Cold forging wire rod and steel part having improved delayed fracture resistance, and method for manufacturing same

Abstract

Disclosed are: a cold forging wire rod and a steel part in which the microstructure is controlled through the alloy composition and manufacturing method, thus enabling cost reduction and enhancing delayed fracture resistance; and a method for manufacturing same. The steel part having improved delayed fracture resistance according to an embodiment of the present invention contains, in weight percent, 0.18-0.25% of C, 0.30-0.50% of Si, 0.35-0.50% of Mn, more than 0% and no more than 0.03% of P, more than 0% and no more than 0.03% of S, 0.45-0.60% of Cr, 0.015-0.03% of Ti, 0.001-0.004% of B, with the remainder comprising Fe and inevitable impurities, and may include at least 90% by volume of auto-tempered martensite.

1 . A steel part with improved delayed fracture resistance comprising:
 in percent by weight (wt %), 0.18 to 0.25% of C, 0.30 to 0.50% of Si, 0.35 to 0.50% of Mn, more than 0 to 0.03% of P, more than 0 to 0.03% of S, 0.45 to 0.60% of Cr, 0.015 to 0.03% of Ti, 0.001 to 0.004% of B, the remainder having Fe and other unavoidable,   wherein autotempered martensite is at least 90% in volume fraction.   
     
     
         2 . The steel part of  claim 1 , wherein carbides in prior austenite grains have an average thickness of 15 nm or less. 
     
     
         3 . The steel part of  claim 1 , wherein tensile strength is at least 1200 MPa. 
     
     
         4 . A method of manufacturing a steel part with improved delayed fracture resistance, the method comprising:
 preparing a steel material including, in percent by weight (wt %), 0.18 to 0.25% of C, 0.30 to 0.50% of Si, 0.35 to 0.50% of Mn, more than 0 to 0.03% of P, more than 0 to 0.03% of S, 0.45 to 0.60% of Cr, 0.015 to 0.03% of Ti, 0.001 to 0.004% of B, the remainder having Fe and other unavoidable;   preparing a wire rod by finish-rolling the steel material;   winding the wire rod;   drawing and then performing spheroidizing heat treatment on the wound wire rod;   shaping the spheroidized heat-treated wire rod into a part; and   austenizing and then quenching the part.   
     
     
         5 . The method of  claim 4 , wherein:
 the finish-rolling is performed at 880 to 980° C., and   wherein the winding is performed at 830 to 930° C.   
     
     
         6 . The method of  claim 4 , wherein the spheroidizing heat treatment is performed at a range of 745 to 765° C. of maximum temperature. 
     
     
         7 . The method of  claim 4 , wherein the austenizing is performed at 870 to 940° C. 
     
     
         8 . The method of  claim 4 , wherein the quenching is performed with a refrigerant of 10 to 80° C. 
     
     
         9 . A wire rod for cold forging comprising:
 in percent by weight (wt %), 0.18 to 0.25% of C, 0.30 to 0.50% of Si, 0.35 to 0.50% of Mn, more than 0 to 0.03% of P, more than 0 to 0.03% of S, 0.45 to 0.60% of Cr, 0.015 to 0.03% of Ti, 0.001 to 0.004% of B, the remainder having Fe and other unavoidable,   wherein the wire rod has a diameter of 5.5 to 20 mm.