Abstract: A steel wire for machine structural parts, including respective predetermined contents of C, Si, Mn, P, S, Al, Cr, N, and iron, wherein when a total content of Cr and Mn (% by mass) in cementite in the metallurgical microstructure is expressed as {Cr+Mn}, a total content of Cr and Mn (% by mass) in steel is expressed as [Cr+Mn], and a C content (% by mass) of the steel is expressed as [C], a concentration ratio {Cr+Mn}/[Cr+Mn] is (0.5[C] +0.040) or more, and an average circular-equivalent diameter of all the cementite is (1.668-2.13[C]) ?m or more and (1.863-2.13[C]) ?m or less.

Abstract: A steel wire for machine structural parts, may include Fe, inevitable impurities, and, by mass: 0.05 to 0.60% C; 0.005 to 0.50% Si; 0.30 to 1.20% Mn; more than 0 to 0.050% P; more than 0 to 0.050% S; 0.001 to 0.10% Al; more than 0 to 1.5% Cr; and more than 0 to 0.02% N. An area of cementite present at ferrite grain boundaries in an area of all cementite of the steel wire may be 32% or more. When a C content (% by mass) of a steel is expressed as [C], an average circular-equivalent diameter of all the cementite is (1.668-2.13 [C]) ?m or more and (1.863-2.13 [C]) ?m or less.