Steel Wire for Cold Forging Having Excellent Low Temperature Impact Properties and Method of Producing the Same

Abstract

Disclosed is a steel wire for cold forging, which has excellent low temperature impact properties, and a method of producing the same. The steel wire consists of 0.10-0.40 wt % C, 1.0 wt % or less of Si, 0.30 - 2.0 wt % Mn, 0.03 wt % or less of P, 0.03 wt % or less of S, and the balance of Fe and impurities. The steel wire has an austenite grain size of 5-20 μm, impact absorption energy of 60 J/cm 2 or more at −40° C., and tensile strength of 70-130 kgf/mm 2 . A steel material for cold forming according present invention has impact toughness that is greatly superior to a conventional spheroidized material or non-heat treated steel at a low temperature of −40° C.

1 . A steel wire for cold forging, which has excellent low temperature impact properties, comprising 0.10-0.40 wt % C, 1.0 wt % or less of Si, 0.30-2.0 wt % Mn, 0.03 wt % or less of P, 0.03 wt % or less of S, and a balance of Fe and impurities, wherein an austenite grain size is 5-20 μm, impact absorption energy is 60 J/cm 2  or more at −40° C., and tensile strength is 70-130 kgf/mm 2 .  
   
   
       2 . The steel wire as set forth in  claim 1 , further comprising at least one component selected from the group consisting of 0.05-2.0 wt % Cr, 0.05-1.5 wt % Mo, and 0.0003-0.0050 wt % B.  
   
   
       3 . A method of producing a steel wire for cold forging, which has excellent low temperature impact properties, comprising: 
 rapidly heating steel, which contains 0.10-0.40 wt % C, 1.0 wt % or less of Si, 0.30-2.0 wt % Mn, 0.03 wt % or less of P, 0.03 wt % or less of S, and a balance of Fe and impurities, to a Ac3 transformation point or higher so that an austenite grain size is 5-20 μm;    cooling the heated steel; and    heat treating the cooled steel in such a way that tensile strength is 70-130 kgf/mm 2  at a tempering parameter (P) ranging from 21,800 to 30,000, which is expressed by a following Equation 1, so that impact absorption energy is 60 J/cm 2  or more at −40° C.,        P= 1.8×( T+ 273)×(14.44+log  t )   Equation 1    wherein, T is a tempering temperature (° C.), and t is a tempering time (sec).    
   
   
       4 . The method as set forth in  claim 3 , wherein the steel further comprises at least one component selected from the group consisting of 0.05-2.0 wt % Cr, 0.05-1.5 wt % Mo, and 0.0003-0.0050 wt % B.