Abstract: Steel with ultrafine grains is produced by altering the transformation from one which normally proceeds with grain boundary nucleation followed by intragranular nucleation at deformation bands and other defects, to one which induces a substantially instantaneous transformation homogeneously over the austenite grain. This is favoured by a reduction or minimisation of grain boundary nucleation, (for example by enlargement of the austenite grain size), prior to or during the transformation. In an embodiment, a partially cooled austenite phase steel is deformed in a single pass at a temperature in the range of 700-950.degree. C. to obtain ferrite grain size of 5 .mu.m or less.