Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02269130 2005-09-13
Title of the Invention
Improved unit of equipments for the high-speed continuous casting of good
quality thin
steel slabs.
Background of the Invention
The present invention relates to an improved apparatus for high-speed
continuous casting
of high quality thin steel slabs.
Conventionally, the continuous casting of the so-called "thin slabs" of steel,
up to 80 mm
thick, has been subject to quality problems, especially for casting at high
speed, e.g.
above 4.5 m/mm.
Such problems result in flaws in the slab surface, i.e., the shell, which is
formed in the
mould, as follows:
~ longitudinal cracks due to the trapping of casting powders;
~ longitudinal and transversal cracks due to a lack of lubricating and
insulating f lm
formed by "slag" (i.e., the product of casting powders being melted and
resolidified);
~ longitudinal cracks due to thermal stresses; and
~ longitudinal cracks due to the copper cooling surfaces of the mould being
discontinuous.
These problems typically affect special steels, but could be at least
partially solved by
reducing the casting speed. However, reducing casting speed would lower
productivity
and accordingly reduce plant efficiency. Another possible solution to the
above problems
could be to use an electromagnetic device, called "EMBR" (ElectroMagnetic
Brake
Ruler), capable of flattening the liquid steel waves rippling along the
meniscus inside of
the mould. However, an EMBR is very expensive and would only partially solve
the
aforementioned problems. Additionally, other problems arise from the
geometrical and
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flow conditions occurring inside the mould, resulting in a reduction of the
operating life
of the casting nozzle (which is dipped in the liquid metal and is usually
called a
"submerged nozzle") and in a reduction of process efficiency.
From the above discussion it should be clear that the quality control problems
can not be
solved in a systematic and satisfying way by independently concentrating on
any one of
the mould, the submerged nozzle and the mould oscillating unit. The above
three
elements are so interconnected in the continuous casting process that to solve
the above-
mentioned quality problems, the three elements must be treated together. Thus,
to find an
effective solution it is important to concentrate on the mould, the submerged
nozzle, and
the mould oscillating unit as a single group.
Summary of the Invention
It is an object of the present invention to provide a casting unit that
overcomes the above-
identified problems when continuously casting thin slabs at high speed.
The improved casting unit of the present invention generally has the
characteristics
recited in claim 1.
Brief Description of the Several Views of the Drawings
Additional advantages and characteristics of a casting unit according to the
present
invention will be evident from the following detailed description of the
preferred
embodiment when examined in combination with the attached drawings, wherein:
FIG. 1 shows a diagrammatic side view of a casting unit according to the
invention
illustrating the various components of the casting unit;
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FIG. 2 shows a view of only the sole upper part of the mould, combined with
the
submerged nozzle, in the direction of arrow II of FIG. 1;
FIGS. 3a-3c show the same diagrammatic view, in a cross-section, taken along
line III--
III of FIG. 2, at the meniscus, or top surface of the liquid steel level, in
order to
particularly show the geometrical relation that the mould and the submerged
nozzle must
satisfy to form a casting unit according to the present invention;
FIG. 4 shows a perspective view of the mould, diagrammatically represented
with respect
to a set of Cartesian axes;
FIGS. Sa and Sb show two diagrammatic views of the mould of FIG. 4, wherein
the ideal
envelope of the cooling system pipes is represented in longitudinal section
through a
plane parallel to the y and z axes of FIG. 4, and along line B--B of FIG. Sa,
respectively.
Detailed Description of the Invention
With reference to the drawings, FIG. 1 is a diagrammatic view of the casting
unit
according to the present invention which preferably includes a mould 1, a dip
casting
nozzle 2 (hereinafter always referred to as "submerged nozzle") and an
oscillator 3
(which is hydraulically driven and is fastened to the mould body so as not to
interfere
with the casting line). FIG. 1 shows the area occupied by the liquid steel
between the
submerged nozzle 2 and the surrounding copper walls (i.e. the two "channels"
4).
Many of the problems that occur when casting thin slabs (as opposed to
traditional
thicker slabs) result from the fact that (assuming that the volumetric flow
rate of molten
steel is constant) a reduction in slab thickness increases the amount of slab
surface
contacting the mould walls within a given amount of time and thus, an
increased amount
of lubricating "slag" is necessary.
