How sheet steel is made

A slab thicker than a cinder block is squeezed into a ribbon a few millimeters thick and coiled like tape.

stations: 7
things upstream: 27
raw sources: 13
steps from the ground: 5

The short version

Sheet steel starts as a cast slab, 180 to 250 millimeters thick, that is reheated and rolled through a hot strip mill into a long coil. The coil is pickled in acid to remove scale, rolled again cold to make it thinner and stronger, annealed to make it workable, and sometimes coated with zinc.

Heat the slab through

Slabs from the caster go straight to the rolling mill if they are still hot. Cold ones are soaked in a gas-fired pit or furnace first. The steel must stay above its recrystallization temperature, so the grains reform as fast as they are squashed.

Goes in
Steel
Squeeze it into strip

The slab passes through a train of roll stands, each pressing it thinner and longer than the last. Hot-rolled steel has little built-in direction in its strength and few locked-in stresses.
Wind it into coils

The output from a strip mill is coiled. Coils are either sold as hot-rolled sheet or fed to the cold mill, and a coil holds a very long strip.
Dissolve the scale

Hot steel grows a dark crust of mill scale. Acid baths strip it, so the cold rolls meet clean metal and do not press the scale into the surface.
Roll it cold

At room temperature, the steel is rolled much thinner. This smooths the surface, holds tighter tolerances and makes the metal stronger by strain hardening, by as much as 20 percent. A full-hard grade is reduced in thickness by 50 percent.
Anneal it soft again

Cold rolling leaves the steel hard and stiff. Heating it in a controlled furnace restores its ductility, so it can later be pressed into a car door or folded into a duct.
Skin-pass and coat

A last light squeeze, called a skin pass, sets the surface finish. Much sheet is then dipped in molten zinc, which is hot-dip galvanizing, so a scratch in the coat still does not let rust start.

Goes in
Zinc

End of the lineSheet steel Next it goes into electric motor, electrical transformer, refrigerator and 22 more.

Worth repeating

Squashing steel cold at room temperature can make it up to 20 percent stronger through strain hardening, with no heat treatment at all.
Zinc protects steel even where the coating is scratched, because zinc corrodes first, within about 5 millimeters of the damage.