Method for manufacturing hollow wind power main shaft by using large-specification round steel continuous casting billet

Abstract

The invention provides a method for manufacturing a hollow wind power main shaft through a large-specification round steel continuous casting billet. High-performance manufacturing is achieved through a four-stage process. The preparation method comprises the following steps: firstly, preparing a continuous casting round billet by adopting an electromagnetic stirring continuous casting process, wherein the components are controlled as follows: 0.38-0.42% of C, 1.10-1.25% of Cr, 0.15-0.25% of Mo and 0.08-0.12% of V; multi-directional forging of axial upsetting and conical anvil spinning is implemented through the three-directional pressurizing device, and the porosity of the wind power main shaft is reduced in combination with radial constraining force. After a blank is pre-bored through a solid punch, skew rolling reaming is carried out through a double-roller asynchronous rolling mill, and the reaming rate is controlled to achieve uniform hole wall structures. The gradient heat treatment stage comprises normalizing matched with atomized steam cooling, and axial magnetic field applying during tempering, and the final product can achieve excellent mechanical property and grain size. Through the synergistic effect of multidirectional forging and component optimization, the technical problems of high porosity of the continuous casting billet and alloy segregation are solved.

Description

Technical Field

[0001] The invention relates to a method for manufacturing a hollow wind turbine main shaft, in particular to a method for manufacturing a hollow wind turbine main shaft by using a large-size round steel continuous casting billet. Background Art

[0002] Wind turbines, which convert wind energy into clean electricity, have become widely used. Their structure typically includes a hollow main shaft, often constructed as a single piece to reduce connection points, better transmit and distribute loads, increase overall structural strength, and extend the life of the wind turbine.

[0003] Traditional wind turbine main shafts mostly use the die-cast steel ingot forging process, but the riser of the die-cast steel ingot accounts for as much as 20%-30%. Removing the riser results in a low effective utilization rate of the steel ingot.

[0004] For example, patent application publication number CN117259627A discloses a method for manufacturing hollow wind turbine main shafts from large-diameter continuously cast billets. To improve material utilization, this method involves upsetting a continuously cast round billet, then ring-rolling the billet, boring and stretching the billet, and then tempering it. However, when the upsetting-forging ratio is ≤2.5, the equivalent strain of the billet is less than 1.8, making it impossible to fully close the inherent looseness of the continuously cast billet, resulting in residual porosity exceeding 0.1%.

[0005] Furthermore, existing technologies generally rely on adding 0.3-0.8% nickel to compensate for the hardenability of wind turbine main shafts. Various alloying elements, particularly nickel, segregate during the manufacturing process. For example, the difference in nickel content between the center and edge of the finished wind turbine main shaft wall thickness can reach 0.2%. This segregation leads to excessive hardenability in high-Ni areas, forming coarse martensite with grain sizes exceeding 50μm, which reduces the material's yield strength. Meanwhile, low-Ni areas exhibit insufficient hardenability, forming untempered bainite. This results in localized hardness fluctuations in the wind turbine main shaft and reduces impact toughness. Summary of the Invention

[0006] The purpose of the present invention is to overcome the shortcomings of the existing technology. On the one hand, it provides a method for manufacturing hollow wind turbine main shafts by continuously casting large-sized round steel billets, which solves the industry pain point of the existing technology of high residual porosity of continuous cast round billets; on the other hand, it alleviates the element segregation phenomenon in the component alloys through the combination of specific composition design and technology.

[0007] To achieve the above objectives, the present invention provides the following technical solutions: A method for manufacturing a hollow wind turbine main shaft from a large-size round steel continuous casting billet comprises the following steps: Step a: preparing a continuous casting round billet with a diameter of ≥800 mm by using an electromagnetic stirring continuous casting process; Step b: performing multi-directional forging on the continuously cast round billet; Step c: performing a composite boring forming process on the continuously cast round billet after multi-directional forging; Step d: performing gradient heat treatment on the continuous casting round billet after the composite boring.

[0008] Furthermore, in step a, the chemical composition of the continuously cast round billet is as follows by weight: 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%, the remainder being Fe and unavoidable impurities, and satisfying [O]<15ppm, [H]<1.5ppm.

[0009] Furthermore, in step a, the electromagnetic stirring parameters are: frequency 2-5 Hz, current intensity 300-500 A, and the distance between the stirrer and the surface of the ingot is maintained at 50-80 mm.

