Bending forming machining process for automobile sheet metal parts

Abstract

The invention belongs to the technical field of machining, and relates to an automobile sheet metal part bending forming machining technology which comprises the steps that a cold-roll steel plate is preheated, the preheated cold-roll steel plate is bent for three times, the first time of bending is carried out till 50%-60% of the design angle, then blowing and cooling are carried out, the second time of bending is carried out till 80%-90% of the design angle, and the third time of bending is carried out till 50%-60% of the design angle is reached. Then blowing and cooling, bending to a design angle for the third time, and then blowing and cooling; reciprocating vibration is conducted on the cold-roll steel sheet in the angular bisector direction of the bent cold-roll steel sheet, and cooling water is sprayed to the cold-roll steel sheet; and then the cold-roll steel sheet is subjected to reciprocating vibration in the bending line direction of the bent cold-roll steel sheet, and air cooling is conducted on the cold-roll steel sheet. Through cooperation of staged bending and gradient cooling, the forming precision and the structural stability of the automobile sheet metal part are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of machining, and relates to a bending and forming process for automotive sheet metal parts. Background Art

[0002] In the field of automotive manufacturing, the bending and forming process of sheet metal parts is a core link in the production of body structural parts (such as car doors, fenders, hoods, etc.). The traditional cold stamping process completes the bending in one go through a mold. Although the efficiency is relatively high, it faces problems such as large springback and concentrated residual stress. Especially for materials such as high-strength steel, the dimensional deviation due to elastic recovery after bending can reach 3% - 8%, which requires relying on subsequent manual trimming or adding compensation molds, significantly increasing the production cost. The hot forming technology developed in recent years can improve the formability by heating to the austenitizing temperature, but it has defects such as high energy consumption, large equipment investment, serious surface oxidation, and easy grain coarsening during high-temperature treatment, reducing the toughness of the material. Some processes attempt to use step-by-step bending combined with staged cooling, but during multiple bending processes, the cold-rolled steel sheet has a sharp increase in the subsequent deformation resistance due to work hardening, and it is prone to cracking at the bending line or local thinning exceeding the tolerance. Conventional intermediate annealing treatment requires interrupting the production line and reheating, seriously affecting the production efficiency. In addition, in the existing technology, the vibration stress relief process mostly uses a single direction or fixed parameters, making it difficult to synchronously eliminate the residual stress in the bending area and the whole sheet. Improper control of the water temperature and flow rate during spray cooling is likely to cause the superposition of quenching stress. The excessive use of volatile organic solvents in the anti-adhesive agent formulation not only poses safety hazards but also forms residues at high temperatures, affecting the coating adhesion. How to achieve the balance of high-precision forming, low springback control, and efficient continuous production is still the key problem restricting the process upgrade of automotive sheet metal parts. Summary of the Invention

[0003] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a bending and forming process for automotive sheet metal parts, which significantly improves the forming accuracy and structural stability of automotive sheet metal parts through the cooperation of staged bending and gradient cooling.

[0004] To achieve this purpose, the present invention adopts the following technical solutions: The present invention provides a bending and forming process for automotive sheet metal parts, and the bending and forming process for automotive sheet metal parts includes: (I) Preheating the cold-rolled steel sheet, and performing three bends on the preheated cold-rolled steel sheet. The first bend is to 50 - 60% of the designed angle, followed by purging and cooling. The second bend is to 80 - 90% of the designed angle, followed by purging and cooling. The third bend is to the designed angle, followed by purging and cooling; (II) Reciprocally vibrate the cold-rolled steel sheet along the angle bisector direction of the bent cold-rolled steel sheet, and spray cooling water on the cold-rolled steel sheet; then reciprocally vibrate the cold-rolled steel sheet along the fold line direction of the bent cold-rolled steel sheet, and air-cool the cold-rolled steel sheet to obtain a formed part.

[0005] Through the synergistic effect of staged bending and compound vibration cooling, the forming accuracy and structural stability of automotive sheet metal parts are significantly improved. The three-step progressive bending controls the rate of work hardening of the material while causing the stress to form a gradient distribution along the thickness direction, controlling the springback amount within ±0.5°. The compound vibration cooling process uses differential vibrations in the angle bisector and fold line directions, uses high-frequency vibrations to break dislocation entanglements, low-frequency vibrations to release longitudinal stress, and combines water cooling and air cooling to greatly reduce residual stress and significantly improve fatigue life.

[0006] The design of three-step progressive bending combined with gradient cooling effectively alleviates the internal stress accumulation of the material. When the first bend reaches 50-60% of the designed angle, the cold-rolled steel sheet is in a high-temperature state. At this time, rapid blowing and cooling form a slightly hardened support on the surface layer, retaining the internal plastic ability and providing a basis for subsequent deformation. The second bend reaches 80-90% of the designed angle. During this process, the ductility of the cold-rolled steel sheet weakens with the decrease in temperature, but the staged forming avoids excessive grain stretching caused by a single large-angle bend. The third bend reaches the designed angle, and the cold-rolled steel sheet has established a preliminary structural memory. Combined with a lower bending speed, the microstructure of the cold-rolled steel sheet is more stable in its oriented arrangement.

[0007] The stress distribution in the formed part is further optimized through the compound vibration cooling process. The high-frequency vibration along the angle bisector direction is combined with water cooling. Using the micro-impact effect generated by the penetration of water molecules, the grain boundary slip and recombination in the bent area of the formed part are promoted, reducing local stress concentration. At this time, the spray water flow forms a dynamic temperature field on the surface of the formed part, which not only avoids the quenching embrittlement caused by traditional immersion cooling but also strengthens the metal dislocation pinning effect through continuous phase transformation. Subsequently, it is switched to the low-frequency vibration along the fold line direction in cooperation with air cooling to specifically release the residual stress extending along the fold line, enabling the overall stress field of the formed part to be redistributed in balance.

[0008] As a preferred technical solution of the present invention, in step (I), the thickness of the cold-rolled steel sheet is 2-3 mm, for example, it can be 2.0 mm, 2.1 mm, 2.2 mm, 2.3 mm, 2.4 mm, 2.5 mm, 2.6 mm, 2.7 mm, 2.8 mm, 2.9 mm or 3.0 mm, but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0009] In some optional examples, the preheating process of the cold-rolled steel sheet is carried out in two stages, including: The cold-rolled steel plate is heated to a first preheating temperature at a first heating rate and held; then it is continuously heated to a second preheating temperature at a second heating rate and held, and then bent.

[0010] The present invention adopts a staged preheating treatment, creating ideal plastic deformation conditions for the subsequent three bends. In the first preheating stage, the cold-rolled steel plate is heated to 150 - 170°C at a relatively low heating rate (3 - 5°C / min). During this process, the metal lattice of the cold-rolled steel plate gradually absorbs heat, and dislocations begin to migrate slowly, but no obvious recrystallization behavior is triggered. During the 10 - 20 min holding stage, the temperature gradient in the thickness direction of the cold-rolled steel plate is effectively removed, and the temperature difference between the surface layer and the core is controlled within ±5°C, avoiding local stress mutation caused by uneven heating during subsequent bending.

[0011] In the second preheating stage, a rapid heating rate of 5 - 8°C / min is adopted to prompt the cold-rolled steel plate to break through the work-hardening critical point faster. When the temperature reaches 180 - 200°C, part of the stored deformation energy inside the cold-rolled steel plate is released, and dislocation tangles begin to dissociate to form slip bands, but a large amount of carbide precipitation phases have not yet dissolved. This structural state enables the cold-rolled steel plate to have sufficient plastic flow ability while maintaining a certain original strength. During the 15 - 25 min holding process, the cold-rolled steel plate completes dynamic recovery, and a stable subgrain structure is formed inside the grains, significantly reducing the work-hardening rate during subsequent bending.

