Preparation process of high-strength screw for automobile seat

Abstract

The invention belongs to the technical field of automobile fastener production, and particularly relates to a preparation process of a high-strength screw for an automobile seat. According to the technology, Cr-Mo-Ni-based microalloyed steel is adopted, and a thin-sheet martensite structure containing 10%-15% of stable retained austenite is obtained through VIM-ESR ultra-clean smelting, planetary cross wedge rolling short-process hot working and Q-A-P-C heat treatment including quenching, isothermal deformation, graded isothermal treatment, deep cooling and double tempering. According to the high-safety and environment-friendly automobile seat screw and the manufacturing method thereof, gradient nitriding, the AlCrSiN nanometer multi-layer coating and the graphene hole sealing layer are matched, laser local destressing and lossless full-inspection control are supplemented, and the manufactured screw has the tensile strength larger than or equal to 1350 MPa, the low-temperature impact toughness larger than or equal to 35 J, the salt mist corrosion resistance larger than or equal to 1000 h, the excellent hydrogen embrittlement resistance and the excellent fatigue life, and the high-safety and environment-friendly manufacturing requirements of a new generation of automobile seats are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of automotive fastener production, and particularly relates to a preparation process of high-strength screws for automotive seats. Background Art

[0002] Automobile seats must maintain a rigid connection with the vehicle body floor during collisions or sudden decelerations to ensure that the occupant restraint system established by seat belts and airbags can transmit impact loads along the designed route. The fixing device usually uses 4 - 8 high-strength bolts / screws, and their failure will directly lead to the overall dislocation of the seat, and then cause high-risk consequences such as secondary collisions or occupant ejection. Regulations such as ISO13216, FMVSS207 / 210, and UN-R17 have put forward strict requirements for the static tension, fatigue, and cyclic tightening performance of seat fixing screws: 1. Tensile strength ≥ 1200 MPa (grade 12.9), yield ratio ≥ 0.80; 2. No cracks shall appear after 10 6 cycles of environmental alternation between - 40°C and 85°C; 3. The specified torque - clamping force relationship shall still be satisfied after more than 15 times of assembly and disassembly.

[0003] With the improvement of the lightweight of passenger cars, the multi-directional electric adjustment of seats, and the requirements for lateral collision restraint, the industry has begun to raise the target strength to 1300 - 1400 MPa, and at the same time requires better toughness, corrosion resistance, and hydrogen embrittlement resistance. However, traditional remanufactured steels such as 35CrMo and 40CrNiMo and their conventional "quenching + tempering + cold rolling + phosphating" routes are difficult to achieve a balance among safety redundancy, service life, and green manufacturing.

[0004] Currently, the most widely used are medium-carbon quenched and tempered steels such as 35CrMo and 40CrNiMo. Through the combined solution strengthening of about 0.35% C and more than 0.3% Mo and Cr, a strength level of 1200 MPa can be obtained in a φ10mm cross-section. However, after quenching and tempering, the martensite laths of such steels are thick, and the impact toughness at - 40°C is often lower than 25 J; at the same time, the high Mo content increases the cost and broadens the temper brittleness range (450 - 550°C), which is not conducive to dimensional stability.

[0005] There are also medium-manganese boron microalloy steels. Boron steels such as 20MnTiB and 23MnB5 can reach 1100 MPa through deep hardenability with 20 - 30 ppm B, but limited by C < 0.25%, they cannot meet the strength above grade 12.9. At the same time, some studies have shown that if BN inclusions are formed without solution in the austenite grain boundary, it is more unfavorable for fatigue and hydrogen embrittlement. Summary of the Invention

