Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention concerns a process for treatment of rolled steel, in
particular concrete reinforcement bar steel, prestressing steel or the like
for increasing the resistance to corrosion by application of a coating of a
non-ferrous metal, preferably æinc, where the rolled steel is initially
descaled from the rolling heat and then the coating applied to the hot
rolled steel, reformation of scale being prevented.
Rolled steel products are used inter alia in the construction industry as
structural steel, here primarily in the form of concrete reinforcement bar
steel, as prestressing steel for prestressed concrete, as well as for guys
for ground and rock anchors, etc.
When used as reinforcement for concrete structural components, as
non-prestressed or prestressed reinforcement or as tension bar for a ground
or rock anchor, a bond generally exists between the reinforcing steel and
the concrete. This occurs in the case of reinforced concrete and in the
case of instant bond reinforced concrete as a direct result of the
reinforcing elements being embedded in the concrete and being closely
surrounded by it, in the case of prestressed concrete with subsequent bond
by virtue of the fact that the prestressing ducts necessary to maintain
longitudinal mobility of the prestressing elements are subsequently injected
with cement mortar.
In the manufacture of cement, an alkaline solution with a pH value of over
12 occurs as a result of the dissolution of the lime particles contained in
the cement. This results in a protective layer of iron o~ide forming on the
surface of the reinforcing elements which passivates the stesl surface and
protects it from surface corrosion. For static as well as structural
reasons, the reinforcing elements are usually located in the surface zones
of the components, where they must maintain a certain minimum distance from
the outer face, the "concrete cover". As a result of the air pollution and
in the case of bridge structures, particularly as a result of the increasing
use of de-icing salt to remove snow and ice, it is possible for chlorides
dissolved in these media to penetrate to the steel in the case of inadequate
and even frequently in the case of adequate concrete cover, where they
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destroy th~ passivating layer and can thus attack the steel directly. A
similar situation applies to the tension bars of ground and rock anchors,
where the corrosive effect on the steel is caused by ground moisture, water
and acidic soil constituents. For this reason, a need exists for additional
corrosion protection measures for such rolled steel products used in
construction engineering.
Quite ganerally, a process is known for coating metals, particularly ferrous
metals, with a thin layer of zinc as rust protection. In this connection,
various processes are known. For the purpose of hot dip zinc coating, the
parts to be protected are dipped in a bath of molten zinc, which can also,
if necessary, be alloyed with aluminium. For electrolytic galvanizing,
sulphate, sulphate chloride or potassium cyanide-caustic potash bathS are
used. Processes are also known for applying metal coatings by spraying on
the metal in liquid form, the metal being supplied to a spray gun in wire or
powder form, rendered molten by oxygen-combustible gas mixtures and atomized
by compressed air. All these processes are costly, particularly for
treatment of mass-produced items of considerable length manufactured in one
continuous operation, such as concrete reinforcement bar steels or
prestressing steels and can be used not at all or only with involvement of
considerable labour and expense. In addition, there is the fact that the
products to be coated must of course be cleaned before application of such a
coating. Rolled steel products, in particular, must have the layer of
scale, adhering to them from the rolling process, removed; this is usually
done by sand blasting, pickling or similar treatment.
In the case of rolled steel products produced by a continuous process, it
has also become known practice to descale the rolling stock directly
following the rolling process, preventing re-formation of scale and
utilizing the rolling heat to provide it with a corrosion resistant &oating
of, for example, zinc (US-PS 2 442 485). In this instance, descaling takes
place in the final roll stand as the result of a pronounced reduction in the
cross sectional area of the rolling stock combined with elongation, as a
result of which the brittle scale breaks up, so that the rolling stock
leaves the roll stand essentially free from scale. In order to maintain
this state, the rolling stock, after leaving the roll stand at a temperature
of approx. 980 to 760 degrees C, is passed through An enclosed chamber in
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which the carbon monoxide is combusted to form carbon dioxide. Directly
afterwards, the rolling stock passes through another chamber in which it is
coated, for example, with ~inc in the form of zinc powde} which fuses on as
a result of the heat still retained by the rolling stock. As however, when
leaving this chamber, the coated rolling stock is still at a temperature of
approx. 700 to 600 C, it must be cooled rapidly immediately after coating to
prevent the zinc powder applied combusting to form zinc oxide for example.
This process is again relatively costly. Keeplng the descaled surface of
the rolling stock clean by means of an atmosphere of deoxidizing gas
requires not only storing, supplying ~he latter and maintaining a
deoxidizing atmosphere in a suitably sealed chamber, but also maintaining a
suitable temperature before the rolling stock is cooled down rapidly after
coating by the use of other media.
