Flow control in continuous slab casting? ABB approaches closed loop peak performance

The stakes for sustainability of the steel industry are high, but complex processes and legacy assets will take time to transform. In the near term, advanced flow control during steel casting is a vital step in optimizing quality, minimizing rejects and thereby reducing industry emissions. By tuning processes to produce consistently better quality steel, flow control will, says physicist Martin Sedén, “substantially increase the yield of the casting process, reduce the carbon footprint of steel and improve the handling of our natural resources.”

Sedén is an R&D Engineer at ABB’s Metallurgy business unit. His goal? To merge sensor and automation technologies with dynamic and powerful electromagnetic stirring and braking devices to achieve the ultimate in flow control and positively impact safety, steel quality, energy reductions and waste minimization in steelmaking processes.

The demand for clean, spotless and special steels is growing in diverse applications such as automotive, electrical motors, tooling and manufacturing, as well as structures and general infrastructure. Steel plants produce more specialized steels than ever before, and to maintain competitiveness, the end product quality is vital.

ABB’s Flow Control Mold (FC Mold), is already the most advanced technology of its kind, offering simultaneous mold stirring and braking from one fixed position for optimal process control in thick slab casting. Its development is based on long experience including 140+ installations for slab mold globally. It is also the chosen platform for major steelmakers such as POSCO, JFE, Hyundai Steel Company, Tata Steel and Thyssenkrupp, and is integrated into the casting equipment of all major original equipment manufacturers (OEM) – SMS Group, Primetals, Danieli and SPCO, among them.

Modern and emerging innovations are enabling electromagnetic flow control devices to deliver even greater performance improvements in steelmaking processes. Sedén is applying his skills in numerical simulations and product development to maximize the flexibility of these ABB technologies and ultimately harness their potential as the main component to realize closed-loop control in continuous slab casting.

“Tailored to individual manufacturer’s processes, electromagnetic flow control devices offer many different operating modes,” he says. “Importantly, they can be gradually and accurately controlled, which makes them the perfect contributor to a process in which conditions are constantly changing.”

^{_{With 140+ electromagnetic stirring and braking installations for slab mold, ABB's technologies are proven to improve productivity and end-product quality.}}

Sedén’s work in R&D for ABB’s Metallurgy team began in 2005 and since then he has co-authored several scientific papers aimed at bringing the industry closer to automatic, closed loop control in continuous slab casting. His early research encompassed magnetic fields in the steel mold; mold monitoring; the nexus between temperature measuring, stirring and braking the flow of the melt; the role of fiber-optic temperature sensing; and the need to eliminate asymmetric flow of molten steel into continuous casting molds, which has resulted in multiple technology patents.

“Producing premium grade steel on a large scale is a complex process sensitive to several critical obstacles,” explains Sedén. Focusing on one step alone – that of solidifying steel in a continuous caster – he says, “cracks, uneven material composition, non-metallic contaminants, and melt spills and other safety issues are just some of the many risk factors.”

Several of these issues are governed by the flow of the molten steel in the mold, which is inherently difficult to steer from the outside due to the intense heat of molten metal and the continuous nature of the process.  “Inner modifications of the flow can only be achieved by electromagnetic forces that penetrate the fluid,” says Sedén. “Electromagnetic flow control devices facilitate tuning and optimization of the flow of the casting melt in order to avoid potential quality issues.”

^{_{Watch ABB's Martin Sedén as he explains in simple terms how electromagnetic stirring technology works.}}

The fact that modern continuous casters typically use a variety of feedstocks to produce a range of steel products, says Sedén, “adds to the need for dynamic control of the molten flow with respect to the variety of casting conditions and range of undesirable process deviations that may occur.”

What’s valuable about ABB’s existing mold flow control solutions is that they already assist in producing stable, more robust steel quality over time, minimizing the need to scrap or downgrade substandard materials. Asymmetric flow in continuous slab casting, for example, is one event which can affect steel quality to the point where it no longer attracts premium prices.

