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 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,

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),

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 %.

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 according to claim 2 , the method comprising:

making molten steel having the chemical composition with said one or more of Ti, Ca, Zr and Te;

continuously casting the molten steel to obtain a cast piece;

blooming the cast piece to obtain a steel piece;

scarfing all faces of the steel piece after the blooming at 2 mm or more from a surface; and

performing a steel bar rolling after holding the steel piece after the scarfing at a heating temperature of 1000° C. to 1150° C. for 7000 seconds or shorter.

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

making molten steel having the chemical composition;

continuously casting the molten steel to obtain a cast piece;

blooming the cast piece to obtain a steel piece;

scarfing all faces of the steel piece after the blooming at 2 mm or more from a surface; and

performing a steel bar rolling after holding the steel piece after the scarfing at a heating temperature of 1000° C. to 1150° C. for 7000 seconds or shorter.