Non-oriented electrical steel sheet and its manufacturing method

Abstract

To provide a non-oriented electrical steel sheet that reduces manufacturing costs, increases productivity and yield, and improves core loss. The non-oriented electrical steel sheet of the present invention has an area fraction of crystal grains that are less than 1 / 3 of the average crystal grain size of less than 5%, and a dislocation density of 10 12 / m 2 excess 10 16 / m 2 The fraction of the area where dislocations are concentrated is less than 5% of the total area, The area fraction of crystal grains having a grain size that is more than three times the average grain size is less than 5%.

Description

[Technical Field]

[0001] The present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof, and more particularly to a non-oriented electrical steel sheet and a manufacturing method thereof. [Background technology]

[0002] Electrical steel sheets are used in transformers, motors, and electronic devices. Unlike ordinary carbon steels, which emphasize workability as a material for machine parts, electrical steel sheets are functional products that emphasize electrical properties. These electrical properties include iron loss, magnetic flux density, magnetic permeability, and space factor. Electrical steel sheets are characterized by low iron loss and high magnetic flux density, magnetic permeability, and space factor. The electrical steel sheets are broadly divided into grain-oriented and non-oriented electrical steel sheets. The grain-oriented electrical steel sheets are formed by utilizing the irregular grain growth phenomenon called secondary recrystallization, which results in a Goos texture {110} <001> The non-oriented electrical steel sheet has excellent magnetic properties in the rolling direction due to the texture formed throughout the steel sheet. The non-oriented electrical steel sheet has uniform magnetic properties in all directions on the rolled sheet.

[0003] Non-oriented electrical steel sheets are mainly used in motors that convert electrical energy into mechanical energy. To achieve high efficiency in this energy conversion process, non-oriented electrical steel sheets must have excellent magnetic properties. Additionally, industrial structures are being restructured worldwide to focus on environmentally friendly, low-carbon industries in preparation for a carbon-neutral era. In line with this trend, automobiles are rapidly being replaced by electric vehicles (EVs), and the drive motors used in these EVs account for more than half of the electric energy consumed. This has led to a continuous increase in demand for non-oriented electrical steel sheets, which are used as the core material for these drive motors. Against this backdrop, improving the iron loss of non-oriented electrical steel sheets has become a crucial issue for increasing the efficiency of these drive motors.

[0004] In the case of general non-oriented electrical steel sheets, the magnetic properties are evaluated mainly by the iron loss at 50 Hz and the iron loss at 400 Hz. The iron loss means the energy loss that occurs at a specific magnetic flux density and frequency, so the iron loss at 50 Hz is considered to be the energy loss that occurs at normal frequencies, and the iron loss at 400 Hz is considered to be the energy loss that occurs at high frequencies. When a non-oriented electrical steel sheet with low iron loss is used for the motor core, heat loss generated in the motor core is reduced, resulting in a highly efficient motor. Since the motor is driven through inverter control and operates at various driving speeds, one way to manufacture a more efficient motor is to reduce iron loss from normal frequencies to high frequencies or even ultra-high frequencies.

[0005] One method used to improve the iron loss of non-oriented electrical steel sheets is to add alloying elements such as silicon (Si), aluminum (Al), and manganese (Mn) to minimize impurities and reduce the thickness of the steel sheet. The addition of these alloying elements increases the resistivity of the steel, thereby reducing eddy current loss and overall iron loss. In addition, minimizing the impurities changes the magnetic domain structure in the steel sheet to one that generates less loss during magnetization, and reduces the number of closed magnetic domains fixed by inclusions and precipitates in the steel sheet, thereby improving iron loss.Furthermore, reducing the thickness of the steel sheet reduces eddy current loss, which increases in proportion to the square of the thickness, thereby effectively reducing iron loss.

[0006] Depending on the motor's design intent, not only the overall iron loss but also the iron loss in the rolling direction in particular is an important factor. Generally, when the magnetization direction does not rotate, such as in large rotating machines or linear motors, or in motors that use reluctance during magnetization, magnetic steel sheets with improved iron loss in the rolling direction are used. However, among the technologies used to improve iron loss, increasing the amount of alloy increases production costs due to rising alloy raw material costs and a sharp decline in cold rolling properties, making it impossible to increase the amount of alloy beyond the current level, while reducing the thickness of the steel sheet increases heat treatment and rolling times, resulting in a sharp increase in production costs and a sharp decline in production volume per hour. Furthermore, reducing impurities reduces productivity due to limitations in steelmaking technology and increased refining time, and increases costs due to strict restrictions on the alloy raw material components, making it difficult to reduce iron loss.

[0007] Furthermore, in improving iron loss in the rolling direction, other than the above-mentioned methods, methods for improving iron loss only in the rolling direction are very limited, making it difficult to apply them to actual manufacturing processes. As such, the conventional techniques increase manufacturing costs and reduce productivity and yield, so research is needed into technologies that can solve the above-mentioned problems and effectively reduce iron loss. Summary of the Invention [Problem to be solved by the invention]

[0008] The technical problem to be solved by the present invention is to provide a non-oriented electrical steel sheet that reduces manufacturing costs, increases productivity and yield, and improves core loss. Another technical problem to be solved by the present invention is to provide a method for manufacturing a non-oriented electrical steel sheet having the above advantages. [Means for solving the problem]

[0009] The non-oriented electrical steel sheet of the present invention has an area fraction of crystal grains having a grain size less than 1 / 3 of the average grain size of less than 5%, and a dislocation density of 10 12 / m 2 excess 10 16 / m 2 The area fraction of crystal grains having a surface area of ​​less than 5% of the total surface area is characterized in that:

[0010] It is preferable that the area fraction of crystal grains having a grain size exceeding three times the average grain size is less than 5%. The steel sheet may contain, by weight, Si: 0.1 to 6.5%, Al: 0.001 to 6.5%, Mn: 0.01 to 20%, C: 0.0010 to 0.015%, N: 0.0003 to 0.001%, S: 0.0003 to 0.001%, Ti: 0.0003 to 0.001%, with the remainder being Fe and unavoidable impurities. The average crystal grain size is preferably 40 to 250 μm and the thickness is preferably 0.03 to 0.5 mm.

