WO2009082155A1

WO2009082155A1 - Apparatus and method for refining magnetic domain of electrical steel sheet

Info

Publication number: WO2009082155A1
Application number: PCT/KR2008/007597
Authority: WO
Inventors: Sang-Yun Cha; Jae-Kwan Kim; Oh-Yeoul Kwon
Original assignee: Posco
Priority date: 2007-12-26
Filing date: 2008-12-23
Publication date: 2009-07-02
Also published as: JP2011511152A; JP5606923B2

Abstract

Disclosed herein is an apparatus for refining a magnetic domain of an electrical steel sheet and a method of refining a magnetic domain of an electrical steel sheet. The apparatus for refining a magnetic domain of an electrical steel sheet includes: a laser beam emission unit; a toroidal mirror for converting the laser beam emitted from the laser beam emission unit into a first elliptical laser beam; a switch mirror for transferring the first elliptic laser beam transferred from the toroidal mirror to a pair of scan mirrors; the pair of scan mirrors for scanning the first elliptical laser beam transferred from the switch mirror and then transferring the scanned first elliptical laser beam; a cylindrical mirror for converting the first elliptical laser beam transferred from the pair of scan mirrors into a second elliptical laser beam and then irradiating the electrical steel sheet with the second elliptical laser beam.

Description

APPARATUS AND METHOD FOR REFINING MAGNETIC DOMAIN OF ELECTRICAL STEEL SHEET

Technical Field

[1] The present invention relates to an apparatus for manufacturing a grain-oriented electrical steel sheet having low iron loss and low noise, and, more particularly, to an apparatus for refining a magnetic domain of an electrical steel sheet, a method of refining a magnetic domain of an electrical steel sheet using the apparatus, and an electrical steel sheet whose magnetic domain is refined using the method. Background Art

[2] A grain-oriented electrical steel sheet has a { 110}<001> oriented texture developed in a rolling direction. U.S. Patent No. 1,965,559 first disclosed a method of manufacturing the grain-oriented electrical steel sheet. Thereafter, the method has been improved, and concurrently new methods have been proposed. It is common in the manufacture of a grain-oriented electrical sheet that precipitates or grain boundary segregation elements functioning to inhibit the growth of primarily -recrystallized grains, called inhibitors, are used. This inhibitor serves to inhibit the growth of primarily- recrystallized grains and to grow { 110}<001> oriented crystal grains of the non-grown primarily-recrystallized grains. This phenomenon is referred to as secondary recrystal- lization. As such, the most important factor in the method of manufacturing the grain- oriented electrical steel sheet is that { 110}<001> oriented secondarily -recrystallized grains are developed in a rolling direction.

[3] However, in a real electrical steel sheet, (110) plane of each crystal grain is slightly inclined from the plane of the electrical steel sheet, and [001] direction of each crystal grain is also slightly inclined even in a rolling direction. Generally, the magnetic properties of an electrical steel sheet are greatly influenced by the inclination, and thus researchers in this field are making every effort to decrease iron loss by approximating every crystal grain to (11O)[OOl] ideal orientation. As a result, currently, an electrical steel sheet having a low iron loss in which W17/50 is nearly 0.85 watt/kg in the case where the electrical steel sheet has a thickness of 0.23 mm is being industrially produced. Here, W17/50 is iron loss at a magnetic flux density of 1.7 Tesla and a frequency of 50 Hz.

[4] However, it is limited to decrease iron loss only by approximating every crystal grain to (HO)[OOl] ideal orientation. Generally, iron loss depends on crystal grain size as well as crystal orientation. The crystal grain size in the grain-oriented electrical steel sheet is in the range of several millimeters to several centimeters in order to obtain the aforementioned (HO)[OOl] crystal orientation. Currently, this crystal grain size cannot be arbitrarily adjusted by researchers. The reason for this is that the (11O)[OOl] crystal orientation must be first obtained through a long-period heat treatment process. Therefore, researchers have developed technologies for artificially adjusting the crystal grain size. Among them, there was disclosed a method of scratching the surface of an electrical steel sheet with tools having sharp tips, such as knives, iron brushes and the like, using a final product having been secondarily recrystallized, finally insulated, tension-coated and then cured (refer to U.S Pat. No. 3,647,575).

