Grain-oriented electrical steel sheet and its manufacturing method

Abstract

The present invention relates to a method for producing a grain-oriented electrical steel sheet that can suppress the formation of subgrain boundaries and improve magnetic properties. [Solution] A method for producing a grain-oriented electrical steel sheet according to one embodiment of the present invention includes the steps of: hot rolling a slab containing, by weight, 2.5-4.0% Si, 0.03-0.09% C, 0.015-0.040% Al, 0.04-0.15% Mn, 0.01% or less (excluding 0%) S, and 0.002-0.012% N, with the remainder being Fe and other inevitably mixed impurities, to produce a hot-rolled sheet; cold rolling the hot-rolled sheet to produce a cold-rolled sheet; performing primary recrystallization annealing on the cold-rolled sheet; and performing secondary recrystallization annealing on the cold-rolled sheet after the primary recrystallization annealing.

Description

[Technical Field]

[0001] The present invention relates to a grain-oriented electrical steel sheet and a manufacturing method thereof, and more particularly to a manufacturing method thereof that controls the tension applied to the steel sheet during the insulating coating layer formation process to suppress the formation of subgrain boundaries and improve magnetic properties. [Background technology]

[0002] Generally, grain-oriented electrical steel sheets are steel sheets containing silicon, and the grain orientation is {100} <001> This refers to electrical steel sheets that have a texture aligned in the {100} direction and have extremely excellent magnetic properties in the rolling direction. <001> Obtaining texture is possible through a combination of various manufacturing processes, and in particular, the composition of the steel slab, as well as the series of processes including heating, hot rolling, hot-rolled sheet annealing, primary recrystallization annealing, and secondary recrystallization annealing, must be very strictly controlled.

[0003] Specifically, grain-oriented electrical steel sheets suppress the growth of primary recrystallized grains, and the {100} <001> The secondary recrystallized structure obtained by selectively growing grains with the {100} orientation is what gives the alloy its excellent magnetic properties, so the growth inhibitor for the primary recrystallized grains is more important. <001> One of the main issues in grain-oriented electrical steel manufacturing technology is to allow grains with a certain orientation texture to grow preferentially.

[0004] Primary grain growth inhibitors that satisfy the above conditions and are currently widely used industrially include MnS, AlN, MnSe, etc. Specifically, MnS, AlN, and MnSe contained in a steel slab are reheated at high temperatures for a long period of time to form a solid solution, and then hot-rolled. During the subsequent cooling process, the elements are formed as precipitates with the appropriate size and distribution, which can be used as the growth inhibitors.

[0005] However, this method has the drawback of requiring the steel slab to be heated at high temperatures. Recently, efforts have been made to improve the magnetic properties of grain-oriented electrical steel sheets by heating the steel slab at low temperatures. To achieve this, the addition of antimony (Sb) to grain-oriented electrical steel sheets has been proposed, but this method has been criticized for the problem that the grain size becomes uneven and coarse after the final high-temperature annealing, resulting in poor transformer noise quality.

[0006] Meanwhile, to minimize power loss in grain-oriented electrical steel sheets, an insulating coating (or tensile coating layer) is typically formed on the surface. The insulating coating must have high electrical insulation, excellent adhesion to the substrate, and a uniform color without defects. In addition, with the recent strengthening of international standards for transformer noise and intensifying competition in the related industry, research into the magnetostriction phenomenon is needed to reduce noise from the insulating coating of grain-oriented electrical steel sheets.

[0007] Specifically, when a magnetic field is applied to electrical steel sheets used as transformer cores, they repeatedly contract and expand, inducing vibrations that cause vibration and noise in the transformer. Generally, grain-oriented electrical steel sheets have an insulating coating formed on the steel sheet and a forsterite-based base coating. The difference in the thermal expansion coefficients of the insulating coating is used to apply tensile stress to the steel sheet, thereby improving iron loss and reducing noise caused by magnetostriction. However, there are limitations to meeting the noise levels currently required for high-quality grain-oriented electrical steel sheets.

[0008] Meanwhile, wet coating is known as a method for reducing the 90° magnetic domains in grain-oriented electrical steel sheets. Here, 90° magnetic domains refer to regions with magnetization perpendicular to the direction of the applied magnetic field. The fewer the 90° magnetic domains, the less magnetostriction there is. However, the conventional wet coating method has drawbacks: it lacks the noise reduction effect of applying tensile stress, and it requires a thick coating, which can lead to poor space factor and efficiency in transformers.

[0009] Other known methods for imparting high tensile strength to the surface of grain-oriented electrical steel sheets include vacuum deposition coating methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). However, these coating methods are difficult to commercialize, and grain-oriented electrical steel sheets manufactured using these methods have the disadvantage of having inferior insulation properties. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention provides a method for manufacturing grain-oriented electrical steel sheets. Specifically, the method controls the tension applied to the steel sheet during the insulating coating layer formation process to suppress the formation of subgrain boundaries, thereby improving magnetic properties. [Means for solving the problem]

[0011] According to one embodiment of the present invention, there is provided an electrical steel sheet comprising an electrical steel sheet substrate containing 2.0 to 7.0 wt. % Si and 0.01 to 0.07 wt. % Sb, with the remainder being Fe and other inevitable impurities, and an insulating coating layer disposed on the electrical steel sheet substrate, wherein the insulating coating layer contains pores having a grain size of 10 nm or more, and the electrical steel sheet substrate has subgrains present in a region (A) within 1500 μm from the center of the pore in the RD direction and in a region (B) 50 to 100 μm from the surface of the electrical steel sheet substrate toward the interior of the electrical steel sheet substrate, the subgrains having a crystal orientation of {110} <001> The subgrains form an angle of 1° to 15° with respect to the grains, and the area fraction of the subgrains in the ND cross section is 5% or less.

