Posco of South Korea, the world's fourth-largest steelmaker, has moved to strengthen the security of its coal supplies by joining ArcelorMittal and China's Citic on the share register of Macarthur Coal. The three groups together now control almost half of the A$4.2bn (US$4bn) Australian group, making it virtually impossible for one of them to launch a successful takeover bid for Macarthur without the others' agreement.