Low-temperature impact performance control process for low-temperature bolt steel

Abstract

The invention relates to the technical field of research and development of low-temperature bolt steel, and discloses a low-temperature impact performance control process for low-temperature bolt steel. Comprising the steps that S1, in the steelmaking refining stage, purity and uniformity control is conducted, S2, casting blank homogenization treatment is conducted, stress relief and structure regulation are conducted, S3, a heat treatment core process is conducted, structure and toughness balance regulation is conducted, and S4, online detection and dynamic feedback are conducted, and performance stability is guaranteed. By accurately controlling the element proportion of carbon (C), silicon (Si), manganese (Mn), chromium (Cr) and molybdenum (Mo) in steel and combining with an optimized heat treatment process (such as 860 DEG C oil quenching and 640-650 DEG C high-temperature tempering, and the heat preservation time is prolonged to 120 minutes or more), the V-shaped notch impact energy of the material in the environment of-101 DEG C stably reaches 35 J or more, the impact energy of part of samples can reach 50 J or more, and the impact energy of the part of samples can reach 50 J or more. The problem that the low-temperature impact performance is insufficient after wire hardening and tempering is thoroughly solved, and it is ensured that the brittle failure resistance of the bolt is remarkably improved in the ultralow-temperature environment.

Description

Technical Field

[0001] This invention relates to the field of low-temperature bolt steel research and development technology, specifically a process for controlling the low-temperature impact performance of low-temperature bolt steel. Background Technology

[0002] Cryogenic stud bolts, as core connectors for cryogenic pressure vessels, valves, flanges, and pipe fittings, are widely used in extreme environments such as chemical and petroleum industries, bridge engineering, and wind power generation equipment. They must withstand alternating loads and impact stresses for extended periods in temperatures of -100℃ and below, and their service safety directly impacts the stable operation of industrial plants. These bolts place stringent requirements on the comprehensive performance of the materials: they must possess not only high strength (tensile strength ≥ 860 MPa, yield strength ≥ 725 MPa) and good plasticity (elongation ≥ 16%, reduction of area ≥ 50%), but more importantly, they must maintain excellent impact toughness at ultra-low temperatures (V-notch impact energy ≥ 27 J at -101℃) to avoid cryogenic brittle fracture failure.

[0003] Currently, the industry faces multiple technical challenges in the research and production of low-temperature bolt steel: Insufficient stability of low-temperature impact performance: Existing wires often meet the mechanical properties after heat treatment but fail to meet the low-temperature impact performance requirements, especially the impact value fluctuates greatly at -101℃, making it difficult to meet the stringent service requirements.

[0004] The challenge of balancing composition and cost: Traditionally, nickel (Ni) alloys (such as 40CrNiMoA) are often used to improve low-temperature toughness. Although this can improve toughness, it significantly increases the cost of materials. If the content of elements such as manganese (Mn), chromium (Cr), and molybdenum (Mo) is adjusted to replace nickel, the strength and toughness may be unbalanced due to unreasonable composition design.

[0005] Poor adaptability of heat treatment process: Heat treatment parameters have a significant impact on low-temperature performance. In the existing process, the quenching temperature, tempering temperature and holding time are not accurately controlled. For example, if the tempering temperature is too low (below 620℃), it is easy to cause insufficient impact energy, while if the temperature is too high, it may reduce the strength, making it difficult to achieve a precise balance between strength and toughness.

[0006] Insufficient control of material purity: High content of gases (oxygen, hydrogen) and harmful elements (phosphorus, sulfur) in steel, or excessive non-metallic inclusions, can become stress concentration sources at low temperatures, exacerbating the risk of brittle fracture. Traditional refining processes cannot meet the requirements for ultra-low temperature service by controlling purity.

[0007] With the increasing safety requirements of cryogenic equipment in industries such as chemical and wind power, as well as the industry's demand for cost reduction and efficiency improvement, developing a set of R&D processes for cryogenic stud bolt steel that can stably control cryogenic impact performance, balance mechanical properties and costs, and adapt to industrial production has become the key to solving industry pain points. Summary of the Invention

[0008] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a low-temperature impact performance control process for low-temperature bolt steel, which has the advantages of significantly improving low-temperature impact toughness and ensuring safe service in extreme environments, thus solving the problem of insufficient stability in low-temperature impact performance.

