Steel brow is nice, but not great. I think one small change could both make it a little bit stronger, and also serve to make it a lot more fun in terms of the mental image it brings up.
Simply change the description from “Hits to the head no longer cause critical damage to this character” to “Hits to the head no longer cause critical damage to this character, and hits will always hit the head if it has more armor remaining”
Now that’s using your head.