Abstract:
A Ni-Fe-Cr soft magnetic alloy essentially consisting of 40-50% Ni, 0.5-5% Cr and balance Fe and satisfying the following conditions: 
     50≦(Ni%)+4×(Cr%)≦60; 
     S+O+B≦0.008%; 
     S≦0.003%; 
     O≦0.005%; and 
     B≦0.005%; 
     has excellent magnetic characteristics for magnetic core materials.

Description:
FIELD OF THE INVENTION 
     This invention relates to a soft-magnetic Ni-Fe-Cr (nickel-iron-chromium) alloy suitable for magnetic core materials wherein high magnetic permeability and high saturated magnetic flux density are required. 
     BACKGROUND OF THE INVENTION 
     Ni-Fe alloys having high magnetic permeability are widely used as materials for magnetic cores such as cores of transformers for communication instruments, small motors, clocks, watches and the like. For such core materials, excellent magnetic permeability and high saturated magnetic flux density are required as direct current magnetic characteristics. For example, materials for clock cores and yokes should have a magnetic permeability (μ m ) of not less than 35,000 and a saturated magnetic flux density (B 10 ) of not less than 11,000 G. 
     Conventionally, 45%-Ni Permalloy (trademark), which has the most excellent magnetic permeability and saturated magnetic flux density among the Ni-Fe magnetic alloys, is used as magnetic core materials to satisfy the above-mentioned requirements. Recently, however, it is desirable to make compact magnetic cores for various devices and the requirements for high performance magnetic cores are getting more and more severe. Under the circumstances, magnetic materials having improved magnetic permeability and saturated magnetic flux density are needed. 
     Japanese Laid-Open Patent Publication No. 142749/87 describes an attempt to improve the magnetic properties of magnetic materials by reducing O and S contents. Japanese Laid-Open Patent Publication No. 227065/87 describes another attempt wherein Mo is added and P and S contents are limited. 
     Alloys of 80%-Ni Permalloy series (JIS-PC corresponding to ASTM A753) exhibit the highest magnetic permeability and have the maximum magnetic permeability (μ m ) of not less than 100,000, which is much higher than that achieved by alloys of JIS-PB series. However, the saturated magnetic flux density B 10  of the former is not satisfactory being at the level about 7000 G. Furthermore, JIS-PC alloys are expensive because they contain no less than about 80% of expensive Ni and the application thereof is limited due to this economical factor. 
     Accordingly, the object of the present invention is to provide an inexpensive soft magnetic alloy containing a reduced amount of Ni which is provided with the maximum magnetic flux density B 10  of not less than 11,000 and the maximum magnetic permeability (μ m ) comparable to that of JIS-PC. 
     The inventors conducted extensive studies in search for a Ni-Fe soft magnetic alloy so as to achieve the above-mentioned object and found that a Ni-Fe-Cr soft magnetic alloy comprising 40-52% of Ni, 0.5-5% of Cr, not more than 0.003% of S, not more than 0.005% of O, not more than 0.005% of B and balance iron has a high saturated magnetic flux density B 10  and a high maximum magnetic permeability (μ m ) of not less than 100,000. 
     SUMMARY OF THE INVENTION 
     The present invention provides a Ni-Fe-Cr soft magnetic alloy having excellent magnetic characteristics for magnetic core materials which essentially consists of: 
     Ni: 40-52% 
     Cr: 0.5-5% 
     S≦0.003% 
     O≦0.005% 
     B≦0.005% 
     and balance Fe 
     and satisfies the following conditions: 
     50≦(Ni%)+4×(Cr%)≦60; and 
     S+O+B≦0.008%. 
     In the alloy of the present invention Si, Al (useful for deoxidizing agents) and Mn (useful for deoxidizing and desulfuring agents) may be contained up to 2% in total. 
     Cr: Cr is an element effective for improving the maximum magnetic permeability (μ m ). This effect does not appear well with less than 0.5% Cr, while the saturated magnetic flux density B 10  decreases when the Cr content is excessive. Accordingly, the Cr content in the alloy of the present invention is limited to the range of 0.5-5%, preferably 1-4%, more preferably 1.5-3%. 
     Ni: Ni is an element effective for improving the saturated magnetic flux density B 10 . It is observed that the saturated magnetic flux density B 10  tends to decrease when the Cr content is less than 40%. The effect of the addition of Ni in an amount of 0.5-5% improving magnetic properties is remarkable when the Ni content exceeds 40%. However, both the saturated magnetic flux density B 10  and the maximum magnetic permeability (μ m ) show a tendency to decrease as the Ni content increases over 52%. Accordingly, the Ni content in the alloy of the present invention is limited to the range of 40-52%, preferably 42-51%, more preferably 44-50%. 
     Furthermore, addition of a large amount of Ni in the alloy raises the price of the alloy and is not advantageous. Accordingly, the Ni content in the alloy of the present invention is limited in the range of 40-52%. 
