High-strength and high-ductility austenitic stainless steel with bimodal structure and preparation method thereof

Abstract

This invention provides a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel and its preparation method, relating to the technical field of medical-grade austenitic stainless steel preparation. The chemical composition of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel, by mass percentage, is: C 0.02-0.07%, Si 0.4-0.6%, Mn 1.3-1.9%, Cr 15.5-18.6%, Cu 3.5-4.1%, Ni 7.3-9.4%, N≤0.05%, S≤0.02%, P≤0.03%, with the remainder being Fe and unavoidable impurities. This invention employs an "aging + cold rolling + flash annealing" process, which is simple to operate. Large-reduction cold rolling deformation yields deformation-induced martensite and residual deformed austenite; subsequent flash annealing produces a bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and high antibacterial properties. It not only achieves a synergistic effect of ultra-high strength and plasticity, but also possesses excellent antibacterial properties, with an antibacterial rate of >99% after 24 hours of contact with Staphylococcus aureus. In summary, this material represents a dual breakthrough in both mechanical and antibacterial properties.

Description

Technical Field

[0001] This invention relates to the technical field of medical austenitic stainless steel preparation, and in particular to a high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel and its preparation method, which is suitable for medical devices, public health protection and other scenarios with stringent requirements for material strength and antibacterial properties. Background Technology

[0002] The recent outbreaks of several major pandemics have made reducing the risk of infection from contact with contaminated surfaces a key global concern. Austenitic stainless steel, due to its significant cost-effectiveness and ease of integration with antibacterial properties, has broad application prospects in biosafety protection in the medical, industrial, and public sectors. However, in many medical and everyday applications, antibacterial materials not only need to possess highly effective antibacterial capabilities but also meet excellent mechanical property requirements to reduce economic losses and safety risks caused by medical device malfunctions. Examples include handrails and door handles that need to withstand continuous friction, and orthodontic archwires and molar brackets that need to withstand the compressive loads of the oral cavity. However, the inherently low strength of traditional austenitic stainless steel (typical yield strength in the 200-300 MPa range) greatly limits its application in lightweight structures and safety-critical components such as medical devices.

[0003] To achieve a synergistic improvement in strength and ductility, microstructure design is considered a key breakthrough, with the construction of heterostructures showing great potential. Heterostructured materials (HSMs) can achieve excellent mechanical property matching through heterogeneous deformation-induced (HDI) strengthening mechanisms. Compared with traditional technologies such as severe plastic deformation (SPD), HSMs have advantages such as large-scale production, strong scalability, and controllable cost. More importantly, HDI strengthening can coexist with the HDI work hardening effect, which can significantly coordinate the balance between strength and ductility compared to homogeneous materials. Regarding antibacterial properties, various metal nanoparticles (NPs) in nature possess antibacterial activity, among which silver (Ag) and copper (Cu) nanoparticles have been widely reported to have antibacterial effects against various microorganisms such as bacteria, viruses, fungi, and algae. Compared to silver, copper is easier to obtain and cheaper. Recent research further confirms that copper has superior antibacterial properties. Its antibacterial mechanism mainly relies on the continuous release of copper ions from the matrix. These ions can destroy bacterial cell membranes, catalyze the generation of oxidative free radicals, and rapidly inactivate microorganisms through a direct "contact-kill" effect.

[0004] Chinese patent CN113355607A discloses antibacterial and antitoxic austenitic stainless steel and its preparation method. By adding Ag and rare earth elements, the material has broad-spectrum antibacterial and antitoxic properties, with a hardness ≤210HV and plasticity ≥40%. However, it lacks high strength and plasticity design, has a relatively conventional microstructure, and insufficient strength, making it difficult to adapt to scenarios with high requirements for material strength.

[0005] Chinese patent CN118422071A discloses a heterogeneous antibacterial stainless steel and its preparation method. H&ASS, prepared by the "aging + cold rolling + annealing" process, can achieve an antibacterial rate of over 99% against Staphylococcus aureus within 24 hours, but its yield strength is only about 500 MPa and its strength-ductility product is about 26 GPa%, indicating poor mechanical properties.

[0006] Chinese patent CN116287998A discloses a medical high-strength antibacterial austenitic stainless steel, its preparation method and application. It achieves a tensile strength of 1350-1630 MPa by subjecting cold-rolled copper-containing austenitic stainless steel to low-temperature aging treatment, which strengthens the cold-rolled defects and the copper-rich phase. While maintaining excellent antibacterial properties of 99.1-99.9%, the stainless steel has a low strength-ductility product and severely insufficient formability, which limits its application scenarios.

[0007] Chinese patent CN120290985A discloses corrosion-resistant and antibacterial stainless steel and its preparation method. By adding Cu and N elements and combining them with heat treatment, the antibacterial rate of the material is ≥99.8% and the corrosion resistance is good. However, the strength and plasticity do not reach the high strength and plasticity synergy standard, which cannot meet the application scenarios with strict requirements for both strength and plasticity.

[0008] Chinese patent CN119392120A discloses antibacterial stainless steel and its preparation method. By adding Ce element, the material has an antibacterial rate of ≥99.5% against Staphylococcus aureus and a tensile strength of ≥730MPa. However, it does not form a unique microstructure, the plasticity index does not clearly meet the requirements of high strength and plasticity matching, and the synergistic optimization of strength and plasticity is not achieved. Summary of the Invention

[0009] The main objective of this invention is to address the technical problems in existing medical austenitic antibacterial stainless steel preparation technologies, such as the inability to synergistically improve strength, plasticity, and antibacterial properties. Therefore, this invention proposes a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel and its preparation method, which can solve the aforementioned problems.

[0010] A high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, wherein the chemical composition of the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel is as follows (by mass percentage): C 0.02-0.07%, Si 0.4-0.6%, Mn 1.3-1.9%, Cr 15.5-18.6%, Cu 3.5-4.1%, Ni 7.3-9.4%, N≤0.05%, S≤0.02%, P≤0.03%, with the remainder being Fe and unavoidable impurities.

[0011] Optionally, the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel is produced by an "aging + cold rolling + flash annealing" process. The microstructure consists of a heterogeneous bimodal structure with a volume fraction of 70-80% reverse shear austenite and a volume fraction of 20-30% recrystallized deformed austenite, with an average grain size of 2.5-4.2 μm. Furthermore, high-density nano-Cu precipitates with a size of 30-50 nm are distributed in the matrix.

[0012] Optionally, the density of the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel is 7.92-7.97 g / cm³. 3 The hardness is 210-230HV, the tensile strength is >720MPa, the yield strength is >420MPa, the yield ratio is 0.58-0.60, the uniform elongation is >47%, the total elongation is >55%, the strength-ductility product is >41GPa%, and the antibacterial rate is >99%.

[0013] A method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, comprising the following preparation steps:

[0014] S1. Melting and casting: Weigh and melt the raw materials according to the composition ratio of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel, and then cast the steel ingot at high temperature.

[0015] S2, High-temperature homogenization + hot forging: The steel billet of S1 is heated to perform high-temperature homogenization treatment, and then hot forged to obtain homogenized steel forgings.

[0016] S3, High-temperature solution treatment + multi-pass hot rolling + water quenching: The homogenized steel forging of S2 is first subjected to high-temperature solution treatment, and after removing the iron oxide scale from the furnace, it is subjected to multi-pass hot rolling, and then water quenched to room temperature to obtain hot-rolled plate.

