JP7334673B2 - Non-oriented electrical steel sheet and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a non-oriented electrical steel sheet, and more particularly to a non-oriented electrical steel sheet with low core loss in a high frequency range. The present invention also relates to a method for manufacturing the non-oriented electrical steel sheet.

Motors used for electric vehicles and hybrid electric vehicles are driven in a high frequency range of 400 Hz to 2 kHz from the viewpoint of miniaturization and high efficiency. Therefore, a non-oriented electrical steel sheet used as a core material for such a motor is required to have a low core loss in a high frequency range.

Therefore, in order to reduce the iron loss in the high frequency range, various methods such as addition of alloying elements such as Si and Al and reduction of plate thickness have been investigated.

For example, Patent Literature 1 and Patent Literature 2 propose controlling the Si concentration distribution in the sheet thickness direction by subjecting a steel sheet to siliconizing annealing.

JP-A-11-293422 JP 2013-155397 A

According to conventional methods such as those proposed in Patent Documents 1 and 2, a certain improvement in iron loss in the high frequency range is seen, but it is still not sufficient, and further reduction in iron loss in the high frequency range is achieved. is required.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a non-oriented electrical steel sheet with further reduced core loss in a high frequency range.

The inventors of the present invention have obtained the following findings (1) to (3) as a result of intensive studies on methods for solving the above problems.

(1) In conventional techniques such as those proposed in Patent Documents 1 and 2, the silicon diffusion treatment is performed in an atmosphere with an N ₂ partial pressure close to 100% in order to reduce costs. By performing the siliconizing diffusion treatment in such a high _N2 partial pressure atmosphere, the steel sheet is nitrided and the N content in the steel sheet is significantly increased.

(2) There is a correlation between the average N content in the steel sheet after the siliconizing treatment and the iron loss in the high frequency range. The iron loss in can be kept low.

(3) In order to suppress the average N content to 0.0020% by mass or less, the siliconizing treatment and the diffusion treatment should be performed in an atmosphere having an _N2 partial pressure of 70% or less.

The present invention has been made based on the above knowledge, and the gist and configuration thereof are as follows.

1. A non-oriented electrical steel sheet having a Si content distribution in which the Si content continuously decreases from the surface toward the center of the plate thickness,
Having a component composition containing Si and N, the balance being Fe and inevitable impurities,
Plate thickness: t is 0.01 to 0.35 mm,
Average N content of total plate thickness: [N] is 0.0020% by mass or less,
The average Si content in the surface layer portion defined as a region where the Si content is equal to or higher than the average Si content of the entire plate thickness: [Si] ₁ is 2.5 to 7.0% by mass,
The average Si content in the inner layer portion defined as a region where the Si content is less than the average Si content of the entire plate thickness: [Si] ₀ is 1.5 to 5.0% by mass,
The multilayer ratio t ₁ /t defined as the ratio of the total thickness of the surface layer portion to the plate thickness t: t ₁ is 0.10 to 0.70,
ΔSi defined as the difference ([Si] ₁ −[Si] ₀ ) between the average Si content in the surface layer: [Si] ₁ and the average Si content in the inner layer: [Si] ₀ is 0.0. A non-oriented electrical steel sheet with a content of 5 to 4.0% by mass.

2. 1. The above 1, wherein the maximum magnetic flux density: 1.0 T, the iron loss at a frequency of 1 kHz: W _10/1k (W/kg), and the plate thickness: t (mm) satisfy the following formula (1): Non-oriented electrical steel sheet.
W10 _/1k ≦15+138×t (1)

3. The component composition further comprises
Sn: 0.10% by mass or less,
3. The non-oriented electrical steel sheet according to 1 or 2 above, containing at least one selected from the group consisting of P: 0.10% by mass or less, and Sb: 0.10% by mass or less.

4. The component composition further comprises
Al: 1.20% by mass or less,
Ti: 0.0100% by mass or less,
4. The non-oriented electrical steel sheet according to any one of 1 to 3 above, containing at least one selected from the group consisting of Nb: 0.0100% by mass or less and V: 0.0100% by mass or less.

5. A method for producing a non-oriented electrical steel sheet according to any one of 1 to 4 above,
Including a siliconizing and diffusion treatment process in which the steel plate is subjected to siliconizing treatment and diffusion treatment,
The siliconizing treatment is performed in an atmosphere containing one or both of nitrogen and a rare gas and SiCl ₄ and having an N ₂ partial pressure of 70% or less,
A method for producing a non-oriented electrical steel sheet, wherein the diffusion treatment is performed in an atmosphere having a partial pressure of N ₂ of 70% or less.

