Nippon Steel & Sumitomo Metal Corporation

Isuzu Motors Limited and Nippon Steel Corporation

Osamu Ohyama

Hiromasa Takada

Shinya Teramoto

Hirohito Eto

Tetsuroh Hashiguchi

A rolled steel bar for machine structural use includes a predetermined chemical composition comprising, by mass %, C: 0.45% to 0.65%, Si: higher than 1.00% to 1.50%, Mn: higher than 0.40% to 1.00%, P: 0.005% to 0.050%, S: 0.020% to 0.100%, V: 0.08% to 0.20%, Ti: 0% to 0.050%; Ca: 0% to 0.0030%, Zr: 0% to 0.0030%, Te: 0% to 0.0030%, and a remainder including Fe and impurities, wherein the impurities include: Cr: 0.10% or lower, Al: lower than 0.01%, and N: 0.0060% or lower. In the rolled steel bar for machine structural use, K1 obtained from 'K1=C+Si/7+Mn/5+1.54&#xd7;V' is 0.95 to 1.05, K2 obtained from 'K2=139&#x2212;28.6&#xd7;Si+105&#xd7;Mn&#x2212;833&#xd7;S&#x2212;13420&#xd7;N' is more than 35, a Mn content and a S content satisfy Mn/S&#x2265;8.0, and a total decarburized depth of a surface layer is 500 &#x3bc;m or less.

Rolled steel bar for machine structural use and method of producing the same

A rolled steel bar for machine structural use includes a predetermined chemical composition comprising, by mass %, C: 0.45% to 0.65%, Si: higher than 1.00% to 1.50%, Mn: higher than 0.40% to 1.00%, P: 0.005% to 0.050%, S: 0.020% to 0.100%, V: 0.08% to 0.20%, Ti: 0% to 0.050%; Ca: 0% to 0.0030%, Zr: 0% to 0.0030%, Te: 0% to 0.0030%, and a remainder including Fe and impurities, wherein the impurities include: Cr: 0.10% or lower, Al: lower than 0.01%, and N: 0.0060% or lower. In the rolled steel bar for machine structural use, K1 obtained from 'K1=C+Si/7+Mn/5+1.54&#xd7;V' is 0.95 to 1.05, K2 obtained from 'K2=139&#x2212;28.6&#xd7;Si+105&#xd7;Mn&#x2212;833&#xd7;S&#x2212;13420&#xd7;N' is more than 35, K3 obtained from &#x201c;K3=137&#xd7;C&#x2212;44.0&#xd7;Si&#x201d; is 10.7 or more, a Mn content and a S content satisfy Mn/S&#x2265;8.0, and a total decarburized depth in a surface layer is 500 &#x3bc;m or less.

A steel for machine structural use, essentially having the following chemical composition:

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Hokkaido, Japan

Osamu Ohyama