Very thin soft magnetic alloy strips and magnetic core and electromagnetic apparatus made therefrom

A thin Co-based amorphous alloy strip is produced, the conditions for production being controlled to those specified by the invention. The thin strip has an extremely small thickness and few pinholes. The extremely small thickness of less than 4.8 .mu.m notably enhances soft magnetic properties such as permeability and core loss in the high frequency range. Additionally, magnetic cores and electromagnetic apparatuses can be produced from the thin Co-based amorphous alloy strips.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Now, the present invention will be described more specifically below with 
reference to working examples. 
Now, the first aspect of this invention, namely the method for the 
production of an extremely thin soft magnetic alloy strip will be 
described in detail below. FIG. 1 is a diagram illustrating the 
construction of an apparatus for the production of a thin soft magnetic 
alloy strip embodying the method of this invention for the production of a 
thin soft magnetic alloy strip. 
With reference to this diagram, a vacuum chamber 10 is provided with a gas 
supply system 12 and a discharge system 14. Inside this vacuum chamber 10, 
a single-roll mechanism 40 consisting mainly of a cooling roll 20 capable 
of being cooled to a prescribed temperature and controlled to a prescribed 
peripheral speed and a raw material melting container 30. 
In the lower part of the raw material melting container 30 is disposed a 
nozzle 32 which opens in the direction of a peripheral surface 22 of the 
cooling roll 20. The shape of the orifice of this nozzle 32 is rectangular 
as illustrated in FIG. 2. The short side of the rectangular cross section 
of the orifice falls parallelly to the circumferential direction of the 
cooling roll 20. The long side a and the short side b of the orifice of 
the nozzle 32 are to be set in accordance with the particular raw material 
to be used. As showed in FIG. 3, the nozzle 32 are set so the appropriate 
distance c between the nozzle 32 and the peripheral surface 22 of the 
working roll 20 can be formed. This distance c can be varied depending on 
the particular raw material to be used. The angle of ejection onto the 
cooling roll 20 is not limited to 90.degree.. 
An induction heating coil 34 is disposed on the outer periphery of the raw 
material melting container 30 and is used for melting the raw material to 
be introduced. The molten raw material is ejected through the nozzle 32 
onto the peripheral surface 22 of the cooling roll 20. 
In producing an extremely thin Co-based amorphous alloy strip by the use of 
the apparatus for the production of a thin soft magnetic alloy strip 
constructed as described above, the raw material for a Co-based alloy 
composition represented by the aforementioned general formula: 
EQU (Co.sub.1-a A.sub.a).sub.100-b X.sub.b (I) 
is first introduced into the raw material melting container 30 and melted 
therein. 
In the composition of the formula (I) mentioned above, A represents an 
element which is effective in enhancing the thermal stability and 
improving the magnetic properties. When A is selected from among Mn, Fe, 
Ni, Cr, Mo, W, V, Nb, Ta, Ti, Zr, Hf, Cu, and the platinum-group elements, 
any value of a exceeding 0.3 is practically undesirable because this 
excess of the value goes to lower the Curie point. When A is Fe or Ni, any 
value of a exceeding 0.5 prevents the magnetic properties from being 
improved. X represents an element essential for the produced thin alloy 
strip to assume an amorphous phase. When the content of this element is 
less than 10 atomic % or not less than 35 atomic %, to obtain an amorphous 
phase becomes difficult. 
Where the thin alloy strip is expected to possess particularly satisfactory 
high frequency properties so as to fit utility in a saturable reactor, a 
noise filter, main transformer, choke coil, or a magnetic head, for 
example, it is desirable to use a raw material of an alloy composition 
represented by the following general formula: 
EQU (Co.sub.1-m-n L.sub.m M.sub.n).sub.100-o (Si.sub.l-p B.sub.p).sub.0 (IV) 
wherein L stands for at least one element selected from the class 
consisting of Fe and Mn, M for at least on element selected from the class 
consisting of Ti, V, Cr, Ni, Cu, Zr, Nb, Mo, Hf, Ta, W and the 
platinum-group elements, and m, n, o, and p for numbers satisfying the 
following formulas, 0.03.ltoreq.m.ltoreq.0.15, 0.ltoreq.n.ltoreq.0.10, 20 
atomic %.ltoreq.0.ltoreq.35 atomic % and 0.2.ltoreq.P.ltoreq.1.0. 
