Grain-oriented electrical steel sheet and its manufacturing method

Abstract

The present invention provides a grain-oriented electrical steel sheet that can prevent surface damage to the electrical steel sheet and has excellent iron loss characteristics, and a manufacturing method thereof. [Solution] One embodiment of the present invention provides a grain-oriented electrical steel sheet comprising an electrical steel sheet substrate, a glass coating layer located on the electrical steel sheet substrate, and an insulating coating layer located on the glass coating layer, wherein linear deformations are formed on the surface of the electrical steel sheet, and the electrical steel sheet on which the deformations are formed has a natural corrosion potential value of (-)350mV or more.

Description

[Technical Field]

[0001] The present invention relates to a grain-oriented electrical steel sheet and a manufacturing method thereof, and more specifically to a grain-oriented electrical steel sheet that has been subjected to secondary recrystallization and that has excellent iron loss characteristics and that is prevented from surface damage by irradiating the surface of the electrical steel sheet with superimposed lasers having different wavelengths, and a manufacturing method thereof. [Background technology]

[0002] Grain-oriented electrical steel sheets have excellent magnetic properties and are used as iron core materials for transformers. These grain-oriented electrical steel sheets are produced through the rolling and annealing processes unique to the electrical steel sheet manufacturing process. <001> This creates a Goss texture, which is recrystallized in the direction of the crystal orientation, throughout the steel sheet. In response to climate change, the world is steadily tightening greenhouse gas emission calculation standards. In the case of transformer cores, factors that affect greenhouse gas emission calculation standards are related to the improvement in efficiency when using electrical steel sheets. Furthermore, the efficiency of transformer cores is determined by the iron loss and magnetic flux density of the electrical steel sheet, i.e., its magnetic properties. The magnetic flux density of electrical steel sheet is determined by the concentration of crystal axes that are easy to magnetize in the crystal structure. In other words, the higher the crystal orientation, the higher the magnetic flux density, so the manufacturing process of the electrical steel sheet can have a significant impact.

[0003] The iron loss of electrical steel sheets is measured at a maximum magnetic flux density of 1.7T and a magnetic field of 50Hz frequency, and is generally referred to as the guaranteed iron loss value of the core material. However, when designing transformers, a value of W15 / 50W / kg measured at a maximum magnetic flux density of 1.5T and a magnetic field of 50Hz frequency is sometimes used. The lower the iron loss value, the more efficient the transformer. Therefore, in the case of electrical steel sheets, the higher the magnetic flux density and the lower the iron loss, the more efficient the transformer core can be used. Among these, magnetic flux density is considered a more important indicator, as the process technology required to ensure high magnetic flux density has advanced to the point where it supports transformer efficiency through the upward leveling of the electrical steel sheet manufacturing process.

[0004] Iron loss is divided into eddy current loss and hysteresis loss. Hysteresis loss tends to decrease as magnetic flux density increases, making eddy current loss an important factor in controlling overall iron loss in grain-oriented electrical steel sheets. Eddy current loss, which is part of iron loss, is divided into classical eddy current loss and anomalous eddy current loss. Classical eddy current loss is proportional to the thickness of the steel sheet, so the thinner the steel sheet, the less classical eddy current loss there is. Therefore, controlling anomalous eddy current loss is an important technique for reducing iron loss.

[0005] Eddy current loss, a type of iron loss, decreases as the inter-wall spacing of 180° magnetic domains, which are magnetic domains in the rolling direction, narrows. Therefore, iron loss can be reduced by refining the magnetic domains of electrical steel sheets. Refining magnetic domains in electrical steel sheets refers to the process of applying physical stimuli to crystal grains with a single magnetic domain characteristic to separate them into multiple magnetic domains. Methods for refining magnetic domains include laser irradiation, electron beam irradiation, plasma treatment, etching, and roll pressing. These magnetic domain refinement processes are classified as permanent magnetic domain refinement and temporary magnetic domain refinement, depending on whether the magnetic domain refinement effect is maintained after stress relief annealing (SRA).

[0006] In terms of the series of manufacturing processes for the electrical steel sheet, the magnetic domain refinement step may be carried out before the decarburization step or after the insulating coating step.

[0007] Meanwhile, due to factors such as product transportation, it takes a long time for the manufactured electrical steel sheets to be shipped in coil form and processed into the final core. During this transportation period or during processing into the core, there is a possibility that the portions of the electrical steel sheet that are physically stimulated to refine the magnetic domains may corrode. Corrosion occurring at the portions of the electrical steel sheet surface that are physically stimulated means that the insulating coating on the surface peels off, exposing the base material of the electrical steel sheet. If this is used as is in an iron core, the insulating coating formed on the surface of the electrical steel sheet may be destroyed, causing electrical current to flow between the upper and lower laminated cores, which could lead to a transformer explosion. Therefore, even if physical stimulation is applied to the surface of the electrical steel sheet to refine the magnetic domains, it is necessary to apply the stimulation within a range that does not damage the insulating coating. Summary of the Invention [Problem to be solved by the invention]

[0008] This invention relates to a grain-oriented electrical steel sheet and a manufacturing method thereof. More specifically, it relates to a method for manufacturing a grain-oriented electrical steel sheet that has excellent iron loss characteristics and prevents surface damage by irradiating the surface of an electrical steel sheet that has undergone secondary recrystallization with superimposed lasers having different wavelengths. [Means for solving the problem]

[0009] A method for refining magnetic domains in a grain-oriented electrical steel sheet according to one embodiment of the present invention involves irradiating the surface of the electrical steel sheet with a superimposed laser beam, which comprises a first beam spot formed by irradiating a first laser beam of a first wavelength and a second beam spot formed by irradiating a second laser beam of a second wavelength, and which is controlled so that the first beam spot is partially or completely positioned within the second beam spot, to form a deformed portion.

[0010] In this case, it is preferable that the first laser of the superimposed laser beam is a short wavelength laser, and the second laser is a long wavelength laser having a wavelength longer than that of the short wavelength laser. According to another embodiment of the present invention, a method for refining magnetic domains in a grain-oriented electrical steel sheet comprises irradiating a superimposed laser beam, which is obtained by simultaneously superimposing a first laser and a second laser, onto the surface of an electrical steel sheet that has undergone secondary recrystallization in a direction perpendicular to or oblique to the rolling direction, thereby forming a linear deformed portion. Here, the beam shape of the first laser or the second laser is preferably either elliptical or circular.

[0011] Preferably, the first laser is any one of an optical fiber laser, a YAG laser, or a disk laser, and the second laser is a CO2 laser. When an optical fiber laser is used as the first laser, its beam spot is preferably elliptical or circular with a width of 10 to 200 μm and a length equal to or greater than the length of the CO2 laser beam spot. When a CO2 laser is used as the second laser, its beam spot is preferably elliptical with a width of 100 to 400 μm and a length of 0.4 to 20 mm.

[0012] When the surface of the steel sheet is irradiated with the above-mentioned superimposed laser beams, the irradiation interval is preferably 2 to 10 mm and the scanning speed is preferably 0.1 to 300 m / sec. The tilt direction of such superimposed laser beams is preferably ±10° or less. Here, it is preferable that the output of the optical fiber laser as the first laser is 10 to 150 W, the output of the YAG laser is 10 to 250 W, and the output of the CO2 laser as the second laser is 200 to 500 W.

