Controlled rolling and controlled cooling process for non-tempered steel C70S6

Abstract

The invention relates to the technical field of ferrous metallurgy, and discloses a controlled rolling and controlled cooling process for non-conditioned steel C70S6. The controlled rolling and controlled cooling process comprises the steps of S1, a heating procedure, S2, a rough rolling procedure, S3, a finish rolling procedure, S4, a controlled cooling procedure, S5, a cooling post-treatment procedure and S6, finished product inspection. The steel billet is uniform in austenitizing, the casting structure is thoroughly crushed, the austenite grain size is controlled to be 6.0-9.0 grade, the phenomenon of coarse grains or mixed grains is effectively avoided, the target structure of pearlite + ferrite (less than or equal to 10%) is uniform in distribution, the carbon segregation is less than or equal to + / -15%, and the problem of expansion fracture performance fluctuation caused by component segregation and uneven structure is solved; according to the process, surface quality and size precision optimization is achieved, high-pressure water descaling (24-30 MPa) is combined with scaling treatment (the allowance is 0.3-0.8 mm), and oxide scale and surface crack defects are thoroughly removed.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a controlled rolling and controlled cooling process for non-cooled steel C70S6. Background Technology

[0002] C70S6 non-adjustable steel, as the core material of fracture-resistant connecting rods, is widely used in the automotive engine field. Its performance directly determines the fracture quality and service life of the connecting rod. This steel grade requires high strength, low toughness, narrow composition range control, and good fracture-resistant brittle fracture characteristics, while also meeting the requirements of machinability and microstructure uniformity. However, existing production processes have many technical challenges: during the heating stage, improper heating rates and insufficient homogenization often lead to uneven austenitization of the billet, resulting in coarse grains and significant compositional segregation, which poses a hidden danger for subsequent microstructure control; during rough rolling, incomplete descaling with high-pressure water leaves residual iron oxide scale, which can easily cause surface cracks, folds, and other defects, and unreasonable reduction distribution can lead to insufficient microstructure fragmentation, affecting the subsequent finish rolling effect; during finish rolling, insufficient dimensional accuracy control can easily lead to problems such as diameter deviation and excessive out-of-roundness, while excessively long residence time in the high-temperature zone can result in an excessively thick decarburized layer on the surface, reducing the surface hardness of the steel; during controlled cooling, uneven cooling rates can easily generate harmful structures such as bainite, leading to an imbalance in strength and toughness, and defects such as slag shedding and uneven fracture surfaces during expansion fracture; in the post-cooling treatment process, insufficient slow cooling and insufficient peeling allowance result in residual internal stress and micro-cracks on the surface not being completely removed, ultimately affecting the product qualification rate and service stability. In addition, the domestic production of this steel grade also faces problems such as poor compositional stability and low yield, making it difficult to meet the stringent requirements of the automotive industry for fracture-resistant connecting rod materials. Summary of the Invention

[0003] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a controlled rolling and cooling process for non-heat-treated C70S6 steel, which has advantages such as significantly improved microstructure uniformity and solves the problem of uneven austenitization of steel billets caused by improper heating rate and insufficient heat homogenization during the heating stage.

[0004] (II) Technical Solution To achieve the aforementioned goal of significantly improving the uniformity of the microstructure, the present invention provides the following technical solution: a controlled rolling and cooling process for non-heated steel C70S6, comprising an S1 heating process, an S2 rough rolling process, an S3 finishing rolling process, an S4 controlled cooling process, an S5 cooling post-treatment process, and an S6 finished product inspection process. The S1 heating process includes an S101 billet pretreatment, an S102 furnace loading and heating, an S103 homogenization control, and an S104 temperature monitoring and unloading. The S2 roughing process includes S201 initial rolling conditions, S202 descaling operation, S203 reduction distribution, S204 rolling speed and equipment adjustment, and S205 final rolling temperature control. The S3 finishing rolling process includes S301 initial rolling and pass control, S302 dimensional accuracy control, S303 rolling speed and cooling, S304 final rolling temperature and decarburization control, and S305 post-rolling surface inspection. Among them, the S4 cooling process includes S401 first stage water cooling (rapid cooling), S402 second stage air cooling (uniform cooling), S403 final cooling temperature and insulation, and S404 cooling equipment maintenance. Among them, the S5 cooling post-processing process includes S501 slow cooling treatment, S502 peeling treatment and S503 flaw detection. Among them, S6 finished product inspection includes S601 metallographic structure, S602 mechanical properties, S603 chemical composition, S604 gas content and S605 identification and storage.

