Wire rod having excellent cold forgeability and manufacturing method therefor
Disclosed are a wire rod and a manufacturing method therefor. The wire rod comprises in percentage by weight: 0.02 to 0.15% of C; 0.05 to 0.3% of Si; 0.5 to 1.2% of Mn; 0.3 to 0.9% of Cr; 0.02% or less of P; 0.02% or less of S; 0.01 to 0.05% of sol. Al; 0.01% or less of N; Fe as the remainder; and unavoidable impurities, wherein the wire rod satisfies following formulas 1 and 2, wherein, when the hardness of the wire rod measured in 1/2d position and in 1/4d position in the diameter direction of the wire rod is Hv, 1/2d (Hv) and Hv, 1/4d (Hv), respectively (here, d is the diameter of the wire). ( Hv, 1/2d +Hv, 1/4d )/2≤150 [Formula 1] Hv, 1/2d /Hv, 1/4d ≤1.2 [Formula 2]
1. A wire rod, comprising:
carbon (C): 0.02 wt % to 0.14 wt %, silicon (Si): 0.05 wt % to 0.3 wt %, manganese (Mn): 0.5 wt % to 1.2 wt %, chrome (Cr): 0.3 wt % to 0.9 wt %, phosphorus (P): 0.02 wt % or less, sulfur (S): 0.02 wt % or less, soluble aluminum (sol. Al): 0.01 wt % to 0.05 wt %, nitrogen (N): 0.008 wt % or less, iron (Fe) as a remainder, and unavoidable impurities,
wherein, the wire rod satisfies Formula 1 and Formula 2, when the hardness of the wire rod, measured in a 1/2d position and in a 1/4d position in the diameter direction of the wire rod, are Hv, 1/2 (HV) and Hv, 1/4 (HV), respectively,
( Hv 1/2d +Hv, 1/4d )/2≤150 [Formula 1]
Hv, 1/2d /Hv 1/4d ≤1.2 [Formula 2]
where d is the diameter of the wire.
2. The wire rod of claim 1 , wherein the unavoidable impurities include titanium (Ti), niobium (Nb), and vanadium (V), and are suppressed to 0.02 wt % or less of Ti, as well as 0.06 wt % or less of Nb and V in total.
3. The wire rod of claim 1 , wherein a carbon equivalent (Ceq) defined by Equation 1 is 0.23 or more and 0.33 or less,
Ceq =[C]+[Si]/9+[Mn]/5+[Cr]/12 [Equation 1]
where each of [C], [Si], [Mn], and [Cr] is the content (%) of a corresponding element.
4. The wire rod of claim 1 , wherein the wire rod satisfies Formula 3,
1.2≤[Mn]/[Cr]≤1.8 [Formula 3]
where each of [Mn] and [Cr] is the content (%) of a corresponding element.
5. The wire rod of claim 1 , wherein the wire rod satisfies Formula 4,
9.33[C]+0.97[Mn]+0.51[Cr]+0.26[Nb]+0.66[V]≤2.1 [Formula 4]
where each of [C], [Mn], [Cr], [Nb], and [V] is the content (%) of a corresponding element.
6. The wire rod of claim 1 , including ferrite and pearlite, as a microstructure.
7. The wire rod of claim 1 , including ferrite of 70 area % or more (excluding 100 area %) and pearlite of 30 area % or less (excluding 0 area %), as a microstructure.
8. The wire rod of claim 6 , wherein an average grain size of the ferrite is 10 μm to 30 μm.
9. The wire rod of claim 1 , wherein, during wire drawing in a drawing amount (D) of 5% to 25%, hardness of the wire rod after the wire drawing satisfies Formula 5,
( Hv, 1 −10≤( Hv, D,1/2d +Hv, D,1/4d )/2≤ Hv, 1 +10 [Formula 5]
where, Hv, 1 is “(Hv, 1/2D +Hv, 1/4D )/2+85.45×{1−exp (−D/11.41)}”, and Hv, D,1/2d and Hv, D,1/4d are hardness of the wire rod, measured in a 1/2d position and a 1/4d position in the diameter direction of the wire rod after the wire drawing, respectively.
10. A method for manufacturing a wire rod, comprising:
heating a billet, including carbon (C): 0.02 wt % to 0.14 wt %; silicon (Si): 0.05 wt % to 0.3 wt %; manganese (Mn): 0.5 wt % to 1.2 wt %; chrome (Cr): 0.3 wt % to 0.9 wt %; phosphorus (p): 0.02 wt % or less; sulfur (S): 0.02 wt % or less; soluble aluminum (sol. Al): 0.01 wt % to 0.05 wt %; nitrogen (N): 0.008 wt % or less; iron (Fe) as a remainder and unavoidable impurities, in which a carbon equivalent (Ceq) defined by Equation 1 is 0.23 or more and 0.33 or less, and which satisfies Formula 3 and Formula 4;
obtaining the wire rod by hot-rolling the billet having been heated under the conditions of a finish rolling temperature of 900° C. to 1000° C.; and
cooling, after winding the wire rod,
Ceq =[C]+[Si]/9+[Mn]/5+[Cr]/12 [Equation 1]
1.2≤[Mn]/[Cr]≤1.8 [Formula 3]
9.33[C]+0.97[Mn]+0.51[Cr]+0.26[Nb]+0.66[V]≤2.1 [Formula 4]
where each of [C], [Si], [Mn], [Cr], [Nb], and [V] is the content (%) of a corresponding element, and
wherein, during the winding, a winding temperature is 800° C. to 900° C.
11. The method for manufacturing a wire rod of claim 10 , wherein the unavoidable impurities include titanium (Ti), niobium (Nb), and vanadium (V), and are suppressed to 0.02 wt % or less of Ti, as well as 0.06 wt % or less of Nb and V in total.
12. The method for manufacturing a wire rod of claim 10 , wherein, during the heating, a heating temperature is 1050° C. to 1250° C.
13. The method for manufacturing a wire rod of claim 10 , wherein, during the cooling, a cooling rate is 0.1° C./sec to 1° C./sec.
14. The wire rod of claim 7 , wherein an average grain size of the ferrite is 10 μm to 30 μm.