Non-oriented electrical steel sheet and its manufacturing method

Abstract

The present invention provides a non-oriented electrical steel sheet having excellent magnetic properties regardless of frequency, and a manufacturing method thereof, by adding an appropriate amount of Cr element to the steel sheet and adjusting the conditions of the cold rolling and final annealing processes to form a Cr-enriched layer inside the steel sheet. [Solution] The composition contains, by weight, 2.5-3.8% Si, 0.1-1.5% Al, 0.1-2.0% Mn, and 0.01-0.15% Cr, with the balance being Fe and unavoidable impurities; Satisfying the following formula 1, [Formula 1] [Cr]>([Al]+[Mn]) / [Si] / 10 (In formula 1, [Cr], [Al], [Mn] and [Si] represent the contents (wt%) of Cr, Al, Mn and Si, respectively.) Further containing one or more of P: 0.005 to 0.08% by weight, Sn: 0.01 to 0.08% by weight, and Sb: 0.005 to 0.05% by weight, It is characterized by further containing one or more of C: 0.0040% by weight or less, S: 0.0040% by weight or less, N: 0.0040% by weight or less, and Ti: 0.0040% by weight or less.

Description

[Technical field]

[0001] The present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof, and more particularly to a non-oriented electrical steel sheet having excellent magnetic properties regardless of frequency, which is obtained by adding an appropriate amount of Cr element to a steel sheet and adjusting the conditions of the cold rolling and final annealing processes to form a Cr-enriched layer inside the steel sheet, and a manufacturing method thereof. [Background technology]

[0002] Efficient use of electrical energy is a major issue for improving the global environment, including energy conservation, reducing fine dust generation, and reducing greenhouse gas emissions. Currently, more than 50% of the total electrical energy generated is consumed by electric motors, so high efficiency of electric motors is essential for efficient use of electricity. In recent years, interest in highly efficient drive motors has increased with the rapid development of the field of environmentally friendly automobiles (hybrids, plug-in hybrids, electric vehicles, fuel cell vehicles). Furthermore, with awareness of and government regulations regarding high efficiency in high-efficiency motors for home appliances and super premium motors for heavy electrical equipment, the demand for efficient use of electrical energy is higher than ever before.

[0003] On the other hand, in order to improve the efficiency of electric motors, optimization in all areas from material selection to design, assembly, and control is extremely important. In particular, in terms of materials, the magnetic properties of electromagnetic steel sheets are the most important, and there is a high demand for low iron loss and high magnetic flux density. For automobile drive motors and air conditioner compressor motors, which need to be driven not only in the commercial frequency range but also in the high frequency range, the property of low iron loss at high frequencies is extremely important. To obtain such high-frequency low iron loss properties, it is important to improve the magnetic permeability, and in particular, it is required that the amount of decrease in magnetic permeability is small even as the frequency increases. Good magnetic permeability is an essential property for obtaining low iron loss at high frequencies, since magnetization is fast even under magnetizing force, and low high-frequency dependency of magnetic permeability means that motor efficiency does not decrease suddenly even when the motor rotates at a higher speed. Normally, electrical steel sheets are manufactured by adding large amounts of resistivity elements such as Si, Al, and Mn to reduce the grain size and eddy current loss. On the other hand, since eddy currents pass only through the surface layer of the steel sheet as the frequency increases, increasing the resistivity elements in the surface layer or controlling the grain size of the surface layer to be small can reduce the frequency dependence of the magnetic permeability and improve high-frequency iron loss. However, because conventional manufacturing methods produce steel sheets that are uniform in the thickness direction, the frequency dependence of the magnetic permeability inevitably becomes large, and the method of diffusing resistivity elements on the surface using CVD or other methods has limitations that make it difficult to use commercially due to excessive cost increases. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a non-oriented electrical steel sheet and a manufacturing method thereof. Specifically, it is to provide a non-oriented electrical steel sheet having excellent magnetic properties regardless of frequency by adding an appropriate amount of Cr element to a steel sheet and adjusting the conditions of the cold rolling and final annealing processes to form a Cr-enriched layer inside the steel sheet, thereby providing a non-oriented electrical steel sheet and a manufacturing method thereof. [Means for solving the problem]

[0005] The non-oriented electrical steel sheet of the present invention contains, by weight, 2.5-3.8% Si, 0.1-1.5% Al, 0.1-2.0% Mn, 0.01-0.15% Cr, with the remainder being Fe and unavoidable impurities, and satisfies the following formula 1. [Formula 1] [Cr]>([Al]+[Mn]) / [Si] / 10 (In formula 1, [Cr], [Al], [Mn] and [Si] represent the contents (wt%) of Cr, Al, Mn and Si, respectively.)