[0011] The iron loss (W10 / 400) and the thickness (t) of the non-oriented electrical steel sheet can satisfy the following formula 1. <Expression 1> W10 / 400 Iron loss (W / kg)<6+(t / 0.04) 1.1 (In the above formula 1, t means the thickness (mm) of the non-oriented electrical steel sheet.) The iron loss (W15 / 50) and the thickness (t) of the non-oriented electrical steel sheet can satisfy the following formula 2. <Expression 2> W15 / 50 Iron loss (W / kg)<0.7+(t / 0.03) 1 / 5 (In the above formula 2, t means the thickness (mm) of the non-oriented electrical steel sheet.)

[0012] The method for producing a non-oriented electrical steel sheet of the present invention includes a step of hot-rolling a slab to produce a hot-rolled steel sheet, a step of cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet, and a cold-rolled steel sheet annealing step of annealing the cold-rolled steel sheet, wherein the cold-rolled steel sheet annealing step is performed at a temperature of 650°C or higher in the rolling direction (RD direction) of the coil at a pressure of more than 0.01 to 1.0 kgf / mm. 2 The direction of the tension applied to the cold-rolled steel sheet forms an angle of 3° or less with the rolling direction (RD direction) of the coil and an angle of more than 87° and not more than 93° with the normal direction of the rolling surface (ND direction) of the cold-rolled steel sheet.

[0013] The slab may contain, by weight, Si: 0.1 to 6.5%, Al: 0.001 to 6.5%, Mn: 0.01 to 20%, C: 0.0010 to 0.015%, N: 0.0003 to 0.01%, S: 0.0003 to 0.01%, Ti: 0.0003 to 0.01%, with the remainder being Fe and unavoidable impurities. The method may further include a hot-rolled steel sheet annealing step of heating the hot-rolled steel sheet, and the hot-rolled steel sheet annealing step may be a step of heating the hot-rolled steel sheet to 850 to 1,150°C. The step of annealing the cold-rolled steel sheet may include a heating step of heating the cold-rolled steel sheet to 820°C or higher, and a cooling step of cooling the cold-rolled steel sheet from 820 to 900°C to 750 to 820°C. In the cold-rolled steel sheet annealing step, the temperature-raising step is preferably performed within 60 seconds. In the cold-rolled steel sheet annealing step, the cooling step may be performed for a time period of 5 seconds or more. In the annealing step of the cold-rolled steel sheet, the cooling step is preferably performed such that the sheet surface is cooled perpendicular to the direction of gravity. The cold-rolled steel sheet annealing step may be performed in a reducing atmosphere. [Effects of the Invention]

[0014] The non-oriented electrical steel sheet according to the present invention reduces and improves iron loss in the rolling direction by controlling the area ratio of the crystal grain size, specifically the area fraction of the crystal grain size that is 1 / 3 of the average crystal grain size, and the dislocation density, and at the same time, it is possible to provide a non-oriented electrical steel sheet in which the crystal grains are uniformly distributed and the dislocation density is uniform. The method for producing a non-oriented electrical steel sheet according to the present invention can provide a method for producing a non-oriented electrical steel sheet having the above-mentioned advantages by controlling the cooling rate and the direction and magnitude of tension in the heat treatment step to distribute homogenized crystal grains within the steel sheet. [Brief explanation of the drawings]

[0015] [Figure 1]FIG. 1a shows the arrangement of a steel sheet during the annealing of a cold-rolled steel sheet according to one embodiment of the present invention, and FIGS. 1b and 1c show plan views of the direction of travel of the steel sheet and the direction of tension, respectively. [Figure 2] Figure 2a and Figure 2b show cross-sectional views of the steel plate's travel direction and the direction of movement of the steel plate due to the tension direction, respectively. [Figure 3] FIG. 3 illustrates a dislocation dense region according to one embodiment of the present invention. [Figure 4] 4a to 4e are enlarged views showing regions where dislocations are concentrated in an example of the present invention and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0016] Terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited to these. These terms are used to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention. The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular form includes the plural form unless the context clearly dictates otherwise. As used in the specification, the meaning of "comprising" embodies certain properties, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other properties, regions, integers, steps, operations, elements, and / or components. When a part is referred to as being "on" another part, it can mean that it is directly on top of the other part, or there can be other parts between them. In contrast, when a part is referred to as being "directly on" another part, there are no other parts between them.

[0017] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted as having an ideal or very formal meaning unless defined. Furthermore, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight. In one embodiment of the present invention, the term "additionally containing an additional element" means that an additional amount of the additional element is contained in place of the remaining iron (Fe). In the present invention, the Goss orientation is {110} in Miller indices. <001> The Cube orientation corresponds to {100} in Miller indices. <001> This means the direction corresponding to.

