In the current study,a 0.3C-2Si-2Mn-0.28Mo (in wt%) steel with high hardenability was deformed at a relatively low temperature followed by isothermal static phase transformation.This novel thermomechanical processing made it possible to successfully produce an ultrafine ferrite grained structure (～2 μm) in the absence of both dynamic phase transformation and controlled cooling.The use of a model Ni-30Fe austenitic alloy showed that the low temperature deformation induced very fine intragranular defects throughout the microstructure,which would then act as fine spaced ferrite nucleation sites at an early stage of phase transformation.As a result,the coarsening of ferrite was extremely limited during isothermal phase transformation,resulting a very fine ferrite grained structure;even nanoscale in the region of the prior austenite grain boundary. In the current study, a 0.3C-2Si-2Mn-0.28Mo (in wt%) steel with high hardenability was deformed at a relatively low temperature followed by isothermal static phase transformation. This novel thermomechanical processing made it possible to successfully produce an ultrafine ferrous grained structure (~ 2 μm) in the absence of both dynamic phase transformation and controlled cooling. use of a model Ni-30 Fe austenitic alloy showed that the low temperature deformation induced very fine intragranular defects throughout the microstructure, which would then act as fine spaced ferrite nucleation sites at an early stage of phase transformation. As a result, the coarsening of ferrite was extremely limited during isothermal phase transformation, resulting a very fine ferrite grained structure; even nanoscale in the region of the prior austenite grain boundary.