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Abstract

An object of the present invention is to provide an untempered free-machining steel to be nitrided, not containing Pb, capable of securing a predetermined strength and hardness and having machinability higher than steel containing Pb; and nitrided products. To be more specific, the present invention relates to a bainite-type untempered free-machining steel to be nitrided containing C: 0.05 to 0.8 wt %, Si: 0.01 to 2.5 wt %, Mn: 0.1 to 3.5 wt %, P: 0.001 to 0.2 wt %, S: 0.01 to 0.2 wt %, Cr: 1.0 to 3.5 wt %, V: 0.1 to 0.5 wt %, Al: 0.001 to 0.020 wt %, Ca: 0.0005 to 0.02 wt %, and O: 0.0005 to 0.01 wt % and the residue with an alloy composition of unavoidable impurities and Fe as its basic alloy composition, provided that a content of each of the Mn, the S and the Cr satisfy the formula: ([Mn]-55.times.[S]/32+[Cr])>2.0, in which an occupation area of a sulfide inclusion present in contact with an oxide inclusion containing 8 to 62 wt % of CaO and containing not less than 1.0 wt % of Ca is not less than 2.0.times.10.sup.-4 mm.sup.2 per visual field area of 3.5 mm.sup.2. The present invention also relates to a method of manufacturing the above bainite-type untempered free-machining steel to be nitrided and nitrided products manufactured by the said manufacturing method.

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First Claim