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to exemplary embodiments thereof, so that those skilled in the art will be able to easily understand and practice the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein. The non-oriented electrical steel sheet of the present invention contains, by weight%, 0.1 to 6.5% Si, 0.001 to 6.5% Al, 0.01 to 20% Mn, 0.0010 to 0.0150% C, and 0.0003 to 0.01% each of one or more of N, S, and Ti, with the remainder consisting of Fe and unavoidable impurities. The reasons for limiting the components of the non-oriented electrical steel sheet will be explained below.

[0019] Si:0.1~6.5wt% Silicon (Si) is a component that increases the resistivity of a material and reduces iron loss, and is used as a deoxidizer in the steelmaking process. Silicon is an element that is inevitably added in the electrical steel sheet manufacturing process, and forms oxides during the manufacturing process. The Si content is preferably 0.1 to 6.5 wt %. More preferably, the Si content is 1.0 to 4.5 wt %. If the silicon content is too high, the brittleness of the material increases, the rolling productivity drops sharply, and a thick oxide layer harmful to magnetic properties is formed, which may inhibit core loss due to the internal oxides. Furthermore, if the silicon content is too high, secondary phases may be formed, significantly deteriorating magnetic properties. On the other hand, if the silicon content is too low, low-temperature Si oxides may be formed, deteriorating core loss.

[0020] Al:0.001~6.5wt% Aluminum (Al), like silicon, increases the resistivity of the material and reduces iron loss, and can be used as a powerful deoxidizer in steelmaking. The aluminum content is preferably 0.001 to 6.5 wt %. More preferably, the aluminum content is 0.1 to 2.0 wt %.

[0021] Mn:0.01~20% by weight Manganese (Mn) is an element that can increase the resistivity of a material and improve core loss, and can also play a role in forming sulfides in steel. The manganese content is preferably 0.01 to 20 wt %. More preferably, the manganese content is 0.01 to 6.5 wt %. Even more preferably, the manganese content is 0.01 to 2.0 wt %. The non-oriented electrical steel sheet may contain, by weight percent, one or more of N, S, and Ti in an amount of 0.0003 to 0.001 weight percent each. Specifically, it may contain at least one of N, S, and Ti, and more preferably contains all of N, S, and Ti.

[0022] C:0.0005~0.015% by weight Carbon (C) is an element that is inevitably contained in the manufacturing process of non-oriented electrical steel sheet, and it plays a role in homogenizing the rolled structure in the steel during rolling. Specifically, the carbon content is 0.0005 to 0.015 wt %, and more preferably 0.0015 to 0.004 wt %. If the carbon content is too high, carbides are formed, which hinder the movement of magnetic domains and require additional energy for magnetization, whereas if the carbon content is too low, the material becomes non-uniform during rolling, resulting in non-uniform grain size in the recrystallized structure.

[0023] N:0.0003~0.010wt% Nitrogen (N) not only forms fine AlN precipitates inside the steel sheet, but also combines with other impurities to form fine precipitates, suppressing grain growth and reducing iron loss or improving strength. The nitrogen content is preferably 0.0003 to 0.010 wt %. More preferably, the nitrogen content is 0.0003 to 0.004 wt %.

[0024] S:0.0003~0.010% by weight Sulfur (S) forms fine precipitates of MnS, which deteriorates magnetic properties and hot workability, so it is preferable to control the content to maintain a low level. The sulfur content may be 0.0003 to 0.010 wt %. More preferably, the sulfur content is 0.0003 to 0.004 wt %. If the sulfur content is too high, cracks may occur during continuous casting. On the other hand, if the sulfur content is too low, the steel sheet may have favorable properties, but the production cost may increase because selected raw materials must be used to control the sulfur content below the lower limit.

[0025] Ti:0.0003~0.010wt% Titanium (Ti) has a strong tendency to form precipitates inside the steel sheet, forming fine carbides, nitrides, or sulfides inside the steel sheet, which may inhibit grain growth and thereby deteriorate core loss. The titanium content is preferably 0.0003 to 0.010 wt %. More preferably, the titanium content is 0.0003 to 0.003 wt %. If the titanium content is too high, deterioration of core loss may become a problem, whereas if the titanium content is too low, it may be necessary to use selected raw materials to control the titanium content, resulting in a problem of manufacturing costs.

[0026] The non-oriented electrical steel sheet of the present invention comprises the balance Fe and unavoidable impurities. The unavoidable impurities are impurities that are mixed in during the steelmaking stage and the manufacturing process of the non-oriented electrical steel sheet, and as they are widely known in the art, detailed description thereof will be omitted. The present invention does not exclude the addition of elements other than the above-mentioned alloy components, and various elements may be included within a range that does not impair the technical concept of the present invention. When an additional element is further included, it may be included as a substitute for the balance Fe. The non-oriented electrical steel sheet of the present invention having the above-mentioned composition has the following physical properties.

[0027] The non-oriented electrical steel sheet of the present invention preferably has an average grain size of 40 to 250 μm. The grains of the non-oriented electrical steel sheet are characterized by being uniformly distributed. This is because the grain boundaries are the areas where dislocations concentrate within the steel sheet, and the structural stability of the grain boundaries can be obtained from a uniform distribution of grain size. More preferably, the average grain size is 50 to 120 μm. To obtain the grain size distribution, individual grains enclosed by grain boundaries are identified from a microscopic photograph, and the area of ​​each grain is calculated. The diameter of each grain can then be expressed as the corresponding ECD (Equivalent Circle Diameter). In this case, the grain diameter distribution can be obtained using the ECD, and the average grain diameter can be calculated by calculating the arithmetic mean.

