Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
METHOD AND SYSTEM OF TREATING A STAINLESS STEEL STRIP,
ESPECIALLY FOR A PICKLING TREATMENT
BACKGROUND
[0001] The present invention relates to a method for treating a steel strip
especially for a
pickling treatment of the steel strip, by means of a treatment liquid in a
treatment station, the
treatment station comprising a treatment tank.
[0002] Furthermore, the present invention relates to a system for
treating a steel strip,
especially for a pickling treatment of the steel strip, by means of a
treatment liquid in a
treatment station, the treatment station comprising a treatment tank.
[0003] The steel strip to be treated according to the method of the
present invention and
in a system of the present invention is stainless steel.
[0004] The presence of oxide scale on the surface of steel strip is
formed during high
temperature processing. The term oxide scale generally refers to the chemical
compounds of
iron and oxygen, as well as the chemical compounds of iron alloying elements,
e.g.
chromium and oxygen, formed on the surface of the steel by exposure to air
while the metal
is at an elevated temperature. Chemical compounds thus formed include iron
oxides, such
as FeO, Fe203 and Fe304, oxides of alloying elements such as Cr03, NiO, Si02
and
complex oxide spinals like FeCr204, NiFe204, Fe2SiO4 and others. During
annealing,
stainless steel grades are heated up to a certain temperature (850-1150 C
depending on
steel grade) and are kept at this temperature for some time to soften the
metal in order to
release the work hardening induced by hot and cold rolling. A uniform grain
structure is
obtained depending on the annealing temperature, and oxide scale is formed on
the surface.
Underneath the surface of stainless steel, a chromium-depleted zone is formed,
which is
different for austenitic, ferritic and duplex stainless steel grades.
[0005] Processes are required in the stainless steel industry to remove
that oxide scale
and the chromium depleted layer to obtain technological products, but that has
to be
achieved with a minimum loss of base material. Pickling is the process of
chemically
removing of oxide scale from the surface of a metal by the action of water
solution of
inorganic acids. The stainless steel is widely pickled in diluted sulfuric or
hydrochloric acid.
For pickling of the stainless steel, a mixture of nitric acid and hydrofluoric
acid is widely
applied. The rate of pickling is affected by numerous variables, including the
steel-based
constituents and type and adherence of oxide to be removed. Pickling solution
temperature,
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acid concentration, reaction product concentration, turbulence flow
conditions, immersion
time and presence or absence of inhibitors and accelerators influence the rate
of acid attack.
Because of production factors including pickling speed, quality and
efficiency, as well as
reduced attack of HCI on base metal, hydrochloric acid has effectively
displaced sulfuric acid
as the acid of choice in industrial large-scale pickling lines for stainless
steel. While the rate
of pickling increases in direct proportion to the concentration of the acid,
the influence of
temperature is much more pronounced. On the other hand, certain metals, such
as cooper,
chromium and nickel, retard the rate of pickling when they occur in the steel
base, since the
scale bearing these alloying metals inhibits acid attack. Elements like
aluminum and silicon
form refractory-type oxides, which in turn lower the solubility rate of the
oxide in the pickling
acid. The thickness of the oxide scale varies considerably with practice in
rolling mills. E.g.
loose coiling permits greater atmospheric penetration into the wraps, with
corresponding
heavier oxide formation on the edge areas. In addition, coiling temperature
affects the
adherence of the oxide and determines how easy or difficult it is to remove.
The lower coiling
temperatures makes oxide removal easier, at higher coiling temperatures longer
pickling
times are required. E.g. at the coiling temperature of 750 C, double pickling
time is required
compared to the coiling temperature of 570 C.
[0006] Like with carbon steel, the stainless steel also oxidizes
following hot rolling and
coiling. The oxide scale layer formed on the surface of the hot rolled
stainless steel strip
contains the alloying elements and is very tightly adhering on the surface,
which makes the
de-scaling or pickling of stainless steel very difficult as compared to the
carbon steels. To
achieve efficient and thorough surface oxide removal from a stainless steel
strip, more
severe processing techniques must be used which substantially increase
processing time
and operational costs. Frequently, to effect complete oxide scale removal,
chemical pickling
of stainless steel strip must be preceded by mechanical de-scaling, e.g. by
shot-blasting
and/or scale breaking. Often nowadays additional methods of Pre-Pickling is
applied to
soften the oxide scale. E.g. hot rolled stainless steel is conventionally pre-
pickled in hot
sulfuric acid and the cold rolled stainless steel is pre-pickled
electrolytically in neutral
electrolyte of Na2SO4 solution prior the main pickling in mixed acid. In today
state-of-the-art
practice, the Mechanical de-scaling (MD) and Pre-Pickling (PP) can remove the
oxide scale
layer only, the chromium¨depleted zone and partly the base material can be
only removed
by the final pickling process (FP).
[0007] The pickling process most commonly used for stainless steel
involves the use of
a mixture of nitric and hydrofluoric acid, the mutual concentrations of which
vary according to
the type of stainless steel to be pickled (austenitic, ferritic, martensitic,
duplex...), its surface
characteristics and its past processing history. When processing of 300 and
400 series in the
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same line, various acid compositions (Mix I and Mix II) and various acid
mixture
temperatures are required. Since austenitic steel grades are pickled at 50-65
C, the most of
ferritic and martensitic stainless steel grades generate exothermal reaction
during the
pickling, which require cooling facilities in the pickling line in order to
keep the acid mixture
temperature in the range of 35-40 C .Although the process enables excellent
pickling results
to be obtained, it has the very serious drawback that it creates considerable
and substantial
ecological problems due to the use of these particular acids. The hydrofluoric
acid is
extremely corrosive and a harmful environmental pollutant. The nitric acid is
the source of
highly polluting nitrogen oxide (N0x) vapors which are emitted into the
atmosphere and
which are highly aggressive towards metals and nonmetals with which they come
into
contact. In addition, high nitrate levels exist in the rinse water and in the
spent pickling baths
and create a major disposal problem. The elimination of NOx vapors in the air
by catalytic
DENOX plants and nitrates in the neutralized waste water creates considerable
plant
operational problems, very high investment costs for equipment, high
maintenance demand
and very high operational costs. As a result, there has been considerable
interest in
researching and developing stainless steel pickling processes and plants which
do not use
either nitric acid or hydrofluoric acid and which are ecologically safe and
environmental-
friendly.
