A Mold Simulator for the Continuous Casting of Steel:
Part I. The Development of a Simulator
A. BADRI, T.T. NATARAJAN, C.C. SNYDER, K.D. POWERS, F.J. MANNION,
and A.W. CRAMB
Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of
continuously cast steel, originate in the continuous casting mold. Therefore, a detailed knowledge of
initial solidification behavior of steel in a continuous casting mold is necessary because it determines
the surface quality of continuously cast slabs. In order to develop an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed to investigate
heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used
to obtain solidified steel shells of different grades of steel under conditions similar to those found in
industrial casting operations. The resulting cast surface morphologies were compared with industrial
slabs and were found to be in good agreement, indicating that it is possible to simulate the continuous
casting process by a laboratory scale simulator.
I. INTRODUCTION
ONE of the difficulties in studying the effects of operational parameters on the initial solidification behavior of
steel in a continuous casting mold is the interdependence
among different variables. It is not always feasible to conduct controlled experiments on an industrial continuous caster
that will allow the effects of different operational parameters on the initial solidification of steel to be studied due to
practical constraints. Therefore, most of the information
developed on the formation of defects during the continuous casting of steels is collected under uncontrolled conditions. In the past, this constraint has led to the development
of different types of mold simulators to study various aspects
of continuous casting.
Mold simulators can generally be divided into four types—
dip tests, static molds, dip simulators, and small-scale casters. The major issue in designing mold simulators is to ensure
that the apparatus and the experiment are a true simulation
of reality. This has led to the development of experimentspecific simulators that simulate the conditions in a casting
mold to different degrees. For example, to study the effects
of mold fluxes on the heat transfer between steel and a
copper mold, Machingawuta et al.[1] developed a dip-type
simulator specifically for that purpose. Another dip-type simulator was used by Bouchard et al.[2] to investigate the effects
of mold surface conditions on the heat-transfer rate and attendant surface quality of the cast product. These dip simulators involved chilled plates that were immersed into a molten
metal bath without any of the sophistication of continuous
caster systems, such as oscillation and shell extraction. The
dip simulators are very useful for determining fundamental
A. BADRI is with Shell Oil, Malaysia. T.T. NATARAJAN, Senior
Research Engineer, C.C. SNYDER, Senior Technician, and K.D. POWERS,
Project Analyst, are with the U.S. Steel Research and Technology Center,
Monroeville, PA 15140. F.J. MANNION, General Manager, is with U.S.
Steel, Slovakia. A.W. CRAMB is with the Department of Metallurgical
and Materials Engineering, Carnegie Mellon University, Pittsburgh, PA
15213. Contact e-mail: [email protected]
Manuscript submitted February 4, 2004.
METALLURGICAL AND MATERIALS TRANSACTIONS B
interactions in the continuous casting process, but are not
true simulators since they do not mimic the dynamic nature
of continuous casting.
Related to the dip test simulators are the bottom-pouring
molds, which are in essence similar to dip-type mold simulators, with the exception that the bottom-pouring simulators
have the metal contained in the mold, instead of having the
mold dipped into the metal. This configuration has the advantage that it is easier to observe the surface of the casting
during solidification. Tomono et al.[3] used a bottom-filling
mold to investigate the behavior of the liquid steel meniscus
during casting and projected the results to explain the formation of oscillation marks. Wray[4] developed a simulator
to determine the mechanisms by which surface features
formed on chill cast surfaces and provided a classification
of the different types of features that could be formed.
Stemple et al.[5] used a bottom-pouring configuration to
investigate the formation of ripple marks on the surfaces of
continuously cast products. It was emphasized that the bottom-pouring simulator could only be used to investigate phenomena unrelated to mold oscillation, since the experimental
apparatus did not have provision for oscillation. Even so,
Stemple et al. were able to observe the motion of the meniscus and provided an explanation for the formation of ripple
marks. Nishida et al.[6] developed a mold simulator with the
novel addition of an in-situ tool to measure the distortion of
the shell from the mold wall. This was done to determine
the dynamics of air gap formation and the resulting effect
on the steel-mold heat transfer. Again, these were incomplete simulators of the continuous casting process.
