Steel wire for use in bolts that has excellent cold headability and resistance to delayed fracture after quenching and tempering, and bolt

A steel wire for use in bolts that has excellent cold headability and resistance to delayed fracture after quenching and tempering is provided. This steel wire, for use in bolts and having excellent cold headability and resistance to delayed fracture after quenching and tempering, contains, in mass%, C: 0.3-0.6%, Si: 1.0-3.0%, Mn: 0.10-1.5%, P: more than 0% and less than or equal to 0.020%, S: more than 0% and less than or equal to 0.020%, Cr: 0.3-1.5%, Al: 0.02-0.10%, and N: 0.001-0.02%, the remainder being iron and unavoidable impurities, wherein the ferrite grain size number is 6 to 12 in the position d×1/4 in the steel wire, d being the diameter of the steel wire, the ratio of carbides having an aspect ratio of 2.0 or less to all carbides in the d×1/4 position of the steel wire is 70% or greater, and the amount of C in the position at a depth of 0.1 mm from the surface is 60-120% of the amount of C in the base material.