Non-oriented electrical steel sheet and method for producing same

Abstract

A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention comprises the steps of: providing a steel material containing silicon (Si), manganese (Mn), and aluminum (Al); carrying out hot rolling on the steel; carrying out first heat treatment on the hot-rolled steel before winding; winding the steel material subjected to the first heat treatment; carrying out uncoiling and cold rolling on the coiled steel; and the cold-rolled steel is annealed.

Description

Technical Field

[0001] The present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same, and more particularly, to an efficient non-oriented electrical steel sheet and a method for manufacturing the same. Background Art

[0002] Electrical steel sheets can be divided into oriented electrical steel sheets and non-oriented electrical steel sheets according to their magnetic properties. Oriented electrical steel sheets exhibit excellent magnetic properties, especially in the rolling direction of the steel sheets (because they are manufactured to be easily magnetized in the rolling direction), and are therefore mostly used as cores for large, medium and small transformers requiring low core loss and high magnetic permeability. On the other hand, non-oriented electrical steel sheets have uniform magnetic properties regardless of the direction of the steel sheets, and are therefore generally used as core materials for small motors, small power transformers, stabilizers, etc. Related literature includes Korean Patent Publication No. 2015-0001467A. Summary of the invention

[0003] Technical issues

[0004] The present invention provides a highly efficient non-oriented electrical steel sheet and a method for manufacturing the same.

[0005] However, the above description is an example, and the scope of the present invention is not limited thereto.

[0006] Technical Solution

[0007] According to one aspect of the present invention, there is provided a method for manufacturing a non-oriented electrical steel sheet, the method comprising: providing a steel material comprising silicon (Si), manganese (Mn) and aluminum (Al); hot rolling the steel material; performing a first heat treatment on the hot rolled steel material before coiling; coiling the steel material subjected to the first heat treatment; uncoiling and cold rolling the coiled steel material; and cold annealing the cold rolled steel material.

[0008] The steel material may contain 2.0 to 4.0 wt% Si, 0.1 to 0.5 wt% Mn, 0.3 to 0.9 wt% Al, greater than 0 wt% and not more than 0.003 wt% carbon (C), greater than 0 wt% and not more than 0.015 wt% phosphorus (P), greater than 0 wt% and not more than 0.003 wt% sulfur (S), greater than 0 wt% and not more than 0.003 wt% nitrogen (N), greater than 0 wt% and not more than 0.003 wt% titanium (Ti), and the balance of iron (Fe) and unavoidable impurities.

[0009] Before the hot rolled steel material is cooled to room temperature, a first heat treatment may be performed at a temperature higher than the coiling temperature.

[0010] Hot rolling can be carried out at a slab reheating temperature (SRT) of 1110°C to 1150°C and a finishing exit temperature (FDT) of 860°C to 900°C, the first heat treatment can include maintaining a temperature of 850°C to 1000°C for 5 to 10 minutes, and coiling can be carried out at a temperature of 550°C to 650°C.

[0011] The cold annealing may include annealing at a heating rate of 20° C. / s or more, an annealing start temperature of 900° C. to 1100° C., and a holding time of 30 seconds to 120 seconds, and cooling at a cooling rate of 30° C. / s or more.

[0012] After the steel is coiled, it can be cold rolled without preliminary annealing.

[0013] According to another aspect of the present invention, there is provided a nonoriented electrical steel sheet comprising 2.0 to 4.0 wt % of silicon (Si), 0.1 to 0.5 wt % of manganese (Mn), 0.3 to 0.9 wt % of aluminum (Al), more than 0 wt % and not more than 0.003 wt % of carbon (C), more than 0 wt % and not more than 0.015 wt % of phosphorus (P), more than 0 wt % and not more than 0.003 wt % of sulfur (S), more than 0 wt % and not more than 0.003 wt % of nitrogen (N), more than 0 wt % and not more than 0.003 wt % of titanium (Ti), and the balance of iron (Fe) and inevitable impurities, wherein a ratio of the area fraction of inclusions having an average diameter of 5 μm or more to the area fraction of inclusions having an average diameter of 2 μm or less among inclusions constituting a final microstructure is greater than 0.3, and the nonoriented electrical steel sheet has a core loss (W) of 12.5 W / kg or less. 10 / 400 ) and a magnetic flux density of 1.66T or greater (B 50 ).

