Alloy sheet for shadow mask and method for manufacturing thereof

Abstract: An alloy sheet for making a shadow mask consisting essentially of 34 to 38 wt.% Ni, 0.1 wt.% or less Si, 0.003 wt.% or less B, 0.003 wt.% or less O, less than 0.002 wt.% N, 0.05 to 3 wt.% Cr and the balance being Fe and inevitable impurities; said alloy sheet after annealing before press-forming having 0.2% proof stress of 27.5 kgf/mm 2 or less; and a gathering degree of {211} plane on a surface of said alloy sheet being 16% or less. A method for manufacturing an alloy sheet for shadow mask comprising the steps of: (a) preparing a hot-rolled sheet containing Fe, Ni and Cr; (b) annealing said hot-rolled sheet in a temperature range of 910 to 990°C; (c) cold-rolling said annealed hot-rolled sheet to produce a cold-rolled sheet: (d) a final recrystallization annealing step of annealing said cold-rolled sheet subjected to the cold-rolling; (e) a finish cold-rolling step of cold-rolling the cold-rolled sheet subjected to the final recrystallization annealing at the cold-rolling reduction ratio in response to an average austenite grain size D (µm) yieleded by the final recrystallization annealing, the cold-rolling reduction ratio R (%) satisfying the equations below; 16 ≦ R≦75, 6.38D-133.9 ≦ R ≦ 6.38D-51.0 (f) a stress relief annealing step of annealing the cold-rolled sheet subjected to the finish cold rolling; (g) a softening annealing step of annealing said cold-rolled sheet subjected to the finish cold-rolling in a temperature range of 700 to less than 800 °C for 0.5 to less than 60 min. before press-forming and on conditions satisfying the equation below; T ≧ -48.1 logt + 785, where T( °C) is the temperature and t (min.) is the time of the annealing.

Description: Background of the Invention Field of the Invention The present invention relates to an alloy sheet for making a shadow mask having high press-formability and a method of manufacturing the same. Description of the Related Art A recent trend of up-grading color television to high definition TV has employed Fe-Ni alloy containing 34 to 38wt.% Ni as the alloy for making a shadow mask to suppress color-phase shift. Compared with low carbon steel which has long been used as a shadow mask material, conventional Fe-Ni alloy has a considerably lower thermal expansion coefficient. Accordingly, a shadow mask made of conventional Fe-Ni alloy raises no problem of color-phase shift coming from the thermal expansion of the shadow mask even when an electron beam heats the shadow mask. Common practice in making alloy sheets for shadow masks includes the following steps. An alloy ingot is prepared by a continuous casting process or ingot-making process. The alloy ingot is subjected to slabbing, hot-rolling, cold-rolling, and annealing to form an alloy sheet. The alloy sheet for the shadow mask is then processed usually in the following steps to form a shadow mask. (1) The alloy sheet is photo-etched to form passage-holes for the electron beam on the alloy sheet for the shadow mask. A thin alloy sheet for a shadow mask perforated by etching is hereinafter referred to as "flat mask". (2) The flat mask is subjected to annealing. (3) The annealed flat mask is pressed into the curved shape of a cathode ray tube. (4) The press-formed flat mask is assembled to form a shadow mask which is then subjected to a blackening treatment. The shadow mask which is prepared by cold-rolling, recrystallization annealing, or by further slight finishing rolling after recrystallization annealing, has greater strength than conventional low carbon steel. Accordingly, such a conventional Fe-Ni alloy is subjected to softening-annealing (annealing before press-forming) at a temperature of 800°C or more before press-forming to make the grains of which it is composed coarse. After the softening-annealing, a warm-press is applied to carry out spheroidal forming. The temperature of 800°C or more is, however, relatively high. Therefore, from the viewpoint of efficiency and economy, the development of a manufacturing method to obtain a strength as low as in a material which is softening-annealed at 800°C or more, by softening-annealing at 800°C or less has been a waited. Responding to this desire, a prior art proposal was made in JP-A-H3-267320 (the term JP-A- referred to herein signifies an unexamined Japanese patent publication). This prior art employs cold-rolling, recrystallization annealing, finish cold-rolling and softening annealing. The finish cold-rolling is conducted at a reduction ratio of 5 to 20%. The temperature of the softening annealing is below 800°C, more specifically at 730°C for 60 min. This prior art produces a sheet having sufficiently low strength to give good press-forming performance with a 0.2% proof stress of 9.5 kgf/mm 2 (10 kgf/mm 2 or less) at 200°C. EP-A-0 552 800 discloses metallic sheets for shadow masks. The sheets are formed from an alloy which may contain 34-38% wt. Ni and 0.05% wt. or less Cr, the balance being Fe. However, the prior art does not achieve the quality required to perform satisfactory warm press-forming. Shadow masks prepared by the prior art were found to gall the die and to exhibit cracks generated at the edge of the shadow masks. Nevertheless, cathode ray tube manufacturers try to carry out softening annealing at a lower temperature and in a shorter time than the conventional level described above, aiming to improve work efficiency and economy. The target annealing time is 40 min or less, and in some cases, as short as 2 min. However, if such annealing conditions are applied to the prior art, the galling of dies during press-forming becomes severe and cracks on the shadow mask increase to raise a serious quality problem. Summary of the Invention The object of the present invention is to provide an alloy sheet for making a shadow mask having high press-formability and a method of manufacturing the same. To achieve this object, the present invention provides an alloy sheet for making a shadow mask as given in claims 1 and 8. Preferred embodiments are given in the dependent claims 2-7 and 9-13. The present invention also provides a method of manufacturing an alloy sheet for a shadow mask comprising the steps of: (a) preparing a hot-rolled sheet with the composition as given in claims 1 or 8; (b) annealing said hot-rolled sheet in a temperature range of 910 to 990°C; (c) cold-rolling