Non-oriented electrical steel sheet, method for manufacturing same, and motor core comprising same

Abstract

The present invention provides a non-oriented electrical steel sheet, a method for manufacturing the same, and a motor core including the same, the non-oriented electrical steel sheet according to one embodiment of the present invention containing 2.8% to 4.0% by weight of Si, 0.5% to 1.7% by weight of Al, and 0.3% to 2.0% by weight of Mn, and containing a remainder of Fe and unavoidable impurities. Furthermore, a surface layer is present from the surface of the steel sheet toward the inside of the steel sheet, an insulating coating layer is present on the surface of the steel sheet, and the weight ratio of Al to Mn, i.e., Al / Mn, in the insulating coating layer and the surface layer is 1 to 10.

Description

Technical Field

[0001] One embodiment of the present invention relates to a non-oriented electrical steel sheet, a method for manufacturing the same, and a motor core including the same. Specifically, one embodiment of the present invention relates to a non-oriented electrical steel sheet, a method for manufacturing the same, and a motor core including the same, which appropriately adjust the dew point temperature during stress relief annealing (SRA) to adjust the surface layer formed on the steel sheet. Background Art

[0002] Recently, as disasters caused by climate change have increased, countries around the world have announced carbon neutrality strategies for 2050. The total carbon emissions in 2020 reached 3.9 billion tons, of which the emissions from internal combustion engines accounted for 24% and reached 940 million tons. Therefore, there is a great demand for achieving carbon neutrality in this field through the electrification of internal combustion engines. To this end, in the mobility field, electrification led by electric vehicles is rapidly underway. In new mobility, the characteristics required of drive motors are to increase the driving range and raise the maximum speed. This is directly related to the low iron loss characteristics of electrical steel sheets. When the iron loss of the electrical steel sheet is low, the efficiency can be further improved to further increase the driving range. Therefore, the high-frequency low iron loss characteristics of electrical steel sheets are a necessary option. For this purpose, Si is usually contained in large amounts in electrical steel sheets, and elements such as Al, Mn, and Cr are added in large amounts to ensure high-frequency low iron loss.

[0003] However, in addition to the method of reducing iron loss by adding a large amount of specific resistance elements such as Si, Al, Mn, and Cr, it is necessary to refine the crystal grain size or leave an unrecrystallized part to improve the strength, and then improve the iron loss by growing the crystal grain size through the SRA process. Summary of the Invention

[0004] (I) Technical Problems to be Solved

[0005] One embodiment of the present invention aims to provide a non-oriented electrical steel sheet, a method for manufacturing the same, and a motor core including the same. Specifically, one embodiment of the present invention aims to provide a non-oriented electrical steel sheet, a method for manufacturing the same, and a motor core including the same, which appropriately adjust the dew point temperature during stress relief annealing (SRA) to adjust the surface layer formed on the steel sheet.

[0006] (II) Technical Solutions

[0007] An non-oriented electrical steel sheet according to an embodiment of the present invention contains, by weight%, 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn, and contains the balance of Fe and inevitable impurities. Further, there is a surface layer extending from the surface of the steel sheet toward the inside of the steel sheet, and an insulating coating layer is present on the surface of the steel sheet. In the insulating coating layer and the surface layer, the weight ratio of Al to Mn (Al / Mn) is 1 to 10.

[0008] The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain one or more of C, N, S, Ti, Nb, and V in a content of 0.005% by weight or less, respectively.

[0009] The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain one or more of P in a content of 0.005% by weight or less, Cu in a content of 0.005% by weight to 0.2% by weight, Cr in a content of 0.01% by weight to 0.5% by weight, Sn in a content of 0.06% by weight or less, Sb in a content of 0.06% by weight or less, Ni in a content of 0.05% by weight or less, and Zn in a content of 0.01% by weight or less.

[0010] The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain one or two or more of Bi, Pb, Ge, and As, and the respective contents are 0.200% by weight or less or the total content is 0.200% by weight or less.

[0011] The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain one or more of Mo in a content of 0.03% by weight or less, B in a content of 0.0050% by weight or less, Ca in a content of 0.0050% by weight or less, and Mg in a content of 0.0050% by weight or less.

[0012] The specific resistance of the non-oriented electrical steel sheet according to an embodiment of the present invention may be 50 μΩ·cm or more.

[0013] The average grain size of the non-oriented electrical steel sheet according to an embodiment of the present invention may be 50 μm to 200 μm.

[0014] A method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention includes: a step of hot rolling a slab containing, by weight%, 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn, and containing the balance of Fe and inevitable impurities to manufacture a hot-rolled sheet; a step of cold rolling the hot-rolled sheet to manufacture a cold-rolled sheet; a step of annealing the cold-rolled sheet, which is a cold-rolled sheet annealing step; a step of forming an insulating coating layer on the annealed cold-rolled sheet; and a step of stress-relief annealing the steel sheet formed with the insulating coating layer.

[0015] The stress relief annealing step includes a heating step of heating the steel sheet to the soaking temperature and a soaking step. In the heating step, the heating rate is 10°C / min to 50°C / min in the range of 300°C to 500°C, the dew point of the heating step is 10°C to 50°C, and the dew point of the soaking step is 0°C to 35°C.

