Edge cracking mechanism in two dual-phase advanced high strength steels

One of the manufacturing issues restricting the applications of advanced high-strength steels is edge cracking during forming. Computer simulations using the conventional forming limit curve (FLC) as a failure criterion often fail to predict edge cracking. A new failure criterion is needed to assess the edge stretchability in simulations and applications, subsequently understanding the fracture mechanism in the microstructural scale is a prerequisite. In this study, the edge fracture mechanisms of two selected dual-phase steels (designated as DP980 and IBF980) with identical chemistry but different edge cracking behaviors are studied by controlled edge tension tests and scanning electron microscopy (SEM). The results show that the edge cracking behaviors are essentially fractures propagated from pre-existing microcracks introduced by the shearing process. The dominant edge fracture mechanism is decohesion between the martensite and ferrite phases. This study employs the interfacial strength between ferrite and martensite as an index to predict the material edge cracking resistance. The interfacial strength is calculated to be 1070 MPa for IBF980 and 854 MPa for DP980 using void-nucleation models. This result indicates that IBF980 has a higher resistance to edge cracking and subsequently a higher local formability, which is consistent with the higher hole expansion ratio values observed. Refining the martensite particle size and minimizing banded martensite structures are effective approaches to increase the local formability.