[0001]The present invention relates to a non oriented electrical steel sheet which is mainly used for core materials for electrical equipment and which is excellent in fatigue strength and magnetic characteristics.
10 B ackground Art [0002]
In recent years, in the field of electrical equipment, especially rotating machines, small and medium size transformers, electrical components, and the like in which the non oriented electrical steel sheet is used for core materials thereof, it is eagerly demanded to
15 enhance the efficiency and to reduce the size, due to the movement of global
environmental conservation represented by global power reduction, energy saving, CO2 emission reduction, and the like. Under the social situation, it is demanded to improve the performance for the non oriented electrical steel sheet. [0003]
20 In general, a motor consists of a stator and a rotor. In recent years, the interior
permanent magnet motor (hereinafter, referred to as "IPM motor") in which permanent magnets are included inside the rotor is mainly used as the drive motor for electric vehicles, hybrid electric vehicles, and the like, and the technological development thereof is proceeded for higher efficiency, higher output, higher speed rotation, and smaller size.
25 [0004]

2 In order to improve the performance of the IPM motor, it is necessary to bring
the stator close to the permanent magnets inside the rotor, and thus, it is necessary to
reduce the distance from the outer edge of the rotor core to the permanent magnets inside
the rotor. On the other hand, when being rotated, centrifugal force caused by the rotated
5 permanent magnets is applied to the outer edge of the rotor core, and the force thereby
increases with high speed rotation. Thus, the strength of a part between the outer edge
of the rotor core and the slot for the permanent magnets (hereinafter, referred to as
"bridge part"), especially the fatigue strength, is important. For instance, with respect to
the above, the following techniques are disclosed.
10 [0005]
Patent Document 1 discloses the technique to increase the strength of the electrical steel sheet itself which is used for the rotor core. Patent Document 2 discloses the technique to conduct work hardening and quench hardening in order to strengthen the predetermined part, because the part which needs to be strengthened in the rotor core is
15 the bridge part as mentioned above. Patent Document 3 discloses the technique to reinforce the rotor from the outside with a ring and the like, in order to increase the strength of the entire rotor core. [0006] However, the technique of Patent Document 1 has a disadvantage such that the
20 punchability of the blank of the rotor core deteriorates because the strength of the
electrical steel sheet itself is increased. The decrease in punchability causes a decrease in accuracy of the blank when being punched, a decrease in punching speed, a damage of punching die, or the like. The technique of Patent Document 2 increases the cost because an additional process to only strengthen the bridge part is necessary when
25 producing the rotor core. Moreover, the technique of Patent Document 3 increases the

3 cost because the ring to reinforce the rotor from the outside is necessary.
[0007]
Therefore, it is desired to develop a technique for increasing the strength,
especially the fatigue strength, of the predetermined part without increasing the strength
5 of the electrical steel sheet itself and without adding an additional process.
[0008]
As mentioned above, the centrifugal force caused by rotating the motor is
repeatedly applied to the bridge part of the rotor core, and thus, it is necessary to increase
the fatigue strength at the bridge part. As a typical method for improving the fatigue
10 strength, there is a method for hardening the surface of steel (sheet).
[0009]
As the method for hardening the surface, for instance, there are transformation
strengthening of steel itself represented by quenching and the like, precipitation
strengthening to form the second phase by nitriding, carburizing, and the like, and work
15 hardening to induce the strain by shot peening and the like. However, for the above, the
additional process is necessary.
[0010]
In the past, for the non oriented electrical steel sheet, the technique which
simultaneously improves both the fatigue strength and the magnetic characteristics
20 without adding an additional process has not been established.
Related Art Documents
Patent Documents
[0011]
25 [Patent Document 1] Japanese Patent (Granted) Publication No. 5000136

4 [Patent Document 2] Japanese Patent (Granted) Publication No. 4160469
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2013-115899
[Patent Document 4] Japanese Patent (Granted) Publication No. 3307897
5 [Patent Document 5] Japanese Patent (Granted) Publication No. 4116748
[Patent Document 6] Japanese Patent (Granted) Publication No. 4116749
Non-Patent Documents
[0012]
[Non-Patent Document 1] Tetsu-to-Hagane, Vol.66 (1980), No.7, pl000-pl009
10 [Non-Patent Document 2] Materia, Vol.50 (2011), No.3, pl26-pl28
Summary of Invention Technical Problem to be Solved [0013]
15 The present invention has been made in consideration of the above mentioned
situations. An object of the invention is to simultaneously improve both the fatigue strength and the magnetic characteristics without adding an additional process to the conventional producing method for the non oriented electrical steel sheet. Specifically, the object of the invention is to provide a non oriented electrical steel sheet excellent in
20 the fatigue strength and the magnetic characteristics and also excellent in cost.
Solution to Problem [0014]
In order to solve the above problem, the present inventors have made a thorough 25 investigation to form a hardened surface layer for a silicon steel sheet which is a base

5 steel sheet of the non oriented electrical steel sheet by utilizing producing processes of
the non oriented electrical steel sheet. As a result, it is found that, an internally oxidized
layer is formed in a surface of the silicon steel sheet by favorably combining steel
compositions and producing conditions, the surface is hardened by controlling the
5 hardness of the internally oxidized layer, and thereby, the fatigue strength can be
increased.
[0015]
Herein, as disclosed in Patent Documents 4 to 6, when the thickness of the
internally oxidized layer is thickened, iron loss in high frequency is adversely affected.
10 Thus, the present inventors have made a thorough investigation such that oxides in the
internally oxidized layer and thickness of the internally oxidized layer are controlled,
hardness of the internally oxidized layer is controlled, and thereby, the fatigue strength
and the magnetic characteristics are improved at the same time.
[0016]
15 As a result, it is found that, by conducting heat conservation treatment during
cooling after hot rolling for a steel sheet with adjusted steel composition and by
controlling conditions of the heat conservation treatment properly, it is possible to control
the oxides in the internally oxidized layer and the average thickness of the internally
oxidized layer, and it is possible to control the hardness of the internally oxidized layer.
20 Specifically, it is found that it is possible to obtain the non oriented electrical steel sheet
in which the fatigue strength and the magnetic characteristics are improved at the same
time without adding an additional process.
[0017]
An aspect of the present invention employs the following.
25 [0018]

6 (1) A non oriented electrical steel sheet according to an aspect of the present
invention consists of a silicon steel sheet and an insulation coating, characterized in that
the silicon steel sheet contains, as a chemical composition, by mass%,
more than 2.00 to 4.00% of Si,
5 0.10 to 3.00% of Al,
0.10 to 2.00% of Mn,
0.0030% or less of C,
0.050% or less of P,
0.005% or less of S,
10 0.005% or less of N,
0to0.40%ofSn,
0tol.00%ofCu,
0to0.40%ofSb,
0 to 0.0400% of REM,
15 0 to 0.0400% of Ca,
0 to 0.0400% of Mg, and
a balance consisting of Fe and impurities,
when viewing a cross section whose cutting direction is parallel to a thickness
direction and when a central area is a thickness range of 5/8 to 3/8 of the silicon steel
20 sheet, a vickers hardness in the central area is 120 to 300 Hv, and
when viewing the cross section, the silicon steel sheet includes an internally
oxidized layer containing SiCh in a surface thereof, an average thickness of the internally
oxidized layer is 0.10 to 5.0 |im, and a vickers hardness in the internally oxidized layer is
1.15 to 1.5 times as compared with the vickers hardness in the central area.
25 (2) In the non oriented electrical steel according to (1), the silicon steel sheet

7 may contain, as the chemical composition, by mass%, at least one selected from a group
consisting of
0.02 to 0.40% of Sn,
0.10 to 1.00% of Cu, and
5 0.02 to 0.40% of Sb.
(3) In the non oriented electrical steel according to (1) or (2), the silicon steel
sheet may contain, as the chemical composition, by mass%, at least one selected from a
group consisting of
0.0005 to 0.0400% of REM,
10 0.0005 to 0.0400% of Ca, and
0.0005 to 0.0400% of Mg.
(4) In the non oriented electrical steel according to any one of (1) to (3), the
vickers hardness in the internally oxidized layer may be 155 Hv or more.
(5) In the non oriented electrical steel according to any one of (1) to (4), the
15 average thickness of the internally oxidized layer may be 0.55 |im or more.
Effects of Invention [0019]
According to the above aspects of the present invention, it is possible to provide 20 the non oriented electrical steel sheet excellent in the fatigue strength and the magnetic characteristics and also excellent in cost.
Brief Description of Drawings
[0020]
25 Fig. 1 is a cross sectional illustration of a non oriented electrical steel sheet