[0009] (II) Technical Solution To achieve the aforementioned goal of significantly improving low-temperature impact toughness and ensuring safe service in extreme environments, this invention provides the following technical solution: a low-temperature impact performance control process for low-temperature bolt steel, comprising the following processes: S1 steelmaking and refining stage: purity and uniformity control; S2 billet homogenization treatment: stress relief and microstructure regulation; S3 heat treatment core process: microstructure and toughness balance regulation; and S4 online detection and dynamic feedback: performance stability assurance. The S1 steelmaking and refining stage: purity and uniformity control includes S101 converter smelting basic control, S102 LF furnace refining optimization, and S103 double vacuum degassing treatment. Among them, the S2 billet homogenization treatment: stress relief and microstructure control includes S201 continuous casting process control and S202 billet slow cooling and heating treatment. Among them, the core process of S3 heat treatment: the balance control of microstructure and toughness includes S301 precision quenching process and S302 high temperature tempering and heat preservation. Among them, S4 online detection and dynamic feedback: performance stability assurance includes S401 full-item performance detection and S402 dynamic adjustment of process parameters.

[0010] Preferably, the S101 converter smelting basic control includes: End-point control of steel tapping: Carbon content is precisely controlled within the target range, phosphorus content is ≤0.015%, and tapping temperature is 1560~1580℃ to avoid excessively high temperatures that could lead to grain coarsening; Slag blocking and steel tapping: Pneumatic slag blocking technology is adopted to ensure that the slag amount is ≤5kg / t. When 1 / 3 of the steel is tapped, a composite deoxidizer (aluminum particles + silicon-calcium alloy) is added along the steel flow, and the initial deoxidation efficiency is ≥90%.

[0011] Preferably, the refining process in the S102LF furnace is optimized as follows: Total refining time ≥ 60 min, producing high-basicity white slag (basicity 5.0~6.0), slag quantity 12~15 kg / t, white slag holding time ≥ 35 min, deep deoxidation and desulfurization, controlling sulfur content ≤ 0.010%; The process adopts a diffusion deoxidation + wire feeding composite process: first, silicon carbide diffusion deoxidation is added (dosage ≥ 50 kg / furnace), and then calcium wire is fed (calcium wire feeding amount 2.5~3.0 m / t). The non-metallic inclusion levels in the steel are controlled: Class A, Class B, Class C, and Class D inclusions are all ≤ 1.0 level.

[0012] Preferably, the S103 dual vacuum degassing treatment: The RH+VD dual vacuum degassing process is adopted: the RH furnace is in-station time ≥30min, vacuum is drawn to ≤67Pa and maintained for ≥15min, and soft blowing argon time is 15~20min; the VD furnace secondary degassing time is ≥25min, and the vacuum degree is ≤50Pa, ensuring that the gas content in the steel is: [H]≤1.0ppm, [O]≤12ppm.

[0013] Preferably, the S201 continuous casting process is controlled as follows: Full-process protective casting: Low-silicon protective slag (melting point 1150~1180℃, viscosity 0.8~1.2Pa・s) is used, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. Castings that exceed the tolerance are directly rejected. Process parameter optimization: pull speed 0.7~0.8m / min, specific water volume 0.25~0.30L / kg, turn on the crystallizer electromagnetic stirring (250~300A / 2.0Hz) and the end electromagnetic stirring (350~400A / 5.0Hz), and combine with the light reduction process (total reduction 3~5mm) to suppress center segregation.

[0014] Preferably, the S202 billet undergoes slow cooling and heating treatment: After the continuously cast billets exit the machine, they are immediately stacked closely together and a stepped slow cooling process is adopted: first, they are held at 600~650℃ for ≥12h, and then cooled to room temperature at a rate of ≤50℃ / h to ensure that the internal stress release rate is ≥90%; After slow cooling, the billet undergoes surface flaw detection (magnetic particle inspection). Only billets without cracks or folding defects can proceed to the next process.

