Non-oriented electrical steel sheet and method for producing same

Abstract

A non-oriented electrical steel sheet according to one embodiment of the present invention comprises, in wt%, 1.5 to 4.5% of Si, 0.1 to 2.0% of Al, and 0.1 to 2.0% of Mn, with the remainder comprising Fe and unavoidable impurities, and comprises an oxide layer present from the surface toward the inside, the length of an oxide layer interruption portion in which the thickness of the oxide layer is 2 nm or less in a cross section including the rolling direction in the steel sheet is 5-500 nm per 200 [mu] m in the rolling direction.

Description

Technical Field

[0001] One embodiment of the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same. Specifically, one embodiment of the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same, wherein a suitable oxide layer is formed on the surface of the steel sheet by adjusting the dew point and tension during a pre-cold rolling annealing process, thereby improving its magnetic properties. Background Technology

[0002] Non-oriented electrical steel sheets are mainly used in motors that convert electrical energy into mechanical energy. To achieve high efficiency in the conversion process, non-oriented electrical steel sheets need to have excellent magnetic properties. In recent years, in particular, with the increasing attention paid to environmentally friendly vehicles that replace internal combustion engines with electric motors, the demand for non-oriented electrical steel sheets used as core materials for drive motors has been continuously increasing. Therefore, non-oriented electrical steel sheets with both excellent magnetic properties and strength are required.

[0003] The magnetic properties of non-oriented electrical steel sheets are mainly evaluated using iron loss and magnetic flux density. Iron loss refers to the energy loss that occurs at a specific magnetic flux density and frequency, while magnetic flux density refers to the degree of magnetization obtained under a specific magnetic field. Lower iron loss allows for the manufacture of more energy-efficient motors under the same conditions, while higher magnetic flux density enables motor miniaturization or reduces copper losses. Therefore, using non-oriented electrical steel sheets with low iron loss and high magnetic flux density allows for the manufacture of drive motors with excellent efficiency and torque, thereby improving the driving range and output of environmentally friendly vehicles.

[0004] Depending on the operating conditions of the motor, the properties of the non-oriented electrical steel sheet to be considered will also change. As a standard for evaluating the properties of non-oriented electrical steel sheets used in motors, the iron loss W15 / 50 under a 1.5T magnetic field at a commercial frequency of 50Hz is widely adopted. However, for non-oriented electrical steel sheets with a thickness of less than 0.35mm used in environmentally friendly automotive drive motors, low magnetic fields of 1.0T or less and magnetic properties at high frequencies above 400Hz are often more important. Therefore, in many cases, the W10 / 400 iron loss is used to evaluate the properties of the non-oriented electrical steel sheet. Furthermore, as the rotational speed increases, strength characteristics, which were previously less important, are now considered crucial.

[0005] To improve the magnetic properties of non-oriented electrical steel sheets, alloying elements such as Si, Al, and Mn are commonly added. By adding these elements, the resistivity of the steel increases, reducing eddy current losses and lowering total iron losses. Furthermore, these alloying elements, acting as substitutes dissolved in the iron, exert a strengthening effect, thereby increasing strength. On the other hand, as the amount of Si, Al, and Mn added increases, the magnetic flux density decreases, leading to increased brittleness. If a certain amount is added, cold rolling becomes impossible, hindering industrial production. In particular, for electrical steel sheets, thinner sheets offer better high-frequency iron losses, but the reduced rollability due to brittleness becomes a fatal problem. The maximum known sum of Si, Al, and Mn content for industrial production is approximately 4.5% by weight. Beyond this, by optimizing the content of trace elements, the highest grade of non-oriented electrical steel sheets with excellent magnetic properties and strength can be produced.

[0006] However, when large amounts of high resistivity alloying elements such as Si, Al, Mn, and Cr are added, a decrease in magnetic flux density occurs. This is particularly problematic for materials with high magnetic flux density, which are essential for applications like electric vehicle drive motors that continuously demand lightweight construction.

[0007] To address this, a method for improving properties by thinning hot-rolled plates was proposed, along with a method for improving magnetic properties by incorporating high-alumina content and undergoing two annealing and two rolling processes. Additionally, a method for achieving hot-rolled thinning through a thin slab manufacturing process was also proposed.

[0008] However, methods for reducing the thickness of hot-rolled plates are difficult to mass-produce in conventional hot-rolling processes due to increased rolling load. Furthermore, the addition of high-alumina steel, along with double annealing and double rolling processes, has been shown to improve magnetic properties to some extent, but {110} <001> The Gaussian texture is also highly developed, which degrades the axial characteristics of the motor and greatly increases surface defects caused by the addition of high aluminum. Summary of the Invention

[0009] (a) Technical problems to be solved One embodiment of the present invention provides a non-oriented electrical steel sheet and a method for manufacturing the same. Specifically, one embodiment of the present invention provides a non-oriented electrical steel sheet and a method for manufacturing the same, wherein a suitable oxide layer is formed on the surface of the steel sheet by adjusting the dew point and tension during a pre-cold rolling annealing process, thereby improving its magnetic properties.

[0010] (II) Technical Solution According to an embodiment of the present invention, the non-oriented electrical steel sheet comprises, by weight %, Si: 1.5 to 4.5%, Al: 0.1 to 2.0%, Mn: 0.1 to 2.0%, with the balance including Fe and unavoidable impurities, and includes an oxide layer present from the surface inward direction. The length of the oxide layer interruption portion with a thickness of less than 2 nm on the cross section of the steel sheet in the rolling direction is 5 to 500 nm per 200 μm in the rolling direction.

[0011] The thickness of the oxide layer can be 15 to 50 nm.

