Non-tempered steel with small material anisotropy and excellent strength, toughness and machinability, and method for producing the same

摘要

Provided is a non heat-treated steel that requires no particular controls over cooling rates and no aging treatments after hot working, that can sufficiently increase tensile strength, yield strength and toughness even at lightly deformed parts, and furthermore, that has excellent material anisotropy and machinability as an alloy steel for machine structures. In other words, it is a non heat-treated steel that contains: C: more than 0.05 mass% to less than 0.10 mass%; Si: 1.0 mass% or less; Mn: more than 2.2 mass% to 5.0 mass%; S: less than 0.020 mass%; Cu: more than 1.0 mass% to 3.0 mass%; Ni: 3.0 mass% or less; Cr: 0.01 to 2.0 mass%; Al: 0.1 mass% or less; Ti: 0.01 to 0.10 mass%; B: 0.0003 to 0.03 mass%; N: 0.0010 to 0.0200 mass%; O: 0.0060 mass% or less; and the balance Fe and inevitable impurities, and that the steel structure is bainitic having block structures at 10% or more in area ratios. IMAGE