Non-directional electromagnetic steel sheet

Abstract

The objective is to reduce iron loss in Si gradient magnetic materials mainly used in motor applications. The non-oriented electrical steel sheet 1 consists of an inner layer 10 and surface layers 20 located on both sides of the inner layer 10, and has a component composition in mass%, with an average Si content ([Si]1) of the surface layer 20, an average Si content ([Si]0) of the inner layer 10, and predetermined amounts of C, Mn, Al, P, S, N, and O, with the remainder being Fe and unavoidable impurities, and the difference ΔSi between [Si]1 and [Si]0 is 0.10% to 2.50%, and the steel sheet 1 has an oxide film 30 on its surface with a magnetite (Fe3O4) content (C_mag) of 65% or more, and the average thickness tc of the oxide film 30 is 5 nm to 120 nm.

Description

Technical Field

[0001] The present invention relates to a non-oriented electromagnetic steel sheet, particularly a non-oriented electromagnetic steel sheet having a Si concentration gradient in the plate thickness direction.

Background Art

[0002] For example, motors for drones and fan motors for servers have come to be driven in a high-frequency range of 400 Hz to 1 kHz from the viewpoints of miniaturization, weight reduction, and high efficiency. Non-oriented electromagnetic steel sheets used as core materials for such motors are required to have low high-frequency iron loss and high magnetic flux density. s

[0003] In order to reduce high-frequency iron loss, increasing the specific resistance is effective, and thus conventionally, the development of high-Si steel has been carried out. However, since Si is a non-magnetic element, there is a problem that the magnetic flux density and saturation magnetization decrease.

[0004] As a method for achieving both reduction of high-frequency iron loss and high magnetic flux density, a so-called Si-gradient magnetic material that controls the Si concentration gradient in the plate thickness direction has been developed. For example, in Patent Document 1, there is a concentration gradient of Si in the plate thickness direction, the Si concentration on the surface of the steel sheet is higher than the Si concentration at the center of the plate thickness of the steel sheet, and the portion with a Si concentration of 5 to 8% is 10% or more of the plate thickness in the plate thickness depth direction from both surfaces of the steel sheet, and the Si concentration at the center of the plate thickness is 3.4% or more. By providing such a gradient in the Si content, it is possible to reduce eddy current loss while maintaining high saturation magnetization.

Prior Art Documents

Patent Documents

[0005] [[ID= thirty]]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, when a material with only a Si concentration gradient in the thickness direction is used as a core material for drones, vacuum cleaner motors, etc. (for example, frequency: 400Hz~1kHz, magnetic flux density: 0.8~2.0T), the proportion of hysteresis loss is large due to the high magnetic flux density used, and iron loss is not sufficiently reduced. Therefore, the present invention aims to reduce iron loss in Si gradient magnetic materials mainly used for motor applications. [Means for solving the problem]

[0007] To address the above-mentioned problems and achieve low iron loss, the inventors focused on the influence of the components on the magnetic properties of Si gradient magnetic materials and conducted extensive research. As a result, they discovered that low iron loss can be achieved by forming an oxide film on the surface of the steel plate in which the proportion of magnetite (Fe3O4) (C_mag) is 65% or more.

[0008] This invention is based on the aforementioned findings, and its gist is as follows.

[0009] [1] A non-oriented electrical steel sheet comprising an inner layer and surface layers located on both sides of the inner layer, Based on the plate thickness t (mm), when the position of one surface of the non-oriented electrical steel sheet is defined as 0 and the position of the other surface as t, the surface layer is defined as the region in the range of 0 to t / 4 and 3t / 4 to t. When the aforementioned inner layer is defined as a region greater than t / 4 and less than 3t / 4, In mass percent, The average Si content ([Si]1) of the surface layer is 3.50% or more and 7.00% or less. The average Si content ([Si]0) of the inner layer is 2.00% or more and 6.50% or less. The average content across the entire plate thickness, C: 0.010% or less, Mn: 2.0% or less, Al: 0.10% or less, P: 0.20% or less, S: 0.0050% or less, N: 0.0080% or less, contains O: 0.0050% or less, and the balance consists of Fe and unavoidable impurities, having a component composition of ΔSi defined as the difference between [Si]1 and [Si]0 ([Si]1 - [Si]0) is 0.10% or more and 2.50% or less, the surface of the non-oriented electrical steel sheet has an oxide film with a magnetite (Fe3O4) ratio (C_mag) of 65% or more, a non-oriented electrical steel sheet having an average thickness tc of the oxide film of 5 nm or more and 120 nm or less.

[0010] [2] The component composition further includes, in terms of the average content over the entire plate thickness, in mass%, Sn: 0.10% or less, Sb: 0.10% or less, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 0.10% or less, Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.03% or less, including at least one selected from the group consisting of the non-oriented electrical steel sheet according to [1].

