Process for producing carbonitrided part

Abstract

A process for producing a carbonitrided part comprising the steps of preparing a base steel part, having a composition comprising, in mass percent, C: 0.10 to 0.24%, Si: 0.15 to 1.0%, Mn: 0.30 to 1.0%, Cr: 0.40 to 2.0%, S: 0.05% or less, with the balance being Fe and impurities and performing the following steps 1-4 in sequence. Step 1 is carburizing the base steel part under a carburizing atmosphere at a temperature of 900 to 950° C. Step 2 is carbonitriding the base steel part carburized according to step 1 under a carbonitriding atmosphere at a temperature of 800 to 900° C. with a nitrogen potential of 0.2 to 0.6%. Step 3 is quenching the base steel part carbonitrided according to step 2. Step 4 is tempering the base steel part quenched according to step 3 at a temperature of more than 250° C. to not more than 350° C.

Description

[0001]This application is a continuation of the international application PCT / JP2009 / 070152 field on Dec. 1, 2009, the entire content of which is herein incorporated by reference.TECHNICAL FIELD[0002]The present invention relates to a part subjected to carbonitriding treatment (hereinafter referred to as “carbonitrided part”) and a process for producing carbonitrided parts. More specifically, the present invention relates to a carbonitrided part suitable for a power transmission component requiring excellent rolling contact fatigue strength, in particular, a large strength to pitting and excellent abrasion strength, and a process for producing the carbonitrided parts.BACKGROUND ART[0003]Power transmission components such as a gear used for a transmission of a car and a pulley for a belt-type Continuously Variable Transmission (CVT) are conventionally produced as follows: an alloy steel for machine structural use defined in JIS G 4053 (2003) is formed into a predetermined shape by pr...

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Benefits of technology

[0043]The carbonitrided part of the present invention has excellent abrasion strength and high pitting strength. Hence, in order to realize weight saving of a car directly linked to the improvement of energy efficiency, the said carbonitrided part can be used in power transmission components such as gear for a transmission and pulley for a belt-type continuously variable transmission of a car requiring more miniaturization and higher strength. In addition thereto, the carbonitrided part of the present invention can be produced by a method of the present invention, and a material of the carbonitrided part is a low-cost steel with less content of Mo of an expensive alloy element or without addition of Mo. Thus, it is possible to realize the reduction of production costs in comparison with the conventional power transmission components.

Problems solved by technology

In recent years, requirements on energy efficiency of a car have been becoming severe more and more.

Regarding the “carbonitriding,” there are “gas carbonitriding” where ammonia gas is mixed in a carburizing atmosphere to undergo carburizing and nitriding at the same time and the like, and nitrogen is thought to have an effect of enhancing a so-called “temper softening strength.” However, nitrogen has an effect of suppressing diffusion of carbon, in addition, since nitriding treatment is conducted at a lower temperature than that of carburizing treatment, there has been a problem that hardening depth becomes small.

Further, nitrogen is an austenite-stabilizing element, and lowers an Ms point in the same way as C, thus, retained austenite tends to be present, and there has also been a problem that it is difficult to obtain hard martensite.

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