How steel is made

A blast of pure oxygen burns the carbon out of molten iron in under 40 minutes and turns it into steel.

stations: 6
things upstream: 24
raw sources: 12
steps from the ground: 4

The short version

Most steel starts as molten pig iron poured into a vessel called a converter. A water-cooled lance blows nearly pure oxygen onto the metal, burning off excess carbon and other impurities, while lime collects them as slag. The steel is tapped into a ladle, adjusted with alloys, and cast into a continuous red-hot strand that is cut into slabs.

Strip the sulfur from the iron

Hot metal from the blast furnace is poured into a ladle. If the steel must be clean, a lance pushes several hundred kilograms of powdered magnesium into the iron, and the sulfur turns into a crust that is raked off.

Goes in
Pig iron Magnesium
Tip it into the converter

The converter is a tilting vessel lined with heat-proof brick. Scrap steel, about 25 to 30 percent of the load, goes in first, then the liquid iron. The ratio matters, because the scrap has to be melted by the heat the process itself makes.

Goes in
Steel scrap
Blow in pure oxygen

The vessel is stood upright and a water-cooled lance with a copper tip goes down to within a few feet of the bath. It blows oxygen of about 99 percent purity at supersonic speed. The carbon ignites, the temperature rises to about 1700 °C, and lime added alongside it forms slag that absorbs the impurities.

about 20 minutes of blowing

Goes in
Oxygen gas Quicklime
Tap it into a ladle

The vessel is tilted and the steel pours through a tap hole into a ladle, leaving the slag behind to be dumped into pots. A sample is taken first, since the carbon content must now be a few tenths of a percent.
Alloy and degas in the ladle

The ladle is where the grade is set. Manganese and other alloying elements are added, oxygen and dissolved gases are removed, and argon or nitrogen is bubbled up from below to stir the alloys in evenly.

Goes in
Manganese Argon
Cast a continuous strand

The ladle empties through a holding bath called a tundish into an open-bottomed, water-cooled copper mold. A skin freezes on the strand while the middle is still liquid. Rollers pull it down and around a curve, and torches cut it into slabs, blooms or billets.

up to 1.4 m of slab per minute

End of the lineSteel Next it goes into toothbrush, lithium-ion cell, electric car battery and 61 more.

Worth repeating

A modern oxygen converter turns up to 400 tons of iron into steel in less than 40 minutes, where an open-hearth furnace needed 10 to 12 hours.
More than 95 percent of the world's steel is now cast through a continuous caster, not poured into individual ingot molds.
To strip sulfur from molten iron, steelmakers lance several hundred kilograms of powdered magnesium into it, in a violent reaction that gives off heat.