Patent ID: 6858095

Claim:
In a method for producing a thick grain-oriented electrical steel sheet with excellent properties, the method comprising: preparing a slab consisting essentially of, by weight, 0.025-0.075% of C, 2.5-4.5% of Si, 0.025-0.035% of acid soluble Al, 0.0060-0.0086% of N, 0.070-0.161% of Mn, 0.005-0.029% of S, one or more elements selected from the group consisting of Se, Sb, Cu, Nb, Cr, Sn, Ti and Bi, and the balance being iron and unavoidable impurities; heating the slab to a temperature not higher than 1,300° C., hot rolling the slab to a hot-rolled sheet, optionally annealing the hot-rolled sheet, cold rolling the hot rolled sheet to a cold-rolled sheet by a reduction ratio of not less than 80% by using a one stage cold rolling or two or more stages of cold rolling with intermediate annealing, said cold rolling providing the sheet with a final thickness of 0.36-1.00 mm, decarburization annealing the cold-rolled sheet for decarburization of the sheet at a temperature of 700-1,000° C., treating the cold-rolled sheet for nitriding by using NH 3 gas, and final annealing, setting C-content to not greater than 0.0050% by weight after decarburization annealing of the sheet, setting total N-content to 0.010-0.027% by weight after nitriding treatment of the sheet in NH 3 gas following said decarburization annealing, coating the nitrided sheet with an annealing separation agent consisting essentially of MgO and subjecting the coated sheet to final annealing as a coil having a coil inside diameter within a range of 200-1500 mm to obtain grains of a selected diameter, its grains of a diameter exceeding 5 mm having a crystal orientation deviation (Δθ) of 0.2-4 degrees in relation to that at the grain center, and a post-final-annealing SF value of less 0.80, where SF is defined as SF =(grain area×4π)/(grain boundary length) 2 , whereby a magnetic flux density B 8 of the sheet is not less than 1.83 T and core loss W 17/50 (w/kg) of the sheet is not more than 3.3×t+0.35.