Non-oriented electrical steel sheet and its manufacturing method

Abstract

According to one embodiment of the present invention, the non-oriented electrical steel sheet comprises, by weight %, 3.0 to 4.0% Si, 0.3 to 1.5% Al, 0.1 to 0.6% Mn, 0.006 to 0.1% of one or more of Sn and Sb, 0.0015 to 0.0040% C, 0.01 to 0.03% Cr, 0.003 to 0.008% Cu, 0.0005 to 0.0025% Mg, with the balance being Fe and unavoidable impurities.

Description

Technical Field

[0001] 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 improves the magnetic properties of the non-oriented electrical steel sheet and its manufacturing method by appropriately adjusting the contents of Sb, Sn, Cu, Cr, and Mg. Background Technology

[0002] Electric motors, which convert electrical energy into mechanical energy, primarily use non-oriented electrical steel sheets. To achieve high efficiency in the energy conversion process, the properties of electrical steel sheets need further improvement.

[0003] Especially recently, with environmental protection technologies gaining significant attention, improving the efficiency of electric motors, which account for a large portion of electricity consumption, is considered crucial. Therefore, methods to simultaneously enhance the magnetic properties and strength of non-oriented electrical steel sheets have attracted considerable interest.

[0004] The magnetic properties of non-oriented electrical steel 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 achieved 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 allows for smaller motors or reduced copper losses. Therefore, manufacturing non-oriented electrical steel sheets with low iron loss and high magnetic flux density is important.

[0005] On the other hand, the characteristics of non-oriented electrical steel sheets should also take into account the operating conditions of the motor. As a general standard for evaluating the characteristics of non-oriented electrical steel for motors, the iron loss W15 / 50 under a 1.5T magnetic field at a 50Hz commercial frequency is widely adopted. However, not all motors used for various purposes consider W15 / 50 iron loss to be the most important factor, and the iron loss at different frequencies or under applied magnetic fields should be evaluated based on the main operating conditions.

[0006] In particular, for non-oriented electrical steel sheets used in drive motors of recent vehicles, the magnetic properties at low magnetic fields of 1.0T or below and high frequencies above 400Hz are often important. In this case, the properties of non-oriented electrical steel sheets are evaluated using W10 / 400 iron loss.

[0007] To improve the magnetic properties of non-oriented electrical steel sheets, a common method is to add alloying elements such as silicon (Si). Adding these elements increases the resistivity of the steel; as resistivity increases, eddy current losses decrease, thus reducing total iron loss. However, increasing the amount of Si added leads to disadvantages such as decreased magnetic flux density and increased brittleness. Excessive Si content makes cold rolling impossible and hinders industrial production. While thinner electrical steel sheets achieve reduced iron loss, the reduced rollability due to brittleness becomes a critical problem. To further improve the resistivity of the steel, elements such as al (Al) and manganese (Mn) can be added to produce the highest grade of non-oriented electrical steel sheets with excellent magnetic properties.

[0008] In non-oriented electrical steel used in electric vehicle drive motors, high-frequency iron losses above 400Hz are crucial. As the frequency increases, the proportion of eddy current losses in iron losses increases, which is beneficial for improving resistivity and reducing thickness. However, as the steel sheet thickness decreases, the cold rolling reduction rate increases, leading to the development of the {111} / / ND texture, which becomes a cause of magnetic degradation. To mitigate this, if the cold rolling reduction rate is reduced by decreasing the thickness of the hot-rolled sheet, the shape of the steel sheet cannot be fully controlled during cold rolling, resulting in increased thickness deviation in the width direction and dimensional defects in the motor core. Furthermore, as the steel sheet thins, the coil length increases, thus increasing the operation time of the continuous annealing process and reducing annealing productivity.

[0009] To address these issues, methods have been proposed to improve magnetic properties by thoroughly removing impurities during the steelmaking process to create ultra-clean steel or by adding specific elements to reduce inclusions and precipitates in the steel. However, these methods are limited in application due to constraints imposed by commercial production conditions. Furthermore, methods have been proposed to improve texture by controlling annealing temperature or atmosphere and controlling the strain rate of the steel plate during rolling. However, these techniques are rarely used in practice due to increased manufacturing costs, decreased productivity, and unsatisfactory results. Summary of the Invention

[0010] (a) Technical problems to be solved

[0011] 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 improves the magnetic properties of the non-oriented electrical steel sheet and its manufacturing method by appropriately adjusting the contents of Sb, Sn, Cu, Cr, and Mg.

