U.S. patent application number 16/089734 was filed with the patent office on 2019-04-18 for electrical steel sheet and method of producing the same.
The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Tatsuhiko Hiratani, Yoshihiko Oda, Yoshiaki Zaizen.
Application Number | 20190112697 16/089734 |
Document ID | / |
Family ID | 59964631 |
Filed Date | 2019-04-18 |
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United States Patent
Application |
20190112697 |
Kind Code |
A1 |
Hiratani; Tatsuhiko ; et
al. |
April 18, 2019 |
ELECTRICAL STEEL SHEET AND METHOD OF PRODUCING THE SAME
Abstract
An electrical steel sheet includes a surface part in which a Si
concentration in the steel sheet changes continuously from a high
Si concentration to a low Si concentration in a thickness direction
of the steel sheet from a surface of the steel sheet, as defined by
a symmetry plane located at the center of the steel sheet in the
thickness direction, a boundary part in which the Si concentration
changes discontinuously, and an inner part in which the Si
concentration does not change substantially in the thickness
direction of the steel sheet, the inner part including the center
of the steel sheet in the thickness direction, wherein the
electrical steel sheet has a stress distribution such that an
in-plane tensile stress is generated in the surface part and an
in-plane compressive stress is generated in the inner part.
Inventors: |
Hiratani; Tatsuhiko; (Tokyo,
JP) ; Oda; Yoshihiko; (Tokyo, JP) ; Zaizen;
Yoshiaki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
59964631 |
Appl. No.: |
16/089734 |
Filed: |
March 29, 2017 |
PCT Filed: |
March 29, 2017 |
PCT NO: |
PCT/JP2017/013027 |
371 Date: |
September 28, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/02 20130101;
C21D 6/008 20130101; C23C 10/08 20130101; C21D 1/74 20130101; C21D
2211/001 20130101; C21D 2211/005 20130101; C22C 38/002 20130101;
C22C 38/04 20130101; C23C 10/60 20130101; C21D 8/12 20130101; C22C
38/06 20130101; C22C 38/004 20130101; C21D 8/1255 20130101; C22C
38/001 20130101; C21D 9/46 20130101; H01F 1/14775 20130101 |
International
Class: |
C23C 10/08 20060101
C23C010/08; C21D 9/46 20060101 C21D009/46; C21D 6/00 20060101
C21D006/00; C21D 1/74 20060101 C21D001/74; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C23C 10/60 20060101
C23C010/60 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2016 |
JP |
2016-071619 |
Claims
1.-6. (canceled)
7. An electrical steel sheet comprising: a surface part in which a
Si concentration in the steel sheet changes continuously from a
high Si concentration to a low Si concentration in a thickness
direction of the steel sheet from a surface of the steel sheet, as
defined by a symmetry plane located at the center of the steel
sheet in the thickness direction, a boundary part in which the Si
concentration changes discontinuously, and an inner part in which
the Si concentration does not change substantially in the thickness
direction of the steel sheet, the inner part including the center
of the steel sheet in the thickness direction, wherein the
electrical steel sheet has a stress distribution such that an
in-plane tensile stress is generated in the surface part and an
in-plane compressive stress is generated in the inner part, an
average aspect ratio of crystal grains included in the surface part
defined as a ratio of a dimension of the crystal grains in a
direction parallel to the surface of the steel sheet to a dimension
of the crystal grains in a direction (depth direction)
perpendicular to the surface of the steel sheet, being 0.7 or more
and 4.0 or less, the average aspect ratio is the average of aspect
ratios of 50 or more crystal grains and, when a crystal grain
included in the surface part extends to the inner part beyond the
boundary part, the dimension of the crystal grain in the direction
(depth direction) perpendicular to the surface of the steel sheet
includes a portion of the crystal grain which is included in the
inner part.
8. The electrical steel sheet according to claim 7, wherein the
thickness of the surface part is 10% to 40% of the thickness of the
steel sheet.
9. The electrical steel sheet according to claim 7, wherein the
average Si concentration in the surface part is 2.5% to 6.5% by
mass and the average Si concentration in the inner part is 2.0% or
less by mass.
10. The electrical steel sheet according to claim 8, wherein the
average Si concentration in the surface part is 2.5% to 6.5% by
mass and the average Si concentration in the inner part is 2.0% or
less by mass.
11. The electrical steel sheet according to claim 7, wherein a
tensile stress of 50 to 200 MPa is generated in the surface part in
the direction parallel to the surface of the steel sheet, and a
compressive stress of 50 to 200 MPa is generated in the inner part
in the direction parallel to the surface of the steel sheet.
12. The electrical steel sheet according to claim 8, wherein a
tensile stress of 50 to 200 MPa is generated in the surface part in
the direction parallel to the surface of the steel sheet, and a
compressive stress of 50 to 200 MPa is generated in the inner part
in the direction parallel to the surface of the steel sheet.
13. The electrical steel sheet according to claim 9, wherein a
tensile stress of 50 to 200 MPa is generated in the surface part in
the direction parallel to the surface of the steel sheet, and a
compressive stress of 50 to 200 MPa is generated in the inner part
in the direction parallel to the surface of the steel sheet.
14. The electrical steel sheet according to claim 10, wherein a
tensile stress of 50 to 200 MPa is generated in the surface part in
the direction parallel to the surface of the steel sheet, and a
compressive stress of 50 to 200 MPa is generated in the inner part
in the direction parallel to the surface of the steel sheet.
15. The electrical steel sheet according to claim 7, the electrical
steel sheet having a thickness of 0.03 to 0.5 mm.
16. The electrical steel sheet according to claim 8, the electrical
steel sheet having a thickness of 0.03 to 0.5 mm.
17. The electrical steel sheet according to claim 9, the electrical
steel sheet having a thickness of 0.03 to 0.5 mm.
18. The electrical steel sheet according to claim 10, the
electrical steel sheet having a thickness of 0.03 to 0.5 mm.
19. The electrical steel sheet according to claim 11, the
electrical steel sheet having a thickness of 0.03 to 0.5 mm.
20. The electrical steel sheet according to claim 12, the
electrical steel sheet having a thickness of 0.03 to 0.5 mm.
21. The electrical steel sheet according to claim 13, the
electrical steel sheet having a thickness of 0.03 to 0.5 mm.
22. The electrical steel sheet according to claim 14, the
electrical steel sheet having a thickness of 0.03 to 0.5 mm.
23. A method of producing an electrical steel sheet comprising:
heating a steel sheet to 1100.degree. C. to 1250.degree. C. in a
non-oxidizing atmosphere to transform the steel sheet into the
austenite phase, the steel sheet having a composition containing,
by mass, C: 0.020% or less, Si: 0.15% to 2.0%, Mn: 0.05% to 2.00%,
P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, and N: 0.01%
or less, with the balance being Fe and inevitable impurities;
subsequently causing Si to penetrate a surface of the steel sheet
at 1100.degree. C. to 1250.degree. C. in a non-oxidizing atmosphere
containing 10 mol % or more and less than 45 mol % silicon
tetrachloride to transform a surface layer of the steel sheet into
the ferrite phase; subsequently holding the steel sheet for a
predetermined amount of time at 1100.degree. C. to 1250.degree. C.
in a non-oxidizing atmosphere that does not contain Si until a
thickness of a surface part that is in the ferrite phase reaches
10% to 40% of the thickness of the steel sheet, while maintaining
the austenite phase in an inner part; and subsequently cooling the
steel sheet to 400.degree. C. at an average cooling rate of 5 to
30.degree. C./s.
Description
TECHNICAL FIELD
[0001] This disclosure relates to an electrical steel sheet used to
produce iron cores included in high-frequency transformers,
reactors, motors and the like for power electronics and a method of
producing the electrical steel sheet.
BACKGROUND
[0002] The iron loss of an electrical steel sheet consists of the
hysteresis loss of the electrical steel sheet, which is strongly
dependent on a precipitate included in the steel, the size of
crystal grains of the steel, the texture of the steel and the like,
and the eddy-current loss of the electrical steel sheet, which is
strongly dependent on the thickness, specific resistance, magnetic
domain structure and the like of the steel sheet.
[0003] In common electrical steel sheets, the content of impurities
in the steel is reduced to a minimum level to facilitate the growth
of crystal grains and thereby reduce hysteresis loss.
[0004] At the commercial frequency (50/60 Hz), the hysteresis loss
of an electrical steel sheet accounts for a large part of the iron
loss of the electrical steel sheet. At high frequencies of a few
kilohertz or higher, on the other hand, the eddy-current loss of
the electrical steel sheet becomes dominant, since eddy-current
loss increases in proportion to the square of frequency while
hysteresis loss increases in proportion to frequency.
[0005] In addition, with increases in the operating frequencies of
switching devices used in the field of power electronics, there has
been a strong demand for a reduction in the high-frequency iron
loss of an electrical steel sheet used for producing iron cores
included in transformers, reactors, motors and the like.
[0006] To meet the above demand, there has been an attempt to
reduce the eddy-current loss of a steel sheet by reducing the
thickness of the steel sheet to 0.2 mm or less, which is smaller
than the thicknesses of common electrical steel sheets (0.3 to 0.5
mm), or by increasing the content of element that increases the
specific resistance of steel such as Si or Al in the steel
sheet.
[0007] Switching devices having an operating frequency of a few
kilohertz to 50 kilohertz have been used in a power source having a
relatively large capacity not only in the fields of automobiles and
air conditioners, but also in the field of new energy sources such
as photovoltaic power generation. Accordingly, an iron core
material having a further low iron-loss at high frequencies has
been anticipated.
[0008] In the field of power sources described above, ultrathin
electrical steel sheets having a thickness of 0.1 mm or less,
high-Si electrical steel sheets, dust cores formed of a compact of
iron powder and the like have been used. In the field of small
capacity, for example, Mn--Zn ferrite, which has a specific
resistance several orders of magnitude higher than the specific
resistances of soft magnetic metal materials, has been used.
[0009] However, in view of a possible further increase in operating
frequency in the future, even an ultrathin electrical steel sheet
having a thickness of 0.1 mm does not always have a sufficiently
low eddy-current loss. It is not easy to produce a high-Si
electrical steel sheet having a Si concentration of more than 4% by
mass because such a steel sheet is hard and brittle. Since a dust
core has a significantly higher hysteresis loss than an electrical
steel sheet, the iron loss of a dust core considerably increases at
frequencies of a few kilohertz. While Mn--Zn ferrite has a markedly
low eddy-current loss, the saturation magnetic flux density of
Mn--Zn ferrite is 0.5 T at most, which is significantly lower than
the saturation magnetic flux density (2.0 T) of common electrical
steel sheets. Therefore, when a power source having a large
capacity is prepared using Mn--Zn ferrite, the size of the core
needs to be increased disadvantageously.
