Non-oriented electromagnetic steel sheet

Abstract

The present invention addresses the problem of reducing the iron loss of a non-oriented electromagnetic steel sheet used in motor applications for electric vehicles and hybrid electric vehicles. Provided is a non-oriented electromagnetic steel sheet 1 comprising an inner layer part 10 and surface layer parts 20 positioned on both sides of the inner layer part 10, wherein: the steel sheet 1 has a component composition containing, in terms of mass%, predetermined amounts of Si, C, Mn, Al, P, S, N, and O, with the balance consisting of Fe and inevitable impurities; the difference between the average Si content of the surface layer parts 20 and the average Si content of the inner layer part 10 is 0.10% or less; the steel sheet 1 has, on a surface thereof, an oxide film 30 in which the ratio (C_mag) of magnetite (Fe3O4) is 65% or more; and the average thickness tc of the oxide film 30 is 5-120 nm.

Description

Non-oriented electrical steel sheet

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

[0002] Motors used in electric vehicles, hybrid electric vehicles, etc. are driven at high frequencies of 400 Hz to 1 kHz in order to achieve compactness and high efficiency. Therefore, the non-oriented electrical steel sheets used as the core material of such motors are required to have low high-frequency iron loss and high magnetic flux density.

[0003] Reducing the high-frequency iron loss of non-oriented electrical steel sheets can be effectively achieved by reducing their thickness or increasing their resistivity. However, reducing the thickness of non-oriented electrical steel sheets increases the costs of rolling and annealing, and increasing their resistivity reduces their rollability and workability. For these reasons, there is a need to establish new methods for reducing iron loss.

[0004] As a method for reducing iron loss other than thinning or increasing specific resistance, so-called Si gradient magnetic materials have been developed, which control the Si concentration gradient in the sheet thickness direction. For example, Patent Document 1 discloses an electrical steel sheet having a Si concentration gradient in the sheet thickness direction, in which the Si concentration at the steel sheet surface is higher than the Si concentration at the center of the sheet thickness, the area with an Si concentration of 5 to 8% extends from both surfaces of the steel sheet in the sheet thickness direction to 10% or more of the sheet thickness, and the Si concentration at the center of the sheet thickness is 3.4% or more.

[0005] Another known method for reducing iron loss is to improve hysteresis loss by smoothing the irregularities on the surface of an electrical steel sheet. For example, Patent Document 2 discloses a method for smoothing the surface of a grain-oriented electrical steel sheet and reducing hysteresis loss by removing an oxide film by pickling or the like and then performing chemical polishing or mechanical polishing. Patent Document 3 also discloses a method for removing an oxide film and achieving a mirror finish on the surface by incorporating a Bi chloride into the annealing separator of the grain-oriented electrical steel sheet.

[0006] JP-A-11-293422 JP-A-64-83620 Patent No. 2647333

[0007] However, materials with a Si concentration gradient in the sheet thickness direction, such as those described in Patent Document 1, exhibit reduced eddy current loss but increased hysteresis loss. Therefore, when used as an iron core material for electric vehicles or hybrid electric vehicles (e.g., frequency: 400 Hz to 1 kHz, magnetic flux density: 1.0 to 2.0 T), the proportion of hysteresis loss is high and iron loss is not sufficiently reduced. Furthermore, the method disclosed in Patent Document 2 has the problem of reduced coating adhesion because the oxide coating is removed and the surface becomes smooth. Furthermore, the method disclosed in Patent Document 3 requires the application of an annealing separator and annealing for several tens of hours, making it unsuitable for non-oriented electrical steel sheets.

[0008] Therefore, an object of the present invention is to reduce iron loss in non-oriented electrical steel sheets used mainly for motors of electric vehicles and hybrid electric vehicles.

[0009] To solve the above problems, the inventors have conducted extensive research focusing on the influence of components on the magnetic properties of non-oriented electrical steel sheets. ３ O ４ It was found that by forming a coating of ) it is possible to reduce iron loss.

[0010] The present invention has been made based on the above findings, and the gist and configuration thereof are as follows.

