Non-oriented electrical steel sheet and method for manufacturing same

Abstract

To provide a nonoriented silicon steel sheet simultaneously excellent in strength and core loss, and to provide its production method.SOLUTION: 3.0% to 4.0% of silicon (Si), 0.1% to 1.5% of aluminum (Al), 0.1% to 0.5% of manganese (Mn), 2% to 20% of Mn, 0.006% to 0.1% of a sum of tin (Sn) and antimony (Sb), 0.0010% to 0.0050% of carbon (C), 0.0003% to 0.0050% of each of one or more of nitrogen (N), sulfur (S), titanium (Ti), niobium (Nb), and vanadium (V), and a balance of iron (Fe) and inevitable impurities, wherein an area fraction of grains having a grain size of 10% or less of a thickness of the steel sheet is 0.5% or more, A number fraction of crystal grains is 20% or more, and an average crystal grain size from a center layer to a surface layer in a thickness direction of the steel sheet satisfies a relational expression of D (surface) / D (center) ≥ 0.6. Wherein D (surface) represents an average grain size of a region from a surface layer to a 1 / 4 layer in a thickness direction of the steel sheet, and D (center) represents an average grain size of a region from a center layer to a 1 / 4 layer in the thickness direction of the steel sheet. ) SELECTED DRAWING: None

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 in which strength and magnetic properties are simultaneously improved by adjusting components that increase strength and precisely controlling a manufacturing process that uniformly forms fine crystal grains, and a manufacturing method thereof. [Background technology]

[0002] Non-oriented electrical steel sheets are mainly used in motors that convert electrical energy into mechanical energy. To achieve high efficiency in this energy conversion process, non-oriented electrical steel sheets must have excellent magnetic properties.

[0003] In preparation for a carbon-neutral era, countries around the world are restructuring their industrial structures to focus on environmentally friendly, low-carbon production. Amid this trend, the automotive industry is rapidly replacing internal combustion engines with electric vehicles, and the drive motors used in these vehicles account for more than half of their electrical energy consumption. Interest in and demand for non-oriented electrical steel sheets, which are used as the core material for such drive motors, is growing. Against this backdrop, there has been growing interest in methods for simultaneously improving the magnetic properties and strength of non-oriented electrical steel sheets as a way to increase the efficiency of drive motors. The magnetic properties of non-oriented electrical steel sheets are primarily evaluated by their iron loss and magnetic flux density. Iron loss refers to the energy loss that occurs at a specific magnetic flux density and frequency, while magnetic flux density refers to the degree of magnetization achieved under a specific magnetic field. Lower iron loss allows for the manufacture of more energy-efficient motors under the same conditions, while higher magnetic flux density allows for the manufacture of more compact motors and reduced copper loss. Therefore, it is important to develop non-oriented electrical steel sheets with low iron loss and high magnetic flux density. Using non-oriented electrical steel sheets with these properties allows for the manufacture of drive motors with excellent efficiency and torque, thereby improving the driving range and power output of electric vehicles.

[0004] Meanwhile, the properties of non-oriented electrical steel must also take into account the operating conditions of the motor. The commonly used standard for evaluating the properties of non-oriented electrical steel used in motors is the iron loss (W15 / 50) when a 1.5 T magnetic field is applied at a commercial frequency of 50 Hz. However, for non-oriented electrical steel sheets with a thickness of 0.35 mm or less used in the drive motors of environmentally friendly vehicles, magnetic properties are often important in low magnetic fields of 1.0 T or less and at high frequencies of 400 Hz or more, so the W10 / 400 iron loss method is used to evaluate the properties of non-oriented electrical steel. Non-oriented electrical steel sheets for the drive motors of highly efficient, eco-friendly vehicles are required to have not only excellent magnetic properties but also excellent strength. Drive motors for eco-friendly vehicles are typically designed with permanent magnets inserted into the rotor, and for inserted permanent magnet motors to achieve excellent performance, the permanent magnets must be positioned on the outside of the rotor so that they are as close to the stator as possible. However, if the strength of the magnetic steel sheet is weak, there is a risk that the permanent magnets inserted in the rotor will become detached due to centrifugal force when the motor rotates at high speed. Therefore, to ensure the performance and durability of the motor, magnetic steel sheet with high strength is required. In particular, considering the temperature rise caused by the operation of the motor, excellent strength at temperatures between 120 and 200°C is required.

