A composition optimization process to improve the cold heading performance of ML40Cr steel

Abstract

This invention relates to the interdisciplinary field of metal material smelting and pressure processing, and discloses a composition optimization process to improve the cold heading performance of ML40Cr steel. The process includes S1 composition design optimization, S2 raw material pretreatment process, S3 smelting process optimization, S4 continuous casting process synergistic optimization, S5 billet finishing and inspection, S6 rolling and heat treatment synergistic optimization, and S7 finished product composition and performance verification. This process achieves precise and controllable composition and optimized balance between plasticity and strength: through precise control of core elements (C: 0.38-0.43%, Si: 0.15-0.25%) and strict limitation of harmful elements (P≤0.012%, S≤0.004%), combined with Al element to refine grains, the steel composition qualification rate is ≥99.5%, effectively balancing strength and cold heading plasticity. The cold heading deformation is increased to ≥65% without cracking, completely solving the problem of frequent cold heading cracks in traditional processes.

Description

Technical Field

[0001] This invention relates to the interdisciplinary field of metal material smelting and pressure processing, specifically to a composition optimization process for improving the cold heading performance of ML40Cr steel. Background Technology

[0002] ML40Cr steel, as a core raw material for high-end fasteners, is widely used in key fields such as automobiles and machinery. Its cold heading performance directly determines the forming qualification rate and service life of the end products. However, there are many technical bottlenecks in the existing production process, which seriously restrict the stability of its cold heading process: On the one hand, the precision of composition control is insufficient, the fluctuation range of core elements (C, Si, Mn, Cr) is large, and the control of harmful elements (P, S, O, N, H) is not strict, resulting in an imbalance between the strength and plasticity of the steel, which is prone to cracking due to stress concentration during cold heading; on the other hand, the raw material pretreatment process is imperfect, the desulfurization of molten iron is incomplete, and the impurity content of scrap steel is high, resulting in insufficient purity of molten steel and excessive inclusion levels, which further aggravates the risk of cold heading cracks.

[0003] In the smelting and continuous casting stages, traditional processes lack precise methods for fine-tuning the composition and removing gases, resulting in poor uniformity of the molten steel composition. Furthermore, unreasonable control of crystallizer parameters, cooling rate, and protective slag during continuous casting leads to defects such as pinholes, nodules, surface cracks, shrinkage cavities, and porosity in the cast billets. These defects will continue to expand during subsequent rolling and cold heading processes.

[0004] Furthermore, inadequate billet finishing and poor compatibility of rolling heating and controlled cooling process parameters result in uneven carbide distribution and coarse grains in the finished product microstructure, making it difficult to meet the required level of spherical carbides. Ultimately, this leads to insufficient cold heading deformation capacity of the steel (prone to cracking when deformation is below 60%), failing to meet the stringent requirements of high-end fasteners for material processing stability and performance consistency. Meanwhile, competitors in the industry have achieved narrow composition control, high-purity production, and synergistic optimization of the entire process, further highlighting the competitive shortcomings of existing processes. Summary of the Invention

[0005] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a solution with advantages such as precise and controllable composition and optimized balance between plasticity and strength. It solves the problems of traditional processes lacking precise means for fine-tuning composition and gas removal, resulting in poor uniformity of molten steel composition, and unreasonable control of crystallizer parameters, cooling rate, and protective slag during continuous casting.

