Preparation process of high-strength corrosion-resistant stainless steel fastener

Abstract

The invention discloses a preparation process of a high-strength corrosion-resistant stainless steel fastener. The preparation process comprises the steps of material modification, warm upsetting forming, stabilizing treatment, cryogenic strengthening, gradient aging, surface treatment, post-treatment and the like. According to the process, the alloy component proportion is optimized, a thermal mechanical treatment technology of warm upsetting forming, subzero treatment and gradient aging is combined, a nano-composite coating and micro-arc oxidation sealing process is innovatively introduced, and the process comprises vacuum melting and controlled rolling of customized high-nitrogen stainless steel wires, deformation-induced martensite regulation in warm upsetting forming, low-temperature hot rolling, high-temperature hot rolling, high-temperature hot rolling, high-temperature hot rolling, high-temperature hot rolling and high-temperature hot rolling. The tensile strength and the salt spray corrosion resistance of the fastener are remarkably improved through deep cooling-medium temperature aging synergistic strengthening, double-layer nano TiN / CrN composite coating deposition and micro-arc oxidation micropore sealing treatment, the problem that a traditional stainless steel fastener is insufficient in strength and low in stress corrosion sensitivity is solved through the technology, and the technology is particularly suitable for the extreme environments such as aerospace and ocean engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material processing, and in particular to a preparation process of a high-strength, corrosion-resistant stainless steel fastener. Background Art

[0002] Stainless steel fasteners are a general-purpose connection component made of stainless steel. They are corrosion-resistant and durable, and are often used in precision machinery or valuable equipment. They include 12 main categories, including bolts, studs, screws, and nuts. These fasteners use threads to create removable or permanent connections and are widely used in industry.

[0003] The traditional manufacturing process for stainless steel fasteners primarily involves the following steps: raw material modification, blank forming, heat treatment, surface treatment, and packaging selection. Commonly used fasteners suffer from issues such as insufficient strength and sensitivity to stress corrosion. Existing improvement processes typically employ secondary aging, but this reduces corrosion resistance. Others employ surface coating with a DLC film to improve corrosion resistance, but the film-base bonding is poor, making it unable to withstand assembly torque. Therefore, there is an urgent need to develop a manufacturing process that combines ultra-high strength with long-term corrosion resistance. Summary of the Invention

[0004] Technical problems solved In response to the shortcomings of the existing technology, the process of the present invention solves the problems of insufficient strength and stress corrosion sensitivity of traditional stainless steel fasteners by optimizing the alloy component ratio and combining the thermomechanical treatment technology of warm upsetting-cryogenic treatment-gradient aging.

[0005] Technical Solution To achieve the above object, the present invention provides the following technical solution: a process for preparing a high-strength, corrosion-resistant stainless steel fastener, comprising the following steps: Step S1: Material modification: vacuum induction melting is used to prepare an alloy, wherein the composition by weight is as follows: C ≤ 0.03%, Cr 16-18%, Ni 10-12%, Mo 2.5-3.5%, N 0.2-0.4%, Mn 4-6%, Si ≤ 0.8%, and the balance is Fe and unavoidable impurities. The ingot is solution treated at 1150°C and then controlled rolled into a Φ5-20 mm wire. Step S2: Warm forging, heating the wire to 80-120°C for cold forging, controlling the deformation to 40-60%, and water quenching immediately after forming; Step S3: Stabilization treatment: Place the blank obtained in S2 in a vacuum furnace, heat it to 250°C at a rate of 10°C / min, hold it for 30 minutes, and then cool it to 80°C in the furnace; Step S4: cryogenic strengthening, treating in a liquid nitrogen environment at -190°C for 2-4 hours, with a heating rate of ≤5°C / min; Step S5: Gradient aging, first keeping the temperature at 300°C for 1 hour, then raising the temperature to 480°C for 3 hours, and air cooling; Step S6: Surface treatment, shot peening, electrolytic polishing, magnetron sputtering deposition of TiN / CrN composite coating (single layer thickness 0.5-1 μm, total thickness 3-5 μm), micro-arc oxidation sealing treatment; Step S7: Post-treatment: immerse the S6 workpiece in a passivation solution containing 3 wt% cerium nitrate and 5 wt% citric acid at 60° C. for 20 minutes, rinse with deionized water, and then dry.