[0016] The slab may further contain one or more of C, N, S, Ti, Nb, and V in a content of 0.005% by weight or less.

[0017] The slab may further contain one or more of P at 0.005% by weight or less, Cu at 0.005% by weight to 0.2% by weight, Cr at 0.01% by weight to 0.5% by weight, Sn at 0.06% by weight or less, Sb at 0.06% by weight or less, Ni at 0.05% by weight or less, and Zn at 0.01% by weight or less.

[0018] The slab may further contain one or two or more of Bi, Pb, Ge, and As, each in a content of 0.200% by weight or less or the total content thereof being 0.200% by weight or less.

[0019] The slab may further contain one or more of Mo at 0.03% by weight or less, B at 0.0050% by weight or less, Ca at 0.0050% by weight or less, and Mg at 0.0050% by weight or less.

[0020] In the annealing step of the cold-rolled sheet, the annealing temperature may be 750°C to 850°C, and the annealing time may be 10 seconds to 60 seconds.

[0021] After annealing the cold-rolled sheet, the average grain size of the annealed cold-rolled sheet may be 10 μm to 30 μm.

[0022] After annealing the cold-rolled sheet, the non-recrystallized fraction may be 1 area% to 15 area%.

[0023] After annealing the cold-rolled sheet, there may be a surface layer extending from the surface of the cold-rolled sheet toward the inside of the cold-rolled sheet, and the thickness of the surface layer may be 0.0001 μm to 0.2 μm.

[0024] After annealing the cold-rolled sheet, the following formula 1 may be satisfied.

[0025] [Formula 1]

[0026] 50 ≤ [Average crystal grain size (μm)] × [Annealing time of cold-rolled sheet (seconds)] / [Non-recrystallized area fraction (%)] ≤ 500

[0027] In the heating step, the heating rate and the dew point may satisfy the following formula 2.

[0028] [Formula 2]

[0029] 0.7 ≤ [Heating rate (°C / min)] / [Dew point (°C)] ≤ 2.5

[0030] The motor core according to an embodiment of the present invention includes a rotor formed by laminating a plurality of non-oriented electrical steel sheets and a stator formed by laminating a plurality of non-oriented electrical steel sheets. The non-oriented electrical steel sheet in the rotor contains 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight, and contains the balance of Fe and inevitable impurities. The non-oriented electrical steel sheet in the stator contains 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight, the balance of Fe and inevitable impurities. There is a surface layer from the surface of the steel sheet toward the inside of the steel sheet. In the surface layer, the weight ratio of Al to Mn (Al / Mn) is 1 to 10.

[0031] The average grain size of the non-oriented electrical steel sheet in the rotor can be 10 μm to 30 μm.

[0032] The differences in the contents of Si, Al, and Mn between the non-oriented electrical steel sheets included in the stator and the rotor can be 0.2% by weight or less, respectively.

[0033] (III) Beneficial effects

[0034] The non-oriented electrical steel sheet according to an embodiment of the present invention has uniform and excellent tensile strength after annealing of the cold-rolled sheet.

[0035] Moreover, the non-oriented electrical steel sheet according to an embodiment of the present invention has excellent iron loss and insulation after SRA.

[0036] Finally, the non-oriented electrical steel sheet according to an embodiment of the present invention can contribute to the manufacture of motors for environmentally friendly vehicles, motors for high-efficiency household appliances, and ultra-high-value motor cores by using the same steel sheet, using the one without SRA treatment as the rotor and the one after SRA treatment as the stator. Description of the drawings

[0037] Figure 1 is a schematic side cross-sectional view of a non-oriented electrical steel sheet according to an embodiment of the present invention. Detailed implementation manners

[0038] The terms first, second, third, etc. are used to describe various parts, components, regions, layers, and / or elements, but are not limited thereto. These terms are only used to distinguish one part, component, region, layer, or element from another part, component, region, layer, or element. Thus, without departing from the scope of the present invention, the first part, component, region, layer, or element described below may be referred to as the second part, component, region, layer, or element.

[0039] The technical terms used herein are only used to refer to specific embodiments and are not intended to limit the present invention. Unless the contrary is clearly shown, the singular forms used herein also include the plural forms. The meaning of "including" used in the specification specifies specific characteristics, regions, integers, steps, actions, elements, and / or components, and does not exclude the existence or addition of other characteristics, regions, integers, steps, actions, elements, and / or components.

[0040] When referring to a part "above" or "over" another part, this may mean directly above or over the other part, or there may be other parts between them. In contrast, when referring to a part "directly above" another part, there are no other parts between them.

[0041] All terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which the present invention pertains, unless otherwise defined. Terms defined in commonly used dictionaries are additionally interpreted to have meanings consistent with the relevant technical literature and the content currently disclosed, and should not be construed as ideal or overly formal meanings unless they are defined.

[0042] And, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

[0043] In one embodiment of the present invention, the meaning of further including an additional element means including the additional element by replacing the balance of iron (Fe) according to the amount of the additional element added.

[0044] Hereinafter, embodiments of the present invention will be described in detail so that one of ordinary skill in the technical field to which the present invention pertains can easily implement it. However, the present invention can be implemented in various different forms and is not limited to the embodiments described herein.

