Non-oriented electrical steel sheet and its manufacturing method

Abstract

An object of the present invention is to provide a non-oriented electrical steel sheet having excellent magnetic properties and fatigue limit, which can be preferably used for iron cores of environmentally friendly automobile drive motors, and a method for producing the same. [Solution] The non-oriented electrical steel sheet of the present invention contains, 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%, the sum of Sn and Sb: 0.005 to 0.1%, with the remainder being Fe and other unavoidable impurities, and is characterized in that the ratio (D0.9 / D0.0) of the average crystal grain diameter in the center (D0.0) to the average crystal grain diameter in the surface layer (D0.9) is 0.55 to 0.85.

Description

[Technical Field]

[0001] The present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof, and more particularly to a non-oriented electrical steel sheet that can be preferably used as an iron core for an environmentally friendly automobile drive motor, and a manufacturing method thereof. [Background technology]

[0002] Non-oriented electrical steel sheets are primarily used in motors that convert electrical energy into mechanical energy, and excellent magnetic properties are required to ensure high efficiency during this process. In particular, with the recent rise in attention to environmentally friendly automobiles that are driven by motors instead of internal combustion engines, demand for non-oriented electrical steel sheets used as the core material of drive motors is increasing, leading to a demand for non-oriented electrical steel sheets that have both excellent magnetic properties and strength.

[0003] The magnetic properties of non-oriented electrical steel sheets are primarily evaluated by 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 production of more energy-efficient motors under the same conditions, while higher magnetic flux density allows for the miniaturization of motors and the reduction of copper loss. Therefore, non-oriented electrical steel sheets with low iron loss and high magnetic flux density can be used to create drive motors with excellent efficiency and torque, thereby improving the mileage and power output of environmentally friendly automobiles.

[0004] The characteristics of non-oriented electrical steel that should be considered vary depending on the motor's operating conditions. The commonly used standard for evaluating the properties of non-oriented electrical steel used in motors is W15 / 50, which is the iron loss when a 1.5T magnetic field is applied at a commercial frequency of 50Hz. However, for non-oriented electrical steel sheets with a thickness of 0.35mm or less used in environmentally friendly automobile drive motors, magnetic properties at low magnetic fields of 1.0T or less and high frequencies of 400Hz or more are often important, so the properties of non-oriented electrical steel sheets are sometimes evaluated using W10 / 400 iron loss or B1 magnetic flux density.

[0005] Non-oriented electrical steel sheets for eco-friendly automobile traction motors require excellent fatigue limits as well as magnetic properties. Because eco-friendly automobile traction motors operate for long periods at various rotational speeds depending on the driving conditions of the vehicle, the fatigue limit of the electrical steel sheet is one of the factors that determine the vehicle's lifespan. In particular, eco-friendly automobile traction motors are designed with permanent magnets inserted into the rotor, and the inserted permanent magnets are subject to a force that tries to separate them due to sustained centrifugal force during operation, requiring electrical steel sheets with a high fatigue limit.

[0006] The most common method for simultaneously improving the magnetic and fatigue properties of non-oriented electrical steel sheets is to add alloying elements such as Si, Al, and Mn. The addition of these alloying elements increases the steel's resistivity, thereby reducing eddy current loss and lowering overall iron loss. Furthermore, the alloying elements dissolve in iron as substitutional elements, strengthening the steel and increasing fatigue strength. However, the addition of alloying elements such as Si, Al, and Mn has drawbacks: the magnetic flux density decreases and brittleness increases as the amount increases. Adding more than a certain amount makes cold rolling impossible, making commercial production impossible. While thinner electrical steel sheets improve high-frequency iron loss, the reduction in rollability due to brittleness is a critical issue. The maximum total Si, Al, and Mn content that can be commercially produced is approximately 4.5%. By optimizing the trace element content, the highest quality non-oriented electrical steel sheets with excellent magnetic properties and strength can be produced.

[0007] Depending on the motor design intent, electrical steel sheets with higher fatigue strength but slightly lower magnetic properties may be used. Methods for manufacturing electrical steel sheets for such applications include precipitation of interstitial elements and grain size reduction. Mainly to miniaturize motors to increase rotational speed or enhance the effectiveness of permanent magnets inserted in rotors, rotors are manufactured using electrical steel sheets with significantly improved strength, even if the magnetic properties of the steel sheet are somewhat degraded. While forming fine precipitates containing interstitial solid solution elements such as C, N, and S is effective in improving strength, it has the disadvantage of rapidly deteriorating core loss and reducing motor efficiency. Furthermore, grain size reduction has the disadvantage of increasing the non-uniformity of the steel sheet material due to the inclusion of unrecrystallized portions, resulting in greater product quality variation.

