Rolled (up) steel

Before it rolls up on the customer’s premises, strip steel from Salzgitter is rolled up itself. The coiler at the end of the hot mill train winds the sometimes still glowing steel into coils that are then delivered to the customer by road or rail.

 “The coil parameters are specified by the customer,” says Salzgitter Flachstahl operations manager Sebastian Fuhs, before going on to explain why a larger coil tends to be more practical: The larger and heavier such a "roll of steel”, the better as a rule for the customer, who is then required to change the coils in the production plant less often. Which saves time and money.

A coil weighs 24 t on average and is up to 2 m in width. The coiler in the SZFG hot mill is actually designed for coils weighing up to 35 t, but coils of this weight are rarely ordered. They are too heavy to be shipped by truck, and not every customer can accept material delivered on a railcar, nor handle coils that weigh so much.

Each coil weighs almost as much as the slab that served as input stock. The slab is heated in the reheating furnace to around 1,200 °C, descaled and then passed forward and back through the roughing stand to reduce it to strip. Only when it reaches the finishing train does the material achieve its final dimensions: what began as a 4.80 by 12.4 m slab here becomes hot rolled strip up to 2,000 m in length. The rolled strip reaches the coiler at a speed of up to 70 km/h, passing on its way through the cooling line where it is cooled to the predetermined coiling temperature. Despite this, the steel still has a temperature of 590 °C when it is grabbed by the coiler mandrel. That is no problem, as the maximum is as a high as 740 °C.

Coiling is the last important stage of production in the hot mill. There are three coilers installed in Salzgitter, the youngest dating from the year 2010 being specially designed for wide, high-strength coils with a thickness of up to 25 mm. The steel often still glows red as it wraps itself at high speed around the mandrel. Specific quantities of water are sprayed onto the material to be coiled. Sparks fly and a cloud of steam obscures the view. Despite this, complex automated systems detect the beginnings and ends of the material, allowing the coiler mandrel to spread and grab them with precision. In fact, the precision is so exceptional that a strip change takes just seven seconds.
As soon as a coil is fully wound, it is removed from the coiler, strapped and marked. At this point the steel is still so hot that a human hand can barely approach it – which is in any case unnecessary here in the hot mill.

Everything is fully automated. Just one employee in the control room keeps track of the production process – either by direct line of sight from the platform or via the screens that display all the requisite plant and process data. In his office outside of the workshop, Sebastian Fuhs can also track all of the processes on-screen. “Here in the hot mill we produce around 12,600 t per day, and the storage area behind the mill can hold around 100,000 t,” he explains.

A freshly wound coil must be left to cool for up to three days before the surface can be treated at the pickling line. For this purpose it must be fully unwound and then rewound. Consequently, the steel that makes its way by road or rail from Salzgitter to the customer is well wound as well as rolled.