Non-oriented electrical steel sheet and method for manufacturing same

Abstract

The present invention relates to a non-oriented electrical steel sheet and a method for manufacturing same, and more specifically, to a non-oriented electrical steel sheet that may preferably be used as an iron core for a drive motor for an environmentally friendly automobile or the like, and a method for manufacturing same. One aspect of the present invention has the purpose of providing a non-oriented electrical steel sheet with excellent magnetism and fatigue limit, and a method for manufacturing same.

Description

Technical Field

[0001] The present disclosure relates to a non-oriented electrical steel sheet and a method for manufacturing the same, and more particularly, to a non-oriented electrical steel sheet which may be preferably used as an iron core of a driving motor for an eco-friendly vehicle, and a method for manufacturing the same.Background Art

[0002] A non-oriented electrical steel sheet may be mainly used in a motor converting electrical energy into mechanical energy, and excellent magnetic properties may be required to achieve high efficiency in the process. In particular, as eco-friendly vehicles driven by motors, instead of internal combustion engines, have recently gained attention, demand for a non-oriented electrical steel sheet used as a driving motor core material has increased, and to this end, a non-oriented electrical steel sheet having excellent magnetic properties and strength may be necessary.

[0003] Magnetic properties of a non-oriented electrical steel sheet may be mainly evaluated by iron loss and magnetic flux density. Iron loss may refer to energy loss occurring at a specific magnetic flux density and frequency, and magnetic flux density may refer to the degree of magnetization obtained under a specific magnetic field. The lower the iron loss, the more energy-efficient the motor may be manufactured under the same conditions, and as magnetic flux density increases, the motor may have a reduced size or copper loss may be reduced. Therefore, a driving motor with excellent efficiency and torque may be manufactured using a non-oriented electrical steel sheet having low iron loss and high magnetic flux density, thereby improving a driving range and output of an eco-friendly vehicle.

[0004] The characteristics of the non-oriented electrical steel sheet to be considered may also vary depending on operating conditions of a motor. As a general criteria for evaluating the characteristics of a non-oriented electrical steel sheet used in a motor, W15 / 50, which is the iron loss when a 1.5T magnetic field is applied at a commercial frequency of 50Hz, has been widely used. However, in a non-oriented electrical steel sheet having a thickness of 0.35mm or less used in an eco-friendly vehicle driving motor, magnetic characteristics may often be important at low fields of 1.0T or less and high frequencies of 400Hz or higher, such that the characteristics of a non-oriented electrical steel sheet may be evaluated using W10 / 400 iron loss or B1 magnetic flux density.

[0005] A non-oriented electrical steel sheet for an eco-friendly vehicle driving motor may require excellent fatigue limit as well as magnetic properties. Since an eco-friendly vehicle driving motor may be driven for long periods of time at various rotation speeds depending on the driving conditions of a vehicle, the fatigue limit of an electrical steel sheet may be one of the factors determining a lifespan of the vehicle. In particular, an eco-friendly vehicle driving motor may be designed with permanent magnets inserted into a rotor, and since the inserted permanent magnets exert a force to separate by continuous centrifugal force during driving, an electrical steel sheet having a high fatigue limit may be required.

[0006] A method commonly used to simultaneously increase magnetic properties and fatigue properties of a non-oriented electrical steel sheet may be to add alloy elements such as Si, Al, and Mn. The addition of these alloy elements may increase resistivity of steel, which may reduce the eddy current loss and may thus lower the overall iron loss. Also, alloy elements may be dissolved as substitutional elements in iron, and may causing a strengthening effect, thereby enhancing fatigue strength. However, as the amount of alloy elements such as Si, Al, and Mn added increases, magnetic flux density may deteriorate and brittleness may increase, and when a predetermined amount or more is added, it may be impossible to perform cold rolling, such that commercial production may be impossible. In particular, as a thickness of an electrical steel sheet decreases, the highfrequency iron loss may become excellent, but the decrease in rollability due to brittleness may become a serious problem. The maximum value of the total content of Si, Al, and Mn which may be commercially produced may be known to be approximately 4.5%, and also, by optimizing the content of trace elements, a top-quality non-oriented electrical steel sheet having excellent magnetism and strength may be produced.

[0007] Depending on a design intention of a motor, an electrical steel sheet having higher fatigue strength may also be used even when magnetic properties are relatively low. Methods for manufacturing an electrical steel sheet as above may include a method of using precipitation of interstitial elements and a method of reducing a grain size. Mainly, when miniaturizing a motor to increase a rotation speed or to increase the effect of a permanent magnet inserted into a rotor, a rotor manufactured with electrical steel having significantly improved strength may be used even when magnetic properties of the electrical steel are deteriorated. In this case, when micro-precipitates including interstitial solid-solution elements such as C, N, and S are formed, the effect of strength improvement may be desirable, but iron loss may become worse rapidly, which may lower the efficiency of the motor. Also, the method of reducing a grain size may have the disadvantage of increasing unevenness of a steel sheet material due to the inclusion of unrecrystallized regions, which may increase the quality deviation of the product.

