Heat resisting bearing steel
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a bearing steel, more particularly, to a steel suitable for the material of bearings to be used at a high temperature. 
     2. State of the Art 
     Bearings supporting a main rotating shaft of a gas turbine are used under very severe conditions of a high temperature in addition to a very high peripheral speed. 
     Needless to say, a heat engine exhibits a higher efficiency when it is operated at a higher temperature, and therefore, it is desired to raise the operation temperature of a turbine up to the allowable limit. The bearings are also requested to have a heat resistance durable at a temperature of 300° C. or more. Further, as the results of higher speed and larger size, the peripheral velocity of the bearings becomes inevitably higher. Bearings are required to have high reliability to secure safety operation of rotational machinery for a long period of time. 
     The representative heat resistant bearing steel presently being used is AISI-M50 steel, which consists of 0.77-0.85% C, up to 0.35% Si, up to 0.35% Mn, 3.75-4.25% Cr, 4.00-4.50% Mo and 0.90-1.10% V, and the balance being Fe and inevitable impurities. 
     It is demanded to give higher reliability under severe working conditions to this material or other steels of similar alloy compositions. 
     SUMMARY OF THE INVENTION 
     It is the object of this invention to satisfy the above demand and provide a heat resisting bearing steel which secures high reliability. 
     The heat resisting bearing steel of the present invention to achieve this object consists essentially of 0.6-0.9% C, up to 0.5% Si, up to 0.5% Mn, 3.0-5.5% Cr, 3.0-5.0% Mo and 0.5-1.5% V, and the balance being Fe and inevitable impurities; and the content of large Al 2  O 3  inclusions therein with 10 micron or larger length being restricted to 1.5 ppm or less. 
     The typical composition of this alloy is, in the above ranges, in accordance with the above noted composition of AISI-M50 steel, which consists of 0.77-0.85% C, up to 0.35% Si, up to 0.35% Mn, 3.75-4.25% Cr, 4.00-4.50% Mo and 0.90-1.10% V, and the balance being Fe and inevitable impurities. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     All the figures illustrate the data of working examples of the present invention; wherein, 
     FIG. 1 is a graph made by plotting the relation between the O contents in the bearing steel and the rolling fatigue life L 10 , and 
     FIG. 2 is a graph made by plotting the relation between the contents of large size Al 2  O 3  in the bearing steel and the rolling fatigue life L 10 . 
    
    
     DETAILED EXPLANATION OF PREFERRED EMBODIMENTS 
     To reduce the content of large Al 2  O 3  inclusion with 10 micron or larger length to 1.5 ppr: or less, it is necessary first to reduce the quantities of Al and O which produce Al 2  O 3 , and it is recommended to adopt a suitable steel refining method such as ladle refining (LF), vacuum arc furnace remelting (VAR), or VAR preceded by vacuum induction furnace melting (VI), or vacuum degassing (RH). 
     In addition to the above, it is desirable to allow sufficient standing time of the refining process to allow floating separations of the inclusions. Of course, the selection of refractory to be used for refining is important. A smaller ingot size is advantageous to avoid aggregation of inclusion of huge particles. 
     The alloy composition of the present heat resisting bearing steel is limited to the percentages as stated above for the following reasons: 
     C : 0.6-0.9%, preferably 0.77-0.85% 
     Content of 0.6% or more of C is necessary to obtain a necessary hardness for a bearing. However, increased amounts of the C-content heightens the retained austenite resulting in lowering of hardness, and therefore, C-content should not exceed 0.9%. The suitable range of C-content is relatively narrow as stated above. 
     Si: up to 0.5%, preferably up to 0.35% 
     Silicon is necessary as a deoxidizer, however, too much Si deteriorates the clearness, and reduces toughness, and is restricted to the above range. 
     Cr: 3.0-5.5%, preferably 3.75-4.25% 
     Mo: 3.0-5.5%, preferably 4.00-4.50% 
     V: 0.5-1.5%, preferably 0.90-1.10% 
     These elements form carbides to impart wear resistance to the composition, therefore, they should be added in the amounts of at least their respective lower limits. Cr and Mo function to improve the hardenability. Additional benefits are not achieved using amounts larger than the upper values set forth above. Therefore, it is advisable to limit the amounts to the upper limits in consideration of economy. 
     Large Al 2  O 3  inclusions with 10 micron or larger length: 1.5 ppm or less. 
     The inventor pursued the factors which cause failures such as flaking in heat resisting bearings from the material side, and first investigated the relation between O-content and rolling fatigue life considering the bad effects of all the oxide inclusions. However, no interrelations therebetween where found is seen in the examples described later. Moreover, it was clarified that there is no interrelation between the total quantity of Al 2  O 3  inclusions, which consist the major part of the oxide inclusions, and the rolling fatigue life. Finally, investigation was made paying attention to the size of the Al 2  O 3  inclusions, and found that the quantity of large size Al 2  O 3  inclusions with the length of 10 micron or larger is a key factor, and it was verified that 1.5 ppm is the critical value for the allowable quantity. 
