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Embrittlement behaviors were studied in a cast steel that bears a relatively large proportion of alloying elements (Mn, Cu, Nb, Ni) at intermediate temperatures. A low-carbon cast steel was heated to 1673 K (1,400 °C) of 10 K/s, then cooled to 673–873 K of 10 K/s. Tensile tests were performed at the target temperature. Reduction of area decreased abruptly; it was smallest at 823 K (550 °C), then increased gradually as the tensile testing temperature decreased. At 823 K, the fracture surface showed intergranular fracture with small craters. Further study on the fracture behavior determined that upper bainite structure existing cementite at the prior austenite grain boundaries is crucial for the embrittlement. Segregated phosphorus triggers initial micro-crack generation at the grain boundary cementite/matrix interface. We suggest that intergranular fracture of a low-carbon steel at intermediate temperature occurs by formation of cementite forms at prior austenite grain boundaries, then active segregation of P at the GB cementite/matrix interface; microcracks then form starting at this interface; S segregates at the resulting free surfaces and thereby nucleates formation of micro-cracks, which grow and coalesce until a sufficient number of them link, then fracture occurs. To fabricate a crack-free slab, its temperature should be controlled to avoid the upper bainite temperature range at the unbending zone where stress is applied during continuous casting.