Precipitation and fine structure in medium-carbon vanadium and vanadium/niobium microalloyed steels

Abstract

Hot-rolled and continuously cooled, medium-carbon microalloyed steels containing 0.2 or 0.4 pct C with vanadium (0.15 pct) or vanadium (0.15 pct) plus niobium (0.04 pct) additions were investigated with light and transmission electron microscopy. Energy dispersive spectroscopy in a scanning transmission electron microscope was conducted on precipitates of the 0.4 pct C steel with vanadium and niobium additions. The vanadium steels contained fine interphase precipitates within ferrite, pearlite nodules devoid of interphase precipitates, and fine ferritic transformation twins. The vanadium plus niobium steels contained large Nb-rich precipitates, precipitates which formed in cellular arrays on deformed austenite substructure and contained about equal amounts of niobium and vanadium, and V-rich interphase precipitates. Transformation twins in the ferrite and interphase precipitates in the pearlitic ferrite were not observed in either of the steels containing both microalloying elements. Consistent with the effect of higher C concentrations on driving the microalloying precipitation reactions, substructure precipitation was much more frequently observed in the 0.4C-V-Nb steel than in the 0.2C-V-Nb steel, both in the ferritic and pearlitic regions of the microstructure. Also, superposition of interphase and substructure precipitation was more frequently observed in the high-C-V-Nb steel than in the similar low-C steel.