[0005] Preferably, the pretreatment of the S101 steel billet is as follows: before loading into the furnace, the surface of the steel billet is thoroughly inspected to remove surface iron oxide scale, cracks, inclusions and defects. If the defect depth exceeds 0.5mm, it needs to be ground in advance to avoid the defects from expanding after heating. The end face of the steel billet needs to be flat and without obvious tilt to ensure uniform heating. S102 Furnace Loading and Heating: The loading temperature is ≤400℃, and a stepped heating mode is adopted to avoid thermal stress cracks in the billet due to excessive temperature difference; the heating rate is controlled at 120-160℃ / h, the low temperature section (200-400℃) is held for 60-90min to eliminate residual stress inside the billet; the medium temperature section (400-800℃) is held for 90-120min to gradually increase the core temperature of the billet; the high temperature section (800-1210℃) is held for 120-180min to promote composition homogenization. S103 heat soaking control: The temperature of the heat soaking section is stabilized at 1150-1210℃, and the holding time is 240-400min, which is adjusted according to the cross-sectional size of the billet (the larger the cross-section, the longer the holding time); the residual oxygen content of the furnace gas is controlled at 0.3%-4.0%, and a reducing or weak oxidizing atmosphere is maintained by adjusting the air-fuel ratio to reduce decarburization on the surface of the billet; S104 Temperature Monitoring and Unloading: Multi-point thermometers are used to monitor the surface and core temperature of the billet in real time, and the temperature difference is controlled within ≤30℃; the unloading temperature is strictly controlled at 1040-1100℃, and the heating rate is reduced 30 minutes before unloading to ensure uniform billet temperature and avoid local overheating that leads to coarse grains.

[0006] Preferably, the S201 initial rolling conditions are: the initial rolling temperature is matched with the furnace exit temperature and controlled at 1040-1100℃. When the billet temperature is lower than 1020℃, it needs to be returned to the heating furnace to be heated again to avoid excessive rolling force or cracks caused by low-temperature rough rolling. S202 Descaling Operation: Set up 2-3 passes of high-pressure water descaling, with a descaling pressure of 24-30 MPa and a water flow rate ≥150 m³ / h. 3 / h, to ensure thorough removal of secondary iron oxide scale generated during the heating process; the descaling nozzle needs to be inspected and cleaned regularly to ensure accurate spray angle and avoid surface defects caused by iron oxide scale residue; S203 Reduction Distribution: The total reduction is controlled at 60%-70%, and the reduction per pass is distributed as follows: 15%-25% for the first 2-3 passes (large reduction to break the casting structure), and gradually reduced to 10%-15% for subsequent passes to avoid uneven deformation of the billet due to excessive reduction in a single pass; a 5-8s interval is set between adjacent passes to ensure stable temperature of the rolled piece.

[0007] Preferably, the S204 rolling speed and equipment adjustment are as follows: the rolling speed is gradually increased to 3.0-5.0 m / s, and the speed is matched according to the changes in the cross-section of the rolled piece to avoid steel piling or pulling; the mill rigidity is monitored in real time, and the roll gap is adjusted according to the changes in rolling force to ensure that the dimensional tolerance of the intermediate billet is controlled within ±2.0 mm; S205 final rolling temperature control: The final rolling temperature of roughing is controlled at 950-1000℃. By adjusting the rolling rhythm and pass interval, the temperature of the final rolling is kept away from excessively high temperature which may cause abnormal growth of austenite grains, or too low temperature which may affect the plasticity of finishing rolling.

[0008] Preferably, the S301 initial rolling and pass control is as follows: the finishing rolling initial rolling temperature is controlled at 900-950℃, connecting with the roughing rolling final rolling temperature to avoid excessive temperature fluctuations; the pass reduction is allocated as 8%-15%, and the reduction in the last two passes is reduced to 5%-8%, focusing on ensuring dimensional accuracy; the number of finishing rolling passes is adjusted according to the finished product specifications (generally 5-8 passes). S302 Dimensional Accuracy Control: The finished product dimensions are monitored in real time using an online diameter gauge, and the roll gap is dynamically adjusted. The diameter tolerance of the finished product is controlled within +0.10-+0.60mm, and the out-of-roundness is ≤0.50mm. The wear of the rolls is checked regularly, and the rolls are replaced or repaired in time when the wear exceeds 0.3mm. S303 rolling speed and cooling: The finishing rolling speed is controlled at 5.0-7.0 m / s, optimized according to the finished product specifications (higher speed for small specifications, lower speed for large specifications); the rolls are cooled by circulating water, and the cooling water temperature is controlled at 30-45℃ to avoid the impact of roll thermal expansion on dimensional accuracy; S304 Finishing Rolling Temperature and Decarburization Control: The finishing rolling temperature is strictly controlled at 820-880℃, which is achieved by adjusting the rolling speed and pass interval; shortening the residence time of the rolled piece in the high-temperature zone reduces surface decarburization, while controlling the residual oxygen content of the furnace gas, and controlling the decarburized layer depth to ≤0.6% of the finished product diameter.

[0009] Preferably, the S305 post-rolling surface inspection involves setting up an online surface inspection device after finishing rolling to check for surface cracks, folds, scratches, and other defects. Any defects found are marked in a timely manner and subsequently treated by grinding.