According to the present invention, it is possible to provide a non-oriented electrical steel sheet with further reduced iron loss in a high frequency range.

1 is a schematic diagram showing the structure of a non-oriented electrical steel sheet in one embodiment of the present invention. FIG. FIG. 4 is a schematic diagram showing an example of a Si content profile in the thickness direction of a non-oriented electrical steel sheet. 4 is a graph showing the correlation between the difference in Si content (ΔSi) between the surface layer portion and the inner layer portion and the iron loss: W _10/1k (W/kg). Fig. 2 is a graph showing the correlation between the _N2 partial pressure during siliconizing treatment: p (%) and the average N content in the entire plate thickness: [N] (mass%). 4 is a graph showing the correlation between the average N content in the total plate thickness: [N] (% by mass) and the iron loss: W _10/1k (W/kg). A graph showing the correlation between the multi-layer ratio defined as the ratio of the total thickness of the surface layer to the thickness of the non-oriented electrical steel sheet: _t and the iron loss: W _10/1k (W / kg) be.

A method for carrying out the present invention will be specifically described below. In addition, the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.

[Non-oriented electrical steel sheet]
FIG. 1 is a schematic diagram showing the structure of a non-oriented electrical steel sheet in one embodiment of the present invention. Moreover, FIG. 2 is a schematic diagram showing an example of the Si content profile in the plate thickness direction of the non-oriented electrical steel sheet. The vertical axis in FIG. 2 indicates the position in the plate thickness direction, with 0 representing one surface of the non-oriented electrical steel sheet and t representing the other surface of the non-oriented electrical steel sheet.

As shown in FIG. 2, the non-oriented electrical steel sheet 1 of the present invention (hereinafter sometimes simply referred to as "steel sheet") has a Si content in which the Si content continuously decreases from the surface toward the center of the thickness. It has a content distribution. The Si content distribution may be a distribution in which the Si content changes continuously throughout the thickness direction of the steel plate, but for example, the Si content changes continuously on the surface side of the steel plate, and the central portion of the plate thickness may have a constant Si content distribution.

Here, if the region where the Si content is equal to or more than the average Si content of the entire plate thickness is defined as the surface layer portion, and the region where the Si content is less than the average Si content of the entire plate thickness is defined as the inner layer portion, as shown in FIG. It can be said that the non-oriented electrical steel sheet 1 of the present invention comprises an inner layer portion 10 and surface layer portions 20 provided on both sides of the inner layer portion 10 .

[Component composition]
First, the chemical composition of the non-oriented electrical steel sheet of the present invention will be described. In the following description, "%" representing the content of each element represents "% by mass" unless otherwise specified.

The non-oriented electrical steel sheet of the present invention has a chemical composition containing Si and N with the balance being Fe and unavoidable impurities.

(Average Si content)
In the non-oriented electrical steel sheet of the present invention, each of the surface layer portion and the inner layer portion has the average Si content described below. Here, a region in which the Si content is equal to or greater than the average Si content in the entire plate thickness is defined as the surface layer portion, and a region in which the Si content is less than the average Si content in the entire plate thickness is defined as the inner layer portion.

Average Si content in surface layer: 2.5 to 7.0%
Si is an element that has the effect of increasing the electrical resistance of the steel sheet and reducing the eddy current loss. If the average Si content ([Si] ₁ ) in the surface layer is less than 2.5%, eddy current loss cannot be effectively reduced. Therefore, the average Si content in the surface layer is 2.5% or more, preferably 3.0% or more, and more preferably more than 3.5%. On the other hand, if the average Si content in the surface layer portion exceeds 7.0%, not only the magnetic flux density decreases due to the decrease in saturation magnetization, but also the manufacturability decreases. Therefore, the Si content in the surface layer is 7.0% or less, preferably less than 6.5%, and more preferably 6.0% or less.

As described above, the average Si content in the surface layer portion of 2.5 to 7.0% means that the average Si content of the surface layer portion (first surface layer portion) on one surface of the non-oriented electrical steel sheet It means that the Si content is 2.5 to 7.0% and the average Si content of the surface layer portion (second surface layer portion) of the other surface is 2.5 to 7.0%. In general, the component composition of the first surface layer portion and the component composition of the second surface layer portion may be the same, but they may be different.