Particularly the use of at least one element selected from among Cr, Mo, 
and W as M in the composition of the formula (IV) is effective in 
decreasing the thickness of the strip to extremity. 
Then, the vacuum chamber 10 is evacuated to a reduced pressure of not 
higher than 10.sup.-4 Torr. The molten alloy composition is subsequently 
ejected under a pressure in the range of 0.015 to 0.025 kg/cm.sup.2 
through the nozzle onto the peripheral surface 22 of the cooling roll 20 
operated at a controlled peripheral speed in the range of 20 to 50 m/sec, 
to rapidly quench the molten alloy and obtain a thin Co-based amorphous 
alloy strip 40. 
The upper limit, 10.sup.-4 Torr, fixed for the pressure to be used for the 
atmosphere in which the molten metal is ejected is critical because the 
thin amorphous alloy strip 40 containing only very few pinholes and 
measuring less than 4.8 .mu.m in thickness is not easily produced when the 
pressure is lower vacuum (worse) than 10.sup.-4 Torr. If the peripheral 
speed of the cooling roll 20 is less than 20 m/sec, the thin strip 
measuring less than 4.8 .mu.m in thickness is obtained with difficulty. If 
the peripheral speed exceeds 50 m/sec, the possibility of the thin strip 
being broken during the course of production is increased and the 
production of the thin strip cannot be continued. Particularly where the 
thin strip measuring not less than 5 mm in width is to be produced, the 
peripheral speed is desired to be in the range of 20 to 40 m/sec, 
preferably 20 to 35 m/sec. If the pressure for the ejection of the molten 
metal is less than 0.015 kg/cm.sup.2, it often happens that the ejection 
itself fails to occur. Conversely, if the pressure exceeds 0.025 
kg/cm.sup.2, the thin strip measuring less than 4.8 .mu.m in thickness is 
produced only with difficulty. 
The cooling roll 20 to be used herein is formed of a Fe-based alloy, 
preferably a Cr-containing Fe-based alloy such as, for example, tool 
steel. By the use of this cooling roll 20, the produced thin strip 
acquires improved surface smoothness and it is made possible to produce an 
extremely thin strip of fine state. 
The long side a of the rectangular cross section of the orifice of the 
nozzle 32 functions to determine the width of the produced thin strip and 
has no specific restriction except for the requirement that they should 
measure not less than 2 mm. The short side b is an important factor for 
determining the thickness of the thin strip and is set in the range of 
0.07 to 0.13 mm. If the short side b is less than 0.07 mm, the molten 
metal is ejected only with extreme difficulty. Conversely, if the short 
side b exceeds 0.13 mm, the thin strip measuring less than 4.8 .mu.m in 
thickness cannot be produced. Preferably, the short side b is in the range 
of 0.08 to 0.12 mm. 
Then, the distance c between the leading end of the nozzle 32 and the 
cooling roll 20 is set in the range of 0.05 to 0.20 mm. the reason for 
this range is that the thin strip is not easily obtained with desirable 
surface quality if this distance c is less than 0.05 mm and the thin strip 
measuring less than 4.8 .mu.m is not obtained easily if this distance 
exceeds 0.20 mm. 
By rapidly quenching the molten metal while fulfilling the conditions 
mentioned above, the thin Co-based amorphous alloy strip 40 measuring less 
than 4.8 .mu.m can be obtained. 
The thin Co-based amorphous alloy strip obtained as described above is 
coiled or superposed one ply over another to form a magnetic core, 
subjected to a heat treatment performed for the relief of strain at a 
temperature below crystallizing temperature to the Curie point, and then 
cooled. The cooling speed is required to fall in the range between 
0.5.degree. C./min and the speed of quenching in water, preferably in the 
range of 1.degree. to 50.degree. C./min. Thereafter, the cooled core may 
be given an additional heat treatment in the presence of a magnetic field 
(in the direction of the axis of the thin strip, the direction of the 
width, the direction of the plate thickness, or the rotary magnetic field) 
as occasion demands. The atmosphere in which this heat treatment is 
performed is not critical. An inert gas such as N.sub.2 or Ar, a vacuum, a 
reducing atmosphere such as of H.sub.2, or the ambient air may be used. 
The reason for setting the limit of less than 4.8 .mu.m for the thickness 
of the thin Co-based amorphous alloy strip is that the thin strip exhibits 
particularly desirable magnetic properties in the high frequency range of 
MHz, for example. 
