Electrical steel sheet and manufacturing method therefor

Abstract

An electrical steel sheet according to one embodiment of the present invention has, among all the grains, at least 70 area% of grains having a ratio (D2 / D1) of the diameter (D2) of an inscribed circle to the diameter (D1) of a circumscribed circle of at least 0.5 and has a difference (△AI) of 0.5-7 wt% between the Al content at the point 25 μm inward from a surface of the steel sheet and the average Al content in the entire steel sheet.

Description

Electrical steel sheet and method of manufacturing the same

[0001] One embodiment of the present invention relates to an electrical steel sheet and a method for manufacturing the same. Specifically, one embodiment of the present invention relates to an axial motor electrical steel sheet with excellent magnetic properties and processability, and a method for manufacturing the same, by appropriately controlling the grain shape and diffusing Al from the outside.

[0002] Non-oriented electrical steel is used as a core material in rotating machinery such as motors and generators, as well as in stationary machinery such as small transformers, playing a crucial role in determining the energy efficiency of electrical equipment. Recently, stricter motor efficiency regulations have led to a significant increase in the use of high-efficiency motors. To improve the efficiency of these motors, it is necessary to reduce iron losses or copper losses. These iron and copper losses are significantly influenced by the magnetic properties of the electrical steel used as the motor core material. Therefore, motor manufacturers are increasingly using electrical steel with low iron losses instead of conventional steel with high iron losses. To reduce copper losses, methods such as lowering the design magnetic flux density or reducing the excitation current at the design flux are used; however, to utilize the latter method, it is necessary to increase the magnetic flux density of the electrical steel. In particular, electrical steel with high magnetic flux density offers the advantage of enhancing torque, which allows motors with frequent on / off cycles to generate high output quickly. To manufacture electrical steel sheets with high magnetic flux density, the silicon content must be lowered; however, a low silicon content—an element that increases resistivity—presents a problem of high iron loss. Therefore, there is a need to develop electrical steel sheets with characteristics of low iron loss and high magnetic flux density.

[0003] From a structural perspective of the motor, the torque in a radial motor is typically generated in proportion to the product of the square of the rotor diameter and the length of the cylindrical stator. Therefore, to increase the torque, the length of the motor's cylindrical stator is increased, which leads to the problem of an increase in the overall volume of the motor.

[0004] Unlike conventional radial motor structures, the stator and rotor of an axial flux motor are arranged parallel to each other in the axial direction. Magnetic force is transmitted axially through the stator to the rotor, generating torque, and the magnitude of the torque is proportional to the cube of the diameter of the cylindrical rotor. In other words, it is a structure capable of generating high torque even in a small space. Since the magnetic force generated by the current applied to the stator is transmitted axially to the rotor magnets, the application of magnetic materials with particularly excellent unidirectional magnetic properties is required.

[0005] The electrical steel sheet described in the Korean patent (KR10-1605791 B1) can be used in axial motors due to its excellent magnetic flux density and iron loss; however, because its grain shape is columnar, there is a high possibility of defects in the cutting edge during punching. When manufacturing motors using electrical steel sheets, the product is processed by punching; however, plastic deformation and residual stress inevitably occur at the cutting edge during this process. The resulting plastic deformation causes non-uniformity in the lamination of the electrical steel sheets, while residual stress degrades magnetic properties. To improve performance in processing steps that include such punching, an electrical steel sheet with excellent processability is required. Generally, it is known that materials with smaller grain sizes are advantageous for punchability, but materials with larger grain sizes are advantageous in terms of magnetic properties.

[0006] One embodiment of the present invention relates to an electrical steel sheet and a method for manufacturing the same. Specifically, one embodiment of the present invention provides an axial motor electrical steel sheet with excellent magnetism and a method for manufacturing the same by appropriately controlling the grain shape and diffusing Al from the outside.

[0007] An electrical steel sheet according to one embodiment of the present invention has at least 70% of the total grain area in which the ratio (D2 / D1) of the diameter of the circumscribed circle (D1) to the diameter of the inscribed circle (D2) is 0.5 or more, and the difference (△Al) between the Al content at a point 25 µm inward from the surface of the steel sheet and the average Al content in the entire steel sheet is 0.5 to 7 weight%.

[0008] An electrical steel sheet according to one embodiment of the present invention may comprise, in weight percent, Si: 0.3% to 4.0%, C: 0.15% or less (excluding 0%), Al: 0.1% to 3.0%, and the remainder being Fe and unavoidable impurities.