[0017] S4, Aging + Cold Rolling: The hot-rolled sheet of S3 is aged, cooled to room temperature with water, and then cold-rolled to obtain the cold-rolled sheet.

[0018] S5, Quenching Expansion Tester Flash Annealing: The cold-rolled sheet of S4 is placed in an improved quenching expansion tester, heated rapidly and held at the temperature, and then rapidly cooled to room temperature with nitrogen to obtain a bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties.

[0019] S6. Mechanical property testing + quasi-in-situ analysis: Tensile specimens of bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties (S4 grade) are cut and polished. The polished tensile specimens are used for mechanical property testing. For specimens for quasi-in-situ testing, the surface needs to be polished to a mirror finish by grinding with sandpaper of different grits from small to large. Then, it is polished with a silica gel suspension and followed by quasi-in-situ analysis using electron backscatter diffraction (EBSD).

[0020] S7. Antibacterial performance test and analysis: Square pieces of S4 bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties were cut and tested according to ISO 22196-2011 "Determination of antibacterial properties of plastics and other non-porous surfaces".

[0021] Optionally, the melting temperature in S1 is 1500-1600℃, the casting temperature is 650-720℃, and the size of the steel ingot is 95×95×115-95×95×135mm.

[0022] Optionally, the heating rate of the high-temperature homogenization treatment in S2 is 15-30℃ / s, the temperature is 1100-1200℃, and the holding time is 20-24h; the size of the homogenized steel forging is 70×70×90-70×70×110mm.

[0023] Optionally, the heating rate of the high-temperature solution treatment in S3 is 10-20℃ / s, the temperature is 1150-1200℃, and the holding time is 5-7h; the initial rolling temperature of the multi-pass hot rolling is 1130-1180℃, the final rolling temperature is 930-970℃, and the thickness of the hot-rolled plate is 5.5-6.2mm.

[0024] Optionally, the heating rate of the aging treatment in S4 is 10-20℃ / s, the temperature is 800-950℃, and the holding time is 15-25h; the total reduction rate of cold rolling is 65-75%; the thickness of the cold-rolled sheet is 1.5-2.0mm; the microstructure after cold rolling contains 70-80% deformation-induced martensite and 20-30% residual deformed austenite by volume, and high-density nano-Cu precipitates with a size of 30-50nm are distributed.

[0025] Optionally, the heating rate for rapid heating in S5 is 70-100℃ / s, the holding temperature is 830-880℃, and the holding time is 1-10s; the cooling rate for rapid nitrogen cooling is 50-80℃ / s; the average grain size of austenitic stainless steel with bimodal nano / ultrafine grain structure in the microstructure of the high-strength, high-plasticity, and high-antibacterial properties is 1.5-2.0μm in the fine-grained region and 3.0-4.0μm in the coarse-grained region.

[0026] Optionally, in S6, the sandpaper used ranges from 240# to 2000#, and the silica gel suspension is used for vibration polishing for 6 hours. The gauge length of the tensile specimen after polishing is 15 mm. For the quasi-in-situ tensile test, the parallel section length of the specimen is 4 mm, and it is marked with dots using a surface Vickers hardness tester to ensure the consistency of discontinuous strain and the test area. The strain values ​​are set to 10%, 20%, 30%, and 40%. In S7, the square pieces are cut to a size of 10×10×1.9 mm. For the antibacterial performance test, the specimen must first be sterilized with ultraviolet light, and 25 μL of a 1×10 concentration is added. 6 CFU·mL -1 The antibacterial rate was calculated using the plate count method after the bacteria had been in contact with the sample for 24 hours.

[0027] Optionally, the test can be conducted according to ISO 22196-2011 "Determination of antimicrobial properties of plastics and other nonporous surfaces": the sample is first sterilized by ultraviolet light, then placed in a 24-well plate, and tested at 37°C and 70% relative humidity with 25 μL of a 1×10⁻⁶ solution. 6 CFU·mL -1 The bacterial suspension was inoculated onto the sample surface, ensuring complete spread and full contact with the material surface (coverage area 1 cm²). After incubating the 24-well plate at 37°C for 24 hours, the bacterial suspension on the sample surface was diluted with 2.5 mL of PBS solution. After shaking to mix, 100 μL of the diluted solution was spread onto LB agar plates and incubated at 37°C for another 24 hours. After removal, the colony status was photographed and the colony count was recorded. A PE membrane was used as a blank control group. Three replicates were set up for each group. The antibacterial rate was calculated using the following formula:

[0028] K = (Nc - Ns) / Nc × 100%

[0029] In the formula, K is the antibacterial rate of the experimental sample, Nc is the number of colonies on the LB agar plate of the control group, and Ns is the number of colonies on the LB agar plate of the experimental group.

[0030] Technical principle of the invention:

[0031] Based on existing technologies, this invention further studies, designs, and prepares a bimodal austenitic stainless steel with high-density nanoscale Cu precipitation. The performance advantages of this material stem from three synergistic effects: First, the presence of numerous nanocrystals and submicron grains in the fine-grained region significantly enhances the material's yield strength. Second, during the early and middle stages of plastic deformation, nanoscale and submicron grains undergo significant martensitic phase transformation, exhibiting a significant transformation-induced plasticity (TRIP) effect, thus endowing the material with a high work hardening rate. Meanwhile, the micron-sized grains in the coarse-grained region possess high mechanical stability during the early and middle stages of plastic deformation, delaying the TRIP effect until the later stages of large plastic deformation. Therefore, this multi-stage TRIP effect allows the work hardening rate to be maintained at a high level, ultimately achieving a synergistic improvement in both strength and plasticity, with a strength-plasticity product reaching 41 GPa. Third, the uniform distribution of high-density nanoscale Cu precipitation gives the stainless steel excellent antibacterial properties of >99%.

[0032] Specifically, this invention uses 304 austenitic stainless steel as a base and selects Cu as the antibacterial element—Cu serves as a raw material for metal nanoparticles with excellent antibacterial activity, offering significant cost advantages. On one hand, this invention achieves a rapid shear-mechanism reversal transformation from deformation-induced martensite to austenite using a simple flash annealing process, resulting in a bimodal microstructure with randomly distributed coarse and fine grains. The nano / submicron fine-grained region has high-density grain boundaries, making it easier to induce martensite nucleation at the grain boundaries during deformation, resulting in a significant TRIP effect. Meanwhile, the micron-sized coarse-grained region exhibits strong mechanical stability in the early and middle stages of deformation, making it less prone to martensitic transformation. In the later stages of plastic deformation, sufficient strain energy and crystal defects such as dislocations accumulate, promoting the large-scale generation of deformation-induced martensite. Therefore, this multi-stage TRIP effect, coupled with a large amount of copper precipitation, ensures that the work hardening rate remains at a high level, allowing the material to undergo continuous plastic deformation and significantly improving uniform elongation, thus resulting in excellent comprehensive mechanical properties. On the other hand, the aging treatment results in the precipitation of highly dense and uniformly distributed nano-Cu. When bacteria adhere to the substrate, they release a large number of copper ions, rapidly disrupting the bacterial cell walls and quickly killing the bacteria, thus exhibiting excellent antibacterial properties. In summary, this invention achieves a dual combination of mechanical properties and biological antibacterial properties.