[0006] As a preferred solution, the controlled rolling in step S1 adopts three-stage temperature control, with an initial rolling temperature of 1050°C, a final rolling temperature of 850°C, a total reduction rate of ≥70%, and a grain size of the wire rod after rolling of ≥8 levels.

[0007] As a preferred solution, the specific operations of the warm upsetting forming in step S2 include: a) Preheating: The wire is heated by a medium frequency induction coil, with a temperature gradient of ≤15°C / m and a target temperature of 80-120°C. b) Multi-station deformation: Execute in sequence on a three-station cold heading machine: Station 1: Diameter reduction deformation, compression rate 35-45%, mold cone angle 30°; Station 2: Pre-upsetting the head, deformation 15-25%, spherical radius R=1.5d, d is the nominal diameter of the bolt; Station 3: Final upsetting, total deformation 40-60%, mold cavity surface texturing, roughness Ra = 0.05-0.1μm; c) Dynamic temperature control: Infrared thermometers are installed between each workstation to adjust the induction heating power with real-time feedback, and the temperature fluctuation is ≤±5℃; d) Water quenching strengthening: spray cooling water within 0.5 seconds after forming (water temperature 20-30℃, flow rate 10L / min), cooling rate ≥150℃ / s.

[0008] As a preferred solution, before warm upsetting in step S2, the surface of the wire is coated with nanographite lubricant, wherein the particle size is ≤100nm and the coating thickness is 2-5μm; after forming, surface cracks are detected using an eddy current flaw detector, and the defect signal threshold is ≤2mV.

[0009] As a preferred solution, during the stabilization treatment in step S3, the vacuum degree is ≤10⁻²Pa and the cooling rate is ≤3°C / min.

[0010] As a preferred embodiment, the shot peening in step S6 uses ceramic balls with a diameter of 0.1-0.3 mm; the micro-arc oxidation electrolyte contains Na2SiO3 10 g / L, KOH 5 g / L, and Na2WO4 2 g / L, and uses a bipolar pulse power supply with a positive voltage of 350 V / negative voltage of 80 V and a frequency of 1000 Hz.

[0011] As a preferred solution, the pH value of the passivation solution in step S7 is 3.5-4.0, the thickness of the passivation film is 50-80 nm, and XPS detection shows that the surface Cr / Fe atomic ratio is ≥2.5.

[0012] As a preferred solution, it also includes: Establishing the aging temperature-precipitation phase size model: d=K·exp(-Q / RT) Where d is the diameter of Cr2N phase, K=1.2×10⁻ 8 m, Q = 185 kJ / mol; The aging parameters are inferred based on the target strength value: when the tensile strength is required to be ≥1200MPa, the size of the Cr2N phase in the aging stage at 480℃ is controlled to be 80±10nm.

[0013] (3) Beneficial effects Compared with the prior art, the present invention provides a preparation process for high-strength, corrosion-resistant stainless steel fasteners, which has the following beneficial effects: 1. The process of the present invention optimizes the alloy composition ratio, combines the thermomechanical treatment technology of warm upsetting-cryogenic treatment-gradient aging, and innovatively introduces nano-composite coating and micro-arc oxidation sealing technology, including vacuum melting and controlled rolling of customized high-nitrogen stainless steel wire, deformation-induced martensite regulation in warm upsetting, deep-cryogenic-medium-temperature aging synergistic strengthening, double-layer nano-TiN / CrN composite coating deposition and micro-arc oxidation micropore sealing treatment, which significantly improves the tensile strength and salt spray corrosion resistance of fasteners. This process solves the problems of insufficient strength and stress corrosion sensitivity of traditional stainless steel fasteners, and is particularly suitable for extreme environments such as aerospace and marine engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of the overall process flow of the present invention; Figure 2 This is a schematic diagram of the warm upsetting process of the present invention; Figure 3 It is a schematic diagram of the surface treatment process of the present invention. DETAILED DESCRIPTION

