[0001]The present invention relates to a method of manufacturing a grain-oriented
electrical steel sheet having excellent magnetic characteristics suitable as an iron core
material for a transformer.
10 Priority is claimed on Japanese Patent Application No. 2019-5083, filed January
16, 2019, the content of which is incorporated herein by reference.
[Background Art]
[0002]
A grain-oriented electrical steel sheet is mainly used for a transformer. Since a
15 transformer is continuously excited for a long period of time from installation to disposal
and continues to generate an energy loss, the energy loss generated when the transformer
is magnetized by an alternating current, that is, an iron loss is a main indicator for
determining the performance of the transformer. Generally, a grain-oriented electrical
steel sheet includes a base steel sheet that contains 7% by mass or less of Si and has a
20 texture controlled such that an orientation of each grain matches a {110} <001>
orientation referred to as Goss orientation and an insulation coating for imparting
insulation to the base steel sheet.
[0003]
Many methods have been proposed so far to reduce the iron loss of a grain25
oriented electrical steel sheet. For example, a method of increasing alignment in the
2
Goss direction in the texture of the base steel sheet, a method of increasing the amount of
a solid solution element such as Si, which increases the electrical resistance, in the base
steel sheet, a method of reducing the sheet thickness of the base steel sheet, and the like
are known.
5 [0004]
Further, it is known that application of tension to the base steel sheet is an
effective method for reducing the iron loss. To apply tension to the base steel sheet, it is
effective to form a coating made of a material having a thermal expansion coefficient
smaller than that of the base steel sheet on a surface of the base steel sheet at a high
10 temperature.
[0005]
In a final annealing process of the base steel sheet, a forsterite coating generated
by the reaction of an oxide present on the surface of the base steel sheet with an
annealing separator can give tension to the base steel sheet. Since an unevenness is
15 present at an interface between the forsterite coating and the base steel sheet, the
forsterite coating also functions as an intermediate coating that enhances the adhesion
between the insulating coating and the base steel sheet due to the anchor effect of the
unevenness.
[0006]
20 A method of forming an insulating coating by baking a coating liquid mainly
composed of colloidal silica and phosphate, which is disclosed in Patent Document 1, has
a significant effect of applying tension to the base steel sheet and is effective in reducing
the iron loss. Therefore, a general method of manufacturing a grain-oriented electrical
steel sheet is to execute insulation coating with phosphate as a main component in a state
25 in which the forsterite coating occurring in the final annealing process remains. In the
3
specification of the present application, an insulation film capable of applying tension as
well as insulation properties to the base steel sheet is referred to as a tension-insulation
coating.
[0007]
On the other hand, in recent years, it has be 5 come clear that the forsterite coating
inhibits the movement of a magnetic wall and adversely affects the iron loss. In a grainoriented
electrical steel sheet, a magnetic domain varies with the movement of the
magnetic wall under an alternating magnetic field. The smooth movement of the
magnetic wall is effective in reducing the iron loss, but the movement of the magnetic
10 wall is hindered due to the presence of the unevenness at the interface between the
forsterite coating and the base steel sheet, and as a result, it has been found that the iron
loss reducing effect due to applying tension is canceled out and a sufficient iron loss
reducing effect cannot be obtained.
[0008]
15 Therefore, a technique for suppressing generation of a forsterite coating and
smoothing the surface of the base steel sheet has been developed. For example, in Patent
Documents 2 to 5, a technique for smoothing the surface of the base steel sheet without
generating a forsterite coating in final annealing by controlling the dew point of the
atmosphere of decarburization annealing and using alumina as the annealing separator is
20 disclosed.
[0009]
Further, in Patent Document 6, a technique for manufacturing a grain-oriented
silicon steel sheet in which an inorganic mineral coating composed of forsterite and the
like is not present on the surface of the base steel sheet using an annealing separator
25 containing 5% by weight or more and 30% by weight or less of magnesia with respect to
4
the total weight of alumina and the magnesia as the annealing separator is disclosed.
[Citation List]
[Patent Document]
[0010]
5 [Patent Document 1]
Japanese Unexamined Patent Application, First Publication No. S48-039338
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. H07-278670
[Patent Document 3]
10 Japanese Unexamined Patent Application, First Publication No. H11-106827
[Patent Document 4]
Japanese Unexamined Patent Application, First Publication No. H07-118750
[Patent Document 5]
Japanese Unexamined Patent Application, First Publication No. 2003-268450
15 [Patent Document 6]
International Application Publication No. WO 2002-088403
[Summary of the Invention]
[Problems to be Solved by the Invention]
[0011]
20 Unless the forsterite coating is formed on the surface of the base steel sheet as in
the above-mentioned related art, the unevenness that hinders the movement of the
magnetic wall disappears from the surface of the base steel sheet, and thus it is
considered that the iron loss of the grain-oriented electrical steel sheet can be improved.
However, even with these techniques, the effect of improving the iron loss could not be
25 sufficiently obtained.
5
[0012]
The present invention is made in view of the above circumstances, and an object
of the present invention is to reduce the iron loss of the grain-oriented electrical steel
sheet in which a forsterite coating is not present between the base steel sheet and the
tension-insulation c 5 oating as compared with in the related art.
[Means for Solving the Problem]
[0013]
To solve the above problems, the present inventors have conducted a diligent
study on a cause of not obtaining a sufficient effect of reducing the iron loss in a case in
10 which a grain-oriented electrical steel sheet in which a forsterite coating is not present
between the base steel sheet and the tension-insulation coating is manufactured. As a
result, it was found that a large number of needle-like inclusions are present in a surface
layer region of the base steel sheet in the grain-oriented electrical steel sheet in which a
sufficient effect of reducing the iron loss was not obtained.
15 The present inventors postulate that these needle-like inclusions are a cause of
movement of the magnetic wall being hindered, that is, a cause of the iron loss being
adversely affected. As a result of further study by the present inventors, it was found that
by controlling components and an application amount of the annealing separator under
specific conditions, it is possible to suppress generation of the needle-like inclusions in
20 the surface layer region of the base steel sheet, and it is possible to reduce the iron loss of
the grain-oriented electrical steel sheet in which a forsterite coating is not present
between the base steel sheet and the tension-insulation coating as compared with in the
related art.