Now, typical examples of the manufacture of the thin Co-based amorphous 
alloy strip will be described below. 
EXAMPLE 1 
An alloy composition represented by the formula, (Co.sub.0.95 
Fe.sub.0.05).sub.95 Mo.sub.5).sub.75 (Si.sub.0.5 B.sub.0.5).sub.25, was 
prepared and placed in a raw material melting container and melted 
therein. The nozzle used herein had a rectangular orifice measuring 10.3 
mm.times.0.10 mm (a.times.b) and the distance c between the nozzle and the 
cooling roll was 0.1 mm. The cooling roll was made of Fe. 
Then, the vacuum chamber was evacuated to 5.times.10.sup.-5 Torr and the 
molten alloy composition was ejected under pressure of 0.02 kg/cm.sup.2 
through the nozzle onto the peripheral surface of the cooling roll 
operated at a controlled peripheral speed of 33 m/sec, to rapidly quence 
the molten metal and produce a thin Co-based amorphous strip. 
Thus, a long thin amorphous strip possessing satisfactory surface quality 
and measuring 4.7 .mu.m in thickness and 10 mm in width was obtained. 
The long very thin Co-based amorphous strip thus obtained was coiled, then 
subjected to the optimum heat treatment at a temperature below the 
crystallizing temperature, and tested for the frequency characteristic of 
initial permeability and for the high-frequency core loss. 
FIG. 4 shows the frequency characteristic of initial permeability in an 
excited magnetic field of 2 mOe. For comparison, the results obtained 
similarly of a thin Co-based amorphous alloy strip using the same 
composition and measuring 15 .mu.m in thickness are also shown in the 
diagram. 
It is clearly noted from the diagram that the effect of the plate thickness 
conspicuously manifested when the permeability exceeded 100 kHz. The thin 
Co-based amorphous alloy strip 4.7 .mu.m in thickness produced in the 
present example exhibited higher degrees of permeability at 1 MHz and 10 
MHz than the thin strip produced for comparison, indicating that the thin 
strip of this invention exhibits highly satisfactory permeability even in 
the high frequency range. 
The core loss of the thin strip of this example at 1 MHz under the 
condition of 1 kG of excited magnetic amplitude was about one half of that 
of the strip of a plate thickness of 15 .mu.m. The rectangular ratio of 
the thin strip was almost 100% at a frequency above 500 kHz, indicating 
that this thin strip was useful in a saturable reactor, for example. 
EXAMPLE 2 
Thin Co-based amorphous alloy strips were produced by following the 
procedure of Example 1, excepting varying alloy compositions indicated in 
Table 1 were used as starting materials and varying conditions of 
manufacture similarly indicated in Table 1 were used. 
Comparative experiments indicated in the same table produced thin strips of 
the same compositions as those of the example, with some or other of the 
manufacturing conditions of this invention deviated from the respective 
ranges specified by this invention. 
TABLE 1 
__________________________________________________________________________ 
Degree 
of Orifice size 
Peripheral Injection 
Plate 
vacuum 
of nozzle 
Material 
speed of roll 
Gap pressure 
thickness 
Alloy composition 
(Torr) 
(a .times. bmm) 
of roll 
(m/sec) 
(cmm) 
(kg/cm.sup.2) 
(.mu.m) 
__________________________________________________________________________ 
Example 2 
Sample 1 
(Co.sub.0.91 Fe.sub.0.05 Mo.sub.0.04).sub.75 
5 .times. 10.sup.-5 
15 .times. 0.10 
SKD roll 
36 0.10 
0.02 4.0 
Comparative 
Sample 1 
(Si.sub.0.55 B.sub.0.45).sub.25 
5 .times. 10.sup.-2 
" " " " " 5.8* 
Experiment 
Sample 2 5 .times. 10.sup.-5 
15 .times. 0.30 
" " " " 10.1 
2 Sample 3 " 15 .times. 0.10 
