Methods for manufacturing wire rod for cold forging and screw part, having excellent drilling characteristics

Abstract

The present disclosure relates to methods for manufacturing a wire rod for cold forging and a screw part, which may manufacture with only quenching heat treatment a part with excellent drilling characteristics, and which may specifically manufacture a part to be used for mechanical coupling of automotive parts manufactured with different materials.

1 . A method for manufacturing a wire rod for cold forging having excellent drilling characteristics, the method comprising:
 providing a billet including, in percent by weight (wt %), 0.30 to 0.50% of carbon (C), 0.30 to 0.50% of silicon (Si), 0.35 to 0.75% of manganese (Mn), 0.40 to 0.70% of chromium (Cr), 0.010 to 0.050% of titanium (Ti), 0.01 to 0.05% of aluminum (Al), 0.0010 to 0.0050% boron (B), 0.002 to 0.020% of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities;   heating the billet to a temperature of 900 to 1200° C. and finish hot rolling the billet to at a temperature of 800 to 1000° C. prepare a wire rod:   coiling the wire rod at a temperature of 700 to 900° C. to control an average crystal grain size of austenite to 30 μm or less; and   cooling the coiled wire rod at a rate of 0.4 to 1.0° C./s.   
     
     
         2 . The method according to  claim 1 , wherein a microstructure of the prepared wire rod includes, in area fraction, 45 to 60% of ferrite and 40 to 55% of pearlite. 
     
     
         3 . A method for manufacturing a screw part, the method comprising:
 softening heat-treating a wire rod at a temperature of 600 to 800° C., the wire rod comprising, in percent by weight (wt %), 0.30 to 0.50% of carbon (C), 0.30 to 0.50% of silicon (Si), 0.35 to 0.75% of manganese (Mn), 0.40 to 0.70% of chromium (Cr), 0.010 to 0.050% of titanium (Ti), 0.01 to 0.05% of aluminum (Al), 0.0010 to 0.0050% boron (B), 0.002 to 0.020% of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, wherein a microstructure includes, in area fraction, 45 to 60% of ferrite and 40 to 55% of pearlite, and an average crystal grain size of austenite is 30 μm or less;   cold forging the softening heat-treated wire rod to form a screw part shape having a body diameter of 3 to 6 mm;   heating the screw part shape at a temperature of 850 to 950° C. for 500 to 4,000 seconds to control the average crystal grain size of austenite to 15 μm or less; and   quenching the wire rod in a refrigerant at a temperature of 20 to 100° C. after the heating.   
     
     
         4 . The method according to  claim 3 , wherein the quenching is controlled to have a microstructure including, in an area fraction, 70% or more of autotempered martensite, 0.1 to 5.0% of bainite, 1 to 28% of fresh martensite, and 0.1 to 1.0% of retained austenite. 
     
     
         5 . The method according to  claim 3 , wherein an average thickness of a carbide precipitated in crystal grains of prior austenite is controlled to 20 nm or less in the quenching. 
     
     
         6 . The method according to  claim 3 , wherein the screw part has a hardness of 500 HV or more at room temperature. 
     
     
         7 . A screw part comprising a microstructure including, in area fraction, 70% or more of autotempered martensite, 0.1 to 5% of bainite, 1 to 28% of fresh martensite, and 0.1 to 1% of retained austenite,
 wherein an average thickness of a carbide precipitated in crystal grains of prior austenite is 20 nm or less, and the screw part has a hardness of 500 HV or more at room temperature.