HIGH-STRENGTH STEEL BOLT HAVING EXCELLENT RESISTANCE FOR DELAYED FRACTURE AND METHOD FOR PRODUCING THE SAME

摘要

A bolt for use in connection of steel structures and for use as automotive parts and components, and a method for producing the same are disclosed. More specifically, provided is a steel bolt which is capable of achieving high strength simultaneously with an excellent delayed fracture resistance, by appropriate control of the steel microstructure. The bolt has a composition comprising (i) 0.35-0.55 wt% of carbon, 0.05-2.0 wt% of silicon, 0.1-0.8 wt% of manganese, 0.001-0.004 wt% of boron, 0.3-1.5 wt% of chromium, 0.005 wt% and less of oxygen (T.O), 0.015 wt% and less of phosphorous, 0.010 wt% and less of sulfur, and the balance of Fe with inevitable impurities; and further comprising (ii) at least one selected from the group consisting of 0.05-0.5 wt% of vanadium, 0.05-0.5 wt% of niobium, 0.1-0.5 wt% of nickel, 0.1-1.5 wt% of molybdenum and 0.01-0.1 wt% of titanium, wherein the bolt has an internal structure composed of ferrite and tempered martensite, and a content of ferrite in the internal structure is 3 to 10% in terms of an area fraction. The present invention can provide a high-strength bolt which is capable of simultaneously achieving an excellent delayed fracture resistance and a high strength without addition of large amounts of alloying elements, and also exhibits no deterioration of notch toughness. Further, the present invention can provide a method for preparing such a bolt which is simply and conveniently carried out without complicated heat treatment processes.