| 摘要 |
The present invention provides an induction-hardened hollow driving shaft that comprises, as a raw material, a steel pipe that contains, by mass %, 0.30 to 0.47% C, 0.5% or less Si, 0.3 to 2.0% Mn, 0.018% or less P, 0.015% or less S, 0.15 to 1.0% Cr, 0.001 to 0.05% Al, 0.005 to 0.05% Ti, 0.004% or less Ca, 0.01% or less N, 0.0005 to 0.005% B and 0.0050% or less O (oxygen) and the balance Fe and impurities and of which Beff defined by an equation (a) or (b) below is 0.0001 or more, wherein a prior austenite grain size number (JIS G0551) after the hardening is 9 or more. Here, in the case of Neff=N−14×Ti/47.9≧0, Beff=B−10.8×(N−14×Ti/47.9)/14 . . . (a), and, in other cases, Beff=B . . . (b). According to the present invention, a hollow driving shaft that is simultaneously provided with excellent cold workability, hardenability, toughness and torsional fatigue strength and can exert stable fatigue lifetime can be obtained and can be widely utilized.
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