Enhancing resistance to hydrogen embrittlement in high-strength hot-press-forming steel sheets through Al-Si-Zn coating optimization

This study addresses critical issues of hydrogen embrittlement (HE) and liquid metal embrittlement (LME) in Al-Si-coated and Zn-coated hot-press-forming (HPF) steel sheets, respectively, by proposing an innovative approach of incorporating (24–30) wt% Zn into the conventional Al-Si (AS) coating to mitigate HE issues while leveraging the absence of LME associated with Zn coating. To the best of the authors' knowledge, no other paper has considered this subject. The optimal Al-Si-Zn (ASZ) sheet formed a surface oxide layer and Zn-enriched zone, acting as local trap sites to reduce H intrusion, while the AS sheet lacked this zone, leading to continuous H intrusion. The ASZ sheet reached complete H emission in 4 weeks, whereas significant H remained in the AS sheet even after 4 weeks due to higher H intrusion. This result emphasizes the crucial role of Zn addition in impeding H diffusion and ensuring excellent resistance to HE. Bending angles at peak load, crucial for assessing bending and sheet forming properties, were adversely affected by diffusible H contents within the sheets. When austenitized at 900 °C for 3 min and then cooled to 800 °C for tensile testing, the ASZ sheet experienced abrupt premature fracture due to selective Zn diffusion from the liquid alloy to grain boundaries, inducing LME. However, prolonged austenitization to 6 min mitigated LME, with no expected issues on LME. These findings underscore the significance of the tailored Al-Si-Zn coating in enhancing resistance to HE, making it suitable for broad applications in high-strength HPF steel sheets.