You’ll notice that most scholarly words in Japanese are built from the onyomi readings of kanji, either because the words themselves were imported from Chinese or because the words were created by scholarly types later in history (like “kagaku” for “chemistry”).

In the case of most metals, I’d guess that the Japanese had words for steel and the like before Chinese was introduced, which is why it sounds like two words but the kanji is written as one. Note that “hagane” is the kunyomi for the “steel” character. The onyomi is “kou”, which is probably the original Chinese.

Linguistics is cool-ass stuff.

waka