[0010] The electromagnetic stirring continuous casting process in step a refers to a method of driving the directional flow of molten steel through the Lorentz force generated by an alternating electromagnetic field, which is used to inhibit 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.

[0011] Furthermore, the step b includes: Axial upsetting: The continuous casting round billet is upset using a three-way pressurizing device at 1200±20℃, with an axial forging ratio of 2.8-3.2, and a radial restraining force of 15-20% of the axial pressure is applied simultaneously; Radial spinning: A conical anvil with a cone angle of 30-45° is used to spin the continuous casting round billet for 3-5 passes, with a single pass deformation of 8-12%.

[0012] The forging ratio in step b refers to the ratio of the billet's height after compression to its original height, reflecting the metal's flow rate. During radial spinning, the anvil angle influences the axial / radial flow ratio. A smaller anvil angle promotes radial deformation, while a single-pass deformation of 8-12% controls grain breakage and avoids the risk of cracking due to excessive deformation.

[0013] Furthermore, the three-way pressurizing device includes upper and lower hydraulic punches and an adjustable lateral restraint module, and the lateral restraint force is regulated in real time by a proportional valve.

[0014] Furthermore, the step c includes: Pre-boring: Use a φ300-400mm solid punch to bore the axial end face of the continuous casting round billet at a temperature of 950-1000℃ to form a through-hole structure; Cross-rolling hole expansion: The continuous casting round billet with end boring is expanded by a twin-roll asynchronous rolling mill at a speed of 45-60r / min, with an asynchronous ratio of 1:1.05-1.12 and an expansion rate of ≤50mm / min.

[0015] The diameter of the solid punch in step c matches the inner diameter of the final hollow shaft. It's worth noting that a punch diameter that's too small will result in excessive deformation during subsequent hole expansion. During the cross-rolling hole expansion process, the twin rolls of the twin-roll asynchronous rolling mill are positioned on the inner and outer sides of the material in the direction of the wall thickness. There's a certain speed difference between the upper and lower rolls to achieve shear strain in the wall thickness of the hollow spindle, promoting uniform flow of metal in the hole wall. The hole expansion rate is controlled below 50 mm / min to control the metal's recrystallization rate and avoid grain coarsening.

[0016] Furthermore, the step d includes: Normalizing treatment: After holding at 920 ± 10℃ for 2-3 hours, the continuous casting round billet is cooled to 300℃ using a mixture of atomized water and air at a cooling rate of 15-20℃ / s; Tempering treatment: The cooled continuous casting round billet is heated to 650 ± 10℃ and kept at this temperature for 4-6 hours before air cooling. During the air cooling process, a 0.5-1.0T constant magnetic field is applied along the axial direction of the continuous casting round billet.

[0017] Step d uses atomized water vapor as a mixed medium to normalize the wind turbine main shaft. The purpose of normalizing is to achieve grain refinement, microstructure homogenization, and carbide dispersion by controlling the decomposition process of austenite. For large-scale continuous casting billets, insufficient air cooling speed can lead to the formation of coarse grains and increased segregation of alloying elements. If air cooling with a low cooling rate is used, it is easy to cause the transformation time of austenite to ferrite / pearlite to be too long, which in turn leads to a grain size of more than 50μm, corresponding to ASTM material grade below level 4, and the yield strength is reduced to below 700MPa. In addition, during slow cooling, carbide-forming elements such as Cr and Mo are prone to enrichment and segregation at the grain boundaries.

[0018] Furthermore, the atomized water-gas mixture has an air-water volume ratio of 3:1-5:1, an atomization pressure of 0.3-0.5 MPa, and the nozzles for the atomized water-gas mixture are evenly distributed in a circular, inward direction. In step d, a relatively high air volume is used to maintain the cooling rate of the wind turbine main shaft, thereby avoiding bainite / martensite transformation, suppressing the formation of coarse ferrite, and preventing cracks.

[0019] Furthermore, the final product produced by the present invention satisfies: Yield strength ≥800MPa, impact energy at -40℃ ≥60J; ultrasonic flaw detection complies with GB / T 6402-2008 standard Level II requirements; grain size ≥GB / T 6394-2017 standard Level 7.

[0020] Furthermore, after the radial spinning stage, the continuous casting round billet is subjected to a deformation heat treatment, and after being kept at 750-780°C for 1-2 hours, it is cooled to 500°C at a rate of 10-15°C / min and then immediately subjected to a boring operation.

