Role of vanadium in dual-phase steels

Abstract

The effects of varying the annealing temperature and cooling rate on the strength, ductility, and microstructure of a plain carbonsteel, and a similar steel containing 0·06%V, are reported. The transformation of austenite to ferrite during cooling is slower in the vanadium steel than in the plain carbon steel. Any combination of elongation and UTS which can be obtained for the vanadium steel can also be obtained for the plain carbon steel, if this steel is exposed to a sufficiently higher cooling rate than the vanadium steel. An optimum combination of elongation and UTS is reached for an optimum cooling rate. After annealing at 840°C the optimum combination of elongation and UTS is reached for the vanadium steel after forced-air cooling (30 KS⁻¹), and for the plain carbon steel after quenching in brine at 99°C (80 KS⁻¹). These cooling methods produce the same microstructural constituents and the same combination of elongation and UTS in both steels. About 30% of the ‘second phase’ is then retained austenite. The optimum combination of elongation and UTS is improved when the annealing temperature is raised in the ferrite-austenite region.