Non-oriented electromagnetic steel sheet and method for producing the same

Abstract

The method for manufacturing a non-oriented electromagnetic steel sheet according to an embodiment of the present invention includes the steps of providing a steel material containing silicon (Si), manganese (Mn), and aluminum (Al), hot rolling the steel material, performing a first heat treatment before winding up the hot-rolled steel material, winding up the first heat-treated steel material, uncoiling the wound steel material and cold rolling it, and performing a cold rolling annealing treatment on the cold-rolled steel material.

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 a high-efficiency non-oriented electrical steel sheet and a method for manufacturing the same.

Background Art

[0002] Electrical steel sheets can be classified into oriented electrical steel sheets and non-oriented electrical steel sheets according to their magnetic properties. Oriented electrical steel sheets are manufactured so that magnetization is easy in the rolling direction of the steel sheet, and have particularly excellent magnetic properties in the rolling direction. Therefore, they are mainly used as cores of large, medium-sized, and small transformers that require low iron loss and high magnetic permeability. On the contrary, non-oriented electrical steel sheets have uniform magnetic properties regardless of the direction of the steel sheet, and are widely used as core materials for small motors, small power transformers, ballasts, etc. As a prior art document, there is Korean Patent Laid-Open No. 10-2015-0001467.

Summary of the Invention

Problems to be Solved by the Invention

[0003] The technical problem to be achieved by the present invention is to provide a high-efficiency non-oriented electrical steel sheet and a method for manufacturing the same.

[0004] However, such problems are exemplary and do not limit the scope of the present invention.

Means for Solving the Problems

[0005] According to one aspect of the present invention for solving the above problems, a method for manufacturing a non-oriented electromagnetic steel sheet includes a step of providing a steel material containing silicon (Si), manganese (Mn), and aluminum (Al), a step of hot-rolling the steel material, a step of performing a first heat treatment before winding up the hot-rolled steel material, a step of winding up the first heat-treated steel material, a step of uncoiling the wound steel material and cold-rolling it, and a step of subjecting the cold-rolled steel material to a cold-rolling annealing treatment.

[0006] In the method for manufacturing the non-oriented electromagnetic steel sheet, the steel material can contain 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, carbon (C): more than 0 and 0.003% by weight or less, 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 balance of iron (Fe) and other inevitable impurities.

[0007] In the method for manufacturing the non-oriented electromagnetic steel sheet, the step of performing the first heat treatment is carried out before cooling the hot-rolled steel material to room temperature, and the temperature of the step of performing the first heat treatment may be higher than the temperature of the step of winding up.

[0008] In the step of hot-rolling in the method for manufacturing the non-oriented electromagnetic steel sheet, the reheat temperature (SRT) is 1110 to 1150 °C, the finish rolling temperature (FDT) is 860 to 900 °C, the step of performing the first heat treatment includes a step of maintaining at a temperature of 850 to 1000 °C for 5 to 10 minutes, and the temperature of the step of winding up may be 550 to 650 °C.

[0009] In the method for manufacturing the non-oriented electromagnetic steel sheet, the step of performing the cold-rolling annealing treatment can include a step of annealing under the conditions of a heating rate: 20 °C / s or more, an annealing start temperature: 900 to 1100 °C, and a holding time: 30 to 120 seconds, and a step of cooling under the condition of a cooling rate: 30 °C / s or more.

[0010] In the method for manufacturing the non-oriented electromagnetic steel sheet, the cold rolling step can be characterized in that cold rolling is performed without performing preliminary annealing treatment after the step of winding up the steel material.

[0011] The non-oriented electromagnetic steel sheet according to another aspect of the present invention for solving the above problems is a non-oriented electromagnetic steel sheet containing 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, carbon (C): more than 0 and 0.003% by weight or less, 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 balance iron (Fe) and other inevitable impurities, 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 the inclusions constituting the final fine structure is greater than 0.3, and it has an iron loss (W10 / 400) of 12.5 W / kg or less and a magnetic flux density (B50) of 1.66 T or more.

