Basic oxygen steelmaking

Oxygen converter being charged at ThyssenKrupp steel mill in Duisburg

To put it simply, basic oxygen steelmaking is a means of producing steel by using oxygen. The word “basic” refers to the pH of the refractories lining the ladle interior, which are normally composed of calcium oxide or magnesium oxide.4 This process is one of the worlds leading methods for producing steel, and since its implementation, many methods have developed which better utilize oxygen steelmaking for improved efficiency and quality. Current processes can produce around 350 tons of steel in less than 40 minutes. The basic oxygen furnace was first developed in 1952 and was called the LD (Linz-Donawitz) process. This process utilized similar mechanics to the Bessemer process except the LD process used oxygen as fuel, which was introduced into the top of the ladle. The process would reach many parts of the world and develop into many variations. Presently three general classifications for oxygen steelmaking have been established and are denoted as the top-blown, bottom blown, and combined blowing processes. The first process, top-blown, is named for how oxygen is introduced into the system. This process is also known as the Linz-Donawitz process (LD), Basic Oxygen Furnace process (BOF), or the Basic Oxygen Process (BOP). In this process, oxygen is blown at supersonic speeds through a water-cooled lance that enters the top or “roof” of the ladle. The lance can range from 60-70 feet in length and contains 3-5 nozzles. The introduction of oxygen in this way is important because it allows for the formation of the slag emulsion, as well as, sustaining the reactions required to produce steel. This remains the most common method of oxygen steelmaking. Figure 1 depicts the construction of the top-blown vessel, also referred to as a ladle, and the reactions/compositions within the vessel during operation. An advantage to the top-blown process is the fact that the refractories wear more evenly upon design than other processes which reduce the amount of down-time. This process is also able to most effectively use slag splashing. Bottom blowing or bottom stirring operations can experience clogging due to the deposition of slag onto gas entryways. In some operations, the vessels have been converted back to top-blown processes to alleviate clogging. The bottom-blown oxygen process did not become useful until the early 1970s when shrouded tuyeres were developed that could withstand the heat and wear associated with transporting oxygen into the vessel. This process is commonly known as an Oxygen Bottom Maxhutte process (OBM) or the Quick-Quiet Basic Oxygen Process (Q-BOP) among other names and uses several tuyeres, typically 8-12, which run along the bottom of the ladle in evenly dispersed rows. Figure 2 shows an OBM process with tuyeres and a cross-section showing their location. Each tuyere is composed of two concentric pipes. An inner pipe carries the oxygen to the charge material while an outer pipe carries the coolant that protects the tuyere from overheating. Methane (natural gas) or propane is typically used as a coolant, although some use fuel oil. These fuel choices can vary depending upon availability, price, and the process.1 As the gas breaks down, causing an endothermic reaction, it forms “mushroom” accretions which protect the tuyere from wear. After the oxygen blow is completed, nitrogen gas is blown through the tuyeres so that plugging does not occur. To further improve process efficiency, however, some bottom blowing processes have introduced a stationary top lance into the system which allows for less wear to the bottom refractories. The charging of bottom blowing poses some great advantages over top blowing operations due to its ability to melt scrap up to 2 feet in size. This allows the process to utilize many types of scrap which lowers preparation costs. In addition, solid scrap does not remain within the vessel after converting it to liquid. Dolomitic lime can be charged into the ladle through bins located at the top of the vessel or as a mixture with burnt lime through tuyeres. The remaining raw charge materials are similar to the BOF process, however. Another main advantage found in bottom blowing is that the oxygen reacts directly with carbon and silicon within liquid iron to form a stronger reaction. This stronger reaction results in fewer oxides remaining in the metal following the blow. Higher residual manganese can also be attained through bottom blowing. These improved reaction rates allow steels with 0.015-0.020% to require less bath and slag oxidation as well as no vacuum decarburization. Overall, similar steel grades can be produced in comparison to the top-blown processes as well. The bottom refractories and tuyeres require frequent replacing as one unit and an increased amount of downtime about the top-blown process. The design of the process vessel is more complicated due to the need for changing out the bottom refractories, as well as, the nozzles. The 1980s brought the introduced of the combination blowing processes including the LBE (Lance Bubbling Equilibrium) and K-BOP (Kawasaki) processes. These processes, along with many others, are composed of a retractable supersonic lance-like those used in the top-blown processes and tuyeres or porous plugs similar to the bottom-blown process which are embedded within the refractory at the bottom of the ladle and are used for stirring. This modified top-blown configuration results in a lower operating cost due to the stirring of the liquid steel from the bottom. This is because the bottom stirring reduces the occurrence of FeO in the slag, resulting in a greater Fe yield within the steel. Bottom stirring also increases the formation of the slag layer which provides better thermal insulation for the steel bath. The slag layer can further be increased with the injection of lime into the bath. Three prominent configurations of combination blowing operations exist. The first configuration consists of a top lance with the addition of permeable elements or porous plugs (LBE process). An advantage of this process is the fact that steel can not intrude on the pores even while gas pressure is not present. A problem arises, however, when a lime/slag agglomeration can cover the plugs and inhibit adequate stirring. An application of this process is the injection of nitrogen through the plugs to stir the bath and bring it closer to equilibrium.2 The second configuration found in combined blowing operations is the use of a top lance with the addition of cooled bottom tuyeres and is also known as the Kawasaki Basic Oxygen Process (K-BOP). The process varies from the first configuration in that the bottom permeable elements or porous plugs are replaced by cooled tuyeres which are similar to that of the bottom-blown process. This configuration proves to be more reliable than the first configuration due to the reduced maintenance required by the tuyeres. To assure that blockage does not occur, the gas flow must remain throughout the process. The third configuration consists of a top lance with the addition of uncooled bottom tuyeres. This process was designed to introduce large quantities of inert gas through each nozzle. This alleviates the need for cooling and produces strong stirring at the bottom of the vessel. A drawback to this system, however, is that air and oxygen can not be fed through the tuyeres due to a major decrease in their lifespan. Like the bottom-blown processes, the combined blowing processes suffer from the need to change out the bottom refractories and tuyeres at a greater rate than the other refractories. They also suffer from the top-blown process’s need for larger vertical space due to the oxygen lance height. With the evolution of oxygen steelmaking throughout the years, it is clear that many choices are present to fill the needs of different situations. In either case, the need for greater process quality and efficiency will always be present. And with the fulfillment of these needs, new processes will develop that will continue to improve the way steel is produced using oxygen. Basic oxygen steelmaking (BOS, BOP, BOF, or OSM), also known as Linz–Donawitz-steelmaking or the oxygen converter process is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic because fluxes of burnt lime or dolomite, which are chemical bases, are added to promote the removal of impurities and protect the lining of the converter.