[0008] To address the above problems, cited document 1 attempted to create a non-oriented electrical steel sheet having excellent magnetism and strength by controlling a cooling rate during the final annealing process, but it may be difficult to be applied to the mass production process due to an increase in material unevenness caused by inclusion of unrecrystallized regions. Also, most of the existing techniques proposed to simultaneously improve magnetism and strength may not be used due to reasons such as increased manufacturing costs, decreased productivity and yield, and insufficient improvement effects.[Prior art]

[0009] (Cited document 1) International Laid-Open Patent Publication No. 2009-128428Detailed description of present disclosureTechnical problems to solve

[0010] An aspect in the present disclosure is to provide a non-oriented electrical steel sheet, and a method for manufacturing the same.

[0011] A preferable aspect in the present disclosure is to provide a non-oriented electrical steel sheet having excellent magnetic properties and fatigue limit, and a method for manufacturing the same.Solution to Problem

[0012] According to an embodiment of the present disclosure, a non-oriented electrical steel sheet includes, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities, and a ratio (D0.9 / D0.0) between an average grain diameter (D0.0) of a central portion and an average grain diameter (D0.9) of a surface portion is 0.55 to 0.85,

[0013] where the surface portion indicates a region from a surface of the steel sheet to 1 / 10t (t: thickness of steel material), and the central portion indicates a region other than the surface portion.

[0014] The non-oriented electrical steel sheet may further include one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.

[0015] The non-oriented electrical steel sheet may further include one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.

[0016] The non-oriented electrical steel sheet may further include 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.

[0017] The surface portion may have 0.02 to 0.20 / µm 2< carbides having a diameter of 20 to 200 nm.

[0018] The non-oriented electrical steel sheet may have an average grain diameter of 50 to 100 µm.

[0019] The non-oriented electrical steel sheet may have a thickness of 0.1 to 0.35 mm.

[0020] The non-oriented electrical steel sheet may have a fatigue limit of 310 MPa or higher at 90°C.

[0021] The non-oriented electrical steel sheet may have a magnetic flux density (B1) of 1.12T or higher, a magnetic flux density (B50) of 1.67T or higher, and an iron loss (W10 / 400) of 11.4W / Kg or lower.

[0022] According to another embodiment of the present disclosure, a method for manufacturing a non-oriented electrical steel sheet includes heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities, finishing hot-rolling the heated slab and obtaining a hot-rolled sheet; hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more; pickling the hot-rolled sheet annealed hot-rolled sheet, cold-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a cold-rolled sheet; and final annealing the cold-rolled sheet.

[0023] According to another embodiment of the present disclosure, a method for manufacturing a non-oriented electrical steel sheet includes heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities; finishing hot-rolling the heated slab and obtaining a hot-rolled sheet; hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more; pickling the hot-rolled sheet annealed hot-rolled sheet, warm-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a warm-rolled sheet; and final annealing the warm-rolled sheet.

[0024] The slab may further include one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.

[0025] The slab may further include one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.

[0026] The slab may further include 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.

[0027] The heating the slab may be performed at 1050 to 1200°C.

[0028] The finishing hot-rolling may be performed at 800 to 950°C.

[0029] The final annealing may be performed at 850°C or lower.

[0030] The final annealing may be performed in an atmosphere in which hydrogen (H 2 ) and nitrogen (N 2 ) gases are mixed.Advantageous Effects of Invention

[0031] According to an aspect of the present disclosure, a non-oriented electrical steel sheet and a method for manufacturing the same may be provided.

[0032] According to a preferable aspect of the present disclosure, a non-oriented electrical steel sheet having excellent magnetic properties and fatigue limit and a method for manufacturing the same.Best Mode for Invention

[0033] Hereinafter, a non-oriented electrical steel sheet according to an embodiment of the present invention may be described. First, an alloy composition may be described. The content of the alloy composition described below may be indicated in weight% unless otherwise indicated.Si: 3.3 to 3.8%

[0034] Si may reduce iron loss by increasing resistivity of a material, and may increase strength through solid-solution strengthening. When the content of Si is less than 3.3%, the effect of addressing iron loss and improving strength may be insufficient. When the content of Si exceeds 3.8%, brittleness of the material may increase, which may rapidly reduce rolling productivity, and surface portion oxide layers and oxides which may be harmful to magnetic properties may be formed. Accordingly, the content of the Si may range from 3.3 to 3.8% preferably.Al: 0.4 to 1.5%

[0035] Al may reduce iron loss by increasing resistivity of the material, and may increase strength through solid-solution strengthening. When the content of Al is less than 0.4%, fine nitrides may be formed, such that it may be difficult to obtain the effect of improving magnetic properties. When the content of Mn exceeds 1.5%, nitrides may be excessively formed, which may deteriorate magnetic properties and may cause problems in the entirety of processes such as steelmaking and continuous casting, such that productivity may be significantly reduced. Accordingly, the content of Al may have a range of 0.4 to 1.5% preferably. A lower limit of the Al content may more preferably be 0.6%. An upper limit of the Al content may more preferably be 1.3%.Mn: 0.2 to 1.5%

