Towards Quality in BOF Steel Refining Trends and Innovative Plant Concepts for Ladle Treatment

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1 Towards Quality in BOF Steel Refining Trends and Innovative Plant Concepts for Ladle Treatment Christian Redl, Christian Buchmaier, Matthias Pfennig, Alexander Weigl INTECO Melting and Casting Technologies GmbH Austria ABSTRACT Over the last years ladle metallurgy becomes an important factor in BOF plants. The driving force is the steady development and increasing demand for high quality steel grades fulfilling highest technical requirements. This leads to a rising share of vacuum treated steel. New alloying concepts and stringent quality targets requires ladle furnace treatment. The design, erection and installation of additional ladle treatment units such as ladle furnaces or vacuum treatment plants into existing and fully operating mills require thorough planning, flexible and innovative plant concepts and excellent project management in close cooperation with the customer in order to meet project time, budget and performance requirements. The paper presents recent developments in vacuum treatment plants and ladle furnace technology for utmost operation availability. Selected examples of realized projects illustrate different plant concepts customized for the client s specific situation regarding available space, operation safety, plant logistics and metallurgical targets. 1. INTRODUCTION During the last years the requirements on steel properties were steadily increasing. This led to new alloying concepts, very narrow chemical target analysis and very stringent demands on steel cleanliness. The request for vacuum treated heats tremendously increased from 25 % up to 75 % [1-5] and more during only two decades. Therefore steel producers expanded vacuum treatment capacities by installing new degassing units. To cope with accompanying challenges in steel plant logistics and to open new possibilities to reach more and more stringent quality targets ladle metallurgy becomes an important factor in BOF plants. The key equipment in this field is the ladle furnace. 2. VACUUM TREATMENT The main reason for installing degassing unit is clearly to lower the gas content of the melt. Achievable results are e.g. a hydrogen content below 1.2 ppm as well as a nitrogen content lower than 40 ppm. Also the production of ULC grades with carbon content as low as 10 ppm is possible by a combination of vacuum treatment and oxygen blowing for efficient decarburization. Two main plant concepts prevail in modern steel industry: Vacuum tank degassing (VD/VOD) Recirculatory treatment (mainly RH type degasser; DH degasser can barely found nowadays) Figure 1 RH degassing Although the achievable metallurgical results are almost the same for both types the RH degasser is in most cases the preferred solution for BOF shops due to the following reasons: Fast degassing in time with BOF tapping cycle No ladle freeboard required, therefore the full heat size can be treated One restriction compared to a VD plant (which is the preferred equipment for EAF shops) is the limited possibility to remove Sulphur. This could principally be solved by using an injection lance system which has the disadvantage of higher refractory wear in the vacuum vessel. However the optimum solution is to transfer desulphurization to the ladle furnace, which is perfectly suited for this task. 2.1 RH DEGASSER Most plants are designed as single treatment stations (see Figure 2), sometimes also RH plants with two treatment positions are installed (see Figure 3)

2 7.EFRS 7 th INTERNATIONAL IRON & STEEL SYMPOSIUM / "Let s Talk Steel Now..." Figure 4 Hydraulic lifting system Figure 2 RH single treatment stations Figure 5 Split type vessel Figure 3 RH double treatment station Immersion of the snorkels into the melt can either be done by lowering the vacuum vessel or lifting the ladle. As a possible example a hydraulic ladle lifting device is shown in Figure 4. This solution is characterized by a small footprint and easy maintenance possibility. Alternatively lifting could also be done by winch system. Lifting the ladle (instead of lowering the vacuum vessel) has the advantage that no moveable parts in the vacuum system (which are potential points of leakages) are necessary. One of the key components is the top lance (Figure 6) which is used for decarburization and chemical heating by oxygen blowing. In combination with the burner function easy removal of skulls can be achieved as well as vessel heating and temperature holding between treatments is possible. The vacuum vessel itself can either be designed as mono block or split type design. Using a split type vessel allows rapid and easy exchange of the lower vessel part and snorkels, which are prone to high refractory wear. Snorkel dimensioning is done in a way to ensure efficient circulation rates. Figure 6 Oxygen lance and burner system for twin RH plant UCTEA CHAMBER OF METALLURGICAL AND MATERIALS ENGINEERS S TRAINING CENTER