Effect of the Invention

[0012] According to an embodiment of the present invention, a high-efficiency non-oriented electromagnetic steel sheet and a method for manufacturing the same can be provided.

[0013] Of course, the scope of the present invention is not limited by such effects.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Best Mode for Carrying Out the Invention

[0015] The manufacturing method of the non-oriented electromagnetic steel sheet according to an embodiment of the present invention will be described in detail. The terms described below are terms appropriately selected in consideration of the functions in the present invention, and the definitions of such terms must be made based on the content throughout this specification.

[0016] Generally, electromagnetic steel sheets are classified into oriented electromagnetic steel sheets and non-oriented electromagnetic steel sheets. In the case of oriented electromagnetic steel sheets, they are mainly used for static devices such as transformers, and in the case of non-oriented electromagnetic steel sheets, they are widely used for rotating machines that rotate like motors. The characteristics of electromagnetic steel sheets can be evaluated by magnetic flux density and iron loss. The magnetic flux density is mainly B50, and in the case of iron loss, generally W15 / 50 is evaluated, but when high-frequency characteristics are required like in electric vehicles, it is evaluated by W10 / 400. B50 indicates the magnetic flux density at 5000 A / m, W15 / 50 indicates the iron loss at 50 Hz and 1.5 T, and W10 / 400 indicates the iron loss at 400 Hz and 1.0 T.

[0017] Due to the policy of reducing CO2 emissions to prevent global warming, existing internal combustion engine vehicles are being rapidly replaced by environmentally friendly vehicles (hybrid electric vehicles (HEVs), electric vehicles (EVs)), especially electric vehicles (EVs). In line with the increasing demand for such electric vehicles (EVs), the energy conversion efficiency of drive motors for electric vehicles is being improved, and for this purpose, excellent magnetic properties of the motor core material are required. The non-oriented electrical steel sheet used as the motor core material plays a role in converting electrical energy into mechanical energy in rotating equipment, and for energy conservation, it is important to have its magnetic properties, namely, low iron loss and high magnetic flux density. In particular, for improving the efficiency of motors during high-speed driving where energy loss is large, non-oriented electrical steel sheets with low high-frequency (400 Hz) iron loss are required. In response to such needs, the development of non-oriented electrical steel sheet products has been carried out by adding elements such as silicon (Si), manganese (Mn), and aluminum (Al) to improve the specific resistance and to enable the thinning of the material. However, when the alloying elements such as silicon (Si), manganese (Mn), and aluminum (Al) increase, rolling becomes difficult, the cold rolling recovery rate decreases, thinning becomes difficult, and when thinning the thickness of the electrical steel sheet, high production technology is required, the production unit price increases, and there is a problem of decreased productivity.

[0018] Figure 1 is a flowchart showing a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention.

[0019] Referring to Figure 1, the method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention includes a step (S10) of providing a steel material containing silicon (Si), manganese (Mn), and aluminum (Al), a step (S20) of hot rolling the steel material, a step (S30) of performing a first heat treatment before winding up the hot-rolled steel material, a step (S40) of winding up the first heat-treated steel material, a step (S50) of uncoiling the wound steel material and cold rolling it, and a step (S60) of performing a cold rolling annealing treatment on the cold-rolled steel material.

[0020] Steel material supply step (S10)

[0021] The steel material input into the hot rolling process is a steel material for manufacturing a non-oriented electrical steel sheet. For example, it contains silicon (Si): 2.0 to 4.0 wt%, manganese (Mn): 0.1 to 0.5 wt%, aluminum (Al): 0.3 to 0.9 wt%, carbon (C): more than 0 and 0.003 wt% or less, phosphorus (P): more than 0 and 0.015 wt% or less, sulfur (S): more than 0 and 0.003 wt% or less, nitrogen (N): more than 0 and 0.003 wt% or less, titanium (Ti): more than 0 and 0.003 wt% or less, and the balance of iron (Fe) and other inevitable impurities.

