From the Background: "The Trump Administration has conducted multiple investigations under Section 232 of the Trade Expansion Act of 1962 (19 U.S.C. § 1862, as amended) to determine if certain imports threaten to impair national security. President Trump acted after three separate investigations found potential threats; two additional investigations are ongoing. Prior to the Trump Administration, the last time a president imposed tariffs or other trade restrictions under Section 232 was in 1986, based on a 1983 investigation into imports of machine tools. Recent action under Section 232 has generated active debate in Congress and at the multilateral level, including legislative initiatives to amend the congressional delegation of authority under Section 232 to the President."
CRS In Focus, IF10667
Congressional Research Service: https://crsreports.congress.gov/