[5] After this, as a more advanced technology for artificially adjusting the crystal grain size, there was disclosed a method of decreasing iron loss by providing a discontinuous plane such as a crystal grain boundary by irradiating the surface of an electrical steel sheet with a laser. This laser is an Nd-YAG (neodymium- yttrium-aluminum-garnet) laser, and is operated at a Q-swith mode. However, this method is problematic in that, when an electrical steel sheet is irradiated with a laser beam, a coated layer formed on the surface of the electrical steel sheet is vaporized by the laser beam, so that a coating process must be performed again, thereby decreasing productivity.

[6] Thereafter, as a laser scanning apparatus, there was disclosed a laser irradiation apparatus for reciprocating a laser beam in the direction of the width of an electrical steel sheet using a plane mirror, but this laser irradiation apparatus was not further improved any more. The reasons for this are as follows. First, one spot of the plane mirror is continuously irradiated with a laser beam, so that the plane mirror reflects a part of laser energy but absorbs the other part thereof, with the result that the temperature of the inner side of the electrical steel sheet is increased, thereby easily damaging the surface of the electrical steel sheet. That is, since thermal problems with the plane mirror were not solved, this technology was not further improved any more. Second, due to the reciprocating motion of the plane mirror, irradiated lines are formed on the electrical steel sheet in a zigzag manner, so that the intervals between the irradiated lines are continuously changed, with the result that the irradiation effect cannot be uniformly obtained. Third, since the plane mirror does not reciprocate rapidly, productivity is low.

[7] In order to solve the problems deriving from the plane mirror, a laser irradiation apparatus using a polygon mirror was proposed. In this laser irradiation apparatus, since a circular rotator provided thereon with several plane mirrors rotates, each of the plane mirrors irradiate the surface of an electrical steel sheet with a laser beam, and then another plane mirror adjacent to the plane mirror receives the laser beam and then irradiates the surface of the electrical steel sheet with the received laser beam. However, the laser irradiation apparatus using the polygon mirror is problematic in that, since the rotator has a diameter of 300 mm or more and is provided with several tens of expensive plane mirrors, noise is serious, and the price is high. Further, the laser irradiation apparatus using the polygon mirror is problematic in that, since an Nd- YAG laser operated at a Q-swith mode is used as a laser beam source, the deterioration of the coating of an electrical steel sheet cannot be prevented.

[8] Further, when a laser beam is focused into a parallel beam through a collimate lens, the parallel beam is incident onto a polygon mirror, and then the incident parallel beam is focused through an Fθ-lens and then applied onto a steel sheet. Since the focused laser beam is diffused in proportion to distance, there is a limitation in the increase of the distance between the polygon mirror and the Fθ-lens, and thus the irradiation distance of the laser beam using the polygon mirror is several tens of millimeters at most. Therefore, in the laser irradiation apparatus using the polygon mirror, since the irradiation width using the polygon mirror is several tens of millimeters, assuming that the width of the electrical steel sheet is 1000 mm, at least ten polygon mirrors and lasers corresponding to the number of the polygon mirrors are required. Further, the boundaries between the irradiation regions are dually irradiated, thus deteriorating the improvement effect of an iron loss. Disclosure of Invention Technical Problem

[9] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an apparatus for decreasing an iron loss and simultaneously reducing magnetostriction, which is a cause of noise, by irradiating the surface of a grain-oriented electrical steel sheet with a laser beam, and, more particularly, to an apparatus for refining a magnetic domain of an electrical steel sheet, in which a laser beam is applied to the width of the electrical steel sheet using two lasers and six plane mirrors, and thus the magnetic domain of the electrical steel sheet can be effectively refined, and in which a coated layer is not deteriorated, and thus the recoating of the electrical steel sheet is not required. Technical Solution

[10] In order to accomplish the above object, an aspect of the present invention provides an apparatus for refining a magnetic domain of an electrical steel sheet, including an optical system, the optical system including: a laser beam emission unit for generating a laser beam; a switch mirror for receiving the laser beam emitted from the laser beam emission unit and then alternately transferring the laser beam while changing a transfer direction; and a pair of scan mirrors for scanning the laser beam alternately transferred from the switch mirror and then alternately applying the laser beam to an electrical steel sheet. [11] In the apparatus, the pair of scan mirrors reciprocates right and left in opposite directions.

