Oxygen converter being charged at ThyssenKrupp

steel mill in Duisburg

Basic oxygen steelmakingFrom Wikipedia, the free encyclopedia

Basic oxygen steelmaking (BOS, BOP, BOF, and OSM), alsoknown as Linz-Donawitz-Verfahren steelmaking or the oxygen

converter process[1] is a method of primary steelmaking in whichcarbon-rich molten pig iron is made into steel. Blowing oxygenthrough molten pig iron lowers the carbon content of the alloy andchanges it into low-carbon steel. The process is known as basic dueto the type of refractoriescalcium oxide and magnesium oxidethat line the vessel to withstand the high temperature of the moltenmetal.

The process was developed in 1948 by Robert Durrer andcommercialized in 19521953 by Austrian VOEST and AMG.The LD converter, named after the Austrian towns Linz andDonawitz (a district of Leoben) is a refined version of the Bessemerconverter where blowing of air is replaced with blowing oxygen. Itreduced capital cost of the plants, time of smelting, and increasedlabor productivity. Between 1920 and 2000, labor requirements in the industry decreased by a factor of 1,000, from more

than 3 worker-hours per tonne to just 0.003.[2] The vast majority of steel manufactured in the world is produced using the

basic oxygen furnace; in 2000, it accounted for 60% of global steel output.[2] Modern furnaces will take a charge of iron ofup to 350 tons and convert it into steel in less than 40 minutes, compared to 1012 hours in an open hearth furnace.

Contents

1 History

2 Process

3 Variants

4 References

4.1 Bibliography

5 External links

History

The basic oxygen process developed outside of traditional "big steel" environment. It was developed and refined by a singleman, Swiss engineer Robert Durrer, and commercialized by two small steel companies in allied-occupied Austria, which

had not yet recovered from the destruction of World War II.[3]

In 1858, Henry Bessemer patented a steelmaking process involving oxygen blowing for decarburizing molten iron (UK

Patent No. 2207).[3] For nearly a hundred years commercial quantities of oxygen were not available at all or were too

expensive, and the invention remained unused.[3] During World War II German (C. V. Schwartz), Belgian (John Miles) andSwiss (Durrer and Heinrich Heilbrugge) engineers proposed their versions of oxygen-blown steelmaking, but only Durrer

and Heilbrugge brought it to mass-scale production.[3]

In 1943, Durrer, formerly a professor at the Berlin Institute of Technology, returned to Switzerland and accepted a seat on

the board of Roll AG, the country's largest steel mill.[3] In 1947 he purchased the first small 2.5-ton experimental converter

from the U. S., and on April 3, 1948 the new converter produced its first steel.[3] The new process could conveniently

process large amounts of scrap metal with only a small proportion of primary metal necessary.[4] In the summer of 1948

Roll AG and two Austrian state-owned companies, VOEST and AMG, agreed to commercialize the Durrer process.[4]

By June 1949, VOEST developed an adaptation of Durrer's process, known as the LD (Linz-Donawitz) process.[5][6] In

December 1949, VOEST and AMG committed to building their first 30-ton oxygen converters.[6] They were put into

operation in November 1952 (VOEST in Linz) and May 1953 (AMG, Donawitz)[6] and temporarily became the leading

edge of the world's steelmaking, causing a surge in steel-related research.[7] Thirty-four thousand businesspeople and

engineers visited the VOEST converter by 1963.[7] The LD process reduced processing time and capital costs per ton of

steel, contributing to the competitive advantage of Austrian steel.[5] VOEST eventually acquired the rights to market the

new technology.[6] However, errors made by the VOEST and the AMG management in licensing their technology made

control over its adoption in Japan impossible and by the end of the 1950s the Austrians lost their competitive edge.[5]

The original LD process consisted in blowing oxygen over the top of the molten iron through the water-cooled nozzle of avertical lance. In the 1960s steelmakers introduced bottom-blown converters and introduced inert gas blowing for stirring

the molten metal and removing the phosphorus impurities.[2]

In the Soviet Union, some experimental production of steel using the process was done in 1934, but industrial use washampered by lack of efficient technology to produce liquid oxygen. In 1939, the Russian physicist Pyotr Kapitsa perfectedthe design of the centrifugal turboexpander. The process was put to use in 1942-1944. Most turboexpanders in industrialuse since then have been based on Kapitsa's design and centrifugal turboexpanders have taken over almost 100 percent of

the industrial gas liquefaction and in particular the production of liquid oxygen for steelmaking.[8]

The big American steelmakers caught up late with the new technology; the first oxygen converters in the United States werelaunched at the end of 1954 by McLouth Steel in Trenton, Michigan, which accounted for less than 1 per cent of the

national steel market.[2] U.S. Steel and Bethlehem Steel introduced the oxygen process only in 1964.[2] By 1970 half of

world's and 80% of Japan's steel output was produced in oxygen converters.[2] In the last quarter of the 20th century basicoxygen converters were gradually replaced by the electric arc furnace. In Japan the share of LD process decreased from

80% in 1970 to 70% in 2000; worldwide share of the basic oxygen process stabilized at 60%.[2]

Process

Basic oxygen steelmaking is a primary steelmaking process for converting the molten pig iron into steel by blowing oxygenthrough a lance over the molten pig iron inside the converter. The converter used for steel making is called as BasicOxygen Furnace because of the exothermic heat generated by the oxidation reactions during blowing.

