The production and mechanical properties of ultrafine ferrite

A new thermomechanical process has been developed to produce ultrafine (1 mu m) equiaxed ferrite grains in hot rolled steel strip. This process is remarkably simple and is applicable to a wide range of steel chemistries, including low and high carbon and microalloyed steels. Strips are reheated to produce a coarse austenite grain size, then rolled in a single pass at or just above the austenite to ferrite transformation temperature. It is suggested that the observed refinement is due to strain induced transformation from austenite to ferrite. The requirements for this appear to be high strain induced by shear in the strip surface layers, and thermal gradients created by heavy quenching of the strip surface by the work rolls. The yield strength was markedly higher than conventionally processed strip, although there was little work hardening even though total elongation of over 20% was achieved.