Steel having finely dispersed inclusions

Abstract

An object of the invention is to provide a steel having good fatigue life characteristics and acoustic characteristics by eliminating the harmful effects of oxide based inclusions. The object is attained by the following steel. A steel having at least a part comprising: REM of which amount meets formula (1); and REM-containing inclusions of which number meets formula (2); wherein the concentration of Al20 3 in the REM-containing inclusions is 30 mass % or less (including 0%); Formula (1); −30<REM(ppm)−(T.O(ppm)×280/48)<50, Formula (2); number of REM-containing inclusions/total number of inclusions >0.8, wherein the inclusions of Formula (2) having an equivalent diameter of 1 μm or more.

1 . A steel having at least a part comprising: 
 rare earth metal (REM) in an amount meeting formula (1); and REM-containing inclusions in a number meeting formula (2); wherein a concentration of Al 2 O 3  in the REM-containing inclusions is 0-30 mass %;      −30<REM(ppm)−(T.O.(ppm)×280/48)<50  formula (1)  number of REM-containing inclusions/total number of inclusions>0.8  formula (2)    wherein the inclusions in formula (2) have an equivalent diameter of 1 μm or more.    
   
   
       2 . The steel according to  claim 1 , wherein REM is at least one selected from the group consisting of Ce, La, Nd and Pr.  
   
   
       3 . The steel according to  claim 2 , wherein REM comprises at least Ce, La, Nd and Pr.  
   
   
       4 . The steel according to  claim 1  or  2 , wherein the steel meets at least one of following four conditions: 
 a) content of Al 0-0.05 mass %;    b) content of total oxygen (T.O.)=0.005 mass %;    c) content of S=0.003 mass %; and    d) content of Ti=0.001 mass %.    
   
   
       5 . The steel according to  claim 4 , wherein the steel has a content of Al 0-0.05 mass %.  
   
   
       6 . The steel according to  claim 4 , wherein the steel has a content of total oxygen (T.O.)=0.005 mass %.  
   
   
       7 . The steel according to  claim 4 , wherein the steel has a content of S=0.003 mass %.  
   
   
       8 . The steel according to  claim 4 , wherein the steel has a content of Ti=0.001 mass %.  
   
   
       9 . The steel according to  claim 1  or  2 , wherein the steel has a rolling fatigue life (L 10 ) which is greater than 3.2 times the L 10  of essentially the same steel which has been prepared without added REM.  
   
   
       10 . The steel according to  claim 9 , wherein the steel has a rolling fatigue life (L 10 ) which is at least 7.6 times greater than the L 10  of essentially the same steel which has been prepared without added REM.  
   
   
       11 . The steel according to  claim 9 , wherein the steel has a rolling fatigue life (L 10 ) which is greater than 3.2 up to and including 9.2 times the L 10  of essentially the same steel which has been prepared without added REM.  
   
   
       12 . The steel according to  claim 1 , wherein the steel has no added calcium or magnesium.  
   
   
       13 . The steel according to  claim 4 , wherein the steel is a bearing steel.  
   
   
       14 . The steel according to  claim 13 , wherein the bearing steel is shaped for use in a hard disk drive or audiovisual equipment.  
   
   
       15 . A method for manufacturing a steel: 
 said method comprising the steps of: 
 preparing a molten steel,  
 adding rare earth metal (REM) and oxygen to the molten steel,  
 casting the molten steel into said steel;  
 wherein the steel has at least a part comprising  
 REM in an amount meeting formula (1); and REM-containing inclusions in a number meeting formula (2); wherein a concentration of Al 2 O 3  in the REM-containing inclusions is 0-30 mass %;  
   −30<REM(ppm)−(T.O.(ppm)×280/48)<50  formula (1)  number of REM-containing inclusions/total number of inclusions>0.8  formula (2)  
 wherein the inclusions in formula (2) have an equivalent diameter of 1 μm or more.  
   
   
   
       16 . The method according to  claim 15 , wherein the REM is added during RH vacuum degassing treatment of the molten steel.  
   
   
       17 . The method according to  claim 16 , wherein the aluminum is added to the molten steel prior to addition of REM.  
   
   
       18 . The method according to  claim 16 , wherein air is essentially completely sealed after RH treatment.