To incorporate further sophistication into the experiment,
several researchers have constructed more complex dip-type
experiments in which the mold is equipped with oscillation
drives and a mechanism for the extraction of the solidified
shell to simulate continuous casting. This type of mold simulator is quite versatile and has been used by Saucedo[7] to
investigate the initial solidification phenomena. The simulated castings exhibited the typical surface morphologies of
industrial cast slabs, and the results were used to propose a
mechanism of oscillation mark formation. The work also
included a comprehensive survey of the various hypotheses
VOLUME 36B, JUNE 2005—355
proposed in the literature for oscillation mark formation.
Suzuki et al.[8] designed simulation experiments on shell formation and mold flux consumption and also presented findings on the formation mechanism of oscillation marks. These
simulators simulated the dynamic nature of continuous casting but were not true simulators of the heat-transfer conditions that could be found in the steel plant.
The next step in complexity of mold simulators was to
build a scale model of an actual continuous casting machine,
with the liquid contained in the mold. These mold simulators include various levels of the complexity found in industrial machines, and are generally used as pilot casters to
investigate particular conditions that cannot be studied with
any of the previous simulators or even on an actual caster.
One of the earliest reported experiments was that of Savage
and Pritchard,[9] who built a mold to investigate billet rupture during continuous casting. Singh and Blazek[10] constructed a similar model to study the effects of heat transfer
and shell formation on surface rippling in low-carbon steels.
Building further on this idea, and to determine the various
factors affecting mold heat transfer, Blazek et al.[11] built a
simulator that had mold plates modified to allow for variations in the water cooling flow configuration. To investigate
the effects of high oscillation frequencies on oscillation marks
and mold flux consumption, Yasunaka et al.[12] constructed
a simulator with a modified oscillation drive that allowed
the mold to be oscillated at frequencies up to 50 Hz. To
illustrate the importance of using a simulator instead of an
actual casting machine, the authors found that a danger with
high-frequency oscillation is that there exists the possibility that resonance can occur, leading to catastrophic failure
of the machine.
The state of the liquid steel meniscus is often considered
to be important in the formation of oscillation marks, but it
is exceedingly difficult and dangerous to attempt to observe
the liquid steel meniscus directly in an industrial caster. To
attempt visual observation, Matsushita et al.[13] constructed
a simulator with a quartz window near the meniscus. The
experiments yielded important information on the distortion
of the meniscus during the oscillation cycle as affected by
the casting speed. In another effort to improve the surface
quality of continuously cast slabs, Itoyama et al.[14] built
a simulator with horizontal oscillation in addition to the commonly utilized vertical oscillation, and found that the depth
of oscillation marks decreased with the use of horizontal
oscillation.
In order to conduct comprehensive studies on the effects
of operational parameters on casting quality, it is often necessary that the casting parameters be varied independently
of each other. With this goal in view, a dip-type simulator
of a continuous caster was developed in this study, with
capabilities for mold oscillation and continuous shell extraction, and with the cooling conditions and capacity that are
found in industrial casting machines. The mold simulator
developed in this study is unique in the geometric form of
the mold and in the sophistication of the sensor instrumentation. Additionally, this study used sensor configurations
that were optimized to detect small changes in temperature
and displacement in the system. The main objective of this
article is to present a description of the system and several
examples of useful data that can be obtained during a normal trial, including subsecond temperature variations, heat
356—VOLUME 36B, JUNE 2005
fluxes related to initial solidification, and surface profiles.
Using these experimental data, the validity of hypotheses of
oscillation mark formation can be tested. The simulation
capability of the mold simulator itself was verified by obtaining solidified shells of different grades of steel under conditions that would be commonly seen in industrial operations.
The surface quality of the simulated shells was then compared against that of industrial slabs to ensure reproducibility.
It is shown that the mold simulator does indeed replicate the
surface features seen in a slab cast under industrial conditions.
II. EXPERIMENTAL TECHNIQUE
Figure 1 is a schematic of the molten steel in a continuous casting mold. The mold is cooled by water flowing
through the grooves and acts as a heat sink. The steel solidifies against the copper mold and increases in thickness as
it moves down the length of the mold. The steel shell is
about 12 mm in thickness when it exits the mold. The mold
is oscillated to prevent the sticking of the steel shell to the
copper mold, and this oscillation promotes the infiltration
of a film of liquid flux between the shell and the mold. Furthermore, the liquid mold flux on top of the liquid steel solidifies where it contacts the copper mold and gradually builds
up a flux/slag rim. It has been theorized that the oscillation
of the copper mold is responsible for the formation of oscillation marks.[3]
A mold simulator provides an ideal laboratory system for
the study of initial solidification of steel in a continuous
casting mold. The depth and width of the oscillation marks
can be easily modified by changing the mold oscillation
cycle, oscillation stroke, and casting speed. In addition, the
effects of different mold fluxes on the initial heat transfer
can be studied easily without interrupting the normal production operations at the plant.