8 according to an embodiment of the present invention.
Fig. 2 is a flow chart illustrating a producing method for the non oriented electrical steel sheet according to the embodiment.
Fig. 3 is a cross sectional illustration showing a situation such that an internally 5 oxidized layer is formed in a base steel sheet for the non oriented electrical steel sheet according to the embodiment.
Detailed Description of Preferred Embodiments
[0021]
10 Hereinafter, a preferable embodiment of the present invention is described in
detail. However, the present invention is not limited only to the configuration which is disclosed in the embodiment, and various modifications are possible without departing from the aspect of the present invention. In addition, the limitation range as described below includes a lower limit and an upper limit thereof. However, the value expressed 15 by "more than" or "less than" does not include in the limitation range. "%" of the amount of respective elements expresses "mass%". [0022]
First, the limitation reasons in regard to the chemical composition of the silicon steel sheet which is the base steel sheet for the non oriented electrical steel sheet 20 according to the embodiment (hereinafter, it may be referred to as "the present electrical steel sheet") are explained.
[0023]
In the embodiment, the silicon steel sheet contains, as a chemical composition, 25 base elements, optional elements as necessary, and a balance consisting of Fe and

9 impurities.
[0024]
In the embodiment, Si, Al, and Mn are the base elements (main alloying
elements) in the chemical composition of the silicon steel sheet.
5 [0025]
more than 2.00 to 4.00% of Si
Si (silicon) is an element which has the effect of reducing the eddy current loss
by increasing the electrical resistance, and thereby reducing the iron loss. Moreover, Si
is the element which has the effect of improving the tensile strength and the fatigue
10 strength by increasing the yield ratio of the steel sheet because of the solute strengthening.
Moreover, as explained below, Si is the element necessary for forming SiCh in the
internally oxidized layer and for hardening the surface of the steel sheet.
[0026]
When the Si content is 2.00% or less, it is difficult to obtain the above effect and
15 to harden the internally oxidized layer. Thus, the Si content is to be more than 2.00%.
The Si content is preferably 2.10% or more, more preferably 2.30% or more, and further
more preferably 2.60% or more. On the other hand, when the Si content is more than
4.00%, the magnetic flux density decreases, the operability for the cold rolling and the
like deteriorates, and the production cost increases. Thus, the Si content is to be 4.00%
20 or less. The Si content is preferably 3.70% or less, and more preferably 3.40% or less.
[0027]
0.10 to 3.00% of Al
In common with Si, Al (aluminum) is an element which has the effect of
reducing the eddy current loss by increasing the electrical resistance, and thereby
25 reducing the iron loss. However, Al is the element whose effect of increasing the

10 hardness is small as compared with that of Si. Moreover, Al is the element which has
the effect of improving the magnetic flux density by increasing B50 / Bs which is the ratio
of the magnetic flux density B50 to the saturation magnetic flux density Bs.
[0028]
5 When the Al content is less than 0.10%, the addition effect is not sufficiently
obtained. Thus, the Al content is to be 0.10% or more. The Al content is preferably
0.30% or more, more preferably 0.50% or more, and further more preferably 0.60% or
more. On the other hand, when the Al content is more than 3.00%, the magnetic flux
density decreases because the saturation magnetic flux density decreases, and the tensile
10 strength and the fatigue strength decrease because the yield ratio decreases. Thus, the
Al content is to be 3.00% or less. The Al content is preferably 2.70% or less, and more
preferably 2.40% or less.
[0029]
0.10 to 2.00% of Mn
15 Mn (manganese) is an element which has the effect of reducing the eddy current
loss by increasing the electrical resistance and of suppressing the formation of {111}
<112> texture which is undesirable for magnetic characteristics.
[0030]
When the Mn content is less than 0.10%, the addition effect is not sufficiently
20 obtained. Thus, the Mn content is to be 0.10% or more. The Mn content is preferably
0.15% or more, more preferably 0.20% or more, further more preferably more than
0.60%, and further more preferably 0.70% or more. On the other hand, when the Mn
content is more than 2.00%, the grain growth during annealing is suppressed, and the
iron loss deteriorates. Thus, the Mn content is to be 2.00% or less. The Mn content is
25 preferably 1.70% or less, and more preferably 1.50% or less.

11
[0031]
In the embodiment, the silicon steel sheet contains the impurities as the chemical
composition. The impurities correspond to elements which are contaminated during
industrial production of steel from ores and scrap that are used as a raw material of steel,
5 or from environment of a production process. For instance, the impurities are elements
such as C, P, S, and N. It is preferable that the impurities are limited as follows in order
to sufficiently obtain the effects of the embodiment. Moreover, since it is preferable
that the amount of respective impurities is low, a lower limit of the respective impurities
does not need to be limited, and the lower limit may be 0%.
10 [0032]
0.0030% or less of C
C (carbon) is an impurity element which causes the deterioration of the iron loss and the magnetic aging. When the C content is more than 0.003%, the iron loss deteriorates, and the magnetic aging occurs excessively. Thus, the C content is to be 15 0.0030% or less. The C content is preferably 0.0020% or less, and more preferably 0.0010% or less. The lower limit thereof includes 0%. However, it is difficult to control the content to be 0% due to production technology. The practical lower limit thereof is substantially 0.0001%. [0033] 20 0.050% or less of P
Although P (phosphorus) may contribute to the improvement of the tensile strength, P is an impurity element which embrittles the steel sheet. When the P content is more than 0.050%, the steel sheet including 2.00% or more of Si becomes brittle significantly. Thus, the P content is to be 0.050% or less. The P content is preferably 25 0.030% or less, and more preferably 0.020% or less. The lower limit thereof includes

12 0%. However, it is difficult to control the content to be 0% due to production
technology. The practical lower limit thereof is substantially 0.002%.
[0034]
0.005% or less of S
5 S (sulfur) is an impurity element which forms fine sulfides such as MnS, and
thus, suppresses the recrystalhzation and the grain growth during final annealing. When
the S content is more than 0.005%, the recrystallization and the grain growth during final
annealing are suppressed significantly. Thus, the S content is to be 0.005% or less.
The S content is preferably 0.003% or less, and more preferably 0.002% or less. The
10 lower limit thereof includes 0%. However, it is difficult to control the content to be 0%
due to production technology. The practical lower limit thereof is substantially
0.0003%.
[0035]
0.005% or less of N
15 N (nitrogen) is an impurity element which forms fine nitrides such as A1N, and
thus, suppresses the recrystallization and the grain growth during final annealing. When
the N content is more than 0.005%, the recrystallization and the grain growth during final
annealing are suppressed significantly. Thus, the N content is to be 0.005% or less.
The N content is preferably 0.003% or less, and more preferably 0.002% or less. The
20 lower limit thereof includes 0%. However, it is difficult to control the content to be 0%
due to production technology. The practical lower limit thereof is substantially
0.0005%.
[0036]
In the embodiment, the silicon steel sheet may contain the optional element in
25 addition to the base elements and the impurities described above. For instance, as

13 substitution for a part of Fe which is the balance described above, as the optional element,
the steel sheet may contain Sn, Cu, Sb, REM, Ca, and Mg. The optional elements may
be contained as necessary. Thus, a lower limit of the optional element does not need to
be limited, and the lower limit may be 0%. Moreover, even if the optional element may
5 be contained as impurities, the above mentioned effects are not affected.
[0037]
0to0.40%ofSn
0tol.00%ofCu
0to0.40%ofSb
10 Sn (tin), Cu (copper), and Sb (antimony) are elements which have the effect of
suppressing the formation of {111} <112> texture which is undesirable for magnetic characteristics, of suppressing the oxidation of steel sheet surface, and of controlling the grain growth to be uniform. In addition, Sn, Cu, and Sb are elements which have the effect of favorably controlling the thickness of the internally oxidized layer for the hot
15 rolled steel sheet. [0038]
When the Sn content is more than 0.40%, when the Cu content is more than 1.00%, or when the Sb content is more than 0.40%, the addition effect is saturated, the grain growth during final annealing are suppressed, and the steel sheet becomes brittle
20 during cold rolling due to the decrease in workability. Thus, the Sn content is to be
0.40% or less, the Cu content is to be 1.00% or less, and the Sb content is to be 0.40% or less. The Sn content is preferably 0.30% or less, and more preferably 0.20% or less. The Cu content is preferably 0.60% or less, and more preferably 0.40% or less. The Sb content is preferably 0.30% or less, and more preferably 0.20% or less.
25 [0039]

