Image Number 2 for United States Patent #4708680.
A color picture tube comprises a shadow mask which is formed to, oppose at a small gap therefrom, a phosphor screen formed on the inner surface of a panel. The shadow mask comprises a sheet of a nickel-containing iron alloy which contains 0.1% by weight or less of manganese and having an austenite grain number of 7 or less both within and at surfaces of the sheet, the austenite grain number being defined by JIS G 0551 of the Japanese Industrial Standards. The iron alloy sheet is annealed in a vacuum of 10.sup.-1 Torr or less at a temperature of 1,000.degree. C. or higher prior to formation into the shadow mask.