Large-sized hot-rolled round steel for wind power pin shaft and manufacturing method thereof

Abstract

The application provides a large-size hot-rolled round steel for a wind power pin shaft and a manufacturing method thereof. A Si, Mn and Cr composite alloying process is adopted to ensure good mechanical properties of the steel and stabilize the grain size of the steel. Through stable control of a smelting process, the oxygen content in the steel is controlled to be less than 10 ppm, and the inclusion is less than or equal to 3.0 grade. A single-pass large reduction rolling process is adopted in the rolling process to ensure the compactness of the core structure of the steel, so that the round steel can meet the use requirements of high load and long service life of a wind power gear box.

Description

Technical Field

[0001] This invention relates to the field of composite steel technology, and in particular to a hot-rolled round steel bar for large-size wind turbine pin shafts and its manufacturing method. Background Technology

[0002] As a core transmission component of wind turbine generators, the technological development of wind turbine gearboxes is closely related to the overall progress of the wind power industry. Currently, wind power equipment is evolving towards larger scale, deeper waters, higher altitudes, and more diverse environmental adaptability, which places higher demands on the reliability, efficiency, power density, and service life of gearboxes. The industry as a whole is showing a significant trend towards informatization, digitalization, refinement, and intelligence, aiming to address the challenges posed by complex and harsh operating conditions at sea and in high mountains through full lifecycle data management and intelligent operation and maintenance.

[0003] In the technological evolution path, continuous material innovation and manufacturing process optimization are key to improving gearbox performance. Leading domestic and international companies are committed to upgrading gear steel materials and innovating production processes through joint research and development. For example, the use of specific continuous casting processes to replace traditional casting methods for producing high-performance gear steel billets has made progress in improving material purity and microstructure uniformity, providing a higher-quality material foundation for the industrial production of gearboxes. At the same time, industry manufacturers are continuously optimizing the meshing performance of gear pairs by integrating advanced gear design theories, new high-strength wear-resistant materials, and precision manufacturing and shaping technologies. These efforts not only help achieve the localization of key material supply but also significantly contribute to improving transmission efficiency, reducing vibration and noise, and extending service life, thereby improving the overall power density and operational reliability of gearboxes.

[0004] In the transmission chain of a wind turbine gearbox, planetary pins are one of the key fundamental load-bearing components. During operation, they endure complex alternating stresses caused by wind load fluctuations, while also overcoming additional loads caused by manufacturing and assembly errors, system deformation under stress, and temperature variations. These operating conditions directly affect the load-sharing performance and fatigue life of the planetary gear train; therefore, the overall performance of the pins is crucial.

[0005] Currently, planetary pins are generally manufactured using a forging process with specific medium-carbon alloy steel. Compared to rolling, this traditional forging method has several limitations, including relatively high production costs, limited material utilization, less efficient production pace, and the potential introduction of internal quality risks that are difficult to completely eliminate during the forging process. Furthermore, forging imposes certain constraints on the adaptability of parts to size and shape, often requiring more subsequent machining steps, and presents higher demands in terms of process quality control and environmental impact. Summary of the Invention

[0006] This application provides a hot-rolled round steel bar for large-size wind turbine pin shafts and its manufacturing method, in order to solve the problems that existing hot-rolled round steel bars cannot fully meet the requirements of wind turbine gearboxes and the complexity of existing manufacturing processes.

[0007] Firstly, this application provides a large-size hot-rolled round steel bar for wind turbine shafts, wherein the chemical composition of the hot-rolled round steel bar by weight percentage is: C: 0.38–0.45%, Si: 0.17–0.40%, Mn: 0.60–1.00%, Cr: 0.90–1.50%, Mo: ≤0.15%, Ni: ≤0.1%, P: ≤0.020%, S: ≤0.015%, Alt: 0.015–0.045%, Cu: ≤0.20%, Ti: ≤0.010%, O: ≤0.0010%, N: ≤0.01%, H: ≤0.0015%, with the remainder being Fe and impurities.

[0008] Preferably, C: 0.41-0.44%.