The process was developed in 1948 by Swiss engineer Robert Durrer and commercialized in 1952–1953 by the Austrian steelmaking company VOEST and ÖAMG. The LD converter, named after the Austrian towns Linz and Donawitz (a district of Leoben) is a refined version of the Bessemer converter where blowing of air is replaced with blowing oxygen. It reduced capital cost of the plants, time of smelting, and increased labor productivity. Between 1920 and 2000, labor requirements in the industry decreased by a factor of 1,000, from more than three man-hours per metric ton to just 0.003. The majority of steel manufactured in the world is produced using the basic oxygen furnace. In 2000, it accounted for 60% of global steel output.

Modern furnaces will take a charge of iron of up to 400 tons and convert it into steel in less than 40 minutes, compared to 10–12 hours in an open hearth furnace.

History

The basic oxygen process developed outside of traditional "big steel" environment. It was developed and refined by a single man, 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.

In 1856, Henry Bessemer patented a steelmaking process involving oxygen blowing for decarbonizing molten iron (UK Patent No. 2207). For nearly 100 years commercial quantities of oxygen were not available or were too expensive, and the invention remained unused. During WWII German (Karl Valerian Schwarz), Belgian (John Miles) and Swiss (Durrer and Heinrich Heilbrugge) engineers proposed their versions of oxygen-blown steelmaking, but only Durrer and Heilbrugge brought it to mass-scale production.

