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, comprises 3.0-3.8 wt% of silicon (Si), 0.2-0.4 wt% of manganese (Mn), 0.8-1.5 wt% of aluminum (Al), 0.01 wt% or less of carbon (C), 0.01 wt% or less of sulfur (S), 0.08 wt% or less of phosphorus (P), 0.01 wt% or less of nitrogen (N), 0.01 wt% or less of titanium (Ti), and 0.01-0.1 wt% of at least one of tin (Sn) and antimony (Sb), with the remainder being iron (Fe) and other unavoidable impurities, and satisfies a surface defect index (Dt) according to formula 1 ranging from 120 to less than 350, thereby enabling the realization of a non-oriented electrical steel sheet with low iron loss and a method for manufacturing same.

Description

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

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

[0002] In line with recent global policies to reduce carbon dioxide emissions to combat global warming, conventional internal combustion engine vehicles are rapidly being replaced by eco-friendly vehicles such as hybrid, electric, and hydrogen-powered vehicles. Eco-friendly vehicles utilize electric motors to generate the necessary driving force and significantly reduce harmful exhaust gas emissions, thereby minimizing environmental pollution.

[0003] As interest in and demand for eco-friendly vehicles grows, so does the demand for electric motors, which power these vehicles. Electric motors use electricity to generate the power needed for vehicles. Energy efficiency, which allows them to operate for longer periods of time using the same amount of energy, is a crucial technological element.

[0004] To improve the energy efficiency of such electric motors, it is essential to improve the magnetic performance of non-oriented electrical steel, which is used as the core material. Non-oriented electrical steel exhibits consistent magnetic properties in all directions, regardless of rolling direction. To improve energy efficiency, core loss must be reduced and magnetic flux density increased.

[0005] In particular, reducing iron loss is crucial. Iron loss represents the energy loss that occurs during the magnetization process of a material and can be expressed as the energy loss occurring at a specific magnetic flux density and frequency. Iron loss can be divided into hysteresis loss, which occurs due to the magnetization process itself, and eddy current loss, which is generated by the eddy current induced during magnetization.

[0006] Conventionally, methods for reducing iron loss involved adding alloying elements that increase the resistivity of steel, such as silicon (Si), manganese (Mn), and aluminum (Al). However, this method suffers from reduced rollability, making it difficult to thin the material.

[0007] Additionally, additive elements such as carbon (C), sulfur (S), nitrogen (N), and titanium (Ti) can combine with each other to form precipitates. These finely deposited precipitates can impede the movement of magnetic domains, thereby degrading magnetic properties.

[0008] Meanwhile, surface defects formed on the surface of the steel plate can impede magnetic flow and deteriorate magnetic properties.

[0009] Therefore, there is a need for a technology to manufacture non-oriented electrical steel sheets with excellent magnetic properties and an excellent surface shape by controlling the amount of alloying elements added, precipitates, and process elements.

[0010] [Prior Art Literature]

[0011] [Patent Document]

[0012] (Patent Document 1) Republic of Korea Patent No. 10-2513317

[0013] The present invention has been devised to solve the above problems, and the object of the present invention is to provide a non-oriented electrical steel sheet and a method for manufacturing a non-oriented electrical steel sheet having excellent magnetic properties by controlling surface defects of the steel sheet.

[0014] The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the description below.

[0015] According to one embodiment of the present invention, a non-oriented electrical steel sheet contains 3.0 to 3.8 wt% of silicon (Si), 0.2 to 0.4 wt% of manganese (Mn), 0.8 to 1.5 wt% of aluminum (Al), 0.01 wt% or less of carbon (C), 0.01 wt% or less of sulfur (S), 0.08 wt% or less of phosphorus (P), 0.01 wt% or less of nitrogen (N), 0.01 wt% or less of titanium (Ti), and 0.01 to 0.1 wt% of at least one of tin (Sn) and antimony (Sb), the remainder being iron (Fe) and other unavoidable impurities, and has a surface defect index (D) according to the following Equation 1: t ) is 120 or more and less than 350.

[0016] [Formula 1]

[0017] D t = (D n ) 2 / 3 + 4(D p ) + 10(D a )

[0018] (However, in the above formula 1, D t is the surface defect index, D n is the number of surface defects, D p is the average depth of surface defects, D a represents the average surface defect area.)

[0019] Additionally, the insulation resistance may be greater than 1.

[0020] Also, iron loss (W 10 / 400 ) may be less than 16W / kg.

[0021] A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention comprises a first step of preparing a steel material including the above-described alloy component, a second step of hot-rolling the steel material to form a hot-rolled steel sheet, a third step of hot-rolling and annealing the hot-rolled steel sheet, a fourth step of cold-rolling the hot-rolled steel sheet subjected to the third step to form a cold-rolled steel sheet, and a fifth step of cold-rolling and annealing the cold-rolled steel sheet in a reducing atmosphere.

[0022] In addition, the fifth step can be annealed in a mixed atmosphere composed of 10% by volume or more of hydrogen (H2) and the remainder of nitrogen (N2).

[0023] In addition, the steel material that performed the above 5th step has a surface defect index (D) according to the above equation 1 t ) can be greater than or equal to 120 and less than or equal to 350.

[0024] Additionally, the steel material that has undergone the above-mentioned fifth step may have an insulation resistance of 1 or more.

