Conventionally, as heat-resisting steels for exhaust valves of automobile engine valves, there have widely been used 21-4N steel (JIS specification: SUH35), that is, a high Mn heat-resisting steel, and improved steels thereof which are good high temperature strength and oxidation resistance, and low cost.
For the face part of engine valves, high wear resistance is required because of continual contact with a valve seat. Accordingly, for the face part of valves using the 21-4N steel or improved steels thereof, usually, the built-up of Stellite etc. is done to thereby reinforce the hardness and wear resistance at high temperatures.
Moreover, as a valve material used for portions exposed to a higher load, there is used in part a precipitation strengthening-type heat-resisting alloy including a lot of Ni and having an enhanced high temperature strength by precipitating γ′ (gamma prim) being an intermetallic compound, or NCF751 being a super heat-resisting alloy. However, since these alloys contain a lot of Ni, there is such a problem of increasing the cost.
However, as the result of the tightening of environmental regulations in recent years, the efficiency and power of gasoline engines are enhanced to raise the combustion temperature, and therefore, a request is placed for a heat-resisting steel for valves which is low cost and excellent in high temperature strength as compared with the above-described heat-resisting alloys.
In order to answer the request, Japanese Patent Application Laid-Open No. 2001-323323 (Patent Document 1) proposes a production method of an engine valve, in which a base material formed by adding appropriately Mo, Nb and V besides C, N, Mn, Ni and Cr to a base of inexpensive Fe-based heat-resisting steel, and suppressing as much as possible the use of expensive raw materials such as Ni is used, which is subjected to a solution heat treatment at 1100 to 1180° C. and, after that, is subjected to forging in a temperature range of 700 to 1000° C. to form a valve having been subjected to an aging treatment of accumulating residual strain by machining intended for strain age hardening, thereby enhancing the hardness of the face part of the engine valve to 400 HV or more and controlling the overaging and softening even in the use in high temperature regions.
Furthermore, Japanese Patent Application Laid-Open No. 2002-294411 (Patent Document 2) and Japanese Patent Application Laid-Open No. 3-177543 (Patent Document 3) propose engine valve materials obtained by adding, as an improved material of 21-4N steel being a high Mn heat-resisting steel, alloying elements such as Mo, W, Nb and V to thereby promote solid solution strengthening or precipitation strengthening and to improve high temperature strength and wear resistance.