Preparation method of novel high-strength corrosion-resistant metastable austenitic stainless steel

Abstract

The invention relates to a preparation method of novel high-strength corrosion-resistant metastable austenitic stainless steel, the designed stainless steel has a heterostructure of surface nanocrystalline-internal hardened state deformation induced martensite + austenite.The preparation method of the novel high-strength corrosion-resistant metastable austenitic stainless steel comprises the following steps that firstly, an austenitic stainless steel plate is subjected to deep cooling rolling treatment, and deformation induced martensite is introduced into the steel plate; a double-phase structure of austenite and deformation-induced martensite is obtained; then, the surface of the steel plate is subjected to melting treatment through a laser melting technology, and a nanometer equiaxed crystal layer is formed on the surface; and then, the steel plate is subjected to low-temperature aging treatment, residual stress in a fused layer structure is eliminated, nanoscale dispersed rich Cr is generated in deformation induced martensite to be separated out, and the hardness of the martensite is improved. Through the coupling effect of the process, the novel high-strength corrosion-resistant metastable austenite stainless steel is prepared.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of stainless steel heterogeneous material production, and particularly relates to a preparation method of a new high-strength corrosion-resistant metastable austenitic stainless steel based on surface laser melting, cryogenic rolling and low-temperature aging. BACKGROUND

[0002] Austenitic stainless steel has been widely used in marine engineering, medical device manufacturing, chemical equipment, etc. due to its excellent corrosion resistance, forming performance and good biocompatibility, and has become an important material indispensable to modern industry. However, the room temperature structure of austenitic stainless steel is coarse austenite, which leads to low yield strength and cannot be applied to high-strength components. With the continuous growth of economy and the rapid development of technology, the engineering equipment in various fields has put forward higher requirements for the light weight and service safety of components, which makes the low-strength austenitic stainless steel unable to meet the production needs of high-strength components, limiting the development of austenitic stainless steel. Therefore, how to improve the yield strength of austenitic stainless steel has become a research hotspot.

[0003] Among various strengthening methods, fine-grain strengthening and deformation strengthening can effectively improve the yield strength of austenitic stainless steel and have become the main means to improve the yield strength of austenitic stainless steel. However, due to the high Cr and Ni content of austenitic stainless steel, the austenite grains have very high thermal stability, and cannot be refined by traditional cyclic phase transformation. Only by relying on the mechanical stability of austenite, large plastic deformation can be performed on the austenite to induce the phase transformation of martensite, and the inverse phase transformation of deformation-induced martensite can be used for grain refinement. This method requires large plastic deformation process, which requires high equipment and high energy consumption, and the obtained structure uniformity is poor, leading to unstable performance of the experimental steel and limiting the large-scale application of this means. Deformation strengthening mainly introduces dislocations, deformation-induced martensite and other deformation substructures into austenite grains through plastic deformation, uses substructures to block dislocation movement, and improves the yield strength of the experimental steel. However, the excellent corrosion resistance of austenitic stainless steel is due to the formation of a passivation film on its surface, and the introduction of deformation substructures destroys the structure and stability of the original passivation film, resulting in a serious decline in the corrosion resistance of the experimental steel after deformation strengthening. In addition, the strength improvement effect is positively correlated with the deformation amount, and the larger the deformation amount, the better the strength improvement effect. However, deformation will damage the plasticity of the experimental steel, and the larger the deformation degree, the poorer the plasticity. It can be seen that the deformation strengthening method also has certain deficiencies in improving the yield strength of austenitic stainless steel.