[0005] A commonly used method for simultaneously increasing the magnetic properties and strength of non-oriented electrical steel sheets is to add alloying elements such as Si, Al, Mn, etc. If the resistivity of the steel increases through the addition of these alloying elements, eddy current loss can be reduced, thereby lowering overall iron loss. In addition, alloying elements dissolve as substitutional elements for iron, enhancing the strength through their strengthening effect. However, the increased addition of alloying elements such as Si, Al, and Mn has the disadvantage of decreasing magnetic flux density and increasing brittleness. Adding more than a certain amount makes cold rolling difficult, making commercial production impossible. While thinner electrical steel sheets offer better high-frequency core loss, the reduction in rollability due to brittleness is a critical issue. The maximum combined Si, Al, and Mn content that allows commercial production is known to be approximately 4.5%. By optimizing the content of other trace elements, it is possible to produce the highest-quality non-oriented electrical steel sheets with excellent magnetic properties and strength. Depending on the motor design intent, electrical steel sheets may be manufactured with improved strength even if their magnetic properties are slightly deteriorated, and methods for manufacturing such electrical steel sheets include a method using the precipitation of interstitial elements and a method using reduced grain size. Mainly when miniaturizing motors to increase rotational speed or to enhance the effectiveness of permanent magnets inserted in rotors, rotors manufactured with electrical steel sheets with significantly increased strength may be used, even if their magnetic properties are slightly deteriorated.

[0006] In this case, forming fine precipitates containing interstitial solid solution elements such as C, N, and S significantly increases strength, but has the disadvantage of rapidly deteriorating iron loss and potentially reducing motor efficiency. Furthermore, reducing the grain size has the disadvantage of increasing the non-uniformity of the steel sheet material due to the inclusion of unrecrystallized parts, resulting in greater quality deviations in mass-produced products. To solve these problems, there is a method of manufacturing non-oriented electrical steel sheets that simultaneously increase magnetic properties and strength by controlling the cooling rate in the final annealing process. However, steel sheets manufactured by this method have problems such as the inclusion of unrecrystallized portions, resulting in non-uniform material properties and making them difficult to apply to mass production. In addition, most of the techniques proposed so far to simultaneously improve magnetism and strength have been ignored due to the reasons that they increase manufacturing costs, decrease productivity and yield, or are insufficient in improving effects. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a non-oriented electrical steel sheet and a manufacturing method thereof, specifically a non-oriented electrical steel sheet having excellent strength and core loss properties, which is obtained by carefully controlling the pre-annealing and final annealing processes so that a sufficient number of fine crystal grains are present in the steel sheet, resulting in excellent yield strength, and at the same time, the fine crystal grains are uniformly distributed to the center layer without being concentrated in the surface layer. [Means for solving the problem]

[0008] The non-oriented electrical steel sheet of the present invention is characterized in that it contains, by weight, 3.0 to 4.0% Si, 0.1 to 1.5% Al, 0.1 to 0.5% Mn, 2 to 20% Cr of the Mn content, 0.006 to 0.1% Sn and Sb in total, 0.0010 to 0.0050% C, and 0.0003 to 0.0050% each of one or more of N, S, Ti, Nb, and V, with the remainder being Fe and unavoidable impurities, in which the area fraction of crystal grains having a crystal grain size of 10% or less of the thickness of the steel sheet is 0.5% or more, the number fraction of the crystal grains is 20% or more, and the average crystal grain size from the center layer to the surface layer in the thickness direction of the steel sheet satisfies the relationship of the following formula 1. Here, Equation 1 satisfies D(surface) / D(center)≧0.6, where D(surface) represents the average grain size in the region from the surface layer to 1 / 4 of the thickness of the steel plate, and D(center) represents the average grain size in the region from the center layer to 1 / 4 of the thickness of the steel plate.

