Note: Descriptions are shown in the official language in which they were submitted.
CA 02622817 2008-03-17
SI/cs 051196W0
22 September 2006
PROCESS TO MANUFACTURE A CORROSION-RESISTANT FLAT STEEL
PRODUCT
The invention relates to a process for manufacturing
corrosion-resistant flat steel products, which are provided
at least with a first zinc-containing coating, and a second
coating lying thereon, which is based on pure magnesium or
a magnesium alloy. Such processes are used for example to
produce sheet steel, which due to its optimised corrosion
resistance is particularly suitable for use in the
construction, domestic appliance or motor vehicle
industries.
Coatings, which in the predominant number of applications,
consist of zinc or zinc alloys are applied to sheet steel
in order to improve its corrosion resistance. Such zinc or
zinc alloy coatings, due to their barrier and cathodic
protection effect, ensure very good corrosion resistance of
the coated sheet steel. However despite the quality
already achieved up till now, higher and higher
requirements in the corrosion resistance and general
characteristics of coated sheet steel are demanded by the
processors.
At the same time apart from the heavy cost pressure there
is a need for better workability of coated sheet steel. In
particular surface qualities optimised in relation to the
respective intended purpose are demanded.
These demands in practice cannot be met alone by increasing
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the coating thickness, since on the one hand this is
countermanded by economic and ecological reasons and on the
other hand increasing the coating thickness involves a
general degradation in the formability of sheet steel
galvanized in this way.
Galvanized sheet steel is usually converted to consumer
articles by forming, joining, organic coating (for example
painting) or similar processes. Particularly in the field
of motor vehicle body construction the bonding together of
preformed steel parts is gaining acceptance. A further
important factor is the formability of the coatings, that
is to say their ability to withstand even greater
transforming stresses, as they occur for example in the
case of deep-drawing, without serious damage. None of
these demands can be met to the same degree with
conventional pure-galvanized products. Rather,
conventionally coated sheet steel usually has particularly
good characteristics as regards a certain requirement
feature, while shortcomings must be accepted as regards
other requirement features.
Thus for example hot-dip galvanized sheet steel is
characterised by high corrosion resistance in both the
unpainted as well as in the painted state. Although
electro-galvanized sheet steel, in comparison to hot-dip
galvanized sheet steel, generally has a further improved
surface quality and equally improved bonderizing-ability in
preparation for paint finishing, it must be considered that
the production of electro-galvanized sheet steel is more
cost-intensive than hot-dip galvanizing due to higher
energy consumption and the waste disposal requirements,
which the wet chemical process entails.
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An improvement in the performance characteristics of
galvanized sheet steel can be obtained by applying a second
layer, which is based on pure magnesium or a magnesium
alloy, to the first protective layer formed by galvanizing.
A characteristic combination is achieved by application of
this second magnesium-containing layer, wherein the
characteristics of the first zinc-containing layer and the
second magnesium-based layer are optimally enhanced.
In order to be able to utilize this optimum characteristic
combination of the different layers to the full, the
coating process is preferably carried out in such a way
that breakdown of the layers is avoided. For this reason a
diffusion or convection layer is formed between the zinc-
containing and the magnesium-based layer, which ensures the
magnesium-containing layer adheres firmly to the zinc
layer.
A process, which permits a second layer to be applied to a
sheet steel previously coated with a corrosion-protective
coating, is for example disclosed by the German Patent DE
195 27 515 Cl or the corresponding European Patent EP 0 756
022 Bl. The corrosion-resistant sheet steel manufactured
by this process has enhanced forming and spot weld ability.
For this reason the sheet steel provided with the zinc
layer by hot-dip galvanizing or electro-galvanizing is
firstly cleaned mechanically or chemically. By means of a
suitable PVD (physical vapour deposition) process, a top
layer is then deposited on the previously zinc-coated steel
substrate. Afterwards the strip coated in this way
undergoes heat treatment, which is carried out for at least
ten seconds within a temperature range of 300 C - 400 C
in an inert gas or oxygen-lean atmosphere. As the result
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of this heat treatment the metal of the coating diffuses
into the first zinc-containing corrosion protective layer
lying on the steel substrate.
In order to be able to precisely control the diffusion
process and to achieve good uniformity of the top layer,
the sheet steel in the course of the prior art process,
before the vacuum coating, undergoes vacuum pre-treatment
by ion bombardment or plasma treatment. The galvanized
steel substrate to be plated with the second layer of metal
is fine-cleaned and conditioned by this pre-treatment so
that the metal, deposited in the subsequent PVD process, is
distributed widely and densely as a thin layer over the
entire zinc coating. Corresponding fine cleaning is
necessary, according to the statements of the professional
world, particularly if a magnesium-based coating is applied
as an external layer to galvanized sheet steel in order to
improve its bonding and painting performance.
