Patent ID: 7815750

Claim:
A method of production of a steel soft nitrided machine part comprising the steps of: preparing a steel material of a composition containing, by mass %, C: 0.15 to 0.30%, Si: 0.03 to 1.00%, Mn: 0.20 to 1.5%, S: 0.04 to 0.06%, Cr: 0.01 to 0.5%, Mo: 0.40 to 1.5%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, V: 0.2 to 0.4%, Ni: 0.05 to 1.5%, N: 0.002to 0.0048%, and the balance of Fe and unavoidable impurities, limiting P to 0.02% or less in said unavoidable impurities, limiting a value of Ceq., defined by the following equation (1) where the C content (%) is indicated by [C], the Si content (%) by [Si], the Mn content (%) by [Mn], the P content (%) by [P], the S content (%) by [S], the Cr content (%) by [Cr], the Mo content (%) by [Mo], the V content (%) by [V], and the Ni content (%) by [Ni], to 0.65 to 0.85, having a value of DI, defined by the following equation (2), of 80 to 155, having a value of log Kp, defined by the following equation (3), of 2.5 to 8, and further having a relationship between the Si content and Mn content satisfying the following equation (4), heating this to 1150 to 1280° C., then hot forging the material to the shape of the part, and cooling it after forging by a 0.5 to 1.5° C./sec cooling rate to obtain a hot forged part having a micrometallic structure in which a ratio of a bainite base structure is more than 50%; machining said hot forged part, then; soft nitriding the machined hot forged part under temperature conditions of 550 to 650° C. for 30 minutes or more, Ceq . = [ C ] + 1 10 × [ Si ] + 2 11 × [ Mn ] + 1 5 × [ Cr ] + 1 3 ⁡ [ V ] + 1 6 × [ Mo ] ( 1 ) DI = 8.65 × [ C ] × ( 1 + 0.64 × [ Si ] ) × ( 1 + 4.10 × [ Mn ] ) × ( 1 + 2.83 × [ P ] ) × ( 1 - 0.62 × [ S ] ) × ( 1 + 2.33 × [ Cr ] ) × ( 1 + 0.52 × [ Ni ] ) × ( 1 + 3.14 × [ Mo ] ) ( 2 ) log ⁢ ⁢ Kp = 0.597 × [ C ] - 0.100 × [ Si ] + 1.395 × [ Mn ] + 0.395 × [ Ni ] + 1.295 × [ Cr ] + 3.730 × [ Mo ] - 0.869 ( 3 ) 2.9 × [ Si ] + [ Mn ] ≧ 2.0 . ( 4 )