[0010] Preferably, the first stage of S401 water cooling (rapid cooling) has the following characteristics: water cooling section length ≥ 8m, inlet temperature 800-850℃, cooling rate 5-15℃ / s, and cooling time 10-20s; the cooling intensity is adjusted according to the finished product specifications (higher cooling rate for smaller specifications, lower cooling rate for larger specifications) by controlling the water flow rate and the number of nozzles opened; during the water cooling process, the rolled piece is ensured to cool evenly to avoid cracks caused by excessively rapid local cooling; S402 Second Stage Air Cooling (Uniform Cooling): The air cooling line is ≥25m long, with an inlet temperature of 600-650℃ and a cooling rate of 2-8℃ / s. Forced convection air cooling is adopted, and the wind speed is controlled at 5-12m / s. The air cooling line is equipped with a zoned air control device to adjust the wind speed in each zone according to the temperature distribution of the rolled piece, ensuring that the cross-sectional cooling temperature difference is ≤30℃. S403 Final Cooling Temperature and Insulation: The final cooling temperature is strictly controlled between 500-550℃ and monitored in real time by an infrared thermometer. When the temperature is below 500℃, the air cooling intensity is reduced, and when it is above 550℃, the air speed is increased. After final cooling, an insulation section (length ≥10m) is set up to allow the temperature to drop naturally to below 400℃, further eliminating internal stress.

[0011] Preferably, the S404 cooling equipment maintenance includes: regularly cleaning the water-cooled nozzles and air-cooled pipes to avoid blockages that could lead to uneven cooling; checking the cooling water quality to ensure that the suspended solids content is ≤50mg / L to prevent scale buildup in the nozzles from affecting the cooling effect.

[0012] Preferably, the S501 slow cooling treatment involves: the steel after final cooling being placed in a slow cooling pit or slow cooling stack, with a slow cooling temperature ≤300℃ and a slow cooling time of 12-24 hours, which is adjusted according to the steel specifications (the slow cooling time for large specifications is extended to 24-36 hours); during the slow cooling process, the steel should be protected from rain or rapid cooling to prevent secondary stress. S502 peeling process: The peeling process is carried out using a centerless lathe, with a peeling allowance of 0.3-0.8mm to ensure complete removal of the surface decarburized layer, iron oxide scale and micro cracks; after peeling, the surface roughness of the steel is controlled at Ra≤3.2μm, and the diameter tolerance is adjusted to +0.05-+0.30mm; S503 Flaw Detection: Ultrasonic and magnetic particle combined flaw detection is adopted, and the Class A flaw detection standard in GB / T4162 is implemented. Ultrasonic flaw detection is used to detect internal defects (shrinkage cavities, inclusions, cracks), and magnetic particle flaw detection is used to detect surface and near-surface defects (microcracks, folds). Products that fail the flaw detection must be peeled again or rejected.

[0013] Preferably, the S6 finished product inspection: Sampling inspections are conducted on steel that has passed flaw detection, including: S601 metallographic structure: pearlite + ferrite (ferrite content ≤10%), grain size 6.0-9.0 grade; S602 mechanical properties: Brinell hardness 240-265HBW, tensile strength ≥750MPa, yield strength ≥450MPa; S603 chemical composition: carbon segregation ≤ ±15%, non-metallic inclusions (according to ASTM E45 standard) Class A ≤ 3.5, Class D ≤ 1.0; S604 gas content: nitrogen content 120-200ppm, oxygen content ≤15ppm; S605 Identification and Storage: Qualified products should be identified by specifications and batch number and stored in a dry and ventilated warehouse to prevent moisture and rust. Products from different batches should be stacked separately to prevent confusion and ensure traceability.

[0014] (III) Beneficial Effects Compared with the prior art, the present invention provides a controlled rolling and controlled cooling process for non-cooled C70S6 steel, which has the following beneficial effects: 1. The controlled rolling and cooling process of the non-heat-treated steel C70S6 significantly improves the uniformity of the microstructure: through stepped heating, precise heat homogenization control and reasonable reduction distribution, the billet is uniformly austenitized, the casting microstructure is completely broken, the austenite grain size is controlled at 6.0-9.0 grade, effectively avoiding coarse grains or mixed grains, the target microstructure of pearlite + ferrite (≤10%) is uniformly distributed, and carbon segregation is ≤±15%, solving the problem of fluctuation in fracture performance caused by compositional segregation and microstructure inhomogeneity.

[0015] 2. The controlled rolling and cooling process of this non-adjustable steel C70S6 achieves optimized surface quality and dimensional accuracy: high-pressure water descaling (24-30MPa) combined with peeling treatment (residual 0.3-0.8mm) thoroughly removes iron oxide scale and surface crack defects, with a surface roughness Ra≤3.2μm; real-time monitoring by online diameter measuring instrument and dynamic adjustment of roll gap ensure that the diameter tolerance of the finished product is controlled within +0.05-+0.30mm, and the out-of-roundness is ≤0.50mm, eliminating the problem of incomplete filling of forgings caused by undersized dimensions and greatly improving product adaptability.