[0012] (II) Technical Solution

[0013] According to one embodiment of the present invention, the non-oriented electrical steel sheet comprises, by weight %, 3.0 to 4.0% Si, 0.3 to 1.5% Al, 0.1 to 0.6% Mn, 0.006 to 0.1% of one or more of Sn and Sb, 0.0015 to 0.0040% C, 0.01 to 0.03% Cr, 0.003 to 0.008% Cu, 0.0005 to 0.0025% Mg, with the balance being Fe and unavoidable impurities.

[0014] According to an embodiment of the present invention, the non-oriented electrical steel sheet satisfies the following formula 1.

[0015] [Formula 1]

[0016] 0.66≤([Sn]+[Sb]) / ([Cr]+[Cu]+[Mg])≤2

[0017] (In Formula 1, [Sn], [Sb], [Cr], [Cu], and [Mg] represent the contents (by weight%) of Sn, Sb, Cr, Cu, and Mg, respectively.)

[0018] According to one embodiment of the present invention, the non-oriented electrical steel sheet further comprises one or more of N, S, Ti, Nb and V, each in an amount of 0.0003 to 0.0030% by weight.

[0019] According to one embodiment of the present invention, the non-oriented electrical steel sheet further comprises one or more of P: 0.005 to 0.05 wt%, Mo: 0.001 to 0.01 wt%, and Ni: 0.005 to 0.04 wt%.

[0020] According to one embodiment of the present invention, the non-oriented electrical steel sheet has an average grain size of 55 to 75 μm.

[0021] According to an embodiment of the present invention, a non-oriented electrical steel sheet has an oxide layer extending from the surface to the interior of the steel sheet, and the thickness of the oxide layer can be 10 to 50 nm.

[0022] The oxide layer contains 1.0 to 30 wt% Al and 0.5 to 10.0 wt% Si.

[0023] The weight ratio of Al content to Si content in the oxide layer is 5 to 20.

[0024] Within a depth of 2 μm from the surface of the steel plate inward, the distribution density of AlN precipitates with diameters ranging from 10 to 500 nm is 3 precipitates / mm². 2 the following.

[0025] The thickness of the steel plate is 0.10 to 0.35 mm.

[0026] A method for manufacturing non-oriented electrical steel sheet according to an embodiment of the present invention comprises: hot rolling a slab to manufacture a hot-rolled sheet, wherein the slab comprises, by weight %, 3.0 to 4.0% Si, 0.3 to 1.5% Al, 0.1 to 0.6% Mn, 0.006 to 0.1% of one or more of Sn and Sb, 0.0015 to 0.0040% C, 0.01 to 0.03% Cr, 0.003 to 0.008% Cu, 0.0005 to 0.0025%, and the balance comprising Fe and unavoidable impurities, satisfying Formula 1 below;

[0027] The steps of cold rolling hot-rolled sheet to produce cold-rolled sheet; and

[0028] The step of performing final annealing on the cold-rolled sheet.

[0029] 0.66≤([Sn]+[Sb]) / ([Cr]+[Cu]+[Mg])≤2

[0030] (In Formula 1, [Sn], [Sb], [Cr], [Cu], and [Mg] represent the contents (by weight%) of Sn, Sb, Cr, Cu, and Mg, respectively.)

[0031] Prior to the step of manufacturing the hot-rolled sheet, a further step of heating the hot-rolled sheet at a temperature of 1200°C is included.

[0032] In the process of manufacturing hot-rolled plates, the finishing rolling temperature is above 800℃.

[0033] Following the step of manufacturing the hot-rolled sheet, the process further includes an annealing step of the hot-rolled sheet at 850°C to 1150°C.

[0034] The final annealing step involves annealing at a homogenization temperature above 900°C for at least 15 seconds.

[0035] The final annealing step is carried out in an atmosphere containing less than 40 vol% hydrogen (H2) and more than 60 vol% nitrogen, with a dew point of 0 to -40°C.

[0036] (III) Beneficial Effects

[0037] According to one embodiment of the present invention, by providing a non-oriented electrical steel sheet with excellent high-frequency iron loss, it is helpful to improve the performance of environmentally friendly automotive drive motors using the highest quality non-oriented electrical steel sheets. Attached Figure Description

[0038] Figure 1 This is a cross-sectional schematic diagram of a non-oriented electrical steel sheet according to one embodiment of the present invention. Detailed Implementation

[0039] 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, and / or segment from another. Therefore, without departing from the scope of the invention, the first part, component, region, layer, and / or segment described below can also be described as a second part, component, region, layer, and / or segment.

