Effect of cold deformation and multiphase treatment conditions an low-carbon, low-silicon multiphase steel

This paper presents multiphase (MP) treatments of a low-C, low-Si cold rolled steel. Despite the much lower content of Si compared to a typical TRIP steel, up to about 8 pct of retained austenite (gamma(r)) with 1.2% carbon content can be obtained. Increasing prior cold deformation (i.e. decrease of parent austenite grain size) accelerates the transformation to bainite resulting in a decrease of the volume fraction of residual austenite (gamma(r) + martensite). Tensile strength of MP steel intercritically annealed at high temperature increases with higher cold reduction degree due to the smaller grain size of the present phases. On the contrary, the ductility and strength-ductility balance deteriorate because the banded structure becomes more pronounced and the gamma(r) volume fraction diminishes. Decreasing intercritical annealing temperature results in an increasing gamma(r) fraction and a uniform distribution of second phases. Hence, the ductility and strength-ductility balance are improved. Crystallographic preferred orientation is evident in the ferrite and martensite and its extent increases with higher cold deformation.