[0015] Preferably, the S301 precision quenching process: Step-by-step heating and quenching: preheating section (400~450℃, holding for 30~40min) → heating section (650~700℃, holding for 40~50min) → quenching heating section (860~870℃, holding time adjusted according to specifications: φ≤30mm, holding for 40~50min, φ>30mm, holding for 60~70min), ensuring austenite homogenization and grain size ≤8; Optimized quenching medium: Use a special fast-cooling oil (kinematic viscosity 40~50 mm). 2 / s (flash point ≥180℃), cooling rate controlled at 30~40℃ / s, ensuring hardenability ≥95%, and obtaining uniform martensitic structure.

[0016] Preferably, the S302 high-temperature tempering and heat preservation are: Precise tempering temperature control: High-temperature tempering at 640~660℃ with a holding time of 120~150min (increased according to specifications: 15min extension for every 10mm increase in diameter) ensures that carbides are fully precipitated and evenly distributed; Warm holding and slow cooling: After tempering, hold at 500~550℃ for 30~40min, then cool to below 200℃ at a rate of ≤30℃ / h to eliminate residual stress and transform into a uniform tempered sorbite structure (sorbitization rate ≥95%).

[0017] Preferably, the S401 full-item performance test includes: Three to five samples are randomly selected from each batch for mechanical property testing: tensile strength, yield strength, elongation, and reduction of area, to ensure compliance with basic mechanical requirements. Low-temperature impact testing: V-notch impact tests are conducted at -101℃, with 3 samples per group. The average impact energy is ≥38J, and the minimum single sample energy is ≥30J. Metallographic analysis is performed on samples with lower impact values ​​(30~35J) to ensure the absence of network carbides and coarse grain defects.

[0018] Preferably, the process parameters in S402 are dynamically adjusted: If the impact value is lower than the target value (<38J), prioritize checking the uniformity of quenching temperature (deviation ≤±5℃) and tempering holding time (extend by 10~15min); if the impact value of a single sample is <30J, trace the billet segregation situation, adjust the continuous casting light reduction parameters or increase the holding time of white slag in the LF furnace refining process; Establish a database linking heat treatment parameters, microstructure, and properties. Use AI algorithms to predict impact performance and enable intelligent fine-tuning of process parameters (such as tempering temperature ±5℃ and holding time ±10min).

[0019] (III) Beneficial Effects Compared with the prior art, the present invention provides a process for controlling the low-temperature impact performance of low-temperature bolt steel, which has the following beneficial effects: 1. This low-temperature impact performance control process for low-temperature bolt steel significantly improves low-temperature impact toughness and ensures safe service in extreme environments. By precisely controlling the proportions of carbon (C), silicon (Si), manganese (Mn), chromium (Cr), and molybdenum (Mo) elements in the steel, combined with optimized heat treatment processes (such as oil quenching at 860℃ + high-temperature tempering at 640~650℃, with the holding time extended to more than 120 minutes), the V-notch impact energy of the material at -101℃ can stably reach more than 35J (far exceeding the standard requirement of 27J), and the impact energy of some samples can reach more than 50J. This completely solves the problem of insufficient low-temperature impact performance after wire rod quenching and tempering, ensuring that the bolt's resistance to brittle fracture is significantly improved in ultra-low temperature environments.

[0020] 2. The low-temperature impact performance control process of this low-temperature bolt steel achieves a precise balance between strength and toughness, and its mechanical properties fully meet the standards. During the research and development process, the composition design was optimized (such as controlling the carbon content at 0.38~0.43% to avoid a decrease in toughness, and adjusting the chromium and molybdenum content to enhance hardenability). Combined with the stepped heating quenching and warm tempering process, the tensile strength of the material is stabilized at 970~1010MPa, the yield strength reaches 880~910MPa, while the elongation is maintained at 18~20% and the reduction of area reaches 54~58%. It fully meets and exceeds the standards and user requirements for strength and plasticity, and achieves the synergistic optimization of "high strength-high toughness".

[0021] 3. The low-temperature impact performance control process for this low-temperature bolt steel optimizes the composition design to reduce costs, balancing economy and performance. By scientifically adjusting the content of manganese, chromium, and molybdenum alloying elements to replace the reliance on nickel in traditional processes, the amount of precious metals used is reduced while ensuring low-temperature toughness, thus lowering material production costs. At the same time, the refining process improves the purity of the material (phosphorus ≤ 0.015%, sulfur ≤ 0.010%), avoiding performance fluctuations caused by excessive inclusions, indirectly reducing the scrap rate, and further reducing the overall cost of industrial production.