[0040] 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. The word "comprising" as used in the specification can specifically refer to a particular feature, domain, integer, step, action, element, and / or component, but does not exclude the presence or addition of other features, domains, integers, steps, actions, elements, components, and / or groups.

[0041] 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.

[0042] Additionally, unless otherwise specified, % indicates weight, and 1 ppm is 0.0001 wt%.

[0043] In one embodiment of the present invention, the inclusion of additional elements refers to the replacement of a portion of the remaining iron (Fe) by additional elements, the replacement amount being equivalent to the amount of additional elements added.

[0044] 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.

[0045] Embodiments of the present invention will be described in detail below to enable those skilled in the art to implement the invention. However, the invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0046] According to one embodiment of the present invention, the non-oriented electrical steel sheet, by weight percent, comprises Si: 3.0 to 4.0%, Al: 0.3 to 1.5%, Mn: 0.1 to 0.6%, one or more of Sn and Sb: 0.006 to 0.1%, C: 0.0015 to 0.0040%, Cr: 0.01 to 0.03%, Cu: 0.003 to 0.008%, Mg: 0.0005 to 0.0025%, with the balance comprising Fe and unavoidable impurities.

[0047] The reasons for the compositional restrictions on non-oriented electrical steel sheets are explained below.

[0048] Si: 3.0 to 4.0% by weight

[0049] Silicon (Si) increases the resistivity of materials, reduces iron loss, and improves strength through solid solution strengthening. If the amount of Si added is too small, the effect on improving iron loss and strength may be insufficient. When the amount of Si added is too large, rolling productivity drops rapidly due to increased brittleness of the material, and there is a problem of forming oxide layers and surface oxides that are detrimental to magnetism. Therefore, 3.0 to 4.0 wt% Si may be included. More specifically, 3.1 to 3.8 wt% Si may be included.

[0050] Al: 0.3 to 1.5% by weight

[0051] Aluminum (Al) plays a crucial role in increasing resistivity and reducing iron loss, and also enhances strength through solid solution strengthening. If too little Al is added, the improvement in magnetic properties will be difficult to achieve due to the formation of fine nitrides or the inability to form a dense surface oxide layer. If too much Al is added, excessive nitride formation may occur, potentially degrading the magnetism and causing problems in all processes, including steelmaking and continuous casting, thus reducing productivity. Therefore, 0.30 to 1.50% by weight of Al may be included. More specifically, 0.40 to 1.30% by weight may be included.

[0052] Mn: 0.1 to 0.6% by weight

[0053] Manganese (Mn) improves iron loss by increasing resistivity and forms sulfides. Insufficient Mn content leads to the formation of fine sulfides, causing magnetic degradation; excessive Mn content results in the precipitation of excessive fine MnS, promoting the formation of an unfavorable {111} texture and causing a rapid decrease in magnetic flux density. Therefore, the content can be from 0.1% to 0.6% by weight of Mn. More specifically, it can be from 0.2% to 0.5% by weight.

[0054] One or more of Sn and Sb: 0.006 to 0.100% by weight

[0055] Tin (Sn) and antimony (Sb) segregate on the surface and grain boundaries of the steel sheet, inhibiting surface oxidation during annealing, hindering element diffusion through grain boundaries, and impeding recrystallization of the {111} / / ND orientation, thus contributing to improved texture. If the amount of Sn and Sb added is too small, the aforementioned effects are insufficient. When the amount of Sn and Sb added is too large, the increased grain boundary segregation leads to decreased toughness, potentially resulting in reduced productivity compared to improved magnetism. Therefore, it may include 0.006 to 0.100 wt% of one or more of Sn and Sb. More specifically, it may include 0.010 to 0.070 wt%. "One or more of Sn and Sb" means that when Sn or Sb is included alone, it refers to their individual content; when Sn and Sb are included simultaneously, it refers to the total amount of Sn and Sb.

[0056] C: 0.0015 to 0.0040% by weight

[0057] Carbon (C) causes magnetic aging and combines with other impurity elements to form carbides, reducing magnetism, hindering dislocation slip, and thus increasing strength. Therefore, the lower the carbon (C) content, the better. However, in one embodiment of the invention, because appropriate amounts of Cr, Cu, and Mg are included, even if the content is higher than a certain amount, it does not affect the magnetism. Therefore, it can be contained at 0.0015% by weight or more. Specifically, it can be contained at 0.0015 to 0.0040% by weight. More specifically, it can be contained at 0.0020 to 0.0035% by weight.