[0009] The steel plate may have an average crystal grain size of 50 to 150 μm. It is preferable that the steel plate has a yield strength YS(RT) at room temperature of 400 MPa or more and a yield strength YS(150°C) at 150°C of 340 MPa or more. The steel plate preferably has a thickness of 0.10 to 0.35 mm.

[0010] The method for producing a non-oriented electrical steel sheet of the present invention includes the steps of: producing a slab containing, by weight, 3.0 to 4.0% Si, 0.1 to 1.5% Al, 0.1 to 0.5% Mn, 2 to 20% Cr of the Mn content, 0.006 to 0.1% Sn and Sb in total, 0.0010 to 0.0050% C, and 0.0003 to 0.0050% each of one or more of N, S, Ti, Nb, and V, with the balance being Fe and unavoidable impurities; heating the slab and hot-rolling it to produce a hot-rolled steel sheet; and heating the hot-rolled steel sheet to 950 to 1,150°C, followed by rolling the slab at 40°C. a hot-rolled steel sheet pre-annealing step including a first pre-annealing step in which the temperature is maintained at 850 to 950°C within 20 seconds after the first pre-annealing step, and a second pre-annealing step in which the temperature is changed to 850 to 950°C within 20 seconds after the first pre-annealing step, and the second pre-annealing step is maintained for 20 seconds or more; a step in which the hot-rolled steel sheet is cold-rolled after the hot-rolled steel sheet pre-annealing step, to produce a cold-rolled steel sheet; and a final annealing step including a first final annealing step in which the cold-rolled steel sheet is heated to 900°C or higher in a mixed atmosphere of hydrogen (H2) and nitrogen (N2) and then maintained at 60 seconds or less, and a second final annealing step in which the temperature is maintained at 650 to 850°C for 15 seconds or more.

[0011] The slab can be heated to a temperature of up to 1,200°C. In the step of producing the hot-rolled steel sheet, the finish hot rolling is preferably performed at 800° C. or higher. In the first pre-annealing stage, the hot-rolled steel sheet is preferably heated at a temperature increase rate of 10° C. / s or more.

[0012] In the first final annealing step, the cold-rolled steel sheet may be heated at a temperature rising rate of 25° C. / s or more. In the final annealing stage, the cold-rolled steel sheet is subjected to annealing at 0.75 kgf / mm 2 It is preferable to raise the temperature while applying the following tension in the rolling direction. [Effects of the Invention]

[0013] According to one embodiment of the present invention, the non-oriented electrical steel sheet and its manufacturing method can provide a non-oriented electrical steel sheet having excellent yield strength due to the presence of a sufficiently large number of fine crystal grains in the steel sheet, and at the same time, excellent strength and iron loss can be achieved by carefully controlling the pre-annealing and final annealing processes so that the fine crystal grains are uniformly distributed to the center layer without being concentrated in the surface layer. When non-oriented electrical steel sheets having such excellent strength and core loss properties are used in the drive motors of top-class environmentally friendly automobiles, the performance of the drive motors can be significantly improved. DETAILED DESCRIPTION OF THE INVENTION

[0014] Terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention. The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. As used herein, the term "comprising" refers to the inclusion of certain features, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0015] When we say that an element is "on" another element, it means that it is directly on top of the other element, or that there is another element between them. In contrast, when we say that an element is "directly on top of" another element, it means that there are no other elements between them. Unless otherwise specified, % means % by weight, and 1 ppm means 0.0001% by weight. In one embodiment of the present invention, the term "additionally containing an additional element" means that the remaining iron (Fe) is replaced by the additional amount of the additional element. Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless otherwise defined.

[0016] While the present invention may be embodied in various different forms, it is to be understood that the invention is not limited to the specific embodiments set forth herein, and that the invention may be embodied in various different forms, without departing from the spirit or scope of the present invention. A non-oriented electrical steel sheet according to one embodiment of the present invention contains, by weight, 3.0 to 4.0% Si, 0.1 to 1.5% Al, 0.1 to 0.5% Mn, 2 to 20% Cr of the Mn content, 0.006 to 0.1% Sn and Sb in total, 0.0010 to 0.0050% C, 0.0003 to 0.0050% each of one or more of N, S, Ti, Nb, and V, and the remainder being Fe and unavoidable impurities. The reasons for limiting the components of the non-oriented electrical steel sheet will be explained below.