[0014]
25 The present invention is made based on the above findings, and the gist thereof is
6
as follows.
[0015]
(1) A method of manufacturing a grain-oriented electrical steel sheet according to an
aspect of the present invention includes a process of executing hot rolling on a slab to
obtain a hot-rolled sheet; a process of executing 5 hot-rolled sheet annealing on the hotrolled
sheet to obtain an annealed hot-rolled sheet; a process of executing cold rolling on
the annealed hot-rolled sheet to obtain a cold-rolled sheet; a process of executing
decarburization annealing on the cold-rolled sheet to obtain a decarburization annealed
sheet; a process of applying an annealing separator containing alumina as a main
10 component to the decarburization annealed sheet; and a process of executing final
annealing on the decarburization annealed sheet to which the annealing separator is
applied, wherein the annealing separator contains 28% to 50% by mass of MgO, and
wherein an application amount of the annealing separator is 6.0 to 14.0 g/m2 per one
surface of the decarburization annealed sheet.
15 [0016]
(2) In the method of manufacturing a grain-oriented electrical steel sheet according to
(1), a BET specific surface area of the alumina may be 3.0 to 10.0 m2/g.
[0017]
(3) In the method of manufacturing a grain-oriented electrical steel sheet according to (1)
20 or (2), the slab may contain, as a chemical composition, in % by mass, C: 0.085% or less,
Si: 0.80% to 7.00%, Mn: 0.05% to 1.00%, acid-soluble Al: 0.010% to 0.065%, S: 0.01%
or less, N: 0.004% to 0.012%, B: 0.0005% to 0.0080%, P: 0% to 0.50%, Ni: 0% to
1.00%, Sn: 0% to 0.30%, Sb: 0% to 0.30%, Cu: 0% to 0.40%, Cr: 0% to 0.30%, Bi: 0%
to 0.01%, and the remainder of Fe and impurities.
25 [Effects of the Invention]
7
[0018]
According to the aspect of the present invention, it is possible to reduce the iron
loss of the grain-oriented electrical steel sheet in which a forsterite coating is not present
between the base steel sheet and the tension-insulation coating as compared with in the
5 related art.
[Brief Description of Drawings]
[0019]
Fig. 1 is a ternary phase diagram of Al2O3-MgO-SiO2.
Fig. 2 is a diagram showing the relationship between the amount of MgO in an
10 annealing separator and the number of pieces of mullite.
Fig. 3 is a diagram showing the relationship between the amount of MgO in an
annealing separator and an iron loss (W17/50).
Fig. 4 is a diagram showing the relationship between an application amount of an
annealing separator per one surface and the number of pieces of mullite.
15 [Embodiment(s) for implementing the Invention]
[0020]
A method of manufacturing a grain-oriented electrical steel sheet according to an
embodiment of the present invention (hereinafter referred to as the present manufacturing
method) includes a hot rolling process, a hot-rolled sheet annealing process, a cold
20 rolling process, a decarburization annealing process, an annealing separator applying
process, and a final annealing process.
The hot rolling process is a process of executing hot rolling on a slab having a
predetermined chemical composition to obtain a hot-rolled sheet. The hot-rolled sheet
annealing process is a process of executing hot-rolled sheet annealing on the hot-rolled
25 sheet to obtain an annealed hot-rolled sheet. The cold rolling process is a process of
8
executing cold rolling on the annealed hot-rolled sheet to obtain a cold-rolled sheet. The
decarburization annealing process is a process of executing decarburization annealing on
the cold-rolled sheet to obtain a decarburization annealed sheet. The annealing separator
applying process is a process of applying an annealing separator containing alumina as a
main component to the decarburization a 5 nnealed sheet. The final annealing process is a
process of executing final annealing on the decarburization annealed sheet to which the
annealing separator is applied.
The details of each step will be described later, but the present manufacturing
method, to suppress generation of needle-like inclusions in a surface layer region of a
10 base steel sheet of the grain-oriented electrical steel sheet that is a final product, is
characterized in that the following two manufacturing conditions are satisfied.
(Condition 1) The annealing separator containing alumina as a main component contains
28% to 50% by mass of MgO.
(Condition 2) An application amount of the annealing separator is 6.0 to 14.0 g/m2 per
15 one surface of the decarburization annealed sheet.
[0021]
Hereinafter, the present manufacturing method will be described.
[0022]
The present inventors postulate that one of the causes why an iron loss cannot be
20 sufficiently reduced in a grain-oriented electrical steel sheet in which a forsterite coating
is not present between a base steel sheet and a tension-insulation coating is the generation
of inclusions that adversely affect magnetism during the final annealing. Therefore, the
present inventors have taken a sample from a grain-oriented electrical steel sheet having
a large iron loss (inferior) such that a cross section (a C cross section) orthogonal to a
25 rolling direction of the base steel sheet is exposed and have observed the cross section of
9
the sample with an optical microscope.
[0023]
As a result, in a case of a grain-oriented electrical steel sheet having a large iron
loss, it was found that a large number of needle-like inclusions are present in the surface
layer region of the base steel sheet appearing 5 in the C cross section, more specifically, a
region having a length of 10 μm from a surface of the base steel sheet toward the inside
of the base steel sheet in a sheet thickness direction of the base steel sheet. Furthermore,
it was found that the needle-like inclusions were mullite (3Al2O3·2SiO2). These
observation results are the findings underlying the present invention.
10 [0024]
In the manufacturing of the grain-oriented electrical steel sheet, the
decarburization annealing is performed for the purpose of removing C (carbon) contained
in the cold-rolled sheet before the final annealing. In the decarburization annealing, C
contained in the cold-rolled sheet is removed, and at the same time, an oxide film of SiO2
15 is formed on a surface of the cold-rolled sheet. A steel sheet obtained by such
decarburization annealing, that is, a cold-rolled sheet in which C is removed, and an
oxide film of SiO2 is formed on a surface thereof is referred to as a decarburization
annealed sheet. After the decarburization annealing, an annealing separator containing
alumina as a main component is applied to the decarburization annealed sheet having an
20 oxide film of SiO2 for the purpose of preventing seizure of the decarburization annealed
sheet wound in a coil shape during the final annealing. Then, the final annealing is
performed on the decarburization annealed sheet to which the annealing separator is
applied.
