Forging method of extra-large-section 42CrMo4 bearing steel

Abstract

The invention relates to the technical field of bearing steel forging, in particular to a forging method of extra-large-section 42CrMo4 bearing steel, which comprises the following steps of: when a bearing steel forge piece is heated before forging, controlling the pre-forging temperature of a bearing steel blank by adopting a staged gradient temperature control method; the temperature close to the surface layer of the bearing steel forging is relatively high, and the temperature of the inner core part of the bearing steel forging is relatively low, so that the 42CrMo4 bearing steel forging with the extra-large section has a synergistic state of high plasticity of the surface layer beneficial to deformation and high strength of the core part beneficial to crack inhibition before forging; and the forging defect caused by fast heat loss of the surface layer during subsequent forging is avoided, and meanwhile, the overburning or non-diathermanous defect is avoided by optimally setting a reasonable temperature gradient. The forging quality and the forging heating production efficiency can be improved, and the energy consumption of forging manufacturing is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of bearing steel forging, and specifically relates to a forging method for extra-large section 42CrMo4 bearing steel. Background Art

[0002] 42CrMo4 is a high-strength alloy structural steel, which is widely used in manufacturing mechanical parts requiring relatively high strength, toughness, and wear resistance. For large 42CrMo4 bearing steel, it is mainly used to produce key components in large or heavy mechanical equipment, such as large bearings, gears, etc. in wind power spindle components.

[0003] There are the following deficiencies in the forging heating of conventional 42CrMo4 bearing steel: First, in order to ensure that the entire forging (including the core) reaches the target temperature and compensate for the heat loss during the forging process, a relatively high heating temperature is usually adopted. For large forgings, due to the high heating temperature and the need for a long heating and holding time, it will lead to waste of energy and also reduce the production efficiency.

[0004] Second, the entire forging (including the core) is in a relatively high-temperature state for a long time, there is a risk of grain coarsening, resulting in a decrease in the quality of the forging.

[0005] Third, due to the forging being heated as a whole, large thermal stresses will occur during the cooling process of the forging, resulting in possible defects such as deformation and cracking. Summary of the Invention

[0006] In order to solve the above problems, the present invention proposes a forging method for extra-large section 42CrMo4 bearing steel, aiming to improve the quality of the forging and the production efficiency of forging heating, and reduce the energy consumption of forging manufacturing. The specific technical solutions are as follows: A forging method for extra-large section 42CrMo4 bearing steel. When the bearing steel forging is preheated before forging, a staged gradient temperature control method is used to control the pre-forging temperature of the bearing steel billet, so that the bearing steel forging forms a gradient temperature field with a relatively high temperature near the surface layer and a relatively low temperature in the internal core, that is, a high outside and low inside temperature field. Thus, the extra-large section 42CrMo4 bearing steel forging has a synergistic state of high surface plasticity conducive to deformation and high core strength conducive to suppressing cracks before forging, and avoids forging defects caused by rapid heat dissipation on the surface layer during subsequent forging. At the same time, reasonable temperature gradients are optimized to avoid overburning or incomplete through-heating defects.

[0007] As one of the optimized schemes of the staged gradient temperature control method for the bearing steel billet in the present invention, the staged gradient temperature control method for the bearing steel billet includes the following heating and temperature control strategies: (1)Division of temperature control regions of forgings: According to the depth of the distance from the outside to the inside of the forging, the forging is successively divided into three temperature control regions: the surface region, the shallow region, and the core region; (2)Preheating in stages before forging: The forging is preheated in stages, specifically divided into the following preheating stages: Full-region preheating stage: The forging is placed in a heating furnace, heated and insulated, and the whole forging is heated to a temperature slightly lower than the Ac3 temperature; Shallow-region temperature-rising preheating stage: A directional skin heating device is used to heat the surface region and the shallow region of the forging simultaneously, so that the temperatures of the surface region and the shallow region of the forging increase by a predetermined temperature value relative to the core region; Surface-region re-preheating temperature-rising stage: A directional skin heating device is used to heat the surface region of the forging alone, so that the temperature of the surface region of the forging increases by a predetermined temperature value relative to the shallow region, so that the forging forms a gradient temperature field with a high outside and a low inside.

[0008] As the second optimization scheme of the method for controlling the temperature in stages and in gradients of the bearing steel billet in the present invention, the method for controlling the temperature in stages and in gradients of the bearing steel billet includes the following heating and temperature control strategies: (1)Division of temperature control regions of forgings: According to the depth of the distance from the outside to the inside of the forging, the forging is successively divided into four temperature control regions: the surface region, the shallow region, the deep region, and the core region; (2)Preheating in stages before forging: The forging is preheated in stages, specifically divided into the following preheating stages: Full-region preheating stage: The forging is placed in a heating furnace, heated and insulated, and the whole forging is heated to a temperature slightly lower than the Ac3 temperature; Deep-region temperature-rising preheating stage: A directional skin heating device is used to heat the surface region, the shallow region, and the deep region of the forging simultaneously, so that the temperatures of the surface region, the shallow region, and the deep region of the forging increase by a predetermined temperature value relative to the core region; Shallow-region temperature-rising preheating stage: A directional skin heating device is used to heat the surface region and the shallow region of the forging simultaneously, so that the temperatures of the surface region and the shallow region of the forging increase by a predetermined temperature value relative to the deep region; Surface-region re-preheating temperature-rising stage: A directional skin heating device is used to heat the surface region of the forging alone, so that the temperature of the surface region of the forging increases by a predetermined temperature value relative to the shallow region, so that the forging forms a gradient temperature field with a high outside and a low inside.

[0009] During the full - area preheating stage, sufficient heat preservation is required to improve the material properties. For the subsequent heating of the layered areas, the heating time needs to be strictly controlled to achieve short - time heating, and over - burning should be avoided. Eventually, a good temperature gradient is formed inside the forging.

[0010] The above - mentioned temperature - gradient control strategy of setting the temperature of the forging to be higher outside and lower inside is beneficial to compensating for the heat loss during forging and making use of the characteristic that heat will transfer to the inside during the forging process, so that the temperature of the core part of the forging during forging is moderate and the austenite transformation is uniform. Thus, it can not only avoid the coarsening of grains caused by too high a temperature of the core part of the forging, which reduces the performance of the forging, but also avoid a series of problems such as insufficient plasticity, easy cracking, and poor heat treatment effect caused by too low a temperature of the core part of the forging.

[0011] Preferably, the directional skin - effect heating device includes a forging transfer conveyor line arranged between the heating furnace and the forging press for transferring the forging in the heating furnace to the forging press, and an inverted - U - shaped magnetic - induction heating tunnel covering the upper position of the forging transfer conveyor line for heating the forging while transporting it. When the forging moves from the heating furnace to the forging press direction on the forging transfer conveyor line, the inverted - U - shaped magnetic - induction heating tunnel sequentially conducts directional heating and temperature - rising on each layer area of the forging, so as to make the forging form a gradient temperature field with a higher temperature outside and a lower temperature inside.

