Non-oriented electrical steel sheet and its manufacturing method

Abstract

A non-oriented electrical steel sheet according to an embodiment of the present invention contains more than 0 and 0.003% by weight of carbon (C), 2.0 to 4.0% by weight of silicon (Si), 0.1 to 0.5% by weight of manganese (Mn), 0.3 to 0.9% by weight of aluminum (Al), more than 0 and 0.015% by weight of phosphorus (P), more than 0 and 0.003% by weight of sulfur (S), more than 0 and 0.003% by weight of nitrogen (N), more than 0 and 0.003% by weight of titanium (Ti), and the remainder being iron (Fe) and other unavoidable impurities, and satisfies the following mathematical formula 1 in its final microstructure: Formula 1: 0.00172[A]-0.0266[B]<2.0 (However, the above [A] is 10 x 10 mm 2 [B] is the average number of secondary phase particles in a steel sheet cross section having an area of 2 μm or more, and [B] is the volume fraction (unit: %) of particles having an average size of 2 μm or more among the secondary phase particles.

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 with improved magnetic properties and a manufacturing method thereof. [Background technology]

[0002] Electrical steel sheets can be divided into oriented and non-oriented electrical steel sheets based on their magnetic properties. Oriented electrical steel sheets are manufactured to facilitate magnetization in the rolling direction of the steel sheet, and have particularly excellent magnetic properties in the rolling direction. Therefore, they are primarily used as iron cores for large, medium-sized, and small transformers, which require low iron loss and high magnetic permeability. In contrast, non-oriented electrical steel sheets have uniform magnetic properties regardless of the direction of the steel sheet, and are therefore widely used as iron core materials for small electric motors, small power transformers, and stabilizers. With the recent trend toward increasing the efficiency and minimizing the size of electrical equipment to save energy, research is underway to minimize iron loss in non-oriented electrical steel sheets. For example, current internal combustion engine vehicles are rapidly being replaced by electric vehicles (EVs) due to policies to reduce carbon dioxide emissions to prevent global warming. Electric vehicles (EVs) must generate large torque when accelerating or at low speeds, and rotate at high speeds (over 200Hz) when traveling at constant or high speeds. Therefore, the non-oriented electrical steel sheet that is used as the iron core material for the motor must simultaneously satisfy high magnetic flux density and low iron loss.

[0003] Prior art documents include Korean Patent Publication No. 10-2015-0001467. Summary of the Invention [Problem to be solved by the invention]

[0004] The technical problem to be achieved by the present invention is to provide a non-oriented electrical steel sheet that simultaneously satisfies high magnetic flux density and low core loss.

[0005] However, these challenges are exemplary and are not intended to limit the scope of the present invention. [Means for solving the problem]

[0006] In order to solve the above problems, a non-oriented electrical steel sheet according to one embodiment of the present invention contains more than 0 and 0.003% by weight of carbon (C), 2.0 to 4.0% by weight of silicon (Si), 0.1 to 0.5% by weight of manganese (Mn), 0.3 to 0.9% by weight of aluminum (Al), more than 0 and 0.015% by weight of phosphorus (P), more than 0 and 0.003% by weight of sulfur (S), more than 0 and 0.003% by weight of nitrogen (N), more than 0 and 0.003% by weight of titanium (Ti), and the remainder being iron (Fe) and other unavoidable impurities, and satisfies the following mathematical formula 1 in its final microstructure:

[0007] Formula 1: 0.00172[A]-0.0266[B]<2.0

[0008] (However, the above [A] is 10 x 10 mm 2 [B] is the average number of secondary phase particles in a steel sheet cross section having an area of 2 μm or more, and [B] is the volume fraction (unit: %) of particles having an average size of 2 μm or more among the secondary phase particles.

[0009] The non-oriented electrical steel sheet according to one embodiment of the present invention has a thickness of 0.25 to 0.35 mm, and a magnetic flux density (B 50 ) is 1.66T or more, and iron loss (W 10 / 400 ) may be 12.5W / Kg or less.

[0010] The non-oriented electrical steel sheet according to an embodiment of the present invention may have an average crystal grain size of 80 to 150 μm.