[0010] In response, Japanese Examined Patent Application
Publication No. 6-45881 discloses a method of reducing the iron
loss of an electrical steel sheet at high frequencies. In this
method, a 6.5-mass % Si steel sheet is produced by a siliconizing
process. In that technique, a 3-mass % Si steel sheet having a
thickness of 0.05 to 0.3 mm is caused to react with a silicon
tetrachloride gas at a high temperature to increase the Si
concentration in the steel. This is because a 6.5-mass % Si steel
sheet has a specific resistance about double the specific
resistance of a 3-mass % Si steel sheet, which enables an effective
reduction in eddy current loss, and is advantageously used in a
high-frequency application. In addition, since the magnetostriction
of the 6.5-mass % Si steel sheet is substantially zero, the level
of noise generated by an iron core may be markedly reduced.
[0011] Japanese Examined Patent Application Publication No. 5-49744
discloses a steel sheet in which the Si concentration changes in
the thickness direction, that is, a "Si-gradient steel sheet", that
can be produced by, in a siliconizing process, pausing uniform
diffusion of Si upon the Si concentration in the surface layer
reaching 6.5% by mass. The Si-gradient steel sheet has a lower iron
loss at high frequencies than a steel sheet having a uniform Si
concentration.
[0012] In Japanese Unexamined Patent Application Publication No.
2005-240185, a difference (maximum-minimum) in Si concentration in
the thickness direction of the steel sheet, the Si concentration in
the surface layer, and a difference in Si concentration between the
front and rear surfaces of the steel sheet are specified to reduce
the iron loss of a Si-gradient steel sheet at high frequencies. It
is described that, in particular, iron loss can be minimized when
the Si concentration in the surface layer is 6.5% by mass.
[0013] In general, an electrical steel sheet containing 3% by mass
or more Si does not transform into the austenite phase (.gamma.
phase) even when heated to a high temperature and remains in the
ferrite phase (.sigma. phase) until a liquid phase is formed.
Therefore, the above-described siliconizing process is entirely
performed in the .alpha. phase.
[0014] Japanese Unexamined Patent Application Publication No.
2000-328226 discloses an electrical steel sheet for motors having
high workability and excellent high-frequency properties and in
which the average Si concentration over the entire thickness of the
steel sheet is 0.5% to 4% by mass, which is at a low level. The
electrical steel sheet is produced by siliconizing only the surface
layer of a steel sheet containing less than 3% by mass Si at
900.degree. C. to 1000.degree. C.
[0015] Japanese Patent No. 5533801 and Japanese Patent No. 5648335
disclose a technique in which excellent magnetic properties are
achieved by diffusing a ferrite-formation element from the surface
of a steel sheet toward the inner austenite phase to transform the
austenite phase into the ferrite phase and form a microstructure
strongly accumulated in a particular crystal plane.
[0016] Japanese Unexamined Patent Application Publication No.
2015-61941 discloses a technique in which excellent magnetic
properties are achieved by creating a portion of a steel sheet
having a composition capable of causing .alpha.-.gamma.
transformation and at which an element other than Fe is
concentrated, the portion extending partially in the thickness
direction, and thereby reducing the residual stress generated in
the surface of the steel sheet.
[0017] Japanese Patent No. 5655295 discloses that it is possible to
markedly reduce eddy-current loss by siliconizing a low-carbon
steel sheet in the temperature range of 1050.degree. C. to
1250.degree. C., which is the austenite phase region, and cooling
the siliconized steel sheet while only the Si concentration in the
surface layer is maintained to be high to produce a Si-gradient
steel sheet.
[0018] Japanese Patent No. 5644680 discloses a technique in which a
steel sheet containing 0.003% to 0.02% by mass C which can be
transformed into the austenite phase when heated to a high
temperature is siliconized to produce a clad electrical steel sheet
having excellent magnetic properties.
[0019] As described above, iron loss is the sum of hysteresis loss
and eddy-current loss. It is known that, the higher the excitation
frequency, the higher the proportion of eddy-current loss in the
total iron loss. The higher the specific resistance of a material,
the higher the resistance to eddy current passing through the
material. Therefore, a material having a high specific resistance
is used to produce a core used at high frequencies.
[0020] Known examples of elements that increase the specific
resistance of a steel sheet include Si, Al, Cr, and Mn. In general,
Si is primarily added to an electrical steel sheet to increase the
specific resistance of the electrical steel sheet. However, if the
Si concentration in the material exceeds 4% by mass, the material
becomes significantly brittle, which makes it difficult to
cold-roll the material. Accordingly, the maximum amount of Si added
to a steel sheet is normally set to about 4% by mass. To further
increase the specific resistance of a steel sheet, 1% to 4% by mass
of Al and Cr are further added to the steel sheet.
[0021] However, adding large amounts of alloying elements to a
steel sheet increases the costs and reduces the saturation magnetic
flux density of the material. For example, while a 3-mass % Si
steel has a saturation magnetic flux density of 2.03 T, adding 1%
by mass Al and 3% by mass Cr to the 3-mass % Si steel reduces the
saturation magnetic flux density of the steel to about 1.80 T.
[0022] A material for cores used at high frequencies is commonly
designed in consideration of a certain amount of direct-current
component of the excitation current and magnetic saturation of the
material caused by a high current that may instantaneously pass
through the material. It is necessary to increase the size of the
core to compensate for the reduction in the saturation magnetic
flux density of the material.
[0023] In Japanese Examined Patent Application Publication No.
6-45881, after a 3-mass % Si steel sheet has been rolled to a final
thickness, silicon tetrachloride is sprayed to the steel sheet at a
high temperature in the final annealing treatment. That
siliconizing process enables production of a 6.5-mass % Si steel
sheet, which has been difficult to produce by rolling. The 6.5-mass
% Si steel sheet, which has a specific resistance about double that
of a 3-mass % Si steel sheet, is suitably used to produce an iron
core used at high frequencies.
[0024] However, when the 6.5-mass % Si steel sheet is used to
produce iron cores in practice, the material, that is, the 6.5-mass
% Si steel sheet, needs to be subjected to slitting, pressing,
bending or the like and, in these steps, cracking and chipping are
likely to occur. Therefore, high-yield production of cores requires
a sophisticated processing technique. In addition, since the Si
content in the 6.5-mass % Si steel sheet is high, the saturation
magnetic flux density of the steel sheet is about 1.80 T, which is
at a low level.
[0025] In Japanese Examined Patent Application Publication No.
5-49744 and Japanese Unexamined Patent Application Publication No.
2005-240185, a steel sheet in which the Si concentration changes in
the thickness direction, that is, a Si-gradient steel sheet, is
described. The Si-gradient steel sheet has more excellent
high-frequency properties than a 6.5-mass % Si steel sheet. While
the Si concentration in the surface layer of the Si-gradient steel
sheet is about 6.5%, which is at a high level, the Si concentration
in the sheet-thickness center layer about 3% to 4% by mass, which
is at a low level, and the average Si concentration over the entire
steel sheet is low. Therefore, the Si-gradient steel sheet has
higher workability than a 6.5-mass % Si steel sheet and has a high
saturation magnetic flux density of 1.85 to 1.90 T.
[0026] In that technique, since the siliconizing process is
performed in the ferrite single-phase in which diffusion rate is
basically high, while Si contained in the gas phase permeates the
surface layer of the steel sheet, Si quickly diffuses to the inside
of the steel sheet. When the Si-gradient steel sheet is an
ultrathin steel sheet, Si atoms may reach the center of the steel
sheet in the thickness direction during the siliconizing process
and, consequently, the Si concentration over the entire steel sheet
may be increased.
[0027] In Japanese Unexamined Patent Application Publication No.
2000-328226, a material containing less than 3% Si is used to
produce a steel sheet in which the Si concentration changes in the
thickness direction to reduce the average Si concentration over the
entire steel sheet and produce a high-frequency low iron-loss
material having good workability.
[0028] A material containing Si at a low concentration can be
transformed into the austenite (.gamma.) phase at high
temperatures. In the technique disclosed in Japanese Unexamined
Patent Application Publication No. 2000-328226, when the material
is siliconized at a high temperature exceeding 1000.degree. C.,
that is, in the .gamma. phase, cracking may occur at the
.gamma./.alpha. transformation interface in the surface layer.
Accordingly, the siliconizing process is performed in the
temperature range of 900.degree. C. to 1000.degree. C., in which
the austenite phase is hardly formed.
[0029] However, the above siliconizing process is an extension of
the known siliconizing process performed in the .alpha. phase, and
a reduction effect in eddy-current loss which can be achieved by
the above siliconizing process will also be within an expected
range.
[0030] In Japanese Patent No. 5533801 and Japanese Patent No.
5648335, the soft magnetic properties of a steel sheet are enhanced
by diffusing a ferrite-formation element from the surface of the
steel sheet toward the inner austenite phase and forming a
particular texture through the use of the .gamma..fwdarw..alpha.
transformation. However, while the change in texture markedly
affects hysteresis loss, which accounts for a part of iron loss, it
does not markedly affect eddy-current loss. Therefore, it appears
that changing the texture of a steel sheet is not an effective way
to reduce eddy-current loss which accounts for a large part of iron
loss at high frequencies. On the contrary, developing a texture
effective to reduce hysteresis loss may increase the width of
magnetic domain and, consequently, increase abnormal eddy-current
loss.
[0031] In Japanese Unexamined Patent Application Publication No.
2015-61941, the soft magnetic properties of a steel sheet are
enhanced by changing the concentration of an element other than Fe
in the thickness direction of the steel sheet and limiting the
residual stress generated in the surface of the steel sheet to a
low level. However, the method in which residual stress is reduced
to limit an increase in the hysteresis loss of a soft magnetic
material has been practiced for a long time. Moreover, the
relationship between a reduction in residual stress and a reduction
in eddy-current loss is not clear.
