High-performance non-quenched and tempered steel bar for fan shaft and manufacturing method of high-performance non-quenched and tempered steel bar

Abstract

The invention relates to a high-performance non-quenched and tempered steel bar for a fan shaft and a manufacturing method of the high-performance non-quenched and tempered steel bar, and belongs to the field of iron-based alloys. The steel comprises the following chemical components in percentage by weight: 0.37 to 0.43 percent of C, 0.30 to 0.60 percent of Si, 1.20 to 1.70 percent of Mn, less than or equal to 0.025 percent of P, 0.030 to 0.070 percent of S, less than or equal to 0.50 percent of Cr, 0.10 to 0.30 percent of Ni, less than or equal to 0.30 percent of Mo, less than or equal to 0.30 percent of Cu, 0.010 to 0.030 percent of Al, 0.005 to 0.025 percent of Ti, 0.05 to 0.15 percent of V, 0.0100 to 0.0200 percent of N, less than or equal to 0.0002 percent of H, less than or equal to 0.0015 percent of O and the balance of Fe and inevitable impurity elements. The manufacturing process comprises the steps of converter / electric furnace smelting, LF furnace refining, RH / VD furnace vacuum degassing, continuous casting, casting blank reheating, cogging rolling, flame cleaning, intermediate blank finishing, intermediate blank heating, bar rolling, cooling and stress relief tempering. The total cross-section structure of the bar is a ferrite and pearlite structure. The bending degree per meter is smaller than or equal to 0.5 mm, the surface defect depth is smaller than or equal to 0.10 mm, and the internal quality meets the A-level requirement of the GB / T4162 standard.

Description

Technical Field

[0001] This invention belongs to the field of iron-based alloys, specifically relating to non-quenched and tempered steel bars for wind turbine shafts and their manufacturing methods. Background Technology

[0002] With the transformation of the global energy structure, the increasing demand for renewable energy, and the growing environmental awareness, the wind turbine industry market has shown rapid development in recent years. Especially in fields such as metallurgy and mining, environmental dust removal, and heating and ventilation, wind turbines, as important energy conversion equipment, have been widely used.

[0003] Fan products mainly consist of important components such as a motor, fan shaft, impeller, and casing. Among these, the fan shaft, as a key component for torque transmission, connects the fan impeller and the motor rotor, responsible for transferring the power output from the motor to the impeller. Therefore, the fan shaft must have sufficient mechanical strength and precision to successfully transmit motor power and prevent vibration or rotor-stator rubbing during motor operation. Wear or bending of the fan shaft will affect the fan's operational accuracy and service life. Therefore, fan shafts are mostly made of medium carbon steel or medium carbon alloy steel, forged, heat-treated, and then machined.

[0004] Currently, commonly used raw materials for fan shafts in the industry include steel grades such as 45 steel, 40Cr, and 42CrMo. However, the mechanical properties of quenched and tempered alloy steel often only fully manifest themselves after quenching and tempering heat treatment. Not only is component deformation unavoidable and straightening necessary, but machining and assembly accuracy cannot be strongly guaranteed. Furthermore, for large-capacity fans, quenching and tempering heat treatment cannot penetrate to the core of the material, resulting in uneven cross-sectional properties. This often leads to premature failure of the motor shaft at its weakest points, especially at splines and shaft seats, during service. Moreover, quenching and tempering heat treatment often easily causes stress deformation of the fan shaft, necessitating the addition of straightening processes to improve straightness. Heat treatment consumes a large amount of energy. Therefore, the current fan shaft industry mainly suffers from long production cycles, high production costs, and low yield rates, severely restricting the energy conservation and environmental protection requirements of the fan industry, and urgently requiring technological innovation. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for manufacturing high-strength and high-toughness non-quenched and tempered steel 38MnVS6 for wind turbine shafts. This method involves optimizing the chemical composition of non-quenched and tempered steel, utilizing micro-alloying composite technology with Cu, Ni, V, and Ti, and processes such as controlled rolling and controlled cooling. This achieves a method for manufacturing high-performance non-quenched and tempered steel bars for wind turbine shafts, replacing quenched and tempered steel, shortening the processing flow, and reducing production costs.

