Process for improving spheroidizing annealing and cold bending performance of round-link chain steel

Abstract

The application relates to the technical field of metal material processing, and discloses a spheroidizing annealing and cold bending performance improvement process for steel for round-link chains, which comprises the following steps: S1, a pretreatment process step; S2, a spheroidizing annealing core process step; S3, a cooling optimization process step; S4, a subsequent strengthening process step; S5, a quality control and adjustment step; and S6, a process stability guarantee step.The process realizes accurate and controllable spheroidizing organization: through a segmented heating and temperature spheroidizing synergistic process, a nitrogen-hydrogen mixed protective atmosphere is matched, so that the spheroidizing rate of the steel is greater than or equal to 90%, the proportion of spheroidal cementite is greater than or equal to 85%, the particle size is uniformly controlled within 1-3 microns, and the core pain point of uneven spheroidizing in the traditional process is completely solved.The process realizes significant improvement of the cold bending performance: the spheroidizing uniformity optimization is combined with the introduction of residual compressive stress through shot blasting strengthening, and internal stress is eliminated through low-temperature tempering, so that no visible cracks are generated in the 180-degree cold bending test, the process is suitable for subsequent drawing and cold bending ring forming processing, the forming cracking risk is greatly reduced, and the chain forming qualified rate is improved.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, specifically to a spheroidizing annealing and cold bending performance improvement process for steel used in circular link chains. Background Technology

[0002] The steel used for round link chains requires multiple cold working processes, including drawing, cold bending into rings, and welding. The uniformity of its spheroidized structure and its cold bending performance directly determine the forming qualification rate and service safety. In existing technologies, the spheroidizing annealing process mostly adopts a single-stage heating and cooling mode, lacking a precise design for segmented heating and isothermal spheroidization. This results in a low spheroidization rate of cementite (often below 85%) and uneven distribution, large fluctuations in microhardness, and easy cracking due to stress concentration during cold bending. At the same time, improper control of the protective atmosphere during annealing, with excessive oxygen content in the furnace, can easily lead to excessive surface decarburization, further weakening the cold bending plasticity. This makes it difficult to meet the stringent requirements of subsequent drawing deformation and cold bending into rings of the round link chain, and in severe cases, it can lead to insufficient strength at the welded joints after chain forming or breakage failure during service. Summary of the Invention

[0003] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a spheroidizing annealing and cold bending performance improvement process for steel used in circular links chains. This process has advantages such as precise and controllable spheroidization structure, and solves the problem that spheroidizing annealing processes often adopt a single-stage heating and cooling mode, lacking a precise design for segmented heating and isothermal spheroidization, which leads to low cementite spheroidization rate.

[0004] (II) Technical Solution To achieve the above-mentioned goal of precise and controllable spheroidization structure, the present invention provides the following technical solution: a spheroidizing annealing and cold bending performance improvement process for steel for circular links, including S1 pretreatment process step, S2 spheroidizing annealing core process step, S3 cooling optimization process step, S4 subsequent strengthening process step, S5 quality control and adjustment step, and S6 process stability assurance. The S1 pretreatment process step includes S101 surface cleaning treatment, S102 straightening and shaping, and S103 preheating and hydrogen removal. Among them, the core process steps of S2 spheroidizing annealing include S201 segmented heating process and S202 warm spheroidizing process; The S3 cooling optimization process includes S301 first-stage cooling, S302 second-stage cooling, and S303 furnace exit cooling. The subsequent strengthening process steps of S4 include shot blasting strengthening (S401), low temperature tempering (S402), and precision drawing adaptation (S403). The S5 quality control and adjustment steps include S501 key indicator testing and S502 process adjustment mechanism.

[0005] Preferably, the surface cleaning treatment of S101 is carried out by mechanical peeling and shot blasting. The thickness of the oxide scale removed by peeling is ≥0.2mm, the shot blasting pressure is 0.4-0.7MPa (0.5-0.6MPa for material basis optimization), and the shot blasting time is 8-15min, to ensure that the surface is free of oil, rust and inclusions, and the surface roughness Ra≤1.6μm.