[0021] Deformation heat treatment is a composite process that combines plastic deformation with heat treatment, achieving microstructural control through strain-induced phase transformation. When held at 750-780°C, dislocations generated during spinning climb and rearrange, forming a subgrained structure. Subsequent cooling allows residual strain energy to drive carbide nucleation and refine the precipitated phase.

[0022] The advantages and beneficial effects of the present invention are: This invention utilizes a multi-directional forging process to achieve higher equivalent strain in continuously cast round billets, significantly reducing porosity compared to existing technologies and improving material density and mechanical properties. During the upsetting process, a 15-20% radial restraining force is simultaneously applied to the billet, effectively suppressing stress concentration caused by lateral metal flow and strengthening grain boundary bonding.

[0023] The present invention uses solid punch pre-boring combined with double-roll asynchronous rolling mill expansion to accurately control the expansion rate. Through the coupling effect of temperature field and strain field, the rheological reorganization of the hole wall metal is completed at a high temperature of 950-1000°C, so that the grain size uniformity of the hollow part is improved, and the problem of grain coarsening in the wall thickness direction caused by traditional boring is solved.

[0024] The present invention adopts a Cr-Mo-V alloy system and replaces nickel with carbide-forming elements, so that the Cr content fluctuation along the wall thickness direction of the finished hollow spindle is less than 0.08, and the Mo segregation index is reduced to below 1.15. Compared with the technical solution containing Ni, the segregation degree of the alloy elements is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a preparation process flow chart of the present invention; Figure 2 It is a structural schematic diagram of the three-way pressurizing device of the present invention; In the picture: 1-upper and lower hydraulic punches, 2-lateral restraint module. DETAILED DESCRIPTION

[0026] The present invention provides a method for manufacturing a hollow wind turbine main shaft from a large-scale round steel continuous casting billet. In step a, a Cr-Mo-V alloy system is used to replace the traditional Ni-based alloy, and hardenability is controlled based on the synergistic effect of carbide-forming elements. Electromagnetic stirring technology is used in the continuous casting stage, and the Lorentz force generated by the alternating magnetic field is used to drive the flow of molten steel, inhibit dendrite growth, and refine 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, thereby reducing the macrosegregation of the Cr / Mo / V elements. At the same time, the oxygen and hydrogen content is strictly controlled to reduce the tendency of loose defects to form at the source.

[0027] The skin depth control equation of electromagnetic stirring and molten steel is consistent with: ( =2πf) Where: , magnetic field penetration depth (m); f, stirring frequency (Hz); , magnetic permeability of molten steel (H / m); , conductivity (A / m).

[0028] During the manufacturing process of continuous casting 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 of ≥800 mm.

[0029] When f = 2 Hz, δ ≈ 120 mm, full-section stirring can be achieved, Cr / Mo element segregation can be suppressed, and the solute distribution coefficient k (grain boundary / intra-grain concentration ratio) can be reduced.

[0030] In step b, a combined plastic deformation strategy of axial upsetting and radial spinning is employed. A three-dimensional pressurizing device simultaneously applies axial pressure and radial restraint to achieve metal flow reorganization under a three-dimensional stress state. The equivalent strain generated by axial upsetting effectively closes the central shrinkage cavity of the continuous casting billet. Radial spinning, through 3-5 cumulative deformation passes, introduces shear strain, breaking up coarse grains and eliminating element segregation.

[0031] The equivalent strain equation of multi-directional forging conforms to:

[0032] Where: is the true strain of the hollow spindle axial upsetting, which is calculated as ln( H 0 / H 1 ), H 0 / H 1 The value range of is the upsetting ratio, which is 2.8-3.2. , is the true strain of the hollow spindle radial spinning, which is calculated as ln(1.08-1.12). It is necessary to explain that 、 and The vectors are perpendicular to each other and are dimensionless during the calculation process.

[0033] As a further explanation, axial pressure and radial constraint force can be achieved through the upper and lower hydraulic punches 1 and the lateral constraint module 2. Each component can be connected to hydraulic cylinders, servo push rods, cylinders and other components to radially and / or axially squeeze the wind turbine main shaft toward its center to shrink, constrain, or deform.

