Enhancing mold flow control in continuous slab casting process

A pilot has been carried out at a European slab caster to measure mold flow conditions online, in real-time, in an effort to unlock a higher level of process insight, improve operation of the FC Mold electromagnetic flow control device and produce cleaner steel, faster and at lower cost

STEEL PRODUCTION CASE STUDY

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Electromagnetic flow control is a well-proven technology for improving quality in continuous slab casting. Electromagnetic stirring, based on the principle of travelling magnetic fields, has been shown as particularly efficient for flow acceleration, temperature homogenization and inclusion washing in surface/subsurface regions of the strand. Different configurations of static magnetic fields have also been utilized to brake and stabilize high speed processes. For varying casting conditions such as throughput, slab format, slag type, steel grade, argon flow injection, SEN type and immersion depth mold level and so on, different settings are needed to create the best possible flow conditions. The most appropriate settings can be found through casting trials where an experience database can slowly be built up, or by numerically simulating the process with modern computer technology and software. Both methods, however, are very time-consuming.
Powerful analysis of casting conditions and detection of mold flow pattern characteristics by means of high-resolution temperature measurements opens up new possibilities for enhancing mold flow control.

Digital solution to monitor and visualize real-time mold data for mold flow prediction and much more with fiber-optic mold temperature measurement technology - ABB Abilty™ Optimold Monitor


The challenge
  • How to reduce defects, rejects, steel downgrades?
  • How to gain real-time process insights allowing electromagnetic devces to improve mold fluid flow?
  • How to reduce time and effort needed to find appropriate settings?
The solution
  • Fiber-optic mold plate temperature measurement with ABB Ability™ Optimold Monitor
  • Detecting patterns in meniscus shape and dynamics, flow velocity and flow asymmetry
  • Detecting local thermal phenomena

The benefits
  • Monitor mold flow
  • Get early warnings on deviations
  • Access process insights that give new opportunities to adjust operation of your electromagnetic actuator and improve mold flow control
  • Produce cleaner steel, faster and at lower cost
"The interest in electromagnetic devices has steadily increased over the years, with the recognized potential for improving cast quality of slabs"
"A single broad face copper plate has been equipped with optical FBG technology, having a staggering 2660 temperature measuring points"
"In the first 8 months of operation, more than 2000 heats were cast using the mold equipped with the fiber-optic mold temperature measurement system"

Measurement set-up and future extension possibilities

Schematic set-up of ABB Ability™ Optimold Monitor pilot and potential future extension to ABB Ability™ Optimold Control

The core of the ABB Ability™ Optimold Monitor pilot is a set of 38 measuring optical fibers. One end is mounted into evenly spaced vertical holes in the top half of a new copper broad face plate and the other end is spliced into a multi-strand harsh environment collector cable in a connection box mounted on the side of the mold plate. At the mold end of each fiber, 70 Fiber Bragg Gratings (FBGs) have been accurately positioned every 5mm covering the upper broad face down to 375mm from the top of the copper. Each FBG acts as a single temperature sensor, equipping the broad face plate with a total of 2,660 sensors.

Using robust, expanded beam connectors, four 15m long connector cables are plugged in to an intermediate box positioned close to the caster. From the intermediate box, the optical transmission is carried by a 125m transport cable, which connects all the caster fibers to interrogators in a climate-controlled room. The light source in the interrogator sends a broadband light pulse into the fibers. Reflections caused by the FBGs are picked up by the interrogator unit and translated by software to temperature data for all sensor points. Temperature data is scanned and treated online for the entire mold plate twice every second.

In a future extension, the ABB Ability™ Optimold Monitor pilot sensor system can be connected to an electromagnetic actuator to allow for automatic flow control by means of electromagnetic device regulation using  ABB Ability™ Optimold Control. 

High resolution temperature mappings

The slab caster where the ABB AbilityTM Optimold Monitor pilot was installed, is also equipped with an ABB FC Mold, a two-level electromagnetic braking device controlling the flow of molten steel in the mold. During a trial, the FC Mold’s magnetic fields were varied, as the ABB AbilityTM Optimold Monitor scanned the thermal status of the mold. Time-averaged temperature mappings show that a strong magnetic field (a) results in a relatively homogeneous temperature distribution. For a vanishing top magnetic field (c), temperature distribution becomes inhomogeneous with accentuated hotspots close to the narrow faces.
Time averaged OptiMold monitor 2D heat maps for different FC Mold magnetic field configurations (Bbottom/Btop)

Meniscus shape along the width of the mold for different FC mold settings

Meniscus shape has been estimated based on measured temperature distributions and the resulting time averaged calculated meniscus shape for the FC Mold configurations described above for the 1100mm width at 1.9m/min.