[0028] If the average grain diameter is outside the upper limit of the range, the grain size in the steel may vary widely. A grain size distribution with a wide variation may result in more dislocations forming around larger grains, creating subgrain boundaries within the grains. If the average grain diameter is outside the lower limit of the range, the proportion of grain boundaries in the entire material may increase, making magnetization difficult.

[0029] The non-oriented electrical steel sheet may have an area fraction of crystal grains having a grain size less than 1 / 3 of the average crystal grain size of less than 5%. The crystal grains having a grain size less than 1 / 3 of the average crystal grain size refer to crystal grains having a grain size smaller than 1 / 3 of the average crystal grain size calculated as above. The area fraction of the grain size less than 1 / 3 of the average grain size means the ratio of the area occupied by grain sizes 1 / 3 of the average grain size in the entire structure of the non-oriented electrical steel sheet. Texture fraction can be measured using the X-ray diffraction pole figure, neutron diffraction, X-ray transmission analysis, or electron microscope EBSD. The area fraction of crystal grains with misorientation within 15 degrees from the center of the Goss and Cube orientations can be calculated.

[0030] If the surface area fraction of the crystal grain size less than 1 / 3 of the average crystal grain size is too high, magnetization in a low magnetic field becomes difficult, resulting in a problem of low magnetic permeability. The area fraction of the crystal grain size exceeding three times the average crystal grain size may be less than 5%. The crystal grain size exceeding three times the average crystal grain size means a crystal grain size having a crystal grain size that is more than three times the average crystal grain size calculated as above. The area fraction of grains having a grain size more than three times the average grain size means the ratio of the area of ​​grains having a grain size more than three times the average grain size in the entire structure of the non-oriented electrical steel sheet. If the area fraction of grains having a grain size more than three times the average grain size is excessively high, there is a problem that a difference in dislocation distribution between grains occurs, and displacement is concentrated in locally coarse grains, resulting in a significant increase in iron loss.

[0031] The sum of the fraction of crystal grains having the Goss orientation and the fraction of crystal grains having the Cube orientation in the non-oriented electrical steel sheet can exceed 5%. The dislocation density of non-oriented electrical steel sheets is 10 12 / m 2 excess 10 16 / m 2 The fraction of the area where dislocations are concentrated is less than 5% of the total area. Dislocation density can be measured using a TEM (Transmission Electron Microscope), or more simply, it can be calculated using a SEM (Scanning Electron Microscope). Dislocation density can be measured using the line intercept method.

[0032] By controlling the tension during annealing according to the present invention, it is possible to control the formation of dislocations around grain boundaries after annealing is completed. Dislocation formation at high temperatures occurs when dislocations from regions several tens of nanometers away from the grain boundaries are prevented from being emitted to the grain boundaries and instead align internally at sub-grain boundaries. The greater the tension during annealing, the greater the number of dislocations forming sub-grain boundaries, thereby increasing the misorientation angle between regions separated by sub-grain boundaries. As the misorientation angle increases, additional energy consumption occurs during magnetization, resulting in deterioration of magnetism. When there are a sufficient number of grain boundaries and the size of each grain is uniform, dislocation generation due to tension during annealing is suppressed by the dislocation accommodation of the grain boundaries.

[0033] The thickness of the non-oriented electrical steel sheet is preferably 0.03 to 0.5 mm, and more preferably 0.15 to 0.3 mm. If the thickness is too thick, the tension during annealing varies depending on the thickness of the sheet, and the difference in tension between the surface and the center generates tension in the thickness direction, which may result in the formation of multiple dislocations.On the other hand, if the thickness is too thin, there is a problem that tension control in the annealing furnace becomes industrially impossible.

[0034] The iron loss (W10 / 400) of the non-oriented electrical steel sheet at a high frequency of 400 Hz and 1.0 T and the thickness (t) of the non-oriented electrical steel sheet can satisfy the following formula 1. <Expression 1> W10 / 400 Iron loss (W / kg)<6+(t / 0.04) 1.1 (In the above formula 1, t means the thickness of the non-oriented electrical steel sheet) By satisfying the formula 1, it is possible to provide a non-oriented electrical steel sheet that is excellent in magnetic properties such as iron loss. If the formula 1 is not satisfied, there is a problem that it is not possible to obtain a steel sheet that is excellent in high-frequency iron loss relative to the sheet thickness.

[0035] The non-oriented electrical steel sheet can satisfy the following formula 2 in terms of the iron loss at a normal frequency of 50 Hz and 1.5 T and the thickness (t) of the non-oriented electrical steel sheet. <Expression 2> W15 / 50 Iron loss (W / kg)<0.7+(t / 0.03) 1 / 5 (In the above formula 2, t means the thickness of the non-oriented electrical steel sheet) By satisfying the formula 2, it is possible to provide a non-oriented electrical steel sheet with excellent magnetic properties such as iron loss. If the formula 2 is not satisfied, iron loss will deteriorate under high magnetic flux density conditions, and motor loss at high torque will increase significantly.

[0036] The non-oriented electrical steel sheet can satisfy the following formula 3 in terms of the iron loss at a normal frequency of 50 Hz and 1.5 T and the thickness (t) of the non-oriented electrical steel sheet. <Expression 3> W15 / 50 rolling direction iron loss (W / kg)<0.6+(t / 0.03) 1 / 6 (In the above formula 3, t means the thickness of the non-oriented electrical steel sheet) By satisfying the formula 3, it is possible to provide a non-oriented electrical steel sheet with excellent magnetic properties such as iron loss. If the formula 3 is not satisfied, motor loss at high torque increases when using split cores made by cutting parts of motor components in the rolling direction and assembling them.

