The relation between the supercooled solidification macro- and microstructures of castings and cooling rate during solidification obtained by casting simulation was investigated. In the simulation of pure Al castings under the critical solid fraction of 0.99, the formation of significantly high cooling rate zones corresponding to fine crystal grain structures was observed. In the cross-sectional observation of flake graphite cast iron castings and their simulation under critical solid fractions ranging from 0.6 to 0.99, the correspondence between D-type graphite structures and higher cooling rate regions was also observed. In addition, there was a case suggesting that high cooling rate zones under the critical solid fraction of 0.99 may correspond to the generation of inverse chill in spheroidal graphite cast iron castings. The critical solid fraction when the release of most latent heat completes corresponds to the timing when considerable solutes are discharged from the region to the surrounding unsolidified area during non-equilibrium solidification. This suggests that the cooling rate distribution under critical solid fractions near 1 such as 0.99 reflects the ease of constitutional supercooling due to the liquid concentration in the vicinity of the solid-liquid interface, which causes the supercooled solidification macro- and microstructure such as fine crystal grains, the D-type graphite structure, and inverse chill.
