Mobility analysis of the austenite to ferrite transformation in Nb microalloyed steel by phase field modelling

Abstract

It is well-known that the solute drag effect due to dissolved Nb and the pinning effect due to NbC in Nb microalloyed steel cause retardation of the grain growth during austenitisation and of the transformation during cooling. These effects strongly depend on the austenitisation-temperature. But there is little numerical research on both effects simultaneously so far. In this study the mobility behaviour during austenite (y) to ferrite (a) transformation was investigated by phase field modelling, showing that the aly boundary velocity during cooling increases with increasing austenitisation-temperature. These phenomena are caused by both pinning and solute drag effects. The effect of pinning decreases as the austenitisation-temperature increases because NbC dissolves with austenitisation treatment at high temperature. On the other hand, the strength of the solute drag effect is the highest for intermediate austenitisation-temperature, where the aly boundary velocity during cooling is in the intermediate range, and the strength is determined by the effects of both the concentration of dissolved Nb atoms and the aly boundary velocity. These features are quantitatively discussed from the simulation results. © 2008 ISIJ.