[0001]The present invention relates to a coating agent for forming a grain-oriented electrical steel
sheet coating and a method for manufacturing grain-oriented electrical steel sheet.
BACKGROUND
[0002]
15 Grain-oriented electrical steel sheet has a texture having 110<001> as its main
orientation and is extensively used as a material for cores of transformers. In particular, a
material with a small iron loss is required so as to reduce energy loss.
[0003]
In PTL 1, as a means for reducing the iron loss of grain-oriented electrical steel sheet, the
20 method of laser beam radiation over a surface of steel sheet after batch annealing to impart local
strain and thereby subdivide the magnetic domains is disclosed.
[0004]
In PTL 2, a means for subdividing the magnetic domains where this effect is not lost even
after stress relief annealing after working the cores (stress relieving annealing) is disclosed.
25 [0005]
On the other hand, an iron alloy containing iron and silicon has a large magnetic anisotropy,
so if imparting external tension, subdivision of the magnetic domains occurs and the eddy
current loss, the main element in iron loss, is made to decrease. In particular, to reduce the iron
loss of grain-oriented electrical steel sheet containing 5% or less of silicon, it is known to be
30 effective to impart tension to the steel sheet. This tension is imparted by the coating formed on
the surface.
[0006]
Grain-oriented electrical steel sheet is given tension of about 10 MPa in the case of a
thickness of 0.23 mm by two layers of coatings: a primary coating comprised of mainly forsterite
35 formed by a reaction of oxides on the surface of the steel sheet and an annealing separator in the
batch annealing process and a secondary coating comprised of mainly amorphous solids formed
2
by annealing a coating solution comprised of mainly colloidal silica and a phosphate disclosed in
PTL 3 etc.
[0007]
In the case of the above such conventional coating, while it is possible to increase the
coating weight to impart a f 5 urther larger tension and the possibility remains of improvement of
iron loss by increase of tension, making the coating thicker than the present so as to increase the
tension imparted is not preferable since it causes a decrease in the lamination factor. For this
reason, a coating excellent in adhesion and able to impart a large tension to steel sheet thinly
without causing a decrease in the lamination factor has been desired.
10 [0008]
As opposed to this, in PTL 4, grain-oriented electrical steel sheet having a coating mainly
comprised of aluminum borate crystals on its surface has been proposed.
In order for a certain coating to become a high tension coating, a high Young’s modulus
and a small coefficient of thermal expansion of the coating are required. In general, crystals are
15 higher in Young’s modulus than amorphous solids. A coating comprised of aluminum borate is
higher in Young’s modulus than a conventional amorphous coating comprised of silica and a
phosphate since the main constituent is crystalline. A coating comprised of aluminum borate is
also sufficiently low in coefficient of thermal expansion, so together with the effect of the
Young’s modulus, can give a higher tension than a coating such as disclosed in PTL 3.
20 However, in the art of PTL 4, the solids concentration of the coating solution for forming
the coating is low, so there was the problem that bubbling occurred at the time of drying and
annealing on the coating and coating defects were formed.
[0009]
To prevent such coating defects, in PTL 5, it is disclosed that it is necessary to raise the
25 concentration of an aluminum oxide precursor so as to raise the solids concentration. A method
combining the three conditions of addition, strong stirring, and warming of a glue to stabilize the
viscosity of the coating solution becoming a problem at that time is described. Note that if
raising the concentration of boric acid, gelling is caused, so it is shown that increasing the
amount of boric acid is not preferable. In PTL 6, a coating agent for forming grain-oriented
30 electrical steel sheet containing compounds forming boron and aluminum sources, an organic
solvent having compatibility with water, and water is disclosed. Further, in PTL 7, a method
using a particulate dispersion with a high solids concentration as the coating solution for the
purpose of raising the rate of temperature rise at the time of drying the coating solution is
disclosed. That is, this method is a method of using a particulate dispersion using 12 to 26 wt%
35 of soluble boric acid converted to boron oxide and raising the temperature in the temperature
region where boric acid precipitates by a relatively fast rate so as to inhibit the formation of
3
coarse boric acid crystals at the time of drying after coating the dispersion. However, boric acid
is soluble in water if in a concentration of its solubility or less, but with the method of PTL 7, a
solution containing boric acid in a concentration above its solubility is used, so unavoidably
undissolved boric acid is present in the particulate dispersion. If undissolved boric acid is present
in the particulate 5 dispersion, the undissolved boric acid will easily settle, so it will become
difficult to maintain a state where the coating solution is uniformly mixed and as a result
obtaining a high tension coating will become difficult. A coating solution which easily settles is
an unstable coating solution. Inconveniences arise when using this for production.
10 [CITATIONS LIST]
[PATENT LITERATURE]
[0010]
[PTL 1] Japanese Unexamined Patent Publication No. 58-26405
[PTL 2] Japanese Unexamined Patent Publication No. 62-86175
15 [PTL 3] Japanese Unexamined Patent Publication No. 48-39338
[PTL 4] Japanese Unexamined Patent Publication No. 6-65754
[PTL 5] Japanese Unexamined Patent Publication No. 9-263951
[PTL 6] Japanese Unexamined Patent Publication No. 7-278828
[PTL 7] Japanese Unexamined Patent Publication No. 9-272983
20
SUMMARY
[TECHNICAL PROBLEM]
[0011]
as mentioned above, a coating comprised of aluminum borate is sufficiently low in
25 coefficient of thermal expansion, so can give a higher tension than a conventional amorphous
coating comprised of silica and a phosphate. However, a coating agent for forming an aluminum
borate coating had the weak point of being low in solids concentration. Specifically, the solids
concentration in a conventional coating agent of a tension coating comprised of a phosphate and
amorphous silica is 20 mass% or so, but with a coating agent for forming an aluminum borate
30 coating, raising the solids concentration to 10 mass% or so was the limit.
