A metastable austenitic stainless steel and a method for producing and using the same

Abstract

The application provides a metastable austenitic stainless steel and a preparation method and application thereof. The metastable austenitic stainless steel comprises the following mass percentage of element components: C 0.05-0.15%, Si 0.3-0.7%, Mn 6.0-9.0%, Cr 16.5-19.0%, Ni 1.0-3.0% and N 0.20-0.30%, and the balance is Fe; wherein, 0<8Ni-Mn<30, and the element symbols in the formula represent the mass percentage of the element multiplied by 100. The metastable austenitic stainless steel provided by the application has high strength in a solid solution annealing state and a cold work hardening state, and the content of deformation-induced martensite is low when the high strength is reached in the cold work hardening state; the metastable austenitic stainless steel can reach different strength levels through one composition and different processing modes, and meets the performance requirements of various automobile parts.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel technology, specifically relating to a metastable austenitic stainless steel, its preparation method, and its application. Background Technology

[0002] Automotive exterior parts and deep-drawn components often use 300 series austenitic stainless steel, such as 301 and 304, which offer good corrosion resistance and formability. While 300 series austenitic stainless steel possesses excellent corrosion resistance due to the presence of expensive alloying elements like Ni and Mo, its high and fluctuating price makes it difficult to use extensively in automobiles. The more economical 400 series ferritic stainless steel, however, struggles to meet the complex forming and strength requirements.

[0003] Mo- and Ni-reducing austenitic stainless steels offer good corrosion resistance while maintaining a certain level of strength and formability, making them widely used in the automotive industry. These Mo- and Ni-reducing austenitic stainless steels are typical metastable austenitic stainless steels. After solution treatment, their main microstructure at room temperature is single-phase austenite, while cold deformation treatment generates a large amount of deformation-induced martensite, increasing strength. However, current metastable austenitic stainless steels exhibit a high content of deformation-induced martensite when reaching high strength, leading to higher residual stress and a greater risk of delayed cracking, necessitating further improvements. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a metastable austenitic stainless steel, its preparation method, and its applications. This metastable austenitic stainless steel can achieve different strength levels through different processing methods, realizing the normalization of automotive stainless steel. Furthermore, after cold work hardening, this metastable austenitic stainless steel can achieve high strength with a relatively low deformation-induced martensite content.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides a metastable austenitic stainless steel, wherein the metastable austenitic stainless steel comprises the following elemental composition by mass percentage:

[0007] C 0.05-0.15%, Si 0.3-0.7%, Mn 6.0-9.0%, Cr 16.5-19.0%, Ni 1.0-3.0% and N 0.20-0.30%, balance Fe;

[0008] Where 0 < 8Ni - Mn < 30, the element symbol in this formula represents the mass percentage of the element × 100.

[0009] In this invention, the mass percentage of C can be 0.05%, 0.055%, 0.06%, 0.065%, 0.07%, 0.075%, 0.08%, 0.085%, 0.09%, 0.095%, 0.1%, 0.105%, 0.11%, 0.115%, 0.12%, 0.125%, 0.13%, 0.135%, 0.14%, 0.145%, or 0.15%, etc.

[0010] The mass percentage of Si can be 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, or 0.7%, etc.

[0011] The mass percentage of Mn can be 6.0%, 6.2%, 6.3%, 6.5%, 6.6%, 6.8%, 7%, 7.2%, 7.3%, 7.5%, 7.6%, 7.8%, 8%, 8.2%, 8.3%, 8.5%, 8.6%, 8.8%, or 9%, etc.

[0012] The mass percentage of Cr can be 16.5%, 16.8%, 17%, 17.2%, 17.5%, 17.8%, 18%, 18.2%, 18.5%, 18.8%, or 19%, etc.

[0013] The mass percentage of Ni can be 1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.8%, 2%, 2.2%, 2.3%, 2.5%, 2.6%, 2.8%, or 3%, etc.

[0014] The mass percentage of N can be 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, or 0.3%, etc.

[0015] Ni and Mn are both austenite-forming elements. Mn can lower the critical quenching rate of stainless steel, increase the stability of austenite during cooling, inhibit the decomposition of austenite, and ensure that the austenite formed at high temperatures is stably maintained at room temperature. However, higher levels of Mn will reduce the corrosion resistance of stainless steel.

[0016] Nitrogen (N) is a strong austenite-forming element that can significantly improve the strength of stainless steel and enhance its corrosion resistance.