said annealed hot-rolled sheet to produce a cold-rolled sheet; (d) final recrystallization annealing of said cold-rolled sheet; (e) finish cold-rolling the cold-rolled and recrystallization annealed sheet at a cold-rolling reduction ratio R(%), in response to an average austenite grain size D (µm) yielded by the final recrystallization annealing, satisfying the equations below; 16 ≤ R ≤ 75 6.38D-133.9 ≤ R ≤ 6.38D-51.0 (f) stress relief annealing the sheet after subjecting it to the finish cold rolling; (g) softening annealing the cold-rolled sheet at a temperature of from 700 to less than 800°C for 0.5 to less than 60 min., before press-forming and under conditions satisfying the equation below; T ≥ - 48.1 log t + 785, wherein T(°C) is the temperature and t (min.) is the time of the annealing. The term "favorable press-formability" of the present invention means having excellent shape freezing performance, having a good fit with dies (free of galling of dies), and generating no cracks in materials during press-forming. Brief Description of the Drawings Fig. 1 is a graph showing the relation between 0.2 % proof stress after annealing before press-forming, the gathering degree of {211} planes and crack generation during press-forming according to the invention; Fig. 2 is a graph showing the relation between the gathering degree of {211} planes after annealing before press-forming, the elongation perpendicular to the rolling direction and the annealing temperature of the hot-rolled sheet according to the invention; Fig. 3 is a graph showing the relation between the average austenite grain size before finishing cold-rolling, the finish cold-rolling reduction ratio and 0.2 % proof stress after annealing before press-forming, according to the invention; Fig. 4 is a graph showing the relation between the conditions for annealing before press-forming, 0.2 % proof stress after annealing before press-forming and the gathering degree of {211} planes according to the invention. Description of the invention and its preferred Embodiments The present invention asks for a specific range of yield strengths in order to improve shape fixability during warm press-forming and to suppress crack generation in alloy sheets. The yield strength is represented by 0.2% proof stress of 27.5 kgf/mm 2 or less at ambient temperature after softening annealing before press-forming (hereinafter referred to as "annealing before press-forming"). A 0.2% proof stress of 27.5-kgf/mm 2 or less further improves the shape fixability. The gist of the present invention is as follows. (a), Growth of crystal grains is enhanced during annealing before press-forming by specifying the content of B and O. Coarsening of crystal grains causes a low yield strength. (b), Fitness to dies during press-forming is improved by specifying the content of Si and N to suppress galling of dies. (c), Generation of cracks during press-forming is suppressed by controlling the degree of {211} planes on the thin alloy sheet after annealing before press-forming. The invention is described in greater detail in the following with reasons for limiting the ranges of the chemical composition of the alloy. To prevent color-phase shift, Fe-Ni alloy sheet for shadow masks has to have an upper limit to the average thermal expansion coefficient of approximately 3.0 x 10 -6 / °C in the temperature range of 30 to 100°C. The average thermal expansion coefficient depends on the content of Ni in the alloy sheet. The Ni content which satisfies the above limitation of average thermal expansion coefficient is in a range of 34 to 38wt.%. More preferably, the Ni content which further decreases average thermal expansion coefficient is in the range of 35 to 37wt.%, and most preferably in the range of 35.5 to 36.5wt.%. Usually Fe-Ni alloy includes Co as one of the inevitable impurities. A Co content of 1 wt.% or less does not affect the characteristics. An Ni content which satisfies the above described range is also employed. The Fe-Ni-Cr alloy sheet of the present invention may include 1 wt.% or less Co. However, when the Co content is over 1 wt.% to 7 wt.%, the Ni content which satisfies the above limitation of the average thermal expansion coefficient is in the range of 28 to 38 wt.%. Consequently, the Ni content is 28 to 38 wt.% when the Co content is over 1 wt.% to 7 wt.% in the Fe-Ni-Co-Cr alloy sheet. The Co and Ni content to further improve the characteristics is in a range of 3 to 6 and 30 to 33 wt.%, respectively. As over 7 wt.% Co increases the thermal expansion coefficient, the upper limit of Co content is defined as 7 wt.%. Chromium is an element that enhances corrosion resistance, but degrades thermal expansion characteristics. The Cr content is required to be in a range that improves corrosion resistance and affords thermal expansion characteristics within the desired limits. Accordingly the Cr content is defined to be 0.05 to 3.0 wt.% or over 0.05 to 3.0 wt.%. A Cr content of 0.05 wt.% or less cannot improve the corrosion resistance, on the other hand, over 3.0 wt.% can not give the thermal expansion characteristics required in the present invention. Oxygen is one of the inevitable impurities. Increasing the content of O increases the non-metallic oxide content of the alloy, which suppresses the growth of crystal grains during annealing before press-forming, particularly when annealed below 800°C and for less than 60 min, as specified in this invention. If the content of O exceeds 0.0030%, then it considerably suppresses the growth of crystal grains, and 0.2% proof stress after annealing before press-forming exceeds 27.5 kgf/mm 2 . At the same time, the corrosion resistance deteriorates. The upper limit of the O content is not specially limited, but it is selected to 0.003% from the economy of the ingot-making process. The lower limit of the O content is not specifically limited, but it is selected to 0.001% from the economy of the ingot making process. Boron enhances the hot-workability of the alloy. An excess of B induces the segregation of B at the boundaries of recrystallized grains formed during annealing before press-forming, which inhibits the free migration of grain boundaries and results in the suppression of grain growth and causes the limits of 0.2% proof stress after annealing before press-forming to be exceeded. In particular, under the annealing conditions before press-forming which are specified in this invention, the suppression of grain growth is strong, but this does not uniformly affect all grains, so a severely mixed grain structure appears accompanied by an irregular elongation of the material during press-forming. Boron also increases the gathering degree of {211} planes after annealing, which causes cracks on the skirt of the material. A boron content above 0.0030wt.