Non-oriented electrical steel sheet and method for manufacturing non-oriented electrical steel sheet

Abstract

A non-oriented electrical steel sheet according to one embodiment of the present invention includes, in weight%: 2.0 - 3.5% of silicon (S) 0.8 - 1.5% of aluminum (Al); 0.1 - 0.5% of manganese (Mn); 0.005% or less (excluding 0) of carbon (C); 0.005% or less (excluding 0) of nitrogen (N); and the balance of iron (Fe) and other inevitable impurities, and includes a coating layer on the surface, in which the average thickness (Tavg) of the coating layer is 100 nm or less, and the thickness deviation (Tdev) of the coating layer is 10 nm or less.

Description

[TECHNICAL FIELD]

[0001] The present invention relates to a non-oriented electrical steel sheet and a method for manufacturing a non-oriented electrical steel sheet.[BACKGROUND ART]

[0002] Non-oriented electrical steel is a steel grade which has higher amounts of silicon (Si), aluminum (Al), and manganese (Mn) than ordinary carbon steel in order to achieve superior electromagnetic properties. The magnetic property of the non-oriented electrical steel sheet is mainly evaluated by the magnetic flux density and the iron loss. The magnetic flux density refers to a degree of magnetization obtained under a specific magnetic field and the iron loss refers to an energy loss generated at a specific magnetic flux density and a frequency. The higher the magnetic flux density, the larger the magnetic field can be made with the same energy so that the energy efficiency may be increased. Further, the lower the iron loss, the less the energy lost as heat so that the energy efficiency may be increased.

[0003] Recently, as the environmental regulations have been strengthened worldwide, the automobile industry is moving toward developing eco-friendly vehicles such as hybrid vehicles and electric vehicles. In accordance with this trend toward vehicle electrification, the magnetic properties of the non-oriented electrical steel sheet used as the core material for automotive motors are becoming increasingly important. Accordingly, the technical demands for reducing the iron loss under a specific frequency environment are being increased.

[0004] In the related art, a content of silicon (Si) is increased to increase a specific resistance of the material itself, thereby reducing the eddy current loss. However, if the content of silicon (Si) is excessive, there is a problem in that the brittleness is significantly increased to deteriorate the rollability.

[0005] Aluminum (Al) is a ductile material so that even though the content thereof is increased, the rollability is not deteriorated. However, in the case of steel having a higher content of aluminum (Al), residual nitrogen (N) or oxygen (O) in the steel material reacts during the annealing process to form a coating layer configured by nitride (AlN) or oxide (Al 2 O 3 ). If a thick coating layer is formed on the steel material, the iron loss is increased, but the coating adhesiveness is deteriorated.

[0006] Accordingly, it is necessary to develop a technology which is capable of controlling a thickness of the coating layer which affects the magnetic property, in the non-oriented electrical steel sheet including aluminum (Al).[DISCLOSURE] [TECHNICAL PROBLEM]

[0007] The present invention has been devised to solve the problems as described above and an object of the present invention is to provide a non-oriented electrical steel sheet which improves the magnetic property so as to have a low iron loss and a higher magnetic flux density, by controlling a thickness of the coating layer and a manufacturing method of the non-oriented electrical steel sheet.

[0008] Objects of the present invention are not limited to the above-mentioned objects, and other objects, which are not mentioned above, can be clearly understood by those skilled in the art from the following descriptions.[TECHNICAL SOLUTION]

[0009] According to an aspect of the present invention, a non-oriented electrical steel sheet includes, in weight%, 2.0 to 3.5% of silicon (S), 0.8 to 1.5% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.005% or less (excluding 0) of carbon (C), 0.005% or less (excluding 0) of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, and includes a coating layer on the surface and in which an average thickness (T avg ) of the coating layer is 100 nm or less and the thickness deviation (T dev ) of the coating layer is 10 nm or less.

[0010] The average thickness (T avg ) of the coating layer may satisfy [Equation 1]. T avg = ∑ i = 1 N T i N + ∑ i = 1 nt T i , nt nt 2 (in which N is the number of measurement points, T i is a thickness at an i-th measurement point, nt is the number of peaks in the measurement point, and T i,nt is a thickness of an i-th peak.)

[0011] A thickness deviation (T dev ) of the coating layer may satisfy [Equation 2]. T dev = T max − T avg (in which T max is a maximum value of measured T i .)

[0012] The thickness may satisfy 0.35 mm or less.

[0013] The grain size may satisfy 100 to 130 µm.

[0014] A yield strength may satisfy 300 MPa or higher and a tensile strength may satisfy 400 MPa or higher.

[0015] An iron loss (W 10 / 400 ) may satisfy 13.0 W / kg or lower and a magnetic flux density (B 50 ) may satisfy 1.60 T or higher.

[0016] The non-oriented electrical steel sheet may further include, in weight%, 0.015% or less of phosphorus (P), 0.003% or less of sulfur (S), and 0.005% or less of titanium (Ti).

[0017] According to an aspect of the present invention, a non-oriented electrical steel sheet includes, in weight%, 2.0 to 3.5% of silicon (S), 0.8 to 1.5% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.005% or less (excluding 0) of carbon (C), 0.005% or less (excluding 0) of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, and includes a coating layer on the surface and in which the thickness deviation (T dev ) of the coating layer satisfies 100 nm or less and the thickness deviation (T dev ) of the coating layer satisfies [Equation 2]. T dev = T max − T avg (in which T avg is an average thickness of the coating layer and T max is a maximum value of measured thickness of the coating layer.)

[0018] The average thickness (T avg ) of the coating layer may satisfy 100 nm or less.