[0017] Cr is beneficial for improving the corrosion resistance of stainless steel.

[0018] Controlling 0 < 8Ni - Mn < 30 is beneficial to improving the corrosion resistance and cold work hardening strength of stainless steel.

[0019] This invention, through the specific proportions of the aforementioned elements, yields a metastable austenitic stainless steel. This stainless steel can achieve different strength levels through various processing methods, thus normalizing the performance of automotive stainless steel. After cold working, this metastable austenitic stainless steel can achieve high strength with a relatively low deformation-induced martensite content.

[0020] In some embodiments of the present invention, the metastable austenitic stainless steel further includes one or more of the following elemental components in mass percentage: Mo 0.01-2.5%, Cu 0.01-3.5%, V 0-0.2%.

[0021] In this invention, the mass percentage of Mo can be 0.01%, 0.03%, 0.05%, 0.08%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, or 2.5%, etc.

[0022] The mass percentage of Cu can be 0.01%, 0.03%, 0.05%, 0.08%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3%, 3.3%, or 3.5%, etc.

[0023] The mass percentage of V can be 0%, 0.02%, 0.03%, 0.05%, 0.06%, 0.08%, 0.1%, 0.12%, 0.13%, 0.15%, 0.16%, 0.18%, or 0.2%, etc.

[0024] Mo can make the passivation film on the surface of stainless steel denser, thereby improving the stainless steel's resistance to chloride ion corrosion.

[0025] Cu can improve the deep-drawing performance of stainless steel, and can also play a role in precipitation strengthening. After special treatment, it can also play a bactericidal role.

[0026] In addition, the content of P and S should be controlled in this invention, and the lower the content, the better, so as to prevent the material toughness from being reduced.

[0027] In some embodiments of the present invention, the martensitic transformation temperature Md30 of the metastable austenitic stainless steel at 30% cold deformation to form 50% martensite satisfies -16.5℃≤Md30≤40℃; Md30 can be, for example, -16.5℃, -16℃, -15℃, -13℃, -10℃, -8℃, -5℃, -2℃, 0℃, 2℃, 5℃, 8℃, 10℃, 12℃, 15℃, 18℃, 20℃, 22℃, 25℃, 28℃, 30℃, 32℃, 35℃, 38℃ or 40℃, etc.

[0028] Wherein, Md30=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo, and the element symbols in this formula represent the mass percentage of the element ×100.

[0029] In this invention, by controlling Md30 within the above-mentioned range, it is beneficial to the cold rolling and forming of stainless steel coils, and at the same time, it is beneficial to the formation of deformation-induced martensite under different cold work hardening states, thereby obtaining metastable austenitic stainless steels with different strength levels.

[0030] In some embodiments of the present invention, the pitting corrosion resistance equivalent value PREN of the metastable austenitic stainless steel is >15.0; for example, it can be 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30, etc.

[0031] In the formula PREN = Cr + 3.3Mo - Mn + 30N, the element symbol represents the mass percentage of that element × 100.

[0032] In this invention, by controlling PREN > 15.0, it is beneficial to improve the corrosion resistance of stainless steel, so that it meets the requirement of no red rust after at least 720 hours of neutral salt spray test.

[0033] In some embodiments of the present invention, the metastable austenitic stainless steel is in a solution-annealed state, and the microstructure of the metastable austenitic stainless steel in the solution-annealed state includes: more than 95 vol% austenite, 0.01-5 vol% δ ferrite and less than 0.5 vol% carbonitrides; wherein the grain size of the austenite is ≤25 μm.

[0034] In some embodiments of the present invention, the metastable austenitic stainless steel is in a cold-worked state, and the microstructure of the metastable austenitic stainless steel in the cold-worked state with a reduction rate of 30% comprises: 59.5-90 vol% austenite, 10-40 vol% martensite, and less than 0.5 vol% carbonitrides.

[0035] The metastable austenitic stainless steel provided by this invention can achieve high strength in the cold-worked state with a relatively low content of deformation-induced martensite. The low martensite content helps to maintain optimal strength and elongation in the cold-worked state, ensuring that cracking does not occur during subsequent cold forming (such as stamping, bending, etc.).