[0011] A method for producing a non-oriented electrical steel sheet according to one embodiment of the present invention for solving the above problems includes: (a) providing a steel material containing more than 0% by weight of carbon (C) and not more than 0.003% by weight of silicon (Si), 2.0 to 4.0% by weight of manganese (Mn), 0.1 to 0.5% by weight of aluminum (Al), more than 0.015% by weight of phosphorus (P), more than 0.015% by weight of sulfur (S), more than 0.003% by weight of nitrogen (N), and more than 0.003% by weight of titanium (Ti), with the remainder being iron (Fe) and other unavoidable impurities; (b) hot-rolling the steel material; (c) coiling the hot-rolled steel material and then pre-annealing the coiled steel material without cooling it to room temperature; (d) cold-rolling the pre-annealed steel material; and (e) cold-rolling annealing the cold-rolled steel material.

[0012] In the method for producing the non-oriented electrical steel sheet, the step (c) may include a step of coiling at a coiling temperature (CT) of 550 to 650°C, and a step of annealing at 850 to 950°C for 10 to 30 hours.

[0013] In the method for producing the non-oriented electrical steel sheet, the pre-annealing treatment may be performed in a batch annealing furnace (BAF) instead of in an annealing and pickling line (APL).

[0014] In the method for manufacturing the non-oriented electrical steel sheet, the hot rolling step can include a step of reheating the steel material at a reheating temperature (SRT) of 1110 to 1150°C, and a step of hot rolling at a finish rolling temperature (FDT) of 860 to 900°C.

[0015] In the method for manufacturing the non-oriented electrical steel sheet, the step of performing the cold rolling annealing treatment includes a step of annealing under conditions of a temperature rise rate of 10°C / s or more, an annealing temperature of 900 to 1100°C, and a maintenance time of 30 to 120 seconds, and a step of cooling under conditions of a cooling rate of 20°C / s or more.

[0016] In the method for manufacturing the non-oriented electrical steel sheet, the final microstructure realized after the steps (a) to (e) satisfy the following mathematical formula 1.

[0017] Formula 1: 0.00172[A]-0.0266[B]<2.0

[0018] (However, the above [A] is 10 x 10 mm 2 [B] is the average number of secondary phase particles in a steel sheet cross section having an area of 2 μm or more, and [B] is the volume fraction (unit: %) of particles having an average size of 2 μm or more among the secondary phase particles. [Effects of the Invention]

[0019] According to the embodiments of the present invention, it is possible to provide a non-oriented electrical steel sheet that simultaneously satisfies high magnetic flux density and low core loss, and a method for manufacturing the same.

[0020] Of course, the scope of the present invention is not limited to such effects. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a flowchart schematically illustrating a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] A non-oriented electrical steel sheet and a manufacturing method thereof according to an embodiment of the present invention will now be described in detail. The terms used below have been appropriately selected in consideration of the functions of the present invention, and the definitions of such terms should be based on the content of this specification as a whole.

[0023] Generally, factors that affect the magnetic properties of non-oriented electrical steel sheets include the chemical composition, steel sheet thickness, microstructure, insulating coating layer, and texture. These various factors are affected by the conditions of the manufacturing process of non-oriented electrical steel sheets. Non-oriented electrical steel sheets are manufactured through the following processes: steelmaking / continuous casting → hot rolling → heat treatment → cold rolling → heat treatment and coating. By optimizing the conditions of each of these processes, electrical steel sheets with excellent magnetic properties can be manufactured.

[0024] Non-oriented electrical steel sheets are primarily used as materials for transformers and rotating machinery such as motors. Recently, environmental issues have led to increased demands for environmental conservation and improved energy efficiency. In particular, with the shift from internal combustion engines to electric vehicles and hybrid vehicles, there is a demand for improved magnetic properties in non-oriented electrical steel sheets to improve the efficiency of electric vehicle drive motors. Non-oriented electrical steel sheets are required to have magnetic properties such as high magnetic flux density and low iron loss.

