A Mold Simulator for the Continuous Casting of Steel: Part I

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 interactions in the continuous casting process, but are not true simulators since they do not mimic the dynamic nature ONE of the difficulties in studying the effects of oper- of continuous casting. ational parameters on the initial solidification behavior of Related to the dip test simulators are the bottom-pouring steel in a continuous casting mold is the interdependence molds, which are in essence similar to dip-type mold sim- among different variables. It is not always feasible to con- ulators, with the exception that the bottom-pouring simulators duct controlled experiments on an industrial continuous caster have the metal contained in the mold, instead of having the that will allow the effects of different operational parame- mold dipped into the metal. This configuration has the advan- ters on the initial solidification of steel to be studied due to tage that it is easier to observe the surface of the casting practical constraints. Therefore, most of the information during solidification. Tomono et al.[3] used a bottom-filling developed on the formation of defects during the continu- mold to investigate the behavior of the liquid steel meniscus ous casting of steels is collected under uncontrolled condi- during casting and projected the results to explain the for- tions. In the past, this constraint has led to the development mation of oscillation marks. Wray[4] developed a simulator of different types of mold simulators to study various aspects to determine the mechanisms by which surface features of continuous casting. formed on chill cast surfaces and provided a classification Mold simulators can generally be divided into four types— of the different types of features that could be formed. dip tests, static molds, dip simulators, and small-scale cast- Stemple et al.[5] used a bottom-pouring configuration to ers. The major issue in designing mold simulators is to ensure investigate the formation of ripple marks on the surfaces of that the apparatus and the experiment are a true simulation continuously cast products. It was emphasized that the bot- of reality. This has led to the development of experiment- tom-pouring simulator could only be used to investigate phe- specific simulators that simulate the conditions in a casting nomena unrelated to mold oscillation, since the experimental mold to different degrees. For example, to study the effects apparatus did not have provision for oscillation. Even so, of mold fluxes on the heat transfer between steel and a [1] Stemple et al. were able to observe the motion of the menis- copper mold, Machingawuta et al. developed a dip-type cus and provided an explanation for the formation of ripple simulator specifically for that purpose. Another dip-type sim- [6] [2] marks. Nishida et al. developed a mold simulator with the ulator was used by Bouchard et al. to investigate the effects novel addition of an in-situ tool to measure the distortion of of mold surface conditions on the heat-transfer rate and atten- the shell from the mold wall. This was done to determine dant surface quality of the cast product. These dip simula- the dynamics of air gap formation and the resulting effect tors involved chilled plates that were immersed into a molten on the steel-mold heat transfer. Again, these were incom- metal bath without any of the sophistication of continuous plete simulators of the continuous casting process. caster systems, such as oscillation and shell extraction. The To incorporate further sophistication into the experiment, dip simulators are very useful for determining fundamental 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 A. BADRI is with Shell Oil, Malaysia. T.T. NATARAJAN, Senior shell to simulate continuous casting. This type of mold sim- [7] Research Engineer, C.C. SNYDER, Senior Technician, and K.D. POWERS, ulator is quite versatile and has been used by Saucedo to Project Analyst, are with the U.S. Steel Research and Technology Center, investigate the initial solidification phenomena. The simu- Monroeville, PA 15140. F.J. MANNION, General Manager, is with U.S. lated castings exhibited the typical surface morphologies of Steel, Slovakia. A.W. CRAMB is with the Department of Metallurgical and Materials Engineering, Carnegie Mellon University, Pittsburgh, PA industrial cast slabs, and the results were used to propose a 15213. Contact e-mail: [email protected] mechanism of oscillation mark formation. The work also Manuscript submitted February 4, 2004. included a comprehensive survey of the various hypotheses

