Method for manufacturing grain-oriented electrical steel sheet

A method for manufacturing a grain-oriented electrical steel sheet, the method comprising:

• a hot rolling step of heating and hot rolling a slab including, as a chemical composition, by mass %,
• C:
• 0.020% to 0.150%,
• Si:
• 3.00% to 4.00%,
• Mn:
• 0.01% to 0.50%,
• S:
• 0.0010% to 0.0400%,
• acid-soluble Al:
• 0.010% to 0.050%,
• N:
• 0.002% to 0.020%,
• Bi:
• 0.0000% to 0.0200%,
• P:
• 0.000% to 0.100%,
• Sn:
• 0.00% to 0.50%,
• Cu:
• 0.00% to 0.50%,
• Cr:
• 0.00% to 0.50%,
• Sb:
• 0.00% to 0.20%,
• Mo:
• 0.00% to 0.10%,
• Nb:
• 0.0000% to 0.0200%,
• B:
• 0.0000% to 0.0200%,
• Te:
• 0.0000% to 0.0200%,
• Ni:
• 0.00% to 0.20%,
• Se:
• 0.0000% to 0.0200%, and
• a balance being Fe and impurities to obtain a hot-rolled steel sheet;
• a hot-band annealing step of subjecting the hot-rolled steel sheet to hot-band annealing to obtain a hot-band annealed sheet and then immersing the hot-band annealed sheet in a pickling solution;
• a cold rolling step of subjecting the hot-band annealed sheet to cold rolling to obtain a cold-rolled steel sheet;
• a decarburization annealing step of subjecting the cold-rolled steel sheet to decarburization annealing to obtain a decarburization-annealed steel sheet;
• a separator applying step of applying an annealing separator containing MgO, Al2O3, and a chloride to a surface of the decarburization-annealed steel sheet;
• a finish annealing step of subjecting the decarburization-annealed steel sheet coated with the annealing separator to finish annealing to obtain a finish-annealed sheet;
• a surface treatment step of pickling a surface of the finish-annealed sheet to obtain a surface-treated steel sheet;
• a thermal oxidation-reduction annealing step of subjecting the surface-treated steel sheet to two-stage annealing to obtain a reduction annealed steel sheet;
• and
• a tension-applying insulating coating forming step of forming a tension-applying insulating coating containing a phosphate and silica on a surface of the reduction annealed steel sheet, wherein
• the thermal oxidation-reduction annealing step includes
• a primary temperature rising process of heating the surface-treated steel sheet after the surface treatment step to raise the temperature of the surface-treated steel sheet,
• a primary annealing process performed subsequent to the primary temperature rising process in which a steel sheet temperature is held at a constant temperature in a range of 800° C. to 900° C. for a constant holding time in a range of 10 seconds to 200 seconds in an atmosphere having a constant value in a range of an oxygen potential PH2O/PH2 of 0.10 to 10.00, and
• a secondary annealing process performed after the primary annealing process in which the steel sheet temperature is held at a constant temperature in a range of 900° C. to 1100° C. for a constant holding time in a range of 3 seconds to 100 seconds in an atmosphere where an oxygen potential PH2O/PH2 is constant and less than 0.1000 and a dew point is constant and less than-20° C.