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Stretch-flange-formability is an important property for ultra high strength steel (UHSS) sheets for pressforming. In this study, microscopic deformation behaviors during punching and following stretch-flangeforming were investigated using three types of 980 MPa grade UHSS sheets with either two ferrite/martensite dual-phase structures or a martensite single-phase structure in order to clarify how the microstructure affects the stretch-flange-formability of UHSS sheets. The results of this investigation revealed following conclusions. Microscopic plastic-flow or micro-void density generated by punching is not the dominant factor of the stretch-flange-formability of UHSS sheets. During hole-expanding, cracks were mainly initiated at the fractured surface part and the cracks became longer and deeper from the punched surface with the increase of hole-expanding ratio. Deep cracking resistance in this process is important to improve the stretchflange-formability. The existence of strain gradient induced by hole punching is considered to be one of the reason for the highest hole-expanding ratio of the martensite single phase steel. During hole-expanding, the micro-cracks propagate mostly along the phase interfaces in the dual-phase steel sheets in the case of poor stretch-flange-formability, while the micro-cracks are tend to propagate through ferrite and martensite phases in the case of high stretch-flange-formability. The analysis of the hardness of ferrite and martensite suggests that the difference in hardness is the dominant factor of the stretch-flange-formability of the dualphase steel. In addition, the volume fractions of phases also influence the formability.

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Synopsis :The risk of delayed fracture should be evaluated when applying ultra high strength steel sheets to automotive parts. 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 affecting the hydrogen embrittlement of high strength steel bolts. In this study, the influence of plastic strain, as well as stress and diffusible hydrogen content, on hydrogen embrittlement of steel sheets was quantitatively studied by using an 1180 MPa grade cold rolled steel sheet. Plastic strain was introduced by U-shape bending. Stress was applied by tightening the bent specimen with a bolt. Then, hydrogen was introduced by immersing in hydrochloric acid. The time to fracture and the content of diffusible hydrogen entered into steel were investigated. The fracture was promoted by severe deformation, 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 was 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 hydrogen cracking conditions and the service conditions of the parts on the 3D space.

When ultra high strength steel (UHSS) sheets with tensile strength over 980 MPa are applied in automobiles, there is a risk that a type of hydrogen embrittlement fracture called delayed fracture may occur while a vehicle is in use. This paper summarizes the effects of stress, strain, diffusible hydrogen content and the forming mode on the hydrogen embrittlement resistance of UHSS sheets for automotive applications. In this study, 1 180 MPa grade ferrite-martensite dual phase steel was used. This material was evaluated by the U-bending and drawn cup methods. It was concluded that high strain, high stress and high diffusible hydrogen content reduced hydrogen embrittlement resistance.In addition, an improved immersion-type hydrogen charging method using an ammonium thiocyanate (NH4SCN) aqueous solution was introduced in this paper. The NH4SCN solution enables control of the diffusible hydrogen content from low to high concentrations using the NH4SCN concentration, and dissolution of the specimens during immersion in NH4SCN was minimal, making it possible to maintain substantially the same surface condition as before immersion.KEY WORDS: delayed fracture; equivalent strain; equivalent stress; diffusible hydrogen; U-bending; drawn cup; ammonium thiocyanate; immersion-type hydrogen charging method.