Wire rod for non heat-treated mechanical part, steel wire for non heat-treated mechanical part, and non heat-treated mechanical part

1. A steel wire for a non heat-treated mechanical part, the steel wire comprising, as a chemical composition, by mass %,

C: 0.18% to 0.65%,

Si: 0.05% to 1.5%,

Mn: 0.50% to 2.0%,

Cr: 0% to 1.50%,

Mo: 0% to 0.50%,

Ti: 0% to 0.050%,

Al: 0% to 0.050%,

B: 0% to 0.0050%,

Nb: 0% to 0.050%,

V: 0% to 0.20%,

P: limited to less than or equal to 0.030%,

S: limited to less than or equal to 0.030%,

N: limited to less than or equal to 0.0050%,

O: limited to less than or equal to 0.01%, and

a remainder of Fe and impurities;

wherein a structure includes, by volume %, a bainite of greater than or equal to 75×[C %]+25, and a remainder of one or more of a ferrite and a pearlite when an amount of C is set to [C %] by mass %;

when a diameter of the steel wire is set to D 2 mm, an area from a surface of the steel wire to a depth of 0.1×D 2 mm toward a center line of a cross section is set as a second surface layer area of the steel wire, and an average aspect ratio of a bainite block in the second surface layer area of the steel wire is set to R1 in the cross section parallel to a longitudinal direction of the steel wire,

the R1 is greater than or equal to 1.2;

when the diameter of the steel wire is set to D 2 mm, an area from a surface of the steel wire to a depth of 0.1×D 2 mm toward a center of a cross section is set as a third surface layer area of the steel wire, an area from the depth of 0.25×D 2 mm to the center of the cross section is set as a third center portion of the steel wire, an average grain size of a bainite block in the third surface layer area of the steel wire is set to P S3 μm, and an average grain size of a bainite block in the third center portion of the steel wire is set to P C3 μm in the cross section perpendicular to the longitudinal direction of the steel wire,

the P S3 satisfies Expression (c),

P S3 ≤20/ R 1  (c),

and the P S3 and the P C3 satisfy Expression (d),

P S3 /P C3 ≤0.95  (d);

a standard deviation of a grain size of the bainite block in the structure is less than or equal to 8.0 μm; and

a tensile strength is in a range of 800 MPa to 1600 MPa.

2. The steel wire for a non heat-treated mechanical part according to claim 1 , the steel wire comprising, as the chemical composition, by mass %,

C: 0.18% to 0.50%, and

Si: 0.05% to 0.50%.

3. The steel wire for a non heat-treated mechanical part according to claim 1 , the steel wire comprising, as the chemical composition, by mass %,

C: 0.20% to 0.65%,

wherein the structure includes, by volume %, the bainite of greater than or equal to 45×[C %]+50 when the amount of C is set to [C %] by mass %.

4. The steel wire for a non heat-treated mechanical part according to claim 1 , the steel wire comprising, as the chemical composition, by mass %,

B: less than 0.0005%,

wherein F1 obtained by Expression (b) is greater than or equal to 2.0,

F 1=0.6×[C %]−0.1×[Si %]+1.4×[Mn %]+1.3×[Cr %]+3.7×[Mo %]  (b),

when the amount of C is set to [C %], an amount of Si is set to [Si %], an amount of Mn is set to [Mn %], an amount of Cr is set to [Cr %], and an amount of Mo content is set to [Mo %] by mass %.

5. The steel wire for a non heat-treated mechanical part according to claim wherein the R1 is less than or equal to 2.0.

6. The steel wire for a non heat-treated mechanical part according to claim 1 , wherein the structure includes, by volume %, the bainite of greater than or equal to 45×[C %]+50.

7. A wire rod for a non heat-treated mechanical part, the wire rod comprising, as a chemical composition, by mass %,

C: 0.18% to 0.65%,

Si: 0.05% to 1.5%,

Mn: 0.50% to 2.0%,

Cr: 0% to 1.50%,

Mo: 0% to 0.50%,

Ti: 0% to 0.050%,

Al: 0% to 0.050%,

B: 0% to 0.0050%,

Nb: 0% to 0.050%,

V: 0% to 0.20%,

P: limited to less than or equal to 0.030%,

S: limited to less than or equal to 0.030%,

N: limited to less than or equal to 0.0050%,

O: limited to less than or equal to 0.01%, and

a remainder of Fe and impurities;

wherein a structure includes, by volume %, a bainite of greater than or equal to 75×[C %]+25, and a remainder of one or more of a ferrite and a pearlite without a martensite when an amount of C is set to [C %] by mass %;

an average grain size of a bainite block of the structure is in a range of 5.0 μm to 20.0 μm, and a standard deviation of a grain size of the bainite block is less than or equal to 15.0 μm; and

when a diameter of the wire rod is set to D 1 mm, an area from a surface of the wire rod to a depth of 0.1×D 1 mm toward a center of a cross section is set as a first surface layer area of the wire rod, an area from the depth of 0.25×D 1 mm to the center of the cross section is set as a first center portion of the wire rod, an average grain size of a bainite block in the first surface layer area is P S1 μm, and an average grain size of a bainite block in the first center portion is P C1 μm in the cross section perpendicular to a longitudinal direction of the wire rod,

the P S1 and the P C1 satisfy Expression (a),

P S1 /P C1 ≤0.95  (a).

8. The wire rod for a non heat-treated mechanical part according to claim 7 , the wire rod comprising, as the chemical composition, by mass %,

C: 0.18% to 0.50%, and

Si: 0.05% to 0.50%.