[0034] In step c, a 300-400mm φ solid punch is used for pre-boring at temperatures between 950°C and 1000°C, reducing the boring resistance through thermoplastic deformation. Subsequently, a twin-roll asynchronous rolling mill is used for cross-rolling expansion, utilizing the shear strain generated by the speed differential between the inner and outer rolls to promote uniform metal flow within the hole wall. This process controls the recrystallization process through temperature-strain coupling, improving the uniformity of the grain size distribution within the hole wall by over 40%, thereby resolving the problem of grain gradients across the wall thickness associated with conventional boring.

[0035] The shear strain rate equation for cross-rolling hole expansion is:

[0036] Where, , the speed difference between the outer roller and the inner roller (m / s), h is the wall thickness of the hollow spindle (m), is the shear strain rate (s -1 ). When the asynchronous ratio between the two rollers is 1:1.05-1.12, the shear strain rate is 0.8-1.2s -1 , the shear strain rate matches the recrystallization rate of the alloy, which has the effect of reducing the standard deviation of the pore wall grain size by about 40%.

[0037] As a preferred embodiment, step c can be combined with thermomechanical treatment to refine the grains through strain-induced phase transformation to improve the mechanical properties of the product, such as yield strength and impact energy.

[0038] In the normalizing stage of step d, atomized water-gas mixed cooling is used to suppress the coarsening of austenite at an appropriate cooling rate while avoiding the quenching stress caused by martensite transformation. During the tempering process, an axial steady magnetic field is applied to promote the orientation of carbide precipitation phase by magnetostrictive effect, so that Cr 23 The dispersion of C6 carbides is improved.

[0039] In step d, the stretching effect of tempering in a magnetic field environment complies with:

[0040] Where, is the dimensionless magnetostriction coefficient, and M is the magnetization intensity. Under a magnetic field of 0.5-1.0 T, M is approximately 1.6*10⁶ A / m. Under the influence of a magnetic field, carbides can be induced to precipitate along the (110) crystal plane, reducing the spacing of the precipitated phases from 150 nm to approximately 80 nm. This technical effect can effectively increase the impact energy of the hollow spindle.

[0041] As a further illustration 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 dispersed carbides are formed, the stability of austenite is enhanced, and the hardenability compensation of the Ni alloy element is achieved. Specifically, Cr compensates for the decrease in hardenability caused by the lack of Ni through the dual effects of solid solution strengthening and carbide precipitation. The diffusion coefficient of Mo in austenite is low, which can inhibit grain boundary segregation and alleviate the segregation problem of traditional alloys. V forms uniformly distributed carbides during the forging and heat treatment stages through the strain-induced precipitation mechanism, compensating for the loss of toughness caused by the lack of Ni. Cr 23 C6 provides matrix strengthening, 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.

[0042] The following embodiments are further described in conjunction with the accompanying drawings and examples. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0043] Example 1 A method for manufacturing a hollow wind turbine main shaft from a round steel continuous casting billet comprises the following steps: Step a: A vertical continuous casting machine is used to prepare a continuous casting round billet with a diameter of 800 mm. The operating parameters of the continuous casting machine are: frequency of 3 Hz, current of 380 A, and a stirrer 60 mm away from the billet surface.

[0044] The composition of the continuous casting billet, by mass percentage, is as follows: 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] = 12 ppm, [H] = 1.4 ppm.

[0045] Step b: axially upsetting the continuous casting billet with a forging ratio of 3.0 using a three-way pressing device (lateral constraint force 18% axial pressure) at 1180°C; After axial upsetting, the continuous casting billet was spun in four passes using a 40° conical anvil, with a deformation of 10% per pass.

[0046] Step c: Use a φ350mm solid punch to punch a center through hole at the axial end face of the continuous casting billet at 980°C to form a hollow main shaft.

[0047] A double-roll asynchronous rolling mill is used to insert the inner roller into the center of the hollow main shaft, and the outer roller fits the outer wall of the hollow main shaft. The inner roller is then rolled at a speed of 55r / min and the asynchronous ratio between the outer roller and the inner roller is 1:1.08 for hole expansion. The outer roller and the inner roller rotate clockwise and counterclockwise respectively, and the hole expansion rate is controlled at 45mm / min.

[0048] Step d: The expanded hollow spindle was normalized at 930°C for 2.5 hours. The spindle was then cooled to 300°C at a rate of 18°C / s using an atomizing medium with an atomizing pressure of 0.5 MPa and an air-to-water ratio of 4:1. The cooled spindle was then heated to 660°C for 5 hours and then air-cooled. A 0.8T magnetic field was applied along the axis of the spindle during the air-cooling process.