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, JUNE 2005—355 proposed in the literature for oscillation mark formation. fluxes related to initial solidification, and surface profiles. Suzuki et al.[8] designed simulation experiments on shell for- Using these experimental data, the validity of hypotheses of mation and mold flux consumption and also presented find- oscillation mark formation can be tested. The simulation ings on the formation mechanism of oscillation marks. These capability of the mold simulator itself was verified by obtain- simulators simulated the dynamic nature of continuous casting solidified shells of different grades of steel under con- ing but were not true simulators of the heat-transfer condi- ditions that would be commonly seen in industrial operations. tions that could be found in the steel plant. The surface quality of the simulated shells was then com- The next step in complexity of mold simulators was to pared against that of industrial slabs to ensure reproducibility. build a scale model of an actual continuous casting machine, It is shown that the mold simulator does indeed replicate the with the liquid contained in the mold. These mold simula- surface features seen in a slab cast under industrial conditions. tors 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 II. EXPERIMENTAL TECHNIQUE any of the previous simulators or even on an actual caster. One of the earliest reported experiments was that of Savage Figure 1 is a schematic of the molten steel in a continu- and Pritchard,[9] who built a mold to investigate billet rup- ous casting mold. The mold is cooled by water flowing ture during continuous casting. Singh and Blazek[10] con- through the grooves and acts as a heat sink. The steel solid- structed a similar model to study the effects of heat transfer ifies against the copper mold and increases in thickness as and shell formation on surface rippling in low-carbon steels. it moves down the length of the mold. The steel shell is Building further on this idea, and to determine the various about 12 mm in thickness when it exits the mold. The mold factors affecting mold heat transfer, Blazek et al.[11] built a is oscillated to prevent the sticking of the steel shell to the simulator that had mold plates modified to allow for varia- copper mold, and this oscillation promotes the infiltration tions in the water cooling flow configuration. To investigate of a film of liquid flux between the shell and the mold. Fur- the effects of high oscillation frequencies on oscillation marks thermore, the liquid mold flux on top of the liquid steel solid- and mold flux consumption, Yasunaka et al.[12] constructed ifies where it contacts the copper mold and gradually builds a simulator with a modified oscillation drive that allowed up a flux/slag rim. It has been theorized that the oscillation the mold to be oscillated at frequencies up to 50 Hz. To of the copper mold is responsible for the formation of oscil- illustrate the importance of using a simulator instead of an lation marks.[3] actual casting machine, the authors found that a danger with A mold simulator provides an ideal laboratory system for high-frequency oscillation is that there exists the possibil- the study of initial solidification of steel in a continuous ity that resonance can occur, leading to catastrophic failure casting mold. The depth and width of the oscillation marks of the machine. can be easily modified by changing the mold oscillation The state of the liquid steel meniscus is often considered cycle, oscillation stroke, and casting speed. In addition, the to be important in the formation of oscillation marks, but it effects of different mold fluxes on the initial heat transfer is exceedingly difficult and dangerous to attempt to observe can be studied easily without interrupting the normal pro- the liquid steel meniscus directly in an industrial caster. To duction operations at the plant. 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 Fig. 1—Schematic sketch of liquid steel in a continuous caster mold.

356—VOLUME 36B, JUNE 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B Mold Simulator The cooling water is fed into the mold from the cooling The mold simulator developed in this study is an inverse- water manifold, as shown in Figure 2. type mold, where the steel solidifies around the mold, instead In order to simulate continuous casting, the mold assem- of the mold surrounding the solidifying steel. Figure 2 is a bly is fitted with an extraction mechanism, which is fabri- schematic sketch of the mold simulator stage, which con- cated from 6.25-mm-thick steel plates. The extractor pulls sists of several distinct modules to simulate the casting the solidifying steel shell in the casting direction (down- process. wards). This exposes liquid steel to the water-cooled cop- The different physical modules of the simulator include per mold at the meniscus and allows the formation of a new the mold assembly, the extraction mechanism, the stabi- steel shell. The extractor is designed so that only one face lization 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.

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Fig. 2—Schematic sketch of the mold simulator stage. Fig. 3—The copper mold assembly and extractor mechanism.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, JUNE 2005—357 Fig. 4—Steps in building the mold assembly. of the mold is exposed to the liquid steel, as seen in Fig- The mold is connected to an oscillating stage so that the ure 3. This allows a controlled exposure of the mold hot mold oscillates sinusoidally in the vertical direction about face to the liquid steel while protecting the other faces of the meniscus position. A slotted cam is used to convert the the mold. rotational motion of the motor into a linear vertical sinu- The process of solidification and extraction of the steel soidal oscillation of the rectangular mold. The extractor is shell displaces some liquid steel, and so the stabilization sys- attached to drive shafts powered by stepper motors. During tem moves the main stage upward with time to maintain the the experiment, the drive shafts push the steel shell down liquid steel meniscus at a constant level (about 150 mm from with respect to the meniscus and expose steel to the copper the bottom) with respect to the copper mold. All of the mold to allow the formation of “new” steel shell. The mold sensor systems, data and control cables, and drive systems motion is independent of the extractor motion. This allows are protected from the steel bath by a heat shield. the incorporation of negative strip time, which can be defined

