Nippon Steel Sumitomo Metal Corporation

Honda Motor Co., Ltd.

Masayuki Horimoto

Hideki Imataka

Yuya Gyotoku

Kosuke Tanaka

Akihito Ninomiya

Yoshinari Okada

Akira Shiga

Masaki Amano

Naoyuki Sano

Yoshihiro Daitoh

Gen Kato

Mitsuru Fujimoto

Masaomi Nakaoka

Yusuke Usui

A steel for cold forging/nitriding has, by mass percent, C: 0.01 to 0.15%, Si&#x2264;0.35%, Mn: 0.10 to 0.90%, P&#x2264;0.030%, S&#x2264;0.030%, Cr: 0.50 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.10%, N&#x2264;0.0080%, and O&#x2264;0.0030%, further according to need a specific amount of one or more elements selected from Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se and Sb, with the balance being Fe and impurities, and further satisfying the conditions of [399&#xd7;C+26&#xd7;Si+123&#xd7;Mn+30&#xd7;Cr+32&#xd7;Mo+19&#xd7;V&#x2264;160], [20&#x2264;(669.3&#xd7;logC&#x2212;1959.6&#xd7;logN&#x2212;6983.3)&#xd7;(0.067&#xd7;Mo+0.147&#xd7;V)&#x2264;80], [140&#xd7;Cr+125&#xd7;Al+235&#xd7;V&#x2265;160] and [90&#x2264;511&#xd7;C+33&#xd7;Mn+56&#xd7;Cu+15&#xd7;Ni+36&#xd7;Cr+5&#xd7;Mo+134&#xd7;V&#x2264;170] are excellent in cold forgeability and machinability after cold forging.

Steel for cold forging/nitriding, steel material for cold forging/nitriding, and cold-forged/nitrided component

A case hardening steel material having a chemical composition consists of, by mass percent, C: 0.15 to 0.23%, Si: 0.01 to 0.15%, Mn: 0.65 to 0.90%, S: 0.010 to 0.030%, Cr: 1.65 to 1.80%, Al: 0.015 to 0.060%, and N: 0.0100 to 0.0250%, further containing, as necessary, one or more kinds selected from Cu and Ni of predetermined amounts, the balance being Fe and impurities; 25&#x2266;Mn/S&#x2266;85, 0.90&#x2266;Cr/(Si+2Mn)&#x2266;1.20, and 1.16Si+0.70Mn+Cr&#x2267;2.20; P, Ti and O in the impurities being P&#x2266;0.020%, Ti&#x2266;0.005%, and O&#x2266;0.0015%; and having a structure consisting of 20 to 70% in an area ratio being ferrite; and the portion other than the ferrite being one or more kinds of pearlite and bainite. The steel material is used suitably as a raw material of the carburized part such as a CVT pulley shaft.

Case hardening steel material

A steel material for nitriding has a composition comprising, by mass percent, C: more than 0.15% and not more than 0.35%, Si&#x2266;0.20%, Mn: 0.10 to 2.0%, P&#x2266;0.030%, S&#x2266;0.050%, Cr: 0.80 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.06%, N&#x2266;0.0080%, O&#x2266;0.0030%, and optionally one or more elements of Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se and Sb, the balance being Fe and impurities. The composition satisfies the conditions of [20&#x2266;(669.3&#xd7;logC&#x2212;1959.6&#xd7;logN&#x2212;6983.3)&#xd7;(0.067&#xd7;Mo+0.147&#xd7;V)&#x2266;80] and [140&#xd7;Cr+125&#xd7;Al+235&#xd7;V&#x2267;160]. The microstructure is a ferritic-pearlitic structure, a ferritic-bainitic structure, or a ferritic-pearlitic-bainitic structure. The area fraction of ferrite is 20% or more and the precipitate content of V is 0.10% or less.

Steel material for nitriding and nitrided component

A steel comprises, by mass percent, C: 0.10 to 0.15%, Si: not less than 0.02% and less than 0.10%, Mn: more than 0.90% and not more than 2.50%, P&#x2266;0.030%, S&#x2266;0.050%, Cr: 0.80 to 2.0%, V: 0.05 to 0.50%, Al: 0.01 to 0.07%, N&#x2266;0.0080%, O&#x2266;0.0030%, and one or more selected from Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se and Sb, the balance being Fe and impurities. The composition satisfies [35&#x2266;Mn/S&#x2266;200], [20&#x2266;(669.3&#xd7;logC&#x2212;1959.6&#xd7;logN&#x2212;6983.3)&#xd7;(0.067&#xd7;Mo+0.147&#xd7;V)&#x2266;80], [140&#xd7;Cr+125&#xd7;Al+235&#xd7;V&#x2267;160] and [150&#x2266;511&#xd7;C+33&#xd7;Mn+56&#xd7;Cu+15&#xd7;Ni+36&#xd7;Cr+5&#xd7;Mo+134&#xd7;V&#x2266;200].

A method for producing a steel component by performing the treatments of the following sequentially steps 1 and 2 to a steel material containing, by mass %, C: 0.15 to 0.25%, Si: 0.01 to 0.10%, Mn: 0.50 to 0.80%, S: 0.003 to 0.030%, Cr: 0.80 to 1.20%, Mo: 0.30 to 0.45%, Al: 0.015 to 0.050%, and N: 0.010 to 0.025%, wherein Cr/Mn is 1.3 to 2.4, the balance being Fe and impurities, and contents of P and O among the impurities are P: not more than 0.010%, and O: not more than 0.0020%:

Method for producing steel component

A rolled steel bar or a wire rod for hot forging capable of coping with both bending/surface fatigue strength of components and machinability at a high level includes: a composition containing, in mass %, C: 0.1 to 0.25%, Si: 0.01 to 0.10%, Mn: 0.4 to 1.0%, S: 0.003 to 0.05%, Cr: 1.60 to 2.00%, Mo: 0.10% or less (including 0%), Al: 0.025 to 0.05%, and N: 0.010 to 0.025%, where a value of fn1 represented in a following formula (1) satisfies 1.82&#x2266;fn1&#x2266;2.10: fn1=Cr+2&#xd7;Mo (1); impurities containing P: 0.025% or less, Ti: 0.003% or less, and O (oxygen): 0.002% or less; and a cross section in which a maximum value/a minimum value of an average ferrite grain diameter is 2.0 or less when measurement by observation is randomly carried out in 15 visual fields with an area per visual field set to be 62500 &#x3bc;m.

Rolled steel bar or wire for hot forging

A method for producing a carbonitrided member comprising the steps of carburizing a steel part having a composition of, by mass percent, C: 0.10 to 0.35%, Si: 0.15 to 1.0%, Mn: 0.30 to 1.0%, Cr: 0.40 to 2.0%, S: 0.05% or less with the balance being Fe and impurities in a carburizing atmosphere of 900 to 950&#xb0; C. The carburized steel part is carbonitrided in a carbonitriding atmosphere with the temperature of 800 to 900&#xb0; C. and the nitrogen potential of 0.2 to 0.6%. The carbonitrided steel part is qhenched and then shot peened. The shot peening treatment may be further performed while heating to a temperature not higher than 350&#xb0; C. or immediately after heating to a temperature not higher than 350&#xb0; C. After quenching, the shot peening treatment may be further performed after tempering in the temperature range exceeding 250&#xb0; C. and not higher than 350&#xb0; C.

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Tokyo, Japan

Masayuki Horimoto