Effect of chromium and molybdenum addition on the matching mechanism of strength-ductility-toughness of low carbonmedium manganese steel

Low carbon medium manganese steel with chromium and molybdenum addition (4MnCM), alongside a control group without these additions (4Mn), was developed. The quenching (860 degrees C) and tempering (620 degrees C) heat treatment process was applied to both steel groups. The influence of Cr and Mo on the microstructure evolution and mechanical properties was examined. The findings revealed that, the low-temperature impact energy values (KV2) of 4MnCM at -60 and -84 degrees C were enhanced by 104 and 107 J, in comparison to 4Mn. The yield and tensile strength saw an increase of 100 MPa. The higher low-temperature toughness of 4MnCM is attributed, on one hand, to the addition of Cr and Mo, which inhibiting deformation twinning through enhances the stacking fault energy (SFE) of reversed austenite. On the other hand, it is due to the absence of coarse epsilon-martensite (epsilon- m) along the crack propagation path. Additionally, the phase transformation of reversed austenite occurs at the crack nucleation stage during the impact process of both steels, which helps alleviate stress concentration and increases the energy required for crack formation. The primary reason for the higher strength of 4MnCM lies in the significant enhancement of precipitation strengthening effects through the precipitation of M6C, M23C6 and Nb/TiC co-precipitation with M6C.