U.S. steelmakers, already stung from tariff-related business moves that have backfired, are facing an unexpected new woe: a bad vegetable harvest.

United States Steel Corp. and ArcelorMittal SA ramped up production and improved the quality of the tin-coated steel sheet they make for food cans, expecting domestic demand to increase after foreign metal became more expensive following the 25% duty the U.S. applied to those imports. But imports of steel for cans have hardly dropped since the tariff was implemented in March 2018....