[0033] The above technical solution has at least the following advantages compared with the existing technology:

[0034] The present invention proposes a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel and its preparation method, which can solve the technical problems existing in the preparation technology of medical austenitic antibacterial stainless steel, such as the inability to synergistically improve the strength, plasticity, and antibacterial properties.

[0035] The present invention describes a bimodal austenitic stainless steel with high strength, plasticity, and high antibacterial properties. The addition of Cu not only appropriately increases the stacking fault energy of the austenitic stainless steel and improves the stability of austenite, but more importantly, it also enhances the properties of copper ions (Cu). 2+ It has a strong bactericidal ability and the Cu content meets medical-grade standards, realizing the innovative design concept of "medical-engineering integration".

[0036] This invention discloses a high-strength, high-plasticity, and high-antibacterial-performance bimodal austenitic stainless steel and its preparation process. The process employs "aging + cold rolling + flash annealing," firstly causing the uniform precipitation of high-density nano-Cu, then controlling the proportion of deformation-induced martensite and deformed austenite, and finally utilizing the difference in nucleation rates between the two during annealing to obtain a bimodal-scale nano / ultrafine-grained austenitic microstructure. On the one hand, the bimodal-scale nano / ultrafine-grained structure exhibits high mechanical stability and excellent strength-plasticity matching, with a multi-stage TRIP effect increasing the total elongation to 58%. On the other hand, the precipitation of high-density nano-Cu in the matrix can rapidly disrupt bacterial cell membranes, achieving an excellent antibacterial performance of >99% within 24 hours.

[0037] This invention, through innovative organizational design and process optimization, not only effectively solves the problem of insufficient synergistic improvement of strength and plasticity of austenitic stainless steel under existing processes, but also enables the material to have efficient and long-lasting antibacterial properties. It breaks through the bottleneck that traditional austenitic stainless steel cannot be widely used in medical, industrial and public fields due to its inherent low strength. The process is highly operable and widely applicable, significantly expanding the application scenarios of austenitic stainless steel.

[0038] In summary, compared to traditional methods for preparing hot-work die steel, the method of this invention achieves highly efficient antibacterial properties through the precise addition of Cu. Utilizing a process of "aging + cold rolling + flash annealing," a bimodal nano / ultrafine austenitic microstructure with high-density nano-Cu precipitation is constructed. This microstructure exhibits high mechanical stability, demonstrates a typical multi-stage TRIP effect during deformation, and maintains both high strength and excellent plastic deformation capacity while retaining superior antibacterial properties. This method is simple, easy to operate, environmentally friendly, low-cost, and highly efficient, facilitating large-scale industrial production and widespread application. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1This is a schematic diagram of the process for preparing a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel according to the present invention.

[0041] Figure 2 This is an EBSD grain size distribution diagram of a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel according to Embodiment 1 of the present invention.

[0042] Figure 3 This is a TEM microstructure image of a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel according to Embodiment 1 of the present invention.

[0043] Figure 4 This is a microstructure of Cu precipitation in a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel according to Embodiment 1 of the present invention, wherein the small image in the upper right corner is a high-resolution magnified image of the Cu precipitation microstructure.

[0044] Figure 5 These are engineering stress-strain curves of a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel according to Embodiments 1-4 of the present invention.

[0045] Figure 6 These are bacterial morphology images of a plate after inoculation and incubation for 24 hours of a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel according to Examples 1-4 of the present invention. Detailed Implementation

[0046] The technical solution of the present invention will now be described with reference to the accompanying drawings.

[0047] In embodiments of the present invention, words such as "exemplarily," "for example," etc., are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" in the present invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the word "exemplary" is intended to present the concept in a concrete manner. Furthermore, in embodiments of the present invention, the meaning expressed by "and / or" can be both, or either one.

[0048] In the embodiments of the present invention, the terms "image" and "picture" may sometimes be used interchangeably. It should be noted that when the distinction is not emphasized, their intended meanings are consistent.

[0049] In this embodiment of the invention, sometimes a subscript such as W1 may be written in a non-subscript form such as W1. When the difference is not emphasized, the meaning they express is the same.

[0050] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0051] A high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, wherein the chemical composition of the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel is as follows (by mass percentage): C 0.02-0.07%, Si 0.4-0.6%, Mn 1.3-1.9%, Cr 15.5-18.6%, Cu 3.5-4.1%, Ni 7.3-9.4%, N≤0.05%, S≤0.02%, P≤0.03%, with the remainder being Fe and unavoidable impurities.

[0052] Specifically, the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel is produced using an "aging + cold rolling + flash annealing" process. Its microstructure consists of a heterogeneous bimodal structure with a volume fraction of 70-80% reverse shear austenite and a volume fraction of 20-30% recrystallized deformed austenite, with an average grain size of 2.5-4.2 μm. Furthermore, high-density nano-Cu precipitates with a size of 30-50 nm are distributed in the matrix.

[0053] Specifically, the density of the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel is 7.92-7.97 g / cm³. 3 The hardness is 210-230HV, the tensile strength is >720MPa, the yield strength is >420MPa, the yield ratio is 0.58-0.60, the uniform elongation is >47%, the total elongation is >55%, the strength-ductility product is >41GPa%, and the antibacterial rate is >99%.

[0054] A method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, wherein the method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel is combined with... Figure 1 The preparation steps include the following:

[0055] S1. Melting and casting: Weigh and melt the raw materials according to the composition ratio of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel, and then cast the steel ingot at high temperature.

[0056] S2, High-temperature homogenization + hot forging: The steel billet of S1 is heated to perform high-temperature homogenization treatment, and then hot forged to obtain homogenized steel forgings.

[0057] S3, High-temperature solution treatment + multi-pass hot rolling + water quenching: The homogenized steel forging of S2 is first subjected to high-temperature solution treatment, and after removing the iron oxide scale from the furnace, it is subjected to multi-pass hot rolling, and then water quenched to room temperature to obtain hot-rolled plate.

[0058] S4, Aging + Cold Rolling: The hot-rolled sheet of S3 is aged, cooled to room temperature with water, and then cold-rolled to obtain the cold-rolled sheet.

[0059] S5, Quenching Expansion Tester Flash Annealing: The cold-rolled sheet of S4 is placed in an improved quenching expansion tester, heated rapidly and held at the temperature, and then rapidly cooled to room temperature with nitrogen to obtain a bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties.

[0060] S6. Mechanical property testing + quasi-in-situ analysis: Tensile specimens of bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties (S4 grade) are cut and polished. The polished tensile specimens are used for mechanical property testing. For specimens for quasi-in-situ testing, the surface needs to be polished to a mirror finish by grinding with sandpaper of different grits from small to large. Then, it is polished with a silica gel suspension and followed by quasi-in-situ analysis using electron backscatter diffraction (EBSD).

[0061] S7. Antibacterial performance test and analysis: Square pieces of S4 bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties were cut and tested according to ISO 22196-2011 "Determination of antibacterial properties of plastics and other non-porous surfaces".

[0062] Specifically, the smelting temperature in S1 is 1500-1600℃, the casting temperature is 650-720℃, and the dimensions of the steel ingot are 95×95×115-95×95×135mm.

[0063] Specifically, the heating rate of the high-temperature homogenization treatment in S2 is 15-30℃ / s, the temperature is 1100-1200℃, and the holding time is 20-24h; the size of the homogenized steel forging is 70×70×90-70×70×110mm.

