Steel is a flexible, high-strength iron alloy with good formability. Its characteristics can be modified to a great extent via different processes; for instance, its hardness can be increased to almost that of diamond. Several ways of steel production are known, with one of the most widespread ways all over the world being the basic-oxygen steel-making process also applied at ISD DUNAFERR, which is an oxidizing melting process. During this process, materials are charged into a large basic oxygen furnace (converter) that has refractory lining. The main charge materials – liquid hot metal and solid steel scrap – are put into the converter together with fluxing agents. Then oxygen is blown in through the upper hole, via a water-cooled injection lance, which leads to oxidizing melting at a high temperature, the final product of which is crude steel. During blowing, the majority of the charge carbon content burns out, and silicon content is totally oxidized together with some of the manganese and phosphorus content. Upon getting into contact with the oxygen present in the air, the CO-gas developed during oxidation burns and turns into CO, and is transferred into the wet scrubber. The heat content of the BOF gas is used for steam production. Oxides of silicon, manganese and phosphorus become components of the slag created by the blowing process. The sulphur content of the charge is bound in the slag in the form of CaS. Inactive gas (argon or nitrogen) flushing is possible through the flushing blocks at the converter bottom during the entire manufacturing process.
The steel charge is deoxidized and alloyed during tapping, and then its target chemical composition is set at the ladle metallurgical station. Primary slag is entrapped during tapping. Argon flushing is used during the ladle metallurgical treatment to decrease the inclusion content of steel, which can be modified by injecting Ca-cored wires. The oxygen level and temperature of the charge can be controlled.
Basic oxygen steel-making began in Dunai Vasmű (Danube Ironworks) in August 1981. Two basic oxygen furnaces with the capacity of 135 tons each operate in the Converter Plant of the Steelworks of ISD DUNAFERR Zrt., where production runs in 2/1 operating mode, alternately. Production in 2/2 operating mode is possible during the period of converter change. Annual capacity: 1.6 million tons of steel.
Continuous casting
100% of the steel produced in the converter is cast in the continuous casting mill. Twin-stand, vertical casting machines operating with crystallizers of rectangular section were commissioned at Danube Ironworks in 1973 and 1974, respectively.
During continuous casting, the casting ladle full of steel is lifted by cranes to the casting level, where it is turned into casting position by the ladle turning fork. When the ladle slide valve is open, liquid steel flows into the tundish, in which the level of steel is controlled by plugs. The crystallization of the steel strand starts in the copper-walled, water-cooled crystallizers, and while going through the back-up rolls in the secondary cooling zone, the strand sets across its entire cross-section with the help of water sprayed on it using compressed air. The strand is cut to the required length using natural gas-oxygen cutting torches. Afterwards, the slab is forwarded by the tilter to the discharging roller table, where identifying characters are painted on the slab side. The slabs are transferred by rail to the reheating furnaces in the Hot Rolling Mill. The continuous casting process is fully automatized, and the liquid steel stream is protected against reoxidation. The capacity of the tundish is 20 tons. Slabs, the base material of sheet rolling, have a thickness of 230 mm, and their width may vary from 860 to 1550 mm in eight different sizes. Length ranges that can be cut are: 3000 - 4200 mm and 7000 - 8400 mm. Annual casting capacity: 1.7 million tons.