Average Si content in inner layer: 1.5 to 5.0%
If the Si content ([Si] ₀ ) in the inner layer is less than 1.5%, the high-frequency core loss increases. Therefore, the average Si content of the inner layer is set to 1.5% or more. On the other hand, if the average Si content in the inner layer portion exceeds 5.0%, there arises a problem that the core cracks during punching. Therefore, the average Si content of the inner layer is set to 5.0% or less, preferably 4.0% or less.

(Difference in Si content)
ΔSi defined as the difference ([Si] ₁ - [Si] ₀ ) between the average Si content in the surface layer: [Si] ₁ and the average Si content in the inner layer: [Si] ₀ In order to examine the effects on the magnetic properties of grain-oriented electrical steel sheets, non-oriented electrical steel sheets with different ΔSi were produced by the following procedure, and their magnetic properties were evaluated. In addition, the multilayer ratio, the surface layer Si, and the inner layer Si are controlled while setting [Si] ₀ to a value close to 2.5% or more and close to 2.5% so that the average Si of the total plate thickness is 3.3%. did The multilayer ratio ranged from 0.18 to 0.53.

First, a steel slab having a chemical composition containing Si: 2.5%, N: 0.0015%, and the balance being Fe and unavoidable impurities was hot-rolled into a hot-rolled steel sheet. The hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 950° C.×30 s, and then the steel sheet after the hot-rolled sheet annealing was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.2 mm. Next, the cold-rolled steel sheets were subjected to silicon diffusion treatment to produce non-oriented electrical steel sheets having a ΔSi of 1.5 to 4.5%. In the siliconizing treatment, first, siliconizing treatment is performed at a temperature of 1200° C. in an atmosphere of 70% N ₂ partial pressure, 15% SiCl ₄ partial pressure, and 15% Ar partial pressure, and then SiCl ₄ partial pressure is removed. After substitution with Ar, diffusion treatment was performed in an atmosphere of N ₂ partial pressure of 70% and Ar partial pressure of 30%. An attempt was made to produce a non-oriented electrical steel sheet with a ΔSi of 5.0%, but the steel sheet broke during production.

In addition, under the same conditions except that a steel slab having a chemical composition containing Si: 2.8 to 3.2%, N: 0.0015%, and the balance being Fe and inevitable impurities, ΔSi was A non-oriented electrical steel sheet of 0.2 to 1.0% was produced.

A test piece having a width of 30 mm and a length of 180 mm was taken from each of the obtained non-oriented electrical steel sheets, and the Epstein test was performed to evaluate the magnetic properties. In the Epstein test, an L-direction test piece was taken so that the length direction of the test piece was in the rolling direction (L direction), and a test piece was taken so that the length direction was perpendicular to the rolling direction (C direction). Equal amounts of the C-direction test pieces were used to evaluate the average values of the magnetic properties in the L-direction and the C-direction.

In FIG. 3, ΔSi defined as the difference ([Si] ₁ - [Si] ₀ ) between the average Si content in the surface layer: [Si] ₁ and the average Si content in the inner layer: [Si] ₀ (% by mass) and iron loss at 1.0 T, 1 kHz: W _10/1k (W/kg).

As can be seen from the results shown in FIG. 3, iron loss can be kept low when ΔSi is 0.5% by mass or more and 4.0% by mass or less. This is considered to be due to the following reasons. That is, when the Si content of the surface layer is higher than that of the inner layer, the magnetic permeability of the surface layer is higher than that of the inner layer. As a result, the magnetic flux concentrates on the surface layer and the eddy current loss is reduced. However, if ΔSi is excessively large, the difference in lattice constant and the difference in magnetostriction between the surface layer portion and the inner layer portion increase accordingly. As a result, the stress applied when the steel sheet is magnetized increases, resulting in an increase in hysteresis loss.

For the above reasons, in the present invention, the difference between the average Si content in the surface layer: [Si] ₁ and the average Si content in the inner layer: [Si] ₀ ([Si] ₁ - [Si] ₀ ) is The defined ΔSi is 0.5 to 4.0% by mass.

Average N content: 0.0020% or less In the present invention, it is important to set the average N content: [N] in the total plate thickness of the non-oriented electrical steel sheet to 0.0020% or less. The reason for the limitation will be described below.

In order to further reduce the iron loss of non-oriented electrical steel sheets, the present inventors have been studying, and found that the amount of N is high in non-oriented electrical steel sheets with large iron loss, and that the amount of N in the slab stage is high. It was found that even when the N content is low, the N content in the non-oriented electrical steel sheet after the siliconization diffusion treatment is high. That is, it was found that the steel sheet was nitrided by the siliconizing diffusion treatment, and the N content in the steel sheet was significantly increased.