[0025] In addition, the steel that performed the above 5th step has a core loss (W 10 / 400 ) may be less than 16W / kg.

[0026] According to one embodiment of the present invention, by controlling surface defects of a steel sheet, a non-oriented electrical steel sheet and a method for manufacturing a non-oriented electrical steel sheet having excellent magnetic properties can be implemented.

[0027] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

[0028] Figure 1 shows the iron loss (W) according to the average number of surface defects by the average depth of the surface defects. 10 / 400 ) is a graph showing the change.

[0029] Figure 2 is a flowchart schematically illustrating a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention.

[0030] Figure 3 is a graph showing the change in the number of surface defects according to the internal oxidation thickness.

[0031] Figure 4 shows the iron loss (W) according to the number of surface defects. 10 / 400 ) is a graph showing the changes.

[0032] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings so that those skilled in the art can easily implement the present invention. However, the present invention is not limited or restricted by the following embodiments.

[0033] Additionally, when a component (or region, layer, portion, etc.) is referred to as being "on," "connected to," or "coupled to" another component, it means that it can be directly placed / connected / coupled to the other component, or that a third component may be placed between them.

[0034] Terms such as "include" or "have" should be understood to specify the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but not to exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0035] In order to clearly explain the present invention, a detailed description of a part that is irrelevant to the description or a related known technology that may unnecessarily obscure the gist of the present invention has been omitted, and when adding reference signs to components of each drawing in this specification, the same or similar reference signs are attached to the same or similar components throughout the specification.

[0036] In addition, terms and words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0037] Unless otherwise specified, the notation 'A to B' for numerical values ​​A and B means 'A or more and B or less'. In such notation, if a unit is attached only to numerical value B, the unit shall be applied to numerical value A as well.

[0038] Also, unless otherwise stated, 1 ppm is 0.0001 wt%.

[0039]

[0040] Non-oriented electrical steel sheet

[0041] A non-oriented electrical steel sheet according to one embodiment of the present invention contains 3.0 to 3.8 wt% of silicon (Si), 0.2 to 0.4 wt% of manganese (Mn), 0.8 to 1.5 wt% of aluminum (Al), 0.01 wt% or less of carbon (C), 0.01 wt% or less of sulfur (S), 0.08 wt% or less of phosphorus (P), 0.01 wt% or less of nitrogen (N), 0.01 wt% or less of titanium (Ti), and 0.01 to 0.1 wt% of at least one of tin (Sn) and antimony (Sb), with the remainder being iron (Fe) and other unavoidable impurities.

[0042] Hereinafter, the role and content of alloy elements included in a non-oriented electrical steel sheet according to one embodiment of the present invention will be described in detail.

[0043]

[0044] Silicon (Si)

[0045] Silicon (Si) is a key additive in electrical steel, increasing the steel's resistivity and reducing core loss. However, too little silicon can insufficiently improve core loss. However, excessive silicon can reduce magnetic flux density and, due to increased brittleness, compromise cold-rollability. Therefore, appropriate adjustment of the silicon content is necessary.

[0046] If the silicon content is less than 3.0 wt%, the above-described effects cannot be expected, and if the silicon content exceeds 3.8 wt%, problems such as a decrease in magnetic flux density and an increase in brittleness may occur. Therefore, the non-oriented electrical steel sheet according to one embodiment of the present invention may contain 3.0 to 3.8 wt% of silicon.

[0047]

[0048] manganese (Mn)

[0049] Manganese (Mn), like silicon (Si), is an element that improves magnetic properties by increasing resistivity and reducing iron loss. It is also added to enhance the fraction of textures with favorable magnetic properties. Furthermore, manganese can combine with sulfur (S) present in steel to form sulfides such as MnS.

[0050] When the manganese content is less than 0.2 wt%, fine MnS precipitates are formed to inhibit grain growth, and when the manganese content exceeds 0.4 wt%, coarse MnS precipitates are formed to reduce the magnetic flux density. Therefore, the non-oriented electrical steel sheet according to one embodiment of the present invention may contain manganese in an amount of 0.2 to 0.4 wt%.

[0051]

[0052] Aluminum (Al)

[0053] Aluminum (Al), along with silicon (Si) and manganese (Mn), increases resistivity and reduces iron loss, making it a key additive in electrical steel. Aluminum can also play a role in reducing magnetic anisotropy, thereby reducing magnetic deviation.

[0054] When the aluminum content is less than 0.8 wt%, the increase in resistivity is not sufficient, which may increase high-frequency iron loss, and may form fine nitrides, which may increase the deviation of magnetic properties. When the aluminum content exceeds 1.5 wt%, excessive nitrides may be formed, which may reduce the magnetic flux density and cause a decrease in cold rolling properties. In addition, adding excessive aluminum may cause nozzle clogging problems during the casting process. Therefore, the non-oriented electrical steel sheet according to one embodiment of the present invention may contain 0.8 to 1.5 wt% of aluminum.

[0055]

[0056] carbon (C)

[0057] Carbon can increase iron loss by combining with titanium (Ti) to form carbides such as TiC. If the carbon content exceeds 0.01 wt%, self-aging can occur, degrading magnetic properties. Therefore, a non-oriented electrical steel sheet according to one embodiment of the present invention may contain carbon in an amount of 0.01 wt% or less.

