The Solidification Of Steel Ingots

Steel has been chosen as the metal whose solidification will be used to tie in the principles discussed in the previous papers. Although steel is the most important [ ] practical example that could be chosen, its solidification is complicated by the presence of many elements added either intentionally or present as impurities. The liquid steel bath in an open-hearth furnace contains carbon, manganese, phosphorus, sulphur, and many residual alloying elements such as nickel, copper, and molybdenum. The bath also contains oxygen, the concentration of which is a function of the carbon content, as shown in Fig. I. Deoxidizers such as silicon and aluminum may be added either before or after tapping depending on the grade of steel being made. There are four types of steel ingots: Killed, semikilled, capped, and rimmed, and these differ from each other in their state of oxidation. Each type of steel has advantages in the production of specific steel products, and it should be emphasized that no one type should be considered superior to the others. Fully deoxidized steels, known as killed steels, have little or no gas evolution on solidification. When the steel solidifies in the mold, shrinkage occurs which causes a large void known as "pipe." To minimize the amount of metal that has to be discarded on account of pipe, a big-end-up mold is used together with a refractory "hot top" which supplies molten steel to the main body of the ingot while solidification proceeds, Fig. 2. A section through a 32x32 in. ingot is shown in Fig. 3. The "hot top" volume is about 15 pct of the ingot, and the yield from killed steel in billet form is about 8o pct of the ingot weight. High quality machinery and tool steels are rolled from killed-steel ingots, but at the present time this represents less than 20 pct of the total steel production in the United States. In order to reduce cost and to increase