Therefore, in order to investigate the relationship between the magnetic properties and the N content of steel sheets after siliconization diffusion treatment, siliconization diffusion treatment was performed at various _N2 partial pressures to produce non-oriented electrical steel sheets. The correlation between the average N content of the non-oriented electrical steel sheets obtained and the iron loss in the high frequency region was investigated. Specific procedures are described below.

First, a steel slab having a chemical composition containing Si: 2.0%, N: 0.0014%, and the balance being Fe and unavoidable impurities was hot-rolled to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet was subjected to hot-rolled steel annealing at 950°C for 30 seconds. Thereafter, the steel sheet after the hot-rolled sheet annealing was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.2 mm. Next, the cold-rolled steel sheets were subjected to siliconizing diffusion treatment at various _N2 partial pressures: p to obtain non-oriented electrical steel sheets. In the siliconizing diffusion treatment, first, the siliconizing treatment is performed by a chemical vapor deposition (CVD) method at 1200° C. in an atmosphere of _SiCl4 partial pressure: 10.0%, _N2 partial pressure: p, balance Ar. Si was deposited on the surface of the steel sheet, and then diffusion treatment was performed at 1200° C. in an atmosphere in which SiCl ₄ was replaced with Ar to diffuse Si. The ΔSi of the non-oriented electrical steel sheet was set to 1.0%, and the multi-layer ratio described later was set to 0.3.

After that, the average N content: [N] in the total plate thickness of the finally obtained non-oriented electrical steel sheet was measured by ICP (Inductively Coupled Plasma) emission spectrometry. A test piece having a width of 30 mm and a length of 180 mm was taken from each of the obtained non-oriented electrical steel sheets, and the Epstein test was performed to evaluate the magnetic properties. In the Epstein test, an L-direction test piece was taken so that the length direction of the test piece was in the rolling direction (L direction), and a test piece was taken so that the length direction was perpendicular to the rolling direction (C direction). Equal amounts of the C-direction test pieces were used to evaluate the average values of the magnetic properties in the L-direction and the C-direction.

FIG. 4 is a graph showing the correlation between the N ₂ partial pressure: p (%) during the siliconizing diffusion treatment and the average N content: [N] (mass %) in the entire sheet thickness. Moreover, FIG. 5 is a graph showing the correlation between the average N content: [N] (% by mass) and the iron loss: W _10/1k (W/kg) in the entire sheet thickness.

From the results shown in FIG. 4, it can be seen that there is a correlation between the _N2 partial pressure during the siliconizing diffusion treatment: p and the average N content of the non-oriented electrical steel sheet: [N]. From the results shown in FIG. 5, it can be seen that the iron loss increases as the average N content: [N] of the non-oriented electrical steel sheet increases. This is probably because the higher the [N], the greater the distortion of the crystal lattice due to dissolved N, which hinders domain wall motion. Therefore, in the present invention, [N] is set to 0.0020% or less in order to reduce iron loss. Further, from FIG. 4, it can be seen that p should be 70% or less in order to make [N] 0.0020% or less.

From the above results, the non-oriented electrical steel sheet in one embodiment of the present invention has a chemical composition containing Si and N, the balance being Fe and inevitable impurities, and having the following (a) to (c) shall meet the conditions.
(a) Average N content in total plate thickness: [N] is 0.0020% by mass or less.
(b) Average Si content in the surface layer portion defined as a region where the Si content is equal to or higher than the average Si content of the entire sheet thickness: [Si] ₁ is 2.5 to 7.0% by mass.
(c) Average Si content in the inner layer portion defined as a region where the Si content is less than the average Si content of the entire plate thickness: [Si] ₀ is 1.5 to 5.0% by mass.

Next, optional additive elements that the non-oriented electrical steel sheet of the present invention can optionally contain will be described. Note that the optional additive elements described below are elements that do not have to be added, so the lower limit of the content of each element may be 0%.

In one embodiment of the present invention, the chemical composition of the non-oriented electrical steel sheet may further optionally contain at least one selected from the group consisting of Sn, P, and Sb.

Sn: 0.10% or less By adding Sn, the texture can be greatly improved, the magnetic flux density can be improved, and the hysteresis loss can be further reduced. Further, by adding Sn, nitriding of the steel sheet can be suppressed, and an increase in iron loss can be suppressed. However, when the Sn content exceeds 0.10%, the effect is saturated, and in addition, the manufacturability is lowered and the cost is increased. Therefore, when Sn is added, the Sn content should be 0.10% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of enhancing the effect of adding Sn, the Sn content is preferably 0.005% or more.

