U.S. patent application number 14/130806 was filed with the patent office on 2014-09-18 for method for producing a grain-oriented electrical steel flat product intended for electrotechnical applications.
This patent application is currently assigned to THYSSENKRUPP ELECTRICAL STEEL GMBH. The applicant listed for this patent is Christof Holzapfel, Thorsten Krenke, Ludger Lahn, Heiner Schrapers. Invention is credited to Christof Holzapfel, Thorsten Krenke, Ludger Lahn, Heiner Schrapers.
Application Number | 20140261895 14/130806 |
Document ID | / |
Family ID | 46508021 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140261895 |
Kind Code |
A1 |
Schrapers; Heiner ; et
al. |
September 18, 2014 |
Method for Producing a Grain-Oriented Electrical Steel Flat Product
Intended for Electrotechnical Applications
Abstract
The invention relates to a method for producing a grain-oriented
steel flat product for electrotechnical applications, wherein, in a
production step, "decarburising and nitriding annealing" is carried
out in two stages. The first stage of the annealing process extends
over a first time interval, which comprises heating the cold strip
starting from a start temperature to a first target annealing
temperature and holding it at this target annealing temperature,
and the second stage of the annealing process extends over a second
time interval, in which the cold strip is heated to a second target
annealing temperature and subsequently held at this target
annealing temperature.
Inventors: |
Schrapers; Heiner;
(Duisburg, DE) ; Krenke; Thorsten; (Duisburg,
DE) ; Holzapfel; Christof; (Gelsenkirchen, DE)
; Lahn; Ludger; (Moers, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schrapers; Heiner
Krenke; Thorsten
Holzapfel; Christof
Lahn; Ludger |
Duisburg
Duisburg
Gelsenkirchen
Moers |
|
DE
DE
DE
DE |
|
|
Assignee: |
THYSSENKRUPP ELECTRICAL STEEL
GMBH
Gelsenkirchen
DE
|
Family ID: |
46508021 |
Appl. No.: |
14/130806 |
Filed: |
July 4, 2012 |
PCT Filed: |
July 4, 2012 |
PCT NO: |
PCT/EP2012/063039 |
371 Date: |
June 2, 2014 |
Current U.S.
Class: |
148/208 |
Current CPC
Class: |
C21D 8/1233 20130101;
C21D 8/1277 20130101; C22C 38/008 20130101; C22C 38/38 20130101;
C22C 38/02 20130101; C21D 8/1283 20130101; C21D 8/1272 20130101;
C22C 38/04 20130101; C22C 38/06 20130101; C21D 8/1211 20130101;
C21D 8/1255 20130101; C21D 8/1205 20130101; C22C 38/34 20130101;
C22C 38/001 20130101; C22C 38/20 20130101; C21D 8/1222
20130101 |
Class at
Publication: |
148/208 |
International
Class: |
C21D 8/12 20060101
C21D008/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 2011 |
DE |
10 2011 107 304.7 |
Claims
1. A method for producing a grain-oriented electrical steel flat
product, comprising the following production steps: a) producing a
steel melt which contains, in addition to iron and unavoidable
impurities, (in % wt.): Si: 2.5-4.0%, C: 0.02-0.1%, Al:
0.01-0.065%, N: 0.003-0.015%, and optionally: up to 0.30 % Mn, up
to 0.05% Ti, up to 0.3 % P, one or more elements from the group S,
Se in contents of which the total is at most 0.04%, one or more
elements from the group As, Sn, Sb, Te, Bi with contents of up to
0.2% in each case, one or more elements from the group Cu, Ni, Cr,
Co, Mo with contents of up to 0.5% in each case, one or more
elements from the group B, V, Nb with contents of up to 0.012% in
each case, b) casting the melt into a strand in a continuous
casting machine, c) separating at least one thin slab from the cast
strand, d) heating the thin slab to a temperature between
1050.degree. C. and 1300.degree. C., e) hot rolling the thin slab
into a hot strip having a thickness of 0.5-4.0 mm in a hot-rolling
train, 2QB7015.DOC Page 6 f) cooling the hot strip, g) coiling the
hot strip into a coil, h) cold rolling the hot strip into a cold
strip having a final thickness of 0.15-0.50 mm, i) decarburising
and nitriding annealing of the cold strip obtained, j) applying an
annealing separator onto the surface of the annealed cold strip,
and k) final annealing of the cold strip provided with the
annealing separator to form a Goss texture, wherein the cold strip
is annealed in at least two stages in the course of production step
i), the first stage of this annealing process extends over a first
time interval and comprises heating the cold strip starting from a
start temperature to a first target annealing temperature and
subsequently holding the heated cold strip at the target annealing
temperature, the second stage of the annealing process extends over
a second time interval, within which the cold strip is firstly
heated to a second target annealing temperature and is subsequently
held at this target annealing temperature, the first target
annealing temperature is 10-50.degree. C. lower than the second
target annealing temperature, and the duration of the first time
interval is 30-70% of an entire duration of the annealing treatment
comprising the first time interval and the second time
interval.
2. The method according to claim 1, wherein the first target
annealing temperature is 10-30.degree. C. lower than the second
target annealing temperature.
