The word equation for the combustion of steel wool, which is primarily composed of iron, is:

$$ 4Fe_{(s)} + 3O_{2(g)} \longrightarrow 2Fe_2O_{3(s)} $$

In this reaction, solid iron (Fe) reacts with oxygen gas (O₂) to produce solid iron(III) oxide (Fe₂O₃), commonly known as rust.