Dual phase structure formed by partial reversion of cold-deformed martensite

Effect of prior deformation and heating rate on the dual phase (DP) structure formed by partial reversion of cold-rolled martensite was investigated in a low carbon steel (0.15%C–1.0%Mn). The steel plate was quenched after austenitization to obtain full martensitic structure and then cold-rolled with varying reductions. The cold-rolled specimens were continuously heated at a slow (0.083 K/s) or fast (100 K/s) heating rate up to a temperature above A1 point to partially form reversed austenite. Increasing rolling reduction rate or lowering heating rate enhanced recrystallization on heating before the onset of reversion, while the undeformed martensite never caused recrystallization irrespective of heating rate. The matrix of DP structure was changed from tempered martensite to equiaxed ferrite through the recrystallization, which resulted in a large difference in the distribution of fresh martensite (reversed austenite). Tensile testing revealed that the excellent strength-elongation balance was obtained in the DP steel produced from undeformed martensite, while higher strength was realized in the steel with prior deformation. With increasing the rolling reduction and the heating rates, the grain size of recrystallized ferrite becomes finer and the tensile strength is more increased. It was also suggested that the competition between recrystallization and reversion during continuous heating could be predicted by the modified tempering parameter.