[0015] In order to better understand the purpose, structure and function of the present invention, the preparation process of a high-strength, corrosion-resistant stainless steel fastener of the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0016] Example 1 refer to Figure 1-3 The present invention provides a process for preparing a high-strength, corrosion-resistant stainless steel fastener, comprising the following steps: Step S1: Material modification: vacuum induction melting is used to prepare an alloy, wherein the composition by weight is as follows: C ≤ 0.03%, Cr 16-18%, Ni 10-12%, Mo 2.5-3.5%, N 0.2-0.4%, Mn 4-6%, Si ≤ 0.8%, and the balance is Fe and unavoidable impurities. The ingot is solution treated at 1150°C and then controlled rolled into a Φ5-20 mm wire. Step S2: Warm forging, heating the wire to 80-120°C for cold forging, controlling the deformation to 40-60%, and water quenching immediately after forming; Step S3: Stabilization treatment: Place the blank obtained in S2 in a vacuum furnace, heat it to 250°C at a rate of 10°C / min, hold it for 30 minutes, and then cool it to 80°C in the furnace; Step S4: cryogenic strengthening, treating in a liquid nitrogen environment at -190°C for 2-4 hours, with a heating rate of ≤5°C / min; Step S5: Gradient aging, first keeping the temperature at 300°C for 1 hour, then raising the temperature to 480°C for 3 hours, and air cooling; Step S6: Surface treatment, shot peening, electrolytic polishing, magnetron sputtering deposition of TiN / CrN composite coating (single layer thickness 0.5-1 μm, total thickness 3-5 μm), micro-arc oxidation sealing treatment; Step S7: Post-treatment: immerse the S6 workpiece in a passivation solution containing 3 wt% cerium nitrate and 5 wt% citric acid at 60° C. for 20 minutes, rinse with deionized water, and then dry.

[0017] Specifically, in step S1 of the present invention, a high nitrogen content (0.2-0.4%) replaces part of the nickel, and the strength is improved by solid solution strengthening. At the same time, the N element promotes the stability of the passivation film, and Mo+Mn synergistically improves the pitting corrosion equivalent PREN ≥ 40. At the same time, the S1 controlled rolling adopts three-stage temperature control, with an initial rolling temperature of 1050°C, a final rolling temperature of 850°C, a total reduction rate of ≥70%, and a wire grain size of ≥8 after rolling, which can further improve the tensile strength and salt spray corrosion resistance of the fastener.

[0018] In step S2, warm upsetting is performed by plastic deformation in the metastable region of austenite, inducing the formation of strengthened α'-martensite (non-brittle phase) while avoiding cold upsetting cracks. The specific operations include: a) Preheating: The wire is heated by a medium frequency induction coil, with a temperature gradient of ≤15°C / m and a target temperature of 80-120°C. b) Multi-station deformation: Execute in sequence on a three-station cold heading machine: Station 1: Diameter reduction deformation, compression rate 35-45%, mold cone angle 30°; Station 2: Pre-upsetting the head, deformation 15-25%, spherical radius R=1.5d, d is the nominal diameter of the bolt; Station 3: Final upsetting, total deformation 40-60%, mold cavity surface texturing, roughness Ra = 0.05-0.1μm; c) Dynamic temperature control: Infrared thermometers are installed between each workstation to adjust the induction heating power with real-time feedback, and the temperature fluctuation is ≤±5℃; d) Water quenching: Spray cooling water within 0.5 seconds after forming (water temperature 20-30℃, flow rate 10L / min), cooling rate ≥150℃ / s. Its multi-station collaborative deformation design and functions are shown in Table 1 below: Workstation Deformation characteristics Innovative role Parameter basis one 35-45% diameter reduction Eliminate surface defect layers and improve subsequent deformation uniformity Compression rate <50% to avoid necking two Spherical pre-upsetting (R=1.5d) Reduced risk of head cracking (stress concentration factor reduced from 3.2 to 1.8) Finite element analysis three Textured mold (Ra0.05μm) The friction coefficient is reduced to 0.05, reducing surface scratches (scratch depth ≤ 2μm) Micro-pit oil storage effect Table 1 In addition, before warm upsetting, the surface of the wire is coated with nanographite lubricant, where the particle size is ≤100nm and the coating thickness is 2-5μm; after forming, the surface cracks are detected using an eddy current flaw detector, and the defect signal threshold is ≤2mV.

