The Role of Phase Hardness Differential on the Non-uniform Elongation of a Ferrite-Martensite Dual Phase Steel

This study involved a commercial hot-rolled dual-phase (DP) steel consisting of martensite (similar to 10 pct) and ferrite phases. The harder lath martensite was located at the grain boundaries and triple junctions of the equiaxed ferrite grains. Tempering and high-pressure torsion (HPT) were used to alter the phase hardness differential Delta H (where Delta H = H-Martensite - H-Ferrite) of the DP. The relationship between Delta H and non-uniform elongation, epsilon(NU) , or post-necking ductility under tensile deformation, was then explored. Tempering softened predominantly the martensite, while HPT increased the ferrite hardness. Both led to a reduction in Delta H. A drop in Delta H in the tempered DP resulted in a steady increase and eventual saturation in epsilon(NU) . On the other hand, a Delta H decrease in the HPT specimens showed an initial increase in epsilon(NU) followed by a drop. Strain analysis, with optical digital image correlation during tensile deformation of the tempered DP samples, clearly related the formation of strain localization with Delta H. In particular, severity of strain localization during necking scaled linearly with Delta H. This study thus brought out a potential relationship among the phase hardness differential (Delta H), severity of strain localizations and post-necking ductility(epsilon(NU)).