NEW YORK, June 18, 2025 /PRNewswire/ -- APi Group Corp. (NYSE: APG) will replace United States Steel Corp. (NYSE: X) in the S&P MidCap 400 effective prior to the opening of trading on Tuesday, June 24. Nippon Steel Corp. (TSE: 5401) acquired United States Steel in a deal that closed today.Following is a summary of the changes that will take place prior to the open of trading on the effective date: Effective Date  Index Name  Action  Company Name  Ticker  GICS Sector   June 24, 2025   S&P MidCap 4 ...