[0012] According to an embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of the following: P: less than 0.1% by weight and excluding 0%; C: less than 0.005% by weight and excluding 0%; S: less than 0.005% by weight and excluding 0%; Ti: less than 0.004% by weight and excluding 0%; N: less than 0.005% by weight and excluding 0%.

[0013] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of Sn, Sb, Bi, Pb, Ge and As, with each or their total content being 0.005 to 0.200 by weight.

[0014] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of the following: Cu: 0.005 to 0.2 wt%, Cr: 0.01 to 0.5 wt%, Ni: less than 0.05 wt% and excluding 0%, Zn: less than 0.01 wt% and excluding 0%, and Co: less than 0.05 wt% and excluding 0%.

[0015] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of the following: Mo: less than 0.03 wt% and excluding 0%; B: less than 0.0050 wt% and excluding 0%; V: less than 0.0050 wt% and excluding 0%; Ca: less than 0.0050 wt% and excluding 0%; Nb: less than 0.0050 wt% and excluding 0%; Zr: less than 0.005 wt% and excluding 0%; Te: less than 0.01 wt% and excluding 0%; and Mg: less than 0.0050 wt% and excluding 0%.

[0016] In a cross-section from the surface to a depth of 100 nm, the density of nitride particles with a diameter of 10 to 200 nm can be 4 particles / μm. 2 the following.

[0017] A method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention comprises: hot rolling a slab to manufacture a hot-rolled steel sheet, wherein the slab comprises, by weight %, 1.5 to 4.5% Si, 0.1 to 2.0% Al, 0.1 to 2.0% Mn, with the balance comprising Fe and unavoidable impurities; setting the dew point of the steel sheet at -70 to -40°C and applying a dew pressure of 3.0 to 5.0 kgf / mm². 2 The steps include: a pre-annealing step of annealing under tension; a step of cold rolling annealed steel sheet to manufacture cold-rolled sheet; and a cold-rolled sheet annealing step of annealing the cold-rolled sheet.

[0018] The slab may also contain one or more of the following: P: less than 0.1% by weight and excluding 0%; C: less than 0.005% by weight and excluding 0%; S: less than 0.005% by weight and excluding 0%; Ti: less than 0.004% by weight and excluding 0%; N: less than 0.005% by weight and excluding 0%.

[0019] The slab may also contain one or more of Sn, Sb, Bi, Pb, Ge and As, with each or their combined content ranging from 0.005 to 0.200 by weight.

[0020] The slab may also contain one or more of the following: Cu: 0.005 to 0.2 wt%, Cr: 0.01 to 0.5 wt%, Ni: less than 0.05 wt% and excluding 0%, and Zn: less than 0.01 wt% and excluding 0%.

[0021] The slab may also contain one or more of the following: Mo: less than 0.03 wt% and excluding 0%; B: less than 0.0050 wt% and excluding 0%; V: less than 0.0050 wt% and excluding 0%; Ca: less than 0.0050 wt% and excluding 0%; Nb: less than 0.0050 wt% and excluding 0%; Zr: less than 0.005 wt% and excluding 0%; Te: less than 0.01 wt% and excluding 0%; Co: less than 0.05 wt% and excluding 0%; and Mg: less than 0.0050 wt% and excluding 0%.

[0022] After manufacturing hot-rolled steel sheets, subsequent steps can be performed while the hot-rolled steel sheets still have residual oxide scale.

[0023] The process may include a pre-cold rolling step of the hot-rolled sheet before the pre-annealing step of cold rolling.

[0024] In the pre-cooling rolling step, the reduction rate can be 25% to 65%.

[0025] The soaking temperature for the pre-annealing step before cold rolling can be 800 to 1100℃.

[0026] In the process of manufacturing cold-rolled sheets, the reduction rate can be 55% to 70%.

[0027] The annealing process for cold-rolled steel sheets can be carried out at a homogenization temperature of 850 to 1100°C in a dew point atmosphere below 0°C.

[0028] (III) Beneficial Effects According to an embodiment of the present invention, the non-oriented electrical steel sheet does not suffer from surface degradation, thus further improving magnetic properties.

[0029] Ultimately, the non-oriented electrical steel sheet according to one embodiment of the present invention can help manufacture environmentally friendly automotive motors, high-efficiency home appliance motors, and ultra-high-end electric motors. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a cross-section of a non-oriented electrical steel sheet according to an embodiment of the present invention. Detailed Implementation

[0031] The terms "first," "second," "third," etc., are used to describe 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. Therefore, without departing from the scope of the invention, the first part, component, region, layer, or segment described below can also be described as a second part, component, region, layer, or segment.

[0032] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, the singular forms used herein are intended to include the plural forms as well. As used in the specification, "comprising" can specifically refer to a feature, field, integer, step, action, element, and / or component, but does not exclude the presence or addition of other features, fields, integers, steps, actions, elements, and / or components.

[0033] If one part is described as being on top of another part, then other parts can exist directly on top of or in between the other part. When one part is described as being directly on top of another part, there are no other parts in between.

[0034] In addition, unless otherwise specified, % means weight, 1 ppm is 0.0001 weight.

[0035] In one embodiment of the present invention, the additional element refers to the additional element replacing the balance of iron (Fe), and the amount of replacement is equivalent to the amount of additional element added.

[0036] Although not otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries should be interpreted as having the same meaning as disclosed in relevant technical literature and herein, and should not be interpreted in an idealized or overly formal sense.

[0037] The embodiments of the present invention will be described in detail below to enable those skilled in the art to implement the invention. However, the present invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0038] According to one embodiment of the present invention, the non-oriented electrical steel sheet comprises, by weight %, 1.5 to 4.5% Si, 0.1 to 2.0% Al, 0.1 to 2.0% Mn, with the balance being Fe and unavoidable impurities.

