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|DEVELOPMENTS IN STEELMAKING|
|A Brief Historical Review||By Supriya Das Gupta, CMD, M. N. Dastur & Company (P) Ltd.|
Steel has been indispensable to the development of the world at large. There has never been a let up in the efforts to develop and/or improve steelmaking process technologies. New ideas to widen the acceptance of established processes, speeding up of production and reducing the costs have been the prime movers of such developmental efforts. The concept of steelmaking, therefore, has witnessed dramatic changes and new technologies have replaced the older ones, from time to time.
We will briefly dwell on the revolutionary changes that have taken place in steelmaking technology since the time steel came to be produced on a large scale commercially; their contributions to overall world crude steel production over the same period; and more importantly, the major technological innovations they have been undergoing or are likely to undergo in the future. The address will also briefly touch upon the current steelmaking scenario in India.
From Bessemer to BOF Steelmaking
The earliest methods of steel production were the blister, shear and crucible. The origin of the cementation – blister steel is lost in antiquity. In spite of the progress in the art of steelmaking, crucible steel is produced in very much the same manner as it was many years ago. Its products are usually high-grade alloy and special steels. The first crucible steel works is reported to have been set up in Sheffield in 1740 by a clockmaker called Huntsman. The process was so successful that it ruled the field in steel production until Bessemer’s invention.
The bottom-blown acid Bessemer process, patented in 1856, was the first large-scale method for the rapid production of liquid steel by refining molten iron. The process yielded successful results with the use of low-phosphorus and high-manganese iron ores. The Bessemer process was the original pneumatic steelmaking process and became popular in those countries, which produced low phosphorus iron that was refined in acid lined converters as in the United States.
The bottom blown basic pneumatic process or the Thomas (or basic Bessemer) process, which used basic lining and basic flux in the converter, enabled the use of the pneumatic method for refining molten iron produced from high-phosphorus iron ores in many European countries.
The Bessemer processes, both acid and basic, dominated the world steel production scenario during the last two decades of the 19th century and until about 1905. By 1950, the share of Bessemer processes reduced to only 14% and in 1978 production from these processes dwindled down to only 3 million tonnes. The process was declared obsolete and was phased out in the following year. This was the result of the development of the Open Hearth process.
Open Hearth (Siemens-Martin) Process
The open hearth or the Siemens-Martin process, originally known as the pig-and-ore process was the next milestone in the development of steelmaking. Like other processes for purifying pig iron, the main principle of the open-hearth process was that of oxidation. The process proved that it was possible to oxidise the carbon in liquid iron using iron ore. In other respects also, this new process was unique. The high temperature achieved through fuel burning caused the solid metallics (pig iron and scrap) to melt and helped retain the final product in its molten state. This was free of entrapped slag.
Open hearth became the predominant steelmaking process for six decades from 1910 to 1970. It was only after a lapse of more than 80 years since its first industrial application, open hearth’s contribution to world steel production reached its highest at about 80 per cent. In terms of quantity, the peak production of open hearth was 279 million tonnes in 1964. However, with the advent of oxygen steelmaking, which could produce steel at a faster rate, open-hearth steelmaking gradually began to phase out. By 1970, its share had already dipped to 39 per cent. In 1997, the open hearths worldwide produced a paltry 46 million tonnes that is only about 6 per cent of the world crude steel production. By 2001 its share plunged to about 36 million tonnes, accounting for about 4.3% of global steel make. This process is expected to be phased out completely in the near future.
Basic Oxygen Process
The advent of oxygen steelmaking began with the successful development of the LD process at Linz and Donawitz in Austria in 1952. With the appearance of the LD process on the industrial stage, the importance of the open-hearth process declined and currently the basic oxygen processes are contributing about 60 per cent of the world steel production. The top blown LD process also led to the development of several other combined – and bottom-blown oxygen processes such as LDAC, OLP, OBM, LWS, QBOP and EOF.
Over the years, combination blowing processes, too, have been developed to control the oxygen pneumatic steelmaking process. These processes indicate that 60-100 per cent of the oxygen required to refine the steel is blown through a top mounted lance (as in the conventional LD or BOF) while additional gases such as argon, nitrogen, carbon dioxide or air is blown through bottom mounted tuyeres or permeable brick elements. There are, at present many different combination blowing processes, which differ in the type of bottom gas used, the flow rates of bottom gas that can be attained and the equipment used to introduce the bottom gas into the furnace.
In 1967-68, the method of bottom blowing of converter was tested at the works of the Eisenwerke-Gesellschaft Maximilianshutte of Germany. This new process consisted of blowing of high phosphorus hot metal through the converter bottom, with protection of oxygen jets by a coolant – natural gas. Fine powdered lime is also blown together with oxygen. This was called the Oxygen Boden Maxhutte (OBM) process. In France, this bottom blown converter process has been modified by using liquid fuel to protect and cool oxygen. This has been called the Loire Wendel Sprunck (LWS) process.
