METHOD OF PRODUCTION OF GRAIN-ORIENTED ELECTRICAL STEEL SHEET WITH HIGH MAGNETIC FLUX DENSITY
TECHNICAL FIELD
The present invention relates to a method of
producing grain-oriented electrical steel sheet able to be used as a soft magnetic material for a core of a transformer or other electrical equipment by low temperature slab heating.
BACKGROUND ART
Grain-oriented electrical steel sheet is a steel
sheet containing not more than 7% Si comprising crystal grains aligned in the {110}<001> orientation. Control of the crystal orientation in the production of such grainoriented
electrical steel sheet is realized utilizing the
catastrophic grain growth phenomenon called "secondary recrystallization".
As one method for controlling this secondary
recrystallization, the method of completely dissolving a coarse precipitates at the time of heating a slab before hot rolling, then forming finely precipitate called an "inhibitor" in the hot rolling and the subsequent annealing process is being industrially practiced. With this method, to cause the precipitate to completely
dissolve, it is necessary to heat the slab to a high temperature of 1350°C to 1400°C or more. This temperature is about 200°C higher than the slab heating temperature of ordinary steel. A special heating furnace is' therefore necessary for this. Further, there are the problems that
the amount of the molten scale is large etc.
Therefore, R&D on the production of grain-oriented
electrical steel sheet by low temperature slab heating
35 have been carried; out.
As the method for production of low temperature slab
heating, for example Komatsu et al. disclose the method
_ 2 -
of using {Al,Si)N formed by nitridation as the inhibitor
in Japanese Patent Publication (B2) No. 62-45285.
Further, Kobayashi et al. disclose as the method of
nitridation at that time the method of nitridation in a
5 strip form after decarburization annealing in Japanese
Patent Publication (A) No. 2-77525. The present inventors
reported on the behavior of nitrides in the case of
nitridation in a strip form in "Materials Science Forum",
204-206 (1996), pp. 593-598.
10 Further, the inventors showed that in such a method
of production of grain-oriented electrical steel sheet by
low temperature slab heating, no inhibitor is formed at
the time of decarburization annealing, so adjustment of
the primary recrystallized structure in the
15 decarburization annealing is important for the control of
secondary recrystallization and that if the coefficient
of variation of the distribution of grain size in the
primary recrystallized grain structure becomes larger
than 0.6 and the grain structure becomes inhomogeneous,
20 the secondary recrystallization becomes unstable in
Japanese Patent Publication (B2) No. 8-32929.
Furthermore, the inventors engaged in research on
the control factor of secondary recrystallization, that
is, the primary recrystallized structure, and inhibitor,
25 and as a result discovered that {411} oriented grains in
the primary recrystallized structure have an effect on
the preferential growth of the {110}<001> secondary
recrystallized grains and showed, in Japanese Patent
Publication (A) No. 9-256051, that by adjusting the
30 {111}/{411} ratio of the primary recrystallized texture
after decarburization annealing to 3.0 or less, then
performing the nitridation to strengthen the inhibitor,
it is possible to produce grain-oriented electrical steel
sheet high in magnetic flux density industrially stably
35 and showed that as a method for control of the grain
structure after primary recrystallization at this time,
for example, there is the method of controlling the
- 3 -
heating rate in the process of temperature elevation in
the decarburizing annealing step to 12°C/s or more.
After this, it was learned that the above method of
controlling the heating rate is very effective as a
5 method of controlling the grain structure after primary
recrystallization. The inventors proposed, in Japanese
Patent Publication (A) No. 2002-60842, the method of
rapidly heating the steel sheet in the process of
temperature elevation in the decarburization annealing
10 process up to a predetermined temperature in the range
from the region of 600°C or less to 750 to 900°C by a
heating rate of 40°C/s or more so as to control the
I{111}/I{411} ratio in the grain structure after
decarburization annealing to 3 or less and adjusting the
15 amount of oxygen of the oxidized layer of the steel sheet
in the subsequent annealing to 2.3 g/m2 or less to
stabilize the secondary recrystallization.
Here, 1(111} and I{411} are the ratios of grains
with {111} and {411} planes parallel to the sheet surface
20 and show values of diffraction strengths measured at the
sheet thickness 1/10 layer by X-ray diffraction
measurement.
In the above method, rapid heating up to a
predetermined temperature in the range of 750 to 9O0°C by
25 a heating rate of 40°C/s or more is necessary. Regarding
the heating means for this, modified decarburization
annealing facilities using radiant tubes utilizing
conventional ordinary radiant heat etc., the method of
utilizing lasers or other high energy heat sources,
30 induction heating, electrical heating apparatuses, etc.
may be mentioned, but among these heating methods, in
particular induction heating is advantageous in the
points that it has a high freedom of heating rate,
enables heating without contact with the steel sheet, and
35 is relatively easy to install in decarburization
annealing furnaces.
- 4 -
In this regard, when using induction heating to heat
electrical steel sheets, it is difficult to heat
electrical steel sheet to a temperature of the Curie
point or more, since the sheets are thin, when the
5 temperature becomes close to the Curie point, the current
penetration depth of the eddy current becomes deeper, the
eddy current circling the front surface in the strip
width direction cross-section is cancelled out at the
front and rear, and the eddy current no longer flows.
10 The Curie point of grain-oriented electrical steel
sheet is about 750°C, so even if using induction heating
for heating to a temperature up to this, for heating to a
temperature above this, it is necessary to use another
means to take the place of the induction heating, for
15 example, electrical heating.
However, using another heating means in combination
loses the advantage in facilities of use of induction
heating. Also, for example, with electrical heating,
contact with the steel sheet becomes necessary. There was
20 therefore the problem that the steel sheet was scratched.
For this reason, when the end of the rapid heating
region is 750 to 900°C as shown in Japanese Patent
Publication (A) No. 2002-60842, there was the problem
that it was not possible to snf f i r-.i «=>nt 1 y pmjoy_fchfi
25 advantages of induction heating.
DISCLOSURE OF THE INVENTION
Therefore, the present invention has as its object,
when using low temperature slab heating for producing
grain-oriented electrical steel sheet, to make the
30 temperature region for control of the heating rate in the
temperature elevation process of the decarburization
annealing for improving the grain structure after primary
recrystallization after decarburizing annealing a range
able to be heated by just induction heating and thereby
35 solve the above problem.
