Non-oriented electrical steel sheet and method for producing same

Abstract

The present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same, the non-oriented electrical steel sheet having an area fraction of crystal grain diameters that are less than 1 / 3 times the average crystal grain diameter of less than 5%, a dislocation density of more than 1012 / m2 and not more than 1016 / m2, and a fraction of an area in which dislocation is concentrated that may be less than 5% of the overall area.

Description

Technical Field

[0001] The present invention relates to electrical steel sheets. More specifically, the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same. Background Art

[0002] Electrical steel sheets are products used as materials for transformers, motors, and electronic devices. Different from ordinary carbon steels that emphasize workability such as mechanical properties, electrical steel sheets are functional products that emphasize electrical properties. The electrical properties are characteristics such as iron loss, magnetic flux density, magnetic permeability, and duty ratio. For the electrical steel sheets, it is characterized by low iron loss, high magnetic flux density, magnetic permeability, and duty ratio.

[0003] The electrical steel sheets are mainly divided into oriented electrical steel sheets and non-oriented electrical steel sheets. The oriented electrical steel sheets utilize an abnormal grain growth phenomenon called secondary recrystallization to form a Goss texture ({110}<001> texture) throughout the steel sheet, thereby having excellent magnetic properties in the rolling direction. The non-oriented electrical steel sheets are electrical steel sheets having uniform magnetic properties in all directions of the rolled sheet.

[0004] For the non-oriented electrical steel sheets, they are mainly used for motors that convert electrical energy into mechanical energy. In order to achieve high efficiency in such an energy conversion process, the magnetic properties of the non-oriented electrical steel sheets must be excellent.

[0005] In addition, globally, in order to respond to the carbon neutral era, the industrial structure is shifting towards environmentally friendly and low-carbon industries. With the development of this trend, in the automotive field, internal combustion engine vehicles are being rapidly replaced by electric vehicles, and the drive motors for the electric vehicles account for more than half of the electrical energy consumption, and the demand for non-oriented electrical steel sheets used as the core material of the drive motors is increasing continuously. Against this background, in order to improve the efficiency of the drive motors, it is very important to improve the iron loss of the non-oriented electrical steel sheets.

[0006] For general non-oriented electrical steel sheets, the magnetic properties are mainly evaluated by the iron loss at 50 Hz and the iron loss at 400 Hz. The iron loss refers to the energy loss that occurs at a specific magnetic flux density and frequency. Therefore, the iron loss at 50 Hz is regarded as the energy loss that occurs at a conventional frequency, and the iron loss at 400 Hz is regarded as the energy loss that occurs at a high frequency.

[0007] It is known that when a low-iron-loss non-oriented electrical steel sheet is used in the core of the motor, the heat loss generated in the core of the motor is reduced, so that a high-efficiency motor can be manufactured. The motor driven by controlling an inverter operates at various driving speeds. Therefore, reducing the iron loss from the general frequency to the high frequency and even higher ultra-high frequency is one of the solutions for manufacturing a more efficient motor.

[0008] In order to improve the iron loss of the non-oriented electrical steel sheet, one of the methods adopted is to add alloying elements such as silicon (Si), aluminum (Al), and manganese (Mn), minimize impurities, and reduce the thickness of the steel sheet. By adding the alloying elements, as the resistivity of the steel increases, the eddy current loss decreases, thereby reducing the overall iron loss.

[0009] In addition, when minimizing the impurities, it is transformed into a form with less loss during the magnetization of the magnetic domain structure in the steel sheet, and the number of closed magnetic domains fixed by inclusions or precipitates in the steel sheet decreases, so the iron loss is improved. In addition, when reducing the thickness of the steel sheet, the eddy current loss that increases in proportion to the square of the thickness decreases, so the iron loss can be effectively reduced.

[0010] According to the design intention of the motor, not only the overall iron loss, especially the iron loss in the rolling direction, may be an important factor. Generally, there are cases where the magnetization direction does not rotate, such as in large rotating machines or linear motors, or cases where electrical steel sheets with improved iron loss in the rolling direction are used in motors driven by reluctance during magnetization.

[0011] However, among the technologies adopted to improve the iron loss, increasing the amount of alloy will lead to an increase in the cost of alloy raw materials and a sharp decline in cold rolling properties, so the production cost increases, and there is a problem that the amount of the alloy cannot be increased beyond the current level. Reducing the thickness of the steel sheet will lead to an increase in heat treatment and rolling time, so the production cost rises sharply, and there is a problem that the hourly production volume drops sharply. In addition, for reducing impurities, there are problems of decreased productivity with the limitations of steelmaking technology and the increase in refining time, and problems such as increased cost due to strict control of the alloy raw material composition, so it is difficult to reduce the iron loss.

[0012] In addition, for improving the iron loss in the rolling direction, in addition to the aforementioned methods, the methods only for improving the iron loss in the rolling direction are very limited, so it is difficult to apply in actual manufacturing processes. As described above, the existing technologies often lead to an increase in manufacturing cost or a decrease in productivity and yield, so it is necessary to study technologies that can solve the aforementioned problems and effectively reduce the iron loss. Summary of the Invention

[0013] (1) Technical Problems to be Solved

[0014] The technical problem to be solved by the present invention is to provide a non-oriented electrical steel sheet to reduce the manufacturing cost, increase productivity and yield, and improve the iron loss.

[0015] 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.

[0016] (2) Technical Solutions

[0017] For the non-oriented electrical steel sheet according to an embodiment of the present invention, the area fraction of grains with a particle size less than 1 / 3 times the average grain size may be less than 5%, and the dislocation density exceeds 10 12 / m 2 and does not exceed 10 16 / m 2 and the area fraction of grains with a particle size exceeding 3 times the average grain size may be less than 5% of the total area. In one embodiment, the area fraction of grains with a particle size exceeding 3 times the average grain size may be less than 5%.

