Grain-oriented electrical steel sheet and method for refining its magnetic domains

Abstract

By irradiating the surface of a steel sheet with two or more superimposed lasers, sufficient groove depth is ensured while simultaneously drastically reducing melting by-products such as hill-up sputter, thereby securing grain-oriented electrical steel sheets with excellent iron loss characteristics and a method for refining their magnetic domains. [Solution] The present invention provides a method for refining magnetic domains in grain-oriented electrical steel sheets, comprising the step of forming grooves by superimposing and irradiating with beam spots of two or more lasers having different beam spot shapes, wherein the lasers include a first laser and a second laser, the first beam spot of the beam of the first laser and the second beam spot of the beam of the second laser overlap by 10% or more, and the energy density of the beam of the first laser is 1.1 to 4.0 times that of the beam of the second laser.

Description

[Technical Field]

[0001] The present invention relates to grain-oriented electrical steel sheets and a method for refining their magnetic domains, and more specifically, to grain-oriented electrical steel sheets having excellent iron loss characteristics, achieved by irradiating the surface of the steel sheet with two or more superimposed lasers to ensure sufficient groove depth while simultaneously dramatically reducing melting by-products such as hill-up and sputter, and a method for refining their magnetic domains. [Background technology]

[0002] Grain-oriented electrical steel sheets have excellent magnetic properties and are commonly used as core materials for transformers. The manufacture of such grain-oriented electrical steel sheets involves unique rolling and annealing processes that are specific to the electrical steel sheet manufacturing process. <001> A Goss texture, which is recrystallized in a specific orientation, is formed throughout the steel sheet. To address climate change, the world is increasingly strengthening its greenhouse gas emission classifications. In the case of transformer cores, the factors influencing the greenhouse gas emission classification are directly related to the efficiency of transformers using electrical steel sheets. The efficiency of the transformer core is determined by the iron loss and magnetic flux density of the electrical steel sheets, i.e., the magnetic properties.

[0003] The magnetic flux density of electrical steel sheets is influenced by the degree to which easily magnetized crystal axes are clustered in the crystal structure, i.e., the higher the crystal orientation, the higher the magnetic flux density. Therefore, the manufacturing process of electrical steel sheets can have a significant impact. According to the IEC60404-3 standard, the iron loss of electrical steel sheets is measured in watts when a magnetic field with a maximum magnetic flux density of 1.7T and a frequency of 50Hz is applied. 17 / 50 The [W / kg] value is called the guaranteed iron loss value of the core material, and this value is used as a measure of the iron loss of the electrical steel sheet. However, when designing a transformer, the W measured when a magnetic field of 50Hz frequency is applied at a lower maximum magnetic flux density of 1.5T is used. 15 / 50 The [W / kg] value is sometimes used. In transformers, efficiency is evaluated as being better the lower the iron loss value. Therefore, in the case of electrical steel sheets, the higher the magnetic flux density and the lower the iron loss of the steel sheet, the more efficient it can be used as a transformer core. Of these, the process technology to ensure high magnetic flux density through upward leveling in the manufacturing process of electrical steel sheets has developed to the extent that it can support the efficiency of transformers, so iron loss is now evaluated as a more important indicator.

[0004] Iron loss is classified into eddy current loss and hysteresis loss. Since hysteresis loss tends to decrease with increasing magnetic flux density, eddy current loss plays a crucial role in controlling overall iron loss in grain-oriented electrical steel sheets. Eddy current loss is further classified into classical eddy current loss and abnormal 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 abnormal eddy current loss is an important technique for reducing iron loss. Of these iron losses, eddy current losses decrease as the spacing between magnetic domain walls in the 180° magnetic domain (the magnetic domain in the rolling direction) narrows. Therefore, iron losses can be reduced by miniaturizing the magnetic domains of the electrical steel sheet.

[0005] In electrical steel sheets, refining magnetic domains refers to the process of separating a single magnetic domain-based crystal grain into multiple magnetic domains by applying physical stimulation. Methods for refining magnetic domains include laser irradiation, electron beam irradiation, plasma treatment, etching, or roll pressing. After such a treatment, the process is classified into permanent and temporary magnetic domain refinement depending on whether the magnetic domain refinement effect is maintained after stress relaxation annealing (SRA).

[0006] Among magnetic domain refinement technologies, those that can ensure the effect of reducing iron loss even after stress-relief annealing (SRA) are called permanent magnetic domain refinement technologies. Permanent magnetic domain refinement technologies include laser methods, groove transcription methods using rotating wedge-shaped knife rolls, and etching methods that form grooves by electrochemical etching in solution, all of which are applied to cold-rolled sheets or steel sheets that have completed secondary recrystallization.

