Punch forming process of automobile left front door lock reinforcing plate

Abstract

The invention relates to the technical field of automobile part manufacturing, and discloses a punch forming process of an automobile left front door lock reinforcing plate, which comprises the following steps: firstly, coating a temperature-sensitive gas-liquid phase change lubricating isolation medium on the surface of a plate, then carrying out subcritical gradient induction heating, and controlling the heating temperature to be lower than an austenitizing temperature to prevent oxidation; and performance gradients of plasticity and rigidity are established in different areas of the plate. In the stamping process, the medium is heated and gasified to form a gas film between the plate and the die so as to reduce friction. After the mold is closed to form a closed cavity, a cryogenic medium is injected for quenching, thermal shrinkage of the plate is promoted, meanwhile, the gasification medium in the cavity is condensed and subjected to phase change to generate negative pressure, and the plate is tightly adsorbed to the mold surface. According to the method, the formability is improved through gas film lubrication, resilience of the ultra-high strength steel is restrained through the synergistic effect of condensation negative pressure adsorption and base body heat shrinkage, and the problem that the forming precision of complex curved surface parts is low is solved.

Description

Technical Field

[0001] This invention relates to the field of automotive parts manufacturing technology, specifically to a stamping process for a reinforcing plate for the left front door lock of an automobile. Background Technology

[0002] As the automotive industry increasingly demands lightweighting and crash safety, ultra-high-strength steels such as DP1180 are being used more and more widely in vehicle body structural components. The left front door lock reinforcement plate, as a critical connecting and load-bearing component, has a complex geometry and strict requirements for dimensional accuracy and mechanical properties.

[0003] Currently, the manufacturing of such ultra-high strength steel plates mainly employs cold stamping or traditional hot forming processes, but numerous technical bottlenecks remain in actual production. Regarding forming precision control, the extremely high yield strength of ultra-high strength steel leads to a significant release of internal stress during cold stamping, making the springback after unloading difficult to predict and control. Often, multiple trial moldings and mold repairs are required to barely meet tolerance requirements. Even with traditional hot forming processes, although deformation resistance is reduced, the cooling process of the sheet metal within the mold relies primarily on mechanical contact heat transfer, lacking a mechanism to actively constrain the sheet metal's fit against the mold surface. This results in warping deformation due to uneven cooling or phase transformation volume changes, ultimately leading to poor dimensional stability of the final parts.

[0004] In terms of tribology during the forming process, ultra-high-strength steel sheets and mold surfaces experience intense friction under high contact pressure. Existing lubrication methods are prone to failure under high pressure and high temperature environments, leading to impaired flow of the sheet material in large deformation areas such as deep drawing, making it highly susceptible to cracking or necking. Simultaneously, direct metal-to-metal contact exacerbates wear on the mold surface, easily causing scratches or roughening on the part surface, affecting product quality and shortening the mold maintenance cycle.

[0005] Furthermore, in terms of heating and material property control, traditional hot forming typically involves heating the sheet metal to its austenitizing temperature (above 900°C) for uniform heating across the entire surface. This high-temperature treatment results in severe oxide scale formation on the steel surface, which not only deteriorates the workshop environment but also necessitates additional cleaning processes such as shot blasting after stamping, increasing manufacturing costs. More importantly, uniform heating and quenching transform the entire part into an extremely hard martensitic structure, leading to insufficient ductility in areas requiring drilling or flanging during subsequent assembly. This makes the parts prone to processing cracks and fails to meet the differentiated strength and plasticity requirements of complex structural components in different areas. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a stamping process for a reinforcing plate for the left front door lock of an automobile. This process solves the problems of low dimensional accuracy due to large springback during the stamping of ultra-high strength steel, high frictional resistance that easily leads to part breakage and mold wear, and the fact that traditional heating processes easily produce oxide scale and cannot meet the differentiated performance requirements of different parts of the part.

[0007] This invention provides a stamping process for a reinforcing plate for the left front door lock of an automobile, employing the following technical solution:

[0008] A stamping process for a reinforcing plate for the left front door lock of an automobile includes the following steps:

[0009] S1. Blank pretreatment: A temperature-sensitive gas-liquid phase change lubricating isolation medium is coated on the surface of the sheet to obtain a pretreated sheet with a surface coating.

[0010] S2, Subcritical gradient induction heating: The pre-treated sheet material with surface coating is heated in zones, the target temperature of the large deformation zone is set as the first target temperature, the target temperature of the high rigidity zone is set as the second target temperature, and the first target temperature is lower than the austenitizing temperature of the sheet material, so as to obtain a heated sheet material with temperature gradient.

[0011] S3, Dynamic air film assisted stamping: The heated plate with temperature gradient is transferred to the stamping die for stamping, and the temperature-sensitive gas-liquid phase change lubricating isolation medium is heated and vaporized to form an air film between the heated plate with temperature gradient and the die.

[0012] S4. Sealing and Cavity Construction: Before the stamping slide reaches the bottom dead center, the sealing structure set around the mold is closed to construct a sealed cavity surrounding the sheet metal being stamped and deformed.

[0013] S5, Transient rapid cooling and negative pressure adsorption: During the pressure holding stage, a cryogenic medium is injected into the mold. The sheet metal being stamped and deformed undergoes thermal shrinkage, and the vaporized temperature-sensitive gas-liquid phase change lubricating and isolating medium inside the sealed cavity condenses and undergoes phase change, forming a negative pressure inside the sealed cavity.

