The strength of interstitial free steels is increased by up to 100% and the ductile to brittle transition temperature is decreased by up to 100.degree. C. by warm finish rolling in the single phase ferrite region below A.sub.rl to effect ferrite dynamic recrystallization of the steel microstructure to a ferrite structure of grain size having a grain size of up to 5 .mu.m, and especially an ultra fine grain size of 1 to 2 .mu.m; the method may be employed in various hot working methods including strip and rod mills, planetary hot rolling and extrusion.