[0036] Mn may improve iron loss by increasing resistivity of the material and may form sulfides. When the content of Mn is less than 0.2%, sulfides may be finely formed, which may deteriorate magnetic properties. When the Mn content exceeds 1.5%, fine MnS may be excessively precipitated, formation of {111} texture, which is unfavorable for magnetic properties, may be promoted, such that magnetic flux density may decrease rapidly. Accordingly, the Mn content may range from 0.2 to 1.5% preferably. A lower limit of the Mn content may more preferably be 0.3%. An upper limit of the Mn content may more preferably be 1.3%.C: 0.0025% or less (excluding 0%)

[0037] C may cause magnetic aging and may combine with other impurity elements and may form carbides, thereby hindering grain boundary or domain wall movement and deteriorating magnetic properties. When the C content exceeds 0.0025%, fine carbides may increase rapidly, which may seriously deteriorate magnetic properties. Accordingly, the content of C may range from 0.0025% or less (excluding 0%) preferably.S: 0.005% or less (excluding 0%)

[0038] S may form fine precipitates, MnS, which may deteriorate magnetic properties and hot workability. When the content of S exceeds 0.005%, it may be difficult to ensure sufficient magnetic properties and hot workability. Accordingly, the content of S may preferably range from 0.005% or less (excluding 0%). The content of S may more preferably be 0.003% or less.N: 0.005% or less (excluding 0%)

[0039] N may form fine AlN precipitates in a base material, and may also combine with other impurities and may form fine precipitates, such that grain growth and domain wall movement may be inhibited, and iron loss may worsen. When the N content exceeds 0.005%, fine nitrides may increase rapidly, such that iron loss may worsen. Accordingly, the N content may preferably have a range of 0.005% or less (excluding 0%). The N content may more preferably be 0.002% or less.Ti: 0.003% or less (excluding 0%)

[0040] Ti may have a relatively strong tendency to form precipitates in steel, and may form fine carbides, nitrides, or sulfides in a base material, such that grain growth and domain wall movement may be inhibited and iron loss may thus worsen. When the content of Ti exceeds 0.003%, it may be difficult to obtain sufficient magnetic properties. Accordingly, the content of Ti may preferably have a range of 0.003% or less (excluding 0%). The content of Ti may more preferably be 0.0025% or less, and even more preferably 0.002% or less.Nb: 0.003% or less (excluding 0%)

[0041] Nb may have a relatively strong tendency to form precipitates in steel, and may form fine carbides, nitrides, or sulfides in a base material, such that grain growth and domain wall movement may be inhibited and iron loss may thus worsen. When the content of Nb exceeds 0.003%, it may be difficult to obtain sufficient magnetic properties. Accordingly, the content of Nb may preferably have a range of 0.003% or less (excluding 0%). The content of Nb may more preferably be 0.0025% or less, and even more preferably 0.002% or less.V: 0.003% or less (excluding 0%)

[0042] V may have a relatively strong tendency to form precipitates in steel, and may form fine carbides, nitrides, or sulfides in a base material, such that grain growth and domain wall movement may be inhibited and iron loss may thus worsen. When the V content exceeds 0.003%, it may be difficult to obtain sufficient magnetic properties. Accordingly, the V content may preferably have a range of 0.003% or less (excluding 0%). The V content may more preferably be 0.0025% or less, and even more preferably 0.002% or less.W: 0.0005 to 0.0050%

[0043] W may affect the behavior of carbide in steel, and may promote grain size gradient by inducing differences in carbide formation behavior depending on a thickness layer at a temperature of 700°C or lower. When the W content is less than 0.0005%, it may be difficult to induce differences in carbide formation behaviors between the surface portion and the central portion. When the W content exceeds 0.0050%, carbide formation between the surface portion and the central portion may be promoted, which may seriously deteriorate magnetic properties. Accordingly, the W content may preferably have a range of 0.0005 to 0.0050%. A lower limit of the W content may more preferably be 0.001%. An upper limit of the W content may more preferably be 0.0040%.Sn and Sb: 0.005 to 0.1% in total

[0044] Sn and Sb may segregate at the initial grain boundary during final recrystallization annealing and may suppress development of {111} orientation, which may deteriorate magnetic properties. When the total content of Sn and Sb is less than 0.005%, it may be difficult to suppress the development of {111} orientation. When the total content of Sn and Sb exceeds 0.1%, it may deteriorate surface quality, which may degrade product productivity. Accordingly, the total content of Sn and Sb may preferably range from 0.005 to 0.1%. A lower limit of the total content of Sn and Sb may more preferably be 0.015%. An upper limit of the total content of Sn and Sb may more preferably be 0.08%.