[0008] Today pickling lines are designed as shallow tank turbulence
installations
comprising of several consecutive pickling tanks. The steel strip is pulled or
pushed through
the treatment tanks. The complete pickling section is arranged as a cascade,
i.e. the fresh or
regenerated acid is added to the last treatment tank (i.e. the most downstream
treatment
tank according to the direction of movement of the steel strip) and is then
processed in a
countercurrent flow to the strip transport direction in order to maximize the
use of the pickling
acid. At the entry and exit of the treatment tanks, wringer rolls are
installed to remove the
pickling acid from the strip to the greatest possible extend in order to
enhance the cascade
effect. German patent disclosure DE 40 31 234 describes this technology.
[0009] Inside the treatment tank, the pickling acid is injected on both
sides of the tank
creating a high turbulence between the strip surface and the pickling acid.
[0010] The pickling acid is then overflowing from the treatment tank to a
circulation tank
from where it is again injected into the treatment tank by means of pumps. The
high
turbulence reduces the thickness of the liquid boundary layer on the strip
surface resulting in
an improved media and energy exchange and consequently reducing the required
pickling
time.
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[0011] Another well known pickling method is the spray pickling,
wherein the pickling
acid is directly sprayed onto the strip surface using several spray nozzles
installed both
above and below the steel strip, cf. e.g. document DE 42 28 808 Al. The
pickling acid is then
collected in a circulation tank from where it is pumped to the spray nozzles
and sprayed on to
the strip surface again. The spray nozzles are typically operated at a
pressure above 1 bar.
Due to the high impulse of the pickling acid sprayed onto the strip surface,
the pickling
efficiency and consequently the pickling time can be further improved. However
this
technology has never been used commercially in strip pickling lines.
[0012] The use of hydrochloric acid as a pickling agent for stainless
steel pickling allows
the realization of pickling mechanisms of both the removal of oxide scale and
the chromium-
depleted zone. The pickling of stainless steel in hydrochloric acid is a
combined process of
reduction and oxidation.
The chemical dissolution of steel in HCI is as follows:
Fe + 2 HCI 4 FeCl2 + H2
The base metal, Fe, is dissolved by the oxidizing agents, mainly FeCI3:
2 FeCI3 + Fe 4 3 FeCl2
Oxidation reaction to produce the required oxidizing agent is as follows:
4 FeCl2 + 02 + 4 HCI 4 4 FeCI3 + 2 H20
A minimum proper FeCI3 concentration is required for the pickling process of
stainless steel.
This is today typically reached by adding H202 to the pickling liquid.
[0013] In the context of the present invention, chlorides of iron and
chlorides of other
metals (especially chromium) are collectively referred to by the term MeCI.
[0014] Laboratory tests, carried out for different steel grades, have
proven for HCI
containing pickling solution, that the pickling speed of spray-pickling is up
to five times higher
compared to the shallow tank turbulence technology. In addition, the spray
nozzles used in
the spray pickling technology create fine droplets with a high surface which
are in direct
contact with air. The air, in particular the oxygen contained in the air,
dissolves in the pickling
acid and oxidizes the FeCl2 together with the HCI forming FeCI3. Therefore
using HCI in a
spray pickling section to treat stainless steel has the advantage, that no
H202 is needed to
create FeCI3. However, in pure spray pickling tanks, the formation of FeCI3
can be too high
(to reach 60 g/I and above), making the whole pickling process difficult to
control, with a high
risk of over pickling the metal strip or causing inacceptable roughness of the
strip surface.
Another drawback of the increased FeCI3 concentration in the pickling acid is
the effect on
the regeneration process of the spent pickling acid. Spent pickling acid
containing HCI is
typically regenerated using the pyrohydrolysis process. In this process FeCl2
and FeCI3 are
converted back to HCI and Fe203. FeCI3 however has a much lower evaporation
temperature
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than FeCl2 and evaporates in the pyrohydrolysis reactor causing very fine
Fe203 particles
below 1 pm in size when converted to Fe203. These fine particles are difficult
to remove from
the process off-gases causing high dust emissions.
SUMMARY
[0015] It is therefore an object of the present invention to provide
method and a system
for an improved steel strip treatment, especially pickling, such that fixed
investment as well
as maintenance costs are reduced, the treatment and pickling process is
realized
comparatively quickly, with high quality, and in an environmentally friendly
manner.
[0016] The object of the present invention is achieved by a method for
treating a steel
strip, especially for a pickling treatment of the steel strip, by means of a
treatment liquid in a
treatment station, the treatment station comprising a treatment tank with a
spray section and
an immersion section, and the treatment station comprising a common collection
means for
the treatment liquid,
wherein the steel strip comprises stainless steel and is a continuous steel
strip being oriented
substantially horizontally, both in its longitudinal and transverse
directions,
wherein the steel strip has a top surface and a bottom surface,
wherein the method comprises transporting the steel strip continuously through
the treatment
station in a transport direction, the transport direction being parallel to
the longitudinal
direction of the steel strip, such that
-- in a first step, the treatment liquid is sprayed onto the top surface of
the steel strip and
onto the bottom surface of the steel strip while the steel strip being in the
spray section of the
treatment station,
-- in a second step, the steel strip is immersed in the treatment liquid while
the steel strip
being in the immersion section of the treatment station,
wherein, while treating the steel strip, the treatment liquid is continuously
pumped out of the
common collection means and through both the spray section and the immersion
section of
the treatment station, wherein spraying of the treatment liquid onto the top
and bottom
surfaces of the steel strip is provided using spray nozzles.
[0017] According to the present invention, it is thereby advantageously
possible to
provide a treatment station that requires comparatively low installation costs
as well as
reduced maintenance costs. The present invention is related to a process for
chemical or
electrochemical treatment of the surface of stainless steel, preferably strip
shaped, wherein
the material is treated with a pickling solution, preferably containing HCI,
in one or more
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treatment tanks to remove an oxide scale layer which was previously formed
during the hot
rolling process of the metal strip (steel strip). This treatment is needed to
reach a clean
surface for either further processing it in a cold rolling process or for
direct commercial use.
