物質・材料研究機構では，湿間多パス溝口ール圧延により，バルク材のフェライト粒径を1 μ m以下に微細化する技術を確立している。この技術により作製した超微細フェライトーセメンタイト組織鋼の疲労特性について研究を進めているが，超微細フェライトーセメンタイト組織鋼の疲労強度はフェライト・パーライト鋼よりも高く，焼き戻しマルテンサイト鋼と同等の特性を示すことが明らかとなっている。%Rotating bending fatigue tests were conducted for a series of plasma-nitrided Fe-C-Mn steels in order to investigate the effect of manganese contents. 5 types of steels were prepared, i.e., 0.0Mn, 0.5Mn, 1.0Mn, 2.0Mn and 4.0Mn. Microstructures of the steels were ferrite-pearlite except 4.0Mn whose microstructure was martensite. 2.0Mn and 4.0Mn of the nitrided specimens had a hardened layer of 0.5 to 1 mm in depth, while the no hardening was observed in 0.0Mn, 0.5Mn and l.OMn of the nitrided specimens. In case of 0.0Mn, 0.5Mn and l.OMn, the nitrided specimen showed surface fracture and increase of fatigue strength was very small. The 2.0Mn nitrided specimen showed surface fracture and fatigue strength were largely improved by plasma-nitriding. On the other hand, in case of 4.0Mn, the nitrided specimen showed internal fracture beneath the hardened layer and the fatigue strength were lower than that of the 2.0Mn nitrided specimen.
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