Metal 2001 May Ostrava, Czech Republic. CONARC - Innovative Steelmaking by SMS Demag By: K. Schmale; K.-H. Deppner; Dr. P.

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1 Metal 2001 May Ostrava, Czech Republic CONARC - Innovative Steelmaking by SMS Demag By: K. Schmale; K.-H. Deppner; Dr. P. Meierling SMS Demag AG, Steelmaking Division Wolfgang-Reuther-Platz D Duisburg Germany Abstract The charge mix of an Electric Arc Furnace is mainly determined by the steel quality or grade to be produced. As the quality demands are steadily increasing, highest flexibility in terms of charging scrap, DRI and hot metal, the use of different energy sources and the production of a wide range of steel qualities is of extremely importance for the producer. The SMS Demag CONARC process was developed for using any kind and mix of raw materials like hot metal, DRI and scrap to ensure highest quality requirements for the production of all steel grades covering a wide range from carbon to stainless steel. This report will show that the CONARC process is the best tailor-made solution for steelmaking, allowing maximum flexibility without any equipment modifications when producing carbon or stainless steel. 1

2 CONARC - Innovative Steelmaking by SMS Demag In response to the challenges in the steelmaking market, SMS Demag presents a new combined technology that requires just one investment in both process lines for carbon and stainless steel production: - CONARC for Carbon Steel - CONARC for Stainless Steel Before discussing these two technological solutions we would like to emphasise the requirements for some highquality steel products and their consequences for the charged raw material as well as the technological steelmaking process to be selected. The requirements for maximum tolerable tramp elements (Cu + Cr + Ni + Mo) range from 0.13 % for deep-drawing steel for automotive exposed body, cans and screen masks to % for heavy plates for the shipbuilding industry, boilers and welded pipes up to -0.8 % for seamless tubes for gas and oil drilling applications (figure 1). These tramp element requirements have a direct influence on the material that can be charged in the steelmaking plant. Tramp element levels of, e.g., 0,13 % or 0,35 % can only be realised by using large percentages of virgin materials such as DRI, HBI, pig iron and hot metal. Scrap used as main charge material needs to be blended with a significant amount of virgin materials. It is obvious that steel mills based on scrap as charge material can hardly fulfil the stringent requirements imposed by most of the more sophisticated high-quality steel products. The technological process selected for the steelmaking plant (figure 1) is greatly determined by these tramp element requirements and the starting material charged. For products that require about 80% or more virgin hot metal the Oxygen Blowing Furnace (BOF) is applied with good economic results. For material charges with a hot-metal content of only up to 30%, the Electric Arc Furnace process should be selected. Here both alternatives, the AC or DC Furnace, can be applied depending on the capacity of the electrical power network. The most flexible of all processes is the CONARC process in which the starting material may on the one hand be chosen according to the requirements of the finished product and on the other hand according to the economics of raw materials and energy sources available. 2

3 The CONARC CS Process for Carbon Steel CONARC is the most flexible process to produce high-quality steel in a combination of the EAF process and BOF process. As you can see in figure 2, the CONARC process can operate with a variety of different starting materials such as hot metal, scrap, pig iron and DRI. The twin-vessel process used a top-blowing lance and electrically arcing electrodes that can alternatively be used in both identical vessels. Depending on the finished-product requirements, the CONARC process is followed by a ladle furnace or a vacuum degassing unit. Figure 3 shows the references for CONARC systems. The process unit at Saldanha Steel, South Africa, consists of two 170-ton vessels and has a capacity of about 1.35 million tons per year. Start-up was in So far the process exceeds all expectations in terms of flexibility of the charged raw materials. A further CONARC process unit is in operation at Ispat Industries, India. This CONARC consists of two 180-ton vessels with a final capacity of 3 million tons per year, using 2 CONARC units in a second step. Both CONARC process units at Saldanha as well as at Ispat operate upstream state-of-the-art CSP plants. 3

4 In figure 4 you can see the schematic principle of the process sequences. The process starts with the oxygen-blowing phase, which is followed by the electric arcing phase. The whole process operates in sequences interchangeable in both vessels, starting with hot metal charging, then the oxygen lance is put into operation and starts blowing. At the end of the blowing phase, solid charge materials like scrap and/or DRI are charged into the vessel, the electrodes are swung in the operating position and arcing starts. At the end of arcing, deslagging takes place. Finally the liquid steel is tapped into the ladle. At Saldanha Steel the relation between hot metal and DRI as charged material was varied over a wide range. In production (figure 5), the relation of hot metal and DRI covers a range from 30 to 70 % up to 75 to 25 %. The graph shows that: a power consumption of less than 200 kwh/t has been achieved, the consumption of oxygen was adjusted to the increasing amount of hot metal, the tap-to-tap time representing the capacity of the CONARC process was kept constant over the full raw-material range. This new process for the production of high-quality carbon steel has been successfully introduced in the market and has opened a new avenue in terms of process flexibility. 4

5 The CONARC SS process for Stainless Steel The successful results of the CONARC CS process for carbon steel gave rise to the idea of introducing this innovative process also in the production of stainless steel. The conventional stainless steelmaking process as shown in figure 6 starts either with scrap or with hot metal from a blast furnace and ferro-alloys. The hot metal has to be treated in a special process for decarburisation, dephosphorisation and desiliconisation (DDDunit) to satisfy the requirements of stainless steel production. Depending on the finished product mix and its average carbon content, the Duplex or Triplex route is to be selected. The Duplex route, represented by a one-step decarburisation, is best suited for carbon levels > 4 ppm. If ultra low carbon levels (< 2 ppm - superferrite) represent a greater part of the product mix, the Triplex process with its two-step decarburisation is the most economic process route. The new CONARC SS process for stainless steel (figure 7) that combines the EAF with the stainless-steel converter opens up a new economic window for stainless steel production. All types of raw materials can be charged, and even products with an extralow carbon content (< 2 ppm) can be produced using a VOD vacuum tank degasser. 5

6 The process sequence as shown in figure 8 is totally reverse to that of carbon steel production. As in conventional stainless steel production, the process starts with melting-down of the scrap and ferroalloys in an EAF. The second phase, refining of the steel, takes place in the same vessel with oxygen blowing in combination with bottom stirring. The complete process is performed in only one vessel which has outstanding economic advantages for the total production. In comparison with the conventional Duplex or Triplex processes the CONARC SS process yields a significantly higher productivity (up to 30 %) due to shorter tap-to-tap times for the entire process (figure 9). Based on 150-ton melting units, capacities of more than 1.1 million tons per year can be produced. Calculations demonstrate that production cost savings of up to 50 US$ per ton and a reduced specific plant investment of about 20% in comparison with conventional stainless steel routes can be expected. Conclusion The SMS Demag CONARC process is the new technology for the most economic way of steelmaking given its extremely high flexibility in terms of: - charging of any kind and mix of raw materials - the use of different energy sources - the production of all steel grades covering a wide range from carbon to stainless steel. The CONARC process is the best tailor-made solution for steelmaking, allowing maximum flexibility without any equipment modifications when producing carbon or stainless steel. 6