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Abstract

A steel wire rods and bars for machine structural use having, in the as-hot-rolled state, the same cold workability as a conventional steel wire rods and bars subjected to softening annealing after hot rolling, and a production method thereof are provided. This hot-rolled wire rods and bars usable for machine structural use without annealing comprises, in terms of mass %, C: 0.1 to 0.5%, Si: 0.01 to 0.5%, Mn: 0.3 to 1.5% and the balance of Fe and unavoidable impurities and if desired, comprises strengthening elements, wherein the microstructure of steel is composed of ferrite and pearlite, the ferrite grain size is No. 11 or more as defined in JIS G 0552, a granular carbide having an equivalent-circle diameter of 2 μm or less and an aspect ratio of 3 or less is contained in an area ratio of 5 to 40%, and the steel wire rods and bars has a tensile strength TS (MPa)≦573×Ceq+257 and a reduction of area RA (%)≧-23×Ceq+75 (wherein Ceq=C+Si/7+Mn/5+Cr/9+Mo/2).

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First Claim