[0032] In Japanese Patent No. 5655295, a remarkable reduction in
eddy-current loss is achieved by siliconizing a low-carbon steel
containing more than 0.02% by mass C in a high-temperature range
higher than 1050.degree. C. and creating a specific stress
distribution in the resulting Si-gradient steel sheet such that an
in-plane tensile stress is generated in the surface layer and an
in-plane compressive stress is generated in the inner layer.
However, a complex transformation structure extends around the
center of the steel sheet in the thickness direction and,
consequently, the direct-current magnetic properties of the steel
sheet are considerably poor when the steel sheet is used as an
electrical steel sheet.
[0033] For example, the magnetic flux density B8 that corresponds
to a magnetizing force of 800 A/m in a magnetization curve is about
0.75 T at most. The dimensions of a core material used in practice
are determined in accordance with the magnetic flux density at
which the differential permeability starts rapidly decreasing in
the magnetization curve, that is, the height of the shoulder of the
B-H curve. The B8 value is likely to be used as an index of such a
magnetic flux density. Therefore, a material having poor
direct-current magnetic properties and a low B8 value is
substantially not suitable to reduce the size of a core even if the
saturation magnetic flux density of the material is high.
[0034] In Japanese Patent No. 5644680, when an impact force similar
to the force generated in shearing process is applied to the steel
sheet, crystal grains included in the surface part were cracked
along the grain boundaries in the thickness direction. In addition,
cracking occurred at the boundary between the surface part and the
inner part. This resulted in variations in soft magnetic
properties. In fact, although samples were prepared under the same
conditions, the degree of variations in the soft magnetic
properties of the samples was large in some cases. The degree of
variations in soft magnetic properties was likely to increase
particularly when the C content was 0.005% by weight or less. With
a recent increase in the use of switching devices having a high
operating frequency of 10 k to 50 kHz in the production of power
sources having a relatively large capacity which are included in
hybrid vehicles, electric vehicles, photovoltaic power generation
systems and the like, there has been a demand for a practical
material having a high saturation magnetic flux density, a low
iron-loss at high frequencies, and consistent properties. In this
regard, the variations in magnetic properties are unfavorable.
[0035] Accordingly, it could be helpful to provide an electrical
steel sheet having a high saturation magnetic flux density and a
low iron-loss at high frequencies and a method of producing the
electrical steel sheet.
SUMMARY
[0036] We thus provide:
[1] An electrical steel sheet including, with a symmetry plane
being the center of the steel sheet in the thickness direction, a
surface part in which the Si concentration in the steel sheet
changes continuously from a high Si concentration to a low Si
concentration in the thickness direction of the steel sheet from
the surface of the steel sheet, a boundary part in which the Si
concentration changes discontinuously, and an inner part in which
the Si concentration does not change substantially in the thickness
direction of the steel sheet, the inner part including the center
of the steel sheet in the thickness direction, the electrical steel
sheet having a stress distribution such that an in-plane tensile
stress is generated in the surface part and an in-plane compressive
stress is generated in the inner part, the average aspect ratio of
crystal grains included in the surface part, that is, the ratio of
the dimension of the crystal grains in a direction parallel to the
surface of the steel sheet to the dimension of the crystal grains
in a direction (depth direction) perpendicular to the surface of
the steel sheet, being 0.7 or more and 4.0 or less,
[0037] wherein the average aspect ratio is the average of aspect
ratios of 50 or more crystal grains and, when a crystal grain
included in the surface part extends to the inner part beyond the
boundary part, the dimension of the crystal grain in the direction
(depth direction) perpendicular to the surface of the steel sheet
is determined taking a portion of the crystal grain which is
included in the inner part into account.
[2] The electrical steel sheet described in [1], wherein the
thickness of the surface part is 10% to 40% of the thickness of the
steel sheet. [3] The electrical steel sheet described in [1] or
[2], wherein the average Si concentration in the surface part is
2.5% to 6.5% by mass and the average Si concentration in the inner
part is 2.0% or less by mass. [4] The electrical steel sheet
described in any one of [1] to [3], wherein a tensile stress of 50
to 200 MPa is generated in the surface part in the direction
parallel to the surface of the steel sheet, and wherein a
compressive stress of 50 to 200 MPa is generated in the inner part
in the direction parallel to the surface of the steel sheet. [5]
The electrical steel sheet described in any one of [1] to [4], the
electrical steel sheet having a thickness of 0.03 to 0.5 mm. [6] A
method of producing an electrical steel sheet, the method
including: heating a steel sheet to 1100.degree. C. to 1250.degree.
C. in a non-oxidizing atmosphere to transform the steel sheet into
the austenite phase, the steel sheet having a composition
containing, by mass, C: 0.020% or less, Si: 0.15% to 2.0%, Mn:
0.05% to 2.00%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or
less, and N: 0.01% or less, with the balance being Fe and
inevitable impurities; subsequently causing Si to penetrate the
surface of the steel sheet at 1100.degree. C. to 1250.degree. C. in
a non-oxidizing atmosphere containing 10 mol % or more and less
than 45 mol % silicon tetrachloride to transform a surface layer of
the steel sheet into the ferrite phase; subsequently holding the
steel sheet for a predetermined amount of time at 1100.degree. C.
to 1250.degree. C. in a non-oxidizing atmosphere that does not
contain Si until the thickness of the surface part that is in the
ferrite phase reaches 10% to 40% of the thickness of the steel
sheet, while maintaining the austenite phase in the inner part;
and
[0038] subsequently cooling the steel sheet to 400.degree. C. at an
average cooling rate of 5 to 30.degree. C./s.
[0039] Hereinafter, when referring to the composition of steel, "%"
denotes "% by mass" unless otherwise specified.
[0040] An electrical steel sheet having a high saturation magnetic
flux density and a low iron-loss at high frequencies may be
produced. It is possible to produce an electrical steel sheet
having a high saturation magnetic flux density, a low iron-loss at
high frequencies, and consistent properties. Consequently, an iron
core material that enables a reduction in the size of
high-frequency transformers and the like may be provided.
[0041] Our steel sheets can be suitably used to produce an iron
core included in a high-frequency transformer, a reactor, a motor
or the like for power electronics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a diagram illustrating the basic structure of a
Si-gradient steel sheet.
[0043] FIG. 2 is a diagram illustrating the relationship between
the average aspect ratio, b/a, of crystal grains included in the
surface part and iron loss.
[0044] FIG. 3 is a diagram illustrating the aspect ratio, b/a, of a
crystal grain included in the surface part.
DETAILED DESCRIPTION
[0045] Our steel sheets and methods are described in detail
below.
[0046] We conducted extensive studies of a method of producing an
electrical steel sheet having a high saturation magnetic flux
density and a low iron-loss at high frequencies. First, attention
was focused on the Si-gradient steel sheet illustrated in FIG. 1,
which can be used as an electrical steel sheet. The Si-gradient
steel sheet illustrated in FIG. 1 is an electrical steel sheet that
includes, with a symmetry plane being the center of the steel sheet
in the thickness direction, a surface part in which the Si
concentration in the steel sheet changes continuously from a high
Si concentration to a low Si concentration in the thickness
direction from the surface, a boundary part in which the Si
concentration changes discontinuously, and an inner part in which
the Si concentration does not change substantially in the thickness
direction, the inner part including the center of the steel sheet
in the thickness direction. The Si-gradient steel sheet has a
specific stress distribution such that an in-plane tensile stress
is generated in the surface part and an in-plane compressive stress
is generated in the inner part, which reduces iron loss at high
frequencies.
[0047] To reduce the iron loss of the Si-gradient steel sheet,
samples each of which included a surface part constituted of a
different form of crystal grains were prepared and the properties
of the samples were determined. Specifically, a cold-rolled steel
sheet having a thickness of 0.2 mm which contained, by mass, C:
0.0024%, Si: 0.6%, Mn: 0.12%, P: 0.008%, S: 0.003% or less, Al:
0.003%, N: 0.003%, and the balance being Fe and inevitable
impurities was prepared. Specimens having a width of 50 mm and a
length of 200 mm were taken from the cold-rolled steel sheet. The
specimens were siliconized and subjected to a diffusion treatment.
The conditions under which the siliconizing process and the
diffusion treatment were performed were adjusted such that the
amount of silicon used, that is, the amount of silicon added to the
steel sheet in the siliconizing process, was 2.4%.+-.0.2% or less
and the ratio of the thickness, ds, of the surface part, that is,
the Si-concentrated layer, to the thickness, d0, of the steel sheet
was 30%.+-.3% or less. Subsequent to the siliconizing process and
diffusion treatment of the samples, the width of the samples was
reduced to 30 mm by shearing both ends of the samples. The samples
were subsequently subjected to a magnetic measurement in accordance
with a method (Epstein test method) conforming to JIS C2550 with a
small single-sheet testing frame. After the magnetic measurement
had been terminated, the samples were further sheared.
Subsequently, the microstructure of a cross section of each of the
samples was determined with an optical microscope, and the Si
distribution in the sample in the thickness direction was
determined by EPMA.
[0048] The form of crystal grains included in the surface part can
be adjusted by changing the conditions under which the siliconizing
process is performed. For example, the higher the temperature at
which the siliconizing process is performed within the austenite
temperature range of the material (steel sheet) or the lower the
concentration of the silicon tetrachloride gas, the higher the
likelihood of the size of crystal grains included in the surface
layer increasing in the direction parallel to the surface of the
steel sheet. In contrast, the lower the temperature at which the
siliconizing process is performed within the austenite temperature
range of (steel sheet) or the higher the concentration of the
silicon tetrachloride gas, the higher the likelihood of the size of
crystal grains included in the surface part increasing in the
thickness direction of the steel sheet.
[0049] As illustrated in FIG. 3, the dimension of a crystal grain
included in the surface part which is measured in the direction
parallel to the surface (hereinafter, this direction may be
referred to as "parallel-to-surface direction") is denoted by b,
and the dimension of the crystal grain which is measured in the
thickness direction of the steel sheet (hereinafter, this direction
may be referred to as "perpendicular-to-surface direction" or
"depth direction") is denoted by a. The above dimensions of each of
50 or more crystal grains included in the surface layer were
measured, and the aspect ratio, b/a, of each of the crystal grains
was calculated. The average thereof was considered to be the
representative value (average aspect ratio, b/a, of crystal grains
included in the surface part) of the sample. FIG. 3 is a
cross-sectional view of the steel sheet which is taken in the
L-direction (rolling direction), schematically illustrating the
aspect ratio, b/a, of a crystal grain included in the surface part.
