도금층 미세조직 제어에 따른 핫 스탬핑 강의 수소 취성 저항성 및 파괴 메커니즘 연구

Abstract

Hot-press-forming (HPF) steels have gained great attention as automotive reinforcement parts but are susceptible to hydrogen embrittlement (HE) during the process due to hydrogen being in the solidified coating. Al-Si coating is essential for preventing oxidation and decarburization, but the high reactivity of Al with moisture promotes H uptake at high temperatures. The coating layer also prevents the out-diffusion of absorbed H atoms, leading to H-induced degradation of sheet formability and tensile properties, especially bendability. To address this issue, this study modified the Al-Si coating structures in two ways. Firstly, the intermetallic layer structure in the coating was changed by adjusting the plating amount and austenitizing conditions. The effects of coating structures on bendability and H desorption were investigated by interrupted bending tests, H-permeation tests, and thermal desorption analyses. A 33-μm-thick multiple coating structure composed of Fe2Al5, FeAl, and ferrite layers was produced by immersion in an Al-10%Si bath and the subsequent HPF process (930 °C for 6 min). On the other hand, the reduced Al-Si adhesion amount increased time and temperature (950 °C for 30 min) produced a 30-µm-thick body-centered-cubic (BCC)-based coating structure composed of FeAl and ferrite layers. The BCC-based crystal structure, reduced Al content in the FeAl layer, and coarsened ferrite grains effectively enhanced H diffusivity and suppressed H-induced degradation. The softened FeAl and thick ferrite layers also improved bendability by allowing for large strain accommodation of bending deformation. Secondly, the addition of Sb was suggested, based on its role in adjusting the relative diffusivity of Fe and Al/Si and 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 increased fraction of voids effectively prevented H intrusion. This study proposes an optimal Al-Si coating design that enhances both bendability and resistance to H-induced degradation for secure HPF steel applications. The voids provided emission routes for diffusible H atoms, thereby improving the resistance to bending crack propagation. Unlike previous studies, which mainly modified the base material to change H behavior, HE can be improved while maintaining excellent mechanical properties of the base material by controlling the coating layer. Since no additional steps are required in terms of the process, it is efficient and can ensure wide application of high-strength HPF steel.