[0025]
25 Since mullite is a composite oxide of alumina (Al2O3) and SiO2, it is considered
10
that the mullite is generated and remains because SiO2 formed by the decarburization
annealing is not sufficiently removed during the final annealing.
[0026]
In the first place, SiO2 formed by the decarburization annealing is adsorbed by
alumina having a high BET specific surface 5 area during the final annealing and SiO2 is
removed by the washing and removing of the annealing separator with water. Therefore,
it is considered that insufficiency of the application amount of the annealing separator is
a factor that SiO2 formed by the decarburization annealing is not sufficiently removed.
[0027]
10 That is, it is considered that there is a limit to the amount of SiO2 that can be
adsorbed per unit weight of alumina, the application amount of the annealing separator is
insufficient, all of the SiO2 is not adsorbed and removed, and SiO2 remains on the surface
of the steel sheet. As a result, it is considered that mullite is generated by the reaction of
Al that has risen from the inside of the decarburization annealed sheet toward the surface
15 of the steel sheet during the final annealing (Al generated due to the decomposition of
AlN functioning as an inhibitor) with SiO2 remaining on the surface of the
decarburization annealed sheet and remains inside the decarburization annealed sheet
(particularly, the surface layer region near the surface of the decarburization annealed
sheet).
20 [0028]
The present inventors have diligently examined the component composition and
the application amount of the annealing separator for suppressing the generation of
mullite based on the technical idea that the generation of mullite is suppressed by the
adjustment of the component composition and the application amount of the annealing
25 separator. As a result, it was found that it is possible to suppress the generation of mullite
11
by adding MgO to the annealing separator containing alumina as a main component at a
specific proportion and controlling the application amount of the annealing separator
within a specific range.
[0029]
Fig. 1 shows a ternary phase diagram of Al2O3 5 (alumina)-MgO-SiO2. As shown
in Fig. 1, theoretically, if MgO is present in a proportion of 50 mol% (28% by mass) or
more with respect to alumina, mullite is not generated. Therefore, the present inventors
have investigated the relationship between an addition amount of MgO to the annealing
separator containing alumina as a main component and the number of pieces of mullite
10 generated in the surface layer region of the base steel sheet (the steel sheet obtained after
the final annealing of the decarburization annealed sheet).
[0030]
The decarburization annealed sheet having a sheet thickness of 0.23 mm was used
as a test material, and the annealing separator containing alumina as a main component
15 was applied to the decarburization annealed sheet at an application amount of 8.0 g/m2
per one surface while the addition amount of MgO is varied in the range of 0% to 80%
by mass. After the annealing separator was dried, the final annealing was executed on
the decarburization annealed sheet, and thus a grain-oriented electrical steel sheet in
which the forsterite coating is not present on the surface of the base steel sheet (the steel
20 sheet obtained after the final annealing was executed to the decarburization annealed
sheet) was obtained. The final annealing was executed on the decarburization annealed
sheet to which the annealing separator was applied in a state in which the decarburization
annealed sheets were stacked.
[0031]
25 After an excess annealing separator was removed from the grain-oriented
12
electrical steel sheet obtained in such a manner by the washing with water, a 20 mm
square test piece was taken, and a cross section (a C cross section) orthogonal to a rolling
direction of the test piece was polished with a diamond buff. After that, the test piece
was observed at a magnification of 1000 times using an optical microscope, and the
number of needle-like inclusions each ha 5 ving a length of 1 μm or more which are present
in a region (an observation region) having a length of 10 μm from the surface of the steel
sheet toward the inside of the base steel sheet in the sheet thickness direction of the base
steel sheet and a length of 20 mm in a sheet width direction of the base steel sheet was
measured. The needle-like inclusion is defined as an inclusion in which the maximum
10 major axis/maximum minor axis of the inclusion is 10 times or more.
[0032]
Next, an iron loss W17/50 of each of the test pieces having different levels in the
amount of MgO in the annealing separator was measured. The average of the measured
values at 10 points was taken as the iron loss W17/50 of the test piece.
15 [0033]
The results of these measurements are shown in Figs. 2 and 3. Fig. 2 is a diagram
showing the relationship between the amount of MgO in the annealing separator and the
number of pieces of mullite. Fig. 3 is a diagram showing the relationship between the
amount of MgO in the annealing separator and the iron loss (W17/50).
20 [0034]
As shown in Fig. 2, when the amount of MgO in the annealing separator is 28%
by mass or more, mullite is not generated. On the other hand, as shown in Fig. 3, it is
understood that when the amount of MgO in the annealing separator is in the range of
28% by mass or more, the iron loss is less than 1.00 W/kg, and the effect of reducing the
25 iron loss is obtained, however, when the amount of MgO in the annealing separator
13
exceeds 50% by mass, the iron loss becomes 1.00 W/kg or more, which is inferior.
[0035]
To clarify the cause, the surface of the obtained steel sheet was analyzed by XRD.
As a result, it was confirmed that forsterite is detected at the level at which the amount of
MgO is 54% by mass or more, and that the X 5 RD peak height of forsterite increases as the
amount of MgO increases. From this, it is considered that when the amount of MgO in
the annealing separator exceeds 50% by mass, mullite is not generated (see Fig. 2), but
on the other hand, forsterite is generated and the iron loss characteristics become inferior.
[0036]
10 Next, the annealing separator containing alumina as a main component and 45%
by mass of MgO was applied to the decarburization annealed sheet having a sheet
thickness of 0.23 mm. The application amount of the annealing separator was varied in
the range of 5.0 to 15.0 g/m2 per one surface. A plurality of the decarburization annealed
sheets to which the annealing separator is applied for drying were stacked and subjected
15 to the final annealing to prepare grain-oriented electrical steel sheets.
[0037]
After an excess annealing separator was removed from the grain-oriented
electrical steel sheet obtained in such a manner by the washing with water, a 20 mm
square test piece was taken, and a cross section (a C cross section) orthogonal to a rolling
20 direction of the test piece was polished with a diamond buff. After that, the test piece
was observed at a magnification of 1000 times using an optical microscope, and the
number of needle-like inclusions each having a length of 1 μm or more which are present
in a region (an observation region) having a length of 10 μm from the surface of the steel
sheet toward the inside of the base steel sheet in the sheet thickness direction of the base
25 steel sheet and a length of 20 mm in a sheet width direction of the base steel sheet was
14
measured.
