Phase constitution effect on the ductility of low alloy multiphase transformation induced plasticity steels

The effect of phase constitution on the ductility of a low-alloy transformation induced plasticity (TRIP)-assisted steel was studied. The processing parameters were selected in such a way to obtain microstructures with systematically different volume fractions of ferrite, bainitic ferrite, martensite, and retained austenite, while keeping the grain size of ferrite and morphology of bainitic ferrite almost the same among various cases. Uniaxial tensile testing was performed at room temperature to examine the strength and ductility for each microstructure. It was observed that the ratio of carbon content to volume fraction of retained austenite is a better indicator of uniform strain than only carbon content or volume fraction. Regardless, martensite was found to be the main factor controlling the ductility. An almost linear relationship between tensile ductility and the volume fraction of martensite was observed up to 40% of martensite. The results also suggested that maximizing ferrite is a more direct design strategy for good ductility since increasing ferrite content reduces martensite volume fraction.