[0064] Specifically, the heating rate of the high-temperature solution treatment in S3 is 10-20℃ / s, the temperature is 1150-1200℃, and the holding time is 5-7h; the initial rolling temperature of the multi-pass hot rolling is 1130-1180℃, the final rolling temperature is 930-970℃, and the thickness of the hot-rolled plate is 5.5-6.2mm.

[0065] Specifically, the heating rate of the aging treatment in S4 is 10-20℃ / s, the temperature is 800-950℃, and the holding time is 15-25h; the total reduction rate of cold rolling is 65-75%; the thickness of the cold-rolled sheet is 1.5-2.0mm; the microstructure after cold rolling contains 70-80% deformation-induced martensite and 20-30% residual deformed austenite by volume, and high-density nano-Cu precipitates with a size of 30-50nm are distributed.

[0066] Specifically, in S5, the heating rate for rapid heating is 70-100℃ / s, the holding temperature is 830-880℃, and the holding time is 1-10s; the cooling rate for rapid nitrogen cooling is 50-80℃ / s; and in the microstructure of the bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and high antibacterial properties, the average grain size of austenite in the fine-grained region is 1.5-2.0μm, and the average grain size of austenite in the coarse-grained region is 3.0-4.0μm.

[0067] Specifically, in S6, the sandpaper used ranges from 240# to 2000#, and the sample is vibrated and polished with silica gel suspension for 6 hours. The gauge length of the tensile specimen after polishing is 15 mm. For the quasi-in-situ tensile test, the parallel section length of the specimen is 4 mm, and it is marked with dots using a surface Vickers hardness tester to ensure the consistency of discontinuous strain and the test area. The strain values ​​are set to 10%, 20%, 30%, and 40%. In S7, the square pieces are cut to a size of 10×10×1.9 mm. For the antibacterial performance test, the sample must first be sterilized with ultraviolet light, and 25 μL of a 1×10 concentration is added. 6 CFU·mL -1 The antibacterial rate was calculated using the plate count method after the bacteria had been in contact with the sample for 24 hours.

[0068] Specifically, the test was conducted according to ISO 22196-2011 "Determination of antimicrobial properties of plastics and other nonporous surfaces": the sample was first sterilized by ultraviolet light, then placed in a 24-well plate, and tested at 37°C and 70% relative humidity with 25 μL of a 1×10⁻⁶ solution. 6 CFU·mL -1 The bacterial suspension was inoculated onto the sample surface, ensuring complete spread and full contact with the material surface (coverage area 1 cm²). After incubating the 24-well plate at 37°C for 24 hours, the bacterial suspension on the sample surface was diluted with 2.5 mL of PBS solution. After shaking to mix, 100 μL of the diluted solution was spread onto LB agar plates and incubated at 37°C for another 24 hours. After removal, the colony status was photographed and the colony count was recorded. A PE membrane was used as a blank control group. Three replicates were set up for each group. The antibacterial rate was calculated using the following formula:

[0069] K = (Nc - Ns) / Nc × 100%

[0070] In the formula, K is the antibacterial rate of the experimental sample, Nc is the number of colonies on the LB agar plate of the control group, and Ns is the number of colonies on the LB agar plate of the experimental group.

[0071] Example 1

[0072] This embodiment describes a high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel. The chemical composition of this high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, by mass percentage, is as follows: C 0.035%, Si 0.43%, Mn 1.39%, Cr 17.6%, Cu 4.08%, Ni 8.33%, N 0.04%, S 0.007%, P 0.025%, with the remainder being Fe and unavoidable impurities.

[0073] A method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, comprising the following preparation steps:

[0074] S1. Melting and casting: The raw materials are weighed and melted according to the composition ratio of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel. The vacuum induction melting temperature is 1550℃, the casting temperature is 680℃, and then the steel ingot is obtained by high-temperature casting. The size of the steel ingot is 95×95×120mm.

[0075] S2, High-temperature homogenization + hot forging: The steel billet of S1 is heated to undergo high-temperature homogenization treatment. The heating rate of the high-temperature homogenization treatment is 20℃ / s, the temperature is 1200℃, and the holding time is 24h; then it is hot forged with a forging ratio of 1.2:1 to obtain homogenized steel forgings; the size of the homogenized steel forgings is 70×70×100mm.

[0076] S3, High-Temperature Solution Treatment + Multi-Pass Hot Rolling + Water Quenching: The homogenized steel forgings of S2 are first subjected to high-temperature solution treatment. The heating rate of the high-temperature solution treatment is 15℃ / s, the temperature is 1150℃, and the holding time is 5h, so that the alloying elements are fully dissolved at high temperature. After removing the iron oxide scale from the furnace, multi-pass hot rolling is performed. The initial rolling temperature of the multi-pass hot rolling is 1150℃, and the final rolling temperature is 950℃. Then, it is water quenched to room temperature to obtain hot-rolled plate. The thickness of the hot-rolled plate is 5.9mm.

[0077] S4, Aging + Cold Rolling: The hot-rolled S3 sheet is aged at a heating rate of 15℃ / s, a temperature of 850℃, and a holding time of 20h. After water cooling to room temperature, it is then cold-rolled with a total reduction rate of 69%, resulting in a cold-rolled sheet with a thickness of 1.9mm. The microstructure after cold rolling contains 75% deformation-induced martensite and 25% residual deformed austenite by volume, and also features high-density nano-Cu precipitates with a size of 45nm.

[0078] S5. Flash annealing in a quenching expander: The cold-rolled sheet of S4 was placed in a modified quenching expander, heated rapidly and held at a temperature of 100℃ / s, with a holding temperature of 850℃ and a holding time of 4s. Subsequently, it was rapidly cooled to room temperature with nitrogen at a cooling rate of 65℃ / s, resulting in a bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and antibacterial properties. The average grain size of austenite in the fine-grained region of the bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and antibacterial properties was 1.8μm, and the average grain size of austenite in the coarse-grained region was 3.6μm.

[0079] S6. Mechanical Property Testing + Quasi-In-Situ Analysis: Tensile specimens were cut from the S4 bimodal nano / ultrafine crystalline austenitic stainless steel, which possesses high strength, plasticity, and antibacterial properties. The surfaces were then polished to a bright finish. The polished tensile specimens were used for mechanical property testing, with a gauge length of 15 mm. For specimens undergoing quasi-in-situ testing, the surface was sequentially smoothed using sandpaper ranging from 240# to 2000#, then mechanically polished to a mirror finish. This was followed by vibratory polishing with a silica gel suspension for 6 hours, and then quasi-in-situ analysis was performed using electron backscatter diffraction (EBSD). The parallel section length of the quasi-in-situ tensile test specimens was 4 mm. Markings were made using a Vickers hardness tester to ensure consistency in discontinuous strain and the test area. Strain values ​​were set at 10%, 20%, 30%, and 40%.

[0080] S7. Antibacterial performance test and analysis: 10×10×1.9mm square pieces of bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties from S4 were cut. The surface of the sample was smoothed by sanding with 240#~2000# sandpaper and then mechanically polished to a mirror finish. The sample was then tested according to ISO 22196-2011 "Determination of antibacterial properties of plastics and other non-porous surfaces".

[0081] For antibacterial performance testing, the sample must first be sterilized by ultraviolet light, and then 25 μL of a 1×10⁻⁶ solution is added. 6 CFU·mL -1 The antibacterial rate was calculated using the plate count method after the bacteria had been in contact with the sample for 24 hours.