[0013] A grain-oriented electrical steel sheet according to another embodiment of the present invention includes an electrical steel sheet substrate and an insulating coating layer located on the electrical steel sheet substrate, and one or more linear deformation portions are formed on the surface of the electrical steel sheet substrate, and the natural corrosion potential value of such an electrical steel sheet is (-)350 mV or more.

[0014] In this case, a glass coating layer may be further formed between the electrical steel sheet substrate and the insulating coating layer. The natural corrosion potential of such an electrical steel sheet is preferably (-)50 to (-)300 mV. Meanwhile, a resolidified layer is formed on the surface of the insulating coating layer. The thickness of this resolidified layer is preferably within 20% of the cross-sectional thickness of the insulating coating layer. The electrical steel sheet according to one embodiment of the present invention preferably has an improvement rate of W15 / 50 iron loss of 6% or more, and an improvement rate of W17 / 50 iron loss of 9% or more.

[0015] A grain-oriented electrical steel sheet according to another embodiment of the present invention includes an electrical steel sheet substrate and an insulating coating layer located on the electrical steel sheet substrate, in which one or more linear deformation portions are formed on the surface of the electrical steel sheet, and a re-solidified layer is formed on the surface of the insulating coating layer above the linear deformation portions.

[0016] In this case, a glass coating layer may be further formed between the electrical steel sheet substrate and the insulating coating layer. The thickness of the insulating coating layer above the deformed portion is preferably 60% or more of the thickness of the insulating coating layer above the region where the deformed portion is not formed. The resolidified layer preferably contains silicon oxide as a main component, and the insulating coating layer preferably contains phosphate or colloidal silica as a main component. The thickness of the resolidified layer is preferably within 20% of the cross-sectional thickness of the insulating coating layer. [Effects of the Invention]

[0017] According to one embodiment of the present invention, by using superimposed lasers with different wavelengths to perform optimal magnetic domain refinement, it is possible to further improve the magnetic properties and sufficiently suppress damage to the surface of the steel sheet. According to another embodiment of the present invention, it is possible to easily increase the average output power by using a long wavelength laser, thereby ensuring the reliability of the processing line, and at the same time, it is possible to minimize the magnetic domains by irradiating with a short wavelength laser, thereby effectively improving the magnetism. When using only a CO laser to improve the iron loss of grain-oriented electrical steel sheet, there is a possibility that deviations in the improvement effect of iron loss characteristics will occur depending on the thickness of the insulating coating. However, when a different type of laser according to the present invention is used in combination, the improvement effect of iron loss can be maintained and magnetic deviations due to the coating can be suppressed. In a magnetic domain refinement method according to one embodiment of the present invention, a CO laser is used as a long-wavelength laser to stably preheat a steel sheet without destroying the coating layer, and an optical fiber laser is used as a short-wavelength laser to precisely induce residual stress due to thermoelastic deformation in the steel sheet to a width required for forming closure domains without considering the thickness of the coating layer, thereby enabling accurate magnetic domain refinement. In another embodiment of the magnetic domain refinement method of the present invention, laser beams with different wavelengths are irradiated simultaneously in an overlapping state onto the surface of a steel sheet on which secondary recrystallization has been completed, thereby maximizing thermal shock in the thickness direction even under low laser output conditions, thereby providing a directional magnetic domain refinement product with excellent iron loss in both low and high magnetic fields. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a graph showing the light absorptance of a steel sheet depending on the laser wavelength. [Figure 2] 1A and 1B are schematic diagrams illustrating the concept of magnetic domain refinement for forming a deformation portion using superimposed lasers with different wavelengths according to an embodiment of the present invention. [Figure 3] FIG. 2 is a schematic diagram showing a case where a steel plate is scanned with beam spots of superimposed lasers having different wavelengths according to an embodiment of the present invention. [Figure 4] FIG. 2 is a schematic diagram showing energy density when a steel plate is scanned with beam spots of superimposed lasers having different wavelengths according to an embodiment of the present invention. [Figure 5] FIG. 2 is a diagram showing a working electrode of an apparatus for measuring the natural corrosion potential value of a steel plate according to an embodiment of the present invention. [Figure 6] FIG. 1 is a diagram showing an apparatus for measuring the natural corrosion potential value of a steel plate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Thus, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention. The terminology used herein is merely for the purpose of describing particular embodiments and is not intended to limit the present invention. The singular form "a," "an," or "an" includes the plural form unless the context clearly dictates otherwise. As used herein, the term "comprising" refers to a specific feature, region, integer, step, operation, element, and / or component and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0020] When we say that a part is "on" another part, it means that it is directly on top of the other part, or there may be other parts between them. In contrast, when we say that a part is "directly on top of" another part, there are no other parts between them. All terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless otherwise defined.

[0021] Although the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein, the present invention will be described in detail below so that those skilled in the art can easily practice the present invention.

[0022] In one embodiment of the present invention, the surface of an electrical steel sheet is irradiated with a superimposed laser beam having different wavelengths to prevent surface damage and to impart excellent iron loss characteristics. One preferred method for improving the iron loss properties of grain-oriented electrical steel sheets is to refine magnetic domains using a laser.

[0023] The magnetic domain refinement process for grain-oriented electrical steel sheets involves irradiating the electrical steel sheet with a laser along the entire length in a direction perpendicular or inclined to the rolling direction (RD direction) of the steel sheet, i.e., in the width direction (TD direction) of the steel sheet, to form deformed portions 10 or linear deformed portions, as shown in Fig. 2. Here, linear deformed portions include not only solid lines but also intermittent lines such as dotted lines and dashed lines, and also include all deformed portions that are substantially linear, including those that have a zigzag shape when viewed microscopically but a straight line shape when viewed macroscopically.

[0024] The formation of deformed portions in steel sheets by laser irradiation refers to the deformation of the crystal lattice caused by thermal shock due to laser irradiation. This deformation occurs during the process of rapid localized heating and immediate cooling of the steel sheet by the laser. The heating rate of the steel sheet is proportional to the laser energy density (power density) per unit time. However, the deformation of the crystal lattice due to thermal shock during laser irradiation increases as the total laser irradiation energy increases. Therefore, if the steel sheet is irradiated with more energy than is necessary for magnetic domain refinement, the heat source in excess of that required for closure domain formation diffuses to the surrounding area, resulting in increased magnetic deformation. Therefore, the deformation of the crystal lattice due to thermal shock during laser irradiation requires exactly the amount of lattice deformation energy required to form closure domains. To suppress thermal diffusion, it is preferable to irradiate the laser energy to a narrow area for a short period of time.

[0025] The interaction conditions between the laser beam and the steel sheet are affected by the laser characteristics and the laser beam absorption rate of the steel sheet. The laser beam absorption rate is affected by the surface roughness of the steel sheet, the temperature of the steel sheet, the absorption characteristics of the coating on the steel sheet surface, and the laser wavelength. However, when the manufacturing conditions of the grain-oriented electrical steel sheet on which the coating is formed are constant, the surface roughness of the steel sheet, the temperature of the steel sheet, and the absorption characteristics of the coating on the steel sheet surface should also be constant, and in this case, the laser beam absorption rate of the steel sheet depends on the laser wavelength.

