Non-oriented electrical steel sheet and method for producing same

Abstract

A non-oriented electrical steel sheet according to one embodiment of the present invention comprises, in weight%, 1.5 to 4% of Si, 0.1 to 2% of Al, and 0.05 to 2% of Mn, with the remainder comprising Fe and unavoidable impurities. A non-oriented electrical steel sheet according to one embodiment of the present invention has a non-oriented electrical steel sheet having a cross-section from {110} lt; 001gt, 001gt; the area fraction of crystal grains having an orientation within 15 degrees is 10% or less.

Description

Technical Field

[0001] One embodiment of the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same. Specifically, one embodiment of the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same, which suppresses the {110}<001> texture by performing hot rolling twice, thereby ensuring the strength of the steel sheet before stress relief annealing and ensuring the magnetic properties of the steel sheet after stress relief annealing. Background Art

[0002] Non-oriented electrical steel sheets are important core materials required for converting electrical energy into mechanical energy in rotating equipment. In order to save energy, their magnetic properties, namely low and high-frequency iron losses and high magnetic flux density, are very important. Iron loss is the energy that disappears as heat during the energy conversion process, so the lower the iron loss, the more effective it is. The magnetic flux density is the force that generates power, and the higher the magnetic flux density, the more effective it is. In addition, when rotating at a speed of several thousand to tens of thousands of revolutions per minute (RPM), in order to suppress vibration and improve durability, ensuring strength and reducing the thickness deviation of the electrical steel sheet are beneficial for fabricating a symmetric core.

[0003] Recently, as a countermeasure to address the shortage of fossil fuels and reduce greenhouse gases, the technology of converting traditional internal combustion engine vehicles into HEV (hybrid electric vehicle) / EV (electric vehicle) is rapidly developing. These HEV / EVs are vehicles that can reduce the use of traditional internal combustion engine fuels, namely gasoline or diesel, by changing part or all of the driving methods to electric motors, and at the same time can achieve better fuel efficiency.

[0004] The motors used in such vehicles need to generate a large torque at low speed or during acceleration and rotate at high speed during constant speed and high-speed driving. Therefore, as the core material of the motor, the non-oriented electrical steel sheet needs to have a high magnetic flux density and high strength at low speed rotation, and at high speed rotation, in addition to high magnetic flux density and low high-frequency iron loss, it also needs to have a small thickness variation to suppress vibration. Generally, high-frequency iron loss refers to the iron loss at frequencies above 200 Hz, but in non-oriented electrical steel sheets used for automobiles, the value of W10 / 400 is usually adopted. Especially due to the characteristics of rotating equipment, the iron loss and magnetic flux density in the circumferential direction are important.

[0005] Generally, for the rotor of a motor, factors such as magnetic flux density, strength, and thickness deviation are emphasized, while for the stator, high-frequency low iron loss is emphasized. However, when using grain refinement to improve strength, the iron loss deteriorates. Therefore, different materials are used for the rotor and stator respectively, resulting in an increase in cost. Thus, after manufacturing the stator and rotor using grain-fine electrical steel sheets, only the stator is subjected to customer heat treatment. As a result, using the same material, the rotor can have high strength while the stator can have low high-frequency iron loss. However, if this process is carried out, the texture will deteriorate, resulting in an excessive reduction in magnetic flux density after customer heat treatment. In addition, resistivity elements such as Si, Al, Mn, etc. added to improve high-frequency iron loss will cause a further reduction in magnetic flux density. Therefore, materials that continuously require lightweighting, such as environmentally friendly electric vehicle drive motors, need to have a relatively high magnetic flux density.

[0006] To solve these problems, a method has been proposed to thin the thickness of the hot-rolled sheet to less than 2.0 mm to improve characteristics. In addition, a method to improve magnetism by adding high aluminum and undergoing two annealing processes and two rolling processes has been proposed. Additionally, a method to achieve hot-rolled thinning through a thin slab manufacturing method has been proposed. However, the method of reducing the hot-rolled thickness is limited by the roll force in the hot-rolling process and is difficult to apply to mass production. Moreover, in the hot-rolling process without tension operation, as the reduction ratio increases, there is a problem of increased thickness deviation caused by roll bending. The two annealing and two rolling processes have improved in some characteristics, but due to the relatively large cost increase factor and the over-developed Goss texture, there is a tendency for circumferential characteristics to deteriorate. Summary of the Invention

[0007] (I) Technical Problems to be Solved

[0008] An embodiment of the present invention aims to provide an non-oriented electrical steel sheet and a manufacturing method thereof. Specifically, an embodiment of the present invention aims to provide an non-oriented electrical steel sheet and a manufacturing method thereof, which suppress the {110}<001> texture by performing two hot-rollings, thereby ensuring the strength of the steel sheet before stress relief annealing and ensuring the magnetism of the steel sheet after stress relief annealing.

[0009] (II) Technical Solutions

[0010] For the non-oriented electrical steel sheet according to an embodiment of the present invention, by weight%, the non-oriented electrical steel sheet contains 1.5 to 4% of Si, 0.1 to 2% of Al, and 0.05 to 2% of Mn, and the balance contains Fe and inevitable impurities. The area fraction of grains having an orientation within 15° from {110}<001> is 10% or less.

