Different roles of reversed austenite, athermal martensite and tempered martensite on low-temperature toughness in ultra-low carbon medium Mn steel

The -40 degrees C impact energy of 0.02C-7Mn steel strongly depended on the grain boundary (GB) wetting transition. No a/a GBs were wetted when the tempering temperature (TT) was blow Ac1, but Mn segregation embrittled the GBs resulting in the intergranular fracture mode. Ductile fracture mode was formed by transformation induced plasticity (TRIP) effect of RA after tempered between 580 degrees C and 650 degrees C, where RA was the dominant second solid phase. As the dominant second solid phases, athermal a-martensite and athermal e-martensite damaged toughness to some extent after tempered at 675 degrees C and 700 degrees C. The portion of completely wetting a/a GBs reached about 80% at 700 degrees C, and athermal a-martensite had lower toughness loss than that was induced by athermal e-martensite at 675 degrees C. (c) 2021 Elsevier B.V. All rights reserved.