In FIG. 3, a and b denote the maximum dimension of each of the
crystal grains measured in the thickness direction and the maximum
dimension of the crystal grain measured in the direction parallel
to the surface, respectively. Although the aspect ratio of a
crystal grain does not change whether the measurement is done in
the L-direction (the rolling direction) or the C-direction (the
width direction), evaluations are made using aspect ratios measured
in the L-direction.
[0050] FIG. 2 illustrates the relationship between the average
aspect ratio, b/a, of crystal grains included in the surface part
(in FIG. 2, this ratio is abbreviated as "average aspect ratio in
surface part b/a") and iron loss. The samples prepared in the test
had a ratio, b/a, of 0.5 to 4.5. However, in the test where the
composition of the material, thickness of the steel sheet, the
amount of silicon added, and the thickness of the surface part were
set to be identical, the iron losses of the samples were generally
above a certain level regardless of the average aspect ratio of
crystal grains included in the surface part, and reduction effect
in iron loss were not confirmed. On the other hand, we found that
whether the degree of variations in iron loss is large or small is
clearly distinguished in accordance with the average aspect ratio
of crystal grains included in the surface part. That is, we
confirmed that the degree of variations in iron loss is large when
the average aspect ratio of crystal grains included in the surface
part is significantly small or large, while the degree of
variations in iron loss is small when the average aspect ratio of
crystal grains included in the surface part falls within a
particular range.
[0051] We also confirmed that a test similar to the above test was
conducted using samples having different steel compositions and
different thicknesses. On the other hand, when the composition of
the material used, the thickness of samples, and the distribution
of Si concentration in the samples were changed, the average iron
loss of samples and the degree of variations in iron loss were
changed. Accordingly, a plurality of samples each of which included
a surface layer composed of crystal grains having a different
average aspect ratio were prepared while the composition of the
material, thickness of the steel sheet, the amount of silicon
added, and the thickness of the surface layer were set to be
identical, the average iron loss, m, of the samples and the
standard deviation, .sigma., of iron loss were determined, and the
degree of variations in iron loss was considered small when the
coefficient of variation, .sigma./m, was less than 10%. As a
result, we found that the degree of variations in iron loss can be
reduced when the average aspect ratio of crystal grains included in
the surface part is 0.7 or more and 4.0 or less.
[0052] Although the direct relationship between iron loss and the
average aspect ratio of crystal grains included in the surface part
is not clear, the results of observation of cross sections of the
samples with a loupe confirm that cracking and chipping occurred in
crystal grains included in the surface parts of samples having a
high iron loss, while cracking and chipping hardly occurred in
samples having an average iron loss. Since the likelihood of
cracking and chipping varies with the average aspect ratio of
crystal grains included in the surface part, we believe that the
average aspect ratio of crystal grains included in the surface part
may affect the degree of variations in iron loss. The results of
observation of the microstructures included in the cross sections
of the samples also confirm that cracking occurred at the boundary
part between the surface part and the inner part in some of the
samples. The occurrence of such defects was significant in samples
in which the average aspect ratio of crystal grains included in the
surface part was significantly low or high. In contrast, such
defects were not likely to occur when samples in which the average
aspect ratio of crystal grains included in the surface part fell
within a specific range were sheared. This presumably reduced the
degree of variation in iron loss. The results of further detail
observation of microstructure confirmed that cracking is likely to
occur at the grain boundaries present in the surface part and grain
size when the average aspect ratio of crystal grains included in
the surface part is small, that is, when the crystal grains
included in the surface part have a slender shape elongated in the
thickness direction of the steel sheet and that cracking is likely
to occur at the boundary part interposed between the surface part
and the inner part when the average aspect ratio of crystal grains
included in the surface part is large, that is, when the crystal
grains included in the surface part have a slender shape elongated
in the direction parallel to the surface. The samples in which the
occurrence of the above defects was significant had a high iron
loss.
[0053] The reasons for the limitation of the basic structure of the
steel sheet are described.
[0054] The electrical steel sheet is a Si-gradient steel sheet
produced by heating a steel sheet containing Si at a low
concentration to the high-temperature austenite phase, increasing
the Si concentration in the surface layer by siliconizing process
and diffusion treatment, transforming the surface layer into the
ferrite phase, and cooling the steel sheet such that the austenite
phase having a low Si concentration remains in the inner layer. The
electrical steel sheet includes, with a symmetry plane being the
center of the steel sheet in the thickness direction, a surface
part in which the Si concentration in the steel sheet changes
continuously from a high Si concentration to a low Si concentration
in the thickness direction from the surface, a boundary part in
which the Si concentration changes discontinuously, and an inner
part in which the Si concentration does not change substantially in
the thickness direction, the inner part including the center of the
steel sheet in the thickness direction. This enables the electrical
steel sheet to achieve both high saturation magnetic flux density
and low iron-loss at high frequencies. The inner part in which the
Si concentration does not change substantially in the thickness
direction and that includes the center of the steel sheet in the
thickness direction is a part of the steel sheet extending from the
boundary part to the center of the steel sheet in the thickness
direction and in which the difference between the maximum and
minimum Si concentrations in the region between two boundary parts
is less than .+-.0.1%. The boundary part in which the Si
concentration changes discontinuously is a part of the steel sheet
in which the Si concentration changes by 0.2% or more in a region
having a thickness of .+-.1 .mu.m or less and the minimum Si
concentration in the surface part and the maximum Si concentration
in the inner part occur discontinuously. The electrical steel sheet
has a stress distribution such that an in-plane tensile stress is
generated in the surface part and an in-plane compressive stress is
generated in the inner part. It is possible to reduce eddy-current
loss and iron loss at high frequencies through the use of the above
stress distribution.
[0055] As described above, the electrical steel sheet has a Si
concentration distribution with a symmetry plane being the center
of the steel sheet in the thickness direction. If the distribution
of Si concentration in the steel sheet extending from the front
surface to the rear surface is asymmetrical, the steel sheet may
become significantly warped and the shape of the steel sheet may
become degraded. Furthermore, the stress distribution such that an
in-plane tensile stress is generated in the surface part and an
in-plane compressive stress is generated in the inner part, which
is unique to the Si-gradient steel sheet, may become asymmetrical
with respect to the center plane of the steel sheet in the
thickness direction and, consequently, the reduction effect in
eddy-current loss may be limited. In consideration of the shape of
the steel sheet and the reduction in iron loss at high frequencies,
the difference between the Si concentrations at the front and rear
surfaces of the steel sheet is desirably minimized and is
preferably 0.2% or less.
[0056] As described above, the electrical steel sheet, that is, the
Si-gradient steel sheet produced by performing siliconizing process
in the austenite phase, includes a discontinuous Si-concentration
distribution region formed as a result of .gamma./.alpha.
transformation, that is, the boundary part (Si concentration gap)
in which the Si concentration changes discontinuously. The boundary
part is a part of the steel sheet in which the Si concentration
changes 0.1% or more per 1 micrometer in the thickness direction of
the steel sheet (concentration gradient of 0.1%/.mu.m or more),
that is, in which the Si concentration changes by 0.2% or more in a
region within .+-.1 .mu.m or less in the thickness direction.
[0057] The Si concentration gap, which exists in the boundary part
interposed between the surface part and the inner part, enables
magnetic flux to concentrate at the surface part and thereby
suitably reduces eddy-current loss. However, since the stress
distribution rapidly changes in the boundary part, cracking is
likely to occur at the interfaces upon an impact force similar to
the force generated in shearing process being applied to the steel
sheet. Although such cracks do not lead to the fracture of the
material because they do not propagate over the entire steel sheet
and remain in a narrow region, the cracks cause variations in
magnetic properties and, in particular, variations in iron loss.
With consideration of the application of the material to the
practical use, it is considered necessary to minimize variations in
the properties of the Si-gradient steel sheet having a
Si-concentration distribution that is discontinuous at the
interface between the surface part and the inner part and having a
steep stress distribution.
[0058] To address the above issues, the average aspect ratio of
crystal grains included in the surface part, that is, the ratio of
the dimension of crystal grains measured in the parallel-to-surface
direction to the dimension of the crystal grains measured in the
perpendicular-to-surface direction (depth direction), is specified.
The average aspect ratio of crystal grains included in the surface
part is limited to 0.7 or more and 4.0 or less. This reduces the
degree of variations in iron loss and enables the specific
stabilization to be achieved.
Average Aspect Ratio of Crystal Grains Included in Surface Part:
Ratio of Dimension of Crystal Grains in Parallel-to-Surface
Direction to Dimension of Crystal Grains in
Perpendicular-to-Surface Direction (Depth Direction) is 0.7 or More
and 4.0 or Less
[0059] As described above, as a result of our extensive studies, we
found that the average aspect ratio, b/a, of crystal grains
included in the surface part is the factor significantly important
to the Si-gradient steel sheet. If the ratio, b/a, is less than
0.7, cracking and chipping may occur at the boundaries of crystal
grains included in the surface part upon the steel sheet being
sheared and, consequently, the degree of variations in iron loss
may be increased to a significant level. If the ratio, b/a, is more
than 4.0, cracking is likely to occur at the boundary part
interposed between the surface part and the inner part upon the
steel sheet being sheared and, consequently, the degree of
variations in iron loss may be increased to a significant level.
When the ratio, b/a, is 0.7 or more and 4.0 or less, such cracking
hardly occurs and it is possible to reduce the degree of variations
in iron loss to a markedly low level.
[0060] The average aspect ratio is the average of the aspect ratios
of 50 or more crystal grains. When a crystal grain included in the
surface part extends to the inner part beyond the boundary part,
the dimension of the crystal grain in the perpendicular-to-surface
direction (depth direction) is determined taking a portion of the
crystal grain which is included in the inner part into account.
[0061] The texture of the surface part and the texture of the inner
part are not limited and may be a microstructure constituted of
crystal grains randomly oriented or highly accumulated in a
particular plane or orientation. When the electrical steel sheet,
in which the Si concentration distributions in the surface part and
the inner part are clearly different from each other, is
constituted of randomly oriented crystal grains, cracking is less
likely to occur at the crystal grains included in the surface layer
having a high Si concentration and at the boundary part in which
the Si concentration changes discontinuously upon the steel sheet
being, for example, sheared, because the dislocation movements of
the crystal grains are averaged. Accordingly, the crystal grains
are preferably randomly oriented.