[0012] As a preferred technical solution of the present invention, the first heating rate is 3 - 5°C / min, for example, it can be 3.0°C / min, 3.2°C / min, 3.4°C / min, 3.6°C / min, 3.8°C / min, 4.0°C / min, 4.2°C / min, 4.4°C / min, 4.6°C / min, 4.8°C / min or 5.0°C / min, but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0013] In some alternative examples, the first preheating temperature is 150 - 170°C, for example, it can be 150°C, 152°C, 154°C, 156°C, 158°C, 160°C, 162°C, 164°C, 166°C, 168°C or 170°C, but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0014] In some alternative embodiments, it is held at the first preheating temperature for 10 to 20 minutes, for example, it can be 10 minutes, 11 minutes, 12 minutes, 13 minutes, 14 minutes, 15 minutes, 16 minutes, 17 minutes, 18 minutes, 19 minutes or 20 minutes, but is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0015] In some alternative embodiments, the second heating rate is 5 to 8 °C / min, for example, it can be 5.0 °C / min, 5.5 °C / min, 6.0 °C / min, 6.5 °C / min, 7.0 °C / min, 7.5 °C / min or 8.0 °C / min, but is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0016] In some alternative embodiments, the second preheating temperature is 180 to 200 °C, for example, it can be 180 °C, 182 °C, 184 °C, 186 °C, 188 °C, 190 °C, 192 °C, 194 °C, 196 °C, 198 °C or 200 °C, but is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0017] The present invention specifically defines the second preheating temperature as 180 to 200 °C. This temperature range is exactly in the transition region of the cold-rolled steel sheet from elastic deformation to plastic deformation. The activity of the lattice structure inside the material is excited but not to the extent of drastic reconstruction. When the first bending is carried out within this temperature range, the cold-rolled steel sheet retains a certain degree of tensile strength and obtains sufficient ductility to withstand an angular deformation of 50 to 60% without generating stress concentration.

[0018] When the second preheating temperature is lower than 180 °C, the plasticity of the cold-rolled steel sheet is insufficient, which will directly damage the continuity of bending. The residual cold-rolling stress inside the cold-rolled steel sheet during the first bending is superimposed with the newly added deformation stress, easily forming invisible microcracks at the root of the bending line. These microcracks are sharply amplified during the second bending, resulting in stress release gaps in local areas, manifested as fish-scale-like wrinkles at the edge of the bending line.

[0019] When the second preheating temperature exceeds 200 °C, the high temperature promotes an increase in the thickness of the oxide layer on the surface of the cold-rolled steel sheet. These oxide particles are pressed into the material matrix during the first bending. When the second bending is carried out, the areas where these oxide particles are located become stress concentration sources, resulting in symmetrically distributed crescent-shaped dark cracks on both sides of the bending line. In addition, a preheating temperature above 200 °C will induce abnormal coarsening of carbide particles. These hard phases with a diameter exceeding 200 nm become crack propagation channels during the cooling process of the third bending, significantly reducing the fatigue life of the finally obtained formed part.

[0020] In some alternative examples, keep warm at the second preheating temperature for 15 to 25 minutes. For example, it can be 15 minutes, 16 minutes, 17 minutes, 18 minutes, 19 minutes, 20 minutes, 21 minutes, 22 minutes, 23 minutes, 24 minutes or 25 minutes. However, it is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0021] As a preferred technical solution of the present invention, in step (Ⅰ), after the first bending, blow the surface of the cold-rolled steel plate with compressed air at 0.5 to 0.6 MPa. For example, it can be 0.5 MPa, 0.51 MPa, 0.52 MPa, 0.53 MPa, 0.54 MPa, 0.55 MPa, 0.56 MPa, 0.57 MPa, 0.58 MPa, 0.59 MPa or 0.6 MPa, until the surface temperature of the cold-rolled steel plate drops to 130 to 150 °C. For example, it can be 130 °C, 132 °C, 134 °C, 136 °C, 138 °C, 140 °C, 142 °C, 144 °C, 146 °C, 148 °C or 150 °C. However, it is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0022] In some alternative examples, the bending speed of the first bending is 12 to 15 mm / s. For example, it can be 12 mm / s, 12.5 mm / s, 13 mm / s, 13.5 mm / s, 14 mm / s, 14.5 mm / s or 15 mm / s. However, it is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0023] As a preferred technical solution of the present invention, in step (Ⅰ), before the second bending starts, spray an anti-sticking agent on the surface of the cold-rolled steel plate, let it stand for 30 to 60 s to form a film, and then perform the second bending. For example, it can be 30 s, 35 s, 40 s, 45 s, 50 s, 55 s or 60 s. However, it is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0024] In some alternative examples, the spraying amount of the anti-sticking agent is 0.1 to 0.2 g / cm 2 , for example, it can be 0.1 g / cm 2 , 0.11 g / cm 2 , 0.12 g / cm 2 , 0.13 g / cm 2 , 0.14 g / cm 2 , 0.15 g / cm 2 , 0.16 g / cm 2 , 0.17 g / cm 2 , 0.18 g / cm 2, 0.19 g / cm 2 or 0.2 g / cm 2 , but not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0025] As a preferred technical solution of the present invention, the anti-sticking agent includes graphite powder, molybdenum disulfide, boron nitride, hydroxypropyl methylcellulose, absolute ethanol, acetone, and deionized water.

[0026] In some alternative examples, based on the mass fraction of the anti-sticking agent being 100 wt%, the anti-sticking agent includes the following components with the following mass fractions: Graphite powder 3 - 5 wt%; Molybdenum disulfide 2 - 3 wt%; Boron nitride 1.5 - 2.5 wt%; Hydroxypropyl methylcellulose 1.5 - 2.5 wt%; Absolute ethanol 10 - 12 wt%; Acetone 5 - 8 wt%; The balance is deionized water.

[0027] Among them, the mass fraction of graphite powder can be 3.0 wt%, 3.2 wt%, 3.4 wt%, 3.6 wt%, 3.8 wt%, 4.0 wt%, 4.2 wt%, 4.4 wt%, 4.6 wt%, 4.8 wt% or 5.0 wt%, the mass fraction of molybdenum disulfide can be 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, 2.5 wt%, 2.6 wt%, 2.7 wt%, 2.8 wt%, 2.9 wt% or 3.0 wt%, the mass fraction of boron nitride can be 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt% or 2.5 wt%, the mass fraction of hydroxypropyl methylcellulose can be 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt% or 2.5 wt%, the mass fraction of absolute ethanol can be 10 wt%, 10.2 wt%, 10.4 wt%, 10.6 wt%, 10.8 wt%, 11 wt%, 11.2 wt%, 11.4 wt%, 11.6 wt%, 11.8 wt% or 12 wt%, the mass fraction of acetone can be 5.0 wt%, 5.5 wt%, 6.0 wt%, 6.5 wt%, 7.0 wt%, 7.5 wt% or 8.0 wt%, but not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0028] During the second bending process, the anti-sticking agent mainly plays a dual role of interfacial lubrication and stress buffering. When the cold-rolled steel plate completes the first bending and cools down to 130 - 150 °C, a micron-scale oxide layer is formed on the material surface due to rapid cooling. At this time, the anti-sticking film layer formed by spraying the anti-sticking agent can effectively block the direct contact between the cold-rolled steel plate and the mold, significantly reducing the friction coefficient between the cold-rolled steel plate and the mold. At the same time, a certain amount of bending stress is absorbed through the deformation of the anti-sticking film layer.

