UNTEMPERED STEEL FOR NITRIDING, AND CRANK SHAFT

To provide an untempered steel for nitriding that can secure sufficient warpage correctability while securing prescribed fatigue strength, without adding any new process causing increase in production cost.SOLUTION: The untempered steel for nitriding comprises, in mass%, C of 0.20-0.50%, Si of 0.05-0.60%, Mn of 0.80-1.70%, S of 0.001-0.20%, Cu of 0.60-1.20%, Ni of 0.01-1.00%, Cr of 0.05-0.50%, Ti of 0.002-0.040%, N of 0.005-0.040%, and the balance consisting of Fe and inevitable impurities, satisfying formula (1) defined by F1≥0.30 and formula (2) defined by F2≥11.9, where F1=0.20[C]+0.04[Mn]+0.18[Cu]+[Cr] and F2=-13[C]-25[Si]+3[Mn]+2[Cu]+5[Ni]-15[Cr]+5, and mainly consists of a ferrite/pearlite two-phase structure.SELECTED DRAWING: None 【課題】製造コストの増加に繋がるような新たな工程を追加することなく所定の疲労強度を確保しつつ十分な曲げ矯正性を確保することが可能な窒化用非調質鋼を提供する。【解決手段】窒化用非調質鋼は、質量％で、Ｃ：０．２０〜０．５０％、Ｓｉ：０．０５〜０．６０％、Ｍｎ：０．８０〜１．７０％、Ｓ：０．００１〜０．２０％、Ｃｕ：０．６０〜１．２０％、Ｎｉ：０．０１〜１．００％、Ｃｒ：０．０５〜０．５０％、Ｔｉ：０．００２〜０．０４０％、Ｎ：０．００５〜０．０４０％、残部がＦｅ及び不可避的不純物であり、且つ下記式（１），式（２）を満たし、主としてフェライト・パーライトの２相組織からなる。Ｆ１≧０．３０・・式(１)、Ｆ２≧１１．９・・式(２)但しＦ１＝0.20[C]＋0.04[Mn]＋0.18[Cu]＋[Cr]Ｆ２＝−13[C]−25[Si]＋3[Mn]＋2[Cu]＋5[Ni]−15[Cr]＋5【選択図】 なし