Thickness of Surface Part: 10% to 40% of Thickness of Steel Sheet
(Preferable Condition)
[0062] If the thickness of the surface part is less than 10% of the
thickness of the steel sheet, the surface part may become almost
magnetically saturated, which results in a reduction in magnetic
permeability, when the excitation magnetic flux density is low. As
a result, the inner part also starts becoming magnetized, which
limits the reduction effect in eddy-current loss. On the other
hand, if the thickness of the surface part is more than 40% of the
thickness of the steel sheet, a large part of the steel sheet
extending from the surface to a region around the center of the
steel sheet in the thickness direction becomes magnetized and,
consequently, a magnetic flux distribution similar to that formed
in a Si-uniform material is formed, which limits the reduction
effect in eddy current. To effectively reduce eddy-current loss in
the Si-gradient steel sheet, it is important to accumulate a
magnetic flux at a specific region of the surface layer. For the
above reasons, the thickness of the surface part is preferably 10%
or more and 40% or less and is more preferably 20% or more and 35%
or less of the thickness of the steel sheet.
Average Si Concentration in Surface Part: 2.5% to 6.5% (Preferable
Condition)
[0063] If the average Si concentration in the surface part is less
than 2.5%, the reduction effect in eddy current may fail to be
achieved at a sufficient level. If the average Si concentration in
the surface part exceeds 6.5%, the likelihood of cracking in the
surface layer may rapidly increase. Accordingly, the average Si
concentration in the surface part is preferably 2.5% to 6.5%.
Average Si Concentration in Inner Part: 2.0% or Less (Preferable
Condition)
[0064] If the average Si concentration in the inner part exceeds
2.0%, the likelihood of a discontinuous Si concentration
distribution (boundary part) being formed at the boundary between
the surface part and the inner part is small and, consequently, the
reduction effect in eddy-current loss may fail to be achieved at a
sufficient level. Accordingly, the average Si concentration in the
inner part is preferably 2.0% or less. On the other hand, if the
average Si concentration in the inner part is less than 0.15%,
crystal grains included in the surface part are likely to grow in a
slender shape elongated in the thickness direction of the steel
sheet, the average aspect ratio, b/a, of crystal grains included in
the surface part is likely to be less than 0.7 even when the
conditions under which the siliconizing process and the conditions
under which the diffusion treatment are performed are adjusted and,
consequently, cracking is likely to occur in the surface layer.
Accordingly, the average Si concentration in the inner part is
preferably 0.15% or more.
Si Concentration Gap in Boundary Part: 0.4% or More (Preferable
Condition)
[0065] When the Si concentration gap in a region of the boundary
part, which separates the surface part and the inner part from each
other, the region extending .+-.1 .mu.m or less in the thickness
direction of the steel sheet, is 0.4% or more, the reduction effect
in eddy-current loss increases by 10% or more compared to when the
Si concentration distribution is made completely uniform. If the Si
concentration gap in the boundary part is less than 0.4%, the
accumulation of magnetic flux at the surface part may fail to be
achieved at a sufficient level because also the inner part is
likely to be magnetized and, consequently, the reduction effect in
eddy-current loss may fail to be achieved at a sufficient level.
Accordingly, the Si concentration gap in the boundary part is
preferably 0.4% or more. The minimum Si concentration in the
boundary part corresponds to the Si concentration in the inner
part. The maximum Si concentration in the boundary part corresponds
to a possible minimum Si concentration in the surface part (a
phase) which may occur in the temperature range in which the
siliconizing process and the diffusion treatment are performed.
Surface Part: Tensile Stress of 50 to 200 MPa in Direction Parallel
to Surface, Inner Part: Compressive Stress of 50 to 200 MPa in
Direction Parallel to Surface (Preferable Condition)
[0066] A stress distribution such that a tensile stress is
generated in the surface part and a compressive stress is generated
in the inner part is created to reduce eddy-current loss. It is
preferable to set the tensile stress generated in the surface part
to be 50 MPa or more and the compressive stress generated in the
inner part to be 50 MPa or more to reduce eddy-current loss at a
significant level (by 10% or more) compared to a Si-uniform steel
sheet having the same thickness and the same average Si
concentration. If the tensile stress generated in the surface part
exceeds 200 MPa and the compressive stress generated in the inner
part exceeds 200 MPa, severe cracking may occur during shearing
process, which increases the degree of variations in iron loss,
even when the aspect ratio of crystal grains included in the
surface part falls within the desired range. Accordingly, the
tensile stress generated in the surface part is preferably 50 to
200 MPa, and the compressive stress generated in the inner part is
preferably 50 to 200 MPa. The above internal stresses are
determined using the radius of curvature of warpage observed when a
Si-gradient steel sheet that is not warped substantially is
chemically polished such that a portion of the steel sheet
extending from one of the surfaces to the central portion in the
sheet thickness direction is removed.
Thickness of Steel Sheet: 0.03 to 0.5 mm (Preferable Condition)
[0067] The smaller the thickness of the steel sheet, the larger the
reduction in eddy-current loss. However, reducing the thickness of
the steel sheet to be less than 0.03 mm increases the production
costs for rolling and the loads placed on the working of the core
material and the assembly of cores. On the other hand, if the
thickness of the steel sheet exceeds 0.5 mm, large amounts of time
may be required to siliconize the surface of the steel sheet and
perform the diffusion treatment to achieve adequate Si
distribution. If the thickness of the steel sheet exceeds 0.5 mm,
in the production of cores, cracking is likely to occur in a shear
plane of the steel sheet and, consequently, the degree of
variations in the properties of the steel sheet may be increased.
Accordingly, the thickness of the steel sheet is preferably 0.03 to
0.5 mm.
[0068] The above-described electrical steel sheet can be produced
by heating a steel sheet to 1100.degree. C. to 1250.degree. C. in a
non-oxidizing atmosphere to transform the steel sheet into the
austenite phase, the steel sheet having a composition containing,
by mass, C: 0.020% or less, Si: 0.15% to 2.0%, Mn: 0.05% to 2.00%,
P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, and N: 0.01%
or less, with the balance being Fe and inevitable impurities;
subsequently causing Si to penetrate the surface of the steel sheet
at 1100.degree. C. to 1250.degree. C. in a non-oxidizing atmosphere
containing 10 mol % or more and less than 45 mol % silicon
tetrachloride to transform a surface layer of the steel sheet into
the ferrite phase; subsequently holding the steel sheet for a
predetermined amount of time at 1100.degree. C. to 1250.degree. C.
in a non-oxidizing atmosphere that does not contain Si until the
thickness of the surface part that is in the ferrite phase reaches
10% to 40% of the thickness of the steel sheet, while maintaining
the austenite phase in the inner part; and subsequently cooling the
steel sheet to 400.degree. C. at an average cooling rate of 5 to
30.degree. C./s.
[0069] The reasons for the limitation of the composition of the
material that is to be siliconized are described below.
C: 0.020% or Less
[0070] It is preferable to minimize the C concentration in the
material to enhance soft magnetic properties. If the C
concentration exceeds 0.020%, the pearlite structure, the bainite
structure, and the martensite structure, which increase the
coercive force and hysteresis loss of the steel sheet, are likely
to be formed in the inner part having a low Si concentration while
cooling is performed subsequent to the siliconizing process and the
diffusion treatment. Accordingly, the C concentration in the
material is limited to 0.020% or less. While the lower limit for
the C concentration is not specified, a steel having an excessively
low solute C concentration is likely to undergo intergranular
fracture similarly to ultralow-carbon steel. Accordingly, the C
concentration is preferably 0.0005% to 0.020%.
Si: 0.15% to 2.0%
[0071] If the Si concentration in the material is less than 0.15%,
slender crystal grains that are elongated in the thickness
direction of the steel sheet and have an aspect ratio of less than
0.7 are likely to be formed in the surface layer during the
siliconizing process and the diffusion treatment. This increases
the likelihood of cracking occurring during shearing process and
the degree of variations in iron loss. On the other hand, if the Si
concentration in the material exceeds 2.0%, the likelihood of the
discontinuous Si concentration distribution (boundary part) being
created at the boundary between the surface part and the inner part
is small and, consequently, the reduction effect in eddy-current
loss may fail to be achieved at a sufficient level.
[0072] Accordingly, the Si concentration in the material is limited
to 0.15% to 2.0%.
Mn: 0.05% to 2.00%
[0073] Mn is an element effective to improve the toughness of
steel. In steel, Mn bonds to S and precipitates in the form of MnS.
If the Mn concentration in the material is less than 0.05%, the
intergranular segregation of S may occur, which increases the
likelihood of intergranular fracture occurring in the crystal
grains included in the surface part having a high Si concentration.
Mn is also an element that stabilizes the austenite phase. If the
Mn concentration in the material exceeds 2.00%, a large
transformation strain is likely to remain in the inner part when
the inner part is transformed from the austenite phase to the
ferrite phase during the cooling process performed subsequent to
the siliconizing process and the diffusion treatment. This
transformation strain disturbs the stress distribution created in
the Si-gradient steel sheet and thereby limits the reduction effect
in eddy current. Accordingly, the Mn concentration in the material
is limited to 0.05% to 2.00%.
P: 0.1% or Less
[0074] P is an element effective to strengthen steel, but also an
element that causes embrittlement. Moreover, Mn may segregate at
the phase-transformation interfaces. When the P concentration is
0.1% or less, the occurrence of intergranular cracking in the
surface part and the occurrence of cracking in the boundary part
can be reduced substantially to an insignificant level.
Accordingly, the P concentration in the material is limited to 0.1%
or less.
S: 0.01% or Less
[0075] Since S is an element that is likely to segregate at grain
boundaries, it is preferable to minimize the S concentration to
prevent embrittlement. When the S concentration is 0.01% or less,
the occurrence of cracking is reduced to a substantially
insignificant level. Accordingly, the Si concentration in the
material is limited to 0.01% or less.
Al: 0.1% or Less
[0076] Similarly to Si, Al is an element that increases the
specific resistance of steel. There are not a few cases where Al is
added to an electrical steel sheet in combination with Si. While Si
is an element that reduces the lattice spacing of Fe crystals, Al
is an element that increases the lattice spacing of Fe crystals.
Conversely, adding Al to the Si-gradient steel sheet
disadvantageously mitigates the stress distribution suitable for
reducing eddy current, which is formed by the addition of Si.
