Dilatometric Analysis of Phase Fraction during Austenite Decomposition into Banded Microstructure in Low-Carbon Steel
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TRODUCTION
DILATOMETRIC curves of carbon steels exhibit characteristic changes during the phase transformation on thermal cycles due to the difference of atomic volume among the phases involved in the transformation. Many dilatometric analysis models have been proposed to extract the phase fractions from the dilatometric curve, including the lever rule and other analysis methods considering the carbon enrichment in austenite.[1–11] Recently, the authors suggested an analysis model that could further take into account the nonisotropic volume change during the transformation.[12] For the evaluation of the phase fraction, most of the previous dilatometric studies employed a dilatometric curve that was measured along a single specimen orientation, assuming that the dilatometric analysis would yield almost the same results regardless of the measured orientation. Those approaches are reasonable when the specimen has a homogeneous microstructure and thus exhibits similar dilatation behaviors for any orientation. However, it is not clear that they still give reliable results when the material shows distinctly different dilatation behavior depending on the specimen orientation. Microstructural bands, which refer to the alternating layers of ferrite and second phase, are commonly observed in hot-rolled, low-carbon steels. The segregaDONG-WOO SUH, Senior Researcher, CHANG-SEOK OH, Principal Researcher, and SUNG-JOON KIM, Principal Researcher, are with the Department of Advanced Metallic Materials, Korea Institute of Materials Science, Kyungnam, 641-010 Korea. Contact e-mail: [email protected] HEUNG NAM HAN, Assistant Professor, is with the School of Materials Science and Engineering, Seoul National University, Seoul, 151-742 Korea. Manuscript submitted June 15, 2007. Article published online October 27, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
tion of substitutional alloying elements during solidification and the elongation of the segregated region during hot rolling are known to be responsible for their development.[13,14] It was reported that the microstructural bands had a detrimental effect on toughness properties as a result of orientation-dependent impact energy.[15] Recent investigations revealed that the dilatation behavior of low-carbon steels varied with the specimen orientation when the microstructural bands evolved during the transformation.[16–18] It was confirmed that the orientation dependence of dilatometric curves was associated with the microstructural directionality. However, it has been rarely investigated whether the conventional dilatometric analysis could provide reliable phase fractions from the dilatometric curves varying with the measured orientation. The study of Kop et al.[16] is probably the only work that reported the dilatometric analysis while taking into account the orientation dependence of dilatometric curves. They attempted to consider the effect of the microstructural directionality by introducing a correction factor for the strain of each constituent phase, even though their approach ha
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