Transmission Electron Microscopy Analysis of Yielding in Ultrafine-Grained Medium Mn Transformation-Induced Plasticity S
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THE development of advanced high strength steel (AHSS) grades with property combinations required for modern automotive body designs is driven by the need for vehicle mass reduction, fuel efficiency, and low greenhouse gas emissions, while improving passenger safety.[1–4] The first-generation AHSS automotive grades, such as dual-phase, transformation-induced plasticity and complex-phase steels, had multi-phase microstructures with varying volume fractions of the low temperature austenite decomposition products including bainite, martensite, and carbon-enriched austenite. Secondgeneration AHSS include high Mn fully austenitic steels, such as twinning-induced plasticity (TWIP) steel, which exhibit combinations of high strength with superior ductility.[5–7] However, room temperature stabilization of the fully austenitic microstructure of TWIP steels requires high contents of Mn, Al, and C. In addition, several processing and application issues need to be addressed before large-scale industrial production and use of TWIP steel in automotive manufacturing are considered. There is therefore considerable interest in the BRUNO C. DE COOMAN, Professor, and SEAWOONG LEE, Graduate Student, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, South Korea. Contact e-mail: [email protected] PAUL GIBBS, Graduate Student, and DAVID K. MATLOCK, Professor, are with the Advanced Steel Products and Processing Center, Colorado School of Mines, Golden, CO. Manuscript submitted September 11, 2012. Article published online February 9, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
development of third-generation medium Mn multiphase steel grades containing 5 to 10 wt pct Mn with large volume fractions of meta-stable retained austenite. The medium Mn steels have an ultrafine-grained (UFG) microstructure[8–11] and routinely achieve combinations of ultimate tensile stress (UTS) and total elongation (TE) superior to those of the first-generation AHSS grades. UFG microstructures are achieved by martensite reversion during an intercritical annealing heat treatment after cold rolling. Steels with a sub-micron grain size typically achieve very high strengths, but they have very limited strain hardening and, as a consequence, low elongations. The UFG medium Mn TRIP steels containing 5 to 10 wt pct Mn have superior mechanical properties due to the occurrence of a deformation-induced martensitic transformation. The size of the UFG retained austenite islands and the partitioning of Mn and C to austenite during intercritical annealing are the two main contributions to the austenite stability, and mechanical stabilization of the austenite does not contribute to the austenite stability due to the very low dislocation density of the austenite grains.[12–15] The processing path-dependent tensile properties and work hardening behavior of UFG medium Mn TRIP steel are not yet fully understood. Localization of plastic flow is often observed during the deformation of medium Mn TRIP steel after interc
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