3 As steam is usually available in appropriate quantity and quality in integrated mills most RH plants are equipped with steam ejector vacuum pumps. As against the trend in electric steel mill towards dry mechanical pumps this is barely considered in BOF shops. Nevertheless it is worth to mention the modern mechanical pumps systems are of course suitable also for RH operation. In a summary it is obvious that the major decision factor for a specific plant concept and corresponding equipment configuration is obviously the required treatment capacity as well as the available treatment time to follow the CCM sequences. As many installations are incorporated into an existing plant (as so called brown field projects) the available space is also of utmost importance. To ensure smooth engineering and subsequent installation in narrow spaces nowadays a 3D- Laser scan of existing shops and surrounding equipment prior to project start is almost mandatory. 3.2 Basic furnace design To cope with challenging installation situations in narrow spaces as well as high productivity requirements led to a variety of layout solutions and equipment configurations. Below two different furnace design concepts are shown exemplarily. Figure 8 illustrates a furnace with fixed portal and one treatment station, whereas Figure 9 depicts a twin ladle furnace with swiveling portal. A side view showing the hanging electrode portal of the twin LF is given in Figure 10. The obvious advantage of the twin ladle furnace is the flexibility offered by two treatment positions. This is a versatile solution enabling heating of one ladle and simultaneously having the possibility for other ladle treatments (e.g. wire feeding or material injection). Additionally the hanging portal gives the advantage of little space requirement but also allows the ladle to pass through underneath the transformer from one to the other bay. 3. LADLE FURNACE 3.1 Tasks The reasons to install a ladle furnace in a BOF shop are manifold, the most important ones are the following: The tapping temperature in the BOF can be decreased, which yields a considerable increase in refractory lining of the converter Due to lower tapping temperature dephosphorization in the BOF is enhanced Very low Sulphur contents can be achieved by injection of desulphurization agents or slag treatment Precise adjustment of temperature and chemical composition to achieve optimal casting conditions and meet narrow product specifications The ladle furnace acts as an important buffer and helps to synchronize between BOF, RH and CCM Higher level of steel cleanliness can be achieved by appropriate stirring practice using porous plugs in the ladle. Figure 8 Standard LF with single treatment position Figure 9 Twin LF with hanging portal Figure 7 Ladle furnace -143-

4 7.EFRS 7 th INTERNATIONAL IRON & STEEL SYMPOSIUM / "Let s Talk Steel Now..." Figure 10 "Hanging" electrode portal for Twin LF 3.3 Ladle furnace auxiliaries Beside the selection of the basic furnace concept as shown above the auxiliary equipment is of utmost important to reach the targets of ladle furnace treatments. Usually modern LF installations consists of following auxiliaries: Injection systems (Figure 11) for lime, coal, CaSi etc to enhance metallurgical reactions such as desulphurization Wire feeding machines with up to 8 strands for very accurate material additions (Figure 12) Gas stirring system for ladle purging through porous plugs in the ladle bottom. An emergency top lance adds production safety as stirring is possible when the porous plug fails (or if the plant is still using a ladle without porous plugs) Figure 12 Wire feeder for 8 strands installed at a Twin LF 3.3 Roof design For ladle furnaces using injection systems a roof design with flat inner surface have proven to be an optimal solution to minimize sticking of splashed materials on the roof (Figure 13). Optimized adjustment of ladle rim and fume collection chamber offgas at the roof, a regulation damper in the dedusting system in combination with state-of-the-art electrode regulation system ensures minimum carbon and nitrogen pick up as well as low electrical energy and electrode consumption. Figure 13 Ladle furnace roof with flat inner surface Figure 11 Injection system 4. PROCESS AUTOMATION AND PRODUCTION MANAGEMENT SYSTEMS The required features of today s steel plant automation systems are much more than simple data collection and processing, plant control etc. Nowadays production UCTEA CHAMBER OF METALLURGICAL AND MATERIALS ENGINEERS S TRAINING CENTER

