Ferrite grain coarsening during transformation of thermomechanically processed C–Mn–Nb austenite

The ferrite grain size of low carbon steel is known to be refined by hot rolling in the austenite phase field at temperatures too low for recrystallisation to occur. The strain thus retained in the austenite increases ferrite nucleation density and in current models of grain refinement it is assumed that each nucleus becomes a grain in the fully transformed microstructure. In this paper it is shown that, in a heavily deformed C–Mn–Nb austenite, ferrite grains impinged, then coarsened during the initial stages of transformation during continuous cooling. The final ferrite grain size was not established until 35% of transformation had occurred. It is suggested, firstly, that ferrite grain refinement due to controlled rolling cannot be modelled simply from observed increases in nucleation density and, secondly, that deformation of austenite has considerably greater potential for grain refinement than is commonly observed, provided that coarsening of the ferrite during transformation can be limited. © 1992 The Institute of Materials.