Soft-magnetic nickel-iron-chromium alloy
FIELD OF THE INVENTION 
     This invention relates to a soft-magnetic nickel-iron-chronium (Ni-Fe-Cr) alloy suitable for magnetic shielding wherein high magnetic permeability is required. 
     BACKGROUND OF THE INVENTION 
     High magnetic permeability Ni-Fe alloys are widely used as magnetic shielding materials such as materials for casings of magnetic heads, magnetic shielding plates for cassette tape digitizers, etc. For such magnetic shielding materials, high magnetic permeability in the low frequency bands is required as an alternating current magnetic characteristic. Often inductance specific magnetic permeability μ L  of not less than 9000 at 0.3 kHz or of not less than 4500 at 1 kHz is required. Also, with respect to direct current magnetism, often high characteristics such as coercive force Hc of not more than 0.10 Oe and saturated magnetic flux density Bs of not less than 3000 G are required. 
     Therefore, 80% Ni Permalloy (JIS-PC(corresponding to ASTM A753)), which contains Mo, Cr, Cu, etc. and has the highest magnetic permeability among the Ni-Fe magnetic alloys, is widely used as magnetic shielding materials. However, this alloy has a disadvantage that it is expensive, because the alloy contains no less than 80% of expensive Ni as well as the more expensive Mo. Therefore, there is a demand for an inexpensive magnetic alloy which has magnetic characteristics comparable with those of JIS-PC alloy. 
     The principal object of the present invention is to provide a novel soft-magnetic alloy which is provided with alternating current magnetic characteristics of the same level as those of JIS-PC alloy or better and yet is inexpensive. 
     We studied magnetic properties of a number of Fe-Ni . magnetic alloys and found that alloys comprising 35- 40% Ni, 5-14% Cr and balance Fe have alternating current magnetic characteristics, such as magnetic permeability, of the same level as those of JIS PC alloys or JIS PB alloys (45% Ni Permalloy) or better in spite that the Ni content is far less than the latter. 
     SUMMARY OF THE INVENTION 
     This invention provides a soft-magnetic nickel-iron-chromium (Ni-Fe-Cr) alloy having excellent alternating current magnetic characteristics, which essentially consists of: 
     35-40% Ni 
     5-14% Cr 
     and 
     balance Fe and unavoidable impurities, and satisfies the relations: 
     3(Ni%)-5(Cr%)≦80 
     and 
     (Ni%)-(Cr%)≦25 
     The alloy should preferably satisfy the following conditions. The contents of the impurity elements S, 0 and B should be 
     S≦0.003% 
     O≦0.005% 
     B≦0.005% 
     and that 
     S+O+B≦0.008% 
     Preferably, the B content should be not more than 0.002%. 
     In the alloy of the present invention Si and Al which are used for deoxidation and Mn which is used for deoxidation and desulfurization can be contained up to 1% in total. 
     In the alloy of the present invention, Cr is effective for reducing the coercive force and increases the magnetic permeability under alternating current. Such effect does not well appear with less than 5% Cr. The magnetic permeability is saturated at around 13-14% Cr. 
     Ni enhances the alternating current magnetic characteristics caused by addition of Cr when contained in an amount of around 35% or more. With less content of Ni, inductance specific magnetic permeability μ L  decreases. On the other hand, addition of a larger amount of Ni not only raises the price of the alloy but also decreases inductance specific magnetic permeability μ L . The upper limit of the Ni content will be around 40%. 
     With respect to the contents of Ni and Cr, the following condition must be satisfied 
     47≦3(Ni%)=5(Cr%)≦80 
     in order that the alloy is provided with inductance specific magnetic permeability, which is one of the alternating current magnetic characteristics, of the same level as that of the JIS-PC alloy or better. In addition, the following relation also must be satisfied 
     (Ni%)-(Cr%)≧25 
     in order that direct current saturated magnetic flux density Bs, which is a significant factor for magnetic shielding materials, is 3000 G or more, since the direct current saturated magnetic flux density decreases with increase of the Cr content. 
