Rolled Rod for Cold-Forged Thermally Refined Article

Abstract

The rolled rod for cold-forged thermally refined articles according to the present invention has a chemical composition consisting, in mass %: 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%, and O: 0.0020% or less, with the balance being Fe and impurities, and that satisfies Formula (1) and Formula (2), wherein a total area fraction of pro-eutectoid ferrite and pearlite is 90% or more, 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)

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.