[0018] According to an embodiment of the present invention, instead of
the steel strip
comprising stainless steel, it is also preferred that the steel strip consists
of stainless steel.
[0019] According to the present invention, it is preferred that the
process for chemical or
electrochemical treatment of the surface of stainless steel is conducted using
a pickling
solution containing HCI as the only pickling agent, wherein the advantages of
spray pickling
are used to a maximum extend. Furthermore the process shall be controllable
minimizing the
risk of over-pickling so that the process can be realized in commercially used
industrial scale
pickling lines.
[0020] This drastically shortening of the pickling time in case of
spray and turbulence
pickling can be explained by a very thin laminar boundary layer, which is much
more thinner
than in case of turbulence dip process. The drive force of heat, mass and
momentum
transfer across the boundary layer is faster, since the thickness of this
layer is significantly
reduced.
[0021] According to the present invention, the spent acid of such a
process is of a
quality such that it can be treated in regeneration plants without additional
investment
considering in particular the FeCI3 concentration in such spent acid.
[0022] The present invention is also directed to the possibility to revamp
existing pickling
lines, in particular the treatment tanks, and to use more efficient pickling
technology with
improved efficiency while re-using existing equipment in order to reduce
installation costs, as
for example acid circulation circuits etc. can be reused.
[0023] According to the present invention, it is advantageously
possible that such
requirements can be achieved by the present invention, comprising a pickling
process using
an HCI containing pickling solution as the only pickling acid, wherein the
material to be
treated (i.e. the steel strip) is processed horizontally through one or more
treatment tanks
which are ¨ in case of more than one treatment tank ¨ operated as a pickling
cascade.
[0024] According to the present invention, each single treatment tank
(of the treatment
station) of the above described process comprises of one spray pickling zone
and one dip
pickling zone arranged as one unit using one common circulation circuit, i.e.
one common
circulation tank (common collection means) with several pump circuits as
required. All
pickling acid coming from the dip section and the spray section are collected
and mixed in
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the common circulation tank (common collection means) and pumped back to the
above
mentioned two pickling sections (of the treatment tank of the treatment
station). Inside the
single pickling tank (treatment tank), a guide roll underneath the strip
located between the
spray and pickling section might be required to better position the steel
strip inside the
treatment tank. Typically, a wringer roll unit ¨ as it is typically installed
between two pickling
sections ¨ is not required. Preferably the first section of the treatment tank
is a spray section
while the second section of the dip pickling type, preferably with high
efficiency such as
shallow tank turbulence technology.
[0025] According to the present invention, the steel strip is treated ¨
in the treatment
tank of the treatment station ¨ by means of a treatment liquid such that the
same treatment
liquid is used both in the spray section and in the immersion section of the
treatment tank.
Advantageously, it is thereby possible to realize the treatment station
(having both the spray
section and the immersion section) in a more cost effective manner as the same
common
collection means (as well as at least a part of the circulation system) can be
used for both the
spray section and the immersion section, hence reducing the costs for
realizing the
possibility to treat the steel strip by means of both the spray section and
the immersion
section.
[0026] According to the present invention, the steel strip comprises
stainless steel and is
a continuous steel strip being oriented substantially horizontally, both in
its longitudinal and
transverse directions, at least at the treatment station. This means that
steel strip is mostly
horizontally oriented in its transverse direction but is allowed to be sagging
in its longitudinal
direction. The height variation through the treatment station of the steel
strip in its longitudinal
direction may reach, e.g., up to 0,5 m. Preferably, also between the treatment
station or
between the plurality of treatment stations, the height variations of the
steel strip in its
longitudinal direction are also comprised up to 0,5 m. Generally, it is
preferred according to
the present invention that the height variation of the steel strip in its
longitudinal direction is
comprised between up to 0,5 m throughout the complete treatment system, that
potentially
(and typically) comprises a plurality of treatment stations one after the
other in the transport
direction of the steel strip.
[0027] According to the present invention, the treatment liquid is sprayed
¨ in a first step
and by means of nozzles ¨ onto the top surface of the steel strip and onto the
bottom surface
of the steel strip while the steel strip is in the spray section of the
treatment station. In second
step (that is not necessarily subsequent to the first step but could also be
preceding the first
step), the steel strip is immersed in the treatment liquid while the steel
strip is in the
immersion section of the treatment station. For the treatment of the steel
strip, the treatment
liquid of the treatment station is continuously pumped out of the common
collection means
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(of that treatment station) and through both the spray section and the
immersion section of
the treatment tank, wherein spraying of the treatment liquid onto the top and
bottom surfaces
of the steel strip is provided using spray nozzles.
[0028] According to the present invention, two pickling technologies
are directly
combined in one treatment tank (i.e. in one treatment station), i.e. using
physically the same
pickling acids (or the same treatment liquid) in both pickling sections (i.e.
in both the spray
section and the immersion section of the considered treatment station), as
described. By
doing so, the concentration of FeCI3 can be kept below a critical level
throughout the entire
pickling process, guaranteeing a uniform pickling result without the risk of
over-pickling.
Furthermore the spent acid of such process can be easily regenerated in
regeneration plants
without additional investment to reach the legally required emission values,
especially
regarding dust emissions.
[0029] According to the present invention, the efficiency of the
treatment process (or
pickling process) is increased. Tests have proven that a certain increase in
the FeCI3
concentration reduces the pickling time also for the dip pickling process.
Consequently the
process according to the present invention uses the advantage of the high
efficient spray
pickling process while the efficiency of the dip pickling process is improved
as well, due to
the common use of the pickling acid (i.e. the same treatment liquid is used
both in the spray
section and the immersion section of one and the same treatment station), and
the
consequently increased FeCI3 level. Of course, in (the typical) case that more
than one
treatment stations are used in a pickling line or steel strip pickling
installation, this does not
mean that the same treatment liquid is used in all of such different treatment
stations. To the
contrary, in case of a plurality of treatment stations (i.e. having each a
treatment tank
comprising a spray section and an immersion section), a different treatment
liquid is normally
used for a different treatment station; however within the same treatment
station/treatment
tank, the same treatment liquid is used for both kinds of pickling processes
(spray and dip
pickling). Thereby, it is advantageously possible that the drawbacks of a
comparatively high
concentration in FeCI3 can be avoided that would typically arise in case of
combining spray
pickling and dib pickling using different treatment liquids in the same
treatment station.