[0038]
The results are shown in Fig. 4. Fig. 4 is a diagram showing the relationship
between the application amount of the annealing separator per one surface and the
number of pieces of mull 5 ite. From Fig. 4, it is understood that when the application
amount of the annealing separator per one surface is less than 6.0 g/m2, “a needle-like
inclusion (mullite) having a length of 1 μm or more” is generated.
[0039]
According to the ternary phase diagram of Al2O3-MgO-SiO2 shown in Fig. 1,
10 when MgO is present in a proportion of 50 mol% (28% by mass) or more with respect to
alumina, mullite is not generated, and thus in a case in which the addition amount of
MgO is 45% by mass, mullite will not be generated. However, as shown in Fig. 4, when
the application amount of the annealing separator containing 45% by mass of MgO per
one surface is less than 6.0 g/m2, “a needle-like inclusion (mullite) having a length of 1
15 μm or more” is generated. The reason for this can be considered as follows.
[0040]
(x) When the application amount of the annealing separator is small, the adsorption and
removal of SiO2 by Al2O3 of the annealing separator become insufficient during the final
annealing.
20 (y) During the final annealing, Al generated by the decomposition of AlN (an inhibitor) is
added to the Al component of the annealing separator, the ratio of MgO in the annealing
separator is relatively reduced, and the component composition of the annealing
separator is shifted to a mullite generation region (see Fig. 1).
[0041]
25 Therefore, it is important to sufficiently adsorb and remove SiO2 with Al2O3 of
15
the annealing separator during the final annealing in suppressing the generation of
mullite, and for that purpose, it is necessary not only to control the addition amount of
MgO in the annealing separator to 28% by mass or more but also to control the
application amount of the annealing separator to 6.0 g/m2 or more. When the application
amount of the annealing separator exceeds 14.0 5 g/m2, the application effect becomes
saturated and the manufacturing cost increases, and thus the application amount of the
annealing separator is set to 14.0 g/m2 or less.
[0042]
As described above, the present inventors have found that by controlling the
10 addition amount of MgO in the annealing separator containing alumina as a main
component and the application amount of the annealing separator to a specific range, it is
possible to suppress the generation of the needle-like inclusions (mullite) in the surface
layer region of the base steel sheet of the grain-oriented electrical steel sheet, and thus it
is possible to realize the reduction of the iron loss of the grain-oriented electrical steel
15 sheet.
Based on the above-mentioned study results by the present inventors, the present
manufacturing method is characterized in that the following two manufacturing
conditions are satisfied.
(Condition 1) The annealing separator containing alumina as a main component contains
20 28% to 50% by mass of MgO.
(Condition 2) An application amount of the annealing separator is 6.0 to 14.0 g/m2 per
one surface of the decarburization annealed sheet.
[0043]
Hereinafter, the above-mentioned features (manufacturing conditions) of the
25 present manufacturing method will be described.
16
[0044]

As shown in Fig. 2, when the amount of MgO in the annealing separator is 28%
by mass or more, mullite is not generated, and as shown in Fig. 3, the iron loss W17/50 is
less than 1.00 W/kg, which is s 5 uperior. Therefore, the amount of MgO in the annealing
separator is set to 28% by mass or more. It is preferably 32% by mass or more, and more
preferably 35% by mass or more.
[0045]
On the other hand, as shown in Fig. 3, when the amount of MgO in the annealing
10 separator exceeds 50% by mass, the iron loss W17/50 becomes 1.00 W/kg or more, which
is inferior. Therefore, the amount of MgO in the annealing separator is set to 50% by
mass or less. It is preferably 48% by mass or less, and more preferably 45% by mass or
less.
[0046]
15
As shown in Fig. 4, when the application amount of the annealing separator
containing 45% by mass of MgO per one surface is less than 6.0 g/m2, “a needle-like
20 inclusion (mullite) having a length of 1 μm or more” is generated, and thus the adhesion
amount of per unit area on one surface after application and drying of the annealing
separator (the application amount of the annealing separator on one surface of the
decarburization annealed sheet) is set to 6.0 g/m2 or more. It is preferably 7.0 g/m2 or
more, and more preferably 8.0 g/m2 or more.
25 [0047]
17
On the other hand, when the application amount of the annealing separator
exceeds 14.0 g/m2, the application effect becomes saturated and the manufacturing cost
increases, and thus the application amount of the annealing separator is set to 14.0 g/m2
or less. It is preferably 13.0 g/m2 or less, and more preferably 12.0 g/m2 or less.
5 [0048]
Next, basic processes of the present manufacturing method will be described.
[0049]
Molten steel having a predetermined chemical composition is cast by a usual
method to obtain a silicon steel slab. The chemical composition of the silicon steel slab
10 is not limited to a specific composition as long as the magnetic characteristics and the
mechanical characteristics required for the grain-oriented electrical steel sheet can be
obtained, but an example of the chemical composition of the silicon steel slab is as
follows. For example, the silicon steel slab contains, as a chemical composition, in % by
mass, C: 0.085% or less, Si: 0.80% to 7.00%, Mn: 0.05% to 1.00%, acid-soluble Al:
15 0.010% to 0.065%, N: 0.004% to 0.012%, S: 0.01% or less, and B: 0.0005% to 0.0080%.
[0050]
C: 0.085% or less
C is an element effective for controlling a primary recrystallization structure, but
it adversely affects the magnetic characteristics, and thus it is to be removed by the
20 decarburization annealing before the final annealing. When the amount of C exceeds
0.085%, the decarburization annealing time becomes long and the productivity decreases,
and thus the amount of C is set to 0.085% or less. The amount of C is preferably 0.070%
or less, and more preferably 0.050% or less.
[0051]
25 The lower limit of the amount of C includes 0%, but when the amount of C is
18
reduced to less than 0.0001%, the manufacturing cost increases significantly, and thus
0.0001% is substantially the lower limit of the amount of C in the practical steel sheet.
In the grain-oriented electrical steel sheet, the amount of C is usually reduced to about
0.001% or less by the decarburization annealing.
