Steel for nitriding and nitrided component

Abstract

A steel for nitriding having a chemical composition consisting of, by mass percent, C: 0.07-0.14%, Si: 0.10-0.30%, Mn: 0.4-1.0%, S: 0.005-0.030%, Cr: 1.0-1.5%, Mo: ≦0.05% (including 0%), Al: 0.010% or more to less than 0.10%, V: 0.10-0.25%, optionally at least one element selected from Cu: ≦0.30% and Ni: ≦0.25% [0.61Mn+1.11Cr+0.35Mo+0.47≦2.30], and the balance of Fe and impurities. P, N, Ti and O among the impurities are P: ≦0.030%, N: ≦0.008%, Ti: ≦0.005%, and O: ≦0.0030%. The steel is easily subjected to cutting before nitriding and suitable for use as an automobile ring gear. The nitrided component having a surface hardness of 650-900 HV, core hardness being ≧150 HV, and effective case depth of ≧0.15 mm has excellent bending fatigue strength and surface fatigue strength although the content of Mo is as low as ≦0.05% and has a small amount of expansion caused by nitriding.

1 . A steel for nitriding having a chemical composition consisting of, by mass percent, C: 0.07 to 0.14%, Si: 0.10 to 0.30%, Mn: 0.4 to 1.0%, S: 0.005 to 0.030%, Cr: 1.0 to 1.5%, Mo: 0.05% or less (including 0%), Al: 0.010% or more to less than 0.10%, and V: 0.10 to 0.25%, Fn1 expressed by Formula (1) is 2.30 or less, and the balance of Fe and impurities, wherein P, N, Ti and O among the impurities are P: 0.030% or less, N: 0.008% or less, Ti: 0.005% or less, and O: 0.0030% or less:
   Fn1=0.61Mn+1.11Cr+0.35Mo+0.47V  (1)
   
       where, the symbol of each element in Formula (1) represents the content thereof in mass percent. 
     
     
         2 . The steel for nitriding according to  claim 1  having a chemical composition containing, in lieu of a part of Fe, at least one element selected from, by mass percent, Cu: 0.30% or less and Ni: 0.25% or less. 
     
     
         3 . A nitrided component having the chemical composition according to  claim 1 , in which the surface hardness is 650 to 900 in Vickers hardness, the core hardness is 150 or higher in Vickers hardness, and the effective case depth is 0.15 mm or larger.