[0058] Cr: 0.0100 to 0.0300% by weight

[0059] While chromium (Cr) has a weak tendency to form fine precipitates, it can hinder the formation of a surface Al oxide layer, leading to the formation of Cr-based carbides and thus deteriorating magnetic properties. Insufficient Cr content results in an excessively thick Al oxide layer, or the formation of circular oxides or nitrides on the surface, also causing magnetic degradation. Excessive Cr content makes it difficult to form a dense oxide layer, potentially leading to magnetic degradation. Therefore, 0.0100% to 0.0300% by weight of Cr may be included. More specifically, 0.0120% to 0.0275% by weight of Cr may be included.

[0060] Cu: 0.0030 to 0.0080 wt%

[0061] Copper (Cu) is an element that can form sulfides at high temperatures, and its addition in large quantities can affect the composition of the surface oxide layer. When added in appropriate amounts, it can coarsen fine CuS or MnCuS precipitates, leading to improved magnetism. Therefore, it may contain 0.0030% to 0.0080% by weight of Cu. More specifically, it may contain 0.0040% to 0.0077% by weight.

[0062] Mg: 0.0005 to 0.0025% by weight

[0063] Magnesium (Mg) is an element that primarily combines with sulfur to form sulfides, which can affect the oxide layer on iron-based surfaces. Therefore, it may contain 0.0005% to 0.0025% by weight of Mg. More specifically, it may contain 0.0008% to 0.0020% by weight.

[0064] According to an embodiment of the present invention, the non-oriented electrical steel sheet satisfies the following formula 1.

[0065] [Formula 1]

[0066] 0.66≤([Sn]+[Sb]) / ([Cr]+[Cu]+[Mg])≤2

[0067] More specifically, the value of Equation 1 can be between 0.68 and 1.95.

[0068] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of N, S, Ti, Nb and V, each in an amount of 0.0003 to 0.0030% by weight.

[0069] N: 0.0003 to 0.0030% by weight

[0070] Nitrogen (N) not only forms fine AlN precipitates within the steel sheet, but also combines with other impurities to form fine precipitates that inhibit grain growth and worsen iron loss. Therefore, it may contain 0.0003 to 0.0030% by weight of N. Specifically, it may contain 0.0005 to 0.0025% by weight.

[0071] S: 0.0003 to 0.0030% by weight

[0072] Sulfur (S) forms fine precipitates such as MnS, CuS, and (Mn, Cu)S, which degrade magnetic properties and hot workability; therefore, its content is best controlled at a low level. Thus, it may contain 0.0003 to 0.0030% by weight of S. Specifically, it may contain 0.0005 to 0.0025% by weight.

[0073] Ti: 0.0003 to 0.0030% by weight

[0074] Titanium (Ti) has a very strong tendency to form precipitates in steel, creating fine carbides, nitrides, or sulfides within the base metal, inhibiting grain growth and worsening iron loss. Therefore, the steel may contain less than 0.0040% by weight of Ti. Specifically, it may contain less than 0.002%.

[0075] Nb: 0.0003 to 0.0030% by weight

[0076] Niobium (Nb) reduces iron loss by inhibiting grain growth and domain wall movement through the formation of fine carbides or nitrides within the matrix. Therefore, it may contain less than 0.0040% by weight of Ti. Specifically, it may contain less than 0.002%.

[0077] V: 0.0003 to 0.0030% by weight

[0078] Vanadium (V) reduces iron loss by inhibiting grain growth and domain wall movement through the formation of fine carbides or nitrides within the matrix. Therefore, it may contain less than 0.0040% by weight of V. Specifically, it may contain less than 0.002%.

[0079] According to one embodiment of the present invention, the non-oriented electrical steel sheet may further contain one or more of P: 0.005 to 0.05 wt%, Mo: 0.001 to 0.01 wt%, and Ni: 0.005 to 0.04 wt%.

[0080] P: 0.0005 to 0.050% by weight

[0081] Phosphorus (P) segregates on the surface and grain boundaries of the steel sheet, inhibiting surface oxidation during annealing, hindering element diffusion through grain boundaries, and impeding recrystallization of the {111} / / ND orientation, thus improving the texture. If too little P is added, the effect may be insufficient. If too much P is added, hot workability may decrease, and productivity may decrease compared to the improvement in magnetic properties. Therefore, when further containing P, it may contain 0.005 to 0.050% by weight of P. More specifically, it may further contain 0.007 to 0.045% by weight of P.