[0007] The surface transfer method is a groove formation method using a rotating knife roll, so when the steel plate thickness exceeds a critical value or becomes thinner, it is difficult to maintain the roll durability due to changes in applied load, making it difficult to precisely control the groove depth at high speed. The chemical etching method forms grooves by the anodic reaction of the steel plate in a solvent, so it is difficult to form grooves at high speed. In the laser method, when forming grooves using partial melting of the steel plate, a high-power oscillator is required when forming grooves at high speed, and it is necessary to use a brush after groove formation or introduce a step of applying an oxide before groove formation in order to remove hill-up that occurs around the groove by physical or chemical means. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The present invention provides grain-oriented electrical steel sheets and a method for refining their magnetic domains. More specifically, the present invention provides grain-oriented electrical steel sheets and a method for refining their magnetic domains by irradiating the surface of a steel sheet with two or more superimposed lasers to ensure sufficient groove depth while simultaneously dramatically reducing melting by-products such as hill-up sputter, thereby providing grain-oriented electrical steel sheets and a method for refining their magnetic domains that have excellent iron loss characteristics. [Means for solving the problem]

[0009] The present invention is characterized in that the method for refining the magnetic domains of a grain-oriented electrical steel sheet includes the step of forming grooves by superimposing and irradiating two or more laser beam spots having different beam spot shapes.

[0010] The laser includes a first laser and a second laser, and the first beam spot of the first laser beam and the second beam spot of the second laser beam may overlap by 10% or more. Furthermore, the laser includes a first laser and a second laser, and the energy density of the first laser beam may be 1.1 to 4.0 times that of the second laser beam. The first and second lasers can be selected from CO2 lasers, fiber optic lasers, YAG lasers, ruby ​​lasers, sapphire lasers, disk lasers, diode lasers, or UV lasers. The first and second lasers may each have an output power of 10 to 2000 watts. The first laser and the second laser may have different wavelengths.

[0011] At the superimposed position, the interval between the irradiation time of the first laser beam and the irradiation time of the second laser beam may be 16 ms or less. The grooves may be in a linear form that extends in a direction intersecting the rolling direction. The groove depth may be 5 to 15% of the thickness of the electrical steel sheet. The longitudinal direction of the groove can form an angle of 75 to 105° with the rolling direction. Two to ten grooves can be formed intermittently along the direction perpendicular to the rolling of the electrical steel sheet. [Effects of the Invention]

[0012] According to the present invention, by performing optimal magnetic domain refinement using superimposed lasers, magnetism can be further improved, and the generation of melting by-products such as hill-up and sputter on the steel plate surface can be sufficiently suppressed. Further, according to the present invention, using a long-wavelength laser makes it easy to achieve a higher average output, ensuring the reliability of the processing line, and at the same time, co-irradiation with a short-wavelength laser minimizes magnetic domain formation, which can effectively improve magnetism. [Brief Description of the Drawings]

[0013] [Figure 1] It is a graph showing the light absorptivity of steel sheets depending on laser wavelength. [Figure 2] It is a schematic diagram showing the concept of magnetic domain refinement by forming grooves using superimposed lasers according to the present invention. [Figure 3] It is a schematic diagram showing a beam spot of a superimposed laser according to the present invention. [Figure 4] It is a schematic diagram showing a beam spot of a superimposed laser according to another embodiment of the present invention. [Figure 5] It is a schematic diagram showing a case where laser irradiation is performed twice with a time interval that is not for superimposed lasers according to the present invention. [Mode for Carrying Out the Invention]

[0014] 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 only used to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, 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 for the purpose of referring to specific embodiments only and is not intended to limit the present invention. As used herein, the singular form also includes the plural form unless the context clearly indicates the contrary. As used in the specification, the meaning of "comprise" embodies a specific characteristic, region, integer, step, operation, element, and / or component, and does not exclude the existence or addition of other characteristics, regions, integers, steps, operations, elements, and / or components.

[0015] When one part is described as being "on top of" or "above" another part, it means that it is either directly on top of or above the other part, or that the other part is involved between them. In contrast, when one part is described as being "directly on top of" another part, there is no other part in between them. Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as that generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have the meaning corresponding to the relevant technical literature and the content now disclosed, and are not interpreted in their ideal or highly formal sense unless otherwise defined.

[0016] The embodiments of the present invention will be described below in detail so that they can be easily implemented by a person with ordinary skill in the art to which the present invention pertains. However, the present invention can be realized in a variety of different forms and is not limited to the embodiments described herein. The present invention aims to impart excellent iron loss characteristics to an electrical steel sheet by irradiating the surface of the sheet with a superimposed laser to ensure sufficient groove depth while suppressing the formation of melting by-products such as hill-up and sputter on the steel sheet surface. The present invention includes the step of forming a groove by superimposing and irradiating beam spots of two or more lasers having different beam spot shapes. In grain-oriented electrical steel sheets, one preferred method for improving iron loss is to refine the magnetic domains by forming grooves on the steel sheet surface using a laser.

