Structure-properties relationship of ultra-fine grained V-microalloyed dual phase steels

The effect of vanadium microalloying on ultra-high strength dual phase (DP) ferrite-martensite steel microstructure and properties was studied. It was found that the addition of 0.14wt% V to a Fe-0.18C-1.5Mn-0.3Si-0.008N reference alloy introduced very significant ferrite grain size refinement in the cold rolled and annealed state. During continuous annealing the initial ferrite to austenite transformation kinetics were strongly retarded, however under slow cooling both pearlite and bainite transformations were suppressed indicating increased hardenability. After cold rolling and intercritical annealing at 750⁰C intense V(C,N) precipitates (mean radius 3.7nm) were observed in the ferrite phase whereas precipitates were scarce in martensite (austenite) and much larger (mean radius 6.7nm). Significant gains in YS, UTS and work hardening rate were observed at low martensite fractions due to a combination of selective precipitation strengthening and grain refinement of ferrite. However, at higher martensite fractions (> 45%) the YS, UTS and work hardening rate became lower than the reference, primarily due to softening of the martensite. The latter was attributed to the fixing of solute carbon by V(C,N). The net increase in tensile strength with martensite content of the vanadium alloy was ~ 4MPa/%α’ compared to ~ 16MPa/%α’ for the reference alloy. A recently developed size-sensitive mean field structure-properties model was extended to capture these microalloying effects. At iso-tensile strength both the fracture strain and hole expansion behaviour of the new microalloyed steel showed improved performance over the reference.