[0006] Aiming at the technical pain points of the existing automotive seat fixing screw materials, such as poor strength-ductility balance, insufficient corrosion resistance and hydrogen embrittlement resistance, high processing energy consumption and great environmental protection pressure, a preparation process for high-strength screws for automotive seats is provided, which integrates alloy microalloying, short-process thermo-mechanical coupling rolling, step-by-step Q-A-P-C heat treatment, gradient surface strengthening and laser stress relief.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: A preparation process for high-strength screws for automotive seats, which successively includes: 1) Clean melting: Using a double process of vacuum induction melting-electroslag remelting, melting alloy elements with the following mass percentages into an ingot: C 0.26 - 0.34%, Si 0.45 - 0.70%, Mn 0.90 - 1.40%, Cr 0.90 - 1.30%, Mo 0.25 - 0.45%, Ni 0.30 - 0.70%, V 0.05 - 0.12%, Ti 0.02 - 0.08%, Nb 0.015 - 0.05%, B 0.001 - 0.005%, N ≤ 0.008%, and the balance is Fe and unavoidable impurities, where P ≤ 0.012% and S ≤ 0.010%; 2) Thermo-mechanical co-production rolling: Heating the ingot to 1200 - 1280 °C, directly forming φ8 - φ12 mm wire rods by planetary cross wedge rolling and cooling directly through a water-cooling bed at a temperature 50 - 80 °C higher than the Ar3 temperature to obtain a pearlite-tempered sorbite duplex structure with an average grain size ≤ 10 µm; 3) Cold heading forming and medium-temperature isothermal annealing: After cold heading the wire rods into screw blanks, isothermal annealing at 600 - 640 °C for 15 - 25 min to eliminate work hardening and promote the precipitation of dispersed carbonitrides; 4) Step-by-step Q-A-P-C heat treatment: 4.1) Primary quenching: Heating the screws to 840 - 880 °C, holding for 5 - 10 min and then quenching in oil to 120 - 160 °C; 4.2) Austenite isothermal deformation: Implementing 3 - 8% radial compression deformation on the screws at 120 - 160 °C; 4.3) Stepwise isothermal: Heating the screws to 260 - 300 °C and holding for 10 - 25 min; 4.4) Cryogenic treatment: Insulating at -70 - -90 °C for 20 - 40 min; 4.5) Secondary tempering: Double tempering at 460 - 500 °C, 45 - 75 min each time; 5) Online laser local tempering stress relief: Synchronously scanning in the symmetric area of the thread root to make the residual tensile stress ≤ 30 MPa.

[0008] Preferably, during the melting process in step 1), the total oxygen content is controlled to be ≤15 ppm to ensure the effective solid solution of element B at the austenite grain boundaries.

[0009] Preferably, immediately after the planetary cross-rolling in step 2), a cooling rate of 30 - 60 °C / s is achieved in the air-water mist composite cooling zone to inhibit the growth of austenite grains and reduce the energy consumption by at least 15%.

[0010] Preferably, the deformation amount of the austenite isothermal deformation in step 4.2) and the subsequent stepwise isothermal time satisfy the following formula: ε×t≈0.3 - 0.8 min to obtain a target retained austenite volume fraction of 10 - 15%.

[0011] Preferably, during the secondary tempering in step 4.5), the synchronous precipitation of NbC, VC, and TiCN is utilized to achieve a secondary hardening peak, and the precipitation particle size is controlled within 15 - 40 nm.

[0012] Preferably, the core structure of the screw is fine lath martensite + 10 - 15% stable retained austenite, and the surface is successively an ε-Fe2-3N / γ'-Fe4N composite nitrided layer, an AlCrSiN nano-multilayer coating, and a graphene sealing layer.

[0013] Preferably, the gradient hardness ratio of the nitrided layer is 1.5 - 1.8, and the hardness of the nitrided layer is ≥900 HV 0.05 , and the AlCrSiN coating is deposited by multi-arc ion source - magnetron sputtering composite PVD with a substrate bias voltage of -60 - 90 V to make the coating grain size ≤60 nm.

[0014] Preferably, the process further includes the following surface engineering treatments: a) Vacuum nitriding at a nitriding temperature of 550 - 590 °C for 3 - 5 h to obtain a gradient hardened layer depth of 18 - 22 µm; b) Physical vapor deposition of an AlCrSiN nano-multilayer coating with a thickness of 2 - 4 µm; c) A graphene-based solid lubricant sealing layer with a thickness of 10 - 80 nm.

[0015] Furthermore, the present invention also provides an alloy steel for high-strength screws of automotive seats, which is composed of the following components by mass percentage: C 0.26 - 0.34%, Si 0.45 - 0.70%, Mn 0.90 - 1.40%, Cr 0.90 - 1.30%, Mo 0.25 - 0.45%, Ni 0.30 - 0.70%, V 0.05 - 0.12%, Ti 0.02 - 0.08%, Nb 0.015 - 0.05%, B 0.001 - 0.005%, N ≤0.008%, P ≤0.012%, S ≤0.010%, and the balance is Fe and unavoidable impurities.

[0016] Furthermore, the present invention also provides high-strength screws for vehicle seats obtained by the described process, and the screws meet the following requirements: tensile strength ≥ 1350 MPa, yield ratio 0.83 - 0.87, impact energy absorption at -40°C ≥ 35 J, no red rust after salt spray test (ISO9227) ≥ 1000 h, and no failure after cyclic assembly ≥ 15 times.

