Method for manufacturing a hollow wind turbine main shaft from a large-sized round steel continuous casting billet

Abstract

The application provides a method for manufacturing hollow wind power main shafts from large-diameter round steel continuous casting blanks, which realizes high-performance manufacturing through a four-stage process. First, an electromagnetic stirring continuous casting process is used to prepare continuous casting round blanks, with the composition controlled to be C 0.38-0.42%, Cr 1.10-1.25%, Mo 0.15-0.25% and V 0.08-0.12%. Multidirectional forging is implemented through a three-way pressurizing device to perform axial upsetting and conical anvil spinning, combined with radial restraint force, to reduce the porosity of the wind power main shaft. After the blank is pre-bored by a solid punch, inclined rolling hole expansion is performed by a double-roller asynchronous rolling mill, and the hole expansion rate is controlled to realize uniform hole wall structure. The gradient heat treatment stage includes normalizing combined with atomized water gas cooling, and an axial magnetic field is applied during tempering, so that the final product can realize excellent mechanical properties and grain size. Through the synergistic effect of multidirectional forging and composition optimization, the technical problems of high porosity of continuous casting blanks and alloy segregation are solved.

Description

Technical Field

[0001] This invention relates to a method for manufacturing hollow wind turbine main shafts, specifically a method for manufacturing hollow wind turbine main shafts using large-size round steel continuous casting billets. Background Technology

[0002] Wind turbines are a clean energy source that converts wind energy into electrical energy, and their application is already quite widespread. The structure of a wind turbine typically includes a hollow main shaft. The hollow main shaft often uses a one-piece structure to reduce connection points, better transmit and distribute loads, improve the overall structural strength, and extend the service life of the wind turbine.

[0003] Traditional wind turbine main shafts mostly use die casting steel ingot forging process, but the riser ratio of die casting steel ingot is as high as 20%-30%, and removing the riser results in low effective utilization rate of steel ingot.

[0004] As shown in the invention patent application with publication number CN117259627A, the method for manufacturing hollow wind turbine main shafts using large-diameter continuously cast billets includes upsetting continuously cast round billets, then rolling the billets in annular shape, followed by boring, stretching, and heat treatment tempering to improve material utilization. However, when the upsetting-to-forging ratio is ≤2.5, the equivalent strain on the billet is less than 1.8, which means the billet cannot completely close the inherent porosity problem of the continuously cast billet, which has a residual porosity >0.1%.

[0005] Furthermore, existing technologies generally rely on adding 0.3-0.8% nickel metal to compensate for the hardenability of wind turbine main shafts. However, various alloying elements, represented by Ni, can segregate during manufacturing. For example, the difference in Ni content between the center and the edge of the finished wind turbine main shaft wall thickness can reach 0.2%. This segregation leads to excessive hardenability in the high-Ni region, forming coarse martensite with a grain size exceeding 50 μm, resulting in a decrease in the material's yield strength. Conversely, insufficient hardenability in the low-Ni region leads to the formation of untempered bainite, causing localized hardness fluctuations and a decrease in impact toughness in the wind turbine main shaft. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art. Firstly, it provides a method for manufacturing hollow wind turbine main shafts using large-size round steel continuous casting billets, solving the industry pain point of high residual porosity in continuous casting billets. Secondly, it alleviates the element segregation phenomenon in the component alloy by combining specific composition design and technology.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0008] A method for manufacturing hollow wind turbine main shafts from large-size round steel continuous casting billets includes the following steps:

[0009] Step a: Prepare continuously cast round billets with a diameter ≥ 800 mm using electromagnetic stirring continuous casting process;

[0010] Step b: Perform multi-directional forging treatment on the continuously cast round billet;

[0011] Step c: Perform composite boring forming process on the multi-directional forged continuous casting round billet;

[0012] Step d: Perform gradient heat treatment on the continuously cast round billet after the composite boring is completed.

[0013] Furthermore, in step a, the chemical composition of the continuously cast round billet by weight percentage is: C, 0.38-0.42%; Si, 0.25-0.35%; Mn, 0.75-0.90%; Cr, 1.10-1.25%; Mo, 0.15-0.25%; V, 0.08-0.12%; Al, 0.02-0.05%; P≤0.012%, S≤0.003%, with the balance being Fe and unavoidable impurities, and satisfying [O]<15ppm, [H]<1.5ppm.

