Niobium-iron rectangular hysteresis magnetic alloy
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of our copending U.S. patent application Ser. No. 536,635 filed Dec. 26, 1974 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a rectangular hysteresis magnetic alloy consisting of iron and niobium and more particularly to a rectangular hysteresis magnetic alloy consisting of iron and niobium as main ingredients and at least one element selected from the group consisting of vanadium, tantalum, chromium, molybdenum, tungsten, nickel, copper, cobalt, titanium, zirconium, silicon, aluminum, germanium, tin, antimony, beryllium, manganese, cerium and carbon as subingredients. 
     At present, magnetic alloys exhibiting a rectangular hysteresis loop and having high residual induction and coercive force of more than 2 oersteds are usually used as a magnetic material for memory elements, ferreed switches, latching relays and the like in electromagnetic devices. The manufacture of these articles may require a high temperature working operation such as glass sealing and the like. Therefore, it is desired that these alloys have a good workability and stable magnetic properties even at an elevated temperature (about 800° C). 
     As the magnetic material satisfying such requirements, there have been used iron-carbon series alloy, iron-manganese series alloy, iron-cobalt series alloy, iron-nickel series alloy and the like. In the Fe--C and Fe--Mn series alloys, however, the magnetic properties are considerably degraded by heating at an elevated temperature although they are cheap and have a good workability. On the other hand, the Fe--Co and Fe--Ni series alloys contain large amounts of expensive cobalt and nickel, respectively, and require a high working operation, so that they are economically unsatisfactory. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide magnetic alloys having excellent rectangular hysteresis loop, high residual induction, high forgeability and high workability. 
     The inventors have made various investigations on magnetic alloys having a rectangular hysteresis loop and found that alloys comprising iron and niobium, as will be mentioned hereinafter, exhibit an excellent rectangular hysteresis loop and have high residual induction, high forgeability and stable magnetic properties even at an elevated temperature. 
     Namely, the present invention provides magnetic alloys having an excellent rectangular hysteresis loop and a coercive force of more than 2 oersteds. These alloys are preferably useful as magnetic materials in the form of a thin wire or sheet for the manufacture of the above described electromagnetic devices requiring a rectangular hysteresis loop. 
     According to one embodiment of the present invention, the magnetic alloy consists of 0.5-10% by weight of niobium and the balance of iron and contains a small amount of impurities. The preferably alloy consists of 2-8% by weight of niobium and the balance of iron. 
     According to another embodiment of the present invention, the magnetic alloy consists of 0.5-10% by weight of niobium, 0.01-60% by weight in total amount of at least one element selected from the group consisting of 0-10% of vanadium, 0-25% of tantalum, 0-25% of chromium, 0-20% of molybdenum, 0-10% of tungsten, 0-30% of nickel, 0-20% of copper, 0-45% of cobalt, 0-5% of titanium, 0-5% of zirconium, 0-5% of silicon, 0-5% of aluminum, 0-5% of germanium, 0-5% of tin, 0-5% of antimony, 0-3% of beryllium, 0-15% of manganese, 0-2% of cerium and 0-1.5% of carbon and the balance of iron and contains a small amount of impurities. The preferable alloy of this embodiment consists of 2-8% by weight of niobium, 0.01-60% by weight in total amount of at least one element selected from the group consisting of 0-7% of vanadium, 0-15% of tantalum, 0-15% of chromium, 0-10% of molybdenum, 0-7% of tungsten, 0-20% of nickel, 0-7% of copper, 0-35% of cobalt, 0-3% of titanium, 0-3% of zirconium, 0-3% of silicon, 0-3% of aluminum, 0-3% of germanium, 0-3% of tin, 0-3% of antimony, 0-2% of beryllium, 0-7% of manganese, 0-1.5% of cerium and 0-1% of carbon and the balance of iron. 
