Application of hot press forming process to manufacture an automotive part and its finite element analysis considering phase transformation plasticity

An automotive flex plate that is installed in the automotive engine and delivers a torque to a transmission is manufactured by hot press forming technique. By this technique, significant increase of strength through quenching of heated high carbon SK5 steel sheet and higher dimensional stability after forming through the press operation at high temperature can be attainable. The indirect method with a pre-forming step and direct oil quenching to attain uniform and fast cooling rate are employed considering cooling characteristics of the SK5 steel and large thickness of the flex plate. A new tool design is proposed for the hot press forming process, and an optimized heat treatment condition is determined by studying two frequently utilized heat treatments: austempering and quenching with tempering heat treatments. By introducing designed tools and selected heat treatment condition, the target product can be successfully manufactured, which satisfies two main manufacturer's specifications: high hardness and good dimensional accuracy. Moreover, finite element analysis, which considers transformation induced plasticity (TRIP) during phase transformations, is performed to understand the thermo-mechanical behavior of hot press formed sheet. The analysis verifies that phase transformations play significant roles in strengthening by transforming hard martensitic phase and in reducing dimensional change by additional plastic deformation during phase transformations.