US President Donald Trump has signed an order doubling tariffs on steel and aluminium imports from 25% to 50%.
The move hikes import taxes on the metals, which are used in everything from cars to canned food, for the second time since March.
Trump has said the measures, which came into effect on Wednesday, are intended to secure the future of the American steel industry.
Critics say the protections could wreak havoc on steel producers outside the US, spark retaliation from trade partners, and come at a punishing cost for American users of the metals.
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Hours before he hiked the duties, many firms directly affected could hardly believe the plan was moving forward, hoping it would turn out to be temporary or some kind of negotiating ploy.
Even as Trump signed the orders, the UK was granted a carve-out from the measures, leaving duties on its steel and aluminium at 25%, a move Trump said reflected its ongoing trade discussions with the US.
"Always the question with Mr Trump is, is this a tactic or is this a long-term plan?" said Rick Huether, chief executive of Independent Can Co, a Maryland-based business, which brings in steel from Europe and turns it into decorative cookie tins, popcorn boxes, and other products.
Trump's moves earlier this year had prompted him to put investments on hold and raise prices. He said he feared his customers would turn to alternatives such as plastic or paper boxes due to all of the uncertainty.
The US is the biggest importer of steel in the world, after the European Union, getting most of the metal from Canada, Brazil, Mexico and South Korea, according to the US government.
During his first term, Trump imposed tariffs of 25% on steel and 10% on aluminium, citing a law that gives him authority to protect industries considered vital to national security.
But many imports ultimately escaped the duties after the US struck trade deals with allies and granted exemptions to certain imports at the request of firms.
Trump ended those carve-outs in March, saying he was unhappy with the way the protections had been weakened.
At Friday's rally at the US Steel factory, he said he wanted to make tariffs so high that US businesses would have no alternative but to buy from American suppliers.
"Nobody's going to get around that," he said of the 50% rate. "That means that nobody's going to be able to steal your industry. It's at 25% - they can get over that fence. At 50%, they can no longer get over the fence."
Steel and aluminium manufacturers in Canada, a major supplier of the metals to the US, have said the doubled tariffs will devastate their industry.
Lisa Hepfner, a member of parliament from the city of Hamilton, a major steel hub, said the tariffs mean "thousands of jobs lost", and "the end of the industry". She called for Canada to amp up its retaliation.
But Canada's Prime Minister Mark Carney told reporters on Wednesday he would "take some time" before responding, citing ongoing "intensive discussions" with the Trump administration on trade.