     In order to make the magnetic alloy of the present invention, suitable amounts of starting materials comprising 0.5-10 wt. % of Nb and the balance of Fe are first melted in a suitable melting furnace in air, preferably in a non-oxidizing atmosphere or in a vacuum. Alternately, a given amount of 0.01-60 wt. % in total amount of at least one element selected from the group consisting of 0-10% of V, 0-25% of Ta, 0-25% of Cr, 0-20% of Mo, 0-10% of W, 0-30% of Ni, 0-20% of Cu, 0-45% of Co, 0-5% of Ti, 0-5% of Zr, 0-5% of Si, 0-5% of Al, 0-5% of Ge, 0-5% of Sn, 0-5% of Sb, 0-3% of Be, 0-15% of Mn, 0-2% of Ce and 0-1.5% of C may be added as subingredients together with iron and niobium. Then, the resulting molten mass is added with a small amount (less than 1%) of a deoxidizer and desulfurizer such as manganese, silicon, aluminum, titanium, calcium alloy, magnesium alloy and the like to remove impurities therefrom as far as possible, and thoroughly stirred to obtain a molten alloy having a homogeneous composition. 
     Next, the thus obtained molten alloy is poured into a mold having an adequate shape and size to form a sound ingot. This ingot is made into a suitable form, for example a rod or a plate by forging or hot working at an elevated temperature and if necessary, annealed at a temperature above 400° C. Then, the rod or plate is subjected to cold working in a working ratio of more than 50% by swaging, drawing, rolling or the like to form an article of a desired form, for example a wire having a diameter of 0.5-1 mm or sheet having a thickness of 0.1-0.2 mm. The thus cold worked article is heated at a temperature about 400° C in air, preferably in a non-oxidizing atmosphere or in a vacuum for more than 1 minute to obtain a cold worked, heat treated magnetic alloy having an excellent rectangular hysteresis loop and a coercive force of more than 2 oersteds. 
     The above mentioned cold working acts to make the preferred orientation of alloy crystal even, and particularly the effect by the cold working is remarkable at a working ratio of more than 50%. Furthermore, the heating which follows the cold working serves to improve the rectangular hysteresis loop through removal of working strain, recrystallization, transformation precipitation and the like, and particularly the effect by the heating is remarkable at a temperature above 400° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing magnetic properties of iron-niobium alloys which contain different amounts of niobium, which alloys were subjected to cold working at a working ratio of 98% and then heated at 650° C for 2 hours; and 
     FIG. 2 is a graph showing magnetic properties of iron-niobium alloy containing 4% Nb when it is subjected to cold working at a working ratio of 98% and then heated at various temperatures. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following examples are given in illustration of this invention and are not intended as limitations thereof. 
     EXAMPLE 1 
     Preparation of Fe--Nb Alloy Specimen (Composition Fe : 96%, Nb : 4%) 
     As a starting material, electrolytic iron of 99.9% purity and niobium of 99.8% purity were used. The starting materials were charged in a total amount of 700g into an alumina crucible and melted in a high frequency induction electric furnace in air and then thoroughly stirred to obtain a homogeneous molten alloy. Then, the molten alloy was poured into a mold having a hole of 25 mm diameter and 170 mm height to form an ingot. This ingot was forged at about 1000° C to a rod of 4 mm diameter, which was annealed at 1000° C for 1 hour, cooled with water and then cold drawn to a wire of 0.5 mm diameter. In this case, the working ratio (reduction of area) was 98%. The thus obtained wire is cut to lengths of 1 m with each length being wound in a coil to form a specimen. 
     Then, the specimens were subjected to several heat treatments to obtain characteristic features of coercive force Hc, residual induction Br and squareness ratio (Br/B 100 ) as shown in the following Table 1. 
     The term&#34;squareness ratio&#34; used herein is expressed by a percentage of residual induction Br to magnetic flux density B 100  when magnetic field is 100 oersteds (i.e. Br/B 100  × 100) unless indicated otherwise. For some of the alloys, the &#34;squareness ratio&#34; is expressed as Br/B 200 , Br/B 300 , or Br/B 400  wherein the magnetic flux densities of B 200 , B 300  and B 400  are created by magnetic fields of 200, 300 and 400 oersteds, respectively. 
     