[0019] The average thickness (T avg ) of the coating layer may satisfy [Equation 1]. T avg = ∑ i = 1 N T i N + ∑ i = 1 nt T i , nt nt 2 (in which N is the number of measurement points, T i is a thickness at an i-th measurement point, nt is the number of peaks in the measurement point, and T i,nt is a thickness of an i-th peak.)

[0020] The coating layer may include aluminum oxide (Al 2 O 3 ).

[0021] An iron loss (W 10 / 400 ) of the finished product may satisfy 13.0 W / kg or lower and a magnetic flux density (B 50 ) may satisfy 1.60 T or higher.

[0022] According to an aspect of the present invention, a method for manufacturing a non-oriented electrical steel sheet includes the steps of: (a) preparing a steel material which is a half-finished product; (b) hot rolling the sheet material, thereby forming a hot-rolled steel sheet; (c) hot rolling annealing and acid pickling the hot-rolled steel sheet, (d) cold rolling the hot-rolled steel sheet which was subject to the step (c), thereby forming a cold-rolled steel sheet; and (e) forming a finished product by cold rolling annealing and coating the cold-rolled steel sheet, and in the step (e), in volume%, a hydrogen (H 2 ) concentration is 30% or higher and the cold rolling annealing is performed at the atmosphere of the balance of nitrogen (N 2 ) and inevitable impurity, and if the hydrogen (H 2 ) concentration is 30% or higher and 50% or lower, a residual oxygen concentration is 300 ppm or lower and if the hydrogen (H 2 ) concentration is higher than 50% and 100% or lower, the residual oxygen concentration is 800 ppm or lower.

[0023] In the step (e), a temperature rising rate may satisfy 5 to 30°C / s, an annealing temperature may satisfy 800 to 1000°C, and an annealing time may satisfy 40 to 100 seconds.

[0024] The step (e) may include: a step of controlling a residual oxygen concentration in a cold rolling annealing furnace; and a step of forming a hydrogen atmosphere in the cold rolling annealing furnace.

[0025] The finished product may include, in weight%, 2.0 to 3.5% of silicon (S), 0.8 to 1.5% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.005% or less (excluding 0) of carbon (C), 0.005% or less (excluding 0) of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities.

[0026] An iron loss (W 10 / 400 ) of the finished product may satisfy 13.0 W / kg or lower and a magnetic flux density (B 50 ) may satisfy 1.60 T or higher.

[0027] A coating layer may be formed on the surface of the finished product, an average thickness (T avg ) of the coating layer may be 100 nm or less and the thickness deviation (T dev ) of the coating layer may satisfy 10 nm or less.

[0028] The average thickness (T avg ) of the coating layer may satisfy [Equation 1] and a thickness deviation (T dev ) of the coating layer may satisfy [Equation 2]. T avg = ∑ i = 1 N T i N + ∑ i = 1 nt T i , nt nt 2 (in which N is the number of measurement points, T i is a thickness at an i-th measurement point, nt is the number of peaks in the measurement point, and T i,nt is a thickness of an i-th peak.) T dev = T max − T avg (in which T max is a maximum value of measured thickness of the coating layer.)[ADVANTAGEOUS EFFECTS]

[0029] According to the embodiment of the present invention, a non-oriented electrical steel sheet with an improved magnetic property having a lower iron loss and a higher magnetic flux density can be manufactured.

[0030] The effects of the present invention are not limited to the aforementioned effects, and other effects, which are not mentioned above, will be apparently understood by a person skilled in the art from the recitations of the claims.[DESCRIPTION OF DRAWINGS]

[0031] FIG. 1 is a cross-sectional view schematically illustrating a non-oriented electrical steel sheet according to one embodiment of the present invention. FIG. 2 is a cross-sectional view schematically illustrating a specimen used to measure a thickness of a coating layer on a non-oriented electrical steel sheet according to one embodiment of the present invention. FIG. 3 is a flowchart illustrating a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention. [BEST MODE]

[0032] Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which embodiments of the invention are shown. However, the present invention is not restricted or limited by the following embodiments.

[0033] When a component (or an area, a layer, or a portion) is described as being "placed on", "connected to" or "coupled to" another component, it should be understood that it may be directly placed on / connected to / coupled to the other component, but there may be another component therebetween.

[0034] It should be understood that a term "include" or "have" indicates that a feature, a number, a step, an operation, a component, a part or a combination thereof described in the specification is present, but does not exclude a possibility of presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof, in advance.

[0035] In order to clearly describe the present invention, detailed descriptions of parts which are unrelated to the description or well-known related technologies which may unnecessarily obscure the gist of the present invention will be omitted. Further, when reference numerals are denoted to components of each drawing in the present specification, throughout the specification, the same or like components are denoted by the same or like reference numerals.

[0036] Further, terms or words used in the specification and the claims should not be restrictively analyzed as a general and dictionary meaning and should be analyzed as a meaning and a concept which conform to the technical spirit of the present invention based on a principle that the inventor can appropriately define a concept of a term in order to describe his / her own invention by the most method.

[0037] Unless otherwise specified, the notation "A ~ B" with respect to numerical values A and B refers to A or more and B or less. In this notation, when a unit is attached only to the numerical value B, the corresponding unit is also applied to the numerical value A.

[0038] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.Non-oriented electrical steel sheet

[0039] The non-oriented electrical steel sheet according to the embodiment of the present invention is manufactured by a steel material including 2.0 to 3.5 wt% of silicon (Si), 0.8 to 1.5 wt% of aluminum (Al), 0.1 to 0.5 wt% of manganese (Mn), 0.005 wt% or lower of carbon (C), 0.005 wt% or lower of nitrogen (N), 0.003 wt% or lower of sulfur (S), 0.015 wt% or lower of phosphor (P), 0.005% or lower of titanium (Ti), and the balance of iron (Fe) and other inevitable impurities and may include the same alloying constituent in the electrical steel sheet which is a finished product.

