basic oxygen process

a type of conversion of pig iron into steel without the consumption of fuel by blowing the pig iron in the converter from above with industrial-grade oxygen. The expediency of using oxygen in the production of steel by the converter method was pointed out as early as 1876 by the Russian metallurgist D. K. Chernov. Pure oxygen was first used for blowing molten pig iron from below by the Soviet engineer N. I. Mozgovoi in 1936. Experiments were performed in 1939–41 at the Moscow Machine-tool Construction Plant on the blowing of pig iron from above in a 1.5–ton ladle to produce steel for intricate-shape casting. The basic oxygen process was tested for the first time on an industrial scale in Austria in 1952. The first production plant in the USSR was started up in Dnepropetrovsk at the Petrovskii Metallurgical Works in 1956.

The basic oxygen process is performed in a converter with a basic tar-dolomite lining and a closed bottom. Oxygen is fed under a pressure of more than 1 meganewton per sq m (MN/m²), or 10 kilograms-force per sq cm (kgf/cm²), by a water-cooled tuyere through the nose of the converter. Lime is added to the converter at the start of blowing to form a basic slag that binds phosphorus. As a result of blowing, the impurities in the pig iron (silicon, manganese, and carbon) oxidize, liberating a considerable quantity of heat, leading to a decrease in the impurity levels in the metal and an increase in temperature, maintaining the metal in the molten state. When the carbon content reaches the desired level (the quantity of carbon is determined from the time elapsed from the start of blowing and the quantity of oxygen consumed), the blowing is stopped and the tuyere is withdrawn from the converter. The blowing cycle usually lasts 15–22 min. The resulting metal contains excess oxygen in solution; therefore, the final stage of the process is deoxidation of the metal. The progress of the basic oxygen process (the sequence of the reactions of oxidation of impurities in the pig iron) depends on the temperature regime of the process and is controlled either by the amount of blowing or by the addition of “coolants” to the converter (scrap, iron ore, or limestone). The temperature of the metal during tapping is about 1600°C.

The use of oxygen instead of air blowing in the conversion process has made possible the production of steel with a low nitrogen content (0.002–0.006 percent). The basic oxygen process is characterized by a significant increase in the quantity of heat received by the bath per unit of oxidizing element, since no heat is consumed on heating nitrogen, which is introduced into the bath during blowing with air. In connection with this, the processing of pig iron with a low silicon and phosphorus content, as well as the processing of large amounts of scrap (up to 25 percent) or ore (up to 5 percent), becomes possible.

The treatment of pig iron with a jet of oxygen directed at the surface of the bath provides a number of advantages as compared to blowing from below. The high-temperature oxygen jet directed at the surface of the bath causes intense heating of the slag, making possible earlier formation of the active slag and regulation of its content of iron oxides, which improves the conditions for dephosphorization of the metal. The removal of phosphorus is possible with a high carbon content of the bath. In the basic Bessemer process, however, dephosphorization takes place only at the end of the melt, at a low carbon content (0.04–0.06 percent). The high temperature of the basic oxygen process promotes an intense oxidation of carbon; therefore, the content of dissolved oxygen in the metal is reduced to 0.005–0.01 percent. The consumption of oxygen per ton of pig iron is about 53 cu m in the basic oxygen process. For steel of equal quality, the basic oxygen process provides a saving of 20–25 percent in capital expenditures, a reduction of 2–4 percent in the prime cost of the steel, and an increase of 25–30 percent in labor productivity as compared to the open-hearth process.

Over the period from 1965 to 1971, steel production in oxygen converters in the USSR was increased from 4 to 23.2 million tons per year, or by a factor of 5.8. The increase in production of converter steel is accompanied by an increase in converter capacity. From the point of view of engineering, an increase in converter capacity does not produce any additional difficulties in carrying out the melting. For this reason, large converters are used for the production not only of low-carbon steel but also of medium-carbon and high-carbon steel, low-alloy steel, and alloy steel.