International Journal of Meterials Sciences

Present study concerns the correlation of volume fraction (V_M) and morphology of martensite with the mechanical properties of dual-phase (DP) steel in the high martensite regime (V_M > 0.25). A low-carbon microalloyed steel was subjected to three different heat treatment schedules namely, intermediate quenching (I_Q), step quenching (S_Q) and intercritical annealing (I_A) with an aim to vary the morphology of the constituent phases; while the V_M was also varied from 0.22 to 0.74 by changing the intercritical annealing temperature. The hardness, tensile and impact properties as well as morphologies of fracture surfaces of these steels were studied and compared. Among the different microstructures, the I_Q treatment forms very fine and fibrous martensite uniformly distributed in the ferrite matrix, while the S_Q treatment yields blocky and banded ferrite and martensite and the I_A treatment evolves martensite inlands distributed in the grain boundaries and triple points of polygonal ferrite. In the high martensitic region, I_Q sample has yielded the best combination of strength and ductility. Moreover, the I_Q sample also exhibited much higher impact toughness value compared to S_Q and I_A samples. A carefully conducted comparison of structure-property relationship of present DP microstructures indicates that the morphology of martensite offered by I_Q treatment is more conducive for the development of high strength DP steels capable to offer adequate formability and toughness by increasing the V_M.