[0012] The ratio (y / z) of the subgrain's length in the TD (y) to the grain's length in the ND (z) may be 1.5 or less.

[0013] The crystal orientation is {110} in the region (B) of 50 to 100 μm from the surface of the electrical steel sheet substrate toward the interior of the electrical steel sheet substrate. <001> The Goss grains are less than 1° from the ND cross section, and the average grain size (L G ) to the average grain size of subgrains (L S ) ratio (L S / L G ) may be 0.20 or less.

[0014] There may be 1 to 300 pores with a particle size of 10 nm or more per mm in the RD direction.

[0015] A fine grain interface layer may be present from the surface of the electrical steel sheet substrate toward the interior of the electrical steel sheet substrate, and the fine grain interface layer may have an average crystal grain size of 0.1 to 5 μm.

[0016] The fine grain interface layer may have a residual stress in the RD direction of -10 to -1000 MPa.

[0017] The thickness of the fine grain interface layer may be 0.1 to 5 μm.

[0018] A base coating layer may further be included between the electrical steel sheet substrate and the insulating coating layer.

[0019] The base coating layer may have a residual stress in the RD direction of -50 to -1500 MPa.

[0020] The thickness of the base coating layer may be 0.1 to 15 μm.

[0021] The insulating coating layer may have a residual stress in the RD direction of -10 to -1000 MPa.

[0022] The insulating coating layer may have a thickness of 0.1 to 15 μm.

[0023] The electrical steel sheet substrate may have a residual stress in the RD direction of 1 to 50 MPa.

[0024] A method for manufacturing a grain-oriented electrical steel sheet according to one embodiment of the present invention includes the steps of: manufacturing a grain-oriented electrical steel sheet; applying an insulating coating layer-forming composition on the grain-oriented electrical steel sheet; and heat-treating the grain-oriented electrical steel sheet to form an insulating coating layer on the grain-oriented electrical steel sheet, wherein a tension of 0.2 to 0.7 kgf / mm is applied to the steel sheet in the step of forming the insulating coating layer. 2 It is characterized in that:

[0025] With respect to the entire length of the steel plate, the maximum value (MA) and minimum value (MI) of the tension can satisfy the following formula 2. [Formula 2] [MI] ≥ 0.5 × [MA] The step of forming the insulating coating layer may be performed by heat treatment at a temperature of 550 to 1100°C. [Effects of the Invention]

[0026] The grain-oriented electrical steel sheet according to an embodiment of the present invention can improve magnetic properties by suppressing subgrains that adversely affect magnetic properties. The grain-oriented electrical steel sheet according to an embodiment of the present invention may have increased residual stress in the base coating layer, the insulating coating layer, and the fine grain interface layer, thereby improving magnetic properties. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 2 is a schematic diagram of a TD cross section of a steel plate according to an embodiment of the present invention. [Figure 2] 1 is an electron backscatter diffraction (EBSD) photograph of the steel sheet produced in Example 1. [Figure 3] FIG. 10 is a diagram showing a coating tension calculation method using a radius of curvature. [Figure 4] FIG. 10 is a diagram showing the slope in the measurement of residual stress. DETAILED DESCRIPTION OF THE INVENTION

[0028] The grain-oriented electrical steel sheet of the present invention includes an electrical steel sheet base material containing 2.0 to 7.0 wt % Si, 0.01 to 0.07 wt % Sb, and the remainder being Fe and other unavoidable impurities. In this specification,Terms such as "first," "second," and "third" are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a first part, component, region, layer, or section described below can be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0029] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. As used in the specification, the term "comprising" refers to the inclusion of specific features, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components. When a part is referred to as being "on" or "above" another part, it means that it is directly on or above the other part, or there may be other parts between them. In contrast, when a part is referred to as being "directly on" another part, there are no other parts between them.

[0030] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention belongs. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the currently disclosed content, and are not interpreted as having an ideal or very formal meaning unless otherwise defined. Also, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight. In one embodiment of the present invention, the inclusion of an additional element means that the remaining iron (Fe) is replaced by the additional amount of the additional element.

[0031] Although the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein, the present invention will be described in detail below so that those skilled in the art can easily practice the present invention.

[0032] FIG. 1 is a schematic diagram showing a TD cross section of a grain-oriented electrical steel sheet according to an embodiment of the present invention. As shown in FIG. 1, a grain-oriented electrical steel sheet 100 according to one embodiment of the present invention includes an electrical steel sheet substrate 10 and an insulating coating layer 30 located on the electrical steel sheet substrate 10 . The insulating coating layer 30 is formed by applying an insulating coating layer-forming composition containing a solvent onto a steel sheet and then heat-treating the steel sheet. At this time, as the solvent evaporates at high temperature, pores 31 are inevitably formed in the insulating coating layer 30. If the pores 31 become larger than 10 nm, stress applied to the steel sheet will concentrate in the lower part of the pores 31, forming subgrains 11. This has a negative effect on magnetic properties compared to Goss grains, which are the main grains of grain-oriented electrical steel sheets, and is therefore preferably suppressed as much as possible.

