Refining And Casting 261 Process Description

Ladle metallurgical furnace (LMF) processes are used to refine molten steel from the BOF or EAF prior to ingot or continuous casting. These processes include the following steps:

1. Reheating. Arc reheating or oxygen injection is used to adjust the temperature of steel to levels needed for uninterrupted sequential casting.

2. Refining. The first step in refining is deoxidation of the steel with ferromanganese, ferro-silicon, silicomanganese, and aluminum. The second step is desulfurization, which is required for steel grade, which requires low sulfur content. The third step involves addition of ferroalloys and fluxes to the molten steel. The fourth step involves inclusion modification in which the steel in the ladle is stirred by argon gas bubbling to obtain a homogeneous bath temperature, composition and the removal of nonmetallic inclusions. Inclusions responsible for clogging nozzles during the continuous casting process may be removed by calcium treatment.1

3. Degassing. In vacuum degassing, molten steel is subjected to a vacuum for composition and temperature control, deoxidation, degassing (hydrogen removal), decarburization, and the removal of impurities. While the molten steel is under vacuum, elements that have a relatively higher vapor pressure (such as manganese and zinc) volatilize and exit with the gases.14 The argon-oxygen decarburization (AOD) is the predominant method used for the manufacture of stainless steel.1

In continuous casting, the molten steel is solidified into a semifinished shape (i.e., billet, bloom, or slab) for subsequent rolling in the finishing mill. Continuous casting eliminates the need for conventional processes like the ingot teeming process and directly casting steel into semifinished shapes.14 The continuous process has higher yields, quality, and productivity compared with the ingot process, as well as higher energy efficiency.1 The molten steel is delivered in ladles and poured into a reservoir or tundish, from which it is released into the molds of the casting machine. As it descends through the molds, the metal is cooled and emerges with a hardened outer shell. As the semifinished shapes proceed on the runout table, the center also solidifies, providing the semifinished shape.14

In ingot casting, the molten steel is teemed into ingot molds followed by cooling and stripping the ingots out of the molds. The ingots are then heated to a uniform temperature in soaking pits to prepare them for rolling. The heated ingots are removed from the pits and rolled into slabs, blooms, or billets. Figure 2.6 illustrates the refining and casting processes, Table 2.12 gives the major inputs

Combustion emissions

APC dust

Particulate

Sludge

4 Mill

Wastewater scale

Sludge

Wastewater

Combustion emissions

APC dust

Particulate

Sludge

4 Mill

Wastewater scale

Sludge

Wastewater

FIGURE 2.6 Refining and casting flow diagram. (From Energetics, Inc., Energy and Environmental Profile of the U.S. Iron and Steel Industry, DOE/EE-0229, U.S. Department of Energy, Washington, DC, 2000.)
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