                       Table 1                                                     
______________________________________                                    
                                   Squareness                             
                Coercive Residual  ratio                                  
                force    induction Br/B.sub.100                           
Heat treatment  Hc(Oe)   Br(G)     (%)                                    
______________________________________                                    
Cold worked at a working                                                  
ratio of 98%    32       12,000    76.5                                   
After heated at 450° C                                             
for 30 hours in a vacuum,                                                 
                18       13,500    83.0                                   
cooled in furnace                                                         
After heated at 650° C                                             
for 2 hours in a vacuum,                                                  
                28       15,200    98.0                                   
cooled in furnace                                                         
After heated at 850° C                                             
for 1 hour in a vacuum,                                                   
                19       14,400    93.3                                   
cooled in furnace                                                         
After heated at 1,100° C                                           
for 1 hour in a vacuum,                                                   
                8        13,500    88.8                                   
cooled in furnace                                                         
______________________________________                                    
 
    
     EXAMPLE 2 
     Preparation of Fe--Nb Alloy Specimen (Composition Fe : 93%, Nb : 7%) 
     As a starting material, iron and niobium of the same purities as in Example 1 were used. The specimen was prepared in the same manner as described in Example 1. The specimen was subjected to several heat treatments to obtain characteristic features as shown in the following Table 2. 
     
                       Table 2                                                     
______________________________________                                    
                                   Squareness                             
                Coercive Residual  ratio                                  
                force    induction Br/B.sub.100                           
Heat treatment  Hc(Oe)   Br(G)     (%)                                    
______________________________________                                    
Cold worked at a working                                                  
ratio of 98%    54       10,500    75.2                                   
After heated at 500° C                                             
for 20 hours in a vacuum,                                                 
                24       12,600    85.4                                   
cooled in furnace                                                         
After heated at 650° C                                             
for 2 hours in a vacuum,                                                  
                36       14,800    97.0                                   
cooled in furnace                                                         
After heated at 900° C                                             
for 1 hour in a vacuum,                                                   
                28       14,100    92.1                                   
cooled in furnace                                                         
After heated at 1,100° C                                           
for 30 minutes in a                                                       
                12       12,900    88.0                                   
vacuum cooled in furnace                                                  
______________________________________                                    
 
    
     FIG. 1 shows the coercive force Hc, residual induction Br and squareness ratio Br/B 100  of Fe--Nb alloy containing variable amount of niobium when the alloy is worked at a working ratio of 98% and heated at 650° C in a vacuum for 2 hours. As seen from this FIG., the larger niobium content will increase the coercive force, but reduces the residual induction. The squareness ratio is more than 90% independent of the niobium content. However, when the niobium content is less than 0.5%, the coercive force is less than 2 oersteds, and when the content exceeds 10%, the working of the alloy is difficult. 
     FIG. 2 shows the coercive force Hc, residual induction Br and squareness ratio Br/B 100  of an Fe--Nb alloy which contains 4% niobium when the alloy is cold worked at a working ratio of 98% and then heated at various temperatures for 2 hours. As seen from this FIG., the squareness ratio is more than 80% when the heating is carried out at a temperature above 400° C and less than 80% when the heating is carried out at a temperature below 400° C. The latter case becomes unsuitable as a magnetic alloy requiring a rectangular hysteresis loop. One of the features of the cold worked, heat treated alloy of the present invention is that the squareness ratio is more than 80% even in the heating at an elevated temperature. 
     EXAMPLE 3 
     Preparation of Alloy Specimen No. 28 (Composition Fe : 86.3%, Nb: 3.2%, Ta: 10.5%) 
     As a starting material, iron and niobium of the same impurities as in Example 1 and tantalum of 99.9% purity were used. The specimen was prepared in the same manner as described in Example 1. The specimen was subjected to several heat treatments to obtain characteristic features as shown in the following Table 3. 
     