[0030] According to the present invention, the design of the treatment line
or pickling line
is done in such a way that it is advantageously possible that the treatment
stations or
treatment tanks can easily replace existing treatment tanks in case of a
required revamp (or
refurbishment) while the circulation circuits can be reused. This is mainly
attributed to the fact
that the spray pickling technology and the dip pickling technology (i.e. the
spray section and
the immersion section) are combined in one treatment tank (i.e. as part of one
treatment
tank).
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[0031] The design of the present invention also allows the possibility
to operate the
treatment tanks without an additional (external) circulation tank ¨ or common
collection
means ¨ (i.e. external or separate to the treatment tank). In such an
embodiment, the
treatment tank itself, in particular the area underneath the spray section,
and, if required, also
underneath the dip section, is used as circulation tank (or common collection
means), i.e. the
circulation tank (or common collection means) is realized in a manner
integrated with the
treatment tank. This is advantageous for the replacement (refurbishment) of
deep bath
treatment tanks which have often been operated without circulation circuits.
In this case only
the pump circuit needs to be added while the circulation tank is incorporated
(or integrated)
in the treatment tank.
[0032] According to a preferred embodiment of the present invention,
the spray section
comprises an effective spray length in parallel to the longitudinal direction
of the steel strip
such that ¨ during the first step ¨ the top and bottom surfaces of the steel
strip receive the
treatment liquid while being located within the effective spray length,
wherein the immersion
section comprises an effective immersion length in parallel to the
longitudinal direction of the
steel strip such that ¨ during the second step ¨ the steel strip is immersed ¨
with its top and
bottom surfaces ¨ in the treatment liquid while being located within the
effective immersion
length, wherein the effective spray length and the effective immersion length
are provided
having a ratio of between and including 30:70 to 70:30, especially a ratio of
50:50.
[0033] According to the present invention, it is thereby advantageously
possible to
flexibly adapt process parameters of a pickling line to fit best with the
intended operative use
after construction. By means of defining the length of the immersion section
(at a given
transport speed of the steel strip through the pickling line), the time is
defined during which
the treatment liquid is effectively treating the steel strip in the immersion
section. By means
of defining the length of the spray section (equally at a given transport
speed of the steel strip
trough the pickling line), the maximum time of spray pickling is defined in
relation to the dip
pickling time.
[0034] According to another preferred embodiment of the present
invention, the effective
spray length and hence the ratio of the effective spray length vs. the
effective immersion
length is varied by activating only a part of the spray nozzles.
[0035] According to the present invention, it is thereby advantageously
possible to vary
the spray pickling time even during operational use of the pickling line, i.e.
by de-activating a
part of the spray nozzles. By selectively activating and/or de-activating
groups of spray
nozzles, is it advantageously possible according to the present invention,
that also the
manner or the intensity of the spray pickling step can be varied in
operational use of the
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pickling line, e.g. by using only every second spray nozzle such that spray
pickling is less
intensive in the spray section.
[0036] According to a preferred embodiment of the present invention,
the spray section
is located ¨ along the transport direction of the steel strip ¨ upstream with
respect to the
immersion section. According to an alternative preferred embodiment of the
present
invention, the spray section is located ¨ along the transport direction of the
steel strip ¨
downstream with respect to the immersion section.
[0037] According to the present invention, it is thereby advantageously
possible to
provide the possibility of different pickling line architectures. E.g., it is
advantageously
possible (in case that at least two treatment stations are used) to provide
both treatment
stations such that the spray section is located upstream with respect to the
immersion
section (i.e. the steel strip passes the spray section first and afterwards
the immersion
section): This results in a pickling sequence of the kind of a spray and dip
pickling (using a
first treatment liquid) in the first (or upstream) treatment station, followed
by a spray and dip
pickling (using a second treatment liquid) in the second (or downstram)
treatment station.
Alternatively, it is also advantageously possible (in case that at least two
treatment stations
are used) to provide the first treatment station such that the spray section
is located
downstream with respect to the immersion section (i.e. the steel strip passes
the immersion
section first and afterwards the spray section), and to provide the second
treatment station
such that the spray section is located upstream with respect to the immersion
section (i.e. the
steel strip passes the spray section (of the second treatment station) first
and afterwards the
dip section (of the second treatment station)): This results in a pickling
sequence of the kind
of a dip and spray pickling (using a first treatment liquid) in the first (or
upstream) treatment
station, followed by a spray and dip pickling (using a second treatment
liquid) in the second
(or downstream) treatment station. Of course, these building blocks of two
treatment stations
can be either repeated or combined with other treatment stations or
configurations of
treatment stations.
[0038] According to a preferred embodiment of the present invention,
the method
comprises using ¨ besides using the treatment liquid in the treatment station
¨ a further
treatment liquid in a further treatment station, the further treatment station
comprising a
further treatment tank with a further spray section and a further immersion
section, and the
further treatment station comprising a further common collection means for the
further
treatment liquid,
wherein the method comprises transporting the steel strip continuously through
the further
treatment station in the transport direction such that
-- in a third step, the further treatment liquid is sprayed onto the top
surface of the steel
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strip and onto the bottom surface of the steel strip while the steel strip
being in the further
spray section of the further treatment station,
-- in a fourth step, the steel strip is immersed in the further treatment
liquid while the steel
strip being in the further immersion section of the further treatment station,
wherein, while treating the steel strip, the further treatment liquid is
continuously pumped out
of the further common collection means and through both the further spray
section and the
further immersion section of the further treatment station , wherein spraying
of the further
treatment liquid onto the top and bottom surfaces of the steel strip is
provided using further
spray nozzles, wherein the third and fourth steps are preceding the first and
second steps or
are subsequent to the first and second steps.
