A Short History
In 1947, the steel producer and engineering firm Dorman, Long and Co started construction of the Lackenby Steelworks, which included a universal beam mill, a continuous billet mill and a rod mill, on a 600 acres site located between the Cleveland Steelworks and Redcar. Dorman, Long and Co were established in 1876 and by 1914 they had a workforce of about 20,000 and were a dominant firm on Teesside and a major British steel producer. In 1921 and 1924 their engineering department won the contracts to build the Sydney Harbour Bridge and the New Tyne Bridge in Newcastle-upon-Tyne. The company was nationalised under the “Iron and Steel Act” in 1951 and became part of the Iron and Steel Corporation of Great-Britain. In 1963, Dorman, Long and Co, having been denationalised in the late 1950s, brought a stabbing and blooming mill into operation to serve a new universal plate mill. Finally, in 1967 the company became part of British Steel.
In 1971 the Basic Oxygen Steel (BOS) Making Plant opened at Lackenby followed by a continuous casting complex comprising a bloom casting machine, comissioned in 1972, and a slab casting machine which came into production in the early part of 1973. In addition to the BOS plant and the continuous casting complex, the site also included a beam mill, a rod and bar mill and a slab mill, all of which had been commissioned at an earlier stage.
The BOS plant, which consists of three main buildings, namely the scrap plant, the converter shop and the mould preparation bay, began operations in 1971, as a two-converter shop, a third converted having been added in early 1972. In the early 1970s the molten iron was delivered to the BOS plant from the Cleveland blast furnaces in torpedo ladles and transferred into 200-tonne ladles for charging into the converter by one of the two 330-tonne charging cranes (Work on the (still) existing blast furnace at the Redcar Steelworks only started in 1973 and finished in 1979).
In 1988 British Steel was privatised to form the British Steel Plc, which, in 1999, merged with the Dutch steel make Koninklijke Hoogovens to form Corus Group. Corus utilised the Lackenby BOS plant for producing steel, using the iron produced at the company’s Redcar blast furnace. In 2007 Tata Steel bought Corus Group and only 2 years later the blast furnace at Redcar is mothballed. In 2011 the steelworks change hands one more time, having been purchased by the Thai-based Sahaviriya Steel Industries (SSI) for $469 million. Only 4 years later SSI (UK) enters into administration because of liquidation because of difficult trading conditions and a drop in the steel prices. As now other buyers were found, Redcar site, the BOS plant and the continuous casting facility at the Lackenby site were closed down.
Basic Oxygen Steelmaking
Basic Oxigen Steelmaking (BOS) also known as “Linz-Donawitz Steelmaking” or the “Oxygen Converter Process” is a method of steelmaking in which carbon-rich pig-iron, i.e. the crude iron obtained from smelting iron ore in a blast furnace, is transformed into steel. In essence, the process consists of blowing oxygen through the pig iron which lowers the carbon content and removes other impurities from the iron thus turning it. The term ‘basic’ refers to the refractory linings of the furnace which are made of alkaline materials (dolomite and magnesite) as well as to the purifying agents (also known as fluxes) that are added to the crude iron in order to promote the removal of impurities and protect the lining of the converter. Refractory linings must have specific properties to withstand high temperatures, the corrosive action of the highly oxidized and basic slags, and abrasion during charging and blowing. Basic Oxigen Steelmaking is basically a refined version of the Bessemer Process where the blowing of air has been replaced with the blowing of pure oxygen.
Let us have a closer look at the BOS process. The primary raw materials for the BOS converter generally consist of hot metal (~80%) from the blast furnace and steel/metal scrap. Filling the converter with the raw ingredients is called charging, which is facilitated by the fact that the converter can be tilted sideways. For charging, the converter is tilted about 45 degrees towards the charging bay and the pre-weighted scrap charge is charged into the mouth of the converter from a scrap charging box with the help of the charging crane. The hot metal is immediately poured directly onto the scrap from a transfer ladle. Charging the converter takes of the order of a couple of minutes.
After charging the converter is set upright and a water-cooled, copper tipped lance with 3–7 nozzles is lowered into it, delivering high-purity oxygen at supersonic speeds. The lance “blows” 99% pure oxygen over the hot metal, igniting the carbon dissolved in the steel, to form carbon monoxide and carbon dioxide causing the temperature to rise to about 1700 °C. This melts the scrap, lowers the carbon content of the molten iron and removes other unwanted chemical elements. Shortly after the ignition of the carbon the calcined lime/dolomite fluxes, which form the slag, are dropped in the converter from overhead bins. It is necessary to form a fluid slag early in the process, since the slag prevents sparking, i.e. the ejection of liquid metal from the converter due to the impact of the oxygen jet. This whole process takes about 50 minutes and at the end of it a steel ladle is positioned in the ladle car under the furnace. The converter is then tilted towards the tapping bay, and liquid steel pours through the tap hole from under the floating slag, into the ladle below. This process is called tapping. After tapping the steel into the ladle, the converter is rotated upside down for the tapping of the slag into the slag pot. Then the converter is returned to the upright position and the whole process basically begins again.