[0017] The magnetic domain refinement process for grain-oriented electrical steel sheets involves irradiating the sheet with a laser in a direction intersecting the rolling direction (RD direction), as illustrated in Figure 2, to form grooves 10. The grooves can be point-like or continuous linear grooves, with Figure 2 illustrating linear grooves. Here, linear grooves include not only solid lines but also intermittently connected grooves such as dotted and dashed lines, and also grooves that appear zigzag at a microscopic level but are straight at a macroscopic level, encompassing virtually all grooves that form a linear shape. Groove formation in steel plates using a laser involves melting and vaporizing a portion of the steel plate with laser irradiation, leaving the molten and vaporized portion as a groove. In this case, the depth of the groove is proportional to the energy density (power density) of the laser per unit time and is also affected by the laser absorption rate of the steel plate surface.

[0018] However, the depth of the grooves during laser irradiation increases with increasing total laser irradiation energy. However, if more energy than necessary is irradiated onto the steel plate, melting by-products such as hill-up and sputter will remain on the surface of the steel plate, which adversely affect magnetism and must be suppressed. Therefore, it is preferable to irradiate a large amount of energy over the smallest possible area for a short period of time with a high energy absorption rate during laser irradiation. Furthermore, the interaction conditions between the laser beam and the steel sheet are influenced by the characteristics of the laser and the absorption rate of the laser beam to the steel sheet. The absorption rate of the laser beam is influenced by the surface roughness of the steel sheet, the temperature of the steel sheet, the absorption characteristics of the steel sheet surface, and the laser wavelength. However, when the manufacturing conditions of the steel sheet are kept constant across the entire surface, the surface roughness, temperature, and absorption characteristics of the steel sheet surface are constant, and in this case, the absorption rate of the laser beam to the steel sheet will follow the wavelength of the laser.

[0019] In other words, assuming constant manufacturing conditions for steel sheets, the laser absorption rate is affected by the laser wavelength. As shown in Figure 1, the laser absorption rate of steel sheets is approximately 35-40% when the wavelength is short (e.g., a YAG or fiber laser of 1.06 μm), while it is relatively low at approximately 5-10% when the wavelength is long (e.g., a CO2 laser of 10.6 μm). Thus, in the process of refining the magnetic domains of grain-oriented electrical steel sheets using lasers, using short-wavelength lasers is more efficient than using long-wavelength lasers in order to ensure stable iron loss characteristics.

[0020] On the other hand, the surface of electrical steel sheets subjected to magnetic domain refinement treatment may have an insulating coating made mainly of phosphate and silica, several to tens of micrometers thick. These coatings absorb relatively little of the laser beam from short-wavelength lasers (e.g., YAG or fiber lasers with a wavelength of 1.06 μm), but show significant absorption from long-wavelength lasers (e.g., CO2 lasers with a wavelength of 10.6 μm). As a result, the thickness of the insulating coating must be considered when using long-wavelength lasers, but the thickness of the insulating coating does not need to be considered as much when using short-wavelength lasers as it does when using long-wavelength lasers. As described above, short-wavelength lasers and long-wavelength lasers have different characteristics. Therefore, when short-wavelength and long-wavelength lasers are used simultaneously in superposition, only the advantages of each laser applied to magnetic domain refinement are preferentially exhibited without side effects, thereby inducing a synergistic effect.

[0021] Here, using superimposed lasers means using two or more lasers with different beam spot shapes to irradiate the surface of the steel plate, so that the spot of one laser beam formed on the surface of the steel plate is partially or completely located within the spot of another laser beam. Therefore, in this invention, superimposed lasers with different wavelengths include not only those in which the spots of one laser beam are completely superimposed within the spot of another laser beam, but also those in which they are partially superimposed. On the other hand, regarding the irradiation time, if there is a time interval, the surface of the steel plate cools rapidly during that interval, making it difficult to obtain the effect of superimposed laser irradiation. Therefore, it is preferable to irradiate with two or more lasers simultaneously, and specifically, the interval between the irradiation time of the first laser beam and the irradiation time of the second laser beam may be 16 ms or less. Figure 3 shows an example in which the first laser beam 21 and the second laser beam 22 are partially superimposed, and Figure 4 shows an example in which the first laser beam 21 and the second laser beam 22 are fully superimposed. As mentioned above, Figure 5 shows the case where the laser beams are not superimposed at a specific point in time. If the interval between irradiation times at a specific location exceeds 16 ms, it is defined as not being irradiated with superimposed beams.

[0022] In this invention, "beam spot" refers to the beam spot on the surface of the steel plate. Figure 3 schematically shows the first beam spot of the first laser beam 21 and the second beam spot of the second laser beam 22. The first beam spot of the first laser beam and the second beam spot of the second laser beam overlap by 10% or more. 10% or more means that the width (B) of the first laser beam overlaps by 10% or more. 1W ) and width of the second laser beam (B 2W ) for the width of the superimposed region (O W This refers to the ratio of the width of the first laser beam (B). In Figure 3, the width of the first laser beam (B) is shown. 1W This is the case when ) is small, and in this case the superposition ratio is O W / B 1W It can be calculated using this method.

