Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-07-17
2002-12-31
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S330000, C420S094000, C420S098000
Reexamination Certificate
active
06500281
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to an Fe—Ni alloy material used for a shadow mask subjected to fine etching. More particularly, the present invention relates to an Fe—Ni alloy material used for a shadow mask, which enables through-holes for passing an electron beam to be formed by etching, having improved uniformity of diameter. The present invention relates to the Fe—Ni alloy material used for a shadow mask, having etched through-holes with improved uniformity of diameter.
2. Description of Related Art
Heretofore, mild steel has been generally used for the shadow mask of a CRT. However, when the CRT is continuously operated, the temperature of the shadow mask rises due to the radiation of an electron beam. As a result of thermal expansion of the shadow mask, coincidence of the fluorescent material and the irradiation point of an electron beam is not maintained, thereby resulting in color deviation. When the color image tube is operated, one third or less of the electron beam passes through the apertures, while the rest of the electron beam is irradiated and impinged on the shadow mask, elevating its temperature.
Accordingly, Fe—Ni alloy having a small coefficient of thermal expansion, referred to as “36 Alloy” has been used in recent years from the viewpoint of color deviation in the field of a shadow mask used for a CRT.
In the production process of the Fe—Ni alloy shadow-mask material, a predetermined Fe—Ni alloy is vacuum-melted, for example, in a VIM furnace or ladle-refined in LF, and then cast into an ingot. The alloy is forged and then hot-rolled into a slab. The oxide scale on the surface of the slab is removed. Cold-rolling and annealing (recrystallizing annealing) are repeated. After the final annealing, the final cold-rolling is carried out to finish the sheet to a predetermined thickness, i.e., 0.3 mm or less. Thereafter, slitting is carried out to a predetermined width. After degreasing of the so-produced material for a shadow mask, photoresist is applied on both surfaces of the material. A pattern is printed on the photoresist and then developed. The etching is then carried out with an etchant. The material is then cut into separate flat masks. The flat masks are annealed in non-oxidizing atmosphere so as to impart press formability. In the case of the pre-annealing method, the annealing is applied to the finally rolled material prior to the etching. Pressing into a spherically shape is carried out. Finally, the spherically-shaped mask is degreased and is then subjected to blackening treatment in steam or combustion-gas atmosphere to form a black oxide film on the surface. The shadow mask is produced as above.
The finally cold-rolled material, which is or has been subjected to etching for forming the through-holes for passing an electron beam, is herein collectively referred to as the material used for a shadow mask. The flat mask is, therefore, included in the material used for a shadow mask. The material, on which the through-holes have been formed, but which is not yet press-formed, is also included in the material used for a shadow mask.
The through-holes for passing an electron beam are formed in the shadow mask by means of the well known etching usually using a ferric chloride aqueous solution. Before the etching, the well known photolithography technique is applied in such a manner that the photoresist mask is delineated to form a number of apertures in the circular form having, for example, 80 &mgr;m of diameter on one of the surfaces of the alloy strip and to form a number of apertures in the circular form having, for example, 180 &mgr;m of diameter on the coincident positions of the other surface of the alloy strip. The aqueous-solution of ferric chloride in the form of spray is blown onto the alloy strip.
The shadow mask, on which minute apertures are densely arranged, is obtained by the etching mentioned above. Local variation in etching conditions results in deviation of the diameter of apertures. When such variation becomes excessive, color shift occurs in a Braun tube mounting such shadow mask. Such mask is, therefore, unacceptable. In the production of shadow masks, the yield has heretofore been lowered and hence the cost has been increased due to variation of the aperture diameter.
Various considerations have heretofore been made to improve the etching formability of through-holes. Japanese Unexamined Patent Publication No. 05-311357 is related to improvement of the material and proposes to control the texture degree of the {100} plane on the rolling plane to less than 35% and hence randomize the crystal orientation. Japanese Unexamined Patent Publication No. 5-311358 describes to limit the total length of inclusions in the rolling direction per unit area of the parallel cross-section to the rolling direction. In addition, Japanese Unexamined Patent Publication No. 7-207415 describes that the etching formability of through-holes is improved by means of limiting the Mn and S concentrations as well as the Si and C concentrations, and also by controlling the cleanliness of the oxide-based inclusions of the cross section of the material.
The present inventors carried out intensive research and discovered that the local etching failure of through-holes for passing the electron beam described below cannot be prevented by means of controlling the texture and limiting the inclusions. Excessive etching of the apertures as compared with the neighboring apertures may occur, resulting in etching failure. As a result of the failure in local etching, the diameter of through-holes for passing an electron beam varies. This etching failure discovered by an inventor is a phenomenon that, when the shadow mask, in which the through-holes for passing an electron beam has been formed by means of etching, is observed in such a manner that an observer sees the light through the mask, the vicinity of apertures appears light and shines.
FIG. 1
is an enlarged drawing of a normal aperture, while
FIG. 2
is an enlarged view of an abnormal aperture. When the wall of the normal and abnormal apertures are observed, the inclination angle of the wall is seen smaller in the abnormal aperture (
FIG. 4
) than that of the normal aperture (FIG.
3
). Because of very local etching failure around the periphery of an abnormal aperture, the aperture diameter tends to be greater than the target value.
SUMMARY OF INVENTION
It is an object of the present invention to provide an Fe—Ni alloy material, which has through-holes formed by etching, without variation of the diameter which is attributable to local etching failure of the Fe—Ni alloy material during etching to form through-holes for passing an electron beam.
The present inventors carried out intensive research to attain the object mentioned above from a novel point of view not found in the prior art, particularly the reasons for the local corrosion anomaly mentioned above. As a result, it is found that fine precipitates and inclusions present in the Fe—Ni alloy material exert great influence upon the etching of through-holes for passing the electron beam. Such local etching failure and hence the diameter variation of etched apertures are difficult occur in the Fe—Ni alloy material, in which a large number of fine precipitates and inclusions are present in the material as a whole. It was found that, when the precipitates and inclusions from 0.01 &mgr;m to 5 &mgr;m in size are present on the surface of material at a frequency of 2000 or more per mm
2
, the precipitates and inclusions are effective for suppressing the above mentioned variation.
The components of the precipitates and inclusions were identified. The identified precipitates and inclusions are nitrides such as BN, TiN, AlN and the like, oxides such as MnO, MgO, CaO, TiO, Al
2
O
3
, SiO
2
and the like, sulfides such as MnS, CaS, MgS
2
and the like, and carbides such as TiC, SiC and the like. When a sample is immersed in the acidic solution such as dilute hydrochloric acid, dilute sulfuric acid solution, and the sampl
Nippon Mining & Metals Co., Ltd.
Yee Deborah
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