[0029] The present invention adds 3 - 5 wt% of graphite powder. As a layered structure material, the graphite powder is arranged in parallel in the anti-sticking film layer to form a directional lubrication channel. This solid lubrication mechanism converts sliding friction into interlayer shear.

[0030] The present invention adds 2 - 3 wt% of molybdenum disulfide. The hexagonal crystal system structure of molybdenum disulfide will activate the active sites of sulfur atoms in the range of 130 - 150 °C, forming a weak chemical adsorption with the iron elements on the surface of the cold-rolled steel plate. This instantaneous bonding can not only prevent the anti-sticking film layer from falling off but also does not affect the demolding separation after forming.

[0031] The present invention adds 1.5 - 2.5 wt% of boron nitride. The introduction of boron nitride enhances the high-temperature resistance of the anti-sticking film layer, and its high thermal conductivity can quickly conduct out the frictional heat at the bending contact point, avoiding the decomposition and failure of the anti-sticking film layer caused by excessive local temperature.

[0032] The present invention adds 1.5 - 2.5 wt% of hydroxypropyl methylcellulose. As a water-soluble polymer, hydroxypropyl methylcellulose constructs a three-dimensional network skeleton through molecular chain entanglement during the film-forming process.

[0033] The compound solvent system of 10 - 12 wt% of absolute ethanol and 5 - 8 wt% of acetone realizes the rapid volatilization of the anti-sticking agent. The high volatility of acetone promotes the surface drying of the anti-sticking agent within 30 - 60 s, and the slow-release effect of ethanol ensures the uniformity of the film-forming process, avoiding pinhole defects caused by too fast solvent volatilization.

[0034] As a preferred technical solution of the present invention, in step (Ⅰ), after the second bending, compressed air with a pressure of 0.3 - 0.5 MPa is used to blow the surface of the cold-rolled steel plate. For example, it can be 0.3 MPa, 0.32 MPa, 0.34 MPa, 0.36 MPa, 0.38 MPa, 0.4 MPa, 0.42 MPa, 0.44 MPa, 0.46 MPa, 0.48 MPa or 0.5 MPa, until the surface temperature of the cold-rolled steel plate drops to 100 - 120 °C. For example, it can be 100 °C, 102 °C, 104 °C, 106 °C, 108 °C, 110 °C, 112 °C, 114 °C, 116 °C, 118 °C or 120 °C, but it is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0035] In some optional examples, the bending speed of the second bending is 8 - 10 mm / s. For example, it can be 8.0 mm / s, 8.2 mm / s, 8.4 mm / s, 8.6 mm / s, 8.8 mm / s, 9.0 mm / s, 9.2 mm / s, 9.4 mm / s, 9.6 mm / s, 9.8 mm / s or 10.0 mm / s. However, it is not limited to the listed values only, and other unlisted values within this numerical range are equally applicable.

[0036] The present invention specifically limits the bending speed of the second bending to 8 - 10 mm / s. When the second bending occurs, the surface temperature of the cold-rolled steel sheet has dropped to 130 - 150 °C, and the internal grain structure is in a semi-hardened state. The appropriate bending speed allows sufficient time for stress redistribution of the material, avoiding both local strain concentration caused by rapid bending and excessive temperature reduction caused by slow bending.

[0037] When the bending speed of the second bending is lower than 8 mm / s, it will result in too long a bending time. When 80 - 90% of the designed angle is completed, the surface temperature of the cold-rolled steel sheet has dropped below 100 °C, and the cold work hardening effect is significantly enhanced. At this time, when a large-angle bending is carried out, the ability of the material to resist deformation increases sharply, and visible tensile folds are easily formed outside the bend line. At the same time, the slow bending speed causes the anti-adhesive film layer to be under continuous extrusion for too long, and the molybdenum disulfide in the anti-adhesive film layer will undergo directional migration, resulting in local lubrication failure and finally leaving intermittent indentation-like defects at the bending part of the formed part.

[0038] When the bending speed of the second bending exceeds 10 mm / s, the high-speed bending makes the slip movement of metal grains unable to proceed fully, forming a dislocation pile-up zone at the root of the bend line of the formed part. These microscopic defects will become stress release channels during the subsequent cooling process, resulting in a certain degree of angle regression of the formed part and unable to reach the designed angle. At the same time, the too-fast bending speed will damage the integrity of the anti-adhesive film layer, and the anti-adhesive film layer will undergo shear fracture under the impact load, resulting in the exposure of the surface of the cold-rolled steel sheet and direct contact with the mold, and the instantaneous friction coefficient will increase rapidly. This abnormal friction will leave scratch stripes parallel to the bending direction on the surface of the formed part.

[0039] As a preferred technical solution of the present invention, in step (Ⅰ), after the third bending, the surface of the cold-rolled steel sheet is purged with compressed air at 0.2~0.3 MPa, for example, it can be 0.2 MPa, 0.21 MPa, 0.22 MPa, 0.23 MPa, 0.24 MPa, 0.25 MPa, 0.26 MPa, 0.27 MPa, 0.28 MPa, 0.29 MPa or 0.3 MPa, until the surface temperature of the cold-rolled steel sheet drops to 70~80 °C, for example, it can be 70 °C, 71 °C, 72 °C, 73 °C, 74 °C, 75 °C, 76 °C, 77 °C, 78 °C, 79 °C or 80 °C, but it is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0040] In some alternative examples, the bending speed of the third bending is 5~8 mm / s, for example, it can be 5.0 mm / s, 5.5 mm / s, 6.0 mm / s, 6.5 mm / s, 7.0 mm / s, 7.5 mm / s or 8.0 mm / s, but it is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0041] The present invention specifically defines the bending speed of the third bending as 5~8 mm / s. During the third bending process, the material has been in a semi-hardened state after the first two deformations and multiple temperature drops. By bending slowly, the grains have enough time for orderly slip recombination, which can not only use the initially formed hardened structure on the surface layer to suppress springback, but also achieve angle fine-tuning through the limited plasticity retained in the core. The deformation process at this bending speed promotes the formation of a gradient distribution of stress in the thickness direction of the material. The synergistic effect of the surface hardened zone and the core plastic zone ensures both dimensional stability and disperses the stress concentration points to a wider area, effectively avoiding micro-defects caused by local overload.

[0042] When the bending speed is lower than 5 mm / s, the cold-rolled steel sheet bears the die pressure for a long time in a low-temperature environment, and an abnormal hardened zone is generated due to continuous compression on the inner side of the bending line, significantly changing the uniformity of the material properties. In addition, the slow bending speed makes it difficult for the frictional heat at the contact surface between the die and the material to dissipate quickly, resulting in the recovery of the already formed metastable structure due to local heating, and the ability of the material to resist deformation fluctuates. Eventually, the formed part is prone to angle relaxation under dynamic loads, directly affecting the fitting accuracy during assembly.

[0043] When the bending speed exceeds 8 mm / s, high-frequency mechanical disturbances are likely to activate the dislocation movement at micro-defects, resulting in a radial microcrack network at the root of the bending line. At the same time, the inertial force brought by rapid bending will also interfere with the integrity of the anti-sticking agent film, generating instantaneous friction at the bending line and forming a visible surface damage zone. These damaged areas become corrosion channels during the subsequent cooling process, significantly reducing the environmental tolerance of the formed part. In addition, too fast a bending speed shortens the time for the internal stress of the material to redistribute, leading to an unbalanced residual stress distribution.