% significantly suppresses grain growth, and the 0.2% proof stress exceeds 27.5 kgf/mm 2 . Also irregular elongation during press-forming appears, and the degree of {211} planes exceeds the upper limit specified in this invention. Based on these findings, the upper limit of B content is defined as 0.0030 wt.%. Silicon is used as the deoxidizer during ingot-making of the alloy. An Si content of above 0.10 wt.% reduces the corrosion resistance and forms an oxide film of Si on the surface of alloy during annealing before press-forming. The oxide film degrades the fit between die and alloy sheet during press-forming and results in the galling of die by the alloy sheet. Consequently, the upper limit of Si content is specified as 0.10 wt.%. Less Si content improves the fit of die and alloy sheet. The lower limit of the Si content is limited to a practical value of 0.001 wt.% or more from the economy of the ingot-making process. Nitrogen is an element unavoidably entering into the alloy during the ingot-making process. A nitrogen content of more than 0.0020wt.% increases the concentration of N on the surface of alloy during annealing before press-forming. The concentration of N on the surface of the alloy degrades the fit of the die and alloy sheet and makes the alloy sheet gall the die. Consequently, the upper limit of the N content is specified as 0.0020wt.%. The lower limit of N content is limited to, the practical value of 0.0001 wt.% or more from the economy of the ingot-making process. Preferably, the composition further contains 0.0001 to 0.010 wt.% C and 0.001 to 0.50 wt.% Mn. As described above, the control of the chemical composition of the alloy and of 0.2% proof stress after annealing before press-forming specified in this invention reduces galling of the alloy to dies during press-forming and gives a superior shape fixability. However, regarding to press-forming quality, there remains the problem of crack generation in press-formed materials. To cope with this problem, the inventors studied the relation between crack generation in the material during press-forming and crystal orientation during press-forming, by changing the crystal orientation of the alloy sheet in various directions using alloy sheets having chemical composition and 0.2% proof stress in the range specified in this invention, and found that effective conditions to suppress crack generation in the alloy material are to control the gathering degree of {211} planes to be maintained at or below a specified value, as well as to control the 0.2% proof stress after annealing before press-forming to keep it at or below a specified level. Fig. 1 shows the relation between crack generation on an alloy sheet during press-forming, the gathering degree of {211} planes, and 0.2% proof stress for an alloy sheet having chemical composition specified in the present invention. The gathering degree of {211} planes is determined from the relative X-ray intensity ratio of (422) diffraction planes of the alloy sheet after annealing before press-forming divided by the sum of the relative X-ray diffraction intensity ratio of the (111), (200), (220), (311), (331), and (420) diffraction planes where the (422) diffraction plane has the equivalent factor to that of the {211} plane. Fig. 1 clearly shows that the case where 0.2% proof stress does not exceed 27.5 kgf/mm 2 and where the gathering degree of {211} planes does not exceed 16%, does not induce crack generation in the alloy sheet during press-forming, which fact indicates the effect of this invention. Based on this finding, the invention specifies 16% or less for the gathering degree of {211} planes as the condition for suppressing crack generation in the alloy sheet. The alloy sheet of the present invention is manufactured by the following processes. A hot-rolled sheet having the above described chemical composition is annealed, subjected to a process including cold-rolling, final recrystallization annealing and finish cold-rolling, followed by stress relief annealing and annealing before press-forming. The processes will be described in detail. The hot-rolled sheet has to be annealed in the specified temperature range to maintain the degree of {211} planes at 16% or less. A hotrolled sheet which satisfies the conditions of chemical components specified in the present product claims is annealed at different temperatures, subjected to a process including cold-rolling, recrystallization annealing (at 890 °C for 1 min.), finish cold-rolling (at 21% of reduction ratio), stress relief annealing and annealing before press-forming (at 750°C for 20 min) to obtain the desired alloy sheet. As a comparative example, a hot-rolled strip which had not been annealed was treated under the same conditions as above. Fig. 2 shows the relation between the gathering degree of {211} planes, elongation perpendicular to the rolling direction, and annealing temperature of the alloy sheet treated by the processes above. According to Fig. 2, the gathering degree of {211} planes was 16% or less when the annealing temperature of the hot-rolled sheet was 910 to 990°C. Consequently, this invention specifies the temperature for annealing the hot-rolled sheet in the range of 910 to 990 °C to assure a degree of {211} planes of 16% or less. To achieve the satisfactory degree of {211} planes in this invention, uniform heat treatment of the slab after slabbing is not preferable. For example, when a uniform heat treatment is carried out at 1200°C or more for 10 hours or more, the degree of {211} planes exceeds the range specified in this invention. Therefore, such a heat treatment must be avoided. The mechanism of crack generation during press-forming under the conditions above 16% of the gathering degree of {211} planes is not clear. Fig. 2 shows the trend that a high degree of {211} planes gives a low elongation perpendicular to the rolling direction. Increasing the degree of {211} planes decreases the elongation perpendicular