The call from a supervisor came to the steel shop floor in Sheffield, England, on Thursday afternoon: The tariffs were off. “Everything had changed for us,” said Richard Bott, as he stood near stacks of steel slabs still radiating waves of heat from the mill.

In a trade deal with Britain announced with much fanfare on Thursday, President Trump agreed to lift the 25 percent tariffs on steel that had posed a dire threat to Britain’s struggling industry and to Mr. Bott’s employer, Marcegaglia Stainless Sheffield.

The cavernous plant is one of the last remaining large steel-making facilities in a city that since the 18th century was a hub of innovation in the industry.

The plant is now old and dusty, but it is in some ways avant-garde. It uses an electric furnace, a technology the government wants other mills to adopt to reduce emissions, to melt piles of sparkling stainless steel scrap into molten metal.

Marcegaglia, part of a family-owned company in Italy, ships more than 100 million pounds’ (or about $133 million) worth of steel from Britain to the United States annually, around a quarter of the country’s steel exports.