Abstract: 220 mm steel blocks were cut at the edge of quenching testing slab of peritectic steel from curving section to straightening section, and then its surface quality and microstructure is anlysized. The result shows that transverse comer cracks appear at straightening section. With the extending of strand from curving section to straightening section,ferrite film at grain boundary of austenite become thicker and clear,and the grain size of austenite is in the range of 1.0 mm to 1.5 mm. Owing to the coarse grain of austenite and thick fragile ferrite film, transverse comer cracks spread along the grain boundary of austenite. With increasing cooling water in single narrow side of mould from original 30 〜32 m³/h to 34 ~36 m³/h,and shut the side sprayer of inner arc of straightening sections, transverse comer cracks slab is effectively controlled.