Case hardening steel, carburized component, and manufacturing method of case hardening steel

Abstract

A case hardening steel includes, as a chemical composition, by mass %, C: 0.10% to 0.30%, Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S: 0.003% to 0.020%, Cr: 0.40% to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to 0.20%, Bi: 0.0001% to 0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0% to 0.50%, B: 0% to 0.0050%, Nb: 0% or more and less than 0.040%, P: limited to 0.050% or less, N: limited to 0.0060% or less, O: limited to 0.0025% or less, and a remainder including an iron and impurities, and satisfies Ti/S≥6.0, in which, in a longitudinal section, a maximum diameter of Ti-based precipitates predicted by extreme value statistics under a condition that an inspection standard area is 100 mm 2 , a number of inspections is 16 visual fields, and an area where prediction is performed is 30,000 mm 2 , is 40 μm or less.

1 . A case hardening steel comprising, as a chemical composition, by mass %,
 C: 0.10% to 0.30%,   Si: 0.02% to 1.50%,   Mn: 0.30% to 1.80%,   S: 0.003% to 0.020%,   Cr: 0.40% to 2.00%,   Al: 0.005% to 0.050%,   Ti: 0.06% to 0.20%,   Bi: 0.0001% to 0.0050%,   Mo: 0% to 1.50%,   Ni: 0% to 3.50%,   V: 0% to 0.50%,   B: 0% to 0.0050%,   Nb: 0% or more and less than 0.040%,   P: limited to 0.050% or less,   N: limited to 0.0060% or less,   O: limited to 0.0025% or less, and   a remainder including an iron and impurities,   wherein an Expression (1) is satisfied, and   in a longitudinal section, a maximum diameter of Ti-based precipitates predicted by extreme value statistics under a condition that an inspection standard area is 100 mm 2 , a number of inspections is 16 visual fields, and an area where a prediction is performed is 30,000 mm 2 , is 40 μm or less,
   Ti/S≥6.0   Expression (1)
 
   where Ti in the Expression (1) represents a Ti content by mass %, and S in the Expression (1) represents a S content by mass %.   
     
     
         2 . The case hardening steel according to  claim 1  comprising, as the chemical composition, by mass %, one or more selected from the group consisting of
 Mo: 0.02% to 1.50%, 
 Ni: 0.10% to 3.50%, 
 V: 0.02% to 0.50%, 
 B: 0.0002% to 0.0050%, and 
 Nb: more than 0% and less than 0.040%. 
 
     
     
         3 . The case hardening steel according to  claim 1 ,
 wherein a metallographic structure contains a bainite, and   a structure fraction of the bainite is 30% or less.   
     
     
         4 . The case hardening steel according to  claim 1 ,
 wherein the metallographic structure contains a ferrite, and   a grain size number of the ferrite is No. 8 to No. 11 specified in JIS G 0552.   
     
     
         5 . A carburized component comprising:
 the case hardening steel according to  claim 1 .   
     
     
         6 . A manufacturing method of a case hardening steel comprising:
 a heating process of heating steel including, as a chemical composition, by mass, C: 0.10% to 0.30%, Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S: 0.003% to 0.020%, Cr: 0.40% to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to 0.20%, Bi: 0.0001% to 0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0% to 0.50%, B: 0% to 0.0050%, Nb: 0% or more and less than 0.040%, P:   limited to 0.050% or less, N: limited to 0.0060% or less, O: limited to 0.0025% or less, and a remainder including an iron and impurities, and satisfies an Expression (2), at a temperature of 1150° C. or higher for a holding time of ten minutes or longer; and   a hot rolling process of hot rolling the steel into a wire rod or a steel bar,
   Ti/S≥6.0   Expression (2)
 
   where Ti in the Expression (2) represents a Ti content (mass %), and S in the Expression (2) represents a S content (mass %).   
     
     
         7 . The manufacturing method of a case hardening steel according to  claim 6  comprising:
 a cooling process of slow cooling the wire rod or the steel bar at a cooling rate of 1.00° C./s or less in a temperature range of 800° C. to 500° C. after the hot rolling process. 
 
     
     
         8 . The manufacturing method of a case hardening steel according to  claim 6 ,
 wherein, in the hot rolling process, a finish temperature is set to 840° C. to 1000° C.   
     
     
         9 . The case hardening steel according to  claim 2 ,
 wherein a metallographic structure contains a bainite, and   a structure fraction of the bainite is 30% or less.   
     
     
         10 . The case hardening steel according to  claim 2 ,
 wherein the metallographic structure contains a ferrite, and   a grain size number of the ferrite is No. 8 to No. 11 specified in JIS G 0552.   
     
     
         11 . The case hardening steel according to  claim 3 ,
 wherein the metallographic structure contains a ferrite, and   a grain size number of the ferrite is No. 8 to No. 11 specified in JIS G 0552.   
     
     
         12 . The case hardening steel according to  claim 9 ,
 wherein the metallographic structure contains a ferrite, and   a grain size number of the ferrite is No. 8 to No. 11 specified in JIS G 0552.   
     
     
         13 . A carburized component comprising:
 the case hardening steel according to  claim 2 .   
     
     
         14 . A carburized component comprising:
 the case hardening steel according to  claim 3 .   
     
     
         15 . A carburized component comprising:
 the case hardening steel according to  claim 4 .   
     
     
         16 . A carburized component comprising:
 the case hardening steel according to  claim 9 .   
     
     
         17 . A carburized component comprising:
 the case hardening steel according to  claim 10 .   
     
     
         18 . A carburized component comprising:
 the case hardening steel according to  claim 11 .   
     
     
         19 . A carburized component comprising:
 the case hardening steel according to  claim 12 .   
     
     
         20 . The manufacturing method of a case hardening steel according to  claim 7 , wherein, in the hot rolling process, a finish temperature is set to 840° C. to 1000° C.