[0014] S6. Pressure holding, shaping and demolding: Stop injecting the cryogenic medium and release the seal, open the mold and take out the molded left front door lock reinforcement plate.

[0015] By adopting the above technical solution, and by combining the phase change characteristics of the temperature-sensitive medium with the sealing negative pressure technology, the following technical effects are achieved:

[0016] This process solves the molding problem through the synergistic effect of multiple physical fields. The specific mechanism of action is as follows:

[0017] Dynamic film lubrication mechanism: During the stamping deformation stage, the temperature-sensitive gas-liquid phase change lubrication isolation medium on the sheet metal surface absorbs the heat of the sheet metal itself and vaporizes. The high-pressure gas generated by vaporization forms a micron-sized gas film between the sheet metal and the die surface. This gas film physically isolates the metal surfaces of the sheet metal and the die, transforming traditional solid friction into gas viscous friction, reducing the coefficient of friction, improving material flow in large deformation areas, and preventing tearing and surface scratches.

[0018] Subcritical gradient plasticity control: By controlling the heating temperature below the austenitizing temperature (subcritical region), uneven volume expansion and contraction caused by phase transformation are avoided, while preventing high-temperature surface oxidation. Partial heating allows the deep-drawing region to achieve higher plasticity, while the mounting holes and hard spots maintain higher yield strength, meeting the differentiated stiffness and ductility requirements of different parts of the reinforcing plate.

[0019] Negative pressure adsorption and thermal shrinkage synergistic shaping mechanism: This is the core of the process for controlling rebound, and it consists of two physical processes:

[0020] Process 1 (Phase Change-Induced Negative Pressure): When the cryogenic medium is injected into the mold runner, the temperature of the mold cavity drops sharply. The gaseous medium (derived from the vaporization of the lubricating medium) that originally filled the sealed cavity rapidly condenses into a liquid or solid state upon contact with the cold wall. According to the ideal gas law, the volume shrinks drastically when the gaseous state changes to a liquid state, causing the pressure in the sealed cavity to drop instantaneously, forming a high-vacuum negative pressure state. This negative pressure generates an adsorption force pointing towards the mold surface, tightly adhering the sheet metal to the mold surface and forcibly suppressing the sheet metal's springback tendency.

[0021] Process Two (Substrate Thermal Shrinkage): The cryogenic medium simultaneously removes heat from the sheet metal, causing the metal substrate to thermally shrink. Under the constraint of negative pressure adsorption, this thermal shrinkage is transformed into bonding stress along the mold normal, further locking the geometric dimensions of the part.

[0022] Preferably, in step S2, the first target temperature of the large deformation zone is 450℃-550℃, the second target temperature of the high rigidity zone is 25℃-150℃, and the heating rate is 30℃ / s-50℃ / s.

[0023] By adopting the above technical solution, the first target temperature is set in the range of 450℃-550℃, which is within the warm forming range of the steel plate. This softens the matrix to reduce deformation resistance and avoids grain coarsening. The higher heating rate reduces heat conduction to unheated areas, ensuring clear boundaries of the temperature gradient.

[0024] Preferably, in step S3, the stamping speed is 100mm / s-250mm / s; in step S5, the holding pressure applied in the holding stage is 1000kN-2000kN, the cooling rate of the sheet surface is 80℃ / s-150℃ / s, and the duration of the holding stage is 5.0s-10.0s.

[0025] By adopting the above technical solutions, the higher stamping speed can shorten the residence time of the sheet metal in the air, reduce heat loss, and ensure that the medium vaporizes in large quantities during deformation to maintain the air film pressure. The high cooling rate and moderate holding time ensure the timely formation of negative pressure and the stability of sheet metal shaping.

[0026] Preferably, in step S2, the partitioned heating is performed using a multi-channel induction heating station, and the large deformation zone corresponds to the deep drawing zone, while the high rigidity zone corresponds to the mounting hole and hard point zone.

[0027] By adopting the above technical solution, multi-channel induction heating can independently control the power output of different areas, realize the precise construction of the temperature field of complex shaped parts, ensure that the deep-drawn parts have sufficient ductility, and retain the original strength of the hard points.

[0028] Preferably, in step S4, the closing action of the sealing structure precedes the closing action of the mold surface; in step S5, the cryogenic medium is introduced into the mold flow channel using a pulse injection method.

[0029] By adopting the above technical solution, the sealing structure is closed first to ensure that the high-pressure gas generated by the vaporization of the medium at the end of the stamping stage does not leak, thus storing the working fluid for subsequent condensation under negative pressure. Pulsed injection disrupts the laminar boundary layer in the cooling channel, improving heat exchange efficiency and causing the mold surface temperature to drop rapidly.

[0030] Preferably, the temperature-sensitive gas-liquid phase change lubricating isolation medium used in step S1 is made of components comprising the following weight percentages: matrix film-forming component: 30.0wt%-45.0wt%; gradient volatile phase change component: 50.0wt%-65.0wt%; functional additive component: 3.0wt%-5.0wt%; the boiling point of the gradient volatile phase change component is lower than the carbonization temperature of the matrix film-forming component.