Cu roll 
" " " 7.9 
Sample 4 " " SKD roll 
17 " " 7.6 
Sample 5 " " " 36 0.30 
" 8.3 
Sample 6 " " " " 0.10 
0.05 6.5 
Example 2 
Sample 2 
(Co.sub.0.91 Fe.sub.0.05 Cr.sub.0.04).sub.75 
5 .times. 10.sup.-5 
15 .times. 0.10 
SKD roll 
36 0.10 
0.02 3.7 
Comparative 
Sample 7 
(Si.sub.0.6 B.sub.0.4).sub.25 
5 .times. 10.sup.-2 
" " " " " 5.5* 
Experiment 
Sample 8 5 .times. 10.sup.-5 
15 .times. 0.30 
" " " " 9.8 
2 Sample 9 " 15 .times. 0.10 
Cu roll 
" " " 7.7 
Sample " " SKD roll 
17 " " 7.6 
10 
Sample " " " 36 0.30 
" 8.0 
11 
Sample " " " " 0.10 
0.05 6.4 
12 
Example 2 
Sample 3 
(Co.sub.0.95 Fe.sub.0.05).sub.74 
5 .times. 10.sup.-5 
15 .times. 0.10 
SKD roll 
36 0.10 
0.02 4.6 
Comparative 
Sample 
(Si.sub.0.6 B.sub.0.4).sub.26 
5 .times. 10.sup.-2 
" " " " " 6.8 
Experiment 
13 
2 Sample 5 .times. 10.sup.-5 
15 .times. 0.30 
" " " " 10.5 
14 
Sample " 15 .times. 0.10 
Cu roll 
" " " 8.9 
15 
Sample " " SKD roll 
17 " " 8.0 
16 
Sample " " " 36 0.30 
" 9.6 
17 
Sample 8 " " " " 0.10 
0.05 7.3 
Example 2 
Sample 4 
(Co.sub.0.905 Fe.sub.0.05 Nb.sub.0.02 Cr.sub.0.25).sub.75 
8 .times. 10.sup.-5 
20 .times. 0.12 
SKD roll 
30 0.12 
0.015 
4.4 
Sample 5 
(Si.sub.0.5 B.sub.0.5).sub.25 
7 .times. 10.sup.-5 
25 .times. 0.10 
" 25 0.15 
0.020 
4.0 
Sample 6 4 .times. 10.sup.-5 
30 .times. 0.09 
" 25 0.15 
0.020 
3.7 
__________________________________________________________________________ 
*Pinholes contained 
It is clearly noted from Table 1 that an extremely thin Co-based amorphous 
alloy strip measuring less than 4.8 .mu.m in thickness and possessing a 
fine state devoid of a pinhole could not be obtained when any one of the 
conditions of manufacture deviated from the relevant range specified by 
this invention. 
EXAMPLE 3 
Thin strips were produced by following the procedure of Example 1, 
excepting an alloy composition represented by the formula, (Co.sub.0.95 
Fe.sub.0.05).sub.95 Cr.sub.5).sub.75 (Si.sub.0.5 B.sub.0.5).sub.25, was 
used instead and the conditions of manufacture were varied from those of 
Example 1. Consequently, thin Co-based amorphous alloy strips measuring 
variously in the range of 3.0 to 10.2 .mu.m in thickness. The thin strips 
had a fixed width of 5 mm. 
Then, the thin amorphous alloy strips thus obtained were insulated with 
MgO, wound in the form of a toroidal core 12 mm in outermost diameter and 
8 mm in inner diameter, annealed at a temperature not exceeding the 
crystallizing temperature and exceeding the curie point, and then cooled 
at a cooling speed of 3.degree. C./min, to produce magnetic cores. 
The magnetic cores thus obtained were tested for core loss at varying 
frequencies between 1 MHz and 5 MHz by the use of a magnetic property 
evaluating apparatus. The results were as shown in FIG. 5. During the 
test, the magnetic flux density was fixed at 1 KG. 
It is clearly noted from the diagram that the core loss decreased in 
proportion as the plate thickness decreased and that in the magnetic flux 
density of 1 kG the core loss value of the plate thickness of less than 
4.8 .mu.m in f=2 MHz is smaller than the value in f=500 kHz (3(w/cc)), of 
the plate thickness of 20 .mu.m Co-based amorphous alloy which is used 
practically at present time. It is indicated that these thin strips were 
highly advantageous for use in the high frequency range. 
Now, the second aspect of this invention, namely the method for the 
production of an extremely thin soft magnetic alloy strip, will be 
described more specifically below. The apparatus used for this production 
was configured similarly to the apparatus of production illustrated in 
FIG. 1. The conditions for manufacture were different. 