[0039] According to the present invention, it is thereby advantageously
possible to
combine at least two inventive treatment stations in a pickling line. Of
course, it is also
possible and preferred according to the present invention to combine such two
inventive
treatment stations with a conventional treatment station (i.e. having solely a
spray section or
solely an immersion section in a treatment tank) or with a plurality of
conventional treatment
stations. In such an architecture of the pickling line, the two inventive
treatment station are
either located directly subsequent one after the other along the transport
direction of the steel
strip or the combination with one or a plurality of conventional treatment
stations is provided
such that the treatment station (or the first treatment station) is located
upstream according
to the transport direction of the steel strip with respect to a conventional
treatment station (or
with respect to a plurality of conventional treatment stations) and downstream
with respect to
this or these conventional treatment station(s) is located the further
treatment station (or
second treatment station) according to the present invention.
[0040] According to another preferred embodiment of the present
invention, the
treatment liquid and/or the further treatment liquid comprises
-- hydrochloric acid in a concentration ranging from and including 150 g/I to
and including
250 g/I and
-- FeCI3 in a concentration ranging from and including 10 g/I to and including
35 g/I,
especially in a concentration ranging from and including 15 g/I to and
including 30 g/I or
especially in a concentration ranging from and including 19 g/I to and
including 26 g/I and,
-- MeCl2 in a concentration ranging from and including 30 g/I to and including
300 g/I,
especially in a concentration ranging from and including 30 g/I to and
including 60 g/I or in a
concentration ranging from and including 130 g/I to and including 180 g/I or
in a
concentration ranging from and including 230 g/I to and including 300 g/I.
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[0041] According to the present invention, it is thereby advantageously
possible to
combine a high efficiency of the pickling process while retaining the
possibility to
comparatively easily regenerated the used pickling acids (treatment liquids).
[0042] The present invention also relates to a system for treating a
steel strip, especially
for a pickling treatment of the steel strip, by means of a treatment liquid in
a treatment
station, the system comprising the treatment station, wherein the treatment
station comprises
a treatment tank with a spray section, an immersion section, and the treatment
station
comprising a common collection means for the treatment liquid,
wherein the steel strip comprises stainless steel and is a continuous steel
strip being oriented
substantially horizontally, both in its longitudinal and transverse
directions,
wherein the steel strip has a top surface and a bottom surface,
wherein the system is configured to transport the steel strip continuously
through the
treatment station in a transport direction, the transport direction being
parallel to the
longitudinal direction of the steel strip, such that
-- the treatment liquid is sprayed onto the top surface of the steel strip and
onto the bottom
surface of the steel strip while the steel strip being in the spray section of
the treatment
station,
-- the steel strip is immersed in the treatment liquid while the steel strip
being in the
immersion section of the treatment station,
wherein the system is configured such that the treatment liquid is
continuously pumped out of
the common collection means and through both the spray section and the
immersion section
of the treatment station, wherein the system comprises spray nozzles such that
the treatment
liquid is sprayed onto the top and bottom surfaces of the steel strip using
the spray nozzles.
[0043] According to the present invention, it is thereby advantageously
possible to
provide a system (or a treatment station) that requires comparatively low
installation costs as
well as reduced maintenance costs. According to the present invention, it is
advantageously
possible to combine the advantages of spray pickling and dip pickling and to
minimize the
risk of over-pickling. It is furthermore advantageous that the spent acid of
such a system is of
a quality such that it can be treated in regeneration plants without
additional investment
considering in particular the FeCI3 concentration in such spent acid.
[0044] According to an embodiment of the present invention, instead of
the steel strip
comprising stainless steel, it is also preferred that the steel strip consists
of stainless steel.
[0045] According to a preferred embodiment of the present invention ¨
especially
regarding the inventive system ¨, the spray section comprises an effective
spray length in
parallel to the longitudinal direction of the steel strip such that the top
and bottom surfaces of
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the steel strip receive the treatment liquid while being located within the
effective spray
length, wherein the immersion section comprises an effective immersion length
in parallel to
the longitudinal direction of the steel strip such that the steel strip is
immersed ¨ with its top
and bottom surfaces ¨ in the treatment liquid while being located within the
effective
immersion length, wherein the effective spray length and the effective
immersion length are
provided having a ratio of between and including 30:70 to 70:30, especially a
ratio of 50:50.
[0046] According to the present invention, it is thereby advantageously
possible to XXX
[0047] According to a preferred embodiment of the present invention ¨
especially
regarding the inventive system ¨, the spray section is located ¨ along the
transport direction
of the steel strip ¨ upstream with respect to the immersion section. According
to an
alternative preferred embodiment of the present invention ¨ especially
regarding the
inventive system ¨, the spray section is located ¨ along the transport
direction of the steel
strip ¨ downstream with respect to the immersion section.
[0048] According to the present invention, it is thereby advantageously
possible to
flexibly adapt process parameters of a pickling line to fit best with the
intended operative use
after construction.
[0049] According to a preferred embodiment of the present invention ¨
especially
regarding the inventive system ¨, the common collection means for the
treatment liquid of
both the spray section and the immersion section is a collection means
separated from the
treatment tank of the treatment station.
[0050] According to the present invention, it is thereby advantageously
possible to build
the treatment tank in a very cost effective manner such that especially the
volume of the
treatment tank is comparably small (and hence less treatment liquid is to be
used). The
treatment liquid is pumped through the common collection means (or circulation
tank) that is
separated from the treatment tank.
[0051] According to a preferred embodiment of the present invention ¨
especially
regarding the inventive system ¨, the common collection means for the
treatment liquid of
both the spray section and the immersion section is a collection means
integrated with the
treatment tank of the treatment station, especially integrated such that the
bottom part of the
treatment tank forms the common collection means.
[0052] According to the present invention, it is thereby advantageously
possible to
realize the treatment station in a very cost effective manner as no separate
common
collection means (or circulation tank) is required.
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[0053] According to a preferred embodiment of the present invention ¨
especially
regarding the inventive system ¨, the system comprises ¨ besides the treatment
liquid in the
treatment station ¨ a further treatment liquid in a further treatment station,
the further
treatment station comprising a further treatment tank with a further spray
section and a
further immersion section, and the further treatment station comprising a
further common
collection means for the further treatment liquid,
wherein the system is configured such that the steel strip is transported
continuously through
the further treatment station in the transport direction such that
-- the further treatment liquid is sprayed onto the top surface of the steel
strip and onto the
bottom surface of the steel strip while the steel strip being in the further
spray section of the
further treatment station,
-- the steel strip is immersed in the further treatment liquid while the steel
strip being in the
further immersion section of the further treatment station,
wherein the system is configured such that the further treatment liquid is
continuously
pumped out of the further common collection means and through both the further
spray
section and the further immersion section of the further treatment station,
wherein the system
comprises further spray nozzles such that the further treatment liquid is
sprayed onto the top
and bottom surfaces of the steel strip using the further spray nozzles.