[0012]
If the solids concentration of the coating agent is low, if trying to secure the required
coating thickness, the problem arises that the drying process for removing the moisture takes
time. If rapidly raising the temperature so as to shorten the drying time, there was the problem
35 that coating defects formed due to bubbling etc. If such coating defects are formed, the tension
decreases and the adhesion of the coating to the base metal steel sheet decreases. The inventors
4
studied this and clarified that to solve such a problem, it is necessary to make the solids
concentration of the coating agent for forming the aluminum borate coating the same extent as a
conventional coating agent of a tension coating comprised of a phosphate and amorphous silica.
[0013]
In the art described in PTL 5 for raising 5 the solids concentration of an aluminum borate
coating solution, it was only possible to raise the solids concentration of the aluminum borate
coating solution to approximately 19 mass% as maximum. On the other hand, in the art
described in PTL 6, sometimes gas was generated derived from the organic solvent at the time of
drying the coating solution. Coating defects were liable to be induced. Further, in the method in
10 PTL 7, it was not possible to obtain a stable coating solution with a high solids concentration.
[0014]
The present invention has as its object the provision of a coating agent for forming a grainoriented
electrical steel sheet able to form an aluminum borate coating high in adhesion and
large in tension and a method for manufacturing the grain-oriented electrical steel sheet.
15
[SOLUTION TO PROBLEM]
[0015]
The inventors discovered that to obtain a coating solution of a solids concentration
sufficient for suppressing coating defects, a high concentration boric acid solution may be used
20 and that the problem of gelling in the high concentration boric acid solution in PTL 5 can be
avoided by adjusting the pH of the coating solution. Further, they discovered that to raise the
solids concentration, it is possible to use an aqueous solution of boric acid dissolved in heated
water.
The present invention was made based on the above findings and has as its gist the
25 following:
[0016]
(1) A coating agent for forming a grain-oriented electrical steel sheet coating comprising
an aluminum source containing aluminum oxide and/or an aluminum oxide precursor
compound,
30 a boron source containing a borate of an alkali metal, and
silicon oxide and/or a silicon oxide precursor in an amount, converted to silicon oxide, of 5
mass% or more and 10 mass% or less with respect to a total mass of the aluminum source and
boron source,
the aluminum source and the boron source contained so that the B contained in the boron
35 source and the Al contained in the aluminum source become, converted to molar ratio, Al/B: 0.5
to 2.0,
5
solid concentration of a total of the aluminum source and the boron source being 20 mass%
or more and 38 mass% or less, and
pH being 2.0 or more and 6.0 or less.
(2) The coating agent for forming the grain-oriented electrical steel sheet coating according to
(1) 5 , wherein the boron source contains boric acid.
(3) The coating agent for forming the grain-oriented electrical steel sheet coating according to
(1) or (2), wherein the alkali metal contains at least one of sodium and potassium.
(4) The coating agent for forming the grain-oriented electrical steel sheet coating according to
any one of (1) to (3), further containing one or more types of inorganic acids selected from the
10 group comprising nitric acid and hydrochloric acid and/or one or more types of organic acids
selected from the group comprising acetic acid, citric acid, and oxalic acid.
(5) A method for manufacturing grain-oriented electrical steel sheet having a process of
forming an aluminum borate coating using a coating agent for forming the grain-oriented
electrical steel sheet coating according to any one of (1) to (4).
15
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0017]
As mentioned above, according to the present invention, it is possible to provide a coating
agent for forming a grain-oriented electrical steel sheet coating able to form an aluminum borate
20 coating high in adhesion and large in tension and a method for manufacturing the grain-oriented
electrical steel sheet.
DESCRIPTION OF EMBODIMENT
[0018]
25 Below, the present invention will be mentioned in detail based on a preferred embodiment
of the present invention.
1. Coating Agent for Forming Grain-Oriented Electrical Steel Sheet Coating
First, a coating agent for forming the grain-oriented electrical steel sheet coating according
to the present embodiment (below, also simply called “the coating agent”) will be mentioned.
30 [0019]
Investigation of inventors
First, before explaining the constituents of the coating agent for forming the grain-oriented
electrical steel sheet coating according to the present embodiment, the studies of the inventors
leading to the present invention will be mentioned.
35 The inventors first thought to increase the solids concentration in the coating agent by using
a borate of an alkali metal having a high solubility in water. However, as mentioned above, the
6
inventors were confronted with the two problems that if increasing the concentration of boric
acid in the coating agent, the coating agent would gel and the adhesion would decrease.