[0026] In other words, when the manufacturing conditions for steel sheet are constant, the laser absorption rate is affected by the laser wavelength. As shown in Figure 1, the laser absorption rate of steel sheet is approximately 35-40% when the wavelength is short (e.g., YAG or fiber laser at 1.06 μm), but is relatively low at approximately 5-10% when the wavelength is long (e.g., CO2 laser at 10.6 μm). Thus, in the process of refining the magnetic domains of grain-oriented electrical steel sheet using a laser, using a short-wavelength laser is more efficient than using a long-wavelength laser to ensure stable iron loss characteristics.

[0027] On the other hand, the surface of the electrical steel sheet that is the target of magnetic domain refinement treatment is coated with an insulating coating several to several tens of micrometers thick, primarily composed of phosphate and silica. These coatings have been shown to absorb relatively little of the laser beam from short-wavelength lasers (e.g., YAG or fiber lasers at 1.06 μm), but to absorb significantly more of the laser beam from long-wavelength lasers (e.g., CO2 lasers at 10.6 μm). As a result, the thickness of the insulating coating must be taken into consideration with long-wavelength lasers, but the thickness of the insulating coating does not need to be considered as much with short-wavelength lasers as with long-wavelength lasers. As described above, short-wavelength lasers and long-wavelength lasers have different characteristics. Therefore, when short-wavelength lasers and long-wavelength lasers are used in combination, the advantages of each laser applied to magnetic domain refinement are primarily exhibited without any side effects, resulting in a synergistic effect.

[0028] Here, the use of superimposed different lasers means that two or more lasers with different wavelengths are used as laser beams irradiated onto the surface of the steel sheet, and the spot of one laser beam formed on the surface of the steel sheet is partially or entirely located within the spot of another laser beam. Therefore, in one embodiment of the present invention, the use of superimposed lasers with different wavelengths includes not only the complete overlap of the spot of one laser beam with the spot of another laser beam, but also the partial overlap of the spot of one laser beam.

[0029] As shown in Figures 2 to 4, in the superimposed laser 30 used for magnetic domain refinement according to one embodiment of the present invention, the short wavelength laser is the main laser and is referred to as the first laser A, and the long wavelength laser is an auxiliary laser with a wavelength longer than that of the first laser and is referred to as the second laser B.

[0030] The first laser A, which is a short-wavelength laser, can be a laser with a relatively short wavelength, such as an optical fiber (Er-Fiber, Yb-Fiber, Tm-Fiber) laser, a YAG (Nd:YAG, Yb:YAG) laser, a ruby ​​laser, a sapphire laser, etc. In addition, the first laser A can also be a disk laser (1.03 μm), a diode laser (0.808 to 0.980 μm), or a UV laser (0.150 to 0.355 μm).

[0031] The second laser B, which is a long-wavelength laser, is a laser with a wavelength relatively longer than the short-wavelength laser. For example, a CO2 laser is preferable as the second laser. However, if a UV laser (0.150 to 0.355 μm) is used as the first laser, which is a short-wavelength laser, any laser with a longer wavelength than the first laser can be used as the second laser. For example, if a UV laser (0.150 to 0.355 μm) is used as the first laser, a YAG laser can also be used as the second laser.

[0032] The magnetic domain refinement method using the superimposed laser 30 will be described in more detail below, taking as an example a case where an optical fiber laser is used as the first laser A, which has a short wavelength, and a CO2 laser is used as the second laser B, which has a long wavelength. The optical fiber laser used as the first laser A has a short wavelength that has a relatively high laser absorption rate in steel sheets, so it is possible to irradiate a narrow area for a short time with incident energy sufficient to accurately form closure domains and induce residual stress due to lattice deformation and thermoelastic deformation. Furthermore, because the optical fiber laser used as the first laser A has a narrow incident energy range, it is possible to suppress thermal diffusion to the surrounding area and minimize unnecessary thermal deformation.

[0033] On the other hand, the CO2 laser (second laser B) can be used with an average output ranging from several hundred watts to several kilowatts or more, depending on the steel sheet speed, and can easily induce carbonization deformation in the irradiated area of ​​the steel sheet. Furthermore, the CO2 laser (second laser B) has a high absorption rate for insulating coatings made of phosphate and silica, allowing it to stably pass through the coating layer. Therefore, the CO2 laser (second laser B) can stably induce carbonization deformation in the steel sheet without destroying the insulating coating layer, making it suitable for a type of preheating role. However, because the CO2 laser (second laser B) has a low laser absorption rate for steel sheets, it is preferable to irradiate the steel sheet with the laser to a level that induces carbonization deformation but does not induce permanent deformation.

[0034] In other words, if a long-wavelength laser such as a CO2 laser (second laser B) is used, the area subjected to thermal shock on the steel sheet will be too wide, preventing magnetic domain refinement. Therefore, a fiber optic laser with a relatively short wavelength is used as the first laser A as the main laser for magnetic domain refinement, and a CO2 laser with a relatively long wavelength is used as an auxiliary laser that serves as a kind of preheating function to induce thermal deformation of the steel sheet. The reason why a fiber optic laser with a short wavelength was selected for first laser A and used as the main laser for magnetic domain refinement is that the high laser absorption rate on the surface of the steel sheet forms strong compressive stress areas where the laser is irradiated, and the magnetoelastic energy is reduced in these compressive stress areas, making it easy to form lancet magnetic domains (closure domains).

[0035] In this case, magnetic domain refinement forms 180° magnetic domains (opposite polarity to lancet magnetic domains) in the surface direction using magnetoelastic energy, and 90° magnetic domains are formed in the thickness direction to reduce the magnetoelastic energy, thereby narrowing the spacing between the magnetic domains and resulting in reduced anomalous eddy current loss. As described above, the magnetic domain refinement method according to one embodiment of the present invention uses a short-wavelength optical fiber laser as the first laser to precisely induce residual stress due to thermoelastic deformation in the steel sheet to the width required to form closure domains, enabling accurate magnetic domain refinement, while using a long-wavelength CO2 laser as the second laser to stably preheat the steel sheet without destroying the coating layer.

[0036] In addition, the short-wavelength optical fiber laser (first laser A) can form a small final beam width, which has the advantage of improving laser absorption in the steel sheet, but the depth of field is relatively short. However, the long-wavelength CO2 laser (second laser B) has the advantage of a wide final beam width, which has the advantage of relatively low laser absorption in the steel sheet, but a deep depth of field. Therefore, when these two laser beams are simultaneously superimposed and irradiated, the laser absorption in the steel sheet can be further increased.

[0037] In this case, the beam spot of the first laser A (optical fiber laser) irradiated onto the surface of the steel sheet is preferably approximately circular, with a diameter of 10 to 200 μm. The width of the optical fiber laser beam spot (length in the "RD direction" in Figure 2) is 10 to 200 μm, and its length (length in the "TD direction" in Figure 2) can be equal to or greater than the length of the beam spot of the CO2 laser (second laser B). If the beam width of the optical fiber (first laser A) is reduced to less than 10 μm, the energy density is concentrated in a narrow area, which can result in deterioration of magnetic flux density and iron loss, and the optical system structure becomes complex. Furthermore, if the beam width of the optical fiber (first laser A) is increased by more than 200 μm, the thermal effect in the longitudinal direction of the steel sheet can increase, which can result in a decrease in magnetic flux density, which is undesirable.

