Case-hardening steel superior in tooth surface fatigue strength, gear using the same, and method of production of the same

Abstract

The present invention provides case-hardening steel superior in tooth surface fatigue strength and a gear using the same used for parts of automobiles, construction machines, industrial machines, etc., that is case-hardening steel superior in tooth surface fatigue strength containing, by wt %, C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 0.3 to 2.0%, S: 0.005 to 0.05%, Cr: 1.0 to 2.6%, Mo: 0.8 to 4.0%, V: 0.1 to 0.3%, Al: 0.001 to 0.2%, and N: 0.003 to 0.03%, limiting P to 0.03% or less, and having as a balance iron and unavoidable impurities, and satisfies the following expression, 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) is 100 or more, and a gear comprised of case-hardening steel and having an X-ray diffraction half width at a depth of 50 mm from the surface of the gear of 6.4 degrees or more.

1 - 8 . (canceled)  
   
   
       9 . Case-hardening steel superior in tooth surface fatigue strength characterized by containing, by wt %, 
 C: 0.1 to 0.3%,    Si: 1.0 to 2.0%,    Mn: 0.3 to 2.0%,    S: 0.005 to 0.05%,    Cr: 1.0 to 2.6%,    Mo: 0.8 to 4.0%,    V: 0.1 to 0.3%,    Al: 0.001 to 0.2%, and    N: 0.003 to 0.03%,    limiting P to 0.03% or less, and    having a balance of iron and unavoidable impurities, wherein satisfying the following expression (1).      31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%)≧100   (1)    
   
   
       10 . Case-hardening steel superior in tooth surface fatigue strength as set forth in  claim 9  wherein said steel further includes, by wt %, one or two of 
 Nb: 0.2% or less and    Ti: 0.2% or less    
   
   
       11 . A gear superior in tooth surface fatigue strength characterized in that it comprises steel as set forth in  claim 9  and has an X-ray diffraction half width at a depth of 50 μm from the gear surface of 6.4 degrees or more when forming the steel to a gear shape and carburizing or carbonitriding the same, the “X-ray diffraction half width” referred to here meaning the half width of the peak when using a micro-area X-ray residual stress measurement system (Cr lamp) to measure the α-Fe (211) plane over 60 seconds.  
   
   
       12 . A gear superior in tooth surface fatigue strength as set forth in  claim 11 , wherein said gear further includes, by wt %, one or two of 
 Nb: 0.2% or less and    Ti: 0.2% or less    
   
   
       13 . A gear superior in tooth surface fatigue strength as set forth in  claim 11 , characterized in that the amount of Si, Cr, Mo and V are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to 1.2%, and V: 0.10 to 0.25%, and satisfies the following expression (2) instead of the expression (1):  
       37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%)=100˜150   (2)  
   
   
       14 . A gear superior in tooth surface fatigue strength as set forth in  claim 12 , characterized in that the amount of Si, Cr, Mo and V are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to 1.2%, and V: 0.10 to 0.25%, and satisfies the following expression (2) instead of the expression (1):  
       37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%)=100˜150   (2)  
   
   
       15 . A method of production of a gear superior in tooth surface fatigue strength characterized by forming the steel as set forth in  claim 14  to a gear shape, then subjecting it to vacuum carburization or vacuum carbonitridation at a heating temperature of 900 to 1050° C. in range.