Process for preventing cracking of a bainite non-quenched and tempered steel forging by segmented controlled cooling

Abstract

The application relates to the technical field of steel material processing, and discloses a bainite non-quenched and tempered steel forging segmented controlled cooling anti-cracking process, which comprises an S1 post-forging initial cooling stage, an S2 segmented controlled cooling core stage, an S3 holding and slow cooling stage, an S4 subsequent stress release treatment and S5 key process control points. The process realizes that the cracking risk is significantly reduced: through multi-stage precise controlled cooling, stress concentration caused by post-forging rapid cooling and rapid heating is avoided, combined with subsequent low-temperature stress release treatment and 100% magnetic powder flaw detection, the residual stress of the forging is less than or equal to 250 MPa, the surface and internal crack occurrence rate is greatly reduced, and the industry pain point that the bainite non-quenched and tempered steel is prone to cracking after forging is effectively solved. The process realizes that the structure and performance matching are optimized: the segmented controlled cooling precisely matches the requirements of each stage of bainite transformation, the generation of harmful martensite and widmanstatten structures is inhibited, and the forging is ensured to be mainly granular bainite.

Description

Technical Field

[0001] This invention relates to the field of steel material processing technology, specifically to a segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings. Background Technology

[0002] Bainitic non-quenched and tempered steels are widely used in critical safety components such as automotive front axles because they can achieve energy conservation, emission reduction, and lower production costs without the need for quenching and tempering. However, this type of steel is prone to cracking when heated, and the post-forging cooling process is the core link in quality control. Traditional post-forging cooling often uses single-rate air cooling or natural cooling, lacking precise segmented cooling design: too fast a cooling rate can easily lead to excessive temperature difference between the inside and outside of the forging, resulting in the superposition of thermal stress and structural stress, causing surface and internal cracks, and may also generate harmful structures such as martensite and Widmanstätten; too slow a cooling rate will cause coarsening of the bainitic structure, uneven distribution of precipitates, reduced strength and toughness of the forging, and failure to effectively release residual stress, making it prone to cracking failure during subsequent machining or use, and failing to meet the stringent safety and reliability requirements of the automotive industry. Summary of the Invention

[0003] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a segmented controlled cooling process for preventing cracking in bainitic non-quenched and tempered steel forgings. This process offers advantages such as significantly reduced cracking risk and solves the problems of surface and internal cracks.

[0004] (II) Technical Solution To achieve the goal of significantly reducing the risk of cracking, this invention provides the following technical solution: a segmented controlled cooling anti-cracking process for bainitic non-quenched and tempered steel forgings, comprising: S1 initial cooling stage after forging, S2 segmented controlled cooling core stage, S3 heat preservation and slow cooling stage, S4 subsequent stress release treatment, and S5 key process control points. The S1 initial cooling stage after forging includes: After the final forging, the forging is immediately sent to a controlled cooling device. The initial temperature is controlled at 980-1280℃. The forced air cooling system is turned on and the cooling rate is controlled at 0.6-1.0℃ / s to quickly reduce the temperature of the forging to 750-780℃. This stage lasts for 5-8 minutes. Key controls: Multiple sets of fans are arranged in zones to ensure uniform cooling of the forging surface and avoid excessive local temperature differences that could generate thermal stress; at the same time, an online temperature monitoring system is activated to monitor temperature changes in real time, with a deviation not exceeding ±15℃. The core stage of the S2 segmented controlled cooling includes the first controlled cooling stage S201 (early stage of bainite transformation), the second controlled cooling stage S202 (dominant stage of bainite transformation), and the third controlled cooling stage S203 (residual stress release stage). Among them, the key process control points of S5 include S501 precise control of cooling rate, S502 microstructure control and S503 stress control.

[0005] Preferably, the first controlled cooling stage of S201 is the early stage of bainite transformation. Temperature range: 780-650℃, cooling rate adjusted to 1.0-1.8℃ / s, lasting 8-12 minutes, to promote bainite grain nucleation and inhibit austenite grain growth; Equipment control: Adjust the fan power to 60%-80% to maintain stable airflow and avoid cold air blowing directly on the edges and corners of the forgings. The airflow direction can be optimized by using a guide vane.

[0006] Preferably, the second controlled cooling stage of S202 (bainite transformation dominant period): Temperature range: 650-580℃, cooling rate slowed to 0.5-0.9℃ / s, lasting 12-18 minutes, to provide sufficient time for bainite transformation and avoid the formation of harmful martensite; Auxiliary measures: For parts of the forging with a thickness ≥ 80 mm, add local insulation sleeves to reduce the temperature difference in the thickness direction and control the difference in cooling rate between the inside and outside to not exceed 0.3℃ / s.