[0044] As a preferred technical solution of the present invention, in step (Ⅱ), the reciprocating vibration frequency of the cold-rolled steel plate along the angular bisector direction is 20 - 30 Hz. For example, it can be 20 Hz, 21 Hz, 22 Hz, 23 Hz, 24 Hz, 25 Hz, 26 Hz, 27 Hz, 28 Hz, 29 Hz or 30 Hz, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0045] The present invention specifically limits the reciprocating vibration frequency of the cold-rolled steel plate along the angular bisector direction to 20 - 30 Hz. Within this range, it can effectively stimulate the coordinated slip movement between grains. The alternating stress field generated by high-frequency vibration prompts the dislocations accumulated in the bending area to rearrange, eliminating local stress concentration points and promoting the uniform dispersion of carbide particles at the grain boundaries. During the vibration process, the tiny plastic deformation layer formed on the material surface also plays a role in suppressing crack propagation, significantly extending the fatigue life of the formed part.

[0046] When the vibration frequency is lower than 20 Hz, the mechanical energy input is insufficient to break the dislocation locking state inside the material. The stress wave amplitude generated by low-frequency vibration is limited and can only act on the surface layer of the material with a depth of 0.1 - 0.2 mm, unable to reach the high-stress area at the root of the bend.

[0047] When the vibration frequency exceeds 30 Hz, the energy density applied by high-frequency vibration per unit time exceeds the tolerance range of grain boundary slip, resulting in a nano-scale microcrack network at the bending line. These micro-defects gradually connect under the thermal stress during subsequent cooling treatment, forming an invisible crack band parallel to the bending direction, leading to a significant decline in the fatigue performance of the formed part. At the same time, the thermo-mechanical coupling effect induced by high-frequency vibration accelerates the oxidation of the material surface, forming an unevenly thick oxide skin layer on both sides of the bending line. The presence of the oxide skin layer makes the formed part prone to coating peeling risks during subsequent spraying processes.

[0048] In some optional examples, the amplitude of the cold-rolled steel sheet along the angle bisector direction is 0.3~0.5mm, for example, it can be 0.3mm, 0.32mm, 0.34mm, 0.36mm, 0.38mm, 0.4mm, 0.42mm, 0.44mm, 0.46mm, 0.48mm or 0.5mm, but is not limited to the listed values, and other unlisted values within this numerical range are also applicable.

[0049] In some optional examples, the spray volume of the cooling water is 0.8~1.2L / min, for example, it can be 0.8L / min, 0.85L / min, 0.9L / min, 0.95L / min, 1.0L / min, 1.05L / min, 1.1L / min, 1.15L / min or 1.2L / min, but is not limited to the listed values, and other unlisted values within this numerical range are also applicable.

[0050] In some optional examples, the temperature of the cooling water is 15~25℃, for example, it can be 15℃, 16℃, 17℃, 18℃, 19℃, 20℃, 21℃, 22℃, 23℃, 24℃ or 25℃, but is not limited to the listed values, and other unlisted values within this numerical range are also applicable.

[0051] In some optional embodiments, cooling water is sprayed on the surface of the cold-rolled steel plate until its surface temperature drops to 40-50°C, for example, it can be 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C or 50°C, but is not limited to the listed values, and other unlisted values within this numerical range are also applicable.

[0052] As a preferred technical solution of the present invention, in step (III), the reciprocating vibration frequency of the cold-rolled steel plate along the bending line direction is 10-20 Hz, for example, it can be 10 Hz, 11 Hz, 12 Hz, 13 Hz, 14 Hz, 15 Hz, 16 Hz, 17 Hz, 18 Hz, 19 Hz or 20 Hz, but is not limited to the listed values, and other values not listed within this numerical range are also applicable.

[0053] This invention specifically limits the reciprocating vibration frequency of the cold-rolled steel sheet along the bend line to 10-20 Hz. Within this range, the combined tensile and compressive stresses extending along the bend line are released in a targeted manner. The vibration energy is transmitted through periodic loads, prompting the grains on both sides of the bend line to align in an orderly manner along the deformation direction. This eliminates a significant amount of residual stress and reduces the material's anisotropy index. This treatment method results in a more uniform deformation capability for the formed part when subjected to multi-directional loads. Furthermore, the appropriate vibration frequency avoids excessive mechanical energy input, protects the dense dislocation structure already formed at the bend line root, and reduces the fatigue crack growth rate.

[0054] When the vibration frequency is below 10Hz, the energy input intensity cannot exceed the elastic response range of the material. The stress waves generated by low-frequency vibration mainly act on the surface of the material at a depth of 0.05-0.1mm, with little impact on the high-stress area at the core of the bend line. The resulting molded parts are prone to asymmetric deformation on both sides of the bend line during alternating load testing. In addition, excessively low vibration frequencies prolong the stress relaxation time, causing dynamic strain aging in the material after the vibration treatment. This significantly reduces the elongation of the bend line area and severely weakens the impact resistance of the molded parts.

[0055] When the vibration frequency exceeds 20Hz, the excessive mechanical vibration disrupts the established microstructural balance. The number of cyclic loads applied per unit time by high-frequency vibration exceeds the material's dislocation recombination rate, leading to dislocation pileups at the bend line. Furthermore, high-frequency vibration triggers localized temperature increases, altering the formation of the oxide film on the material surface, resulting in unevenly thick oxide layers at the bend line edges. These weakly oxidized areas are prone to coating bubbles during the subsequent spraying process, making them susceptible to corrosion during use, significantly shortening the service life of the molded parts.

[0056] In some optional examples, the amplitude of the cold-rolled steel plate along the bending line direction is 0.2~0.4mm, for example, it can be 0.2mm, 0.22mm, 0.24mm, 0.26mm, 0.28mm, 0.3mm, 0.32mm, 0.34mm, 0.36mm, 0.38mm or 0.4mm, but is not limited to the listed values, and other unlisted values within this numerical range are also applicable.

[0057] In some optional instances, the wind speed of the air cooling is 4~6m / s, for example, it can be 4.0m / s, 4.2m / s, 4.4m / s, 4.6m / s, 4.8m / s, 5.0m / s, 5.2m / s, 5.4m / s, 5.6m / s, 5.8m / s or 6.0m / s, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0058] Vibration along the angle bisector direction and spray cooling mainly act on stress reconstruction and microstructure optimization inside the material. When the cold-rolled steel plate is bent three times, a V-shaped stress concentration zone will be formed in the bent area. The grains in this area are arranged in a tensile manner along the bend line, while the matrix on both sides retains the original rolling structure. At this time, applying high-frequency vibration of 20 - 30 Hz along the angle bisector direction is equivalent to applying periodic alternating stress on the material symmetry axis. This vibration stimulation in a specific direction can effectively break the dislocation tangles formed at the root of the bend. While vibrating, spray cooling water at 15 - 25 °C to cool down. On the one hand, the dynamic pressure brought by the water flow impact promotes the slip and reorientation of surface grains. On the other hand, the temperature gradient caused by water cooling drives dislocations to migrate towards sub-boundaries. Through the thermo-mechanical coupling effect, the residual stress inside the bend angle is significantly reduced, and at the same time, carbide particles are uniformly precipitated at the grain boundaries, forming a dispersion distribution with a size less than 200 nm.