358—VOLUME 36B, JUNE 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B Fig. 6—Physical description of the problem domain.

niques to be fully developed before application on the mold simulator. Based on the preceding heat-transfer studies, the mold Fig. 5—Dimensions of mold copper plate with thermocouple locations was instrumented with 12 grounded T-type thermocouples (dimensions in millimeters). Note: The oval shown at the right is not exactly at various elevations to detect casting events on the hot face the same as that shown at the left. 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 as the portion of the mold oscillation cycle during which the meniscus location of 150 mm. Figure 5 shows the physical mold moves downward faster than the shell. The translat- locations of the thermocouples with respect to the menis- ing stage is controlled so that the meniscus remains about cus. These thermocouples are located 133, 140, 146, 152, 150 mm from the bottom of the mold during an experiment. 159, and 165 mm from the bottom of the mold. The molds In order to characterize the heat transfer, the mold was were machined with 1.59-mm-diameter 50-mm-deep ther- instrumented with thermocouples to observe the transient mocouple voids with an orientation parallel to the mold sur- variation of the temperature and heat flux during the cast- face to install the thermocouples. The dual-lead T-type ing period. The instrumentation of the mold with thermo- thermocouples used in the experiments have magnesium couples was preceded by a comprehensive study[15] of oxide insulation and a stainless steel sheath with an outer temperature measurements in conducting solids. For exam- diameter of 0.5 mm. The thermocouples were fitted with ple, it is known that when a temperature sensor is inserted metal collars to ensure a good fit in the void so that the posi- into a conducting solid, the void created for the sensor and the tions of the tips were well defined, and the tips were cov- sensor material itself can introduce errors into the measured ered with a heat sink compound to enhance heat transfer to temperature signal. These errors were studied to determine the thermocouple tip. In addition to the thermocouples, lin- how best to install the sensors in the mold. Additionally, the ear velocity displacement transducers (LVDTs) were used response of the material, as deduced from the temperature to monitor the motions of the mold and the extractor mech- sensors, was studied under conditions of transient high ther- anism. By attaching both LVDTs to the same reference point, mal fluxes to determine the ability of the sensor to dis- motions of the extractor and mold were measured relative criminate between different functional forms of surface heat to the same frame of reference. The instrumentation of the flux variation. A heat-transfer simulator was also used to mold resulted in a complete characterization of heat-transfer confirm the ability of subsurface thermocouples to measure phenomena with respect to the motion of the mold. The small variations in temperature due to oscillations in the sur- National Instruments Labview software was used to acquire face heat flux. The resulting conceptual models assisted in temperature data at 60 Hz for the duration of the experiment the development of a greater understanding of meniscus heat to detect phenomena occurring within individual oscilla- transfer, which finally led to the ability to interpret the heat- tion cycles. transfer data obtained from the mold simulator experiments. The temperature data acquired from the mold thermo- A heat flux simulator was built that allowed heat fluxes couples were used to develop an estimate of the heat flux of up to 1 MW/m2 to be applied to a copper mold. A variety through the mold during the initial solidification of the steel of thermocouple designs were modeled to determine the opti- shell using the one-dimensional inverse heat conduction pro- mum method to allow transient heat fluxes to be measured gram developed by Beck.[16] A typical form of the physical accurately. This study allowed such issues as hole size, posi- problem which Beck’s method is designed to solve is shown tioning, thermocouple attachment, and data acquisition tech- in Figure 6. It is sufficient to use only one internal body

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, JUNE 2005—359 temperature and one boundary condition to determine the thermocouple 5.0 mm away from the heated surface was unknown boundary condition. used to define the subdomain. In the mold simulator experiments, the known boundary Before using this program and the domain decomposition condition is the convective cooling of the mold by water approach to calculate heat fluxes from experimental data, flowing through cooling channels. While there are correla- the reliability of the program was tested rigorously using tions for determining the heat-transfer coefficient for the data from simulations. This was done to ensure that the pro- cooling channels, another thermocouple was introduced into gram would calculate results that were accurate and precise the domain to provide a well-defined boundary condition. for the waveforms of interest in this work, and to examine The domain of the problem shown in Figure 7 can be decom- the effects of the known boundary conditions and signal posed into two domains by creating an interface S at the sec- noise. For more details, interested readers can refer to the ond thermocouple. The boundary condition at this point work of Badri.[15] 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 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 (Fig- ures 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 Fig. 7—Problem domain including two thermocouples. initial steel shell during extraction. In the casting phase (Fig- ure 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 Fig. 8—Domain decomposition yielding a known value boundary condition. mold, using a contact profilometer.

