A bending and forming process for automobile sheet metal parts

Abstract

The present invention belongs to the field of mechanical processing technology and relates to a bending and forming process for automobile sheet metal parts. The bending and forming process comprises: preheating a cold-rolled steel plate, bending the preheated cold-rolled steel plate three times, bending the cold-rolled steel plate to 50-60% of the design angle for the first time, followed by purging and cooling, bending the cold-rolled steel plate to 80-90% of the design angle for the second time, followed by purging and cooling, and bending the cold-rolled steel plate to the design angle for the third time, followed by purging and cooling; reciprocatingly vibrating the cold-rolled steel plate along the angle bisector direction of the bent cold-rolled steel plate and spraying cooling water on the cold-rolled steel plate; then reciprocatingly vibrating the cold-rolled steel plate along the bending line direction of the bent cold-rolled steel plate and air-cooling the cold-rolled steel plate. The present invention significantly improves the forming accuracy and structural stability of automobile sheet metal parts by combining staged bending with gradient cooling.

Description

Technical Field

[0001] The invention belongs to the technical field of mechanical processing and relates to a bending and forming processing technology for automobile sheet metal parts. Background Art

[0002] In the automotive industry, sheet metal bending is a core process in the production of structural components such as doors, fenders, and hoods. Traditional cold stamping processes, which achieve high efficiency by using dies to perform bending in a single pass, suffer from significant springback and residual stress concentration. In particular, elastic recovery after bending can lead to dimensional deviations of up to 3%-8% for materials such as high-strength steel, necessitating manual trimming or the addition of compensating dies, significantly increasing production costs. Recent advances in hot forming technologies can improve formability by heating to austenitizing temperatures, but these technologies suffer from high energy consumption, significant equipment investment, and significant surface oxidation. High-temperature treatments can also cause grain coarsening and reduce material toughness. Some processes have attempted to combine step-by-step bending with staged cooling. However, during the multiple bending processes, the cold-rolled steel sheet experiences work hardening, leading to a sharp increase in subsequent deformation resistance, which can lead to cracking along the bend line or localized thinning. Conventional intermediate annealing requires interrupting the production line and reheating, significantly impacting production efficiency. Furthermore, existing vibration stress relief processes often employ a single direction or fixed parameters, making it difficult to simultaneously eliminate residual stress in both the bend zone and the entire sheet. Improper control of water temperature and flow rate during spray cooling can easily lead to the accumulation of sudden cooling stresses. Excessive use of volatile organic solvents in anti-sticking agent formulations not only poses a safety hazard but also forms residues at high temperatures, impairing coating adhesion. Achieving a balance between high-precision forming, low rebound control, and efficient continuous production remains a key challenge hindering the upgrading of automotive sheet metal processing technology. Summary of the Invention

[0003] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a bending and forming processing technology for automobile sheet metal parts, which significantly improves the forming accuracy and structural stability of automobile sheet metal parts through the combination of staged bending and gradient cooling.

[0004] To achieve this object, the present invention adopts the following technical solutions:

[0005] The present invention provides a bending and forming process for automobile sheet metal parts, which includes:

[0006] (I) Preheating the cold-rolled steel sheet, bending the preheated cold-rolled steel sheet three times, bending the sheet to 50-60% of the design angle for the first time, then blowing and cooling, bending the sheet to 80-90% of the design angle for the second time, then blowing and cooling, and bending the sheet to the design angle for the third time, then blowing and cooling;

[0007] (II) The cold-rolled steel plate is vibrated back and forth along the direction of the angle bisector of the bent cold-rolled steel plate, and cooling water is sprayed on the cold-rolled steel plate; then, the cold-rolled steel plate is vibrated back and forth along the direction of the bending line of the bent cold-rolled steel plate, and the cold-rolled steel plate is air-cooled to obtain a formed part.

[0008] This invention significantly improves the forming accuracy and structural stability of automotive sheet metal parts through the synergistic effect of staged bending and composite vibration cooling. Three progressive bending steps control the material's work hardening rate while creating a gradient distribution of stress along the thickness direction, keeping the springback within ±0.5°. The composite vibration cooling process employs differentiated vibrations along the angle bisector and the bending line, using high-frequency vibration to break dislocation entanglements and low-frequency vibration to release longitudinal stress. Combined with water and air cooling, this significantly reduces residual stress and significantly improves fatigue life.

[0009] The design of three progressive bends combined with gradient cooling effectively alleviates the internal stress accumulation of the material. During the first bend to 50-60% of the design angle, the cold-rolled steel sheet is at a high temperature. At this time, rapid purging and cooling form a slightly hardened support on the surface, retaining the internal plasticity and providing a basis for subsequent deformation. The second bend is to 80-90% of the design angle. During this process, the ductility of the cold-rolled steel sheet decreases as the temperature decreases, but the staged forming avoids excessive grain stretching caused by a single large-angle bend. By the third bend to the design angle, the cold-rolled steel sheet has established preliminary structural memory, and combined with a lower bending speed, the directional arrangement of the cold-rolled steel sheet's microstructure is more stable.

[0010] The stress distribution in the molded part is further optimized through a composite vibration cooling process. High-frequency vibration along the angle bisector, combined with water cooling, leverages the micro-impact effect of water molecule penetration to promote grain boundary slip reorganization in the bend area of ​​the molded part, reducing localized stress concentration. At this point, the spraying water creates a dynamic temperature field on the molded part's surface, avoiding the sudden embrittlement caused by traditional immersion cooling while strengthening the metal's dislocation pinning effect through continuous phase transformation. Subsequently, a switch to low-frequency vibration along the bend line, combined with air cooling, specifically releases residual stress extending along the bend line, rebalancing the overall stress field of the molded part.

[0011] As a preferred technical solution of the present invention, in step (I), the thickness of the cold-rolled steel plate is 2 to 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. Other values ​​not listed within this numerical range are also applicable.

[0012] In some optional examples, the preheating process of the cold-rolled steel sheet is divided into two stages, including:

[0013] The cold-rolled steel sheet is heated to a first preheating temperature at a first heating rate and kept warm; then the cold-rolled steel sheet is heated to a second preheating temperature at a second heating rate and kept warm, and then bent.

