The new taxes are to go into effect in 15 days.
The president signed the measures in a ceremony at the White House on Thursday. Workers employed by U.S. steel and aluminum manufacturers attended the event.
The president, however, is permitting exceptions to the tariffs for metal imports from Canada and Mexico. Both countries are currently negotiating with U.S. officials to change the North American Free Trade Agreement (NAFTA).》
The president spoke at a cabinet meeting Thursday morning. He said U.S. officials "have a right" to drop or add countries to the list of exceptions. Officials said those exemptions would be made on a "case by case" and "country by country" basis.
Earlier, Trump wrote on Twitter, "We have to protect & build our Steel and Aluminum Industries while at the same time showing great flexibility and cooperation toward those that are real...and treat us fairly on both trade and the military."
Talk of the tariffs brought warnings of a trade war from Europe and China. One European Union official even said the EU would target goods from areas governed by Trump's Republican Party.
Reaction from China
The president has argued that the tariffs would prevent low-cost imports, especially from China, from hurting U.S. industries and jobs.
In Beijing, Chinese Foreign Minister Wang Yi warned a "trade war" would hurt all parties and called for negotiations. He added, "China will, of course, make a proper and necessary response."
China had a record $375.2 billion goods surplus with the United States last year.
Trade tensions between the world's two largest economies have risen since Trump took office in 2017. Although China exports little steel to the United States, its huge steel production has driven down the price of steel across the world.
In Brussels, European Commission Vice President Jyrki Katainen said the EU has told the U.S. that tariffs were a bad idea. He said the U.S. lost thousands of jobs when it ordered tariffs on steel imported from Europe in the 2000s.
Another official was firm in his response. "If Donald Trump puts in place the measures this evening, we have a whole arsenal at our disposal with which to respond," said Peter Moscovici of France. He serves as the European Commissioner for Economic and Financial Affairs.
The European measures could include tariffs on American oranges, tobacco and bourbon, he said. Some of the goods are produced in parts of the United States where Trump is popular.
Moscovici said he wanted American lawmakers to know that imposing tariffs "would be a ‘lose-lose' situation."
Internal opposition to tariffs
Trump's plan to tax steel and aluminum imports has faced growing opposition from congressional Republicans and American businesses. Many were worried about possible effects on the economy.
More than 100 Republican lawmakers wrote to Trump, "We urge you to reconsider the idea of broad tariffs to avoid unintended negative consequences to the U.S. economy and its workers."
Tom Donohue is president of the United States Chamber of Commerce, an organization representing American businesses. He said, "We urged the administration to take this risk [of a trade war] seriously."
On Tuesday, Gary Cohn, the president's chief economic advisor, announced that he would leave the White House. Cohn has long argued for free trade and against tariffs.