5 management systems (like IMAS INTECO metals application suite) covers a wide range of functionalities such as (see Figure 14) Level 2 Process Automation Level 3 MES Functionalities Level 4 Integration While the use of robots is kind of standard in the automotive or electronic industry unfavorable ambient conditions (like heat and heavy dust) combined with maybe conservative mindset prevents its procession into the steel industry. However trend towards robotisation is irresistible and led to sophisticated technical solutions also in harsh environments. An example (as shown in Figure 16) is the complete automatic handling of samples, hydrogen probes and temperature measurements. Figure 14 Automation Pyramid 5. SAFETY CONSIDERATIONS Nowadays occupational safety is becoming more and more important. Severe accidents in modern European steel plants are down to a minimum thanks to strict safety regulations and recent equipment developments. Operation of the plants from one common control room via production management systems (as shown above in Figure 14) these days is standard. For process observation CCTV systems with dozens of cameras are common practice and reduces physical presence of operator in endangered areas drastically. Key transfer system (as e.g. installed at the Secondary Metallurgy No 4 at voestalpine Stahl Linz [2]) further prevents unauthorized access to plant components and additionally provides safety to people working on plant components from unintentionally caused plant movements. Formerly necessary manual operations like coupling purging gas connection can easily be replaced by proven automatic gas coupling systems as depicted in Figure 15. Figure 16 Roboter for temperature measurement and sampling at the ladle furnace 6. REFERENCE PROJECTS During the past 15 years INTECO executed a number of projects with the aim to install refining units at BOF shops. voestalpine Stahl Linz Secondary Metallurgy #3 & #4 One of the first projects was the engineering for the Secondary Metallurgy No 3 for voestalpine Stahl Linz, which was started up in December This project comprised the installation of a ladle furnace and a RH degasser [1]. The success of the plant concept was proven by the decision of voestalpine to further invest into the extension of secondary steelmaking capacities for the top most quality segment which leds to Secondary Metallurgy No 4 started up in The background was another increasing demand in vacuum treated steel (from 60 % up to 75 % [2, 3]) and concurrent higher production flexibility. As reported by Gantner et al [2, 3] the new plant shows excellent metallurgical results. Saarstahl AG Figure 15 Automatic gas coupling system Saarstahl AG contracted INTECO in 2011 with the engineering and supply of a new secondary metallurgical complex. This complex comprises two Twin Ladle Furnaces and one RH plant. Each Twin LF is equipped with an 8 strand wire feeder, sampling devices, emergency stirring -145-

