Formability of Ultrafine-Grained Interstitial-Free Steels
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THE fast-growing micro-technology applications require the implementation of smaller products utilizing punching and forming technology. Therefore, miniaturization of parts and plastic-forming applications are gaining attention for applications on a small scale.[1] The coarse grain sizes of the materials to be used in scaled down plastic deformation applications yield a series of problems due to the size effects including loss of ductility and strength, higher frictional forces, variability in product dimensions, and free surface roughening.[2] Ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) with grain sizes in the range of 100 to 1000 nm have received considerable attention due to their higher strength combined with an adequate ductility compared to their coarse-grained (CG) counterparts.[3–13] Among the available SPD methods, equalchannel angular extrusion (ECAE) is considered to be the most capable one to control the resulting ONUR SARAY, Student, and GENCAGA PURCEK, Professor, are with the Department of Mechanical Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey. Contact e-mail: [email protected], [email protected] IBRAHIM KARAMAN, Professor, is with the Department of Materials Science and Engineering, Texas A&M University, College Station, TX. HANS J. MAIER, Professor formerly with the Institut fu¨r Werkstoffkunde, University of Paderborn, 33095 Paderborn, Germany, is now with the Institut fu¨r Werkstoffkunde, Leibniz Universita¨t Hannover, 30823 Garbsen, Germany. Manuscript submitted August 2, 2012. Article published online May 11, 2013 4194—VOLUME 44A, SEPTEMBER 2013
microstructural features of the UFG materials like grain size, grain morphology, and grain boundary character.[14,15] UFG materials have been identified as promising candidates for micro-forming processes due to their submicron-sized structures.[1,2] However, there is lack of knowledge regarding post-SPD processing of shaping of UFG metals, which limits the practical application of UFG metals. A limited number of studies have addressed the promising performance of UFG materials for micro-forming applications. UFG aluminum demonstrated more uniform flow during microextrusion[16] and lower surface roughness after microbulge tests.[17] Furthermore, UFG copper produced by ECAE yielded a higher punch load and slightly lower limiting drawing ratio.[18] Also, the notable decrease in tensile ductility with increase in strength after ECAE was less pronounced for biaxial ductility.[18] Interstitial-free steels (IF-steels) contain very low amounts of interstitials like C, N, and O in their ferritic microstructure.[19] The decreased interstitial content results in excellent formability and high normal anisotropy, which provide benefits for sheet metal forming.[20,21] However, the low strength of conventional IFsteels due to the lack of interstitials is a drawback for many applications. Given their mono-phase ferritic microstructure, useful strengthening methods are grain refinement via plastic deformation and
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