Non-oriented electrical steel sheet and method of manufacturing same

Abstract

The non-oriented electrical steel sheet according to one embodiment of the present invention includes, by wt%, 2.0% to 5.0% of Si and 0.005% or less (excluding 0%) of C and the balance of Fe and inevitable impurities, wherein grains having a ratio (D2 / D1) of the diameter (D2) of an inscribed circle to the diameter (D1) of a circumscribed circle of 0.5 or greater with respect to the surface of the steel sheet account for 95 area% or greater of all grains, and the difference between the Si contents at depths of 20 μm and 100 μm, both from the surface of the steel sheet, is 1.0 wt% or greater.

Description

Non-oriented electrical steel sheet and manufacturing method thereof

[0001] One embodiment of the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same. Specifically, the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same, which includes a decarburization annealing process between multiple cold rolling processes and a cold rolling process, and which improves magnetism in the rolling direction by diffusing silicon (Si) from the outside to the inside.

[0002] Non-oriented electrical steel sheets are used as core materials in rotating machines such as motors and generators and stationary machines such as small transformers, and play an important role in determining the energy efficiency of electrical equipment.

[0003] Recently, due to strengthened motor efficiency regulations, the use of high-efficiency motors has increased significantly.

[0004] To improve the efficiency of these motors, either the iron loss or the copper loss must be reduced.

[0005] These iron and copper losses are significantly affected by the magnetic properties of the electrical steel used in the motor core. Therefore, motor manufacturers are increasingly using low-iron-loss electrical steel instead of the traditional high-iron-loss electrical steel.

[0006] In order to reduce copper loss, a method is used to lower the design magnetic flux density than before or to lower the excitation current at the design magnetic flux. However, in order to use the latter method, it is necessary to improve the magnetic flux density of the electrical steel sheet.

[0007] High-flux-density electrical steel, in particular, offers the advantage of improved torque, enabling motors with frequent on / off cycles to generate high outputs quickly. Manufacturing high-flux-density electrical steel requires a reduced silicon content, but this low content, a resistivity-increasing element, leads to high core loss. Therefore, the development of electrical steel with both low core loss and high flux density is necessary.

[0008] Structurally, torque in a typical radial motor is proportional to the square of the rotor diameter times the length of the cylindrical stator. Therefore, to increase torque, the length of the motor's cylindrical stator must be increased, which increases the overall motor volume.

[0009] Meanwhile, unlike the conventional radial motor structure, the axial flux motor has a stator and rotor arranged axially parallel, so that magnetic force is transmitted to the rotor in the axial direction through the stator, generating torque. The magnitude of the torque is proportional to the cube of the cylindrical rotor diameter. In other words, it can be said to be a structure that can generate high torque even in a small space. Since the magnetic force generated by the current applied to the stator is transmitted to the rotor magnet in the axial direction, the application of a magnetic material with particularly excellent magnetic properties in one direction is required.

[0010] The normal oriented electrical steel sheet has a crystal orientation of {110}. <001> It is a soft magnetic material with excellent magnetic properties in the rolling direction, composed of crystal grains with the so-called Goss orientation. This grain-oriented electrical steel sheet is manufactured by rolling the final thickness through slab heating, hot rolling, hot-rolled sheet annealing, and cold rolling, and then going through primary recrystallization annealing and high-temperature annealing for secondary recrystallization formation.

[0011] Since MgO is coated on the surface after decarburization annealing and a metal oxide layer (also called a base coating layer) mainly composed of Mg2SiO4 is formed during high-temperature annealing, unlike non-oriented electrical steel sheets used in conventional motor cores, there is a problem that the mold is damaged during the punching process due to the metal oxide layer on the surface.

[0012] In addition, since the conventional oriented electrical steel sheet has a high fraction of Goss crystal grains, it has excellent one-way characteristics when applied to an axial flux motor, but since the strength of the magnetic force lines in the direction perpendicular to the rolling direction is weak, there is a problem in that the efficiency of the motor is reduced when used for an axial flux motor.