[0017] Due to the adoption of the above technical solution, compared with the traditional "35CrMo - quenching and tempering - phosphating" system of existing 12.9 - grade vehicle seat screws, the present invention has achieved remarkable technical effects in multiple dimensions such as performance, safety, and green manufacturing through an integrated process combination of "ultra - clean micro - alloy steel + planetary cross - wedge rolling short process + step - by - step Q - A - P - C heat treatment + gradient surface engineering + laser local stress relief", which are specifically reflected in the following aspects: 1. Significantly improved comprehensive mechanical properties: Tensile strength Rm ≥ 1350 MPa, yield ratio 0.83 - 0.87, meeting and exceeding the design requirements of 13.9 - grade seat fixing screws; impact energy absorption Ak at -40°C ≥ 35 J, and the low - temperature toughness is increased by about 40% compared with the comparative sample; room - temperature fatigue limit σw ≥ 480 MPa (stress ratio = 0.1, 10 7 cycles), and the high - cycle fatigue life is increased by about 50%.

[0018] 2. Excellent corrosion resistance and adaptability to cyclic assembly and disassembly The gradient structure of ε - Fe2 - 3N / γ'-Fe4N composite nitrided layer (18 - 22 µm) + AlCrSiN nano - multilayer coating (2 - 4 µm) + graphene sealing film (10 - 80 nm) enables the screws to pass the 1000 - h salt spray test without red rust; the low friction coefficient μ = 0.10 ± 0.02 and stable scatter < 8% enable the screws to complete ≥ 15 times of assembly and disassembly without failure.

[0019] 2. Enhanced hydrogen embrittlement resistance and delayed fracture ability Ultra - low oxygen ([O] ≤ 15 ppm) + V / Ti / Nb / B synergistic fine - grain refinement to inhibit inclusions and hydrogen traps; 10 - 15% stable retained austenite generates the TRIP effect under service stress to absorb hydrogen stress concentration; laser local tempering makes the residual tensile stress at the thread root ≤ 30 MPa, and comprehensively reduces the hydrogen - induced delayed crack failure rate to < 0.02 ppm.

[0020] 3. Resistance to temper softening and dimensional stability The 15 - 40 nm NbC / VC / TiCN particles precipitated during the secondary tempering in the step - by - step Q - A - P - C process still maintain a high interface pinning force at 500°C, and the temper softening ΔHRC is reduced by 3 points; cryogenic treatment combined with deformation - induced austenite refinement of lath martensite controls the dimensional change amount |Δl / l| ≤ 0.03%.

[0021] 4. Energy consumption and carbon emissions reduction, manufacturing greener The short process of planetary cross wedge rolling - hot charging cooling bed is ≥15% more energy - saving than the traditional multi - pass continuous rolling - secondary heating; the gradient coating system without Cr(VI) complies with EU ELV and REACH regulations; the carbon footprint calculation in the whole production process is reduced by about 0.25tCO2e / t finished product.

[0022] 5. Zero - defect quality assurance Multi - frequency eddy current + acoustic emission 100% online non - destructive testing makes the failure rate of finished products PPM < 5; the full - process digital traceability meets the management requirements of special characteristics (SC / CC) for functional safety fasteners in IATF16949.

[0023] In summary, the present invention has achieved comprehensive technical breakthroughs in high strength, high toughness, long life, strong corrosion resistance, low hydrogen embrittlement, and low carbonization. It can provide high - performance screw products that are safer, more reliable and in line with the trend of green manufacturing for the new - generation automotive seat fixing system without significantly increasing costs, and has significant industrial promotion value. Description of the drawings

[0024] Figure 1 It is the TEM photo of Example 1; among them, tempered martensite: dark - gray lath matrix, width ≈ 80–150nm; retained austenite: light - gray film - like phase, about 12%; NbC / VC: bright - white dot - like nano - precipitates, particle size 20–30nm, uniformly pinning the dislocation and martensite lath interfaces. Detailed implementation manners

[0025] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0026] Test Example 1 Influence of alloy formula on products I. Preparation methods of examples and comparative examples The alloy formulas (mass percentages, with the balance being Fe + impurities) of the embodiments and comparative examples of the present invention are as follows: Table 1 Alloy formulas of examples and comparative examples Specimen C Si Mn Cr Mo Ni V Ti Nb B / ppm N / % Example 1 0.30 0.60 1.20 1.05 0.32 0.50 0.08 0.04 0.03 35 0.006 Example 2 0.28 0.55 1.10 0.95 0.28 0.40 0.06 0.05 0.02 30 0.006 Example 3 0.32 0.65 1.30 1.20 0.40 0.55 0.10 0.04 0.04 40 0.007 Comparative Example A 0.35 0.30 0.70 1.00 0.30 — — — — — 0.005 Comparative Example B 0.23 0.25 1.40 0.40 0.05 — — 0.04 — 25 0.006 Comparative Example C 0.38 0.40 0.45 5.00 1.30 — — — — — 0.004 Comparative Example D 0.30 0.60 1.20 1.05 0.32 0.50 — — — — 0.006 Comparative Example E 0.30 0.60 1.20 1.05 0.32 — 0.08 0.04 0.03 35 0.006 All specimens were completed in the same workshop. Except for the differences in the comparative examples specifically noted, the equipment, operators, and testing institutions were kept consistent to exclude systematic errors.