[0014] Furthermore, in step a, the electromagnetic stirring parameters are: frequency 2-5Hz, current intensity 300-500A, and the distance between the stirrer and the surface of the billet is maintained at 50-80mm.

[0015] The electromagnetic stirring continuous casting process in step a refers to the method of driving the molten steel to flow in a directional manner by the Lorentz force generated by the alternating electromagnetic field. It is used to suppress dendrite growth and refine the solidification structure. The frequency range corresponds to the penetration depth under different molten steel viscosities, and the current intensity determines the stirring intensity.

[0016] Furthermore, step b includes:

[0017] Axial upsetting: The continuously cast round billet is upset using a three-way pressure device at 1200±20℃, with an axial forging ratio of 2.8-3.2, and a radial constraint force of 15-20% of the axial pressure is applied simultaneously;

[0018] Radial spinning: The continuously cast round billet is spun and deformed in 3-5 passes using a conical anvil with a cone angle of 30-45°, with a single pass deformation of 8-12%.

[0019] The forging ratio in step b refers to the ratio of the height of the billet after compression to its original height, which reflects the degree of metal flow. During radial spinning, the anvil cone angle affects the axial / radial flow ratio of the metal. A smaller anvil cone angle is used to promote radial deformation of the metal, and a single-pass deformation amount of 8-12% is used to control the degree of grain breakage and avoid the risk of cracking that may be caused by excessive deformation.

[0020] Furthermore, the three-way pressurization device includes upper and lower hydraulic punches and an adjustable lateral constraint module, with the lateral constraint force being adjusted in real time via a proportional valve.

[0021] Furthermore, step c includes:

[0022] Pre-boring: A φ300-400mm solid punch is used to bore through holes on the axial end face of the continuously cast round billet at a temperature of 950-1000℃;

[0023] Skew rolling expansion: The continuously cast round billet with end face boring is expanded by a twin-roll asynchronous mill at a speed of 45-60 r / min, with an asynchronous ratio of 1:1.05-1.12 and an expansion rate of ≤50 mm / min.

[0024] In step c, the diameter of the solid punch matches the inner diameter of the final hollow shaft. It's important to note that an excessively small punch diameter will lead to excessive deformation during subsequent hole expansion. During the skew rolling hole expansion process, the two rolls of the twin-roll asynchronous mill are positioned on the inner and outer sides of the material's wall thickness, respectively. A certain speed difference exists between the upper and lower rolls to achieve shear strain on the hollow spindle's wall thickness, promoting uniform metal flow through the hole. Here, the hole expansion rate is controlled below 50 mm / min to control the metal's recrystallization rate and prevent grain coarsening.

[0025] Furthermore, step d includes:

[0026] Normalizing treatment: After holding at 920 ± 10℃ for 2-3 hours, the continuous casting round billet is cooled to 300℃ using an atomized water-air mixture at a cooling rate of 15-20℃ / s;

[0027] Tempering treatment: After cooling, the continuously cast round billet is heated to 650 ± 10℃ and held for 4-6 hours before air cooling. During the air cooling process, a constant magnetic field of 0.5-1.0T is applied along the axial direction of the continuously cast round billet.

[0028] Step d uses atomized water vapor as a mixing medium to perform normalizing treatment on the wind turbine main shaft. The purpose of normalizing treatment is to refine grains, homogenize the microstructure, and disperse carbides by controlling the austenite decomposition process. For large-size continuously cast billets, insufficient air cooling rate can lead to the formation of coarse grains and exacerbated segregation of alloying elements. If air cooling with a low cooling rate is used, the transformation time from austenite to ferrite / pearlite is easily prolonged, resulting in grain sizes exceeding 50 μm, corresponding to ASTM material grades below level 4, and yield strength reduced to below 700 MPa. In addition, during slow cooling, carbide-forming elements such as Cr and Mo tend to accumulate and segregate at grain boundaries.