P: 0.10% or less Similar to Sn, the addition of P greatly improves the texture, improves the magnetic flux density, and further reduces the hysteresis loss. However, when the P content exceeds 0.10%, the effect is saturated and the manufacturability is lowered. Therefore, when P is added, the P content should be 0.10% or less. On the other hand, the lower limit of the P content is not particularly limited, but from the viewpoint of enhancing the effect of adding P, the P content is preferably 0.010% or more.

Sb: 0.10% or less As with Sn and P, the addition of Sb greatly improves the texture, improves the magnetic flux density, and further reduces the hysteresis loss. However, when the Sb content exceeds 0.10%, the effect is saturated, and in addition, the manufacturability is lowered and the cost is increased. Therefore, when adding Sb, the Sn content should be 0.10% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but from the viewpoint of increasing the effect of adding Sb, the Sb content is preferably 0.005% or more.

In another embodiment of the present invention, the chemical composition of the non-oriented electrical steel sheet may further optionally contain at least one selected from the group consisting of Al, Ti, Nb, and V.

Al: 1.20% or less Al is an element that has the effect of increasing specific resistance, and by adding Al, eddy current loss can be reduced, and iron loss can be further reduced. However, when the Al content exceeds 1.20%, the toughness deteriorates. Therefore, when adding Al, the Al content is made 1.20% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of enhancing the effect of adding Al, the amount of Al added is preferably 0.01% or more.

Ti: 0.0100% or less Ti reacts with N and C in steel to form nitrides and carbides. Therefore, the addition of Ti makes the crystal grains finer, making it possible to further reduce iron loss. However, if a large amount of Ti is added, the hysteresis loss increases due to excessive formation of nitrides and carbides, resulting in an increase in core loss. Therefore, when Ti is added, the Ti content is made 0.0100% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but from the viewpoint of increasing the effect of adding Ti, the Ti content is preferably 0.0030% or more.

Nb: 0.0100% or less Nb reacts with N and C in steel to form nitrides and carbides. Therefore, the addition of Nb makes the crystal grains finer, making it possible to further reduce iron loss. However, when a large amount of Nb is added, hysteresis loss is increased due to excessive formation of nitrides and carbides, resulting in an increase in core loss. Therefore, when Nb is added, the Nb content is made 0.0100% or less. On the other hand, the lower limit of the Nb content is not particularly limited, but from the viewpoint of increasing the effect of adding Nb, the Nb content is preferably 0.0030% or more.

V: 0.0100% or less V reacts with N and C in steel to form nitrides and carbides. Therefore, the addition of V makes the crystal grains finer, making it possible to further reduce iron loss. However, if a large amount of V is added, the hysteresis loss is increased due to excessive formation of nitrides and carbides, so the core loss is increased. Therefore, when V is added, the V content is made 0.0100% or less. On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of increasing the effect of adding V, the V content is preferably 0.0030% or more.

[Thickness]
If the non-oriented electrical steel sheet is too thin, it becomes difficult to handle in manufacturing processes such as cold rolling and annealing, resulting in increased manufacturing costs. Therefore, the thickness t of the non-oriented electrical steel sheet is set to 0.01 mm or more. On the other hand, if the steel plate is too thick, the eddy current loss increases and the total iron loss increases. Therefore, t is set to 0.35 mm or less.

[multilayer ratio]
Next, in order to examine the influence of the multi-layer ratio t ₁ /t _, which is defined as the ratio of the total thickness of the surface layer: t to the thickness of the non-oriented electrical steel sheet: t, on the magnetic properties, the multi-layer Non-oriented electrical steel sheets with different ratios were produced by the following procedure, and their magnetic properties were evaluated. Here, the "total thickness of the surface layers" refers to the sum of the thicknesses of the surface layers provided on both sides of the non-oriented electrical steel sheet. Further, the surface layer portion is defined as a region in which the Si content is equal to or higher than the average Si content of the entire plate thickness, as described above.

