The role of nanoscale strain-induced defects in the sharp increase of low-temperature toughness in low-carbon and low-alloy steels

In the paper it is studied how the modes of thermal treatment of low-carbon steels alloyed with manganese (09G2S) and with manganese, vanadium, and niobium (10G2FBYu) affect their hierarchical structure and toughness in the low-temperature region from +20 down to −70 °C. A fine-grained structure, quasi-homogeneous lattice curvature, and nanoscale mesoscopic structural states arising due to radial-shear rolling at 850 °C are shown to be responsible for the formation of a nonequilibrium nanoscale bainitic structure, being a highly effective damping factor in a deformed material. The pearlitic steel structure is equilibrium and is formed within a translation-invariant crystal lattice, while the bainitic structure results from nanoscale mesoscopic structural states in interstices of lattice curvature zones. A spatial change in the lattice curvature is accompanied by a synchronous transformation of the nanoscale mesoscopic structural states and structural geometry of the bainitic phase. That is why the toughness of the steel with bainitic structure is very high down to the temperature −70 °C. Scratch testing is used to estimate the possibility of elastic recovery of bainitic phases in deformed steels.