Study of Microstructural Evolution of Press Hardening Steels using Dilatometer and In-situ Studies for a Simulated Hot Stamping Condition

Press hardening grades are widely used in automotive industries for safety-critical structural parts due to their unique combination of high strength, excellent formability, and crash performance. Considering various scenarios on thermo-mechanical profiles in different hot stamping lines, achieving the targeted strength and ductility / fracture strain in the hot-stamped parts is still challenging to some hot stampers for some grades. In this investigation, a dilatometer study for Usibor®1500 and two emerging grades Usibor®2000 and Ductibor®1000 under a given hot stamping condition was conducted with consideration of the entire hot-stamping processes (i.e., austenitization, blank transfer, forming and final quenching from 700°C) to understand the differences in critical cooling rates, and evolution of microstructures. Influence of large forming strain (15%) on final properties is also examined for Ductibor®1000 and Ductibor®500 by DIL 805 A/D dilatometer under tensile deformation mode. In-situ observation of microstructural evolution during hot stamping process for Usibor®1500 is explored using Confocal scanning laser microscope to uncover some physical phenomena for further refinement of hot stamping practices.