[0049] Example 2 A method for manufacturing a hollow wind turbine main shaft from a round steel continuous casting billet comprises the following steps: Step a: A vertical continuous casting machine is used to prepare a continuous casting round billet with a diameter of 1000 mm. The operating parameters of the continuous casting machine are: frequency of 5 Hz, current of 450 A, and a stirrer 50 mm away from the billet surface.

[0050] The composition of the continuous casting billet is as follows by mass: 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] = 10 ppm, [H] = 1.2 ppm.

[0051] Step b: axially upsetting the continuous casting billet with a forging ratio of 3.2 using a three-way pressing device (lateral constraint force 20% of axial pressure) at 1200°C; After axial upsetting, the continuous casting billet was spun five times using a 30° conical anvil, with a single-pass deformation of 8%.

[0052] Step c: Use a φ400mm solid punch to punch a center through hole at the axial end face of the continuous casting billet at 950°C to form a hollow main shaft.

[0053] A double-roll asynchronous rolling mill is used to insert the inner roller into the center of the hollow main shaft, and the outer roller fits the outer wall of the hollow main shaft. The inner roller is then rolled at a speed of 60r / min and the asynchronous ratio between the outer roller and the inner roller is 1:1.10 for hole expansion. The outer roller and the inner roller rotate clockwise and counterclockwise respectively, and the hole expansion rate is controlled at 30mm / min.

[0054] Step d: The expanded hollow spindle was normalized at 930°C for 2.5 hours. The spindle was then cooled to 300°C at a rate of 20°C / s using an atomizing medium with an atomizing pressure of 0.3 MPa and a 5:1 air-to-water ratio. The cooled spindle was then heated to 660°C for 5 hours and then air-cooled. A 1.0T magnetic field was applied along the axis of the spindle during the air-cooling process.

[0055] Example 3 A method for manufacturing a hollow wind turbine main shaft from a round steel continuous casting billet comprises the following steps: Step a: A vertical continuous casting machine is used to prepare a continuous casting round billet with a diameter of 1200 mm. The operating parameters of the continuous casting machine are: frequency of 5 Hz, current of 450 A, and a stirrer 50 mm away from the billet surface.

[0056] The composition of the continuous casting billet, by mass percentage, is as follows: 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] = 15 ppm, [H] = 1.4 ppm.

[0057] Step b: axially upsetting the continuously cast billet at a forging ratio of 2.8 using a three-way pressing device (lateral constraint force 15% of axial pressure) at 1220°C; After axial upsetting, the continuous casting billet was spun in four passes using a 40° conical anvil, with a deformation of 12% per pass.

[0058] Step c: The continuous casting blank is then subjected to thermomechanical treatment: heat preservation at 750°C for 2 hours, followed by air cooling at a rate of 10-15°C / min to 500°C. A φ400mm solid punch is then used to punch a central through hole in the axial end face of the continuous casting blank at 1000°C to form a hollow spindle.

[0059] A double-roll asynchronous rolling mill is used to insert the inner roller into the center of the hollow main shaft, and the outer roller fits the outer wall of the hollow main shaft. The inner roller is then rolled at a speed of 50r / min and the asynchronous ratio between the outer roller and the inner roller is 1:1.12 for hole expansion. The outer roller and the inner roller rotate clockwise and counterclockwise respectively, and the hole expansion rate is controlled at 30mm / min.

[0060] Step d: The expanded hollow spindle was normalized at 930°C for 2 hours. The spindle was then cooled to 300°C at a rate of 15°C / s using an atomizing medium with an atomizing pressure of 0.4 MPa and an air-to-water ratio of 3:1. The cooled spindle was then heated to 660°C and held for 4 hours, followed by air cooling. A 0.5T magnetic field was applied along the axis of the spindle during the air cooling process.

[0061] Example 4 A method for manufacturing a hollow wind turbine main shaft from a round steel continuous casting billet comprises the following steps: Step a: A vertical continuous casting machine is used to prepare a continuous casting round billet with a diameter of 1500 mm. The operating parameters of the continuous casting machine are: frequency of 5 Hz, current of 450 A, and a stirrer 50 mm away from the billet surface.

[0062] The composition of the continuous casting billet, by mass percentage, is as follows: 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] = 18 ppm, [H] = 1.7 ppm.

[0063] Step b: axially upsetting the continuous casting billet with a forging ratio of 3.1 using a three-way pressing device (lateral constraint force 18% of axial pressure) at 1220°C; After axial upsetting, the continuous casting billet was spun five times using a 45° conical anvil, with a single-pass deformation of 10%.

