  Experimental findings in the literature reveal that DP steels with identical
strength and composition, but different microstructures, exhibit inconsistent
trends under either necking-controlled or damage-controlled ductility tests. A
special case of this phenomenon is referred to in the forming community as the
cut-edge failure or edge cracking issue of DP steels. It is observed that
globally ductile microstructures are prone to premature damage at cut edges
while a comparatively less ductile DP steel performs better under the locally
applied deformation at such edges. In this paper, a systematical statistical
study is conducted to analyse this paradox from a micromechanical point of
view. The obtained results qualitatively confirm the experimental observations
and allowed us to explain/rationalize them, as follows. To reach a higher
necking-controlled ductility, i.e. global ductility, the mechanical contrast
between ferrite and martensite must be increased. Since martensite reveals a
very high initial hardening and fast saturation, activation of martensite
plasticity will lead to early necking in the microstructure and should
therefore be avoided; a high mechanical phase contrast has this effect.
However, at the same time a higher mechanical phase contrast is detrimental for
the local ductility. More heterogeneity in the microstructure causes the
generation of higher local plastic strains and triaxial stresses which leads to
early damages. Therefore, in DP steel, increasing the global ductility may come
at the expense of decreasing local ductility, and vice versa. The martensite
hardening behavior is the main factor governing this phenomenon, and therefore
also constitutes the key to processing improved steels that combine the best of
both.
