Non-oriented electrical steel sheet and method of manufacturing the same

Abstract

A method for manufacturing non-oriented electrical steel sheet according to an embodiment of the present invention includes: hot rolling a slab to manufacture a hot-rolled steel sheet; hot-rolling annealing the hot-rolled steel sheet; cold rolling the annealed hot-rolled steel sheet once; decarburizing annealing the cold-rolled steel sheet once; cold rolling the decarburized annealed steel sheet a second time; and non-oxidizing annealing the cold-rolled steel sheet in a non-oxidizing atmosphere.

Description

Technical Field

[0001] One embodiment of the present invention relates to non-oriented electrical steel and a method for manufacturing the same, specifically to a method for manufacturing non-oriented electrical steel and a method thereof that improves the magnetic properties of the rolling direction by including multiple cold rolling and decarburization annealing processes between cold rolling. Background Technology

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

[0003] In recent years, with the strengthening of motor efficiency regulations, the use of high-efficiency motors has increased significantly.

[0004] To improve the efficiency of this type of motor, it is necessary to reduce iron losses or copper losses.

[0005] Iron loss and copper loss are largely influenced by the magnetic properties of the electrical steel sheet used as the core material of motors. Therefore, motor manufacturers tend to use electrical steel sheets with lower iron loss instead of existing ones with higher iron loss.

[0006] To reduce copper losses, methods such as reducing the design magnetic flux density or reducing the excitation current under the design magnetic flux are adopted. However, to adopt the latter method, it is necessary to increase the magnetic flux density of the electrical steel plate.

[0007] In particular, electrical steel sheets with high magnetic flux density have the advantage of increasing torque, thus enabling motors with frequent on / off cycles to output high power in a short time. To manufacture electrical steel sheets with high magnetic flux density, the silicon (Si) content needs to be reduced; however, as Si is an element that increases resistivity, reducing its content leads to increased iron losses. Therefore, it is necessary to develop electrical steel sheets with both low iron losses and high magnetic flux density characteristics.

[0008] From a structural perspective, in a conventional radial motor, the torque is directly proportional to the product of the square of the rotor diameter and the length of the cylindrical stator. Therefore, to increase the torque, the length of the cylindrical stator needs to be increased, which leads to an increase in the overall size of the motor.

[0009] On the other hand, axial flux motors differ structurally from traditional radial flux motors. In axial flux motors, the stator and rotor are arranged parallel along the axial direction. Magnetic force is transmitted axially from the stator to the rotor, generating torque. The magnitude of the torque is proportional to the cube of the diameter of the cylindrical rotor. In other words, this structure can output high torque even in confined spaces. Because the magnetic force generated by the current applied to the stator is transmitted axially to the rotor magnets, magnetic materials with exceptionally excellent unidirectional magnetization characteristics are required.

[0010] Ordinary grain-oriented electrical steel sheet is a soft magnetic material with a so-called Goss orientation of {110}<001} grains, exhibiting excellent magnetic properties in the rolling direction. For this type of grain-oriented electrical steel sheet, the slab is heated and then hot-rolled, hot-rolled and annealed, and cold-rolled to the final thickness. It is then subjected to a first recrystallization anneal and a high-temperature annealing process to induce secondary recrystallization.

[0011] After decarburization annealing, MgO is coated on the surface, and a metal oxide layer (or base coating) with Mg2SiO4 as the main component is formed during high-temperature annealing. Therefore, unlike non-oriented electrical steel sheets commonly used for motor cores, the surface metal oxide layer causes problems with die damage during the punching process.

[0012] In addition, ordinary grain-oriented electrical steel sheets contain a large number of Gaussian grains, thus exhibiting excellent magnetic properties in the rolling direction, but poor magnetic properties in the rolling perpendicular direction. When used in axial flux motors, this results in a decrease in motor efficiency. Summary of the Invention

[0013] (a) Technical problems to be solved 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, wherein the magnetic properties in the rolling direction are improved by including multiple cold rolling processes and a decarburization annealing process between cold rolling.

