非調質ボルト用鋼の開発

概要

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A study has been carried out on the development of microalloyed steel for JIS 8.8 class cold headed fasteners. Newly developed microalloyed steel is characterized by its unique alloying design, controlled rolling and controlled cooling technologies. New grade, low carbon-manganese steel containing precipitation hardening and grain refining elements: 0.14C-1.45Mn-0.1V-Al-Nb-N, is strengthened by finely precipitated vanadium carbo-nitrides in ferrite-pearlite matrix at cooling after wire rolling. In wire rolling, low temperature heating and rolling are combined to obtain the fine grained microstructure aiming at high ductility.New grade exhibits the strength of 64∼76kgf/mm2 as rolled condition and is strengthened to 85∼96kgf/mm2 by cold drawing with the reduction of 24%. The microstructure is composed of fine grained ferrite and pearlite, the grain size of which is about 5μm. Cold headed and threaded M6 fasteners show tensile strength of 83∼93kgf/mm2 and meet the class 8.8 specifications. Furthermore, it exhibits almost the same mechanical properties as those of conventional hardened-tempered carbon steel, S45C. New grade microalloyed steel has been applied class 8.8 automotive fasteners and 120 to 150 tons of wire per month are mass produced now.
大同特殊鋼株式会社の論文