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. In 1947 he purchased the first small 2.5-ton experimental converter from the US, and on April 3, 1948 the new converter produced its first steel. The new process could conveniently process large amounts of scrap metal with only a small proportion of primary metal necessary. In the summer of 1948 Roll AG and two Austrian state-owned companies, VOEST and ÖAMG, agreed to commercialize the Durrer process.

By June 1949, VOEST developed an adaptation of Durrer's process, known as the LD (Linz-Donawitz) process. In December 1949, VOEST and ÖAMG committed to building their first 30-ton oxygen converters. They were put into operation in November 1952 (VOEST in Linz) and May 1953 (ÖAMG, Donawitz) and temporarily became the leading edge of the world's steelmaking, causing a surge in steel-related research. Thirty-four thousand businesspeople and engineers visited the VOEST converter by 1963. The LD process reduced processing time and capital costs per ton of steel, contributing to the competitive advantage of Austrian steel. VOEST eventually acquired the rights to market the new technology. Errors by the VOEST and the ÖAMG management in licensing their technology made control over its adoption in Japan impossible. By the end of the 1950s, the Austrians lost their competitive edge.

In the original LD process, oxygen was blown over the top of the molten iron through the water-cooled nozzle of a vertical lance. In the 1960s, steelmakers introduced bottom-blown converters and introduced inert gas blowing for stirring the molten metal and removing phosphorus impurities.

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

Big American steelmakers were late adopters of the new technology. The first oxygen converters in the US were launched at the end of 1954 by McLouth Steel in Trenton, Michigan, which accounted for less than 1% of the national steel market. U.S. Steel and Bethlehem Steel introduced the oxygen process in 1964. By 1970, half of the world's and 80% of Japan's steel output was produced in oxygen converters. In the last quarter of the 20th century, use of basic oxygen converters for steel production was gradually, partially replaced by the electric arc furnace using scrap steel and iron. 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%.

Process

Principle of a LD converter

Cross-section of a basic oxygen furnace

The outside of a basic oxygen steelmaking plant at the Scunthorpe steel works.

Basic oxygen steelmaking is a primary steelmaking process for converting molten pig iron into steel by blowing oxygen through a lance over the molten pig iron inside the converter. Exothermic heat is 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 700–1,000 kilopascals (100–150 psi) 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 a 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 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 important. The 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, 0.2–0.8% Si, 0.08%–0.18% P, and 0.01–0.04% S, all of which can be oxidised by the supplied oxygen except sulphur.(which requires reducing conditions)
4. The vessel is then set upright and a water-cooled, copper tipped lance with 3–7 nozzles is lowered 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, causing the temperature to rise to about 1700 °C. 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 (an undesirable element) and other gases in air do not react with the charge, and decrease efficiency of furnace.
5. Fluxes (burnt lime or dolomite) are fed into the vessel to form slag, to maintain basicity above 3 and absorb 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 0.3–0.9% C, 0.05–0.1% Mn, 0.001–0.003% Si, 0.01–0.03% S and 0.005–0.03% P.
6. The BOS vessel is tilted towards the slagging 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 impart special properties required by the customer. Sometimes argon or nitrogen is bubbled into the ladle to make the alloys mix correctly.
7. 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, with a false bottom that can be detached and repaired, are still in use. Modern converters have a fixed bottom with plugs for argon purging. The Energy Optimization Furnace (EOF) is a BOF variant associated with a scrap preheater where the sensible heat in the off-gas is used for preheating scrap, located above the furnace roof.

The lance used for blowing has undergone changes. Slagless lances, with a long tapering copper tip, have been employed to avoid jamming of the lance during blowing. Post-combustion lance tips burn the CO generated during blowing into CO₂ and provide additional heat. For slag-free tapping, darts, refractory balls and slag detectors are employed. Modern converters are fully automated with auto blowing patterns and sophisticated control systems.

See also

• AJAX furnace, transitional oxygen based open hearth technology

References

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. 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. 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. Transaction Publishers. ISBN 0-7658-0679-7. pp. 290–322.

External links

• Basic Oxygen Steelmaking module at steeluniversity.org, including a fully interactive simulation
• Basic Oxygen Steelmaking cost model showing typical cost structure for liquid steel