[0018] In one embodiment, by weight%, the non-oriented electrical steel sheet may contain 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 contains Fe and inevitable impurities. In one embodiment, the average grain size may be 40 to 250 μm. In one embodiment, the thickness may be 0.03 to 0.5 mm.

[0019] In one embodiment, the iron loss (W10 / 400) of the non-oriented electrical steel sheet and the thickness (t) may satisfy the following formula 1.

[0020] <Formula 1>

[0021] W10 / 400 iron loss (W / kg) < 6 + (t / 0.04) 1.1

[0022] In the above formula 1, t represents the thickness (mm) of the non-oriented electrical steel sheet.

[0023] In one embodiment, the iron loss (W15 / 50) of the non-oriented electrical steel sheet and the thickness (t) may satisfy the following formula 2.

[0024] <Formula 2>

[0025] W15 / 50 iron loss (W / kg) < 0.7 + (t / 0.03) 1 / 5

[0026] In the above formula 2, t represents the thickness (mm) of the non-oriented electrical steel sheet.

[0027] A method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention may include: a step of hot rolling a slab to manufacture a hot-rolled steel sheet; a step of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and a step of annealing the cold-rolled steel sheet, in the step of annealing the cold-rolled steel sheet, a tension greater than 0.01 to less than 1.0 kgf / mm is applied along the rolling direction (RD direction) of the coiled sheet at a temperature of 650 °C or higher. 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 coiled sheet and forms an angle of more than 87 and not more than 93° with the normal direction (ND direction) of the rolling surface of the cold-rolled steel sheet. In one embodiment, by weight%, the slab may include 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 includes Fe and inevitable impurities.

[0028] In one embodiment, it may further include a step of annealing the hot-rolled steel sheet, and the step of annealing the hot-rolled steel sheet may be a step of heating the hot-rolled steel sheet to 850 to 1150 °C. In one embodiment, the step of annealing the cold-rolled steel sheet may include a heating-up step of heating the cold-rolled steel sheet to 820 °C or higher and a cooling step of cooling from 820 to 900 °C to 750 to 820 °C.

[0029] In one embodiment, in the step of annealing the cold-rolled steel sheet, the heating-up step may be performed for a time within 60 seconds. In one embodiment, in the step of annealing the cold-rolled steel sheet, the cooling step may be performed for a time of 5 seconds or more.

[0030] In one embodiment, in the step of annealing the cold-rolled steel sheet, the cooling step may cool the plate surface perpendicular to the direction of gravity. In one embodiment, the step of annealing the cold-rolled steel sheet may be annealing in a reducing environment.

[0031] (III) Beneficial effects

[0032] For a non-oriented electrical steel sheet according to an embodiment of the present invention, by controlling the area ratio of the grain diameter, specifically the grain diameter area fraction of 1 / 3 times the average grain and the dislocation density, the iron loss in the rolling direction is reduced and improved, and at the same time, the grain distribution is uniformly formed, and a non-oriented electrical steel sheet with a uniform dislocation density can be provided.

[0033] According to another embodiment of the present invention, a method for manufacturing a non-oriented electrical steel sheet controls the cooling rate, the direction and magnitude of the tension in the heat treatment process, so that the homogenized grains are distributed in the steel sheet, and a method for manufacturing a non-oriented electrical steel sheet having the aforementioned advantages can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1a FIG. 6 is a schematic diagram of the arrangement of the steel sheet in the annealing step of the cold-rolled steel sheet according to an embodiment of the present invention, Figure 1b and Figure 1c FIGS. 7A and 7B are top views showing the traveling direction of the steel sheet and the moving direction of the steel sheet according to the tension direction, respectively.

[0035] Figure 2a and Figure 2b FIGS. 8A and 8B are cross-sectional views showing the traveling direction of the steel sheet and the moving direction of the steel sheet according to the tension direction, respectively.

[0036] Figure 3 FIG. 9 shows a region with dense dislocations according to an embodiment of the present invention.

[0037] Figures 4a to 4e FIG. 10 magnifies and shows the regions with dense dislocations according to an embodiment and a comparative example of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

[0038] The terms first, second, third, etc. are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, or segment from another part, component, region, layer, or segment. Therefore, without departing from the scope of the present invention, the first part, component, region, layer, or segment described below may also be described as the second part, component, region, layer, or segment.

[0039] The terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. Unless otherwise clearly indicated to the contrary in the context, the singular forms used herein are also intended to include the plural forms. The term "comprising" used in the specification may specifically refer to a certain property, field, integer, step, action, element, and / or component, but does not exclude the existence or addition of other properties, fields, integers, steps, actions, elements, and / or components.

[0040] If a part is described as being on another part, there may be other parts directly on or between the other part. When a part is described as being directly on another part, there are no other parts therebetween.

[0041] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. For terms defined in a dictionary, they should be interpreted as having a meaning consistent with the relevant technical literature and the content disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0042] In addition, unless otherwise specifically stated, % represents weight %, and 1 ppm is 0.0001 weight %. In one embodiment of the present invention, further comprising an additional element means that the additional element replaces the remaining iron (Fe), and the replacement amount is equivalent to the addition amount of the additional element.

[0043] Furthermore, in the present invention, the Goss orientation refers to the orientation with Miller indices corresponding to {110}<001>, and the Cube orientation refers to the orientation with Miller indices corresponding to {100}<001>.

[0044] Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention belongs can easily implement the present invention. However, the present invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0045] An non-oriented electrical steel sheet according to an embodiment of the present invention, in weight %, the non-oriented electrical steel sheet comprises Si: 0.1 to 6.5 weight %, Al: 0.001 to 6.5 weight %, Mn: 0.01 to 20 weight %, C: 0.0010 to 0.0150 weight %, and one or more of N, S, Ti: each 0.0003 to 0.01 weight %, and the balance comprises Fe and inevitable impurities.