[0017] [Si:3.0~4.0wt%] Silicon (Si) increases the resistivity of the material, reducing iron loss, and increases the strength of the steel sheet through solid solution strengthening. If too little Si is added, the effects of improving iron loss and strength may be insufficient. If too much Si is added, the brittleness of the material increases, rolling productivity drops sharply, and there is a risk of forming an oxide layer and oxides in the surface layer that are harmful to magnetic properties. Therefore, it is preferable that the Si content be 3.0 to 4.0 wt %. More preferably, it should be 3.1 to 3.8 wt %.

[0018] [Al:0.1~1.5wt%] Aluminum (Al) increases the resistivity of the material, reducing iron loss, while increasing the strength of the steel sheet through solid solution strengthening. If too little Al is added, fine nitrides may form, making it difficult to obtain the magnetic improvement effect. If too much Al is added, excessive nitrides may be formed, degrading magnetic properties and causing problems in all processes, including steelmaking and continuous casting, significantly reducing productivity. Therefore, it is recommended that the Al content be 0.10 to 1.50 wt%.

[0019] [Mn:0.1~0.5wt%] Manganese (Mn) increases the material's resistivity, improving iron loss, and plays a role in forming sulfides. If too little Mn is added, fine sulfides are formed, causing magnetic deterioration. On the other hand, if too much Mn is added, it has a negative effect on the formation of an oxide layer on the steel sheet surface, causing excessive precipitation of fine MnS, which promotes the formation of a {111} texture that is unfavorable to magnetic properties, resulting in a rapid decrease in magnetic flux density. Therefore, it is recommended that the Mn content be 0.1 to 0.5 wt%.

[0020] [Cr: 2~20% of Mn content] Chromium (Cr) forms a thin, dense oxide layer on the surface of steel sheet, suppressing the formation of fine precipitates in the surface layer. If too little Cr is added, the effect of forming a dense oxide layer is not achieved. On the other hand, if too much Cr is added, excessive carbides are formed inside the steel sheet, deteriorating core loss. More specifically, if the Cr content is less than 2% of Mn, a coarse oxide layer is formed on the steel sheet surface, deteriorating magnetic properties. If the Cr content exceeds 20% of Mn, carbonitrides are formed, increasing the fraction of fine crystal grains and deteriorating magnetic properties. Therefore, it is recommended that the Cr content be 2-20% of Mn.

[0021] [Total of Sn and Sb: 0.006 to 0.100 wt%] Tin (Sn) and antimony (Sb) segregate at the surface and grain boundaries of steel sheets to suppress surface oxidation during annealing, prevent element diffusion through grain boundaries, and delay the development of {111} texture. If too little Sn or Sb is added, the above effects may not be fully achieved. If too much Sn or Sb is added, the amount of segregation at grain boundaries increases, which may reduce toughness and reduce productivity for improving magnetic properties. Therefore, it is recommended that a total of 0.006 to 0.100 wt% of Sn and Sb be included. The total of Sn and Sb refers to the content of either Sn or Sb alone when Sn or Sb is included alone, and to the combined amount of Sn and Sb when both Sn and Sb are included.

[0022] [C:0.0010~0.0050wt%] Carbon (C) causes magnetic aging and combines with other impurity elements to form carbides, which reduces magnetic properties, but it also plays a role in preventing dislocation movement and improving strength. If the C content is controlled to be too high, the fraction of fine carbides increases, reducing magnetic properties, while if the C content is controlled to be too low, productivity drops excessively. Therefore, it is recommended that the C content be 0.0010 to 0.0050 wt%. The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain at least one of N, S, Ti, Nb, and V in an amount of 0.0003 to 0.0050 wt % each.

[0023] [N:0.0003~0.0050wt%] Nitrogen (N) not only forms fine AlN precipitates inside the steel sheet, but also combines with other impurities to form fine precipitates, which inhibit grain growth and worsen iron loss, but also has the effect of improving strength. If the N content is too high, the nitride fraction increases, causing a rapid deterioration in iron loss, while if the N content is too low, the magnetic flux density decreases. Therefore, the N content should be 0.0003 to 0.0050 wt%. More preferably, the N content is 0.0025 wt% or less.

