Journal
ACTA MATERIALIA
Volume 60, Issue 6-7, Pages 3022-3033Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.actamat.2012.02.007
Keywords
TRIP-assisted steels; Computational thermodynamics; Equal channel angular pressing (ECAP); Phase transformations; Mechanical behavior
Funding
- US National Science Foundation, Division of Civil, Mechanical, and Manufacturing Innovation [0900187]
- Div Of Civil, Mechanical, & Manufact Inn
- Directorate For Engineering [0900187] Funding Source: National Science Foundation
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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 degrees 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 (similar to 1300 MPa) with good uniform elongation (similar to 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. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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