Steel melted under a slag with a basic reaction, and in a furnace with a basic bottom and lining.
In specific steelmaking processes, the matter of acid versus basic steelmaking should be examined, because an understanding of these two terms is metallurgically important to both steelmaking and welding. The terms acid and basic are derived from the kind of refractory lining and the slag used in a process. Most of the non-metallic compounds that are used in making refractory furnace linings or employed as a flux or slag can be classified as having either acid or basic (alkaline characteristics when heated to the temperatures encountered in steelmaking. A material is classified by noting any tendency on its part to react with a strongly basic material like lime (CaO) or a decidedly acid material like silica (Si02). Dissimilar materials will react or attack each other while similar materials will not. A furnace operating with a basic-type slag will have a refractory lining made of basic materials, whereas afurnace using an acid-type slag will have a lining of acid materials. If an acid slag is used in a basic-lined furnace, the slag would quickly attack and damage the furnace lining. The common acid materials involved in steel-melting are silica (Si02) and phosphorus pentoxide (P205), while the basic materials are lime (CaO), burnt dolomite (MgO, CaO), iron oxide (FeO), and manganese oxide (MnO).
The important difference between acid and basic steelmaking processes is in their respective ability to rid the molten metal bath of residual phosphorus and sulfur. In the acid steelmaking furnace, there is no significant removal of phosphorus and sulfur because the acid slag cannot react chemically with these two elements. The charge of raw materials as a whole must meet the same maximum requirements specified for these two elements in the finished steel. This means high-grade ore and steel scrap must be used. For the most part, an acid-lined furnace functions mainly as a furnace to melt a charge, remove carbon, and hold the molten bath while nonmetallics rise from it and become part of the slag.