Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels

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Formation processes of as-cast γ grain structures during casting of hyperperitectic carbon steels with 0.15-0.45 mass% carbon concentrations have been studied by means of a rapid unidirectional solidification technique. In steels with 0.15-0.41 mass% carbon concentrations, coarse columnar γ grains (CCG) with a minor axis diameter of 1-3 mm developed along the direction of temperature gradient. In a steel with 0.38 mass% carbon, importantly, columnar γ grains (CG) whose minor axis diameter is less than 500 μm form before the formation of CCG and the grain structure discontinuously changes from CG to CCG. The fraction of CG region increases with increase in the carbon concentration. In the samples with the carbon concentration higher than 0.43 mass%, the as-cast structure consists of CG over almost the entire ingots. Analyses of a relation between γ grain and dendrite structures and their crystallographic orientations indicate that the formation of CG originates from the primary solidification of γ phase instead of δ phase. This is supported by numerical analysis of the dendrite growths.