[0022] Hereinafter, the roles and contents of exemplary composition components to which the method for manufacturing a non-oriented electrical steel sheet according to the technical idea of the present invention can be applied will be described. However, the method for manufacturing a non-oriented electrical steel sheet according to the technical idea of the present invention is not limited by such examples of composition ranges, and can be extended to any composition range capable of performing the functions of a non-oriented electrical steel sheet.

[0023] Silicon (Si): 2.0 to 4.0 wt%

[0024] Silicon (Si) is a component that increases the specific resistance and reduces iron loss (eddy current loss), and is a main additive element. When the addition amount of silicon is less than 2.0 wt%, it becomes difficult to obtain a desired high-frequency low iron loss value, and as the addition amount increases, the magnetic permeability and magnetic flux density decrease. Also, when the addition amount of silicon exceeds 4.0 wt%, brittleness increases, cold rolling becomes difficult, and productivity decreases.

[0025] Manganese (Mn): 0.1 to 0.5 wt%

[0026] Manganese (Mn) increases the resistivity together with silicon and improves the structure. When manganese is added in an amount exceeding 0.5% by weight, coarse MnS precipitates are formed and magnetic properties deteriorate, such as a decrease in magnetic flux density. Further, when the manganese content exceeds 0.5% by weight, the reduction in iron loss is small compared to the addition amount, while a significant decrease in cold rolling property occurs. Further, when the manganese content is less than 0.1% by weight, the composition range of manganese can be adjusted to 0.1 to 0.5% by weight in that it can form fine MnS precipitates and suppress the growth of crystal grains.

[0027] Aluminum (Al): 0.3 to 0.9 wt%

[0028] Aluminum (Al) is a component that increases the resistivity together with silicon and reduces iron loss (eddy current loss), and is a main additive element. Aluminum plays a role in reducing magnetic anisotropy and reducing magnetic variation. Aluminum reacts with nitrogen to induce precipitation of AlN. When the aluminum content is less than 0.3% by weight, it is difficult to expect the above-described effects, and it may form fine nitrides and increase the variation in magnetic properties. When the aluminum content exceeds 0.9% by weight, a decrease in cold rolling property occurs, excessive nitrides are formed, the magnetic flux density decreases, and magnetic properties deteriorate.

[0029] Carbon (C): More than 0 and 0.003 wt% or less

[0030] Carbon (C) is an element that forms carbides such as TiC and NbC and increases iron loss, and the less the better, and it is limited to 0.003% by weight or less. When the carbon content exceeds 0.003% by weight, magnetic aging occurs and magnetic properties deteriorate, and when it is 0.003% by weight or less, the magnetic aging phenomenon is suppressed.

[0031] Phosphorus (P): More than 0 and 0.015 wt% or less

[0032] Phosphorus (P) is a grain boundary segregation element and an element that promotes the development of the microstructure. When the phosphorus content exceeds 0.015% by weight, the growth of grains is inhibited due to the segregation effect, the magnetic properties deteriorate, and the cold rolling property decreases.

[0033] Sulfur (S): More than 0 and 0.003 wt% or less

[0034] Sulfur (S) forms precipitates such as MnS and CuS, increasing the iron loss and inhibiting the growth of grains. Therefore, it is added as low as possible and limited to 0.003% by weight or less. When the sulfur content exceeds 0.003% by weight, the problem of increased iron loss occurs.

[0035] Nitrogen (N): More than 0 and 0.003 wt% or less

[0036] Nitrogen (N) forms precipitates such as AlN, Tin, and NbN, increasing the iron loss and inhibiting the growth of grains. Therefore, it is added as low as possible and limited to 0.003% by weight or less. When the nitrogen content exceeds 0.003% by weight, the problem of increased iron loss occurs.