5 [0052]
Si: 0.80% to 7.00%
Si is an element that increases the electrical resistance of the steel sheet and
improves the iron loss characteristics. When the amount of Si is less than 0.80%, γ
transformation occurs during the final annealing and the crystal orientation of the steel
10 sheet is impaired, and thus the amount of Si is set to 0.80% or more. The amount of Si is
preferably 1.50% or more, and more preferably 2.50% or more.
[0053]
On the other hand, when the amount of Si exceeds 7.00%, the workability is
lowered and cracks occur during rolling, and thus the amount of Si is set to 7.00% or
15 less. The amount of Si is preferably 5.50% or less, and more preferably 4.50% or less.
[0054]
Mn: 0.05% to 1.00%
Mn is an element that prevents cracking during hot rolling and combines with S
and/or Se to form MnS that functions as an inhibitor. When the amount of Mn is less
20 than 0.05%, the addition effect is not sufficiently exhibited, and thus the amount of Mn is
set to 0.05% or more. The amount of Mn is preferably 0.07% or more, and more
preferably 0.09% or more.
[0055]
On the other hand, when the amount of Mn exceeds 1.00%, precipitation and
25 dispersion of MnS become non-uniform, a required secondary recrystallization structure
19
cannot be obtained, and a magnetic flux density decreases, and thus the amount of Mn is
set to 1.00% or less. The amount of Mn is preferably 0.80% or less, and more preferably
0.06% or less.
[0056]
5 Acid-soluble Al: 0.010% to 0.065%
Acid-soluble Al is an element that combines with N to generate (Al, Si) N that
functions as an inhibitor. When the amount of acid-soluble Al is less than 0.010%, the
addition effect is not sufficiently exhibited and secondary recrystallization does not
proceed sufficiently, and thus the amount of acid-soluble Al is set to 0.010% or more.
10 The amount of acid-soluble Al is preferably 0.015% or more, and more preferably
0.020% or more.
[0057]
On the other hand, when the amount of acid-soluble Al exceeds 0.065%,
precipitation and dispersion of (Al, Si) N become non-uniform, a required secondary
15 recrystallization structure cannot be obtained, and a magnetic flux density decreases, and
thus the amount of soluble Al is set to 0.065% or less. The amount of acid-soluble Al is
preferably 0.050% or less, and more preferably 0.040% or less.
[0058]
N: 0.004% to 0.012%
20 N is an element that combines with Al to form AlN that functions as an inhibitor,
but on the other hand, it is also an element that forms blisters (voids) in the steel sheet
during the cold rolling. When the amount of N is less than 0.004%, the formation of AlN
is insufficient, and thus the amount of N is set to 0.004% or more. The amount of N is
preferably 0.006% or more, and more preferably 0.007% or more.
25 [0059]
20
On the other hand, when the amount of N exceeds 0.012%, there is a concern that
blisters (voids) may be generated in the steel sheet during the cold rolling, and thus the
amount of N is set to 0.012% or less. The amount of N is preferably 0.010% or less, and
more preferably 0.009% or less.
5 [0060]
S: 0.01% or less
S is an element that combines with Mn to form MnS that functions as an inhibitor.
[0061]
When the amount of S exceeds 0.01%, precipitation and dispersion of MnS
10 become non-uniform after purification, a desired secondary recrystallization structure
cannot be obtained, a magnetic flux density decreases, and a hysteresis loss deteriorates,
or MnS after purification remains, and the hysteresis loss deteriorates. The lower limit is
not particularly set, but the amount of S is preferably 0.003% or more. The amount of S
is more preferably 0.007% or more.
15 [0062]
B: 0.0005% to 0.0080%
B is an element that combines with N and complex-precipitates with MnS to form
BN that functions as an inhibitor.
[0063]
20 When the amount of B is less than 0.0005%, the addition effect is not sufficiently
exhibited, and thus the amount of B is set to 0.0005% or more. The amount of B is
preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand,
when the amount of B exceeds 0.0080%, precipitation and dispersion of BN become nonuniform,
a required secondary recrystallization structure cannot be obtained, and a
25 magnetic flux density decreases, and thus the amount of B is set to 0.0080% or less. The
21
amount of B is preferably 0.0060% or less, and more preferably 0.0040% or less.
[0064]
In the silicon steel slab, the remainder excluding the above elements is Fe and
impurities. Impurities are elements that are inevitably mixed in from a steel raw material
and/or in a steelmaking process and 5 are acceptable elements as long as they do not impair
the characteristics of the grain-oriented electrical steel sheet.
[0065]
Further, the silicon steel slab may contain one or two or more of Cr: 0.30% or
less, Cu: 0.40% or less, P: 0.50% or less, Ni: 1.00% or less, Sn: 0.30% or less, Sb: 0.30%
10 or less, and Bi: 0.01% or less within a range in which the magnetic characteristics of the
grain-oriented electrical steel sheet are not impaired and other characteristics can be
enhanced. Since these elements do not have to be contained, the lower limit is 0.
[0066]
In the hot rolling process, hot rolling is executed on a slab having the above
15 chemical composition to obtain a hot-rolled sheet. Hot rolling conditions are not
particularly limited, and ordinary conditions can be used. The hot-rolled sheet obtained
by the hot rolling process is wound in a coil shape.
[0067]
Before the slab is subjected to hot rolling, the slab may be heated to a temperature
20 of more than 1300°C to sufficiently make inhibitor components of MnS and AlN into
solid solution. Further, from the viewpoint of productivity and manufacturing cost, the
slab may be heated to about 1250°C on the premise that the inhibitor is enhanced by
nitriding treatment in the subsequent step.
[0068]
25 In the hot-rolled sheet annealing process, the hot-rolled sheet in a coil shape is
22
rewound to a strip-shaped hot-rolled sheet, and then the hot-rolled sheet annealing is
executed on the strip-shaped hot-rolled sheet to obtain an annealed hot-rolled sheet. Hotrolled
sheet annealing conditions are not particularly limited, and ordinary conditions can
be used. In the cold rolling process, the cold rolling is executed on the annealed hotrolled
sheet once or twice or more to obtain a 5 cold-rolled sheet having a final sheet
thickness. In this cold rolling process, the cold rolling may be executed on the annealed
hot-rolled sheet two or more times with intermediate annealing interposed therebetween
to obtain a cold-rolled sheet. In the annealing performed before the final (last) cold
rolling, homogenization of the crystal structure is performed. Cold rolling conditions are
10 not particularly limited, and ordinary conditions can be used.