[0011] [3] The component composition further includes, in terms of the average content over the entire plate thickness, in mass%, B: 0.0020% or less, Ti: 0.010% or less, Nb: 0.0050% or less, V: 0.050% or less, Pb: 0.020% or less, As: 0.020% or less, Zn: 0.010% or less, [[ID=​​​​​​​ The non-oriented electrical steel sheet according to [1] or [2] above.

[0012] [4] The non-oriented electrical steel sheet according to any one of [1] to [3] above, wherein the coating rate of the oxide film is 70% or more.

[0013] [5] The non-oriented electrical steel sheet according to any one of [1] to [4] above, wherein the sheet thickness t is 0.01 mm or more and 0.35 mm or less. [Effect of the Invention]

[0014] According to the present invention, it is possible to achieve a reduction in iron loss in a Si-gradient magnetic material mainly used for motor applications. [Brief Description of the Drawings]

[0015] [Figure 1] It is a schematic diagram showing the structure of the non-oriented electrical steel sheet of the present invention. [Figure 2] It is a schematic diagram showing an example of the Si content distribution in the thickness direction of the non-oriented electrical steel sheet of the present invention. [Figure 3] It is an enlarged schematic diagram near the surface of the non-oriented electrical steel sheet shown in FIG. 1. [Figure 4] It is a diagram showing the relationship between the average thickness tc of the oxide film and the iron loss W10 / 2k at a frequency of 2 kHz and a maximum magnetic flux density of 1.0 T. [Embodiments for Carrying Out the Invention]

[0016] [[ID=4I]]Hereinafter, a method for implementing the present invention will be specifically described. The following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.

[0017] Figure 1 is a schematic diagram showing the structure of the non-oriented electrical steel sheet of the present invention. In the non-oriented electrical steel sheet 1 shown in Figure 1 (hereinafter sometimes simply referred to as "steel sheet"), with respect to the total thickness t (mm) of the steel sheet 1, the position of one surface of the steel sheet 1 is defined as 0, and the position of the other surface is defined as t. The regions in the range of 0 to t / 4 and 3t / 4 to t are defined as the surface layer 20, and the region greater than t / 4 and less than 3t / 4 is defined as the inner layer 10. Then, as shown in Figure 1, the steel sheet 1 consists of the inner layer 10 and the surface layer 20 provided on both sides of the inner layer 10.

[0018] Figure 2 is a schematic diagram showing an example of the Si content distribution in the thickness direction of a non-oriented electrical steel sheet 1. In Figure 2, the vertical axis indicates the position in the thickness direction, where 0 represents the position on one surface of the non-oriented electrical steel sheet 1, and t represents the position on the other surface of the non-oriented electrical steel sheet 1. As shown in Figure 2, the steel sheet 1 has a Si content distribution in which the Si content decreases continuously from the surface toward the center of the thickness. Note that the above Si content distribution may be a distribution in which the Si content changes continuously over the entire thickness direction of the steel sheet 1, or it may be a Si content distribution that changes continuously in the surface layer of the steel sheet 1 and remains constant in the inner layer.

[0019] [Oxide film] Figure 3 is an enlarged view of the vicinity of the surface of the non-oriented electrical steel sheet 1 shown in Figure 1. As shown in Figure 3, in the steel sheet 1 of the present invention, an oxide film 30 is provided on both surfaces of the steel sheet 1, with an average thickness tc of 5 nm to 120 nm and a magnetite (Fe3O4) content (C_mag) of 65% or more. As shown in Figure 3, an additional film such as an insulating film 40 may be provided on the surface of the oxide film 30 as needed. The reason for providing the above oxide film 30 will be explained below.

[0020] The inventors, in an effort to further reduce iron loss in the non-oriented electrical steel sheet 1, were investigating the steel sheet composition and heat treatment methods when they discovered that iron loss was significantly reduced in steel sheet 1 with an oxide film formed on the surface of the steel sheet having a magnetite content (C_mag) of 65% or more. This is thought to be due to the following reasons.

[0021] In other words, during the manufacturing process of steel plates, fine irregularities are formed on the surface of the steel plate, and these irregularities hinder the movement of magnetic domains, increasing hysteresis loss. When the above oxide film 30 is formed on the surface of the steel plate, the fine irregularities on the surface of the steel plate are oxidized, and the surface of the steel plate is smoothed. When the surface of the steel plate is smoothed, the resistance when magnetic domains move decreases, and thus the hysteresis loss decreases. As a result, it is thought that the iron loss of the steel plate 1 on which the magnetite film 30 is formed on the surface has decreased significantly.

[0022] Furthermore, while aluminum oxide films and other iron-based oxide films such as wustite (FeO) and hematite (Fe2O3) have uneven thickness, magnetite forms a thin, uniform scale on the steel sheet surface. Therefore, it is believed that when an oxide film with a magnetite content (C_mag) of 65% or more is formed on the steel sheet surface, it can effectively reduce fine irregularities on the steel sheet surface and lower iron loss.