[0014] (II) Technical Solution A method for manufacturing non-oriented electrical steel sheet according to an embodiment of the present invention includes: hot rolling a slab to manufacture a hot-rolled steel sheet; performing a first cold rolling on the hot-rolled steel sheet; performing decarburization annealing on the first cold-rolled steel sheet; performing a second cold rolling on the decarburized annealed steel sheet; and performing a non-oxidizing annealing on the second cold-rolled steel sheet in a non-oxidizing atmosphere.

[0015] By weight percent, the slab may contain 0.3% to 4.0% Si, 0.03% to 0.4% C, and the balance Fe and unavoidable impurities.

[0016] The slab may also contain less than 0.1% by weight of Mn and less than 0.005% by weight of S.

[0017] The manufacturing method may further include a hot-rolled steel plate annealing step, wherein the hot-rolled steel plate annealing step includes a decarburization process.

[0018] The annealing process for hot-rolled steel sheets can be carried out at temperatures ranging from 850°C to 1000°C and at dew point temperatures below 70°C.

[0019] The decarburization annealing step can be performed at temperatures ranging from 750°C to 1000°C and dew point temperatures ranging from 25°C to 70°C.

[0020] The decarburization annealing step can be performed in the austenitic single-phase region or in the region where a ferrite and austenitic composite phase exists.

[0021] After the decarburizing annealing step, the carbon content in the steel plate can be less than 0.005% by weight.

[0022] The decarburization annealing step and the secondary cold rolling step can be repeated more than twice.

[0023] For a single cold rolling step, the reduction rate can be 70% to 80%.

[0024] The non-oxidative annealing step can be performed at a homogenization temperature of 750 to 1050°C for 60 seconds to 5 minutes.

[0025] The non-oxidative annealing step can be performed in an atmosphere with a dew point temperature below -20°C.

[0026] According to an embodiment of the present invention, a non-oriented electrical steel sheet, wherein, with {110} <001> The area fraction of grains at angles less than 15° can be 20% to 60%, compared to {100} <001> The area fraction of grains with an angle of less than 15° can be 5% to 20%.

[0027] Among all grains, grains with a ratio of circumscribed circle diameter (D1) to inscribed circle diameter (D2) of 0.5 or higher (D2 / D1) can account for more than 95% of the area.

[0028] Of all the grains, the fraction of grains with a grain size between 30 μm and 200 μm can be more than 80% of the area.

[0029] Electrical steel sheets may contain 0.3% to 4.0% Si, less than 0.005% and excluding 0% C, and the balance Fe and unavoidable impurities, by weight%.

[0030] Electrical steel sheets may also contain less than 0.1% by weight of Mn and less than 0.005% by weight of S.

[0031] (III) Beneficial Effects According to one embodiment of the present invention, the non-oriented electrical steel sheet has excellent magnetism in all directions, and is particularly suitable for axial flux motors due to its excellent magnetism in the rolling direction.

[0032] Furthermore, manufacturing time can be relatively shortened due to the continuous manufacturing process, which can improve productivity. Attached Figure Description

[0033] Figure 1 This is an image of the surface of a non-oriented electrical steel sheet made of No. 2 steel, analyzed using EBSD.

[0034] Figure 2 This is an image of the surface of a non-oriented electrical steel sheet made of No. 2 steel, analyzed using EBSD. Detailed Implementation

[0035] The terms "first," "second," "third," etc., are used to describe 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. Therefore, without departing from the scope of the invention, the first part, component, region, layer, or segment described below can also be described as a second part, component, region, layer, or segment.

[0036] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, the singular forms used herein are intended to include the plural forms as well. The word "comprising" as used in the specification can specifically refer to a particular feature, domain, integer, step, action, element, and / or component, but does not exclude the presence or addition of other features, domains, integers, steps, actions, elements, components, and / or groups.

[0037] When one part is described as being on top of another part, there can be other parts directly on top of the other part or in between. When one part is described as being directly on top of another part, there are no other parts in between.

[0038] Although not otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries should be interpreted as having the same meaning as disclosed in relevant technical literature and herein, and should not be interpreted in an idealized or overly formal sense.

