Micro-structure regulation and control wind power main shaft steel as well as heat treatment preparation method and application thereof

Abstract

The invention relates to the technical field of metallurgy, in particular to microstructure regulation and control wind power main shaft steel and a heat treatment preparation method and application thereof. The wind power main shaft steel is prepared through the processes of smelting, LF refining, VD refining, continuous casting and rolling, profiling forging and performance heat treatment. The wind power main shaft steel comprises C, Si, Mn, Nb, Cr, Mo, Al, V, Ti, N, P, S, Fe and inevitable impurities. According to the wind power main shaft steel, the proportion of small-angle grain boundaries smaller than 10 degrees is larger than 65%; and a micro-nano-sized (Nb, V) N precipitated phase exists at the grain boundary. The tensile strength of the wind power main shaft steel exceeds 900 MPa, the elongation exceeds 14%, and the wind power main shaft steel has good performance.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and in particular to a microstructure-controlled wind turbine main shaft steel and a heat treatment preparation method and application thereof. Background Art

[0002] With the growth of global energy demand and the increasing severity of environmental problems, the search for clean and renewable energy has become a common focus of attention for countries around the world. As an important clean energy source, wind power generation can not only reduce dependence on fossil fuels and reduce greenhouse gas emissions, but also has broad development prospects. The progress of wind power generation technology, especially the research and development of large wind turbines, is of great significance to promoting the transformation of energy structure and achieving sustainable development goals.

[0003] In wind turbine generator sets, the wind turbine main shaft occupies an indispensable position due to its core support role and key transmission function. As a key component connecting the wind rotor and the gearbox or generator, the wind turbine main shaft undertakes the important task of smoothly transmitting the kinetic energy captured by the wind rotor to subsequent components, and ultimately realizes the conversion of mechanical energy into electrical energy. Its stable and reliable working state directly determines the efficiency and stability of the entire wind power generation system. Therefore, the design, manufacture and maintenance of the wind turbine main shaft have become one of the important factors to ensure the efficient operation of wind power generation projects.

[0004] The wind turbine main shaft is the backbone of the wind turbine and is one of the core elements that determine its performance. Given that wind turbines are mostly installed in areas with harsh environmental conditions, such as offshore platforms or high-altitude land, the wind turbine main shaft must be able to adapt to and resist various complex working conditions, including but not limited to extreme temperature fluctuations, high humidity, salt spray erosion, and unstable wind changes. In addition, with the rapid development of wind power generation technology, the unit capacity of wind turbines has continued to climb to higher megawatt levels. This not only indicates that the power production capacity of a single wind turbine has been significantly improved, but also puts forward more stringent requirements for the wind turbine main shaft: larger size, heavier load, and stronger durability. Especially under high-intensity wind conditions, the wind turbine main shaft needs to withstand huge rotational stress, which places extremely high demands on its structural strength and material properties.

[0005] In order to meet these challenges, wind turbine main shaft materials are mostly made of alloy steel with good comprehensive mechanical properties. However, the strength and other properties of the alloy are jointly affected by many factors, including element composition, process parameters, metallographic microstructure, etc. Among them, grain boundary energy is an important influencing factor of the mechanical properties of alloy steel, and grain boundary energy is significantly affected by the grain boundary angle. Usually, the grain boundaries of adjacent grains with a phase difference of less than 10° are called small-angle grain boundaries; the grain boundaries of adjacent grains with a phase difference of more than 10° are called large-angle grain boundaries. Increasing the proportion of particles in small-angle grain boundaries is conducive to reducing the grain boundary energy of the alloy and improving the performance of the alloy. Patent document CN114381641A provides an Al-Mg-Zn-RE alloy containing high-density small-angle grain boundaries, but the strength is about 300MPa, which is relatively low. CN114686776A provides an extra-thick steel plate with small-angle grain boundaries with a phase difference of 3°~15° and large-angle grain boundaries with a phase difference of more than 15° distributed in the crystal, but the strength is less than 700MPa, which is relatively low. Summary of the invention