[0004] The above analysis reveals that although fine-grain strengthening and deformation strengthening can effectively improve the yield strength of austenitic stainless steel, they both have deficiencies and cannot meet the demand of various industrial fields for high-performance austenitic stainless steel. Therefore, new processes for improving the yield strength of austenitic stainless steel should be developed to improve the yield strength while maintaining the excellent corrosion resistance and certain plasticity of austenitic stainless steel. SUMMARY

[0005] The present application aims at the problem that the existing deformation strengthening method improves the yield strength of austenitic stainless steel while causing serious reduction of plasticity and corrosion resistance, and innovatively proposes a new process based on the cooperation of surface laser melting and cryogenic rolling and low-temperature aging coupling to realize the preparation of high-strength high-plasticity high-corrosion-resistance stainless steel. The production process is as follows: first, the cryogenic rolling process is used to introduce a certain amount of deformation-induced martensite into the austenitic stainless steel structure with small deformation; then, the nanosecond laser melting technology is used to melt the upper and lower surfaces of the steel plate to obtain a nanocrystalline equiaxed crystal surface layer (grain size of 300-500 nm) on the surface; then, the low-temperature aging process is used to eliminate the residual tensile stress of the melting layer and promote the generation of nanometer dispersed Cr-rich precipitates in the deformation-induced martensite. Through the above process, a heterogeneous structure with surface nanocrystalline-internal hardened deformation-induced martensite and austenite mixed distribution is obtained, which realizes high corrosion resistance by using surface nanocrystalline equiaxed crystal, improves the yield strength of the steel plate by using internal hardened deformation-induced martensite, and ensures plasticity by using residual austenite, thereby realizing the high-strength high-plasticity high-corrosion-resistance design of stainless steel.

[0006] In order to achieve the above purpose, the present application is realized by the following means:

[0007] The first aspect of the present application provides a preparation method of a new type of high-strength corrosion-resistant metastable austenitic stainless steel, comprising the following steps:

[0008] (1) cryogenic treatment is performed on an austenitic stainless steel plate with a thickness of d, the cryogenic temperature is -196 to -80℃, the cryogenic time is (10-20)*d minutes, then rolling deformation is performed at the cryogenic temperature, and the total deformation amount is 5-10%, wherein the thickness d is in mm;

[0009] (2) laser melting treatment is performed on the surface of the rolled steel plate, the laser pulse frequency used is 160-380KHz, the pulse width is 12-32ns, the spot diameter is 15-90μm, the laser power density is 20-65kW / cm 2 , the overlap rate is 10-30%, the laser scanning speed is 200-600mm / s, the light-on delay time is 20μs, the light-off delay time is 180μs, the corner delay time is 80μs, and the laser scanning number is 1-3 times;

[0010] (3) argon is used to protect the laser melting process, and the oxygen content in the melting environment is controlled to be 160-340PPm;

[0011] (4) during the melting process, a low-temperature backing plate is arranged below the steel plate to cool the steel plate through the low-temperature backing plate to realize ultra-fast cooling of the molten pool, and the temperature of the low-temperature backing plate is -150 to -70℃;

[0012] (5) aging treatment of the steel plate at a temperature of 300-450℃ for (30-60)*d minutes.

[0013] Preferably, the austenitic stainless steel plate in step (1) is a 321 austenitic stainless steel plate.

[0014] Preferably, the cryogenic temperature in step (1) is -196 to -100℃, the cryogenic time is (15-20)*d minutes, and the total deformation is 6-8%.

[0015] Preferably, the laser scanning speed in step (2) is 300-500 mm / s, the laser pulse frequency is 200-350 kHz, the spot diameter is 30-60 μm, and the laser power density is 25-40 KW / cm 2 , and the overlap rate is 15-25%.

[0016] Preferably, the oxygen content in step (3) is 180-300 PPM.

[0017] Preferably, the temperature of the low-temperature pad in step (4) is -120 to -90℃

[0018] Preferably, the aging temperature in step (5) is 380-440℃, and the aging time is (40-60)*d minutes.

[0019] The second aspect of the present application provides a novel high-strength corrosion-resistant metastable austenitic stainless steel with a surface layer of 300-500 nm equiaxed crystals and an internal "sandwich" structure of deformed austenite and hardened martensite.

