Formation mechanism of coarse columnar γ grains in as-cast hyperperitectic carbon steels

The formation mechanism of as-cast coarse columnar γ grain (CCG) structure in hyperperitectic carbon steels is investigated by means of a rapid unidirectional solidification method. This method achieves cooling conditions similar to those in the vicinity of a practically continuously cast slab surface. The microstructural observation of the quenched samples indicates that the CCG structure develops from the mold side along the direction of the temperature gradient. In the solidifying samples, fine columnar γ grains (FCG) always exist ahead of the CCG region. Instead of continuous growth into large grains, FCG always shrink and vanish as a result of the growth of CCG initially formed near the mold side. Therefore, the grain size at a fixed point in the ingot discontinuously changes from the FCG to the CCG. The validity of this process is supported by numerical analyses. This finding is in marked contrast to the assumption made in conventional grain growth analysis on the CCG structure.

► We examine the formation process of as-cast coarse columnar γ grains (CCG) in steels.
► Fine columnar γ grains (FCG) exist ahead of the CCG region during the solidification.
► The FCG do not continuously grow into the CCG and they always shrink.
► We find that the CCG develop by the mechanism of the discontinuous grain growth.