Effect of strain on formation of ultrafine ferrite in surface of hot rolled microalloyed steel

Abstract

The phenomenon of ultra grain refinement of ferrite in surface layers of hot rolled strip has been studied in a low carbon, niobium microalloyed steel. Wedge specimens were used, to vary the nominal equivalent strain applied during rolling from zero to approximately unity, and the cooling rate after rolling was varied from ~ 20 to 1 K s -1. In contrast with previous work, which contended that a very coarse austenite grain size and a low rolling temperature near the Ar3 were essential to obtain ultrafine ferrite in surface layers, such ultrafine layers were observed after rolling coarse austenite at up to 150 K above the Ar3 and after rolling fine grained austenite near the Ar3. In the case of coarse grained austenite, a critical nominal rolling strain needed to be exceeded to trigger the surface layer phenomenon, upon which cooling rate had little effect on the surface layer's grain size. Refining the prior austenite grain size had the further beneficial effect of refining the grain size at the centre of the rolled product, for example to 2·6 μm, while the surface layer was refined to 0·7 μm.