However, the adverse effect is not produced when the Al
concentration is 0.1% or less. Accordingly, the Al concentration in
the material is limited to 0.1% or less. While the lower limit for
the Al concentration is not specified, limiting the Al
concentration to be less than 0.002% increases formation of a
microstructure including crystal grains having various sizes, which
increases iron loss. While the upper limit for the Al concentration
is also not limited, it is advantageous to limit the Al
concentration to 0.01% or less in consideration of workability.
Accordingly, the Al concentration is preferably 0.002% to
0.01%.
N: 0.01% or Less
[0077] Adding N at a concentration of more than 0.01% increases
iron loss. Accordingly, the N concentration is limited to 0.01% or
less.
[0078] The balance includes Fe and inevitable impurities.
[0079] A preferable production method is described below.
[0080] A slab having the above-described composition is heated,
hot-rolled, and repeatedly cold-rolled to form a steel sheet having
a predetermined thickness. Intermediate annealing may be performed
once or twice or more between the cold-rolling steps. Finish
annealing may optionally be performed. The steel sheet is heated to
1100.degree. C. to 1250.degree. C. in a non-oxidizing atmosphere to
be transformed into the austenite phase. Subsequently, Si is caused
to penetrate the surface of the steel sheet at 1100.degree. C. to
1250.degree. C. in a non-oxidizing atmosphere containing 10 mol %
or more and less than 45 mol % silicon tetrachloride to transform
the surface layer (to the depth 5% to 40% of the thickness of the
steel sheet) of the steel sheet into the ferrite phase. Then, the
steel sheet is held for a predetermined amount of time at
1100.degree. C. to 1250.degree. C. in a non-oxidizing atmosphere
that does not contain Si until the thickness of the surface layer
that is in the ferrite phase reaches 10% to 40% of the thickness of
the steel sheet, while the austenite phase is maintained in the
inner part. Subsequently, the steel sheet is cooled to 400.degree.
C. at an average cooling rate of 5 to 30.degree. C./s.
[0081] As described above, the high-temperature steel sheet that is
in the austenite phase is subjected to the siliconizing process and
the diffusion treatment to transform only the surface part into a
high-Si ferrite phase while maintaining the inner part to be in the
austenite phase and, subsequently, the steel sheet is cooled to
room temperature to transform the inner part into the ferrite
phase. Through the above-described process, an electrical steel
sheet including, with a symmetry plane being the center of the
steel sheet in the thickness direction, a surface part in which the
Si concentration in the steel sheet changes continuously from a
high Si concentration to a low Si concentration in the thickness
direction of the steel sheet from the surface of the steel sheet, a
boundary part in which the Si concentration changes
discontinuously, and an inner part in which the Si concentration
does not change substantially in the thickness direction of the
steel sheet, the inner part including the center of the steel sheet
in the thickness direction can be produced.
[0082] The conditions under which the siliconizing process is
performed are one of the elements important to produce the
electrical steel sheet. Examples of a method of causing Si to
penetrate the steel sheet (for siliconizing the steel sheet)
include publicly known methods such as a gas-phase siliconizing
process, a liquid-phase siliconizing process, and a solid-phase
siliconizing process. A Si-containing gas used in the process is
not limited and is preferably, for example, one or two or more
gases selected from silicon tetrachloride, trichlorosilane,
dichlorosilane, monosilane, and disilane. Hereinafter, a gas-phase
siliconizing process in which the steel sheet is heated in a
non-oxidizing atmosphere and a silicon tetrachloride gas is used is
described.
[0083] In the gas-phase siliconizing process, it is possible to
control the amount of Si added to the steel sheet and the Si
concentration distribution in the steel sheet by adjusting the
concentration of the silicon tetrachloride gas in the non-oxidizing
atmosphere such as, nitrogen or argon, the temperature at which the
reaction is performed in the non-oxidizing atmosphere, the amount
of time for which the reaction is performed in the non-oxidizing
atmosphere, the temperature at which the subsequent diffusion
treatment is performed in a non-oxidizing atmosphere that does not
contain a silicon tetrachloride gas, and the amount of time for
which the diffusion treatment is performed. To add a predetermined
amount of Si to the steel sheet in a short time, it is preferable
to produce the steel sheet using a silicon tetrachloride gas at a
high temperature and a high concentration. To adjust the amount of
Si added to the steel sheet and the Si concentration distribution
in the steel sheet with high accuracy, it is preferable to produce
the steel sheet using a silicon tetrachloride gas at a low
temperature and a low Si concentration.
[0084] Where the siliconizing process is performed in the
high-temperature austenite phase, it is possible to change the form
of the crystal grains included in the surface part by adjusting the
conditions under which the siliconizing process is performed and
the conditions under which the diffusion treatment is performed.
For example, the concentration of silicon tetrachloride in the
non-oxidizing atmosphere has been set to about 50 to 75 mol % in
consideration of the efficiency of the siliconizing process.
However, when the silicon tetrachloride concentration is increased
to the above level, the siliconizing rate is increased and the
crystal grains included in the surface layer which has been
transformed into the ferrite phase are likely to grow in the
thickness direction of the steel sheet and have a small aspect
ratio b/a. When the siliconizing process is performed using silicon
tetrachloride at a concentration of more than 45 mol %, surface
layer grains having an average aspect ratio, b/a, of crystal grains
included in the surface part of less than 0.7 are likely to be
formed. Conversely, when the silicon tetrachloride concentration is
low, the surface-layer grains are likely to grow in the direction
parallel to the surface of the steel sheet and have a large aspect
ratio. When the siliconizing process is performed using silicon
tetrachloride at a concentration of less than 10 mol %, surface
layer grains having an average aspect ratio, b/a, of crystal grains
included in the surface part of more than 4.0 are likely to be
formed. Accordingly, the silicon tetrachloride concentration is
limited to 10 mol % or more and less than 45 mol % to adjust the
aspect ratio b/a of crystal grains included in the surface layer of
the Si-gradient steel sheet to be 0.7 or more and 4.0 or less and
to thereby reduce the occurrence of defects in shearing process and
the degree of variations in iron loss.
[0085] If the siliconizing process is performed at less than
1100.degree. C., the sufficient tensile strength may fail to be
generated in the surface part and, consequently, the reduction
effect in eddy current may be limited. On the other hand, if the
siliconizing process is performed at more than 1250.degree. C., a
liquid phase may disadvantageously be formed in a portion of the
surface part which has the highest Si concentration. This may lead
to the rupture, wrinkling, and warpage of the steel sheet.
Accordingly, the temperature at which the siliconizing process is
performed is limited to 1100.degree. C. to 1250.degree. C.
[0086] Subsequent to the siliconizing process, a diffusion
treatment in which the steel sheet is maintained for a
predetermined amount of time at 1100.degree. C. to 1250.degree. C.
in a non-oxidizing atmosphere that does not contain Si until the
thickness of the surface part that is in the ferrite phase reaches
a predetermined thickness is performed. Specifically, the diffusion
treatment is performed until the thickness of the surface part that
is in the ferrite phase reaches 10% to 40% of the thickness of the
steel sheet.
[0087] Subsequent to the siliconizing process and the diffusion
treatment, the steel sheet is cooled to 400.degree. C. at an
average cooling rate of 5 to 30.degree. C./s. If the average
cooling rate is less than 5.degree. C./s, relaxation of the
internal stress may occur and, consequently, the reduction effect
in eddy-current loss may fail to be achieved at a sufficient level.
On the other hand, if rapid cooling is performed at a cooling rate
of more than 30.degree. C./s, the microstructure of the inner part
of the steel sheet may become distorted in various directions. This
significantly degrades soft magnetic properties. Accordingly, to
achieve good direct-current magnetic properties, it is necessary to
perform cooling at an average cooling rate of 5 to 30.degree. C./s
until the temperature reaches at least 400.degree. C.
Example 1
[0088] Our steel sheets and methods are described more in detail
with reference to Examples below.
[0089] Ingots having the compositions described in Table 1 with the
balance being Fe and inevitable impurities were heated to
1100.degree. C., hot-rolled to a thickness of 2.3 mm, and then
cold-rolled to a thickness of 0.2 mm. Specimens that were to be
siliconized and had a width of 50 mm and a length of 150 mm were
taken from each of the resulting cold-rolled steel sheets. The
specimens were heated in an argon atmosphere from room temperature
to the temperature range of 1100.degree. C. to 1225.degree. C., in
which the austenite phase is formed, while the specimens were
transported. Subsequently, an argon gas containing silicon
tetrachloride at a concentration of 8% to 66% by volume was charged
into the furnace, and a siliconizing process was performed at the
same temperature as above for 1 to 6 minutes. Then, the atmosphere
in the furnace was replaced with a non-oxidizing atmosphere that
was an argon gas that did not contain silicon tetrachloride, and a
diffusion treatment was performed at 1100.degree. C. to
1250.degree. C. for 2 to 30 minutes. The amount of silicon used,
that is, the amount of Si added to each of the steel sheets, was
adjusted by changing the concentration of silicon tetrachloride in
the atmosphere and the amount of time during which the treatment
was performed. The thickness of the surface part, which is to be
transformed from the austenite phase into the ferrite phase by
diffusing Si from the surface, was adjusted by changing the amount
of time during which the siliconizing process was performed and the
amount of time during which the diffusion treatment was performed.
In the subsequent step, the Si concentration distribution in a
cross section of each of the steel sheets was determined by EPMA
(electron beam microanalyzer). For each of Test Nos., 12 samples
having the same shape were prepared under the same treatment
conditions.
[0090] The samples that had been subjected to the siliconizing
process and the diffusion treatment were then transported in a
nitrogen atmosphere to the room temperature region to be cooled to
400.degree. C. or less at an average cooling rate of 15.degree.
C./s. The samples were removed when the temperature reached
100.degree. C. or less. We confirmed that samples prepared under
the same conditions contained the same amount of silicon by
determining the change in the mass of each of the samples which
occurred during the treatments.
[0091] One of the 12 samples taken from each of Test Nos. was again
heated in an argon atmosphere and subjected to an additional
heating treatment in the ferrite phase region of 900.degree. C.
until the Si distribution in the sample became uniform in the
thickness direction of the steel sheet.