[0006] (II) Technical Solution To achieve the above-mentioned precise control of composition and optimization of plasticity and strength balance, the present invention provides the following technical solution: a composition optimization process to improve the cold heading performance of ML40Cr steel, including S1 composition design optimization, S2 raw material pretreatment process, S3 smelting process optimization, S4 continuous casting process synergistic optimization, S5 billet finishing and inspection, S6 rolling and heat treatment synergistic optimization, and S7 finished product composition and performance verification, wherein the process includes S101 precise control of core elements, S102 strict restriction of harmful elements, and S103 supplementation of beneficial elements; Among them, the S2 raw material pretreatment process includes S201 molten iron pretreatment and S202 scrap steel screening; Among them, the S3 smelting process optimization includes S301 converter smelting, S302 LF refining fine-tuning and S303 VD vacuum degassing; Among them, the collaborative optimization of the S4 continuous casting process includes S401 crystallizer parameters, S402 casting speed and cooling control, and S403 protective slag control. Among them, S5 billet finishing and inspection includes S501 heat treatment, S502 surface finishing and S503 low magnification inspection. Among them, the synergistic optimization of S6 rolling and heat treatment includes the synergistic effects of S601 heating process, S602 controlled rolling and cooling and S603 spheroidizing annealing. Among them, the S7 finished product composition and performance verification includes S701 composition detection and S702 performance verification.

[0007] Preferably, the core elements of S101 are precisely controlled: the carbon (C) content is controlled at 0.38-0.43% to ensure the balance between the strength and cold heading plasticity of the steel; silicon (Si) is 0.15-0.25% to avoid excessive amounts affecting the cold heading formability; manganese (Mn) is 0.60-0.75% to refine the grains and improve toughness; and chromium (Cr) is 0.90-1.05% to enhance hardenability and wear resistance.

[0008] Preferably, the harmful elements in S102 are strictly limited: phosphorus (P) ≤ 0.012% and sulfur (S) ≤ 0.004% to reduce crack sensitivity during cold heading; oxygen (O) ≤ 8ppm, nitrogen (N) ≤ 40ppm and hydrogen (H) ≤ 0.4ppm to reduce internal defects caused by gas inclusions.

[0009] Preferably, the beneficial element supplement in S103 is aluminum (Al) 0.020-0.035%, which plays a role in refining grains and improving the plasticity and cold working properties of steel.

[0010] Preferably, the S201 molten iron pretreatment adopts the KR desulfurization process, with a desulfurization rate of ≥90%, ensuring that the sulfur content of the molten iron before entering the furnace is ≤0.003%, thus avoiding the accumulation of harmful elements; S202 scrap steel screening: Select high-quality scrap steel with low phosphorus and low sulfur content, requiring phosphorus ≤0.010% and sulfur ≤0.005%, and prevent the introduction of oil stains, rust and other impurities to ensure the purity of raw materials.

[0011] Preferably, the S301 converter smelting process involves controlling the final carbon content at 0.08-0.12%, phosphorus ≤0.010%, and tapping temperature at 1620-1650℃ to avoid uneven composition caused by over-blowing. S302 LF Refining and Fine-tuning: Utilizing computer-aided composition fine-tuning technology, ensuring C element fluctuation ≤ ±0.015% and other alloying element fluctuations ≤ ±0.025%; refining time 60-90 minutes, with composition and temperature uniformized by argon gas stirring at a flow rate of 0.3-0.6 m³ / h. 3 / h; S303 VD vacuum degassing: vacuum degree ≤67Pa, holding time 20-30 minutes, efficiently removes gas from steel, ensuring that the purity of the molten steel after degassing meets the requirement that inclusions A / B / C / D / DS are all ≤0.8.

[0012] Preferably, the parameters of the S401 crystallizer are: the crystallizer water flow rate is adjusted to 110-220 m³ / h according to the steel material specifications (Φ6-42mm). 3 / h, adopting sinusoidal vibration mode, vibration frequency 80-120 times / min; crystallizer electric stirring parameters 200-320A / 2-3.5Hz, end electric stirring 180-400A / 6-18Hz, to refine billet grains; S402 Casting Speed ​​and Cooling Control: Continuous casting speed is 1.4-2.2m / min to match the requirements of different billet specifications; the secondary cooling water supply adopts a weak cooling mode with a specific water volume of 0.35-0.50L / kg to reduce the temperature difference between the inside and outside of the billet and avoid thermal stress cracks. S403 protective slag control: Medium carbon protective slag is selected to control the total slag thickness in the crystallizer to 25-30mm and the liquid slag thickness to 6-9mm, thereby improving the surface lubrication effect of the cast billet and reducing pinhole and nodule defects.

