Properties of Ultra-fine Grained V-microalloyed Dual Phase Steels

The effect of vanadium microalloying on the microstructure and properties of a high strength dual phase steel was studied.Compared to the Fe-0.186C-1.5Mn-0.3Si-0.008 N reference alloy it was found that the addition of 0.14 %wt.V resulted in:①A very significant ferrite grain size refinement in the cold rolled and annealed state.②During continuous annealing the initial ferrite to austenite transformation kinetics was strongly retarded.③On slow cooling the ferrite start transformation temperature was slightly increased but the pearlite and bainite transformations were suppressed,resulting in a net overall increase in hardenability.④Undissolved cementite was observed at intercritical annealing temperatures ≤740 ℃.⑤After annealing at 750 ℃/180 s intense V（C,N） precipitation（mean diameter 7.4 nm） was observed in the ferrite phase whereas precipitates were scarce in martensite （austenite） and much larger （mean diameter 13.4 nm）.⑥The increase in tensile strength with martensite fraction of the reference alloy was ~16 MPa/%. However,the vanadium alloy showed a much lower sensitivity to martensite content（~4MPa/%） due to grain refinement and selective strengthening of the ferrite phase,and actually became softer than the reference steel at martensite fractions ＞ 45%.At iso-tensile strength the fracture strain and hole expansion coefficient of the V alloy showed some improvement over the reference steel,however the gains were much smaller than those obtained by tempering.