First, the raw materials for a Fe-based alloy composition represented by 
the aforementioned formula: 
EQU Fe.sub.100-c-d D.sub.c Y.sub.d (II) 
or, particularly for the production of a thin Fe-based microcrystalline 
alloy strip, the raw material for a Fe-based alloy composition represented 
by the general formula: 
EQU Fe.sub.100-e-f-g-h-i-j E.sub.e G.sub.f J.sub.g Si.sub.h B.sub.i Z.sub.j 
(III) 
was placed in the raw material melting container 30 and melted therein. 
Here, D in the formula (II) shown above represents an element effective in 
the enhancement of thermal stability and the improvement of magnetic 
properties. Then, Y represents an element essential for the impartation of 
an amorphous texture to the thin strip. If the content of this element, Y, 
is less than 15 atomic % or exceeds 30 atomic %, the crystallizing 
temperature is unduly low and the sample obtained from the alloy 
composition is adulterated by inclusion of a crystalline portion. 
Then, E (Cu or Au) in the aforementioned formula (III) represents an 
element effective in improvement of the corrosionresistance, preventing 
crystalline grains from being coarsened, and improving the soft magnetic 
properties such as core loss and permeability. It is particularly 
effective in the education of the bcc phase at low temperatures. If the 
amount of this element is unduly small, the effects mentioned above are 
not obtained. Conversely, if this amount is unduly large, the magnetic 
properties are degraded. Suitably, therefore, the content of E is in the 
range of 0.1 to 8 atomic %. Preferably, this range is from 0.1 to 5 atomic 
%. 
G (at least one element selected from the class consisting of the elements 
of Group IVa, the elements of Group Va, the elements of Group VIa, and the 
rare-earth elements) is an element for effectively uniformizing the 
diameter of crystalline grains, diminishing magnetostriction and magnetic 
anisotropy, improving the soft magnetic properties, and also improving the 
magnetic properties against temperature changes. The combined addition of 
G and E (Cu, for example) allows the stabilization of the bcc phase to be 
attained over a wide range of temperature. If the amount of this element, 
G, is unduly small, the aforementioned effects are not attained. 
Conversely, if this amount is unduly large, the amorphous phase can not be 
obtained during the course of manufacture and, what is more, the saturated 
magnetic flux density is unduly low. The content of G, therefore, is 
suitably in the range of 0.1 to 10 atomic %. Preferably, this range is 
from 1 to 8 atomic %. 
As concerns the effects of a varying element as E, in addition to the 
effects mentioned above, the elements of Group IVa are effective in 
widening the ranges of conditions of the heat treatment for the attainment 
of the optimum magnetic properties, the elements of Group Va are effective 
in improving the resistance to embrittlement and improving the workability 
as for cutting, and the elements of Group VIa are effective in improving 
the corrosionresistance and improving the surface quality. 
Among the elements mentioned above, Ta, Nb, W, and Mo are particularly 
effective in improving the soft magnetic properties and V is conspicuously 
effective in improving the resistance to embrittlement and the surface 
quality. These elements are, therefore, constitute themselves preferred 
choices. 
J (at least one element selected from the class consisting of Mn, Al, Ga, 
Ge, In, Sn, and the platinum-group elements) is an element effective in 
improving the soft magnetic properties or the corrosion resistant 
properties. If the amount of this element is unduly large, the saturated 
magnetic flux density is not sufficient. Thus, the upper limit of this 
amount is fixed at 10 atomic %. Among the elements of this class, Al is 
particularly effective in promoting fine division of crystalline grains, 
improving the magnetic properties, and stabilizing the bcc phase, Ge is 
effective in stabilizing the bcc phase, and the platinum-group elements 
are effective in improving the corrosion resistant properties. 
Si and B are elements effective in obtaining amorphous phase during the 
course of manufacture, improving the crystallizing temperature, and 
promoting the heat treatment for the improvement of the magnetic 
properties. Particularly, Si forms a solid solution with Fe as the main 
component of microcrystalline grains and contributes to diminishing 
magnetostriction and magnetic anisotropy. If the amount of Si is less than 
12 atomic %, the improvement of the soft magnetic properties is not 
conspicuous. If this amount exceeds 25 atomic %, the rapidly quenching 
effect is not sufficient, the educed crystalline grains are relatively 
coarse on the order of .mu.m, and the soft magnetic properties are not 
satisfactory. Further, Si is an essential element for the construction of 
a super lattice. For the appearance of this super lattice, the content of 
Si is preferably in the range of 12 to 22 atomic %. If the content of B is 
less than 3 atomic %, the educed crystalline grains are relatively coarse 
and do not exhibit satisfactory properties. If this content exceeds 12 
atomic %, B is liable to form a compound of B in consequence of the heat 
treatment and the soft magnetic properties are not satisfactory. 