[0011] [1] A non-oriented electrical steel sheet comprising an inner layer portion and surface layer portions located on both sides of the inner layer portion, the non-oriented electrical steel sheet having a component composition containing, by mass%, Si: 3.50% or more and 7.00% or less, 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, O: 0.0050% or less, with the balance being Fe and inevitable impurities, the surface layer portion is defined as a region ranging from 0 to t / 4 and from 3t / 4 to t from the surface of the non-oriented electrical steel sheet, based on the sheet thickness t (mm), and the inner layer portion is defined as a region ranging from more than t / 4 to less than 3t / 4 from the surface of the non-oriented electrical steel sheet, and the average Si content ([Si]) of the surface layer portion is １ ) and the average Si content of the inner layer portion ([Si] ０) is less than 0.10%, and magnetite (Fe) is present on the surface of the non-oriented electrical steel sheet. ３ O ４ ) ratio (C_mag) is 65% or more, and the average thickness t ｃ A non-oriented electrical steel sheet having a grain size of 5 nm or more and 120 nm or less.

[0012] [2] The non-oriented electrical steel sheet according to [1], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of 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, as an average content across the entire sheet thickness.

[0013] [3] The non-oriented electrical steel sheet according to [1] or [2], wherein the chemical composition further includes at least one selected from the group consisting of, in average contents across the entire sheet 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, Co: 0.10% or less, Ge: 0.30% or less, and Ga: 0.030% or less.

[0014] [4] The non-oriented electrical steel sheet according to any one of [1] to [3], wherein the coverage of the oxide film is 70% or more.

[0015] [5] The non-oriented electrical steel sheet according to any one of [1] to [4], wherein the sheet thickness t is 0.01 mm or more and 0.35 mm or less.

[0016] According to the present invention, it is possible to realize low iron loss in non-oriented electrical steel sheets used mainly for motor applications in electric vehicles and hybrid electric vehicles.

[0017] 1 is a schematic diagram showing the structure of a non-oriented electrical steel sheet according to the present invention; and FIG. 2 is an enlarged schematic diagram showing the surface vicinity of the non-oriented electrical steel sheet shown in FIG. 1. The average thickness tc of the oxide film and the iron loss (W １０／８００ ) is a diagram showing the relationship between

[0018] A method for carrying out the present invention will be specifically described below. Note that the following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

[0019] Fig. 1 is a schematic diagram showing the structure of a non-oriented electrical steel sheet of the present invention. In the non-oriented electrical steel sheet 1 (hereinafter sometimes simply referred to as "steel sheet") shown in Fig. 1, the total thickness t (mm) of the steel sheet 1 is taken as the reference, and the position of one surface of the steel sheet 1 is defined as 0, and the position of the other surface of the steel sheet 1 is defined as t. The regions ranging from 0 to t / 4 and 3t / 4 to t are defined as surface layer portions 20, and the region ranging from greater than t / 4 to less than 3t / 4 is defined as inner layer portions 10. As shown in Fig. 1, the steel sheet 1 is composed of inner layer portions 10 and surface layer portions 20 provided on both sides of the inner layer portion 10.

[0020] [Oxide Film] Fig. 2 is an enlarged view of the surface vicinity of the non-oriented electrical steel sheet 1 shown in Fig. 1. As shown in Fig. 2, in the steel sheet 1 of the present invention, magnetite (Fe ３ O ４ 2, an additional coating such as an insulating coating 40 may be provided on the surface of the oxide coating 30 as needed. The reason for providing the oxide coating 30 will now be described.

[0021] The inventors have been studying steel sheet components and heat treatment methods in order to further reduce the iron loss of non-oriented electrical steel sheet 1, and have found that steel sheet 1, on whose surface an oxide film with a magnetite ratio (C_mag) of 65% or more is formed, exhibits a significant reduction in iron loss. This is thought to be due to the following reasons.