[0009] Preferably, 7.18≤1.7Si+4.4Mn+3.2Cr+8.32Mo≤7.8, where the element symbols represent the mass percentage content of the corresponding elements in the steel.

[0010] Secondly, this application also provides a method for manufacturing large-size hot-rolled round steel for wind turbine shafts, the method being used to manufacture any of the aforementioned hot-rolled round steel, the method comprising: Scrap steel and molten iron are used as raw materials for electric arc furnace smelting. The weight percentage of C at the final stage of the electric arc furnace is controlled to be ≥0.08%, and the weight percentage of P at the final stage is ≤0.010%. Pure aluminum ingots are added to the steel for deoxidation during the tapping process, and the weight percentage of Alt in the steel is controlled to be 0.03-0.04% after reaching the refining stage. The proportion of molten iron is 65-75%. After the molten steel reaches the refining stage, diffusion deoxidation is carried out using carbon raisers, silicon carbide, aluminum granules, etc. Aluminum wire is added in the early stage of refining to ensure that the AL content in the steel reaches 0.035-0.055% before tapping the steel and that the white slag in the refining process is maintained for ≥30 minutes. The slag amount is stabilized at 0.5-0.6 kg / ton of steel, and the steel after refining is slag-removed. After the slag removal is completed, the VD furnace is vacuum treated. During the VD furnace vacuum treatment, the VD furnace is kept below 67 Pa for ≥15 min, and the argon flow rate is controlled at 70±10 NL / min in a single line. After the vacuum is broken, an appropriate amount of calcium line is not introduced. After refining, the molten steel is subjected to continuous casting with full protection during the casting process, and the nitrogen increase in the molten steel during the continuous casting process is controlled to be ≤3ppm. After continuous casting, the molten steel is treated with a cold charging heating process to form a billet. The total heating time is controlled between 9.0 and 14.0 hours, of which the soaking time is ≥2.5 hours and the soaking temperature is controlled between 1200 and 1240℃. After the billet is processed, the molten steel is slowly cooled to obtain hot-rolled round steel.

[0011] Preferably, during the continuous casting process: The superheat of molten steel is controlled at 15-30℃, the specific water content is 0.08-0.13L / kg, the ratio of the foot roll section, the second cooling section 1, and the second cooling section 2 is 6:4:3, the electromagnetic stirring current of the crystallizer is ≤200A, and the electromagnetic stirring current at the end is ≥350A.

[0012] Preferably, during the billet processing: The steel is rolled using a rough rolling process followed by a finish rolling process. The initial rolling temperature is controlled at 1070–1180℃, the reduction per pass in the rough rolling is ≥50mm, and the final rolling temperature is controlled at 930–1050℃. The steel is kept warm before sawing.

[0013] Preferably, the VD furnace vacuum treatment includes: Vacuum degassing is performed using a VD furnace. After breaking the vacuum, the soft blowing time of molten steel is controlled to be 15-25 minutes.

[0014] Preferably, the slow cooling treatment includes controlling the temperature of the upper cooling bed at 650-750°C.

[0015] Preferably, the continuous casting process is carried out using a large round billet continuous casting machine, and the cross-section of the billet is φ800~1200mm.

[0016] Preferably, the rolling specification is φ270~350mm.

[0017] As can be seen from the above, this application provides a hot-rolled round steel bar for large-size wind turbine pin shafts and its manufacturing method. It adopts a Si, Mn, and Cr composite alloying process to ensure good mechanical properties of the steel and stabilize the grain size of the steel. Through stable control of the smelting process, the oxygen content in the steel is controlled below 10 ppm, and the comprehensive inclusion level is less than or equal to 3.0. The rolling process adopts a single-pass large reduction rolling process to ensure the compactness of the steel core structure, thereby enabling the round steel bar to meet the high load and long service life requirements of wind turbine gearboxes. Attached Figure Description

[0018] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart illustrating a method for manufacturing a large-size hot-rolled round steel bar for wind turbine shafts according to this application; Figure 2 This is a low-magnification schematic diagram of a hot-rolled round steel billet according to one embodiment; Figure 3 for Figure 2 A comparison diagram; Figure 4 This is a schematic diagram of the microstructure of hot-rolled round steel in one embodiment; Figure 5 for Figure 4 A comparison diagram; Figure 6 This is a schematic diagram of the average grain size of hot-rolled round steel in one embodiment; Figure 7 for Figure 6 A comparison diagram. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] It should be noted that the brief descriptions of terms in this application are only for the convenience of understanding the embodiments described below, and are not intended to limit the embodiments of this application. Unless otherwise stated, these terms should be understood in their ordinary and common meaning.