[0039] In addition, unless otherwise specified, % means weight, 1 ppm is 0.0001 weight.

[0040] In one embodiment of the present invention, the inclusion of additional elements refers to the replacement of a portion of the remaining iron (Fe) by additional elements, the replacement amount being equivalent to the amount of additional elements added.

[0041] Embodiments of the present invention will be described in detail below to enable those skilled in the art to implement the invention. However, the invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0042] A method for manufacturing non-oriented electrical steel sheet according to an embodiment of the present invention includes: hot rolling a slab to manufacture a hot-rolled steel sheet; hot-rolling annealing the hot-rolled steel sheet; cold rolling the annealed hot-rolled steel sheet once; decarburizing annealing the cold-rolled steel sheet once; cold rolling the decarburized annealed steel sheet a second time; and non-oxidizing annealing the cold-rolled steel sheet in a non-oxidizing atmosphere.

[0043] The following sections will describe each step in detail.

[0044] First, the slab is hot-rolled.

[0045] By weight percent, the slab may contain 0.3% to 4.0% Si, 0.03% to 0.4% C, and the balance Fe and unavoidable impurities.

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

[0047] Si improves iron loss by reducing the magnetic anisotropy and increasing the resistivity of electrical steel sheets. When the Si content is below 0.3 wt%, iron loss deteriorates; when the Si content exceeds 4.0 wt%, brittleness increases. Therefore, the Si content in the slab and in the non-oriented electrical steel sheet after the final non-oxidative annealing step can be from 0.3 to 4.0 wt%. Specifically, the Si content can be from 0.4 to 3.0 wt%. More specifically, the Si content can be from 0.5 to 2.0 wt%.

[0048] Regarding carbon (C), during the decarburization annealing process, in order for the Gaussian grains in the surface layer to diffuse towards the center, a process is required for carbon in the center to escape towards the surface layer. Therefore, the C content in the slab can be 0.03 to 0.4% by weight. More specifically, the C content in the slab can be 0.15 to 0.3% by weight. Furthermore, in the non-oriented electrical steel sheet finally obtained after decarburization, the carbon content can be 0.0050% by weight or less. Specifically, it can be 0.002% by weight or less. More specifically, it can be 0.0005 to 0.0020% by weight.

[0049] The slab may also contain less than 0.1% by weight of Mn and less than 0.005% by weight of S.

[0050] Mn and S form MnS precipitates, which hinder the growth of Gaussian grains diffusing towards the center during decarburization. Therefore, it is preferable not to add Mn and S. However, considering the unavoidable amount mixed in during the steelmaking process, in the slab and the non-oriented electrical steel sheet after the final annealing step, Mn and S can be controlled to be less than 0.1 wt% for Mn and less than 0.005 wt% for S.

[0051] The balance includes Fe and unavoidable impurities. Unavoidable impurities are those introduced during the steelmaking process and the manufacturing process of non-oriented electrical steel sheets; these impurities are well-known in the art and therefore detailed descriptions are omitted. Specifically, since elements such as Al, N, Ti, Mg, and Ca react with oxygen in the steel to form oxides, strong suppression is required; therefore, each component can be controlled to below 0.005% by weight. In one embodiment of the invention, the addition of other elements besides the aforementioned alloying components is not excluded; various elements can be included without affecting the technical concept of the invention. When additional elements are further included, they replace a portion of the Fe in the balance.

[0052] More specifically, by weight percent, the slab may consist of 0.3% to 4.0% Si, 0.03% to 0.4% C, and the balance Fe and unavoidable impurities.

[0053] Before hot rolling, the slab can be heated. The slab heating temperature can be 1050°C to 1350°C, which is higher than the conventional heating temperature. When the slab is reheated, if the temperature is high, the hot-rolled microstructure will coarsen, thus causing adverse effects on magnetism. However, in the manufacturing method of non-oriented electrical steel sheet according to an embodiment of the present invention, because the carbon content is higher than that of conventional processes, even if the slab is reheated at a higher temperature, the hot-rolled microstructure will not coarsen. Reheating at a temperature higher than the conventional level is beneficial for hot rolling. However, if the reheating temperature is too high, the solid solubility of precipitates such as TiN and AlN increases, and a large number of fine precipitates will be distributed during cooling, thereby hindering the growth of surface grains.

