Ultrafine ferrite grains produced by tempering cold-rolled martensite in low carbon and microalloyed steels

Ultrafine ferrite grains as small as a few hundred nano meters were obtained, without severe plastic deformation, by tempering cold-rolled martensite in a low carbon and a microalloyed steel. A multilevel subdivision mechanism responsible for the formation of ultrafine ferrite grains in cold-rolled martensite was discussed. It involves subdividing firstly a prior austenite grain into several martensite packets by phase transformation and then further subdividing the martensite structure into ultrafine cell blocks by plastic deformation. The relatively large misorientation between the ultrafine cell blocks achieved at a moderate strain level in martensite may be attributed to the interaction between the transformation introduced and the deformation introduced dislocations. Ultrafine ferrite grains were developed from the cell blocks during tempering at the temperature range from 500 to 600 degrees C for 60 min. It was also demonstrated that the microalloying precipitates can effectively pin down the movement of dislocations and grain boundaries and thus, can increase the thermo stability of the ultrafine grained microstructure.