Wire rod and steel wire for spring, spring with improved strength and fatigue limit, and method for manufacturing same

Abstract

Disclosed are a wire rod and a steel wire for a spring, a spring with improved strength and fatigue limit, and a method for manufacturing the same. The disclosed wire rod for a spring with improved strength and fatigue limit according to an embodiment includes, in percent by weight (wt %): 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr≤1.8% and 0.05 at %≤Mo+W≤0.15 at % may be satisfied.

1 . A wire rod for a spring with improved strength and fatigue limit, the wire rod comprising, in percent by weight (wt %), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities,
 wherein Mn+Cr≤1.8% is satisfied,   0.05 at %≤Mo+W≤0.15 at % is satisfied,   a proportion (wt %) of an area satisfying one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0% is 10% or less within an area of 1 mm 2  of a central region of a cross-section perpendicular to a lengthwise direction.   
     
     
         2 . The wire rod according to  claim 1 , wherein the wire rod comprises, in an area fraction, 80% or more of a pearlite structure and the balance of a bainite structure or a martensite structure. 
     
     
         3 . The wire rod according to  claim 1 , wherein a prior austenite average grain size is 20 μm or less. 
     
     
         4 . The wire rod according to  claim 1 , wherein the number of a carbonitride having a maximum diameter of 15 μm or more distributed in a cross-section parallel to a lengthwise direction within a surface depth of 1 mm is less than 2 per cm 2 . 
     
     
         5 . The wire rod according to  claim 1 , wherein a tensile strength is 1,400 MPa or less, and a reduction of area is 35% or more. 
     
     
         6 . A method for manufacturing a wire rod for a spring with improved strength and fatigue limit, the method comprising:
 preparing a bloom by continuously casting a molten steel including, in percent by weight (wt %), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities;   heating the bloom at a temperature of 1,200° ° C. or above and rolling the bloom to prepare a billet;   heating the billet at a temperature of 1,030° ° C. or above and rolling the billet at a temperature of 1,000° ° C. below to prepare a wire rod;   coiling the rolled wire rod at a temperature of 800 to 900° C.; and   cooling the coiled wire rod at a seed of 0.5 to 2ºC/sec.   
     
     
         7 . The method according to  claim 6 , wherein the continuously casting process comprises performing soft reduction with a total rolling reduction 20 mm or more. 
     
     
         8 . The method according to  claim 7 , wherein the soft reduction is performed to allow each roll to roll by reducing 4 mm or less and have a cumulative rolling reduction of 60% or more at a solidification fraction of 0.6 or more. 
     
     
         9 . A steel wire for a spring with improved strength and fatigue limit, the steel wire comprising, in percent by weight (wt %), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities,
 wherein Mn+Cr≤1.8% is satisfied,   0.05 at %≤Mo+W≤0.15 at % is satisfied, and   the steel wire comprises, in an area fraction, 85% or more of a tempered martensite structure and the balance of an austenite structure.   
     
     
         10 . The steel wire according to  claim 9 , wherein a prior austenite average grain size is 15 μm or less. 
     
     
         11 . The steel wire according to  claim 9 , wherein the number of a carbonitride having a maximum diameter of 15 μm or more distributed in a cross-section parallel to the lengthwise direction within a surface depth of 1 mm is less than 2 per cm 2 . 
     
     
         12 . The steel wire according to  claim 9 , wherein the number of carbides in an area of 100 μm 2  is from 10 to 50, the maximum diameter of the carbide is from 5 to 50 nm, and a content of V or Nb is 10 at % or more. 
     
     
         13 . The steel wire according to  claim 9 , wherein a tensile strength is 2,100 MPa or more, and a reduction of area is 45% or more. 
     
     
         14 . A method for manufacturing a steel wire for a spring with improved strength and fatigue limit, the method comprising:
 performing LA heat treatment on the wire rod according to  claim 1 ;   drawing the LP heat-treated wire rod to prepare a steel wire; and   performing QT heat treatment on the steel wire,   wherein the LP heat treatment comprises:   a primary austenizing process of heating to a temperature of 950 to 1100° ° C. within 3 minutes and maintaining for 3 minutes or less; and   a process of passing the primarily austenized wire rod through a Pb bath at a temperature of 650 to 700° ° C. within 3 minutes.   
     
     
         15 . The method according to  claim 14 , wherein, in the LP heat treatment, a pearlite transformation completion time is less than 130 seconds.