[0033] In one embodiment of the present invention, the positional correlation between the pores 31 and the subgrains 11 and the cause of the formation of the subgrains 11 are analyzed to minimize the formation of the subgrains 11. In FIG. 1, pores 31 and subgrains 11 are shown schematically. As shown in Figure 1, subgrains 11 are present below the pores 31. All subgrains 11 in the steel sheet substrate 10 are present in specific regions below the pores 31. However, not all pores 31 have subgrains 11 below them, and there may be pores 31 below which no subgrains 11 exist.

[0034] Hereinafter, each component according to one embodiment of the present invention will be described in detail.

[0035] The electrical steel sheet substrate 10 refers to a portion of the grain-oriented electrical steel sheet 100 excluding the base coating layer 20 and the insulating coating layer 30 . In one embodiment of the present invention, this is expressed by the pores 31 in the insulating coating layer 30 and the subgrains 11 in the electrical steel sheet substrate 10, regardless of the alloy composition of the electrical steel sheet substrate 10. The alloy composition of the electrical steel sheet substrate 10 will now be described in more detail. The electrical steel sheet substrate 10 contains 2.0 to 7.0 weight percent Si, 0.01 to 0.10 weight percent Sn, 0.01 to 0.07 weight percent Sb, 0.020 to 0.040 weight percent Al, 0.01 to 0.20 weight percent Mn, 0.005 weight percent or less C, 0.005 weight percent or less N, and 0.005 weight percent or less S, with the remainder being Fe and other unavoidable impurities.

[0036] Si:2.0~7.0wt% Silicon (Si) increases the resistivity of steel and reduces iron loss. If the Si content is too low, the resistivity of the steel will decrease, deteriorating iron loss characteristics. Furthermore, there may be a phase transformation interval during secondary recrystallization annealing, making the secondary recrystallization unstable. If the Si content is too high, the steel will become brittle, making cold rolling difficult. Therefore, the Si content can be adjusted within the aforementioned range. More specifically, Si may be contained in an amount of 2.5 to 5.0 wt.%.

[0037] Sn:0.01~0.10wt% Tin (Sn) is a grain boundary segregating element that hinders the movement of grain boundaries, so it is used as a grain growth inhibitor. <001> It is an important element in strengthening the grain growth inhibition force, as it promotes the formation of Goss grains with the right orientation and allows secondary recrystallization to develop well. If the Sn content is too low, the effect is reduced, while if the Sn content is too high, excessive grain boundary segregation occurs, increasing the brittleness of the steel sheet and causing sheet breakage during rolling. Therefore, the Sn content can be adjusted within the above range. More specifically, Sn may be contained in an amount of 0.02 to 0.08 wt%.

[0038] Sb:0.01~0.05wt% Antimony (Sb) is {110} <001> Sb is an element that promotes the formation of Goss crystal grains in the desired orientation. If the Sb content is too low, it will not be effective as a Goss crystal grain formation promoter. If the Sb content is too high, it will segregate on the surface, suppressing the formation of an oxide layer and causing surface defects. Therefore, the Sb content can be adjusted within the above range. More specifically, Sb may be included in an amount of 0.02 to 0.04 wt%.

[0039] Al:0.020~0.040wt% Aluminum (Al) is an element that ultimately becomes nitrides in the form of AlN, (Al,Si)N, and (Al,Si,Mn)N and acts as an inhibitor. If the Al content is too low, sufficient inhibitor effect cannot be expected. On the other hand, if the Al content is too high, Al-based nitrides precipitate and grow excessively coarsely, resulting in insufficient inhibitor effect. Therefore, the Al content can be adjusted within the above range. More specifically, Al may be included in an amount of 0.020 to 0.030 wt%.

[0040] Mn:0.01~0.20wt% Manganese (Mn), like Si, increases resistivity and reduces iron loss. It also reacts with Si to form (Al, Si, Mn)N precipitates, inhibiting primary recrystallization grain growth and promoting secondary recrystallization. However, excessive Mn content promotes austenite phase transformation during hot rolling, reducing the size of primary recrystallized grains and destabilizing secondary recrystallization. Furthermore, excessively low Mn content can insufficiently enhance the austenite fraction during hot rolling and reheating, increasing the amount of precipitate solid solution. This can lead to refinement of precipitates and the formation of MnS, preventing primary recrystallization grains from becoming excessively large. Therefore, the Mn content can be adjusted within the aforementioned range.

[0041] C: 0.005% by weight or less In embodiments of the present invention, carbon (C) is an element that does not significantly contribute to improving the magnetic properties of grain-oriented electrical steel sheets, so it is preferable to eliminate it as much as possible. However, if present at a certain level or above, it promotes the austenite transformation of steel during rolling and refines the hot-rolled structure during hot rolling, helping to form a uniform, fine structure. The C content in the slab is preferably 0.04 wt% or more. However, excessive C content can form coarse carbides that are difficult to remove during decarburization, so it may be 0.07 wt% or less. Decarburization occurs during the primary recrystallization annealing process, and the final grain-oriented electrical steel sheet substrate after decarburization contains 0.005 wt% or less.

