document20

Copyright © 2003, The AISE Steel Foundation, Pittsburgh, PA. All rights reserved. 1

20.1 IntroductionInstruments have been used on casters since the early days of continuous casting and, as is shownin Fig. 20.1, they can be found on every major component of a caster between the turret, or ladlecar, and the run-out. The main functions of caster instruments are to:

• Measure the parameters that are utilized for controlling the performance ofmechanical and metallurgical functions of casting.

• Assign a quality rating for each cast section.

• Diagnose operating and machine problems.

Chapter 20

Instrumentation

Mustafa R. Ozgu, Senior Research Consultant, Bethlehem Steel Corp.

ladleweight- slag carry over

tundish- weight- steel temperature- steel depth

mold- copper temperature- steel level- oscillation- slab-mold friction- slag cover depth- in-gap slag thickness- wall deformation- steel flow velocity- water ∆T and flow rate

containment- strand surface temperature- strand shape and bulging- final solidification point and segment/roll loads- roll bending and temperature- roll gap, allignment and rotation- cast speed and strand tracking

run-out- length, width and weight- hot surface quality

Fig. 21.1 Parameters measured on casters.

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2 Copyright © 2003, The AISE Steel Foundation, Pittsburgh, PA. All rights reserved.

• Develop knowledge that correlates product quality and productivity to caster designand operation.

The number and sophistication of instruments used on casters has been growing rapidly.1–3 Themain reasons for the rapid growth are the ever-increasing demands for higher productivity and as-cast product quality, and the availability of the modern on-line digital computer. This is particu-larly true for slab casters, where quality and productivity demands are the most stringent. Early on,emphasis was placed on mold instrumentation because mold practices and parameters have themost impact on product quality and productivity. However, lately, significant progress has beenmade in developing and applying instrumentation on the ladle, tundish, containment and runout.The text of this chapter is not meant to be an authoritative treatise on the history, theory and devel-opment, but rather a review of instruments used on casters. Sample measurement data and analy-ses results from several applications are reviewed as well.

20.2 LadleIn a continuous casting operation, the ladle is used to transfer liquid steel from the steelmakingshop to the caster with aim chemistry, temperature, cleanliness and slag cover. At the caster, steelfrom the ladle is teemed into the tundish through a nozzle in the bottom of the ladle. It is desirableto monitor three parameters on the ladle as a function of time to properly control the teeming oper-ation. The first is the weight of steel in the ladle, which is routinely measured with load cells onthe turret arms or the ladle car. The second is the temperature of steel flowing into the tundish.Direct measurement of steel temperature in the ladle with instrumentation on the ladle itself is notpractical, because the ladle is a mobile unit that moves through harsh environments and thus is notconducive to temperature instrumentation. Instead, ladle temperatures are inferred from tundishtemperatures, which are continuously or intermittently measured. The third is the slag carryoverfrom the ladle to the tundish. As the end of the ladle drainage is approached, vortex formationabove the ladle nozzle and slag carryover into the tundish can occur. The mixing of slag with steelcan contaminate the steel in the tundish and cause an increase in the inclusion content of the castproduct. It is possible to minimize the transfer of slag from the ladle to the tundish using one ofthe following slag detection methods:

• Visual observation.

• Tare weight monitoring.

• Rate of teeming change.4,5

• Opto-electronic monitoring of stream surface.6

• Vibration analysis of the ladle shroud.7,8

• Electromagnetic methods.9–12

The ability to visually detect slag carryover into the tundish is highly dependent on operator expe-rience. This can lead to the carryover of varying amounts of slag before detection and correctiveaction. Tare weight monitoring relies on correct ladle, ladle cover and slag weight estimates and isthus inherently inaccurate. The rate of teeming change monitors the variation in the rate of ladleweight change. A change from metal teeming to slag and metal teeming can be indicated by a rapiddecrease in the teeming rate. The method relies on the accuracy of the load cells and can thus leadto errors at low teeming rates. It can also result in false alarms if the teeming rate is affected by themovements of the ladle flow control device or by the ladle. The opto-electronic method cannotobserve the core of the stream through which large amounts of slag can be carried over. The vibra-tion analysis method is based on vibration monitoring of the ladle shroud, using an accelerometerattached to the shroud manipulator arm. As is shown in Fig. 20.2, the amplitude of processed sig-nal from the accelerometer increases with the onset of entrained slag flow through the shroud. A

Instrumentation


system incorporating slag alarm threshold points can be set up to activate an alarm and shut theflow off when the signal exceeds the threshold. Although the system is sensitive to environmentalnoise and hence is prone to inaccuracies, it can be used to reduce ladle slag carryover to thetundish. Because the accelerometer stays on the manipulator arm and can be used on the incomingladle, the vibration analysis technique may appear attractive and preferable to other techniques thatrequire sensor installation on each ladle.

The electromagnetic method is the most widely used ladle slag flow detection method. It employsa transmitter and a receiver coil located around the exit nozzle in the ladle bottom, a pre-amplifierand a control unit. Two types of devices are used. In the first, the transmitter and receiver coils areopposite each other. In the second, the transmitter and receiver coils are concentrically oriented inthe same housing. The sensor induces an electromagnetic field in the stream flowing through theexit nozzle and measures the resultant eddy currents. Because slag has a significantly lower elec-trical conductivity than liquid steel, the eddy currents induced in the slag are smaller than thoseinduced in the steel. Hence, a transition from metal to slag and metal flow results in a rapid changein the output signal and the initiation of an alarm. Fig. 20.3 shows typical ladle weight and elec-tromagnetic slag sensor output signals from a slab caster installation where the slide gate shutoffis manually activated upon the initiation of the alarm.10 As a result of the use of electromagneticsensors, yield improvements ranging between 0.5 and 1.5%, and significant reduction in down-graded slabs attributable to ladle-to-tundish slag carryover have been reported by several steelplants.

Other ladle sensors that are under development include an ultrasonic probe for the early detectionof vortex formation13 and a microwave-driven slag thickness device.14

20.3 TundishThe tundish is an intermediate vessel between the ladle and the mold and is used mainly to per-form the following functions:

• Deliver liquid steel to the mold(s) at a controlled rate.

• Maintain a steady supply of liquid steel to the mold(s) during a ladle change.

• Remove nonmetallic inclusions from liquid steel before delivery to the mold(s).

• Facilitate the control of steel superheat by means of plasma preheaters.

Time (seconds)

Am

plitu

de in

dex

25 50 75 100 125 150

1.0

0.5

0

slag detectedwith accelerometer

slag visuallydetected by operator

Fig. 20.2 Typical processed accelerometer signal obtained during ladle teeming. From Ref. 8.

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In order to properly perform the above functions and control the caster operation, it is essential toknow the temperature and depth of liquid steel in the tundish. Temperature can be measured in twoways. The first is the manual immersion of disposable thermocouples. This method is reliable andis used in many caster installations. Its main drawback is that, if not exercised at sufficient fre-quency, rapid temperature changes can go unnoticed. Unexpected increases in steel temperaturecan result in breakouts, and rapid temperature losses can lead to freeze-off and cast terminations.Because of the push toward stable caster operation and reduced labor costs, continuous tempera-ture measurement systems have been developed and are in use at several caster installations.12,15–17

The sensor used for continuous temperature measurement is comprised of a Pt-Pt/Rh thermocou-ple embedded in a refractory tube, and it is immersed into the steel bath through an opening in thetundish cover. Fig. 20.4 shows continuous temperature measurement from such a sensor, alongwith tundish weight and cast speed or throughput.16 The effect of tundish level (or weight) ontundish temperature profile can be clearly seen by examining the difference in profiles duringtundish tube changes E1 and E2. When the tundish weight drops to 41,000 kg, about 25% of thetundish wall hot surface is exposed to the atmosphere. Consequently, significant heat loss and thustemperature drop occur from the tundish walls, which must be recovered upon refill. The extent ofthe temperature loss depends on the flow rate from the tundish, the length of time during which thetundish walls are exposed, and the wall area that is exposed to the atmosphere. Such temperaturelosses during ladle, mold width and tundish tube changes can be reduced through the continuousmeasurement of tundish temperature. Other potential benefits of continuous tundish temperaturemeasurement are caster automation and accurate correlation of steel temperature to as-cast prod-uct quality.

The depth of liquid steel in the tundish is either inferred from weight measurements with load cellson the tundish car or directly measured with electromagnetic sensors installed in the tundish bot-tom or the vertical walls. Knowledge of liquid steel depth from these measurements is used to:

• Safely fill the tundish during the start of a cast.

• Maintain a stable tundish level during steady-state casting.

• Assure a minimum tundish level during ladle changes to avoid downgrades.

• Safely drain the tundish to a very low level during grade changes, tundish changes(tundish flys) and cast terminations, without allowing any slag to flow into themold.

2 sec

ladle slidegate position (%)

ladle weight (tons)

electromagnetic slag signal (%)

alarm

100

80

60

40

20

0

Time

Fig. 20.3 Ladle slag carryover, weight and slidegate signals. From Ref. 10.

Instrumentation


Most of the caster operations use weight monitoring with load cells for bath height control becauseload cells are easy to install and maintain on the tundish car. Furthermore, load cells stay on thetundish car and can be used on the incoming tundish. The major disadvantage of weight monitor-ing is that it relies on correct estimates of tundish, tundish cover and slag weights, and is thus inher-ently inaccurate. The inaccuracies are particularly worrisome during draindowns, because they canresult in slag drainage into the mold or excessive residual steel in the tundish. Slag in the mold cancause breakouts. An excessive amount of residual steel in the tundish compromises yield during atundish fly or increases the length of the mixed-grade zone during a grade change. For improveddraindown control, some caster operations use a bobber or a ceramic ball in the tundish in con-junction with the weight measurement system. The bobber or ceramic ball is positioned over thetundish nozzle. The bobber is used to indicate bath level. The ceramic ball is used to prevent vor-texing and slag carryover into the mold during the draining of the tundish.18

Some caster operations use electromagnetic sensors in the bottom of the tundish for direct mea-surement of steel level.19,20 The sensor assembly consists of a primary and a secondary coil that areconcentrically arranged in stainless steel housing. As Fig. 20.5 shows, the housing is installedbetween the refractory lining and the tundish bottom shell. An alternating current applied to the pri-mary coil generates an electromagnetic field in the liquid steel. The electromagnetic field, in turn,generates eddy currents that attenuate the primary field in proportion to the steel level. By thismeans, the height of the slag/metal interface above the refractory bottom in the vicinity of the sen-sor can be measured very accurately in the range of 0–200 mm. The thickness of the refractory lin-ing is compensated for. The electromagnetic sensor, in conjunction with an optimized tundishbottom design and an automatic drain control system, enables the operators to consistently drain thetundish to very low levels without allowing slag to flow into the mold. Other significant benefits of

Tem

pera

tue

(˚C

)C

ast s

peed

(m

/min

)

Time (hours)

1566

1552

1538

1.18

0.64

0

55,000

41,000

27,000

0 2 4 6 8

Tundish weight (kg)

(a)(a)

(a) (a) (a)

(h)

(h)

(e1) (e2)

(g)

(f)

weight

Temperature

Cast speed

2810 kg/min(6.2 klds/min)

(a) - ladle change(e) - tundish tube change(f ) - mold width change

(g) - machine slowdown for late ladle connection(h) - reduced tundish level prior to tundish tube change

Fig. 20.4 Tundish tempera-ture, weight and cast speedvariations. From Ref. 16.

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low tundish levels during transitions are increased yield during tundish exchanges and cast termi-nations, reduced mixed-grade-zone length during grade changes, and improved tundish life due tothe ability to cast longer sequences with the same tundish.

