On the nature of the bimodal grain size distribution after intercritical deformation of a carbon–manganese steel

Laboratory rolling trials have been performed to investigate the development of microstructure during and after intercritical rolling. The finishing temperature was varied over a wide range, and samples were taken after quenching, following the last pass and after air cooling and coiling, following the last pass. In the study, light optical microscopy, electron microscopy and orientation imaging mapping (OIM) techniques were used to reveal the recrystallization and recovery behavior after finishing in the austenitic–ferritic two-phase region. The large grains that were observed in the bimodal grain size distribution after intercritical rolling are the result of the combination of transformation of deformed austenite into undeformed ferrite (transformation-induced (TI) nucleation of ferrite) and rapid ferrite grain growth into the deformed ferrite grains. The apparent small grains in the bimodal grain size distribution are subgrains and, hence, the result of extensive recovery of deformed ferrite.