[0054] According to the present invention, it is thereby advantageously
possible to
combine at least two inventive treatment stations in a pickling line. Of
course, it is also
possible and preferred according to the present invention to combine such two
inventive
treatment stations with a conventional treatment station (i.e. having solely a
spray section or
solely an immersion section in a treatment tank) or with a plurality of
conventional treatment
stations.
[0055] According to a preferred embodiment of the present invention, the
system
comprises ¨ besides the treatment liquid in the treatment station and the
further treatment
liquid in the further treatment station ¨ a third treatment liquid in a third
treatment station, the
third treatment station comprising a third treatment tank with a third spray
section and a third
immersion section, and the third treatment station comprising a third common
collection
means for the third treatment liquid.
[0056] According to the present invention, it is thereby advantageously
possible to
combine at least three inventive treatment stations in a pickling line. Of
course, it is also
possible and preferred according to the present invention to combine such
three inventive
treatment stations with a conventional treatment station (i.e. having solely a
spray section or
solely an immersion section in a treatment tank) or with a plurality of
conventional treatment
stations.
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[0057] According to a further preferred embodiment of the present
invention, the system
comprises ¨ besides the treatment liquid in the treatment station, the further
treatment liquid
in the further treatment station, and the third treatment liquid in the third
treatment station ¨ a
fourth treatment liquid in a fourth treatment station, the fourth treatment
station comprising a
fourth treatment tank with a fourth spray section and a fourth immersion
section, and the
fourth treatment station comprising a fourth common collection means for the
fourth
treatment liquid. According to other embodiments, also the combination of five
treatment
stations according to the present invention is possible and preferred
according to the present
invention.
[0058] According to another preferred embodiment of the present invention,
the
treatment liquid and/or the further treatment liquid and/or the third
treatment liquid comprises
-- hydrochloric acid in a concentration ranging from and including 150 g/I to
and including
250 g/I and
-- FeCI3in a concentration ranging from and including 10 g/I to and including
35 g/I,
especially in a concentration ranging from and including 15 g/I to and
including 30 g/I or
especially in a concentration ranging from and including 19 g/I to and
including 26 g/I and,
-- FeCl2 in a concentration ranging from and including 30 g/I to and including
300 g/I,
especially in a concentration ranging from and including 30 g/I to and
including 60 g/I or in a
concentration ranging from and including 130 g/I to and including 180 g/I or
in a
concentration ranging from and including 230 g/I to and including 300 g/I.
[0059] According to the present invention, it is thereby advantageously
possible to
combine a high efficiency of the pickling process while retaining the
possibility to
comparatively easily regenerated the used pickling acids (treatment liquids).
[0060] These and other characteristics, features and advantages of the
present
invention will become apparent from the following detailed description, taken
in conjunction
with the accompanying drawings, which illustrate, by way of example, the
principles of the
invention. The description is given for the sake of example only, without
limiting the scope of
the invention. The reference figures quoted below refer to the attached
drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0061] Figure 1 schematically illustrates a treatment system comprising
three different
treatment stations, each one being configured according to the present
invention, i.e. having
a treatment tank with both a spray section and an immersion section such that
a common
collection means and the same treatment liquid is used for the treatment of
the steel strip in
both the respective spray section and the immersion section.
[0062] Figure 2 schematically illustrates a first embodiment of a
treatment station having
a treatment tank and a common collection means separated from the treatment
tank, the
treatment tank having its spray section and its immersion section to treat the
steel strip with a
common treatment liquid circulating between ¨ and within ¨ the common
collection means on
the one hand, and the spray and immersion sections on the other hand.
[0063] Figure 3 schematically illustrates a second embodiment of a
treatment station
having a treatment tank and a common collection means separated from the
treatment tank,
the treatment tank having its spray section and its immersion section to treat
the steel strip
with a common treatment liquid circulating between ¨ and within ¨ the common
collection
means on the one hand, and the spray and immersion sections on the other hand.
DETAILED DESCRIPTION
[0064] The present invention will be described with respect to particular
embodiments
and with reference to certain drawings but the invention is not limited
thereto but only by the
claims. The drawings described are only schematic and are non-limiting. In the
drawings, the
size of some of the elements may be exaggerated and not drawn on scale for
illustrative
purposes.
[0065] Where an indefinite or definite article is used when referring to a
singular noun,
e.g. "a", "an", "the", this includes a plural of that noun unless something
else is specifically
stated.
[0066] Furthermore, the terms first, second, third and the like in the
description and in
the claims are used for distinguishing between similar elements and not
necessarily for
describing a sequential or chronological order. It is to be understood that
the terms so used
are interchangeable under appropriate circumstances and that the embodiments
of the
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invention described herein are capable of operation in other sequences than
described of
illustrated herein.
[0067]
Figure 1 schematically illustrates a treatment system comprising three
different
treatment stations 3, 31, 32 as an example of a pickling line according to the
present
invention. In one possible and preferred implementation of the pickling line
for the treatment
of a steel strip 1, represented in Figure 1, all three treatment stations 3,
31, 32 represent
treatment stations according to the present invention, i.e. having a treatment
tank with both a
spray section and an immersion section such that a common collection means and
the same
treatment liquid is used for the treatment of the steel strip in both the
respective spray section
and the immersion section. In this implementation or architecture of the
pickling line, all three
treatment stations 3, 31, 32 are realized either according to a first
embodiment of the present
invention, represented in Figure 2 for the exemplary case of the treatment
station being
represented by reference sign 3, or according to a second embodiment of the
present
invention, represented in Figure 3 for the exemplary case of the treatment
station being
represented by reference sign 3. Alternatively, a part of the three treatment
stations 3, 31, 32
is or are realized according to the first embodiment of the present invention
(Figure 2) and
another part is or are realized according to the second embodiment of the
present invention
(Figure 3). In the context of the present invention, the terms 'treatment
station' and 'first
treatment station' as well as 'further treatment station' and 'second
treatment station' are
used synonymously and only aim to differentiate the treatment stations from
one another.
