Influence of martensite volume fraction and cooling rate on the properties of thermomechanically processed dual phase steel

Thermomechanical controlled processing (TMCP) simulations were preformed using a flat compression test, in order to develop a comprehensive understanding of the effect of the martensite volume fraction (MVF) and the cooling rate on the phase transformation behavior, microstructural evolution, mechanical properties and bake hardening (BH) ability of thermomechanically processed dual phase (DP) steel. The cooling schedule affected the microstructural constituents (polygonal ferrite and martensite) and phase transformation behavior of the DP steel. The relationship between the volume fraction of the martensite, the morphology and distribution of phases present in the microstructure, and the mechanical properties of DP steel was studied in detail. A higher MVF and an increasing cooling rate after the last deformation to the fast cooling start temperature (TFC) raised both the strength and the bake hardenability of the DP steel. Moreover, a correlation between MVF and bake hardening effect was found. A larger MVF led to an increase of BH levels.