Fig. 1—Schematic sketch of liquid steel in a continuous caster mold.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Mold Simulator
The mold simulator developed in this study is an inversetype mold, where the steel solidifies around the mold, instead
of the mold surrounding the solidifying steel. Figure 2 is a
schematic sketch of the mold simulator stage, which consists of several distinct modules to simulate the casting
process.
The different physical modules of the simulator include
the mold assembly, the extraction mechanism, the stabilization system, and the oscillation mechanism. The mold
assembly consists of a pair of grooved copper plates and a
stainless steel baffle that separates the inlet and outlet water,
as shown in Figure 3. In this work, the mold surface is flat
instead of cylindrical, and is constructed from actual mold
plates previously used at the U.S. Steel Gary Works. This
flat plate configuration has nickel plating on the hot face,
and the cold face is grooved with cooling channels. Figure 4
shows the assembly of a typical mold used in the mold simulator and the placement of the stainless steel baffle that
allows the circulation of cooling water. The assembled parts
are Tungsten inert gas (TIG)-welded to form a unit, after
which the unit is pressurized with water and checked for
leaks. Figure 5 shows the dimensions of the copper plates
and of the cooling grooves. Figure 5 also shows the location
of the meniscus with respect to the bottom of the mold and
the locations of thermocouples with respect to the meniscus.
The cooling water is fed into the mold from the cooling
water manifold, as shown in Figure 2.
In order to simulate continuous casting, the mold assembly is fitted with an extraction mechanism, which is fabricated from 6.25-mm-thick steel plates. The extractor pulls
the solidifying steel shell in the casting direction (downwards). This exposes liquid steel to the water-cooled copper mold at the meniscus and allows the formation of a new
steel shell. The extractor is designed so that only one face
(a)
(b)
Fig. 2—Schematic sketch of the mold simulator stage.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 3—The copper mold assembly and extractor mechanism.
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Fig. 4—Steps in building the mold assembly.
of the mold is exposed to the liquid steel, as seen in Figure 3. This allows a controlled exposure of the mold hot
face to the liquid steel while protecting the other faces of
the mold.
The process of solidification and extraction of the steel
shell displaces some liquid steel, and so the stabilization system moves the main stage upward with time to maintain the
liquid steel meniscus at a constant level (about 150 mm from
the bottom) with respect to the copper mold. All of the
sensor systems, data and control cables, and drive systems
are protected from the steel bath by a heat shield.
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The mold is connected to an oscillating stage so that the
mold oscillates sinusoidally in the vertical direction about
the meniscus position. A slotted cam is used to convert the
rotational motion of the motor into a linear vertical sinusoidal oscillation of the rectangular mold. The extractor is
attached to drive shafts powered by stepper motors. During
the experiment, the drive shafts push the steel shell down
with respect to the meniscus and expose steel to the copper
mold to allow the formation of “new” steel shell. The mold
motion is independent of the extractor motion. This allows
the incorporation of negative strip time, which can be defined
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 6—Physical description of the problem domain.
Fig. 5—Dimensions of mold copper plate with thermocouple locations
(dimensions in millimeters). Note: The oval shown at the right is not exactly
the same as that shown at the left.
as the portion of the mold oscillation cycle during which the
mold moves downward faster than the shell. The translating stage is controlled so that the meniscus remains about
150 mm from the bottom of the mold during an experiment.
In order to characterize the heat transfer, the mold was
instrumented with thermocouples to observe the transient
variation of the temperature and heat flux during the casting period. The instrumentation of the mold with thermocouples was preceded by a comprehensive study[15] of
temperature measurements in conducting solids. For example, it is known that when a temperature sensor is inserted
into a conducting solid, the void created for the sensor and the
sensor material itself can introduce errors into the measured
temperature signal. These errors were studied to determine
how best to install the sensors in the mold. Additionally, the
response of the material, as deduced from the temperature
sensors, was studied under conditions of transient high thermal fluxes to determine the ability of the sensor to discriminate between different functional forms of surface heat
flux variation. A heat-transfer simulator was also used to
confirm the ability of subsurface thermocouples to measure
small variations in temperature due to oscillations in the surface heat flux. The resulting conceptual models assisted in
the development of a greater understanding of meniscus heat
transfer, which finally led to the ability to interpret the heattransfer data obtained from the mold simulator experiments.