[0040] Hereinafter, a role and a content of each alloying element included in the non-oriented electrical steel sheet according to one embodiment of the present invention will be described in detail.Silicon (Si)

[0041] Silicon (Si) is a component which improves the magnetic property by lowering the iron loss by increasing the specific resistance and is a major additive element in the electrical steel sheet. If the content of silicon is below a predetermined range, the increased amount of specific resistance is not sufficient so that it may be difficult to obtain a desired low iron loss value. In contrast, if the content of silicon exceeds the predetermined range, the brittleness of the material is increased to reduce cold rollability and the plate rupture may be caused during the coiling and rolling processes, which may degrade the productivity. Further, the larger the added amount of silicon, the lower the magnetic permeability and the magnetic flux density.

[0042] Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 2.0 to 3.5 wt% of silicon.Aluminum (Al)

[0043] Aluminum (Al) increases the specific resistance, together with silicon (Si), to reduce the iron loss, thereby improving the magnetic property. Further, aluminum is a ductile material so that unlike silicon (Si), even if aluminum is excessively added, the rollability is not degraded so that the workability may be improved during the cold rolling.

[0044] Aluminum is coupled to nitrogen (N) in a gas atmosphere in the annealing furnace or the steel to form nitride (for example, AlN) or is coupled to oxygen (O) to form oxide (for example, Al 2 O 3 ). Accordingly, a coating layer configured by at least any one of oxide and nitride may be formed on a surface of the steel material.

[0045] If the content of aluminum is less than a predetermined range, the specific resistance is insufficient to increase a high frequency iron loss, thereby reducing the magnetic property. In contrast, if the content of aluminum exceeds the predetermined range, the coating layer is excessively formed to degrade the magnetic property and the coatability.

[0046] Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 0.8 to 1.5 wt% of aluminum.Manganese (Mn)

[0047] Manganese (Mn) increases the specific resistance, together with silicon (Si) to reduce the iron loss, thereby improving the magnetic property. Further, the manganese reacts with sulfur (S) to form sulfide, such as MnS, to suppress the grain growth. If the content of manganese is less than the predetermined range, the effect of increasing the specific resistance is reduced, thereby increasing a high frequency iron loss. In contrast, when the content of manganese exceeds the predetermined range, coarse precipitate is formed to reduce the magnetic flux density. Further, the coarse secondary phase is formed to increase {112} texture unfavorable to magnetization, to degrade the magnetic property.

[0048] Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 0.1 to 0.5 wt% of manganese.Carbon (C)

[0049] Carbon (C) is an element which is effective to increase the strength. However, carbon is coupled to titanium (Ti) or niobium (Nb) to form carbide, such as TiC or NbC, thereby increasing the iron loss. If the content of carbon exceeds 0.005 wt%, it may cause magnetic aging to deteriorate the magnetic property. Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 0.005 wt% or less of carbon.Nitrogen (N)

[0050] Nitrogen (N) is an element which contributes to the strength and corrosion resistance of steel and stabilizes austenite to improve toughness of steel. However, if the content of nitrogen exceeds 0.005 wt%, nitrogen is coupled to aluminum (Al) and titanium (Ti) to form precipitate, such as aluminum nitride (AlN) or titanium nitride (TiN) and increase the iron loss. Further, nitrogen suppresses the grain growth so that the nitrogen is desirably added as small as possible. Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 0.005 wt% or less of nitrogen (N).Phosphorus (P)

[0051] Phosphorus (P) is a grain boundary segregated element which contributes to improvement of texture to increase the specific resistance and lower the iron loss. However, if the content of phosphorus exceeds 0.015 wt%, grain refinement may be caused and textures unfavorable to the magnetism may be formed, simultaneously. Further, excessive grain boundary segregation is caused to degrade the cold rollability. Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 0.015 wt% or less of phosphorus.Sulfur (S)

[0052] Sulfur (S) is an impurity element which is inevitably contained during the manufacturing process. If a large amount of sulfur is added, brittleness may be caused. Further, sulfur forms the precipitate, such as MnS, to increase iron the loss and suppress the grain growth. Therefore, it is desirable to add sulfur as small as possible. Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 0.003 wt% or less of sulfur.Titanium (Ti)

[0053] Titanium (Ti) has a high tendency to form precipitate in the steel and is coupled to carbon (C) or nitrogen (N) to form refined precipitate, such as TiC or TiN, to suppress the grain growth. If titanium exceeds 0.005 wt%, a fraction of precipitate is increased and the {112} texture unfavorable to magnetization is formed to deteriorate the magnetic property. Accordingly, the non-oriented electrical steel sheet according to the embodiment of the present invention may include 0.005 wt% or less of titanium.

[0054] A balance other than the above-described components of steel may include Fe and inevitable impurities. The inevitable impurities are impurities mixed during a steelmaking step and a manufacturing process of an electrical steel sheet and are widely known in the art so that a detailed description thereof will be omitted.

[0055] In the embodiment of the present invention, addition of an element other than the above-described alloying constituents is not excluded, but various elements may be included without departing from the technical spirit of the present invention. When an additional element is further included, Fe which is the balance may be replaced.

[0056] FIG. 1 is a cross-sectional view schematically illustrating a non-oriented electrical steel sheet according to one embodiment of the present invention.

[0057] Referring to FIG. 1, the non-oriented electrical steel sheet according to the embodiment of the present invention may include a base layer 1 and a coating layer 2. For example, the coating layer 2 may include aluminum oxide (Al 2 O 3 ).