[0045] In one embodiment of the present invention, the dew point temperature is appropriately adjusted during the stress relief annealing (SRA) process to adjust the surface layer and the insulating coating layer formed on the steel plate, thereby reducing iron loss and improving insulation.

[0046] The nonoriented electrical steel sheet according to an embodiment of the present invention includes, by weight %, 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn, and the balance of Fe and inevitable impurities.

[0047] First, the reasons for limiting the composition of the non-oriented electrical steel sheet will be described.

[0048] Si: 2.8 wt% to 4.0 wt%

[0049] Silicon (Si) plays a role in increasing the resistivity of the material to reduce iron loss, so it is necessary to add Si in a relatively large amount. If too little Si is added, the effect of improving high-frequency iron loss may be minimal. If too much Si is added, the hardness of the material increases, resulting in poor production efficiency and stamping properties, so it is not preferred. More specifically, 3.0 wt% to 3.7 wt% Si can be included.

[0050] Al: 0.5 wt% to 1.7 wt%

[0051] Aluminum (Al) plays a role in increasing the resistivity of the material to reduce iron loss, so a large amount of Al needs to be added. If too little Al is added, it will not have the effect of reducing high-frequency iron loss, and nitrides will be finely formed, which may deteriorate magnetic properties. If too much Al is added, it will cause the problem of changing the physical properties of the mold flux during the continuous casting process, which may greatly reduce production efficiency. More specifically, 0.7 wt% to 1.5 wt% Al can be included.

[0052] Mn: 0.3 wt% to 2.0 wt%

[0053] Manganese (Mn) plays a role in increasing the resistivity of the material to improve iron loss and forming sulfides. If too little Mn is added, MnS is finely precipitated and may deteriorate magnetic properties. If too much Mn is added, it promotes the formation of a

[111] texture that is unfavorable to magnetic properties, which may cause a sharp decrease in magnetic flux density. More specifically, 0.5 wt% to 1.5 wt% of Mn may be included.

[0054] Resistivity 50μΩ∙cm or more

[0055] The specific resistance is preferably as large as possible in order to reduce eddy current loss in a high-frequency rotator. However, if it becomes too large, the magnetic flux density may deteriorate. In one embodiment of the present invention, the specific resistance can be calculated by 13.25 + 11.3×([Si] + [Al] + [Mn] / 2). At this time, [Si], [Al], and [Mn] respectively represent the contents (weight %) of Si, Al, and Mn. The higher the specific resistance, the more effective it is in reducing iron loss. If the specific resistance is too low, it is difficult to be used as a high-efficiency motor due to deteriorated iron loss. More specifically, the specific resistance can be from 50 μΩ∙cm to 90 μΩ∙cm. More specifically, the specific resistance can be from 60 μΩ∙cm to 85 μΩ∙cm.

[0056] The non-oriented electrical steel sheet according to one embodiment of the present invention may further contain one or more of P of 0.005 wt% or less, Cu of 0.005 wt% to 0.2 wt%, Cr of 0.01 wt% to 0.5 wt%, Sn of 0.06 wt% or less, Sb of 0.06 wt% or less, Ni of 0.05 wt% or less, and Zn of 0.01 wt% or less.

[0057] P: 0.005 wt% or less

[0058] Phosphorus (P) is a grain boundary segregation element. If too much P is added, the strength uniformity in the rolling direction and the direction perpendicular to rolling may deteriorate due to delayed recrystallization. More specifically, it may contain P of 0.0001 wt% to 0.0030 wt%.

[0059] Cu: 0.005 wt% to 0.200 wt%

[0060] Copper (Cu) functions to form sulfides together with Mn. When Cu is also added, if too little Cu is added, CuMnS may precipitate finely and the magnetic properties may deteriorate. If too much Cu is added, high-temperature brittleness may occur and cracks may form during continuous casting or hot rolling. More specifically, it may contain Cu of 0.01 wt% to 0.10 wt%.

[0061] Cr: 0.010 wt% to 0.50 wt%

[0062] Chromium (Cr) functions to increase the specific resistance to improve iron loss. If too little Cr is added, the effect of increasing the specific resistance may be insufficient. If too much Cr is contained, the magnetic flux density may be reduced. More specifically, when Cr is also contained, it may contain Cr of 0.050 to 0.20 wt%.

[0063] Sn: 0.06 wt% or less

[0064] Tin (Sn) is a segregation element in the grain boundaries, which inhibits the diffusion of nitrogen through the grain boundaries, inhibits the {111} texture (which is harmful to magnetism), and increases the {100} texture (which is beneficial to magnetism), and thus is added to improve magnetic properties. If too much Sn is added, the grain growth is hindered, the magnetism is reduced, and the rolling properties deteriorate. Therefore, Sb can be added within the aforementioned range. More specifically, 0.005% by weight to 0.060% by weight of Sb can be included. More specifically, 0.01% by weight to 0.05% by weight of Sb can be included.

[0065] Sb: 0.06% by weight or less

[0066] Antimony (Sb) is a segregation element in the grain boundaries, which inhibits the diffusion of nitrogen through the grain boundaries, inhibits the {111} texture (which is harmful to magnetism), and increases the {100} texture (which is beneficial to magnetism), and thus is added to improve magnetic properties. If too much Sb is added, the grain growth is hindered, the magnetism is reduced, and the rolling properties deteriorate. Therefore, Sb can be added within the aforementioned range. More specifically, 0.005% by weight to 0.060% by weight of Sb can be included. More specifically, 0.01% by weight to 0.05% by weight of Sb can be included.

