PROCESS FOR PRODUCING HIGH-STRENGTH MOLDING, HIGH-STRENGTH MOLDING OBTAINED THEREBY AND HIGH-STRENGTH MACHINE SCREW

摘要

A steel of ultrafine structure having a ferrite structure of = 500 nm average crystal grain diameter is prepared by subjecting a steel material to warm working and further to cold working, thereby producing a steel excelling in strength and ductility. By forming this steel, there can be produced a machine screw, or the like, having an extremely thin portion of = 0.25 mm. In the production of machine screws, as the thickness of head portion is reduced, the thickness of the thinnest portion of engaging grooves is reduced. Thus, not only is a minimum reinforcement formed at the neck bottom portion but also framing thereof is made of the steel of ultrafine structure. Consequently, there can be accomplished production of a screw that ensures the fundamental performance of screw, being capable of preventing any delayed fracture after tightening, and that even in the form of screw with its head partition thickness minimized and with its nominal diameter reduced, has satisfactory strength, thereby providing screws permitting parts assembly for portable electronic equipment, etc. In this processing, there is no need of carrying out softening treatment, such as spheroidizing annealing, of screw material. Further, after working into machine screws, there is no need of carrying out surface hardening heat treatment or toughening treatment, such as quenching or tempering.