Sedén explains, “You have hundreds of tons of molten steel pouring through a nozzle into a water-cooled copper mold where it starts to solidify from the sides, becoming increasingly solid in a continuous stream.” The nozzle itself is a crucial component in distributing the flow in the mold. An ill-designed nozzle, irregular nozzle geometry or off-centered nozzle position are all known potential contributors to asymmetric mold flow. "We are collaborating with suppliers of submerged entry nozzles to minimize asymmetric biased flows and maximize flow control capabilities utilizing the combined design of nozzles and electromagnetic flow devices." Nevertheless, contaminants in the molten mass can stick and build up unevenly in the nozzle, causing asymmetric (unbalanced) flows that negatively influence the uniformity and therefore quality of the solidified product.

“Ideally,” he says, "as we cannot predict this type of imbalance, we would like the system to automatically identify the deviation and react in time to stabilize the process or homogenize the molten mass flows, by activating or adjusting the actions of electromagnetic flow control devices.”

^{_{ABB’s existing mold flow control technologies already assist in producing more stable and high-quality steel production, minimizing the need to scrap or downgrade substandard materials.}}

Challenges to achieving ultimate control of the process include that each steelmaker may produce several different steel alloys, “different recipes”, says Sedén, that all behave differently during casting. Manufacturers may also run the caster in different formats – for example, wide or narrow – and at different speeds, which affects the rate of solidification and the conditions in the mold. These changes in the process need to be addressed, ideally by software adjusting to both programmed parameters and real-time data.

“What we're trying to get to,” says Sedén, “is a system that can aid operators in running these machines more efficiently, and in a safer way, identifying potential issues and reacting to them more quickly than the most experienced human can.”

He refers to the full closed-loop control system as the “holy grail” of steel manufacturing.

Some 20 or 30 years ago, Sedén says, “electromagnetic stirrers, or EMS, were manually operated – turned on or off with little nuance.” The next step implemented by ABB was parametric control, which, for example, allowed metals manufacturers to adjust the EMS in real time to varying casting speeds. “With that signal in the system, you could then automatically connect the EMS to apply a greater stirring strength if the casting speed was lower, for example.”

Today, the FC Mold Control's data patterns are fine-tuned based on quality system feedback as well as numerical simulations to deliver automatic online control of the FC Mold. This type of flow control is highly beneficial in dealing with planned variations in a caster, but it is still an open-loop one-way system which doesn’t capture unexpected, undesirable process alterations. The optimal process control solution is a closed-loop system that constantly adjusts to continuous sensor feedback from monitoring equipment that detects deviations from desired process characteristics.

The next generation of sensors will allow faster measurement of a wider range of influential melt factors with higher spatial resolution. Software will accurately interpret and link the signals. “We’re working to ensure that our technologies are so agile and flexible that they can react to whatever features come up” says Sedén.“Designing the appropriate electromagnetic flow control devices and putting together the mathematical algorithms that take care of the control – that's where I'm active.”

^{_{ABB’s vision is to achieve automatic, real-time, closed-loop control of the FC Mold to unlock even greater performance in continuous slab casting.}}

Sedén says that for him the electromagnetic attraction of ABB Metallurgy is “in the combination of working with technology development but also seeing its applied form.” In many other large companies, he adds, “I would be researching within a small, detailed, well-defined scope, but I wouldn’t get to see the whole chain of events. At ABB I’m developing technology in response to customer needs and optimizing products in response to their feedback. It’s a demanding position to be in but extremely rewarding when you succeed.”

Sedén recalls the first time he visited a steel plant, “The dimensions of the undertaking are awe inspiring, like when you arrive at the mountains, or first see an ocean horizon. Experiencing the scale of steel production impresses upon you that the output of these factories must affect people and the planet, just by its sheer volume.

He says that “from an emotional point of view, that’s what got me hooked on metal production”. Today, “that rationally translates into trying to make a difference to carbon emissions. Steelmaking is one of the bigger polluters in the world, and we are contributing to the solution.”