[0039] The reasons for the compositional restrictions on non-oriented electrical steel sheets will be described below.

[0040] Si: 1.5 to 4.5% by weight Silicon (Si) serves to increase the resistivity of materials, reduce iron loss, and improve strength through solid solution strengthening. If too little Si is added, the improvement in iron loss and strength may be insufficient. If too much Si is added, the material becomes more brittle, rolling productivity decreases sharply, and a surface oxide layer and oxides that are detrimental to magnetism may form. Therefore, Si can be contained in quantities of 1.5 to 4.5% by weight. More specifically, it can be contained in quantities of 2.0 to 4.3% by weight. More specifically, it can be contained in quantities of 2.5 to 4.2% by weight.

[0041] Al: 0.1 to 2.0% by weight Aluminum (Al) serves to increase the resistivity of materials, reduce iron loss, and improve strength through solid solution strengthening. If too little Al is added, fine nitrides form, making it difficult to achieve the desired magnetic improvement. If too much Al is added, excessive nitride formation leads to magnetic degradation, causing problems in all processes, including steelmaking and continuous casting, and potentially resulting in a significant decrease in productivity. Therefore, Al can be present in quantities from 0.1 to 2.0% by weight. More specifically, it can be present in quantities from 0.2 to 1.6% by weight. More specifically, it can be present in quantities from 0.3 to 1.5% by weight.

[0042] Mn: 0.1 to 2.0% by weight Manganese (Mn) serves to increase the resistivity of materials, improve iron loss, and form sulfides. If too little Mn is added, fine sulfide particles form, leading to magnetic degradation. If too much Mn is added, excessive fine MnS precipitation promotes the formation of a {111} texture, which is detrimental to magnetism, resulting in a sharp decrease in magnetic flux density. Therefore, Mn can be present in quantities from 0.1 to 2.0 wt%. More specifically, it can be present in quantities from 0.2 to 1.6 wt%. More specifically, it can be present in quantities from 0.3 to 1.5 wt%.

[0043] According to an embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of the following: P: less than 0.1% by weight and excluding 0%; C: less than 0.005% by weight and excluding 0%; S: less than 0.005% by weight and excluding 0%; Ti: less than 0.004% by weight and excluding 0%; N: less than 0.005% by weight and excluding 0%.

[0044] P: less than 0.1% by weight Phosphorus (P) is a grain boundary segregating element that can increase magnetic flux density, but if too much is added, it increases the brittleness of the steel plate and worsens its weldability. More specifically, P can be present in amounts from 0.0001 to 0.0500% by weight.

[0045] C: less than 0.005% by weight Carbon (C) causes magnetic aging and combines with other impurity elements to form carbides, thereby hindering the movement of grain boundaries or magnetic domain walls and potentially leading to deterioration of magnetic properties. More specifically, C may contain 0.0001 to 0.003% by weight.

[0046] S: less than 0.005% by weight Sulfur (S) forms fine precipitates of MnS and CuS, which may lead to deterioration of magnetic properties and hot-rolling processability. More specifically, S may contain 0.0001 to 0.0030% by weight.

[0047] Ti: less than 0.004% by weight Titanium (Ti) has a very strong tendency to form precipitates in steel, and it forms fine carbides, nitrides, or sulfides within the base material, thereby inhibiting grain growth and domain wall movement, which may lead to iron loss degradation. More specifically, Ti may contain 0.0001 to 0.003% by weight.

[0048] N: less than 0.005% by weight Nitrogen (N) not only forms fine AlN precipitates within the matrix, but also combines with other impurities to form fine precipitates, inhibiting grain growth and domain wall movement, potentially leading to deterioration of iron losses. More specifically, N may contain 0.0001 to 0.0030% by weight.

[0049] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of Sn, Sb, Bi, Pb, Ge and As, with each or their total content being 0.005 to 0.200 by weight.

[0050] Sn and Sb The role of tin (Sn) and antimony (Sb) is to segregate at grain boundaries during the initial stage of final recrystallization annealing, thereby suppressing the development of {111} orientation, which leads to magnetic degradation. If too much Sn and Sb are added, it hinders the recovery and growth of coarse extended bands, potentially leading to surface quality deterioration. Therefore, one or more of Sn and Sb can be further added within the aforementioned range. More specifically, Sn can be present in 0.005 to 0.200% by weight, or Sb can be present in 0.005 to 0.200% by weight.

[0051] Bi, Pb, Ge and As When bismuth (Bi), lead (Pb), germanium (Ge), and arsenic (As) are further added, segregation occurs at grain boundaries. During cold rolling, this alleviates stress concentration at the grain boundaries and suppresses stress during subsequent recrystallization annealing processes. <111> / / Recrystallization of ND-oriented grains increases magnetic flux density. When these elements are added appropriately, the aforementioned effects can be further achieved. However, if the content is too high, excessive segregation will occur, thereby inhibiting grain growth and potentially leading to a decrease in magnetic flux density and iron loss.

[0052] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of the following: Cu: 0.005 to 0.2 wt%, Cr: 0.01 to 0.5 wt%, Ni: less than 0.05 wt% and excluding 0%, Zn: less than 0.01 wt% and excluding 0%, and Co: less than 0.05 wt% and excluding 0%.

[0053] Cu: 0.005 to 0.200% by weight The role of copper (Cu) is to form sulfides with Mn. If too little Cu is added, fine (Cu·Mn)S precipitates, potentially leading to magnetic degradation. If too much Cu is added, it can cause high-temperature brittleness, leading to cracking during continuous casting or hot rolling. More specifically, Cu can be present in amounts from 0.01 to 0.10% by weight.