3. The method according to claim 1, wherein the duration of the
first time interval is 30-60% of the entire duration of the
annealing treatment.
4. The method according to claim 1, wherein the heating rate, at
which the cold strip is heated from the start temperature to the
first target annealing temperature in the first annealing stage, is
25-500.degree. C./s.
5. The method according to claim 4, wherein the heating rate is at
least 200.degree. C.
6. The method according to claim 5, wherein the cold strip is
inductively heated.
7. The method according to claim 1, wherein in the production step
i) the first and second annealing stages are completed following
one another and a further annealing step is subsequently carried
out, in which the cold strip is subjected to decarburising and
nitriding annealing.
8. The method according to claim 7, wherein the first and second
annealing stages are carried out in production step i) as a pure
decarburisation annealing process.
Description
[0001] The invention relates to a method for producing
grain-oriented electrical steel flat products intended for
electrotechnical applications. Such electrical steel flat products
are also, in practice, referred to as grain-oriented "electrical
sheets" or grain-oriented "electrical strips".
[0002] Grain-oriented electrical steel flat products have special
magnetic properties and are produced by means of an elaborate
production process. The base material for electrical steel flat
products is a silicon steel sheet. The metallurgical properties of
the material, the deformation degrees of the rolling processes and
the parameters of the heat treatment steps are coordinated such
that targeted recrystallisation processes take place. These
recrystallisation processes result in a "Goss texture" which is
typical for the material, in which the direction of easiest
magnetisability is in the rolling direction of the finished
strips.
[0003] Electrical steel flat products, in which the grains do not
have a distinct alignment, are to be differentiated from
grain-oriented electrical sheet or strip of the kind in question
here. In such non-grain-oriented electrical strip or sheet, the
magnetic flux is not fixed in any specific direction, so that
identical magnetic properties form in all directions (isotropic
magnetisation).
[0004] Grain-oriented electrical strip or sheet of the kind in
question here, in contrast, has a strongly anisotropic magnetic
behaviour. This can be attributed to a uniform orientation of the
grains (crystallites) of the microstructure. This crystallographic
texture is achieved by means of an effective grain growth selection
effected by corresponding measures in the production process. The
aim is to obtain an electrical steel flat product after final
annealing, which takes place at the end of the production process,
in which the grains have a low misorientation and hence have an
almost ideal texture.
[0005] Grain-oriented electrical strip is particularly suitable for
applications, in which particularly high requirements are imposed
on the magnetic properties, as is the case, example, when building
transformers.
[0006] A relatively large number of methods are known for producing
high-grade grain-oriented electrical sheet.
[0007] With the so-called "low-heating method" described in EP 0
910 676 B1, high permeable, grain-oriented electrical sheets can be
produced having an optimised distribution of properties. This
method is characterised by a slab heating temperature below
1250.degree. C. Due to this comparatively low temperature,
aluminium nitrides, which are brought fully into solution during
the high-temperature annealing step carried out at the end of the
production process, are only partly dissolved and precipitated
again. Consequently, electrical strip produced according to the
low-heating process has a weaker inherent inhibition than material
produced by means of the conventional process path via
high-temperature slab heating.
[0008] The purpose of particle inhibition is to suppress the grain
growth in the primary microstructure of the cold strip during and
after decarburisation annealing. Controlled abnormal grain growth
in the temperature range from 950-1100.degree. C. is only to take
place during final coarse grain annealing, in which the cold strips
are annealed at temperatures of up to 1200.degree. C., in order to
make a high texture sharpness with Goss orientation (001) (110)
possible.
[0009] After decarburisation annealing, an ideal equilibrium state
between driving and restoring forces has to be set, so that optimum
abnormal grain growth with high texture sharpness begins. The
driving force for the grain growth during coarse grain annealing is
the grain boundary energy stored in the microstructure. This is
essentially determined by the grain size after primary
recrystallisation.
[0010] Due to the weaker inherent inhibition with the low-heating
method, the average primary grain size is greater after a
decarburisation annealing treatment than with the conventional
method and is subject to greater fluctuations through the cold
process. Hence, the driving force for the abnormal grain growth is
generally lower. On the other hand, a restoring force opposing the
abnormal grain growth is determined by the non-magnetic
precipitations (inhibitors) precipitated in the cold strip.
Therefore, it is essential to have many finely distributed
particles present. In the case of the low-heating method, the
relevant particles are not, however, produced in the hot strip, but
before, after or during decarburisation annealing or during the
heating phase of final annealing in the course of a variety of
nitriding processes.
[0011] In the course of the processes described in EP 0 950 119 B1
and EP 0 950 120 B1, via the hot-rolling process an inhibition
strength Iz is set by nitrides and sulphides in such a way that the
primary grain growth is inhibited during the cold process even at
higher temperatures. The slabs are heated to temperatures from
1100.degree. C. to 1320.degree. C. before hot rolling. A nitriding
treatment carried out simultaneously with the decarburisation
annealing treatment at temperatures between 850 and 1050.degree. C.
in an atmosphere containing ammonia enables the direct formation of
aluminium nitrides. The subsequent coarse grain annealing does not
have to be modified compared to the conventional production path
for the manufacture of grain-oriented electrical strip.
