WEB A microstructural insight into the inconsistent behavior of dual-phase steels observed under different formability tests.Friday (25.09.2020) 13:05 - 13:20 M: Modelling and Simulation 2 Part of:
The formability of sheet metal is constrained either by localized necking or shear fracture, which are termed respectively as global and local ductility measures. Under a necking-controlled formability test, dual-phase (DP) steels show higher strain hardening and their ductility surpasses other steel grades of the same strength. However, under a localized deformation such as shearing, necking is stabilized and the same DP steel, in contrast, exhibits very poor damage resistance. The observed inconsistent trend is called edge cracking issue of DP steels known as the higher tendency of a DP sheet metal to damage initiation due to further loading at a sheared edge. Even though the literature addresses this phenomenon to incompatibility of mechanical properties between constituent phases, triggering micro-scale localization and early fracture, there is still no clear explanation for the pronounced mismatch of formability responses, and hence for the edge cracking issue. Here, we conduct a systematic analysis of the artificial two-phase microstructures of DP steel to first, understand the underlying mechanisms of edge cracking and second, to propose a way to improve it. The simplified micro-scale modeling scheme pave the way to systematically reproduce the literature observations and investigate them in detail. Initial results indicate that in order to achieve higher ductility under a necking-controlled formability test, the activation of martensite plasticity should be delayed. While avoiding martensite plasticity demands higher mechanical phase contrast which in turn leads to more heterogeneous stress-strain distribution between phases and early failure under applied local deformation. This formability trade-off can be suppressed only by increasing strain hardening capability of the martensite islands. Based on simulation results, as the strain hardening of martensite reaches the same levels as of ferrite, the inconsistency of global and local ductility fades away.