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A new combined LF-VD plant for SSAB Oxelösund equipped with twin tanks on movable cars to maximise productivity is meeting the tight product quality requirements and has a new-generation, thermodynamics based metallurgical model to ensure the required end point precision. Edited by R Sellan*, M Rinaldi* & R Coughlan**
STRUCTURAL steel plates with high yield strengths can be produced by thermomechanical (TM) rolling and accelerated cooling, resulting in a fine-grained material with excellent ductility and high strength. The final microstructure and mechanical properties depend on the steel composition, the rolling parameters and the cooling conditions. Due to the low residual element requirements, the steels for these grades are generally produced through the BOF route, thus implying process times of about 40 minutes. It is therefore necessary to have a Ladle Furnace - Vacuum Degasser (LF-VD) station that guarantees high precision of the final chemical composition and temperature, as well as a high throughput, in order to match the BOF treatment times.
Combined LF-VD station
Danieli Centro Met and Danieli Automation have designed a new combined LF-VD plant for SSAB Oxelösund. The salient features of the combined LF-VD station are the twin tank cars containing the ladles that move to various positions under the multiple operational roofs. The station is aligned along the main meltshop crane axis, with the following sequence of possible tank car positions - Park #1, VD #1, LF, VD #2 and Park #2. Normally, tank car #1 will range from Park # 1 to the LF, but if necessary, providing that tank car #2 is withdrawn to Park #2, tank car #1 can then reach the VD #2 position also. The inverse also applies to tank car #2.
Problems of a traditional VD
One of the most difficult problems in operating a meltshop that includes a stand-alone VD remote from the LF is matching the BOF (or EAF) cycle times, with the VD station even assuming full availability of cranes. The caster, if multi-strand, faced with a delay in the arrival of the next heat of a sequence has the option of closing one or more strands and/or reducing the casting speed to accommodate this. For singlestrand slab casters though, and especially thinslab ones, these options are simply not available. Some...