6 7.EFRS 7 th INTERNATIONAL IRON & STEEL SYMPOSIUM / "Let s Talk Steel Now..." lance etc. Alloying is done by a 34 bin material handling system. The entire plant was commissioned in Remarkable results regarding lifetime increase of converter refractories (+ 39 %) due to lower tapping temperatures have been observed [4]. thyssenkrupp Steel Europe The most recent ladle furnace installation is a 265 t Twin LF for thyssenkrupp Steel Europe AG for the German enterprise s plant in Duisburg Beeckerwerth [5]. This new furnace is equipped with all kinds of technological innovations and is completely encased with noise insulation panels to reduce the noise level. Again one challenge was the design works to fit the furnace into the narrowest space into the existing building whilst steel production in the surrounding area continues. First heat successfully commences in summer Figure 18 Electric Energy Consumption [6] Isedemir In spring 2019 Turkish steel producer Isdemir, the only local integrated mill that produces both flat and long product, started a project with INTECO to install a new 200 t RH plant [7]. To fulfill all requirements of modern steel refining it will be equipped with wire feeding machines, top lance as well as manipulators for temperature measurement and sampling. A complex material handling system with individually weighed bunkers ensures exact dosing of slag builders and alloys. The supply is completed by an additional ladle treatment station and a powder feeding station, which is well integrated in the production flow. 7. CONCLUSION Summarizing INTECO s experiences gained during the recent projects leds to following conclusion: Figure 17 Twin ladle furnace Pnomarenko et al [6] published first operational results showing low energy consumption as well as electrode consumption well below 4.5 g/kwh. The furnace roof has been designed to avoid any unnecessary nitrogen pick up. During operation this concept has been proven as the maximum N-pick up during treatment is non-critical. Plant layout concepts are usually dictated by narrow space constraints and at once keeping the running production of the BOF shop uninterrupted. Up-to-date 3D engineering in combination with smart plant design and 3D scanning of existing surrounding is therefore indispensable. Proper vacuum pump design to safely reach minimum pump down times and low vacuum pressure (usually below 5 minutes to reach 0.67 mbar) Adequate dimensioning of ladle furnace power supply to ensure sufficient heating rates (up to 5 C/min) and simultaneously low electrode (values well below 4 g/kwh have been achieved) and power consumption values (typically below kwh/t/ C). Beside the basic plant requirement as mentioned above equipment configurations which have been proven UCTEA CHAMBER OF METALLURGICAL AND MATERIALS ENGINEERS S TRAINING CENTER

7 successful for production of high quality steel grades are the following: Connection of the refining units to a material handling system equipped with sometimes more than 30 bins for numerous (ferro-) alloys and slag conditioners. Installation of wire feeding machines with up to 8 lines at the ladle furnace to enable flexible and highly precise addition of material Gas purging (with automatic coupling system) is inevitably at ladle furnace for further enhancing steel quality Injection system for e.g. lime, CaC 2, CaSi at the ladle furnace to enhance desulphurization (down to below 10 ppm) Automatic temperature and sampling devices (including hydrogen measurement) for both LF and RH. The nowadays standard manipulators are most likely be replaced in future by fully autonomous robots. Strict compliance with highest safety and environmental standards is compulsory REFERENCES 1. Gantner, A., Egger, M., Lehner, J., Experiences with the new Secondary Metallurgy No. 3 at voestalpine Stahl Linz. Proceedings 6 th European Oxygen Steelmaking Conference, 2011, September, Stockholm, Sweden 2. Gantner, A., From SekMet 3 to SekMet 4 - Developments at voestalpine Stahl GmbH Linz. Proceedings INTECO Technical Symposium, 2018, July 5th, Bruck/Mur, Austria. 3. Ganter, A., Reisinger, P., Grimm, G., Egger, M., Rössler, R., Secondary Metallurgy No 4 - The Answer to Future Quality Demands at voestalpine Stahl GmbH. Proceedings ESTAD, 2017, June, Vienna, Austria; p Heinen, A., Deckers, D., Schreiner, M., Kläser, D., Ladutkin, D., Commissioning of the new Secondary Steelmaking Facilities at Saarstahl AG. Proceedings 7 th European Oxygen Steelmaking Conference, 2014, September, Trinec, Czech Republic. 5. Brand, T., Future Possibilities of Steel Treatment at thyssenkrupp Steel Europe. Proceedings INTECO Technical Symposium, 2018, July 5th, Bruck/Mur, Austria. 6. Ponamarenko, A., Brand, T., Löcken, M., Arns, M., Meiß, S., Dolle, B., The first production results of a new ladle furnace in Oxygen Steel Plant No. 2. Proceedings ESTAD 2019, June, Dusseldorf, Germany. 7. n.n. An Order from a steel giant Isdemir orders 200 t RH plant. BHM 2019 (7), p