Which element is added to iron to make steel?

Carbon, and remarkably little of it. Steel is iron with somewhere between about 0.05 and two per cent carbon; go much above that and you have cast iron, which is hard and brittle and shatters rather than bends. The difference between a girder and a drainpipe is a fraction of a per cent of one element.

It works because carbon atoms are small enough to sit in the gaps of the iron crystal lattice and get in the way. Metals deform when planes of atoms slide over one another, and the intruders jam that sliding. Quenching hot steel in water freezes the structure into a distorted, extremely hard form; reheating it gently - tempering - lets some of the stress out and trades a little hardness for the toughness that stops it snapping.

It stayed expensive for millennia because getting the carbon content right was guesswork. Crucible methods in India and Central Asia produced excellent steel in small quantities from about 500 BC, and the material we are surrounded by only arrived with Henry Bessemer's process in the 1850s, which blew air through molten iron to burn the carbon out in bulk. The price fell by something like eighty per cent, and railways, skyscrapers and shipbuilding all followed from it.