[0037] The iron loss (W10 / 400) of the non-oriented electrical steel sheet at a high frequency of 400 Hz and 1.0 T and the thickness (t) of the non-oriented electrical steel sheet can satisfy the following formula 4. <Expression 4> W10 / 400 rolling direction iron loss (W / kg)<5+(t / 0.04) 1.1 (In the above formula 4, t means the thickness of the non-oriented electrical steel sheet) By satisfying the formula 4, it is possible to provide a non-oriented electrical steel sheet with excellent magnetic properties such as iron loss. On the other hand, if the formula 4 is not satisfied, when using a split core made by cutting a portion of a motor component in the rolling direction and assembling these, the loss of the motor increases significantly as the rotation speed increases.

[0038] According to another embodiment of the present invention, a method for manufacturing a non-oriented electrical steel sheet includes the steps of hot-rolling a slab to manufacture a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, and annealing the cold-rolled steel sheet. The step of hot-rolling the slab may involve hot-rolling the slab to have the alloy composition of the present invention. The alloy composition of the slab has been described above in relation to the steel composition of the non-oriented electrical steel sheet, and therefore a duplicated description will be omitted. The alloy composition during the manufacturing process of the non-oriented electrical steel sheet is substantially the same as that of the final product. The step of hot-rolling the slab may include a step of heating the slab. The heating temperature of the slab is not limited, but may be, for example, 1,200°C or less. If the slab heating temperature is excessively high, precipitates present in the slab, such as AlN and MnS, may be redissolved and then finely precipitate during hot-rolling and annealing, suppressing grain growth and reducing magnetic properties.

[0039] The heated slab is then hot-rolled to produce a hot-rolled steel sheet, which is preferably produced to a thickness of 1 to 3 mm. In the step of hot rolling the slab, the finish rolling temperature is 700°C or higher, and more preferably, the finish rolling temperature is 800 to 1,000°C. After the step of producing the hot-rolled steel sheet, a step of annealing the hot-rolled steel sheet may be included. The hot-rolled steel sheet annealing step may be performed by heating the hot-rolled steel sheet to 850 to 1,150°C.

[0040] After the step of producing the hot-rolled steel sheet, the method may include a step of pickling the annealed hot-rolled steel sheet. After the step of producing the hot-rolled steel sheet, the method may further include a step of cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet. The cold rolling can be final rolled to a thickness of 0.03 to 0.5 mm. The cold rolling step may further include an intermediate annealing step between a plurality of cold rolling steps. After the step of cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, the step of annealing the cold-rolled steel sheet may be included. The step of annealing the cold-rolled steel sheet includes a heating step of heating the steel sheet and a cooling step of cooling the steel sheet.

[0041] The step of annealing the cold-rolled steel sheet may include annealing the cold-rolled steel sheet at a cracking temperature of 820 to 1150°C. If the cracking temperature is outside the upper limit of the temperature range, coarse crystal grains are formed, causing the crystal grain size distribution to deviate from the range of the present invention, resulting in a significant increase in regions with high dislocation density. If the cracking temperature is outside the lower limit of the temperature range, incompletely recrystallized crystal grains remain, resulting in a deterioration in magnetic properties. The step of annealing the cold-rolled steel sheet may include a temperature-raising step of heating the cold-rolled steel sheet to 820°C or higher, and a cooling step of cooling the cold-rolled steel sheet from 820-900°C to 750-820°C. More preferably, the step of annealing the cold-rolled steel sheet may include a temperature-raising step of heating the cold-rolled steel sheet to 850°C or higher, and a cooling step of cooling the cold-rolled steel sheet from 850-900°C to 750-850°C.

[0042] In the annealing step of the cold-rolled steel sheet, the temperature-raising step is preferably performed within 60 seconds, since if it is performed for a longer time than this, the grain size distribution may be outside the range of the present invention, resulting in a wide variation in grain size distribution. In the annealing step of the cold-rolled steel sheet, the cooling step is preferably performed for a time of 5 seconds or more. If the cooling step is performed for a time shorter than the above time, stress in the thickness direction may be generated due to thermal contraction, and thermal stress may be generated in the width direction of the sheet due to difference in sheet surface cooling. In the annealing of the cold-rolled steel sheet, the cooling step may include cooling the sheet surface perpendicular to the direction of gravity.

[0043] During the annealing stage of cold-rolled steel sheets, the temperature is 650°C or higher, and the pressure is 0.01 to 1.0 kgf / mm in the rolling direction (RD direction) of the coil. 2 More preferably, the tension is 0.05 to 0.8 kgf / mm. 2 If the tension falls outside the upper limit of the range, the area ratio of regions with high dislocation density due to the tension increases significantly. On the other hand, if the tension falls outside the lower limit of the range, the sheet may not move in a horizontal plane or the sheet may not be able to be analyzed due to complex deformation stresses.