[0013] One embodiment of the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same. Specifically, the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same, which includes a decarburization annealing process between multiple cold rolling processes and a cold rolling process, and in which silicon (Si) is diffused from the inside to the outside, thereby improving magnetism in the rolling direction.

[0014] A non-oriented electrical steel sheet according to one embodiment of the present invention contains, in wt%, Si: 2.0% to 8.0%, C: 0.005% or less (excluding 0%), and the remainder being Fe and unavoidable impurities, and includes grains having a ratio (D2 / D1) of a diameter of a circumscribed circle (D1) to a diameter of an inscribed circle (D2) of 0.5 or more with respect to the surface of the steel sheet, in an area of ​​95% or more of the total grains, and a difference between the Si content at a depth of 20 μm from the surface of the steel sheet and the Si content at a depth of 100 μm is 1.0 wt% or more.

[0015] The proportion of crystal grains having a crystal grain size of 30 ㎛ to 500 ㎛ may be 80% or more.

[0016] A non-oriented electrical steel sheet according to one embodiment of the present invention may further include Mn: 0.1 wt% or less and S: 0.005 wt% or less.

[0017] A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention comprises the steps of: hot-rolling a slab containing, in wt%, Si: 0.3% to 2.5%, C: 0.05% to 0.4%, and the remainder being Fe and unavoidable impurities, to manufacture a hot-rolled steel sheet; a step of first cold-rolling the hot-rolled steel sheet; a step of decarburization annealing the first cold-rolled steel sheet; a step of second cold-rolling the steel sheet on which decarburization annealing is completed; a cold-rolled sheet annealing step of annealing the steel sheet on which the second cold-rolling is completed; a step of coating the annealed steel sheet with a composition containing Fe-Si powder to form a Si coating layer; and a diffusion annealing step of annealing the steel sheet on which the Si coating layer is formed for 8 to 45 minutes.

[0018] The slab may further contain Mn: up to 0.1 wt% and S: up to 0.005 wt%.

[0019] The step of annealing the hot-rolled steel sheet may further include a step of annealing the hot-rolled steel sheet, and the step of annealing the hot-rolled steel sheet may include a decarburization process.

[0020] The hot-rolled plate annealing step can be performed at a temperature of 850℃ to 1000℃ and a dew point temperature of 70℃ or lower.

[0021] The decarburization annealing step can be performed at a temperature of 750°C to 1000°C and a dew point temperature of 25°C to 70°C.

[0022] The decarburization annealing step may include a first decarburization annealing step annealing at a dew point temperature of 25°C to 70°C and a second decarburization annealing step annealing at a dew point temperature of -10°C to 20°C.

[0023] The decarburization annealing step can be performed in the austenite single-phase region or in the region where a composite phase of ferrite and austenite exists.

[0024] After the decarburization annealing step, the carbon content in the steel sheet may be 0.005 wt% or less.

[0025] The decarburization annealing step and the secondary cold rolling step can be repeated two or more times.

[0026] The first cold rolling step and the second cold rolling step may each have a reduction ratio of 50 to 70%.

[0027] The cold-rolled sheet annealing step may include a first cold-rolled sheet annealing step in which annealing is performed at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower, and a second cold-rolled sheet annealing step in which annealing is performed at a temperature of 1000°C to 1200°C and in an atmosphere containing H250 volume% or more.

[0028] After the cold-rolled sheet annealing step, a pickling step may be further included using an acid aqueous solution of 5 to 50 wt% at a temperature of 50 to 100°C for 20 to 100 seconds.

[0029] In the step of forming the Si coating layer, the composition may further include MgO.

[0030] In the step of forming the Si coating layer, the Fe-Si powder may contain 20 to 80 wt% of Si.

[0031] In the diffusion annealing step, the soaking temperature can be 1050 to 1200°C.

