| 摘要 |
PROBLEM TO BE SOLVED: To provide a rolled steel bar or wire rod for hot forging in which bending fatigue strength and surface fatigue strength are high and heat treatment strain is reduced after carburizing and quenching or carbonitriding quenching.SOLUTION: A rolled steel bar or wire rod for hot forging has a chemical composition including 0.1 to 0.3% C, 0.01 to 0.6% Si, 0.4 to 1.0% Mn, 0.003 to 0.05% S, 0.80 to 2.00% Cr, ≤0.50% (including 0%) Mo, 0.01 to 0.05% Al, 0.010 to 0.025% N, and the balance comprising Fe and impurities, in which ≤0.025 P, ≤0.003% Ti and ≤0.002% O are satisfied in the impurities, a difference between the maximum value and the minimum value of [550-361×C-39×Mn-20×Cr-5×Mo] value in specified 17 parts in the cross-sectional face is ≤10, microstructure is made of ferrite-pearlite, ferrite-pearlite-bainite or ferrite-bainite, and a ratio between the maximum value and the minimum value of ferrite average grain sizes upon 15 visual field observation measurement at random in an area of 62,500 μmper visual field in the cross-sectional face is ≤2.0. |