[0054] For hot rolling, hot-rolled sheets with a thickness of 1.50 to 4.00 mm can be produced so that an appropriate rolling rate can be applied in the final cold rolling step to achieve the final product thickness. More specifically, hot-rolled sheets with a thickness of 1.80 to 2.60 mm can be produced.

[0055] There are no particular restrictions on hot rolling temperature or cooling temperature, but as an example of pursuing excellent magnetic properties, the hot rolling end temperature can be set below 950°C, and the coiling can be carried out at a temperature below 600°C by water quenching.

[0056] Following hot rolling, an annealing step can be performed on the hot-rolled steel sheet. This annealing may include a decarburization process. Specifically, the hot-rolled sheet can be annealed at a temperature of 850°C to 1000°C and a dew point temperature below 70°C. More specifically, annealing can be performed at a dew point temperature of -70°C to 70°C. After the aforementioned annealing, further annealing can be performed at a temperature of 1000°C to 1200°C and a dew point temperature below 0°C. Pickling can be performed after the hot-rolled sheet annealing.

[0057] Next, a cold rolling process is carried out to produce cold-rolled steel sheets.

[0058] In the conventional manufacturing process of grain-oriented electrical steel sheets, it is known that single cold rolling with a reduction rate close to 90% is effective. This is because only Gaussian grains create an environment favorable for grain growth during single recrystallization. However, according to an embodiment of the present invention, the manufacturing method of non-oriented electrical steel sheets does not utilize the abnormal grain growth of Gaussian-oriented grains, but rather diffuses the resulting Gaussian grains inward through decarburization annealing and cold rolling. Therefore, it is advantageous to form a surface layer with a large distribution of Gaussian-oriented grains. Furthermore, for use as an axial flux motor (AFM) core, a structure in which Gaussian-oriented grains and cubic-oriented grains are mixed in an appropriate proportion is beneficial for optimizing the magnetic circuit distribution. Therefore, a reduction rate that can also achieve cubic orientation can be adopted.

[0059] Therefore, if a reduction rate of 70% to 80% is used during cold rolling, a large amount of Gaussian texture will form on the surface. More specifically, the reduction rate can be 72% to 80%.

[0060] Next, the cold-rolled steel sheet undergoes decarburization annealing. This decarburization annealing step can be performed in the austenitic single-phase region or in the region where a composite phase of ferrite and austenite exists. Specifically, 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 mixture of hydrogen and nitrogen. After decarburization annealing, the carbon content in the steel sheet can be less than 0.005% by weight. More specifically, annealing can be performed at a temperature of 750°C to 900°C. The annealing time can be from 60 seconds to 5 minutes, more specifically from 90 to 300 seconds.

[0061] During this decarburizing annealing process, the grain size on the surface of the electrical steel sheet grows coarser, while the internal grains remain as fine microstructure. After this decarburizing annealing, the average grain diameter can be 150 μm to 250 μm. At this point, the grains are surface ferrite grains. Furthermore, the grain diameter refers to the diameter of a hypothetical circle with the same area as the grain. The reference plane is a plane parallel to the rolling vertical plane (TD plane).

[0062] Next, the decarburized and annealed steel sheet undergoes a second cold rolling process. Since the second cold rolling is identical to the first, a detailed description is omitted. The reduction rate in the second cold rolling step can be 50% to 80%, more specifically 50% to 70%.

[0063] The aforementioned decarburization annealing step and secondary cold rolling step can be repeated more than twice. By repeating the process more than twice, a large amount of Gaussian and cubic textures can be formed on the surface layer. In one embodiment of the present invention, Gaussian texture refers to a texture at an angle of less than 15° to the Gaussian orientation, that is, the rolling surface (ND surface) and rolling direction (RD direction) of the steel plate are perpendicular to the {110} grains. <001> Grains oriented at an angle of less than 15°. Cubic texture refers to grains with an orientation similar to {100}. <001> Grains with an orientation angle of less than 15°.

