We Claim: [Claim 1]
A method for refining magnetic domains of a grain-oriented electrical steel sheet, comprising:
a step of preparing a grain-oriented electrical steel sheet; and
a step of forming a groove by irradiating a quasi-continuous laser beam of which a duty is from 98.0 to 99.9 % on a surface of the grain-oriented electrical steel sheet,
wherein the duty is a ratio (Tb/Ta) of [a time of irradiation with an output of 10 % or more of a maximum output (Pmax)] Tb to [an output modulation cycle time] Ta in a time diagram for an output of a laser beam.
[Claim 2]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, wherein
a frequency of the quasi-continuous laser beam is from 100 Hz to 8 kHz.
[Claim 3]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, wherein
a ratio P/W of an average output P of a laser beam for a quasi-continuous laser beam width W is from 30 W/mm to 300 W/mm.
[Claim 4]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, wherein
a ratio W/L of a beam width W in a steel sheet rolling direction to a beam length L in a steel sheet width direction of the quasi-continuous laser beam is from 0.1 to 0.86.
[Claim 5]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, wherein
the laser has a TEMoo mode, and M2 of a beam quality factor is 1.0 to 1.1.

[Claim 6]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, wherein
3 to 8 grooves are intermittently formed from in the width direction of the steel sheet.
[Claim 7]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, wherein
the grooves are linear and are formed at an angle from 82° to 98° with respect to a rolling direction of the electrical steel sheet.
[Claim 8]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, wherein
a depth D of the groove is formed from 3 % to 8 % of the thickness of the steel sheet.
[Claim 9]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 1, further comprising,
after the step of forming the groove:
a step of forming an oxide layer on the surface of the steel sheet through decarburization annealing or nitride annealing; and
a step of coating an annealing separator on the surface of the steel sheet formed with the oxide layer and forming non-metal oxide layer on the surface of the steel sheet through high temperature annealing.
[Claim 10]
The method for refining magnetic domains of the grain-oriented electrical steel sheet of claim 9, further comprising,
after the step of forming the non-metal oxide layer,
a step of forming an insulating coating layer on the non-metal oxide layer.