[0067] Ni: 0.05% by weight or less

[0068] Nickel (Ni) reacts with impurity elements to form fine sulfides, carbides, and nitrides, which may have an adverse effect on magnetism. More specifically, 0.001% by weight to 0.03% by weight of Ni can be included.

[0069] Zn: 0.01% by weight or less

[0070] When the content of zinc (Zn) is excessive, it may act as an impurity and cause magnetic degradation. Therefore, Zn can also be added within the aforementioned range. More specifically, 0.001% by weight to 0.005% by weight of Zn can be included.

[0071] The non-oriented electrical steel sheet according to an embodiment of the present invention may further include one or more of Bi, Pb, Ge, and As, and their respective contents are 0.200% by weight or less, or the total content is 0.200% by weight or less.

[0072] When the above-described elements are additionally added, segregation occurs at the grain boundaries, relieving stress concentration at the grain boundaries during cold rolling, thereby suppressing the recrystallization of <111> / / ND-oriented grains during subsequent recrystallization annealing, and thus enabling an increase in magnetic flux density. If these elements are added appropriately, the above-described effects can be additionally obtained, but if these elements are contained in excessive amounts, a large amount of segregation occurs and grain growth is suppressed, which may instead lead to deterioration of magnetic flux density and iron loss. More specifically, one or more of Bi, Pb, Ge, and As may also be contained, and their respective contents are 0.0001 wt% to 0.200 wt% or their total content is 0.0001 wt% to 0.200 wt%. More specifically, 0.001 wt% to 0.100 wt% of these elements may also be contained. 0.005 wt% to 0.050 wt% of these elements may also be contained.

[0073] The non-oriented electrical steel sheet according to an embodiment of the present invention may also contain one or more of Mo at 0.03 wt% or less, B at 0.0050 wt% or less, Ca at 0.0050 wt% or less, and Mg at 0.0050 wt% or less.

[0074] These elements may react with C, S, N, etc. inevitably contained to form fine carbides, nitrides, or sulfides, thereby having an adverse effect on magnetism, and thus the upper limits of these elements can be defined as described above.

[0075] Other impurities

[0076] In addition to the above-described elements, impurities such as carbon (C), sulfur (S), nitrogen (N), titanium (Ti), niobium (Nb), and vanadium (V) that are inevitably mixed in may also be contained.

[0077] Since C, N, and Ti play a role in forming carbonitrides and hindering magnetic domain movement, these elements can be restricted. Since S can form sulfides and cause deterioration of grain growth property, its upper limit can be restricted. These elements can be contained at a content of 0.0050 wt% or less, respectively.

[0078] N combines with Ti, Nb, and V to form nitrides and plays a role in reducing grain growth property.

[0079] C reacts with N, Ti, Nb, V, etc. to form fine carbides and plays a role in hindering grain growth property and magnetic domain movement.

[0080] S deteriorates grain growth property by forming sulfides.

[0081] In the case of further containing impurity elements as described above, one or more of C, S, N, Ti, Nb, and V may be contained in a content of 0.005% by weight or less.

[0082] The average grain size of the non-oriented electrical steel sheet according to an embodiment of the present invention may be 50 μm to 200 μm. When the average grain size is appropriately adjusted, the magnetic properties can be further improved. In an embodiment of the present invention, the grain size may be measured for a plane parallel to the surface of the steel sheet. More specifically, with respect to the entire thickness t of the steel sheet, the measurement may be performed at a thickness in the range of 1 / 4t to 3 / 4t. The grain size is a virtual circle having the same area as the area of the grain, and the diameter of this circle is defined as the grain size. The average grain size may be measured by dividing the area to be measured by the number of grains present in that area. More specifically, the average grain size of the non-oriented electrical steel sheet according to an embodiment of the present invention may be 60 μm to 150 μm. In an embodiment of the present invention, the average grain size without further specification represents the average grain size after SRA.

[0083] Figure 1 Fig. shows a schematic side cross-sectional view of a non-oriented electrical steel sheet according to an embodiment of the present invention. Figure 1 The non-oriented electrical steel sheet is only used to illustrate the present invention, and the present invention is not limited thereto. Therefore, the structure of the non-oriented electrical steel sheet can be variously deformed.

[0084] As Figure 1 shown, the non-oriented electrical steel sheet 100 according to an embodiment of the present invention includes a surface layer 20 and a steel sheet base material 10 other than the surface layer in the direction from the surface of the steel sheet toward the inside of the steel sheet. An insulating coating layer 30 is present on the surface of the steel sheet.

[0085] In an embodiment of the present invention, the insulating coating layer 30 and the surface layer 20 are composed of Al and Mn oxides. When Al and Mn oxides are appropriately formed in the insulating coating layer 30 and the surface layer 20 during stress relief annealing, the insulation and magnetic properties can be improved simultaneously. In the insulating coating layer 30, the Mn oxide may be the main component, and in the surface layer 20, the Al oxide may be the main component.