[0023] Furthermore, the inventors investigated the relationship between the thickness of the oxide film 30 and iron loss. The specific procedure is described below. First, a sample of steel slab having the components of sample symbol A in Table 1 was hot-rolled and subjected to hot-rolled sheet annealing at 950°C for 30 seconds. Next, cold-rolling was performed to produce a cold-rolled steel sheet with a thickness of 0.1 mm, and then the surface of the obtained cold-rolled steel sheet was mechanically polished to a thickness of 1 μm on both sides. Subsequently, using the polished steel sheet, silicate treatment was performed at 1200°C in an atmosphere consisting of N2 gas and SiCl4 by the CVD method to increase the Si content of the surface layer of the steel sheet. Next, a diffusion treatment (heat treatment) was performed at 1200°C for 30 seconds in an N2 atmosphere to diffuse the Si from the surface layer of the steel sheet into the interior. At this time, the dew point of the atmosphere during silicate treatment was controlled to -70°C, and the dew point of the atmosphere during diffusion treatment was set to -70 to -30°C to produce the steel sheet.

[0024] Using the method described above, non-oriented electrical steel sheets were fabricated with target values ​​of 6.5% average Si content ([Si]1) in the surface layer and 5.0% average Si content ([Si]0) in the inner layer. Samples for transmission electron microscopy (TEM) observation were taken from the steel sheets fabricated using the method described above, and the cross-sectional direction of the steel sheets was observed using bright-field TEM images. Micro-electron diffraction using TEM and energy-dispersive X-ray analysis (EDX) measurements were performed on the oxide film formed on the surface of the steel sheets to determine the composition of the formed oxide film. As a result, it was confirmed that an oxide film with a magnetite content (C_mag) of 65% or more had been formed.

[0025] The average thickness tc of the oxide film was determined by cutting five samples from a steel plate at 50 mm intervals in the width direction, observing the cross-sections of these samples using a TEM (bright-field imaging, 100,000x magnification), measuring the thickness of the oxide film from each sample, and evaluating it as the average value. If no oxide film was observed, the thickness was set to 0.

[0026] Furthermore, a test specimen measuring 30 mm in width and 280 mm in length was taken from each of the aforementioned steel plates, and the iron loss was measured in the Epstein test in accordance with JISC2550-1:2011 at a maximum magnetic flux density of 1.0 T and a frequency of 2 kHz. 10 / 2k (W / kg) was measured. In the Epstein test described above, equal amounts of L-direction specimens, taken so that the length of the specimen is in the rolling direction (L direction), and C-direction specimens, taken so that the length of the specimen is in the direction perpendicular to the rolling direction (C direction), were used, and the average values ​​of the magnetic properties in the L direction and C direction were evaluated.

[0027] Figure 4 shows the average thickness tc of the oxide film and the iron loss W at a frequency of 2 kHz and a maximum magnetic flux density of 1.0 T. 10 / 2kThe relationship is shown. It can be seen that excellent iron loss is observed in the range of tc from 5 nm to 120 nm. The mechanism for this is thought to be as follows: In the range of tc less than 5 nm, the thickness of the coating is small relative to the irregularities on the surface of the steel sheet, so the smoothing is insufficient and iron loss cannot be fully reduced. However, in the range of tc from 5 nm to 120 nm, the surface of the steel sheet is smoothed by the coating, so iron loss decreases. Furthermore, in the range of tc exceeding 120 nm, the thickness of the coating is not uniform, making it difficult to sufficiently homogenize the surface of the steel sheet, and it is thought that iron loss increases.

[0028] Based on the above-mentioned findings, in the present invention, the average thickness tc of the oxide film 30 is set to be between 5 nm and 120 nm. Preferably, the average thickness tc of the oxide film 30 is between 5 nm and 100 nm.

[0029] The coverage rate of the oxide film 30 on the steel sheet surface is preferably 70% or more. In this invention, the above coverage rate is the ratio of the area on the surface of the non-oriented electrical steel sheet 1 where the oxide film 30 is formed. As a specific method for measuring the coverage rate, five or more samples were cut from the non-oriented electrical steel sheet 1 at 50 mm intervals in the width direction of the sheet, and then the cross-section of each sample was photographed near the steel sheet surface using a TEM bright-field image. Points were then marked on the steel sheet surface of the photographed images at 100 nm intervals, and the ratio of the total number of points on which the oxide film 30 exists was defined as the coverage rate. Typical test conditions included an acceleration voltage of 200 kV, an irradiation current of 100 nA, an imaging resolution of 1 k × 1 k pixels, a magnification of 30,000 times, and a total of 25 or more measurement points.

[0030] When the coverage rate of the oxide film 30 on the surface of the non-oriented electrical steel sheet 1 is 70% or higher, iron loss can be effectively reduced, and as a result, motor efficiency can be further improved. The coverage rate is more preferably 80% or higher. On the other hand, since a higher coverage rate is desirable, there is no upper limit, and it can be 100%.