[0044] FIG. 1a shows the arrangement of a steel sheet during the annealing of a cold-rolled steel sheet according to one embodiment of the present invention, and FIGS. 1b and 1c show plan views of the direction of travel of the steel sheet and the direction of tension, respectively. Referring to FIG. 1a, during annealing of a cold-rolled steel sheet, the cold-rolled steel sheet 10 is placed on rolls 30 disposed in an annealing furnace 20. The rolls 30 rotate, and the cold-rolled steel sheet 10 is annealed while moving. Tension is applied to the cold-rolled steel sheet during annealing, and this tension is calculated based on the forward force applied to the thickness and width of the steel sheet. Specifically, the tension applied to the sheet during furnace cooling is measured. However, if measurement is difficult due to the structure of the annealing furnace in an actual manufacturing process, the tension can be calculated by dividing the difference between the tension measured at the exit of the annealing furnace and the tension measured at the entry of the annealing furnace by the cross-sectional area of ​​the sheet within the annealing furnace. In this case, the cross-sectional area of ​​the sheet within the annealing furnace can be considered to be the cross-sectional area at the exit of the annealing furnace.

[0045] FIG. 1b shows a case where the direction of travel of the cold-rolled steel sheet 10 and the direction of tension are applied in the same direction. Referring to FIG. 1b, the direction in which the tension is applied (TSD1_1, Tensile Stress Direction 1_1) may generally be the same as the direction of travel (D1) of the cold-rolled steel sheet. In FIG. 1c, the direction of tension (D2) of the cold-rolled steel sheet 10 and the tensile direction (TSD1_2, Tensile Stress Direction 1-2) can form an angle within a predetermined range. During the cold-rolled steel sheet annealing step, the direction of tension (TSD1_2) applied to the cold-rolled steel sheet can form an angle of 3° or less with the rolling direction (RD) of the coil. If this angle is too large, the stress applied to the sheet surface during cooling will not be uniform across the sheet width, resulting in significant variations in the location of high dislocation density regions within the sheet. Furthermore, compared to when the direction of tension (TSD) and the rolling direction of the coil are the same, when the direction of tension (TSD) and the rolling direction of the coil form an angle of 3° or less, the stress applied to the sheet surface during cooling will be uniform across the sheet width, preventing high stress from concentrating in specific locations.

[0046] Figure 2a and Figure 2b show cross-sectional views of the steel plate's travel direction and the direction of movement of the steel plate due to the tension direction, respectively. Figure 2a shows the tension direction (TSD) and the direction normal to the steel sheet surface (SVD) when the direction of tension applied to the cold-rolled steel sheet during the annealing step forms an angle of more than 87° and less than 93° with the normal direction to the rolling surface (ND). It can be seen that the tension direction (TSD) and the direction normal to the steel sheet surface (SVD) are perpendicular when the angle is within this range. Figure 2b shows the case where the lower and upper limits of the range deviate from the normal by more than 3°. This indicates that complex stress is applied in the thickness direction of the plate, and the tension direction (TSD) and the direction normal to the steel plate surface (SVD) are not consistent, resulting in a problem of stress inconsistency. As described above, in the annealing step of the cold-rolled steel sheet, the direction of the tension is set within the above range in the rolling direction (RD direction) of the coil and the normal direction of the rolling surface (ND direction) of the cold-rolled steel sheet to minimize the pulling force in the rolling direction, and also minimize friction in the cooling zone taking into consideration friction generated as the sheet moves through the annealing furnace and length change due to thermal expansion, thereby making it possible to manufacture a non-oriented electrical steel sheet having a uniform average grain size and excellent magnetic properties due to good iron loss.

[0047] The cold-rolled steel sheet annealing step can be performed in a reducing atmosphere, which may contain at least one of hydrogen (H2), nitrogen (N2), and an inert gas. By performing the annealing in the reducing atmosphere, it may be possible to manufacture a non-oriented electrical steel sheet with excellent core loss. Hereinafter, specific examples of the present invention will be described. However, the following examples are merely specific examples of the present invention, and the present invention is not limited to the following examples.

[0048] Slab Composition Table 1 below shows the composition of the slab. Slabs were produced using the components shown in Table 1 below, with the balance being Fe and unavoidable impurities. The slabs were then heated at 1,180°C and hot-rolled at a finishing temperature of 880°C to produce hot-rolled steel sheets with a thickness of 2.0 mm. The hot-rolled steel sheets were annealed under the pre-annealing conditions shown in Table 1 below. Specifically, the pre-annealing is a step of heating the hot-rolled steel sheets at the temperatures shown in Table 1 below.

[0049] [Table 1]

[0050] The hot-rolled and annealed steel sheets prepared according to Table 1 were cold-rolled to the thicknesses shown in Table 2, and then cold-rolled annealing was performed under the conditions shown in Table 2. The reducing atmosphere used for cold-rolled annealing was a mixture of 80% nitrogen and 20% hydrogen. Annealing performed in the reducing atmosphere with this composition is indicated by "O," and annealing not performed in the reducing atmosphere is indicated by "X." The heating time refers to the annealing time at 850°C or higher, and the cooling time refers to the time required to cool from 850°C to 800°C.

[0051] [Table 2]

[0052] The average grain size and area fraction by grain size were calculated using an optical microscope for cold-rolled and annealed sheets, and the fraction of high dislocation density regions in the steel sheet was measured using ECCI under an electron microscope. The iron loss of the manufactured steel sheet was also measured in the rolling direction, and the results are shown in Table 3. As described above, the average grain size was determined by identifying individual grains enclosed by grain boundaries in a microscopic structural photograph, determining the area of ​​each grain, and then using the corresponding ECD (Equivalent Circle Diameter) to indicate the diameter of each grain. The distribution of grain diameters was then obtained using the ECD, and the arithmetic average was calculated to determine the average grain size. In Table 3 below, the 1 / 3 grain size fraction means the area fraction of grains having a grain diameter that is less than 1 / 3 the average grain diameter of the steel sheet, and the 3x grain size fraction means the area fraction of grains having a grain diameter that is more than 3x the average grain diameter of the steel sheet.