Despite the characteristic improvements attainable by using
the method described in DE 195 27 515 Cl or EP 0 756 022
Bl, this process has not become generally accepted in
practice. This is due inter alia to the high construction
and operating costs, which are incurred when setting up and
maintaining a production line designed for executing the
prior art process. These are caused inter alia because a
large part of the stages in the prior art process must be
carried out under vacuum, in order to manufacture flat
steel products plated with at least a zinc coating and a
surface layer applied thereon, which meet the strict
requirements of the users. Furthermore on an industrial
scale it has proven difficult, in the case of economic
continuous production, within the narrow time window
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prescribed in DE 195 27 515 Cl, to heat the strip to 300 -
400 C with homogeneous temperature distribution over the
strip profile.
The object of the invention was to create a process, which
permits economical production of corrosion-resistant sheet
steel with good performance characteristics for certain
application purposes.
This object was achieved on the basis of the prior art
described above through a process for manufacturing a flat
steel product made from corrosion-resistant steel, wherein
according to the invention a zinc-containing coating is
applied by electro-galvanizing to a flat steel product,
wherein the flat steel product if required is finally
cleaned mechanically and/or chemically, wherein a second
magnesium-based coating is applied directly to the finally
cleaned zinc-containing coating by means of physical vapour
deposition and wherein under normal atmosphere after
application of the second coating, post heat treatment of
the coated flat steel product is carried out for forming a
diffusion or convection layer between the zinc-containing
and the magnesium-based coating, at a heat treatment
temperature of 320 C to 335 C.
In accordance with the invention, the steel substrate,
which is a flat product such as strip or sheet, made from
low carbon steel, is firstly galvanized in a conventional
way and then cleaned mechanically or chemically in a way,
which is equally conventional. Mechanical or chemical
cleaning in this case can take place alternatively or in
combination, in order to ensure the surface of the zinc
coating is as free as possible of grease and loosely
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adhering zinc material or other residues.
For the invention it is essential that the galvanized flat
steel product is completely clean at the end of this
cleaning. Thus deviating from the notion, prevailing up
until now in the professional world that such an
intermediate step is indispensable, with the process
according to the invention no further fine cleaning takes
place before the magnesium-containing coating is deposited
on the zinc-layer. Instead according to the invention the
flat steel product, plated with the zinc layer, is fed in
the purely mechanically or chemically final cleaned state
into the physical vapour deposition, where it is provided
with the magnesium-containing external layer.
Surprisingly it has also been shown that a previously
galvanized steel sheet or strip, provided in such a way
with a magnesium layer, while dispensing with prior
reactive plasma cleaning, apart from a surface quality
optimised in relation to its optical appearance possesses a
bonding performance, which meets all requirements arising
in the practical use of such sheet steel.
A test for evaluating bonding performance of coated sheet
steel, used in the motor vehicle and steel-making industry,
is the so-called "adhesive bead test".
In this test a commercially available structural adhesive,
suitable for bonding body components, is applied to the
previously degreased surface to be examined. The adhesive
is applied in the form of two parallel adhesive beads with
a height of 4 - 5 mm and a width of about 10 mm. In order
to ensure standard conditions, the geometry of the bead is
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then adjusted by means of a template. After the adhesive
has hardened, possibly assisted by heat, the sheet steel is
bent at an angle of approx. 1000. Due to tension between
the adhesive and the coating surface, produced by bending,
in this case the adhesive bead usually firstly breaks
vertically to the specimen surface and then peels away
along the specimen surface.
In the case of coated sheet steel with poor bonding
performance peeling away takes place in the transient area
between the individual coatings or between the lowest
coating and the steel substrate. With the method of
production according to the invention on the other hand the
peeling action, if it occurs at all, is limited to the
border between the free surface of the outer lying coating
or to the area of the adhesive bead itself. That is to
say, despite simplification of the process achieved by the
invention, in the case of sheet steel provided according to
the invention with a zinc-magnesium plating system, the
applied coatings adhere so firmly amongst themselves and to
the steel substrate, that in the adhesive bead bending
test, the adhesive does not peel away in the coatings or
between the coatings and the steel substrate, but at most
between the adhesive and the coating or only in the
adhesive itself. The quality of an adhesive bond produced
with a flat product according to the invention thus only
depends on the bonding performance of the adhesive on the
surface of the coating. Chipping or lifting of the plating
system applied to the steel substrate is reliably
prevented, despite fine cleaning being dispensed with
according to the invention before vapour deposition of the
magnesium layer, due to the heat treatment carried out
according to the invention, following application of the
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magnesium coating.