[0012] Preferably, the inverted - U - shaped magnetic - induction heating tunnel includes a first inverted - U - shaped magnetic - induction heating tunnel module arranged in sections for heating the deep - layer area of the forging, a second inverted - U - shaped magnetic - induction heating tunnel module for heating the shallow - layer area of the forging, and a third inverted - U - shaped magnetic - induction heating tunnel module for heating the surface - layer area of the forging. And the first inverted - U - shaped magnetic - induction heating tunnel module, the second inverted - U - shaped magnetic - induction heating tunnel module, and the third inverted - U - shaped magnetic - induction heating tunnel module are arranged adjacent to each other in sequence according to the conveying direction of the forging. Among them, the first inverted - U - shaped magnetic - induction heating tunnel module is connected to a first intermediate - frequency heating power supply, the second inverted - U - shaped magnetic - induction heating tunnel module is connected to a second intermediate - frequency heating power supply, and the third inverted - U - shaped magnetic - induction heating tunnel module is connected to a high - frequency heating power supply. The first intermediate - frequency heating power supply, the second intermediate - frequency heating power supply, and the high - frequency heating power supply respectively heat the forging in sequence with different power frequencies, so as to form different skin - effect heating depths, and make the forging form a gradient temperature field with a higher temperature outside and a lower temperature inside.

[0013] Preferably, the power frequency of the first intermediate - frequency heating power supply is 1 kHz - 5 kHz, the power frequency of the second intermediate - frequency heating power supply is 5 kHz - 10 kHz, and the power frequency of the high - frequency heating power supply is above 10 kHz.

[0014] Preferably, the power frequency of the first intermediate frequency heating power supply is 1 kHz, achieving heating and temperature rise of about 20°C in a region with a skin depth of approximately 50 mm.

[0015] Preferably, the power frequency of the second intermediate frequency heating power supply is 5 kHz, achieving heating and temperature rise of about 60°C in a region with a skin depth of approximately 22 mm.

[0016] Preferably, the power frequency of the high-frequency heating power supply is 10 kHz, achieving heating and temperature rise of about 120°C in a region with a skin depth of approximately 16 mm.

[0017] Through the above three-stage heating and temperature rise control, the temperature distribution of the forging from the outside to the inside is as follows: The core temperature is T0; The deep layer temperature T1 = T0 + 20°C = T0 + 20°C; The shallow layer temperature T2 = T0 + 20°C + 60°C = T0 + 80°C; The surface layer temperature T3 = T1 = T0 + 20°C + 60°C + 120°C = T0 + 200°C.

[0018] Preferably, for 42CrMo4 bearing steel, the core temperature T0 can be set to the Ac3 line temperature (800°C - 850°C), or a temperature about 20°C below the Ac3 line (800°C - 850°C), that is, 780°C - 800°C.

[0019] Preferably, different stratified regions of the forging are selectively heated according to different current frequencies. High-frequency induction heating is mainly used for surface heating, while low frequency can penetrate deeper. By adjusting the parameters of the induction heater (such as power, frequency), the surface of the workpiece can reach the required working temperature, while the interior remains relatively low but high enough to support subsequent forging processing.

[0020] Preferably, the inverted U-shaped magnetic induction heating tunnel module includes a number of inverted U-shaped hollow conductive tubes arranged at intervals. Each of the inverted U-shaped hollow conductive tubes is sequentially connected end to end through a hollow conductive connecting tube to form an inverted U-shaped heating coil as a whole. Each section of the inverted U-shaped heating coil module is arranged adjacent to each other to form the inverted U-shaped magnetic induction heating tunnel; wherein, circulating cooling water flows through the hollow tube body of the inverted U-shaped heating coil.

[0021] Preferably, the inverted U-shaped heating coil is bent from a copper tube with a rectangular cross-section or a copper tube with a circular cross-section.

[0022] Preferably, the number of turns of the coil of the first inverted U-shaped magnetic induction heating tunnel module (the number of inverted U-shaped hollow conductive tubes) is greater than that of the second inverted U-shaped magnetic induction heating tunnel module, and the number of turns of the coil of the second inverted U-shaped magnetic induction heating tunnel module is greater than that of the coil in the third inverted U-shaped magnetic induction heating tunnel module.

[0023] In the present invention, an inverted U-shaped heat preservation cover is arranged on one side of the periphery of the inverted U-shaped magnetic induction heating tunnel; the inverted U-shaped magnetic induction heating tunnel is fixed on the inner wall of the inverted U-shaped heat preservation cover.

[0024] Preferably, the wall of the inverted U-shaped heat preservation cover is a water-cooled wall.

[0025] Preferably, a number of infrared thermometers can be arranged on the inverted U-shaped heat preservation cover to monitor the heating and temperature rising effects of the segmented inverted U-shaped magnetic induction heating tunnel modules on the forging, so as to achieve precise control of heating and temperature rising.

[0026] In order to enhance the effect of magnetic induction directional heating, a number of U-shaped magnetic conduction blocks with their openings facing the surface of the forging are sleeved on the inverted U-shaped heating coil and are arranged adjacent to each other in sequence.

[0027] Considering that the skin effect of high-frequency induction heating is relatively strong and the current is mainly concentrated near the surface of the workpiece. Therefore, when working at high frequency, the distance between the induction coil and the forging can be set relatively small. While the penetration depth of medium-frequency induction heating is relatively large, in order to make the magnetic field cover the surface of the workpiece more evenly, the distance between the induction coil and the forging can be appropriately increased.

[0028] Preferably, the forging transfer and conveying line includes a pair of support guide rails arranged at the opening position below the inverted U-shaped magnetic induction heating tunnel along the conveying direction of the forging, a moving seat movably arranged between the pair of support guide rails, a moving seat traction device arranged below the moving seat for driving the moving seat to move back and forth along the pair of support guide rails, a pair of roller shafts rotatably arranged on the moving seat for placing the cylindrical forging, and a roller shaft rotation driving device arranged on the moving seat for driving the roller shafts to rotate.

[0029] By arranging the roller shaft rotation driving device as described above, the cylindrical forging can be conveyed and freely rotated at the same time, thereby improving the heating uniformity of the inverted U-shaped magnetic induction heating tunnel for the forging.

[0030] Preferably, the roller shaft rotation driving device includes a driving motor arranged on the moving seat, a driving sprocket arranged on the motor shaft of the driving motor, a driven sprocket arranged on the roller shaft, and a transmission chain connected between the driving sprocket and the driven sprocket.

[0031] As a further improvement of the present invention, composite planar spiral magnetic induction heating coils for heating the end faces of cylindrical forgings are respectively erected on both sides of the moving base. The composite planar spiral magnetic induction heating coils are formed by sequentially and spacedly sleeving three types of planar spiral magnetic induction heating coils. The three types of planar spiral magnetic induction heating coils include a first planar spiral magnetic induction heating coil for heating the deep region of the end face of the cylindrical forging, a second planar spiral magnetic induction heating coil for heating the shallow region of the end face of the cylindrical forging, and a third planar spiral magnetic induction heating coil for heating the surface layer region of the end face of the cylindrical forging. Among them, the first planar spiral magnetic induction heating coil is connected to a first intermediate frequency heating power supply, the second planar spiral magnetic induction heating coil is connected to a second intermediate frequency heating power supply, and the third planar spiral magnetic induction heating coil is connected to a high-frequency heating power supply.