[0036] Secondly, the present invention provides a method for preparing metastable austenitic stainless steel as described in the first aspect, wherein the metastable austenitic stainless steel is in a solution-annealed state, and the preparation method includes the following steps performed sequentially:

[0037] Smelting, casting, hot rolling, first annealing, first pickling, first cold rolling, second annealing and second pickling; or, smelting, casting, hot rolling, first annealing, first pickling, first cold rolling and bright annealing;

[0038] Alternatively, the metastable austenitic stainless steel is in a cold-work hardened state, and the preparation method includes the following steps performed sequentially: smelting, casting, hot rolling, first annealing, first pickling, first cold rolling, second annealing, second pickling, and second cold rolling; or, smelting, casting, hot rolling, first annealing, first pickling, first cold rolling, bright annealing, and second cold rolling.

[0039] In this invention, unless otherwise specified, annealing is performed in air. Bright annealing is performed in an inert atmosphere. Using bright annealing instead of the second annealing and second pickling directly yields a BA (Bright Annealing) surface.

[0040] In some embodiments of the present invention, the hot rolling step includes: heating the steel billet obtained after casting in a heating furnace, with the furnace temperature controlled at 1200-1250°C, the heating time controlled at 200-240 min, the initial rolling temperature controlled at 1100-1150°C, and then roughing and finishing the billet to the required thickness.

[0041] In some embodiments of the present invention, the temperature of the first annealing is 1100-1150℃ (e.g., it can be 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, 1150℃, etc.), the annealing time is 3-10min (e.g., it can be 3min, 4min, 5min, 6min, 7min, 8min, 9min or 10min, etc.), and the cooling method is air cooling.

[0042] In some embodiments of the present invention, the preparation method further includes: performing mechanical descaling between the first annealing and the first pickling. Mechanical descaling can be performed using a shot blasting machine.

[0043] In some embodiments of the present invention, the first pickling step includes: sulfuric acid pickling and mixed acid pickling, wherein the concentration of the sulfuric acid is 250-400 g / L (e.g., 250 g / L, 260 g / L, 280 g / L, 300 g / L, 320 g / L, 330 g / L, 350 g / L, 360 g / L, 380 g / L, or 400 g / L, etc.), and the mixed acid includes nitric acid and hydrofluoric acid, wherein the concentration of the nitric acid is 140 g / L. The concentration of hydrofluoric acid is 10-25 g / L (e.g., it can be 140 g / L, 150 g / L, 160 g / L, 180 g / L, 200 g / L, 220 g / L, 230 g / L, or 240 g / L, etc.), and the concentration of hydrofluoric acid is 10-25 g / L (e.g., it can be 10 g / L, 12 g / L, 13 g / L, 15 g / L, 16 g / L, 18 g / L, 20 g / L, 22 g / L, 23 g / L, or 25 g / L, etc.).

[0044] In some embodiments of the present invention, the reduction rate of the first cold rolling is 55-65%; for example, it can be 55%, 56%, 58%, 60%, 62%, 63%, or 65%, etc.

[0045] In some embodiments of the present invention, the temperature of the second annealing is 1100-1150℃ (e.g., it can be 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, 1150℃, etc.), the annealing time is 1-3 min (e.g., it can be 1 min, 1.2 min, 1.5 min, 1.8 min, 2 min, 2.2 min, 2.5 min, 2.8 min, or 3 min, etc.), the cooling rate is 90-110℃ / s (e.g., it can be 90℃ / s, 92℃ / s, 95℃ / s, 98℃ / s, 100℃ / s, 102℃ / s, 105℃ / s, 108℃ / s, or 110℃ / s, etc.), and the cooling method is a combination of air cooling and water cooling.

[0046] In some embodiments of the present invention, the second pickling step includes: sodium sulfate electrolysis and mixed acid pickling; wherein, in the sodium sulfate electrolysis, the concentration of sodium sulfate is 160-200 g / L (e.g., it can be 160 g / L, 170 g / L, 180 g / L, 190 g / L, or 200 g / L, etc.), and the current is 3000-4000 A (e.g., it can be 3000 A, 3200 A, 3300 A, 3500 A, 3600 A, 3800 A, or 4000 A, etc.); the mixed acid package The mixture includes nitric acid and hydrofluoric acid, wherein the concentration of the nitric acid is 140–240 g / L (e.g., 140 g / L, 150 g / L, 160 g / L, 180 g / L, 200 g / L, 220 g / L, 230 g / L, or 240 g / L, etc.), and the concentration of the hydrofluoric acid is 10–25 g / L (e.g., 10 g / L, 12 g / L, 13 g / L, 15 g / L, 16 g / L, 18 g / L, 20 g / L, 22 g / L, 23 g / L, or 25 g / L, etc.).