[0029] Furthermore, the gas-water volume ratio of the atomized water-air mixture is 3:1-5:1, the atomization pressure is 0.3-0.5 MPa, and the nozzles of the atomized water-air mixture are arranged circumferentially inward and evenly distributed. Step d also employs a relatively high gas volume to maintain the cooling rate of the wind turbine main shaft, which can both avoid bainite / martensite transformation and suppress the formation of coarse ferrite, thus preventing crack formation.

[0030] Furthermore, the final product produced by this invention satisfies:

[0031] Yield strength ≥ 800 MPa, impact energy at -40℃ ≥ 60 J; ultrasonic testing meets the requirements of Class II of GB / T 6402-2008 standard; grain size ≥ Class 7 of GB / T 6394-2017 standard.

[0032] Furthermore, after the radial spinning stage, the continuously cast round billet undergoes deformation heat treatment. After holding at 750-780℃ for 1-2 hours, it is cooled to 500℃ at a rate of 10-15℃ / min and then immediately subjected to boring.

[0033] Deformation heat treatment refers to a composite process that combines plastic deformation with heat treatment, achieving microstructure control through strain-induced phase transformation. During holding at 750-780℃, dislocations generated by spinning undergo climb and rearrangement, forming a subcrystalline structure. Subsequent cooling allows residual strain energy to drive carbide nucleation, refining the precipitated phases.

[0034] The advantages and beneficial effects of this invention are as follows:

[0035] This invention enables continuously cast round billets to achieve higher equivalent strain through a multi-directional forging process, significantly reducing the porosity of the billets compared to existing technologies, thereby improving material density and mechanical properties. During the upsetting process of the continuously cast round billets, a radial constraint force of 15-20% is applied simultaneously, effectively suppressing stress concentration caused by lateral metal flow and resulting in tighter grain boundary bonding.

[0036] This invention employs a solid punch for pre-boring combined with a twin-roll asynchronous mill to precisely control the hole expansion rate. Through the coupling effect of the temperature field and strain field, the rheological reorganization of the hole wall metal is completed at a high temperature of 950-1000℃, thereby improving the uniformity of grain size in the hollow part and solving the problem of grain coarsening in the wall thickness direction caused by traditional boring.

[0037] This invention employs a Cr-Mo-V alloy system, in which carbide-forming elements synergistically replace nickel, resulting in a Cr content fluctuation of <0.08 along the wall thickness direction in the hollow spindle product and a Mo segregation index reduced to below 1.15. Compared with Ni-containing technical solutions, the segregation degree of alloying elements is significantly improved. Attached Figure Description

[0038] Figure 1This is a flowchart of the preparation process of the present invention;

[0039] Figure 2 This is a schematic diagram of the three-way pressurization device of the present invention;

[0040] In the picture:

[0041] 1-Upper and lower hydraulic punches, 2-Lateral restraint module. Detailed Implementation

[0042] This invention provides a method for manufacturing hollow wind turbine main shafts from large-size round steel continuous casting billets. Step a replaces the traditional Ni-based alloy with a Cr-Mo-V alloy system, achieving hardenability control based on the synergistic effect of carbide-forming elements. Electromagnetic stirring technology is employed during the continuous casting stage, utilizing the Lorentz force generated by an alternating magnetic field to drive the flow of molten steel, suppressing dendrite growth and refining the solidification structure. By controlling the distance between the stirrer and the billet surface, the electromagnetic field penetration depth is ensured to match the viscosity of the molten steel, reducing macroscopic segregation of Cr / Mo / V elements. Simultaneously, the oxygen and hydrogen content is strictly controlled to reduce the tendency for porosity defects from the source.

[0043] The electromagnetic stirring equation matches the skin depth control equation for molten steel:

[0044] ( =2πf)

[0045] In the formula: , magnetic field penetration depth (m); f, stirring frequency (Hz); , Permeability of molten steel (H / m); Electrical conductivity (A / m).

[0046] In the manufacturing process of continuously cast round billets, the penetration depth δ is controlled by adjusting the frequency f, so that the magnetic field covers the core area of ​​the billet with a diameter ≥ 800 mm.

[0047] When f = 2 Hz, δ≈120 mm, full-section stirring can be achieved, Cr / Mo element segregation can be suppressed, and the solute partition coefficient k (grain boundary / intragranular concentration ratio) can be reduced.

