Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CAMM2746W0 (English translation of PCT application as filed).doc
= WO 2006/096942
METHOD FOR THE CONTINUOUS CASTING OF A METAL WITH IMPROVED
MECHANICAL STRENGTH AND PRODUCT OBTAINED BY THE METHOD
Field of the invention
[0001] The present invention relates to a new method
for the continuous casting of a molten metal, in particular
steel, that allows to obtain an intermediate product such as
a slab, billet, wire, etc. before subsequent thermomechanical
treatment such as lamination, continuous annealing, etc.,
such that its chemical composition is modified by the
addition of elements in order to give it greater mechanical
strength.
15 [0002] The following description makes more specific
reference to the continuous casting of steel. However, this
choice is only an example and does not entail any limitation
of the invention.
[0003] The invention also relates to the product with
improved mechanical features obtained by the method.
State of the art
[0004] The technique of the continuous casting of
steel is well known. It essentially consists in feeding
molten steel from a ladle or from a tundish into a cooled
copper or copper-alloy mould called "continuous casting ingot
mould", the latter being open at its bottom end, and in
extracting from this opening an ingot in the form of a partly
solidified continuous sheet.
30 [0005] In general, the molten steel is fed into the
ingot mould by means of at least one nozzle, i.e. a generally
tubular element positioned between the tundish and the ingot
mould. The bottom end of the nozzle is usually provided with
one or two outlet apertures located on the axis of the nozzle
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or on the sides, and cbmes out belbw the level that is free
= . of molten steel present in the ingot' mould.
[0006] Figure 1 shows a casting device as in the state of the art
(patent BE 1014063) fitted between a continuous casting mould 1
.5 and a casting ladle or tundish 2 with an outlet pipe 3.
The
outlet pipe 3 is equipped with a flow regulator such as a stopper
4 or a sliding tray. A nozzle 5, essentially in the form of a
cylinder and possibly with an oval cross-section, attached to the
fl tundish, is positioned .above the mould 1 and plunges into it.
The top base of the cylinder is in contact with the pipe 3. This
base is equipped with an aperture matching the inner aperture of
the pipe. The nozzle 5 comprises in its lower part at least one
= communication eye 8 to allow the steel to pass through to the
mould.
In the upper part of the nozzle 5, there is a
distribution device in the form of a dome 6, whose top surface is
-
slightly sloped, preferably at an angle greater than 100 relative
to the horizontal. An injection device is positioned in such a
way as to introduce finely divided solid particles or particles
in powder form 15 under the dome 6, using a non-oxidizing gas as
a vector. This device also comprises an incoming flow of argon 7
and its device for measuring flow rate 9 and pressure 10, a
container for powder or finely divided particles 11 and its
weight-measurement device 12, and lastly a supplier pipe in the
nozzle 13, as well as an injector 14.
[0007]
Developments of the nozzles are also known that
are intended to achieve improved cooling of the too-hot
molten steel coming from the tundish. The aim is to obtain
steel 'the form of a 'paste upon its entry into the ingot
mould. These nozzles may, it particular comprise a heat
exchanger with a water-cooled copper tube or even a. deflector ,
or a dome. The latter has the purpOse of forcing -the
fl .30 overheated steel to trickle down.in a thin layer along the
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-walls of the nbzzle',- which' allows tici significantly increase
the area of thermal exchange. The cooling of the conduit
- ensures the removal of the excess heat from the Steel- and .
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causes the appearance of a solid fraction which turns the
steel into a paste upon its entry into the ingot mould. The
introduction of a protective gas under pressure, for example
argon, in the conduit causes an overload that prevents any
air flow by the molten steel, which would lead to its
oxidisation or to the formation of alumina and the clogging
of the nozzle. This technique described in patent EP-B-269-
180 is called casting with a hollow jet or by means' of a 1-1L7N
or hollow jet nozzle.
Another development, described in patent EP-B-
6-05 379, relates to the injection into the hollow jet of some
quantity of finely divided metal material by using a non-
oxidising gas as a vector at a slightly higher pressirre
relative to atmospheric -pressure in order to prevent any
entry. of air. Depending on the case, the aim is to obtain
refinement of the solidification structure by creating new
solidification seeds or a modification of the basic chemical
composition of the steel.
[0008] A continuous
casting nozzle with a rotating jet
is also known, as described in patent BE-A-101 20 37, and
composed of a vertical conduit with a distribution device or
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dome in its upper part, whose function is also to divert the
metal entering the nozzle towards the internal surface of
said conduit and which comprises three arms arranged in a
star pattern relative to the nozzle axis and canted relative
to the horizontal. These arms are configured so as to impart
a helicoidal rotary motion along the inner wall to the molten
steel. The molten steel then comes out through two side
outlets in the nozzle at a speed that is significantly lower
than that obtained with a conventional nozzle with the same
flow, which improves the quality of the ingots extracted
(less inclusions and less gas bubbles).