[0031] Furthermore, the proportion of magnetite (C_mag) in the oxide film 30 is 65% or more. In this invention, the proportion of magnetite in the oxide film 30 is the area ratio of magnetite in the oxide film 30 as seen from the cross-section, obtained by observing the oxide film 30 from the cross-sectional direction of the steel sheet using a bright-field TEM image. When the proportion of magnetite in the oxide film 30 is 65% or more, the iron loss of the non-oriented electrical steel sheet 1 can be effectively reduced. It is preferable that the proportion of magnetite in the oxide film 30 be 80% or more. On the other hand, since a higher proportion of magnetite in the oxide film 30 is preferable, there is no upper limit, and it can be 100%.

[0032] [Component composition] Next, the component composition of the non-oriented electrical steel sheet 1 of the present invention will be described. In the following description, the "%" representing the content of each element will represent "mass%" unless otherwise specified. Furthermore, unless otherwise specified, the content of each element will be the average content across the entire sheet thickness.

[0033] The non-oriented electrical steel sheet 1 of the present invention has a component composition comprising Si, C, Mn, Al, P, S, N, and O, with the remainder being Fe and unavoidable impurities. Here, unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objective of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of impurities include H, Ti, or Co.

[0034] The average Si content ([Si]1) of the surface layer 20 is between 3.50% and 7.00%. Si is an element that increases the electrical resistance of steel sheets and reduces eddy current losses. Due to the skin effect, eddy currents tend to concentrate in the surface layer 20 of the electrical steel sheet 1, and iron loss can be effectively reduced by increasing the Si content in the surface layer 20 compared to the inner layer 10. If [Si]1 is less than 3.50%, eddy current losses cannot be effectively reduced. Therefore, it is preferable that [Si]1 be 3.50% or more, and preferably 4.50% or more. When performing general cold rolling, if the Si content exceeds 4.50%, the sheet becomes prone to fracture, so it is necessary to perform treatments such as warm rolling or silica diffusion treatment. In order to manufacture steel sheets with [Si]1 exceeding 7.00% by silica diffusion treatment, there is a problem that the silica diffusion treatment takes a long time, and also a problem that the elongation of the material decreases significantly, making it easier for defects to occur in the iron core manufacturing process. Therefore, it is preferable that [Si]1 be 7.00% or less, and preferably 6.70% or less.

[0035] Furthermore, the statement that [Si]1 is 3.50% or more and 7.00% or less means that the [Si]1 in the surface layer 20 (first surface layer) on one side of the non-oriented electrical steel sheet 1 is 3.50% or more and 7.00% or less, and the [Si]1 in the surface layer 20 (second surface layer) on the other side of the non-oriented electrical steel sheet 1 is 3.50% or more and 7.00% or less. The [Si]1 in the first surface layer and the [Si]1 in the second surface layer may be the same or different.

[0036] The average Si content ([Si]0) of the inner layer 10 is between 2.00% and 6.50%. If the [Si]0 is less than 2.00%, eddy current losses cannot be effectively reduced. Therefore, it is preferable that the [Si]0 be 2.00% or higher, and preferably 2.50% or higher. On the other hand, if the [Si]0 exceeds 6.50%, the elongation of the material decreases significantly. Therefore, it is preferable that the [Si]0 be 6.50% or lower, and preferably 6.20% or lower.

[0037] ΔSi([Si]1-[Si]0) is between 0.10% and 2.50% ΔSi is defined as the difference ([Si]1-[Si]0) between the average Si content in the surface layer 20 and the average Si content in the inner layer 10. By setting ΔSi to 0.10% or more, iron loss in the high-frequency range of the non-oriented electrical steel sheet 1 can be effectively reduced. On the other hand, if ΔSi exceeds 2.50%, the difference in lattice constants between the surface layer 20 and the inner layer 10 becomes large, which increases the internal stress of the steel sheet 1 and increases iron loss. For this reason, it is preferable to set ΔSi to be between 0.10% and 2.50%, and between 0.30% and 2.00%.

[0038] C: 0.010% or less The carbon (C) contained in steel sheet 1 is a harmful element that forms carbides, causing magnetic aging and degrading iron loss characteristics. Therefore, the carbon content in steel sheet 1 should be 0.010% or less, and preferably 0.007% or less. On the other hand, there is no particular lower limit to the carbon content, but from the viewpoint of suppressing decarburization costs in the refining process, it is preferable that the carbon content be 0.0001% or more.

[0039] Mn: 2.0% or less Mn is an element that increases the resistivity of steel sheet 1 and reduces iron loss. Furthermore, from the viewpoint of suppressing the fine deposition of sulfides, it is preferable that the Mn content be 0.01% or more. On the other hand, if the Mn content exceeds 2.0%, slab cracking and other problems occur, worsening the operability in the steelmaking process. For this reason, the Mn content should be 2.0% or less.

