An unexpected feature of the stress–strain diagram of dual-phase steel

The microstructure of low alloyed ferritic–martensitic dual-phase steels as used for deep drawn parts in automotive applications consists of coarse grained hard martensitic inclusions embedded in a soft ferritic matrix. In tensile tests commercially produced dual-phase steels show an unexpected dependence of their initial yield behaviour on the content of martensite. The impact of the thermomechanical history on the mechanical properties of the material is demonstrated by means of a fully three-dimensional finite element analysis. A work-hardened ferritic skeleton formed during rapid cooling connects the martensitic inclusions and governs the initial stages of plastic deformation. The model correctly predicts both the experimentally observed dependence of the proof stress on the amount of martensite and a lower initial slope of the stress–strain diagram.