The electromagnetic sensors installed in the tundish bottom have a limited measurement range of0–200 mm and thus cannot be used to measure and control tundish level during steady-state cast-ing and ladle exchanges. Instead, the electromagnetic sensors can be installed on the sidewalls ofthe tundish, as depicted in Fig. 20.6.21,22 The transmitter and receiver coils can be installed onopposite walls, on adjacent wide and narrow walls, or side by side on the same wall. A tundish levelmeasurement and control system incorporating electromagnetic sensors on the tundish bottom andvertical sidewalls may be preferable to the weight measurement system in some caster installa-tions.

Instruments such as displacement transducers, flow meters and pressure gauges are routinely usedon the tundish to measure and control slidegate or stopper rod position, argon flows to the flowcontrol system, and submerged tundish entry nozzle depth.

1. safety lining2. spray lining3. impact pad4. electromagnetic level sensor

SEN SEN

34

1

2

Fig. 20.5 Installation of electromagnetic level sensor in tundish bottom. From Refs. 19 and 21.

levelsensorloop

levelsensorloop

SEN

flexiblecable

preampelectronicunit andsoftware

maintransformer

incomingpower

instrument panel

controlpanel

notebookcomputer

Fig. 20.6 Electromagnetic level sensors in tundish vertical walls. From Ref. 20.

Instrumentation


20.4 Mold

20.4.1 IntroductionThe mold is the most complex and critical component of a continuous caster. The functions of themold are to:

• Act as a substrate for the formation of a thin, hot and crack-free solid shell.

• Form the shape of the final product.

• Extract heat from the strand at very high rates.

• Facilitate the separation of nonmetallics from the solidifying shell.

• Allow adequate production rates without breakouts.

Since the early days of continuous casting, extensive studies have been conducted by numerousresearchers to correlate the functions of the mold to its design and operation. As a result of thesestudies, both the design and operation of the conventional caster mold have been improved to thepoint where almost all steel grades can now be continuously cast with the desired surface qualityand at high productivity rates. Significant improvements have also been made and continue to bemade in the design and operation of the medium-thickness and thin-slab caster molds. The devel-opment and use of sensors have played a crucial role in the improvements made in mold designand operation. Because of the criticality of the mold and the relative ease with which sensors canbe installed on it, a large number of mold parameters are now monitored for control and automa-tion, on-line quality prediction and analysis in conventional as well as in medium-thickness andthin-slab casting. The following sections describe the mold parameters that are monitored and thevarious sensors used for monitoring.

20.4.2 Copper TemperaturesFrom the early days of continuous casting, numerous researchers have measured the copper tem-peratures on caster molds.23–41 The objectives of the measurements varied, but in general they wereto:

• Evaluate mold powders and practices.

• Analyze mold heat transfer and strand solidification.

• Evaluate mold design.

• Predict mold/strand sticking.

• Measure liquid steel pool level.

In all cases, thermocouples were used for copper temperature measurements, because thermocou-ples are inexpensive and easy to install, and their signals are easy to interpret. The thermal behav-ior of all four walls between the top and bottom of a mold can be analyzed relatively easily bylacing the copper plates with thermocouples. There are two methods of installing thermocouplesin mold coppers. In the first and most commonly used method, the thermocouples are introducedfrom the backside of the copper plates parallel to the path of heat flow (perpendicular to the hotface). The advantage of this method is the ease of thermocouple installation. The disadvantage isthat, because the thermocouple holes are parallel to the path of heat flow, they affect the flow ofheat and thus can result in erroneous temperature measurements. This method is suitable for rou-tine relative temperature monitoring whereby the measurements are used for sticker breakout pre-dictions, mold level measurement and other automation purposes. In the second method, thethermocouples are introduced from the top or bottom of the copper plates and perpendicular to the

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path of heat flow (parallel to the hot face).30,34,35,38 The method is not suitable for routine installa-tion of thermocouples because of the cost associated with the drilling of the thermocouple holesand the difficulties of handling thermocouple lead wires. However, it is the preferred method onmolds that are specially instrumented for thermal analysis and evaluations because the disturbancecaused by the thermocouples on heat flow—and thus absolute temperature measurement errors—are small.

Thermocouples may be of the sheathed or intrinsic type. In sheathed thermocouples, at the mea-surement point a junction (“bead”) is formed between two wires of dissimilar metals, and the entirearrangement is encased in a protective sheath. With intrinsic thermocouples, the mold copper itselfis used as one of the thermocouple metal components, while a second dissimilar metal, such asconstantan, usually in the form of an electrically insulated rod, is welded or threaded to the desiredmeasurement point in the mold copper. Sheathed thermocouples are more expensive and difficultto install than intrinsic thermocouples. However, they are more accurate than intrinsic thermocou-ples. Intrinsic thermocouples may be subject to electrical ground loop problems because of the pos-sibility of variable electrical resistance paths between the mold coppers and the system groundpotential of the measuring device. Response time of either type of thermocouple is determinedmostly by the effective size of the “bead.” For general use, such as temperature, heat flux andsticker breakout prevention monitoring, response times of the order of one second are adequate.

Two different instrumentation and analysis methods are used to determine the heat flux and thetemperature variation along the heat path between the mold hot face and the cooling channels. Inthe first method, two thermocouples are used at different distances from the hot face along the pathof heat flow.30,34 Fig. 20.7 shows the installation of dual thermocouples on the wide and narrowwalls parallel to the hot faces of a slab caster mold.34 The heat flux and temperature distributionbetween the hot face and the water channels are then simply calculated from the gradient of thetemperatures measured at the two points. In the second method, the inverse boundary solutionmethod is used to calculate the local heat flux and temperature distribution from temperature mea-surements at a single point between the hot face and the cooling channel.38

615 485 635 615

bottom

narrowwall

102152229

457

711

900

narrowwall

259wide wall

waterchannel

thermocouple holes

21.6 11.5 hot face

section oftop view

dimensions: mm

Fig. 20.7 Dual termnocouple installation on wide and narrow walls parallel to hot face. From Ref. 34.

Instrumentation


Fig. 20.8 shows the variations of hot-face temperatures with vertical distance below the mold topresultant from the two-thermocouple method shown in Fig. 20.7.34 The hot-face temperatures werecalculated from wall temperature measurements made along the mid-plane of the loose and narrowwalls when casting low-carbon strip and medium-carbon plate grades. As expected, the tempera-tures on both the wide and narrow walls decreased with increasing distance below the mold top.This is caused by three factors. First, as the distance below the meniscus increases the thickness ofthe solid shell and its resistance to heat transfer from the liquid core to the mold increases. Second,solid fraction of the slag layer between the slab and the mold increases with distance below themold top, thus increasing the resistance to heat flow from the slab to the mold. Third, the fluid flowactivity in the liquid core decreases with the vertical distance which, in turn, decreases the heattransfer from the liquid core to the solid shell. Because the liquid steel streams from the bifurcatedtundish nozzle are directed toward the narrow walls, they affect higher heat transfer on and throughthe narrow side shell. As a result, copper temperatures on the narrow walls were higher than thosemeasured on the wide walls. From Fig. 20.8 it can also be observed that, when casting low-carbonstrip grade slabs, the mold hot-face temperatures exceeded 350°C, beyond which it is suggestedthat there is an increased potential for sticking.42

Thermocouples are also being used on medium-thickness and thin-slab caster molds to improvemold design, select mold powders, develop casting practices and control the operation of thecaster.40,41 Figs. 20.9 and 20.10 show sample temperatures measured on the broad face coppers of

narrow wall

loose wide wall

loose wide wall

various trials}Distance from mold top (mm)

Mol

d w

all h

ot fa

ce te

mpe

ratu

re (

˚C)

medium carbon platespeed: 1.02 − 1.29 m/min.width: 1930.4 − 1955.8 mm

low carbon stripspeed: 1.27 − 1.62 m/min.width: 990.6 − 1435.1 mm

400

300

200

1000

300

200

100

0 300 600 900

Fig. 20.8 Variation of moldhot-face temperatures withvertical distance from moldtop. From Ref. 34.

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a funnel-shaped thin-slab caster mold during the casting of stainless steel. The two broad faceswere instrumented with a total of 144 thermocouples. The specific objectives of the temperaturemeasurements were to analyze early solidification, meniscus waviness, mold lubrication and shellsticking in the mold. Fig. 20.9 shows the copper temperature variation with vertical distance fromthe mold top in the funnel and parallel areas of the mold. The variation of copper temperatures inthe width direction at 100 mm below the meniscus is shown in Fig. 20.10. As the figure illustrates,the temperatures on the two sides of the tundish nozzle are rather uniform. Such temperature mea-surements were helpful in improving mold powders and practices to achieve uniform mold tem-peratures and heat removal.

parallel area

funnel area

Thermocouple location from mold top (mm)

Tem

pera

ture

(˚C

)300

200

100

00 100 200 300 400 500 600

Fig. 20.9 Variation of copper temperatures with vertical distance from mold top in the funnel and parallel areas of a thin-slab caster mold. From Ref. 41.

Tem

pera

ture

(˚C

)

300

200

100

0−580 −460 −380 −280 −140 20 140 280 380 460 580

Thermocouple locations in horizontal plane 100 mm below the meniscus (mm)

Fig. 20.10 Variation of copper temperatures in width direction on either side of the tundish nozzle of a thin-slab caster mold.From Ref. 41.

Instrumentation


Today, thermocouples are standard instrumentation on almost every modern caster mold. They areroutinely used to predict mold sticker breakouts, which is explained in Chapter 19. Mold thermo-couples are also used for process automation, such as automatic cast start-up and resumptionthrough level sensing and confirmation. Attempts are also being made to use mold thermocouplesfor quality prediction and control during the casting of plate-grade slabs.26,31,32 Although the ther-mocouple data show good correlation between copper temperatures and longitudinal slab crack-ing,32 the correlation has not been strong enough for routine on-line quality prediction and as-castslab disposition.

20.4.3 Steel LevelNext to copper temperatures, perhaps the most critical mold parameter is the level of the liquidsteel pool in the mold.43–52 Maintaining a constant steel level in the mold is crucial for casting prod-uct with good surface quality, reducing the occurrence of breakouts, and process automation. Thevarious methods that are commercially available or have been attempted for steel bath level mea-surement in the mold include:

• Eddy current probe.1,43,50–52

• Electromagnetic cassette.44

• Radioactive source.44

• Thermocouples in mold copper.1,44,46

• Optical devices.1

• Link-arm type magnetic flux meter.44

• Telescopic magnetic flux meter.44

The eddy current probe, the electromagnetic cassette and the radioactive source are the most com-monly used methods. The thermocouple, optical and flux meter methods have limited or no indus-trial acceptance.

The eddy current probe is suspended over the mold and generates an electromagnetic field that isdirected into the mold. The electromagnetic field induces eddy currents in a near-surface layer ofthe liquid metal in the mold via the primary coil. This eddy current produces a secondary field,which, in turn, induces voltages in the sensor’s secondary coil. The magnitude of the eddy current,and thus the induced voltage, is dependent on the actual distance between the sensor and the steelsurface. The actual liquid steel level in the mold can be detected independently of any slag andpowder layer on top of the liquid steel. Measurement accuracy with the suspended probe has beenreported to be better than ± 2 mm at casting rates up to 7 tons/minute.51 The measurements are notaffected by copper temperature and copper coating such as nickel. However, it is not suited forauto-start detection because the probe is brought over the mold after the initial filling of the moldis over and the mold level stabilizes.

The electromagnetic cassette entails a transmitter and a receiver coil, which are mounted side byside on top of the fixed wide mold wall. This method differs from the suspended sensor method inthat the transmitter’s electromagnetic field induces eddy currents in the exposed face of the copperwall. These eddy currents generate their own magnetic fields, which are detected by the receivercoil. Because the exposed surface area of the copper wall changes as the level of liquid steel in themold changes, the strength of the signal at the receiver coil is dependent on the level of liquid steelin the mold. The sensor signal is also dependent on the temperature of the copper wall. The tem-perature effect becomes pronounced on molds that are coated with nickel at the top end. Althoughthe coils in the cassettes can be oriented to offset the effect of nickel coating, experience shows thatthe nickel can cause a gradual drift in the mold level, which needs to be periodically readjusted.