[0045] The non-oriented electrical steel sheet of the present invention may further include one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.P: 0.1% or less

[0046] P may be a grain boundary segregation element, which may delay recrystallization and may deteriorate strength uniformity in the rolling direction and the vertical rolling direction, and thus, an upper limit may be limited to 0.1%. More specifically, the P content may be 0.0001 to 0.1%. More specifically, the P content may be 0.001 to 0.05%.Cr: 0.01 to 0.5%

[0047] Cr may improve iron loss by increasing resistivity. When the Cr content is less than 0.01%, the effect of resistivity improvement may not be sufficient. When the Cr content exceeds 0.5%, magnetic flux density may decrease. More specifically, the Cr content may be 0.02 to 0.3%.Ni: 0.05% or less

[0048] Ni may react with impurity elements and may form fine sulfides, carbides, and nitrides, which may detrimentally affect magnetic properties, and thus, an upper limit may be limited to 0.05%. More specifically, the content of Ni may be 0.0001 to 0.050%. More specifically, the content of Ni may be 0.001 to 0.030%.Cu: 0.005 to 0.2%

[0049] Cu may form sulfides with Mn. When the content of Cu is less than 0.005%, (Cu·Mn)S may be finely precipitated and may deteriorate magnetic properties. When the content of Cu exceeds 0.2%, high-temperature embrittlement may occur, which may form cracks during casting or hot rolling. More specifically, the content of Cu may be 0.010 to 0.1%.Zn: 0.01% or less

[0050] Zn may act as an impurity and may deteriorate magnetic properties, and thus, an upper limit may be limited to 0.01%. More specifically, the content of Zn may be 0.0001 to 0.01%. More specifically, the content of Zn may be 0.001 to 0.008%.

[0051] The non-oriented electrical steel sheet of the present invention may further include one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.Mo: 0.03% or less

[0052] Since the Mo may inevitably react with C, S, N, or the like, which may be inevitably added, and may form fine carbides, nitrides, or sulfides, which may adversely affect magnetic properties. Thus, an upper limit thereof may be limited to 0.03%.B: 0.002% or less

[0053] B may form inclusions in steel, and may deteriorate magnetic properties. When the content of B exceeds 0.002%, it may be difficult to ensure excellent magnetic properties. Accordingly, the content of B may preferably have a range of 0.002% or less (excluding 0%). The B content may more preferably be 0.0005% or less.Mg: 0.005% or less

[0054] The Mg may form inclusions in steel, and may deteriorate magnetic properties. When the content of Mg exceeds 0.005%, it may be difficult to ensure excellent magnetic properties. Accordingly, the content of Mg may preferably have a range of 0.005% or less (excluding 0%). The Mg content may more preferably be 0.002% or less.Ca: 0.005% or less

[0055] The Ca may react with C, S, N, or the like, which may be inevitably included, and may form fine carbides, nitrides, or sulfides, which may adversely affect magnetic properties, and thus, an upper limit may be limited to 0.005%.Zr: 0.005% or less

[0056] The Zr may form inclusions in steel, and may deteriorate magnetic properties. When the content of Zr exceeds 0.005%, it may be difficult to ensure excellent magnetic properties. Accordingly, the content of Zr may preferably have a range of 0.005% or less (excluding 0%). The Zr content may more preferably be 0.002% or less.

[0057] The non-oriented electrical steel sheet of the present invention may further include 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.

[0058] When the aforementioned elements are additionally added, the elements may segregate at the grain boundary, may relieve stress concentration at the grain boundary during cold-rolling, and may suppress recrystallization of <111> / / ND orientation grains in the subsequent recrystallization annealing process, thereby improving magnetic flux density. When they are added appropriately, the aforementioned effect may be additionally obtained, but when they are included excessively, a large amount of segregation may occur, which may suppress grain growth and may deteriorate magnetic flux density and iron loss. More specifically, 0.0001 to 0.20% of one or more of Bi, Pb, Ge and As may be included individually or in combination. More specifically, 0.001 to 0.10% of one or more of Bi, Pb, Ge and As may be included individually or in combination.

[0059] A remainder of the present disclosure is iron (Fe). However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded. A person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.

