GB598201A - A process of producing insulative coatings of silicon steel sheets - Google Patents

Abstract

598,201. Insulative coatings. ENCINDERING & METALS CO., Ltd. (American Rolling Mill Co.). July 2, 1945, No. 16793. [Class 56] A tightly adherent insulative coating is produced on the surfaces of silicon steel for power transformers and like magnetic purposes, by preferentially oxidizing silicon in the sheet stock at and adjacent the surfaces thereof to form silica, coating the surfaces of said stock with a magnesia bearing substance and in a heat treatment causing the silica so formed to migrate to the surface of the steel and there to combine with the magnesia to form an adherent layer of glassy substance, the thickness of which is dependent upon the quantity of silica therein primarily derived from the silicon steel. Silicon steel, of say 0.05-4.00 per cent silicon, is treated according to the method of Specification 558,558, [Group XXII], to an open anneal at temperatures of 1350-1650‹ F. in a hydrogen bearing atmosphere containing 2-35 per cent by volume of water vapour to decarbonize the steel and to oxidize the silicon without oxidizing the iron. A two-minute heat treatment at a temperature of 1500-1600‹ F. and a hydrogen bearing atmosphere of 15 per cent water vapour may be used for 29-gauge material in which the carbon content has thereby been reduced to as low as 0.01 per cent and the desired silica developed at the surface. The sheets or coils are then coated with a magnesia slurry comprising an aqueous suspension of a mixture of light and " non-hydrating " magnesia, which has been found to provide an even coat, and box annealed. The charge is slowly heated to 1100‹ F. to drive off the chemically combined water from the magnesium hydroxide, a hydrogen bearing atmosphere being introduced at 850‹ F. to sweep out the water vapour and prevent oxidation of iron. The temperature is gradually raised to 2100‹ F. during which period the developed silica segregates at the surface of the steel and is maintained at this temperature until the silica combines with the magnesia to form a glass. The furnace is slowly cooled to 1000‹ F., the sheet removed and excess magnesia, which prevented sticking during the heating process, brushed off. Specifications 527,525, [Group XXII], 531,599, 545,938, [Group XXII], and 560,667 also are referred to.