[0054] Cr: 0.01 to 0.50% by weight Chromium (Cr) is added to increase resistivity and improve iron loss. If too little Cr is added, the resistivity increase may be insufficient. If too much Cr is added, it may lead to a decrease in magnetic flux density. More specifically, Cr can be present in amounts ranging from 0.050 to 0.20% by weight.

[0055] Ni: less than 0.05% by weight Nickel (Ni) reacts with impurity elements to form fine sulfides, carbides, and nitrides, which can adversely affect magnetism. More specifically, Ni may contain 0.001 to 0.03% by weight.

[0056] Zn: less than 0.01% by weight If the zinc (Zn) content is too high, it may act as an impurity and cause a decrease in magnetic properties. Therefore, Zn can be added further within the aforementioned range. More specifically, Zn can be present in amounts from 0.001 to 0.005% by weight.

[0057] Co: less than 0.05% by weight Cobalt (Co) does not form fine precipitates that reduce the magnetism of steel sheets, but it increases high-temperature strength and may lead to poor shape of hot-rolled coils.

[0058] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further comprise one or more of the following: Mo: less than 0.03 wt% and excluding 0%; B: less than 0.0050 wt% and excluding 0%; V: less than 0.0050 wt% and excluding 0%; Ca: less than 0.0050 wt% and excluding 0%; Nb: less than 0.0050 wt% and excluding 0%; Zr: less than 0.0050 wt% and excluding 0%; Te: less than 0.0100 wt% and excluding 0%; and Mg: less than 0.0050 wt% and excluding 0%.

[0059] Mo: less than 0.030% by weight If excessive molybdenum (Mo) is added, it may suppress the segregation of segregating elements and reduce the texture improvement effect. Therefore, Mo can be contained in amounts of 0.03 wt% or less, with no particular lower limit, but it can be contained in amounts of 0.001 wt% or more because it improves texture through segregation at the surface and grain boundaries. More specifically, Mo can be contained in amounts of 0.001 to 0.010 wt%. More specifically, Mo can be contained in amounts of 0.005 to 0.010 wt%.

[0060] B: Less than 0.0050% by weight If excessive boron (B) is added, it may form inclusions in the steel, causing magnetic degradation. Therefore, B can be contained in amounts up to 0.005% by weight, with no particular limit on the lower limit, but due to steelmaking costs, it can be 0.0001% by weight. More specifically, B can be contained in amounts from 0.0001 to 0.0030% by weight.

[0061] V: less than 0.0050% by weight Vanadium (V) exhibits a strong tendency to precipitate in steel, forming fine carbides or nitrides within the base metal. This inhibits grain growth and domain wall movement, leading to deterioration of iron losses. Therefore, the V content can be below 0.0050% by weight, with no particular lower limit, but due to steelmaking costs, it can be as low as 0.0003% by weight. That is, V can contain 0.0003 to 0.0050% by weight. More specifically, V can contain 0.0003 to 0.0030% by weight.

[0062] Ca: less than 0.0050% by weight Calcium (Ca) has a very strong tendency to form precipitates in steel and forms fine sulfides inside the base metal, which inhibits grain growth and magnetic domain wall movement, thus leading to iron loss deterioration.

[0063] Nb: less than 0.0050% by weight Niobium (Nb) has a very strong tendency to form precipitates in steel, and it forms fine carbides or nitrides within the base metal, inhibiting grain growth and domain wall movement, thus leading to deterioration of iron loss. Therefore, the Nb content can be below 0.0050% by weight, with no particular lower limit, but due to steelmaking costs, it can be 0.0003% by weight. That is, Nb can be present from 0.0003 to 0.0050% by weight. More specifically, Nb can be present from 0.0003 to 0.0030% by weight.

[0064] Zr: less than 0.0050% by weight Adding excessive amounts of zirconium (Zr) can lead to inclusions and other defects in the steel, causing magnetic degradation. Therefore, Zr can be present in quantities of less than 0.005% by weight, with no particular lower limit, but due to steelmaking costs, it can be as low as 0.0001% by weight. That is, Zr can be present in quantities from 0.0001 to 0.0050% by weight. More specifically, it can be present in quantities from 0.0005 to 0.0030% by weight.

[0065] Te: less than 0.0100% by weight Tellurium (Te) diffuses into the oxide layer on the surface of hot-rolled coils, increasing the coefficient of friction between the oxide layer and the rolling mill rolls, while also accumulating in the lower part of the oxide layer, thereby increasing hardness. Therefore, tellurium can be added to allow the oxide layer, which breaks off during rolling, to detach without being pressed into the base material. If too little Te is added, the effect may be insignificant. If too much Te is added, the oxide layer is easily detached, and the base material directly contacts the rolling mill rolls, thus reducing the effect and generating excessive deformation bands within the steel sheet during cold rolling, potentially leading to the development of a detrimental {111} / / ND texture. More specifically, tellurium can be contained in amounts from 0.0001 to 0.007% by weight.

[0066] Mg: less than 0.0050% by weight Magnesium (Mg) is an element that primarily combines with sulfur to form sulfides, which may affect the oxide layer on the surface of the base iron. Therefore, Mg can be contained in amounts up to 0.0050% by weight, with no particular lower limit, but due to steelmaking costs, it can be as low as 0.0001% by weight. That is, Mg can be contained from 0.0001 to 0.0050% by weight. More specifically, it can be contained from 0.0005 to 0.0030% by weight.

