12 of the steel sheet. In this case, the distance D2 between the groove 20 and the thermal shock portion 30 is defined as the distance D2 between an imaginary line and the thermal shock portion 30 based on the imaginary line projected onto the other surface thereof by making the groove 20 symmetrical to the thickness center of the steel sheet. Except that the thermal shock portion 30 is formed on the other surface 12, since it is the same as described in the embodiment of the present invention, a duplicate description will be omitted.

FIG. 1 to FIG. 3 illustrate the case in which one thermal shock portion 30 is formed within the distance D1 between the grooves, that is, D3/D1 is about 1, but the present invention is not limited thereto.

For example, as shown in FIG. 4, a case in which D3/D1 is less than 1 is also possible. More specifically, the distance D3 between the thermal shock portions may be 0.2 to 0.5 times the distance D1 between the grooves. In this case, as described above, the average values of the distances D1 and D2 may satisfy the aforementioned range. More specifically, the distance D2 between the groove 20 and the thermal shock portion 30 may be 0.2 to 0.4 times the distance D1 between the grooves 20. For example, when four thermal shock portions 30 are formed within the distance D1 between the grooves (D3/D1 is 0.25), and each distance D2 is 0.25 times, 0.5 times, 0.25 times, and 0 times D1, the calculated average D2 is 0.25 times D1.

In addition, on the contrary, a case in which D3/D1 is larger than 1 is possible. More specifically, the distance D3 between the thermal shock portions may be 2 to 2.8 times the distance D1 between the grooves.

As shown in FIG. 1, the groove 20 means a portion of a surface of the steel sheet removed by laser irradiation. In FIG. 1, the shape of the groove 20 is illustrated as a wedge shape, but this is merely an example, and the groove may be formed in various shapes such as a quadrangular shape, a trapezoidal shape, a U-shape, a semi-circular shape, and a W-shape.

FIG. 5 illustrates a schematic view of the groove 20 according to the embodiment of the present invention. A depth (H_G) of groove 20 may be 3 to 5% of the thickness of the steel sheet. When the depth (H_G) of the groove is too shallow, it is difficult to obtain a proper iron loss improvement effect. When the depth (H_G) of the groove is too deep, texture characteristics of the steel sheet 10 are significantly changed due to strong laser irradiation, or a large amount of hill-up and spatter are formed, so that magnetic properties may be deteriorated. Therefore, it is possible to control the depth of the groove 20 in the above-described range.

As shown in FIG. 5, a solidified alloy layer 40 formed at a lower portion of the groove 20 may be included, and the solidified alloy layer 40 may have a thickness of 0.1 μm to 3 μm. By properly controlling the thickness of the solidified alloy layer 40, only spike domains are formed in the grooves after a final insulation coating without affecting secondary recrystallization formation. When the thickness of the solidified alloy layer 40 is too thick, it affects the recrystallization during the first recrystallization, so the Goss integration degree of the secondary recrystallization after the secondary recrystallization annealing is decreased, so even if the secondary recrystallized steel sheet is irradiated with a laser, iron loss improvement effect characteristics may not be secured. The solidified alloy layer contains recrystallization with an average grain diameter of 1 to 10 μm, and is distinguished from other parts of the steel sheet.

As shown in FIG. 5, the insulation coating layer 50 may be formed on the groove 20.

FIG. 2 and FIG. 4 illustrate that the length direction and the rolling direction (RD direction) of the groove 20 and the thermal shock portion 30 form a right angle, but the present invention are not limited thereto. For example, the length direction and the rolling direction of the groove 20 and the thermal shock portion 30 may form an angle of 75 to 88°. When forming the above-described angle, it may contribute to improving the iron loss of the grain-oriented electrical steel sheet.

FIG. 2 and FIG. 4 illustrate that the groove 20 and the thermal shock portion 30 are continuously formed along the rolling vertical direction (TD direction), but the present invention is not limited thereto. For example, the groove 20 and the thermal shock portion 30 may be intermittently formed at 2 to 10 along the rolling vertical direction (TD direction) of the steel sheet. As described above, when they are intermittently formed, they may contribute to improving the iron loss of the grain-oriented electrical steel sheet.

