WASHINGTON, April 19 - US President Donald Trump on Thursday will sign a directive asking for a speedy probe into whether imports of foreign-made steel are hurting U.S. national security, two administration officials said on Wednesday.Trump is to sign the memorandum related to section 232 of the Trade Expansion Act of 1962 at a White House event that is expected to include leaders of some US steel companies. The law allows the president to impose restrictions on imports for reasons of national security.There are national security implications, one of the officials said, from imports of steel alloys that are used in products such as the armor plating of ships and require a lot of expertise to create and produce.Commerce Secretary Wilbur Ross launched the probe on Wednesday night. Trump's directive will ask Ross to conduct it "with all deliberate speed and deliver the results to the president with his recommendations," a second official said.