[0014] Beneficial effects

[0015] According to the embodiments of the present invention, an efficient non-oriented electrical steel sheet and a method for manufacturing the same can be provided.

[0016] However, the scope of the present invention is not limited to the above-mentioned effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 FIG. 4 is a flow chart of a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention.

[0018] Figure 2 The following is a flow chart of a method for manufacturing a non-oriented electrical steel sheet according to a comparative example of the present invention.

[0019] Figure 3This is a microscopic image of inclusions having an average diameter of 2 μm or less in the sample of Test Example 1.

[0020] Figure 4 This is a microscopic image of inclusions having an average diameter of 5 μm or more in the sample of Test Example 5. DETAILED DESCRIPTION

[0021] A method for manufacturing a nonoriented electrical steel sheet according to an embodiment of the present invention will now be described in detail. Terms used herein are appropriately selected according to their functions in the present invention, and the definitions of these terms should be based on the entire contents of this specification.

[0022] Electrical steel sheets are generally divided into oriented electrical steel sheets and non-oriented electrical steel sheets. Oriented electrical steel sheets are mostly used in stationary machinery such as transformers, while non-oriented electrical steel sheets are often used in rotating machinery such as motors. The performance of electrical steel sheets can be evaluated based on magnetic flux density and core loss. Magnetic flux density is mostly expressed in B 50 Evaluation, core loss is generally expressed in W 15 / 50 However, when high frequency characteristics are required in electric vehicles, the core loss is expressed in W. 10 / 400 Evaluate. B 50 Indicates the magnetic flux density at 5000A / m, W 15 / 50 represents the core loss at 50Hz and 1.5T, W 10 / 400 Represents the core loss at 400Hz and 1.0T.

[0023] Due to regulations aimed at reducing CO2 emissions to combat global warming, existing internal combustion engine vehicles are being rapidly replaced by environmentally friendly vehicles such as hybrid electric vehicles (HEVs) and electric vehicles (EVs), more specifically electric vehicles. As the demand for electric vehicles increases, the energy conversion efficiency of electric vehicle drive motors continues to improve, so motor core materials are required to have excellent magnetic properties. Non-oriented electrical steel sheets used as motor core materials are used in rotating machinery to convert electrical energy into mechanical energy, and magnetic properties (i.e., low core loss and high magnetic flux density) are essential for energy saving. In particular, non-oriented electrical steel sheets with low core loss at high frequencies (e.g., 400 Hz) are required to improve motor efficiency during high-speed operation where energy loss is high. In order to meet these requirements, non-oriented electrical steel sheet products are being developed by adding elements such as silicon (Si), manganese (Mn) and aluminum (Al) to increase resistivity or reduce material thickness. However, when alloying elements such as Si, Mn and Al are increased, the rolling process may not be properly performed, resulting in a decrease in the cold rolling recovery rate and making it difficult to reduce the thickness, and when reducing the thickness of the electrical steel sheet, advanced production technology is required, production costs increase, and productivity decreases.

[0024] Figure 1FIG. 4 is a flow chart of a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention.

[0025] Reference Figure 1 , a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention includes: providing a steel material containing silicon (Si), manganese (Mn) and aluminum (Al) (S10); hot rolling the steel material (S20); performing a first heat treatment on the hot-rolled steel material before coiling (S30); coiling the steel material subjected to the first heat treatment (S40); uncoiling and cold rolling the coiled steel material (S50); and cold annealing the cold-rolled steel material (S60).

[0026] Provide steel (S10)

[0027] The steel material provided to the hot rolling process is a steel material for manufacturing a non-oriented electrical steel sheet, and contains, for example, 2.0 to 4.0 wt% of Si, 0.1 to 0.5 wt% of Mn, 0.3 to 0.9 wt% of Al, more than 0 wt% and not more than 0.003 wt% of carbon (C), more than 0 wt% and not more than 0.015 wt% of phosphorus (P), more than 0 wt% and not more than 0.003 wt% of sulfur (S), more than 0 wt% and not more than 0.003 wt% of nitrogen (N), more than 0 wt% and not more than 0.003 wt% of titanium (Ti), and the balance of iron (Fe) and inevitable impurities.

[0028] The functions and contents of exemplary components applicable to the method for producing a non-oriented electrical steel sheet according to the technical features of the present invention will now be described. However, the method for producing a non-oriented electrical steel sheet according to the technical features of the present invention is not limited to the following exemplary compositions, and can be extended to any composition capable of achieving the functions of the non-oriented electrical steel sheet.