[0064] Next, the steel sheet that has undergone secondary cold rolling is annealed in a non-oxidizing atmosphere.

[0065] In one embodiment of the invention, annealing is performed immediately after the second cold rolling in a non-oxidizing atmosphere, thus omitting the additional decarburization annealing. If decarburization annealing is performed after the second cold rolling, the cubic texture in the steel sheet will transform into a Gaussian texture, and the remaining cubic texture may be insufficient. Therefore, although the magnetism in the rolling direction can be improved, the magnetism in other directions will be significantly reduced.

[0066] The non-oxidative annealing step can be performed at a homogenization temperature of 750 to 1050°C for 60 seconds to 5 minutes.

[0067] The purpose of the non-oxidative annealing step is to allow the grains to grow to a certain size. This improves magnetic properties, particularly high-frequency iron loss. If the annealing temperature is too low or the time is too short, Gaussian and cubic grains may not develop properly. If the annealing temperature is too high or the time is too long, the Gaussian and cubic grains will not maintain the fine size beneficial to high-frequency iron loss, but will continue to grow, potentially leading to a decrease in the fraction of Gaussian and cubic grains during growth. More specifically, the soaking temperature for non-oxidative annealing can be 800 to 1000°C, more specifically 900 to 970°C.

[0068] The secondary non-oxidizing annealing step can be performed at a dew point temperature below -20°C. If the dew point temperature is too high, an oxide layer will form on the surface, which may adversely affect the magnetism. More specifically, annealing can be performed in an atmosphere with a dew point temperature of -50 to -30°C and a hydrogen concentration of 99% by volume or higher.

[0069] After a single cold rolling step, the process can be carried out continuously up to the non-oxidizing annealing step. A continuous process refers to a batch process where the steel sheet is rolled into a coil and then annealed. As mentioned above, the decarburizing annealing step to the non-oxidizing annealing step is completed within a few minutes, thus enabling a continuous process.

[0070] After completing the non-oxidative annealing step, in the finally manufactured non-oriented electrical steel sheet, with {110} <001> The area fraction of grains at angles less than 15° can be 20% to 60%, compared to {100} <001> The area fraction of grains with an angle of less than 15° can be 5% to 20%.

[0071] At this point, the area fraction of the grains is measured using a plane parallel to the rolling surface (ND surface) as a reference, and measurements can be taken at 1 / 4t to 3 / 4t of the steel plate thickness. The measurement method is not particularly limited, but EBSD can be used for measurement.

[0072] In one embodiment of the invention, the moderate presence of Gaussian and cubic grains in the steel plate enhances magnetism in all directions, particularly further enhancing magnetism in the rolling direction, thereby enabling effective use in axial flux motors. More specifically, with {110} <001> The area fraction of grains with an angle of less than 15° can be 40% to 55%, compared with {100} <001> The area fraction of grains with an angle of less than 15° can be 7% to 15%.

[0073] In one embodiment of the present invention, grains with a ratio (D2 / D1) of circumscribed circle diameter (D1) to inscribed circle diameter (D2) of 0.5 or higher account for more than 95% of all grains.

[0074] Furthermore, the area fraction of grains with a grain size of 30 μm to 200 μm can be 80% or more in all grains. This is because, in one embodiment of the invention, decarburization annealing and non-oxidation annealing are completed in a short time. As in the manufacture of conventional oriented electrical steel sheets, when subjected to long-term annealing of more than one hour through batch annealing, the average grain size increases to more than 5 mm, which is significantly different in shape and size from the grain size of non-oriented electrical steel sheets according to an embodiment of the invention. More specifically, the fraction of grains with a grain size of 30 μm to 200 μm in all grains can be 90% to 99%. These fine grains contribute to improving high-frequency iron loss. The grain fraction refers to the area fraction, which can be obtained by calculating the area of ​​grains with the corresponding grain size in any cross-section of the steel sheet. More specifically, it can be measured based on the vertical surface (ND surface) of the rolled steel sheet. For the grain size, a virtual circle with the same area as the grain can be assumed, and the calculation can be performed using the diameter of this circle.