[0082] Mo: 0.001 to 0.01% by weight

[0083] Molybdenum (Mo) segregation on the steel plate surface and at grain boundaries plays a role in improving texture. If too little Mo is added, the development of the {111} texture leads to deterioration of magnetic properties. If too much Mo is added, it inhibits the segregation of Sn and P, resulting in a reduced texture-improving effect. Therefore, when further incorporating Mo, 0.001 to 0.01% by weight of Mo can be included.

[0084] Ni: 0.005 to 0.04% by weight

[0085] Nickel (Ni) increases the ductility of steel and promotes the segregation of Sn and P. However, if too much Ni is added, the magnetic flux density may decrease rapidly. Therefore, when further Ni is included, its content can be from 0.005 to 0.04% by weight.

[0086] The balance includes Fe and unavoidable impurities. Unavoidable impurities are those introduced during the steelmaking process and the manufacturing process of grain-oriented electrical steel sheets; these impurities are well-known in the art and therefore omitted in detail. In one embodiment of the invention, the addition of other elements besides the aforementioned alloy composition is not excluded, and various elements may be included without affecting the technical concept of the invention. When additional elements are further included, they replace a portion of the Fe in the balance.

[0087] As unavoidable impurities, such as B and Zr, it is necessary to include B: less than 0.002% by weight and Zr: less than 0.005% by weight.

[0088] Figure 1 This shows a cross-section of a non-oriented electrical steel sheet according to an embodiment of the present invention. For example... Figure 1 As shown, an oxide layer 20 exists from the surface of the electrical steel sheet 100 inwards. The electrical steel sheet 100 with the oxide layer 20 removed is the electrical steel sheet base material 10.

[0089] Because the electrical steel sheet 100 is exposed to oxygen during the manufacturing process, oxygen in the atmosphere can penetrate into the interior of the steel sheet, and there may be an oxygen concentration gradient from the surface to the interior.

[0090] The oxide layer 20 and the base material 10 can be divided into an oxide layer 20 with an oxygen content of 40% by weight or more and a base material 10 with an oxygen content of less than 40% by weight. The thickness of the oxide layer 20, distinguished in this way, can be 10 to 50 nm. Thus, since an oxide layer 20 of appropriate thickness is formed, the diffusion of nitrogen from the annealing atmosphere into the base material is suppressed, and the formation of fine nitrides is inhibited, thereby improving magnetism. The thickness of the oxide layer 20 may vary across the entire steel plate surface; in one embodiment of the invention, the thickness of the oxide layer 20 refers to the average thickness within the steel plate.

[0091] In addition to the oxygen present due to oxygen permeation during the manufacturing process, the oxide layer 20 also contains a large amount of Al that has been concentrated due to diffusion from the parent material 10. On the other hand, due to the increase in Al and O, the Si content may be relatively reduced.

[0092] Specifically, oxide layer 20 may contain 1.0 to 30 wt% Al and 0.5 to 10.0 wt% Si. More specifically, oxide layer 20 may contain 40 to 70 wt% O, 1 to 30 wt% Al, 0.5 to 10.0 wt% Si, and the balance Fe and unavoidable impurities. Thus, by forming an Al-concentrated oxide layer, the formation of round oxides or fine nitrides within the substrate is suppressed, thereby improving magnetism. Similar to O, in the case of Al, a concentration gradient exists from the substrate to the surface, the range representing the average content in oxide layer 20.

[0093] The weight ratio of Al content to Si content in oxide layer 20 can be from 5 to 20. Thus, as the Al content in oxide layer 20 increases, a dense oxide layer can be formed to suppress the formation of fine precipitates beneath the surface during the final annealing process, thereby obtaining excellent magnetic properties. More specifically, the weight ratio of Al content to Si content in oxide layer 20 can be from 7.0 to 17.0.

[0094] According to one embodiment of the present invention, the average grain size of the non-oriented electrical steel sheet can be 55 to 75 μm. Within this range, the non-oriented electrical steel sheet exhibits superior magnetic properties.

[0095] Grain size can be calculated as (measured area ÷ number of grains)^0.5. The grain size can be measured using a plane parallel to the rolling surface (ND surface) as a reference, and can be measured within the base material 10. Specifically, the average grain diameter can be 60 to 70 μm.