[0025] To reduce iron loss in non-oriented electrical steel sheets, either the thickness of the steel sheet must be reduced or the resistivity of the steel sheet must be increased. Reducing the thickness of steel sheets requires advanced production technology and reduces productivity during the process. This increases costs during the processing and lamination of motor cores. To increase the resistivity of steel sheets, high-alloying elements such as silicon (Si), aluminum (Al), and manganese (Mn) can be added. However, the addition of these alloying elements makes cold rolling difficult. For example, in conventional electrical steel sheet processes, if the Si content exceeds 3.5 wt%, fracture occurs during cold rolling, requiring adjustment of the composition. Furthermore, if the Si content exceeds 3.5 wt% in non-oriented electrical steel sheet processes, the resistivity cannot exceed 60 μΩ·cm.

[0026] In order to meet the high magnetic flux density and low core loss requirements of non-oriented electrical steel sheets for high-efficiency electric vehicles and to increase the specific resistance, optimal alloy adjustment and advanced processing technology are required.

[0027] In terms of optimal alloy adjustment, Si, Al, and Mn are the main alloying elements that increase resistivity, and in addition to the combination of these alloys, elements such as Cr, Cu, and Ni that can improve cold rolling properties are also being considered. However, it is difficult to find the optimal conditions because they change the magnetic and mechanical properties.

[0028] In terms of the manufacturing process for non-oriented electrical steel sheets, the Annealing and Pickling Line (APL) process is known to be essential for improving the magnetic flux density and core loss of non-oriented electrical steel sheets containing more than 3.5% silicon by weight. While texture can be improved by controlling the heating rate, magnetic inferiority occurs due to inhomogeneity in the microstructure, so control of the microstructure of the hot-rolled structure is necessary from the intermediate APL process.

[0029] The present invention describes a non-oriented electrical steel sheet having high magnetic flux density and low iron loss value by controlling the microstructure of the hot-rolled and annealed material through process conditions suitable for mass production, and a method for manufacturing the same.

[0030] The following provides specific details of a non-oriented electrical steel sheet with improved magnetic properties and a method for manufacturing the same.

[0031] steel plate

[0032] A non-oriented electrical steel sheet according to one embodiment of the present invention comprises more than 0 to 0.003 wt% carbon (C), 2.0 to 4.0 wt% silicon (Si), 0.1 to 0.5 wt% manganese (Mn), 0.3 to 0.9 wt% aluminum (Al), more than 0 to 0.015 wt% phosphorus (P), more than 0 to 0.003 wt% sulfur (S), more than 0 to 0.003 wt% nitrogen (N), more than 0 to 0.003 wt% titanium (Ti), and the balance being iron (Fe) and other unavoidable impurities. The role and content of each component in the non-oriented electrical steel sheet will be described below.

[0033] Carbon (C): More than 0 and 0.003% by weight or less

[0034] Carbon (C) is an element that increases iron loss by forming carbides such as TiC and NbC, and the less carbon the better, so it is limited to 0.003 wt% or less. If the carbon content exceeds 0.003 wt%, magnetic aging occurs, degrading magnetic properties, but if the carbon content is 0.003 wt% or less, the magnetic aging phenomenon is suppressed.

[0035] Silicon (Si): 2.0 to 4.0% by weight

[0036] Silicon (Si) is a major additive element that increases resistivity and reduces eddy current loss. If the amount of silicon added is less than 2.0 wt%, it becomes difficult to achieve the desired low core loss value, and as the amount added increases, the magnetic permeability and magnetic flux density decrease. Also, if the amount of silicon added exceeds 4.0 wt%, the brittleness increases, making cold rolling difficult and reducing productivity.

[0037] Manganese (Mn): 0.1 to 0.5% by weight

[0038] Manganese (Mn), along with silicon, increases resistivity and improves texture. At less than 0.1 wt%, manganese forms fine MnS precipitates, inhibiting grain growth. Addition of more than 0.5 wt% results in the formation of coarse MnS precipitates, reducing magnetic flux density and otherwise degrading magnetic properties. Furthermore, manganese content exceeding 0.5 wt% results in a smaller reduction in iron loss compared to the added amount, but significantly reduces cold rolling properties.