                       Table 3                                                     
______________________________________                                    
                                   Squareness                             
                Coercive Residual  ratio                                  
                force    induction Br/B.sub.100                           
Heat treatment  Hc(Oe)   Br(G)     (%)                                    
______________________________________                                    
Cold worked at a working                                                  
ratio of 98%    68       10,700    74.0                                   
After heated at 450° C                                             
for 30 hours in a vacuum,                                                 
                44       12,600    87.1                                   
cooled in furnace                                                         
After heated at 650° C                                             
for 5 hours in a vacuum,                                                  
                52       14,200    98.2                                   
cooled in furnace                                                         
After heated at 850° C                                             
for 1 hour in a vacuum,                                                   
                48       14,000    96.8                                   
cooled in furnace                                                         
After heated at 1,100° C                                           
for 1 hour in a vacuum,                                                   
                19       13,800    95.4                                   
cooled in furnace                                                         
______________________________________                                    
 
    
     EXAMPLE 4 
     Preparation of Alloy Specimen No. 202 (Composition Fe : 93.8%, Nb : 3.2%, Al : 3.0%) 
     As a starting material, iron and niobium of the same purities as in Example 1 and aluminum of 99.8% purity were used. The specimen was prepared in the same manner as described in Example 1. The specimen was subjected to several heat treatments to obtain characteristic features as shown in the following Table 4. 
     
                       Table 4                                                     
______________________________________                                    
                                   Squareness                             
                Coercive Residual  ratio                                  
                force    induction Br/B.sub.100                           
Heat treatment  Hc(Oe)   Br(G)     (%)                                    
______________________________________                                    
Cold worked at a working                                                  
ratio of 98%    72       11,000    76.0                                   
After heated at 500° C                                             
for 20 hours in a vacuum,                                                 
                45       12,700    85.2                                   
cooled in furnace                                                         
After heated at 750° C                                             
for 2 hours in a vacuum,                                                  
                54       14,600    97.6                                   
cooled in furnace                                                         
After heated at 900° C                                             
for 1 hour in a vacuum,                                                   
                49       14,400    96.6                                   
cooled in furnace                                                         
After heated at 1,100° C                                           
for 30 minutes in a                                                       
                21       14,000    93.9                                   
vacuum, cooled in furnace                                                 
______________________________________                                    
 
    
     Moreover, characteristic features of representative alloys of the present invention are shown in the following Tables 5 and 6. 
     
                                           Table 5                                 
__________________________________________________________________________
                Cold                  Residual                            
                                           Squareness                     
     Composition                                                          
                working                                                   
                     Heating                                              
                            Heating                                       
                                 Coercive                                 
                                      induc-                              
                                           ratio                          
Specimen                                                                  
     (%)        ratio                                                     
                     temperature                                          
                            time force                                    
                                      tion Br/B.sub.100                   
No.  Fe Nb      (%)  (° C)                                         
                            (hr) Hc(Oe)                                   
                                      Br(G)                               
                                           (%)                            
__________________________________________________________________________
15   91.2                                                                 
        4.5                                                               
          v  4.3                                                          
                95   600    10   36   15,300                              
                                           97.5                           
28   86.3                                                                 
        3.2                                                               
          Ta 10.5                                                         
                98   650    5    52   14,200                              
                                           98.2                           
42   90.4                                                                 
        5.0                                                               
          Cr 4.6                                                          
                99   600    15   44   15,100                              
                                           97.8                           
57   91.8                                                                 
        4.2                                                               
          Mo 4.0                                                          
                98   600    10   41   15,400                              
                                           97.3                           
70   91.5                                                                 
        4.0                                                               
          W  4.5                                                          
                95   650    5    38   14,800                              
                                           97.5                           
83   84.6                                                                 
        3.5                                                               
          Ni 11.6                                                         
                98   550    15   65   14,500                              
                                           96.4                           
105  92.0                                                                 
        4.5                                                               
          Cu 3.5                                                          
                98   600    5    57   15,200                              
                                           97.6                           
129  92.7                                                                 
        3.8                                                               
          Co 13.5                                                         
                95   650    7    60   16,300                              
                                           97.9                           
143  93.8                                                                 
        4.2                                                               
          Ti 2.0                                                          
                96   700    3    46   15,000                              
                                           96.7                           
162  95.5                                                                 
        3.0                                                               
          Zr 1.5                                                          
                97   650    10   43   14,700                              
                                           97.0                           
187  94.0                                                                 
        4.0                                                               
          Si 2.0                                                          
                98   750    5    38   15,200                              
                                           96.3                           
202  93.8                                                                 
        3.2                                                               
          Al 3.0                                                          
                98   750    2    54   14,600                              
                                           97.6                           
226  94.3                                                                 
        3.5                                                               
          Ge 2.2                                                          
                98   600    10   46   15,300                              
                                           95.7                           
240  96.0                                                                 
        2.5                                                               
          Sn 1.5                                                          
                95   600    5    35   15,100                              
                                           96.4                           
259  95.5                                                                 
        3.0                                                               
          Sb 1.5                                                          
                95   600    5    40   15,000                              
                                           97.0                           
276  95.9                                                                 
        3.1                                                               
          Be 1.0                                                          
                95   600    3    42   15,200                              
                                           97.8                           
294  91.8                                                                 
        3.2                                                               
          Mn 5.0                                                          
                98   600    3    53   14,800                              
                                           96.7                           
317  95.7                                                                 
        4.0                                                               
          Ce 0.3                                                          
                95   600    10   48   15,500                              
                                           95.5                           
334  96.1                                                                 
        3.5                                                               
          C  0.4                                                          
                95   550    3    57   15,100                              
                                           94.6                           
__________________________________________________________________________
 
