Non-heat treated wire rod excellent in strength and cold workability and method for manufacturing same

Abstract

Disclosed is a non-heat treated wire rod, comprising, as percentage by weight: C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and the balance being Fe and unavoidable impurities, wherein the non-heat treated wire rod includes ferrite and pearlite microstructures, and wherein the phase fraction of the pearlite satisfies the following relational expressions 1 and 2, and the average lamellar spacing of the pearlite satisfies the following relational expressions 3 and 4. VP 2 /VP 1 ≤1.4   [Relational expression 1] 50≤(15 VP 1 +VP 2 )/16≤70   [Relational expression 2] DL 1 /DL 2 ≤1.4   [Relational expression 3] 0.1≤(15 DL 1 +DL 2 )/16≤0.3   [Relational expression 4]

1 . A non-heat treated wire rod comprising:
 by wt %, C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and a balance of Fe and unavoidable impurities,   wherein the non-heat treated wire rod includes ferrite and pearlite microstructures, and the phase fraction of the pearlite satisfies relational expressions 1 and 2, and the average lamellar spacing of the pearlite satisfies relational expressions 3 and 4.
     VP   2   /VP   1 ≤1.4   [Relational expression 1]
 
   50≤(15 VP   1   +VP   2 )/16≤70   [Relational expression 2]
 
     DL   1   /DL   2 ≤1.4   [Relational expression 3]
 
   0.1≤(15 DL   1   +DL   2 )/16≤0.3,   [Relational expression 4]
 
   where VP 1  and VP 2 , respectively, refer to, in a cross-section perpendicular to the longitudinal direction of the wire rod, a pearlite fraction (area %) in the region from the surface of the wire rod to a ⅜ D position in the diameter (D) direction of the wire rod, and a pearlite fraction (area %) in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod, and DL 1  and DL 2 , respectively, refer to, in the cross-section perpendicular to the longitudinal direction of the wire rod, the average lamellar spacing (μm) of the pearlite in the region from the surface of the wire rod to the ⅜ D position in the diameter (D) direction of the wire rod, and the average lamellar spacing (μm) of the pearlite in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod.   
     
     
         2 . The non-heat treated wire rod of  claim 1 , wherein a deviation in strength of the pearlite satisfies relational expression 5.
   ( VP   2   /VP   1 )×(√( DL   1   /DL   2 ))≤1.5   [Relational expression 5]
   
     
     
         3 . The non-heat treated wire rod of  claim 1 , wherein the unavoidable impurities include Ti, and an amount of Ti is limited to, by wt %, 0.005% or less. 
     
     
         4 . The non-heat treated wire rod of  claim 1 , wherein a carbon equivalent is 0.6 or more and 0.7 or less. 
     
     
         5 . The non-heat treated wire rod of  claim 1 , wherein, in the cross-section perpendicular to the longitudinal direction of the wire rod, the average composition of an oxide-based inclusion in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod satisfies relational expressions 6 to 8.
   30≤[Al 2 O 3 ]≤70   [Relational expression 6]
 
   20≤[SiO 2 ]≤40   [Relational expression 7]
 
   10≤[CaO]+[MgO]≤20,   [Relational expression 8]
 
 where Al 2 O 3 , SiO 2 , CaO, and MgO each refer to the content (wt %) of a corresponding inclusion. 
 
     
     
         6 . The non-heat treated wire rod of  claim 5 , wherein the average diameter of the oxide-based inclusion is 8 μm or less. 
     
     
         7 . The non-heat treated wire rod of  claim 5 , wherein the maximum diameter of the oxide-based inclusion is 15 μm or less. 
     
     
         8 . A method for manufacturing a non-heat treated wire rod comprising:
 heating, at a heating temperature for 1,200-1,300° C., a bloom comprising, by wt %, C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and a balance of Fe and unavoidable impurities, and having a carbon equivalent of 0.6 or more and 0.7 or less, maintaining the bloom at the heating temperature for 240 minutes or more, and subjecting the bloom to steel rolling to obtain billets;   reheating the billets, and then subjecting the billet to wire rod rolling to obtain a wire rod; and   coiling the wire rod, and then cooling the wire rod at a rate of 0.3-1° C./s.   
     
     
         9 . The method of  claim 8 , wherein the unavoidable impurities include Ti, and an amount of Ti is limited to, by wt %, 0.005% or less. 
     
     
         10 . The method of  claim 8 , wherein a reheating temperature for the billets ranges from 1,050-1,200° C. 
     
     
         11 . The method of  claim 8 , wherein a coiling temperature for the wire rod ranges from 750-900° C.