Title of Invention

"A STEEL SHEET FOR A SHADOW MASK"

Abstract

The object of the present invention is to provide a low cost steel sheet for a shadow mask and a shadow mask which has a stable etchability. [Means of Solution ] A steel sheet for a shadow mask consists essentially of not less than 0. 015wt% of Cu and the balance being of Fe. At the same time, the said steel sheet is made of aluminum killed steel containing a more amount of Cu than that of S by not less than 0.02wt% so as to further improve etchability. The said steel sheet is obtained by being subjected to hot rolling, descaling, annealing, primary cold-rolling, decarburization-annealing and finishing cold-rolling, accordingly. A shadow mask made of the said steel sheet has an excellent etchability. A color picture tube incorporating this shadow mask can attain a clear, high quality picture.

Full Text

[Detailed description of the Invention] {Technological Field] The present invention relates to a steel sheet for a shadow mask for use in a color picture tube, a shadow mask, and a color picture tube incorporating the shadow mask. More particularly, it relates to a steel sheet for a shadow mask made of aluminum killed steel having excellent etchability in forming dot holes of the shadow mask and also excellent press-formability capable of inhibiting stretcher strains during press-forming the steel sheet into a flat mask, and it relates to the shadow mask made thereof and the color picture tube incorporating the shadow mask. [Problem to be solved by the Invention] The present invention relates to a steel sheet for shadow mask, using Al-killed steel containing Cu of 0.015 1.0 wt%, S of 0.025 wt% or less, Cu -S> 0.02 wt% and the balance being Fe, and H-factor value defined by (H depth) / (side-H) being 2.89 or more. As a material for a shadow mask for use in a color picture tube ( hereinafter referred to as CRT ) a thin steel sheet made of an invar alloy or aluminum killed steel is used. The material for a shadow mask made of aluminum killed steel is produced by the steps of hot-rolling aluminum killed steel, pickling same for descaling, and subjecting same to annealing, subsequently cold-rolling, decarburization-annealing, and finishing cold-rolling. The thus obtained thin aluminum killed steel sheet is pert orated to form dot holes by the use of a photoetching method so that a flat mask can be produced. The flat mask is subjected to the steps of annealing, press-forming into a desired shape, and blackening, and then it is incorporated into a CRT. A shadow mask serves as an anode for generating electron beam, with a voltage supplied, and as an iris diaphragm contraction for allowing the electron beam that have passed through the dot holes to be projected onto dots of fluorescent coating spread over a front panel. As for the latter role, the dot holes directly affect image visibility, irregular color, or irregular luminance of a picture displayed on the CRT, and therefore they require extremely high dimensional accuracy. A dot hole comprises a small hole diameter portion (hereinafter referred to as a small dot) provided on a surface of a thin plate-like mask sheet facing a cathodic side, a large hole diameter portion (hereinafter referred to as a large dot) provided on another surface of the mask sheet facing the front panel, and a Joint hole portion (Break Through Hole, hereinafter referred to as Br Th hole) at which the small dot and the large dot meet each other. The function of the iris diaphragm for the electron beam is substantially effected by this Br Th hole. An etching process of a steel sheet for a shadow mask is carried out as follows. First, a steel sheet is subjected to a pretreatment such as degreasing and has both surfaces coated with a resist and dried. Subsequently, a dotrholes master pattern is exposed and developed to be printed on the steel sheet so that only portions of the steel sheet corresponding to the dot holes of a shadow mask can be exposed. Then, ferric chloride solution is sprayed for etching to perforate the portions of the steel sheet corresponding to the dot holes. Gennerally,a steel sheet for shadow mask has a thickness of 100 to 250 Mm. The above mentioned Br Th hole is positioned near to the large dot from the small dot by 15 to 25 Mm in the thickness direction of the steel sheet, and its diameter is approximately 120 Mm for a high definition shadow mask, and approximately 140Mm for a normal definition shadow mask. A single sheet of shadow mask should be provided with several hundreds of thousands of Br Th holes perforated and these Br Th holes should be strictly controlled to have dimensional accuracy for their positions, diameter, roundness, and so on. Accordingly, in order to obtain Br Th holes having accurate and uniform shapes, it is indispensable not only to strictly control etching conditions such as the concentration