[0029] Si: 2.0 wt% to 4.0 wt%

[0030] Si is a main element added as a component to increase resistivity and reduce core loss (or eddy current loss). When the content of Si is less than 2.0 wt%, it is not easy to achieve the low core loss value required at high frequencies, and when the content increases, the magnetic permeability and magnetic flux density decrease. When the content of Si is more than 4.0 wt%, brittleness increases, making cold rolling difficult and reducing productivity.

[0031] Mn: 0.1 wt% to 0.5 wt%

[0032] Mn increases resistivity and improves texture together with Si. When Mn is added in excess of 0.5 wt%, coarse MnS precipitates are formed, thereby deteriorating magnetic properties, such as reduced magnetic flux density. In addition, when the content of Mn is greater than 0.5 wt%, the reduction in core loss is small compared to the addition amount, and cold rolling properties are significantly deteriorated. Because fine MnS precipitates may be formed and grain growth may be inhibited when the content of Mn is less than 0.1 wt%, the composition of Mn may be controlled to 0.1 wt% to 0.5 wt%.

[0033] Al: 0.3 wt% to 0.9 wt%

[0034] Al is a main element added together with Si as a component for increasing resistivity and reducing core loss (or eddy current loss). Al is used to reduce magnetic deviation by reducing magnetic anisotropy. Al induces AlN precipitation when combined with N. When the content of Al is less than 0.3 wt%, the above effect may not be easily expected, and fine nitrides may be formed to increase the deviation of magnetic properties, and when the content of Al is greater than 0.9 wt%, cold rolling properties deteriorate, excessive nitride formation occurs, resulting in reduced magnetic flux density and deteriorated magnetic properties.

[0035] C: greater than 0 wt% and not more than 0.003 wt%

[0036] C is an element that forms carbides such as TiC and NbC and increases core loss, so the less the better. The content of C is limited to 0.003 wt% or less. When the content of C is more than 0.003 wt%, magnetic aging occurs and the magnetic properties deteriorate, while when the content of C is 0.003 wt% or less, magnetic aging is suppressed.

[0037] P: greater than 0 wt% and not more than 0.015 wt%

[0038] P is a grain boundary segregation element and is an element for forming a texture. When the content of P is more than 0.015 wt%, grain growth is inhibited, magnetic properties deteriorate, and cold rolling properties are reduced due to the segregation effect.

[0039] S: greater than 0 wt% and not more than 0.003 wt%

[0040] S forms precipitates such as MnS and CuS to increase core loss and inhibit grain growth, so the less the better. The S content is limited to 0.003 wt% or less. When the S content is more than 0.003 wt%, the core loss increases.

[0041] N: greater than 0 wt% and not more than 0.003 wt%

[0042] N forms precipitates such as AlN, TiN and NbN to increase core loss and inhibit grain growth, so the less the better. The content of N is limited to 0.003 wt% or less. When the content of N is more than 0.003 wt%, the core loss increases.

[0043] Ti: more than 0 wt% and not more than 0.003 wt%

[0044] Ti forms fine precipitates such as TiC and TiN and inhibits grain growth. Ti deteriorates magnetic properties, so the less the better. The content of Ti is limited to 0.003 wt% or less. When the content of Ti is greater than 0.003 wt%, magnetic properties deteriorate.

[0045] Hot rolling (S20)

[0046] The steel material having the above composition is hot rolled. The steel material may be hot rolled under the conditions of a slab reheating temperature (SRT) of 1000°C to 1200°C (more strictly, 1110°C to 1150°C) and a finishing outlet temperature (FDT) of 860°C to 900°C (S20).

[0047] When the SRT is higher than 1200°C (more strictly, 1150°C), precipitates such as C, S, and N in the slab may be redissolved, and fine precipitates may appear during subsequent rolling and annealing, thereby inhibiting grain growth and deteriorating magnetic properties. When the SRT is lower than 1000°C (more strictly, 1110°C), the rolling load may increase and the final product may have a high core loss.

[0048] After hot rolling the steel (S20), the thickness of the hot rolled sheet may be, for example, 1.8 mm to 2.6 mm. Since the cold rolling reduction rate increases and the texture deteriorates when the hot rolled sheet is thick, the thickness may be controlled to be 2.6 mm or less.

