Predicting Forming Limit of Hot Stamping Boron Steel using a Modified Zener-Hollomon Parameter based Ductile Failure Criterion

This study combines a recently proposed ductile failure criterion with a modified Zener-Hollomon parameter to predict the forming limit of a boron steel at hot stamping temperatures. The Zener-Hollomon parameter is modified to include a parameter that accounts for the non-linear variation of strain rate during hot stamping. The combined ductile failure criterion takes into account the critical damage at localized necking or at fracture as a function of strain path, initial sheet thickness and strain rate variation. The process of determining various parameters in the failure criterion is explained. The capability of the ductile failure criterion is demonstrated by predicting forming limit of a boron steel in an isothermal Nakajima dome test. The criterion is further demonstrated by predicting fracture in hot stamping a B-Pillar part (NUMISHEET Benchmark #3 2008).