| Summary: | 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. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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