Steel for Rzeppa-type universal joint retainer and its manufacturing method

Abstract

The present invention relates to a steel for a Rzeppa type universal joint retainer and a method for producing the same, which belongs to the field of special steel refining technology. The chemical composition of the steel is as follows (wt%): C: 0.10-0.25%, Si: 0.20-0.40%, Mn: 0.40-0.65%, Cr: 0.40-0.70%, B: 0.0003-0.0025%, Ti: 0.010-0.035%, Mo: 0.30-0.45%, N: 0.0050-0.0100%, S≦0.015%, P≦0.025%, Ni≦0.25%, Cu≦0.30%, Al: 0.015-0.035%, O≦0.0010%, As≦0.04%, Sn≦0.03%, Sb≦0.005%, Pb≦0.002%, and the balance is Fe and unavoidable impurities. The microstructure of the steel is bainite, and the austenite grain size is ≧6. The production process is primary molten steel refining → molten steel refining → molten steel vacuum degassing → continuous casting → hot rolling → finishing. The chemical composition of this steel is optimized to reduce the alloy cost. In addition, the hardenability is close to that of 20CrMnTi. The steel produced by combining this chemical composition and production method has better strength and toughness than 20CrMnTi, and the overall performance meets the requirements for steel for Rzeppa type universal joint retainer.

Description

[Technical Field]

[0001] The present invention relates to the field of special steel refining technology, and more particularly to steel for Rzeppa type universal joint retainers and a method for manufacturing the same. [Background technology]

[0002] The constant velocity universal joint commonly used in passenger cars today is the Rzeppa type universal joint, whose role is to transmit engine power from the transmission to the two front wheels, enabling the car to travel at high speeds. It is mainly composed of key components such as a ball housing, star cover, retainer (ball cage), and steel balls. Because constant velocity universal joints transmit heavy driving torque, the heavier the load they bear, the greater the demand for high transmission precision. At the same time, as they are a safety component of automobiles, the demands on product quality are also high.

[0003] As competition in the modern automobile industry becomes more intense, the requirements for automobile power, operability, comfort, and safety are becoming higher, and in addition, there are also requirements for energy and environmental measures. Therefore, when designing important functional parts of automobiles, important indicators such as safety, functionality, economy, and emissions must be taken into consideration comprehensively. Therefore, higher requirements are being placed on materials, and while ensuring performance, there is a need for materials to be lighter. As for materials for automobile universal joints, since the parts play the roles of transmission and support and are subjected to long-term repeated load stress, the materials must have sufficient wear resistance, fatigue resistance, and excellent toughness.

[0004] During operation of a Rzeppa-type universal joint, especially when the load is complex and the joint rotates at high speeds, the retainer is subjected to large centrifugal forces, shocks, and vibrations, resulting in significant sliding friction between the retainer and the rolling elements and generating a large amount of heat. The combined force and heat can lead to retainer failure, and in severe cases, the retainer can even burn or break. Therefore, the retainer material must have excellent thermal conductivity, excellent wear resistance, a small coefficient of friction, a relatively low density, a certain combination of strength and toughness, relatively good elasticity and rigidity, an expansion coefficient similar to that of the rolling elements, and excellent processing capabilities. Furthermore, the retainer must also have a certain degree of corrosion resistance because it is exposed to chemical media such as lubricants, lubricant additives, organic solvents, and coolants.

[0005] Currently, the material commonly used for Rzeppa-type universal joint retainers is 20CrMnTi, which has fine, uniform grains after carburization and relatively good surface tensile strength and bending fatigue strength. The core has sufficient strength and toughness, improving wear resistance. However, the manufacturing cost is quite high and there is little margin for strength and toughness. Summary of the Invention

[0006] The steel material of the present invention is a newly designed material based on 20CrMnTi. By optimizing the contents of the alloying elements Mn and Cr, while adding B and Mo, and perfecting the production process, it achieves hardenability similar to that of 20CrMnTi under the premise of optimizing costs. In addition, the strength and toughness of the product are superior to those of 20CrMnTi, and it ultimately meets the requirements for a carburizing steel for Rzeppa-type universal joint retainers.

