CASE-HARDENED STEEL SUPERIORIN COLD WORKABILITY, MACHINABILITY, AND FATIGUE CHARACTERISTICS AFTER CARBURIZED QUENCHING AND METHOD OF PRODUCTION OF SAME
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.