Development of a Slab Intermix Model for Casting Dissimilar Steel Grades at SSAB Mobile

摘要

It is a common practice in the steel industry to cast as many heats as possible in a sequence to maximize productivity. However, in some cases, steel grades with vastly dissimilar chemical compositions may create intermix or transition slabs which are not suitable for application to either of the grades being cast. Therefore, minimizing and proper dispositioning of intermix slabs are critical for productivity, quality and yield. This requires the development of a suitable practice for identifying the intermix region during casting. A typical intermix process (without a flying tundish and grade separator operation) includes steel grade mixing in the liquid region in both the tundish and casting strand. The mixing starts in the tundish when a ladle with new grade is opened. Consequently, the grade mixing extends to the liquid region of the cast mold and strand. Initial studies of grade intermixing were conducted in the early 1990s. The methods employed to study and quantify intermix phenomena in both the tundish and the mold included water modeling experiments, numerical simulations, and direct measurements. From these studies, it has been shown that various operational parameters such as chemical composition, tundish geometry, tundish filling rate, operating level, casting speed, and slab dimensions affect the location and length of intermix slabs. For a steel mill, practical ways to improve intermix casting may include: 1. Scheduling: Since not every grade is intermixable, pre-evaluations are required for all of the potential intermix grades before casting. Well-planned intermixable grades can reduce the length of the intermix region; 2. Chemistry Control and Optimization of Casting Parameters: The steel chemistry gap between two different grades can be minimized by adjusting the heat chemistry during refining within the acceptable ranges for each element. This strategy may result in reducing the intermix length. In addition, optimized casting parameters can minimize intermix length further; and 3. Slab Cutting and Dispositioning: In an ideal situation, the intermix zone is contained in one slab; therefore, monitoring the position of the intermix zone in real time may aid in optimizing slab cutting and improving slab dispositioning.