Abnormal grain growth of austenite in a V-Nb microalloyed steel

An investigation was made of the grain-growth kinetics of the V-Nb microalloyed steel austenite phase at temperatures in the range 1173–1373 K, relevant for the understanding of the steel thermomechanical processing. The fine precipitation of vanadium- and niobium-rich particles plays an important role in helping the alloy to maintain a fine grain structure, which was characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. It was found that at temperatures where the fine precipitate particles are stable, the grain-growth process is slow and the mode of grain growth is normal. At higher temperatures where the precipitates are prone to dissolve, the abnormal grain-growth mode dominates. All the observations were comprised in a time-temperature map of the grain-growth modes. The map was built up by employing extensive line-intercept measurements of grain sizes at different temperatures and annealing times, followed by numerical calculations yielding curves of grain-growth rate as a function of grain size at different time intervals. The present experimental observations were found consistent with the results of calculations and predictions of the earlier theoretical work.