[0092] One of the surfaces of another one of the 12 samples was
covered with an adhesive label, and a portion of the sample that
extended from the other surface to the center of the sample in the
thickness direction was removed by chemical polishing with
hydrofluoric acid. The results of observation of warpage of the
sample confirmed that the sample had a stress distribution such
that a tensile stress was generated in the surface part and a
compressive stress was generated in the inner part.
[0093] Each of the other 10 samples was subjected to a precision
shearing machine for thin sheets to cut both ends of the sample at
positions 10 mm from the respective ends in the width direction
with an appropriate blade clearance. Hereby, single-sheet samples
for magnetic property evaluation which had a width of 30 mm were
prepared. In the magnetic measurement, a single-sheet testing frame
with which a sample having a width of 30 mm and a length of 100 mm
can be excited and the magnetic properties of the sample can be
evaluated was used and the iron loss (W1/10 k) of each of the
samples was measured in accordance with a method (Epstein test
method) conforming to JIS C2550.
[0094] Subsequent to the above measurement, the samples were cut
with a high-speed rotary cutter for microstructure testing. The
microstructure of each of the samples was determined with an
optical microscope. Furthermore, the Si concentration distribution
in each of the samples in the thickness direction was determined by
EPMA.
[0095] In the above-described manner, the average Si concentration
in the inner part, the Si concentration at the surface of the steel
sheet, the average Si concentration in the surface part, the ratio
of the thickness of the surface part to the thickness of the steel
sheet, the average aspect ratio of crystal grains included in the
surface part, the Si concentration gap in the boundary part,
saturation magnetic flux density Bs, the average m of iron losses
at high frequencies W1/10 k of samples excited at a magnetic flux
density of 0.1 T and 10 kHz, the standard deviation .sigma.
thereof, and the coefficient of variation .sigma./m were measured.
In addition, the iron loss W1/10 k of the sample (Si-uniform
material) having a uniform Si concentration was measured, and the
ratio of the average iron loss of the Si-gradient material
determined above to the iron loss of the Si-uniform material was
calculated for each of Test Nos. Table 2 summarizes the
results.
TABLE-US-00001 TABLE 1 Composition (mass %) Steel type C Si Mn P S
sol. Al N A 0.0023 0.18 0.40 0.012 0.008 0.021 0.0023 B 0.0024 0.65
0.18 0.007 0.006 0.009 0.0022 C 0.0026 1.36 0.32 0.007 0.005 0.005
0.0026
TABLE-US-00002 TABLE 2 Ratio of Siliconizing conditions thickness
ds of Average Silicon Average Si Si Average Si surface part to
aspect Siliconizing tetrachloride concentration concentration
concentration thickness d0 of ratio in Test Steel temperature
concentration in inner part at surface in surface steel sheet
surface No. type [.degree. C.] [mol %] [mass %] [mass %] part [mass
%] ds/d0 [%] part b/a 1 A 1225 66 0.18 6.17 3.4 33 0.4 2 A 1175 43
0.18 6.06 3.6 28 0.9 3 A 1175 31 0.18 6.27 3.8 28 2.4 4 A 1130 22
0.18 6.65 4.1 20 3.3 5 B 1200 54 0.65 6.24 4.3 30 0.5 6 B 1175 41
0.65 6.51 4.5 30 0.8 7 B 1175 35 0.65 6.32 4.4 31 1.5 8 B 1150 18
0.65 6.48 4.6 33 3.6 9 B 1130 8 0.65 6.68 4.7 27 4.3 10 B 1100 12
0.65 6.34 4.5 17 3.5 11 C 1200 51 1.36 6.42 5.1 32 0.6 12 C 1180 41
1.36 6.23 4.9 29 0.9 13 C 1150 25 1.36 6.09 4.7 27 2.7 14 C 1150 21
1.36 6.04 4.8 28 3.4 15 C 1160 9 1.36 6.57 5.2 27 4.6 16 C 1100 28
1.36 6.24 5.0 18 2.1 Si- concentration gap in Saturation Iron loss
W.sub.1/10k Iron loss boundary magnetic Standard Coefficient ratio
to Si- Test part flux density Average: m deviation of variation
uniform No. [mass %] Bs [T] [W/kg] .sigma. .sigma./m [%] material
Remarks 1 1.6 2.06 14.9 1.52 10.2 0.88 Comparative example 2 1.6
2.07 13.3 0.61 4.6 0.70 Example 3 1.6 2.07 12.9 0.31 2.4 0.73
Example 4 1.6 2.09 13.5 0.36 2.7 0.83 Example 5 1.2 2.03 14.2 1.48
10.4 0.85 Comparative example 6 1.2 2.03 12.4 0.58 4.7 0.72 Example
7 1.2 2.03 12.6 0.43 3.4 0.74 Example 8 1.2 2.01 12.6 0.33 2.6 0.73
Example 9 1.2 2.03 14.3 1.53 10.7 0.81 Comparative example 10 1.2
2.07 13.7 0.37 2.7 0.77 Example 11 0.5 1.98 13.8 1.46 10.6 0.86
Comparative example 12 0.5 2.00 11.9 0.66 5.5 0.71 Example 13 0.5
2.01 11.7 0.35 3.0 0.67 Example 14 0.5 2.01 11.4 0.33 2.9 0.69
Example 15 0.5 2.00 13.6 1.53 11.2 0.82 Comparative example 16 0.5
2.04 12.4 0.52 4.2 0.78 Example
[0096] The results described in Table 2 confirm that, in our
Examples, where the average aspect ratio of crystal grains included
in the surface part was 0.7 or more and 4.0 or less, iron loss at
high frequencies was low, the coefficient of variations in iron
loss was 2.4% to 5.5%, which is small. That is, the degree of
variations in iron loss was small.
[0097] In contrast, in the Comparative examples, where the average
aspect ratio of crystal grains included in the surface part was
less than 0.7 or more than 4.0, the coefficient of variation was
more than 10%. That is, the degree of variations in iron loss was
large.
[0098] Furthermore, the ratio of, to the iron loss of the sample
having a uniform Si concentration, the average iron loss of the
other samples was 0.9 or less. This confirms that the samples
prepared in our Examples, in which the specific Si concentration
distribution was set to create a stress distribution such that a
tensile stress is generated in the surface part and a compressive
stress is generated in the inner part, had a lower iron loss than
the samples having a uniform Si concentration.
Example 2
[0099] Ingots having the compositions described in Table 3 with the
balance being Fe and inevitable impurities were heated to
1100.degree. C., hot-rolled to a thickness of 2.3 mm, and then
cold-rolled to a thickness of 0.5 to 0.08 mm. Specimens that were
to be siliconized and had a width of 50 mm and a length of 150 mm
were taken from each of the resulting cold-rolled steel sheets. The
specimens were heated in an argon atmosphere from room temperature
to the temperature range of 1200.degree. C., in which the austenite
phase is formed, while the specimens were transported.
Subsequently, an argon gas containing silicon tetrachloride at a
concentration of 8% to 57% by volume was charged into the furnace,
and a siliconizing process was performed at the same temperature as
above for 1 to 10 minutes. Then, the atmosphere in the furnace was
replaced with a non-oxidizing atmosphere that was an argon gas that
did not contain silicon tetrachloride, and a diffusion treatment
was performed at 1200.degree. C. for 2 to 40 minutes. The amount of
silicon used, that is, the amount of Si added to each of the steel
sheets, was adjusted by changing the concentration of silicon
tetrachloride in the atmosphere and the amount of time during which
the treatment was performed. The thickness of the surface part,
which is to be transformed from the austenite phase into the
ferrite phase by diffusing Si from the surface, was adjusted by
changing the amount of time during which the siliconizing process
was performed and the amount of time during which the diffusion
treatment was performed. In the subsequent step, the Si
concentration distribution in a cross section of each of the steel
sheets was determined by EPMA (electron beam microanalyzer). For
each of Test Nos., 11 samples having the same shape were
prepared.
[0100] The samples that had been subjected to the above treatments
were transported in a nitrogen atmosphere to the room temperature
region to be cooled to 400.degree. C. or less at a cooling rate of
15.degree. C./s. The samples were removed when the temperature was
reduced to 100.degree. C. or less. We confirmed that samples
prepared under the respective conditions contained the same amount
of silicon by determining the change in the weight of each of the
samples which occurred during the treatments.
[0101] One of the 11 samples taken from each of Test Nos. was
covered with an adhesive label, and a portion of the sample that
extended from the other surface to the center of the sample in the
thickness direction was removed by chemical polishing with
hydrofluoric acid. The results of observation of warpage of the
sample confirmed that the sample had a stress distribution such
that a tensile stress was generated in the surface part and a
compressive stress was generated in the inner part.
[0102] Each of the other 10 samples was subjected to a precision
shearing machine for thin sheets to cut both ends of the sample at
positions 10 mm from the respective ends in the width direction
with an appropriate blade clearance. Hereby, single-sheet samples
for magnetic property evaluation which had a width of 30 mm were
prepared. In the magnetic measurement, a single-sheet testing frame
with which a sample having a width of 30 mm and a length of 100 mm
can be excited and the magnetic properties of the sample can be
evaluated was used and the iron loss (W1/10 k) of each of the
samples was measured in accordance with a method (Epstein test
method) conforming to JIS C2550.
[0103] Subsequent to the above measurement, the samples were cut
with a high-speed rotary cutter for microstructure testing. The
microstructure of each of the samples was determined with an
optical microscope. Furthermore, the Si concentration distribution
in each of the samples in the thickness direction was determined by
EPMA.
[0104] In the above-described manner, the Si concentration at the
surface of the steel sheet, the average Si concentration in the
surface part, the ratio of the thickness of the surface part to the
thickness of the steel sheet, the average aspect ratio of crystal
grains included in the surface part, the Si concentration gap in
the boundary part, the average m of iron losses at high frequencies
W1/10 k of samples excited at a magnetic flux density of 0.1 T and
10 kHz, the standard deviation .sigma. thereof, and the coefficient
of variation .sigma./m were measured. Table 4 summarizes the
results.