[12] Further, the switch mirror alternately transfers the laser beam to the pair of scan mirrors by sending the laser beam to each of the pair of scan mirrors only when each of the pair of scan mirrors operates in a predetermined direction and by blocking the laser beam when each of the pair of scan mirrors operates in a direction opposite to the operation direction of each of the pair of scan mirrors.

[13] In this case, the apparatus further includes: a torodial mirror provided between the laser beam emission unit and the switch mirror, the toroidal mirror serving to convert the laser beam transferred from the laser beam emission unit into an elliptical laser beam and to transfer the elliptical laser beam to the switch mirror; and a cylindrical mirror provided over the electrical steel sheet, the cylindrical mirror serving to convert the elliptical laser beam transferred from the pair of scan mirrors into a differently shaped elliptical laser beam and to apply the differently shaped elliptical laser beam to the electrical steel sheet.

[14] The apparatus is provided with a plurality of optical systems to double an irradiation range of the laser beam in a width direction of the electrical steel sheet.

[15] In the apparatus, each of the pair of scan mirrors adheres on a copper support, and the copper support is provided therein with minute tubes to allow cooling water to pass therethrough.

[16] Further, in the apparatus, nitrogen for cooling is sprayed on a surface of each of the pair of scan mirrors to cool the scan mirror and to prevent the scan mirror from being dirtied by dust.

[17] Further, in the apparatus, a carbon dioxide laser having a continuous wave mode is used as the laser to prevent a coated layer formed on the electrical steel sheet from being damaged, and a reflecting mirror is provided between the laser beam emission unit and the toroidal mirror, the reflecting mirror serving to reflect the laser beam and transfer the reflected laser beam to the toroidal mirror.

[18] Another aspect of the present invention provides a method of refining a magnetic domain of an electrical steel sheet, in which the electrical steel sheet is irradiated with a laser beam and thus scratches having no zigzag line pattern are formed on the electrical steel sheet, comprising: alternately transferring the laser beam to a pair of scan mirrors reciprocating right and left; and irradiating the electrical steel sheet with the laser beam by alternately moving the pair of scan mirrors in a scan direction.

[19] The method of refining a magnetic domain of an electrical steel sheet further includes: converting the laser beam into an elliptical laser beam to allow a long axis of the elliptical laser beam to be paralleled to an irradiation direction of the laser beam and thus to improve an irradiation rate of the laser beam. [20] In the method, the converting of the laser beam into an elliptical laser beam is performed two or more times to make a shape of the spot of the laser beam more minute.

[21] In this case, the converting of the laser beam into an elliptical laser beam is performed by transferring the laser beam to a curved toroidal mirror or a curved cylindrical mirror.

[22] A further aspect of the present invention provides an electrical steel sheet, coated with an insulation film, whose magnetic domain is refined, on which a plurality of scratches is formed by a laser beam in a width direction of the electrical steel sheet, wherein the electrical steel sheet is formed thereon with continuous linear scratches having a length of 300 mm or more, and the insulation film is not damaged by the scratches.

[23] In this case, the magnetic domain of the electrical steel sheet is refined at a steel sheet moving velocity of 20 mpm or more. Further, the electrical steel sheet is provided with the scratches having no zigzag line pattern. Further, the electrical steel sheet has a width of 1000 mm or more, and is formed thereon with one to four scratches in the width direction thereof.

[24] A still further aspect of the present invention provides an electrical steel sheet, manufactured using the method.

Advantageous Effects

[25] The apparatus for refining a magnetic domain of a grain-oriented electrical steel sheet according to the present invention is advantageous in that an iron loss is decreased 10% or more and a coated layer is not deteriorated. Further, the apparatus for refining a magnetic domain of a grain-oriented electrical steel sheet according to the present invention is advantageous in that, unlike a conventional large- sized polygon mirror type, laser beams are continuously applied on the electrical steel sheet by infinitesimally small-sized plane scan mirrors and switch mirrors which communicate with each other, so that continuous scratches of 300 mm or more are formed on the electrical steel sheet without formation of zigzag line patterned scratches, with the result that the electrical steel sheet can be widely irradiated, and in that a toroidal mirror and a cylindrical mirror are used together with minimum lasers and mirrors (switch mirrors and scan mirrors), and thus the irradiation scope of the electrical steel sheet can be doubled and the refinement of the magnetic domain of the electrical steel sheet can keep up with rapid line speed. Furthermore, the apparatus for refining a magnetic domain of a grain-oriented electrical steel sheet according to the present invention is advantageous in that, since the apparatus is not noisy and is very easily maintained and repaired, it is environment- friendly and can greatly reduce production costs compared to conventional apparatuses. Brief Description of the Drawings

