Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CONTINUOUSLY ANNEALING AND PREPARING STRIP OF
HIGH-STRENGTH STEEL FOR THE PURPOSE OF HOT-DIP
GALVANISATING IT
Field of the invention
[0001] The present invention relates to a new method
for continuously annealing and preparing a strip of high-
strength steel with a view to coating it by hot dipping in
a bath of molten metal, preferably by galvanisation or a
treatment known as "galvannealing."
[0002] The technical area considered here is that of
the galvanisation in continuous motion, in a coating bath
of zinc or of a zinc alloy, of high-alloy strips of steel,
more particularly HSS steel (high strength steels). These
special steels, reputed to be difficult to galvanise, are
for example steels that may comprise a level of alloy
elements (aluminium, manganese, silicon, chromium, etc.) of
up to 2%- or more, stainless steels, "dual phase", TRIP,
TWIP (up to 25% Mn and 3% Al), etc. These steel strips are
generally intended to be cut and formed at a later stage by
pressing, folding, etc. for applications in the
construction or automobile sector for example.
State of the art
[0003] It is well known that some steels do not
respond well to galvanisation or to a galvannealing
treatment given their specific surface reactivity. The
ability to galvanise essentially depends on the proper
elimination of the residues of rolling oil and on the
prevention of excessive surface oxidation before the
immersion in the bath of molten metal. Thus, a lack of
wettability of molten zinc on shades of high-alloy steels
may be encountered during the continuous galvanisation
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process. This decrease in wetting of zinc is explained by
the presence of a layer of selective oxides in the outer
layer at the surface of the strip ("outermost surface").
These selective oxides are created by the segregation of
the alloy elements and their oxidation by water steam
during the continuous annealing prior to the immersion in
the bath of zinc. The water steam is generated at this
point by the reduction of iron oxide, always present in
cold-rolled bars, by the hydrogen contained in the
atmosphere of annealing furnaces.
[0004] Consequently, there have been attempts to
eliminate the selective oxidation on the outside or to make
it migrate to the inside of the steel, to 1 or 2 m beneath
the outer layer of the surface, in order to allow the
presentation of a layer of practically pure metallic iron
to the molten zinc, regardless of the alloy composition and
favouring the attachment of the zinc or zinc-alloy coating.
This result may be obtained by various methods:
- increasing the dewpoint while maintaining a high
temperature (for example JP-A-2005/068493), in such a
way as to shift the selective oxidation of the alloy
elements from the outside to the inside;
- total oxidation of the iron during the heating stage by
increasing e.g. the ratio of air/combustible gas in the
direct flame burners of the furnace, then reduction by
hydrogen to metallic iron while maintaining a high
temperature (for example JP-A-2005/023348, JP-A-07
034210, etc.) or reduction by the free carbon of the
steel which diffuses, if need be, through the oxide
layer and exchanges oxygen on its surface (see for
example BE-A-1 014 997);
- pre-deposition of iron or nickel (for example JP-A-
04 280925, JP-A-2005/105399).
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[0005] These methods generally entail working under
steel-reducing atmosphere during the stage of maintaining
at high temperature, which requires a low dewpoint and a
high level of hydrogen (up to 75% of the gas of the
atmosphere), which is an expensive gas. They all allow to
improve the "galvanisability" of high-strength steels with
significant but nevertheless insufficient efficiency, above
all in the case of some steels with, for example, high
silicon levels (about 1.5% by weight). Moreover, the
methods requiring pre-deposition are very costly.
[0006] According to one example of a method already
known in the state of the art, premises for annealing and
preparing a steel strip for galvanisation typically
comprise in the flow direction of the strip:
- a first (pre-)heating section to ensure the heating of
the strip up to a temperature that allows to form an
oxide film of suitable thickness (about 50 nanometres)
for subsequent reduction; this section is under an
atmosphere that was rendered oxidising by the addition
of air or oxygen, for example in the form of an
air/combustible gas mixture in the case of a direct-
flame furnace or the addition of air only in the case of
a radiant furnace;
- a second annealing section, separated from the heating
section by a conventional airlock, where the strip is
maintained at the high annealing temperature and that is
under inert and over-pressurised atmosphere in order to
prevent the penetration of the gases of the heating
section;
- a third reduction section, also separated from the
second section by a conventional airlock, under an
atmosphere that is slightly depressurised compared with
the preceding section but that is slightly over-
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pressurised relative to ambient pressure; this section
is intended to complete the annealing cycle (end of the
temperature-maintenance period), to cool the strip and
possibly to cause overaging before it is transferred to
the bath of molten metal through an immersion pump; in
this zone, the oxide layer created in the first section
is ideally completely reduced by a hydrogen/inert gas
atmosphere with a very low dewpoint.