[0022] It should be noted that, in this application, the terms "exemplary" or "for example" are used to indicate that something is being described as an example, illustration, or illustration. Any embodiment or design described as "exemplary" or "for example" in this application should not be construed as being more preferred or advantageous than other embodiments or design solutions. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0023] The first aspect of this embodiment provides a large-size hot-rolled round steel bar for wind turbine shafts, wherein the chemical composition of the hot-rolled round steel bar by weight percentage is as follows: C: 0.38–0.45%, Si: 0.17–0.40%, Mn: 0.60–1.00%, Cr: 0.90–1.50%, Mo: ≤0.15%, Ni: ≤0.1%, P: ≤0.020%, S: ≤0.015%, Alt: 0.015–0.045%, Cu: ≤0.20%, Ti: ≤0.010%, O: ≤0.0010%, N: ≤0.01%, H: ≤0.0015%, with the remainder being Fe and impurities.

[0024] Specifically, in this embodiment, in order to ensure the hardenability of the J1.5 position, the preferred C composition is controlled at 0.41-0.44%.

[0025] Meanwhile, to ensure the hardenability of the J25 position, the other alloy compositions must meet the following requirements: 7.18≤1.7Si+4.4Mn+3.2Cr+8.32Mo≤7.8; In the formula, the element symbol represents the mass percentage of the corresponding element in the steel.

[0026] Meanwhile, after being processed by the user through turning, heat treatment, etc., the average grain size of the round steel is ≥7 grade and the range is <3 grade.

[0027] The properties of the above-mentioned hot-rolled round steel are as follows: Rm (tensile strength) ≥ 980 MPa; Rel (yield strength) ≥ 785 MPa; A (elongation after fracture) ≥9%; Z (reduction of area) ≥ 45%; Impact absorption energy / J (23℃±2℃)≥47 (KU2 (impact absorption energy measured using a U-notch (Charpy U-notch) specimen, in joules (J))); J represents the ambient temperature during the impact test; Impact absorption energy / J (-40℃±2℃) ≥ 21KV2 (Impact absorption energy measured using a V-notch (Charpy V-notch) specimen, in joules (J).

[0028] V-notches are more sensitive to stress concentration and are often used to evaluate a material's sensitivity to notches and its low-temperature toughness.

[0029] Hardenability, J1.5: 56-61 HRC, J5: 53-60 HRC, J25: 31-42 HRC; In steel, inclusions of type A are ≤1.5 grade, inclusions of type B are ≤1.0 grade, inclusions of type C are ≤0.5 grade, inclusions of type D are 1.0 grade, and inclusions of type Ds are ≤1.5 grade.

[0030] UT flaw detection meets the requirement that a single defect is ≤φ3mm and there are no continuous defects.

[0031] Figure 1 This is a flowchart illustrating a method for manufacturing a large-size hot-rolled round steel bar for wind turbine shafts according to this application.

[0032] See Figure 1 It can be seen that, on the other hand, this embodiment also provides a method for manufacturing hot-rolled round steel for large-size wind turbine shafts, the method comprising: S1, using scrap steel and molten iron as raw materials for electric furnace smelting, controlling the weight percentage of C at the electric furnace endpoint to be ≥0.08% and the weight percentage of P at the endpoint to be ≤0.010%, adding pure aluminum ingots to the steel for deoxidation during the tapping process, and controlling the weight percentage of Alt in the steel to be 0.03-0.04% after reaching the refining stage; the proportion of molten iron is 65-75%.

