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 (epsilon(c)) and saturated strain (epsilon(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.