Deformation is a key element in most thermomechanical processing routes for steel; therefore, understanding how deformation affects phase transformation kinetics is essential for effective microstructure control and for tailoring mechanical properties. In this study, different levels of compressive strain (ε2 = 0.0, 0.3 and 0.5), followed by continuous cooling at different rates (1, 10, 50 and >100 K/s), were applied to a Nb-microalloyed low-carbon steel (0.05C-1.3Mn-0.042Nb-0.2Si-0.010Ti-0.03Al-0.0040N-Bal. Fe, wt.%). To quantify the influence of deformation in the austenite on the transformed product, the resulting microstructure, grain size, grain boundary character, and hardness were analyzed and the combined results were used to construct a continuous cooling transformation (CCT) diagram.

Results show that with increasing deformation, the pearlite fraction of the slow-cooled samples decreases. Samples with higher cooling rates and increasing deformation exhibit increased variability in the resulting microstructures, due to shifts in the CCT curves. The results show that increasing deformation in the austenite generally leads to grain refinement in the transformed phases for all cooling rates. However, despite the reduction in grain size with increasing deformation, the hardness decreases with increasing deformation for all cooling conditions. For the slow-cooled samples, it is associated with a decrease in the pearlite fraction. However, for the samples processed at high cooling rates, this behavior is explained by the nature of the microstructural constituents present in the samples, which, they evolve from martensite to bainite and eventually to ferrite with increasing deformation. Grain boundary analysis reveals that the effect of deformation on total grain boundary length strongly depends on the cooling rate. At low cooling rates, deformation promotes the formation of low-angle grain boundaries (LAGBs), and at high cooling rates, the grain boundary length decreases with increasing deformation, attributed to changes in the resulting microstructures.

In the CCT diagram, deformation shifts the ferrite transformation curves to shorter times and locally to higher temperatures, while the bainite curves are also shifted to shorter times, but locally to lower temperatures. The martensitic transformation is suppressed to a lower temperature with increasing deformation. Overall, the results demonstrate that prior deformation has a pronounced effect on the microstructural evolution and phase transformation kinetics of a Nb-microalloyed low-carbon steel.