[0008] To solve these problems, Patent Document 1 attempts to produce a non-oriented electrical steel sheet that simultaneously exhibits excellent magnetic properties and strength by controlling the cooling rate during the final annealing process, but this approach has the drawback of being difficult to apply to mass production due to the increased material non-uniformity caused by the inclusion of unrecrystallized portions. In addition, most of the techniques previously proposed to simultaneously improve magnetic properties and strength have been unable to be put to practical use due to factors such as increased manufacturing costs, reduced productivity and yield, and insufficient improvement effects. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2009 / 128428 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a non-oriented electrical steel sheet and a method for manufacturing the same.

[0011] An object of the present invention is to provide a non-oriented electrical steel sheet having excellent magnetic properties and fatigue limit, and a method for producing the same. [Means for solving the problem]

[0012] The non-oriented electrical steel sheet of the present invention contains, by weight%, 3.3 to 3.8% Si, 0.4 to 1.5% Al, 0.2 to 1.5% Mn, 0.0025% or less (excluding 0%) C, 0.005% 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%) V, 0.0005 to 0.0050% W, and 0.005 to 0.1% in total of Sn and Sb, with the remainder being Fe and other unavoidable impurities, and is characterized in that the ratio (D0.9 / D0.0) of the average crystal grain diameter in the center (D0.0) to the average crystal grain diameter in the surface layer (D0.9) is 0.55 to 0.85. (However, the surface layer portion refers to the region from the surface of the steel plate to 1 / 10t (t: thickness of the steel material), and the center portion refers to the region outside the surface layer portion.)

[0013] The non-oriented electrical steel sheet may further contain 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.

[0014] The non-oriented electrical steel sheet may further contain 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.

[0015] The non-oriented electrical steel sheet may further contain one or more of Bi, Pb, Ge and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

[0016] The surface layer has carbides with a diameter of 20 to 200 nm at a density of 0.02 to 0.20 particles / μm 2 May be present.

[0017] The non-oriented electrical steel sheet may have an average crystal grain diameter of 50 to 100 μm.

[0018] The non-oriented electrical steel sheet may have a thickness of 0.1 to 0.35 mm.

[0019] The non-oriented electrical steel sheet may have a fatigue limit at 90°C of 310 MPa or more.

[0020] The non-oriented electrical steel sheet may have a magnetic flux density (B1) of 1.12 T or more, a magnetic flux density (B50) of 1.67 T or more, and an iron loss (W10 / 400) of 11.4 W / Kg or less.

[0021] The method for producing a non-oriented electrical steel sheet of the present invention contains, 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%, and the total of Sn and Sb: 0. The method includes the steps of heating a slab containing 0.05 to 0.1% of Cu, with the balance being Fe and other inevitable impurities; finish hot rolling the heated slab to obtain a hot-rolled sheet; annealing the hot-rolled sheet so that the hot-rolled sheet has an average grain diameter of 250 μm or more; pickling the hot-rolled sheet that has been annealed, and then cold-rolling the hot-rolled sheet so that the surface temperature of the steel sheet includes a range of 400°C or more to obtain a cold-rolled sheet; and final annealing the cold-rolled sheet.

[0022] The method for producing a non-oriented electrical steel sheet of the present invention contains, 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%, and the total of Sn and Sb: 0. The method includes the steps of heating a slab containing 0.05 to 0.1% of Cu, with the balance being Fe and other inevitable impurities; finish hot rolling the heated slab to obtain a hot-rolled sheet; annealing the hot-rolled sheet so that the hot-rolled sheet has an average grain diameter of 250 μm or more; pickling the annealed hot-rolled sheet, and then warm-rolling the hot-rolled sheet so that the surface temperature of the steel sheet includes a range of 400°C or more to obtain a warm-rolled sheet; and final annealing the warm-rolled sheet.

[0023] The slab may further contain 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.

[0024] The slab may further contain 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.

[0025] The slab may further contain one or more of Bi, Pb, Ge and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

[0026] The slab heating can be carried out at 1050 to 1200°C.