[0020]
First, regarding the first problem of the gelling of the coating agent, it was surmised that
this occurred due to the alkali borate of an alkali m 5 etal being added to the acid-stable alumina
sol or other aluminum source and the environment of the alumina sol or other aluminum source
changing to a neutral side. If the coating agent gels before coating, a normal coating cannot be
formed, so it is necessary to avoid this. To avoid gelling due to the above such reasons, it is
sufficient to maintain the environment of dispersion or dissolution of the alumina sol or other
10 aluminum source at the acidic side. The inventors discovered that this can be solved by adding
an acid to the coating agent.
[0021]
Next, regarding the second problem of the deterioration of the adhesion, it was guessed that
this occurred due to addition of the alkali metal to the aluminum borate coating. This is believed
15 to have occurred due to splitting of glassy network believed to be present in the aluminum borate
coating in addition to the aluminum borate crystal substance. The inventors surmised that this
glassy substance was comprised of boric acid glass formed by the boron contained in excess in
the coating agent.
[0022]
20 The composition of the aluminum and boron in the coating agent can be designed so that
the amount of boron becomes larger than the stoichiometric composition of the aluminum borate
crystals. If increasing the amount of boron, the effect of improvement of the tension and the
effect of improvement of the adhesion are secured. In this case, the inventors surmised that the
excess boron formed a glassy substance in the aluminum borate coating and contributed to
25 securing adhesion of the coating and the steel sheet.
[0023]
If potassium or another monovalent metal element is contained in this glassy substance, the
network structure of the glass is destroyed and as a result it is believed that the adhesion of the
aluminum borate coating is impaired. The inventors thought that such a mechanism degrading
30 the adhesion was in action and tried to solve the problem by supplementing the glass forming
elements. They studied various means for solving this and as a result discovered that by
optimizing the ratio of the aluminum and boron in the coating solution and adding silicon oxide
into the aluminum borate coating, it was possible to secure adhesion. Specifically, they
discovered that by adjusting the ratio of the aluminum and boron in the coating agent to a
35 composition with an excess of boron compared with the past and adding a suitable quantity of
silicon oxide, an improvement in the adhesion could be achieved.
7
[0024]
Therefore, the coating agent for forming the grain-oriented electrical steel sheet coating
according to the present embodiment contains an aluminum source containing aluminum oxide
and/or an aluminum oxide precursor compound, a boron source containing a borate of an alkali
metal, silicon oxide and/5 or a silicon oxide precursor in 5 mass% or more and 10 mass% or less
converted to silicon oxide with respect to the total mass of the aluminum source and boron
source, and water, the aluminum source and the boron source contained so that the B contained
in the boron source and the Al contained in the aluminum source become, by molar ratio, Al/B:
0.5 to 2.0, the total solids concentration of the aluminum source and the boron source being 20
10 mass% or more and 38 mass% or less, and pH being 2.0 or more and 6.0 or less.
Below, the constituents etc. contained in the coating agent will be mentioned in detail.
[0025]
Aluminum source
The aluminum source of the coating agent contains aluminum oxide and/or an aluminum
15 oxide precursor compound. The aluminum oxide precursor compound is not particularly limited
so long as able to form aluminum oxide in the aluminum borate coating formed. For example, a
hydrate of aluminum oxide expressed by Al2O3 mH2O such as boehmite, aluminum hydroxide,
etc. may be mentioned. Among these, single types alone or two or more types in combination
may be used.
20 [0026]
The aluminum source may be dispersed in the coating agent, but it may also be dissolved in
the coating agent. Normally, the aluminum source is dispersed in the coating agent. The
aluminum source is preferably particle-like in form so as to stably disperse in the coating agent.
In this case, the average particle size (D50) based on the volume of the aluminum source by the
25 laser diffraction scattering method is, for example, 0.005 m or more and 1.0 m or less,
preferably 0.015 m or more and 0.7 m or less.
[0027]
Further, the aluminum source may be added to the coating agent in the form of a sol. By
using a particulate dispersion called a sol, a thin, uniform, and good adhesion aluminum borate
30 coating is obtained. As such a sol, for example, alumina sol, boehmite sol, etc. may be
mentioned. Boehmite sol and alumina sol are particularly suitable from the viewpoints of work
efficiency or price etc.
[0028]
Further, the content of the aluminum source in the coating agent is not particularly limited
35 so long as able to satisfy the later mentioned solids concentration and ratio with the boron
source, but for example can be made 1 mass% or more and 25 mass% or less, preferably 2
8
mass% or more and 20 mass% or less.
[0029]
Boron source
The boron source of the coating agent contains a borate of an alkali metal.
A borate of an alkali metal has an extremely 5 high solubility with respect to water or another
solvent of the coating agent and enables manufacture of a coating agent with a high solids
concentration.
The alkali metal is not particularly limited. Lithium, sodium, potassium, rubidium, cesium,
and francium may be mentioned. Among these, one or two or more types in combination may be
10 used. Among these, sodium and potassium have high solubility with respect to the solvent of the
coating agent when made borates and, further, are advantageous from the viewpoint of
manufacturing costs.