[0059] Subsequently, switch to vibration along the bend line direction and cooperate with air cooling, which is mainly used to improve the dimensional stability of the formed part. The low-frequency vibration of 10 - 20 Hz is transmitted along the length direction of the bend line. Its vibration waveform forms reverse interference with the elastic recovery trend of the material, which can effectively neutralize the accumulation of elastic potential energy during unloading and springback, and greatly improve the angular accuracy of the bend line. The air cooling process with a wind speed of 4 - 6 m / s realizes the slow phase transformation of the formed part. Compared with the rapid quenching of spray water cooling, the temperature field formed by air cooling is more gentle, enabling the dislocation network to gradually complete recombination during the cooling process, and finally forming a stable dislocation cell structure inside the formed part. This structural feature makes the elastic modulus distribution of the formed part more uniform and the anisotropy index significantly reduced.

[0060] Compared with the prior art, the beneficial effects of the present invention are as follows: Through the synergistic effect of staged bending and compound vibration cooling, the present invention significantly improves the forming accuracy and structural stability of automotive sheet metal parts. The three-step progressive bending controls the work hardening rate of the material while forming a gradient distribution of stress along the thickness direction, and controls the springback amount within ±0.5°. The compound vibration cooling process uses differential vibration in the angle bisector and bend line directions, uses high-frequency vibration to break dislocation tangles, low-frequency vibration to release longitudinal stress, and cooperates with water cooling and air cooling to significantly reduce the residual stress and significantly improve the fatigue life. Description of the Drawings

[0061] Figure 1 It is a flowchart of the bending and forming process of the automotive sheet metal part provided by Embodiments 1 - 14 of the present invention. Detailed Embodiments

[0062] The technical solution of the present invention will be described in detail below in conjunction with specific embodiments and their accompanying drawings. The embodiments described herein are specific specific embodiments of the present invention and are used to illustrate the concept of the present invention; these descriptions are all explanatory and exemplary and should not be construed as limiting the embodiments of the present invention and the protection scope of the present invention. Except for the embodiments described herein, those skilled in the art can also adopt other obvious technical solutions based on the content disclosed in the claims and the specification of this application. These technical solutions include technical solutions that make any obvious substitutions and modifications to the embodiments described herein.

[0063] Embodiment 1 This embodiment provides a bending and forming process for automotive sheet metal parts, as Figure 1 shown. The bending and forming process for automotive sheet metal parts specifically includes the following steps: (1) Preheat a cold-rolled steel plate with a thickness of 2 mm, raise the temperature of the cold-rolled steel plate to 150 °C at a heating rate of 3 °C / min and keep it warm for 20 min, and then continue to raise the temperature to 180 °C at a heating rate of 5 °C / min and keep it warm for 25 min; (2) Bend the preheated cold-rolled steel plate three times. First, perform the first bend at a bending speed of 12 mm / s, bend to 50% of the designed angle for the first time. After the first bend, blow the surface of the cold-rolled steel plate with compressed air at 0.5 MPa until the surface temperature of the cold-rolled steel plate drops to 130 °C; Subsequently, spray an anti-sticking agent on the surface of the cold-rolled steel plate. The spraying amount of the anti-sticking agent is 0.1 g / cm 2 , and let it stand for 30 s to form a film after spraying. Calculated based on the mass fraction of the anti-sticking agent being 100 wt%, it includes the following components with the following mass fractions: Graphite powder 3 wt%; Molybdenum disulfide 2.5 wt%; Boron nitride 2 wt%; Hydroxypropyl methylcellulose 2 wt%; Absolute ethanol 12 wt%; Acetone 7 wt%; The balance is deionized water; After the anti-sticking agent forms a film, perform the second bend at a bending speed of 8 mm / s, bend to 80% of the designed angle for the second time. After the second bend, blow the surface of the cold-rolled steel plate with compressed air at 0.3 MPa until the surface temperature of the cold-rolled steel plate drops to 100 °C; Finally, perform the third bend at a bending speed of 5 mm / s, bend to the designed angle for the third time. After the third bend, blow the surface of the cold-rolled steel plate with compressed air at 0.2 MPa until the surface temperature of the cold-rolled steel plate drops to 70 °C; After the three - time bending is completed, reciprocating vibration is carried out on the cold - rolled steel plate along the angle - bisector direction of the bent cold - rolled steel plate. The vibration frequency is set to 20 Hz, and the amplitude is set to 0.3 mm. During the vibration, cooling water at 15 °C is sprayed onto the cold - rolled steel plate at a spraying rate of 0.8 L / min until the surface temperature of the cold - rolled steel plate drops to 40 °C; Subsequently, stop the reciprocating vibration along the angle - bisector direction and turn off the cooling - water spraying. Then, carry out reciprocating vibration on the cold - rolled steel plate along the bending - line direction of the bent cold - rolled steel plate. The vibration frequency is set to 10 Hz, and the amplitude is set to 0.2 mm. During the vibration, blow - cool the cold - rolled steel plate with a wind speed of 4 m / s until the cold - rolled steel plate drops to room temperature to obtain the formed part.

[0064] Example 2 This example provides a bending and forming process for automotive sheet metal parts. As Figure 1 shown, the bending and forming process for automotive sheet metal parts specifically includes the following steps: (1) Preheat the cold - rolled steel plate with a thickness of 2.2 mm. Heat the cold - rolled steel plate to 155 °C at a heating rate of 3.5 °C / min and keep it warm for 18 min. Subsequently, continue to heat it to 185 °C at a heating rate of 6 °C / min and keep it warm for 22 min; (2) Carry out three - time bending on the pre - heated cold - rolled steel plate. First, carry out the first bending at a bending speed of 13 mm / s until 52% of the designed angle. After the first bending, blow - sweep the surface of the cold - rolled steel plate with compressed air at 0.52 MPa until the surface temperature of the cold - rolled steel plate drops to 135 °C; Subsequently, spray an anti - sticking agent on the surface of the cold - rolled steel plate. The spraying amount of the anti - sticking agent is 0.12 g / cm 2 , and let it stand for 40 s to form a film after spraying. Calculated by the mass fraction of the anti - sticking agent being 100 wt%, it includes the following components with the following mass fractions: Graphite powder 4 wt%; Molybdenum disulfide 3 wt%; Boron nitride 1.5 wt%; Hydroxypropyl methylcellulose 2.2 wt%; Absolute ethanol 10 wt%; Acetone 8 wt%; The rest is deionized water; [[ID=3S]]After the anti - sticking agent forms a film, carry out the second bending at a bending speed of 8.5 mm / s until 82% of the designed angle. After the second bending, blow - sweep the surface of the cold - rolled steel plate with compressed air at 0.35 MPa until the surface temperature of the cold - rolled steel plate drops to 105 °C; Finally, the third bending is carried out at a bending speed of 6 mm / s until the third bending reaches the designed angle. After the third bending, the surface of the cold-rolled steel plate is purged with compressed air at 0.22 MPa until the surface temperature of the cold-rolled steel plate drops to 72 °C; (3) After the three bends are completed, the cold-rolled steel plate is reciprocally vibrated along the angle bisector direction of the bent cold-rolled steel plate. The vibration frequency is set to 22 Hz and the amplitude is set to 0.35 mm. During the vibration, cooling water at 18 °C is sprayed onto the cold-rolled steel plate at a spraying rate of 0.9 L / min until the surface temperature of the cold-rolled steel plate drops to 42 °C; Subsequently, stop the reciprocal vibration along the angle bisector direction and turn off the cooling water spraying. Then, the cold-rolled steel plate is reciprocally vibrated along the bend line direction of the bent cold-rolled steel plate. The vibration frequency is set to 12 Hz and the amplitude is set to 0.25 mm. During the vibration, the cold-rolled steel plate is cooled by blowing air at a wind speed of 4.5 m / s until the cold-rolled steel plate reaches room temperature, obtaining the formed part.