A heat flux simulator was built that allowed heat fluxes
of up to 1 MW/m2 to be applied to a copper mold. A variety
of thermocouple designs were modeled to determine the optimum method to allow transient heat fluxes to be measured
accurately. This study allowed such issues as hole size, positioning, thermocouple attachment, and data acquisition techMETALLURGICAL AND MATERIALS TRANSACTIONS B
niques to be fully developed before application on the mold
simulator.
Based on the preceding heat-transfer studies, the mold
was instrumented with 12 grounded T-type thermocouples
at various elevations to detect casting events on the hot face
of the mold. The thermocouples were arranged in two
columns—1.5 and 5.0 mm from the hot face—of six rows.
The six rows were in the immediate vicinity of the aim
meniscus location of 150 mm. Figure 5 shows the physical
locations of the thermocouples with respect to the meniscus. These thermocouples are located 133, 140, 146, 152,
159, and 165 mm from the bottom of the mold. The molds
were machined with 1.59-mm-diameter 50-mm-deep thermocouple voids with an orientation parallel to the mold surface to install the thermocouples. The dual-lead T-type
thermocouples used in the experiments have magnesium
oxide insulation and a stainless steel sheath with an outer
diameter of 0.5 mm. The thermocouples were fitted with
metal collars to ensure a good fit in the void so that the positions of the tips were well defined, and the tips were covered with a heat sink compound to enhance heat transfer to
the thermocouple tip. In addition to the thermocouples, linear velocity displacement transducers (LVDTs) were used
to monitor the motions of the mold and the extractor mechanism. By attaching both LVDTs to the same reference point,
motions of the extractor and mold were measured relative
to the same frame of reference. The instrumentation of the
mold resulted in a complete characterization of heat-transfer
phenomena with respect to the motion of the mold. The
National Instruments Labview software was used to acquire
temperature data at 60 Hz for the duration of the experiment
to detect phenomena occurring within individual oscillation cycles.
The temperature data acquired from the mold thermocouples were used to develop an estimate of the heat flux
through the mold during the initial solidification of the steel
shell using the one-dimensional inverse heat conduction program developed by Beck.[16] A typical form of the physical
problem which Beck’s method is designed to solve is shown
in Figure 6. It is sufficient to use only one internal body
VOLUME 36B, JUNE 2005—359
temperature and one boundary condition to determine the
unknown boundary condition.
In the mold simulator experiments, the known boundary
condition is the convective cooling of the mold by water
flowing through cooling channels. While there are correlations for determining the heat-transfer coefficient for the
cooling channels, another thermocouple was introduced into
the domain to provide a well-defined boundary condition.
The domain of the problem shown in Figure 7 can be decomposed into two domains by creating an interface S at the second thermocouple. The boundary condition at this point
S then applies to the two subdomains. This decomposition
is shown in Figure 8. In this case, the first thermocouple
was 1.5 mm away from the heated surface, while the second
Fig. 7—Problem domain including two thermocouples.
Fig. 8—Domain decomposition yielding a known value boundary condition.
360—VOLUME 36B, JUNE 2005
thermocouple 5.0 mm away from the heated surface was
used to define the subdomain.
Before using this program and the domain decomposition
approach to calculate heat fluxes from experimental data,
the reliability of the program was tested rigorously using
data from simulations. This was done to ensure that the program would calculate results that were accurate and precise
for the waveforms of interest in this work, and to examine
the effects of the known boundary conditions and signal
noise. For more details, interested readers can refer to the
work of Badri.[15]
III. EXPERIMENTAL PROCEDURE
A typical experimental run of the mold simulator involves
the heating and melting of a charge of ultra-low carbon steel
in a 200-kg induction furnace under an argon atmosphere.
