5th International High Manganese Steel Conference - HMNS 2022

The strength of ausformed martensite is higher than that of as-quenched martensite. This is believed to be because the dislocation introduced by ausforming is inherited after quenching, and the dislocation density of ausformed martensite is higher than that of as-quenched martensite. However, there is little studies about the detailed relationship between the mechanical properties and microstructure (dislocation density, grain size, carbides, etc.). In this study, the microstructure of ausformed martensite was investigated in medium Mn steel with and without C (Fe-5Mn-0.1C and Fe-5Mn-0.02C alloys, respectively), and then, the strengthening mechanism of ausformed martensite was discussed. In this study, a medium Mn steel was prepared because the diffusional transformation does not occur during ausforming due to its excellent hardenability. In Fe-5Mn-0.02C alloy, the block size decreases by ausforming, while the hardness and dislocation density hardly varied by ausforming, which means that the hardness of ausformed martensite does not depend on the block size. On the other hand, in Fe-5Mn-0.1C alloy, the hardness and dislocation density increase with increasing the thickness reduction during ausforming. The carbides did not be observed with transmission electron microscope. These results suggest that the hardening by ausforming is mainly due to the increasing of the dislocation density enhanced by C. The effect of C on the dislocation introduction in ausformed martensite was discussed in terms of the interaction between C atoms and dislocations.