Similarly, the Klockner Maxhutte Stahlerzeugungsverfahren (KMS) process developed by Klockner of Germany used the principles similar to the OBM process and also injected powdered coal from the bottom for more scrap melting. Later on, this KMS process was modified for 100 per cent scrap melting and came to be known as the Klockner Stahlverfahren (KS) process, which has now been declared obsolete.
In 1971, the United States Steel Corporation acquired a license for a process of bottom blown converter operation at their works in Chicago. They developed a system for introducing measured quantities of ground lime into the oxygen jet and neutral gas was supplied on stopping of blow in order to prevent flooding of the tuyeres by the metal. This modified process came to be known as the Quiet or Quality-Basic Oxygen Process (Q-BOP). All these processes have been combined under the generic name of bottom-blown converter processes.
Energy optimisation furnace (EOF)
The EOF started commercial production in 1988. The EOF is essentially a batch oxygen steelmaking process with high post-combustion, with coal additions and extensive scrap pre-heating. The process theoretically can charge upto 100 per cent scrap, but typically 40-60 per cent scrap is used. Hot metal is charged into the vessel followed by scrap from the lower pre-heating chamber. The post-combustion gas is used to pre-heat the scrap and therefore, the energy from post-combustion does not have to be completely transferred to the bath. EOF has been operating in Brazil for over ten years and there are two 30-tonne furnaces. There was an 80-tonne furnace at Tata Steel in India, which has since, been shut down as also a 60-tonne furnace at AFS in Italy. Even though two more EOF plants have been commissioned in late 1998 at Hospet by Kalyani Mukand and at Salem by SISCOL with respective capacities of 1 X 40 tonnes and 1 X 30 tonnes, their performance is yet to be evaluated.
The process has been reasonably well proven using a scrap charge upto 60 per cent only. The yield with a 50:50 charge mix is 92-94 per cent, which is similar to that of a BOF. Consequently, a higher percentage of scrap in the charge mix will lead to lower yield. This is the basic reason why the EOF process has found limited application in the long run as compared to the BOF route.
Electric Arc Furnace Process
The first commercial electric arc furnace was put into operation in 1899. The most interesting feature of this process is that though it has not brought about a dramatic change, it is the only process that has survived and continued to grow slowly and steadily all through the 20th century. In 2001, it accounted for about 285 million tonnes of crude steel production, which was about 33% of the world crude steel output.
The history of the development of steelmaking processes during the last one-and-half centuries brings out that for production of tonnage steels, the amenability of a process to accept a variety of metallic charge, high productivity and low cost of production are the deciding factors. The open-hearth process, which had dominated the steelmaking scene for six decades, had a particular advantage of using large amounts of scrap available after the two World Wars. History also indicates that efforts to develop new processes continue at all times. However, many of these processes find limited industrial application under specific local conditions, while several others fail to establish their commercial viability. Besides, there are modifications to some of the major processes to meet local needs or raw materials availability. Occasionally, two or three of the standard steelmaking processes have also been employed in conjunction with each other. Typical examples of these developments are Bertrand-Thiel process, Talbot continuous process, Twynam process, Perrin process, Graef Rotor process, Duplex and Triplex processes, Twin-hearth process etc. However, the contribution of these processes to world steel production has generally been very small.
The share of the major processes in the world crude steel make is shown in Fig.-1. The contribution of the major processes towards global crude steel production over the last five decades is given in Fig.-2.
Trends in Technology
In generic terms, two steelmaking processes are available today, namely the basic oxygen furnace and the electric arc furnace. Given this present scenario, for mass production of steel, these two processes would continue to dominate the scene in the foreseeable future. It is to be noted that while the basic oxygen process is more dependent on the use of hot metal, the electric arc furnace shows a greater flexibility in this regard.
Basic Oxygen Steelmaking
Generally speaking, the BOF operates with high hot metal charge. There have also been developments to utilise high proportions of scrap and other solid metallics in the charge. However, these applications are and would be restricted to specific plants under special conditions.
There have been many improvements in the design of the converter vessel and associated systems. The specific volume of the vessels in new plants is commonly adopted as 0.9 – 1.0 cu m per tonne of liquid steel and the height to diameter ratio has risen to 1.4 to 1.6. The vessel suspension system has been modified from earlier bracket design to a more reliable design based on tension element units or suspension discs to eliminate risks of shocks and impacts during converter rotation. The oxygen blowing intensity has also been increased to about 4.0 N cu m per tonne per minute and the oxygen blowing period is restricted to 15 – 18 minutes, irrespective of the size of the converter.
