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...

AI Technical Summary

Benefits of technology

The present invention provides a steel that can be easily cut before nitriding and has minimal expansion during nitriding. The nitrided component produced using this steel has high bending fatigue strength and surface fatigue strength even at a low content of expensive element Mo (0.05 mass %).

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 transformation cannot be applied.

However, in the case where the hardness is increased by containing a large amount of alloying element, the cutting becomes difficult to perform.

However, since the case depth is shallow, a high surface fatigue strength cannot be provided.

Also, if the surface hardness is too high, the damage against a pair-gear becomes undesirably high.

However, since Mo is an expensive element, the use of a large amount of Mo is unfavorable in terms of economy.

Therefore, even in nitriding, the amount of heat treatment distortion undesirably increases.

In particular, an automobile ring gear poses a problem even if being subjected to slight heat treatment distortion because the automobile ring gear is nitrided after having been machined into a thin-wall final shape and having been subjected to gear cutting.

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