Deputy Steel is a robot guard of Megaton in Fallout 3. He's a Mister Gutsy robot model who guards the Megaton Armory. As soon as you enter the armory he will become hostile, whether he can see you or not.
If Deputy Weld is destroyed, Steel will take his place guarding the front gate.
The terminal at the back of the Armory can allow the deactivation of Deputy Steel, but only for a short time.