[0006] The non-oriented electrical steel sheet of the present invention includes a Cr-enriched layer formed from the surface of the steel sheet toward the inside of the steel sheet to a thickness of 1 / 50 or less of the total thickness of the steel sheet, and a base material, and the average grain size in the Cr-enriched layer is 50 to 95% of the average grain size in the base material. It may further contain one or more of P: 0.005 to 0.08% by weight, Sn: 0.01 to 0.08% by weight, and Sb: 0.005 to 0.05% by weight. It may further contain one or more of C: 0.0040% by weight or less, S: 0.0040% by weight or less, N: 0.0040% by weight or less, and Ti: 0.0040% by weight or less. It may further contain one or more of Mo: 0.03 wt.% or less, B: 0.0050 wt.% or less, V: 0.0050 wt.% or less, Ca: 0.0050 wt.% or less, Nb: 0.0050 wt.% or less, and Mg: 0.0050 wt.% or less. It may further include an insulating layer located on a surface of the steel sheet. The frequency dependency (α) of magnetic permeability in the range of 200 Hz to 800 Hz can be −5 or more. However, the frequency dependence of magnetic permeability (α) is determined by measuring the magnetic permeability at 200 Hz, 400 Hz, 600 Hz, and 800 Hz under a magnetic flux density of 1 T and finding the average slope (H / m / Hz). The resistivity may be 45 μΩ·cm or more.

[0007] A method for producing a non-oriented electrical steel sheet includes the steps of hot rolling a slab containing, by weight, 2.5-3.8% Si, 0.1-1.5% Al, 0.1-2.0% Mn, 0.01-0.15% Cr, with the balance being Fe and unavoidable impurities, and satisfying the following formula 1 to produce a hot-rolled sheet, cold rolling the hot-rolled sheet to produce a cold-rolled sheet, and final annealing the cold-rolled sheet. [Formula 1] [Cr]>([Al]+[Mn]) / [Si] / 10 (In formula 1, [Cr], [Al], [Mn] and [Si] represent the contents (wt%) of Cr, Al, Mn and Si, respectively.) In the step of producing a cold-rolled sheet, the maximum rolling speed is 10 m / s or more, and the surface temperature of the steel sheet can be maintained at 150° C. or more for 3 minutes or more. In the final annealing step, the steel sheet may be cooled from the soaking temperature to 700° C. at a cooling rate of 10 to 40° C. / s. The method may further include a step of heating the slab to 1100 to 1250° C. before the step of producing the hot-rolled sheet. After the step of producing the hot-rolled sheet, the method may further include a step of annealing the hot-rolled sheet at 850 to 1150°C. The reduction rate in the cold rolling stage may be 70 to 95%. In the final annealing stage, the material may be soaked at 800 to 1070°C. Effect of the Invention

[0008] According to the non-oriented electrical steel sheet of the present invention, by adding an appropriate amount of Cr to the steel sheet and forming a Cr-enriched layer, the frequency dependency of magnetic permeability can be significantly reduced. When the non-oriented electrical steel sheet of the present invention is used to manufacture a motor, the motor can be driven with a small current even at high speeds, resulting in excellent motor efficiency. Ultimately, the non-oriented electrical steel sheet of the present invention contributes to the production of environmentally friendly motors for automobiles, highly efficient motors for home appliances, and super-premium class electric motors. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic cross-sectional side view of a non-oriented electrical steel sheet according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Terms such as first, second and third are used to describe various parts, components, regions, layers and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Thus, a first part, component, region, layer or section described below can be referred to as a second part, component, region, layer or section without departing from the scope of the present invention. The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the invention. As used herein, the singular forms include the plural forms unless the context clearly indicates otherwise. As used in the specification, the meaning of "comprising" is to embody certain 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 said to be "on" or "on top of" another part, this means that it is directly on or above the other part, or there may be other parts intervening between them. Conversely, when a part is said to be "directly on" another part, there are no other parts intervening between them. 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 the present invention belongs. Terms defined in commonly used dictionaries are additionally analyzed to have a meaning that fits the relevant technical literature and the present disclosure, and are not interpreted as being overly ideal or formal unless otherwise defined. Moreover, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight. In one embodiment of the present invention, the term "additionally contain an additional element" refers to the inclusion of an additional amount of the additional element in place of the remaining iron (Fe).