After the charge is molten, the chemistry and temperature
of the melt are adjusted to aim values and enough mold flux
powder is added to the surface so that there will be a layer
of molten flux approximately 6.5-mm thick on top of the
liquid steel after melting. Following the melting of the mold
flux powder, the levels of the liquid steel and mold slag are
measured to ensure that the meniscus will be located at a
particular level on the mold (150 mm from the bottom of
the mold). Samples of the steel and slag are taken for analysis, and then the liquid steel is heated to slightly above the
desired casting temperature. When the aim temperature is
reached, the main stage is lowered into the steel bath (Figures 9(a) through (c)). During the descent of the main stage,
the oscillator motor is turned on. After the main stage reaches
a preset depth (Figure 9(d)), it is held for 3 seconds to form
an initial shell on the mold. This pause allows for the formation of a shell sufficiently strong to prevent tearing of the
initial steel shell during extraction. In the casting phase (Figure 9(e)), the extractor is lowered an additional 3 in. at
constant velocity while the mold continues to oscillate about
the meniscus to simulate the continuous casting of a 3-in.
length of steel shell. The main stage moves to compensate
for any additional displacement of the liquid level so that
the meniscus is maintained at the same level with respect to
the mold. At the end of the casting phase (Figure 9(f)), the
entire assembly is withdrawn from the furnace and the shell
is allowed to cool (Figures 9(g) through (i)). The profile of
this motion is shown in Figure 10(a), while the corresponding
velocity profile is shown in Figure 10(b).
Additional samples of the liquid steel and mold flux are
taken to analyze for any change in composition. After the
shell has completely cooled, the portion of the shell that
solidified adjacent to the copper mold is cut away. Figure 11
shows a schematic sketch of a cutaway shell from a typical
mold simulator run. The solidified shell is removed from
the mold while an attempt is made to keep the mold flux
film intact on the mold surface. As an example, the shell
surface from an ultra-low carbon grade casting is shown in
Figure 12. The shell is shown just after it was removed from
the mold. Good slag infiltration between the copper mold
and the steel shell can be seen. Subsequently, the surface
profile is measured along the centerline of the steel shell,
corresponding to the location of the thermocouples in the
mold, using a contact profilometer.
METALLURGICAL AND MATERIALS TRANSACTIONS B
(a)
(b)
(c)
(d )
(e)
(f)
Fig. 9—Digital images showing the progress of the experiment.
METALLURGICAL AND MATERIALS TRANSACTIONS B
VOLUME 36B, JUNE 2005—361
(g)
(h)
(i)
Fig. 9—(Continued). Digital images showing the progress of the experiment.
(a)
(b)
Fig. 10—Mold and steel shell displacement and velocity during an experiment.
362—VOLUME 36B, JUNE 2005
METALLURGICAL AND MATERIALS TRANSACTIONS B
IV. RESULTS AND DISCUSSION
Using the mold simulator, experimental runs were conducted for several grades of steel with most of the runs
focused on ultra-low carbon steel. Table I summarizes the
typical chemical composition for the experimental runs, while
Table II summarizes the operating parameters. Some of the
typical information that can be obtained during the operation of the mold simulator includes temperature history, the
associated heat flux at the hot face of the mold, the surface
profile of the cast shell measured using a contact profilometer, and flux film characteristics.
The surface profiles of ultra-low, peritectic, and medium
(hyperperitectic) carbon steels are shown below to enumer-
Fig. 11—Schematic sketch of an expected steel shell from a mold simulator run.
ate the differences between different grades of steel. Furthermore, the shells obtained from mold simulator runs are
compared with industrial samples. This is an important step
because it reveals whether the cast shells are in fact representative of industrial cast slabs. The surface profile of the
solidified steel shell can be analyzed in conjunction with the
measured temperatures to obtain insight into the solidification history of the shell surface. Such an analysis is the main
topic of a subsequent article.[17]
Typical temperature traces, as recorded by thermocouples
just above and below the meniscus, are shown in Figure 13.
The thermocouples below the meniscus measure higher temperatures because of the direct contact of the mold surface
with the liquid steel, and they also register the variations in
temperature due to mold oscillation. The temperature traces
measured by all of the thermocouples have roughly the same
form. The initial temperature of the mold is ambient temperature. As the mold is immersed into the liquid steel bath,
the temperature rises. However, as the mold enters the bath
and the liquid steel begins to solidify on the hot face, there
is also an increase in the resistance to further heat transfer,
which results in a decrease in the temperature measured by
the thermocouples. During the extraction phase of the casting process, the temperature rises as the mold is exposed to
fresh liquid steel at the meniscus, and the oscillation in temperature reflects the changing position of the mold with
respect to the meniscus. At the end of the casting stage, the
mold is withdrawn from the liquid steel and the thermocouples show a rapid decrease in mold temperature.