[0042] N: 0.005% by weight or less Nitrogen (N) is an element that reacts with Al and other elements to refine crystal grains. When appropriately distributed, this helps ensure an appropriate grain size for the primary recrystallization by appropriately refining the structure after cold rolling, as described above. However, excessive N content can result in excessive refinement of the primary recrystallization grains. As a result, the driving force for grain growth during secondary recrystallization increases due to the fine grains, which can lead to the growth of grains with undesirable orientations. Furthermore, excessive N content is undesirable because it takes a long time to remove it during the final annealing process. Therefore, the upper limit of the nitrogen content can be set to 0.005 wt.%. The amount of nitrogen may increase due to nitriding during the primary recrystallization process. This is then removed again during the secondary recrystallization annealing process, so the amount of nitrogen in the slab and the final grain-oriented electrical steel sheet substrate 10 can be the same.

[0043] S: 0.005% by weight or less, If the sulfur (S) content exceeds 0.005% by weight, it will re-dissolve and precipitate finely during heating of the hot-rolled slab, reducing the size of the primary recrystallized grains and lowering the secondary recrystallization start temperature, resulting in a deterioration of magnetic properties. Furthermore, it takes a long time to remove the dissolved S in the secondary soaking zone of the final annealing process, reducing the productivity of grain-oriented electrical steel sheets. On the other hand, if the S content is low (0.005% or less), the initial crystal grain size before cold rolling will become coarse, resulting in the {110} nucleated in the deformation band during the primary recrystallization process. <001> The number of oriented crystal grains is increased, and therefore, in order to reduce the size of secondary recrystallized grains and improve the magnetic properties of the final product, it is preferable that the S content be 0.005 wt % or less.

[0044] The remainder includes Fe and inevitable impurities. Inevitable impurities are elements that are inevitably added during the steelmaking and grain-oriented electrical steel sheet manufacturing processes, and as this is widely known, a detailed explanation of the inevitable impurities will be omitted.

[0045] In one embodiment of the present invention, elements other than the above alloy components may be added, and various elements may be included within a range that does not impair the technical concept of the present invention. When an additional element is further included, the remaining Fe is replaced with the additional element.

[0046] As shown in FIG. 1, subgrains 11 exist within an electrical steel sheet substrate 10 . Subgrain 11 has a crystal orientation of {110} <001> The Goss grains are distinguished from the remaining Goss grains excluding the subgrains by the angle of 1° to 15° from the grains. <001> The crystal orientation is expressed by the Miller index.

[0047] In one embodiment of the present invention, the subgrains 11 are located below the pores 31. Specifically, the subgrains 11 are present in a region (A) within 1500 μm from the pore center in the RD direction and in a region (B) 50 to 100 μm from the surface of the electrical steel sheet substrate toward the interior of the electrical steel sheet substrate. In FIG. 1, the positions defined as region A and region B are represented by dotted rectangles. Specifically, the entire region of the subgrains 11 may be included in the positions defined as region A and region B. In one embodiment of the present invention, the subgrains 11 are present only in the aforementioned regions, and no subgrains 11 are present in the remaining portions.

[0048] In one embodiment of the present invention, the magnetic property can be improved by suppressing the subgrains 11. Specifically, the area fraction of the subgrains in the ND cross section may be 5% or less. If the area fraction of the subgrains 11 is excessively large, the magnetic property will be deteriorated. More specifically, the area fraction of the subgrains in the ND cross section may be 0.1 to 5%. Even more specifically, it may be 1 to 3%. The ND cross section means a plane perpendicular to the ND direction.

[0049] The subgrains 11 have a grain size of 1 to 500 nm, and can be distinguished from the remaining Goss grains by their grain size. Specifically, the average grain size of the Goss grains excluding the subgrains may be 5 to 100 mm. This is the grain size in the ND cross section of the grains. More specifically, the subgrains 11 have a grain size of 10 to 250 nm, and the average grain size of the Goss grains excluding the subgrains may be 10 to 50 mm.

[0050] The average grain size of Goss crystal grains on the ND plane (L G ) to the average grain size of subgrains (L S ) ratio (L S / L G ) may be 0.20 or less. More specifically, it may be 0.10 or less.

[0051] In one embodiment of the present invention, the grain size refers to the diameter of an imaginary circle having the same area as the grain size. In one embodiment of the present invention, the electrical steel sheet substrate 10 may have a residual stress in the RD direction of 1 to 50 MPa. The residual stress in this range is due to the base coating layer 20 and the insulating coating layer 30 present on the electrical steel sheet substrate 10. The presence of residual stress in the above range imparts coating tension to the base steel, improving its magnetic properties. Specifically, the electrical steel sheet substrate 10 may have a residual stress in the RD direction of 16.0 to 30.0 MPa. The residual stress of the electrical steel sheet substrate 10 can be calculated as the value at which the sum of the residual stresses of the fine-grained interface layer 12, the base coating layer 20, and the insulating coating layer 30 (described below) becomes zero.

[0052]

number

[0053] t i : thickness of each layer σ i : Residual stress in each layer i: Base coating layer / fine grain interface layer / base steel plate

[0054] As shown in Fig. 1, a fine grain interface layer 12 may be present from the surface of an electrical steel sheet substrate 10 toward the interior of the electrical steel sheet substrate. This fine grain interface layer 12 may have an average crystal grain size of 0.1 to 5 µm. The fine grain interface layer 12 is formed due to the influence of non-uniform surface energy. The thickness of the fine grain interface layer 12 may be 0.1 to 5 μm. If the fine grain crystal layer 12 is too thick, it is advantageous to reduce its thickness by degrading its magnetic properties. More specifically, the thickness of the fine grain interface layer 12 may be 0.5 to 3 μm.

