| abstract |
Chemical component is mass%, C: 0.1-0.6%, Si: 0.01-1.5%, Mn: 0.05-2.5%, Al: 0.015-0.3% , N: 0.004 to 0.015%, may further contain a predetermined optional element, the balance is iron and impurities, and P and S in the impurities are P: 0.035% or less, S: Steel wire rod or steel bar of 0.025% or less, and the surface layer region from the surface to a depth of 15% of the cross-sectional radius is ferrite having an average particle diameter of 1 to 15 μm and an average aspect ratio of 2 or less. , A steel structure that is spheroidized cementite having an average particle diameter of 0.1 to 2 μm, and an internal region from the surface to a depth of 25% of the cross-sectional radius to the center includes ferrite having an average particle diameter of 15 to 40 μm, It is a steel structure composed of pearlite and / or spheroidized cementite and has a surface scale. Circumferential surface roughness Ra of the surface after removal of the is at 4μm or less, the steel wire rod or steel bar depth of the grain boundary oxidized layer on the surface is 30μm or less. |