Patent ID: 7833360

Claim:
A method of producing a grain-oriented electrical steel sheet having excellent magnetic properties comprising the steps of: heating a steel slab consisting essentially of, in mass %, C: 0.025 to 0.09%, Si: 2.5 to 4.0%, acid-soluble Al: 0. 022 to 0.033%, N: 0.003 to 0.006%, 0.008 to 0.018% of S equivalent (Seq) with Seq =S+0.405Se, Mn: 0.03 to 0.10%, Ti≦0.005%, and a balance of Fe and unavoidable impurities, to a temperature of 1280° C. or more; hot-rolling the heated steel slab into a hot-rolled steel strip; optionally conducting hot-rolled steel strip annealing; cold rolling the hot-rolled steel strip to a final sheet thickness by a plurality of rolling passes with intermediate annealing or heat-treating the cold rolled steel strip one or more times before final cold-rolling and having a final cold-rolling reduction ratio of 83% to 92%; decarburization-annealing the cold rolled steel strip combined with primary recrystallization, wherein PH 2 O/PH 2 in the first half part of the decarburization annealing is controlled to 0.30 to 0.70 with soaking at a temperature of 810 to 890° C. for 60 to 200 seconds and PH 2 O/PH 2 , in the last half part of the decarburization annealing is controlled to 0.20 or less, thereby making the circular equivalent average grain diameter of the primary recrystallization grains 7 μm to less than 18μm; and controlling the oxygen concentration (So) contained in the decarburized steel strip to 450 to 700 ppm, where So is calculated based on a steel strip having a thickness of 0.30 mm, nitriding the decarburized steel strip as it travels in a mixture of hydrogen, nitrogen and ammonia until the total nitrogen content in the nitrided steel strip after nitriding is at 0.013 to 0.024%; coating an annealing separator composed of MgO onto the nitrided steel strip; and secondary recrystallization annealing the coated steel strip in a coiled form such that, when the temperature at a hottest coil outer periphery point is between room temperature and 950° C., PH 2 O/PH 2 is controlled to 0.01 to 0.15 in a hydrogen atmosphere containing 25 to 75% of nitrogen, or when the temperature of the hottest coil outer periphery point exceeds 950° C. during secondary recrystallization annealing PH 2 O/PH 2 is controlled to less than 0.01; where So is the numerical value obtained by multiplying the actual oxygen analysis value S (ppm) by t /0.30, namely, So (ppm) =S×t /0.30, and where t (mm) is the actual steel strip thickness.