Steel is a mixture. It is a mixture because it contains atoms and molecules of varying types, making intermolecular bonds that are not as strong as those found in compounds.
A compound, like an element, is considered to be a pure substance. This is because compounds have a molecular composition that is identical throughout, in terms of its molecules and/or ions.
Mixtures are the same in any proportion they can be reduced to or increased to, whereas the addition of other atoms to a pure substance changes what it is entirely. Mixtures can be created through stirring and heat, whereas compounds require more complicated chemical reactions.