Figure 14 shows typical temperature traces measured by
all of the thermocouples during the casting stage of an ultralow carbon grade of steel. The labels 5.25F, 5.25B, etc. refer
to the locations of thermocouples. The numeric value denotes
the distance of the thermocouple from the bottom of the mold
Fig. 12—Example of a mold flux film (left) and steel shell (right) from an ultra-low carbon steel trial.
METALLURGICAL AND MATERIALS TRANSACTIONS B
VOLUME 36B, JUNE 2005—363
Table I. Typical Chemical Compositions
Element
Pct
Pct
Pct
Pct
Pct
carbon
manganese
silicon
sulfur
nitrogen
Ultra-Low
Carbon Steel
Peritectic
Steel
Medium
Carbon Steel
0.0046
0.46
0.11
0.0089
0.0057
0.065
0.95
0.22
0.0075
0.0047
0.175
1.17
0.31
0.027
0.0069
Table II. Operating Parameters
Stroke (mm)
Oscillation frequency (Hz)
Casting/extraction speed (mm/s)
6.3
1.3
12.7
Fig. 15—Temperatures measured by thermocouples at the meniscus during solidification of an ultra-low carbon steel grade.
Fig. 16—The heat flux calculated using the temperature data from the thermocouples at the meniscus during solidification of an ultra-low carbon steel
grade.
Fig. 13—Thermocouple temperature traces during immersion and casting
of steel using the mold simulator.
Fig. 14—Mold thermocouple data during the casting stage of an ultra-low
carbon steel grade.
in inches, while the character (F or B, meaning front or back)
refers to the distance of the thermocouple tip from the hot
face. The F refers to thermocouple tips located about 1.5 mm
from the hot face, while B refers to the thermocouple tips
located 5.0 mm from the hot face. It can be seen that the
individual oscillations in temperature due to the oscillation
of the mold relative to the meniscus have been resolved. The
trials indicate that any phenomenon causing a temperature
364—VOLUME 36B, JUNE 2005
change greater than 0.1 °C can be identified with the current
thermocouple instrumentation of the mold.
Figure 15 shows a close-up view of the temperature data
recorded by the thermocouples at the meniscus, from which
the associated heat flux at the meniscus shown in Figure 16
is derived using the one-dimensional inverse heat conduction
program developed by Beck. The heat flux plotted is the
horizontal heat flux in the area of the meniscus and is not
the total heat removed from the steel in the meniscus area.
In the meniscus area, the heat flux is multidimensional and
transient. Calculation of multidimensional heat conduction
using inverse techniques is a very complicated issue and its
discussion is beyond the scope of this work. Figure 16 and
subsequent plots are shown to illustrate that heat fluxes in
the meniscus area can be measured and that, even in the
one-dimensional solution, one can adequately resolve transient behavior in the heat flux during the oscillation cycle.
In this study, unfiltered and unaltered heat flux data calculated directly from thermal measurement are shown. It is
felt that other methods of calculation of heat flux would only
change the numbers and not the variation of the heat flux
values as a function of oscillation cycle.
An image of the surface of the ultra-low carbon steel shell
and the associated contact profile measurement are shown in
Figure 17. From the surface profile measurement, it can be
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 18—Mold thermocouple data during casting stage of a peritectic steel
grade.
(a)
Fig. 19—Temperatures measured by thermocouples at the meniscus during solidification of a peritectic steel grade.
(b)
Fig. 17—(a) Photograph and (b) measured profile of shell surface for an
ultra-low carbon steel grade.
seen that this grade of steel has peaks that are rounded between
oscillation marks. Furthermore, the oscillation marks in the
ultra-low carbon grade can be described as being composed
of peaks and subpeaks. In other words, each oscillation mark
is bracketed by these sharp peaks, and within each oscillation
mark, there is an irregularity referred to here as a subpeak.
Figures 18 through 20 show the temperature and heat flux
graphs for the peritectic grade of steel. Figure 21 is a contact profile measurement of the surface of the steel shell cast
by the mold simulator. The surface profile measurement
indicates that this particular grade of steel has several plateauMETALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 20—The heat flux calculated using the temperature data from the thermocouples at the meniscus during solidification of a peritectic steel grade.
shaped features between oscillation marks and that the shape
of the oscillation marks is sharply defined.
