Abstract: Axial cracking were found on the surface of some 40MnBH steel bars. Inclusion testing, blue brittleness testing and EPMA analysis were applied to analyse the cause of the cracks. The results showed that the cracks were caused by the protective slag entrapment of continuous casting. The high continuous casting speed led to the turbulent flow of the hot melt in the mould, resulting in the entrapment of the protective slag that has absorbed lots of deoxidation products, such as Al2O3 and SiO2.