Abstract
In order to apply the high strength steel sheet with tensile strength is above 1200MPa, it is necessary to assess the delayed fracture susceptibility caused by hydrogen entering from the environment where steel components are exposed, and also necessary to adopt proper evaluation method of the delayed fracture. In this study, to clarify the influence of various experimental methods on delayed fracture susceptibility of high tensile steel sheets, slow strain rate technique (SSRT), constant load test (CLT) and conventional strain rate technique (CSRT) were demonstrated using commercial high strength steel sheet, and the fracture surfaces were observed. The results show that fracture stress decreased with the increase in the diffusible hydrogen content, and the fracture limit curves of SSRT and CLT were almost the same, whereas that of CSRT shifted to higher stress level. The area fraction of brittle fracture of SSRT, which is the sum of intergranular and quasi-cleavage fracture, increasing with the diffusible hydrogen content, is almost the same as that of CLT, however the area fraction of brittle fracture of CSRT is lower. It is supposed that SSRT and CLT owe to accumulated hydrogen concentration and CSRT owes to accumulated less than that of the others. Thus, both SSRT and CLT are suitable methods to assess the delayed fracture susceptibility; SSRT is superior to the point of testing time, and CLT is superior to the point of reproducing the environments. However, CSRT is suitable to classify materials susceptibility of the delayed fracture immediately on the condition of high diffusible hydrogen content.