[0032] A non-oriented electrical steel sheet according to one embodiment of the present invention has excellent magnetism in all directions and at the same time excellent magnetism in the rolling direction, and thus can be particularly usefully used for an axial flux motor.

[0033] Additionally, since it can be manufactured through a continuous process, the manufacturing time can be relatively shortened and productivity can be improved.

[0034] The terms first, second, and third, etc., are used to describe, but are not limited to, various parts, components, regions, layers, and / or sections. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0035] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include plural forms as well, unless the context clearly dictates otherwise. The word "comprising," as used herein, specifies particular features, regions, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0036] When a part is referred to as being "on" or "over" another part, it can be directly on or over the other part, or there may be other parts intervening. Conversely, when a part is referred to as being "directly on" another part, there are no other parts intervening.

[0037] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0038] Also, unless otherwise stated, % means weight%, and 1 ppm is 0.0001 weight%.

[0039] In one embodiment of the present invention, the inclusion of additional elements means including the remaining iron (Fe) in an amount equivalent to the additional amount of the additional elements.

[0040] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0041]

[0042] A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention comprises the steps of: hot-rolling a slab containing, by weight %, Si: 0.3% to 2.5%, C: 0.05% to 0.4%, and the remainder being Fe and unavoidable impurities, to manufacture a hot-rolled steel sheet; a step of first cold-rolling the hot-rolled steel sheet; a step of decarburization annealing the first cold-rolled steel sheet; a step of second cold-rolling the steel sheet on which decarburization annealing is completed; a cold-rolled sheet annealing step of annealing the steel sheet on which the second cold-rolling is completed; a step of coating the annealed steel sheet with a composition containing Fe-Si powder to form a Si coating layer; and a diffusion annealing step of annealing the steel sheet on which the Si coating layer is formed for 8 to 45 minutes.

[0043] Below, each step is explained in detail.

[0044] First, the slab is hot rolled.

[0045] The slab may contain, by weight %, Si: 0.3% to 2.5%, C: 0.05% to 0.4%, and the remainder being Fe and unavoidable impurities.

[0046] The reasons for limiting the composition are as follows.

[0047] Si lowers the magnetic anisotropy of electrical steel sheets and increases resistivity, thereby improving core loss. If the Si content is too low, the core loss becomes poor, and if it is too high, brittleness increases. Therefore, the Si content in the non-oriented electrical steel sheet after the slab and cold-rolled sheet annealing steps may be 0.3% to 2.5 wt%. More specifically, the Si content may be 0.5 to 2.0 wt%. Even more specifically, the Si content may be 1.0 to 1.8 wt%.

[0048] Carbon (C) content in the slab may be 0.1 to 0.4 wt% because the Goss crystal grains in the surface layer need to diffuse to the center during decarburization annealing, and the C in the center needs to escape to the surface layer. More specifically, the C content in the slab may be 0.15 to 0.3 wt%. In addition, the carbon content in the final non-oriented electrical steel sheet manufactured after decarburization is completed may be 0.0050 wt% or less. More specifically, it may be 0.003 wt% or less.

[0049] The slab may further contain Mn: up to 0.1 wt% and S: up to 0.005 wt%.

[0050] Mn and S form MnS precipitates, which inhibit the growth of Goss grains that diffuse toward the center during the decarburization process. Therefore, it is preferable not to add Mn or S. However, considering the amount that is unavoidably mixed during the steelmaking process, the Mn and S contents in non-oriented electrical steel sheets after the slab and final annealing stages can be controlled to Mn: 0.1 wt% or less and S: 0.005 wt% or less, respectively.