[0060] As for the non-oriented electrical steel sheet of the present invention, a ratio (D0.9 / D0.0) between an average grain diameter (D0.0) of a central portion and an average grain diameter (D0.9) of a surface portion may be 0.55 to 0.85 preferably. The surface portion may have 0.02 to 0.20 / µm 2< of carbides having a diameter of 20 to 200 nm. By appropriately controlling the grain fraction of the surface portion and the grain fraction of the surface portion with optimized carbide distribution, good magnetic properties may be assured by the central portion grain having excellent aggregate structure, and also excellent fatigue properties may be assured by the surface portion grain. In particular, when the fine carbide of the surface portion is appropriately distributed, fatigue properties at a temperature higher than room temperature may be further improved. When the ratio (D0.9 / D0.0) between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion is less than 0.55, iron loss may be deteriorated due to an increase in hysteresis loss. When the ratio (D0.9 / D0.0) between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion exceeds 0.85, stress may be concentrated at the grain boundary positioned at the surface portion and the fatigue limit may be lowered. A lower limit of the ratio (D0.9 / D0.0) between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion may more preferably be 0.60. An upper limit between the ratio (D0.9 / D0.0) of the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion may more preferably be 0.80. When the number density of carbides having a diameter of 20 to 200 nm is less than 0.02 / µm 2< , the surface portion grains may grow to a similar size to that of the central portion, which may lower the fatigue limit. When the number density of carbides having a diameter of 20 to 200 nm exceeds 0.20 / µm 2< , the surface portion grains may become excessively fine and may hinder domain wall movement, such that magnetic properties may be deteriorated. Meanwhile, the surface portion may indicate a region from the surface of the steel sheet to 1 / 10t (t: thickness of steel material), and the central portion may indicate a region other than the surface portion. In the present invention, the type of carbide may not be specifically limited, and as an example, a precipitate in which one or more elements of Ti, Nb, V, W are combined with carbon may be formed.

[0061] The non-oriented electrical steel sheet of the present invention may have an average grain diameter of 50 to 100 µm. When the average grain diameter is less than 50 µm, hysteresis loss may increase rapidly and iron loss may worsen. When the average grain diameter exceeds 100 µm, stress may be concentrated at the grain boundary and the fatigue limit may be lowered.

[0062] As described above, the non-oriented electrical steel sheet of the present invention described above may have a thickness of 0.1 to 0.35 mm. Also, the non-oriented electrical steel sheet of the present invention may have a fatigue limit of 310 MPa or higher at 90°C. Also, the non-oriented electrical steel sheet of the present invention may have a magnetic flux density (B1) of 1.12 T or higher, a magnetic flux density (B50) of 1.67 T or higher, and an iron loss (W10 / 400) of 11.4 W / Kg or lower. In the present invention, the higher values of fatigue limit, magnetic flux density (B1) and magnetic flux density (B50) at 90°C may be advantageous, and thus, there is no particular limitation on an upper limit thereof. A lower value of iron loss (W10 / 400) may be advantageous, and thus, there is no particular limitation on a lower limit thereof. Meanwhile, electrical characteristics may be based on the case in which the thickness of the non-oriented electrical steel sheet is 0.25 mm.

[0063] Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention may be described.

[0064] First, a slab may be heated. The heating the slab may be performed at 1050 to 1200°C. When the slab heating temperature is lower than 1050°C, the shape may be poor after finishing rolling. When the slab heating temperature exceeds 1200°C, precipitates such as AlN and MnS may be re-precipitated in a fine size after re-dissolution, which may significantly deteriorate magnetic properties. A lower limit of the slab heating temperature may more preferably be 1100°C. An upper limit of the slab heating temperature may more preferably be 1170°C.

[0065] Thereafter, the heated slab may be finishing hot-rolled and a hot-rolled sheet may be obtained. The finishing hot-rolling may be performed at 800 to 950°C. When the finishing hot-rolling temperature is lower than 800°C, the coil shape may become poor due to deterioration in processability caused by an increase in deformation resistance. When the finishing hot-rolling temperature exceeds 950°C, an oxide layer may be excessively formed on the coil surface, which may cause defects.

[0066] Thereafter, the hot-rolled sheet may be hot-rolled sheet annealed to have an average grain diameter of 250 µm or more. When the average grain diameter of the hot-rolled sheet annealed hot-rolled sheet is less than 250 µm, it may be difficult to assure a difference in grain sizes between the central portion and the surface portion after final annealing, such that it may be difficult to assure both magnetic properties and the fatigue limit. In the present invention, the specific configuration for controlling the average grain diameter of the hot-rolled sheet annealed hot-rolled sheet to 250 µm or more may not be particularly limited. However, as an example, a method of controlling the annealing temperature and annealing time, or the like, may be used.

[0067] Thereafter, the hot-rolled sheet annealed hot-rolled sheet may be pickled and cold-rolled such that a section in which the surface temperature of the steel sheet is 400°C or higher may be included, and a cold-rolled sheet may be obtained. By including a section in which the surface temperature of the steel sheet is 400°C or higher during cold-rolling, a temperature and deformation structure advantageous for forming carbides may be formed on the surface portion, and the deformation structure occurring during cold-rolling may be partially recovered in the central portion, thereby obtaining an effect of generating a difference in grain sizes between the surface portion and the central portion. In the present invention, the higher surface temperature of the steel sheet may be advantageous during cold-rolling, there is no particular limitation on an upper limit thereof. However, an upper limit of the surface temperature of the steel sheet during cold-rolling may be, for example, 550°C.