[0096] In one embodiment of the invention, the density of AlN precipitates on the surface can be reduced by appropriately controlling the alloy composition. Specifically, the distribution density of AlN precipitates with a diameter of 10 to 500 nm at a depth of less than 2 μm from the surface of the steel plate inward can be 3 precipitates / mm². 2 Therefore, by reducing the distribution density of AlN inclusions, fine precipitates that hinder domain wall movement can be suppressed, thereby contributing to improved magnetic properties. More specifically, the distribution density of AlN precipitates can be from 0.5 to 2.5 inclusions / mm². 2 At this point, the diameter of AlN can be measured using a plane parallel to the rolling surface (ND surface) as a reference. The diameter of AlN can be the diameter of an assumed circle, which is an assumed circle with the same area as AlN.

[0097] The thickness of the steel plate can be from 0.10 to 0.35 mm.

[0098] As described above, in one embodiment of the present invention, magnetism can be improved by proposing an optimal alloy composition and improving precipitation characteristics. Specifically, the iron loss (W) of non-oriented electrical steel sheets... 10 / 400 The flux density (B50) can be below 12.5 W / kg, and the magnetic flux density (B50) can be above 1.650 T. Iron loss (W 10 / 400 ) refers to the iron loss required to induce a magnetic flux density of 1.0T at a frequency of 400Hz. Magnetic flux density (B 50 ) is the magnetic flux density induced in 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 below 11.6 W / kg, and the magnetic flux density (B50) can be above 1.660 T.

[0099] A method for manufacturing non-oriented electrical steel sheet according to an embodiment of the present invention includes: a step of manufacturing a slab; a step of hot rolling the slab to manufacture a hot-rolled sheet; a step of cold rolling the hot-rolled sheet to manufacture a cold-rolled sheet; and a step of final annealing the cold-rolled sheet.

[0100] The following is a detailed explanation of each step.

[0101] First, the slab is manufactured.

[0102] Regarding the alloy composition of the slab, the alloy composition of the non-oriented electrical steel sheet has already been described, so a repeat description is omitted. The alloy composition does not substantially change during the manufacturing process of the non-oriented electrical steel sheet; therefore, the alloy composition of the non-oriented electrical steel sheet and the slab is actually the same.

[0103] The slab, by weight percent, comprises Si: 3.0 to 4.0%, Al: 0.3 to 1.5%, Mn: 0.1 to 0.6%, one or more of Sn and Sb: 0.006 to 0.1%, C: 0.0015 to 0.0040%, Cr: 0.01 to 0.03%, Cu: 0.003 to 0.008%, Mg: 0.0005 to 0.0025%, with the balance comprising Fe and unavoidable impurities.

[0104] Since other added elements have already been described in the alloy composition of non-oriented electrical steel sheets, repeated descriptions are omitted.

[0105] The slab can be heated before hot rolling. There is no limit to the heating temperature of the slab, 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 reducing magnetism.

[0106] Next, hot-rolled sheets are manufactured by hot rolling the slab. The thickness of the hot-rolled sheet can be 2 to 2.3 mm. In the manufacturing process of the hot-rolled sheet, the final rolling temperature can be above 800°C. Specifically, it can be 800–1000°C. The hot-rolled sheet can be coiled at a temperature below 700°C.

[0107] After the manufacturing process of the hot-rolled sheet, an annealing step may be included. In this case, the annealing temperature can be between 850 and 1150°C. If the annealing temperature is too low, the microstructure will not grow or will be very small, making it difficult to obtain a magnetically favorable texture during cold rolling followed by annealing.

[0108] If the annealing temperature is too high, the grains may overgrow and the surface defects of the plate may become excessive. Annealing of hot-rolled plates is performed as needed to improve the orientation favorable to magnetism and can be omitted. Annealed hot-rolled plates can be pickled.

[0109] Next, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet. The thickness of the cold-rolled sheet can be from 0.10 to 0.35 mm. During the cold rolling process, the reduction rate can be adjusted to 85% or higher. Specifically, the reduction rate can be from 85% to 95%. If the reduction rate is too low, thickness deviations may occur in the width direction of the steel sheet.

[0110] Next, the cold-rolled sheet undergoes final annealing. Annealing can be performed by maintaining a soaking temperature above 900°C for at least 15 seconds. The iron loss of non-oriented electrical steel sheets is closely related to grain diameter, therefore annealing can be carried out at appropriate temperatures and times. More specifically, annealing is performed at a soaking temperature of 950 to 1100°C for 30 to 150 seconds.