[0007] Of course, simpler or more complex annealing
furnaces are also known that typically comprise between one
and four separate sections for achieving the functions of
(pre-)heating, temperature maintenance, cooling, overaging,
etc., respectively.
Aims of the invention
[0008] The present invention aims to provide a
solution that allows to overcome the drawbacks of the state
of the art.
[0009] In particular, the invention aims to provide
a method for annealing and preparing high-strength steels
for galvanisation that is more economical, the latter being
achieved with or without accompanying heat treatment of a
galvannealing type.
[0010] The invention also aims to allow the
preparation of high-strength steels for galvanisation that
are free of brittleness defects.
[0011] In particular, the invention aims to provide
an annealing method under confined atmosphere that is free
of added hydrogen.
[0012] One additional aim of the invention is to
prevent the selective oxidation of alloy elements in the
outermost layer of the strip surface during the total
oxidation stage in the course of the continuous annealing
preceding cooling and immersion in the bath of zinc.
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Main characteristic features of the invention
[0013] The present invention relates to a method for
the continuous annealing and preparation of a strip of
high-strength steel with a view to its hot-dip coating in a
5 bath of molten metal, according to which said strip of
steel is treated in at least two sections comprising
successively, if considered in the flow direction of the
strip:
- a"heating and temperature-maintenance" section in which
the strip is heated, then maintained at a given
annealing temperature under oxidising atmosphere with an
air (or oxygen)/non-oxidising or inert gas mixture in
order to form a thin oxide film on the surface of the
strip, whose thickness, preferably between 0.02 and
0.2 m, is controlled, said heating of the strip being
achieved either by a direct flame or by radiation;
- a "cooling and transfer" section in which, before it is
transferred into the coating bath, the strip, which is
at least annealed, is cooled and undergoes complete
reduction to metallic iron of the iron oxide present in
the oxide layer formed in the heating and temperature-
maintenance section, under reducing atmosphere with a
mixture of low level of hydrogen and inert gas, both
said sections being separated from each other by a
conventional airlock;
wherein the oxidising atmosphere is at least partially
separated from the reducing atmosphere, wherein a
controlled level of oxygen is maintained in the heating and
temperature-maintenance section at between 50 and 1,000 ppm
and wherein a controlled level of hydrogen is maintained in
the cooling and transfer section at a value lower than 4%
and preferably lower than 0.5%.
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[0014] Complete reduction of the iron oxide should
be understood as its reduction of at least 98%.
[0015] As an advantage, the controlled oxygen level
is maintained in the heating and temperature-maintenance
section at between 50 and 400 ppm.
[0016] According to a first preferred embodiment of
the invention, the oxidising atmosphere is separated from
the reducing atmosphere by over-pressurising the oxidising
atmosphere so that the oxygen introduced by the strip into
the cooling and transfer zone through the airlock
completely reacts, because of this overpressure, with the
hydrogen contained in the cooling atmosphere by forming
steam.
[0017] According to a second preferred embodiment of
the invention, the hydrogen present in the cooling and
transfer section, introduced into the hot gaseous flow
directed upstream, is allowed to react with the oxygen
coming from the heating and temperature-maintenance section
in order to form steam. In this case, the cooling and
transfer section is maintained at overpressure compared
with the heating and temperature-maintenance section. Since
the high-pressure gas cannot escape towards the bath of
molten metal, it returns to the heating and temperature-
maintenance zone.
[0018] According to the invention, the control of
the oxygen content of the oxide layer formed in the heating
and temperature-maintenance section is obtained either by
modifying the gaseous mixture with the combustion air
feeding the direct-flame heating means or by controlled
injection of the air (or oxygen)/inert gas mixture in the
case of radiation or induction heating.
[0019] The non-oxidising or inert gas is preferably
nitrogen or argon.
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[0020] As an advantage, the molten metal is zinc or
one of its ailoys.
[0021] As a further advantage, the heating and
temperature-maintenance zone is free of any reducing
atmosphere.
[0022] The method for hot-dip coating is preferably
galvanisation or a galvannealing treatment.
[0023] Still according to the invention, the
atmosphere both in the heating and temperature-maintenance
section and in the cooling and transfer section has a
dewpoint lower than or equal to -10 C and preferably -20 C.
[0024] According to a preferred embodiment, the
strip is heated up to a temperature between 650 C and
1,200 C, which includes the maintenance temperature.
[0025] According to another preferred embodiment,
the strip is then cooled to a temperature higher than 450 C
at a cooling speed between 10 and 100 C/s.
Description of a preferred embodiment of the invention
[0026] One economical method, proposed according to
the invention, aims to implement the annealing stage in
preparation for galvanisation without the addition of
hydrogen, a gas which is ten times as expensive as a more
common gas such as nitrogen and which is moreover the cause
of serious brittleness defects in strong steels.
