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Study of the kinetics of precipitation, dissolution and coarsening of aluminum nitride in low carbon steels Cheng, Leon Man Lung

Abstract

Owing to its significant effect on grain growth and recrystallization, both of which directly influence hardenability, ductility, texture formation and the mechanical properties of steels, the precipitation behavior of aluminum nitride (A1N) in steels is becoming of increasing practical importance. To improve the current understanding of the mechanisms and kinetics of precipitation, dissolution, growth and coarsening of A1N during the thermomechanical processing of low carbon steels, a comprehensive experimental and theoretical study has been carried out. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) have been used, with the aid of the carbon extraction replica technique, to relate the observed morphologies, spatial distribution and particle size distribution of A1N precipitates in a low carbon, Al-killed steel to their thermal history. Experimental studies have been conducted on the precipitation, dissolution, growth and coarsening of A1N precipitates during isothermal annealing, reheating or continuous cooling of the steel at 650°C to 1350°C. The influence of prestraining on the kinetics of isothermal precipitation of A1N has also been studied using the Gleeble 1500 thermomechanical simulator and the stress relaxation technique. Generalized, theoretically based models have been developed in the present project to predict the kinetics of precipitation, dissolution, growth and coarsening of nitrides or carbides in low carbon or microalloyed steels undergoing thermomechanical treatments. The precipitation model takes into account the equilibrium thermodynamic properties of the systems, the precipitate morphology, the misfit strain energy (coherent or incoherent) and the interfacial energy. The precipitation kinetics is formulated using classical nucleation and growth theories. Both homogeneous and heterogeneous nucleation are considered. In the present work, the model is applied to study the precipitation behavior of A1N in low carbon, Al-killed steels during isothermal annealing and continuous cooling in both the austenite and ferrite regions. A theoretical model has also been developed to predict the dissolution, growth and coarsening behaviors and to simulate the size distribution evolution of nitrides and carbides in steels undergoing various heat treatment by using numerical integration methods on a multi-particle system. In this model, dissolution and coarsening are being treated as one continuous, simultaneous process. A set of diffusion, equilibrium and mass balance equations are solved simultaneously for each individual particle. The local equilibrium at the interface, curvature effects and diffusion along grain boundaries are taken into account. These models provide a physical basis for interpreting the influence of the thermomechanical variables on the stabilization of second phase particles in industrial hot strip mills. Theoretical predictions are in good agreement with the experimental measurements. These results lend support to several aspects of the models and to the assumptions incorporated in the models.

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