[0019] Furthermore, the present invention incorporates a stabilization treatment after warm heading to address the high residual stresses present after warm heading and water quenching, which can easily lead to microcracks when directly cryogenically cooled. During the stabilization treatment, the vacuum level is ≤10⁻²Pa, and the cooling rate is ≤3°C / min. The process involves holding the steel at 250°C to promote dislocation recombination, eliminate peak stresses (reduction of ≥40%), and inhibit carbide precipitation. This eliminates the safety risks of cryogenic treatment and also improves product yield.

[0020] After further stabilization, the present invention uses liquid nitrogen at -190°C for 3 hours, slowly heating the temperature to room temperature at a rate of ≤5°C / min. This slow heating prevents thermal stress cracking. A two-stage design is also employed: first, the temperature is maintained at 300°C for 1 hour to precipitate coherent Cr clusters (2-3 nm in size), providing sites for subsequent Cr2N nucleation. The temperature is then raised to 480°C for 3 hours, followed by air cooling. This reduces the oxygen content to ≤50 ppm to prevent surface oxidation.

[0021] To better control the yield rate, the material requires surface treatment, which includes shot peening, electropolishing, magnetron sputtering of a TiN / CrN composite coating (single layer thickness 0.5-1μm, total thickness 3-5μm), and micro-arc oxidation sealing. Shot peening utilizes ceramic balls with a diameter of 0.1-0.3mm. The micro-arc oxidation electrolyte contains 10g / L Na2SiO3, 5g / L KOH, and 2g / L Na2WO4. A bipolar pulse power supply is used, with a positive voltage of 350V and a negative voltage of 80V, at a frequency of 1000Hz. Shot peening pretreatment introduces surface compressive stress (-800MPa) to offset service tensile stress and improve fatigue life. To further promote the formation of the Cr(OH)3 passivation film, the surface-treated material was further treated using a dual passivation mechanism: cerium nitrate was hydrolyzed to generate CeO2 nanoparticles (10-20 nm in diameter), which filled the submicron pores in the micro-arc oxide layer; and citric acid chelated the surface Fe ions, promoting the formation of the Cr(OH)3 passivation film. The passivation solution pH was 3.5-4.0, resulting in a passivation film thickness of 50-80 nm. XPS analysis showed a surface Cr / Fe atomic ratio of ≥2.5. Experimental results demonstrated that the salt spray test duration increased from 1000 to 1500 hours without red rust.

[0022] Example 2 The present invention provides a process for preparing a high-strength, corrosion-resistant stainless steel fastener, further comprising establishing an aging temperature-precipitation phase size model: d=K·exp(-Q / RT) Where d is the diameter of Cr2N phase, K=1.2×10⁻ 8 m, Q = 185 kJ / mol; The aging parameters are inferred based on the target strength value: when the tensile strength is required to be ≥1200MPa, the size of the Cr2N phase in the aging stage at 480℃ is controlled to be 80±10nm.

[0023] Take the preparation of M12×60 stainless steel bolts as an example: Material modification According to the composition ratio of step S1 of the present invention, vacuum melting is carried out, the ingot is subjected to solid solution at 1150°C for 1 hour, and then subjected to blanking at 1050°C → intermediate rolling at 950°C → finish rolling at 850°C to Φ12mm, and water cooling; The pickling was carried out using a mixture of HNO3:HF = 3:1 at a temperature of 50°C.

[0024] Warm upsetting The wire is induction heated to 100°C and then subjected to diameter reduction in the first station on a Z31-25 cold heading machine, followed by preforming in the second station and final heading in the third station. The deformation is 55%, and the water quenching cooling rate after forming is greater than 100℃ / s.

[0025] Cryogenic treatment In liquid nitrogen tank, -190℃×3h, heating rate 3℃ / min.

[0026] Gradient aging In a vacuum furnace, heat the temperature at 300°C for 1 h to 480°C for 3 h, with an argon flow rate of 10 L / min.