[0037] Titanium (Ti): More than 0 and 0.003 wt% or less

[0038] Titanium (Ti) forms fine precipitates such as TiC and TiN, inhibiting the growth of grains. Since the magnetic properties become inferior as more titanium is added, it is added as low as possible and limited to 0.003% by weight or less. When the titanium content exceeds 0.003% by weight, the problem of deteriorated magnetic properties occurs.

[0039] Hot rolling step (S20)

[0040] The steel material having the above-described composition will undergo a hot rolling process. The step (S20) of hot rolling the steel material can be performed under the conditions of a reheat temperature (SRT): 1000 to 1200 °C (strictly, 1110 to 1150 °C) and a finish rolling temperature (FDT): 860 to 900 °C.

[0041] When the slab reheating temperature exceeds 1200 °C (strictly, 1150 °C), precipitates such as C, S, and N in the slab redissolve, and fine precipitates are generated in subsequent rolling and annealing processes, which may suppress the growth of crystal grains and deteriorate the magnetic properties. When the slab reheating temperature is less than 1000 °C (strictly, 1110 °C), the rolling load increases, and there may be a problem that the iron loss becomes high in the final product.

[0042] After performing the step (S20) of hot rolling the steel material, the thickness of the hot-rolled sheet may be, for example, 1.8 to 2.6 mm. The greater the thickness of the hot-rolled sheet, the greater the reduction rate of cold rolling becomes, and the microstructure becomes inferior. Therefore, it is preferable to control the thickness to 2.6 mm or less.

[0043] First heat treatment step (S30)

[0044] Before coiling the hot-rolled steel material, the step (S30) of performing the first heat treatment can be carried out. The step (S30) of performing the first heat treatment can be continuously carried out before cooling the hot-rolled steel material to room temperature. After the first heat treatment, the steel material can be cooled to room temperature. The first heat treatment can be an annealing heat treatment. The temperature of the step (S30) of performing the first heat treatment may be higher than the temperature of the subsequent coiling step (S40). For example, when the coiling temperature is 550 to 650 °C, the step (S30) of performing the first heat treatment can include maintaining the temperature at 850 to 1000 °C for 5 to 10 minutes. When the first heat treatment temperature is less than 850 °C, fine inclusions such as nitrides are formed from the surface layer, and the inclusions do not grow sufficiently, so the magnetic properties of the final product become inferior. On the contrary, when the first heat treatment temperature exceeds 1000 °C, not only the distribution of inclusions but also the crystal grains grow excessively, the variation in crystal grain size becomes severe, and a lot of oxidation occurs, which has an adverse effect on the final product. Therefore, in order to obtain a uniform fine structure of the hot-rolled sheet, the first heat treatment temperature can be adjusted to 850 to 1000 °C.

[0045] Winding step (S40)

[0046] The first heat-treated steel material may be coiled under the condition that the coiling temperature (CT) is 550 to 650 °C. When the coiling temperature is less than 550 °C, there is no annealing effect on the steel material, so the grain growth does not occur. When the coiling temperature exceeds 650 °C, oxidation may increase during cooling, so the pickling property may deteriorate.

[0047] Cold rolling step (S50)

[0048] The step (S50) of uncoiling the coiled steel material and performing cold rolling is carried out. The reduction ratio of cold rolling is 80 to 85%, and the thickness of the cold-rolled steel material may be 0.35 mm or less (strictly, 0.25 mm or less). In order to impart rollability, the sheet temperature can be raised to 150 to 200 °C for warm rolling.

[0049] Normally, after hot rolling and coiling and before cold rolling, a preliminary annealing treatment can be performed as an APL (Annealing and Pickling Line) step of annealing and pickling the hot-rolled sheet. For example, the preliminary annealing treatment may include a step of annealing under the conditions of a heating rate of 20 °C / s or more, an annealing temperature of 950 to 1100 °C, and a holding time of 30 to 120 seconds, a step of cooling under the condition of a cooling rate of 30 °C / s or more, and a step of pickling treatment.