[0027] The finish hot rolling can be performed at 800 to 950°C.

[0028] The final annealing can be carried out at 850°C or less.

[0029] The final annealing may be performed in an atmosphere of a mixture of hydrogen (H2) and nitrogen (N2) gases. [Effects of the Invention]

[0030] According to the present invention, a non-oriented electrical steel sheet and a method for manufacturing the same can be provided.

[0031] According to the present invention, it is possible to provide a non-oriented electrical steel sheet having excellent magnetic properties and fatigue limit, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0032] Hereinafter, a non-oriented electrical steel sheet according to an embodiment of the present invention will be described. First, the alloy composition will be described. The contents of the alloy composition described below are in weight percent unless otherwise specified.

[0033] Si: 3.3 to 3.8% Si increases the resistivity of the material, reduces iron loss, and enhances strength through solid solution strengthening. If the Si content is less than 3.3%, the iron loss and strength improvement effects are insufficient. If the Si content exceeds 3.8%, the brittleness of the material increases, rolling productivity drops sharply, and a surface oxide layer and oxides that are harmful to magnetic properties are formed. Therefore, the Si content is preferably in the range of 3.3 to 3.8%.

[0034] Al: 0.4 to 1.5% Al increases the resistivity of the material, reducing iron loss, and enhances strength through solid solution strengthening. If the Al content is less than 0.4%, fine nitrides are formed, making it difficult to obtain the effect of improving magnetic properties. If the Mn content exceeds 1.5%, excessive nitrides are formed, degrading magnetic properties and causing problems in all processes, such as steelmaking and continuous casting, significantly reducing productivity. Therefore, the Al content is preferably in the range of 0.4 to 1.5%. The lower limit of the Al content is more preferably 0.6%. The upper limit of the Al content is even more preferably 1.3%.

[0035] Mn: 0.2 to 1.5% Mn increases the resistivity of the material, improves iron loss, and plays a role in forming sulfides. If the Mn content is less than 0.2%, fine sulfides are formed, causing magnetic deterioration. If the Mn content exceeds 1.5%, fine MnS precipitates excessively, promoting the formation of a {111} texture that is unfavorable to magnetic properties, resulting in a rapid decrease in magnetic flux density. Therefore, the Mn content is preferably in the range of 0.2 to 1.5%. The lower limit of the Mn content is more preferably 0.3%. The upper limit of the Mn content is even more preferably 1.3%.

[0036] C: 0.0025% or less (excluding 0%) C causes magnetic aging and combines with other impurity elements to form carbides, which impede the movement of grain boundaries or domain walls, thereby degrading magnetic properties. If the C content exceeds 0.0025%, the number of fine carbides increases rapidly, resulting in a significant deterioration of magnetic properties. Therefore, the C content is preferably 0.0025% or less (excluding 0%).

[0037] S: 0.005% or less (excluding 0%) S forms fine precipitates of MnS, which deteriorates magnetic properties and hot workability. If the S content exceeds 0.005%, it is difficult to ensure sufficient magnetic properties and hot workability. Therefore, the S content is preferably 0.005% or less (excluding 0%). It is more preferable that the S content be 0.003% or less.

[0038] N: 0.005% or less (excluding 0%) N not only forms fine AlN precipitates inside the base material, but also combines with other impurities to form fine precipitates, which inhibit grain growth and domain wall motion, thereby worsening iron loss. If the N content exceeds 0.005%, the number of fine nitrides increases rapidly, resulting in a significant deterioration in iron loss. Therefore, the N content is preferably 0.005% or less (excluding 0%). It is more preferable that the N content be 0.002% or less.

[0039] Ti: 0.003% or less (excluding 0%) Ti has a strong tendency to form precipitates in steel, forming fine carbides, nitrides, or sulfides inside the base material, which inhibit grain growth and domain wall motion, thereby deteriorating iron loss. If the Ti content exceeds 0.003%, it is difficult to obtain sufficient magnetic properties. Therefore, the Ti content is preferably 0.003% or less (excluding 0%). The Ti content is more preferably 0.0025% or less, and even more preferably 0.002% or less.

[0040] Nb: 0.003% or less (excluding 0%) Nb has a strong tendency to form precipitates in steel, forming fine carbides, nitrides, or sulfides inside the base material, which inhibit grain growth and domain wall motion, thereby deteriorating iron loss. If the Nb content exceeds 0.003%, it is difficult to obtain sufficient magnetic properties. Therefore, the Nb content is preferably 0.003% or less (excluding 0%). The Nb content is more preferably 0.0025% or less, and even more preferably 0.002% or less.