[0007] Specifically, the microstructure of the steel material researched and developed in this application is bainite, with an austenite grain size of ≥ 6, a yield strength of ≥ 850 MPa, a tensile strength of ≥ 1080 MPa, an elongation rate of ≥ 10%, and a Charpy impact strength of AK at room temperature. U≥ 55J. The hardenability of the steel end is evaluated according to GB / T 225, with J5 meeting 35-42 HRC, J9 meeting 25-35 HRC, and J13 meeting 20-30 HRC. The band structure of the steel is graded according to GB / T 13299, with the band structure not exceeding level 2.0. Nonmetallic inclusions are graded according to Method A in GB / T 10561, with the following requirements: A fine ≤ 1.5, A coarse ≤ 1.0, B fine ≤ 1.5, B coarse ≤ 0.5, C fine = 0, C coarse = 0, D fine ≤ 1.0, D coarse ≤ 0.5, Ds ≤ 1.5. The macrostructure of the steel, as evaluated according to ASTM E381, is graded as follows: C ≤ 2.0, R ≤ 2.0, S ≤ 2.0.

[0008] The steel material of the present invention, which satisfies the above performance requirements, ultimately meets the requirements for use as a steel material for Rzeppa-type universal joint retainers.

[0009] The present invention employs the following technical approach to solve the above problems: A steel for a Rzeppa type universal joint retainer, the chemical composition of which is as follows (wt%): C: 0.10-0.25%, Si: 0.20-0.40%, Mn: 0.40-0.65%, Cr: 0.40-0.70%, B: 0.0003-0.0025%, Ti: 0.010-0.035%, Mo: 0.30-0.45%. %, N: 0.0050 to 0.0100%, S≦0.015%, P≦0.025%, Ni≦0.25%, Cu≦0.30%, Al: 0.015 to 0.035%, O≦0.0010%, As≦0.04%, Sn≦0.03%, Sb≦0.005%, Pb≦0.002%, and the balance being Fe and unavoidable impurities.

[0010] The basis for setting the chemical components of the present application is as follows.

[0011] 1) Determination of C content C is an essential element for ensuring the wear resistance of steel materials. Increasing the carbon content in steel enhances its martensitic transformation ability, improving its hardness and strength, and thereby improving its wear resistance. However, too much C content is detrimental to the toughness of the steel. Furthermore, too much C content can cause serious central C segregation, affecting the toughness of the core of the steel material. In the present invention, the C content is controlled to 0.10 to 0.25%.

[0012] 2) Determination of Si content Silicon is a key element in the present invention. Silicon dissolves in the ferrite phase and has a fairly strong solid-solution strengthening effect, significantly improving the strength of ferrite, but at the same time reducing the plasticity and toughness of ferrite. The range of silicon content set in the present invention is 0.20 to 0.40%.

[0013] 3) Determination of Mn content Mn is a deoxidizing element in the steelmaking process and is effective in strengthening steel, performing solid solution strengthening. Mn also increases the hardenability of steel and can improve the thermal processing performance of steel. Mn can eliminate the effects of S (sulfur). Mn can form MnS, which has a high melting point, together with S during steel refining, and can further weaken and eliminate the adverse effects of S. However, if the Mn content is high, the toughness of the steel decreases. In the present invention, the Mn content is controlled to 0.40 to 0.65%.

[0014] 4) Determination of Cr content Cr is a carbide-forming element and can improve the hardenability, wear resistance, and corrosion resistance of steel. However, if the Cr content is too high, the hardness of the steel material will be too high, which will be disadvantageous for customers in processing and use. After comprehensive analysis, the Cr content range in the present invention has been determined to be 0.40 to 0.70%.

[0015] 5) Determination of Al content Al is added to steel as a deoxidizing element, not only reducing the dissolved oxygen in molten steel, but also forming fine aluminum nitride inclusions dispersed by Al and N, thereby refining the crystal grains. However, if the Al content is too high, brittle inclusions such as large particles of Al2O3 are likely to form during the molten steel refining process, reducing the purity of the molten steel and affecting the service life of the finished product. In this invention, the Al content range is determined to be 0.015 to 0.035%.

