Method of manufacturing grain-oriented electrical steel sheet

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

heating a silicon steel slab at 1280° C. or below, the silicon steel slab containing, in % by mass, Si: 0.8% to 7%, and acid-soluble Al: 0.01% to 0.065%, with a C content of 0.085% or less, a N content of 0.012% or less, a Mn content of 1% or less, and a S equivalent Seq., defined by “Seq.=[S]+0.406×[Se]” where [S] being S content (%) and [Se] being Se content (%), of 0.015% or less, a Cu content of 0.4% or less, and the balance of Fe and unavoidable impurities;

hot rolling the heated silicon steel slab so as to obtain a hot-rolled steel strip;

annealing the hot-rolled steel strip so as to obtain an annealed steel strip;

cold rolling the annealed steel strip so as to obtain a cold-rolled steel strip;

decarburization annealing the cold-rolled steel strip so as to obtain a decarburization-annealed steel strip wherein primary recrystallization occurs during the decarburization annealing;

coating an annealing separating agent on the decarburization-annealed steel strip; and

finish annealing the decarburization-annealed steel strip so as to cause secondary recrystallization, wherein

the method further comprises performing a nitriding treatment in which a N content of the decarburization-annealed steel strip is increased between start of the decarburization annealing and occurrence of the secondary recrystallization in the finish annealing,

the hot rolling the heated silicon steel slab comprises:

finish rolling with a finish temperature of 950° C. or below; and

starting cooling within 2 seconds after completion of the finish rolling, and coiling at 700° C. or below,

a heating rate of the hot-rolled steel strip within the temperature range from 800° C. to 1000° C. in the annealing the hot-rolled steel strip is 5° C./sec or above, and

a cooling rate over a duration from the completion of the finish rolling up to a start of the coiling is 10° C./sec or above and 16° C./sec or below.

2. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 , wherein a cumulative reduction in the finish rolling is 93% or above.

3. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 , wherein a cumulative reduction in the last three passes in the finish rolling is 40% or above.

4. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2 , wherein a cumulative reduction in the last three passes in the finish rolling is 40% or above.

5. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 , wherein the silicon steel slab further contains Cu: 0.05% to 0.4% by mass.

6. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2 , wherein the silicon steel slab further contains Cu: 0.05% to 0.4% by mass.

7. The method of manufacturing a grain-oriented electrical steel sheet according to claim 3 , wherein the silicon steel slab further contains Cu: 0.05% to 0.4% by mass.

8. The method of manufacturing a grain-oriented electrical steel sheet according to claim 4 , wherein the silicon steel slab further contains Cu: 0.05% to 0.4% by mass.

9. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 , wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

10. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2 wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

11. The method of manufacturing a grain-oriented electrical steel sheet according to claim 3 wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

12. The method of manufacturing a grain-oriented electrical steel sheet according to claim 4 wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

13. The method of manufacturing a grain-oriented electrical steel sheet according to claim 5 wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

14. The method of manufacturing a grain-oriented electrical steel sheet according to claim 6 wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

15. The method of manufacturing a grain-oriented electrical steel sheet according to claim 7 wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

16. The method of manufacturing a grain-oriented electrical steel sheet according to claim 8 wherein the silicon steel slab further contains, in % by mass, at least one selected from the group consisting of Cr: 0.3% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and Bi: 0.01% or less.

17. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 , wherein the cooling rate is 10° C./sec or above and 14° C./sec or below.