Effect of austenite grain size on transformation of nanobainite and its mechanical properties

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 µm to 3 µm 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 harmful to the toughness of the steel remains at the same time. When grain is in medium dimension (18 µm), the best impact toughness of 51 J/cm2 occurs with corresponding strength of 2034 MPa and ductility of 14%.