Daniel 2:33
his legs were of iron; his feet were part of iron and part of clay.
Daniel 2:40-43
And the fourth kingdom shall be as strong as iron. Since iron crushes and smashes all things; and as the iron that shatters all these, it will crush and shatter.
Daniel 7:7-8
After this I looked in the night visions, and behold, a fourth beast, frightening and terrifying, and very strong. And it had great iron teeth; it devoured and broke in pieces, and stamped the rest with its feet. And it was different from all the beasts before it; and it had ten horns.
Daniel 7:19-26
Then I wanted to know the truth of the fourth beast, which was different from all the others, very frightening, whose teeth were of iron, and his nails of bronze; who devoured, broke in pieces, and stamped the rest with his feet;