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Because the cassettes are located on top of the copper plate, they can be used for automatic start-up; a “blip” in the sensor signal indicates the start of steel flow into the mold and thus the initia-tion of mold filling. As with the eddy current probe, the depth of the slag cover on top of the liquidsteel has negligible effect on the measured electromagnetic signal.

In the radioactive source method, a γ-ray emitter and a detector are installed in opposite moldwalls. The accuracy of the method is better than ± 5 mm and is used on several casters throughoutthe world. However, it has the handling and disposal problems associated with radioactive materi-als. Another disadvantage of the radioactive source method is that the output signal is affected bythe thickness of the mold powder on top of the steel pool.

As-cast product quality requirements dictate mold level variation to be kept under tight control. Acommon measure is the standard deviation of the mold level, with a typical good value being at orbelow 2 mm in a 20-second window. The signal-to-distance relationship for the eddy current andelectromagnetic sensors is highly nonlinear. Fig. 20.11 shows typical signal outputs of an eddy cur-rent probe and an electromagnetic sensor as a function of liquid steel depth below the top of themold copper. When using measuring equipment that treats these sensors as providing a linear out-put, care must be exercised in the calibration and signal interpretation procedure. Otherwise, sub-stantial errors and apparent nonrepeatable performance may result. The errors increase withincreasing sensor nonlinearity and with departure from a targeted operating window.

The effect of the mold level calibration procedure on the instantaneous level and level range vari-ation will be described in the following example. Before the start of each casting sequence, a typ-ical calibration procedure would adjust the electrical output of each sensor to two standard valuesat two fixed calibration points. Then, a straight line would be used between these two points toapproximate the mold level. Fig. 20.12 shows such a calibration where the output at the two cali-bration points of 50 mm and 100 mm are fixed at 33.3 and 66.7% of the total signal output range.The desired range of operation is between the two calibration points. With this arrangement andthe assumption that the signal response varies linearly as shown by the dotted line, it is clear thataccurate representation occurs only at the two calibration points. When the actual mold level isbetween these two points, the signal response will be higher than that predicted by a linear rela-tionship. Consequently, through the linear equation, the mold level will be interpreted as beinglower in the mold than it actually is. The situation is reversed outside the range of the calibrationpoints. In general, the situation may not be serious because the feature of importance for mold levelcontrol is not the actual instantaneous level, but rather the range of variation of level. For properrepresentation of this range, the local slope of the sensor curve at its instantaneous position is

electromagneticsensor

eddy currentsensor

Sig

nal (

coun

ts)

Distance from mold top (mm)

4096

3072

2048

1024

00 25 50 75 100 125 150

Fig. 20.11 Input range of eddy current and electromagnetic sensors.

Instrumentation


important. Obviously, the local slope of each sensor curve is the closest to that of the assumedstraight line midway between the two calibration points, and deviates as the level moves away fromthis central position. Therefore, when operating at a true mold level midway between the calibra-tion points, the reported level will be greater than actual. However, the level variation about thispoint will have a small error because the slope of the actual level variation is very close to that ofthe linear curve. This discussion holds for electrical systems that make the assumption of linearsignal response, and it is also true of older mold-level measuring equipment. However, the recenttrend has been to use digital equipment, which facilitates the programming of a nonlinear responsecurve so that accurate interpretations of both level and level variation are simultaneously obtained.

For level control and process automation, the measured level signal is fed into a control system thatacts either on the tundish nozzle opening via a sliding gate or a stopper rod, or on the withdrawalspeed.

Fluctuations of the free surface of molten steel in the mold resulting from mold oscillation and re-circulating flows can cause powder entrapment and thus defects at the product subsurface. A studywas conducted to establish the relationship between mold oscillation and surface wave motion nearthe narrow wall in a slab caster mold.47 In the study, fiber optics were used to observe the menis-cus through a quartz glass window mounted in a cutout in the mold top corner, as shown in Fig.20.13. The casting conditions during the trials were:

• Cast speed = 0.53–1.60 m/min.

• Mold oscillation = ± 6 mm, 68–127 cpm, near-sinusoidal.

• Mold size = 250 x 920 mm.

• Grade = Low-carbon aluminum-killed.

The trials showed that:

• Contrary to the generally accepted belief, the meniscus does not slide freely alongthe mold wall and is not stationary when viewed from a stationary position. Rather,it fluctuates in phase with mold oscillation, as depicted in Fig. 20.14.

eddy currentsensor

idealizedlinear sensor

electromagneticsensor

Ran

ge (

%)

Distance from mold top (mm)

0 25 50 75 100 125 150

100.0

83.3

66.7

50.0

33.3

16.7

0.0

Fig. 20.12 Calibration curves for eddy current and electromagnetic sensors.

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quartz windowoptical fiber

SEN

mold

displacemant transducer

data scanner

recorder monitor

Fig. 20.13 Apparatus used for meniscus observation. From Ref. 47.

Abs

olut

edi

spla

cem

ent

of m

enis

cus

Mol

d di

spla

cem

ent (

mm

)

casting speed: 1.60 m/minoscilation freq: 2.11 Hzoscilation amplitude: + 6 mm −

10 mm from narrow wall

3 mm from narrow wall

5 mm

Time (seconds)

0 1.0 2.0

6

0

-6

Fig. 20.14 Meniscus behavior. From Ref. 47.

Instrumentation


• The magnitudes of the fluctuations are close to mold displacement and increase asthe casting speed increases, as illustrated in Fig. 20.15.

• The shape of the meniscus is always convex upward, but the radius of curvature ofthe meniscus varies with the displacement of the mold during an oscillation cycle.This is illustrated in Fig. 20.16.

Abs

olut

edi

spla

cem

ent

of m

enis

cus

(mm

)(a) upstroke

casting speed0.53 m/min1.60 m/min

castingspeed0.53 m/min1.60 m/min

(b) downstroke

Abs

olut

edi

spla

cem

ent

of m

enis

cus

(mm

)

displacement of mold (mm)

5

0−6 −3 0 3 6

6 3 0 −3 −60

5

Fig. 20.15 Relationshipsbetween mold displacementand meniscus displacement.From Ref. 47.

time upper dead point

lower dead point

narrowwall

narrowwall time

(a) downstroke (b) upstroke

5 mm

Fig. 20.16 Change in meniscus profileat casting speed of 0.53 m/min. FromRef. 47.

Casting Volume


In a recent study, two suspended level sensors were used to study level fluctuations in a slab castermold.48 As is shown in Fig. 20.17, one of the sensors was installed at 120 mm from the narrowwall. The other was located either at 50 mm away from the tundish nozzle on the same side as thefirst sensor, or on the other side of the tundish nozzle at 60 mm from the opposite narrow wall.Observations when casting at 1.3–1.4 m/min showed that rapid level fluctuations, which can haveamplitudes of ± 20 mm, occur in small-sized slabs and that these fluctuations can cause surfacedefects. In larger sections, the level fluctuations have smaller amplitudes and occur at lower fre-quencies. Measurements also showed that the waves propagate from one side of the mold to theother in the width direction; the phenomenon is referred to as “pumping.” The effect of slab widthon the principal frequency of the level fluctuations is shown in Fig. 20.18. The problem was partlysolved by installing a band rejection filter on the control system. Another benefit of the dual sen-sor tests was the establishment of favored places for the single level measurement sensor.

sensor locations

SEN

SENlevelsensor

levelsensor

levelsensor

wave motion

2 1 2 1

50 mm 120 mm 60mm 120 mm

Fig. 20.17 Mold free surface fluc-tuations. From Ref. 48.

1

0.9

0.8

0.7

0.6

0.5700 900 1100 1300 1500 1700 1900

Freq

uenc

y (H

z)

Mold width (mm)

Fig. 20.18 Effect of moldwidth on mold level fluctuationfrequency. From Ref. 48.

Instrumentation


The incidence and depth of subsurface inclusions resulting from mold level variation increase withcasting speed.50 In several slab caster operations, the in-mold electromagnetic brake (EMBr) isused to suppress the mold level fluctuations when casting at high speeds.50,52–60 Plant data indicatethat the EMBr decreases the surface wave fluctuations significantly and improves slab quality.However, it can adversely affect slab quality if the magnetic field strength is not properly adjustedfor the various cast widths and speeds.

20.4.4 Mold OscillationMold oscillation was introduced into continuous casting in 1949 to prevent shell sticking to themold by providing sufficient lubrication between the strand and the mold.61,62 Today, mold oscil-lation is common practice on all conventional continuous casters. Numerous studies have shownthat the surface quality of the product is highly dependent on the shape of the oscillation curve(e.g., sinusoidal, triangular), stroke and frequency.63 Until recently, conventional casters utilizedmechanical oscillators and sinusoidal oscillation curves. However, most new and rebuilt casters areutilizing hydraulic oscillators because the profile, frequency and stroke of the oscillation curve canbe easily changed and customized for each grade.59,64–68

The stroke and frequency of oscillation can vary from one installation to the next, and the oscilla-tion practice can be different for various steel grades at a given caster installation. However, oncethe optimum oscillation practices are established, it is desirable that they remain constant. Thisrequires that the quality of oscillation and the physical condition of the oscillator be checked andcorrected periodically. Several methods are used to check the condition of the oscillator and thequality of mold oscillation. These include the manual pencil trace, human touch, periodic trialswith displacement transducers and accelerometers,30,69,70 and on-line continuous monitoring withaccelerometers.71–73 Displacement transducers have moving parts and are thus prone to failureunder the harsh caster environment. On the other hand, accelerometers do not have moving parts.Furthermore, they have high sensitivity and accuracy and are thus the best suited for oscillationmonitoring. A system comprised of four triaxial accelerometers mounted at each mold or moldtable corner, a data acquisition and analysis subsystem, and a personal computer can yield all theparameters that are essential for complete assessment of the quality of oscillation and the condi-tion of the oscillator.73 Each triaxial accelerometer sensor consists of three individual accelerome-ters oriented in the vertical and two horizontal directions perpendicular to the mold narrow andbroad faces. The acquired acceleration signal is integrated to obtain time-based velocity and dis-placement data. The Fast Fourier Transformation (FFT) technique is utilized to convert the time-based mold acceleration data and the calculated mold velocity and displacement data into discretefrequency component.74 The procedure is repeated for acceleration signals collected in the threedirections from each triaxial accelerometer sensor on the four mold or mold table corners. It hasbeen demonstrated that the system can produce the following oscillation parameters:

• Displacement, velocity and acceleration curves in three directions.

• Peak-to-peak displacement, velocity and acceleration values in three directions.

• Primary oscillation frequency.

• Secondary (extraneous) oscillation frequencies in three directions.

• Negative strip time.

• Negative strip ratio.

• Rise/fall ratio (i.e., peak-to-peak time up/peak-to-peak time down).

• Phase (lead or lag of one mold table corner relative to the other in the vertical direc-tion expressed in degrees).

Casting Volume


1200

800

400

0

−400

−800

−12000 0.4 0.8 1.2 1.6 2

Time (seconds)

Acc

eler

atio

n (m

in/s

ec2 )

Fig. 20.19 Mold vertical acceleration versus time. From Ref. 73.

Fequency (cpm)

Pea

k-to

-pea

k ac

cele

ratio

n(m

m/s

ec2 )

0 400 800 1200 1600 2000

1500

1000

500

0

Fig. 20.20 Mold vertical acceleration versus frequency. From Ref. 73.

Time (seconds)

0 0.4 0.8 1.2 1.6 2

80

60

40

20

0

−20

−40

−60

−80

Vel

ocity

(m

m/s

ec)

Fig. 20.21 Mold vertical velocity versus time. From Ref. 73.