[0006] The technical solution adopted by this invention to solve the above problems is as follows: a high-performance non-quenched and tempered steel bar for wind turbine shafts and its manufacturing method. The bar specifications are generally Ф20-60mm, with a bend per meter ≤0.5mm. Non-metallic inclusions are rated according to GB / T10561 standards, meeting the following requirements: A fine ≤3.0, A coarse ≤2.0, B fine ≤2.0, B coarse ≤1.0, C fine ≤1.0, C coarse ≤1.0, D fine ≤2.0, D coarse ≤1.0. Austenite grain size is rated according to GB / T6394 standards, meeting a grade ≥6.0. After tempering and stress relief, the bar is sampled and tested according to GB / T228, meeting the following mechanical properties: yield strength ≥600MPa, tensile strength ≥900MPa, elongation ≥15%, reduction of area ≥33%, impact strength ≥50J. Surface defect depth is ≤0.10mm, and internal quality meets the GB / T4162 standard Grade A requirements.

[0007] To ensure performance requirements, the high-performance non-quenched and tempered steel for industrial fan shafts has the following chemical composition by weight percentage: C: 0.37–0.43%, Si: 0.30–0.60%, Mn: 1.20–1.70%, P: ≤0.025%, S: 0.030–0.070%, Cr: ≤0.50%, Ni: 0.10–0.30%, Mo: ≤0.30%, Cu: ≤0.30%, Al: 0.010–0.030%, Ti: 0.005–0.025%, V: 0.05–0.15%, N: 0.0100–0.0200%, H ≤0.0002%, O ≤0.0015%, with the balance being Fe and unavoidable impurities.

[0008] The rationale for the design of the chemical composition of the high-performance non-quenched and tempered steel for industrial fan shafts in this invention: C: Increases the strength and hardness of the material, but reduces the plasticity and impact toughness of the steel. In order to achieve a reasonable balance of the comprehensive properties of the material, such as strength and toughness, the C content in this invention is 0.37-0.43%.

[0009] Si: As a deoxidizing element and a basic solid solution strengthening element, it improves yield strength, hardenability and surface wear resistance. Therefore, the Si content of the steel of this invention is determined to be in the range of 0.30 to 0.60%.

[0010] Mn: Manganese forms a solid solution with iron, increasing the strength and hardness of ferrite and austenite in steel. Manganese's ability to stabilize austenite structure is second only to nickel, and it strongly increases the hardenability of steel. However, manganese tends to increase grain coarsening in steel and is sensitive to temper brittleness. Excessive manganese content increases the cracking sensitivity of steel. In this invention, the manganese content is controlled at 1.20-1.70%.

[0011] P: Phosphorus is a harmful element in steel. It easily segregates at grain boundaries, causing segregation, increasing the temper brittleness of steel, and significantly reducing the plasticity and toughness of grain boundaries. In this invention, the phosphorus content is controlled at ≤0.025%.

[0012] S: The addition of sulfur can improve the machinability of steel, but it will cause hot brittleness and reduce the plasticity and toughness of steel. However, considering the requirements of the part for machinability, the S content range is determined to be 0.030 to 0.070%.

[0013] Cr: Chromium increases the hardenability of steel and has a secondary hardening effect, which can improve the strength and hardness of steel, but it has a deteriorating effect on the plasticity and toughness of steel and leads to an increase in temper brittleness. The chromium content in this invention is controlled at ≤0.50%.

[0014] Ni plays two roles in bar stock: improving hardenability and enhancing impact toughness, ultimately increasing fatigue strength. However, the high price of Ni increases the cost of steel. To ensure the performance of the steel in this invention, the Ni content is controlled at 0.10–0.30%.

[0015] Mo: Molybdenum increases the hardenability of materials, which can improve the strength and tempering stability of steel. Molybdenum can also improve the ductility, toughness and wear resistance of steel. In addition, molybdenum improves the hot strength of steel. In this invention, the molybdenum content is controlled at ≤0.30%.

[0016] Cu: Copper can improve the strength of steel, especially its yield strength ratio, meaning that steel can better resist deformation when subjected to external forces. With increasing copper content, the room temperature impact toughness of steel also improves, as does its fatigue strength, which is particularly important for applications requiring repeated loading. In this invention, the copper content is controlled to ≤0.30%.

[0017] Al: When added to steel as a deoxidizer, aluminum mainly plays a deoxidizing role. Its deoxidizing ability is stronger than that of Si and Mn. In addition, it can refine the grains and fix nitrogen in the steel, thereby improving the impact toughness of the steel. However, if the content is too high, it will impair the hot working performance of the steel and easily form large non-metallic inclusions, increasing the brittleness of the steel. In this invention, the aluminum content is controlled at 0.010 to 0.030%.

