Nippon Steel & Sumitomo Metal Corporation

Shunta Homma

Atsushi Monden

Shingo Yamasaki

Kei Miyanishi

Koichi Banno

A steel bar according to one embodiment of the present invention includes predetermined chemical composition, wherein a quenching deflection in a cross section is 1.5 mm or less, wherein &#x394;max and &#x394;min is 1.5 mm or less, wherein a structure in a surface layer area includes 10 area % or less of ferrite and a remainder including one or more selected from a group consisting of a bainite and a martensite, wherein an average value of the grain size of a bcc phase in the surface layer area is 1.0 to 10.0 &#x3bc;m, wherein an average value of the grain size of the bcc phase in a center area is 1.0 to 15.0 &#x3bc;m, wherein a hardness of a region of which a depth from the surface is 50 &#x3bc;m is Hv200 to Hv500, and wherein a total decarburized layer thickness DM-T is 0.20 mm or less.

Steel bar

A steel wire rod or steel bar as hot-rolled, including: by mass %: C: 0.1 to 0.6%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.5%, Al: 0.015 to 0.3%, and N: 0.0040 to 0.0150%, and P: limited to 0.035% or less and S: limited to 0.025% or less, and the balance substantially consisting of iron and unavoidable impurities, wherein a depth of d (mm) from the surface of the surface layer region with 20 HV 0.2 or more higher, relative to HV 0.2 that is the average hardness in the region where the depth from the surface is from sectional radius R&#xd7;0.5 (mm) to the center satisfies the formula (1); the steel structure of the surface layer region has a ferrite fraction of 10% or less by area ratio, with the balance being one or two or more of martensite, bainite and pearlite; the steel structure where the depth from the surface is from the sectional radius R&#xd7;0.5 (mm) to the center is ferrite-pearlite or ferrite-bainite; and the surface roughness Ra in the circumferential direction when scales adhering to the surface have been removed is 4 &#x3bc;m or less.

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Narashino, Japan

Shunta Homma