[0009]
The continuous casting of steel-based products
with a mixed chemical or bi-component composition has also
aroused great interest in a large number of specific
applications, both for long and flat products (for example
reduction of the silicon level at the surface of the slabs,
in order to improve the suitability of laminated products to
galvanisation; modification of the carbon content at the
surface of peritectic steels to improve their casting flow;
casting of products whose mechanical properties vary along
their thicknesses, such as for instance great strength at the
surfaces and high ductility in the cores, etc.). The term bi-
component refers to products with a chemical composition of
steel that varies depending on its position in the product
studied, for example varying in the skin compared with the
core. To meet this requirement, the Applicant proposed in
international patent application WO-A-02/30598 a continuous
casting nozzle comprising a distribution device with a dome
in its top part, designed to separate the molten steel into
two streams, an inner stream and an outer stream, in two
physically well-separated zones. A means for injecting a gas,
liquid or finely divided solid material (a powder with a
particle size typically greater than 100 microns) under the
dome into the inner zone allows the formation of a steel with
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a chemical composition that is different to that of the basic
steel, cast in the outer zone.
[0010] In addition, it is known that traditional
thermomechanical treatments aimed at improving the mechanical
features of a steel, for example by its microstructure
(martensite, bainite, etc.) or by endogenous precipitation,
have the drawback that the structure of the steel finally
obtained may be adversely affected by thermal post-treatment
of the product (for example welding, galvanisation, etc.). It
would therefore be desirable, at least in some cases, to be
able to cast directly a product with a structure, and hence
mechanical properties, that are stable throughout any
subsequent treatment that the product might undergo.
Aims of the invention
[0011] The present invention aims to provide a
solution that allows to overcome the drawbacks of the state
of the art.
[0012] The present invention aims in particular to
provide a method of continuous casting that allows to produce
slabs or billets of a modified chemical composition adapted
to give the steel greater mechanical strength before
lamination.
[0013] The invention notably aims to obtain a steel of
homogeneous chemical composition and/or stabilised structure
relative to a lamination process and/or thermomechanical
treatment subsequent to casting.
[0014] One particular aim of the present invention is
to exploit the hollow-jet technique in order to inject finely
divided ceramic particles through the continuous casting
nozzle.
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Main characteristic elements of the invention
[0015] A first aim of the present invention relates to
a method for the continuous casting of a metal, in the form
of a hollow jet in a nozzle positioned between a ladle or a
5 tundish and a continuous casting ingot mould, said nozzle
comprising in its upper part a distribution device capable of
diverting at least part of the molten metal arriving at the
inlet of the nozzle towards an inner wall in the nozzle
before it enters the ingot mould, said method comprising the
injection in an internal volume of the hollow jet of finely
divided solid material, characterised in that the finely
divided solid material comprises nanoparticles of technical
ceramic, of a characteristic size lower than 200nm, and
preferably lower than 100nm.
15 [0016] Advantageously, the nanoparticles of technical
ceramic comprise nanoparticles of oxides, nitrides, carbides,
borides, silicides and/or compounds thereof.
[0017] The oxides are preferably A1203, Ti02, Si02,
MgO, Zr02 or Y203.
20 [0018] As a further advantage, the size of the
nanoparticles is between 10 and 100nm.
[0019] Still according to the invention, the quantity
of nanoparticles incorporated into the molten metal is lower
than or equal to 5%, and preferably between 0.1 and 1% by
25 weight of cast metal.
[0020] According to a preferred embodiment of the
invention, the ceramic nanoparticles injected into the
internal volume of the hollow jet of the nozzle are in
suspension in a non-oxidising gas, preferably argon, said gas
30 being at slightly higher pressure relative to atmospheric
pressure and at most equal to the static pressure of the cast
metal upon its entry into the ingot mould.
[0021] According to another preferred embodiment of
the invention, the ceramic nanoparticles are injected into
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the internal volume of the hollow jet of the nozzle by means
of a mechanical conveyance device such as a worm screw.
[0022] As a particular advantage, the nanoparticles
are conglomerated prior to their injection into the nozzle
into microparticles of a size essentiallyy between 10 and
1,000 microns, and preferably between 100 and 200 microns.
[0023] Still advantageously, prior to their injection
into the nozzle, the nanoparticles are conglomerated into a
metal matrix made of the same metal or of a different metal
to the cast metal.
[0024] The cast metal is preferably molten steel and
the metal matrix is an iron matrix or the metal matrix
comprises a alloy metal other than iron.
[0025] As a further advantage, the conglomeration of
the nanoparticles is obtained by mixing ceramic nanoparticles
with micrometric iron particles, i.e. particles of a size
greater than 10 microns, and preferably less than 20 microns.