[0033] Specifically, in this embodiment, scrap steel and molten iron are used as raw materials, with the proportion of molten iron entering the furnace being 65-75%. The carbon content at the end of the electric furnace is controlled to be ≥0.08%, and the phosphorus content at the end of the furnace is controlled to be ≤0.010%. During the tapping process of the electric furnace, an appropriate amount of pure aluminum ingots are added to the steel for deoxidation, and the Alt content in the steel is controlled to be 0.03-0.04% after refining.

[0034] The method further includes: S2. After the molten steel reaches the refining stage, diffusion deoxidation is carried out using carbon raisers, silicon carbide, aluminum particles, etc. Aluminum wire is added in the early stage of refining to ensure that the AL content in the steel reaches 0.035-0.055% before refining and the white slag in the refining process is maintained for ≥30 minutes.

[0035] Specifically, in this embodiment, after the molten steel reaches the refining stage, diffusion deoxidation is carried out using carbon raisers, silicon carbide, aluminum particles, etc. Aluminum wire is added in the early stage of refining to ensure that the aluminum content in the steel reaches 0.035-0.055% before LF tapping, and the white slag in refining is maintained for ≥30 minutes.

[0036] The method further includes: S3, stabilize the slag amount at 0.5-0.6 kg / ton of steel, and perform slag removal on the refined steel. After slag removal, perform vacuum treatment in the VD furnace. During the vacuum treatment, the VD furnace is kept below 67 Pa for ≥15 min, and the argon flow rate is controlled at 70±10 NL / min in a single line. After breaking the vacuum, no appropriate amount of calcium line is introduced.

[0037] Specifically, in this embodiment, before VD, slag is removed and the slag amount is stabilized at 0.5-0.6 kg / ton of steel. During the VD process, the pressure is kept below 67 Pa for ≥15 min. At this time, the argon flow rate is controlled at 70±10 NL / min in a single channel. After breaking the air, an appropriate amount of calcium wire is fed in and the soft blowing time is ≥20 min.

[0038] The method further includes: S4, after refining, is a continuous casting process with full protection for the molten steel, and the nitrogen increase in the molten steel during the continuous casting process is controlled to be ≤3ppm.

[0039] Specifically, in this embodiment, the continuous casting process adopts a full-process protective casting process to control the nitrogen increase in molten steel during the continuous casting process to ≤3ppm; The superheat of molten steel is controlled at 15-30℃, the specific water content is 0.08-0.13L / kg, the ratio of the foot roll section, the second cooling section 1, and the second cooling section 2 is 6:4:3, the electromagnetic stirring current of the crystallizer is ≤200A, and the electromagnetic stirring current at the end is ≥350A.

[0040] The method further includes: S5, after continuous casting, uses cold charging heating process to process the molten steel into billets. The total heating time is controlled between 9.0 and 14.0 hours, of which the soaking time is ≥2.5 hours and the soaking temperature is controlled between 1200 and 1240℃. S6, after completing the billet treatment, the molten steel is subjected to slow cooling treatment to obtain hot-rolled round steel.

[0041] Specifically, in this embodiment, the billet adopts a cold charging heating process, and the total heating time is controlled between 9.0 and 14.0 hours, of which the soaking time is ≥2.5 hours and the soaking temperature is controlled between 1200 and 1240℃. The steel is rolled using a rough rolling process followed by a finish rolling process. The initial rolling temperature is controlled at 1070–1180℃, the reduction per pass in the rough rolling is ≥50mm, and the final rolling temperature is controlled at 930–1050℃. The steel is kept warm before sawing.

[0042] The continuous casting process uses a large round billet continuous casting machine, with the billet cross-section ranging from φ800 to 1200 mm.

[0043] The rolling specifications are φ270~350mm.

[0044] Carbon is one of the most important elements determining the properties of steel, directly affecting its strength, hardness, plasticity, and weldability. As a major strengthening element, carbon can form solid solutions (such as ferrite and austenite) and carbides (such as cementite Fe3C) with iron, significantly improving the strength and hardness of steel. Carbon can also improve the hardenability of steel and enhance the effectiveness of heat treatment.