[0006] In view of the above problems, the purpose of the present invention is to provide a microstructure-controlled wind turbine main shaft steel and its heat treatment preparation method and application, so as to improve the strength, elongation, etc. of the wind turbine main shaft steel. Therefore, the technical solution of the present invention is as follows: In a first aspect, the present invention provides a wind turbine main shaft steel, the element composition of the wind turbine main shaft steel being, by mass percentage, C: 0.18~0.32%; Si: 0.21~0.38%; Mn: 1.11~1.34%; Nb: 0.41~0.49%; Cr: 1.82~1.96%; Mo: 0.36~0.41%; Al: ≤0.01%; V: 0.36~0.45%; Ti: 0.04~0.08%; N: 0.010~0.014%; P: ≤0.01%; S: ≤0.01%; the remainder being Fe and unavoidable impurities; the proportion of small-angle grain boundaries less than 10° in the wind turbine main shaft steel is greater than 65%.

[0007] Preferably, the proportion of small-angle grain boundaries less than 10° in the wind turbine main shaft steel reaches 67% to 69%. The proportion of small-angle grain boundaries can be obtained by irradiating the surface of the sample crystal with X-ray diffraction to obtain the grain orientation information of the material, thereby obtaining statistics.

[0008] Preferably, there are micro-nano sized (Nb, V)N precipitated phases at the grain boundaries of the wind turbine main shaft steel. The micro-nano sized precipitated phases can be obtained by conventionally preparing metal microstructures for microscopic observation and detection.

[0009] In a second aspect, the present invention provides a method for preparing the above-mentioned wind turbine main shaft steel, comprising the following steps: S1 Smelting: S11 adds raw materials into a furnace for smelting to produce molten iron; S12 blowing oxygen into the molten iron obtained in step S11, and adding quicklime for slag making; S13: removing impurities by slag blocking to obtain molten steel, wherein the element composition of the molten steel is as follows by mass percentage: C: 0.04-0.06%; Si: 0.07-0.13%; Mn: 0.33-0.55%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; S2 LF Refining: S21 transferring the molten steel obtained in step S13 to the LF refining furnace; S22: heating the molten steel in step S21 and adding ferrovanadium and ferroniobium master alloys to adjust the chemical composition; S23 is stirred by blowing argon, and then aluminum wire is fed for deoxidation; S24 adds quicklime and fluorite, makes slag, and obtains refined molten steel; the element composition of the refined molten steel is calculated by mass percentage as follows: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.001-0.003%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and unavoidable impurities; S3 VD Refining: S31: transferring the refined molten steel obtained in step S24 to a VD vacuum furnace and performing a vacuum treatment; S32 Bottom blowing nitrogen breaks the vacuum and adds nitrogen to the molten steel; S33 is fed with calcium wire to further purify the molten steel to obtain high nitrogen casting molten steel; the element composition of the high nitrogen casting molten steel is as follows by mass percentage: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.010-0.014%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; S4 Continuous Casting and Rolling: S41: pouring the high nitrogen casting molten steel obtained in step S33 into a crystallizer for continuous casting to obtain a casting round billet; S42: the cast round billet obtained in step S41 is subjected to a blanking process, and then rolled, wherein the rolling is deformed twice by horizontal rolling and longitudinal rolling, and the cast round billet is then hot-sent to a steel ingot at 900° C. to 950° C.; S5 Profile Forging: S51 The hot-delivered steel ingot is heated to 1260°C~1280°C and kept at this temperature for 5h~6h to ensure uniform heating; then, two repeated stretching-upsetting operations are performed in sequence to obtain a repeatedly stretched-upsetting steel ingot; S52: the steel ingot repeatedly stretched and upset in step S51 is returned to the furnace for heating and then punched; S53: the forged piece after punching in step S52 is returned to the furnace for heating, and then the shaft body is stretched and rounded to complete the shaft body forming, thereby obtaining a rolled round steel; S6 Performance Heat Treatment: S61: placing the rolled round steel obtained in step S53 into a slow cooling pit for multi-stage slow cooling treatment to obtain multi-stage slow cooling rolled round steel; S62 The multi-stage slow cold rolled round steel of step S61 is taken out and cooled to room temperature to obtain wind turbine main shaft steel.

[0010] Preferably, the smelting temperature in step S1 is 1600° C.-1650° C., and the smelting time is based on the C content reaching the standard.

