US3909311A

US3909311A - Shadow mask for use in color picture tube and method for fabricating same

Info

Publication number: US3909311A
Application number: US494905A
Authority: US
Inventors: Junichi Yamada; Etuzo Terashima; Akira Takai
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1974-08-05
Filing date: 1974-08-05
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30

Abstract

A shadow mask for use in the color picture tube and a method of fabricating the same are disclosed in which developement of the stretcher strain phenomenon is prevented by reducing the diameter of crystal grains of iron sheet material used for the shadow mask.

Description

United States Patent 11 1 Yamada et a1.

SHADOW MASK FOR USE IN COLOR PICTURE TUBE AND METHOD FOR FABRICATING SAME Inventors: Junichi Yamada; Etuzo Terashima;

Akira Takai, all of Mobara Japan Assignee: Hitachi, Ltd., Japan Filed: Aug. 5, 1974 Appl. No.: 494,905

U.S. Cl. l48/l2.l; 96/36; 148/36 Int. Cl. C21D 9/46 Field of Search 148/12 R, 12.1; 96/36,

References Cited UNITED STATES PATENTS 3.510.335 5/1970 Mcars ..148/12.1

-- ANNEALING ROLLING Prinmry E.\'aminerW. Stallard [451 Sept. 30, 1975 Frantzen 96/36.]

Labuna et al. 96/361 Attorney, Agent, or FirmCraig & Antonelli ABSTRACT A shadow mask for use in the color picture tube and a method of fabricating the same are disclosed in which developement of the stretcher strain phenomenon is prevented by reducing the diameter of crystal grains of iron sheet material used for the shadow mask.

ANNEALING -/-O SKIN PASS Mb CUTTING IRON SHEET ROLLlNG STEPS L WINDING SENSITIZER CQATING EXPOSJRE DEVELOPMENT HJRNING H-Ul'O-ETCHING STEPS L ETCHING MASK-SHAPING STEPS 4 Claims, 6 Drawing Figures PRESS- SHAPING BLACKEN [N6 MASK ASSEMBLDGE US. Patent Sept. 30,1975

ANNEALING ROLLING CUTTING WINDING SENSITIZER COATING EXPOSURE DEVELOPMENT BURNING ETCHING Sheet 1 0f 5 MASK-Sl-iAPING STEPS ANNEALING ROLLER LEVELLER PRESS-SHAPING BLACKEN l NG MASK ASSEMBLAGE FIG. 2

TENSILE sTREssa (kg/mm m 04 9 O O I I l I IO 20 3O 4O EXTENSIONAL STRAIN e (/o) US. Patent Sept. 30,1975 Sheet 3 of 5 3,909,311

F I G 3 I' I ANNEALING ROLLING ANNEALING a SKIN PASS b ROLLER I LLEVELLER I PRESS- SHAPING BLACKEN l NG EXPOSURE I I S SIEMBLI GE DEVELOPMENT BU RN I NG ETCHING CUTTING WINDING SENSI TIZER COATING PHOTO-ETCHING STEPS U.S. Patent Sept. 30,1975 Sheet 4 of 5 3,909,311

F I G 4 ANNEALING TEMPERATION (ANNEALING TIME OF IO MINUTES) s'sosboebo'rbomebo HBSINI'IN EIZIS NIVEIS) V\LISV BZIS NIVHQ "IVlSAHQ SHADOW MASK FOR USE IN COLOR PICTURE TUBE AND METHOD FOR FABRICATING SAME The present invention relates to a shadow mask for use in the television picture tube and a method of fabricating the same, or more in particular to a method of fabricating the shadow mask used in the color picture tube in which a superior picture is produced by minimizing displacement of shadow mask apertures which otherwise might lead to what is called the stretcher strain phenomenon.

The prior art to the present invention and the present invention will be described with reference to the accompanying drawings, in which:

FIG. 1 is a flow diagram showing steps of fabricating the conventional shadow mask;

FIG. 2 is a stress-strain curve for the conventional shadow mask;

FIG. 3 is a flow diagram for explaining the Steps of fabricatiang the shadow mask according to the present invention;

FIG. 4 is a diagram showing the relationship between the annealing temperature and crystal grain size;

FIG. 5 is a diagram showing the relationship between crystal grain size and strength; and

FIG. 6 is a diagram showing stressstrain curves before and after a skin pass step.

Generally, the conventional color picture tube of shadow mask type employs, of all the magnetic metals, an iron sheet as a shadow mask material for reasons of workability and cost, and the shadow mask of an iron sheet is fabricated according to the steps shown in FIG. 1.