To solve the above problem, the method of production
of grain-oriented electrical steel sheet of the present
- 5 -
invention provides:
(1) A method of production of grain-oriented
electrical steel,sheet comprising heating a silicon steel
material containing, by mass%, Si: 0.8 to 7%, C: 0.085%
5 or less, acid soluble Al: 0.01 to 0.065%, and N: 0.012%
or less at a temperature of 1280°C or less, then hot
rolling it, annealing the obtained hot rolled sheet, then
cold rolling it once or cold rolling it several times
with intermediate annealing to obtain steel sheet of the
10 final sheet thickness, decarburization annealing this
steel sheet, then coating an annealing separator,
applying final annealing, and applying treatment to
increase an amount of nitrogen of the steel sheet from
the decarburization annealing to the start of secondary
15 recrystallization in the final annealing, characterized
by performing the annealing of the hot rolled sheet by
heating the sheet up to a predetermined temperature of
1000 to 1150°C to cause recrystallization, then annealing
it by a temperature of 850 to 1100°C lower than that
20 temperature to thereby control a lamellar spacing in the
grain structure after annealing to 20 urn or more and by
heating in the temperature elevation process in the
decarburization annealing of the steel sheet by a rate of
40°C/s or more in the temperature range of a steel sheet
25 temperature of 550°C to 720°C.
Here, "lamellar structures", as shown in FIG. 1,
refer to a layered structures split by the transformation
phases or crystal grain boundaries and parallel to the
rolling surface, while the "lamellar spacing" is the
30 average spacing between these lamellar structures.
(2) A method of production of grain-oriented
electrical steel sheet comprising heating a silicon steel
material containing, by mass%, Si: 0.8 to 7%, C: 0.085%
or less, acid soluble Al: 0.01 to 0.065%, and N: 0.012%
35 or less at a temperature of 1280°C or less, then hot
rolling it, annealing the obtained hot rolled sheet, then
- 6 -
cold rolling it once or cold rolling it several times
with intermediate annealing to obtain steel'sheet of the
final sheet thickness, decarburization annealing this
steel sheet, then coating an annealing separator,
5 applying final annealing, and applying treatment to
increase an amount of nitrogen of the steel sheet from
the decarburization annealing to the start of secondary
recrystallization of the final annealing, characterized
by, in the annealing process of the hot rolled sheet,
10 decarburizing the steel sheet to 0.002 to 0.02 mass% of
the amount of carbon before decarburization•annealing to
thereby control a lamellar spacing in the surface layer
grain structure after annealing to 20 urn or more and by
heating in the temperature elevation process in the
15 decarburization annealing of the steel sheet of the final
sheet thickness by a heating rate of 40°C/s or more in the
temperature range of a steel sheet temperature of 550°C to
720°C.
Here, the "surface layer" of the "surface layer
20 grain structure" refers to the region from the outermost
surface part to 1/5 the total sheet thickness, while the
"lamellar spacing" is the average spacing of lamellar
structures parallel to the rolling surface in this
region-:
25 Further, in the invention of the above (1) or (2),
(3) the present invention is further characterized
by heating in the temperature evaluation process in the
decarburization annealing of the steel sheet by a heating
rate of 50 to 250°C/s between a steel sheet temperature of
30 550°C to 720°C.
(4) the present invention is further characterized
by heating in the temperature elevation process in the
decarburization annealing of the steel sheet by a heating
rate of 75 to 125°C/s between a steel sheet temperature of
35 550°C to 720°C.
(5) the present invention is further characterized
~ 1 -
by performing the heating of the steel sheet in the
temperature range of a steel sheet temperature of 550°C to
720°C when decarburization annealing said steel sheet by
induction heating.
5 (6) the present invention is further characterized
by, making the temperature range for heating by said
heating rate in the temperature elevation process in the
decarburization annealing, to be from Ts (°C) to 720°C,
making it the following range from Ts (°C) to 720°C in
10 accordance with the heating rate H (°C/s) from room
temperature to 500°C:
H^15: Ts^550
1514/27[A1}. (
(9) the present invention is further characterized
by increasing the amount of nitrogen [N] of said steel
sheet in accordance with an amount of acid soluble Al
30 [Al] of the steel sheet so as to satisfy the formula
[N]>2/3[A1].
(10) the present invention is further characterized
by, when coating said annealing separator, coating an
annealing separator mainly comprised of alumina and
35 performing the final annealing.
- 8 -
(11) the present invention is further characterized
in that said silicon steel material further contains, by
mass%, one or more of Mn: 1% or less, Cr: 0.3% or less,
Cu: 0.4% or less, P: 0.5% or less, Sn: 0.3%'or less, Sb:
5 0.3% or less, Ni: 1% or less, and S and Se in a total of
0.015% or less.
The present invention uses low temperature slab
heating for the production of grain-oriented electrical
steel sheet during which it anneals the hot rolled sheet
10 in the above two temperature ranges or decarburizes the
hot rolled sheet at the time of annealing in the above
way to control the lamellar spacing and thereby rapidly
heat the sheet in the temperature elevation process of
the decarburizing annealing to improve the primary
15 recrystallized grain structure after decarburizing
annealing. At this time, the upper limit of the
temperature for maintaining the heating rate high can be
made a lower temperature range enabling heating by
induction heating, so the heating can be performed more
20 easily and grain-oriented electrical steel sheet superior
in magnetic properties can be produced more easily.
For this reason, since the heating can be performed
by induction heating, the degree of freedom of the
heating rate is high, the heating is possible without
25 contact with the steel sheet, installation in the
decarburization annealing furnace is relatively easy, and
other advantageous effects are obtained.
In the present invention, further, by adjusting the
oxidation degree in the decarburization annealing or the
30 amount of nitrogen of the steel sheet in the above way,
even when raising the heating rate of the decarburization
annealing, the secondary recrystallization can be
performed more stably.
Further, in the present invention, by adding the
35 above elements to the silicon steel material, it is
possible to further improve the magnetic properties etc.
in accordance with the added elements. By using an
- 9 -
annealing separator mainly comprised of alumina at the
time of final annealing, it is possible to produce
mirror-surface grain-oriented electrical steel sheet.
BRIEF DESCRIPTION OF THE DRAWINGS.
5 FIG. 1 is a view showing the lamellar structure in a
grain structure before cold rolling at a cross-section
parallel to the rolling direction (sheet thickness 2.3
mm) .
FIG. 2 is a view showing the relationship between
10 the lamellar spacing of the grain structure before cold
rolling and the magnetic flux density (B8) of a sample
obtained by annealing the hot rolled sheet in two stages
of temperature ranges.
FIG. 3 is a.view showing the relationship between a
15 first annealing temperature and the magnetic flux density
(B8) of a sample obtained by annealing the hot rolled
sheet in two stages of temperature ranges.
FIG. 4 is a,view showing the relationship between
the heating rate in a temperature range of 550 to 720°C
20 during temperature elevation in decarburization annealing
and the magnetic.flux density (B8) of a sample obtained
by annealing the hot rolled sheet in two stages of
temperature ranges.
FIG. 5 is a view showing the relationship between
25 the lamellar spacing of the surface layer grain structure
before cold rolling and the magnetic flux density (B8) of
a sample decarburized at the time of annealing the hot
rolled sheet.
