Credit agency Fitch has cut ArcelorMittal's long-term issuer default rating to BB+, just below investment grade, due to the challenging outlook for Western European steel markets in 2013. ArcelorMittal, which makes about 6-7 percent of the world's steel, says demand in Europe had fallen 29 percent since 2007 when the financial crisis started. Goldman Sachs and RBC Capital Markets were advising ArcelorMittal on the deal, while Morgan Stanley is advising the POSCO consortium. (Reporting by Joyce Lee and Denny Thomas; Editing by Michael Perry)