[0012] In contrast, in the case of the method described in EP 0 219
611 B1, the nitriding is carried out after primary
recrystallisation but before the abnormal grain growth begins.
Here, the nitriding can be effected by means of an atmosphere
having a nitriding capability or by means of a nitrogen-donating
adhesion protection additive.
[0013] Specifically in the case of the method with an
ammonia-containing atmosphere, in which the nitriding temperature
is below 850.degree. C., silicon-manganese nitrides are present
close to the surface after nitriding (Materials Science Forum
204-206 (1996), 143-154). Due to their lower thermodynamic
stability they dissolve during the heating phase of coarse grain
annealing. Then, the nitrogen diffuses into the steel matrix and
recombines with the free aluminium present there to form aluminium
nitride (Materials Science Forum 204-206 (1996), 593-598). The
aluminium nitrides formed in this was are thereupon the effective
inhibitors for the secondary grain growth. Although the inhibition
is weaker compared to the conventional process used for producing
grain-oriented electrical sheet, it enables a full secondary
recrystallisation at higher temperatures with a greater secondary
grain size in the finished strip (TMS Proceedings 3 (2008),
49-54).
[0014] However, a disadvantage of this procedure is that modified
time-temperature cycle of the coarse grain annealing process is
required. The dissolving of the silicon-manganese nitrides and the
new formation of AlN through nitrogen diffusion take place at
temperatures between 700 to 800.degree. C. In order to fully
facilitate this critical process step, when carrying out the
previously explained method an isothermal holding stage of as least
four hours is required during the heating phase of coarse grain
annealing. This not only causes the total duration of the process
to be considerably lengthened but also results in increased
production costs.
[0015] In addition to the previously explained prior art, a method
for producing high-grade grain-oriented electrical strip based on
thin slab continuous casting is known from EP 1 752 549 A1, in
which the production steps are coordinated such that an electrical
sheet having optimised magnetic properties is obtained using
conventional aggregates. In the course of this, the aim is to
prevent the formation of nitridic precipitations before and during
hot rolling as far as possible, so that the possibility of
producing such precipitations in a controlled manner during cooling
of the hot strip can be exploited to a large extent. Specifically,
firstly a steel is melted for this purpose which contains, in
addition to iron and unavoidable impurities, (in % mass) Si:
2.5-4.0%, C: 0.02-0.10%, Al 0.01-0.065%, N: 0.003-0.015%,
optionally up to 0.30% Mn, up to 0.05% Ti, up to 0.3% P, one or
more elements from the group S, Se in contents of which the total
is at most 0.04%, one or more elements from the group As, Sn, Sb,
Te, Bi with contents of up to 0.2% in each case., one or more
elements from the group Cu, Ni, Cr, Co, Mo with contents of up to
0.5% in each case, and one or more elements from the group B, V, Nb
rich contents of up to 0.012% in each case. The melt composed in
this way is then treated in a vacuum system or a ladle furnace in a
secondary metallurgical step and subsequently continuously cast
into a strand. Thin slabs are separated from the strand obtained in
this way and are subsequently heated to a temperature between
1050.degree. C. and 1300.degree. C. in a furnace situate in-line.
The dwell time in the furnace is at most 60 mins. After the thin
slabs have been heated, the thin slabs are hot rolled into hot
strip having a thickness of 0.5-4.0 mm in a multiple-stand
hot-rolling train situated in-line. During hot rolling, the first
forming pass is carried out at a temperature of 900-1200.degree. C.
with a deformation degree of more than 40%. Furthermore, at least
the two forming passes following the rolling at 900-1200.degree.
C., are rolled in the two-phase mixed region (.alpha.-y) during hot
rolling. Finally, in the last hot-rolling forming pass, the pass
reduction is at most 30%. Following hot rolling, the hot strip
obtained in this way is cooled and coiled into a coil. Optionally,
the hot strip can subsequently be annealed after coiling or before
cold rolling. Afterwards, the hot strip is cold rolled into a cold
strip having a final thickness of 0.25 mm to 0.50 mm. The cold
strip obtained, is then subjected to recrystallisation and
decarburisation annealing. In addition to decarburisation
annealing, the strip can also be nitrided in an NH.sub.3-containing
atmosphere at temperatures above 850.degree. C. After an annealing
separator has subsequently been applied onto the surface of the
cold strip subjected to an annealing treatment, the cold strip
coated in this way is subjected to a recrystallisation final
annealing treatment to form a Goss texture. Equally optionally, the
finally annealed cold strip can subsequently also be provided with
electrical insulation and finally stress-relieved
[0016] In EP 0 378 131 B1, the importance of the average grain size
and also its variance is indicated. In addition to an optimum
average grain size, it is thus particularly important that the
deviation from the average grain size in the sheet is slight. This
results from the fact that grain growth processes Lake place in a
more uncontrolled manner due to the lower inhibition (Materials
Science Forum 204-206 (1996), 623-628). Consequently, under
unfavourable processing conditions grains can grow which have no
Goss orientation, but at high temperatures are not capable of
growth and contribute to fine-grain formation.
