-
to form a liquid
slag pool which protects the surface
of the steel from oxidation
-
to provide thermal
insulation which prevents the surface
of the steel from freezing
-
to lubricate the
newly-formed shell
-
to control the
horizontal heat transfer between shell
and mould
-
to absorb inclusions
from the steel
The most important
of the five functions of the mould flux
are providing lubrication and the right
level of heat transfer. However the
continuous casting process is a very
complicated process and “everything
counts”.
The
casting powder must be ‘tailored’
to perform satisfactorily for the casting
conditions.
The
casting powder cannot act on its own
and satisfactory operation requires
that other casting variables should
be carefully adjusted, for example:
-
good steelmaking
practice (minimise Al2O3 formation,
inclusion engineering)
-
good mould level
control
-
careful design
and positioning of the SEN to ensure
good temperature and steel flow pattern
at meniscus
-
mould oscillation
characteristics.
Modern
synthetic powders usually contain:
-
raw materials with
similar melting points (eg lithium
feldspar, sodium feldspar, wollastonite)
to provide more uniform melting
-
a minimum number
of raw materials to achieve the desired
composition so as to keep the recipe
simple and simplify quality assurance.
Spray
dried powders were introduced to provide
better quality control and minimise
health hazard.
Pulverised
powders do have the following disadvantages:
For
the production of Spray dried spherical
powders, the various powdered raw materials
are weighed and mixed with water for
> 5 hours to form a ‘slurry’
or suspension. It is then sieved and
pumped into a stirrer (to prevent segregation)
and is then fed to an atomiser in a
spray tower. Fine spherical droplets
are formed. Hot air is introduced and
the evaporation of water in droplets
is very rapid since the drops have a
high ratio surface area/ volume.
The
result is dry spherical hollow particles,
which are then sieved, cooled, sieved
and packaged.
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Chemical
composition of mould powders
Continuous
casting powders are made up of several
components as shown in the following:
| Glass
formers |
SiO2
|
17
- 56% |
Basic
Oxides |
CaO |
22
- 45% |
| Al2O3
|
0
- 13% |
MgO |
0
- 10% |
| B2O3
|
0
- 19% |
BaO |
0
- 10% |
| Fe2O3
|
0
- 6% |
SrO |
0
- 5% |
| Alkali |
Na2O |
0 -
25% |
Fluidiser |
F |
2
- 15% |
| Li2O |
0 -
5% |
MnO |
0
- 5% |
| K2O |
0 -
2% |
Melting
rate control |
C |
2
- 20% |
The
effects of these various components
on the viscosity, melting point and
solidification temperature are summarised
in the following table
| Increase |
Viscosity |
Solidification
Point |
Melting
Point |
| CaO |
Decrease |
Increase |
Increase |
| SiO2 |
Increase |
Decrease |
Decrease |
| CaO
/ SiO2 |
Decrease |
Increase |
Increase |
| Al2O3 |
Increase |
Decrease |
Increase |
| Na2O |
Decrease |
Decrease |
Decrease |
| F |
Decrease |
Increase |
Decrease |
| Fe2O3 |
Decrease |
Decrease |
Decrease |
| MnO |
Decrease |
Decrease |
Decrease |
| MgO |
Decrease |
Decrease |
Decrease |
| B2O3 |
Decrease |
Decrease |
Decrease |
| BaO |
Decrease |
Decrease |
Decrease |
| Li2O |
Decrease |
Decrease |
Decrease |
| TiO2 |
No Change |
Increase |
Increase |
| K2O |
Decrease |
Decrease |
Decrease |
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Functions
of mould powders
Powders
are added continuously to the mould
and form 4 layers over the steel surface:
-
raw powder layer
-
sintered layer
-
mushy layer (mixture
of slag globules and carbon particles)
-
a pool of liquid
slag
Chemical
protection
The
presence of a liquid slag pool above
the meniscus and carbon particles in
the sintered and powder layers prevent
oxidation of the steel surface
Thermal
insulation
The
principal factors affecting thermal
insulation is the bulk density.
Lubrication
of the steel
Liquid
slag infiltrates into the gap between
the solid slag film formed against the
water-cooled mould and provides lubrication
for the solidified steel shell. It is
desirable that this layer of slag should
remain molten through the whole length
of the mould.
Uniform
heat transfer between steel and mould
Longitudinal
cracking is known to be related to both
the excessive magnitude and variability
of the heat flux. The heat flux is controlled
by the slag film formed between the
steel and mould.
The
principal factors affecting the heat
transfer between steel and mould are
-
thickness of the
slag layer, this increases with increasing
solidification or break temperature
and viscosity of the flux
-
the crystallisation
tendency of the slag film that acts
as a good shield to the radiant heat
-
interfacial resistance
caused by the air gap formed resulting
from the steel shrinkage.
Absorption
of inclusions
The
principal non-metallic inclusions formed
in the continuous casting process are
Al2O3 and TiO2, in Ti-containing grades.
Alumina is formed principally by oxidation
of the steel particularly in Al-killed
grades; consequently the powder has
to maintain its properties after alumina
absorption.
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Physical
properties of fluxes
The
physical properties of the casting flux
are important to the success of the
casting process
Melting
rate
Carbon,
in the form of either carbon black,
graphite or coke dust is added to the
mould powder to control its melting
rate and to provide insulation to both
the steel and the liquid slag.
Carbon
particles are non-wetting to molten
slag and thus separate the mineral particles
and slow down the agglomeration of molten
slag globules. Thus the more C, the
more time required to agglomerate, the
slower the melting rate.
