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

1. 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 frorn the following Expression 2 is more than 35,

K3 obtained from the following Expression 3 is 10.7 or more,

a Mn content and a S content satisfy the following Expression 4,

a total decarburized depth in a surface layer is 500 μm or less,

K 1=C+Si/7+Mn/5+1.54×V  (Expression 1),

K 2=139−28.6×Si+105×Mn−833×S−13420×N  (Expression 2),

K 3=137×C−44.0×Si  (Expression 3),

Mn/S≥8.0  , and(Expression 4)

C, Si, Mn V, S, and N in the Expressions 1 to 4 represent the amounts of the respective elements in mass %.

2. The rolled steel bar for machine structural use according to claim 1 , wherein the chemical composition further comprising, by mass %,

one or more selected from the group consisting of

Ti: 0.010% to 0.050%,

Ca: 0.0005% to 0.0030%,

Zr: 0.0005% to 0.0030%, and

Te: 0.0005% to 0.0030%.

3. A method of producing the rolled steel bar for machine structural use having the chemical composition according to claim 2 , the method comprising:

making molten steel having said chemical composition;

continuously casting the molten steel to obtain a cast piece having a cross-sectional area of 40000 cm 2 or less; and

subsequently to the continuous casting, heating the cast piece to a temperature range of 1000° C. to 1150° C. and holding the cast piece in the temperature range for 7000 seconds or shorter and performing a steel bar rolling.

4. A method of producing the rolled steel bar for machine structural use having the chemical composition according to claim 1 , the method comprising:

making molten steel having said chemical composition;

continuously casting the molten steel to obtain a cast piece having a cross-sectional area of 40000 cm 2 or less; and

subsequently to the continuous casting, heating the cast piece to a temperature range of 1000° C. to 1150° C. and holding the cast piece in the temperature range for 7000 seconds or shorter and performing a steel bar rolling.