[0030]
As the boric acid constituent forming the borate of an alkali metal, orthoboric acid, meta15
boric acid, tetra-boric acid, or other oxoacids of boron may be mentioned. Among these, one or
two or more types in combination may be used. Among these, tetra-boric acid is high in
solubility with respect to the solvent of the coating agent when made a borate and can suitably
contribute to increase of the solids concentration of the coating agent. Here, if trying to raise the
concentration of boric acid by just boric acid without using a borate of an alkali metal, since
20 boric acid has a low solubility with respect to water, undissolved boric acid remains in the
aqueous solution. If such a boric acid aqueous solution stops being stirred, the undissolved boric
acid settles resulting in a boric acid aqueous solution unstable as a boric acid source for a coating
solution. Note that whether a boric acid aqueous solution is unstable can be easily judged by the
presence of any sediment in the aqueous solution since boric acid settles if stirring is stopped.
25 The coating agent according to the present embodiment can be made an aqueous solution with
no undissolved boric acid present.
[0031]
As specific examples of preferable combinations of borates of an alkali metal, sodium
tetraborate, potassium tetraborate, lithium tetraborate, etc. may be mentioned. In particular, due
30 to the high solubility with respect to water, the borate of an alkali metal preferably contains
sodium tetraborate and potassium tetraborate, more preferably potassium tetraborate.
[0032]
Further, as the boron source, in addition to the above-mentioned borate of an alkali metal,
other boron sources can be used in a range enabling the later-mentioned solids concentration to
35 be maintained. As such other boron sources, orthoboric acid, meta-boric acid, tetra-boric acid, or
other oxoacid of boron (boric acid), boron oxide represented by B2O3 , etc. may be mentioned.
9
Among these, single types alone or two or more types in combination may be used. Among
these, orthoboric acid represented as H3BO3 is preferable from the viewpoint of work
efficiency and costs.
[0033]
Further, the content of the boron source 5 in the coating agent is not particularly limited so
long as able to satisfy the later mentioned solids concentration and ratio with the aluminum
source, but for example it may be 5 mass% or more and 30 mass% or less. In particular, as the
method for raising the solids content of the sodium borate aqueous solution, the method is
known of mixing boric acid and sodium tetra-borate in a weight ratio of 1:1.25, making them
10 completely dissolve in 80C or more water, then cooling down to room temperature (2515C)
to obtain a sodium poly-borate aqueous solution. If using this method, it is possible to obtain an
aqueous solution containing a boron source with a higher solids concentration than the case of
mixing these at room temperature.
[0034]
15 Here, as mentioned above, the coating agent according to the present embodiment contains
a larger amount of the boron source than the aluminum source compared with the past.
Specifically, the coating agent contains the aluminum source and the boron source to give a
molar ratio Al/B of 0.5 to 2.0. Due to this, a glassy network is sufficiently formed in the
aluminum borate coating which is formed and the adhesion is improved. Here, if the boron
20 source is too small, there is no effect of improvement of the adhesion. On the other hand, if the
boron source is too large, the tension decreases and the waterproofness of the aluminum borate
coating deteriorates causing rust to form.
[0035]
Note that, by satisfying the above-mentioned molar ratio of the aluminum source and the
25 boron source, the adhesion of the aluminum borate coating is improved, but with just satisfying
the molar ratio, the adhesion is not sufficiently improved. As mentioned later, the adhesion of
the aluminum borate coating is sufficiently improved by containing silicon oxide and/or a silicon
oxide precursor in the coating agent.
[0036]
30 Further, the total solids concentration of the aluminum source and the boron source in the
coating agent is 20 mass% or more and 38 mass% or less. Here, the “solids concentration” is the
concentration of the total mass of the aluminum source and the boron source in the coating
agent. The aluminum source is evaluated converted to aluminum oxide (Al2O3 ), while the
boron source is evaluated converted to orthoboric acid (H3BO3 ). The solids concentration is the
35 weight percent of the weights of the aluminum oxide and orthoboric acid in the overall amount
of these plus the weights of the solvent and acids. The coating agent according to the present
10
embodiment can achieve the above such solids concentration by containing a borate of an alkali
metal as a boron source and containing a predetermined amount of acid mentioned later. By the
total solids concentration of the aluminum source and the boron source being large in this way, it
is possible to form an aluminum borate coating which is high in adhesion and large in tension.
Further, in the coating agent according to the 5 present embodiment, the gelling of the coating
agent and the decrease in the adhesion which were conventional problems are also prevented.
[0037]
If the total solids concentration of the aluminum source and the boron source is less than 20
mass%, the solids concentration becomes lower. As a result, if trying to secure the required
10 coating thickness, excessive time is required for the process of removing the solvent (drying
process). If rapidly raising the temperature to shorten the drying time, bubbling etc. causes
coating defects to end up being formed. The above solids concentration is preferably 25 mass%
or more.
[0038]
15 If the total solids concentration of the aluminum source and the boron source is over 38
mass%, the coating solution becomes easier to gel and becomes unstable. The above solids
concentration is preferably 35 mass% or less.
[0039]
Silicon oxide and silicon oxide precursor
20 Further, the coating agent contains silicon oxide and/or a silicon oxide precursor. The
silicon oxide and/or silicon oxide precursor contributes to formation of a glassy network in the
aluminum borate coating and contributes to improvement of the adhesion of the obtained
aluminum borate coating.
[0040]
25 The silicon oxide is not particularly limited. Various types of known silicon oxide can be
used. In particular, colloidal silica is excellent in dispersion in the coating agent.