[0038] Meanwhile, the beam spot of the long-wavelength CO laser (second laser B) irradiated onto the surface of the steel sheet is preferably an ellipse with a beam width (length in the "RD" direction in FIG. 2) of 100 to 400 μm and a beam length (length in the "TD" direction in FIG. 2) of 0.4 to 20 mm. A circular beam spot with a radius of 100 μm or more can also be used for the long-wavelength CO laser. Forming a CO laser beam width of 100 μm or less is undesirable because it requires a complex mirror optical system, as with an optical fiber laser. A beam width of 400 μm or more is undesirable because it increases the thermal effect in the longitudinal direction of the steel sheet, resulting in a decrease in magnetic flux density. The reason for limiting the size of the beam spot of the long-wavelength CO laser (second laser B) is to consider the range within which the thermal deformation effect of the laser beam acting on the steel sheet can be maintained when the laser is scanned at high speed on the surface of a rapidly moving steel sheet. The simultaneous superposition of different lasers with different wavelengths according to one embodiment of the present invention will now be described in more detail.

[0039] 2 to 4, the use of an optical fiber laser as the first laser A and a CO2 laser as the second laser B in a superimposed manner means that the first laser A, which has a small beam spot, is irradiated onto the surface of the steel sheet, and is controlled so that the beam spot is located within the range of the second laser beam B, which has a larger beam spot. In other words, the beams are said to be "superimposed" when the optical fiber laser beam as the first laser A is completely located anywhere within the range of the CO2 laser beam as the second laser B, which has a larger beam spot, when the different laser beam spot 20 irradiated onto the surface of the steel sheet is viewed from a plane as shown in Figure 3. This also means that the optical fiber laser beam as the first laser A is partially located within the range of the CO2 laser beam as the second laser B.

[0040] 4, it is preferable that the position (b) of the Gaussian shape (a, b, c) of the first laser beam A having a short wavelength is simultaneously located at a position where the intensity of the second laser beam B having a long wavelength is high, and in this case, in the present invention, this means that the superimposed lasers 20 of different wavelengths irradiated onto the surface of the steel sheet are "superimposed simultaneously." As for the laser beam emission mode used in one embodiment of the present invention, it is preferable to use a continuous wave laser that continuously generates laser light for both the first laser A and the second laser B, but a pulse laser can also be used.

[0041] Furthermore, the quality of the laser beams used is preferably a Gaussian mode TEM00 for both the first laser A and the second laser B, but a multi-transverse mode TEM01 can also be used. However, since the superimposed laser beam 20 of different wavelengths irradiated onto the surface of the steel sheet according to one embodiment of the present invention minimizes the thermal effect in the length direction of the steel sheet while maximizing the thermal shock in the thickness direction, the beam shape and beam quality of each laser are not specifically limited.

[0042] Meanwhile, the output of the optical fiber laser serving as the first laser A is preferably 10 to 150 W, and the output of the CO₂ serving as the second laser B is preferably 200 to 500 W. When a YAG laser is used as the first laser A, its output is preferably 10 to 250 W. These output ranges for each laser indicate the laser output conditions when the steel sheet travels at a speed of 15 mpm, and it is preferable to optimally control the laser output value according to the travel speed of the steel sheet.

[0043] When the surface of a steel sheet is irradiated with the laser beam 30, which is a combination of the first laser A and the second laser B, the interval between the beams is preferably 2 to 10 mm, the angle of inclination relative to the rolling direction is preferably ±10° or less, and the scanning speed is preferably 0.1 to 300 m / sec. The electromagnetic steel sheet used here is preferably an electromagnetic steel sheet that has undergone secondary recrystallization. If the irradiation interval between the superimposed laser beams 20 irradiated onto the surface of the steel sheet is too narrow, less than 2 mm, the influence of the heat-affected zone increases, resulting in deterioration of magnetic flux density and iron loss. If the irradiation interval is 10 mm or more, the thermal shock effect required to ensure magnetic domain refinement is reduced, making it difficult to achieve the desired effect.

[0044] Furthermore, when the superimposed laser beam 20 is irradiated onto the surface of the steel sheet, it can be irradiated perpendicular to the rolling direction of the steel sheet or in a direction inclined thereto, and the angle inclined to the rolling direction is preferably ±10° or less. If the superimposed laser beam is irradiated at an angle inclined beyond this angle, the desired magnetic domain refinement effect may not be achieved. Furthermore, the scanning speed of the superimposed laser is the same as the traveling speed of the steel sheet, and as the traveling speed increases, the scanning speed must also increase, so it is preferably 0.1 to 300 m / sec, and the speed is exemplified as a value under the condition of 15 mpm.

[0045] Another embodiment of the present invention provides a grain-oriented electrical steel sheet comprising an electrical steel sheet substrate and an insulating coating layer disposed on the electrical steel sheet substrate, wherein one or more linear deformation portions are formed on the surface of the electrical steel sheet substrate, and the natural corrosion potential of the steel sheet is (-)350 mV or more. The grain-oriented electrical steel sheet may further comprise a glass coating layer between the electrical steel sheet substrate and the insulating coating layer. Here, the glass coating layer is primarily composed of forsterite, and the insulating coating layer is primarily composed of phosphate and colloidal silica. In the present invention, the term "main component" refers to, in the case of forsterite, a single-sided coating amount of 0.7 g / m2 of oxygen on the surface of the steel sheet. 2 In the case of phosphate insulating coating, the amount of coating on one side of the steel sheet is 0.1g / m 2 The colloidal silica in the insulating coating is 0.1g / m based on the amount applied on one side of the steel plate surface. 2 This means that the above is included.

[0046] The spontaneous corrosion potential value is measured by the open circuit potential measurement method commonly used by engineers. The spontaneous corrosion potential value is measured using the open circuit potential (OCP) method. A spontaneous corrosion potential value of "0" indicates no corrosion, and a more negative (-) value indicates more corrosion. The spontaneous corrosion potential value is preferably measured after 600 seconds (10 minutes) in a 3.5% NaCl solution maintained at a constant temperature of 30°C.

[0047] In addition, the natural corrosion potential value of the steel sheet having the linear deformation portion 10 according to the present invention being (-)350mV or more means a value between "0" and (-)350mV, which means that corrosion does not occur easily. At this time, the linear deformation portion formed on the electrical steel sheet means that at least one linear deformation portion is formed on the exposed surface of the specimen to be measured, which is exposed to the electrolyte.

[0048] In one embodiment of the present invention, the natural corrosion potential of the electrical steel sheet is preferably (-)50 to (-)300 mV. If the natural corrosion potential of the electrical steel sheet is (-)50 mV or higher, the formation of linear deformation portions may be incomplete, and the required iron loss characteristics may not be ensured. If the natural corrosion potential of the electrical steel sheet is (-)300 mV or lower, even if linear deformation portions are formed, the corrosion characteristics may be somewhat reduced.

[0049] Meanwhile, in one embodiment of the present invention, the linear deformation portion 10 refers to a deformation portion formed by a beam spot 20 in which a portion with a large beam width produced by the second laser B having a long wavelength and a portion with a small beam width produced by the first laser A having a short wavelength are overlapped when a superimposed laser beam 20 in which laser beams having different wavelengths are superimposed is irradiated onto the surface of the steel sheet, and the width of the deformation portion has a value equal to or close to the beam width or beam diameter of the first laser beam A having a short wavelength. In addition, above the linear deformation portion 10 formed on the steel sheet by irradiating the superimposed laser beam 30, i.e., on the surface of the insulating coating layer directly contacting the superimposed laser beam, a resolidified layer is formed in which the insulating coating layer is partially melted and then resolidified.

