In Sweden they are building a prototype of the Hybrit process, using hydrogen to reduce the ore. The plan is to convert the blast furnaces in Sweden and Finland

In Sweden they are building a prototype of the Hybrit process, using hydrogen to reduce the ore. The plan is to convert the blast furnaces in Sweden and Finland within a couple of decades. Now this isn't a particularly low tech way of going about it, and producing the hydrogen requires massive amounts of electricity. So probably not an option if you're bootstrapping civilization after a collapse.