[0075] In one embodiment of the present invention, compared to the alloy composition of non-oriented electrical steel sheets, the effect can be achieved through a unique manufacturing process and the unique fine structure generated by this process. Further explanation of the alloy composition of the non-oriented electrical steel sheet: According to one embodiment of the present invention, the non-oriented electrical steel sheet may contain, by weight percent, 0.3% to 4.0% Si, less than 0.005% and excluding 0% C, and the balance Fe and unavoidable impurities. Furthermore, the non-oriented electrical steel sheet according to one embodiment of the present invention may also contain less than 0.1% Mn and less than 0.005% S. Since the alloy composition of the non-oriented electrical steel sheet is the same as that of the aforementioned slab, repeated descriptions are omitted.

[0076] In one embodiment of the invention, the non-oriented electrical steel sheet exhibits excellent magnetic properties, particularly in the rolling direction and the direction perpendicular to rolling. Specifically, the average magnetic flux density (B0) in the rolling direction and the direction perpendicular to rolling is... 50 The value can be 1.78 to 1.88 T. Furthermore, the average iron loss (W) in the rolling direction and perpendicular to the rolling direction... 15 / 50 The magnetic flux density (B) in the rolling direction can be 1.5 to 2.0 W / kg. 50 The iron loss (W) in the rolling direction can be from 1.95 to 2.05 T. 15 / 50 The value can range from 0.80 to 1.5 W / kg.

[0077] Meanwhile, in one embodiment of the present invention, the magnetic flux density deviation of the non-oriented electrical steel sheet in the rolling direction and the rolling perpendicular direction is relatively small. Specifically, the deviation can be 5.0% to 15.0%. More specifically, the deviation can be 10.0% to 14.5%. The deviation can be calculated using the following formula: (magnetic flux density in the rolling direction - magnetic flux density in the rolling perpendicular direction) / magnetic flux density in the rolling direction × 100.

[0078] Magnetic flux density (B) 50 The magnetic flux density induced in a magnetic field of 5000 A / m is called the iron loss (W). 15 / 50 The value is the magnitude of the iron loss induced under conditions of 1.5 Tesla and 50 Hz (W / kg).

[0079] Specific embodiments of the present invention are described below. However, the following embodiments are merely one specific embodiment of the present invention, and the present invention is not limited to the following embodiments.

[0080] Example 1 The slab was heated at 1150°C, containing 2.0% Si, 0.085% C, and the balance Fe and unavoidable impurities by weight. It was then hot-rolled to the thicknesses shown in Table 1, followed by hot-rolled sheet annealing at 920°C. The sheet was then cooled, pickled, and cold-rolled at the reduction rates shown in Table 1 to produce cold-rolled sheets. The cold-rolled sheets were then heat-treated at a dew point of 55°C according to the decarburization annealing temperature and time shown in Table 1. They were then cold-rolled again at a reduction rate of 56% to produce cold-rolled sheets with a thickness of 0.22 mm. Non-oxidative annealing was performed for 2 minutes at 950°C in a mixed atmosphere of hydrogen and nitrogen and at a dew point of -30°C. The final C content, magnetic flux density, iron loss, and magnetic flux density deviation of the samples after non-oxidative annealing were measured and summarized in Table 2.

[0081] The grain area fraction was measured on the ND surface using EBSD.

[0082] For magnetic properties, a single-sheet tester was used for measurement.

[0083] Table 1 Table 2 As shown in Tables 1 and 2, it can be confirmed that during hot rolling, single cold rolling, decarburization annealing, and non-oxidation annealing, a large number of cubic and Gaussian grains are formed depending on the conditions, thereby enhancing the magnetism in all directions and in the rolling direction.

[0084] On the other hand, for No. 1, No. 5 and No. 6, whose single cold rolling reduction rate exceeded the range, it can be confirmed that no large number of cubic grains or Gaussian grains were generated, and therefore the magnetic properties were poor.

[0085] For No. 9 and No. 12, which were annealed at excessively high temperatures during decarburization, a dense oxide layer was formed on the surface, resulting in delayed decarburization and slow grain growth. This made it difficult to form Gaussian and cubic orientations, thus leading to poor magnetic properties.