[0016] 3. The controlled rolling and cooling process of the non-adjusted steel C70S6 achieves satisfactory and stable fracture performance: the segmented controlled cooling process precisely regulates the cooling rate to avoid bainite formation, ensuring that the steel's strength (tensile strength ≥750MPa), toughness, and hardness (240-265HBW) are matched, the nitrogen content is stable at 120-200ppm, the decarburized layer depth is ≤0.6% of the finished product diameter, the plastic deformation of the fracture surface is small during fracture, and the slag shedding rate is controlled within 2%, meeting the requirements of the connecting rod fracture process for brittle fracture of the material.

[0017] 4. The controlled rolling and cooling process for this non-rolled C70S6 steel achieves thorough elimination of internal stress and improves quality reliability: the slow cooling treatment (12-36h) after cooling and the design of the heat preservation section effectively release the internal stress generated during rolling and cooling, reducing the risk of secondary cracks; ultrasonic + magnetic particle combined flaw detection (GB / T4162 Class A standard) comprehensively investigates internal and surface defects, and non-metallic inclusions (Class A ≤ 3.5, Class D ≤ 1.0) are effectively controlled, significantly improving the product qualification rate.

[0018] 5. The controlled rolling and controlled cooling process of the non-heat-treated steel C70S6 achieves a balance between production efficiency and economy: the process parameters are scientifically adapted to different specifications of products, the rolling process is stable and controllable, and the scrap and rework caused by defects are reduced; the optimized process does not require additional heat treatment, reducing production costs, while the material performance is stable, meeting the long-term supply requirements of automotive OEMs for parts materials and enhancing market competitiveness. Detailed Implementation

[0019] 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.

