Influence of the prior austenite microstructure on the transformation products obtained for C-Mn-Nb steels after continuous cooling

In the present work, the ferrite grain sizes after transformation from non-deformed (recrystallized) and work-hardened austenite have been considered. Two C-Mn-Nb steels have been deformed by multipass torsion tests carried out at temperatures, both above and below the determined T(nr) (non recrystallization temperatures) for these steels. The influence of the prior austenite grain size, the retained strain and the cooling rate on the final ferrite microstructure has been analysed. Austenite grain sizes variying from 28 to 125 mu m with retained strains ranging from 0 (non-deformed austenite) to 2 and cooling rates of I and 5 s(-1) have been considered. It has been observed that higher cooling rates tend to produce acicular microstructures, and that this effect is enhanced at the lower retained strains. It has been shown that the reduction of austenite grain size and an increase of retained strain, provide a higher density of nucleation sites, which leads to a refinement of the ferrite. A similar effect is produced by increasing the cooling rate from 1 to 5 s(-1).