14 The lower limits of Sn, Cu, and Sb are not particularly limited, and may be 0%.
In order to favorably obtain the above effects, the Sn content may be 0.02% or more, the
Cu content may be 0.10% or more, and the Sb content may be 0.02% or more. The Sn
content is preferably 0.03% or more, and more preferably 0.05% or more. The Cu
5 content is preferably 0.20% or more, and more preferably 0.30% or more. The Sb
content is preferably 0.03% or more, and more preferably 0.05% or more.
[0040]
In the embodiment, it is preferable that the silicon steel sheet contains, as the
chemical composition, by mass%, at least one selected from the group consisting of
10 0.02 to 0.40% of Sn,
0.10 to 1.00% of Cu, and
0.02 to 0.40% of Sb.
[0041]
0 to 0.0400% of REM
15 0 to 0.0400% of Ca
0 to 0.0400% of Mg
REM (Rare Earth Metal), Ca (calcium), and Mg (magnesium) are the elements
which have the effects of fixing S as sulfides or oxysulfides, of suppressing the fine
precipitation of MnS and the like, and of promoting the recrystallization and grain
20 growth during final annealing.
[0042]
When REM, Ca, and Mg exceed 0.0400%, the sulfides or oxysulfides are
excessively formed, and the recrystallization and grain growth during final annealing are
suppressed. Thus, the REM content, the Ca content, and the Mg content are to be
25 0.0400% or less respectively. The respective contents are preferably 0.0300% or less

15 and more preferably 0.0200% or less.
[0043]
The lower limits of REM content, Ca content, and Mg content are not
particularly limited, and may be 0%. The REM content, the Ca content, and the Mg
5 content may be 0.0005% or more in order to obtain the above effects preferably. The
respective contents are preferably 0.0010% or more and more preferably 0.0050% or
more.
[0044]
In the embodiment, it is preferable that the silicon steel sheet contains, as the
10 chemical composition, by mass%, at least one selected from the group consisting of
0.0005 to 0.0400% of REM,
0.0005 to 0.0400% of Ca, and
0.0005 to 0.0400% of Mg.
[0045]
15 Herein, REM indicates a total of 17 elements of Sc, Y and lanthanoid, and is at
least one of them. The above REM content corresponds to the total content of at least
one of these elements. Industrially, misch metal is added as the lanthanoid.
[0046]
The steel composition as described above may be measured by typical analytical
20 methods for steel. For instance, the steel composition may be measured by using
ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometer: inductively
coupled plasma emission spectroscopy spectrometry). In addition, C and S may be
measured by the infrared absorption method after combustion, N may be measured by the
thermal conductometric method after fusion in a current of inert gas, and O may be
25 measured by, for instance, the non-dispersive infrared absorption method after fusion in a

16 current of inert gas.
[0047]
The above chemical composition is that of the silicon steel sheet. When the
non oriented electrical steel sheet to be the measurement sample has the insulation
5 coating and the like on the surface, the above chemical composition is obtained after
removing the coating.
[0048]
As a method for removing the insulation coating and the like of the non oriented
electrical steel sheet, for instance, the following method is exemplified. First, the non
10 oriented electrical steel sheet having the insulation coating and the like is immersed in
sodium hydroxide aqueous solution, sulfuric acid aqueous solution, and nitric acid
aqueous solution in this order. The steel sheet after the immersion is washed. Finally,
the steel sheet is dried with warm air. Thereby, it is possible to obtain the silicon steel
sheet from which the insulation coating is removed.
15 [0049]
Next, in regard to the non oriented electrical steel sheet according to the
embodiment, the internally oxidized layer of the silicon steel sheet is explained.
[0050]
Fig. 1 is a cross sectional illustration of the non oriented electrical steel sheet
20 according to the embodiment. When viewing a cross section whose cutting direction is
parallel to a thickness direction, the non oriented electrical steel sheet 1 according to the
embodiment includes the silicon steel sheet 11, and the insulation coating 15 arranged on
the silicon steel sheet 11. The silicon steel sheet includes the internally oxidized layer
13 in the surface thereof. The internally oxidized layer 13 includes SiCh 131. Herein,
25 the internally oxidized layer is a region where oxide phase of Si and the like is dispersed

17 in the form of particles or layers inside the silicon steel sheet.
[0051]

The internally oxidized layer includes Si02. In the embodiment, by finely and
5 densely precipitating Si02 in the internally oxidized layer and by controlling the hardness
of the internally oxidized layer, it is possible to obtain the effect of improving the fatigue
strength.
[0052]
In order to finely and densely precipitate Si02 in the internally oxidized layer,
10 the steel sheet needs to contain more than 2.00% of Si. In addition, the heat
conservation treatment during cooling after hot rolling needs to be favorably controlled.
[0053]

Average thickness of internally oxidized layer : 0.10 to 5.0 |im
15 When the average thickness of the internally oxidized layer is less than 0.10 |im,
it is difficult to obtain the effect of improving the fatigue strength. Thus, the average
thickness of the internally oxidized layer is to be 0.10 |im or more. The average
thickness of the internally oxidized layer is preferably more than 0.5 |im, more preferably
0.55 |im or more, further more preferably 0.6 |im or more, further more preferably 0.7
20 |im or more, and further more preferably 1.0 |im or more. On the other hand, when the
average thickness of the internally oxidized layer is more than 5.0 |im, the magnetic
characteristics, specifically the iron loss, deteriorates. Thus, the average thickness of
the internally oxidized layer is to be 5.0 |im or less. The average thickness of the
internally oxidized layer is preferably 4.0 |im or less, and more preferably 3.0 |im or less.
25 [0054]

18
In the embodiment, the vickers hardness in the internally oxidized layer is
controlled to be higher than the vickers hardness in the central area of the steel sheet.
Specifically, in the embodiment, the fatigue strength is improved not by increasing the
5 hardness of the electrical steel sheet in itself but by increasing only the hardness of the
predetermined region.
[0055]

Vickers hardness in central area of steel sheet: 120 to 300 Hv
10 When viewing the cross section whose cutting direction is parallel to the
thickness direction, the central area is a thickness range of 5/8 to 3/8 of the silicon steel
sheet. When the vickers hardness in the central area is less than 120 Hv, sufficient
fatigue strength is not obtained. Thus, the vickers hardness in the central area is to be
120 Hv or more. The vickers hardness in the central area is preferably 150 Hv or more,
15 and more preferably 170 Hv or more.
[0056]
On the other hand, when the vickers hardness in the central area is more than
300 Hv, the entire steel sheet is excessively hard, and the punchability deteriorates.
Thus, the vickers hardness in the central area is to be 300 Hv or less. The vickers
20 hardness in the central area is preferably 270 Hv or less, and more preferably 250 Hv or
less.
[0057]
Herein, it is possible to control the vickers hardness in the central area by the
solid solution strengthening of Si, Al, and Mn to Fe and by the grain size after final
25 annealing. The Si content, the Al content, and the Mn content may be determined, and

19 the grain size after final annealing may be determined, depending on the required
magnetic characteristics, the required workability during cold rolling, the production cost,
and the like. Herein, the grain size influences the magnetic characteristics, especially
the iron loss.
5 [0058]

Vickers hardness in internally oxidized layer : 1.15 times or more as compared with
vickers hardness in central area
It is possible to increase the fatigue strength by finely and densely precipitating
10 Si02 in the internally oxidized layer and by controlling the hardness of the internally
oxidized layer. Specifically, in the embodiment, the vickers hardness in the internally
oxidized layer is higher than the vickers hardness in the central area of the steel sheet.
[0059]
When the vickers hardness in the internally oxidized layer is less than 1.15 times
15 as compared with the vickers hardness in the central area, it is difficult to sufficiently
obtain the effect of improving the fatigue strength. Thus, the vickers hardness in the
internally oxidized layer is 1.15 times or more as compared with the vickers hardness in
the central area. The vickers hardness in the internally oxidized layer is preferably 1.20
times or more, and more preferably 1.25 times or more.
20 [0060]
The upper limit of the vickers hardness in the internally oxidized layer is not
particularly limited for the improvement of the fatigue strength. Substantial maximum
of the vickers hardness in the internally oxidized layer may be 1.5 times as compared
with the vickers hardness in the central area.
25 [0061]