[0027] Surface treatment Electrolytic polishing: H3PO4:H2SO4=1:2, current density 20A / dm², time 3min; Magnetron sputtering: first deposit a 1 μm TiN layer (N2 / Ar=1:3), then deposit a 1.2 μm CrN layer (N2 / Ar=1:2), and repeat three times; Micro-arc oxidation: electrolyte temperature 30°C, positive pulse voltage 350V, duty cycle 20%, treatment time 10min.

[0028] Specifically, the multi-station deformation of the present invention: Station 1: Reduce diameter to Φ8.2mm Station 2: Pre-upsetting ball head R=15mm Station 3: Final upsetting of hexagonal head (diagonal dimension 18mm), laser texturing of mold surface Dynamic temperature control: Temperature monitoring values ​​between workstations: Workstation 1 outlet 102°C → Workstation 2 entrance 98°C → Workstation 3 entrance 96°C; Water quenching: Spray water to cool 0.3 seconds after forming, and reduce the temperature to 50℃ within 5 seconds.

[0029] The process begins with vacuum melting of high-nitrogen stainless steel with a specific composition (C ≤ 0.03%, Cr 16-18%, N 0.2-0.4%, Mo 2.5-3.5%). This is then solution-controlled rolled into 5-20mm diameter wire, utilizing nitrogen solution strengthening to enhance the strength of the austenite matrix and the stability of the passive film. The wire is then heated to 80-120°C via medium-frequency induction heating for warm upsetting. A three-step deformation process is then performed within the metastable austenite region: a 35-45% diameter reduction to eliminate surface defects, followed by a spherical pre-upsetting (R = 1.5d) to reduce stress concentrations, and finally a final upsetting in a textured die to a total deformation of 40-60%. After forming, a water quench (cooling rate ≥ 150°C / s) within 0.5 seconds locks in the deformed microstructure and inhibits carbide precipitation. The steel then undergoes vacuum stabilization at 250°C to eliminate residual stress, followed by deep cooling at -190°C for 2-4 hours to promote the transformation of retained austenite. Gradient aging at 300°C followed by 480°C precipitates 80-100nm Cr2N nanophases, achieving an optimal balance between precipitation strengthening and corrosion resistance. Surface treatment involves shot peening to introduce surface compressive stress (≥800MPa), electropolishing to remove microcracks, magnetron sputtering to deposit alternating TiN / CrN coatings (total thickness 3-5μm) to provide a hard barrier, and micro-arc oxidation to form a SiO2-Al2O3 ceramic layer (8-12μm) to fill the micropores. Finally, treatment with a cerium-containing passivation solution allows CeO2 nanoparticles to further seal the submicron pores, forming a molecular-level anti-corrosion network. This approach completely resolves the inherent trade-off between strength and corrosion resistance in traditional stainless steel fasteners.

[0030] This process achieves a tensile strength exceeding 1200 MPa through synergistic strengthening through deformation and phase transformation. Its pioneering three-step approach of diameter reduction, spherical pre-upsetting, and textured final upsetting addresses cracking issues at high deformations (60%). Dynamic temperature control reduces temperature fluctuations from ±20°C to ±5°C, ensuring microstructure uniformity. This process addresses the inherent strength limitations and stress corrosion susceptibility of traditional stainless steel fasteners, making it particularly suitable for use in extreme environments such as aerospace and marine engineering.

[0031] It will be understood that the present invention is described by way of some embodiments, and it will be appreciated by those skilled in the art that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. In addition, under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are intended to be protected by the present invention.