[0058]

[0059] Yellow (S)

[0060] Sulfur (S) is an impurity element that is inevitably contained during the manufacturing process, and adding large amounts can cause brittleness. Furthermore, it can combine with manganese (Mn), copper (Cu), and other elements to form precipitates such as MnS and CuS, thereby increasing iron loss. Therefore, it is desirable to add as little S as possible. A non-oriented electrical steel sheet according to one embodiment of the present invention may contain sulfur in an amount of 0.005 wt% or less.

[0061]

[0062] Person (P)

[0063] Phosphorus is a grain boundary segregation element, and if added excessively, problems such as grain growth inhibition, deterioration of magnetic properties, and reduction in cold rolling properties may occur due to the segregation effect. Therefore, it is preferable to add as little as possible, and the non-oriented electrical steel sheet according to one embodiment of the present invention may contain phosphorus at 0.08 wt% or less.

[0064]

[0065] Nitrogen (N)

[0066] Nitrogen (N) can combine with aluminum (Al) or titanium (Ti) to form precipitates such as AlN and TiN, which can increase iron loss and inhibit grain growth. Therefore, it is desirable to add as little as possible. The non-oriented electrical steel sheet according to one embodiment of the present invention may contain nitrogen in an amount of 0.01 wt% or less.

[0067]

[0068] Titanium (Ti)

[0069] Titanium (Ti) is an element with a strong tendency to form precipitates in steel, and combines with carbon (C) or nitrogen (N) to form precipitates such as TiC and TiN. These precipitates can inhibit grain growth. As the amount of titanium added increases, the fraction of precipitates increases, and this can form a texture that is unfavorable for magnetization, thereby deteriorating magnetic properties. Therefore, it is preferable to add as little as possible, and the non-oriented electrical steel sheet according to one embodiment of the present invention may contain titanium in an amount of 0.01 wt% or less.

[0070]

[0071] In particular, in the case of the above sulfur (S), nitrogen (N) and titanium (Ti), since they may form sulfides, nitrides and carbides, respectively, and thus have a negative effect on magnetism, it is preferable that their contents be controlled to 0.01 wt% or less.

[0072]

[0073] Tin (Sn) and antimony (Sb)

[0074] Tin (Sn) and antimony (Sb) are precipitate elements that form precipitates on the surface of steel sheets, and can be concentrated in the surface layer of the steel sheet. At this time, the concentrated tin and antimony can reduce iron loss by suppressing nitrogen adsorption and nitride formation. However, if the total amount of any one or more of tin and antimony is less than 0.01 wt%, the above-described effect cannot be expected, and if it exceeds 0.1 wt%, excessive precipitate formation may cause grain boundaries to become embrittled, thereby reducing fatigue resistance. Therefore, the non-oriented electrical steel sheet according to one embodiment of the present invention may include any one or more of tin and antimony in a total amount of 0.01 to 0.1 wt%.

[0075]

[0076] In addition to the aforementioned steel components, the remainder may contain iron and unavoidable impurities. Unavoidable impurities are impurities introduced during the steelmaking process and the manufacturing process of non-oriented electrical steel sheets. Since these impurities are widely known in the field, a detailed description will be omitted.

[0077] In one embodiment of the present invention, the addition of elements other than the aforementioned alloy components is not excluded, and various elements may be included within a range that does not impair the technical spirit of the present invention. When additional elements are included, they may be included to replace the remaining Fe.

[0078]

[0079] A non-oriented electrical steel sheet satisfying the above-described alloy composition and manufactured using the non-oriented electrical steel sheet manufacturing method described below has a surface defect index (D) according to the following equation 1 t ) is 120 or more and less than 350.

[0080] [Formula 1]

[0081] D t = (D n ) 2 / 3 + 4(D p ) + 10(D a )

[0082] However, in the above formula 1, D t is the surface defect index, D n is the average number of surface defects, D p is the average depth of surface defects, D a represents the average surface defect area. Here, the above D n Silver dog / m 2 , D p is ㎛, D a is ㎛ 2 The values ​​measured in units are used, and the surface defect index (Dt) is calculated using values ​​excluding the units of the above variables. The above D n , D p and D a The measurement method will be described later.

[0083] Here, a surface defect may be a defect in which foreign matter detached from the steel sheet during transport by a hearth roll after heat treatment becomes lodged between the hearth roll and the steel sheet surface, resulting in a partial peeling or impression of the steel sheet surface. The shape of the surface defect may be a metal peeling off and bending, or a square shape.

[0084] The above surface defects are one of the factors that affect the magnetic properties of non-oriented electrical steel sheets. More specifically, surface defects can affect the magnetic properties of non-oriented electrical steel sheets by impeding the magnetic flux flow or affecting the insulating film coating properties. For example, if the surface defect area is 50㎛, 2 In addition, if the depth is 10㎛ or more, it may interfere with the magnetic flux flow of the electrical steel plate, thereby increasing the iron loss.

[0085] Therefore, in the present invention, the average number of surface defects (D) which is a characteristic of the surface defects n ), average depth of surface defects (D p ) and average surface defect area (D a ) is the surface defect index (D) of the above equation 1 t) is expressed as. Accordingly, in one embodiment of the present invention, the surface defect index (D t ) can quantify the degree of influence of surface defects formed on non-oriented electrical steel sheets on magnetic properties.