[0067] The balance includes Fe and unavoidable impurities. Unavoidable impurities are those introduced during the steelmaking process and the manufacturing process of the non-oriented electrical steel sheet; these impurities are well-known in the art and therefore omitted in detail. In one embodiment of the invention, in addition to the aforementioned alloy composition, the addition of elements is not excluded, and various elements may be included without prejudice to the technical concept of the invention. When additional elements are further included, they replace a portion of the Fe in the balance.

[0068] As described above, in one embodiment of the present invention, the magnetism can be improved by appropriately adjusting the alloy composition of the steel plate and appropriately forming an oxide layer on the surface of the steel plate.

[0069] Figure 1 The diagram shows a cross-section of a non-oriented electrical steel sheet according to an embodiment of the present invention.

[0070] like Figure 1 As shown, it may include an oxide layer 10 that exists from the surface of the non-oriented electrical steel sheet 100 toward the interior.

[0071] Regarding oxide layer 10, during the manufacturing process of electrical steel sheet, oxygen penetrates into the steel sheet to form oxide layer 10.

[0072] Oxide layer 10 is defined as the portion of the steel plate surface containing more than 20% oxygen by weight. For the detection and thickness of oxide layer 10, after processing the TD surface of the specimen with FIB, observation with TEM, and EDS analysis, the portion containing more than 20% oxygen by weight can be identified as an oxide layer. At this time, if the steel plate specimen does not have an insulating coating, or if an insulating coating has been formed, a specimen with the insulating coating removed can be used. To reduce measurement errors based on location, the specimen can be measured along the RD direction for a minimum length of 200 μm, and the average value is taken.

[0073] like Figure 1 As shown, there exists an oxide layer interruption in the steel plate with a thickness of less than 2 nm on a cross-section in the rolling direction. The length of this interruption is DC. L It can be 5 to 500 nm per 200 μm in the rolling direction. For example... Figure 1As shown, multiple oxide layer interruptions can exist within the sample. In this case, the sum of the lengths of all oxide layer interruptions falls within the aforementioned range. If the length of the oxide layer interruption is too short, the overall thickness of the oxide layer increases, creating an uneven shape at the interface between the oxide layer and the substrate, which may lead to problems with magnetic properties. If the length of the oxide layer interruption is too long, fine nitrides will be promoted to form in the region below 100 nm from the center of the substrate in the oxide layer, which may also lead to problems with magnetic properties. More specifically, the length of the interruption (DC...) L The value can be 50 to 400 nm per 200 μm in the rolling direction. The measurement and determination of the interrupted portion can be performed using the same method as the aforementioned measurement and determination of the oxide layer.

[0074] The oxide layer 10 and the oxide layer interruption can be properly formed by adjusting the dew point and tension during the pre-rolling annealing process. For a more specific method, it is described below in connection with the manufacturing method of non-oriented electrical steel sheets.

[0075] The thickness of oxide layer 10 can be 15 to 50 nm. If the thickness of oxide layer 10 is too thin, Al concentration in oxide layer 10 cannot proceed properly, and the aforementioned AlN suppression effect may not be fully achieved. If the thickness of oxide layer 10 is too thick, oxygen will penetrate into the steel plate in large quantities, which may lead to a deterioration in magnetism. More specifically, the thickness of oxide layer 10 can be 20 to 30 nm.

[0076] Due to the surface concentration of Al, the oxide layer 10 can contain more than 20% by weight of Al. More specifically, Al can contain 20 to 60% by weight. Apart from Al and O, the remaining alloy composition is the same as that of the aforementioned non-oriented electrical steel sheet. The oxide layer 10 is very thin relative to the overall thickness of the non-oriented electrical steel sheet 100, and therefore has no substantial impact on the alloy composition of the non-oriented electrical steel sheet 100.

[0077] As described above, the presence of oxide layer 10 suppresses the formation of nitrides near the surface. Specifically, in a cross-section from the surface to a depth of 100 nm, the density of nitride particles with a diameter of 10 to 200 nm can be 4 particles / μm. 2 The particle size and number density of nitrides can be measured based on a cross-section (TD plane) perpendicular to the rolling direction (TD direction) of the steel plate. As a measurement method, after preparing the sample using a replication method, it can be observed using a TEM. For particle size, a virtual circle with an area equal to the area occupied by the nitrides is assumed, and the diameter of this circle is taken as the particle size.

[0078] As described above, in one embodiment of the present invention, magnetism can be improved by appropriately controlling the steel composition and appropriately forming an oxide layer. Specifically, based on a thickness of 0.25 mm, the iron loss (W) of the non-oriented electrical steel sheet is...10 / 400 The flux density (B50) can be below 12.5 W / kg. Additionally, the magnetic flux density (B50) can be above 1.67 T. Iron loss (W... 10 / 400 (This refers to the iron loss when a magnetic flux density of 1.0T is excited at a frequency of 400Hz.) Magnetic flux density (B) 50 ) is the magnetic flux density induced under a magnetic field of 5000 A / m. More specifically, the iron loss (W) of non-oriented electrical steel sheets 10 / 400 The flux density (B50) can be from 10.0 to 12.0 W / kg. More specifically, it can be from 10.5 to 11.5 W / kg.

[0079] A method for manufacturing non-oriented electrical steel sheet according to an embodiment of the present invention includes: hot rolling a slab to manufacture a hot-rolled steel sheet; a pre-rolling annealing step of annealing the steel sheet; cold rolling the annealed steel sheet to manufacture a cold-rolled sheet; and a cold-rolled sheet annealing step of annealing the cold-rolled sheet.

[0080] The following describes each step in detail.

[0081] First, the slab is hot-rolled.