FIG. 6 is a view showing the relationship between
30 the heating rate of the temperature range of 550 to 720°C
during temperature elevation in decarburization annealing
and the magnetic flux density (B8) of a sample
decarburized at the time of annealing the hot rolled
sheet.
35 BEST MODE FOR CARRYING OUT INVENTION
The inventors thought that when heating a silicon
steel material containing, by mass%, Si: 0.8 to 7%, C:
- 10 -
0.085% or less, acid soluble Al: 0.01 to 0.065%, and N:
0.012% by a temperature of 1280°C or less, then hot
rolling it, annealing the obtained hot rolled sheet, then
cold rolling it once or cold rolling it a plurality of
5 times with intermediate annealing to obtain steel sheet
of the final sheet thickness, decarburization annealing
the steel sheet, then coating it with an annealing
separator and final annealing it and nitriding the steel
sheet from the decarburization annealing to the start of
10 secondary recrystallization of the final annealing so as
to produce grain-oriented electrical steel sheet, the
lamellar spacing in the grain structure of the hot rolled
sheet after annealing might have an effect on the grain
structure after primary recrystallization and that even
15 if lowering the temperature for suspending rapid heating
at the time of decarburization annealing (even if
suspending it before the temperature at which primary
recrystallization occurs), the ratio of {411} grains in
the primary recrystallized texture might be raised, and
20 changed the annealing conditions of hot rolled sheet in
various ways to investigate the relationship of the
lamellar spacing in the grain structure after annealing
of the hot rolled sheet with the magnetic flux density B8
of the steel sheet after secondary recrystallization and
25 the effect of the heating rate at different temperatures
in the temperature elevation process of the
decarburization annealing on the magnetic flux density
B8.
As a result, they obtained the discovery that, in
30 the process of annealing the hot rolled sheet, when
heating the sheet at a predetermined temperature to cause
it to recrystallize, then further annealing it by a
temperature lower than that temperature to control the
lamellar spacing of the grain structure after annealing
35 to 20 jim or more, the temperature range with the large
change in structure in the temperature elevation process
- 11 -
of the decarburization annealing process is 700 to 720°C
and that by making the heating rate in the temperature
range of 550°C to 720°C including that temperature range
40°C/s or more, preferably 50 to 250°C/s, more preferably
5 75 to 125°C/s, it is possible to control the primary
recrystallization so that the ratio of the I{111}/I{411}
of the texture after decarburization annealing becomes a
predetermined value or less and possible to stably
promote a secondary recrystallized structure and thereby
10 completed the present invention.
Here, the "lamellar spacing" is the average spacing
of the layered structures parallel to the rolling surface
called "lamellar structures".
Below, the experiment by which this discovery was
15 obtained will be explained.
First, the inventors investigated the relationship
between the annealing conditions of the hot rolled sheet
and the magnetic;flux density B8 of samples after final
annealing.
20 FIG. 2 shows the relationship between the lamellar
spacing of the grain structure in samples before cold
rolling and the magnetic flux density B8 of samples after
final annealing. The samples used here were obtained by
heat±ng^~a slab containing, by mass%, Si~l 3.3%, Cl 0.045
25 to 0.065%, acid soluble Al: 0.027%, N: 0.007%, Mn: 0.1%,
and S: 0.008% anp! having a balance of Fe and unavoidable
impurities by a temperature of 1150°C, then hot rolling it
to a 2.3 mm thickness, then heating this to 1120°C to
cause it to recrystallize, then annealing the hot rolled
30 sheet in two stages of annealing at a temperature of 800
to 1120°C, cold rolling the hot rolled sheet to a 0.22 mm
thickness, then heating it by a heating rate of 15°C/s to
550°C, heating it1 by a heating rate of 40°C/s to the
temperature range of 550 to 720°C, then further heating it
35 by a heating rate of 15°C/s for decarburizing annealing at
- 12 -
a temperature of 830°C, then annealing it in an ammoniacontaining
atmosphere to increase the nitrogen in the
steel sheet for nitridation, then coating it with an
annealing separator mainly comprised of MgO, then final
5 annealing it. The lamellar spacing was adjusted by
changing the amount of C and the second temperature in
the two-stage hot rolled sheet annealing.
As clear from FIG. 2, it is learned that a high
magnetic flux density of a B8 of 1.91 T or more is
10 obtained at a lamellar spacing of 20 urn or more.
Further, the inventors analyzed the primary
recrystallized texture of decarburization annealed sheets
of samples giving a B8 of 1.91T or more and as a result
confirmed that in all samples, the value of I{111}/I{411}
15 was 3 or less.
Still further, FIG. 3 shows the relationship between
the first heating temperature in the case of heating by
two stages in the hot rolled sheet annealing and the
magnetic flux density B8 of the samples after final
20 annealing.
The samples used here were prepared in the same way
as the case of FIG. 2 except for making the first
temperature in the temperatures of the hot rolled sheet
annealing 900°C to 1150°C and the—second temperature
25 920°C. Note that the heating rate when heating to the
first temperature was made 5°C/s and 10°C/s.
As clear from FIG. 3, it is learned that a high
magnetic flux density of a B8 of 1.91T or more is
obtained at the first hot rolled sheet annealing
30 temperature of 1000°C to 1150°C.
Further, the inventors analyzed the primary
recrystallized texture of decarburization annealed sheets
of samples giving a B8 of 1.91T or more and as a result
confirmed that in all samples, the value of I{111}/!{411}
35 was 3 or less.
Next, the inventors investigated the heating
- 13 -
conditions at the time of decarburization annealing
giving steel sheets of a high magnetic flux density (B8)
under conditions of a lamellar spacing of the grain
structure in the samples before cold rolling of 20 \im or
5 more.
Cold rolled samples prepared in the same way as in
the case of FIG. 2 except for making the C content
0.055%, making the first hot rolled sheet annealing
temperature 1120°C, making the second hot rolled sheet
10 annealing temperature 920°C, and making the lamellar
spacing 25 jxm were decarburization annealed while
changing the heating rate of the temperature range of 550
to 720°C at the time of decarburization annealing in
various ways during the temperature elevation. Further,
15 the magnetic flux densities B8 of the samples after final
annealing were measured.
From FIG. 4, it is learned that if controlling the
heating rate at the temperatures in the temperature range
of 550°C to 720°C in the temperature elevation process of
20 the decarburization annealing to 40°C/s or more,
electrical steel sheet having a magnetic flux density
(B8) of 1.91T or more is obtained, while if controlling
the heating rate' to a range of 50 to 250°C/s, more
preferably 75 to 125°C/s, electrical steel sheet with a
25 further higher magnetic flux density of a B8 of 1.92T or
more is obtained.