[0082] After the above processing, the microstructure is a bimodal nano / ultrafine grain structure with a large amount of nanoscale Cu precipitation, consisting of 80% reverse-transformed austenite and 20% recrystallized austenite. The EBSD grain size distribution diagram is shown below. Figure 2 As shown, the average grain size of the reverse-transformed austenite in the fine-grained region is 1.8 μm, and the average grain size of the deformed austenite in the coarse-grained region is 3.6 μm. Figure 3TEM images of bimodal stainless steel are shown, exhibiting a coexistence of coarse and fine grains, accompanied by the appearance of annealing twins; high-density nano-Cu precipitation and high-resolution images are also presented. Figure 4 As shown, the size is approximately 45 nm; the engineering stress-strain curve is as follows. Figure 5 As shown, the density is 7.93 g / cm³. 3 The stainless steel has a hardness of 215HV, a yield strength of 427MPa, a tensile strength of 728MPa, a yield-to-tensile ratio of 0.587, a uniform elongation of 48.5%, a total elongation of 57.2%, and a strength-ductility product of 41.64GPa%, exhibiting an ultra-high strength-ductility balance. This stainless steel also demonstrates antibacterial effects against Staphylococcus aureus. Figure 6 As shown, there is almost no bacterial growth on the sample surface, and the antibacterial rate is >99%.

[0083] Quasi-in-situ observations were conducted on the content and distribution of deformation-induced martensite in the fine-grained region and the coarse-grained region of the reverse-transformed austenite under different deformation amounts within the same area. When the deformation amount was 11.3%, almost no deformation-induced martensite was generated in either the fine-grained or coarse-grained region. When the strain reached 20.2%, the fine-grained region began to generate more deformation-induced martensite at the grain boundaries, exhibiting a significant TRIP effect. At this point, the martensite generated in the coarse-grained region was still minimal, exhibiting high mechanical stability. When the strain reached 29.2%, a large amount of martensite was generated in the coarse-grained region at shear bands, twin boundaries, and grain boundaries, also beginning to exhibit a significant TRIP effect. When the strain continued to increase to 40.5%, the content of deformation-induced martensite increased sharply, and the multi-stage TRIP effect kept the work hardening rate at a high level for a long time.

[0084] Example 2

[0085] This embodiment describes a high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel. The chemical composition of this high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, by mass percentage, is as follows: C 0.064%, Si 0.58%, Mn 1.43%, Cr 16.8%, Cu 3.73%, Ni 8.92%, N 0.03%, S 0.008%, P 0.013%, with the remainder being Fe and unavoidable impurities.

[0086] A method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, comprising the following preparation steps:

[0087] S1. Melting and casting: The raw materials are weighed and melted according to the composition ratio of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel. The vacuum induction melting temperature is 1520℃, the casting temperature is 650℃, and then the steel ingot is obtained by high-temperature casting. The size of the steel ingot is 95×95×115mm.

[0088] S2, High-temperature homogenization + hot forging: The steel billet of S1 is heated to undergo high-temperature homogenization treatment. The heating rate of the high-temperature homogenization treatment is 15℃ / s, the temperature is 1200℃, and the holding time is 24h; then it is hot forged with a forging ratio of 1.15:1 to obtain homogenized steel forgings; the size of the homogenized steel forgings is 70×70×100mm.

[0089] S3, High-Temperature Solution Treatment + Multi-Pass Hot Rolling + Water Quenching: The homogenized steel forgings of S2 are first subjected to high-temperature solution treatment at a heating rate of 10℃ / s, a temperature of 1165℃, and a holding time of 6h, so that the alloying elements are fully dissolved at high temperature. After removing the iron oxide scale from the furnace, they are subjected to multi-pass hot rolling at an initial rolling temperature of 1130℃ and a final rolling temperature of 930℃. They are then water quenched to room temperature to obtain hot-rolled plates. The thickness of the hot-rolled plates is 5.5mm.

[0090] S4, Aging + Cold Rolling: The hot-rolled S3 sheet was aged at a heating rate of 10℃ / s, a temperature of 800℃, and a holding time of 22h. After water cooling to room temperature, it was cold-rolled with a total reduction rate of 67%, resulting in a cold-rolled sheet with a thickness of 1.8mm. The microstructure after cold rolling contained 72% deformation-induced martensite and 28% residual deformed austenite by volume, and also contained high-density nano-Cu precipitates with a size of 36nm.

[0091] S5. Flash annealing in a quenching expander: The cold-rolled sheet of S4 was placed in a modified quenching expander, heated rapidly and held at a temperature of 90℃ / s, with a holding temperature of 860℃ and a holding time of 4s. Subsequently, it was rapidly cooled to room temperature with nitrogen at a cooling rate of 50℃ / s, resulting in a bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and antibacterial properties. The average grain size of austenite in the fine-grained region of the bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and antibacterial properties was 1.9μm, and the average grain size of austenite in the coarse-grained region was 3.8μm.

[0092] S6. Mechanical Property Testing + Quasi-In-Situ Analysis: Tensile specimens were cut from the S4 bimodal nano / ultrafine crystalline austenitic stainless steel, which possesses high strength, plasticity, and antibacterial properties. The surfaces were then polished to a bright finish. The polished tensile specimens were used for mechanical property testing, with a gauge length of 15 mm. For specimens undergoing quasi-in-situ testing, the surface was sequentially smoothed using sandpaper ranging from 240# to 2000#, then mechanically polished to a mirror finish. This was followed by vibratory polishing with a silica gel suspension for 6 hours, and then quasi-in-situ analysis was performed using electron backscatter diffraction (EBSD). The parallel section length of the quasi-in-situ tensile test specimens was 4 mm. Markings were made using a Vickers hardness tester to ensure consistency in discontinuous strain and the test area. Strain values ​​were set at 10%, 20%, 30%, and 40%.

[0093] S7. Antibacterial performance test and analysis: 10×10×1.8mm square pieces of bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties from S4 were cut. The surface of the sample was smoothed by sanding with 240#~2000# sandpaper and then mechanically polished to a mirror finish. The sample was then tested according to ISO 22196-2011 "Determination of antibacterial properties of plastics and other non-porous surfaces".

[0094] For antibacterial performance testing, the sample must first be sterilized by ultraviolet light, and then 25 μL of a 1×10⁻⁶ solution is added. 6 CFU·mL -1 The antibacterial rate was calculated using the plate count method after the bacteria had been in contact with the sample for 24 hours.

[0095] After undergoing an aging + cold rolling + annealing process, the microstructure is a bimodal nano / ultrafine grain structure with a large amount of nanoscale Cu precipitation, consisting of 78% reverse-transformed austenite and 22% recrystallized austenite. The average grain size of the reverse-transformed austenite in the fine-grained region is 1.9 μm, and the average grain size of the deformed austenite in the coarse-grained region is 3.8 μm. High-density nano-Cu precipitation and high-resolution images are shown below. Figure 4 As shown, the size is approximately 36 nm. In this embodiment, after the steel was in contact with Staphylococcus aureus for a period of time, almost no bacteria grew on the surface, demonstrating an antibacterial rate >99%; the density of the steel in this embodiment is 7.92 g / cm³. 3 It has a hardness of 210HV, a yield strength of 421MPa, a tensile strength of 722MPa, a yield-to-tensile ratio of 0.583, a uniform elongation of 49.6%, a total elongation of 58.3%, and a strength-ductility product of 42.64GPa%, exhibiting an ultra-high strength-ductility balance.

