Process for improving yield strength of wind turbine main shaft

Abstract

This invention discloses a process for improving the yield strength of wind turbine main shafts. The chemical composition of the main shaft steel, by mass percentage, includes: C 0.38–0.45%, Si 0.20–0.40%, Mn 0.50–0.80%, Cr 0.90–1.20%, Mo 0.15–0.25%, Ni 0.40–0.60%, Cu≤0.05%, P≤0.010%, S≤0.002%, O≤0.0015%, N≤0.006%, H≤0.0002%, with the balance being Fe and unavoidable impurities. The tempering method includes normalizing pretreatment, austenitizing heating, pre-cooling treatment, PAG quenching liquid staged cooling, and tempering. During the tempering stage, a nitrogen-ethanol-dimethyl disulfide mixed atmosphere is pulsedly injected into the furnace. After the dimethyl disulfide decomposes at high temperature, it reacts with Mn in the steel to form a dispersed MnS nano-precipitate phase on the surface. This invention enables the main shaft steel to achieve a room temperature yield strength ≥750 MPa, an impact energy ≥150 J at -40℃, and excellent resistance to hydrogen-induced cracking, thus meeting the long-term use requirements of large-megawatt offshore wind turbines under complex operating conditions.

Description

Technical Field

[0001] This invention relates to the field of wind turbine technology, specifically to a process method for improving the yield strength of wind turbine main shafts. Background Technology

[0002] As the global wind power industry rapidly develops towards larger-scale and offshore applications, the single-unit capacity of wind turbine generators continues to increase. As the core load-bearing component that transmits wind loads from the blades, the wind turbine main shaft faces increasingly stringent operating conditions. During operation, the main shaft endures complex alternating loads and multiaxial stresses, which determine the reliability and service life of the wind turbine generator.

[0003] Domestic and international research and practice indicate that improving the performance of wind turbine main shafts primarily relies on optimizing the material system, controlling purity, and precisely regulating the heat treatment process. Leading international companies have accumulated extensive experience in ultra-high purity smelting, microalloying design, precipitation strengthening mechanisms, and process simulation. They reduce the content of harmful elements and minimize fatigue crack initiation caused by non-metallic inclusions through technologies such as vacuum degassing and electroslag remelting. Simultaneously, they add microalloying elements such as vanadium, niobium, and titanium, utilizing the carbonitride precipitation effect to achieve fine-grain strengthening and precipitation strengthening. At the microstructure level, they pursue high proportions of low bainite and tempered sorbite, resulting in strong and tough structures, and precisely control the phase transformation process through thermodynamic simulation. However, the segregation of micron-sized inclusions in the core region of the main shaft is a significant problem, with uneven inclusion density distribution along the axial direction. An increase in the number of inclusions reduces the material's elongation and tensile strength. Furthermore, 42CrMo4 steel wind turbine main shafts face a risk of cracking during quenching.

[0004] Offshore wind turbine main shaft bearings need to meet the stringent requirement of serving for more than 25 years under extreme conditions such as heavy loads, high and low temperatures, corrosion, and radiation. Therefore, there is still room for improvement in the existing hollow main shaft steel. Summary of the Invention

[0005] The object of the present invention is to overcome at least one of the deficiencies described in the prior art.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows.

[0007] In a first aspect, the present invention provides a process for improving the yield strength of wind turbine main shafts, wherein the chemical composition of the main shaft steel comprises, by mass percentage: C: 0.38–0.45%; Si: 0.20–0.40%; Mn: 0.50–0.80%; Cr: 0.90–1.20%; Mo: 0.15–0.25%; Ni: 0.40–0.60%; Cu: ≤0.05%; P: ≤0.010%; S: ≤0.002%; O: ≤0.0015%; N: ≤0.006%; H: ≤0.0002%; Balance: Fe and unavoidable impurities; After the main shaft steel is quenched and tempered, the austenite grain size in its microstructure reaches level 8 or above, and the non-metallic inclusion levels meet the following requirements: Class A fine series ≤ 0.5 level, Class D fine series ≤ 0.5 level, Class D coarse series ≤ 0.5 level, and all other types of inclusions ≤ 0 level. The chemical composition of the spindle steel was determined according to GB / T 4336-2016, wherein O, N, and H were determined according to GB / T 11261-2006, GB / T 20124-2006, and GB / T 223.82-2018, respectively; the grain size was determined according to GB / T 6394-2017; and the inclusions were evaluated according to GB / T 10561-2005.

[0008] As a preferred technical solution, the room temperature mechanical properties of the spindle steel, as measured according to GB / T 228.1-2021, meet the following requirements: The specified plastic elongation strength Rp0.2 ≥ 750 MPa, Tensile strength Rm ≥ 900 MPa Elongation at break (A) ≥ 16.0%, The reduction of area Z ≥ 58%.

[0009] As a preferred technical solution, the low-temperature impact toughness of the spindle steel, as measured according to GB / T 229-2020, meets the following requirements: Impact absorption energy at -20℃ ≥ 160 J Impact absorption energy at -30℃ ≥ 165 J Impact absorption energy at -40℃ ≥ 150 J.

[0010] As a preferred technical solution, the room temperature impact toughness of the spindle steel, as measured according to GB / T 229-2020, satisfies the following: Room temperature shock absorption energy ≥ 170 J.

[0011] As a preferred technical solution, the resistance to hydrogen-induced cracking of the spindle steel, as determined by GB / T 8650-2015, meets the following requirements: After immersion in solution A, the average crack length ratio (CLR) was ≤ 0.5%. The average crack thickness ratio (CTR) is ≤ 0.15%. The average crack sensitivity rate (CSR) is ≤ 0.001%.

[0012] As a preferred technical solution, the spindle steel, after quenching and tempering, achieves a metallographic structure that meets the Grade 1 sorbite structure specified in GB / T 13320-2007.

[0013] Secondly, the present invention provides a method for quenching and tempering the aforementioned spindle steel, comprising the following steps: S1. The forged spindle steel is pre-normalized, heated to 880-900℃ and held for 3-5 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 840~860℃ at a rate of ≤80℃ / h. The holding time is calculated based on the wall thickness of 1.5~2.0 min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 790-810℃, and the pre-cooling time is controlled at 8-15 min. S4. Quickly transfer the pre-cooled spindle steel into the quenching medium for staged cooling. S5. Temper the quenched spindle steel by heating it to 590-620℃ and holding it for 3.0-4.0 min / mm based on the effective thickness. After holding, air cool it to room temperature.