[0014] The present invention adopts a staged preheating treatment to create ideal plastic deformation conditions for the subsequent three bending steps. In the first preheating stage, a relatively low heating rate (3-5°C / min) is used to heat the cold-rolled steel sheet to 150-170°C. During this process, the metal lattice of the cold-rolled steel sheet gradually absorbs heat, and dislocations begin to migrate slowly, but no obvious recrystallization behavior has yet to be triggered. During the 10-20 minute insulation stage, the temperature gradient in the thickness direction of the cold-rolled steel sheet is effectively eliminated, and the temperature difference between the surface and the core is controlled within ±5°C, thus avoiding local stress mutations caused by uneven heating during subsequent bending.

[0015] In the second preheating stage, a rapid temperature increase of 5-8°C / min is used to accelerate the cold-rolled steel sheet through the critical work hardening point. When the temperature reaches 180-200°C, the deformation energy stored in the cold-rolled steel sheet is partially released, and dislocation entanglements begin to dissociate to form slip bands. However, the carbide precipitation phase has not yet dissolved significantly. This structural state enables the cold-rolled steel sheet to have sufficient plastic flow ability while maintaining a certain level of original strength. During the 15-25 minute holding period, the cold-rolled steel sheet completes dynamic recovery, forming a stable subgrain structure within the grains, significantly reducing the work hardening rate during subsequent bending.

[0016] As a preferred technical solution of the present invention, the first heating rate is 3~5℃ / min, for example, it can be 3.0℃ / min, 3.2℃ / min, 3.4℃ / min, 3.6℃ / min, 3.8℃ / min, 4.0℃ / min, 4.2℃ / min, 4.4℃ / min, 4.6℃ / min, 4.8℃ / min or 5.0℃ / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0017] In some optional instances, the first preheating temperature is 150~170℃, for example, it can be 150℃, 152℃, 154℃, 156℃, 158℃, 160℃, 162℃, 164℃, 166℃, 168℃ or 170℃, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0018] In some optional examples, the first preheating temperature is kept warm for 10 to 20 minutes, for example, 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 the numerical range are also applicable.

[0019] In some optional examples, the second heating rate is 5~8℃ / min, for example, it can be 5.0℃ / min, 5.5℃ / min, 6.0℃ / min, 6.5℃ / min, 7.0℃ / min, 7.5℃ / min or 8.0℃ / min, but is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0020] In some optional instances, the second preheating temperature is 180~200℃, for example, it can be 180℃, 182℃, 184℃, 186℃, 188℃, 190℃, 192℃, 194℃, 196℃, 198℃ or 200℃, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0021] The present invention specifically limits the second preheating temperature to 180~200℃. This temperature range is exactly in the transition zone from elastic deformation to plastic deformation of the cold-rolled steel plate. The activity of the internal lattice structure of the material is stimulated but does not reach the level of drastic reconstruction. When the first bending is performed within this temperature range, the cold-rolled steel plate retains a certain degree of tensile strength and obtains sufficient ductility to withstand 50~60% angular deformation without stress concentration.

[0022] When the second preheating temperature is lower than 180°C, the plasticity of the cold-rolled steel plate is insufficient, which will directly destroy the continuity of the bending. During the first bending, the residual cold-rolling stress inside the cold-rolled steel plate is superimposed on the newly added deformation stress, which can easily form microcracks invisible to the naked eye at the root of the bending line. These microcracks are sharply amplified during the second bending, resulting in stress release gaps in local areas, which appear as fish-scale wrinkles at the edge of the bending line.

[0023] When the second preheat temperature exceeds 200°C, the high temperature increases 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 bend. During the second bend, the area containing these oxide particles becomes a source of stress concentration, resulting in the appearance of symmetrical crescent-shaped dark cracks on both sides of the bend line. In addition, preheating temperatures above 200°C induces abnormal coarsening of carbide particles. These hard phases with diameters exceeding 200nm become crack propagation channels during the cooling process of the third bend, significantly reducing the fatigue life of the resulting formed part.

[0024] In some optional examples, the second preheating temperature is kept warm for 15 to 25 minutes, for example, 15 minutes, 16 minutes, 17 minutes, 18 minutes, 19 minutes, 20 minutes, 21 minutes, 22 minutes, 23 minutes, 24 minutes or 25 minutes, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0025] As a preferred technical solution of the present invention, in step (I), after the first bending, the surface of the cold-rolled steel sheet is blown with compressed air of 0.5-0.6 MPa, for example, 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 sheet drops to 130-150°C, for example, 130°C, 132°C, 134°C, 136°C, 138°C, 140°C, 142°C, 144°C, 146°C, 148°C or 150°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0026] In some optional instances, the bending speed of the first bend is 12~15mm / s, for example, it can be 12mm / s, 12.5mm / s, 13mm / s, 13.5mm / s, 14mm / s, 14.5mm / s or 15mm / s, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0027] As a preferred technical solution of the present invention, in step (I), before the second bending begins, an anti-sticking agent is sprayed onto the surface of the cold-rolled steel sheet, and a film is formed after standing for 30 to 60 seconds, and then the second bending is performed. For example, it can be 30 seconds, 35 seconds, 40 seconds, 45 seconds, 50 seconds, 55 seconds or 60 seconds, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0028] In some optional embodiments, the spraying amount of the anti-sticking agent is 0.1~0.2g / cm 2 , for example, it can be 0.1 g / cm 2 , 0.11g / cm 2 , 0.12g / cm 2 , 0.13g / cm 2 , 0.14g / cm 2 , 0.15g / cm 2 , 0.16g / cm 2 , 0.17g / cm 2 , 0.18g / cm 2, 0.19g / cm 2 or 0.2g / cm 2 , but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0029] As a preferred technical solution of the present invention, the anti-sticking agent includes graphite powder, molybdenum disulfide, boron nitride, hydroxypropyl methylcellulose, anhydrous ethanol, acetone and deionized water.

