ROD FOR HIGH STRENGTH NON-HEAT-TREATED UPSET HEAD BOLT, MANUFACTURING METHOD THEREFOR, AND METHOD FOR MANUFACTURING UPSET HEAD BOLT USING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a rod used for manufacturing an upset head bolt superior in delayed fracture resistance, in spite of having as high or higher tensile strength as 900 N/mm² after the rod in as-hot-rolled state is draw-treated and cold-forged.;SOLUTION: The rod comprises is a two-phase structure of pearlite and ferrite, of which the ferrite fraction is 30-70% by area rate. Furthermore, an average particle size of carbides/nitrides in the ferrite is controlled to be 50 nm or less, and the number of carbides/nitrides with average sizes of 50 nm or less is controlled to be 50/μm² or more.;COPYRIGHT: (C)2003,JPO