[0082] The alloy composition of the slab has already been described in the previous section on the alloy composition of non-oriented electrical steel sheets, so it will not be repeated here. The alloy composition does not substantially change during the manufacturing process of non-oriented electrical steel sheets; therefore, the alloy composition of non-oriented electrical steel sheets and slabs is essentially the same.

[0083] Specifically, by weight percent, the slab contains Si: 1.5 to 4.5%, Al: 0.1 to 2.0%, Mn: 0.1 to 2.0%, with the balance including Fe and unavoidable impurities.

[0084] Other additional elements are already described in the alloy composition of non-oriented electrical steel sheets, so a repeating description is omitted.

[0085] Before hot rolling, the slab can be heated. The heating temperature of the slab is not limited, but it can be heated to below 1200℃. If the slab is heated to too high a temperature, precipitates such as AlN and MnS present in the slab will precipitate finely again during hot rolling and annealing after solution treatment, thus inhibiting grain growth and potentially leading to a decrease in magnetic properties.

[0086] Next, the slab is hot-rolled to produce a hot-rolled sheet. The thickness of the hot-rolled sheet can be from 0.8 to 2.0 mm. In the process of producing the hot-rolled sheet, the final rolling temperature can be above 800°C. Specifically, it can be from 800 to 1000°C. For hot-rolled sheets, coiling can be performed at a temperature above 600°C. More specifically, the thickness of the hot-rolled sheet can be from 0.9 to 1.8 mm.

[0087] After manufacturing hot-rolled steel sheets, subsequent steps can be performed with residual oxide scale remaining on the hot-rolled steel sheets. That is, after hot rolling, oxide scale removal processes such as pickling, sandblasting, or surface grinding can be omitted, and subsequent steps can be performed. Because cold rolling is performed without pickling, the friction between the rolling mill rolls and the steel sheet increases, resulting in shear deformation in addition to planar deformation during rolling, and specific orientations are developed during recrystallization annealing. In one embodiment of the invention, oxide scale refers to the portion of the steel sheet surface where elements such as Fe, Al, and Si combine with oxygen to form a phase different from the base material. Residual oxide scale refers to oxide scale with a thickness of at least 1 μm remaining on the hot-rolled sheet. Here, oxide scale thickness refers to the sum of the thicknesses of oxide scale generated on both surfaces of the steel sheet. If the residual oxide scale thickness is too thin, the effect caused by the residual oxide scale may not be fully realized. Even if the oxide scale thickness is thicker, the effect will not improve, and there is a problem of reduced steel sheet yield. More specifically, residual oxide scale with a thickness of 0.1 to 1 μm is permissible.

[0088] In one embodiment of the present invention, after manufacturing the hot-rolled steel sheet, a pre-cold rolling annealing step for annealing the hot-rolled steel sheet can be performed directly. Alternatively, after pre-cooling the hot-rolled steel sheet, a pre-cold rolling annealing step for annealing the pre-cooled rolled sheet can be performed.

[0089] Pre-cooling rolling differs from cold rolling as described below because it is a rolling step in the following process: rolling to an intermediate thickness rather than the final product thickness, then performing intermediate annealing, and finally cold rolling to the final product thickness.

[0090] For pre-cooling rolling, a reduction rate of 25% to 65% can be used to improve the final cold-rolled productivity and grain size in the final product sheet. Alternatively, if rolling productivity is not a concern, the invention can also be carried out in a reversible mill. The pre-cooled rolled sheet can have a thickness of 0.5 to 1.5 mm. More specifically, the reduction rate can be 30% to 60% and the thickness can be 0.8 to 1.3 mm.

[0091] The reduction rate in pre-cooling rolling can be calculated as (thickness of steel plate before rolling – thickness of steel plate after rolling) / thickness of steel plate before rolling. If the reduction rate in the pre-cooling rolling step is too low, the rolling load during the final cold rolling will increase, thus reducing productivity. An increase in the final reduction rate may promote finer grains. <111> The issue of / ND orientation recrystallization. On the other hand, if the reduction rate is too high, the cold rolling load increases, and the possibility of sheet breakage also increases.

[0092] The pre-cooling rolling step can be performed at a temperature between 60 and 300°C. At this temperature, the temperature of the steel sheet naturally increases due to friction between the steel sheet and the rolls, or it can be increased by external heating. If the temperature is too low, the rolling load will increase significantly, causing the steel sheet to slide between the rolls instead of being rolled, potentially leading to problems such as twisting. If the temperature is too high, a thick oxide layer will form on the surface of the steel sheet, deteriorating its magnetic properties and potentially causing problems such as rolling oil ignition. More specifically, it is preferable to perform the process at a temperature between 70 and 250°C. The aforementioned temperatures refer to the temperature of the steel sheet.

[0093] As mentioned above, the pre-cooling rolling step can be omitted if necessary.

[0094] Next, in the pre-cold rolling annealing step, the hot-rolled steel sheet or pre-cooled rolled sheet is annealed. In one embodiment of the invention, by adjusting the dew point and tension in the pre-cold rolling annealing step, an oxide layer 10 can be appropriately formed.

[0095] Specifically, the dew point can be -70 to -40°C. If the dew point is too low, the oxide layer of the final product will be excessively interrupted, or fine nitrides will be generated in large quantities near the surface, potentially causing problems with magnetic properties. If the dew point is too high, an excessively thick oxide layer will form, or fine oxide particles will form near the surface, potentially causing problems with magnetic properties. More specifically, the dew point can be -65 to -45°C. More specifically, the dew point can be the dew point of the atmosphere during the homogenization process.