[0020] Traditional deformation strengthening methods for improving the yield strength of austenitic stainless steel suffer from a significant decrease in the steel's plasticity and corrosion resistance, posing a major challenge to their application. The severe reduction in plasticity is attributed to the introduction of a large amount of deformation-induced martensite and dislocation substructures into the austenite grains during plastic deformation, reducing the number of mobile dislocations within the austenite grains and increasing the resistance to dislocation movement. To address this, the inventors, based on theories related to the mechanical stability of austenite, proposed using cryogenic rolling technology instead of traditional room temperature rolling. Compared to room temperature rolling, cryogenic rolling results in lower mechanical stability of austenite, making it easier to induce martensitic phase transformation during deformation. Therefore, compared to traditional deformation processes, cryogenic rolling can introduce the same amount of martensite with a smaller deformation amount. Simultaneously, under the influence of a smaller deformation amount, the remaining austenite in the steel sheet undergoes less work hardening, retaining work hardening capacity and thus exhibiting higher plasticity. The severe reduction in corrosion resistance is attributed to the introduction of deformation substructures, which disrupts the structure and stability of the original passivation film in austenitic stainless steel. To address this issue, this invention employs nanosecond laser surface melting and solidification technology. Leveraging the high energy density and short duration of nanosecond lasers, coupled with rapid heat dissipation, ultra-rapid cooling of a small molten pool is achieved, resulting in a nanoscale equiaxed austenitic surface structure. The grain size has a beneficial influence on the corrosion resistance of the austenitic stainless steel passivation film (smaller grain size results in better corrosion resistance), further enhancing the steel plate's corrosion resistance. However, the ultra-rapid cooling of the surface structure obtained during laser melting and solidification leads to significant residual tensile stress, which can reduce the steel plate's plasticity and corrosion resistance. To address this, this invention incorporates a low-temperature annealing process to eliminate this residual stress within the surface melted structure. Furthermore, low-temperature annealing promotes the formation of dispersed nano-rich Cr precipitation in the internal martensite structure, further enhancing the steel plate's strength. Based on the above analysis, this invention proposes a coupled preparation process of deep cryogenic rolling-surface laser melting-low temperature aging. Through the synergistic effect of multiple processes, a two-phase heterostructure of surface nanocrystals-internal hardened martensite and deformed austenite is prepared, which solves the problem of severely reduced plasticity and corrosion resistance faced by deformation strengthening methods in improving the yield strength of austenitic stainless steel.

[0021] The present invention has the following advantages over the prior art:

[0022] (1) This invention proposes a heterostructure design with a mixed distribution of surface nano-equiaxed crystals and internal hardened martensite and deformed austenite. This design enables the control of the hardening process of surface nanocrystals and martensite, providing a new idea for the design of advanced high-strength corrosion-resistant stainless steel.

[0023] (2) Compared with traditional deformation strengthening technology, the present invention effectively improves the yield strength of austenitic stainless steel without seriously reducing the plasticity and corrosion resistance of austenitic stainless steel, thus providing a new direction for the design of high-performance stainless steel.

[0024] (3) The application first proposes a coupling preparation process of cryogenic rolling-surface laser melting-low temperature aging, the new process is developed and designed based on the traditional process, has low equipment requirement, low energy consumption, and obtains uniform microstructure and performance. DETAILED DESCRIPTION

[0025] In order to make the object, technical scheme and effect of the application more clear and explicit, the application is further described below with reference to examples. It should be understood that the specific examples described herein are only used to explain the application, and are not used to limit the application.

[0026] Example 1

[0027] A preparation method of a new type of high-strength corrosion-resistant metastable austenitic stainless steel, comprising the following steps:

[0028] (1) First, a 321 austenitic stainless steel plate with a thickness of 2 mm is subjected to cryogenic treatment, the cryogenic temperature is -196℃, the cryogenic time is 20 minutes, then rolling deformation is carried out at the cryogenic temperature, and the total deformation amount is 5%;

[0029] (2) Then, the deformed steel plate is subjected to laser melting surface treatment, the laser pulse frequency used is 160KHz, the pulse width is 12ns, the spot diameter is 15μm, the laser power density is 20kW / cm 2 , the overlap rate is 30%, the laser scanning speed is 200mm / s, the light-on delay time is 20μs, the light-off delay time is 180μs, the corner delay time is 80μs, and the laser scanning number is 3 times;