[0039] Aluminum (Al): 0.3 to 0.9% by weight

[0040] Aluminum (Al) is a major additive element that, together with silicon, increases resistivity and reduces eddy current loss. Aluminum, combined with nitrogen, induces the precipitation of AlN. If the aluminum content is less than 0.3 wt%, the above effects are difficult to achieve. If the aluminum content exceeds 0.9 wt%, the cold rolling processability deteriorates, the magnetic flux density decreases, and the magnetic properties deteriorate.

[0041] Phosphorus (P): More than 0 and 0.015% by weight or less

[0042] Phosphorus (P) is a grain boundary segregating element that develops texture. If the phosphorus content exceeds 0.015 wt%, the segregation effect inhibits grain growth, degrades magnetic properties, and reduces cold rolling properties.

[0043] Sulfur (S): More than 0 and 0.003% by weight or less

[0044] Sulfur (S) forms precipitates such as MnS and CuS, which increase iron loss and inhibit grain growth, so its content should be kept as low as possible, limited to 0.003 wt% or less. If the sulfur content exceeds 0.003 wt%, the iron loss will increase.

[0045] Nitrogen (N): Over 0 and 0.003% by weight or less

[0046] Nitrogen (N) forms precipitates such as AlN, TiN, and NbN, which increase iron loss and inhibit grain growth, so its content should be limited to 0.003 wt% or less. If the nitrogen content exceeds 0.003 wt%, the iron loss will increase.

[0047] Titanium (Ti): Over 0% and up to 0.003% by weight

[0048] Titanium (Ti) forms fine precipitates such as TiC and TiN, which inhibits the growth of crystal grains. The more titanium is added, the more the magnetic properties deteriorate, so the amount should be kept as low as possible, limited to 0.003 wt% or less. If the titanium content exceeds 0.003 wt%, the magnetic properties will deteriorate.

[0049] The non-oriented electrical steel sheet according to an embodiment of the present invention, having the above-described composition of alloy elements, satisfies the following mathematical formula 1 in the final microstructure.

[0050] Formula 1: 0.00172[A]-0.0266[B]<2.0

[0051] Here, the [A] is 10 x 10 mm 2 [B] is the average number of secondary phase particles in a steel sheet cross section having an area of 2 μm or more, and [B] is the volume fraction (unit: %) of the secondary phase particles having an average size of 2 μm or more. In the final microstructure, the secondary phase particles may include inclusion particles and / or precipitate particles.

[0052] The non-oriented electrical steel sheet according to one embodiment of the present invention having the above-mentioned composition has a thickness of 0.25 to 0.35 mm and a magnetic flux density (B 50 ) is 1.66T or more, and iron loss (W 10 / 400 ) may be 12.5 W / Kg or less. In the non-oriented electrical steel sheet, the average crystal grain size may be 80 to 150 μm. The mechanical properties of the non-oriented electrical steel sheet are a yield strength (YP): 400 MPa or more and a tensile strength (TS): 500 MPa or more.

[0053] Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention having the above-described composition and properties will be described.

[0054] Manufacturing method for non-oriented electrical steel sheet

[0055] FIG. 1 is a flowchart schematically showing a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention.

[0056] Referring to FIG. 1 , a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention includes: (a) a steel sheet containing more than 0% by weight and not more than 0.003% by weight of carbon (C), 2.0 to 4.0% by weight of silicon (Si), 0.1 to 0.5% by weight of manganese (Mn), 0.3 to 0.9% by weight of aluminum (Al), more than 0% by weight and not more than 0.015% by weight of phosphorus (P), more than 0% by weight and not more than 0.003% by weight of sulfur (S), more than 0% by weight and not more than 0.003% by weight of nitrogen (N), more than 0% by weight and not more than 0.003% by weight of titanium (Ti), and the remainder being iron (b) providing a steel material comprising (Fe) and other unavoidable impurities (S10); (b) hot-rolling the steel material (S20); (c) coiling the hot-rolled steel material and then subjecting it to pre-annealing in the coiled state without cooling it to room temperature (S30); (d) cold-rolling the pre-annealed steel material (S40); and (e) cold-rolling annealing the cold-rolled steel material (S50).

[0057] The composition of the steel material in the step of providing the steel material (S10) has been described in detail above. The steel material may have the shape of a slab.