[0011] Preferably, the LF refining temperature in step S2 is 1720°C~1770°C, the argon flow rate is 200NL / min~300NL / min, the amount of aluminum wire fed is 4kg / ton~5kg / ton, the amount of quicklime supplemented is 8.6kg / ton~8.9kg / ton, the amount of fluorite supplemented is 1.8kg / ton~2.0kg / ton, and the refining time is calculated as 17min / ton~19min / ton.

[0012] Preferably, the vacuum degree of VD refining in step S3 is ≤55Pa, the refining temperature is 1780°C~1800°C, the nitrogen flow rate is 200NL / min~300NL / min, the amount of calcium wire fed is 0.1kg / ton~0.12kg / ton, and the refining time is calculated as 16min / ton~19min / ton.

[0013] Preferably, in step S4, the first rolling temperature of the transverse rolling is 1150℃~1200℃, and the reduction rate is 15%~17%; the first rolling temperature of the longitudinal rolling is 1100℃~1150℃, and the reduction rate is 12%~14%; the second rolling temperature of the transverse rolling is 1050℃~1100℃, and the reduction rate is 10%~12%; the second rolling temperature of the longitudinal rolling is 1000℃~1050℃, and the reduction rate is 8%~10%.

[0014] Preferably, in step S51, the hot-delivered steel ingot is heated to 1260°C~1280°C at a rate of 50°C / h~60°C / h, and kept warm for 5h~6h, followed by the first drawing and the first upsetting, with a drawing ratio of 7~8.5, an upsetting ratio of 7.3~7.6, and an end temperature of the first drawing-upsetting step of 1000~1050°C; followed by the second drawing and the second upsetting, with a drawing ratio of 3.8~4.2, an upsetting ratio of 3.8~4.1, and an end temperature of the second drawing-upsetting step of 750°C~780°C.

[0015] Preferably, the punched forging obtained in step S52 is returned to the furnace and heated to 1180°C~1200°C for shaft lengthening. The shaft lengthening is carried out according to the anvil width ratio of 0.5~0.7, the pressing amount is 80mm~100mm, the feed amount is 0.4~0.6 of the anvil width, and the shaft lengthening is carried out by turning the shaft clockwise by 90°.

[0016] Preferably, the forging obtained in step S52 is subjected to rounding treatment at a temperature range of 750° C. to 960° C., with an anvil width ratio of 0.7 to 0.8 and a reduction of 8 mm to 16 mm.

[0017] Preferably, in step S6, the rolled round steel is quickly placed in a slow cooling pit and sealed with a cover for 5h~8h for slow cooling, then the cover is opened and air is blown in for 15min~20min to form air cooling, and when the temperature drops to 300~400°C, it is sealed with a cover for 8h~10h for slow cooling, and finally taken out and placed in the air to cool naturally to room temperature.

[0018] In a third aspect, the present invention provides use of the aforementioned wind turbine main shaft steel in the preparation of a wind turbine main shaft.

[0019] In a fourth aspect, the present invention provides a wind turbine main shaft, which is made of any of the aforementioned wind turbine main shaft steels.

[0020] The present invention has the following technical effects or advantages: 1. The wind turbine main shaft steel of the present invention and the wind turbine main shaft steel prepared by the method of the present invention have the characteristics of low grain boundary energy regulated by multi-scale microstructure. With the coordinated regulation of the preparation process of smelting-LF refining-VD refining-continuous casting and rolling-profile forging-performance heat treatment, through the control of the composition of Nb and V, the smelting control is used to form a supersaturated solid solution, and the rolling stress and strain are used to control the precipitation and reaction of supersaturated solute atoms to form metal nitrides; temperature control and cooling control promote the mutual dissolution reaction of metal nitrides at the grain boundaries to form thermodynamically stable composite nitrides, pin the grain boundaries, form large-proportion small-angle grain boundaries, and reduce the grain boundary energy of the wind turbine main shaft steel. The specific principles are: (1) During the preparation of the wind turbine main shaft steel of the present invention, its composition contains Nb: 0.41-0.49%, V: 0.36-0.45% and N: 0.010%-0.014%. During the VD vacuum refining process, the solubility of Nb and V in iron increases significantly with the increase of temperature, and they are added to the molten iron in the form of an intermediate alloy. At the smelting temperature, Nb and V can be dissolved in a large amount in the iron matrix, and then during the pouring and solidification process, due to rapid cooling, Nb and V form a supersaturated solid solution in the steel.