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to exemplary embodiments thereof, so that those skilled in the art can easily practice the present invention. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. By adding an appropriate amount of Cr element to the non-oriented steel sheet of the present invention and adjusting the conditions of the cold rolling and final annealing processes to form a Cr-enriched layer inside the steel sheet, magnetic properties are improved regardless of frequency. A non-oriented electrical steel sheet according to an embodiment of the present invention contains, by weight, 2.5-3.8% Si, 0.1-1.5% Al, 0.1-2.0% Mn, and 0.01-0.15% Cr, with the balance being Fe and unavoidable impurities.

[0012] First, the reasons for limiting the components of non-oriented electrical steel sheets will be explained. Si:2.5~3.8wt% Silicon (Si) plays a role in increasing the resistivity of the material and reducing the iron loss. If too little Si is added, the effect of improving high-frequency iron loss is insufficient, and if too much Si is added, the hardness of the material increases, resulting in poor productivity and punchability, which is not preferable. More specifically, Si may be included in an amount of 2.6 to 3.5 wt %.

[0013] Al: 0.1~1.5% by weight Aluminum (Al) increases the resistivity of the material and reduces iron loss. If too little Al is added, it is ineffective in reducing high frequency iron loss and may cause fine nitrides to form, reducing magnetic properties. Conversely, if too much is added, it may cause problems in all processes, including steelmaking and continuous casting, significantly reducing productivity. Therefore, Al is added within the above-mentioned range. More specifically, Al may be contained in an amount of 0.3 to 1.0 wt%.

[0014] Mn:0.1~2.0wt% Manganese (Mn) increases the resistivity of the material, improves core loss, and forms sulfides. If too little Mn is added, MnS may precipitate finely and reduce magnetic properties. Conversely, if too much Mn is added, it may promote the formation of {111} texture, which is detrimental to magnetic properties, and reduce magnetic flux density. Therefore, Mn is added within the above-mentioned range. More specifically, Mn may be contained in an amount of 0.2 to 1.5 wt%.

[0015] Specific resistance 45 μΩ cm or more Resistivity is a value calculated from 13.25 + 11.3 x ([Si] + [Al] + [Mn] / 2), where [Si], [Al], and [Mn] indicate the content (weight%) of Si, Al, and Mn, respectively. The higher the resistivity, the lower the iron loss. If the resistivity is too low, the iron loss will be poor, making it difficult to use as a high-efficiency motor. More specifically, the resistivity can be 50 to 80 μΩ·cm.

[0016] Cr:0.010~0.150wt% Chromium (Cr) increases the resistivity of the material and reduces iron loss, and by controlling the cold rolling conditions and final annealing conditions, it can be enriched on the surface to form a Cr-enriched layer. If the Cr content is too low, the surface enrichment effect does not occur, and if too much Cr is added, it is distributed evenly throughout the entire thickness rather than being enriched on the surface. More specifically, Cr can be contained in a range of 0.030 to 0.100 wt%. The non-oriented electrical steel sheet of the present invention satisfies formula 1. [Cr]>([Al]+[Mn]) / [Si] / 10 (In formula 1, [Cr], [Al], [Mn] and [Si] represent the contents (wt%) of Cr, Al, Mn and Si, respectively.) Equation 1 is a relational equation that defines the correlation between Cr and Al, Mn, and Si. When equation 1 is not satisfied, i.e., when Cr or Si is contained in a small amount or Al or Mn is contained in an excessive amount, various solid solutions are formed and the Cr-enriched layer is not properly formed, making it difficult to achieve the object of the present invention to improve magnetic properties regardless of frequency.

[0017] The non-oriented electrical steel sheet of the present invention may further contain one or more of 0.005 to 0.08% by weight of P, 0.01 to 0.08% by weight of Sn, and 0.005 to 0.05% by weight of Sb. When additional elements are further contained, they may be contained in place of the balance Fe. P:0.005~0.08wt% Phosphorus (P) is concentrated on the surface and plays a role in controlling the fraction of the internal oxide layer. If the amount of P added is too small, it is difficult to form a uniform internal oxide layer. If the amount of P added is too large, the melting point of the Si-based oxide fluctuates, and the internal oxide layer is rapidly formed. Therefore, the P content is controlled to be within the above-mentioned range. More specifically, P may be contained in an amount of 0.005 to 0.07% by weight.