TABLE-US-00003 TABLE 3 Composition (mass %) Steel type C Si Mn P S
sol. Al N B 0.0024 0.65 0.18 0.007 0.006 0.009 0.0022
TABLE-US-00004 TABLE 4 Siliconizing Ratio of conditions thickness
ds of Average Silicon Si Average Si surface part to aspect
tetrachloride Thickness concentration concentration thickness d0 of
ratio in Test concentration of steel at surface in surface steel
sheet surface No. [mol %] sheet [mm] [mass %] part [mass %] ds/d0
[%] part b/a 17 52 0.08 6.31 4.26 30 0.5 18 41 0.08 6.15 4.17 32
0.9 19 23 0.08 6.43 4.32 28 2.5 20 16 0.08 6.27 4.24 30 3.2 21 57
0.25 6.14 4.17 32 0.4 22 42 0.25 6.21 4.21 30 0.8 23 26 0.25 6.18
4.19 32 2.1 24 9 0.25 6.41 4.31 28 4.3 25 35 0.35 6.07 4.12 32 1.2
26 28 0.35 6.23 4.22 30 1.8 27 8 0.35 6.09 4.14 32 4.4 28 30 0.50
6.08 4.13 28 1.8 Iron loss W.sub.1/10k Si-concentration Standard
Coefficient Test gap in boundary Average: m deviation of variation
No. part [mass %] [W/kg] .sigma. .sigma./m [%] Remarks 17 1.2 8.6
0.92 10.7 Comparative example 18 1.2 7.7 0.34 4.4 Example 19 1.2
7.5 0.22 2.9 Example 20 1.2 7.8 0.28 3.6 Example 21 1.2 20.1 2.24
11.1 Comparative example 22 1.2 18.7 0.91 4.9 Example 23 1.2 18.4
0.72 3.9 Example 24 1.2 21.3 2.53 11.9 Comparative example 25 1.2
24.5 1.26 5.1 Example 26 1.2 23.8 1.33 5.6 Example 27 1.2 24.1 2.95
12.2 Comparative example 28 1.2 38.6 2.16 5.6 Example
[0105] The results described in Table 4 confirm that, in our
Examples, where the aspect ratio of crystal grains included in the
surface part was 0.7 or more and 4.0 or less, iron loss at high
frequencies was low, the coefficient of variations in iron loss was
about 5%, which is small. That is, the degree of variations in iron
loss was small. In contrast, in the Comparative examples, where the
aspect ratio of crystal grains included in the surface layer was
less than 0.7 or more than 4.0, the coefficient of variation was
more than 10%. That is, the degree of variations in iron loss was
large.
Example 3
[0106] Ingots having the compositions described in Table 5 with the
balance being Fe and inevitable impurities were heated to
1100.degree. C., hot-rolled to a thickness of 2.3 mm, and then
cold-rolled to a thickness of 0.2 mm. Specimens that were to be
siliconized and had a width of 50 mm and a length of 150 mm were
taken from each of the resulting cold-rolled steel sheets. The
specimens were heated in an argon atmosphere from room temperature
to the temperature range of 1100.degree. C. to 1250.degree. C., in
which the austenite phase is formed, while the specimens were
transported. Subsequently, an argon gas containing silicon
tetrachloride at a concentration of 10% to 30% by volume was
charged into the furnace, and a siliconizing process was performed
at the same temperature as above for 1 to 6 minutes. Then, the
atmosphere in the furnace was replaced with a non-oxidizing
atmosphere that was an argon gas that did not contain silicon
tetrachloride, and a diffusion treatment was performed at
1100.degree. C. to 1250.degree. C. for 2 to 30 minutes. The amount
of silicon used, that is, the amount of Si added to each of the
steel sheets, was adjusted by changing the concentration of silicon
tetrachloride in the atmosphere and the amount of time during which
the treatment was performed. The thickness of the surface part,
which is to be transformed from the austenite phase into the
ferrite phase by diffusing Si from the surface, was adjusted by
changing the amount of time during which the siliconizing process
was performed and the amount of time during which the diffusion
treatment was performed. In the subsequent step, the Si
concentration distribution in a cross section of each of the steel
sheets was determined by EPMA (electron beam microanalyzer). For
each of Test Nos., 12 samples having the same shape were
prepared.
[0107] The samples that had been subjected to the above treatments
were transported in a nitrogen atmosphere to the room temperature
region to be cooled to 400.degree. C. or less at a cooling rate of
15.degree. C./s. The samples were removed when the temperature was
reduced to 100.degree. C. or less. We confirmed that samples
prepared under the respective conditions contained the same amount
of silicon by determining the change in the weight of each of the
samples which occurred during the treatments.
[0108] One of the 12 samples taken from each of Test Nos. was again
heated in an argon atmosphere and subjected to an additional
heating treatment in the ferrite phase region of 900.degree. C.
until the Si distribution in the sample became uniform in the
thickness direction of the steel sheet.
[0109] One of the surfaces of another one of the 12 samples was
covered with an adhesive label, and a portion of the sample that
extended from the other surface to the center of the sample in the
thickness direction was removed by chemical polishing with
hydrofluoric acid. The results of observation of warpage of the
sample confirmed that the sample had a stress distribution such
that a tensile stress was generated in the surface part and a
compressive stress was generated in the inner part.
[0110] Each of the other 10 samples was subjected to a precision
shearing machine for thin sheets to cut both ends of the sample at
positions 10 mm from the respective ends in the width direction
with an appropriate blade clearance. Hereby, single-sheet samples
for magnetic property evaluation which had a width of 30 mm were
prepared. In the magnetic measurement, a single-sheet testing frame
with which a sample having a width of 30 mm and a length of 100 mm
can be excited and the magnetic properties of the sample can be
evaluated was used and the iron loss (W1/10 k) of each of the
samples was measured in accordance with a method (Epstein test
method) conforming to JIS C2550.
[0111] Subsequent to the above measurement, the samples were cut
with a high-speed rotary cutter for microstructure testing. The
microstructure of each of the samples was determined with an
optical microscope. Furthermore, the Si concentration distribution
in each of the samples in the thickness direction was determined by
EPMA.
[0112] Subsequent to the above measurement, the samples were cut
with a high-speed rotary cutter for microstructure testing. The
microstructure of each of the samples was determined with an
optical microscope. Furthermore, the Si concentration distribution
in each of the samples in the thickness direction was determined by
EPMA.
[0113] In the above-described manner, the average Si concentration
in the inner part, the Si concentration at the surface of the steel
sheet, the average Si concentration in the surface part, the ratio
of the thickness of the surface part to the thickness of the steel
sheet, the average aspect ratio of crystal grains included in the
surface part, the Si concentration gap in the boundary part,
saturation magnetic flux density Bs, the average m of iron losses
at high frequencies W1/10 k of samples excited at a magnetic flux
density of 0.1 T and 10 kHz, the standard deviation .sigma.
thereof, and the coefficient of variation .sigma./m were measured.
In addition, the iron loss W1/10 k of the sample (Si-uniform
material) having a uniform Si concentration was measured, and the
ratio of the average iron loss of the Si-gradient material
determined above to the iron loss of the Si-uniform material was
calculated for each of Test Nos. Table 6 summarizes the
results.
TABLE-US-00005 TABLE 5 Composition (mass %) Steel type C Si Mn P S
sol. Al N A 0.0023 0.18 0.40 0.012 0.008 0.021 0.0023 B 0.0024 0.65
0.18 0.007 0.006 0.009 0.0022 C 0.0026 1.36 0.32 0.007 0.005 0.005
0.0026 D 0.0038 1.66 0.65 0.006 0.006 0.008 0.0024 E 0.0035 2.06
0.93 0.005 0.008 0.012 0.0026
TABLE-US-00006 TABLE 6 Ratio of thickness ds of Average Average Si
Si Average Si surface part to aspect concentration concentration
concentration thickness d0 of ratio in Si-concentration Test Steel
in inner part at surface in surface part steel sheet surface gap in
boundary No. type [mass %] [mass %] [mass %] ds/d0 [%] part b/a
part [mass %] 29 A 0.18 6.2 3.9 32 1.8 1.6 30 B 0.65 6.4 4.1 28 2.3
1.2 31 B 0.65 6.3 4.7 18 1.5 1.2 32 B 0.65 6.5 5.5 8 1.2 1.2 33 C
1.36 6.3 4.3 28 1.8 0.5 34 C 1.36 6.1 4.3 36 2.2 0.5 35 C 1.36 5.6
3.1 42 2.6 0.5 36 D 1.66 6.4 4.5 30 1.8 0.2 37 E 2.06 6.1 4.4 32
2.2 0.1 Saturation magnetic Iron loss W.sub.1/10k Iron loss flux
Standard Coefficient ratio to Si- Reduction Test density Average: m
deviation of variation uniform effect in No. Bs [T] [W/kg] .sigma.
.sigma./m [%] material iron loss Remarks 29 2.04 13.6 0.61 4.5 0.72
.circle-w/dot. Example 30 2.04 12.7 0.38 3.0 0.67 .circle-w/dot.
Example 31 2.07 14.9 0.61 4.1 0.75 .circle-w/dot. Example 32 2.11
16.4 0.56 3.4 0.93 .largecircle. Example 33 2.03 11.9 0.28 2.4 0.64
.circle-w/dot. Example 34 2.00 12.8 0.36 2.8 0.69 .circle-w/dot.
Example 35 2.03 15.5 0.38 2.5 0.91 .largecircle. Example 36 2.00
13.1 0.51 3.9 0.84 .circle-w/dot. Example 37 1.99 17.1 0.43 2.5
0.99 X Comparative example Reduction effect in iron loss (iron loss
ratio to Si-uniform material): .circle-w/dot. Large (0.90 or less),
.largecircle. Small (more than 0.90 and 0.95 or less), X None (more
than 0.95)
[0114] The samples in which the ratio ds/d0 of the thickness of the
surface part to the thickness of the steel sheet, which is a
preferable condition, was less than 10% or more than 40% had a low
iron-loss. However, the reductions in iron loss were smaller than
reductions in the iron loss of the samples having a ratio ds/d0 of
10% to 40%. On the other hand, the ratio of the iron loss of the
sample in which the Si concentration gap in the boundary part was
0.1% to the iron loss of the sample having a uniform Si
concentration was close to 1. That is, the iron loss of the sample
was not reduced substantially by the formation of the Si
concentration distribution.
[0115] In our Examples, where the ds/d0 ratio was 10% or more and
40% or less, the Si concentration gap in the boundary part was 0.2%
or more, and the average aspect ratio of crystal grains included in
the surface part was 0.7 or more and 4.0 or less, iron loss was
reduced by 10% or more compared to when the Si concentration was
made uniform. In addition, the coefficient of variation was less
than 10%. That is, the degree of variations in iron loss was
reduced to a sufficiently low level.
* * * * *