Grain-oriented electrical steel sheet and manufacturing method thereof

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

forming an oxide layer at a surface of a steel strip;

nitriding the steel strip;

after forming the oxide layer and nitriding the steel strip, coating the steel strip with an annealing separating agent containing MgO; and

after coating the steel strip, forming a forsterite based glass coating film using the oxide layer at the surface of the steel strip,

wherein the step of forming the forsterite based glass coating film comprises:

heating the coated steel strip by starting at a first temperature of 850° C. or less and increasing the first temperature to a second temperature of 1000° C. to 1100° C. in a mixed gas atmosphere consisting essentially of H 2 gas and N 2 gas in which a rate of N 2 gas is 20 volume % or more; and

next switching the atmosphere into H 2 gas atmosphere at the second temperature of 1000° C. to 1100° C.,

wherein an oxygen potential P (H 2 O)/P (H 2 ) is set to be 0.05 to 0.3 when the temperature is 850° C. or less during the heating in the mixed gas atmosphere.

2. The manufacturing method of a grain-oriented electrical steel sheet according to claim 1 , further comprising:

making primary recrystallization occur in the steel strip during the time period of forming the oxide layer; and

making secondary recrystallization occur in the steel strip during the time period of forming the forsterite based glass coating film.