[0047] In some embodiments of the present invention, the bright annealing is carried out in an inert atmosphere, the bright annealing temperature is 1100-1150℃ (e.g., 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, 1150℃, etc.), the annealing time is 1-3 min (e.g., 1 min, 1.2 min, 1.5 min, 1.8 min, 2 min, 2.2 min, 2.5 min, 2.8 min, or 3 min, etc.), the cooling rate is 90-110℃ / s (e.g., 90℃ / s, 92℃ / s, 95℃ / s, 98℃ / s, 100℃ / s, 102℃ / s, 105℃ / s, 108℃ / s, or 110℃ / s, etc.), and the cooling method is a combination of air cooling and water cooling.

[0048] Generally, as the reduction rate increases during cold rolling, the content of deformation-induced martensite increases, and the strength of metastable austenitic stainless steel increases. In this invention, there are no special restrictions on the reduction rate of the second cold rolling; those skilled in the art can select the reduction rate according to the required stainless steel strength.

[0049] In some embodiments of the present invention, the preparation method further includes the following steps: acid washing and bright annealing after solution annealing.

[0050] Thirdly, the present invention provides an application of metastable austenitic stainless steel as described in the first aspect, wherein when the metastable austenitic stainless steel is in a solution-annealed state, the metastable austenitic stainless steel is used in automobile fuel tanks, battery pack underbody plates, stainless steel wheel hubs, cross and longitudinal beams, seats, interior and / or exterior trim.

[0051] When the metastable austenitic stainless steel is in a cold-worked state, it is used for battery pack underbody protection plates, front and rear bumpers, and / or vehicle body panels.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] The metastable austenitic stainless steel provided by this invention has a yield strength of over 400 MPa, a tensile strength of over 750 MPa, an elongation of over 50% in the solution-annealed state, a pitting potential (Epit, according to GB / T 17899) of over 230 mV, and no intergranular corrosion susceptibility. In the cold-worked state, when the tensile strength reaches 1300 MPa, the reduction rate does not exceed 30%, the elongation is over 10%, the martensite content is below 40%, and there is no intergranular corrosion susceptibility.

[0054] The metastable austenitic stainless steel provided by this invention can be widely used in automotive components such as fuel tanks, battery pack underbody plates, stainless steel wheel hubs, cross and longitudinal beams, interior and exterior trim, front and rear bumpers, and body panels under solution annealing and cold work hardening with different reduction rates. This eliminates the need for the traditional process of overall electrophoretic treatment after automotive sheet metal or galvanized sheet metal forming, saving costs while also reducing processes and pollution. Attached Figure Description

[0055] Figure 1 Metallographic micrograph of metastable austenitic stainless steel in solution annealed state provided in Example 2 of the present invention;

[0056] Figure 2 Metallographic micrograph of metastable austenitic stainless steel in the cold-worked state provided in Embodiment 2 of the present invention. Detailed Implementation

[0057] The technical solution of the present invention will be further described below through specific embodiments and in conjunction with the accompanying drawings. Those skilled in the art should understand that the specific embodiments described are merely illustrative of the present invention and should not be considered as specific limitations thereof.

[0058] Examples 1-8 and Comparative Examples 1-3

[0059] Examples 1-8 and Comparative Examples 1-3 each provide a metastable austenitic stainless steel, the preparation methods of which are as follows:

[0060] Solution-annealed state:

[0061] (1) Smelting and casting: Smelting, AOD (argon-oxygen refining), LF (ladle refining) and continuous casting into slabs are carried out in sequence according to the composition of metastable austenitic stainless steel.

[0062] (2) Hot rolling: The steel billet obtained after casting is heated in a walking beam furnace at a temperature of 1200℃ for 220 minutes and a rolling temperature of 1120℃. After rough rolling and finish rolling to the required thickness, it is coiled.

[0063] (3) First annealing and first pickling: Annealing is carried out in a horizontal annealing furnace at a temperature of 1120℃ for 3 minutes, and the cooling method is air cooling. After annealing, the steel coil is mechanically descaled by shot blasting, and then pickled in a sulfuric acid section and a mixed acid section (nitric acid + hydrofluoric acid) to obtain a steel coil with a surface that meets the requirements; the concentration of sulfuric acid in the sulfuric acid section is 300g / L, the concentration of nitric acid in the mixed acid section is 160g / L, and the concentration of hydrofluoric acid is 15g / L.

