US President Donald Trump has said he's going to double the tariff rate on steel to 50 per cent, a dramatic increase that could further push up prices for a metal used to make housing, autos and other goods.

Trump was speaking at US Steel's Mon Valley Works–Irvin Plant in West Mifflin, Pennsylvania, on Friday to announce investments by Japan's Nippon Steel.

The price of steel products has increased roughly 16 per cent since Trump became president, according to the government's producer price index.

Trump is holding a rally in Pennsylvania to celebrate a details-to-come deal for Japan-based Nippon Steel to invest in US Steel, which he says will keep the iconic American steelmaker under US-control.

Employees, Trump supporters, local officials and others streamed into one of the massive warehouses on the grounds of the Irvin finishing plant to hear Trump speak about the deal.

Giant American flags hung from the ceiling and a sign read, “The Golden Age".

Steelworkers in orange hard hats and work clothes milled about, and part of the warehouse's cement floor was packed with huge rolled coils of shiny steel sheet produced at the plant and used for appliances, doors and other applications.

Though Trump initially vowed to block the Japanese steelmaker's bid to buy Pittsburgh-based US Steel, he changed course and announced an agreement last week for what he described as “partial ownership” by Nippon.

It's not clear, though, if the deal his administration helped broker has been finalized or how ownership would be structured.