[0046] The reasons for the composition limitations of the non-oriented electrical steel sheet are described below.

[0047] Si: 0.1 to 6.5 weight %

[0048] Silicon (Si) plays a role in increasing the resistivity of the material to reduce iron loss and is a component as a deoxidizer in the steelmaking process. In addition, the silicon is an element inevitably added in the manufacturing process of the electrical steel sheet and is an element that forms oxides in the manufacturing process. The content of the Si can be 0.1 to 6.5 weight %. Specifically, the content of the Si can be 1.0 to 4.5 weight %.

[0049] If the content of the silicon is too high, the brittleness of the material increases, the rolling productivity drops sharply, a thick oxide layer harmful to magnetism forms, and internal oxides may damage the iron loss. In addition, if the content of the silicon is too high, the magnetism may be greatly deteriorated due to the formation of a secondary phase. If the content of the silicon is too low, there is a problem of iron loss deterioration due to the formation of low-temperature Si oxides.

[0050] Al: 0.001 to 6.5 wt%

[0051] Similar to the silicon, aluminum (Al) plays a role in increasing the resistivity of the material to reduce the iron loss and can be used as a strong deoxidizer in steelmaking. The content of the aluminum can be 0.001 to 6.5 wt%. Specifically, the content of the aluminum can be 0.1 to 2.0 wt%.

[0052] Mn: 0.01 to 20 wt%

[0053] Manganese (Mn) is an element that improves the resistivity of the material to improve the iron loss and can play a role in forming sulfides in the steel. The content of the manganese can be 0.01 to 20 wt%. Specifically, the content of the manganese can be 0.01 to 6.5 wt%. More specifically, the content of the manganese can be 0.01 to 2.0 wt%.

[0054] In one embodiment, the non-oriented electrical steel sheet may contain one or more of N, S, and Ti by weight%, and their contents are 0.0003 to 0.001 wt% respectively. Specifically, it may contain at least one of N, S, and Ti, and more specifically, it may also contain N, S, and Ti at the same time.

[0055] C: 0.0005 to 0.015 wt%

[0056] Carbon (C) is an element inevitably contained in the manufacturing process of the non-oriented electrical steel sheet and may play a role in homogenizing the rolling structure in the steel during rolling. Specifically, the content of carbon can be 0.0005 to 0.015 wt%, and more specifically, it can contain 0.0015 to 0.004 wt%.

[0057] If the content of the carbon is too high, the movement of magnetic domains is hindered due to the formation of carbides, and there is a problem of additional energy required for magnetization when the material is magnetized. If the content of the carbon is too low, there is a problem that the grain size of the recrystallized structure becomes uneven due to the non-uniformity of the material during rolling.

[0058] N: 0.0003 to 0.010 wt%

[0059] Nitrogen (N) not only forms fine AlN precipitates inside the steel sheet, but also combines with other impurities to form fine precipitates, inhibiting grain growth, which may lead to deterioration of iron loss or can increase strength. The content of the nitrogen may be from 0.0003 to 0.010% by weight. Specifically, the content of the nitrogen may be from 0.0003 to 0.004% by weight.

[0060] S: 0.0003 to 0.010% by weight

[0061] Sulfur (S) forms fine precipitates of MnS, leading to deterioration of magnetic properties and hot rolling workability, and is preferably controlled to maintain a low content. The content of the sulfur may be from 0.0003 to 0.010% by weight. Specifically, the content of the sulfur may be from 0.0003 to 0.004% by weight.

[0062] If the content of the sulfur is too high, there is a problem that cracks may occur during continuous casting. When the content of the sulfur is too low, although it may be ideal in terms of the properties of the steel sheet, it is necessary to use selected raw materials to control the content of the sulfur below the lower limit value, so there is a problem in manufacturing cost.

[0063] Ti: 0.0003 to 0.010% by weight

[0064] Titanium (Ti) has a strong tendency to form precipitates inside the steel sheet, and forms fine carbides, nitrides, or sulfides inside the steel sheet, inhibiting grain growth, which may lead to deterioration of iron loss. The content of the titanium may be from 0.0003 to 0.010% by weight. Specifically, the content of the titanium may be from 0.0003 to 0.003% by weight.

[0065] If the content of the titanium is too high, there may be a problem of deterioration of iron loss. If the content of the titanium is too low, it is necessary to use selected raw materials to control the content of the titanium, so there is a problem in manufacturing cost.

[0066] An non-oriented electrical steel sheet according to an embodiment of the present invention contains Fe and inevitable impurities as the balance. Regarding the inevitable impurities, they are impurities mixed in during the steelmaking step and the manufacturing process of the non-oriented electrical steel sheet, and these impurities are well known in the art, so specific descriptions are omitted. In an embodiment of the present invention, in addition to the aforementioned alloy components, the addition of other elements is not excluded, and various elements may be included within the scope not affecting the technical idea of the present invention. When additional elements are further included, a part of the Fe as the balance is replaced.

[0067] An non-oriented electrical steel sheet according to an embodiment of the present invention having the aforementioned components has the physical properties described below.

[0068] An non-oriented electrical steel sheet according to an embodiment of the present invention may have an average grain size of 40 to 250 μm. The grains of the non-oriented electrical steel sheet have a characteristic of uniform distribution. This is because the places where dislocations concentrate in the steel sheet are grain boundaries, and the structural stability of the grain boundaries can be obtained from the uniform grain size distribution. Specifically, the average grain size may be 50 to 120 μm.

