Abstract: A 550 MPa grade high strength and high ductility steel for automotive square tubes was developed. The microstructure and mechanical properties of hot rolled steel strip and plane, fillet and weld of square tube were studied. The results show that the microstructure of hot rolled steel strip was composed of uniform and fine ferrite and a small amount of pearlite, and a large number of (Nb,Ti)C phases with size of 1-110 nm were precipitated in the matrix. The microstructures of the square tube plane and fillet were basically the same as that of the hot rolled steel. The dislocation density in the steel increased after tube forming, dislocation plugging occurred at the grain boundary and the second phase, and dislocation walls and cells were formed at the fillet. Work hardening occurred in the steel after tube forming. The yield strength, tensile strength and yield ratio of the square tube plane, fillet and weld were increased in sequence, while the elongation decreased sequentially, and the toughness at the weld was relatively poor.