[0030] (3) The laser melting process is protected by argon, and the oxygen content in the melting environment is controlled at 340PPm;

[0031] (4) During the melting process, the steel plate is cooled by a low-temperature pad plate arranged below the steel plate to realize ultra-fast cooling of the molten pool, and the temperature of the low-temperature pad plate is -70℃;

[0032] (5) Finally, the steel plate is subjected to low-temperature aging treatment, the aging temperature is 450℃, and the aging time is 120 minutes.

[0033] Example 2

[0034] A preparation method of a new type of high-strength corrosion-resistant metastable austenitic stainless steel, comprising the following steps:

[0035] (1) First, a 321 austenitic stainless steel plate with a thickness of 2 mm is subjected to cryogenic treatment, the cryogenic temperature is -80℃, the cryogenic time is 40 minutes, then rolling deformation is carried out at the cryogenic temperature, and the total deformation amount is 10%;

[0036] (2) Then, the deformed steel plate is subjected to laser surface melting treatment, the laser pulse frequency used is 380 KHz, the pulse width is 32 ns, the spot diameter is 90 μm, the laser power density is 65 kW / cm 2 , the overlap rate is 10%, the laser scanning speed is 600 mm / s, the light-on delay time is 20 μs, the light-off delay time is 180 μs, the corner delay time is 80 μs, and the laser scanning times is 1;

[0037] (3) The laser melting process is protected by argon, and the oxygen content in the melting environment is controlled at 160 PPM;

[0038] (4) In the melting process, the low-temperature cushion plate arranged below the steel plate is used to cool the steel plate, so as to realize the ultra-fast cooling of the molten pool, and the temperature of the low-temperature cushion plate is-150 ℃;

[0039] (5) Finally, the steel plate is subjected to low-temperature aging treatment, the aging temperature is 300 ℃, and the aging time is 60 minutes.

[0040] Example 3

[0041] A preparation method of a novel high-strength corrosion-resistant metastable austenitic stainless steel, comprising the following steps:

[0042] (1) First, the 321 austenitic stainless steel plate with a thickness of 2 mm is subjected to cryogenic treatment, the cryogenic temperature is-120 ℃, the cryogenic time is 30 minutes, and then the cryogenic temperature is subjected to rolling deformation, and the total deformation is 7%;

[0043] (2) Then, the deformed steel plate is subjected to laser surface melting treatment, the laser pulse frequency used is 240 KHz, the pulse width is 20 ns, the spot diameter is 50 μm, the laser power density is 40 kW / cm 2 , the overlap rate is 20%, the laser scanning speed is 400 mm / s, the light-on delay time is 20 μs, the light-off delay time is 180 μs, the corner delay time is 80 μs, and the laser scanning times is 2;

[0044] (3) The laser melting process is protected by argon, and the oxygen content in the melting environment is controlled at 280 PPM;

[0045] (4) In the melting process, the low-temperature cushion plate arranged below the steel plate is used to cool the steel plate, so as to realize the ultra-fast cooling of the molten pool, and the temperature of the low-temperature cushion plate is-100 ℃;

[0046] (5) Finally, the steel plate is subjected to low-temperature aging treatment, the aging temperature is 400 ℃, and the aging time is 90 minutes.

[0047] The present application can be prepared by the preparation method described in the above embodiments 1-3 to obtain a new type of high-strength corrosion-resistant metastable austenitic stainless steel with a 300-500 nm equiaxed crystal as a surface layer and a "sandwich" structure of deformed austenite and hardened martensite inside.

[0048] In addition, the austenitic stainless steel plate can also be an austenitic stainless steel plate with a thickness d of 1 mm, 3 mm, 6 mm or other thicknesses.