First, a steel slab having a chemical composition containing Si: 2.0%, N: 0.0018%, and the balance being Fe and unavoidable impurities was hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 950° C.×30 s, and then the steel sheet after the hot-rolled sheet annealing was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.1 mm. Next, the cold-rolled steel sheet was subjected to a siliconizing diffusion treatment to obtain a non-oriented electrical steel sheet. In the siliconizing diffusion treatment, first, siliconizing treatment is performed by the CVD method at a temperature of 1200° C. in an atmosphere of N ₂ partial pressure of 70% and SiCl ₄ partial pressure of 30% to deposit Si on the surface of the steel sheet. Diffusion treatment was performed at 1200° C. in an atmosphere in which the partial pressure of SiCl ₄ was replaced with Ar to diffuse Si. In this experiment, non-oriented electrical steel sheets having various multi-layer ratios were produced by adjusting the diffusion treatment time. The average Si content in the surface layer of the non-oriented electrical steel sheet: [Si] ₁ was 4.5%, and ΔSi was 2.5%.

A test piece having a width of 30 mm and a length of 180 mm was taken from each of the obtained non-oriented electrical steel sheets, and the Epstein test was performed to evaluate the magnetic properties. In the Epstein test, an L-direction test piece was taken so that the length direction of the test piece was in the rolling direction (L direction), and a test piece was taken so that the length direction was perpendicular to the rolling direction (C direction). Equal amounts of the C-direction test pieces were used to evaluate the average values of the magnetic properties in the L-direction and the C-direction.

FIG. 6 shows the correlation between the multi-layer ratio (t ₁ /t) and iron loss W _10/1k (W/kg). From this result, it can be seen that the iron loss is greatly reduced when the multi-layer ratio is 0.10 to 0.70. This reduction in iron loss is considered to be due to the following reasons. First, when the multi-layer ratio is less than 0.10, the eddy current concentrated in the surface layer portion cannot be effectively reduced because the ratio of the surface layer portion with high resistance is low. On the other hand, when the layer ratio exceeds 0.70, the magnetic permeability difference between the surface layer and the inner layer becomes small, so that the magnetic flux penetrates into the inner layer and eddy current loss also occurs from the inner layer. Therefore, iron loss can be reduced by setting the multi-layer ratio to 0.10 to 0.70. For the above reasons, the multi-layer ratio (t ₁ /t) is set to 0.10 to 0.70 in the present invention.

[Iron loss]
In the present invention, the iron loss (total iron loss) at frequency: 1 kHz, maximum magnetic flux density: 1.0 T: W _10/1k (W/kg), and the plate thickness: t (mm) are defined as follows (1 ) is preferably satisfied.
W10 _/1k ≦15+138×t (1)

When the iron loss of the non-oriented electrical steel sheet satisfies the relationship of the above formula (1), the heat generation of the stator core manufactured using the non-oriented electrical steel sheet is suppressed, and as a result, the motor efficiency can be further improved. . Since the iron loss depends on the plate thickness, the above formula (1) defines the upper limit of the iron loss in consideration of the effect of the plate thickness.

In an electromagnetic steel sheet, iron loss usually increases when the magnetic flux density is increased, so a general motor core is designed so that the magnetic flux density is about 1.0T. On the other hand, the non-oriented electrical steel sheet of the present invention has a high magnetic flux density and a low iron content, which are contradictory properties, by controlling the chemical composition and multilayer ratio of the surface layer portion and the inner layer portion of the steel sheet as described above. It is compatible with loss.

[Production method]
The non-oriented electrical steel sheet of the present invention is not particularly limited, but can be produced using a siliconizing method. When the siliconizing method is used, for example, a steel sheet having a constant Si content in the thickness direction is subjected to a siliconizing diffusion treatment to increase the Si content in the surface layers on both sides of the steel sheet. The method of siliconizing diffusion treatment is not particularly limited, and any method can be used. For example, it is possible to perform a siliconizing treatment for depositing Si on the surface of the steel sheet by a CVD method, and then perform a diffusion treatment for diffusing Si into the interior of the steel sheet by performing heat treatment. The Si content in the surface layer portion and the inner layer portion can be controlled by adjusting the deposition amount of Si by the CVD method and the heat treatment conditions for the diffusion treatment. A non-oriented electrical steel sheet obtained by siliconizing diffusion treatment by the CVD method has, for example, a Si content profile in the sheet thickness direction as shown in FIG.

Conventionally, the siliconizing diffusion treatment has been performed in an atmosphere with an N ₂ partial pressure close to 100% in order to reduce costs. By performing the siliconizing diffusion treatment in such a high _N2 partial pressure atmosphere, the steel sheet is nitrided and the N content in the steel sheet is significantly increased. Therefore, in the present invention, the siliconizing treatment and the diffusion treatment are performed in an atmosphere having an _N2 partial pressure of 70% or less. Thereby, the average N content: [N] of the obtained non-oriented electrical steel sheet is set to 0.0020% by mass or less, and as a result, iron loss can be reduced.