9. The wire rod for a non heat-treated mechanical part according to claim 7 , the wire rod comprising, as the chemical composition, by mass %,

C: 0.20% to 0.65%,

wherein the structure includes, by volume %, the bainite of greater than or equal to 45×[C %]+50 when the amount of C is set to [C %] by mass %.

10. A non heat-treated mechanical part having a cylindrical axis, the mechanical part comprising, as a chemical composition, by mass %,

C: 0.18% to 0.65%,

Si: 0.05% to 1.5%,

Mn: 0.50% to 2.0%,

Cr: 0% to 1.50%,

Mo: 0% to 0.50%,

Ti: 0% to 0.050%,

Al: 0% to 0.050%,

B: 0% to 0.0050%,

Nb: 0% to 0.050%,

V: 0% to 0.20%,

P: less than or equal to 0.030%,

S: less than or equal to 0.030%,

N: less than or equal to 0.0050%,

O: less than or equal to 0.01%, and

a remainder of Fe and impurities;

wherein a structure includes, by volume %, a bainite of greater than or equal to 75×[C %]+25, and a remainder of one or more of a ferrite and a pearlite when an amount of C is set to [C %] by mass %;

when a diameter of an axis is set to D 3 mm, an area from a surface of the axis to a depth of 0.1×D 3 mm toward a center line of a cross section is set as a fourth surface layer area of the mechanical part, and an average aspect ratio of a bainite block in the fourth surface layer area of the mechanical part is set to R2 in the cross section parallel to a longitudinal direction of the axis,

the R2 is greater than or equal to 1.2;

when the diameter of the axis is set to D 3 mm, an area from a surface of the axis to a depth of 0.1×D 3 mm toward a center of a cross section is set as a fifth surface layer area of the mechanical part, an area from the depth of 0.25×D 3 mm to the center of the cross section is set as a fifth center portion of the mechanical part, an average grain size of a bainite block in the fifth surface layer area of the mechanical part is set to P S5 μm, and an average grain size of a bainite block in the fifth center portion of the mechanical part is set to P C5 μm in the cross section perpendicular to the longitudinal direction of the axis,

the P S5 satisfies Expression (e),

P S5 ≤20 /R 2  (e),

and the P S5 and the P C5 satisfy Expression (f),

P S5 /P C5 <0.95  (f);

a standard deviation of a grain size of the bainite block in the structure is less than or equal to 8.0 μm; and

a tensile strength is in a range of 800 MPa to 1600 MPa.

11. The non heat-treated mechanical part according to claim 10 , wherein

the R2 is greater than or equal to 1.5, and

the tensile strength is in a range of 1200 MPa to 1600 MPa.

12. The non heat-treated mechanical part according to claim 11 , wherein the non heat-treated mechanical part is a bolt.

13. The non heat-treated mechanical part according to claim 10 ,

wherein the non heat-treated mechanical part is a bolt.

14. The non heat-treated mechanical part having a cylindrical axis, the mechanical part comprising, as a chemical composition, by mass %,

C: 0.18% to 0.65%,

Si: 0.05% to 1.5%,

Mn: 0.50% to 2.0%,

Cr: 0% to 1.50%,

Mo: 0% to 0.50%,

Ti: 0% to 0.050%,

Al: 0% to 0.050%,

B: 0% to 0.0050%,

Nb: 0% to 0.050%,

V: 0% to 0.20%,

P: less than or equal to 0.030%,

S: less than or equal to 0.030%,

N: less than or equal to 0.0050%,

O: less than or equal to 0.01%, and

a remainder of Fe and impurities;

wherein a structure includes, by volume %, a bainite of greater than or equal to 75×[C %]+25, and a remainder of one or more of a ferrite and a pearlite when an amount of C is set to [C %] by mass %;

when a diameter of an axis is set to D 3 mm, an area from a surface of the axis to a depth of 0.1×D 3 mm toward a center line of a cross section is set as a fourth surface layer area of the mechanical part, and an average aspect ratio of a bainite block in the fourth surface layer area of the mechanical part is set to R2 in the cross section parallel to a longitudinal direction of the axis,

the R2 is greater than or equal to 1.2;

when the diameter of the axis is set to D 3 mm, an area from a surface of the axis to a depth of 0.1×D 3 mm toward a center of a cross section is set as a fifth surface layer area of the mechanical part, an area from the depth of 0.25×D 3 mm to the center of the cross section is set as a fifth center portion of the mechanical part, an average grain size of a bainite block in the fifth surface layer area of the mechanical part is set to P S5 μm, and an average grain size of a bainite block in the fifth center portion of the mechanical part is set to P c5 μm in the cross section perpendicular to the longitudinal direction of the axis,

the P S5 satisfies Expression (e),

P S5 ≤20 /R 2  (e),

and the P S5 and the P C5 satisfy Expression (f),

P S5 /P C5 ≤0.95  (f);

a standard deviation of a grain size of the bainite block in the structure is less than or equal to 8.0 μm; and

a tensile strength is in a range of 800 MPa to 1600 MPa, which is obtained by performing a cold working on the steel wire according to claim 1 .

15. The non heat-treated mechanical part according to claim 14 , wherein the D 2 and the D 3 are equivalent to each other.

16. The non heat-treated mechanical part according to claim 15 , wherein the non heat-treated mechanical part is a bolt.

17. The non heat-treated mechanical part according to claim 14 , wherein

the R2 is greater than or equal to 1.5, and

the tensile strength is in a range of 1200 MPa to 1600 MPa.

18. The non heat-treated mechanical part according to claim 17 , wherein the non heat-treated mechanical part is a bolt.

19. The non heat-treated mechanical part according to claim 14 ,

wherein the non heat-treated mechanical part is a bolt.