[0032] Preferably, a shielding cover for preventing the influence of the inverted U-shaped magnetic induction heating tunnel is provided at the outer circle part and the back of the composite planar spiral magnetic induction heating coil away from the end face of the cylindrical forging.

[0033] Preferably, the shielding cover is a water-cooled wall shielding cover.

[0034] Preferably, the composite planar spiral magnetic induction heating coil is fixed inside the shielding cover.

[0035] Preferably, each planar spiral magnetic induction heating coil in the composite planar spiral magnetic induction heating coil is formed by planar spiral winding of a hollow conductive tube, and circulating cooling water flows through the tube during operation.

[0036] Preferably, a number of U-shaped magnetic conduction blocks are successively and adjacently sleeved on each planar spiral magnetic induction heating coil, and the openings of the U-shaped magnetic conduction blocks face the end face of the cylindrical forging.

[0037] In order to facilitate the picking and placing of the forging on the moving base and prevent interference, telescopic cylinders are respectively provided on both sides of the lower part of the moving base, and the shielding cover is connected to the telescopic rods on the telescopic cylinders, so as to realize the longitudinal movement of the composite planar spiral magnetic induction heating coil built in the shielding cover.

[0038] Preferably, a connecting column is erected between the telescopic rod on the telescopic cylinder and the shielding cover, and a distance measuring sensor for detecting the distance between the composite planar spiral magnetic induction heating coil and the end face of the cylindrical forging is provided on the connecting column.

[0039] Preferably, the telescopic cylinder is a servo electric cylinder. By means of the servo electric cylinder, the distance between the composite planar spiral magnetic induction heating coil and the end face of the cylindrical forging can be finely adjusted to realize the optimization of the heating distance, so as to adapt to the precise control of the skin effect heating depth under different frequency power supplies.

[0040] A forging method of extra-large cross-section 42CrMo4 bearing steel, comprising the following steps: S1. Preheating the whole area of the forging: Put the forging into the heating furnace, heat and keep the forging warm, and heat and keep the whole forging warm to a temperature about 20°C below the Ac3 line (800°C - 850°C), that is, 780°C - 800°C; S2. Loading the forging: Open the furnace door of the heating furnace, and load the forging in the heating furnace onto a pair of roller shafts on the moving seat of the forging transfer conveyor line through the loader set beside the heating furnace; S3. Heating and warming up the forging in different areas while transferring: Start the roller rotation drive device to make the cylindrical forging rotate freely between a pair of roller shafts, and at the same time start the moving seat traction device to drive the moving seat to move, so that the freely rotating cylindrical forging moves towards the forging press on the forging transfer conveyor line. When the forging passes through the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module and the third inverted U-shaped magnetic induction heating tunnel module in sequence, turn on the corresponding inverted U-shaped magnetic induction heating tunnel modules in turn to achieve heating and transferring of the forging. By applying different power frequencies to the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module and the third inverted U-shaped magnetic induction heating tunnel module, different skin heating depths are formed on the outer circle of the cylindrical forging in each heating stage, so that a gradient temperature field with a high outer and low inner is formed on the forging; among them, when the cylindrical forging passes through the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module and the third inverted U-shaped magnetic induction heating tunnel module in sequence on the forging transfer conveyor line, the first planar spiral magnetic induction heating coil, the second planar spiral magnetic induction heating coil and the third planar spiral magnetic induction heating coil in the composite planar spiral magnetic induction heating coil are also turned on in sequence to heat the end face of the cylindrical forging, and by applying different power frequencies to each planar spiral magnetic induction heating coil, different skin heating depths are formed on the end face of the cylindrical forging in each heating stage, so that a gradient temperature field with a high outer and low inner is formed on the forging; S4. Forging: The forging that has been heated in stages and gradients on the forging transfer conveyor line finally reaches the position of the forging press. The forging manipulator beside the forging press transfers the forging to the forging press, and the forging press and the forging manipulator cooperate with each other to perform forging operations.

[0041] Preferably, the moving seat traction device is a traction transmission belt driven by a variable frequency motor. The lower end of the moving seat is connected to the traction transmission belt, and by changing the speed of the variable frequency motor, the variable speed movement of the forging on the forging transfer conveyor line is realized.

[0042] Preferably, the speed of the forging moving forward on the forging transfer conveyor line gradually transitions from slow to fast to ensure the heating and temperature rise effect of deeper parts.

[0043] In addition, the heating and temperature rise effect of each layer area can also be ensured by setting and adjusting the power of the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module, and the third inverted U-shaped magnetic induction heating tunnel module.

[0044] Preferably, in the step S3, during the process of the forging being transported and heated in sub-regions, when the cylindrical forging passes through the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module, and the third inverted U-shaped magnetic induction heating tunnel module in sequence on the forging transfer conveyor line, the distance between the composite planar spiral magnetic induction heating coil and the end face of the cylindrical forging is dynamically adjusted; among them, a larger distance is set when the forging is located in the first inverted U-shaped magnetic induction heating tunnel module, a smaller distance is set when the forging is located in the third inverted U-shaped magnetic induction heating tunnel module, and the distance when the forging is located in the second inverted U-shaped magnetic induction heating tunnel module is between the larger distance and the smaller distance.

[0045] The beneficial effects of the present invention are as follows: First, for a forging method of a super-large section 42CrMo4 bearing steel of the present invention, by adopting a staged gradient temperature control method to control the pre-forging temperature of the bearing steel billet, a temperature gradient is formed inside the forging where the temperature of the outer layer part is higher than that of the inner core part, which is beneficial to maintaining the plasticity of the workpiece surface during forging. At the same time, by using the characteristics of the relatively high temperature inside and the transfer of high-temperature heat from the outer layer to the relatively low-temperature area of the core to promote the overall deformation ability of the material, optimize the fluidity and forming performance of the material, reduce the energy consumption of forging heating, and ensure the quality of the forging.

[0046] Second, for a forging method of a super-large section 42CrMo4 bearing steel of the present invention, a forging preheating method combining in-furnace heating and an out-of-furnace forging transfer conveyor line is adopted. The directional skin heating device of the forging transfer conveyor line uses an inverted U-shaped magnetic induction heating tunnel with a staged skin heating effect. The inverted U-shaped magnetic induction heating tunnel is divided into multiple independent heating zones, and each zone is responsible for different heating depths, so as to efficiently achieve the effect of skin heating while transporting, so that the forging forms a multi-layer temperature gradient from the outside to the inside when it reaches the forging press. It can not only effectively compensate for the heat loss during the transfer process of the forging, but also optimize the fluidity and forming performance of the material, thereby improving the quality of forging.