The
powder bed above the steel meniscus
consists of four layers, powder, sinter,
mushy (mixture of slag globules and
carbon particles) and molten slag pool.
The
melting rate is controlled by several
factors
-
the vertical heat
flux density which is affected by
various casting parameters such as
casting speed, superheat, turbulence,
etc.
-
the free carbon
content of the powder
-
the type of carbon
and particle size of carbon
-
the carbonate content
of the mould flux (as the CO2
developed from carbonates decomposition
causes turbulence in the mould powder
layers so favouring its melting)
-
the presence of
exothermic agents which can raise
the melting rate.
Viscosity
The viscosity of the casting powder
is a key physical property since it
influences:
-
the lubrication
of the steel shell
-
slag entrapment
(lower with higher viscosity)
-
depth of oscillation
marks (lower with higher viscosity)
-
SEN erosion (higher
with lower viscosity)
Several
investigators have determined the effects
of different additions on the viscosities
of mould powders:
-
Al2O3
additions cause increase in viscosity
since they favour in a prone polymerised
slag
-
Li2O,
Na2O and CaF2
cause a decrease in viscosity since
they result in a more depolymerised
melt
-
Additions of TiO2
result in a slight decreases in viscosity
up to the solubility limit of about
10% beyond which the viscosity increases
because of the solids present.
Addition of ZrO2 probably
have a similar effect.
Measurements
of viscosity as a function of temperature
are made only when a new powder is designed.
Usually values of viscosity are calculated
by means of mathematical models.
Break (Tbr)
Solidification (Tsol) and Melting (Tmelt)
Temperatures
-
The
break temperature is the
temperature below which there is a
marked change in viscosity and represents
the point where liquid lubrication
starts to break down.
Break temperatures are obtained from
viscosity measurements during the
cooling cycle. Break temperatures
are dependent upon the cooling rate
and are usually quoted for a cooling
rate of 10°C/min but cooling rate
in the mould could be > 50 times
that. Consequently, Tbr values in
the mould could be substantially lower
than those quoted for 10°C/min.
It
should be noted that high-viscosity
fluxes such as those used in the billet-casting
have no break temperature since they
form super-cooled liquids.
-
The
solidification temperature
is the temperature where the flux
starts solidifying (formation of crystalline
phase) and is sometimes called Crystallisation
temperature. It is usually measured
during cooling cycle in Differential
Thermal Analysis (DTA) or Differential
Scanning Calorimetry (DSC) measurements.
-
The
melting temperature is the
temperature where flux becomes totally
fluid and is usually measured on the
heating cycle in DTA/DSC experiments.
Differential
Thermal Analysis are made only when
a new powder is designed. Usually, the
characteristic temperatures (softening,
melting and fluidity) are determined
by means of Heating Microscope.
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Mould
powders characteristics
As
the mould powders performance influences
both continuous casting operations and
slabs surface quality, mould powders
need to have the following fundamental
characteristics:
-
melting temperature
suitable to guarantee, at a fixed
casting temperature, a sufficient
slag fluidity for the infiltration
into the gap between steel and mould.
The casting powders are essentially
mixtures of oxides, present as different
mineralogical phases, which do not
melt simultaneously; for this reason
the heating of these powders give
rise to a temperature melting range
instead of a melting temperature
-
a viscosity suitable
for both the infiltration into a very
narrow gap and minimisation of the
friction between steel and mould
-
a melting rate
suitable to guarantee a proper molten
slag reserve
-
convenient values
of the break and solidification temperatures
that are very important for their
influence on the slag lubricating
ability and heat flux regulation.
When a slag changes from the molten
phase to doughy and then glassy phase,
it maintains a certain lubricating
ability due to the fact that a glass
is an undercooled liquid. When a slag
changes from the molten phase to a
crystalline one, its lubricating ability
is completely lost
Concernings
the heat flux regulation it has to be
remembered that a glassy slag layer
between steel and mould does not shield
the radiant heat and it is suitable
to cast steels not subjected to longitudinal
cracks. A slag with high tendency to
the crystallisation forms a good shield
to the radiant heat and it is recommended
for casting of steels generally subjected
to longitudinal cracks.
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How
to read a mould powder technical data
sheet
Chemical
Analysis
-
The basicity index
(IB = CaO/SiO2) gives an
idea of the slag tendency to crystallisation:
- IB > 1.0 generally means that
the powder has a certain tendency
to crystallisation and so it is best
suited to cast steels subjected to
longitudinal cracks;
- IB < 1.0 generally means that
the powder is prone to form a glassy
film between mould and steel, so the
powder has good lubricant ability
but does not shield the radiant heat.
-
The Free Carbon
content indicates the melting rate
of the powder: low free C corresponds
to high melting rate and consequently
high molten pool depth (keeping constant
operative parameters and powder viscosity).
-
The characteristic
temperatures (softening, melting and
fluidity), determined by means of
Heating Microscope are important,
especially the fluidity temperature
which determines the thickness of
the slag rim. When this temperature
is too high it can cause a big slag
rim that can inhibit the slag inflow
into the channel between mould and
steel.
-
Viscosity calculated
as a function of temperature. Generally,
the viscosity has to be lower for
higher casting speed to guarantee
a good lubrication.
Free
Carbon content and viscosity are the
most important parameters in determining
the molten slag pool depth.
Free
Carbon content controls how much liquid
slag is produced; viscosity controls
how much liquid slag infiltrates; in
practice that means that if you need
to reduce the viscosity the free Carbon
content has to be reduced too, in order
to keep constant the liquid slag pool
depth.
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