Effects of Constituent Hardness on Formability of Dual Phase Steels

Abstract

A commercially produced 0.15% carbon dual phase (DP) steel with 1081 MPa tensile strength is modified to create eight additional conditions using tempering or tempering plus cold‐rolling, processes that are shown to primarily affect constituent (martensite and ferrite) hardness as measured with nanoindentation. The effects of different martensite/ferrite hardness ratios are evaluated using tensile and hole expansion testing. Lower martensite/ferrite hardness ratios (increased similarity between ferrite and martensite hardness) are interpreted to improve strain sharing, which suppresses plastic strain localization to higher stresses for yield strength and results in higher hole expansion ratios. Microstructure‐level strain maps created using digital image correlation on deformed tensile samples illustrate that plastic strains heterogeneously develop, with the largest local strains residing in ferrite at interfaces with martensite, locations consistent with preferential void nucleation sites in DP steels. Metallographic assessments of the extent of void formation in the necked regions of failed tensile samples show that for a given local thickness strain, the extent of void formation is significantly less in steels with lower martensite/ferrite hardness ratios, further illustrating the importance of controlling martensite/ferrite hardness ratios to optimize forming responses of DP steels.