[0058] However, the present invention is not limited thereto and the coating layer 2 may be configured by an aluminum oxide (Al 2 O 3 ) layer and aluminum nitride (AlN) interposed therebetween or configured by aluminum nitride (AlN). Alternatively, the coating layer may include other oxides, such as silicon oxide (Si 2 O 3 ) or nitride.

[0059] FIG. 2 is a cross-sectional view schematically illustrating a specimen used to measure a thickness of a coating layer of a non-oriented electrical steel sheet according to one embodiment of the present invention. Hereinafter, the coating layer 2 illustrated in FIG. 1 will be described in more detail.

[0060] In the non-oriented electrical steel sheet according to one embodiment of the present invention, an average thickness (T avg ) of the coating layer may satisfy Equation 1. T avg = ∑ i = 1 N T i N + ∑ i = 1 nt T i , nt nt 2 ≤ 100 nm

[0061] In Equation 1, N refers to the number of points which are measured by the measurement experiment, T i is a thickness of the coating layer at an i-th measurement point, nt is the number of peaks in the measurement point, and T i,nt is a thickness of an i-th peak.

[0062] Referring to FIG. 2, in order to measure a thickness of the coating layer of the non-oriented electrical steel sheet according to the embodiment of the present invention, a specimen may be prepared. T i value may be calculated by measuring a thickness of the coating layer with a predetermined interval (L / (N-1) along a direction D1. Here, N is the number of measurement points and is a natural number of 2 or larger.

[0063] According to the embodiment of the present invention, the thickness of the coating layer may be measured by nondestructive methods, such as laser or ultrasound test, but is not limited thereto.

[0064] The peak T k may be defined as a point where a thickness of the coating layer is increased and then starts to be reduced with respect to the direction D1 or a point which the thickness of the coating layer is reduced and then starts to be increased.

[0065] Specifically, the peak T k may be defined as a point where a thickness of the coating layer at a center point k is larger or smaller than thicknesses of the coating layer of other points (k-1, k+1) adjacent to both sides among the three adjacent points (k-1, k, k+1) along the direction D1. Here, k refers to an arbitrary natural number greater than or equal to 2 and less than N. In the present embodiment, the peak (T k ) includes both end values T 1 and T N .

[0066] According to the embodiment of the present invention, the number nt of the peaks may be 30 to 40% of the number N of measurement points, but is not limited thereto.

[0067] In Equation 1, an average thickness T avg of the coating layer may be calculated by including not only an arithmetic mean of thicknesses T i at each measurement point, but also an arithmetic mean of the thickness (T i,nt ) at the peak so that a more reliable measured average thickness T avg of the coating layer may be obtained.

[0068] The average thickness T avg of the coating layer is controlled to be 100 nm or less by the above-described Equation 1 so that the magnetic property and the coatability of the electrical steel sheet may be improved.

[0069] In the non-oriented electrical steel sheet according to one embodiment of the present invention, a thickness deviation (T dev ) of the coating layer may satisfy Equation 2. T dev = T max − T avg ≤ 10 nm

[0070] In Equation 2, T avg refers to an average thickness of the coating layer and may be calculated by the above-described Equation 1. However, the present invention is not limited thereto and the average thickness (T avg ) of the coating layer may be an arithmetic mean of the thickness T i of the coating layer measured at each measurement point. T max may refer to the largest value of the measured thickness T i of the coating layer.

[0071] The thickness deviation of the coating layer is controlled by Equation 2 so that the coating layer is uniformly formed to further improve the magnetic property and the coatability of the electric steel sheet.

[0072] Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described in detail.Method for manufacturing non-oriented electrical steel sheet

[0073] FIG. 3 is a flowchart illustrating a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention.

[0074] Hereinafter, the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described with reference to FIG. 3.

[0075] The method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention includes the steps of: (a) a first step of preparing a steel material which is a half- finished product; (b) a second step of hot rolling the steel material prepared in the first step (a), to form a hot-rolled steel sheet, (c) a third step of hot rolling annealing and acid pickling the hot-rolled steel sheet, (d) a fourth step of cold rolling the hot-rolled steel sheet which was subject to the third step (c) to form a cold-rolled steel sheet; and (e) a fifth step of cold rolling annealing and coating the cold-rolled steel sheet, thereby forming a finished product.

[0076] In the fifth step (e) according to the embodiment of the present invention, the cold rolling annealing may be performed under an atmosphere where a hydrogen (H 2 ) concentration is 30 vol.% or larger. This will be described below.

[0077] Hereinafter, individual steps of the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described in detail.

[0078] The first step (a) of preparing a steel material which is a half-finished product is a step of preparing a steel material having the above-described alloying composition range to manufacture a non-oriented electrical steel sheet which is a finished product. To be more specific, the alloying constituent is designed in the above-described alloying composition range to manufacture a half-finished product. The half-finished product may be a slab, but is not necessarily limited thereto. Further, the slab is manufactured by a known process in the art, such as a steelmaking process or a continuous casting process.

[0079] In the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention, in the second step (b), the steel material on which the first step (a) is performed is hot-rolled to form a hot-rolled steel sheet. The second step (b) may include a reheating step, a hot rolling step, and a coiling step.

[0080] First, the reheating step is performed prior to the hot rolling step to reheat the steel material for the subsequent process. To be more specific, in the reheating step, the steel material is charged into a heating furnace to uniformly heat the steel material to easily perform plastic deformation.

[0081] The steel material may be reheated at a temperature condition of 1000 to 1200°C. If the reheating temperature is below 1000°C, a deformation resistance is increased during the hot rolling and the rolling load is increased, thereby deteriorating the rollability. In contrast, if the reheating temperature exceeds 1200°C, precipitate in the steel material, such as carbon (C), sulfur (S), or nitrogen (N) is redissolved to cause refined precipitates in the subsequent rolling and annealing processes to suppress the grain growth and degrade the magnetic property. Accordingly, in the present invention, the steel material may be reheated at a temperature of 1000 to 1200°C.

