I used to always do what I could to make sure I always had easy access to steel in my forts, because I thought that's the only way to defeat large sieges (without walling/bridging yourself in).
Then I realized iron is good enough, so always did my best to make sure I had iron on my embarks, and it was still a pita to find it.
But now the last fort I played, I struck copper pretty early on and just went and equipped my dwarves in that, and it worked just as well as iron or steel. Now, I did have 4 legendary military dwarves by the time the siege came, but it was just 4 of them in copper vs 20-30 invaders. But when those invaders went through my fort entrance door where I had my military stationed, they RAN. They simply ran off the map. The ones who didn't they were slaughtered.
So yeh, like someone in this thread already mentioned, the skill of the dwarf is more important than what armour they are wearing.
But for other creatures that aren't goblins, I think you really do need some better metal. For example I sent that same legendary dwarf to fight a cave crocodile in the caverns, and for a minute there I thought he's going to get eaten, because it was taking a far longer time to kill the croc than a goblin. Turns out all his attacks were just bruising the croc until he got a lucky hit. The only thing I think that kept him alive was that he kept dodging all of the croc's attacks.