SURFACE HARDENED HIGH STRENGTH PARTS SMALL IN HEAT TREATING STRAIN

PURPOSE:To dvelop a steel for high strength machine structural use requiring surface hardness and small in heat treating strains by icorporating specified amounts of V, Mo, Al, N or the like into steel. CONSTITUTION:Machine structural parts are produced by case hardening steel obtd. by incorprating steel having a fundamental compsn. constituted of, by weight, 0.03 to 0.3% C, 0.03 to 0.35% Si, 0.3 to 3% Mn, 0.1 to 0.5% V, 0.015 to 0.06% Al and 0.002 to 0.02% N ro furthermore incorporated with 0.03 to 1.5% Cr, 0.02 to 2% Ni, 0.02 to 2% Cu, 0.005 to 0.1% Ti and 0.005 to 0.1% Nb independently or compositely. The structure of thee surface layer face of the parts is formed of a mixed one of martensite and retained austenite, the structure of the core part is formed of a mixed one of ferrite and martensite, the volume ratio of the ferrite is regulated to $40%, V compounds having 0.5 is regulated, by which the high strength machine structural parts whose surface has high hardness and free from heat treating strains can be obtd.