[0082] Next, the hot rolling step may be performed. The hot rolling step may include rough rolling and finishing rolling processes. Here, during the rough rolling process, the steel material may be created as a rolled material with an appropriate shape, thickness, and width and during the finishing rolling process, the steel material may be adjusted to have a predetermined thickness and width and be rolled to have a satisfactory surface and shape at a correct finishing temperature.

[0083] At this time, the finishing rolling temperature may be 860 to 900°C. If the finishing rolling temperature is lower than 860°C, the rolling load is increased to deteriorate the rollability. Further, if the finishing rolling temperature exceeds 900°C, there may be a problem in that the strength is sharply lowered. Accordingly, the finishing rolling temperature is desirably 860 to 900°C.

[0084] A thickness of the hot-rolled steel sheet which was subject to the hot rolling step is desirably 1.8 to 2.6 mm. The larger the thickness of the hot-rolled steel sheet, the lower the thickness reduction rate during the cold rolling so that the texture may be deteriorated and the magnetic property may be degraded. In contrast, if the thickness of the hot-rolled steel sheet is excessively thin, a thickness obtained after the cold rolling is not sufficient, which may cause a shape defect when it is applied to the product.

[0085] Further, after the hot rolling step, a coiling step may be performed. It means a step of making the hot-rolled steel sheet formed by the hot rolling a coil shape so as to be easily stored and carried.

[0086] The coiling step may be performed at a temperature of 550 to 650°C. If the coiling temperature is lower than 550°C, a size of the grain is too small so that the grain does not sufficiently grow even after hot rolling annealing and if the coiling temperature exceeds 650°C, refined precipitates are increased to degrade the magnetic property. Accordingly, the coiling temperature is desirably 550 to 650°C.

[0087] Next, the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention may perform the third step (c) of hot rolling annealing and acid pickling the hot-rolled steel sheet which was subject to the second step. The third step (c) may include a hot rolling annealing step and an acid pickling step.

[0088] The hot rolling annealing step may be performed to ensure the uniformity of the refined structure of the steel material on which the hot rolling is performed and a cold rollability.

[0089] In the hot rolling annealing step, the heat treatment may be performed in the temperature range of 950 to 1100°C and a range of 30 to 150 seconds. At this time, a temperature rising rate is 20°C or higher and a cooling rate is 20°C or higher.

[0090] If the hot rolling annealing temperature is lower than 950°C, the stretched cast structure remaining after the hot rolling remains to cause the refined structure and at this time, the small grains are formed, thereby degrading the magnetic property of the finished product. In contrast, if the annealing temperature exceeds 1100°C, the grain excessively grows to cause unbalance of the texture of the finished product, thereby excessively generating oxidation on the hot-rolled steel sheet to reduce the magnetic property. Further, the hot rolling annealing time may be ranged from 30 to 150 seconds so as to form an appropriate grain size at each temperature condition.

[0091] Next, the acid pickling step of removing an oxide layer formed on the surface of the hot-rolled steel sheet which was subject to the hot rolling annealing with an acid pickling agent may be performed. In the acid pickling step, an acid pickling agent produced by mixing sulfuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid solely or in combination may be used, but is not limited thereto.

[0092] Next, the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention may perform the fourth step (d) to cold roll the hot-rolled steel sheet which was subject to the third step (c) to form a cold-rolled steel sheet.

[0093] The fourth step (d) may be a process of rolling the hot-rolled steel sheet under the temperature condition of 150 to 200°C to further reduce the thickness of the steel sheet. To be more specific, the cold rolling may be a process of rolling the hot-rolled steel sheet to have a thickness and a width which satisfy the specification of the finished product.

[0094] The fourth step (d) may be performed with a thickness reduction rate of 80 to 90% and the thickness of the cold-rolled steel sheet formed by the cold rolling may be 0.35 mm or less, and more desirably 0.25 mm or less.

[0095] Next, the fifth step (e) of cold rolling annealing and coating the cold-rolled steel sheet, thereby forming a finished product may be performed. The fifth step (e) may include a cold rolling annealing step and a coating step.

[0096] In the cold rolling annealing step, the heat treatment may be performed at the temperature of 800 to 1100°C in the range of 40 to 100 seconds.

[0097] If the cold rolling annealing temperature is lower than 800°C, the grain size is minute to increase the hysteresis loss and if the cold rolling annealing temperature exceeds 1100°C, the grain size is coarse to increase the eddy current loss. Further, during the cold rolling annealing, the temperature rising rate may be 5 to 30°C / s and the cooling rate may be 30°C / s or higher.

[0098] During the cold rolling annealing step, the cold-rolling annealing furnace may be operated in a gas atmosphere of 30 vol% or more of hydrogen (H 2 ), with the balance of nitrogen (N 2 ) and other inevitable impurities.

[0099] In order to form the hydrogen (H 2 ) atmosphere, the cold rolling annealing step includes a step of controlling the residual oxygen concentration in the cold rolling annealing furnace and a step of forming a hydrogen atmosphere in the cold rolling annealing furnace by injecting hydrogen (H 2 ) gas and nitrogen (N 2 ) gas into the cold rolling annealing furnace.

[0100] For example, the step of controlling a residual oxygen concentration may include a process of removing oxygen O 2 after forming the inside of the cold rolling annealing furnace in a vacuum (or a sub-vacuum) state, but it is not limited thereto.

[0101] During the cold rolling annealing step, if the cold rolling annealing is performed under the atmosphere of hydrogen (H 2 ) concentration of 30% or lower, the oxide layer or the nitride layer is formed on the surface to be thick, thereby increasing the iron loss. Accordingly, the cold rolling annealing may be performed under the atmosphere of hydrogen (H 2 ) concentration of 30% or higher.

