Steel, carburized steel component, and method for manufacturing carburized steel component

Abstract

A steel according to an aspect of the present invention has a chemical composition within a predetermined range, in which a hardenability index Ceq ranges from greater than 7.5 to smaller than 44.0, an AlN precipitation index I AlN ranges from greater than 0.00030 to smaller than 0.00110, a metallographic structure includes ferrite ranging from 85 to 100 area %, an average distance between sulfides, which are observed in a cross section parallel to a rolling direction of the steel and have an equivalent circle diameter ranging from 1 μm or greater to smaller than 2 μm, is shorter than 30.0 μm, and a presence density of the sulfides, which are observed in the cross section parallel to the rolling direction of the steel and have an equivalent circle diameter ranging from 1 μm or greater to smaller than 2 μm, is 300 pieces/mm 2 or more.

1 . A steel, wherein
 a chemical composition comprises, by mass %,   C: 0.07% to 0.13%,   Si: 0.0001% to 0.50%,   Mn: 0.0001% to 0.80%,   S: 0.0050% to 0.0800%,   Cr: more than 1.30% to 5.00% or less,   B: 0.0005% to 0.0100%,   Al: 0.070% to 0.200%,   N: 0.0030% to 0.0100%,   Bi: more than 0.0001% to 0.0100% or less,   Ti: 0.020% or less,   P: 0.050% or less,   O: 0.0030% or less,   Nb: 0% to 0.1000%,   V: 0% to 0.20%,   Mo: 0% to 0.500%,   Ni: 0% to 1.000%,   Cu: 0% to 0.500%,   Ca: 0% to 0.0030%,   Mg: 0% to 0.0030%,   Te: 0% to 0.0030%,   Zr: 0% to 0.0050%,   a rare earth metal: 0% to 0.0050%,   Sb: 0% to 0.0500%, and   a remainder including Fe and impurities,   wherein a hardenability index Ceq obtained by substituting the amount of each element in the chemical composition indicated by mass % in Expression 1 ranges from greater than 7.5 to smaller than 44.0,   wherein an AlN precipitation index I AlN  obtained by substituting the amount of each element in the chemical composition indicated by mass % in Expression 2 ranges from greater than 0.00030 to smaller than 0.00110,   wherein a metallographic structure includes ferrite ranging from 85 to 100 area %,   wherein an average distance between sulfides, which are observed in a cross section parallel to a rolling direction of the steel and have an equivalent circle diameter ranging from 1 μm or greater to smaller than 2 μm, is shorter than 30.0 μm, and   wherein a presence density of the sulfides, which are observed in the cross section parallel to the rolling direction of the steel and have an equivalent circle diameter ranging from 1 μm or greater to smaller than 2 μm, is 300 pieces/mm 2  or more,
   Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)  (Expression 1)
 
     I   AlN =Al×(N—Ti×(14/48))  (Expression 2).
 
   
     
     
         2 . The steel according to  claim 1 ,
 wherein the chemical composition includes, by mass %, one or more selected from the group consisting of   Nb: 0.0020% to 0.1000%,   V: 0.002% to 0.20%,   Mo: 0.005% to 0.500%,   Ni: 0.005% to 1.000%,   Cu: 0.005% to 0.500%,   Ca: 0.0002% to 0.0030%,   Mg: 0.0002% to 0.0030%,   Te: 0.0002% to 0.0030%,   Zr: 0.0002% to 0.0050%,   a rare earth metal: 0.0002% to 0.0050%, and   Sb: 0.0020% to 0.0500%.   
     
     
         3 . A carburized steel component comprising:
 a steel portion; and   a carburized layer, which is a region on an outer surface of the steel portion and which has Vickers hardness of HV 550 or higher,   wherein a thickness of the carburized layer ranges from greater than 0.40 mm to smaller than 2.00 mm,   wherein an average Vickers hardness at a position in a depth of 50 μm from a surface of the carburized steel component ranges from HV 650 or higher to HV 1,000 or lower,   wherein an average Vickers hardness at a position in a depth of 2.0 mm from the surface of the carburized steel component ranges from HV 250 or higher to HV 500 or lower,   wherein a chemical composition of the steel portion includes, by mass %,
 C: 0.07% to 0.13%, 
 Si: 0.0001% to 0.50%, 
 Mn: 0.0001% to 0.80%, 
 S: 0.0050% to 0.0800%, 
 Cr: more than 1.30% to 5.00% or less, 
 B: 0.0005% to 0.0100%, 
 Al: 0.070% to 0.200%, 
 N: 0.0030% to 0.0100%, 
 Bi: more than 0.0001% to 0.0100% or less, 
 Ti: 0.020% or less, 
 P: 0.050% or less, 
 O: 0.0030% or less, 
 Nb: 0% to 0.1000%, 
 V: 0% to 0.20%, 
 Mo: 0% to 0.500%, 
 Ni: 0% to 1.000%, 
 Cu: 0% to 0.500%, 
 Ca: 0% to 0.0030%, 
 Mg: 0% to 0.0030%, 
 Te: 0% to 0.0030%, 
 Zr: 0% to 0.0050%, 
 a rare earth metal: 0% to 0.0050%, 
 Sb: 0% to 0.0500%, and 
 a remainder including Fe and impurities, 
   wherein a hardenability index Ceq obtained by substituting the amount of each element in the chemical composition of the steel portion indicated by mass % in Expression 3 ranges from greater than 7.5 to smaller than 44.0,   wherein an AlN precipitation index I AlN  obtained by substituting the amount of each element in the chemical composition indicated by mass % in Expression 4 ranges from greater than 0.00030 to smaller than 0.00110,   wherein an average distance between sulfides, which are observed in a cross section parallel to a rolling direction of the carburized steel component and have an equivalent circle diameter in the steel portion ranging from 1 μm or greater to smaller than 2 μm, is shorter than 30.0 μm, and   wherein a presence density of the sulfides, which are observed in the cross section parallel to the rolling direction of the carburized steel component and have an equivalent circle diameter in the steel portion ranging from 1 μm or greater to smaller than 2 μm, is 300 pieces/mm 2  or more,
   Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)  (Expression 3)
 
