Comparison of Hydrogen Embrittlement Resistance of High Strength Steel Sheets Evaluated by Several Methods

In steels with tensile strength of 1 000 MPa and higher, delayed fracture, which means fracture of ultra-high strength steels caused by hydrogen, may occur even under atmospheric corrosion conditions. The F13T grade high strength bolt is well-known for a delayed fracture accident.1) Based on this background, round bar or square bar specimens have mainly been used in this research field. Recently, however, the demand for delayed fracture evaluation of automotive steel sheets has increased in response to increasing application of ultra-high strength steel sheets with tensile strength of 1 000 MPa and higher to high strength automobile bodies. In order to evaluate the risk of delayed fracture, it is generally considered that resistance to hydrogen embrittlement (hereinafter, HE), which means the HE susceptibility of steels and the hydrogen entry property, should be evaluated.2) The focus of this study is HE susceptibility. There are many test methods for HE evaluation, for example, the Constant Load Test (CLT),2,3) Slow Strain Rate Test (SSRT),4–10) Comparison of Hydrogen Embrittlement Resistance of High Strength Steel Sheets Evaluated by Several Methods