[0014] As a preferred technical solution, in step S4, the quenching medium is a PAG polymer quenching liquid, and the specific steps include: S41. The initial temperature of the quenching fluid is controlled at 20-30℃. After the workpiece is immersed, full-frequency stirring is turned on. The cooling time is controlled at 8-10 min per 100mm effective thickness. S42. Turn off the agitator and let the workpiece stay in the static quenching liquid for 3 to 5 minutes; S43. Restart the agitator and continue cooling until the workpiece surface temperature is ≤100℃. The total quenching time is controlled at 12-15 min per 100 mm of effective thickness. S44. During the quenching process, the temperature of the quenching liquid is maintained at 20-40℃ through a heat exchange system.

[0015] As a preferred technical solution, during step S42 when the agitator is turned off and the workpiece remains in the static quenching liquid, compressed air is synchronously injected into the quenching liquid. The air source pressure is controlled at 0.3 to 0.5 MPa, and the air supply time is 2 to 4 minutes, forming a microbubble layer on the surface of the workpiece.

[0016] As a preferred technical solution, in S5, when the furnace temperature rises to 400-450℃, a nitrogen-ethanol-dimethyl disulfide mixed atmosphere is pulsedly injected into the furnace. The volume ratio of nitrogen, ethanol and dimethyl disulfide is 100:10:1 to 100:15:2, the pulse cycle is 15 min / time, and each pulse lasts for 3-5 minutes.

[0017] The advantages and beneficial effects of this invention are as follows: Based on 42CrMo4, the low-temperature toughness is improved by increasing the Ni content, while the P, S, and gaseous elements are controlled at low levels, reducing the impact of non-metallic inclusions on the matrix. The tempering method includes normalizing pretreatment, austenitizing heating, pre-cooling treatment, PAG quenching liquid staged cooling, and tempering. During tempering heating, a nitrogen-ethanol-dimethyl disulfide mixed atmosphere is pulsedly injected into the furnace. Dimethyl disulfide decomposes at high temperature to produce active sulfur atoms, which react with Mn elements in the steel to form dispersed MnS nano-precipitates in situ in the surface micro-regions. These precipitates pin grain boundaries and dislocations, further improving yield strength, and simultaneously forming a self-lubricating layer on the surface.

[0018] The spindle steel of this invention exhibits the following room temperature mechanical properties: specified plastic elongation strength Rp0.2 ≥ 750 MPa, tensile strength Rm ≥ 900 MPa, elongation after fracture A ≥ 16.0%, and reduction of area Z ≥ 58%. It also demonstrates excellent resistance to hydrogen-induced cracking, meeting the requirements for long-term operation of large-megawatt offshore wind turbines under complex conditions. Detailed Implementation

[0019] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below. It is to be understood that the specific embodiments described herein are merely illustrative of this application and not intended to limit it. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0020] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0021] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly or implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0022] This invention provides a process and a tempering method for improving the yield strength of wind turbine main shafts. The chemical composition of the main shaft steel, by mass percentage, includes: C 0.38–0.45%, Si 0.20–0.40%, Mn 0.50–0.80%, Cr 0.90–1.20%, Mo 0.15–0.25%, Ni 0.40–0.60%, Cu ≤0.05%, P ≤0.010%, S ≤0.002%, O ≤0.0015%, N ≤0.006%, H ≤0.0002%, with the balance being Fe and unavoidable impurities. After tempering, the austenite grain size in the main shaft steel reaches grade 8 or higher, and the non-metallic inclusion levels meet the following requirements: Class A fine inclusions ≤0.5, Class D fine inclusions ≤0.5, Class D coarse inclusions ≤0.5, and all other inclusion types ≤0. The tempering method includes normalizing pretreatment, austenitizing heating, precooling treatment, PAG quenching liquid staged cooling and tempering steps, wherein in the tempering stage, a nitrogen-ethanol-dimethyl disulfide mixed atmosphere is pulsedly injected into the furnace.

[0023] This invention solves the technical problem of quenching cracking in large wind turbine main shafts when achieving a balance between high strength and excellent low-temperature toughness through the refined design and control of the component system. At the same time, it further improves the yield strength by generating nano-precipitates in situ.

[0024] The chemical composition design of the spindle steel described in this invention follows the principle of synergistic control of multi-element microalloying and ultra-high purity. Carbon is a fundamental element ensuring the strength and hardenability of the steel, and its content is controlled within the range of 0.38% to 0.45%. If the carbon content is too low, the matrix strength is insufficient, making it difficult to meet the yield strength requirement of ≥750 MPa; if the carbon content is too high, it will reduce weldability and toughness, and increase the tendency for quenching cracking. The chromium content is 0.90% to 1.20%. Chromium enhances the matrix strength through solid solution strengthening and simultaneously forms Cr3C2 type carbides with carbon, enhancing the wear resistance of the material. The molybdenum content is 0.15% to 0.25%. Molybdenum can significantly refine the grains, improve the hardenability and tempering stability of the steel, suppress temper brittleness, and enable the material to maintain excellent strength and toughness after high-temperature tempering. The nickel content is 0.40% to 0.60%. Its main purpose is to improve low-temperature toughness. By lowering the ductile-brittle transition temperature, the spindle can still maintain an impact absorption energy of over 150 J at -40℃. The silicon content is controlled between 0.20% and 0.40%. Silicon acts as a deoxidizer during steelmaking and also has a certain solid solution strengthening effect, but excessive content will deteriorate toughness. The manganese content is between 0.50% and 0.80%. Manganese can improve hardenability and form MnS with sulfur to improve machinability, but its content needs to be controlled to avoid segregation.

