Ladle Metallurgy & Continuous Casting
In the steel industry, ladles are subject to cyclic and changing thermal conditions along with varying loads. Steel processed through a vacuum tank degasser (without heating capability) must be heated to higher temperatures during ladle refining to avoid freezing during vacuum treatment. The mechanical effect from the load and prolonged high-temperature exposures can result in creep damage. To avoid damage and extend the life of the ladle shell, it is important to ensure that the shell temperature does not exceed the specified design maximum. In this study, a mathematical modeling method was employed to investigate the effect of refractory configurations on the ladle shell temperature under various conditions. An optimum ladle refractory configuration was established to ensure that ladle shell temperatures are always within the design range.
Investigation into irregular submerged-entry nozzle (SEN) erosion at Steel Dynamics Inc. – Flat Roll Group Butler Division’s thin-slab casters revealed a significant error in the caster mold level control systems. Upon correcting the error, numerous operational benefits have been realized. Accurate mold level control has allowed for the development of an optimized SEN wear program, which has increased average sequence length and reduced yield loss due to tundish skulls. Improved mold level control has also reduced the occurrence of sticker breakouts, decreased the number of slab downgrades for longitudinal cracks and allowed for increased casting speeds on some grades.
Numerical models have been conducted to understand the effects of fluid flow and temperature distribution in the appearance of longitudinal cracks in peritectic steel grades. Computed fluid flow patterns and temperature distributions delivered by different submerged-entry nozzle (SEN) designs have been correlated with data from the plant. Asymmetric flow patterns in the mold lead to the occurrence of longitudinal cracks at transient events during the operation, such as changes in casting speed, superheat or SEN swaps, and even at steady operating conditions. SEN features such as excessive positive ports angle and cup bottom on the bore body increase the chances for developing longitudinal cracks. A reduced port angle and pyramid bottom on the bore of the SEN resulted in a more uniform fluid flow and temperature distribution near the meniscus region.
Mold thermal data was investigated from two thin-slab casting facilities that employed funnel molds instrumented with thermocouples that extended into the lower part of the mold. Under specific operating conditions, regular “sawtooth-like” temperature fluctuations were observed in the lower part of the funnel area. Analysis of cap-off flux film samples and mold temperature data suggests that the temperature fluctuations are caused by the fracture, withdrawal and regrowth of the crystalline layer of the flux film. The effects of this flux film fracture and reformation on the heat flux profile in the mold are investigated.
Cold-start ladle shroud requires very good thermal spalling resistance. Silica raw materials such as fused silica and zirconia mullite materials are widely used because their low thermal expansion characteristics improve thermal spalling resistance. However, silica raw materials have poor erosion resistance. This study applies low/non-silica alumina-carbon materials that have superior erosion resistance to cold-start ladle shroud, together with carbon-free inner liner. Carbon-free inner liner has low thermal conductivity and acts as a heat insulator to reduce thermal shock. The ladle shrouds were successfully implemented at actual casters.
ArcelorMittal Burns Harbor utilizes a hot rolling process to reheat semi-finished steel slabs nearly to their melting point before reaching 13 successive rolling mill stands driven by motors, coiling up steel sheet for transport to the next process. After exiting the finishing mills, the steel is carried through 10 banks of low-pressure, high-volume water sprays that cool the strip to between 1,000°F and 1,250°F. The cooling system was reliant on antiquated line starters for control of the constant speed pumps powering the process. Constant speed pumps led to energy waste that could be prevented through the application of adjustable frequency drives. This paper offers a case study outlining the modernization of the laminar cooling control system at the Burns Harbor mill, which delivered annual operational cost savings in excess of US$500,000.