[0069] To obtain the distribution of the grain sizes, each grain surrounded by grain boundaries is specified in a micrograph of the microstructure, the area of each grain is calculated, and the diameter of each grain is represented by the corresponding ECD (Equivalent Circle Diameter). At this time, the distribution of the grain diameters can be obtained from the ECD, and the average grain diameter can be calculated by arithmetic mean.

[0070] When the average grain diameter exceeds the upper limit value of the range, the dispersion in terms of the size of the grain sizes in the steel may become larger. In a grain size distribution with a large dispersion, more dislocations will be formed around the large grains, and there may be a problem of the formation of sub grain boundaries in the grains. When the average grain diameter is less than the lower limit value of the range, the fraction of the grain boundaries in the overall material becomes larger, and there is a problem of difficulty in magnetization.

[0071] In one embodiment, for the non-oriented electrical steel sheet, the area fraction of grains with a grain size less than 1 / 3 times the average grain size may be less than 5%. The grains with a grain size less than 1 / 3 times the average grain size refer to the grain sizes that are less than 1 / 3 times the grain size calculated previously with respect to the average grain size.

[0072] The area fraction of the grain sizes less than 1 / 3 times the average grain size refers to the area ratio occupied by the grain sizes that are 1 / 3 times the average grain size in the overall microstructure of the non-oriented electrical steel sheet.

[0073] For the measurement of the texture fraction, it can be calculated using an X-ray diffraction pole figure (Pol Figure), or by neutron diffraction, or using X-ray transmission analysis, or it can also be analyzed by EBSD of an electron microscope. The area fraction of grains with a misorientation within 15 degrees from the centers of the Goss orientation and the Cube orientation can be calculated.

[0074] When the proportion of the area fraction of grain sizes smaller than 1 / 3 times the average grain size is too large, it is difficult to magnetize under a low magnetic field, and there is a problem of reduced magnetic permeability. In one embodiment, the area fraction of grain sizes larger than 3 times the average grain size can be less than 5%. The grain size larger than 3 times the average grain size refers to a grain size that is larger than 3 times the calculated average grain size with respect to the previously calculated average grain size.

[0075] The area fraction of grains with a grain size exceeding 3 times the average grain size refers to the area ratio of grains with a grain size exceeding 3 times the average grain size in the overall structure of the non-oriented electrical steel sheet. When the proportion of the area fraction of grain sizes larger than 3 times the average grain size is too large, the dislocation distribution between grains is different, and the displacement is concentrated in locally coarse grains, resulting in a problem of a significant increase in iron loss.

[0076] In one embodiment, the sum of the fraction of grains with Goss orientation and the fraction of grains with Cube orientation in the non-oriented electrical steel sheet can be greater than 5%.

[0077] In one embodiment, the dislocation density of the non-oriented electrical steel sheet exceeds 10 12 / m 2 and does not exceed 10 16 / m 2 , and the area fraction of the concentrated dislocations can be less than 5% of the overall area. For the measurement of the dislocation density, TEM (Transmission Electron Microscope) can be used for measurement, and more conveniently, SEM (scanning electron microscope) can be used for calculation. For the dislocation density, the line intercept method can be used for measurement.

[0078] By controlling the tension in the annealing according to the present invention, the dislocations formed around the grain boundaries after annealing can be controlled. The formation of dislocations at high temperatures starts from a region dozens of nanometers away from the grain boundaries, and the release of dislocations towards the grain boundaries is suppressed and arranged towards the internal sub-grain boundaries. The greater the tension during annealing, the more the amount of dislocations constituting the sub-grain boundaries, so the orientation deviation angle between the regions on both sides of the sub-grain boundaries increases. Due to the increase in this deviation angle, additional energy consumption will occur during magnetization, resulting in magnetic degradation. When there are enough grain boundaries and the sizes of the grains are uniform, the dislocations generated by the tension during annealing will be suppressed by the absorption of the dislocations at the grain boundaries.

[0079] In one embodiment, the thickness of the non-oriented electrical steel sheet can be 0.03 to 0.5 mm. Specifically, the thickness of the non-oriented electrical steel sheet can be 0.15 to 0.3 mm.

[0080] When the thickness is too thick, the tension during annealing varies according to the plate thickness. Due to the tension difference between the surface part and the central part, tension is generated in the thickness direction, and there is a problem of forming complex dislocations. When the thickness is too thin, there is a problem that the tension control of the annealing furnace cannot be achieved industrially.

[0081] In one embodiment, for non-oriented electrical steel sheets, the iron loss (W10 / 400) at 400 Hz and 1.0 T of the non-oriented electrical steel sheets and the thickness (t) may satisfy the following formula 1.

[0082] <Formula 1>

[0083] W10 / 400 iron loss (W / kg) < 6 + (t / 0.04) 1.1

[0084] In the above formula 1, t represents the thickness of the non-oriented electrical steel sheet.

[0085] By satisfying the above formula 1, a non-oriented electrical steel sheet with excellent magnetic properties such as iron loss can be provided. If the above formula 1 is not satisfied, there is a problem that a steel sheet with excellent high-frequency iron loss cannot be obtained with respect to the plate thickness.

[0086] In one embodiment, for non-oriented electrical steel sheets, the iron loss at 50 Hz and 1.5 T of the non-oriented electrical steel sheets and the thickness (t) may satisfy the following formula 2.

[0087] <Formula 2>

[0088] W15 / 50 iron loss (W / kg) < 0.7 + (t / 0.03) 1 / 5

[0089] In the above formula 2, t represents the thickness of the non-oriented electrical steel sheet.

[0090] By satisfying the above formula 2, a non-oriented electrical steel sheet with excellent magnetic properties such as iron loss can be provided. If the above formula 2 is not satisfied, the iron loss under high magnetic flux density conditions deteriorates, and there is a problem that the motor loss under high torque increases significantly.

[0091] In one embodiment, for the non-oriented electrical steel sheets, the iron loss at 50 Hz and 1.5 T of the non-oriented electrical steel sheets and the thickness (t) may satisfy the following formula 3.