[0027] 1. Melting and ingot casting (general steps) 1) Charge preparation Pure iron, Fe-Cr, Fe-Mo, Fe-V, Fe-Nb alloys, electrolytic Ni, industrial pure Si, low-phosphorus and low-sulfur pig iron; proportioned according to the target composition; B is added in the form of Fe-B alloy (20%B) in the secondary feeding stage.

[0028] 2) Vacuum induction melting (VIM) The furnace shell is evacuated to 6 Pa; 30 kPa Ar protection is added; the melting temperature is 1600 ± 20 °C, refined for 12 min, and [C] and [O] are controlled; a 90 mm × 90 mm square ingot is cast; after cooling, it is peeled by 5 mm.

[0029] 3) Electroslag remelting (ESR) Slag: CaF2–Al2O3–CaO; slag temperature 1600 °C; remelting ratio 6:1, remelting rate 150 kg / h -1 ; After refining, [O] ≤ 15 ppm and [S] ≤ 10 ppm are measured.

[0030] 2. Table 2 Hot working process Specimen Heating system Deformation route Outfeed controlled cooling Examples 1–3, Comparative Examples D / E 1250°C × 40 min One-step forming of φ10 mm by planetary cross wedge rolling <![CDATA[Ar3+ Blow-through and water mist cooling bed at 60 °C, 30–60 °C s -1 > Comparative Example A Same as above Multi-pass continuous rolling on 6 stands (φ22 → φ10), total reduction 83% Air cooling Comparative Example B 1200°C × 30 min Continuous rolling Air cooling Comparative Example C 1200°C × 40 min Quick forging + continuous rolling Air cooling The grain size is measured on the hot-worked bars: Example ≈ ASTM10-11; Comparative Example A ≈ 7; C ≈ 6.

[0031] 3. Cold heading forming and intermediate annealing (applicable to all specimens) 1) Cold heading machine: six dies and six punches, 160 t; one-time forming of external hexagonal flange screws (M10 × 1.25).

[0032] 2) Intermediate isothermal annealing: 620 °C × 20 min (Example 1 / 2; Comparative Examples B and E take 600 °C × 15 min; Example 3 takes 640 °C × 25 min); furnace cooling to < 200 °C for discharging.

[0033] 4. Table 3 Heat treatment differences Heat treatment route Specific parameters Applicable specimen Q-A-P-C double tempering (the present invention) ① 840°C / 7 min → oil quenching to 130°C; ② Compressive deformation of 5% at 130°C (vertical 600 kN hydraulic press); ③ Isothermal partitioning at 285°C / 20 min; ④ Deep cooling at -80°C / 30 min; ⑤ Tempering at 480°C × 60 min × 2 Examples 1–3; Comparative Example E Single tempering (for comparison) 860°C / 7 min oil quenching → 560°C / 90 min tempering Comparative Examples A, D Applicable boron steel tempering 870°C / 10 min oil quenching → 580°C / 90 min tempering Comparative Example B Age hardening (hot work steel) 1040°C oil quenching → 550°C / 2 h → 620°C / 4 h Comparative Example C 5. Table 4 Surface engineering Steps Parameters Specimen used Vacuum nitriding <![CDATA[560℃×4h, p(N2+H2)=5mbar]]> All specimens PVD AlCrSiN Multi-arc ion source + magnetron sputtering composite; target ratio Al:Cr:Si = 65:30:5; substrate -80 V; temperature 480°C; thickness 3 µm All specimens, (Comparative Example F is nitriding + phosphating) Graphene sealing Room temperature plasma enhanced chemical deposition 50 nm Examples 1-3 II. Test methods 1. Table 5 Mechanical properties Items Standards Tensile test (room temperature) GB / T228.1-2021, round specimen Φ5 mm, parallel section 25 mm Elongation after fracture Same as above Impact test GB / T229-2020 V-notch at -40°C Hardness <![CDATA[GB / T231.1-2018 (HRC), Metallographic Microhardness HV 0.05 > 2. Microstructure and phase analysis Grain size: GB / T6394-2017, optical counting.

[0034] Retained austenite: XRD integration method (Cu-Kα, step 0.02°).

[0035] Precipitated particles: TEM (JEM-2100), EDS qualitative analysis.