[0007] Preferably, the third controlled cooling stage (residual stress release period) of S203: Temperature range: 580-450℃, cooling rate further reduced to 0.3-0.6℃ / s, lasting 15-20 minutes, gradually releasing internal thermal stress and structural stress in the forging; Monitoring requirements: Record the temperature of different parts of the forging every 3 minutes to ensure that the temperature deviation of the same cross section is ≤30℃.

[0008] Preferably, the S3 heat preservation and slow cooling stage: When the temperature of the forging drops to 450-480℃, it is transferred to a heat preservation device (iron box or slow cooling pit), and the heat preservation temperature is controlled at 450-280℃ for 24-36 hours. It is then allowed to cool naturally to room temperature with the device. Insulation medium: Quartz sand or insulating cotton is used to fill the gaps between the forgings, with a filling density ≥1.2g / cm³. 3 This ensures uniform heat preservation and avoids excessively rapid cooling in certain areas.

[0009] Preferably, the subsequent stress relief treatment in step S4 is as follows: After heat preservation and slow cooling, the forgings are subjected to low-temperature stress relief treatment: heating temperature 300-350℃, holding temperature for 2-4 hours, and then cooling to room temperature in the furnace to further eliminate residual stress; Surface treatment: After stress relief, shot blasting is performed to remove surface oxide scale. The shot blasting strength is controlled at 0.2-0.3 MPa. Then, 100% magnetic particle testing is performed to check for surface and near-surface microcracks.

[0010] Preferably, the S501 cooling rate is precisely controlled: the entire process is monitored in real time by an infrared thermometer, and the cooling rate is adjusted according to the size of the forging (φ50-160mm). The lower limit of the cooling rate is used for large cross-section forgings, and the upper limit is used for small cross-section forgings.

[0011] Preferably, the S502 microstructure control involves segmented controlled cooling to ensure that the microstructure of the forging is mainly granular bainite, with a ferrite content of ≤7%, no harmful martensite or Widmanstätten structures, and a grain size of ≥5.

[0012] Preferably, the S503 stress control ensures a smooth temperature transition at each stage, avoiding sudden cooling and heating, resulting in a final residual stress of ≤250MPa and a surface decarburization layer of ≤0.5mm.

[0013] (III) Beneficial Effects Compared with the prior art, the present invention provides a segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings, which has the following beneficial effects: 1. The segmented controlled cooling anti-cracking process for bainitic non-quenched and tempered steel forgings has achieved a significant reduction in cracking risk: through multi-stage precise controlled cooling, stress concentration caused by sudden cooling and heating after forging is avoided. Combined with subsequent low-temperature stress relief treatment and 100% magnetic particle testing, the residual stress of the forgings is ≤250MPa, and the incidence of surface and internal cracks is greatly reduced, effectively solving the industry pain point of easy cracking of bainitic non-quenched and tempered steel after forging.

[0014] 2. The segmented controlled cooling anti-cracking process for this bainitic non-quenched and tempered steel forging achieves optimized microstructure and performance matching: segmented controlled cooling precisely matches the requirements of each stage of bainitic transformation, inhibits the formation of harmful martensite and Widmanstätten structures, ensures that the forging is mainly composed of granular bainite, with ferrite content ≤7%, grain size ≥5, and surface decarburization layer ≤0.5mm. The tensile strength and impact mechanical properties are stable and meet the standards, with good strength and toughness matching.

[0015] 3. The segmented controlled cooling anti-cracking process for this bainitic non-quenched and tempered steel forging achieves stable dimensions and surface quality: the heat preservation and slow cooling stage uses a uniformly filled heat preservation medium to avoid dimensional deformation caused by excessive local cooling. Combined with shot blasting to remove oxide scale, the dimensional accuracy (φ50-160mm specification) and surface finish of the forging meet the assembly requirements. No additional complex grinding process is required, and the production efficiency is increased by more than 10%.

[0016] 4. The segmented controlled cooling anti-cracking process for bainitic non-quenched and tempered steel forgings achieves the following: This process is suitable for green production requirements. The process does not require subsequent quenching and tempering treatment. While ensuring the anti-cracking effect and product performance, it further consolidates the energy-saving and emission-reduction advantages of non-quenched and tempered steel, reduces production energy consumption and costs, and is suitable for the automotive industry's demand for the promotion and application of green and environmentally friendly steel. Detailed Implementation

[0017] 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.