[0086] For annealing temperatures of 13 to 15 where the decarburization temperature was too low, it can be confirmed that the carbon activity required for decarburization is reduced, resulting in delayed decarburization. Consequently, grain growth slows down, and the formation of Gaussian and cubic grains is delayed, resulting in poorer magnetism.

[0087] For annealing times of 16 to 18, which are too short for decarburization, it can be confirmed that decarburization is incomplete, and therefore grain growth is also incomplete. The formation of Gaussian and cubic grains is delayed, resulting in poor magnetic properties.

[0088] This invention is not limited to the above-described embodiments and / or examples, and can be prepared in various different ways. Those skilled in the art will understand that this invention can be implemented in other specific ways without changing the technical concept or essential features of the invention. Therefore, it should be understood that the above-described embodiments and / or examples are exemplary in all respects and are not restrictive.

Claims

1. A manufacturing method of a non-oriented electrical steel sheet, comprising: a step of subjecting a slab to hot rolling to produce a hot-rolled steel sheet; a step of subjecting the hot-rolled steel sheet to primary cold rolling; a step of subjecting the steel sheet after the primary cold rolling to decarburization annealing; a step of subjecting the steel sheet after the decarburization annealing to secondary cold rolling; and a non-oxidizing annealing step of subjecting the steel sheet after the secondary cold rolling to annealing in a non-oxidizing atmosphere.

2. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the slab contains, in mass%, 0.3 to 4.0% of Si, 0.03 to 0.4% of C, and the balance of Fe and unavoidable impurities.

3. The manufacturing method of a non-oriented electrical steel sheet according to claim 2, wherein: the slab further contains 0.1 mass% or less of Mn and 0.005 mass% or less of S.

4. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, further comprising: a step of subjecting the hot-rolled steel sheet to hot-rolled sheet annealing, the hot-rolled sheet annealing step includes a decarburization process.

5. The manufacturing method of a non-oriented electrical steel sheet according to claim 4, wherein: the hot-rolled sheet annealing step is performed at a temperature of 850°C to 1000°C and a dew point temperature of 70°C or lower.

6. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the 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.

7. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the decarburization annealing step is performed in an austenite single phase region or a region where ferrite and austenite coexist.

8. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the amount of carbon in the steel sheet after the decarburization annealing step is 0.005 mass% or less.

9. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the decarburization annealing step and the secondary cold rolling step are repeated two or more times.

10. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the reduction rate for the primary cold rolling step is 70 to 80%.

11. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the non-oxidizing annealing step is performed at a soaking temperature of 750°C to 1050°C for 60 seconds to 5 minutes.

12. The manufacturing method of a non-oriented electrical steel sheet according to claim 1, wherein: the non-oxidizing annealing step is performed in an atmosphere where the dew point temperature is -20°C or lower.

13. A non-oriented electrical steel sheet, wherein: the area fraction of grains at an angle of 15° or less to {110} <001> is 20 to 60%, and the area fraction of grains at an angle of 15° or less to {100} <001> is 5 to 20%.

14. The non-oriented electrical steel sheet according to claim 13, wherein: the difference in magnetic flux density between the rolling direction and the direction perpendicular to the rolling is 10% or more and 20% or less.

15. The non-oriented electrical steel sheet according to claim 13, wherein: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Of all the grains, those with a ratio of circumscribed circle diameter (D1) to inscribed circle diameter (D2) of 0.5 (D2 / D1) account for more than 95% of the area.

16. The non-oriented electrical steel sheet according to claim 13, wherein, Of all the grains, those with a diameter of 30 μm to 200 μm account for more than 80% of the area.

17. The non-oriented electrical steel sheet according to claim 13, wherein, The electrical steel sheet contains, by weight percent, 0.3% to 4.0% Si, less than 0.005% and less than 0% C, and the balance Fe and unavoidable impurities.

18. The non-oriented electrical steel sheet according to claim 17, wherein, The electrical steel sheet also contains less than 0.1% by weight of Mn and less than 0.005% by weight of S.

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