The impact of Nb on dynamic microstructure evolution of an Nb-Ti microalloyed steel

Dynamic recrystallization (DRX) grain size of a Nb-Ti microalloyed steel is investigated at various strain rates and temperatures. Electron microscopy reveals that Nb preferably precipitates on TiN particles at temperature range of T < 1100 °C. As a result, considerable data scatterings are observed for the power law relationship between the normalized DRXed grain size (DDRX/b) and the Zener-Hollomon parameter (Z), the peak strain (εp) and the normalized steady state stress (σss/E). Much better fittings are achieved, however, when distinct analyzes are carried out within the two temperature ranges of T < 1100 °C and T ≥ 1100 °C. It is found that the well-known universal relationship suggested earlier for DDRX/b does not agree with the present results when they are analyzed separately at the two ranges of temperatures. Through this separation, an approach is developed for quantitative analysis of DRX retardation by Nb solute atoms. Moreover, the results show that separate analyses at the two temperature ranges result in almost similar dependencies of DDRX/b, εp and σss/E as a power function of Z with indexes in the range of 0.11-0.15.