[0055] The fine-grained interface layer 12 may have a RD residual stress of -10 to -1000 MPa. In this case, the negative sign indicates the stress that the fine-grained interface layer 12 imparts to the electrical steel sheet substrate 10. More specifically, the fine-grained interface layer 12 may have a RD residual stress of -100 to -500 MPa. Even more specifically, the fine-grained interface layer 12 may have a RD residual stress of -400 to -500 MPa.

[0056] As shown in FIG. 1 , a grain-oriented electrical steel sheet 100 according to an embodiment of the present invention may further include a base coating layer 20 located between the electrical steel sheet substrate 10 and the insulating coating layer 30 . The base coating layer 20 is formed when an oxide layer formed during the primary recrystallization process reacts with components in the annealing separator to form the coating layer. The base coating layer 20 improves adhesion between the insulating coating layer 30 and the electrical steel sheet substrate 10, and, together with the insulating coating layer 30, provides insulation to the grain-oriented electrical steel sheet 100.

[0057] The components of the base coating layer 20 are not particularly limited, but if the annealing separator contains MgO, it may contain forsterite (Mg2SiO4). The base coating layer 20 can be omitted if necessary. That is, the electrical steel sheet substrate 10 and the insulating coating layer 30 may be in direct contact with each other. The thickness of the base coating layer 20 may be 0.1 to 15 μm. If the thickness of the base coating layer 20 is excessively thin, it will not be able to adequately perform the aforementioned functions of insulation and improving adhesion to the insulating coating layer 30. If the base coating layer 20 is excessively thick, the space factor will decrease and adhesion to the insulating coating layer 30 may also decrease. More specifically, the thickness of the base coating layer 20 may be 0.5 to 3 μm. The RD residual stress of the base coating layer 20 may be −50 to −1500 MPa. More specifically, it may be −500 to −1000 MPa. Even more specifically, it may be −760 to −1000 MPa.

[0058] As shown in Figure 1, the insulating coating layer 30 is located on the electrical steel sheet substrate 10. When the base coating layer 20 is located on the electrical steel sheet substrate 10, the insulating coating layer 30 is located on the base coating layer 20. The insulating coating layer 30 provides insulation to the grain-oriented electrical steel sheet 100 and at the same time provides tension to the electrical steel sheet substrate 10, thereby improving iron loss. The insulating coating layer 30 may be made of a material that can impart insulating properties to the surface of the electrical steel sheet 100. Specifically, it may contain phosphate (H3PO4).

[0059] The insulating coating layer 30 is formed by applying an insulating coating layer-forming composition containing a solvent onto a steel sheet and then heat-treating it. During this process, as the solvent evaporates at high temperatures, pores 31 are inevitably formed in the insulating coating layer 30. The pores 31 refer to empty spaces in the corresponding areas. There may be 1 to 300 pores with a diameter of 10 nm or more per mm in the RD direction. More specifically, there may be 1 to 30 pores per mm. In this case, the diameter of the pores can be measured based on the ND or TD plane. The number of pores can be measured based on the TD plane.

[0060] There are 1 to 30 subgrains per pore with a grain size of 10 nm or more. As mentioned above, there may be no subgrains 11 in the regions (A, B) below the pore 31, and two or more subgrains 11 may be present. However, there may be no subgrains 11 other than the regions (A, B) below the pore 31. The thickness of the insulating coating layer 30 may be 0.1 to 15 μm. If the insulating coating layer 30 is too thin, it will not be able to adequately perform the insulating function described above. If the insulating coating layer 30 is too thick, the space factor will be low and the adhesion to the steel sheet substrate 10 may be reduced. More specifically, the thickness of the insulating coating layer 30 may be 1.0 to 5.0 μm.

[0061] The residual stress in the RD direction of the insulating coating layer 30 may be −10 to −1000 MPa, or more specifically, −70 to −500 MPa. A method for manufacturing a grain-oriented electrical steel sheet according to one embodiment of the present invention includes the steps of manufacturing a grain-oriented electrical steel sheet; applying an insulating coating layer-forming composition on the grain-oriented electrical steel sheet; and heat-treating the grain-oriented electrical steel sheet to form an insulating coating layer-forming composition on the grain-oriented electrical steel sheet.

[0062] Each step will be explained in detail below. First, a grain-oriented electrical steel sheet is manufactured. In this case, the grain-oriented electrical steel sheet may be formed with a base coating layer 20 or may not be formed with a grain-oriented electrical steel sheet having only the electrical steel sheet substrate 10.

[0063] Grain-oriented electrical steel sheets without a base coating layer 20 can be manufactured by various methods. For example, the composition of the annealing separator can be adjusted, or the base coating layer 20 can be formed and then removed by a physical or chemical method.

[0064] In one embodiment of the present invention, a technical feature is that the tension applied to the steel sheet is adjusted in the step of forming the insulating coating layer, and various known methods can be used to manufacture the grain-oriented electrical steel sheet.

[0065] An example of a method for manufacturing a grain-oriented electrical steel sheet before forming an insulating coating layer will be described below. The method for manufacturing a grain-oriented electrical steel sheet may further include the steps of hot-rolling a slab to manufacture a hot-rolled sheet; cold-rolling the hot-rolled sheet to manufacture a cold-rolled sheet; performing primary recrystallization annealing on the cold-rolled sheet; and performing secondary recrystallization annealing on the cold-rolled sheet after the primary recrystallization annealing.

