F. Wallner, E. Fritz, VOEST-ALPINE Industrieanlagenbau GmbH & Co, Linz/Austria

"We secure our future by honoring our past." -Roman Empress Augusta, 15 A. D.

 
Reflections on Developments of the LD Process
 

Fifty years ago the first LD steelmaking plant in the world was put into operation at Vereinigte Osterreichische Eisen- und Stahlwerke (VOEST) in Linz. A half year later the second LD plant went into operation at Österreichisch-Alpine Montangesellschaft (ÖAMG) in Donawitz.

Why did it take so long to put an idea into practice that Henry Bessemer-the Leonardo da Vinci of metallurgy-had had a hundred and fifty years earlier? The use of pure oxygen as a refining gas was already mentioned in his British patent numbers 356 and 1292. Why was it not possible to develop a converter route that would utilize pure oxygen even after the Linde-Fränkl process was discovered in 1928 for the large-scale production of oxygen, and since this time a number of inventors, metallurgists and companies had tried to develop an oxygen-converter process? Such a technology would have been urgent at the time for reasons of productivity, quality and costs, not least because of Word War II, which ensued soon thereafter.

In his first attempt to refine hot metal with air, Henry Bessemer melted pig iron in an externally heated fire-clay crucible, deeply submerged a fire-clay blowpipe from above and blew air into the metal bath. In a patent letter from the year 1855 he writes, " ... a current of air ... is then to be forced into the fluid metal and allowed to bubble up through it" (Figure 1a). In his subsequent attempts, Bessemer poured hot metal from a cupola furnace into a fixed converter and blew air through the metal bath from six tuyeres laterally arranged near the bottom of the converter (Figure 1b). Finally, Bessemer developed the "movable converter," in which air was blown from below through the metal bath through tuyeres arranged in the bottom of the converter

Bessemer was strongly convinced that the metal bath and the slag must be intensively mixed together with refining gas in order to keep the metallurgical reactions going. He wrote the following in his autobiography:

"Hence, to carry out the Bessemer process successfully, a temperature must be obtained very considerably above the mere melting temperature of malleable iron; and in order to secure this it is necessary to drive powerful streams of air into the metal, so as to divide it into innumerable tiny globules diffused throughout the whole body of iron under treatment which, for the time being, may be likened to a fluid sponge with the active combustion of carbon with oxygen going on in every one of its myriads of ever-changing cavities."

All of the metallurgists and inventors concerned with the development of an oxygen-blowing process between 1930 and 1950 more or less followed the concepts put forward by Henry Bessemer. Attempts at oxygen bottom blowing were carried out during this time (Figure 2a). Attempts were also made at blowing the stream of oxygen with such force into the metal bath that it penetrated deeply into the hot metal "as a solid body" (Figure 2b). Furthermore, attempts were made at blowing the oxygen onto the bath at an angle (Figure 2c). None of these attempts led to a result that could be put into industrial practice because either the refractory brick lining was subject to heavy wear, the tuyeres were damaged, dangerous retardation of boiling occurred, and/or the desired steel quality was not achieved.

This experience finally led to the recognition, however, that a change in the paradigm was necessary in order to develop an industrially sound production technology (Figure 2d).

In June of 1949, VOEST in Linz made their first attempts at top-blowing oxygen in an adapted two-ton Bessemer converter. The first trials soon showed that deep penetration of the oxygen stream and its kinetic energy were not required to agitate the bath and slag mechanically. It was soon recognized that the refining reactions themselves are sufficient. The vertical stream of oxygen directed at the center of the bath surface must penetrate the slag zone in order to be able to react with the metal bath. Very high temperatures occur in the area of impingement, the so-called hot spot, from which the circulation of the bath is initiated. The bath circulation is accentuated by the development of carbon monoxide. The tremendous amount of gas that occurs forms an emulsion with the liquid slag and the metal droplets that are sheared from the bath surface by the force of the oxygen jet. The gas-metal-slag emulsion generates large surface areas that dramatically increase the rates of the refining reactions (Figure 3). These characteristics essentially form the basis for large-scale industrial application:
1. High reaction speeds allow high production rates.
2. The refractory lining and oxygen nozzle are at a sufficient distance from the hot spot and are thus not damaged.
3. The refining processes with pure oxygen ensure the high quality of the steel.
4. The simplicity in the process and systems results in low investment and operation costs.

Further trials in a 15-ton pilot plant in Linz and in 5-ton and 10-ton vessels in Donawitz confirmed the industrial feasibility, economic profitability and the possibility to produce the required steel grades for the wide product mix already offered by these two companies.

