for a time of about 1 min this anneal makes it possible to obtain a fine grain of a size between 8 astm and 11 astm which is also necessary for the quality of chemical perforation finally the sheet is planished under tension planishing under tension causes a small plastic deformation of the sheet which has the effect of slightly degrading the permeability and the coercive field of the metal but this planishing is essential in order to obtain perfect planarity in the sheet necessary for the manufacture of shadow masks the sheet thus treated has a yield stress of 350 mpa a tensile strength of 650 mpa a coercive field h c of about 0 1 a cm and a saturation induction b s greater than 0 7 tesla it is important to note that when the sheet is subjected to a tension of approximately 200 mpa the coercive field remains unchanged at about 0 1 a cm with the alloys according to the prior art when the softened sheet is deformed by the operation of planishing under tension and then subjected to stresses the magnetic properties are degraded more markedly than with the alloy according to the invention as a result the final coercive field is approximately three times higher and the permeability three times lower than with the alloy according to the invention by way of example sheets were manufactured with five alloys according to the invention identified as a b c d and e and two sheets.