[0049] First heat treatment (S30)

[0050] Before coiling, the hot rolled steel may be subjected to a first heat treatment (S30). Before the hot rolled steel is cooled to room temperature, the first heat treatment (S30) may be continuously performed. After the first heat treatment, the steel may be cooled to room temperature. The first heat treatment may be annealing. The first heat treatment (S30) may be performed at a temperature higher than the temperature of the subsequent coiling (S40). For example, when the coiling temperature is 550°C to 650°C, the first heat treatment (S30) may include maintaining a temperature of 850°C to 1000°C for 5 to 10 minutes. When the first heat treatment temperature is lower than 850°C, fine inclusions such as nitrides may form on the surface layer, and the magnetic properties of the final product may deteriorate due to insufficient growth of the inclusions. On the other hand, when the first heat treatment temperature is higher than 1000°C, inclusions may be formed and the grains may grow excessively, resulting in severe deviations in grain size and severe oxidation, which may have a negative impact on the final product. Therefore, in order to obtain a uniform microstructure of the hot-rolled plate, the first heat treatment temperature can be controlled to be 850°C to 1000°C.

[0051] Coiling (S40)

[0052] The first heat-treated steel may be coiled at a coiling temperature (CT) of 550° C. to 650° C. When the CT is below 550° C., the steel does not experience an annealing effect, so the grains do not grow; when the CT is above 650° C., oxidation may be aggravated during cooling, so pickling properties may deteriorate.

[0053] Cold rolled (S50)

[0054] The coiled steel is uncoiled and cold rolled (S50). The cold rolling reduction ratio may be 80% to 85%, and the thickness of the cold rolled steel may be 0.35 mm or thinner (more strictly, 0.25 mm or thinner). In order to improve rollability, the plate temperature may be increased to 150° C. to 200° C. for warm rolling.

[0055] Generally, after hot rolling and coiling and before cold rolling, the hot rolled sheet may be subjected to preliminary annealing (e.g., annealing and pickling line (APL)). For example, the preliminary annealing may include annealing at a heating rate of 20°C / s or more, an annealing temperature of 950°C to 1100°C, and a holding time of 30 seconds to 120 seconds, cooling at a cooling rate of 30°C / s or more, and then pickling.

[0056] However, in the present invention, it was found that since the hot-rolled steel material was subjected to the first heat treatment as described above before coiling, it could be directly cold-rolled without preliminary annealing after removing the oxide layer formed on the surface using a pickling solution.

[0057] Cold annealing (S60)

[0058] The cold-rolled steel material may be cold annealed. The cold annealing (S60) may include: annealing at a heating rate of 20°C / s or more, an annealing temperature of 900°C to 1100°C, and a holding time of 30 seconds to 120 seconds, and cooling at a cooling rate of 30°C / s or more. In the cold annealing, when the annealing temperature is lower than 900°C, fine grains may be formed, thereby increasing hysteresis loss, and when the annealing temperature is higher than 1100°C, coarse grains may be formed, thereby increasing eddy current loss.

[0059] The cold-rolled sheet obtained after cold rolling is cold annealed. A temperature that achieves an optimum grain size is adopted in consideration of core loss reduction as well as mechanical properties. In the cold annealing, heating is performed under mixed atmosphere conditions to prevent surface oxidation and nitration. The surface is further smoothed in a mixed atmosphere of nitrogen and hydrogen. When the cold annealing temperature is lower than 900°C, fine grains may be formed, thereby increasing hysteresis loss, and when the cold annealing temperature is higher than 1100°C, coarse grains may be formed, thereby increasing eddy current loss.

[0060] Meanwhile, a coating process may be performed after the final cold annealing to form an insulating coating. By forming the insulating coating, stampability may be improved and insulation may be ensured. The insulating coating formed on and below the cold-rolled material may have a thickness of about 1 μm to 2 μm.

[0061] The nonoriented electrical steel sheet manufactured using the above method may be a nonoriented electrical steel sheet comprising 2.0 wt% to 4.0 wt% of Si, 0.1 wt% to 0.5 wt% of Mn, 0.3 wt% to 0.9 wt% of Al, more than 0 wt% to not more than 0.003 wt% of C, more than 0 wt% to not more than 0.015 wt% of P, more than 0 wt% to not more than 0.003 wt% of S, more than 0 wt% to not more than 0.003 wt% of N, more than 0 wt% to not more than 0.003 wt% of Ti, and the balance of Fe and inevitable impurities, wherein the average grain size is 80 μm to 150 μm, and the ratio of the area fraction of inclusions having an average diameter of 5 μm or more to the area fraction of inclusions having an average diameter of 2 μm or less among inclusions constituting the final microstructure is greater than 0.3. The nonoriented electrical steel sheet may have a core loss (W) of 12.5 W / kg or less. 10 / 400 ), 1.66T or greater magnetic flux density (B 50 ), a yield strength of 400 MPa or greater and a tensile strength of 500 MPa or greater.

