Steel.—The steel-making processes at present in extensive use for the production of mild steel consist essentially of a process of oxidation by which the carbon and certain of the impurities of pig iron are eliminated. The oxidising process, however, usually leaves the metal in an over-oxidised condition and subsequent reduction of the excess of oxide is necessary. It will be seen that this procedure is analogous to that adopted in the treatment of copper mattes, which are first over-oxidised and then subjected to a reducing treatment by "poling." In the case of steel, re duction by the direct action of carbon or by the equivalent of "poling" is not used, but de-oxidation is effected by the addition to the steel, just before it is tapped from the furnace, or even in the ladle after tapping but before the metal is run into the moulds, of certain "ferro-alloys." These usually contain a con siderable amount of carbon to serve for re-carburising the steel to the desired degree, and also either or both manganese and silicon. Sometimes aluminium is also used as a de-oxidant. These substances have a sufficiently strong affinity for oxygen to rob the iron of that element, although this de-oxidation is never complete. The products of this reaction—oxide of manganese and silica, or—more probably—a silicate of iron and manganese— form more or less fusible slag particles and, if the steel is given time to settle, these rise to the surface and are eliminated. As a rule, however, they remain in suspension in the steel and form un desirable non-metallic enclosures.
Two methods of oxidising the molten pig-iron are employed. In one, known as the Bessemer process, air is blown through or over the molten iron until the character of the resulting flame shows that oxidation has been carried far enough. The burning of the silicon and carbon content of the pig iron generates enough beat to raise the molten metal to a very high temperature, suf ficient to allow it to be run out of the converter into a ladle and then into moulds. In the "Open Hearth" process on the other
hand, the pig-iron, usually mixed with a considerable amount of steel scrap, is melted on the hearth of a large furnace regener atively fired so as to maintain a very high temperature. It was, in fact, the invention of the Siemens regenerative furnace which, by making it possible to produce and maintain steel-melting temperatures easily and economically, made this type of process possible. Once the charge is molten it is oxidised by the addition of an iron-oxide ore, the oxygen of this ore combining vigorously with the carbon and silicon of the pig iron in the charge. A slag layer is formed above the layer of molten metal and protects it from direct interaction with the atmosphere of the furnace.
The reactions which occur in both types of oxidation process depend upon the material with which the furnace is lined. At the high temperatures employed the composition of the slag must be adapted to that of the lining, otherwise rapid attack would result. In the "acid" processes the lining of the open-hearth furnace or of the Bessemer converter is siliceous in character: the name "acid" refers to the acidic character of the silica which predominates in these linings and which must, therefore, be kept high in the slag also. Under such a slag it is possible to remove or lower the silicon and the carbon content of the charge, but it is not possible to purify the resulting steel in regard to its content of phosphorus or sulphur. It follows that, for the pro duction of high-class steel in the acid process the pig-iron em ployed must itself be sufficiently low in sulphur and phosphorus, and this stringent condition—in view of the fact that the finished steel is frequently required to carrry less than o.o5% of these impurities—sets a limit on the range of pig-iron and there fore of ores which can be employed.
Basic Steel.—In the "basic" processes the furnace linings are made of magnesia and lime, bonded together with tar or similar material and containing very little silica. The slags used with these basic linings must also be correspondingly basic, i.e., low in silica. A mixture of lime and magnesia, even with a small amount of iron oxide would not be sufficiently fusible, but the acidic element is provided by phosphoric acid, derived from the phosphorus of the charge. The basic slag is thus a highly basic lime-magnesia phosphate and tends to absorb phosphorus from the iron so long as oxidising conditions are maintained—for instance, by the frequent addition of iron oxide in the form of ore. Under reducing or less strongly oxidising conditions, how ever, there is a tendency for phosphorus to be reduced by the iron and to be returned from the slag to the charge. In the basic process, any silicon present in the charge rapidly forms silicates with the bases of the slag and would, if present in sufficient amount, attack the furnace lining. It will thus be seen that while in the acid process phosphorus is extremely undesirable, while silicon is needed, the reverse holds for the basic processes where pig-irons can be used which have a high phosphorus content, while high silicon is undesirable. There is one other point of general importance in which the two processes differ; it is much more difficult—on account of the danger of returning phosphorus to the molten steel—to effect thorough de-oxidation in the basic process than in the acid. Badly made basic steel, therefore, is liable to be over-oxidised and unsatisfactory, although when properly operated produces material of excellent quality.