[0011] The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain one or more of Cr of 0.5 wt% or less, Cu of 0.2 wt% or less, P of 0.1 wt% or less, Sn of 0.06 wt% or less, and Sb of 0.06 wt% or less.

[0012] 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 of 0.005 wt% or less.

[0013] The average grain size of the non-oriented electrical steel sheet according to an embodiment of the present invention may be 10 to 25 μm.

[0014] The non-oriented electrical steel sheet according to an embodiment of the present invention may satisfy the following formula 1.

[0015] Yield strength (MPa) ≥ 140 + 100 × [Si] + 35 × ([Al] + [Mn])

[0016] Wherein, in formula 1, [Si], [Al], and [Mn] respectively represent the contents (wt%) of Si, Al, and Mn.

[0017] The non-oriented electrical steel sheet according to an embodiment of the present invention may satisfy the following formula 2.

[0018] [Formula 2]

[0019] Circumferential magnetic flux density (B 50 , Tesla) ≥ 1.88 + 0.1 × t - 0.067 × [Si] - 0.0458 × [Al] - 0.022 × [Mn]

[0020] Wherein, in formula 2, [Si], [Al], and [Mn] respectively represent the contents (wt%) of Si, Al, and Mn, and t represents the thickness (mm) of the steel sheet.

[0021] The non-oriented electrical steel sheet according to an embodiment of the present invention may satisfy the following formula 3 after stress relief annealing at a temperature of 700 to 850 °C for 10 to 300 minutes.

[0022] [Formula 3]

[0023] Circumferential magnetic flux density (B 50 , Tesla) ≥ 1.85 + 0.1 × t - 0.067 × [Si] - 0.0458 × [Al] - 0.022 × [Mn]

[0024] Circumferential iron loss (W 10 / 400 , W / kg) ≤ 6 + 4000 × t 2 / (13 + 11×([Si] + [Al] + 0.5×[Mn])

[0025] Among them, in Formula 3, [Si], [Al], and [Mn] respectively represent the contents (weight %) of Si, Al, and Mn, and t represents the thickness (mm) of the steel sheet.

[0026] A method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention includes: a first hot rolling step of hot rolling a slab to manufacture a first hot rolled sheet, wherein, by weight %, the slab contains 1.5 to 4% of Si, 0.1 to 2% of Al, and 0.05 to 2% of Mn, and the balance contains Fe and inevitable impurities; a step of coiling the first hot rolled sheet; a second hot rolling step of hot rolling the first hot rolled sheet at a temperature of 700 to 1000 °C with a reduction ratio of 20 to 50% to manufacture a second hot rolled sheet; a step of cold rolling the second hot rolled sheet to manufacture a cold rolled sheet; and a cold rolled sheet annealing step of annealing the cold rolled sheet at a temperature of 710 to 820 °C.

[0027] The first hot rolling step may include a rough rolling step and a finish rolling step.

[0028] The thickness of the first hot rolled sheet may be 1.8 to 2.5 mm.

[0029] After the coiling step, the coiled sheet may be cooled to below 700 °C.

[0030] After the coiling step, it may further include a first hot rolled sheet annealing step of annealing at a temperature of 700 to 1000 °C.

[0031] The thickness of the second hot rolled sheet may be 1.2 to 1.8 mm.

[0032] After the second hot rolling step, it may further include a second hot rolled sheet annealing step of annealing the second hot rolled sheet at a temperature of 850 to 1150 °C.

[0033] After the cold rolled sheet annealing step, it may further include a step of stress relief annealing at a temperature of 700 to 850 °C for 10 to 300 minutes.

[0034] (III) Beneficial effects

[0035] The non-oriented electrical steel sheet according to an embodiment of the present invention, by reducing the thickness deviation in the width direction caused by shape correction, reducing the cold rolling reduction ratio, and suppressing the generation of Goss texture, has excellent strength and circumferential average magnetic flux density before customer heat treatment (stress relief annealing, SRA), and has excellent circumferential average magnetic flux density and high-frequency iron loss after customer heat treatment (stress relief annealing, SRA).

[0036] Finally, the non-oriented electrical steel sheet according to an embodiment of the present invention uses the same steel sheet, without undergoing SRA treatment as a rotor, and after undergoing SRA treatment as a stator, which helps to manufacture motors for environmentally friendly vehicles, motors for high-efficiency household appliances, and ultra-high-end motor cores. Detailed Description

[0037] The terms first, second, third, etc. are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, or segment from another part, component, region, layer, or segment. Therefore, without departing from the scope of the present invention, the first part, component, region, layer, or segment described below can also be described as the second part, component, region, layer, or segment.

[0038] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. Unless the context clearly gives the opposite meaning, the singular forms used herein are also intended to include the plural forms. The term "comprising" used in the specification can specifically refer to a certain property, field, integer, step, action, element, and / or component, but does not exclude the existence or addition of other properties, fields, integers, steps, actions, elements, and / or components.

[0039] If a part is described as being on another part, there may be other parts directly on the other part or there may be other parts in between. When a part is described as being directly on another part, there are no other parts in between.