[0045] Silicon is both an important alloying element in steel and a commonly used deoxidizer in the steelmaking process. It possesses strong deoxidizing capabilities, reducing oxide inclusions in steel and improving its quality. Silicon delays the transformation of austenite to ferrite and pearlite, shifting the C-curve (isothermal transformation curve) to the right and extending the stability of supercooled austenite, thus allowing for complete hardening even at slower cooling rates. When silicon coexists with manganese, their effect on hardenability is far greater than the simple superposition of individual elements, exhibiting a multiplicative relationship. For example, in silicon-manganese steel, the combined effect of the two significantly shifts the C-curve to the right, greatly enhancing hardenability.

[0046] Manganese is an indispensable element in steelmaking, possessing deoxidizing, desulfurizing, and strengthening functions. It improves the strength, hardness, and wear resistance of steel while maintaining good toughness. It significantly enhances the hardenability of steel, enabling uniform hardening of large-section parts.

[0047] Chromium is one of the most crucial alloying elements in steel, playing a central role in improving its mechanical properties, corrosion resistance, wear resistance, and thermal stability. Chromium atoms integrate into the iron matrix (such as ferrite or austenite), causing lattice distortion and hindering dislocation movement, thereby enhancing the material's yield strength and tensile strength. Chromium can slow down the decomposition rate of austenite, allowing steel to achieve a martensitic structure even under slower cooling conditions, making it suitable for the quenching treatment of large-section parts.

[0048] Furthermore, in some embodiments, the specific steps of LF refining in step S2 are as follows: using lime and refining pre-melted slag to make refining slag, controlling the Al2O3 content in the refining slag to be 25-32%, controlling the slag basicity R to be within the range of 5-7, using silicon carbide and carburizing agent for diffusion deoxidation during the refining process, the ∑Fe content in the final refining slag to be ≤0.5%, and the refining time of molten steel in LF to be 45-55 min.

[0049] For example: The steel for wind turbine shafts is initially smelted in a 120-ton electric arc furnace using high-quality self-circulating scrap steel, with 65-75% iron added. The final carbon content is ≥0.08%, and the final phosphorus content is ≤0.010%. The tapping temperature is 1628-1640℃. The electric arc furnace produces 115-120 tons of steel per tap. During tapping, 600 kg of lime, 400 kg of refining pre-melted slag, and 120 kg of pure aluminum blocks are added, along with alloys according to the required composition.

[0050] After the LF is in place, use carburizing agent and silicon carbide for diffusion deoxidation. The amount of silicon carbide is 80-110 kg, added in small amounts multiple times. After diffusion deoxidation is completed, add the alloy according to the target composition. The steel refining cycle is 50-60 minutes.

[0051] Before the molten steel enters the VD station, slag is removed, 1 / 3 of the slag is removed, and vacuum treatment is carried out. The pressure is maintained below 67 Pa for 15-18 minutes. After breaking the vacuum, 30-60 m of calcium wire is fed per furnace. After soft blowing for 3 minutes, a sample of the finished product is taken for analysis. The total soft blowing time after breaking the vacuum is ≥20 minutes. After the temperature is measured to be appropriate, the steel is loaded.

[0052] Table 1 shows the continuous casting process parameters of this invention; Table 2 shows the continuous casting process parameters of this invention; Table 3 shows the smelting analysis composition of this invention; Table 4 shows the gas testing results of this invention; Table 5 shows the mechanical property testing results of steel in this invention; Table 6 shows the hardenability testing results of steel in this invention; Table 7 shows the grain size rating of this invention; and Table 8 shows the flaw detection results of this invention.

[0053] Table 1 Parameters of the continuous casting process in the embodiments of the present invention

[0054] Table 2 Low-magnification rating of cast billets

[0055] Table 3 Parameters of the rolling process in the embodiments of the present invention

[0056] Table 4. Smelting analysis components in embodiments of the present invention

[0057] Table 5 Gas testing results of embodiments of the present invention (ppm)

[0058] Table 6. Test results of mechanical properties of steel in embodiments of the present invention.

