Improved mechanical properties of V-microalloyed dual phase steel by enhancing martensite deformability

A good combination of ultimate tensile strength (UTS) up to 1365 MPa and total strain to failure (StF) to 15.5 % has been achieved due to deformable martensite in the invented vanadium-microalloyed dual-phase (DP) steel, which was manufactured by two-stage annealing of cold rolled steel strip. The employed extensive characterizations revealed that the ductile martensitic phase in this DP steel differentiated from ordinarily low-carbon martensitic lath in both morphology and lattice structure. Complex coherent orientation relationships between ferrite, reverse austenite, martensitic phase and vanadium carbide (VC) do exist, leading to a new martensitic transformation mechanism and resultant dual-phase microstructure. Besides, a detailed characterization including essential phase transformation analysis in combination with in situ TEM observation, shows that, all the essential processing including recrystallization, reverse austenitic and martensitic transformation, in debt to the particular effects of VC, can be recognized as phase transformations with higher thermodynamic driving force and higher kinetic energy barrier as compared to previously common processing, which actually changes the microstructure and, indirectly leads to higher strength and higher ductility. This synergy of thermodynamics and kinetics can be generalized to improve mechanical properties of present steels.