[0068] In the present invention, there is no particular limitation on a specific configuration in which a section in which the surface temperature of the steel sheet is 400°C or higher during cold-rolling is included. However, as an example, a method of charging into a box furnace provided between passes during cold-rolling, a method of heating the rolls to a high temperature during rolling, a method of heating through an induction heating device provided before and after the rolling, or the like, may be used.

[0069] Thereafter, the cold-rolled sheet may be final annealed. The final annealing may be performed at 850°C or lower. When the final annealing temperature is lower than 850°C, recrystallization may not occur sufficiently, and magnetic properties of the steel sheet may deteriorate significantly. The final annealing may be performed in an atmosphere in which hydrogen (H 2 ) and nitrogen (N 2 ) gases are mixed.

[0070] Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention may be described.

[0071] The method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention may satisfy most of the manufacturing conditions described above, but instead of the process of pickling the hot-rolled sheet annealed hot-rolled sheet, cold-rolling the steel sheet such that the section in which the surface temperature of the steel sheet is 400°C or higher is included, and obtaining a cold-rolled sheet, a process of pickling the hot-rolled sheet annealed hot-rolled sheet, warm-rolling the steel sheet such that the section in which the surface temperature of the steel sheet is 400°C or higher is included, and obtaining a warm-rolled sheet. By including the section in which the surface temperature of the steel sheet is 400°C or higher during the warm-rolling, the effect of simultaneously improving magnetic properties and fatigue limit by generating a difference in microstructures between the surface portion and the central portion of the steel sheet during the final recrystallization annealing process may be obtained. In the present invention, the higher surface temperature of the steel sheet may be advantageous during warm-rolling, and thus, there is no particular limitation on an upper limit thereof. However, an upper limit of the surface temperature of the steel sheet during warm-rolling may be, for example, 550°C.Mode for Invention

[0072] Hereinafter, the present disclosure may be described more specifically through embodiments. However, it should be noted that the embodiments below are merely intended to describe the present disclosure in greater detail based on embodiments, and are not intended to limit the scope of the rights of the present disclosure. This may be because the scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.(Embodiment)

[0073] A slab having an alloy composition described in Table 1 below was heated at 1150°C, and hot-rolled at a finishing hot-rolling temperature of 900°C, thereby manufacturing a hot-rolled sheet having a thickness of 2.0 mm. Thereafter, the hot-rolled sheet was hot-rolled sheet annealed (at 1150°C for 120 seconds) under the conditions described in Table 2 below to obtain an average grain diameter, and cold-rolled, thereby manufacturing a cold-rolled sheet having a thickness of 0.25 mm. In this case, during the cold-rolling, the steel sheet was put in a box furnace set to the conditions described in Table 2 below between passes, and maintained for 5 minutes. Thereafter, the cold-rolled sheet was finally annealed at 1000°C for 100 seconds in an atmosphere in which hydrogen (H 2 ) and nitrogen (N 2 ) gases are mixed, and a non-oriented electrical steel sheet was manufactured.

[0074] For the non-oriented electrical steel sheet manufactured as above, the average grain diameter of the hot-rolled sheet annealed hot-rolled sheet, the ratio between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion (D0.9 / D0.0), the number density of carbides having a diameter of 20 to 200 nm in the surface portion, the average grain diameter, the fatigue limit at 90°C, and the electrical properties were measured, and the results are listed in Tables 2 and 3.

[0075] The average grain diameter of the hot-rolled sheet annealed hot-rolled sheet was measured using an optical microscope on a rolling vertical direction cross-section (TD plane) of the hot-rolled sheet after hot-rolled sheet annealing the hot-rolled sheet.

[0076] The average grain diameter (D0.0) of the central portion was measured on the surface (ND plane) of 1 / 2t (t: thickness of steel material) and 1 / 10t (t: thickness of steel material) of the non-oriented electrical steel sheet using an optical microscope.

[0077] The number density of carbides having a diameter of 20 to 200 nm in the surface portion was measured on the surface (ND plane) of 1 / 10t (t: thickness of steel material) of the non-oriented electrical steel sheet using a scanning electron microscope (TEM).

[0078] The average grain diameter was measured on the rolling vertical direction cross-section (TD plane) of the non-oriented electrical steel sheet using an optical microscope.

[0079] As for the cold-rolled sheet surface temperature during cold-rolling, the highest temperature measured at the time of ejection of the steel sheet during cold rolling and the temperature measured at the exit of the rolling roll was listed.

[0080] The fatigue limit at 90°C was obtained by drawing an SN diagram through a fatigue test at a temperature of 90°C with a stress ratio of 0.05 and a frequency of 50Hz.