More specifically, the siliconizing treatment is performed in an atmosphere containing one or both of nitrogen and a rare gas and SiCl ₄ and having an N ₂ partial pressure of 70% or less. The atmosphere during the siliconizing treatment may be an atmosphere comprising one or both of nitrogen and a rare gas, and SiCl ₄ , and having an N ₂ partial pressure of 70% or less.

The time required for the siliconizing treatment depends on the partial pressure of SiCl ₄ as the Si source. Therefore, the SiCl ₄ partial pressure may be appropriately adjusted according to the Si content and the time required for production.

Moreover, the diffusion treatment is performed in an atmosphere having a partial pressure of N ₂ of 70% or less. The diffusion treatment is preferably performed in an atmosphere containing one or both of nitrogen and a rare gas and having an _N2 partial pressure of 70% or less. Incidentally, the _N2 partial pressures during the siliconizing treatment and the diffusion treatment may be the same or different.

At least one selected from the group consisting of Ar, He, and Ne, for example, is preferably used as the rare gas.

(Example 1)
In order to confirm the effects of the present invention, non-oriented electrical steel sheets were produced by the procedure described below, and their magnetic properties were evaluated.

(steel slab)
First, Si: 2.0% to 3.8%, N: 0.0005% to 0.0015%, and optionally at least one selected from the group consisting of Sn, P, and Sb, and the balance is Fe and unavoidable impurities. However, for comparison, Comparative Example No. 21, a steel slab with an N content of 0.0023% was used. Also, the Si content in the steel slab used is equal to the average Si content in the inner layer portion of the finally obtained non-oriented electrical steel sheet: [Si] ₀ . Also, the contents of Sn, P, and Sb in the steel slab used do not change throughout the manufacturing process, and thus are equal to the contents in the finally obtained non-oriented electrical steel sheet.

(Hot rolling, hot-rolled sheet annealing)
The steel slab was hot-rolled into a hot-rolled steel sheet, and then the hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 950°C for 30s.

(cold rolling)
Next, the hot-rolled steel sheet after the hot-rolled sheet annealing was cold-rolled to obtain a cold-rolled steel sheet having a thickness t shown in Table 1.

(Siliconizing diffusion treatment)
The obtained cold-rolled steel sheet was subjected to a siliconizing diffusion treatment to obtain a non-oriented electrical steel sheet. Specifically, first, a siliconizing treatment was performed by a CVD method at 1200° C. in an atmosphere containing one or both of N ₂ and a rare gas and SiCl ₄ to deposit Si on the surface of the steel sheet. The composition of the atmosphere during the siliconizing treatment was as shown in Table 1. Next, diffusion treatment (heat treatment) is performed at 1200° C. in an atmosphere in which SiCl ₄ of the components contained in the atmosphere is replaced with N ₂ or a rare gas to diffuse Si deposited on the surface of the steel sheet to the inside. Ta. For the replacement of SiCl ₄ , the same kind of rare gas was used when the siliconizing atmosphere contained a rare gas, and N ₂ was used when the siliconizing atmosphere did not contain a rare gas.

However, the steel sheets of Comparative Examples Nos. 21, 23, and 24 were not subjected to treatment after cold rolling because they were broken during the cold rolling process. It is believed that these steel sheets broke because the content of at least one of Sn, P, and Sb was excessive.

Also, Comparative Example No. The steel sheet No. 40 was broken during coil winding after the siliconization diffusion treatment, so the product was not evaluated. No. Steel sheet No. 40 is considered to have broken because the average Si content in the surface layer after siliconizing treatment was high.

(Si content/N content)
The obtained non-oriented electrical steel sheet was embedded in a carbon mold, and an EPMA (Electron Probe Micro Analyzer) was used to measure the Si content distribution in the cross section in the thickness direction. From the obtained Si content distribution, the average value of the Si content in the entire plate thickness of the non-oriented electrical steel sheet was calculated, and the portion where the Si concentration was higher than the average value was the surface layer portion, and the portion where the Si concentration was lower than the average value was the inner layer portion. . From the obtained results, the average Si content in the surface layer: [Si] ₁ , the average Si content in the inner layer: [Si] ₀ , and ΔSi were determined. Also, the average N content of the total thickness of the non-oriented electrical steel sheet: [N] was measured using ICP emission spectrometry. The measurement results are also shown in Table 1.

As described above, the average Si content: [Si] ₀ in the inner layer of the non-oriented electrical steel sheet was equal to the Si content in the steel slab used.

