Precipitate morphology in MnS-AlN-inhibited high permeability grain oriented silicon steel

A transmission electron microscope study of precipitation in an MnS-AlN-inhibited 3% silicon steel is described. The effects of various hot rolled band heat treatments are related to precipitate morphologies developed by subsequent cold rolling and recrystallization in a decarburizing anneal. The hot rolled band heat treatments consisted of solutioning at 1120°C, air cooling to selected temperatures and quenching. Quenching at temperatures higher than about 650°C resulted in a uniformly distributed fine structure (approx. 100Å) imaged by strain contrast and believed to be vacancy discs stabilized by carbon diffusion. The vacancy discs did not occur in hot rolled bands quenched from temperatures below 650°C. Cold rolled and recrystallized structures derived from high quench temperatures contained clusters of very fine precipitate particles. The clusters consisted mostly of spheroidal MnS particles with some angular AlN particles. The tendency for particle clustering decreased with decreasing quench temperature, and non-uniform coarsening of the MnS particles occurred both within clusters and from cluster to cluster. Quenching from 540°C and below resulted in essentially random particle distribution and in the more frequent occurrence of angular AlN particles. The absence of particle clusters in cold rolled and recrystallized structures corresponded approximately with the disappearance of vacancy discs as quench temperature was decreased in the original heat treatments.