[0021] (2) During the preparation process of the wind power main shaft steel of the present invention, after two deformations of transverse rolling and longitudinal rolling (the first rolling temperature of transverse rolling is 1150℃~1200℃, the reduction rate is 15%~17%, the first rolling temperature of longitudinal rolling is 1100℃~1150℃, the reduction rate is 12%~14%, the second rolling temperature of transverse rolling is 1050℃~1100℃, the reduction rate is 10%~12%, the second rolling temperature of longitudinal rolling is 1000℃~1050℃, the reduction rate is 8%~10%), under the action of gradient compressive stress and strain, a large number of dislocations are generated in the matrix and subgrain boundaries are formed, and under rolling conditions (1000℃~1200℃, pressure processing), Nb, V and N atoms gather at the dislocations under the stress gradient and precipitate to form NbN and VN. The reaction formula is: Formation of VN: V+N→VN; Formation of NbN: Nb+N→NbN.

[0022] (3) During the preparation of the wind turbine main shaft steel of the present invention, during the performance heat treatment process, on the one hand, because the free energy of the solid solution is usually lower than the free energy of a single compound, (Nb, V) N is more stable than VN and NbN alone. On the other hand, VN and NbN are both transition metal nitrides with similar crystal structures. The lattice constants of VN and NbN are 4.38Å and 4.39Å, respectively, and the atomic radii of Nb and V are similar (the atomic radius of Nb is about 1.46Å, and the atomic radius of V is about 1.35Å). They can replace each other's positions in the lattice at high temperatures (under the temperature condition of 500℃~700℃), and have the conditions for mutual reaction to form a (Nb, V) N solid solution, thereby finally forming a micro-nano (Nb, V) N precipitation phase at the grain boundary, promoting the formation of a high proportion of small-angle grain boundaries and reducing the grain boundary energy of the material.

[0023] 2. Low-angle grain boundaries are usually composed of arrays of dislocations. These dislocations form a dense network at the grain boundaries, increasing the dislocation density inside the material. High-density dislocations hinder the movement of other dislocations, thereby increasing the strength of the material. Dislocations are prone to pileup near low-angle grain boundaries. Dislocation pileup can prevent further slip of dislocations, thereby increasing the strength of the material. The formation of low-angle grain boundaries is usually accompanied by finer grains, which plays a role in grain refinement. Smaller grains mean more grain boundary area. Grain boundaries have a strong hindering effect on the movement of dislocations, thereby increasing the strength of the material. The energy of low-angle grain boundaries is lower, which makes them easier to move and rearrange during deformation. This property helps the material maintain a higher strength during deformation.

[0024] 3. Low-angle grain boundaries are usually composed of arrays of dislocations, which can slip more easily under the action of external forces. This slip helps to disperse stress and reduce local stress concentration, thereby improving the plasticity of the material. Dislocations can proliferate more easily near low-angle grain boundaries. The proliferation of dislocations can provide more slip paths, so that the material can better absorb energy during deformation and enhance plasticity. Low-angle grain boundaries have less obstructive effect on dislocations, so they do not hinder dislocation movement as strongly as high-angle grain boundaries. This makes it easier for dislocations to slip during deformation, thereby improving plasticity. The presence of low-angle grain boundaries can reduce the total energy of grain boundaries, making it easier for grain boundaries to move and rearrange during deformation. This grain boundary softening effect helps the material to deform uniformly and reduce the formation and expansion of cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The metallographic microstructure diagram and partial enlarged diagram of the wind turbine main shaft steel of Example 1 of the present invention; Figure 2 This is a statistical diagram of the proportion of small-angle grain boundaries in the wind turbine main shaft steel in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with specific embodiments. It should be understood that the specific embodiments are only used to explain the present invention and are not used to limit the scope of protection of the present invention.

[0027] In the following examples, the performance test of the wind turbine main shaft steel is analyzed and tested according to GB / T 228.1-2021. The metal microstructure is conventionally prepared and tested according to the metal microstructure inspection method of GB / T 13298-2015. The X-ray diffraction method is used to irradiate the surface of the sample crystal to obtain the grain orientation information of the material.