[0050] However, in the present invention, since the first heat treatment described above is performed before coiling the hot-rolled steel material, it has been confirmed that the cold rolling process can be immediately performed after removing the oxide layer formed on the surface through the pickling solution without performing the preliminary annealing treatment.

[0051] Cold rolling annealing treatment step (S60)

[0052] The cold-rolled steel material can be subjected to cold rolling and annealing treatment. The step (S60) of the cold rolling and annealing treatment may include an annealing step under the conditions of a heating rate of 20°C / s or more, an annealing temperature of 900 to 1100°C, and a holding time of 30 to 120 seconds, and a cooling step under the condition of a cooling rate of 30°C / s or more. In the cold rolling and annealing treatment, if the annealing temperature is less than 900°C, the grain size is fine, so the hysteresis loss may increase. If the annealing temperature exceeds 1100°C, the grain size becomes coarse, and the eddy current loss may increase.

[0053] Cold rolling and annealing is performed on the cold-rolled sheet obtained after cold rolling. Considering the improvement of iron loss and mechanical properties, the temperature for deriving the optimal grain size is applied. In order to prevent surface oxidation and nitridation during cold rolling and annealing, heating is carried out under the conditions of a mixed atmosphere. The surface state is further smoothed through a mixed atmosphere of nitrogen and hydrogen. If the cold rolling and annealing temperature is less than 900°C, the grain size is fine, so the hysteresis loss may increase. If the cold rolling and annealing temperature exceeds 1100°C, the grain size becomes coarse, and the eddy current loss will increase.

[0054] On the other hand, after the final cold rolling and annealing, a coating process can be performed to form an insulating coating layer. By forming the insulating coating layer, the punching property can be improved and the insulation can be ensured. The thickness of the insulating coating layers formed on the upper and lower parts of the cold-rolled material may be about 1 to 2 μm.

[0055] The non-oriented electrical steel sheet embodied by the above-described manufacturing method is a non-oriented electrical steel sheet containing 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, carbon (C): more than 0 and 0.003% by weight or less, 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 balance of iron (Fe) and other inevitable impurities. The average size of crystal grains is 80 to 150 μm. Among the inclusions constituting the final microstructure, 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 is greater than 0.3, and it has an iron loss (W10 / 400) of 12.5 W / kg or less and a magnetic flux density (B50) of 1.66 T or more. The yield strength may be 400 MPa or more, and the tensile strength may be 500 MPa or more.

[0056] In the present invention, by controlling the microstructure of a hot-rolled annealed material through mass-producible process conditions, a non-oriented electrical steel sheet having a high magnetic flux density and a low iron loss value and a manufacturing method thereof are provided.

[0057] Referring to FIG. 2, the manufacturing method of the non-oriented electrical steel sheet according to the comparative example of the present invention generally includes steps (S10, S20, S40) of reheating a slab and then hot-rolling to produce a hot-rolled sheet, an APL (Annealing and Pickling Line) step (S45) of annealing and pickling the hot-rolled sheet, a step (S50) of cold-rolling the hot-rolled annealed material after the APL step to produce a cold-rolled sheet, and an ACL (Annealing and Coating Line) step (S60) of finally annealing the cold-rolled sheet. In a high-alloy non-oriented electrical steel sheet, the APL process is essential. Through the APL process, cold rollability is imparted to improve the magnetic properties of the final product. Since the magnetic quality of the final product is greatly affected by the APL process, control of the microstructure is essential. If the size of fine precipitates and inclusions cannot be controlled through heat treatment operations, the movement of magnetic walls due to the fine precipitates and inclusions is delayed, and as a result, the magnetic properties may become inferior.

