Non-oriented electrical steel sheet and its manufacturing method

Abstract

The present invention relates to a method for manufacturing a hot-rolled steel sheet, the method comprising the steps of hot-rolling a slab containing, by weight, carbon (C): more than 0% and 0.005% or less, silicon (Si): 2.0% or more and 4.0% or less, manganese (Mn): 0.1% or more and 0.5% or less, aluminum (Al): 0.9% or more and 1.5% or less, phosphorus (P): more than 0% and 0.015% or less, sulfur (S): more than 0% and 0.005% or less, nitrogen (N): more than 0% and 0.005% or less, titanium (Ti): more than 0% and 0.005% or less, the balance being iron (Fe) and unavoidable impurities, and A method for manufacturing a non-oriented electrical steel sheet includes: a pre-annealing step; a cold-rolling step of cold-rolling the pre-annealed hot-rolled annealed sheet; and a cold-roll annealing step of cold-rolled the cold-rolled sheet, wherein the cold-roll annealing step includes a first heating section, a second heating section, and a soaking section, wherein in the first heating section, the cold-rolled sheet is heated from a start temperature to a recrystallization temperature at a first average heating rate, and in the second heating section, the cold-rolled sheet is heated from the recrystallization temperature to a target temperature at a second average heating rate that is faster than the first average heating rate.

Description

[Technical Field]

[0001] The present invention relates to a non-oriented electrical steel sheet and a method for producing the same. [Background technology]

[0002] Recently, there has been an increasing demand for environmental conservation and improved energy efficiency. In particular, the shift from internal combustion engine vehicles to electric vehicles or hybrid vehicles is accelerating.

[0003] Non-oriented electrical steel sheet is a material that has uniform magnetic properties in all directions regardless of the rolling direction, and for energy efficiency, it is necessary to reduce iron loss and increase magnetic flux density.

[0004] The manufacturing process of non-oriented electrical steel sheet varies depending on the silicon (Si) content, but if the silicon (Si) content exceeds 2.0 wt%, the brittleness increases and fractures occur during cold rolling. Therefore, an APL (Annealing and Picking Line) process is required before the final cold rolling.

[0005] Related technologies include Korean Patent Publication No. 10-2021-0094027 (Name of invention: Manufacturing method for non-oriented electrical steel sheet). Summary of the Invention [Problem to be solved by the invention]

[0006] According to an embodiment of the present invention, a non-oriented electrical steel sheet having improved magnetic properties can be manufactured by controlling the temperature rise rate in the cold rolling annealing step. [Means for solving the problem]

[0007] In one embodiment of the present invention, the method includes the steps of hot rolling a slab containing, in weight percent, carbon (C): more than 0% and 0.005% or less, silicon (Si): 2.0% or more and 4.0% or less, manganese (Mn): 0.1% or more and 0.5% or less, aluminum (Al): 0.9% or more and 1.5% or less, phosphorus (P): more than 0% and 0.015% or less, sulfur (S): more than 0% and 0.005% or less, nitrogen (N): more than 0% and 0.005% or less, titanium (Ti): more than 0% and 0.005% or less, the balance being iron (Fe) and unavoidable impurities; and a pre-annealing step of pre-annealing a hot-rolled sheet; a cold-rolling step of cold-rolling the pre-annealed hot-rolled annealed sheet; and a cold-rolling annealing step of cold-rolling the cold-rolled sheet, wherein the cold-rolling annealing step includes a first heating section, a second heating section, and a soaking section, wherein in the first heating section, the cold-rolled sheet is heated from a start temperature to a recrystallization temperature at a first average heating rate, and in the second heating section, the cold-rolled sheet is heated from the recrystallization temperature to a target temperature at a second average heating rate that is faster than the first average heating rate.

[0008] In this embodiment, the first average heating rate may be greater than 5° C. / s and less than 20° C. / s.

[0009] In this embodiment, the second average heating rate may be 15° C. / s or more and 30° C. / s or less.

[0010] In this embodiment, the recrystallization temperature may be 750°C to 800°C.

[0011] In this embodiment, the target temperature may be 850°C to 1,050°C.

[0012] In this embodiment, the cold-rolling and annealing step may further include a cooling section, in which the cold-rolled and annealed sheet may be cooled at a cooling rate of 30° C. / s or more.

[0013] In this embodiment, the texture of the non-oriented electrical steel sheet <111> / / ND orientation fraction can be less than 30%.

[0014] In this embodiment, the texture of the non-oriented electrical steel sheet <100> / / ND orientation fraction can be 20% or more.

[0015] In this embodiment, the average grain size of the non-oriented electrical steel sheet may be 100 μm or more and 130 μm or less.