[0053] The texture fraction was also analyzed using EBSD on the cross section of the plate. To measure the fraction over a sufficient area, the cross section of the plate was stacked and the texture fraction was measured over a 10mm x 5mm area. The area fraction of grains with misorientation within 15 degrees from the center of the Goss and Cube orientations was calculated. To measure the area fraction of the grains, the areas of more than 2,000 grains with the average grain size were measured and the area fraction was calculated statistically. Magnetic properties were measured using an Epstein specimen according to the IEC 60404 standard. The Epstein specimen was a rectangular shape with a length of 305 mm and a width of 30 mm. The Epstein test for non-oriented electrical steel sheets involved inserting half of the specimen cut longitudinally in the rolling direction and half of the specimen cut longitudinally perpendicular to the rolling direction into a measuring instrument to determine iron loss. Iron loss in the rolling direction was measured by cutting all specimens to 305 mm in the rolling direction and 30 mm perpendicular to the rolling direction, and then inserting the specimens into an Epstein frame.

[0054] In addition, dislocation density was calculated using electron channeling contrast images taken with a scanning electron microscope (FE-SEM). Dislocation density was calculated using the line intercept method according to the following equation (5). <Formula 5> Dislocation density=2N / lt In Equation 5, N is the number of dislocations contacting the randomly drawn line, l is the length of the randomly drawn line, and t is the depth of the image. In ECCI, the value is proportional to the electron beam strength, specimen composition, and current strength. When steel is measured using a 15 kV electron beam, the depth is about 70 nm, so the density was calculated taking this into consideration.

[0055] FIG. 3 illustrates a dislocation dense region according to one embodiment of the present invention. Referring to FIG. 3, when the sheet surface of the non-oriented electrical steel sheet of Steel Type 2 according to the embodiment of the present invention is measured using ECCI, a region where dislocations are concentrated due to strain, for example, the red circle region in FIG. 2, can be identified. 4a to 4e are enlarged views of areas where dislocations are concentrated in steel type 2 and steel type 10, which are comparative examples of the present invention. 4a and 4b, it can be seen that local strain occurs in the image at a 2 μm accumulation, and FIG. 3c, when FIGS. 3a and 3b are enlarged at a 1 μm accumulation, it can be seen that the dislocation density can be specifically calculated in the ECCI image. Figures 3d and 3e show the results for steel type 10 of the present invention, and are enlarged regions of the non-shaded areas on the ECCI image. Unlike Figures 3a to 3c, no dense dislocation regions can be identified. Based on the above, Table 3 below shows the following:

[0056] [Table 3]

[0057] When the slab composition, pre-annealing conditions, and cold-rolled sheet annealing conditions described in Tables 1, 2, and 3 satisfy the invention requirements, the steel sheet has uniform grain size and no dislocation concentrations within the steel sheet, resulting in excellent iron loss in the rolling direction and excellent average iron loss in both the rolling direction and the direction perpendicular to the rolling direction. Table 4 below shows the results of a slab containing 3.4 wt% Si, 0.8 wt% Al, 0.5 wt% Mn, 0.002 wt% N, 0.002 wt% S, 0.002 wt% Ti, with the balance being Fe and unavoidable impurities. This slab was heated to 1,150°C and then hot-rolled at a finishing temperature of 900°C to produce a hot-rolled steel sheet with a thickness of 1.8 mm. The hot-rolled steel sheet was pre-annealed at 1,050°C. This was cold-rolled to 0.3 mm and subjected to cold-rolled sheet annealing in a reducing atmosphere by raising the temperature to 850°C or higher over 11 seconds and cooling from 850°C to 800°C over 10 seconds. In addition to the above conditions, the other conditions were the same as those listed in Table 4 below, and the average crystal grain size, the fraction of crystal grains less than 1 / 3 the size of the average crystal grain size, and the fraction of crystal grains more than three times the average crystal grain size were measured, and the area of ​​the region where dislocations were densely packed was calculated. In addition, magnetic properties were measured and listed in Table 4.

[0058] [Table 4]

[0059] Table 4 above shows that the dislocation-dense regions during cold-rolled sheet annealing vary significantly depending on the tension and the angle with the sheet surface, resulting in significant changes in the rolling direction iron loss and average iron loss. Table 5 below shows the results of a slab containing 3.4% Si, 0.8% Al, 0.5% Mn, 0.002% N, 0.002% S, and 0.002% Ti, with the remainder being Fe and unavoidable impurities. This slab was heated to 1,150°C and hot-rolled at a finishing temperature of 900°C to produce a 1.8 mm thick hot-rolled steel sheet. The hot-rolled steel sheet was pre-annealed at 1,050°C. This was then cold-rolled to 0.3 mm and subjected to cold-rolled sheet annealing, in which the temperature was raised to above 850°C in a reducing atmosphere over 12 seconds and then cooled from 850°C to 800°C over 10 seconds. In addition to the above conditions, other conditions were also set forth in Table 5 below, and the average crystal grain size, the fraction of crystal grains less than 1 / 3 the average crystal grain size, and the fraction of crystal grains more than three times the average crystal grain size were measured, and the area of ​​the region densely packed with dislocations was calculated. Furthermore, the magnetic properties were measured and are listed in Table 5 below.

