CARBONITRIDING STEEL MATERIAL AND CARBONITRIDED STEEL MATERIAL

To provide a carbonitriding steel material high in surface fatigue strength and sufficiently high in high temperature tempering hardness, and a carbonitrided steel material obtained by carbonitriding the carbonitriding steel material.SOLUTION: A carbonitriding steel material including C, Si, Mn, P, S, Cu, Ni, Cr, Mo, Al, N and the remainder consisting of Fe and inevitable impurities satisfies a surface C concentration (mass%)+12/14x a surface N concentration (mass%)=0.6 to 1.4, and 129.7805×[Cr(mass%)]-76.9797×[Cr (mass%)]2+339.3375x[a surface N concentration (by mass%)]-539.345×[a surface N concentration (mass%)] 2+181.4983×[Cr (mass%)]×[a surface N concentration (mass%)]+437.6799>560 by carbonitriding. A carbonitrided steel material having a hardness of 560 HV or more in a portion of a depth of 0.05 mm from the surface is obtained when tempered at 500°C after the carbonitriding.SELECTED DRAWING: Figure 2 【課題】面疲労強度が高く、かつ、高温焼戻し硬さが十分に高い浸炭窒化処理用鋼材およびそれを浸炭窒化処理してなる浸炭窒化鋼材の提供。【解決手段】Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ａｌ、Ｎを含有し、残部はＦｅおよび不可避的不純物からなり、浸炭窒化を施すことで、表面Ｃ濃度（質量％）＋１２／１４×表面Ｎ濃度（質量％）＝０．６～１．４を満たし、１２９．７８０５×［Ｃｒ（質量％）］－７６．９７９７×［Ｃｒ（質量％）］2＋３３９．３３７５×［表面Ｎ濃度（質量％）］－５３９．３４５×［表面Ｎ濃度（質量％）］2＋１８１．４９８３×［Ｃｒ（質量％）］×［表面Ｎ濃度（質量％）］＋４３７．６７９９＞５６０を満たし、前記浸炭窒化を施した後に５００℃で焼戻し処理を施した場合に、表面から０．０５ｍｍの深さの部分における硬さが５６０ＨＶ以上である浸炭窒化鋼材が得られる、浸炭窒化処理用鋼材。【選択図】図２