[0016] In another embodiment of the present invention, the alloy contains, in weight percent, carbon (C): more than 0% and not more than 0.005%, silicon (Si): 2.0% or more and 4.0% or less, manganese (Mn): 0.1% or more and not more than 0.5%, aluminum (Al): 0.9% or more and not more than 1.5%, phosphorus (P): more than 0% and not more than 0.015%, sulfur (S): more than 0% and not more than 0.005%, nitrogen (N): more than 0% and not more than 0.005%, titanium (Ti): more than 0% and not more than 0.005%, the balance being iron (Fe) and unavoidable impurities, and the alloy contains the alloy containing ... <111> / / ND orientation fraction is 30% or less, and the texture <100> / / Non-oriented electrical steel sheet with an ND orientation fraction of 20% or more is provided.

[0017] In this embodiment, the non-oriented electrical steel sheet may have an iron loss (W10 / 400 standard) of 13.0 W / kg or less and a magnetic flux density (B50 standard) of 1.68 T or more.

[0018] In this embodiment, the non-oriented electrical steel sheet may have a yield strength (YP) of 400 MPa or more and a tensile strength (TS) of 500 MPa or more.

[0019] Other aspects, features, and advantages beyond those described above will become apparent from the following detailed description of the invention, the claims, and the drawings. [Effects of the Invention]

[0020] According to one embodiment of the present invention, a non-oriented electrical steel sheet having improved magnetic properties can be manufactured by controlling the temperature rise rate in the cold rolling and annealing step. However, the scope of the present invention is not limited to such an effect. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a flowchart schematically illustrating a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention. [Figure 2] 1 is a diagram showing the state depending on the composition of silicon (Si). [Figure 3] 1 is a diagram showing the magnetization velocity for each orientation of the texture. [Figure 4] <100> Orientation and <111> 1 is a diagram showing an orientation hysteresis loop. [Figure 5] 1 is a diagram showing magnetic flux density depending on the orientation of the texture. [Figure 6A] This is a diagram of the microstructure observed by EBSD at different heat treatment temperatures. [Figure 6B] This is a diagram of the microstructure observed by EBSD at different heat treatment temperatures. [Figure 6C] This is a diagram of the microstructure observed by EBSD at different heat treatment temperatures. [Figure 6D] This is a diagram of the microstructure observed by EBSD at different heat treatment temperatures. [Figure 6E] This is a diagram of the microstructure observed by EBSD at different heat treatment temperatures. [Figure 6F] This is a diagram of the microstructure observed by EBSD at different heat treatment temperatures. [Figure 6G] This is a diagram of the microstructure observed by EBSD at different heat treatment temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention can be modified in various ways and can have various embodiments, and specific embodiments are illustrated in the drawings and will be described in detail in the detailed description. The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the embodiments described in detail below with the drawings. However, the present invention is not limited to the embodiments described below, and can be embodied in various forms.

[0023] In the following embodiments, terms such as first and second are not used in a limiting sense but are used to distinguish one component from another.

[0024] In the following embodiments, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0025] In the following embodiments, terms such as "comprise" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0026] In the following embodiments, when a part such as a film, region, or component is said to be on or above another part, this includes not only the case where it is directly on top of the other part, but also the case where another film, region, component, etc. is interposed between them.

[0027] In the drawings, the size of components may be exaggerated or reduced for the sake of convenience of explanation. For example, the size and thickness of each component shown in the drawings are arbitrarily shown for the sake of convenience of explanation, and the present invention is not necessarily limited to what is shown.

[0028] In this specification, "A and / or B" refers to A, B, or A and B. In addition, in this specification, "at least one of A and B" refers to A, B, or A and B.

[0029] In the following embodiments, the term "extending in a first or second direction" means that the wiring extends not only in a straight line but also in a zigzag or curved line along the first or second direction.

[0030] In the following embodiments, "plan view" means a view of a target portion from above, and "cross-sectional view" means a view of a target portion cut vertically from the side. In the following embodiments, "superimposition" includes "plan view" and "cross-sectional view" superimposition.

[0031] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description with reference to the drawings, the same or corresponding elements are designated by the same reference numerals.

[0032] FIG. 1 is a flow chart that schematically illustrates a method for producing a non-oriented electrical steel sheet according to one embodiment of the present invention.

[0033] Referring to FIG. 1, a method for manufacturing a non-oriented electrical steel sheet according to an embodiment includes a hot rolling step (S100), a pre-annealing step (S200), a cold rolling step (S300), a cold rolling annealing step (S400), and a coating step (S500).

