TMS ONLINE | TMS PUBLICATIONS | SITE MAP An Article from the January 2002 JOM-e: A Web-Only Supplement to JOM

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EXPERIMENTAL

The new thin slab caster at Algoma Steel's Direct Strip Production Complex (DSPC) employs a straight funnel mold with an effective length of 1.1 meters. The slab width cast can be varied from 800 mm to 1,640 mm and the slab thickness is 90 mm at mold exit, soft reduced to 72 mm. The mold is instrumented with 194 thermocouples—90 thermocouples per broad face arranged in ten rows and nine columns and seven thermocouples per narrow face. The first row of thermocouples on the broad face and narrow faces are 20 mm and 65 mm below the meniscus, respectively.

Mold temperatures are recorded every two seconds and represented in the form of temperature contours for the broad and narrow faces. The contour maps, in essence, provide a snapshot of the mold hot face temperatures over the entire area and exhibit a hot or a cold region depending on the conditions in the mold. The colors to represent shell temperatures have been picked to approximate reality.

A localized hot region (relative to neighboring areas) is usually due to shell adhering to the mold wall—a classic symptom of an impending sticker. In addition, direct metal impingement on the mold walls also shows up as hot regions. This can be used to monitor bleeders that can heal if caught in time, submerged entry nozzle (SEN) alignment with respect to broad faces, and SEN rupture.

Colder regions exhibit themselves as a columnar front, as can be seen in one of the examples presented, and signifies a loss of shell contact with the mold wall. This 'flexing' of the shell either in the off-corner regions or on the narrow faces results in localized shell thinning (Figure 1). The regions of the slab with a thinner shell can cause a longitudinal broad face crack of varying severity or in the worst, rupture causing a breakout.

The mold hot face temperatures were further used in developing breakout detection algorithms. The temperature differential between mold inlet and outlet water temperature (delta T) was converted to integral strand heat flux that in turn was used to estimate a shell thickness expressed as a shell thickness index. The integral mold heat flux was computed as well.

All this data—thermal map, heat flux values, shell thickness index, text, and audio diagnostics from the breakout prediction algorithms-are presented to the operators in a composite format (called the Hemy Vision) as shown in Figure 2.

The actual Hemy Vision format the operator sees has better definition and clarity than the examples shown here. One significant feature of this system is its ability to detect bleeders (slow breakouts). In these instances, the control system initiates an automatic termination of the cast, thereby minimizing equipment damage. Audio alarms accompany all visual alarms. A "replay" mode is used for post-mortem analysis of any cast, good or bad. The dark dots in the thermal maps represent thermocouples that are out of service.

Figure 1. Broad face shell thinning due to loss of shell contact with the mould wall.		Figure 2. The Hemy Vision screen format displayed to the operators.

RESULTS

The instrumented mold is routinely used at the DSPC on all production runs. In the following sections, six 'live' examples of unstable process conditions in steady and transient casting states are presented to illustrate the diagnostic capabilities of the control system.

Longitudinal Broad Face Crack

The origination of a longitudinal broad face crack is very evident in this example of a low-carbon, high-manganese vanadium bearing grade at a narrow width of 1,280 mm, casting steady state at a casting speed of 3.0 m/min. (Animation 1). The cold column G on the outer broad face indicates a severe crack in that region. The operator initiated a 'sticker' sequence—a momentary slowing of the cast to 0.7 m/min., and recovered out of a potential breakout situation. Although the Hemy Vision alarms at the onset of a broad face crack, it is still the operator's call to take a corrective action. In this instance, his response was timely and appropriate.

SEN Rupture

The progressive degradation of the symmetrical flow from the SEN ports in this cast of a Peritectic grade exhibits itself in Animation 2. The failure of the SEN is not surprising given the cast was into the eighth or ninth heat of a sequence. Note that this video is a combination of various snapshots taken over five minutes of casting. Although there is currently no logic within Hemy Vision to detect a SEN failure, this phenomenon can be readily inferred, as shown in this example.