360—VOLUME 36B, JUNE 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B (a) (b)

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Fig. 9—Digital images showing the progress of the experiment.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, JUNE 2005—361 (g) (h)

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Fig. 9—(Continued). Digital images showing the progress of the experiment.

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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 ate the differences between different grades of steel. Fur- thermore, the shells obtained from mold simulator runs are Using the mold simulator, experimental runs were con- compared with industrial samples. This is an important step ducted for several grades of steel with most of the runs because it reveals whether the cast shells are in fact repre- focused on ultra-low carbon steel. Table I summarizes the sentative of industrial cast slabs. The surface profile of the typical chemical composition for the experimental runs, while solidified steel shell can be analyzed in conjunction with the Table II summarizes the operating parameters. Some of the measured temperatures to obtain insight into the solidifica- typical information that can be obtained during the opera- tion history of the shell surface. Such an analysis is the main tion of the mold simulator includes temperature history, the topic of a subsequent article.[17] associated heat flux at the hot face of the mold, the surface Typical temperature traces, as recorded by thermocouples profile of the cast shell measured using a contact pro- just above and below the meniscus, are shown in Figure 13. filometer, and flux film characteristics. The thermocouples below the meniscus measure higher tem- The surface profiles of ultra-low, peritectic, and medium peratures because of the direct contact of the mold surface (hyperperitectic) carbon steels are shown below to enumer- 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 ultra- low carbon grade of steel. The labels 5.25F, 5.25B, etc. refer Fig. 11—Schematic sketch of an expected steel shell from a mold simu- to the locations of thermocouples. The numeric value denotes lator run. 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

Ultra-Low Peritectic Medium Element Carbon Steel Steel Carbon Steel Pct carbon 0.0046 0.065 0.175 Pct manganese 0.46 0.95 1.17 Pct silicon 0.11 0.22 0.31 Pct sulfur 0.0089 0.0075 0.027 Pct nitrogen 0.0057 0.0047 0.0069

Table II. Operating Parameters

Stroke (mm) 6.3 Fig. 15—Temperatures measured by thermocouples at the meniscus dur- Oscillation frequency (Hz) 1.3 ing solidification of an ultra-low carbon steel grade. Casting/extraction speed (mm/s) 12.7

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. 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 Fig. 14—Mold thermocouple data during the casting stage of an ultra-low the meniscus area can be measured and that, even in the carbon steel grade. one-dimensional solution, one can adequately resolve transient behavior in the heat flux during the oscillation cycle. in inches, while the character (F or B, meaning front or back) In this study, unfiltered and unaltered heat flux data calcu- refers to the distance of the thermocouple tip from the hot lated directly from thermal measurement are shown. It is face. The F refers to thermocouple tips located about 1.5 mm felt that other methods of calculation of heat flux would only from the hot face, while B refers to the thermocouple tips change the numbers and not the variation of the heat flux located 5.0 mm from the hot face. It can be seen that the values as a function of oscillation cycle. individual oscillations in temperature due to the oscillation An image of the surface of the ultra-low carbon steel shell of the mold relative to the meniscus have been resolved. The and the associated contact profile measurement are shown in trials indicate that any phenomenon causing a temperature Figure 17. From the surface profile measurement, it can be

364—VOLUME 36B, JUNE 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B Fig. 18—Mold thermocouple data during casting stage of a peritectic steel grade.

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Fig. 19—Temperatures measured by thermocouples at the meniscus during solidification of a peritectic steel grade.

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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 Fig. 20—The heat flux calculated using the temperature data from the ther- mark, there is an irregularity referred to here as a subpeak. mocouples at the meniscus during solidification of a peritectic steel grade. Figures 18 through 20 show the temperature and heat flux graphs for the peritectic grade of steel. Figure 21 is a con- shaped features between oscillation marks and that the shape tact profile measurement of the surface of the steel shell cast of the oscillation marks is sharply defined. by the mold simulator. The surface profile measurement Figures 22 through 24 show the variation of temperature indicates that this particular grade of steel has several plateau- and heat flux values during the course of an experiment for

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, JUNE 2005—365 a medium carbon (hyperperitectic) grade of steel. Finally, an average baseline component and a time-varying compo- Figure 25 shows the measured surface profile of a medium nent. The time-varying component of the heat flux has a carbon steel shell from the mold simulator experiment. This magnitude approximately 10 pct of the average baseline 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

Fig. 22—Mold thermocouple data during the casting stage of a medium carbon steel grade.