of the ferric chloride solution in the etching process but also to improve etchability of the steel sheet for shadow mask itself. Several techniques for improving etchability of aluminum killed steel sheet has been proposed in the disclosures such as Laid-Open Japanese Patent No. Sho-58-81926, No. Hei~6~330167, and NcC Sho-63~286524] ,but no technique has concretely taught the improvement of etchability of a steel sheet for shadow mask itself. [Problem to be solved by the Invention] As described above, a technological problem of producing a CRT shadow mask consists in the improvement of etchability of a steel sheet for shadow mask. Sulfur is well known as an element included in steel which impairs etchability of aluminum killed steel sheet, and a technique in which a sulfur content in steel is reduced has been disclosed. Normally, sulfur is removed by desulfurization in iron manufacturing and steel manufacturing processes. In a steel manufacturing operation for producing aluminum killed steel for shadow mask, since sulfur reduction is given priority, the yield of molten steel is sacrificed, resulting in the raise of material cost. Therefore, a producing method in which sulfur content in steel is merely reduced is incapable of providing a low cost aluminum killed steel sheet for shadow mask with improved etchability and moderate price. Accordingly, an object of the present invention is to provide a low cost aluminum killed steel sheet for shadow mask having stable etchability. More specifically, the reduction of sulfur obstructing etchabitity, that is the above mentioned object, can be achieved by keeping the yield of molten iron in the desulfurization process maintained at a normal extent so as not to press upon material cost and adding other low cost alloy elements to the steel so that the remaining noxious sulfur can be made innoxious. {Means for solving the Problem] A steel sheet for a shadow mask as claimed in claim 1 is characterized in that the steel sheet is made of aluminum killed steel consisting essentially of not less than 0.015 wt%)of Cu and the balance being Fe. A steel sheet for a shadow mask as claimed in claim 2 is characterized in that the steel sheet is made of aluminum killed steel containing a more amount of Cu than that of S by not less than 0.02 wt%. A shadow mask as claimed in claim 3 is characterized in that the shadow mask is made of a steel sheet as claimed in claim 1 or 2. A color picture tube as claimed in claim 4 is characterized in that the color picture tube incorporates a shadow mask as claimed in claim 3. [Preferred embodiment of the Invention] According to the present invention, a steel sheet for a shadow mask is made of aluminum killed steel in which Cu is solidly and uniformly dissolved so that, the steel sheet for shadow mask attains remarkably improved etchability when sprayed with a ferric chloride solution. The effect of Cu being solidly dissolved in the steel sheet is thus; if a sulfur content in the steel is not more than 0.025 wt% that is substantially the amount of S included in normal aluminum killed steel, the solidly dissolved Cu not only compensates the impaired etchability due to sulfur but surpasses the impairment due to sulfur so that the steel sheet for shadow mask can attain more improved and stable etchability. Further, in case where the sulfur content in the steel is not more than 0.025 wt%. if a Cu addition amount is larger than the sulfur content by 0.02 wt% or more, the improvement in etchability of the steel effected by Cu addition is further distinguished. In order to quantatively evaluate the above mentioned effect, one of well known means is a method for determining etch factor. In the present invention, steel sheets were evaluated for their etchability based on this etch factor. Determining of etch factor comprises the steps of etching one side surface of a steel sheet, and calculating a ratio of etching depth to side etching. etch factor= etching depth/ side etching • • • (1) As expressed by the above equation (1), if a material has excellent etchability, it is more etched in a depth direction (thickness direction), while less etched in a side direction (surface direction) so that etch factor shows a greater value. In contrast, a material having inferior etchability is more etched in a side direction, so etch factor shows a smaller value. Accordingly, when a material having large etch factor is etched on condition that the dot holes may have a fixed diameter, the depth of the dot holes can be increased. Namely, the material having large etch factor is not only excellent in etchability for perforation but also capable of satisfying the requirement of high definition shadow mask whose pitch between dot holes is extremely narrow. In addition, etch factor affects a shape of Br Th hole which is the most important to CRT' s picture quality. The larger etch