[0025] The key feature of this invention's composition system lies in the extreme control of impurity elements. Phosphorus and sulfur, as harmful elements in steel, cause cold brittleness and hot brittleness, respectively. This invention controls P to ≤0.010% and S to ≤0.002%, lower than the allowable content of conventional 42CrMo4 steel, reducing the cutting effect of non-metallic inclusions on the matrix. Controlling the gaseous elements oxygen, nitrogen, and hydrogen reduces the formation of oxide inclusions, which are the main initiation sources of fatigue cracks. A nitrogen content of ≤0.006% avoids excessive aluminum nitride precipitation leading to decreased toughness, and simultaneously forms fine AlN particles with residual aluminum in the steel, pinning grain boundaries. Studies have shown that when the aluminum content is 0.015–0.025% and the aluminum-nitrogen ratio is ≥3, the average grain size of the principal axis can reach 7.5–8.0 grades. A hydrogen content of ≤0.0002% effectively prevents white spot defects and hydrogen-induced delayed cracking, especially in large forgings where the harmful effects of hydrogen are more pronounced. Copper is controlled to ≤0.05% to avoid hot brittleness.

[0026] After tempering and quenching with the above-mentioned composition system, the microstructure of the spindle steel exhibits the following characteristics: the austenite grain size reaches level 8 or higher, indicating grain refinement. According to the theory of fine-grained strengthening, the finer the grains, the larger the grain boundary area, the greater the resistance to dislocation slip, and the higher the yield strength. Simultaneously, fine-grained structures can also improve toughness, making it an effective way to achieve a balance between strength and toughness. Regarding the control of non-metallic inclusions, the fine inclusions in Class A (sulfides) are ≤0.5 level, the fine and coarse inclusions in Class D (spherical oxides) are both ≤0.5 level, and the inclusions in Class B (alumina), Class C (silicates), and Class DS (single-particle spherical inclusions) are all 0 level.

[0027] The tempering method of the present invention includes the following steps: S1, pre-normalizing the forged spindle steel, heating it to 880-900℃ and holding it for 3-5 hours, then air-cooling it to room temperature; S2, loading the spindle steel into a heat treatment furnace, heating it to 840-860℃ at a rate of ≤80℃ / h, with the holding time calculated based on a wall thickness of 1.5-2.0 min / mm; S3, after the holding period, removing the spindle steel from the furnace and pre-cooling it in air, with the pre-cooling termination temperature at 790-810℃ and the pre-cooling time controlled at 8-15 min; S4, rapidly transferring the pre-cooled spindle steel into a quenching medium for graded cooling; S5, tempering the quenched spindle steel, heating it to 590-620℃, with the holding time calculated based on an effective thickness of 3.0-4.0 min / mm, and air-cooling it to room temperature after the holding period.

[0028] In the above-mentioned tempering method, the purpose of normalizing pretreatment is to refine the coarse grains after forging, eliminate forging stress, improve the uniformity of the microstructure, and provide a good microstructure basis for subsequent quenching. Heating to 880–900℃ and holding for 3–5 hours ensures complete austenitization while avoiding overheating that could lead to grain coarsening. Austenitization heating uses a slow heating rate of ≤80℃ / h, mainly considering the large cross-sectional dimensions of large forgings; rapid heating would generate significant thermal stress and could even lead to cracking. The selection of an austenitizing temperature of 840–860℃ is based on the fact that the austenite transformation temperature of 42CrMo4 steel is approximately 758℃; 840–860℃ ensures complete austenitization while avoiding excessively high temperatures that could lead to grain coarsening and quenching deformation. The holding time is calculated based on a wall thickness of 1.5–2.0 min / mm to ensure the core reaches the set temperature and completes the homogenization of alloying elements.

[0029] After austenitizing and heat treatment, the spindle is removed from the furnace and pre-cooled in air to 790–810°C for 8–15 minutes. The mechanism of pre-cooling is that when large forgings are directly quenched, there is a huge temperature difference between the surface and the core, leading to a sharp increase in thermal stress and making them prone to quenching cracks. Air pre-cooling appropriately lowers the surface temperature of the workpiece, reducing the initial temperature difference when entering the quenching medium, thereby reducing thermal stress. Lowering the quenching temperature from 860°C to 810°C effectively reduces the tendency to crack. The pre-cooling termination temperature of 790–810°C is still higher than the austenite transformation temperature of 42CrMo4 steel, ensuring that proeutectoid ferrite does not precipitate during pre-cooling, thus avoiding affecting the microstructure and properties after quenching.

[0030] The quenching and cooling process utilizes PAG polymer quenching fluid for staged cooling. PAG quenching fluid is a water-soluble polymer quenching medium with cooling characteristics between water and oil; different cooling rates can be obtained by adjusting the concentration. This invention provides refined control over the PAG quenching and cooling process, specifically including: S41, the initial temperature of the quenching fluid is controlled at 20–30°C; after immersion of the workpiece, full-frequency stirring is activated, and the cooling time is controlled at 8–10 min per 100 mm effective thickness; S42, the stirrer is turned off, allowing the workpiece to remain in the static quenching fluid for 3–5 min; S43, the stirrer is restarted, and cooling continues until the workpiece surface temperature is ≤100°C, with the total quenching time controlled at 12–15 min per 100 mm effective thickness; S44, during the quenching process, the quenching fluid temperature is maintained at 20–40°C through a heat exchange system.

[0031] In the aforementioned staged cooling design, the first stage (S41) involves rapid cooling with full-frequency stirring, allowing the workpiece surface to quickly pass through the pearlite transformation zone and obtain martensite, ensuring the depth of the hardened layer. Studies have shown that when 42CrMo4 steel uses water or 3-5% PAG as the quenching medium, the hardenability of the material can reach over 97%. The second stage (S42) involves turning off the stirrer, allowing the workpiece to remain in the static liquid. The residual heat in the core is used to self-temper the martensite already formed on the surface, reducing the brittleness of the surface martensite and alleviating the thermal stress between the core and the surface. After the stirring is turned off, a stable polymer film forms on the workpiece surface, further slowing the cooling rate. The third stage (S43) involves restarting the stirring to continue cooling to ≤100℃, ensuring the martensitic transformation is fully completed. Throughout the quenching process, the liquid temperature is maintained between 20-40℃ through a heat exchange system to prevent a decrease in cooling capacity due to increased liquid temperature.

[0032] In some embodiments, to further reduce quenching distortion of large wind turbine main shafts, during step S42 when the agitator is turned off and the workpiece remains in the static quenching liquid, compressed air is simultaneously injected into the quenching liquid. The air source pressure is controlled at 0.3–0.5 MPa, and the air supply time is 2–4 min, forming a microbubble layer on the workpiece surface. The mechanism of injecting compressed air is that microbubbles adhere to the workpiece surface, forming a local air film, further reducing the cooling rate in that area, thereby adjusting the cooling uniformity of different parts of the workpiece and reducing distortion caused by uneven cooling.

