The steel maker bought about 31.6 million shares in Macarthur Coal, the firm said in a statement.
It included 9.05 million company shares acquired from Talbot Group Holdings and another 22.09 million shares from Tinkler Investments.
The share purchases were conducted through open market transactions between April 24 and May 21. Macarthur's main product is low volatile pulverised coal injection coal used in steel production.