[0065] Example 3 This example provides a bending and forming process for automotive sheet metal parts, as Figure 1 shown. The bending and forming process for automotive sheet metal parts specifically includes the following steps: (1) Preheat the 2.5-mm-thick cold-rolled steel plate. Raise the temperature of the cold-rolled steel plate to 160 °C at a heating rate of 4 °C / min and hold for 15 min. Subsequently, continue to raise the temperature to 190 °C at a heating rate of 6 °C / min and hold for 20 min; (2) Perform three bends on the preheated cold-rolled steel plate. First, carry out the first bend at a bending speed of 13 mm / s until 55% of the designed angle is reached. After the first bend, purge the surface of the cold-rolled steel plate with compressed air at 0.55 MPa until the surface temperature of the cold-rolled steel plate drops to 140 °C; Subsequently, spray an anti-adhesive agent on the surface of the cold-rolled steel plate. The spraying amount of the anti-adhesive agent is 0.15 g / cm 2 , and after spraying, let it stand for 40 s to form a film. Calculated based on the mass fraction of the anti-adhesive agent being 100 wt%, it includes the following components with the following mass fractions: Graphite powder 3.5 wt%; Molybdenum disulfide 2.8 wt%; Boron nitride 1.8 wt%; Hydroxypropyl methylcellulose 2.5 wt%; Absolute ethanol 12 wt%; Acetone 6 wt%; The balance is deionized water; After the anti - sticking agent forms a film, perform the second bending at a bending speed of 9 mm / s, bend to 85% of the designed angle for the second time. After the second bending, blow the surface of the cold - rolled steel plate with compressed air at 0.4 MPa until the surface temperature of the cold - rolled steel plate drops to 110 °C; Finally, perform the third bending at a bending speed of 6 mm / s, bend to the designed angle for the third time. After the third bending, blow the surface of the cold - rolled steel plate with compressed air at 0.25 MPa until the surface temperature of the cold - rolled steel plate drops to 75 °C; (3)After the three - time bending is completed, perform reciprocating vibration on the cold - rolled steel plate along the angle - bisector direction of the bent cold - rolled steel plate. The vibration frequency is set to 25 Hz, and the amplitude is set to 0.4 mm. During the vibration, spray cooling water at 20 °C on the cold - rolled steel plate at a spraying rate of 1 L / min until the surface temperature of the cold - rolled steel plate drops to 45 °C; Subsequently, stop the reciprocating vibration along the angle - bisector direction and turn off the cooling - water spraying. Perform reciprocating vibration on the cold - rolled steel plate along the bend - line direction of the bent cold - rolled steel plate. The vibration frequency is set to 15 Hz, and the amplitude is set to 0.3 mm. During the vibration, blow - cool the cold - rolled steel plate with a wind speed of 5 m / s until the cold - rolled steel plate drops to room temperature to obtain the formed part.

[0066] Example 4 This example provides a bending and forming process for automotive sheet metal parts. As Figure 1 shown, the bending and forming process for automotive sheet metal parts specifically includes the following steps: (1)Preheat the cold - rolled steel plate with a thickness of 2.8 mm, raise the temperature of the cold - rolled steel plate to 165 °C at a heating rate of 4.5 °C / min and keep it warm for 12 min, and then continue to raise the temperature to 195 °C at a heating rate of 7 °C / min and keep it warm for 18 min; (2)Perform three - time bending on the pre - heated cold - rolled steel plate. First, perform the first bending at a bending speed of 14 mm / s, bend to 58% of the designed angle for the first time. After the first bending, blow the surface of the cold - rolled steel plate with compressed air at 0.58 MPa until the surface temperature of the cold - rolled steel plate drops to 145 °C; Subsequently, spray an anti - sticking agent on the surface of the cold - rolled steel plate. The spraying amount of the anti - sticking agent is 0.18 g / cm 2 , and let it stand for 50 s to form a film. Calculated based on the mass fraction of the anti - sticking agent being 100 wt%, it includes the following components with the following mass fractions: Graphite powder 4.5 wt%; Molybdenum disulfide 2.7 wt%; Boron nitride 2.5 wt%; Hydroxypropyl methylcellulose 1.5 wt%; Absolute ethanol 11.5 wt%; Acetone 6 wt%; The balance is deionized water; After the anti-sticking agent forms a film, the second bending is carried out at a bending speed of 9.5 mm / s until 88% of the designed angle is reached. After the second bending, the surface of the cold-rolled steel plate is purged with compressed air at 0.45 MPa until the surface temperature of the cold-rolled steel plate drops to 115 °C; Finally, the third bending is carried out at a bending speed of 7 mm / s until the designed angle is reached. After the third bending, the surface of the cold-rolled steel plate is purged with compressed air at 0.28 MPa until the surface temperature of the cold-rolled steel plate drops to 78 °C; (3)After the three bends are completed, the cold-rolled steel plate is vibrated reciprocally along the angle bisector direction of the bent cold-rolled steel plate. The vibration frequency is set to 28 Hz and the amplitude is set to 0.45 mm. During the vibration, cooling water at 22 °C is sprayed onto the cold-rolled steel plate at a spraying rate of 1.1 L / min until the surface temperature of the cold-rolled steel plate drops to 48 °C; Subsequently, the reciprocal vibration along the angle bisector direction is stopped, the cooling water spraying is turned off, and the cold-rolled steel plate is vibrated reciprocally along the bending line direction of the bent cold-rolled steel plate. The vibration frequency is set to 18 Hz and the amplitude is set to 0.35 mm. During the vibration, the cold-rolled steel plate is cooled by blowing air at a wind speed of 5.5 m / s until the cold-rolled steel plate drops to room temperature, obtaining the formed part.

[0067] Example 5 This example provides a bending and forming process for automotive sheet metal parts, as Figure 1 shown. The bending and forming process for automotive sheet metal parts specifically includes the following steps: (1)Preheat a 3-mm-thick cold-rolled steel plate, raise the temperature of the cold-rolled steel plate to 170 °C at a heating rate of 5 °C / min and hold for 10 min, and then continue to raise the temperature to 200 °C at a heating rate of 8 °C / min and hold for 15 min; (2)Perform three bends on the preheated cold-rolled steel plate. First, perform the first bend at a bending speed of 15 mm / s until 60% of the designed angle is reached. After the first bend, purge the surface of the cold-rolled steel plate with compressed air at 0.6 MPa until the surface temperature of the cold-rolled steel plate drops to 150 °C; Subsequently, spray an anti-sticking agent on the surface of the cold-rolled steel plate. The spraying amount of the anti-sticking agent is 0.2 g / cm 2 , and after spraying, let it stand for 60 s to form a film. Calculated based on the mass fraction of the anti-sticking agent being 100 wt%, it includes the following components with the following mass fractions: Graphite powder 5 wt%; Molybdenum disulfide 2 wt%; Boron nitride 2.5 wt%; Hydroxypropyl methylcellulose 1.8 wt%; Absolute ethanol 12 wt%; Acetone 5 wt%; The balance is deionized water; After the anti - sticking agent forms a film, perform the second bending at a bending speed of 10 mm / s, bend to 90% of the designed angle for the second time. After the second bending, blow the surface of the cold - rolled steel plate with compressed air at 0.5 MPa until the surface temperature of the cold - rolled steel plate drops to 120 °C; Finally, perform the third bending at a bending speed of 8 mm / s, bend to the designed angle for the third time. After the third bending, blow the surface of the cold - rolled steel plate with compressed air at 0.3 MPa until the surface temperature of the cold - rolled steel plate drops to 80 °C; (3)After the three - time bending is completed, reciprocally vibrate the cold - rolled steel plate along the angle bisector direction of the bent cold - rolled steel plate. The vibration frequency is set to 30 Hz, and the amplitude is set to 0.5 mm. During the vibration process, spray cooling water at 25 °C onto the cold - rolled steel plate at a spraying rate of 1.2 L / min until the surface temperature of the cold - rolled steel plate drops to 50 °C; Subsequently, stop the reciprocating vibration along the angle bisector direction and turn off the cooling water spraying. Reciprocally vibrate the cold - rolled steel plate along the fold line direction of the bent cold - rolled steel plate. The vibration frequency is set to 20 Hz, and the amplitude is set to 0.4 mm. During the vibration process, blow - cool the cold - rolled steel plate at a wind speed of 6 m / s until the cold - rolled steel plate drops to room temperature to obtain the formed part.