[0041] V: 0.003% or less (excluding 0%) V has a strong tendency to form precipitates in steel, forming fine carbides, nitrides, or sulfides inside the base material, which inhibit grain growth and domain wall motion, thereby deteriorating iron loss. If the V content exceeds 0.003%, it is difficult to obtain sufficient magnetic properties. Therefore, the V content is preferably 0.003% or less (excluding 0%). The V content is more preferably 0.0025% or less, and even more preferably 0.002% or less.

[0042] W: 0.0005 to 0.0050% W is an element that affects the behavior of carbides within steel, and induces differences in carbide formation behavior across thickness layers, particularly at temperatures below 700°C, thereby promoting a grain size gradient. If the W content is less than 0.0005%, it is difficult to induce differences in carbide formation behavior between the surface layer and the center. If the W content exceeds 0.0050%, carbide formation in the surface layer and the center is promoted, resulting in significant deterioration of magnetic properties. Therefore, the W content is preferably in the range of 0.0005 to 0.0050%. The lower limit of the W content is more preferably 0.001%. The upper limit of the W content is even more preferably 0.0040%.

[0043] Sn and Sb total: 0.005 to 0.1% Sn and Sb play a role in suppressing the development of the {111} orientation, which segregates at grain boundaries during the initial stage of final recrystallization annealing and deteriorates magnetic properties. If the total content of Sn and Sb is less than 0.005%, it is difficult to suppress the development of the {111} orientation. If the total content of Sn and Sb exceeds 0.1%, the surface quality deteriorates, resulting in poor product productivity. Therefore, the total content of Sn and Sb is preferably in the range of 0.005 to 0.1%. The lower limit of the total content of Sn and Sb is more preferably 0.015%. The upper limit of the total content of Sn and Sb is even more preferably 0.08%.

[0044] The non-oriented electrical steel sheet of the present invention may further contain 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.

[0045] P: 0.1% or less P acts as a grain boundary segregating element and can delay recrystallization, deteriorating strength uniformity in the rolling direction and the direction perpendicular to the rolling direction, so its upper limit is limited to 0.1%. More specifically, the P content may be 0.0001 to 0.1%. Even more specifically, the P content may be 0.001 to 0.05%.

[0046] Cr: 0.01 to 0.5% Cr plays a role in increasing resistivity and improving iron loss. If the Cr content is less than 0.01%, the effect of increasing resistivity may be insufficient. If the Cr content exceeds 0.5%, the magnetic flux density may decrease. More specifically, the Cr content may be 0.02 to 0.3%.

[0047] Ni: 0.05% or less Ni reacts with impurity elements to form fine sulfides, carbides, and nitrides, which have a detrimental effect on magnetic properties, so the upper limit is limited to 0.05%. More specifically, the Ni content may be 0.0001 to 0.050%. Even more specifically, the Ni content may be 0.001 to 0.030%.

[0048] Cu: 0.005 to 0.2% Cu plays a role in forming sulfides together with Mn. If the Cu content is less than 0.005%, fine (Cu Mn)S precipitates, which can deteriorate magnetic properties. If the Cu content exceeds 0.2%, high-temperature brittleness occurs, which can lead to crack formation during continuous casting or hot rolling. More specifically, the Cu content can be 0.010 to 0.1%.

[0049] Zn: 0.01% or less Zn acts as an impurity and can deteriorate magnetic properties, so its upper limit is limited to 0.01%. More specifically, the Zn content can be 0.0001 to 0.01%. Even more specifically, the Zn content can be 0.001 to 0.008%.

[0050] The non-oriented electrical steel sheet of the present invention may further contain 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.

[0051] Mo: 0.03% or less The above-mentioned Mo reacts with the unavoidably contained C, S, N, etc. to form fine carbides, nitrides or sulfides, which may adversely affect the magnetic properties, so the upper limit is set to 0.03%.

[0052] B: 0.002% or less The B content forms inclusions in the steel, which causes deterioration of magnetic properties. If the B content exceeds 0.002%, it is difficult to ensure excellent magnetic properties. Therefore, the B content is preferably 0.002% or less (excluding 0%). The B content is more preferably 0.0005% or less.