[0048] Step b employs a combined plastic deformation strategy of axial upsetting and radial spinning. A axial pressure and radial constraint force are applied simultaneously through a triaxial pressurization device to achieve metal flow reorganization under three-dimensional stress. The equivalent strain generated by axial upsetting can effectively close the shrinkage cavity in the center of the continuously cast billet; radial spinning introduces shear strain through 3-5 passes of cumulative deformation, breaking up coarse grains and eliminating elemental segregation.

[0049] The equivalent strain equation for multi-directional forging conforms to:

[0050]

[0051] In the formula: The true strain of the axial upsetting of the hollow spindle is calculated as ln( H 0 / H 1 ), H 0 / H 1 The value range is the upsetting ratio, that is, 2.8-3.2. , This is the true strain of the radial spinning of the hollow spindle, calculated as ln(1.08-1.12). It is necessary to clarify that... , and The vectors between them are perpendicular and are dimensionless during the calculation.

[0052] As further explained, axial pressure and radial constraint force can be achieved through upper and lower hydraulic punches 1 and lateral constraint module 2. Each component can be connected to hydraulic cylinders, servo push rods, air cylinders and other components to compress the wind turbine main shaft along its center in the radial and / or axial directions, causing it to contract, constrain, or deform.

[0053] Step c involves pre-boring at a high temperature of 950-1000℃ using a φ300-400mm solid punch to reduce the boring resistance through thermoplastic deformation. Subsequently, a twin-roll asynchronous mill is used for skew rolling to expand the hole, utilizing the shear strain generated by the speed difference between the inner and outer rolls to promote uniform metal flow along the hole wall. This process controls the recrystallization process through temperature-strain field coupling, improving the uniformity of the hole wall grain size distribution by more than 40%, thus solving the grain gradient problem in the wall thickness direction caused by traditional boring.

[0054] The shear strain rate equation for skew rolling expansion is:

[0055]

[0056] In the formula, The difference in rolling speed between the outer and inner rollers (m / s) is given by h, where h is the wall thickness of the hollow spindle (m). Shear strain rate (s) -1 When the asynchronous ratio between the two rollers is 1:1.05-1.12, the shear strain rate is 0.8-1.2 s⁻¹. -1 Matching the shear strain rate with the recrystallization rate of the alloy can reduce the standard deviation of the pore wall grain size by about 40%.

[0057] As a preferred implementation, step c can be combined with deformation heat treatment to refine the grains through strain-induced phase transformation, thereby improving the product's mechanical properties such as yield strength and impact energy.

[0058] In step d, the normalizing stage employs atomized water-air mixed cooling to suppress austenite coarsening at an appropriate cooling rate, while avoiding quenching stress caused by martensite transformation. During tempering, an axial steady magnetic field is applied to promote the orientation and alignment of carbide precipitates using the magnetostrictive effect, thus increasing the Cr content. 23 The dispersion of C6-type carbides is improved.

[0059] In step d, the stretching effect of tempering under a magnetic field environment conforms to:

[0060]

[0061] In the formula, Let M be the magnetostriction coefficient (dimensionless) and M be the magnetization intensity. Under a magnetic field of 0.5-1.0T, the value of M is approximately 1.6*106A / m. Under the action of the magnetic field, carbides can be induced to precipitate along the (110) crystal plane, and the spacing between the precipitated phases is reduced from 150nm to about 80nm. This technique can effectively improve the impact energy of the hollow spindle.

[0062] As a further explanation of the present invention, by increasing the C content to 0.38-0.42%, combined with the synergistic effect of Cr, Mo, and V, fine and dispersed carbides are formed, enhancing the stability of austenite and achieving hardenability compensation by replacing Ni alloying elements. Specifically, Cr compensates for the decrease in hardenability caused by Ni deficiency through a dual effect of solid solution strengthening and carbide precipitation. Mo has a low diffusion coefficient in austenite, which can suppress grain boundary segregation and alleviate the segregation problem of traditional alloys. V forms uniformly distributed carbides during forging and heat treatment through a strain-induced precipitation mechanism, compensating for the toughness loss caused by Ni deficiency. Cr 23 C6 provides matrix reinforcement, Mo2C suppresses temper brittleness, and VC refines grains; the three work together to reduce the carbide spacing from 200nm in traditional processes to 80-120nm.

