SOLIDIFICATION OF MIDDLE CARBON-STEEL ON THE SMOOTH OR ROUGH MOLD SURFACE

In order to investigate the nucleation phenomena on the surface of mol d wall, two types of surface condition of chill blocks have been immer sed in the molten steel. When the mold A, of which surface is flat, wa s immersed, irregular shell was obtained and very-fine-solidified stru cture which locates in the thicker region was observed on the chill bl ock side of the shell. On the other hand, when the mold B, of which su rface is rough, was immersed, uniform shell was obtained and regular c olumnar dendritic structure was observed. The shape of very-fine-solid ified structure (mold A) is hemisphere. The degree of recalescense for nucleation dose to the flat chill block measured by fine thermocouple was 20K, which is larger than that close to the rough chill block. Th ese results may come from the difference oi nucleation phenomena on th e mold surface. The details are followings. : When the flat chill bloc k is immersed, nucleation on the surface is difficult to occur, theref ore undercooling becomes large, In the same time the undercooled regio n becomes large. The dendrites, growing rapidly in the largely underco oled, are very fine and they correspond to the thicker part of shell. Finally shell with irregular thickness is obtained. When the rough chi ll block is immersed, nucleation is easy to perform, therefore underco oling for nucleation is small and uniform columnar dendritic structure is observed.