     The contents of impurity elements such as S, O, B, P, N, etc. should be as low as possible from the viewpoint of improvement of magnetic characteristics. Especially, S, O and B impair the coarsening of crystal grains in magnetic annealing and decreases inductance specific magnetic permeability μ L . Therefore, it is desirable that the alloy composition satisfies the following conditions 
     S≦0.003%, O≦0.005%, B≦0.005 and 
     S+O+B≦0.008% 
     in order to increase the μ L  value at low frequencies, especially of 0.3 kHz. 
     The alloy of the present invention is usually annealed in a hydrogen atmosphere. When the B content is not more than 0.002%, the alloy can be annealed in vacuo instead of an hydrogen atmosphere with same effect. 
     In the present invention, the preferred content range of Ni is 36-39% and the more preferred content range is 36-38 %. The preferred content range of the Cr content is 7-12% and the more preferred content range is 8-10%. 
     The alloy of the present invention has excellent alternating current magnetic characteristics and satisfies direct current magnetic characteristics required for magnetic shielding materials, and yet is inexpensive. This alloy is suitable as a magnetic shielding material for various magnetic shielding members including magnetic head casings. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     FIG. 1 is a diagram which shows the relation between the contents of Ni and Cr and the inductance specific magnetic permeability μ L . 
     FIG. 2 is a diagram which shows the relation between the contents of Ni and Cr and the coercive force Hc and the saturated magnetic flux density Bs. 
     FIG. 3 is a diagram which shows the claimed composition range of the present invention. 
     FIG. 4 is a diagram which shows the influence of the content of S+O+B to inductance specific magnetic permeability μ L  at 0.3 kHz. 
     FIG. 5 is a diagram which shows the influence of the content of S+O +B to inductance specific magnetic permeability μ L  at 1 kHz. 
    
    
     SPECIFIC DESCRIPTION OF THE INVENTION 
     Ingots of alloys the compositions of which are indicated in Table 1 were respectively prepared by vacuum melting, and made into 0.4 mm thick sheets by means of ordinary hot rolling and cold rolling. Annular pieces having an external diameter of 10 mm and an internal diameter of 6 mm were cut out of these sheets. They were annealed at 1100° C. for 1 hour in a hydrogen atmosphere and then cooled. Coercive force Hc, saturated magnetic flux density Bs and inductance specific magnetic permeability μ L  of the thus obtained specimens were measured in accordance with the test methods stipulated in JIS C2531. The results are shown in Table 2. 
     FIG. 1 shows inductance specific magnetic permeability (μ L ) values at 0.3 kHz and 1 kHz out of all the measurement values of all the specimens. As is apparent from FIG. 1, when the alloy contains 35-40% Ni, inductance specific magnetic permeability of the alloy increases with increase of the Cr content and has alternating current magnetic characteristics of the same level as those of JIS-PC alloy or better within the domain surround by solid lines. 
     FIG. 2 shows values of saturated magnetic flux density (Bs) values and coercive force (Hc) values out of direct current magnetic characteristics. As is apparent from FIG. 2, the alloy satisfies saturated magnetic flux density (Bs) of ≧3000 G, which is required for magnetic shielding materials in the domain below the solid line. All the samples have a coercive force Hc of ≦0.10 and this increases with increase of the Cr content. 
     Also, as shown in Table 2, the Bs value does not vary with the same contents of Ni and Cr. However, FIGS. 4 and 5 show that the μ L  value improves if the contents of S, O and B are reduced to S+O+B≦0.008%. 
     As has been described above, a magnetic alloy, which is provided with direct current magnetic characteristics required for magnetic shielding materials and has excellent alternating current magnetic characteristics of the same level as those of JIS-PC alloy, can be obtained by defining the alloy composition as indicated by FIG. 3. Further, an alloy having excellent alternating current magnetic characteristics can be obtained by reducing the content of S+O+B≦0.008% as shown in FIGS. 4 and 5. 
