Rapid evaluation of the critical condition for hydrogen-induced cracking in ultrahigh-strength automotive steel sheets: A semiquantitative investigation based on U-bend specimens

The production of automotive steel plates, which involves pickling, electroplating, and welding among other steps, introduces hydrogen. Additionally, during the service life of components fabricated using such plates, electrochemical corrosion occurs, and hydrogen atoms are introduced. The hydrogen atoms are adsorbed and diffused into the interior of the metal, accumulating at any defects. Automotive parts are commonly fabricated using cold forming techniques. The hydrogen embrittlement sensitivity of ultrahigh-strength automotive sheets (UHSSs) is influenced by the stress, concentration of diffusible hydrogen, and plastic strain resulting from the cold forming process. The standard methods for quantitative studies on the UHSS hydrogen embrittlement sensitivity, such as the constant-load testing and low-strain-rate testing, require a specialized equipment and extended experimental period, making them impractical for original automotive equipment manufacturers. This study focuses on the use of the quenching and partitioning (QP) steel QP1180, which contains residual austenite, and martensite steel MS1500, which has a low deformation ability but high strength, as research objects. The equivalent strain and stress were controlled by adjusting the bending radius and U-bend opening distance, while the diffusible hydrogen concentration was charged through immersion. The fracture time of the U-bend sample was recorded, and the critical diffusible hydrogen concentration for hydrogeninduced cracking was measured. Additionally, the stress distribution during the U-bending process was simulated using the finite-element method. To evaluate the hydrogen embrittlement resistances of QP1180 and MS1500, the area enclosed by the load stress and equivalent strain (bending radius) and fracture time were semiquantitatively determined. Finally, the hydrogeninduced delayed fracture space regions of QP1180 and MS1500 were determined in the threedimensional space, with equivalent strain, load stress, and diffusible hydrogen concentration as coordinate axes. This provided a convenient and reliable means of determination of the critical conditions for a safe service and rapid evaluation of the hydrogen embrittlement sensitivity of ultrahigh-strength automotive sheets.