Definition of a ladle change strategy to avoid slag entrapment and to control inclusion population

High quality products such as tin plate steels for the packaging industry and IF steels for automotive industry are very demanding in terms of internal cleanliness. Problems arise with detection of surface or internal defects or with cracking occurring at the customer. Cleanliness problems encountered are related to the ladle slag, to the tundish slag or to mold slag. Inclusions of ladle and tundish slags are affecting the transition slabs due to the ladle change process. Their size may be comparable to the plate thickness. As a consequence, for the most severe applications, these transition slabs are downgraded from the order. Steel quality is always influenced during transient periods such as a ladle change but also at the start and end of casting, as well as undesired casting events e. g. delayed opening of the new ladle, reduction of casting speed or clogging are important. The main conclusions of the previous studies is that a more profound knowledge is necessary concerning transient flow conditions in order to provide an improved utilization of the tundish to complement ladle metallurgy. The general objectives of this multi-national multi-partners research project focused on ladle changeover period, involving ARCELOR Research, MEFOS, BFI, SIDMAR and COCKERILL SAMBRE, are: • to quantify the inclusion size distribution originating from ladle and tundish slags in industrial tundish and products for various pouring conditions, • to define a ladle changeover strategy in terms of ladle emptying and tundish refilling and geometry of tundish filling area in order to minimize ladle and tundish slag entrapment into the mould and product, • to develop an on-line quality rating model for internal cleanliness, which assesses the quality loss at transient situations such as ladle or tundish changeover and at undesired casting events like delayed opening of the new ladle or reduction of normal casting speed. Main activities and achievements may be summarized as following. (1) Inclusion size distribution originating from ladle and tundish slags in industrial tundish and products for various pouring conditions (ARCELOR Research, MEFOS, COCKERILL Sambre) The main characteristics of inclusions during ladle changeover are as follows: • both ladle and tundish slags accumulated in the tundish are responsible for defects, after demonstration with tracing: lanthanum has been found in inclusions found in tin plate when tundish slag was traced, • the size distribution in both liquid steel in tundish, semis and final products ranges from about 15 μm to up to about 500 μm but most of slaggy inclusions are small: 80% lower than 50 μm. (2) Ladle changeover strategy to minimize ladle and tundish slag entrapment into the mould and product