First, a shadow mask material in the form of iron sheet belt approximately 700 mm wide and approximately 0.3 mm thick is annealed, rolled into the approximate thickness of 0.18 mm by means of a twohigh tandem rolling mill, cut into the width of about 550 mm and taken up in a roll. The rolled iron sheet belt is cut into a plurality of iron sheets each about 400 kg and transferred to the photoetching step. In the pho toetching step, each iron sheet is coated with a sensitizer on both sides and cut into a plurality of sheets again each about 50 kg. Glass reference patterns preformed with predetermined dots or slots are closely attached onto both sides of the iron sheet, so that the iron sheet is exposed to ultraviolet ray and developed with a hot-water spray. After the predetermined dotos and slots have been formed, the sheet is subjected to a burning process at about 320C for about 5 minutes to heat and harden the remaining photosensitive film. The sheet is etched by spraying a ferric chloride solution thereby to cut apertures of predetermined size. Thus a flat base sheet of the shadow mask is obtained. This base sheet is annealed for 10 minutes at 940C, smoothed with a roller lcveler and applied through a press to form predetermined curvatures and side walls. Finally, the shadow mask surface is blackened to form a layer of triferric tetraoxide for the purpose of anticorrosion.

In the above-described conventional steps of shadow mask fabrication, the process of rolling the iron sheet 0.30 mm thick into the thickness of 0.18 mm after annealing involves a rolling reduction of as large as 40 (the rolling reduction being defined as the ratio of the thickness reduction of a sheet to the original thickness thereof in a rolling process), so that crystal grains of the iron sheet as viewed from the section thereof in parallel to the direction of rolling are spindle-shaped, very fine and have an elongation of l to 2 Since the elongation of at least 3 is required for the step of pressshaping, however, the iron sheet thus rolled into the thickness of 0.18 mm will be broken as shown by curve 1 of FIG. 2, if press-shaped without any preliminary protective measure. As such a protective measure, the step of annealing is added as a preliminary step for the press-shaping. As a result of the annealing, the crystal grain diameter in the iron sheet section, as expressed in ASTM ferrite grain size number, changes to 4 or 5 as compared with 10 or 11 for the original sheet material. This represents an elongation of about 30 which eliminates the risk of the iron sheet being broken during the press-shaping step as will be seen from curve 2 in FIG. 2. Even though the larger the crystal grain diameter, the higher the annealing temperature, the annealing time of more than 10 minutes is not very effective because of the small weight of the shadow mask base sheet. In other words, in the case of crystal grain lower than 3, for example, 2 or 1 in ASTM ferrite grain size number, the diameter of the grains is so large that the surface of the shadow mask base sheet roughens for deterioration of its quality. Further, the heat required for prolonged annealing causes excessive consumption of furnace materials, thereby adversely affecting economy of cost. A low annealing temperature, on the other hand, results in less growth of grain diameter in the iron sheet section. Even though it is a known fact that a roller leveller process following an annealing process reduces the yield point extension as shown by curve 3 of FIG. 2, grains larger than 5 or 6 in ASTM ferrite grain size number, for example, those of 7 or 8, are not reduced below 2 in yield point extension even if they are applied through a roller leveller repeatedly many times, for example, 50 times. As a result, during the press-shaping process accompanied by a maximum elongation of 3 what is called the stretcher strain phenomenon presents itself conspicously in which the elongation is different at different points of the iron sheet, ranging from 1 to 3 thereby leading to the disadvantages of partial lack of uniformity of both mask aperture pitches and sizes. A shadow mask with such mask apertures also Iasks uniformity in transmission of electron beams and the resulting color picture tube is a product so low in commercial value as to have uneveness in white color and brightness. Specifically, in the event that the mask apertures are arranged at a pitch of 600 ,u, the difference of a small 2 p. in pitch causes lack of transmission uniformity. In fact, this severe rule also supplies to the mask aperture diameter. In the case of mask apertures 200 pt in diameter, for example, an error of 2 ,u. will result in lack of transmission uniformity if it occurs with several successive apertures.

In the conventional shadow mask comprising an iron sheet with crystal grains of 4 or 5 in ASTM grain size number, the shadow mask material becomes thinner by being melted on one hand and mask apertures are enlarged on the other hand during the well known postetching stop. (Especially, in the shadow mask for use in the conventional color picture tube of black matrix type or non-black matrix type, for example, postaccelerlation type, the post etching may be carried out.) Thus the strength of the shadow mask material is reduced, with the result that when a localized large beam current brightens part of the viewing screen in actuating the color picture tube incorporating such a shadow mask, shadow mask portions about 60 mm inwardly of the periphery thereof, unlike the central and peripheral portions, are often distorted, thereby causing relative displacement of the landing positions of electron beams onto the screen and phosphor materials and hence the image thereat shows an entirely different color. Specifically, when a beam current of 2 mA flows in an area within 60 mm in diameter at the portions in question, a displacement of about 140 t occurs in electron beam landing on the screen in the case of a inches black matrix color picture tube of 110 deflection type.

The object of the present invention is to provide a shadow mask for use in a color picture tube comprising an iron sheet material whose grain size is reduced without giving rise to any stretcher strain phenomenon and a method of fabricating the same.

According to one aspect of the invention, there is provided an improved method of fabricating a shadow mask for use in a color picture tube comprising the processes of rolling, photo-etching and mask-shaping, characterized in that the rolling process includes a first annealing step for annealing a shadow mask material, a rolling step for rolling the annealed material into a desired thickness, a second annealing step for annealing the rolled material, and a skin-pass step for skin-passing the material applied through the second annealing step.

On embodiment of the present invention will be described more in detail below.