[0006] Preferably, the S102 straightening and shaping is performed using a multi-roller straightener with a straightening pressure of 5-12 MPa. After straightening, the straightness deviation of the wire rod is ≤0.3 mm / m (material base ≤0.4 mm / m optimized tightening), avoiding the influence of bending stress on the spheroidization uniformity.

[0007] Preferably, the S103 preheating and hydrogen removal process involves a preheating temperature of 350-450℃ (380-420℃ is optimized and expanded based on the material base), a holding time of 60-120 min, and a heating rate of ≤120℃ / h, which removes residual hydrogen from the material and reduces the risk of cold bending cracking.

[0008] Preferably, the S201 segmented heating process is as follows: The first stage of heating: from room temperature to 650-700℃ (670-690℃ for material basis, optimized and expanded), with a heating rate of 80-100℃ / h, to avoid excessive heating that could lead to structural stress. Second stage of heating: Continue heating to 720-780℃ (740-760℃ for material base, optimized and expanded), heating rate 60-80℃ / h, holding for 2-4h (2.5-3.5h for material base, optimized and expanded), to promote the initial spheroidization of cementite; Heat preservation stage: Heat up to 790-840℃ (800-820℃ for material base, optimized and expanded), heat preservation for 4-8 hours (5-7 hours for material base, optimized and expanded). During the heat preservation process, a nitrogen + hydrogen mixed protective atmosphere (85%-95% nitrogen + 5%-15% hydrogen) is introduced. The oxygen content in the furnace is ≤0.05% to prevent surface oxidation and decarburization.

[0009] Preferably, the S202 warm spheroidizing process: Temperature: Reduce to 700-740℃ (710-730℃ for material basis optimization and expansion), hold for 3-6 hours to ensure full spheroidization of cementite and refine the size of spherical particles (target 1-3μm). Atmosphere control: Maintain a protective atmosphere with positive pressure (0.02-0.05MPa), and periodically replace the atmosphere to avoid carbon potential fluctuations in the furnace that could lead to decarburization or carbon enrichment.

[0010] Preferably, in the first stage of cooling S301: after the initial heating, the furnace is cooled to 550-600℃ at a rate of 15-30℃ / h (20-25℃ / h for material-based applications, optimized and expanded) to prevent the growth or decomposition of spherical cementite. S302 second-stage cooling: continue cooling to 300-350℃, cooling rate 30-50℃ / h, shortening the cooling cycle while ensuring tissue stability; S303 furnace cooling: Remove from the furnace at 300-350℃ and air cool to room temperature. Avoid wind or rain during air cooling to prevent excessive surface temperature difference from causing stress.

[0011] Preferably, the S401 shot blasting strengthening uses steel shot with a diameter of 0.2-0.5 mm, a shot blasting pressure of 0.3-0.6 MPa, and a shot blasting time of 5-10 min to remove surface oxide scale and introduce residual compressive stress to improve cold bending performance. S402 Low-Temperature Tempering: For products with higher requirements, low-temperature tempering is performed after shot blasting at a temperature of 180-220℃ (190-210℃ for material basis optimization and expansion), holding for 90-150 minutes, and then cooling to room temperature with the furnace to stabilize the spherical cementite structure and reduce internal stress. S403 precision drawing adapter: drawing deformation is controlled at 10%-25% (15%-20% based on material, optimized and expanded), drawing speed is 3-8m / min, drawing oil lubrication is used to reduce surface scratches, further refine the microstructure and improve cold bending coordination.