[0111] The final annealing can be carried out in an atmosphere containing less than 40% by volume of hydrogen and more than 60% by volume of nitrogen, with a dew point of 0 to -40°C. Specifically, the final annealing can be carried out in an atmosphere containing 5 to 40% by volume of hydrogen and 60% to 95% by volume of nitrogen. During the final annealing process, an average grain diameter of 55 to 75 μm can be formed, and all processed structures formed in the previous cold rolling step (i.e., more than 99%) can be recrystallized.

[0112] After final annealing, an insulating film can be formed. This insulating film can be treated with organic, inorganic, or organic / inorganic composite film layers, or with other insulating films.

[0113] The invention will be described in more detail below by way of examples. However, these examples are merely illustrative and the invention is not limited to the examples described herein.

[0114] Example 1

[0115] Slabs were prepared according to Tables 1 and 2, with the balance including Fe and unavoidable impurities.

[0116] The slab was heated to 1,150°C and hot-rolled at a finishing temperature of 830°C to prepare a hot-rolled sheet with a thickness of 2.3 mm. The hot-rolled sheet was annealed at 1030°C for 100 seconds and then cold-rolled to a thickness of 0.27 mm. It was then subjected to recrystallization annealing at 950°C for 88 seconds.

[0117] Table 3 shows the oxide layer thickness, Al and Si content in the oxide layer, AlN distribution density in the surface layer, W10 / 400 iron loss, and B50 magnetic flux density for each sample.

[0118] The oxide layer thickness is determined by preparing a smooth cross-section using FIB treatment and then photographing it at high magnification using TEM. The average value of the oxide layer thickness is measured at more than 10 points on the surface of the base material.

[0119] For AlN, the steel plate surface was ground to a thickness of 1 μm, and a high-magnification TEM image was taken to measure 2500 μm. 2 The number of AlNs in the above regions is summarized in Table 3.

[0120] Magnetic properties such as magnetic flux density and iron loss were measured. Each sample was cut into 5 pieces (60mm wide × 60mm long). A single sheet tester was used to measure the magnetic flux density and iron loss in the rolling direction and perpendicular to the rolling direction. The average values ​​were used as the magnetic flux density and iron loss. Here, W10 / 400 represents the iron loss induced at a magnetic flux density of 1.0T at a frequency of 400Hz, and B50 represents the magnetic flux density induced in a magnetic field of 5000A / m.

[0121] Table 1

[0122]

[0123] Table 2

[0124]

[0125] Table 3

[0126]

[0127] As shown in Tables 1 to 3, by appropriately controlling the alloy composition of A4, B4, C3, C4, D3, and D4, an oxide layer is appropriately formed, resulting in a small amount of AlN. Therefore, it can be confirmed that the magnetic properties are excellent.

[0128] Conversely, because the Cr content in Al is too low, the oxide layer cannot be properly formed, resulting in a large amount of AlN, which confirms poor magnetic properties.

[0129] It can be confirmed that the Mg content of A2 is too low, failing to form a proper oxide layer and resulting in a large amount of AlN, leading to poor magnetism.

[0130] In A3, the value of Equation 1 is too large, failing to form a suitable oxide layer and resulting in a large amount of AlN, which confirms poor magnetic properties.

[0131] B1 contains a large amount of Sn and Sb. The value of Equation 1 is too large, which fails to form a proper oxide layer and forms a large amount of AlN, resulting in poor magnetism.

[0132] Because B2 contains a large amount of Mg, it does not form a proper oxide layer, resulting in a large amount of AlN and thus poor magnetism.

[0133] In B3, the value of Equation 1 is too small, so an appropriate oxide layer is not formed, resulting in a large amount of AlN and poor magnetism.

[0134] C1 contains a large amount of Cu, so a proper oxide layer is not formed, resulting in a large amount of AlN and poor magnetism.

[0135] C2 contains small amounts of Sn and Sb. The value of Equation 1 is too small, so an appropriate oxide layer is not formed, resulting in a large amount of AlN and poor magnetism.

[0136] The Cu content in D1 is too low, so an appropriate oxide layer is not formed, resulting in a large amount of AlN and poor magnetism.

[0137] The excessive Cr content in D2 prevents the formation of a proper oxide layer, resulting in the formation of a large amount of AlN and thus poor magnetic properties.

[0138] The D5 has too little Al content, so an appropriate oxide layer is not formed, resulting in poor magnetism.

[0139] This invention can be implemented in various ways and is not limited to the embodiments described above. 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.