[0016] 6) Determination of B content B can improve the hardenability of steel, increase the high-temperature strength of steel, and strengthen the grain boundaries in steel. In the present invention, the range of B content is determined to be 0.0003 to 0.0025%.

[0017] 7) Determination of Mo content Molybdenum refines the grain size of steel, improving its hardenability and high-temperature performance, and maintaining sufficient strength and creep resistance at high temperatures. Adding molybdenum to steel improves its mechanical properties and also suppresses the brittleness of alloy steels caused by tempering. However, since molybdenum is a ferrite-forming element, a high molybdenum content can lead to the appearance of ferrite δ phase and other brittle phases, which can reduce toughness. Therefore, the Mo content range in this invention is set to 0.30 to 0.45%.

[0018] 8) Determination of Ti content Titanium is a strong deoxidizer in steel. Titanium can make the internal structure of steel denser and refine the crystal grains. However, Ti can form titanium carbonitride inclusions in steel, which are hard and angular inclusions that seriously affect the fatigue life of the material. Therefore, in this invention, the Ti content range is determined to be 0.01 to 0.035%.

[0019] 9) Determination of N content Nitrogen can improve the strength, low-temperature toughness, and weldability of steel, and increase its aging sensitivity. Adding an appropriate amount of aluminum to steel produces stable AlN and inhibits the formation and precipitation of Fe4N, thereby improving the aging resistance of steel and inhibiting the growth of austenite grains and refining the grains. However, nitrogen can form nitride nonmetallic inclusions together with alloying elements in steel, and more importantly, it reduces the effectiveness of the alloying elements. A high nitrogen content in steel increases the strength of the steel but decreases its impact toughness. In this invention, the N content is determined to be 0.0050 to 0.0100%.

[0020] 10) Determination of O content The oxygen content indicates the total amount of oxide inclusions, and these oxide embrittlement inclusions limit and affect the service life of the finished product. Numerous tests have shown that reducing the oxygen content is significantly beneficial for improving the purity of steel, especially for reducing the content of oxide embrittlement inclusions in steel. In the present invention, the oxygen content range is determined to be ≦0.0010%.

[0021] 11) Determination of P and S content P causes serious segregation during solidification in steel, and when P dissolves in ferrite, it distorts and coarsens the grains and increases cold brittleness. In this invention, the P content range is determined to be ≦0.025%. S causes hot brittleness in steel and reduces the ductility and toughness of steel, but S can improve the cutting performance of steel, so in this invention, the S content range is determined to be ≦0.015%.

[0022] 12) Determination of As, Sn, Sb, and Pb contents Trace elements such as As, Sn, Sb, and Pb are all low-melting-point non-ferrous metals that, when present in steel, can cause soft spots on the surface of components and uneven hardness. Therefore, they are considered harmful elements in steel, and the content ranges for these elements are set as follows: As≦0.04%, Sn≦0.03%, Sb≦0.005%, and Pb≦0.002%.

[0023] The manufacturing process for the steel used in the above-mentioned Rzeppa-type universal joint retainers is as follows: electric furnace or converter (primary refining) → LF extra-furnace refining → VD or RH vacuum degassing → continuous casting → rolling → finishing → packaging and warehousing.

[0024] The main features of the manufacturing process are as follows:

[0025] (1) Molten steel refining section: In the primary refining process, high-quality molten iron, scrap steel, and auxiliary raw materials are used to reduce the content of harmful elements in the molten steel. During the refining process, deoxidation is strengthened, and the carbon content at the end of the tapping point of the electric furnace or converter is controlled to 0.05-0.15%. Aluminum iron is added during the tapping process to perform pre-deoxidation, creating favorable conditions for subsequent deoxidation. After tapping, slag scraping technology is used to scrape off harmful slag.