Instrumentation


Figs. 20.19 through 20.23 show sample outputs of the triaxial accelerometer-based mold conditionmonitoring system.73 The data was collected on a straight slab caster mold during casting. Themeasurements showed that the actual frequency and stroke were 112.2 cpm and 8.77 mm, com-pared with the aim values of 112.0 cpm and 8.8 mm. Horizontal mold movements were 0.24 mmperpendicular to the broad face and 0.25 mm perpendicular to the narrow face. Phase was 0.0degree, and rise/fall ratio was 0.977. The measured parameters in this trial were all within theacceptable ranges; thus, no corrective maintenance action was taken. Ideally, the acceleration-fre-quency spectrum presented in Fig. 20.20 should show only the primary oscillation frequency of112 cpm. However, numerous secondary (extraneous) frequencies of higher values are observed,the most significant having a value of about 550 cpm. It might be possible to develop a correlationbetween the secondary oscillation frequencies and oscillator component repair, mold powder char-acteristics and product quality. However, the development of such a correlation would requiredesigned powder trials, and long-term trending of oscillator repairs and extraneous oscillation fre-quencies.

Casting practices, oscillator equipment, maintenance practices and quality standards vary from oneinstallation to the next. For example, a survey of several major slab casting operations in NorthAmerica and Europe revealed that the allowable mold movement in the horizontal directions variesbetween 0.2 and 0.5 mm to avoid corner and longitudinal cracks. Therefore, each caster operationshould develop its own maintenance and quality criteria to be incorporated into an on- or off-linemold oscillation monitoring system.

Time (seconds)

Dis

plac

emen

t (m

m)

6

4

2

0

−2

−4

−60 0.4 0.8 1.2 1.6 2

Fig. 20.22 Mold vertical displace-ment versus time. From Ref. 73.

to broad faceto narrow face

Dis

plac

emen

t (m

m)

Time (seconds)

0 0.4 0.8 1.2 1.6 2

0.3

0.2

0.1

0

−0.1

−0.2

−0.3

⊥⊥

Fig. 20.23 Mold horizontal displace-ment versus time. From Ref. 73.

Casting Volume


20.4.5 Mold/Strand FrictionInterfacial friction between the strand and the mold affects surface quality and caster productivity.Depending on shell strength, if friction is not maintained below a critical level, shell sticking andtearing can occur. Also, if friction is not low enough, the desired casting speed and productivitylevels might not be attained. Mold/strand friction is dependent on mold powder characteristics;steel grade; mold oscillation curve, e.g., sinusoidal vs. triangular; and oscillation parameters suchas frequency, stroke, and duration of positive and negative strip times. In an effort to determine theproper combination of mold powders and practices that yield the best mold lubrication for differ-ent steel grades, several measurement and analysis methods have been applied or proposed toquantify mold friction. These include:

• Strain gauges in oscillator arms.24,30,64,75

• Plurality of load cells under the mold support points.35,76,77

• Measuring the pressure variation between the inlet and outlet of the hydraulic cylin-der in a hydraulic oscillator.65,67

• Measuring and analyzing the oscillator drive motor current.78,79

• Measuring the withdrawal roll current.80

• Accelerometer mounted on the side of the mold.81

Irrespective of the method used, friction is calculated from the difference between measurementsmade during casting and when oscillating with no casting. To illustrate the point, a recent study inwhich strain gauge bridges were used for slab/mold friction calculations will be cited. In this par-ticular study,30 the strain gauges were installed in two pins connecting the mold table to the oscil-lator arms, as shown in Fig. 20.24. The intent was to evaluate and optimize mold powders for stripand plate grade slabs by comparing mold/slab interfacial friction and other mold parameters whencasting with various mold powders. Mold/slab friction forces were obtained from pin forces and

moldtable

pin

eccentric

LVDT

instrumented pin

straingaugebridges thermocouple

pin

Fig. 20.24 Schematic of strain gauge and displacement transducer installation on mold table oscillator. From Ref. 30.

Instrumentation


mold displacements measured during casting and cold oscillation tests. In the cold tests, the moldwas oscillated at frequencies ranging between 18 and 130 cpm as though a cast were made.

Force and displacement measurements under cold and casting conditions are shown in Fig. 20.25.The force and displacement measurements were used to calculate the “work” expended by the pinsto move the mold and mold table in a complete oscillation cycle from:

(Eq. 20.1)

Pin work over a cycle was then divided by the full mold stroke to determine a “work-averaged” pinforce, and the difference between the work-averaged force during casting and cold oscillation testat the same oscillation frequency was defined as the average friction force. Friction per unit areawas then determined by dividing the average friction force by the total slab surface area in themold. That is:

(Eq. 20.2)

(Eq. 20.3)

The method may best be understood by examining Fig. 20.26, which shows the pin work per cycleduring casting and cold tests. Fig. 20.27 shows friction force per unit slab area in the mold versuscasting speed for three different mold powders. As expected, friction decreases as cast speedincreases. In the study, an attempt was made to calculate mold/slab friction from peak-to-peakstrain gauge measurements. However, this yielded inconsistent and sometimes negative frictionvalues.

Another technique that can be used to calculate strand/mold friction from load cell signals is theapplication of Fast Fourier Transformation (FFT) on load cell signals measured during casting andwhen oscillating with no casting.24,75

20.4.6 Mold Powder Film Thickness and Molten Pool DepthMold heat removal is strongly affected by the thickness of the powder film in the gap between themold and the strand. Insufficient or uneven flow of mold powder into the strand/mold interfacialgap can cause surface cracks on the cast product. On the other hand, an excessively thick film ofpowder can result in low heat removal rates and thus low caster productivity. One of the major

Friction =Average friction force

Slab area in mold

Average friction force =Pin work during casting Pin work under cold- cconditions

Mold stroke

Pin work over a cycle Pin force Mold displacement( )= ¥Ú

cold casting200

190

180

170

5.0

2.5

0

−2.5

−5.00 1 2 3 0 1 2 3

Time (seconds)

Pin force(kN)

Displacement(mm)

Fig. 20.25 Mold displacementand pin force under cold andcasting conditions. From Ref.30.

Casting Volume


factors that control the flow rate and uniformity of the film is the depth of the molten powdercover above the steel pool. It is thus desirable to measure the depth of the molten powder coverand correlate it to the thickness of the in-gap powder film. The depth of the molten powder coveris usually measured by wire burnoff. The thickness of the powder film is measured by recoveringlayers of powder from the mold hot face during major transitions when the mold level decreasesor from the mold exit. However, these methods are manual and inherently inaccurate.

An on-line measuring system comprised of an in-gap powder thickness gauge and a molten pow-der pool depth gauge was developed and applied on a slab caster.82 The film thickness gauge isthermal radiation-based and is installed below the mold. The molten powder cover gauge is an eddycurrent device, which is suspended above the mold and utilizes the difference in the electrical resis-tance of liquid steel and molten slag. Both gauges are of noncontacting type. Measurements madeon a slab caster mold are shown in Fig. 20.28. The results showed that:

pin force(kN)

Displacement (mm)

Friction(kN/m2) = A2−A1

[mold stroke] x [slab area in mold]

-5 -2.5 0 2.5 5

200

190

180

170

160

A1 A2 castingwork

coldwork

Fig. 20.26 Pin work under castingand cold conditions. From Ref. 30.

Cast speed (m/min)

0.7 0.9 1.1 1.3 1.5 1.7

mold powders

Mold/slabfriction(kN/m2)

low carbon

16

12

8

4

0

Fig. 20.27 Mold/slab friction versus cast speed. From Ref. 30.

Instrumentation


• The accuracy is ± 0.1 mm for the film gauge and ± 2.0 mm for the molten powderdepth gauge.

• Changes in cast speed and powder viscosity cause variations in film thickness.

• Variations in film thickness affect strand surface cracks.

Sustained performance of the devices in the hostile caster environment is yet to be proven.

20.4.7 Mold Wall DeformationThe continuous caster mold is known to sustain large thermal stresses and undergo sizable defor-mation during casting.83,84 The deformation of the mold is critical because it distorts the shape ofthe strand and can thus impact surface quality. Furthermore, in slab casters, a gap is neededbetween the broad and narrow faces to facilitate width changes while casting. Excessive deforma-tion of the wide walls, in combination with the thermal expansion of the narrow walls, can resultin the damaging of the wide wall hot faces and the edges of the narrow walls. The deformation ofthe wide walls can sometimes be severe enough to restrain the movement of the narrow walls andthus prevent size changes while casting. It is thus desirable to know the extent of mold wall defor-mation under various casting conditions.

Displacement transducers are commonly used to measure mold wall deformation during casting.Fig. 20.29 shows the installation of linear displacement transducers on a straight slab caster moldat Bethlehem Steel’s Burns Harbor plant to study wide wall and mold cavity deformation duringthe casting of 254 x 1933-mm slabs. Five transducers were installed on the backside of each wideside water jacket at the lower end of the mold, and two in the mold cavity at the mold ends. Mea-surements were made while casting at a steady-state speed of 1.14 m/min and just prior to auto-matic size change while casting at 0.6 m/min. The results are shown in Fig. 20.29 and indicatethat:

• The fixed and loose sides deformed differently during steady-state casting and justprior to size change; the deformed shapes were convex into the mold cavity.

• The middle of the fixed side moved into the cavity. The middle of the loose sideeither stayed in the original position or moved back from the original position.

• During steady-state casting, the mold cavity thickness decreased by 0.96 mm in themiddle of the mold and increased by about 2.00 mm at the ends of the mold. Dur-ing size changes, because of lower casting speed and lower copper temperatures, thewalls sustained smaller deformation.

In-g

ap s

lag

thic

knes

s (m

m)

Powder consumption (kg/m2)M

olten powder pool depth (m

m)

filmpool

0 0.2 0.4 0.6 0.8 1

12

10

8

6

4

2

1

0.8

0.6

0.4

0.2

0

Fig. 20.28 Relationship among in-gap slagthickness, molten powder pool depth andpowder consumption. From Ref. 82.

Casting Volume


The plant measurementsshown in Fig. 20.29 werematched with mathematicallycalculated deformations ofthe mold backplate.84

Dynamic changes of the cav-ity thickness at the two endsof the Burns Harbor straightmold during a seven-hourcampaign are shown in Fig.20.30. The measurementswere made as part of a majorundertaking to understand thecauses of corner cracks onplate grade slabs. Slab dimen-sions were 254 x 1270 mm.As the figure shows, the cav-ity thickness opened by about2.50 mm at both ends duringsteady-state casting. Duringtundish tube changes, the cav-ity thickness openingdecreased significantlybecause of speed reductionand the resultant decrease incopper temperatures. Aftercast terminations, the wallsmoved back to their cold posi-tions. The wall deformationsmeasured in these studieswere not large enough torestrain the movement of thenarrow walls during in-castwidth changes.

2.00 mm 1.91 mm

0.00 mm 0.51 mm 0.96 mm 0.50 mm 0.05 mmfixed

Displacement transducers

1.10 mm 0.38 mm 0.00 mm 0.38 mm 1.22 mmloose

displacement transducers

1.27 mm 1.27 mm

0.00 mm 0.36 mm 0.51 mm 0.30 mm 0.05 mmfixed


0.96 mm 0.46 mm 0.25 mm 0.51 mm 1.10 mmloose


(a) six minutes after start-up; cast speed = 1.14 m/min

(b) just prior to width change; cast speed = 0.6 m/min

Fig. 20.29 Measurement of in-cast mold deformation.

Time (hours)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

00 1 2 3 4 5 6 7

Ope

ning

(m

m)

1.50

1.00

0.50

0

Cast speed (m

/min.)

cast speed

in-cast width change

2

1

cap-off

1 2

Fig. 20.30 Variation of moldgap opening during a seven-hour cast.