[0034] In a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention, a semi-finished product to be hot-rolled is a slab. The semi-finished slab can be obtained through a continuous casting process after molten steel having a predetermined composition is obtained through a steelmaking process.

[0035] In one embodiment, the slab contains, by weight, carbon (C): greater than 0% and less than 0.005%, silicon (Si): 2.0% to 4.0%, manganese (Mn): 0.1% to 0.5%, aluminum (Al): 0.9% to 1.5%, phosphorus (P): greater than 0.015%, sulfur (S): greater than 0% and less than 0.005%, nitrogen (N): greater than 0% and less than 0.005%, titanium (Ti): greater than 0% and less than 0.005%, the balance being iron (Fe), and unavoidable impurities.

[0036] Carbon (C) is a component that forms carbides such as TiC and NbC, increasing iron loss. In one embodiment, carbon (C) is included in an amount greater than 0% and less than 0.005% by weight based on the total weight of the slab. If carbon (C) is included in an amount greater than 0.005% by weight based on the total weight of the slab, magnetic aging may occur, degrading the magnetic properties of the manufactured non-oriented electrical steel sheet. If carbon (C) is included in an amount greater than 0% and less than 0.005% by weight based on the total weight of the slab, magnetic aging is suppressed.

[0037] FIG. 2 is a diagram showing the state according to the composition of silicon (Si).

[0038] As shown in Figure 2, when the silicon (Si) content is 2.0 wt% or more, a ferrite single phase can be maintained throughout the entire region without any phase transformation. On the other hand, when the silicon (Si) content is less than 2.0 wt%, an austenite phase exists in some regions, and a phase transformation occurs during heat treatment at a target temperature of approximately 950°C, as described below. If a phase transformation occurs during the heat treatment process, the texture orientation distribution will change, so it is preferable to limit the silicon (Si) content to a composition range that does not cause any phase transformation.

[0039] Silicon (Si) is a component that increases resistivity and reduces eddy current loss. In one embodiment, silicon (Si) is included in an amount of 2.0% to 4.0% by weight based on the total weight of the slab. If silicon (Si) is included in an amount less than 2.0% based on the total weight of the slab, it is difficult to achieve a low core loss value. Meanwhile, as the silicon (Si) content in the slab increases, magnetic permeability and magnetic flux density decrease. Furthermore, if silicon (Si) is included in an amount greater than 4.0% based on the total weight of the slab, brittleness increases, cold rolling ability deteriorates, and productivity may decrease.

[0040] Manganese (Mn), together with silicon (Si), is a component that increases resistivity and improves texture. In one embodiment, manganese (Mn) is included in an amount of 0.1% to 0.5% by weight based on the total weight of the slab. When manganese (Mn) is included in an amount less than 0.1% by weight based on the total weight of the slab, fine MnS precipitates are formed, suppressing grain growth. On the other hand, when manganese (Mn) is included in an amount greater than 0.5% by weight based on the total weight of the slab, coarse MnS precipitates are formed, reducing magnetic flux density and other magnetic properties. When manganese (Mn) is included in an amount of 0.1% to 0.5% by weight based on the total weight of the slab, the microstructure and texture within the slab (or non-oriented electrical steel sheet) can be controlled.

[0041] Aluminum (Al), together with silicon (Si), increases resistivity and reduces eddy current loss. Aluminum (Al) also reduces magnetic anisotropy and magnetic deviation. In one embodiment, aluminum (Al) is included in an amount of 0.9% to 1.5% by weight of the total slab. If the aluminum (Al) content is less than 0.9% by weight of the slab, it is difficult to achieve a low core loss value. Furthermore, fine nitrides may be formed, increasing magnetic property deviation. On the other hand, if the aluminum (Al) content exceeds 1.5% by weight of the slab, cold rolling performance may be reduced, and excessive nitrides may be formed, reducing magnetic flux density and degrading magnetic properties.

[0042] Phosphorus (P) is a grain boundary segregation element that develops texture. In one embodiment, phosphorus (P) is contained in an amount of more than 0% and not more than 0.015% by weight based on the total weight of the slab. If phosphorus (P) is contained in an amount of more than 0.015% based on the total weight of the slab, the segregation effect can inhibit grain growth, degrade magnetic properties, and reduce cold rolling ability.

[0043] Sulfur (S) forms precipitates such as MnS and CuS, which increase iron loss and suppress grain growth. In one embodiment, sulfur (S) is contained in an amount of more than 0% and not more than 0.005% by weight based on the total weight of the slab. If sulfur (S) is contained in an amount of more than 0.005% based on the total weight of the slab, precipitates such as MnS and CuS are formed, which increases iron loss and suppresses grain growth.