    
     
         Table 6
    Cold Heating   Residual Squareness  Speci- Composition working
 temper- Heating Coercive induc- ratio men (%) ratio ature time force
 tion Br/B.sub.100 No. Fe Nb V Ta Cr Mo W Ni Cu Co Ti Zr Si Al Ge Sn Sb
 Be Mn Ce C (%) (°
  C) (hr) Hc(Oe) Br(G) (%)                           361 74.3 5.2 -- --
 -- 5.5 -- -- -- 15.0 -- -- -- -- -- -- -- -- -- -- -- 98 700 5 96 16,800 9
 7.2  385 71.0 3.5 3.0 -- -- -- -- -- -- 19.5 -- -- -- 3.0 --  -- -- --
 -- -- -- 96 700 3 105 17,700 a 94.8 407 57.2 7.0 -- -- 20.5 -- -- 10.3
 -- -- -- -- -- -- -- -- -- -- 5.0 -- -- 90 600 2 72 15,300 95.5 428 54.7
 4.3 1.5 -- -- 10.0 -- -- 4.5 25.0 -- -- ---- -- -- -- -- -- -- -- 96 750
 5 270 16,500 b 95.0 455 41.7 3.0 2.1 5.7 -- -- -- 18.0 -- 27.5 -- -- --
 -- 2.0 -- -- -- -- -- -- 85 550 10 310 15,000 c 92.3 473 53.2 5.8 1.0 --
 -- 5.0 -- -- 15.0 20.0 -- -- -- -- -- -- -- -- -- -- -- 90 700 5 195
 16,400 a 95.6 496 67.0 5.0 0.5 -- -- 7.0 -- -- 2.0 18.5 -- -- -- -- --
 -- -- -- -- -- -- 96 650 3 124 17,600 a 96.5 513 53.5 4.5 2.0 -- 15.0 --
 -- -- -- 23.0 -- -- 1.0 1.0 -- -- -- -- -- -- -- 90 700 7 110 16,400 a
 97.2 535 47.7 2.3 3.0 -- -- 7.0 -- 8.5 -- 28.0 -- -- -- -- -- -- -- --
 3.0 0.5 -- 90 600 10 265 16,800 b 94.6 564 71.6 4.2 2.0 -- -- -- 5.0 --
 -- 13.0 1.2 -- 1.0 -- -- -- 1.0 -- 1.0 -- -- 85 650 3 73 15,100 95.3 589
 61.2 3.1 -- -- 8.0 -- -- 7.0 -- 20.0 -- -- -- -- -- -- -- -- -- 0.5 0.2
 90 700 2 88 16,500 95.5 611 56.9 6.0 -- -- -- -- 3.5 5.0 -- 23.0 -- --
 2.0 -- 3.0 -- -- 0.5 -- -- 0.1 85 700 5 107 17,200 a 93.7 626 75.0 3.5
 1.0 -- -- 3.0 -- -- -- 15.5 1.0 -- -- -- -- 1.0 -- -- -- -- -- 90 700 3
 80 16,200 95.5 643 59.6 4.0 -- 5.3 -- -- 7.0 -- 2.1 20.0 -- 1.0 -- -- --
 -- 1.0 -- -- -- -- 90 750 5 75 17,500 96.8 671 78.5 1.0 -- 10.0 -- 5.0
 -- -- -- -- -- 1.0 -- -- -- 1.0 -- 0.5 3.0 -- -- 90 700 2 62 15,800 95.7
 690 72.3 4.2 -- -- -- -- 5.7 10.0 -- -- 2.3 -- -- -- -- -- -- -- 5.0 0.5
 -- 85 600 5 130 14,200 a 90.5
 a Br/B.sub.200
 b Br/B.sub.300
 c Br/B.sub.400
 