[0096] Quasi-in-situ observations were conducted on the content and distribution of deformation-induced martensite in the fine-grained region and the coarse-grained region of the reverse-transformed austenite under different deformation amounts within the same area. When the deformation amount was 11.3%, almost no deformation-induced martensite was generated in either the fine-grained or coarse-grained region. When the strain reached 20.2%, the fine-grained region began to generate more deformation-induced martensite at the grain boundaries, exhibiting a significant TRIP effect. At this point, the martensite generated in the coarse-grained region was still minimal, exhibiting high mechanical stability. When the strain reached 29.2%, a large amount of martensite was generated in the coarse-grained region at shear bands, twin boundaries, and grain boundaries, also beginning to exhibit a significant TRIP effect. When the strain continued to increase to 40.5%, the content of deformation-induced martensite increased sharply, and the multi-stage TRIP effect kept the work hardening rate at a high level for a long time.

[0097] Example 3

[0098] This embodiment describes a high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel. The chemical composition of this high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, by mass percentage, is as follows: C 0.055%, Si 0.41%, Mn 1.57%, Cr 15.7%, Cu 3.58%, Ni 7.88%, N 0.04%, S 0.007%, P 0.018%, with the remainder being Fe and unavoidable impurities.

[0099] A method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, comprising the following preparation steps:

[0100] S1. Melting and casting: The raw materials are weighed and melted according to the composition ratio of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel. The vacuum induction melting temperature is 1565℃, the casting temperature is 695℃, and then the steel ingot is obtained by high-temperature casting. The size of the steel ingot is 95×95×105mm.

[0101] S2, High-temperature homogenization + hot forging: The steel billet of S1 is heated to undergo high-temperature homogenization treatment. The heating rate of the high-temperature homogenization treatment is 25℃ / s, the temperature is 1200℃, and the holding time is 24h; then it is hot forged with a forging ratio of 1.05:1 to obtain homogenized steel forgings; the size of the homogenized steel forgings is 70×70×100mm.

[0102] S3, High-Temperature Solution Treatment + Multi-Pass Hot Rolling + Water Quenching: The homogenized steel forgings of S2 are first subjected to high-temperature solution treatment. The heating rate of the high-temperature solution treatment is 18℃ / s, the temperature is 1175℃, and the holding time is 6h, so that the alloying elements are fully dissolved at high temperature. After removing the iron oxide scale from the furnace, multi-pass hot rolling is performed. The initial rolling temperature of the multi-pass hot rolling is 1140℃, and the final rolling temperature is 960℃. Then, it is water quenched to room temperature to obtain hot-rolled plate. The thickness of the hot-rolled plate is 5.7mm.

[0103] S4, Aging + Cold Rolling: The hot-rolled S3 sheet is aged at a heating rate of 18℃ / s, a temperature of 840℃, and a holding time of 24h. After water cooling to room temperature, it is cold rolled with a total reduction rate of 70%, resulting in a cold-rolled sheet with a thickness of 1.7mm. The microstructure after cold rolling contains 77% deformation-induced martensite and 23% residual deformed austenite by volume, and high-density nano-Cu precipitates with a size of 34nm are distributed.

[0104] S5. Flash annealing in a quenching expander: The cold-rolled sheet of S4 was placed in a modified quenching expander, heated rapidly and held at a temperature of 95℃ / s, with a holding temperature of 845℃ and a holding time of 4s. Subsequently, it was rapidly cooled to room temperature with nitrogen at a cooling rate of 70℃ / s, resulting in a bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and antibacterial properties. The average grain size of austenite in the fine-grained region of the bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and antibacterial properties was 1.6μm, and the average grain size of austenite in the coarse-grained region was 3.1μm.

[0105] S6. Mechanical Property Testing + Quasi-In-Situ Analysis: Tensile specimens were cut from the S4 bimodal nano / ultrafine crystalline austenitic stainless steel, which possesses high strength, plasticity, and antibacterial properties. The surfaces were then polished to a bright finish. The polished tensile specimens were used for mechanical property testing, with a gauge length of 15 mm. For specimens undergoing quasi-in-situ testing, the surface was sequentially smoothed using sandpaper ranging from 240# to 2000#, then mechanically polished to a mirror finish. This was followed by vibratory polishing with a silica gel suspension for 6 hours, and then quasi-in-situ analysis was performed using electron backscatter diffraction (EBSD). The parallel section length of the quasi-in-situ tensile test specimens was 4 mm. Markings were made using a Vickers hardness tester to ensure consistency in discontinuous strain and the test area. Strain values ​​were set at 10%, 20%, 30%, and 40%.

[0106] S7. Antibacterial performance test and analysis: 10×10×1.7mm square pieces of bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties were cut from S4. The surface of the sample was smoothed by grinding with 240#~2000# sandpaper and then mechanically polished to a mirror finish. The sample was then tested according to ISO 22196-2011 "Determination of antibacterial properties of plastics and other non-porous surfaces".

[0107] For antibacterial performance testing, the sample must first be sterilized by ultraviolet light, and then 25 μL of a 1×10⁻⁶ solution is added. 6 CFU·mL -1 The antibacterial rate was calculated using the plate count method after the bacteria had been in contact with the sample for 24 hours.

[0108] After undergoing an aging, cold rolling, and annealing process, the microstructure is a bimodal nano / ultrafine grain structure with abundant nanoscale Cu precipitation, consisting of 79% reverse-transformed austenite and 21% recrystallized austenite. The average grain size of the reverse-transformed austenite in the fine-grained region is 1.6 μm, and the average grain size of the deformed austenite in the coarse-grained region is 3.1 μm. High-density nano-Cu precipitation and high-resolution images are shown below. Figure 4 As shown, the size is approximately 34 nm. In this embodiment, after the steel was in contact with Staphylococcus aureus for a period of time, almost no bacteria grew on the surface, demonstrating an antibacterial rate >99%; the density of the steel in this embodiment is 7.93 g / cm³. 3 It has a hardness of 221HV, a yield strength of 432MPa, a tensile strength of 733MPa, a yield-to-tensile ratio of 0.589, a uniform elongation of 47.6%, a total elongation of 56.4%, and a strength-ductility product of 41.17GPa%, exhibiting an ultra-high strength-ductility balance.

[0109] Quasi-in-situ observations were conducted on the content and distribution of deformation-induced martensite in the fine-grained region and the coarse-grained region of the reverse-transformed austenite under different deformation amounts within the same area. When the deformation amount was 11.3%, almost no deformation-induced martensite was generated in either the fine-grained or coarse-grained region. When the strain reached 20.2%, the fine-grained region began to generate more deformation-induced martensite at the grain boundaries, exhibiting a significant TRIP effect. At this point, the martensite generated in the coarse-grained region was still minimal, exhibiting high mechanical stability. When the strain reached 29.2%, a large amount of martensite was generated in the coarse-grained region at shear bands, twin boundaries, and grain boundaries, also beginning to exhibit a significant TRIP effect. When the strain continued to increase to 40.5%, the content of deformation-induced martensite increased sharply, and the multi-stage TRIP effect kept the work hardening rate at a high level for a long time.

[0110] Example 4

[0111] This embodiment describes a high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel. The chemical composition of this high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, by mass percentage, is as follows: C 0.043%, Si 0.52%, Mn 1.88%, Cr 18.2%, Cu 3.94%, Ni 7.45%, N 0.02%, S 0.005%, P 0.021%, with the remainder being Fe and unavoidable impurities.

