Abstract: In order to optimize the composition and microstructure properties of 600 MPa cold-pressed axle housing steel, and further improve the fatigue service life of the axle, this paper uses a thermal simulation test machine to prefabricate the welding heat affected coarse grained area (CGHAZ) structure of the axle housing steel. The impact toughness of CGHAZ was obtained; the softening characteristics of CGHAZ were examined by a Vickers hardness tester; the fatigue crack growth rate of CGHAZ was tested by an electro-hydraulic servo fatigue tester; the laser scanning confocal microscope (CLSM), high temperature Laser microscopy (HTLM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) studied the structural evolution of CGHAZ, the morphology of M/A, the distribution of large-angle grain boundaries, and the propagation of fatigue secondary cracks and their trends. Studies have shown that the brittle-ductile transition temperature of bridge shell steel using the Nb-V composition system is lower than −20 ℃; when t_8/5 ≤ 15 s, the microstructure in the coarse grain region does not soften and fatigue secondary cracks occur at large angle grain boundaries. Deflection, the fatigue crack growth rate is the lowest compared to Mn-Ti and Ti-Nb systems.