Stainless steel is a valuable material. But a relatively large amount of scrap is generated during production when the rolling mill has to be stopped in order to replace worn-out work rolls with new ones. Idling the rolling mill leads to thickness defects in the final product, so that it does not meet quality standards. Hans-Joachim Felkl (58) from Erlangen has developed a new process for replacing rolls that allows the material to retain its nominal thickness. A cold rolling mill that uses the new process can achieve an increase of at least seven percent in throughput and prevent the scrapping of 6,000 metric tons of finished product per year.

To make thin stainless steel sheets, the material is run as a strip through a rolling mill in which the metal is worked by up to five rolling stands. The steel strip becomes thinner as it passes from one rolling stand to the next. When their surface and contour are no longer good enough, the two work rolls of a rolling stand have to be replaced and refurbished in order to produce stainless steel without defects. This involves idling the entire mill and using a special structure to pull the work rolls concerned out of the rolling stand. When the mill starts up again, the strip of stainless steel is not rolled uniformly thin at the spot where the rolling process was interrupted. This piece must be cut out and melted down as scrap. Felkl calculates that an average stainless steel rolling mill generates almost 6,000 metric tons of such material every year, which represents a value of €4.5 million. In addition, the idling of the mill results in a loss of production that the inventor estimates to be about 62,000 metric tons per year. If the mill didn’t have to stop, about seven percent more throughput could be achieved, resulting in additional earnings of approximately €4 million.

“In my opinion, there doesn’t have to be any production downtime,” says the inventor, himself an expert in control engineering for rolling mills. His invention is a new strategy for the rolling process and its control technology that makes it possible to replace worn-out work rolls without idling the mill. For this, the rolling mill requires an additional rolling stand. “The idea is that the rolling mill continues to run while one pair of rolls is replaced,” says Felkl. The extra rolling stand takes over the work of the one not in use. “The difficult part is rolling out the thick wedge that’s created at the point in the stainless steel strip where the rolling stand is removed,” says Felkl. To accomplish this, the rolling stand with the new rolls that will be encountered by the wedge must be synchronized with the speed of the steel strip and be deliberately brought to bear. The steel strip is under great tension in the rolling mill. “The motors of the rolling mill are very powerful,” says the specialist for controls in automated rolling mills. “The trick is to coordinate all the control units of the mill in such a way that the steel strip doesn’t tear when the thick section is being rolled out.” With Felkl’s control strategy, which coordinates approximately 20 control units, the forces are equalized in such a way that the rolling mill keeps running smoothly and the stainless steel is of uniform quality. “Another advantage of this solution is that there’s always a spare rolling stand available,” says Felkl. That makes it possible to minimize the huge losses that steel rolling mills suffer when a rolling stand has to be taken out of service for repair.

To date, no rolling mill has implemented this specific invention of Felkl’s. “The additional rolling stand represents a very large investment, but it’s one that pays for itself quickly,” says Felkl. He developed this strategy at the suggestion of the sales department, which had been looking for innovative solutions for the economic optimization of rolling mills. Felkl knows that the principle is sound, based on his experience with similar complex solutions that have already been implemented and “a simulation in my head”. Whereas his coworkers mainly use computers to simulate developments, Felkl likes to imagine the new processes. And of course, he also checks the exact processes thoroughly on a computer. Felkl began working at Siemens right after completing his studies in electrical engineering at the Technische Universität Stuttgart. For ten years, he put new installations into operation in basic industry. “During that time, I developed a very thorough understanding of what works in an installation and what doesn’t,” he says. This knowledge grew further in the course of his subsequent experience as sales team leader for cold rolling mills. Since 1999, Felkl has been crafting customized automation solutions for rolling mills as Lead Engineer in the Industry Sector in Erlangen. During this time, he has registered 28 inventions. His ideas have led to 27 patent families, comprising a total of 54 individual patents.

A native Swabian, Felkl rides a bicycle to stay in shape. “But usually that’s limited to riding the bike to work,” he says with a laugh. “It’s only 42 kilometers, after all.” He is “typically Swabian” in his attitude toward work, he says. What does that mean? “You live to work, instead of the other way around,” says the inventor, with a tinge of regret.
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