Heavy gauge steel plates are often hot rolled, at least partially, in the α-γ two-phase region for strength reasons or inevitably due to the temperature gradient across the plate thickness. In any case, the evolution of final properties is very depending on the exact rolling conditions in this intercritical range and a better understanding of intercritical rolling is of increasing importance.
To understand better the material behavior and in particular the microstructural changes when rolling intercritically, systematic trials have been performed in lab using a generic lean CMn-steel. In this work, a single pass at a temperature corresponding to 50% ferrite/austenite was given. The target rolling temperature was determined by dilatometry and torsion testing on beforehand. To perform the intercritical deformation, the plates were reheated to 900°C followed by air cooling to the target temperature where 0, 20, 40, 60 or 80% of deformation was given and followed by air cooling or quenching to ambient temperature.
Depending on the strength of the separate phases, austenite and ferrite grains act differently towards the applied macroscopic deformation. Therefore, light optical microscopy, SEM and in-depth EBSD investigations have been performed to monitor in detail the microstructural changes such as grain size, aspect ratios, texture, Kernel average misorientation etc. Those were correlated with the evolution of mechanical properties, i.e. strength and Charpy impact toughness. Specific attention was paid to the softening mechanisms in each phase as function of the applied deformation and a link was made with the real industrial conditions when intercritical rolling is applied.