[0066] The slabs may contain Si: 2.0 to 7.0 wt%, Sn: 0.01 to 0.10 wt%, Sb: 0.01 to 0.07 wt%, Al: 0.020 to 0.040 wt%, Mn: 0.01 to 0.20 wt%, C: 0.04 to 0.07 wt%, N: 10 to 50 wt ppm, S: 0.001 to 0.005 wt%, with the remainder being Fe and other unavoidable impurities. First, a slab is hot rolled to produce a hot-rolled sheet.

[0067] In the following, the alloy components of the slab are the same as those of the electrical steel sheet substrate 10 except for the C content, so a duplicated explanation will be omitted.

[0068] Prior to producing the hot-rolled sheet, the process may further include a step of heating the slab to 1230°C or less. This step allows the precipitates to be partially dissolved. Furthermore, the columnar crystal structure of the slab is prevented from growing coarsely, preventing cracks from occurring across the width of the sheet during the subsequent hot rolling process, thereby improving yield. If the slab heating temperature is excessively high, the surface of the slab may melt, requiring repairs to the heating furnace and shortening the furnace's lifespan. More specifically, the slab may be heated at 1130-1200°C. It is also possible to hot-roll the continuously cast slab directly without heating it. In the step of producing a hot-rolled sheet, a hot-rolled sheet having a thickness of 1.8 to 2.3 mm can be produced by hot rolling.

[0069] After the hot-rolled sheet is manufactured, the method may further include a step of annealing the hot-rolled sheet, which may be performed by heating the hot-rolled sheet to a temperature of 950 to 1,100°C, soaking it at a temperature of 850 to 1,000°C, and then cooling it. Next, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet. Cold rolling can be done through one strong cold rolling pass or multiple passes. During rolling, warm rolling is performed at a temperature of 200~300℃ at least once to impart a pass aging effect, allowing for production with a final thickness of 0.14~0.25mm. The cold-rolled sheet undergoes decarburization and recrystallization of the deformed structure during the primary recrystallization annealing process, and nitriding treatment using nitriding gas.

[0070] Next, the cold-rolled sheet undergoes primary recrystallization annealing. It can be decarburized or nitrided during the primary recrystallization annealing process. The primary recrystallization annealing step can be performed at a temperature of 800 to 900°C. If the temperature is too low, the primary recrystallization may not occur or the nitriding may not proceed smoothly. If the temperature is too high, the primary recrystallization may grow too large, resulting in poor magnetic properties.

[0071] Decarburization can be carried out in an atmosphere with an oxidation power (PH2O / PH2) of 0.5 to 0.7. By decarburizing, the steel sheet can contain carbon of 0.005 wt% or less, more specifically 0.003 wt% or less.

[0072] Next, an annealing separator is applied to the cold-rolled steel sheet that has undergone the primary recrystallization annealing, followed by secondary recrystallization annealing. Various separators can be used as the annealing separator. For example, an annealing separator mainly composed of MgO can be applied. At this time, after the secondary recrystallization annealing, a base coating layer 20 containing forsterite is formed.

[0073] The purpose of secondary recrystallization annealing is to remove the {110} <001> The purpose of secondary recrystallization annealing is to remove impurities that impair texture formation and magnetic properties. In the temperature-raising section before secondary recrystallization occurs, a mixture of nitrogen and hydrogen gas is maintained to protect nitrides, which act as grain growth inhibitors, allowing secondary recrystallization to develop well, and after secondary recrystallization is complete, impurities can be removed by maintaining the material in a 100% hydrogen atmosphere for a long period of time.

[0074] After the secondary recrystallization annealing step, a flattening annealing step may be included. Returning to the description of the manufacturing process of a grain-oriented electrical steel sheet according to an embodiment of the present invention, an insulating coating layer-forming composition is applied to the grain-oriented electrical steel sheet. In an embodiment of the present invention, various insulating coating layer-forming compositions may be used and are not particularly limited. For example, an insulating coating layer-forming composition containing phosphate may be used.

[0075] Next, the grain-oriented electrical steel sheet is heat-treated to form an insulating coating layer on the grain-oriented electrical steel sheet. During this heat treatment, the solvent volatilizes at high temperatures, inevitably forming pores 31 in the insulating coating layer 30. At this time, stress applied to the steel sheet concentrates below the pores 31, forming subgrains 11. In one embodiment of the present invention, the formation of subgrains 11 is minimized by adjusting the tension applied to the steel sheet during the process of forming the insulating coating layer.

[0076] Specifically, the tension applied to the steel sheet during the insulating coating layer formation step is 0.20 to 0.70 kgf / mm 2 is. If the tension applied to the steel sheet is too low, scratches may occur on the surface, resulting in poor corrosion resistance. If the tension applied to the steel sheet is too high, a large number of subgrains 11 may be formed, which may have a negative effect on magnetic properties. More specifically, the tension is set to 0.20 to 0.50 kgf / mm. 2 More specifically, it may be 0.3 to 0.47 kgf / mm 2 In this case, the tension is an average tension in the longitudinal direction of the steel sheet measured at the exit side of the heat treatment process.

[0077] In the step of forming the insulating coating layer, the tension applied to the steel sheet may vary depending on the longitudinal direction (RD direction). In one embodiment of the present invention, the difference between the maximum tension (MA) and the minimum tension (MI) is minimized across the entire length of the steel sheet, thereby appropriately controlling the residual stress applied to each layer and suppressing the formation of subgrains 11. Specifically, the maximum value (MA) and minimum value (MI) of the tension over the entire length of the steel plate satisfy the following formula 2.