Optionally, as an element for promoting the conversion of the crystalline 
texture of the thin strip to the amorphous texture, Z (C, N, or P) may be 
contained in the alloy composition in an amount of not more than 10 atomic 
%. 
The total amount of Si, B, and the element contributing to the conversion 
into the amorphous texture is desired to be in the range of 15 to 30 
atomic %. For the acquisition of highly satisfactory soft magnetic 
properties, Si and B are desired to be sued in such amounts as to satisfy 
the relation, Si/B.gtoreq.1. 
Particularly when the content of Si is in the range of 13 to 21 atomic %, 
the diminution of magnetostriction, .lambda.s, close to 0 is attained, the 
deterioration of the magnetic properties by resin mold is eliminated, and 
the outstanding soft magnetic properties aimed at are effectively 
manifested. 
The effect of this invention is not impaired when the Fe-based soft 
magnetic alloy mentioned above contains in a very small amount such 
unavoidable impurities as 0 and S which are contained in ordinary Fe-based 
alloys. 
Then, after the vacuum chamber 10 has been evacuated to a reduced pressure 
of not higher than 10.sup.-2 Torr or filled with a He atmosphere of not 
higher than 60 Torrs, the molten alloy composition is ejected under a 
pressure of not more than 0.03 kg/cm.sup.2 through the nozzle 32 onto the 
peripheral surface of the cooling roll 20 operated at a controlled 
peripheral speed of not less than 20 m/sec, to quench the molted metal and 
produce a thin amorphous strip 40. 
The reason for setting the upper limit of the reduced pressure or the 
pressure of the atmosphere of inert gas at 10.sup.-2 Torr or 60 Torrs is 
that particularly in the production of a thin strip of a large width 
exceeding 1.5 mm, the thin strip having a sufficient small thickness, 
excelling in surface quality, and containing no pinhole is obtained when 
the upper limit is not surpassed. If this upper limit is surpassed, the 
produced thin strip acquires a laterally undulating surface, abounds with 
pinholes, and fails to acquire a thickness of not more than 10 .mu.m. The 
peripheral speed is required only to exceed 20 m/sec. In view of the 
facility of manufacture of the thin strip, however, this peripheral speed 
is desired to be not more than 50 m/sec. Then, the pressure for the 
ejection of the molten alloy is required only not to exceed 0.03 
kg/cm.sup.2, desirably not more than 0.025 kg/cm.sup.2, and more desirably 
not more than 0.02 kg/cm.sup.2. If this pressure is less than 0.001 
kg/cm.sup.2, the ejection of the molten metal is not easily attained. 
The cooling roll 20 is desired to be made of a Cu-based alloy (such as, for 
example, brass). Where the plate thickness of the thin strip to be 
produced is not more than 8 .mu.m, the cooling roll 20 may be made of a 
Fe-based alloy. The cooling roll made of the materials allows the produced 
thin strip to acquire improved surface quality and fine quality. 
The long side a of the rectangular cross section of the orifice of the 
nozzle 32 determines the width of the produced thin strip. It is required 
only to exceed 2 mm. The short side b constitutes itself an important 
value for determining the plate thickness of the thin strip. For the sake 
of the production of this thin strip in an extremely small thickness of 
not more than 0.15 mm, the value of b is desired to be not more than 0.2 
mm, preferably not more than 0.15 mm. In due consideration of the 
ejectability of the molten metal, however, the value of b is desired to be 
not less than 0.07 mm. 
The distance c between the leading end of the nozzle 32 and the cooling 
roll 20 is not more than 0.2 mm. The reason for this upper limit is that 
the strip is not easily obtained in an extremely small thickness if this 
distance exceeds 0.20 mm. If this distance c is unduly small, the produced 
thin strip suffers from inferior surface quality. Thus, the distance is 
desired to be not less than 0.05 mm. 
By quenching the molten metal faithfully under the conditions described 
above, the thin strip 40 of an amorphous state is obtained in a thickness 
of not more than 10 .mu.m. 