[0030] In some optional examples, based on the mass fraction of the anti-sticking agent being 100 wt %, the anti-sticking agent includes the following components in mass fractions:

[0031] Graphite powder 3~5wt%;

[0032] Molybdenum disulfide 2~3wt%;

[0033] Boron nitride 1.5~2.5wt%;

[0034] Hydroxypropyl methylcellulose 1.5~2.5wt%;

[0035] Anhydrous ethanol 10~12wt%;

[0036] Acetone 5~8wt%;

[0037] The rest was deionized water.

[0038] Among them, the mass fraction of graphite powder can be 3.0wt%, 3.2wt%, 3.4wt%, 3.6wt%, 3.8wt%, 4.0wt%, 4.2wt%, 4.4wt%, 4.6wt%, 4.8wt% or 5.0wt%, the mass fraction of molybdenum disulfide can be 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt%, 2.5wt%, 2.6wt%, 2.7wt%, 2.8wt%, 2.9wt% or 3.0wt%, the mass fraction of boron nitride can be 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt% or 2.5wt%, hydroxypropyl methyl The mass fraction of cellulose can be 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt% or 2.5wt%; the mass fraction of anhydrous ethanol can be 10wt%, 10.2wt%, 10.4wt%, 10.6wt%, 10.8wt%, 11wt%, 11.2wt%, 11.4wt%, 11.6wt%, 11.8wt% or 12wt%; and the mass fraction of acetone can be 5.0wt%, 5.5wt%, 6.0wt%, 6.5wt%, 7.0wt%, 7.5wt% or 8.0wt%, but is not limited to the listed values, and other values ​​not listed within this numerical range are equally applicable.

[0039] During the second bending process, the anti-sticking agent mainly plays the dual role of interface lubrication and stress buffering. When the cold-rolled steel plate completes the first bending and is cooled to 130~150℃, a micron-level oxide layer is formed on the surface of the material 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, greatly 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.

[0040] The present invention adds 3-5wt% graphite powder. The graphite powder serves as a layered structural material and 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.

[0041] The present invention adds 2-3 wt% of molybdenum disulfide. The hexagonal crystal structure of molybdenum disulfide activates the active sites of sulfur atoms in the range of 130-150°C, forming weak chemical adsorption with the iron element on the surface of the cold-rolled steel plate. This instantaneous bonding can prevent the anti-sticking film layer from falling off and will not affect the demolding separation after molding.

[0042] The present invention adds 1.5-2.5wt% of boron nitride. The introduction of boron nitride enhances the high temperature resistance of the anti-adhesive film layer. Its high thermal conductivity can quickly extract the friction heat of the bending contact point, avoiding the decomposition and failure of the anti-adhesive film layer caused by excessive local temperature.

[0043] The present invention adds 1.5-2.5 wt% of hydroxypropyl methylcellulose, which serves as a water-soluble polymer and constructs a three-dimensional network skeleton through molecular chain entanglement during the film-forming process.

[0044] The compound solvent system of 10-12wt% anhydrous ethanol and 5-8wt% acetone achieves rapid volatilization of the anti-sticking agent. The high volatility of acetone enables the anti-sticking agent to complete surface drying within 30-60 seconds. The slow-release effect of ethanol ensures the uniformity of the film-forming process and avoids pinhole defects caused by rapid solvent volatilization.

[0045] As a preferred technical solution of the present invention, in step (I), after the second bending, the surface of the cold-rolled steel sheet is purged with compressed air of 0.3-0.5 MPa, for example, 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 sheet drops to 100-120°C, for example, 100°C, 102°C, 104°C, 106°C, 108°C, 110°C, 112°C, 114°C, 116°C, 118°C or 120°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0046] In some optional instances, the bending speed of the second bending is 8~10mm / s, for example, it can be 8.0mm / s, 8.2mm / s, 8.4mm / s, 8.6mm / s, 8.8mm / s, 9.0mm / s, 9.2mm / s, 9.4mm / s, 9.6mm / s, 9.8mm / s or 10.0mm / s, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0047] The present invention specifically limits the bending speed of the second bending to 8~10mm / s. During the second bending, the surface temperature of the cold-rolled steel plate has dropped to 130~150℃, and the internal grain structure is in a semi-hardened state. The appropriate bending speed allows the material sufficient time to redistribute stress, thereby avoiding local strain concentration caused by rapid bending and preventing excessive temperature drop caused by slow bending.

[0048] When the bending speed for the second bend is slower than 8mm / s, the bending time becomes excessively long. By the time 80-90% of the designed angle is achieved, the surface temperature of the cold-rolled steel sheet has dropped below 100°C, significantly enhancing the cold work hardening effect. At this point, when bending at large angles, the material's resistance to deformation increases dramatically, easily forming visible tensile wrinkles on the outside of the bend line. Furthermore, the slow bending speed causes the release film layer to withstand continuous compression for an extended period of time, leading to directional migration of molybdenum disulfide in the film and localized lubrication failure, ultimately leaving intermittent indentation-like defects at the bends of the formed part.

[0049] When the bending speed during the second bend exceeds 10mm / s, the high speed prevents the metal grains from fully sliding, forming dislocation accumulation zones at the base of the bend line of the molded part. These microscopic defects act as stress release channels during the subsequent cooling process, causing the molded part to experience a certain degree of angular retreat, failing to achieve the designed angle. Furthermore, excessive bending speeds can damage the integrity of the anti-adhesive film layer, causing it to shear under impact loads, exposing the cold-rolled steel sheet surface and bringing it into direct contact with the mold. This results in a rapid increase in the friction coefficient, and this abnormal friction can leave scratches on the molded part surface parallel to the bending direction.

[0050] 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 of 0.2-0.3 MPa, for example, 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, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C or 80°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0051] In some optional instances, the bending speed of the third bend is 5~8mm / s, for example, it can be 5.0mm / s, 5.5mm / s, 6.0mm / s, 6.5mm / s, 7.0mm / s, 7.5mm / s or 8.0mm / s, but it is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0052] The present invention specifically limits the bending speed for the third bend to 5-8 mm / s. During this third bend, the material is already in a semi-hardened state after the first two deformations and multiple cooling steps. Slow bending allows sufficient time for the grains to undergo orderly slip and reorganization, effectively suppressing springback by utilizing the initially formed hardened structure on the surface and enabling fine-tuning of the angle through the limited plasticity retained in the core. The deformation process at this bending speed promotes a gradient stress distribution through the thickness of the material. The synergistic effect of the hardened surface zone and the plastic core zone ensures dimensional stability while dispersing stress concentration points over a wider area, effectively avoiding micro-defects caused by localized overload.