Therefore, it is learned that, in the process of
annealing the hot rolled sheet, by heating to a
predetermined temperature of 1000 to 1150°C to cause
30 recrystallization, then annealing at a lower temperature
than this of 850 to 1100°C to control the lamellar spacing
in the grain structure after annealing to 20 am or more,
even if making the temperature range for rapid heating in
the temperature elevation process of the decarburization
35 annealing process a steel sheet temperature of a range of
- 14 -
550°C to 720°C, it is possible to raise the ratio of the
grains of the {411} orientation, possible, as shown in
Japanese Patent Publication (B2) Wo. 8-32929, to make the
ratio of I{ 111}/I {411} 3 or less, and possible to stably
5 produce grain-oriented electrical steel sheet with a high
magnetic flux density.
In the above way, since it was confirmed that
control of the lamellar spacing to 20 urn or more in the
grain structure after hot rolled sheet annealing is
10 effective, the inventors also studied other means for
controlling the lamellar spacing to 20 (am or more.
As a result, the inventors discovered from
experiments similar to the experiments for finding FIGS.
2 and 4 that by decarburization annealing the amount of
15 carbon of the steel sheet before decarburizing in the
annealing process of the hot rolled sheet to 0.002 to
0.02 mass%, it is possible to make the lamellar spacing
20 um or more in the surface layer grain structure after
annealing and, even if doing so, by similarly making the
20 heating rate in the temperature range of 550°C to 720°C in
the temperature elevation process of the decarburization
annealing after cold rolling 40°C/s or more, it is
possible to control the primary recrystallization so that
the ratio of the I{111}/I{411} of the texture after
25 decarburization annealing becomes a predetermined value
or less and possible to stably promote a secondary
recrystallized structure.
Here, "lamellar spacing" is the average spacing of
the layered structures parallel to the rolling surface
30 called "lamellar structures". Further, the "surface
layer" of the surface layer grain structure means the
region from the surfacemost part to 1/5 of the sheet
total thickness.
FIG. 5 shows the relationship between the lamellar
35 spacing before cold rolling and the magnetic flux density
B8 of the samples after final annealing in which lamellar
- 15 -
spacing of the surface layer grain structure after
annealing were changed by decarburization in the
processing of hot rolled sheet annealing. Note that
lamellar spacing of the surface layer was adjusted by
5 changing the steam partial pressure of the atmospheric
gas in the annealing of the hot rolled sheet performed at
1100°C so that the difference in amounts of carbon before
and after decarburization became a range of 0.002 to 0.02
mass%.
10 As will be clear from FIG. 5, it is learned that
even when decarburizing the hot rolled sheet in the
process of annealing it so as to make the lamellar
spacing of the surface layer 20 jam or more, a high
magnetic flux density B8 of 1.91T or more is obtained.
15 Further, FIG. 6 shows the relationship between the
heating rate of the temperature range of 550 to 720°C
during temperature elevation at the time of
decarburization annealing and the magnetic flux density
B8 of samples after final annealing which were prepared
20 in the same way by adjusting the oxidation degree of the
atmospheric gas in the hot rolled sheet annealing to make
the lamellar spacing of the surface layer grain structure
25 um.
From FIG. &} it is learned-th~a~tr~e~ven when
25 controlling the lamellar spacing by decarburization in
the process of annealing hot rolled sheet, if the heating
rate in the temperature range of 550°C to 720°C in the
temperature elevation process of the decarburization
annealing is 40°C/s or more, electrical steel sheet with a
30 high magnetic flux density is obtained.
The reason why the lamellar spacing in the grain
structure after hot rolled sheet annealing causes the
{411}, {111} texture to change is still not clear, but
currently is believed to be as follows. It is known that
35 there are preferential nucleation sites and they are
different due to the orientation of recrystallization.
- 16 -
Supposing that in the cold rolling process, {411} nuclei
are formed inside the lamellar structure and {111} nuclei
are formed near the lamellar parts at {111}, it is
possible to explain the phenomenon of the change of the
5 ratio of crystal orientation of {411} and {111} after
primary recrystallization by control of the lamellar
spacing of the crystal structure before cold rolling.
The present invention created based on the above
discoveries will be successively explained below.
10 First, the reasons for limitation of the ingredients
of the silicon steel material used in the present
invention will be explained.
The present invention uses as a material a silicon
steel slab for grain-oriented electrical steel sheet
15 containing at least, by mass%, Si: 0.8 to 7%, C: 0.085%
or less, acid soluble Al: 0.01 to 0.065%, and N: 0.012%
or less and having a balance of Fe and unavoidable
impurities as a basic composition of ingredients and if
necessary containing other ingredients. The reasons for
20 limitation of the ranges of content of the ingredients
are as follows.
If the amount of Si is increased, the electrical
resistance rises and the core loss characteristic is
improved. However, if added over 7%, cold rolling becomes
25 extremely difficult and the sheet ends up cracking at the
time of rolling. The value more suited for industrial
production is 4.8% or less. Further, if smaller than
0.8%, at the time of final annealing, y transformation
occurs and the crystal orientation of the steel sheet
30 ends up being impaired.
C is an element effective in controlling the primary
recrystallized structure, but has a detrimental effect on
the magnetic properties, so decarburization is necessary
before final annealing. If C is greater than 0.085%, the
35 decarburization annealing time becomes longer and the
productivity in industrial production is impaired.
The acid soluble Al is an essential element which
- 17 -
bonds with N in the present invention to form (Al,Si)N
functioning as an inhibitor. The 0.01 to 0.065% where the
secondary recrystallization stabilizes is made the range
of limitation.
5 N, if over 0.012%, causes holes called "blisters" in
the steel sheet at the time of cold rolling, so is made
not to exceed 0.012%.
In the present invention, the slab material may
include, in addition to the above ingredients, in
10 accordance with need at least one type of element of Mn,
Cr, Cu, P, Sn, Sb, Ni, S, and Se in amounts, by mass%, of
Mn of 1% or less, Cr of 0.3% or less, Cu of 0.4% or less,
P of 0.5% or less, Sn of 0.3% or less, Sb of 0.3% or
less, Ni of 1% or less, and a total of S and Se of 0.015%
15 or less. That is,
Mn has the effect of raising the specific
resistivity and reducing the core loss. Further, for the
purpose of preventing cracking in hot rolling, it is
preferably added in an amount of Mn/(S+Se)>4 in relation
20 to the total amount of S and Se. However, if the amount
of addition exceeds 1%, the magnetic flux density of the
product ends up falling.
Cr is an element effective for improving the
oxidized layer in decarburizing annealing and forming a
25 glass film and is added in a range of 0.3% or less.