[0092] <Formula 3>

[0093] W15 / 50 rolling direction iron loss (W / kg) < 0.6 + (t / 0.03) 1 / 6

[0094] In the above formula (3), t represents the thickness of the non-oriented electrical steel sheet.

[0095] By satisfying the above formula (3), a non-oriented electrical steel sheet with excellent magnetic properties such as iron loss can be provided. If the above formula (3) is not satisfied, when a part of the motor component is cut in the rolling direction and assembled into a split core, there is a problem of increased motor loss under high torque.

[0096] In one embodiment, for the non-oriented electrical steel sheet, the iron loss (W10 / 400) at 400 Hz and 1.0 T of the non-oriented electrical steel sheet and the thickness (t) may satisfy the following formula (4).

[0097] W10 / 400 rolling direction iron loss (W / kg) < 5 + (t / 0.04) 1.1

[0098] In the above formula (4), t represents the thickness of the non-oriented electrical steel sheet.

[0099] By satisfying the above formula (4), a non-oriented electrical steel sheet with excellent magnetic properties such as iron loss can be provided. If the above formula (4) is not satisfied, when a part of the motor component is cut in the rolling direction and assembled into a split core, there is a problem that the loss of the motor increases greatly when the rotational speed increases.

[0100] A method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention includes: a step of hot rolling a slab to manufacture a hot-rolled steel sheet; a step of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and a step of annealing the cold-rolled steel sheet, which is a cold-rolled steel sheet annealing step.

[0101] The step of hot rolling the slab may hot roll the slab that satisfies the alloy composition of the present invention. Regarding the alloy composition of the slab, it has been described in the steel components of the above-mentioned non-oriented electrical steel sheet, so the repeated description is omitted. The alloy composition in the manufacturing process of the non-oriented electrical steel sheet is basically the same as that of the final product.

[0102] In the step of hot rolling the slab, a step of heating the slab may be included. In the step of heating the slab, the heating temperature is not limited, but specifically it can be heated to below 1200 °C. When the heating temperature of the slab is too high, precipitates such as AlN and MnS present in the slab redissolve and then finely precipitate during hot rolling and annealing, thereby inhibiting grain growth and possibly causing a decrease in magnetism.

[0103] Then, the heated slab is hot rolled to manufacture a hot-rolled steel sheet. The thickness of the hot-rolled steel sheet that can be manufactured is 1 to 3 mm.

[0104] In one embodiment, in the step of hot rolling the slab, the finish rolling temperature can be 700 °C or higher. Specifically, the finish rolling temperature can be 800 to 1000 °C.

[0105] After the step of manufacturing the hot-rolled steel sheet, a hot-rolled sheet annealing step of heating the hot-rolled steel sheet can be included. In one embodiment, the hot-rolled sheet annealing step can heat the hot-rolled steel sheet to 850 to 1150 °C.

[0106] In one embodiment, after the step of manufacturing the hot-rolled steel sheet, a step of pickling the annealed hot-rolled steel sheet can be included.

[0107] After the step of manufacturing the hot-rolled steel sheet, a step of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet can be included. For the cold rolling, it can be finally rolled to a thickness of 0.03 to 0.5 mm. In one embodiment, for the cold rolling step, an intermediate annealing step can also be included between multiple cold rollings.

[0108] After the step of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, a step of annealing the cold-rolled steel sheet can be included. The step of annealing the cold-rolled steel sheet can include a heating-up step of heating up and a cooling step of cooling.

[0109] In one embodiment, the step of annealing the cold-rolled steel sheet can include a step of annealing the cold-rolled steel sheet at a soaking temperature of 820 to 1150. When exceeding the upper limit value of the soaking temperature, due to the formation of coarse grains, the distribution of grain sizes exceeds the scope of the invention, and there is a problem that the area with a high dislocation density greatly increases. When exceeding the lower limit value of the soaking temperature, unrecrystallized grains remain, and there is a problem of magnetic deterioration.

[0110] In one embodiment, the step of annealing the cold-rolled steel sheet can include a heating-up step of heating the cold-rolled steel sheet to 820 °C or higher and a cooling step of cooling from 820 to 900 °C to 750 to 820 °C. Specifically, the step of annealing the cold-rolled steel sheet can include a heating-up step of heating the cold-rolled steel sheet to 850 °C or higher and a cooling step of cooling from 850 to 900 °C to 750 to 850 °C.

[0111] In one embodiment, in the cold-rolled steel sheet annealing step, the heating-up step can be performed for a time within 60 seconds. If the execution time is too long and exceeds this time, the distribution of grains exceeds the scope of the invention, and there is a problem of generating a widely dispersed grain size distribution.

[0112] In one embodiment, during the annealing step of the cold-rolled steel sheet, the cooling step can be performed for a time of more than 5 seconds. If the execution time is less than this time, thermal shrinkage causes stress in the thickness direction, and there is a problem of thermal stress caused by the difference in the cooling of the plate surface in the width direction of the steel sheet. In one embodiment, during the annealing step of the cold-rolled steel sheet, the cooling step can include a step of cooling the plate surface perpendicular to the direction of gravity.

[0113] In one embodiment, during the annealing step of the cold-rolled steel sheet, a tension greater than 0.01 to less than 1.0 kgf / mm can be applied in the rolling direction (RD direction) of the coiled sheet at a temperature above 650 °C. 2 Specifically, the tension can be in the range of 0.05 to 0.8 kgf / mm. 2 When the range of the tension exceeds the upper limit value, there is a problem that the area ratio of the region with a high dislocation density caused by the tension greatly increases. When the range of the tension exceeds the lower limit value, there is a problem that the traveling direction of the steel sheet is not on the horizontal plane or complex deformation stress makes it impossible to explain the stress of the steel sheet.

