Effect of austenite grain size and accelerated cooling start temperature on the transformation behaviors of multi-phase steel

Abstract

The transformation behaviors and microstructures of a low carbon multi-phase steel were investigated by the simulation of deformation-relaxation-accelerated cooling processing, using a Gleeble 3500 thermal-mechanical simulator. A pre-treatment of solid solution at 1200°C was implemented to minimize the influence on transformation from solid solution/precipitation qualities of 0.08%Nb in this steel. On this basis, the effect of austenite grain size and accelerated cooling start temperature were studied individually. The results indicated that the transformation of ferrite in multi-phase steel could be significantly promoted by the refinement of austenite grains and the increase of relaxation time, while the hard phase, such as lath bainite or martensite, could still be obtained with the following accelerated cooling. In contrast, more uniform lower temperature transformed microstructure could form from coarse grain austenite. The potential benefit of austenite grain size on adjusting the proportion of phases in multiphase steel was also discussed.