[0031] By employing the above technical solution, and by limiting the proportions of each component and the relationship between its boiling point and carbonization temperature, it is ensured that the medium preferentially undergoes physical phase change (vaporization) to provide a gas film when heated, rather than undergoing chemical decomposition (carbonization) that leads to coking. The gradient volatile phase change component, as the main gas-generating agent, ensures sufficient gas generation to fill the cavity; the matrix film-forming component ensures coating integrity at low temperatures and provides auxiliary lubrication at high temperatures.

[0032] Preferably, the matrix film-forming component is composed of polyalkylene glycol and polyvinyl alcohol; the gradient volatile phase change component is composed of deionized water, anhydrous ethanol and n-butanol; and the functional additive component is composed of sodium molybdate dihydrate, polyethylene glycol octylphenyl ether and triethanolamine borate.

[0033] By adopting the above technical solution, polyalkylene glycol exhibits excellent high-temperature lubricity; the gradient evaporation system composed of water, ethanol, and n-butanol can vaporize stepwise at different temperature ranges (from low to high), ensuring the continuity of gas film formation throughout the entire heating and stamping process. Additives such as sodium molybdate provide extreme pressure anti-wear and rust prevention functions.

[0034] Preferably, in step S1, the sheet material is DP1180 cold-rolled duplex steel sheet, and the dry film thickness after coating and drying is 15μm-25μm.

[0035] By adopting the above technical solution, a dry film thickness of 15μm-25μm stores a sufficient amount of phase change material, which is enough to maintain the existence of the gas film throughout the stamping process, while avoiding material waste or mold carbon buildup caused by excessive film thickness.

[0036] Preferably, in step S4, the sealing structure is selected from one of a high-temperature resistant silicone rubber sealing ring, a flexible metal bellows, or a composite sealing device. By adopting the above technical solutions, these sealing structures are resistant to high temperatures and have elastic compensation capabilities, enabling them to adapt to gap changes during mold closing and maintain the airtightness of the cavity.

[0037] Preferably, in step S5, the cryogenic medium is liquid nitrogen or R410a refrigerant.

[0038] By adopting the above technical solution, liquid nitrogen or R410a has an extremely low boiling point and a huge latent heat of vaporization. After being injected into the mold, it can instantly absorb a large amount of heat, causing the gas in the cavity to condense rapidly. It is the key medium for achieving transient cooling and establishing negative pressure.

[0039] This invention provides a stamping process for a reinforcing plate of the left front door lock of an automobile. It has the following advantages:

[0040] 1. This invention achieves negative pressure shaping by constructing a sealed mold cavity and utilizing the phase change characteristics of the medium. During the pressure holding stage, the gaseous medium inside the mold condenses due to deep cryogenic cooling, and the volume shrinkage creates a negative pressure environment inside the cavity. The resulting adsorption force forces the sheet metal to adhere tightly to the mold surface. Simultaneously, combined with the physical thermal shrinkage of the sheet metal substrate under rapid cooling, this overcomes the springback problem caused by the release of internal stress after stamping DP1180 ultra-high strength steel, ensuring the accuracy of the final product's contour.

[0041] 2. This invention utilizes the high-temperature vaporization effect of a temperature-sensitive medium to establish a gas film lubrication mechanism. During the stamping process, the high-pressure gas generated by the heating and vaporization of the medium forms an isolation gas film between the sheet metal and the die, transforming the traditional metal contact friction into gas viscous friction, which significantly reduces frictional resistance. This not only improves the fluidity of the material in the deep drawing region and prevents the parts from breaking or scratching the surface, but also reduces the wear of the die surface.

[0042] 3. This invention adopts a subcritical gradient induction heating process to control the heating temperature below the austenitizing temperature, avoiding the generation of high-temperature oxide scale and eliminating the need for subsequent cleaning processes. At the same time, through zoned temperature control, the deep drawing zone obtains high plasticity after softening, while the mounting holes and hard spot areas maintain a lower temperature to preserve the high yield strength of the base material, thus meeting the differentiated requirements of stiffness and ductility for different parts of the reinforcing plate. Detailed Implementation

[0043] The technical solutions of the present invention will be clearly and completely described below with reference to embodiments, comparative examples, and test examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] Preparation Examples 1-3:

[0045] Preparation Example 1:

[0046] This preparation example provides a temperature-sensitive gas-liquid phase change lubricating isolation medium, the raw material composition of which is: matrix film-forming component (phase A) 45.0 wt%, gradient volatile phase change component (phase B) 50.0 wt%, and functional auxiliary component (phase C) 5.0 wt%. Phase A consists of polyalkylene glycol and polyvinyl alcohol in a mass ratio of 4:1; phase B consists of deionized water, anhydrous ethanol, and n-butanol in a mass ratio of 80:15:5; and phase C consists of sodium molybdate dihydrate, polyethylene glycol octylphenyl ether, and triethanolamine borate in a mass ratio of 2.0:0.5:1.5. The preparation includes the following steps:

[0047] Heat the deionized water of the formula to 85°C, slowly add polyvinyl alcohol powder, and dissolve it for 2 hours at a stirring speed of 500 rpm until a transparent liquid is formed. Then cool it down to 50°C, add polyalkylene glycol, and stir evenly to obtain phase A premixed liquid.

[0048] Anhydrous ethanol and n-butanol were mixed evenly in a sealed container at room temperature, and then mixed with the remaining deionized water to obtain phase B solution.

[0049] Cool the A-phase premix to below 30°C, and slowly add the B-phase solution dropwise while stirring. After the addition is complete, add the components of the C-phase.