[0018] Sn:0.01~0.08wt% Tin (Sn) segregates on the surface and grain boundaries of the steel sheet to suppress surface oxidation during annealing and improve the texture. If too little Sn is added, the effect is insufficient. If too much Sn is added, it segregates on the grain boundaries, reducing toughness and decreasing productivity for improving magnetic properties, which is not preferable. More specifically, Sn can be contained in an amount of 0.02 to 0.07% by weight.

[0019] Sb:0.005~0.05wt% Antimony (Sb) segregates on the surface and grain boundaries of steel sheets to suppress surface oxidation during annealing and improve texture. If too little Sb is added, this effect is lost, and if the amount is 0.05% or more, it is undesirable because it segregates on the grain boundaries, reducing the toughness of the material and reducing productivity for improving magnetic properties. More specifically, Sb can be contained in an amount of 0.01 to 0.03% by weight.

[0020] The non-oriented electrical steel sheet of the present invention may further contain one or more of Mo: 0.03% by weight or less, B: 0.0050% by weight or less, V: 0.0050% by weight or less, Ca: 0.0050% by weight or less, Nb: 0.0050% by weight or less, and Mg: 0.0050% by weight or less. These react with the inevitably contained C, S, N, etc. to form fine carbides, nitrides, or sulfides, which can adversely affect magnetic properties, so the upper limit is set as described above.

[0021] Other impurities In addition to the elements mentioned above, impurities such as carbon (C), sulfur (S), nitrogen (N), and titanium (Ti) may be included that are inevitably mixed in. C, N, and Ti are restricted because they form carbonitrides and play a role in preventing magnetic domain movement, and S forms sulfides and deteriorates grain growth, so its upper limit is set. Each of these elements may be contained in an amount of 0.0040 wt% or less. N combines with Ti, Nb, and V to form nitrides, which acts to reduce grain growth. C reacts with N, Ti, Nb, V, etc. to form fine carbides, which serve to hinder grain growth and magnetic domain movement. S forms sulfides and impairs grain growth. When the alloy further contains impurity elements, it may contain one or more of C, S, N, Ti, Nb, and V in an amount of 0.004 wt % or less.

[0022] Fig. 1 shows a schematic side cross-sectional view of a non-oriented electrical steel sheet according to the present invention. The non-oriented electrical steel sheet in Fig. 1 is merely for the purpose of illustrating the present invention, and the present invention is not limited thereto. Therefore, the structure of the non-oriented electrical steel sheet can be modified in various ways. 1, a non-oriented electrical steel sheet 100 according to an embodiment of the present invention includes a Cr-enriched layer 12 formed from the surface of the steel sheet toward the inside of the steel sheet to a thickness of 1 / 50 or less of the total thickness of the steel sheet, and a base material 11. The inclusion of Cr-enriched layer 12 has the effect of increasing resistivity, so that there is little change in magnetic permeability even when the frequency is increased.

[0023] The thickness d of the Cr-enriched layer 12 may be 1 / 50 or less of the total thickness of the steel sheet. If the Cr-enriched layer 12 is formed too thick, the amount of concentrated Cr decreases, and the grain size in the Cr-enriched layer 12 does not become sufficiently small. More specifically, the thickness d of the Cr-enriched layer 12 is 1 / 100 to 1 / 50 of the total thickness of the steel sheet. A Cr concentration gradient exists from the surface to the inside. The Cr-enriched layer 12 exists in an area of ​​1 / 50 or less of the entire thickness of the steel sheet, and the Cr-enriched layer 12 may contain a larger amount of Cr than the steel sheet substrate 11. More specifically, it may contain more than 0.15 wt%. In this case, the Cr content means the average content with respect to the entire thickness of the Cr-enriched layer 12. The contents of the remaining elements may be the same as the element contents in the non-oriented electrical steel sheet 100 described above. Since the Cr-enriched layer 12 is formed to be thinner than the thickness of the entire electrical steel sheet, the Cr content in the substrate 11 may be substantially the same as the Cr content in the non-oriented electrical steel sheet 100.