[0033] In the tempering process, the tempering temperature is raised to 590–620℃, and the holding time is calculated based on an effective thickness of 3.0–4.0 min / mm. The purpose of tempering is to eliminate quenching stress, adjust the microstructure to obtain tempered sorbite, and achieve an optimal balance between strength and toughness. After quenching, 42CrMo4 steel yields approximately 10% martensite, with a maximum hardness of 50.9 HRC. After high-temperature tempering, the martensite transforms into tempered sorbite, and the hardness decreases to approximately 30 HRC. This invention introduces atmosphere treatment during the tempering process: when the furnace temperature reaches 400–450℃, a nitrogen-ethanol-dimethyl disulfide mixed atmosphere is pulsed into the furnace. The volume ratio of nitrogen, ethanol, and dimethyl disulfide is 100:10:1 to 100:15:2, with a pulse cycle of 15 min / time, each lasting 3–5 min.

[0034] Dimethyl disulfide undergoes thermal decomposition at high temperatures. This process occurs through homolytic cleavage of CS bonds, generating sulfur-containing free radicals. Within the temperature range of 400–450°C, dimethyl disulfide decomposes to produce reactive sulfur atoms, which react in situ with Mn elements in the steel matrix, forming a dispersed MnS nanoprecipitate phase in the surface micro-region. The MnS nanoprecipitate phase is typically nanoscale in size, distributed at grain boundaries and within grains. By pinning grain boundaries and dislocations, it can further improve yield strength. While sulfur is conventionally considered a harmful element, this invention generates nanoscale MnS in situ on the surface through the decomposition of dimethyl disulfide, avoiding the formation of coarse MnS inclusions in traditional steelmaking processes, thus turning a potential hazard into a beneficial one. Simultaneously, the MnS layer formed on the surface possesses self-lubricating properties, improving the frictional performance of the spindle and bearing mating surfaces. Nitrogen is used as the carrier gas, and ethanol decomposition produces a reducing atmosphere, preventing oxidation of the workpiece surface. The pulsed injection design avoids excessive reaction due to excessive atmosphere concentration, ensuring a uniform distribution of the MnS precipitate phase.

[0035] In some embodiments, to obtain better low-temperature impact toughness, the room-temperature mechanical properties of the spindle steel, as determined according to GB / T 228.1-2021, meet the following requirements: specified plastic elongation strength Rp0.2 ≥ 750 MPa, tensile strength Rm ≥ 900 MPa, elongation after fracture A ≥ 16.0%, and reduction of area Z ≥ 58%.

[0036] In some embodiments, the low-temperature impact toughness of the main shaft steel, as measured according to GB / T 229-2020, meets the following requirements: impact absorption energy ≥160 J at -20℃, impact absorption energy ≥165 J at -30℃, and impact absorption energy ≥150 J at -40℃. Large offshore wind turbine main shafts need to operate in a -40℃ low-temperature environment, and conventional 42CrMo4 steel typically struggles to consistently achieve an impact energy exceeding 150 J at this temperature.

[0037] In some embodiments, the hydrogen-induced cracking resistance of the spindle steel, as determined by GB / T 8650-2015, meets the following requirements: after immersion in solution A, the average crack length ratio (CLR) is ≤0.5%, the average crack thickness ratio (CTR) is ≤0.15%, and the average crack sensitivity ratio (CSR) is ≤0.001%. This excellent resistance to hydrogen-induced cracking stems from: ultra-low P and S content reducing the formation of inclusions such as MnS, thus preventing hydrogen from accumulating at the inclusion interface and causing cracking; uniform and fine microstructure reducing local hydrogen segregation; and the MnS nano-precipitated layer formed on the surface, which to some extent blocks hydrogen penetration. Solution A can be, as stated in the standard, a 5.0% (w / w) sodium chloride aqueous solution or a 0.5% (w / w) acetic acid solution.

[0038] In some embodiments, after quenching and tempering, the microstructure of the spindle steel reaches the Grade 1 sorbite structure specified in GB / T 13320-2007. The Grade 1 sorbite structure is characterized by uniform and fine tempered sorbite, without free ferrite, network structure, or coarse carbides, which is the ideal microstructure obtained by quenching and tempering.

[0039] Through the synergistic effect of the above-mentioned technical solutions, this invention enables the wind turbine main shaft steel to achieve the following comprehensive performance: room temperature yield strength ≥750 MPa, low-temperature impact energy ≥150 J at -40℃, and excellent resistance to hydrogen-induced cracking. The achievement of these performance indicators relies on the rationality of the composition design, strict control of purity, and refined regulation of the heat treatment process.

[0040] The present invention will be further described below through specific embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0041] Example 1 A tempering method for improving the yield strength of wind turbine main shafts is disclosed, which is used to treat the main shaft steel. The chemical composition of the main shaft steel, by mass percentage, includes: C 0.41%, Si 0.28%, Mn 0.65%, Cr 1.05%, Mo 0.19%, Ni 0.48%, Cu 0.03%, P 0.008%, S 0.0015%, O 0.0012%, N 0.0045%, H 0.00015%, with the balance being Fe and unavoidable impurities.

[0042] Includes the following steps: S1. The forged spindle steel is pre-normalized, heated to 890℃ and held for 4 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 850℃ at a rate of 60℃ / h. The holding time is calculated based on a wall thickness of 1.8min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 800℃ and the pre-cooling time is controlled within 12 minutes. S4. Quickly transfer the pre-cooled spindle steel into the quenching medium for graded cooling. The quenching medium is PAG polymer quenching fluid. Specific steps include: S41. Control the initial temperature of the quenching fluid at 25℃. After immersing the workpiece, start full-frequency stirring. Control the cooling time at 9 min per 100mm effective thickness. S42. Turn off the stirrer and let the workpiece stay in the static quenching fluid for 4 min. S43. Turn the stirrer back on and continue cooling until the workpiece surface temperature is ≤100℃. Control the total quenching time at 13 min per 100mm effective thickness. S44. During the quenching process, maintain the quenching fluid temperature at 25~35℃ through the heat exchange system. S5. Temper the quenched spindle steel by heating it to 605℃ and holding it for 3.5 min / mm of effective thickness. After holding, air cool it to room temperature.