Where the thin Fe-based microcrystalline alloy strip is to be produced 
thereafter, the thin amorphous layer obtained as described above is 
subjected to a heat treatment at a suitable temperature exceeding the 
crystallizing temperature of the amorphous alloy for a period in the range 
of 10 minutes to 15 hours. This heat treatment allows the thin amorphous 
strip to effect precipitation of not more than 1000 .ANG. microcrystalline 
grains and acquire improved magnetic properties. Optionally, the thin 
Fe-based microcrystalline alloy strip may be given an additional heat 
treatment in the presence of a magnetic field (in the direction of the 
axis of the thin strip, the direction of the width, the direction of the 
thickness, or in the rotary magnetic field). The kind of the atmosphere in 
which this heat treatment is carried out is not critical. The heat 
treatment effectively proceeds in the insert gas such as N.sub.2 or Ar, in 
the vacuum, in the reducing atmosphere such as of H.sub.2, or in the 
ambient air, for example. 
The microcrystalline grains not more than 1,000 .ANG. in diameter present 
in the thin Fe-based microcrystalline alloy strip obtained as described 
above are desired to be such that they exist therein in an area ratio in 
the range of 25 to 95%. If the area ratio of the microcrystalline grains 
is unduly small, namely if the area ratio of the amorphous is unduly 
large, the core loss is large, the permeability low, and the 
magnetostriction large. Conversely, if the area ratio of the 
microcrystalline grains is unduly large, the magnetic properties are 
unsatisfactory. The preferable ratio of presence of the microcrystalline 
grains in the alloy is in the range of 40 to 90% as area ratio. Within 
this range, the soft magnetic properties are obtained particularly stably. 
The reason for setting the upper limit of the thickness of the thin 
Fe-based microcrystalline alloy strip at 10 .mu.m is that the magnetic 
properties in the high frequency range such as of MHz are highly 
satisfactory and the resistance to embrittlement is improved when this 
upper limit is observed. The improvement of the resistance to 
embrittlement is prominent when the thickness is restricted below 8 .mu.m. 
In the production of the thin Fe-based amorphous alloy strip, the thin 
strip in an amorphous state is subjected to a heat treatment at a 
temperature not exceeding the crystallizing temperature of the amorphous 
alloy. 
Now, the production of the thin Fe-based microcrystalline alloy strip will 
be described specifically below with reference to typical examples. 
EXAMPLE 4 
An alloy composition represented by the formula, Fe.sub.72 Cu.sub.1 V.sub.6 
Si.sub.13 B.sub.8, was prepared, placed in the raw material melting 
container, and melted therein. 
The nozzle used herein had a rectangular orifice measuring 5.2 
mm.times.0.15 mm (a.times.b). The distance c between the nozzle and the 
cooling roll was 0.15 mm. The cooling roll was made of a Cu alloy. 
Then, after the vacuum chamber had been evacuated to 5.times.10.sup.-5 
Torr, the molten alloy composition was ejected under a pressure of 0.025 
kg/cm.sup.2 through the nozzle onto the peripheral surface of the cooling 
roll operated under a controlled peripheral speed of 42 m/sec, to quench 
the molten metal and obtain a thin strip. 
The thin strip thus obtained measured 5 mm in width and 7.8 .mu.m in 
thickness and possessed an amorphous state. 
Then, the thin strip was wound in a toroidal core with 12 mm outermost 
diameter and 8 mm inner diameter). This core was subjected to a heat 
treatment in an atmosphere of N.sub.2 at 570.degree. C. for two hours. 
The core after the heat treatment was measured for magnetic core loss, and 
frequency characteristic of initial permeability by the use of a U 
function meter and a LCR meter. 
FIG. 6 shows the frequency characteristic of the initial permeability in an 
excited magnetic field of 2 mOe. For comparison, the results similarly 
obtained of a thin Fe-based microcrystalline alloy strip using the same 
alloy composition and possessing a thickness of 18 .mu.m are shown in the 
diagram. 
It is clearly noted from the diagram that the effect of plate thickness on 
permeability appeared conspicuously at a high frequency exceeding 100 kHz. 
The test results on core loss were as shown in Table 2 below, indicating 
the extreme decrease in plate thickness was evidently effective. 
