Nucleation sites for ultrafine ferrite produced by deformation of austenite during single-pass strip rolling

An austenitic Ni-30 wt pet Fe alloy, with a stacking-fault energy and defor mation characteristics similar to those of austenitic low-carbon steel at e levated temperatures, has been used to examine the defect substructure with in austenite deformed by single-pass strip rolling and to identify those fe atures most likely to provide sites for intragranular nucleation of ultrafi ne ferrite in steels. Samples of this alloy and a 0.095 wt pet C-1.58Mn-0.2 2Si-0.27Mo steel have been hot rolled and cooled under similar conditions, and the resulting microstructures were compared using transmission electron microscopy (TEM), electron diffraction, and X-ray diffraction. Following a single rolling pass of similar to 40 pet reduction of a 2 mm strip at 800 degreesC, three microstructural zones were identified throughout its thickn ess. The surface zone (of 0.1 to 0.4 mm in depth) within the steel comprise d a uniform microstructure of ultrafine ferrite, while the equivalent zone of a Ni-30Fe alloy contained a network of dislocation cells, with an averag e diameter of 0.5 to 1.0 mum. The scale and distribution and, thus, nucleat ion density of the ferrite grains formed in the steel were consistent with the formation of individual ferrite nuclei on cell boundaries within the au stenite, In the transition zone, 0.3 to 0.5 mm below the surface of the ste el strip, discrete polygonal ferrite grains were observed to form in parall el, and closely spaced "rafts" traversing individual grains of austenite. B ased on observations of the equivalent zone of the rolled Ni-30Fe alloy, th e ferrite distribution could be correlated with planar defects in the form of intragranular microshear bands formed within the deformed austenite duri ng rolling. Within the central zone of the steel strip, a bainitic microstr ucture, typical of that observed after conventional hot rolling of this ste el, was observed following air cooling. In this region of the rolled Ni-30F e alloy, a network of microbands was observed, typical of material deformed under plane-strain conditions.