Method of manufacturing grain-oriented electrical steel sheet and hot-rolled steel sheet for grain-oriented electrical steel sheet

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

• preparing a steel slab having a chemical composition containing
• C:
• 0.005 mass % to 0.085 mass %,
• Si:
• 2.00 mass % to 4.50 mass %,
• Mn:
• 0.03 mass % to 1.00 mass %,
• sol.Al:
• 0.008 mass % or more and less than 0.030 mass %, and
• N:
• 0.004 mass % to 0.009 mass %, and
• further containing either or both of S:
• 0.0005 mass % to 0.02 mass % and Se:
• 0.0005 mass % to 0.02 mass %, with the balance being Fe and inevitable impurities,
• subjecting the steel slab to slab heating to a temperature of higher than a γ-phase precipitation temperature and 1380° C. or lower, the γ-phase precipitation temperature being determined by equilibrium calculation,
• next, subjecting the steel slab to rough rolling including at least two passes of rolling at a temperature of (temperature at which γ-phase fraction reaches its maximum −20° C.) or higher with an introduced sheet thickness true strain Ft of 0.50 or more for each pass to obtain a rough-rolled sheet, the temperature at which γ-phase fraction reaches its maximum being determined by equilibrium calculation,
• next, subjecting the rough-rolled sheet to finish rolling where a rolling finish temperature is 900° C. or higher to obtain a hot-rolled sheet,
• next, cooling the hot-rolled sheet for 1 second or longer at a cooling rate of 70° C./s or higher within 2 seconds after an end of the finish rolling,
• coiling the hot-rolled sheet obtained after cooling at a coiling temperature of 600° C. or lower,
• next, subjecting the hot-rolled sheet to hot-rolled sheet annealing for soaking at a soaking temperature of 750° C. or higher and (1080−5Y) ° C. or lower to obtain a hot-rolled and annealed sheet, where Y (%) is a recrystallization ratio of a sheet thickness central layer of the hot-rolled sheet obtained after coiling and Y is 12% or higher and 66% or less,
• next, subjecting the hot-rolled and annealed sheet to cold rolling once, or twice or more with intermediate annealing performed therebetween, to obtain a cold-rolled sheet with a final sheet thickness,
• next, subjecting the cold-rolled sheet to primary recrystallization annealing to obtain a primary recrystallization annealed sheet, and
• next, subjecting the primary recrystallization annealed sheet to secondary recrystallization annealing to obtain a grain-oriented electrical steel sheet, wherein
• the sheet thickness true strain gt is calculated by the following equation (1)
• εt=−ln (sheet thickness after rolling/sheet thickness before rolling) (1).