The introduction of LD steel was not accompanied by any problems for deep-drawing grades because no limiting standards were applicable. The official norms and standards prescribed open-hearth or EAF steel for components which in the case of their failure could pose dangers to human life, cause severe material damage or harm the environment. Thomas and Bessemer grades enjoyed only a limited degree of acceptance. In 1955 the Austrian standards granted the LD process same status as the open-hearth and EAF processes based on proofs of performance and the experience gained in processing and applying LD steel. Standards outside Austria, especially in countries with Thomas steelmaking plants, initially designated the LD process as an air-refining method (Windfrischverfahren), and it took up to fifteen years-in some cases even longer-to completely abolish this discrimination. For this reason, among others, VÖEST decided to buy shares in a German shipping company and commissioned the first ship to be built completely of Linz-Donawitz steel (sheets, profiles, rivets, forgings and castings). The ship Linzertor was launched in 1958, making the final breakthrough for unlimited acceptance of LD steel in the field of shipbuilding.

The competition at the time naturally played an important role. Existential fears began to grow at the Thomas steelmaking plant as soon as it became clear that LD steel was of superior quality. It is important to note that fifteen million tons of Thomas steel were produced during 1958 in Germany, France, Belgium and Luxembourg, which amounted to 60% of the annual production of these countries. Suddenly open-hearth steel was also subject to market competition with other steels which were either of comparable or superior quality, but which could be produced at substantially lower cost. The invention of the LD process was acknowledged in accordance with patent law everywhere in the world, with one exception. Henry Bessemer had had similar experiences in the United States, but the solution worked out for him was much more favorable.

Fifty years later, the development of the LD process and its immediate large-scale industrial implementation are still recognized as great and unique achievements. The first trial heat in the small-scale converter on 3 June 1949 and the first heat in the 30-ton vessel of the LD steelmaking plant in Linz on 27 November 1952 were only made possible by the entrepreneurial spirit and the competence of the metallurgists coupled with the experience of the plant builders at VÖEST. All this was accompanied by the courage and vision of the executive managers. The year 1952 was not only the beginning of the large-scale application of the LD process but it was also the year in which the cornerstone was symbolically laid for today's VOEST-ALPINE Industrieanlagenbau.

 
The Impact of the LD Process on Steelmaking throughout the World
 

The fast-paced victory march of the LD process is explained by the high potential of further development and optimization that are inherent in this method of steelmaking. Another important aspect is that the invention was made at just the right time. Many a good idea is not crowned with success because it comes either too soon or too late.

The development of the LD process was evolutionary; however, its implementation and impact on the entire steelmaking industry was revolutionary. Upstream and downstream process steps were affected in a sustainable manner. Large-scale blast furnaces became necessary. Developments in the field of secondary metallurgy were greatly accelerated, because they brought not only qualitative improvements but also economic advantages in connection with the LD process. The LD heats with their short tap-to-tap times and low degrees of scatter, as well as the purity and homogeneity of the liquid steel were the basis for rapid general implementation of continuous-casting technology as well. The LD process was even one of the first and most important technologies into which process-controlled automation systems were integrated in metallurgical facilities.

 
Process Flexibility
 

The LD process has proven to be extremely flexible with respect to the processed materials. It was highly advantageous that the technology was first developed for the utilization of Austrian iron ore because of its low P content. The processing of hot metal with a high P content was much more difficult. The best solution was found in top-blowing lime powder together with the oxygen. The LD-AC process was developed by ARBED and CNRM (Centre National de Recherges Metallurgiques). Both processes offer the possibility of cost-optimizing the charging materials because the scrap portion may range between 0% and 30%.

Table 1: Minimum contents of different elements achievable through LD and secondary metallurgical processes >

The LD process has also proven a high degree of flexibility with respect to the produced steel grades and quality. As early as fifty years ago it was possible to achieve low P and S contents for mass-production steels as it was specified for special steels. Hot-metal preprocessing, appropriate input materials and slag practice in the LD process, including bottom stirring, slag separation at tap and secondary metallurgical treatment of the liquid steel, all together make it possible to meet purity specifications that were held to be utopian at the time the LD process was introduced (Table 1).

 
Productivity
 

The most important further development was the increase in productivity of the process, because global steel consumption increased steeply during the second half of the past century. The population explosion, reconstruction after World War II, wide-spread wealth in developed countries and weapons re-armament were the reasons that the worldwide steel production has increased in fifty years from 200 to more than 800 million tons (Figure 5). Around 1970 the annual rate of increase in production lessened along with a strong negative trend in the annual per-capita steel production rate.