[26] FIG. 1 is a schematic view showing a process of refining a magnetic domain of an electrical steel sheet according to an embodiment of the present invention;

[27] FIG. 2 is a view for explaining an apparatus for refining a magnetic domain of an electrical steel sheet according to an embodiment of the present invention;

[28] FIG. 3 is a view for explaining the change in shape of the apparatus for refining a magnetic domain of an electrical steel sheet according to an embodiment of the present invention;

[29] FIG. 4 is a view showing the state in which a cylindrical mirror is obliquely disposed to an electrical steel sheet according to an embodiment of the present invention; and

[30] FIG. 5 is a graph showing the improvement effect of the iron loss of an electrical steel sheet depending on laser power. Best Mode for Carrying Out the Invention

[31] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings such that those skilled in the art can easily carry out the technical ideas of the present invention. However, the present invention is not limited to the following embodiments, and can be embodied in other forms. The following embodiments are provided to allow full understanding of the scope and spirit of the present invention and to sufficiently transmit the technical idea of the present invention to those skilled in the art.

[32] In the embodiments of the present invention, although the terms "first, second and the like" are used in order to describe the constituents of the present invention, the constituents of the present invention must not be limited to the terms. These terms are only used in order to distinguish predetermined constituents from other constituents.

[33] Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.

[34] FIG. 1 is a schematic view showing a process of refining a magnetic domain of an electrical steel sheet according to an embodiment of the present invention. Referring to FIG. 1, a steel sheet 2 is unrolled from a pay-off reel 1 and forms a path line with a first pinch roll 3 and a second pinch roll 3'. Thus, the steel sheet 2 is unrolled from the pay-off reel 1 and then rewound on a take-up reel 14. A first laser beam 9 emitted from a first laser beam emission unit 6 reaches a pair of first scan mirrors 12. The pair of first scan mirrors 12 reciprocates right and left, and simultaneously transfers the first laser beam 9 to a cylindrical mirror 5 which is a parabolic reflector.

[35] The cylindrical mirror 5 applies the laser beam onto the steel sheet 2 passing under the cylindrical mirror 5, thus refining a magnetic domain of the steel sheet 2. In the same manner as this, the magnetic domain of the steel sheet 2 is refined by a second laser beam emission unit 7, a second laser beam 8, a pair of second scan mirrors 13 and the cylindrical mirror 5.

[36] The steel sheet irradiated with the laser beam passes through a continuous iron loss measuring device 30, and simultaneously the iron loss of the steel sheet is measured. Before the steel sheet 2 passes through a magnetic domain refining apparatus, an X-ray thickness measurer 4 is disposed on the steel sheet 2. The iron loss per unit weight of the steel sheet 2 can be displayed using the measured iron loss and thickness signals. Thus, workers can realize the effect of the refinement of the magnetic domain of the steel sheet 2.

[37] Hereinafter, an apparatus for refining a magnetic domain of an electrical steel sheet according to an embodiment of the present invention will be described in detail with reference to FIG. 2.

[38] The apparatus 100 for refining a magnetic domain of an electrical steel sheet includes a first optical system and a second optical system. The first optical system and second optical system respectively apply laser beams over 1/2 of the entire region of the steel sheet 2 in the width direction of the steel sheet 2. For example, the first optical system and second optical system apply laser beams over 600 mm of the entire length of the steel sheet 2 in the width direction of the steel sheet 2. The first optical system includes a first laser beam emission unit 6, a pair of first reflecting mirrors 15 and 16, a first toroidal mirror 17, a first switch mirror 11, a pair of first scan mirrors 12, and a cylindrical mirror 5. The second optical system includes a second laser beam emission unit 7, a pair of second reflecting mirrors 18 and 19, a second toroidal mirror 20, a second switch mirror 10, a pair of second scan mirrors 12, and the cylindrical mirror 5.

[39] Since the first optical system is operated in the same manner as the second optical system, only the first optical system is described below.