     
                                           TABLE 1                                 
__________________________________________________________________________
(wt %)                                                                    
No.                                                                       
   Ni Cr   C  Si Mn Al P  N   S   O   B   S + O + B                       
__________________________________________________________________________
 1 34.4                                                                   
      6.9  0.01                                                           
              0.21                                                        
                 0.43                                                     
                    0.011                                                 
                       0.014                                              
                          0.0025                                          
                              0.0022                                      
                                  0.0038                                  
                                      0.0025                              
                                          0.0085                          
 2 34.7                                                                   
      13.1 0.02                                                           
              0.29                                                        
                 0.50                                                     
                    0.007                                                 
                       0.011                                              
                          0.0031                                          
                              0.0022                                      
                                  0.0053                                  
                                      0.0005                              
                                          0.0080                          
 3 35.2                                                                   
      11.8 0.02                                                           
              0.18                                                        
                 0.52                                                     
                    0.010                                                 
                       0.013                                              
                          0.0033                                          
                              0.0032                                      
                                  0.0018                                  
                                      0.0043                              
                                          0.0093                          
 4 *1                                                                     
   35.6                                                                   
      5.7  0.02                                                           
              0.20                                                        
                 0.62                                                     
                    0.019                                                 
                       0.010                                              
                          0.0033                                          
                              0.0033                                      
                                  0.0028                                  
                                      0.0026                              
                                          0.0087                          
 5 *1                                                                     
   36.4                                                                   
      7.7  0.01                                                           
              0.20                                                        
                 0.57                                                     
                    0.015                                                 
                       0.008                                              
                          0.0021                                          
                              0.0025                                      
                                  0.0032                                  
                                      0.0038                              
                                          0.0095                          
 6 *1                                                                     
   36.2                                                                   
      9.5  0.01                                                           
              0.28                                                        
                 0.55                                                     
                    0.008                                                 
                       0.012                                              
                          0.0022                                          
                              0.0028                                      
                                  0.0051                                  
                                      0.0005                              
                                          0.0084                          
 7 36.4                                                                   
      14.0 0.02                                                           
              0.19                                                        
                 0.42                                                     
                    0.023                                                 
                       0.011                                              
                          0.0027                                          
                              0.0019                                      
                                  0.0021                                  
                                      0.0010                              
                                          0.0050                          
 8 36.6                                                                   
      5.1  0.01                                                           
              0.11                                                        
                 0.45                                                     
                    0.018                                                 
                       0.010                                              
                          0.0018                                          
                              0.0015                                      
                                  0.0038                                  
                                      0.0030                              
                                          0.0083                          
 9 *1                                                                     
   37.0                                                                   
      7.8  0.02                                                           
              0.15                                                        
                 0.61                                                     
                    0.012                                                 
                       0.007                                              
                          0.0021                                          
                              0.0018                                      
                                  0.0041                                  
                                      0.0025                              
                                          0.0084                          
10 37.8                                                                   
      5.9  0.02                                                           
              0.18                                                        
                 0.57                                                     
                    0.007                                                 
                       0.007                                              
                          0.0019                                          
                              0.0043                                      
                                  0.0028                                  
                                      0.0044                              
                                          0.0115                          
11 *1                                                                     
   38.0                                                                   
      7.8  0.01                                                           
              0.18                                                        
                 0.48                                                     
                    0.005                                                 
                       0.011                                              
                          0.0031                                          
                              0.0020                                      
                                  0.0028                                  
                                      0.0059                              
                                          0.0107                          
12 *1                                                                     
   37.7                                                                   
      10.1 0.01                                                           
              0.25                                                        
                 0.39                                                     
                    0.007                                                 
                       0.015                                              
                          0.0018                                          
                              0.0028                                      
                                  0.0030                                  
                                      0.0025                              
                                          0.0083                          
13 *1                                                                     
   38.0                                                                   
