A metallurgical interpretation of the static recrystallization kinetics of an intercritically deformed C-Mn steel
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27/4/04
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A Metallurgical Interpretation of the Static Recrystallization Kinetics of an Intercritically Deformed C-Mn Steel HAIWEN LUO, JILT SIETSMA, and SYBRAND VAN DER ZWAAG The austenite recrystallization kinetics in the intercritical region of a C-Mn steel is investigated by means of stress relaxation tests. It is found that the Avrami exponent, n, decreases significantly with decreasing temperature, i.e., with increasing ferrite fraction. This behavior deviates from that of austenite recrystallization in the purely austenitic state, in which case the Avrami exponent is constant and independent of temperature. To resolve the origin of the changing Avrami exponent, the influence of the austenite/ferrite interface boundary area and that of the spatial variation of the plastic strain in the austenite grains is modeled quantitatively. The modeling results seem to indicate that the strain heterogeneity rather than the reduced interface boundary length is responsible for the decreasing Avrami exponent with decreasing temperature.
I.
INTRODUCTION
MODERN high-quality steel strip production requires a detailed understanding of the softening kinetics during the interpass cycles in hot strip rolling to ensure accurate thickness and microstructure control. Hence, in the past, numerous studies[1–5] have been devoted to the softening kinetics after deformation of fully austenitic microstructures as a function of temperature, strain, and strain rate. From these studies, it emerges that the recrystallization kinetics can be described by the Johnson–Mehl–Avrami–Kolmagorov (JMAK) model, which is one of the most frequently used classical models for the analysis of the kinetics of recrystallization and phase transformations.[6] Furthermore, it turns out that the Avrami exponent for austenite recrystallization is close to 3 or 4 as JMAK theory predicts only in some lightly deformed fined-grained texture-free materials of a uniform grain size, most often varies between 1 and 2 for the various C-Mn steel grades, but shows no or only a slight dependence on temperature or deformation conditions. In contrast, much less detailed knowledge exists on the softening kinetics during intercritical rolling, which is a relatively new rolling procedure applied in direct strip rolling operations or as a variant of classical thin hot-strip rolling. In intercritical rolling, the microstructure to be deformed consists of a mixture of ferrite and austenite. The starting structure therefore differs from that in austenitic rolling in that it now contains austenite-ferrite interface boundaries rather than just austenite grain boundaries. These austenite-ferrite grain boundaries may play a different role in nucleation of recrystallization than the austenitic grain boundaries, since strain gradients can be expected near the interface boundary.[7,8] The dual-phase microstructure of the soft ferrite grains and the
HAIWEN LUO, Postdoctoral Researcher, Netherlands Institute for Metals Research, 2628 AL Delft, The Netherlands, is with the D
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