Steels are extremely versatile materials, and their properties (particularly strength, ductility and toughness) can be changed very significantly through heat treatment.
The crystal structure of steel changes with temperature. This is through the phase transformation from ferrite to austenite, which occurs typically between 700°C and 950°C, depending on the alloy composition. The nucleation and growth of the new phases can be used to control the crystal grain size. This is a very important consequence of heat treatment. Other phase transformations can also be used (such as martensite and precipitation of carbides) to control the microstructure.
A very important property that is not affected very much by heat treatment, is the stiffness, which is also known as Young's Modulus. This is because stiffness is controlled by the bonds between the iron atoms, which are not changed by the heat treatment.