ホットスタンプ部品の曲げ圧壊挙動と鋼材の機械的特性との相関

摘要

Lately, hot-stamping steel with tensile strength of 1470 MPa grade is being increasingly used for BIW. A study has been conducted on the steel's mechanical properties having strong correlation with cracking behavior upon side impact crash. This study was conducted on test steels that have different chemical compositions and surface conditions and that are affected differently by heat treatment. The parts made of these steels were subjected to three-point bending crash tests, for which a "Crash Index" was defined to quantitatively evaluate the size of cracks generated upon side impact crash. The Crash Index showed no clear correlation with the total elongation based on a/the JIS No.5 tensile test, nor with the hole expansion ratio, the mechanical properties commonly used for steel. Rather, the index was found to correlate well with the bending angle based on the VDA238-100 bending test standardized by German Association of the Automotive Industry (VDA: Verband der Automobilindustrie e. V.) and with a load-lowering behavior (called the "Post Uniform Slope") newly defined in this study for the VDA bending test, as well as with the local fracture strain calculated from the results of notched tensile tests. Another point discussed is the relationship between Post Uniform Slope and microstructural factors of the material.