factor provides a better roundness of the Br Th hole, allowing the CRT to attain a sharp and high quality picture. The present invention is now explained in detail below. First, a producing method is explained. An aluminum killed steel sheet for a shadow mask for use in a CRT of the present invention is prepared by the step of subjecting molten steel which is produced through a normal iron manufacturing process and steel manufacturing process, or another molten steel produced from steel scraps or others through an electric induction melting process, to a ladle refining process and vacuum degassing process so as to be Al~deoxidized. The steel sheet is adjusted for its components so as to have an alloy composition, after solidification, consisting essentially of not less than 0.015 wt% of Cu and the balance being Fe and inevitable impurities, or an alloy composition consisting essentially of Cu, the amount of which is more than S content by 0.02 wt%, and the balance being Fe and inevitable impurities. A continuously casted slab which is formed by a normal continuous casting process or ingot which is formed by a normal ingot casting process is hot-rolled to produce a hot-rolled steel sheet. The hot-rolled steel sheet is subjected to descaling and cold-rolling to have a prescribed thickness, and wound up in a open coil shape so as not to come into contact with each other own surfaces, and then subjected to decarburization-annealing in a box annealing furnace. Incidentally, the annealing furnace is not limited to the box annealing furnace but if a continuous annealing furnace has the same level capacity, the latter furnace may be used. The decarburization-annealing is conducted under an oxygen-free environment at a controlled dew point with a heat history of normally (630 to 700)°C (5 to 20)hburs + (700 to 860)°C (5 to 10)hours. The steel sheet after decarburization should contain a carbon amount reduced to not more than 0.001 wt%. Subsequently, the steel sheet is finished by cold-rolling to obtain a desired thickness of a shadow mask. At the same time, surface of the steel sheet is so finished that they may attain a desirable surface roughness, considering adhesion of a resist film that is to be coated over it. The following description concerns elements to be added to the aluminum killed steel sheet for a shadow mask of a CRT of the present invention and inevitable impurities, and the reason the addition amounts or contents of those components should be limited. Copper,which solidly and uniformly dissolves into steel, remarkably effects an improvement in etchability of the steel. In particular, since it has an effect of unif ormalizing an etching rate micro- and macroscopically, etched surfaces become smoother and exhibit a finer texture. This effect depends on a Cu addition amount. As the Cu addition amount increases, surface roughness of the etched steel sheet surface, e.g. , Ra value becomes markedly small and the steel sheet surface tends to exhibit a fine texture. Accordingly, the Cu addition amount is limited to 0.015.wt% or more. A less amount of Cu than 0.015 wt% effects only little improvement in etchability of the steel sheet, so the lower limit of Cu addition amount is 0.015 wt%. More preferably it is 0.018 wt% or more, and further desirably 0.020 wt% or more. The upper limit of Cu addition amount is not specified, however, when exceeding 1.0 wt%, it becomes a main cause to deteriorate an etching solution, resulting in a lowered etching rate. Therefore, if particularly necessary, the upper limit of Cu addition amount is determined to be 1.0 wt%, more preferably 0.95 wt% or less, and further desirably 0. 90 wt% or less. Further, since the addition of Cu compensates the impaired effect of etchability of the steel sheet due to sulfur, a more amount of Cu than the sulfur content is added to the steel sheet. Concretely, copper is added to the steel sheet such that a difference between Cu addition amount and sulfur content may be larger than 0.02 wt%. The difference may be preferably 0.022 wt% or more, and 0.023 wt% or more further desirably. Remaining in a steel, sulfur causes an etched surface of the steel sheet to be roughened. Therefore, sulfur is an element that impairs etchability of the steel sheet. Further, sulfur induces brittleness of steel and deteriorates a physical property of steel, so it is desirable that a sulfur.Content is as little as possible. If necessary, it is limited to 0.030 wt% or less, more preferably 0.028 wt% or less, and further desirably 0.025 wt% or less. Other inevitable impurities include trace elements such as C, Si, Mn, P, N, Al normally contained in steel materials, and alloy elements such as Cr, Ni, Mo, Ti, V, Nb, W contained in iron scraps used for part of molten material. Carbon is decarburized during decarburization-annealing in the producing process of the aluminum killed steel