The microstructure evolution of medium manganese steel (Fe-5Mn-2Si-0.1C (wt%)) during thermo-mechanical processing in ferrite + austenite two-phase region was investigated by in situ neutron diffraction analysis and microstructure observations. When the specimens were isothermally held at a temperature of 700 °C, the fraction of reversely transformed austenite increased gradually with an increase in the isothermal holding time. However, it did not reach the equilibrium fraction of austenite even after isothermal holding for 10 ks. On the other hand, the fraction of reversely transformed austenite increased rapidly after the compressive deformation at a strain rate of 1 s−1 at 700 °C and reached the equilibrium state during subsequent isothermal holding for around 3 ks. Moreover, microstructure observations suggested that the austenite, which was reversely transformed during isothermal holding at 700 °C, exhibited film-like shape and existed between pre-existing martensite laths. In contrast, when the compressive deformation was applied during isothermal holding at 700 °C, most of the reversely transformed austenite had globular shapes with grain sizes less than 1 μm.
Medium manganese steel Neutron diffraction Austenite reverse transformation Thermo-mechanical processing
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The neutron experiment at the Materials and Life Science Experimental Facility of the J-PARC was performed under a user program (Proposal No. 2016E0003, 2017E0001, 2017A0129). This study was financially supported by the Elements Strategy Initiative for Structural Materials (ESISM) through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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