[0040] Although not otherwise defined, the meanings of all terms (including technical terms and scientific terms) used herein are the same as those commonly understood by those of ordinary skill in the art to which the present invention pertains. For terms defined in a dictionary, they should be interpreted as having a meaning consistent with the relevant technical literature and the content disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0041] In addition, unless otherwise specifically stated, % represents weight %, and 1 ppm is 0.0001 weight %.

[0042] In an embodiment of the present invention, further including additional elements means that the additional elements replace the remaining iron (Fe), and the replacement amount is equivalent to the addition amount of the additional elements.

[0043] Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement the present invention. However, the present invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0044] An non-oriented electrical steel sheet according to an embodiment of the present invention, in terms of weight%, the non-oriented electrical steel sheet contains 1.5 to 4% of Si, 0.1 to 2% of Al, and 0.05 to 2% of Mn, and the balance contains Fe and inevitable impurities.

[0045] First, the reasons for the composition limitations of the non-oriented electrical steel sheet will be described.

[0046] Si: 1.5 to 4.0 wt%

[0047] Silicon (Si) plays a role in increasing the resistivity of the material to reduce iron loss, so a relatively large amount needs to be added. If too little Si is added, the effect of improving high-frequency iron loss may be negligible. If too much Si is added, the hardness of the material increases, and the productivity and punching properties deteriorate, so it is not preferred. More specifically, Si may contain 2.5 to 3.7 wt%.

[0048] Al: 0.1 to 2.0 wt%

[0049] Aluminum (Al) plays a role in increasing the resistivity of the material together with Si to reduce iron loss, and may form nitrides and oxides that are harmful to magnetism. If too little Al is added, there is no effect on reducing high-frequency iron loss, and fine nitrides are formed, which may lead to magnetic deterioration. If too much Al is added, problems such as changing the physical properties of the mold powder occur during the steelmaking and continuous casting processes, which may lead to a significant reduction in productivity. More specifically, Al may contain 0.5 to 1.5 wt%.

[0050] Mn: 0.05 to 2.00 wt%

[0051] Manganese (Mn) plays a role in increasing the resistivity of the material together with Si and Al to improve iron loss and form sulfides. If too little Mn is added, fine precipitation of MnS may occur, which may lead to magnetic deterioration. If too much Mn is added, it promotes the formation of a

[111] texture that is not conducive to magnetism, and Mn reduces the fraction of Fe, and the magnetic flux density may decrease sharply. More specifically, Mn may contain 0.5 to 1.5 wt%.

[0052] The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain one or more of Cr of 0.5 wt% or less, Cu of 0.2 wt% or less, P of 0.1 wt% or less, Sn of 0.06 wt% or less, and Sb of 0.06 wt% or less.

[0053] Cr: 0.50 wt% or less

[0054] Chromium (Cr) plays a role in improving iron loss by increasing the resistivity. If the Cr content is too high, the magnetic flux density may decrease. More specifically, when further containing Cr, it may contain 0.01 to 0.50 wt%. More specifically, it may contain 0.050 to 0.20 wt%.

[0055] Cu: 0.200 wt% or less

[0056] Copper (Cu) plays a role in forming sulfides together with Mn. If too much Cu is added, high-temperature brittleness will occur, and cracks may form during continuous casting or hot rolling. More specifically, Cu may also contain 0.005 to 0.200 wt%. More specifically, it may contain 0.01 to 0.10 wt%.

[0057] P: 0.10 wt% or less

[0058] Phosphorus (P) is mostly dissolved in the steel, showing the effect of improving iron loss. When further adding P, if too much is added, it segregates at the grain boundaries, reducing the toughness of the material, which may lead to poor productivity and blanking performance. More specifically, P may contain 0.001 to 0.100 wt%. More specifically, it may contain 0.005 to 0.050 wt%.

[0059] Sn: 0.06 wt% or less

[0060] Tin (Sn) segregates at the grain boundaries and the surface, playing a role in improving the texture of the material and suppressing surface oxidation. Therefore, tin can be added to improve the magnetism. If too much Sn is added, the grain boundary segregation is severe, the surface quality deteriorates, the hardness increases, which may cause the cold-rolled sheet to break and lead to a decrease in rollability. Therefore, Sn can be further added within the aforementioned range. More specifically, Sn may contain 0.01 to 0.06 wt%. More specifically, it may contain 0.02 to 0.05 wt%.

[0061] Sb: 0.06 wt% or less

[0062] Antimony (Sb) segregates at the grain boundaries and the surface, playing a role in improving the texture of the material and suppressing surface oxidation. Therefore, antimony can be added to improve the magnetism. If too much Sb is added, the grain boundary segregation is severe, the surface quality deteriorates, the hardness increases, which may cause the cold-rolled sheet to break and lead to a decrease in rollability. Therefore, Sb can be further added within the aforementioned range. More specifically, Sb may contain 0.01 to 0.06 wt%. More specifically, it may contain 0.02 to 0.05 wt%.

[0063] 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 of 0.005 wt% or less.

[0064] C, N, and Ti can be restricted because the formation of carbonitrides hinders the movement of magnetic domains, and the upper limit of S can be restricted because the formation of sulfides results in poor grain growth. These elements can each be contained in an amount of 0.0050% by weight or less.

[0065] N combines with Ti, Nb, and V to form nitrides and plays a role in reducing grain growth.

[0066] C reacts with N, Ti, Nb, V, etc. to form fine carbides, thereby playing a role in hindering grain growth and the movement of magnetic domains.