[0024] [S:0.0003~0.0050wt%] Sulfur (S) should be kept low because it forms fine precipitates of MnS, which deteriorates magnetic properties and hot workability. Therefore, the S content should be 0.0003 to 0.0050 wt %. More preferably, the S content is 0.0025 wt % or less.

[0025] [Ti:0.0003~0.0050wt%] Titanium (Ti) has a strong tendency to form precipitates inside the steel sheet, forming fine carbides, nitrides, or sulfides inside the steel sheet, which inhibit grain growth and deteriorate core loss. Therefore, the Ti content must be controlled to 0.005% or less, and more preferably 0.002% or less.

[0026] [Nb:0.0003~0.0050wt%] Niobium (Nb) forms fine carbides or nitrides inside the steel sheet, inhibiting grain growth and domain wall motion, thereby deteriorating core loss. Therefore, the Nb content must be controlled to 0.005% or less, and more preferably 0.002% or less.

[0027] [V:0.0003~0.0050wt%] Vanadium (V) forms fine carbides or nitrides inside the steel sheet, inhibiting grain growth and domain wall motion, thereby deteriorating core loss. Therefore, the V content must be controlled to 0.005% or less, and more preferably 0.002% or less.

[0028] The non-oriented electrical steel sheet according to an embodiment of the present invention inevitably contains elements such as B, Mo, Mg, and Zr during the manufacturing process. Even trace amounts of these elements can form inclusions inside the steel sheet and deteriorate the magnetic properties, so it is preferable to control the amounts of B to 0.0002 wt% or less, Mo to 0.01 wt% or less, Mg to 0.005 wt% or less, and Zr to 0.005 wt% or less. The non-oriented electrical steel sheet according to one embodiment of the present invention contains the balance Fe and inevitable impurities. The inevitable impurities are impurities that are mixed in during the steelmaking stage and the manufacturing process of the grain-oriented electrical steel sheet, and as they are widely known in the art, detailed description thereof will be omitted. In one embodiment of the present invention, the addition of elements other than the above-mentioned alloy components is not excluded, and various elements may be included within a range that does not impair the technical concept of the present invention. When an additional element is further included, it is included as a substitute for the balance Fe. The non-oriented electrical steel sheet according to one embodiment of the present invention having the above composition has the following physical properties.

[0029] A non-oriented electrical steel sheet according to an embodiment of the present invention has an average grain size of 50 to 150 μm. The grain diameters of the grains occupying 10% or less of the thickness of the steel sheet are 0.5% or more in area fraction and 20% or more in number fraction. The average grain size D(center) in the region from the center layer to the ¼ layer in the thickness direction of the steel sheet and the average grain size D(surface) in the region from the surface layer to the ¼ layer in the thickness direction satisfy the relationship in the following formula 1: D(surface) / D(center)≧0.6---[Formula 1] Thus, if a steel sheet is manufactured so that the grain diameter and grain distribution in the thickness direction within the steel sheet satisfy the relationship defined by Equation 1, the yield strength and core loss of the manufactured non-oriented electrical steel sheet are improved simultaneously. This fact was confirmed by the inventors through experiments in which different steel sheet compositions and manufacturing processes were repeated.

[0030] The reason why the yield strength and iron loss were simultaneously improved by controlling the crystalline structure of the steel sheet in this way is believed to be because the Si, Al, Mn, and Cr contents, which affect the oxidation behavior in the surface layer of the steel sheet, were precisely controlled, and at the same time, the manufacturing process conditions according to one embodiment of the present invention, i.e., the hot-rolled steel sheet pre-annealing and the cold-rolled steel sheet final annealing conditions, were precisely controlled in two stages, thereby forming a sufficient number of fine crystal grains, each with a diameter of 10% or less of the steel sheet thickness. It was confirmed that by simultaneously refining the crystal grains of the steel sheet in this way and uniformly distributing them between the surface and center, the yield strength at room temperature (YS(RT)) was 400 MPa or more, and the yield strength at 150°C (YS(150°C)) was 340 MPa or more, and at the same time, the iron loss was also excellent.