[0024] Thus, the Cr-enriched layer 12 contains a larger amount of Cr than the substrate 11, which causes the crystal grains in the Cr-enriched layer 12 to be finer than those in the substrate 11. The Cr content and fine crystal grains in the Cr-enriched layer 12 reduce the change in magnetic permeability even when the frequency is increased due to the skin effect, in which eddy currents flow along the surface layer. Specifically, the average grain size in the Cr-enriched layer may be 50 to 95% of the average grain size in the substrate. Specifically, the average grain size in the Cr-enriched layer means the average grain size at the middle thickness (d / 2) of the Cr-enriched layer 12, and the average grain size in the substrate means the average grain size at the middle thickness (t / 2) of the steel sheet. The reference plane for measuring the grains may be a plane parallel to the rolled surface (ND surface). More specifically, the average grain size in the Cr-enriched layer is 80 to 93% of the average grain size in the substrate. More specifically, the average crystal grain size in the Cr-enriched layer 12 may be 55 to 90 μm, and the average crystal grain size in the substrate 11 may be 60 to 100 μm. 1, an insulating layer 20 may be further formed on the Cr-concentrated layer 12. The insulating layer 20 is formed on the surface of the Cr-concentrated layer 12, i.e., outside the steel sheet, and is distinguished from the Cr-concentrated layer 12. The insulating layer 20 may have a thickness of 0.7 to 1.0 μm. The insulating layer 20 is widely known in the technical field of non-oriented electrical steel sheets, and therefore a detailed description thereof will be omitted.

[0025] In the present invention, the frequency dependency of magnetic permeability can be significantly reduced. Specifically, the frequency dependency (α) of magnetic permeability in the range of 200 Hz to 800 Hz can be −5.0 or more. In this case, the frequency dependence of the magnetic permeability (α) is calculated as follows: ([Minimum permeability in the 200Hz~800Hz range]-[Maximum permeability in the 200Hz~800Hz range]) Since the frequency dependency (α) of magnetic permeability is small, when the non-oriented electrical steel sheet according to one embodiment of the present invention is used to manufacture a motor, the motor can be driven with a small current even at high speeds, resulting in excellent motor efficiency. The frequency dependence (α) of the magnetic permeability can be between −4.5 and −1.0. The magnetic permeability at 200Hz may be 9500-11000H / m. The magnetic permeability at 400Hz may be 9000-10000H / m. The magnetic permeability at 600Hz may be 8500-9500H / m. The magnetic permeability at 800Hz may be 7500-9000H / m.

[0026] The method for producing a non-oriented electrical steel sheet of the present invention includes the steps of hot rolling a slab containing, by weight, 2.5-3.8% Si, 0.1-1.5% Al, 0.1-2.0% Mn, 0.01-0.15% Cr, with the balance being Fe and unavoidable impurities, and satisfying the following formula 1 to produce a hot-rolled sheet, cold rolling the hot-rolled sheet to produce a cold-rolled sheet, and final annealing the cold-rolled sheet. [Formula 1] [Cr]>([Al]+[Mn]) / [Si] / 10 (In formula 1, [Cr], [Al], [Mn] and [Si] represent the contents (wt%) of Cr, Al, Mn and Si, respectively.) Each step will be described in detail below.

[0027] First, a slab is manufactured. The reason for limiting the addition ratio of each component in the slab is the same as the reason for limiting the composition of the non-oriented electrical steel sheet described above, so a duplicated explanation will be omitted. The composition of the slab does not substantially change during the manufacturing process such as hot rolling, hot-rolled sheet annealing, cold rolling, and final annealing described below, so the composition of the slab and the composition of the non-oriented electrical steel sheet are substantially the same. The slab may be heated before the step of producing the hot-rolled sheet. Specifically, the slab is charged into a heating furnace and heated to 1100 to 1250°C. When heated to a temperature exceeding 1250°C, the precipitates are remelted and finely precipitated after hot rolling. The heated slab is hot-rolled to 2-2.3 mm to produce a hot-rolled sheet. The finish rolling temperature in the hot-rolled sheet production step can be 800-1000°C. After the step of producing the hot-rolled sheet, the method may further include a step of annealing the hot-rolled sheet. The annealing temperature of the hot-rolled sheet may be 850 to 1150°C. If the annealing temperature of the hot-rolled sheet is less than 850°C, the structure does not grow or grows finely, so the effect of increasing the magnetic flux density is small, and if the annealing temperature exceeds 1150°C, the magnetic properties are rather reduced and the rolling workability is deteriorated due to deformation of the sheet shape. More specifically, the temperature range may be 950 to 1125°C. More specifically, the annealing temperature of the hot-rolled sheet is 900 to 1100°C. The annealing of the hot-rolled sheet is performed to increase the orientation favorable to magnetism as necessary, and may be omitted.

