High performance low cost steels with ultrafine grained and multi-phased microstructure

Ultrafine grained ferrite was obtained through tempering cold rolled martensite with an average grain size of 200–400 nm in a low carbon and a microalloyed steel. Thermal and mechanical stability of the two steels was studied. Due to the pinning effect of microalloyed precipitates on the movement of dislocations and grain boundaries, the recrystallization and grain growth rate were retarded, and the thermal stability of ultrafine grained microstructure was improved. The ultrafine grained ferritic steel was strengthened, but its strain hardening rate was reduced. It seems that the tiny carbide precipitates have no significant effect on work hardening rate. The ultrafine grained ferrite+martensite dual phase microstructure was obtained in the microalloyed steel through intercritically annealing cold rolled martensite. The resulting multiphase microstructure has a tensile strength higher than 1.0 GPa with a yield ratio lower than 0.7. Another type of multiphase microstructure with nanoscaled lath bainite+retained austenite was obtained through an isothermal heat treatment in low temperature bainite transformation region in high carbon steel. The tensile strength was as high as 1.64 GPa with a yield ratio of 0.84. Supported by the National Natural Science Fundation of China (Grant No. 50634030)