[0031] A method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention includes the steps of hot-rolling a slab to manufacture a hot-rolled steel sheet, pre-annealing the hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, and final annealing the cold-rolled steel sheet. First, the slab is hot rolled. The alloy composition of the slab has been explained in the alloy composition of the non-oriented electrical steel sheet, so a duplicate explanation will be omitted. Since the alloy composition does not substantially change during the manufacturing process of the non-oriented electrical steel sheet, the alloy composition of the non-oriented electrical steel sheet and the slab is substantially the same. Specifically, the slab contains, by weight, Si: 3.0-4.0%, Al: 0.1-1.5%, Mn: 0.1-0.5%, Cr: 2-20% of the Mn content, Sn and Sb total: 0.006-0.1%, C: 0.0010-0.0050%, one or more of N, S, Ti, Nb and V each 0.0003-0.0050%, and the remainder consisting of Fe and unavoidable impurities. Other additional elements have been explained in the alloy components of the non-oriented electrical steel sheet, so duplicate explanations will be omitted.

[0032] The slabs may be heated before being hot rolled. Although there are no restrictions on the heating temperature of the slab, it is recommended that the slab be heated to 1,200°C or less. If the heating temperature of the slab is excessively high, precipitates such as AlN and MnS present in the slab may redissolve and then precipitate finely during hot rolling and annealing, suppressing grain growth and reducing magnetic properties. Next, the slab is hot rolled to produce a hot rolled steel sheet. In the stage of producing the hot-rolled steel sheet, the finish rolling temperature is preferably 800° C. or higher, more specifically, 800 to 1,000° C. The hot-rolled steel sheet is coiled at a temperature of 700° C. or lower. After the step of producing the hot-rolled steel sheet, the method further includes a step of annealing the hot-rolled steel sheet, wherein the annealing is performed in two steps. First, the primary pre-annealing of the hot-rolled steel sheet is preferably performed by heating the hot-rolled steel sheet to 950-1,150°C at a temperature increase rate of 10°C / s or more and then maintaining that temperature for 40 seconds or more. After completing the primary pre-annealing in this manner, the furnace atmosphere is changed to 850-950°C within 20 seconds and maintained for 20 seconds or more to perform the secondary pre-annealing.

[0033] Such two-stage pre-annealing of hot-rolled steel sheets is carried out to increase the crystal orientation advantageous for magnetic properties and to form appropriate crystal grain sizes. The annealed hot-rolled steel sheet is preferably subsequently subjected to pickling. Next, the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. The cold rolling is preferably final rolling to a thickness of 0.1 mm to 0.35 mm. Next, the cold-rolled steel sheet is subjected to final annealing, which is also carried out in two stages. First, the cold-rolled steel sheet was subjected to a pressure of 0.75 kgf / mm in the rolling direction of the steel sheet in a mixed atmosphere of hydrogen (H2) and nitrogen (N2). 2 With the following tension applied, the steel sheet is heated to 900°C or higher at a temperature increase rate of 25°C / s or higher. The soaked state is maintained for 60 seconds or less, and the cold-rolled steel sheet is subjected to the first final annealing step.

[0034] Then, the temperature in the furnace is controlled to 650 to 850°C and maintained for 15 seconds or more to carry out the second final annealing step. Since the iron loss of non-oriented electrical steel sheets is closely related to the size of the crystal grains, the final annealing step is divided into two steps and annealing is carried out while precisely controlling the steps. In the final annealing stage, the atmosphere in the furnace can be a mixture of hydrogen (H2) and nitrogen (N2).

