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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.

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