Answer:
Steel is almost all iron. It has a touch of carbon in it (to make the iron turn into steel) and can have bits of this and that to produce an alloy that is specific to a task. Some cobalt and heat treating will get it really hard and wear resistant. Add 11% or so of chromium and presto! you'll get stainless steel. Get the lowdown by using the link to our friends at Wikipedia. The science of steel is a broad and deeply involved topic from alloys to heat treating. Life as we know it would not be the same without steel.