[0018] Ti: Titanium has a strong affinity for nitrogen, oxygen, and carbon in steel, forming compounds such as TiC and TiN. These compounds exhibit high stability at high temperatures and can inhibit grain growth in steel, thereby refining the grain size. Titanium in steel can react with sulfur, reducing the hot brittleness caused by sulfur, similar to Mn, which helps improve the tempering stability of steel. This invention controls the titanium content to be between 0.005% and 0.025%.

[0019] V: Vanadium exists in steel in the form of carbides. Its main function is to refine the steel’s microstructure and grains, and improve the steel’s strength and toughness. Vanadium can also increase the tempering stability of quenched steel and reduce overheating sensitivity. In this invention, the vanadium content is controlled at 0.05-0.15%.

[0020] Nitrogen (N) has a solid solution strengthening effect, which can improve the strength and hardness of steel. Nitrogen-containing ferritic steel, during tempering after rapid cooling or prolonged exposure to room temperature, can undergo precipitation hardening due to the precipitation of ultramicroscopic nitrides, thus increasing the steel's strength and hardness. Nitrogen can combine with aluminum and titanium in steel to form aluminum nitride and titanium nitride. These compounds have high stability at high temperatures and can inhibit grain growth, thereby refining the grain size. In this invention, the nitrogen content is controlled at 0.0100–0.0200%.

[0021] O: Oxygen is a harmful element in steel. Oxygen exists in steel in the form of inclusions such as FeO, MnO, SiO2, and Al2O3, which reduces the strength and plasticity of steel, and has a serious impact on the fatigue strength and impact toughness of steel. The oxygen content of steel is controlled at ≤0.0015% in this invention.

[0022] H: Hydrogen is a harmful element in steel. The presence of dissolved hydrogen in steel can cause defects such as hydrogen embrittlement and white spots. Hydrogen drastically reduces the plasticity, toughness and fatigue strength of steel, and in severe cases, it can cause hydrogen-induced cracking and brittle fracture. The hydrogen content in this invention is controlled at ≤0.0002%.

[0023] The specific production process for steel bars used in this invention is as follows: converter / electric furnace smelting → LF furnace refining → RH / VD furnace vacuum degassing → continuous casting → billet reheating → billet rolling → flame cleaning → intermediate billet finishing → intermediate billet heating → bar rolling → cutting → cooling → stress-relief tempering → straightening → rolling → polishing → flaw detection (ultrasonic flaw detection, magnetic flux leakage flaw detection) → inspection → packaging → warehousing. The main processes are described below.

[0024] 1) Smelting: The C content in the primary refining furnace is controlled at ≥0.15%, and the molten steel temperature is ≥1650℃. During the tapping process, pre-melted slag, strong deoxidizer, and some alloys are added to the ladle. The primary refining ladle is hoisted to the LF refining furnace by a crane, and argon is blown in by the bottom blowing device. 200kg of lime and other slag-forming materials are added, and electricity is supplied for reheating. During the refining process, powdered diffusion deoxidizer is added to the slag surface to keep the slag white or grayish-white. The refining time is greater than 30 minutes. Five minutes before tapping, all alloy elements are finely adjusted to the target value. The refined ladle is hoisted to the RH / VD station by a crane, and the molten steel is degassed under high vacuum for no less than 15 minutes. After degassed, the ladle is hoisted to the soft blowing argon position, and the soft blowing time is controlled at 15-30 minutes.

[0025] 2) Continuous casting: The ladle is hoisted to the continuous casting ladle turret. The entire casting process is protected during pouring, controlling the superheat of the molten steel in the tundish to 15-25℃. Automatic control of the crystallizer liquid level, electromagnetic stirring in the crystallizer, and end-point electromagnetic stirring are employed. The casting speed is controlled at 0.50-0.70 m / min to obtain qualified continuously cast billets. The billets are then slowly cooled in the pit at ≥620℃ or cooled to 400-550℃ through a slow cooling hood before being hot-sent to the rolling mill heating furnace.

[0026] 3) Rolling: After heating the continuous casting billet to 1150-1250℃, it is rolled. The total heating time is more than 5.0 hours, the soaking time is more than 1.5 hours, the initial rolling temperature is 1000-1100℃, and the final rolling temperature is ≤950℃. It is rolled into an intermediate billet of 200mm×200mm to 230mm×230mm. The initial rolling process adopts flame cleaning to melt and remove 1-4mm of metal from the surface of the billet at high temperature.