[0063] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0064] Example 1

[0065] A method for manufacturing hollow wind turbine main shafts from round steel continuous casting billets includes the following steps:

[0066] Step a: Prepare a continuous casting round billet with a diameter of 800mm using a vertical continuous casting machine. The operating parameters of the continuous casting machine are: frequency of 3Hz, current of 380A, and agitator distance of 60mm from the billet surface.

[0067] The composition of the continuously cast billet prepared by mass percentage is: C 0.38%, Si 0.30%, Mn 0.85%, Cr 1.10%, Mo 0.15%, V 0.08%, Al 0.02%, with the balance being iron and impurities. [O]=12ppm, [H]=1.4ppm.

[0068] Step b: Axial upsetting of the continuously cast billet with a forging ratio of 3.0 is performed at 1180℃ using a three-way pressurizing device (lateral constraint force 18% axial pressure);

[0069] After axial upsetting, the continuously cast billet is spun in 4 passes using a 40° conical anvil, with a deformation of 10% per pass.

[0070] Step c: Use a φ350mm solid punch to punch a central through hole on the axial end face of the continuously cast billet at 980℃ to form a hollow spindle.

[0071] A twin-roll asynchronous mill is used to insert the inner roll into the center of the hollow main shaft, and the outer roll is attached to the outer wall of the hollow main shaft. The inner roll is then expanded at a speed of 55 r / min and the asynchronous ratio between the outer roll and the inner roll is 1:1.08. The outer roll and the inner roll rotate clockwise and counterclockwise respectively, and the expansion rate is controlled at 45 mm / min.

[0072] Step d: The hollow spindle with the expanded hole is placed at 930℃ for 2.5 hours for normalizing treatment. Then, the hollow spindle is cooled to 300℃ using an atomizing medium with an atomization pressure of 0.5 MPa and an air-to-water ratio of 4:1 at a cooling rate of 18℃ / s. The cooled hollow spindle is then heated to 660℃ and held for 5 hours, followed by air cooling. During the air cooling process, a magnetic field of 0.8T is simultaneously applied along the axial direction of the hollow spindle.

[0073] Example 2

[0074] A method for manufacturing hollow wind turbine main shafts from round steel continuous casting billets includes the following steps:

[0075] Step a: A 1000mm diameter continuous casting round billet is prepared using a vertical continuous casting machine. The operating parameters of the continuous casting machine are: frequency 5Hz, current 450A, and the agitator is 50mm away from the surface of the billet.

[0076] The composition of the continuously cast billet prepared by mass percentage is: C 0.40%, Si 0.25%, Mn 0.85%, Cr 1.18%, Mo 0.20%, V 0.10%, Al 0.03%, with the balance being iron and impurities. [O]=10ppm, [H]=1.2ppm.

[0077] Step b: Axial upsetting of the continuously cast billet with a forging ratio of 3.2 is performed at 1200℃ using a three-way pressurization device (lateral constraint force 20% axial pressure);

[0078] After axial upsetting, the continuously cast billet is spun in 5 passes using a 30° conical anvil, with a single pass deformation of 8%.

[0079] Step c: Use a φ400mm solid punch to punch a central through hole on the axial end face of the continuously cast billet at 950℃ to form a hollow spindle.

[0080] A twin-roll asynchronous mill is used to insert the inner roll into the center of the hollow main shaft, and the outer roll is attached to the outer wall of the hollow main shaft. The inner roll is then expanded at a speed of 60 r / min and the asynchronous ratio between the outer roll and the inner roll is 1:1.10. The outer roll and the inner roll rotate clockwise and counterclockwise respectively, and the expansion rate is controlled at 30 mm / min.

[0081] Step d: The hollow spindle with the expanded hole is placed at 930℃ for 2.5 hours for normalizing treatment. Then, the hollow spindle is cooled to 300℃ using an atomizing medium with an atomization pressure of 0.3 MPa and an air-to-water ratio of 5:1 at a cooling rate of 20℃ / s. The cooled hollow spindle is then heated to 660℃ and held for 5 hours, followed by air cooling. During the air cooling process, a magnetic field of 1.0T is simultaneously applied along the axial direction of the hollow spindle.