Typically, the naming convention typically (but not necessarily) relates to
the location of a
treatment station along the transport direction of the steel strip, the
transport direction being
represented by reference sign 2. In the implementation represented in Figure
1, a treatment
station (or first treatment station) 3 is located upstream of a further
treatment station (or
second treatment station) 31. The further treatment station (or second
treatment station) 31
is located upstream of a third treatment station 32. The treatment station (or
first treatment
station) 3 comprises a treatment tank (or first treatment tank) 4, and a
common collection
means (or first common collection means) 5. The further treatment station (or
second
treatment station) 31 comprises a further treatment tank (or second treatment
tank) 41, and a
further common collection means (or second common collection means) 51. The
third
treatment station 32 comprises a third treatment tank 42, and a third common
collection
means 52. In the exemplary implementation of the pickling line represented in
Figure 1
(where all treatment stations are configured according to the present
invention), all tree
treatment stations 3, 31, 32 each have a spray section and an immersion
section as part of
their respective treatment tank 4, 41, 42, i.e. the treatment station (or
first treatment station) 3
has a spray section (or first spray section) and an immersion section (or
first immersion
section) using a treatment liquid (or first treatment liquid), the further
treatment station (or
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second treatment station) 31 has a further spray section (or second spray
section) (not
depicted in Figure 1) and a further immersion section (or second immersion
section) (not
depicted in Figure 1) using a further treatment liquid (or second treatment
liquid), and the
third treatment station 32 has a third spray section (not depicted in Figure
1) and a third
immersion section (not depicted in Figure 1) using a third treatment liquid.
For the exemplary
case of the treatment station 3 (first treatment station 3), a first and a
second embodiment of
the present invention is schematically shown in Figures 2 and 3.
[0068] Figure 2 schematically illustrates the first embodiment of the
treatment station 3
having the treatment tank 4 and the common collection means 5 separated from
the
treatment tank 4, the treatment tank 4 having its spray section 13 and its
immersion section
14 to treat the steel strip 1 with a common treatment liquid circulating
between ¨ and within ¨
the common collection means 5 on the one hand, and the spray and immersion
sections 13,
14 on the other hand.
[0069] Figure 3 schematically illustrates a second embodiment of a
treatment station 3
having the treatment tank 4 and the common collection means 5 separated from
the
treatment tank 4, the treatment tank 4 having its spray section 13 and its
immersion section
14 to treat the steel strip 1 with a common treatment liquid circulating
between ¨ and within ¨
the common collection means 5 on the one hand, and the spray and immersion
sections 13
on the other hand.
[0070] Figures 1, 2 and 3 combined illustrate the inventive treatment
process and
system (or treatment station) for chemical or electrochemical treatment of the
surface of the
steel strip 1, the steel strip 1 being a stainless steel strip. The steel
strip 1 is first horizontally
transported through the treatment stations 3, 31, 32 in which the steel strip
1 is treated with a
treatment liquid in the form of a pickling acid, normally containing HCI. At
least one of the
treatment stations 3, 31, 32 (or their respective treatment tanks 4, 41, 42)
comprises a spray
pickling section (as represented in Figures 2 and 3 as spray section 13 of the
treatment
station 3) and a dip pickling section (as represented in Figures 2 and 3 as
immersion section
14 of the treatment station 3) according to the present invention. Figure 1
shows an
exemplary implementation with three treatment station 3, 31, 32 (each having a
treatment
tank) as a preferred embodiment, however the number of treatment stations (and
treatment
tanks) is at least one and is not limited to three.
[0071] All treatment stations 3, 31, 32 comprise a common collection
means,
respectively (i.e. the respective treatment tanks 4, 41, 42 are connected to
respective
common collection means (or circulation tanks) 5, 51, 52), wherein the common
collection
means 5, 51, 52 are either (i.e. potentially for each treatment station 3, 31,
32 differently)
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realized as separate tanks as shown in the first embodiment represented in
Figure 2 or are
realized as common collection means 5, 51, 52 integrated in the respective
treatment tank 4,
41,42 as shown in Figure 3.
[0072] In the exemplary embodiment shown in Figure 1, the common
collection means
(or circulation tanks) 5, 51, 52 are operated as a cascade, i.e. the fresh or
regenerated acid
(i.e. the treatment liquid) is added (cf. reference sign 54) to the last
common collection
means (or last circulation tank) 52 ¨ i.e. being related to the most
downstream treatment
station 32 according to the transport direction 2 of the steel strip 1 ¨ and
is consequently
transferred to the other common collection means (or circulation tanks) in
counter direction to
the strip transport direction 2. Thereby, the level of free acid is the
highest in the third
treatment liquid (circulating in the third treatment station 32), the level of
free acid is medium
in the further treatment liquid (second treatment liquid) (circulating in the
further (second)
treatment station 31), and lowest in the treatment liquid (first treatment
liquid) (circulating in
the (first) treatment station 3. Finally the spent acid is removed (reference
sign 55) from the
(first) common collection means (or (first) circulation tank) 5. After the
pickling treatment in
the three treatment stations 3, 31, 32 (in the exemplary implementation shown
in Figure 1),
the steel strip 1 is further processed in section 6 which comprises a rinse
section and a dryer,
if required.
[0073] According to the first embodiment of the common collection means
(or circulation
tank) 5, shown in Figure 2, the treatment station 3 comprises the treatment
tank 4 with a
separate common collection means 5 (or separate circulation tank 5). At the
entry and exit
section, wringer rolls 12 are installed to remove pickling acid from the strip
and to guide the
steel strip 1 inside the treatment tank 4. The wringer roll 12 in the entry
section is only used
when the treatment tank is the first tank in the pickling process like the
treatment station 3 in
Figure 1. The following treatment stations (or treatment tanks), like
treatment stations 31, 32
in Figure 1, do not need such wringer roll 12. In the exemplary representation
of Figure 2 (i.e.
not necessarily), the first part (according to the transport direction of the
steel strip 1) of the
treatment tank 4 is a spray pickling section 13 or spray section 13, followed
by a dip pickling
section 14 or immersion section 14. In the spray pickling section 13, spray
nozzles 15 are
mounted above and below the surface of the steel strip 1. The pickling acid
(or treatment
liquid) is pumped from the circulation tank 5 (or common collection means 5)
by means of
pumps 17, 18 to both the spray pickling section 13 and the dip pickling
section 14. In one or
more of the pump circuits, a heat exchanger 19 is installed to heat the
pickling acid
(treatment liquid) to the required temperature. If required, a guide roll 20
can be installed
between the spray pickling section13 and the dip pickling section 14 in order
to reduce the
slack of the strip.
