Quantification of Large Deformation with Punching in Dual Phase Steel and Change of its’ Microstructure – Part III: Micro-tensile Behavior of Pre-strained Dual-phase Steel

The deformation behavior of inhomogeneous microstructures developed by pre-straining was studied by micro-tensile testing to elucidate the cause of low hole expandability of ferrite-martensite dual-phase (DP) steels. Slip bands developed in the ferritic phase, when the DP steel was cold-rolled (CR) at a reduction of 60% in thickness; in the 88% CR microstructure, ultrafine ferrite grains with a strong texture were locally observed. While the nanohardness increased with increasing pre-strain in the ferritic phase, it was invariable in the martensitic phase. Tensile tests using micrometer-sized specimens with ferritic and martensitic phases revealed that the ultrafine-grained ferritic microstructure exhibited high yield strength but low ductility when compared to the slip band ferritic microstructure. While a shear type fracture occurred without necking in the former, the latter exhibited a chisel-edge type failure. Without ultra-grain refinement by pre-straining, the inhibition of slip transfer by the interphase boundary was a major contributor to the strengthening in the DP steel. The ductility loss of the severely deformed DP steel was presumably attributed to localized strain into the ultrafine-grained ferritic microstructure.