It's a blow to countries that send steel and aluminum to the United States.
The president following through on a campaign pledge to impose punishing new tariffs.
President Trump said, "Twenty-five percent for steel. It will be 10 percent for aluminum. And it will be for a long period of time."
The move, designed to make prices for American steel and aluminum more competitive with countries that produce them cheaper.
Industry leaders say it will grow domestic steel production and add jobs.
CEO of Majestic Steel Todd Leebow said, "I believe we have to stop the bleeding that the domestic steel industry has experienced for quite a while now."
But it could make things you buy, like cars and washing machines more expensive.
That's what spooked investors Thursday.
The DOW closing down more than 400 points.
Earmon Javers said, "The winners here are steel companies in the United States who sell steel. The losers are any company that buys steel. That means they are going to past those costs on to their customers."
The US buys more steel than it makes.
And while the president's tariffs are aimed at China they could hurt Canada even more.
Sen. Orrin Hatch of Utah said, "I hope the President doesn't really do this because if he does it's just going to be a huge tax on American citizens"
A calculated risk President Trump appears willing to take.
The new tariffs are expected to be finalized next week.
Meaning there is still time for the president to change his mind on who they will target and by how much.