TABLE 2 
______________________________________ 
Plate Core loss (mW/cc) 
thickness 
f = 100kHz 
f = 1MHz 
(.mu.m) 
B = 2 kG B = 1 kG 
______________________________________ 
Example 4 7.8 80 1350 
Comparative Experiment 4 
18 350 4600 
______________________________________ 
The thin Fe-based microcrystalline alloy strips of Example 4 and 
Comparative Experiment 4 were subjected to a bending test. This test was 
carried out by disposing a given thin heat-treated Fe-based 
microcrystalline alloy strip in a bent state between tow plates, narrowing 
the distance between the two plates until the bent sample broke, measuring 
the distance, l, between the two plates at the time of breakage of the 
sample, and calculating the following formula using the found distance 
##EQU1## 
(wherein t stands for the average thickness of the sample thin strip by 
gravimetric method based on 
##EQU2## 
The value resulting from the calculation was .epsilon.=5.times.10.sup.-3 
for the thin Fe-based microcrystalline alloy strip of Example 4 and 
.epsilon.=2.times.10.sup.-4 for that of Comparative Experiment 4. This 
fact clearly indicates that the resistance to embrittlement was improved 
by the extreme decrease of plate thickness. .epsilon. is not less than 
1.times.10.sup.-3, preferably not less than 3.times.10.sup.-3. 
EXAMPLE 5 
Thin amorphous strips were produced by following the procedure of Example 
4, excepting varying alloy compositions indicated in Table 3 were used 
instead and the conditions of production were varied as indicated in Table 
3. Then, the thin strips were wound to produce cores and the cores were 
heat-treated similarly. 
TABLE 3 
__________________________________________________________________________ 
Degree Peripher- Plate 
of Orifice size 
al speed Injection 
thick- 
Iron Permea- 
Value of 
vacuum 
of nozzle 
of roll 
Gap pressure 
ness 
loss *1 
bilith 
brittleness 
Alloy composition 
(Torr) 
(a .times. bmm) 
(m/sec) 
(cmm) 
(kg/cm.sup.2) 
(.mu.m) 
(mW/cc) 
*2 (.epsilon.) 
__________________________________________________________________________ 
Ex- 
Sample 
Fe .sub.73 Cu.sub.1 Nb.sub.4 Si.sub.14 B.sub.8 
8 .times. 10.sup.-5 
15 .times. 0.12 
38 0.15 
0.025 
6.9 1240 1200 4.8 .times. 
10.sup.-3 
am- 
1 
ple 
Sample 
Fe.sub.72 Cu.sub.1.5 Mo.sub.3 Si.sub.13.5 B.sub.10 
1 .times. 10.sup.-4 
20 .times. 0.15 
35 0.12 
0.020 
6.0 1120 1280 8.5 .times. 
10.sup.-3 
5 2 
Sample 
Fe.sub.74 Cu.sub.2 Ta.sub.4 Si.sub.14 B.sub.6 
5 .times. 10.sup.-5 
20 .times. 0.10 
40 0.15 
0.020 
5.4 1030 1350 7.8 .times. 
10.sup.-3 
3 
Sample 
Fe.sub.72 Cu.sub.1 W.sub.3 Si.sub.13 B.sub. 6 
2 .times. 10.sup.-4 
20 .times. 0.12 
32 0.10 
0.015 
6.0 1150 1250 6.0 .times. 
10.sup.-3 
4 
Sample 
Fe.sub.75 Cu.sub.1 Ti.sub.5 Si.sub.13 B.sub.6 
5 .times. 10.sup.-5 
20 .times. 0.10 
40 0.15 
0.020 
5.9 1100 1300 6.0 .times. 
10.sup.-3 
5 
Sample 
Fe.sub.71 Cu.sub.2 Zr.sub.5 Si.sub.14 B.sub.8 
5 .times. 10.sup.-5 
20 .times. 0.10 
40 0.15 
0.020 
6.2 1100 1280 6.5 .times. 
10.sup.-3 
6 
Sample 
Fe.sub.72 Cu.sub.0.8 Hf.sub.4 Si.sub.14 B.sub.9.2 
8 .times. 10.sup.-5 
15 .times. 0.12 
38 0.15 
0.025 
7.1 1300 1190 4.9 .times. 
10.sup.-3 
7 
__________________________________________________________________________ 
*1: Under the conditions of 1MHz and 0.1T 
*2: Under the conditons of 10MHz 
It is clearly noted form Table 3 that thin Fe-based microcrystalline alloy 
strips of fine quality measuring not more than 10 .mu.m in thickness and 
containing few pinholes were obtained by first preparing thin strips of an 
amorphous state under the conditions invariably falling in the ranges 
specified by this invention and then heat-treating these thin amorphous 
strips. It is also clear that they satisfied the requirements for low core 
loss and high permeability in the high frequency range.