Bronze Ingots -> Rings/Rivets

After that, you have two routes. You can take advantage of dated recipes by doing Iron Nuggets -> Iron Squares then using Iron Square recipes essentially all the way to 50, but you'll need a lot of Buffalo Leather Straps later on. Or you can use new recipes and essentially ride the ore -> ingot -> rings/rivets/plates train all the way to 50, ending with Darksteel.