[0043] After quenching and tempering, the austenite grain size in the microstructure of the main shaft steel reaches grade 8.5, and the non-metallic inclusions meet the following grades: grade 0.5 for fine A series, grade 0.5 for fine D series, grade 0.5 for coarse D series, and grade 0 for all other types of inclusions. The metallographic structure reaches the grade 1 sorbite structure specified in GB / T 13320-2007.

[0044] The spindle steel prepared in this embodiment meets the following room temperature mechanical properties as measured by GB / T 228.1-2021: specified plastic elongation strength Rp 0.2785 MPa, tensile strength Rm 945 MPa, elongation after fracture A 17.5%, and reduction of area Z 61%. The low-temperature impact toughness, measured by GB / T 229-2020, meets the following: impact energy absorbed at -20℃ 178 J, -30℃ 172 J, -40℃ 165 J, and room temperature 192 J. The hydrogen-induced cracking resistance, measured by GB / T 8650-2015, meets the following: average crack length ratio (CLR) 0.35%, average crack thickness ratio (CTR) 0.10%, and average crack sensitivity ratio (CSR) 0.0005%.

[0045] Example 2 A tempering method for improving the yield strength of wind turbine main shafts is used to treat the main shaft steel, wherein the chemical composition of the main shaft steel, by mass percentage, includes: C 0.42%, Si 0.32%, Mn 0.70%, Cr 1.12%, Mo 0.21%, Ni 0.52%, Cu 0.04%, P 0.009%, S 0.0018%, O 0.0013%, N 0.0050%, H 0.00018%, with the balance being Fe and unavoidable impurities.

[0046] Includes the following steps: S1. The forged spindle steel is pre-normalized, heated to 885℃ and held for 4.5 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 845℃ at a rate of 70℃ / h. The holding time is calculated based on a wall thickness of 1.7min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 805℃ and the pre-cooling time is controlled within 10 minutes. S4. The pre-cooled spindle steel is quickly transferred to the quenching medium for graded cooling. The quenching medium is PAG polymer quenching liquid. The specific steps include: S41. The initial temperature of the quenching liquid is controlled at 22℃. After the workpiece is immersed, full-frequency stirring is turned on. The cooling time is controlled at 8.5 min per 100mm effective thickness; S42. The stirrer is turned off, and the workpiece is kept in the static quenching liquid for 3.5 min. During this period, compressed air is injected into the quenching liquid simultaneously. The air source pressure is controlled at 0.4 MPa, and the air circulation time is 3 min, forming a microbubble layer on the surface of the workpiece; S43. The stirrer is turned on again, and the workpiece surface temperature is continued to be cooled until the temperature is ≤100℃. The total quenching time is controlled at 13.5 min per 100mm effective thickness; S44. During the quenching process, the temperature of the quenching liquid is maintained at 22~32℃ through the heat exchange system. S5. Temper the quenched spindle steel by heating it to 610℃ and holding it for 3.2 min / mm of effective thickness. When the furnace temperature reaches 420℃, inject a nitrogen-ethanol-dimethyl disulfide mixed atmosphere into the furnace in a pulsed manner. The volume ratio of nitrogen, ethanol and dimethyl disulfide is 100:12:1.5. The pulse cycle is 15 min / time, and each pulse lasts for 4 minutes. After holding, air cool to room temperature.

[0047] After quenching and tempering, the austenite grain size in the microstructure of the main shaft steel reaches grade 8.5, and the non-metallic inclusions meet the following grades: grade 0.5 for fine A series, grade 0.5 for fine D series, grade 0.5 for coarse D series, and grade 0 for all other types of inclusions. The metallographic structure reaches the grade 1 sorbite structure specified in GB / T 13320-2007.

[0048] The spindle steel prepared in this embodiment meets the following room temperature mechanical properties as measured by GB / T 228.1-2021: specified plastic elongation strength Rp 0.2792 MPa, tensile strength Rm 955 MPa, elongation after fracture A 17.2%, and reduction of area Z 60%. The low-temperature impact toughness, measured by GB / T 229-2020, meets the following: impact energy absorbed at -20℃ 182 J, -30℃ 175 J, -40℃ 168 J, and room temperature impact energy 196 J. The hydrogen-induced cracking resistance, measured by GB / T 8650-2015, meets the following: average crack length ratio (CLR) 0.32%, average crack thickness ratio (CTR) 0.09%, and average crack sensitivity ratio (CSR) 0.0004%.

[0049] Example 3 A tempering method for improving the yield strength of wind turbine main shafts is disclosed, which is used to treat the main shaft steel. The chemical composition of the main shaft steel, by mass percentage, includes: C 0.39%, Si 0.25%, Mn 0.60%, Cr 0.98%, Mo 0.17%, Ni 0.45%, Cu 0.02%, P 0.006%, S 0.0012%, O 0.0010%, N 0.0040%, H 0.00012%, with the balance being Fe and unavoidable impurities.

[0050] Includes the following steps: S1. The forged spindle steel is pre-normalized, heated to 895℃ and held for 3.5 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 855℃ at a rate of 50℃ / h. The holding time is calculated based on a wall thickness of 1.9min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 795℃ and the pre-cooling time is controlled within 14 min. S4. Quickly transfer the pre-cooled spindle steel into the quenching medium for graded cooling. The quenching medium is PAG polymer quenching fluid. The specific steps include: S41. Control the initial temperature of the quenching fluid at 28℃. After immersing the workpiece, start full-frequency stirring. Control the cooling time at 9.5 min per 100 mm effective thickness. S42. Turn off the stirrer and let the workpiece stay in the static quenching fluid for 4.5 min. S43. Turn the stirrer back on and continue cooling until the workpiece surface temperature is ≤100℃. Control the total quenching time at 14 min per 100 mm effective thickness. S44. During the quenching process, maintain the quenching fluid temperature at 28~38℃ through the heat exchange system. S5. Temper the quenched spindle steel by heating it to 595℃. The holding time is calculated based on an effective thickness of 3.8 min / mm. When the furnace temperature reaches 430℃, inject a nitrogen-ethanol-dimethyl disulfide mixed atmosphere into the furnace in a pulsed manner. The volume ratio of nitrogen, ethanol and dimethyl disulfide is 100:13:1.8. The pulse cycle is 15 min / time, and each pulse lasts for 3.5 min. After the holding time is completed, air cool to room temperature.