[0102] Specifically, if the cold rolling annealing is performed in a state in which the atmosphere of the inside of the cold rolling annealing furnace is hydrogen (H 2 ) concentration of 30% or higher and 50% or lower, the concentration of the residual oxygen (O 2 ) may be 300 ppm or less, and more desirably, may be 50 ppm or higher and 285 ppm or lower. If the concentration of the residual oxygen (O 2 ) is lower than 50 ppm, the cost for removing oxygen (O 2 ) in the cold rolling annealing furnace may be increased.

[0103] In the above-described condition, if the concentration of residual oxygen (O 2 ) exceeds 300 ppm, an oxide layer is formed on the surface to be thick, thereby increasing the iron loss.

[0104] If the cold rolling annealing is performed in a state in which the atmosphere of the inside of the cold rolling annealing furnace is hydrogen (H 2 ) concentration higher than 50% and 100% or lower, reduction of the cold-rolled steel sheet due to hydrogen (H 2 ) is caused more than the case that the concentration of hydrogen (H 2 ) is less than 50%.

[0105] Accordingly, when the cold rolling annealing is performed at higher than 50% of the hydrogen (H 2 ) concentration, the thickness of the oxide layer may be thinner than the case in which the cold rolling annealing is performed at 50% or lower of hydrogen (H 2 ) concentration.

[0106] Specifically, in the above-described condition, if the concentration of residual oxygen (O 2 ) exceeds 800 ppm, an oxide layer is formed on the surface to be thick, thereby increasing the iron loss. In contrast, if the concentration of the residual oxygen (O 2 ) is lower than 50 ppm, the cost for removing oxygen (O 2 ) in the cold rolling annealing furnace may be increased.

[0107] Accordingly, if the cold rolling annealing is performed in a state in which the atmosphere of the inside of the cold rolling annealing furnace is hydrogen (H 2 ) concentration of higher than 50% and 100% or lower, the concentration of the residual oxygen (O 2 ) may be 800 ppm or less, and more desirably, may be 50 ppm or higher and 740 ppm or lower.

[0108] The coating step may be performed after the cold rolling annealing step. The coating step may be performed to improve the punching property and ensure the insulating property of the non-oriented electrical steel sheet and form an insulating film on a surface of the cold-rolled steel sheet which was subject to the cold rolling annealing step.

[0109] According to the embodiment of the present invention, in the electrical steel sheet manufactured by the above-described manufacturing process, an average thickness (T avg ) of a coating layer formed on the surface may be 100 nm or less and a thickness deviation (T dev ) of the coating layer may be 10 nm or less. Accordingly, the magnetic property and the coatability of the electrical steel sheet may be improved.

[0110] The non-oriented electrical steel sheet manufactured by the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention may have an iron loss (W 10 / 400 ) which is 13.0 W / kg or lower at 400 Hz and 1.0 T and a magnetic flux density (B 50 ) of 1.60 T or higher.

[0111] The non-oriented electrical steel sheet manufactured by the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention may have a yield strength (YS) of 300 MPa or larger, a tensile stress (TS) of 400 MPa or larger, and a grain size of 100 to 130 µm.Comparative Example and Example

[0112] Hereinafter, desirable Examples and Comparative Examples are proposed for better understanding of the present invention. However, the following Examples and Comparative Examples are provided to help the understanding of the present invention, but the present invention is not limited by the following Examples.

[0113] In Table 1, alloying element compositions of Comparative Examples and Examples are represented. In Table 2, results obtained by measuring a cold annealing condition, a type of a coating layer, an average thickness (T avg ), a thickness deviation (T dev ) of the coating layer, and a magnetic property of the present embodiment are represented.

[0114] In Comparative Examples and Examples, the steel material having the alloying composition represented in Table 1 was used. Further, the hot rolling step was performed at a reheating temperature of 1150°C, a rolling end temperature of 890°C, and a coiling temperature of 610°C so that a thickness of the finished hot-rolled plate was 2.0 mm.

[0115] Further, the hot rolling annealing step was performed at a temperature rising rate of 20°C / s, and a temperature of 1000°C for 60 seconds and the cooling rate was 25°C / s. The acid pickling processing was performed on the steel sheet on which the hot rolling annealing was finished.

[0116] The cold rolling step was performed at a temperature of 180°C with a thickness reduction rate of 87.5% so that a thickness of the finished cold-rolled plate was 0.25 mm.

[0117] The cold rolling annealing step was performed at a temperature rising rate of 30°C / s, and a temperature of 1000°C for 60 seconds and the cooling rate was 40°C / s. The coating treatment was performed on the steel sheet on which the cooling was finished.

[0118] Further, the manufacturing process of Comparative Examples and Examples of the present invention was controlled by the same condition within the range described in the above-described method for manufacturing a non-oriented electrical steel sheet according to the embodiment of the present invention with the control variable, other than the above-described conditions and the process conditions represented in Table 2.

[0119] In the following Table 2, the average thickness (T avg ) and the thickness deviation (T dev ) of the coating layer were measured with a specimen with a length of 10 mm from Examples and Comparative Examples. In Table 2, the average thickness (T avg ) of the coating layer was calculated by applying N = 30 to Equation 1. In the following Table 2, the thickness deviation (T dev ) of the coating layer was calculated by Equation 2.