[0058] In the method for manufacturing a non-oriented electromagnetic steel sheet according to an embodiment of the present invention, after the hot rolling process of the non-oriented electromagnetic steel sheet is completed and before being cooled to a coiling temperature or lower, a first heat treatment step (S30) is continuously performed. At this time, the step of performing the first heat treatment may include a step of maintaining at a temperature of 850 to 1000°C for 5 to 10 minutes. Through the first heat treatment operation, APL untreated operation is possible, and a more excellent electromagnetic steel sheet can be manufactured.

[0059] In the present invention, by applying the heat treatment history described above, a microstructure is ensured 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. In this case, it was confirmed that an iron loss (W10 / 400) of 12.5 W / kg or less and a magnetic flux density (B50) of 1.66 T or more can be achieved simultaneously. The above-described method for analyzing the inclusion distribution is realized by laminating the final annealed material and analyzing a cross-section with an area of 10×10 mm2, and by utilizing SEM BSD-mode to analyze the distribution of inclusions of all sizes due to the difference in light and dark.

[0060] The final product realized by the method for manufacturing a non-oriented electromagnetic steel sheet according to the present invention has a uniform microstructure, so that the magnetic properties are further improved. At this time, the iron loss is realized to be 12.5 W / kg or less based on W10 / 400, and the magnetic flux density is 1.65 T or more. More preferably, the iron loss can be realized to be 12.0 W / kg or less based on W10 / 400, and the magnetic flux density is 1.66 T or more.

[0061] As described above, in order to reduce iron loss in non-oriented electrical steel sheets, it is necessary to reduce the sheet thickness and increase the specific resistance. However, when reducing the sheet thickness, high production technology is required, productivity during the process decreases, and costs increase during the fabrication and lamination of motor cores. Also, to increase the specific resistance, high alloying elements such as Si, Al, and Mn are added, but the addition of such alloying elements makes cold rolling difficult. The non-oriented electrical steel sheets for high-efficiency electric vehicles must satisfy high magnetic flux density and low iron loss, and optimal alloy adjustment and advanced process technology are necessary to increase the specific resistance.

[0062] In terms of alloy adjustment, Si, Al, and Mn, which increase the specific resistance, are the main alloying elements. Not only this combination of alloys, but also elements such as Cr, Cu, and Ni, which can improve cold rollability, are considered. However, it is difficult to find the optimal conditions because they change the magnetic and mechanical properties.

[0063] In terms of the manufacturing process of non-oriented electrical steel sheets, it is known that for non-oriented electrical steel sheets with 3% or more of Si, the APL (Annealing and Pickling Line) process is essential to improve the magnetic flux density and iron loss of the final product. Although the grain structure can be improved by controlling the heating rate, since a magnetic inferiority phenomenon occurs due to non-uniformity of the fine structure, it is essential to control the fine structure of the hot-rolled structure from the APL, which is an intermediate process.

[0064] Considering such points, in the present invention, during the hot rolling process of the electrical steel sheet, after hot rolling is finished and before it is cooled to room temperature, a heat treatment operation of the hot-rolled material is performed. Through such a heat treatment operation of the hot-rolled material, recrystallization / growth of the crystal grains of the hot-rolled material occurs, and cold rolling can be performed without the APL operation. Therefore, the process can be simplified and productivity can be increased, and excellent non-oriented electrical steel sheets can be manufactured.

[0065] Experimental example

[0066] Hereinafter, preferred experimental examples are presented to facilitate the understanding of the present invention. However, the following experimental examples are merely for facilitating the understanding of the present invention, and the present invention is not limited by the following experimental examples.

[0067] 1. Composition of test piece

[0068] In this experimental example, a test piece having the composition of alloy elements in Table 1 (unit: wt%) is provided.