[0023] The width of the laser beam refers to the length of the laser beam in the direction (Y direction) perpendicular to the groove length direction (or the laser irradiation traveling direction, X direction). The length of the laser beam refers to the length of the laser beam in the groove length direction (or the laser irradiation traveling direction, X direction). In FIG. 3, the length of the first laser beam (B 1L ) and the length of the second laser beam (B 2L ) are shown. FIG. 4 schematically shows that the first beam spot of the first laser beam 21 is located within the second beam spot of the second laser beam 22, and in this case, the superposition ratio is calculated as 100%.

[0024] Meanwhile, in the present invention, the groove formed by irradiation with the superimposed laser has an inverted bell shape. The first laser and the second laser can be selected from CO₂ lasers, optical fiber lasers, YAG lasers, ruby lasers, sapphire lasers, disk lasers, diode lasers, or UV lasers. More specifically, as the first laser (A) which is a short-wavelength laser, a laser with a relatively short wavelength can be used, for example, optical fiber (Er-Fiber, Yb-Fiber, Tm-Fiber) lasers, YAG (Nd:YAG, Yb:YAG) lasers, ruby lasers, sapphire lasers, etc. can be used. In addition, as such a first laser, 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) can also be used.

[0025] Furthermore, as the second laser, which is a long-wavelength laser, a laser with a wavelength relatively longer than the short-wavelength laser can be used. For example, a CO2 laser is preferred as the second laser. However, if a UV laser (0.150~0.355 μm) is used as the first laser, which is a short-wavelength laser, then any laser with a longer wavelength than the first laser can be used as the second laser. For example, if a UV laser (0.150~0.355 μm) is used as the first laser, a YAG laser can also be used as the second laser.

[0026] The following will provide a more detailed explanation of the magnetic domain refinement method using superimposed lasers 30, using as an example a case where a fiber optic laser is used as the first laser with a short wavelength and a CO2 laser is used as the second laser with a long wavelength. The first laser, a fiber optic laser, uses a short wavelength laser with a relatively high laser absorption rate for steel plates, making it possible to irradiate a narrow area with sufficient incident energy for groove formation for a short duration. Furthermore, because the fiber optic laser used as the first laser has a narrow incident energy range, the formation of melting byproducts can be minimized. However, irradiation with the first laser alone is insufficient to irradiate enough energy for groove formation.

[0027] On the other hand, the second laser, the CO2 laser, can be used at high power levels ranging from several hundred watts to several kilowatts or more, depending on the steel sheet speed. Therefore, the second laser, the CO2 laser, is suitable for a kind of preheating role. However, because the second laser (B), the CO2 laser, has a low laser absorption rate for steel sheets, an excessive amount of energy must be administered in order to form grooves with the CO2 laser alone, and a large amount of melting byproducts are formed in this process. Therefore, a fiber optic laser with a relatively short wavelength is used as the primary laser for groove formation, while a CO2 laser with a relatively long wavelength is used as an auxiliary laser that serves a kind of preheating function. In this case, by refining the magnetic domains, 180° magnetic domains (opposite poles of lancet domains) are formed in the surface direction by magnetoelastic energy, and 90° magnetic domains are formed in the plate thickness direction to reduce magnetoelastic energy. As a result, the spacing between magnetic domains becomes narrower, and consequently, the anomalous eddy current loss is reduced.

[0028] The main laser is preferably located in a region where the auxiliary laser's cross-sectional beam intensity is 25% or higher. More preferably, the main laser is located in a region where the auxiliary laser's cross-sectional beam intensity is 30% or higher. When the main laser located on the final steel plate surface is in a region where the auxiliary laser's cross-sectional beam intensity is 25% or higher, the laser absorption rate of the main laser can be maximized. In other words, if the main laser is located in a region where the beam intensity is less than 25%, there is a disadvantage in that the laser absorption rate of the steel plate surface by the main laser cannot be significantly increased.

[0029] As described above, the magnetic domain refinement method according to the present invention uses a short-wavelength optical fiber laser as the first laser and a long-wavelength CO2 laser as the second laser to form grooves 10 of sufficient depth while minimizing the formation of melting byproducts. At this time, the beam spot of the optical fiber laser, which is the short-wavelength first laser irradiated onto the surface of the steel plate, is preferably approximately circular in shape, and its diameter B W1 B L1 The width B may be 5 to 500 μm. Also, the beam spot of an optical fiber laser has a width B W1 Its length B is 5 to 500 μm. L1 It can be used even if its length is less than or greater than the length of the CO2 laser beam spot, which is the second laser.

