Abstract: It is found that the transverse corner cracks occurring on the medium cabon hypo-peritectic steel CC slabs produced by BAOSHAN STEEL are type of inter-γ granular fractures occurring mainly at temperature below 850℃. Hot ductility measurement of three medium carbon steels indicates that, under the strain rate of 4×10^-3/s, there exist two fow ductility temperature regions for the tested steels within the temperture range from steel melting points to 700℃, i.e. Region I from the melting points to 1350℃ and Region Ⅲ from 850℃ to 750℃. In Reginn Ⅲ, the ductility loss is mainly resulted from the precipitations of nitrudes of Al, Nb, etc. at the austenite boundaries in single γ phase tempeniture range and the formation of the film-like proeutectoid ferrites along the γgrain boundaries in γ+α two-phase temperature range. By taking measures of precisely controlling steel N, Al concentrations and adopting mild secondary cooling pattem to raise the slab unbending temperature out off the low ductility Region Ⅲ, the occurrence of the transverse corner cracks on medium carbon hypoperiteetic steel CC slabs has been significantly reduced.