Further, as the silicon oxide precursor, a compound able to form silicon oxide, for example,
a silane compound, may be mentioned. The silane compound is not particularly limited, but, for
example, tetraethoxysilane or other alkoxysilane or another silicon oxide precursor etc. may be
30 mentioned. Among these, single types alone or two or more types in combination may be used.
Alternatively, partially hydrolyzed products of these silane compounds may also be used.
[0041]
Further, the total content of the silicon oxide and silicon oxide precursor in the coating
agent is 5 mass% or more and 10 mass% or less converted to silicon oxide with respect to the
35 total mass of the aluminum source and the boron source. Due to this, it is possible to
simultaneously make the adhesion and tension of the obtained aluminum borate coating
11
excellent.
[0042]
As opposed to this, if the total content of the silicon oxide and silicon oxide precursor is
less than the above lower limit value, the adhesion of the obtained aluminum borate coating
becomes inferior. The total content of the silicon oxide a 5 nd silicon oxide precursor is preferably
6 mass% or more converted to silicon oxide with respect to the total mass of the aluminum
source and the boron source.
[0043]
Further, if the total content of the silicon oxide and silicon oxide precursor is over the above
10 upper limit value, there is an effect on the formation of aluminum borate and as a result the
tension of the obtained aluminum borate coating becomes inferior. The total content of the
silicon oxide and silicon oxide precursor is preferably 8 mass% or less converted to silicon oxide
with respect to the total mass of the aluminum source and the boron source.
[0044]
15 Further, with just containing silicon oxide and/or a silicon oxide precursor, the adhesion of
the aluminum borate coating is not sufficiently improved. It is only after satisfying the molar
ratio of the aluminum source and boron source such as mentioned above and further containing
silicon oxide and/or a silicon oxide precursor that the adhesion of the aluminum borate coating
becomes sufficient.
20 [0045]
Acid
The coating agent contains an ordinary acid. Here, in this Description, the “acid” means an
acid defined in the Brønsted-Lowry acid-base theory and means a substance donating protons.
By the coating agent containing such an acid, the pH of the coating agent can be adjusted to the
25 later mentioned range, the dispersion stability and solubility of the aluminum source in the
coating agent are improved, and gelling of the coating agent is prevented.
[0046]
As such an acid, nitric acid, hydrochloric acid, or other inorganic acids and acetic acid,
citric acid, oxalic acid, and other organic acids may be mentioned. Among these, one or two or
30 more types in combination may be used. Among these, as the acid, one which is degraded or
vaporized at the formation of the aluminum borate coating, for example, during heating, is
preferable.
[0047]
As such an acid which is degraded or vaporized, one or more types of inorganic acids
35 and/or organic acids selected from the group comprised of nitric acid and hydrochloric acid or
one or more types of organic acids selected from the group comprised of acetic acid, citric acid,
12
and oxalic acid may be mentioned. Therefore, the coating agent preferably contains one or more
types of acids selected from these.
[0048]
The content of the acid in the coating solution is not particularly limited so long as enabling
the pH of the coating agent to be ma 5 intained in a suitable range (2.0 or more and 6.0 or less) and
can be suitably adjusted in accordance with the targeted pH.
[0049]
Solvent
Further, the coating agent includes a solvent. The solvent functions as a solvent dissolving
10 the constituents and functions as a dispersant causing the constituents to disperse.
[0050]
The above such solvent is not particularly limited, but water or an alcohol, ketone, ether,
hydrocarbon, etc. may be mentioned. Among these, one or two or more types in combination can
be used.
15 In particular, as the solvent, water is preferable from the viewpoint of work efficiency and
the effect of suppression of defects at the time of drying and also the excellent dispersion and
solubility of the different constituents.
[0051]
Above, the pH of the coating agent is 2.0 or more and 6.0 or less. If the pH of the coating
20 agent is in the above range, the aluminum source can be stably dispersed and dissolved.
[0052]
As opposed to this, if the pH in the coating agent is over the above upper limit value, the
dispersion stability and solubility of the aluminum source cannot be improved and the coating
agent ends up gelling. As a result, when coating and drying the coating agent on the steel sheet,
25 cracks and voids frequently occur in the aluminum borate coating and other fine coating defects
are formed making it impossible to obtain a sound coating and as a result sufficient tension can
no longer be obtained. The pH of the coating agent is preferably 5.0 or less.
[0053]
On the other hand, if the pH in the coating agent is less than the above lower limit value,
30 conversely the coating solution becomes unstable. As a result, when coating and drying the
coating agent on the steel sheet, cracks and voids frequently occur in the aluminum borate
coating and other fine coating defects are formed making it impossible to obtain a sound coating
and as a result sufficient tension can no longer be obtained. The pH of the coating agent is
preferably 3.0 or more.
35 [0054]
Note that, the above-mentioned pH can, for example, be realized by addition of an acid. As
13
one example, it can be realized by addition of a solution of an acid with pH of 2.0 or less in 5.0
mass% or more and 10.0 mass% or less.
[0055]
According to the coating agent according to the present embodiment mentioned above, it is
possible to prevent gelling of the coating agent and 5 the decrease in adhesion of the aluminum
borate coating while increasing the solids concentration of the boron source and the aluminum
source. For this reason, when forming an aluminum borate coating of a sufficient coating
thickness, the time required for drying after coating the coating agent on the steel sheet is greatly
shortened. Further, it is possible to ease the temperature at the time of drying and other drying
10 conditions and possible to keep coating defects from being formed. As a result, when using the
coating agent according to the present embodiment, it is possible to form an aluminum borate
coating high in adhesion and large in tension.
