ALLOY STEEL FOR MACHINE STRUCTURAL USE

PROBLEM TO BE SOLVED: To provide an alloy steel for machine structural use which has high fatigue strength, is excellent in machinability, particularly in chipping resistance and can secure a stable life of a tool. SOLUTION: The alloy steel for machine structural use contains, by mass, 0.15-0.25% C, ≥0.15% to 1.25% to ≤1.80% Cr, 0.005-0.035% Al and ≥0.010-0.025% N, wherein Si and Cr satisfy relationship: 1.50≤Si+Cr≤1.85, the balance comprising Fe and impurities, wherein P and O in the impurities respectively satisfy relationships: P≤0.020% and O≤0.0020%, further, 95% or more of the structure is composed of a ferrite-pearlite structure and, moreover, the proportion of ferrite occupied in the structure is 40 to 60%. The alloy steel for machine structural use may contain at least one kind of Mo, Ni, Nb and Ca which satisfy [Mn+Mo+Ni≤1.30] each in a specific amount. COPYRIGHT: (C)2011,JPO&INPIT