[0081] Among the electrical properties, magnetic flux density (B1), magnetic flux density (B50), and iron loss (W10 / 400) were measured by collecting 5 samples measuring width of 60 mm and length of 60 mm from a non-oriented electrical steel sheet, measuring the rolling direction and the vertical rolling direction using a single sheet tester, and calculating the average value thereof. Magnetic flux density (B1) and magnetic flux density (B50) indicate magnetic flux density induced in a magnetic field of 100 A / m and 5000 A / m, respectively, and iron loss (W10 / 400) may indicate iron loss when a magnetic flux density of 1.0 T is induced at a frequency of 400 Hz. [Table 1]Steel typeSiAlMnCNSTiNbVWSn+SbCompar ative steel 13.351.000.200.003 10.000 90.001 70.000 70.000 70.000 80.001 30.078Compar ative steel 23.351.000.200.001 60.001 40.001 70.003 70.000 70.000 70.001 20.039Compar ative steel 33.351.000.200.001 70.001 40.002 30.000 80.0010.001 10.000 20.074Compar ative steel 43.351.000.200.001 80.001 60.001 00.001 50.001 80.001 70.004 60.120Invent ive steel 13.351.000.200.000 90.001 20.001 60.001 60.000 90.001 40.001 50.056Invent ive steel 23.351.000.200.001 80.000 90.000 80.001 60.001 40.001 50.001 10.084Invent ive steel 33.351.000.200.001 40.000 70.001 10.001 70.001 40.000 90.003 80.092Invent ive steel 43.351.000.200.000 70.000 80.002 10.000 90.001 20.001 50.001 00.076Compar ative steel 53.550.700.500.001 60.005 90.001 70.001 10.001 80.001 60.002 20.012Compar ative steel 63.550.700.500.001 00.001 80.001 80.001 70.004 10.001 20.002 90.052Compar ative steel 73.550.700.500.000 90.001 40.000 90.001 40.001 80.000 70.002 40.003Invent ive steel 53.550.700.500.002 30.000 70.001 80.000 90.001 40.000 70.002 30.018Invent ive steel 63.550.700.500.001 70.001 80.001 20.000 80.001 10.000 70.004 20.049Invent ive steel 73.550.700.500.001 00.001 80.001 20.000 90.000 70.000 80.003 30.008Invent ive steel 83.550.700.500.001 60.000 90.001 00.001 40.001 60.001 40.004 00.033Invent ive steel 93.550.700.500.001 70.001 80.001 70.001 70.001 80.001 70.004 20.054Compar ative steel 83.750.500.600.001 60.001 60.006 20.000 30.000 90.001 40.002 80.056Compar ative steel 93.750.500.600.001 40.001 50.001 10.001 80.001 80.003 60.004 10.080Compar ative steel 103.750.500.600.001 80.000 90.001 70.000 90.001 80.000 90.005 90.054Invent ive steel 103.750.500.600.001 40.000 90.001 40.001 80.001 40.000 70.002 60.017Invent ive steel 113.750.500.600.001 20.001 50.001 10.001 70.000 70.000 80.001 60.022Invent ive steel 123.750.500.600.001 10.000 70.000 30.000 20.001 70.000 30.002 20.069Invent ive steel 133.750.500.600.001 80.001 60.001 80.000 90.001 00.001 50.001 30.037Invent ive steel 143.750.500.600.001 60.000 90.000 90.000 90.001 60.001 60.002 90.038 [Table 2] Classif icationSteel typeAverage grain diameter of hot-rolled sheet annealed hot rolled sheet (µm)Surface temperatu re of cold-rolled sheet surface temperatu re during cold-rolling (°C)Average grain diameter (D0.0) of central portionAverage grain diameter (D0.9) of surface portionAverage grain diameter (D0.9) of surface portion / average grain diameter (D0.0) of central portionCompara tive example 1Compara tive steel 129843865330.51Compara tive example 2Compara tive steel 227741076390.51Compara tive example 3Compara tive steel 3284450107940.88Compara tive example 4Compara tive steel 4314449106540.51Inventi ve example 1Inventi ve steel 126643195660.69Inventive example 2Inventive steel 228141892730.79Inventi ve example 3Inventi ve steel 331141470540.77Inventi ve example 4Inventi ve steel 428442281550.68Compara tive example 5Compara tive steel 527344068310.46Compara tive example 6Compara tive steel 628643271370.52Compara tive example 7Compara tive steel 728441786430.50Compara tive example 8Inventi ve steel 522244094880.94Inventi ve example 5Inventi ve steel 6273440109840.77Inventi ve example 6Inventi ve steel 730044277630.82Inventi ve example 7Inventi ve steel 831042192750.82Inventi ve example 8Inventi ve steel 931444886520.60Compara tive example 9Compara tive steel 826845059300.51Compara tive example 10Compara tive steel 9307441110570.52Compara tive example 11Compara tive steel 1028244078210.27Compara tive example 12Inventi ve steel 1028438489810.91Inventi ve example 9Inventi ve steel 1131142991730.80Inventi ve example 10Inventi ve steel 1230842679580.73Inventi ve example 11Inventi ve steel 1327241188660.75Inventi ve example 12Inventi ve steel 14269417107740.69 [Table 3] Classif icationSteel typeNumber density of carbides having a diameter of 20 to 200 nm within surface portion (µm 2< )Averag e grain size (µm)Fatigue limit at 90°C (MPa)Magneti c flux density (B1) (T)Magneti c flux density (B50) (T)Iron loss (W10 / 400) (W / Kg)Compara tive example 1Compar ative steel 10.35442980.931.6412.5Compara tive example 2Compar ative steel 20.25672930.941.6412.4Compara tive example 3Compar ative steel 30.01982850.931.6412.5Compara tive example 4Compar ative steel 40.26872870.951.6412.6Inventi ve example 1Invent ive steel 10.11843181.131.6711.3Inventi ve example 2Invent ive steel 20.07863141.141.6711.4Inventi ve example 3Invent ive steel 30.06643161.131.6711.2Inventi ve example 4Invent ive steel 40.14753131.121.6711.3Compara tive example 5Compar ative steel 50.27452970.941.6412.5Compara tive example 6Compar ative steel 60.24562900.931.6412.3Compara tive example 7Compar ative steel 70.19662870.941.6412.5Compara tive example 8Invent ive steel 50.01912830.951.6412.6Inventi ve example 5Invent ive steel 60.13913121.131.6711.4Inventi ve example 6Invent ive steel 70.14703151.121.6711.4Inventi ve example 7Invent ive steel 80.06813141.131.6711.3Inventi ve example 8Invent ive steel 90.08683151.121.6711.2Compara tive example 9Compar ative steel 80.37412990.951.6412.6Compara tive example 10Compar ative steel 90.28902850.951.6412.4Compara tive example 11Compar ative steel 100.44493060.941.6412.5Compara tive example 12Invent ive steel 100.01852940.931.6412.5Inventi ve example 9Invent ive steel 110.13893131.121.6711.1Inventi ve example 10Invent ive steel 120.04713151.131.6711.3Inventi ve example 11Invent ive steel 130.15783141.121.6711.1Inventi ve example 12Invent ive steel 140.11893151.141.6711.2