[0044] Nitrogen (N) forms precipitates such as AlN, TiN, and NbN, which increase iron loss and suppress grain growth. In one embodiment, nitrogen (N) is contained in an amount greater than 0% and less than 0.005% by weight based on the total weight of the slab. If nitrogen (N) is contained in an amount greater than 0.005% based on the total weight of the slab, precipitates such as AlN, TiN, and NbN are formed, which increases iron loss and suppresses grain growth.

[0045] Titanium (Ti) forms precipitates such as TiC and TiN, which inhibits grain growth. In one embodiment, titanium (Ti) is included in an amount greater than 0% and less than 0.005% by weight based on the total weight of the slab. If titanium (Ti) is included in an amount greater than 0.005% based on the total weight of the slab, precipitates such as TiC and TiN may form, which may degrade the magnetic properties.

[0046] In the hot rolling step (S100), a slab is reheated and then hot-rolled to obtain a hot-rolled sheet. First, in the hot rolling step (S100), a slab can be reheated. If the slab heating temperature is excessively high, precipitates such as C, S, and N in the slab are reused and fine precipitates are formed in the subsequent rolling and annealing steps. This inhibits grain growth and deteriorates magnetic properties. On the other hand, if the slab heating temperature is excessively low, the rolling load increases during hot rolling, resulting in poor rollability. In one embodiment, the slab reheating temperature in the hot rolling step (S100) is about 1,000°C to about 1,200°C.

[0047] In addition, in the hot rolling step (S100), the reheated slab can be rolled at a predetermined finishing delivery temperature. In one embodiment, the finishing delivery temperature (FDT) in the hot rolling step (S100) is about 860°C to about 900°C.

[0048] In the hot rolling step (S100), the hot rolled sheet can be cooled to a predetermined coiling temperature (CT) and then coiled. In one embodiment, the coiling temperature is about 550°C to about 650°C.

[0049] In one embodiment, the thickness of the hot-rolled sheet after hot rolling is about 1.8 mm to about 2.6 mm. If the thickness of the hot-rolled sheet exceeds about 2.6 mm, the cold rolling reduction increases, which may deteriorate the texture.

[0050] After the hot rolling step (S100), a pre-annealing step (S200) is performed. In the pre-annealing step (S200), the hot-rolled sheet that has been coiled and cooled can be pre-annealed. In this case, the pre-annealed hot-rolled sheet is also called a hot-rolled annealed sheet. Through the pre-annealing step (S200), uniformity of the microstructure of the hot-rolled sheet and cold rolling properties can be ensured.

[0051] The pre-annealing step (S200) is performed at an annealing temperature of about 950°C to about 1,100°C, a holding time of about 30 seconds to about 120 seconds, and a heating rate of about 20°C / s or more. If the annealing temperature in the pre-annealing step (S200) is too low, fine inclusions such as carbides and nitrides may form in the surface layer and not grow sufficiently, which may result in a deterioration in the magnetic properties of the final product. On the other hand, if the annealing temperature in the pre-annealing step (S200) is too high, not only the inclusion distribution but also the crystal grains may grow excessively, resulting in large crystal grain size deviation and excessive oxidation, which may adversely affect the final product.

[0052] In the pre-annealing step (S200), the hot-rolled annealed steel sheet can be cooled at a cooling rate of about 30°C / s. At this time, the hot-rolled annealed steel sheet is cooled to about 200°C to about 250°C. In addition, an oxide layer formed on the surface of the hot-rolled annealed steel sheet after the pre-annealing step (S200) can be removed using a pickling solution.

[0053] After the pre-annealing step (S200), the cold rolling step (S300) is performed. In the cold rolling step (S300), the hot-rolled and annealed sheet may be cold-rolled. In this case, the cold-rolled hot-rolled and annealed sheet may also be referred to as a cold-rolled sheet. In the cold rolling step (S300), the pickled hot-rolled and annealed sheet may be cold-rolled to a thickness of about 0.35 mm or less. In this case, to impart rollability, the sheet temperature (e.g., the temperature of the hot-rolled and annealed sheet) may be increased to about 150°C to about 200°C, and then warm rolling may be performed. The final reduction in the cold rolling step (S300) is about 80% to about 85%.

[0054] FIG. 3 shows the magnetization velocity for each orientation of the texture. <100> direction, <110> Orientation, and <111> 1 is a diagram showing magnetization velocity in azimuth.