[0022] That is, during the manufacturing process of the steel sheet, fine irregularities are formed on the surface of the steel sheet, and these irregularities hinder the movement of magnetic domains, increasing hysteresis loss. When the oxide film 30 is formed on the surface of the steel sheet, the fine irregularities on the surface of the steel sheet are oxidized, smoothing the surface of the steel sheet. When the steel sheet surface is smoothed, the resistance to the movement of magnetic domains decreases, thereby reducing hysteresis loss. As a result, it is believed that the iron loss of the steel sheet 1 having the magnetite film 30 formed on its surface is significantly reduced.

[0023] In addition, the oxide film of Al, and wustite (FeO), hematite (Fe ２ O ３ While other iron-based oxide films such as those made of magnetite are non-uniform in thickness, magnetite forms a thin, uniform scale on the surface of a steel sheet. For this reason, it is believed that when an oxide film made mainly of magnetite is formed on the surface of a steel sheet, it can effectively reduce the fine irregularities on the steel sheet surface and lower iron loss.

[0024] Furthermore, the inventors investigated the relationship between the thickness of the oxide film 30 and iron loss. The specific procedure is explained below. First, a steel slab sample having the composition 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 the surface of the obtained cold-rolled steel sheet was ground by 1 μm on both sides by mechanical polishing. Next, using the ground steel sheet, N ２ gas, and SiCl ４ The steel sheet was siliconized at 1200°C by CVD in an atmosphere containing N ２ The steel sheet was subjected to a diffusion treatment (heat treatment) at 1200°C for 30 seconds in an atmosphere to diffuse Si from the surface layer of the steel sheet to the interior. At this time, the dew point of the atmosphere in the siliconizing treatment was controlled to -70°C, and the dew point of the atmosphere in the diffusion treatment was controlled to -70 to -30°C, thereby producing the steel sheet.

[0025] Using the above method, the target value for the Si content was set to 5.0%, and the average Si content ([Si] １ ) and the average Si content of the inner layer portion 10 ([Si] ０ ) and the difference ΔSi ([Si]１ -[Si] ０ ) less than 0.10 mass % were produced.

[0026] Samples for transmission electron microscope (TEM) observation were taken from each of the steel sheets produced by the above method, and the cross-sectional direction of the steel sheets was observed using TEM bright-field images. Microelectron diffraction using TEM and energy dispersive X-ray analysis (EDX) were performed on the oxide film formed on the steel sheet surface to determine the composition of the formed oxide film. As a result, it was confirmed that an oxide film with a magnetite ratio (C_mag) of 65% or more was formed.

[0027] The average thickness tc of the oxide film was evaluated by cutting five samples from the steel sheet at 50 mm intervals in the sheet width direction, observing the cross sections of the samples using a TEM bright field image (magnification: 100,000 times), measuring the thickness of the oxide film for each sample, and averaging the measured values. In addition, when no oxide film was observed, the thickness was recorded as 0.

[0028] Further, test pieces of 30 mm in width and 280 mm in length were taken from each of the steel plates, and in accordance with JIS C2550-1:2011, an Epstein test was carried out at a maximum magnetic flux density of 1.0 T and a frequency of 800 Hz to measure the iron loss: W １０／８００ In the Epstein test, equal amounts of L-direction test pieces, which were taken so that the length direction of the test pieces was the rolling direction (L-direction), and C-direction test pieces, which were taken so that the length direction of the test pieces was 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.

[0029] FIG. 3 shows the relationship between the average thickness tc of the oxide film and the iron loss (W １０／８００) shows the relationship between the thickness and the surface roughness. It can be seen that excellent iron loss is exhibited when tc is in the range of 5 nm or more and 120 nm or less. The mechanism behind this is thought to be as follows. That is, when tc is in the range of less than 5 nm, the thickness of the coating is small compared to the irregularities on the steel sheet surface, so smoothing is insufficient and iron loss cannot be fully reduced. However, when tc is in the range of 5 nm or more and 120 nm or less, the coating smoothes the steel sheet surface, so iron loss decreases. Furthermore, when tc exceeds 120 nm, the thickness of the coating is no longer uniform, making it difficult to sufficiently uniformize the steel sheet surface, which is thought to result in increased iron loss.