[0013] Preferably, the S501 stacking cooling treatment involves stacking cooling time ≥ 24 hours after the billet is removed from the production line to ensure sufficient slow cooling, eliminate internal residual stress, and achieve a furnace exit temperature ≤ 300℃. S502 Surface Finishing: After shot blasting, the billet undergoes magnetic particle testing and ultrasonic testing. Surface cracks and pinhole defects are repaired by grinding to a depth of 0.3-1.2mm, ensuring that the surface scratch depth is ≤0.025mm. S503 Low-magnification inspection: 1-2 billets are randomly selected from each furnace for low-magnification inspection. The shrinkage cavity should be ≤0.5 grade, the general porosity and central porosity should be ≤0.8 grade, and there should be no white bright bands or crack defects.

[0014] Preferably, the S601 heating process involves a pre-rolling heating temperature of 1050-1100℃ and a holding time of 90-120 minutes to ensure uniform composition of the billet and avoid overheating that could lead to coarse grains. S602 controlled rolling and cooling: The final rolling temperature is 850-900℃, and a segmented cooling mode is adopted with a cooling rate of 5-15℃ / s to obtain a uniform pearlite + ferrite (P+F) structure, which lays the foundation for subsequent spheroidizing annealing. S603 Spheroidizing Annealing Synergy: Referencing the optimized spheroidizing annealing process, ensure that the spherical carbide level reaches 4-5, and the carbide particle diameter is ≤4μm, thereby improving the cold heading deformation capability.

[0015] Preferably, the S701 component testing involves batch-by-batch testing of the finished product's chemical composition to ensure that both core elements and harmful elements are within the design range, with a component qualification rate ≥99.5%. S702 performance verification: Sampling was conducted for hardness testing (160-200HB), tensile testing (yield strength 340-440MPa, tensile strength 540-640MPa, elongation after fracture ≥16%), and no cracking was observed when the deformation in the cold heading test was ≥65%, meeting the requirements for cold heading processing of high-end fasteners.

[0016] (III) Beneficial Effects Compared with the prior art, the present invention provides a composition optimization process for improving the cold heading performance of ML40Cr steel, which has the following beneficial effects: 1. This composition optimization process improves the cold heading performance of ML40Cr steel. This process achieves precise control of composition and optimized balance between plasticity and strength: through precise control of core elements (C: 0.38-0.43%, Si: 0.15-0.25%) and strict limitation of harmful elements (P≤0.012%, S≤0.004%), combined with Al element to refine grains, the steel composition qualification rate is ≥99.5%, effectively balancing strength and cold heading plasticity. The cold heading deformation is increased to ≥65% without cracking, completely solving the problem of frequent cold heading cracks in traditional processes.

[0017] 2. The composition optimization process for improving the cold heading performance of ML40Cr steel significantly improves the purity of molten steel: after KR desulfurization (desulfurization rate ≥90%), screening of high-quality scrap steel and VD vacuum degassing (vacuum degree ≤67Pa) treatment, the gas content (O≤8ppm, N≤40ppm, H≤0.4ppm) and inclusion level (A / B / C / D / DS all ≤0.8 grade) in the steel are greatly reduced, avoiding internal defects caused by gas inclusions and large particle inclusions, and improving the stress transfer uniformity during cold heading.

[0018] 3. The composition optimization process for improving the cold heading performance of ML40Cr steel achieves a comprehensive improvement in billet quality: through the synergistic optimization of the continuous casting process (molder parameter adaptation, weak cooling mode, and medium-carbon protective slag control) and billet finishing (shot blasting, double flaw detection, and grinding), the defects of pinholes, nodules, and cracks on the billet surface are reduced by more than 80%, shrinkage cavities are ≤0.5 grade, porosity is ≤0.8 grade, and there is no white bright band, providing high-quality billets for subsequent processing and reducing the risk of rolling rejection and cold heading failure.