[0036] 3. Table 6 Fatigue and cyclic assembly / disassembly Items Methods High-cycle axial fatigue <![CDATA[GB / T 3075-2008, R = 0.1, frequency 80 Hz, N = 10 7 Weekly judgment for shutdown]]> Cyclic assembly and disassembly <![CDATA[ISO16047, pre-tightening force 70kN, installation and disassembly rate 30r / min -1 > 4. Table 7 Environmental adaptability Test Standards Judgment Salt spray ISO9227 neutral salt spray at 35°C, 5% NaCl Time to appearance of red rust Hydrogen embrittlement GMW14872; applied cathode potential -1000 mV (NHE), load 72% Rp0.2; observed for 200 h Delayed cracking PPM 5. Residual stress X-ray sin 2 ψ method (Protoi XRD); Test location: Thread root 1-2.

[0037] III. Data processing For tensile and impact tests, take the average of 5 samples; for fatigue, calculate σw by the upper / lower limit method; Hydrogen embrittlement PPM = Number of cracked parts / Number of tested parts × 10 6 ; Statistical significance: α = 0.05, t-test to compare Example 1 with each comparative example.

[0038] IV. Experimental data 1. Table 8 Specimen microstructure (metallographic statistics) Specimen Grain size ASTM Retained austenite / % Feeder crystal carbonitride particle size / nm Example 1 10–11 12.5 22 Example 2 10 11.3 20 Example 3 10 13.2 25 Comparative Example A 7 2.1 — Comparative Example B 8 3.4 — Comparative Example C 6 — — Comparative Example D 9 1.9 90 Comparative Example E 10 12.1 26 2. Table 9 Mechanical and service properties Performance indicators Example 1 Example 2 Example 3 Comparative Example A Comparative Example B Comparative Example C Comparative Example D Comparative Example E Tensile strength Rm / MPa 1390(±12) 1340 1425 1225 1045 1305 1160 1365 Yield ratio Rp0.2 / Rm 0.84 0.83 0.85 0.82 0.78 0.82 0.79 0.84 Elongation after fracture A / % 13.2 14.0 12.5 9.0 18.0 7.2 7.8 12.9 Impact energy Ak at -40°C / J 38 40 35 24 29 18 20 25 <![CDATA[High-cycle fatigue limit σw / MPa (10 7 )]]> 505 490 515 350 295 415 335 455 Neutral salt spray life / h (red rust) ≥1000 ≥1000 ≥1000 ≥1000* 480 960 ≥1000* ≥1000 Delayed crack rate / ppm 0.01 0.02 0.02 0.18 0.34 0.09 0.32 0.08 * Comparative examples A and D use the same surface system, so the salt spray index meets the standard, but the hydrogen embrittlement failure rate is high.

[0039] V. Result discussion 1. Example 1-3 vs. Comparative example A / B / C Strength: The average of the examples increases by 13-25%; Low-temperature toughness: Increases by 45-110%; Fatigue limit: Increases by 22-71%; Hydrogen embrittlement failure rate: Decreases by more than one order of magnitude.

[0040] 2. Contribution of microalloying (Example 1 vs. Comparative example D) The absence of V / Ti / Nb / B leads to coarse secondary hardening particles (90 nm), severe temper softening, a 17% decrease in R m and a 18 J decrease in Ak, and the hydrogen embrittlement rate increases by 32 times, verifying the necessity of nano-precipitation and deep hardenability design.

[0041] 3. Role of Ni in resisting hydrogen embrittlement (Example 1 vs. Comparative example E) After removing Ni, the strength is similar, but the hydrogen embrittlement failure rate increases from 0.01 ppm to 0.08 ppm, indicating that Ni has a significant effect on improving hydrogen diffusion and crack initiation.

[0042] 4. Retained Austenite and TRIP Effect The retained austenite in the range of 11–13% in the examples is significantly superior to Comparative Examples A / D (~2%). It can transform into fine plate martensite under fatigue and impact loading to absorb energy. Too high or too low retained austenite is not conducive to the comprehensive performance.

[0043] 5. Conclusion The three examples fully cover the alloy composition range of the present invention, and the upper and lower limit ratios can meet the targets of strength ≥1350 MPa, low-temperature toughness ≥35 J, and salt spray resistance ≥1000 h.

[0044] The five comparative examples prove that only increasing C-Cr-Mo (Comparative Example A), introducing B without solid solution / precipitation synergy (B), using high-Cr hot work steel (C), removing microalloying elements (D), or lacking Ni (E) cannot achieve the multiple advantages of strength-toughness-corrosion resistance-hydrogen embrittlement resistance realized by the present invention simultaneously.

[0045] Below, the alloy of "Example 1" (0.30% C, Cr-Mo-Ni + V / Ti / Nb / B microalloying) is selected to exclude formula interference.

[0046] Single variable: Each comparative example only changes one key link in the invention process chain, and the remaining steps are consistent with the examples, so as to intuitively present the contribution of this link to the final performance and resource efficiency.

[0047] Evaluation dimensions: Comprehensively investigate mechanical properties, fatigue life, corrosion resistance, hydrogen embrittlement, disassembly and assembly torque scatter, residual stress, and energy consumption / carbon emission.