[0055] Referring to Figure 3, <100> direction, <110> Orientation, and <111> Among the directions, <100> It can be seen that the magnetization speed in the azimuth direction is the fastest. <100> direction, <110> Orientation, and <111> Among the directions, <100> It can be seen that the azimuth direction is the easiest to magnetize. <100> Orientation and <111> Among the directions, <100> The direction <111> It can be confirmed that this has advantages in terms of magnetic properties compared to the azimuth direction.

[0056] Figure 4 shows <100> Orientation and <111> 1 is a diagram showing an orientation hysteresis loop.

[0057] In Figure 4, the area enclosed by the hysteresis loop represents the energy loss per unit volume. That is, the larger the area enclosed by the hysteresis loop, the greater the energy loss per unit volume.

[0058] Referring to Figure 4, <100> The area of ​​the hysteresis loop of the orientation is <111> It can be confirmed that the area is smaller than the area of ​​the azimuthal hysteresis loop. <100> The direction <111> It can be seen that the iron loss is lower and the magnetic flux density is higher than in the axial direction.

[0059] FIG. 5 is a diagram showing magnetic flux density as a function of the orientation of the texture.

[0060] Referring to Figure 5, <111> The average magnetic flux density in the direction is the lowest, <100> It can be seen that the average magnetic flux density in the direction of the grain boundary is the highest. <100> When the orientation increases, the magnetic flux density of the non-oriented electrical steel sheet containing the same can increase.

[0061] Generally, when a cold-rolled sheet is heat-treated, the microstructure of the cold-rolled annealed sheet (or final annealed sheet) is formed through the recovery, recrystallization, and growth processes. Furthermore, when heat-treated near the recrystallization temperature, nucleation and growth for recovery and recrystallization occur in competition. During the heat treatment process, the heating rate affects the recovery, nucleation, and recrystallization processes.

[0062] Since the accumulated deformation energy varies depending on the position, if the heating rate is fast, recrystallization and growth will occur preferentially in the area with high deformation energy. <111> / / ND orientation is formed in large amounts, and regions with low deformation energy are relatively delayed and can disappear during the growth stage.

[0063] The texture after the final cold rolling is composed of two orientations, α-fiber and γ-fiber. During the heat treatment in the cold rolling annealing stage (S400), when passing through the recrystallization temperature zone, nucleation and recrystallization proceed preferentially at the γ-fiber position where the deformation energy is relatively high. At this time, there is a problem that the magnetic properties are unfavorable at this position. <111> / / ND-oriented grains were formed and preferentially formed <111> / / The ND texture tends to grow preferentially in the growth stage after recrystallization. Therefore, the texture of the final cold-rolled and annealed sheet <111> / / ND orientation is strongly expressed, resulting in deterioration of iron loss and magnetic flux density.

[0064] On the other hand, if the heating rate is slow, recovery and recrystallization occur uniformly throughout the entire region, resulting in <100> / / ND direction and <111> / / ND orientation will grow through competition.

[0065] Therefore, by knowing the time when recrystallization is completed, it is possible to control the rate of temperature increase before the recrystallization temperature is reached, thereby controlling the orientation of the texture.

[0066] Figures 6A through 6G are electron backscatter diffraction (EBSD) photographs of the microstructures at different heat treatment temperatures. Specifically, Figure 6A is an EBSD photograph of the microstructure at a heat treatment temperature of 600°C, Figure 6B is an EBSD photograph of the microstructure at a heat treatment temperature of 650°C, Figure 6C is an EBSD photograph of the microstructure at a heat treatment temperature of 700°C, Figure 6D is an EBSD photograph of the microstructure at a heat treatment temperature of 750°C, Figure 6E is an EBSD photograph of the microstructure at a heat treatment temperature of 800°C, Figure 6F is an EBSD photograph of the microstructure at a heat treatment temperature of 850°C, and Figure 6G is an EBSD photograph of the microstructure at a heat treatment temperature of 950°C.

[0067] 6A to 6G, when a cold-rolled sheet is heat-treated, recrystallization is completed at about 800°C.

[0068] Therefore, the temperature rise rate at about 800° C. or less can be controlled to control the orientation of the texture.

[0069] After the cold rolling step (S300), a cold rolling annealing step (S400) is performed. In the cold rolling annealing step (S400), the cold rolled cold rolled sheet may be annealed. In this case, the annealed cold rolled sheet is also referred to as an annealed cold rolled sheet.

[0070] In one embodiment, the cold roll annealing step (S400) includes a temperature rising section, a soaking section, and a cooling section. The temperature rising section includes a first temperature rising section and a second temperature rising section. That is, the cold roll annealing step (S400) includes a first temperature rising section, a second temperature rising section, a soaking section, and a cooling section. In this regard, the temperature rising section refers to a section in which the cold rolled sheet is heated to increase its temperature, the soaking section refers to a section in which the cold rolled sheet is soaked to a target temperature, and the cooling section refers to a section in which the soaked cold rolled sheet is cooled.