[0017] Finally, in EP 0 392 534 B1 the eligible atmospheres for the
decarburisation annealing are described in detail. In this
connection, it is painted out that at the beginning of the
decarburisation annealing and nitriding annealing the partial
pressure p.sub.H20/p.sub.H2 must be lowered, in order to set a
suitable oxide layer. The result of this process is that a
satisfactory glass film is formed during coarse grain
annealing.
[0018] Against this background of the previously explained prior
art, the object of the invention was to specify a method, by means
of which grain-oriented electrical steel flat products can be
produced in a simple mariner wish an optimum uniform distribution
of the grain size.
[0019] This object was achieved according to the invention by a
method which comprises the measures specified in Claim 1.
[0020] Advantageous embodiments of the invention are specified in
the dependent claims and are explained in detail below together
with the general concept of the invention.
[0021] In accordance with the previously explained prior art, a
method according to the invention for producing a grain-oriented
electrical steel flat product intended for electrotechnical
applications comprises the following production steps: [0022] a)
producing a steel melt which contains, in addition to iron and
unavoidable impurities, (in % wt.) Si: 2.5-4.0%, C: 0.02-0.1%, Al:
0.01-0.065%, N: 0.003-0.015% and in each case optionally up to
0.30% Mn, up to 0.05% Ti, up to 0.3% P, one or more elements from
the group S, Se in contents of which the total is at most 0.04%,
one or more elements from the group As, Sn, Sb, Te, Bi with
contents of up to 0.2% in each case, one or more elements from the
group Cu, Ni, Cr, Co, Mo with contents of up to 0.5% in each case,
one or more elements, from the group B, V, Nb with contents of up
to 0.012% an each case, [0023] b) casting the melt into a strand in
a continuous casting machine, [0024] c) separating at least one
thin slab from the cast strand, [0025] d) heating the thin slab to
a temperature between 1050.degree. C.: and 1300.degree. C., [0026]
e) hot rolling the than slab into a hot strap having a thickness of
0.5-4.0 mm in a hot-rolling train, [0027] f) cooling the hot strip,
[0028] g) coiling the hot strip into a coil, [0029] h) cold rolling
the hot strip into a cold strip having a final thickness of
0.15-0.50 mm, [0030] i) decarburising and nitriding annealing of
the cold strip obtained, [0031] j) applying an annealing separator
onto the surface the annealed cold strip [0032] and [0033] k) final
annealing of the cold strip provided with the annealing separator
to form a Goss texture.
[0034] Of course, additional production steps, which are usually
required in the conventional production of grain-oriented
electrical strips or sheets, can be carried out during production
of the electrical steel flat product. These include, for example, a
single or multi-stage hot strip annealing treatment carried out
between the production steps g) and h), thermal flattening of the
cold strip and application of an insulation layer, which can be
carried out within the framework of the method according to the
invention using and taking into account the parameters known from
the prior art.
[0035] It is essential for the invention that the cold strip in the
course of production step i) "decarburising and nitriding annealing
of the cold strip obtained" is subjected to decarburisation and
nitriding annealing in at least two stages.
[0036] According to the invention, the first stage of this
annealing process extends over a first time interval which
comprises heating the cold strip starting from a start temperature
to a first target annealing temperature and subsequently holding it
at this target annealing temperature.
[0037] According to the invention, the second stage of the
annealing process extends in a corresponding manner over a second
time interval, within which the cold strip is firstly heated to a
second target annealing temperature and is subsequently held at
this target annealing temperature.
[0038] According to the invention, the first target annealing
temperature is 10-50.degree. C. lower than the second target
annealing temperature. At the same time, according to the
invention, the duration of the first time interval is 30-70% of the
entire duration of the annealing treatment comprising the first
time interval ant the second time interval.
[0039] The invention proceeds from the finding that a cold strip,
in which, on the one hand, the grains have an optimum average grain
size, and in which, on the other hand, the deviation of the grain
size of the individual trains from the average grain size is
slight, can be produced by a "staged annealing process" which is
carried out in at least two stages during production step i).
[0040] In practice, this can be achieved by conveying the cold
strip, obtained after cold rolling, for decarburising and nitriding
annealing in a continuous pass through a continuous annealing
furnace, which is divided into at least two zones, a target
annealing temperature being set according to the invention in the
front zone of the furnace first passed through which is
10-50.degree. C. lower than the target annealing temperature in the
second zone of the furnace subsequently passed through by the cold
strip, wherein the duration of the time interval, within which the
first annealing stage takes place, is 30-70% of the entire duration
of the decarburising and nitriding annealing. Excessive grain
growth of the orientations which are unfavourable for the Goss
texture formation is suppressed by means of the temperature
difference, predefined according to the invention, between the
first and second stages of the decarburising and nitriding
annealing and the times provided according to the invention for the
two stages of this annealing process. In this way, the cold strip
microstructure obtained after the annealing, with the same average
grain size which is set by the annealing carried out a higher
annealing temperature in the rear furnace zone, has a significantly
smaller variance and hence makes homogenous secondary grain growth
possible during final annealing carried out at a higher
temperature.