[0028] The hot-rolled sheet is then pickled and cold-rolled to the desired sheet thickness. Depending on the thickness of the hot-rolled sheet, a rolling reduction of 70-95% is applied, and the sheet is cold-rolled to a final thickness of 0.2-0.65 mm. To achieve the desired rolling reduction, one cold rolling pass or two or more cold rolling passes with intermediate annealing may be performed. In the step of producing a cold-rolled sheet, the maximum rolling speed may be 10 m / s or more. If the maximum rolling speed is low, the speed at which Cr diffuses to the steel sheet surface is low, and a Cr-enriched layer is not properly formed. More specifically, the maximum rolling speed may be 10 to 20 m / s. During the cold-rolled sheet manufacturing step, the steel sheet surface temperature is maintained at 150°C or higher for 3 minutes or more. The steel sheet temperature can be increased by using residual heat from the hot-rolled sheet manufacturing step or the hot-rolled sheet annealing step, or by supplying heat from an external source. If the time for the steel sheet surface temperature to be 150°C or higher is not adequately secured, the Cr diffuses slowly to the steel sheet surface and a Cr-enriched layer is not properly formed. More specifically, the steel sheet surface temperature is maintained at 150°C or higher for 3 to 7 minutes.

[0029] The cold-rolled sheet undergoes final annealing. In the final annealing stage, the sheet is soaked at 800~1070℃. If the soaking temperature is too low, recrystallization does not occur sufficiently, and if the soaking temperature is too high, the grain size becomes too large, resulting in poor high-frequency core loss. When cooling after soaking, cool from the soaking temperature to 700°C at a cooling rate of 10 to 40°C / s. If the cooling rate is too fast, there will not be enough time for Cr to concentrate in the surface layer, and if the cooling rate is too slow, the grain size will grow excessively, resulting in poor high-frequency iron loss. More specifically, cool at a rate of 15 to 35°C / s. Thereafter, the method may further include forming an insulating layer. A method for forming an insulating layer is widely known in the technical field of non-oriented electrical steel sheets, and therefore a detailed description thereof will be omitted.

[0030] Hereinafter, preferred examples and comparative examples of the present invention will be described. However, the following examples are only preferred examples of the present invention, and the present invention is not limited to the following examples. Example 1 A slab with the composition shown in Table 1 below was manufactured. The elements other than those listed in Table 1, such as C, S, N, Ti, Nb, and V, were all controlled to 0.003 wt% or less, with the remainder being Fe. The slab was heated to 1150°C and hot-finish rolled at 850°C to produce a hot-rolled sheet with a thickness of 2.0 mm. The hot-rolled sheet was annealed at 1100°C for 4 minutes and then pickled. It was then cold-rolled to a thickness of 0.25 mm, and then final annealing was performed. The maximum cold rolling speed, maintenance time above 150°C, final annealing soaking temperature, average cooling rate from soaking to 700°C, and final steel sheet thickness were adjusted as shown in Table 2. The crystal grain size was measured at 1 / 50 of the total thickness, and at 1 / 2 of the total thickness. The results are summarized in Table 2 below. The magnetic permeability of the manufactured non-oriented electrical steel sheets was measured at 200 Hz, 400 Hz, 600 Hz, and 800 Hz, and the results are summarized in Table 3 below. In addition, the frequency dependency was calculated and summarized in Table 3 below.

[0031] [Table 1]

[0032] [Table 2]

[0033] [Table 3]

[0034] As shown in Tables 1 to 3, in the examples that satisfy the alloy composition and manufacturing process conditions, it can be confirmed that the crystal grain size in the Cr-enriched layer is appropriately formed and the magnetic permeability is constant even when the frequency changes. In contrast, in steel types 1, 3, 4, 6, 7, and 8, which do not satisfy the alloy composition requirements, the size of the crystal grains in the Cr-enriched layer is not properly formed, and it can be confirmed that the change in magnetic permeability due to frequency change is large. In addition, it can be confirmed that in the steel types 10, 11, and 12 that do not satisfy the manufacturing process conditions, the crystal grain size in the Cr-enriched layer is not formed appropriately, and the change in magnetic permeability due to frequency change is large.

