Steel for machine structural purposes for surface hardening use and steel parts for machine structural purposes and method of production of same

Abstract

A steel for surface hardening for machine structural use which is characterized by containing, in terms of mass %, 0.30-0.60% C, 0.02-2.0% Si, 0.35-1.5% Mn, 0.001-0.5% Al, 0.05-2.0% Cr, 0.001-1.0% Sn, 0.0001-0.021% S, 0.0030-0.0055% N, 0.01-2.0% Ni, 0.01-2.0% Cu, up to 0.030% P, and up to 0.005% O, with the remainder comprising Fe and incidental impurities, the contents of Sn, Cu, Ni, Mn, and S satisfying relationships (1) and (2).−0.19≦0.12×Sn+Cu−0.1×Ni≦0.15  (1)60≦Mn / S≦300  (2)

Description

TECHNICAL FIELD[0001]The present invention relates to steel for machine structural purposes for surface hardening use which is applied to power transmission parts of automobiles etc., in particular parts which require high surface fatigue strength under a corrosive environment, for example, gears, continuously variable transmissions, constant velocity joints, hubs, bearings, to steel parts for machine structural purposes, and to a method of production of those parts.BACKGROUND ART[0002]Parts for machine structural purposes, for example, gears of automatic transmissions and sheaves of continuously variable transmissions, constant velocity joints, hubs, and other power transmission parts are required to have a high surface fatigue strength.[0003]In the past, for the above parts, case hardened steels with C of around 0.2% (JIS SCr420, SCM420, etc.) have generally been used for the material, while a hardened layer of martensite with C of around 0.8% has been formed on the surface of the...

AI Technical Summary

Benefits of technology

The present invention provides a steel for machine structural purposes that can be surface hardened for use in parts that require high surface pressure fatigue strength in a corrosive environment. This steel has superior surface fatigue strength and corrosion resistance, as well as good temper softening resistance and machineability compared to conventional soft nitrided induction hardened steel parts. The steel also has a sufficient lubrication film for the working surfaces. The use of this steel in power transmission parts of automobiles, for example, can increase output and lower costs.

Problems solved by technology

However, carburization is treatment which takes a long time of 5 to 10 hours, in some cases more than 10 hours, along with the austenite transformation at the high temperature of around 950° C., so heat treatment deformation (quenching distortion) due to the crystal grain coarsening unavoidably becomes greater.

If increasing the amount of C of the steel material, the hardness of the inside, where no improvement of the surface fatigue strength is required, rises and the machineability remarkably deteriorates, so it is not possible to just increase the amount of C of the steel material.

However, with just ferritic nitrocarborizing treatment, the depth of the hardened layer is small, so it is difficult to apply ferritic nitrocarborizing to the production of transmission gears etc. to which a high surface pressure is applied.

However, with the above method, the surface hardness is high, but the concentration of N in the nitrided layer is low, so the hardness at a high temperature is low, a sufficient softening resistance cannot be obtained at the surface of a gear etc. which becomes a high temperature at the time of operation, and a high surface fatigue strength cannot be obtained.

However, there are insufficient elements which have a high affinity with N and which break down and diffuse nitrides, so high temperature heating is required.

Therefore, an oxide layer is formed at the steel material surface and the mechanical properties deteriorate.

Further, with the above method, formation of a thick compound layer is not considered, so it is not possible to obtain a good surface fatigue strength under a high surface pressure.

However, with this art, nitriding is performed at a high temperature of 600° C. or more, so the compound layer is thin and, furthermore, the N concentration is low, so the amount of N which diffuses upon breakdown of the compound at the time of induction hardening is small.

In the above art, it is possible to form a compound layer by nitriding, but it is difficult to form a thick, high N concentration nitrided layer.

For this reason, even if combined with induction hardening, it is not possible to form a nitrided layer with a high softening resistance and a good surface fatigue strength.

However, in the above parts for machine structural purposes, the mechanical strength, for example, the durability under a lubrication environment containing water, is insufficient.

Further, machineability is not considered.

Therefore, deterioration of the machineability cannot be avoided.

As a result, the productivity falls and the manufacturing cost rises.

Images