    
     As understood from the above FIGS. 1 and 2, and Tables 1 to 6, the alloys according to the present invention, that is either Fe--Nb alloys alone or in admixture with 0.01-60 wt. % in total amount of at least one element selected from the group consisting of V, Ta, Cr, Mo, W, Ni, Cu, Co, Ti, Zr, Si, Al, Ge, Sn, Sb, Mn, Ce and C are magnetic alloys having a coercive force of more than 2 oersteds, a high residual induction and an excellent rectangular hysteresis loop when the alloy has been subjected to cold working at a working ratio of more than 50% and then heat treated by heating at a temperature above 400° C. 
     Furthermore, even if the above magnetic alloy is further heated or subjected to cold working, the rectangular hysteresis loop is not easily transformed. Accordingly, the alloy of the present invention is useful for the manufacture of articles requiring the glass sealing or additional working after a final heat treatment. 
     As mentioned above, the magnetic properties of the alloy according to the present invention are obtained by subjecting the alloy to cold working at a working ratio of more than 50% and then to a heat treatment by heating it at a temperature above 400° C. Of course, the good rectangular hysteresis loop is obtained even if the cold working and heating are carried out repeatedly. 
     In the alloys shown in Examples 1 to 4, FIGS. 1 and 2 and Tables 5 and 6, metals having a relatively high purity, such as Ta, Cr, Mo, W, Mn, V, Ti, Al, Si, Ce, C and the like are used. However, even if economically useful and commercially available ferroalloys and Misch metals are used instead of these metals, and if deoxidization and desulfurization are sufficiently effected during the melting, substantially the same magnetic properties and workability as in the case of using the elemental metal can be obtained. 
     As mentioned above, the alloys of the present invention have an excellent rectangular hysteresis loop and a large coercive force, so that they are suitable as a magnetic material for not only the aforesaid electromagnetic devices requiring the rectangular hysteresis loop but also as a material for a core of a hysteresis motor. 
     In the present invention, the reason why the composition of the alloy is limited to the ranges as mentioned above is due to the fact that when the composition is within the aforesaid range, the coercive force is more than 2 oersteds, the rectangular hysteresis loop is excellent and the workability is good, but when the composition deviates from this range, the magnetic properties are degraded and the working becomes very difficult so as to be improper to use as a magnetic alloy having a rectangular hysteresis loop as understood from each Example, FIGS. 1 and 2 and Tables 5 and 6. 
     Thus, the alloy, which consists of 0.5-10 wt. % Nb and the balance of Fe, has a coercive force of more than 2 oersteds and an excellent rectangular hysteresis loop. The alloy suffers only a small amount of degradation of the magnetic properties even by heating at an elevated temperature and has excellent forgeability and workability. The addition of Ta, Cr, Mo, W, Ni, Cu, Co, Ti, Zr, Al, Sn, Sb, Be, Mn, Ce and/or C to this Fe--Nb alloy will improve the rectangular hysteresis loop and coercive force. The addition of Ti, Al, Si, Ge or V to this Fe--Nb alloy reduces the degradation of magnetic properties by heating at an elevated temperature. The addition of Ta, Cr or Ni to this Fe--Nb alloy improves the forgeability and workability.