Converter Steelmaking

Converter steelmaking is the main step in ore-based steel production using blast furnace hot metal and steel scrap as basic raw materials. About 70% of steel is nowadays produced via different variants of basic oxygen converters.The converter process was developed in the middle of the nineteenth century using bottom-blown air for oxidation. Oxygen converting was developed about one century later, in the 1950s, first by applying oxygen top-blowing through lance and little later as oxygen bottom-blowing through nozzles. Toward the end of the past century, several new converter technologies were developed by combining top- and bottom-blowing of oxygen and inert gas, argon, or nitrogen. The converter-blowing technique, process control methodology, furnace construction, and long-life lining play key roles in optimal production of crude steel in various converter processes including the optimal achievement of desired end composition and temperature of the steel.In this chapter, basic phenomena occurring in the converter and controlling the process are presented and discussed. These phenomena comprise chemical reactions in liquid metal, slag, and gas phases and at their interfaces, heat effects, and fluid flow and mass transfer phenomena in different reaction zones as well as in the entire system. Oxidation of carbon and other impurities in the iron melt is controlled mainly by mass transport from the oxygen jet to the iron melt in the converter. Other constraints are the thermodynamic affinities of individual oxidation reactions controlling oxygen distribution between the competing oxidation reactions in the oxidation environments, iron melt surface, slag, and surface layers of the melt.Converting stainless steel differs from that of carbon grades as the crude steel melt is rich in chromium. To minimize the oxidation of chromium, the oxidation potential must be controlled and decreased along the lowering carbon content. This can be done by diluting oxygen with inert gas, Ar/N₂ as is done in AOD process or by decreasing the total pressure by vacuum. Fundamentals of stainless converting will be shortly discussed too.