STEEL WIRE FOR BOLT, BOLT, AND PRODUCTION METHOD THEREFOR

摘要

Provided are a high-strength bolt having a tensile strength of 1,000 to 1,400 MPa and exhibits the same level of the delayed fracture resistance as that of the standard alloy steel even when omitting or reducing Mo and Cr from the SCM steel and the SCr steel, a steel wire for bolts used to manufacture such a high-strength bolt, and useful methods for manufacturing them. The steel wire for bolts in the present invention satisfies the predetermined chemical composition. When a rate of a B content at D 0 /4 part in the steel wire for bolts is 100% where D 0 is a diameter of the steel wire for bolts, a ratio of a B content at a surface of the steel wire for bolts is 75% or less on average, and a difference between a maximum value and a minimum value of the ratio is 25% or less. A prior austenite crystal grain size number in a region from the surface to a depth of 100 µm of the steel wire for bolts is No. 8 or more.