Method of producing grain-oriented electrical steel sheet very excellent in magnetic properties

Abstract

The invention provides a method of producing a grain-oriented electrical steel sheet of the complete solid solution nitrided type that is good in glass film formation and excellent in magnetic properties, which method comprises: C: 0.025 to 0.09%, hot-rolling the steel slab containing Si: 2.5 to 4.0% and acid-soluble Al into a hot-rolled steel strip; controlling the rate at which N contained in the hot-rolled steel strip is precipitated as AlN to a precipitation rate of 20% or less; conducting hot-rolled strip annealing and cold rolling conducting decarburization-annealing combined with primary recrystallization by during the former part of the process in an atmosphere whose PHO/PHis 0.30 to 0.70 and then during the latter part thereof in an atmosphere whose PHO/PHis 0.20 or less, thereby making the circular equivalent average grain diameter of the primary recrystallization grains 7 μm to less than 18 μm; nitriding the strip as it travels in a mixed gas of hydrogen, nitrogen and ammonia; controlling the steel strip oxygen concentration before secondary recrystallization annealing calculated based on strip thickness of 0.30 mm (oxygen content: So) to 450 ppm to 700 ppm inclusive; applying a coat of annealing separator; and then conducting secondary recrystallization annealing in an atmosphere that, while the temperature at the hottest coil outer periphery point is between room temperature and 950° C., is controlled to a nitrogen atmosphere containing oxygen: 25 to 75% wherein the balance is hydrogen and PHO/PHis 0.01 to 0.15.