[0062] The present invention provides a non-oriented electrical steel sheet and a method for manufacturing the same, wherein the non-oriented electrical steel sheet can achieve high magnetic flux density and low core loss by controlling the microstructure of a thermally annealed material on the basis of process conditions that can be mass-produced.

[0063] Reference Figure 2 A method for manufacturing a non-oriented electrical steel sheet according to a comparative example of the present invention generally includes: producing a hot-rolled sheet by reheating a slab and then hot rolling (S10, S20 and S40); performing preliminary annealing (e.g., APL) on the hot-rolled sheet (S45); producing a cold-rolled sheet by cold rolling the thermally annealed material (S50); and performing final annealing (e.g., annealing and coating line (ACL)) on the cold-rolled sheet (S60). The APL process is essential for high-alloy non-oriented electrical steel sheets. Through the APL process, cold rollability is imparted and the magnetic properties of the final product are enhanced. Since the magnetic quality of the final product is severely affected by the APL process, precise structural control is essential. When the size of fine precipitates and inclusions is not controlled by heat treatment, domain wall motion may be delayed by the fine precipitates and inclusions, resulting in poor magnetic properties.

[0064] In the method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention, after hot rolling the non-oriented electrical steel sheet, a first heat treatment (S30) is continuously performed before the hot rolled material is cooled to a temperature lower than the coiling temperature. In this case, the first heat treatment may include maintaining a temperature of 850° C. to 1000° C. for 5 to 10 minutes. Due to the first heat treatment, APL can be omitted and an electrical steel sheet with better properties can be manufactured.

[0065] In the present invention, it was found that by applying the above-mentioned heat treatment, a microstructure in which the ratio of the area fraction of inclusions having an average diameter of 5 μm or more to the area fraction of inclusions having an average diameter of 2 μm or less among the inclusions constituting the final annealed material is greater than 0.3 can be ensured, and in this case, a core loss (W) of 12.5 W / kg or less can be achieved at the same time. 10 / 400 ) and a magnetic flux density of 1.66T or greater (B 50 Using the backscattered electron detector (BSD) mode of a scanning electron microscope (SEM), a 10 × 10 mm2 image of the layered structure of the final annealed material was analyzed based on brightness and contrast. 2 Distribution of inclusions of all sizes on a cross section.

[0066] The final product obtained by the method for manufacturing a non-oriented electrical steel sheet of the present invention has a uniform microstructure and improved magnetic properties. In this case, a core loss (W) of 12.5 W / kg or less can be achieved. 10 / 400) and a magnetic flux density of 1.65 T or higher, more specifically a core loss of 12.0 W / kg or less (W 10 / 400 ) and a magnetic flux density of 1.66T or higher.

[0067] As mentioned above, in order to reduce the core loss of non-oriented electrical steel sheets, it is necessary to reduce the sheet thickness and increase the resistivity. However, reducing the sheet thickness requires advanced production technology, which will reduce productivity during processing and increase processing and lamination costs in motor core production. In addition, although high alloy elements such as Si, Al and Mn are added to increase resistivity, the addition of these alloy elements complicates cold rolling. Non-oriented electrical steel sheets for high-efficiency electric vehicles need to achieve high magnetic flux density and low core loss, requiring optimized alloy composition and advanced processing technology to increase resistivity.

[0068] In terms of alloy composition, Si, Al, and Mn are the main alloying elements used to increase resistivity. In addition to combinations of these elements, elements that can improve cold rollability, such as Cr, Cu, and Ni, are also considered. However, these elements change magnetic properties and mechanical properties, so it may not be easy to find the optimal conditions.

[0069] As for the manufacturing process of non-oriented electrical steel sheets, it is known that APL is essential to achieve high magnetic flux density and low core loss in the final product of non-oriented electrical steel sheets containing more than 3% Si. Although the texture can be improved by controlling the heating rate, the magnetic properties will deteriorate due to the uneven microstructure, so microstructural control of the hot-rolled structure is essential starting from the intermediate process of APL.

