Annealing behavior of a ferritic stainless steel subjected to large-strain cold working
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Y. Kimura and Y. Mishima Department of Materials Science and Engineering, Tokyo Institute of Technology, 4295 Nagatsuta, Yokohama 226-8502, Japan (Received 26 June 2007; accepted 16 August 2007)
Mechanisms of microstructure evolution during annealing after cold working were studied in an Fe-15%Cr ferritic stainless steel, which was processed by bar rolling/swaging to various total strains ranging from 1.0 to 7.3 at ambient temperature. Two types of recrystallization behavior were observed depending on the cold strain. An ordinary primary (discontinuous) recrystallization developed in the samples processed to conventional strains of 1.0–2.0. On the other hand, rapid recovery at early annealing resulted in ultrafine-grained microstructures in the larger strained samples that continuously coarsened on further annealing. Such annealing behavior was considered as continuous recrystallization. I. INTRODUCTION
The mechanical properties of engineering materials are strongly affected by their microstructures. Desired microstructures in metallic materials can be developed by recrystallization annealing. The widely used phenomenon is primary recrystallization, which takes place during the annealing of cold-worked materials.1–3 The primary recrystallization development follows the partial recovery of deformation microstructures that is discussed as an incubation period of recrystallization. Then, the new grains nucleate and grow out consuming the workhardened structures during the recrystallization process; therefore, this is considered to be a discontinuous mechanism of microstructure evolution. The regularities of primary recrystallization (e.g., recrystallized grain size, texture, effects of cold strain, and temperature) have been investigated for conventional processing methods. Recrystallization commonly develops on heating above a critical temperature and changes deformation textures. The recrystallization kinetics increase with increasing cold strain, leading to finer recrystallized microstructures. Recently, the interest in recrystallization behavior was rekindled for various metals and alloys that were subjected to very large strain deformations.4–17 In the asprocessed state, such materials are characterized by
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0398 3042 J. Mater. Res., Vol. 22, No. 11, Nov 2007 http://journals.cambridge.org Downloaded: 11 Mar 2015
strain-induced submicrocrystalline structures with largely misoriented grains/subgrains. In certain cases, the large strained materials were shown to be essentially stable against any discontinuous grain growth during subsequent annealing. Uniform microstructures with large fractions of high-angle grain boundaries were discussed as a necessary condition leading to gradual (continuous) grain coarsening.5,8,10,18–20 However, structural mechanisms responsible for the development of uniform annealed microstructures in severely deformed materials were not studied in sufficient detail. Despite the lowered d
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