[0009] An electrical steel sheet according to one embodiment of the present invention may further include Mn: 0.1 wt% or less and S: 0.005 wt% or less.

[0010] In one embodiment of the present invention, the electrical steel sheet may have a fraction of grains with a grain diameter of 30 μm to 200 μm among the total grains of 80 area% or more.

[0011] The electrical steel sheet according to one embodiment of the present invention is {110} <001> The area fraction of grains forming an angle of 15˚ or less is 20 to 60%, and {100} <001> The area fraction of crystal grains forming an angle of 15˚ or less may be 5 to 20%.

[0012] In one embodiment of the present invention, the difference in magnetic flux density between the rolling direction and the rolling direction perpendicular to the electrical steel sheet may be 10% to 20%.

[0013] A method for manufacturing an electrical steel sheet according to one embodiment of the present invention comprises the steps of: hot rolling a slab to produce a hot-rolled steel sheet; first cold rolling the hot-rolled steel sheet; decarburizing annealing the steel sheet that has been first cold rolled; second cold rolling the steel sheet that has been decarburized; a non-oxidizing annealing step of annealing the steel sheet that has been second cold rolled in a non-oxidizing atmosphere; a coating step of forming a coating layer containing Al on one or both sides of the steel sheet that has been non-oxidizing annealed; and a diffusion annealing step of diffusion annealing Al on the coated steel sheet.

[0014] The slab may contain, in weight percent, Si: 0.3% to 4.0%, C: 0.01% to 0.4%, Al: 0.0100% or less (excluding 0%), and the remainder being Fe and unavoidable impurities.

[0015] The slab may further contain Mn: 0.1 wt% or less and S: 0.005 wt% or less.

[0016] A method for manufacturing an electrical steel sheet according to one embodiment of the present invention further includes a step of annealing a hot-rolled steel sheet, and may include a decarburization process in the step of annealing the hot-rolled steel sheet.

[0017] The step of annealing the hot-rolled plate can be performed at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower.

[0018] The decarburization annealing step can be performed at a temperature of 750°C to 1000°C and a dew point temperature of 25°C to 70°C.

[0019] After the second cold rolling step, a decarburization annealing step may be further included.

[0020] Prior to the non-oxidizing annealing step, the carbon content in the steel sheet may be 0.15 weight% or less.

[0021] The decarburization annealing step and the secondary cold rolling step may be repeated two or more times.

[0022] The first cold rolling step and the second cold rolling step may each have a reduction rate of 50 to 80%.

[0023] The non-oxidizing annealing step can be performed at a cracking temperature of 750 to 1050°C for 60 seconds to 5 minutes.

[0024] The non-oxidizing annealing step can be performed in an atmosphere with a dew point temperature of -0℃ or lower.

[0025] The thickness of the coating layer containing Al is 1 to 15 μm.

[0026] The diffusion annealing step can be performed at a cracking temperature of 800 to 1150°C for 60 seconds to 30 minutes.

[0027] According to one embodiment of the present invention, the magnetism in the rolling direction is excellent, so it can be usefully utilized as an iron core for an axial motor.

[0028] According to one embodiment of the present invention, it has excellent processability and can be usefully utilized as an iron core for an axial motor.

[0029] According to one embodiment of the present invention, due to the diffusion of Al, it has excellent magnetism and can be usefully utilized as an iron core for an axial motor.

[0030] In addition, since it can be manufactured through a continuous process, manufacturing time can be relatively shortened and productivity can be improved.

[0031]

[0032] 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 solely to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section without departing from the scope of the present invention.

[0033] The technical terms used herein are for the reference of specific embodiments only and are not intended to limit the invention. The singular forms used herein include plural forms unless phrases clearly indicate otherwise. As used in the specification, the meaning of “comprising” specifies certain characteristics, areas, integers, steps, actions, elements, and / or components, and does not exclude the presence or addition of other characteristics, areas, integers, steps, actions, elements, and / or components.

[0034] When it is stated that one part is "on" or "on" another part, it may be directly on or on the other part, or another part may be involved in between. In contrast, when it is stated that one part is "directly on" another part, no other part is interposed in between.

[0035] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or highly formal sense unless otherwise defined.

[0036] Also, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

[0037] In one embodiment of the present invention, the meaning of including additional elements is that the remainder of iron (Fe) is replaced by an amount of the additional element.

[0038]

[0039] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0040]

[0041] An electrical steel sheet for an axial motor according to one embodiment of the present invention has at least 70% of the total grain area in which the ratio (D2 / D1) of the diameter of the circumscribed circle (D1) to the diameter of the inscribed circle (D2) is 0.5 or more, and the difference (△Al) between the Al content at a point 25 µm inward from the surface of the steel sheet and the average Al content in the entire steel sheet is 0.5 to 7.0 weight%.