[0071] In one embodiment, the cold-rolled cold-rolled sheet is heated in the heating section of the cold-rolling annealing step (S400). At this time, the heating section includes a first heating section and a second heating section. That is, the heating section is divided into the first heating section and the second heating section. The average heating rates of the first heating section and the second heating section are different from each other.

[0072] In one embodiment, in the first heating section, the cold-rolled sheet is heated (or heated) from the starting temperature to the recrystallization temperature at a first average heating rate. The starting temperature is room temperature. For example, the starting temperature is about 15°C to about 25°C. The recrystallization temperature is the temperature at which recrystallization of the texture in the cold-rolled sheet is completed. For example, the recrystallization temperature is about 750°C to about 800°C. However, the present invention is not limited thereto.

[0073] Furthermore, the first average heating rate is greater than about 5°C / s and less than about 20°C / s. More preferably, the first average heating rate is greater than about 10°C / s and less than about 15°C / s. If the first average heating rate is less than about 5°C / s, the low heating rate can cause excessive grain growth, increasing eddy current loss and resulting in little improvement in magnetic properties. In addition, as the heat treatment time increases, productivity and process costs can increase. On the other hand, if the first average heating rate is greater than about 20°C / s, the heating rate to the recrystallization temperature is too high (e.g., too fast), resulting in fine primary recrystallization. <111> / / ND orientation texture is preferentially formed and grown. <111> / / The ND fraction increases, which may result in increased core loss and decreased magnetic flux density.

[0074] Therefore, when the first average heating rate is more than about 5°C / s and less than about 20°C / s (or 5°C / s to 20°C / s), a non-oriented electrical steel sheet with excellent magnetic properties can be manufactured. Specifically, when the heating rate at or below the temperature at which the recrystallization of the microstructure is completed is more than about 5°C / s and less than about 20°C / s, <100> / / ND direction and <111> / / ND orientation can grow while competing with each other, so <100> / / The proportion of ND orientation increases, and the non-oriented electrical steel sheet manufactured through this can have low core loss and high magnetic flux density.

[0075] In one embodiment, in the second heating section, the cold-rolled sheet heated (or heated) through the first heating section is heated (or heated) from the recrystallization temperature to the target temperature at a second average heating rate. As described above, the recrystallization temperature is the temperature at which recrystallization of the texture in the cold-rolled sheet is completed. For example, the recrystallization temperature is about 750°C to about 800°C. However, the present invention is not limited thereto.

[0076] In one embodiment, the target temperature is the temperature at which the heated (or heated) cold-rolled sheet is soaked (or annealed), and is about 850°C to about 1,050°C. If the target temperature in the cold-rolling annealing step (S400) is too low, the grain size may be too fine and hysteresis loss may increase. On the other hand, if the target temperature in the cold-rolling annealing step (S400) is too high, the grain size may be too large and eddy current loss may increase.

[0077] In one embodiment, the second average heating rate is greater than the first average heating rate. That is, the average heating rate in the second heating section is greater than the average heating rate in the first heating section. The second average heating rate is greater than about 15°C / s and less than about 30°C / s. More preferably, the second average heating rate exceeds the first average heating rate but is less than about 30°C / s. If the second average heating rate is the same as or less than the first average heating rate, productivity and process costs may increase as the heat treatment time increases. On the other hand, if the second average heating rate exceeds about 30°C / s, <111> / / The ND fraction increases, which can result in increased core losses and decreased magnetic flux density. This will be discussed in more detail later.

[0078] Therefore, when the second average heating rate is about 15° C. / s or more and about 30° C. / s or less, a non-oriented electrical steel sheet with excellent magnetic properties is produced.

[0079] In one embodiment, the target temperature holding time is about 40 seconds to about 200 seconds, but the present invention is not limited thereto. For example, the total time for performing the cold rolling annealing step (S400) may be about 40 seconds to about 200 seconds.

[0080] Next, in the cooling section, the cold-rolled annealed steel sheet can be cooled at a cooling rate of about 30°C / s or more, and the cold-rolled annealed steel sheet is cooled to about 200°C to about 250°C.

[0081] In one embodiment, the cold rolling annealing step (S400) is performed in a mixed atmosphere of nitrogen and hydrogen, specifically, in a gas atmosphere consisting of about 5% to about 40% by volume of hydrogen and the remainder being nitrogen.

