The essential role of niobium in high manganese austenitic steel for application in liquefied natural gas tanks

Five high manganese austenitic steels with different niobium additions were designed, and the essential role of niobium in high manganese austenitic steel has been elucidated by means of electron back-scatter diffraction, transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy. Regardless of niobium content, NbC carbides can be formed during hot rolling in the temperature range of 1100-970 degrees C, whereas these NbC carbides are at 100-nm scale. Furthermore, the dynamic strain aging can be effectively restrained once the niobium is added. It is found that the niobium addition of 0.05 wt% greatly lowers tensile ductility and deteriorates cryogenic impact toughness, whereas both tensile ductility and cryogenic impact toughness are slightly sacrificed with further increasing niobium content. In addition, it is found that the grain refinement effect of niobium in high manganese austenitic steels is rather weak. Moreover, there is a steep increase in yield strength only when the niobium content reaches 0.29 wt%, and the sharp increase in yield strength is due to the formation of heterogeneous microstructure consisting of recrystallized or strong recovered grains and dislocation tangled or lath structured grains, indicating that the essential role of niobium in high manganese austenitic steels is that it suppresses recovery and recrystallization, and its solid-solution strengthening, grain refinement, and precipitation hardening effects are relatively weak.