Influence of Sheared Edge on Hydrogen Embrittlement Resistance in an Ultra-high Strength Steel Sheet

Automotive parts made from steel sheets normally have sheared edges, which have been reported to decrease the hydrogen embrittlement (HE) resistance of ultra-high strength steel (UHSS) sheets. However, the mechanism on the detrimental effect of the sheared edge on HE resistance is not yet clearly understood. In this study, the influence of the edge condition in UHSS sheets on the HE property was investigated using a 1180 MPa grade steel sheet. The HE resistance of specimens with the edges ground or as-sheared was evaluated by the U-bend method. Two types of as-sheared specimens, which had been bent so that either the burnished surface or the fracture surface was the outer side, were prepared. The specimens with the ground edges did not fracture under any conditions. The fracture stress of the fracture surface specimens was significantly lower than that of the burnished surface specimens. Microcracks were observed at the edge of the specimens except for the ground specimens, and larger microcracks were observed in the fracture surface specimens. Fracture stress drastically decreased as the microcrack length increased. The threshold of the stress intensity factor K decreased with increasing diffusible hydrogen content. When the threshold stress intensity factor at each diffusible hydrogen content was defined as KH, the fracture condition was described as K > KH. The reason why the fracture stress in the as-sheared specimen decreased was considered to be that K increased due to the microcracks introduced by bending.