You can't limit what materials an item is made out of when you give a civilization access to it through [WEAPON:ITEM_KATANA] or anything, but you can limit materials by making it only available through custom reactions. It would also be more realistic to how katanas are actually made (maybe, I'm not an expert on katanas) because I don't think the metal is folded and then made into a katana, I think it's folded directly into a katana shape. I might be wrong though. Whether or not it's like that it can be made to either be made from pig iron directly or have a seperate pig iron folding reaction, and it will limit the materials available for it to be made out of. There is a limitation in that only your fortress will produce them. You won't find them using them if they're attacking you, selling them to you in caravans, or storing them in mead halls in adventurer mode. If they're not supposed to be playable you'll just need to go with the [WEAPON:ITEM_KATANA] method, but if they're playable then you have some options. If they're not playable then you could also restrict their access to metals by removing the ore smelting job and then giving them a reaction to make whatever material you want to use and they'll have infinite amounts of that and no other metals so they'll only use that. That however has some other problems with it meaning that everything will be made out of that so if they make armor it will be made out of that, or if they have any non-katana weapons they'll be made out of that. Basically there's no way to do it perfectly, but there's a lot of ways to do it imperfectly.