[0026] During refining, new synthetic slag is added to the LF refining furnace while strengthening the deoxidation during the refining process. During the refining process, silicon carbide and aluminum are used for deoxidation, and white slag is formed as quickly as possible in the early stages of refining. The white slag is maintained for 25 minutes or more, and the aluminum content throughout the refining process is controlled to be between 0.025% and 0.045%, thereby ensuring the deoxidation effect.

[0027] Since the steel material in this application is a type with high crack sensitivity, vacuum degassing is strengthened, and the treatment time under high vacuum (133 Pa or less) is set to ≥ 15 minutes to ensure that the harmful gas H ≤ 2 ppm. After vacuum degassing, silicon calcium wire is applied to perform inclusion modification treatment, and after vacuum degassing, soft argon blowing is performed for a long period of time to ensure sufficient floating of inclusions. The soft argon blowing time is set to ≥ 25 minutes.

[0028] (2) Continuous casting section: Anti-oxidation protection throughout the continuous casting process (i.e., isolating the molten steel from air) reduces the amount of inclusions in the steel. Furthermore, the selection of high-quality resistant materials and control techniques to reduce contamination of the molten steel by external inclusions strengthen control over the production process. During the continuous casting process, electromagnetic stirring and soft reduction technologies are employed. By adjusting the pressure distribution of the soft reduction rollers, the molten steel is fully filled at the center of the steel as it solidifies, avoiding shrinkage cavities. Strengthening the crystallization device and end-stage electromagnetic stirring alters the solidification flow field of the molten steel, improving the internal structure of the continuously cast molten steel and reducing segregation. Low superheat pouring is used during continuous casting, and the superheat is controlled between 10 and 30°C, effectively improving and reducing component segregation in the continuously cast billet. Continuously cast billets with dimensions of 300mm x 300mm or larger are continuously cast, matching the chemical composition of the finished steel. The continuous casting billet must be cooled slowly in the pit to prevent cracks in the continuous casting billet. The cooling time should not be less than 24 hours. After that, the continuous casting billet is sent to a step-type heating furnace to be heated, and then rolled into a target material.

[0029] (3) Rolled section: Before rolling, the continuous cast billet is placed in a furnace and heated. The temperature of the preheating zone is controlled between 600 and 850°C, the temperature of the heating zone between 950 and 1100°C, and the temperature of the soaking zone between 1150 and 1200°C. To ensure uniform heating of the billet, the total heating time is at least 240 minutes, with the soaking zone time at least 180 minutes. The starting temperature for rolling is controlled between 950 and 1050°C, and the finish rolling temperature is controlled between 800 and 900°C. The entire rolling process is carried out in the austenite single-phase region. To achieve austenite-to-bainite transformation after rolling, slow cooling is not recommended between the end of finish rolling and the cooling bed. This prevents the appearance of coarse ferrite grains, which would reduce the strength and toughness of the steel. However, cooling should not be too rapid to prevent the appearance of martensite, which would reduce the toughness of the steel. In the present invention, the cooling rate in the temperature-reducing region from the end of finish rolling to the cooling bed is set to 10-15°C / s, while the rolling speed is relatively slowed to control the time the steel passes through this region, allowing the steel to fully transform into bainite. The final cooling bed temperature for the steel is controlled to 600-650°C. At this point, the bainite transformation of the steel's metal structure is nearly complete, and the subsequent cooling bed can cool it at a normal cooling rate. The cooling rate is 15-20°C / min, after which the steel is removed from the line and subjected to subsequent straightening and flaw detection to obtain the target bar product.

[0030] Compared with the prior art, the advantages of the present invention are as follows:

[0031] 1) The steel material of the present invention is based on 20CrMnTi and has been redesigned with reduced contents of the alloying elements Mn and Cr, as well as the harmful element Ti. Because reducing the Mn and Cr contents inevitably affects the hardenability of steel, the steel material of the present invention also contains trace amounts of B and a certain amount of Mo, improving hardenability and ensuring that the hardenability of the steel is comparable to that of 20CrMnT. Furthermore, the purity of the steel is improved by reducing Ti, which tends to form hard, non-deformable inclusions, and adding N, which also refines grains. At the same time, the grain size of the steel material of the present invention is equivalent to that of 20CrMnTi. Furthermore, by controlling the transformation of the metal structure during the rolling process, the formation of a bainite structure is ensured in the finished steel material, ensuring that the strength and toughness of the steel material are not less than those of 20CrMnTi. Ultimately, the steel material meets the requirements for Rzeppa-type universal joint retainers.

