Steel=Iron+Carbon+Chromium+others
Base part of steel is iron. Pure iron is kinda "soft" and easily reshapable, because crystal structure within iron resists little to iron atoms interacting with one another, leading to dislocations.
In steel, however, iron is adulterated with certain amounts of carbon and other elements which together harden the iron and thus prevent atom dislocations, or in other words, steel is more rigid and less ductible than pure iron.
Controlling carbon amount and other alloying elements can achieve different phases of iron and enhance certain qualities.
In steel heat treatment, there are some famous phases including Austentitle, Martensite, Pearlite and Sorbite.