[0041] In this way, the method according to the invention succeeds
in minimising a variance in the grain sizes which arose in the
course of the cold-rolling process. Thus, overall, the outcome from
the preceding cold process is stabilised with respect to
fluctuations in grain size distribution. In this way, after the
annealing treatment carried out according to the invention in at
least two stages subsequent to the cold rolling, an electrical
steel flat product produced according to the invention has a
crystallographic texture, by means of which homogenous secondary
grain growth is optimally ensured during final high-temperature
annealing.
[0042] The invention is way combines the procedure known from the
low-heating process with modern thin slab manufacture which is
carried out according to the known casting-rolling process which is
characterised by a continuous manufacturing sequence. As a result,
with the procedure according to the invention an electrical steel
flat product is obtainable which has optimum magnetic properties in
relation to the typical uses for grain-oriented electrical sheets
or strips.
[0043] When nitriding and decarburising annealing carried out
according to the invention in at least two stages is referred to
here, this does not mean that combined nitriding and
decarburisation necessarily always has to take place in both stages
of this annealing process.
[0044] Instead, the first stage of this annealing process carried
out according to the invention can also be executed as a pure
heating stage and the decarburisation and nitriding can take place
in the second stage. It is equally possible for decarburisation to
be carried out over both annealing stages and for residual
decarburisation and nitriding to be subsequently carried out in a
further annealing step. Alternatively, the decarburisation and
nitriding can take place allocated successively over the at least
two stages of the annealing process carried out according to the
invention. Finally, it is also possible to let at least one of the
annealing stages completed according to the invention to take place
without decarburisation or nitriding and only complete the
decarburisation and nitriding in an annealing step following the
two stages of the annealing process according to the invention.
[0045] Accordingly, within the framework of the invention, in
production step i)1.i the first and second stages of the annealing
process can, in practice, be completed following one another and
subsequently a further annealing step carried out, in which the
cold strip is subjected to decarburising and nitriding annealing.
The first and second stages of the annealing process in production
step i) can be carried out taking into account the for these
annealing stages according to the invention with respect to the
position of the temperature levels and the time slice for the first
annealing stage in relation to the overall time for the annealing
stages. Afterwards, a further annealing step is then carried out,
in which decarburisation and nitriding is carried out in a
conventional way. Therefore, overall, in the case of this variant
of the invention, at least Three sub-annealing steps are
successively completed in the course of production step i), wherein
the specifications according to the invention apply for the first
two annealing steps and the third step comprising the nitriding can
be completed in a conventional way.
[0046] Practical tests have shown that optimum properties of an
electrical steel flat product produced according to the invention
result if the target annealing temperature of the first stage is
10-30.degree. C., lower than the target annealing temperature of
the second annealing stage.
[0047] There is likewise a favourable effect on the outcome of the
annealing step carried out according to the invention in at least
two stages if the duration of the first time interval is limited to
30-60% of the entire duration of the annealing treatment.
[0048] The cold strip should be heated to the target temperature of
the first annealing stage as quickly as possible. During the
heating phase of the decarburisation annealing and nitriding
annealing, the cold-formed strip initially passes through a
recovery. Then, the primary recrystallisation begins. At higher
temperatures and with longer annealing times, grain growth
processes also occur. In order to provide as much stored energy as
possible for the recrystallisation, the temperature range of the
recovery should be passed through quickly. For this purpose, one
advantageous embodiment of the invention makes provision for the
heating rate, at which the cold strip is heated from the start
temperature to the first target annealing temperature in the first
annealing stage, to be 25-500.degree. C./s. In the case of
conventional heating, the heating rate is typically 30-70.degree.
C./s. With a view to a particularly good primary recrystallisation
and as a consequence thereof optimum production results, it can,
however, also be advantageous to set particularly fast heating
rates of 200-500.degree. C./s. In practice, such a fast heating
rate, particularly in the case of manufacture carried out in a
continuous pass, can be achieved by inductive rapid heating taking
place at the entrance to the respective continuous furnace, in
which the cold strip is heated by the effect of an electromagnetic
field induced into the strip.
[0049] The invention is explained in more detail below by means of
exemplary embodiments. [0050] Diag. 1 shows a schematic
illustration of the temperature course T over the annealing time t
for a conventionally annealed electrical steel strip (curve A) and
an electrical steel strip according to the invention (curve B);
[0051] Diag. 2 shows the polarisation at 800 A/m in Tesla for two
differently composed electrical steel sheets S1, S2, plotted via
the ratio t.sub.1/t.sub.2 of the duration of the time interval
t.sub.1, provided for the first annealing stage in the annealing
process according to the invention, to the entire duration t.sub.2
of the annealing process.
[0052] Four steel melts S1-S4 having the compositions specified in
Table 1 were continuously cast into a 63 mm thick strand after a
secondary metallurgical treatment carried out in a ladle furnace
and a vacuum system.
[0053] Thin slabs were separated from the strand also in the
conventional way. After equalisation annealing in an equalisation
furnace at 1165.degree. C., these thin slabs were de-scaled and in
the finishing train hot rolled to a final thickness of 2.34 mm and
coiled into a coil.