[0053] Mg: 0.005% or less The Mg forms inclusions in the steel, which causes deterioration of magnetic properties. If the Mg content exceeds 0.005%, it is difficult to ensure excellent magnetic properties. Therefore, the Mg content is preferably 0.005% or less (excluding 0%). The Mg content is more preferably 0.002% or less.

[0054] Ca: 0.005% or less The Ca content is set to an upper limit of 0.005% because Ca reacts with the unavoidably contained C, S, N, etc. to form fine carbides, nitrides, or sulfides, which can adversely affect magnetic properties.

[0055] Zr: 0.005% or less Zr forms inclusions in steel, which deteriorates magnetic properties. If the Zr content exceeds 0.005%, it is difficult to ensure excellent magnetic properties. Therefore, the Zr content is preferably 0.005% or less (excluding 0%). The Zr content is more preferably 0.002% or less.

[0056] The non-oriented electrical steel sheet of the present invention may further contain one or more of Bi, Pb, Ge and As in an amount of 0.20% or less (excluding 0%), respectively or in total.

[0057] When the above-mentioned elements are further added, they segregate at the grain boundaries, alleviating stress concentration at the grain boundaries during cold rolling and allowing the steel to be recrystallized during the subsequent recrystallization annealing process. <111> / / Improves magnetic flux density by suppressing recrystallization of ND-oriented crystal grains. When these elements are added appropriately, the above-mentioned effects can be further achieved. However, if they are added in excessive amounts, a large amount of segregation occurs, suppressing crystal grain growth and potentially deteriorating magnetic flux density and core loss. More specifically, the alloy may contain one or more of Bi, Pb, Ge, and As in an amount of 0.0001 to 0.20%, each or a total of 0.001 to 0.10%, respectively. Even more specifically, the alloy may contain one or more of Bi, Pb, Ge, and As in an amount of 0.001 to 0.10%, each or a total of 0.001 to 0.10%.

[0058] The remaining component is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, the contents of all of them will not be specifically mentioned in this specification.

[0059] In the non-oriented electrical steel sheet of the present invention, the ratio (D0.9 / D0.0) of the average grain diameter (D0.0) at the center to the average grain diameter (D0.9) at the surface layer is preferably 0.55 to 0.85. The surface layer preferably has 0.02 to 0.20 particles / μm of carbides having a diameter of 20 to 200 nm. 2In this way, by appropriately controlling the crystal grain fraction in the surface layer with optimized carbide distribution and the crystal grains in the central portion, it is possible to ensure good magnetic properties due to the central crystal grains having an excellent texture, while at the same time ensuring excellent fatigue properties due to the surface layer crystal grains. In particular, when the fine carbides in the surface layer are appropriately distributed, it is possible to achieve even better fatigue properties at temperatures higher than room temperature.

[0060] If the ratio (D0.9 / D0.0) of the average grain diameter (D0.0) in the center to the average grain diameter (D0.9) in the surface layer is less than 0.55, there is a disadvantage that hysteresis loss increases and iron loss deteriorates. If the ratio (D0.9 / D0.0) of the average grain diameter (D0.0) in the center to the average grain diameter (D0.9) in the surface layer exceeds 0.85, there is a disadvantage that stress concentrates at the grain boundaries located in the surface layer, resulting in a decrease in fatigue limit. The lower limit of the ratio (D0.9 / D0.0) of the average grain diameter (D0.0) in the center to the average grain diameter (D0.9) in the surface layer is more preferably 0.60. The upper limit of the ratio (D0.9 / D0.0) of the average grain diameter (D0.0) in the center to the average grain diameter (D0.9) in the surface layer is more preferably 0.80.

[0061] The number density of the above carbides with a diameter of 20 to 200 nm is 0.02 pieces / μm 2 If the number density of the carbides with a diameter of 20 to 200 nm is less than 0.20 particles / μm, the surface grains grow to a size similar to that of the central grains, resulting in a decrease in fatigue limit. 2 If the carbide content exceeds 1 / 10t, the surface layer crystal grains become too fine, hindering domain wall motion and resulting in a deterioration of magnetic properties. The surface layer refers to the region from the surface of the steel sheet to 1 / 10t (t: thickness of the steel), and the center refers to the region outside the surface layer. In the present invention, the type of carbide is not particularly limited, and may be, for example, a precipitate in the form of a combination of carbon and one or more elements selected from Ti, Nb, V, and W.