[0086] Below, with reference to Figure 1, the surface defect index (D t ) and the average number of surface defects (D n ), average depth of surface defects (D p ) and average surface defect area (D a ) will be explained in more detail.

[0087] Figure 1 shows the iron loss (W) according to the average number of surface defects by the average depth of the surface defects. 10 / 400 ) is a graph showing the change.

[0088] Referring to Figure 1, the surface defect index (D t ) Number of surface defects (D) n ) and average depth of surface defects (D p ) value increases, it can be seen that the iron loss increases. In addition, the number of surface defects (D n ) exceeds a certain number, it can be seen that the iron loss tends to increase rapidly, and accordingly, the surface defect index (D t ) is expressed as a quadratic function.

[0089] At this time, the average number of surface defects (D n ), average depth of surface defects (D p ) and the average surface defect area (Da) have different degrees of influence on the iron loss of non-oriented electrical steel sheets, so the coefficients were differentiated. In particular, the surface defect area (D a ) on the iron loss. The average depth of surface defects (D) p ) has a greater effect on iron loss, so a relatively large coefficient was used.

[0090] As described above, the surface defect index (D t) affects the iron loss of non-oriented electrical steel sheets, and it is necessary to control its value in order to manufacture non-oriented electrical steel sheets with excellent magnetic properties.

[0091] The above surface defect index (D t ) is less than 120, the insulating film coating property of the non-oriented electrical steel sheet may deteriorate, and the insulation resistance may actually decrease. In particular, if the insulation resistance is less than 1, it may have a negative effect on the iron loss, so the surface defect index (D t ) is important to control the surface defect index (D) to 120 or higher. On the other hand, the surface defect index (D t ) exceeds 350, surface defects may impede the flow of magnetic flux within the non-oriented electrical steel sheet, resulting in poor iron loss. In addition, excessive surface defects may reduce the marketability of the non-oriented electrical steel sheet.

[0092] Therefore, the non-oriented electrical steel sheet according to one embodiment of the present invention has the surface defect index (D t ) can be greater than or equal to 120 and less than or equal to 350.

[0093] Additionally, the surface defect index (D t ) can be controlled to be 1 or more and less than 350, so that the insulation resistance can be 1 or more.

[0094] The alloy composition and surface defect index (D) described above t ), the non-oriented electrical steel sheet manufactured by the non-oriented electrical steel sheet manufacturing method described later can have excellent magnetic properties. Preferably, the iron loss (W 10 / 400 ) may be a non-oriented electrical steel sheet having low iron loss characteristics of less than 16 W / kg.

[0095] Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described in detail.

[0096]

[0097] Method for manufacturing non-oriented electrical steel sheet

[0098] Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described with reference to FIG. 2.

[0099] Figure 2 is a flowchart schematically illustrating a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention.

[0100] A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention includes a first step of preparing steel, a second step of hot-rolling the steel sheet to form a hot-rolled steel sheet, a third step of hot-rolling and annealing the hot-rolled steel sheet, a fourth step of cold-rolling the hot-rolled steel sheet subjected to the third step to form a cold-rolled steel sheet, and a fifth step of cold-rolling and annealing the cold-rolled steel sheet in a reducing atmosphere.

[0101] At this time, the steel contains 3.0 to 3.8 wt% of silicon (Si), 0.2 to 0.4 wt% of manganese (Mn), 0.8 to 1.5 wt% of aluminum (Al), 0.01 wt% or less of carbon (C), 0.01 wt% or less of sulfur (S), 0.08 wt% or less of phosphorus (P), 0.01 wt% or less of nitrogen (N), 0.01 wt% or less of titanium (Ti), and 0.01 to 0.1 wt% of at least one of tin (Sn) and antimony (Sb), with the remainder containing iron (Fe) and other unavoidable impurities.

[0102] The alloy composition has been previously described, so a detailed explanation will be omitted. Furthermore, since the alloy composition does not substantially change during the manufacturing process described below, the alloy composition of the steel material and the alloy composition of the final product, the non-oriented electrical steel sheet, are substantially identical.

[0103] Hereinafter, each step of a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described in detail.

[0104] The first step (S1) of preparing steel is the step of preparing a semi-finished product for manufacturing the final product, a non-oriented electrical steel sheet. More specifically, this step may be a step of manufacturing a semi-finished product by designing the alloy components within the alloy composition range according to one embodiment of the present invention. The semi-finished product may be a slab, but is not necessarily limited thereto. Furthermore, the slab may be manufactured using a steelmaking process known in the art, such as a casting process.

[0105] A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention may, after the first step (S1), perform a second step (S2) of hot-rolling the steel material to form a hot-rolled steel sheet. More specifically, the second step (S2) may include a reheating process, a hot-rolling process, and a coiling process.

[0106] First, the reheating process may be performed prior to the hot rolling process to reheat the steel for subsequent processing. Specifically, this may be a step of loading the steel into a heating furnace and uniformly heating it, thereby facilitating plastic deformation.

