Abstract
Applications of ultra high strength steel sheets to automotive body have been expanding year after year. Hydrogen embrittlement (HE) is one of the problems of ultra high strength steels. Various methods are used as the evaluation method of HE resistance. In this study, the critical HE conditions obtained by SSRT, CSRT and 4-point bending test were compared by using the same materials, which have tempered martensite microstructure with the composition of a SCM435 or an V added steel with many hydrogen trapping precipitations. Specimens were charged with hydrogen by the cathodic charging method. The specimen used in the SSRT and the CSRT was machined with notches on the both sides of a parallel part and the stress concentration factor (Kt) of the specimens was 4.26 or 1.76. The specimen used in the 4-point bending test was coupon shape. The critical HE conditions evaluated with the average applied stress and the average hydrogen content of the specimen were different depending on the test methods. HE conditions were also evaluated with the local stress and the local accumulated diffusible hydrogen content at the fracture initiation point. The critical condition evaluated by the 4-point bending test was located in the higher stress and higher hydrogen content region compared with the critical conditions obtained by the CSRT and the SSRT.