[0078] [Formula 2] [MI] ≥ 0.5 × [MA] If Equation 2 is not satisfied and there is a large deviation in tension in the longitudinal direction (RD) of the steel plate, localized non-uniformity increases, residual stress is not properly adjusted, and a large amount of subgrains 11 is formed.

[0079] In conventional processes, the line speed varies widely during the flattening annealing process, resulting in large tension deviations along the length of the steel sheet (RD), leading to localized unevenness. Specifically, laser welding is performed at the minimum line speed to join the tail of the preceding coil and the top of the following coil at the flattening annealing entry point. Once welding is complete, the line speed is increased to increase productivity of the final product, resulting in large tension deviations.

[0080] More specifically, as the speed change range of the bridle roll and hearth roll increases due to changes in line speed, there is a large tension deviation in the length direction (RD) of the steel sheet at the high temperatures inevitably involved in flattening annealing, which increases localized non-uniformity and prevents proper adjustment of residual stress. As a result, the minimum tension (MI) could only be less than 0.5 × [MA].

[0081] There are various methods for reducing the difference between the maximum (MA) and minimum (MI) tension values, but in one embodiment of the present invention, for example, a method of controlling the bridle roll and the hearth roll speed can be used. Specifically, the bridle roll control is a method of controlling the feedback tension by following the tension meter value. More specifically, it is a method of controlling the bridle roll speed to reduce the difference between the maximum and minimum tension values.

[0082] More specifically, the hearth roll control is a bridle roll speed follow-up feedforward tension control method. More specifically, the tension can be controlled by a control method that decreases the tension as the hearth roll speed increases in order to reduce the difference between the maximum and minimum tension values. In one embodiment of the present invention, even if the line speed fluctuates during the planarization annealing process, the tension can be controlled within a specific range while simultaneously reducing the difference between the maximum and minimum tension values ​​(MA and MI).

[0083] In the step of forming the insulating coating layer, the heat treatment temperature may be 550 to 1100°C. At the above temperature, fewer pores 31 are generated and the insulating coating layer 30 can be appropriately provided with residual stress. Preferred examples and comparative examples of the present invention are described below. However, the following examples are merely preferred embodiments of the present invention, and the present invention is not limited to the following examples. [Example]

[0084] The steel contained 3.4 wt% Si, 0.05 wt% Sn, 0.02 wt% Sb, 0.02 wt% Al, 0.10 wt% Mn, 0.05 wt% C, 0.002 wt% N, and 0.001 wt% S, with the remainder being Fe and other unavoidable impurities. The steel was vacuum melted to produce an ingot, which was then heated at 1150°C for 210 minutes and hot-rolled to produce a 2.0 mm thick hot-rolled sheet. After pickling, the sheet was cold-rolled to a thickness of 0.220 mm. The cold-rolled sheet was maintained at a temperature of approximately 800-900°C in a humid atmosphere of 50v% hydrogen and 50v% nitrogen and an ammonia mixed gas atmosphere, and was subjected to decarburization and nitriding annealing heat treatment so that the carbon content was increased to 30ppm or less and the total nitrogen content to 130ppm or more.

[0085] The steel sheet was coated with MgO as an annealing separator and then coiled and subjected to final annealing. Final annealing was performed in a mixed atmosphere of 25v% nitrogen and 75v% hydrogen up to 1200°C. After reaching 1200°C, the temperature was maintained in a 100% hydrogen atmosphere for 10 hours or more, followed by furnace cooling.

[0086] An insulating coating layer-forming composition containing phosphate and silica was applied to this steel plate, and the steel plate was heat-treated at a temperature of about 820° C. for 2 hours to form an insulating coating layer. When forming the insulating coating layer, the average tension at the exit side was adjusted as shown in Table 1 below. The pores, subgrains, and other grain characteristics of the manufactured grain-oriented electrical steel sheets are summarized in Table 1, and the properties and core loss of the interface layer, base coating layer, and insulating coating layer are summarized in Table 2.

[0087] It was confirmed that all subgrains were located only in specific regions below the pores. The number of pores was measured only for pores with a diameter of 10 nm or more. The subgrain fraction was measured by electron backscatter diffraction (EBSD) method for volume per unit area.

[0088] Iron loss (W17 / 50) and magnetic flux density (B8) were measured immediately after forming the insulating coating layer and after heat treatment at 820°C for two hours, which simulates stress relief annealing. Iron loss was measured using the single sheet measurement method at 1.7 Tesla and 50 Hz. Magnetic flux density induced in a magnetic field of 800 A / m was also measured. The residual stress of the insulating coating layer was measured using a 3D curvature measurement device (ATOS core 45). The insulating coating layer on one side was removed and the amount of bending of the steel plate was measured.

[0089] The insulation properties were measured on the top of the coating using a Franklin measuring instrument according to the ASTM A717 international standard.