[0019] 4. The composition optimization process for improving the cold heading performance of ML40Cr steel achieves enhanced microstructure uniformity and processing stability: the combined use of rolling heating (1050-1100℃) and controlled rolling and cooling (final rolling 850-900℃), along with spheroidizing annealing, results in a uniform P+F microstructure in the finished product, with spheroidal carbide levels stable at 4-5, particle diameter ≤4μm, hardness controlled at 160-200HB, and mechanical property dispersion reduced by 30%, ensuring consistency and stability of cold heading in mass production.

[0020] 5. The composition optimization process for improving the cold heading performance of ML40Cr steel enhances product competitiveness: the optimized finished product has a yield strength of 340-440MPa, a tensile strength of 540-640MPa, and an elongation after fracture of ≥16%. All performance indicators meet the requirements of high-end fasteners. At the same time, the optimization of the entire process reduces raw material loss and subsequent repair costs, improves the product's cost-effectiveness, and can effectively cope with industry competitive pressure. Detailed Implementation

[0021] 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.

[0022] This invention provides a technical solution, specifically a composition optimization process for improving the cold heading performance of ML40Cr steel, including the following process, S1 composition design optimization: S101 core elements are precisely controlled: carbon (C) content is controlled at 0.38-0.43% to ensure a balance between the steel's strength and cold heading plasticity; silicon (Si) is 0.15-0.25% to avoid excessive amounts affecting cold heading formability; manganese (Mn) is 0.60-0.75% to refine grains and improve toughness; chromium (Cr) is 0.90-1.05% to enhance hardenability and wear resistance. S102 strictly limits harmful elements: phosphorus (P) ≤ 0.012% and sulfur (S) ≤ 0.004% to reduce crack sensitivity during cold heading; oxygen (O) ≤ 8ppm, nitrogen (N) ≤ 40ppm and hydrogen (H) ≤ 0.4ppm to reduce internal defects caused by gas inclusions; S103 beneficial element supplement: Aluminum (Al) 0.020-0.035%, which plays a role in refining grains and improving the plasticity and cold working properties of steel; S2 raw material pretreatment process: S201 molten iron pretreatment: adopts KR desulfurization process, with a desulfurization rate of ≥90%, ensuring that the sulfur content of molten iron before entering the furnace is ≤0.003%, avoiding the accumulation of harmful elements; S202 scrap steel screening: Select high-quality scrap steel with low phosphorus and low sulfur, requiring phosphorus ≤0.010% and sulfur ≤0.005%, and prevent the introduction of oil, rust and other impurities to ensure the purity of raw materials; S3 smelting process optimization: S301 converter smelting: final carbon content controlled at 0.08-0.12%, phosphorus ≤0.010%, tapping temperature 1620-1650℃, to avoid uneven composition caused by overblowing; S302 LF Refining and Fine-tuning: Utilizing computer-aided composition fine-tuning technology, ensuring C element fluctuation ≤ ±0.015% and other alloying element fluctuations ≤ ±0.025%; refining time 60-90 minutes, with composition and temperature uniformized by argon gas stirring at a flow rate of 0.3-0.6 m³ / h. 3 / h; S303 VD vacuum degassing: vacuum degree ≤67Pa, holding time 20-30 minutes, efficiently removes gas from steel, ensuring that the purity of the molten steel after degassing meets the requirement that inclusions A / B / C / D / DS are all ≤0.8. Collaborative optimization of S4 continuous casting process: S401 crystallizer parameters: Adjust the crystallizer water flow rate to 110-220 m³ / h according to the steel material specifications (Φ6-42mm). 3 / h, adopting sinusoidal vibration mode, vibration frequency 80-120 times / min; crystallizer electric stirring parameters 200-320A / 2-3.5Hz, end electric stirring 180-400A / 6-18Hz, to refine billet grains; S402 Casting Speed ​​and Cooling Control: Continuous casting speed is 1.4-2.2m / min to match the requirements of different billet specifications; the secondary cooling water supply adopts a weak cooling mode with a specific water volume of 0.35-0.50L / kg to reduce the temperature difference between the inside and outside of the billet and avoid thermal stress cracks. S403 protective slag control: Medium carbon protective slag is selected to control the total slag thickness in the crystallizer to 25-30mm and the liquid slag thickness to 6-9mm, thereby improving the surface lubrication effect of the billet and reducing pinhole and nodule defects. S5 billet finishing and inspection: S501 stacking cooling treatment: The stacking cooling time after the billet comes off the production line