Influence of Phosphorus on Solidification Structure in Continuously Cast 0.1 mass% Carbon Steel.

In 0.1 mass% carbon steels with phosphorus contents ranging from 0.01 to 0.2 mass%, slabs were continuously cast to a thickness of 100 mm with a laboratory scale caster. Their macro- and micro-structures were characterized, focusing on the effects of phosphorus addition on the structural evolution during solidi-fication and subsequent cooling. Cast slabs of high phosphorus steels have a fine columnar-γ-grain structure. The mean width of the columnar grain was approximately half of that in the cast slabs without the phosphorus addition. Dispersed globular α grains were observed in the α grain structure of high phosphorus steels. The globular grains evolved at the phosphorus-rich spots in the prior-γ grain. The micro-segregation of phosphorus results in these structural evolutions. Since the phosphorus enrichment stabilizes bcc (δ or α) phase locally at the inter-dendritic region, the phosphorus-rich spot makes δ phase retained to lower temperature for the δ/γ transformation, and provides a predominant site for the γ/α transformation. In the high phosphorus casts, therefore, the dispersed δ phase are thought to pin the γ grain growth more effectively in the δ+γ region when the completion of the δ/γ transformation is remarkably suppressed by the phosphorus segregation.