Case-hardening steel and case-hardened steel member

Abstract

There is provided a case-hardening steel in which in inclusion evaluation using an extreme value statistical method, when an estimated area S is 30,000 mm 2 , an estimated value of the maximum size (√area) S of sulfide-based inclusions present in the estimated area S is 49 μm or less and an estimated value of the maximum size (√area) Ox of oxide-based inclusions present in the estimated area S is 80 μm or less, and the number of sulfide-based inclusions having a length of more than 20 μm and a thickness of more than 2 μm per 1 mm 2 is limited to 200 or less,

1 . A case-hardening steel comprising, as a chemical composition, by mass %:
 C: 0.05% to 0.30%;   Si: 0.40% to 1.5%;   Mn: 0.2% to 1.0%;   S: 0.001% to 0.050%;   Cr: 1.0% to 2.0%;   Mo: 0.02% to 0.8%;   Al: 0.001% to 0.20%;   N: 0.003% to 0.03%;   Nb: 0% to 0.10%;   Cu: 0% to 0.2%;   Ni: 0% to 1.5%;   V: 0% to 0.20%;   Ca: 0% to 0.0050%;   Mg: 0% to 0.0050%;   Sb: 0% to 0.050%;   P: limited to 0.030% or less;   O: limited to 0.0020% or less;   Ti: limited to 0.005% or less; and   a balance consisting of Fe and impurities,   wherein the following Equations (1) and (2) are satisfied,   in inclusion evaluation using an extreme value statistical method, when an estimated area S is 30,000 mm 2 , an estimated value of the maximum size (√area) S  of sulfide-based inclusions present in the estimated area S is 49 μm or less and an estimated value of the maximum size (√area) Ox  of oxide-based inclusions present in the estimated area S is 80 μm or less, and   the number of sulfide-based inclusions having a length of more than 20 μm and a thickness of more than 2 p.m per 1 mm 2  is limited to 200 or less,
   12×Si(%)+25×Mn(%)+Cr(%)+2×Mo(%)≦25   (1),
 
   31×Si(%)+15×Mn(%)+23×Cr(%)≧50   (2),
 
   here, Si(%), Mn(%), Cr(%) and Mo(%) represent the amount of each element by mass % in Equations (1) and (2).   
     
     
         2 . The case-hardening steel according to  claim 1 , comprising, as the chemical composition, by mass %:
 Nb: 0.015% to 0.10%.   
     
     
         3 . The case-hardening steel according to  claim 1 , comprising, as the chemical composition, by mass %:
 Si: 0.55% to 1.5%.   
     
     
         4 . The case-hardening steel according to  claim 1 , comprising, as the chemical composition, by mass %, either or both of,
 Cu: 0.001% to 0.2%; and   Ni: 0.001% to 1.5%.   
     
     
         5 . The case-hardening steel according to  claim 1 , comprising, as the chemical composition, by mass %:
 V: 0.01% to 0.20%.   
     
     
         6 . The case-hardening steel according to  claim 1 , comprising, as the chemical composition, by mass %, either or both of,
 Ca: 0.0001% to 0.0050%; and   Mg: 0.0001% to 0.0050%.   
     
     
         7 . The case-hardening steel according to  claim 1 , comprising, as the chemical composition, by mass %:
 Sb: 0.0001% to 0.050%.   
     
     
         8 . The case-hardening steel according to  claim 1 ,
 wherein the microstructure has a spheroidized carbide structure.   
     
     
         9 . A case-hardened steel member made from the case-hardening steel according to  claim 1 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.   
     
     
         10 . A case-hardened steel member made from the case-hardening steel according to  claim 8 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.   
     
     
         11 . The case-hardening steel according to  claim 2 ,
 wherein the microstructure has a spheroidized carbide structure.   
     
     
         12 . The case-hardening steel according to  claim 3 ,
 wherein the microstructure has a spheroidized carbide structure.   
     
     
         13 . The case-hardening steel according to  claim 4 ,
 wherein the microstructure has a spheroidized carbide structure.   
     
     
         14 . The case-hardening steel according to  claim 5 ,
 wherein the microstructure has a spheroidized carbide structure.   
     
     
         15 . The case-hardening steel according to  claim 6 ,
 wherein the microstructure has a spheroidized carbide structure.   
     
     
         16 . The case-hardening steel according to  claim 7 ,
 wherein the microstructure has a spheroidized carbide structure.   
     
     
         17 . A case-hardened steel member made from the case-hardening steel according to  claim 2 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.   
     
     
         18 . A case-hardened steel member made from the case-hardening steel according to  claim 3 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.   
     
     
         19 . A case-hardened steel member made from the case-hardening steel according to  claim 4 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.   
     
     
         20 . A case-hardened steel member made from the case-hardening steel according to  claim 5 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.   
     
     
         21 . A case-hardened steel member made from the case-hardening steel according to  claim 6 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.   
     
     
         22 . A case-hardened steel member made from the case-hardening steel according to  claim 7 , wherein
 a surface hardened layer that is formed by a carburizing quenching and tempering treatment or a carburizing nitriding quenching and tempering treatment.