[0042] First, an electrical steel sheet for an axial motor according to one embodiment of the present invention comprises Si: 0.3% to 4.0%, C: 0.15% or less (excluding 0%), Al: 0.1% to 3.0%, and the remainder being Fe and unavoidable impurities.

[0043] Si: 0.3 to 4.0 wt%

[0044] Silicon (Si) improves iron loss by lowering the magnetic anisotropy of electrical steel sheets and increasing resistivity. If the Si content is too low, iron loss becomes inferior, and if it is too high, brittleness increases. More specifically, the Si may contain 1.0 to 3.0 weight percent.

[0045] C: 0.15 wt% or less

[0046] Since a process is required for carbon (C) to escape from the center to the surface layer so that Goss grains in the surface layer diffuse to the center during intermediate decarburization annealing, the carbon content in the slab may be 0.010 to 0.400 wt%. More specifically, the carbon content in the slab may be 0.100 to 0.300 wt%. Additionally, the carbon content in the steel sheet after the decarburization annealing step completed may be 0.150 wt% or less. More specifically, it may be 0.100 wt% or less. More specifically, it may be 0.050 wt% or less. More specifically, it may be 0.010 wt% or less. More specifically, it may be 0.005 wt% or less.

[0047] Al: 0.1 to 3.0 wt%

[0048] Aluminum (Al) helps improve magnetism in conventional non-oriented electrical steel sheets by increasing resistivity similar to Si and lowering iron loss. In one embodiment of the present invention, oxidation by Al occurs in the slab and the steel sheet before Al diffusion annealing during the decarburization annealing process, which promotes the growth of crystal grains with orientations harmful to magnetism, so it can be controlled to 100 ppm or less in the slab state.

[0049] In one embodiment of the present invention, Al may be included in the steel sheet by diffusion annealing Al from the outside after non-oxidizing annealing. If too little Al is included, it is difficult to sufficiently obtain the effects of the present invention. If too much Al is included in the steel sheet, brittleness may deteriorate due to the formation of FeAl regular phases. More specifically, Al is included in an amount of 0.60 to 2.90 weight%. In one embodiment of the present invention, the Al content may vary depending on the thickness of the steel sheet, and the Al content can be calculated by assuming that uniform Al exists throughout the entire thickness of the steel sheet. That is, it refers to the average Al content with respect to thickness.

[0050] In one embodiment of the present invention, after non-oxidizing annealing, Al is diffusion annealed from the outside, so that the Al content at the surface of the steel plate and the center of the steel plate may differ. That is, the Al content at the surface of the steel plate may be higher than the Al content at the center. Specifically, the difference (△Al) between the Al content at a point 25 μm inward from the surface of the steel plate and the Al content at the center of the steel plate is 0.5 to 7.0 wt%. If this difference in content is too small, it can be assumed that Al is contained within the slab during the steelmaking process rather than diffusion, or that Al diffusion annealing was performed for too long. If Al diffusion is performed for a long time, orientation crystals that adversely affect magnetism may grow in the crystal grains of the steel plate, thereby adversely affecting magnetism. More specifically, the difference in Al content (△Al) may be 0.70 to 6.50 wt%. The Al content by thickness can be measured using a Glow Discharge Spectrometer (GDS), and to reduce error, measurements can be taken at more than 10 locations and the average can be obtained.

[0051] Specifically, the Al content at a point 25 μm inward from the surface of the steel plate may be 1.00 to 10.00 weight%. More specifically, it may be 1.50 to 8.70 weight%.

[0052] The average content of Al in the entire steel sheet may be 0.500 wt% to 3.000 wt%. More specifically, it may be 0.730 wt% to 2.900 wt%. An electrical steel sheet for an axial motor according to one embodiment of the present invention may further include Mn: 0.1 wt% or less and S: 0.005 wt% or less.

[0053] Mn and S form MnS precipitates, which hinder the growth of Goss grains that diffuse into the center during the decarburization process. Therefore, it is preferable not to add Mn and S. However, considering the amount inevitably incorporated during the steelmaking process, Mn and S in the slab and steel sheet after the non-oxidizing annealing step can be controlled to Mn: 0.1 wt% or less and S: 0.005 wt% or less, respectively. More specifically, Mn: 0.0001 to 0.0500 wt% and S: 0.0001 to 0.0030 wt% may be included. More specifically, Mn: 0.0010 to 0.0300 wt% may be included.

