The effect of the increased strain per pass during the hot-rolling and its effect on the tensile properties of V-Mo and Cr-V-Mo microalloyed dual-phase steels

It is well known that the thermomechanical processing parameters affect the mechanical properties of dual-phase (DP) steels, but optimization still remains a key challenge. This work studied two interphase precipitation-strengthened DP steels based on V, Mo, and V, Mo, Cr microalloy additions. Hot-rolling was performed with a strain per pass of 0.2 and 0.4 before isothermal transformation at temperatures between 600 and 690 °C to determine the effect of these process parameters on microstructure, particularly interphase precipitation. The microstructure was carefully correlated with the mechanical properties. It was found that a higher strain during hot rolling increases the transformation kinetics of austenite to ferrite and also increases the volume fraction of the interphase precipitation within it, leading to higher strength values. The reaustenitization temperature before the isothermal transformation also plays an important role, with increased temperature reducing the segregation banding effect and increasing the amount of ferrite. The optimal reaustenitization temperature depends on the composition, being 1250 °C for the microalloyed steel with Cr additions and 1150 °C for the Cr-free microalloyed steel. A new method was used to calculate the yield strength of these DP microalloyed steels, including the strengthening contributors of each phase and the banding effect brought into one single expression that matches the experimental results.