[0020] This solution provides a technical approach, specifically a controlled rolling and cooling process for non-heated steel C70S6, including the following process: S1 heating step: Process objective: To achieve uniform austenitization of steel billets, completely break down the casting structure, control the austenite grain size within a reasonable range, reduce compositional segregation, and lay a solid foundation for subsequent microstructure refinement and performance regulation during rolling. Process parameters and key operating points: S101 billet pretreatment: Before loading into the furnace, a comprehensive inspection of the billet surface is carried out to remove surface iron oxide scale, cracks, inclusions and defects. If the defect depth exceeds 0.5mm, it needs to be ground in advance to avoid the defects from expanding after heating. The end face of the billet must be flat and without obvious tilt to ensure uniform heating. S102 Furnace Loading and Heating: The loading temperature is ≤400℃, and a stepped heating mode is adopted to avoid thermal stress cracks in the billet due to excessive temperature difference; the heating rate is controlled at 120-160℃ / h, the low temperature section (200-400℃) is held for 60-90min to eliminate residual stress inside the billet; the medium temperature section (400-800℃) is held for 90-120min to gradually increase the core temperature of the billet; the high temperature section (800-1210℃) is held for 120-180min to promote composition homogenization. S103 heat soaking control: The temperature of the heat soaking section is stabilized at 1150-1210℃, and the holding time is 240-400min, which is adjusted according to the cross-sectional size of the billet (the larger the cross-section, the longer the holding time); the residual oxygen content of the furnace gas is controlled at 0.3%-4.0%, and a reducing or weak oxidizing atmosphere is maintained by adjusting the air-fuel ratio to reduce decarburization on the surface of the billet; S104 Temperature Monitoring and Unloading: Multi-point thermometers are used to monitor the surface and core temperature of the billet in real time, and the temperature difference is controlled within ≤30℃; the unloading temperature is strictly controlled at 1040-1100℃, and the heating rate is reduced 30 minutes before unloading to ensure uniform billet temperature and avoid local overheating that leads to coarse grains. S2 rough rolling process: Process objective: By rationally allocating the reduction amount, the cross-section of the billet is initially reduced, the austenite grains are further broken up, the uniformity of the structure is improved, and an intermediate billet with the required dimensional accuracy is provided for the finishing rolling process, while avoiding rolling defects. Process parameters and key operating points: S201 initial rolling conditions: The initial rolling temperature should be matched with the furnace exit temperature and controlled at 1040-1100℃. When the billet temperature is below 1020℃, it needs to be returned to the heating furnace to reheat, in order to avoid excessive rolling force or cracks caused by low-temperature rough rolling. S202 Descaling Operation: Set up 2-3 passes of high-pressure water descaling, with a descaling pressure of 24-30 MPa and a water flow rate ≥150 m³ / h. 3 / h, to ensure thorough removal of secondary iron oxide scale generated during the heating process; the descaling nozzle needs to be inspected and cleaned regularly to ensure accurate spray angle and avoid surface defects caused by iron oxide scale residue; S203 Reduction Distribution: The total reduction is controlled at 60%-70%, and the reduction per pass is distributed as follows: 15%-25% for the first 2-3 passes (large reduction to break the casting structure), and gradually reduced to 10%-15% for subsequent passes to avoid uneven deformation of the billet due to excessive reduction in a single pass; a 5-8s interval is set between adjacent passes to ensure stable temperature of the rolled piece; S204 Rolling Speed ​​and Equipment Adjustment: The rolling speed is gradually increased to 3.0-5.0 m / s, and the speed is matched according to the changes in the cross-section of the rolled piece to avoid steel piling or pulling; the mill rigidity is monitored in real time, and the roll gap is adjusted according to the changes in rolling force to ensure that the dimensional tolerance of the intermediate billet is controlled within ±2.0 mm; S205 final rolling temperature control: The final rolling temperature of roughing is controlled at 950-1000℃. By adjusting the rolling rhythm and pass interval, the final rolling temperature is avoided from being too high, which would cause abnormal growth of austenite grains, or too low, which would affect the plasticity of finishing rolling. S3 finishing rolling process: Process objective: Precise control of finished product cross-sectional dimensions, refinement of austenite grains, optimization of microstructure, and ensuring uniformity of mechanical properties of steel are achieved. At the same time, strict control of decarburization layer depth and surface quality is maintained to meet subsequent processing requirements. Process parameters and key operating points: S301 Initial Rolling and Pass Control: The initial rolling temperature of the finishing mill is controlled at 900-950℃, connecting with the final rolling temperature of the roughing mill to avoid excessive temperature fluctuations; the reduction per pass is 8%-15%, and the reduction in the last two passes is reduced to 5%-8%, with a focus on ensuring dimensional accuracy; the number of finishing mill passes is adjusted according to the finished product specifications (generally 5-8 passes). S302 Dimensional Accuracy Control: The finished product dimensions are monitored in real time using an online diameter gauge, and the roll gap is dynamically adjusted. The diameter tolerance of the finished product is controlled within +0.10-+0.60mm, and the out-of-roundness is ≤0.50mm. The wear of the rolls is checked regularly, and the rolls are replaced or repaired in time when the wear exceeds 0.3mm. S303 rolling speed and cooling: The finishing rolling speed is controlled at 5.0-7.0 m / s, optimized according to the finished product specifications (higher speed for small specifications, lower speed for large specifications); the rolls are cooled by circulating water, and the cooling water temperature is controlled at 30-45℃ to avoid the impact of roll thermal expansion on dimensional accuracy; S304 Finishing Rolling Temperature and Decarburization Control: The finishing rolling temperature is strictly controlled at 820-880℃, which is achieved by adjusting the rolling speed and pass interval; shortening the residence time of the rolled piece in the high-temperature zone reduces surface decarburization, while controlling the residual oxygen content of the furnace gas, and controlling the decarburized layer depth to ≤0.6% of the finished product diameter; S305 Post-rolling surface inspection: After finishing rolling, an online surface inspection device is installed to check for surface cracks, folds, scratches and other defects. Defects are marked in time and subsequently treated by grinding. S4 controlled cooling process: Process objective: By controlling the microstructure transformation of steel through segmented cooling, a target microstructure of pearlite + a small amount of ferrite (≤10%) is obtained, avoiding the formation of harmful bainite microstructure, ensuring the matching of steel strength, toughness and fracture performance, and reducing internal stress. Process parameters and key operating points: S401 First-stage water cooling (rapid cooling): Water cooling section length ≥ 8m, inlet temperature 800-850℃, cooling rate 5-15℃ / s, cooling time 10-20s; Adjust the cooling intensity according to the finished product specifications (higher cooling rate for smaller specifications, lower cooling rate for larger specifications), by controlling the water flow and the number of nozzles opened; Ensure uniform cooling of the rolled piece during water cooling process to avoid localized excessively rapid cooling that could cause cracks. S402 Second Stage Air Cooling (Uniform Cooling): The air cooling line is ≥25m long, with an inlet temperature of 600-650℃ and a cooling rate of 2-8℃ / s. Forced convection air cooling is adopted, and the wind speed is controlled at 5-12m / s. The air cooling line is equipped with a zoned air control device to adjust the wind speed in each zone according to the temperature distribution of the rolled piece, ensuring that the cross-sectional cooling temperature difference is ≤30℃. S403 Final Cooling Temperature and Insulation: The final cooling temperature is strictly controlled between 500-550℃ and monitored in real time by an infrared thermometer. When the temperature is below 500℃, the air cooling intensity is reduced, and when it is above 550℃, the air speed is increased. After final cooling, an insulation section (length ≥10m) is set up to allow the temperature to drop naturally to below 400℃, further eliminating internal stress. S404 Cooling Equipment Maintenance: Regularly clean water-cooled nozzles and air-cooled pipes to prevent blockages that could lead to uneven cooling; check the cooling water quality to ensure that the suspended solids content is ≤50mg / L to prevent scale buildup in the nozzles from affecting the cooling effect. S5 cooling post-processing steps: Process objective: Completely eliminate internal stress, remove surface defects and decarburized layer, and ensure the internal and surface quality of steel through rigorous flaw detection to meet delivery requirements; Process parameters and key operating points: S501 Slow Cooling Treatment: After final cooling, the steel is placed in a slow cooling pit or slow cooling stack. The slow cooling temperature is ≤300℃, and the slow cooling time is 12-24h, which is adjusted according to the steel specifications (the slow cooling time for large specifications is extended to 24-36h). During the slow cooling process, avoid rain or rapid cooling of the steel to prevent secondary stress. S502 peeling process: The peeling process is carried out using a centerless lathe, with a peeling allowance of 0.3-0.8mm to ensure complete removal of the surface decarburized layer, iron oxide scale and micro cracks; after peeling, the surface roughness of the steel is controlled at Ra≤3.2μm, and the diameter tolerance is adjusted to +0.05-+0.30mm; S503 Flaw Detection: Ultrasonic and magnetic particle combined flaw detection is adopted, and the Class A flaw detection standard in GB / T4162 is implemented; ultrasonic flaw detection is used to detect internal defects (shrinkage cavities, inclusions, cracks), and magnetic particle flaw detection is used to detect surface and near-surface defects (microcracks, folds). Products that fail the flaw detection must be peeled off again or rejected. S6 Finished Product Inspection: Sampling inspections are conducted on steel that has passed flaw detection, including: S601 metallographic structure: pearlite + ferrite (ferrite content ≤10%), grain size 6.0-9.0 grade; S602 mechanical properties: Brinell hardness 240-265HBW, tensile strength ≥750MPa, yield strength ≥450MPa; S603 chemical composition: carbon segregation ≤ ±15%, non-metallic inclusions (according to ASTM E45 standard) Class A ≤ 3.5, Class D ≤ 1.0; S604 gas content: nitrogen content 120-200ppm, oxygen content ≤15ppm; S605 Identification and Storage: Qualified products should be identified by specifications and batch number and stored in a dry and ventilated warehouse to prevent moisture and rust; products from different batches should be stacked separately to prevent confusion and ensure traceability. Furthermore, this process significantly improves the uniformity of the microstructure: through stepped heating, precise heat homogenization control, and reasonable reduction distribution, the billet is austenitized uniformly, the casting microstructure is completely broken up, the austenite grain size is controlled at 6.0-9.0 grade, effectively avoiding coarse grains or mixed grain phenomena, the target microstructure of pearlite + ferrite (≤10%) is uniformly distributed, and carbon segregation is ≤±15%, solving the problem of fluctuation in fracture performance caused by compositional segregation and microstructure inhomogeneity; Furthermore, this process optimizes surface quality and dimensional accuracy: high-pressure water descaling (24-30MPa) combined with peeling treatment (0.3-0.8mm allowance) thoroughly removes iron oxide scale and surface crack defects, achieving a surface roughness Ra≤3.2μm; real-time monitoring by an online diameter gauge and dynamic adjustment of the roll gap ensure that the finished product diameter tolerance is controlled within +0.05-+0.30mm, and the out-of-roundness is ≤0.50mm, eliminating the problem of incomplete filling of forgings caused by undersized dimensions and significantly improving product adaptability; Furthermore, this process achieves satisfactory and stable expansion fracture performance: the segmented controlled cooling process precisely regulates the cooling rate, avoids bainite formation, ensures the matching of steel strength (tensile strength ≥750MPa), toughness and hardness (240-265HBW), stabilizes nitrogen content at 120-200ppm, decarburized layer depth ≤0.6% of finished product diameter, and minimizes plastic deformation of the fracture surface and controls slag shedding rate within 2% during expansion fracture, meeting the requirements of the connecting rod expansion fracture process for brittle fracture of materials; Furthermore, this process achieves complete elimination of internal stress and improves quality reliability: the slow cooling treatment (12-36h) and the heat preservation section design after cooling effectively release the internal stress generated during rolling and cooling, reducing the risk of secondary cracks; ultrasonic + magnetic particle combined flaw detection (GB / T4162 Class A standard) comprehensively investigates internal and surface defects, and non-metallic inclusions (Class A ≤ 3.5, Class D ≤ 1.0) are effectively controlled, significantly improving the product qualification rate; Furthermore, this process achieves a balance between production efficiency and economy: the process parameters are scientifically adapted to different product specifications, the rolling process is stable and controllable, reducing scrap and rework caused by defects; the optimized process does not require additional heat treatment, reducing production costs, while the material properties are stable, meeting the long-term supply requirements of automotive OEMs for parts materials and enhancing market competitiveness.