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[0074] According to the second embodiment of the common collection
means (or
circulation tank) 5, shown in Figure 3, the treatment station 3 comprises the
treatment tank 4
with an integrated common collection means 5. The other components of the
treatment
station 3 are analogous to the description of Figure 2.
[0075] While specific embodiments of the invention have been shown and
described in
detail to illustrate the inventive principles, it will be understood that the
invention may be
embodied otherwise without departing from such principles.
[0076] For example, the pickling line is configured for a maximum width
of the steel strip
1 of 1890 mm, a maximum speed of the steel strip 1 of 85 m/min. Furthermore
exemplarily,
the distance of the spray nozzles 15 to the steel strip 1 (both from the spray
nozzles to the
top surface 1' of the steel strip 1, and to the bottom surface 1" of the steel
strip 1) is 200 mm
or approximately 200 mm. Additionally, the distance of the spray nozzles 15 to
each other in
the lateral direction of the steel strip 1 corresponds to 200 mm or
approximately 200 mm.
Additionally, the distance of the spray nozzles 15 to each other in the
longitudinal direction of
the steel strip 1 corresponds to 500 mm or approximately 500 mm. The treatment
liquid is
preferably pumped out of the spray nozzles having a pressure of between and
including 1
bar to and including 3 bar, and the amount of treatment liquid per spray
nozzle is preferably
12 l/min or approximately 12 l/min. For example, the total number of spray
nozzles per
treatment station corresponds to 306 or approximately 306, and the amount of
pumped
treatment liquid per treatment station corresponds to 220 m3/h or
approximately 220 m3/h.
[0077] Test trials were carried out in a pilot plant. The pilot plant
consisted of two
treatment stations (each having a treatment tank) both arranged as described
in the present
invention with a first spray pickling section followed by a dip pickling
section in each of the
treatment tanks. The treatment tanks were designed so that the length of both
sections was
approximately the same. The pickling acid used was HCI with a concentration of
approx. 200
g/I total acid in both tanks. The material treated during the test runs were
different austenitic
steel grades such as AISI 304 and 316. The test results have proven that the
pickling time
could be reduced by 40 ¨ 45% compared to the conventional pickling process
using dip
pickling with shallow tank turbulence technology, while the FeCI3
concentration was
constantly below 30 g/I which is considered to be uncritical as far as the
acid regeneration
process is concerned. All tested materials showed uniform pickling results
without any signs
of over-pickling.
[0078] In another test using the same pilot plant, the material was
treated with a reduced
temperature of the pickling acid (treatment liquid). The results showed that
the temperature
could be reduced from 90 C down to 70 C while still reaching the same
pickling time as for
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the conventional dip pickling process with shallow tank turbulence technology.
This result is
equivalent to a 20 % reduction of the energy which is needed to keep the
process
temperature in the pickling process.
[0079] As an example of the operation of the system and especially of
the use of the
treatment liquids as a cascade, an example is given of the concentration
values for an
example of using three treatment stations in a pickling line:
In the first treatment station 3, the concentration of HCI is in the range of
between and
comprising 201 g/I to and comprising 215 g/I, the concentration of MeCl2 is in
the range of
between and comprising 270 g/I to and comprising 286 g/I, the concentration of
FeCI3 is in
the range of between and comprising 23 g/I to and comprising 29 g/I. The
temperature of the
treatment liquid is in the range of between and comprising 87 C to and
comprising 89 C.
In the second treatment station 31, the concentration of HCI is in the range
of between and
comprising 204 g/I to and comprising 214 g/I, the concentration of MeCl2 is in
the range of
between and comprising 141 g/I to and comprising 149 g/I, the concentration of
FeCI3 is in
the range of between and comprising 19 g/I to and comprising 23 g/I. The
temperature of the
treatment liquid is in the range of between and comprising 91 C to and
comprising 93 C.
In the third treatment station, the concentration of HCI is in the range of
between and
comprising 190 g/I to and comprising 201 g/I, the concentration of MeCl2 is in
the range of
between and comprising 40 g/I to and comprising 50 g/I, the concentration of
FeCI3 is in the
range of between and comprising 20 g/I to and comprising 22 g/I. The
temperature of the
treatment liquid is in the range of between and comprising 88 C to and
comprising 91 C.
[0080] Various tests were made with different qualities of the hot
rolled stainless steel
strips. Tests were done majority with austenitic AISI 304 and 316 grades as
well as with
ferritic AISI 409 and 430 grades, but also with other steel grades. The best
results were
achieved with steel that has been either shot-blasted or the scale-broken off
prior to pickling
in hydrochloric acid. In these trials some of the stainless steel grades were
compared under
the same pickling conditions. In all trials combined spray pickling and dip
pickling in one
treatment step using a common circulation system gave the shortest pickling
time and
excellent surface quality while the FeCI3 concentration could be kept at a
level below 30g/I
Significantly reduction of the pickling time were achieved for heavy to pickle
steel grades as
this is the case of coils coiled at higher coiling temperature e.g. > 700 C.
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REFERENCE SIGNS
1 steel strip
2 transport direction of the steel strip
3 treatment station
4 treatment tank (of the treatment station)
5 common collection means (of the treatment station)
12 wringer roll(s)
13 spray section (of the treatment station)
14 immersion section (of the treatment station)
spray nozzles (of the treatment station)
17, 18 pumps
19 heat exchanger
guide roll(s)
15 31 further treatment station
41 further treatment tank (of the further treatment
station)
51 further common collection means (of the further
treatment station)
32 third treatment station
42 third treatment tank (of the third treatment station)
20 52 third common collection means (of the third treatment
station)
54 feeding of fresh treatment liquid
55 removing of used treatment liquid