Unlike the groove 20, the thermal shock portion 30 is indistinguishable from other surfaces of the steel sheet. The thermal shock portion 30 is a portion that is etched in a form of a groove when immersed in hydrochloric acid at a concentration of 5% or more for 10 minutes or more, and may be distinguished from other surface portions of the steel sheet. Alternatively, the thermal shock portion 30 may be distinguished in that it has a difference in Vickers hardness (HV) of 10 to 120 from the surface of the steel sheet in which the groove 20 or the thermal shock portion 30 is not formed. In this case, the hardness measurement method thereof may measure the hardness of the thermal shock portion and a portion not subjected to thermal shock with micro-hardness by a nanoindenter. That is, it means nano Vickers hardness (HV).

The magnetic domain refining method of the grain-oriented electrical steel sheet according to the embodiment of the present invention includes: preparing the grain-oriented electrical steel sheet 10: forming the groove 20 by irradiating a laser on one surface or both surfaces of the grain-oriented electrical steel sheet 10 in a direction crossing the rolling direction (RD direction); and forming the thermal shock portion 30 by irradiating a laser on one surface or both surfaces of the grain-oriented electrical steel sheet 10 in a direction crossing the rolling direction (RD direction).

First, the grain-oriented electrical steel sheet 10 is prepared. The magnetic domain refining method according to the embodiment of the present invention has features in shapes of the groove 20 and the thermal shock portion 30, thus the grain-oriented electrical steel sheet for the magnetic domain refining may be used without limitation. Particularly, an effect of the present invention is realized regardless of an alloy composition of the grain-oriented electrical steel sheet. Therefore, a detailed description of the alloy composition of the grain-oriented electrical steel sheet will be omitted.

In the embodiment of the present invention, as the grain-oriented electrical steel sheet, a grain-oriented electrical steel sheet rolled with a predetermined thickness through hot rolling and cold rolling from a slab, may be used.

Next, one surface 11 of the grain-oriented electrical steel sheet is irradiated with a laser in a direction crossing the rolling direction (RD direction) to form the groove 20.

In this case, energy density (Ed) of the laser may be 0.5 to 2 J/mm². When the energy density is too small, the groove 20 having an appropriate depth is not formed, and thus it is difficult to obtain an effect of ameliorating iron loss. In contrast, even when the energy density is too large, the groove 20 having too large a depth is formed, and thus it is difficult to obtain an effect of ameliorating iron loss.

FIG. 6 shows a schematic diagram of a shape of a laser beam. In the forming of the groove, a beam length L of the laser in a rolling vertical direction (TD direction) of the steel sheet may be 50 to 750 μm. When the beam length L in the rolling vertical direction (TD direction) is too short, a time during which the laser is irradiated is too short, so that an appropriate groove may not be formed, and it is difficult to obtain an effect of ameliorating iron loss. In contrast, when the beam length L in the rolling vertical direction (TD direction) is too long, a time during which the laser is irradiated is too long, so that the groove 20 having too large a depth is formed, and it is difficult to obtain an effect of ameliorating iron loss.

A beam width W of the laser in the rolling direction (RD direction) of the steel sheet may be 10 to 30 μm. When the beam width W is too short or long, a width of the groove 20 may be short or long, and thus an appropriate magnetic domain refining effect may not be obtained.

FIG. 6 illustrates that the shape of the beam is elliptical, but the shape thereof may be spherical or rectangular, and is not limited thereto.

As a laser, a laser with 1 kW to 100 kW power may be used, and a laser of a Gaussian Mode, a Single Mode, and a Fundamental Gaussian Mode may be used. It is a TEMoo-shaped beam, and an M2 value may have a value ranging from 1.0 to 1.2.

Next, the thermal shock portion 30 is formed, by irradiating a laser on one surface or both surfaces of the grain-oriented electrical steel sheet 10 in a direction crossing the rolling direction (RD direction).

