The Formation of Complex Microstructures After Different Deformation Modes in Advanced High-Strength Steels
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THE development of advanced high strength steels (AHSS) for the automotive industry has been a major theme over the past decade or so. This has led to the introduction of steels with complex microstructures that provide a better balance of strength and ductility. The two main steel types that have been developed to the stage of commercial production are dual phase (DP) and transformation induced plasticity (TRIP) steels.[1,2] The DP steels consist of a microstructure of ferrite and martensite, whereas the TRIP steels contain ferrite, bainite, retained austenite, and martensite. Both the microstructures offer enhanced ductility over their precipitation-hardened ferrite equivalent strength grades. In the case of DP steels, this is due to the reduction in yield strength through the introduction of dislocations into the ferrite as a result of the volume change associated with the formation of martensite.[3] The martensite then provides strengthening at larger strains, essentially through a composite effect. Hence, the combination of low yield strength and high tensile strength provides high levels of work hardening and reduced localized deformation in sheet forming.[4] For the TRIP steels, the situation is more complex. A large component of the increased ductility comes from the TRIP effect associated with deformation-induced transformation of the retained austenite to martensite.[5,6] The bainite and the newly formed martensite then produce a composite hardening at higher strains. Depending on the steel composition and manufacturing routes, there may also be some martensite present in the initial microstructure leading to a combined DP/TRIP effect. ILANA TIMOKHINA, Senior Research Academic, and PETER HODGSON, ARC Laureate Fellow, Alfred Deakin Professor, Director, are with the Institute for Frontier Materials, Deakin University, Geelong, VIC, Australia. Contact e-mail: ilana.timokhina@ deakin.edu.au ELENA PERELOMA, Director, is with UOW Electron Microscopy Center, Engineering Materials Institute, University of Wollongong, Wollongong, NSW, Australia. Manuscript submitted October 2, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
There have been some works to characterize the formability, fatigue, and crash behavior of these steels in the as-received condition; i.e., after processing but before any forming operations.[7–9] However, these steels are cold formed by stamping, or hydroforming, or other processes prior to their incorporation into the final component. Then, this cold working is known to affect also the performance of the steel. For example, in a TRIP steel, testing of the as-received material will involve the effect of the retained austenite, whereas in the formed part, the volume fraction of retained austenite in the same steel will approach zero and be replaced by martensite. In the current work, two key aspects are considered. The first is to understand how the room-temperature stamping of an industrial component affects the dislocation structures at various strain levels. This involved the stamping of a larg
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