Structure of continuously cooled low-carbon vanadium steels

Abstract

The structures of four 0.15 pct carbon steels containing vanadium, nitrogen, and aluminum separately and together were studied systematically, with the help of transmission electron microscopy, by cooling suitable steels at four different rates ranging from 120 °C/min to 3.6 °C/ min from temperatures giving a common austenite grain size of 35 μm. Except for the steel containing only vanadium and that containing only aluminum and nitrogen cooled at the fastest rate used, the observed microstructures were all essentially mixtures of polygonal ferrite and expected amounts for pearlite. For all the steels studied, except the one containing aluminum and nitrogen, it was found that general precipitation was more common than interphase precipitation, although the extent of the latter increased at lower cooling rates. Moreover, in some cases, both general and interphase precipitation were present in the same area. The presence of aluminum was observed to enhance the formation of interphase precipitates at all cooling rates, and the spacing between parallel rows of precipitates increased as the cooling rate was decreased. The dislocation density was high at all cooling rates in all the steels, but it was found to decrease with decreasing cooling rates. Very fine precipitates were found in all the steels, except the steel containing aluminum and nitrogen. At the fast cooling rates, the segregation of vanadium and interstitial elements, which led to locally lower transformation temperatures and higher supersaturations, resulted in clusters of fine particles of vanadium carbonitride, V(C, N). At the slower cooling rates, all the steels showed severe heterogeneity in precipitate morphology which was more pronounced in the steel containing aluminum and nitrogen, while a needlelike morphology of V(C, N) precipitate was occasionally found in steels containing either vanadium and nitrogen or vanadium, nitrogen, and aluminum. As the cooling rate decreased, particle coarsening and growth occurred, causing a reduction in the number of particles/unit area. The coarsening rate of V(C,N) in the presence of aluminum is considerably lower than that of vanadium carbide, VC, or of V(C, N) in the absence of aluminum. Because of the unfavorable precipitation kinetics, any aluminum nitride (A1N) formed during cooling did not nucleate separately but was deposited on the pre-existing A1N particles, thus causing them to be coarsened very rapidly with decreasing cooling rate.