     The contents of Ni and Cr should satisfy the condition represented by the formula: 
     50≦(Ni%)+4×(Cr%)≦60 
     so that the maximum magnetic permeability (μ m ) may be comparable to or greater than that of JIS-PC alloys. 
     It is desirable to reduce the contents of impurity elements S, O and B as much as possible in order to improve magnetic properties. These impurity elements decrease the maximum magnetic permeability (μ m ) hindering the growth of crystal grains and impairing the orientation of thereof. Therefore, the alloy composition should satisfy the following conditions: S≦0.003%, O≦0.005%, B≦0.005% and S+O+B≦0.008%, preferably. S≦0.003%, O≦0.003%, B≦0.003%. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     FIG. 1 is a diagram which shows the relation between the contents of Ni, Cr, S, O and B and the maximum magnetic permeability (μ m ). 
     FIG. 2 is a diagram which shows the area of the composition defined in the claims. 
    
    
     DESCRIPTION OF THE INVENTION 
     Features and effects of the present invention will be more clearly illustrated by way of the following examples. 
     Ingots of alloys of the compositions indicated in Table I were prepared by vacuum melting. Each of the ingots were hot-rolled and cold-rolled in an ordinary manner to form a 0.5 mm thick sheet. 
     Test pieces in the annular form having an diameter of 45 mm and an inner diameter of 33 mm were cut out from the cold-rolled sheets, subjected to magnetic annealing at 1100° C. for an hour in the hydrogen atmosphere and then cooled. The maximum magnetic permeability (μ m ) and saturation magnetic flux density of the each test piece were measured following the test methods stipulated in JIS C2531. The results are also shown in Table I. 
     The relation of the maximum permeability (μ m ) to the Ni content was studied for all the test pieces. The results are shown in FIG. 1. As can be seen from FIG. 1, the maximum permeability (μ m ) is improved by Cr when the Ni content is in the range of 40-52%. It was also confirmed that the alloys having compositions within the area surrounded by broken line in FIG. 1 has the maximum permeability (μ m ) comparable to or better than that of JIS-PC alloys when the contents of S, O and B are limited so that they satisfy the condition: S+O+B≦0.008%. 
     No significant effect of the reduction of impurities S, O and B on the magnetic flux density (B10) was observed. The magnetic flux density (B10) is not less than 11,000 G when the Ni content is 40-52% and the Cr content is not more than 5%. 
     FIG. 2 shows the area wherein the Ni and Cr contents satisfy the conditions of the present invention. An improved Ni-Fe-Cr alloy having a saturated magnetic flux density B 10  not less than 11,000 G and a maximum magnetic permeability (μ m ) not less than 100,000 which are required for core materials can be obtained when the Ni and Cr contents are selected in the hatched area in FIG. 2 and the impurities are reduced so that S+O+B may be not more than 0.008%. 
     As described above, Ni-Fe-Cr soft magnetic alloys having magnetic properties required for magnetic cores were provided according to the present invention by defining the Cr and Ni contents in a specific relation and limiting impurities including S, O and B. Furthermore, the Ni-Fe-Cr soft magnetic alloys of the present invention do not contain such expensive metals as Ni and Mo in a large amount and accordingly can be prepared in a low cost. 
     
                                           TABLE I__________________________________________________________________________Composition and Magnetic Properties of Test PiecesComposition (wt %)                                        (Ni % +Specimen                                 S + B +                                         Magnetic Properties                                                     4 Cr %)No.  Ni Cr     C  Si Mn Al P  N   S   O   B   O    SFD.sup.(1)                                               MMP.sup.(2)                                                     Value__________________________________________________________________________ 1   39.1   2.5     0.01        0.20           0.51              0.010                 0.005                    0.0022                        0.0014                            0.0021                                0.0022                                    0.0057                                         12.400 G                                               78.000                                                     49.1 2*  41.0   4.1     0.02        0.19           0.48              0.007                 0.006                    0.0020                        0.0013                            0.0022                                0.0009                                    0.0044                                         12.000 G                                               115.000                                                     57.4 3   41.1   4.0     0.01        0.18           0.62              0.019                 0.010                    0.0024                        0.0045                            0.0020                                0.0005                                    0.0070                                         11.900 G                                               75.000                                                     57.1 4   40.9   4.9     0.01        0.21           0.55              0.011                 0.009                    0.0019                        0.0023                            0.0037                                0.0010                                    0.0070                                         11.000 G                                               88.000                                                     60.5 5   42.2   tr.     0.01        0.22           0.54              0.006                 0.005                    0.0020                        0.0008                            0.0024                                0.0033                                    0.0065                                         14.300 G                                               40.000                                                     42.2 6   42.1   1.0     0.01        0.15           0.44              0.005                 0.004                    0.0016                        0.0020                            0.0021                                0.0025                                    0.0066                                         13.500 G                                               53.000                                                     46.1 7   42.0   1.0     0.02        0.18           0.49              0.014                 0.005                    0.0021                        0.0025                            0.0028                                0.0045                                    0.0098                                         13.600 G                                               45.000                                                     46.0 8*  41.9   2.4     0.01        0.19           0.50              0.011                 0.006                    0.0017                        0.0017                            0.0022                                0.0010                                    0.0049                                         12.300 G                                               120.000                                                     51.5 9   42.0   2.5     0.02        0.20           0.51              0.007                 0.011                    0.0022                        0.0013                            0.0055                                0.0007                                    0.0075                                         12.200 G                                               80.000                                                     52.010   44.0   1.1     0.02        0.20           0.52              0.006                 0.005                    0.0023                        