20 The vickers hardness in the internally oxidized layer is to be 1.15 times or more
as compared with the vickers hardness in the central area, and thus, may be 138 Hv or
more. The vickers hardness in the internally oxidized layer is preferably 155 Hv or
more, more preferably 180 Hv or more, and further more preferably 200 Hv or more.
5 The vickers hardness in the internally oxidized layer is preferably 400 Hv or less, and
more preferably 300 Hv or less.
[0062]
The observation of the microstructure and the measurement of the hardness of
the internally oxidized layer and the central area of the silicon steel sheet as explained
10 above may be conducted by typical observation and measurement methods. For
instance, the following method may be employed.
[0063]
The specimens are cut out from the non oriented electrical steel sheet so that the
cutting direction is parallel to the thickness direction (specifically, the specimens are cut
15 out so that the cross section is parallel to the thickness direction and is perpendicular to
the rolling direction). The cross-sectional structure of the cross section is observed with
SEM (Scanning Electron Microscope) at a magnification at which each layer is included
in the observed visual field. For instance, in observation with a reflection electron
composition image (COMP image), it is possible to infer a constituent phase in the
20 cross-sectional structure. For instance, in the COMP image, the silicon steel sheet can
be distinguished as light color, Si02 in the internally oxidized layer as dark color, and the
insulation coating as intermediate color. As necessary, the constituent phase may be
identified in detail by quantitatively analyzing the chemical composition using
SEM-EDX (energy dispersive X-ray spectroscopy).
25 [0064]

21 Moreover, it is possible to identify whether or not the internally oxidized layer is
included in a surface area of the silicon steel sheet by SEM and SEM-EDX.
Specifically, it is confirmed whether or not the region where SiCh is observed is included
from an interface between the silicon steel sheet and an upper layer thereof toward a
5 depth direction of the silicon steel sheet. SiCh may be identified as the precipitate in
which the atomic ratio of Si and O is approximately 1 : 2 in the observed visual field by
EDX. For instance, in the observed visual field, a straight line along the thickness
direction is set as a reference line, and then, it is confirmed whether or not the region
where SiCh is observed is included on the reference line. When the region where SiCh
10 is observed is included in the silicon steel sheet, the region is judged to be the internally
oxidized layer. Moreover, the line segment (length) of the region on the reference line
may be judged to be the thickness of the internally oxidized layer.
[0065]
The average thickness of the internally oxidized layer may be determined as
15 follows. An area of approximately 100 |im or more in a planar direction in the steel
sheet is observed using SEM. Ten lines or more of the above reference lines are set at
even intervals, and the thickness of the internally oxidized layer is measured on each
reference line. An average of the obtained thicknesses of the internally oxidized layer is
regarded as the average thickness of the internally oxidized layer.
20 [0066]
Herein, when it is needed to observe a micro area which is smaller than a spatial
resolution of SEM in order to identify SiCh or to determine the average thickness of the
internally oxidized layer, a transmission electron microscope (TEM) may be used.
[0067]
25 The vickers hardness may be measured on the basis of a method disclosed in JIS

22 Z 2244 : 2009. When the vickers hardness in the internally oxidized layer is measured,
an indentation for the vickers hardness needs to be within the internally oxidized layer.
In the case, the measuring load is preferably within 9.8 x 10"5 to 9.8 x 10~2 N.
[0068]
5 The vickers hardness in the internally oxidized layer may be measured
according to the thickness of the internally oxidized layer, and can be accurately
measured when the load is appropriately set so that the maximum size of the indentation
is applied within the thickness of the internally oxidized layer. In order to accurately
measure the vickers hardness in the internally oxidized layer, the load may be more than
10 the above range of the load.
[0069]
For the measurement of the vickers hardness, the indentation size is generally
measured using an optical microscope. In order to accurately measure the vickers
hardness, the indentation size may be measured at a magnification of 1000-fold or more
15 using the electron microscope such as SEM.
[0070]
On the other hand, it is preferable that the vickers hardness in the central area of
the steel sheet is measured by the same load as that applied for measuring the vickers
hardness in the internally oxidized layer. In the case, the indentation size is small as
20 compared with grain size of the steel sheet. Thus, it is preferable that the indentation is
applied away from a grain boundary, and then the indentation size is measured.
[0071]
In the measurement of the vickers hardness specified in JIS, the measuring load
is provided from 1 gf (9.8 x 10~2 N). However, when the vickers hardness is measured,
25 it is preferable that the load is precisely controlled, is reduced, and is set so that the

23 indentation size becomes within the internally oxidized layer. Herein, when it is needed
to observe a micro area which is smaller than the spatial resolution of the optical
microscope or SEM for measuring the vickers hardness, the hardness value measured by
a nanoindentation method may be converted to the vickers hardness.
5 [0072]
Next, a producing method for the non oriented electrical steel sheet according to
the embodiment is explained.
[0073]
Fig. 2 is a flow chart illustrating a producing method for the non oriented
10 electrical steel sheet according to the embodiment. In the embodiment, the silicon steel
sheet is obtained by casting molten steel with an adjusted composition, by being
hot-rolled, by being heat-conservation-treated during cooling after hot rolling, by being
pickled, by being cold-rolled, and then by being final-annealed. Further, the non
oriented electrical steel sheet is obtained by forming the insulation coating on the silicon
15 steel sheet.
[0074]
The formation of the internally oxidized layer is explained. Fig. 3 is a cross
sectional illustration showing a situation such that the internally oxidized layer is formed
in the base steel sheet. Fig. 3(A) shows a situation after hot rolling, Fig. 3(B) shows a
20 situation after heat conservation treatment, Fig. 3(C) shows a situation after pickling, and
Fig. 3(D) shows a situation after cold rolling.
[0075]
As shown in Fig. 3(A), by hot rolling, an externally oxidized layer 17 is formed
on the surface of the base steel sheet 11. Subsequently, as shown in Fig. 3(B), by the
25 heat conservation treatment during cooling after hot rolling, oxygen diffuses from the

24 externally oxidized layer 17 to the base steel sheet 11, and the internally oxidized layer
13 is formed. At this time, it is preferable to finely and densely precipitate SiCh 131 in
the internally oxidized layer 13 by controlling conditions of the heat conservation
treatment.
5 [0076]
Subsequently, as shown in Fig. 3(C), by pickling, the externally oxidized layer
17 on the surface of the base steel sheet 11 is removed. At this time, in order to
improve the magnetic characteristics, a part of the internally oxidized layer 13 may be
removed by pickling, and thereby, the thickness of the internally oxidized layer 13 may
10 be controlled. Furthermore, as shown in Fig. 3(D), by cold rolling, the internally
oxidized layer 13 in the surface of the base steel sheet 11 is extended in the rolling
direction L. After cold rolling, the internally oxidized layer 13 may be remained.
Alternatively, when the thickness of the internally oxidized layer 13 is excessive, a part
of the internally oxidized layer 13 may be removed by pickling, and thereby, the
15 thickness of the internally oxidized layer 13 may be controlled.
[0077]
Thereafter, for instance, the final annealing is conducted in the atmosphere
including nitrogen and hydrogen, the recrystallization and the grain growth are proceeded
in the base steel sheet, and thereby, it is possible to obtain the silicon steel sheet in which
20 the internally oxidized layer containing SiCh is included in the surface thereof.
[0078]
The insulation coating may be formed on the surface of the silicon steel sheet.
The insulating coating is generally a coating called a semi-organic coating. For instance,
a coating including chromic acid and organic resin disclosed in Non-Patent Document 1
25 or a coating including phosphate and organic resin disclosed in Non-Patent Document 2

25 is general. The amount of the insulation coating is preferably 0.1 to 5 gm"2 per one side.
[0079]
In the non oriented electrical steel sheet according to the embodiment, the
silicon steel sheet includes the internally oxidized layer, the internally oxidized layer
5 includes SiCh, the average thickness of the internally oxidized layer is 0.10 to 5.0 |im, the
vickers hardness in the central area of the steel sheet is 120 to 300 Hv, and the vickers
hardness in the internally oxidized layer is 1.15 to 1.5 times as compared with the vickers
hardness in the central area.
[0080]
10 The silicon steel with the above features may be produced by the following
method for instance.
[0081]

A cast piece with the adjusted chemical composition is heated and hot-rolled.
15 At this time, in order to suppress the deterioration of the iron loss caused by
solid-soluting and precipitating the sulfides and the like in steel, the heating temperature
is to be 1200°C or less. Moreover, in order to ensure the final temperature of 900°C or
more, the heating temperature is to be 1080°C or more.
[0082]
20 When the final temperature of hot rolling is low, hot workability deteriorates,
and thickness accuracy in the transverse direction of the steel sheet deteriorates. Thus,
the lower limit of the final temperature is to be 900°C. On the other hand, when the
final temperature of hot rolling is more than 1000°C, {100} texture which is favorable
for the magnetic characteristics decreases. Thus, the upper limit of the final temperature
25 is to be 1000°C.