[0082] As indicated in Tables 1 to 3, in inventive examples 1 to 12 satisfying the alloy composition and manufacturing conditions proposed in the present invention, the microstructure and carbide conditions which the present invention aims to obtain were satisfied, such that excellent magnetic properties and fatigue limit were assured.

[0083] In comparative examples 1 to 7 and 9 to 11, the alloy composition proposed in the present invention was not satisfied, such that a microstructure or carbide conditions which the present invention aims to obtain were not satisfied, and accordingly, magnetic properties and fatigue limit were low.

[0084] In comparative examples 8 and 12, the microstructure or carbide conditions which the present invention aims to obtain were not satisfied, and accordingly, magnetic properties and fatigue limit were low.

Claims

1. A non-oriented electrical steel sheet, comprising: by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities, wherein a ratio (D0.9 / D0.0) between an average grain diameter (D0.0) of a central portion and an average grain diameter (D0.9) of a surface portion is 0.55 to 0.85, where the surface portion indicates a region from a surface of the steel sheet to 1 / 10t, where t is a thickness of steel material, and the central portion indicates a region other than the surface portion.

2. The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet further includes one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.

3. The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet further includes one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.

4. The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet further includes 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.

5. The non-oriented electrical steel sheet of claim 1, wherein the surface portion has 0.02 to 0.20 / µm2 carbides having a diameter of 20 to 200 nm.

6. The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has an average grain diameter of 50 to 100 µm.

7. The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has a thickness of 0.1 to 0.35 mm.

8. The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has a fatigue limit of 310 MPa or higher at 90°C.

9. The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has a magnetic flux density (B1) of 1.12T or higher, a magnetic flux density (B50) of 1.67T or higher, and an iron loss (W10 / 400) of 11.4W / Kg or lower.

10. A method for manufacturing a non-oriented electrical steel sheet, the method comprising: heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities; finishing hot-rolling the heated slab and obtaining a hot-rolled sheet; hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more; pickling the hot-rolled sheet annealed hot-rolled sheet, cold-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a cold-rolled sheet; and final annealing the cold-rolled sheet.

11. A method for manufacturing a non-oriented electrical steel sheet, the method comprising: heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities; finishing hot-rolling the heated slab and obtaining a hot-rolled sheet; hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more; pickling the hot-rolled sheet annealed hot-rolled sheet, warm-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a warm-rolled sheet; and final annealing the warm-rolled sheet.

12. The method of claim 10 or 11, wherein the slab further includes one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.

13. The method of claim 10 or 11, wherein the slab further includes one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.

14. The method of claim 10 or 11, wherein the slab further includes 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.

15. The method of claim 10 or 11, wherein the heating the slab is performed at 1050 to 1200°C.

16. The method of claim 10 or 11, wherein the finishing hot-rolling is performed at 800 to 950°C.

17. The method of claim 10 or 11, wherein the final annealing is performed at 850°C or lower.

18. The method of claim 10 or 11, wherein the final annealing is performed in an atmosphere in which hydrogen (H2) and nitrogen (N2) gases are mixed.

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