Cu is an element effective for raising the specific
resistivity and reducing the core loss. If the amount of
addition is over 0.4%, the effect of reduction of the
core loss becomes saturated. This becomes a cause of the
30 surface defect of "bald spots" at the time of hot
rolling.
P is an element effective for raising the specific
resistivity and reducing the core loss. If the amount of
addition is over 0.5%, a problem arises in the
35 rollability.
Sn and Sb are well known grain boundary segregating
elements. The present invention contains Al, so depending
- 18 -
on the conditions of the final annealing, sometimes the
moisture released from the annealing separator causes the
Al to be oxidized and the inhibitor strength to fluctuate
at the coil position and the magnetic properties
5 fluctuates by the coil position. As one countermeasure,
there is the method of preventing oxidation by adding
these grain boundary segregating elements. For this
reason, these can be added in ranges of 0.30% or less. On
the other hand, if over 0.30%, the steel becomes
10 difficult to oxidize at the time of decarburizing
annealing, formation of a glass film becomes
insufficient, and the decarburizing annealing ability is
remarkably impaired.
Ni is an element effective for raising the specific
15 resistivity and reducing the core loss. Further, it is an
element effective v/hen controlling the metal structure of
the hot rolled sheet to improve the magnetic properties.
However, if the amount of addition exceeds 1%, the
i
secondary recrystallization becomes unstable.
20 In addition, S and Se have a detrimental effect on
the magnetic properties, so the total amount is
preferably made 0.015% or less.
Next, the production conditions of the present
invention will be explained.
25 The silicon steel slab having the above composition
of ingredients is obtained by producing the steel by a
converter, electric furnace, etc., vacuum degassing the
molten steel in accordance with need, then continuously
casting or making ingots, then cogging. After this, the
30 slab is heated before hot rolling. In the present
invention, the slab heating temperature is made 1280°C or
less to avoid the above problems of high temperature slab
heating.
The silicon steel slab is usually cast to a
35 thickness of a range of 150 to 350 mm, preferably a
thickness of 220 to 280 mm, but it may also be a socalled
thin slab of a range of 30 to 70 mm. In the case
- 19 -
of a thin slab, there is the advantage that it is not
necessary to roughly rolled process the steel to an
intermediate thickness at the time of producing hot
rolled sheet.
5 The slab heated by the above temperature is next hot
rolled and made a hot rolled sheet of the required sheet
thickness,
In the present invention, (a) this hot rolled sheet
is heated to a predetermined temperature of 1000 to 1150°C
10 to cause recrystallization, then is annealed at a
temperature lower than this of 850 to 1100°C for the
necessary time. Alternatively, (b) it is decarburized in
the process of annealing this hot rolled sheet so that
the difference in amount of carbon of the steel sheet
15 before and after decarburization becomes 0.002 to 0.02
mass%.
By doing this, the lamellar spacing of the grain
structure of the steel sheet after annealing (or steel
sheet surface layer) is controlled to 20 um or more.
20 When annealing as in (a), the first annealing
temperature range is made 1000 to 1150°C because a steel
sheet of a magnetic flux density of B8 of 1.91T or roore
is obtained when recrystallized in this range as shown in
FIG. 3, while the second annealing temperature range is
25 made 850 to 1100°C lower than the first temperature
because, as shown in FIG. 2, this is necessary for making
the lamellar spacing 20 jam or more.
As more preferable conditions, the first annealing
temperature is 1050 to 1125°C and the second annealing
30 temperature is 850°C to 950°C.
The first annealing, from the viewpoint of promoting
recrystallization of the hot rolled sheet, is performed
at 5°C/s or more, preferably 10°C/s or more. At a high
temperature of 1100°C or more, the annealing should be
35 performed for 0 second or more, while at a low
- 20 -
temperature of 1000°C or so, it is performed for 30
seconds or more. Further, the second annealing time, from
the viewpoint of controlling the lamellar structure,
should be 20 seconds or more. After the second annealing,
5 from the viewpoint of maintaining the lamellar structure,
the sheet should be cooled by a cooling rate of an
average 5°C/s or more, preferably 15°C/s or more.
Note that annealing a hot rolled sheet in two stages
is described in Japanese Patent Publication (A) No. 2005-
10 226111 as well, but the method of production of grainoriented
electrical steel sheet described in this
publication is a combination of the method of causing the
inhibitor to finely precipitate by the hot rolling
process etc. explained in the section on the background
15 art and the method of forming an inhibitor by nitridation
after decarburization annealing. The object* of this
annealing is the adjustment of the state of the
inhibitor. That is not related at all to the fact that,
like in the present invention, when using the latter
20 method to produce grain-oriented electrical steel sheet,
annealing the hot rolled sheet in two stages so as to
control the lamellar spacing in the grain structure after
annealing enables the ratio of grains of an orientation
enabling easy secondary recrystallizatlon after primary
25 recrystallizatlon to be increased even if making the
range of rapid heating in the temperature elevation
process of decarburizing annealing a lower temperature
range.
Further, when decarburizing the sheet in the process
30 of annealing the hot rolled sheet as in (b),
as the treatment method, the method of introducing
steam into the atmospheric gas to adjust the oxidation
degree and, further, the method of coating a
decarburization accelerator (for example, K2CO3 or Na2CC>3)
35 on the surface of the steel sheet or another known method
may be used.
The amount of decarburization at that time
- 21 -
(difference of amounts of carbon of steel sheet before
and after decarburization) is made a range of 0.002 to
0.02 mass%, preferably a range of 0.003 to 0.008 mass% to
control the lamellar spacing of the surface1 layer. If the
5 amount of decarburization is less than 0.002 mass%, there
is no effect on the lamellar spacing of the surface,
while if 0.02 mass% or more, there is a detrimental
effect on the texture of the surface part.
The hot rolled sheet controlled to a lamellar
10 spacing of 20 urn or more in this way is then cold rolled
once or two or more times with intermediate annealing to
obtain the final sheet thickness. The number of times of
cold rolling is suitably selected considering the level
of characteristics and cost of the product desired. At
15 the time of cold rolling, making the final cold rolling
rate 80% or more is necessary for promoting the {411} and
{111} or other primary recrystallization orientation.
The cold rolled steel sheet is decarburization
annealed in a moist atmosphere so as to remove the C
20 contained in the steel. At that time, by making the ratio
of I{111}/I{411} in the grain structure after
decarburization annealing 3 or less and then increasing
the nitrogen before causing the secondary
recrystallization,—it is possible to stably produce a
25 product with a high magnetic flux density.
As the method for controlling the primary
recrystallization after this decarburization annealing,
the heating rate in the temperature elevation process of
the decarburizing annealing step is adjusted. The present
30 invention is characterized by the point of rapid heating
between a steel sheet temperature of at least 550°C to
720°C by a heating rate of 40°C/s or more, preferably 50
to 250°C/s, more preferably 75 to 125°C/s.