[0030] Based on the above-described results, in the present invention, the average thickness tc of the oxide coating 30 is set to 5 nm or more and 120 nm or less, and preferably set to 5 nm or more and 100 nm or less.

[0031] The coverage of the oxide film 30 on the steel sheet surface is preferably 70% or more. In the present invention, the coverage refers to the proportion of the surface area of ​​the non-oriented electrical steel sheet 1 on which the oxide film 30 is formed. A specific method for measuring the coverage is to cut five or more samples from the non-oriented electrical steel sheet 1 at 50 mm intervals in the sheet width direction, photograph the cross section of each sample near the steel sheet surface using a TEM bright-field image, and then mark dots at 100 nm intervals on the steel sheet surface in the photograph. The coverage is defined as the proportion of the oxide film 30 present on the dots relative to the total number of dots. Typical test conditions are an acceleration voltage of 200 kV, a probe current of 100 nA, a photographing resolution of 1 k × 1 k pixels, a magnification of 30,000 times, and a total of 25 or more measurement points.

[0032] When the coverage of the oxide film 30 on the surface of the non-oriented electrical steel sheet 1 is 70% or more, iron loss can be effectively reduced, and as a result, motor efficiency can be further improved. The coverage is more preferably 80% or more. However, since a higher coverage is desirable, there is no upper limit and it can be 100%.

[0033] Furthermore, the proportion of magnetite in the oxide film 30 (C_mag) is 65% or more. In the present invention, the proportion of magnetite in the oxide film 30 is the area ratio of magnetite in the cross section of the oxide film 30, which is obtained by observing the oxide film 30 in the cross-sectional direction of the steel sheet using a TEM bright-field 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. The proportion of magnetite in the oxide film 30 is preferably 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%.

[0034] [Composition] Next, the composition of the non-oriented electrical steel sheet 1 of the present invention will be described. In the following description, "%" representing the content of each element represents "mass %" unless otherwise specified. Furthermore, the content of each element represents the average content across the entire sheet thickness unless otherwise specified.

[0035] The non-oriented electrical steel sheet 1 of the present invention has a chemical composition containing Si, C, Mn, Al, P, S, N, and O, with the balance being Fe and inevitable impurities. Here, inevitable impurities refer to impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be present to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of impurities include H, Ti, and Co.

[0036] Si: 3.50% or more and 7.00% or less. Si is an element that increases the electrical resistance of steel sheets and reduces eddy current loss. If the Si content is less than 3.50%, eddy current loss cannot be effectively reduced. Therefore, the Si content is set to 3.50% or more, preferably 4.00% or more, and more preferably 4.50% or more. When performing general cold rolling, if the Si content exceeds 4.50%, the steel sheet becomes prone to fracture, so treatments such as warm rolling or siliconizing diffusion treatment are required. To manufacture a steel sheet with a Si content exceeding 7.00% by siliconizing diffusion treatment, the siliconizing diffusion treatment requires a long time. Furthermore, the elongation of the material is significantly reduced, which increases the likelihood of defects in the iron core manufacturing process. Therefore, the Si content is set to 7.00% or less, preferably 6.70% or less.

[0037] ΔSi ([Si] １ -[Si] ０ ) is less than 0.10% ΔSi is the average Si content ([Si] １ ) and the average Si content in the inner layer portion 10 ([Si] ０ ) and the difference ([Si] １ -[Si] ０ ) is a value defined as. Under relatively high frequency conditions (1 kHz or higher), by providing a difference in Si concentration in the sheet thickness direction, eddy current loss can be reduced by the skin effect. On the other hand, when used as an iron core material for motors for electric vehicles, etc., as envisioned by the present invention, the proportion of hysteresis loss is large and iron loss is not sufficiently reduced. For this reason, ΔSi is set to less than 0.10%. There is no particular lower limit for ΔSi, but excessive reduction leads to increased processing time and costs, so ΔSi is preferably set to 0.005% or more.

[0038] C: 0.010% or less C contained in steel sheet 1 is a harmful element that forms carbides, causes magnetic aging, and deteriorates iron loss characteristics. Therefore, the C content in steel sheet 1 is set to 0.010% or less, and preferably 0.007% or less. On the other hand, although there is no particular lower limit for the C content, from the viewpoint of suppressing decarburization costs in the refining process, the C content is preferably set to 0.0001% or more.