[0053] When bending speeds are lower than 5mm / s, the cold-rolled steel sheet is subjected to die pressure for extended periods at low temperatures. This constant pressure creates an abnormally hardened zone on the inner side of the bend line, significantly altering the uniformity of the material's properties. Furthermore, the slow bending speed prevents the frictional heat at the die-material interface from dissipating quickly. This localized heating causes the metastable structure to recover, fluctuating the material's ability to resist deformation. The resulting molded part is prone to angular relaxation under dynamic loads, directly impacting the precision of assembly.

[0054] When bending speeds exceed 8 mm / s, high-frequency mechanical disturbances easily activate dislocation motion at microscopic defects, leading to a network of radial microcracks at the base of the bend line. Simultaneously, the inertial forces caused by rapid bending interfere with the integrity of the anti-adhesive film, generating instantaneous friction at the bend line and forming visible surface damage zones. These damaged areas become corrosion channels during the subsequent cooling process, significantly reducing the environmental tolerance of the molded part. Furthermore, excessively fast bending speeds shorten the time for internal stress redistribution in the material, resulting in an unbalanced residual stress distribution.

[0055] As a preferred technical solution of the present invention, in step (II), the reciprocating vibration frequency of the cold-rolled steel plate along the angle 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, and other values ​​not listed within this numerical range are also applicable.

[0056] This invention specifically limits the reciprocating vibration frequency of the cold-rolled steel sheet along the angle bisector to 20-30 Hz. Within this range, it effectively stimulates coordinated slip motion between grains. The alternating stress field generated by high-frequency vibration prompts the realignment of dislocations accumulated in the bending region, eliminating localized stress concentration points and promoting the uniform dispersion of carbide particles at grain boundaries. During the vibration process, a microscopic plastic deformation layer forms on the material surface, inhibiting crack propagation and significantly extending the fatigue life of the formed part.

[0057] 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 of the material to a depth of 0.1 to 0.2 mm, and cannot reach the high stress area at the root of the bend.

[0058] When the vibration frequency exceeds 30Hz, the energy density applied per unit time by high-frequency vibration exceeds the tolerance of grain boundary slip, resulting in a network of nanoscale microcracks at the bend line. These microscopic defects gradually connect under the thermal stress of the subsequent cooling process, forming invisible crack bands parallel to the bending direction, which leads to a significant decrease in the fatigue performance of the molded part. At the same time, the thermomechanical coupling effect caused by high-frequency vibration accelerates the oxidation of the material surface, forming an oxide layer of uneven thickness on both sides of the bend line. The presence of the oxide layer increases the risk of coating delamination during the subsequent spraying process.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] Vibration and spray cooling along the angle bisector primarily contribute to stress reconstruction and microstructural optimization within the material. After three bends, a V-shaped stress concentration zone forms within the bend region, where grains are stretched along the bend line, while the matrix on either side retains its original rolled structure. Applying high-frequency vibrations of 20-30 Hz along the angle bisector is equivalent to applying periodic alternating stress along the material's symmetry axis. This specific vibrational stimulation effectively breaks up the dislocation tangles formed at the bend root. Simultaneously with vibration, spraying cooling water at 15-25°C promotes cooling. The dynamic pressure from the water impact induces slip reorientation of surface grains, while the temperature gradient created by the water cooling drives dislocation migration toward subgrain boundaries. This thermal-mechanical coupling significantly reduces residual stress within the bend angle and promotes the uniform precipitation of carbide particles at the grain boundaries, forming a dispersed distribution less than 200 nm.

[0070] The process then switches to vibration along the bend line, combined with air cooling, primarily to improve the dimensional stability of the molded part. Low-frequency vibrations of 10-20 Hz are transmitted along the length of the bend line. The vibration waveform interferes with the material's elastic recovery tendency, effectively neutralizing the elastic potential energy accumulated during unloading and rebound, significantly improving the angular accuracy of the bend line. Air cooling at wind speeds of 4-6 m / s achieves a slow phase transition in the molded part. Compared to the rapid quenching achieved with water spray, the temperature field created by air cooling is more gradual, allowing the dislocation network to gradually reorganize during cooling, ultimately forming a stable dislocation cell structure within the molded part. This structural feature results in a more uniform distribution of the elastic modulus and a significantly reduced anisotropy index.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] This invention significantly improves the forming accuracy and structural stability of automotive sheet metal parts through the synergistic effect of staged bending and composite vibration cooling. Three progressive bending steps control the material's work hardening rate while creating a gradient distribution of stress along the thickness direction, keeping the springback within ±0.5°. The composite vibration cooling process employs differentiated vibrations along the angle bisector and the bending line, using high-frequency vibration to break dislocation entanglements and low-frequency vibration to release longitudinal stress. Combined with water and air cooling, this significantly reduces residual stress and significantly improves fatigue life. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 Flowchart of the automotive sheet metal bending and forming processing technology provided in Examples 1-14 of the present invention. DETAILED DESCRIPTION

[0074] The technical solutions of the present invention are described in detail below in conjunction with specific embodiments and their accompanying drawings. The embodiments described herein are specific embodiments of the present invention and are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be understood as limiting the embodiments of the present invention and the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can also adopt other obvious technical solutions based on the contents disclosed in the claims of this application and its specification, including technical solutions that adopt any obvious replacements and modifications to the embodiments described herein.

