Damage Behavior in Modern Automotive High Strength Dual Phase Steels During Uniaxial Tensile Deformation

In the present research, damage mechanisms during room temperature uniaxial tensile testing of two different modern high strength dual phase steels,DP780 and DP980, were studied. Detailed microstructural characterization of the strained and sectioned samples was performed by scanning electron microscopy (SEM). The results revealed that interface decohesion, especially at the triple junctions of ferrite-ferrite-martensite, was the most probable mechanism for void nucleation. Also, it was found that ductile fracture in these steels was nucleation controlled such that just before ductile fracture incidence, a high density of voids would nucleate or a sudden accelerated void nucleation could happen. Microscopic observations as well as statistical analysis confirmed this phenomenon. Moreover, damage analysis suggested that the void nucleation rate was higher in DP980 than DP780 steel. It seemed to be highly influenced by the morphology and distribution of martensite particles within the ferrite matrix.