Results indicate that the stronger top magnetic fields ensure a lower wave peak close to the narrow faces, whereas the meniscus wave height is much more pronounced for the un-braked top domain. An estimate of the dynamic meniscus fluctuations also indicates a stabilization over the entire width of the mold with the application of magnetic fields.

Meniscus speed control

A set of numerical simulations of molten steel flow in the mold was produced to illustrate the causes of the observed meniscus shapes and flow speeds.

The CFD-simulated meniscus shapes (shown here on the right) are very similar to the ABB Ability™ Optimold Monitor's estimated meniscus shapes. As the top magnetic field is missing, the flow pattern shows a strong upward directed flow along the upper part of the narrow faces. This vertical momentum pushes the meniscus wave upward and creates a high crest of potential energy close to the narrow face; energy that is converted into kinetic energy in the form of meniscus flow speed towards the SEN. For the stronger top magnetic field, the upper recirculation loop is restricted in speed and hence has a flatter meniscus with lower flow speeds.

100% / 57%

69% / 0%

Calculated meniscus flow speeds for different FC Mold configurations

Meniscus flow speed can be calculated based on temperature distribution and meniscus dynamics. By implementing algorithms for meniscus profile and meniscus flow speed in the ABB Ability™ Optimold system analysis unit, the deduced in-mold flow speeds can be monitored over time.

The variation of the calculated meniscus flow speed on the left and right sides is given over a two-minute trial period. The connection between the top magnetic field and meniscus general flow speed level is apparent. It is also clear that the unbraked high flow speeds undergo large fluctuations over time. A stable meniscus flow level around 0.3 m/s is in this casting sequence found for an FC Mold field setting of 57%/100% (top/bottom).

Monitored left and right hand side estimated meniscus wave heights (hL and hR) for last 2 heats before tundish and SEN change

Asymmetric mold flows can easily be monitored by separate left/right indicators of meniscus wave height or flow speed.

The meniscus wave height has been monitored over the last two heats of a casting sequence before tundish and SEN change

Before the ladle change at 17:14, temperature distribution and meniscus wave are relatively symmetric. In the last part of the sequence, asymmetry becomes more pronounced, biasing the meniscus wave on the left side. It can be concluded that clogging affects the flow pattern symmetry in the last stages of casting. These results reveal the ABB Ability™ Optimold Monitor’s potential in conjunction with a flow control device for counteracting and controlling in-mold flow asymmetries as soon as they are detected by the temperature measurements. A modern generation FC Mold has the ability to control left and right side magnetic fields independently and consequently apply different electro-magnetic forces to the mold flow on the left and right sides, and is a perfect fit for ABB Ability™ Optimold Control to regulate symmetry as well as the appropriate flow speed levels in real time.
Schematic configuration of ABB Ability™ Optimold Control

By means of high-resolution temperature measurements the ABB Ability™ Optimold Monitor can allow for powerful analysis of casting conditions and detection of mold flow pattern characteristics such as meniscus shape, flow velocity and flow asymmetry. This opens up new possibilities for enhancing mold flow control, dampening or accelerating flow speeds as well as controlling flow asymmetry. What’s more, the ABB Ability™ Optimold Monitor can extend functionality and performance of conventional thermocouple systems and detect local thermal phenomena. Fiber-optics measure undisturbed in the presence of magnetic fields.

Getting to the stage where a plant can gain maximum results from digitalization demands planning a business relevant and executable path
Digital transformation in steel industry

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Continuous casting real-time optimization

Reduce defects - typically by 50%, reject ratio and downgrades - in some cases by 80% and allow for higher casting speed with FC Mold. Use advanced software to close the loop and achieve automated control and further optimization of the continuous casting process in real-time
Electromagnetic stirring and braking devices for slab casting

Boost overall productivity and cost-efficiency. Access new markets within the higher quality segment
Automated, real-time control and optimization of the casting process

ABB Ability™ Optimold Control closes the loop between electromagnetic actuators and sensors to allow for real-time, automatic online control of mold fluid flow in slab casting
69% / 0%

100% / 57%

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