In Endhaven, steel is king and there's no obscurium or unobtanium in sight. "What's with that?" my shill in the audience asks?
When I began my Endhaven series, I fully intended to have adamantium and other exotic metals. They seemed fun. They seemed cool. They looked like they would work well in the story up until I got to the point where I actually needed to know what these metals did and why they were so important. Needing a bit of inspiration, I hit the books and surveyed the literature on metals, especially steel. Whatever this fanciful metal was, it had to work better than steel.
Here's a hint: NOTHING works better than steel. OK, that's a simplification, but it's pretty much true in the case of weapons. If you want a sword today, you are going to buy one out of steel because, after a thousand years, steel is still better than anything out there. It's not just that steel is hard, or that steel is flexible, or that it weights enough to give you a good punch without weighing too much. It's because it does all three of those things so well that any challenger has to do better. Steel was picked as the metal of choice for weapons because it hit the sweet spot so well.
Steel armor also does pretty well, to the point where we put it on soldier's heads through two world wars, and still insert it into body armor today to ward against sharp weapons. Where steel doesn't do well is against bullets, but you already knew that. If there were no bullets out there, I'd lay good money that we'd still be using steel in armor. (If you know something that's better in terms of weight vs. protection, speak up.)
Steel was developed before 1000 AD, but it took a while for the technology to spread. Smiths who knew how to make steel kept that knowledge secret so that they could corner whatever market they could. Up until the industrial revolution, the manufacture of steel was incredibly labor intensive, and therefore incredibly expensive. Once the blast furnace hit, the cost of iron plummeted, soon followed by steel. What was once rare became far more common.
So you can imagine what an unbelievable advantage that a group of steelmakers would have if they invented the blast furnace 500 years sooner than anyone else. They could undercut all their competition with lower prices while raking in a monster share of the profit. That's a pretty nice place to be.
One would also imagine that the secret of steel production would be worth an incalculable amount of money, so whoever had it would defend it vigorously, if not kill to keep it, like so many ancient technological secrets. (Read about silk sometime.)
With all this setup coming out so engaging, why would I go and invent a new metal? Steel is already the wonder metal. I literally could not make up better. So, I threw out adamantium and embraced steel.