Evaluation methods of hydrogen embrittlement for ultra-high strength steel sheets for automobiles

Abstract

Automotive applications of ultra-high strength steel (UHSS) sheets with tensile strength of 980MPa or higher have been increasing year after year. UHSS sheets are now used in a variety of parts which are produced by various forming modes, not limited to simple large-radius bending. When using UHSS sheets, it is well known that particular attention must be paid to hydrogen embrittlement (HE), especially under heavy deformation, because HE susceptibility increases with increasing strain. In this paper, the critical HE conditions for 1180MPa grade cold-rolled dual phase (ferrite-martensite) steel (1180DP) and hot-rolled martensitic steel (MS-W 1200) were investigated with specimens formed by bending and drawing. HE evaluation tests were conducted by immersing specimens of these materials in solutions of various pH. HE conditions were evaluated with diffusible hydrogen, stress and strain introduced in the specimens, as these are the main factors influencing HE. When tested by the U-bend method, the safe region of 1180DP was slightly wider than that of MS-W 1200. On the other hand, with the drawn cup method, the safe region of MS-W 1200 was wider than that of 1180DP. The critical conditions for 1180DP with each method were compared to investigate universal evaluation parameters for HE applicable to ultra-high strength steels. Copyright © 2010 MS&T'10®.