[40] In the first optical system, a first circular laser beam 9 is emitted from the first laser beam emission unit 6. A laser beam emitted from carbon dioxide (CO₂) laser having a continuous wave mode, the laser beam being not diffused in progress, may be used as the first circular laser beam 9, and a collimate lens for focusing the first circular laser beam 9 is not required. The first circular laser beam 9 reaches the first toroidal mirror 17 after its route is changed by the pair of first reflecting mirrors 15 and 16. The toroidal mirror 17, which is a curved mirror, serves to convert the first circular laser beam 9 into a first elliptical laser beam 21. The first elliptical laser beam 21 is transferred from the toroidal mirror 17 to the first switch mirror 11. The first switch mirror 11 alternately transfers the first elliptical laser beam 21 to the pair of first scan mirrors 12 (12a and 12b). The two first scan mirrors 12a and 12b reciprocate in opposite directions. That is, the moment that one 12a of the first scan mirrors 12 receives the first elliptical laser beam 21 from first switch mirror 11 in order to performing a scanning from the left side to right side of the cylindrical mirror 5, the other 12b of the first scan mirrors 12 sends the first elliptical laser beam 21 to the cylindrical mirror 5 and then returns from the right side to left side of the cylindrical mirror 5.

[41] Since the switch mirror 11 sends the first elliptical laser beam 21 to the first scan mirrors 12 when the first scan mirrors 12 transfer the first elliptical laser beam 21 to the cylindrical mirror 5 in only one direction and blocks the first elliptical laser beam 21 when the first scan mirrors 12 transfer the first elliptical laser beam 21 to the cylindrical mirror 5 in the direction opposite thereto, the switch mirror 11 can prevent the first elliptical laser beam 21 from being irradiated in zigzags.

[42] Since the first elliptical laser beam 21 transmitted from the first scan mirrors 12 is not diffused in progress, the distance between the first scan mirrors 12 and the cylindrical mirror 5 can be increased, so that the irradiation distance of the first elliptical laser beam 21 to the cylindrical mirror 5 can also be increased, with the result that continuous scratches can be formed on the steel sheet.

[43] Further, the irradiation distance of the first elliptical laser beam 21 to the cylindrical mirror 5 can be easily adjusted by changing the reciprocating rotation angle of the first scan mirror 12. In this case, it is preferred that continuous scratches of 300 mm or more be formed on the steel sheet by determining the reciprocating rotation angle of the first scan mirror 12 such that the irradiation distance of the first elliptical laser beam 21 to the cylindrical mirror 5 is increased. Therefore, since four or less continuous scratches are formed on a steel sheet having a width of 1000 mm or more, preferably about 1200 mm, the magnetic domain of the steel sheet can be efficiently refined by four or less lasers and mirrors (switch mirrors, toroidal mirrors and cylindrical mirrors) and four or less pairs of scan mirrors, thereby maximizing production efficiency and reducing production costs.

[44] The first elliptical laser beam 21 reaching the cylindrical mirror 5 is converted into a second elliptical laser beam 22 by the cylindrical mirror 5. The steel sheet 2 is irradiated with the second elliptical laser beam 22, and thus the magnetic domain of the steel sheet 2 is refined.

[45] In the embodiments of the present invention, since laser beams are continuously applied to mirrors (a toroidal mirror, a switch mirror, scan mirrors, a cylindrical mirror, and the like), it is required to cool the mirrors in order to prevent the increase in the temperature of the mirrors and it is required to prevent the mirrors from being dirtied by dust. Cooling water may be used in order to cool the mirrors. For example, the mirrors can be cooled by adhering the mirrors on a support, forming minute tubes in the support and then circulating cooling water through the minute tubes (or, when the mirrors themselves are bodies, cooling water may flow around the mirrors themselves), and the cooling capacity of the mirrors can be adjusted by controlling the flow rate of cooling water. Meanwhile, air curtains may be formed on the surfaces of the mirrors in order to prevent the surfaces of the mirrors irradiated with a laser beam from being dirtied by dust. For example, both the dirtying of the mirrors by dust and the increase in temperature of the mirrors can be prevented by spraying nitrogen for cooling onto the surfaces of the mirrors.