[0082] After the cold rolling and annealing step (S400), the coating step (S500) is performed. In the coating step (S500), a coating layer can be formed on the annealed cold rolled and annealed sheet. By forming the coating layer through the coating step (S500), punching properties can be improved and insulation properties can be ensured.

[0083] The non-oriented electrical steel sheet manufactured by the method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention has an average grain size of about 100 μm to about 130 μm. The manufactured non-oriented electrical steel sheet may have an iron loss (W10 / 400 basis) of about 13.0 W / kg or less and a magnetic flux density (B50 basis) of about 1.68 T or more. In addition, the manufactured non-oriented electrical steel sheet may have a yield strength (YP) of about 400 MPa or more and a tensile strength (TS) of about 500 MPa or more.

[0084] <Experimental Example> The present invention will be described in more detail through the following experimental examples. However, the following experimental examples are intended to further explain the present invention, and the scope of the present invention is not limited by the following experimental examples. The following experimental examples can be appropriately modified and changed by those skilled in the art within the scope of the present invention.

[0085] [Table 1]

[0086] [Table 2]

[0087] Table 1 is a slab composition table including main components and impurities. Example 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3 were all produced using the slabs in Table 1.

[0088] Referring to Tables 1 and 2, a slab containing the chemical composition shown in Table 1 was heated to approximately 1,150°C and hot-rolled under conditions of an FDT (finish rolling temperature) of approximately 890°C and a CT (coiling temperature) of approximately 610°C to produce a hot-rolled sheet having a thickness of approximately 2.0 mm. The hot-rolled hot-rolled sheet was then pre-annealed at 1,050°C for 60 seconds and then pickled. The hot-rolled annealed sheet was then cold-rolled to produce a cold-rolled sheet having a thickness of approximately 0.25 mm, and cold-rolled annealing was performed at the first average heating rate, second average heating rate, and target temperature shown in Table 2. The target temperature holding time was approximately 30 seconds, the cooling rate was 30°C / s, and the cold-rolled annealing was performed in a mixed atmosphere of 30% hydrogen and 70% nitrogen.

[0089] [Table 3]

[0090] Table 3 shows the crystal grain sizes of Example 1, Example 2, and Comparative Examples 1 to 3. <111> / / ND orientation fraction (area %), <100> / / This is a table showing the measurement results of ND orientation fraction (area %), iron loss, and magnetic flux density. <111> / / ND azimuth fraction, and <100> The ND orientation fraction was measured using EBSD, and the measured data was calculated using TSL OIM analysis software. However, the present invention is not limited to this method. Magnetic measurements were performed using a single sheet tester (SST), and the iron loss and magnetic flux density values ​​were measured in the L and C directions and then averaged. In Table 3, B50 is the magnetic flux density at 5000 A / m, and W10 / 400 is the iron loss at a frequency of 400 Hz and a magnetic flux density of 1.0 Tesla.

[0091] Referring to Tables 2 and 3, the slower the first average heating rate in the first heating section of the cold rolling annealing step (S400), <111> / / The azimuth fraction decreases, <100> / / It can be seen that the ND orientation fraction increases. However, if the first average heating rate in the first heating section of the cold rolling annealing step (S400) is too small, the grains grow excessively, and the low heating rate increases the heat treatment time, which can reduce productivity and increase process costs.

[0092] In addition, in the case of Example 1 and Example 2, the crystal grain size is 123 μm and 117 μm, respectively, and it can be confirmed that the grain size satisfies the condition of about 100 μm or more and about 130 μm or less. <111> / / ND orientation fraction is 30% or less in both cases, and the texture <100> / / It can be confirmed that the ND orientation fraction is 20% or more. In addition, in the case of Example 1 and Example 2, it can be confirmed that the iron loss is 13.0 W / kg or less and the magnetic flux density is 1.68 T or more.

[0093] If the first average heating rate is more than 5°C / s and less than 20°C / s, the grain size and texture <111> / / ND orientation fraction, texture <100> / / It can be confirmed that the ND orientation fraction, iron loss, and magnetic flux density meet the desired conditions.

[0094] Specifically, when the first average heating rate is more than 5°C / s and less than 20°C / s, the crystal grain size is about 100 μm or more and about 130 μm or less, and the texture is <111> / / ND orientation fraction decreases, and texture <100> / / ND orientation fraction increases. As discussed above, the texture <100> / / ND direction is <111> / / Since it has better magnetic properties than the ND orientation, if the first average heating rate is more than 5℃ / s and less than 20℃ / s, the texture <111> / / ND orientation fraction decreases, and texture <100> / / As the proportion of ND orientation increases, the iron loss of the manufactured non-oriented electrical steel sheet can be reduced and the magnetic flux density can be improved.

