Dynamic precipitation and coarsening of niobium carbonitrides during the hot compression of HSLA steels

The size distribution of the static precipitates produced in unstrained austenite was determined at 900° C by the method of extraction replication. The effect of deformation on the rate of coarsening of the particles was established up to a true strain of 0.75. Size distributions were also measured in materials containing static precipitates which were undergoing dynamic precipitation at 900°C. The mean size in these materials decreased while dynamic precipitation was occurring and then increased due to dynamic coarsening. The rate of increase duringdynamic ripening was two to three orders of magnitude higher than the rate ofstatic Ostwald ripening reported in the literature. The effect of particle size on the initiation of dynamic recrystallization was also evaluated. The results confirm the general rule that fine particles (d ≃ 6 nm) are more effective than coarser ones (e.g. d ≃ 20 nm) with respect to the retardation of dynamic recrystallization. The necessary fine particles can be nucleated homogeneously in the alpha phase or heterogeneously on gamma phase dislocations. This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metallurgy Committee of The Metallurgical Society of AIME.