[0075] Example 1

[0076] This embodiment provides a bending and forming process for automobile sheet metal parts, such as Figure 1 As shown, the automotive sheet metal bending forming process specifically includes the following steps:

[0077] (1) Preheat the 2 mm thick cold-rolled steel plate by heating it to 150 °C at a heating rate of 3 °C / min and keeping it at that temperature for 20 min, then continue heating it to 180 °C at a heating rate of 5 °C / min and keeping it at that temperature for 25 min;

[0078] (2) The preheated cold-rolled steel sheet was bent three times. First, the first bending was performed at a bending speed of 12 mm / s to 50% of the design angle. After the first bending, the surface of the cold-rolled steel sheet was blown with 0.5 MPa compressed air until the surface temperature of the cold-rolled steel sheet dropped to 130 °C.

[0079] Then, anti-sticking agent was sprayed on the surface of the cold-rolled steel plate. The spraying amount of anti-sticking agent was 0.1g / cm 2 After spraying, the film is left to stand for 30 seconds to form. Taking the mass fraction of the anti-sticking agent as 100wt%, it includes the following components by mass fraction:

[0080] Graphite powder 3wt%;

[0081] Molybdenum disulfide 2.5wt%;

[0082] Boron nitride 2wt%;

[0083] Hydroxypropyl methylcellulose 2wt%;

[0084] Anhydrous ethanol 12wt%;

[0085] Acetone 7wt%;

[0086] The rest is deionized water;

[0087] After the anti-sticking agent forms a film, the second bending is performed at a bending speed of 8 mm / s to 80% of the design angle. After the second bending, the surface of the cold-rolled steel plate is blown with 0.3 MPa compressed air until the surface temperature of the cold-rolled steel plate drops to 100°C.

[0088] Finally, the third bending is performed at a bending speed of 5 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 0.2 MPa compressed air until the surface temperature of the cold-rolled steel plate drops to 70°C.

[0089] (3) After the three bendings are completed, the cold-rolled steel plate is subjected to reciprocating vibration 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 process, cooling water at 15°C is sprayed on the cold-rolled steel plate at a spray rate of 0.8 L / min until the surface temperature of the cold-rolled steel plate drops to 40°C.

[0090] Subsequently, the reciprocating vibration along the angle bisector direction is stopped, and the cooling water spray is turned off. The cold-rolled steel plate is reciprocated 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 process, the cold-rolled steel plate is blown and cooled at a wind speed of 4 m / s until the cold-rolled steel plate drops to room temperature to obtain a formed part.

[0091] Example 2

[0092] This embodiment provides a bending and forming process for automobile sheet metal parts, such as Figure 1 As shown, the automotive sheet metal bending forming process specifically includes the following steps:

[0093] (1) Preheat the 2.2 mm thick cold-rolled steel plate by heating it to 155 °C at a heating rate of 3.5 °C / min and keeping it at that temperature for 18 min, then continue heating it to 185 °C at a heating rate of 6 °C / min and keeping it at that temperature for 22 min;

[0094] (2) The preheated cold-rolled steel sheet was bent three times. First, the first bending was performed at a bending speed of 13 mm / s to 52% of the design angle. After the first bending, the surface of the cold-rolled steel sheet was purged with 0.52 MPa compressed air until the surface temperature of the cold-rolled steel sheet dropped to 135 °C.

[0095] Then, anti-sticking agent was sprayed on the surface of the cold-rolled steel plate. The spraying amount of anti-sticking agent was 0.12g / cm 2After spraying, the film is left to stand for 40 seconds to form. Taking the mass fraction of the anti-sticking agent as 100wt%, it includes the following components in mass fractions:

[0096] Graphite powder 4wt%;

[0097] Molybdenum disulfide 3wt%;

[0098] Boron nitride 1.5wt%;

[0099] Hydroxypropyl methylcellulose 2.2wt%;

[0100] Anhydrous ethanol 10wt%;

[0101] Acetone 8wt%;

[0102] The rest is deionized water;

[0103] After the anti-sticking agent forms a film, the second bending is performed at a bending speed of 8.5 mm / s to 82% of the design angle. After the second bending, the surface of the cold-rolled steel plate is purged with 0.35 MPa compressed air until the surface temperature of the cold-rolled steel plate drops to 105°C.

[0104] Finally, the third bending is performed 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 0.22 MPa compressed air until the surface temperature of the cold-rolled steel plate drops to 72°C.

[0105] (3) After the three bendings, the cold-rolled steel plate was vibrated back and forth along the angle bisector of the bent cold-rolled steel plate. The vibration frequency was set to 22 Hz and the amplitude was set to 0.35 mm. During the vibration process, cooling water at 18°C ​​was sprayed on the cold-rolled steel plate at a spray rate of 0.9 L / min until the surface temperature of the cold-rolled steel plate dropped to 42°C.

[0106] Subsequently, the reciprocating vibration along the angle bisector direction was stopped, and the cooling water spray was turned off. The cold-rolled steel plate was vibrated reciprocatingly along the bending line direction of the bent cold-rolled steel plate. The vibration frequency was set to 12 Hz and the amplitude was set to 0.25 mm. During the vibration process, the cold-rolled steel plate was blown and cooled at a wind speed of 4.5 m / s until the cold-rolled steel plate cooled to room temperature to obtain a formed part.

[0107] Example 3

[0108] This embodiment provides a bending and forming process for automobile sheet metal parts, such as Figure 1 As shown, the automotive sheet metal bending forming process specifically includes the following steps:

[0109] (1) Preheat the 2.5 mm thick cold-rolled steel plate by heating it to 160 °C at a heating rate of 4 °C / min and keeping it at that temperature for 15 min, then continue heating it to 190 °C at a heating rate of 6 °C / min and keeping it at that temperature for 20 min;

[0110] (2) The preheated cold-rolled steel sheet was bent three times. First, the first bending was performed at a bending speed of 13 mm / s to 55% of the design angle. After the first bending, the surface of the cold-rolled steel sheet was purged with 0.55 MPa compressed air until the surface temperature of the cold-rolled steel sheet dropped to 140 °C.

