Those tariffs were imposed under Trump's authority given to him by Section 232 of the Trade Expansion Act of 1962 which, in brief, allows the Executive to impose tariffs if a foreign nation's trade practices are affecting national security. Crucially, this determination is not made by the president, but rather the Department of Commerce.

Here is one such report from 2018 that was the impetus for the steel tariffs: https://www.bis.doc.gov/index.php/232-steel