[0050] The mixture was transferred to a high-shear emulsifier and processed at 2500 rpm for 15 minutes, followed by degassing at -0.09 MPa vacuum for 10 minutes.

[0051] Preparation Example 2:

[0052] This preparation example provides a temperature-sensitive gas-liquid phase change lubricating isolation medium, the raw material composition of which is: matrix film-forming component (phase A) 37.0 wt%, gradient volatile phase change component (phase B) 60.0 wt%, and functional auxiliary component (phase C) 3.0 wt%. Phase A consists of polyalkylene glycol and polyvinyl alcohol in a mass ratio of 5:1; phase B consists of deionized water, anhydrous ethanol, and n-butanol in a mass ratio of 75:22:3; and phase C consists of sodium molybdate dihydrate, polyethylene glycol octylphenyl ether, and triethanolamine borate in a mass ratio of 1.5:0.3:1.0. The process includes the following steps:

[0053] Heat the deionized water of the formula to 85°C, slowly add polyvinyl alcohol powder, and dissolve it for 2 hours at a stirring speed of 500 rpm until a transparent liquid is formed. Then cool it down to 50°C, add polyalkylene glycol, and stir evenly to obtain phase A premixed liquid.

[0054] Anhydrous ethanol and n-butanol were mixed evenly in a sealed container at room temperature, and then mixed with the remaining deionized water to obtain phase B solution.

[0055] Cool the A-phase premix to below 30°C, and slowly add the B-phase solution dropwise while stirring. After the addition is complete, add the components of the C-phase.

[0056] The mixture was transferred to a high-shear emulsifier and processed at 2500 rpm for 18 minutes, followed by degassing at -0.09 MPa vacuum for 10 minutes.

[0057] Preparation Example 3:

[0058] This preparation example provides a temperature-sensitive gas-liquid phase change lubricating isolation medium, the raw material composition of which is: matrix film-forming component (phase A) 30.0 wt%, gradient volatile phase change component (phase B) 65.0 wt%, and functional auxiliary component (phase C) 5.0 wt%. Phase A consists of polyalkylene glycol and polyvinyl alcohol in a mass ratio of 6:1; phase B consists of deionized water, anhydrous ethanol, and n-butanol in a mass ratio of 70:28:2; and phase C consists of sodium molybdate dihydrate, polyethylene glycol octylphenyl ether, and triethanolamine borate in a mass ratio of 2.0:0.5:1.5. The preparation includes the following steps:

[0059] Heat the deionized water of the formula to 85°C, slowly add polyvinyl alcohol powder, and dissolve it for 2 hours at a stirring speed of 500 rpm until a transparent liquid is formed. Then cool it down to 50°C, add polyalkylene glycol, and stir evenly to obtain phase A premixed liquid.

[0060] Anhydrous ethanol and n-butanol were mixed evenly in a sealed container at room temperature, and then mixed with the remaining deionized water to obtain phase B solution.

[0061] Cool the A-phase premix to below 30°C, and slowly add the B-phase solution dropwise while stirring. After the addition is complete, add the components of the C-phase.

[0062] The mixture was transferred to a high-shear emulsifier and processed at 2500 rpm for 20 minutes, followed by degassing at -0.09 MPa vacuum for 10 minutes.

[0063] Examples 1-3:

[0064] Example 1:

[0065] This embodiment provides a stamping process for a reinforcing plate for the left front door lock of an automobile, which uses the temperature-sensitive gas-liquid phase change lubricating isolation medium obtained in Preparation Example 1, and includes the following steps:

[0066] Pre-treatment of billet: DP1180 cold-rolled duplex steel sheet with a thickness of 1.4 mm was selected. After cleaning and drying, the medium obtained in Example 1 was uniformly coated on both sides of the sheet by roller coating. The thickness of the dry film after drying was controlled to be 15 μm.

[0067] Subcritical gradient induction heating: The pre-treated sheet metal is fed into a multi-channel induction heating station for zoned heating, and the target temperature of the large deformation zone (deep drawing zone) is set. The target temperature for the high-rigidity area (mounting holes and hard spots) is set at 450℃. The temperature is 25℃ (maintaining room temperature), and the heating rate is controlled at 30℃ / s;

[0068] Dynamic air film assisted stamping: The heated sheet metal is transferred into the stamping die within 3.0s, and the slide moves the upper die downward at a stamping speed of 100mm / s to perform stamping; during this process, the volatile components in the medium are heated and vaporized, forming an air film between the sheet metal and the die;

[0069] Sealing and cavity construction: Before the slider reaches the bottom dead center, the high-temperature resistant silicone rubber sealing rings set around the mold close before the mold surface, thus constructing the mold cavity into a sealed cavity isolated from the atmosphere.

[0070] Transient rapid cooling and dual stress compensation: When the slider reaches the bottom dead center and a holding pressure of 1000kN is applied, liquid nitrogen is pulsedly injected into the mold flow channel as a cryogenic medium, so that the cooling rate of the sheet surface reaches 80℃ / s; during this process, the sheet undergoes thermal shrinkage, and the vaporized temperature-sensitive gas-liquid phase change lubricating isolation medium in the sealed cavity undergoes condensation phase change, establishing a negative pressure adsorption environment in the cavity;

[0071] Pressure holding and shaping: Under the combined action of 1000kN mechanical pressure and negative pressure adsorption, the pressure is held for 5.0s. Then, the deep cryogenic medium is stopped and the seal is released. The slider moves upward to open the mold and the molded left front door lock reinforcement plate is taken out.

