I've intentionally embarked in areas with no metals at all, and done well. Embarks with gold and flux, but no weapon-grade metals are actually fairly easy. Trade gold goods for every scrap of metal and steel the caravans bring, and ask for more. Ask for hematite, limonite and magnetite stones, iron, pig iron and steel bars, and all the crafts, weapons, armor, musical instruments, anvils, and anything else that a magnet will stick to. Crank out gold crafts/goblets/statues/whatever, plus get your cook trained up and make masterwork roasts. You can easily buy out everything a caravan brings with good gold & food industries. Buy all the wonderfully crafted stuff the caravan brings, then toss it all in the melting pots. Also grab copper/bronze stuff to melt for use in training smiths. If you're short on trade goods and can't buy everything, skip on the decorated stuff. But it's not hard to get to the point where you can afford those as well.
Also collect goblinite (or humanite if you've managed to convince them to make non-caravan deliveries). Dodge-fall traps are good for harvesting it, but cage traps or just a bunch of weapon traps work as well.
Girlinhat recommended just using one smelter for melting because fractions are recorded for each smelter, but I've found that one smelter ends up being a bottleneck, and I use multiple smelters for melting objects. On average, each will have half a bar of fractional melts, but once you've filled up the pipeline, you don't lose any more. So in the long run it doesn't really hurt you, and it speeds up throughput considerably.
Have one forge that only allows dabblers to use it, and with repeating orders for copper or bronze armor bits. Have enough orders so your manager can't add more to that forge when you want something good built. Then have everyone in the fort who doesn't specialize in a moodable skill have armorsmithing turned on. Dwarf Therapist is useful for keeping track of this, just make sure the option for "Highlight moodable cells in labor/skill columns" is turned on. So all your haulers and farmers will come to that forge, crank out some crappy stuff, and raise their armorsmithing skill to be their highest moodable skill. Once a dwarf has armorsmithing as its highest moodable skill, turn it off for that dwarf to avoid having him/her waste more time/metal. Eventually one of them will get a non-possession strange mood and you'll get a legendary armorsmith.
Within a year or three you should be able to crank out masterwork steel armor for everyone in your military. No exploits needed.
Well, no exploits, unless you consider being able to sell a stack of pig tallow roasts for thousands of Urists to be an exploit. But selling gold crafts is certainly not an exploit. Or being able to harvest all the metal from a siege without any real risk. Harvesting goblinite without traps would make this much more difficult. But marksdwarves in towers armed with bone bolts can still collect a fair amount of goblinite relatively safely (as long as no enemy bowmasters show up). But you don't need to use any clear exploits like melting metals to get more than you started with.
Keith