[0039] Mn: 2.0% or less Mn is also an element that increases the resistivity of the steel sheet 1 and reduces iron loss. From the viewpoint of suppressing fine precipitation of sulfides, the Mn content is preferably 0.01% or more. On the other hand, if the Mn content exceeds 2.0%, slab cracking and other problems occur, deteriorating the operability of the steelmaking process. Therefore, the Mn content is set to 2.0% or less. If the Mn content is 0.2% or more, if the amount of base material ground is insufficient, the enriched layer in the surface layer cannot be removed, oxides are formed, and the formation of an oxide film 30 having a magnetite proportion (C_mag) of 65% or more is inhibited. Therefore, from the viewpoint of manufacturing stability, the Mn content is preferably 0.2% or less. More preferably, it is 0.1% or less.

[0040] Al: 0.10% or less Like Si, Al is a useful element that increases the resistivity of steel and reduces iron loss. However, Al is more easily oxidized than Fe, and if the Al content exceeds 0.10%, Al-based oxides are formed preferentially over magnetite. Unlike magnetite coatings, these Al-based oxides are formed unevenly, making it impossible to eliminate surface irregularities on the steel sheet and to sufficiently reduce iron loss. For this reason, the Al content is set to 0.10% or less. Reducing the Al content to 0.010% or less improves the texture and increases the magnetic flux density. For this reason, when magnetic flux density is important, the Al content is preferably set to 0.010% or less. The Al content is more preferably set to 0.003% or less. Although there is no particular lower limit for the Al content, an excessive reduction increases refining costs, so the Al content is preferably set to 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 embrittled and rolling becomes difficult. For this reason, the P content is set to 0.20% or less. Although there is no particular restriction on the lower limit of the P content, from the viewpoint of suppressing the cost of dephosphorization in the refining process, the P content is preferably set to 0.001% or more.

[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, inhibit grain growth during annealing, and adversely affect 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. Therefore, the S content is set to 0.0050% or less, the N content is set to 0.0080% or less, and the O content is set to 0.0050% or less. While the lower limits of the S content, N content, and O content are not particularly specified, from the viewpoint of reducing costs in the refining process, it is preferable that the S content be 0.0002% or more, the N content be 0.0005% or more, and the O content be 0.0005% or more.

[0043] In one embodiment of the present invention, the composition of the non-oriented electrical steel sheet 1 may optionally contain, in addition to the above components, one or more elements 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 have the effect of significantly improving the texture, increasing the magnetic flux density, and further reducing hysteresis loss. However, if the Sn content and Sb content exceed 0.10%, the effect saturates, and manufacturability and costs increase. Therefore, when Sn and Sb are added, the Sn content and Sb content are set to 0.10% or less. When Sn and Sb are contained, there are no particular restrictions on the lower limits of their respective contents, but in order to more easily obtain the above effects, it is preferable that the Sn and Sb contents are 0.001% or more. It is more preferable that the Sn and Sb contents are 0.005% or more.

[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 content, Ni content, and Cr content are 2.0% or more, the effect of reducing iron loss saturates, resulting in increased costs. Therefore, when Cu, Ni, and Cr are added, the Cu content, Ni content, and Cr content are set to 2.0% or less. When Cu, Ni, and Cr are contained, the lower limits of their respective contents are not particularly limited, but in order to more easily obtain the above-mentioned effects, it is preferable that the Cu, Ni, and Cr contents are each 0.01% or more. It is more preferable that the Cu, Ni, and Cr contents are each 0.05% or more.

[0046] Mo: 0.10% or less Mo is an element that has the effect of improving the toughness of the steel plate 1. However, if the Mo content exceeds 0.10%, this effect saturates and the alloy cost simply increases. Therefore, when Mo is added, the Mo content is set to 0.10% or less. When Mo is contained, there is no particular lower limit for the content, but from the viewpoint of making it easier to obtain the above-mentioned effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.002% or more.