[0072] Example 2:

[0073] This embodiment provides a stamping process for a reinforcing plate of the left front door lock of an automobile, which uses the temperature-sensitive gas-liquid phase change lubricating isolation medium obtained in Preparation Example 2, and includes the following steps:

[0074] Pre-treatment of billet: Select DP1180 cold-rolled duplex steel sheet with a thickness of 1.4 mm, clean and dry it, and then uniformly coat the medium obtained in Example 2 on both sides of the sheet using a roller coating method, controlling the dry film thickness to be 20 μm after drying.

[0075] Subcritical gradient induction heating: The pre-treated sheet metal is fed into a multi-channel induction heating station for zoned heating, and the target temperature of the large deformation zone (deep drawing zone) is set. Set the target temperature for the high-rigidity area (mounting holes and hard spot area) to 500℃. The temperature is 100℃, and the heating rate is controlled at 40℃ / s;

[0076] Dynamic air film assisted stamping: The heated sheet metal is transferred into the stamping die within 2.5s, and the slide moves the upper die downward at a stamping speed of 180mm / s to perform stamping; during this process, the volatile components in the medium are heated and vaporized, forming an air film between the sheet metal and the die;

[0077] Sealing and cavity construction: Before the slider reaches the bottom dead center, the flexible metal bellows sealing structure set around the mold closes before the mold surface, constructing the mold cavity into a sealed cavity isolated from the atmosphere.

[0078] Transient rapid cooling and dual stress compensation: When the slider reaches the bottom dead center and applies a holding pressure of 1500kN, high-pressure R410a refrigerant is pulsedly injected into the mold flow channel as a deep cryogenic medium, so that the cooling rate of the sheet surface reaches 100℃ / s; during this process, the sheet undergoes thermal shrinkage, and at the same time, the vaporized temperature-sensitive gas-liquid phase change lubricating isolation medium in the sealed cavity undergoes condensation phase change, establishing a negative pressure adsorption environment in the cavity;

[0079] Pressure holding and shaping: Under the combined action of 1500kN mechanical pressure and negative pressure adsorption, the pressure is held for 7.5s. Then, the deep cryogenic medium is stopped and the seal is released. The slider moves upward to open the mold and the molded left front door lock reinforcement plate is taken out.

[0080] Example 3:

[0081] This embodiment provides a stamping process for a reinforcing plate for the left front door lock of an automobile, which uses the temperature-sensitive gas-liquid phase change lubricating isolation medium obtained in Preparation Example 3, and includes the following steps:

[0082] Pre-treatment of billet: DP1180 cold-rolled duplex steel sheet with a thickness of 1.4 mm was selected. After cleaning and drying, the medium obtained in Example 3 was uniformly coated on both sides of the sheet by roller coating. The thickness of the dry film after drying was controlled to be 25 μm.

[0083] Subcritical gradient induction heating: The pre-treated sheet metal is fed into a multi-channel induction heating station for zoned heating, and the target temperature of the large deformation zone (deep drawing zone) is set. The target temperature for the high-rigidity area (mounting holes and hard spots) is set at 550℃. The temperature is 150℃, and the heating rate is controlled at 50℃ / s.

[0084] Dynamic air film assisted stamping: The heated sheet metal is transferred into the stamping die within 2.0s, and the slide moves the upper die downward at a stamping speed of 250mm / s to perform stamping; during this process, the volatile components in the medium are heated and vaporized, forming an air film between the sheet metal and the die;

[0085] Sealing and cavity construction: Before the slider reaches the bottom dead center, the high-temperature resistant composite sealing device set around the mold closes before the mold surface, constructing the mold cavity into a sealed cavity isolated from the atmosphere.

[0086] Transient rapid cooling and dual stress compensation: When the slider reaches the bottom dead center and a holding pressure of 2000kN is applied, liquid nitrogen is pulsedly injected into the mold flow channel as a deep cryogenic medium, so that the cooling rate of the sheet surface reaches 150℃ / s; during this process, the sheet undergoes thermal shrinkage, and the vaporized temperature-sensitive gas-liquid phase change lubricating isolation medium in the sealed cavity undergoes condensation phase change, establishing a negative pressure adsorption environment in the cavity;

[0087] Pressure holding and shaping: Under the combined action of 2000kN mechanical pressure and negative pressure adsorption, the pressure is held for 10.0s. Then, the deep cryogenic medium is stopped and the seal is released. The slider moves upward to open the mold and the molded left front door lock reinforcement plate is taken out.

[0088] Comparative Examples 1-5:

[0089] Comparative Example 1:

[0090] This comparative example provides a conventional cold stamping forming process. The differences from Example 2 are: subcritical gradient induction heating is not performed; the sheet metal is directly stamped at room temperature (25°C); a temperature-sensitive gas-liquid phase change lubricating isolation medium is not used; instead, commercially available ordinary stamping and drawing oil is applied; sealing, cavity construction, and cryogenic medium injection are not performed; conventional water-cooled molds are used for natural cooling and pressure holding. All other aspects are the same.

