Up to now,little research has been reported about the bainite
transformation characteristic with an ultrafine prior austenite grain
size .In this work,refining austenite grain size from a coarse one
(53 mm)to an ultrafine one was(3 mm) was designed,and the effect
of austenite grain size on microstructure ,transformation rate and
the mechanical properties of a nanobainite steel were studied.