[0048] Statistical reliability: At least 50 M10×1.25 screws are prepared for each process; the average value of 5 to 10 samples is taken for all performance results, and the standard deviation is given.

[0049] Influence of Process Improvement on Products in Test Example 2 This test verification test focuses on the five core links of "ultra-clean microalloyed steel + planetary cross wedge rolling short process + step-by-step Q-A-P-C heat treatment + gradient surface engineering + laser local stress relief", aiming to conduct systematic and quantifiable comparisons: 1) Confirm the necessity of the creative links - gradually peel off or replace the key steps, and observe the changes in mechanical properties, fatigue life, corrosion resistance, hydrogen embrittlement sensitivity, and energy consumption, proving that the absence of any single link will cause significant performance degradation.

[0050] 2) Define the process window - use the upper and lower limit examples to verify the rationality and robustness of the claim parameter range, ensuring that stable compliance can still be achieved within the fluctuations allowed by the industrial scale.

[0051] 3) Evaluate the synergy effect - Examine the comprehensive contribution of the "fine-grained blank - retained austenite - nano-precipitation - gradient surface layer - stress relief" chain, and quantify the improvement extent of each link on safety reliability and green manufacturing indicators.

[0052] Overall idea: Keep the material constant (select the alloy in Example 1), only change the process flow, and prepare at least 50 M10×1.25 screws for each process; take the average value of 5 to 10 for all performance results and give the standard deviation.

[0053] 1. Overview of the process flow in Table 10 Process section Benchmark conditions of the present invention Variable direction Main performance correlation A Hot working One-step forming by planetary cross wedge rolling + hot charging (grain size ASTM10–11) Traditional multi-stand continuous rolling + air cooling Grain size, energy consumption, residual stress B Pretreatment Intermediate temperature isothermal annealing at 600–640°C / 20 min Spheroidizing annealing at 720°C for 2 h Dispersion degree of carbonitride, cold heading crack C Core heat treatment "Quenching-Isothermal Deformation-Graded Isothermal-Cryogenic-Binary Tempering” (Q-A-P-C) Single-step quenching and tempering or defectizing Q-A-P Strength-toughness balance, retained austenite, secondary hardening D Surface engineering ε / γ′ nitrided layer + 3µm AlCrSiN + graphene hole sealing Nitriding + phosphating or single-layer TiN Salt spray, torque coefficient, hydrogen blocking effect E Stress reduction Laser local tempering (σ_res ≤ 30MPa) No treatment or overall tempering Hydrogen embrittlement, fatigue crack source density F Quality inspection Multi-frequency eddy current + acoustic emission 100% Only eddy current spot check Failure rate PPM, traceability 2. Examples (3 groups) in Table 11 Serial number Name Example I (benchmark) Fully in accordance with the median value of the full window of claim 1: Planetary wedge cross rolling → 620°C / 20 min → 840°C oil quenching - 130°C 5% deformation - 285°C / 20 min - −80°C / 30 min - 480°C × 60 min × 2 → Nitriding 560°C / 4 h → AlCrSiN 3µm → Graphene 50 nm → Laser stress relief → NDT full inspection Example II (low-temperature window verification) Deformation of 8% at Ms + 10°C (120°C) in Q-A-P-C, grading at 270°C / 25 min, cryogenic treatment at -90°C / 40 min; others are the same as the benchmark Example III (high-temperature window verification) Final oil quenching temperature of 160°C, deformation of 3%, grading at 300°C / 15 min, cryogenic treatment at -70°C / 20 min; tempering at 500°C × 45 min × 2 3. Comparative examples (5 groups) in Table 12 Serial number Name Differences from Example I Comparative example A (traditional rolling) Multi-pass continuous rolling (φ22 → φ10) + air cooling; other steps are the same as Example I Prove the contribution of fine grains and energy saving in planetary wedge cross rolling Comparative example B (lack of annealing) Cancel the 600°C isothermal annealing; directly enter heat treatment after cold heading Evaluate the residual stress and brittle cracks in cold heading Comparative example C (single-step quenching and tempering) Replace Q-A-P-C with 860°C / oil quenching + 560°C / 90 min single-step quenching and tempering Verify the strength-toughness-fatigue advantages of multi-stage heat treatment Comparative example D (nitriding + phosphating) The surface layer is changed to Zn-Mn phosphating and wax coating after nitriding Evaluate the effect of nano-coating and hole sealing on salt spray and torque Comparative example E (without laser stress relief) Omit local tempering; others are the same as Example I Directly compare hydrogen embrittlement and fatigue crack origin 4. Test items and methods in Table 13 Test Standard / equipment Evaluation index Tensile GB / T228, MTS-250kN R_m, Rp0.2, A Impact GB / T229, −40°C Akv High-cycle fatigue <![CDATA[GB / T3075, R = 0.1, 10 7 weeks]]> <![CDATA[σ w > Neutral salt spray ISO9227 Appearance time of red rust Assembly and disassembly torque ISO16047 (15 times) <![CDATA[μ,σ μ > Delayed fracture GMW14872, 200h PPM Residual stress XRD sin²ψ <![CDATA[σ res > Energy consumption Measured by electricity meter + gas meter <![CDATA[kWh·t⁻¹, kgCO2e·t⁻¹]]> 5. Test results in Table 14 Index Example I Example II Example III Comparative example A Comparative example B Comparative example C Comparative example D Comparative example E <![CDATA[R m / MPa]]> 1390 1365 1410 1240 1350 1250 1385 1390 A / % 13.2 14.1 12.6 9.2 10.3 8.9 13.1 13.3 <![CDATA[σ w / MPa]]> 505 490 515 365 420 350 505 440 Tempering softening ΔHRC 3.1 3.2 3.0 6.4 5.8 6.6 3.1 3.1 Salt spray / h ≥1000 ≥1000 ≥1000 ≥1000 ≥1000 ≥1000 730 ≥1000 μ (15 times) 0.10±0.01 0.10±0.01 0.10±0.01 0.11±0.02 0.12±0.02 0.10±0.01 0.14±0.03 0.10±0.01 <![CDATA[σ res / MPa]]> 28 30 27 95 92 94 29 95 Hydrogen embrittlement PPM 0.01 0.02 0.02 0.18 0.22 0.15 0.08 0.16 Energy consumption kWh·t⁻¹ 840 840 840 1000 840 840 840 840 <![CDATA[CO2ekg·t⁻¹]]> 1480 1480 1480 1740 1480 1480 1480 1480 6. Attribution of technical effects 1) In Comparative Example A, due to grain coarsening (ASTM7) and air-cooled pitting, σ w decreases by 27%, and A k decreases by 35%, with 19% higher energy consumption; highlighting the energy-saving and fine-grained value of the planetary wedge cross-rolling short process.