[0069]

Table 1

[0070] Referring to Table 1, the composition of the non-oriented electrical steel sheet according to the experimental example satisfies silicon (Si): 2.0 to 4.0 wt%, manganese (Mn): 0.1 to 0.5 wt%, aluminum (Al): 0.3 to 0.9 wt%, carbon (C): more than 0 and 0.003 wt% or less, phosphorus (P): more than 0 and 0.015 wt% or less, sulfur (S): more than 0 and 0.003 wt% or less, nitrogen (N): more than 0 and 0.003 wt% or less, titanium (Ti): more than 0 and 0.003 wt% or less, and the balance being iron (Fe). The slab having the above composition was reheated to 1150 °C, hot-rolled under the condition that the finish rolling temperature (FDT) was 890 °C, and then a hot-rolled sheet having a thickness of 2.0 mm was produced. After the hot rolling was completed, heat treatment was continuously performed under various temperature conditions. Then, cold rolling was performed to produce a cold-rolled sheet having a thickness of 0.25 t, and final annealing was performed at 980 °C for 40 seconds. Thereafter, the final product was manufactured through a coating process. The final annealing atmosphere was a mixed atmosphere of 30% hydrogen - 70% nitrogen. At this time, the heating rate was 20 °C / s and the cooling rate was 30 °C / s.

[0071] 2. Process conditions and evaluation of physical properties

[0072] Table 2 shows the process conditions of this experimental example and the iron loss (W10 / 400) and magnetic flux density (B50) resulting therefrom. Figure 3 is a photograph of inclusions with an average diameter of 2 μm or less in the test piece of Experimental Example 1, and Figure 4 is a photograph of inclusions with an average diameter of 5 μm or more in the test piece of Experimental Example 5.

[0073] Among the heat treatment methods disclosed in Table 2, the first heat treatment corresponds to the heat treatment step (S30) according to the embodiment of the present invention described with reference to Figure 1, and APL corresponds to the pre-annealing step (S45) according to the embodiment of the present invention described with reference to Figure 2. That is, the APL process means a pre-annealing (APL) treatment applied after hot rolling and before cold rolling. The steps of the pre-annealing treatment include annealing under the conditions of a heating rate of 20 °C / s or more, an annealing temperature of 950 to 1100 °C, a holding time of 30 to 120 seconds, and cooling under the condition of a cooling rate of 30 °C / s or more.

[0074] Also, the item of inclusions disclosed in Table 2 is the distribution of inclusions in the final annealed material, showing the area fraction (area %) of inclusions with an average size of 2 μm or less or 5 μm or more and their ratio. Also, the magnetic properties of the final product were measured through an SST (single sheet tester), and the iron loss value and magnetic flux density value were measured in the L direction and the C direction and then averaged.

[0075]

Table 2

[0076] Referring to Table 2, in Experimental Examples 1, 2, 3, and 4, the heat treatment step (S30) of the hot-rolled material disclosed in FIG. 1 was applied, but the heat treatment temperature was below 850 to 1000 °C. In this case, it can be confirmed 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 among the inclusions constituting the final fine structure is less than 0.3, and it can be confirmed that an iron loss (W10 / 400) of 12.5 W / kg or less and a magnetic flux density (B50) of 1.66 T or more cannot be achieved. That is, when the heat treatment temperature of the hot-rolled material is low, the ratio of inclusions with a diameter of 2 μm or less increases, and the relationship 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 is greater than 0.3 is not satisfied, and an iron loss (W10 / 400) of 12.5 W / kg or less and a magnetic flux density (B50) of 1.66 T or more cannot be satisfied.

[0077] In Experimental Examples 11, 12, and 13, the heat treatment step (S30) of the hot-rolled material disclosed in FIG. 1 was not applied, and the preliminary annealing step (S45) disclosed in FIG. 2 was applied. In this case, it can be confirmed 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 among the inclusions constituting the final fine structure is less than 0.3, and it can be confirmed that an iron loss (W10 / 400) of 12.5 W / kg or less cannot be achieved.