Figures 22 through 24 show the variation of temperature
and heat flux values during the course of an experiment for
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a medium carbon (hyperperitectic) grade of steel. Finally,
Figure 25 shows the measured surface profile of a medium
carbon steel shell from the mold simulator experiment. This
grade exhibits poorly defined peaks and relatively smooth
plateaus. It was found that the geometry of the oscillation
marks in medium carbon steel appears to be a function of
the mold flux used. The morphologies of the oscillation
marks for ultra-low carbon, peritectic, and medium carbon
steel grades are summarized in Figure 26.
From the heat flux graphs, it can be seen that the total
heat flux at the meniscus can be considered as the sum of
an average baseline component and a time-varying component. The time-varying component of the heat flux has a
magnitude approximately 10 pct of the average baseline
Fig. 22—Mold thermocouple data during the casting stage of a medium
carbon steel grade.
(a)
Fig. 23—Temperatures measured by thermocouples at the meniscus during solidification of a medium carbon steel grade.
(b)
Fig. 21—(a) Photograph and (b) measured profile of shell surface for a
peritectic steel grade.
366—VOLUME 36B, JUNE 2005
Fig. 24—Heat flux calculated using the temperature data from the thermocouples at the meniscus during solidification of a medium carbon steel
grade.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 26—Morphologies of oscillation marks from three types of steel grades.
(a)
(b)
Fig. 25—(a) Photograph and (b) measured profile of shell surface for a
medium carbon steel grade.
component. The average heat flux for the peritectic grade
is less than that for the ultra-low carbon and medium
carbon grades. The heat flux data indicate that it is possiMETALLURGICAL AND MATERIALS TRANSACTIONS B
ble to resolve the one-dimensional heat flux profiles from
temperature measurements for peritectic grades that are
traditionally viewed as difficult to interpret due to the nonuniformity of shell thickness caused by volume changes accompanying the peritectic phase transformation. This technique
can be used to determine the exact carbon content where
this rippling becomes problematic and also to determine the
relationship between the mold flux chemistry and surface
quality.
For the conclusions deduced from the results of the mold
simulator experiments to be applicable to industrial operations, the cast shells from the mold simulator must be shown
to be similar to those cast industrially. This was accomplished by comparing the surface profiles of narrow faces
of industrially cast slabs with those of the mold simulator
shells. The comparisons of surface profiles for different
grades of steel are shown in Figures 27 through 29. These
figures show that there is reasonable similarity between the
narrow faces of industrially cast slabs and the shells from
the mold simulator for different grades of steel. The oscillation mark morphology changes with the composition of
the steel, and these morphologies change in the same way
in the mold simulator as they do in the industrially cast slabs.
In addition to using profile measurements to show similarity to industrial slabs, the surface profiles of ultra-low
carbon steel were also analyzed for two characteristics of
the oscillation marks, the pitch and the depth. The pitch of
the oscillation mark is the distance between two consecutive oscillation marks. Ideally, if one oscillation mark were
formed in each oscillation cycle, the pitch of the oscillation marks would be equal to the theoretical value vc/f,
where vc is the casting speed and f is the frequency of oscillation. In the mold simulator trials, there is a distribution
of oscillation mark pitch values. The measured values were
compared against the published data of Cramb and Mannion,[18] as shown in Figure 30. It was found that the distribution of oscillation mark pitch measurements conforms
to a Gaussian distribution and that the peak is located at
the theoretical value. The spread in the data about the theoretical calculated value is expected, because the spacing
of oscillation marks does not depend uniquely on the casting speed and oscillation frequency, but is actually determined by the relative velocity between the shell and the
meniscus. Since the meniscus level is not absolutely constant, but varies slightly with time about the mean position,
there is a spread in the measured values. The simulation
quality of the mold simulator is again confirmed by the fact
VOLUME 36B, JUNE 2005—367
(a)
(b)
Fig. 27—(a) Comparison of surfaces of ultra-low carbon steel from the narrow face of a slab and the shell from the mold simulator. (b) Comparison of
surface profiles.
that the distribution of the pitch measurements is similar
to that found in the industrial measurements reported by
Cramb and Mannion.