Fig. 23—Temperatures measured by thermocouples at the meniscus dur- (a) ing solidification of a medium carbon steel grade.

(b) Fig. 24—Heat flux calculated using the temperature data from the ther- Fig. 21—(a) Photograph and (b) measured profile of shell surface for a mocouples at the meniscus during solidification of a medium carbon steel peritectic steel grade. grade.

366—VOLUME 36B, JUNE 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B Fig. 26—Morphologies of oscillation marks from three types of steel grades.

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 nonuni- formity of shell thickness caused by volume changes accom- panying 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 (a) 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 consecu- tive 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 Man- nion,[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 the- (b) oretical calculated value is expected, because the spacing of oscillation marks does not depend uniquely on the cast- Fig. 25—(a) Photograph and (b) measured profile of shell surface for a ing speed and oscillation frequency, but is actually deter- medium carbon steel grade. mined by the relative velocity between the shell and the meniscus. Since the meniscus level is not absolutely con- component. The average heat flux for the peritectic grade stant, but varies slightly with time about the mean position, is less than that for the ultra-low carbon and medium there is a spread in the measured values. The simulation carbon grades. The heat flux data indicate that it is possi- quality of the mold simulator is again confirmed by the fact

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, JUNE 2005—367 (a)

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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 analyze the solidification and heat-transfer phenomena in to that found in the industrial measurements reported by the industrial mold. Cramb and Mannion. In addition to the pitch of the oscillation marks, the depths were also measured. The distribution of the measured depths V. SUMMARY is shown in Figure 31, and also appears to conform to a An apparatus was successfully designed and constructed Gaussian distribution. The average depth of the oscillation to simulate the mold of a continuous casting machine. The marks is about 275 m, but there were a few instances in main features of the mold simulator the following: which the depth was much larger, up to 700 m. The quality of the measured distribution would increase with an (1) a copper mold designed using actual mold plates; increase in sample size. The distribution of the depths is (2) an extracting mechanism, which allows continuous casting; comparable to that measured by Cramb and Mannion, as can (3) a translating stage to maintain the meniscus at a con- be seen in Figure 31. stant level on the mold; The experimental data show that the measured charac- (4) a sinusoidal oscillating drive that allows the mold to teristics of the oscillation marks agree well with industrial oscillate independently of the shell; and slab profiles and the reported measurements of Cramb and (5) sensors that measure in-mold temperatures, bath tem- Mannion, again showing that the mold simulator does indeed perature, steel shell displacement, and mold displacement. simulate the conditions of industrial continuous casting machines. This is in addition to the comparison between the The temperature data obtained were converted to heat flux surface profiles of mold simulator shells and industrial slab values using the one-dimensional heat conduction program surfaces, which confirmed the ability of the mold simulator developed by Beck. to reproduce the surface features of industrially cast slabs. The mold simulator was used to obtain steel shells of vary- This confirmation justifies the use of the mold simulator to ing composition in order to confirm that it could indeed act

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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 the mold simulator is typical of what is seen on the narrow profile was measured using a contact profilometer. It was face of a slab. This allows valid conclusions to be deduced determined that mold simulator shells exhibited surface fea- from results of the mold simulator experiments. Last but not tures similar to those of industrially cast slabs. It was found least, the mold flux film between the mold and the shell can that the features on the surfaces of the cast slabs varied with be retrieved intact after an experiment, something that has steel composition, and that the changes in these features were not been accomplished in earlier studies. 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, ACKNOWLEDGMENTS and can therefore be used to conduct experiments reflective of conditions in an industrial caster. Furthermore, the distri- The authors thank the United States Steel Corporation and bution of depth and pitch measurements of oscillation marks the former Bethlehem Steel Corporation (now part of ISG) on mold simulator shells compared favorably with measure- for their financial support of this project. Additionally, we ments by Cramb and Mannion of the oscillation marks on the thank G. Biddle, J. Sadecky, and R.C. Evans for their assis- narrow faces of slabs. Therefore, the mold simulator can be tance with apparatus design and construction. In addition, used to investigate phenomena affecting the surface quality the authors deeply appreciate the assistance of Falcon of cast shells and also the castability of various steel grades. Foundries in welding the copper mold plates. The unique feature of this mold simulator is the instru- The material in this paper is intended for general infor- mentation of the apparatus, which permits the resolution of mation only. Any use of this material in relation to any spe- the temperature and heat flux variations within the period cific application should be based on independent examination of a single oscillation cycle. Furthermore, the shell cast by and verification of its unrestricted availability for such use,