to the rolling direction and lowers the fracture limit, presumably inducing cracks. To keep the gathering degree of {211} planes of 16% or less and to maintain the 0.2% proof stress after annealing before press-forming of 27.5 kgf/mm 2 or less, the control of the conditions of finish cold rolling (reduction ratio of finish cold-rolling ), and of the conditions of annealing before press-forming are also important. Hot-rolled alloy strips having the composition above were subjected to annealing (in the temperature range of 910 to 990 °C), cold-rolling, recrystallization annealing, finish cold-rolling, stress relief annealing and annealing before press-forming (at 750°C for 20 min.) to produce alloy sheets. The alloy sheets were tested for tensile strength to determine 0.2% proof stress (the values are shown in the parentheses in Fig. 3). Fig. 3 shows the relation between the 0.2% proof stress, reduction ratio of finish cold-rolling and average austenite grain size before finish cold-rolling. In this test, the specified austenite grain size was obtained by varying the temperature of recrystallization annealing before finish cold-rolling. A 0.2% proof stress of 27.5 kgf/mm 2 or less is obtained as shown in Fig. 3 at the reduction ratio of finish cold-rolling R (R%): [16-75%, 6.38D - 133.9 ≤ R ≤ 6.38D - 51.0], where D = austenite grain size (µm) before finish cold-rolling. In the case of R < 16% or R < [6.38D - 133.9], the conditions specified in this invention for annealing before press-forming give insufficient recrystallization, insufficient growth of recrystallized grain, and 0.2% proof stress of more than 27.5 kgf/ mm 2 , and result in an unsatisfactory alloy sheet. If R > 75% or R > 6.38D - 51.0, then the conditions specified in this invention for annealing before press-forming allow 100% recrystallization but give an excess frequency of nucleation during recrystallization, which reduces the size of the recrystallized grains. In that case, the 0.2% proof stress exceeds 27.5 kgf/mm 2 , and the alloy sheet has unsatisfactory qualities. For the above described reasons, the conditions which achieve 0.2% proof stress of 27.5 kgf/mm 2 or less by annealing before press-forming specified in this invention are specified such that R (%), the reduction ratio of finish cold-rolling, satisfies equations (1a) and (1b) described below according to the average austenite grain size before finish cold-rolling. 16 ≤ R ≤ 75 6.38D - 133.9 ≤ R ≤ 6.38D - 51.0 An adequate value for the reduction ratio of finish cold-rolling (R%) specified above in response to the austenite grain size (D µm) before finish cold-rolling produces a gathering degree of {211} planes on the surface of alloy sheet after annealing before press-forming at or below 16%. The structure control of the alloy sheet of the present invention is realized by controlling the frequency of nucleation during recrystallization, through texture control of the alloy during hot-rolled sheet annealing and controlling the reduction ratio of finish cold-rolling in response to the grain size before finish cold rolling. Fig. 3 shows that further reduction of 0.2% proof stress after annealing before press-forming is achieved by optimizing the reduction ratio of finish cold-rolling (R%). In concrete terms, by controlling the value of the reduction ratio of finish cold-rolling to satisfy equations (2a) and (2b), that is so that, the value is in region II in Fig. 3, the 0.2% proof stress can be 27.5 kgf/mm 2 or less. 21 ≤ R ≤ 70 6.38D - 122.6 ≤ R ≤ 6.38D - 65.2 Furthermore, by controlling the value of the reduction ratio to satisfy equations (3a) and (3b), that is, so that the value is in region III, the 0.2% proof stress can be 26.5 kgf/mm 2 or less. 26 ≤ R ≤ 63 6.38D - 108.0 ≤ R ≤ 6.38D - 79.3 For the above described reasons, the present invention specifies the reduction ratio of finish cold-rolling R(%) to satisfy equations (2a) and (2b) above, in response to the average austenite grain size D (µm) before finish cold-rolling to obtain 0.2% proof stress of 27.0 kgf/mm 2 or less, and specifies the reduction ratio of finish cold-rolling R(%) to satisfy equations (3a) and (3b) above, in response to the average austenite grain size D (µm) before finish cold-rolling to obtain 0.2% proof stress of 26.5 kgf/mm 2 or less. The average austenite grain size specified by the relation to reduction ratio of finish cold-rolling, R, is obtained by annealing a hot-rolled sheet followed by cold-rolling and annealing in the temperature range of 860 to 950°C for 0.5 to 2 min. Fig. 4 shows the relation between annealing temperature before press-forming (T), annealing time (t), 0.2% proof stress after annealing before press-forming and the gathering degree of {211} planes of an alloy sheet manufactured by a process including annealing hot-rolled sheets in the temperature range of 910 to 990°C, cold-rolling, recrystallization annealing, finish cold-rolling, stress relief annealing and annealing before press-forming and by controlling the conditions such as chemical composition, annealing conditions and the reduction ratio of finish cold-rolling in response to the average austenite grain size before finish cold-rolling to satisfy the specification of the present invention. As clearly shown in Fig. 4, though the hot-rolled sheet annealing conditions, austenite grain size before finish cold-rolling, and finish cold-rolling reduction ratio stay within the ranges specified in this invention and the temperature of annealing before press-forming has the relation of T < -53.8 log t + 806, satisfactory recrystallization is not achieved, 0.2% proof stress exceeds 27.5 kgf/mm 2 and the gathering degree of {211} planes exceeds 16%, such that these values do not satisfy the ranges specified in the present invention. When the temperature of annealing before press-forming, T, exceeds 800°C or when the time of annealing before press-forming, t, exceeds 60min., the gathering degree of {211} planes increases so as to be higher than 16%, which is also inadequate. Consequently, to obtain a value of 0.2% proof stress and the gathering degree of {211} planes specified in the present invention, this invention specifies the temperature of annealing before press-forming, T (°C), to be less than 800°C, and the annealing time, t, before press-forming, to be less than 60 min. and T >= -48.1 log t + 785. The annealing before