[0051] After quenching and tempering, the austenite grain size in the microstructure of the main shaft steel reaches grade 8.0, and the non-metallic inclusions meet the following grades: Grade A fine series 0.5, Grade D fine series 0.5, Grade D coarse series 0.5, and all other types of inclusions are grade 0. The metallographic structure reaches the grade 1 sorbite structure specified in GB / T 13320-2007.

[0052] The spindle steel prepared in this embodiment meets the following room temperature mechanical properties as measured by GB / T 228.1-2021: specified plastic elongation strength Rp 0.2 775 MPa, tensile strength Rm 935 MPa, elongation after fracture A 17.8%, and reduction of area Z 62%. The low-temperature impact toughness, measured by GB / T 229-2020, meets the following: impact energy absorbed at -20℃ 175 J, -30℃ 168 J, -40℃ 160 J, and room temperature 188 J. The resistance to hydrogen-induced cracking, measured by GB / T 8650-2015, meets the following: average crack length ratio (CLR) 0.38%, average crack thickness ratio (CTR) 0.12%, and average crack sensitivity ratio (CSR) 0.0006%.

[0053] Example 4 A tempering method for improving the yield strength of wind turbine main shafts is disclosed, which is used to treat the main shaft steel. The chemical composition of the main shaft steel, by mass percentage, includes: C 0.43%, Si 0.35%, Mn 0.75%, Cr 1.15%, Mo 0.23%, Ni 0.55%, Cu 0.04%, P 0.009%, S 0.0019%, O 0.0014%, N 0.0055%, H 0.00019%, with the balance being Fe and unavoidable impurities.

[0054] Includes the following steps: S1. The forged spindle steel is pre-normalized, heated to 880℃ and held for 5 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 840℃ at a rate of 75℃ / h. The holding time is calculated based on a wall thickness of 1.6min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 810℃ and the pre-cooling time is controlled within 9 minutes. S4. The pre-cooled spindle steel is quickly transferred to the quenching medium for graded cooling. The quenching medium is PAG polymer quenching liquid. The specific steps include: S41. The initial temperature of the quenching liquid is controlled at 26℃. After the workpiece is immersed, full-frequency stirring is turned on. The cooling time is controlled at 8.5 min per 100mm effective thickness; S42. The stirrer is turned off, and the workpiece is kept in the static quenching liquid for 3 min. During this period, compressed air is injected into the quenching liquid simultaneously. The air source pressure is controlled at 0.45 MPa, and the air circulation time is 2.5 min, forming a microbubble layer on the surface of the workpiece; S43. The stirrer is turned on again, and the workpiece surface temperature is continued to be cooled until the temperature is ≤100℃. The total quenching time is controlled at 12.5 min per 100mm effective thickness; S44. During the quenching process, the temperature of the quenching liquid is maintained at 26~36℃ through the heat exchange system. S5. Temper the quenched spindle steel by heating it to 615℃. The holding time is calculated based on an effective thickness of 3.3 min / mm. When the furnace temperature reaches 410℃, inject a nitrogen-ethanol-dimethyl disulfide mixed atmosphere into the furnace in a pulsed manner. The volume ratio of nitrogen, ethanol and dimethyl disulfide is 100:11:1.2. The pulse cycle is 15 min / time, and each pulse lasts for 4.5 min. After the holding time is completed, air cool to room temperature.

[0055] After quenching and tempering, the austenite grain size in the microstructure of the main shaft steel reaches grade 8.5, and the non-metallic inclusions meet the following grades: grade 0.5 for fine A series, grade 0.5 for fine D series, grade 0.5 for coarse D series, and grade 0 for all other types of inclusions. The metallographic structure reaches the grade 1 sorbite structure specified in GB / T 13320-2007.

[0056] The spindle steel prepared in this embodiment meets the following room temperature mechanical properties as measured by GB / T 228.1-2021: specified plastic elongation strength Rp 0.2788 MPa, tensile strength Rm 950 MPa, elongation after fracture A 17.0%, and reduction of area Z 59%. The low-temperature impact toughness, measured by GB / T 229-2020, meets the following: impact energy absorbed at -20℃ 176 J, -30℃ 170 J, -40℃ 162 J, and room temperature impact energy 190 J. The hydrogen-induced cracking resistance, measured by GB / T 8650-2015, meets the following: average crack length ratio (CLR) 0.34%, average crack thickness ratio (CTR) 0.11%, and average crack sensitivity ratio (CSR) 0.0005%.

[0057] Example 5 A tempering method for improving the yield strength of wind turbine main shafts is disclosed, which is used to treat the main shaft steel. The chemical composition of the main shaft steel, by mass percentage, includes: C 0.40%, Si 0.30%, Mn 0.68%, Cr 1.08%, Mo 0.20%, Ni 0.50%, Cu 0.03%, P 0.007%, S 0.0014%, O 0.0011%, N 0.0048%, H 0.00014%, with the balance being Fe and unavoidable impurities.

[0058] Includes the following steps: S1. The forged spindle steel is pre-normalized, heated to 892℃ and held for 4 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 852℃ at a rate of 65℃ / h. The holding time is calculated based on a wall thickness of 1.8min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 802℃ and the pre-cooling time is controlled within 13 minutes. S4. The pre-cooled spindle steel is quickly transferred to the quenching medium for graded cooling. The quenching medium is PAG polymer quenching liquid. The specific steps include: S41. The initial temperature of the quenching liquid is controlled at 24℃. After the workpiece is immersed, full-frequency stirring is turned on. The cooling time is controlled at 9.2 min / 100mm effective thickness; S42. The stirrer is turned off, and the workpiece is kept in the static quenching liquid for 4 min. During this period, compressed air is injected into the quenching liquid simultaneously. The air source pressure is controlled at 0.35 MPa, and the air circulation time is 3.5 min, forming a microbubble layer on the surface of the workpiece; S43. The stirrer is turned on again, and the workpiece surface temperature is continued to be cooled until the temperature of the workpiece is ≤100℃. The total quenching time is controlled at 13.8 min / 100mm effective thickness; S44. During the quenching process, the temperature of the quenching liquid is maintained at 24~34℃ through the heat exchange system. S5. Temper the quenched spindle steel by heating it to 600℃ and holding it for 3.6 min / mm of effective thickness. When the furnace temperature reaches 440℃, inject a nitrogen-ethanol-dimethyl disulfide mixed atmosphere into the furnace in a pulsed manner. The volume ratio of nitrogen, ethanol and dimethyl disulfide is 100:14:1.9. The pulse cycle is 15 min / time, and each pulse lasts for 3 minutes. After holding, air cool to room temperature.