[0049] Comparative Example 1

[0050] A method for strengthening 321 austenitic stainless steel, comprising the following steps:

[0051] (1) Laser melting surface treatment is performed on the 321 austenitic stainless steel plate, the laser pulse frequency used is 240 KHz, the pulse width is 20 ns, the spot diameter is 50 μm, the laser power density is 40 kW / cm 2 , the overlap rate is 20%, the laser scanning speed is 400 mm / s, the light-on delay time is 20 μs, the light-off delay time is 180 μs, the corner delay time is 80 μs, and the laser scanning number is 2 times;

[0052] (2) Argon is used for protection during the laser melting process to control the oxygen content in the melting environment to be 280 PPM;

[0053] (3) During the melting process, a low-temperature pad is used to cool the steel plate to achieve ultra-fast cooling of the molten pool, and the temperature of the pad is -100℃;

[0054] (4) Finally, the steel plate is subjected to low-temperature aging treatment, and the aging temperature is 400℃ and the aging time is 90 minutes.

[0055] Comparative Example 2

[0056] A method for strengthening 321 austenitic stainless steel, comprising the following steps:

[0057] (1) First, the 321 austenitic stainless steel plate with a thickness of 2 mm is subjected to cryogenic treatment, the cryogenic temperature is -120℃, the cryogenic time is 30 minutes, and then the plate is subjected to rolling deformation at the cryogenic temperature, and the total deformation amount is 7%;

[0058] (2) Finally, the steel plate is subjected to low-temperature aging treatment, and the aging temperature is 400℃ and the aging time is 90 minutes.

[0059] Comparative Example 3

[0060] A method for strengthening 321 austenitic stainless steel, comprising the following steps:

[0061] (1) First, the 321 austenitic stainless steel plate with a thickness of 2 mm is subjected to cryogenic treatment, the cryogenic temperature is -120℃, the cryogenic time is 30 minutes, then the rolling deformation is carried out at the cryogenic temperature, and the total deformation is 7%;

[0062] (2) Then, the deformed steel plate is subjected to laser melting surface treatment, the laser pulse frequency used is 240KHz, the pulse width is 20ns, the spot diameter is 50μm, the laser power density is 40kW / cm 2 , the overlap rate is 20%, the laser scanning speed is 400mm / s, the light-on delay time is 20μs, the light-off delay time is 180μs, the corner delay time is 80μs, and the laser scanning times are 2;

[0063] (3) The laser melting process is protected by argon, and the oxygen content in the melting environment is controlled at 280PPm;

[0064] (4) During the melting process, the steel plate is cooled by using a low-temperature pad to realize ultra-fast cooling of the molten pool, and the temperature of the pad is -100℃.

[0065] Comparative Example 4

[0066] A strengthening method of 321 austenitic stainless steel, comprising the following steps:

[0067] (1) The 321 austenitic stainless steel plate with a thickness of 2 mm is subjected to rolling deformation treatment, the deformation temperature is 25℃, and the total deformation is 7%.

[0068] Verification Example 1

[0069] The commercial 321 austenitic stainless steel, the stainless steel prepared by the examples 1-3 and the comparative examples 1-4 are respectively taken, and the microstructure (whether it is a surface nanocrystalline-heterostructure of internal hardened martensite and deformed austenite), corrosion current density, yield strength and elongation are detected by using the conventional technical method in the art, and the specific detection results are as follows in Table 1.

[0070] Table 1 Detection results of commercial 321 austenitic stainless steel, examples 1-3 and comparative examples 1-4

[0071]

[0072]

[0073] By comparing the performance of examples 1-3 and commercial 321 austenitic stainless steel plate, it is found that the stainless steel subjected to cryogenic rolling-nanosecond laser melting-low temperature aging treatment can greatly improve the yield strength, and can obtain good corrosion resistance and better plasticity.

