Daniel 2:33
its legs made of iron, and its feet made partly of iron and partly of clay.
Daniel 2:40-43
Then there will be a fourth kingdom, as strong as iron. Just as all things are broken to pieces and shattered by iron, so it will shatter and crush everything.
Daniel 7:7-8
"After this, I continued to observe the night visions. And look! there was a fourth awe-inspiring, terrifying, and viciously strong animal! It had large, iron teeth. It devoured and crushed things, and trampled under its feet whatever remained. Different from all of the other previous animals, it had ten horns.
Daniel 7:19-26
"I wanted to learn the precise significance of the fourth animal that was different from all the others, extremely awe-inspiring, with iron teeth and bronze claws, and that had devoured and crushed things, trampling under its feet whatever remained.