[0062] The non-oriented electrical steel sheet of the present invention may have an average grain diameter of 50 to 100 μm. If the average grain diameter is less than 50 μm, hysteresis loss may increase sharply, resulting in poor core loss. If the average grain diameter is more than 100 μm, stress may concentrate at the grain boundaries, resulting in a low fatigue limit.

[0063] The non-oriented electrical steel sheet of the present invention provided as described above may have a thickness of 0.1 to 0.35 mm. The non-oriented electrical steel sheet of the present invention may have a fatigue limit of 310 MPa or more at 90°C. Furthermore, the non-oriented electrical steel sheet of the present invention may have a magnetic flux density (B1) of 1.12 T or more, a magnetic flux density (B50) of 1.67 T or more, and an iron loss (W10 / 400) of 11.4 W / Kg or less. In the present invention, the fatigue limit at 90°C, the magnetic flux density (B1), and the magnetic flux density (B50) are more advantageous as the values ​​are higher, so there are no particular restrictions on their upper limits. The iron loss (W10 / 400) is more advantageous as the value is lower, so there is no particular restriction on its lower limit. Meanwhile, the electrical properties are based on a non-oriented electrical steel sheet with a thickness of 0.25 mm.

[0064] A method for producing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described below.

[0065] First, the slab is heated. The slab heating can be performed at 1050 to 1200°C. If the slab heating temperature is less than 1050°C, the shape may be poor after finish rolling. If the slab heating temperature exceeds 1200°C, precipitates such as AlN and MnS are re-precipitated into fine particles after re-solidification, resulting in significant deterioration of magnetic properties. The lower limit of the slab heating temperature is more preferably 1100°C. The upper limit of the slab heating temperature is even more preferably 1170°C.

[0066] The heated slab is then finish hot rolled to obtain a hot-rolled sheet. The finish hot rolling can be performed at 800 to 950°C. If the finish hot rolling temperature is less than 800°C, there is a drawback in that the workability deteriorates due to an increase in deformation resistance, resulting in a poor coil shape. If the finish hot rolling temperature exceeds 950°C, there is a drawback in that an excessive oxide layer is formed on the coil surface, causing defects.

[0067] The hot-rolled sheet is then annealed to have an average grain diameter of 250 μm or more. If the average grain diameter of the annealed hot-rolled sheet is less than 250 μm, it is difficult to ensure a difference in grain size between the center and the surface layer after final annealing, which is a drawback in that it is difficult to simultaneously ensure magnetic properties and fatigue limit. In the present invention, the specific configuration for controlling the average grain diameter of the annealed hot-rolled sheet to 250 μm or more is not particularly limited. However, as an example, a method of controlling the annealing temperature and annealing time can be used.

[0068] The hot-rolled sheet annealed as described above is then pickled and cold-rolled to obtain a cold-rolled sheet, including a section in which the surface temperature of the steel sheet is 400°C or higher. By including a section in which the surface temperature of the steel sheet is 400°C or higher during the cold rolling, a temperature and deformation structure favorable for the formation of carbides are formed in the surface layer, and the deformation structure generated during cold rolling is partially recovered in the center, resulting in an effect of creating a difference in grain size between the surface layer and the center. In the present invention, the higher the surface temperature of the steel sheet during the cold rolling, the more advantageous it is, so there is no particular limitation on the upper limit. However, the upper limit of the surface temperature of the steel sheet during the cold rolling may be, for example, 550°C.

[0069] In the present invention, the specific configuration for including a section in which the surface temperature of the steel sheet is 400°C or higher during the cold rolling is not particularly limited. However, examples thereof include a method of charging the steel sheet into a box furnace provided between each pass during cold rolling, a method of heating rolls to a high temperature during rolling, and a method of heating the steel sheet via an induction heater provided at the front and rear ends of the rolling mill.

[0070] The cold-rolled steel sheet is then subjected to final annealing. The final annealing can be performed at 850°C or less. If the final annealing temperature is less than 850°C, recrystallization may not occur sufficiently, resulting in significant deterioration of the magnetic properties of the steel sheet. The final annealing can be performed in an atmosphere containing a mixture of hydrogen (H2) and nitrogen (N2) gases.

[0071] Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention will be described.