[0069]
In the decarburization annealing process, the decarburization annealing is
executed on the cold-rolled sheet to obtain a decarburization annealed sheet. In this
decarburization annealing process, by heat-treatment of the cold-rolled sheet in wet
15 hydrogen, the amount of C in the cold-rolled sheet is reduced to an amount that does not
cause deterioration due to magnetic aging in a product steel sheet, and primary
recrystallization occurs in the cold-rolled sheet to prepare for the next secondary
recrystallization. Decarburization annealing conditions are not particularly limited, and
ordinary conditions can be used. An oxide film of SiO2 is formed on the surface of the
20 decarburization annealed sheet obtained by such a decarburization annealing process. In
a case in which a cold-rolled sheet is manufactured from a slab heated to about 1250°C,
after the decarburization annealing, the decarburization annealed sheet is annealed in an
ammonia atmosphere to generate AlN that functions as an inhibitor in the decarburization
annealed sheet.
25 [0070]
23
In the annealing separator applying process, an annealing separator containing
alumina (Al2O3) as a main component is applied to the decarburization annealed sheet for
the purpose of removing SiO2 present on the surface of the decarburization annealed
sheet and preventing seizure in the final annealing process. The annealing separator
containing 5 alumina as a main component contains 28% to 50% by mass of MgO, and the
application amount of the annealing separator is 6.0 to 14.0 g/m2 per one surface of the
decarburization annealed sheet. The decarburization annealed sheet to which the
annealing separator is applied is wound in a coil shape after the annealing separator is
dried.
10 [0071]
As described above, by controlling the amount (the addition amount) of MgO of
the annealing separator containing alumina as a main component to 28% to 50% by mass,
and by controlling the application amount of the annealing separator to 6.0 to 14.0 g/m2
per one surface of the decarburization annealed sheet, it is possible to suppress the
15 generation of the needle-like inclusions (mullite) in the surface layer region of the
decarburization annealed sheet during the final annealing of the decarburization annealed
sheet in the subsequent final annealing process. Further, as a result, it is possible to
reduce the iron loss W17/50 of a final product to a low value of less than 1.00 W/kg.
[0072]
20 Further, to more effectively suppress the generation of the needle-like inclusions
(mullite), preferably, the BET specific surface area of alumina, which is a main
component of the annealing separator, is controlled to 3.0 to 10.0 m2/g. If the BET
specific surface area of alumina is less than 3.0 m2/g, it is difficult to sufficiently adsorb
and remove SiO2, and thus the BET specific surface area of alumina is preferably 3.0
25 m2/g or more. It is more preferably 5.0 m2/g or more.
24
[0073]
On the other hand, if the BET specific surface area of alumina exceeds 10.0 m2/g,
the viscosity of aqueous slurry of the annealing separator increases, application spots are
generated, and a portion where SiO2 cannot be sufficiently adsorbed and removed occurs,
and thus the BET specific surface area 5 of alumina is preferably 10.0 m2/g or less. It is
more preferably 8.0 m2/g or less.
[0074]
In the final annealing process, a base steel sheet of a final product (a grainoriented
electrical steel sheet) is obtained by executing the final annealing on the
10 decarburization annealed sheet in a coil shape to which the annealing separator is
applied. In this final annealing process, secondary recrystallization occurs in the
decarburization annealed sheet by performing the final annealing at a temperature of
1100°C or higher. Final annealing conditions are not particularly limited, and ordinary
conditions can be used. To reduce the hysteresis loss of the final product, purifying and
15 annealing may be executed on the decarburization annealed sheet after the completion of
the secondary recrystallization such that precipitate used as an inhibitor is detoxified.
[0075]
Al moves from the inside of the decarburization annealed sheet toward the
surface during the final annealing, but the amount of MgO of the annealing separator is
20 controlled to 28% to 50% by mass, and the application amount of the annealing separator
is controlled to 6.0 to 14.0 g/m2 per one surface of the decarburization annealed sheet,
and thus it is possible to prevent Al from reacting with SiO2 remaining on the surface of
the decarburization annealed sheet. Further, as a result, it is possible to suppress the
generation of the needle-like inclusions (mullite) in the surface layer region of the
25 decarburization annealed sheet during the final annealing. Further, since the amount of
25
MgO of the annealing separator is limited to 50% by mass or less, it is possible to
suppress the formation of a forsterite coating on the surface of the decarburization
annealed sheet during the final annealing.
[0076]
The needle-like inclusions (mullite) a 5 re not generated in the surface layer region
of the base steel sheet (decarburization annealed sheet after the final annealing) obtained
by the present manufacturing method as described above, and the forsterite coating is not
present on the surface of the base steel sheet. That is, according to the present
manufacturing method, it is possible to obtain a base steel sheet in which two factors that
10 hinder the movement of the magnetic wall are eliminated. Therefore, in a case in which
a grain-oriented electrical steel sheet in which a forsterite coating is not present between
the base steel sheet and the tension-insulation coating is obtained as a final product by the
formation of the tension-insulation coating on the surface of the base steel sheet after the
final annealing process, it is possible to obtain a grain-oriented electrical steel sheet
15 having a lower iron loss as compared with in the related art.
[Examples]
[0077]
Next, examples of the present invention will be described, however, the
conditions in the examples are one condition example employed for confirming the
20 feasibility and effect of the present invention, and the present invention is not limited to
this one condition example. In the present invention, various conditions can be employed
as long as the gist of the present invention does not deviate and the object of the present
invention is achieved.
[0078]
25 (Example 1)
26
A slab having the composition shown in Table 1 was heated to 1100 °C and
subjected to hot rolling to obtain a hot-rolled sheet having a sheet thickness of 2.60 mm,
hot-rolled sheet annealing was executed on the hot-rolled sheet at 1100°C, and then the
hot-rolled sheet was subjected to cold rolling a plurality of times with intermediate
annealing interposed therebetween to be wound a 5 s a cold-rolled sheet having a final sheet
thickness of 0.23 mm.