The slag splashing technology, developed in the late 1980’s, has been successful in improving the converter availability and decreasing the operating costs. This technology is being increasingly adopted.
With regard to the unit size, it is noted that very small size converters of less than 15 tonnes were installed in the past, particularly in China. In the coming years, it is unlikely that such mini converters will be set up. This is mainly because the quality of steel produced in these converters is not suitable for continuous casting. Future converters will possibly have a minimum size of 30/40 tonnes.
With regard to the large size converters, it is unlikely that large units of 250/300 tonnes or more will find wide application. This is because the installation of large size steelmaking facilities in one go may become rare.
Electric Arc Furnace Steelmaking
In the 1980s, single electrode direct current (DC) systems demonstrated some additional advantages as compared to the conventional AC furnaces. In the past 15 years, a large percentage of the new EAFs built have been DC. Commercial furnaces vary in size from 10 tonnes to over 300 tonnes. A typical state-of-the-art furnace is of 150-180 tonnes size.
From 1970-2000, the process has undergone wide changes so as to raise production capacity and to improve product quality considerably. These changes have also aimed at reducing consumption of electrical energy, metallics, alloy inputs, utility and refractory. This again in turn has been possible due to progressive technological upgradation of process parameters; equipment design; operating practices; economics of conversion and reduced generation of pollutants and wastes. The gradual increase in popularity of the EAF process has been possible primarily due to increase in productivity with the introduction of improved equipment design and operating practices such as:
Widening choice of metallics
Conventionally, scrap has been the metallic charge for EAF. Use of virgin metallics such as DRI, HBI and pig iron lower the tramp elements in the charge. Also, these materials are of uniform chemistry and predictable quality. DRI and HBI can also be continuously charged in the electric arc furnace. The use of DRI and HBI is, however, associated with higher power consumption and increased slag volume. Normally, solid pig iron is added upto 40 per cent in the charge. Use of hot heel practice and oxygen lancing, preferably with post combustion lances, are effective means of melting pig iron.
The use of iron carbide has undergone trails in some American plants. Results show that there is some improvement in yield and reduction in specific power consumption. However, the techno-economics of using this charge material is not yet established.
Use of hot metal in the electric arc furnace has opened up a new direction in the development of electric arc furnace steelmaking. Hot metal use improves productivity; reduces power and electrode consumption; lowers melting time etc. It is being practiced with advantage in various plants in South Africa, Japan, Italy and India. This practice is finding increased acceptance by EAF steelmakers. For instance, in the USA, Steel Dynamics plans to utilise DRI produced in rotary hearth furnace as a charge material for their electric furnaces, after melting it in a sub-merged arc furnace.
It is reported that HYLSA has started using hot DRI through pipeline transfer from the DR plant direct to its electric arc furnace. The use of hot DRI from rotary hearth furnaces adopting bucket charging has been proposed in a plant in Thailand. Hot charging by using a special container has been developed and practiced in India.
New concepts of furnace design
With the aforesaid developments in the use of metallic charge, interesting developments have now taken place in the design of the electric arc furnace. The twin-shell EAF of hybrid design also referred to as CONARC by its supplier has been installed in India and South Africa. These furnaces are provided with a single power supply system with rotating electrode arms supplying power to both the shells. Such design will permit use of 50 per cent hot metal as metallic charge. Other designs under development are K-ES, Danarc, Shaft Furnace, Consteel process; Contiarc; Comelt and twin-electrode DC-furnace.
It would be relevant to mention also the emerging continuous steelmaking processes. To name a few: IRSID of France; AISI of USA; WORCA of Australia; Bethlehem of the USA; NRIM of Japan; COSMOS of USA; and IFCON of South Africa. Some of these processes have reached pilot plant stage. It may be noted that most of these processes use hot metal as the main charge material. However, doubts have expressed whether any of these will have a significant share in the world steel production in the immediate future. The history of process metallurgy development shows that it takes time for a process to commercially establish itself and then to grow. There is a considerable time lag between the development of a new process and its reaching maturity. For instance it took some 20 years for BOF to become the lead process; about 30 years for the continuous casting to reach its full spread; and 10 years for thin slab casting to reach 40 million tonnes capacity. In ironmaking also, even after so much of effort, all the processes other than blast furnace today account for only about 7 per cent of the total ore reduced.
Steelmaking in India: The current Scenario
Steelmaking in India in the pre-independence era was confined largely to the private sector where the prevalent processes at that time were the Bessemer, Open hearth and Duplex with ingot casting. It was after independence that public sector steel plants emerged, particularly during the Second Five Year Plan period. It was during this time that only the Rourkela Steel Plant adopted the new LD Steelmaking process whereas all other integrated plants continued with open-hearth technology. The primary objective during those days was to produce tonnage steel for infrastructure development of the country and to meet the existing shortage of steel. Cost competitiveness and profit were not the prime objectives of the industry.
