To provide a method for producing soft-nitrided machine parts made of steel where, even if machining is performed with heat treatment obviated after hot forging, mechanical parts having high fatigue strength can be obtained by performing soft nitriding treatment after that.
Regarding the production method, a steel material having a composition comprising prescribed amounts of Si, Mn, S, Cr, Mo, Nb, Ti, V, Ni and N, and the balance Fe with inevitable impurities, and in which, among the inevitable impurities, the content of P is regulated to a prescribed amount or below, and the value of relational formula of the contents of the elements is controlled to a specific range is heated at 1,150 to 1,280°C, is thereafter hot-forged, so as to be formed, and is thereafter cooled at 0.5 to 1.5°C/s, so as to be a hot-forging having a microstructure in which the ratio of a bainitic structure is controlled to ≥50%, and the hot-forging is subjected to mechanical working, and is thereafter subjected to soft nitriding treatment at 550 to 650°C for ≥30 min.
NAITO KENICHIRO
MIZUNO YOSHIKI
MATSUDA HIDEKI
KOBAYASHI SEIJI
HONDA MOTOR CO LTD
Yasuhiko