[0079]
[Table 1]
Steel No.
Chemical composition of steel slab (% by mass)
C Si Mn Al N S B
A1 0.085 3.45 0.10 0.028 0.0040 0.008 0.0015
A2 0.031 1.21 0.10 0.029 0.0100 0.009 0.0020
A3 0.033 6.52 0.10 0.029 0.0100 0.007 0.0018
A4 0.041 3.45 0.08 0.028 0.0070 0.005 0.0019
A5 0.044 3.33 0.80 0.029 0.0060 0.004 0.0021
A6 0.052 4.52 0.12 0.020 0.0050 0.003 0.0016
A7 0.055 3.12 0.09 0.055 0.0017 0.001 0.0017
A8 0.061 2.81 0.09 0.030 0.0120 0.009 0.0018
A9 0.062 3.12 0.11 0.030 0.0040 0.001 0.0019
A10 0.071 2.92 0.13 0.030 0.0050 0.001 0.0021
[0080]
10 The cold-rolled sheet was rewound, decarburization annealing was executed on
the cold-rolled sheet at 820°C in a moist atmosphere with 75% hydrogen, 25% nitrogen,
and a dew point of 40°C, and then nitrification annealing was executed on a
decarburization annealed sheet for the purpose of the formation of an inhibitor AlN in the
decarburization annealed sheet. Then, aqueous slurry of an annealing separator
15 containing alumina having a BET specific surface area of 3.0 to 10.0 m2/g as a main
component and MgO in an amount of 0% to 80% by mass was applied to a surface of the
decarburization annealed sheet while the application amount per one surface is changed
in the range of 5.0 to 15.0 g/m2, and the decarburization annealed sheet was wound in a
27
coil shape.
[0081]
Final annealing was executed on the decarburization annealed sheet in a coil
shape, to which the above annealing separator was applied to be dried, at 1200°C for 20
hours. An excess annealing separator 5 was removed from the base steel sheet obtained
after the final annealing by the washing with water to obtain a base steel sheet of a grainoriented
electrical steel sheet that has no forsterite coating, has mirror gloss, and has
completed secondary recrystallization.
[0082]
10 A test piece having a 20 mm square was taken from the central portion in a width
direction of the outermost circumference of the grain-oriented electrical steel sheet (base
steel sheet) in a coil shape obtained in such a manner. A cross section (a C cross section)
orthogonal to a rolling direction of the test piece was polished with a diamond buff. A
cross section of one side (20 mm) of the test piece was observed with an optical
15 microscope (1000 times), and the number of needle-like inclusions each having a length
of 1 μm or more present in an observation region having a sheet thickness direction
length of 10 μm and a sheet width direction length of 20 mm was measured. Further, the
iron loss W17/50 of the test piece was measured according to JIS C 2550. The results are
shown in Table 2.
20 [0083]
[Table 2]
No. No.
Steel
No.
Annealing separator Needle-like
inclusion of
1 μm or more
(number/20
mm)
Iron
loss
W17/50
(W/kg)
Presence
or
absence
of
forsterite
Amount of
MgO
(% by mass)
Application
amount
(g/m2)
Invention B1 A1 28 6.0 0 0.98 Absence
28
Example B2 A2 32 6.4 0 0.89 Absence
B3 A3 34 7.1 0 0.87 Absence
B4 A4 35 7.8 0 0.85 Absence
B5 A5 36 8.2 0 0.84 Absence
B6 A6 38 9.0 0 0.83 Absence
B7 A7 40 9.5 0 0.81 Absence
B8 A8 45 10.8 0 0.82 Absence
B9 A9 48 11.1 0 0.84 Absence
B10 A10 50 12.0 0 0.82 Absence
Comparative
Example
b1 A3 0 10.0 54 1.43 Absence
b2 A3 12 10.0 38 1.20 Absence
b3 A3 22 10.0 2 1.24 Absence
b4 A3 60 10.0 0 1.21 Presence
b5 A3 80 10.0 0 1.48 Presence
b6 A3 28 5 49 1.24 Absence
b7 A3 32 4.2 42 1.19 Absence
b8 A3 34 4.5 38 1.17 Absence
b9 A3 35 4.8 14 1.14 Absence
b10 A3 36 4.9 10 1.01 Absence
b11 A3 38 5.1 20 1.11 Absence
b12 A3 40 5 12 1.08 Absence
b13 A3 45 5.1 39 1.21 Absence
b14 A3 48 4.9 58 1.32 Absence
b15 A3 50 5.2 41 1.24 Absence
[0084]
As shown in Table 2, in Invention Examples B1 to B10, the amount of MgO of
the annealing separator is controlled in the range of 28% by mass to 50% by mass, and
the application amount of the annealing separator is controlled in the range of 6.0 to 14.0
g/m2 per one surface, and as a result, the needle-5 like inclusions (mullite) each having a
length of 1 μm or more are not present in the observation area of the base steel sheet, and
the iron loss W17/50 was suppressed to less than 1.00 W/kg.
[0085]
As shown in Table 2, in Comparative Examples b1 to b3, the application amount
10 of the annealing separator is controlled in the range of 6.0 to 14.0 g/m2 per one surface,
but the amount of MgO of the annealing separator is less than 28% by mass, and thus a
plurality of needle-like inclusions (mullite) each having a length of 1 μm or more are
present in the observation region of the base steel sheet, and the iron loss W17/50 increased
to more than 1.00 W/kg.
29
In Comparative Examples b4 and b5, the application amount of the annealing
separator is controlled in the range of 6.0 to 14.0 g/m2 per one surface, but the amount of
MgO of the annealing separator is more than 50% by mass. In this case, the needle-like
inclusions (mullite) each having a length of 1 μm or more are not present in the
observation region of the base 5 steel sheet, but forsterite is generated, and as a result, the
iron loss W17/50 increased to more than 1.00 W/kg.
In Comparative Example b6, the amount of MgO of the annealing separator is
28% by mass or more, but the application amount of the annealing separator is less than
6.0 g/m2 per one surface, and thus a plurality of needle-like inclusions (mullite) each
10 having a length of 1 μm or more are present in the observation region of the base steel
sheet, and the iron loss W17/50 increased to more than 1.00 W/kg.
