Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Tatsusei Tada

Daisuke Hirakami

Makoto Okonogi

Toshiyuki Manabe

Arata Iso

Susumu Sahara

A steel wire for a non heat-treated mechanical part includes, as a chemical composition, by mass %, a predetermined amount of C, Si, Mn, Cr, Mo, Ti, Al, B, Nb, and V, and limited P, S, N, and O and a remainder of Fe and impurities; in which a structure includes, by volume %, a bainite of greater than or equal to 75&#xd7;[C %]+25, and a remainder of one or more of a ferrite and a pearlite when an amount of C is set to [C %] by mass %; when an average aspect ratio of a bainite block in a second surface layer area of the steel wire is set as R1, the R1 is greater than or equal to 1.2; when an average grain size of a bainite block in a third surface layer area of the steel wire is set to P&#x3bc;m, and an average grain size of a bainite block in a third center portion of the steel wire is set to P&#x3bc;m, the Psatisfies Expression (c), and the Pand the Psatisfy Expression (d), a standard deviation of a grain size of the bainite block in the structure is less than or equal to 8.0 &#x3bc;m; and a tensile strength is in a range of 800 MPa to 1600 MPa,&#x2264;20/1&#x2003;&#x2003;(c),&#x2264;0.95&#x2003;&#x2003;(d).

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Muroran, Japan

Tatsusei Tada