STEEL FOR MACHINE STRUCTURAL USE, EXCELLENT IN MACHINABILITY AND QUENCHING CRACK RESISTANCE, AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To obtain a steel for machine structure use, having high torsional strength after induction hardening and tempering and excellent in machinability and quenching crack resistance, by specifying a chemical composition and the structure of a core part, respectively. ;SOLUTION: This steel for machine structural use has a composition consisting of, by mass, 0.35-0.60% C, ≤0.05% Si, 0.65-1.70% Mn, ≤0.020% P, 0.005-0.035% S, ≤0.15% Cr, 0.05-0.50% Mo, 0.01-0.05% Ti, 0.01-0.05% Al, ≤0.01% N, 0.0005-0.0050% B, and the balance Fe and also has a structure containing bainitic phase by 5-30% by area ratio, and further, the above torsional strength is regulated to ≥1400MPa in this steel. The quenching crack resistance of this steel can be improved to a greater extent by regulating the Ms value, defined by equation, Ms=538-317(%C)-33(%Mn)-28(%Cr)-17(%Ni)-11(%Si)-11(% Mo), to ≥360. Moreover, one or more kinds among ≤1.0% Cu, ≤3.5% Ni, 0.01-0.30% V, and 0.005-0.050% Nb can be further incorporated into the steel.;COPYRIGHT: (C)1997,JPO