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.

Description

TECHNICAL FIELD[0001]The present invention relates to a steel for nitriding and a component having been nitrided (hereinafter, referred to as a nitrided component). More particularly, it relates to a steel for nitriding that is suitable for being used as a material for a nitrided component such as an automobile ring gear, which steel is easily subjected to cutting before nitriding, has a high bending fatigue strength and surface fatigue strength after nitriding, and further can suppress expansion (heat treatment distortion) caused by nitriding, and a nitrided component produced by using the steel.BACKGROUND ART[0002]A component used for an automobile transmission is usually subjected to casehardening treatment such as carburizing-quenching, induction hardening, or nitriding from the viewpoint of improvement in bending fatigue strength and surface fatigue strength.[0003]Among these treatments, the “carburizing-quenching” is a treatment in which a low carbon steel is generally used, a...

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Benefits of technology

[0033]For the steel for nitriding of the present invention, cutting before nitriding is easy to perform, and also the amount of expansion caused by nitriding is small. Moreover, the nitrided component pro...

Problems solved by technology

Also, this treatment has a problem that the fatigue strength is decreased with a so-called “nonmartensitic layer” such as a grain boundary oxidized layer or an incompletely quenched layer, which is formed on an outer layer, being a fracture starting point of bending fatigue and the like.

However, the medium carbon steel has a problem of decreased machinability because the hardness thereof is higher than that of the low carbon steel.

Also, this treatment has a problem that a high-frequency heating coil must be prepared for each component.

However, the conventional steel for nitriding has the problems described in the following (1) to (4).

(1) Since nitriding is a treatment in which quenching treatment from a high-temperature austenite zone is not performed, strengthening associated with martensitic transform...

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