[0035] The steel sheet that has been subjected to the above-described series of steps and final annealing has an average grain size of 50 to 150 μm. In addition, the steel sheet preferably has grains with a diameter of 10% or less of the sheet thickness in an area fraction of 0.5% or more and in number fraction of 20% or more. Furthermore, it is preferable that the average grain size D(center) of the grains in the region from the center layer to the 1 / 4 layer in the thickness direction of the steel sheet and the average grain size D(surface) of the grains in the region from the surface layer to the 1 / 4 layer in the thickness direction satisfy the relationship of the following formula 1: D(surface) / D(center)≧0.6---[Formula 1] Steel sheets that have completed the final annealing have fine crystal grains uniformly distributed on the surface and center, and exhibit excellent strength characteristics, with a room temperature yield strength (YS(RT)) of 400 MPa or more and a 150°C yield strength (YS(150°C)) of 340 MPa or more. After final annealing, an insulating coating is formed. The insulating coating may be formed using an organic coating agent, an inorganic coating agent, or an organic-inorganic composite coating agent, or may be formed using other insulating coating agents.

[0036] The present invention will be described in more detail with reference to the following examples, but these examples are merely for illustrative purposes and are not intended to limit the scope of the present invention. Example 1

[0037] Slabs were produced with the components shown in Table 1 and the balance containing Fe and unavoidable impurities. These were heated at 1,150°C and hot rolled at a finishing temperature of 880°C to produce hot-rolled steel sheets with a thickness of 2.0 mm. [Table 1]

[0038] The hot-rolled hot-rolled steel sheet was subjected to hot-rolled steel sheet pre-annealing under the conditions in Table 2, and then cold-rolled to produce a thin cold-rolled steel sheet. This cold-rolled steel sheet was subjected to cold-rolled steel sheet final annealing under the conditions in Table 2. [Table 2]

[0039] The final thickness, average grain size, area fraction and number fraction of fine grains with a diameter of 10% or less of the thickness, D(center), D(surface), D(surface) / D(center), YS(RT), YS150°C), W10 / 400 iron loss, and B50 magnetic flux density of each test specimen were measured and are shown in Table 3. The content of each component was measured by inductively coupled plasma (ICP) wet analysis. The average diameter of crystal grains, the area fraction and number fraction of fine grains were measured by polishing the vertical (TD) cross section of the test piece to 100 mm. 2 The above areas were measured using electron backscatter diffraction (EBSD), then merged using the Merge function of the OIM software, and the average crystal grain size (Average), fine grain area fraction (Area fraction), and fine grain number fraction values ​​obtained when calculating using the Grain Size (diameter) function were used.

[0040] The D (center) value was calculated from the data cropped from the center of the thick layer to 1 / 4 of the thickness in the EBSD measurement data described above, and the D (surface) value was calculated from the data cropped from the surface of the thick layer to 1 / 4 of the thickness. Tensile tests at room temperature and 150°C were conducted in accordance with ISO6892-1 and 2. Magnetic properties such as magnetic flux density and iron loss were measured by cutting five test pieces, each 60 mm wide x 60 mm long, using a single sheet tester to measure the magnetic properties in the rolling direction and perpendicular to the rolling direction, and the average values ​​were shown. In this case, W10 / 400 is the iron loss when a magnetic flux density of 1.0 T is induced at a frequency of 400 Hz, and B50 means the magnetic flux density induced in a magnetic field of 5,000 A / m.

[0041] [Table 3]

[0042] As shown in Table 3, in the case of A7, A8, B7, B8, C7, C8, D7, and D8, which fall within the scope of the present invention, the content of the components was appropriately adjusted, and at the same time, the conditions for the two-stage pre-annealing and the two-stage final annealing were appropriately controlled, so that the size and distribution of the crystal grains were fine and uniform, and as a result, excellent yield strength and magnetic properties were obtained. However, in the case of A1, A5, B1, C6, D1, D3, D5, and D6, one or more of the Cr content, the primary preliminary annealing heating rate, the primary final annealing heating rate, and the soaking temperature are outside the ranges specified by the present invention. Therefore, some of the samples have an excessively small average crystal grain size, and fine crystal grains are concentrated in the surface layer, resulting in poor W10 / 400 properties. Furthermore, in the cases of A2, A3, A4, A6, B2, B3, B4, B5, B6, C1, C2, C3, C4, C5, D1, D2, D3, D4, and D5, the conditions for the two-stage preliminary annealing and the two-stage final annealing were outside the ranges specified by the present invention, and therefore it was confirmed that the average crystal grain size was either excessively large or excessively small, or the fraction of fine crystal grains was low, resulting in insufficient yield strength at room temperature and 150°C.