EXAMPLE 1
[0054] The hot strips produced in the previously described way were
subjected to a two-stage hot-strip annealing process. The annealing
temperature in the first stage of the hot-strip annealing process
was 1090.degree. C., while the annealing temperature in the second
stage was 850.degree. C. Instead of a two-stage hot-strip annealing
process, a single-stage hot-strip annealing process with a
consistently uniform annealing temperature could have been carried
out.
[0055] After hot-strip annealing, the annealed hot strip was cold
rolled in a single stage with a deformation degree of 87% to a
final thickness of 0.285 mm. Sheet samples were separated from the
cold strips obtained in this way.
[0056] A comparison group A of these sheet samples was annealed in
a continuous pass in a continuous annealing furnace. In a first
furnace section first passed through, firstly an annealing step
lasting 150 seconds was carried out at a temperature of 860.degree.
C. under a moist atmosphere consisting of a hydrogen/nitrogen
mixture (p.sub.H20/p.sub.H2=0.50). Then, a second annealing step
lasting 30 seconds was carried out in a second furnace section
passed through following the first furnace section under a moist
atmosphere consisting of an ammonia/hydrogen/nitrogen mixture, in
order to bring about residual decarburisation and nitriding. The
annealing temperature was constantly 910.degree. C. Corresponding
to the embodiment of the invention already mentioned above, which
is important in practice, here the annealing process in production
step i) of the method according to the invention therefore took
place sub-divided into two annealing steps, the first annealing
step of which, following the sub division specified according to
the invention, was again carried out in two annealing stages,
following which conventional decarburising and nitriding annealing
was completed as the second annealing step. Overall, production
step i) was therefore completed here in three successive parts.
[0057] A second group B of sheet samples was, in a corresponding
production sequence, firstly annealed in the course of the first
annealing step in two successive annealing stages according to the
invention and residual decarburisation and nitriding of this second
group B of sheet samples was subsequently carried out in a second
annealing step. Five variants B.s)-B.e) of the two-stage annealing
process according to the invention were tested. In the first
annealing stage, taking place over a first duration t.sub.1, in
each case a target annealing temperature T.sub.1 was set and in the
second annealing stage in each case a target annealing temperature
T.sub.2 was set. The entire duration t.sub.2 of the two
successively completed annealing stages was in this case 150 s. The
first stage of the first annealing step additionally included rapid
heating to the respective target annealing temperature T.sub.1
which was carried out at a heating rate of 40.degree. C./sec.
[0058] In Diag. 1, the temperature course during annealing in the
first annealing step is in each case illustrated via the annealing
time t, on the one hand, in a continuous line for the electrical
sheet samples of group A produced for comparison and, on the other
hand, in a dotted line for one of the variants B.a)-B.e).
[0059] Thus, the first two annealing stages of the variant of the
method according to the invention explained here by way of example
are predominantly used for carrying out decarburisation and are
optimised in this respect in terms of gas composition and
temperature. The decarburisation annealing takes place in two
stages regarding temperature control, namely in such a way that
decarburisation is firstly gently carried out in the front section
which is first passed through, in order to prevent grain
enlargements as far as possible, and in the section subsequently
passed through decarburisation is continued and completed at an
optimum temperature for the effectiveness of the decarburisation
process.
[0060] In contrast, the third annealing stage of the method
according to the invention is optimised with respect to nitriding.
At the same time, residual decarburisation takes place to a minor
degree here. The third annealing stage is essentially optimised
with respect to nitriding by choosing an optimised gas composition,
but it can also mean a temperature adjustment. In Diag. 1, by way
of example, a correspondingly carried out temperature control can
be recognised by a small temperature jump which occurs after the
annealing time t.sub.2 has elapsed.
[0061] Specifically, for carrying out the annealing treatment
variants B.a)-B.e) according to the invention, the first furnace
section of the continuous annealing furnace was divided into two
temperature zones of equal length, which the sheet samples to be
respectively annealed therefore required 75 s to pass through in
each case. Accordingly, these tests, the duration t.sub.1 of the
first annealing stage was 50% of the entire duration t of 150
s.
[0062] In the first temperature zone of the first furnace section
first passed rough by the respective sample, the target annealing
temperature was altered from variant to variant when the tests
according to the invention were carried out, while in the second
temperature zone when the second annealing stage was carried out,
in each case a constant target annealing temperature of 860.degree.
C. was set. The two annealing stages carried out according to the
invention in the first furnace section of the continuous annealing
furnace, were in each case carried out under a moist atmosphere
consisting of a hydrogen/nitrogen mixture
(p.sub.H20/p.sub.H2=0.50), as with the processing of the sheet
samples group A.
[0063] Then, as with the treatment of the comparison samples of
group A, decarburising and nitriding annealing was carried out over
30 seconds under a moist atmosphere consisting of an
ammonia/hydrogen/nitrogen mixture in the second furnace section
following the first furnace section. The target annealing
temperature was also 910.degree. C. here during this in the second
annealing step.
[0064] After annealing, the samples were subsequently coated with
magnesium oxide and finally annealed under an annealing atmosphere
consisting of 50% vol. H2 and 50% vol. N2.