[0096] Additionally, it can be supplied with 3.0 to 5.0 kgf / mm. 2 The tension is crucial. If the tension is too low, the shape of the steel sheet cannot be adequately corrected, resulting in excessive localized oxide layer formation on the final product, which may cause problems with magnetic properties. If the tension is too high, excessive oxide layer breaks will form on the final product, which may also cause problems with magnetic properties. More specifically, the tension can be between 3.3 and 4.7 kgf / mm. 2 Tension can be measured between the bridle rolls at the inlet and outlet of the annealing furnace, and can be measured using a load cell.

[0097] In the pre-rolling annealing step, the soaking temperature can be between 800 and 1100°C. If the annealing temperature is too low, recrystallized structures will not form or will grow finely, resulting in a small increase in magnetic flux density. Conversely, if the annealing temperature is too high, the magnetic properties will decrease, and the rolling operability will deteriorate due to plate-like deformation. More specifically, the temperature range can be between 830 and 1080°C. The soaking time can be between 30 and 300 seconds.

[0098] The aforementioned pre-cold rolling annealing can be performed in vertical or horizontal continuous annealing equipment. If post-hot rolling oxide scale removal is omitted, it can be performed after pre-cold rolling annealing. Oxide scale residue on the final manufactured non-oriented electrical steel sheet will impair magnetism. Furthermore, cold rolling with residual oxide scale can cause it to indent or peel off, creating unevenness and potentially degrading the motor's operational stability and magnetic properties. After pickling, the oxide scale may be completely removed or may remain at a thickness of less than 0.01 μm. Pickling refers not only to acid immersion but also to all methods of physical and chemical descaling. Pickling methods can include acid immersion, sandblasting, or surface grinding.

[0099] Next, the annealed steel sheet is cold-rolled to produce a cold-rolled sheet. At this stage, cold rolling can be performed at a reduction rate of 55% to 70%. If the reduction rate is too low, the deformation energy accumulated within the rolled steel sheet is small, making recrystallization difficult in subsequent annealing processes, resulting in residual rolled microstructure. This can lead to problems with magnetic flux density and iron loss improvement. On the other hand, if the reduction rate is too high, it will hinder the subsequent annealing process from promoting... <111> / / Recrystallization of ND-oriented grains can lead to finer grains, potentially resulting in decreased magnetic flux density and increased iron loss. The reduction rate can be 58% to 67%. For the cold rolling step, either a tandem cold rolling mill or a reverse mill can be used. A tandem cold rolling mill utilizes multiple rolling stands for continuous cold rolling of the steel sheet, while a reverse mill uses 12 or more rolls for discontinuous cold rolling. The final rolled thickness can range from 0.1 mm to 0.35 mm.

[0100] Next, the cold-rolled sheet is annealed. The annealing process for cold-rolled sheet can be carried out in a dew point atmosphere below 0°C. More specifically, annealing can be carried out in a dew point temperature atmosphere ranging from -50°C to -10°C.

[0101] For the annealing process of cold-rolled steel sheets, annealing can be carried out at a soaking temperature of 850 to 1100°C. If the soaking temperature is too low, the grains cannot grow sufficiently, hysteresis losses increase, and iron loss deterioration may occur. If the soaking temperature is too high, eddy current losses increase, and magnetic flux density may drop sharply. More specifically, annealing can be carried out at a temperature of 900 to 1050°C. Soaking can be performed for 10 to 300 seconds.

[0102] During the annealing process of cold-rolled sheet, all (i.e., more than 99%) of the processed structures formed in the cold rolling step can recrystallize.

[0103] After annealing, cold-rolled steel sheets can be coated with an insulating film. The insulating film can be processed into organic films, inorganic films, and organic-inorganic composite films, or it can be treated with other insulating film-forming agents.

[0104] The present invention will be further described in detail below by way of examples. However, the following examples are merely illustrative and the present invention is not limited to the following examples.

[0105] Example 1 A slab was manufactured containing the composition shown in Table 1, with the balance including Fe and unavoidable impurities. This slab was heated to 1150°C and hot-rolled at a finishing temperature of 950°C to produce a hot-rolled plate with the thickness shown in Table 2. For sample number A1, the oxide scale on the hot-rolled plate was completely removed by pickling; oxide scale removal was omitted for the remaining samples.

[0106] Then, annealing of the hot-rolled sheet is omitted, and pre-cold rolling, pre-cold rolling annealing, and cold rolling are performed under the conditions in Table 2 to achieve a final thickness of 0.25 mm. The cold-rolled steel sheet is annealed at a homogenization temperature of 1000°C for 100 seconds.

[0107] For magnetic flux density and iron loss, five 60mm wide × 60mm long samples were cut out. For each sample, the magnetic flux density and iron loss were measured in the rolling direction and the direction perpendicular to the rolling using a single sheet tester, and the average value was expressed.

[0108] At this time, W 10 / 400 The iron loss is calculated when a magnetic flux density of 1.0T is excited at a frequency of 400Hz. (B) 50 It is the magnetic flux density induced under a magnetic field of 5000 A / m.

[0109] For oxide and nitride characteristics, the TD surface of the FIB-processed sample was photographed along the RD direction for a length of more than 300 μm using TEM, and the chemical composition was analyzed using EDS. In a cross-section from the surface to a depth of 100 nm, if the density of nitride particles with a diameter of 10 to 200 nm exceeded 4 particles / μm... 2 If it is 4 / μm, it is represented by ○. 2 The following is represented as X.

[0110] Table 1 Table 2 Table 3 As shown in Tables 1 to 3, for the inventive examples that appropriately adjust the steel composition and process conditions to form an oxide layer with appropriate characteristics, it can be confirmed that they have excellent iron loss and magnetic flux density.

