MECHANISM AND KINETICS OF STRAIN INDUCED PRECIPITATION OF NB(C, N) IN AUSTENITE

A model for strain induced precipitation of Nb(C,N) nucleating on all nodes in the dislocation network is shown to give reasonable agreement with observed particle densities and strengthening at about 5% of equilibrium precipitation. Subsequent growth controlled by volume diffusion, with concurrent coarsening by accelerated pipe diffusion along the dislocation links, accounts for the observed rapid decrease in number of particles per unit volume as precipitation proceeds, with loss of the precipitation strengthening. By pinning the dislocation nodes, particles initially prevent recovery taking place and hence prevent the formation of recrystallisation nuclei. Particle coarsening relaxes these constraints. Multipass finishing, leading to reduced coarsening and repeated nucleation of new particles enhances the precipitation strengthening and the retardation of recrystallisation. The proposed model provides a physical basis for interpreting the influence of the thermomechanical variables in industrial finish rolling.