[0067] S forms sulfides, resulting in poor grain growth.

[0068] More specifically, one or more of C, S, N, Ti, Nb, and V can each be contained in an amount of 0.0040% by weight or less.

[0069] The non-oriented electrical steel sheet according to an embodiment of the present invention may further contain one or more of Mo in an amount of 0.03% by weight or less, B in an amount of 0.0050% by weight or less, V in an amount of 0.0050% by weight or less, Ca in an amount of 0.0050% by weight or less, Nb in an amount of 0.0050% by weight or less, and Mg in an amount of 0.0050% by weight or less.

[0070] These elements react with unavoidably contained C, S, N, etc. to form fine carbides, nitrides, or sulfides, which may have an adverse effect on magnetism. Therefore, the upper limit can be restricted as described above.

[0071] Other impurities

[0072] In addition to the aforementioned elements, unavoidably mixed impurities may be contained. In an embodiment of the present invention, in addition to the aforementioned elements, the inclusion of additional elements is not excluded. When additional elements are included, a part of the remaining Fe is replaced.

[0073] The non-oriented electrical steel sheet according to an embodiment of the present invention has an area fraction of grains with an orientation of {110}<001> within 15° of 10% or less.

[0074] This fine structure is an orientation that has an adverse effect on magnetism. By reducing its quantity, the grains with an orientation favorable to magnetism are relatively increased, thereby improving magnetism.

[0075] For this fine microstructure, the generation of the fine microstructure can be suppressed by performing two hot rolling processes. When performing two hot rollings, the thickness of the second hot-rolled sheet becomes thinner after the two hot rollings, and the cold rolling reduction ratio becomes smaller, thereby suppressing the generation of the fine microstructure. Even after the stress relief annealing process, there is no additional fractional change in the fine microstructure and it remains unchanged. More specific content will be described in the manufacturing process.

[0076] The measurement method is not particularly limited, but X-ray pole figure or EBSD (electron backscatter diffraction) can be used for measurement. The measurement reference plane is not particularly limited and can be the plane perpendicular to the rolling direction (TD plane).

[0077] The average grain size of the steel sheet can be 10 to 25 μm. If the average grain size is too small, the magnetic properties deteriorate excessively and it is impossible to obtain the desired magnetic properties even if stress relief annealing is performed. If the average grain size is too large, it is difficult to appropriately ensure the strength. More specifically, the average grain size of the steel sheet can be 15 to 20 μm. In one embodiment of the present invention, the grain size can be measured with respect to the plane perpendicular to the rolling direction (TD plane) of the steel sheet. More specifically, with respect to the entire thickness t of the steel sheet, the measurement can be performed at a thickness in the range of 1 / 4t to 3 / 4t. For the grain size, assuming a virtual circle having the same area as the grain area, the diameter of the circle is the grain size. The average grain size can be measured by dividing the number of grains present in the area to be measured by the area to be measured. In one embodiment of the present invention, characteristics such as the average grain size and yield strength without separate description refer to the characteristics before SRA.

[0078] After stress relief annealing, the average grain size can be 30 to 300 μm. Stress relief annealing refers to a process in which when manufacturing a motor from non-oriented electrical steel sheets, the steel sheets are blanked and laminated, and then heat treated to remove the residual stress in the steel sheets. Specifically, it can be performed at a temperature of 700 to 850 °C for 10 to 300 minutes.

[0079] As described above, the non-oriented electrical steel sheet according to one embodiment of the present invention has excellent strength and circumferential average magnetic flux density before stress relief annealing, and excellent circumferential average magnetic flux density and high-frequency iron loss after stress relief annealing.

[0080] Specifically, regarding the strength, the following formula 1 can be satisfied.

[0081] [Formula 1]

[0082] Yield strength (MPa) ≥ 140 + 100×[Si] + 35×([Al] + [Mn])

[0083] Wherein, in formula 1, [Si], [Al], and [Mn] each represent the contents (weight %) of Si, Al, and Mn.

[0084] When Si, Al, and Mn are included in the steel sheet, the yield strength is increased. In one embodiment of the present invention, in addition to adding Si, Al, and Mn, the formation of {110}<001> grains is suppressed and the average grain diameter is adjusted, so that the yield strength can be further increased.

[0085] More specifically, the yield strength can be 400 MPa or more. More specifically, it can be 400 to 600 MPa. More specifically, it can be 450 to 550 MPa. For the yield strength, three KS-13A specimens can be respectively fabricated and subjected to a shortening tensile test, and then the yield strength can be measured under the condition of 0.2% offset.

[0086] The non-oriented electrical steel sheet according to one embodiment of the present invention can satisfy the following formula 2.

[0087] [Formula 2]

[0088] Circumferential magnetic flux density (B 50 , Tesla) ≥ 1.88 + 0.1×t - 0.067×[Si] - 0.0458×[Al] - 0.022×[Mn]

[0089] Wherein, in formula 2, [Si], [Al], and [Mn] respectively represent the contents (weight %) of Si, Al, and Mn, and t represents the thickness (mm) of the steel sheet.

