A steel wire for non-heat treated machine parts, the steel wire contains, based on % by mass: C: from 0.20 to 0.40%, Si: from 0.05 to 0.50%, Mn: from 0.50 to 2.00%, Al: from 0.005 to 0.050%, and the balance being Fe and impurities, in which the microstructure contains bainite of (35×[C %]+50)% or more, and when the diameter is defined as D, the average aspect ratio of a bainite grain at a depth of 50 μm in the L cross section is defined as AR, and the average grain size of a bainite grain at a depth of 50 μM in the C cross section is defined as GD, AR is 1.4 or more, (AR)/(the average aspect ratio of a bainite grain at a depth of 0.25D in the L cross section) is 1.1 or more, GD is (15/AR) μm or less, and (GD)/(the average grain size of a bainite grain at a depth of 0.25D in the C cross section) is less than 1.0.
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