The Effect of Solute Nb on the Austenite-to-Ferrite Transformation

Niobium is a widely used micro-alloying element in steels that can retard the austenite-to-ferrite transformation primarily by solute drag when Nb remains in solution. It is critical to develop quantitative models to predict the effect of Nb on the transformation kinetics. In the present work, dedicated continuous cooling transformation (CCT) studies were performed for a low-carbon steel microalloyed with 0.047 wt pct Nb starting from fully recrystallized austenite states with the same grain size but different amounts of Nb in solution. The austenite-to-ferrite transformation kinetics is described from a fundamental perspective by assuming a mixed-mode reaction including solute drag of Nb. Using the solute drag model of Fazeli and Militzer, the intrinsic interface mobility, trans-interface diffusivity of Nb, and its binding energy to the interface have been determined from the CCT data. The interfacial parameters are critically analyzed and compared with independent measurements of diffusion and grain boundary segregation.