[0120] In the following Table 2, the iron loss (W 10 / 400 ) means an iron loss at 400 Hz and 1.0 T and the unit thereof is W / kg. Further, the magnetic flux density (B 50 ) refers to a magnitude of a magnetic flux density induced when a magnetic field of 5000 A / m was applied and Tesla (T) was used as the unit. [Table 1]Wt%CSiMnAlPSNTiContent0.00153.30.20.90.010.00150.00150.0015 [Table 2] Experimental ExampleGas atmosphere [vol%]Residual oxygen [ppm]Type of coating layerT avg [nm]T dev [nm]Iron lossMagnetic flux densityRemarksN 2 H 2 W 10 / 400 [W / kg]B 50 [T] 1100083Nitride layer (AlN)1722813.51.55Comp. Ex. 124221583113.81.55Comp. Ex. 239010170Oxide layer (Al 2 O 3 )1431313.31.58Comp. Ex. 343251852313.41.57Comp. Ex. 458020851241213.11.57Comp. Ex. 565531392513.21.57Comp. Ex. 6770302829510131.6Ex. 183581091513.21.59Comp. Ex. 79604022688912.81.6Ex. 2103191061113.11.59Comp. Ex. 811505018378712.61.62Ex. 3124971031513.11.6Comp. Ex. 91340607373612.31.63Ex. 41457179812.51.61Ex. 515307034066712.21.62Ex. 61673175912.31.62Ex. 7172080180577121.63Ex. 8188131011613.11.59Comp. Ex. 1019109045253611.91.63Ex. 92069357812.21.63Ex. 102101007047511.71.65Ex. 1122560637121.65Ex. 12

[0121] Referring to Tables 1 and 2, in Comparative Examples 1 to 6, the cold rolling annealing was performed in the atmosphere in which hydrogen (H 2 ) concentration was lower than 30%. In Comparative Examples 1 to 6, the average thickness (T avg ) and the thickness deviation (T dev ) of the coating layer exceed 100 nm and 10 nm, respectively, so that the above-mentioned Equations 1 and 2 were not satisfied.

[0122] Accordingly, it is confirmed that the iron loss (W 10 / 400 ) of 13.0 W / kg or lower and the magnetic flux density (B 50 ) of 1.60 T or higher which are target magnetic properties of the present invention were not satisfied.

[0123] In Comparative Examples 7 to 9, the cold rolling annealing was performed in the atmosphere in which hydrogen (H 2 ) concentration was 30% or higher and 50% or lower. At this time, a concentration of the residual oxygen (O 2 ) of Comparative Example 7 was 358 ppm, a concentration of the residual oxygen (O 2 ) of Comparative Example 8 was 319 ppm, and a concentration of the residual oxygen (O 2 ) of Comparative Example 9 was 497 ppm.

[0124] In Comparative Examples 7 to 9, the residual oxygen (O 2 ) concentration inside the cold rolling annealing furnace did not satisfy the range according to the embodiment of the present invention which was 300 ppm or lower, the average thickness (T avg ) and the thickness deviation (T dev ) of the coating layer exceeded 100 nm and 10 nm, respectively, and the above-described Equations 1 and 2 were not satisfied.

[0125] Accordingly, it is confirmed that Comparative Examples 7 to 9 did not satisfy any one of the iron loss(W 10 / 400 ) of 13.0 W / kg or lower and the magnetic flux density (B 50 ) of 1.60 T or higher which are target magnetic properties of the present invention.

[0126] In Comparative Example 10, the cold rolling annealing was performed in the atmosphere in which the hydrogen (H 2 ) concentration was 80%. At this time, a residual oxygen (O 2 ) concentration of Comparative Example 10 was 813 ppm.

[0127] In Comparative Example 10, the residual oxygen (O 2 ) concentration inside the cold rolling annealing did not satisfy the range according to the embodiment of the present invention which was 800 ppm or lower so that an average thickness (T avg ) of the coating layer was 101 nm and the thickness deviation (T dev ) of the coating layer was 16 nm.

[0128] In other words, it is confirmed in Comparative Example 10 that the average thickness (T avg ) and the thickness deviation (T dev ) of the coating layer exceeded 100 nm and 10 nm, respectively so that the above-described Equations 1 and 2 were not satisfied and the iron loss (W 10 / 400 ) of 13.0 W / kg or lower and the magnetic flux density (B 50 ) of 1.60 T or higher which were the target magnetic property of the present invention were not satisfied.

[0129] In contrast, in Examples 1 to 12 according to the embodiment of the present invention, it is confirmed that the hydrogen (H 2 ) concentration inside the cold rolling annealing furnace satisfied the range according to the embodiment of the present invention which was 30% or higher and the concentration of the residual oxygen (O 2 ) also satisfied the above-described condition.

[0130] By doing this, it is confirmed that the average thickness (T avg ) and the thickness deviation (T dev ) of the coating layer were 100 nm or less and 10 nm or less, respectively, to satisfy both Equation 1 and 2. Further, it is confirmed that both the iron loss (W 10 / 400 ) of 13.0 W / kg or lower and the magnetic flux density (B 50 ) of 1.60 T or higher which were the target magnetic properties of the present invention were satisfied to have excellent magnetic properties.

[0131] As described above, the embodiments of the present invention have been described and it is obvious to those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or scope of the present invention. Therefore, it should be understood that the embodiment is not restrictive, but is illustrative and thus the present invention is not limited to the above-described embodiment but may be modified within the scope of the accompanying claims and the equivalent range.[Explanation of Reference Numerals and Symbols]

[0132] 1: Base layer 2: Coating layer L: Length of specimen T: Thickness of coating layer

Claims

1. A non-oriented electrical steel sheet comprising: in weight%, 2.0 to 3.5% of silicon (S), 0.8 to 1.5% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.005% or less (excluding 0) of carbon (C), 0.005% or less (excluding 0) of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities, wherein a coating layer on a surface is included and an average thickness (Tavg) of the coating layer satisfies 100 nm or less and a thickness deviation (Tdev) of the coating layer satisfies 10 nm or less.

