Unraveling the Effect of Microstructure on Edge Ductility of Dual-Phase Steels: A Computational Modelling Study

In dual-phase steels, microstructural characteristics such as phase volume fraction and phase contrast tend to have opposite effects on the ductility measured in hole expansion capacity (HEC) testing as compared to the forming limit curve (FLC). This has lead to a number of paradoxical observations in the literature, in which microstructures which were optimized for ductility in terms of the FLC turned out to perform poorly on HEC and vice versa. This study systematically analyzes the issue by means of microstructural simulations. Artificial, highly idealized two-phase microstructures are constructed with have the same nominal strength, but which achieve this strength by different combinations of martensite volume fraction and hardness. They are subjected to pure shear deformation and based on the computed response their hardening curve and damage resistance are predicted; furthermore, the point of necking in plane-strain tension is predicted based on a Considère-like criterion. If the latter is taken as representative of the FLC and the strain to (local) failure due to damage of the HEC, the paradoxical trend discussed above is reproduced. It may furthermore be traced to the distinct hardening behavior of martensite, with a rapid hardening at low strain levels followed by early saturation.