[0047] Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.03% or less. Ca, Mg, and REM form sulfides to fix S, improve grain growth during stress relief annealing, and contribute to iron loss reduction. However, excessive inclusion of these elements may result in poor economic efficiency. Therefore, when Ca, Mg, and REM are added, the Ca content and Mg content are set to 0.01% or less, and the REM content is set to 0.03% or less. When Ca, Mg, and REM are contained, the lower limits of their respective contents are not particularly limited. However, in order to more easily obtain the above-mentioned effects, it is preferable that the Ca and Mg contents are each set to 0.0001% or more. Furthermore, it is preferable that the REM content be set to 0.001% or more. It is more preferable that the Ca and Mg contents are each set to 0.0005% or more. It is more preferable that the REM content be set to 0.005% or more.

[0048] A non-oriented electrical steel sheet according to one embodiment of the present invention may further contain, in addition to the above components, 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 has the effect of improving the toughness of steel plate. However, if the B content exceeds 0.0020%, iron loss increases significantly. Therefore, when B is contained, the upper limit is set to 0.0020%. When B is contained, the lower limit of the content is not particularly limited, but from the viewpoint of making it easier to obtain the above-mentioned effect, the B content is preferably set to 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 form fine precipitates and refine crystal grains, thereby improving the hardness of the steel sheet. 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. Therefore, when Ti, Nb, V, and Pb are contained, the Ti content should be 0.010% or less, the Nb content should be 0.0050% or less, the V content should be 0.050% or less, and the Pb content should be 0.020% or less. When Ti, Nb, V, and Pb are contained, the lower limit of each content is not particularly limited. However, in order to more easily obtain the above-mentioned effects, it is preferable that the contents of Ti, V, and Pb are each 0.002% or more, and the Nb content is 0.0005% or more.

[0051] As: 0.020% or less As is an element that has the effect of improving the hardness of steel sheet. However, if the As content exceeds 0.020%, the steel becomes embrittled and rolling becomes difficult. Therefore, when As is contained, the As content is set to 0.020% or less. Note that the lower limit of the As content is not particularly limited, but from the viewpoint of suppressing the refining cost in the refining process, the As content is preferably set to 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 embrittled and difficult to roll. Therefore, if Zn is contained, the Zn content is set to 0.010% or less. If Zn is contained, there is no particular lower limit for the Zn content, but from the viewpoint of making the above effect more easily obtainable, the Zn content is preferably set to 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 embrittled and rolling becomes difficult. Therefore, when Co is contained, the Co content is set to 0.10% or less. When Co is contained, there is no particular lower limit for the Co content, but from the viewpoint of making it easier to obtain the above-mentioned effect, the Co content is preferably set to 0.0010% or more.

[0054] Ge: 0.030% or less, Ga: 0.030% or less Both Ge and Ga are elements that significantly improve texture and increase magnetic flux density. However, if the Ge and Ga contents exceed 0.030%, the effects saturate, and manufacturability and costs increase. Therefore, when Ge and Ga are contained, the Ge and Ga contents are each set to 0.030% or less. When Ge and Ga are contained, there are no particular restrictions on the lower limits of their respective contents, but from the viewpoint of more easily achieving the above effects, it is preferable that the Ge and Ga contents are each set to 0.0005% or more.

[0055] When the content of any of the above optional components is less than the respective preferred lower limit values ​​mentioned above, the component is considered to be contained as an unavoidable impurity.

[0056] [Sheet Thickness] If the non-oriented electrical steel sheet 1 is too thin, it becomes difficult to handle during manufacturing processes such as cold rolling and annealing, increasing manufacturing costs. Therefore, the sheet thickness t of the non-oriented electrical steel sheet 1 is preferably 0.01 mm or more. On the other hand, if the steel sheet 1 is too thick, eddy current loss increases, resulting in an increase in total iron loss. Therefore, the sheet thickness t is preferably 0.35 mm or less, and more preferably 0.25 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 800 Hz and a maximum magnetic flux density of 1.0 T. １０／８００ It is preferable that the weight ratio (W / kg) and the plate thickness: t (mm) satisfy the following formula (1): １０／８００ ≦ 8 + 100 × t (1)