[0047] Third, in a forging method of a super-large cross-section 42CrMo4 bearing steel according to the present invention, composite planar spiral magnetic induction heating coils are arranged on both sides of the moving seat. Through the planar spiral structure and inlay setting, the structure of the end face heating device is simplified, and flexible staged heating of the end face of the cylindrical forging is realized; the composite planar spiral magnetic induction heating coils cooperate with the inverted U-shaped magnetic induction heating tunnel, so that multi-layered temperature gradients from the outside to the inside can be formed in both the outer circle part and the end face part of the cylindrical forging, thereby further improving the forging quality.

[0048] Fourth, in a forging method of a super-large cross-section 42CrMo4 bearing steel according to the present invention, the inverted U-shaped magnetic induction heating tunnel utilizes the characteristic that the skin depth of intermediate-frequency induction heating and high-frequency heating decreases with the increase of frequency to form multi-layered temperature gradients on the forging with the external temperature higher than the temperature of the inner core part, which is beneficial to reducing thermal stress and further improving the forging quality. And because the magnetic induction heating time of the inverted U-shaped magnetic induction heating tunnel is short, its heating method combined with the in-furnace heating method makes the in-furnace heating temperature relatively low, thereby reducing the energy consumption of forging heating.

[0049] Fifth, in a forging method of a super-large cross-section 42CrMo4 bearing steel according to the present invention, by setting a roller shaft rotation drive device on the forging transfer conveyor line, the cylindrical forging can be transported and freely rotated at the same time, thereby improving the heating uniformity of the inverted U-shaped magnetic induction heating tunnel for the forging.

[0050] Sixth, in a forging method of a super-large cross-section 42CrMo4 bearing steel according to the present invention, the multi-layered temperature gradients inside the forging are beneficial to slowing down the speed of heat conduction to the core, thereby being beneficial to preventing the risk of forging quality decline caused by grain coarsening due to overheating of the core.

[0051] Seventh, in a forging method of a super-large cross-section 42CrMo4 bearing steel according to the present invention, each inverted U-shaped magnetic induction heating tunnel module in the inverted U-shaped magnetic induction heating tunnel adopts sequential alternate interval operation, and each planar spiral magnetic induction heating coil in the composite planar spiral magnetic induction heating coil adopts sequential alternate interval operation, which can avoid overheating caused by the long-term continuous operation of each module and each planar spiral magnetic induction heating coil, thereby improving the working reliability of the magnetic induction heating equipment. In addition, the distance between the composite planar spiral magnetic induction heating coil and the end face of the cylindrical forging is adjustable. On the one hand, it facilitates the loading and unloading of the forging, and on the other hand, it can optimize the heating effect at different power frequencies and realize the optimal control of the skin heating depth of each layer.

[0052] Eighth, in the forging method of a super-large-section 42CrMo4 bearing steel of the present invention, during the furnace heating and heat preservation stage, the target temperature of the core part is appropriately reduced (lower than the Ac3 temperature, such as 780°C - 800°C) to reserve a temperature rise space for subsequent induction heating, and finally a multi-level gradient temperature difference from the outside to the inside (total temperature difference of 200°C) is formed in the forging. This can improve the formability and plasticity of the surface layer, improve the internal tissue performance, and reduce the cracking risk caused by thermal stress concentration. Description of the Drawings

[0053] Figure 1 is a schematic diagram of the temperature control area division of the forging of a super-large-section 42CrMo4 bearing steel of the present invention; Figure 2 is a schematic diagram of the forging heating process of a forging method of a super-large-section 42CrMo4 bearing steel of the present invention; Figure 3 is a schematic diagram of the structure of a directional skin effect heating device used in the forging method of a super-large-section 42CrMo4 bearing steel of the present invention; Figure 4 is Figure 3 a partial enlarged view of Figure 5 is Figure 3 a cross-sectional view (left view) of the inverted U-shaped magnetic induction heating tunnel part involved in Figure 6 is a schematic diagram of the sleeve setting of a composite planar spiral magnetic induction heating coil; Figure 7 is a schematic diagram of the structure of a magnetic conduction block arranged on a U-shaped heating coil.

[0054] In the figure: 1. Bearing steel forging, 2. Surface area, 3. Shallow layer area, 4. Deep layer area, 5. Core area, 6. Heating furnace, 7. Forging press, 8. Forging transfer and conveying line, 9. Inverted U-shaped magnetic induction heating tunnel, 10. First inverted U-shaped magnetic induction heating tunnel module, 11. Second inverted U-shaped magnetic induction heating tunnel module, 12. Third inverted U-shaped magnetic induction heating tunnel module, 13. First intermediate frequency heating power supply, 14. Second intermediate frequency heating power supply, 15. High frequency heating power supply, 16. Inverted U-shaped hollow conducting tube, 17. Hollow conducting connecting tube, 18. Inverted U-shaped heating coil, 19. Inverted U-shaped heat preservation cover, 20. Infrared thermometer, 21. U-shaped magnetic conduction block, 22. Shielding cover, 23. Support guide rail, 24. Moving seat, 25. Moving seat traction device, 26. Roller shaft, 27. Roller shaft rotation driving device, 28. Traction drive belt, 29. Composite planar spiral magnetic induction heating coil, 30. First planar spiral magnetic induction heating coil, 31. Second planar spiral magnetic induction heating coil, 32. Third planar spiral magnetic induction heating coil, 33. Telescopic cylinder, 34. Variable frequency motor, 35. Driving motor, 36. Transmission chain. Detailed implementation manners

[0055] The following combines the accompanying drawings and embodiments to further describe the detailed implementation manners of the present invention. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and cannot be used to limit the protection scope of the present invention.

[0056] As Figures 1 to 7 shown in the embodiment of a forging method for a super-large cross-section 42CrMo4 bearing steel of the present invention, when the bearing steel forging is preheated, a step-by-step gradient temperature control method is adopted to control the preheating temperature of the bearing steel billet, so that the bearing steel forging forms a gradient temperature field with a relatively high temperature near the surface and a relatively low temperature in the inner core, that is, a high outside and low inside temperature field, so that the super-large cross-section 42CrMo4 bearing steel forging has a synergistic state of high surface plasticity conducive to deformation and high core strength conducive to suppressing cracks before forging, and avoids forging defects caused by rapid heat dissipation on the surface during subsequent forging. At the same time, reasonable temperature gradients are optimized to avoid overburning or incomplete heat penetration defects.

[0057] As one of the optimized solutions of the step-by-step gradient temperature control method for the bearing steel billet in this embodiment, the step-by-step gradient temperature control method for the bearing steel billet includes the following heating and temperature control strategies: (1) Division of the temperature control area of the forging: According to the depth of the distance from the outside to the inside of the forging, the forging 1 is successively divided into three temperature control areas: the surface area 2, the shallow layer area 3 and the core area 5; (2) Step-by-step preheating before forging: The forging is preheated step by step, specifically divided into the following preheating stages: Full - area preheating stage: Put the forging into the heating furnace, heat and keep the forging warm, and heat the whole forging to a temperature slightly lower than the Ac3 temperature; Shallow - layer area temperature - rising preheating stage: Use a directional skin - effect heating device to heat the surface layer area 2 and the shallow - layer area 3 of the forging simultaneously, so that the temperatures of the surface layer area 2 and the shallow - layer area 3 of the forging increase by a predetermined temperature value relative to the core area 5; Surface - layer area reheating and temperature - rising stage: Use a directional skin - effect heating device to heat the surface layer area 2 of the forging alone, so that the surface layer area 2 of the forging increases by a predetermined temperature value relative to the shallow - layer area 3, thus forming a gradient temperature field with a higher outer and lower inner temperature in the forging.