The basic oxygen steel-making process is as follows:

1. Molten pig iron (sometimes referred to as "hot metal") from a blast furnace is poured into a large refractory-lined

container called a ladle;

2. The metal in the ladle is sent directly for basic oxygen steelmaking or to a pretreatment stage. High purity oxygen at a

pressure of 100-150 psi (pounds per inch square) is introduced at supersonic speed onto the surface of the iron bath

through a water-cooled lance, which is suspended in the vessel and kept at few feet above the bath. Pretreatment of

the blast furnace hot metal is done externally to reduce sulphur, silicon, and phosphorus before charging the hot metal

into the converter. In external desulphurising pretreatment, a lance is lowered into the molten iron in the ladle and

Cross-section of a basic oxygen

furnace

Tapping of steel from BOF

several hundred kilograms of powdered magnesium are added and the

sulphur impurities are reduced to magnesium sulphide in a violent

exothermic reaction. The sulfide is then raked off. Similar pretreatments are

possible for external desiliconisation and external dephosphorisation using

mill scale (iron oxide) and lime as fluxes. The decision to pretreat depends

on the quality of the hot metal and the required final quality of the steel.

3. Filling the furnace with the ingredients is called charging. The BOS

process is autogenous, i.e. the required thermal energy is produced during

the oxidation process. Maintaining the proper charge balance, the ratio of

hot metal, from melt, to cold scrap, is therefore very important. BOS

vessel can be tilted up to 360 and is tilted towards the deslagging side for

charging scrap and hot metal. The BOS vessel is charged with steel or iron

scrap (25%-30%) if required. Molten iron from the ladle is added as

required for the charge balance. A typical chemistry of hotmetal charged

into the BOS vessel is: 4% C, 0.20.8% Si, 0.08%0.18% P, and 0.010.04% S all of which can be oxidised by

the supplied oxygen except sulphur (requires reducing condition).

4. The vessel is then set upright and a water-cooled, copper tipped lance with 3-7 nozzles is lowered down into it and

high purity oxygen is delivered 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, and causing the temperature to rise

to about 1700C. This melts the scrap, lowers the carbon content of the molten iron and helps remove unwanted

chemical elements. It is this use of pure oxygen instead of air that improves upon the Bessemer process, as the

nitrogen (a particularly undesirable element) and other gases in air do not react with the charge.[9]

5. Fluxes (burnt lime or dolomite) are fed into the vessel to form slag, to maintain basicity more than 3 and absorbs

impurities during the steelmaking process. During "blowing," churning of metal and fluxes in the vessel forms an

emulsion, that facilitates the refining process. Near the end of the blowing cycle, which takes about 20 minutes, the

temperature is measured and samples are taken. A typical chemistry of the blown metal is 0.30.9% C, 0.050.1%

Mn, 0.0010.003% Si, 0.010.03% S and 0.005-0.03% P.

1. The BOS vessel is tilted towards the deslagging side and the

steel is poured through a tap hole into a steel ladle with basic

refractory lining. This process is called tapping the steel. The

steel is further refined in the ladle furnace, by adding alloying

materials to give the it special properties required by the

customer. Sometimes argon or nitrogen is bubbled into the

ladle to make the alloys mix correctly.

2. After the steel is poured off from the BOS vessel, the slag is

poured into the slag pots through the BOS vessel mouth and

dumped.

Variants

Earlier converters had false bottom, which can be detached and fixed again and they are still in use. Modern convertershave fixed bottom with plugs for argon purging. Energy Optimization Furnace is a BOF variant associated with a scrappreheater where the sensible heat in the off-gas is used for preheating scrap, located above the furnace roof. Lance used forblowing has undergone changes, slagless lances have been employed to avoid jamming of lance during blowing which has along tapering copper tip. Post combustion lance tips burn the CO generated during blowing into CO2 which is anasphyxiant. For slag free tapping, darts, refractory balls and slag detectors are employed. Modern converters are fullyautomated with auto blowing patterns and sophisticated control systems.

References

1. ^ Brock and Elzinga, p. 50.

2. ^a b c d e f g Smil, p. 99.

3. ^a b c d e f Smil, p. 97.

4. ^a b Smil, pp. 97-98.

5. ^a b c Tweraser, p. 313.

6. ^a b c d Smil, p. 98.

7. ^a b Brock and Elzinga, p. 39.

8. ^ Ebbe Almqvist (2002). History of Industrial Gases (First Edition ed.). Springer. p. 165. ISBN 0-306-47277-5.

9. ^ McGannon, p 486

Bibliography

McGannon, Harold E. editor (1971). The Making, Shaping and Treating of Steel: Ninth Edition. Pittsburgh, Pennsylvania:

United States Steel Corporation.

Smil, Vaclav (2006). Transforming the twentieth century: technical innovations and their consequences, Volume 2

(http://books.google.com/books?id=tl23A0mCPLUC). Oxford University Press US. ISBN 0-19-516875-5.

Brock, James W.; Elzinga, Kenneth G. (1991). Antitrust, the market, and the state: the contributions of Walter Adams

(http://books.google.com/books?id=2Xj1qt1daHAC). M. E. Sharpe. ISBN 0-87332-855-8.

Tweraser, Kurt (2000). The Marshall Plan and the Reconstruction of the Austrian Steel Industry 1945-1953. in: Bischof,

Gunther et al. (2000). The Marshall Plan in Austria (http://books.google.com/books?id=pKlWyYA26GMC). Transaction

Publishers. ISBN 0-7658-0679-7. pp. 290322.

External links

Basic Oxygen Steelmaking module at steeluniversity.org (http://www.steeluniversity.org/content/html/eng/default.asp?

catid=24&pageid=2081272110), including a fully interactive simulation

Basic Oxygen Steelmaking cost model (http://www.steelonthenet.com/steel_cost_bof.html) showing typical cost

structure for liquid steel

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Categories: Industrial processes Metallurgy Steelmaking

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