Korea also has a good metal scene. Crash is a good thrash metal band who has had a long career so far and one of the best. Oathean, who is a black metal group, also gained some popularity with overseas listeners, and I've heard alot of good about Vassline, as well.
Magwi and Dokkaebi (whose name comes from a demon in korean mythology, btw) are also good if you dig korean death metal. Also, Jeremy is an amazing group who is popular in the scene as well. Korean christian metal/prog band, but blows away most of the christian artists in this genre in the west, which amazes me. Great guitarist, keyboardist, etc.
I still prefer Japanese metal the most as it is more eclectic, but Korea does have some bands worth checking out. They had alot of good 80s metal groups too but their work is often hard to find aside from maybe Sinawe, who was well known.