[0058] As the second optimization scheme of the staged gradient temperature control method for the bearing steel billet in this embodiment, the staged gradient temperature control method for the bearing steel billet includes the following heating and temperature control strategies: (1) Division of the temperature - controlled areas of the forging: According to the depth of the distance from the outside to the inside of the forging, the forging 1 is successively divided into four temperature - controlled areas: the surface layer area 2, the shallow - layer area 3, the deep - layer area 4, and the core area 5; (2) Pre - heating in stages before forging: Pre - heat the forging in stages, specifically divided into the following pre - heating stages in sequence: Full - area preheating stage: Put the forging into the heating furnace, heat and keep the forging warm, and heat the whole forging to a temperature slightly lower than the Ac3 temperature; Deep - layer area temperature - rising preheating stage: Use a directional skin - effect heating device to heat the surface layer area 2, the shallow - layer area 3, and the deep - layer area 4 of the forging 1 simultaneously, so that the temperatures of the surface layer area 2, the shallow - layer area 3, and the deep - layer area 4 of the forging 1 increase by a predetermined temperature value relative to the core area 5; Shallow - layer area temperature - rising preheating stage: Use a directional skin - effect heating device to heat the surface layer area 2 and the shallow - layer area 3 of the forging simultaneously, so that the temperatures of the surface layer area 2 and the shallow - layer area 3 of the forging increase by a predetermined temperature value relative to the deep - layer area 4; Surface - layer area reheating and temperature - rising stage: Use a directional skin - effect heating device to heat the surface layer area 2 of the forging alone, so that the surface layer area 2 of the forging increases by a predetermined temperature value relative to the shallow - layer area 3, thus forming a gradient temperature field with a higher outer and lower inner temperature in the forging.

[0059] The full - area preheating stage requires sufficient heat preservation to improve the material properties. The subsequent temperature - rising preheating of the layered areas requires strict control of the heating time to achieve heating in a shorter time, and at the same time, avoid over - burning, and finally form a good temperature gradient inside the forging.

[0060] The above temperature gradient control strategy of setting the temperature of the forging to be higher outside and lower inside is beneficial to compensating for the heat loss during forging, and taking advantage of the characteristic that heat will transfer inward during the forging process, so that the temperature of the core of the forging during the forging process is moderate and the austenite transformation is uniform. Thus, it can not only avoid the coarsening of grains caused by too high temperature of the core of the forging and reduce the performance of the forging, but also avoid a series of problems such as insufficient plasticity, easy cracking and poor heat treatment effect caused by too low temperature of the core of the forging.

[0061] Preferably, the directional skin heating device includes a forging transfer conveyor line 8 arranged between the heating furnace 6 and the forging press 7 for transferring the forging in the heating furnace 6 to the forging press 7, and an inverted U-shaped magnetic induction heating tunnel 9 covering the upper position of the forging transfer conveyor line 8 for heating the forging while conveying. When the forging moves from the heating furnace 6 to the forging press 7 direction on the forging transfer conveyor line 8, the inverted U-shaped magnetic induction heating tunnel 9 sequentially conducts directional heating and temperature increase on each layer area of the forging, so as to make the forging form a gradient temperature field with higher outside and lower inside.

[0062] Preferably, the inverted U-shaped magnetic induction heating tunnel 9 includes a first inverted U-shaped magnetic induction heating tunnel module 10 for heating the deep layer area 4 of the forging, a second inverted U-shaped magnetic induction heating tunnel module 11 for heating the shallow layer area 3 of the forging, and a third inverted U-shaped magnetic induction heating tunnel module 12 for heating the surface layer area 2 of the forging, and the first inverted U-shaped magnetic induction heating tunnel module 10, the second inverted U-shaped magnetic induction heating tunnel module 11 and the third inverted U-shaped magnetic induction heating tunnel module 13 are arranged adjacent to each other in sequence according to the conveying direction of the forging; wherein, the first inverted U-shaped magnetic induction heating tunnel module 10 is connected to a first intermediate frequency heating power supply 13, the second inverted U-shaped magnetic induction heating tunnel module 11 is connected to a second intermediate frequency heating power supply 14, the third inverted U-shaped magnetic induction heating tunnel module 12 is connected to a high frequency heating power supply 15, and the first intermediate frequency heating power supply 13, the second intermediate frequency heating power supply 14 and the high frequency heating power supply 15 respectively heat the forging in sequence with different power frequencies, so as to form different skin heating depths, and make the forging form a gradient temperature field with higher outside and lower inside.

[0063] Preferably, the power frequency of the first intermediate frequency heating power supply 13 is 1 kHz to 5 kHz, the power frequency of the second intermediate frequency heating power supply 14 is 5 kHz to 10 kHz, and the power frequency of the high frequency heating power supply 15 is above 10 kHz.

[0064] Preferably, the power frequency of the first intermediate frequency heating power supply 13 is 1 kHz, and it realizes heating and temperature increase of about 20°C in the area range with a skin depth of about 50 mm.

[0065] Preferably, the power frequency of the second intermediate frequency heating power supply 14 is 5 kHz, achieving heating and temperature rise of about 60 °C in a region with a skin depth of about 22 mm.

[0066] Preferably, the power frequency of the high-frequency heating power supply 15 is 10 kHz, achieving heating and temperature rise of about 120 °C in a region with a skin depth of about 16 mm.

[0067] Through the above three-stage heating and temperature rise control, the temperature distribution of the forging from the outside to the inside is as follows: Core temperature T0; Deep layer temperature T1 = T0 + 20 °C = T0 + 20 °C; Shallow layer temperature T2 = T0 + 20 °C + 60 °C = T0 + 80 °C; Surface layer temperature T3 = T1 = T0 + 20 °C + 60 °C + 120 °C = T0 + 200 °C.

[0068] Preferably, for 42CrMo4 bearing steel, the core temperature T0 can be set to the Ac3 line temperature (800 °C - 850 °C), or a temperature about 20 °C below the Ac3 line (800 °C - 850 °C), that is, 780 °C - 800 °C.

[0069] Preferably, different stratification regions of the forging are selectively heated according to different current frequencies. High-frequency induction heating is mainly used for surface heating, while low frequency can penetrate deeper. By adjusting the parameters of the induction heater (such as power, frequency), the surface of the workpiece can reach the required working temperature, while the interior remains relatively low but sufficient to support subsequent forging processes.

[0070] Preferably, the inverted U-shaped magnetic induction heating tunnel modules 10, 11, 12 include a number of inverted U-shaped hollow conductive tubes 16 arranged at intervals. Each of the inverted U-shaped hollow conductive tubes 16 is connected end to end in sequence through a hollow conductive connecting tube 17 to form an inverted U-shaped heating coil 18 as a whole. Each section of the inverted U-shaped heating coil modules 10, 11, 12 is arranged adjacent to each other to form the inverted U-shaped magnetic induction heating tunnel 9; wherein, circulating cooling water flows through the hollow tube body of the inverted U-shaped heating coil 18.

