Abstract: To find out the origin of 20Mn2 axle crack defects during rolling round pipes to square pipes,the formation mechanism and influencing factors of the crack defects were studied by optical microscopy,scanning electron microscopy,and O/N automatic analyzer.It is shown that cracks all appear at the corners of square pipes,where the mass fraction of nitrogen is more than 70×10^-6.Upper bainite is found in the microstructure of cracked specimens and there are non-metal inclusions at the cracks.The crack defects can be effectively decreased through reducing the nitrogen content of liquid steel,enhancing the cooling speed of quenching,reducing inclusions in the steel.