Rolled Rod for Cold-Forged Thermally Refined Article

1 . A rolled rod for cold-forged thermally refined articles comprising a chemical composition consisting of, in mass percent:

C: 0.22 to 0.40%,

Si: 0.35 to 1.5%,

Mn: 0.20 to 0.40%,

P: less than 0.020%,

S: less than 0.015%,

Cr: 0.70 to 1.45%,

Al: 0.005 to 0.060%,

Ti: 0.01 to 0.05%,

B: 0.0003 to 0.0040%,

N: 0.0020 to 0.0080%,

O: 0.0020% or less,

Cu: 0 to 0.50%,

Ni: 0 to 0.30%,

Mo: 0 to 0.05%,

V: 0 to 0.05%, and

Nb: 0 to 0.05%, with the balance being Fe and impurities, and that satisfies Formula (1) and Formula (2),

wherein, in a matrix structure, a total area fraction of pro-eutectoid ferrite and pearlite is 90% or more, and an area fraction of the pro-eutectoid ferrite is 30% or more, and

the rolled rod has a tensile strength of 700 MPa or less,

0.50≤C+Si/10+Mn/5+5Cr/22≤0.85  (1)

Si/Mn>1.0  (2)

where symbols of elements in the formulae are to be substituted by contents of corresponding elements (in mass percent).

2 . The rolled rod for cold-forged thermally refined articles according to claim 1 , wherein the chemical composition contains one or more types selected from a group consisting of:

Cu: 0.02 to 0.50%;

Ni: 0.01 to 0.30%;

Mo: 0.005 to 0.05%; and

V: 0.005 to 0.05%.

3 . The rolled rod for cold-forged thermally refined articles according to claim 1 , wherein the chemical composition contains:

Nb: 0.0015 to 0.05%.

4 . (canceled)

5 . The rolled rod for cold-forged thermally refined articles according to claim 2 , wherein the chemical composition contains Nb: 0.0015 to 0.05%.

6 . A method for producing a rolled rod for cold-forged thermally refined articles, the method comprising:

a billeting process of heating a starting material having the chemical composition according to claim 1 to a temperature of 1200° C. or more and thereafter subjecting the starting material to billeting, so as to produce a billet; and

a finish rolling process of heating the billet to a temperature of 1050° C. or less and thereafter subjecting the billet to finish rolling, so as to produce a rolled rod, wherein

the finish rolling process includes:

a process of subjecting the billet to the finish rolling to produce the rolled rod at a working rate Z defined by Formula (3) set at 5 to 15/sec within a range of a surface temperature of the billet of 750 to 850° C.; and

a process of cooling the rolled rod, immediately after completion of the rolling, at a cooling rate of 0.2 to 5.0° C./sec down to 500° C.,

Z=−ln(1−R)/ t   (3)

where “R” in Formula (3) denotes a reduction of area (%) of the finish rolling and is defined by Formula (4), and “t” denotes a finish rolling duration (sec),

R=(A 0 −A)/A 0   (4)

where “A 0 ” in Formula (4) denotes a cross-sectional area (mm 2 ) of the billet before the finish rolling, and “A” denotes a cross-sectional area (mm 2 ) of the rolled rod after the finish rolling.

7 . A method for producing a rolled rod for cold-forged thermally refined articles, the method comprising:

a billeting process of heating a starting material having the chemical composition according to claim 2 to a temperature of 1200° C. or more and thereafter subjecting the starting material to billeting, so as to produce a billet; and

a finish rolling process of heating the billet to a temperature of 1050° C. or less and thereafter subjecting the billet to finish rolling, so as to produce a rolled rod, wherein

the finish rolling process includes:

a process of subjecting the billet to the finish rolling to produce the rolled rod at a working rate Z defined by Formula (3) set at 5 to 15/sec within a range of a surface temperature of the billet of 750 to 850° C.; and

a process of cooling the rolled rod, immediately after completion of the rolling, at a cooling rate of 0.2 to 5.0° C./sec down to 500° C.,

Z=−ln(1−R)/ t   (3)

where “R” in Formula (3) denotes a reduction of area (%) of the finish rolling and is defined by Formula (4), and “t” denotes a finish rolling duration (sec),

R=(A 0 −A)/A 0   (4)

where “A 0 ” in Formula (4) denotes a cross-sectional area (mm 2 ) of the billet before the finish rolling, and “A” denotes a cross-sectional area (mm 2 ) of the rolled rod after the finish rolling.

8 . A method for producing a rolled rod for cold-forged thermally refined articles, the method comprising:

a billeting process of heating a starting material having the chemical composition according to claim 3 to a temperature of 1200° C. or more and thereafter subjecting the starting material to billeting, so as to produce a billet; and

a finish rolling process of heating the billet to a temperature of 1050° C. or less and thereafter subjecting the billet to finish rolling, so as to produce a rolled rod, wherein

the finish rolling process includes:

a process of subjecting the billet to the finish rolling to produce the rolled rod at a working rate Z defined by Formula (3) set at 5 to 15/sec within a range of a surface temperature of the billet of 750 to 850° C.; and

a process of cooling the rolled rod, immediately after completion of the rolling, at a cooling rate of 0.2 to 5.0° C./sec down to 500° C.,

Z=−ln(1−R)/ t   (3)

where “R” in Formula (3) denotes a reduction of area (%) of the finish rolling and is defined by Formula (4), and “t” denotes a finish rolling duration (sec),

R=(A 0 −A)/A 0   (4)

where “A 0 ” in Formula (4) denotes a cross-sectional area (mm 2 ) of the billet before the finish rolling, and “A” denotes a cross-sectional area (mm 2 ) of the rolled rod after the finish rolling.

9 . A method for producing a rolled rod for cold-forged thermally refined articles, the method comprising:

a billeting process of heating a starting material having the chemical composition according to claim 5 to a temperature of 1200° C. or more and thereafter subjecting the starting material to billeting, so as to produce a billet; and

a finish rolling process of heating the billet to a temperature of 1050° C. or less and thereafter subjecting the billet to finish rolling, so as to produce a rolled rod, wherein

the finish rolling process includes:

a process of subjecting the billet to the finish rolling to produce the rolled rod at a working rate Z defined by Formula (3) set at 5 to 15/sec within a range of a surface temperature of the billet of 750 to 850° C.; and

a process of cooling the rolled rod, immediately after completion of the rolling, at a cooling rate of 0.2 to 5.0° C./sec down to 500° C.,

Z=−ln(1−R)/ t   (3)

where “R” in Formula (3) denotes a reduction of area (%) of the finish rolling and is defined by Formula (4), and “t” denotes a finish rolling duration (sec),

R=(A 0 −A)/A 0   (4)

where “A 0 ” in Formula (4) denotes a cross-sectional area (mm 2 ) of the billet before the finish rolling, and “A” denotes a cross-sectional area (mm 2 ) of the rolled rod after the finish rolling.