In order to increase the production capacities, converter heat sizes of up to 400 tons were built for the LD process and up to 200 tons for the LD-AC process (Figure 6). Dispersing the hot spot with multi-hole tuyeres in larger converters made it possible to keep the oxygen-blowing time independent of the amount of hot metal to be refined. The hourly output of crude steel per converter in a 2/3 operation was boosted from 55 to 650 tons.

In 1950 a total of 80% of the world's steel was produced in the open-hearth furnace. The rest was produced in the Thomas converter and the EAF. Today the LD process accounts for approximately 60%, and the EAF process covers 34% of globally produced steel. The open-hearth furnace only plays a minor role, one that should be lost by the end of this decade for economic and environmental reasons. The Thomas process was completely phased out by the end of the 1970s, and the Bessemer process even before (Figure 7).

 
Environment
 

Environmental aspects were a serious challenge for the LD process even at the time it was industrially implemented. The fineness of the dusts in the LD offgas forced the plant-building companies to develop new dedusting systems. One gram of LD dust has a visible surface area of between 300 and 500 m2. In order to generally avoid the optical effects of "brown fumes," the dust must be cleared from the system to a degree of less than 100 mg per cubic meter (operating conditions). In Linz the responsible technicians opted for a wet-dedusting method, whereas in Donawitz a dry-type process was preferred in light of water shortages.

The challenge became more and more of an opportunity for the LD process as the number of environmental prerequisites grew. Offgas cleaning systems became very expensive as a result of the large amounts of offgas for Thomas steelmaking plants. This was one of the economic reasons that the Thomas steelmaking plants were phased out. As of 1957 there were no longer any open-hearth steelmaking plants built in the United States, because stringent environmental requirements would have made them even less profitable.

Approximately 90% of the current dedusting systems in the world operate on the basis of a wet-type process and have the capacity of meeting a requirement of less than 50 mg dust per mn3. A dust content of less than 10 mg/mn3 can be achieved with electrostatic dry-type dedusting systems. The advantages and disadvantages of both methods are summarized in Table 2. Dry-type dedusting systems have a brighter future because of their higher degree of effectiveness, lower energy consumption, the quality of the cleaned converter offgas for utilization outside the converter process, and because of economical dust-recycling. For this reasons dry-type electric dedusting system has been installed also in the new LD No. 3 steelmaking plant, combined with hot-briquetting and dust recycling.

In the early days of the process, brown fumes indicated that the LD steelmaking plant was in operation. Today, as a result of modern dedusting plants, the operation of the oxygen converter can only be detected by the flare stack. Economic and environmental demands require that the energy in the converter gas and the iron-containing dust be efficiently recycled. In the early years of the LD process the converter offgas was completely combusted through the open hood. At best roughly 300 kg of steam per ton of crude steel and 250 mn3 offgas per ton of crude steel were produced. In the meanwhile the tendency has been toward suppressed combustion and the production figures lie at roughly 80 kg steam/t crude steel and approximately 70-100 mn3 of converter gas/t crude steel with a chemical energy of 0.7 GJ/t crude steel. This corresponds to 15 liters of heating oil per ton of crude steel. The cleaned converter offgas, which is practically free of dust and sulfur, can replace energy sources in metallurgical facilities as well as in steam or combined-cycle power plants. A metallurgical application for reduction processes would make sense.

The sludge occurring in wet-type dedusting plants is generally either dumped or put through an expensive drying process before being recycled. The dust collected in dry-type systems can be hot- or cold-briquetted and directly recycled to the converter. Converter slags are used in the cement industry and in road construction. In some plants it is recycled to the blast furnace. Dumping should be avoided altogether. Converter offgas and converter slags have become valuable by-products that help conserve energy and natural resources when properly utilized.

 
Automation
 

The LD process has proven to be most suitable in achieving the basic objectives of metallurgical-process automation. The automation of the LD converter plant was not only the beginning of comprehensive process controls and increasing automation level in the entire steelmaking route from iron ore to the finished sheet, but also a decisive factor for the further development of the LD process itself. Fifty years ago the procedure of an LD heat was statically planned and recorded by hand in a heat log. During the process an experienced operator dynamically interfered with the process to make corrections in order to achieve the required turndown and tapping conditions of the heat (Figure 8). Today the planning procedures are carried out by a static process model which is much more precise and extensive. The use of a sublance that is equipped with a head for measuring the temperature and thermoanalytical determination of C content as well as for taking samples made it possible to measure the actual status of the melt immediately before tapping. A dynamic process model is used to calculate the correction measures for achieving the correct blowing endpoint. Operational and economical reasons led to a dynamic blowing process control based on a continuous analysis of the converter offgas. This control allows a cyclical calculation of the current C content in the last minute(s) of low-carbon heats.