[0074] By comparing the results of examples 1-3 with those of comparative examples 1-4, it can be concluded that: comparative example 1 does not set the cryogenic rolling process, and fails to introduce hard phase martensite into the steel plate organization, resulting in the failure to effectively improve the yield strength of the steel plate. Comparative example 2 does not set the laser melting process, so that the steel plate surface does not obtain the nano-equiaxed crystal layer, resulting in the steel plate not being improved in corrosion resistance, which is far lower than the present application. Comparative example 3 does not set the low-temperature aging process, and the deformation-induced martensite in the steel plate organization is not hardened, so that the yield strength of the steel plate is lower than that of the present application, and the laser melting layer organization is not stress-relieved, and the residual tensile stress in the organization reduces the corrosion resistance and plasticity of the steel plate. Comparative example 4 is a traditional deformation strengthening process, and the obtained steel plate has high yield strength, low plasticity and low corrosion resistance. The above analysis reveals that the low-temperature rolling, nanosecond laser melting and low-temperature aging process are essential conditions for the design, and the absence of any process cannot make the austenitic stainless steel obtain the best performance.

[0075] In addition, the preparation method described in the present application can be applied to other series of austenitic stainless steel in addition to 300 series austenitic stainless steel, and is also applicable to other grades of 300 series austenitic stainless steel.

[0076] The above specific embodiment part specifically introduces the analysis method involved in the present application. It should be noted that the above introduction is only to help those skilled in the art to better understand the method and idea of the present application, and is not a limitation on the related content. Those skilled in the art can also make appropriate adjustments or modifications to the present application without departing from the principles of the present application, and the above adjustments and modifications should also belong to the protection scope of the present application.

Claims

1. A method for preparing a novel high-strength, corrosion-resistant, metastable austenitic stainless steel, characterized in that, Includes the following steps: (1) The austenitic stainless steel plate with a thickness of d is subjected to deep cryogenic treatment at a temperature of -196 to -80℃ and a time of (10 to 20)*d minutes. Then, it is rolled and deformed at the deep cryogenic temperature with a total deformation of 5 to 10%. The unit of thickness d is mm. (2) The surface of the rolled steel plate is subjected to laser melting and solidification treatment. The laser pulse frequency is 160-380 kHz, the pulse width is 12-32 ns, the spot diameter is 15-90 μm, and the laser power density is 20-65 kW / cm². 2 The overlap rate is 10-30%, the laser scanning speed is 200-600 mm / s, the light-on delay is 20 μs, the light-off delay is 180 μs, the corner delay is 80 μs, and the number of laser scans is 1-3. (3) Argon gas is used to protect the laser melting process and the oxygen content in the melting environment is controlled at 160-340 ppm; (4) During the melting and solidification process, a low-temperature pad is set under the steel plate to achieve ultra-fast cooling of the molten pool. The temperature of the low-temperature pad is -150 to -70℃. (5) The steel plate is subjected to low-temperature aging treatment at a temperature of 300-450℃ for a duration of (30-60)*d minutes.

2. The preparation method according to claim 1, characterized in that, The austenitic stainless steel plate mentioned in step (1) is 321 austenitic stainless steel plate.

3. The preparation method according to claim 1, characterized in that, The cryogenic temperature in step (1) is -196 to -100°C, the cryogenic time is (15 to 20) * d minutes, and the total deformation is 6 to 8%.

4. The preparation method according to claim 1, characterized in that, The laser scanning speed in step (2) is 300–500 mm / s, the laser pulse frequency is 200–350 kHz, the spot diameter is 30–60 μm, and the laser power density is 25–40 KW / cm². 2 The overlap rate is 15%–25%.

5. The preparation method according to claim 1, characterized in that, The oxygen content mentioned in step (3) is 180-300 ppm.

6. The preparation method according to claim 1, characterized in that, The temperature of the low-temperature pad mentioned in step (4) is -120 to -90℃.

7. The preparation method according to claim 1, characterized in that, The aging temperature in step (5) is 380-440℃ and the aging time is (40-60)*d minutes.

8. The preparation method according to any one of claims 1-7, characterized in that, This method yields a novel high-strength, corrosion-resistant, metastable austenitic stainless steel with a surface layer of 300–500 nm equiaxed crystals and an interior of modified austenite and hardened martensite, forming a "sandwich" structure.

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