[0072] A method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention satisfies most of the above-mentioned manufacturing conditions, but instead of the process of obtaining a cold-rolled sheet by pickling a hot-rolled sheet that has been annealed and then cold-rolling the hot-rolled sheet so that the surface temperature of the steel sheet includes a range of 400°C or higher, a process of obtaining a warm-rolled sheet by pickling the hot-rolled sheet that has been annealed and then warm-rolling the hot-rolled sheet so that the surface temperature of the steel sheet includes a range of 400°C or higher is replaced with a process of obtaining a warm-rolled sheet. By including a range of 400°C or higher in the surface temperature of the steel sheet during warm rolling, a difference in microstructure between the surface and center of the steel sheet is generated during the final recrystallization annealing process, thereby achieving excellent magnetic properties and fatigue limit. In the present invention, since a higher surface temperature of the steel sheet during warm rolling is advantageous, there is no particular limitation on the upper limit. However, the upper limit of the surface temperature of the steel sheet during warm rolling may be, for example, 550°C. [Example]

[0073] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.

[0074] (Example) A slab having the alloy composition listed in Table 1 below was heated to 1150°C and then hot rolled at a finish hot rolling temperature of 900°C to produce a 2.0 mm thick hot-rolled sheet. The hot-rolled sheet was then annealed (at 1150°C for 120 seconds) to obtain an average grain diameter under the conditions listed in Table 2 below, and then cold-rolled to produce a 0.25 mm thick cold-rolled sheet. During the cold rolling, the sheet was placed in a box furnace set under the conditions listed in Table 2 below and held for 5 minutes between passes. The cold-rolled sheet was then final annealed at 1000°C for 100 seconds in a hydrogen (H2) and nitrogen (N2) gas mixture atmosphere to produce a non-oriented electrical steel sheet.

[0075] For the non-oriented electrical steel sheets manufactured as described above, the average grain diameter of the hot-rolled sheet after annealing, the ratio (D0.9 / D0.0) of the average grain diameter (D0.0) in the center portion to the average grain diameter (D0.9) in the surface layer portion, the number density of carbides having a diameter of 20 to 200 nm in the surface layer portion, the average grain diameter, the fatigue limit at 90°C, and electrical properties were measured, and the results are shown in Tables 2 and 3.

[0076] The average grain diameter of the annealed hot-rolled sheet was measured using an optical microscope on the cross section (TD plane) of the hot-rolled sheet perpendicular to the rolling direction after annealing.

[0077] The average grain diameter (D0.0) at the center was measured using an optical microscope on the surface (ND surface) of the non-oriented electrical steel sheet at 1 / 2t (t: steel thickness) and 1 / 10t (t: steel thickness).

[0078] The number density of carbides with diameters of 20 to 200 nm in the surface layer was measured using a transmission electron microscope (TEM) on 1 / 10t (t: thickness of steel) of the non-oriented electrical steel sheet.

[0079] The average grain diameter was measured using an optical microscope on a cross section (TD plane) perpendicular to the rolling direction of the non-oriented electrical steel sheet.

[0080] The surface temperature of the cold-rolled sheet during cold rolling was recorded as the highest temperature among the temperatures measured when the steel sheet was stretched between passes of cold rolling and the temperatures measured at the exit side of the rolling rolls.

[0081] The fatigue limit at 90°C was determined by conducting a fatigue test at a temperature of 90°C, a stress ratio of 0.05, and a frequency of 50 Hz, and drawing an SN diagram.

[0082] Among the electrical properties, magnetic flux density (B1), magnetic flux density (B50), and iron loss (W10 / 400) were measured by taking five 60mm wide x 60mm long test pieces from non-oriented electrical steel sheets, measuring them in the rolling direction and perpendicular to the rolling direction using a single sheet tester, and then calculating the average values. Magnetic flux density (B1) and magnetic flux density (B50) refer to the magnetic flux densities induced in a magnetic field of 100A / m and 5000A / m, respectively, and iron loss (W10 / 400) refers to the iron loss when a magnetic flux density of 1.0T is induced at a frequency of 400Hz.

[0083] [Table 1]

[0084] [Table 2]

[0085] [Table 3]

[0086] As shown in Tables 1 to 3 above, in the case of Examples 1 to 12, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, it can be seen that excellent magnetic properties and fatigue limit are ensured as the microstructure and carbide conditions that the present invention aims to obtain are satisfied.

[0087] In the case of Comparative Examples 1 to 7 and 9 to 11, the alloy composition proposed by the present invention is not satisfied, and therefore the conditions for the microstructure or carbide to be obtained by the present invention are not met, and the magnetic properties and fatigue limit are at low levels.

