What? This move is pinned on exactly that being the case.Steel and aluminum are commodities. That means there is zero difference in quality between either metal produced in the US or anywhere else in the world. The "buy American" nonsense doesn't apply here.
https://www.nytimes.com/2018/03/01/business/trump-tariffs.html
Mr. Trump’s authority to impose such sweeping tariffs stems from a Commerce Department investigation that concluded last month that imported metal threatened national security by degrading the American industrial base. The administration has said it wants to combat cheap metals flooding into the United States, particularly from China, but a broad set of tariffs would fall most heavily on allies, especially Canada, which supplies steel and aluminum to American companies as well as the military.
I do disagree with the blanket tariffs though. Canada is an excellent producer. And while that's likely the reason this move inevitably fails I'm sure all these new-found tariff doomsdayers won't acknowledge it.