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

A rolled steel bar for machine structural use having a 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,
• K1 obtained from the following Expression 1 is 0.95 to 1.05,
• K2 obtained from the following Expression 2 is more than 35,
• a Mn content and a S content satisfy the following Expression 3,
• a total decarburized depth of a surface layer is 500 μm or less,
• K1=C+Si/7+Mn/5+1.54×V (Expression 1),
• K2=139−28.6×Si+105×Mn−833×S−13420×N (Expression 2),
• Mn/S≧8.0 (Expression 3), and
• C, Si, Mn, V, S, and N in the Expressions 1 to 3 represent the amounts of the respective elements in mass %.