STEEL FOR MACHINE STRUCTURAL USE EXCELLENT IN MACHINABILITY, COLD FORGEABILITY, FATIGUE STRENGTH CHARACTERISTIC AFTER QUENCH-AND-TEMPER, MEMBER OF THE SAME, AND THEIR PRODUCTION

摘要

PURPOSE:To produce a carbon steel for machine structural use, having machinability equal to or higher than that of conventional Pb-added free cutting steel without deteriorating cold forgeability and excellent in fatigue strength characteristics. CONSTITUTION:A bloom of a steel, having a composition consisting of, by weight, 0.1-1.5% C, 0.5-2.0% Si, 0.1-2.0% Mn, 0.0003-0.0150% B, 0.005-0.1% Al, =0.0030% O, =0.020% P, =0.035% S, 0.0015-0.0150% N, and the balance Fe with inevitable impurities, is heated up to a temp. not lower than the sold solution temps. of BN and AlN, hot-rolled, heated to 300-600 deg.C, held at a temp. in the temp. region for =15min, further heated to 680-740 deg.C, and held for =5hr. By this method, fatigue strength characteristics of the steel after quench-and- temper can be improved.