[46] The change in shape of the apparatus for refining a magnetic domain of an electrical steel sheet according to an embodiment of the present invention will be described with reference to FIG. 3. The first laser beam 9 has a circular shape. The first laser beam 9 is converted into the first elliptical laser beam 21 by the toroidal mirror 17. The first elliptical laser beam 21 is converted into the second longish elliptical laser beam 22 by the cylindrical mirror. Irradiated lines 23 and 24 are alternately formed on the steel sheet 2 by the pair of first scan mirrors 12 and the pair of second scan mirrors 13. According to the embodiments of the present invention, since the refining of the magnetic domain of the steel sheet 2 proceeds using the elliptical laser beam rather than the circular laser beam, it can keep up with rapid line speeds. The reason for this is that the elliptical laser beam can perform the refining of the magnetic domain of the steel sheet 2 for a short period of time compared to the circular laser beam.

[47] Referring to FIG. 4, the pair of first scan mirrors 12 and the pair of second scan mirrors 13 transfer the first elliptical laser beam 21 to the cylindrical mirror 5 along the length direction of the cylindrical mirror 5. The length direction of the cylindrical mirror is oblique to the width direction of the steel sheet 2, and thus the second elliptical laser beam 22 is applied to the steel sheet 2 in a direction parallel to the width direction of the steel sheet 2 (in FIG. 4, an arrow indicates the direction in which the steel sheet proceeds). Therefore, the second elliptical laser beam 22 is not applied to the steel sheet 2 in zigzags, so that the intervals between the irradiated lines are constant, with the result that the effect of the refinement of the magnetic domain of the steel sheet can be uniform.

[48] A method of refining a magnetic domain of an electrical steel sheet using the above apparatus for refining a magnetic domain of an electrical steel sheet will be described below. First, a first circular laser beam 9 is generated from a laser beam emission unit 6 in a state in which first scan mirrors 12a and 12b reciprocate right and left in opposite directions.

[49] Subsequently, the first circular laser beam 9 is transferred to a toroidal mirror 17 and then converted into a first elliptical laser beam 21 by the toroidal mirror 17. In this case, it is preferred that the length of the short axis of the first elliptical laser beam 21 be smaller than the diameter of the first circular laser beam 9. The reason for this is to enable the long axis of a second elliptical laser beam 22 applied to the steel sheet to be formed in the direction of irradiation.

[50] Subsequently, a switch mirror 11 receiving the first elliptical laser beam 21 alternately transfers the first elliptical laser beam 21 to scan mirrors 12a and 12b.

[51] Each of the scan mirrors 12a and 12b scans the first elliptical laser beam 21 alternately transferred from the switch mirror 11 while alternately moving in a scan direction.

[52] Subsequently, in order to suitably adjust the shape of a beam applied to the steel sheet, the first elliptical laser beam 21 is converted into a second elliptical laser beam 22. The second elliptical laser beam 22 is applied to the steel sheet in the width direction of the steel sheet 22, so that the steel sheet is scratched, thereby refining the magnetic domain of the steel sheet.

[53] The steel sheet, the magnetic domain of which is refined by the above method, is formed thereon with linear scratches rather than zigzag scratches, and thus the iron loss thereof is greatly improved.

[54] Further, since the steel sheet is irradiated with an elliptical laser beam in which a long axis is formed in an irradiation direction, the irradiation rate of a laser beam can be improved, and an insulating layer formed on the surface of the steel sheet can be prevented from being damaged during a scratch forming process.

[55] Since a conventional steel sheet has a width of 1000 mm or more, preferably about

1200 mm, five or more scratches are formed on the steel sheet along the width direction of the steel sheet when the length of the scratches is less than 300 mm, thus causing the deterioration of the improvement effect of an iron loss attributable to the refinement of the magnetic domain of the steel sheet. Further, in the refinement of the magnetic domain of the steel sheet, five or more lasers and mirrors (switch mirrors, toroidal mirrors, and cylindrical mirrors) and five or more pairs of scan mirrors are required, thus decreasing production efficiency and increasing maintenance costs. Therefore, it is preferred that one to four continuous scratches having a length of 300 mm or more be formed on a steel sheet having a width of 1000 mm or more, particularly about 1200 mm.

[56] The laser used in the embodiments of the present invention is a carbon dioxide (CO₂) laser having a continuous wave mode. The laser power is adjusted to such an extent that the improvement rate of the iron loss of the steel sheet is good and the coated layer formed on the surface of the steel sheet is not evaporated.