[0091] Comparative Example 2:

[0092] This comparative example provides a conventional hot forming (hot stamping) process. The differences from Example 2 are: in the subcritical gradient induction heating step, the sheet metal is heated to 950°C for complete austenitization and held for 3 minutes; a commercially available high-temperature antioxidant protective coating is used instead of the temperature-sensitive medium of this invention; sealing and cavity construction are not performed, and a closed negative pressure environment is not formed in the mold; conventional cooling water is used for quenching and cooling in the transient rapid cooling and dual stress compensation steps. All other aspects are the same.

[0093] Comparative Example 3:

[0094] This comparative example provides a subcritical stamping process lacking a phase change mechanism. The difference from Example 2 is that the medium used in the billet pretreatment step is replaced with a commercially available high-temperature resistant synthetic polyalphaolefin (PAO) lubricating oil of the same viscosity (this lubricating oil does not undergo significant vaporization phase change at 500°C and has no volatile components). Because the medium does not undergo a phase change, a gas film cannot be formed in the dynamic gas film assisted stamping step, and a negative pressure adsorption effect cannot be generated through condensation in the transient quenching and dual stress compensation steps. Everything else is the same.

[0095] Comparative Example 4:

[0096] This comparative example provides a subcritical stamping process lacking a negative pressure adsorption environment. The difference from Example 2 is that no sealing or cavity construction is performed, no sealing device is installed around the mold, and the cavity remains open to the atmosphere. Therefore, during rapid cooling, although the medium condenses, due to cavity leakage, an effective negative pressure adsorption environment cannot be established, and only the thermal shrinkage effect of the sheet metal itself is retained. Everything else is the same.

[0097] Comparative Example 5:

[0098] This comparative example provides a subcritical stamping process without gradient temperature control. The difference from Example 2 is that the subcritical gradient induction heating step employs a uniform heating method, uniformly heating the entire sheet metal (including the large deformation zone and the high rigidity zone) to 500°C. All other aspects are the same.

[0099] Test Example 1-2:

[0100] Test Example 1: Coupling Characteristics Test of Pressure Evolution and Temperature Field within Mold Cavity

[0101] Experimental description:

[0102] To verify the existence and dynamic evolution of the physical mechanism of lubricant vaporization film formation and closed-loop condensation negative pressure adsorption in the process described in this invention, an embedded pressure-temperature synchronous monitoring system was constructed. The process parameters and media conditions of Example 2 were selected for testing, and Comparative Example 4 (without a sealed structure) was introduced as a control group to eliminate the influence of simple thermal shrinkage on the pressure readings.

[0103] Experimental setup: A micro-sensor was pre-embedded in the non-appearance surface area (flange edge process supplementary surface) of the upper die of the stamping die.

[0104] Pressure monitoring: A high-temperature resistant thin-film pressure sensor (range 0-1MPa, response frequency 5kHz) is used. The sensor probe end face is flush with the mold surface to measure the fluid pressure at the interface between the sheet metal and the mold.

[0105] Temperature monitoring: A K-type armored thermocouple is embedded 2mm away from the pressure sensor to synchronously record the interface temperature change.

[0106] Data acquisition: A multi-channel high-speed data acquisition instrument was used, with the sampling frequency set to 1000Hz, to record data from 1 second before the slider contacts the sheet metal until the end of the pressure holding process.

[0107] Experimental steps:

[0108] The sensor system was zero-point calibrated, and the ambient atmospheric pressure was set to 101.3 kPa.

[0109] Sheet metal coating (medium preparation example 2), gradient heating and stamping operations were performed according to the parameters described in Example 2.

[0110] The data acquisition system is triggered to simultaneously record the cavity pressure and interface temperature during the four stages of slider descent, mold closing, refrigerant injection and pressure holding, and mold opening.

[0111] Replace the mold settings with those of Comparative Example 4 (remove the sealing ring), keep all other process parameters exactly the same, and repeat the above test steps.

[0112] Remove obvious abnormal noise caused by electromagnetic interference and extract data for the effective time period.

[0113] Experimental data:

[0114] The table below records typical monitoring data for Example 2 in a single stamping cycle. The zero point t=0 is defined as the instant the slider contacts the sheet metal.

[0115] Table 1. Dynamic monitoring data record of cavity interface pressure and temperature during the process of Example 2

[0116]

[0117] (Note: The pressure values ​​in the table are absolute pressures. A pressure below 101.3 kPa indicates the generation of a negative pressure vacuum.)

[0118] Conclusions and Mechanism Analysis:

[0119] Based on the temporal characteristics analysis of the monitoring data and combined with the thermodynamic principles of gas-liquid phase change, the following conclusions are drawn:

[0120] During the initial stage of stamping deformation (t=0.25s to 2.50s), monitoring data showed that the pressure inside the mold cavity was significantly higher than the ambient atmospheric pressure, reaching a peak of 165.8 kPa. This phenomenon confirms that the volatile component in the medium (ethanol / water mixture) underwent a violent endothermic vaporization reaction upon contact with the high-temperature sheet metal (>400℃). The generated gas, due to viscous resistance, did not escape immediately, forming a high-pressure, micron-sized air cushion with load-bearing capacity between the sheet metal and the mold interface. This positive pressure environment effectively isolated direct metal contact, confirming the dynamic film lubrication effect described in this invention and providing a physical basis for reducing the forming friction coefficient.