Instrumentation


20.4.8 In-Mold Liquid Steel Flow VelocityThe surface and internal qualities of the cast product and the operability of the caster can beaffected significantly by the flow of metal in the mold. Therefore, it is highly desirable to know thevelocity distribution of liquid steel in the mold. Until recently, researchers relied on physical andmathematical models to study the flow field in the mold because instruments were not available tomeasure the flow velocities of liquid steel under production conditions. Although the researcherswere mostly successful in correlating their model results to the quality of the cast product, theywere not able to simulate critical in-cast events such as the plugging or wear of the tundish tube,and interactions between the liquid steel and the mold powder. Now, devices are available to mea-sure liquid steel velocities in the mold during casting. Of the devices, the most notable is the elec-tromagnetic sensor,85,86 which is schematically shown in Fig. 20.31. The device consists of apermanent magnet mounted on the water jacket, and a sensor mounted on the back of the copperplate. Neither the magnet nor the sensor contact the liquid steel and thus do not disturb the flowfield at the measurement location. As the steel flows through the magnetic field in front of thedevice, it creates electrical currents in the liquid that can be determined from:

(Eq. 20.4)

where

σ = electrical conductivity of liquid steel,

J→

= current density,

V→

= flow velocity of liquid steel, and

B→

= magnetic flux density.

Thus, the velocity of the liquid steel can be determined by measuring the electromagnetic fieldof the induced currents in the vicinity of the sensor. The measurement represents the average

J V B��

= ¥s ( )

castingpowder

cooling water

permanent magnet

MFC - sensor

water jacket

copper wall

liquidsteel

measuredarea

Fig. 20.31 Installation of electromagnetic flow sensor on a caster mold. From Refs. 85 and 86.

Casting Volume


horizontal (or vertical) component of flow velocity in a small volume of liquid steel in the vicin-ity of the sensor.

Four such flow sensors were installed in a 200 x 2700-mm slab caster mold to correlate flow con-ditions in the meniscus region to slab internal quality.85,86 As Fig. 20.32 shows, the sensors were attwo vertical locations on either side of the tundish nozzle. The flow velocities measured in front ofthe lower sensors and the casting speed during a time period of 1.5 hours are shown in Fig. 20.33.The velocities on the two sides of the tundish tube were about the same and varied similarly as thecasting speed was changed. However, when all casting parameters were constant, the flow veloci-ties on either side of the tundish tube varied randomly between about 2 and 40 cm/sec, indicatingthat even during steady-state casting the flow field in the mold is not steady.

The flow pattern in a slab caster mold depends mostly upon the argon flow rate, tundish nozzledesign, tundish nozzle submergence, casting speed and extent of clogging in the tundish nozzle. Asillustrated in Fig. 20.34, the main flow patterns can be described as “single roll,” “double roll” or“meniscus roll.” Water and mathematical modeling indicated that, in the particular mold instru-mented with the flow sensors, small tundish nozzle submergence would produce a “single roll”flow pattern, and large tundish nozzle submergence would produce a “double roll” flow pattern. Toverify this, a trial was conducted in which the tundish nozzle submergence was varied over a 120-mm range, but all other casting parameters were kept constant. The argon flow rate was 80% higherthan in the water model. The flow velocities measured by all four sensors and the tundish nozzleposition are shown in Fig. 20.35. It is clearly seen from the figure that, with all tundish nozzle posi-tions, the flow was directed toward the narrow wall at all four sensor locations, indicating that the

137 mm

270 mm

electromagneticflow sensor

2700 mm

720 mm 720 mm

Fig. 20.32 Flow sensor locations onbroadface of slab caster mold. FromRefs. 85 and 86.

right

left

Flow velocity(cm/sec)

50

25

0

−25

−50

Casting speed(m/min)

1.6

1.2

0.8

0.4

0Time

9 min

Fig. 20.33 Variation of horizontal flow velocities measured by the lower sensor during casting. From Ref. 85.

Instrumentation


flow pattern was always in the “single roll” mode. The velocities on the two sides of the tundishnozzle were about equal and showed negligible change with tundish nozzle submergence, exceptfor the slight increase in flow velocities when the tundish nozzle was in its highest position. Fur-ther trials showed that tundish nozzle port angle had little effect on the flow pattern. Only whencasting with a deeply immersed tundish nozzle and low argon flow rate was the “double roll”mode the dominant flow pattern. Cold mill data showed a correlation between coil surface defectsand mold flow pattern. The coils produced with the “double roll” pattern had no defects, those

double roll

promoted by:low argon rate

deep SEN immersionhigh casting speed

single roll

promoted by:high argon rate

shallow SEN immersionlow casting speed

meniscus roll

promoted by:clogged SEN

Fig. 20.34 Main flow patterns in slab caster mold. From Ref. 86.

Flow velocity(cm/s)

Flow velocity(cm/m)

SEN immersiondepth (mm)

upper sensors right

left

right

left

lower sensors

100

50

0

-50

-10050

25

0

-25

-50

min

max

14 min

50mm

Time

Fig. 20.35 Variations of mold velocities with tundish nozzle submergence under steady-state casting conditions. From Ref.85.

Casting Volume


produced with the “single roll” pattern had slightly more defects than the plant average defect rate,and the coils produced with the “meniscus roll” pattern had the most defects. The “meniscus roll”type of flow pattern is promoted by the clogging of the tundish nozzle. These test results provedthat the electromagnetic flow sensors are viable for on-line use to predict slab quality and to fine-tune mold parameters and practices.

A simpler and more commonly used mold flow measurement technique involves the use of a cir-cular ceramic probe as is schematically shown in Fig. 20.36. The circular ceramic probe isimmersed in the liquid steel below the meniscus. The supporting rod at the upper end of the probeis equipped with a load cell or strain gauge bridge. Two approaches are used. In the first and morecommonly used approach, the strain or load measurement is directly converted to the liquid steelvelocity through the momentum imparted on the probe by the stream. Such an approach was usedin several slab caster operations to see the effects of an electromagnetic brake (EMBr) on moldmeniscus velocities.87–89 The measurement results from two different operations are shown in Table20.1. In one operation, the measurement probe was kept at 155 mm from the narrow wall. In theother, it was held at 300, 250 and 180 mm from the narrow wall. Positive and negative velocity val-ues in Table 20.1 imply flow toward and away from the narrow wall, respectively. Clearly, in bothoperations, the meniscus flow velocities were significantly lower when the EMBr was on com-pared with those when the EMBr was off. When the EMBr was off, the flow velocities variedbetween 4 and 110 cm/sec; and when the EMBr was on, they varied between 1 and 35 cm/sec.

The second approach with the circular ceramic probe was tested in Wood’s metal and in liquid steelunder laboratory conditions.90 In this approach, the linear relation between the velocity of the fluidflow approaching the ceramic probe and the shedding frequency of vortex streets generated behindthe probe is used. The flow around the circular probe is governed by the Reynolds number, whichis defined by:

(Eq. 20.5)

where

V = mean velocity of liquid approaching the cylinder,

D = diameter of cylinder, and

ν = kinematic viscosity of fluid.

Re =V Di

u

moldSEN

refractoryrod

load cell

Fig. 20.36 Refractory probe andload cell arrangement for moldvelocity measurement. From Ref.87.

Instrumentation


If the Reynolds number is larger than 40, vortices are shed in very regular patterns from the cylin-der, as shown in Fig. 20.37. These regular patterns are called Kármán vortex streets, and they resultin oscillation of the refractory probe in a direction perpendicular to the flow direction. The oscil-lation frequency of the cylinder is the same as the shedding frequency of the vortex streets, whichcan be defined by:

(Eq. 20.6)fSt V

D=

i

Table 20.1 Typical Mold Velocity Measurements (from Ref. 87)

Casting Volume


where

f = oscillation frequency and

St = Strouhal number.

The Strouhal number remains nearly constant over the Reynolds number range of 300–200,000.90

Thus, once a setup is constructed and the Strouhal number is determined in off-line trials, it can beused to determine the flow velocities in the actual caster mold by measuring the probe oscillationfrequency f.

The concept was tested inWood’s metal melted at 47°Cin a cylindrical vessel, and inliquid steel melted at 1550°Cin a cylindrical crucible usingan induction furnace. TheKármán vortex probe wasrotated in the melt by usingthe setup shown in Fig. 20.38.The speed of the probe waschanged by means of a vari-able-speed motor on the rota-tion arm. The submergence ofthe refractory probe was 50mm in both Wood’s metal andliquid steel. The test cylinderwas made of sialon, and thesupporting rod was made ofstainless steel. The cross-sec-tion of the central part of thesupporting rod was made rec-tangular to facilitate theattachment of strain gaugebridges. The erosion anddeformation of the sialon testcylinder were negligible, even

flow

D

V

Fig. 20.37 Schematic of Kármán’s vertex street.From Ref. 90.

∆tstrain gauge

supporting rod

circular cylinder

D

50mm

bridge

dynamicmeterstrain

fast Fourier

analyzer

furnace

molten steel

Fig. 20.38 Experimental apparatus for velocity measurement with Kármán vortexprobe. From Ref. 90.

Instrumentation


though the cylinder was submerged in the liquid steel without any preheating. The output of thestrain gauge bridges was processed by means of a Fast Fourier Transformation analyzer (FFT) todetermine the vortex shedding frequency f and subsequently the fluid flow velocity V. The testresults with Wood’s metal and liquid steel are shown in Fig. 20.39. The Strouhal number with bothliquids was 0.15. The measured velocities varied between about 20 and 70 cm/sec.

The ceramic rod is simple and viable for experimental use in the laboratory or on the cast floor tofine-tune mold parameters. However, it is not practical for on-line continuous use in the castermold to predict slab quality.

7.5 Hz

shedding frequency of Kormon vortex

Frequency (Hz)

Mag

nitu

de (

mV

)

100

00 100

molten metal

Wood's metal

molten steel

supporting rod

material ∆t (mm)

stainlesssteel

2.0

1.5

2.0

1.5

∆t

D

h

h = 50 mmd = 5.6 mm

Sialon cylinder

k = 0.15

Molten metal velocity, V/cms-1

20 40 60 80

15

10

5

0

f−D

/cm

s-1

Fig. 20.39 Velocity measurements in liquid steel and Wood’s metal. From Ref. 90.

Casting Volume


20.4.9 Mold Cooling WaterThe flow rate, pressure, and inlet and exit temperatures of mold cooling water are continuouslymeasured and displayed in the caster control room. The difference between the water exit and inlettemperatures, and the water flow rate is used to calculate the heat removed from the mold. Themeasurements and the mold heat removal are then used to control the casting speed, issue alarmsin the event of flow disturbances and call for emergency water in the extreme case of major coolantloss. They can also be used to detect mold problems. For example, if the measurements show lowheat removal from the mold narrow side, this may indicate wrong taper and thus the need for moldmaintenance. The sensors used to measure mold cooling water parameters are off-the-shelf indus-trial instruments and hence will not be elaborated here.

20.5 Containment

20.5.1 IntroductionAfter exiting the mold, the partially solidified strand enters the containment section of the caster.The containment is comprised of a large number of roll assemblies and is by far the largest andmost mechanically complicated part of the casting machine. The main functions of the containmentare to:

• Extract the strand at a controlled speed.

• Guide the strand with top and bottom rolls until the completion of solidification.

• Remove heat from the strand.

• Maintain a stable flow path to yield the desired strand profile and dimensions.

• Perform the required mechanical functions such as the bending, squeezing andstraightening of the strand.

• Allow adequate production rates without yielding surface or internal defects.