[0058] The step (S20) of hot rolling the steel material may include a step of reheating the steel material at a reheating temperature (SRT) of 1110 to 1150°C, and a step of hot rolling the steel material at a finish rolling temperature (FDT) of 860 to 900°C.

[0059] If the slab reheating temperature exceeds 1150°C, precipitates such as carbon (C), sulfur (S), and nitrogen (N) in the slab will re-dissolve, resulting in the formation of fine precipitates during the subsequent rolling and annealing processes, which may inhibit grain growth and deteriorate magnetic properties. If the slab reheating temperature is less than 1110°C, the rolling load will increase.

[0060] The thickness of the hot-rolled steel material may be 1.8 to 2.6 mm. As the thickness of the hot-rolled sheet increases, the reduction ratio of the cold rolling increases, and the texture becomes inferior, so it is preferable to set the thickness to 2.6 mm or less.

[0061] The step (S30) of coiling the hot-rolled steel sheet and then pre-annealing it in a coiled state without cooling it to room temperature includes coiling it at a coiling temperature (CT) of 550 to 650°C and annealing it at 850 to 950°C for 10 to 30 hours. After the annealing heat treatment, the steel sheet is cooled to room temperature. After cooling to room temperature, the oxide layer formed on the surface of the steel sheet is removed through a pickling solution, and then the steel sheet is subjected to a subsequent cold rolling process.

[0062] If the coiling temperature (CT) is less than 550°C, recovery and recrystallization are insufficient, resulting in excessively high potential density and stored energy, while if the coiling temperature (CT) is more than 650°C, significant oxidation occurs during cooling due to the high content of Si, Al, Mn, etc., which can lead to poor pickling properties. In addition, sufficient recovery and recrystallization occurs, reducing potential density and stored energy, resulting in inferior microstructure and texture after APL.

[0063] If the BAF heat treatment is performed after cooling to room temperature after coiling, a large amount of fine precipitates will be generated at low temperatures when cooling to room temperature after coiling, which will affect subsequent processes and have a negative impact on magnetic properties.On the other hand, if the BAF heat treatment is performed without cooling to room temperature after coiling, the heat treatment will occur before the fine precipitates are formed, which is expected to increase the proportion of coarse precipitates.

[0064] On the other hand, if the annealing temperature is lower than 850°C, or if the annealing time is less than 10 hours even within the appropriate annealing temperature range (850-950°C), fine inclusions such as carbides and nitrides are formed in the surface layer of the steel sheet, and the inclusions do not grow sufficiently, resulting in inferior magnetic properties of the final product.Furthermore, insufficient growth of crystal grains leads to the formation of fine crystal grains, resulting in inferior magnetic properties of the final product.

[0065] On the other hand, if the annealing temperature exceeds 950°C, or if the annealing time exceeds 30 hours even within the appropriate annealing temperature range (850-950°C), not only the distribution of inclusions but also the grains grow excessively, resulting in significant variation in grain size and frequent oxidation, which adversely affects the final product.

[0066] On the other hand, as a comparative example of the present invention, the APL (Annealing and Pickling Line) process for annealing and pickling a hot-rolled sheet includes an annealing step under the conditions of a temperature increase rate of 20°C / s or more, an annealing temperature of more than 950°C and less than 1100°C, and a maintenance time of 30 to 120 seconds, a cooling step under the conditions of a cooling rate of 30°C / s or more, and a pickling step.

[0067] In the method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention, the pre-annealing treatment is performed in a batch annealing furnace (BAF) rather than in an annealing and pickling line (APL).

[0068] In the Batch Annealing Furnace (BAF) process, the coil is loaded into the furnace as a unit and processed, so the entire coil is heated at the same time, which can cause variations in material quality due to temperature differences between the inside and outside of the coil. Therefore, by applying a relatively long heat treatment time, such temperature differences can be minimized. The Batch Annealing Furnace (BAF) process can be performed in a 100% nitrogen atmosphere.

[0069] On the other hand, the advantage of the batch annealing furnace (BAF) process is that it is possible to optimize the heat treatment conditions by adjusting the temperature and time to suit the characteristics when performing the heating, holding, and cooling annealing processes. Compared to continuous annealing processes (APL, ACL, etc.), it has the advantage of reducing production costs and oxidation of steel sheets.

