Process for producing grain oriented electrical silicon steel sheet containing aluminium

A process for producing a grain oriented electrical silicon steel sheet containing aluminium and having an extremely high magnetic flux density, which process comprises secondary recrystallization annealing of a silicon sheet containing aluminium, which has been cold rolled and, then, decarburization annealed, in a reducing atmosphere, and which process is characterized in that in said secondary recrystallization annealing operation, during the time of elevating the temperature of said reducing atmosphere to a level within a range of from 850.degree. to 950.degree. C., the content of nitrogen in said reducing atmosphere is limited to such an extent that the partial pressure of nitrogen is 20% or less based on the entire pressure of said reducing atmosphere, and then, during the time of elevating the temperature of said reducing atmosphere from a level at which the secondary recrystallization of said steel sheet is started to a level at which the secondary recrystallization of said steel sheet is completed, the content of nitrogen in said reducing atmosphere is maintained at such a level that the partial pressure of nitrogen is at least 3% based on the entire pressure of said reducing atmosphere.