In the light of this, the invention is based on the technical problem of
being able to coat rolled steel products in a continuous process directly
following the rolling process, utilizing the rolling heat, and at the same
time keeping process and equipment expense to a minimum.
This problem is addressed according to the invention by the ~olled steel being
impinged upon by water under high pressure for the purpose of descaling as a
direct continuation of the rolling process, by the steel surface being
cooled down rapidly to a temperature of less than 600 degrees C by the use
of water directly after descaling to provent re-formation of scale,and by
the coating being applied directly afterwards.
The advantage of the invention lies primarily in the fact that, for the
purpose of descaling as well as for preventing re-formation of scale, and at
tha same time for the purpose of cooling, only a single medium, i.e. water,
is used, whereby cooling can be carried out such that the rolling heat can
bé put to optimum use for the purpose of coating. This ensures that all
process stages or treatment processes can take place as a direct
continuation of the rolling process, that is to say at rolling speed, so
tha~ the finish coated end product leaves the productlon line in the same
way as a normal, untreated rolled product.
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By subjecting the steel surface to water at high pressure for the purpose of
descaling, use is made of the phenomenon that the scale adhering to the
rolled steel from the rolling process, which has been loosened during the
course of deformation of the rolling stock taking place in th~ final roll
stand, e.g. during transition from an oval to a circular cross section,
immediately after the rolling stock has left the roll stand, can be
completely removed with relative ease. The pressure of the water should be
in excess of 200 bar, preferably 300 to 400 bar; it can, if necessary, be
increased to about 1000 bar. This involves only very brief treatment, i.e.
passage of the rolled steel through a spray nozzle which results in only
insignificant reduction in the temperature of the rolled steel.
Whilst descalin~ with water under pressure from rolling heat takes place at
a temperature of about 900 to 1000 degress C, according to the invention,
cooling is carried out as a continuation of this water treatment as a
continuous process by using the same medium, such that the temperature drops
as rapidly as possible below the scaling temperature of 600 degrees C, thus
that no fresh scale can form and temperatures are reached at which the
coatings can be applied. According to the invention, this can be effected
by the rolled steel being passed through a water bath or being sprayed with
water. Suitable temperatures for application of corrosion-resistant coating
are for example with pure aluminium a temperature of approx. 560 to 570
degrees C and with pure zinc a temperature of approx. 415 degrees C;
intermedia~e stages can be used for alloys. Thus, according to the
invention, it is directly possible to use any desired alLoy for the
coatings, as all temperature ranges are passed through, from rolllng heat
down to complete cooling.
Application of the coating can be effected in any desired manner~ The
non-ferrous metal forming the coating can be sprayed on in powder form where
lt uses on; the rolled steel can also however be passed through a bath
containing the molten non-ferrous metal.
Whilst, after descaling, the rolled steel is protected by the water
treatment from fresh scale formation, this protection, until complete
cooling of the rolled stce] has taken place, is in fact provided after
application of the coating by this coating itself. In some cases~ it is
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advisable to apply to the coating of zinc, for example, a further layer of
synthetic resin, e.g. epoxy resin, as additional protection. A process of
this nature can also be readily integrated in the process according to the
invention by utilizing the residual heat.
As the rolled steel according to the Lnvention is cooled after descaling by
treatment with water, this treatment can also take place in the form of
known hardening and tempering processes using water. One of these is, for
e~ample, the process known as the "Tempcore" process for low-carbon and thus
weldable concrete reinforcement bar steels, where the water treatment takes
place such that directly after quenching a surface zone of martensite and
bainite is present in the rolled steel, whilst the heat remaining in the
steel does not cause tempering of the surface zone past the bainite stage
during subsequent cooling. In the case of high-carbon steels suitable for
use ~s prestressing steels, the water treatment is carried out, such that
ehe steels are quenched from a final rolling temperature of between ô60 and
1060 degrees C in such a way that completely martensitic structure if~
formed in the skin layer and the temperature of the skin layer due to
thermal equilibrium is 400 to 500 degrees C in the period between 2 and 6
seconds after commencement of the quenching process. In this way, the
cnpling to be carried out according to the invention can still be utilized
in a particularly advantageous manner to improve the strength of the steels.
A further advantage of the invention lies in the fact that all operations
can be carried out in a continuous process directly following the rolling
process in individual treatment stations located downstream of the final
roll stand. Whilst the rolling process which is of necessity discontinuous
on account of the rolling of billets has been largely standardized up to
leaving the final roll stand, the process according to the invention
displays of course particular advantages when the rolling process is made
into continuous rolling by welding the billetsA This is rendered
particularly successful by the known method of flying flash butt welding of
the billets in the area between leaving the furnace or leaving a three-high
roll stand and the initial pass of the finishing ~ill train.
