Introduction of dendrite fragmentation to microstructure calculation by cellular automaton method

Control of solidification microstructure is necessary because the microstructure affects material characters. In order to predict solidification microstructure, effect of fragmentation was indirectly introduced to the cellular automaton method using V_crit, which is threshold of molten steel flow. Calculations were carried out with various V_crit (200 μms^-1 ≦ V_cri ≦ 1 mms^-1) and the results were compared with the results of casting experiment using high carbon steel. Observed specimens were extracted from upper part and lower part of casting. Equiaxed grains and branched columnar grains were observed in the microstructure of upper specimen and just columnar grains were observed in the micro-structure of lower specimen. Calculation results with V_crit which is greater than 400 μms^-1 show good agreement with microstructure of both observed specimens qualitatively. The microstructures were calculated because flow velocity of molten steel around upper specimen was much greater than flow velocity around lower specimen. There is a possibility that solute transportation which induces the fragmentation occurs even if velocity of molten steel flow was in the order of 10^-4 ms^-1.