In Comparative Examples b7 to b15, the amount of MgO of the annealing
separator is controlled in the range of 28% by mass to 50% by mass, but the application
amount of the annealing separator is less than 6.0 g/m2 per one surface, and thus a
15 plurality of needle-like inclusions (mullite) each having a length of 1 μm or more are
present in the observation region of the base steel sheet, and the iron loss W17/50 increased
to more than 1.00 W/kg.
[0086]
(Example 2)
20 A slab having the composition of Steel No. A5 shown in Table 1 was heated to
1100 °C and subjected to hot rolling to obtain a hot-rolled sheet having a sheet thickness
of 2.60 mm, hot-rolled sheet annealing was executed on the hot-rolled sheet at 1100°C,
and then the hot-rolled sheet was subjected to cold rolling a plurality of times with
intermediate annealing interposed therebetween to be wound as a cold-rolled sheet
25 having a final sheet thickness of 0.23 mm.
30
[0087]
The cold-rolled sheet was rewound, decarburization annealing was executed on
the cold-rolled sheet at 820°C in a moist atmosphere with 75% hydrogen, 25% nitrogen,
and a dew point of 40°C, and then nitrification annealing was executed on a
decarburization a 5 nnealed sheet for the purpose of the formation of an inhibitor Al in the
decarburization annealed sheet.
[0088]
Then, aqueous slurry of an annealing separator containing alumina having a BET
specific surface area changed in the range of 3.0 to 10.0 m2/g as a main component and
10 MgO in an amount of 35% to 48% by mass was applied to a surface of the
decarburization annealed sheet while the application amount per one surface is changed
in the range of 8.2 to 11.2 g/m2, and the decarburization annealed sheet was wound in a
coil shape.
[0089]
15 Final annealing was executed on the decarburization annealed sheet in a coil
shape, to which the above annealing separator was applied to be dried, at 1200°C for 20
hours. An excess annealing separator was removed from the base steel sheet obtained
after the final annealing by the washing with water to obtain a base steel sheet of a grainoriented
electrical steel sheet that has no forsterite coating, has mirror gloss, and has
20 completed secondary recrystallization.
[0090]
A test piece having a 20 mm square was taken from the central portion in a width
direction of the outermost circumference of the grain-oriented electrical steel sheet (base
steel sheet) in a coil shape obtained in such a manner. A cross section (a C cross section)
25 orthogonal to a rolling direction of the test piece was polished with a diamond buff. A
31
cross section of one side (20 mm) of the test piece was observed with an optical
microscope (1000 times), and the number of needle-like inclusions each having a length
of 1 μm or more present in an observation region having a sheet thickness direction
length of 10 μm and a sheet width direction length of 20 mm was measured. Further, the
iron loss W17/50 of the test piece was measured 5 according to JIS C 2550. The results are
shown in Table 3.
[0091]
[Table 3]
No. No.
Steel
No.
Annealing separator BET
specific
surface
area of
alumina
(m2/g)
Needle-like
inclusion of
1 μm or
more
(number/20
mm)
Iron
loss
W17/50
(W/kg)
Presence
or
absence
of
forsterite
Amount
of MgO
(% by
mass)
Application
amount
(g/m2)
Invention
Example
C1 A5 35 8.2 3.0 0 0.88 Absence
C2 A5 38 9.8 4.8 0 0.84 Absence
C3 A5 42 10.1 6.2 0 0.80 Absence
C4 A5 45 10.8 7.5 0 0.77 Absence
C5 A5 48 11.2 10.0 0 0.72 Absence
[0092]
10 As shown in Table 3, it is understood that it is possible to significantly reduce the
iron loss W17/50 by controlling the amount of MgO of the annealing separator in the range
of 28% by mass to 50% by mass, by controlling the application amount of the annealing
separator in the range of 6.0 to 14.0 g/m2 per one surface, and by controlling the BET
specific surface area of alumina, which is a main component of the annealing separator,
15 to 3.0 to 10.0 m2/g. It is considered that this is because the needle-like inclusions are not
generated and the amount of SiO2 adsorbed by alumina increases.
[Industrial Applicability]
[0093]
According to the present invention, it is possible to reduce the iron loss of the
20 grain-oriented electrical steel sheet in which the forsterite coating is not present between
32
the base steel sheet and the tension-insulation coating as compared with in the related art.
Therefore, the present invention is highly applicable in the electrical steel sheet
manufacturing industry and the electrical steel sheet utilization industry.

WE CLAIMS

1. A method of manufacturing a grain-oriented electrical steel sheet comprising:
a process of executing hot rolling on a slab to obtain a hot-rolled sheet;
a process of executing hot-rolled sheet 5 annealing on the hot-rolled sheet to obtain
an annealed hot-rolled sheet;
a process of executing cold rolling on the annealed hot-rolled sheet to obtain a
cold-rolled sheet;
a process of executing decarburization annealing on the cold-rolled sheet to
10 obtain a decarburization annealed sheet;
a process of applying an annealing separator containing alumina as a main
component to the decarburization annealed sheet; and
a process of executing final annealing on the decarburization annealed sheet to
which the annealing separator is applied,
15 wherein the annealing separator contains 28% to 50% by mass of MgO, and
wherein an application amount of the annealing separator is 6.0 to 14.0 g/m2 per
one surface of the decarburization annealed sheet.
2. The method of manufacturing a grain-oriented electrical steel sheet according to claim
20 1, wherein a BET specific surface area of the alumina is 3.0 to 10.0 m2/g.
3. The method of manufacturing a grain-oriented electrical steel sheet according to claim
1 or 2,
wherein the slab contains, as a chemical composition, in % by mass,
25 C: 0.085% or less,
34
Si: 0.80% to 7.00%,
Mn: 0.05% to 1.00%,
acid-soluble Al: 0.010% to 0.065%,
S: 0.01% or less,
5 N: 0.004% to 0.012%,
B: 0.0005% to 0.0080%,
P: 0% to 0.50%,
Ni: 0% to 1.00%,
Sn: 0% to 0.30%,
10 Sb: 0% to 0.30%,
Cu: 0% to 0.40%,
Cr: 0% to 0.30%,
Bi: 0% to 0.01%, and
the remainder of Fe and impurities.