26 [0083]
In order to properly form the internally oxidized layer during the heat
conservation treatment after hot rolling, it is preferable to form the externally oxidized
layer with a thickness of 1 |im or more on the surface of the hot rolled steel sheet during
5 hot rolling. The formation of the externally oxidized layer may be controlled by the
temperature, holding time, and the like of hot rolling.
[0084]

The hot rolled steel sheet is heat-conservation-treated during cooling after hot
10 rolling. In the heat conservation treatment, the grains are coarsened so that the grain
size is 20 |im or more. Moreover, oxygen included in the externally oxidized layer
formed on the surface of the hot rolled steel sheet diffuses into the hot rolled steel sheet,
and thereby, the internally oxidized layer is formed.
[0085]
15 The internally oxidized layer is formed by diffusing oxygen into the steel sheet
during the heat conservation treatment. At this time, the oxygen source is the externally
oxidized layer formed during hot rolling, specifically the externally oxidized layer which
mainly consists of magnetite and wustite, hematite, or the like.
[0086]
20 During cooling after hot rolling, the hot rolled steel sheet is
heat-conservation-treated under conditions such as the atmosphere with oxygen partial
pressure of 10"15 Pa or more, the temperature range of 850°C or less and 700°C or more,
and the time of 10 minutes or more and 3 hours or less. As a result, it is possible to
form the internally oxidized layer in which SiCh is finely and densely precipitated, and
25 possible to favorably control the hardness of the internally oxidized layer.

27 [0087]
When the heat conservation temperature is more than 850°C, the average
thickness of the internally oxidized layer is thickened. As a result, the average
thickness of the internally oxidized layer is more than 5.0 |im even after cold rolling, and
5 thus, the pickling to reduce the thickness of the internally oxidized layer may be
overloaded. Moreover, when the heat conservation temperature is more than 850°C, it
is difficult to finely and densely precipitate S1O2. Thus, the heat conservation
temperature is preferably 850°C or less. On the other hand, in order to finely and
densely precipitate SiCh, although the Si content in steel influences a situation, the heat
10 conservation temperature is preferably 700°C or more, more preferably 750°C or more,
and further more preferably 800°C or more.
[0088]
The heat conservation time is preferably 10 minutes or more, in order to coarsen
the grains to 20 |im or more in the hot rolled steel sheet. Moreover, in order to finely
15 and densely precipitate SiCh, the heat conservation time is preferably 10 minutes or more,
more preferably 20 minutes or more, and further more preferably 30 minutes or more.
On the other hand, the upper limit of the heat conservation time is not particularly limited.
However, when the heat conservation time is excessive, grain boundaries become brittle
near the surface of the steel sheet, and then cracks and fractures tend to occur in the
20 following pickling and cold rolling. Thus, the heat conservation time is preferably 3
hours or less.
[0089]
As the atmosphere during the heat conservation treatment, the oxygen partial
pressure is preferably 10"15 Pa or more. The atmosphere is preferably the mixed
25 atmosphere of inert gas such as nitrogen.

28 [0090]
Herein, it is preferable that the externally oxidized layer of 1 |im or more is
formed during hot rolling, and then, the heat conservation treatment is conducted so that
the surface of the steel sheet is not exposed in the atmosphere of the heat conservation
5 treatment. For instance, the heat conservation treatment is conducted after coiling the
hot rolled steel sheet. In the case, since the sheet surfaces are contacted each other
except for the outermost surface of coil, it is favorably suppressed that the surface of the
steel sheet is exposed in the atmosphere of the heat conservation treatment.
[0091]
10 When the steel sheet contains Sn, Cu, or Sb, these elements suppress to form
and growth the internally oxidized layer, and thus, it is possible to increase the heat
conservation temperature within the above range. In the case, it is possible to favorably
coarsen the grain size, while suppressing the excessive growth of the internally oxidized
layer. Moreover, when the steel sheet contains Sn, Cu, or Sb, by controlling the heat
15 conservation temperature to 800°C or more, it is possible to favorably improve the
magnetic flux density, while forming the internally oxidized layer with favorable
thickness.
[0092]
However, when the heat conservation temperature is excessive high even when
20 the steel sheet contains Sn, Cu, or Sb, the magnetic characteristics may be improved, but
the internally oxidized layer may be excessively thickened. In the case, the amount of
pickling may be controlled during pickling treatment, and thereby, the thickness of the
internally oxidized layer may be appropriately controlled.
[0093]
25 The mechanism such that Sn, Cu, and Sb contained in steel suppress to form and

29 growth the internally oxidized layer is considered as follows. These elements segregate
between the externally oxidized layer and the steel, and thereby, it is suppressed that
oxygen included in the externally oxidized layer diffuses into the steel sheet.
[0094]
5 In conventional technique, the hot rolled steel sheet after hot rolling is cooled to
near room temperature, and thereafter, the hot rolled steel sheet annealing is conducted in
the temperature range of 800 to 1000°C for approximately 1 minute by reheating the steel
sheet. On the other hand, in the embodiment, in order to favorably control the internally
oxidized layer, the hot rolled steel sheet during cooling after hot rolling is
10 heat-conservation-treated under the above conditions. Moreover, the steel sheet after
heat conservation treatment is cooled to near room temperature, and thereafter, is
subjected to pickling and cold rolling without conducting the hot rolled steel sheet
annealing.
[0095]
15
The base steel sheet after the heat conservation treatment is pickled. The
amount of pickling (weight loss after pickling) is controlled depending on the state of the
externally oxidized layer and the internally oxidized layer on the surface of the steel
sheet and on the conditions of acid used for pickling such as type, concentration, and
20 temperature. In the pickling, the externally oxidized layer is made to be dissolved, and
the internally oxidized layer is made to be thinned to the predetermined thickness.
[0096]
For instance, as the method for controlling the amount of pickling to be small, a
method of shortening the pickling time, decreasing the temperature of the pickling
25 solution, or adding a commercially available pickling inhibitor (polyamine or the like) is

30 effective. For instance, the pickling inhibitor includes mainly polyamine, and the
polymer thereof has a property of being easily adsorbed on unshared electron pairs of
iron atoms. The polymer adheres to the surface of the steel sheet, the area in contact
with the acid is reduced, and thus, the pickling rate is reduced. Formic acid and the like
5 are known as additives which enhance the above effect.
[0097]
On the other hand, as the method for controlling the amount of pickling to be
large, a method of prolonging the pickling time, increasing the temperature of the
pickling solution, or adding a commercially available pickling accelerator (sodium
10 thiosulfate or the like) is effective. The pickling accelerator includes chelating agent for
iron atoms, and has a property of easily forming a coordination bond to iron ion. When
the pickling accelerator is included, iron dissolved in the pickling solution is chelated.
As a result, the concentration of iron ions dissolved in the pickling solution does not
increase easily, the dissolution rate of iron does not decrease, and the pickling proceeds.
15 [0098]

The base steel sheet after the pickling is cold-rolled. In order to improve the
magnetic flux density, the reduction of cold rolling is preferably 50 to 90%. The
reduction of cold rolling is the cumulative reduction of cold rolling and is obtained by
20 (thickness of steel sheet before cold rolling - thickness of steel sheet after cold rolling) -f
thickness of steel sheet before cold rolling x 100. It is desirable to determine the
reduction of cold rolling in consideration of the thickness of final product, cold
workability, and the like.
[0099]
25