[0112] A method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, comprising the following preparation steps:

[0113] S1. Melting and casting: The raw materials are weighed and melted according to the composition ratio of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel. The vacuum induction melting temperature is 1585℃, the casting temperature is 705℃, and then the steel ingot is obtained by high-temperature casting. The size of the steel ingot is 95×95×130mm.

[0114] S2, High-temperature homogenization + hot forging: The steel billet of S1 is heated to undergo high-temperature homogenization treatment. The heating rate of the high-temperature homogenization treatment is 30℃ / s, the temperature is 1200℃, and the holding time is 24h; then it is hot forged with a forging ratio of 1.3:1 to obtain homogenized steel forgings; the size of the homogenized steel forgings is 70×70×100mm.

[0115] S3, High-Temperature Solution Treatment + Multi-Pass Hot Rolling + Water Quenching: The homogenized steel forgings of S2 are first subjected to high-temperature solution treatment. The heating rate of the high-temperature solution treatment is 20℃ / s, the temperature is 1190℃, and the holding time is 7h, so that the alloying elements are fully dissolved at high temperature. After removing the iron oxide scale from the furnace, multi-pass hot rolling is performed. The initial rolling temperature of the multi-pass hot rolling is 1160℃, and the final rolling temperature is 970℃. Then, it is water quenched to room temperature to obtain hot-rolled plate. The thickness of the hot-rolled plate is 5.6mm.

[0116] S4, Aging + Cold Rolling: The hot-rolled sheet of S3 is aged at a heating rate of 20℃ / s, a temperature of 870℃, and a holding time of 18h; it is then water-cooled to room temperature and cold-rolled with a total reduction rate of 73%, resulting in a cold-rolled sheet with a thickness of 1.5mm; the microstructure after cold rolling contains 80% deformation-induced martensite and 20% residual deformed austenite, and high-density nano-Cu precipitates with a size of 39nm are distributed.

[0117] S5. Flash annealing in a quenching expander: The cold-rolled sheet of S4 was placed in a modified quenching expander, heated rapidly and held at a temperature of 85℃ / s, with a holding temperature of 830℃ and a holding time of 8s. Subsequently, it was rapidly cooled to room temperature with nitrogen at a cooling rate of 75℃ / s, resulting in a bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and high antibacterial properties. The average grain size of austenite in the fine-grained region of the bimodal nano / ultrafine-grained austenitic stainless steel with high strength, plasticity, and high antibacterial properties was 1.7μm, and the average grain size of austenite in the coarse-grained region was 3.3μm.

[0118] S6. Mechanical Property Testing + Quasi-In-Situ Analysis: Tensile specimens were cut from the S4 bimodal nano / ultrafine crystalline austenitic stainless steel, which possesses high strength, plasticity, and antibacterial properties. The surfaces were then polished to a bright finish. The polished tensile specimens were used for mechanical property testing, with a gauge length of 15 mm. For specimens undergoing quasi-in-situ testing, the surface was sequentially smoothed using sandpaper ranging from 240# to 2000#, then mechanically polished to a mirror finish. This was followed by vibratory polishing with a silica gel suspension for 6 hours, and then quasi-in-situ analysis was performed using electron backscatter diffraction (EBSD). The parallel section length of the quasi-in-situ tensile test specimens was 4 mm. Markings were made using a Vickers hardness tester to ensure consistency in discontinuous strain and the test area. Strain values ​​were set at 10%, 20%, 30%, and 40%.

[0119] S7. Antibacterial performance test and analysis: Cut 10×10×1.5mm square pieces of bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties from S4. After grinding the sample surface with 240#~2000# sandpaper, mechanically polish it to a mirror finish and then test it according to ISO 22196-2011 "Determination of antibacterial properties of plastics and other non-porous surfaces".

[0120] For antibacterial performance testing, the sample must first be sterilized by ultraviolet light, and then 25 μL of a 1×10⁻⁶ solution is added. 6 CFU·mL -1 The antibacterial rate was calculated using the plate count method after the bacteria had been in contact with the sample for 24 hours.

[0121] After undergoing an aging, cold rolling, and annealing process, the microstructure is a bimodal nano / ultrafine grain structure with abundant nanoscale Cu precipitation, consisting of 79% reverse-transformed austenite and 21% recrystallized austenite. The average grain size of the reverse-transformed austenite in the fine-grained region is 1.7 μm, and the average grain size of the deformed austenite in the coarse-grained region is 3.3 μm. High-density nano-Cu precipitation and high-resolution images are shown below. Figure 4As shown, the size is approximately 39 nm. In this embodiment, after the steel was in contact with Staphylococcus aureus for a period of time, almost no bacteria grew on the surface, demonstrating an antibacterial rate >99%; the density of the steel in this embodiment is 2.75 g / cm³. 3 It has a hardness of 226HV, a yield strength of 437MPa, a tensile strength of 740MPa, a yield-to-tensile ratio of 0.591, a uniform elongation of 47.1%, a total elongation of 55.9%, and a strength-ductility product of 41.37GPa%, exhibiting an ultra-high strength-ductility balance.

[0122] Quasi-in-situ observations were conducted on the content and distribution of deformation-induced martensite in the fine-grained region and the coarse-grained region of the reverse-transformed austenite under different deformation amounts within the same area. When the deformation amount was 11.3%, almost no deformation-induced martensite was generated in either the fine-grained or coarse-grained region. When the strain reached 20.2%, the fine-grained region began to generate more deformation-induced martensite at the grain boundaries, exhibiting a significant TRIP effect. At this point, the martensite generated in the coarse-grained region was still minimal, exhibiting high mechanical stability. When the strain reached 29.2%, a large amount of martensite was generated in the coarse-grained region at shear bands, twin boundaries, and grain boundaries, also beginning to exhibit a significant TRIP effect. When the strain continued to increase to 40.5%, the content of deformation-induced martensite increased sharply, and the multi-stage TRIP effect kept the work hardening rate at a high level for a long time.

[0123] The present invention proposes a high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel and its preparation method, which can solve the technical problems existing in the preparation technology of medical austenitic antibacterial stainless steel, such as the inability to synergistically improve the strength, plasticity, and antibacterial properties.

[0124] The present invention describes a bimodal austenitic stainless steel with high strength, plasticity, and high antibacterial properties. The addition of Cu not only appropriately increases the stacking fault energy of the austenitic stainless steel and improves the stability of austenite, but more importantly, it also enhances the properties of copper ions (Cu). 2+ It has a strong bactericidal ability and the Cu content meets medical-grade standards, realizing the innovative design concept of "medical-engineering integration".

[0125] This invention discloses a high-strength, high-plasticity, and high-antibacterial-performance bimodal austenitic stainless steel and its preparation process. The process employs "aging + cold rolling + flash annealing," firstly causing the uniform precipitation of high-density nano-Cu, then controlling the proportion of deformation-induced martensite and deformed austenite, and finally utilizing the difference in nucleation rates between the two during annealing to obtain a bimodal-scale nano / ultrafine-grained austenitic microstructure. On the one hand, the bimodal-scale nano / ultrafine-grained structure exhibits high mechanical stability and excellent strength-plasticity matching, with a multi-stage TRIP effect increasing the total elongation to 58%. On the other hand, the precipitation of high-density nano-Cu in the matrix can rapidly disrupt bacterial cell membranes, achieving an excellent antibacterial performance of >99% within 24 hours.