[0054] The remainder comprises Fe and unavoidable impurities. Unavoidable impurities are those introduced during the steelmaking stage and the manufacturing process of electrical steel sheets; as this is widely known in the field, a detailed explanation is omitted. Specifically, since components such as N, Ti, Mg, and Ca react with oxygen in the steel to form oxides, it is necessary to strongly suppress their presence; thus, each component can be managed to a level of 0.005 weight% or less. In one embodiment of the present invention, the addition of elements other than the aforementioned alloy components is not excluded, and various elements may be included within a scope that does not impair the technical spirit of the present invention. If additional elements are included, they replace the remainder, Fe.

[0055]

[0056] On the surface of the steel plate, there may be 70% or more of the area of ​​grains in which the ratio (D2 / D1) of the diameter of the circumscribed circle (D1) to the diameter of the inscribed circle (D2) is 0.5 or greater. Here, the circumscribed circle refers to the smallest circle among the imaginary circles surrounding the outside of the grain, and the inscribed circle refers to the largest circle among the imaginary circles contained within the grain.

[0057] In the texture of an electrical steel sheet for an axial motor according to one embodiment of the present invention, the crystal grains on the surface grow into the interior of the steel sheet, so round-shaped crystal grains are formed on the surface of the steel sheet. As such, due to the crystal grain shape according to one embodiment of the present invention, superior magnetism can be obtained. More specifically, among the total crystal grains on the surface of the steel sheet, the ratio (D2 / D1) of the circumscribed circle diameter (D1) to the inscribed circle diameter (D2) may be 0.5 or greater, and the crystal grains may be 80% or more in area. More specifically, it may be 90% or more in area.

[0058] In one embodiment of the present invention, the grain shape of the surface is due to the short duration of decarburization annealing and non-oxidation annealing. If the annealing is performed for a long duration of 30 minutes or more through decarburization annealing and subsequent batch annealing, the diameter (D1) of the circumscribed circle becomes significantly larger than that of the inscribed circle (D2), so the ratio cannot be greater than 0.5. Here, the circumscribed circle refers to the smallest circle among the imaginary circles surrounding the outside of the grain, and the inscribed circle refers to the largest circle among the imaginary circles contained within the grain. The grain shape can be analyzed using a computer program after observing the surface (ND surface) of the steel plate with an optical microscope and the observed photograph.

[0059] In one embodiment of the present invention, the grain shape of the surface may have a fraction of grains with a grain diameter of 30 μm to 200 μm among the total grains of 80 area% or more. By forming a large number of grains with a specific range of grain diameters throughout the steel plate in this way, uniform magnetism can be imparted to the entire steel plate. In addition, processability can be secured by appropriately adjusting the grain diameter of the steel plate. The grain diameter can be measured as the diameter of a virtual circle assumed to have an area equal to the area occupied by the grains. More specifically, the fraction of grains with a grain diameter of 30 μm to 200 μm may be 90 area% or more.

[0060] An electrical steel sheet for an axial motor according to one embodiment of the present invention is {110} <001> The area fraction of grains having an orientation within 15° of the orientation may be 20 to 60%. In this case, the measurement reference plane is not limited and, specifically, may be a plane parallel to the rolling plane (ND plane). The fraction of Goss grains contributes to improving magnetic properties, particularly high-frequency iron loss, in the rolling direction (RD direction). More specifically, Goss ({110} <001> The fraction of crystal grains may be 40 to 50%. The area of ​​crystal grains having the corresponding orientation in the cross-section of the steel plate is calculated and considered as the area fraction.

[0061] Also, {100} <001> The area fraction of grains forming an angle of 15˚ or less may be 5 to 20%. {100} <001> The crystal grains are also called cube grains and can improve magnetism in the direction perpendicular to the rolling direction (TD direction) of the steel sheet rolling direction (RD direction). {100} <001> The area fraction of crystal grains forming an angle of 15˚ or less may be 10 to 15%.

[0062] In one embodiment of the present invention, the difference in magnetic flux density between the rolling direction (RD direction, the direction with the best magnetic flux density on the plate surface) and the rolling perpendicular direction (TD direction) may be 10% to 20%. The difference in magnetic flux density is 2×(B RD - B TD ) / (B RD + B TD It can be obtained as follows. In the case of an axial motor, since it is advantageous for the magnetic flux density in the rolling direction to be excellent, such magnetic properties can be obtained by controlling the grain orientation and microstructure shape.

