Effects of Sn addition on the improved hydrogen intrusion and emission behaviors in 1.5-GPa-grade Al-Si-coated hot press forming steels

High-strength hot press forming (HPF) steel sheets are coated mostly by Al-Si to prevent the decarburization or oxidation of sheet surfaces, but become vulnerable to hydrogen embrittlement (HE) as H atoms supplied from moisture in a furnace penetrate through the molten coating into the steel substrate during the HPF process. In this study, 0.1-wt% of Sn was added to a 1.5-GPa-grade reference HPF steel (referred to as 0.1Sn and Ref steels), and their coating microstructures including Sn-enriched zone and Kirkendall and surface voids were examined after laboratory-scale HPF simulation tests. The content of diffusible H atoms immediately after the HPF simulation at 900 °C was 0.111 wt.ppm in the Ref sheet, decreased slowly as the elapsed time increased, and remained to be 0.030 wt.ppm even after 17 days. In the 0.1Sn sheet, it was about half of that of the Ref sheet (0.061 wt.ppm), and decreased rapidly to nil after 3 days. These results indicated that the 0.1Sn steel showed excellent H-intrusion and emission behavior, which would favorably work for better resistance to HE, by optimally controlling the multi-mechanisms of enrichment of Sn solutes and population of coating voids. The presence of Sn-enriched zone and voids played an important role in blocking the H intrusion to the substrate and in accelerating the H emission to the coating surface, respectively.