[0107] At this time, if the reheating temperature is below 1000℃, the rolling load increases, which can make hot rolling difficult. On the other hand, if the reheating temperature exceeds 1150℃, precipitates formed by carbon (C), sulfur (S), nitrogen (N), etc. in the steel are re-dissolved, which can lead to the formation of fine precipitates during the subsequent rolling and annealing processes. These fine precipitates can inhibit grain growth and increase iron loss.

[0108] Therefore, the reheating process according to one embodiment of the present invention can reheat the steel at a temperature of 1000 to 1150°C.

[0109] Next, a hot rolling process may be performed to form a hot-rolled steel sheet by hot-rolling the reheated steel. The hot rolling process may include rough rolling and finish rolling. Here, rough rolling may refer to forming the steel into a rolled material with an appropriate shape, thickness, and width, and finish rolling may refer to adjusting the steel to a specified thickness and width and rolling it at a finishing temperature suitable for the intended use to achieve a good surface and shape.

[0110] At this time, the finishing temperature of the hot rolling process may be carried out at a temperature for uniform tissue formation and appropriate strength enhancement, and preferably, the finishing temperature may be 800 to 900°C. If the finishing temperature is less than 800°C, rolling may be performed in a two-phase region, which may result in the formation of an uneven tissue. If the finishing temperature exceeds 900°C, the problem of a rapid decrease in the strength of the steel may occur.

[0111] Afterwards, a coiling process can be performed to coil the hot-rolled steel sheet formed through the hot rolling process. At this time, the coiling temperature is preferably 500 to 600°C. If the coiling temperature is below 500°C, brittleness increases, potentially leading to sheet fracture. Furthermore, the grain size may become too small, preventing sufficient grain growth even after annealing. Conversely, if the coiling temperature exceeds 600°C, fine precipitates may form, increasing iron loss.

[0112] The thickness of the hot-rolled steel sheet formed through the second step (S2) is preferably 1.6 to 2.3 mm. If the thickness of the hot-rolled steel sheet is excessively thin, less than 1.6 mm, the thickness obtained after cold rolling may be insufficient, which may cause shape defects during product application. On the other hand, if the thickness of the hot-rolled steel sheet exceeds 2.3 mm, the cold rolling reduction rate increases, and the fraction of aggregate structures unfavorable to magnetic properties increases, which may result in inferior magnetic properties.

[0113] According to one embodiment of the present invention, a non-oriented electrical steel sheet can perform a third step (S3) of hot-rolling and annealing the hot-rolled steel sheet after the second step (S2).

[0114] The third step (S3) may be a hot-rolling annealing step performed to ensure uniformity of the microstructure of the steel material subjected to hot rolling. When the silicon content is 3.0 wt% or more, hot-rolling annealing is preferably performed.

[0115] The reasons for performing hot-rolled annealing are as follows. When the silicon content exceeds 3.0 wt%, the α-ferrite single phase is maintained without phase transformation into a solid state during heating from room temperature to the melting point. However, dynamic recrystallization (DRX) does not occur during this process, resulting in the formation of elongated unrecrystallized parts. These unrecrystallized parts have high internal residual stresses and are unstable, which can lead to increased anisotropy and plate failure.

[0116] Therefore, the third step (S3) may be a hot rolling annealing step that performs heat treatment at an appropriate temperature to promote recrystallization of unrecrystallized grains and thereby obtain a uniform equiaxed grain.

[0117] The appropriate temperature for hot-rolling annealing according to the present invention may be 900 to 1100°C. If the hot-rolling annealing temperature is below 900°C, the elongated cast structure may remain after hot rolling, causing microstructural inhomogeneity. Conversely, if the hot-rolling annealing temperature exceeds 1100°C, it may cause an imbalance in the final product's texture and deteriorate its magnetic properties.

[0118] In addition, the hot-rolled annealing may be performed for 30 to 120 seconds to form an appropriate grain size under the above temperature conditions. At this time, the heating rate to the above temperature is preferably 10°C / s or higher, and the cooling rate after the heat treatment may be preferably 20°C / s or higher.

[0119] In addition, the third step (S3) according to the present invention can be performed in an atmosphere with a nitrogen fraction of 80 to 100%. At this time, the remaining gas excluding nitrogen can be filled with hydrogen (H2). When the hydrogen (H2) fraction is 10 to 20%, a surface denitrification reaction can occur, which can lower iron loss and thereby improve magnetic properties. On the other hand, when the nitrogen (N2) fraction is less than 80%, oxidation can occur during heat treatment, which can deteriorate magnetic properties.

[0120] The method for manufacturing a non-oriented electrical steel sheet according to the present invention may, after the third step (S3), perform a fourth step (S4) of cold rolling the hot-rolled steel sheet that has undergone the third step (S3) to form a cold-rolled steel sheet. The fourth step (S4) may be a cold rolling process in which the hot-rolled and annealed hot-rolled steel sheet is rolled at a temperature below the recrystallization temperature to further reduce the thickness of the steel sheet. More specifically, it may be a process in which the hot-rolled steel sheet is rolled to a thickness and width that meet the specifications of the final product.

[0121] In the above cold rolling process, it is preferable to control the first pass reduction ratio to 40 to 50%, the reduction ratios for each of the remaining passes to 30 to 45%, and the total reduction ratio to 81 to 96%. The sheet thickness of the cold rolled steel sheet formed by cold rolling at the above-described reduction ratio may be 0.2 to 0.25 mm.

