Carburized steel and its process of manufacture

Abstract

A carburized steel member is manufactured by specific carburizing, cooling, and quenching steps. The steel member contains: C: 0.1% to 0.4%, Si: 0.35% to 3.0%, Mn: 0.1% to 3.0%, P: 0.03% or less, S: 0.15% or less, Al: 0.05% or less, and N: 0.03% or less, and a content of Cr is less than 0.2%, a content of Mo is 0.1% or less, and remainder is constituted of Fe and unavoidable impurities. A surface layer thereof includes: a first layer having a carbon concentration of 0.60 mass % to 0.85 mass % and including a martensitic structure in which no grain boundary oxide layer caused by Si exists; a second layer having a carbon concentration of 0.1 mass % to 0.4 mass % and including a martensitic structure; and a third layer having a carbon concentration of 0.1 mass % to 0.4 mass % and including no martensitic structure.

1 . A carburized steel member that is manufactured by:
 a carburizing step of forming a carburized layer on a surface layer by heating a steel member to an austenitizing temperature or higher in a carburizing atmosphere where oxygen concentration is lower than that of the atmosphere; a cooling step of, subsequent to the carburizing step, cooling the steel member at a cooling rate slower than a cooling rate at which martensitic transformation occurs, and cooling the steel member to not higher than a temperature at which structural transformation caused by cooling is completed; and a quenching step of heating a desired portion of the steel member with high-density energy to an austenitic region, and thereafter cooling the steel member at a cooling rate of not slower than the cooling rate at which martensitic transformation occurs, the carburized steel member wherein   as a basic chemical composition, the steel member contains, by mass %, C: 0.1% to 0.4%, Si: 0.35% to 3.0%, Mn: 0.1% to 3.0%, P: 0.03% or less, S: 0.15% or less, Al: 0.05% or less, and N: 0.03% or less, and a content of Cr is less than 0.2%, a content of Mo is 0.1% or less, and remainder is constituted of Fe and unavoidable impurities, and   the surface layer of the desired portion on which the quenching step is performed includes:
 a first layer having a carbon concentration higher than the range of the basic chemical composition and including a martensitic structure in which no grain boundary oxide layer caused by Si exists; 
 a second layer located inside the first layer, having a carbon concentration within the range of the basic chemical composition, and including a martensitic structure; and 
 a third layer located inside the second layer, having a carbon concentration within the range of the basic chemical composition, and including no martensitic structure. 
   
     
     
         2 . The carburized steel member according to  claim 1 , wherein the carbon concentration of a surface portion of the first layer is within a range of 0.6 mass % to 0.85 mass % and is gradually lowered to approach the range of the basic chemical composition toward a boundary with the second layer. 
     
     
         3 . The carburized steel member according to  claim 2 , wherein the second layer has a larger thickness than that of the first layer. 
     
     
         4 . The carburized steel member according to  claim 1 , wherein the basic chemical composition further includes Ti: 0.005% to 0.2% and B: 0.0006% to 0.005%. 
     
     
         5 . The carburized steel member according to  claim 1 , wherein Mo content in the basic chemical composition is less than 0.01%. 
     
     
         6 . The carburized steel member according to  claim 1 , wherein the basic chemical composition further includes either or both of Nb: 0.01% to 0.3% and V: 0.01% to 0.2%. 
     
     
         7 . The carburized steel member according to  claim 1 , characterized in that the basic chemical composition further includes Ni: 0.1% to 3.0%. 
     
     
         8 . A method of manufacturing a carburized steel member, comprising:
 a carburizing step of forming a carburized layer on a surface layer by heating a steel member to an austenitizing temperature or higher in a carburizing atmosphere where oxygen concentration is lower than that of the atmosphere;   a cooling step of, subsequent to the carburizing step, cooling the steel member at a cooling rate slower than a cooling rate at which martensitic transformation occurs, and cooling the steel member to not higher than a temperature at which structural transformation caused by cooling is completed; and   a quenching step of heating a desired portion of the steel member with high-density energy to an austenitic region, and thereafter cooling the steel member at a cooling rate of not slower than the cooling rate at which martensitic transformation occurs, wherein   as a basic chemical composition, the steel member contains, by mass %, C: 0.1% to 0.4%, Si: 0.35% to 3.0%, Mn: 0.1% to 3.0%, P: 0.03% or less, S: 0.15% or less, Al: 0.05% or less, and N: 0.03% or less, and a content of Cr is less than 0.2%, a content of Mo is 0.1% or less, and remainder is constituted of Fe and unavoidable impurities.   
     
     
         9 . The method of manufacturing a carburized steel member according to  claim 8 , wherein the cooling step is started immediately after the carburizing step so as not to precipitate solid solution of carbon in austenite created in the carburizing step as graphite grain. 
     
     
         10 . The method of manufacturing a carburized steel member according to  claim 8 , wherein the carburizing step is performed under a condition that a surface carburizing concentration of the steel member after diffusion is 0.8% or less. 
     
     
         11 . The method of manufacturing a carburized steel member according to  claim 10 , wherein
 the steel member on which the carburizing step is performed includes a first portion and a second portion whose diffusion rate of carbon entering when the carburizing treatment is performed is different from that of the first portion because of shapes thereof, the entering carbon in the second portion has a lower diffusion rate than that in the first portion, and   the carburizing step is performed under a condition that a surface carburizing concentration of the first portion is within a range of 0.65±0.1 mass %.   
     
     
         12 . A case hardening steel for the carburized steel member according to  claim 1 , wherein
 the case hardening steel has a basic chemical composition including, by mass %, C: 0.1% to 0.4%, Si: 0.35% to 3.0%, Mn: 0.1% to 3.0%, P: 0.03% or less, S: 0.15% or less, Al: 0.05% or less, and N: 0.03% or less, and   a content of Cr is less than 0.2%, a content of Mo is 0.1% or less, and remainder is constituted of Fe and unavoidable impurities.   
     
     
         13 . The case hardening steel according to  claim 12 , wherein the basic chemical composition further includes Ti: 0.005% to 0.2% and B: 0.0006% to 0.005%. 
     
     
         14 . The case hardening steel according to  claim 12 , wherein the Mo content in the basic chemical composition is less than 0.01%. 
     
     
         15 . The case hardening steel according to  claim 12 , wherein the basic chemical composition further includes either or both of Nb: 0.01% to 0.3% and V: 0.01% to 0.2%. 
     
     
         16 . The case hardening steel according to  claim 12 , wherein the basic chemical composition further includes Ni: 0.1% to 3.0%.