The role of niobium in the retardation of the early stage of austenite recovery in hot-deformed steels

The distribution of Nb at an early stage of austenite recovery in hot-deformed HSLA steels was investigated mainly by APFIM. It was confirmed that the onset of austenite recovery is retarded by the solute atoms of microalloying elements randomly dispersed in the austenite matrix. A simple calculation considering the interaction between solute atoms and defects indicates that the different retardation ability for the onset of austenite recovery of Nb, Ti and V is a result of the difference in impurity diffusion coefficients and in solute–defect interactions. AP analysis of a 0.06 at.% Nb steel revealed that not only single solute atoms but also Nb–N and Nb–C pairs exist at the early stage of recovery. These pairs probably play an important role in the retardation of austenite recovery in hot-deformed steels.