抄録
In order to investigate the nucleation phenomena on the surface of mold wall, two types of surface condition of chill blocks have been immersed in the molten steel. When the mold A, of which surface is flat, was immersed, irregular shell was obtained and very-fine-solidified structure which locates in the thicker region was observed on the chill block side of the shell. On the other hand, when the mold B, of which surface is rough, was immersed, uniform shell was obtained and regular columnar dendritic structure was observed.
The shape of very-fine-solidified structure (mold A) is hemisphere. The degree of recalescense for nucleation close to the flat chill block measured by fine thermocouple was 20K, which is larger than that close to the rough chill block.
These results may come from the difference of nucleation phenomena on the mold surface. The details are followings.:
When the flat chill block is immersed, nucleation on the surface is difficult to occur, therefore undercooling becomes large. In the same time the undercooled region becomes large. The dendrites, growing rapidly in the largely undercooled, are very fine and they correspond to the thicker part of shell. Finally shell with irregular thickness is obtained. When the rough chill block is immersed, nucleation is easy to perform, therefore undercooling for nucleation is small and uniform columnar dendritic structure is observed.
