Role of Nb on Microstructural Evolution during Intercritical Annealing in Low-carbon Steels

We investigated the microstructural evolution during intercritical annealing in Nb-added low-carbon steels, focusing on the synergistic effects of the addition of Nb and the ferrite (alpha) to austenite (gamma) phase transformation on the recrystallization behavior of alpha. Two kinds of specimens, containing 0.02 and 0.05 mass% Nb, were prepared and annealed at the intercritical temperature (750 degrees C) for a long time. The progress of recovery and alpha recrystallization was retarded by increasing the amount of Nb addition during intercritical annealing. Moreover, the progress of alpha recrystallization during intercritical annealing was mainly attributed to continuous recrystallization due to subgrain growth. The fraction of gamma that formed during intercritical annealing increased owing to Nb addition, but it did not increase with increasing amount of Nb addition. These results suggest that the progress of recovery and alpha recrystallization was retarded during intercritical annealing by the addition of Nb, thereby causing the increase in gamma fraction. Furthermore, the increase in gamma fraction led to further retardation of alpha recrystallization and a refinement with the addition of Nb.