The steel null for carburization of the gear which is superior in fatigue

摘要

PURPOSE:To manufacture a gear excellent in fatigue strength by properly regulating respective contents of Mo, V, W, and Cu in the composition of a carburizing steel for gear composed of low alloy steel to allow proper amounts of retained austenite to exist in the surface layer of a gear. CONSTITUTION:At the time of manufacturing a power transmission gear for industrial machinery, etc., a steel which has a composition containing, by weight, 0.10-0.40% C, 0.15% Si, 0.30-2.00% Mn, 0.40-2.00% Cr, 0.30-2.00% Mo, 0.030% P, 0.030% S, 0.10% Cu, 0.0020% O, and at least one kind among 0.010% Ca, 0.10% Te, 0.10% Zr, and 0.10% REM and having the quantitative relations represented by inequality I or a composition further containing 0.03-0.20% V and/or 0.03-1.0% W besides the above components and satisfying the quantitative relations represented by inequality II and in which an aspect ratio l/W between the length (l) and width W of contained sulfide-type inclusions, such as MnS, is regulated to =5 is used. The prepared gear is subjected to carburizing hardening to allow proper amounts of austenite to remain and to increase residual compressive stress, by which the gear excellent in fatigue strength can be manufactured.