[0071] Preferably, the inverted U-shaped heating coil 18 is bent from a copper tube with a rectangular cross-section or a copper tube with a circular cross-section.

[0072] Preferably, the number of turns of the coil of the first inverted U-shaped magnetic induction heating tunnel module 10 (the number of inverted U-shaped hollow conductive tubes 16) is greater than that of the second inverted U-shaped magnetic induction heating tunnel module 11, and the number of turns of the coil of the second inverted U-shaped magnetic induction heating tunnel module 11 is greater than that of the coil in the third inverted U-shaped magnetic induction heating tunnel module 12.

[0073] In this embodiment, an inverted U-shaped heat insulation cover 19 is provided on one side of the periphery of the inverted U-shaped magnetic induction heating tunnel 9; the inverted U-shaped magnetic induction heating tunnel 9 is fixed on the inner wall of the inverted U-shaped heat insulation cover 19.

[0074] Preferably, the wall of the inverted U-shaped heat insulation cover 19 is a water-cooled wall.

[0075] Preferably, a plurality of infrared temperature detectors 20 can be provided on the inverted U-shaped heat insulation cover 19 to monitor the heating and temperature rising effects of the segmented inverted U-shaped magnetic induction heating tunnel modules 10, 11, and 12 on the forging, so as to achieve precise control of the heating and temperature rising.

[0076] In order to enhance the effect of magnetic induction directional heating, a plurality of U-shaped magnetic conduction blocks 21 with their openings facing the surface of the forging 1 are sleeved on the inverted U-shaped heating coil 18 and are arranged adjacent to each other in sequence.

[0077] Considering that the skin effect of high-frequency induction heating is relatively strong and the current is mainly concentrated near the surface of the workpiece. Therefore, when working at high frequencies, the distance between the induction coil and the forging can be set relatively small. While the penetration depth of medium-frequency induction heating is relatively large, in order to make the magnetic field cover the surface of the workpiece more evenly, the distance between the induction coil and the forging can be appropriately increased.

[0078] Preferably, the forging transfer and conveying line 8 includes a pair of supporting guide rails 23 arranged along the conveying direction of the forging at the opening position below the inverted U-shaped magnetic induction heating tunnel 9, a moving seat 24 movably arranged between the pair of supporting guide rails 23, a moving seat traction device 25 arranged at the lower position of the moving seat 24 for driving the moving seat 24 to move back and forth along the pair of supporting guide rails 23, a pair of roller shafts 26 rotatably arranged on the moving seat 24 for placing the cylindrical forging 1, and a roller shaft rotation driving device 27 arranged on the moving seat 24 for driving the roller shafts 26 to rotate.

[0079] By arranging the roller shaft rotation driving device 27 as described above, the cylindrical forging 1 can be conveyed and freely rotated at the same time, thereby improving the heating uniformity of the inverted U-shaped magnetic induction heating tunnel 9 for the forging.

[0080] Preferably, the roller shaft rotation driving device 27 includes a driving motor 35 disposed on the moving seat 24, a driving sprocket disposed on the motor shaft of the driving motor 35, a driven sprocket disposed on the roller shaft 26, and a transmission chain 36 connected between the driving sprocket and the driven sprocket.

[0081] As a further improvement of this embodiment, composite planar spiral magnetic induction heating coils 29 for heating the end faces of the cylindrical forgings 1 are respectively erected on both sides of the moving seat 24. The composite planar spiral magnetic induction heating coil 19 is formed by sequentially and spacedly sleeving three kinds of planar spiral magnetic induction heating coils. The three kinds of planar spiral magnetic induction heating coils include a first planar spiral magnetic induction heating coil 30 for heating the deep region 4 of the end face of the cylindrical forging 1, a second planar spiral magnetic induction heating coil 31 for heating the shallow region 3 of the end face of the cylindrical forging, and a third planar spiral magnetic induction heating coil 32 for heating the surface layer region 2 of the end face of the cylindrical forging 1. Among them, the first planar spiral magnetic induction heating coil 30 is connected to a first intermediate frequency heating power supply 13, the second planar spiral magnetic induction heating coil 31 is connected to a second intermediate frequency heating power supply 14, and the third planar spiral magnetic induction heating coil 32 is connected to a high frequency heating power supply 15.

[0082] Preferably, the composite planar spiral magnetic induction heating coil 29 is provided with a shielding cover 22 at its outer circular part and the back away from the end face of the cylindrical forging to avoid being affected by the inverted U-shaped magnetic induction heating tunnel 9.

[0083] Preferably, the shielding cover 22 is a water-cooled wall shielding cover.

[0084] Preferably, the composite planar spiral magnetic induction heating coil 29 is fixed in the shielding cover 22.

[0085] Preferably, each of the planar spiral magnetic induction heating coils 30, 31, 32 in the composite planar spiral magnetic induction heating coil 29 is formed by planar spiral winding of a hollow conductive tube, and circulating cooling water flows through the tube during operation.

[0086] Preferably, a number of U-shaped magnetic conduction blocks (not shown in the figure) with their openings facing the end face of the cylindrical forging 1 are successively and adjacently sleeved on each of the planar spiral magnetic induction heating coils 30, 31, 32.

[0087] In order to facilitate the picking and placing of the forgings on the moving seat 24 and prevent interference, telescopic cylinders 33 are respectively disposed on both sides of the lower part of the moving seat 24. The shielding cover 22 is connected to the telescopic rods on the telescopic cylinders 33, so as to realize the longitudinal movement of the composite planar spiral magnetic induction heating coil 29 built in the shielding cover 22.

[0088] Preferably, a connecting column is erected between the telescopic rod on the telescopic cylinder 33 and the shielding cover 22, and a distance measuring sensor for detecting the distance between the composite planar spiral magnetic induction heating coil 29 and the end face of the cylindrical forging 1 is arranged on the connecting column.

[0089] Preferably, the telescopic cylinder 33 is a servo electric cylinder. Through the servo electric cylinder, the distance between the composite planar spiral magnetic induction heating coil 29 and the end face of the cylindrical forging 1 can be finely adjusted to optimize the heating distance and adapt to the precise control of the skin heating depth under different frequency power supplies.

