What is added to iron to make steel and give different proportions?

The main ingredient is carbon. Carbon is what makes iron into steel. Other chemicals are added, like chromium, which makes stainless steel. Different amounts of carbon can change the properties. Adding a lot of carbon makes high-carbon steel, which is hard and flexible, used to make knife blades and (lower quality) springs. Adding molybdenum (and chrome, I think) makes chromoly steel, very flexible and strong steel often used in the racing industry.
The element chromium, itself another metal, is added to steel (which is iron with a bit of carbon) to make stainless steel alloys.

Also added: vanadium, manganese, molybdenum, nickel, wolfram, etc.
Chromium and nickel are added to the iron to make stainless steel.
it will be process in heating room