Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-01-04
2002-07-23
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S610000, C148S609000
Reexamination Certificate
active
06423160
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a stainless steel plate for a shadow mask on which an etching process can be performed excellently and warp does not occur easily and to a method for producing the same.
BACKGROUND ART
The main components constituting a color cathode ray tube of a television receiver include an electron gun, a screen for imaging an electron beam, and a shadow mask as an electrode for selecting colors. In general, the shadow mask uses a thin metal plate of a thickness of 0.3 mm or less on which numerous micro-holes are provided regularly and precisely.
Hitherto, as a material of the metal thin plate for a shadow mask, a low carbon aluminum killed steel (hereinafter low carbon steel will be referred to) has been used.
However, in this material, a long time of irradiation with electron beams due to a continuous use causes a thermal expansion, thus distorting the micro holes provided on the plate. As a result, the misalignment of colors, called a doming phenomenon, occurs so that electron beams passing through the micro-holes are mislocated from the predetermined phosphor dots.
In particular, recently, since a large size and high quality of color television, or high accuracy of personal computer displays are demanded, the above-mentioned doming phenomenon becomes a large problem.
Therefore, for such applications of use, Fe—Ni invar alloy (hereinafter, invar alloy will be referred to), which has a small thermal expansion of about {fraction (1/10)} of a common steel, has been used widely.
However, since the invar alloy is an expensive metal material, it is not appropriate from the economical viewpoint.
On the other hand, recently, a flat television in which an image appears on a display screen apparently and is recognized visually has been given attention.
In this method, since a shadow mask is incorporated into a cathode ray tube so that a shadow mask is held with tension applied, the deformation of the shadow mask due to the thermal expansion can be prevented. Therefore, in the material having a coefficient of thermal expansion larger than that of the conventional invar alloy, the doming phenomenon does not easily occur.
However, on the contrary, since high tension is applied to the shadow mask itself, a metal material with high strength is required.
When the shadow mask is incorporated into the cathode ray tube, the shadow mask is subjected to a heating process of about 500° C. with tension applied. Therefore, the shadow mask is required to be produced of a material that is not deformed easily at high temperature.
Furthermore, since a low carbon steel or invar alloy, which has been used conventionally, is poor in corrosion resistance and easily rusts, such materials have to be stored generally in a state where they are coated with a rust-preventive agent. Therefore, a material for a shadow mask that does not form rust easily and has an excellent corrosion resistance even during storage has been highly demanded.
Moreover, JP63-255340A proposes an Fe-based material including 1.0 to 4.0% of Cu (hereinafter, component rate is expressed by %, and % means weight % unless otherwise noted) as a material for a flat tension shadow mask having a high proof strength so that deformation does not occur easily at the time of fabrication or in use and sufficient elastic stretchability so that plastic deformation does not occur due to the thermal distortion in use.
However, although this metal material has 0.2% proof strength of 50 kgf/mm
2
(490 MPa) or more, the coefficient of thermal expansion is substantially the same as that of low carbon steel. Therefore, this material cannot prevent the doming phenomenon sufficiently.
Furthermore, this material has a corrosion resistance substantially the same as that of low carbon steel or invar alloy, and likewise requires coating with rust-preventive agent during the storage.
However, in order to provide micro-holes on a metal thin plate for a shadow mask, it is common to employ a photo-etching process utilizing a corrosion melting phenomenon of the metal. The photo-etching process is carried out by:
(a) degreasing and washing a metal thin plate to form a photosensitive photoresist film on the surface of the metal thin plate and thermosetting a predetermined pattern;
(b) then developing this pattern into the intended form of photoresist patterns;
(c) spraying a solution of ferric chloride on the surface of the metal thin plate on which the photoresist patterns are developed and melting an exposed metal part so as to provide micro-holes; and
(d) finally removing the photoresist film.
Thus, the intended shadow mask can be obtained. However, in the process in which the metal thin plate is subjected to corrosion melting by the etching process, as shown in a cross section in
FIG. 1
, corrosion in the side direction, called side etching (S), simultaneously proceeds in addition to the corrosion in the depth (D) direction. In
FIG. 1
, reference numeral
1
denotes a metal thin plate,
2
denotes a photoresist film, and
3
denotes an etched hole.
Herein, a value obtained by dividing the etching depth (D) by side etching (S) is called an etching factor.
Namely, in the schematic view of the etched cross section of
FIG. 1
, the etching factor (EF) is represented by the following formula 1:
EF=D/S=D/[
(
W−M
)/2] (formula 1)
wherein
M: width of pattern of the photoresist film
W: width of groove after etching process
S: side etching
D: depth after etching process.
In order to provide micro-holes as on a shadow mask by the photo-etching process, the above-mentioned side etching should be as little as possible. Therefore, it is desirable that a metal material has a large etching factor (EF).
Furthermore, if there is a large amount of inclusions in the steel, when the etching process is performed, the neighborhood of the inclusion is nonuniformly dissolved, thus making the porous shape irregular. Therefore, when such a metal is used, it is difficult to provide micro-holes as on a shadow mask. Therefore, it is a necessary condition as a material for a shadow mask that a material includes as few inclusions as possible.
A metal thin plate that is a material for a shadow mask is generally produced by forming a material metal into a plate material, and cold rolling and annealing of the plate material. Since the annealing state may be insufficient in mechanical strength, it is common to perform temper rolling.
Furthermore, the flatness is poor in the metal plate that is subjected to the temper rolling, uniform tension cannot be applied to the plate and the plate is wrinkled. In such a case, occasionally, in order to correct the plate shape, bending and restoring are done repeatedly so as to carry out the shape correction (tension level controller) of the plate.
However, in the metal plate that is subjected to the cold rolling or shape correction as mentioned above, although the plate appears flat, warp occurs as the removal of the plate thickness from one side of the plate by etching process (half etching) proceeds.
In particular, in the metal plate after its shape are corrected, although the flatness is improved as compared with the plate which no processing is performed after cold rolling, the warp may be larger at the time of etching process.
Namely, micro-holes provided on the shadow mask is designed to have a small apertured portion (small hole
4
) at the side of the electron gun (the side where electron beams enter), and a large apertured portion (small hole
5
) at the side of the phosphor screen (the side where electron beams are emitted), so that the electron beams are introduced into the predetermined phosphor screen exactly. However, when micro-holes are provided on the metal plate for a shadow mask produced by a cold rolling such as a temper rolling or a shape correction in accordance with a usual method of etching process, warp tends to occur disadvantageously.
If the shadow mask warps, the disadvantage in working occurs, for example, bending in handling etc. easily occurs during handli
Adachi Kazuhiko
Aoki Masahiro
Arimoto Nozomu
Kita Hayato
Tsuge Shinji
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P,C,
Yee Deborah
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