[0063] In addition, the average magnetic flux density (B) in the rolling direction and the rolling perpendicular direction 50 ) can be 1.85T or more. In addition, the average iron loss in the rolling direction (W 15 / 50 ) may be 1.60 W / kg or less. In addition, the average iron loss (W in the rolling direction and the direction perpendicular to rolling) is 10 / 400) may be 11.5 W / kg or less. More specifically, the magnetic flux density (B) in the rolling direction 50 ) can be 1.87 to 1.98 T. In addition, the average iron loss (W in the rolling direction and the rolling perpendicular direction) 15 / 50 ) can be 1.10 to 1.55 W / kg. In addition, the average iron loss (W in the rolling direction and the rolling perpendicular direction) is 10 / 400 ) can be 9.0 to 11.3 W / kg. In this case, the measurement reference thickness for iron loss can be 0.5 mm.

[0064]

[0065] A method for manufacturing an electrical steel sheet for an axial motor according to one embodiment of the present invention comprises the steps of: manufacturing a hot-rolled steel sheet by hot-rolling a slab; first cold-rolling the hot-rolled steel sheet; decarburizing annealing the steel sheet that has been first cold-rolled; second cold-rolling the steel sheet that has been decarburized; a non-oxidizing annealing step of annealing the steel sheet that has been second cold-rolled in a non-oxidizing atmosphere; a coating step of forming a coating layer containing Al on one or both sides of the steel sheet that has been non-oxidizing annealed; and a diffusion annealing step of diffusion annealing Al on the coated steel sheet.

[0066] First, the slab is hot-rolled.

[0067] The slab may contain, in weight percent, Si: 0.3% to 4.0%, C: 0.01% to 0.4%, and the remainder being Fe and unavoidable impurities.

[0068] The slab may further contain Mn: 0.1 wt% or less and S: 0.005 wt% or less.

[0069] The description of the steel composition of the slab is the same as the description of the electrical steel sheet for axial motors mentioned above, so the redundant description is omitted. Except for Al and C, the steel composition of the steel sheet may be substantially the same as the steel composition of the slab.

[0070] The slab may be heated before hot rolling. The slab heating temperature may be 1050°C to 1350°C, which is higher than the conventional heating temperature. When the temperature is high during slab reheating, there is a problem in that the hot-rolled structure becomes coarse, which adversely affects magnetism. However, the method for manufacturing an electrical steel sheet according to one embodiment of the present invention has a higher carbon content than conventional methods, so the hot-rolled structure does not become coarse even if the slab reheating temperature is high, and by reheating at a higher temperature than in the conventional case, it is advantageous during hot rolling. However, when the reheating temperature is high, the solubility of precipitates such as TiN and AlN increases, and when cooling, a large number of fine precipitates are distributed, which hinders the growth of crystal grains on the surface.

[0071] Hot rolling can be used to produce a hot-rolled plate with a thickness of 1.3 to 4.3 mm by applying an appropriate rolling reduction rate at the final cold rolling stage so that it can be manufactured to the final product thickness. More specifically, it can be manufactured to a thickness of 1.5 to 4.0 mm.

[0072] The hot rolling finish rolling temperature may be 750°C to 1050°C. If the hot rolling finish rolling temperature is too high, surface defects occur, degrading the quality. In addition, if the finish rolling temperature is too low, the rolling load increases, reducing sheet throughput and lowering productivity. More specifically, it may be 800°C to 1030°C.

[0073] The winding temperature can be 600℃ or lower.

[0074] After hot rolling, the process may further include a step of annealing the hot-rolled steel sheet. At this time, the annealing of the hot-rolled sheet may or may not include a decarburization process. Specifically, the annealing of the hot-rolled sheet may be performed at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower. More specifically, the annealing may be performed at a dew point temperature of -70°C to 70°C.

[0075] More specifically, by including a decarburization process during the annealing step of the hot-rolled plate, the grain orientation can be further improved and the magnetism can be enhanced. Specifically, annealing can be performed at a dew point temperature of 25°C to 70°C.

[0076] After annealing the hot-rolled plate, pickling can be performed. The annealing of the hot-rolled plate may be omitted if necessary.

[0077]

[0078] Next, cold rolling is performed to manufacture cold-rolled steel sheets.

[0079] It is known that in the manufacturing process of oriented electrical steel sheets, cold rolling is effective when performed once with a high reduction rate of nearly 90%. This is because it creates an environment favorable for grain growth for only the Goss grains among the primary recrystallized grains. However, since the method for manufacturing an electrical steel sheet for an axial motor according to one embodiment of the present invention does not utilize abnormal grain growth of Goss orientation grains and instead diffuses the Goss grains in the surface layer generated by decarburization annealing and multiple cold rollings inward, it is advantageous to form a large number of Goss orientation grains distributed in the surface layer.