The heating rate has a large effect on the primary
35 recrystallized texture I{lll}/I{411}. In primary
recrystallization, the ease of recrystallization differs
- 22 -
depending on the crystal orientation, so to make
I{111}/I{411} 3 or less, control to a heating rate
enabling easy recrystallization of the {411} oriented
grains is necessary. {411} oriented grains easily
5 recrystallize the most at a speed near 100°C/s, so to make
the I{111}/I{411} 3 or less and stably produce a product
with a magnetic flux density B8 of 1.91T or more, the
heating rate is made 40°C/s or more, preferably 50 to
250°C/s, more preferably 75 to 125°C/s.
10 The temperature range at which heating by this
heating rate is necessary is basically the temperature
range from 550°C to 720°C. Of course, it is also possible
to start the rapid heating by the above heating rate
range from a temperature under 550°C. The lower limit
15 temperature of the temperature range for maintaining this
heating rate at a high heating rate is affected by the
heating cycle in the low temperature region. For this
reason, when making the temperature range where rapid
heating is required the start temperature Ts (°C) to
20 720°C, the range should be made the following Ts (°C) to
720°C in accordance with the heating rate H (°C/s) from
room temperature to 500°C.
H oriented grains
grow preferentially by secondary recrystallization.
When using an annealing separator having alumina as
its main ingredient, as shown in Japanese Patent
Publication (A) No. 2003-268450, an electrical steel
20 sheet with a smoothed (mirror) surface is obtained after
final annealing.
As explained above, in the present invention, when
producing grain-oriented electrical steel sheet by
h^afjng silicon RJ-RRI to a temperature of 1280°C or less,
25 then hot rolling it, annealing the hot rolled sheet, then
cold rolling it once or cold rolling it a plurality of
times with intermediate annealing to obtain the final
sheet thickness, decarburizing annealing it, then coating
an annealing separator and final annealing it and
30 nitriding the steel sheet from the decarburization
annealing to the start of secondary recrystallization of
the final annealing, by (a) annealing the hot rolled
sheet by heating it to a predetermined temperature of
1000 to 1150°C to cause recrystallization, then annealing
35 by a temperature lower than that of 850 to 1100°C or by
(b) decarburizing the hot rolled sheet in annealing so
- 25 -
that the difference in amounts of carbon of the steel
sheet before and after hot rolled sheet annealing becomes
0.002 to 0.02 mass% to thereby control the lamellar space
to 20 um or more in the grain structure of the steel
5 sheet after hot rolled sheet annealing (or surface layer
grain structure) and by heating the cold rolled steel
sheet in the temperature elevation process at the time of
decarburization annealing between a steel sheet
temperature of 550°C to 720°C by a heating rate of 40°C/s
10 or more, preferably 50 to 250°C/s, more preferably 75 to
125°C/s, then performing the decarburization annealing in
the temperature range of 770 to 900°C under: conditions of
an oxidation degree of the atmospheric gas (PH20 /PH2) in
the range of over 0.15 to 1.1 with a time by which the
15 amount of oxygen of the steel sheet becomes 2.3 ,g/m2 or
less and the primary recrystallization grain size becomes
15 um or more, it is possible to produce grain-oriented
electrical steel sheet with a high magnetic flux density
and, further, by using an annealing separator mainly
20 comprised of alumina at the time of final annealing, it
is possible to produce a mirror surface grain-oriented
electrical steel sheet with a high magnetic flux density.
Below, examples of the present invention will be
explained, but the conditions employed in the examples
25 are examples of conditions for confirming the workability
and advantageous effects of the present invention. The
present invention is not limited to this example. The
present invention may employ various conditions insofar
as not departing from the present invention and achieving
30 the object of the present invention.
EXAMPLES
(Example 1)
A silicon steel slab containing, by mass%, Si: 3.3%,
C: 0.06%, acid soluble Al: 0.028%, and N: 0.008% and
35 having a balance of Fe and unavoidable impurities was
heated at a temperature of 1150°C, then hot rolled to a
- 26 -
2.3 mm thickness, then samples (A) were annealed by a
single stage of 1120°C and samples (B) were annealed by
two stages of 1120°C+920°C. These samples were cold rolled
to a 0.22 mm thickness, then heated by heating rates of
5 (1) 15°C/s, (2) 40°C/s, (3) 100°C/s, and (4) 300°C/s to
720°C, then heated by 10°C/s to a temperature of 830°C for
decarburization annealing, then annealed in an ammoniacontaining
atmosphere to increase the nitrogen in the
steel sheet to 0.02%, then coated by an annealing
10 separator mainly comprised of MgO, then final annealed.
The magnetic properties after final annealing of the
obtained samples are shown in Table 1. Note that the
notations of the samples show the combination of the
annealing method and heating rate."
15 Table 1
(Example 2)
A silicon steel slab containing, by mass%, Si: 3.3%,
C: 0.055%, acid soluble Al: 0.027%, N: 0.008%, Mn: 0.1%,
20 S: 0.007%, Cr: 0.1%, Sn: 0.05%, P: 0.03%, and Cu: 0.2%
and having a balance of Fe and unavoidable impurities was
heated to a temperature of 1150°C, then hot rolled to a
2.3 mm thickness, then samples (A) were annealed by one
stage at 1100°C and samples (B) were annealed by two
25 stages at 1100°C+900°C. These samples were cold rolled to
0.22 mm thicknesses, then heated by a heating rate of
40°C/s to 550°C and further heated by heating rates of (1)
- 27 -
l5°C/s, (2) 40°C/s, and (3) 100°C/s to 550 to 720°C, then
further heated by a heating rate of 15°C/s and
decarburization annealed at a temperature of 840°C, then
annealed in an ammonia-containing atmosphere to increase
5 the nitrogen in the steel sheet to 0.02%, then coated
with an annealing separator mainly comprised of MgO, then
final annealed.
The magnetic properties of the obtained samples
after final annealing are shown in Table 2.
10 T^hlfi 2
(Example 3)
A silicon steel slab containing, by mass%, Si: 3.3%,
C: 0.055%, acid soluble Al: 0.027%, N: 0.008%, Mn: 0.1%,
15 S: 0.007%, Cr: 0.1%, Sn: 0.06%, P: 0.03%, and Ni: 0.2%
and having a balance of Fe and unavoidable impurities was
heated to a temperature of 1150°C, then hot rolled to a
2.3 mm thickness, then samples (A) were annealed by a
single stage of 1100°C and samples (B) were annealed by
20 two stages of 1100°C+900°C. These sample were cold rolled
to a 0.22 mm thickness, then heated by a heating rate of
(1) 15°C/s, (2) 40°C/s, (3) 100°C/s, and (4) 200°C/s to
720°C, then heated by a heating rate of 10°C/s for
decarburization annealing to a temperature of 840°C, then
25 annealed in an ammonia-containing atmosphere to increase
the nitrogen in the steel sheet to 0.02%, then coated by
an annealing separator mainly comprised of MgO, then
final annealed.
