Steel for nitrocarburizing and nitrocarburized component, and methods of producing same

Abstract

The steel for nitrocarburizing has a chemical composition that contains C: 0.01% or more and less than 0.20%, Si: 1.0% or less, Mn: 1.5% or more and 3.0% or less, P: 0.02% or less, S: 0.06% or less, Cr: 0.30% or more and 3.0% or less, Mo: 0.005% or more and 0.40% or less, V: 0.02% or more and 0.5% or less, Nb: 0.003% or more and 0.20% or less, Al: 0.010% or more and 2.0% or less, Ti: more than 0.005% and less than 0.025%, N: 0.0200% or less, Sb: 0.0005% or more and 0.02% or less, and the balance consisting of Fe and incidental impurities; and a steel microstructure that contains bainite phase in an area ratio of more than 50%.

1 . A steel for nitrocarburizing comprising:
 a chemical composition that contains, in mass %,
 C: 0.01% or more and less than 0.20%, 
 Si: 1.0% or less, 
 Mn: 1.5% or more and 3.0% or less, 
 P: 0.02% or less, 
 S: 0.06% or less, 
 Cr: 0.30% or more and 3.0% or less, 
 Mo: 0.005% or more and 0.40% or less, 
 V: 0.02% or more and 0.5% or less, 
 Nb: 0.003% or more and 0.20% or less, 
 Al: 0.010% or more and 2.0% or less, 
 Ti: more than 0.005% and less than 0.025%, 
 N: 0.0200% or less, 
 Sb: 0.0005% or more and 0.02% or less, 
 the balance consisting of Fe and incidental impurities, with the chemical composition satisfying either one of the following relations: 
   in a case where the C content is 0.01% or more and 0.10% or less,
   (S/32)/(Ti/48)+(N/14)/(Ti/48)≤13.0, and
 
   in a case where the C content is more than 0.10% and less than 0.20%,
   2(S/32)/(Ti/48)+3(N/14)/(Ti/48)≤35.0; and
 
   a steel microstructure that contains bainite phase in an area ratio of more than 50%.   
     
     
         2 . The steel for nitrocarburizing according to  claim 1 , wherein the steel composition further contains, in mass %, one or more selected from the group consisting of
 B: 0.0100% or less,   Cu: 0.3% or less,   Ni: 0.3% or less,   W: 0.3% or less,   Co: 0.3% or less,   Hf: 0.2% or less,   Zr: 0.2% or less,   Pb: 0.2% or less,   Bi: 0.2% or less,   Zn: 0.2% or less, and   Sn: 0.2% or less.   
     
     
         3 - 4 . (canceled) 
     
     
         5 . A component comprising:
 a core part comprising the chemical composition and the steel microstructure as recited in  claim 1 ; and   a surface layer part comprising a chemical composition with high nitrogen and carbon contents relative to the chemical composition of the core part.   
     
     
         6 . The component according to  claim 5 , wherein precipitates containing V and Nb are dispersed in the bainite phase. 
     
     
         7 . A method of producing a steel for nitrocarburizing, comprising:
 subjecting a steel to hot working with a heating temperature of 950° C. or higher and a finishing temperature of 800° C. or higher, the steel comprising a chemical composition that contains, in mass %,
 C: 0.01% or more and less than 0.20%, 
 Si: 1.0% or less, 
 Mn: 1.5% or more and 3.0% or less, 
 P: 0.02% or less, 
 S: 0.06% or less, 
 Cr: 0.30% or more and 3.0% or less, 
 Mo: 0.005% or more and 0.40% or less, 
 V: 0.02% or more and 0.5% or less, 
 Nb: 0.003% or more and 0.20% or less, 
 Al: 0.010% or more and 2.0% or less, 
 Ti: more than 0.005% and less than 0.025%, 
 N: 0.0200% or less, 
 Sb: 0.0005% or more and 0.02% or less, 
 the balance consisting of Fe and incidental impurities, with the chemical composition satisfying either one of the following relations: 
   in a case where the C content is 0.01% or more and 0.10% or less,
   (S/32)/(Ti/48)+(N/14)/(Ti/48)≤13.0, and
 
   in a case where the C content is more than 0.10% and less than 0.20%,
   2(S/32)/(Ti/48)+3(N/14)/(Ti/48)≤35.0; and
 
   then cooling the steel at a cooling rate of higher than 0.4° C./s at least in a temperature range of 700° C. to 550° C.   
     
     
         8 . The method of producing a steel for nitrocarburizing according to  claim 7 , wherein the steel composition further contains, in mass %, one or more selected from the group consisting of
 B: 0.0100% or less,   Cu: 0.3% or less,   Ni: 0.3% or less,   W: 0.3% or less,   Co: 0.3% or less,   Hf: 0.2% or less,   Zr: 0.2% or less,   Pb: 0.2% or less,   Bi: 0.2% or less,   Zn: 0.2% or less, and   Sn: 0.2% or less.   
     
     
         9 - 10 . (canceled) 
     
     
         11 . A method of producing a component, comprising:
 processing the steel for nitrocarburizing obtainable by the method as recited in  claim 7  into a desired shape; and   then subjecting the steel for nitrocarburizing to nitrocarburizing treatment at 550° C. to 700° C. for 10 minutes or more.   
     
     
         12 . A component comprising:
 a core part comprising the chemical composition and the steel microstructure as recited in  claim 2 ; and   a surface layer part comprising a chemical composition with high nitrogen and carbon contents relative to the chemical composition of the core part.   
     
     
         13 . The component according to  claim 12 , wherein precipitates containing V and Nb are dispersed in the bainite phase. 
     
     
         14 . A method of producing a component, comprising:
 processing the steel for nitrocarburizing obtainable by the method as recited in  claim 8  into a desired shape; and   then subjecting the steel for nitrocarburizing to nitrocarburizing treatment at 550° C. to 700° C. for 10 minutes or more.