[0090] When Si, Al, and Mn are included in the steel sheet, the magnetic flux density will decrease. In one embodiment of the present invention, even if a certain amount of Si, Al, and Mn is added, the formation of {110}<001> grains and the adjustment of the average grain diameter are suppressed, so that the magnetic flux density can be increased.

[0091] More specifically, the circumferential magnetic flux density can be 1.62 T or more. More specifically, it can be 1.65 to 1.80 T. More specifically, it can be 1.68 to 1.75 T. The magnetic flux density (B 50 ) is the magnetic flux density induced under a magnetic field of 5000 A / m. The circumferential direction refers to the circumferential direction of a circle. In one embodiment of the present invention, an annular sample with an outer diameter of 100 mm and an inner diameter of 90 mm can be fabricated by electrical discharge machining, and 10 of them can be stacked and then wound with copper wire to measure the magnetism.

[0092] The non-oriented electrical steel sheet according to one embodiment of the present invention can satisfy the following formula 3 after stress relief annealing at a temperature of 700 to 850 °C for 10 to 300 minutes.

[0093] [Formula 3]

[0094] Circumferential magnetic flux density (B 50, Tesla) ≥ 1.85 + 0.1×t - 0.067×[Si] - 0.0458×[Al] - 0.022×[Mn]

[0095] Circumferential direction iron loss (W 10 / 400 , W / kg) ≤ 6 + 4000×t 2 / (13 + 11×([Si] + [Al] + 0.5×[Mn])

[0096] Among them, in Equation 3, [Si], [Al], and [Mn] respectively represent the contents (weight %) of Si, Al, and Mn, and t represents the thickness (mm) of the steel sheet.

[0097] When stress relief annealing is performed, there is a tendency for the magnetic flux density and iron loss to decrease. In one embodiment of the present invention, by appropriately adjusting the microstructure, the deterioration of the magnetic flux density can be minimized while reducing the iron loss.

[0098] More specifically, the circumferential magnetic flux density (B 50 ) after stress relief annealing can be 1.59 T or more. More specifically, the circumferential magnetic flux density (B 50 ) after stress relief annealing can be 1.62 to 1.78 T. More specifically, the circumferential magnetic flux density (B 50 ) after stress relief annealing can be 1.65 to 1.75 T.

[0099] The circumferential iron loss (W 10 / 400 ) after stress relief annealing can be 13.2 W / kg or less. More specifically, it can be 8.0 to 13.0 W / kg. More specifically, it can be 8.5 to 12.5 W / kg.

[0100] For the non-oriented electrical steel sheet according to one embodiment of the present invention, the thickness deviation can be 2.0% or less. More specifically, the thickness deviation can be 1.0 to 1.8%. For the thickness deviation, after measuring the thickness at the center in the width direction of the steel sheet and at 15 mm from both sides of the width, the difference is divided by the center thickness for calculation. The steel sheet thickness can be 0.10 to 0.35 mm.

[0101] For the non-oriented electrical steel sheet, an insulating coating film can also be provided on the base steel sheet. For the insulating coating film, it is well known, so the specific description is omitted.

[0102] A method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention includes: a first hot rolling step of hot rolling a slab to manufacture a first hot rolled sheet, wherein, by weight %, the slab contains 1.5 to 4% of Si, 0.1 to 2% of Al, and 0.05 to 2% of Mn, and the balance contains Fe and inevitable impurities; a step of coiling the first hot rolled sheet; a second hot rolling step of hot rolling the first hot rolled sheet at a temperature of 700 to 1000 °C with a reduction ratio of 20 to 50% to manufacture a second hot rolled sheet; a step of cold rolling the second hot rolled sheet to manufacture a cold rolled sheet; and a cold rolled sheet annealing step of annealing the cold rolled sheet at a temperature of 710 to 820 °C.

[0103] The following describes each step in detail.

[0104] First, the slab is hot rolled to manufacture a first hot rolled sheet. The reasons for restricting the addition ratios of the respective components in the slab are the same as the reasons for restricting the components of the aforementioned non-oriented electrical steel sheet, so repeated descriptions are omitted. During the following manufacturing processes such as the first hot rolling, the second hot rolling, the cold rolling, the cold rolled sheet annealing, and the stress relief annealing, the components of the slab do not substantially change, so the components of the slab are substantially the same as those of the non-oriented electrical steel sheet.

[0105] Before the step of manufacturing the first hot rolled sheet, the slab can be heated. Specifically, the slab is loaded into a heating furnace and heated to 1100 to 1250 °C. When heated at a temperature higher than 1250 °C, the precipitates will redissolve and may precipitate finely after hot rolling. If the temperature is too low, the deformation resistance during hot rolling will be too large, so it may be difficult to hot roll to an appropriate thickness.

[0106] The first hot rolling step can include a rough rolling step and a finish rolling step. The rough rolling step is a step of manufacturing a bar with a thickness of 20 to 50 mm. The finish rolled bar is rolled to manufacture a hot rolled sheet with a thickness of 1 to 3 mm. The rough rolling and the finish rolling are continuously performed without coiling, which is different from the first hot rolling and the second hot rolling processes of the present invention. In the finish rolling step, the temperature of the steel sheet can be 800 to 1000 °C.