[0114] Figure 1a is a schematic diagram of the arrangement of the steel sheet during the annealing step of the cold-rolled steel sheet according to an embodiment of the present invention. Figure 1b and Figure 1c are respectively top views showing the traveling direction of the steel sheet and the moving direction of the steel sheet according to the tension direction.

[0115] See Figure 1a , when annealing the cold-rolled steel sheet, place the cold-rolled steel sheet 10 on the roller 30 in the annealing furnace 20. As the roller 30 rotates, the cold-rolled steel sheet 10 will move and be annealed. Tension can be applied during the step of annealing the cold-rolled steel sheet, and the tension can be calculated by the forward force applied to the thickness and width of the steel sheet. Specifically, although the tension applied to the plate is measured during cooling in the furnace, in actual manufacturing processes, due to the structural problems of the annealing furnace, when it is difficult to measure, it can be calculated by the method of dividing the difference between the tension measurement value on the outlet side of the annealing furnace and the tension measurement value on the inlet side of the annealing furnace by the cross-sectional area of the steel sheet in the annealing furnace. At this time, the cross-sectional area of the steel sheet in the annealing furnace can be regarded as the cross-sectional area value on the outlet side of the annealing furnace.

[0116] Figure 1b shows the situation where the traveling direction and the tension direction of the cold-rolled steel sheet 10 are applied in the same direction.

[0117] See Figure 1b For the direction TSD 1_1 (tensile stress direction 1_1) for applying the tension, generally, the tension can be applied in the direction consistent with the traveling direction D1 of the cold-rolled steel sheet.

[0118] See Figure 1c , the traveling direction D2 of the cold-rolled steel sheet 10 and the tension direction TSD 1_2 (Tensile stress direction 1 - 2) can form an angle within a predetermined range. In one embodiment, in the annealing step of the cold-rolled steel sheet, the direction TSD of the tension applied to the cold-rolled steel sheet 1_2 can form an angle within 3° with the rolling direction (RD direction) of the coiled sheet. When the angle is too large, the stress acting on the plate surface during cooling is uneven across the plate width, and there is a problem that the dislocation density in the plate varies greatly depending on the position in the plate. In addition, compared with the case where the direction TSD of the applied tension is consistent with the rolling direction of the coiled sheet, when the direction TSD of the applied tension forms an angle within 3° with the rolling direction of the coiled sheet, the stress acting on the plate surface during cooling is uniform across the plate width, thus having the advantage of higher stress concentration at a specific part.

[0119] Figure 2a and Figure 2b are cross-sectional views showing the traveling direction of the steel sheet and the moving direction of the steel sheet according to the tension direction, respectively.

[0120] Figure 2a Show the tension direction (TSD) and the perpendicular direction (SVD) of the steel sheet surface when the direction of the tension applied to the cold-rolled steel sheet in the annealing step of the cold-rolled steel sheet forms an angle greater than 87° and less than 93° with the normal direction (ND direction) of the rolling plane. When the angle range is reached, it can be confirmed that the tension direction (TSD) and the perpendicular direction (SVD) of the steel sheet surface are perpendicular.

[0121] From Figure 2b it can be confirmed that for the angle, when the lower limit value and the upper limit value of the range deviate from the normal by more than 3°, complex stresses are applied in the thickness direction of the steel sheet, and the tension direction (TSD) and the perpendicular direction (SVD) of the steel sheet surface are inconsistent, thus resulting in the problem of uneven stress.

[0122] As described above, in the annealing step of the cold-rolled steel sheet of the present invention, by keeping the direction of the tension, that is, the rolling direction (RD direction) of the coiled sheet and the normal direction (ND direction) of the rolling plane of the cold-rolled steel sheet within the range, while minimizing the pulling force in the rolling traveling direction and minimizing the friction generated when the steel sheet travels in the annealing furnace and considering the friction in the cooling area caused by thermal expansion-induced length changes, an evenly distributed average grain size can be obtained. Due to good iron loss, non-oriented electrical steel sheets with excellent magnetic properties can be manufactured.

[0123] In one embodiment, the annealing step of the cold-rolled steel sheet can be carried out in a reducing environment. The reducing environment can include at least any one of hydrogen (H2), nitrogen (N2), and inert gas. Since annealing is carried out in the reducing environment, a non-oriented electrical steel sheet with excellent iron loss can be manufactured.

[0124] Specific embodiments of the present invention are described below. However, the following embodiments are merely specific embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0125] Slab composition

[0126] Table 1 below shows the composition of the slab. The slab is manufactured according to the composition shown in Table 1 below, and the balance includes Fe and inevitable impurities. Then, the slab is heated to 1180 °C and hot-rolled at a finishing temperature of 880 °C to manufacture a hot-rolled steel sheet with a thickness of 2.0 mm. For the hot-rolled steel sheet, annealing is carried out under the pre-annealing conditions shown in Table 1 below. Specifically, the pre-annealing is a step of heating the hot-rolled steel sheet at the temperature shown in Table 1 below.

[0127] [Table 1]

[0128]

[0129] After cold-rolling the hot-rolled annealed steel sheet manufactured from Table 1 above to the thickness of the steel sheet shown in Table 2 below, cold-rolled sheet annealing is carried out under the conditions shown in Table 2 below. For the reducing environment, when carrying out cold-rolled sheet annealing, 80% nitrogen and 20% hydrogen are mixed and used. When annealing is carried out in the reducing environment thus created, it is indicated as "O", and when not carried out in the reducing environment, it is indicated as "X". The heating time refers to the annealing time at a temperature above 850 °C, and the cooling time refers to the time from 850 °C to 800 °C.

