Crashworthiness evaluation of press hardened steels with different lath-like microstructures.

Abstract

The deployment of Press Hardened Steels (PHSs) in the Body-In-White structures that took place during the two last decades is at the origin of a breakthrough weight saving on vehicles while guaranteeing the safety requirements of the automotive industry. The production of parts with complex shapes and a high strength between 1500-2000MPa was made possible by taking advantage of the hot rheology of austenite combined with the fast cooling obtained by in-die quenching leading to auto-tempered martensitic microstructures. In the present work, PHS1500 was hot stamped with different thermomechanical processes to promote different lath-like microstructures (bainite, tempered martensite…). An innovative approach is proposed to link the complex microstructures formed between thermoregulated dies to the mechanical behaviour and local ductility of these materials during crash-like solicitations. The microstructure has been characterised through a multi-characterization technique approach including light optical microscopy, scanning electron microscopy and X-ray diffraction. The local ductility is assessed with a combination of crack initiation tests (bending, notch tensile test) and crack propagation resistance tests (Essential work of fracture). Despite similar microstructural crystallographic features, lower bainite and auto-tempered martensite can lead to very different strength – local ductility compromise.