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 five separate investigations found potential threats; two additional investigations are ongoing. Prior to the Trump Administration, 1986 was the last time a president imposed tariffs or other trade restrictions under Section 232, based on a 1983 investigation into imports of machine tools. Recent action under Section 232 has generated considerable debate in Congress and at the multilateral level. Some in Congress favor legislative initiatives to amend the congressional delegation of authority."
CRS In Focus, IF10667
Congressional Research Service: https://crsreports.congress.gov/