The model description of the LD process is very precise today, but unfortunately it still is difficult to gather representative and correct information of the process throughout the entire blowing period. We are still lacking proven methods of continuously and directly measuring the temperature and the composition of the bath. The most recent developments of a direct and contactless online measurement method which utilizes a submerged tuyere will help to solve this problem.

In the VAI-CON Temp(r) system the bath temperature is measured pyrometrically online and in the VAI-CON Chem(r) system the chemical composition of the bath is spectrometrically determined online by means of a laser, ultraviolet and infrared mirrors, etc. (Figure 9). The signals are used for an advanced dynamic process control. Automation processes have made an essential contribution to the economics of the LD process and the quality of the produced steels as well as to the environmental compatibility of the technology.

 
Current Status of Oxygen-Converter Steelmaking
 

As already mentioned, the bath and slag in the LD process are predominantly mixed by the carbon monoxide gas. However, this mixing force is slowed down when the C content falls below 0.1%. Overoxidation of the melt and an increase in (Fe) content in the slag are the result.

This as well as the blowing behavior are problems which led to substantial difficulties in the refining operation in the LD-AC converter at the steelmaking plant of Eisenwerkgesellschaft Maximilianshütte GmbH in Sulzbach-Rosenberg, Germany, because of high Si, Mn and P contents in the hot metal. The solution to the problem was found in oxygen bottom blowing with a special oxygen tuyere protected by hydrocarbons. It allows additional injection of solids such as fine-ground lime into the heat. This method protects the tuyeres and refractory lining from premature wear in the region in which the oxygen is injected, thus finally implementing the original idea of bottom blowing with pure oxygen that Henry Bessemer had one hundred and ten years earlier. In comparison to oxygen top blowing, the OBM (Oxygen-Bottom Maxhütte) process has led to a more quiet refining process, lower oxygen, phosphorous and sulfur contents in the steel and most particularly to lower (Fe) contents in the slag, to higher manganese contents and generally better output of the melt.

Based on these experiences, the LD process was improved by bottom stirring with inert gas. The bottom-blowing OBM process was improved by adopting combined top and bottom oxygen blowing (K-OBM). In comparison to the classical LD heats with low carbon content, the iron content in the slag was reduced through bottom stirring by 20-40%, through combined blowing by 40-60% and through bottom blowing by 60-80% (Figure 10).

Today almost all LD converters are equipped with bottom-stirring devices, and combined blowing converters already contribute a substantial portion to worldwide steel production. From 1952 to 2001 a total of 12.3 billion tons of steel were produced in LD converters, and since 1968 roughly 1.1 billion tons of steel have been produced in bottom-blowing converters, the largest share of this according to the K-OBM process (Figure 11). The LD process currently accounts for 85.5%, and the K-OBM process, (including OBM) covers 12% of the entire worldwide converter steel production.

The objective of every competitive steelmaking plant never changes: the economic production of steel of sufficiently high quality and with the least possible burden to the environment. A modern oxygen converter, in order to meet the demands of the current state of technology, should have the following characteristics (Figure 12):

The newly developed online measuring method for temperature and chemical composition of the bath by utilizing a bottom-blowing tuyere combines the following advantages:

 
The Future of Oxygen-Converter Steelmaking
 

What does the future hold for oxygen-converter technology now that the process has reached such a high degree of maturity?

In light of this question, let us discuss the general life cycle of steel-production technologies (Figure 13). After the Bessemer and Thomas processes emerged, the puddling and the crucible processes soon disappeared. The age of ingot steelmaking had begun. At roughly the same time the first open-hearth furnaces were put into operation. This process made it possible to not only process hot metal but scrap as well. This made the process become more and more popular until it practically replaced the two pneumatic processes after one hundred years. The development of the LD process and the oxygen-converter technology was a resurrection of the pneumatic methods which will soon bring the era of the open-hearth furnace to a close.

The replacement of the open-hearth process was strongly supported by the EAF, which has assumed the role of utilizing scrap previously ascribed to the open-hearth furnace. The EAF continues to be the optimum melting unit for scrap. Having increased the productivity and economic feasibility, EAF technology today is the dominating process route for long products. For ecologic and economic reasons this trend will continue with greater emphasis into the future. It was the sponge-iron (DRI, HBI) and/or hot-metal portion of the charge which made it possible for the electric-arc furnace to enter the quality range of flat products.