[0022] 4. This low-temperature impact performance control process for low-temperature bolt steel improves production stability, ensures batch performance consistency, and establishes a full-process control system of "steelmaking refining - billet treatment - heat treatment - testing feedback": by reducing gas content (hydrogen ≤ 1.0 ppm, oxygen ≤ 12 ppm) through double vacuum degassing (RH+VD), eliminating internal stress through continuous casting slow cooling, and combining online testing and dynamic adjustment of process parameters (such as fine-tuning tempering temperature ±5℃ based on impact value feedback), the performance fluctuations between batches are effectively reduced, and the deviation of key indicators such as low-temperature impact energy and strength is controlled within 5%, meeting the stability requirements of large-scale production.

[0023] 5. The low-temperature impact performance control process of this low-temperature bolt steel broadens the application scenarios and adapts to the low-temperature service requirements of multiple fields. The developed low-temperature stud bolt steel not only meets the requirements of ASTM A320 / A320M-22 standard for L7 steel, but also adapts to the low-temperature working conditions of multiple fields such as chemical and petroleum, wind power generation, and bridge engineering through performance upgrades. In particular, it exhibits stable mechanical properties and resistance to brittle fracture in extreme environments below -100℃, providing reliable material protection for the safe operation of low-temperature industrial equipment. Detailed Implementation

[0024] 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.