[0064] (4) First cold rolling: Roll to the required thickness using a conventional continuous rolling mill or a reversible single rolling mill; ensure that the cold rolling reduction rate is 60%.

[0065] (5) Second annealing and second pickling: Annealing is carried out in a horizontal annealing furnace at a temperature of 1120℃ for 2 minutes. The cooling rate is controlled at 100℃ / s, and the cooling method is a combination of air cooling and water cooling. Pickling adopts sodium sulfate electrolysis and mixed acid (nitric acid + hydrofluoric acid) pickling process; wherein, the sodium sulfate concentration in the electrolysis section is 160g / L and the current is 3000A; the nitric acid concentration in the mixed acid section is 160g / L and the hydrofluoric acid concentration is 15g / L.

[0066] Cold-worked state:

[0067] (1) First, obtain solution-annealed stainless steel, using the same method as the above-mentioned preparation method for solution-annealed stainless steel;

[0068] (2) Second cold rolling: Cold rolling is performed on the solution-annealed stainless steel, with a total reduction of 30%.

[0069] The mass percentage (%) of elements in the metastable austenitic stainless steel provided in the above embodiments and comparative examples is shown in Table 1 below.

[0070] Table 1

[0071]

[0072]

[0073] The microstructure and composition of the metastable austenitic stainless steel provided in the above embodiments were analyzed using metallographic methods.

[0074] Among them, the metallographic micrographs of metastable austenitic stainless steel in the solution-annealed state provided in Example 2 are as follows: Figure 1As shown, its microstructure consists of 99.2 vol% austenite, 0.5 vol% δ-ferrite, and 0.3 vol% carbonitrides.

[0075] Metallographic micrographs of metastable austenitic stainless steel in the cold-worked state provided in Example 2 are as follows: Figure 2 As shown, its microstructure consists of 82 vol% austenite, 17.7 vol% martensite, and 0.3 vol% carbonitrides.

[0076] The properties of the metastable austenitic stainless steels provided in the above embodiments and comparative examples were tested using the following methods:

[0077] Yield strength, tensile strength, and elongation: tested according to GB / T 228.1, using A50 standard sample;

[0078] Pitting potential (Epit): Tested according to the method in GB / T 17897;

[0079] Intergranular corrosion susceptibility: Tested according to Method E in GB / T4334;

[0080] The results of the above performance tests are shown in Table 2 below.

[0081] Table 2

[0082]

[0083]

[0084] As can be seen from the test results in Table 2, the metastable austenitic stainless steel provided by this invention has a yield strength of over 400 MPa, a tensile strength of over 750 MPa, an elongation of over 50% in the solution-annealed state, and a pitting potential (Epit, according to GB / T 17897) of over 230 mV. In the cold-worked state with a reduction rate of 30%, the yield strength is over 1000 MPa, the tensile strength is over 1300 MPa, the elongation is over 10%, there is no intergranular corrosion sensitivity, and the martensite content is below 40% when the strength reaches 1300 MPa after cold work hardening.

[0085] In Comparative Example 1, due to excessively high Ni content, excessively low Mn content, and excessively small Md30 value, martensitic transformation was difficult during the secondary cold rolling process, resulting in work hardening to the limit value. The tensile strength of the finished material was less than 1200 MPa, which did not meet the design requirements.

[0086] In Comparative Example 2, due to the excessively large Md30 value, the martensitic transformation was easy and the transformation variables were numerous during the secondary cold rolling process. The elongation of the finished material was lower than the target value of >10%, and the intergranular corrosion sensitivity was poor.

[0087] Comparative Example 3 shows a significant reduction in corrosion resistance due to 8Ni-Mn < 0, especially at the material edges, failing to meet the requirement of no intergranular corrosion.

[0088] The above description is merely a specific embodiment of this disclosure, enabling those skilled in the art to understand or implement it. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not to be limited to the embodiments described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A metastable austenitic stainless steel, characterized in that, The metastable austenitic stainless steel comprises the following elemental composition by mass percentage: C 0.05-0.15%, Si 0.3-0.7%, Mn 6.0-9.0%, Cr 17.5-18%, Ni 1.0-1.3% and N 0.20-0.30%, balance Fe; Where 0 < 8Ni-Mn < 30, the element symbol in this formula represents the mass percentage of the element × 100; The martensitic transformation temperature Md30 of the metastable austenitic stainless steel, which forms 50% martensite under 30% cold deformation, satisfies -16.5℃≤Md30≤40℃. The pitting corrosion resistance equivalent value PREN of the metastable austenitic stainless steel is >15.

