Torsional fatigue hardening steel and case hardened parts

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PROBLEM TO BE SOLVED : To provide a case-hardening steel excellent in torsional fatigue characteristics and suitable as the steel for parts such as the shaft of an automobile transmission, and to provide case hardened parts. SOLUTION : This steel has a composition containing, by mass, 0.1 to 0.4% C, 0.01 to 1.2% Si, 0.2 to 0.65% Mn, 0.005 to 0.15% S, 0.5 to 1.6% Cr, 0.0005 to 0.006% B and 0.015 to 0.1% Al, further containing specified quantity of one or more kinds selected from Te, Ca, Zr, Mg, Y and rare earth elements and moreover containing specified quantity of Ti, Nb or the like, in which the structural fractional ratio of bainite is limited to =8.