[0059] After quenching and tempering, the austenite grain size in the microstructure of the main shaft steel reaches grade 8.0, and the non-metallic inclusions meet the following grades: Grade A fine series 0.5, Grade D fine series 0.5, Grade D coarse series 0.5, and all other types of inclusions are grade 0. The metallographic structure reaches the grade 1 sorbite structure specified in GB / T 13320-2007.

[0060] The spindle steel prepared in this embodiment meets the following room temperature mechanical properties as measured by GB / T 228.1-2021: specified plastic elongation strength Rp 0.2780 MPa, tensile strength Rm 940 MPa, elongation after fracture A 17.3%, and reduction of area Z 60.5%. The low-temperature impact toughness, measured by GB / T 229-2020, meets the following: impact energy absorbed at -20℃ 177 J, -30℃ 171 J, -40℃ 163 J, and room temperature 191 J. The hydrogen-induced cracking resistance, measured by GB / T 8650-2015, meets the following: average crack length ratio (CLR) 0.36%, average crack thickness ratio (CTR) 0.11%, and average crack sensitivity ratio (CSR) 0.0005%.

[0061] Example 6 A tempering method for improving the yield strength of wind turbine main shafts is disclosed, which is used to treat the main shaft steel. The chemical composition of the main shaft steel, by mass percentage, includes: C 0.44%, Si 0.38%, Mn 0.78%, Cr 1.18%, Mo 0.24%, Ni 0.58%, Cu 0.05%, P 0.010%, S 0.0020%, O 0.0015%, N 0.0060%, H 0.00020%, with the balance being Fe and unavoidable impurities.

[0062] Includes the following steps: S1. The forged spindle steel is pre-normalized, heated to 900℃ and held for 3 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 860℃ at a rate of 80℃ / h. The holding time is calculated based on a wall thickness of 1.5min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 790℃ and the pre-cooling time is controlled within 8 minutes. S4. Quickly transfer the pre-cooled spindle steel into the quenching medium for graded cooling. The quenching medium is PAG polymer quenching fluid. Specific steps include: S41. Control the initial temperature of the quenching fluid at 20℃. After immersing the workpiece, start full-frequency stirring. Control the cooling time at 8 min per 100mm effective thickness. S42. Turn off the stirrer and let the workpiece stay in the static quenching fluid for 5 min. During this period, synchronously inject compressed air into the quenching fluid. Control the air source pressure at 0.5 MPa and the air circulation time at 4 min to form a microbubble layer on the workpiece surface. S43. Restart the stirrer and continue cooling until the workpiece surface temperature is ≤100℃. Control the total quenching time at 12 min per 100mm effective thickness. S44. During the quenching process, maintain the quenching fluid temperature at 20~30℃ through the heat exchange system. S5. Temper the quenched spindle steel by heating it to 620℃. The holding time is calculated based on an effective thickness of 3.0 min / mm. When the furnace temperature reaches 400℃, inject a nitrogen-ethanol-dimethyl disulfide mixed atmosphere into the furnace in a pulsed manner. The volume ratio of nitrogen, ethanol and dimethyl disulfide is 100:10:1. The pulse cycle is 15 min / time, and each pulse lasts for 5 minutes. After the holding time is completed, air cool to room temperature.

[0063] After quenching and tempering, the austenite grain size in the microstructure of the main shaft steel reaches grade 8.0, and the non-metallic inclusions meet the following grades: Grade A fine series 0.5, Grade D fine series 0.5, Grade D coarse series 0.5, and all other types of inclusions are grade 0. The metallographic structure reaches the grade 1 sorbite structure specified in GB / T 13320-2007.

[0064] The spindle steel prepared in this embodiment meets the following room temperature mechanical properties as measured by GB / T 228.1-2021: specified plastic elongation strength Rp 0.2782 MPa, tensile strength Rm 942 MPa, elongation after fracture A 17.1%, and reduction of area Z 59.5%. The low-temperature impact toughness, measured by GB / T 229-2020, meets the following: impact energy absorbed at -20℃ 174 J, -30℃ 167 J, -40℃ 159 J, and room temperature 187 J. The hydrogen-induced cracking resistance, measured by GB / T 8650-2015, meets the following: average crack length ratio (CLR) 0.39%, average crack thickness ratio (CTR) 0.13%, and average crack sensitivity ratio (CSR) 0.0007%.

[0065] Comparative Example 1 A tempering method for improving the yield strength of wind turbine main shafts is disclosed, which is used to treat the main shaft steel. The main shaft steel is conventional 42CrMo steel, and its chemical composition by mass percentage includes: C 0.42%, Si 0.28%, Mn 0.75%, Cr 1.10%, Mo 0.18%, Ni 0.10%, Cu 0.06%, P 0.018%, S 0.015%, O 0.0025%, N 0.008%, H 0.0004%, with the balance being Fe and unavoidable impurities.

[0066] The tempering method adopts conventional processes: S1, normalize the forged spindle steel, heat to 880℃ and hold for 4 hours, then air cool; S2, heat to 850℃ and hold, then water quench and oil cool; S3, temper at 580℃ and hold, then air cool.

[0067] Testing revealed that the austenite grain size of the spindle steel is grade 7.0, and the non-metallic inclusions are grade 1.5 (Class A fine series) and grade 1.0 (Class D fine series). Room temperature mechanical properties: Rp 0.2 685 MPa, Rm 860 MPa, A 14.5%, Z 52%. Low temperature impact toughness: -20℃ 68 J, -30℃ 45 J, -40℃ 28 J. Resistance to hydrogen-induced cracking: CLR 2.3%, CTR 0.8%, CSR 0.006%.

[0068] Comparative Example 2 A tempering method for improving the yield strength of wind turbine main shafts is provided for treating main shaft steel, wherein the chemical composition of the main shaft steel is the same as that in Example 1.

[0069] In the tempering method, step S4 did not use staged cooling, but directly used water quenching, and the remaining steps were the same as in Example 1.

