When President Trump placed tariffs on imported aluminum last spring in an effort to protect American producers, European rivals thought their U.S. profits would come under pressure.

But months later European aluminum companies have yet to feel much pain. Imports have long accounted for the majority of raw aluminum consumed in the U.S.—61% in 2017, according to the U.S. Geological Survey—and the tariff doesn’t appear to have hurt that business.

Norway’s...