Effect of Micro-Crack - Crack Depth on Fatigue Property in Zn-Ni - Ni Coated Press Hardened Steel

Press hardened steel ( PHS ) has been applied to many automotive body parts to reduce the weight of the white body, and high corrosive properties are required in PHS. Al-Si - Si coated steel sheets are widely used for protection against scaling and decarburization. Although Al-Si - Si coated steel sheets have better corrosion resistance than bare steels due to the barrier effect,the corrosive protection performance of Al-Si - Si coated steel sheets is not sufficient for use in wet under-body - body areas compared with galvanized or galvannealed steel sheets. Zn coated PHS is appropriate for hot stamped parts in which corrosive protection is required because of its sacrificial protection property. However, micro-cracks - cracks occur in severely deformed areas of Zn-coated - coated parts under some hot stamping conditions, and deep micro-cracks - cracks can affect the fatigue property. In this study, several hat-shaped - shaped samples were prepared, and the depth of micro-cracks - cracks was changed by altering hot stamping conditions. The effects of the micro-crack - crack depth on the fatigue property of Zn-Ni - Ni coated PHS were investigated by a plane bending fatigue test. At the average depth of 20 mu m, the fatigue life of the Zn-Ni - Ni coated samples was almost equal to that of the Al-Si - Si coated sample which was taken from the wall side of the hat-shaped - shaped sample after crash forming. Considering the standard deviation of the micro-crack - crack depth in practical use, the Zn-Ni - Ni coated steel sheets can be used equally with the Al-Si - Si coated steel sheets in fatigue property when the micro-crack - crack depth was restricted to under 10 mu m. Moreover, the impacts of hot deformation conditions on the micro-crack - crack depth were investigated. It was found that the reduction of equivalent plastic strain is effective in lowering the micro-crack - crack depth.