[0040] Al: 0.10% or less Al, like Si, is a useful element that increases the resistivity of steel and reduces iron loss. However, Al is more easily oxidized than Fe, and when the Al content exceeds 0.10%, Al-based oxides are formed preferentially over magnetite. Unlike magnetite coatings, these Al-based oxides are formed unevenly, so they cannot eliminate surface irregularities on the steel sheet and cannot sufficiently reduce iron loss. For this reason, the Al content should be 0.10% or less. Furthermore, reducing the Al content to 0.01% or less improves the texture and increases the magnetic flux density. For this reason, when magnetic flux density is important, it is preferable to have an Al content of 0.01% or less. It is even more preferable to have an Al content of 0.003% or less. There is no specific lower limit for the Al content, but excessive reduction leads to increased refining costs, so it is preferable to have an Al content of 0.0001% or more.

[0041] P:0.20% or less P is a useful element used to adjust the strength of steel. However, if the P content exceeds 0.20%, the steel becomes brittle and difficult to roll. For this reason, the P content should be 0.20% or less. Although there is no specific lower limit for the P content, it is preferable that the P content be 0.001% or more from the viewpoint of suppressing the cost of removing P in the refining process.

[0042] S: 0.0050% or less, N: 0.0080% or less, O: 0.0050% or less S, N, and O are elements that form precipitates in the steel sheet 1, inhibiting grain growth during annealing and adversely affecting the iron loss characteristics. In particular, when the S content exceeds 0.0050%, the N content exceeds 0.0080%, and the O content exceeds 0.0050%, the adverse effects become significant. For this reason, the S content should be 0.0050% or less, the N content 0.0080% or less, and the O content 0.0050% or less. There are no specific lower limits for the S, N, and O content, but from the viewpoint of suppressing costs in the refining process, it is preferable that the S content be 0.0002% or more, the N content 0.0005% or more, and the O content 0.0005% or more.

[0043] In one embodiment of the present invention, the component composition of the non-oriented electrical steel sheet 1 may optionally include, in addition to the above components, one or more selected from Sn, Sb, Cu, Ni, Cr, Mo, Ca, Mg, and REM.

[0044] Sn: 0.10% or less, Sb: 0.10% or less Both Sn and Sb significantly improve texture, increase magnetic flux density, and further reduce hysteresis loss. However, when the Sn and Sb content exceeds 0.10%, the effect saturates, and it leads to decreased manufacturability and increased costs. Therefore, when adding Sn and Sb, the Sn and Sb content should be 0.10% or less. When Sn and Sb are included, there is no particular lower limit for their respective content, but from the perspective of easily obtaining the above effects, it is preferable that the Sn and Sb content be 0.002% or more each. It is more preferable that the Sn and Sb content be 0.010% or more each.

[0045] Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less Cu, Ni, and Cr are elements that increase the resistivity of steel and reduce iron loss. However, if the Cu, Ni, and Cr content exceeds 2.0%, the effect of reducing iron loss saturates, leading to increased costs. Therefore, when adding Cu, Ni, and Cr, the Cu, Ni, and Cr content should be 2.0% or less. When Cu, Ni, and Cr are included, there is no particular lower limit to the content of each element, but from the perspective of easily obtaining the above effects, it is preferable that the content of Cu, Ni, and Cr be 0.01% or more each. It is more preferable that the content of Cu, Ni, and Cr be 0.05% or more each.

[0046] Mo: 0.10% or less Mo is an element that has the effect of improving the toughness of steel sheet 1. However, if the Mo content exceeds 0.10%, the above effect saturates, and only the alloy cost increases. For this reason, when Mo is added, the Mo content should be 0.10% or less. When Mo is included, there is no particular lower limit to the content, but from the perspective of easily obtaining the above effect, it is preferable that the Mo content be 0.001% or more. It is more preferable that the Mo content be 0.002% or more.

[0047] Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.03% or less Ca, Mg, and REM are elements that form sulfides to fix sulfur, improve grain growth during stress-relieving annealing, and contribute to reducing iron loss. However, excessive amounts of these components may worsen economic efficiency. Therefore, when adding Ca, Mg, and REM, the Ca and Mg content should be 0.01% or less, and the REM content should be 0.03% or less. When Ca, Mg, and REM are included, there is no particular lower limit to the content of each, but from the perspective of making it easier to obtain the above effects, it is preferable that the Ca and Mg content be 0.0001% or more each. Furthermore, it is preferable that the REM content be 0.001% or more. It is more preferable that the Ca and Mg content be 0.0005% or more each. Furthermore, it is more preferable that the REM content be 0.005% or more.

[0048] In addition to the above components, the non-oriented electrical steel sheet according to one embodiment of the present invention may optionally further contain one or more elements selected from B, Ti, Nb, V, Pb, As, Zn, Co, Ge, and Ga.