[0086] In an embodiment of the present invention, in the insulating coating layer 30 and the surface layer 20, the weight ratio of Al to Mn (Al / Mn) may be 10 or less. If the amount of Mn in the insulating coating layer 30 is too small and the ratio of Al / Mn is too high, the bonding strength between the insulating coating layer 30 and the surface layer 20 may become weak. More specifically, in the insulating coating layer 30 and the surface layer 20, the weight ratio of Al to Mn (Al / Mn) may be 1 to 8.

[0087] The Mn and Al contents in the insulating coating layer 30 and the surface layer 20 can be measured using GDS. The amounts of Al and Mn can be defined as the areas of the GDS peaks. The boundary between the surface layer 20 and the steel base material 10 can be defined as the intersection point of the location where the oxygen peak decreases and the location where the Fe peak increases.

[0088] In one embodiment of the present invention, the distinction between the insulating coating layer 30 and the surface layer 20 is not necessary, but it can be defined as the inflection point where the slope of the Al peak changes.

[0089] The Al and Mn contents in the insulating coating layer 30 and the surface layer 20 can be appropriately formed by adjusting the heating rate and the dew point during the stress relief annealing process. This will be specifically described in the manufacturing method of the non-oriented electrical steel sheet described later.

[0090] Specifically, the insulation value of the non-oriented electrical steel sheet according to one embodiment of the present invention can be 500 mA or more. More specifically, the insulation value can be 550 mA to 900 mA. The insulation value can be measured using the Franklin test method.

[0091] Moreover, the iron loss ( 10 / 400 ) of the non-oriented electrical steel sheet according to one embodiment of the present invention can be 10.50 W / kg or less. More specifically, it can be 9.0 W / kg to 10.0 W / kg. The magnetic properties can be measured using the Epstein measurement method or the single sheet test (SST) method. At this time, the thickness can be based on 0.25 mm.

[0092] The manufacturing method of the non-oriented electrical steel sheet according to one embodiment of the present invention includes: a step of hot rolling a slab containing 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight%, and containing the balance of Fe and inevitable impurities to manufacture a hot rolled sheet; a step of cold rolling the hot rolled sheet to manufacture a cold rolled sheet; a cold rolled sheet annealing step of annealing the cold rolled sheet; and a step of stress relief annealing the annealed cold rolled sheet.

[0093] The following will be specifically described for each different step.

[0094] First, a slab is manufactured. The addition ratios of the respective components in the slab are defined for the same reasons as the component definitions of the aforementioned non-oriented electrical steel sheet, so the repeated description will be omitted. During the subsequent manufacturing processes such as hot rolling, hot rolled sheet annealing, cold rolling, cold rolled sheet annealing, and stress relief annealing, the components of the slab do not change substantially, so the components of the slab are substantially the same as those of the non-oriented electrical steel sheet.

[0095] Before the step of manufacturing the hot-rolled sheet, the slab can be heated. Specifically, the slab is charged into a heating furnace and heated at 1100°C to 1250°C. When heated at a temperature exceeding 1250°C, the precipitates are remelted, and thus may precipitate finely after hot rolling.

[0096] The heated slab is hot-rolled to 2 mm to 2.3 mm to manufacture a hot-rolled sheet. In the step of manufacturing the hot-rolled sheet, the post-treatment rolling temperature can be 800°C to 1000°C.

[0097] After the step of manufacturing the hot-rolled sheet, it may further include a step of annealing the hot-rolled sheet. At this time, the hot-rolled sheet annealing temperature can be 850°C to 1150°C. If the hot-rolled sheet annealing temperature is less than 850°C, the structure may not grow or grow finely, resulting in a less rising effect of the magnetic flux density. If the annealing temperature is greater than 1150°C, the magnetic properties will instead decrease, and the rolling operation efficiency may deteriorate due to the deformation of the sheet shape. More specifically, the temperature range can be 950°C to 1125°C. More specifically, the annealing temperature of the hot-rolled sheet can be 900°C to 1100°C. The hot-rolled sheet annealing is performed as needed to increase the orientation favorable for magnetism, and it can also be omitted.

[0098] Next, the hot-rolled sheet is pickled and cold-rolled to reach a specified sheet thickness. Different reduction ratios can be adopted according to the hot-rolled sheet thickness, but a reduction ratio of 70% to 95% can be adopted for cold rolling to make the final thickness reach 0.2 mm to 0.65 mm. To adjust the reduction ratio, two or more cold rollings with an intermediate annealing or a single cold rolling in between can be performed.

[0099] The cold-rolled sheet after cold rolling is subjected to cold-rolled sheet annealing.

[0100] In the cold-rolled sheet annealing step, the annealing temperature can be 750°C to 850°C, and the annealing time can be 10 seconds to 60 seconds. If the annealing temperature becomes higher and the soaking time increases, the non-recrystallized part disappears, and sufficient strength cannot be ensured due to the increase in the average crystal grain size, and the strength uniformity in different directions may deteriorate. If the annealing temperature becomes lower and the soaking time is shortened, magnetic degradation may occur due to the improper growth of the grains. More specifically, in the cold-rolled sheet annealing step, the annealing temperature can be 770°C to 830°C, and the annealing time can be 20 seconds to 45 seconds.