press-forming of this invention may be carried before photo-etching. In that case, if the conditions for annealing before press-forming are kept within the ranges specified in this invention, then satisfactory photo-etching quality is secured. In concrete terms, the alloy that has the chemical composition and the gathering degree of the planes specified in the present invention can be etched after annealing before press-forming to obtain a good quality. As for the alloy of the prior art, there is no example that satisfies the conditions described above. Consequently, annealing before press-forming can not be conducted before photo-etching because photo-etching after annealing before press-forming following the conditions of this invention results in poor quality photo-etching. There are other methods to limit the degree of {211} planes on the thin alloy sheet after annealing before press-forming within the ranges specified in the present invention. Examples of these methods are quenching solidification and comprehensive structure control through the control of recrystallization during hot-working. Example 1 A series of ladle refinements produced alloys No 1 through No. 23 having the composition shown in Table 1 and Table 2. Alloys No. 1 through No. 13 and No. 18 through No. 23 were continuously cast into ingots. These continuously casted slabs were subjected to adjusting and hot-rolling (at 1100°C for 3 hrs) to provide hot-rolled sheets. Alloys No. 14 through No. 17 were directly cast into thin plates. These plates were hot-rolled at a 40% reduction ratio, then rolled at 700°C to provide hot-rolled sheets. These hot-rolled sheets were subjected to annealing (at 930°C), cold-rolling, recrystallization annealing, cold-rolling, recrystallization annealing (following the conditions shown in Table 5), finish cold-rolling (at a 21% reduction ratio) and stress relief annealing to provide alloy sheets having a thickness of 0.25 mm. The hot-rolled sheets were fully recrystallized by hot-rolling. The alloy sheets were etched to make flat masks, which were then treated by annealing before press-forming at 750 °C for 20min. to provide materials No. 1 through No. 23. These were press-formed to inspect the press-formability. Table 1 and Table 2 show the average austenite grain size before finish cold-rolling of each material, and Table 3 and Table 4 show the gathering degree of {211} planes, tensile properties and press-formability. The tensile properties (0.2% proof stress and elongation perpendicular to the rolling direction) and gathering degree of {211} planes was inspected after annealing before press-forming. The tensile properties were determined at room temperature. The measurement of the gathering degree of {211} planes was carried with the X-ray diffraction method described above. The corrosion resistance was inspected after unstressing annealing. As shown in Table 3 and Table 4, materials No. 1 through No. 13, which have a chemical composition, a gathering degree of {211} planes, and 0.2% proof stress within the range specified in the present invention, show excellent press-formability and corrosion resistance better than the comparative example described below. Materials No. 1 through No. 17 of the present invention that include Co also show excellent press-formability. On the contrary, materials No. 18 through No. 20 have an Si and Ni content above the upper limit of this invention and raise a problem in fit with the die. Material No. 18 has a corrosion resistance inferior to the material of the present invention. Material No. 19 has an O content above the upper limit of this invention and also a 0.2% proof stress of more than 27.5 kgf/mm 2 , the upper limit, which results in a poor shape fixability and induces crack generation. Material No. 21 is a comparative example having a B content above the upper limit of this invention, which gives a 0.2% proof stress above the upper limit of this invention, 27.5 kgf/mm 2 , reducing shape fixability. These comparative materials give a gathering degree of {211} planes above the upper limit of the present invention leading to cracking in the alloy sheet. Material No. 22 has a Cr content below the lower limit of the present invention. The average austenite grain size before finish cold-rolling of material No. 23 fails to reach the level that satisfies the reduction ratio of finish cold-rolling, which gives a 0.2% proof stress of more than 27.5 kgf/mm 2 , reducing shape fixability and inducing crack generation. The above discussion clearly shows that the Fe-Ni-Cr alloy sheets and Fe-Ni-Co-Cr alloys having high press-formability of the present invention can be prepared by adjusting the chemical composition, gathering degree of {211} planes, and 0.2% proof stress within the ranges specified in this invention. Example 2 Hot-rolled sheets of alloy Nos. 1, 9, and 14, which were used in Example 1, were employed. Annealing the hot-rolled sheets was carried out under various annealing conditions given in Table 5, and no annealing was applied to one material which is also given in the Table. They were subjected to cold-rolling, recrystallization annealing (at 890°C for 1 min.), finish cold rolling (at a 21% reduction ratio), and stress relief annealing to provide alloy sheet having a thickness of 0.25 mm. The flat masks were then treated by annealing before press-forming at 750°C for 15min. to give materials No. 24 through No. 28. The flat masks were press-formed and were tested for press-formability. Table 6 shows the annealing temperature, average austenite grain size before finish cold-rolling and gathering degree of {211} planes, Table 7 shows tensile properties and press-formability. The method for measuring the properties was the same as described above. As shown in Tables 6 and 7, materials No. 24 and No. 25 having a chemical composition and satisfying the conditions specified in the present invention have excellent press-formability. On the contrary, materials No.26 through No. 28 give a hot-rolled sheet annealing temperature above the limit of this invention, and all of these materials give the gathering degree of {211} planes above the upper limit of this invention and generate cracks in the alloy sheet during press-forming. Furthermore, material No. 28 gives a 0.2% proof stress of more than 27.2 kgf/mm 2 and raises the problem of shape fixability during press-forming. Consequently, to