[0027] 4) Finished product rolling: After heating the intermediate billet to 1150-1200℃, it is rolled. The total heating time is more than 4.0 hours, the soaking time is more than 70 minutes, the initial rolling temperature is 1000-1050℃, and the final rolling temperature is ≤900℃, and it is rolled into Φ20mm-60mm round bars.

[0028] 5) Controlled Cooling: The cooling rate of the bars mainly depends on the distance between the bars on the cooling bed, i.e., the step distance of the bars on the cooling bed. To control residual stress and non-equilibrium structures caused by excessively rapid cooling, or poor flatness caused by slow cooling, the cooling step distance of the bars on the cooling bed is controlled at 500-800 mm, and the temperature drop is controlled at 10-20℃ / min to obtain better strength and toughness.

[0029] 6) Tempering heat treatment: The bars are subjected to stress-relief tempering in a continuous tempering furnace at a tempering temperature of 400-600℃ and a holding time of more than 2 hours. After being taken out of the furnace, they are air-cooled.

[0030] 7) Finishing: After tempering, the bars are straightened, machined, and polished, followed by surface and ultrasonic testing. The finished bars have a curvature of ≤0.5mm per meter, a surface defect depth of ≤0.10mm, and internal quality meets the Class A requirements of GB / T4162 standard.

[0031] 8) Inspection: Non-metallic inclusions for each furnace number shall be graded according to GB / T10561 standard and meet the following requirements: A (fine) ≤ 3.0, A (coarse) ≤ 2.0, B (fine) ≤ 2.0, B (coarse) ≤ 1.0, C (fine) ≤ 1.0, C (coarse) ≤ 1.0, D (fine) ≤ 2.0, D (coarse) ≤ 1.0; austenite grain size shall be graded according to GB / T6394 standard and meet the following requirement: ≥ 6.0; after tempering, the bars shall be sampled and inspected according to GB / T228 to ensure the following mechanical properties: yield strength ≥ 600 MPa, tensile strength ≥ 900 MPa, elongation ≥ 15%, reduction of area ≥ 33%, impact strength ≥ 50 J. The internal quality shall meet the GB / T4162 standard Grade A requirements. The microstructure of the entire cross-section of the bars shall be ferrite + pearlite, without non-equilibrium structures such as bainite and martensite.

[0032] Compared with the prior art, the advantages of the present invention are as follows: 1) This invention adopts a new composition design, starting from ensuring the requirements of the fan shaft, taking into account the instantaneous impact during start-up and shutdown and the fatigue performance during high-speed rotation of the fan shaft. While ensuring the mechanical properties of the steel, the proportion of each element is optimized, so that the invented steel has high strength, toughness and fatigue performance, while also having excellent machinability.

[0033] 2) The steel manufactured by this invention is subjected to tempering stress relief treatment after rolling, which can eliminate the internal stress generated by rolling and cooling, and at the same time avoid the superposition of internal stress during machining, which can cause the parts to bend.

[0034] 3) The steel manufactured by this invention employs a special straightening process. While ensuring the steel's straightness meets specifications, reasonable straightening process parameters are used to obtain steel with a high level of straightness. Furthermore, residual stress, straightening roller wear, and straightening machine power consumption are all reduced. High-straightness bars can reduce fan shaft installation errors and lower operating noise.

[0035] 4) The steel manufactured by this invention undergoes a machining process to remove surface defects and ensure the surface quality of the raw materials. During subsequent fan shaft machining, only localized machining of parts such as the bearing housing, assembly end, and spline groove is required. This high surface quality prevents premature fatigue failure of the fan shaft due to surface defects during operation.

[0036] 5) The steel manufactured by this invention can replace the original quenched and tempered steel in the production of fan shafts, reducing energy consumption and costs, obtaining a more stable and uniform microstructure, and achieving green and low-carbon manufacturing. Attached Figure Description

[0037] Figure 1 This is a tempered microstructure diagram of the Φ48mm round steel produced in Embodiment 1 of the present invention.

[0038] Figure 2 This is a diagram of non-metallic inclusions in Φ48mm round steel produced in Embodiment 1 of the present invention.

[0039] Figure 3 This is a tempered microstructure diagram of the Φ25mm round steel produced in Embodiment 2 of the present invention.