[0030] The beam width B of the optical fiber, which is the first laser. W1If the beam width decreases to less than 5 μm, the energy density may be concentrated in a narrow region, which can lead to a decrease in magnetic flux density and iron loss, and the optical system structure becomes more complex. W1 If the width exceeds 500 μm, the groove width in the longitudinal direction of the steel plate increases, and multiple melting by-products such as hill-up and sputter may be formed, potentially causing a decrease in magnetic flux density. More specifically, the width and length of the beam spot may be 10 to 100 μm. On the other hand, the beam spot of the CO2 laser, which is a second long-wavelength laser irradiated onto the surface of the steel plate, has a beam width B. 2W The beam length is 100-400 μm, and the beam length B 2L An elliptical shape with a radius of 0.4 to 20 mm is preferred. Furthermore, the beam spot of a long-wavelength CO2 laser can also be circular with a radius of 100 μm or more.

[0031] The second laser is a CO2 beam with width B 2W To form the magnetic flux layer within 100 μm is undesirable because it requires a complex mirror optical system, similar to that of a fiber optic laser. If the layer exceeds 400 μm, multiple melting byproducts such as hill-up and sputtering may form, potentially leading to a decrease in magnetic flux density. The reason for limiting the beam spot size of the CO2 laser, which is a long-wavelength second laser, is to consider the range over which the effect of the laser beam on the steel plate is maintained when scanning the surface of a steel plate moving at high speed.

[0032] The case in which the lasers according to the present invention are used in superposition will be explained in more detail. As shown in Figures 3 and 4, using the first laser beam 21 and the second laser beam 22 superimposed on the final steel plate surface means controlling them so that the beam spots of the first laser beam 21 and the second laser beam 22 superimpose. In other words, when the laser beam spot 20 irradiated onto the steel plate surface is viewed on a plane as shown in Figure 4, the first laser beam 21 is said to be completely located anywhere within the range of the second laser beam 22, which has a larger beam spot. Furthermore, as shown in Figure 3, the beams are said to be "superimposed" even if the first laser beam 21 is partially located within the range of the second laser beam 22.

[0033] Furthermore, as shown in Figure 5, if the laser beams do not overlap at a specific point in time, they are considered not to be overlapping. However, if the interval between the irradiation time of the first laser beam and the irradiation time of the second laser beam to a specific location is 16 ms or less, they are considered to be irradiated simultaneously. If this time range is exceeded, the steel plate surface cools rapidly, making it difficult to obtain sufficient effect from superimposing the lasers. The time interval refers to the width of the overlapping region after the second laser (or first laser) has been irradiated and the first laser (or second laser) has progressed. W This refers to the time until the value reaches its maximum. In the case of Figure 5, although the laser beams are not superimposed on a specific point in time, if the first laser beam 21 moves to the dotted circle within 16 ms and superimposes the irradiation position of the second laser beam 22, this is considered as superimposed irradiation.

[0034] In this invention, the oscillation mode of the laser beams used is preferably a continuous wave laser for both the first and second lasers, which continuously generates laser light, but a pulse laser can also be used. The energy density of the first laser beam may be 1.1 to 4.0 times that of the second laser beam. As mentioned above, by applying high energy to a small area with a high energy density of the first laser beam, deep grooves can be formed with less melting byproduct. Also, by applying low energy to a large area with a relatively low energy density of the second laser beam, the absorption rate of the first laser beam can be increased. Furthermore, while the Gaussian mode of the TEM00 is preferred for both the first and second laser beams, the multi-transverse mode of the TEMmn can also be used.

[0035] However, the superimposed laser beams 20 of different wavelengths irradiated onto the steel plate surface according to the present invention minimize the formation of grooves in the longitudinal direction of the steel plate, while allowing for the formation of deeper grooves. Therefore, the beam shape and beam quality of each laser are not specifically limited. The first and second lasers may each have an output of 10 to 2000 W. More specifically, the output of the first laser may be 1000 to 2000 W, and the output of the second laser may be 100 to 700 W. These output ranges for each laser indicate the laser output conditions when the steel plate is traveling at a speed of 10 to 30 mpm. The laser output value can be optimally controlled depending on the speed of the steel plate, and the output value may deviate from the above range depending on the speed of the steel plate.

[0036] When the laser beam 30, which is formed by the superposition of the first and second lasers as described above, is irradiated onto the surface of the steel plate, the spacing between the grooves (i.e., the spacing between grooves in the rolling direction of the steel plate) is 2 to 10 mm, the angle between the rolling direction and the laser propagation direction (groove length direction, X direction) is 75 to 105°, and the scanning speed is preferably 0.1 to 300 m / sec. The electrical steel sheet used at this time can be a cold-rolled sheet, a steel sheet after primary recrystallization annealing, or a steel sheet after secondary recrystallization annealing. Furthermore, if the irradiation interval of the superimposed laser beams 20 irradiated onto the steel plate surface becomes excessively narrow, less than 2 mm, the effect of the heat-affected zone becomes large, resulting in inferior magnetic flux density and iron loss. If the irradiation interval is 10 mm or more, the thermal shock effect necessary to ensure the reduction of magnetic domains decreases, making it difficult to achieve the desired effect.

