 2, characterized in that: In step (4), the hot rolling temperature is 950°C-1300°C, the thickness reduced by hot rolling is 70%-85%, and the cooling method is water cooling.

7. The method for preparing high-strength and high-plasticity metastable austenitic stainless steel according to claim 2, characterized in that: In step (5), the heating temperature is 1050°C-1200°C, the holding time is 1h-3h, and the cooling method is water cooling.

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