is ≥24 hours to ensure sufficient slow cooling, eliminate internal residual stress, and the furnace exit temperature is ≤300℃; S502 Surface Finishing: After shot blasting, the billet undergoes magnetic particle testing and ultrasonic testing. Surface cracks and pinhole defects are repaired by grinding to a depth of 0.3-1.2mm, ensuring that the surface scratch depth is ≤0.025mm. S503 Low-magnification inspection: 1-2 billets are randomly selected from each furnace for low-magnification inspection. Shrinkage cavities should be ≤0.5 grade, general porosity and central porosity should be ≤0.8 grade, and there should be no white bright bands or crack defects. S6 rolling and heat treatment synergistic optimization: S601 heating process: heating temperature before rolling is 1050-1100℃, holding time is 90-120 minutes, to ensure the uniformity of the billet composition and avoid overheating that leads to coarse grains; S602 controlled rolling and cooling: The final rolling temperature is 850-900℃, and a segmented cooling mode is adopted with a cooling rate of 5-15℃ / s to obtain a uniform pearlite + ferrite (P+F) structure, which lays the foundation for subsequent spheroidizing annealing. S603 Spheroidizing Annealing Synergy: Referencing the optimized spheroidizing annealing process, ensure that the spheroidized carbide level reaches 4-5, and the carbide particle diameter is ≤4μm, thereby improving the cold heading deformation capability; S7 Finished Product Composition and Performance Verification: S701 Composition Testing: The chemical composition of finished products is tested batch by batch to ensure that both core elements and harmful elements are within the design range, with a composition qualification rate of ≥99.5%. S702 performance verification: Sampling was conducted for hardness testing (160-200HB), tensile testing (yield strength 340-440MPa, tensile strength 540-640MPa, elongation after fracture ≥16%), and no cracking was observed when the deformation in the cold heading test was ≥65%, meeting the requirements for cold heading processing of high-end fasteners; Furthermore, this process achieves precise control over composition and optimized balance between plasticity and strength: through precise control of core elements (C: 0.38-0.43%, Si: 0.15-0.25%) and strict limitation of harmful elements (P≤0.012%, S≤0.004%), combined with Al element to refine grains, the steel composition qualification rate is ≥99.5%, effectively balancing strength and cold heading plasticity, increasing cold heading deformation to ≥65% without cracking, and completely solving the problem of frequent cold heading cracks in traditional processes; Furthermore, this process significantly improves the purity of molten steel: after KR desulfurization (desulfurization rate ≥90%), screening of high-quality scrap steel, and VD vacuum degassing (vacuum degree ≤67Pa) treatment, the gas content (O≤8ppm, N≤40ppm, H≤0.4ppm) and inclusion level (A / B / C / D / DS all ≤0.8 grade) in the steel are greatly reduced, avoiding internal defects caused by gas inclusions and large particle inclusions, and improving the uniformity of stress transfer during cold heading. Furthermore, this process achieves a comprehensive improvement in billet quality: through the synergistic optimization of the continuous casting process (molder parameter adaptation, weak cooling mode, and control of medium-carbon protective slag) and billet finishing (shot blasting, dual flaw detection, and grinding), the defects of pinholes, nodules, and cracks on the billet surface are reduced by more than 80%, shrinkage cavities are ≤0.5 grade, porosity is ≤0.8 grade, and there is no white bright band, providing high-quality billets for subsequent processing and reducing the risk of rolling rejection and cold heading failure; Furthermore, this process improves the uniformity of microstructure and processing stability: the combined use of rolling heating (1050-1100℃) and controlled rolling and cooling (final rolling 850-900℃), along with the spheroidizing annealing process, enables the finished product to obtain a uniform P+F microstructure, with the spherical carbide level stabilized at grade 4-5, particle diameter ≤4μm, hardness controlled at 160-200HB, and mechanical property dispersion reduced by 30%, ensuring the consistency and stability of cold heading in mass production; Furthermore, this process enhances product competitiveness: the optimized finished product has a yield strength of 340-440MPa, a tensile strength of 540-640MPa, and an elongation at break of ≥16%, meeting the requirements of high-end fasteners. At the same time, the optimization of the entire process reduces raw material loss and subsequent repair costs, improves the product's cost-effectiveness, and can effectively cope with industry competitive pressure.