[0065] In Table 2, the target annealing temperature T.sub.1 set in
the first annealing stage in each case, the difference .DELTA.T
between the first target annealing temperature and the target
annealing temperature of the second annealing stage, as well as the
polarisation J.sub.800 at 800 A/m, specified in Tesla, and the core
loss P.sub.1.7, specified in W/kg, at a polarisation of 1.7 T and a
respective frequency of 50 Hz, are listed for each variant a)-e) of
the heat treatment according to the invention. The electrical steel
sheets produced according to the invention, regardless of which of
the variants a)-e) is used to manufacture them, have proved to have
better properties than the samples which undergo an annealing
treatment in the conventional way.
EXAMPLE 2
[0066] Hot strips produced in the above explained way from melt 1
were subjected to a two-stage hot-strip annealing process at
1130.degree. C./900.degree. C. and hot strips of melt 2 were
subjected to a single-stage hot-strip annealing process at
980.degree. C. Afterwards, the hot strips were cold rolled with a
deformation degree of 87% in a single stage into 0.285 mm thick
cold strips. Sheet samples were separated from the cold strips
obtained.
[0067] In this case, likewise for comparison, a group A of
electrical sheet samples obtained from the cold strips was annealed
for a duration of 150 seconds at a temperature of 840.degree. C. in
a moist hydrogen/nitrogen mixture atmosphere
(p.sub.H20/p.sub.H2=0.45). Subsequently, annealing was carried out
at 860.degree. C. for 30 seconds in a moist
ammonia/hydrogen/nitrogen mixture, wherein residual decarburisation
and nitriding were carried out. Subsequently, as in Example 1,
nitriding and residual decarburisation were carried out at
910.degree. C.
[0068] A second group B of samples was annealed in the same
atmosphere according to the invention in two stages in the first
process part of the continuous furnace used. The temperature of the
first furnace cone was set to 810.degree. C. (.DELTA.T=30.degree.
C.). Five variants B.a)-B.e) were also shown in this case. The
annealing tome t.sub.1, until the target annealing temperature was
raised to 840.degree. C. in the second part of the annealing
process, was 120 s in the case of variant B.a) (annealing time
ratio t.sub.1/t.sub.2=80%), 90 s in the case of variant B.b)
(t.sub.1/t.sub.2=60%), 75 s in the case of variant B.c)
(t.sub.1/t.sub.2=50%), 45 s in the case of variant B.e)
(t.sub.1/t.sub.2=30%) and 30 s in the case of variant B.e)
(t.sub.1/t.sub.2=20%). Subsequently, as in Example 1, nitrating and
residual decarburisation were also carried out here at 910.degree.
C.
[0069] The electrical sheet samples were in each case subsequently
coated with magnesium oxide and finally annealed under an annealing
atmosphere consisting of 50% vol. H2 and 50% vol. N2.
[0070] In Diag. 2, the polarisation J.sub.800 over the annealing
time t.sub.1 of the first stage of the annealing process according
to the invention is plotted for the samples produced from melts 1
and 2 according to the invention.
EXAMPLE 3
[0071] Hot strips of melts 1 and 2 were subjected to a single-stage
hot-strip annealing process at 950.degree. C. Subsequently,
single-stage cold rolling into cold strip having a final thickness
of 0.165 mm was carried out. Sheet samples were separated from the
cold strips obtained.
[0072] A first group A of the sample sheets separated from the cold
strip was annealed for a duration of 130 seconds at a temperature
of 880.degree. C. in a moist hydrogen/nitrogen mixture atmosphere
(p.sub.H20/p.sub.H2=0.44). Subsequently, annealing was carried out
at 900.degree. C. for 30 seconds in a moist
ammonia/hydrogen/nitrogen mixture atmosphere. In the course of this
second annealing step, on the one hand, residual decarburisation,
and, on the other hand, nitriding were carried out.
[0073] A second group B of sheet samples was annealed in two stages
under the same atmosphere in the first process part of the
continuous furnace used for the tests reported in the present case,
wherein during the first annealing stage of the annealing process
lasting up to the 70.sup.th second) t.sub.1/t.sub.2.about.55%) a
target annealing temerature of 850.degree. C. was set and then
subsequently in the second annealing stage lasting from the
70.sup.th second to the 130.sup.th second a target annealing
temperature of 880.degree. C. was set. Subsequently, as in Example
1, nitriding and residual decarburisation were also carried out
here at 900.degree. C. in each case.
[0074] After this annealing treatment of the electrical sheet
samples, they are each subsequently coated with magnesium oxide and
finally annealed under an annealing atmosphere consisting of 50%
vol. H2 and 50% vol. N2.
[0075] The magnetic properties J.sub.800 and P.sub.1.7 of the
samples produced according to the invention and for comparison are
summarised in Table 3. The superiority of the products produced
according to the invention has also been proved here.
EXAMPLE 4
[0076] Hot strips produced in the above explained way from melt 3
were subjected to a two-stage hot-strip annealing process at
1070.degree. C./950.degree. C. and cold rolled into cold strip in a
single stage having a final thickness of 0.215 mm. Sheet samples
were separated from the cold strips obtained.
