Study On Hot Stamping Steels With High Formability Index Achieved By Quenching & Partitioning(Q&P) Treatment

The innovation and application of Advanced High Strength Steels(AHSS) used in automobiles, especially structural components, are crucial methods to meet the needs for vehicle weight reduction and safety improvement so that less petroleum can be consumed and drivers and passengers would be better protected. Hot stamping steel is commonly used to strengthen automobiles, while Quenching & Partitioning(Q&P) is a new heat treatment process that could be utilized to improve steels’ mechanical properties and silicon is one of the mostly added alloying elements in steels with property of retarding cementite precipitation. In this work, a one-step Q&P process was applied to commercial B1500 HS hot stamping steel and a new type of Si-modified hot stamping steel called HSS steel with higher silicon content. After achieving the optimized Q&P process parameters, HSS steel was treated by a continuous heat treatment process highlighted with hot stamping + one-step Q&P. The main research contents and results are summarized as follows:1. Study of B1500 HS and HSS steels treated by one-step Q&P processOne-step Q&P process parameters of Quenching Temperature(260℃, 280℃,300℃ and 320℃) and Partitioning time(15s, 30 s and 300s) were designed to investigate their effects on the microstructure and mechanical properties of both commercial B1500 HS and HSS steels after one-step Q&P treatment. Through microstructure observation, tensile test and other techniques, it is found that the commercial B1500 HS steel is not suitable to one-step Q&P process with the mechanical properties being worse than that of water quenching sample. While after the silicon content is added to 0.89%wt, very high product of tensile strength and ductility(more than 20GPa·%) of HSS steel can be achieved at appropriate Q&P parameters with the microstructure comprising of martensite and retained austenite.2. Thermal-mechanical simulation of hot stamping +Q&P heattreatment processBased on the optimum one-step Q&P process parameters obtained in the previous results, a continuous heat treatment process highlighted with hot stamping + Q&P were applied to HSS steel utilizing Gleeble3500 mechanical simulator. The XRD patterns illustrates that the volume fraction of retained austenite is increased with the increase of hot deformation strain. Strain rate of hot deformation also has great impact on the mechanical properties of the HSS steel after treated with lower strain rate sample possessing higher formability index.Overall, since a relative high amount of retained austenite can be obtained in the high silicon content HSS steel after treated by one-step Q&P and hot stamping + Q&P, its mechanical properties are better than those of B1500 HS hot stamping steel.