Abstract: In view of the fact that the HRB400 hot-rolled ribbed steel bars produced by steel mills often break in cold bend operations during winter, the cause of cold bending fracture of HRB400 hot-rolled ribbed bar was analyzed by means of scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS),Thermo-Calc thermodynamic software and optical microscope (OM). The effects of non-metallic inclusion, P content, V/N ratio and metallographic microstructure on fracture of the steel were discussed. The results show that the fracture of the tested steel belongs to brittle fracture, and the cracked large size inclusion is the source of the cracks. The high content of P in the steel increases the ductile-brittle transition temperature and the existence of free nitrogen intensifies the temperature further. Due to the low temperature in winter in northwest China, the working temperature of the steel is likely to be lower than the ductile-brittle transition temperature, which makes the reinforcement in a brittle state. The existence of widmanstatten microstructure increases the brittleness of the steel and provides the possibility for crack propagation. Due to the low temperature in northwest and northeast China, higher requirements should be put forward for the HRB400 hot-rolled ribbed bar used in northwest and northeast China, and new standards should be set for P, S, O and N contents in order to meet the needs of working and using temperature higher than ductile-brittle transformation temperature in winter.

Key words: HRB400 hot-rolled ribbed bar, cold bending fracture, non-metallic inclusion, ductile-brittle transition temperature