[0050] The re-solidified layer formed on the surface of such an insulating coating layer is mainly formed directly below the portion irradiated with the superimposed laser beam 20, and such a re-solidified layer may appear in a linear form intermittently or continuously along the path scanned by the superimposed laser beam 20. When the superimposed laser 20 is irradiated onto the surface of a steel sheet on which an insulating coating has been formed, it is presumed that some of the phosphate forming the insulating coating layer is melted by the superimposed laser and solidifies immediately after the superimposed laser beam passes through.

[0051] The formation of such a partially resolidified layer is a phenomenon that does not occur when the second laser (CO2 laser) or the first laser (optical fiber laser) that make up the superimposed laser beam are irradiated alone. However, when the first laser (optical fiber laser) is irradiated alone at a high output that is sufficient to ensure effective iron loss, a partial resolidified layer appears, but even in this case, the surface of the insulating coating layer is damaged by peeling, such as by the formation of blisters on the surface, and the spontaneous corrosion potential value appears very low. In other words, even if the first laser (optical fiber laser) is used alone to irradiate the surface of the steel sheet and form a deformed area with an effective iron loss value, the natural corrosion potential value at the deformed area is (-)700mV or less, and corrosion may occur.

[0052] Furthermore, when comparing the first laser (optical fiber laser) constituting the superimposed laser with a single optical fiber laser, a partial resolidification layer can be formed even if the output of the first laser (optical fiber laser) constituting the superimposed laser is only approximately 70%. The reason why a partially resolidified layer appears on the insulating coating layer when irradiated with a superimposed laser beam is that the vaporization point of the phosphate that makes up the insulating coating is low, so phosphorus vaporizes first when irradiated with the superimposed laser, and silicon oxide, which consists of Si and O, resolidifies in an amorphous state. When an amorphous resolidified layer forms on the surface of the insulating coating layer in this way, it is thought that the inherent properties of amorphous materials improve corrosion resistance.

[0053] On the other hand, the thickness of the resolidified layer of the insulating coating layer formed by irradiating the surface of the steel sheet with the superimposed laser beam 20 is preferably within 20% of the average cross-sectional thickness of the insulating coating layer. If the thickness of the resolidified layer of the insulating coating is 20% or more of the average cross-sectional thickness, the absolute thickness of the insulating coating layer becomes thin, which may have a detrimental effect on corrosion resistance and may reduce the tension effect of the insulating coating layer, resulting in deterioration of iron loss. Therefore, it is preferable to limit the thickness within this range.

[0054] As described above, when a partially resolidified layer forms on an insulating coating layer, the physical properties of the insulating coating layer change, making the insulating coating layer stable overall and preventing partial peeling and damage. If a partially melted layer forms within 20% of the thickness of the insulating coating layer, this refutes the idea that sufficient laser energy is being applied to the steel sheet without causing significant damage to the insulating coating layer. Furthermore, if a partially resolidified layer forms on more than 20% of the entire surface of the insulating coating layer, this is undesirable because it further increases the damage to the insulating coating layer and eliminates the tensile effect of the insulating coating layer. However, if a partially resolidified layer forms on an insulating coating layer, it can be considered that sufficient laser energy absorption is being applied to the steel sheet.

[0055] When a deformed portion is formed on the electrical steel sheet by irradiating the superimposed laser beam as described above and a partially resolidified layer is formed on the upper insulating coating layer of the steel sheet, the formation of the resolidified layer may cause shrinkage of the insulating coating layer, resulting in a change in the thickness of the insulating coating layer where the deformed portion of the steel sheet is formed. In this case, the thickness of the insulating coating layer above the deformed portion formed on the steel sheet is preferably 60% or more of the thickness of the insulating coating layer above the region where the deformed portion is not formed. If the thickness of the coating layer at the deformed portion formed on the steel sheet is 60% or less, the reduction in coating thickness may result in a deterioration in corrosion resistance and an increase in iron loss due to a tension reduction effect.

[0056] In one embodiment of the present invention, this is intended to prevent peeling of the glass coating as well as the insulating coating to ensure insulation between steel sheets after magnetic domain refinement treatment by thermal deformation using a superimposed laser beam. Meanwhile, when forming a deformed portion by irradiating the surface of an electrical steel sheet with a superimposed laser beam, the improvement rate of W15 / 50 iron loss of such steel sheet is preferably 6% or more. If the W15 / 50 improvement rate is lower than this, the laser absorption rate of the steel sheet is low, making it difficult to expect the desired iron loss reduction effect. Furthermore, when forming a deformed portion by irradiating the surface of an electrical steel sheet with a superimposed laser beam, the improvement rate of W17 / 50 iron loss of such steel sheet is preferably 9% or more. If the W17 / 50 iron loss improvement rate is lower than this, the laser absorption rate of the steel sheet is low, making it difficult to expect the desired iron loss reduction effect.

[0057] Hereinafter, a method for producing a grain-oriented electrical steel sheet and a method for refining magnetic domains according to one embodiment of the present invention will be described in detail.

[0058] [Manufacturing of cold-rolled steel sheets] To manufacture grain-oriented electrical steel sheets, first, a slab of electrical steel sheet substrate is manufactured. The chemical composition and metal structure of the slab are not particularly limited as long as the axis of easy magnetization is aligned in a certain direction and functions as an electrical steel sheet. However, to explain by way of example, the chemical composition of the slab is as follows: In mass%, C: 0.08% or less (0% excluded), Si: 1.0 to 6.5%, Mn: 0.005 to 3.0%, (total of one or more of Nb, V, and Ti): 0.070% or less, (total of one or more of Cr, Sn, and Sb): 2.5% or less, Al: 2.0% or less (0% excluded), (total of one or more of P and S): 0.100% or less (0% excluded), (total of Cu and Sn): 1.0% or less, the total of rare earth elements and other impurities is 0.2% or less, and the remainder is Fe.

[0059] (C: 0.08% or less (excluding 0%)) Carbon (C) is an element that is inevitably mixed into steel, but because it deteriorates magnetic properties due to magnetic aging, its content is preferably controlled to an appropriate level. If the C content in steel sheet is too low, phase transformation does not occur sufficiently during the manufacturing process, resulting in a non-uniform microstructure of the steel sheet and ultimately an unstable secondary recrystallization texture. On the other hand, if the C content is too high, carbides become coarse during the manufacturing process and precipitate in excess. As a result, decarburization does not occur sufficiently, reducing the concentration of the Goss texture and damaging the secondary recrystallization texture. Therefore, the C content of steel sheet is 0.08% or less, more preferably 0.001 to 0.040%.

[0060] (Si: 1.0 to 6.5%) Silicon (Si) is a basic component of grain-oriented electrical steel sheet, and its role is to increase the non-resistivity of the steel sheet and reduce iron loss. If the Si content is less than 1.0%, the non-resistivity decreases, eddy current loss increases, and iron loss characteristics deteriorate, making the effect of adding Si impossible to expect. However, if the Si content is 6.5% or more, the brittleness of the steel sheet increases and toughness decreases, which may cause sheet fracture during the rolling process. In addition, nitrides are not sufficiently formed during the manufacturing process, and sufficient grain suppression force required for secondary recrystallization during the final high-temperature annealing process cannot be secured. Therefore, the Si content is preferably 1.0 to 6.5%.