[0049] B: 0.0020% or less B is an element that improves the toughness of steel plates. However, if the B content exceeds 0.0020%, iron loss increases significantly. For this reason, when B is included, the upper limit should be 0.0020%. When B is included, there is no particular lower limit to the content, but from the perspective of easily obtaining the above effect, it is preferable that the B content be 0.0005% or more.

[0050] Ti: 0.010% or less, Nb: 0.0050% or less, V: 0.050% or less, Pb: 0.020% or less Ti, Nb, V, and Pb are elements that improve the hardness of steel sheets by forming fine precipitates and refining the crystal grains. However, if the Ti content exceeds 0.010%, the Nb content exceeds 0.0050%, the V content exceeds 0.050%, and the Pb content exceeds 0.020%, the hysteresis loss increases significantly. For this reason, when Ti, Nb, V, and Pb are included, the Ti content should be 0.010% or less, the Nb content 0.0050% or less, the V content 0.050% or less, and the Pb content 0.020% or less. When Ti, Nb, V, and Pb are included, there is no particular lower limit for the content of each element, but from the perspective of easily obtaining the above effect, it is preferable that the Ti, V, and Pb content be 0.002% or more, and the Nb content be 0.0005% or more.

[0051] As: 0.020% or less As (As) is an element that improves the hardness of steel sheets. However, if the As content exceeds 0.020%, the steel becomes brittle and difficult to roll. Therefore, when As is included, the As content should be 0.020% or less. While there is no particular lower limit to the As content, from the viewpoint of suppressing refining costs in the refining process, it is preferable that the As content be 0.0005% or more.

[0052] Zn: 0.010% or less Zn is an element that has the effect of coarsening inclusions and reducing iron loss. However, if the Zn content exceeds 0.010%, not only does the above effect saturate, but the steel becomes brittle and difficult to roll. For this reason, when Zn is included, the Zn content should be 0.010% or less. When Zn is included, there is no particular lower limit to the Zn content, but from the perspective of easily obtaining the above effect, it is preferable that the Zn content be 0.0005% or more.

[0053] Co:0.10% or less Co is an element that has the effect of improving magnetic flux density. However, if the Co content exceeds 0.10%, the steel becomes brittle and difficult to roll. For this reason, when Co is included, the Co content should be 0.10% or less. When Co is included, there is no particular lower limit to the Co content, but from the perspective of easily obtaining the above effect, it is preferable that the Co content be 0.0010% or more.

[0054] Ge: 0.030% or less, Ga: 0.030% or less Ge and Ga are both elements that significantly improve texture and increase magnetic flux density. However, if the Ge and Ga content exceeds 0.030% each, the effect saturates, and it leads to decreased manufacturability and increased costs. Therefore, when Ge and Ga are included, their content should be 0.030% or less each. When Ge and Ga are included, there is no particular lower limit to their respective content, but from the perspective of easily obtaining the above effects, it is preferable that the Ge and Ga content be 0.0005% or more each.

[0055] Furthermore, if the content of any of the above optional components is below the preferred lower limit value mentioned above, those components shall be considered to be included as unavoidable impurities.

[0056] [plate thickness] If the non-oriented electrical steel sheet 1 is too thin, handling during manufacturing processes such as cold rolling and annealing becomes difficult, increasing manufacturing costs. Therefore, it is preferable that the thickness t of the non-oriented electrical steel sheet 1 be 0.01 mm or more. On the other hand, if the steel sheet 1 is too thick, eddy current losses increase, and total iron losses increase. Therefore, it is preferable that the thickness t be 0.35 mm or less.

[0057] [Iron loss] The non-oriented electrical steel sheet 1 of the present invention has an iron loss (total iron loss) of W at a frequency of 2 kHz and a maximum magnetic flux density of 1.0 T. 10 / 2k It is preferable that the (W / kg) and the plate thickness: t (mm) satisfy the following formula (1). W 10 / 2k ≤ 30 + 450 × t (1)

[0058] When the iron loss of the non-oriented electrical steel sheet 1 satisfies the relationship in equation (1) above, the heat generation of the stator core made using the corresponding non-oriented electrical steel sheet 1 is suppressed, and as a result, the motor efficiency can be further improved. Note that the iron loss W 10 / 2k Since it depends on the plate thickness t, the iron loss W in equation (1) above takes into account the effect of the plate thickness t. 10 / 2k It specifies an upper limit.

[0059] [Manufacturing method] The non-oriented electrical steel sheet 1 of the present invention is not particularly limited, but can be manufactured using a silicate dilution method. When using a silicate dilution method, for example, the Si content of the surface layer 20 on both sides of the steel sheet can be increased by applying a silicate diffusion treatment to a steel sheet in which the Si content is constant in the thickness direction. The Si content before the silicate diffusion treatment is not particularly limited, but if the Si content exceeds 4.50%, it is necessary to strictly control the temperature and speed during cold rolling, and from the viewpoint of cold rolling efficiency, it is preferable to keep it at 4.50% or less.

