Effect of retained austenite on the fracture behavior of a novel press-hardened steel

Tensile and bending properties are two critical attributes of press hardened steels (PHSs) for automotive body structural components. However, the research on these properties of PHSs with retained austen-ite (RA), which is introduced to improve the mechanical properties, has not been well reported. In this study, the effect of RA on the quasi-static uniaxial tensile and three-point bending behaviors has been systematically investigated for a newly developed Cr and Si alloyed PHS. RA with various degrees of me-chanical stabilities was obtained by tuning the auto-tempering of martensite through the control of the die contact pressure during the press hardening process. Mechanically stable RA provides the optimal combination of tensile and bending performance, while the PHS with RA of poor mechanical stability has a deteriorated bending toughness as manifested by a reduction of bending angle from approximately 61.6 degrees to 56.8 degrees. However, its tensile properties, in contrast, are the best in terms of the product of ulti-mate tensile strength and total elongation (15.9 GPa %). This is mainly attributed to the unstable RA on the outermost surface of the bending sample. The unstable RA can easily transform into brittle marten -site under local plane strain and strain gradient conditions in the bending test compared with uniaxial stress in the tensile test, promoting crack initiation and propagation. Furthermore, the effect of marten-sitic auto-tempering or contact pressure on the quantity and stability of RA are also discussed. This study would provide a useful reference to guide hot stamping process optimization for the newly developed Cr and Si-containing press-hardened steel. (c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

This study systematically investigates the effect of retained austenite (RA) on the tensile and bending properties of a newly developed Cr and Si alloyed press hardened steel (PHS). Mechanically stable RA provides the optimal combination of tensile and bending performance, while RA with poor mechanical stability leads to a reduction in bending toughness. These findings provide useful insights for optimizing the hot stamping process of the Cr and Si-containing PHS.