0.

2. The metastable austenitic stainless steel according to claim 1, characterized in that, The metastable austenitic stainless steel further includes one or more of the following elemental components in mass percentage: Mo 0.01-2.5%, Cu 0.01-3.5%, V 0-0.2%.

3. The metastable austenitic stainless steel according to claim 1 or 2, characterized in that, The metastable austenitic stainless steel is in the solution-annealed state, and the microstructure of the metastable austenitic stainless steel in the solution-annealed state includes: more than 95 vol% austenite, 0.01-5 vol% δ ferrite and less than 0.5 vol% carbonitrides; wherein the grain size of the austenite is ≤25 μm.

4. The metastable austenitic stainless steel according to claim 1 or 2, characterized in that, The metastable austenitic stainless steel is in a cold-worked state, and the microstructure of the metastable austenitic stainless steel in the cold-worked state with a reduction rate of 30% includes: 59.5-90 vol% austenite, 10-40 vol% martensite, and less than 0.5 vol% carbonitrides.

5. A method for preparing metastable austenitic stainless steel as described in any one of claims 1-4, characterized in that, The metastable austenitic stainless steel is in a solution-annealed state, and the preparation method includes the following steps performed sequentially: smelting, casting, hot rolling, first annealing, first pickling, first cold rolling, second annealing and second pickling; or, smelting, casting, hot rolling, first annealing, first pickling, first cold rolling and bright annealing; Alternatively, the metastable austenitic stainless steel is in a cold-work hardened state, and the preparation method includes the following steps performed sequentially: smelting, casting, hot rolling, first annealing, first pickling, first cold rolling, second annealing, second pickling, and second cold rolling; or, smelting, casting, hot rolling, first annealing, first pickling, first cold rolling, bright annealing, and second cold rolling.

6. The preparation method according to claim 5, characterized in that, The hot rolling steps include: heating the steel billet obtained after casting in a heating furnace, controlling the furnace temperature at 1200-1250℃, controlling the heating time at 200-240 min, controlling the initial rolling temperature at 1100-1150℃, and then rough rolling and finish rolling to the required thickness.

7. The preparation method according to claim 5, characterized in that, The first annealing temperature is 1100-1150℃, the annealing time is 3-10 min, and the cooling method is air cooling.

8. The preparation method according to claim 5, characterized in that, The preparation method further includes: mechanical dephosphorization between the first annealing and the first pickling.

9. The preparation method according to claim 5, characterized in that, The first pickling step includes: sulfuric acid pickling and mixed acid pickling, wherein the concentration of sulfuric acid is 250-400 g / L, the mixed acid includes nitric acid and hydrofluoric acid, the concentration of nitric acid is 140-240 g / L, and the concentration of hydrofluoric acid is 10-25 g / L.

10. The preparation method according to claim 5, characterized in that, The reduction rate of the first cold roll is 55-65%.

11. The preparation method according to claim 5, characterized in that, The second annealing temperature is 1100-1150℃, the annealing time is 1-3 min, the cooling rate is 90-110℃ / s, and the cooling method is a combination of air cooling and water cooling.

12. The preparation method according to claim 5, characterized in that, The second pickling step includes: sodium sulfate electrolysis and mixed acid pickling; wherein, in the sodium sulfate electrolysis, the concentration of sodium sulfate is 160-200 g / L and the current is 3000-4000 A; the mixed acid includes nitric acid and hydrofluoric acid, the concentration of nitric acid is 140-240 g / L and the concentration of hydrofluoric acid is 10-25 g / L.

13. The preparation method according to claim 5, characterized in that, The bright annealing is carried out in an inert atmosphere, the temperature of the bright annealing is 1100-1150℃, the annealing time is 1-3 min, the cooling rate is 90-110℃ / s, and the cooling method is a combination of air cooling and water cooling.

14. An application of the metastable austenitic stainless steel as described in any one of claims 1-4, characterized in that, The metastable austenitic stainless steel is in a solution-annealed state and is used in automobiles for fuel tanks, battery pack underbody protection plates, stainless steel wheel hubs, cross and longitudinal beams, seats, interior and / or exterior trim. Alternatively, the metastable austenitic stainless steel is in a cold-worked state, and the metastable austenitic stainless steel is used for the battery pack underbody protection plate, front and rear bumpers and / or body panels of automobiles.

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