In 0.1 mass% carbon steels with phosphorus contents ranging from 0.01 to 0.2 mass%, slabs were continuously cast in a thickness of 100 mm. Cast slabs in high phosphorus steels have a fine columnar-g-grain structure, in which the mean width of columnar g grain is approximately half of that in cast slab without phosphorus addition. The structure change is brought by the local stabilization of bcc at the phosphorus-segregated spot, which has a lowered d / g phase transformation temperature. Hence, in the high phosphorus slabs, it is believed that the dispersed d phase can pin the g grain growth more effectively in the d + g region when the completion of d / g transformation is suppressed by the phosphorus addition as well as by the segregation.