[0070] In view of the above, in the present invention, the hot rolled material is heat treated after hot rolling and before cooling to room temperature. Since the heat treatment of the hot rolled material can induce recrystallization / grain growth in the hot rolled material, cold rolling can be performed without APL. Therefore, the process can be simplified, the productivity can be improved, and a high-quality non-oriented electrical steel sheet can be produced.

[0071] Test Example

[0072] Test examples will now be described in order to better understand the present invention. However, the following test examples are only used to facilitate understanding of the present invention, and the present invention is not limited thereto.

[0073] 1. Sample composition

[0074] The samples provided in this test example have the alloy element composition shown in Table 1 (unit: weight %).

[0075] [Table 1]

[0076] Si Mn Al C P S N Ti Fe 3.35 0.32 0.73 0.0025 0.0052 0.0014 0.0018 0.0011 margin

[0077] Referring to Table 1, the composition of the non-oriented electrical steel sheet according to the test example satisfies Si: 2.0 wt% to 4.0 wt%, Mn: 0.1 wt% to 0.5 wt%, Al: 0.3 wt% to 0.9 wt%, C: greater than 0 wt% and not more than 0.003 wt%, P: greater than 0 wt% and not more than 0.015 wt%, S: greater than 0 wt% and not more than 0.003 wt%, N: greater than 0 wt% and not more than 0.003 wt%, Ti: greater than 0 wt% and not more than 0.003 wt%, and the balance of Fe. A hot-rolled sheet having a thickness of 2.0 mm was produced by reheating the slab having the above composition to 1150°C and hot rolling under the condition of FDT of 890°C. After hot rolling, heat treatment was continuously performed under different temperature conditions. Then, a cold-rolled sheet having a thickness of 0.25t was produced by performing cold rolling, and final annealing was performed at 980°C for 40 seconds. Then, a final product was obtained through a coating process. The final annealing was performed in a mixed atmosphere of 30% hydrogen-70% nitrogen. In this case, a heating rate of 20°C / s and a cooling rate of 30°C / s were used.

[0078] 2. Process conditions and performance evaluation

[0079] Table 2 shows the process conditions, core loss (W 10 / 400 ) and magnetic flux density (B 50 ). Figure 3 To test the microscopic image of inclusions with an average diameter of 2 μm or less in the sample of Example 1, Figure 4 This is a microscopic image of inclusions having an average diameter of 5 μm or more in the sample of Test Example 5.

[0080] In the heat treatment method of Table 2, the first heat treatment corresponds to the above Figure 1 Regarding the heat treatment (S30) according to an embodiment of the present invention, APL corresponds to the above Figure 2 The preliminary annealing (S45) according to the embodiment of the present invention is related. That is, APL refers to the preliminary annealing performed after hot rolling and before cold rolling, and the preliminary annealing includes: annealing at a heating rate of 20°C / s or more, an annealing temperature of 950°C to 1100°C, and a holding time of 30 seconds to 120 seconds, and cooling at a cooling rate of 30°C / s or more.

[0081] The entries related to inclusions in Table 2 show the distribution of inclusions in the final annealed material and show the area fraction (area %) of inclusions having an average diameter of 2 μm or less and the area fraction (area %) of inclusions having an average diameter of 5 μm or more and the ratio therebetween. The magnetic properties of the final product were determined by measuring using a single sheet tester (SST) and averaging the core loss and flux density values ​​in the L and C directions.

[0082] [Table 2]

[0083]

[0084] Referring to Table 2, in Test Examples 1, 2, 3 and 4, the above-mentioned Figure 1 The hot-rolled material was subjected to heat treatment (S30), but the heat treatment temperature was lower than 850° C. to 1000° C. In this case, it was shown that the ratio of the area fraction of inclusions having an average diameter of 5 μm or more to the area fraction of inclusions having an average diameter of 2 μm or less among the inclusions constituting the final microstructure was less than 0.3, and a core loss (W ) of 12.5 W / kg or less was not achieved. 10 / 400 ) and a magnetic flux density of 1.66T or greater (B 50 That is, since the number of inclusions with an average diameter of 2 μm or less increases when the heat treatment temperature of the hot-rolled material is low, the requirement that the ratio of the area fraction of inclusions with an average diameter of 5 μm or more to the area fraction of inclusions with an average diameter of 2 μm or less needs to be greater than 0.3 is not met, and the core loss (W 10 / 400 ) is 12.5W / kg or less and the magnetic flux density (B 50 ) is required to be 1.66T or larger.

