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 (R) 1500 and two emerging grades Usibor (R) 2000 and Ductibor (R) 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 degrees 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 (R) 1000 and Ductibor (R) 500 by DIL 805 A/D dilatometer under tensile deformation mode. In-situ observation of microstructural evolution during hot stamping process for Usibor (R) 1500 is explored using Confocal scanning laser microscope to uncover some physical phenomena for further refinement of hot stamping practices.