Improved strength of a medium -Mn steel by V addition without sacrificing ductility

Abstract

Here, we investigated the effects of adding a micro-alloying element to medium-Mn steel and explored why a resulting sample containing precipitates exhibited higher strength than one without precipitates, without sacrificing ductility. The model alloys comprised steels of Fe-8Mn-0.2C-3Al-(0, 0.2)V (wt.%); they were cold-rolled and intercritically annealed at identical temperatures between 670 and 730 degrees C for 30 min. These annealed steels exhibited two-phase microstructures consisting of ferrite (alpha) and retained austenite (gamma(R)), with a nanoscale globular morphology. Smaller grain sizes and lower volume fractions of gamma(R) were observed in the V-containing specimen, relative to the V-free specimen, owing to the formation of VC precipitates. The latter were mostly formed in the alpha phase, rather than in the gamma(R) phase. These VC precipitates meant that the V-containing steel showed a lower C concentration in gamma(R) and a higher C concentration in alpha than the V-free steel. We propose that such phase compositions enhanced the strain hardening rate during the later stage of mechanical loading due to more active twinning-induced plasticity and dynamic strain aging. These effects resulted in the observed higher tensile strength in the V-containing steel, without sacrificing ductility.