[0085] In Test Examples 11, 12 and 13, the above-mentioned Figure 2 The preliminary annealing (S45) is not performed as described above. Figure 1 In this case, it was shown that the ratio of the area fraction of inclusions having an average diameter of 5 μm or more to the area fraction of inclusions having an average diameter of 2 μm or less among the inclusions constituting the final microstructure was less than 0.3, and a core loss (W 10 / 400 ).

[0086] On the other hand, in Test Examples 5, 6, 7, 8, 9 and 10, the above-mentioned Figure 1The hot-rolled material is subjected to heat treatment (S30), and a heat treatment temperature of 850° C. to 1000° C. and a heat treatment time of 5 minutes to 10 minutes are satisfied. In this case, it is shown that the ratio of the area fraction of inclusions having an average diameter of 5 μm or more to the area fraction of inclusions having an average diameter of 2 μm or less among the inclusions constituting the final microstructure is greater than 0.3, and a core loss (W) of 12.5 W / kg or less is achieved. 10 / 400 ) and a magnetic flux density of 1.66T or greater (B 50 ).

[0087] The above test examples of the present invention show that recrystallization / grain growth can be induced in the hot rolled material by heat treating the hot rolled material after hot rolling and before cooling to room temperature, and thus cold rolling can be performed without APL.

[0088] While the invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the following claims.

Claims

1. A method for manufacturing a non-oriented electrical steel sheet, the method comprising: Providing steel containing silicon (Si), manganese (Mn) and aluminum (Al); Hot rolling of steel; Before coiling, the hot-rolled steel is subjected to a first heat treatment; Coiling the steel material after the first heat treatment; Uncoiling and cold rolling the coiled steel; as well as The cold rolled steel is cold annealed.

2. The method according to claim 1, wherein: The steel material contains: 2.0 wt% to 4.0 wt% Si, 0.1 wt% to 0.5 wt% Mn, 0.3 wt% to 0.9 wt% Al, greater than 0 wt% and not more than 0.003 wt% carbon (C), greater than 0 wt% and not more than 0.015 wt% phosphorus (P), greater than 0 wt% and not more than 0.003 wt% sulfur (S), greater than 0 wt% and not more than 0.003 wt% nitrogen (N), greater than 0 wt% and not more than 0.003 wt% titanium (Ti), and the balance of iron (Fe) and unavoidable impurities.

3. The method according to claim 1, wherein: Before the hot rolled steel material is cooled to room temperature, a first heat treatment is performed at a temperature higher than the coiling temperature.

4. The method according to claim 1, wherein: Hot rolling is carried out at a slab reheating temperature (SRT) of 1110°C to 1150°C and a finishing outlet temperature (FDT) of 860°C to 900°C. The first heat treatment includes maintaining a temperature of 850° C. to 1000° C. for 5 to 10 minutes and coiling at a temperature of 550° C. to 650° C.

5. The method according to claim 1, wherein: The cold annealing includes annealing at a heating rate of 20° C. / s or more, an annealing start temperature of 900° C. to 1100° C., and a holding time of 30 seconds to 120 seconds, and cooling at a cooling rate of 30° C. / s or more.

6. The method according to claim 1, wherein: After the steel is coiled, it is cold rolled without preliminary annealing.

7. A nonoriented electrical steel sheet comprising: 2.0 to 4.0 wt% of silicon (Si), 0.1 to 0.5 wt% of manganese (Mn), 0.3 to 0.9 wt% of aluminum (Al), more than 0 wt% and not more than 0.003 wt% of carbon (C), more than 0 wt% and not more than 0.015 wt% of phosphorus (P), more than 0 wt% and not more than 0.003 wt% of sulfur (S), more than 0 wt% and not more than 0.003 wt% of nitrogen (N), more than 0 wt% and not more than 0.003 wt% of titanium (Ti), and the balance of iron (Fe) and unavoidable impurities, in, Among the inclusions constituting the final microstructure, the ratio of the area fraction of inclusions having an average diameter of 5 μm or more to the area fraction of inclusions having an average diameter of 2 μm or less is greater than 0.3, The non-oriented electrical steel sheet has a core loss (W 10 / 400 ) and a magnetic flux density of 1.66T or greater (B 50 ).

Images