In addition to the pitch of the oscillation marks, the depths
were also measured. The distribution of the measured depths
is shown in Figure 31, and also appears to conform to a
Gaussian distribution. The average depth of the oscillation
marks is about 275 m, but there were a few instances in
which the depth was much larger, up to 700 m. The quality of the measured distribution would increase with an
increase in sample size. The distribution of the depths is
comparable to that measured by Cramb and Mannion, as can
be seen in Figure 31.
The experimental data show that the measured characteristics of the oscillation marks agree well with industrial
slab profiles and the reported measurements of Cramb and
Mannion, again showing that the mold simulator does indeed
simulate the conditions of industrial continuous casting
machines. This is in addition to the comparison between the
surface profiles of mold simulator shells and industrial slab
surfaces, which confirmed the ability of the mold simulator
to reproduce the surface features of industrially cast slabs.
This confirmation justifies the use of the mold simulator to
368—VOLUME 36B, JUNE 2005
analyze the solidification and heat-transfer phenomena in
the industrial mold.
V. SUMMARY
An apparatus was successfully designed and constructed
to simulate the mold of a continuous casting machine. The
main features of the mold simulator the following:
(1) a copper mold designed using actual mold plates;
(2) an extracting mechanism, which allows continuous casting;
(3) a translating stage to maintain the meniscus at a constant level on the mold;
(4) a sinusoidal oscillating drive that allows the mold to
oscillate independently of the shell; and
(5) sensors that measure in-mold temperatures, bath temperature, steel shell displacement, and mold displacement.
The temperature data obtained were converted to heat flux
values using the one-dimensional heat conduction program
developed by Beck.
The mold simulator was used to obtain steel shells of varying composition in order to confirm that it could indeed act
METALLURGICAL AND MATERIALS TRANSACTIONS B
(a)
(b)
Fig. 28—(a) Comparison of surfaces of peritectic steel from the narrow face of a slab and the shell from the mold simulator. (b) Comparison of surface
profiles.
as a simulator of the continuous casting mold. The surface
profile was measured using a contact profilometer. It was
determined that mold simulator shells exhibited surface features similar to those of industrially cast slabs. It was found
that the features on the surfaces of the cast slabs varied with
steel composition, and that the changes in these features were
reflected in the cast shells from the mold simulator. This confirmation was essential in that it shows that the mold simulator is a realistic model of the continuous casting process,
and can therefore be used to conduct experiments reflective
of conditions in an industrial caster. Furthermore, the distribution of depth and pitch measurements of oscillation marks
on mold simulator shells compared favorably with measurements by Cramb and Mannion of the oscillation marks on the
narrow faces of slabs. Therefore, the mold simulator can be
used to investigate phenomena affecting the surface quality
of cast shells and also the castability of various steel grades.
The unique feature of this mold simulator is the instrumentation of the apparatus, which permits the resolution of
the temperature and heat flux variations within the period
of a single oscillation cycle. Furthermore, the shell cast by
METALLURGICAL AND MATERIALS TRANSACTIONS B
the mold simulator is typical of what is seen on the narrow
face of a slab. This allows valid conclusions to be deduced
from results of the mold simulator experiments. Last but not
least, the mold flux film between the mold and the shell can
be retrieved intact after an experiment, something that has
not been accomplished in earlier studies.
ACKNOWLEDGMENTS
The authors thank the United States Steel Corporation and
the former Bethlehem Steel Corporation (now part of ISG)
for their financial support of this project. Additionally, we
thank G. Biddle, J. Sadecky, and R.C. Evans for their assistance with apparatus design and construction. In addition,
the authors deeply appreciate the assistance of Falcon
Foundries in welding the copper mold plates.
The material in this paper is intended for general information only. Any use of this material in relation to any specific application should be based on independent examination
and verification of its unrestricted availability for such use,
VOLUME 36B, JUNE 2005—369
(a)
(b)
Fig. 29—(a) Comparison of surfaces of ultra-low carbon steel from the narrow face of a slab and the shell from the mold simulator. (b) Comparison of
surface profiles.
Fig. 30—Distribution of oscillation mark pitch measurements for an ultralow carbon steel grade.
Fig. 31—Distribution of oscillation mark depth measurements for an ultralow carbon steel grade.
and a determination of suitability for the application by professionally qualified personnel. No license under any United
States Steel Corporation patents or other proprietary interest
is implied by the publication of this paper. Those making
use of or relying upon the material assume all risks and liability arising from such use or reliance.
370—VOLUME 36B, JUNE 2005
METALLURGICAL AND MATERIALS TRANSACTIONS B
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