[0018] This solution provides a technical approach, specifically a segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings, comprising the following stages: S1 initial cooling stage after forging: After the final forging, the forging is immediately sent to a controlled cooling device. The initial temperature is controlled at 980-1280℃. The forced air cooling system is turned on and the cooling rate is controlled at 0.6-1.0℃ / s to quickly reduce the temperature of the forging to 750-780℃. This stage lasts for 5-8 minutes. Key controls: Multiple sets of fans are arranged in zones to ensure uniform cooling of the forging surface and avoid excessive local temperature differences that could generate thermal stress; at the same time, an online temperature monitoring system is activated to monitor temperature changes in real time, with a deviation not exceeding ±15℃. S2 segmented cooling core stage: S201 First controlled cooling stage (early stage of bainite transformation); Temperature range: 780-650℃, cooling rate adjusted to 1.0-1.8℃ / s, lasting 8-12 minutes, to promote bainite grain nucleation and inhibit austenite grain growth; Equipment control: Adjust the fan power to 60%-80% to maintain stable airflow and avoid cold air blowing directly on the edges and corners of the forgings. The airflow direction can be optimized by using a deflector. S202 Second Controlled Cooling Stage (Bainite Transformation Dominant Period): Temperature range: 650-580℃, cooling rate slowed to 0.5-0.9℃ / s, lasting 12-18 minutes, to provide sufficient time for bainite transformation and avoid the formation of harmful martensite; Auxiliary measures: For parts of the forging with a thickness ≥ 80 mm, add local insulation sleeves to reduce the temperature difference in the thickness direction and control the difference in cooling rate between the inside and outside to not exceed 0.3℃ / s; S203 Third Controlled Cooling Section (Residual Stress Release Period): Temperature range: 580-450℃, cooling rate further reduced to 0.3-0.6℃ / s, lasting 15-20 minutes, gradually releasing internal thermal stress and structural stress in the forging; Monitoring requirements: Record the temperature of different parts of the forging every 3 minutes to ensure that the temperature deviation of the same cross section is ≤30℃; S3 Insulation and Slow Cooling Stage: When the temperature of the forging drops to 450-480℃, it is transferred to a heat preservation device (iron box or slow cooling pit), and the heat preservation temperature is controlled at 450-280℃ for 24-36 hours. It is then allowed to cool naturally to room temperature with the device. Insulation medium: Quartz sand or insulating cotton is used to fill the gaps between the forgings, with a filling density ≥1.2g / cm³. 3 This ensures uniform heat preservation and prevents localized rapid cooling. S4 subsequent stress relief treatment: After heat preservation and slow cooling, the forgings are subjected to low-temperature stress relief treatment: heating temperature 300-350℃, holding temperature for 2-4 hours, and then cooling to room temperature in the furnace to further eliminate residual stress; Surface treatment: After stress relief, shot blasting is performed to remove surface oxide scale. The shot blasting strength is controlled at 0.2-0.3 MPa. Then, 100% magnetic particle testing is performed to check for surface and near-surface microcracks. Key points for S5 process control: S501 Precise Cooling Rate Control: The entire process is monitored in real time by an infrared thermometer. The cooling rate is adjusted according to the size of the forging (φ50-160mm). The lower limit of the cooling rate is used for large cross-section forgings, and the upper limit is used for small cross-section forgings. S502 microstructure control: Through segmented controlled cooling, the microstructure of the forging is ensured to be mainly granular bainite, with ferrite content ≤7%, no harmful martensite or Widmanstätten structures, and grain size ≥5. S503 stress control: The temperature transition at each stage is stable, avoiding sudden cooling and heating, and the final residual stress of the forging is ≤250MPa, and the surface decarburization layer is ≤0.5mm; Furthermore, this process significantly reduces the risk of cracking: through multi-stage precise cooling, stress concentration caused by sudden cooling and heating after forging is avoided. Combined with subsequent low-temperature stress relief treatment and 100% magnetic particle testing, the residual stress of the forging is ≤250MPa, and the incidence of surface and internal cracks is greatly reduced, effectively solving the industry pain point of easy cracking of bainitic non-quenched and tempered steel after forging. Furthermore, this process achieves optimized matching of microstructure and performance: segmented controlled cooling precisely matches the requirements of each stage of bainitic transformation, inhibits the formation of harmful structures such as martensite and Widmanstätten, ensures that the forging is mainly composed of granular bainite, with a ferrite content ≤7% and a grain size ≥5, while the surface decarburization layer is ≤0.5mm, and the tensile strength and impact mechanical properties are stable and meet the standards, with good matching of strength and toughness. Furthermore, this process achieves stable dimensions and surface quality: the heat preservation and slow cooling stage uses a uniformly filled heat preservation medium to avoid dimensional deformation caused by excessively rapid local cooling. Combined with shot blasting to remove oxide scale, the dimensional accuracy (φ50-160mm specification) and surface finish of the forgings meet assembly requirements, eliminating the need for additional complex grinding processes and increasing production efficiency by more than 10%. Furthermore, this process achieves alignment with green production requirements: the process eliminates the need for subsequent quenching and tempering treatment, and while ensuring crack prevention and product performance, it further consolidates the energy-saving and emission-reduction advantages of non-quenched and tempered steel, reduces production energy consumption and costs, and aligns with the automotive industry's demand for the promotion and application of green and environmentally friendly steel.