[0059] Table 7. Hardenability test results of steel in the embodiments of the present invention / HRC

[0060] Table 8 Grain size rating of embodiments of the present invention

[0061] Table 9 Flaw Detection Pass Rate

[0062] As shown in Table 1, Table 2 and Figure 2 , Figure 3 As shown, the low-magnification inspection results of the cast billet indicate that the central porosity of Example 1 is grade 0.5 and the shrinkage cavity is grade 0, while the low-magnification central porosity of the cast billet in Comparative Example 1 is grade 1.0 and the shrinkage cavity is grade 0.5. Appropriate electromagnetic stirring parameters are beneficial for improving the core microstructure of the cast billet.

[0063] As shown in Table 2, Table 5 and Figure 4 , Figure 5 As shown, the reduction of area of ​​Example 1 was 58%, the impact energy at room temperature was 68-78 J, and the impact energy at low temperature was 22-28 J. The reduction of area of ​​Comparative Example 3 was 46%, the impact energy at room temperature was 34-40 J, and the impact energy at low temperature was 7-9 J. Furthermore, the original microstructure of the steel in Example 1 was significantly finer and more uniform than that in Comparative Example 3. This indicates that excessively high heating temperatures are detrimental to the control of the microstructure and properties of steel.

[0064] As shown in Tables 8 and 9, the flaw detection pass rates of Comparative Example 1 and Comparative Example 2 are 0% and 85%, respectively. According to Table 2, the soaking time of Comparative Example 1 does not meet the requirement of ≥2.5h, and the soaking time of Comparative Example 2 does not fully meet the requirement of ≥2.5h. At the same time, it can be seen from Table 2 that the low magnification quality of the billet of Comparative Example 1 is significantly worse than that of other examples and comparative examples, which leads to the flaw detection pass rate of 0% for Comparative Example 1. As for Comparative Example 2, since the soaking time does not fully meet the requirement of ≥2.5h, the improvement of the core quality of the billet during the rolling process is limited, resulting in the flaw detection pass rate not reaching 100%.

[0065] As shown in Table 6, the hardenability of Comparative Example 2 is significantly lower, with J1.5 at 53.6 HRC, J5 at 51.9 HRC, and J25 at 28.6 HRC. The main reason is that the controlled C, Mn, Cr and other components are lower, with 1.7Si+4.4Mn+3.2Cr+8.32Mo=6.95.

[0066] like Figure 6 , Figure 7 As shown in Table 7, the average grain size of Example 1 is grade 7, and the average grain size of Comparative Example 2 is grade 6.5. The Al content of Comparative Example 2 is too low, which is not conducive to the stable control of grain size.

[0067] The advantages of this embodiment include: Through a full-process inclusion control technology, the D-type inclusions in steel are stably controlled within level 1.0, and the Ds-type inclusions are controlled within level 1.5.

[0068] By ensuring stable control of the continuous casting and rolling processes, the mechanical properties of large-diameter rolled products with diameters of φ270~350mm were guaranteed to meet the requirements.

[0069] Through continuous casting and rolling process control, the core quality of ultra-large specification rolled materials with diameters of φ270~350mm is guaranteed, and the overall flaw detection quality of the rolled materials meets the requirement that a single defect is ≤φ3mm and there are no continuous defects.

[0070] For ease of explanation, the above description has been provided in conjunction with specific embodiments. However, the discussion in some embodiments is not intended to be exhaustive or to limit the embodiments to the specific forms disclosed above. Various modifications and variations can be obtained based on the above teachings. The selection and description of the above embodiments are for the purpose of better explaining the contents of this disclosure, thereby enabling those skilled in the art to better utilize the embodiments.

Claims

1. A type of hot-rolled round steel for large-size wind turbine shafts, characterized in that, The chemical composition of the hot-rolled round steel, by weight percentage, is as follows: C: 0.38–0.45%, Si: 0.17–0.40%, Mn: 0.60–1.00%, Cr: 0.90–1.50%, Mo: ≤0.15%, Ni: ≤0.1%, P: ≤0.020%, S: ≤0.015%, Alt: 0.015–0.045%, Cu: ≤0.20%, Ti: ≤0.010%, O: ≤0.0010%, N: ≤0.01%, H: ≤0.0015%, with the remainder being Fe and impurities.

