Influence of heating rate on the evolution of post-annealed microstructure in Nb–Ti microalloyed steel: Interplay with initial cold-rolled microstructure

This study focuses on understanding the complex interactions between the initial cold-rolled microstructure and the heating rate, and how these factors influence the post-annealed microstructure, particularly the fraction of polygonal ferrite, in the Nb–Ti microalloyed steel. Key findings include that increasing the heating rate from 3 °C/s to 30 °C/s enhances the kinetics of reverse transformation to austenite, resulting in a larger fraction of new ferrite and thus a higher proportion of polygonal ferrite in the post-annealed microstructure in most cases. Interestingly, in scenarios where recrystallization can occur by altering the initial microstructure, the combined evolution of recrystallized and new ferrite enlarges the fraction of polygonal ferrite at slower heating rates of 3 °C/s. It indicates that the contribution of recrystallization and reverse transformation on the evolution of polygonal ferrite depends on the interplay between initial microstructure and the heating rate. Further, the calculations of the austenite/ferrite interface velocity suggest that the accelerated reverse transformation to austenite, facilitated by increased heating rates, is little affected by kinetic transitions.