Multi-phase microstructure design of a low-alloy TRIP-assisted steel through a combined computational and experimental methodology
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The multiphase constitution of a transformation-induced plasticity (TRIP)-assisted steel with a nominal composition of Fe-1.5Mn-1.5Si-0.3C (wt.%) was designed, utilizing a combination of computational methods and experimental validation, in order to achieve significant improvements in both strength and ductility. In this study, it was hypothesized that a microstructure with maximized ferrite and retained austenite volume fractions would optimize the strain hardening and ductility of multiphase TRIP-assisted steels. Computational thermodynamics and kinetics calculations were used to develop a predictive methodology to determine the processing parameters in order to reach maximum possible ferrite and retained austenite fractions during conventional two-stage heat treatment, i.e. intercritical annealing followed by bainitic isothermal transformation. Experiments were utilized to validate and refine the design methodology. Equal channel angular pressing was employed at a high temperature (950 C) on the as-cast ingots as the initial processing step in order to form a homogenized microstructure with uniform grain/phase size. Using the predicted heat treatment parameters, a multiphase microstructure including ferrite, bainite, martensite and retained austenite was successfully obtained. The resulting material demonstrated a significant improvement in the true ultimate tensile strength (1300 MPa) with good uniform elongation (23%), as compared to conventional TRIP steels. This provided a mechanical property combination that has not been exhibited before by low-alloy first-generation high-strength steels. The developed computational framework for the selection of heat treatment parameters can also be utilized for other TRIP-assisted steels and help design new microstructures for advanced high-strength steels, minimizing the need for cumbersome experimental optimization. 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
author list (cited authors)
Zhu, R., Li, S., Karaman, I., Arroyave, R., Niendorf, T., & Maier, H. J.
complete list of authors
Zhu, R||Li, S||Karaman, I||Arroyave, R||Niendorf, T||Maier, HJ