[0111] Then, anti-sticking agent was sprayed on the surface of the cold-rolled steel plate. The spraying amount of anti-sticking agent was 0.15g / cm 2 After spraying, the film is left to stand for 40 seconds to form. Taking the mass fraction of the anti-sticking agent as 100wt%, it includes the following components in mass fractions:

[0112] Graphite powder 3.5wt%;

[0113] Molybdenum disulfide 2.8wt%;

[0114] Boron nitride 1.8wt%;

[0115] Hydroxypropyl methylcellulose 2.5wt%;

[0116] Anhydrous ethanol 12wt%;

[0117] Acetone 6wt%;

[0118] The rest is deionized water;

[0119] After the anti-sticking agent forms a film, the second bending is performed at a bending speed of 9 mm / s to 85% of the design angle. After the second bending, the surface of the cold-rolled steel plate is blown with 0.4 MPa compressed air until the surface temperature of the cold-rolled steel plate drops to 110°C.

[0120] Finally, the third bending is performed at a bending speed of 6 mm / s until the third bending reaches the designed angle. After the third bending, 0.25 MPa compressed air is used to blow the surface of the cold-rolled steel plate until the surface temperature of the cold-rolled steel plate drops to 75°C.

[0121] (3) After the three bendings, the cold-rolled steel plate was subjected to reciprocating vibration along the angle bisector direction of the bent cold-rolled steel plate. The vibration frequency was set to 25 Hz and the amplitude was set to 0.4 mm. During the vibration process, cooling water at 20°C was sprayed on the cold-rolled steel plate at a spray rate of 1 L / min until the surface temperature of the cold-rolled steel plate dropped to 45°C.

[0122] Subsequently, the reciprocating vibration along the angle bisector direction is stopped, and the cooling water spray is turned off. The cold-rolled steel plate is reciprocated and vibrated along the bending 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 process, the cold-rolled steel plate is blown and cooled at a wind speed of 5 m / s until the cold-rolled steel plate drops to room temperature to obtain a formed part.

[0123] Example 4

[0124] This embodiment provides a bending and forming process for automobile sheet metal parts, such as Figure 1 As shown, the automotive sheet metal bending forming process specifically includes the following steps:

[0125] (1) Preheat the 2.8 mm thick cold-rolled steel plate by heating it to 165 °C at a heating rate of 4.5 °C / min and keeping it at that temperature for 12 min, then continue heating it to 195 °C at a heating rate of 7 °C / min and keeping it at that temperature for 18 min;

[0126] (2) The preheated cold-rolled steel sheet was bent three times. First, the first bending was performed at a bending speed of 14 mm / s to 58% of the design angle. After the first bending, 0.58 MPa compressed air was used to blow the surface of the cold-rolled steel sheet until the surface temperature of the cold-rolled steel sheet dropped to 145 °C.

[0127] Then, anti-sticking agent was sprayed on the surface of the cold-rolled steel plate. The spraying amount of anti-sticking agent was 0.18g / cm 2 After spraying, the film is left to stand for 50 seconds to form. Taking the mass fraction of the anti-sticking agent as 100wt%, it includes the following components in mass fractions:

[0128] Graphite powder 4.5wt%;

[0129] Molybdenum disulfide 2.7wt%;

[0130] Boron nitride 2.5wt%;

[0131] Hydroxypropyl methylcellulose 1.5wt%;

[0132] Anhydrous ethanol 11.5wt%;

[0133] Acetone 6wt%;

[0134] The rest is deionized water;

[0135] After the anti-sticking agent forms a film, the second bending is performed at a bending speed of 9.5 mm / s to 88% of the design angle. After the second bending, 0.45 MPa compressed air is used to blow the surface of the cold-rolled steel plate until the surface temperature of the cold-rolled steel plate drops to 115°C.

[0136] Finally, the third bending is performed at a bending speed of 7 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 0.28 MPa compressed air until the surface temperature of the cold-rolled steel plate drops to 78°C.

[0137] (3) After the three bendings, the cold-rolled steel plate was vibrated back and forth along the angle bisector of the bent cold-rolled steel plate. The vibration frequency was set to 28 Hz and the amplitude was set to 0.45 mm. During the vibration process, cooling water at 22°C was sprayed on the cold-rolled steel plate at a spray rate of 1.1 L / min until the surface temperature of the cold-rolled steel plate dropped to 48°C.

[0138] Subsequently, the reciprocating vibration along the angle bisector direction was stopped, and the cooling water spray was turned off. The cold-rolled steel plate was vibrated reciprocatingly along the bending line direction of the bent cold-rolled steel plate. The vibration frequency was set to 18 Hz and the amplitude was set to 0.35 mm. During the vibration process, the cold-rolled steel plate was blown and cooled at a wind speed of 5.5 m / s until the cold-rolled steel plate cooled to room temperature to obtain a formed part.

[0139] Example 5

[0140] This embodiment provides a bending and forming process for automobile sheet metal parts, such as Figure 1 As shown, the automotive sheet metal bending forming process specifically includes the following steps:

[0141] (1) Preheat the 3 mm thick cold-rolled steel plate by heating it to 170 °C at a heating rate of 5 °C / min and keeping it at that temperature for 10 min, then continue heating it to 200 °C at a heating rate of 8 °C / min and keeping it at that temperature for 15 min;

[0142] (2) The preheated cold-rolled steel sheet was bent three times. First, the first bending was performed at a bending speed of 15 mm / s to 60% of the design angle. After the first bending, the surface of the cold-rolled steel sheet was purged with 0.6 MPa compressed air until the surface temperature of the cold-rolled steel sheet dropped to 150°C.

[0143] Then, anti-sticking agent is sprayed on the surface of the cold-rolled steel plate. The spraying amount of anti-sticking agent is 0.2g / cm 2 After spraying, the film is left to stand for 60 seconds to form. Taking the mass fraction of the anti-sticking agent as 100wt%, it includes the following components in mass fractions:

[0144] Graphite powder 5wt%;

[0145] Molybdenum disulfide 2wt%;

[0146] Boron nitride 2.5wt%;

[0147] Hydroxypropyl methylcellulose 1.8wt%;

[0148] Anhydrous ethanol 12wt%;

[0149] Acetone 5wt%;

[0150] The rest is deionized water;

[0151] After the anti-sticking agent forms a film, the second bending is performed at a bending speed of 10 mm / s to 90% of the designed angle. After the second bending, the surface of the cold-rolled steel plate is blown with 0.5 MPa compressed air until the surface temperature of the cold-rolled steel plate drops to 120°C.