[0012] Preferably, the S501 key indicator detection includes: Spheroidization rate test: Take 3-5 samples from each batch and observe them under a metallographic microscope. A spheroidization rate of ≥90% is considered qualified, and the uniformity of spherical cementite particles is ≤3. Hardness testing: Using an HRB hardness tester, 3 testing points are taken for each pad. The hardness is controlled within 75-88 HRB (78-85 HRB based on material) and the hardness fluctuation is ≤ ±3 HRB. Cold bending test: 180° cold bending (bending mandrel diameter d = specimen diameter a), no visible cracks on the outer surface after bending, 2-3 specimens of different specifications are taken from each batch for testing; Decarburization layer inspection: For Φ8-10mm specifications, the decarburization layer is ≤0.12mm; for Φ10-15mm specifications, it is ≤1.2% D. Metallographic method or hardness method is used for inspection. Batches that exceed the standard need to be reworked. S502 process adjustment mechanism: If the spheroidization rate is <90%, extend the heat preservation time at 790-840℃ for 1-2 hours, or increase the temperature by 5-10℃. If the hardness is >88HRB: reduce the maximum spheroidizing annealing temperature by 10-15℃, or extend the second stage cooling time; If cold bending cracks occur: check the spheroidization rate (≥88%). If the spheroidization is qualified, optimize the shot blasting pressure (+0.1MPa) or increase the low temperature tempering time by 30min, and at the same time check for surface defects. If the decarburization layer exceeds the standard: increase the proportion of hydrogen in the protective atmosphere (+3%-5%), reduce the oxygen content in the furnace to ≤0.03%, or shorten the high-temperature holding time.

[0013] Preferably, the stability of the S6 process is guaranteed: For each heat, the temperature, holding time, and atmosphere parameters at each stage of spheroidizing annealing are recorded, and a linked database of "parameters-spheroidization rate-cold bending performance" is established. The temperature uniformity of the heating furnace is calibrated monthly, and the temperature deviation in each area of ​​the furnace is ≤ ±5℃. Raw material composition control: Ensure Mn content is 1.40%-1.70%, Al content is 0.020%-0.055% (based on material optimization and expansion), harmful impurities P≤0.022%, S≤0.012%, to avoid composition fluctuations affecting spheroidization effect.

[0014] (III) Beneficial Effects Compared with the prior art, the present invention provides a spheroidizing annealing and cold bending performance improvement process for steel used in circular link chains, which has the following beneficial effects: 1. The spheroidizing annealing and cold bending performance improvement process of the steel used in this circular link chain achieves precise and controllable spheroidization structure: through the synergistic process of segmented heating (650-700℃→720-780℃→790-840℃) and warm spheroidization (700-740℃), combined with a nitrogen + hydrogen mixed protective atmosphere, the steel spheroidization rate is ≥90%, the proportion of spherical cementite is ≥85%, and the particle size is uniformly controlled within 1-3μm, completely solving the core pain point of uneven spheroidization in traditional processes.

[0015] 2. The spheroidizing annealing and cold bending performance improvement process of the steel used in this circular link chain significantly improves the cold bending performance: the optimization of spheroidization uniformity combined with shot blasting to introduce residual compressive stress and low-temperature tempering to eliminate internal stress result in no visible cracks in the 180° cold bending test (d=a), making it suitable for subsequent drawing (deformation amount 10%-25%) and cold bending ring forming, greatly reducing the risk of forming cracks and improving the chain forming qualification rate.

[0016] 3. The spheroidizing annealing and cold bending performance improvement process of the steel used in this circular link chain achieves strict control over the decarburized layer: the protective atmosphere is kept under positive pressure (0.02-0.05MPa) throughout the annealing process, and the oxygen content in the furnace is ≤0.05%, which effectively inhibits surface oxidation and decarburization, making the decarburized layer ≤0.12mm for Φ8-10mm specifications and ≤1.2% D for Φ10-15mm specifications, avoiding the decrease in strength and plasticity caused by decarburization, and ensuring the overall performance consistency of the chain after welding.