[0122] A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention may perform a fifth step (S5) of cold-rolling annealing the cold-rolled steel sheet in a reducing atmosphere after the fourth step (S4). The cold-rolling annealing may be performed at an appropriate temperature to improve magnetic and mechanical properties.

[0123] At this time, the appropriate temperature for cold rolling annealing may be 950 to 1100℃. If the cold rolling annealing temperature is lower than 950℃, the crystal grain size after annealing may be small, resulting in poor magnetic flux density and iron loss. On the other hand, if the cold rolling annealing temperature exceeds 1100℃, the precipitates may re-dissolve, forming fine precipitates, which may result in poor magnetic properties, and excessive grain growth may cause a decrease in strength.

[0124] In addition, the cold rolling annealing according to the present invention can be maintained for 30 to 90 seconds under the above temperature conditions, the heating rate to the above temperature is preferably 10°C / s or more, and the cooling rate after heat treatment is preferably 10°C / s or more.

[0125] The fifth step (S5) above can be performed in a reducing atmosphere, and can be performed in a mixed atmosphere composed of hydrogen (H2) and nitrogen (N2). Preferably, it can be performed in an atmosphere in which the hydrogen (H2) fraction is 10% or more and the remainder is composed of nitrogen (N2).

[0126] At this time, when the cold rolling annealing step is performed in a reducing atmosphere with a hydrogen (H2) fraction of 10% or more, the internal oxidation thickness within the steel sheet may change depending on the hydrogen (H2) fraction.

[0127] Hereinafter, this will be described in detail with reference to FIGS. 3 and 4.

[0128] Figure 3 is a graph showing the change in the number of surface defects according to the internal oxidation thickness, and Figure 4 is a graph showing the iron loss (W) according to the number of surface defects. 10 / 400 ) is a graph showing the changes.

[0129] Referring to Figure 3, it can be seen that as the internal oxidation thickness increases, the number of surface defects increases. This is because the harder the internal oxidation thickness, the weaker the steel sheet's hardness. In the case of steel sheets with weak hardness, surface peeling is likely to occur at the part where the steel sheet comes into contact with the hearth roll when being transported by the hearth roll. This peeled surface remains as foreign matter on the hearth roll, and this foreign matter forms surface defects on the steel sheet's surface.

[0130] Referring to Figure 4, as the number of surface defects increases, the iron loss (W 10 / 400 ) can be confirmed to increase. Therefore, as the internal oxidation thickness increases, the number of surface defects increases, and the iron loss (W) increases according to the increased number of surface defects. 10 / 400 ) can be seen to be increasing.

[0131] Therefore, it is important to control the internal oxidation thickness. Preferably, the hydrogen (H2) fraction should be controlled to 10% or more during cold rolling annealing. At this time, if the hydrogen (H2) fraction is less than 10%, nitrogen (N) can penetrate the steel plate surface and form nitrides, which can increase iron loss. More preferably, the hydrogen (H2) fraction should be controlled to 10 to 20%. If the hydrogen (H2) fraction exceeds 20%, the steel plate manufacturing cost increases, which may increase the production unit price and increase the risk of explosion.

[0132] Here, the hydrogen (H2) fraction may refer to the volume fraction of hydrogen (H2) present in the atmosphere during cold rolling annealing.

[0133] A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention may perform a coating step after the fifth step (S5). The coating step may be performed to secure the insulation properties and improve the punchability of the non-oriented electrical steel sheet, and may mean forming an insulating film on the surface of the cold-rolled steel sheet on which the fifth step (S5) has been performed. The coating step may be performed using a process known in the relevant technical field.

[0134] Accordingly, the non-oriented electrical steel sheet according to one embodiment of the present invention can have low iron loss, and preferably has iron loss (W 10 / 400 ) may be less than 16W / kg.

[0135]

[0136] Experimental example

[0137] Below, the composition and operation of the present invention will be explained through experimental examples. However, these examples are provided to aid understanding of the present invention and are not intended to limit the present invention.

[0138]

[0139] (Method for manufacturing non-oriented electrical steel sheet specimens)

[0140] The specimen of the non-oriented electrical steel sheet of the present invention contains 3.3 wt% of silicon (Si), 0.25 wt% of manganese (Mn), 0.9 wt% of aluminum (Al), 0.0019 wt% of carbon (C), 0.0017 wt% of sulfur (S), 0.0008 wt% of phosphorus (P), 0.0017 wt% of nitrogen (N), 0.0016 wt% of titanium (Ti), 0.0008 wt% of tin (Sn), 0.0092 wt% of antimony (Sb), and the remainder of iron (Fe).

[0141] A slab having the above-described alloy composition was manufactured, and the slab was reheated to 1050°C and then hot-rolled to produce a hot-rolled steel sheet having a thickness of 2 mm. The hot-rolled steel sheet was hot-rolled and annealed at 1000°C for 60 seconds in an atmosphere with a nitrogen fraction of 100%. Thereafter, cold-rolled to produce a cold-rolled steel sheet having a thickness of 0.25 mm. The cold-rolled steel sheet was cold-rolled and annealed at 1000°C for 90 seconds, and the cold-rolled steel sheet was cold-rolled and annealed in a mixed atmosphere containing the hydrogen (H2) fractions shown in Table 1 below, and the remaining atmosphere was composed of nitrogen (N2). Thereafter, the cold-rolled and annealed steel sheet was cooled and then an insulating film was coated to produce the steel sheet.