31 The base steel sheet after the cold rolling is final-annealed. The final annealing
is a process of recrystallizing the cold rolled steel sheet and controlling the grain size, in
order to improve the magnetic characteristics, particularly to improve the magnetic flux
density and the iron loss. Atmosphere is important for the final annealing. Since the
5 magnetic characteristics deteriorate when the steel sheet is oxidized, oxygen
concentration in the atmosphere for the final annealing is preferably several tens of ppm
or less.
[0100]
The atmospheric gas is preferably nitrogen or argon, and hydrogen may be
10 mixed as necessary in order to suppress the oxidation of the steel sheet. Herein, when
the hydrogen concentration is excessively increased, the internally oxidized layer is
reduced, and the fine SiCh which improves the fatigue strength is reduced.
[0101]
The final annealing temperature is preferably 700°C or more in which the
15 recrystallization of the steel sheet occurs. When the final annealing temperature is
excessively lower, the recrystallization becomes insufficient. On the other hand, when
final annealing temperature is excessively higher, fine SiCh included in the internally
oxidized layer is grown, and thus, the effect of improving the fatigue strength is not
obtained. Thus, the final annealing temperature is preferably 1150°C or less.
20 [0102]
The insulation coating is formed for the silicon steel sheet after the final
annealing. For instance, the insulation coating may be a coating including chromic acid
and organic resin or a coating including phosphate and organic resin. The amount of the
insulation coating is preferably 0.1 to 5 gm"2 per one side.
25

32 Examples
[0103]
Hereinafter, the effects of an aspect of the present invention are described in
detail with reference to the following examples. However, the condition in the
5 examples is an example condition employed to confirm the operability and the effects of
the present invention, so that the present invention is not limited to the example condition.
The present invention can employ various types of conditions as long as the conditions
do not depart from the scope of the present invention and can achieve the object of the
present invention.
10 [0104]

The molten steel with the adjusted composition was cast, and then, the silicon
steel sheet was produced by controlling the production conditions in each process. The
chemical compositions are shown in Tables 1 and 2, and the production conditions are
15 shown in Tables 3 and 4. In the above production, the hot rolling was conducted under
the conditions such that the heating temperature was 1180°C and the temperature of
outlet side of final rolling was 970°C, and the hot rolled steel sheet with the thickness of
2.0 mm was produced. At this time, the layer with the thickness of approximately 10
|im which consisted of mainly Fe304 was formed on the surface as the externally
20 oxidized layer.
[0105]
The obtained hot rolled steel sheet during cooling after hot rolling is subjected to
the heat conservation treatment in the atmosphere where the oxygen partial pressure was
10"15 Pa or more at the temperature and time shown in Tables 3 and 4. Thereby, the
25 grains were grown to 20 |im or more, and the internally oxidized layer was formed.

33 Herein, the specimen described as "hot rolled steel sheet annealing" in the "heat
conservation treatment" column in Table 4 was cooled to room temperature without the
heat conservation treatment during cooling after hot rolling, and thereafter, the hot rolled
steel sheet annealing was conducted in the atmosphere of 100% nitrogen at 800°C for 60
5 seconds.
[0106]
The steel sheet which was heat-conservation-treated or
hot-rolled-steel-sheet-annealed after hot rolling was subjected to the pickling by being
immersed for 30 seconds in hydrochloric acid (10 mass%) which was at 85°C and which
10 included the additives (0.05mass%) shown in Tables 3 and 4. The steel sheet after
pickling was subjected to the cold rolling whose reduction was 75% in order to obtain the
cold rolled steel sheet with the thickness of 0.5 mm. The cold rolled steel sheet was
subjected to the final annealing at 1000°C for 30 seconds in the atmosphere of 10%
hydrogen and 90% nitrogen. At this time, the dew point of the above atmosphere was
15 -30°C. Moreover, for the silicon steel sheet after final annealing, the phosphate based
insulation coating with the average thickness of 1 |im was formed.
[0107]
Thereafter, the magnetic characteristics (B50 and W15/50), the fatigue strength, the
vickers hardness in the internally oxidized layer, and the vickers hardness in the central
20 area of the steel sheet were measured. The results are shown in Tables 5 and 6.
[0108]
Magnetic Characteristics (B50 and W15/50)
A sample with 55 mm square was cut and taken from the produced non oriented
electrical steel sheet, and then B50 and W15/50 were measured by the single sheet tester
25 (SST), herein B50 indicating the magnetic flux density in units of T (tesla) when the steel

34 sheet be excited under magnetic field strength of 5000 A/m, and W15/50 indicating the
iron loss when the steel sheet be excited under conditions such that 50 Hz and the
magnetic flux density of 1.5 T.
Evaluation Criteria of B50
5 Acceptable: 1.65 T or more
Unacceptable: less than 1.65 T
Evaluation Criteria of W15/50
Acceptable: 3.0 W/kg or less
Unacceptable: more than 3.0 W/kg
10 [0109]
Fatigue Strength
From the produced non oriented electrical steel sheet, a sample corresponding to
No. 5 specimen specified in Annex B of JIS Z 2241: 2011 was taken by electrical
discharge machining along the rolling direction of the steel sheet, and the fatigue test was
15 conducted under the following conditions. A test was conducted in which the stress
ratio was kept constant and accordingly the minimum and maximum stresses were
changed. The stress conditions in which two specimens or more in three specimens
were not fractured by two million repetitions were determined, and the average stress
((minimum stress + maximum stress) -f 2) was defined as the fatigue strength.
20 [0110]
The fatigue test was conducted under conditions such that the average stress
becomes ±10 MPa in each step, the stress conditions in which two specimens or more in
three specimens were not fractured by two million repetitions were determined, and the
average strength at that time was defined as the fatigue strength.
25 Test Conditions

35 Test Method: Partially Pulsating Test
Stress Ratio: 0.05
Frequency: 20Hz
Repetition: 2 million
5 Number of Specimens: 3 pieces in each stress
Evaluation Criteria of Fatigue Strength
Acceptable: 200 MPa or more of average stress
Unacceptable: less than 200 MPa of average stress
[0111]
10 Analysis of Average Thickness of Internally Oxidized Layer and Precipitate in
Internally Oxidized Layer
The cross section of the produced non oriented electrical steel sheet was
polished, the SEM micrograph was taken using the reflection electron composition image
at a magnification of 1000-fold, and the area of approximately 100 |im or more in the
15 planar direction in the steel sheet was observed regarding the front surface and the back
surface of the steel sheet. According to the necessity, the cross section of the produced
non oriented electrical steel sheet was observed by TEM.
[0112]
The observation of the microstructure and the measurement of the hardness of
20 the internally oxidized layer and the central area of the silicon steel sheet were conducted
on the basis of the above method. For the thickness of the internally oxidized layer, the
average was calculated using those of 20 locations. For the vickers hardness, 10
indentations were formed by the measuring load of 0.03 gf (2.94 x 10"3 N) on each of the
internally oxidized layer and the central area, the diagonal length of each indentation
25 (diamond shape) was measured by SEM, and the average was calculated using those of

36 10 locations. According to the necessity, the hardness value measured by the
nanoindentation method was converted to the vickers hardness.
[0113]
The chemical compositions of the produced silicon steel sheets are shown in
5 Tables 1 and 2, and the production conditions and the evaluation results are shown in
Tables 3 to 6. Herein, the chemical compositions of the silicon steel sheets were
substantially the same as those of the molten steels. In the tables, the underlined value
indicates out of the range of the present invention. Moreover, in the tables,"-" with
respect to the chemical composition of silicon steel sheet indicates that no alloying
10 element was intentionally added.
[0114]
As shown in Tables 1 to 6, in the inventive examples of Nos. Bl to B26, the
chemical composition of silicon steel sheet, the internally oxidized layer, and the central
area of the steel sheet were favorably controlled, and thereby the magnetic characteristics
15 and the fatigue strength were excellent for the non oriented electrical steel sheet.
Specifically, in the inventive examples of Nos. Bl to B26, it was possible to obtain the
non oriented electrical steel sheet excellent in the magnetic characteristics and the fatigue
strength without adding the additional process to harden the surface.
[0115]
20 On the other hand, as shown in Tables 2, 4, and 6, in the comparative examples
of Nos. bl to bl4, at least one of the chemical composition of silicon steel sheet, the
internally oxidized layer, and the central area of the steel sheet were not favorably
controlled, and thereby at least one of the magnetic characteristics and the fatigue
strength were not satisfied for the non oriented electrical steel sheet.
25 [0116]