[0017] 4. The spheroidizing annealing and cold bending performance improvement process for the steel used in this circular link chain achieves strong process stability and adaptability: through pretreatment straightening, precise control of segment parameters, and the establishment of a "parameter-spheroidization rate-cold bending performance" linkage database, the microhardness is stabilized at 75-88HRB (fluctuation ≤±3HRB), which can be flexibly adapted to the production of materials with different specifications of Φ8-15mm, reducing batch quality fluctuations and lowering rework costs caused by substandard performance. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] This solution provides a technical approach, specifically a process for improving the spheroidizing annealing and cold bending properties of steel for round link chains, comprising the following steps: S1 Pretreatment process steps: S101 Surface Cleaning Treatment: Mechanical peeling + shot blasting combined treatment is adopted. The thickness of the oxide scale removed by peeling is ≥0.2mm. The shot blasting pressure is 0.4-0.7MPa (0.5-0.6MPa for material basis optimization and expansion). The shot blasting time is 8-15min to ensure that the surface is free of oil, rust and inclusions, and the surface roughness Ra≤1.6μm. S102 Straightening and Shaping: A multi-roller straightener is used with a straightening pressure of 5-12MPa. After straightening, the straightness deviation of the wire rod is ≤0.3mm / m (material base ≤0.4mm / m optimized tightening), avoiding the influence of bending stress on the spheroidization uniformity. S103 preheating and hydrogen removal: preheating temperature 350-450℃ (material base 380-420℃ optimized and expanded), holding time 60-120min, heating rate ≤120℃ / h, to remove residual hydrogen inside the material and reduce the risk of cold bending cracking; S2 spheroidizing annealing core process steps: S201 segmented heating process: The first stage of heating: from room temperature to 650-700℃ (670-690℃ for material basis, optimized and expanded), with a heating rate of 80-100℃ / h, to avoid excessive heating that could lead to structural stress. Second stage of heating: Continue heating to 720-780℃ (740-760℃ for material base, optimized and expanded), heating rate 60-80℃ / h, holding for 2-4h (2.5-3.5h for material base, optimized and expanded), to promote the initial spheroidization of cementite; Heat preservation stage: Heat up to 790-840℃ (800-820℃ for material base, optimized and expanded), heat preservation for 4-8 hours (5-7 hours for material base, optimized and expanded). During the heat preservation process, a nitrogen + hydrogen mixed protective atmosphere (85%-95% nitrogen + 5%-15% hydrogen) is introduced. The oxygen content in the furnace is ≤0.05% to prevent surface oxidation and decarburization. S202 warm spheroidizing process: Temperature: Reduce to 700-740℃ (710-730℃ for material basis optimization and expansion), hold for 3-6 hours to ensure full spheroidization of cementite and refine the size of spherical particles (target 1-3μm). Atmosphere control: Maintain a protective atmosphere with positive pressure (0.02-0.05MPa), and periodically replace the atmosphere to avoid carbon potential fluctuations in the furnace that could lead to decarburization or carbon enrichment; S3 Cooling Optimization Process Steps: S301 First-Stage Cooling: After the initial warm-up phase, the furnace is cooled to 550-600℃ at a rate of 15-30℃ / h (20-25℃ / h for material-specific applications, optimized for expansion), to prevent the growth or decomposition of spherical cementite. S302 second-stage cooling: continue cooling to 300-350℃, cooling rate 30-50℃ / h, shortening the cooling cycle while ensuring tissue stability; S303 furnace cooling: Remove from the furnace at 300-350℃ and air cool to room temperature. Avoid wind or rain during air cooling to prevent excessive surface temperature difference from causing stress. S4 subsequent strengthening process steps: S401 shot blasting: Use steel shot with a diameter of 0.2-0.5mm, shot blasting pressure of 0.3-0.6MPa, shot blasting time of 5-10min to remove surface oxide scale and introduce residual compressive stress to improve cold bending performance; S402 Low-Temperature Tempering: For products with higher requirements, low-temperature tempering is performed after shot blasting at a temperature of 180-220℃ (190-210℃ for material basis optimization and expansion), holding for 90-150 minutes, and then cooling to room temperature with the furnace to stabilize the spherical cementite structure and reduce internal stress. S403 precision drawing adapter: drawing deformation is controlled at 10%-25% (15%-20% based on material, optimized and expanded), drawing speed is 3-8m / min, drawing oil is used for lubrication to reduce surface scratches, further refine the microstructure and improve cold bending coordination; S5 Quality Control and Adjustment Steps: S501 Key Performance Indicators: Spheroidization rate test: Take 3-5 samples