[0032] 2) As described above, the present invention strengthens deoxidation and dehydrogenation during the refining process, while selecting high-quality raw materials to ensure high steel purity. It also employs low superheat casting for continuous casting, and employs electromagnetic stirring and light pressure control to control segregation in the steel, effectively meeting the requirements for steel for Rzeppa-type universal joint retainers. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 is a typical metallographic diagram ×100 of an embodiment of the present invention. [Figure 2] FIG. 2 is a typical metallographic diagram ×100 of the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention will be described in more detail below in connection with the drawings, but the described embodiments are illustrative and are intended to be used to interpret the present invention, and therefore cannot be understood as limitations on the present invention.

[0035] The chemical compositions (wt%) of the steel materials of the examples of the present invention are shown in Tables 1 and 2. They are also compared with the chemical compositions of the comparative steel 20CrMnTi.

[0036] TIFF0007766692000001.tif37157

[0037] TIFF0007766692000002.tif34161

[0038] See Table 3 for data comparing the mechanical properties of the steel materials in each example and comparative example.

[0039] TIFF0007766692000003.tif41161

[0040] The hot-rolled metal structure, band structure, and grain size data for the steel materials of each example and proportional example are shown in Table 4.

[0041] TIFF0007766692000004.tif37154

[0042] Typical structures of the steel materials of the examples of the present invention and the comparative examples are as shown in Figs. 1 and 2, where the structure shown in Fig. 1 is a bainite structure, and the structure shown in Fig. 2 is ferrite + pearlite + bainite.

[0043] The comparison of terminal hardenability performance between the steel materials of each example and comparative example is shown in Table 5, and the unit is HRC.

[0044] TIFF0007766692000005.tif38119

[0045] Table 6 shows a comparison of inclusions in the steel materials of the examples and comparative examples.

[0046] TIFF0007766692000006.tif80156

[0047] Table 7 shows a comparison of macroscopic data for the steel materials of the examples and comparative examples.

[0048] TIFF0007766692000007.tif49139

[0049] The above test results show that the impact and tensile properties of the present invention are superior to those of the comparative example 20CrMnTi, and therefore the strength and toughness of the final product of the present invention are even superior to those of 20CrMnTi. Other performance indicators of the present invention, including hardenability, grain size, inclusions, bands, macro, etc., are close to those of the comparative steel, and all performance indicators can meet the requirements for steel for Rzeppa type universal joint retainers.

[0050] The following will explain in detail the method for producing steel for the Rzeppa type universal joint retainer of each of the above-mentioned embodiments.

[0051] Production process: electric furnace or converter → LF extra-furnace refining → VD or RH vacuum degassing → continuous casting → continuous rolling → finishing → packaging and warehousing.

[0052] When refining molten steel, high-quality molten iron, scrap steel, and auxiliary raw materials are selected, and high-quality deoxidizers and refractory materials are selected. In the electric furnace / converter production process, the carbon at the end of the tapping point in three examples is controlled between 0.05% and 0.15%, and the phosphorus at the end is controlled below 0.020%, respectively, to strengthen deoxidation. The carbon at the end of the tapping point of the electric furnace or converter is controlled to be 0.05% to 0.15%. Aluminum iron is added during the tapping process to perform pre-deoxidation, creating good conditions for subsequent deoxidation. After tapping, slag scraping technology is used to scrape off harmful slag.

[0053] During refining, new synthetic slag is added to the LF refining furnace, and at the same time, deoxidation during the refining process is strengthened. Silicon carbide and aluminum are used for deoxidation during the refining process, and white slag is formed as quickly as possible at the beginning of refining, and the white slag is kept for more than 25 minutes. The aluminum content throughout the refining process is controlled between 0.025% and 0.045%, so that the deoxidation effect is guaranteed.

