High cycle fatigue behavior of titanium microalloyed high-strength beam steels

The realization of an ideal combination of mechanical and fatigue properties is prerequisites for practical application of titanium (Ti) microalloyed steel in automotive field. The fatigue behavior of four Ti microalloyed high-strength beam steels with different Ti contents was systematically studied. The results show that the content of microalloying element Ti has a significant effect on the fatigue properties, especially in the steel with a high Ti content. For the experimental Ti microalloyed steel, inclusion-induced crack initiation is the main fatigue failure mode. Different from general fatigue fracture mechanism in Ti-contained steel, no TiN, which is the most detrimental to fatigue behavior, was found in fatigue crack initiation area. However, the large-sized TiN and oxide complex inclusion with a core–shell structure is the dominant cause of fatigue fracture. Because of the intense-localized deformation at the interface between complex inclusion and matrix, the angular TiN in the outer shell has a serious deteriorating effect on the fatigue properties, which is consistent with the result of the Kernel average misorientation map. Besides, the modification effect of a small amount of MnS on large-sized inclusion is not obvious and has little effect on the fatigue behavior. For more practical guidance, the critical inclusion sizes of the experimental steels were also investigated by experimental extrapolation method. With the increasing tensile strength, the inclusion sensitivity of the experimental steels increases, leading to the small critical inclusion size.