Good-quality scrap is continually becoming more scarce and more expensive, which is the reason that economic conditions are decisive in the selection of the production route. Of course there are developments underway to increase the hot-metal portion in EAFs up to 50% and the scrap portion in the oxygen converter to the same level. An eventual introduction of the hot-air jet from the top in the main decarburization phase of the heat may lead to a significant increase in the transfer of energy to the melt.

However the blast-furnace and oxygen-converter route will remain the future process leader for the production of high-grade steels, especially for flat products. In addition to the scrap problems, a further reason is the four-to-six-times-higher decarburization rate in the oxygen converter as compared to the EAF with the same inner diameter of the unit (Figure 14).

From a global perspective, EAF production may approximate that in the oxygen converter, however, it will not replace it. No new technology is in sight that will replace the oxygen-converter technology. It is necessary that this technology is further improved from an economic and ecological perspective. Among others it will be advantageous to better utilize the following:

The first two LD converters ever built in the world can be seen in the works of voestalpine Stahl Linz. Their appearance, however, is misleading. These pioneers of the LD process have not lost a final battle, but have only given way to more productive units.

Out of the originally very simplistic LD process have grown modern, process-controlled and automated production systems that anable adaptations in the steelmaking technologies in order to meet today's economic and ecological requirements. These technologies allow the production of unalloyed and alloyed steels of the highest quality from hot metal, scrap and/or sponge iron in top or combined oxygen blowing converters.

"We congratulate the LD process on its fifty years of continued success. There is no doubt that the LD process and oxygen-converter technology as such will live to celebrate their hundredth anniversary as well."

 
References
 

1) H.Bessemer: H.Bessemer: Sir Henry Bessemer, F.R.S. An autobiography 1989, The Institute of Metals, London
2)Th.E.Suess: Stahlherstellung nach dem Sauerstoff-Aufblasverfahren VÖEST Jahrbuch 1950/51, Eigenverlag
3) H.Hauttmann: Properties of LD-Steels produced by the LD-Process Bulletin of the Research Institute of the VÖEST, Linz-Donau, August 1951
4)H.Hauttmann: Die Eigenschaften der unberuhigten und beruhigten DL-Stähle Drei Jahre LD-Stahl VÖEST 1953-56, Eigenverlag der VÖEST, Linz-Donau
5) H.Trenkler: Ein Jahrzehnt LD-Verfahren
Schriftenreihe des Bundeskanzleramtes, Verstaatlichte Industrie (IV) 1961, Heft 2
6.) H.Schaden: Probleme der Entstaubung bei LD-Stahlwerken BHM 104 (1959), 18-21
7) O.Cuscoleca et al: Stahlfrischen mit reinem Sauersto Mitteilung der Österreichisch-Alpine Montangesellschaft, Wien 1956
8) F.Wallner: The LD-process - more than a step forward in metallurgy Richard Weck Lecture 1985, The Welding Institute, London
9)E.Michaelis: Geschichtliche Entwicklung und weltweite Einführung des LD-Verfahrens, BHM 137 (1992), 161-169
10)F.Wallner, A.Moser: Qualität und Markteinführung der LD-Stähle, die Basis für den Siegeszug des Verfahrens, BHM 137 (1992), 170-178
11)The Making, Shaping and Treating of Steel, Steelmaking Volume11th Edition 1999, AISE Steel Foundation
12)R.Lightfoot, I.Craig:: Off gas combustion system design AISE Steel Technology, 5/2000, 23-27
13)T.Emi: Neueste Entwicklungen auf dem Gebiet der Massenstahlerzeugung Stahl u. Eisen 100(1980), 998-1011
14) H.Wiesinger et al: Entwicklung und Stand der LD-Stahlwerksplanung und -einrichtingen, BHM 137 (1992) 187-195
15)H.Kreulitsch, W.Krieger: Der LD-Prozess - ein ökologisch optimierte Verfahren BHM 137 (!992) 271-278
16)E.Fritz et al: Converter Steelmaking with Emphasis on LD-Technology BHM 147 (2002) 127-135
17)E.Fritz: Stahlerzeugung nach dem OBM/Q-BOP-Verfahren Fachberichte Hüttenpraxis Metallweiterverarbeitung, 17 (1979), 4 Seiten
18)P.M.Fish: Global steel - a five year outlook Millenium Steel 2002, 16-21
19)J.K.Stone: The origin of modern oxygen steelmaking Steel Times 9/2000, 328-330
20)H.Trenkler, W.Krieger: Metallurgy of iron, Steelmaking 1 Gmelin-Durrer, 4th Edition, Volume 7a,7b, Springer Verlag 1984

Linz, Austria (June 2002)