sheet for shadow mask, so the C content in the steel sheet is normally 0.001 wt% or less. If necessary, a C content in the steel sheet is limited to 0.001 wt% or less. Carbon is solidly dissolved and interstitially present in steel and induces stretcher strains during press-forming of a flat mask, which makes the flat mask suffer uneven elongation. Therefore, the carbon content in the steel sheet should be strictly controlled. Manganese is added as a deoxidizer in the steel manufacturing process. Remaining in a steel, Mn has an effect on solid solution strengthening of the steel sheet. In addition, Mn is an effective component to inhibit hot brittleness of steel. Accordingly, if necessary, a Mn content in the steel is limited to a range within 0.025 to 0.35 wt%. The reason why the lower limit is determined to be 0.025 wt% is as follows In order to form MnS, which is needed for inhibiting the deterioration of physical property of the steel sheet, in case where the maximum sulfur content is 0.025 wt%, at least the same amount of Mn should be present in the steel. The reason why the upper limit is 0.035 wt% is that solid solution of it may lower the productivity of shadow mask. Silicon is reacts with an etching solution, which causes the deterioration of ferric chloride solution. Further, Si is solidly dissolved in steel, which makes steel brittle, so the less Si the better. When Si content in the steel is 0.04 wt% or more, the above mentioned embrittlement is remarkable. Accordingly, Si content in the steel is limited to not more than 0.04 wt%. Phosphorus is an element which makes steel brittle, and therefore the P content in the steel should be strictly controlled. Through a normal steel manufacturing process, a steel in which not more than 0.015 wt% of P is contained can be obtained. Accordingly, if necessary, P content in the steel is limited to 0.015 wt% or less. Nitrogen affects steel similarly to carbon, so the N content in the steel should be strictly controlled. Through the normal steel manufacturing process a steel in which not more than 0.009 wt% of N is contained can surelybe obtained. Therefore, if necessary, N content in the steel is limited to 0.009 wt% or less. Aluminum is used as a deoxidizer in the steel manufacturing process to improve the cleanliness of steel. Further, remaining in steel, Al reacts with N to form A1N, which effectively inhibits uneven elongation caused by N. However, with much more Al contained, the steel is embrittled through solid solution hardening and besides its etchability is impaired. Through the normal steel manufacturing process, a steel in which not more than 0.1 wt% of Al is contained can surely be obtained, so, if necessary, an Al content in the steel is limited to 0.1 wt% or less. Cr, Ni, Mo, Ti, V, Nb, and W are elements which are not positively added to steel but the remainder that are initially present in the scraps or others used for a molten material for aluminum killed steel sheet. These elements are solidly dissolved in steel to effect solid solution hardening, thus improving strength of the steel. Accordingly, if necessary, each element is limited to 0.5 wt% or less, and more preferably a.3 wt% or less. The following is a couple of supplementary explanation on the producing method of the aluminum killed steel sheet for the shadow mask of the present invention. The annealing of the open coil of steel sheet is conducted with the heat history of (630 to 700)*C (5 to 20)hours + (700 to 860)°C (5 to 10)hours, which is based on the following reason. The temperature of (630 to 700)° C in the first annealing step is mainly for the decarburization and the temperature of (700 to 860). C in the second annealing step is for the diffusion and dispersion of segregated elements. When the temperature is not higher than 630° C, the decarburization reaction rate becomes low, and when the temperature is not less than 860° C, the steel sheet becomes deformed so that the steel sheet surfaces are liable to stick to each other. Therefore, the upper limit temperature is determined to be 860° C. However, one step annealing is available at least in view of improving the productivity in the annealing process. When the C content in a hot-rolled steel sheet is approximately 0.04 wt%, substantially all the steel will be composed of a-Fe in the temperature range (600 to 830).C, so that it is possible to accomplish decarburization and diffusion of the segregated elements by only one step decarburization-annealing with the heat history of (600 to 830)'C (5 to 20)hours. [Experimental Examples] The present invention is described more in detail according to examples below. Aluminum killed steel sheets having chemical compositions as shown in Table 1 were produced by the steps of melting, refining, continuous casting, hot-rolling, primary cold-rolling, decarburization-annealing, and secondary cold-rolling. In the refining process, the steel was subjected to sulfur content adjusting, vacuum-degassing, and Al deoxidizing. Steel sheets after hot-rolling were each provided with a thickness of 2.0mm, which were formed into steel sheets each having a thickness of 0. 