To assure the adequate performance of these demanding functions and to correlate the variousfunctions to the design of the containment, numerous measurements have been conducted on thecaster containment. A big challenge in these measurements has been the operational life of the sen-sors in the hostile containment environment. Although success has been limited compared withmold sensors, many sensors and application techniques have been developed. These developmentsenable the measurement and analysis of containment parameters that relate to slab quality, pro-ductivity and maintenance. The following sections summarize the instrumentation available for themeasurement of critical containment parameters.

20.5.2 Strand Surface TemperatureIt is known that secondary cooling practices in the containment have significant effects on prod-uct surface, internal quality and also productivity. As the strand surface cools and reheats betweenrows of spray nozzles and rolls, thermal stresses are generated in the solid shell. If the stresses arehigh enough, cracks can form or existing cracks can worsen on the strand surface. At the sametime, mechanical stresses arise in the shell due to inter-roll bulging under ferrostatic pressure. Theresistance of the shell to ferrostatic pressure and bulging is determined by its temperature andhence by the spray-cooling practice. If the bulging stresses exceed a critical value, internal (refill)cracks can occur at the solidification front. Another potential problem is the occurrence of surfacecracks during the unbending of the strand because of an improper combination of surface tem-perature and unbending stresses. Also, in single-point unbending machines, segregation and cen-terline laminations can originate in medium- and high-carbon grade slabs if complete

Instrumentation


solidification occurs downstream of the unbending point. To avoid such internal defects, the sec-ondary cooling practice and casting speed must be properly controlled to assure complete solid-ification before the unbending.

Because of the strong correlation between product quality, productivity and secondary coolingpractice, the achievement of proper spray practice for all casting conditions and grades has beenone of the major goals of caster operators and researchers. The complex interactions among theoxidized and rough strand surface, rolls, sprays, surface water layers and containment environmentmake it difficult to mathematically or physically determine the proper spray practice for all cast-ing conditions and grades. Hence, best results have been obtained by measuring critical parame-ters on actual casters and analyzing the data with the help of mathematical heat transfermodels.35,91

The biggest challenge in establishing the heat transfer characteristics in the secondary coolingzone has been the accurate measurement of the strand surface temperature. Thermocouples andoptical pyrometers have been used to measure strand surface temperatures. Thermocouples areeither welded onto the strand surface with a welding gun or dragged and embedded onto the strandsurface by feeding it between the strand and a roll upstream of the roll. Thermocouples are inex-pensive and yield a continuous variation of the surface temperature of a particular strand sectionas it goes through the machine. This suits the approaches used in mathematical solidification mod-els. However, thermocouples have two major disadvantages. First, they are subject to large errorsbecause they act as a cooling fin on the strand surface, and they are affected by the water on thestrand surface and the rolls that they come in contact with. Second, they are impractical for gen-erating surface temperature data for a large number of casting practices, widths and grades. Opti-cal pyrometers are preferable because, once compensated for the emissivity and energy absorptionby water, they can be used for continuous temperature measurement at a particular spot on thecaster. Several pyrometers might be needed along the metallurgical length to establish the coolingcharacteristics in the different spray zones.

Laboratory tests have been useful in defining the effects of water flow rate, location in the spray,pyrometer offset distance, presence of steam, surface temperature, and ambient light on the accu-racy of pyrometers.35 Compensation for energy absorption through water can also be accomplishedin the laboratory tests. Table 20.2 summarizes the characteristics of various pyrometer types thatwere first tested under laboratory conditions and later used on two slab casters in a steel mill.35 Inthe mill, in order to eliminate spray water interference and obtain slab surface temperatures that canbe compared to the model calculations, water-flows from metallurgical spray, cross sprays and fogsprays were turned off at the pyrometer location for short periods of time. The pyrometer read thesurface temperature just before the strand entered the spray impact area. Fig. 20.40 compares themeasured surface temperatures under actual casting conditions against those from a mathematicalmodel before and after the spray impact area. The actual and calculated results are within 30°C.

The mathematical solidification models developed from strand surface temperatures are mostlyone-dimensional for practical reasons. Once developed and verified, they can be used to improvecaster operation, productivity and product quality.

20.5.3 Strand Shape and BulgingDevices have been developed and used to measure strand profile to understand and eliminate thecauses of:

• Excessive bulging or concavity on the narrow side just below the mold, whichmight cause breakouts, excessive wear on mold narrow side coppers, and surfacecracks.

• Deformed strand cross-section (e.g., trapezoidal), which might cause corner cracks.

Castin

g Vo

lum

e

34C

opyright © 2003, T

he AIS

E S

teel Foundation, P

ittsburgh, PA.A

ll rights reserved.

Table 20.2 Characteristics of Tested Pyrometers (from Ref. 35)

MinimumMeasurable Effect of Water

Manufacturer Type Sensor Wave Length, µ Temperature, °C Special Features Spray on Measured Temperature

A 2-color PM tube 0.50 – 0.58 900 Flexible light guide Causes high readings

B 2-color PM tube 0.45 – 0.75 830 Sights through tube Causes high readingsSensitive to ambient light

C 2-color PbS cell 1.65 – 2.30 — Sights through tube Not workable in presence ofwater due to hardware problems

D 1-color Si cell 0.4 – 1.1 700 Flexible light guide Causes low readings

E 1-color Si cell 0.9 700 Sights through tube Causes low readings

F 1-color PbS cell 1.6 330 Flexible light guide Causes low readings

Instrumentation


• Bulging between containment rolls that might cause internal cracks at the solidifi-cation front.

Because measurements are made in the spray chamber, where the environment is not conducive toinstrumentation, most shape sensors have been custom-designed physical contact devices involv-ing simple instrumentation. For example, to understand early solidification profile and concavityjust below the mold foot rolls on the narrow side, a shape detector involving two contact rolls anda displacement transducer was built and used on a high-speed slab caster.92 Fig. 20.41 shows theshape detector and the measurement results. The mold taper was changed to make shell growthmore uniform and reduce the narrow side depression to be within 2–4 mm.

When trapezoidal cross-section and corner cracks became a problem on plate grade slabs on acaster equipped with a straight mold, a probe was built and used at the bender exit, as shown inFig. 20.42, to assess slab shape coming out of the bender.93 The probe was equipped with twocurved contacts that were pushed against the slab narrow side and two displacement transducers.When the probe measurements were compared with mathematical calculations, the results showedthat the slabs were being excessively misshaped before exiting the bender. As one of the measuresto prevent slab misshaping and corner cracks, the bender frames were strengthened and the ben-der rebuilding procedure was improved.

Inter-roll bulging and attendant internal cracking at the solidification front has been a major con-cern for caster builders, operators and researchers. Sensors have been used to measure and corre-late bulging to machine design and casting practices. These sensors include custom-designedphysical contact devices,94,95 and more recently a laser bulge-meter.96 Figs. 20.43 and 20.44 showa contact bulge detector and a laser bulge-meter, respectively. Custom-designed bulge deviceshave the disadvantage that they have short operational life and generally can be installed in spe-cific caster locations. On the other hand, noncontacting laser-based bulge-meters have long oper-ational life and can be moved from one location to the next for continuous measurements at

cast speed: 1.3 m/min.slab: 200 x 1168 mm

before spray

after spray

measuredcalculated

Distance from meniscus (m)

Sur

face

tem

pera

ture

(˚C

)1400

1300

1200

1100

]1000

900

800

700

600

5000 5 10 15 20 25 30 35 40

Fig. 20.40 Strand surface temperature profile. From Ref. 35.

Casting Volume


XX

XX

X

X X

contact roll

measuring roll

displacement transducer

slab

air cylinder

(de)

(dc)

(dd)

moltensteel

solidified shell

depression

Depression of narrow face (dd) (mm)

Rat

io o

fre

tard

ed s

hell

grow

th (

de/d

c)

castspeed

1.0 m/min.1.21.41.620

1.0

0.8

0.6

0.4

0.2

0-4.0 0 4.0 8.0 12.0

Fig. 20.41 Measurement of narrow face depression and retared shell growth. From Ref. 92.

Instrumentation


curved contacts

bender rolls

LVDT

pusher rodslab

Fig. 20.42 Schematic of narrow sideshape probe. From Ref. 93.

linear transducerfor roll deflection

linear transducerfor bulging

drive motor

split roll

slab

air cylinder

motor forrotation ofthe device

V

180˚

∅ 150

∅ 80

Fig. 20.43 Mechanical contact type bulge detector. From Ref. 94.

Casting Volume


multiple points on the strand. Experimental and mathematical studies of bulging, and operatingexperiences with early casters, have led to the design of today’s modern slab casters equipped withsplit rolls of small diameter and pitch that can be operated at high casting speeds without the riskof excessive bulging and attendant internal cracking.

20.5.4 Final Solidification Point (Liquid Core) and Segment/RollLoads It is often desirable to know the complete solidification point (or length of liquid core) under dif-ferent casting conditions for quality and productivity considerations. For example, in casters wheresoft reduction is used to avoid the occurrence of centerline segregation, solidification must be con-trolled in the tapered zone of the machine. In machines equipped with single-point unbending,complete solidification must occur before the unbending point to avoid segregation and centerlinelaminations in medium- and high-carbon grade slabs. It is essential that solidification is completedwithin the containment zone under all casting conditions. Strain gauges97–100 and electromagnetictransducers101,102 have been successfully used to measure the end of the liquid core under differentcasting conditions. The results were then used to develop casting practices that would assure thecompletion of solidification at the desired location in the machine. Fig. 20.45 shows the installa-tion of strain gauge bridges in the thickness setting pins on both sides of a caster segment.97 Thesensors were installed at both the inlet and exit ends of the segment. The variation of the segmentopening force with casting speed during about a 2.2-hour cast is shown in Fig. 20.46. In the samefigure, the solid fraction calculated from a one-dimensional mathematical solidification model isalso shown. The objective of Fig. 20.46 is to establish the relationship between the ferrostatic force

laserblackbox motor driver

water-cooledaluminum case

bulgemetermountingframe

segmentframe

optocatergauge probe

stepper motor

gearbox

supportroll

strand surface

PC

Fig. 20.44 Laser bulge-meter. From Ref. 96.

Instrumentation


transmitted to the roll segment and the level of solidification. It is evident from the figure that sud-den changes in segment force occur when the solid fraction is within 0.65–0.70. This implies thatthe liquid becomes isolated within the dendritic network, thus preventing transmission of ferrosta-tic force to the containment rolls. These findings confirm earlier studies that suggest that the appli-cation of soft reduction beyond the point where the solid fraction exceeds 0.65–0.70 might not bebeneficial and can even have detrimental effects on quality.103,104

Segment load measurements with strain gauges showed that spray nozzle performance has a sig-nificant effect on the complete solidification point.98 The results showed that when nonoptimalspray nozzles were decreased from 58 to about 25%, the solidification constant increased by about10%, which enabled about a 20% increase in caster productivity.

Electromagnetic transducers have also been used to detect the liquid core in the containment ofslab casters.101,102 They have the advantage that they can be more easily moved from one locationto the next. Also, they more clearly indicate the tip of the liquid core. However, their longevity inthe hostile containment environment has yet to be proven.

Load cells and strain gauges are sometimes used to understand the causes of mechanical failures,particularly in the machine containment. For example, as part of a major undertaking to under-stand the causes of frequent roll bearing failures, load cells were used to measure the forces sus-tained by rolls in the bow and unbending section of a slab caster.105 As illustrated in Fig. 20.47,

drive sidethickness pins

free sidethickness pins

open

ing

forc

e F o

clamping force F

c

Fnet = Fc − Fo

upper segmentframe

lower segment frame

Fig. 20.45 Installation of straingauges in segment pins. FromRef. 97.

