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One of the largest steel producers in North America recently built a huge rolling mill for steel plates. The new rolling mill is setting new standards regarding the size of the produced steel plates and is one of the most productive steel plants in North America. Besides supplying its customers in the oil and offshore industry with particularly thick and large rolled steel plates, the company also offers surface treatment of the plates as an additional service. The latter was implemented with a Rösler preservation line, type KON 47/3-HD, that was specially designed for rolling mills.

Producing large and heavy steel plates requires a high degree of precision and maximum performance. With a total length of 75 m, a blast media throughput of 3.7 metric tons per minute and an annual capacity of 122,000 metric tons, the preservation line is one of the largest designed and built by Rösler Oberflächentechnik GmbH.

High performance is necessary because of the size of the processed plates. Depending on the rolling thickness of the original steel slab, the plates can be up to 4,300 mm wide, 18,000 mm long and have a max. thickness of 203 mm.

Marcus Witter, Global Sales Expert at Rösler, explained: "These dimensions presented a real technical challenge. The blast cleaning operation for such large and heavy plates, with a maximum weight of over 45 metric tons, demands extra robust equipment that is not only extremely productive, but also works with a high degree of precision. Otherwise, the specified surface cleanliness of SA 2.5 cannot be achieved. The technical requirements and a tight delivery time made this project very challenging."

The position of the Rösler equipment at the very end of the process chain demonstrates its importance for the entire manufacturing operation: The Rösler preservation line offers not just standard surface treatment but an optional additional service. This represents a true value for steel distributors as well as the end users: the customer can choose between a raw plate as it comes from the rolling mill, or a plate that has been only shot blasted or shot blasted and painted.

"Depending on the final use of the steel plate, this choice can be an attractive additional value," said Witter. "For example, plates that must be transported over long distances can be prone to corrosion. In such cases it helps to blast-clean the plates and apply a corrosion-resistant coating. This not only protects the plates during transport, but at the same time, saves two steps in the customer's own production: they can immediately cut and weld the material without having to remove the weldable primer. In the new preservation line, the company is mostly utilizing solvent-based primers as required for ship building and offshore operations. The same primers are also frequently used by traditional steel fabricators. However, because of stricter environmental demands, the Rösler preservation lines can also process water-based lacquers."

A crucial part of the Rösler preservation line is a roller conveyor shot blast machine. The machine entrance is equipped with a blow-off system for removing contaminants from the plates and a pre-heater. The latter heats up the workpieces to the optimal operating temperature required for the subsequent painting operation. The pre-heater, equipped with two efficient gas burners, heats the air to 260 °C and is running in recirculating mode. With a short intermediate roller conveyor, the steel plates are then transferred into the shot blast machine supplied with 10 Gamma 400G turbines, each with an installed power of 30 kW. Depending on the plate width, some turbines can automatically be turned off. This improves the overall equipment efficiency, lowers the wear rate and thus helps reduce the operational costs.

The shot blast machine contains an automatic plate width recognition system that-depending on the actual width-selectively turns off some turbines. For this reason, to ensure an optimal cleaning effect with even distribution of the blast media, the shot blast machine was equipped with an improved blast media distribution and recycling system.

In view of a workpiece weight of 7 metric tons per running meter, Rösler installed a special heavy-duty roller conveyor system. To minimize wear caused by the continuously high blast media throughput of 3.7 metric tons per minute, the entire blast chamber was fabricated from 8 mm thick manganese steel and additionally lined with exchangeable manganese steel plates, also 8 mm thick.

After the shot blasting operation, an integrated cleaning station removes residual blast media and dust from the plates so they are completely clean when entering the automatic paint booth. There, a two-component paint is applied onto the workpiece surface with an airless paint system that is controlled by sensors recognizing the plate width and length as well as the transport speed. This helps minimize overspray, the unnecessary blowing of paint into the booth, resulting in lower paint consumption and higher operational efficiency. After it has been dried, the paint on the steel plates must have a thickness of between 15 and 25 µm (590 to 985 micro inches).

Once the paint has been applied, the plates are transported into the dryer, heated to about 60 °C. There, a special slat conveyor helps ensure that the paint on the lower plate surface is not scratched so the plates leave the dryer with no visible defects. A thermal post-combustion system cleans the exhaust air from the paint booth and dryer, so that the legal emission limits can be easily met.

The Rösler blast and prime line was commissioned at the end of 2023 and since then has been running with no problems, producing excellent results. Because of a very tight time frame for the entire rolling mill, the installation of the blast and prime line represented a real challenge. While the preservation line was installed, the building, including outer walls, roof and other sections, had to be completed. Witter concluded: "This represented a big challenge for the installation of the preservation line. But we did everything in our power to support our customer in meeting the tight project deadline. In the end, we were successful because the project teams on both sides were in daily contact with each other. The atmosphere between the customer and us was always characterized by a high degree of cooperation."

For more information contact:

Rosler Metal Finishing USA, LLC

1551 Denso Road

Battle Creek, MI 49037

269-441-3000

rosler-us@rosler.com