Structural and phase transformations in nanostructured 0.1% C-Mn-V-Ti steel during cold deformation by high pressure torsion and subsequent heating

The structural and phase transformations which take place in low-carbon 0.1% C-Mn-V-Ti steel during deformation by high pressure torsion (HPT) and subsequent heating have been studied using transmission electron microscopy and X-ray structural analysis methods. Whatever the initial state, be it ferritic-pearlitic or martensitic (obtained by quenching from 950°C and 1180°C), HPT at room temperature leads to the formation of a nanosized oriented grain-subgrain structure. The average size of the elements of the grain-subgrain structure in the 0.1% C-Mn-V-Ti steel after severe plastic deformation is larger in the initially ferritic-pearlitic state (95 nm) than in the initially martensitic states (65 and 50 nm after quenching from 950°C and 1180°C, respectively). It is shown that quenching from 950°C and 1180°C causes the formation of the martensite of different fineness, providing, in turn, nanostructures of different dimensions and different levels of the strength properties after HPT. Quenching from 1180°C is conducive to a higher thermal stability of the HPT-induced nanocrystalline structure of the 0.1% C-Mn-V-Ti steel on account of the larger amount of vanadium carbide precipitates. It is found that HPT leads to a considerable increase in the microhardness of the 0.1% C-Mn-V-Ti steel as compared to its microhardness in the initial state (more than 3 times as high in the ferritic-pearlitic state and more than 2 times as high the initially martensitic states).