TABLES

1

STEEL No. CHEMICA L COMPOSITION OF SILICON STEEL S HEET (IN UNITS OFMASS%, BALANCE CONSIS

C Si Mn P S Al N Cu Sn Sb
A1 0.0028 3.0 0.19 0.02 0.0015 0.30 0.0025 - - -
A2 0.0022 2.1 0.59 0.04 0.0014 1.15 0.0022 - - -
A3 0.0021 3.9 0.21 0.03 0.0013 0.20 0.0023 - - -
A4 0.0023 3.0 0.11 0.02 0.0013 0.32 0.0025 - - -
A5 0.0022 2.2 1.95 0.01 0.0010 0.16 0.0020 - - -
A6 0.0021 3.0 0.18 0.04 0.0012 0.30 0.0021 - - -
A7 0.0022 3.0 0.22 0.02 0.0050 0.30 0.0023 - - -
A8 0.0023 3.0 0.21 0.02 0.0012 0.12 0.0023 - - -
A9 0.0018 2.2 0.22 0.03 0.0012 2.95 0.0021 - - -
A10 0.0021 3.0 0.21 0.02 0.0014 0.30 0.0048 - - -
A11 0.0024 3.0 0.20 0.02 0.0012 0.31 0.0022 0.20 - -
A12 0.0023 3.0 0.19 0.02 0.0010 0.29 0.0021 - 0.05 -
A13 0.0021 3.0 0.21 0.02 0.0011 0.29 0.0021 - - 0.03
A14 0.0021 3.0 0.21 0.02 0.0025 0.31 0.0023 - - -
A15 0.0023 3.0 0.20 0.03 0.0026 0.30 0.0020 - - -
A16 0.0020 3.0 0.20 0.03 0.0026 0.28 0.0018 - - -
A17 0.0016 3.0 0.31 0.01 0.0008 0.29 0.0013 - - 0.03
A18 0.0015 3.0 0.29 0.01 0.0009 0.31 0.0011 - 0.03 -
A19 0.0026 2.7 0.20 0.04 0.0018 0.28 0.0020 - - -
A20 0.0019 2.1 0.19 0.02 0.0010 0.33 0.0023 0.50 0.30 -

TABLES

2

STEEL No. CHEMICAL COMPOSITION OF SILICON STEEL S HEET (IN UNITS OFMASS%, BALANCE

C Si Mn P S Al N Cu Sn
A21 0.0010 3.0 0.20 0.01 0.0015 0.29 0.0020 - -
A22 0.0021 3.0 0.19 0.01 0.0015 0.29 0.0019 0.10 -
A23 0.0024 3.0 0.21 0.02 0.0012 0.31 0.0018 - 0.02
A24 0.0023 3.0 0.20 0.01 0.0014 0.28 0.0020 - -
A25 0.0021 3.0 0.19 0.02 0.0044 0.31 0.0022 - -
A26 0.0023 3.0 0.21 0.03 0.0046 0.28 0.0019 - -
a1 0.0035 2.5 0.19 0.02 0.0016 0.30 0.0021 - -
a2 0.0022 18 0.18 0.02 0.0015 1.50 0.0023 - -
a3 0.0021 43 0.21 0.02 0.0015 0.20 0.0022 - -
a4 0.0023 2.5 0.08 0.01 0.0015 0.40 0.0021 - -
a5 0.0018 3.1 2.20 0.01 0.0017 0.21 0.0016 - -
a6 0.0022 3.0 0.19 0.07 0.0012 0.31 0.0019 - -
a7 0.0023 3.0 0.35 0.02 0.0060 0.18 0.0018 - -
a8 0.0025 3.0 0.50 0.02 0.0015 0.08 0.0020 - -
a9 0.0024 2.3 0.20 0.02 0.0015 3.10 0.0020 - -
a10 0.0020 3.1 0.21 0.02 0.0014 0.20 0.0056 - -
a11 0.0024 2.4 0.17 0.02 0.0015 0.31 0.0023 - -
a12 0.0026 3.0 0.19 0.03 0.0017 0.29 0.0019 - -
a13 0.0025 2.9 0.20 0.02 0.0020 0.30 0.0020 - -
a14 0.0023 2.9 0.20 0.02 0.0020 0.30 0.0020 - -

39
TABLES 3

TEST No. STEEL No. HEAT CONSERVATI JN TREATM ENT PICKLING TREATMENT ADDITIVES

HEAT CONSERVATION TEMPERATURE °C TIME MINUTES

B1 A1 HEAT CONSERVATION 850 10 POLYAMINE + FORMIC ACID
B2 A2 HEAT CONSERVATION 750 10 POLYAMINE + FORMIC ACID
B3 A3 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B4 A4 HEAT CONSERVATION 750 10 POLYAMINE + FORMIC ACID
B5 A5 HEAT CONSERVATION 770 10 POLYAMINE + FORMIC ACID
B6 A6 HEAT CONSERVATION 850 10 POLYAMINE + FORMIC ACID
B7 A7 HEAT CONSERVATION 800 10 POLYAMINE + FORMIC ACID
B8 A8 HEAT CONSERVATION 750 10 POLYAMINE + FORMIC ACID
B9 A9 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B10 A10 HEAT CONSERVATION 750 10 POLYAMINE + FORMIC ACID
B11 A11 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B12 A12 HEAT CONSERVATION 730 10 POLYAMINE + FORMIC ACID
B13 A13 HEAT CONSERVATION 750 10 POLYAMINE + FORMIC ACID
B14 A14 HEAT CONSERVATION 730 10 POLYAMINE + FORMIC ACID
B15 A15 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B16 A16 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B17 A17 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B18 A18 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B19 A19 HEAT CONSERVATION 850 170 SODIUM THIOSULFATE
B20 A20 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
[0119] [Table 4]

40
TABLES 4

TEST No. STEEL No. HEAT CONSERVATION TREATMENT PICKLING TREATMENT ADDITIVES

HEAT CONSERVATION TEMPERATURE °C TIME MINUTES

B21 A21 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
B22 A22 HEAT CONSERVATION 830 10 POLYAMINE + FORMIC ACID
B23 A23 HEAT CONSERVATION 830 10 POLYAMINE + FORMIC ACID
B24 A24 HEAT CONSERVATION 830 10 POLYAMINE + FORMIC ACID
B25 A25 HEAT CONSERVATION 800 20 POLYAMINE + FORMIC ACID
B26 A26 HEAT CONSERVATION 800 30 POLYAMINE + FORMIC ACID
b1 a1 HOT ROLLED STEEL SHEET ANNEALING 800 1 POLYAMINE + FORMIC ACID
b2 a2 HEAT CONSERVATION 750 10 POLYAMINE + FORMIC ACID
b3 a3 HEAT CONSERVATION 680 10 POLYAMINE + FORMIC ACID
b4 a4 HEAT CONSERVATION 650 10 POLYAMINE + FORMIC ACID
b5 a5 HEAT CONSERVATION 800 10 POLYAMINE + FORMIC ACID
b6 a6 HEAT CONSERVATION 850 10 POLYAMINE + FORMIC ACID
b7 a7 HEAT CONSERVATION 800 10 POLYAMINE + FORMIC ACID
b8 a8 HEAT CONSERVATION 750 10 POLYAMINE + FORMIC ACID
b9 a9 HEAT CONSERVATION 700 10 POLYAMINE + FORMIC ACID
b10 a10 HEAT CONSERVATION 750 10 SODIUM THIOSULFATE
b11 a11 HEAT CONSERVATION 690 10 POLYAMINE + FORMIC ACID
b12 a12 HEAT CONSERVATION 860 10 POLYAMINE + FORMIC ACID
b13 a13 HOT ROLLED STEEL SHEET ANNEALING 800 1 POLYAMINE + FORMIC ACID
b14 a14 HEAT CONSERVATION 720 8 POLYAMINE + FORMIC ACID
[0120] [Table 5]

41

TABLES 5
TEST No. STEEL No. INTERNALLY OXIDIZED LAYER VICKERS HARDNESS MAGNETIC
FLUX DENSITY
T IRON LOSS
W15/50 W/k$ FATIGUE STRENGTH
MPa NOTE

