Page 55 - Best of STIL 2018 English

Ready to roll
The coiler in the hot strip mill wraps the hot rolled material into a coil
Before its onward journey to customers by truck or rail, the coiler at the end of the hot rolling mill winds the sometimes still glowing Salzgitter strip steel into coils.
“The customer specifies the parameters of a coil,” says Sebastian Fuhs, Works Manager at the hot strip mill at Salzgitter Flachstahl. He then explains why a larger coil usually makes more sense: Generally speaking, a larger and heavier “roll of steel” is better for customers as it means that coils have to be changed less frequently at their own production site. This saves time and therefore money.
A coil weighs some 24 tons on average and is up to 2 meters wide. The coiler at the SZFG hot strip mill is even designed to handle coils of up to
35 tons, though orders for heavy coils like these are relatively rare. Coils of this weight are too heavy for road transport, and not all customers are able to receive material by freight car or handle such heavy coils.
Every coil weighs almost as much as the slab which served as its pre-ma- terial. As a first step in the process, the slab is heated in a heating furnace to approx. 1,200 °C, descaled and then rolled into pre-strip
in the roughing train. Only then, the material gains its
final dimensions in the finishing train: A 4.80 to 12.4 meter slab is transformed into up to 2,000 meters of
hot rolled strip here. The rolled material is fed into the coiler at speeds of up to 70 km/h after passing through the cooling line to cool it down to the designated coiling temperature. The hot steel is nevertheless still up to 590 °C when it is grasped by the coiler mandrel. This poses no problem as it is designed to handle metal with a tempera- ture of up to 740 °C.
Coiling is the last important production step in the hot strip mill. Three coilers currently operate in Salzgitter, and the latest model (installed in 2010) is specially designed
to handle particularly thick and high-strength coils with a thickness of up to 25 mm. The steel is sometimes still red hot as it wraps around the mandrel at high speed. Specific amounts of water are sprayed onto the moving coil during the process. Sparks fly and a cloud of steam billows into the space. Complex automation sensors are able to identify the start and end of the material regardless, ensuring that the coil mandrel system precisely grasps and widens according- ly. This system is so precise that it takes just seven seconds to change strips.
No sooner has a coil been completely wrapped, it is transported off the coiler, bound and marked. The steel is still so hot at this point that it’s almost impossible to go near it, not that that’s necessary here in the hot strip mill anyway – everything is completely automated. An employee only has to fol- low the production process from the operating platform – either directly from the platform or on screens displaying all system and process data. Sebastian Fuhs is also able to closely monitor all processes on the screens in his office outside of the production hall. “We produce around 12,600 tons a day here in the hot strip mill, and the warehouse behind the mill is able to hold around 100,000 tons,” he explains.
A freshly wound coil has to cool down for up to three days before it can un- dergo surface treatment in the pickling plant. This process involves complete- ly unrolling and then recoiling it. The steel is therefore coiled multiple times before undertaking the final leg to customers by rail or road.
STEEL ROLLS 55
Sebastian Fuhs is Works Manager at the hot strip mill at Salzgitter Flachstahl GmbH