He tweeted that the US was “losing billions of dollars on trade” and would find a trade war “easy to win”.
On Thursday he said steel imports would face a 25% tariff and aluminium 10%.
Canada and the EU said they would bring forward their own countermeasures. Mexico, China and Brazil have also said they are considering retaliatory steps.
Critics argue that the tariffs would fail to protect American jobs and would ultimately put up prices for consumers.
The news sent shares in both Asia and Europe down on Friday.
Steel tariffs: What impact will they really have?
What would China do in a US trade war?
Where Trump stands on world trade
‘We win big’
In a tweet early on Friday, Mr Trump said the US will “win big” in a trade war.
Skip Twitter post by @realDonaldTrump
Donald J. Trump
When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win. Example, when we are down $100 billion with a certain country and they get cute, don’t trade anymore-we win big. It’s easy!
The combative posts follow Thursday’s assertion on Twitter that US steel and aluminium industries had been “decimated” by trade policies.
“We must not let our country, companies and workers be taken advantage of any longer,” he said.
The US imports steel from more than 100 nations and brings in four times more steel from abroad than it exports.
Since 2000, the steel industry has suffered, with production dropping from 112m tons to 86.5m tons in 2016, and the number of employees in steel work falling from 135,000 to 83,600 in the same period.
If trade wars really were good and easy to win, the World Trade Organisation probably wouldn’t exist.
Most countries believe that negotiations are best carried out and disputes settled through a rules-based system. Introducing trade barriers on a tit-for-tat basis has the potential to harm companies on both sides.
But that’s unlikely to bother Mr Trump. His campaign rhetoric drew heavily on the perceived threat to traditional US industries from foreign interlopers acting unfairly. He’s simply continuing in that vein.
And it’s unlikely to register much with the steelworkers of Pennsylvania and Indiana. Concerned about their jobs and the future, many will welcome Mr Trump’s comments.
Presentational grey line
What have other nations said?
Mr Trump’s tariff announcement has been met with international condemnation, at a time when the US is renegotiating the North American Free Trade Agreement (Nafta) with Canada and Mexico.
A spokesperson for 10 Downing Street said the UK is “engaging” with the US on what Mr Trump’s tariff announcement means in practice.
“We are particularly concerned by any measures which would impact the UK steel and aluminium industries,” the spokesperson said.
Speaking on BBC Radio 4, UK Steel’s head of policy Richard Warren said the announced move would have a “significant impact” if it went ahead… read more .. http://tinyurl.com/yc93gf6q