[0101] After the cold-rolled sheet annealing, the average crystal grain size of the annealed cold-rolled sheet can be 10 μm to 30 μm. When the average crystal grain size of the cold-rolled sheet is properly formed, both magnetism and strength can be ensured. The average crystal grain size of the cold-rolled sheet can be 15 μm to 25 μm.

[0102] After annealing the cold-rolled sheet, the unrecrystallized fraction can be from 1 area% to 15 area%. When the unrecrystallization of the cold-rolled sheet is appropriately formed, both magnetism and strength can be ensured. In one embodiment of the present invention, unrecrystallization refers to the crystal grain size that extends relatively long like an elongated grain when observing the microstructure, and it can be distinguished from the crystal grain size of spherical recrystallization. More specifically, the unrecrystallized fraction can be from 3 area% to 13 area%.

[0103] After annealing the cold-rolled sheet, there is a surface layer in the direction from the surface of the cold-rolled sheet to the inside of the cold-rolled sheet, and the thickness of the surface layer can be from 0.001 μm to 0.2 μm. During the annealing process of the cold-rolled sheet, the surface layer can be formed by partial oxidation of the steel sheet surface. More specifically, the thickness of the surface layer can be from 0.01 μm to 0.1 μm.

[0104] After annealing the cold-rolled sheet, the difference in tensile strength between the rolling direction and other directions reaches 15 MPa or less, so the uniformity of tensile strength can be ensured (the ratio of tensile strength in the rolling direction / tensile strength in the 45-degree direction > 98%).

[0105] After annealing the cold-rolled sheet, the following Formula 1 can be satisfied.

[0106] [Formula 1]

[0107] 50 ≤ [average crystal grain size (μm)] × [annealing time of cold-rolled sheet (seconds)] / [unrecrystallized area fraction (%)] ≤ 500

[0108] When the above Formula 1 is satisfied, a tensile strength of 630 MPa or more can be obtained, and the strength ratio in different directions (tensile strength in the rolling direction / tensile strength in the 45-degree direction) is 98.0% or more, so it can be made uniform.

[0109] Next, an insulating coating layer is formed on the annealed cold-rolled sheet. Since the method for forming the insulating coating layer is a well-known technology, the specific description thereof is omitted here. Specifically, an insulating coating layer forming composition containing metal phosphate and silica as main components can be coated, and the insulating coating layer can be formed by performing heat treatment. In one embodiment of the present invention, since Mn in the steel sheet diffuses into the insulating coating layer during the stress relief annealing process, after forming the insulating coating layer, the Mn content in the insulating coating layer may be very small. Specifically, the Mn content can be 0.01 wt% or less.

[0110] Next, stress relief annealing is performed on the steel sheet formed with the insulating coating layer. After forming the insulating film, stamping and lamination processes can be carried out. Since this is a well-known technology for this purpose, specific descriptions thereof are omitted herein. During the stamping process, stress is generated in the non-oriented electrical steel sheet, which will have an adverse effect on the magnetic properties of the non-oriented electrical steel sheet. In the case of the stator where magnetic characteristics are relatively important in the motor core, the stress remaining in the steel sheet is removed by stress relief annealing, thereby improving the magnetic properties of the steel sheet. On the other hand, in the case of the rotor where strength characteristics are relatively more important than magnetism, stress relief annealing can be omitted.

[0111] That is, in the case of using the same steel sheet, its use can also be changed to a stator and a rotor according to whether stress relief annealing is carried out or not.

[0112] The stress relief annealing step includes a heating step of heating the steel sheet to the soaking temperature and a soaking step. In the heating step, the heating rate in the range of 300°C to 500°C is 10°C / minute or more, the dew point of the heating step is 10°C to 50°C, and the dew point of the soaking step is 10°C or more. Under the above conditions, an appropriate insulating coating layer 30 and surface layer 20 are formed, so that the insulation and magnetism can be improved simultaneously.

[0113] The heating step is a step of heating the steel sheet to the soaking temperature. The starting temperature is not particularly limited, but it can be room temperature (25°C) to 300°C. In the heating step, the heating rate in the range of 300°C to 500°C can be 10°C / minute or more. It is necessary to appropriately ensure the heating rate so that Al and Mn can be appropriately contained in the insulating coating layer and the surface layer. More specifically, in the heating step, the heating rate in the range of 300°C to 500°C can be 10°C / minute to 50°C / minute.

[0114] In the heating step, the dew point is 10°C to 50°C. If the dew point is too low, the surface layer 20 may not be properly formed. If the dew point is too high, a large amount of Al and Mn oxides are formed in the coating layer and the surface layer 20, and the internal oxidation of the base steel is severe, which may lead to magnetic degradation. More specifically, in the heating step, the dew point can be 15°C to 46°C.

[0115] In the heating step, the heating rate and the dew point can satisfy the following formula 2.

[0116] [Formula 2]

[0117] 0.7 ≤ [Heating rate (°C / minute)] / [Dew point (°C)] ≤ 2.5

[0118] When the relationship between the heating rate and the dew point is appropriately adjusted, the insulating coating layer 30 and the surface layer 20 will be more appropriately formed, and the insulation and magnetism can be improved more simultaneously.