[0051] The remainder includes Fe and unavoidable impurities. Unavoidable impurities are impurities mixed in during the steelmaking step and the manufacturing process of non-oriented electrical steel sheets, and since this is widely known in the field, a detailed description is omitted. Specifically, components such as Al, N, Ti, Mg, and Ca react with oxygen in steel to form oxides, and therefore, strong suppression is required, and therefore, each component can be managed to 0.005 wt% or less. In one embodiment of the present invention, the addition of elements other than the alloy components described above is not excluded, and various elements may be included within a range that does not impair the technical spirit of the present invention. When additional elements are further included, they are included in place of the remainder, Fe.

[0052] The slab can be heated before hot rolling. The slab heating temperature can be 1050℃ to 1350℃, which is higher than the usual heating temperature. When the temperature is high during slab reheating, there is a problem that the hot-rolled structure becomes coarser, which adversely affects magnetism. However, the method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention has a higher carbon content than in the past, so that even if the slab reheating temperature is high, the hot-rolled structure does not become coarser, and by reheating at a higher temperature than in the usual case, it is advantageous during hot rolling. However, when the reheating temperature is too high, the solid solubility of precipitates such as TiN and AlN increases, so that a large number of fine precipitates are distributed during cooling, which acts to hinder the growth of crystal grains on the surface.

[0053] Hot rolling can be used to manufacture hot rolled sheets with a thickness of 1.5 to 4.0 mm by applying an appropriate rolling ratio in the final cold rolling stage so that the final product thickness can be manufactured.

[0054] There are no special restrictions on the hot rolling temperature or cooling temperature, but for example, if the magnetism is excellent, the hot rolling end temperature can be set to 950℃ or lower, and the cooling can be done rapidly with water so that the coiling can be done at 600℃ or lower.

[0055] After hot rolling, a step of hot-rolled sheet annealing may be further included. At this time, the hot-rolled sheet annealing may include a decarburization process. Specifically, the hot-rolled sheet annealing may be performed at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower. More specifically, the annealing may be performed at a dew point temperature of -70°C to 70°C. After the annealing described above, additional annealing may be performed at a temperature of 1000°C to 1200°C and a dew point temperature of 0°C or lower. After the hot-rolled sheet annealing, pickling may be performed.

[0056] Next, primary cold rolling is performed to manufacture cold rolled steel sheets.

[0057] In the manufacturing process of conventional grain-oriented electrical steel sheets, it is known to be effective to perform cold rolling once at a high pressure reduction rate close to 90%. This is because it creates an environment favorable for grain growth of only Goss grains among the primary recrystallized grains. However, the manufacturing method of a non-oriented electrical steel sheet according to an embodiment of the present invention does not utilize the abnormal grain growth of Goss grains, but internally diffuses Goss grains in the surface layer generated by decarburization annealing and cold rolling, so it is advantageous to form a large number of Goss grains distributed in the surface layer.

[0058] Therefore, when cold rolling is performed at a reduction ratio of 50% to 70%, Goss texture may be formed in large numbers in the surface layer. More specifically, it may be 55% to 65%.

[0059] Next, the first cold-rolled steel sheet is decarburized and annealed. The decarburization annealing step can be performed in the austenite single-phase region or in a region where a composite phase of ferrite and austenite exists. Specifically, the annealing can be performed at a temperature of 750°C to 1000°C and a dew point temperature of 25°C to 70°C. Furthermore, the atmosphere can be a mixed gas atmosphere of hydrogen and nitrogen. Furthermore, after decarburization annealing, the carbon content in the steel sheet can be 0.003 wt% or less.

[0060] During this decarburization annealing process, the grain size on the surface of the electrical steel sheet grows coarsely, but the grains inside the electrical steel sheet remain fine-grained. The average grain diameter after this decarburization annealing can be 150 to 250 μm. In this case, the grains are surface ferrite grains. In addition, the grain diameter refers to the diameter of an imaginary circle having the same area as the grains. The reference plane is a plane parallel to the rolling vertical plane (TD plane).

[0061] The decarburization annealing step may include a first decarburization annealing step annealing at a dew point temperature of 25°C to 70°C and a second decarburization annealing step annealing at a dew point temperature of -10°C to 20°C.