[0140] Symbol Explanation

[0141] 100: Non-oriented electrical steel sheet; 10: Base material; 20: Oxide layer

Claims

1. A non-oriented electrical steel sheet, wherein, The non-oriented electrical steel sheet, by weight percent, comprises Si: 3.0 to 4.0%, Al: 0.3 to 1.5%, Mn: 0.1 to 0.6%, one or more of Sn and Sb: 0.006 to 0.1%, C: 0.0015 to 0.0040%, Cr: 0.01 to 0.03%, Cu: 0.003 to 0.008%, Mg: 0.0005 to 0.0025%, with the balance being Fe and unavoidable impurities. An oxide layer exists from the surface of the steel plate to its interior, and the thickness of the oxide layer is 10 to 50 nm. Satisfy the following equation 1, [Formula 1] 0.66≤([Sn]+[Sb]) / ([Cr]+[Cu]+[Mg])≤2 In Formula 1, [Sn], [Sb], [Cr], [Cu] and [Mg] represent the weight % content of Sn, Sb, Cr, Cu and Mg, respectively.

2. The non-oriented electrical steel sheet according to claim 1, wherein, It further comprises one or more of N, S, Ti, Nb and V, each in amounts ranging from 0.0003 to 0.0030% by weight.

3. The non-oriented electrical steel sheet according to claim 1, wherein, It further comprises one or more of P: 0.005 to 0.05 wt%, Mo: 0.001 to 0.01 wt%, and Ni: 0.005 to 0.04 wt%.

4. The non-oriented electrical steel sheet according to claim 1, wherein, The average grain size is 55 to 75 μm.

5. The non-oriented electrical steel sheet according to claim 1, wherein, The oxide layer comprises 1.0 to 30 wt% Al and 0.5 to 10.0 wt% Si.

6. The non-oriented electrical steel sheet according to claim 1, wherein, In the oxide layer, the weight ratio of Al content to Si content is 5 to 20.

7. The non-oriented electrical steel sheet according to claim 1, wherein, Within a depth of 2 μm from the surface of the steel plate inward, the distribution density of AlN precipitates with diameters ranging from 10 to 500 nm is 3 precipitates / mm². 2 the following.

8. The non-oriented electrical steel sheet according to claim 1, wherein, The thickness of the non-oriented electrical steel sheet is 0.10 to 0.35 mm.

9. A method for manufacturing a non-oriented electrical steel sheet, wherein, The manufacturing method comprises: hot rolling a slab to manufacture a hot-rolled plate, wherein the slab, by weight percent, comprises Si: 3.0 to 4.0%, Al: 0.3 to 1.5%, Mn: 0.1 to 0.6%, one or more of Sn and Sb: 0.006 to 0.1%, C: 0.0015 to 0.0040%, Cr: 0.01 to 0.03%, Cu: 0.003 to 0.008%, Mg: 0.0005 to 0.0025%, with the balance being Fe and unavoidable impurities, satisfying Formula 1 below; The step of cold rolling the hot-rolled sheet to manufacture a cold-rolled sheet; as well as The final annealing step is performed on the cold-rolled sheet. An oxide layer exists from the surface to the interior of the steel plate, and the thickness of the oxide layer is 10 to 50 nm. [Formula 1] 0.66≤([Sn]+[Sb]) / ([Cr]+[Cu]+[Mg])≤2 In Formula 1, [Sn], [Sb], [Cr], [Cu] and [Mg] represent the weight % content of Sn, Sb, Cr, Cu and Mg, respectively.

10. The method for manufacturing non-oriented electrical steel sheet according to claim 9, wherein, Prior to the step of manufacturing the hot-rolled sheet, a further step is included in heating the slab at a temperature of 1200°C.

11. The method for manufacturing non-oriented electrical steel sheet according to claim 9, wherein, In the process of manufacturing hot-rolled plates, the finishing rolling temperature is above 800℃.

12. The method for manufacturing non-oriented electrical steel sheet according to claim 9, wherein, Following the step of manufacturing the hot-rolled sheet, the process further includes an annealing step of the hot-rolled sheet at 850°C to 1150°C.

13. The method for manufacturing non-oriented electrical steel sheet according to claim 9, wherein, The final annealing step involves annealing at a homogenization temperature above 900°C for at least 15 seconds.

14. The method for manufacturing non-oriented electrical steel sheet according to claim 9, wherein, The final annealing step is carried out in an atmosphere containing less than 40% by volume hydrogen and more than 60% by volume nitrogen, with a dew point of 0 to -40°C.

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