[0090] Corrosion resistance is measured according to the JIS Z2371 international standard, which indicates the area of ​​rust formed on the surface after 8 hours at 35°C with 5% NaCl. The following diagram shows a method for calculating coating tension using the radius of curvature (Reference: M. Bielawski et al., Surf. & Coat. Techno., 200(2006)2987). Coating tension can be calculated from images measured using software dedicated to 3D scanners. The R values ​​can be measured for specimens before (R2) and after (R1) removal of the phosphate coating layer.

number

[0091] Residual stress in the base coating layer and the fine-grain interface layer was measured using synchrotron XRD equipment. X-ray residual stress measurement uses the distance between lattice planes of crystal grains as a deformation gauge. When a sample is under stress, the distance between lattice planes changes depending on the relative angle between the stress direction and the crystal planes. Due to the Poisson effect, the distance between lattice planes parallel to the tensile direction, i.e., ψ = 0°, is smaller than when stress is zero, while the distance between lattice planes with a ψ angle tilted toward the tensile direction is larger than when stress is zero. X-ray residual stress measures the peak shift due to the tilting angle Ψ. Therefore, X-ray residual stress calculations follow the sin²Ψ method and can be expressed as Equation 2 below.

[0092]

number

[0093] d ψ : d-spacing of the lattice plane whose lattice plane direction is ψ direction d z : d-spacing of the lattice plane when the lattice plane direction is perpendicular to the sample surface d o : d-spacing of stress-free lattice surface

[0094] [Table 1]

[0095] [Table 2]

[0096] [Table 3]

[0097] As shown in Tables 1 to 3, when the tension is properly controlled during the insulating coating layer formation process, subgrains are suppressed, and the residual stress in the fine grain interface layer, base coating layer, and insulating coating layer increases, improving magnetic properties, insulating properties, and corrosion resistance. On the other hand, when the tension is not properly controlled during the insulating coating layer formation process, a large number of subgrains are formed, resulting in poor magnetic properties, insulating properties, and corrosion resistance.

[0098] The present invention is not limited to the above-described embodiments, and can be manufactured in various different forms, and those skilled in the art will understand that the present invention can be embodied in other specific forms without changing the technical concept or essential features of the present invention. Therefore, it should be understood that the above-described embodiments are illustrative in all respects and not limiting. [Explanation of symbols]

[0099] 100: Grain-oriented electrical steel sheet 10:Electromagnetic steel plate base material 11: Subgrain 12: Fine grain interface layer, 20: Base coating layer 30: insulating coating layer, 31: Stoma

Claims

1. An electrical steel sheet substrate comprising: 2.0 to 7.0 wt% Si, 0.01 to 0.10 wt% Sn, 0.01 to 0.07 wt% Sb, 0.020 to 0.040 wt% Al, 0.01 to 0.20 wt% Mn, 0.005 wt% or less C, 0.005 wt% or less N, and 0.005 wt% or less S, with the balance being Fe and other unavoidable impurities; and an insulating coating layer disposed on the electrical steel sheet substrate; the insulating coating layer contains pores with a diameter of 10 nm or more; The electromagnetic steel sheet substrate has subgrains in a region (A) within 1500 μm from the center of the pore in the RD direction and in a region (B) of 50 to 100 μm from the surface of the electromagnetic steel sheet substrate toward the interior of the electromagnetic steel sheet substrate, The subgrains have a crystal orientation that forms an angle of 1° to 15° from {110}<001>; A grain-oriented electrical steel sheet, characterized in that the area fraction of the subgrains in the ND cross section is 5% or less.

2. 2. The grain-oriented electrical steel sheet according to claim 1, wherein the ratio (y / z) of the grain length (y) in the TD direction to the grain length (z) in the ND direction of the subgrains is 1.5 or less.

3. a region (B) of 50 to 100 μm from the surface of the electromagnetic steel sheet substrate toward the interior of the electromagnetic steel sheet substrate includes Goss crystal grains whose crystal orientation is less than 1° from {110}<001>; The average grain size of the Goss crystal grains in the ND cross section (L Ｇ ) to the average grain size of the subgrains (L Ｓ ) ratio (L Ｓ / L Ｇ 3. The grain-oriented electrical steel sheet according to claim 1, wherein the value of σ is 0.20 or less.

4. a fine grain interface layer is present from the surface of the electrical steel sheet toward the interior of the electrical steel sheet substrate, The grain-oriented electrical steel sheet according to any one of claims 1 to 3, characterized in that the fine grain interface layer has an average crystal grain size of 0.1 to 5 µm.

5. The grain-oriented electrical steel sheet according to claim 4, wherein the fine grain interface layer has a residual stress in the RD direction of -10 to -1000 MPa.

6. The grain-oriented electrical steel sheet according to claim 4 or 5, wherein the thickness of the fine grain interface layer is 0.1 to 5 μm.

7. The grain-oriented electrical steel sheet according to any one of claims 1 to 6, further comprising a base coating layer between the electrical steel sheet substrate and the insulating coating layer.

8. The grain-oriented electrical steel sheet according to claim 7, wherein the base coating layer has a residual stress in the RD direction of −50 to −1500 MPa.

9. The grain-oriented electrical steel sheet according to claim 7 or 8, wherein the thickness of the base coating layer is 0.1 to 15 μm.

10. The grain-oriented electrical steel sheet according to any one of claims 1 to 9, characterized in that the insulating coating layer has a residual stress in the RD direction of -10 to -1000 MPa.

11. The grain-oriented electrical steel sheet according to any one of claims 1 to 10, wherein the insulating coating layer has a thickness of 0.1 to 15 µm.

12. The grain-oriented electrical steel sheet according to any one of claims 1 to 11, characterized in that the electrical steel sheet substrate has a residual stress in the RD direction of 1 to 50 MPa.

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