2. The non-oriented electrical steel sheet of claim 1, wherein the average thickness (Tavg) of the coating layer satisfies [Equation 1]: T avg = ∑ i = 1 N T i N + ∑ i = 1 nt T i , nt nt 2 (in which N is the number of measurement points, Ti is a thickness at an i-th measurement point, nt is the number of peaks in the measurement point, and Ti,nt is a thickness of an i-th peak.).

3. The non-oriented electrical steel sheet of claim 2, wherein the thickness deviation (Tdev) of the coating layer satisfies [Equation 2]: T dev = T max − T avg (in which Tmax is a maximum value of measured Ti.).

4. The non-oriented electrical steel sheet of claim 1, wherein the thickness satisfies 0.35 mm or less.

5. The non-oriented electrical steel sheet of claim 1, wherein a grain size satisfies 100 to 130 µm.

6. The non-oriented electrical steel sheet of claim 1, wherein a yield strength is 300 MPa or higher and a tensile strength is 400 MPa or higher.

7. The non-oriented electrical steel sheet of claim 1, wherein an iron loss (W10 / 400) satisfies 13.0 W / kg or lower and a magnetic flux density (B50) satisfies 1.60 T or higher.

8. The non-oriented electrical steel sheet of claim 1, further comprising: in weight%, 0.015% or less of phosphorus (P), 0.003% or less of sulfur (S), and 0.005% or less of titanium (Ti).

9. A non-oriented electrical steel sheet comprising: in weight%, 2.0 to 3.5% of silicon (S), 0.8 to 1.5% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.005% or less (excluding 0) of carbon (C), 0.005% or less (excluding 0) of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities, wherein a coating layer on a surface is included and a thickness deviation (Tdev) of the coating layer satisfies 10 nm or less and a thickness deviation (Tdev) of the coating layer satisfies [Equation 2]: T dev = T max − T avg (in which Tavg is the average thickness of the coating layer and Tmax is a maximum value of measured thickness of the coating layer.).

10. The non-oriented electrical steel sheet of claim 9, wherein the average thickness (Tavg) of the coating layer satisfies 100 nm or less.

11. The non-oriented electrical steel sheet of claim 10, wherein the average thickness (Tavg) of the coating layer satisfies [Equation 1]: T avg = ∑ i = 1 N T i N + ∑ i = 1 nt T i , nt nt 2 (in which N is the number of measurement points, Ti is a thickness at an i-th measurement point, nt is the number of peaks in the measurement point, and Ti,nt is a thickness of an i-th peak.).

12. The non-oriented electrical steel sheet of claim 9, wherein the coating layer includes aluminum oxide (Al2O3).

13. The non-oriented electrical steel sheet of claim 9, wherein an iron loss (W10 / 400) of the finished product satisfies 13.0 W / kg or lower and a magnetic flux density (B50) satisfies 1.60 T or higher.

14. A method for manufacturing a non-oriented electrical steel sheet, comprising steps of: (a) preparing a steel material which is a half-finished product; (b) hot rolling the sheet material, thereby forming a hot-rolled steel sheet; (c) hot rolling annealing and acid pickling the hot-rolled steel sheet; (d) cold rolling the hot-rolled steel sheet which was subject to the step (c), thereby forming a cold-rolled steel sheet; and (e) forming a finished product by cold rolling annealing and coating the cold-rolled steel sheet, wherein in the step (e), in volume%, a hydrogen (H2) concentration is 30% or higher and the cold rolling annealing is performed at an atmosphere of a balance of nitrogen (N2) and inevitable impurity, and if the hydrogen (H2) concentration is 30% or higher and 50% or lower, a residual oxygen concentration is 300 ppm or lower and if the hydrogen (H2) concentration is higher than 50% and 100% or lower, the residual oxygen concentration is 800 ppm or lower.

15. The method for manufacturing a non-oriented electrical steel sheet of claim 14, wherein in the step (e), a temperature rising rate satisfies 5 to 30°C / s, an annealing temperature satisfies 800 to 1000°C, and an annealing time satisfies 40 to 100 seconds.

16. The method for manufacturing a non-oriented electrical steel sheet of claim 14, wherein the step (e) includes a step of controlling a residual oxygen concentration in a cold rolling annealing furnace, and a step of forming a hydrogen atmosphere in the cold rolling annealing furnace.

17. The method for manufacturing a non-oriented electrical steel sheet of claim 14, wherein the finished product includes, in weight%, 2.0 to 3.5% of silicon (S), 0.8 to 1.5% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.005% or less (excluding 0) of carbon (C), 0.005% or less (excluding 0) of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities.

18. The method for manufacturing a non-oriented electrical steel sheet of claim 14, wherein an iron loss (W10 / 400) of the finished product satisfies 13.0 W / kg or lower and a magnetic flux density (B50) satisfies 1.60 T or higher.

19. The method for manufacturing a non-oriented electrical steel sheet of claim 14, wherein a coating layer is formed on a surface of the finished product, an average thickness (Tavg) of the coating layer satisfies 100 nm or less and a thickness deviation (Tdev) of the coating layer satisfies 10 nm or less.

20. The method for manufacturing a non-oriented electrical steel sheet of claim 19, wherein the average thickness (Tavg) of the coating layer satisfies [Equation 1] and the thickness deviation (Tdev) of the coating layer satisfies [Equation 2]: T avg = ∑ i = 1 N T i N + ∑ i = 1 nt T i , nt nt 2 (in which N is the number of measurement points, Ti is a thickness at an i-th measurement point, nt is the number of peaks in the measurement point, and Ti,nt is a thickness of an i-th peak.) T dev = T max − T avg (in which Tmax is a maximum value of measured thickness of the coating layer.).

Images