Masatoshi Shinozaki

FATIGUE OF AUTOMOTIVE HIGH STRENGTH STEEL SHEETS AND THEIR WELDED JOINTS

Fatigue properties in various types of high strength steel sheets for automotive use and their spot and arc welded joints were investigated. In steel sheets and strain-aged sheets, fatigue limit stresses and notch sensitivities depend on not only the tensile strength but also the yield strength. Two methods for improving fatigue strength of spot welded high strength steel joints were found. One is to generate moderate expulsion during welding with a current just above the upper lobe curve and another is to conduct a controlled tempering treatment during the welding schedule. By these methods, fatigue strengths at 10 to the 7th power cycles of spot welded high strength steel joints increase by about twice as compared with those by the conventional welding method.

Spot Weldability and Fatigue Property of Hot-Rolled High-Strength Sheet Steels for Wheel Usage

Spot weldabilities and fatigue properties of hot-rolled high strength sheet steels for wheel use, with tensile strength of 45 to 60 kgf/mm 2 were investigated. The specimens used were phosphorus-niobium bearing steel (P-Nb steel), phosphorus-manganese bearing steel (P-Mn steel) and conventional carbon-manganese bearing steel (C-Mn steel). The P-Nb steel and the P-Mn steel have been developed by the authors for wheel rim use and wheel disc use, respectively

Development of High Strength Steel Sheets for Wheel Rim Use

Three types of hot-rolled high strength steel sheets having tensile strengths of 600 to 800MPa for wheel rim use were developed. For a fundamental study the effects of chemical compositions on the formability in stretch-flanging and bending on the flash butt welded joints were investigated. In precipitation-hardened steels, the main controlling factors are identified as a newly proposed carbon equivalent value, Mn/Si ratio and S content in steel. P-bearing steels and intercritically heat-treated dual-phase steel hoops are able to be used for rim production. Examples of commercial production of rims are presented.