U.S. President Joe Biden is set to block Nippon Steel Corp.’s plan to acquire United States Steel Corp. on national security grounds, media reports said Wednesday. The decision, which is likely to be announced in the coming days, would be an extraordinary rejection of a business proposal from a major company in Japan, a key U.S. security ally. Biden and Vice President Kamala Harris have both expressed opposition to the proposal without providing a detailed rationale, as votes from blue-collar workers in battleground states have become increasingly critical in the U.S. presidential election in …