[0056]
2. Method for Manufacturing Grain-Oriented Electrical Steel Sheet
15 Below, a method for manufacturing the grain-oriented electrical steel sheet according to the
present embodiment will be mentioned. The method for manufacturing the grain-oriented
electrical steel sheet according to the present embodiment includes a process of using the abovementioned
coating agent for forming the grain-oriented electrical steel sheet coating according to
the present embodiment to form an aluminum borate coating..
20 [0057]
Preparation of coating agent for forming grain-oriented electrical steel sheet coating
First, before the above-mentioned process, the coating agent for forming the grain-oriented
electrical steel sheet coating (coating agent) is prepared. The method for manufacturing the
coating agent is not particularly limited, but for example the coating agent can be obtained by
25 mixing the materials forming it. The order of mixing the materials is not particularly limited and
can be suitably set in accordance with work efficiency or the dispersibility and solubility of the
materials.
[0058]
Preparation of base metal steel sheet
30 Next, the base metal steel sheet for formation of the aluminum borate coating is prepared.
As the base metal steel sheet, specifically, (1) steel sheet batch annealed by a conventionally
known method and formed with a forsterite-based primary coating on its surface, (2) steel sheet
from which the primary coating and ancillary formed internal oxide layer are removed by
pickling, (3) steel sheet obtained by heat flattening the steel sheet obtained in the above (2) in a
35 hydrogen-containing atmosphere or steel sheet chemically or electrolytically polished or
otherwise polished, (4) steel sheet coated with a conventionally known annealing separator to
14
which alumina powder inert to formation of a coating or chlorides or other trace additives are
added and batch annealed under conditions not allowing formation of a primary coating steel
sheet or steel sheet flattened on its surface by a method such as (3), or other steel sheet on which
batch annealing has been completed may be prepared. Note that, the base metal steel sheet may
be prepared before or after preparing 5 the above-mentioned coating agent.
[0059]
Formation of aluminum borate coating
Next, the prepared coating agent is used to form an aluminum borate coating on the surface
of the steel sheet. The aluminum borate coating can be formed by coating the coating agent on
10 the surface of the steel sheet, then drying and annealing it.
[0060]
The surface of the steel sheet can be coated by, for example, a roll coater or other coater,
dipping, spraying, or electrophoresis or another conventionally known method.
[0061]
15 The steel sheet after coating the coating agent is dried, then annealed whereby an aluminum
borate coating is formed on the surface of the steel sheet. The annealing can be performed at a
temperature of, for example, 750C or more. If the annealing temperature is less than 750C, the
coated precursor will sometimes not form oxides. Further, since the annealing temperature is
low, sufficient tension is not obtained so this is not preferable. The annealing temperature is
20 preferably 750C or more and 1200C or less, more preferably 800C or more and 1000C or
less.
[0062]
The atmosphere at annealing is preferably nitrogen or another inert gas atmosphere, a
nitrogen-hydrogen mixed atmosphere, or other reducing atmosphere. Air or an atmosphere
25 excessively containing oxygen may cause excessive oxidation of the steel sheet and is not
preferable.
Regarding the dew point of the atmospheric gas, good results were obtained at 0 to 40C.
[0063]
By the above, grain-oriented electrical steel sheet with an aluminum borate coating having a
30 high adhesion and tension can be manufactured.
EXAMPLES
[0064]
Below, the present invention will be mentioned in detail based on examples, but the
35 examples shown below are in the end merely illustrations of the present invention. The present
invention is not limited to only these examples.
15
[0065]
Example 1
Commercially available boric acid (orthoboric acid), potassium tetraborate, or sodium
tetraborate, aluminum oxide (Al2O3 ) powder (average particle size: 0.4 m), a 0.5M nitric acid
aqueous solution, a 5 nd silicon oxide were mixed in the ratios shown in Table 1. Note that the pH
of the nitric acid was 0.5. Slurries used as the coating agents according to Examples 1-1 to 1-6
and Comparative Examples 1-1 to 1-7 were prepared at room temperature as mentioned above.
Note that, as the solvent, water was used. On the other hand, the coating solution of Example 1-6
was prepared by preparing high concentration polyboric acid in the following way. First, 700 g
10 of water was heated to 80C, the amounts of boric acid and sodium tetraborate in Example 1-6
were added, and the result was stirred while maintaining the temperature until these completely
dissolved. After they completely dissolved, the solution was gradually cooled down to room
temperature (30C) to prepare the high concentration polyboric acid solution. To this, aluminum
oxide powder, 0.5M nitric acid aqueous solution, and silicon oxide were added in the amounts in
15 Table 1 and the mixture sufficiently stirred.
[0066]
The prepared coating agents were measured for viscosity and pH. The viscosity was
measured using a B-type viscometer for a 30C coating agent. The pH was measured for a 30C
coating agent using a pH meter. The results are shown in Table 1.