6mm by the primary cold-rolling. The conditions for decarburization-annealing were selected to be 650° C 10 hours + 750° C 6 hours. Through the secondary finishing cold-rolling, the steel sheets were rolled to have a substantially uniform thickness of 0.124~0.126mm. Among the obtained aluminum killed steel sheets as shown in Table 1, those of sample No.l~7 have sulfur and copper contents adjusted according to the limit ranges of the present invention. Comparative examples are shown by sample No. 8~11. Table 2 shows etch factor value, dispersion of them, and difference between the amounts of Cu and S represented by (Cu~S) wt% with regard to each sample shown in Table 1. The etching conditions were as follows. The hole diameter of photoresist pattern dot hole for the surfaces of aluminum killed steel sheet samples was 110Mm. As the etching solution ferric chloride solution was used, which was adjusted to have a concentration of 48.Be (Baume) and a temperature of 70° C, and sprayed with a spray pressure of 0.3MPa for a time of 100 seconds. Etch factor values were measured with regard to 300 holes/each sample to obtain the average and the dispersion (o ). [Table 1] [Table 2] The materials of sample No. 1~7 were each adjusted such that sulfur content was not more than 0.025 wt% and Cu addition amount was within a range of 0.017~0.978 wt%. Those of sample No. 8~ 11 were each provided with an alloy composition including S content and Cu amount which were out of the range according to the present invention. As shown in Table 2, all the materials having alloy compositions according to the present invention exhibit an etch factor value of higher than 2.85. The dispersions are also controlled within a range of 0.12~0.14. Clearly seen from these values, they are excellent in etchability. In the materials of sample No. 8~11, Cu amounts are each not more than 0.011 wt%, which range allows no Cu effect. In these cases, as the sulfur content is lessened, the etch factor is improved. However, even the material of sample No.11, which is aimed at decreasing a sulfur content (0.0051 wt%), exhibits an etch factor value of at most 2.80. From these results it turned out that even if a sulfur content was reduced, the improvement in etchability of the steel sheets had its limit. At the same time, reducing a sulfur content to 0.0051 wt% requires a high refining technique, and inevitably involves a lowered yield of molten steel. As a consequence, it clearly proved that means for economically and stably improving etchability of an aluminum killed steel sheet for shadow mask was to control a sulfur content at a normal extent and add an appropriate amount of copper to the steel. [Effect of the Invention] As explained above, a steel sheet as claimed in claim 1 is made of aluminum killed steel consisting essentially of not less than 0.015 wt% of Cu and the balance being Fe. Further, a steel sheet as claimed in claim 2 is made of aluminum killed steel containing a more amount of copper than that of sulfur by not less than 0.02 wt%. Therefore, these steel sheets are excellent in etchability when etched to perforate dot holes in a flat mask during a producing process of the flat mask. Further, these steel sheets are capable of satisfying the requirement of a high definition shadow mask whose pitch between dot holes is extremely narrow and small, since they have greater etch footer value and side etching at them is smaller. Furthermore, a shadow mask made of these steel sheets is provided with dot holes having Br Th hole of good quality and little dispersion. A color picture tube incorporating this shadow mask can attain a high quality picture. (Tablel) (Table Removed) Chemical compositions of samples (steel sheet) (Table2) (Table Removed) Evaluation results of the characteristics of samples WE CLAIM: 1. A steel sheet for shadow mask, using Al-killed steel containing Cu of 0.015 to 1.0 wt%, S of 0.0047 to 0.025 wt%, Cu - S> 0.02 wt% and the balance being Fe, and H-factor value being 2.89 to 3.11. 2. A shadow mask, using the steel sheet for shadow mask as claimed in claim 1. 3. A steel sheet for a shadow mask substantially as hereinbefore described with reference to the foregoing examples.

Documents:

253-del-1998-abstract.pdf

253-del-1998-claims.pdf

253-del-1998-correspondence-others.pdf

253-del-1998-correspondence-po.pdf

253-del-1998-description (complete).pdf

253-del-1998-form-1.pdf

253-del-1998-form-13.pdf

253-del-1998-form-19.pdf

253-del-1998-form-2.pdf

253-del-1998-form-3.pdf

253-del-1998-form-4.pdf

253-del-1998-form-6.pdf

253-del-1998-gpa.pdf

253-del-1998-petition-137.pdf