[57] It is effective that two laser beams 8 and 9 emitted from the respective laser beam emission units are applied to the steel sheet 2 over 600 mm of the entire length of the steel sheet 2 in the width direction of the steel sheet 2. The reason for this is due to the limitation of the moving velocity of the scan mirrors 10 and 11. According to an embodiment of the present invention, in consideration of the moving velocity of the scan mirrors 10 and 11, the two laser beams 8 and 9 emitted from the respective laser beam emission units are applied to the steel sheet 2 having a width of 1200 mm over 600 mm of the entire length of the steel sheet 2 in the width direction of the steel sheet 2 to form two scratches on the steel sheet 2. In this case, the laser beam 9 emitted from the laser beam emission unit is a circular laser beam having a diameter of about 25 mm, and the circular laser beam having a diameter of about 25 mm is converted into an elliptical laser beam having a long axis of 25 mm and a short axis of 8 mm by the toroidal mirror 17, and the elliptical laser beam having a long axis of 25 mm and a short axis of 8 mm is converted into an elliptical laser beam having a long axis of 8 mm and a short axis of 0.2 mm by the cylindrical mirror 5.

[58] As the result of refining a magnetic domain of a steel sheet under the condition that the moving velocity of the steel sheet is 20 mpm and the interval between scratches is 5mm, continuous linear scratches having a length of 600 mm were formed on the steel sheet without the formation of zigzag line patterned scratches, and the average improvement rate of an iron loss was 10%, which was good. Additionally, even when the process of refining a magnetic domain of a steel sheet was conducted under the condition that the moving velocity of the steel sheet is 30 mpm, 50 mpm and 80 mpm, continuous linear scratches having a length of 600 mm were also formed on the steel sheet without the formation of zigzag line patterned scratches, and the average improvement rate of an iron loss was 10%±2%, which was also good. From these results, according to the refinement of a magnetic domain of a steel sheet of the present invention, it can be seen that the magnetic domain of the steel sheet can be efficiently refined even at high steel sheet moving velocity of 20 to 80 mpm.

[59] FIG. 5 shows the improvement effect of the iron loss of a grain-oriented electrical steel sheet depending on laser power. Samples having a magnetic flux density (BlO) of 1.92 Tesla and an iron loss (W17/50) of 0.90 watt/kg were used in the test. The laser used in the test is a carbon dioxide (CO₂) laser, and the beam mode of the laser is a continuous wave mode. From FIG. 5, it can be seen that when the laser power is 1.1 Kw or more, the improvement rate of the iron loss of the steel sheet is approximately 10%. Further, it can be seen that when the laser power is 2.3 Kw or more, the improvement rate of the iron loss of the steel sheet is good, but the phenomenon in which the coated layer formed on the steel sheet is evaporated occurs. Therefore, it is preferred that the range of laser power be limited to a range of 1.0 to 2.2 kW in order to exhibit sufficient refining effects of the magnetic domain of the steel sheet.

[60] According to the present invention, the apparatus for refining a magnetic domain of an electrical steel sheet is slightly noisy and is very easily maintained and repaired, thereby decreasing costs. Further, the coated layer formed on an electrical steel sheet is not deteriorated, and thus the re-coating of the electrical steel sheet is not required.

Claims

[1] An apparatus for refining a magnetic domain of an electrical steel sheet, comprising an optical system, the optical system comprising: a laser beam emission unit for generating a laser beam; a switch mirror for receiving the laser beam emitted from the laser beam emission unit and then alternately transferring the laser beam while changing a transfer direction; and a pair of scan mirrors for scanning the laser beam alternately transferred from the switch mirror and then alternately applying the laser beam to an electrical steel sheet.

[2] The apparatus for refining a magnetic domain of an electrical steel sheet according to claim 1, wherein the pair of scan mirrors reciprocates right and left in opposite directions.

[3] The apparatus for refining a magnetic domain of an electrical steel sheet according to claim 1, wherein the switch mirror alternately transfers the laser beam to the pair of scan mirrors by sending the laser beam to each of the pair of scan mirrors only when each of the pair of scan mirrors operates in a predetermined direction and by blocking the laser beam when each of the pair of scan mirrors operates in a direction opposite to the operation direction of each of the pair of scan mirrors.