[0078] On the contrary, in Experimental Examples 5, 6, 7, 8, 9, and 10, the heat treatment step (S30) of the hot-rolled material disclosed in FIG. 1 was applied, satisfying the range of heat treatment temperature: 850 to 1000 °C and the range of heat treatment time: 5 to 10 minutes. In this case, it can be confirmed 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 among the inclusions constituting the final fine structure is greater than 0.3, and it can be confirmed that an iron loss (W10 / 400) of 12.5 W / kg or less and a magnetic flux density (B50) of 1.66 T or more can be achieved.

[0079] As described above, through the experimental examples of the present invention, during the hot rolling process of the electromagnetic steel sheet, after the hot rolling is completed, by applying heat treatment to the hot rolled material before it is cooled to room temperature, recrystallization / growth of crystal grains of the hot rolled material is caused, and it was confirmed that cold rolling can be performed without APL work.

[0080] As described above, the embodiments of the present invention have been mainly described, but various changes and modifications can be made at the level of those skilled in the art. As long as such changes and modifications do not depart from the scope of the present invention, it can be said that they belong to the present invention. Therefore, the scope of the rights of the present invention must be determined by the appended claims.

Claims

1. providing a steel material containing silicon (Si), manganese (Mn), and aluminum (Al); hot-rolling the steel material; performing a first heat treatment before coiling the hot-rolled steel material; coiling the first heat-treated steel material; uncoiling the coiled steel material and cold-rolling it; a method for manufacturing a non-oriented electrical steel sheet, comprising cold-rolling annealing the cold-rolled steel material.

2. The steel material contains silicon (Si): 2.0 to 4.0 wt%, manganese (Mn): 0.1 to 0.5 wt%, aluminum (Al): 0.3 to 0.9 wt%, carbon (C): more than 0 and 0.003 wt% or less, phosphorus (P): more than 0 and 0.015 wt% or less, sulfur (S): more than 0 and 0.003 wt% or less, nitrogen (N): more than 0 and 0.003 wt% or less, titanium (Ti): more than 0 and 0.003 wt% or less, and the balance iron (Fe) and other inevitable impurities. The method for manufacturing a non-oriented electrical steel sheet according to Claim 1.

3. The step of performing the first heat treatment is carried out before cooling the hot-rolled steel material to room temperature, and the temperature of the step of performing the first heat treatment is higher than the temperature of the coiling step. The method for manufacturing a non-oriented electrical steel sheet according to Claim 1.

4. In the step of hot-rolling, the reheating temperature (SRT) is 1110 to 1150 °C, and the finish rolling temperature (FDT) is 860 to 900 °C. The step of performing the first heat treatment includes maintaining at a temperature of 850 to 1000 °C for 5 to 10 minutes. The temperature of the coiling step is 550 to 650 °C. The method for manufacturing a non-oriented electrical steel sheet according to Claim 1.

5. The step of cold-rolling annealing includes annealing under the conditions of a heating rate: 20 °C / s or more, an annealing start temperature: 900 to 1100 °C, a holding time: 30 to 120 seconds, and a cooling rate: 30 °C / s or more. The method for manufacturing a non-oriented electrical steel sheet according to Claim 1.

6. The step of cold-rolling is characterized in that cold rolling is performed without performing a preliminary annealing treatment after the step of coiling the steel material. The method for manufacturing a non-oriented electrical steel sheet according to Claim 1.

7. Non-oriented electrical steel sheet containing silicon (Si): 2.0 to 4.0 wt%, manganese (Mn): 0.1 to 0.5 wt%, aluminum (Al): 0.3 to 0.9 wt%, carbon (C): more than 0 and 0.003 wt% or less, phosphorus (P): more than 0 and 0.015 wt% or less, sulfur (S): more than 0 and 0.003 wt% or less, nitrogen (N): more than 0 and 0.003 wt% or less, titanium (Ti): more than 0 and 0.003 wt% or less, and the balance iron (Fe) and other inevitable impurities, 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, Non-oriented electrical steel sheet characterized by having an iron loss (W10 / 400) of 12.5 W / kg or less and a magnetic flux density (B50) of 1.66 T or more.

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