     The heat resisting bearing steel of this invention remarkably extends the rolling fatigue life of such steel in comparison to conventional materials by restricting large size Al 2  O 3  inclusions. The steels of the present invention have a higher reliability because of the reduced fluctuation of the life. 
     Therefore, the material is most suitable for manufacturing bearings to be used at a high peripheral speed and at a high temperature such as main bearings of heat engines including gas turbines. 
     EXAMPLES 
     Steel of the composition as shown in the Table were all manufactured by the process of arc furnace melting, ladle refining, and vacuum degassing. In the production, the steels of the examples were melted with attention to obtaining floating separation of inclusions in ladle refining, and the steels of the controls were melted without particular attention. 
     Al 2  O 3  inclusions of each test sample were quantified in accordance with its size to the four groups: 10 micron or more, 10 to 5 micron, 5 to 2 micron, and 2 to 0.2 micron. The inclusions were analysed in accordance with the hot sulfuric acid method specified in JIS. Total quantity of Al 2  O 3  and the quantity of the large size inclusions of 10 micron or more are indicated in the Table. The thrust rolling fatigue life of the steels were tested under the following conditions: 
     Bearing pressure: 500 kgf/mm 2   
     Rotational speed: 3600 rpm 
     Lubrication Oil: #180 Turbine Oil 
     L 10  (10% failure probability) and L 50  were measured and entered in the Table. These values were plotted on a Weibull probability chart, and the coefficients &#34;e&#34; obtained from the chart were also entered in the Table. 
     FIG. 1 was obtained by plotting the relation between O-contents and L 10  in the Table. This graph indicates no interrelation between O-content and L 10 . 
     On the other hand, FIG. 2 was obtained by plotting the relation between quantities of Al 2  O 3  of the size of 10 micron or more and L 10 , showing distinct interrelations. It was clarified that the bearing steel of this invention which satisfies the condition of 1.5 ppm or less contents of Al 2  O 3  has a distinctively superior rolling fatigue life in comparison with the control steels. 
     The values of Weibull coefficient &#34;e&#34; in the Table are 0.97 to 1.17 for the control steels. On the other hand, a high level of 2.20 to 3.12 was attained by the example steels. 
     
                                           TABLE                                   
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Composition of Alloys    Al.sub.2 O.sub.3 (ppm)                           
                                    Rolling Fatigue Life                  
(weight %, balance Fe)   Inclusions × 10.sup.6                      
                                        cycle                             
                                           Weibull                        
No.                                                                       
   C  Si Mn Cr Mo V  O   Total ≧10 um                              
                                    .sup.L 10                             
                                        L50                               
                                           Coef. &#34;e&#34;                      
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Example steels                                                            
1  0.84                                                                   
      0.20                                                                
         0.22                                                             
            4.01                                                          
               4.15                                                       
                  0.95                                                    
                     0.0009                                               
                         10.5  0.6  49.7                                  
                                        90.9                              
                                           3.12                           
2  0.83                                                                   
      0.22                                                                
         0.23                                                             
            4.07                                                          
               4.20                                                       
                  0.97                                                    
                     0.0006                                               
                         11.8  0.9  19.3                                  
                                        49.0                              
                                           2.02                           
3  0.82                                                                   
      0.22                                                                
         0.24                                                             
            4.04                                                          
               4.17                                                       
                  0.95                                                    
                     0.0006                                               
                         12.4  1.0  24.2                                  
                                        57.6                              
                                           2.19                           
4  0.84                                                                   
      0.21                                                                
         0.29                                                             
            4.06                                                          
               4.12                                                       
                  0.96                                                    
                     0.0009                                               
                         12.2  1.2  36.8                                  
                                        62.3                              
                                           2.36                           
Control steels                                                            
1  0.84                                                                   
      0.22                                                                
         0.28                                                             
            4.17                                                          
               4.35                                                       
                  1.01                                                    
                     0.0008                                               
                         13.4  2.3  4.4 28.9                              
                                           1.00                           
2  0.84                                                                   
      0.12                                                                
         0.27                                                             
            4.16                                                          
               4.50                                                       
                  1.04                                                    
                     0.0005                                               
                         10.8  2.9  4.7 32.7                              
                                           0.97                           
3  0.82                                                                   
      0.20                                                                
         0.26                                                             
            4.25                                                          
               4.19                                                       
                  0.97                                                    
                     0.0006                                               
                          9.8  2.4  12.7                                  
                                        65.4                              
                                           1.12                           
4  0.82                                                                   
      0.17                                                                
         0.27                                                             
            4.17                                                          
               4.18                                                       
                  0.96                                                    
                     0.0006                                               
                         11.1  3.0  4.6 22.8                              
                                           1.17                           
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