[0142] Other process conditions not described in the previous non-oriented electrical steel sheet sample manufacturing method were controlled as control variables and were controlled identically within the range described in the non-oriented electrical steel sheet manufacturing method according to one embodiment of the present invention.

[0143]

[0144] (Method for measuring the properties of non-oriented electrical steel sheets)

[0145] 1. Average number of surface defects

[0146] In order to measure the average number of surface defects of a non-oriented electrical steel sheet specimen, five areas with a size of 5㎛×5㎛ on the surface of the steel sheet were observed using an optical microscope, and the number of surface defects within the areas was measured to calculate the average value.

[0147] 2. Average depth of surface defects

[0148] The entire thickness of the non-oriented electrical steel sheet specimen was observed through an optical microscope, and the depth of surface defects in five areas with a length of 5 μm along the length of the steel sheet was measured, and the average value was calculated. At this time, the depth of the surface defects was measured from the surface of the steel sheet toward the center, and the maximum depth of the surface defects was measured.

[0149] 3. Average surface defect area

[0150] In order to measure the average surface defect area of ​​a non-oriented electrical steel sheet specimen, five areas with a size of 5㎛×5㎛ on the surface of the steel sheet were observed through an optical microscope, and the area of ​​the surface defects in each of the five areas was calculated using a cross-sectional image analysis program such as Image J, and then the average value was obtained.

[0151] 4. Internal oxidation thickness

[0152] The internal oxidation thickness of non-oriented electrical steel specimens was observed using a scanning electron microscope (SEM) on the TD (transverse direction) plane of the steel plate. The internal oxidation thickness was measured five times at random locations, and the average value was calculated. At this time, the internal oxidation thickness was measured from the surface of the steel plate to the point where the internal oxidation layer ends.

[0153] 5. Insulation resistance

[0154] The insulation resistance of non-oriented electrical steel specimens was measured according to the IEC 60404-11 standard. The insulation resistance value was measured by applying current by touching the contact button to the steel plate surface, checking the return current, and using the reduced current amount.

[0155] 6. Iron hand

[0156] The core loss (W10 / 400) of the non-oriented electrical steel sheet specimen of the invention was measured using the Epstein frame test method according to the IEC 60404-2 international standard. At this time, the non-oriented electrical steel sheet was manufactured into a specimen with a length of 300 mm and a width of 30 mm and measured.

[0157]

[0158] (Evaluation results of non-oriented electrical steel sheets)

[0159] The internal oxidation thickness, average number of surface defects (Dn), average depth of surface defects (Dp), average area of ​​surface defects (Da), insulation resistance and core loss (W) of the experimental examples 10 / 400 ) values ​​are shown in Table 1 below. In Table 1 below, 'W 10 / 400 ' means the iron loss value at 400Hz, 1.0T.

[0160] Experimental example No. Cold rolled annealing Hydrogen fraction (%) Internal oxidation thickness Dn (unit / m) 2 )Dp(㎛)Da(㎛ 2 )Dt insulation resistance (Ω-cm) 3 / lam)W 10 / 400119.41311555115.33330.7216.1219.3515150170370.33331.116.6319.311817885397.3333116.8419.271925563284.33331.114.7519.262126055296.3333114.9619.2552428025346.3333115.7719.2152831375130113819.1323035012215113.9919.121334181289.333330.6316.81018.913642315112.33330.7316.91118.8437488120477.33331.116.91218.8139494177558.3333117.11318.7440447112305.33331.115.121418.714151286142.3333113.21518.644452364164.3333113.51618.345156088336.33331.115.681718.21155545158346.3333115.71817.8159528110230.33331.1141917.626065645281114.62017.576276468340.3333115.52117.47646750900.716.852217.32679641424251.116.852317.28689913760.516.72417.170918131120.7316.92516.837191213880.616.942616.647491111820.5816.842716.61751065135428.3333116.912816.5877101518111.33330.7116.92916.52791096124541.33331.117.083016.4681101498187.3333113.893116.3883102361186.33331.113.883216.2184105682339.3333115.73315.6786102256177.3333113.73415.61881067168469.33331.116.893515.488911350102.33330.816.13615.32921212211170.8816.983715.299713830118.33330.89163815.11981482142535.3333117.023914.5111114532117.33330.8816.14014.4311314528113.33330.85164114.331141482148541.3333117.0824214.291722022102323.33331.115.54314.02118144738291.3333114.854413.671271442116349.3333115.84513.61123142885262.33331.114.34613.5412414821118.33330.8916.14713.51127146498419.3333116.894813.49130151768211114.44913.281321698168645.33331.117.35013.11381621146315.3333115.35112.42142163548273.3333114.55212.381481734189421.33331.116.95312.25151175138338.3333115.65412.1914918521354511.116.785511.61621824993031.115.1569.62391931163407.3331.117.2578.12422536149501.3331.117.42587.73123236210695.3331.117.83.