[0021] 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 controlled rolling and controlled cooling process for non-heated steel C70S6, comprising an S1 heating process, an S2 rough rolling process, an S3 finishing rolling process, an S4 controlled cooling process, an S5 post-cooling treatment process, and an S6 finished product inspection process, characterized in that: The S1 heating process includes S101 billet pretreatment, S102 furnace loading and heating, S103 heat soaking control and S104 temperature monitoring and furnace unloading. The S2 roughing process includes S201 initial rolling conditions, S202 descaling operation, S203 reduction distribution, S204 rolling speed and equipment adjustment, and S205 final rolling temperature control. The S3 finishing rolling process includes S301 initial rolling and pass control, S302 dimensional accuracy control, S303 rolling speed and cooling, S304 final rolling temperature and decarburization control, and S305 post-rolling surface inspection. Among them, the S4 cooling process includes S401 first stage water cooling (rapid cooling), S402 second stage air cooling (uniform cooling), S403 final cooling temperature and insulation, and S404 cooling equipment maintenance. Among them, the S5 cooling post-processing process includes S501 slow cooling treatment, S502 peeling treatment and S503 flaw detection. Among them, S6 finished product inspection includes S601 metallographic structure, S602 mechanical properties, S603 chemical composition, S604 gas content and S605 identification and storage.

2. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The pretreatment of S101 steel billet: Before loading into the furnace, the surface of the steel billet is thoroughly inspected to remove iron oxide scale, cracks, inclusions and defects. If the defect depth exceeds 0.5mm, it needs to be ground in advance to avoid the defects from expanding after heating. The end face of the steel billet must be flat and without obvious tilt to ensure uniform heating. S102 Furnace Loading and Heating: The loading temperature is ≤400℃, and a stepped heating mode is adopted to avoid thermal stress cracks in the billet due to excessive temperature difference; the heating rate is controlled at 120-160℃ / h, the low temperature section (200-400℃) is held for 60-90min to eliminate residual stress inside the billet; the medium temperature section (400-800℃) is held for 90-120min to gradually increase the core temperature of the billet; the high temperature section (800-1210℃) is held for 120-180min to promote composition homogenization. S103 heat soaking control: The temperature of the heat soaking section is stabilized at 1150-1210℃, and the holding time is 240-400min, which is adjusted according to the cross-sectional size of the billet (the larger the cross-section, the longer the holding time); the residual oxygen content of the furnace gas is controlled at 0.3%-4.0%, and a reducing or weak oxidizing atmosphere is maintained by adjusting the air-fuel ratio to reduce decarburization on the surface of the billet; S104 Temperature Monitoring and Unloading: Multi-point thermometers are used to monitor the surface and core temperature of the billet in real time, and the temperature difference is controlled within ≤30℃; the unloading temperature is strictly controlled at 1040-1100℃, and the heating rate is reduced 30 minutes before unloading to ensure uniform billet temperature and avoid local overheating that leads to coarse grains.

3. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The S201 initial rolling conditions are as follows: the initial rolling temperature is matched with the furnace exit temperature and controlled at 1040-1100℃. When the billet temperature is lower than 1020℃, it needs to be returned to the heating furnace to reheat, in order to avoid excessive rolling force or cracks caused by low-temperature rough rolling. S202 Descaling operation: set 2-3 times high pressure water descaling, descaling pressure 24-30 MPa, water flow ≥150 m 3 / h, to ensure complete removal of secondary oxide scale generated during heating; the descaling nozzle needs to be checked and cleaned regularly to ensure accurate spray angle and avoid surface defects caused by residual oxide scale; S203 Reduction Distribution: The total reduction is controlled at 60%-70%, and the reduction per pass is distributed as follows: 15%-25% for the first 2-3 passes (large reduction to break the casting structure), and gradually reduced to 10%-15% for subsequent passes to avoid uneven deformation of the billet due to excessive reduction in a single pass; a 5-8s interval is set between adjacent passes to ensure stable temperature of the rolled piece.

4. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The S204 rolling speed and equipment adjustment: the rolling speed is gradually increased to 3.0-5.0 m / s, and the speed is matched according to the changes in the cross-section of the rolled piece to avoid steel piling or pulling; the mill rigidity is monitored in real time, and the roll gap is adjusted according to the changes in rolling force to ensure that the dimensional tolerance of the intermediate billet is controlled within ±2.0 mm; S205 final rolling temperature control: The final rolling temperature of roughing is controlled at 950-1000℃. By adjusting the rolling rhythm and pass interval, the temperature of the final rolling is kept away from excessively high temperature which may cause abnormal growth of austenite grains, or too low temperature which may affect the plasticity of finishing rolling.

5. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The S301 rolling process and pass control are as follows: the finishing rolling temperature is controlled at 900-950℃, connecting with the roughing rolling temperature to avoid excessive temperature fluctuations; the pass reduction is allocated as 8%-15%, and the reduction in the last two passes is reduced to 5%-8%, focusing on ensuring dimensional accuracy; the number of finishing rolling passes is adjusted according to the finished product specifications (generally 5-8 passes). S302 Dimensional Accuracy Control: The finished product dimensions are monitored in real time using an online diameter gauge, and the roll gap is dynamically adjusted. The diameter tolerance of the finished product is controlled within +0.10-+0.60mm, and the out-of-roundness is ≤0.50mm. The wear of the rolls is checked regularly, and the rolls are replaced or repaired in time when the wear exceeds 0.3mm. S303 rolling speed and cooling: The finishing rolling speed is controlled at 5.0-7.0 m / s, optimized according to the finished product specifications (higher speed for small specifications, lower speed for large specifications); the rolls are cooled by circulating water, and the cooling water temperature is controlled at 30-45℃ to avoid the impact of roll thermal expansion on dimensional accuracy; S304 Finishing Rolling Temperature and Decarburization Control: The finishing rolling temperature is strictly controlled at 820-880℃, which is achieved by adjusting the rolling speed and pass interval; shortening the residence time of the rolled piece in the high-temperature zone reduces surface decarburization, while controlling the residual oxygen content of the furnace gas, and controlling the decarburized layer depth to ≤0.6% of the finished product diameter.

6. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The surface inspection of S305 after rolling: After finishing rolling, an online surface inspection device is set up to check for surface cracks, folds, scratches and other defects. If defects are found, they are marked in time and then treated by grinding.

7. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The first stage of S401 water cooling (rapid cooling): water cooling section length ≥ 8m, inlet temperature 800-850℃, cooling rate 5-15℃ / s, cooling time 10-20s; the cooling intensity is adjusted according to the finished product specifications (higher cooling rate for small specifications, lower cooling rate for large specifications), which is achieved by controlling the water flow and the number of nozzles opened; during the water cooling process, the rolled piece is ensured to be cooled evenly to avoid localized rapid cooling that could cause cracks. S402 Second Stage Air Cooling (Uniform Cooling): The air cooling line is ≥25m long, with an inlet temperature of 600-650℃ and a cooling rate of 2-8℃ / s. Forced convection air cooling is adopted, and the wind speed is controlled at 5-12m / s. The air cooling line is equipped with a zoned air control device to adjust the wind speed in each zone according to the temperature distribution of the rolled piece, ensuring that the cross-sectional cooling temperature difference is ≤30℃. S403 Final Cooling Temperature and Insulation: The final cooling temperature is strictly controlled between 500-550℃ and monitored in real time by an infrared thermometer. When the temperature is below 500℃, the air cooling intensity is reduced, and when it is above 550℃, the air speed is increased. After final cooling, an insulation section (length ≥10m) is set up to allow the temperature to drop naturally to below 400℃, further eliminating internal stress.

8. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: Maintenance of the S404 cooling equipment: Regularly clean the water-cooled nozzles and air-cooled pipes to avoid blockages that could lead to uneven cooling; check the cooling water quality to ensure that the suspended solids content is ≤50mg / L to prevent scale buildup in the nozzles from affecting the cooling effect.

9. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The S501 slow cooling treatment: After final cooling, the steel is sent to a slow cooling pit or slow cooling stack. The slow cooling temperature is ≤300℃ and the slow cooling time is 12-24h, which is adjusted according to the steel specifications (the slow cooling time for large specifications is extended to 24-36h). During the slow cooling process, the steel should be protected from rain or rapid cooling to prevent secondary stress. S502 peeling treatment: The peeling process is carried out using a centerless lathe, with a peeling allowance of 0.3-0.8mm to ensure complete removal of the surface decarburized layer, iron oxide scale and micro cracks; after peeling, the surface roughness of the steel is controlled at Ra≤3.2μm, and the diameter tolerance is adjusted to +0.05-+0.30mm; S503 Flaw Detection: Ultrasonic and magnetic particle combined flaw detection is adopted, and the Class A flaw detection standard in GB / T4162 is implemented. Ultrasonic flaw detection is used to detect internal defects (shrinkage cavities, inclusions, cracks), and magnetic particle flaw detection is used to detect surface and near-surface defects (microcracks, folds). Products that fail the flaw detection must be peeled again or rejected.

10. The controlled rolling and controlled cooling process for non-cooled steel C70S6 according to claim 1, characterized in that: The S6 finished product inspection: Sampling inspections are conducted on steel that has passed flaw detection, including: S601 metallographic structure: pearlite + ferrite (ferrite content ≤10%), grain size 6.0-9.0 grade; S602 mechanical properties: Brinell hardness 240-265HBW, tensile strength ≥750MPa, yield strength ≥450MPa; S603 chemical composition: carbon segregation ≤ ±15%, non-metallic inclusions (according to ASTM E45 standard) Class A ≤ 3.5, Class D ≤ 1.0; S604 gas content: nitrogen content 120-200ppm, oxygen content ≤15ppm; S605 Identification and Storage: Qualified products should be identified by specifications and batch number and stored in a dry and ventilated warehouse to prevent moisture and rust. Products from different batches should be stacked separately to prevent confusion and ensure traceability.