[0068] Example 6 This example provides a bending and forming processing technology for automotive sheet metal parts. The difference from Example 1 is that during the pre - heating of the cold - rolled steel plate, one - time pre - heating is adopted, directly heating to 180 °C at a heating rate of 5 °C / min and holding for 25 min. Other process parameters and operation steps are exactly the same as those in Example 1.

[0069] Example 7 This example provides a bending and forming processing technology for automotive sheet metal parts. The difference from Example 1 is that in step (1), one - time bending is adopted, directly bending the pre - heated cold - rolled steel plate to the designed angle. Other process parameters and operation steps are exactly the same as those in Example 1.

[0070] Example 8 This example provides a bending and forming processing technology for automotive sheet metal parts. The difference from Example 1 is that the pre - heated cold - rolled steel plate is bent twice. In this example, the first bending is to 50% of the designed angle, and at the same time, the compressed air blowing and spraying of the anti - sticking agent in Example 1 are retained, omitting the second bending in the original Example 1, and directly bending to the designed angle for the second time. Other process parameters and operation steps are exactly the same as those in Example 1.

[0071] Example 9 This example provides a bending and forming process for automotive sheet metal parts. The difference from Example 1 is that no anti-sticking agent is sprayed on the surface of the cold-rolled steel sheet. After the first bending, it is purged until the surface temperature of the cold-rolled steel sheet drops to 130 °C, and then the second bending is directly carried out. Other process parameters and operation steps are exactly the same as those in Example 1.

[0072] Example 10 This example provides a bending and forming process for automotive sheet metal parts. The difference from Example 1 is that graphite powder is omitted from the anti-sticking agent. Other process parameters and operation steps are exactly the same as those in Example 1.

[0073] Example 11 This example provides a bending and forming process for automotive sheet metal parts. The difference from Example 1 is that molybdenum disulfide is omitted from the anti-sticking agent. Other process parameters and operation steps are exactly the same as those in Example 1.

[0074] Example 12 This example provides a bending and forming process for automotive sheet metal parts. The difference from Example 1 is that boron nitride is omitted from the anti-sticking agent. Other process parameters and operation steps are exactly the same as those in Example 1.

[0075] Example 13 This example provides a bending and forming process for automotive sheet metal parts. The difference from Example 1 is that after the three bends are completed, the cold-rolled steel sheet is reciprocally vibrated along the angle bisector direction of the bent cold-rolled steel sheet, and cooling water is sprayed during the vibration until the cold-rolled steel sheet drops to room temperature to obtain the formed part. The operation process of reciprocally vibrating the cold-rolled steel sheet along the bend line direction is omitted, and the bending speed of the second bend is adjusted to 5 mm / s. Other process parameters and operation steps are exactly the same as those in Example 1.

[0076] Example 14 This example provides a bending and forming process for automotive sheet metal parts. The difference from Example 1 is that after the three bends are completed, the cold-rolled steel sheet is reciprocally vibrated along the bend line direction of the bent cold-rolled steel sheet, and cooling water is sprayed during the vibration until the cold-rolled steel sheet drops to room temperature to obtain the formed part. The operation process of reciprocally vibrating the cold-rolled steel sheet along the angle bisector direction is omitted. Other process parameters and operation steps are exactly the same as those in Example 1.

[0077] The bending angle springback amount, surface Vickers hardness, and dynamic yield strength of the formed parts prepared in Examples 1 - 14 were tested. The test methods are as follows: (1) Bending angle springback amount Use a laser scanner or a coordinate measuring machine (CMM) to measure the actual bending angle, and calculate the bending angle springback according to the following formula: Bending angle springback = Actual bending angle - Designed bending angle.

[0078] (2) Surface Vickers hardness Use a Vickers hardness tester to test the surface Vickers hardness of the formed part with reference to the national standard GB / T 4340.1-2024 "Metallic materials - Vickers hardness test - Part 1: Test method".

[0079] (3) Dynamic yield strength Conduct a dynamic tensile test on the formed part with reference to the national standard GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", with a strain rate of 0.01 s -1 , record the stress-strain curve, and calculate the dynamic yield strength.

[0080] The test data are shown in Table 1.

[0081] Table 1 Test data of the formed parts obtained in Examples 1-14 Bending Angle Springback Amount (°) Surface Vickers Hardness (HV) Dynamic Yield Strength (MPa) Example 1 0.2 275 485 Example 2 0.3 268 478 Example 3 0.1 282 492 Example 4 0.4 271 463 Example 5 0.2 279 488 Example 6 2.1 228 342 Example 7 2.5 214 329 Example 8 2.3 216 357 Example 9 1.9 237 375 Example 10 1.6 235 384 Example 11 1.7 243 408 Example 12 1.5 249 423 Example 13 1.8 231 396 Example 14 2.2 216 351 From the test data of Example 1 and Example 6, it can be seen that the bending angle springback of the formed part prepared in Example 6 is higher than that in Example 1, and the surface Vickers hardness and dynamic yield strength are lower than those in Example 1. This is because in Example 6, the preheating stage is changed to one-time preheating, directly heating up to 180 °C and holding the temperature. The springback is as high as 2.1 °, the hardness is reduced to 228 HV, and the strength is reduced to 342 MPa. This shows the importance of staged preheating. One-time preheating may cause uneven heating, resulting in stress concentration during subsequent bending, increased springback, and insufficient hardening of the material, resulting in a decrease in both hardness and strength.

[0082] From the test data of Example 1 and Example 7, it can be seen that the bending angle springback of the formed part prepared in Example 7 is higher than that in Example 1, and the surface Vickers hardness and dynamic yield strength are lower than those in Example 1. This is because in Example 7, it is adjusted to one-time bending, the springback is as high as 2.5 °, and the hardness and strength also decrease significantly. The advantage of three-stage progressive bending is to gradually release stress and avoid work hardening. One-time bending leads to excessive stress accumulation, serious springback, and entanglement of dislocations inside the material, resulting in a decrease in hardness and strength because the structure is unstable.

