Study of the State of Industrial P 2 O 5 ‐Containing Slags Relevant to Steelmaking Processes Based on a New Thermodynamic Database Developed for CaO–FeO x –P 2 O 5 –SiO 2 –MnO–MgO–Al 2 O 3 Slags − Part I: Ternary and Lower Order Systems