Wire rod for ultrahigh-strength springs, steel wire, and manufacturing method therefor

Abstract

Disclosed in the present specification are: a wire rod for ultrahigh-strength springs, which can be applied to motorcycle suspension springs; a steel wire; and a manufacturing method therefor. According to one embodiment of the disclosed wire rod for ultrahigh-strength springs, the wire rod comprises, by wt %, 0.5-0.7% of C, 0.4-0.9% of Si, 0.3-0.8% of Mn, 0.2-0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less of O, and the balance of Fe and inevitable impurities, wherein in 1 mm2 area of the center of the cross-section perpendicular to the longitudinal direction, the proportion of the area satisfying at least one from among C>0.8%, Si>0.9%, Cr>0.8% and Mn>0.8% by wt % can be 5% or less.

1 . A wire rod for ultrahigh strength spring, the wire rod comprising:
 in percent by weight (wt %), 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable impurities,   wherein a ratio of an area satisfying one or more of C>0.8%, Si>0.9%, Cr>0.8% and Mn>0.8%, in wt %, in an area of 1 mm 2  of a center of a cross-section perpendicular to a longitudinal direction is 5% or less.   
     
     
         2 . The wire rod for ultrahigh strength spring of  claim 1 , further comprising: a surface ferrite decarburized layer having a thickness of 1 to or less. 
     
     
         3 . A method of manufacturing a wire rode for ultrahigh strength spring, the method comprising:
 preparing a bloom by continuous casting of molten steel including, in wt %, 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable impurities; and   rolling the bloom into billets and then into wire rods,   wherein the continuous casting is performed at a casting speed of 0.48 to 0.54 m/min and comprises soft reduction with a total amount of reduction of 15 to 35 mm during the continuous casting.   
     
     
         4 . The method of  claim 3 , wherein the soft reduction is performed so that each roll rolls to 4 mm or less and a cumulative amount of reduction is 60% or more when the solidification fraction is 0.6 or more. 
     
     
         5 . A steel wire for ultrahigh strength spring, the steel wire comprising:
 in percent by weight (wt %), 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable impurities, and   for area fraction, 90% or more of tempered martensite,   wherein a ratio of an area satisfying one or more of C>0.8%, Si>0.9%, Cr>0.8% and Mn>0.8%, in wt %, in an area of 1 mm 2  of a center of a cross-section perpendicular to a longitudinal direction is 5% or less.   
     
     
         6 . The steel wire for ultrahigh strength spring of  claim 5 , wherein the steel wire has a hardness deviation of 25 Hv or less in the cross-section perpendicular to the longitudinal direction except for an area having a depth of 0.5 mm or less from the outermost surface. 
     
     
         7 . The steel wire for ultrahigh strength spring of  claim 5 , wherein the steel wire has a tensile strength of 1750 to 2200 MPa and a cross-section reduction ratio of 40% or more. 
     
     
         8 . A method of manufacturing a steel wire for ultrahigh strength spring, the method comprising:
 drawing the wire rod of  claim 1 ;   heating at 900 to 1000° C. within 10 seconds;   water cooling at high pressure;   heating and tempering at 400 to 500° C. within 10 seconds; and   water cooling.