Making Stainless Steel

The melt shop represents the first step in the production of cold-rolled strip. Scrap and alloying materials are melted in an electric arc furnace. The molten metal is then transferred into a refining vessel.

In the vessel, oxygen, argon and nitrogen are blown into the metal in stages, refining it into stainless steel.

The aim of secondary steelmaking, besides improving homogeneity and cleanness, is to fine-tune the chemical composition and casting temperature of the steel, which are critical to the chemical and physical properties of the finished product. The material is finally cast into slabs - the feedstock for the hot strip mills - on continuous casters.

The slabs produced on the continuous caster are heated in reheating furnaces and then reduced to the dimensions required for hot rolling in roughing and edging mills. The finishing train - the final section of a hot strip mill - then rolls the strip in a single pass to the required final thickness.

The hot rolled strip is the feedstock for the cold rolling mills.

The hot-rolled strip is prepared for cold rolling on stainless steel treatment facilities at the cold strip mills. First the hot-rolled strip is heat treated. Chromium steels are generally annealed in batch furnaces, while chromium-nickel grades are subjected to continuous heat treatment in an annealing and pickling line.

Then the scale is removed from the hot-rolled strip.

Following initial mechanical descaling, the remaining scale is removed from the surface of the hot-rolled strip by pickling liquors. The annealed and pickled hot-rolled strip is now ready for cold rolling.

The thickness of the hot-rolled strip is reduced on cold rolling stands.

Twenty roll stands generate the high reduction forces required for stainless steel and also ensure that the finish and dimensional tolerances are met.

A variety of mechanisms allow the roll gap - the forming zone of the cold rolling stand - to be modified so as to produce cold strip of the desired shape and flatness.

Data from the flatness measuring system and from the rolls are interpreted with the aid of a processor to allow automatic control of the cold rolling process.

This control enables close cold rolling tolerances even at high rolling speeds.

After cold-rolling, the strip is annealed again. This process is performed either on annealing and pickling lines, where the cold strip is heat-treated in an open atmosphere, or in the inert gas atmosphere of the bright annealing line for particularly high surface requirements. The shiny surface of the cold rolled strip is retained - in fact its brightness is actually improved by final heat treatment in an inert gas atmosphere.

In the following processing stages the strip is gently re-rolled, and, if required, tension levelled and trimmed or finished. To achieve final customer specifications with regard to mechanical properties, flatness, finish and brightness, the heat-treated cold strip is gently skin passed on two-roll skin pass mills with polished work rolls.

If required, the strip receives a ground finish on a grinding line.

To meet very high flatness requirements, skinpassed cold strip is treated on tension levelling equipment. This removes any residual stresses which could impair the flatness of the strip.

In the finishing shop the strip is processed according to customer specifications. The coils are cut into narrow strips or sheets on slitting and cut-to-length lines.

Strip and sheet can be provided with a plastic film coating to provide surface protection for further processing by the customer, e.g. deepdrawing.