[0080] Therefore, when cold rolling is performed at a reduction rate of 50% to 80%, a large number of Goss textures may be formed in the surface layer. More specifically, it may be 55% to 75%.

[0081] Next, the steel sheet that has been cold-rolled once is subjected to decarburization annealing. At this time, the decarburization annealing step may be performed in an austenite single-phase region or in a region where a complex phase of ferrite and austenite exists. Specifically, annealing may be performed at a temperature of 750°C to 1000°C and a dew point temperature of 25°C to 70°C. In addition, the atmosphere may be a mixed gas atmosphere of hydrogen and nitrogen. Furthermore, after decarburization annealing, the carbon content in the steel sheet may be 0.15 wt% or less. More specifically, it may be 0.100 wt% or less. More specifically, it may be 0.050 wt% or less. More specifically, it may be 0.010 wt% or less. More specifically, it may be 0.005 wt% or less.

[0082] During this decarburization annealing process, the grain size on the surface of the electrical steel sheet grows coarsely, while the grains inside the electrical steel sheet remain as a fine structure. After this decarburization annealing, the average grain diameter can be 150㎛ to 250㎛. In this case, the grains are surface ferrite grains. Furthermore, the grain diameter refers to the diameter of a hypothetical circle having the same area as the grain. The reference plane is a plane parallel to the rolling vertical plane (TD plane).

[0083] Next, the steel sheet after decarburization annealing is subjected to secondary cold rolling. Since the secondary cold rolling is identical to the primary cold rolling, a detailed explanation is omitted. Through secondary cold rolling, the thickness of the steel sheet can be manufactured to 0.10 to 0.50 mm.

[0084] The aforementioned intermediate annealing step and secondary cold rolling step can be repeated two or more times. By repeating the process two or more times, the thickness of the steel sheet can be further reduced to improve iron loss, and the amount of residual C in the steel sheet can be further reduced to improve magnetism.

[0085] Next, the steel sheet that has completed secondary cold rolling is subjected to non-oxidizing annealing.

[0086] The non-oxidizing annealing step can be performed at a cracking temperature of 750 to 1050°C for 60 seconds to 5 minutes.

[0087] If the cracking temperature is too low, grain growth may not occur sufficiently, and it may be difficult to secure magnetism. More specifically, the cracking temperature may be 800 to 1100°C. In addition, grain growth may occur sufficiently within the aforementioned time range.

[0088] In one embodiment of the present invention, by performing non-oxidizing annealing at a dew point temperature of 0°C or lower, the grain shape in the thickness direction of the steel sheet can be appropriately controlled. For example, if decarburization is performed by setting the dew point temperature high during the non-oxidizing annealing step, the grain length in the thickness direction of the steel sheet increases, and it may be difficult to properly secure processability. More specifically, the cold-rolled sheet can be annealed at a dew point temperature of -50 to 0°C. More specifically, the cold-rolled sheet can be annealed at a dew point temperature of -30 to -20°C. The annealing can be performed in an atmosphere of 99 volume% or more hydrogen.

[0089] The non-oxidizing annealing step can be performed for 60 seconds to 5 minutes. Grain growth can be sufficiently achieved within the aforementioned time range.

[0090] After the first cold rolling step, the process up to the cold-rolled sheet annealing step can be performed as a continuous process. A continuous process means that there is no batch process, such as winding the steel sheet into a coil for annealing. As previously mentioned, a continuous process is possible because the intermediate annealing step or the non-oxidizing annealing step is completed within a few minutes or less.

[0091] Next, a coating layer containing Al is formed on one or both sides of a non-oxidizing annealed steel sheet.

[0092] As a method for forming an Al coating layer, although not particularly limited, it can be formed by applying a coating composition containing an Al component and then heat treating. The Al component refers to any component containing Al that can be used without limitation and may include pure Al, Al alloys, Al oxides, Al hydroxides, etc. The coating composition may be in the form of a slurry containing a solvent for easy dispersion and surface application. The solvent is not particularly limited but may include water or alcohol.

[0093] The thickness of the Al coating layer may be 1.0 to 15.0 μm. More specifically, it may be 2.5 to 12.5 μm.