keep the degree of {211} planes within the range specified in this invention, it is important to carry out the hot-rolled sheet annealing within the range specified in this invention. Marerial No. Alloy No. Hot-rolled sheet annealing temperature (°C) Average austenite grain size before finish cold-rolling (µm) Gathering degree of {211} planes (%) 24 14 930 18 7 25 9 960 17 8 26 1 900 17 31 27 1 1000 18 35 28 1 17 38 Material No. Tensile properties Material for press-forming 0.2% proof stress(kgf/mm 2 ) Elongation perpendicular to the rolling direction (%) Shape fixability Fitwith die Cracking in the alloy sheet 24 27.2 42.1 O ○ ○ None 25 26.9 42.2 O ○ ○ None 26 27.4 37.5 ○ ○ Yes 27 27.5 38.1 ○ ○ Yes 28 27.7 35.12 Δ ○ Yes Example 3 Hot-rolled sheets of alloy Nos. 1, 2, 4, 6, 7, 8, 9, 11, 12, 13 and 14 which were used in Example 1 were employed. These hot-rolled sheets were subjected to a process including annealing (at 930°C), cold-rolling, recrystallization annealing (at the temperature for 1 min. shown in Table 8 and Table 9), finish cold-rolling and stress relief annealing to obtain an alloy sheet having 0.25 mm thickness. The alloy sheets were etched to make flat masks which were then subjected to annealing before press-forming at 750°C for 20 min. to obtain material Nos. 29 through No. 66. These materials were press-formed to determine the press-formability. Table 8 and Table 9 shows the annealing temperature before finish cold-rolling, average austenite grain size before finish cold-rolling, reduction ratio of finishing cold rolling and tensile properties. Table 10 and Table 11 shows the gathering degree of {211} planes and press-formability. The method for measuring properties was the same as described above. Table 8 through Table 11 show that material No. 30 through No. 35, No. 38, No. 41 through 43 and No. 47 through 66, which have chemical compositions and satisfy the conditions of hot-rolled sheet annealing and annealing before press-forming specified in the present invention and give the relation between average austenite grain size before finish cold-rolling and reduction ratio of finish cold-rolling in the regions specified in the present invention, give {211} planes of 16% or less. Of these, materials No. 30, No. 35, No. 38, No. 41, No. 47, No. 49, No. 50, No. 54, No. 60, No. 63 and No. 66 employed reduction ratios of finish cold-rolling, R, (in Region I in Fig. 3) satisfying the above equations (1a) and (1b) to give a 0.2% proof stress of 27.5 kgf/mm 2 or less. Materials No. 31, No. 33, No. 34, No. 43, No. 48, No. 52, No. 55, No. 59 and No. 65 employed reduction ratios of finish cold-rolling, R, (in Region II in Fig. 3) satisfying the above equations (2a) and (2b) to give a 0.2% proof stress of 27.0 kgf/ mm 2 or less. Materials No. 32, No. 42, No. 51, No. 53, No. 56, No. 57, No. 58, No. 61, No. 62 and No. 64 employed reduction ratios of finish cold-rolling, R, (in Region III in Fig. 3) satisfying the above equations (3a) and (3b) to give a 0.2% proof stress of 26.5 kgf/mm 2 or less. All of these materials give a 0.2% proof stress within the limits of this invention and show good press-forming qualities. Accordingly, the decrease in 0.2% proof stress proved to improve the shape fixability. Contrary to the above preferred embodiment, the relation among the average austenite grain size before finish cold-rolling, conditions of hot-rolled sheet annealing and reduction ratio of finish cold-rolling of comparative materials No. 29, No. 36, No. 37, No. 39, No. 40, No. 44, and No. 45 failed to satisfy the conditions specified in the present invention even if they satisfied the conditions of chemical composition, hot-rolled sheet annealing and annealing before press-forming specified in the present invention. They are out of the scope of this invention as regards one of the 0.2% proof stress and the degree of {211} planes or both, and they raise problems with at least one of the shape fixability and crack generation in alloy sheets during press-forming or both. Material No. 64 was treated by annealing before finish cold-rolling at 850°C for 1min. Such an annealing condition gives an austenite grain size of 10.0 µm, so the 0.2% proof stress exceeds 27.5 kgf/mm 2 even if the finish cold-rolling reduction ratio is 15%. These figures cannot provide a shape fixability during press-forming which satisfies the specifications of this invention. As discussed in detail above, even though the conditions for the chemical composition, the hot-rolled sheet annealing, and the annealing before press-forming are kept in the ranges specified in this invention, it is important to keep the austenite grain size before finish cold-rolling and the reduction ratio of finishing cold-rolling within the ranges specified in this invention to obtain satisfactory press-formability. Material No. Alloy No. Gathering degree of {211} planes (%) Press formability Shape fixability Fitwith die Cracking in the alloy sheet 29 1 15 × ○ Yes 30 1 14 ○ ○ None 31 1 9 O ○ ○ None 32 1 14 O ○ ○ None 33 1 16 O ○ ○ None 34 1 13 O ○ ○ None 35 1 5 ○ ○ None 36 1 12 × ○ Yes 37 2 13 Δ ○ Yes 38 1 15 ○ ○ None 39 1 8 × ○ Yes 40 2 21 × ○ None 41 1 8 ○ ○ None 42 1 16 O ○ ○ None 43 1 5 O ○ ○ None 44 1 9 × ○ Yes 45 1 26 × ○ Yes 46 8 20 × ○ Yes 47 2 14 ○ ○ None Material No. Alloy No. Gathering degree of {211} planes (%) Press formability Shape fixability Fitwith die Cracking in the alloy sheet 48 6 13 O ○ ○ None 49 6 10 ○ ○ None 50 6 5 ○ ○ None 51 1 3 O ○ ○ None 52 1 3 O ○ ○ None 53 1 15 O ○ ○ None 54 1 16 ○ ○ None 55 12 9 O ○ ○ None 56 14 9 O ○ ○ None 57 11 12 O ○ ○ None 58 11 13 O ○ ○ None 59 11 16 O ○ ○ None 60 9 7 ○ ○ None 61 9 13 O ○ ○ None 62 9 16 O ○ ○ None 63 13 13 ○ ○ None 64 13 15 O ○ ○ None 65 7 15 O ○ ○ None 66 4 16 ○ ○ None Example 4 Hot-rolled sheets of alloy No. 1, 4, 9, 10, 12, 14, 21 and 22 which were used in Example 1 were employed. These hot-rolled sheets were subjected to a process including annealing (at 930°C), cold-rolling, recrystallization annealing (at 890°C for 1 min.), finish cold-rolling (at a 21% reduction ratio) and stress relief annealing to obtain alloy sheets having a thickness of 0.25 mm. The alloy sheets were etched to make flat masks, which were then subjected to annealing before press-forming under the conditions shown in Table 12 to obtain material No. 67 through No. 84. These materials were press-formed to determine