[0054] Since the steel used in this example is a type with high crack sensitivity, vacuum degassing is strengthened, and the treatment time under high vacuum (133 Pa or less) is set to ≥ 15 minutes to ensure that the harmful gas H ≤ 2 ppm. After vacuum degassing, silicon calcium wire is applied to perform inclusion modification treatment, and after vacuum degassing, soft argon blowing is performed for a long period of time to ensure sufficient floating of the inclusions. The soft argon blowing time is set to ≥ 25 minutes.

[0055] The continuous casting superheat is controlled between 10 and 30°C, and the continuous casting stretching speed is between 0.45 and 0.75 m / min. The dimensions of the continuous casting billet are 300mm x 300mm. The continuously casting billet is slowly cooled in a pit for at least 24 hours. After this, the slowly cooled continuously casting billet is sent to a heating furnace for rolling into target steel. The specific rolling process is as follows: the temperature of the preheating zone is controlled between 600 and 850°C, the temperature of the heating zone is controlled between 950 and 1100°C, and the temperature of the soaking zone is controlled between 1150 and 1200°C. To ensure that the billet is heated evenly, the total heating time is at least 4 hours, and the soaking zone time is at least 3 hours. The rolling start temperature is controlled to 950-1050°C, the finish rolling temperature is controlled to 800-900°C, and after the finish rolling is completed, the steel is cooled at a controlled cooling rate of 10-15°C / min to transform all the austenite structure into bainite, and the temperature of the steel on the cooling bed is controlled to 600-650°C, and after further straightening and flaw detection, the target bar product is obtained.

[0056] The rolling process parameters for each example are as shown in Table 8.

[0057] TIFF0007766692000008.tif34166

[0058] In addition to the above-mentioned embodiments, the present invention also includes other embodiments, and any technical means formed by means of equivalent transformation or equivalent substitution shall fall within the scope of protection of the claims of the present invention.

Claims

1. In the steel material for the Rzeppa type universal joint retainer, the chemical composition wt% of the steel material is as follows: C: 0.10 to 0.25%, Si: 0.20 to 0.40%, Mn: 0.40 to 0.65%, Cr: 0.40 to 0.70%, B: 0.0003 to 0.0025%, Ti: 0.010 to 0.035%, Mo: 0.30 to 0.4 5%, N: 0.0050 to 0.0100%, S≦0.015%, P≦0.025%, Ni≦0.25%, Cu≦0.30%, Al: 0.015 to 0.035%, O≦0.0010%, As≦0.04%, Sn≦0.03%, Sb≦0.005%, Pb≦0.002%, and the balance being Fe and unavoidable impurities, the microstructure of the steel is bainite and the austenite grain size is ≧6; The steel material has a yield strength of 850 MPa or more, a tensile strength of 1080 MPa or more, an elongation rate of 10%, and a Charpy impact strength AK at room temperature. Ｕ ≧55J, The hardenability of the steel material is evaluated by the GB / T 225 method, and satisfies 35 to 42 HRC for J5, 25 to 35 HRC for J9, and 20 to 30 HRC for J13; The band structure of the steel is graded according to GB / T 13299, and the band structure does not exceed level 2.0; Non-metallic inclusions are graded based on the A method in GB / T 10561, and satisfy the following: A fine≦1.5, A coarse≦1.0, B fine≦1.5, B coarse≦0.5, C fine=0, C coarse=0, D fine≦1.0, D coarse≦0.5, Ds≦1.5; The steel material is characterized by satisfying the following in the grading of macrostructure according to ASTM E381: C≦level 2.0, R≦level 2.0, S≦level 2.