Apart from the particularly good bonding performance, the
stone chip resistance of flat steel products coated
according to the invention also meets the requirements
demanded in practice. Thus stone chip resistance, which
corresponds to that of sheet steel coated in the
conventional way, can be ensured for sheet steel coated
according to the invention, particularly while maintaining
the temperature windows of the heat treatment, indicated
below as preferable dependent on the type of zinc coating,
despite reactive plasma cleaning being dispensed with
before physical vapour deposition plating.
Accordingly flat products manufactured according to the
invention are particularly suitable for producing motor
vehicle body components, which are formed by bonding
individual components with one another.
A pre-condition for the good bonding performance achieved
according to the invention is that the steel strip, vapour
deposition plated according to the invention with the
magnesium layer while dispensing with fine cleaning,
undergoes heat treatment following vapour deposition,
during which time it is held within the temperature range
of 320 C to 335 C, in order to form the diffusion or
convection layer between the zinc coating and the magnesium
layer. The temperatures of the heat treatment are
preferably purposefully selected with regard to as good as
possible bonding performance of the finished flat steel
product, so that in each case they lie in the upper
spectrum of the optimum temperature range for the
respective application.
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As regards suitability of the process according to the
invention for economic industrial use, it is of prime
importance that the post heat treatment according to the
invention can be carried out in air. This also contributes
to reducing the capital expense and the costs generally
linked with carrying out the process according to the
invention to a minimum.
The post heat treatment is preferably carried out so that
the coated strip in each case is held for a duration of up
to 15 seconds, in particular 5 - 10 seconds, in the range
of the optimum heat treatment temperature specified by the
invention, so that its surface when leaving the heat-
treatment furnace is at the correct heat treatment
temperature.
Normal measuring instruments, such as temperature sensors
placed abradantly on the strip surface can be used for
measuring the respective treatment temperature; said
measuring instruments are positioned for example in the
discharge region of the furnace at a place, where on the
one hand their signals and function are no longer disturbed
by the operation of the furnace and on the other hand it is
ensured that no substantial cooling of the strip takes
place on leaving the furnace. Suitable positioning of the
measuring instrument is particularly important if an
induction furnace with correspondingly straying
electromagnetic fields is used for post heat treatment.
The zinc is applied by electro-galvanizing, thus optimised
characteristic combinations arise in the case of the flat
products manufactured according to the invention, if the
heat treatment temperature selected during the post heat
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treatment is 320 C to 335 C. When this temperature is
maintained, it is possible to ensure in an especially
reliable way that no Fe-Zn rich phases are formed in the
plating layer, as a result of which the bonding
characteristics of sheet steel coated according to the
invention might be impaired.
Any PVD process, which is already proven in practice for
this purpose, can be used for physical vapour deposition of
the magnesium or the magnesium alloy on the galvanized
steel substrate.
Practical trials have shown that the working results
achieved with the process according to the invention can be
further improved if the sheet steel provided with the zinc-
containing coating, in the course of its final cleaning, is
chemically pre-conditioned by rinsing with a suitable pre-
conditioning agent. For this purpose the galvanized steel
strip can be rinsed with an alkaline solution in the course
of chemical final cleaning.
Likewise with regard to an optimised plating result, it may
be advantageous if the chemical final cleaning for example
comprises pickling the steel substrate by rinsing with an
acid, in particular hydrochloric acid. Following pickling,
rinsing with de-mineralized water can ensue in order to
remove residues, still remaining on the zinc coated sheet
after pickling, as completely as possible.
Further optimisation of the coating result can be achieved
if the steel substrate provided with the zinc-containing
coating has a roughness Ra on its free surface of at least
1.4 pm, in particular 1.4 - 1.6 pm, when entering the
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physical vapour deposition, with roughness levels of more
than 1.4 um being advantageous. Likewise it is
advantageous for optimum adhesion of the magnesium coat to
the zinc coating, if the zinc-coated flat steel product has
a nib rate RPC of at least 60 per cm when entering the
physical vapour deposition. The nib rate RPC and average
roughness Ra are calculated by the contact stylus
procedure, wherein when determining average roughness Ra
the methods used are those indicated in DIN EN ISO
4287:1998 and when determining the nib rate RPC the methods
are those indicated in the Iron and Steel Test Sheet
September 1940.
Furthermore it has proven advantageous for the result of
the physical vapour deposition if the flat steel product,
provided with the zinc-containing coating, before entering
the physical vapour deposition, is heated to above ambient
temperature, however to a temperature below the alloy
temperature or held at this. Practical trials have shown
that the temperatures particularly suitable for this
purpose lie in the range of 230 C - 250 C, in particular
approx. 240 C.