[0062] Next, the steel sheet, which has undergone decarburization annealing, undergoes a second cold rolling process. The second cold rolling process is identical to the first cold rolling process, so a detailed description will be omitted.

[0063] Next, the steel sheet that has undergone secondary cold rolling is annealed into a cold rolled sheet.

[0064] In a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention, cold-rolled sheet annealing can be performed continuously following cold rolling.

[0065] In a method for manufacturing a grain-oriented electrical steel sheet according to one embodiment of the present invention, the final annealing may include a first cold-rolled sheet annealing step of annealing at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower, and a second cold-rolled sheet annealing step of annealing at a temperature of 1000°C to 1200°C and an atmosphere containing 50% by volume or more of H2. In addition, the atmosphere of the second cold-rolled sheet annealing step may contain 90% by volume or more of H2 and a dew point of -25°C or lower.

[0066] Before cold-rolled sheet annealing, the cold-rolled sheet undergoes decarburization annealing, so that the carbon content of the steel remains at least 40 to 60 wt% compared to the carbon content of the slab. Therefore, during the final annealing, in the first cold-rolled sheet annealing stage, carbon is released, and the crystal grains formed in the surface layer diffuse internally. In the first cold-rolled sheet annealing stage, decarburization can be performed so that the carbon content in the steel sheet is 0.005 wt% or less.

[0067] After this, in the second cold-rolled sheet annealing step, a grain structure with the Goss orientation that was diffused in the first cold-rolled sheet annealing step grows. In the method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention, the grain size of the Goss grain structure can be within 1 mm, unlike in the case where the grains grow by conventional abnormal grain growth. Therefore, compared to the conventional non-oriented electrical steel sheet, a grain structure in which a large number of Goss grains are smaller in size can be obtained.

[0068] The steel plate manufactured in this way can contain grains having a ratio (D2 / D1) of the diameter of the circumscribed circle (D1) to the diameter of the inscribed circle (D2) of 0.5 or more, accounting for 95% or more of the total grains. The crystal structure of the steel plate will be described in detail with respect to the non-oriented electrical steel plate described below.

[0069] Next, a pickling step may be added to the steel sheet after cold-rolled annealing. The pickling process removes the oxide layer that naturally forms on the surface of the steel sheet. Removing the oxide layer facilitates the diffusion of silicon (Si).

[0070] The pickling step can utilize an acid solution of 5 to 50 wt%. The acid solution can include an inorganic acid, such as hydrochloric acid, nitric acid, or sulfuric acid. If the acid solution concentration is too low, proper pickling may not occur. Furthermore, if the acid solution concentration is too high, the surface roughness of the steel sheet may increase excessively, adversely affecting magnetism.

[0071] The pickling step can be performed at temperatures between 50 and 100°C. If the temperature is too low, the pickling process may not proceed properly. If the temperature is too high, re-oxidation may occur.

[0072] The pickling step can last from 20 to 100 seconds. If the time is too short, the oxide layer may not be sufficiently removed. If the time is too long, the magnetism may actually deteriorate due to the non-uniformity of the pickling effect within the grains and between grain diameters. More specifically, the pickling step can last from 25 to 50 seconds.

[0073] Next, a composition including Fe-Si powder is coated on a cold-rolled steel sheet that has undergone annealing to form a Si coating layer.

[0074] A composition including Fe-Si powder may be in the form of a slurry including a solvent for easy dispersion and surface application of the Fe-Si powder. The solvent is not particularly limited, but may include water or alcohol.

[0075] Fe-Si powder contains an alloy component including Fe and Si, wherein Si may be contained in an amount of 20 to 80 wt%. Fe-Si powder can be manufactured by mixing Fe powder and Si powder, mixing them well with a rolling mixer, placing a certain amount in a crucible, and then performing high-temperature firing in a furnace capable of controlling the atmosphere gas.