[0043] The present invention is not limited to the examples, and can be manufactured in various different forms, and a person skilled in the art to which the present invention pertains can understand that the present invention can be embodied in other specific forms without changing the technical idea or essential features of the present invention. It should be understood that the above-described examples are illustrative in all respects and are not limiting.

Claims

1. The alloy contains, by weight, 3.0 to 4.0% Si, 0.1 to 1.5% Al, 0.1 to 0.5% Mn, 2 to 20% Cr of the Mn content, 0.006 to 0.1% Sn and Sb in total, 0.0010 to 0.0050% C, and 0.0003 to 0.0050% each of one or more of N, S, Ti, Nb, and V, with the balance being Fe and inevitable impurities; A non-oriented electrical steel sheet characterized in that the area fraction of crystal grains whose grain size is 10% or less of the thickness of the steel sheet is 0.5% or more, the number fraction of the crystal grains is 20% or more, and the average crystal grain size from a central layer to a surface layer in the thickness direction of the steel sheet satisfies the relationship of the following formula 1: D(surface) / D(center)≧0.6---[Formula 1] (In Formula 1, D (surface) represents the average crystal grain size in the region from the surface layer to the ¼ layer in the thickness direction of the steel plate, and D (center) represents the average crystal grain size in the region from the center layer to the ¼ layer in the thickness direction of the steel plate.)

2. The non-oriented electrical steel sheet according to claim 1, characterized in that the average crystal grain size of the steel sheet is 50 to 150 μm.

3. 2. The non-oriented electrical steel sheet according to claim 1, wherein the steel sheet has a yield strength YS(RT) of 400 MPa or more and a yield strength YS(150°C) at 150°C of 340 MPa or more.

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

5. producing a slab containing, in weight percent, Si: 3.0 to 4.0%, Al: 0.1 to 1.5%, Mn: 0.1 to 0.5%, Cr: 2 to 20% of the Mn content, Sn and Sb total: 0.006 to 0.1%, C: 0.0010 to 0.0050%, and one or more of N, S, Ti, Nb and V each in an amount of 0.0003 to 0.0050%, with the balance being Fe and unavoidable impurities; heating the slab and hot rolling it to produce a hot-rolled steel sheet; a hot-rolled steel sheet pre-annealing step including: a first pre-annealing step of heating the hot-rolled steel sheet to 950 to 1,150°C and maintaining the temperature for 40 seconds or more; and a second pre-annealing step of changing the temperature atmosphere to 850 to 950°C within 20 seconds after the first pre-annealing step and maintaining the temperature for 20 seconds or more; After the pre-annealing step, cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and The cold-rolled steel sheet was subjected to hydrogen (H ２ ) and nitrogen (N ２ a final annealing step including a first final annealing step of heating to 900°C or higher in a mixed atmosphere of 100°C or higher and maintaining the temperature for 60 seconds or less, and a second final annealing step of maintaining the temperature at 650 to 850°C for 15 seconds or more; A method for producing a non-oriented electrical steel sheet, comprising:

6. The method for producing a non-oriented electrical steel sheet according to claim 5, wherein the slab is heated to 1,200°C or less.

7. The method for manufacturing a non-oriented electrical steel sheet according to claim 5, wherein in the step of manufacturing the hot-rolled steel sheet, the finish hot rolling is performed at 800°C or higher.

8. The method for manufacturing a non-oriented electrical steel sheet according to claim 5, wherein in the first pre-annealing step, the hot-rolled steel sheet is heated at a temperature rising rate of 10°C / s or more.

9. The method for manufacturing a non-oriented electrical steel sheet according to claim 5, wherein in the first final annealing step, the cold-rolled steel sheet is heated at a temperature rising rate of 25°C / s or more.

10. In the final annealing step, the cold-rolled steel sheet is subjected to a annealing treatment of 0.75 kgf / mm ２ 6. The method for producing a non-oriented electrical steel sheet according to claim 5, wherein the temperature is raised while applying the following tension in the rolling direction.

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