Ultra-high-strength steel wire having excellent resistance to delayed fracture and manufacturing method thereof

Abstract

An ultra-high-strength steel wire rod having excellent resistance to delayed fracture includes, by wt %, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V and a balance of Fe and inevitable impurities. The method includes the steps of heating the above steel composition to 1100° C. or lower and hot rolling at a temperature of 900-1000° C., followed by cooling to 600-650° C. at a prescribed rate, followed by cold drawing at a reduction ratio of 60-80%.

1 . An ultra-high-strength steel wire rod having excellent resistance to delayed fracture, the wire rod comprising, by wt %, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V and a balance of Fe and inevitable impurities. 
     
     
         2 . The ultra-high-strength steel wire rod of  claim 1 , wherein the wire rod has a pearlite structure having a lamellar spacing of 150˜300 nm, in which V(C,N) precipitates are distributed in the ferrite structure of the pearlite. 
     
     
         3 . The ultra-high-strength steel wire rod of  claim 2 , wherein the average particle size of the V(C,N) precipitates is 30 nm or less, and the number of the V(C,N) precipitates is 1×10 9 /mm 2  or more. 
     
     
         4 . The ultra-high-strength steel wire rod of  claim 1 , wherein the wire rod has a diffusible hydrogen content 0.6˜0.9 ppm. 
     
     
         5 . A method for manufacturing an ultra-high-strength steel wire rod having excellent resistance to delayed fracture, the method comprising:
 heating a steel, which includes, by wt %, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V and a balance of Fe and inevitable impurities, to 1100° C. or lower, and hot-rolling the heated steel at a temperature of 900˜1000° C.;   cooling the rolled steel to 600˜650° C. to a rate of 5˜10° C./s; and   cold-drawing the cooled steel at a reduction ratio of 60˜80%.