[0076] The composition may further include MgO in addition to the Fe-Si powder. MgO not only acts as an annealing separator to prevent high-temperature plate adhesion between the steel sheets during Si diffusion into the steel sheet, but also functions as a binder to ensure that the Fe-Si powder adheres well to the steel sheet. When adding MgO, it may be added in an amount of 60 to 200 parts by weight per 100 parts by weight of Fe-Si.

[0077] As described above, in one embodiment of the present invention, the cold-rolled sheet annealing process can be operated as a continuous annealing process, and the first cold rolling step and the Si diffusion annealing step can be performed continuously.

[0078] Next, the steel sheet with the Si coating layer formed is diffusion-annealed for 8 to 45 minutes. The annealing process can be performed at a temperature of 1050 to 1200°C. The annealing time can be 8 to 45 minutes. If the annealing temperature is too low or the annealing time is too short, smooth Si diffusion may not occur. Conversely, if the annealing time is too long, the Si concentration gradient may not be properly formed in the direction of the steel sheet thickness. If the Si concentration gradient does not exist properly, the Si content of the entire steel sheet increases, and the content of the magnetic material, Fe, decreases, resulting in a decrease in the saturation magnetic flux density. This causes the magnetic flux density to decrease even when a magnetic field strength of 5000 A / m is applied, and the iron loss measured at a specific magnetization value can also increase as the magnetic field strength increases to obtain a constant magnetization value. More specifically, the diffusion-annealing time can be 10 to 30 minutes.

[0079]

[0080] A non-oriented electrical steel sheet according to one embodiment of the present invention contains, in wt%, Si: 2.0% to 8.0%, C: 0.005% or less (excluding 0%), and the remainder being Fe and unavoidable impurities, and includes grains having a ratio (D2 / D1) of a diameter of a circumscribed circle (D1) to a diameter of an inscribed circle (D2) of 0.5 or more with respect to the surface of the steel sheet, in an area of ​​95% or more of the total grains, and a difference between the Si content at a depth of 20 μm from the surface of the steel sheet and the Si content at a depth of 100 μm is 1.0 wt% or more.

[0081] Since the alloy composition has been described in the method for manufacturing the non-oriented electrical steel sheet mentioned above, a duplicate description will be omitted. In one embodiment of the present invention, a process for diffusing Si from the outside of the steel sheet is included, so that the Si content of the final manufactured steel sheet is higher than that of the slab. As described below, the non-oriented electrical steel sheet according to one embodiment of the present invention has a Si concentration gradient depending on the thickness of the steel sheet, and unless specifically indicated, the Si content means the average Si content for the entire thickness of the steel sheet.

[0082] The surface of the steel plate contains grains having a ratio (D2 / D1) of the diameter of the circumscribed circle (D1) to the diameter of the inscribed circle (D2) of 0.5 or more, accounting for more than 95% of the total grains. Here, the circumscribed circle refers to the smallest imaginary circle surrounding the outside of the grain, and the inscribed circle refers to the largest imaginary circle contained within the grain.

[0083] The grain structure of the non-oriented electrical steel sheet according to one embodiment of the present invention is such that the crystal grains on the surface grow into the interior of the steel sheet, thereby producing round-shaped crystal grains. Thus, due to the grain structure shape according to one embodiment of the present invention, even superior magnetism can be achieved.

[0084] According to one embodiment of the present invention, the grain size of the crystal grains of the substrate may be 30 µm to 500 µm, which may account for 80% or more of the total grain size. In this way, the texture of the grain structure of one embodiment of the present invention exists in a long shape in the thickness direction compared to a typical non-oriented electrical steel sheet, so that even if diffusion annealing is performed for a short time, Si can be smoothly diffused. As a result, the iron loss of the non-oriented electrical steel sheet is further improved.