- 28 -
The magnetic properties after final annealing of the
obtained samples are shown in Table 3.
Table 3
5 (Example 4)
A silicon steel slab containing, by mass%, Si: 3.3%,
C: 0.055%, acid soluble Al: 0.028%, N: 0.008%, Mn: 0.1%,
Se: 0.007%, Cr: 0.1%, P: 0.03%, and Sn: 0.05% and having
a balance of Fe and unavoidable impurities was heated to
10 a temperature of 1150°C, then hot rolled to a 2.3 mm
thickness, then samples (A) were annealed by a single
stage of 1120°C and samples (B) were annealed by two
stages of 1120°C+900°C. These samples were cold rolled to
a 0.22 mm thickness, then heated by a heating rate of
-3-5 15°C/S to 550°C,—thon further heated by a heating rate of
(1) 15°C/s, (2) 40°C/s, and (3) 100°C/s to 550 to 720°C,
then further heated by a heating rate of 10°C/s for
decarburization annealing at a temperature of 830°C, then
annealed in an ammonia-containing atmosphere to increase
20 the nitrogen in the steel sheet to 0.02, then coated by
an annealing separator mainly comprised of MgO, then
final annealed.
The magnetic properties after final annealing of the
obtained samples are shown in Table 4.
(Example 5)
A silicon steel slab containing, by mass%, Si: 3.3%,
5 C: 0.06%, acid soluble Al: 0.028%, N: 0.008%, Mn: 0.1%,
S: 0.008%, Cr: 0.1%, and P: 0.03% and having a balance of
Fe and unavoidable impurities was heated to a temperature
of 1150°C, then hot rolled to a 2.3 mm thickness, then
annealed by two stages of 1120°C+920°C. Samples were cold
10 rolled to a 0.22 mm thickness, then heated by a heating
rate of 100°C/s to 720°C, then heated by 10°C/s to a
temperature of 830°C for decarburization annealing, then
annealed in an ammonia-containing atmosphere to increase
the nitrogen in the steel sheet to 0.008 to 0.025%, then
15 coated by an annealing separator mainly comprised of MgO,
then final annealed.
The magnetic properties after final annealing of the
obtained samples with different amounts of nitrogen are
shown in Table 5.
20 Tahlo R
- 30 -
acid soluble Al: 0.028%, and N: 0.008% and .having a
balance of Fe and unavoidable impurities was heated to a
temperature of 1150°C, then hot rolled to a 2.3 mm
thickness, then samples (A) were heated by a single stage
5 of 1120°C and samples (B) were heated by two stages of
1120°C+920°C. These samples were cold rolled to a 0.22 mm
thickness, then heated by a heating rate of (1) 15°C/s,
(2) 40°C/s, (3) 100°C/s, and (4) 300°C/s to ;720°C, then
heated by 10°C/s to a temperature of 830°C for
10 decarburization annealing, then annealed in an ammoniacontaining
atmosphere to increase the nitrogen in the
steel sheet to 0.024%, then coated with an annealing
separator mainly comprised of MgO, then final annealed.
The magnetic properties after final annealing of
15 samples are shown in Table 6. When both the hot rolled
sheet annealing and decarburization annealing satisfy the
conditions of the present invention, a high magnetic flux
density is obtained.
20
l_
{Example 7)
A slab containing, by mass%, Si: 3.3%, C: 0.06%,
acid soluble Al: 0.028%, and N: 0.008% and having a
balance of Fe and unavoidable impurities was heated to a
25 temperature of 1150°C, then was hot rolled to a 2.3 mm
thickness, then was annealed at a temperature of 1100°C.
At that time, steam was blown into the atmospheric gas
- 31 - ,
(mixed gas of nitrogen and hydrogen) to decarburize the
surface and change the lamellar spacing of the surface
layer. Samples were cold rolled to a 0.22 mm thickness,
then heated by a heating rate of 100°C/s to 720°C, then
5 heated by 10°C/s to a temperature of 830°C for
decarburization annealing, then annealed in an ammoniacontaining
atmosphere to increase the nitrogen in the
steel sheet to 0.02%, then coated with an annealing
separator mainly comprised of MgO, then final annealed.
10 The magnetic properties after final annealing of the
obtained samples with different lamellar spacings of the
surface layer are shown in Table 7.
Tahla 1
15 (Example 8)
As samples, the steel sheets given a lamellar
spacing of the surface layer of 29 urn after annealing the
hot rolled sheets in Example 7 were used. The samples
were cold rolled to a 0.22 mm thickness, then heated by
20 heating rates of 10 to 200°C/s to 720°C, then heated by
10°C/s to a temperature of 830°C for decarburization
annealing, then annealed in an ammonia-containing
atmosphere to increase the nitrogen in the steel sheet to
0.02%, then coated with an annealing separator mainly
25 comprised of MgO, then final annealed.
The magnetic properties after final annealing of the
samples with different heating rates obtained are shown
in Table 8.
(Example 9)
A slab containing, by mass%, Si: 3.3%, C: 0.055%,
5 acid soluble Al: 0.027%, N: 0.008%, Mn: 0.1%, S: 0.007%,
Cr: 0.1%, Sn: 0.05%, P: 0.03%, and Cu: 0.2% and having a
balance of Fe and unavoidable impurities was heated to a
temperature of 1150°C, then hot rolled to 2.3 mm
thickness, then samples (A) were left as they were, while
10 samples (B) were coated on their surfaces with K2C03, and
the samples were annealed in a dry atmospheric gas of
nitrogen and hydrogen at a temperature of 1080°C. These
samples were cold rolled to 0.22 mm thickness, then
heated by a heating rate of 20°C/s to 550°C, heated by a
15 heating rate of 100°C/s to 550 to 720°C, then heated by a
heating rate of 15°C/s and decarburization annealed at a
temperature of 840°C, then annealed in an ammoniacontaining
atmosphere to increase the nitrogen in the
steel sheet to 0.022%, then coated with an annealing
20 separator mainly comprising MgO, then final annealed.
The magnetic properties after final annealing of the
obtained samples with different lamellar spacings of the
surface layer are shown in Table 9.
Table 9
25
- 33 -
A silicon steel slab containing, by 111333%, Si: 3.3%,
C: 0.055%, acid soluble Al: 0.027%, and N: 0.008% and
having a balance of Fe and unavoidable impurities was
heated to a temperature of 1150°C, then hot rolled to 2.3
5 mm thickness, then annealed at 1110°C. At that time, steam
was blown into the atmospheric gas (mixed gas of nitrogen
and hydrogen) to cause the surface to decarburize and
make the lamellar spacing of the surface layer 26 urn.
