MACHINE STRUCTURAL STEEL EXCELLENT IN DELAYED FRACTURE RESISTANCE

摘要

PURPOSE:To provide a machine structural steel free from the risk of the occurrence of delayed fracture for the prescribed period and having prescribed tensile strength by subjecting a steel in which compositional limit values are controlled to quench-and-temper treatment and forming the steel into a quenched-and- tempered structure. CONSTITUTION:A rolled stock of a steel which has a composition consisting of, by weight, 0.20-0.45% C, 0.50.2.0% Si, 0.50% Mn, =0.015% P, =0.01% S, 0.01-5.0% Cr, 0.01-0.15% Zr, 0.005-0.20% Nb, 0.005-0.10% Al, =0.035% N, and the balance essentially Fe with inevitable impurities and satisfying an inequality is subjected to quench-and-temper treatment so as to be formed into a quenched-and-tempered structure, by which the steel having =125kgf/mm2 tensile strength is prepared. Moreover, it is desirable that 0.01-0.30% V and/or 0.01-0.80% Mo is further incorporated as component element to the above composition.