[0121] During the pressure holding and cooling phase (t > 2.60 s), with the injection of cryogenic medium, the interface temperature dropped sharply from 438.1 °C to 185.7 °C within 1 second, and then continued to drop to room temperature. Simultaneously, the cavity pressure experienced a step-like decrease, falling below atmospheric pressure (92.6 kPa) at t = 3.50 s, and stabilizing in the 22-24 kPa range (i.e., a relative vacuum of approximately -0.08 MPa) around t = 6.00 s.

[0122] Comparing the fluid state equations, it is clear that simple thermal contraction of the gas (Charles' Law) cannot cause such a large pressure drop. This indicates that the gaseous medium in the sealed cavity underwent a phase change (condensation) from the gas phase to the liquid phase, and the gas molar volume collapsed drastically (theoretically shrinking by about 1000 times), thus macroscopically creating a high vacuum environment.

[0123] Data shows that the establishment of negative pressure lags behind the start of cooling by approximately 0.4 seconds, and the duration of negative pressure maintenance covers the entire springback-sensitive period (low-temperature martensitic transformation period). An atmospheric pressure difference of approximately 0.08 MPa acts vertically on the sheet metal surface. Although the absolute value is lower than the mechanical holding pressure, it acts as a uniformly distributed load, effectively eliminating microscopic gaps between the sheet metal and the mold surface. Compared to Comparative Example 4 (where the pressure is consistently maintained around 101 kPa in the unsealed state), this embodiment demonstrates that the combination of the sealing structure and the phase change medium is key to achieving negative pressure adsorption, confirming the effectiveness of this process in physically suppressing sheet metal springback during demolding.

[0124] Test Example 2: Comprehensive Evaluation of Molding Quality and Comparison Test of Dimensional Accuracy

[0125] Experimental description:

[0126] This test aims to quantitatively evaluate the final quality of door lock reinforcement plate samples produced under different process conditions. The test subjects include samples produced in Examples 1-3 and Comparative Examples 1-5. Five samples were randomly selected from each group for testing, and the average value was recorded. The focus was on dimensional accuracy (springback control capability), surface microstructure quality, and molding thickness distribution.

[0127] Experimental equipment and methods:

[0128] 3D Dimension Scanning and Deviation Analysis:

[0129] The entire surface of the sample was scanned using a GOM ATOS Core blue light 3D scanner to obtain point cloud data. The scanned model was then best-fitted with the CATIA digital model.

[0130] Contour deviation measurement:

[0131] Select the normal deviation values ​​for the flange edge and the deep-drawn R-angle area. Positive values ​​indicate that the flange is not properly fitted, while negative values ​​indicate that it is too concave.

[0132] Hole position accuracy measurement:

[0133] Extract the center coordinates of the mounting hard point holes and calculate their vector distance deviation relative to the theoretical position on the digital model.

[0134] Surface roughness and microstructure inspection:

[0135] The surface roughness of the sample was measured in the large deformation area of ​​the sidewall perpendicular to the drawing direction using a Mitutoyo SJ-410 portable roughness meter. Simultaneously, surface oxidation and coating residue were observed visually and with a 50x optical microscope.

[0136] Thinning rate determination:

[0137] Using an Olympus 38DL PLUS ultrasonic thickness gauge, the corner thickness at the maximum drawing depth of the sample was measured, and the maximum thinning rate was calculated.

[0138] Experimental data:

[0139] Table 2 summarizes the test results of key performance indicators for each group of samples. Among them, in Comparative Example 1 (cold stamping), due to the excessive strength of DP1180 material, some samples cracked at the R-corner; the data are taken from the measurements of the uncracked areas.

[0140] Table 2. Summary of Sample Molding Quality and Precision Test Data under Different Process Conditions

[0141]

[0142] Conclusions and Mechanism Analysis:

[0143] Based on the above test data, the effectiveness of the process of this invention and the mechanism of action of each technical feature are analyzed as follows:

[0144] Comparing Example 2 (maximum deviation 0.21 mm) and Comparative Example 4 (maximum deviation 0.78 mm), the only difference is whether a sealed cavity was constructed. At the same heating temperature and cooling rate, Comparative Example 4, relying solely on material thermal shrinkage to counteract springback, still exhibited a shape deviation of nearly 0.8 mm at its flange edge. Example 2, however, introduced a negative pressure adsorption force of approximately 0.08 MPa through sealing and phase change condensation. Although this force is numerically much smaller than mechanical pressure, it acts on the entire sheet surface, effectively eliminating microscopic gaps at the moment of demolding and acting as a pneumatic clamping mechanism. Data shows that the introduction of the negative pressure adsorption mechanism reduced springback deviation by approximately 73%, demonstrating the substantial contribution of this technical feature to improving dimensional accuracy.

[0145] Regarding surface quality data, Comparative Example 1, using conventional cold stamping, exhibited a RaRa value as high as 1.95 μm and showed signs of roughening due to the high contact pressure of the high-strength steel. Example 2 showed a surface roughness reduced to 0.65 μm, superior to Comparative Example 3 (1.10 μm) using non-phase change synthetic oil. This confirms that the micro-air film generated by the vaporization of the medium in the high-temperature stamping zone plays an equivalent role in friction reduction as in air-bearing bearings, effectively isolating the sheet metal from direct micro-protrusion contact with the mold. Furthermore, compared to the severe surface oxidation (black skin) of Comparative Example 2 (conventional hot stamping), this invention utilizes the vaporized medium to displace air within the mold cavity, creating a locally reducing atmosphere. This allows the sample to retain its metallic color after demolding, eliminating the need for subsequent shot blasting and pickling processes.