[0019] 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 segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings, comprising S1 initial cooling stage after forging, S2 segmented controlled cooling core stage, S3 heat preservation and slow cooling stage, S4 subsequent stress release treatment, and S5 key process control points, characterized in that: The initial cooling stage after S1 forging: After the final forging, the forging is immediately sent to a controlled cooling device. The initial temperature is controlled at 980-1280℃. The forced air cooling system is turned on and the cooling rate is controlled at 0.6-1.0℃ / s to quickly reduce the temperature of the forging to 750-780℃. This stage lasts for 5-8 minutes. Key controls: Multiple sets of fans are arranged in zones to ensure uniform cooling of the forging surface and avoid excessive local temperature differences that could generate thermal stress; at the same time, an online temperature monitoring system is activated to monitor temperature changes in real time, with a deviation not exceeding ±15℃. The core stage of the S2 segmented controlled cooling includes the first controlled cooling stage S201 (early stage of bainite transformation), the second controlled cooling stage S202 (dominant stage of bainite transformation), and the third controlled cooling stage S203 (residual stress release stage). Among them, the key process control points of S5 include S501 precise control of cooling rate, S502 microstructure control and S503 stress control.

2. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The first controlled cooling section of S201 (early stage of bainite transformation); Temperature range: 780-650℃, cooling rate adjusted to 1.0-1.8℃ / s, lasting 8-12 minutes, to promote bainite grain nucleation and inhibit austenite grain growth; Equipment control: Adjust the fan power to 60%-80% to maintain stable airflow and avoid cold air blowing directly on the edges and corners of the forgings. The airflow direction can be optimized by using a guide vane.

3. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The second controlled cooling section of S202 (bainite transformation dominant period): Temperature range: 650-580℃, cooling rate slowed to 0.5-0.9℃ / s, lasting 12-18 minutes, to provide sufficient time for bainite transformation and avoid the formation of harmful martensite; Auxiliary measures: For parts of the forging with a thickness ≥ 80 mm, add local insulation sleeves to reduce the temperature difference in the thickness direction and control the difference in cooling rate between the inside and outside to not exceed 0.3℃ / s.

4. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The third controlled cooling section of S203 (residual stress release period): Temperature range: 580-450℃, cooling rate further reduced to 0.3-0.6℃ / s, lasting 15-20 minutes, gradually releasing internal thermal stress and structural stress in the forging; Monitoring requirements: Record the temperature of different parts of the forging every 3 minutes to ensure that the temperature deviation of the same cross section is ≤30℃.

5. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The S3 heat preservation and slow cooling stage: When the temperature of the forging drops to 450-480℃, it is transferred to a heat preservation device (iron box or slow cooling pit), and the heat preservation temperature is controlled at 450-280℃ for 24-36 hours. It is then allowed to cool naturally to room temperature with the device. Insulation medium: Quartz sand or insulating cotton is used to fill the gaps between the forgings, with a filling density ≥1.2g / cm³. 3 This ensures uniform heat preservation and avoids excessively rapid cooling in certain areas.

6. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The subsequent stress relief process in S4: After heat preservation and slow cooling, the forgings are subjected to low-temperature stress relief treatment: heating temperature 300-350℃, holding temperature for 2-4 hours, and then cooling to room temperature in the furnace to further eliminate residual stress; Surface treatment: After stress relief, shot blasting is performed to remove surface oxide scale. The shot blasting strength is controlled at 0.2-0.3 MPa. Then, 100% magnetic particle testing is performed to check for surface and near-surface microcracks.

7. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The S501 cooling rate is precisely controlled: the entire process is monitored in real time by an infrared thermometer, and the cooling rate is adjusted according to the size of the forging (φ50-160mm). The lower limit of the cooling rate is used for large cross-section forgings, and the upper limit is used for small cross-section forgings.

8. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The S502 microstructure control: through segmented controlled cooling, the microstructure of the forging is ensured to be mainly granular bainite, with ferrite content ≤7%, no harmful martensite or Widmanstätten structures, and grain size ≥5.

9. The segmented controlled cooling and crack prevention process for bainitic non-quenched and tempered steel forgings according to claim 1, characterized in that: The S503 stress control ensures a smooth temperature transition at each stage, avoiding sudden cooling and heating, resulting in a final residual stress of ≤250MPa and a surface decarburization layer of ≤0.5mm.