Nucleation sites for ultrafine ferrite produced by deformation of austenite during single-pass strip rolling
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SINCE reducing the grain size of a material, particularly in the range below 10 m in grain diameter, has the potential dual benefits of increasing both the strength and the toughness, the refinement of ferrite grain size has remained a key goal of research into the thermomechanical processing of steels. In recent years, it has been shown that one of the most promising approaches to ferrite grain refinement involves increasing the density of active nucleation sites within austenite grains and inducing nucleation of ferrite at those sites during thermomechanical processing.[1] It has been demonstrated, for example, that it is possible to produce ultrafine (⬍1 to 2 m) ferrite grains in the surface regions of rolled strip in a wide range of steel compositions,[2] by using a novel approach to processing. Key elements in this approach include an initially large austenite grain size (typically 200 m), a high level of friction between the strip surface and the rolls to generate a high level of effective strain that incorporates substantial shear and compressive components, and a strategic choice of temperature for the deformation stage.[2,3] It is presumed[4] that active features within the substructure of the austenite P.J. HURLEY, formerly PhD student, Department of Materials Engineering, Monash University, is Research Associate, Manchester Materials Science Centre, UMIST, and the University of Manchester, Manchester, M1 7HS, United Kingdom. B.C. MUDDLE, Professor and Head of Department, is with the Department of Materials Engineering, Monash University, Melbourne, 3800 Australia. P.D. HODGSON, Head of Department, is with the School of Engineering and Technology, Deakin University, Geelong, Victoria, Australia 3217. Manuscript submitted August 31, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
developed during rolling provide a high density of sites for intragranular nucleation of ferrite and that it is critical that transformation proceed promptly after creation of these sites and before significant recovery can occur. It has, thus, been suggested that the ultrafine ferrite results effectively from a strain-induced transformation of austenite to intragranular ferrite.[4] However, the sites of intragranular nucleation of ferrite have yet to be identified. The present work has, thus, involved detailed microstructural characterization of a low-carbon steel strip rolled using this novel approach and a comparison with the deformation substructure of an austenitic Ni-30 wt pct Fe alloy strip processed in similar fashion. The latter alloy has a stackingfault energy similar to that of low-carbon steel at temperatures where the material is austenitic and has been shown to behave similarly to low-carbon steel during hot working.[5] Using reflected light and transmission electron microscopy (TEM), the substructure of the austenite has been examined to ascertain those features within the substructure most likely to provide potential sites for intragranular ferrite nucleation. II. EXPERIMENTAL METHODS The two alloys use
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