| The use of ultra high strength stamped parts in car body can not only realize the lightweight of automobile, but also improve the anti-impact ability and the safety of the car. The hot stamping technology is the special manufacturing technology for the ultra high strength parts, and the boron steel is one of the main materials. In hot stamping, the boron steel blank is heated to be austenitized completely at first, and then transferred to the forming dies with cooling system quickly. In the hot stamping dies, the blank will be formed and quenched simultaneously. The microstructure transformation from austenite to martensite during quenching greatly improves the strength and hardness of the formed parts. This technology overcomes the problem that the ultra high strength steel is hard to be formed at ambient temperatures. Besides, the hot stamped parts present little springback and high geometric precision. However, the hot stamping technology is more complicated than the traditional stamping technology. The addition factor of temperature in hot stamping not only brings the heating process, but the cooling process and the phase transformation as well. Therefore, the hot stamping technology needs to be further systematicly researched.The process parameter optimization, the heat transfer performances of the boron steel in different metallographic states, the optimal design for the cooling system of the hot stamping dies and the simulation and the tests of the hot stamping process were systematically studied in this paper. The research in this work is advantageous to promote the practical application of the technology. The main contents and conclusions of this paper are as follows:(1) Process parameter optimization for the quenching stage in hot stampingBased on the hot stamping tests of the plate parts, the influence of the quenching parameters such as contact pressure and holding time on the microstructures and the mechanical properties of the formed parts was investigated, and the optimization of contact pressure and holding time was conducted considering the production efficiency. Results show that during hot stamping of 2 mm 22 Mn B5, when the coolant was water in room temperature, the martensitic transformation couldn’t be sufficient for the quenched part if the contact pressure was smaller than 0.3 MPa; when the part was quenched under 0.3 MPa pressure, sufficient martensitic transformation and high strength and hardness of the quenched part could be gained, while the minimum holding time required for complete martensitic transformation was a bit longer; when the part was quenched under 0.3~1.0 MPa pressure, the minimum holding time decreased as the pressure increased; when the part was quenched under more than 1.0 MPa pressure, the minimum holding time changed little with the pressure increasing. Therefore, for the hot stamping process of 2 mm 22 Mn B5, when the coolant is water in room temperature, the optimized pressure during the quenching stage is more than 1.0 MPa, and the minimum holding time is 8 s when the steel is cooled from 800 ℃ under the optimum pressure.(2) Research on the heat transfer performances of the boron sheet metal in different metallographic statesBased on the differential equation of Newton cooling law, the models for calculating the heat transfer coefficient between the blank and the dies and the effective specific heat of the boron steel were established, considering the microstructure change and the latent heat release in hot stamping. Using the models, the evolutions of the coefficients and the effective specific heat of the boron steel in different metallographic states during complete quenching have been investigated through the hot stamping tests of the plate parts. In addition, the influence of the forming conditions on the heat transfer coefficients of the boron steel in different metallographic states was investigated too. Results show that the evolutions of the coefficients in different metallographic states are significantly different. With blank temperature decreasing during quenching, no evident fluctuation emerges in the austenitic state, while a significant sharp increase followed by a gradual decrease is detected in the multiphase state during martensitic transformation and a slight decrease is observed in the martensitic state. Heat transfer coefficient evolution is closely related to the effective specific heat variation. The latent heat released from the austenite to martensite transformation causes the effective specific heat to change. This variation of the effective specific heat leads to significant fluctuations during martensitic transformation. The forming conditions have some influence on the variation of the coefficients in different metallographic states too. With contact pressure increasing and initial die surface temperature decreasing, the heat transfer coefficients of all the metallographic states increase, while the effect of contact pressure is more noticeable.(3) Optimal design for the cooling system of the hot stamping diesA selection criterion for the cooling channel parameters of the hot stamping dies including the channel diameter, the number of the channels and the distance from the channel center to the die contact surface was developed based on the energy conservation law, the turbulence theory and the shape factor method. Besides, a cooling channel structure optimization method based on the difficult degree of cooling was proposed. The cooling channel models of the hot stamping dies for a typical hot stamped anti-collision beam with ultra high strength in car body were designed. 2D hot stamping simulation models with cooling channels for the anti-collision beam were established. Using the models, the effect of the designed cooling systems was measured according to the cooling intensity, uniformity and the die strength by simulation. Based on the simulation results, the structure of the cooling channels was optimized. Results show that all the cooling systems designed based on the method in this paper can meet the requirements of the cooling intensity and the die strength, while cooling systems with smaller cooling channel diameter are recommended to be the better choices in terms of the cooling uniformity. In addition, the cooling channel arrangement can be optimized based on the difficult degree of cooling. According to the temperature distribution of the part quenched by equally arranged cooling channels, different cooling degrees can be divided. For the regions of the parts which are most difficult to be cooled, smaller channel gaps and distances from the channel center to the die contact surface and more channels are recommended, under the precondition of guaranteeing the die strength. For the regions which are relatively more difficult to be cooled, only smaller distances from the channel center to the die contact surface are needed. For the regions which are easy to be cooled, no changes should be done. Simulation results demonstrate that the cooling uniformity and intensity of the cooling system optimized in this way can be both improved.(4) Research on the hot stamping process of the ultra high strength boron sheet metal by simulationA 3D hot stamping simulation model for the anti-collision beam was established. Based on this model, the evolutions of the temperature, thickness, microstructure and mechanical properties of the steel in different stages of hot stamping were investigated, the influence of the process parameters in the stamping and quenching stage on the microstructure and the mechanical properties of the components was researched, and the parameters of the hot stamping process of the anti-collision beam were optimized. Results show that the blank thickness mainly changes in the stamping stage, while the microstructure and the properties mainly change in the quenching stage, and little springback occurs after the component is unloaded. When the blank is formed with high speed for over 500 ℃ initial temperature, the influence of the initial forming temperature on the microstructure and the hardness of the blank is not evident, while the influence on the thickness is obvious. Excessive thinning is easy to appear in the lower initial forming temperature condition. Too low punch speed may lead to excessive heat loss in the stamping stage, which will reduce the formability because of local hardening. Too high punch speed may cause the blank to thin too much too, for the blank undergoes too large plastic deformation at high temperatures. Setting the blankholder in the hot stamping process does not have much influence on the temperature, microstructure, and the hardness of the blank, but reduces the fluidity of the metal greatly, which may bring in serious local deformation and too much thinning, even cracking. Complete martensite phase cannot be obtained if the contact pressure is too low or the holding time is too short during quenching. For the investigated hot stamped beam in this work, the optimum process parameters are 800 ℃ initial forming temperature, 100 mm/s punch speed, no blankholder setting, over 5 MPa quenching pressure and 8 s holding time.(5) Research on the hot stamping process of the ultra high strength boron sheet metal by testBased on the optimal design method for the cooling channels of the hot stamping dies, the hot stamping dies of the anti-collision beam were designed and manufactured. Using the forming dies, the direct and the indirect hot stamping tests were conducted, and the qualified direct and indirect hot stamped parts were achieved, which prove the rationality of the cooling channel design method. Based on the simulation results of the hot stamping process of the anti-collision beam, a series of direct hot stamping tests were carried out, and the influence of the process parameters on hot stamping of the anti-collision beam was investigated by test. The test results were in good agreement with the corresponding simulation results, which verified the accuracy of the hot stamping simulation. |
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