Carburizing steel having excellent cold forgeability and method of manufacturing the same

Abstract

A carburizing steel has a composition containing, in mass %, C: 0.1-0.35%; Si: 0.01-0.22%; Mn: 0.3-1.5%; Cr: 1.35-3.0%; P: 0.018% or less; S: 0.02% or less; Al: 0.015-0.05%; N: 0.008-0.015%; and O: 0.0015% or less, each being contained in an amount within a range satisfying formulas (1), (2) and (3) below, and the balance of the composition being Fe and incidental impurities, and the carburizing steel having microstructures before spheroidizing annealing such that a total microstructure proportion of ferrite and pearlite is 85% or more and an average ferrite grain size is 25 μm or less. 3.1≧{([% Si]/2)+[% Mn]+[% Cr]}≧2.2   (1) [% C]−([% Si]/2)+([% Mn]/5)+2[% Cr]≧3.0   (2) 2.5≧[% Al]/[% N]≧1.7   (3) [% M] represents content (in mass %) of element M.

1 . A carburizing steel having a composition containing, in mass %,
 C: 0.1% to 0.35%,   Si: 0.01% to 0.22%,   Mn: 0.3% to 1.5%,   Cr: 1.35% to 3.0%,   P: 0.018% or less,   S: 0.02% or less,   Al: 0.015% to 0.05%,   N: 0.008% to 0.015%, and   O: 0.0015% or less,   
       each being contained in an amount within a range satisfying the following formulas (1), (2) and (3), and the balance of the composition being Fe and incidental impurities, wherein the carburizing steel has microstructures such that a total microstructure proportion of ferrite and pearlite is 85% or more and an average ferrite grain size is 25 μm or less:
   3.1≧{([% Si]/2)+[% Mn]+[% Cr]}≧2.2   (1)
 
   [% C]−([% Si]/2)+([% Mn]/5)+2[% Cr]≧3.0   (2)
 
   2.5≧[% Al]/[% N]≧1.7   (3)
 
 
       where [% M] represents content (in mass %) of element M. 
     
     
         2 . The carburizing steel according to  claim 1 , wherein the composition further contains, in mass %, at least one element selected from
 Cu: 1.0% or less,   Ni: 0.5% or less,   Mo: 0.5% or less,   V: 0.5% or less, and   Nb: 0.06% or less.   
     
     
         3 . A method of manufacturing a carburizing steel, the method comprising:
 subjecting a steel material to hot working, where the steel material is heated to at least 1160° C. and lower than 1220° C., the steel material having a composition containing, in mass %,   C: 0.1 to 0.35%,   Si: 0.01% to 0.22%,   Mn: 0.3% to 1.5%,   Cr: 1.35% to 3.0%,   P: 0.018% or less,   S: 0.02% or less,   Al: 0.015% to 0.05%,   N: 0.008% to 0.015%, and   O: 0.0015% or less,   
       each being contained in an amount within a range satisfying the following formulas (1), (2) and (3), and the balance of the composition being Fe and incidental impurities;
 after suspending the hot working within a temperature range of Ar 3  point or higher, cooling the steel material to 450° C. or lower; 
 then reheating the steel material to a temperature higher than 900° C., but not higher than 970° C., to resume the hot working; 
 terminating the hot working under a condition of a total reduction rate being 70% or more after the reheating; and 
 then cooling the steel material at a rate of 0.1° C./s to 1.0° C./s from 800° C. to 500° C.:
   3.1≧{([% Si]/2)+[% Mn]+[% Cr]}≧2.2   (1)
 
   [% C]−([% Si]/2)+([% Mn]/5)+2[% Cr]≧3.0   (2)
 
   2.5≧[% Al]/[% N]≧1.7   (3)
 
 
 
       [% M] represents content (in mass %) of element M. 
     
     
         4 . The method of manufacturing a carburizing steel according to  claim 3 , wherein the composition further contains, in mass %, at least one element selected from
 Cu: 1.0% or less,   Ni: 0.5% or less,   Mo: 0.5% or less,   V: 0.5% or less, and   Nb: 0.06% or less.