[0094] Next, the coated steel plate is subjected to Al diffusion annealing. The diffusion annealing step can be performed at a cracking temperature of 800 to 1150°C for 60 seconds to 30 minutes. If the annealing temperature is too low or the annealing time is too short, it is difficult for Al to diffuse sufficiently. If the annealing temperature is too high or the time is too long, crystals that adversely affect magnetism may grow. More specifically, the diffusion annealing step can be performed at a cracking temperature of 850 to 1000°C for 2 minutes to 15 minutes.

[0095] Afterwards, the surface of the steel plate can be pickled and brushed to remove the remaining coating composition.

[0096]

[0097] Specific embodiments of the present invention are described below. However, the following embodiments are merely specific examples of the present invention, and the present invention is not limited to the following embodiments.

[0098]

[0099] Example 1

[0100] A slab containing Si: 2.0% and C: 0.097% by weight, with the remainder being Fe and unavoidable impurities, was heated at a temperature of 1200°C and then hot-rolled to a thickness of 2.0t. Subsequently, the hot-rolled sheet was annealed at a heating zone temperature of 1000°C and a uniform zone annealing temperature of 920°C. Afterward, the steel sheet was cooled, pickled, and cold-rolled with a reduction rate of 75% to produce a cold-rolled sheet with a thickness of 0.5mmt. The cold-rolled sheet was heat-treated at a decarburization annealing temperature of 880°C for 5 minutes at a dew point temperature of 60°C. Afterward, it was cold-rolled again with a reduction rate of 56% to produce a cold-rolled sheet with a thickness of 0.22mmt. Non-oxidizing annealing was performed at a temperature of 950°C for 2 minutes in a mixed gas atmosphere of hydrogen and nitrogen.

[0101] To deposit Al on the surface of a steel plate that had undergone non-oxidizing annealing, a process was applied in which Al was deposited on the surface using a sputtering method and then diffused into the steel within minutes at a temperature of over 1000 degrees. Deposition was performed at a rate of 0.5 µm / min using an Al target on the surface. After coating the steel plate surface with Al, diffusion annealing was performed in an atmosphere of 50% H2, and the Al concentration distribution was controlled by controlling the diffusion time and is shown in Table 1.

[0102] The grain area fraction was measured using EBSD on the ND plane.

[0103] Magnetism was measured using a Single Sheet Tester.

[0104] Steel No. Al Thickness (um) Diffusion Annealing Temperature (°C) Diffusion Annealing Time (min) Al Content (Surface 25um) Al Content (Average) △Al Remarks 10.01 1000 10.01% 0.001% 0.009% Comparative Example 20.12 1000 20.05% 0.002% 0.048% Comparative Example 3 3.00 750 20.20% 0.025% 0.175% Comparative Example 4 3.00 1000 23.50% 0.690% 2.810% Example 5 3.00 750 50.30% 0.034% 0.266% Comparative Example 6 3.00 1000 52.70% 0.730% 1.970% Example Example 7 3.00750100.40%0.041%0.359% Comparative Example 8 3.001000101.50%0.770%0.730% Example 9 3.00750600.40%0.042%0.358% Comparative Example 10 12.00100028.70%2.600%6.100% Example 11 12.00100056.80%2.900%3.900% Example 12 12.001000104.80%2.900%1.900% Example

[0105] River No.{110} <001> Grain fraction (Area %) Grain fraction (Area %) with D2 / D1 of 0.5 or higher Rolling direction B50(T) Rolling direction W15 / 50(W / Kg) Rolling direction W10 / 400(W / Kg) Remarks 14 19 6 1.9 8 1.3 8 11.7 Comparative Example 2 4 19 8 1.9 8 1.3 8 11.8 Comparative Example 3 4 09 9 1.9 3 1.4 4 12.1 Comparative Example 4 4 ​​19 8 1.9 3 1.5 5 10.7 Example 5 4 09 8 1.9 3 1.4 1 2.0 Comparative Example 6 4 19 81.931.3910.3 Example 740991.931.4112.1 Comparative Example 843981.931.2910.2 Example 940961.931.4012.1 Comparative Example 1041981.881.439.6 Example 1141971.881.399.3 Example 1243961.881.199.8 Example

[0106] As shown in Tables 1 and 2, a reduction in iron loss due to Al diffusion can be observed, and in particular, at high frequency iron loss (400 Hz), it is effective to reduce iron loss as the Al content within 25 µm of the surface increases. Additionally, as Al diffusion occurs, a reduction in low frequency iron loss (50 Hz) can also be simultaneously observed.