      11.7 0.01                                                           
              0.22                                                        
                 0.44                                                     
                    0.021                                                 
                       0.010                                              
                          0.0020                                          
                              0.0040                                      
                                  0.0021                                  
                                      0.0010                              
                                          0.0071                          
14 38.0                                                                   
      14.5 0.01                                                           
              0.15                                                        
                 0.46                                                     
                    0.010                                                 
                       0.006                                              
                          0.0021                                          
                              0.0022                                      
                                  0.0025                                  
                                      0.0008                              
                                          0.0055                          
15 39.0                                                                   
      5.8  0.01                                                           
              0.23                                                        
                 0.45                                                     
                    0.008                                                 
                       0.005                                              
                          0.0033                                          
                              0.0015                                      
                                  0.0033                                  
                                      0.0043                              
                                          0.0091                          
16 *1                                                                     
   39.3                                                                   
      7.9  0.02                                                           
              0.19                                                        
                 0.51                                                     
                    0.011                                                 
                       0.006                                              
                          0.0042                                          
                              0.0038                                      
                                  0.0018                                  
                                      0.0040                              
                                          0.0096                          
17 *1                                                                     
   38.9                                                                   
      9.9  0.01                                                           
              0.18                                                        
                 0.51                                                     
                    0.014                                                 
                       0.014                                              
                          0.0029                                          
                              0.0022                                      
                                  0.0032                                  
                                      0.0044                              
                                          0.0098                          
18 *1                                                                     
   38.7                                                                   
      12.5 0.02                                                           
              0.17                                                        
                 0.55                                                     
                    0.009                                                 
                       0.011                                              
                          0.0017                                          
                              0.0017                                      
                                  0.0018                                  
                                      0.0060                              
                                          0.0095                          
19 40.4                                                                   
      6.9  0.02                                                           
              0.25                                                        
                 0.58                                                     
                    0.010                                                 
                       0.008                                              
                          0.0019                                          
                              0.0034                                      
                                  0.0033                                  
                                      0.0005                              
                                          0.0072                          
20 40.9                                                                   
      9.3  0.02                                                           
              0.25                                                        
                 0.48                                                     
                    0.024                                                 
                       0.011                                              
                          0.0026                                          
                              0.0025                                      
                                  0.0022                                  
                                      0.0039                              
                                          0.0088                          
21 41.5                                                                   
      11.5 0.01                                                           
              0.22                                                        
                 0.44                                                     
                    0.008                                                 
                       0.012                                              
                          0.0048                                          
                              0.0018                                      
                                  0.0015                                  
                                      0.0070                              
                                          0.0103                          
22 *2                                                                     
   35.5                                                                   
      5.8  0.01                                                           
              0.18                                                        
                 0.57                                                     
                    0.012                                                 
                       0.009                                              
                          0.0018                                          
                              0.0025                                      
                                  0.0029                                  
                                      0.0005                              
                                          0.0059                          
23 *2                                                                     
   35.7                                                                   
      5.7  0.02                                                           
              0.19                                                        
                 0.60                                                     
                    0.008                                                 
                       0.008                                              
                          0.0024                                          
                              0.0015                                      
                                  0.0011                                  
                                      0.0008                              
                                          0.0034                          
24 35.4                                                                   
      5.8  0.01                                                           
              0.19                                                        
                 0.55                                                     
                    0.007                                                 
                       0.011                                              
                          0.0023                                          
                              0.0028                                      
                                  0.0044                                  
                                      0.0032                              
                                          0.0104                          
25 35.5                                                                   
      5.7  0.01                                                           
              0.21                                                        
                 0.60                                                     
                    0.022                                                 
                       0.008                                              
                          0.0035                                          
                              0.0037                                      
                                  0.0019                                  
                                      0.0005                              
                                          0.0061                          