The share of different processes in the steel production of India is currently as follows:
It will be noted that the share of BOF processes, like in the global scenario, is about 60%. The share of steel produced in electric furnaces in India is 29%, which is somewhat lower than that in the global scenario. The share of the open-hearth furnaces (including twin hearth) at 10% is more than double of that in the global scenario.
The BOF processes practiced in India include conventional top blowing, combination blowing and EOF. Two plants have opted for combination blowing, while five plants have adopted top blowing. The EOF process, accounting for only 3% of total BOF production is practised in two mini-integrated plants.
Electric Arc Furnace-based Plants
The growth of electric steelmaking in India was very modest till the sixties. In the early seventies, with the encouragement of the Government of India, many electric furnace based mini-steel plants were set up in the private sector to meet the acute shortage of steel prevailing then. These plants had the locational advantage of being close to the consuming centres and operated mainly on locally available scrap. Further, the plants had the advantage of low capital investment and lower gestation periods. In view of high power cost and non-availability of good quality scrap at reasonable prices, most of these plants have closed down since.
The liberalisation of India’s economy in 1991, encouraged entrepreneurs to set up medium/large sized electric furnace based plants. Today, integrated plant with DC UHP furnace of 150 tonne capacity with use of sponge iron and twin-shell AC furnace of 180 tonne capacity with use of hot metal as major constituent of metallic charge have come into operation for the production of flat products. There were 188 EAF units in the country with a total installed capacity of about 10.68 million tonnes of crude steel per year in the pre-liberalisation era. The number of working units now, however, is much less, only 36 plants with a working capacity of 5.50 million tonnes per year.
Induction Furnace Units
Since the early eighties, induction furnaces (IF) have made rapid inroads into the steel industry. Today, India is perhaps the only country in the world with substantial contribution to steel production from the IF’s. About 937 IF units were set up with furnace size of upto 10 tonnes mostly for production of pencil ingots for further processing by re-rollers. The working capacity at present, however, is only 4.27 million tonnes per year from 652 units. The IF units which were earlier based on scrap usage, have started using DRI upto about 80 per cent.
Indian steel plants have been known to suffer from inefficient operations, which have often led to low productivity. However, recent operating practices have focused their attention on these issues and it has been possible to improve operations significantly. For example, Visakhapatnam Steel Plant has adopted the practice of simultaneously operating all the installed BOFs. With the use of selected good quality raw materials in the blast furnace, it has been possible to produce high quality hot metal at Vizag. The plant also practices slag splashing. As a result, it has been possible to improve the lining life to 2154 heats for Converter A (best achieved). Also, with 0.88 cum per tonne of liquid steel specific volume of converter, it has been possible to achieve about 3.36 million tonnes liquid steel production during 2002-03, which is 12% over its rated capacity.
Similarly, in the electric arc furnace plants, the use of hot metal has enabled significant reduction in electric power consumption for steel making.
On the basis of development trends, it would appear that the main processes available for steelmaking in the foreseeable future are the BOF and EAF. However, the other processes including some of the emerging processes may find limited local application. The choice of the steelmaking process in future, as in the past, will be location and raw materials specific. There is no single process, which is expected to find universal application.
There are countries, which would continue to favour the adoption of the BOF route. This will particularly be in those countries where cost of energy is high and/or availability of electrical energy is limited. The BOF will also find preference if these countries have the possibility of producing hot metal at reasonable cost and where scrap availability is restricted. This will include countries such as Japan, China, Western Europe, Brazil, India, South Korea, etc. The EAF route will be preferred by countries such as the USA, Venezuela, Middle East, and North Africa where abundant and cheap energy resources are available. The electric arc furnace may also be set up in some countries as a replacement for existing BOF units. This would be particularly so in respect of countries where the integrated steel plants have not been technologically updated and the cost of modernising them are high. This situation is already apparent in countries like Luxembourg, Spain and in USA. In China, the electric arc furnace process will find application together with the BOF. The electric arc furnace may be set up as a replacement of old smaller units and some major installations. However, the BOF will continue to be the workhorse in China in case they have to maintain their present supremacy in the world steel scenario.
The new electric arc furnaces installed will have to choose from amongst the available and developing technologies in respect of metallic change, operating practices and furnace design, taking into account their local needs.
During the last decade, both the EAF and BOF have reinforced their contribution in world steel output by 100 million tonnes each. While this corresponds to a 60 per cent growth for EAF, for BOF it is only 30 per cent. If one were to assume that history will repeat itself in the next decade, with regard to the growth of EAF production, its share in the crude steel output would even then be in the range of about 38 to 40 per cent; which would tend to show that BOF would still continue to retain the lion’s share in world steel output.
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