Improvement of Hydrogen Embrittlement by Void Formation at Coating Layers of Sb-Added Al-Si-Coated Hot-Press-Forming Steels

Abstract

Al-Si coating can prevent the oxidation or decarburization of hot-press forming (HPF) steel surfaces; however, high reactivity of Al with the moisture promotes to uptake H atoms at high temperatures, and the coating prevents the out-diffusion of H atoms absorbed inside the steel. These H atoms trapped within the substrate result in an H-induced degradation of sheet formability as well as tensile properties, called hydrogen embrittlement (HE). In this study, the addition of Sb was suggested, based on its role in adjusting the relative diffusivity of Fe and Al/Si and in activating the formation of Kirkendall and surface voids as strong H-trapping sites inside the coating. The detailed microstructural evolution of voids and coating structures was investigated by a laboratory-scale HPF simulation test to correlate it with the H intrusion/emission behavior after the HPF simulation. Furthermore, the effectiveness of Sb addition in improving resistance to HE was demonstrated through industrial HPF processes and subsequent investigations of bending properties. The reduced diffusivity of Al and Si and the consequently increased fraction of voids effectively prevented the H intrusion. Concurrently, the voids provided emission routes of diffusible H atoms, thereby improving the resistance to bending crack propagation. This favorable role in enhancing resistance to HE suggests that the optimal Al-Si coating design by adjusting the Sb content and void fraction would secure wide applications of high-strength HPF steel sheets.

Keywords: Hot-press-forming (HPF) steel, Hydrogen embrittlement (HE), Sb addition, Al-Si coating, Sb-enriched zone, Intrusion and emission of H atoms