Casting Volume


500

400

300

200

100

0

1.3

0.9

0.5

1.0

0.8

0.6

0.4

0.2

0 2000 4000 6000 8000

solidfriction

opening force

Seg

men

t ope

ning

forc

e (k

N)

Cas

t spe

ed (

m/m

in)

Time (seconds)

Cen

ter

solid

frac

tion

Fig. 20.46 Variation of center solid fraction and segment opening force. From Ref. 97.

bearing

load cell slab

bearingthermocouple

A B C D

E F G H

horizontal1 horizontal

2

horizontal3straightener

bow

bender

foot rolls driven rollsnumbered rollsinstrumentedwith load cellsand thermocouples

54

5567

67 73

74 79

79

Fig. 20.47 Rolls instrumentedwith load cells and thermocou-ples. From Ref. 105.

Instrumentation


the load cells were installed in roll girders directly under the bearing blocks in the bow and in theunbending assemblies of the machine. The objective was to see if the forces sustained by the rollbearings during casting exceeded the dynamic load capacity of the bearings, thus contributing tothe frequent bearing failures. Numerous trials were conducted with strip and plate grade slabs.Slab width varied between 960 and 1950 mm, and casting speed varied between 0.76 and 1.6m/min. Roll loads were monitored during start-ups, cap-offs, steady-state casting and transientssuch as ladle changes, tundish tube changes, tundish flys and slowdowns. From the measured loadcell data, the loads sustained by the bearings were then mathematically calculated. The resultsshowed that the roll loads increased with slab width but were not significantly affected by steelgrade and cast speed. As illustrated in Figs. 20.48 and 20.49, there was little or no differencebetween the loads sustained by the top and bottom roll bearings in the bow and in the straight-ener. The maximum measured bearing load in the straightener was 231 kN, which was only 43%of the rated dynamic load capacity of the bearing. In the bow, the maximum measured bearingload was 133 kN, which was 31% of the rated dynamic load capacity of the bearing. Another sig-nificant observation was that start-up and cap-off slabs, and speed transitions caused only smallsurges or drops in roll loads. This is verified in Fig. 20.50, which shows one load cell output fromeach of three test rolls in the bow and straightener over a 75-hour test period. Three start-ups,three cap-off’s, numerous width changes and speed transitions occurred during the trial period,yet no major surges or drops in load cell outputs were observed. These load cell measurementsled to the conclusion that the cause of the frequent roll bearing failures was not the overloadingof the bearings. Rather, further tests revealed that debris found in the roll bearings was the maincause of the bearing failures.

20.5.5 Roll Bending and TemperatureA conventional slab caster can have 150–250 containment rolls, some of which sustain forces up to100 tons while contacting a slab with a surface temperature of about 1000°C. An idle roll of splitdesign costs in the range of US$8000 to $10,000, and repair costs are high. It has also been estab-lished that roll deflections in excess of 1 mm can cause intercolumnar cracking at the solidification

bearing

load cell

2388 mm

Bea

ring

load

(kN

)

speed: 1.14 m/min.width: 1850 mm

bottom roll 55top roll 54

Roll length

150

120

90

60

30

0

slab

Fig. 20.48 Measured roll bearing loads in bow. From Ref. 105.

Casting Volume


bearing

load cell

2388 mm

Bea

ring

load

(kN

)

speed: 1.0 m/min.width: 1930 mm

bottom roll 67top roll 67

Roll length

slab

250

200

150

100

50

0

Fig. 20.49 Measured roll bearing loads in straightener. From Ref. 105.

Time (hours)

Load

cel

l mea

sure

men

t (kN

)S

peed

(m

/min

)w

idth (m)

0 15 30 45 60 75

bottom roll 79 - load cell C

bottom roll 67 - load cell B

bottom roll 55 - load cell B

speedwidth

- start-up- cap-off

x x xo o o

ox

2

1

0

400

300

200

100

0

2.0

1.5

1.0

Fig. 20.50 Bow and straightener load cell readings during a 75-hour period. From Ref. 105.

Instrumentation


front. Thus, long operational life for reduced cost and stability for improved product quality haveprovided the impetus for researchers to measure roll bending and temperature in the containment.

Rolls bend because of mechanical and thermal interactions with the strand, misalignment withadjacent rolls and the condition of the roll itself. Fig. 20.51 schematically shows the types of bend-ing that can occur on a roll.106 These are the permanent, elastic, in-cast and stoppage bends (bp, be,bic and bs). The permanent bend bp can be measured by turning the roll when cold, and it usuallydetermines the need to change a roll. The elastic, in-cast and stoppage bends can be measured dur-ing casting by using linear displacement transducers contacting the roll surface. The transducermust be housed in a waterproof housing and attached to a water-cooled stable beam. Roll bendingmeasurements were conducted on solid rolls in a slab caster and were found to be highly depen-dent on the type of roll cooling, which can be external, central-bore, peripheral or scrolled.106,107

bp

bebic

bs

transient bend

away fromstrand

intostrand

bp = roll permanent bend (manual turning of room)be = elastic bendbic = incast bendbs = stoppage bend

Fig. 20.51 Schematic illustration of roll bending. From Ref. 106.

roll deflection strand speed

Def

lect

ion

at m

idsp

an (

mm

)

Time (seconds)

deflectioninto strand

Strand speed (m

/min)

2

1.5

1

0.5

0

-0.5

1

1.5

1

0.5

00 100 200 300 400 500 600 700 800 900 1000

Fig. 20.52 Thermal distortion of solid roll during speed transient. From Ref. 108.

Casting Volume


Variation of total bending of solid rolls in the unbending zone with time and casting speed is shownin Fig. 20.52.108 Severe distortion due to nonuniform heating occurs in the shape of the roll duringstrand stoppages. When the strand is stopped, the nonuniformity of the temperature distribution inthe roll causes it to distort and bend toward the strand. When the strand resumes motion, the dis-torted roll wobbles for a few revolutions until its temperature is more uniformly distributed. As Fig.20.52 shows, at the mid-span the total roll bending was found to be about 1.5 mm during strandstoppage. The amount of roll bending that can be tolerated depends on the casting practices and

Tem

pera

ture

(˚C

)

Time (seconds)

start slowdownstrand stopped resume motion

Depth, mm

@ 2.54@ 25.4

@ 6.35@ 76.2

0 100 200 300 400 500 600 700 800 900

650

520

390

260

130

0

Fig. 20.53 Temperature distribution in solid roll body during speed transient.

Max

imum

rol

l sur

face

tem

pera

ture

(˚C

)

Strand speed (m/min)

0 0.25 0.5 0.75 1 1.25 1.5 1.75

300

250

200

150

100

50

0

Fig. 20.54 Effect of speed on maximum roll surface temperature. From Ref. 108.

Instrumentation


steel grades, and it should be determined for each caster installation. Roll bending has not been anissue on state-of-art slab casters, which are equipped with split rolls having multiple supportpoints.

Containment rolls are often replaced because of thermal cracking and spalling at the outer surface,or because of bearing failures under thermal loading. Thus, it is often desirable to measure rollbody and bearing temperatures. This can be accomplished by using thermocouples. For bearingtemperature measurements thermocouples can be inserted through grease lines or groovesmachined into the roll axle, depending upon roll design. For body temperature monitoring specialthermocouple plugs can be fabricated and installed at various locations on the roll body. Fig. 20.53shows temperatures measured by such devices at various depths below the surface of a solid rollduring steady state casting and strand stoppage in a slab caster.108 Fig. 20.54 shows the maximumroll surface temperature measured at various casting speeds in the same caster installation. As thefigure shows, the roll temperature varied nonlinearly with casting speed because of the particularway the secondary water rate is controlled with casting speed. Roll temperature is determined byroll cooling, secondary cooling practices and roll design, which vary from caster to caster. Hence,roll temperatures should be measured at each caster installation or carefully extrapolated from onecaster to the next.

20.5.6 Roll Gap, Alignment and RotationRotating rolls, correct roll gaps and well-aligned rolls are prime requirements for good as-castproduct quality and high machine productivity. Stuck rolls result in gouging in the product surface,which can sometimes go unnoticed. Bad roll gaps and poorly aligned rolls can yield productdefects such as:

• Deformed cross-sections.

• Corner cracks.

• Refill (mid-face) cracks.

• Segregation.

• Open (laminated) centers.

Stuck rolls, incorrect roll gaps and poorly aligned rolls also result in increased machine mainte-nance costs and reduced machine utilization. It is therefore essential that roll alignments, roll gapsand roll rotations be checked and maintained regularly. These functions can be performed manu-ally by using handheld gap tools and templates, but they are time-consuming and require trainedoperators. The better approach is to use an automatic device that can be attached to the starter barchain and that measures roll gap, roll alignment and roll rotation as it is moved up and down themachine. Such a device is referred to as the “gap tool.”109–111

The early versions of gap tools were provided by casting machine builders, and they measured dataon roll gap, roll alignment and stuck rolls. Recent tools can provide additional information on rolleccentricity and spray-water pattern.110 Two versions of gap tools are available. The most com-monly used version is attached to the end of the starter bar chain, replacing the starter bar head,and is drawn through the machine during scheduled maintenance downturns or turnarounds. Theother version, referred to as “on-board gap tool,” is permanently installed on the starter bar chainin between links and is drawn through the machine during each start-up. The type of gap tool cho-sen should be determined by considering the downtime available for gap runs and maintenancefunctions, and by weighing gap tool purchase and maintenance costs against productivity and qual-ity gains that can be achieved through its use.

Gap and alignment tools provide data on the cold status of the machine. However, experienceshows that roll gaps and alignments can significantly change during casting because of roll and

Casting Volume


segment dynamics under thermal and mechanical loading.93,112 Hence, is often desirable to checkthe stability of the roll gaps and alignments during casting. Such in-cast checks enable the under-standing of the causes of product defects and the control of gap tapering on casters that employsoft reduction for improved internal quality. The sensor that is commonly used to assess roll andsegment stability during casting is the linear displacement transducer. Such sensors were used tomeasure hot-gap changes and frame movements under various casting conditions in a slab casterequipped with a straight mold, a bender and a bow.93 The intent was to determine if roll gapdynamics and roll frame movements at the upper end of the machine were the causes of cross-sectional distortion and corner cracking on slabs of plate grades. Cold measurements were madebefore the start of cast to assure that gaps and alignments were within the machine tolerance of ±0.5 mm. As is depicted in Fig. 20.55, the hot measurements showed that the bender and bowmoved out of alignment by as much as 2.5 mm shortly after start-up. Slab inspections showed adefinite correlation between the incidence of corner cracks and the magnitude of the bender-to-bow

Mis

alig

nmen

t (m

m)

Time (hours)

Cast speed (m

/min)

start-up tube change tube change tube change

1

0

-1

-2

-30 1 2 3 4 5

1.0

0.8

0.4

0

Fig. 20.55 Alignment change between bender and bow during casting. From Ref. 93.

Fig. 20.56 Linear variable displacement transducers (LVDTs) used to measure dynamic movements. From Ref. 112.

Instrumentation


hot misalignments. The bender and the bow were mechanically linked to keep them aligned dur-ing casting. This and other measures taken to stabilize roll gaps and roll frames minimized slabcross-sectional deformation and corner cracking.

Displacement transducers were used to understand the causes of mid-face cracks and centerlinelaminations in slabs of high-carbon and plate grades.112 The transducers were installed at the endof the bow and on the straightener section of the slab caster to investigate roll gap and frame sta-bility during casting. Fig. 20.56 shows the installation of the transducers in the bow exit andstraightener inlet areas of the caster. The measured gap changes at the last bow roll pair and thefirst straightener roll pair are shown in Fig. 20.57 as a function of cast speed and time. The gapchanges at both rolls exceeded the allowable limit of ± 0.5 mm significantly, and showed bigswings during speed transients. Several mechanical improvements were implemented to minimizethe large gap changes and other excessive frame movements measured with the displacement trans-ducers. One of the improvements was the removal of the top driven roll at the inlet of the straight-ener from hydraulic cylinders and installation on rigid girders. Fig. 20.58 shows the improvementin gap change profile at the bow exit and straightener inlet area of the machine as a result of themechanical improvements. The dramatic decrease in slab open centerline incidence as a result ofthe study and numerous mechanical improvements is shown in Fig. 20.59.