from each batch and observe them under a metallographic microscope. A spheroidization rate of ≥90% is considered qualified, and the uniformity of spherical cementite particles is ≤3. Hardness testing: Using an HRB hardness tester, 3 testing points are taken for each pad. The hardness is controlled within 75-88 HRB (78-85 HRB based on material) and the hardness fluctuation is ≤ ±3 HRB. Cold bending test: 180° cold bending (bending mandrel diameter d = specimen diameter a), no visible cracks on the outer surface after bending, 2-3 specimens of different specifications are taken from each batch for testing; Decarburization layer inspection: For Φ8-10mm specifications, the decarburization layer is ≤0.12mm; for Φ10-15mm specifications, it is ≤1.2% D. Metallographic method or hardness method is used for inspection. Batches that exceed the standard need to be reworked. S502 process adjustment mechanism: If the spheroidization rate is <90%, extend the heat preservation time at 790-840℃ for 1-2 hours, or increase the temperature by 5-10℃. If the hardness is >88HRB: reduce the maximum spheroidizing annealing temperature by 10-15℃, or extend the second stage cooling time; If cold bending cracks occur: check the spheroidization rate (≥88%). If the spheroidization is qualified, optimize the shot blasting pressure (+0.1MPa) or increase the low temperature tempering time by 30min, and at the same time check for surface defects. If the decarburization layer exceeds the standard: increase the proportion of hydrogen in the protective atmosphere (+3%-5%), reduce the oxygen content in the furnace to ≤0.03%, or shorten the high-temperature holding time; S6 process stability assurance: For each heat, the temperature, holding time, and atmosphere parameters at each stage of spheroidizing annealing are recorded, and a linked database of "parameters-spheroidization rate-cold bending performance" is established. The temperature uniformity of the heating furnace is calibrated monthly, and the temperature deviation in each area of ​​the furnace is ≤ ±5℃. Raw material composition control: Ensure Mn content is 1.40%-1.70%, Al content is 0.020%-0.055% (based on material optimization and expansion), harmful impurities P≤0.022%, S≤0.012%, to avoid composition fluctuations affecting spheroidization effect; Process objectives: Through precise spheroidizing annealing and multi-stage synergistic optimization, the spheroidization rate of the steel used for circular link chains is ≥90% (of which the proportion of spherical cementite is ≥85%), the microhardness is controlled at 75-88 HRB, and no visible cracks are found in the 180° cold bending test (bending mandrel diameter d = sample diameter a); the decarburized layer depth is ≤0.12mm (Φ8-10mm specification) and ≤1.2% D (Φ10-15mm specification), ensuring the compatibility of subsequent drawing, cold bending and welding processes, and improving the chain's fatigue resistance and forming qualification rate; Furthermore, this process achieves precise and controllable spheroidization structure: through the synergistic process of segmented heating (650-700℃→720-780℃→790-840℃) and warm spheroidization (700-740℃), combined with a nitrogen + hydrogen mixed protective atmosphere, the steel spheroidization rate is ≥90%, the proportion of spherical cementite is ≥85%, and the particle size is uniformly controlled within 1-3μm, completely solving the core pain point of uneven spheroidization in traditional processes; Furthermore, this process significantly improves cold bending performance: the optimization of spheroidization uniformity combined with shot blasting to introduce residual compressive stress and low-temperature tempering to eliminate internal stress result in no visible cracks in the 180° cold bending test (d=a), making it suitable for subsequent drawing (deformation amount 10%-25%) and cold bending ring forming, greatly reducing the risk of forming cracks and improving the chain forming qualification rate. Furthermore, this process achieves strict control over the decarburized layer: the protective atmosphere is kept under positive pressure (0.02-0.05MPa) throughout the annealing process, and the oxygen content in the furnace is ≤0.05%, which effectively inhibits surface oxidation and decarburization, making the decarburized layer ≤0.12mm for Φ8-10mm specifications and ≤1.2% D for Φ10-15mm specifications, avoiding the decrease in strength and plasticity caused by decarburization, and ensuring the overall performance consistency of the chain after welding; Furthermore, this process achieves strong stability and adaptability: through pretreatment straightening, precise control of segmented parameters, and the establishment of a "parameter-sphericity-cold bending performance" linkage database, the microhardness is stabilized at 75-88 HRB (fluctuation ≤ ±3 HRB), which can be flexibly adapted to the production of materials of different specifications from Φ8-15mm, reducing batch quality fluctuations and lowering rework costs caused by substandard performance.