[0028] Example 1 A low grain boundary energy wind turbine main shaft steel and preparation method thereof The preparation method of low grain boundary energy wind turbine main shaft steel is as follows: S1 Smelting: S11 adds raw materials into a furnace for smelting to produce molten iron; S12 blowing oxygen into the molten iron obtained in step S11, and adding quicklime for slag making; S13: removing impurities by slag blocking to obtain molten steel, wherein the element composition of the molten steel is as follows by mass percentage: C: 0.04-0.06%; Si: 0.07-0.13%; Mn: 0.33-0.55%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; S2 LF Refining: S21 transferring the molten steel obtained in step S13 to the LF refining furnace; S22: heating the molten steel in step S21 and adding ferrovanadium and ferroniobium master alloys to adjust the chemical composition; S23 is stirred by blowing argon, and then aluminum wire is fed for deoxidation; S24 adds quicklime and fluorite, makes slag, and obtains refined molten steel; the element composition of the refined molten steel is calculated by mass percentage as follows: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.001-0.003%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and unavoidable impurities; S3 VD Refining: S31: transferring the refined molten steel obtained in step S24 to a VD vacuum furnace and performing a vacuum treatment; S32 Bottom blowing nitrogen breaks the vacuum and adds nitrogen to the molten steel; S33 is fed with calcium wire to further purify the molten steel to obtain high nitrogen casting molten steel; the element composition of the high nitrogen casting molten steel is as follows by mass percentage: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.010-0.014%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; S4 Continuous Casting and Rolling: S41: pouring the high nitrogen casting molten steel obtained in step S33 into a crystallizer for continuous casting to obtain a casting round billet; S42: the cast round billet obtained in step S41 is subjected to a blanking process, and then rolled, wherein the rolling is deformed twice by horizontal rolling and longitudinal rolling, and the cast round billet is then hot-sent to a steel ingot at 900° C. to 950° C.; S5 Profile Forging: S51 The hot-delivered steel ingot is heated to 1260°C~1280°C and kept at this temperature for 5h~6h to ensure uniform heating; then, two repeated stretching-upsetting operations are performed in sequence to obtain a repeatedly stretched-upsetting steel ingot; S52: the steel ingot repeatedly stretched and upset in step S51 is returned to the furnace for heating and then punched; S53: the forged piece after punching in step S52 is returned to the furnace for heating, and then the shaft body is stretched and rounded to complete the shaft body forming, thereby obtaining a rolled round steel; S6 Performance Heat Treatment: S61 The rolled round steel obtained in step S53 is quickly placed in a slow cooling pit for multi-stage slow cooling treatment to obtain multi-stage slow cooling rolled round steel; S62 The multi-stage slow cold rolled round steel of step S61 is taken out and cooled to room temperature to obtain wind turbine main shaft steel.

[0029] Three kinds of experimental wind turbine main shaft steels were prepared according to this method. The process parameters and component parameters of each experimental wind turbine main shaft steel are shown in Tables 1 to 3 respectively.

[0030] Table 1 Experimental Example 1 Process parameters and component parameters of wind turbine main shaft steel

[0031]

[0032] Table 2 Experimental Example 2 Process parameters and component parameters of wind turbine main shaft steel

[0033]

[0034] Table 3 Process parameters and component parameters of wind turbine main shaft steel in Experimental Example 3

[0035]

[0036] Example 2 Analysis and testing of wind turbine main shaft steel The following comparative examples were prepared: Comparative Example 1: The difference from Experimental Example 1 is that step S22 adjusts the addition amount of ferrovanadium and ferroniobium intermediate alloys so that the Nb content of step S24, step S33 and the final product wind turbine main shaft steel is 0.52% and the V content is 0.61%. The other steps and parameters are the same as those in Experimental Example 1.

[0037] Comparative Example 2: The difference from Experimental Example 1 is that the continuous casting and rolling process is changed to rolling through 3 times of horizontal rolling and vertical rolling respectively. The rolling temperature of the three horizontal rolling is 1000℃, and the reduction rate is 3%; the rolling temperature of the three vertical rolling is 1050℃, and the reduction rate is 5%.

[0038] Comparative Example 3: The difference from Experimental Example 1 is that the post-treatment process is changed to placing the rolled round steel into a slow cooling pit, covering and sealing it, and cooling it to room temperature at 8-12°C / s instead of using multi-stage slow cooling.