[0061] (Mn: 0.005 to 3.0%) Manganese (Mn) increases resistivity and reduces eddy current loss, thereby reducing overall iron loss. It also reacts with S to form Mn-based sulfides in a lull state and with Si to form (Al, Si, Mn)N precipitates, inhibiting the growth of primary crystals and causing secondary recrystallization. It is also an important element that influences the surface quality of the final product. However, excessively low Mn content can degrade the surface quality of the final product. Furthermore, excessive Mn content significantly increases the austenite phase fraction, damaging the Goss texture and reducing magnetic flux density. It also forms an excessive oxide layer during decarburization annealing, hindering decarburization. Therefore, the Mn content is preferably 0.005 to 3.0%.

[0062] (Total of one or more of Nb, V, and Ti: 0.05% or less) Niobium (Nb), vanadium (V), and titanium (Ti) are elements that react with C and N to form precipitates during the manufacturing process, and if added in excessive amounts, they remain in the steel sheet even after secondary recrystallization annealing, degrading the magnetic properties of the steel sheet. Therefore, it is preferable to control the total content of one or more elements selected from Nb, V, and Ti to 0.05% or less.

[0063] (Total of one or more of Cr, Sn, and Sb: 2.5% or less) Chromium (Cr) is added to promote the formation of Goss texture and reduce iron loss, while Sn is added to inhibit grain growth and ultimately improve magnetic flux density. Antimony (Sb) segregates in the grain system, inhibiting grain growth and stabilizing secondary recrystallization. Because all three elements are interrelated with the formation of secondary recrystallization, it is preferable to limit the total content of Sn, Sb, and Cr to 2.5% or less.

[0064] (Al: 2.0% or less (0% excluded)) In addition to Al-based nitrides precipitated during the manufacturing process, aluminum (Al) combines with N introduced by nitriding during the primary recrystallization process and Al, Si, and Mn present in solid solution in the steel to form nitrides in the form of (Al, Si, Mn)N and AlN, acting as a strong grain growth inhibitor. However, excessive Al content can lead to uneven precipitates, unstable secondary recrystallization, and reduced magnetic properties of the steel sheet, so it is best to add 2.0% or less.

[0065] (Total of one or more of P and S: 0.1% or less (excluding 0%)) Phosphorus (P) segregates in the grain boundaries, hindering the movement of the grain boundaries and also playing a supporting role in suppressing the growth of grains, while excessive addition of S destabilizes the formation of secondary recrystallization. Furthermore, P and S are elements that are inevitably added in the process of manufacturing electrical steel sheets, and it is preferable to control the total amount of P and S to 0.1% or less.

[0066] (Cu+Sn total: 0.1% or less) Copper (Cu) plays a role in improving the texture by being partially dissolved in the crystal grains, and if the Cu+Sn content is excessively high, it can segregate in the crystal grain system and form a liquid at high temperatures, so it is preferable to control the total amount of Cu and Sn to 0.1% or less.

[0067] (Rare earths and other impurities total less than 0.2%) A grain-oriented electrical steel sheet according to one embodiment of the present invention may contain rare earth elements such as cerium (Ce) and praseodymium (Pr) and other impurities. The total amount of any rare earth elements and impurities is preferably 0.2% or less. Rare earth elements and unavoidable impurities refer to impurities that are intentionally added or unavoidably mixed in during the steelmaking and manufacturing process of the grain-oriented electrical steel sheet. Because unavoidable impurities are widely known, detailed explanations will be omitted. In one embodiment of the present invention, additional elements other than the above-described alloy components may be added, and various elements may be included within a range that does not impair the technical concept of the present invention. When additional elements are further included, they may be included to replace the remaining Fe.

[0068] Next, a slab is produced from the steel sheet having the above composition by continuous casting, and is heated and hot-rolled by a conventional method. After hot rolling, the hot-rolled sheet is optionally annealed as needed, and then cold-rolled to produce a cold-rolled steel sheet having a thickness of 0.1 to 0.5 mm. Here, cold rolling can be performed in one pass or two or more passes with intermediate annealing in between.

[0069] [Primary recrystallization annealing] The cold-rolled steel sheet described above is subjected to primary recrystallization annealing through a simultaneous decarbonization-nitriding or decarbonization-post-nitriding process. In the case of primary recrystallization annealing through simultaneous decarbonization-nitriding, the cold-rolled structure deformed during the annealing process undergoes recrystallization, resulting in decarbonization annealing. To achieve this, the steel sheet undergoes decarbonization annealing in a mixed gas atmosphere containing nitrogen, hydrogen, and moisture. In the case of decarbonization-post-nitriding, a nitriding treatment can be performed after decarbonization to introduce nitrogen ions into the steel sheet using ammonia gas.

[0070] When simultaneous decarbonitriding is performed, the cold-rolled steel sheet is loaded into the furnace at a temperature in the range of 700 to 900°C, the dew point temperature of the atmospheric gas is set to 40 to 70°C, and the Fe2SiO4 / SiO2 ratio on the surface is controlled to 0.5 to 3.0 to form an oxide layer on the surface of the electrical steel sheet.

[0071] [Secondary recrystallization annealing] Then, an annealing separator based on MgO is applied to the surface of such an electrical steel sheet. The steel sheet to which the annealing separator has been applied is then heated to 1,000°C or higher and subjected to long-term soaking annealing to induce secondary recrystallization, so that the {110} plane of the steel sheet is parallel to the rolling surface, <001> This results in the formation of a Goss-oriented texture whose direction is parallel to the rolling direction. This final high-temperature annealing process forms a glassy layer containing forsterite on the surface of the steel sheet, and secondary recrystallization occurs inside the steel sheet.

[0072] [Insulating coating formation] An insulating coating layer is formed on the surface of an electrical steel sheet that has undergone secondary recrystallization and is then coated with an insulating coating solution containing colloidal silica and a metal phosphate, either alone or in combination, and then annealed to form a glass coating layer. The method for forming such an insulating coating layer is not particularly limited, and one example is to apply an insulating coating solution containing a phosphate. The insulating coating solution preferably contains colloidal silica and a metal phosphate. In this case, the metal phosphate may be Al phosphate, Mg phosphate, or a combination thereof, and the content of Al, Mg, or a combination thereof may be 15 wt % or more based on the weight of the insulating coating solution.

[0073] [Magnetic domain refinement processing] The surface of the electrical steel sheet on which the glass coating and insulating coating have been sequentially formed through the above-mentioned processes is then irradiated with a superimposed laser beam 20 from the first laser A and the second laser B, to form a linear deformation portion 10 on the surface of the steel sheet. At this time, the irradiation angle of the superimposed laser beams, the quality of the laser beams used, and the type of laser mode are as described above, so detailed explanations will be omitted.

[0074] [Measurement of natural corrosion potential] After forming a deformed portion 10 on the surface of the manufactured electrical steel sheet by irradiating a superimposed laser beam 20, a portion of the steel sheet is cut into a circular shape and the natural corrosion potential is measured. Figures 5 and 6 show an apparatus for measuring the natural corrosion potential. First, to measure the natural corrosion potential, the manufactured electrical steel sheet is cut into a thickness of 0.2 mm or 0.23 mm and a diameter of 15 mm. At this time, at least one linear deformed portion is formed in the circular specimen to be cut, and this linear deformed portion is cut so as to be located at the center of the specimen. The specimen cut in this manner is polished and cleaned in the usual manner to prepare it for measuring the natural corrosion potential.

