"We are going to be imposing a 25 per cent increase. We're going to bring it from 25 per cent to 50 per cent the tariffs on steel into the United States of America, which will even further secure the steel industry in the United States," he said at a rally in Pennsylvania on Friday.
The levy increase will take effect next week.
Trump announced the higher tariffs and talked up an agreement between Nippon Steel and US Steel. Trump said the $US14.9 billion ($A23.2 billion) deal, like the tariff increase, will help keep jobs for steel workers in the US.
He later posted on social media that the increased tariff would also apply to aluminium products and that it would take effect on Wednesday.
The steel tariffs, along with levies on aluminium, were among the earliest put into effect by Trump when he returned to office in January. The tariffs of 25 per cent on most steel and aluminium imported to the US went into effect in March, and he had briefly threatened a 50 per cent levy on Canadian steel but ultimately backed off.
Under the so-called Section 232 national security authority, the import taxes include both raw metals and derivative products as diverse as stainless steel sinks, gas ranges, air conditioner evaporator coils, horseshoes, aluminium fry pans and steel door hinges.
The total 2024 import value for the 289 product categories came to $US147.3 billion ($A229.1 billion) with nearly two-thirds aluminium and one-third steel, according to Census Bureau data retrieved through the US International Trade Commission's Data Web system.
By contrast, Trump's first two rounds of punitive tariffs on Chinese industrial goods in 2018 during his first term totaled $US50 billion ($A78 billion) in annual import value.