[0060] [Table 5]

[0061] As shown in Table 5, it can be seen that there are significant differences in dislocation-dense regions depending on the magnitude of tension between the tension and the sheet surface and the angle between the tension and the sheet surface during final annealing, and therefore the iron loss in the rolling direction and the average iron loss change significantly. Therefore, when the conditions of the present invention are met, a steel sheet with excellent average iron loss and iron loss in the rolling direction can be produced. The average iron loss refers to the iron loss measurement of a conventional non-oriented electrical steel sheet, and is the iron loss result measured by inserting half of the sample in the rolling direction and the other half in the direction perpendicular to the rolling direction into an iron loss tester using the Epstein method. The iron loss in the rolling direction refers to the iron loss value measured by inserting a sample prepared only in the rolling direction, as in the case of grain-oriented electrical steel, into an iron loss tester. Table 6 below shows whether or not Steel Type 1 to Steel Type 11_3 satisfy Formulas 1 to 4 of the present invention.

[0062] [Table 6]

[0063] Referring to Table 6, it can be seen that the examples of the present invention satisfy the formulas 1 to 4, thereby effectively reducing iron loss. In contrast, it was confirmed that the comparative examples do not satisfy at least one of the formulas 1 to 4. As such, it can be seen from Tables 1 to 6 that the present invention can produce steel sheets that are excellent in both rolling direction and average iron loss by satisfying the conditions of the present invention.

[0064] The present invention is not limited to the above-described embodiments and / or examples, but can be manufactured in various different forms, and those skilled in the art will understand that the present invention can be embodied in other specific forms without changing the technical concept or essential characteristics of the present invention. Therefore, it should be understood that the above-described embodiments and / or examples are illustrative in all respects and not limiting.

Claims

1. The area fraction of crystal grains having a grain size less than 1 / 3 of the average crystal grain size is less than 5%; Dislocation density is 10 １２ / m ２ excess 10 １６ / m ２ The area fraction of crystal grains having a crystal grain size of 0.1 to 0.5 mm or less is less than 5% of the total area of ​​the non-oriented electrical steel sheet.

2. 2. The non-oriented electrical steel sheet according to claim 1, wherein the area fraction of crystal grains having a grain size exceeding three times the average crystal grain size is less than 5%.

3. 2. The non-oriented electrical steel sheet according to claim 1, comprising, by weight%, Si: 0.1 to 6.5%, Al: 0.001 to 6.5%, Mn: 0.01 to 20%, C: 0.0010 to 0.015%, N: 0.0003 to 0.001%, S: 0.0003 to 0.001%, Ti: 0.0003 to 0.001%, and the balance being Fe and unavoidable impurities.

4. 2. The non-oriented electrical steel sheet according to claim 1, wherein the average grain size is 40 to 250 μm.

5. 2. The non-oriented electrical steel sheet according to claim 1, wherein the thickness is 0.03 to 0.5 mm.

6. The non-oriented electrical steel sheet according to claim 5, wherein the iron loss (W10 / 400) and the thickness (t) of the non-oriented electrical steel sheet satisfy the following formula 1: <Formula 1> W10 / 400 iron loss (W / kg) < 6 + (t / 0.04) １．１ (In the above formula 1, t means the thickness (mm) of the non-oriented electrical steel sheet.)

7. The non-oriented electrical steel sheet according to claim 5, wherein the iron loss (W15 / 50) and the thickness (t) of the non-oriented electrical steel sheet satisfy the following formula 2: <Formula 2> W15 / 50 iron loss (W / kg) < 0.7 + (t / 0.03) １／５ (In the above formula 2, t means the thickness (mm) of the non-oriented electrical steel sheet.)

8. hot rolling the slab to produce a hot rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and A cold-rolled steel sheet annealing step of annealing the cold-rolled steel sheet is included, In the cold-rolled steel sheet annealing step, At temperatures of 650°C or higher, in the rolling direction (RD direction) of the coil, more than 0.01 to 1.0 kgf / mm ２ Apply tension less than The direction of tension applied to the cold-rolled steel sheet forms an angle of 3° or less with the rolling direction (RD direction) of the coil, a direction normal to the rolling surface of the cold-rolled steel sheet (ND direction) forming an angle of more than 87° and not more than 93° with respect to the direction normal to the rolling surface of the cold-rolled steel sheet.

9. 9. The method for producing a non-oriented electrical steel sheet according to claim 8, wherein the slab contains, by weight %, Si: 0.1 to 6.5%, Al: 0.001 to 6.5%, Mn: 0.01 to 20%, C: 0.0010 to 0.015%, N: 0.0003 to 0.01%, S: 0.0003 to 0.01%, Ti: 0.0003 to 0.01%, and the balance being Fe and unavoidable impurities.

10. 10. The method of claim 8, further comprising: annealing the hot-rolled steel sheet, wherein the annealing comprises heating the hot-rolled steel sheet to a temperature of 850 to 1,150°C.

11. 9. The method of claim 8, wherein the annealing of the cold-rolled steel sheet comprises a heating step of heating the cold-rolled steel sheet to 820°C or more, and a cooling step of cooling the cold-rolled steel sheet from 820 to 900°C to 750 to 820°C.

12. 9. The method of claim 8, wherein the temperature increasing step is performed for a time period of 60 seconds or less in the cold-rolled steel sheet annealing step.

13. 9. The method of claim 8, wherein the cooling step is performed for 5 seconds or more in the cold-rolled steel sheet annealing step.

14. 9. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein in the annealing of the cold-rolled steel sheet, the cooling is performed with the sheet surface perpendicular to the direction of gravity.

15. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein the cold-rolled steel sheet is annealed in a reducing atmosphere.

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