Mechanical properties of an ultrafine grained C-Mn steel processed by warm deformation and annealing

The mechanical properties of an ultrafine grained 0.2%C-Mn steel, processed by large strain warm deformation and subsequent annealing, have been investigated. The microstructure consists of spheroidized cementite particles in an ultrafine ferrite matrix (average grain diameter ≈1.3 μm). The steel shows an improved combination of strength and toughness when compared with corresponding coarse grained specimens. The reasonable ductility of the steel can be attributed to the finely dispersed cementite particles, which also effectively increase the work hardening rate by the accumulation of geometrically necessary dislocations in their vicinity. The lower shelf energy is significantly higher and the ductile-to-brittle transition temperature is lower in the ultrafine grained steel than in comparable coarse grained specimens. This may be due to the joint effect of grain refinement and delamination in the ultrafine grained steel processed by large strain deformation. The delaminations lead to a decrease in triaxiality of the stress state in the impact test samples. The upper shelf energy is slightly reduced in the ultrafine grained steel, which can be attributed to the effect of delamination.