0.0020                            0.0017                                0.0020                                    0.0057                                         13.700 G                                               85.000                                                     48.4 11* 43.9   3.8     0.01        0.16           0.44              0.008                 0.010                    0.0020                        0.0011                            0.0018                                0.0015                                    0.0044                                         12.000 G                                               105.000                                                     59.112   46.0   tr.     0.01        0.21           0.47              0.014                 0.006                    0.0022                        0.0015                            0.0020                                0.0020                                    0.0055                                         15.000 G                                               49.000                                                     46.013   46.0   0.1     0.01        0.25           0.47              0.015                 0.005                    0.0019                        0.0009                            0.0043                                0.0058                                    0.0110                                         15.000 G                                               38.000                                                     46.4 14* 46.1   1.0     0.01        0.20           0.52              0.021                 0.007                    0.0018                        0.0008                            0.0021                                0.0007                                    0.0036                                         14.200 G                                               110.000                                                     50.1 15* 46.0   2.5     0.02        0.21           0.61              0.010                 0.009                    0.0021                        0.0024                            0.0027                                0.0011                                    0.0062                                         13.000 G                                               120.000                                                     56.016   46.2   2.6     0.02        0.19           0.58              0.009                 0.008                    0.0019                        0.0010                            0.0014                                0.0054                                    0.0078                                         12.900 G                                               77.000                                                     56.617   46.2   4.0     0.02        0.18           0.49              0.018                 0.007                    0.0020                        0.0015                            0.0015                                0.0046                                    0.0076                                         11.600 G                                               85.000                                                     62.218   46.0   3.9     0.01        0.15           0.49              0.023                 0.006                    0.0016                        0.0022                            0.0038                                0.0030                                    0.0090                                         11.500 G                                               63.000                                                     61.619   48.0   0.3     0.02        0.21           0.44              0.022                 0.005                    0.0017                        0.0020                            0.0021                                0.0008                                    0.0049                                         14.800 G                                               53.000                                                     49.2 20* 47.9   1.1     0.01        0.21           0.47              0.005                 0.005                    0.0021                        0.0014                            0.0011                                0.0006                                    0.0031                                         14.300 G                                               115.000                                                     52.321   47.9   1.0     0.01        0.22           0.46              0.013                 0.004                    0.0021                        0.0037                            0.0018                                0.0009                                    0.0064                                         14.200 G                                               70.000                                                     51.9 22* 48.0   2.5     0.02        0.21           0.51              0.014                 0.011                    0.0015                        0.0012                            0.0019                                0.0020                                    0.0051                                         13.000 G                                               120.000                                                     58.0 23* 50.1   0.6     0.01        0.19           0.53              0.005                 0.009                    0.0017                        0.0016                            0.0011                                0.0030                                    0.0057                                         14.300 G                                               110.000                                                     51.7 24* 50.2   2.4     0.01        0.20           0.52              0.023                 0.008                    0.0021                        0.0015                            0.0020                                0.0005                                    0.0040                                         13.100 G                                               125.000                                                     59.825   50.0   2.5     0.01        0.20           0.54              0.018                 0.006                    0.0018                        0.0014                            0.0060                                0.0005                                    0.0079                                         13.200 G                                               71.000                                                     60.026   50.1   4.0     0.01        0.18           0.50              0.010                 0.010                    0.0024                        0.0018                            0.0019                                0.0020                                    0.0047                                         11.900 G                                               55.000                                                     66.1 27* 51.9   1.1     0.02        0.19           0.49              0.006                 0.005                    0.0023                        0.0022                            0.0021                                0.0009                                    0.0052                                         14.000 G                                               120.000                                                     56.328   52.0   2.4     0.01        0.20           0.50              0.007                 0.006                    0.0020                        0.0024                            0.0011                                0.0032                                    0.0067                                         13.100 G                                               65.000                                                     61.6__________________________________________________________________________ .sup.(1) SFD: Saturated Flux Density .sup.(2) MMP: Maximum Magnetic Permeability *Alloys of the present invention (Specimen No. 12 is a 46%Ni Permalloy)