[0064] Step c: The continuous casting blank is then subjected to thermomechanical heat treatment: 780°C for 2 hours, followed by air cooling at a rate of 10-15°C / min to 500°C. A φ400mm solid punch is then used to punch a central through hole in the axial end face of the continuous casting blank at 1000°C to form a hollow spindle.

[0065] A double-roll asynchronous rolling mill is used to insert the inner roller into the center of the hollow main shaft, and the outer roller fits the outer wall of the hollow main shaft. The inner roller is then rolled at a speed of 60r / min and the asynchronous ratio between the outer roller and the inner roller is 1:1.12 for hole expansion. The outer roller and the inner roller rotate clockwise and counterclockwise respectively, and the hole expansion rate is controlled at 25mm / min.

[0066] Step d: The expanded hollow spindle was normalized at 910°C for 3 hours. The spindle was then cooled to 300°C at a rate of 20°C / s using an atomizing medium with an atomizing pressure of 0.5 MPa and an air-to-water ratio of 5:1. The cooled spindle was then heated to 640°C for 6 hours and then air-cooled. A 1.0T magnetic field was applied along the axis of the spindle during the air-cooling process.

[0067] Performance testing of the above examples was conducted, and the results are shown in the following table. 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. Impact energy testing was performed using V-notch specimens (10*10*55 mm) at a test temperature of -40±2°C. Ultrasonic flaw detection Level II acceptance criteria are a single defect equivalent diameter ≤ φ4 mm and defect spacing ≥ 30 mm. Grain size was assessed using a comparative method, with Grade 7 corresponding to a grain size of 32 μm and Grade 8 to 22.5 μm. Porosity was clearly displayed using an A1 standard test piece, and the magnetic suspension concentration was 1.5-3.0 g / L.

[0068]

[0069] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for manufacturing a hollow wind turbine main shaft from a large-size round steel continuous casting billet, characterized in that: The following steps are involved: Step a: preparing a continuous casting round billet with a diameter of ≥800 mm by using an electromagnetic stirring continuous casting process; Step b: performing multi-directional forging on the continuously cast round billet; Step c: performing a composite boring forming process on the continuously cast round billet after multi-directional forging; Step d: performing gradient heat treatment on the continuous casting round billet after the composite boring.

2. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that: In the step a, the chemical composition of the continuous casting round billet is as follows by weight: 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%, the balance being Fe and unavoidable impurities, and satisfying [O]<15ppm, [H]<1.5ppm.

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

4. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that: The step b comprises: Axial upsetting: The continuous casting round billet is upset using a three-way pressurizing device at 1200±20℃, with an axial forging ratio of 2.8-3.2, and a radial restraining force of 15-20% of the axial pressure is applied simultaneously; Radial spinning: A conical anvil with a cone angle of 30-45° is used to spin the continuous casting round billet for 3-5 passes, with a single pass deformation of 8-12%.

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

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

7. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that: The step d comprises: Normalizing treatment: After holding at 920 ± 10℃ for 2-3 hours, the continuous casting round billet is cooled to 300℃ using a mixture of atomized water and air at a cooling rate of 15-20℃ / s; Tempering treatment: The cooled continuous casting round billet is heated to 650 ± 10℃ and kept at this temperature for 4-6 hours before air cooling. During the air cooling process, a 0.5-1.0T constant magnetic field is applied along the axial direction of the continuous casting round billet.

8. The method for manufacturing a hollow wind turbine main shaft according to claim 6, characterized in that: The air-water volume ratio of the atomized water-gas mixed medium is 3:1-5:1, the atomization pressure is 0.3-0.5 MPa, and the nozzles of the atomized water-gas mixed medium are annularly inward and evenly distributed.

9. The method for manufacturing a hollow wind turbine main shaft according to claim 1, characterized in that: The final product meets: Yield strength ≥800MPa, -40℃ impact energy ≥60J; Ultrasonic flaw detection complies with GB / T 6402-2008 standard Level II requirements; Grain size ≥ GB / T 6394-2017 standard grade 7.

10. The method for manufacturing a hollow wind turbine main shaft according to claim 4, characterized in that: After the radial spinning stage, the continuous casting round billet is subjected to a deformation heat treatment, kept at 750-780°C for 1-2 hours, and then cooled to 500°C at a rate of 10-15°C / min and immediately bored.

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