[0107] The thickness of the first hot rolled sheet can be 1.8 to 2.5 mm. If the thickness of the first hot rolled sheet is too thick, the rolling load in the subsequent second hot rolling step and cold rolling step will increase, and a large amount of (111) recrystallization texture will be formed, which may have an adverse effect on the magnetic properties. By one-time hot rolling, it is difficult to further reduce the thickness. Even if the thickness is reduced, due to the bending phenomenon of the rolling mill rolls, the thickness deviation in the width direction will increase, affecting the thickness deviation of the finally manufactured steel sheet and possibly resulting in poor shape. More specifically, the thickness of the first hot rolled sheet can be 1.9 to 2.3 mm.

[0108] Next, the first hot-rolled sheet is coiled. In the coiling step, the temperature of the steel sheet can be 500 to 700 °C. After the coiling step, the coiled sheet can be cooled to below 700 °C. After cooling, second hot rolling is performed, which has the advantage of further improving isotropy.

[0109] After the coiling step, it can also include a first hot-rolled sheet annealing step of annealing at a temperature of 700 to 1000 °C for 1 second to 10 hours. Through the first hot-rolled sheet annealing step, the strong cold-rolled deformation band structure in the subsequent rolling step is suppressed, so that the strongly anisotropic Goss texture can be suppressed during the annealing process after subsequent rolling, and isotropy can be achieved. More specifically, it can be annealed for 10 seconds to 600 seconds.

[0110] Next, in the second hot rolling step, the first hot-rolled sheet is hot-rolled at a temperature of 700 to 1000 °C with a reduction ratio of 20 to 50% to manufacture a second hot-rolled sheet. In one embodiment of the present invention, by adding the second hot-rolled sheet manufacturing step, the reduction ratio in cold rolling can be reduced, thereby improving magnetism, and by correcting the shape, the thickness deviation in the width direction of the final product sheet can be reduced. For the reduction ratio, it can be calculated by (thickness of the steel sheet before rolling - thickness of the steel sheet after rolling) / thickness of the steel sheet before rolling × 100.

[0111] If the temperature of the steel sheet in the second hot-rolled sheet is too low, a higher rolling force is required, it is difficult to correct the width-thickness deviation, and then the Goss texture is formed. The magnetism near 45° from the rolling direction of the final product sheet deteriorates rapidly, which may lead to deterioration of the circumferential characteristics. If the steel sheet temperature is too high, the rigidity of the roll itself weakens, the bending intensifies, and then the ductility of the hot-rolled sheet itself becomes too weak and difficult to control, resulting in an increase in thickness deviation instead. More specifically, in the second hot rolling step, the temperature of the steel sheet can be 730 to 980 °C. More specifically, it can be 750 to 950 °C.

[0112] During the second hot rolling, the reduction ratio can be 20 to 50%. If the reduction ratio is too small, the effect of performing the second hot rolling cannot be fully obtained. If the reduction ratio is too high, it is not easy to control the shape, and the thickness deviation increases instead. More specifically, the reduction ratio can be 25 to 45%.

[0113] The thickness of the second hot-rolled sheet can be 1.2 to 1.8 mm. If the thickness of the second hot-rolled sheet is too thick, the rolling load in the subsequent cold rolling step increases, and a large amount of (111) recrystallization texture is formed, which may have an adverse effect on magnetism. If the thickness is too low, it is not easy to control the shape, and the thickness deviation increases instead. More specifically, the thickness of the second hot-rolled sheet can be 1.3 to 1.7 mm.

[0114] After the second hot rolling step, a second hot rolled sheet annealing step of annealing the second hot rolled sheet at a temperature of 850 to 1150 °C for 3 to 600 seconds may be included.

[0115] If the annealing temperature of the second hot rolled sheet is too low, the structure will not grow or will grow microscopically, and the effect of increasing the magnetic flux density will be small. If the annealing temperature is too high, the magnetic properties will instead decrease, and due to the deformation of the sheet shape, the rolling workability may deteriorate. More specifically, the temperature range may be 950 to 1125 °C. The second hot rolled sheet annealing is carried out to increase the orientation favorable for magnetism as needed and may also be omitted.

[0116] Next, the hot rolled sheet is pickled and cold rolled to a predetermined sheet thickness. Depending on the thickness of the hot rolled sheet, different reduction ratios can be adopted, but a reduction ratio of 70 to 85% can be adopted to cold roll to a final thickness of 0.10 to 0.35 mm. To adjust the reduction ratio, one cold rolling or two or more cold rollings including intermediate annealing can be carried out. More specifically, it can be cold rolled to 0.15 to 0.30 mm.

[0117] The cold rolled sheet after cold rolling is subjected to cold rolled sheet annealing.

[0118] In the cold rolled sheet annealing step, the annealing temperature is 710 to 820 °C. If the annealing temperature is too low, unrecrystallized portions remain, and the magnetic flux density cannot be sufficiently ensured. If the annealing temperature is too high, the grains become coarse, and the strength cannot be appropriately ensured. More specifically, in the cold rolled sheet annealing step, the annealing temperature can be 720 to 800 °C, and the annealing time can be 10 to 60 seconds. More specifically, the annealing time can be 20 to 45 seconds.

[0119] After the cold rolled sheet annealing, the unrecrystallized fraction can be 3 to 13 area %. When the unrecrystallized portion of the cold rolled sheet is appropriately formed, both magnetism and strength can be ensured. In one embodiment of the present invention, unrecrystallized refers to a portion that can be distinguished by analyzing the in-lattice orientation distribution by EBSD. More specifically, the unrecrystallized fraction can be 5 to 10 area %.