[0037] Furthermore, when irradiating the steel plate surface with the superimposed laser beam 20, it can be irradiated in a direction perpendicular or inclined to the steel plate rolling direction, and the angle between the rolling direction and the laser propagation direction (groove length direction, X direction) can be 75 to 105°. If this angle range is exceeded, the required magnetic domain refinement effect may not be achieved. Furthermore, the scanning speed of the superimposed laser must be the same as the steel plate's moving speed, and if the moving speed increases, the scanning speed must increase even further. Therefore, a speed of 0.1 to 300 m / sec is preferred, and this speed refers to the value exemplified under the 15 mpm condition.

[0038] On the other hand, as shown in Figures 3 and 4, this means that the first and second laser beams are irradiated simultaneously and in superposition, excluding superposition with a time interval between them, as shown in Figure 5. Simultaneous irradiation means that the time interval between the irradiation of the first laser beam and the irradiation of the second laser beam to a specific location is 16 ms or less. If this time range is exceeded, the steel plate surface cools rapidly, making it difficult to fully obtain the effect of superimposed laser irradiation. The time interval refers to the time between the irradiation of the second laser (or first laser) and the progression of the first laser (or second laser), and the width of the superposition region O W This refers to the time until the value reaches its maximum.

[0039] The grooves may be in a linear form that extends in a direction intersecting the rolling direction. The groove depth may be 5 to 15% of the thickness of the electrical steel sheet. The longitudinal direction of the groove can form an angle of 75 to 105° with respect to the rolling direction. Grooves can be formed intermittently in groups of 2 to 10 along the direction perpendicular to the rolling of the electrical steel sheet. On the other hand, when grooves are formed by irradiating the surface of the electrical steel sheet according to the present invention with a superimposed laser, it is preferable that the W17 / 50 iron loss improvement rate of such a steel sheet is 5.0% or more. More specifically, it is preferable that it be 9% or more.

[0040] The method for manufacturing grain-oriented electrical steel sheets according to the present invention will be described in detail below. [Manufacturing of cold-rolled steel sheets] To manufacture grain-oriented electrical steel sheets, first, slabs of electrical steel sheet base material are produced. The chemical composition and microstructure of the slab are not limited as long as the easy magnetization axes are aligned in a certain direction and it functions as an electrical steel sheet. However, to give an example, the chemical composition of the slab is as follows.

[0041] The composition, by mass%, is as follows: C: 0.08% or less (excluding 0%), Si: 1.0-6.5%, Mn: 0.005-3.0%, (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 (excluding 0%), (total of one or more of P and S): 0.100% or less (excluding 0%), (total of Cu and Sn): 1.0% or less, with a total of 0.2% or less of rare earth elements and other impurities, and the remainder being Fe.

[0042] (C: 0.08% or less (excluding 0%)) Carbon (C) is an element that is inevitably mixed into steel, but it is preferable to control its content to an appropriate level because it worsens the magnetic properties due to magnetic aging. If the C content in the steel sheet is excessively low, phase transformation may not occur sufficiently during the manufacturing process, leading to non-uniformity of the steel sheet's microstructure and ultimately instability of the secondary recrystallized structure. If the C content is excessively high, carbides become coarse during the manufacturing process, resulting in excessive precipitation and insufficient decarburization, which can reduce the degree of accumulation of the Goss texture and damage the secondary recrystallized texture. Therefore, the C content of the steel sheet should be 0.08% or less, more preferably 0.001 to 0.040%.

[0043] (Si: 1.0~6.5%) Silicon (Si) is a fundamental component of grain-oriented electrical steel sheets and plays a role in increasing the resistivity of the steel sheet and reducing iron loss. If the Si content is less than 1.0%, the resistivity decreases, leading to increased eddy current loss and deterioration of iron loss characteristics, making it impossible to expect the Si addition effect. If the Si content is 6.5% or more, the brittleness of the steel sheet increases and the toughness decreases, which may cause sheet fracture during the rolling process. Furthermore, sufficient nitride formation may not occur during the manufacturing process, making it impossible to secure sufficient grain suppression force necessary for secondary recrystallization during the final high-temperature annealing process. Therefore, a Si content of 1.0 to 6.5% is preferable.

[0044] (Mn: 0.005~3.0%) Manganese (Mn) has the effect of reducing overall iron loss by increasing resistivity and decreasing eddy current loss. Not only does it react with sulfur in the raw steel state to form Mn-based sulfides, but it also reacts with nitrogen introduced by nitriding treatment along with silicon to form (Al,Si,Mn)N precipitates, which suppresses the growth of primary recrystallized grains and causes secondary recrystallization. It is an important element that affects the surface quality of the final product. However, if the Mn content is too low, the surface quality of the final product may deteriorate. Also, if the Mn content is too high, the austenite phase fraction increases significantly, damaging the Goss texture, reducing the magnetic flux density, and excessive oxide layer formation during decarburization annealing may hinder decarburization. Therefore, a Mn content of 0.005 to 3.0% is preferable.

