Case-hardened steel superiorin cold workability, machinability, and fatigue characteristics after carburized quenching and method of production of same

Abstract

Cold worked, machined, and carburized quenched case-hardened steel prevented from formation of coarse grains, that is, case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching characterized by limiting, by mass %, S: 0.001 to 0.15%, Ti: 0.05 to 0.2%, Al: 0.04% or less, and N: 0.0050% or less, containing other specific ingredients in specific ranges, furthermore containing one or more of Mg: 0.003% or less, Zr: 0.01% or less, and Ca: 0.005% or less, limiting the amount of precipitation of AlN to 0.01% or less, and having a density d (/mm 2 ) of sulfides with a equivalent circle diameter of over 20 μm and an aspect ratio of over 3 and a content of S [S] (mass %) satisfying d≦1700[S]+20.

1 . Case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching characterized by containing, by mass %,
 C: 0.1 to 0.5%,   Si: 0.01 to 1.5%,   Mn: 0.3 to 1.8%,   S: 0.001 to 0.15%,   Cr: 0.4 to 2.0%, and   Ti: 0.05 to 0.2%,   limiting   Al: 0.04% or less,   N: 0.0050% or less,   P: 0.025% or less,   O: 0.0025% or less,   further having one or more of   Mg: 0.003% or less,   Zr: 0.01% or less, and   Ca: 0.005% or less,   having a balance of iron and unavoidable impurities,   limiting an amount of precipitation of MN to 0.01% or less, and   having a density d (/mm 2 ) of sulfides of a equivalent circle diameter of over 20 μm and an aspect ratio of over 3 and a content of S [S] (mass %) satisfying
     d≦ 1700[ S]+ 20. 
   
     
     
         2 . Case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching as set forth in  claim 1 , characterized by further containing, by mass %,
 Nb: less than 0.04%.   
     
     
         3 . Case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching as set forth in  claim 1 , characterized by further containing, by mass %, one or more of
 Mo: 1.5% or less,   Ni: 3.5% or less,   V: 0.5% or less, and   B: 0.005% or less.   
     
     
         4 . Case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching as set forth in  claim 1 , characterized by limiting a structural fraction of bainite to 30% or less. 
     
     
         5 . Case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching as set forth in  claim 1 , characterized in that a grain size number of ferrite is 8 to 11 as defined by JIS G 0551. 
     
     
         6 . Case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching as set forth in  claim 1 , characterized in that a maximum size of Ti precipitates is 40 μm or less. 
     
     
         7 . A method of production of case-hardened steel superior in cold workability, machinability, and fatigue characteristics after carburized quenching characterized by heating a steel material comprised of the ingredients of  claim 1  to 1150° C. or more, hot working it at a finishing temperature of 840 to 1000° C., and cooling it in a 800 to 500° C. temperature range by 1° C./s or less.