Rolled steel bar or wire rod for hot forging

Abstract

A rolled steel bar or wire rod having hot surface fatigue strength, wear resistance, and machinability even after hot forging has a composition containing C, Si, Mn, S, Cr, Mo (optional), Al, and N, with the balance being Fe and impurities. The chemical composition satisfies that fn1 defined by Formula (1) is 1.60 to 2.10. The structure of the rolled steel bar or wire rod for hot forging includes a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. A maximum value/a minimum value of average ferrite grain size, which is observed and measured randomly in 15 visual fields each having an area of 62500 μm 2 in a cross section, is not more than 2.0. fn1=Cr+2×Mo (1), and each symbol of elements in Formula (1) is substituted by a content (mass %) of a corresponding element.

1 . A rolled steel bar or wire rod for hot forging, comprising:
 a chemical composition comprising, by mass %,
 C: 0.1 to 025%, 
 Si: 0.30 to 0.60%, 
 Mn: 0.50 to 1.0%, 
 S: 0.003 to 0.05%, 
 Cr: 1.50 to 2.00%, 
 Mo: not more than 0.10% (including 0%), 
 Al: 0.025 to 0.05%, 
 N: 0.010 to 0.025%, and the balance being Fe and impurities, wherein 
 the impurities contain P: not more than 0.025%, Ti: not more than 0.003%, and O (oxygen): not more than 0.002%, respectively, and wherein 
 fn1 defined by Formula (1) is 1.60 to 2.10; and 
   a structure consisting of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure, wherein   a maximum value/a minimum value of average ferrite grain size, which is obtained by making a measurement in 15 visual fields each having an area of 62500 μm 2  in a cross section, is not more than 2.0:
     fn 1=Cr+2×Mo  (1)
 
   where each symbol of elements in Formula (1) is substituted by a content (mass %) of a corresponding element.   
     
     
         2 . The rolled steel bar or wire rod for hot forging according to  claim 1 , wherein
 fn1 is not less than 1.80.   
     
     
         3 . The rolled steel bar or wire rod for hot forging according to  claim 1 , further comprising, by mass %,
 Nb: not more than 0.08% in place of a part of Fe.   
     
     
         4 . The rolled steel bar or wire rod for hot forging according to  claim 2 , further comprising, by mass %,
 Nb: not more than 0.08% in place of a part of Fe.