Evaluation of Hydrogen Embrittlement for High Strength Steel Sheets

Steel sheets for automobiles are usually formed into various parts by cold working. Therefore, plastic strain introduced by the cold working must be considered as a factor affecting the hydrogen embrittlement in addition to the applied stress and the content of diffusible hydrogen entered into steels, which are considered as factors in the studies of high strength steel bolts. However, there are few detailed reports investigating the influence of these factors on hydrogen embrittlement of steel sheets quantitatively. In this study, the influence of plastic strain, as well as stress and diffusible hydrogen content, on hydrogen embrittlement of steel sheet was quantitatively studied to evaluate the hydrogen embrittlement susceptibility of steel sheet by using an 1180 MPa grade cold rolled dual phase steel sheet. Plastic strain was introduced by U-shape bending, and stress was applied by tightening the bent specimen with a bolt. Then, hydrogen was introduced by dipping in hydrochloric acid, and the time to fracture and the content of diffusible hydrogen entered into steel during dipping were investigated. The fracture was promoted by severe deformation near the bending limit, and it seemed to be caused by the presence of micro cracks and/or micro voids. The hydrogen cracking conditions region of the steel sheet were mapped in the three-dimensional space with the axes of applied strain, applied stress and diffusible hydrogen content. It was considered that the evaluation of the risk of delayed fracture of automotive parts made of the steel sheet under service environment was possible by a comparison of the 3D map and the service conditions of the parts.