AVERAGE THICKNESS EXISTENCE 0FSi02 INTERNALLY
OXIDIZED
LAYER
Hv CENTRAL AREA
Hv RATIO
OF
HARDNESS

B1 A1 0.15 EXIST 200 170 1.18 1.71 2.5 210 INVENTIVE EXAMPLE
B2 A2 0.6 EXIST 190 150 1.27 1.70 2.3 200 INVENTIVE EXAMPLE
B3 A3 0.4 EXIST 250 215 1.16 1.69 2.2 220 INVENTIVE EXAMPLE
B4 A4 0.8 EXIST 205 170 1.21 1.71 2.2 210 INVENTIVE EXAMPLE
B5 A5 2.0 EXIST 180 140 1.29 1.71 2.2 200 INVENTIVE EXAMPLE
B6 A6 4.8 EXIST 200 170 1.18 1.69 2.5 200 INVENTIVE EXAMPLE
B7 A7 2.0 EXIST 210 170 1.24 1.71 2.5 210 INVENTIVE EXAMPLE
B8 A8 0.6 EXIST 210 175 1.20 1.68 2.4 210 INVENTIVE EXAMPLE
B9 A9 0.5 EXIST 235 195 1.21 1.65 2.4 230 INVENTIVE EXAMPLE
B10 A10 1.0 EXIST 210 170 1.24 1.71 2.5 210 INVENTIVE EXAMPLE
B11 A11 0.5 EXIST 210 170 1.24 1.72 2.1 210 INVENTIVE EXAMPLE
B12 A12 1.0 EXIST 220 175 1.26 1.72 2.0 210 INVENTIVE EXAMPLE
B13 A13 2.0 EXIST 225 170 1.32 1.72 2.1 220 INVENTIVE EXAMPLE
B14 A14 1.2 EXIST 215 170 1.26 1.72 1.9 220 INVENTIVE EXAMPLE
B15 A15 0.6 EXIST 210 170 1.24 1.72 1.9 210 INVENTIVE EXAMPLE
B16 A16 0.8 EXIST 215 175 1.23 1.72 1.9 210 INVENTIVE EXAMPLE
B17 A17 0.5 EXIST 210 170 1.24 1.73 1.8 210 INVENTIVE EXAMPLE
B18 A18 0.5 EXIST 215 175 1.23 1.73 1.8 210 INVENTIVE EXAMPLE
B19 A19 0.6 EXIST 200 170 1.18 1.73 1.8 230 INVENTIVE EXAMPLE
B20 A20 0.3 EXIST 141 122 1.16 1.71 1.9 200 INVENTIVE EXAMPLE
[0121] [Table 6]

42

LU LU LU LU LU LU LU LU LU LU LU LU LU LU
LU LU LU LU LU LU _l _l _l _l _l _l _l _l _l _l _l _l _l _l
1 l l l l l Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_
Q_ Q_ Q_ Q_ Q_ Q_ ^» ^» ^» ^a- ^a- ^a- ^a- ^» ^=- ^» ^» ^a- ^a- ^»-
«< ■< > > > > > > > > > > > > >

> ■> ■> ~> ■> ■>
^— 1— 1— 1— 1— 1— 1— 1— 1— 1— 1— 1— 1— 1— 1—
1— 1— 1— 1— 1— 1— > > > > > Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_ Q_
-z* ■=" ^>- ^>- ^>- ^» ^» ^» ^» ^>- ^>- ^>- ^» ^^ ^» ^>-
C_) CO CO O O O O O C_) CO O O CO CO
CO CO CO CO CO CO CO CO CO CO CO CO CO CO
FATIGUE STRENGTH (0
Q_ o o o o o o O o o o o o o o o o o o o o


■^ CM CM CO CO ■«tf- LO lO "*!- LO CO LO ■^ ■^ CD LO LO r^ LO CO

^ CM CM CM CM CM CM 1 1 CM ^ CM '^ CM CM "^ '^ 1 ^ 1 1
^ o ^ CO w M
O OO ^ CO CO CO cn LO " T CO ■*!■ LO r^
Q:O -
-,— -,— -,— 1— CM CM CO CO CO CO CO CM CO CO CO CO CM CM CM CM
— _i £

£2 >-|-:xtZ o CM *tf" ■f ■^i- O _ *sT CO O) LO CO _ _ LO OS _ _ CM CD CO
t^=3C0 « H r» P» r> r» r» r» CO CO LO CO CO r» r- CO LO r- r- r~ CO CO
OO
CD PM o O) !— co o ■
X CD O LO o o lO O col LO O LO LO LO o O LO LO ** o LO



LO r» r- r» r- r- "
X o o o LO o LO LO o O O LO o LO o LO o O o LO o



T CM CO CO CM CO ■*" CO O LO T ■^ o o CO 05 CO o LO LO

h^rf
CM CM CM CM CM CM ■^ ■I— CO 1— CM CM CM CM CM 'I— ■»- CM ■i- T-
> ^<=>
s LU 1— 1— 1— i— 1— 1— 1— 1— 1— 1— 1— 1— 1— 1— 1— LU 1— 1— 1— 1—
3 g° CO OO OO CO OO OO CO OO OO OO OO OO CO CO CO
OO OO OO OO
S 1— CO OO !_,_ X X X X X X X X X X X X X X X o X X X X
.YOXIDI/ x<=>
LU LU LU LU LU LU LU LU LU LU LU LU LU LU LU LU
LU LU LU LU

LU OO
3 CO LU t CO CO CM CM CM LO CM CM o 0.08 col CM co CO LO Ol CO 001 LO CO
INTERN LU CJ> ^ o o 1 CM CM CM O o CM
Lo| 1 1— CO CM
o LOI o o
CO STEEL No. A21 CM CM < A23 A24 A25 A26 ^ CM (0 CO CO CO LO CO CO
co p-
(0 CO (0 (0 o
(0 (0 CM (0 CO (0 (0
TABLE TEST No. CM
CQ 229 B23 B24 B25 929 -Q CM
_o CO
_Q _Q LO CO _Q J3 CO CD _Q o ]Q CM 5 CO
Industrial Applicability
[0122]
According to the above aspects of the present invention, it is possible to provide the non oriented electrical steel sheet excellent in the fatigue strength and the magnetic characteristics and also excellent in cost. Therefore, it is possible to provide the non oriented electrical steel sheet which is suitable as the core materials for electrical

43 equipment, especially suitable as the core materials for rotating machines, small and
medium size transformers, electrical components, and the like, and especially suitable as
the rotor core of IPM motor. In addition, it is possible to provide the non oriented
electrical steel sheet which sufficiently meets the demand for higher efficiency of
5 electrical equipment, higher speed rotation of rotating machines, and smaller size of
rotating machines. Accordingly, the present invention has significant industrial
applicability.
Reference Signs List
10 [0123]
I NON ORIENTED ELECTRICAL STEEL SHEET
II SILICON STEEL SHEET (BASE STEEL SHEET) 13 INTERNALLY OXIDIZED LAYER
131 Si02
15 15 INSULATION COATING
17 EXTERNALLY OXIDIZED LAYER
L ROLLING DIRECTION

WE CLAIMS

1.A non oriented electrical steel sheet comprising a silicon steel sheet and an
5 insulation coating, characterized in that
the silicon steel sheet contains, as a chemical composition, by mass%,
more than 2.00 to 4.00% of Si,
0.10 to 3.00% of Al,
0.10 to 2.00% of Mn,
10 0.0030% or less of C,
0.050% or less of P,
0.005% or less of S,
0.005% or less of N,
0to0.40%ofSn,
15 0tol.00%ofCu,
0to0.40%ofSb,
0 to 0.0400% of REM,
0 to 0.0400% of Ca,
0 to 0.0400% of Mg, and
20 a balance consisting of Fe and impurities,
when viewing a cross section whose cutting direction is parallel to a thickness direction and when a central area is a thickness range of 5/8 to 3/8 of the silicon steel sheet, a vickers hardness in the central area is 120 to 300 Hv, and
when viewing the cross section, the silicon steel sheet includes an internally 25 oxidized layer containing SiCh in a surface thereof, an average thickness of the internally

45 oxidized layer is 0.10 to 5.0 |im, and a vickers hardness in the internally oxidized layer is
1.15 to 1.5 times as compared with the vickers hardness in the central area.
2. The non oriented electrical steel sheet according to claim 1,
5 wherein the silicon steel sheet contains, as the chemical composition, by mass%,
at least one selected from a group consisting of 0.02 to 0.40% of Sn, 0.10 to 1.00% of Cu, and 0.02 to 0.40% of Sb. 10
3. The non oriented electrical steel sheet according to claim 1 or 2,
wherein the silicon steel sheet contains, as the chemical composition, by mass%,
at least one selected from a group consisting of
0.0005 to 0.0400% of REM,
15 0.0005 to 0.0400% of Ca, and
0.0005 to 0.0400% of Mg.
4. The non oriented electrical steel sheet according to any one of claims 1 to 3,
wherein the vickers hardness in the internally oxidized layer is 155 Hv or more.
20
5. The non oriented electrical steel sheet according to any one of claims 1 to 4,
wherein the average thickness of the internally oxidized layer is 0.55 |im or
more.