Thermo-chemical Fluid Flow Simulation in Hot-Dip Galvanizing: The Evaluation of Dross Build-Up Formation
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TRODUCTION
HOT-DIP galvanizing process is the main metallurgical process for continuous zinc coating of a steel strip, wherein the steel is immersed into a molten zinc alloy operated at temperatures in the range of 450 °C to 480 °C. In fact, 90 pct of the coated steel sheets in the United States are produced by hot-dip galvanizing.[1] The increasing demand for smooth and thin surface coatings from the processing industry, as well as various cost saving targets in the process chain, have pushed plant operation to its limits, so that practical expertise no longer suffices. This has led to the development of physically based simulation models in order to better understand and assess the hot-dip galvanizing process.
GEORG REISS, ANTON ISHMURZIN, CLAUDIA MUGRAUER, WERNER EßL, and WERNER ECKER are with the Materials Center Leoben Forschung GmbH, Roseggerstraße 12, 8700, Leoben, Austria. Contact e-mail: [email protected] JOHANN STRUTZENBERGER, HARALD UNGER, and GERHARD ANGELI are with the voestalpine Stahl GmbH, voestalpine-Straße 3, 4020, Linz, Austria. Manuscript submitted May 7, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The coating process consists of several individual steps, starting from the pre-treatment of the steel sheet in furnaces, over the dipping in the zinc bath, to the wiping of the liquid coating utilizing air knives after exiting the bath, and solidification of the coating in cooling towers.[2] This paper focuses on the zinc bath, but other research for furnaces,[3,4] gas jet wiping[5,6] and solidification modeling[7] has been done as well. The pre-treated steel strip enters the liquid zinc bath from the furnace, with a temperature close to the bath temperature. It is guided through the bath by stabilization rolls. The thickness of the liquid zinc film is adjusted with gas jet wiping knives. A view of the zinc bath geometry can be seen in Figure 1. In order to obtain a good adherence of the liquid zinc on the steel surface, the bath contains aluminum, whose concentration varies from 0.11 to 0.13 wt pct for galvannealed (GA) products and is typically set around 0.2 wt pct for galvanized (GI) products. The consumed zinc and aluminum are supplied to the bath by zinc alloy ingots usually containing 0.5 wt pct of aluminum. Inductive heating compensates for the heat loss across the walls and the surface, as well as the energy required for melting the ingots. Iron dissolves from the steel strip into the zinc aluminum melt forming a ternary Zn-Al-Fe system, which is likely to produce a solid Fe-Al based
Fig. 1—Depiction of computational geometry of operating zinc bath. The steel strip emerges from the furnace and is conducted by stabilization rolls through the zinc bath. After it leaves the zinc bath, the coating thickness is adjusted with gas jet wiping. The bath temperature level is maintained by inductive heating and the bath level is held constant by charging of ingots. In addition, the locations of the measurement positions (1 to 5) and the thermocouples (red dots) for the temperature distribution are
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