[0039] The samples of Experimental Examples 1 to 3 and Comparative Examples 1-3 were subjected to mechanical property tests, X-ray diffraction tests and metal microstructure observations. The results are shown in Tables 4 and Figure 1 , 2 Table 4 Sample test results

[0040] The tensile strength of the wind turbine main shaft steel of Experimental Examples 1-3 exceeds 900 MPa, and the elongation exceeds 10%. The proportion of small-angle grain boundaries less than 10° in the wind turbine main shaft steel of Experimental Examples 1-3 exceeds 65%, reaching 67-69%. Micro-nano-scale dispersed precipitation phases can be observed at the grain boundary positions of the wind turbine main shaft steel of Experimental Examples 1-3, which are NbN precipitation phases and VN precipitation phases. Typical metal microstructure pictures and local enlarged pictures are shown in Figure 1 .

Claims

1. A wind turbine main shaft steel, characterized in that: The element composition of the wind turbine main shaft steel is calculated by mass percentage as follows: C: 0.18~0.32%; Si: 0.21~0.38%; Mn: 1.11~1.34%; Nb: 0.41~0.49%; Cr: 1.82~1.96%; Mo: 0.36~0.41%; Al: ≤0.01%; V: 0.36~0.45%; Ti: 0.04~0.08%; N: 0.010~0.014%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; the proportion of small-angle grain boundaries less than 10° in the wind turbine main shaft steel is greater than 65%.

2. The wind turbine main shaft steel according to claim 1, characterized in that: In the wind turbine main shaft steel, the proportion of small-angle grain boundaries less than 10° reaches 67%~69%.

3. The wind turbine main shaft steel according to claim 1, characterized in that: There are micro-nano sized (Nb, V)N precipitation phases at the grain boundaries of the wind turbine main shaft steel.

4. The method for preparing wind turbine main shaft steel according to any one of claims 1 to 3, characterized in that: The steps include: S1 Smelting: S11 adds raw materials into a furnace for smelting to produce molten iron; S12 blowing oxygen into the molten iron obtained in step S11, and adding quicklime for slag making; S13: removing impurities by slag blocking to obtain molten steel, wherein the element composition of the molten steel is as follows by mass percentage: C: 0.04-0.06%; Si: 0.07-0.13%; Mn: 0.33-0.55%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; S2 LF Refining: S21 transferring the molten steel obtained in step S13 to the LF refining furnace; S22: heating the molten steel in step S21 and adding ferrovanadium and ferroniobium master alloys to adjust the chemical composition; S23 is stirred by blowing argon, and then aluminum wire is fed for deoxidation; S24 adds quicklime and fluorite, makes slag, and obtains refined molten steel; the element composition of the refined molten steel is calculated by mass percentage as follows: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.001-0.003%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and unavoidable impurities; S3 VD Refining: S31: transferring the refined molten steel obtained in step S24 to a VD vacuum furnace and performing a vacuum treatment; S32 Bottom blowing nitrogen breaks the vacuum and adds nitrogen to the molten steel; S33 is fed with calcium wire to further purify the molten steel to obtain high nitrogen casting molten steel; the element composition of the high nitrogen casting molten steel is as follows by mass percentage: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.010-0.014%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; S4 Continuous Casting and Rolling: S41: pouring the high nitrogen casting molten steel obtained in step S33 into a crystallizer for continuous casting to obtain a casting round billet; S42: the cast round billet obtained in step S41 is subjected to a blanking process, and then rolled, wherein the rolling is deformed twice by horizontal rolling and longitudinal rolling, and the cast round billet is then hot-sent to a steel ingot at 900° C. to 950° C.; S5 Profile Forging: S51 The hot-delivered steel ingot is heated to 1260°C~1280°C and kept at this temperature for 5h~6h to ensure uniform heating; then, two repeated stretching-upsetting operations are performed in sequence to obtain a repeatedly stretched-upsetting steel ingot; S52: the steel ingot repeatedly stretched and upset in step S51 is returned to the furnace for heating and then punched; S53: the forged piece after punching in step S52 is returned to the furnace for heating, and then the shaft body is stretched and rounded to complete the shaft body forming, thereby obtaining a rolled round steel; S6 Performance Heat Treatment: S61: placing the rolled round steel obtained in step S53 into a slow cooling pit for multi-stage slow cooling treatment to obtain multi-stage slow cooling rolled round steel; S62 The multi-stage slow cold rolled round steel of step S61 is taken out and cooled to room temperature to obtain wind turbine main shaft steel.