[0020] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for improving the spheroidizing annealing and cold bending performance of steel for circular link chains, comprising the following steps: S1 pretreatment, S2 spheroidizing annealing, S3 cooling optimization, S4 subsequent strengthening, S5 quality control and adjustment, and S6 process stability assurance, characterized in that: The S1 pretreatment process steps include S101 surface cleaning treatment, S102 straightening and shaping, and S103 preheating and hydrogen removal. Among them, the core process steps of S2 spheroidizing annealing include S201 segmented heating process and S202 warm spheroidizing process; The S3 cooling optimization process includes S301 first-stage cooling, S302 second-stage cooling, and S303 furnace exit cooling. The subsequent strengthening process steps of S4 include shot blasting strengthening (S401), low temperature tempering (S402), and precision drawing adaptation (S403). The S5 quality control and adjustment steps include S501 key indicator testing and S502 process adjustment mechanism.

2. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The surface cleaning treatment of S101 is carried out by mechanical peeling and shot blasting. The thickness of the oxide scale removed by peeling is ≥0.2mm. The shot blasting pressure is 0.4-0.7MPa (0.5-0.6MPa for material basis optimization and expansion). The shot blasting time is 8-15min to ensure that the surface is free of oil, rust and inclusions, and the surface roughness Ra≤1.6μm.

3. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The S102 straightening and shaping process employs a multi-roller straightener with a straightening pressure of 5-12 MPa. After straightening, the straightness deviation of the wire rod is ≤0.3 mm / m (material base ≤0.4 mm / m optimized tightening), avoiding the influence of bending stress on the spheroidization uniformity.

4. The spheroidizing annealing and cold bending performance improvement process for steel used in circular link chains according to claim 1, characterized in that: The S103 preheating and hydrogen removal process involves a preheating temperature of 350-450℃ (380-420℃ is optimized and expanded for the basic material), a holding time of 60-120 minutes, and a heating rate of ≤120℃ / h. This process removes residual hydrogen from the material and reduces the risk of cold bending cracks.

5. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The S201 segmented heating process: The first stage of heating: from room temperature to 650-700℃ (670-690℃ for material basis, optimized and expanded), with a heating rate of 80-100℃ / h, to avoid excessive heating that could lead to structural stress. Second stage of heating: Continue heating to 720-780℃ (740-760℃ for material base, optimized and expanded), heating rate 60-80℃ / h, holding for 2-4h (2.5-3.5h for material base, optimized and expanded), to promote the initial spheroidization of cementite; Heat preservation stage: Heat up to 790-840℃ (800-820℃ for material base, optimized and expanded), heat preservation for 4-8 hours (5-7 hours for material base, optimized and expanded). During the heat preservation process, a nitrogen + hydrogen mixed protective atmosphere (85%-95% nitrogen + 5%-15% hydrogen) is introduced. The oxygen content in the furnace is ≤0.05% to prevent surface oxidation and decarburization.

6. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The S202 warm spheroidizing process: Temperature: Reduce to 700-740℃ (710-730℃ for material basis optimization and expansion), hold for 3-6 hours to ensure full spheroidization of cementite and refine the size of spherical particles (target 1-3μm). Atmosphere control: Maintain a protective atmosphere with positive pressure (0.02-0.05MPa), and periodically replace the atmosphere to avoid carbon potential fluctuations in the furnace that could lead to decarburization or carbon enrichment.

7. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The first stage of cooling for S301 involves furnace cooling to 550-600℃ after the initial warm-up phase, at a rate of 15-30℃ / h (20-25℃ / h for material-specific applications, optimized for expansion), to prevent the growth or decomposition of spherical cementite. S302 second-stage cooling: continue cooling to 300-350℃, cooling rate 30-50℃ / h, shortening the cooling cycle while ensuring tissue stability; S303 furnace cooling: Remove from the furnace at 300-350℃ and air cool to room temperature. Avoid wind or rain during air cooling to prevent excessive surface temperature difference from causing stress.

8. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The S401 shot blasting strengthening method uses steel shot with a diameter of 0.2-0.5mm, a shot blasting pressure of 0.3-0.6MPa, and a shot blasting time of 5-10min to remove surface oxide scale and introduce residual compressive stress to improve cold bending performance. S402 Low-Temperature Tempering: For products with higher requirements, low-temperature tempering is performed after shot blasting at a temperature of 180-220℃ (190-210℃ for material basis optimization and expansion), holding for 90-150 minutes, and then cooling to room temperature with the furnace to stabilize the spherical cementite structure and reduce internal stress. S403 precision drawing adapter: drawing deformation is controlled at 10%-25% (15%-20% based on material, optimized and expanded), drawing speed is 3-8m / min, drawing oil lubrication is used to reduce surface scratches, further refine the microstructure and improve cold bending coordination.

9. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The S501 key indicator detection: Spheroidization rate test: Take 3-5 samples from each batch and observe them under a metallographic microscope. A spheroidization rate of ≥90% is considered qualified, and the uniformity of spherical cementite particles is ≤3. Hardness testing: Using an HRB hardness tester, 3 testing points are taken for each pad. The hardness is controlled within 75-88 HRB (78-85 HRB based on material) and the hardness fluctuation is ≤ ±3 HRB. Cold bending test: 180° cold bending (bending mandrel diameter d = specimen diameter a), no visible cracks on the outer surface after bending, 2-3 specimens of different specifications are taken from each batch for testing; Decarburization layer inspection: For Φ8-10mm specifications, the decarburization layer is ≤0.12mm; for Φ10-15mm specifications, it is ≤1.2% D. Metallographic method or hardness method is used for inspection. Batches that exceed the standard need to be reworked. S502 process adjustment mechanism: If the spheroidization rate is <90%, extend the heat preservation time at 790-840℃ for 1-2 hours, or increase the temperature by 5-10℃. If the hardness is >88HRB: reduce the maximum spheroidizing annealing temperature by 10-15℃, or extend the second stage cooling time; If cold bending cracks occur: check the spheroidization rate (≥88%). If the spheroidization is qualified, optimize the shot blasting pressure (+0.1MPa) or increase the low temperature tempering time by 30min, and at the same time check for surface defects. If the decarburization layer exceeds the standard: increase the proportion of hydrogen in the protective atmosphere (+3%-5%), reduce the oxygen content in the furnace to ≤0.03%, or shorten the high-temperature holding time.

10. The spheroidizing annealing and cold bending performance improvement process for steel used in circular links according to claim 1, characterized in that: The stability guarantee of the S6 process: For each heat, the temperature, holding time, and atmosphere parameters at each stage of spheroidizing annealing are recorded, and a linked database of "parameters - spheroidization rate - cold bending performance" is established. The temperature uniformity of the heating furnace is calibrated monthly, and the temperature deviation in each area of ​​the furnace is ≤ ±5℃. Raw material composition control: Ensure Mn content is 1.40%-1.70%, Al content is 0.020%-0.055% (based on material optimization and expansion), harmful impurities P≤0.022%, S≤0.012%, to avoid composition fluctuations affecting spheroidization effect.