[0077] A first group A of the sheet samples was annealed for
duration of 120 seconds at a temperature of 870.degree. C. in an
atmosphere consisting of a moist hydrogen/nitrogen mixture
(p.sub.H20/p.sub.H2=0.51). Subsequently, annealing was carried out
at 910.degree. C. for 30 seconds under an atmosphere consisting of
a moist ammonia/hydrogen/nitrogen mixture, in which, on the one
hand, residual decarburisation, and, on the other hand, nitriding
took place.
[0078] A second group B of sheet samples was annealed according to
the invention in a first annealing step divided, into two stages
according to the invention in the first furnace section of the
continuous annealing furnace used here in a moist hydrogen/nitrogen
mixture with p.sub.H20/p.sub.H2=0.51. In a first annealing stage
lasting up to the 65.sup.th second the target annealing temperature
was set to 850.degree. C., while the target annealing temperature
in the second annealing stage, which lasted from the 70.sup.th
second to the 120.sup.th second, was an 870.degree. C. After the
end of the first annealing step completed in two stages in the
first furnace section on this way, the sheet samples were subjected
to nitriding and residual decarburisation at 910.degree. C. in a
moist ammonia/hydrogen/nitrogen mixture.
[0079] All sheets were subsequently coated with magnesium oxide and
finally annealed under an annealing atmosphere consisting of 50%
vol. H2 and 50% vol. N2.
[0080] In the present example, the first annealing stage of the
first annealing step, as in the previously described examples,
included rapidly heating the sheet samples to the target annealing
temperature of the first annealing stage. In order to show the
effect of the heating rates "HR", in the present Example 4 the
heating rates HR were varied in four different test runs with
otherwise unchanged conditions.
[0081] (Test 4.1: HR=70.degree. C./s; Test 4.2: HR=150.degree.
C./s; Test 4.3: HR=300.degree. C./s; Test 4.4: HR=500.degree.
C./s;
[0082] The magnetic characteristics of the electrical steel shoots
obtained in this way are summarized in Table 4.
TABLE-US-00001 TABLE 1 Si C Al N Mn S Cu Sn Cr Melt [%] [ppm] [ppm]
[ppm] [ppm] [ppm] [ppm] [ppm] [ppm] S1 3.10 470 260 93 1470 78 1950
580 1140 S2 3.32 580 287 105 1390 82 1810 720 780 S3 3.24 720 320
87 1580 92 1470 630 820 S4 2.90 450 348 112 1420 85 1610 1050 930
Details in % wt. or wt. ppm; Remainder iron and unavoidable
impurities
TABLE-US-00002 TABLE 2 Variant B.a) Variant B.b) according to
invention according to invention T.sub.1: 800.degree. C. T.sub.1:
820.degree. C. .DELTA.T: 60.degree. C. .DELTA.T: 40.degree. C.
J.sub.800 [T] P.sub.1.7 [W/kg] J.sub.800 [T] P.sub.1.7[W/kg] Melt 1
1.865 1.317 1.920 1.038 Melt 2 1.845 1.432 1.909 1.089 Melt 3 1.872
1.238 1.914 1.101 Melt 4 1.853 1.365 1.918 1.030 Variant B.c)
Variant B.d) according to invention according to invention T.sub.1:
830.degree. C. T.sub.1: 840.degree. C. .DELTA.T: 30.degree. C.
.DELTA.T: 20.degree. C. J.sub.800 [T] P.sub.1.7[W/kg] J.sub.800 [T]
P.sub.1.7[W/kg] Melt 1 1.931 1.017 1.922 1.048 Melt 2 1.915 1.056
1.910 1.062 Melt 3 1.924 1.088 1.924 1.087 Melt 4 1.923 1.026 1.919
1.031 Variant B.e) Comparison test not according to the invention
according to the invention T.sub.1: 850.degree. C. T.sub.1:
860.degree. C. .DELTA.T: 10.degree. C. .DELTA.T: 0.degree. C.
J.sub.800 [T] P.sub.1.7[W/kg] J.sub.800 [T] P.sub.1.7[W/kg] Melt 1
1.910 1.113 1.904 1.123 Melt 2 1.908 1.077 1.899 1.092 Melt 3 1.915
1.092 1.890 1.116 Melt 4 1.899 1.104 1.882 1.107
TABLE-US-00003 TABLE 3 Reference 880.degree. C. Invention J.sub.800
[T] P.sub.1.7 [W/kg] J.sub.800 [T] P.sub.1.7 [W/kg] Melt 1 1.867
0.882 1.899 0.803 Melt 2 1.873 0.853 1.901 0.779
TABLE-US-00004 TABLE 4 J.sub.800 [T] P.sub.1.7 [W/kg] Test 4.1 HR =
70.degree. C./s 1.879 0.879 according to invention Test 4.2 HR =
150.degree. C./s 1.885 0.853 according to invention Test 4.3 HR =
300.degree. C./s 1.904 0.837 according to invention Test 4.4 HR =
500.degree. C./s 1.903 0.842 according to invention Comparison
samples 1.872 0.894 not according to invention
* * * * *