[0070] Testing revealed microcracks on the surface of the spindle steel, with an austenite grain size of 7.5. Room temperature mechanical properties: Rp0.2 765 MPa, Rm 920 MPa, A 14.2%, Z 48%. Low temperature impact toughness: -20℃ 112 J, -30℃ 89 J, -40℃ 62 J.

[0071] Comparative Example 3 A tempering method for improving the yield strength of wind turbine main shafts is provided for treating main shaft steel, wherein the chemical composition of the main shaft steel is the same as that in Example 1.

[0072] In the conditioning method, step S5 did not involve the injection of a nitrogen-ethanol-dimethyl disulfide mixed atmosphere; the remaining steps were the same as in Example 1.

[0073] The spindle steel was tested and found to have an austenitic grain size of grade 8.0. Room temperature mechanical properties: Rp 0.2 748 MPa, Rm 905 MPa, A 16.8%, Z 58%. Low temperature impact toughness: -20℃ 168 J, -30℃ 158 J, -40℃ 142 J. Resistance to hydrogen-induced cracking: CLR 0.52%, CTR 0.18%, CSR 0.0012%.

[0074] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A process for improving the yield strength of wind turbine main shafts, characterized in that, The chemical composition of the spindle steel, by mass percentage, includes: C: 0.38–0.45%; Si: 0.20–0.40%; Mn: 0.50–0.80%; Cr: 0.90–1.20%; Mo: 0.15–0.25%; Ni: 0.40–0.60%; Cu: ≤0.05%; P: ≤0.010%; S: ≤0.002%; O: ≤0.0015%; N: ≤0.006%; H: ≤0.0002%; Balance: Fe and unavoidable impurities; After the main shaft steel is quenched and tempered, the austenite grain size in its microstructure reaches level 8 or above, and the non-metallic inclusion levels meet the following requirements: Class A fine series ≤ 0.5 level, Class D fine series ≤ 0.5 level, Class D coarse series ≤ 0.5 level, and all other types of inclusions ≤ 0 level. The chemical composition of the spindle steel was determined according to GB / T 4336-2016, wherein O, N, and H were determined according to GB / T 11261-2006, GB / T 20124-2006, and GB / T 223.82-2018, respectively; the grain size was determined according to GB / T 6394-2017; and the inclusions were evaluated according to GB / T10561-2005.

2. The process method for improving the yield strength of wind turbine main shaft according to claim 1, characterized in that, The room temperature mechanical properties of the spindle steel, as measured according to GB / T 228.1-2021, meet the following requirements: The specified plastic elongation strength Rp0.2 ≥ 750 MPa, Tensile strength Rm ≥ 900 MPa Elongation at break (A) ≥ 16.0%, The reduction of area Z ≥ 58%.

3. The process method for improving the yield strength of wind turbine main shaft according to claim 1, characterized in that, The low-temperature impact toughness of the main shaft steel, as measured according to GB / T 229-2020, meets the following requirements: Impact absorption energy at -20℃ ≥ 160 J Impact absorption energy at -30℃ ≥ 165 J Impact absorption energy at -40℃ ≥ 150 J.

4. The process method for improving the yield strength of wind turbine main shaft according to claim 1, characterized in that, The room temperature impact toughness of the main shaft steel, as measured according to GB / T 229-2020, meets the following requirements: Room temperature shock absorption energy ≥ 170 J.

5. The process method for improving the yield strength of wind turbine main shaft according to claim 1, characterized in that, The resistance to hydrogen-induced cracking of the spindle steel, as determined by GB / T 8650-2015, meets the following requirements: After immersion in solution A, the average crack length ratio (CLR) was ≤ 0.5%. The average crack thickness ratio (CTR) is ≤ 0.15%. The average crack sensitivity rate (CSR) is ≤ 0.001%.

6. The process method for improving the yield strength of wind turbine main shaft according to claim 1, characterized in that, After being quenched and tempered, the spindle steel has a metallographic structure that meets the Grade 1 sorbite structure specified in GB / T 13320-2007.

7. A process for improving the yield strength of wind turbine main shafts, used to treat the main shaft steel according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1. The forged spindle steel is pre-normalized, heated to 880-900℃ and held for 3-5 hours, then air-cooled to room temperature; S2. Load the main shaft steel into the heat treatment furnace and heat it to 840~860℃ at a rate of ≤80℃ / h. The holding time is calculated based on the wall thickness of 1.5~2.0 min / mm. S3. After the heat preservation is completed, the main shaft steel is taken out of the furnace and pre-cooled in the air. The pre-cooling termination temperature is 790-810℃, and the pre-cooling time is controlled at 8-15 min. S4. Quickly transfer the pre-cooled spindle steel into the quenching medium for graded cooling. S5. Temper the quenched spindle steel by heating it to 590-620℃ and holding it for 3.0-4.0 min / mm based on the effective thickness. After holding, air cool it to room temperature.

8. The process method according to claim 7, characterized in that, In step S4, the quenching medium is a PAG polymer quenching liquid, and the specific steps include: S41. The initial temperature of the quenching fluid is controlled at 20-30℃. After the workpiece is immersed, full-frequency stirring is turned on. The cooling time is controlled at 8-10 min per 100 mm of effective thickness. S42. Turn off the agitator and let the workpiece stay in the static quenching liquid for 3 to 5 minutes; S43. Restart the agitator and continue cooling until the workpiece surface temperature is ≤100℃. The total quenching time is controlled at 12-15 min per 100mm effective thickness. S44. During the quenching process, the temperature of the quenching liquid is maintained at 20-40℃ through a heat exchange system.

9. The process method according to claim 8, characterized in that, During step S42, while the agitator is turned off and the workpiece remains in the static quenching liquid, compressed air is simultaneously injected into the quenching liquid. The air source pressure is controlled at 0.3–0.5 MPa, and the air supply time is 2–4 min, forming a microbubble layer on the workpiece surface.

10. The process method according to claim 7, characterized in that, In S5, when the furnace temperature rises to 400-450℃, a nitrogen-ethanol-dimethyl disulfide mixed atmosphere is pulsed into the furnace. The volume ratio of nitrogen, ethanol and dimethyl disulfide is 100:10:1 to 100:15:2, the pulse cycle is 15 min / time, and each pulse lasts for 3-5 minutes.