[0126] This invention, through innovative organizational design and process optimization, not only effectively solves the problem of insufficient synergistic improvement of strength and plasticity of austenitic stainless steel under existing processes, but also enables the material to have efficient and long-lasting antibacterial properties. It breaks through the bottleneck that traditional austenitic stainless steel cannot be widely used in medical, industrial and public fields due to its inherent low strength. The process is highly operable and widely applicable, significantly expanding the application scenarios of austenitic stainless steel.

[0127] In summary, compared to traditional methods for preparing hot-work die steel, the method of this invention achieves highly efficient antibacterial properties through the precise addition of Cu. Utilizing a process of "aging + cold rolling + flash annealing," a bimodal nano / ultrafine austenitic microstructure with high-density nano-Cu precipitation is constructed. This microstructure exhibits high mechanical stability, demonstrates a typical multi-stage TRIP effect during deformation, and maintains both high strength and excellent plastic deformation capacity while retaining superior antibacterial properties. This method is simple, easy to operate, environmentally friendly, low-cost, and highly efficient, facilitating large-scale industrial production and widespread application.

[0128] It should be understood that the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. Additionally, the character " / " in this article generally indicates an "or" relationship between the preceding and following related objects, but it can also represent an "and / or" relationship. Please refer to the context for a more accurate understanding.

[0129] In this invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of a single item or a plurality of items. For example, at least one of a, b, or c can represent: a, b, c, ab, ac, bc, or abc, where a, b, and c can be a single item or multiple items.

[0130] It should be understood that, in various embodiments of the present invention, the order of the above-mentioned process numbers does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0131] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel, characterized in that, The chemical composition of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel, by mass percentage, is as follows: C 0.02-0.07%, Si 0.4-0.6%, Mn 1.3-1.9%, Cr 15.5-18.6%, Cu 3.5-4.1%, Ni 7.3-9.4%, N≤0.05%, S≤0.02%, P≤0.03%, with the remainder being Fe and unavoidable impurities.

2. The high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 1, characterized in that, The high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel is produced using an "aging + cold rolling + flash annealing" process. Its microstructure consists of a heterogeneous bimodal structure with a volume fraction of 70-80% reverse shear austenite and a volume fraction of 20-30% recrystallized deformed austenite, with an average grain size of 2.5-4.2 μm. Furthermore, high-density nano-Cu precipitates with a size of 30-50 nm are distributed in the matrix.

3. The high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 1, characterized in that, The density of the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel is 7.92-7.97 g / cm³. 3 The hardness is 210-230HV, the tensile strength is >720MPa, the yield strength is >420MPa, the yield ratio is 0.58-0.60, the uniform elongation is >47%, the total elongation is >55%, the strength-ductility product is >41GPa%, and the antibacterial rate is >99%.

4. A method for preparing a high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 1, characterized in that, The preparation method of the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel includes the following preparation steps: S1. Melting and casting: Weigh and melt the raw materials according to the composition ratio of the high-strength, high-plasticity, and high-antibacterial bimodal austenitic stainless steel, and then cast the steel ingot at high temperature. S2, High-temperature homogenization + hot forging: The steel billet of S1 is heated to perform high-temperature homogenization treatment, and then hot forged to obtain homogenized steel forgings. S3, High-temperature solution treatment + multi-pass hot rolling + water quenching: The homogenized steel forging of S2 is first subjected to high-temperature solution treatment, and after removing the iron oxide scale from the furnace, it is subjected to multi-pass hot rolling, and then water quenched to room temperature to obtain hot-rolled plate. S4, Aging + Cold Rolling: The hot-rolled sheet of S3 is aged, cooled to room temperature with water, and then cold-rolled to obtain the cold-rolled sheet. S5, Quenching Expansion Tester Flash Annealing: The cold-rolled sheet of S4 is placed in an improved quenching expansion tester, heated rapidly and held at the temperature, and then rapidly cooled to room temperature with nitrogen to obtain a bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties. S6. Mechanical property testing + quasi-in-situ analysis: Tensile specimens of bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties (S4 grade) are cut and polished. The polished tensile specimens are used for mechanical property testing. For specimens for quasi-in-situ testing, the surface needs to be polished to a mirror finish by grinding with sandpaper of different grits from small to large. Then, it is polished with a silica gel suspension and followed by quasi-in-situ analysis using electron backscatter diffraction (EBSD). S7. Antibacterial performance test and analysis: Square pieces of S4 bimodal nano / ultrafine crystalline austenitic stainless steel with high strength, plasticity and high antibacterial properties were cut and tested according to ISO 22196-2011 "Determination of antibacterial properties of plastics and other non-porous surfaces".

5. The method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 4, characterized in that, The smelting temperature in S1 is 1500-1600℃, the casting temperature is 650-720℃, and the dimensions of the steel ingot are 95×95×115-95×95×135mm.

6. The method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 4, characterized in that, The heating rate for high-temperature homogenization treatment in S2 is 15-30℃ / s, the temperature is 1100-1200℃, and the holding time is 20-24h; the dimensions of the homogenized steel forgings are 70×70×90-70×70×110mm.

7. The method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 4, characterized in that, The heating rate for high-temperature solution treatment in S3 is 10-20℃ / s, the temperature is 1150-1200℃, and the holding time is 5-7h; the initial rolling temperature for multi-pass hot rolling is 1130-1180℃, the final rolling temperature is 930-970℃, and the thickness of the hot-rolled plate is 5.5-6.2mm.

8. The method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 4, characterized in that, The heating rate for aging treatment in S4 is 10-20℃ / s, the temperature is 800-950℃, and the holding time is 15-25h; the total reduction rate of cold rolling is 65-75%; the thickness of cold-rolled sheet is 1.5-2.0mm; the microstructure after cold rolling contains 70-80% deformation-induced martensite and 20-30% residual deformed austenite by volume, and high-density nano-Cu precipitation with a size of 30-50nm is distributed.

9. The method for preparing high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 4, characterized in that, The heating rate for rapid heating in S5 is 70-100℃ / s, the holding temperature is 830-880℃, and the holding time is 1-10s; the cooling rate for rapid nitrogen cooling is 50-80℃ / s; the average grain size of austenitic stainless steel with a bimodal nano / ultrafine grain structure, which has high strength, plasticity, and high antibacterial properties, is 1.5-2.0μm in the fine grain region and 3.0-4.0μm in the coarse grain region.

10. The method for preparing the high-strength, high-ductility, and high-antibacterial bimodal austenitic stainless steel according to claim 4, characterized in that, In S6, sandpaper of different grits from 240# to 2000# is used. The sample is vibrated and polished with silica gel suspension for 6 hours. The gauge length of the tensile specimen after polishing is 15 mm. For the quasi-in-situ tensile test, the parallel section length of the specimen is 4 mm. Marking is done using a surface Vickers hardness tester to ensure consistency of discontinuous strain and the test area. The strain values ​​are set at 10%, 20%, 30%, and 40%. In S7, the cut square pieces are 10×10×1.9 mm in size. For antibacterial performance testing, the sample must first be sterilized with ultraviolet light, and then 25 μL of a 1×10⁻⁶ concentration is added. 6 The antibacterial rate was calculated using the plate count method after the bacteria were in contact with the sample for 24 hours at a bacterial suspension of CFU·mL⁻¹.

Images