[0146] Regarding hole position accuracy, Example 2 (0.12 mm) is significantly superior to Comparative Example 5 (0.68 mm) and Comparative Example 2 (0.85 mm), which both use full-body heating. Comparative Example 5, by heating the mounting hole area to 500°C, experienced significant thermal expansion and contraction displacement during cooling, leading to hole position drift. In contrast, the gradient heating strategy employed in this invention maintains the high-rigidity area at a lower temperature (around 100°C), resulting in extremely high dimensional stability throughout the molding process and ensuring the final product's assembly accuracy.

[0147] In summary, this invention, through the synergistic effect of medium phase change gas film lubrication, condensation negative pressure adsorption, and gradient thermal field control, solves the problems of large springback and easy cracking in cold forming of high-strength steel, while avoiding the defects of surface oxidation and difficulty in controlling precision in traditional hot forming, thus achieving a comprehensive improvement in forming quality.

Claims

1. A stamping process for a reinforcing plate for the left front door lock of an automobile, characterized in that, Includes the following steps: S1. Blank pretreatment: A temperature-sensitive gas-liquid phase change lubricating isolation medium is coated on the surface of the sheet to obtain a pretreated sheet with a surface coating. S2, Subcritical gradient induction heating: The pre-treated sheet material with surface coating is heated in zones, the target temperature of the large deformation zone is set as the first target temperature, the target temperature of the high rigidity zone is set as the second target temperature, and the first target temperature is lower than the austenitizing temperature of the sheet material, so as to obtain a heated sheet material with a temperature gradient. S3, Dynamic air film assisted stamping: The heated plate with temperature gradient is transferred to the stamping die for stamping, and the temperature-sensitive gas-liquid phase change lubricating isolation medium is heated and vaporized to form an air film between the heated plate with temperature gradient and the die. S4. Sealing and Cavity Construction: Before the stamping slide reaches the bottom dead center, the sealing structure set around the mold is closed to construct a sealed cavity surrounding the sheet metal being stamped and deformed. S5, Transient rapid cooling and negative pressure adsorption: During the pressure holding stage, a cryogenic medium is injected into the mold. The sheet metal being stamped and deformed undergoes thermal shrinkage, and the vaporized temperature-sensitive gas-liquid phase change lubricating and isolating medium inside the sealed cavity condenses and undergoes phase change, forming a negative pressure inside the sealed cavity. S6. Pressure holding, shaping and demolding: Stop injecting the cryogenic medium and release the seal, open the mold and take out the molded left front door lock reinforcement plate.

2. The stamping process for a reinforcing plate for a left front door lock of an automobile according to claim 1, characterized in that, In step S2, the first target temperature of the large deformation zone is 450℃-550℃, the second target temperature of the high rigidity zone is 25℃-150℃, and the heating rate is 30℃ / s-50℃ / s.

3. The stamping process for a reinforcing plate for the left front door lock of an automobile according to claim 1, characterized in that, In step S3, the stamping speed is 100mm / s-250mm / s; in step S5, the holding pressure applied in the holding stage is 1000kN-2000kN, the cooling rate of the sheet surface is 80℃ / s-150℃ / s, and the duration of the holding stage is 5.0s-10.0s.

4. The stamping process for a reinforcing plate for the left front door lock of an automobile according to claim 1, characterized in that, In step S2, the partitioned heating is performed using a multi-channel induction heating station, and the large deformation zone corresponds to the deep drawing zone, while the high rigidity zone corresponds to the mounting hole and hard point zone.

5. The stamping process for a reinforcing plate for a left front door lock of an automobile according to claim 1, characterized in that, In step S4, the closing action of the sealing structure precedes the closing action of the mold surface; in step S5, the cryogenic medium is introduced into the mold flow channel using a pulse injection method.

6. The stamping process for a reinforcing plate for a left front door lock of an automobile according to claim 1, characterized in that, The temperature-sensitive gas-liquid phase change lubricating isolation medium used in step S1 is made of components comprising the following weight percentages: Matrix film-forming components: 30.0wt%-45.0wt%; Gradient volatile phase change components: 50.0wt%-65.0wt%; Functional additive components: 3.0wt%-5.0wt%; The boiling point of the gradient volatile phase change component is lower than the carbonization temperature of the matrix film-forming component.

7. The stamping process for a reinforcing plate for a left front door lock of an automobile according to claim 6, characterized in that, The matrix film-forming component is composed of polyalkylene glycol and polyvinyl alcohol; the gradient volatile phase change component is composed of deionized water, anhydrous ethanol and n-butanol; and the functional additive component is composed of sodium molybdate dihydrate, polyethylene glycol octylphenyl ether and triethanolamine borate.

8. The stamping process for a reinforcing plate for a left front door lock of an automobile according to claim 1, characterized in that, In step S1, the sheet material is DP1180 cold-rolled duplex steel sheet, and the dry film thickness after coating and drying is 15μm-25μm.

9. The stamping process for a reinforcing plate for a left front door lock of an automobile according to claim 1, characterized in that, In step S4, the sealing structure is selected from one of the following: a high-temperature resistant silicone rubber sealing ring, a flexible metal bellows, or a composite sealing device.

10. The stamping process for a reinforcing plate for a left front door lock of an automobile according to claim 1, characterized in that, In step S5, the cryogenic medium is liquid nitrogen or R410a refrigerant.

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