[4] The apparatus for refining a magnetic domain of an electrical steel sheet according to any one of claims 1 to 3, further comprising: a torodial mirror provided between the laser beam emission unit and the switch mirror, the toroidal mirror serving to convert the laser beam transferred from the laser beam emission unit into an elliptical laser beam and to transfer the elliptical laser beam to the switch mirror; and a cylindrical mirror provided over the electrical steel sheet, the cylindrical mirror serving to convert the elliptical laser beam transferred from the pair of scan mirrors into a differently shaped elliptical laser beam and to apply the differently shaped elliptical laser beam to the electrical steel sheet.

[5] The apparatus for refining a magnetic domain of an electrical steel sheet according to any one of claims 1 to 3, wherein the apparatus is provided with a plurality of optical systems to double an irradiation range of the laser beam in a width direction of the electrical steel sheet.

[6] The apparatus for refining a magnetic domain of an electrical steel sheet according to any one of claims 1 to 3, wherein each of the pair of scan mirrors adheres on a copper support, and the copper support is provided therein with minute tubes to allow cooling water to pass therethrough.

[7] The apparatus for refining a magnetic domain of an electrical steel sheet according to any one of claims 1 to 3, wherein nitrogen for cooling is sprayed on a surface of each of the pair of scan mirrors to cool the scan mirror and to prevent the scan mirror from being dirtied by dust.

[8] The apparatus for refining a magnetic domain of an electrical steel sheet according to any one of claims 1 to 3, wherein a carbon dioxide laser having a continuous wave mode is used as the laser to prevent a coated layer formed on the electrical steel sheet from being damaged.

[9] The apparatus for refining a magnetic domain of an electrical steel sheet according to claim 4, wherein a reflecting mirror is provided between the laser beam emission unit and the toroidal mirror, the reflecting mirror serving to reflect the laser beam and transfer the reflected laser beam to the toroidal mirror.

[10] A method of refining a magnetic domain of an electrical steel sheet, in which the electrical steel sheet is irradiated with a laser beam and thus scratches having no zigzag line pattern are formed on the electrical steel, comprising: alternately transferring the laser beam to a pair of scan mirrors reciprocating right and left; and irradiating the electrical steel sheet with the laser beam by alternately moving the pair of scan mirrors in a scan direction.

[11] The method of refining a magnetic domain of an electrical steel sheet according to claim 10, further comprising: converting the laser beam into an elliptical laser beam to allow a long axis of the elliptical laser beam to be paralleled to an irradiation direction of the laser beam and thus to improve an irradiation rate of the laser beam.

[12] The method of refining a magnetic domain of an electrical steel sheet according to claim 11, wherein the converting of the laser beam into an elliptical laser beam is performed two or more times to make a shape of the spot of the laser beam more minute.

[13] The method of refining a magnetic domain of an electrical steel sheet according to claim 11 or 12, wherein the converting of the laser beam into an elliptical laser beam is performed by transferring the laser beam to a curved toroidal mirror or a curved cylindrical mirror.

[14] An electrical steel sheet, coated with an insulation film, whose magnetic domain is refined, on which a plurality of scratches is formed by a laser beam in a width direction of the electrical steel sheet, wherein the electrical steel sheet is formed thereon with continuous linear scratches having a length of 300 mm or more, and the insulation film is not damaged by the scratches. [15] The electrical steel sheet according to claim 14, wherein the electrical steel sheet is provided with the scratches having no zigzag line pattern. [16] The electrical steel sheet according to claim 14, wherein the electrical steel sheet has a width of 1000 mm or more, and is formed thereon with one to four scratches in the width direction thereof. [17] The electrical steel sheet according to any one of claims 14 to 16, wherein the magnetic domain of the electrical steel sheet is refined at a steel sheet moving velocity of 20 mpm or more. [18] An electrical steel sheet whose magnetic domain is refined, on which a plurality of scratches is formed by a laser beam in a width direction of the electrical steel sheet, wherein the electrical steel sheet has a width of 1000 mm or more, and is formed thereon with one to four scratches in the width direction thereof. [19] The electrical steel sheet according to claims 18, wherein the magnetic domain of the electrical steel sheet is refined at a steel sheet moving velocity of 20 mpm or more. [20] An electrical steel sheet, manufactured using the method of any one of claims 10 to 13.