[0026] According to a first preferred method, said
mixture is produced by a pre-mix in a slurry, followed by
drying, crushing, isostatic pressing and further crushing.
[0027] According to a second preferred method, said
mixture is produced by high-energy tapping of the type
"mechanical alloying" so as to incorporate the ceramics into
the iron matrix.
[0028] According to a first advantageous embodiment,
the hollow-jet nozzle used is of the type rotating jet, i.e.
it comprises a vertical conduit having a distribution device
with a dome in its upper part, whose function is to divert
the molten metal entering the nozzle towards the internal
surface of said conduit and which comprises a series of arms
arranged symmetrically in a star pattern relative to the axis
of the nozzle and canted relative to the horizontal, said
arms being arranged to impart a helicoidal rotary motion to
the molten steel along the inner wall of the nozzle.
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[0029] According to another advantageous embodiment,
the hollow-jet nozzle used comprises a distribution device
with a dome in its upper part designed to separate the molten
metal into two streams, an inner stream and an outer stream,
in two physically well-separated zones, the injection of
ceramic nanoparticles under the dome in the inner zone
allowing the formation of a metal with a different chemical
composition to that of the basic metal, cast in the outer
zone.
10 [0030] Alternatively, the injection of ceramic
nanoparticles may be carried out in the outer zone of the
nozzle.
[0031] A second aim of the present invention relates
to a metal, preferably steel, with high mechanical strength
and taking the form after casting of an ingot in a continuous
sheet upon its exit from a continuous casting ingot mould,
specifically obtained by means of the above-described method
and comprising less than one percent by weight of technical
ceramic homogeneously distributed in at least one part of the
ingot.
Description of a preferred embodiment of the invention
[0032] The idea on which the invention is based is to
develop a steel hardened by a fine dispersion of ceramic
particles that give the steel stable properties that do not
deteriorate because of subsequent thermal treatment(s).
[0033] By way of an example, the case of the
continuous casting of steel will be considered.
[0034] It is therefore proposed to cast a standard
basic steel to which is added, as required, a quantity of
particles needed to obtain the strength properties desired.
As an advantage, the addition of particles to the molten
metal is carried out directly at the level of the continuous
casting nozzle since the latter, in the embodiments generally
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used and described above, generally comprises a means for
inserting alloy elements or oxides in at least one fraction
of the molten metal passing through the nozzle.
[0035] According to the invention, the particles added
are ceramic particles. The man skilled in the art knows that
technical or industrial ceramics refer to a class of
manufactured materials that are non-metallic and inorganic.
They are divided into two main groups: the oxides (for
example A1203, Ti02, Si02, MgO, Zr02, Y203, etc.) and the non-
oxides (nitrides, carbides, borides, silicides, etc.).
Moreover, for the requirements of the invention, the ceramic
particles must comply with the following operational
definition: they are of a nanometric size, typically 10-100
nanometres (1nm = 10-9m), and after incorporation into the
molten steel, they are essentially homogeneously distributed
throughout the entire section of the cast product. The "size"
of the particles is meant here as the largest dimension of
the particle. The nanometric nature of the particles for
inclusion is in fact indispensable to the reinforcement of
the product. By contrast, micrometric inclusions constitute
defects, heterogeneous areas that make the product weaker.
[0036] The quantities of nanoparticles added to the
molten steel are maximum 1% by weight.
[0037] The wettability of the particles in the molten
steel is the most important criterion for the choice of
particles and the resolution of this technical problem is at
the heart of the present invention. Homogeneous distribution
of the nanoparticles in the molten steel is indispensable,
which excludes confinement of the powders injected to the
surface of the molten steel.
[0038] According to the invention, the particles may
advantageously be conglomerated up to a size of 100-20011m so
as to be able to be injected through the HJN nozzle.
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[0039] To improve the wettability of the particles in
the molten steel, the nanometric ceramic particles may be
conglomerated in an iron or metal matrix to obtain a compound
whose characteristic final size is 100-200pm. The iron or
metal matrix favours the dispersion of the particles in the
molten steel. In order to obtain this compound, nanometric
ceramic particles are used mixed with micrometric iron
particles (whose size is for example 10 to 20 microns). The
mixture is produced either by:
- mixing into a slurry and then drying, crushing, isostatic
pressing and then re-crushing;
- high-energy tapping (mechanical alloying) to ensure that
the ceramics are incorporated into the iron matrix.
Tapping is an operation that consists in bringing an element
into contact and introducing it into a combination formed of
one or several elements that are different from the first
element by exerting a force on the element.
[0040] Advantageously, these compounds are injected
under gaseous atmosphere in the HJN nozzle (see patent EP-B-
605 379). The heavy turbulence occurring in the nozzle thus
allows good incorporation of the particles into the molten
steel.