[0058] When the iron loss of the non-oriented electrical steel sheet 1 satisfies the relationship of the above formula (1), heat generation in a stator core manufactured using the corresponding non-oriented electrical steel sheet 1 is suppressed, and as a result, motor efficiency can be further improved. １０／８００ Since depends on the plate thickness t, the iron loss W is calculated by taking into account the influence of the plate thickness t in the above formula (1). １０／８００ The upper limit of the

[0059] [Manufacturing Method] The non-oriented electrical steel sheet 1 of the present invention can be manufactured using, but is not particularly limited to, a siliconizing method. When using the siliconizing method, for example, a steel sheet having a constant Si content in the thickness direction can be subjected to a siliconizing diffusion treatment, thereby increasing the Si content in the surface layer 20 on both sides of the steel sheet. The Si content before the siliconizing diffusion treatment is not particularly limited, but if the Si content exceeds 4.50%, the temperature, speed, etc. during cold rolling must be strictly controlled, and from the viewpoint of cold rolling efficiency, it is preferable that the Si content be 4.50% or less.

[0060] The method of siliconizing and diffusion treatment is not particularly limited, and any method can be used. For example, siliconizing treatment can be performed by CVD to increase the Si content on the surface of the steel sheet, followed by heat treatment to diffuse Si into the interior of the steel sheet. When CVD is used, the Si content in the surface layer portion 20 and the inner layer portion 10 can be controlled by adjusting the amount of Si increased by CVD and the heat treatment conditions for the diffusion treatment.

[0061] In the production of the steel sheet 1 of the present invention, prior to the siliconizing and diffusion treatment, a grinding process is performed on the base material, in which the base material is ground by 0.5 μm or more from the surface in the depth direction. The grinding method is not particularly limited, but examples include acid treatment and mechanical polishing. Furthermore, the steel sheet after the grinding process is subjected to a siliconizing process to increase the Si content at the steel sheet surface, followed by a heat treatment to diffuse the Si into the steel sheet. When using the CVD method, the dew point of the atmosphere during the siliconizing process is controlled to −50°C or less, the dew point of the atmosphere during the diffusion process is controlled to −50°C to −30°C, and the annealing temperature for the siliconizing and diffusion processes is set to 1100°C or higher. By performing these processes, Al concentrated on the surface during the base material production stage is removed, and an oxide layer 30 having a magnetite ratio (C_mag) of 65% or more is formed on the steel sheet surface after the siliconizing and diffusion treatment. Furthermore, in the production of the steel sheet 1 of the present invention, the average thickness tc of the oxide layer 30 can be changed by controlling the dew point during the diffusion process.

[0062] In order to confirm the effects of the present invention, non-oriented electrical steel sheets were produced according to the procedure described below, and their magnetic properties were evaluated.

[0063] [Steel Slab] First, steel slab samples designated A to AL were prepared, each having the chemical composition shown in Table 1. The contents 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 steel slabs used did not change throughout the manufacturing process, and were therefore equal to the contents in the finally obtained non-oriented electrical steel sheet.

[0064] [Hot Rolling, Hot-Rolled Sheet Annealing] The steel slab was hot-rolled to form a hot-rolled steel sheet, which was then subjected to hot-rolled sheet annealing at 950° C. for 30 seconds.

[0065] [Cold Rolling] Next, the hot-rolled steel sheets after the hot-rolled sheet annealing were subjected to cold rolling to obtain cold-rolled steel sheets having the sheet thickness t shown in Table 2. However, the steel sheets with sample symbols C, I, and M were not subjected to any treatment after cold rolling because they broke during the cold rolling process. It is believed that these steel sheets broke because the content of at least one of Si, Mn, and P was excessive.

