They are not the "cheapest" but, when humans first learned how to convert iron ore into metallic Fe with carbon in it (creating a very simple steel alloy), the swords they could make were much better than what existed before (the Bronze Age technology was bronze swords which could literally be cut in two with a good steel sword). Once the technology was developed to convert large quantities of iron ore into steels, human technology really took off. Iron and steel can be processed to be ductile (so it is easy to make bars, plates, rods, shapes of all sorts) and then heat treated to make the steel much stronger. This technology is what allowed the Industrial Age to start.
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