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Simulation Steels Praise By Optimizing Casting Process

Fri, 04/03/2009 - 4:18am

By Christian Deville-Cavellin, Ugitech SA

Every industrial process aims to optimize speed. For French stainless steel manufacturer Ugitech SA, that goal translates into running its continuous casting machines at the highest possible speed while maintaining its standard of quality.

These operational speeds, nonetheless, more frequently culminate in the premature cutting of individual pieces of stainless steel from the square bloom coming out of the casts. The steel in the center of the mold, consequently, doesn’t entirely solidify, thereby forming a temporary molten-metal well that can threaten the release of as much as 1.5 tons of liquid steel into the bottom of the metal manufacturer’s vertical casting machinery, causing major damage.

Another disadvantage of premature stainless steel solidification is that it can lead to cracks, depressions, oscillation marks (which are incurred by vertically oscillating molds to effect lubrication) and segregations near product skin. To help avoid these pitfalls, Ugitech decided to purchase COMSOL Multiphysics simulation software, bestowing upon the company the capability to optimize operational temperatures and process speeds for each of the 150 grades of steel the plant produces.

In Ugitech’s casting process, molten steel enters a tapered copper-tube mold, which is vigorously cooled by circulating external water. During this stage, a rigid shell forms and solidifies around the liquefied metal, protecting it against ferrostatic pressure from the molten-metal well inside the strand.

After that, the shell endures a series of water treatments to enhance its strength, while rollers prevent it from bulging. Upon cooling, however, the shell shrinks, resulting in the formation of air gaps. Then, finally, the strand cools down through radiation.

While the location of these air gaps greatly impact final product, proper control of this process remains delicate. For example, if the air gap opens too early, not enough heat can escape from that area of the mold, which produces a thin solid skin, plus the appearance of internal product defects.

On the other hand, if the gap is too thin, the mold can become overly conical. Under these circumstances, friction arises between the strand and copper mold; if friction remains or becomes excessive during the extraction process, shell breakout may occur below the mold.

Casting About For A Solution

Simulation is first necessary to size various mechanical components, such as the coil and magnet in the driver, and examine their respective effects. While those two components offer only unidirectional movement, the panel on the opposite side of the gel can create very complicated waveforms, especially at high frequencies. These waveforms are comprised of movements that may be able to induce sound distortion and frequently shifting modal shapes.

Multiphysics modeling also makes it possible to understand what exactly occurs within the steel bloom as it passes through a continous casting machine. Using COMSOL Multiphysics, along with heat transfer and structural mechanics modules, to compute skin deformation during the solidification process, Ugitech was able develop and verify its model against experimental data roughly within six months.

The simulation, nevertheless, consists of two parts — a pure-heat transfer model that can predict temperatures and phases within the bloom, and a thermomechanical model that can facilitate understanding of the mold/steel interface, as well as explain certain bloom surface defects, which simplifies the process of correcting them.

Part of the difficulty in model setup is due to strong contact non-linearity between the steel and mold. To compound matters, the steel also undergoes phase changes that require the location, and subsequent inclusion in the model, of thermophysical data about each steel grade.

For instance, users can directly include a description of thermal conductivity in COMSOL by applying a tertiary polynomial, based on years of experimental data, yet can also request that COMSOL extrapolate between a table of 40 to 100 data points in one critical temperature range.

Ugitech spends a great deal of time studying various cooling aspects of the model in an effort to increase process speed without impacting product quality or changing end product properties. Because these are considered sensitive matters, however, the company says that it prefers to leave its customers out of the experiment.

In some markets, such as the automotive and nuclear industries, end users run their own steel trials and certifications. Thus making a process change takes considerable thought and planning, but Ugitech insists thats Multiphysics is extremely helpul in terms of both offering understanding and revealing possibilities.

The model inherently contributes to company value, moreover, by not only furnishing staff with the dexterity to assess equipment with clarity, but also by instilling in them a deeper sense of familiarity with the machinery.

One time in fact, the machine’s production engineers asked Ugitech to study the secondary cooling section in order to validate a move of just a few centimeters for improved maintenance access. Even a seemingly minor change like this, though, could significantly impact process.

Regardless, the model endowed the company with the ability to positively confirm position adjustment of the cooling section, while corroborating that no serious consequences would be incurred as a result.

Multiphysics Makes The Grade

After reviewing myriad simulation products, Ugitech, in search of a tool that could apply to a diversity of what-if situations, selected COMSOL ... If not just for its modeling functionality, according to the stainless steel manufacturer, then for its price/performance ratio.

Since the company already had specialized mechanical engineering software, someone had to convince management that the software would be a valuable multiphysics tool. The initial project (a.k.a. testing grounds) involved finding the temperature profile of a moving wire inside a heated tube. With the aid of COMSOL Multiphysics, Ugitech easily and quickly pinpointed this profile and subsequently convinced management of the software’s wide-ranging utility.

Meanwhile, Ugitech has unearthed new ways of implementing and solving problems with this package, mostly due to the power it provides by permitting staff to add any physics to their models. After integrating this approach, productivity drastically improved — more so than with any other simulation software, according to the company.

Ugitech’s first Multiphysics model inspired new ideas in the plant’s process engineers, as well as encouraged them to implement new troubleshooting techniques. Even more poignant, though, is that their exposure to this problem-solving platform has, furthermore, taught them to ask more productive questions.

More and more people Ugitech employees are considering the benefits of increasing simulation initiatives. It is apparent in the simple observation that questions are no longer posed in a yes/no format, such as "Can we calculate that?” but rather as an open-ended inquiry reflective of “What happens when ... Or if ...?”

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