[0023] 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 composition optimization process for improving the cold heading performance of ML40Cr steel, comprising S1 composition design optimization, S2 raw material pretreatment process, S3 smelting process optimization, S4 continuous casting process synergistic optimization, S5 billet finishing and inspection, S6 rolling and heat treatment synergistic optimization, and S7 finished product composition and performance verification, characterized in that: The process includes precise regulation of core elements (S101), strict restriction of harmful elements (S102), and supplementation of beneficial elements (S103). Among them, the S2 raw material pretreatment process includes S201 molten iron pretreatment and S202 scrap steel screening; Among them, the S3 smelting process optimization includes S301 converter smelting, S302 LF refining fine-tuning and S303 VD vacuum degassing; Among them, the collaborative optimization of the S4 continuous casting process includes S401 crystallizer parameters, S402 casting speed and cooling control, and S403 protective slag control. Among them, S5 billet finishing and inspection includes S501 heat treatment, S502 surface finishing and S503 low magnification inspection. Among them, the synergistic optimization of S6 rolling and heat treatment includes the synergistic effects of S601 heating process, S602 controlled rolling and cooling and S603 spheroidizing annealing. Among them, the S7 finished product composition and performance verification includes S701 composition detection and S702 performance verification.

2. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The core elements of S101 are precisely controlled: carbon (C) content is controlled at 0.38-0.43% to ensure the balance between the strength and cold heading plasticity of the steel; silicon (Si) is 0.15-0.25% to avoid excessive amounts affecting cold heading formability; manganese (Mn) is 0.60-0.75% to refine the grains and improve toughness; and chromium (Cr) is 0.90-1.05% to enhance hardenability and wear resistance.

3. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The harmful elements in S102 are strictly limited: phosphorus (P) ≤ 0.012% and sulfur (S) ≤ 0.004% to reduce crack sensitivity during cold heading; oxygen (O) ≤ 8ppm, nitrogen (N) ≤ 40ppm and hydrogen (H) ≤ 0.4ppm to reduce internal defects caused by gas inclusions.

4. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The S103 is supplemented with beneficial elements: aluminum (Al) 0.020-0.035%, which plays a role in refining grains and improving the plasticity and cold working properties of steel.

5. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The S201 molten iron pretreatment adopts the KR desulfurization process, with a desulfurization rate of ≥90%, ensuring that the sulfur content of the molten iron before entering the furnace is ≤0.003%, thus avoiding the accumulation of harmful elements; S202 scrap steel screening: Select high-quality scrap steel with low phosphorus and low sulfur content, requiring phosphorus ≤0.010% and sulfur ≤0.005%, and prevent the introduction of oil stains, rust and other impurities to ensure the purity of raw materials.

6. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The S301 converter smelting process: the final carbon content is controlled at 0.08-0.12%, phosphorus ≤0.010%, and the tapping temperature is 1620-1650℃ to avoid uneven composition caused by overblowing; S302 LF Refining and Fine-tuning: Utilizing computer-aided composition fine-tuning technology, ensuring C element fluctuation ≤ ±0.015% and other alloying element fluctuations ≤ ±0.025%; refining time 60-90 minutes, with composition and temperature uniformized by argon gas stirring at a flow rate of 0.3-0.6 m³ / h. 3 / h; S303 VD vacuum degassing: vacuum degree ≤67Pa, holding time 20-30 minutes, efficiently removes gas from steel, ensuring that the purity of the molten steel after degassing meets the requirement that inclusions A / B / C / D / DS are all ≤0.

8.

7. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The parameters for the S401 crystallizer are as follows: Adjust the water flow rate of the crystallizer to 110-220 m³ / h according to the steel material specifications (Φ6-42mm). 3 / h, adopting sinusoidal vibration mode, vibration frequency 80-120 times / min; crystallizer electric stirring parameters 200-320A / 2-3.5Hz, end electric stirring 180-400A / 6-18Hz, to refine billet grains; S402 Casting Speed ​​and Cooling Control: Continuous casting speed is 1.4-2.2m / min to match the requirements of different billet specifications; the secondary cooling water supply adopts a weak cooling mode with a specific water volume of 0.35-0.50L / kg to reduce the temperature difference between the inside and outside of the billet and avoid thermal stress cracks. S403 protective slag control: Medium carbon protective slag is selected to control the total slag thickness in the crystallizer to 25-30mm and the liquid slag thickness to 6-9mm, thereby improving the surface lubrication effect of the cast billet and reducing pinhole and nodule defects.

8. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The S501 stacking cooling treatment: the stacking cooling time after the billet comes off the production line is ≥24 hours to ensure sufficient slow cooling, eliminate internal residual stress, and the furnace exit temperature is ≤300℃; S502 Surface Finishing: After shot blasting, the billet undergoes magnetic particle testing and ultrasonic testing. Surface cracks and pinhole defects are repaired by grinding to a depth of 0.3-1.2mm, ensuring that the surface scratch depth is ≤0.025mm. S503 Low-magnification inspection: 1-2 billets are randomly selected from each furnace for low-magnification inspection. The shrinkage cavity should be ≤0.5 grade, the general porosity and central porosity should be ≤0.8 grade, and there should be no white bright bands or crack defects.

9. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The S601 heating process: heating temperature before rolling is 1050-1100℃, holding time is 90-120 minutes, to ensure the uniformity of the billet composition and avoid overheating that leads to coarse grains; S602 controlled rolling and cooling: The final rolling temperature is 850-900℃, and a segmented cooling mode is adopted with a cooling rate of 5-15℃ / s to obtain a uniform pearlite + ferrite (P+F) structure, which lays the foundation for subsequent spheroidizing annealing. S603 Spheroidizing Annealing Synergy: Referencing the optimized spheroidizing annealing process, ensure that the spherical carbide level reaches 4-5, and the carbide particle diameter is ≤4μm, thereby improving the cold heading deformation capability.

10. The composition optimization process for improving the cold heading performance of ML40Cr steel according to claim 1, characterized in that: The S701 component testing involves testing the chemical composition of each batch of finished products to ensure that both core and harmful elements are within the design range, with a component qualification rate of ≥99.5%. S702 performance verification: Sampling was conducted for hardness testing (160-200HB), tensile testing (yield strength 340-440MPa, tensile strength 540-640MPa, elongation after fracture ≥16%), and no cracking was observed when the deformation in the cold heading test was ≥65%, meeting the requirements for cold heading processing of high-end fasteners.