[0107] The present invention is not limited to the above embodiments and / or examples but can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without changing the technical concept or essential features of the invention. Therefore, the embodiments and / or examples described above should be understood as illustrative in all respects and not restrictive.

Claims

1. An electrical steel sheet having at least 70% of the total grain area in which the ratio (D2 / D1) of the diameter of the circumscribed circle (D1) to the diameter of the inscribed circle (D2) is 0.5 or more, and the difference (△Al) between the Al content at a point 25㎛ inward from the surface of the steel sheet and the average Al content throughout the steel sheet is 0.5 to 7 weight%.

2. In Paragraph 1, The above electrical steel sheet comprises, in weight percent, Si: 0.3% to 4.0%, C: 0.15% or less (excluding 0%), Al: 0.1% to 3.0%, and the remainder being Fe and unavoidable impurities.

3. In Paragraph 2, Electrical steel sheet further containing Mn: 0.1 wt% or less and S: 0.005 wt% or less.

4. In Paragraph 1, Electrical steel sheet having a fraction of grains with a grain diameter of 30㎛ to 200㎛ of 80 area% or more among all grains.

5. In Paragraph 1, {110} <001> The area fraction of crystal grains forming an angle of 15˚ or less is 20 to 60%, and {100} <001> Electrical steel sheet having an area fraction of grains forming an angle of 15˚ or less of 5 to 20%.

6. In Paragraph 1, Electrical steel sheet having a difference in magnetic flux density between the rolling direction and the rolling perpendicular direction of 10% to 20%.

7. A step of manufacturing hot-rolled steel sheets by hot-rolling a slab; A step of performing a first cold rolling of the above hot-rolled steel sheet; Step of decarburizing and annealing the primary cold-rolled steel sheet; Step of secondary cold rolling of the steel sheet after decarburization annealing is completed; A non-oxidizing annealing step in which the steel sheet, after secondary cold rolling is completed, is annealed in a non-oxidizing atmosphere; A coating step of forming a coating layer containing Al on one or both sides of a non-oxidizing annealed steel sheet; and A method for manufacturing an electrical steel sheet comprising a diffusion annealing step of Al diffusion annealing on a coated steel sheet.

8. In Paragraph 7, A method for manufacturing an electrical steel sheet, wherein the above slab comprises, in weight percent, Si: 0.3% to 4.0%, C: 0.01% to 0.4%, Al: 0.0100% or less (excluding 0%), and the remainder being Fe and unavoidable impurities.

9. In Paragraph 8, A method for manufacturing an electrical steel sheet, wherein the above slab further comprises Mn: 0.1 wt% or less and S: 0.005 wt% or less.

10. In Paragraph 7, The above hot-rolled steel plate further includes the step of annealing the hot-rolled plate, and A method for manufacturing an electrical steel sheet including a decarburization process in the step of annealing the hot-rolled sheet.

11. In Paragraph 10, The above-mentioned hot-rolled sheet annealing step is a method for manufacturing an electrical steel sheet by annealing at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower.

12. In Paragraph 7, The above decarburization annealing step is a method for manufacturing an electrical steel sheet by annealing at a temperature of 750°C to 1000°C and a dew point temperature of 25°C to 70°C.

13. In Paragraph 7, A method for manufacturing an electrical steel sheet, further comprising a step of decarburization annealing after the above-mentioned second cold rolling step.

14. In Paragraph 7, A method for manufacturing an electrical steel sheet having a carbon content of 0.15 weight% or less in the steel sheet prior to the above-mentioned non-oxidizing annealing step.

15. In Paragraph 7, A method for manufacturing an electrical steel sheet in which the above-mentioned decarburization annealing step and the above-mentioned secondary cold rolling step are repeated two or more times.

16. In Paragraph 7, The above first cold rolling step and the above second cold rolling step are a method for manufacturing an electrical steel sheet in which the reduction rate is 50 to 80% each.

17. In Paragraph 7, The above non-oxidizing annealing step is a method for manufacturing an electrical steel sheet by annealing at a cracking temperature of 750 to 1050°C for 60 seconds to 5 minutes.

18. In Paragraph 7, The above non-oxidizing annealing step is a method for manufacturing electrical steel sheets by annealing in an atmosphere with a dew point temperature of 0°C or lower.

19. In Paragraph 7, A method for manufacturing an electrical steel sheet in which the thickness of the coating layer containing the above Al is 1 to 15 μm.

20. In Paragraph 7, The above diffusion annealing step is a method for manufacturing an electrical steel sheet, wherein the annealing is performed at a cracking temperature of 800 to 1150°C for 60 seconds to 30 minutes.