CA1277158C - Steel foil excellent in shielding characteristics to electromagnetic waves - Google Patents

Abstract

ABTRACT
A steel foil which is suited for use as an electromagnetic wave shielding material and which has excellent shielding properties to electromagnetic waves, consists of C ? 0.010%, Mn ? 0.20%, P ? 0.020%, S ? 0.020%, Al ? 0.040%, N ? 0.0040%, remainder Fe and unavoidable impurities. The steel foil of the inven-tion has crystal grains with diameters in the range of 20 to 80 µm and has a sheet thickness of between 20 and 100 µm.

Description

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This invention relates to a steel foil which is suited for use as an electromagnetic wave shielding material, and is excellent in shielding characteristics to elec-tromagnetic waves.
Steel foils have been used in computers, etc~ for avoiding erroneous operation due to external magnetic waves, for protectiny electromagnetic recording data, and as various kinds of shielding materials for protecting agains-t sources of electro-magnetic waves.
There are known electromagnetic shielding materials which are coated with conductive paints on plastic base sheets, and such materials which are mixed with metallic powders or chips into the plastic materials.
However, the former is difficult to process into a base sheet. The base sheet is low in shielding effect because it has no electromagnetic wave shielding characteristics, and the conductive paint involves high cost. On the other hand, the latter also presents difficulties. For example, since the metallic powders or chips are mixed into the plastic materials, the powders or chips are exposed at the surfaces of the resulting products. Also, anti-shock properties are deteriorated.

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In addition, it is necessary to mix large amounts of metallic powders or chips to provide uniform shielding e~fect, thus resulting in high cost.
In view of the foregoing circumstances, there is a need for shielding materials which are lower in cost than conventional materials, have uniform shielding effect, and may be easily applied to various parts of machinery, etc.
The present invention achieves the afore-mentioned and other objects by cold rolling a steel material having a specific chemical composition and a specific range of grain sizes to obtain a steel foil of specific range of thicknesses which has excellent properties as an electromagnetic wave shielding material.
The electromagnetic steel sheet of 3~ Si has been known commercially as having excellent electromagnetic characteristics, and has been broadly used as cores of coils. Unfortunately, this kind of electromagnetic steel sheet is difficult to be cold rolled, and hence requires many intermediate treat-ments for producing steel foils, and accordingly is very expensive. Furthermore, a problem arises about making it into a steel foil.

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The inventors have developed a material which is low in cost, excellent in electromagnetic wave shielding properties and easy to process into a steel foil.
The electromagnetic wave shieldin~ proper-ties S is below, taking as an example, a tubular shielding rnaterial, and the larger is S, the better is the shielding efEect:
S (dB) - R + A + B (1) wherein d is the diameter of the crystal grain; R is the loss by reflection of energy (dB) of the electromagnetic wave when it passes through the shielding material; A is the loss by absorption of energy (dB) of the electromagnetic wave when it passes through the shielding material; and B is the inner reflection of the shielding plate (dB).
; Thus, that the loss of energy of the electromagnetic wave is large when the shielding property is good. Generally, R -~ A ~ B. R and A
are expressed as Eollows:
A = KIT ~ fG~u (2) II J ~f (3) G

wherein jU is the magnetic permeability; f is the Frequency; G is the ratio of conductivity when the conductivity of Cu is l; KI and KII are constants; and t is the thiclcness of the sheet.
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As can be seen from the above equations (1), (2) and (3), the larger is the magnetic perme-ability (lu), the larger is the quantity R + A, and the bet-ter is -the shielding effect. Also, the greater is the thickness (t), A becomes larger and the shielding effect is increased. But if the thickness (t) is too large, the workability is decreased. Thus, overly large thickness is not practical.
An important consideration is to improve the magnetic permeability (~u) to a point of increas-ing the both R and A without spoiling the workabili-ty. The ranges set forth below accomplish the goals desired.
This invention aims to improve the l magnetic permeability and other desired properties of a steel foil.
In accordance with the present invention, there is thus provided a steel foil having excellent shielding properties against electromagnetic waves and consisting of C ' 0.010~, Mn c 0.20~, p c 0.020%, S ~ 0.020~, Al c 0.040%, N ~ 0.0040~, remainder Fe and unavoidable impurities, the steel ~oil having crystal gxains with diameters in the range oE 20 to 80 ~um ancl having a sheet thickness oE between 20 and l/Um :: ' ' . . . ' :
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Features and advantages of the invention will become more readily apparent from the following description of preEerred embodiments, reference being made to the accompanylng drawings, in which:
Fig. 1 is a graph showing the relation between C content and the magnetic permeability ~u for a sample thickness of 50~um;
Fig. 2 is a graph showing the relation between grain diameter and magnetic permeability JU
for a sample thickness of 50 ~m;
Fig. 3 i5 a graph showing the relation between thickness of the steel foil and the amount of spring back; and Fig. 4 is an explanatory view of the spring back.
As seen in Fig. 1, the smaller is the C
content, the greater is the permeability )u, especial-ly the permeability is increased when the C content is less than 0.010~. Thus, in this invention, the C
content is limited to no more than 0.010~. IE greater improvement of the permeability /u is desired, a C
~ content O:e o . oos~ or less is preferred.
Since the elements Mn, P, Al, N and S
adversely influence the magnetic properties, the amounts to be used of such elements should be as small as possible, with the smaller amounts to be preferred. However, if the amounts are extremely small, special treatment would be re~uired and : . - . . .
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production costs would be high and not practical.
Thus, in this invention, the upper limits of these elemen~s have been determined to be as follows: Mn 0.20~, P ~ 0.020%, S 6 0.020%, Al ~ 0.040% and N ' 0.0040%, remainder being Fe and C as in the amounts specified hereinabove.
Fig. 2 shows the relation between the diameter d of the crystal grains and permeab:ility~u.
As seen, the larger is d, the permeability ~u is increased, and especially when d ~ 20 ~m, the perme-ability ~ is increased. Therefore, in the present invention, d ~ 20 ~m, preferably d ~ 30~um.
The magnetic permeability is preferable at grain diameters of more than 30 ~um, and at about 40 Jum magnetic permeability is saturated. This tells that if the sheet has a -thickness of 100 ~m, the upper limit of grain diameter is 80/um. (Note: Fig. 2 :~ is for thickness of 50 pm). Thus, 2 x 50 ~m = lOO~um;
and 2 x 40 ~m = 80 Jum in grain diameter.
.The larger the grain diameter, the greater the magnetic permeability until saturation. Magnetic permeability is deteriorated by even a slight strain.
Especially, if the grain diameter is more than 80% of the thickness, :Eine bending defects are caused in the foil when it is recoiled after annealing. Also, the magnetic permeability is decreased. Thus, for the range of thicknesses desired, the range oE grain sizes is li.mited to between 20 and 80~um.