[0090] A forging method for a super-large cross-section 42CrMo4 bearing steel includes the following steps: S1. Preheating the entire area of the forging: Put the forging into the heating furnace 6, heat and keep the forging warm, and heat and keep the whole forging warm to a temperature about 20°C below slightly lower than the Ac3 line (800°C - 850°C), that is, 780°C - 800°C; S2. Loading the forging: Open the furnace door of the heating furnace 6, and load the forging in the heating furnace 6 onto a pair of roller shafts 26 on the moving seat 24 of the forging transfer conveyor line 8 through the loader arranged beside the heating furnace 6; S3. Transfer and heat the forging in different areas while transferring: Start the roller rotation drive device 27 to make the cylindrical forging 1 rotate freely between a pair of rollers 26. At the same time, start the moving seat traction device 25 to drive the moving seat 24 to move, so that the freely rotating cylindrical forging 1 moves towards the forging press 7 on the forging transfer conveyor line 8. When the forging passes through the first inverted U-shaped magnetic induction heating tunnel module 10, the second inverted U-shaped magnetic induction heating tunnel module 11, and the third inverted U-shaped magnetic induction heating tunnel module 12 in sequence, turn on the corresponding inverted U-shaped magnetic induction heating tunnel modules 10, 11, and 12 in turn to realize heating and transferring the forging at the same time. By applying different power frequencies to the first inverted U-shaped magnetic induction heating tunnel module 10, the second inverted U-shaped magnetic induction heating tunnel module 11, and the third inverted U-shaped magnetic induction heating tunnel module 12, different skin heating depths are formed on the outer circle of the cylindrical forging 1 at each heating stage, so that a gradient temperature field with a high outer and low inner temperature is formed on the forging; among them, when the cylindrical forging 1 passes through the first inverted U-shaped magnetic induction heating tunnel module 10, the second inverted U-shaped magnetic induction heating tunnel module 11, and the third inverted U-shaped magnetic induction heating tunnel module 13 on the forging transfer conveyor line 8 in sequence, the first planar spiral magnetic induction heating coil 30, the second planar spiral magnetic induction heating coil 31, and the third planar spiral magnetic induction heating coil 32 in the composite planar spiral magnetic induction heating coil 29 are also turned on in turn to heat the end face of the cylindrical forging 1, and by applying different power frequencies to each planar spiral magnetic induction heating coil 30, 31, and 32, different skin heating depths are formed on the end face of the cylindrical forging 1 at each heating stage, so that a gradient temperature field with a high outer and low inner temperature is formed on the forging; S4. Forging: The forging that has been heated in stages and gradients on the forging transfer conveyor line 8 finally reaches the position of the forging press 7. The forging manipulator beside the forging press 7 transfers the forging 1 to the forging press 7, and the forging press 7 and the forging manipulator cooperate with each other to perform forging operations.

[0091] Preferably, the moving seat traction device 25 is a traction drive belt 28 driven by a variable-frequency motor 34. The lower end of the moving seat 24 is connected to the traction drive belt 28, and by changing the speed of the variable-frequency motor 34, the variable-speed movement of the forging on the forging transfer conveyor line 8 is realized.

[0092] Preferably, the forward movement speed of the forging on the forging transfer conveyor line 8 gradually transitions from slow to fast to ensure the heating and temperature rise effect of deeper parts.

[0093] In addition, the heating and temperature rise effect of each layer area can also be ensured by setting and adjusting the power of the first inverted U-shaped magnetic induction heating tunnel module 10, the second inverted U-shaped magnetic induction heating tunnel module 11, and the third inverted U-shaped magnetic induction heating tunnel module 12.

[0094] Preferably, during the step S3 of heating and warming up the forging while transporting it in sub-regions, when the cylindrical forging 1 passes through the first inverted U-shaped magnetic induction heating tunnel module 10, the second inverted U-shaped magnetic induction heating tunnel module 11, and the third inverted U-shaped magnetic induction heating tunnel module 12 in sequence on the forging transfer conveyor line 8, the distance between the composite planar spiral magnetic induction heating coil 29 and the end face of the cylindrical forging 1 is dynamically adjusted; wherein, a larger distance is set when the forging is located in the first inverted U-shaped magnetic induction heating tunnel module, a smaller distance is set when the forging is located in the third inverted U-shaped magnetic induction heating tunnel module 12, and the distance when the forging is located in the second inverted U-shaped magnetic induction heating tunnel module 11 is between the larger distance and the smaller distance.

[0095] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the technical principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A forging method of extra-large cross-section 42CrMo4 bearing steel, characterized in that, When the bearing steel forging is preheated before forging, a staged gradient temperature control method is adopted to control the pre-forging temperature of the bearing steel billet, so that the bearing steel forging forms a gradient temperature field with a relatively higher temperature near the surface layer and a relatively lower temperature in the internal core, that is, a temperature field with a higher outside and a lower inside. As a result, the extra-large section 42CrMo4 bearing steel forging has a synergistic state of high surface plasticity conducive to deformation and high core strength conducive to crack inhibition before forging, and avoids forging defects caused by rapid heat dissipation on the surface layer during subsequent forging. At the same time, by optimizing and setting a reasonable temperature gradient, overburning or non-through heating defects are avoided.

2. The forging method of a super-large cross-section 42CrMo4 bearing steel according to claim 1, characterized in that, The staged gradient temperature control method for the bearing steel billet includes the following heating and temperature control strategies: (1)Division of the temperature control area of the forging: According to the depth of the distance from the outside to the inside of the forging, the forging is successively divided into three temperature control areas: the surface layer area, the shallow layer area, and the core area; (2)Staged preheating before forging: The forging is preheated in stages, specifically divided into the following preheating stages: Full area preheating stage: The forging is placed in a heating furnace, heated and insulated, and the whole forging is heated to slightly lower than the Ac3 temperature; Shallow layer area heating-up preheating stage: A directional skin effect heating device is used to heat the surface layer area and the shallow layer area of the forging simultaneously, so that the temperatures of the surface layer area and the shallow layer area of the forging are increased by a predetermined temperature value relative to the core area; Surface layer area reheating and heating-up stage: A directional skin effect heating device is used to heat the surface layer area of the forging alone, so that the temperature of the surface layer area of the forging is increased by a predetermined temperature value relative to the shallow layer area, so that the forging forms a gradient temperature field with a higher outside and a lower inside.

3. A forging method of a super-large cross-section 42CrMo4 bearing steel according to claim 1, characterized in that, The staged gradient temperature control method for the bearing steel billet includes the following heating and temperature control strategies: (1)Division of the temperature control area of the forging: According to the depth of the distance from the outside to the inside of the forging, the forging is successively divided into four temperature control areas: the surface layer area, the shallow layer area, the deep layer area, and the core area; (2)Staged preheating before forging: The forging is preheated in stages, specifically divided into the following preheating stages: Full area preheating stage: The forging is placed in a heating furnace, heated and insulated, and the whole forging is heated to slightly lower than the Ac3 temperature; Deep layer area heating-up preheating stage: A directional skin effect heating device is used to heat the surface layer area, the shallow layer area, and the deep layer area of the forging simultaneously, so that the temperatures of the surface layer area, the shallow layer area, and the deep layer area of the forging are increased by a predetermined temperature value relative to the core area; Shallow layer area heating-up preheating stage: A directional skin effect heating device is used to heat the surface layer area and the shallow layer area of the forging simultaneously, so that the temperatures of the surface layer area and the shallow layer area of the forging are increased by a predetermined temperature value relative to the deep layer area; Surface layer area reheating and heating-up stage: A directional skin effect heating device is used to heat the surface layer area of the forging alone, so that the temperature of the surface layer area of the forging is increased by a predetermined temperature value relative to the shallow layer area, so that the forging forms a gradient temperature field with a higher outside and a lower inside.