the press-formability. Table 12 shows average austenite grain size before finish cold-rolling, conditions of annealing before press-forming, gathering degree of {211} planes, tensile properties and press-formability. Table 12 also show the gathering degree of {211} plane and press-formability. The method for measuring properties was the same as described above. Table 12 shows that materials No. 67, No. 69, No. 70 and No. 76 through No. 84, which satisfy the conditions of chemical composition and hot-rolled sheet annealing, finish cold-rolling (reduction ratio of finish cold rolling), annealing before press-forming (temperature, time) specified in the present invention give a gathering degree of {211} planes of 16% or less. All of these materials give the 0.2% proof stress required in this invention and show high press-forming quality. Contrary to the above preferred embodiment, comparative materials of No. 72 and No. 73 were annealed before press-forming at a temperature and for a time above the upper limit of the present invention though they satisfy the conditions of chemical composition, hot-rolled sheet annealing and finish cold-rolling (reduction ratio of finish cold-rolling) specified in the present invention. They exhibited a gathering degree of {211} planes of 16% or more and cracks were generated. Comparative material No. 63 was annealed before press-forming at a temperature (T) and for a time (t), not satisfying the equation T ≥ -48.1 log t + 785. Comparative material No. 71 was annealed before press-forming for a time above the upper limit of the present invention the annealing temperature T and annealing time t not satisfying the above equation. All of these comparative materials give a 0.2% proof stress of more than 27.5 kgf/mm 2 , and they had problems with shape fixability during press-forming. The degree of {211} planes of these materials exceeded 16%, and cracks were generated in the alloy sheets. Materials No. 74 and No. 75 employed comparative alloys. Even it annealing before press-forming is carried out at 750°C for 50 min., their 0.2% proof stress values exceed 27.5 kgf/mm 2 and they have problems with shape fixability during press-forming. The gathering degree of {211} planes of these materials exceeds 16%, and cracks are generated in the alloy sheets. As described in detail above, even under the conditions where the chemical composition, hot-rolled sheet annealing and reduction ratio of finishing cold-rolling are kept in the range specified in this invention, it is important to keep the conditions of annealing before press-forming within the range specified in this invention to obtain satisfactory press-form quality. Example 5 Hot-rolled sheets of alloy No. 1 and No. 4, which were used in Example 1, were employed. These sheets were subjected to annealing (at 930°C), cold-rolling, recrystallization annealing, cold rolling, recrystallization annealing (at 890°C for 1 min.), finish cold-rolling (at a 21% reduction ratio) and stress relief annealing to obtain alloy sheets having a thickness of 0.25 mm. These alloy sheets were annealed before press-forming under the conditions shown in Table 13 to obtain material No. 85 through No. 87. The alloy sheets were etched to make flat masks. Press-forming was applied to these flat masks then the press-formability was determined. Table 13 shows the average austenite grain size, conditions for annealing before press-forming and gathering degree of {211} planes of each material. Table 14 shows the tensile properties, press-formability and etching performance. Etching performance was determined by visual observation of irregularities appearing on the etched flat masks. The measuring method for each property was the same as those described above. Table 13 and Table 14 indicate that materials No. 85 through No. 87 which satisfy the conditions for chemical composition and manufacturing specified in the present invention give favorable states without irregularities in etching, a degree of {211} planes of 16% or less, and 0.2% proof stress within the range specified in this invention. All of these materials showed excellent press-form quality. Therefore, it is important to adhere to the chemical composition and manufacturing process specified in this invention to obtain satisfactory press-formability. If these conditions are satisfied, an alloy sheet subjected to etching after the annealing before press-forming gives a flat mask having the desired etching performance free of irregularities. Material No. Alloy No. Average austenite grain size before finish cold-rolling (µm) Annealing conditions before press-forming Gathering degree of {211} planes (%) Temperature (°C) Time (min) 85 1 18 750 20 8 86 1 17 790 2 16 87 4 13 720 40 15 Material No. Tensile properties Press-formability 0.2% proof stress (kfg/mm 2 ) Elongation perpendicular to the rolling direction (%) Shape fixability Fitwith die Cracking in the alloy sheet Etching performance 85 26.9 42.6 O ○ ○ None No irregularities 86 27.5 41.3 ○ ○ None No irregularities 87 27.5 4.04 ○ ○ None No irregularities As described in detail in the Examples, alloy sheets having a gathering degree of {211} planes higher than 16% give lower elongation perpendicular to the rolling direction after annealing before press-forming than that of the invention. An increased gathering degree of {211} planes presumably decreases the elongation and induces cracks in the alloy sheets during press-forming.

