Veteran prop du Randt has announced he is retiring after becoming only the fourth player to win two World Cups, after Australians Tim Horan, Jason Little and John Eales.
Montgomery extended South Africa's lead three minutes later after Martin Corry infringed at a ruck, and Steyn -- at 20 the second-youngest player to feature in a final -- made it 15-6 from 47 meters out after Ben Kay blocked Os du Randt in the 62nd minute.