To stably provide a steel material having both of high torsion-fatigue strength and adequate machinability.
This steel material comprises, by mass%, 0.35-0.7% C, 0.30-1.1% Si, 0.2-2.0% Mn, 0.005-0.25% Al, 0.005-0.1% Ti, 0.05-0.6% Mo, 0.0003-0.006% B, 0.06% or less S, 0.02% or less P, 0.2% or less Cr, one or more elements selected among 0.2% or less Te, 0.2% or less Se, 0.3% or less Sn, 0.2% or less Zr, 0.02% or less Ca, 0.03% or less REM, 0.6% or less Pb, 0.3% or less Bi, 0.2% or less Sb, 0.1% or less W and 0.04% or less N, and the balance Fe with unavoidable impurities; has a base metal structure containing at least a bainitic structure and a martensitic structure in an amount of 10 vol.% or more in total; and contains former austenite grains with sizes of 12 μm or less through the whole thickness direction of an induction-hardened layer.
MATSUZAKI AKIHIRO
HAYASHI TORU
TOYOOKA TAKAAKI