[0085] In addition, in one embodiment of the present invention, the magnetism can be further improved through the Si concentration gradient between the surface and the interior. Specifically, the Si content (Si) at a depth of 20 μm from the steel plate surface 20㎛ ) and Si content at 100 μm depth (Si 100㎛ ) difference (Si20㎛ - Si 100㎛ ) is 1.00 wt% or more. More specifically, it may be 1.10 to 2.50%.

[0086] Si content at a depth of 20 μm from the steel plate surface (Si 20㎛ ) is 3.00 to 5.70 wt%, and the Si content at a depth of 100 μm (Si 100㎛ ) can be 2.20 to 3.50 wt%.

[0087] In one embodiment of the present invention, the non-oriented electrical steel sheet has excellent magnetism. Specifically, the average magnetic flux density (B) in the rolling direction and the direction perpendicular to the rolling 50 ) can be 1.90T or more. In addition, the average iron loss in the rolling direction and the direction perpendicular to the rolling (W 10 / 400 ) may be less than or equal to 10.0 W / kg. More specifically, the magnetic flux density in the rolling direction (B 50 ) can be 1.95 to 2.05 T. Iron loss in the rolling direction (W 10 / 400 ) can be 6.5 to 9.9 W / kg.

[0088] Magnetic flux density (B 50 ) means the magnetic flux density induced in a magnetic field of 5000 A / m. Iron loss (W 15 / 50 ) is the magnitude of the iron loss (W / kg) induced under 1.0 Tesla and 400 Hz conditions.

[0089] Specific examples of the present invention are described below. However, the following examples are merely specific examples of the present invention, and the present invention is not limited to the following examples.

[0090]

[0091] Example 1

[0092] A slab containing Si: 1.4%, C: 0.09% by weight and the remainder Fe and unavoidable impurities was heated at a temperature of 1250°C, hot-rolled to a thickness of 1.6 mm, and then hot-rolled at an annealing temperature of 900°C, cooled, pickled, and subjected to a first cold rolling at a reduction ratio of 60% (0.6 mmt).

[0093] The cold-rolled plate was subjected to a first decarburization annealing at an annealing temperature of 870°C and a dew point temperature of 60°C for 120 seconds, then a second decarburization annealing was performed in a hydrogen atmosphere at a dew point temperature of 0°C, and after cooling, a second cold rolling was performed at a reduction ratio of 60% (0.25 mmt).

[0094] Afterwards, during the annealing of the cold-rolled sheet, decarburization annealing was performed for 60 seconds in a wet mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60°C) at a temperature of 900°C, and then heat treatment was performed for 3 minutes in a 100% H2 atmosphere at 1050°C.

[0095] After cold-rolled sheet annealing, the steel sheet was pickled for 20 seconds using a 25 wt% HCl aqueous solution at 80°C. After the pickling process, the steel sheet was coated with a mixture of 40.5% Si content Fe-Si powder, calcined MgO powder, and ethyl alcohol, and then annealed at 1100°C for the times listed in Table 2 below. The magnetic flux density and iron loss were obtained, which are shown in Table 1.

[0096] The Si content by thickness was measured using GDS.

[0097] The magnetic flux density (B50) and iron loss (W10 / 400) were measured using the SST (single sheet test) method, and the rolling direction and the direction perpendicular to the rolling were measured, and the average values ​​are shown in Table 1.

[0098]

[0099] Heat treatment time (min)Si content (weight%)D2 / D1 0.5 or more crystal grain ratio (area%)B50(Tesla)W10 / 400(W / Kg)RemarksSurface 30um center 100um02.032.03962.0511.0Comparative material52.432.10962.0310.7Comparative material103.432.22962.029.7Inventive material154.222.56962.019.1Inventive material204.652.66961.998.4Inventive material255.152.78961.977.5Inventive material305.663.13961.966.9Inventive material605.784.87961.887.8Comparative material1206.155.78961.858.2Comparative material

[0100] As shown in Table 1, it can be confirmed that the iron loss is improved through Si diffusion. However, as the diffusion time increases, the Si concentration gradient by thickness decreases, which results in a deterioration of the magnetic flux density and iron loss.

