Fatigue crack initiation and competitive crack propagation behavior in 500 MPa grade automobile beam steel

The fatigue crack initiation and propagation in Nb–Ti micro-alloyed 500 MPa grade steels that usually be used in the automobile beam were investigated. Reducing the content of C and Mn elements based on the reference steel and adding microalloying elements Nb and Ti to obtain the modified steel. The fatigue strengths of the reference steel and the modified steel are 201 MPa and 225 MPa, respectively. Fatigue cracks in the experimental steel were initiated at surface defects and persistent slip bands (PSBs). The modified steel is more susceptible to initiating cracks at the PSBs than the reference steel. There were three kinds of fracture modes in the experimental steels, which were determined by the competition between carbide and inclusion. As the loading stress and the stress intensity factor (Δk) increase, the fracture mode transforms from cleavage (brittle) to dimple (ductile) fracture. Under the circumstance of low loading stress and low Δk, the fracture mode is dominated by the carbide-induced cleavage fracture mechanism. Under the situation of high loading stress and Δk, the fracture mode is dominated by the inclusion-induced dimple fracture mechanism. In addition, the mixed cleavage-dimple region is formed when the two fracture mechanisms compete in the steel.