2. The hot-rolled round steel for large-size wind turbine shafts according to claim 1, characterized in that, C：0.41～0.44%。 3. The hot-rolled round steel for large-size wind turbine shafts according to claim 1, characterized in that, 7.18≤1.7Si+4.4Mn+3.2Cr+8.32Mo≤7.8, where the element symbols represent the mass percentage of the corresponding element in the steel.

4. A method for manufacturing large-size hot-rolled round steel for wind turbine shafts, the method being used to manufacture the hot-rolled round steel according to any one of claims 1 to 3, characterized in that, The method includes: Scrap steel and molten iron are used as raw materials for electric arc furnace smelting. The weight percentage of C at the final stage of the electric arc furnace is controlled to be ≥0.08%, and the weight percentage of P at the final stage is ≤0.010%. Pure aluminum ingots are added to the steel for deoxidation during the tapping process, and the weight percentage of Alt in the steel is controlled to be 0.03-0.04% after reaching the refining stage. The proportion of molten iron is 65-75%. After the molten steel reaches the refining stage, diffusion deoxidation is carried out using carbon raisers, silicon carbide, aluminum granules, etc. Aluminum wire is added in the early stage of refining to ensure that the AL content in the steel reaches 0.035-0.055% before tapping the steel and that the white slag in the refining process is maintained for ≥30 minutes. The slag amount is stabilized at 0.5-0.6 kg / ton of steel, and the steel after refining is slag-removed. After the slag removal is completed, the VD furnace is vacuum treated. During the VD furnace vacuum treatment, the VD furnace is kept below 67 Pa for ≥15 min, and the argon flow rate is controlled at 70±10 NL / min in a single line. After the vacuum is broken, an appropriate amount of calcium line is not introduced. After refining, the molten steel is subjected to continuous casting with full protection during the casting process, and the nitrogen increase in the molten steel during the continuous casting process is controlled to be ≤3ppm. After continuous casting, the molten steel is treated with a cold charging heating process to form a billet. The total heating time is controlled between 9.0 and 14.0 hours, of which the soaking time is ≥2.5 hours and the soaking temperature is controlled between 1200 and 1240℃. After the billet is processed, the molten steel is slowly cooled to obtain hot-rolled round steel.

5. The method for manufacturing a large-size hot-rolled round steel bar for wind turbine shafts according to claim 4, characterized in that, During continuous casting: The superheat of molten steel is controlled at 15-30℃, the specific water content is 0.08-0.13L / kg, the ratio of the foot roll section, the second cooling section 1, and the second cooling section 2 is 6:4:3, the electromagnetic stirring current of the crystallizer is ≤200A, and the electromagnetic stirring current at the end is ≥350A.

6. The method for manufacturing a large-size hot-rolled round steel bar for wind turbine shafts according to claim 4, characterized in that, During the billet processing: The steel is rolled using a rough rolling process followed by a finish rolling process. The initial rolling temperature is controlled at 1070–1180℃, the reduction per pass in the rough rolling is ≥50mm, and the final rolling temperature is controlled at 930–1050℃. The steel is kept warm before sawing.

7. The method for manufacturing a large-size hot-rolled round steel bar for wind turbine shafts according to claim 4, characterized in that, The vacuum treatment in the VD furnace includes: Vacuum degassing is performed using a VD furnace. After breaking the vacuum, the soft blowing time of molten steel is controlled to be 15-25 minutes.

8. A method for manufacturing hot-rolled round steel for large-size wind turbine shafts according to claim 4, characterized in that, The slow cooling process includes controlling the temperature of the upper cooling bed at 650–750°C.

9. A method for manufacturing hot-rolled round steel for large-size wind turbine shafts according to claim 5, characterized in that, The continuous casting process uses a large round billet continuous casting machine for casting, and the cross-section of the billet is φ800~1200mm.

10. The method for manufacturing a large-size hot-rolled round steel bar for wind turbine shafts according to claim 6, characterized in that, The rolling specifications are φ270~350mm.

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