Daniel 7:19
After this, I required diligently to know the truth concerning the fourth beast, which was so far unlike the other beasts, and so horrible: whose teeth were of iron, and his nails of brass: which devoured and destroyed, and stamped the residue under his feet.
Daniel 7:7
After this I saw in a vision by night, and behold, the fourth beast was grim and horrible, and marvelous strong. It had great iron teeth, it devoured and destroyed, and stamped the residue under feet. It was far unlike the other beasts that were before it: for it had ten horns, whereof I took good heed.
Daniel 2:40-43
The fourth kingdom shall be as strong as iron. For like as iron bruiseth and breaketh all things: Yea even as iron beateth everything down, so shall it beat down and destroy.