[0152] Finally, the third bending is performed at a bending speed of 8 mm / s until the third bending reaches the designed angle. After the third bending, 0.3 MPa compressed air is used to blow the surface of the cold-rolled steel plate until the surface temperature of the cold-rolled steel plate drops to 80°C.

[0153] (3) After the three bendings are completed, the cold-rolled steel plate is subjected to reciprocating vibration 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, cooling water at 25°C is sprayed on the cold-rolled steel plate at a spray rate of 1.2 L / min until the surface temperature of the cold-rolled steel plate drops to 50°C.

[0154] Subsequently, the reciprocating vibration along the angle bisector direction is stopped, and the cooling water spray is turned off. The cold-rolled steel plate is reciprocated along the bending 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, the cold-rolled steel plate is blown and cooled at a wind speed of 6 m / s until the cold-rolled steel plate drops to room temperature to obtain a formed part.

[0155] Example 6

[0156] This embodiment provides a bending and forming processing technology for automobile sheet metal parts. The difference from Example 1 is that the cold-rolled steel plate is preheated once, directly heating to 180°C at a heating rate of 5°C / min and keeping warm for 25 minutes. Other process parameters and operating steps are exactly the same as those in Example 1.

[0157] Example 7

[0158] This embodiment provides a bending and forming process for automobile sheet metal parts. The difference from embodiment 1 is that in step (1), a single bending is adopted to directly bend the preheated cold-rolled steel plate to the designed angle. Other process parameters and operation steps are exactly the same as those in embodiment 1.

[0159] Example 8

[0160] This embodiment provides a bending and forming processing technology for automobile sheet metal parts. The difference from Example 1 is that the preheated cold-rolled steel plate is bent twice. In this embodiment, the first bending is to 50% of the design angle. At the same time, the compressed air purge and anti-sticking agent spraying in Example 1 are retained, and the second bending in the original Example 1 is omitted. The second bending is directly to the design angle. The other process parameters and operating steps are exactly the same as those in Example 1.

[0161] Example 9

[0162] This embodiment provides a bending and forming processing technology for automobile sheet metal parts. The difference from Example 1 is that no anti-sticking agent is sprayed on the surface of the cold-rolled steel plate. After the first bending, the cold-rolled steel plate is blown until the surface temperature drops to 130°C, and then the second bending is performed directly. The other process parameters and operating steps are exactly the same as those in Example 1.

[0163] Example 10

[0164] This embodiment provides a bending and forming process for automobile sheet metal parts. The difference from Example 1 is that graphite powder is omitted from the anti-sticking agent, and other process parameters and operating steps are exactly the same as those in Example 1.

[0165] Example 11

[0166] This embodiment provides a bending and forming process for automobile sheet metal parts. The difference from Example 1 is that molybdenum disulfide is omitted from the anti-sticking agent, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0167] Example 12

[0168] This embodiment provides a bending and forming process for automobile sheet metal parts. The difference from Example 1 is that boron nitride is omitted from the anti-sticking agent, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0169] Example 13

[0170] This embodiment provides a bending and forming processing technology for automobile sheet metal parts. The difference from Example 1 is that after the three bendings are completed, the cold-rolled steel plate is reciprocatingly vibrated along the angle bisector direction of the bent cold-rolled steel plate, and cooling water is sprayed during the vibration process until the cold-rolled steel plate cools to room temperature to obtain a formed part, thereby eliminating the operation process of reciprocatingly vibrating the cold-rolled steel plate along the bending line direction. The bending speed of the second bending is adjusted to 5 mm / s. Other process parameters and operating steps are exactly the same as those in Example 1.

[0171] Example 14

[0172] This embodiment provides a bending and forming processing process for automobile sheet metal parts. The difference from Example 1 is that after the three bendings are completed, the cold-rolled steel plate is reciprocatingly vibrated along the bending line direction of the bent cold-rolled steel plate, and cooling water is sprayed during the vibration process until the cold-rolled steel plate cools to room temperature to obtain a formed part, thereby eliminating the operation process of reciprocatingly vibrating the cold-rolled steel plate along the angle bisector direction. Other process parameters and operating steps are exactly the same as those in Example 1.

[0173] The bending angle rebound, surface Vickers hardness, and dynamic yield strength of the molded parts prepared in Examples 1-14 were tested. The test method is as follows:

[0174] (1) Bending angle rebound

[0175] Use a laser scanner or coordinate measuring machine (CMM) to measure the actual bending angle and calculate the bending angle springback according to the following formula:

[0176] Bending angle rebound = actual bending angle - designed bending angle.

[0177] (2) Surface Vickers hardness

[0178] Use a Vickers hardness tester to test the surface Vickers hardness of the formed parts in accordance with the national standard GB / T 4340.1-2024 "Vickers hardness test of metallic materials Part 1: Test method".

[0179] (3) Dynamic yield strength

[0180] According to the national standard GB / T 228.1-2021 "Tensile testing of metallic materials - Part 1: Test methods at room temperature", the molded parts were subjected to dynamic tensile tests at a strain rate of 0.01s. -1 , record the stress-strain curve and calculate the dynamic yield strength.

[0181] The test data is shown in Table 1.

[0182] Table 1 Test data of molded parts obtained in Examples 1-14

[0183] Bending angle rebound (°) 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

[0184] It can be seen from the test data of Example 1 and Example 6 that the bending angle rebound of the molded 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 the preheating stage in Example 6 is changed to a single preheating, and the temperature is directly raised to 180°C for insulation. The rebound is as high as 2.1°, the hardness is reduced to 228HV, and the strength is reduced to 342MPa. This illustrates the importance of staged preheating. A single preheating may cause uneven heating, resulting in stress concentration during subsequent bending, increased rebound, and insufficient hardening of the material, resulting in reduced hardness and strength.

