Evolution of Columnar γ Grain in Low Carbon Steel and Its Refinement by Additional Elements in Thin Slab Casting Simulator

The evolution of as-cast γ grain in low carbon steel has been investigated using thin slab cast simulator developed in NIMS (National Institute for Materials Science). Fine columnar γ grain evolved from the surface to the center of the ingot was obtained, and second dendrite arm spacing and γ grain size of the ingot along with the distance from the surface showed a good agreement with those of commercially produced 50 mm thick slab by a thin slab caster. Thus, we succeeded to simulate the macro-structure and micro-structure of the thin slab produced by practical thin slab continuous casting process. The γ grain size for the present cooling rate of approximately 4 K/s showed a relatively good agreement with the estimated value by the previously predicted line of dγ=1.52T−0.5 for low carbon steels based on the Classical Grain Growth Model (CGGM).Phosphorus, one of the typical impurities in steel, well known as the strong b.c.c. stabilizer, was added to low carbon steel aiming at the refinement of γ grain by the suppression of its growth. The γ grain size was drastically reduced to about a half of normally cast one even with a little addition of phosphorus lower than 0.05 mass%. It was probably caused by the retardation of γ phase birth by the phosphorus segregation in the inter-dendritic region and the retardation of completion of δ/γ transformation which leads to pinning of the γ grain growth.Moreover, tin, one of the typical impurities in steel scrap, also known as a b.c.c. stabilizer, was tried for addition. In spite of weaker b.c.c. stabilizing ability than phosphorus judging from the shape of iron binary phase diagram, a compatible γ grain refining effect was observed with addition of 0.1 mass% tin. According to these results, b.c.c. stabilizing elements such as phosphorus and tin are found quite useful to micro-structure control during solidification and cooling process.