0. Steel material for Zeppa type universal joint retainers.

2. The chemical composition wt% of the steel is C: 0.10-0.25%, Si: 0.20-0.40%, Mn: 0.40-0.65%, Cr: 0.40-0.70%, B: 0.0003-0.0025%, Ti: 0.010-0.035%, Mo: 0.30-0.45%, N: 0.0050-0.0100%, S≦0.015%, P≦0.025%, Ni≦0.25%, Cu≦0.30%, Al: 0.015-0.035%, O≦0.0010%, As≦0.04%, Sn≦0.03%, Sb≦0.005%, Pb≦0.002%, and the balance is Fe and unavoidable impurities, the microstructure of the steel is bainite and the austenite grain size is ≧6; The steel material has a yield strength of 850 MPa or more, a tensile strength of 1080 MPa or more, an elongation rate of 10% or more, and a Charpy impact strength AKU of 55J or more at room temperature, The hardenability of the steel material is evaluated by the GB / T 225 method, and satisfies 35 to 42 HRC for J5, 25 to 35 HRC for J9, and 20 to 30 HRC for J13; The band structure of the steel is graded according to GB / T 13299, and the band structure does not exceed level 2.0; Non-metallic inclusions are graded based on the A method in GB / T 10561, and satisfy the following: A fine≦1.5, A coarse≦1.0, B fine≦1.5, B coarse≦0.5, C fine=0, C coarse=0, D fine≦1.0, D coarse≦0.5, Ds≦1.5; A method for producing steel material for Rzeppa type universal joint retainers, which satisfies the following criteria in the steel material macrostructure grading according to ASTM E381: C≦Level 2.0, R≦Level 2.0, S≦Level 2.0 The process is as follows: primary refining of molten steel → molten steel refining → molten steel vacuum degassing → continuous casting → hot rolling. In the molten steel refining process, deoxidation is strengthened, the carbon at the end of the primary refining is controlled to 0.05-0.15%, aluminum iron is added during the tapping process to perform preliminary deoxidation of the molten steel to create favorable conditions for subsequent deoxidation, slag is scraped off after tapping to remove harmful slag, synthetic slag is newly added to the molten steel during refining, and at the same time, deoxidation during the refining process is strengthened, silicon carbide and aluminum are used for deoxidation, white slag is formed as quickly as possible in the early stage of refining and the white slag is maintained for 25 minutes or more, the aluminum content in the molten steel throughout the refining process is controlled to be between 0.025% and 0.045% to ensure the deoxidation effect, vacuum degassing is strengthened until the harmful gas H in the molten steel is ≦2 ppm, silicon calcium wire is added after vacuum degassing to perform inclusion modification treatment, and after vacuum degassing, argon soft blowing is performed on the molten steel, the molten steel is stirred to fully float the inclusions, and the argon soft blowing time is ≧25 minutes, The continuous casting billet obtained in the continuous casting process is slowly cooled in a pit, and the slow cooling time is not less than 24 hours. Before rolling, the continuous cast billet is placed in a furnace and heated. The temperature of the preheating zone is set to 600-850°C, the temperature of the heating zone to 950-1100°C, and the temperature of the soaking zone to 1150-1200°C. The total heating time is 240 minutes or more, and the soaking zone time is 180 minutes or more. After being taken out of the furnace, preparation for rolling is made. The rolling start temperature is set to 950-1050°C, and the finish rolling temperature is set to 800-900°C. The entire rolling process is carried out in the austenite single phase region, and in the region from the end of finish rolling to the cooling bed, the cooling rate in the step (a) is controlled to 10-15°C / s, and the time for the steel material to pass through this region is controlled; this cooling process is controlled to transform the macrostructure from austenite to bainite, and to allow the steel material to fully undergo bainite structural transformation; the temperature of the final steel material on the cooling bed is 600-650°C, and the steel material continues to cool on the cooling bed, and finally the steel material is removed from the cooling bed and straightened to obtain a target product. Manufacturing method.

3. 3. The method according to claim 2, wherein the vacuum degassing step comprises holding the molten steel under high vacuum conditions of 133 Pa or less for 15 minutes or more.

4. 3. The manufacturing method according to claim 2, wherein the superheat of the molten steel during continuous casting is controlled to 10 to 30°C.

5. 3. The manufacturing method according to claim 2, wherein the cooling rate of the steel material on the cooling bed is 15 to 20° C. / min.

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