[0082] Example 3

[0083] A method for manufacturing hollow wind turbine main shafts from round steel continuous casting billets includes the following steps:

[0084] Step a: A 1200mm diameter continuous casting round billet is prepared using a vertical continuous casting machine. The operating parameters of the continuous casting machine are: frequency 5Hz, current 450A, and the agitator is 50mm away from the surface of the billet.

[0085] The composition of the continuously cast billet prepared by mass percentage is: C 0.42%, Si 0.35%, Mn 0.75%, Cr 1.25%, Mo 0.15%, V 0.12%, Al 0.03%, with the balance being iron and impurities. [O]=15ppm, [H]=1.4ppm.

[0086] Step b: Axial upsetting of the continuously cast billet with a forging ratio of 2.8 is performed at 1220℃ using a three-way pressurization device (lateral constraint force 15% axial pressure);

[0087] After axial upsetting, the continuously cast billet is spun in 4 passes using a 40° conical anvil, with a single pass deformation of 12%.

[0088] Step c: Next, perform deformation heat treatment: hold at 750℃ for 2 hours, and air cool to 500℃ at a rate of 10-15℃ / min. Then immediately use a φ400mm solid punch to punch a central through hole on the axial end face of the continuously cast billet at 1000℃ to form a hollow spindle.

[0089] A twin-roll asynchronous mill is used to insert the inner roll into the center of the hollow main shaft, and the outer roll is attached to the outer wall of the hollow main shaft. The inner roll is then expanded at a speed of 50 r / min and the asynchronous ratio between the outer roll and the inner roll is 1:1.12. The outer roll and the inner roll rotate clockwise and counterclockwise respectively, and the expansion rate is controlled at 30 mm / min.

[0090] Step d: The hollow spindle with the expanded hole is placed at 930℃ for 2 hours for normalizing treatment. Then, the hollow spindle with the normalizing treatment is cooled to 300℃ using an atomizing medium with an atomization pressure of 0.4 MPa and an air-to-water ratio of 3:1 at a cooling rate of 15℃ / s. The cooled hollow spindle is then heated to 660℃ and held for 4 hours, followed by air cooling. During the air cooling process, a magnetic field of 0.5T is simultaneously applied along the axial direction of the hollow spindle.

[0091] Example 4

[0092] A method for manufacturing hollow wind turbine main shafts from round steel continuous casting billets includes the following steps:

[0093] Step a: A 1500mm diameter continuous casting round billet is prepared using a vertical continuous casting machine. The operating parameters of the continuous casting machine are: frequency 5Hz, current 450A, and the agitator is 50mm away from the surface of the billet.

[0094] The composition of the continuously cast billet prepared by mass percentage is: C 0.39%, Si 0.30%, Mn 0.90%, Cr 1.22%, Mo 0.18%, V 0.09%, Al 0.05%, with the balance being iron and impurities. [O]=18ppm, [H]=1.7ppm.

[0095] Step b: Axial upsetting of the continuously cast billet with a forging ratio of 3.1 is performed at 1220℃ using a three-way pressurization device (lateral constraint force 18% axial pressure);

[0096] After axial upsetting, the continuously cast billet is spun in 5 passes using a 45° conical anvil, with a deformation of 10% per pass.

[0097] Step c: Next, perform deformation heat treatment: hold at 780℃ for 2 hours, and air cool to 500℃ at a rate of 10-15℃ / min. Then immediately use a φ400mm solid punch to punch a central through hole on the axial end face of the continuously cast billet at 1000℃ to form a hollow spindle.

[0098] A twin-roll asynchronous mill is used to insert the inner roll into the center of the hollow main shaft, and the outer roll is attached to the outer wall of the hollow main shaft. The inner roll is then expanded at a speed of 60 r / min and the asynchronous ratio between the outer roll and the inner roll is 1:1.12. The outer roll and the inner roll rotate clockwise and counterclockwise respectively, and the expansion rate is controlled at 25 mm / min.

[0099] Step d: The hollow spindle with the expanded hole is placed at 910℃ for 3 hours for normalizing treatment. Then, the hollow spindle is cooled to 300℃ using an atomizing medium with an atomization pressure of 0.5 MPa and an air-to-water ratio of 5:1 at a cooling rate of 20℃ / s. The cooled hollow spindle is then heated to 640℃ and held for 6 hours, followed by air cooling. During the air cooling process, a magnetic field of 1.0T is simultaneously applied along the axial direction of the hollow spindle.

