Simulation And Experimental Research On Hot Stamping With Q&P Process

In recent years,the demand for safety and energy saving in the automotive industry has been continuously increasing.The hot stamping process of Advanced High Strength Steels(AHSSs)can effectively reduce the weight of parts with only a small loss in strength.Moreover,the hot stamping of traditional boron steels leads to fully martensitic parts with very low post forming plasticity,which evidently reduces the overall mechanical properties and the application of hot stamping parts.The quenching and partitioning(Q&P)process is currently being applied in traditional hot stamping with the aim of improving the post-forming ductility of high strength structural parts.This paper will focus on the two-step Q&P process because hot stamping can be more productive with this process.This Q&P process consists of quenching of the part to a temperature between Ms and Mf,then the part is heated to a partitioning temperature and held for a number of seconds when carbon diffuses from supersaturated martensite to austenite.After partitioning,the part is quenched to room temperature,where a significant portion of austenite is stabilized.The final martensiteaustenite duplex microstructure exhibits high strength(about 1500MPa)and much greater elongation(14.8%)than typical hot stamped boron steel(6.6%).Application of quenching and partitioning process in hot stamping has proven to be an effective method to improve the plasticity of advanced high-strength steels(AHSSs).In this work,the hot stamping and partitioning process of advanced high-strength steel 30CrMnSi2 Nb is investigated on a mould equipped with a PID temperature control system.The influence of quenching temperature and partitioning time on the microstructure and mechanical properties of advanced high-strength steel is investigated.In addition,a model for quenching and partitioning process is applied to predict the carbon diffusion and interface migration during partitioning,which determines the retained austenite volume fraction and final properties of the part.The predicted results of the retained austenite volume fraction are compared with the experimental results to verify the model’s accuracy.The results of the study are as follows:A model for quenching and partitioning process is applied to predict the carbon diffusion and interface migration during partitioning.Then the retained austenite volume fraction after the final quench is predicted.The predicted trends of the retained austenite volume fraction after the final quench agree with the experimental results.In both cases,the volume fraction of retained austenite increases first then decreases with the quenching temperature and partitioning time increasing.The maximum volume fraction of retained austenite is both observed with the quenching temperature of 290℃ and 30 s.Quenching temperature influences both the volume fraction and stability of retained austenite,and then the retained austenite volume fraction after the final quench.The strength,elongation and strength-ductility balance of HFQP samples are all higher than traditional Q&P samples.This demonstrates that the strain during deformation influences more than stress.In general,the deformation in HFQP process is beneficial for the mechanical properties of advanced high-strength steel.