超微細繊維状結晶粒組織を有するフェールセーフボルトの引張変形特性

抄録

1800 MPa-class ultra-high strength steel bar with an ultrafine elongated grain structure was fabricated through multi-pass caliber rolling at 500 °C for a 0.4C-2Si-1Cr-1Mo steel (mass%) with a tempered martensitic structure. The steel bar with an ultrafine elongated grain structure was warm formed into JIS M12 hexagon head bolts with a nominal length of 60 mm, and then their tensile properties were investigated by a wedge tensile test. Bolt heading was performed at 700 °C, and screw part was warm formed at 500 °C with a set of thread-rolling dies. In the ultrafine elongated grain structure, its transverse grain size was measured to be 0.3 μm, and a spheroidized carbide structure and a strong <110>//rolling direction fiber texture were developed through the warm caliber rolling. Such ultrafine elongated grain structure was also maintained in the bolt screw part. When the developed bolts were tensioned, all of them were failed at screw part. Delamination fracture, in which cracks propagated in the longitudinal direction of the bolt, occurred in the thread root. Hence, fracture appearance of the developed bolts was different from that of conventionally quenched and tempered bolts with a tempered martensitic structure. The developed bolts were demonstrated to be ductile and tough compared to the quenched and tempered bolts at ultra-high tensile strength level of 1800 MPa.