5. The method for preparing wind turbine main shaft steel according to claim 4, characterized in that: The smelting temperature in step S1 is 1600° C. to 1650° C., and the smelting time is based on the C content reaching the standard.

6. The method for preparing wind turbine main shaft steel according to claim 4, characterized in that: The LF refining temperature in step S2 is 1720°C~1770°C, the argon flow rate is 200NL / min~300NL / min, the amount of aluminum wire fed is 4kg / ton~5kg / ton, the amount of quicklime supplemented is 8.6kg / ton~8.9kg / ton, the amount of fluorite supplemented is 1.8kg / ton~2.0kg / ton, and the refining time is calculated as 17min / ton~19min / ton.

7. The method for preparing wind turbine main shaft steel according to claim 4, characterized in that: In step S3, the vacuum degree of VD refining is ≤55Pa, the refining temperature is 1780°C~1800°C, the nitrogen flow rate is 200NL / min~300NL / min, the feeding amount of calcium wire is 0.1kg / ton~0.12kg / ton, and the refining time is calculated as 16min / ton~19min / ton.

8. The method for preparing wind turbine main shaft steel according to claim 4, characterized in that: In step S4, the first rolling temperature of the transverse rolling is 1150°C~1200°C, and the reduction rate is 15%~17%; the first rolling temperature of the longitudinal rolling is 1100°C~1150°C, and the reduction rate is 12%~14%; the second rolling temperature of the transverse rolling is 1050°C~1100°C, and the reduction rate is 10%~12%; the second rolling temperature of the longitudinal rolling is 1000°C~1050°C, and the reduction rate is 8%~10%.

9. The method for processing wind turbine main shaft steel according to claim 4, characterized in that: In the step S51, the hot-delivered steel ingot is heated to 1260°C~1280°C at a speed of 50°C / h~60°C / h, and kept warm for 5h~6h, followed by the first drawing and the first upsetting, with a drawing ratio of 7~8.5, an upsetting ratio of 7.3~7.6, and an end temperature of the first drawing-upsetting step of 1000~1050°C; followed by the second drawing and the second upsetting, with a drawing ratio of 3.8~4.2, an upsetting ratio of 3.8~4.1, and an end temperature of the second drawing-upsetting step of 750°C~780°C.

10. The method for preparing wind turbine main shaft steel according to claim 4, characterized in that: The punched forging obtained in step S52 is returned to the furnace and heated to 1180°C~1200°C for shaft lengthening. The shaft lengthening is carried out according to the anvil width ratio of 0.5~0.7, the pressing amount is 80mm~100mm, and the feed amount is 0.4~0.6 of the anvil width. The shaft lengthening is carried out by turning 90° clockwise.

11. The method for preparing wind turbine main shaft steel according to claim 4, characterized in that: The forging obtained in step S52 is subjected to rounding treatment at a temperature range of 750° C. to 960° C., with an anvil width ratio of 0.7 to 0.8 and a reduction of 8 mm to 16 mm.

12. The method for preparing wind turbine main shaft steel according to claim 11, characterized in that: In step S6, the rolled round steel is quickly placed in a slow cooling pit and sealed with a cover for 5h~8h for slow cooling, then the cover is opened and air is blown in for 15min~20min to form wind cooling, and when the temperature drops to 300~400°C, the cover is sealed for 8h~10h for slow cooling, and finally it is taken out and placed in the air to cool naturally to room temperature.

13. Use of the wind turbine main shaft steel as claimed in any one of claims 1 to 3 or the wind turbine main shaft steel prepared by the method for preparing the wind turbine main shaft steel as claimed in any one of claims 4 to 12 in preparing a wind turbine main shaft.

14. A wind turbine main shaft, characterized in that: The wind turbine main shaft is manufactured by using a wind turbine main shaft steel as described in any one of claims 1 to 3 or a wind turbine main shaft steel manufacturing method as described in any one of claims 4 to 12.

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