[0040] Figure 4 This is a diagram of non-metallic inclusions in Φ25mm round steel produced in Embodiment 2 of the present invention. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention. Example 1

[0042] This embodiment relates to a high-performance non-quenched and tempered steel bar for industrial fan shafts with a diameter of φ50mm. Its chemical element composition by mass percentage is as follows: C: 0.38%, Si: 0.43%, Mn: 1.46%, P: 0.015%, S: 0.038%, Cr: 0.15%, Ni: 0.12%, Mo: 0.02%, V: 0.09%, Cu: 0.05%, Al: 0.022%, H: 0.00006%, O: 0.0010%, Ti: 0.014%, with the balance being iron and unavoidable impurity elements.

[0043] The manufacturing process of the non-quenched and tempered steel for the fan shaft of this invention is as follows: The raw materials for smelting are prepared according to the chemical composition of the above-mentioned finished steel product and then subjected to electric furnace smelting, LF refining, and VD vacuum degassing in sequence to obtain molten steel with high purity. After the billet is cast, it is sent to the rolling mill for reheating when the surface temperature drops to 400-550℃. It is rolled into 230mm×230mm intermediate billets by a continuous rolling mill, with the final rolling temperature controlled at 890-930℃. The intermediate billets are then naturally cooled on a cooling bed and removed from the line. The intermediate billets are cold-charged into the furnace, with the soaking temperature controlled at 1150-1180℃. After the finished product is rolled, the steel bars are naturally cooled on a cooling bed. The bars are then subjected to stress-relief tempering in a continuous tempering furnace at a tempering temperature of 480-520℃ and a holding time of more than 2 hours before being removed from the furnace and air-cooled. The bars are then straightened, machined, and polished, followed by surface flaw detection and ultrasonic flaw detection. The finished bar stock has a bending degree of ≤0.5mm per meter, a surface defect depth of ≤0.10mm, and an internal quality that meets the Class A requirements of GB / T4162 standard.

[0044] The steel bars produced by the above manufacturing process have high strength and toughness. Their mechanical properties in the tempered state are detailed in Table 1. Microstructure and non-metallic inclusions are shown in Table 2. Figure 1 , Figure 2 .

[0045] Table 1. Tempered mechanical properties of steel from Example 1 Example 2

[0046] This embodiment relates to a high-performance non-quenched and tempered steel bar for industrial fan shafts with a diameter of φ50mm. Its chemical element composition by mass percentage is as follows: C: 0.42%, Si: 0.52%, Mn: 1.38%, P: 0.012%, S: 0.058%, Cr: 0.12%, Ni: 0.03%, Mo: 0.01%, V: 0.12%, Cu: 0.12%, Al: 0.028%, H: 0.00010%, O: 0.0007%, Ti: 0.022%, with the balance being iron and unavoidable impurity elements.

[0047] The manufacturing process of the steel for the fan shaft without tempering is as follows: The raw materials are prepared according to the chemical composition of the finished steel product and then subjected to electric furnace smelting, LF refining, and VD vacuum degassing to obtain high-purity molten steel. The cast billet is then placed in a slow cooling pit for slow cooling for ≥48 hours. After the surface temperature drops below 150℃, it is sent to a rolling mill for reheating. The billet is rolled into 230mm×230mm intermediate billets on a continuous rolling mill, with the final rolling temperature controlled at 850-900℃. The intermediate billets are then naturally cooled on a cooling bed before being removed from the line. The intermediate billets are then cold-charged into a furnace, with the soaking temperature controlled at 1170-1200℃. After finished product rolling, the steel bars are naturally cooled on a cooling bed. The bars undergo stress-relief tempering in a continuous tempering furnace at a tempering temperature of 450-480℃, with a holding time of at least 2 hours before being air-cooled. The bars are then straightened, machined, and polished, followed by surface flaw detection and ultrasonic flaw detection. The finished bar stock has a bending degree of ≤0.5mm per meter, a surface defect depth of ≤0.10mm, and an internal quality that meets the Class A requirements of GB / T4162 standard.

[0048] The steel bars produced by the above manufacturing process have high strength and toughness. Their mechanical properties in the tempered state are detailed in Table 2. Microstructure and non-metallic inclusions are shown in Table 2. Figure 3 , Figure 4 .

[0049] Table 2. Tempered mechanical properties of steel from Example 2

[0050] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-performance non-quenched tempered steel bar for a fan shaft, characterized by: The chemical composition is as follows: C: 0.37-0.43%, Si: 0.30-0.60%, Mn: 1.20-1.70%, P: ≤0.025%, S: 0.030-0.070%, Cr: ≤0.50%, Ni: 0.10-0.30%, Mo: ≤0.30%, Cu: ≤0.30%, Al: 0.010-0.030%, Ti: 0.005-0.025%, V: 0.05-0.15%, N: 0.0100-0.0200%, H: ≤0.0002%, O: ≤0.0015%, and the balance of Fe and inevitable impurities, and the microstructure of the rod is ferrite + pearlite.