[0130] [Table 2]

[0131]

[0132] Table 3 below shows the average grain size, the area fraction according to grain size, and the fraction of the region with a higher dislocation density in the steel sheet measured by ECCI of the electron microscope for the steel sheet after cold-rolled sheet annealing is completed. In addition, the iron loss of the manufactured steel sheet and the iron loss in the rolling direction are measured and shown in Table 3 below. For the average grain size, as described above, each grain surrounded by grain boundaries is specified in the micrograph of the microscope, the area of each grain is calculated, and the diameter of each grain is represented by the corresponding ECD (Equivalent Circle Diameter). At this time, the distribution of the grain diameter can be obtained through the ECD, and the average grain size can be calculated by arithmetic mean.

[0133] The 1 / 3 grain size fraction in Table 3 below refers to the area fraction of grains with a grain diameter less than 1 / 3 of the average grain diameter of the steel plate, and the 3-fold grain size fraction refers to the area fraction of grains with a grain diameter greater than 3 times the average grain diameter of the steel plate.

[0134] In addition, the texture fraction was analyzed on the cross-section of the steel plate by EBSD. To measure the fraction of a sufficient area, the cross-sections of the steel plate were stacked, and the texture fraction was measured in an area of 10 mm × 5 mm. The area fraction of grains with an orientation difference (Misorientation) within 15 degrees from the center of the Goss orientation and the Cube orientation was calculated. To measure the area fraction of the grains, the areas of more than 2000 grains with the average grain size were measured, and the area fraction was statistically analyzed.

[0135] In addition, to measure the magnetic properties, measurements were carried out using an Epstein specimen in accordance with IEC 60404. The shape of the Epstein specimen is a rectangular shape with a long side of 305 mm and a short side of 30 mm. The Epstein test for non-oriented electrical steel sheets stipulates that half of the specimen cut along the rolling direction and half of the specimen cut along the direction perpendicular to the rolling direction are loaded into the measuring instrument to derive the iron loss. For the iron loss in the rolling direction, all the measured specimens were cut into specimens with a rolling direction of 305 mm and a rolling perpendicular direction of 30 mm, and the specimens were loaded into the Epstein frame for measurement.

[0136] In addition, the dislocation density was calculated using electron channeling contrast imaging (Electronchanneling contrast image) in FE-SEM (scanning electron microscope). For the dislocation density, it was calculated by the line intercept method using the following Equation 5.

[0137] <Equation 5>

[0138] Dislocation density = 2 N / lt

[0139] In the above Equation 5, N represents the number of dislocations in contact with the randomly (Random) drawn line, l represents the length of the randomly drawn line, and t represents the depth shown in the image. ECCI has a value proportional to the intensity of the electron beam, the composition of the specimen, and the intensity of the current. When measuring steel with an electron beam of 15 kV, it has a depth of about 70 nm, so this is taken into account when calculating the density.

[0140] Figure 3 Shows a region with dense dislocations according to an embodiment of the present invention.

[0141] See Figure 3, when measuring the surface of the non-oriented electrical steel sheet of steel grade 2 in the embodiment of the present invention using ECCI, it can be confirmed that the area where dislocations are concentrated is caused by strain, such as the red circular area in Figure 2.

[0142] Figures 4a to 4e The areas where dislocations are concentrated in steel grade 2 of the comparative example of the present invention and steel grade 10 of an embodiment are shown enlarged.

[0143] See Figure 4a and Figure 4b , according to the 2μm cumulative standard, it can be confirmed that local strain causes contrast in the image. See Figure 3 c, according to the 1μm cumulative standard, magnify the Figure 3 a and Figure 3 b, it can be confirmed that on the ECCI image, it is sufficient to specifically calculate the dislocation density.

[0144] Figure 3 d and Figure 3 e are the results of steel grade 10 of an embodiment of the present invention, which are enlarged areas of the non-shadow part in the ECCI image, different from Figure 3 a to Figure 3 c, and the concentrated area of dislocations cannot be confirmed.

[0145] According to the foregoing content, view Table 3 below, as shown below.

[0146] [Table 3]

[0147]

[0148] If the slab composition, pre-annealing conditions, and cold-rolled sheet annealing conditions that meet the invention conditions shown in Table 1, Table 2, and Table 3 above are satisfied, the size of the grains in the steel sheet is uniform, there is no place where dislocations are concentrated in the steel sheet, and an electrical steel sheet with very excellent iron loss in the rolling direction and excellent average iron loss in the rolling direction and the direction perpendicular to the rolling can be manufactured. Manufacture a slab with the following composition shown in Table 4 containing Si: 3.4 wt%, Al: 0.8 wt%, Mn: 0.5 wt%, N: 0.002 wt%, S: 0.002 wt%, Ti: 0.002 wt%, and the balance contains Fe and inevitable impurities. Heat the slab to 1150°C and then hot-roll it to an end temperature of 900°C to manufacture a hot-rolled steel sheet with a thickness of 1.8 mm. Perform pre-annealing on the hot-rolled steel sheet at a temperature of 1050°C, then cold-roll it to 0.3 mm, raise the temperature to a temperature above 850°C within 11 seconds in a reducing environment, and cool from 850°C to 800°C during cooling, taking 10 seconds, thereby implementing cold-rolled sheet annealing.

[0149] Except for the aforementioned conditions, other conditions are the same as those shown in Table 4 below. After measuring the average grain size, the fraction of grains smaller than 1 / 3 of the average grain size, and the fraction of grains larger than 3 times the average grain, the area in the dislocation-dense region is derived. In addition, the magnetic properties are measured and shown in Table 4.

