Study of the kinetics of the recrystallization of cold-rolled low-carbon steel
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TRODUCTION
IT is known that metals that have been work hardened due to deformation will soften when annealed at a suitable temperature. The main process responsible for the softening is recrystallization, which involves the generation of a new grain structure of the metal. Both effects are of major technological importance for the thermomechanical processing of metallic materials. In order to predict the course of the transformation for any given heat, it is necessary to establish the kinetics of the recrystallization process.[1,2,3] Thermal analysis methods such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC) have been widely employed to determine the evolution of the transformed fraction with time (isothermal experiments) and temperature (constantheating-rate experiments). However, these techniques have not been applied extensively to the study of recrystallization in steel. Due to the good time resolution of the calorimetric signals, it is possible to register the course of the reaction under defined experimental conditions. The process kinetics based on these data (from isothermic measurements) is usually interpreted in terms of the classic kinetic relation, developed independently by Johnson and Mehl, Avrami, and Kolmogorov (JMAK).[4,5] Unfortunately, limitations of the model frequently are overlooked. Recrystallization in many cold-worked materials does not yield linear JMAK behavior, and, in many cases, the corresponding exponents (n) fall lower than predicted.[6,7] If growth takes place isotropically in three dimensions and all nuclei are present at time zero (i.e., the transformation is “site saturated”), the value of the exponent is predicted to be 3. If three-dimensional growth occurs but nuclei appear at a constant rate (“continuous nucleation”), the value of the exponent is predicted to be 4. Lower values of n are predicted for growth in less than three dimensions. In many experimental studies, however, low ´ NCHEZ, ProfesC.E. RODRI´GUEZ TORRES, Researcher, and F.H. SA ´ LEZ, Professor, Facultad de sor, Departamento de Fı´sica, and A. GONZA Ingeniera, are with the Universidad Nacional de La Plata, 1900 La Plata, and CONICET, Buenos Aires, Argentina. F. ACTIS and R. HERRERA, Researchers, are with SIDERAR, San Nicola´s, Buenos Aires, Argentina. Manuscript submitted February 27, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
values of the exponent have been found without convincing evidence for growth in less than three dimensions. This problem was reviewed by Rollett et al.,[8] who pointed out that the discrepancy may be caused by nonrandom distributions of the recrystallized nuclei or by a tendency of the growth rate to decrease as recrystallization proceeds. Both of these effects can be expected to arise as a result of nonuniform distribution of stored energy in the cold-worked matrix, due, for example, to the effects of grain orientation on dislocation density. While isothermal experimental analysis techniques are, in most cases, adequate for kinetics analysis, non
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