[0066] [Siliconizing Diffusion Treatment] The obtained cold-rolled steel sheet was subjected to siliconizing diffusion treatment to obtain a non-oriented electrical steel sheet. ２ gas, and SiCl ４ The steel sheet was siliconized at 1200°C by CVD in an atmosphere containing N ２ The steel sheets were subjected to a diffusion treatment (heat treatment) in an atmosphere at a temperature shown in Table 2, thereby diffusing Si from the surface layer of the steel sheets to the interior. The dew point of the atmosphere was controlled to −70° C. for the siliconizing treatment and to the value shown in Table 2 for the diffusion treatment.

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

[0068] As also described above, the contents 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 do not change throughout the manufacturing process, so the contents of these elements in the finally obtained non-oriented electrical steel sheet are equal to the contents in the steel slab used.

[0069] [Oxide Film] Five samples were cut out from the obtained non-oriented electrical steel sheet at 50 mm intervals in the sheet width direction, embedded in a mold so that the cross-sectional direction was visible, and the steel sheet surface was observed using a TEM bright-field image (accelerating 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 TEM bright-field image, and the average value for each sample was taken as tc. If an oxide film could not be confirmed, the thickness was recorded as 0 (mm). In addition, dots were plotted at 100 nm intervals on the TEM bright-field image of the steel sheet surface, and the proportion of oxide film present on the dots relative to the total number of dots was taken as the coverage. Furthermore, the composition of the oxide film was determined from the diffraction pattern measured by microelectron diffraction and the results of EDX measurement, and the main components of the oxide film were investigated. The proportion of magnetite contained in the oxide film (C_mag) was measured from the results and the TEM bright-field image. These measurement results are also shown in Table 2.

[0070] [Iron loss] Furthermore, a test piece having a width of 30 mm and a length of 280 mm was taken from each of the obtained non-oriented electrical steel sheets, and the iron loss was measured in an Epstein test in accordance with JIS C2550-1:2011 at a maximum magnetic flux density of 1.0 T and a frequency of 800 Hz. １０／８００ The magnetic properties (W / kg) were measured. In the Epstein test, equal amounts of L-direction test pieces, which were taken so that the length direction of the test pieces was the rolling direction (L-direction), and C-direction test pieces, which were taken so that the length direction of the test pieces was 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. The measurement results are also shown in Table 2. As can be seen from the results shown in Table 2, the non-oriented electrical steel sheet satisfying the conditions of the present invention had excellent properties, such as low high-frequency iron loss.

[0071]

[0072]

[0073] According to the present invention, it is possible to realize low iron loss in non-oriented electrical steel sheets used in motors for electric vehicles and hybrid electric vehicles.

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

Claims

1. A non-oriented electrical steel sheet comprising an inner layer portion and surface layer portions located on both sides of the inner layer portion, the non-oriented electrical steel sheet having a composition containing, by mass%, Si: 3.50% to 7.00%, 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, and O: 0.0050% or less, with the balance being Fe and inevitable impurities; the surface layer portion is defined as a region ranging from 0 to t / 4 and from 3t / 4 to t from the surface of the non-oriented electrical steel sheet, based on the sheet thickness t (mm); and the inner layer portion is defined as a region ranging from more than t / 4 to less than 3t / 4 from the surface of the non-oriented electrical steel sheet, and the average Si content ([Si]) of the surface layer portion is １ ) and the average Si content of the inner layer portion ([Si] ０ ) is less than 0.10%, and magnetite (Fe) is present on the surface of the non-oriented electrical steel sheet. ３ O ４ ) ratio (C_mag) is 65% or more, and the average thickness t ｃ A non-oriented electrical steel sheet having a grain size of 5 nm or more and 120 nm or less.

2. The non-oriented electrical steel sheet according to claim 1, wherein the chemical composition further includes at least one element selected from the group consisting of, in average contents across the entire sheet 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.

3. The non-oriented electrical steel sheet according to claim 1 or 2, wherein the chemical composition further includes at least one element selected from the group consisting of, in mass%, average contents across the entire sheet thickness: 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, and Ga: 0.030% or less.

4. A non-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the coverage of the oxide film is 70% or more.

5. A non-oriented electrical steel sheet according to any one of claims 1 to 4, wherein the sheet thickness t is 0.01 mm or more and 0.35 mm or less.

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