On cells and microbands formed in an interstitial-free steel during cold rolling at low to medium reductions
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INTERSTITIAL-FREE (IF) steels have become important materials in the automotive industry due to their good press-shop performance. What is required is a high value of work-hardening rate, because this prevents diffuse necking, which leads ultimately to fracture, as well as reasonable ductility and favorable textures, because these determine deep drawability. Clearly, the desired properties in the final annealed sheet, especially the texture, are derived from recrystallization of the deformation microstructure. Hence, in modern manufacturing, microstructural design through computer-based modeling is increasingly important.[1] To achieve this, it is crucial that the rolling microstructures are fully characterized. However, little quantitative knowledge is available regarding the deformation microstructures of bcc materials,[2] and because of this, the present authors have carried out a crystallographic investigation on the evolution of rolling microstructure at low reduction levels in an IF steel, using transmission electron microscopy. The most important result of the first part of the investigation[3] concerns microbands that are lenticular in shape with a habit plane running approximately parallel to {110} planes. Two sets of dislocations comprise the microband walls: one is dominant and has its Burgers vector lying in the microband’s habit plane. The secondary set is much less dense and its slip plane is not coplanar with the microband habit plane. The crystallography and deformation geometry of rolling reveal that the habit plane has the largest Schmid factor, which produces intense slip in the form of dislocation sheets involving cross-slip of screw dislocations. A mechanism involving double cross-slip to form the microbands from this sheet is proposed. This second article, on the evolution of microstructure during rolling of IF steel, addresses the issue of dislocation density and its role in microstructure development, something that is traditionally measured by X-ray diffraction and more Q.Z. CHEN, Research Assistant Professor, and B.J. DUGGAN, Professor of Materials Science and Engineering, are with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong. Contact e-mail: [email protected] Manuscript submitted November 26, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
recently by neutron radiation.[4] Although these methods provide dislocation densities of regions of known orientations, they do not provide information on how the dislocations are stored in such physical organizations as dislocation cell boundaries and microband walls, which are of significance for modeling work hardening.[1] Transmission electron microscopy is widely used where the resolution required is at micron levels, and previous work[5] has shown that it can provide a representative statistical measurement in a local region where the distribution of dislocations is fairly uniform. Therefore, one objective of the present work is to quantitatively characterize rolling microstructure in an IF steel
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