Study on Nucleation and Growth of Columnar Grains in Non-Oriented Steel by Phase Transformation

The nucleation and growth of {100}-textured and "toothed" columnar grains in Fe-0.5% Mn non-oriented electrical steels were analyzed by considering the factors like cooling rate and flowing rate of the hydrogen through phase transformation. The results demonstrate that the formation of {100}-textured and "toothed" columnar grains with high grain orientation spread (GOS) values mainly involved the nucleation of {100}-oriented nuclei with high GOS values on the surface region in the incipient stage of the gamma (austenite) -> alpha (ferrite) transformation and the growth of them caused by both directed growth along normal direction (ND) and faceted growth on the rolling surface (RS) during solid-state transformation. The morphology, size and structure characteristics of columnar grains are explained in terms of the competitions between the nucleation rate and the rate of directed growth along ND or faceted growth on RS of ferrite nuclei. When the nucleation rate of ferrite nuclei is equal to their directed growth rate along ND during the gamma -> alpha transformation, {100}-textured and "toothed" columnar grains with high GOS values are obtained. With a higher nucleation rate of ferrite nuclei than the directed growth rate of {100}-oriented nuclei along ND, three-layer sandwiched structure is formed, including few {100}-textured and "toothed" columnar grain layers with high GOS values on the surface, whereas large numbers of {111}-textured equiaxed grains with low GOS values in the inner layer. In addition, when the faceted growth on the RS of {100}-oriented nuclei is dominant, heavy and coarse "toothed" columnar grains with high GOS values are obtained.