[0100] Performance tests were conducted on the above embodiments, and the results are shown in the table below. The yield strength test specimens were circular specimens with a diameter of 10 mm, a gauge length of 50 mm, and a parallel section length of 60 mm. The impact energy test used V-notch specimens (10*10*55 mm) at a test temperature of -40±2℃. The ultrasonic flaw detection level II acceptance standard was that the equivalent diameter of a single defect ≤ φ4 mm and the defect spacing ≥ 30 mm. Grain size was evaluated using a comparative method; level 7 corresponds to a grain size of 32 μm, and level 8 corresponds to 22.5 μm. Porosity was clearly displayed using A1 type standard specimens, and the magnetic suspension concentration was 1.5-3.0 g / L.

[0101]

[0102] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for manufacturing hollow wind turbine main shafts from large-size round steel continuous casting billets, characterized in that, Includes the following steps: Step a: Prepare continuously cast round billets with a diameter ≥800mm using electromagnetic stirring continuous casting process; the chemical composition of the continuously cast round billets by weight percentage is: C, 0.38-0.42%; Si, 0.25-0.35%; Mn, 0.75-0.90%; Cr, 1.10-1.25%; Mo, 0.15-0.25%; V, 0.08-0.12%; Al, 0.02-0.05%; P≤0.012%, S≤0.003%, with the balance being Fe and unavoidable impurities, and satisfying [O]<15ppm, [H]<1.5ppm; Step b: Perform multi-directional forging on the continuously cast round billet, the multi-directional forging process including: Axial upsetting is performed by upsetting the continuously cast round billet at 1200±20℃ using a three-way pressure device, with an axial forging ratio of 2.8-3.2, and a radial constraint force of 15-20% of the axial pressure is applied simultaneously. Radial spinning is performed by using a conical anvil with a cone angle of 30-45° to spin the continuously cast round billet 3-5 times, with a single-pass deformation of 8-12%. Step c: Perform composite boring forming process on the multi-directional forged continuous casting round billet; Step d: Perform gradient heat treatment on the continuously cast round billet after composite boring, the gradient heat treatment including: After normalizing and holding at 920±10℃ for 2-3 hours, the continuously cast round billet is cooled to 300℃ using an atomized water-air mixture at a cooling rate of 15-20℃ / s. After tempering, the continuously cast round billet that has been cooled is heated to 650±10℃ and held for 4-6 hours before air cooling. During the air cooling process, a constant magnetic field of 0.5-1.0T is applied along the axial direction of the continuously cast round billet.

2. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that, In step a, the electromagnetic stirring parameters are: frequency 2-5Hz, current intensity 300-500A, and the distance between the stirrer and the surface of the billet is maintained at 50-80mm.

3. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that, The three-way pressurization device includes upper and lower hydraulic punches and an adjustable lateral constraint module. The lateral constraint force is adjusted in real time by a proportional valve.

4. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that, Step c includes: Pre-boring: Using a φ300-400mm solid punch at a temperature of 950-1000℃, the axial end face of the continuously cast round billet is bored to form a through hole structure; The continuous casting round billet with end face boring is expanded by skew rolling and a two-roll asynchronous rolling mill at a speed of 45-60 r / min. The asynchronous ratio is 1:1.05-1.12 and the expansion rate is ≤50 mm / min.

5. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that, The gas-water volume ratio of the atomizing water-air mixture is 3:1-5:1, the atomization pressure is 0.3-0.5 MPa, and the nozzles of the atomizing water-air mixture are arranged in a circumferential inward direction and are evenly distributed.

6. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that, The final product meets the following requirements: Yield strength ≥800MPa, impact energy at -40℃ ≥60J; Ultrasonic testing meets the Class II requirements of GB / T 6402-2008 standard; Grain size ≥ GB / T 6394-2017 standard grade 7.

7. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that, After the radial spinning stage, the continuously cast round billet undergoes deformation heat treatment. After holding at 750-780℃ for 1-2 hours, it is cooled to 500℃ at a rate of 10-15℃ / min and then immediately subjected to boring.

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