The tariff on aluminum, prospectively 10 percent, would allow Mr. Bless to restart some production of high-purity aluminum for military use at Century’s plant in Hawesville, Ky., which it partly shut three years ago, he said. He plans to hire 300 people this year on top of the 1,850 he already employs and invest more than $100 million in the smelting operation.

“These are jobs that are sorely needed,” Mr. Bless said.

That is the kind of response that labor unions have been waiting for. Leo W. Gerard, the president of the United Steelworkers union, which also represents aluminum workers, said his members were tired of enduring layoffs because of an onslaught of artificially cheap steel and aluminum produced by “cheaters” in China.

“Some of these idiots that say we are going to start a trade war — well, we are in a trade war now, and we are just sitting back,” Mr. Gerard said. His union represents more than 200,000 Canadian workers, though, and Mr. Gerard said he hoped Mr. Trump would not apply the tariff to Canada.

“We didn’t want to — and didn’t ask the administration to — alienate those countries that don’t cheat,” Mr. Gerard said, citing Canada and European countries among the virtuous.

Economists say it is unlikely that the tariffs would lead to a steel hiring boom. The industry has hemorrhaged jobs over the last half-century, but research suggests that is largely because of technological innovation.