The forming of the groove 20 and the forming of the thermal shock portion 30 described above may be performed without limitation before and after the time. Specifically, after the forming of the groove 20, the thermal shock portion 30 may be formed. In addition, after the forming of the thermal shock portion 30, the groove 20 may be formed. In addition, it is possible to simultaneously form the groove 20 and the thermal shock portion 30.

In the forming of the thermal shock portion 30, the energy density (Ed) of the laser may be 0.02 to 0.2 J/mm². When the energy density is too small, an appropriate thermal shock portion 30 is not formed, and thus it is difficult to obtain an effect of ameliorating iron loss. In contrast, when the energy density is too large, a surface of the steel sheet is damaged, and thus it is difficult to obtain an effect of ameliorating iron loss.

In the forming of the thermal shock portion 30, the beam length L of the laser in the rolling vertical direction (TD direction) of the steel sheet may be 1000 to 15,000 μm, and the beam width W of the laser in the rolling direction (RD direction) of the steel sheet may be 80 to 300 μm.

Since the shapes of the groove 20 and the thermal shock portion 30 are the same as those described above, a redundant description thereof will be omitted.

The magnetic domain refining method of the grain-oriented electrical steel sheet according to the embodiment of the present invention may further include forming an insulation coating layer. The forming of the insulation coating layer may be included after the preparing of the grain-oriented electrical steel sheet, after the forming of the groove, or after the forming of the thermal shock portion. More specifically, it may be included after the forming of the groove. When the insulation coating layer is formed after the forming of the groove, there is an advantage in that the insulating coating may be performed only once. After the insulation coating layer is formed, the forming of the thermal shock portion may be performed. In the case of the thermal shock portion, since damage is not applied to the insulation coating layer, damage to the insulation coating layer is minimized, thereby maximizing corrosion resistance.

A method of forming the insulation coating layer may be used without particular limitation, and for example, the insulation coating layer may be formed by applying an insulation coating solution containing a phosphate. It is preferable to use a coating solution containing colloidal silica and a metal phosphate as the insulating coating solution. In this case, the metal phosphate may be Al phosphate, Mg phosphate, or a combination thereof, and a content of Al, Mg, or a combination may be 15 wt % or more with respect to a weight of the insulating coating solution.

Hereinafter, the present invention will be described in more detail through examples. However, the examples are only for illustrating the present invention, and the present invention is not limited thereto.

Experimental Example 1: Distance of Groove and Thermal Shock Portion

The grain-oriented electrical steel sheet cold-rolled with a thickness of 0.30 mm was prepared. This electrical steel sheet was irradiated with a 1.0 kW Gaussian mode continuous wave laser to form 86° angled grooves with the RD direction. A width W of the laser beam was 20 μm, and a length L of the laser beam was 600 μm. Energy density of the laser was 1.5 J/mm², and a depth of the groove was 12 μm.

The grooves were formed at the distance D1 between the grooves shown in Table 1 below, and the insulation film was formed.

Then, a 1.0 kW Gaussian mode continuous wave laser was irradiated on the electrical steel sheet to form the thermal shock portion. The width W of the laser beam was 200 μm, and the length L of the laser beam was 10,000 μm. The energy density of the laser was 0.16 J/mm².

The thermal shock portions were formed with the distance D2 between the groove and the thermal shock portion and the distance D3 between the thermal shock portions summarized in Table 1 below, and these are summarized in Table 1.

In Table 1 below, the iron loss amelioration rate and the magnetic flux density deterioration rate are shown. The iron loss amelioration rate was calculated as (W₁−W₂)/W₁by measuring iron loss W₁of the electric steel sheet after the groove was formed by irradiating the laser and iron loss W₂of the electric steel sheet after the thermal shock portion was formed by irradiating the laser. The magnetic flux density deterioration rate was calculated as (W₁−W₂)/W₁by measuring a magnetic flux density B₁of the electric steel sheet after the groove was formed by irradiating the laser and a magnetic flux density B