12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Recently, the tendency to improve the fuel efficiency by reducing the weight of the car body and using advanced high strength steel(AHSS) has affected the automotive industry. In order to secure the strength of the AHSS above 1Gpa, various elements such as Mn, Si, and Al are added. Additionally, annealing process is performed at high temperature to form an austenite phase and at high hydrogen concentration atmosphere to cool rapidly. In this process, hydrogen penetrates into the austenite grain boundary, which could cause hydrogen embrittlement. Most of the problems of hydrogen embrittlement have been reported that hydrogen generated during corrosion process penetrates into steel and thus leads to embrittlement, but the effects of hydrogen penetration during heat treatment should be studied. In this study, the effect of atmosphere during annealing process on hydrogen embrittlement of 1470MPa martensitic steel was studied. To determine the hydrogen embrittlement behavior after the heat treatment, the elastic deformation was maintained below the yield strength, and the time up to the fracture was evaluated. The content of the diffusible hydrogen was also evaluated using TDS. The hydrogen embrittlement behavior showed a tendency to vary according to the concentration of hydrogen introduced during annealing process of 1470MPa martensitic steel. In addition, bare(CR) and galvannealed(GA) materials were advantageous in terms of hydrogen embrittlement, and galvanized(GI) materials showed inferior properties. Hydrogen absorbed in the bare steel(CR) immediately exits after heat treatment, and in the case of galvannealed steel(GA), hydrogen is released during the galvannealing process. However, in the case of galvanized steel(GI), hydrogen absorbed into the steel is blocked in the coating layer and thus leads to hydrogen embrittlement behavior. Additionally, diffusible hydrogen analysis showed that the hydrogen content of galvanized steel was relatively higher than that of other steels.