[0083] It can be seen from the test data of Example 1 and Example 8 that the bending angle springback of the formed part prepared in Example 8 is higher than that in Example 1, and the surface Vickers hardness and dynamic yield strength are lower than those in Example 1. This is because in Example 8, the bending process is adjusted to two steps, omitting 80 - 90% of the second bending stage, resulting in a final springback of up to 2.3°. The performance is also poor. One less bending step means insufficient stress release. Especially the second bending stage (80 - 90%) is crucial for controlling the hardening rate. Without this step, the material is more likely to have defects during the final bending, affecting the overall performance.

[0084] It can be seen from the test data of Example 1 and Example 9 that the bending angle springback of the formed part prepared in Example 9 is higher than that in Example 1, and the surface Vickers hardness and dynamic yield strength are lower than those in Example 1. This is because in Example 9, the anti - sticking agent is omitted, the springback increases to 1.9°, the hardness decreases to 237 HV, and the strength decreases to 375 MPa. The function of the anti - sticking agent is lubrication and stress buffering. Especially during the second bending, it prevents the direct friction between the surface oxide layer and the mold. The absence of spraying the anti - sticking agent leads to increased friction, surface damage, uneven stress distribution, increased springback, and also affects the hardness and strength.

[0085] It can be seen from the test data of Example 1, Example 10, Example 11 and Example 12 that the bending angle springback of the formed parts prepared in Example 10, Example 11 and Example 12 is higher than that in Example 1, and the surface Vickers hardness and dynamic yield strength are lower than those in Example 1. This is because in Example 10, Example 11 and Example 12, the graphite powder, molybdenum disulfide, and boron nitride in the anti - sticking agent are removed respectively. The springback is between 1.5 - 1.7°. The hardness and strength both decrease. The absence of graphite powder affects the lubrication channel, the absence of molybdenum disulfide weakens the high - temperature adsorption, and the absence of boron nitride reduces the heat resistance. After the synergistic effect of these components is damaged, the anti - sticking film effect is discounted, resulting in friction and stress problems during bending, and thus affecting the final performance.

[0086] It can be seen from the test data of Example 1 and Example 13 that the bending angle springback of the formed part prepared in Example 13 is higher than that in Example 1, and the surface Vickers hardness and dynamic yield strength are lower than those in Example 1. This is because in Example 13, the vibration along the bending line is omitted, and only the angular bisector vibration is performed. The springback increases to 1.8°. It shows that without the low - frequency vibration along the bending line, the longitudinal residual stress release is insufficient, the springback increases, and at the same time, the hardness and strength also decrease because the composite vibration could originally synergistically optimize the stress distribution.

[0087] It can be seen from the test data of Example 1 and Example 14 that the bending angle springback of the formed part prepared in Example 14 is higher than that in Example 1, and the surface Vickers hardness and dynamic yield strength are lower than those in Example 1. This is because in Example 14, the vibration of the angle bisector is omitted and only the vibration of the bending line is performed. The springback is increased to 2.2°, and the performance is worse. The high-frequency vibration of the angle bisector is the key to solving the dislocation entanglement at the bending root. The lack of it will lead to more serious stress concentration, obvious increase in springback, and significant reduction in hardness and strength.

[0088] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A bending and forming process for automobile sheet metal parts, characterized in that, The bending and forming process of the automotive sheet metal part includes: (I) Preheating the cold-rolled steel sheet, and performing three bends on the preheated cold-rolled steel sheet. The first bend is to 50 - 60% of the designed angle, followed by purging and cooling. The second bend is to 80 - 90% of the designed angle, followed by purging and cooling. The third bend is to the designed angle, followed by purging and cooling; (II) Vibration is reciprocated along the angle bisector direction of the bent cold-rolled steel sheet, and cooling water is sprayed onto the cold-rolled steel sheet; then vibration is reciprocated along the fold line direction of the bent cold-rolled steel sheet, and the cold-rolled steel sheet is air-cooled to obtain the formed part.

2. The bending and forming process of the automotive sheet metal part according to claim 1, characterized in that, In step (I), the thickness of the cold-rolled steel sheet is 2 - 3 mm; The preheating process of the cold-rolled steel sheet is carried out in two stages, including: Heating the cold-rolled steel sheet to the first preheating temperature at the first heating rate and holding the temperature; then continuing to heat to the second preheating temperature at the second heating rate and holding the temperature, and then performing the bend.

3. The bending and forming process of the automotive sheet metal part according to claim 2, characterized in that, The first heating rate is 3 - 5 °C / min; The first preheating temperature is 150 - 170 °C; Hold the temperature for 10 - 20 min at the first preheating temperature; The second heating rate is 5 - 8 °C / min; The second preheating temperature is 180 - 200 °C; Hold the temperature for 15 - 25 min at the second preheating temperature.

4. The bending and forming process of the automotive sheet metal part according to claim 1, characterized in that, In step (I), after the first bend, the surface of the cold-rolled steel sheet is purged with compressed air at 0.5 - 0.6 MPa until the surface temperature of the cold-rolled steel sheet drops to 130 - 150 °C; The bending speed of the first bend is 12 - 15 mm / s.

5. The bending and forming process of the automotive sheet metal part according to claim 1, wherein, In step (I), before the second bend starts, an anti-sticking agent is sprayed on the surface of the cold-rolled steel sheet, and a film is formed after standing for 30 - 60 s, and then the second bend is carried out; The spraying amount of the anti-sticking agent is 0.1~0.2 g / cm 2 .

6. The bending and forming process of the automotive sheet metal part according to claim 5, wherein The anti-sticking agent includes graphite powder, molybdenum disulfide, boron nitride, hydroxypropyl methylcellulose, anhydrous ethanol, acetone and deionized water; Based on the mass fraction of the anti-sticking agent being 100 wt%, the anti-sticking agent includes the following components with the following mass fractions: Graphite powder 3 - 5 wt%; Molybdenum disulfide 2 - 3 wt%; Boron nitride 1.5 - 2.5 wt%; Hydroxypropyl methylcellulose 1.5 - 2.5 wt%; Anhydrous ethanol 10 - 12 wt%; Acetone 5 - 8 wt%; 7. The bending and forming process of the automotive sheet metal part according to claim 1, characterized in that, The balance is deionized water. In step (I), after the second bend, the surface of the cold-rolled steel sheet is purged with compressed air at 0.3 - 0.5 MPa until the surface temperature of the cold-rolled steel sheet drops to 100 - 120 °C; 8. The bending and forming process of the automotive sheet metal part according to claim 1, wherein, The bending speed of the second bend is 8 - 10 mm / s. In step (I), after the third bend, the surface of the cold-rolled steel sheet is purged with compressed air at 0.2 - 0.3 MPa until the surface temperature of the cold-rolled steel sheet drops to 70 - 80 °C; 9. The bending and forming process of the automotive sheet metal part according to claim 1, characterized in that, The bending speed of the third bend is 5 - 8 mm / s. In step (II), the reciprocating vibration frequency of the cold-rolled steel sheet along the angle bisector direction is 20 - 30 Hz; The amplitude of the cold-rolled steel sheet along the angle bisector direction is 0.3 - 0.5 mm; The spraying amount of the cooling water is 0.8 - 1.2 L / min; The temperature of the cooling water is 15 - 25 °C; Spray cooling water on the surface of the cold-rolled steel sheet until its surface temperature drops to 40 - 50 °C.

10. The bending and forming process of the automotive sheet metal part according to claim 1, wherein, In step (Ⅲ), the reciprocating vibration frequency of the cold-rolled steel sheet along the bending line direction is 10 - 20 Hz; The amplitude of the cold-rolled steel sheet along the bending line direction is 0.2 - 0.4 mm; The wind speed of the air cooling is 4 - 6 m / s.

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