[0075] As shown in FIG. 5, the prepared potential measurement specimen 55 is attached to an exposed electrode plate 56 formed inside a specimen holder 51 formed at the end of a working electrode (WE) 50 of a potential measurement device. The specimen 55 is then fixed to the specimen holder 51 using a specimen holder cap 53 with a sealing ring 54 sandwiched between them. An opening 58 is formed in the front of the specimen holder cap 53 so that the specimen 55 to be measured can come into direct contact with the electrolyte. The opening 58 has a diameter of 10 mm. Therefore, when actually measuring the potential, approximately 10 mm of one side of the specimen 55 comes into direct contact with the electrolyte 85.

[0076] 6 shows a spontaneous corrosion potential meter used in one embodiment of the present invention. This meter comprises a meter body 40, a reaction vessel 80, and a working electrode 50 (WE), a reference electrode 60 (RE), and an auxiliary electrode 70 (CE) installed in the reaction vessel. The reaction vessel 80 is sealed and has a constant temperature water inlet 81 and outlet 83 to maintain a constant temperature of the electrolyte 85 introduced therein.

[0077] The materials of each electrode are preferably a high-density carbon electrode for the working electrode 50 (WE), a calomel electrode for the reference electrode 60 (RE), and a platinum electrode for the auxiliary electrode 70 (CE).The main body 40 of the spontaneous corrosion potential measuring instrument is preferably the Reference 600 model from Gamry Instruments, and the measurement conditions are preferably set to a voltage range of ±0.5 V and a scan rate of 0.333 mV / s.

[0078] The electrolyte 85 used in measuring the natural corrosion potential value is a 3.5% NaCl solution, and the electrolyte 85 is maintained at a constant temperature of 30°C. It is preferable to measure the natural corrosion potential value after maintaining the specimen 55 in the electrolyte 85 for 600 seconds (10 minutes) after installation in order to stabilize the initial state between the specimen 55 and the electrolyte 85. Other conditions and characteristics related to the measurement of the natural corrosion potential value can be measured with reference to open circuit natural corrosion potential measurement methods known to those of ordinary skill in the art, such as KS D 0279 and 0238.

[0079] The present invention will be described in more detail below with reference to specific examples, but these examples are merely for illustrative purposes and are not intended to limit the scope of the present invention.

[0080] Experimental example Cold-rolled steel sheets having thicknesses of 0.20 mm and 0.23 mm were manufactured by hot rolling and cold rolling using slabs having the compositions shown in Table 1. In Table 1, element % means weight %.

[0081] [Table 1]

[0082] These cold-rolled steel sheets were subjected to decarburization annealing and nitriding, including primary recrystallization annealing, at 840°C in a wet hydrogen, nitrogen, and ammonia mixed gas atmosphere (dew point temperature 69°C, Fe2SiO4 / SiO2 ratio controlled to 1.2), maintained for 150 seconds. An annealing separator containing MgO was applied to the surface of the steel sheets that had undergone primary recrystallization, and then final high-temperature annealing was performed. The final high-temperature annealing was performed in a mixed atmosphere of 25% by volume nitrogen and 75% by volume hydrogen up to 1,150°C. After reaching 1,150°C, the sheets were maintained in a 100% by volume hydrogen atmosphere for approximately 8 hours, and then furnace cooled.

[0083] A coating solution containing colloidal silica nanoparticles and metal phosphate was applied to the surface of the steel sheet that had completed the secondary recrystallization annealing through the final high-temperature annealing process, and the steel sheet was heat-treated at 870°C for 55 seconds to form an insulating coating layer for grain-oriented electrical steel sheet. A superimposed laser beam 20 from a first laser (fiber optic) and a second laser (CO2) was then simultaneously irradiated to form linear deformations 10 on the surface of the steel sheet. The irradiation interval of the superimposed laser beams 20 irradiated onto the surface of the steel sheet was varied from 4.0 to 7.0 mm, forming linear deformations 10 on the surface of the steel sheet in a direction intersecting the rolling direction.

[0084] The lasers used for magnetic domain miniaturization were an optical fiber laser with an output of 52 to 150 W used as the first laser A, and a CO2 laser with an output of 260 to 500 W used as the second laser B. At this time, the scanning speed of the superimposed laser beam 20 was changed within a range of 0.1 to 300 m / s, and the irradiation angle was set to 1° or less.

[0085] Table 2 below shows the test conditions and measured magnetic values ​​for a 0.2 mm thick steel piece, and Table 3 shows the test conditions and measured magnetic values ​​for a 0.23 mm thick steel sheet. In Tables 2 and 3, the iron loss improvement rates W15 / 50 and W17 / 50 respectively refer to the iron loss improvement rate of the magnetically treated specimen relative to the original sheet.

[0086] [Table 2]

[0087] In Table 2, the laser irradiation method for Comparison 1 is "sequential superposition," which means that the second laser B, a CO2 laser, is first irradiated, and then, after a certain time has passed and the steel plate has cooled to room temperature, the first laser A, an optical fiber laser, is sequentially irradiated.

[0088] [Table 3]

[0089] The natural corrosion potential of the manufactured grain-oriented electrical steel sheets was measured and a salt spray test was also carried out. The natural corrosion potential was measured using the natural corrosion potential measuring device shown in Figures 5 and 6 after cutting the manufactured grain-oriented electrical steel sheets to a diameter of 15 mm for each of the 0.2 mm and 0.23 mm thicknesses. The measured natural corrosion potential values ​​for the grain-oriented electrical steel sheet specimens with a thickness of 0.2 mm are shown in Table 4 below, and for the 0.23 mm thickness specimens are shown in Table 5 below.

[0090] The corrosion properties shown in Tables 4 and 5 below are classified based on the natural corrosion potential and salt spray test results. In Tables 4 and 5, a "◎" indicates a natural corrosion potential of -200 mV or higher and no corrosion occurred after 8 hours of salt spray. A "○" indicates a natural corrosion potential of -200 to -350 mV and no corrosion occurred after 8 hours of salt spray. An "X" indicates a natural corrosion potential of -350 to -850 mV and corrosion occurred after 8 hours of salt spray. The salt spray test was conducted according to the KS D 9502 measuring instrument standard.

[0091] [Table 4]

[0092] [Table 5]

[0093] As shown in Table 2, when the steel plate thickness is 0.2 mm, the scanning interval of the superimposed laser is the same at 5 mm, and the output value of each laser is the same, it can be seen from Experiment 2 that the iron loss improvement rate by the superimposed laser is significantly improved compared to fiber alone in Comparison 8 and CO2 alone in Comparison 2. Furthermore, as can be seen from Table 2, in the case of Comparison 4, where the output value of the single CO2 laser is significantly increased to 400 W compared to the CO2 laser output value of the superimposed laser (260 W), the iron loss improvement rate appears to be lower than in Experiments 1 to 5, which have lower output values, and in the case of Comparison 13, where the output of the optical fiber laser is increased, the iron loss improvement rate appears to be lower than in Experiments 1 to 4.

[0094] As can be seen from Table 3, when the steel plate was 0.23 mm thick and the scanning interval of the superimposed laser was changed to 4 mm, 5 mm, 6 mm, and 7 mm, the iron loss improvement rate was significantly improved in each experimental example in which the superimposed laser according to the present invention was irradiated at each scanning interval. As can be seen from Table 2, in the cases of Comparisons 2 to 7 using only CO2, the iron loss improvement rate was very poor despite the output power (W; 200-500) being within the range of the present invention, but the corrosion resistance was slight, as shown in Comparative Example 33 in Table 4. In the cases of Comparisons 6 and 7 using only CO2, although the output power was outside the range of the present invention (W; 500 or more), the iron loss improvement rate was low and the corrosion resistance was also very poor, as shown in Comparisons 37 and 38 in Table 4.

