Patent ID: 7981223

Claim:
A method for producing an ultra-high magnetic flux density grain-oriented electrical steel sheet excellent in iron loss at high magnetic flux density and film properties, wherein a grain-oriented electrical hot-rolled steel sheet containing, in mass, not more than 0.15% C, 2 to 7% Si, 0.02 to 0.30% Mn, one or both of S and Se in an amount from 0.001 to 0.040% in total, 0.010 to 0.065% acid-soluble Al, 0.0030 to 0.0150% N and 0.0005 to 0.05% Bi as basic components, with the balance consisting of Fe and unavoidable impurities, is subjected to the process of: optionally annealing; cold rolling once or more or cold rolling twice or more with intermediate annealing interposed in between; decarburization annealing; thereafter applying and drying an annealing separator; and finish annealing, characterized by subjecting the steel sheet cold rolled to the final thickness to a heat treatment prior to decarburization annealing, said heat treatment comprising (i) heating to a temperature of 700° C. or higher within 10 sec. or (ii) heating at a heating rate of 100° C./sec. or more, and immediately after (i) or (ii), preliminary annealing for 1 to 20 sec. at 700° C. or higher; wherein said heat treatment is performed in an atmosphere selected from the group consisting of H 2 O and an inert gas; H 2 O and H 2 ; and H 2 O, an inert gas, and H 2 , and the H 2 O partial pressure being controlled in the range from 10 −4 to 6×10 −1 in said temperature range.