[0088] In the case of Comparative Examples 8 and 12, the manufacturing conditions proposed by the present invention were not met, and therefore the conditions for the microstructure or carbides that the present invention aims to obtain were not met, resulting in low levels of magnetism and fatigue limit.

Claims

1. The alloy contains, 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%, and the sum of Sn and Sb: 0.005 to 0.1%, with the balance being Fe and other unavoidable impurities; A non-oriented electrical steel sheet characterized in that the ratio (D0.9 / D0.0) of the average crystal grain diameter (D0.0) in the center portion to the average crystal grain diameter (D0.9) in the surface layer portion is 0.55 to 0.

85. (However, the surface layer portion refers to the region from the surface of the steel plate to 1 / 10t (t: thickness of the steel material), and the center portion refers to the region outside the surface layer portion.)

2. The non-oriented electrical steel sheet according to claim 1, further comprising 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. 2. The non-oriented electrical steel sheet according to claim 1, further comprising 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. 2. The non-oriented electrical steel sheet according to claim 1, further comprising one or more of Bi, Pb, Ge, and As in an amount of 0.20% or less (excluding 0%), each or a total amount of 0.20% or less (excluding 0%).

5. The surface layer has carbides with a diameter of 20 to 200 nm at a density of 0.02 to 0.20 particles / μm ２ The non-oriented electrical steel sheet according to claim 1 , characterized in that:

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

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

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

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

10. heating a slab containing, by weight percent, 3.3 to 3.8% Si, 0.4 to 1.5% Al, 0.2 to 1.5% Mn, 0.0025% or less (excluding 0%) C, 0.005% or less (excluding 0%) S, 0.005% or less (excluding 0%) N, 0.003% or less (excluding 0%) Ti, 0.003% or less (excluding 0%) Nb, 0.003% or less (excluding 0%) V, 0.0005 to 0.0050% W, 0.005 to 0.1% in total of Sn and Sb, with the balance being Fe and other unavoidable impurities; finish hot rolling the heated slab to obtain a hot-rolled sheet; annealing the hot-rolled sheet to have an average grain diameter of 250 μm or more; The hot-rolled sheet is pickled and then cold-rolled so that the surface temperature of the steel sheet is in a range of 400°C or more to obtain a cold-rolled sheet; and final annealing the cold-rolled sheet.

11. heating a slab containing, by weight percent, 3.3 to 3.8% Si, 0.4 to 1.5% Al, 0.2 to 1.5% Mn, 0.0025% or less (excluding 0%) C, 0.005% or less (excluding 0%) S, 0.005% or less (excluding 0%) N, 0.003% or less (excluding 0%) Ti, 0.003% or less (excluding 0%) Nb, 0.003% or less (excluding 0%) V, 0.0005 to 0.0050% W, 0.005 to 0.1% in total of Sn and Sb, with the balance being Fe and other unavoidable impurities; finish hot rolling the heated slab to obtain a hot-rolled sheet; annealing the hot-rolled sheet to have an average grain diameter of 250 μm or more; pickling the hot-rolled sheet after the annealing, and then warm-rolling the hot-rolled sheet so that the surface temperature of the steel sheet includes a section of 400°C or more to obtain a warm-rolled sheet; and final annealing the hot-rolled sheet.

12. The method for producing a non-oriented electrical steel sheet according to claim 10 or 11, wherein the slab further contains 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. 12. The method for producing a non-oriented electrical steel sheet according to claim 10, wherein the slab further contains 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. 12. The method for producing a non-oriented electrical steel sheet according to claim 10, wherein the slab further contains one or more of Bi, Pb, Ge, and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

15. The method for producing a non-oriented electrical steel sheet according to claim 10 or 11, characterized in that the slab heating is carried out at 1050 to 1200°C.

16. The method for producing a non-oriented electrical steel sheet according to claim 10 or 11, characterized in that the finish hot rolling is carried out at 800 to 950°C.

17. The method for producing a non-oriented electrical steel sheet according to claim 10 or 11, wherein the final annealing is performed at 850°C or less.

18. The final annealing is carried out using hydrogen (H ２ ) and nitrogen (N ２ 12. The method for producing a non-oriented electrical steel sheet according to claim 10, wherein the method is carried out in an atmosphere containing a mixed gas of 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11).

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