Claims: Claims for the following Contracting State : GB An alloy sheet for making a shadow mask consisting of 34 to 38 wt.% of Ni, 0.001 to 0.1 wt.% of Si, 0-1 wt.% of Co, 0.003 wt. % or less of B, 0.003 wt.% or less of O, 0.0001 to 0.002 wt.% of N, 0.05 to 3 wt.% of Cr, optionally 0.0001 to 0.01 wt.% of C, optionally 0.001 to 0.50 wt.% of Mn, the balance being Fe and inevitable impurities; said alloy sheet after annealing and before press-forming having a 0.2% proof stress of 27.5 kgf/mm 2 or less; and a gathering degree of the {211} plane on a surface of said alloy sheet of 16% or less. An alloy sheet according to Claim 1, wherein said Ni content is 35 to 37 wt.%. An alloy sheet according to Claim 2, wherein said Ni content is 35.5 to 36.5 wt.%. An alloy sheet according to any of Claims 1 to 3, wherein said O content is 0.001 to 0.003 wt.%. An alloy sheet according to Claim 1, wherein said 0.2% proof stress is 27 kgf/mm 2 or less. An alloy sheet according to Claim 5, wherein said 0.2% proof stress is 26.5 kgf/mm 2 or less. An alloy sheet according to any preceding Claim including no (0 wt.%) Co. An alloy sheet for making a shadow mask consisting of 28 to 38 wt.% of Ni, 0.001 to 0.1 wt.% of Si, 0.003 wt.% or less of B, 0.003 wt.% or less of O, 0.0001 to 0.002 wt.% of N, 0.05 to 3 wt.% of Cr, more than 1 to 7 wt.% of Co, optionally 0.0001 to 0.01 wt.% of C, optionally 0.001 to 0.50 wt.% of Mn, the balance being Fe and inevitable impurities; said alloy sheet after annealing and before press-forming having a 0.2% proof stress of 27.5 kgf/mm 2 or less; and a gathering degree of {211} plane on a surface of said alloy sheet of 16% or less. An alloy sheet according to Claim 8, wherein said Ni content is 30 to 33 wt. %. An alloy sheet according to Claim 8 or Claim 9, wherein said Co content is 3 to 6 wt. %. An alloy sheet according to Claim 9 or Claim 10, wherein said O content is 0.001 to 0.003 wt.%. An alloy sheet according to Claim 8, wherein said 0.2% proof stress is 27 kgf/mm 2 or less. An alloy sheet according to Claim 12, wherein said 0.2% proof stress is 26.5 kgf/mm 2 or less. A method of manufacturing an alloy sheet for a shadow mask comprising the steps of: (a) preparing a hot-rolled sheet with the compositions defined in Claim 1 or Claim 8; (b) annealing said hot-rolled sheet in a temperature range of 910 to 990°C; (c) cold-rolling said annealed hot-rolled sheet to produce a cold-rolled sheet; (d) final recrystallization annealing of said cold-rolled sheet; (e) finish cold-rolling the cold-rolled and recrystallization annealed sheet at a cold-rolling reduction ratio R(%), in response to an average austenite grain size D (µm) yielded by the final recrystallization annealing, satisfying the equations below; 16 ≤ R ≤ 75 6.38D-133.9 ≤ R ≤ 6.38D-51.0 (f) stress relief annealing the sheet after subjecting it to the finish cold rolling; (g) softening annealing the cold-rolled sheet at a temperature of from 700 to less than 800°C for 0.5 to less than 60 min., before press-forming and under conditions satisfying the equation below; T ≥ - 48.1 log t + 785, wherein T(°C) is the temperature and t (min.) is the time of the annealing. A method according to Claim 14, wherein R(%) satisfies the equations below; 21 ≤ R ≤ 70 6.38D-122.6 ≤ R ≤ 6.38D-65.2 A method according to Claim 15, wherein R(%) satisfies the equations below; 26 ≤ R ≤ 63 6.38D-108.0 ≤ R ≤ 6.38D-79.3 A method according to Claim 14, wherein said final recrystallization annealing is performed at a temperature of from 860 to 950°C for 0.5 to 2 min. Claims for the following Contracting States : FR, DE An alloy sheet for making a shadow mask consisting of 34 to 38 wt.% of Ni, 0.001 to 0.1 wt.% of Si, 0-1 wt.% of Co, 0.003 wt. % or less of B, 0.003 wt.% or less of O, 0.0001 to 0.002 wt.% of N, more than 0.05 to 3 wt.% of Cr, optionally 0.0001 to 0.01 wt.% of C, optionally 0.001 to 0.50 wt.% of Mn, the balance being Fe and inevitable impurities; said alloy sheet after annealing and before press-forming having a 0.2% proof stress of 27.5 kgf/mm 2 or less; and a gathering degree of the {211} plane on a surface of said alloy sheet of 16% or less. An alloy sheet according to Claim 1, wherein said Ni content is 35 to 37 wt.%. An alloy sheet according to Claim 2, wherein said Ni content is 35.5 to 36.5 wt.%. An alloy sheet according to any of Claims 1 to 3, wherein said O content is 0.001 to 0.003 wt.%. An alloy sheet according to Claim 1, wherein said 0.2% proof stress is 27 kgf/mm 2 or less. An alloy sheet according to Claim 5, wherein said 0.2% proof stress is 26.5 kgf/mm 2 or less. An alloy sheet according to any preceding Claim including no (0 wt.%) Co. An alloy sheet for making a shadow mask consisting of 28 to 38 wt.% of Ni, 0.001 to 0.1 wt.% of Si, 0.003 wt.% or less of B, 0.003 wt.% or less of O, 0.0001 to 0.002 wt.% of N, more than 0.05 to 3 wt.% of Cr, more than 1 to 7 wt.% of Co, optionally 0.0001 to 0.01 wt.% of C, optionally 0.001 to 0.50 wt.% of Mn, the balance being Fe and inevitable impurities; said alloy sheet after annealing and before press-forming having a 0.2% proof stress of 27.5 kgf/mm 2 or less; and a gathering degree of {211} plane on a surface of said alloy sheet of 16% or less. An alloy sheet according to Claim 8, wherein said Ni content is 30 to 33 wt. %. An alloy sheet according to Claim 8 or Claim 9, wherein said Co content is 3 to 6 wt. %. An alloy sheet according to Claim 9 or Claim 10, wherein said O content is 0.001 to 0.003 wt.%. An alloy sheet according to Claim 8, wherein said 0.2% proof stress is 27 kgf/mm 2 or less. An alloy sheet according to Claim 12, wherein said 0.2% proof stress is 26.5 kgf/mm 2 or less. A method of manufacturing an alloy sheet for a shadow mask comprising the steps of: (a) preparing a hot-rolled sheet with the compositions defined in Claim 1 or Claim 8; (b) annealing said hot-rolled sheet in a temperature range of 910 to 990°C; (c) cold-rolling said annealed hot-rolled sheet to produce a cold-rolled sheet; (d) final recrystallization annealing of said cold-rolled sheet; (e) finish cold-rolling the cold-rolled and recrystallization annealed sheet at a cold-rolling reduction ratio R(%), in response to an average austenite grain size D (µm) yielded by the final recrystallization annealing, satisfying the equations below; 16 ≤ R ≤ 75 6.38D-133.9 ≤ R ≤ 6.38D-51.0 (f) stress relief annealing the sheet after subjecting it to the finish cold rolling; (g) softening annealing the cold-rolled sheet at a temperature of from 700 to less than 800°C for 0.5 to less than 60 min., before press-forming and under conditions satisfying the equation below; T ≥ - 48.1 log t + 785, wherein T(°C) is the temperature and t (min.) is the time of the annealing. A method according to Claim 14, wherein R(%) satisfies the equations below; 21 ≤ R ≤ 70 6.38D-122.6 ≤ R ≤ 6.38D-65.2 A method according to Claim 15, wherein R(%) satisfies the equations below; 26 ≤ R ≤ 63 6.38D-108.0 ≤ R ≤ 6.38D-79.3 A method according to Claim 14, wherein said final recrystallization annealing is performed at a temperature of from 860 to 950°C for 0.5 to 2 min.

Inventors: HIASA MICHIHITO, INOUE TADASHI, TSURU KIYOSHI, HIASA, MICHIHITO, INOUE, TADASHI, TSURU, KIYOSHI
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1993
Main classification: C22C38
IPC codes: H01J9, C22C38, C21D8