[0185] It can be seen from the test data of Example 1 and Example 7 that the bending angle rebound of the molded 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 Example 7 is adjusted to one bending, the rebound is as high as 2.5°, and the hardness and strength are also greatly reduced. The advantage of three gradual bending is that the stress is gradually released to avoid work hardening. One bending leads to excessive stress accumulation and severe rebound. In addition, the internal dislocations of the material are entangled, and the hardness and strength are reduced because the structure is unstable.

[0186] It can be seen from the test data of Example 1 and Example 8 that the bending angle rebound of the molded 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 Example 8 is adjusted to two bends, which omits the second bending stage of 80-90%, resulting in a final rebound of up to 2.3° and poor performance. One less bend means insufficient stress release, especially the second bending stage (80~90%) is critical for controlling the hardening rate. Without this step, the material is more likely to have defects during the final bend, affecting the overall performance.

[0187] It can be seen from the test data of Example 1 and Example 9 that the bending angle rebound of the molded 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 the anti-sticking agent is omitted in Example 9, the rebound is increased to 1.9°, the hardness is reduced to 237HV, and the strength is reduced to 375MPa. The role of the anti-sticking agent is lubrication and stress buffering, especially during the second bending to prevent direct friction between the surface oxide layer and the mold. Failure to spray the anti-sticking agent leads to increased friction and surface damage. At the same time, the stress distribution is uneven, the rebound increases, and the hardness and strength are also affected.

[0188] It can be seen from the test data of Example 1, Example 10, Example 11 and Example 12 that the bending angle rebound of the molded 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 Example 10, Example 11 and Example 12 respectively remove the graphite powder, molybdenum disulfide and boron nitride in the anti-sticking agent, and the rebound is between 1.5 and 1.7°, and the hardness and strength are reduced. The lack of graphite powder affects the lubrication channel, the lack of molybdenum disulfide weakens the high-temperature adsorption, and the lack of boron nitride reduces the heat resistance. After the synergistic effect of these components is destroyed, the anti-sticking film effect is discounted, resulting in friction and stress problems during bending, which in turn affects the final performance.

[0189] It can be seen from the test data of Example 1 and Example 13 that the bending angle rebound of the molded 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 Example 13 omits the vibration along the bending line direction and only performs angle bisector vibration, and the rebound is increased to 1.8°, indicating that the lack of low-frequency vibration along the bending line leads to insufficient release of longitudinal residual stress, increased rebound, and decreased hardness and strength, because composite vibration can synergistically optimize stress distribution.

[0190] It can be seen from the test data of Example 1 and Example 14 that the bending angle rebound of the molded 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 Example 14 omits the angle bisector vibration and only performs the bending line vibration. The rebound is increased to 2.2°, and the performance is worse. The high-frequency vibration of the angle bisector is the key to solving the entanglement at the root of the bend. Its lack will lead to more serious stress concentration, a significant increase in rebound, and a significant reduction in hardness and strength.

[0191] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A bending and forming process for automobile sheet metal parts, characterized in that: The automotive sheet metal bending forming process comprises: (I) preheating a cold-rolled steel plate, bending the preheated cold-rolled steel plate three times, bending the cold-rolled steel plate to 50-60% of a design angle for the first time, followed by blowing and cooling, bending the cold-rolled steel plate to 80-90% of the design angle for the second time, followed by blowing and cooling, and bending the cold-rolled steel plate to the design angle for the third time, followed by blowing and cooling; (II) reciprocatingly vibrating the cold-rolled steel plate along the direction of the angle bisector of the bent cold-rolled steel plate and spraying cooling water on the cold-rolled steel plate; then reciprocatingly vibrating the cold-rolled steel plate along the bending line direction of the bent cold-rolled steel plate and air-cooling the cold-rolled steel plate to obtain a formed part; 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 divided into two stages, including: heating the cold-rolled steel sheet to a first preheating temperature at a first heating rate and holding the temperature; then heating the cold-rolled steel sheet to a second preheating temperature at a second heating rate and holding the temperature, and then bending; The first heating rate is 3-5°C / min; the first preheating temperature is 150-170°C; the temperature is maintained at the first preheating temperature for 10-20 minutes; the second heating rate is 5-8°C / min; the second preheating temperature is 180-200°C; the temperature is maintained at the second preheating temperature for 15-25 minutes; In step (I), before the second bending begins, spray an anti-sticking agent on the surface of the cold-rolled steel sheet, let it stand for 30 to 60 seconds to form a film, and then perform the second bending; the spraying amount of the anti-sticking agent is 0.1 to 0.2 g / cm 2 ; 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 in mass fractions: 3-5 wt% graphite powder; 2-3 wt% molybdenum disulfide; 1.5-2.5 wt% boron nitride; 1.5-2.5 wt% hydroxypropyl methylcellulose; 10-12 wt% anhydrous ethanol; 5-8 wt% acetone; and the remainder is deionized water. The automobile sheet metal part bending forming process controls the springback amount of the automobile sheet metal part within ±0.5°.

2. The automobile sheet metal bending forming process according to claim 1, characterized in that: In step (I), after the first bending, compressed air at 0.5-0.6 MPa is used to blow the surface of the cold-rolled steel plate until the surface temperature of the cold-rolled steel plate drops to 130-150° C.; the bending speed of the first bending is 12-15 mm / s.

3. The automobile sheet metal bending forming process according to claim 1, characterized in that: In step (I), after the second bending, 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.; the bending speed of the second bending is 8-10 mm / s.

4. The automobile sheet metal bending forming process according to claim 1, characterized in that: In step (I), after the third bending, compressed air at 0.2-0.3 MPa is used to blow the surface of the cold-rolled steel plate until the surface temperature of the cold-rolled steel plate drops to 70-80° C.; the bending speed of the third bending is 5-8 mm / s.

5. The automobile sheet metal bending forming process according to claim 1, characterized in that: In step (II), the reciprocating vibration frequency of the cold-rolled steel plate along the angle bisector direction is 20-30 Hz; the amplitude of the cold-rolled steel plate along the angle bisector direction is 0.3-0.5 mm; the spraying rate of the cooling water is 0.8-1.2 L / min; the temperature of the cooling water is 15-25° C.; and the cooling water is sprayed on the surface of the cold-rolled steel plate until its surface temperature drops to 40-50° C.

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

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