[0060] The method of silicate diffusion treatment is not particularly limited and can be carried out by any method. For example, silicate treatment can be performed by increasing the Si content on the surface of the steel sheet using the CVD method, and then a diffusion treatment can be performed by heat treatment to diffuse Si into the interior of the steel sheet. When using the CVD method, the Si content of the surface layer 20 and the inner layer 10 can be controlled by adjusting the amount of Si increased by the CVD method and the heat treatment conditions for the diffusion treatment. The non-oriented electrical steel sheet 1 obtained by silicate diffusion treatment using the CVD method has, for example, a Si content profile in the thickness direction as shown in Figure 2.

[0061] In the manufacturing of the steel sheet 1 of the present invention, prior to the silicate diffusion treatment, the base material is ground to a depth of 0.5 μm or more from the surface. The grinding method is not particularly limited, but examples include acid treatment and mechanical polishing. Furthermore, the steel sheet after the grinding treatment is subjected to a silicate treatment to increase the Si content on the surface of the steel sheet, and then a diffusion treatment is performed by heat treatment to diffuse Si into the interior of the steel sheet. When using the CVD method, the dew point of the atmosphere in the silicate treatment is controlled to be -50°C or lower, and the dew point of the atmosphere in the diffusion treatment is controlled to be -50°C to -30°C, and the annealing temperature of the silicate treatment and diffusion treatment is set to 1100°C or higher. By performing these treatments, Al concentrated on the surface during the base material manufacturing stage is removed, and after the silicate diffusion treatment, an oxide film 30 with a magnetite content (C_mag) of 65% or more is formed on the surface of the steel sheet. In addition, in the manufacturing of the steel sheet 1 of the present invention, the average thickness tc of the oxide film 30 can be changed by controlling the dew point in the diffusion treatment. [Examples]

[0062] To confirm the effects of the present invention, non-oriented electrical steel sheets were manufactured using the procedure described below, and their magnetic properties were evaluated.

[0063] [Steel slab] First, steel slab samples with sample symbols A to AL having the component compositions shown in Table 1 were prepared. The content of C, Mn, Al, Sn, P, S, Sn, Sb, Cu, Ni, Cr, Mo, Ca, Mg, REM, B, Ti, Nb, V, Pb, As, Zn, Co, Ge, and Ga in the used steel slabs does not change throughout the manufacturing process and is therefore equal to the content in the final non-oriented electrical steel sheet.

[0064] [Hot rolling, hot-rolled sheet annealing] The aforementioned steel slab was hot-rolled to form a hot-rolled steel sheet, and then the hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 950°C for 30 seconds.

[0065] [Cold rolling] Next, the hot-rolled steel sheets, after annealing, were subjected to cold rolling to produce cold-rolled steel sheets with the thickness t shown in Table 2. However, steel sheets with sample symbols C, G, and K fractured during the cold-rolling process and therefore did not undergo any further processing after cold rolling. It is believed that these steel sheets fractured because they contained an excessive amount of at least one of the elements Si, Mn, and P.

[0066] [Silicon Diffusion Treatment] Non-oriented electrical steel sheets were obtained by subjecting the resulting cold-rolled steel sheets to a silicate diffusion treatment. Specifically, first, the Si content in the surface layer of the steel sheet was increased by silicate treatment using the CVD method at 1200°C in an atmosphere consisting of N2 gas and SiCl4. Next, the Si in the surface layer of the steel sheet was diffused into the interior by diffusion treatment (heat treatment) in an N2 atmosphere at the temperatures listed in Table 2. Furthermore, the dew point of the atmosphere was controlled to -70°C for the silicate treatment and to the values ​​listed in Table 2 for the diffusion treatment.

[0067] [Si content] The obtained non-oriented electrical steel sheets were embedded in carbon molds, and the Si content distribution in the thickness direction cross-section was measured using an electron probe microanalyzer (EPMA). Using the total thickness of the steel sheet t (mm) as a reference, one surface position was defined as 0, and the other surface position as t. The regions from 0 to t / 4 and 3t / 4 to t were defined as the surface layer, and the region greater than t / 4 and less than 3t / 4 was defined as the inner layer. From the Si content distribution obtained from the EPMA, the average Si content in the surface layer, the average Si content in the inner layer, and the difference between these values ​​(ΔSi) were calculated. The measurement results are shown in Table 2.

[0068] Furthermore, as also mentioned above, the content of C, Mn, Al, Sn, P, S, Sn, Sb, Cu, Ni, Cr, Mo, Ca, Mg, REM, B, Ti, Nb, V, Pb, As, Zn, Co, Ge, and Ga does not change throughout the manufacturing process, so the content in the final non-oriented electrical steel sheet is equal to the content in the steel slab used.