[0054] 2) In Comparative Example B, the residual tensile stress after cold heading without annealing is >150 MPa, causing thread rolling microcracks and amplifying hydrogen embrittlement, and the PPM soars to 0.22; indicating the necessity of 600°C medium-temperature annealing in "stress relief + precipitation control".

[0055] 3) In Comparative Example C, single tempering lacks retained austenite and secondary hardening particles, resulting in a double decrease in strength and fatigue and doubling of temper softening; directly proving the creative contribution of Q-A-P-C multi-stage heat treatment.

[0056] 4) In Comparative Example D, the porosity of the phosphating layer is high, the notch potential of hexavalent chromium increases, the salt spray life is shortened by 27%, and the torque scatter doubles; indicating the synergistic corrosion resistance and friction stability mechanism of nano-AlCrSiN + graphene sealing.

[0057] 5) In Comparative Example E, the σ at the thread root res returns to 95 MPa, the fatigue limit drops by 13%, and the hydrogen embrittlement rate increases by one order of magnitude; confirming the key value of laser local thermal-cold cycle stress relief for safety screws.

[0058] 7. Conclusion Examples I - III fully fall within the parameter range of the claims. Even within the upper and lower limit window, they always meet the requirements of "tensile strength ≥ 1350 MPa, impact energy ≥ 35 J, salt spray resistance ≥ 1000 h, PPM ≤ 0.02, energy consumption ≤ 850 kWh·t" -1" target.

[0059] Five comparative examples respectively eliminate or degrade the key steps in the inventive process chain, all of which result in significant deterioration of at least one core performance, and cannot meet the safety regulations and green manufacturing requirements simultaneously.

[0060] Therefore, each innovative link in the inventive process chain is irreplaceable for the overall technical effect and has obvious synergistic gains, jointly constituting a creative improvement over the prior art.

[0061] The above is the description of the embodiments of the present invention. Through the above description of the disclosed embodiments, those skilled in the art can implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel points disclosed herein.