[0070] The cold rolling step (S40) may include cold rolling at a rolling reduction of 80 to 85%, and the thickness of the steel material after cold rolling may be 0.35 mm or less. In cold rolling, the pickled hot-rolled sheet is subjected to final cold rolling to a thickness of 0.25 mm or more and 0.35 mm or less. In order to impart rollability, the temperature of the sheet may be increased to 150 to 200°C and then warm-rolled.

[0071] The step (S50) of performing the cold rolling annealing treatment is an ACL (Annealing and Coating Line) step of final annealing the cold rolled sheet, and includes a step of annealing under the conditions of a temperature increase rate of 10°C / s or more, an annealing temperature of 900 to 1100°C, and a maintenance time of 30 to 120 seconds, and a step of cooling under the conditions of a cooling rate of 20°C / s or more.

[0072] Cold rolling annealing is performed on the cold rolled sheet obtained after cold rolling. The temperature is selected to derive the optimum grain size, taking into consideration the improvement of iron loss and mechanical properties. During cold rolling annealing, the material is heated in a mixed atmosphere to prevent surface oxidation and nitridation. A mixed atmosphere of nitrogen and hydrogen further smooths the surface. If the cold rolling annealing temperature is less than 900°C, the grain size will be fine, which may increase hysteresis loss. If the cold rolling annealing temperature is more than 1100°C, the grain size will become coarse, which will increase eddy current loss.

[0073] After the final annealing, a coating process can be carried out to improve punchability and ensure insulation.

[0074] The final microstructure of the non-oriented electrical steel sheet realized by performing the above steps satisfies the following Equation 1.

[0075] Formula 1: 0.00172[A]-0.0266[B]<2.0

[0076] Here, the [A] is 10 x 10 mm 2 [B] is the average number of secondary phase particles in a steel sheet cross section having an area of 2 μm or more, and [B] is the volume fraction (unit: %) of the secondary phase particles having an average size of 2 μm or more. In the final microstructure, the secondary phase particles may include inclusion particles and / or precipitate particles.

[0077] The non-oriented electrical steel sheet realized by the above steps has a thickness of 0.25 to 0.35 mm and a magnetic flux density (B 50 ) is 1.66T or more, and iron loss (W 10 / 400 ) may be 12.5 W / Kg or less. In the non-oriented electrical steel sheet, the average crystal grain size may be 80 to 150 μm. The mechanical properties of the non-oriented electrical steel sheet are a yield strength (YP): 400 MPa or more and a tensile strength (TS): 500 MPa or more.

[0078] Experimental example

[0079] In the following, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are merely provided to aid in understanding the present invention, and the present invention is not limited to the following experimental examples.

[0080] 1. Composition of test specimen

[0081] In this experimental example, a test piece having the composition of alloy elements (unit: wt %) shown in Table 1 was provided.

[0082] [Table 1]

[0083] Referring to Table 1, the composition of the non-oriented electrical steel sheet according to the experimental example is carbon (C): more than 0 and not more than 0.003% by weight, silicon (Si): 2.0 to 4.0% by weight, manganese (Mn): 0.1 to 0.5% by weight, aluminum (Al): 0.3 to 0.9% by weight, phosphorus (P): more than 0 and not more than 0.015% by weight, sulfur (S): more than 0 and not more than 0.003% by weight, nitrogen (N): more than 0 and not more than 0.003% by weight, titanium (Ti): more than 0 and not more than 0.003% by weight, and the remainder is iron (Fe).

[0084] 2. Evaluation of process conditions and physical properties

[0085] Table 2 shows the process conditions and physical properties of the manufacturing method for non-oriented electrical steel sheet according to the experimental example of the present invention. Slabs having the composition shown in Table 1 were heated to 1140°C and hot-rolled at a finish rolling temperature of 880°C to produce hot-rolled sheets with a thickness of 2.0 mm. After hot rolling, the sheets were successively heat-treated at various temperatures shown in Table 2. They were then cold-rolled to produce cold-rolled sheets with a thickness of 0.25 mm, which were then final-annealed at 975°C for 50 seconds. The final products were then produced through a coating process. The final-annealing atmosphere was a mixed atmosphere of 30% hydrogen and 70% nitrogen. The heating rate was 20°C / s and the cooling rate was 30°C / s. The inclusion distribution of each test specimen after final-annealing was also observed. The magnetic properties of the final product were measured by using an SST (Single Sheet Tester) to measure the iron loss value and magnetic flux density value in the L direction parallel to the rolling direction and the C direction perpendicular to the rolling direction, and then calculating the average value.