[0025] This solution provides a technical solution, specifically a process for controlling the low-temperature impact performance of low-temperature bolt steel, including the following process: S1 steelmaking and refining stage: purity and uniformity control; S101 converter smelting basic control: End-point control of steel tapping: Carbon content is precisely controlled within the target range, phosphorus content is ≤0.015%, and tapping temperature is 1560~1580℃ to avoid excessively high temperatures that could lead to grain coarsening; Slag-blocking and tapping: Pneumatic slag-blocking technology is adopted to ensure that the slag discharge is ≤5kg / t. When 1 / 3 of the steel is tapped, a composite deoxidizer (aluminum particles + silicon-calcium alloy) is added along the steel flow, with an initial deoxidation efficiency of ≥90%. S102LF Furnace Refining Optimization: Total refining time ≥ 60 min, producing high-basicity white slag (basicity 5.0~6.0), slag quantity 12~15 kg / t, white slag holding time ≥ 35 min, deep deoxidation and desulfurization, controlling sulfur content ≤ 0.010%; The process employs a diffusion deoxidation + wire feeding composite process: first, silicon carbide diffusion deoxidation is added (dosage ≥ 50 kg / furnace), followed by calcium wire feeding (calcium wire feeding amount 2.5~3.0 m / t), controlling the non-metallic inclusion levels in the steel: Class A, Class B, Class C, and Class D inclusions are all ≤ 1.0 level; S103 Dual Vacuum Degassing Treatment: The RH+VD dual vacuum degassing process is adopted: the RH furnace is in-station time ≥30min, vacuum is drawn to ≤67Pa and maintained for ≥15min, and soft blowing argon time is 15~20min; the VD furnace secondary degassing time is ≥25min, and the vacuum degree is ≤50Pa, ensuring that the gas content in the steel is: [H]≤1.0ppm, [O]≤12ppm; S2 billet homogenization treatment: stress relief and microstructure control; S201 Continuous Casting Process Control: Full-process protective casting: Low-silicon protective slag (melting point 1150~1180℃, viscosity 0.8~1.2Pa・s) is used, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. Castings that exceed the tolerance are directly rejected. Process parameter optimization: pull speed 0.7~0.8m / min, specific water volume 0.25~0.30L / kg, turn on the crystallizer electromagnetic stirring (250~300A / 2.0Hz) and the end electromagnetic stirring (350~400A / 5.0Hz), and combine with the light reduction process (total reduction 3~5mm) to suppress center segregation; Slow cooling and heating treatment of S202 billet: After the continuously cast billets exit the machine, they are immediately stacked closely together and a stepped slow cooling process is adopted: first, they are held at 600~650℃ for ≥12h, and then cooled to room temperature at a rate of ≤50℃ / h to ensure that the internal stress release rate is ≥90%; After slow cooling, the billet undergoes surface flaw detection (magnetic particle inspection). Only if there are no cracks or folding defects can it proceed to the next process. S3 heat treatment core process: balance control of microstructure and toughness; S301 Precision Quenching Process: Step-by-step heating and quenching: preheating section (400~450℃, holding for 30~40min) → heating section (650~700℃, holding for 40~50min) → quenching heating section (860~870℃, holding time adjusted according to specifications: φ≤30mm, holding for 40~50min, φ>30mm, holding for 60~70min), ensuring austenite homogenization and grain size ≤8; Optimized quenching medium: Use a special fast-cooling oil (kinematic viscosity 40~50 mm). 2 / s, flash point ≥180℃), cooling rate controlled at 30~40℃ / s, ensuring hardenability ≥95%, to obtain uniform martensitic structure; S302 High-Temperature Tempering and Insulation: Precise tempering temperature control: High-temperature tempering at 640~660℃ with a holding time of 120~150min (increased according to specifications: 15min extension for every 10mm increase in diameter) ensures that carbides are fully precipitated and evenly distributed; Warm holding and slow cooling: After tempering, hold at 500~550℃ for 30~40min, then cool to below 200℃ at a rate of ≤30℃ / h to eliminate residual stress and transform into a uniform tempered sorbite structure (sorbitization rate ≥95%). S4 online detection and dynamic feedback: ensuring performance stability; S401 Full Performance Test: Three to five samples are randomly selected from each batch for mechanical property testing: tensile strength, yield strength, elongation, and reduction of area, to ensure compliance with basic mechanical requirements. Low-temperature impact testing: V-notch impact tests are conducted at -101℃, with 3 samples per group. The average impact energy is ≥38J, and the minimum single sample energy is ≥30J. Metallographic analysis is performed on samples with lower impact values ​​(30~35J) to ensure the absence of network carbides and coarse grain defects. S402 process parameters dynamic adjustment: If the impact value is lower than the target value (<38J), prioritize checking the uniformity of quenching temperature (deviation ≤±5℃) and tempering holding time (extend by 10~15min); if the impact value of a single sample is <30J, trace the billet segregation situation, adjust the continuous casting light reduction parameters or increase the holding time of white slag in the LF furnace refining process; Establish a database linking heat treatment parameters, microstructure, and properties, and use AI algorithms to predict impact performance to achieve intelligent fine-tuning of process parameters (such as tempering temperature ±5℃ and holding time ±10min). Key technological innovations: Purity control through dual vacuum + stepped slow cooling: reducing gas content through dual vacuum degassing and eliminating stress through stepped slow cooling, thereby reducing brittleness-inducing factors from the source and improving the basic toughness of the material; Step-up quenching and warm tempering work together to avoid uneven microstructure caused by rapid heating. High-temperature tempering combined with warm heat preservation promotes uniform precipitation of carbides, achieving a precise balance between strength and toughness. A dynamic feedback mechanism throughout the entire process: Low-temperature impact performance is correlated in real time with smelting, continuous casting, and heat treatment parameters, and the process is optimized through intelligent algorithms to ensure that the performance fluctuation between batches is ≤5%; Furthermore, this process significantly improves low-temperature impact toughness, ensuring safe service in extreme environments. By precisely controlling the proportions of carbon (C), silicon (Si), manganese (Mn), chromium (Cr), and molybdenum (Mo) in the steel, combined with optimized heat treatment processes (such as oil quenching at 860℃ + high-temperature tempering at 640~650℃, with the holding time extended to more than 120 minutes), the V-notch impact energy of the material at -101℃ can stably reach more than 35J (far exceeding the standard requirement of 27J), and the impact energy of some samples can reach more than 50J. This completely solves the problem of insufficient low-temperature impact performance of wire after heat treatment, ensuring that the bolt's resistance to brittle fracture is significantly improved in ultra-low temperature environments. Furthermore, this process achieves a precise balance between strength and toughness, with all mechanical properties meeting the standards. During the research and development process, the composition design was optimized (such as controlling the carbon content at 0.38~0.43% to avoid a decrease in toughness, and adjusting the chromium and molybdenum content to enhance hardenability). Combined with a stepped heating quenching and warm tempering process, the tensile strength of the material is stabilized at 970~1010MPa, the yield strength reaches 880~910MPa, while the elongation is maintained at 18~20% and the reduction of area reaches 54~58%. This fully meets and exceeds the standards and user requirements for strength and plasticity, achieving a synergistic optimization of "high strength-high toughness". Furthermore, this process optimizes the composition design to reduce costs, balancing economy and performance. By scientifically adjusting the content of manganese, chromium, and molybdenum alloying elements to replace the reliance on nickel in traditional processes, the amount of precious metals used is reduced while ensuring low-temperature toughness, thus lowering material production costs. At the same time, the refining process improves the purity of the material (phosphorus ≤ 0.015%, sulfur ≤ 0.010%), avoiding performance fluctuations caused by excessive inclusions, indirectly reducing the scrap rate, and further reducing the overall cost of industrial production. Furthermore, this process enhances production stability, ensures batch performance consistency, and establishes a comprehensive control system encompassing "steelmaking refining - billet treatment - heat treatment - testing feedback": by reducing gas content (hydrogen ≤ 1.0 ppm, oxygen ≤ 12 ppm) through dual vacuum degassing (RH+VD), eliminating internal stress through slow cooling in continuous casting, and combining online testing with dynamic adjustment of process parameters (such as fine-tuning tempering temperature ±5℃ based on impact value feedback), it effectively reduces performance fluctuations between batches, keeping the deviation of key indicators such as low-temperature impact energy and strength within 5%, thus meeting the stability requirements of large-scale production; Furthermore, this process broadens application scenarios and adapts to the low-temperature service requirements of multiple fields. The developed low-temperature stud bolt steel not only meets the requirements of ASTM A320 / A320M-22 standards for L7 steel, but also adapts to low-temperature working conditions in multiple fields such as chemical and petroleum, wind power generation, and bridge engineering through performance upgrades. In particular, it exhibits stable mechanical properties and resistance to brittle fracture in extreme environments below -100℃, providing reliable material protection for the safe operation of low-temperature industrial equipment.

