Influence of dynamic recovery and dynamic recrystallization on coarsening of Nb(CN) in Nb microalloyed steel

The influence of deformation on the rate of coarsening of Nb(CN) was studied in 0·018%Nb steel by carbon extraction replication. Size distributions were determined in samples containing static precipitates which were also undergoing dynamic precipitation at 925°C. The mean particle radius r̄ decreased while dynamic precipitation was taking place and then increased due to dynamic coarsening. It was found that the dynamic coarsening rate parameter K′ increased with increasing mean particle radius r̄_o (r̄_o is the mean radius after dynamic precipitation is complete and at which particle coarsening begins). In terms of the static model for particle growth, the apparent dependence of K′ on r̄_osuggests that the particle/matrix interfacial energy increases with r̄_o The particle size distributions were also measured after increasing amounts of strain preceded by the completion of static precipitation at 900, 925, and 975°C. At 900°C, for example, the mean radius of static precipitates was 23·5 nm, which was doubled during deformation to a strain of 0·6. By contrast, at 975°C, although the initial mean radius was 30 nm, straining to 0·60 only increased the particle size by ∼33%. The observation that the lower the temperature of straining, the greater the rate of particle growth is explained in terms of a pipe diffusion coarsening model, in which the higher density of dislocations and the larger residence time of individual dislocations in contact with particles at the lower temperature are principally responsible for the higher rate of coarsening. It was also observed that the rate of coarsening at a given temperature is significantly higher during the early stages of straining (when dynamic recovery is the predominant softening mechanism) than at higher strains (when softening is produced principally by dynamic recrystallization). This difference is also ascribed to the differences in dislocation density in these two regions and to the effect of the substructure on the rate of diffusion.