These samples were cold rolled to a 0.22 mm thickness,
10 then heated in an atmosphere comprised of nitrogen and
hydrogen having an oxidation degree of 0.59 by a heating
rate of 100°C/s to 720°C, then heated by 10°C/s to a
temperature of 830°C for decarburization annealing, then
annealed in an ammonia-containing atmosphere to increase
15 the nitrogen in the steel sheet to 0.008 to 0.026%, then
coated with an annealing separator mainly comprised of
MgO, then final annealed.
The magnetic properties after final annealing of the
obtained samples with different amounts of nitrogen are
20 shown in Table 10.
(Example 11)
As samples, the cold rolled sheets of the sheet
25 thickness of 0.22 mm used in Example 10 were heated in an
atmospheric gas comprised of nitrogen and hydrogen with
an oxidation degree of 0.67 by heating rates of 50°C/s to
750°C, then were heated by 15°C/s to a temperature of 780
to 830°C for decarburization annealing, then annealed in
- 34 -
an ammonia-containing atmosphere to increase the nitrogen
in the steel sheet to 0.021%, then coated with an
annealing separator mainly comprised of MgO, then final
annealed.
5 The magnetic properties after final annealing of the
obtained samples with different primary recrystallization
grain sizes are shown in Table 11.
Table 11
10 (Example 12)
A silicon steel slab containing, by mass%, Si: 3.3%,
C: 0.06%, acid soluble Al: 0.028%, N: 0.008%, Mn: 0.1%,
S: 0.008%, Cr: 0.1%, and P: 0.03% and having a balance of
Fe and unavoidable impurities was heated to a temperature
15 of 1150°C, hot rolled to 2.3 mm thickness, then annealed
in two stages of 1120°C+920°C and cold rolled to 0.22 mm
thickness. Its cold rolled sheets were heated by a
heating rate of (A) 15°C/s and (B) 50°C/s until
temperatures of (1) 500°C, (2) 550°C, and (3) 600°C, then
20 were heated by a heating rate of 100°C/s to 720°C and
further heated by 10°C/s to a temperature of 830°C for
decarburization annealing. Next, they were annealed in an
ammonia-containing atmosphere to increase the nitrogen in
the steel sheet to 0.024%, then coated with an annealing
25 separator mainly comprised of MgO, then final annealed.
The magnetic properties after final annealing are
shown in Table 12. By increasing the low temperature
region heating rate, it is learned that excellent
magnetic properties are obtained even if raising the
30 start temperature for heating by 100°C/s to 600°C.
INDUSTRIAL APPLICABILITY
The present invention uses low temperature slab
5 heating to produce grain-oriented electrical steel sheet
during which annealing the hot rolled sheet by two stages
of temperature ranges so as to lower the upper
temperature limit of the control range of the heating
rate in the temperature elevation process of the
10 decarburizing annealing, performed to improve the grain
structure after the primary recrystallization after
decarburization annealing, and to enable heating by only
induction heating, so can perform that heating more
easily using induction heating and can more stably
15 produce grain-oriented electrical steel sheet high in
magnetic flux density and superior in magnetic
properties-; for this reason, it has great industrial
applicability.

We claim:
1. A method of production of grain-oriented electrical steel sheet comprising heating a silicon steel material containing, by mass%, Si: 0.8 to 7%, C: 0.085% or less, acid soluble Al: 0.01 to 0.065%, and N: 0.012% or less, and optionally at least one selected from Mn: 1% or less, Cr: 0.3% or less, Cu: 0.4% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and S and Se in a total of 0.015% or less, and a balance consisting of Fe and unavoidable impurities, at a temperature of 1280°C or less, then hot rolling it, annealing the obtained hot rolled sheet, then cold rolling it once or cold rolling it several times with intermediate annealing to obtain steel sheet of the final sheet thickness, decarburization annealing this steel sheet, then coating an annealing separator, applying final annealing, and applying treatment to increase an amount of nitrogen of the steel sheet from the decarburization annealing to the start of secondary recrystallization of the final annealing, wherein,
in the annealing process of the hot rolled sheet, decarburizing the steel sheet to 0.002 to 0.02 mass% of the amount of carbon before decarburization annealing to thereby control a lamellar spacing in the surface layer grain structure after annealing to 20 µm or more and
performing only an induction heating in the temperature elevation process in the decarburization annealing of the steel sheet of the final sheet thickness by a heating rate of 40°C/s or more, in the temperature range of a steel sheet temperature of 550°C to 720°C.

2. A method of production of grain-oriented electrical steel sheet as claimed in claim 1, wherein heating in the temperature evaluation process in the decarburization annealing of the steel sheet by a heating rate of 50 to 250°C/s in the temperature range of a steel sheet temperature of 550°C to 720°C.

3. A method of production of grain-oriented electrical steel sheet as claimed in claim 1, wherein heating in the temperature elevation process in the decarburization annealing of the steel sheet by a heating rate of 75 to 125°C/s in the temperature range of a steel sheet temperature of 550°C to 720°C.

4. A method of production of grain-oriented electrical steel sheet as claimed in any one of claims 1 to 3, wherein, making the temperature range for heating by said heating rate in the temperature elevation process in the decarburization annealing, to be from Ts (°C) to 720°C, making it the following range from Ts (°C) to 720°C in accordance with the heating rate H (°C/s) from room temperature to 500°C:
H=15: Ts=550
15 < H: Ts = 600

5. A method of production of grain-oriented electrical steel sheet as claimed in any one of claims 1 to 4, wherein, performing said decarburization annealing in a time interval so that the amount of oxygen of the steel sheet becomes 2.3 g/m2 or less, and the primary recrystallization grain size becomes 15 µm or more, in a temperature range of 770 to 900°C under the conditions where the oxidation degree (PH2O/PH2) of the atmospheric gas is in a range of over 0.15 to 1.1.

6. A method of production of grain-oriented electrical steel sheet as claimed in any one of claims 1 to 5, wherein increasing the amount of nitrogen [N] of said steel sheet in accordance with an amount of acid soluble Al [Al] of the steel sheet so as to satisfy the formula [N]=14/27[Al].

7. A method of production of grain-oriented electrical steel sheet as claimed in claim 6, wherein increasing the amount of nitrogen [N] of said steel sheet in accordance with an amount of acid soluble Al [Al] of the steel sheet so as to satisfy the formula [N]=2/3[Al].

8. A method of production of grain-oriented electrical steel sheet as claimed in any one of claims 1 to 7, wherein, when coating said annealing separator, coating an annealing separator mainly comprised of alumina and performing the final annealing.