Also, as also mentioned above, since the contents of Sn, P, and Sb do not change throughout the manufacturing process, the contents in the final non-oriented electrical steel sheet are equal to the contents in the used steel slab. Therefore, Table 1 lists the contents of each element in the steel slabs used as the contents of Sn, P, and Sb in the non-oriented electrical steel sheets.

(multilayer ratio)
From the Si content distribution obtained by the EPMA measurement, the multi-layer ratio t ₁ /t defined as the ratio of the total thickness of the surface layer portion: t ₁ to the plate thickness: t was calculated. Table 1 also shows the values of the multi-layer ratios obtained.

(iron loss)
Further, a test piece having a width of 30 mm and a length of 180 mm was taken from each of the obtained non-oriented electrical steel sheets, and an Epstein test was performed to determine the maximum magnetic flux density: 1.0 T, frequency: 1 kHz iron loss: W _{10 /1k} (W/kg) was measured. In the Epstein test, an L-direction test piece was taken so that the length direction of the test piece was in the rolling direction (L direction), and a test piece was taken so that the length direction was perpendicular to the rolling direction (C direction). Equal amounts of the C-direction test pieces were used to evaluate the average values of the magnetic properties in the L-direction and the C-direction. The measurement results are also shown in Table 1.

As can be seen from the results shown in Table 1, the non-oriented electrical steel sheets satisfying the conditions of the present invention had excellent properties such as low high-frequency iron loss.

(Example 2)
Next, a non-oriented electrical steel sheet was produced under the same conditions as in Example 1 except that a steel slab having the chemical composition shown in Table 2 was used and the siliconizing treatment was performed in the atmosphere shown in Table 3, The obtained non-oriented electrical steel sheets were evaluated in the same manner as in Example 1. The evaluation results are also shown in Table 3.

Since the contents of Al, Ti, Nb, and V do not change throughout the manufacturing process, the contents in the finally obtained non-oriented electrical steel sheet are equal to the contents in the steel slab used.

As can be seen from the results shown in Table 3, adding at least one of Al, Ti, Nb, and V can further reduce iron loss.

1 non-oriented electrical steel sheet 10 inner layer part 20 surface layer part

Claims (4)

A non-oriented electrical steel sheet having a Si content distribution in which the Si content continuously decreases from the surface toward the center of the plate thickness,
Having a component composition containing Si and N, the balance being Fe and inevitable impurities,
Plate thickness: t is 0.01 to 0.35 mm,
Average N content of total plate thickness: [N] is 0.0020% by mass or less,
The average Si content in the surface layer portion defined as a region where the Si content is equal to or higher than the average Si content of the entire plate thickness: [Si] ₁ is 2.5 to 6.0 % by mass,
The average Si content in the inner layer portion defined as a region where the Si content is less than the average Si content of the entire plate thickness: [Si] ₀ is 1.5 to 5.0% by mass,
The multilayer ratio t ₁ /t defined as the ratio of the total thickness of the surface layer portion to the plate thickness t: t ₁ is 0.10 to 0.70,
ΔSi defined as the difference ([Si] ₁ −[Si] ₀ ) between the average Si content in the surface layer: [Si] ₁ and the average Si content in the inner layer: [Si] ₀ is 0.0. 5 to 4.0% by mass,
A non -oriented electrical steel sheet in which the maximum magnetic flux density: 1.0 T, the iron loss at a frequency of 1 kHz: W _10/1k (W/kg), and the thickness: t (mm) satisfy the following formula (1): .
W10 _/1k ≦15+138×t (1) The component composition further comprises
Sn: 0.10% by mass or less,
The non-oriented electrical steel sheet according to claim 1, containing at least one selected from the group consisting of P: 0.10% by mass or less, and Sb: 0.10% by mass or less. The component composition further comprises
Al: 1.20% by mass or less,
Ti: 0.0100% by mass or less,
The non-oriented electrical steel sheet according to claim 1 or 2, containing at least one selected from the group consisting of Nb: 0.0100% by mass or less and V: 0.0100% by mass or less. A method for producing a non-oriented electrical steel sheet according to any one of claims 1 to 3,
Including a siliconizing and diffusion treatment process in which the steel plate is subjected to siliconizing treatment and diffusion treatment,
the siliconizing treatment is performed in an atmosphere containing one or both of nitrogen and a rare gas and _SiCl4 , and having a _N2 partial pressure of 70% or less;
A method for producing a non-oriented electrical steel sheet, wherein the diffusion treatment is performed in an atmosphere having a _N2 partial pressure of 70% or less.

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