[0120] Next, stress relief annealing is carried out on the annealed cold rolled sheet. After the cold rolled sheet annealing, an insulating film formation, blanking, and lamination process can be carried out. This is well known, so specific descriptions are omitted. Stress will be generated in the non-oriented electrical steel sheet during the blanking process, which has an adverse effect on the magnetism of the non-oriented electrical steel sheet. For the stator where the magnetic properties are relatively important in the motor core, the residual stress in the steel sheet is removed by stress relief annealing to improve the magnetism of the steel sheet. On the other hand, for the rotor where the strength characteristics are relatively more important than magnetism, stress relief annealing can be omitted.

[0121] That is to say, although the same steel sheet is used, depending on whether stress relief annealing is carried out, it can be used for different purposes such as stators and rotors.

[0122] The stress relief annealing step can be carried out at a temperature of 700 to 850 °C for 10 to 300 minutes.

[0123] 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 rotor may laminate the non-oriented electrical steel sheets before the aforementioned stress relief annealing, and the stator may laminate the non-oriented electrical steel sheets after the aforementioned stress relief annealing.

[0124] 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, so specific descriptions are omitted.

[0125] In an embodiment of the present invention, using the same non-oriented electrical steel sheet, the rotor and the stator can be manufactured simultaneously, further improving the manufacturing efficiency.

[0126] The motor core may have an insulating coating film sandwiched between the steel sheets. For the insulating coating film, it is well known, so specific descriptions are omitted.

[0127] 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.

[0128] Example 1

[0129] Slabs with the manufacturing components shown in Table 1 below are manufactured. Ti, Nb, V, etc. other than the components shown in Table 1 are each controlled to be 0.003 wt% or less, and the balance is Fe.

[0130] The slab is heated to 1150 °C and subjected to the first hot rolling according to the inlet side thickness shown in Table 2 below. The second hot rolling is carried out under the conditions shown in Table 2, and the second hot rolled sheet is annealed at 1100 °C for 60 seconds. After pickling the hot rolled annealed sheet, it is cold rolled to the thickness shown in Table 2, annealed for 1 minute at the annealing temperature shown in Table 2 in an atmosphere of 20 vol% hydrogen and 80 vol% nitrogen, and then the circumferential average magnetic properties, strength, and thickness deviation are measured. Then, after heat treatment (stress relief annealing) at 800 °C in a nitrogen atmosphere for 1 hour, the circumferential average magnetic properties are measured again. For the circumferential average magnetic properties, an annular sample with an outer diameter of 100 mm and an inner diameter of 90 mm can be made by electrical discharge machining, 10 of them are laminated, and then copper wires are wound to measure the magnetism. For the yield strength, three KS-13A specimens are respectively made to perform a shortening tensile test, and then measured under a 0.2% offset condition. For the thickness deviation, the thickness at the center of the coiled sheet and the thickness at 15 mm from both sides of the width are measured, and the difference is divided by the center thickness to calculate.

[0131] For Gaussian fractions, the steel sheet before stress relief annealing can be measured by EBSD (electron backscatter diffraction) (offset by 15 degrees).

[0132] [Table 1]

[0133]

[0134] [Table 2]

[0135]

[0136]

[0137] [Table 3]

[0138]

[0139]

[0140] As shown in Tables 1 to 3, when the temperature and reduction ratio are appropriately adjusted during two hot rolling processes and the temperature is appropriately adjusted during cold rolled sheet annealing, the {110}<001> texture is less formed and the thickness deviation is moderate. In addition, excellent magnetic flux density and yield strength are obtained before SRA, and excellent magnetic flux density and iron loss are obtained after SRA.

[0141] On the other hand, if the annealing temperature of the cold rolled sheet is low, the magnetic flux density before SRA is poor. If the annealing temperature of the cold rolled sheet is high, the yield strength before SRA is poor. If the temperature during two hot rolling processes is low, the magnetic flux density before and after SRA is poor. If the temperature during two hot rolling processes is high, the thickness deviation increases. If the reduction ratio during two hot rolling processes is small, the magnetic properties before and after SRA are poor. If the reduction ratio during two hot rolling processes is large, the thickness deviation becomes large.

[0142] Example 2

[0143] A slab with the manufacturing components shown in Table 4 below is manufactured. C, S, N, Ti, Nb, V, etc. other than the components shown in Table 4 are respectively controlled below 0.003 wt%, and the balance is Fe.

[0144] The slab is heated to 1130 °C and the first hot rolling is carried out to 2.3 mm. Then, the coiled sheet after coiling is reheated to 730 °C by induction heating, and then the second hot rolling is carried out to 1.6 mm, and hot rolled sheet annealing is carried out at 1050 °C. After pickling the hot rolled annealed sheet, it is cold rolled to 0.25 mm, and after cold rolled sheet annealing for 1 minute in an atmosphere of 20 vol% hydrogen and 80 vol% nitrogen at 790 °C, heat treatment (stress relief annealing) is carried out for 1 hour in a nitrogen atmosphere at 800 °C, and the magnetic properties and yield strength before and after SRA are measured in the same way as in Example 1.