[0111] On the other hand, without proper adjustment of the steel composition or process conditions, and without proper formation of an oxide layer, poor iron loss and magnetic flux density can be confirmed.

[0112] This invention can be implemented in various ways and is not limited to the embodiments described herein. Those skilled in the art will understand that the invention can be implemented in other specific ways without altering its technical concept or essential features. Therefore, it should be understood that the above embodiments are exemplary in all respects and are not restrictive.

[0113] [Explanation of reference numerals in the attached figures] 100: Non-oriented electrical steel sheet; 10: Oxide layer

Claims

1. A non-oriented electrical steel sheet, wherein, By weight percent, the non-oriented electrical steel sheet comprises Si: 1.5 to 4.5%, Al: 0.1 to 2.0%, Mn: 0.1 to 2.0%, with the balance including Fe and unavoidable impurities. It also contains an oxide layer that extends from the surface inwards. The length of the oxide layer interruption in the cross section of the steel plate containing the rolling direction is 5 to 500 nm for every 200 μm of the oxide layer thickness.

2. The non-oriented electrical steel sheet according to claim 1, wherein, The thickness of the oxide layer is 15 to 50 nm.

3. The non-oriented electrical steel sheet according to claim 1, wherein, The non-oriented electrical steel sheet further comprises one or more of the following: P: less than 0.1% by weight and excluding 0%; C: less than 0.005% by weight and excluding 0%; S: less than 0.005% by weight and excluding 0%; Ti: less than 0.004% by weight and excluding 0%; N: less than 0.005% by weight and excluding 0%.

4. The non-oriented electrical steel sheet according to claim 1, wherein, The non-oriented electrical steel sheet further comprises one or more of Sn, Sb, Bi, Pb, Ge and As, with each or their combined content ranging from 0.005 to 0.200 by weight.

5. The non-oriented electrical steel sheet according to claim 1, wherein, The non-oriented electrical steel sheet further comprises one or more of the following: Cu: 0.005 to 0.2 wt%, Cr: 0.01 to 0.5 wt%, Ni: less than 0.05 wt% and excluding 0%, Zn: less than 0.01 wt% and excluding 0%, and Co: less than 0.05 wt% and excluding 0%.

6. The non-oriented electrical steel sheet according to claim 1, wherein, The non-oriented electrical steel sheet further comprises one or more of the following: Mo: less than 0.03% by weight and excluding 0%; B: less than 0.0050% by weight and excluding 0%; V: less than 0.0050% by weight and excluding 0%; Ca: less than 0.0050% by weight and excluding 0%; Nb: less than 0.0050% by weight and excluding 0%; Zr: less than 0.005% by weight and excluding 0%; Te: less than 0.01% by weight and excluding 0%; and Mg: less than 0.0050% by weight and excluding 0%.

7. The non-oriented electrical steel sheet according to claim 1, wherein, In a cross-section from the surface to a depth of 100 nm, the density of nitride particles with a diameter of 10 to 200 nm is 4 particles / μm. 2 the following.

8. A method for manufacturing a non-oriented electrical steel sheet, comprising: The step of hot rolling a slab to produce a hot-rolled steel sheet, wherein the slab comprises, by weight %: Si: 1.5 to 4.5%, Al: 0.1 to 2.0%, Mn: 0.1 to 2.0%, with the balance comprising Fe and unavoidable impurities; The steel plate was subjected to a dew point of -70 to -40°C and an application of 3.0 to 5.0 kgf / mm. 2 The pre-annealing step of cold rolling is performed under tension; The steps of cold rolling annealed steel sheets to produce cold-rolled sheets; and The annealing step for the cold-rolled sheet.

9. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, The slab further comprises one or more of the following: P: less than 0.1% by weight and excluding 0%; C: less than 0.005% by weight and excluding 0%; S: less than 0.005% by weight and excluding 0%; Ti: less than 0.004% by weight and excluding 0%; N: less than 0.005% by weight and excluding 0%.

10. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, The slab also contains one or more of Sn, Sb, Bi, Pb, Ge and As, with each or their combined content ranging from 0.005 to 0.200 by weight.

11. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, The slab further comprises one or more of the following: Cu: 0.005 to 0.2 wt%, Cr: 0.01 to 0.5 wt%, Ni: less than 0.05 wt% and excluding 0%, Zn: less than 0.01 wt% and excluding 0%, and Co: less than 0.05 wt% and excluding 0%.

12. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, The slab contains one or more of the following: Mo: less than 0.03% by weight and excluding 0%; B: less than 0.0050% by weight and excluding 0%; V: less than 0.0050% by weight and excluding 0%; Ca: less than 0.0050% by weight and excluding 0%; Nb: less than 0.0050% by weight and excluding 0%; Zr: less than 0.005% by weight and excluding 0%; Te: less than 0.01% by weight and excluding 0%; and Mg: less than 0.0050% by weight and excluding 0%.

13. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, After the hot-rolled steel sheet is manufactured, subsequent steps are performed while the hot-rolled steel sheet still has residual oxide scale.

14. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, Prior to the pre-annealing step of cold rolling, a step of pre-cold rolling the hot-rolled plate is also included.

15. The method for manufacturing non-oriented electrical steel sheet according to claim 14, wherein, In the pre-cooling rolling step, the reduction rate is 25% to 65%.

16. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, The homogenization temperature of the pre-annealing step before cold rolling is 800 to 1100°C.

17. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, In the process of manufacturing the cold-rolled sheet, the reduction rate is 55% to 70%.

18. The method for manufacturing non-oriented electrical steel sheet according to claim 8, wherein, The annealing step of the cold-rolled sheet is carried out in a dew point atmosphere below 0°C at a homogenization temperature of 850 to 1100°C.

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