26 35.6                                                                   
      5.7  0.02                                                           
              0.19                                                        
                 0.59                                                     
                    0.021                                                 
                       0.006                                              
                          0.0017                                          
                              0.0015                                      
                                  0.0021                                  
                                      0.0059                              
                                          0.0095                          
27 *2                                                                     
   37.9                                                                   
      7.8  0.01                                                           
              0.18                                                        
                 0.45                                                     
                    0.023                                                 
                       0.010                                              
                          0.0021                                          
                              0.0012                                      
                                  0.0013                                  
                                      0.0010                              
                                          0.0035                          
28 *2                                                                     
   38.0                                                                   
      7.9  0.02                                                           
              0.22                                                        
                 0.53                                                     
                    0.005                                                 
                       0.012                                              
                          0.0022                                          
                              0.0014                                      
                                  0.0025                                  
                                      0.0030                              
                                          0.0069                          
29 *2                                                                     
   38.0                                                                   
      7.8  0.01                                                           
              0.21                                                        
                 0.55                                                     
                    0.006                                                 
                       0.006                                              
                          0.0031                                          
                              0.0026                                      
                                  0.0038                                  
                                      0.0010                              
                                          0.0074                          
30 37.8                                                                   
      7.9  0.01                                                           
              0.19                                                        
                 0.55                                                     
                    0.005                                                 
                       0.011                                              
                          0.0028                                          
                              0.0038                                      
                                  0.0019                                  
                                      0.0045                              
                                          0.0102                          
31 38.0                                                                   
      7.9  0.01                                                           
              0.20                                                        
                 0.54                                                     
                    0.011                                                 
                       0.007                                              
                          0.0017                                          
                              0.0015                                      
                                  0.0059                                  
                                      0.0005                              
                                          0.0079                          
32 37.9                                                                   
      7.8  0.01                                                           
              0.20                                                        
                 0.63                                                     
                    0.024                                                 
                       0.009                                              
                          0.0020                                          
                              0.0041                                      
                                  0.0056                                  
                                      0.0040                              
                                          0.0137                          
PB 46.2                                                                   
      0.1  0.01                                                           
              0.21                                                        
                 0.45                                                     
                    0.008                                                 
                       0.012                                              
                          0.0030                                          
                              0.0024                                      
                                  0.0030                                  
                                      0.0011                              
                                          0.0065                          
PC 79.0                                                                   
      (Mo; 4.1)                                                           
           0.01                                                           
              0.25                                                        
                 0.51                                                     
                    0.010                                                 
                       0.005                                              
                          0.0010                                          
                              0.0020                                      
                                  0.0025                                  
                                      0.0040                              
                                          0.0085                          
__________________________________________________________________________
 *1 alloy of Claim 1                                                      
 *2 alloy of Claim 2                                                      
 PB, PC = JIS alloys                                                      
 
    
     
                       TABLE 2                                                     
______________________________________                                    
                   μL                                                  
No.     Hc (Oe)  Bs (G)      0.3 kHz                                      
                                    1 kHz                                 
______________________________________                                    
 1      0.07     5300        6200   3700                                  
 2      0.03      900        7200   3900                                  
 3      0.02     2700        8500   4200                                  
 4 *1   0.07     7700        9400   5400                                  
 5 *1   0.03     6700        12500  5300                                  
 6 *1   0.02     4700        13700  5400                                  
 7      0.02     1400        8500   4100                                  
 8      0.08     8500        6800   3900                                  
 9 *1   0.03     6800        11500  4900                                  
10      0.05     8800        8500   4300                                  
11 *1   0.03     7200        10300  4500                                  
12 *1   0.02     5500        14400  5800                                  
13 *1   0.01     3700        14900  6000                                  
14      0.02     1700        7800   3900                                  
15      0.04     9400        8000   4000                                  
16 *1   0.03     8200        9700   4500                                  
17 *1   0.02     5800        11800  5500                                  
18 *1   0.01     3600        13500  6100                                  
19      0.06     9500        7800   4000                                  
20      0.04     7900        8500   4200                                  
21      0.03     6100        8800   4100                                  
22 *2   0.05     7600        11700  6100                                  
23 *2   0.05     7700        11900  6000                                  
24      0.07     7700        9600   5500                                  
25      0.07     7600        9300   5500                                  
26      0.07     7700        9400   5300                                  
27 *2   0.01     7200        13800  5400                                  
28 *2   0.01     7200        14200  5200                                  
29 *2   0.02     7300        14100  5200                                  
30      0.03     7200        10100  4600                                  
31      0.03     7300        10300  4500                                  
PB      0.15     15000       3800   2100                                  
PC      0.02     8200        9300   4500                                  
______________________________________