<title>Fatigue mechanism of elemental alloyed and nitrided ultra-low carbon steel sheets</title>

Ultra-low carbons steels, commonly known as interstitial free steels are widely used especially for automobile plates because of their excellent deep-draw ability and low deformation resistance has been studied. The influence of alloy elements and nitriding on fatigue properties of ultra-low carbon steel have been investigated using 4 kinds of materials of non-nitrided specimens and nitrided ones, respectively. For non-nitrided specimens, fatigue strength at 1x10⁷ cycles of material added with Niobium element is greater compared to the material added with Titanium element. In addition, the fatigue strength of nitrided specimens is generally higher than that of non-nitrided specimens. This is related to the hardness distribution of the specimens. In the case of the nitrided specimen, the maximum hardness is recorded at the surface and decreased gradually toward the center of the specimen. Accordingly, the fatigue limit and strength of the nitrided specimens are determined by internal properties of the materials due to the influence of nitriding processes to the hardness properties at the surface as well as internal layer. For non-nitrided and nitrided specimens, none of the microcracks are emanated at the curve root but from the inside of specimens and surface crack growth for non-nitrided specimens revealed irregularities. However, in the case of the nitrided specimens, it is difficult to observe the cracks from the surface as the microcrack initiated internally.