Claims

1. A preparation process for high-strength screws for automotive seats, characterized in that, This process sequentially includes: 1) Clean melting: Using a double process of vacuum induction melting - electroslag remelting, melting alloy elements in the following mass percentages into an ingot: C 0.26 - 0.34%, Si 0.45 - 0.70%, Mn 0.90 - 1.40%, Cr 0.90 - 1.30%, Mo 0.25 - 0.45%, Ni 0.30 - 0.70%, V 0.05 - 0.12%, Ti 0.02 - 0.08%, Nb 0.015 - 0.05%, B 0.001 - 0.005%, N ≤ 0.008%, with the balance being Fe and unavoidable impurities, where P ≤ 0.012% and S ≤ 0.010%; 2) Thermomechanical combined rolling: Heating the ingot to 1200 - 1280 °C, directly forming φ8 - φ12 mm wire rods through planetary cross wedge rolling and cooling directly on a water cooling bed at a temperature 50 - 80 °C higher than the Ar3 temperature to obtain a pearlite - tempered sorbite duplex structure with an average grain size ≤ 10 µm; 3) Cold heading forming and medium - temperature isothermal annealing: After cold heading the wire rods into screw blanks, isothermal annealing at 600 - 640 °C for 15 - 25 min to eliminate work hardening and promote the precipitation of dispersed carbonitrides; 4) Step - by - step Q - A - P - C heat treatment: 4.1) Primary quenching: Heating the screw to 840 - 880 °C, holding for 5 - 10 min, and then quenching in oil to 120 - 160 °C; 4.2) Austenite isothermal deformation: Implementing 3 - 8% radial compressive deformation on the screw at 120 - 160 °C; 4.3) Step - by - step isothermal: Heating the screw to 260 - 300 °C and holding for 10 - 25 min; 4.4) Cryogenic treatment: Insulating at - 70 - 90 °C for 20 - 40 min; 4.5) Secondary tempering: Double tempering at 460 - 500 °C, each time for 45 - 75 min; 5) Online laser local tempering to remove stress: Synchronously scanning in the symmetric area of the thread root to make the residual tensile stress ≤ 30 MPa.

2. The preparation process of a high-strength screw for an automotive seat according to claim 1, characterized in that, During the melting process in step 1), control the total oxygen content ≤ 15 ppm to ensure the effective solid solution of B element at the austenite grain boundary.

3. The preparation process of a high-strength screw for an automotive seat according to claim 1, characterized in that, Immediately after planetary cross wedge rolling in step 2), achieve a cooling rate of 30 - 60 °C / s in the wind - water mist composite cooling zone to inhibit the growth of austenite grains and reduce energy consumption by at least 15%.

4. The preparation process of a high-strength screw for an automotive seat according to claim 1, characterized in that, The deformation amount of austenite isothermal deformation in step 4.2) and the subsequent step - by - step isothermal time satisfy the following formula: ε × t ≈ 0.3 - 0.8 min to obtain a target residual austenite volume fraction of 10 - 15%.

5. The preparation process of a high-strength screw for an automotive seat according to claim 1, characterized in that, During secondary tempering in step 4.5), achieve a secondary hardening peak through the synchronous precipitation of NbC, VC, and TiCN, and control the precipitation particle size within 15 - 40 nm.

6. The preparation process of a high-strength screw for an automotive seat according to claim 1, characterized in that, The core structure of the screw is fine - lamellar martensite + 10 - 15% stable residual austenite, and the surface is successively an ε - Fe2 - 3N / γ'-Fe4N composite nitrided layer, an AlCrSiN nano - multilayer coating, and a graphene sealing layer.

7. The preparation process of a high-strength screw for an automotive seat according to claim 6, characterized in that, The gradient hardness ratio of the nitrided layer is 1.5 to 1.8, and the hardness of the nitrided layer is ≥900 HV 0.05 , the AlCrSiN coating is deposited by multi-arc ion source-magnetron sputtering composite PVD, and the substrate bias voltage is -60 to -90 V, so that the grain size of the coating is ≤60 nm.

8. The preparation process of a high-strength screw for an automotive seat according to claim 6, characterized in that, This process also includes the following surface engineering treatments: a) Vacuum nitriding, nitriding temperature 550 - 590 °C, time 3 - 5 h, obtaining a gradient hardened layer depth of 18 - 22 µm; b) Physical vapor deposition of AlCrSiN nano-multilayer coating with a thickness of 2 - 4 µm; c) Graphene-based solid lubricant sealing layer with a thickness of 10 - 80 nm.

9. An alloy steel for high-strength screws of an automotive seat, characterized in that It is composed of the following components by mass percentage: C 0.26 - 0.34%, Si 0.45 - 0.70%, Mn 0.90 - 1.40%, Cr 0.90 - 1.30%, Mo 0.25 - 0.45%, Ni 0.30 - 0.70%, V 0.05 - 0.12%, Ti 0.02 - 0.08%, Nb 0.015 - 0.05%, B 0.001 - 0.005%, N ≤ 0.008%, P ≤ 0.012%, S ≤ 0.010%, and the balance is Fe and unavoidable impurities.

10. An automotive seat high-strength screw obtained by the process according to any one of claims 1 - 8, wherein the screw satisfies: tensile strength ≥ 1350 MPa, yield ratio 0.83 - 0.87, impact absorption energy at -40°C ≥ 35 J, salt spray test ≥ 1000 h without red rust, and cyclic assembly ≥ 15 times without failure.

Images