2. The high performance non-quenched tempered steel bar for a fan shaft according to claim 1, characterized by: The specifications of the rod are Ф20-60mm, the bending degree is ≤0.5mm per meter, the surface defect depth is ≤0.10mm, and the internal quality meets the A-level requirement of GB / T4162 standard.

3. The high performance non-quenched tempered steel bar for a fan shaft according to claim 1, characterized by: The non-metallic inclusions meet the following requirements: A-fine ≤3.0 level, A-coarse ≤2.0 level, B-fine ≤2.0 level, B-coarse ≤1.0 level, C-fine ≤1.0 level, C-coarse ≤1.0 level, D-fine ≤2.0 level, and D-coarse ≤1.0 level according to GB / T10561 standard; the austenite grain size meets ≥6.0 level according to GB / T6394 standard; and the mechanical properties meet the following requirements: yield strength ≥600MPa, tensile strength ≥900MPa, elongation ≥15%, reduction of area ≥33%, and impact ≥50J after stress relief of the rod.

4. A method of manufacturing the steel bar of claim 1, characterized by: The method comprises the following steps: 1) smelting: first, smelting raw materials in turn by converter / electric furnace, LF furnace refining, and RH / VD furnace vacuum degassing to obtain high-purity molten steel; 2) continuous casting: the ladle is hoisted to the continuous casting ladle turret, the whole process of steel pouring is protected, the crystallizer liquid level automatic control, crystallizer electromagnetic stirring, and end electromagnetic stirring process are adopted, the casting blank is slowly cooled in the pit or cooled by a slow cooling cover to 400-550°C, and then is hot sent to the rolling heating furnace; 3) breakdown rolling: after the continuous casting blank is heated to 1150-1250°C, it is rolled, the total heating time is 5.0 hours or more, the soaking time is 1.5 hours or more, the breakdown rolling temperature is 1000-1100°C, the finish rolling temperature is ≤950°C, the blank is rolled into a 200mm×200mm to 230mm×230mm intermediate blank, flame cleaning is adopted in the breakdown process, high-temperature melting cutting is performed on the surface 1-4mm metal of the blank, and surface defects are removed; 4) finished product rolling: after the intermediate blank is heated to 1150-1200°C, it is rolled, the total heating time is 4.0 hours or more, the soaking time is 70 minutes or more, the breakdown rolling temperature is 1000-1050°C, the finish rolling temperature is ≤900°C, and the blank is rolled into a Ф20mm-60mm round rod; 5) controlled cooling: the cooling step distance of the rod on the cooling bed is controlled to be 500-800mm, and the temperature drop is controlled to be 10-20°C / min; 6) tempering heat treatment: the rod is tempered to remove stress.

5. The method of claim 4, wherein: The content of the control end point C in the initial refining furnace in step 1) is ≥0.15%, the molten steel temperature is ≥1650℃, the pre-melted slag, strong deoxidizer and part of the alloy are added in the tundish during the tapping process; the initial refining tundish is lifted by the crane to the LF refining furnace, the bottom blowing device is connected to blow in argon, the slagging material is added, the power is sent for reheating, the powder diffusion deoxidizer is added on the slag surface during the refining process to keep the slag white or gray, the refining time is greater than 30 minutes, all the alloy elements are adjusted to the target value by fine adjustment 5 minutes before the tapping; the tundish after the completion of the refining is lifted by the crane to the RH station, the degassing time of the molten steel under the high vacuum degree is not less than 15 minutes; after the degassing is completed, the tundish is lifted to the soft argon blowing position, and the soft blowing time is controlled to be 15-30 minutes.

6. The method of claim 4, wherein: In step 2), the molten steel superheat degree in the tundish during the continuous casting process is controlled to be 15-25℃, and the drawing speed is controlled to be 0.50-0.70 m / min.

7. The method of claim 4, wherein: In step 5), the tempering temperature is set to be 400-600℃, the holding time is more than 2h, and the natural cooling is performed.

8. The method of claim 4, wherein: In step 6), the bar after the completion of the tempering is subjected to the precision straightening and the rail grinding, and then the surface flaw detection and the ultrasonic flaw detection are performed.

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