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, JUNE 2005—369 (a)

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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 ultra- Fig. 31—Distribution of oscillation mark depth measurements for an ultra- low carbon steel grade. low carbon steel grade. and a determination of suitability for the application by pro- is implied by the publication of this paper. Those making fessionally qualified personnel. No license under any United use of or relying upon the material assume all risks and lia- States Steel Corporation patents or other proprietary interest bility arising from such use or reliance.

370—VOLUME 36B, JUNE 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B REFERENCES 9. J. Savage and W.H. Pritchard: J. Iron Steel Inst. London, 1954, vol. 178, pp. 269-77. 1. N.C. Machingawuta, S. Bagha, and P. Grieveson: Steelmaking Conf. 10. S.N. Singh and K.E. Blazek: J. Met., 1974, vol. 26 (10), pp. 17-27. Proc., ISS-AIME, Warrendale, PA, 1991, vol. 74, pp. 163-70. 11. K.E. Blazek, I.G. Saucedo, and H.T. Tsai: Steelmaking Conf. Proc., 2. D. Bouchard, F.G. Hamel, J.P. Nadeau, S. Bellemare, F. Dreneau, D.A. 1988, vol. 71, ISS-AIME, Warrendale, PA, pp. 411-21. Tremblay, and D. Simard: Metall. Mater. Trans. B, 2001, vol. 32B, 12. H. Yasunaka, T. Mori, H. Nakata, F. Kamei, and S. Hanada: Steelmaking pp. 111-18. Conf. Proc., ISS-AIME, Warrendale, PA, 1986, vol. 69, pp. 497-502. 3. H. Tomono, P. Ackermann, W. Kurz, and W. Heinemann: Solidifica- 13. A. Matsushita, K. Isogami, M. Temma, T. Ninomiya, and K. Tsutsumi: tion Technology in the Foundry and Cast House, The Metals Society, Trans. Iron Steel Inst. Jpn., 1988, vol. 28 (7), pp. 531-34. Warwick, 1983, Book 273, pp. 524-31. 14. S. Itoyama, H. Tozawa, T. Mochida, K. Kurokawa, T. Matsukawa, and 4. P.J. Wray: Metall. Trans. B, 1981, vol. 12B, pp. 167-76. K. Sorimachi: Iron Steel. Inst. Jpn. Int., 1998, vol. 38 (5), pp. 461-68. 5. D.K. Stemple, E.N. Zulueta, and M.C. Flemings: Metall. Trans. B, 15. A.B. Badri: Ph.D. Dissertation, Carnegie Mellon University, Pittsburgh, 1982, vol. 13B, pp. 503-09. PA, 2003. 6. Y. Nishida, W. Droste, and S. Engler: Metall. Trans. B, 1986, vol. 17B, 16. J.V. Beck: “IHCP1D: A Program for Calculating Surface Heat Fluxes pp. 833-44. from Transient Temperatures Inside Solids,” Version 5.31, Beck Engi- 7. I.G. Saucedo: Continuous Casting, vol. 9, Initial Solidification and neering Consultants Company, Houston, TX, 1997. Strand Surface Quality of Peritectic Steels, ISS-AIME, Warrendale, 17. A.B. Badri, T.T. Natarajan, K.D. Powers, C.C. Snyder, F.J. Mannion, PA, 1997, pp. 131-41. M. Byrne, and A.W. Cramb: Metall. Mater. Trans. B, 2005, vol. 36B, 8. M. Suzuki, H. Mizukami, T. Kitagawa, H. Kawakami, S. Uchida, pp. 373-83. and Y. Komatsu: Iron Steel Inst. Jpn. Int., 1991, vol. 31 (3), 18. A.W. Cramb and F.J. Mannion: Steelmaking Conf. Proc., ISS-AIME, pp. 254-61. Warrendale, PA, 1985, vol. 68, pp. 349-59.

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