"We can not allow this to happen as it has for many years!" the president also tweeted on Wednesday. And that's the ongoing threat to U.S.jobs and living standards.
The signing ceremony for the proclamation issued at the White House was attended by several steel and aluminium workers, some of whom spoke of how excessive "dumping" of steel and aluminium imports had negatively affected their jobs and families.
European Commission vice-president Jyrki Katainen said Friday that the EU is ready to take the U.S.to the World Trade Organization (WTO) court if some worst scenario happens. "We want to get as much clarity as possible." and that the European Union is "an ally, not a threat" to the US. On the very day the deal was signed, Fast praised the salvaging of the Trans Pacific Partnership with 10 other nations - including Japan, Australia and Mexico - that was put in jeopardy when the US pulled out previous year.
"We will be making a decision as to who they are - we have a very close relationship with Australia", he said.
Canada and Mexico were already granted exemptions, although Trump has hinted that he will cancel them if NAFTA renegotiation talks don't make progress soon.
Trump critic and vocal free trader, Senator Jeff Flake (R-Ariz.), said he would announce a legislation that would nullify the tariffs.
For one thing, the case-by-case approach can't easily be squared with the government's earlier national-security rationale - which might have helped the plan conform to the letter (though not the spirit) of global trade rules.
Trump reiterated that the tariff would be 25 per cent on steel and 10 per cent on aluminum.
"I was very pleased the President was able to confirm he would not have to impose tariffs on Australian steel and aluminium", he said.
"The retail industry is extremely concerned by the administration's apparent desire to ignite a trade war, where the net losers will be the very people the president wants to help".
Trump's envoy also met Malmstroem and Seko separately. "It meant in practice that in the US they lost thousands and thousands of jobs", Katainen said.
The contentious tariffs come into effect in 15 days.
Flavio Volpe, president of the Automotive Parts Manufacturers' Association said on Friday that he had been informed about cases of steel stockpiling.
The USFIA says it strongly opposes tariffs, and other barriers to trade that ignore 21st-century supply chains and business operations.
Trump expressed his dissatisfaction with USA trade deficits with Japan and the European Union in Twitter posts Saturday, though he did not specifically refer to steel trade with Japan. But one White House aide said Wednesday night that the event had been canceled - because the policy was not yet finalized.
Responding to a question, Trump said the United States is negotiating NAFTA with Mexico and Canada, which will be exempted from the steel and aluminum tariff.
West Australian Premier Mark McGowan said he believed Mr Trump's actions could still have some unforeseen and wide-ranging consequences, given his state exports iron ore to countries like China and Japan.