[0095] However, in the case of Comparative Example 1, the first average heating rate was 5°C / s, and the low heating rate caused excessive growth of crystal grains. The low heating rate also prolonged the heat treatment time, which may result in reduced productivity and increased process costs.

[0096] In addition, in the cases of Comparative Example 2 and Comparative Example 3, the first average heating rate is 20°C / s or more, <111> / / The ND orientation fraction is large and the texture is <100> / / It can be seen that the ND orientation fraction is low, resulting in high iron loss and low magnetic flux density.

[0097] Therefore, when the first average heating rate in the first heating section of the cold rolling annealing step (S400) is greater than about 5°C / s and less than about 20°C / s, more preferably greater than about 10°C / s and less than about 15°C / s, the manufactured non-oriented electrical steel sheet may have an iron loss (W10 / 400 basis) of about 13.0 W / kg or less and a magnetic flux density (B50 basis) of about 1.68 T or more. In other words, when the first average heating rate in the first heating section of the cold rolling annealing step (S400) is greater than about 5°C / s and less than about 20°C / s, a non-oriented electrical steel sheet with excellent magnetic properties is manufactured.

[0098] Although the present invention has been described with reference to the embodiments shown in the drawings, these are merely illustrative, and those skilled in the art will recognize that various modifications and equivalent embodiments are possible therefrom. Therefore, the true technical scope of protection of the present invention should be determined by the technical spirit of the claims.

Claims

1. hot rolling a slab containing, by weight, carbon (C): more than 0% and not more than 0.005%, silicon (Si): 2.0% to 4.0%, manganese (Mn): 0.1% to 0.5%, aluminum (Al): 0.9% to 1.5%, phosphorus (P): more than 0% and not more than 0.015%, sulfur (S): more than 0% and not more than 0.005%, nitrogen (N): more than 0% and not more than 0.005%, titanium (Ti): more than 0% and not more than 0.005%, the balance being iron (Fe) and unavoidable impurities; pre-annealing the hot-rolled sheet; cold rolling the pre-annealed hot-rolled annealed sheet; and cold-rolling annealing the cold-rolled sheet, The cold rolling annealing step includes a first heating section, a second heating section, and a soaking section, In the first heating section, the cold-rolled sheet is heated from the start temperature to the recrystallization temperature at a first average heating rate, In the second heating section, the cold-rolled sheet is heated from the recrystallization temperature to the target temperature at a second average heating rate that is faster than the first average heating rate.

2. The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the first average heating rate is more than 5°C / s and less than 20°C / s.

3. The method for producing a non-oriented electrical steel sheet according to claim 2, wherein the second average heating rate is 15°C / s or more and 30°C / s or less.

4. The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the recrystallization temperature is 750°C to 800°C.

5. The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the target temperature is 850°C to 1,050°C.

6. The cold rolling annealing step further includes a cooling section, The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the cold-rolled annealed steel sheet is cooled at a cooling rate of 30°C / s or more in the cooling section.

7. The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the <111> / / ND orientation fraction of the texture of the non-oriented electrical steel sheet is 30% or less.

8. The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the texture of the non-oriented electrical steel sheet has a <100> / / ND orientation fraction of 20% or more.

9. The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the average grain size of the non-oriented electrical steel sheet is 100 μm or more and 130 μm or less.

10. A non-oriented electrical steel sheet, The alloy contains, by weight, carbon (C): more than 0% and not more than 0.005%, silicon (Si): 2.0% or more and 4.0% or less, manganese (Mn): 0.1% or more and 0.5% or less, aluminum (Al): 0.9% or more and 1.5% or less, phosphorus (P): more than 0% and not more than 0.015%, sulfur (S): more than 0% and not more than 0.005%, nitrogen (N): more than 0% and not more than 0.005%, titanium (Ti): more than 0% and not more than 0.005%, and the balance being iron (Fe) and unavoidable impurities, A non-oriented electrical steel sheet having a texture with a <111> / / ND orientation fraction of 30% or less and a texture with a <100> / / ND orientation fraction of 20% or more.

11. The non-oriented electrical steel sheet according to claim 10, wherein the non-oriented electrical steel sheet has an iron loss (W10 / 400 standard) of 13.0 W / kg or less and a magnetic flux density (B50 standard) of 1.68 T or more.

12. 11. The non-oriented electrical steel sheet according to claim 10, wherein the non-oriented electrical steel sheet has a yield strength (YP) of 400 MPa or more and a tensile strength (TS) of 500 MPa or more.

Images