Comprehensive Analysis of the Effect of Ausforming on the Martensite Start Temperature in a Fe-C-Mn-Si Medium-Carbon High-Strength Bainite Steel

The comprehensive effect of strain and ausforming temperature on the martensite start temperature (M_S) of a medium-carbon bainite steel was investigated by thermal simulation, optical microscope, scanning electron microscope, etc. It is already known that small strain increases the M_S, while larger strain decreases the M_S. However, the effect of ausforming temperature on the M_S has not been reported and clarified. In this study, the concepts of critical strain (ε_c) and saturated strain (ε_s) are proposed. The M_S at the critical strain is equal to the M_S of the nondeformed specimen. The saturation strain, which is first observed, is the strain value, and the M_S does not further decrease with the increasing strain. The results show that the M_S depends on the strain amount of ausforming but is not affected by the ausforming temperature. Moreover, with the increase of strain amount and ausforming temperature, the length of the martensite laths decreases. In addition, the hardness of the specimen increases with the increase of the ausforming strain amount, whereas the ausforming temperature has little effect on the hardness.