[0145] [Table 4]

[0146]

[0147] As shown in Table 4, when the contents of Si, Al, and Mn are appropriately adjusted, excellent magnetic flux density and yield strength are achieved before SRA, and excellent magnetic flux density and iron loss are achieved after SRA.

[0148] On the other hand, if the contents of Si, Al, and Mn are too low or too high, slab cracking occurs, or the magnetic properties are poor before / after SRA, or the yield strength is poor before SRA.

[0149] The present invention can be implemented in various different ways and is not limited to the above embodiments. Those of ordinary skill in the technical field to which the present invention pertains can understand that the present invention can be implemented in other specific ways without changing the technical idea and essential features of the present invention. Therefore, it should be understood that the above embodiments are exemplary in all aspects and not restrictive.

Claims

1. An non-oriented electrical steel sheet, wherein, by weight %, the non-oriented electrical steel sheet contains 1.5 to 4% of Si, 0.1 to 2% of Al, and 0.05 to 2% of Mn, and the balance contains Fe and inevitable impurities, the area fraction of grains having an orientation within 15° from {110}<001> is 10% or less.

2. The non-oriented electrical steel sheet according to claim 1, wherein, the non-oriented electrical steel sheet further contains one or more of Cr of 0.5 wt% or less and except 0%, Cu of 0.2 wt% or less and except 0%, P of 0.1 wt% or less and except 0%, Sn of 0.06 wt% or less and except 0%, and Sb of 0.06 wt% or less and except 0%.

3. The non-oriented electrical steel sheet according to claim 1, wherein, the non-oriented electrical steel sheet further contains one or more of C, N, S, Ti, Nb, and V of 0.005 wt% or less and except 0%.

4. The non-oriented electrical steel sheet according to claim 1, wherein, the average grain diameter is 10 to 25 μm.

5. The non-oriented electrical steel sheet according to claim 1, wherein, the non-oriented electrical steel sheet satisfies the following formula 1, [Formula 1] Yield strength (MPa) ≥ 140 + 100×[Si] + 35×([Al] + [Mn]) wherein, in Formula 1, [Si], [Al], and [Mn] each represent the contents (wt%) of Si, Al, and Mn, respectively.

6. The non-oriented electrical steel sheet according to claim 1, wherein, the non-oriented electrical steel sheet satisfies the following formula 2, [Formula 2] Circumferential direction magnetic flux density (B 50 , Tesla) ≥ 1.88 + 0.1×t - 0.067×[Si] - 0.0458×[Al] - 0.022×[Mn] wherein, in Formula 2, [Si], [Al], and [Mn] each represent the contents (wt%) of Si, Al, and Mn, respectively, and t represents the thickness (mm) of the steel sheet.

7. The non-oriented electrical steel sheet according to claim 1, wherein, the non-oriented electrical steel sheet satisfies the following formula 3 after stress relief annealing at a temperature of 700 to 850 °C for 10 to 300 minutes, [Formula 3] Circumferential direction magnetic flux density (B 50 , Tesla) ≥ 1.85 + 0.1 × t - 0.067 × [Si] - 0.0458 × [Al] - 0.022 × [Mn] Circumferential iron loss (W 10 / 400 , W / kg) ≤ 6 + 4000 × t 2 / (13 + 11 × ([Si] + [Al] + 0.5 × [Mn]) wherein, in Formula 3, [Si], [Al], and [Mn] each represent the contents (wt%) of Si, Al, and Mn, respectively, and t represents the thickness (mm) of the steel sheet.

8. A method for manufacturing a non-oriented electrical steel sheet, comprising: a first hot rolling step of hot rolling a slab to manufacture a first hot rolled sheet, wherein, by weight %, the slab contains 1.5 to 4% of Si, 0.1 to 2% of Al, and 0.05 to 2% of Mn, and the balance contains Fe and inevitable impurities; a step of coiling the first hot rolled sheet; a second hot rolling step of hot rolling the first hot rolled sheet at a temperature of 700 to 1000 °C with a reduction ratio of 20 to 50% to manufacture a second hot rolled sheet; a step of cold rolling the second hot rolled sheet to manufacture a cold rolled sheet; and a cold rolled sheet annealing step of annealing the cold rolled sheet at a temperature of 710 to 820 °C.

9. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein, the first hot rolling step includes a rough rolling step and a finish rolling step.

10. The method for manufacturing a non-oriented electrical steel sheet according to claim 8, wherein, The thickness of the first hot-rolled sheet is 1.8 to 2.5 mm.

11. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, After the coiling step, the coiled sheet is cooled to a temperature below 700 °C.

12. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, After the coiling step, it further includes a first hot-rolled sheet annealing step of annealing at a temperature of 700 to 1000 °C.

13. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, The thickness of the second hot-rolled sheet is 1.2 to 1.8 mm.

14. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, After the second hot-rolling step, it further includes a second hot-rolled sheet annealing step of annealing the second hot-rolled sheet at a temperature of 850 to 1150 °C.

15. The method for manufacturing an non-oriented electrical steel sheet according to claim 8, wherein, After the cold-rolled sheet annealing step, it further includes a step of stress relief annealing at a temperature of 700 to 850 °C for 10 to 300 minutes.

Images