     I   AlN =Al×(N—Ti×(14/48))  (Expression 4).
 
   
     
     
         4 . The carburized steel component according to  claim 3 ,
 wherein the chemical composition of the steel portion includes, by mass %, one or more selected from the group consisting of   Nb: 0.0020% to 0.1000%,   V: 0.002% to 0.20%,   Mo: 0.005% to 0.500%,   Ni: 0.005% to 1.000%,   Cu: 0.005% to 0.500%,   Ca: 0.0002% to 0.0030%,   Mg: 0.0002% to 0.0030%,   Te: 0.0002% to 0.0030%,   Zr: 0.0002% to 0.0050%,   a rare earth metal: 0.0002% to 0.0050%, and   Sb: 0.0020% to 0.0500%.   
     
     
         5 . A method for manufacturing the carburized steel component according to  claim 3 , the method comprising:
 cold plastic working a steel wherein   a chemical composition comprises, by mass %,   C: 0.07% to 0.13%,   Si: 0.0001% to 0.50%,   Mn: 0.0001% to 0.80%,   S: 0.0050% to 0.0800%,   Cr: more than 1.30% to 5.00% or less,   B: 0.0005% to 0.0100%,   Al: 0.070% to 0.200%,   N: 0.0030% to 0.0100%,   Bi: more than 0.0001% to 0.0100% or less,   Ti: 0.020% or less,   P: 0.050% or less,   O: 0.0030% or less,   Nb: 0% to 0.1000%,   V: 0% to 0.20%,   Mo: 0% to 0.500%,   Ni: 0% to 1.000%,   Cu: 0% to 0.500%,   Ca: 0% to 0.0030%,   Mg: 0% to 0.0030%,   Te: 0% to 0.0030%,   Zr: 0% to 0.0050%,   a rare earth metal: 0% to 0.0050%,   Sb: 0% to 0.0500%, and   a remainder including Fe and impurities,   wherein a hardenability index Ceq obtained by substituting the amount of each element in the chemical composition indicated by mass % in Expression 1 ranges from greater than 7.5 to smaller than 44.0,   wherein an AlN precipitation index I AlN  obtained by substituting the amount of each element in the chemical composition indicated by mass % in Expression 2 ranges from greater than 0.00030 to smaller than 0.00110,   wherein a metallographic structure includes ferrite ranging from 85 to 100 area %,   wherein an average distance between sulfides, which are observed in a cross section parallel to a rolling direction of the steel and have an equivalent circle diameter ranging from 1 μm or greater to smaller than 2 μm, is shorter than 30.0 μm, and   wherein a presence density of the sulfides, which are observed in the cross section parallel to the rolling direction of the steel and have an equivalent circle diameter ranging from 1 μm or greater to smaller than 2 μm, is 300 pieces/mm 2  or more,
   Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)  (Expression
 
     I   AlN =Al×(N—Ti×(14/48))  (Expression 2);
 
   cutting the steel after the cold plastic working; and   carburizing or carbonitriding the steel after the cutting.   
     
     
         6 . The method for manufacturing the carburized steel component, according to  claim 5 , further comprising:
 quenching, or quenching and tempering after the carburizing or the carbonitriding.   
     
     
         7 . The method for manufacturing the carburized steel component, according to  claim 5 , wherein
 the chemical composition includes, by mass %, one or more selected from the group consisting of   Nb: 0.0020% to 0.1000%,   V: 0.002% to 0.20%,   Mo: 0.005% to 0.500%,   Ni: 0.005% to 1.000%,   Cu: 0.005% to 0.500%,   Ca: 0.0002% to 0.0030%,   Mg: 0.0002% to 0.0030%,   Te: 0.0002% to 0.0030%,   Zr: 0.0002% to 0.0050%,   a rare earth metal: 0.0002% to 0.0050%, and   Sb: 0.0020% to 0.0500%.