Meniscus Tear

Animation 3 is an example of a high-carbon grade cast to a width of 1,567 mm which clearly exhibits one of the elements of process instability that at times can occur at start cast. At 2.7 min. into the cast, the meniscus region at the outer south corner was beginning to collapse. The operator slowed the cast down momentarily to 0.5 m/min. and recovered from a potential breakout situation. With unstable process conditions inherent at the start of a cast—cold hot face, excessive mold-level fluctuations, and unsteady mold heat removal—such a recovery would not have been possible without the level of sophistication employed in the Hemy Vision software. As a further enhancement, logic to detect this condition and trigger an automatic slow-down of the casting speed was implemented in December 2001.

Animation 1. An example of a low-carbon, high-manganese vanadium bearing grade at a narrow width of 1,280 mm, casting steady state at a casting speed of 3.0 m/min. To best experience this presentation, you should employ the latest version of RealPlayer.		Animation 2. The progressive degradation of the symmetrical flow from the SEN ports in a cast of a Peritectic grade. Note that this video is a combination of various snapshots taken over five minutes of casting. To best experience this presentation, you should employ the latest version of RealPlayer.		Animation 3. An example of process instability that can occur at start cast, as shown in a high-carbon grade cast to a width of 1,567 mm. To best experience this presentation, you should employ the latest version of RealPlayer.

Loss of Taper

The loss of narrow face taper in this cast of a medium-width, low-carbon grade (Animation 4) resulted in the loss of contact of the solidifying shell with the north narrow face and, as a consequence, insufficient heat removal, exhibited by a steep drop in narrow face-shell thickness index. The shell-thickness index has not been incorporated in the breakout-detection system as of yet and, therefore, no alarms were generated.

Narrow Face Shell Detachment

Animation 5 provides an excellent example of narrow face losing contact with the mold wall on a Cb HSLA, narrow width, cast at a steady-state casting speed of 3.6 m/min. A lower north narrow hot-face temperature in comparison to south narrow face and a progressive drop in shell-thickness values indicate a loss of contact and local shell thinning that could have potentially caused a breakout. Although logic to detect this condition had been incorporated within the Hemy Vision software, it was not active during this run.

Broad Face Shell Detachment

The off-corner detachment of the inner broad face in the region of column C (Animation 6) is evident in this example of a near-peritectic grade cast. The cast width is narrow, at 972 mm. The flexing of the shell extends from the meniscus to half the effective length of the mold, causing localized shell thinning (see Figure 2). The edge detachment algorithms recognized the symptom and automatically initiated a slowdown of the casting speed to 2.1 m/min. It can be seen that the recovery to steady-state casting was seamless and smooth.

Animation 4. The loss of narrow face taper in a cast of a medium-width, low-carbon grade, which resulted in the loss of contact of the solidifying shell with the north narrow face. To best experience this presentation, you should employ the latest version of RealPlayer.		Animation 5. An example of the narrow face losing contact with the mold wall on a Cb HSLA, narrow width, cast at a steady-state casting speed of 3.6 m/min. To best experience this presentation, you should employ the latest version of RealPlayer.		Animation 6. The off-corner detachment of the inner broad face in a near-peritectic grade cast. It can be seen that the recovery to steady-state casting was seamless and smooth.To best experience this presentation, you should employ the latest version of RealPlayer.

CONCLUSIONS

The features incorporated in the Hemy Vision system combine in-situ observation of casting conditions with heat flux and temperature-based algorithms. Given the very short residence time in the mold of the solidifying shell due the high speed employed in the casting of thin slabs, the diagnostic capabilities provided by this system have been found to be invaluable in the early detection and correction of unstable casting conditions. As a result, the incidences of breakouts have reduced dramatically and new casting practices have been easy to implement.

ACKNOWLEDGEMENTS

The authors would like to thank the management of Algoma Steel for granting permission to publish this work. The help and support from the DSPC operating management, operating crews, maintenance staff and support engineers is gratefully acknowledged. And finally, D. McFarlane's assistance with capturing and recording the video images that made this publication possible.

For more information, contact P. Hemy, Algoma Steel Inc., Manufacturing Technology Department, Sault Ste. Marie, Ontario, Canada; (705) 945-3019; e-mail philhemy@hotmail.com.