Rolled wire rod and manufacturing method thereof

1. A rolled wire rod comprising, as a chemical component, by mass %:

C: 0.95% to 1.10%;

Si: 0.10% to 0.70%;

Mn: 0.20% to 1.20%; and

Cr: 0.90% to 1.60%,

the balance consisting of Fe and unavoidable impurities,

wherein a metallographic structure of the rolled wire rod includes, by an area ratio, 90% to 100% of pearlite, 0% to 5% of proeutectoid cementite, and 0% to 10% of degenerate pearlite having granular cementite,

an average size of a pearlite block of the pearlite is 1.0 μm to 15 μm and a maximum size of the pearlite block is 1 to 4 times the average size, and

wherein a tensile strength of the rolled wire rod in a unit of MPa is equal to or less than a TS 1 of an upper limit of a strength shown in the following Expression 1:

TS 1=1500×(1−0.5×ln( D 0 /5.5))−150×exp( d p /15−1)−120×(exp( A θ/ 5)−1)−80×exp( A p ′/10−1)  (Expression 1)

wherein,

D 0 represents a wire size of the rolled wire rod in a unit of mm,

d p represents the average size of the pearlite block in a unit of μm,

A θ represents an area ratio of the proeutectoid cementite in a unit of %, and

A p ′ represents an area ratio of the degenerate pearlite in a unit of %.

2. The rolled wire rod according to claim 1 ,

wherein the chemical component further includes, by mass %, any one kind or both kinds of,

Mo: more than 0% and equal to or less than 0.25%, and

B: more than 0% and equal to or less than 0.0025%.

3. The rolled wire rod according to claim 1 ,

wherein a maximum value of a size perpendicular to a longitudinal direction of the proeutectoid cementite is 1.5 μm or less.

4. The rolled wire rod according to claim 1 ,

wherein when a radius of the rolled wire rod in a cross section perpendicular to the longitudinal direction is set to r in a unit of mm, on an inner side of a region surrounded by ½ r from the center of the cross section, a coarse lamellar in which a lamellar spacing is 0.5 μm or more is 0% to 10% in the pearlite by an area ratio.

5. The rolled wire rod according to claim 1 ,

wherein a variation in the tensile strength of the rolled wire rod is in a range of −150 MPa to +150 MPa with the average value of the tensile strength set as a reference.

6. A method of manufacturing a rolled wire rod, the method comprising:

a finish rolling process of shaping a rolled wire rod by using steel having the chemical component according to claim 1 at a starting temperature of a finish rolling set to 700° C. to 850° C.;

a coiling process of coiling the rolled wire rod in a ring shape at a coiling temperature set to 650° C. to 850° C. after the shaping of the rolled wire rod; and

a cooling process of cooling the rolled wire rod to a temperature range of 500° C. to 600° C. at a cooling rate of 0.5° C./sec to 3° C./sec after the coiling of the rolled wire rod,

wherein when a temperature of a not-dense area at which ring overlapping of the rolled wire rod during the cooling of the rolled wire rod is less is set to Tn in a unit of ° C., and a temperature of a dense area at which the ring overlapping is much is set to Td, Td/Tn<1.20 is satisfied.

7. The rolled wire rod according to claim 2 ,

wherein a maximum value of a size perpendicular to a longitudinal direction of the proeutectoid cementite is 1.5 μm or less.

8. The rolled wire rod according to claim 2 ,

wherein when a radius of the rolled wire rod in a cross section perpendicular to the longitudinal direction is set to r in a unit of mm, on an inner side of a region surrounded by ½ r from the center of the cross section, a coarse lamellar in which a lamellar spacing is 0.5 μm or more is 0% to 10% in the pearlite by an area ratio.

9. The rolled wire rod according to claim 2 ,

wherein a variation in the tensile strength of the rolled wire rod is in a range of −150 MPa to +150 MPa with the average value of the tensile strength set as a reference.

10. A method of manufacturing a rolled wire rod, the method comprising:

a finish rolling process of shaping a rolled wire rod by using steel having the chemical component according to claim 2 at a starting temperature of a finish rolling set to 700° C. to 850° C.;

a coiling process of coiling the rolled wire rod in a ring shape at a coiling temperature set to 650° C. to 850° C. after the shaping of the rolled wire rod; and

a cooling process of cooling the rolled wire rod to a temperature range of 500° C. to 600° C. at a cooling rate of 0.5° C./sec to 3° C./sec after the coiling of the rolled wire rod,

wherein when a temperature of a not-dense area at which ring overlapping of the rolled wire rod during the cooling of the rolled wire rod is less is set to Tn in a unit of ° C., and a temperature of a dense area at which the ring overlapping is much is set to Td, Td/Tn<1.20 is satisfied.

11. The rolled wire rod according to claim 3 ,

wherein when a radius of the rolled wire rod in a cross section perpendicular to the longitudinal direction is set to r in a unit of mm, on an inner side of a region surrounded by ½ r from the center of the cross section, a coarse lamellar in which a lamellar spacing is 0.5 μm or more is 0% to 10% in the pearlite by an area ratio.

12. The rolled wire rod according to claim 3 ,

wherein a variation in the tensile strength of the rolled wire rod is in a range of −150 MpA to +150 MPa with the average value of the tensile strength set as a reference.

13. The rolled wire rod according to claim 4 ,

wherein a variation in the tensile strength of the rolled wire rod is in a range of −150 MPa to +150 MPa with the average value of the tensile strength set as a reference.