Nippon Steel Corp. said Friday it has sent a letter to U.S. President Joe Biden, as its $14.1 billion buyout of United States Steel Corp. is at risk of falling through. The letter, signed by Nippon Steel Chairman and CEO Eiji Hashimoto and U.S. Steel President and CEO David Burritt, was sent on Sunday. Biden is reportedly preparing to block the deal, citing potential national security risks related to foreign ownership. Nippon Steel declined to provide details of the letter. The buyout plan, announced in December, has become a political issue in the United States, as Biden and Vice President K…