Abstract: The hot ductility of SS400, Q235B, Q345B steels produced by CSP process was investigated with a Gleeble 1500 testing machine, the results show that the steels have two low ductility zones. The first zone was founded in the range from T_m to 1310℃ and the second zone in the range from 850 to 725℃. Low ductility and strength in the first zone is due to the presence of liquid films in interdendritic regions which result from sulfur, phosphorus and oxygen segregation between dendrites. The two main reasons for low ductility in the second zone are AIN precipitation and network-shape ferrite formation at grain boundaries. Based on the results, some measures were given to improve the hot ductility of medium carbon steel for preventing slab cracks.