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The kinetics of austenite formation during continuous heating of a multi-phase steel Cho, Yeong-Bong


DP (Dual-Phase) and TRIP (Transformation Induced Plasticity) steels, also known as multiphase steels, have been attracting a growing interest in the development of lighter automobiles owing to their excellent combination of strength and ductility. TRIP steels can usually be generated from a standard cold-rolled ferrite-pearlite grade by a two stage continuous annealing process; continuous heating and intercritical annealing, followed by subsequent cooling and austempering. The material is first intercritically annealed in the ferrite/ austenite coexistence region, during which the ferrite matrix recrystallizes and austenite is created. Austempering is then performed and some upper bainite is formed, which, in turn, stabilizes the remaining austenite even down to room temperature. Most studies have been focused so far on the second stage of the thermal scheme due to the fact that the steel properties depend primarily on the transformation processes following austenitization. However, the phase transformations occurring upon heating is of profound importance. The state of the microstructure after heating; i.e., volume fraction, shape, distribution and chemical composition of the austenite grains, has a great influence on the kinetics of the phase transformation during cooling and on the subsequent mechanical properties of the steel. Furthermore, the kinetics of the reverse transformation to austenite determine the time and temperature required for either intercritical heat treatment or normalization. The objective of the present study is to characterize and understand the reaustenitization kinetics from pearlite-ferrite structure during continuous heating. Futher, a mathematical model based on the Avrami equation and the additivity principle has been adopted in modeling the pearlite-ferrite to austenite transformation during continuous heating.

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