Comparative Study on Damage Evolution during Sheet Metal Forming of Steels DP600 and DP1000

Two different dualphase steels of significantly different strength properties are compared with respect to microstructural configuration, chemical composition, mechanical properties, and ductile damage mechanisms. The investigated steel grades DP600 and DP1000 show remarkable differences in terms of martensite phase fractions, yield and ultimate tensile strength, and ductile damage evolution behaviour. In particular it turns out that steel DP600 experiences a relatively early ductile damage initiation, but only a moderate rate of damage evolution, whereas for steel DP1000, contrary behaviour is observed. This steel shows a comparably late ductile damage initiation with rapid subsequent damage evolution. This behaviour is expressed by the ratio of fracture strain over strain at damage initiation. Its origin lies in both the martensite phase fraction and the individual strength properties of the ferritic and the martensitic phases, which will provoke inhomogeneous plastic strain distributions in the materials' microstructures. The more pronounced these differences are, the higher the local plastic strain peaks become, and the earlier ductile damage initiation happens. Nevertheless, with decreasing strength of the ferritic phase, its ability to withstand even large plastic strains without fracturing is strongly promoted, so that the fracture strain in steel DP600 can be shifted to higher values even though the material has undergone early damage initiation.