20 [0067]
The prepared coating agent was allowed to stand without stirring for 30 minutes. After that,
it was coated on grain oriented silicon steel sheet containing Si in 3.2 mass%, having a thickness
of 0.23 mm, and finished being batch annealed (with a forsterite primary coating) to give a
weight of coating after annealing of 4.5 g/m2 . This was dried and annealed at 850C for 60
25 seconds. Here, the atmosphere when drying and annealing was made a nitrogen atmosphere
containing hydrogen in 10 vol% and the dew point was made 30C. Due to the above, grainoriented
electrical steel sheets having an aluminum borate coating according to Examples 1-1 to
1-6 and Comparative Examples 1-1 to 1-7 were obtained.
[0068]
30 The obtained grain-oriented electrical steel sheets were evaluated for adhesion of the
aluminum borate coating and coating tension.
For the adhesion of the aluminum borate coating, the adhesion of a sample when the sheet
was wound around a 20 mm cylinder and there was no peeling of the coating was evaluated as
good and otherwise was evaluated as poor.
35 For measurement of the coating tension, the coating on one side of a steel sheet formed
with a aluminum borate coating was removed and the tension was calculated from the curvature
16
of the steel sheet. The coating was removed using a sodium hydroxide aqueous solution. A
sample with a coating tension of 12 MPa or more was evaluated as good while a case not
satisfying this was evaluated as poor.
The above results are shown in Table 1.
5 [0069]
As shown in Table 1, in the coating agents according to Examples 1-1 to 1-6, the target
solids concentrations (concentration of total mass of aluminum source and boron source in
coating agent) were obtained. In the grain-oriented electrical steel sheets according to Examples
1-1 to 1-6 manufactured using the same, it can be understood that aluminum borate coatings
10 excellent in adhesion and high in tension were formed.
Steel sheets manufactured using the coating agents according to Comparative Examples 1-1
and 1-2 not achieving the target solids concentration were low in coating tension. This was
guessed to be because coating defects formed due to bubbling etc. at the time of drying the
coating agent. On the other hand, Comparative Example 1-7 not containing an alkali metal had a
15 high solids concentration of 30%, but was not sufficient in tension. This was guessed to be
because it was a composition not containing a borate of an alkali metal, so the charged boric acid
exceeded the solubility, so the boric acid remained precipitated, the solution became unstable in
uniformity, and the precipitated boric acid settled while stopping stirring for 30 minutes resulting
in the coating of the intended composition not being able to be obtained.
20 [0070]
17
[Table 1]
Table 1
Coating agent Properties
Mixed substances and amounts of mixture Al/B
molar
ratio
Concentration
of total weight
of aluminum
source and
boron source in
coating agent
(wt%)
Ratio of silicon
oxide to total
weight of
aluminum
source and
boron source
(wt%)
Properties of
coating agent
Aluminum borate coating
Aluminum
source
Boron source Acid (0.5M
nitric acid
aqueous
solution)
(g)
Boron
oxide (g)
Solvent
Aluminum
oxide (g)
Boric
acid
(g)
Potassium
tetraborate
tetrahydrate (g)
Sodium
tetraborate
decahydrate (g)
Water (g) Viscosity
(mPas)
pH Coating
tension
Adhesion
Ex. 1-1 100 95 182 0 177 24 1193 0.5 20 7 60 4.3 Good Good
Ex. 1-2 100 0 232 85 282 24 1088 0.5 20 7 30 4.1 Good Good
Ex. 1-3 100 0 300 0 268 24 613 0.5 28 7 30 4.3 Good Good
Ex. 1-4
96 0 72 0 73 11 179 2.0 38 7 50 3.4
Good Good
Comp. Ex. 1-1 100 0 75 0 144 11 843 2.0 14 7 100 6.5 Poor Poor
Comp. Ex. 1-2 100 0 300 0 300 24 1804 0.5 14 7 100 6.7 Poor Poor
Comp. Ex. 1-3 100 0 375 0 345 28 596 0.4 30 7 30 4.2 Poor Poor
Comp. Ex. 1-4 100 0 68 0 68 11 220 2.2 35 7 40 4.0 Poor Poor
Ex. 1-5 100 0 300 0 234 17 605 0.5 29 5 30 4.4 Good Good
Ex. 1-6 100 133 0 169 234 34 565 0.5 30 10 30 4.0 Good Good
Comp. Ex. 1-5 100 0 300 0 232 10 606 0.5 29 3 40 4.5 Poor Poor
Comp. Ex. 1-6 100 0 300 0 241 51 598 0.5 29 15 50 4.2 Poor Poor
Comp. Ex. 1-7 100 61 0 0 0 0 375 2.0 30 0 20 3.1 Poor Good
18
[0071]
Example 2
Aqueous solutions of various acids comprised of commercially available boric acid
(orthoboric acid), potassium tetraborate, aluminum oxide (Al2O3 ) powder (average particle
size: 0.4 m), silicon oxide, and nitric 5 acid (0.1M, pH 1.0), hydrochloric acid (0.1M, pH 0.9),
acetic acid (0.5M, pH 1.9), citric acid (0.2M, pH 2.0), and oxalic acid (0.1M, pH 1.5) were
mixed in the ratios shown in Table 2 to prepare slurries as the coating agents of Examples 2-1 to
2-5 and Comparative Examples 2-1 to 2-2. Note that, as the solvent, water was used.
[0072]
10 The prepared coating agents were measured for viscosity and pH. The viscosity was
measured using a B-type viscometer for a 30C coating agent. The pH was measured for a 30C
coating agent using a pH meter. The results are shown in Table 2.
