Formation of ultra-fine ferrite in hot rolled strip: potential mechanisms for grain refinement

A novel single-pass hot strip rolling process has been developed in which u ltra-fine (<2 <mu>m) ferrite grains form at the surface of hot rolled strip in two low carbon steels with average austenite grain sizes above 200 mum. Two experiments were performed on strip that had been re-heated to 1250 de greesC for 300 s and air-cooled to the rolling temperatures. The first invo lved hot rolling a sample of 0.09 wt.%C-1.68Mn-0.22Si-0.27Mo steel (steel A ) at 800 degreesC, which was just above the Ar, of this sample, while the s econd involved hot rolling a sample of 0.11C-1.68Mn-0.22Si steel (steel B) at 675 degreesC, which is just below the Ar, temperature of the sample. Aft er air cooling, the surface regions of strip of both steel A and B consiste d of ultra-fine ferrite grains which had formed within the large austenite grains, while the central regions consisted of a bainitic microstructure. I n the case of steel B. a network of allotriomorphic ferrite delineated the prior-austenite grain boundaries throughout the strip cross-section. Based on results from optical microscopy and scanning/transmission electron micro scopy, as well as bulk X-ray texture analysis and microtextural analysis us ing Electron Back-Scattered Diffraction (EBSD), it is shown that the ultra- fine ferrite most likely forms by a process of rapid intragranular nucleati on during. or immediately after, deformation. This process of inducing intr agranular nucleation of ferrite by deformation is referred to as strain-ind uced transformation. (C) 2001 Elsevier Science B.V. All rights reserved.