[0069] [Oxide film] Five samples were cut from the obtained non-oriented electrical steel sheet at 50 mm intervals in the width direction. These samples were embedded in a mold so that the cross-section was visible, and the steel sheet surface was observed using bright-field imaging with a TEM (acceleration voltage 200 kV, irradiation current 100 nA, imaging resolution 1 k × 1 k pixels). The thickness of the oxide film formed on the steel sheet surface was measured from the bright-field TEM image, and the average value for each sample was defined as tc. If no oxide film was observed, the thickness was set to 0 (mm). Points were also plotted at 100 nm intervals on the bright-field TEM image of the steel sheet surface, and the percentage of points on which an oxide film was present was defined as the coverage rate. Furthermore, the composition of the oxide film was determined from the diffraction pattern measurement by micro-electron diffraction and the results of EDX measurement, and the main components of the oxide film were investigated. In addition, the proportion of magnetite (C_mag) contained in the oxide film was measured from the results and the bright-field TEM image. These measurement results are shown in Table 2.

[0070] [Iron loss] Furthermore, a 30mm wide, 280mm long test specimen was taken from each of the obtained non-oriented electrical steel sheets, and the iron loss was measured in the Epstein test in accordance with JISC2550-1:2011 at a maximum magnetic flux density of 1.0T and a frequency of 2kHz. 10 / 2k (W / kg) was measured. In the Epstein test described above, equal amounts of L-direction specimens, taken with the length direction of the specimen aligned with the rolling direction (L direction), and C-direction specimens, taken with the length direction of the specimen aligned perpendicular to the rolling direction (C direction), were used, and the average values ​​of the magnetic properties in the L and C directions were evaluated. The measurement results are shown in Table 2. As can be seen from the results shown in Table 2, the non-oriented electrical steel sheet that satisfies the conditions of the present invention had excellent properties, such as low high-frequency iron loss.

[0071] [Table 1]

[0072] [Table 2] [Industrial applicability]

[0073] According to the present invention, it is possible to achieve low iron loss in Si gradient magnetic materials mainly used in motor applications. [Explanation of Symbols]

[0074] 1 Non-oriented electrical steel sheet 10 Inner layer 20 Surface layer 30 Oxide film 40 Insulating coating

Claims

1. A non-oriented electrical steel sheet comprising an inner layer and surface layers located on both sides of the inner layer, With the plate thickness t (mm) as the reference, when the position of one surface of the non-oriented electrical steel sheet is set to 0 and the position of the other surface is set to t, the surface layer is defined as the region in the range of 0 to t / 4 and 3t / 4 to t. When the aforementioned inner layer is defined as a region greater than t / 4 and less than 3t / 4, In mass percent, The average Si content of the surface layer ([Si] １ ) is between 3.50% and 7.00%, The average Si content of the inner layer ([Si] ０ ) is between 2.00% and 6.50%, The average content across the entire plate thickness, C: 0.010% or less, Mn: 2.0% or less, Al: 0.10% or less, P: 0.20% or less, S: 0.0050% or less, N: 0.0080% or less, The composition has the following characteristics: O: contains 0.0050% or less, with the remainder being Fe and unavoidable impurities. The above [Si] １ and [Si] ０ The difference between ([Si]) １ - [Si] ０ ΔSi, defined as ), is between 0.10% and 2.50%. On the surface of the non-oriented electrical steel sheet, magnetite (Fe ３ O ４ The oxide film has a proportion (C_mag) of 65% or more. Non-oriented electrical steel sheet having an average thickness tc of the oxide film of 5 nm or more and 120 nm or less.

2. The above component composition is further expressed as the average content across the entire plate thickness. In mass percent, Sn: 0.10% or less, Sb: 0.10% or less, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 0.10% or less Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.03% or less, Includes at least one selected from the group consisting of, The non-oriented electrical steel sheet according to claim 1.

3. The above component composition is further expressed as the average content across the entire plate thickness. In mass percent, B: 0.0020% or less, Ti: 0.010% or less, Nb: 0.0050% or less, V: 0.050% or less, Pb: 0.020% or less, As: 0.020% or less, Zn: 0.010% or less, Co: 0.10% or less, Ge: 0.030% or less, Ga: 0.030% or less, Includes at least one selected from the group consisting of, Non-oriented electrical steel sheet according to claim 1 or 2.

4. The non-oriented electrical steel sheet according to claim 1 or 2, wherein the coverage rate of the oxide film is 70% or more.

5. The non-oriented electrical steel sheet according to claim 1 or 2, wherein the plate thickness t is 0.01 mm or more and 0.35 mm or less.

6. The non-oriented electrical steel sheet according to claim 3, wherein the coverage rate of the oxide film is 70% or more.

7. The non-oriented electrical steel sheet according to claim 3, wherein the plate thickness t is 0.01 mm or more and 0.35 mm or less.

8. The non-oriented electrical steel sheet according to claim 4, wherein the plate thickness t is 0.01 mm or more and 0.35 mm or less.

9. The non-oriented electrical steel sheet according to claim 6, wherein the plate thickness t is 0.01 mm or more and 0.35 mm or less.

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