Microstructure and mechanical properties of C-Si-Mn(-Nb) TRIP steels aftersimulated thermomechanical processing

Continuous and discontinuous cooling tests were performed using a quench de formation dilatometer to develop a comprehensive understanding of the struc tural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanica l processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainit e transformation temperatures for cooling rates from 10 to 90 K s(-1). The influence of niobium on the transformation kinetics was also investigated, Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the fina l microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, whic h influences the mechanical properties of TRIP steels. The optimum mechanic al properties were obtained after isothermal holding at 400 degreesC in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.