If you actually mean forging, it was almost certainly invented independently (not only in sub-Saharan Africa, but in many places around the world). Forging - shaping metal by hammering it, whether hot or cold - is a relatively straightforward process and is low-tech (you need a rock to use as a hammer, and another rock to use as an anvil), and typically predates smelting - the extraction of metals from ores - and often by a long time. For forging, pre-smelting, the metal that is shaped is often copper or gold, but can be iron (usually meteoric iron, but sometimes telluric (i.e., terrestrial native iron)).

If you mean smelting rather than forging, the answer is that we don't know. There is no certainty that there were any independent re-inventions of iron smelting, and that iron smelting might have spread throughout the world from one initial discovery (probably in Anatolia). Iron smelting appears to have spread into both Europe and East Asia by diffusion, rather than having been independently invented. There is a stronger case to be made for the independent invention of iron smelting in sub-Saharan Africa than in Europe.

For copper smelting, we have definite independent discovery, with independent development in Eurasia (possibly Eastern Europe) and South America. Again, similar to the case for iron, copper smelting might have spread through Europe, Asia, and Africa from one initial discovery.