[0150] [Table 4]

[0151]

[0152] As can be confirmed from Table 4 above, due to the relationship between the tension and the angle with the plate surface during the annealing of the cold-rolled plate, there are significant differences in the dislocation-dense regions, so the iron loss in the rolling direction and the average iron loss change greatly. A slab is manufactured with the following composition: Si: 3.4%, Al: 0.8%, Mn: 0.5%, N: 0.002%, S: 0.002%, Ti: 0.002%, and the balance contains Fe and unavoidable impurities. The slab is heated to 1150 °C and hot-rolled to a finishing temperature of 900 °C to manufacture a hot-rolled steel plate with a thickness of 1.8 mm. The hot-rolled steel plate is pre-annealed at a temperature of 1050 °C, then cold-rolled to 0.3 mm, heated to a temperature above 850 °C within 12 seconds in a reducing atmosphere, and cooled from 850 °C to 800 °C in 10 seconds during cooling, thereby performing cold-rolled plate annealing.

[0153] Except for the aforementioned conditions, other conditions are the same as those shown in Table 5 below. After measuring the average grain size, the fraction of grains smaller than 1 / 3 of the average grain size, and the fraction of grains larger than 3 times the average grain, the area of the dislocation-dense region is derived. In addition, the magnetic properties are measured and shown in Table 5 below.

[0154] [Table 5]

[0155]

[0156] As shown in Table 5 above, it can be confirmed that there are significant differences in the dislocation-dense regions in terms of the magnitude of the tension and the tension with the plate surface, and the angle between the tension and the plate surface during the final annealing, so the iron loss in the rolling direction and the average iron loss change greatly. Therefore, when the conditions of the present invention are met, a steel plate with excellent average iron loss and iron loss in the rolling direction can be manufactured. The average iron loss refers to the iron loss measured for a conventional non-oriented electrical steel sheet. Half of the specimen in the Epstein measurement method is in the rolling direction, and the remaining half is in the direction perpendicular to the rolling direction, and the iron loss results are measured by loading them into an iron loss measuring instrument. The iron loss in the rolling direction refers to the iron loss value measured by preparing a specimen only in the rolling direction and loading it into an iron loss measuring instrument, such as for an oriented electrical steel sheet.

[0157] Table 6 below shows whether Steel Grades 1 to 11_3 meet the formulas 1 to 4 of the present invention.

[0158] [Table 6]

[0159]

[0160] Referring to Table 6 above, it can be confirmed that the embodiments of the present invention satisfy the above formulas (1) to (4), thereby effectively reducing iron loss. On the other hand, the comparative examples do not satisfy at least one of the above formulas (1) to (4). Thus, through Tables 1 to 6, it is confirmed that steel sheets excellent in both the rolling direction and average iron loss can be manufactured because the conditions of the present invention are satisfied. The present invention can be implemented in various different ways and is not limited to the above embodiments and / or examples. Those of ordinary skill in the technical field to which the present invention pertains can understand that the present invention can be implemented by other specific means without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the above embodiments and / or examples are exemplary in all aspects and not restrictive.

Claims

1. An non-oriented electrical steel sheet, wherein, the area fraction of grains having a grain size less than 1 / 3 times the average grain size is less than 5%; The dislocation density exceeds 10 12 / m 2 and does not exceed 10 16 / m 2 The area fraction of grains with a dislocation density exceeding 10 12 / m 2 and not exceeding 10 16 / m 2 is less than 5% of the total area.

2. The non-oriented electrical steel sheet according to claim 1, wherein, the area fraction of grains having a grain size exceeding 3 times the average grain size is less than 5%.

3. The non-oriented electrical steel sheet according to claim 1, wherein, by weight%, the non-oriented electrical steel sheet contains 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 contains Fe and inevitable impurities.

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

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

6. The non-oriented electrical steel sheet according to claim 5, wherein, the iron loss (W10 / 400) of the non-oriented electrical steel sheet and the thickness (t) satisfy the following formula 1, <Formula 1> The iron loss (W / kg) of W10 / 400 < 6 + (t / 0.04) 1.1 In the above formula 1, t represents 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) of the non-oriented electrical steel sheet and the thickness (t) satisfy the following formula 2, <Formula 2> Iron loss of W15 / 50 (W / kg) < 0.7+(t / 0.03) 1 / 5 In the above formula 2, t represents the thickness (mm) of the non-oriented electrical steel sheet.

8. A method for manufacturing a non-oriented electrical steel sheet, comprising: a step of hot rolling a slab to manufacture a hot rolled steel sheet; a step of cold rolling the hot rolled steel sheet to manufacture a cold rolled steel sheet; and a step of annealing the cold rolled steel sheet for annealing the cold rolled steel sheet, in the step of annealing the cold rolled steel sheet, Apply a tension greater than 0.01 to less than 1.0 kgf / mm in the rolling direction (RD direction) of the rolled sheet at a temperature above 650 °C 2 to it. 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 coiled sheet, and forms an angle exceeding 87° and not exceeding 93° with the normal direction (ND direction) of the rolling surface of the cold rolled steel sheet.

9. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein, by weight%, the slab contains 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 contains Fe and inevitable impurities.

10. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein, the manufacturing method further includes a step of annealing the hot rolled steel sheet for heating the hot rolled steel sheet, and the step of annealing the hot rolled steel sheet is a step of heating the hot rolled steel sheet to 850 to 1150 °C.

11. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein, The step of annealing the cold-rolled steel sheet includes a heating-up step of heating the cold-rolled steel sheet to a temperature above 820 °C and a cooling step of cooling from 820 to 900 °C to 750 to 820 °C.

12. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, in the step of annealing the cold-rolled steel sheet, the heating-up step is carried out for a time within 60 seconds.

13. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, in the step of annealing the cold-rolled steel sheet, the cooling step is carried out for a time of 5 seconds or more.

14. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, in the step of annealing the cold-rolled steel sheet, the cooling step cools the sheet surface perpendicular to the direction of gravity.

15. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, the step of annealing the cold-rolled steel sheet is carried out in a reducing environment.

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