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Thus, the importance of forming the proper amount of lubricating slag
increases when the
thickness of the slabs is reduced because the slab contact surface is
inversely proportional
with the thickness of the slab. Thus, the thinner the slab is the greater the
amount of
contact between the liquid steel in the mould and the mould walls per unit
time. The
increased amount of contact between the liquid steel in the mould and the
mould walls
results in an increased need for slag. However, the interface in the mould
between the
molten steel and the casting powders (i.e., the area over which slag is
introduced) is
reduced along the middle portion of the top surface of the liquid steel where
the slag is
formed, due to the reduced thickness of the mould and area occupied by the
submersed
nozzle.
Although this problem may be partially solved by using specific casting
powders which
are capable of enhancing slag formation, conventionally configured submerged
nozzles
and mould walls are not able to maintain the required equilibrium between the
molten
slag formed by melting casting powders and the slag consumed by the slab
forming
process.
The thin mould of the present invention is capable of containing a reliable,
i.e.
sufficiently thick, submerged nozzle, and the mould preferably has its large
walls formed
with copper plates. The walls have a profile that (when viewed in the
horizontal plane,
around the meniscus level) exactly matches the profile of the submerged nozzle
and thus,
keeps a constant normal distance between the submerged nozzle and the walls.
The
relative geometry of the mould and the casting nozzle are a part of the
present invention.
To quantify the various geometries the following terms are used: A1, A2, S1
and S2.
Referring to FIG. 3b, the area A1 is the area between the casting nozzle and
the mould
walls directly above and below (as viewed in FIG. 36) the casting nozzle. The
length S 1
is the distance along the perimeter of the mould adjacent to area A 1.
Referring to FIG. 3c, the area A2 is generally equal to the total area of the
mould (as
viewed in FIG. 3c) subtracting area A1 and subtracting the area of the casting
nozzle (as
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viewed in FIG. 3c). The length S2 is equal to the length of the perimeter of
the mould
subtracting S 1.
With reference to FIGS. 3a, 3b and 3c, a normal distance is chosen so that the
ratio A1/S1
(see FIG. 2) is similar to the ratio A2/S2. The present invention requires
that A 1 /S 1 is
approximately the same as A2/S2, measured outside the submerged nozzle region
(see
FIG. 3c). Thus the equation to be satisfied is:
0.9 < (A 1 /S 1 )/(A2/S2) < 1.1, and preferably=1
For example, for a mould being 1300x65 mm and having a submerged nozzle 300 mm
wide (with a reliable thickness of 60 mm as indicated in FIGS. 3b and 3c), the
optimal
ratio A1/Sl=A2/S2 is equal to 30. Such a ratio (once the dimensions of the
submerged
nozzle and the thickness of the smaller sides have been fixed) may be used for
determining the desired mould profile in the horizontal plane at the meniscus
level of the
liquid steel in the mould. Alternatively, if the dimensions of the mould
profile are known,
the ratio may be used for determining the required profile of the submerged
nozzle.
This geometrical configuration is also important for the flow of molten steel
in the
meniscus region, since the "channels" 4 which are located between the
submerged nozzle
and the copper mould walls will be sufficiently large to prevent vortex
formation due to
the acceleration of the streams converging in the middle from the mould's
smaller sides.
Vortex formation often causes the casting powders to be trapped, resulting in
the
improper generation of slag which results in the above-mentioned defects.
Preferably, the mould used in the casting unit of the present invention has a
bend in the
longitudinal direction, as detailed in European patent 0705152 (which
disclosed a mould
having a nearly infinite bending radius in the upper region for a better
arrangement of the
submerged nozzle), while providing for the bending of the slab being formed
inside the
mould with an exit on the arc-shaped casting guide other than the vertical.
This
advantageously reduces the height of the casting unit and accordingly the
ferrostatic
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forces and the risk of slab swellings. According to the aforementioned patent
application,
the bending is graded in a progressive and uniform way from the infinite
radius of the
mould inlet to the bending radius R~ corresponding to the casting guide (FIG.
1 ), thereby
preventing both exceeding stresses on the solidified external shell of the
slab and the
possibility of an imperfect contact with the copper walls of the mould.
In order to adequately solve the above problems, the unit for cooling the
mould plates is
especially important and has to be capable of withstanding the high heat
fluxes typically
occurring in the formation of thin slabs (up to 3 MW/m2, average value on the
entire
cooling surface of the mould). At the same time, cooling is preferably
enhanced in the
meniscus region in order to prevent copper cracks and to prevent thermal
stresses from
forming in the slab.