Displacement transducers were used by others to investigate dynamic roll movements and roll gapdistortions in the straightener area,113 and also stability of gap tapering in the soft reduction area114

of slab casters.

Cas

t spe

ed (

m/m

in)

Gap

cha

nge

(mm

)

Time (hours)

bow exit

gap closing

gap opening

straightener inletgap closing

gap opening

2.0

1.5

1.0

0.5

0

-0.5

0

0.5

1.0

1.5

0

0.5

1.0

1.5

2.0

2.50 2 4 6 8 10 12 14 16 18

Fig. 20.57 Cast speed and rollgap changes at bow exit andstraightener inlet. From Ref. 112.

Casting Volume


Today, on modern high-productivity slab casters, displacement transducers are routinely used toautomatically change slab thickness between casting sequences.68 The objective is to avoid longoutages, which would otherwise be taken to manually change roll gap settings. They are also usedin conjunction with on-line solidification models to dynamically monitor and automatically changethe tapering of roll gaps to assure that complete solidification occurs in the tapered section of themachine.68 This helps to improve the internal quality of slabs of critical grades.

20.5.7 Cast Speed and Strand TrackingCast speed is the most basic and fundamental parameter that is measured and controlled on a con-tinuous caster. Cast speed is commonly measured with pulse encoders on drive roll motors. Themeasurement location varies from one caster to the next but is usually at the lower end of themachine where the strand is surely in contact with the drive roll. In some installations, a drive roll

bow exitstraightener inlet

stabilizedroll

after rollenhancement

gap openingtolerance = ± 0.5 mm

before rollenhancement

Gap

ope

ning

(m

m)

2

1.5

1

0.5

0

Fig. 20.58 Roll gap changes at the bow-to-straightener transition before and after straightener roll enhancement From Ref.112.

MAMJJASONDJFMAMJJASONDJFMAMJJASON

Ope

n ce

nter

inde

x

Month

1991 1992 1993

1.0

0.8

0.6

0.4

0.2

0

machine enhancements androll tapering

Fig. 20.59 Decrease in the incidence ofslab open centers. From Ref. 112.

Instrumentation


is designated for primary speed measurement, and one or more other drive rolls are used for com-parison and backup measurements. Strand tracking is also carried out with pulse encoders, usuallyon the drive rolls that are designated for speed measurement and control.

20.6 Slab Processing Area (Runout)

20.6.1 IntroductionThe caster runout begins at the end of the containment (or the last horizontal strand guide) and isequipped with bottom rolls only. In the runout, the completely solidified section is cut into orderedlengths, weighed, stamped with an identification number, inspected, stacked into piles and shippedfor rolling. Other functions such as sample cutting for metallurgical testing (macro etching or sul-fur printing), crop cutting and tail cutting are also performed in the runout.

One of the major challenges for caster runout operators is to provide correct information on weightand dimensions for each cut section to enable direct application of product to intended orders. Sec-tions with wrong weights or dimensions are inventoried for later applications, which is undesirable,particularly in mills that utilize direct or hot charging. Weight is routinely and reliably measuredwith scales before the cut sections are piled at the end of the runout. However, providing accurateinformation on dimensions is complex because of strand thermal shrinkage and creep under ferro-static pressure below the mold. In most of the caster installations, dimensional information is pro-vided through physical measurements on the hot sections in the runout. In others, dimensionalinformation is provided through simple algorithms. Another big challenge for caster operators is toprovide accurate quality information for each section. Quality information is provided mostly bycomputerized on-line models, which make quality predictions by monitoring and comparing actualcaster parameters against those in lookup tables. Over the last 20 years, several devices have beendeveloped for direct measurement of quality on hot sections in the runout. The following sectionssummarize the various devices that are available for measuring dimensions and quality on hot sec-tions in the runout.

20.6.2 Length,Width and Weight

Three methods are commonly used to measure the length of the as-cast product before the cuttingoperation begins. The first is by tracking the position of the strand and the position of the torchmachine. The strand position is tracked by means of speed measurements on one or more driverolls in the caster containment section. The position of the torch machine is tracked by means ofan encoder that is tied to a rack-and-pinion mechanism connected to the torch machine. From thestrand and torch machine positions, the distance between the head of the strand and the torches iscalculated. When that distance equals the ordered length, the torch machine is clamped onto thestrand and the cutting operation is started. The second method entails the use of low-level lasers.The third length measurement method involves the use of a wheel on the torch machine that con-tacts the narrow surface of the strand and directly measures the distance between the head of thestrand and the torch machine. The measurement wheel is water-cooled, and the wheel rotation isconverted to linear distance by means of an encoder. When properly maintained and regularly cal-ibrated, all three methods yield length within the design tolerance, which is usually ± 1% of theaim length. For example, manual length measurements on numerous hot slabs showed that only2% of the lengths measured with the wheels were outside the length tolerance of ± 76.2 mm.115

The width of the slab in the runout is different from that at mold exit mostly because of thermalshrinkage and creep under ferrostatic pressure.115 Other contributors to width variation includeincorrect mold setup and roll gap profile. Width variance from one slab to the next is largely depen-dent on grade, secondary water practice and cast speed. Five methods are used to determine slab

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width in the runout. These are predictive algorithms, manual measurements with handheld calipersat the pilers, on-line physical measurements with contact devices on or around the torch machine,line-scan cameras, and lasers. Predictive algorithms are either experience-based or statisticallydeveloped. Both involve grade, secondary water practice and speed (or residence time in contain-ment).115,116 Manual measurements with handheld calipers are conducted intermittently at the pil-ers. When the difference between the measured and ordered widths exceeds an allowable limit, thedifference is keyboard-entered as a correction and remains in effect on all subsequent sections untilthe next correction. On-line contact devices are either built in-house or supplied by torch machinebuilders. In either case, a mechanical device from either side of the slab moves in and contacts thenarrow side of the slab. Slab width is calculated from distance traveled from home position by eachdevice. Accuracy can be within ± 2.5 mm and is highly dependent on buildup or wear on the con-tact points, the shape of the slab narrow sides and the condition of the encoder. Lasers are gainingpopularity because they are accurate and require low maintenance.115

Predictive algorithms, manual measurements with handheld calipers and on-line contact devicesprovide single-point width information on each slab and are not conducive for identifying taperedslabs. Lasers and line-scan cameras provide continuous width measurement, thus enabling casteroperators to identify tapered slabs. The major disadvantage of the camera systems is that theyrequire light banks under the slab to highlight the dark (cold) edges of the slab. This can rendercamera systems maintenance intensive and impractical for use on casters.

20.6.3 Hot Surface QualityIn most modern slab caster installations, a computerized quality control system is used to assign aquality rating for each slab. The rating is based upon the comparison of actual process parameters,such as mold level and casting speed, which are monitored during casting against standards in pre-defined disposition tables. The concept is good and works adequately. However, it has several

mearuringequipment

continuous castermaster control system

cooling watersupply

descallingequipment

crackmarking

manipulatorcontrolsystem

= CC = AW

= ME = DE = CM

=akcontrol equipment

maincontrolpanel

printer

localcontrolpanel

over

all c

ontr

ol s

yste

m

-A11 -A61

-A12

HC

= MC

Fig. 20.60 Eddy current-basedsurface inspection system. FromRef. 118.

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drawbacks. First, it is as good as the informa-tion in the disposition tables, which are gener-ally configured through statistical analysis ofcasting and quality data. The tables can betuned for improved accuracy or new qualityspecifications, but this requires production tri-als that are costly and time-consuming, and itinvolves lag time in the actual implementationof practice changes. Second, the dispositiontables are conservatively designed forimproved quality assurance, which can lead tothe rejection of acceptable product. Third, attimes, defective slabs can flow through thesystem unnoticed. In an attempt to overcomethese drawbacks and to achieve 100% qualityassurance in an economical manner, severaldevices have been developed for on-line qual-ity inspection of hot slabs in the runout. Basedon the principle utilized, the devices can bebroken into three predominant groups: electro-magnetic, optical and ultrasonic.

Fig. 20.60 shows one of the earliest hot-slab surface inspection methods that is based on the eddycurrent principle and was first installed in Domnarvet, Sweden117,118 It consists of a robust water-cooled sensor that is moved back and forth on a beam, which is mounted transverse to the castingdirection. The sensor consists of a coil that is mounted in a housing, about 5 mm above the slab.On detection of a crack, a paint mark is made on the slab. The sensor scans only the upper surfaceof the slab. Inspection of the slab surface is done above the Curie temperature of 768°C. High-pres-sure water is used to descale the slab surface. Several grades of steel, including medium-carbon,silicon, boron and HSLA steels, were tested on-line. The system can detect longitudinal, transverseand star cracks that are deeper than 2 mm. Reportedly, it also detects scabs, slag spots and double-skin effects. Fig. 20.61 shows typical output signal as a function of average crack depth. In a testof 200 slabs, agreement between manual and on-line inspection was 95%.

Some of the other eddy current devices developed for hot-surface inspection are described in Refs.119–125. The features that are common to most of these eddy current systems are that the slab sur-face temperature must be above the Curie point, the surface must be descaled, and only the top sur-face can be inspected. The system described in Refs. 124 and 125 is used for the detection oftransverse edge cracks on each side and is controlled by a robot. The slab surface temperature mustbe below 500°C. System reliability is better than 99% in detecting corner cracks that are deeperthan 2 mm.

Optical surface inspection systems employ industrial TV cameras (ITV), charge coupled devices(CCD), line-scan or TV cameras, laser scanning and photographic methods.126–132 Fig. 20.62 showsthe system described in Ref. 126, which was installed on a slab caster to inspect both the top andbottom surfaces of slabs. Optical systems are suitable for the inspection of large surface areas butdo not indicate crack depth and are not suitable for hairline or short cracks. Also, the surface mustbe properly cleaned, which is generally satisfied by high-pressure water systems that make it dif-ficult to maintain uniform temperatures across slab width.

Ultrasonic133 and electromagnetic-ultrasonic134 methods were developed and tested to detect inter-nal defects, such as primary pipe, centerline lamination, refill cracks and segregation. In the ultra-sonic method, water is employed as a coupling medium between a combination transmitter/transducer head and the test slab. In the electromagnetic/ultrasonic method, transmitting and

Out

put s

igna

lCrack depth (mm)

6

4

2

01.0 1.6 2.0 2.5

Fig. 20.61 Eddy current strength versus crack depth. FromRef. 188.

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receiving probes are held close to either side of the slab broad faces. The transmitting probes trans-mit an ultrasonic wave through the thickness of the slab, which interacts with a stationary magneticfield in the receiver and generates an induced current. The presence of internal defects is indicatedby the attenuation of the induced current.

Internal defects are associated with passline irregularities, bulging, machine operation and sec-ondary water practices. Generally, they occur in plate-grade slabs, which constitute a small frac-tion of continuous casting, and are not caused by the more frequent mold perturbations. Hence,while being important, they are of a lesser concern in the industry than surface defects. As a result,ultrasonic and electromagnetic/ultrasonic devices for internal defect detection have received lessattention than eddy current devices for surface defect detection.

20.7 AcknowledgmentsThe author expresses his gratitude to Dr. Donovan N. Rego of Bethlehem Steel’s Homer ResearchLaboratories for his numerous contributions. The help provided by Robert H. Klotz and Nancy A.Reszek is also greatly appreciated.

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signalprocessor

dataprocessor

videosignal

VTR

processcomputer

magneticdisk

floppydisk

line printer

CRT console

hardcopyunit

CCD

CCDtransfertable

slab

surfacepre-conditioningmachine

Fig. 20.62 Optical surface inspec-tion system. From Ref. 126.

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