Nanobainite steels display an excellent combination of outstanding strength and ductility.However,the
transformation rate and impact toughness of nanobainite still need to be improved. In this work,the
effect of refining austenite grain size to an ultrafine one on bainite transformation and its mechanical
properties were investigated using a steel with composition of 0.8C-1.5Si-2.0Mn-1.0Cr-0.24Mo-1.0Al-
1.6Co .The results show that refining austenite grain from 53 mm to 3 mm can accelerate the bainite
transformation significantly,but almost half amount of the bainite is no longer typical nanostructured
bainite, and plenty of undesirable blocky austenite which is harmul to the toughness of the steel remains
at the same time. When grain is in medium dimension(18 mm), the best impact toughness of 51J/cm2
occurs with corresponding strength of 2034 MPa andductility of 14%.