Method of manufacturing case-hardened parts with little distortion in heat treatment and superior strength in bending fatigue

A manufacturing method of a case-hardened material with little distortion during heat treatment comprising an uncarburized core and a carburized surface layer, comprising the steps of carburizing, carbonitriding or carburizing followed by carbonitriding parts made of a case-hardening steel, whose chemical composition is C:0.03-0.27 wt. %, Si:

• 0.05-0.35 wt. %, Mn:
• 0.3-2.0 wt. %, S:
• 0.3 wt. % and less, Ni:
• 0.4-3.0 wt. %, Mo:
• 0.1-1.0 wt. %, Al:
• 0.015-0.10 wt. %, V:
• 0.03-0.5 wt. %, and N:
• 0.004-0.02 wt. %, the balance Fe and inevitable impurities, wherein the C-equivalent represented by the following formula is in the range 0.5-1.2 (wt. %),
• C-equivalent=[C]+(1/7.2) [Si]+(1/4.8) [Mn]+(1/7.5) [Ni]+(1/6.0) [Cr]+(1/5.8) [Mo]+(1/1.5) [V]
• wherein, in the above formula, each bracketed element represents the content thereof in weight percentage,
• cooling said parts at a rate of from 100.degree. C./min. to 1.degree. C./minute, and then reheating and quenching in a manner selected from the following heat treatment conditions (1), (2) or (3) to form a carburized surface layer having a mixed phase structure comprising martensite and retained austenite and an uncarburized core having a mixed phase structure comprising ferrite and martensite or ferrite, martensite and bainite,
• (1) heating said parts at a temperature which is greater than the Ac.sub.3 transformation point of the carburized surface layer having a C-content of at least 0.4% by weight but which is less than 850.degree. C., retaining the parts at said temperature and then quenching,
• (2) heating and retaining said parts at said temperature designated in case (1) above, cooling to a temperature not less than 600.degree. C. and then quenching,
• (3) heating and retaining said parts at said temperature designated in case (1) above, cooling to a temperature not less than 600.degree. C., retaining said parts at said temperature not less than 600.degree. C., and then quenching.