Explain how the addition of carbon affects iron's malleability?

When a force is applied to a piece of iron, the layers of iron atoms can slip over each other without changing the metallic structure. This explains the malleability of iron.

The size of iron atom and that of carbon atom are quite different. After the addition of carbon to iron to make steel, the lattice of the steel formed contains different sizes of atoms, and this makes slipping of layers of atoms over each other become difficult. Therefore, the malleability of iron is reduced.

Adding carbon causes the iron to be less malleable, but stronger and harder.