[0045] (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 carbon and nitrogen during the manufacturing process to form precipitates. However, if added in excessive amounts, they can 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 amount of one or more elements selected from Nb, V, and Ti to 0.05% or less.

[0046] (Total of one or more of Cr, Sn, and Sb: 2.5% or less) Chromium (Cr) is added to promote the formation of a Goss texture and reduce iron loss, while sn is added to suppress grain growth and ultimately improve magnetic flux density. Antimony (Sb) segregates at grain boundaries, suppressing grain growth and stabilizing secondary recrystallization. Since all three of these elements are interrelated in the formation of the secondary recrystallized structure, it is preferable to control the total amount of Sn, Sb, and Cr to 2.5% or less.

[0047] (Al: 2.0% or less (excluding 0%)) 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 (Al, Si, Mn)N and AlN nitrides, acting as a strong grain growth inhibitor. However, if the Al content is excessively high, the precipitates become non-uniform, the formation of secondary recrystallization becomes unstable, and the magnetic properties of the steel sheet deteriorate. Therefore, it is preferable to add Al to 2.0% or less.

[0048] (Total of one or more of P and S: 0.1% or less (excluding 0%)) Phosphorus (P) segregates at grain boundaries, hindering grain boundary movement and simultaneously playing a supporting role in suppressing grain growth. S, if added in excessive amounts, destabilizes secondary recrystallization. Furthermore, P and S are elements that are inevitably added during the manufacturing process of electrical steel sheets, and it is preferable to control the total amount of P and S to 0.1% or less.

[0049] (Cu+Sn total: 0.1% or less) Copper (Cu) plays a role in improving the texture by partially dissolving within the crystal grains. If the Cu+Sn content is excessive, it may segregate at the grain boundaries and form a liquid phase at high temperatures. Therefore, it is preferable to control the total amount of Cu and Sn to 0.1% or less.

[0050] (Total amount of rare earth elements and other impurities is 0.2% or less) The grain-oriented electrical steel sheet according to the present invention may contain rare earth elements such as cerium (Ce) and praseodymium (Pr), as well as other impurities, and preferably the total amount of any rare earth elements and impurities present is 0.2% or less. Rare earth elements and unavoidable impurities refer to impurities that are intentionally added or inevitably mixed in during the steelmaking and manufacturing processes of grain-oriented electrical steel sheets. Since unavoidable impurities are widely known, a detailed explanation will be omitted. Rather than excluding the addition of elements other than the alloy components mentioned above in this invention, a variety of elements may be included as long as it does not impair the technical concept of this invention. If additional elements are included, they are included in place of the remainder Fe.

[0051] Next, a slab is produced from a steel sheet having the above composition by continuous casting. After that, it is heated and hot-rolled in the usual manner, and if necessary, hot-rolled sheet annealing is performed, followed by cold rolling to produce a thickness in the range of 0.1 to 0.5 mm. Here, cold rolling can be performed once or two or more times with intermediate annealing in between.

[0052] [Primary recrystallization annealing] The aforementioned cold-rolled steel sheet is subjected to primary recrystallization annealing by either simultaneous decarburization and nitriding or a post-decarburization nitriding process. In the case of primary recrystallization annealing by simultaneous decarburization and nitriding, the cold-rolled microstructure, which has been deformed during the annealing process, undergoes decarburization annealing including recrystallization. For this reason, the process is carried out in a mixed gas atmosphere containing nitrogen, hydrogen, and moisture. In the case of post-decarburization nitriding, a nitriding treatment can also be performed using ammonia gas after decarburization to introduce nitrogen ions into the steel sheet. When performing simultaneous decarburization and nitriding, the cold-rolled steel sheet charged into the furnace is heated to a temperature of 700-900°C, the dew point temperature of the atmospheric gas is set to 40-70°C, and the Fe2SiO4 / SiO2 ratio on the surface is controlled to 0.5-3.0 to form an oxide layer on the surface of the electrical steel sheet.

[0053] [Secondary recrystallization annealing] Subsequently, an annealing and separating agent based on MgO is applied to the surface of such electrical steel sheets, and then the temperature is raised to over 1,000°C and crack annealing is performed for a long time to induce secondary recrystallization, so that the {110} plane of the steel sheet is parallel to the rolling plane. <001> This results in the formation of a texture with a Goss orientation parallel to the rolling direction. This final high-temperature annealing process creates a glass coating layer containing forsterite on the surface of the steel sheet, and secondary recrystallization occurs inside the steel sheet.

[0054] [Insulating coating formation] After secondary recrystallization of the steel sheet, an insulating coating layer is formed on the surface of the electrical steel sheet, which has been coated with a colloidal silica and metal phosphate insulating coating solution (either individually 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, as an example, the insulating coating layer can be formed by applying an insulating coating solution containing phosphate. It is preferable to use a coating solution containing 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 relative to the weight of the insulating coating solution may be 15% by weight or more.

