On the “Pattern” Occuring in the Outer Part of the Continuously Cast Al-Si Killed Steel Blooms

In the circum ferential part of continuously cast blooms, there is a zone of more intensive colouration in macro-etching than parts of the cross-section. This zone is called the “pattern”. In order to make clear the formation mechanism of the “pattern”, the casting structure and distribution of sulfide inclusions are investigated. The results obtained are as follows: (1) The “pattern” is obseved more clearly in Al-Si killed steel than in Si killed steel. (2) In the “pattern”, sulfide structures are very fine and the ratio of interdendritic area is higher than other parts. (3) In the “pattern”, sulfide inclusions precipitated at the interdendritic area are very fine and numerous. (4) When the upward-streaming type immersion nozzle is used, the solidification rate in the lower part of the mold in the bloom caster is higher than that in the spray zone. It is concluded that the “pattern” of the continuously cast bloom is formed in the rapid solidification zone in the mold. (5) The reason why Al-Si killed steel exhibits the “pattern” more clearly is explained by the role of Al2O3 as nucleation sites for solidification. (6) Any difference in mechanical properties between the “pattern” and other parts is not observed.