The invention therefore makes available a process, which
can be carried out particularly economically in a
continuously running operation and provides a product that
due to its surface quality and bonding performance is
particularly suitable for producing components of motor
vehicle bodies with application of joining techniques, such
as inter-bonding.
The invention is described in detail below on the basis of
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two exemplary embodiments.
Exemplary embodiment 1
A module for PVD plating and post heat treatment has been
integrated into an existing conventional plant for
continuous steel strip electro-galvanizing behind the
conventional lines used for galvanizing and in front of the
plant for final treatment of the finish-coated steel strip.
The steel strip firstly electro-galvanized in the known way
in the conventional galvanizing lines of the plant,
converted in this manner, after the galvanizing process and
final cleaning likewise carried out in the conventional
plant, is fed into the module for PVD plating and post heat
treatment, where it is PVD plated and post heat treated.
Afterwards the steel strip is returned to the conventional
plant, in which for example it is phosphatized and oiled
within the context of final treatment.
Steel qualities, which are typical of motor vehicle
manufacture, are considered as material for the steel
strip, processed in this plant and having normal
dimensions. It has proven particularly advantageous if the
average roughness of the cold rolled steel used for the
electro-galvanized sheet lies at the upper limit of the
motor vehicle-standard Ra specification for external parts
of 1.1 - 1.6 pm. A further increase in the Ra value above
2 pm would be advantageous as regards the adhesive power of
the coating and the bonding performance associated
therewith, but under economic criteria at present it does
not appear expedient since today such a product would not
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comply with the specifications of the motor vehicle
customers.
A nib rate value RPC of > 60 per cm is preferred. Both
values can also be positively influenced during the
electro-galvanizing process. A further possibility of
controlling these values consists of a cementation process
as the ultimate stage of final cleaning.
At strip speeds of 20 - 180 metres per minute the steel
strip is firstly provided conventionally by way of
electrolysis on either side with a zinc deposit of 3 pm in
vertically arranged electrolysis cells by means of soluble
anodes. After rinsing and drying the now galvanized steel
strip, the galvanized substrate is thoroughly finally
cleaned and prepared for application of the magnesium-
containing coating.
In order to optimise the result of subsequent physical
vapour deposition however, it may be advantageous, as part
of the final cleaning, to include pickling of the
galvanized steel strip, wherein the steel strip is kept in
each case for 5 seconds in a 0.5 % hydrochloric acid bath
heated to 20 C. In order to neutralize the acid, the steel
strip was then rinsed with de-mineralized water.
The steel strip cleaned this way, after passing through
several compression phases, enters a vacuum chamber, in
which without any further treatment stage magnesium
physical vapour deposition is carried out by means of a PVD
process using a commercial JET evaporator. In order to
ensure a constant magnesium thickness of 300 nm at varying
strip speeds, the JET evaporator by suitable heat or
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mechanical means is able to supply evaporation rates of
between 6 pm x metre per minute and 54 pm x metre per
minute. Via a further number of compression phases the
steel strip, now also plated with a magnesium layer, is
then again conveyed to normal atmosphere.
Treatment by means of NIR emitters is used in this case for
post heat treatment. The heating-up time here depends on
the strip speed, but can be varied by switching off
individual modules. The peak temperature of the heat
treatment according to the invention is 327 C 7K. In
order to reliably maintain this narrow temperature window
under the conditions of industrial application, a special
image-rendering pyrometric process is used, which makes it
possible to accurately control the temperature heat
treatment according to the invention locally and with
respect to time. Different steel substrates and coating
conditions in this case may cause deviating emissivities,
so that extensive calibration is necessary.
After a free strip run of 10 metres, the steel strip is
cooled down by means of water. The residual heat in the
strip is controlled so that the strip dries independently.
Fig. 1 as an inverted illustration shows an FE-SEM
photograph of a cross slice specimen of steel strip coated
according to the invention and heat-treated at a
temperature of 332 C. The advantageous layered structure,
with the steel substrate S, the zinc layer Z applied
thereon by electro-galvanizing and the magnesium-containing
Zn-Mg coating M lying on the zinc layer Z, is clearly
recognizable there. The layer, to be seen above the coating
M, is bedding-in compound E, which was required for
preparing the cross slice.
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Exemplary embodiment 2.
Under the same process conditions at a strip speed of 36
metres per minute as well as with an evaporation rate,
increased to 96 um x metre per minute by suitable
constructional means, of the evaporator at a strip speed of
64 metres per minute, magnesium deposits of 1500 nm were
achieved and thermally alloyed according to the invention.
The advantageous forming of the zinc-magnesium alloy
coating was also demonstrated in these tests.
