Steels are by far the most widely-used of metallic materials, and significant effort is put into improving their manufacture to achieve desirable properties in the most cost-effective way, or to achieve new combinations of properties. Mechanical properties are controlled by a combination of alloying elements, mechanical and thermal processing, all of which affect the physical structure of the steel across different length scales.

The susceptibility of iron-carbon alloys to solid-state transformations enables their structure to be altered by heat treatment.

The development of steel has had a huge impact on civilisation, i've collated an incomplete History of steel, and some discussion of the future.

Probably better to look at wikipedia page on steel.

Steels are part of our everyday lives, but often their contribution seems to be taken for granted. References to steel by the media include from Stalin and Margret 'The Iron Lady' Thatcher (bad?), to Superman - the man of steel (good?). Steel was a main character in the recent Quentin Tarantino movie Kill Bill where the characters enjoyed nothing more than hacking body parts off each other with their 'Hanzo' swords.

There are pages here on bainite, martensite and on carbides.