With reference to FIG. 4, when considering the specific normal heat flux (dq")
between
the surface of the casting product and the mould, the following equation can
be used:
dq" =dq/dA [W/m2
This heat flux is partially a function of the local surface temperature on the
hot surface of
the copper plates, which is dependent upon the distance from the pipes wherein
the
cooling water flows.
Referring to FIG. 4, (a system of Cartesian axes x, y, z is superimposed on
the mould,
wherein the z axis extends toward the mould bottom and the complex surface
formed by
the mould is defined as f(x,y,z)=0) the local surface temperature varies
according to the
equation t=t[f(x,y,z)].
The heat flux dq~, must be kept as constant as possible along a horizontal
line (wherein
z=z~,) belonging to the mould surface (i.e. the temperature t must be kept
virtually
constant along such a line) whereby:
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t=t~.f~x>Y~ zo~J = to
The above equation is obtained by keeping every point of the copper hot
surface at
generally the same normal distance Nd (which is measured along the
perpendicular with
respect to the hot surface) from the ideal surface envelope E of all the ends
of the cooling
pipes W (FIGS. Sa, Sb). Thus, Nd is constant, and experimentally it has been
found that
this constant value optimally ranges from 10 to 25 mm in order to have the
aforementioned conditions for the cooling system.
As for the submerged nozzle, besides the aforementioned dimensional conditions
with
respect to the mould, it is preferably designed to allow the optimal behavior
of the molten
steel flow, while taking into account gradual shell formation and the life of
the
submerged nozzle. In fact, it is known that, upon decreasing of the slab
thickness, the
problems concerning the motions of the liquid inside the mould increase,
resulting in the
formation of stationary waves in the meniscus region and thus a local
reduction of the
thickness of the liquid slag, which adversely affects the lubrication and the
insulation of
the shell of the slab being solidified.
The submerged nozzle for thin slabs, which is detailed in patent application
PCT/IT-
97/00135, has geometrical characteristics which result in castings having a
low energy, a
high probability of energy dissipation inside the liquid volume of the slab,
improved flow
(thereby preventing vortex formation and powder trapping), and an improved
liquid metal
level control in the mould. Furthermore the feed is steady, the flow is
substantially split
into two streams and the surfaces inside the submerged nozzle are preserved to
keep the
same shape as at the beginning of the continuous casting. Since oxide deposits
are
negligible, these good flow conditions result in a reduced amount of external
mechanical
erosion of the nozzle in the meniscus region.
According to the present invention, the optimized design of the apparatus
includes the
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ratio between the amplitude of the stationary wave (measured in mm) and the
casting
speed in m/min never exceeding 5, with an average value of 3.3.
Furthermore, the standard deviation measured for the sampled signal of the
cast level in
the mould (ML), indicated as stdDEV(ML), is usually within the following
range:
stdDEV(ML)=0.7-1.5 mm
Finally, the oscillator 3 is a critical factor for the surface quality of the
slab and the
reliability of the continuous casting process. With reference to FIG. 1, the
oscillator 3
may be formed of a framework 3a being hinged to the floor and driven by a
hydraulic
servocontrol 5. Framework 3a is also hinged to a mould support 3b, thus
forming a kind
of quadrilateral together with a set of 3c fitted into both ends.
The control of the oscillator is managed by a program logic controller
allowing the
oscillator to change the oscillation parameters of the wave shape (e.g. the
wave
amplitude) between ~2 and ~10 mm. The controller continuously records the
actual value
of the casting speed so as to control the oscillation frequency based on the
above
parameters. Maximum oscillation frequencies have been obtained as high as 480-
520
strokes/mm, for the first natural frequency of the entire dynamic system of
16.7 Hz. The
flexibility is such that the oscillation parameters may be adjusted to obtain
an optimal
lubrication and surface quality depending on the casting speed.
Alternatively, the oscillator may be of the so-called "resonance" type with
the mould
being directly mounted upon flexure springs (without any lever system) and
oscillated by
a hydraulic servocontrol at a frequency close to the natural frequency of the
elastic
system.
Possible additions and/or modifications may be made by those skilled in the
art to the
above described and illustrated embodiment without departing from the scope of
the
invention. In particular, the mould itself may have in the vertical plane a
profile other
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than the one disclosed in European patent 0705152 and the submerged nozzle may
be
different than the one disclosed and claimed in application PCT/IT-97/00135,
provided
that the aforementioned geometrical relations are complied with.
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