We know that steel is a type of iron. But the ferrous % is different in different types of iron like wroght(Carbon (0.1-0.25)%) iron, cast iron(>2.1% carbon), mild steel(0.3-0.6% carbon). The coefficient of thermal expansion of concrete is similar to that of steel i.e., Steel = 7.3 x 10^(-6). Concrete=8.0 x 10^(-6). so we use steel as a reinforce material.
(B091640(M.NAVEEN KUMAR) from Civil Engineering)