In this process pig-iron and ore are the ingredients employed to produce steel by fusion upon the open hearth of a regenerative gas furnace. The pig metal is first melted upon the hearth of the furnace, and after having been raised to a steel-melting temperature, rich and pure ore (such as Mokta ore1) and limestone are added gradually, whereby a reaction is established between the oxygen of the ferrous oxide and the carbon and silicon contained in the metal. The silicon is thus converted into silicic acid, which with the lime forms a fusible slag, whereas the carbon in combining with oxygen escapes as carbonic acid, causing a powerful ebullition in the bath.
According to another modification of the process, the iron ore is treated in a separate rotatory furnace with carbonaceous material, and converted into balls of malleable iron, which are transferred in the heated condition from the rotatory to the bath of the steel-melting furnace. This latter process is suitable for the production of steel of very high quality, because the impurities, such as sulphur and phosphorus, in the ore are separated from the metal in the rotatory furnace.2
In another important modification of the same process, which is known generally as the Siemens-Martin Process, a bath of highly-heated pig metal is prepared in the furnace, and three or four time? its weight of scrap-iron or steel is gradually added (preferably in a highly-heated condition) and dissolved in the fluid bath.
1 From Algeria.
Towards the end of these various operations samples are taken from the bath in order to ascertain the percentage of carbon still remaining in the metal, and ore is added in small quantities to reduce the carbon to about 1/10th per cent At this stage of the process the furnace contains from 6 to 12 tons of fluid malleable iron, to which siliceous iron, spiegeleisen, or ferro-manganese is added in such proportions as to produce steel of the required degree of hardness. The metal is thereupon discharged, either by tapping into a ladle, or more generally directly into ingot moulds by the ascensional process.
This material has come rapidly into use for shipbuilding, and modifications of the same have been extensively used for rails, tyres, boilers, forgings for engines, wire rods, etc.
It is also suitable for bridges, roofs, and engineering work generally.