[0119] The soaking step is a step of keeping the steel sheet constant at the soaking temperature. The soaking temperature can be 700°C to 850°C. The soaking time can be 10 minutes to 300 minutes. In the soaking step, the dew point can be 10°C or more. By adjusting the dew point temperature in the aforementioned heating step, the insulating coating layer 30 and the surface layer 20 can be appropriately formed. More specifically, in the soaking step, the dew point can be 10°C to 50°C.

[0120] The motor core according to an embodiment of the present invention includes a rotor composed of a plurality of non-oriented electrical steel sheets laminated and a stator composed of a plurality of non-oriented electrical steel sheets laminated. The non-oriented electrical steel sheet in the rotor contains 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight%, and contains the balance of Fe and inevitable impurities. The non-oriented electrical steel sheet in the stator contains 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight%, and contains the balance of Fe and inevitable impurities. There is a surface layer from the surface of the steel sheet toward the inside of the steel sheet, and there is an insulating coating layer on the surface of the steel sheet. In the insulating coating layer and the surface layer, the weight ratio of Al to Mn (Al / Mn) is 1 to 10.

[0121] For the rotor, its characteristics are the same as those of the non-oriented electrical steel sheet before SRA annealing. For the stator, its characteristics are the same as those of the non-oriented electrical steel sheet after SRA annealing. Therefore, the specific description thereof is omitted here.

[0122] In an embodiment of the present invention, the rotor and the stator can be manufactured simultaneously using the same non-oriented electrical steel sheet, thereby further improving the manufacturing efficiency.

[0123] When the rotor and the stator are manufactured simultaneously using the same non-oriented electrical steel sheet, the differences in the contents of Si, Al, and Mn between the non-oriented electrical steel sheets included in the stator and the rotor can be 0.2% or less, respectively.

[0124] An insulating film can be interposed between the steel sheets of the motor core. Since it is a well-known technology for the insulating film, the specific description thereof is omitted here.

[0125] The preferred embodiments and comparative examples of the present invention are described below. However, the following embodiments are only a preferred embodiment of the present invention, and the present invention is not limited to the following embodiments.

[0126] Example 1

[0127] A slab having the composition shown in Table 1 below was manufactured. C, S, N, Ti, Nb, V, etc., other than the components described in Table 1, were all controlled to be 0.003 wt% or less, and the balance was Fe.

[0128] The slab was heated at 1150 °C and hot-worked and post-treated by rolling at 850 °C to produce a hot-rolled sheet with a sheet thickness of 2.0 mm. The hot-rolled sheet was annealed at 1100 °C for 4 minutes and then pickled. Subsequently, cold rolling was performed to produce a cold-rolled sheet with a thickness of 0.25 mm, and cold-rolled sheet annealing was carried out under the conditions tabulated in Table 2 below.

[0129] The properties of the cold-rolled sheet after annealing (i.e., the rotor) steel sheet are shown in Table 2.

[0130] The crystal grain size was irradiated using an optical microscope, and the non-recrystallized fraction was measured using SEM-EBSD.

[0131] The tensile strength was measured using a tensile tester in accordance with JIS specifications.

[0132] Subsequently, a phosphate-based insulating coating composition without added Cr was coated on the annealed cold-rolled sheet. After forming an insulating coating, SRA annealing was carried out under the conditions of Table 3. The properties of the steel sheet after SRA annealing (i.e., the stator) are shown in Table 3.

[0133] The insulation was measured using a Franklin tester.

[0134] The magnetic properties were measured using a Single Sheet tester.

[0135] The composition of the insulating coating layer and the surface layer was measured using GDS. The amounts of Al and Mn were defined as the areas of the GDS peaks.

[0136] Table 1

[0137]

[0138]

[0139] Table 2

[0140]

[0141]

[0142] Table 3

[0143]

[0144]

[0145] As shown in Tables 1 to 3, it can be confirmed that when the alloy composition and surface layer properties are appropriately adjusted, the iron loss and insulation are improved simultaneously.

[0146] Moreover, it can be confirmed that the tensile strength and tensile strength uniformity before SRA can be ensured after annealing of the cold-rolled sheet.

[0147] In the case of the inventive example, Mn-O mainly exists in the insulation coating layer, and Al-O mainly exists in the surface layer. When observed according to the area ratio of GDS, when Al / Mn exceeds 10, almost no Mn oxide exists in the insulation coating layer and the surface layer. When Mn-O is sufficiently generated in the insulation coating layer and the surface layer, the Al / Mn value falls within an appropriate range and has excellent insulation and magnetic properties.

[0148] The present invention is not limited to the above-described embodiments, but can be made into various different forms. Those of ordinary skill in the technical field to which the present invention pertains should understand that it can also be implemented in other specific forms without changing the technical idea or essential features of the present invention. Therefore, the above-described embodiments are to be considered in all respects as illustrative and not restrictive.

Claims

1. An non-oriented electrical steel sheet contains 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight, and contains the balance of Fe and inevitable impurities. There is a surface layer extending from the surface of the steel sheet towards the interior of the steel sheet. There is an insulating coating layer on the surface of the steel sheet. In the insulating coating layer and the surface layer, the weight ratio of Al to Mn is 1 to 10.