[0086] In Table 2, the Z value is the result of calculation: 0.00172 × [A] - 0.0266 × [B], where [A] is an index showing the total number of inclusions in the final annealed material, and is 2 [B] is an index showing the volume fraction of coarse inclusions, and is the volume fraction (unit: %) of particles having an average size of 2 μm or more among the secondary phase particles.

[0087] [Table 2]

[0088] Experimental Examples 4, 7, and 10 are examples of the present invention, and satisfy the condition of annealing at 850 to 950°C for 10 to 30 hours in the batch annealing furnace (BAF) annealing (S30) step of FIG. 1. In this case, the Z value is smaller than 2, and the iron loss (W 10 / 400 ) can be confirmed to be less than 12.5W / Kg.

[0089] In contrast, Experimental Examples 1 and 2 are below the annealing temperature range of 850 to 950°C in the batch annealing furnace (BAF) annealing (S30) step of FIG. 1, and do not meet this requirement. Experimental Example 12 is above the annealing temperature range of 850 to 950°C in the batch annealing furnace (BAF) annealing (S30) step of FIG. 1, and does not meet this requirement. On the other hand, Experimental Examples 3, 6, and 9 satisfy the annealing temperature range of 850 to 950°C in the batch annealing furnace (BAF) annealing (S30) step of Figure 1, but do not satisfy the annealing time range of 10 to 30 hours. Experimental Examples 5, 8, and 11 satisfy the annealing temperature range of 850 to 950°C in the batch annealing furnace (BAF) annealing (S30) step of Figure 1, but do not satisfy the annealing time range of 10 to 30 hours.

[0090] In summary, Experimental Examples 1, 2, 3, 5, 6, 8, 9, 11, and 12 have a larger average number of secondary phase particles than the previous Examples (Experimental Examples 4, 7, and 10). Furthermore, Experimental Examples 1, 2, and 12 do not satisfy the annealing temperature range of 850 to 950°C, and have a lower volume fraction of secondary phase particles with an average size of 2 μm or more than the previous Examples (Experimental Examples 4, 7, and 10).

[0091] The above-mentioned Experimental Examples 1, 2, 3, 5, 6, 8, 9, 11, and 12 are comparative examples of the present invention, and the Z value is greater than 2 and the iron loss (W10 / 400 ) can be confirmed to exceed 12.5W / Kg.

[0092] For example, when Experimental Example 8 and Experimental Example 10 are compared, even though [B], which is the volume fraction value of secondary phase particles with an average size of 2 μm or more, is the same, the number of inclusions in the final annealed material is 10 × 10 mm 2 If the average number of secondary phase particles in the steel sheet cross section of the area [A] is even smaller, the Z value will be smaller than 2, and the iron loss (W 10 / 400 ) is lower.

[0093] In addition, when comparing Experimental Example 11 and Experimental Example 10, the number of inclusions in the final annealed material was measured using a 10 × 10 mm 2 Even if [A], which is the average number of secondary phase particles in a steel sheet cross section of an area of [A], is similar, if [B], which is the volume fraction of secondary phase particles with an average size of 2 μm or more as an index showing the volume fraction of coarse inclusions, is larger, the Z value will be smaller than 2, and the iron loss (W 10 / 400 ) is lower.

[0094] The present invention has been described above with reference to a non-oriented electrical steel sheet and a manufacturing method thereof. It has been confirmed that, in the manufacturing process of non-oriented electrical steel sheet, the hot rolling annealing heat treatment process is not performed in an Annealing and Pickling Line (APL) but in a Batch Annealing Furnace (BAF), but by adjusting the appropriate temperature and maintenance time, the microstructure can be controlled and the magnetic properties of the final annealed material can be improved.

