I'm asking this question out of curiosity. In general, what was the level of technology like in central Africa, just before the scramble for Africa?
If only we could limit this question to specifics. Be it regional, cultural or any other qualification that would limit the scope. As axsvl77 put it "Africa is a very big place".
But since there is often a certain downward looking attitude or bias when talking about the technology of "Africans", that is a degrading outlook on anything south of the Sahara before European contact and colonisation, it may be most interesting to add just one counter example:
There was an African people that appears to have been really centuries ahead in steel technology compared to Europe or Asia.
These were – or are – the Haya people:
He was led to a tree which was said to rest on the spot of an ancestral furnace used to forge steel. A group of elders were later tasked with the challenge of recreating the forges.
At this time they were the only ones to remember the practice, which had fallen into disuse due in part to the abundance of steel flowing into the country from foreign sources. In spite of the lack of practice the elders were able to create a furnace using mud and grass which when burned provided the carbon needed to transform the iron into steel. Later investigation of the land yielded 13 other furnaces similar in design to the re-creation set up by the elders. This process is very similar to open hearth furnace steelmaking.
These furnaces were carbon-dated and were found to be as old as 2000 years. Steel of similar quality did not appear in Europe until several centuries later.
That is a technology and a level of it not usually found in other parts of Africa, or for that matter in Europe that early. But that is just one example to illustrate the impossibility of ascribing a "general level" of anything to a whole continent at that time.
Note that the quality of the Haza steel may remain somewhat disputed, but the existence of steel production and widespread use of furnaces for that purpose, also by other peoples, is apparently not in question any more.
Coastal sub-Saharan Africa was in Iron Age, towards the interior it was mixed with Copper-Bronze Age with some remote, primitive societies still in Stone Age.
Iron metallurgy in sub-Saharan Africa has long history. Before colonization it was never on level of Europe of China, and there is no evidence of blast furnaces before scramble for Africa. What is certain is that iron (as tools or weapons) was traded in coastal ports of sub-Saharan Africa, so it was widespread and known material, as evidenced for example by this well-known Zulu spear .
Copper (and bronze) metallurgy is somewhat debated topic , with arguments that it started before or at the same time as iron metallurgy. Copper ore is not widespread in Africa, yet copper and bronze objects were found all over the continent, in forms of ritual objects and jewellery, but also as tools and weapons. Copper and bronze are easier to process (if you have ore, of course) than iron, so most of these objects were made with locally produced metal.
Finally, you would have groups like Pygmies in very remote areas with very little contact with metals. Their tools and weapons were often made of wood hardened by fire and stones (sharpened or blunt). Even if they did posses metal objects they didn't know how to make them.