Feb 18 Shares in Usinas Siderurgicas de Minas Gerais SA slumped on Thursday after shareholders failed to agree on a plan to help the money-losing Brazilian steelmaker weather a slumping domestic economy and alleviate its swelling debt burden. On a call to discuss fourth-quarter results, executives said controlling shareholders Nippon Steel & Sumitomo Metal Corp and Techint Group are studying steps to bolster Belo Horizonte, Brazil-based Usiminas.