[0161] Referring to Table 1 above, in the cases of Experimental Example 1, Experimental Example 9, Experimental Example 10, Experimental Example 21, Experimental Example 23, Experimental Example 24, Experimental Example 25, Experimental Example 26, Experimental Example 28, Experimental Example 35, Experimental Example 36, Experimental Example 37, Experimental Example 39, Experimental Example 40, and Experimental Example 46, it can be confirmed that the surface defect index (Dt) is less than 120. At this time, the insulation film coating property is reduced, so that the insulation resistance is 1.0 or less and the iron loss is 16 W / kg or more. Continuing with reference to Table 1 above, in the cases of Experimental Example 2, Experimental Example 3, Experimental Example 11, Experimental Example 12, Experimental Example 22, Experimental Example 27, Experimental Example 29, Experimental Example 34, Experimental Example 38, Experimental Example 41, Experimental Example 47, Experimental Example 49, Experimental Example 52 and Experimental Example 54, it can be confirmed that the surface defect index (Dt) exceeds 350. At this time, the surface defect affects the magnetic properties of the steel plate, so that the iron loss (W 10 / 400 ) It can be confirmed that the value does not satisfy 16W / kg or less.

[0162] Referring to Table 2 above, it can be confirmed that in Experimental Examples 56, 57, and 58, the hydrogen (H2) fraction in the atmosphere during cold rolling annealing is less than 10%, and the surface defect index (Dt) exceeds 350. At this time, it can be confirmed that the magnetic properties are inferior due to the surface defects. More specifically, it can be confirmed that the iron loss exceeds 16 W / kg.

[0163] On the other hand, in the case of Experimental Examples 4 to 8, Experimental Examples 13 to 20, Experimental Examples 30 to 33, Experimental Examples 42 to 45, Experimental Examples 48, Experimental Examples 50, Experimental Examples 51, Experimental Examples 53 and Experimental Examples 55, it can be confirmed that the surface defect index (Dt) is included in the range of 120 or more and less than 350. At this time, the iron loss (W) targeted in the present invention 10 / 400 ) It can be confirmed that the value is less than 16W / kg.

[0164] As described above, preferred embodiments of the present invention have been described. It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms, in addition to the embodiments described above, without departing from the spirit or scope thereof. Therefore, the above-described embodiments should be considered illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

Claims

1. Contains 3.0 to 3.8 wt% of silicon (Si), 0.2 to 0.4 wt% of manganese (Mn), 0.8 to 1.5 wt% of aluminum (Al), 0.01 wt% or less of carbon (C), 0.01 wt% or less of sulfur (S), 0.08 wt% or less of phosphorus (P), 0.01 wt% or less of nitrogen (N), 0.01 wt% or less of titanium (Ti), and 0.01 to 0.1 wt% of at least one of tin (Sn) and antimony (Sb), with the remainder containing iron (Fe) and other unavoidable impurities. Surface defect index (D) according to the following equation 1 t ) Non-oriented electrical steel sheet having a melting point of 120 or more and less than 350. [Formula 1] D t = (D n ) 2 / 3 + 4(D p ) + 10(D a ) (However, in the above formula 1, D t is the surface defect index, D n is the number of surface defects, D p is the average depth of surface defects, D a represents the average surface defect area.) 2. In paragraph 1, Non-oriented electrical steel sheet with an insulation resistance of 1 or more.

3. In paragraph 1, Iron Loss (W 10 / 400 ) Non-oriented electrical steel sheet with a strength of less than 16 W / kg.

4. The first step is to prepare the steel; A second step of hot rolling the above steel to form a hot-rolled steel plate; The third step of hot-rolling and annealing the above hot-rolled steel sheet; A fourth step of forming a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet that has undergone the third step; and Including a fifth step of cold rolling annealing the above cold rolled steel sheet in a reducing atmosphere, The above steel is, A method for manufacturing a non-oriented electrical steel sheet, comprising 3.0 to 3.8 wt% of silicon (Si), 0.2 to 0.4 wt% of manganese (Mn), 0.8 to 1.5 wt% of aluminum (Al), 0.01 wt% or less of carbon (C), 0.01 wt% or less of sulfur (S), 0.08 wt% or less of phosphorus (P), 0.01 wt% or less of nitrogen (N), 0.01 wt% or less of titanium (Ti), and 0.01 to 0.1 wt% of at least one of tin (Sn) and antimony (Sb), with the remainder being iron (Fe) and other unavoidable impurities.

5. In paragraph 4, The fifth step above is, A method for manufacturing a non-oriented electrical steel sheet by annealing in a mixed atmosphere consisting of 10% by volume or more of hydrogen (H2) and the remainder of nitrogen (N2).

6. In paragraph 4, The steel material that has undergone the above 5th step has a surface defect index (D) according to the following equation 1 t ) A method for manufacturing a non-oriented electrical steel sheet having a melting point of 120 or more and less than 350. [Formula 1] D t = (D n ) 2 / 3 + 4(D p ) + 10(D a ) (However, in the above formula 1, D t is the surface defect index, D n is the number of surface defects, D p is the average depth of surface defects, D a represents the average surface defect area.) 7. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet having an insulation resistance of 1 or more, wherein the steel sheet having the above 5th step performed is a steel sheet.

8. In paragraph 4, The steel that has undergone the above 5 steps has a core loss (W 10 / 400 ) A method for manufacturing a non-oriented electrical steel sheet having a strength of less than 16 W / kg.

Images