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The thickness of the steel foil is between 20 and 100 ~m. If the thickness exceeds this upper limit, it would be difficult to attach the foil to machine as a simple sheet layer, such label or the like.
Fiy. 3 is a graph showing the relation between the thickness of the steel foil and the amount of spring back. The amount oE spring back is, as shown in Fig. 4, an opening amount ~ of a connecting part after the foil X has been formed as a magnetic shielding cylinder of a diameter D = 30mm and a height h = 50mm.
As seen in Fig. 3, if the thickness is more than 100 lum, spring back ~ would be larger, so that difficulties are caused in the production process, and hence thicknesses greater than 100 ~m would not be practical. Thus, the upper limit of thickness of the steel foil is 100 /um. On the other hand, if the lower limit of thickness is less -than 20 ~m, the value of A (energy absorption) would be smaller as shown in the above equation (2), and it would be difficult to maintain the desired shielding properties concurrently with improvement oE magnetic permeability alone. Hence, the lower li~it of thickness is determined to be 20 lum.
The steel material of above controlled composltion is subjec-ted to a cold rolling and an annealing to obtain a steel foil of the desired i: "f' ' ' ' : ' ' ' : ', ,. " ' ", ' " ,' , : '' ' , ' .' ' ~, .

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thickness. The reduction may be appropriately selected between 30 and 90% in relation with the diame-ter of crystal grains. Furthermore, the produced steel foil may be subjec-ted to an annealing at tem-pera-tures between 600C and 950C to provide crystal grains having the desired diameter.
'rhe electromagnetic properties of steel foils according to the invention are compared with comparative examples in the following Table:

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O Ul 10 ~ N CO ~ ~ N 1~ a~ ~1 0 ~
r-l ~ ~ r-l N U) O O r-l O U) ~ o N ~D U~ U) ~1 ~
a~ ) o u~ ~r u) U) I` ~ t~
r-i O O ~-1 0 0 0 r-i O O U) r-i O O r~
~ ~ O N - O C~ O ~ N ~D N ~I h X ~_ r l N N N ~ ) r~ N ~) N N ~1 a) h h h a) ~1 ~ -$ x x x x x r~ o ~ O C O U) O U~ O U) . ,~
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p~ 0 3 d~ lo~ = = = = = U) = ~ = = = = C) IJ~ S~
~ h ,;> ~ O = = O O u~ ~ U) = = = =: = rl ~ o U) rf) N ~ a o o o o tq ~ a ~ U h a = f ~ u 0 ~I N ~ d ' U) ~ o ~ O ~--1 N

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` ' ', ' . . ' ' ~ ' In the above Table, the steel foils according to the invention show the high permeability for the samples whlch were cold rolled between 30 to 90~, have a grain size of 20 to 80 jum, a thickness of 5 20 -to 100 ~m, and were annealed at between 600 C. to 950 C. for one hour.
With respect to usage, the steel foil may be used as a base sheet and coated with an adhesive and then used as a label, or a plurality of laminated foils may be coated with adhesive and then used as a label. I~ the foil is formed to be used as a label, it : would have the advantage of being easily attached to various parts of a machine, such as the more inacces-sible corners, the outside, the inside etc.
As mentioned above, the steel foil of the invention has high permeability so that it may be used as an electromagnetic wave shieldir.g material as it is. When it is used as a base sheet to be coated with a paint having electromagnetic wave shielding proper-ties, a higher shielding effect is obtained.

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Claims (3)

1. A steel foil having excellent shielding properties against electromagnetic waves and consist-ing of C ? 0.010%, Mn ? 0.20%, P ? 0.020%, S ?
0.020%, Al ? 0.040%, N ? 0.0040%, remainder Fe and unavoidable impurities, said steel foil having crystal grains with diameters in the range of 20 to 80 µm and having a sheet thickness of between 20 and 100 µm.

2. A steel foil according to claim 1, wherein the diameters of the crystal grains are more than 30 µm.

3. A steel foil according to claims 1 or 2, wherein the C content is 0.005% or less.