[0026] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for controlling the low-temperature impact performance of low-temperature bolt steel, comprising the following processes: S1 steelmaking and refining stage: purity and uniformity control; S2 billet homogenization treatment: stress relief and microstructure regulation; S3 core heat treatment process: microstructure and toughness balance regulation; and S4 online detection and dynamic feedback: performance stability assurance, characterized in that: The S1 steelmaking refining stage includes purity and uniformity control, which includes S101 basic control of converter smelting, S102 LF furnace refining optimization, and S103 dual vacuum degassing treatment. Among them, the S2 billet homogenization treatment: stress relief and microstructure control includes S201 continuous casting process control and S202 billet slow cooling and heating treatment. Among them, the core process of S3 heat treatment: the balance control of microstructure and toughness includes S301 precision quenching process and S302 high temperature tempering and heat preservation. Among them, S4 online detection and dynamic feedback: performance stability assurance includes S401 full-item performance detection and S402 dynamic adjustment of process parameters.

2. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The basic control of S101 converter smelting: End-point control of steel tapping: Carbon content is precisely controlled within the target range, phosphorus content is ≤0.015%, and tapping temperature is 1560~1580℃ to avoid excessively high temperatures that could lead to grain coarsening; Slag blocking and steel tapping: Pneumatic slag blocking technology is adopted to ensure that the slag amount is ≤5kg / t. When 1 / 3 of the steel is tapped, a composite deoxidizer (aluminum particles + silicon-calcium alloy) is added along the steel flow, and the initial deoxidation efficiency is ≥90%.

3. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The S102LF furnace refining optimization: Total refining time ≥ 60 min, producing high-basicity white slag (basicity 5.0~6.0), slag quantity 12~15 kg / t, white slag holding time ≥ 35 min, deep deoxidation and desulfurization, controlling sulfur content ≤ 0.010%; The process adopts a diffusion deoxidation + wire feeding composite process: first, silicon carbide diffusion deoxidation is added (dosage ≥ 50 kg / furnace), and then calcium wire is fed (calcium wire feeding amount 2.5~3.0 m / t). The non-metallic inclusion levels in the steel are controlled: Class A, Class B, Class C, and Class D inclusions are all ≤ 1.0 level.

4. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The S103 dual vacuum degassing treatment: The RH+VD dual vacuum degassing process is adopted: the RH furnace is in-station time ≥30min, vacuum is drawn to ≤67Pa and maintained for ≥15min, and soft blowing argon time is 15~20min; the VD furnace secondary degassing time is ≥25min, and the vacuum degree is ≤50Pa, ensuring that the gas content in the steel is: [H]≤1.0ppm, [O]≤12ppm.

5. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The S201 continuous casting process control: Full-process protective casting: Low-silicon protective slag (melting point 1150~1180℃, viscosity 0.8~1.2Pa・s) is used, and the fluctuation of the liquid level in the crystallizer is controlled within ±3mm. Castings that exceed the tolerance are directly rejected. Process parameter optimization: pull speed 0.7~0.8m / min, specific water volume 0.25~0.30L / kg, turn on the crystallizer electromagnetic stirring (250~300A / 2.0Hz) and the end electromagnetic stirring (350~400A / 5.0Hz), and combine with the light reduction process (total reduction 3~5mm) to suppress center segregation.

6. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The slow cooling and heating treatment of the S202 billet: After the continuously cast billets exit the machine, they are immediately stacked closely together and a stepped slow cooling process is adopted: first, they are held at 600~650℃ for ≥12h, and then cooled to room temperature at a rate of ≤50℃ / h to ensure that the internal stress release rate is ≥90%; After slow cooling, the billet undergoes surface flaw detection (magnetic particle inspection). Only billets without cracks or folding defects can proceed to the next process.

7. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The S301 precision quenching process: Step-by-step heating and quenching: preheating section (400~450℃, holding for 30~40min) → heating section (650~700℃, holding for 40~50min) → quenching heating section (860~870℃, holding time adjusted according to specifications: φ≤30mm, holding for 40~50min, φ>30mm, holding for 60~70min), ensuring austenite homogenization and grain size ≤8; Quenching medium optimization: Use a special fast-cooling oil (kinematic viscosity 40~50 mm). 2 / s (flash point ≥180℃), cooling rate controlled at 30~40℃ / s, ensuring hardenability ≥95%, and obtaining uniform martensitic structure.

8. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The S302 high-temperature tempering and heat preservation: Precise tempering temperature control: High-temperature tempering at 640~660℃ with a holding time of 120~150min (increased according to specifications: 15min extension for every 10mm increase in diameter) ensures that carbides are fully precipitated and evenly distributed; Warm holding and slow cooling: After tempering, hold at 500~550℃ for 30~40min, then cool to below 200℃ at a rate of ≤30℃ / h to eliminate residual stress and transform into a uniform tempered sorbite structure (sorbitization rate ≥95%).

9. The low-temperature impact performance control process for low-temperature bolt steel according to claim 1, characterized in that: The S401 full performance test: Three to five samples are randomly selected from each batch for mechanical property testing: tensile strength, yield strength, elongation, and reduction of area, to ensure compliance with basic mechanical requirements. Low-temperature impact testing: V-notch impact tests are conducted at -101℃, with 3 samples per group. The average impact energy is ≥38J, and the minimum single sample energy is ≥30J. Metallographic analysis is performed on samples with lower impact values ​​(30~35J) to ensure the absence of network carbides and coarse grain defects.

10. The process for controlling the low-temperature impact performance of low-temperature bolt steel according to claim 1, characterized in that: The S402 process parameters are dynamically adjusted: If the impact value is lower than the target value (<38J), prioritize checking the uniformity of quenching temperature (deviation ≤±5℃) and tempering holding time (extend by 10~15min); if the impact value of a single sample is <30J, trace the billet segregation situation, adjust the continuous casting light reduction parameters or increase the holding time of white slag in the LF furnace refining process; Establish a database linking heat treatment parameters, microstructure, and properties. Use AI algorithms to predict impact performance and enable intelligent fine-tuning of process parameters (such as tempering temperature ±5℃ and holding time ±10min).