4. A forging method of a 42CrMo4 bearing steel with an extra-large cross-section according to claim 2 or 3, characterized in that, The described directional skin effect heating device includes a forging transfer conveyor line arranged between the heating furnace and the forging press for transferring the forgings in the heating furnace to the forging press, and an inverted U-shaped magnetic induction heating tunnel covering the upper position of the forging transfer conveyor line for heating the forgings while conveying them. When the forgings move from the heating furnace to the forging press direction on the forging transfer conveyor line, the inverted U-shaped magnetic induction heating tunnel sequentially performs directional heating and temperature increase on each layer area of the forgings, so that the forgings form a gradient temperature field with a higher outer part and a lower inner part.

5. The forging method of a 42CrMo4 bearing steel with an extra-large cross-section according to claim 4, characterized in that, The inverted U-shaped magnetic induction heating tunnel includes a first inverted U-shaped magnetic induction heating tunnel module arranged in sections for heating the deep layer area of the forgings, a second inverted U-shaped magnetic induction heating tunnel module for heating the shallow layer area of the forgings, and a third inverted U-shaped magnetic induction heating tunnel module for heating the surface layer area of the forgings. The first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module, and the third inverted U-shaped magnetic induction heating tunnel module are arranged adjacent to each other in sequence according to the conveying direction of the forgings. Among them, the first inverted U-shaped magnetic induction heating tunnel module is connected to a first intermediate frequency heating power supply, the second inverted U-shaped magnetic induction heating tunnel module is connected to a second intermediate frequency heating power supply, and the third inverted U-shaped magnetic induction heating tunnel module is connected to a high-frequency heating power supply. The first intermediate frequency heating power supply, the second intermediate frequency heating power supply, and the high-frequency heating power supply respectively use different power frequencies to heat the forgings in sequence, so as to form different skin effect heating depths, and make the forgings form a gradient temperature field with a higher outer part and a lower inner part.

6. The forging method of a 42CrMo4 bearing steel with an extra-large cross-section according to claim 5, characterized in that, The inverted U-shaped magnetic induction heating tunnel module includes a number of inverted U-shaped hollow conductive tubes arranged at intervals. Each of the inverted U-shaped hollow conductive tubes is sequentially connected end to end through a hollow conductive connecting tube to form an inverted U-shaped heating coil as a whole. Each section of the inverted U-shaped heating coil module is arranged adjacent to each other in sequence to form the described inverted U-shaped magnetic induction heating tunnel. Among them, circulating cooling water flows through the hollow tube body of the inverted U-shaped heating coil.

7. The forging method of a super-large-section 42CrMo4 bearing steel according to claim 6, characterized in that, An inverted U-shaped heat preservation cover is arranged on one side of the periphery of the inverted U-shaped magnetic induction heating tunnel. The inverted U-shaped magnetic induction heating tunnel is fixed on the inner wall of the inverted U-shaped heat preservation cover. A number of U-shaped magnetic conduction blocks with their openings facing the surface of the forgings are sleeved on the inverted U-shaped heating coil and are arranged adjacent to each other in sequence.

8. A forging method of a 42CrMo4 bearing steel with an extra-large cross-section according to claim 5, characterized in that, The forging transfer conveyor line includes a pair of support guide rails arranged at the opening position below the inverted U-shaped magnetic induction heating tunnel along the conveying direction of the forgings, a moving seat movably arranged between the pair of support guide rails, a moving seat traction device arranged below the moving seat for driving the moving seat to move back and forth along the pair of support guide rails, a pair of roller shafts rotatably arranged on the moving seat for placing cylindrical forgings, and a roller shaft rotation driving device arranged on the moving seat for driving the roller shafts to rotate.

9. A forging method of a 42CrMo4 bearing steel with a super-large cross-section according to claim 8, characterized in that, On both sides of the moving base, there are vertically arranged composite planar spiral magnetic induction heating coils for heating the end faces of cylindrical forgings. The composite planar spiral magnetic induction heating coils are formed by sequentially and alternately sleeving three types of planar spiral magnetic induction heating coils. The three types of planar spiral magnetic induction heating coils include a first planar spiral magnetic induction heating coil for heating the deep region of the end face of the cylindrical forging, a second planar spiral magnetic induction heating coil for heating the shallow region of the end face of the cylindrical forging, and a third planar spiral magnetic induction heating coil for heating the surface layer region of the end face of the cylindrical forging. Among them, the first planar spiral magnetic induction heating coil is connected to a first intermediate frequency heating power supply, the second planar spiral magnetic induction heating coil is connected to a second intermediate frequency heating power supply, and the third planar spiral magnetic induction heating coil is connected to a high frequency heating power supply.

10. A forging method of a 42CrMo4 bearing steel with an extra-large cross-section according to claim 9, characterized in that, The steps are as follows: S1. Preheating of the entire region of the forging: Place the forging in a heating furnace, heat and keep the forging warm, and heat and keep the entire forging warm to a temperature slightly lower than the Ac3 line (800 °C - 850 °C) by about 20 °C, that is, 780 °C - 800 °C. S2. Loading of the forging: Open the furnace door of the heating furnace, and load the forging in the heating furnace onto a pair of roller shafts on the moving base of the forging transfer conveyor line through a loader arranged beside the heating furnace. S3. Heating and temperature rising in sub-regions while the forging is being transported: Start the roller rotation drive device to make the cylindrical forging rotate freely between the pair of roller shafts. At the same time, start the moving base traction device to drive the moving base to move, so that the freely rotating cylindrical forging moves towards the forging press direction on the forging transfer conveyor line. When the forging passes through the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module, and the third inverted U-shaped magnetic induction heating tunnel module in sequence, the corresponding inverted U-shaped magnetic induction heating tunnel modules are alternately started in sequence to achieve heating and transportation of the forging. By applying different power frequencies to the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module, and the third inverted U-shaped magnetic induction heating tunnel module, different skin heating depths are formed on the outer circle of the cylindrical forging in each heating stage, so that a gradient temperature field with a higher outer part and a lower inner part is formed in the forging. Among them, when the cylindrical forging passes through the first inverted U-shaped magnetic induction heating tunnel module, the second inverted U-shaped magnetic induction heating tunnel module, and the third inverted U-shaped magnetic induction heating tunnel module in sequence on the forging transfer conveyor line, the first planar spiral magnetic induction heating coil, the second planar spiral magnetic induction heating coil, and the third planar spiral magnetic induction heating coil in the composite planar spiral magnetic induction heating coil are also alternately started in sequence to heat the end face of the cylindrical forging, and by applying different power frequencies to each planar spiral magnetic induction heating coil, different skin heating depths are formed on the end face of the cylindrical forging in each heating stage, so that a gradient temperature field with a higher outer part and a lower inner part is formed in the forging. S4. Forging: The forgings that have been heated in stages and gradients on the forging transfer conveyor line finally reach the position of the forging press. The forging manipulator beside the forging press transfers the forgings onto the forging press, and the forging press and the forging manipulator cooperate with each other to perform the forging operation.

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