The effect of niobium in solid solution on the transformation kinetics of bainite

A study is made of the effect of niobium in solid solution in microalloyed austenite on the kinetics of transformation to bainite upon cooling, using a combination of dilatometry, scanning and transmission electron microscopy. The steels investigated are three laboratory melts with systematic variations in niobium concentration, and an industrial microalloyed grade. Results indicate that increasing niobium content retards the bainite transformation but that, while significantly, this effect is rather small. A much more dramatic effect on bainite formation is produced by allowing a small amount of niobium carbo-nitride precipitation in the austenite before cooling.