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.