[0095] Furthermore, in Table 2, for Comparative Examples 9 to 13, which are single fiber, the iron loss improvement rate is very poor when the output (W; 10 to 150) is within the range of the present invention, and the corrosion characteristics are also poor, as in Comparative Examples 40 to 44 in Table 4. However, as an exception, in Table 2, for Comparative Example 8, which has a low single fiber output of 52 W, the iron loss improvement rate is significantly low, but the corrosion characteristics are good, as in Comparative Example 39 in Table 4. This tendency is also seen in Table 3, which shows the results of experiments on steel plate with a thickness of 0.23 mm, and shows similar results, as in Table 5, for single CO2 and single fiber. Furthermore, in Table 2, for Comparative Example 1, in which different lasers are sequentially superimposed, the iron loss improvement rate is poor.

[0096] Meanwhile, as can be seen from Tables 4 and 5, the results of measuring the natural corrosion potential values ​​show the following characteristics for both 0.20mm and 0.23mm thick electrical steel sheets. That is, when CO2 laser irradiation is used alone, at an output of 400W or more, where a relatively high iron loss improvement rate is achieved, the measured natural corrosion potential value is low at -650mV or less, indicating poor corrosion characteristics. Furthermore, when fiber laser irradiation is used alone, at an output of 72W or more, where a certain degree of iron loss improvement rate can be ensured, the natural corrosion potential value is low at -700mV or less, indicating poor corrosion characteristics. However, when fiber laser and CO2 laser irradiation are superimposed, not only is a sufficient iron loss improvement rate ensured, but the natural corrosion potential value is also sufficiently ensured, indicating excellent corrosion characteristics.

[0097] In addition to these results, in the cases of Comparison 39 and Comparison 54, where the fiber laser was irradiated alone, the natural corrosion potential value was -120mV or higher, indicating that the corrosion resistance was high. Similar results can also be seen in Comparison 33 and Comparison 45, where the CO2 laser was irradiated alone at 260W output.

[0098] The present invention is not limited to the examples, and can be manufactured in various different forms, and a person skilled in the art to which the present invention pertains should understand that the present invention can be embodied in other specific forms without changing the technical concept or essential characteristics of the present invention. Therefore, the above-described examples should be understood to be illustrative in all respects and not limiting. [Explanation of symbols]

[0099] 10 Deformation section 20 Superimposed laser beam 30 Superimposed Laser 40 Main Unit 50 working electrodes 51 Specimen holder 53 Specimen holder cap 54 Sealing ring 55 Sample 56 Electrode plate 58 Open mouth 60 Reference electrode 70 Auxiliary electrode 80 reactors 81 Constant temperature water inlet 83 Outlet 85 Electrolyte

Claims

1. a first beam spot formed by irradiation with a first laser beam having a first wavelength; a second beam spot formed by irradiation with a second laser beam having a second wavelength, a superimposed laser beam in which the first beam spot is controlled to be positioned partly or entirely within the second beam spot, A method for refining magnetic domains in grain-oriented electrical steel sheets, characterized by irradiating the surface of the electrical steel sheet with radiation to form deformed portions.

2. 2. The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 1, wherein the first laser of the superimposed laser beam is a short-wavelength laser, and the second laser is a long-wavelength laser having a wavelength longer than that of the short-wavelength laser.

3. 3. The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 2, wherein the superimposed laser beam, obtained by simultaneously superimposing the first laser and the second laser, is irradiated onto the surface of the electrical steel sheet that has undergone secondary recrystallization in a direction perpendicular to or inclined from the rolling direction, thereby forming linear deformed portions.

4. The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 2 , wherein the beam shape of the first laser or the second laser is either elliptical or circular.

5. The first laser is one of an optical fiber laser, a YAG laser, and a disk laser, and the second laser is a CO ２ The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 2, wherein the method is performed by a laser.

6. The beam spot of the first laser, the optical fiber laser, has a width of 10 to 200 μm and a length of 10 μm to 200 μm. ２ The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 5, wherein the shape is an ellipse or a circle having a length equal to or less than or greater than the length of the laser beam spot.

7. The second laser is CO ２ 6. The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 5, wherein the laser beam spot is an elliptical beam spot having a width of 100 to 400 [mu]m and a length of 0.4 to 20 mm.

8. 2. The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 1, wherein the irradiation interval of the superimposed laser beam is 2 to 10 mm, and the scanning speed is 0.1 to 300 m / sec.

9. 4. The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 3, wherein the inclination direction of the superimposed laser beam is ±10° or less.

10. The output of the first laser, an optical fiber laser, is 10 to 150 W, the output of the YAG laser is 10 to 250 W, and the output of the second laser, a CO ２ 6. The method for refining magnetic domains in a grain-oriented electrical steel sheet according to claim 5, wherein the laser output is 200 to 500 W.

11. an electromagnetic steel sheet substrate; an insulating coating layer located on the electrical steel sheet substrate, A grain-oriented electrical steel sheet characterized in that one or more linear deformation portions are formed on the surface of the electrical steel sheet substrate, and the electrical steel sheet has a natural corrosion potential value of (-)350 mV or more.

12. The grain-oriented electrical steel sheet according to claim 11, further comprising a glass coating layer formed between the electrical steel sheet substrate and the insulating coating layer.

13. 12. The grain-oriented electrical steel sheet according to claim 11, wherein the electrical steel sheet has a natural corrosion potential value of (-) 50 to (-) 300 mV.

14. The grain-oriented electrical steel sheet according to claim 11, wherein a re-solidified layer is formed on the surface of the insulating coating layer.

15. The grain-oriented electrical steel sheet according to claim 14, wherein the thickness of the resolidified layer is within 20% of the cross-sectional thickness of the insulating coating layer.

16. The grain-oriented electrical steel sheet according to claim 11, wherein the electrical steel sheet has an improvement rate of W15 / 50 iron loss of 6% or more.

17. The grain-oriented electrical steel sheet according to claim 11, wherein the improvement rate of W17 / 50 iron loss of the electrical steel sheet is 9% or more.

18. an electromagnetic steel sheet substrate; an insulating coating layer located on the electrical steel sheet substrate, A grain-oriented electrical steel sheet, characterized in that one or more linear deformed portions are formed on the surface of the electrical steel sheet, and a re-solidified layer is formed on the surface of the insulating coating layer above the linear deformed portions.

19. The grain-oriented electrical steel sheet according to claim 18, wherein a glass coating layer is formed between the electrical steel sheet substrate and the insulating coating layer.

20. 19. The grain-oriented electrical steel sheet according to claim 18, wherein the thickness of the insulating coating layer above the deformed portion is 60% or more of the thickness of the insulating coating layer above an area where no deformed portion is formed.

21. The grain-oriented electrical steel sheet according to claim 18, wherein the resolidified layer contains silicon oxide as a main component, and the insulating coating layer contains phosphate or colloidal silica as a main component.

22. The grain-oriented electrical steel sheet according to claim 18, wherein the thickness of the resolidified layer is within 20% of the cross-sectional thickness of the insulating coating layer.

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