超高強度薄鋼板の耐水素脆化特性に及ぼすせん断端面の影響

Abstract

Automotive parts made of steel sheets normally have sheared edges, which have been reported to decrease 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 has not been clearly understood yet. In this study, the influence of the edge condition in UHSS sheets on HE property was investigated using an 1180MPa grade steel sheet. HE resistance of specimens with the edges ground or as-sheared was evaluated by the U-bend method. Two types of as-sheared specimens with the burnished surface or the fracture surface bent to be the outer side were prepared. The specimens with the ground edges did not fracture at any conditions. Fracture stress of fracture surface specimens was significantly lower than that of burnished surface specimens. Micro cracks were observed at the edge of the specimens except for the ground specimens, and larger micro cracks were observed in the fracture surface specimen. Fracture stress was drastically decreased with the increase in micro crack length. Threshold of stress intensity factor K was decreased with increasing the diffusible hydrogen content. When threshold stress intensity factor at each diffusible hydrogen content was defined as KH, fracture condition was described as K > KH. The reason why the fracture stress in the as-sheared specimen decreased was considered that K increased due to the micro crack introduced by the bending.