[0101]

[0102] The present invention is not limited to the above-described embodiments and / or examples, but can be manufactured in various different forms. Those skilled in the art will understand that the present invention can be implemented in other specific forms without altering the technical spirit or essential characteristics of the present invention. Therefore, the above-described embodiments and / or examples should be understood as illustrative in all respects and not restrictive.

Claims

In weight %, Si: 2.0% to 5.0%, C: 0.005% or less (excluding 0%), and the remainder is Fe and inevitable impurities. The crystal grains having a ratio (D2 / D1) of the diameter of the circumscribed circle (D1) and the diameter of the inscribed circle (D2) of 0.5 or more on the surface of the steel plate comprise at least 95% of the total crystal grains, A non-oriented electrical steel sheet having a difference of 1.0 wt% or more between the Si content at a depth of 20 ㎛ from the steel sheet surface and the Si content at a depth of 100 ㎛. In the first paragraph, A method for manufacturing a non-oriented electrical steel sheet having a grain size of 30 ㎛ to 500 ㎛ in a ratio of 80 area% or more. In the second paragraph, A method for manufacturing a non-oriented electrical steel sheet further comprising Mn of 0.1 wt% or less and S of 0.005 wt% or less. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, by weight%, Si: 0.3% to 2.5%, C: 0.05% to 0.4%, and the remainder being Fe and unavoidable impurities; A step of first cold rolling the above hot-rolled steel plate; A step of decarburization annealing the first cold rolled steel sheet; A step of performing secondary cold rolling on a steel plate that has undergone decarburization annealing; Cold rolled sheet annealing step for annealing steel sheets that have undergone secondary cold rolling; A step of forming a Si coating layer by coating a composition including Fe-Si powder on a steel plate on which annealing is completed; and A method for manufacturing a non-oriented electrical steel sheet, comprising a diffusion annealing step of annealing a steel sheet having a Si coating layer formed thereon for 8 to 45 minutes. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein the above slab further contains Mn: 0.1 wt% or less and S: 0.005 wt% or less. In paragraph 4, Further comprising a step of annealing the hot rolled steel sheet, A method for manufacturing a non-oriented electrical steel sheet including a decarburization process in the step of annealing the hot-rolled sheet. In Article 6, A method for manufacturing a non-oriented electrical steel sheet, wherein the step of annealing the hot-rolled sheet is performed at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein the above decarburization annealing step is performed at a temperature of 750°C to 1000°C and a dew point temperature of 25°C to 70°C. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet in which the above decarburization annealing step is performed in an austenite single-phase region or a region in which a composite phase of ferrite and austenite exists. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet having a carbon content of 0.005 wt% or less in the steel sheet after the above decarburization annealing step. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein the above-mentioned decarburization annealing step and the above-mentioned secondary cold rolling step are repeated two or more times. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein the first cold rolling step and the second cold rolling step each have a reduction ratio of 50 to 70%. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein the cold-rolled sheet annealing step includes a first cold-rolled sheet annealing step in which annealing is performed at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower, and a second cold-rolled sheet annealing step in which annealing is performed at a temperature of 1000°C to 1200°C and in an atmosphere containing H250 volume% or more. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, further comprising, after the cold-rolled sheet annealing step, a step of pickling at a temperature of 50 to 100°C for 20 to 100 seconds using an acid aqueous solution of 5 to 50 wt%. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein in the step of forming the Si coating layer, the composition further contains MgO. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein in the step of forming the Si coating layer, the Fe-Si powder contains 20 to 80 wt% of Si. In paragraph 4, A method for manufacturing a non-oriented electrical steel sheet, wherein in the above diffusion annealing step, the cracking temperature is 1050 to 1200°C.