| 摘要 |
PROBLEM TO BE SOLVED: To stably provide a steel material having a combination of high torsional fatigue strength and satisfactory delayed fracture resistance. SOLUTION: The steel material has a composition comprising, by mass%, 0.35-0.7% C, 0.30-1.1% Si, 0.2-2.0% Mn, 0.005-0.25% Al, 0.05-0.6% Mo, 0.0003-0.006% B, 0.06 mass% or less S, 0.02% or less P, 0.2% or less Cr, one or two elements selected from 0.06-0.10% Ti and 0.25-0.50% V, and the balance Fe with unavoidable impurities; has a base metal structure containing at least a bainite structure and a martensite structure in an amount of 10 vol.% or more in total; and has a former austenite grain size of 5μm or smaller in an induction-hardened layer through the whole thickness direction of the hardened layer. COPYRIGHT: (C)2005,JPO&NCIPI
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