[0073]
The prepared coating agents were coated on oriented silicon steel sheets containing Si in
15 3.2 mass%, having a thickness of 0.23 mm, and finished being batch annealed (with a forsterite
primary coating) to give weights of coating after annealing of 4.5 g/m2 . These were dried and
annealed at 850C for 60 seconds. Here, the atmosphere when drying and annealing was made a
nitrogen atmosphere containing hydrogen in 10 vol% and the dew point was made 30C. Due to
the above, grain-oriented electrical steel sheets having an aluminum borate coating according to
20 Examples 2-1 to 2-5 and Comparative Examples 2-1 to 2-2 were obtained.
[0074]
The obtained grain-oriented electrical steel sheets were evaluated for adhesion of the
aluminum borate coating and tension of the coating.
For the adhesion of the aluminum borate coating, the adhesion of a sample when the sheet
25 was wound around a 20 mm cylinder and there was no peeling of the coating was evaluated as
good and otherwise was evaluated as poor.
For measurement of the coating tension, the coating on one side of a steel sheet formed
with a aluminum borate coating was removed and the tension was calculated from the curvature
of the steel sheet. The coating was removed using a sodium hydroxide aqueous solution. A
30 sample with a coating tension of 12 MPa or more was evaluated as good while a case not
satisfying this was evaluated as poor.
The above results are shown in Table 2.
[0075]
As shown in Table 2, in the coating agents according to Examples 2-1 to 2-5, the target
35 solids concentrations were obtained. In the grain-oriented electrical steel sheets according to
Examples 2-1 to 2-5 manufactured using the same, it can be understood that aluminum borate
19
coatings excellent in adhesion and high in tension were formed.
[0076]
20
[Table 2]
Table 2
Coating agent Properties
Composition Al/B
molar
ratio
Concentration
of total weight
of aluminum
source and
boron source
in coating
agent (wt%)
Ratio of silicon
oxide to total
weight of
aluminum
source and
boron source
(wt%)
Coating agent Aluminum borate coating
Aluminum
source
Boron source Acid (aqueous solution) Silicon
oxide
(g)
Solvent
Aluminum
oxide
(g)
Boric
acid (g)
Potassium
tetraborate
tetrahydrate
(g)
Type Amount
(g)
Water (g) Viscosity
(mPas)
pH Coating
tension
Adhesion
Ex. 2-1 100 95 183 0.1M nitric acid 445 24 845 0.5 21 7 40 3.2 Good Good
Ex. 2-2 100 95 183 0.1M hydrochloric acid 445 24 845 0.5 21 7 20 3.1 Good Good
Ex. 2-3 100 95 183 0.1M acetic acid 445 24 845 0.5 21 7 70 4.2 Good Good
Ex. 2-4 100 95 183 0.2M citric acid 445 24 845 0.5 21 7 80 5.0 Good Good
Ex. 2-5 100 95 183 0.1M oxalic acid 445 24 845 0.5 21 7 90 5.2 Good Good
Comp. Ex. 2-1 100 95 183 0.1M nitric acid 90 24 1200 0.5 21 7 310 8.0 Poor Poor
Comp. Ex. 2-2 100 95 183 0.1M nitric acid 902 24 388 0.5 21 7 5 1.8 Poor Poor
21
[0077]
Above, a preferred embodiment of the present invention or more was mentioned in detail,
but the present invention is not limited this example. A person having ordinary knowledge in the
field of art to which the present invention belongs clearly could conceive of various changes or
corrections within the scope of the technical i 5 dea described in the claims. These also are
understood as including in the technical scope of the present invention.

WE CLAIMS

A coating agent for forming a grain-oriented electrical steel sheet coating comprising
an aluminum source containing aluminum oxide and/or an aluminum oxide precursor
5 compound,
a boron source containing a borate of an alkali metal, and
silicon oxide and/or a silicon oxide precursor in an amount, converted to silicon oxide, of 5
mass% or more and 10 mass% or less with respect to a total mass of the aluminum source and
boron source,
10 the aluminum source and the boron source contained so that the B contained in the boron
source and the Al contained in the aluminum source become, converted to molar ratio, Al/B: 0.5
to 2.0,
a solids concentration of a total of the aluminum source and the boron source being 20
mass% or more and 38 mass% or less, and
15 pH being 2.0 or more and 6.0 or less.
[Claim 2]
The coating agent for forming the grain-oriented electrical steel sheet coating according to
claim 1, wherein the boron source contains boric acid.
20
[Claim 3]
The coating agent for forming the grain-oriented electrical steel sheet coating according to
claim 1 or 2, wherein the alkali metal contains at least one of sodium and potassium.
25 [Claim 4]
The coating agent for forming the grain-oriented electrical steel sheet coating according to
any one of claims 1 to 3, further containing one or more types of inorganic acids selected from
the group comprising nitric acid and hydrochloric acid and/or one or more types of organic acids
selected from the group comprising acetic acid, citric acid, and oxalic acid.
30
[Claim 5]
A method for manufacturing grain-oriented electrical steel sheet having a process of
forming an aluminum borate coating using a coating agent for forming the grain-oriented
electrical steel sheet coating according to any one of claims 1 to 4.