Effect of volume fraction of bainite on strain hardening behavior and deformation mechanism of F/B multi-phase steel

Five ferrite/bainite (F/B) multi-phase experimental steels with different volume fraction of bainite were processed by thermo-mechanical controlled processing (TMCP). The strain hardening behavior and deformation mechanism of F/B multi-phase steel were studied by modified C-J analysis, Hollomon analysis and numerical simulation method. The study indicates that the deformation mechanism in stage I and stage II are directly related to elastic-plastic deformation of ferrite and uniform strain (deformation) of ferrite and bainite, respectively. As the volume fraction of bainite increases, the deformation mechanism of ferrite and bainite changes from incoordinate deformation to coordinate deformation, which will cause the strain difference between ferrite and bainite being lessened gradually, as well as might be the reason for the transition from three-stage to two-stage in modified C-J analysis. The deformation mechanism was verified experimentally. Moreover, the effect of volume fraction of bainite on yield stress/tensile stress ratio (yield ratio), strain hardening exponent and uniform elongation were explained by the division and strain hardening capability of the stages in modified C-J analysis.