[0027] The invention aims to achieve perfect
galvanisation for all shades of strong steel. To prevent
oxidation of the alloy elements on the outermost surface,
one proposal is to inject an air/nitrogen mixture into the
furnace during the entire cycle of (pre-)heating and
maintenance of the bar at high temperature.
[0028] This method therefore does not require the
separation of the atmosphere in the entire heating/
temperature-maintenance part, as is the case in other
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methods (for example JP-A-2003/342645) where low-pressure
reactive zones are incorporated into this part of the
furnace.
[0029] The oxygen of the air/nitrogen mixture will
have the effect of creating two simultaneous and competing
reactions in the annealing section:
- oxidation of the iron by the oxygen on the outermost
surface with an increase in the iron oxide by the
diffusion of iron at the surface. Thus, as long as a
thin layer of iron oxide persists on the surface of the
bar, the alloy elements, with the exception of
manganese, are blocked at the steel/iron oxide
interface;
- subsequent reduction of the iron oxide by diffusion of
the free carbon towards the steel/iron oxide interface.
[0030] The alloy elements also participate in the
reduction of the iron oxide when they migrate to the steel/
iron oxide interface.
[0031] The air/nitrogen atmosphere of the heating/
temperature-maintenance part must however be separated and
partially isolated from the non-oxidising atmosphere of the
strip cooling and transfer stages as far as the bath of
zinc. To this end, the oxidising atmosphere will preferably
be maintained at high pressure compared with the non-
oxidising atmosphere in such a way that the oxygen
introduced by the bar completely reacts with the hydrogen
contained in the atmosphere of the cooling section.
[0032] In such a configuration, a steel comprising
i.a. 1.2% aluminium will, for example, be heated and
annealed to a temperature of 800 C in an atmosphere with
100 ppm of oxygen in nitrogen. At the end of the
temperature maintenance, which lasts one minute, the bar is
cooled to 500 C at a speed of 500C/s in an atmosphere with
4% hydrogen and 0.1% water steam, which corresponds to a
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dewpoint of -200C. This bar is then immersed at a
temperature of 470 C into a bath of zinc with 0.2%
aluminium and maintained at 460 C. After a 3-second
immersion, the coating is wringed so as to leave an 8- m
zinc layer. Such a zinc deposit is then perfectly wetting
and has adherence qualities that are comparable to those
obtained for an ordinary low-carbon steel.
[0033] To cite another example, the same method may
be applied to a steel with i.a. 1.5% silicon. However, in
this case, it will be necessary to increase the oxygen
level to 300 ppm during the heating/temperature-maintenance
stage in order to obtain a comparable result. This increase
in the oxygen level is necessary since silicon delays the
diffusion of iron by providing a silicon oxide barrier at
the steel/iron oxide interface.
[0034] Another way of working is to allow the usual
flow to establish itself from the bath of zinc to the
heating section and to allow the very low level of hydrogen
(<0.5g) of the transfer/cooling section to react with the
oxygen of the heating/temperature-maintenance part in order
to form water steam. Extra oxygen may be added at the exit
from the temperature-maintenance section to neutralise the
entry of hydrogen, the levels implemented always being
positioned very far from the danger zone, i.e. the
explosive zone (4% H2 in the air).
[0035] Indeed, a high hydrogen level is not
necessary in the cooling section since the carbon of the
steel will be sufficient to reduce the thin layer of iron
oxide created in the heating/temperature-maintenance part
and the metallic iron thus prepared will ensure good
wettability by zinc during the immersion of the bar in the
bath.
[0036] To be effective, this method will have to
provide a means for controlling the oxygen level in the
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furnace within the range of 50 to 1,000 ppm. In fact, a
too-low level will not allow to create a layer of iron
oxide sufficiently impervious to the diffusion of the alloy
elements towards the outermost surface and a too-high level
5 of oxygen will produce a too-thick iron-oxide layer that
will not be reduced during the cooling and transfer stages
leading towards the bath of zinc. This oxygen level will
preferably be within a range of 50 to 400 ppm.
[0037] The present invention has a certain number of
10 advantages, including in particular the fact that:
- far less hydrogen than in the state of the art, and
perhaps even none, is added in the heating/temperature-
maintenance zone, which represents major operational
saving and guarantees the production of a high-strength
steel with fewer brittleness defects;
- the heating section is no longer separated from the
section in which the annealing temperature is
maintained, which allows to dispense with an airlock as
well as to avoid any duplication of the control
equipment for the gaseous atmosphere;
- this method is much more effective than the methods
known in the state of the art as regards the adherence
of the coating or the wettability of the strip;
- the gaseous atmosphere used is less damaging to the
equipment (e.g. the radiant tubes), in particular
following the reduction of its hydrogen level.