[0095] Although the present invention has been described above with reference to the preferred embodiments, various modifications and variations may be made by those skilled in the art. Such modifications and variations are within the scope of the present invention. Therefore, the scope of the present invention should be determined by the appended claims.

Claims

1. a non-oriented electrical steel sheet containing more than 0 and 0.003% by weight or less of carbon (C), 2.0 to 4.0% by weight of silicon (Si), 0.1 to 0.5% by weight of manganese (Mn), 0.3 to 0.9% by weight of aluminum (Al), more than 0 and 0.015% by weight or less of phosphorus (P), more than 0 and 0.003% by weight or less of sulfur (S), more than 0 and 0.003% by weight or less of nitrogen (N), more than 0 and 0.003% by weight or less of titanium (Ti), with the remainder being iron (Fe) and other unavoidable impurities; A non-oriented electrical steel sheet that satisfies the following formula 1 in its final microstructure: Formula 1: 0.00172[A]-0.0266[B]<2.0 (However, the above [A] is 10 x 10 mm ２ [B] is the average number of secondary phase particles in a steel sheet cross section having an area of 2 μm or more, and [B] is the volume fraction (unit: %) of particles having an average size of 2 μm or more among the secondary phase particles.

2. The thickness of the electromagnetic steel sheet is 0.25 to 0.35 mm, and the magnetic flux density (B ５０ ) is 1.66T or more, and iron loss (W １０／４００ 2. The non-oriented electrical steel sheet according to claim 1, wherein the tensile strength is 12.5 W / Kg or less.

3. 2. The non-oriented electrical steel sheet according to claim 1, wherein the average crystal grain size of the non-oriented electrical steel sheet is 80 to 150 μm.

4. (a) providing a steel material consisting of carbon (C): more than 0 and 0.003% by weight or less, silicon (Si): 2.0 to 4.0% by weight, manganese (Mn): 0.1 to 0.5% by weight, aluminum (Al): 0.3 to 0.9% by weight, phosphorus (P): more than 0 and 0.015% by weight or less, sulfur (S): more than 0 and 0.003% by weight or less, nitrogen (N): more than 0 and 0.003% by weight or less, titanium (Ti): more than 0 and 0.003% by weight or less, and the remainder being iron (Fe) and other unavoidable impurities; (b) hot rolling the steel material; (c) coiling the hot-rolled steel material, and then performing a preliminary annealing treatment in the coiled state without cooling it to room temperature; (d) cold rolling the pre-annealed steel material; (e) performing cold rolling annealing on the cold-rolled steel material.

5. 5. The method for producing a non-oriented electrical steel sheet according to claim 4, wherein step (c) comprises the steps of coiling at a coiling temperature (CT) of 550 to 650°C and annealing at 850 to 950°C for 10 to 30 hours.

6. 5. The method for manufacturing a non-oriented electrical steel sheet according to claim 4, wherein the pre-annealing treatment is performed in a batch annealing furnace (BAF) rather than in an annealing and pickling line (APL).

7. 5. The method for producing a non-oriented electrical steel sheet according to claim 4, wherein the hot rolling step includes a step of reheating the steel material at a reheating temperature (SRT) of 1110 to 1150°C, and a step of hot rolling the steel material at a finish rolling temperature (FDT) of 860 to 900°C.

8. 5. The method for producing a non-oriented electrical steel sheet according to claim 4, wherein the step of performing the cold rolling annealing treatment includes a step of annealing under conditions of a heating rate of 10°C / s or more, an annealing temperature of 900 to 1100°C, and a maintenance time of 30 to 120 seconds, and a step of cooling under conditions of a cooling rate of 20°C / s or more.

9. The method of claim 4, wherein a final microstructure achieved after steps (a) to (e) satisfies the following equation: Formula 1: 0.00172[A]-0.0266[B]<2.0 (However, the above [A] is 10 x 10 mm ２ [B] is the average number of secondary phase particles in a steel sheet cross section having an area of 2 μm or more, and [B] is the volume fraction (unit: %) of particles having an average size of 2 μm or more among the secondary phase particles.

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