[0035] The present invention is not limited to the above embodiment, and can be manufactured in various different forms, and a person having ordinary skill in the art to which the present invention pertains can understand that the present invention can be embodied in other specific forms without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the above embodiment is illustrative in all respects and not limiting. [Explanation of symbols]

[0036] 11 Steel plate base material 12Cr enriched layer 20 Insulating layer 100 Non-oriented electrical steel sheet

Claims

1. In weight percent, it contains 2.5 to 3.8% Si, 0.1 to 1.5% Al, 0.1 to 2.0% Mn, and 0.01 to 0.15% Cr, with the balance being Fe and unavoidable impurities, and satisfies the following formula 1: The steel plate includes a Cr-enriched layer and a base material formed in a thickness of 1 / 50 or less of the total thickness of the steel plate from the surface of the steel plate toward the inside of the steel plate, The average crystal grain size in the Cr-enriched layer is 50 to 95% of the average crystal grain size in the substrate, The Cr-enriched layer contains a larger amount of Cr than the base material, and the Cr-enriched layer contains more than 0.15 wt. % of Cr. [Formula 1] [Cr]>([Al]+[Mn]) / [Si] / 10 (In formula 1, [Cr], [Al], [Mn] and [Si] represent the contents (wt%) of Cr, Al, Mn and Si, respectively.)

2. 2. The non-oriented electrical steel sheet according to claim 1, further comprising one or more of P: 0.005 to 0.08 wt %, Sn: 0.01 to 0.08 wt %, and Sb: 0.005 to 0.05 wt %.

3. 3. The non-oriented electrical steel sheet according to claim 1, further comprising one or more of C: 0.0040% by weight or less, S: 0.0040% by weight or less, N: 0.0040% by weight or less, and Ti: 0.0040% by weight or less.

4. 4. The non-oriented electrical steel sheet according to claim 1, further comprising one or more of Mo: 0.03% by weight or less, B: 0.0050% by weight or less, V: 0.0050% by weight or less, Ca: 0.0050% by weight or less, Nb: 0.0050% by weight or less, and Mg: 0.0050% by weight or less.

5. 5. The non-oriented electrical steel sheet according to claim 1, further comprising an insulating layer located on a surface of the steel sheet.

6. The non-oriented electrical steel sheet according to any one of claims 1 to 5, characterized in that the frequency dependency (α) of magnetic permeability in the range of 200 Hz to 800 Hz is -5 or more.

7. The non-oriented electrical steel sheet according to any one of claims 1 to 6, characterized in that the non-oriented electrical steel sheet has a resistivity of 45 µΩ·cm or more.

8. A step of producing a hot-rolled sheet by hot rolling a slab containing, in weight percent, 2.5 to 3.8% Si, 0.1 to 1.5% Al, 0.1 to 2.0% Mn, and 0.01 to 0.15% Cr, with the balance being Fe and unavoidable impurities, and satisfying the following formula 1: cold rolling the hot rolled sheet to produce a cold rolled sheet; final annealing of the cold rolled sheet; In the step of producing the cold-rolled sheet, the maximum rolling speed is 10 m / s or more, and the surface temperature of the steel sheet is maintained at 150 ° C. or more for 3 minutes or more; The method for manufacturing a non-oriented electrical steel sheet, wherein the final annealing step comprises cooling from the soaking temperature to 700° C. at a cooling rate of 10 to 40° C. / s. [Formula 1] [Cr]>([Al]+[Mn]) / [Si] / 10 (In formula 1, [Cr], [Al], [Mn] and [Si] represent the contents (wt%) of Cr, Al, Mn and Si, respectively.)

9. The method of claim 8, further comprising the step of heating the slab to 1100 to 1250° C. before the step of producing the hot-rolled sheet.

10. The method for producing a non-oriented electrical steel sheet according to claim 8 or 9, further comprising the step of annealing the hot-rolled sheet at 850 to 1150° C. after the step of producing the hot-rolled sheet.

11. The method for producing a non-oriented electrical steel sheet according to any one of claims 8 to 10, characterized in that the rolling reduction in the step of producing the cold rolled sheet is 70 to 95%.

12. The method for producing a non-oriented electrical steel sheet according to any one of claims 8 to 11, characterized in that in the final annealing step, the heating is performed at 800 to 1070°C.

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