In FIG. 3 showing a flow diagram for explaining the fabricating steps according to an embodiment of the invention, those sections which are identical to those shown in FIG. 1 have the same functions as those of the corresponding sections in FIG. 1; and it should be noted that the annealing step after etching must not be adopted and the roller leveller step may be eliminated from the present invention.

In embodying the present invention, as a preparatory step for the conventional press shaping step, a second annealing step and a skin-pass step as shown by a and b respectively in FIG. 3 are inserted between the step of rolling the iron sheet into thickness, say, 0.18 mm and the step of cutting it to width, say, to 543 mm. The annealing step a is accomplished at a temperature from 550 to 750C or preferably from 660C to 700C. The annealing of the iron sheet at this temperature for about 10 minutes causes the spindle-shaped crystal grains rolled into the thickness of 0.18 mm to be recrystallized and finally settle, and as will be seen from FIG. 4 showing results of an experiment conducted on the relationship between annealing temperatures and grain size, they change to 7 or 8 in ASTM grain size number. It will also be seen from FIG. 5 showing the relationship between grain size and strength that crystal grains with ASTM grain number of 7 or 8 have a high strength and the surface of the iron sheet is not uneven.

The skin pass step b, on the other hand, involves a rolling reduction of 0.5 to 5 an optimum reduction being 1.5 to 2.0 The slight rolling effected by the skin-pass process eliminates the yield point extension 4a of the stress-strain curve 4 prior to the skin-pass process as shown in FIG. 6, so that the extension becomes about 45 after the skin-pass process as shown by the stress-strain curve 5, thereby preventing any case of breakage during the press-shaping step. It is important to follow the annealing step a and skin-pass step b in this order. If the press-shaping step is entered immediately after the annealing step a, for example, the stretcher strain phenomenon is not eliminated; nor is it possible to do away with it during the roller leveller step. On the other hand, the rolling reduction in a skinpass as high as 10 makes so hard a surface of the iron sheet that there is too great an amount of spring-back after the press-shaping step making it impossible to obtain a predetermined curvature. Again, therefore, the optimum rolling reduction in skin-pass is 1.5 to 2.0

In addition to the black matrix tube of post-etching type as mentioned above, the present invention is applicable with equal effect to those black matrix tubes in which post-etching is not effected but a light source is appropriately operated to form graphite holes small in diameter by the use of a shadow mask with apertures large in diameter as well as to color picture tubes other than black matrix type. That is to say, in these addi tional fields of applications where the thickness of the shadow mask is not reduced nor aperture diameter enlarged during color picture tube fabrication steps, it is possible to achieve the same high strength of crystal grains small in size as in the present invention, thereby making a reduction in quality due to shadow mask distortion a rarity. Furthermore, in spite of the fact that the steps of fabricating color picture tubes with a shaping mask involve three and four applications thereof through the baking furnace and exhausing furnace, the size of grains in the shadow mask section remains un changed.

It will be apparent from the foregoing description that according to the present invention a shadow mask is made of an iron sheet with grains having a

size

7 or 8 in ASTM grain size number in a shadow mask section, making possible a high-quality color picture tube with stable characteristics which develops no distortion.

What is claimed is:

1. In a method of fabricating a shadow mask for use in a color picture tube comprising the processes of rolling, photo-etching and mask-shaping; the improvement of said rolling process comprising a first annealing step for annealing a shadow mask material, a rolling step for rolling said annealed material into a desired thickness, a second annealing step for annealing said rolled material, and a skin-pass step for skin-passing said material after said second annealing step.

2. The method of fabricating a shadow mask according to claim 1, in which said second annealing step is effected at a temperature in the range from 600C to 700C to achieve ASTM ferrite grain size number 7 to 8 of the crystal grains of said shadow mask material.

3. The method of fabricating a shadow mask according to claim 1, in which said second rolling step involves a rolling reduction of 1.5 to 2.0

4. A shadow mask for use in a color picture tube, made by the method of claim 1 in which the grain size in a given section of an iron sheet used as a material of said shadow mask is 7 to 8 in ASTM ferrite grain size number.

Claims

1. IN A METHOD OF FABRICATING A SHADOW MASK FOR USE IN A COLOR PICTURE TUBE COMPRISING THE PROCESSES OF ROLLING, PHOTOETCHING AND MASK-SHAPING THE IMPROVEMENT OF SAID ROLLING PROCESS COMPRISING A FIRST ANNEALING STEP FOR ANNEALING A SHADOW MASK MATERIAL, A ROLLING FOR ROLLING SAID ANNEALED MATERIAL INTO A DESIRED THICKNESS, A SECOND ANNEALING STEP FOR ANNEALING SAID ROLLED MATERIAL AND A SKIN-PASS STEP FOR SKINPASSING SAID MATERIAL AFTER SAID SECOND ANNEALING STEP.

2. The method of fabricating a shadow mask according to claim 1, in which said second annealing step is effected at a temperature in the range from 600*C to 700*C to achieve ASTM ferrite grain size number 7 to 8 of the crystal grains of said shadow mask material.

3. The method of fabricating a shadow mask according to claim 1, in which said second rolling step involves a rolling reduction of 1.5 % to 2.0 %.