ABNORMAL GRAIN-GROWTH OF AUSTENITE IN A V-NB MICROALLOYED STEEL

An investigation was made of the grain-growth kinetics of the V-Nb mic roalloyed steel austenite phase at temperatures in the range 1173-1373 K, relevant for the understanding of the steel thermomechanical proce ssing. The fine precipitation of vanadium-and niobium-rich particles p lays an important role in helping the alloy to maintain a fine grain s tructure, which was characterized using optical microscopy, scanning e lectron 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 norma l. 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 gr ain sizes at different temperatures and annealing times, followed by n umerical calculations yielding curves of grain-growth rate as a functi on of grain size at different time intervals. The present experimental observations were found consistent with the results of calculations a nd predictions of the earlier theoretical work.