2. The non-oriented electrical steel sheet according to claim 1, wherein it further contains one or more of C, N, S, Ti, Nb, and V in a content of 0.005% by weight or less respectively.

3. The non-oriented electrical steel sheet according to claim 1, wherein it further contains one or more of P in a content of 0.005% by weight or less, Cu in a content of 0.005% to 0.2% by weight, Cr in a content of 0.01% to 0.5% by weight, Sn in a content of 0.06% by weight or less, Sb in a content of 0.06% by weight or less, Ni in a content of 0.05% by weight or less, and Zn in a content of 0.01% by weight or less.

4. The non-oriented electrical steel sheet according to claim 1, wherein it further contains one or two or more of Bi, Pb, Ge, and As, and their respective contents are 0.200% by weight or less or their total content is 0.200% by weight or less.

5. The non-oriented electrical steel sheet according to claim 1, wherein it further contains one or more of Mo in a content of 0.03% by weight or less, B in a content of 0.0050% by weight or less, Ca in a content of 0.0050% by weight or less, and Mg in a content of 0.0050% by weight or less.

6. The non-oriented electrical steel sheet according to claim 1, wherein the specific resistance is 50 μΩ·cm or more.

7. The non-oriented electrical steel sheet according to claim 1, wherein the average grain size is 50 μm to 200 μm.

8. A method for manufacturing a non-oriented electrical steel sheet, which comprises: a step of hot rolling a slab containing 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight, and containing the balance of Fe and inevitable impurities to produce a hot rolled sheet; a step of cold rolling the hot rolled sheet to produce a cold rolled sheet; a cold rolled sheet annealing step of annealing the cold rolled sheet; a step of forming an insulating coating layer on the annealed cold rolled sheet; and a step of stress relief annealing the steel sheet formed with the insulating coating layer, wherein the stress relief annealing step includes a heating step of heating the steel sheet to the soaking temperature and a soaking step, in the heating step, the heating rate in the range of 300°C to 500°C is 10°C / minute to 50°C / minute, the dew point in the heating step is 10°C to 50°C, and the dew point in the soaking step is 0°C to 35°C.

9. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein the slab further contains one or more of C, N, S, Ti, Nb, and V in a content of 0.005% by weight or less respectively.

10. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein The slab further contains one or more of P of 0.005 wt% or less, Cu of 0.005 wt% to 0.2 wt%, Cr of 0.01 wt% to 0.5 wt%, Sn of 0.06 wt% or less, Sb of 0.06 wt% or less, Ni of 0.05 wt% or less, and Zn of 0.01 wt% or less.

11. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein the slab further contains one or two or more of Bi, Pb, Ge, and As, and their respective contents are 0.200 wt% or less or their total content is 0.200 wt% or less.

12. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein the slab further contains one or more of Mo of 0.03 wt% or less, B of 0.0050 wt% or less, Ca of 0.0050 wt% or less, and Mg of 0.0050 wt% or less.

13. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein in the annealing step of the cold-rolled sheet, the annealing temperature is 750 °C to 850 °C, and the annealing time is 10 seconds to 60 seconds.

14. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein after annealing the cold-rolled sheet, the average grain size of the annealed cold-rolled sheet is 10 μm to 30 μm.

15. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein after annealing the cold-rolled sheet, the non-recrystallized fraction is 1 area% to 15 area%.

16. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein after annealing the cold-rolled sheet, there is a surface layer in the direction from the surface of the cold-rolled sheet to the inside of the cold-rolled sheet, and the thickness of the surface layer is 0.0001 μm to 0.2 μm.

17. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein after annealing the cold-rolled sheet, the following formula 1 is satisfied: Formula 1 50 ≤ [average crystal grain size (μm)] × [cold-rolled sheet annealing time (seconds)] / [non-recrystallized area fraction (%)] ≤ 500.

18. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein in the heating step, the heating rate and the dew point satisfy the following formula 2: Formula 2 0.7 ≤ [heating rate (°C / minute)] / [dew point (°C)] ≤ 2.

5.

19. An electric motor core, wherein it includes a rotor composed of a plurality of stacked non-oriented electrical steel sheets and a stator composed of a plurality of stacked non-oriented electrical steel sheets, the non-oriented electrical steel sheet in the rotor contains Si of 2.8% to 4.0%, Al of 0.5% to 1.7%, and Mn of 0.3% to 2.0% by weight, and contains the balance of Fe and inevitable impurities, The non-oriented electrical steel sheet in the stator contains 2.8% to 4.0% of Si, 0.5% to 1.7% of Al, and 0.3% to 2.0% of Mn by weight, and contains the balance of Fe and inevitable impurities. There is a surface layer from the surface of the steel sheet toward the inside of the steel sheet, and an insulating coating layer exists on the surface of the steel sheet. In the insulating coating layer and the surface layer, the weight ratio of Al to Mn, i.e., Al / Mn, is 1 to 10.

20. The motor core according to claim 19, wherein, The average grain size of the non-oriented electrical steel sheet in the rotor is 10 μm to 30 μm.

21. The motor core according to claim 19, wherein, The differences in the contents of Si, Al, and Mn between the non-oriented electrical steel sheets included in the stator and the rotor are each 0.2% by weight or less.

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