[0055] [Groove formation and magnetic domain refinement treatment] The groove formation and magnetic domain refinement methods are as described above, so a detailed explanation will be omitted. The magnetic domain refinement treatment can be performed during the following steps: after cold rolling, after primary recrystallization annealing, after secondary recrystallization annealing, or after insulating film formation. More specifically, it can be performed after cold rolling and before primary recrystallization annealing.

[0056] The present invention will be described in more detail below through specific examples. However, these examples are merely illustrative and the present invention is not limited thereto. [Examples]

[0057] Experimental Example 1 Cold-rolled steel sheets with a thickness of 0.20 mm were manufactured by hot-rolling and cold-rolling using slabs having the composition shown in Table 1 below. In Table 1, % refers to weight percent.

[0058] [Table 1]

[0059] The steel plate was moved at a speed of 0.83 m / s, and a fiber laser was used as the primary laser, with a CO2 laser used as an auxiliary laser. The fiber laser beam formed on the steel plate had a width (in the rolling direction) of 10 μm and formed an elliptical beam, while the CO2 laser formed an elliptical beam with a width of 150 μm. By aligning the centers of the laser beams and simultaneously irradiating the steel plate in the width direction, linear grooves with an average depth of 20 μm were formed at 3 mm intervals. Steel sheets with grooves formed by laser irradiation were subjected to decarburization and nitriding, followed by MgO coating, high-temperature annealing, and planar annealing to form a surface insulating coating layer. Subsequently, after stress relaxation annealing heat treatment, the magnetism of the steel sheets was measured using a single-sheet magnetic tester (SST). For the original sheets, cold-rolled sheets at the area closest to the groove formation were subjected to decarburization and nitriding, high-temperature annealing, and planar annealing to form a surface insulating coating layer, followed by stress relaxation annealing heat treatment, and the magnetism was measured using a single-sheet magnetic tester to determine the iron loss and magnetic flux density of the original sheets.

[0060] [Table 2]

[0061] As shown in Table 2, when the first and second lasers were superimposed, a satisfactory improvement in iron loss was not achieved.

[0062] Experimental Example 2 The procedure was carried out in the same manner as in Example 1, but with the laser irradiation interval changed to 2.5 mm and the beam overlap ratio changed as shown in Table 3 below. In Comparative Example 4, the second laser was irradiated after a 1-second interval following the first laser irradiation.

[0063] [Table 3]

[0064] As shown in Table 3, it can be confirmed that iron loss improvement can be achieved when the first and second lasers are superimposed. On the other hand, it can be confirmed that when a single laser is used, or when a long time has passed before laser irradiation, iron loss actually deteriorates, and the insulating properties become inferior. The present invention is not limited to the embodiments described above, and can be manufactured in a variety of different forms. Those with ordinary skill in the art to which the present invention pertains should understand that the invention can be implemented in other specific forms without altering the technical idea or essential features of the invention. Therefore, the embodiments described above should be understood in all respects as illustrative and not limiting. [Explanation of Symbols]

[0065] 10 Grooves 20 laser beam spots 21. First Beam Spot 22 Second Beam Spot 100 grain-oriented electrical steel sheet

Claims

1. A method for refining the magnetic domains of a grain-oriented electrical steel sheet, characterized by including a step of forming grooves by superimposing and irradiating two or more laser beam spots having different beam spot shapes.

2. The method for refining magnetic domains of a grain-oriented electrical steel sheet according to claim 1, wherein the lasers include a first laser and a second laser, and the first beam spot of the beam of the first laser and the second beam spot of the beam of the second laser overlap by 10% or more.

3. The method for refining magnetic domains of a grain-oriented electrical steel sheet according to claim 1, characterized in that the laser includes a first laser and a second laser, and the energy density of the beam of the first laser is 1.1 to 4.0 times that of the beam of the second laser.

4. The aforementioned lasers include a first laser and a second laser, and the first laser and the second laser are CO ２ The method for refining magnetic domains of a grain-oriented electrical steel sheet according to claim 1, characterized in that the laser is selected from a laser, optical fiber laser, YAG laser, ruby ​​laser, sapphire laser, disk laser, diode laser, or UV laser.

5. The method for refining magnetic domains of grain-oriented electrical steel sheets according to claim 1, characterized in that the laser includes a first laser and a second laser, and the first laser and the second laser each have an output of 10 to 2000 W.

6. The method for refining magnetic domains of a grain-oriented electrical steel sheet according to claim 1, characterized in that the laser includes a first laser and a second laser, and the wavelengths of the first laser and the second laser are different from each other.

7. The method for refining magnetic domains of grain-oriented electrical steel sheets according to claim 1, characterized in that the lasers include a first laser and a second laser, and the interval between the irradiation time of the beam of the first laser and the irradiation time of the beam of the second laser at the superposition position is 16 ms or less.

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