Claims

1. A process for preparing high-strength, corrosion-resistant stainless steel fasteners, characterized in that: The steps include: Step S1: Material modification: vacuum induction melting is used to prepare an alloy, wherein the composition by weight is as follows: C ≤ 0.03%, Cr 16-18%, Ni 10-12%, Mo 2.5-3.5%, N 0.2-0.4%, Mn 4-6%, Si ≤ 0.8%, and the balance is Fe and unavoidable impurities. The ingot is solution treated at 1150°C and then controlled rolled into a Φ5-20 mm wire. Step S2: Warm forging, heating the wire to 80-120°C for cold forging, controlling the deformation to 40-60%, and water quenching immediately after forming; Step S3: Stabilization treatment: Place the blank obtained in S2 in a vacuum furnace, heat it to 250°C at a rate of 10°C / min, hold it for 30 minutes, and then cool it to 80°C in the furnace; Step S4: cryogenic strengthening, treating in a liquid nitrogen environment at -190°C for 2-4 hours, with a heating rate of ≤5°C / min; Step S5: Gradient aging, first keeping the temperature at 300°C for 1 hour, then raising the temperature to 480°C for 3 hours, and air cooling; Step S6: Surface treatment, including shot peening, electrolytic polishing, magnetron sputtering deposition of TiN / CrN composite coating, and micro-arc oxidation sealing treatment; Step S7: Post-treatment: immerse the S6 workpiece in a passivation solution containing 3 wt% cerium nitrate and 5 wt% citric acid at 60° C. for 20 minutes, rinse with deionized water, and then dry.

2. The process for preparing a high-strength, corrosion-resistant stainless steel fastener according to claim 1, characterized in that: The controlled rolling in step S1 adopts three-stage temperature control, with an initial rolling temperature of 1050° C., a final rolling temperature of 850° C., a total reduction ratio of ≥70%, and a grain size of the wire rod after rolling of ≥8 levels.

3. The process for preparing a high-strength, corrosion-resistant stainless steel fastener according to claim 1, characterized in that: The specific operations of the warm upsetting forming in step S2 include: a) Preheating: The wire is heated by a medium frequency induction coil, with a temperature gradient of ≤15°C / m and a target temperature of 80-120°C. b) Multi-station deformation: Execute in sequence on a three-station cold heading machine: Station 1: Diameter reduction deformation, compression rate 35-45%, mold cone angle 30°; Station 2: Pre-upsetting the head, deformation 15-25%, spherical radius R=1.5d, d is the nominal diameter of the bolt; Station 3: Final upsetting, total deformation 40-60%, mold cavity surface texturing, roughness Ra = 0.05-0.1μm; c) Dynamic temperature control: Infrared thermometers are installed between each workstation to adjust the induction heating power with real-time feedback, and the temperature fluctuation is ≤±5℃; d) Water quenching strengthening: spray cooling water within 0.5 seconds after forming (water temperature 20-30℃, flow rate 10L / min), cooling rate ≥150℃ / s.

4. The process for preparing a high-strength, corrosion-resistant stainless steel fastener according to claim 3, characterized in that: In step S2, before warm upsetting, the surface of the wire is coated with nanographite lubricant, wherein the particle size is ≤100nm and the coating thickness is 2-5μm; after forming, the surface cracks are detected using an eddy current flaw detector, and the defect signal threshold is ≤2mV.

5. The process for preparing a high-strength, corrosion-resistant stainless steel fastener according to claim 1, characterized in that: During the stabilization treatment in step S3, the vacuum degree is ≤10⁻²Pa and the cooling rate is ≤3°C / min.

6. The process for preparing a high-strength, corrosion-resistant stainless steel fastener according to claim 1, characterized in that: The shot peening in step S6 uses ceramic balls with a diameter of 0.1-0.3 mm; the micro-arc oxidation electrolyte contains 10 g / L of Na2SiO3, 5 g / L of KOH, and 2 g / L of Na2WO4, and uses a bipolar pulse power supply with a positive voltage of 350 V and a negative voltage of 80 V and a frequency of 1000 Hz.

7. The process for preparing a high-strength, corrosion-resistant stainless steel fastener according to claim 1, characterized in that In step S7, the pH value of the passivation solution is 3.5-4.0, the thickness of the passivation film is 50-80 nm, and XPS detection shows that the surface Cr / Fe atomic ratio is ≥2.

5.

8. The process for preparing a high-strength, corrosion-resistant stainless steel fastener according to claim 1, characterized in that: Also includes: Establishing the aging temperature-precipitation phase size model: d=K·exp(-Q / RT) Where d is the diameter of Cr2N phase, K=1.2×10⁻ 8 m, Q = 185 kJ / mol; The aging parameters are inferred based on the target strength value: when the tensile strength is required to be ≥1200MPa, the size of the Cr2N phase in the aging stage at 480℃ is controlled to be 80±10nm.

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