Abstract
The thermal simulation experiments with different deformation amounts and cooling rates of low carbon Nb-microalloyed steel were performed using Gleeble 1500 thermomechanical simulator. The observations by OM and TEM reveal that when undeformed austenite continuously cooled at 10 to 30°C/s, bainitic ferrite nucleated mainly at austenite grain boundaries, grown up like parallel lath into the bulk from the grain boundaries of austenite, and original austenite grain boundaries could be seen. Compared with the undeformed austenite, when austenite deformed by 40% at 880°C and continuously cooled at 10 to 30°C/s, the austenite grain boundaries were occupied by polygonal ferrite because deformation can increase the deformation energy of austenite grain. Therefore, bainitic ferrite only nucleated in the grain of austenite, and original austenite grain boundaries can not be seen.