The global market for flat products with highly

Transcription

1 Setting new standards in operator safety and product quality at voestalpine s CC7 slab caster In September 2011 a new slab caster was commissioned at voestalpine Stahl GmbH, Linz, Austria. The caster produces the highest quality slabs in a wide range of thicknesses and widths and is equipped with the latest technologies such as SMART bender, width-adjustable spray nozzles and process automation models, such as the Dynacs 3D secondary cooling system and DynaGap Soft Reduction. Uniquely, the plant has two LiquiRob robots which perform all the dangerous work activities and allow monitoring of the casting process from the safety of the control room. r Fig 1 Schematic of caster No 7 Authors: Andreas Eichinger, Ewald Reisenberger, Josef Watzinger and Jürgen Meisel Siemens VAI Metals Technologies GmbH The global market for flat products with highly sophisticated requirements like ultra high strength or sour gas resistance, has increased continually over the past decades. To achieve high strength, together with low alloying element content, a high deformation ratio of the rolling mill slabs is required the thicker the final flat product, the thicker the slabs needed to reach correct deformation ratio. High strength combined with sour gas resistance, especially for pipeline and offshore applications, needs excellent steel cleanliness and internal cast slab quality. At the new caster at voestalpine Stahl Linz (see Figure 1), the latest state-ofthe-art technology has been installed to ensure this high quality. Table 1 shows some basic parameters. Knowledge of the physical properties of different steel groups, from ultra low carbon to peritectic and up to ultra high carbon, is extremely important for optimum caster design, on which Siemens VAI has been and is continually focused. Thermal expansions, peritectic reactions, crack formation phenomena, as well as correct calculation of the solidification temperature of various steel grades have been the subject of many investigations by Siemens VAI and its partners. During the casting process mechanical and thermal loads act on the solidifying strand shell which are resulting from [1]: ` Contraction and phase transformation ` Temperature gradients along the surface or across the shell Production capacity: 1.2Mt/yr Thicknesses: 225/285/355mm Heat size: 177t Width range: 740 2,200mm Slab weight max. 40t Cut length: m Machine radius: 10m Metallurgical length: 35.3m Max. casting speed: 2.0m/min Start-up: 20 September, 2011 r Table 1 CC7 main caster design parameters 66

2 STEELMAKING AND CASTING ` Friction between strand and mould ` Bending and straightening ` Bulging ` Soft reduction OPERATIONAL RESULTS Historically, only vertical casters were dedicated to the production of thick slabs, with a high requirement on surface and internal quality. With the development of bow type casters with straight moulds and intelligent soft reduction solutions, together with air mist secondary cooling systems, this dependence has lessened. To obtain the best slab quality, caster No. 7 has been equipped with adjustable spray nozzles (3D Sprays) and EcoStar rollers. In combination with the latest generation of process automation models, such as Dynacs 3D, DynaPhase and DynaGap Soft Reduction, it is possible to reach the best surface and internal quality to fulfil the toughest quality standards for all kinds of steel groups. A wide range of steels have been cast to date, as shown in Figure 2. Figure 3 shows accumulated production since first heat in September The caster immediately exceeded all expectations, as the first heats were of marketable quality, and, since 31 October, 2011, the caster has been operating at full capacity. In September 2012, the record value of the first million tonnes of steel was reached, comprising 5,955 heats, 38,799 slabs with an average slab weight of 25.9t and 10m length the distance from Vienna to Munich! DESIGN FEATURES AND TECHNOLOGICAL PACKAGES With the installed 3D Sprays linked with the new Dynacs 3D calculation model, it is possible to adjust the correct water amount for each cooling zone over the whole slab width. Figure 4 shows the schematic layout of a Smart Segment, equipped with the adjustable sprays [2]. Control of the nozzle spray position and cooling intensity using these features to prevent corner overcooling involves optimising the cooling strategy. There are three nozzle position control strategies involving distance of spray water to the slab corner based on: ` the position in the machine based on the calculated shell thickness (solidus temperature) ` the age of the strand at the corresponding position. r Fig 2 Product mix during the start-up phase r Fig 3 Accumulated productions since first heat r Fig 4 Schematic layout of a segment equipped with 3D Sprays This system gives the possibility to avoid low ductility or brittle temperature zones at the slab corners. The avoidance of critical temperature ranges, especially for slab thicknesses more than 250mm, is of key importance. Due to this flexible adjustment of the spray nozzles, it is possible to optimise surface temperature profile according to the slab width along the relevant cooling zones [3]. a 67

3 r Fig 5 Solidification front (a) before optimisation of spray pattern and nozzle position (b) after optimisation Furthermore, the adjustable 3D Spray system creates a more uniform solidification profile, which enables higher efficiency to be reached during Dynamic Soft Reduction [4]. An example of a simulated optimised solidification front is seen in Figure 5. A top view of the centre cross-section of the strand is shown. The area of the liquid phase is indicated in yellow, mushy in grey and solid in red. By optimisation, the solidification front is flattened and the variation of end of solidification across the width is reduced. Using Siemens VAI s simulation tools it is possible to prepare off-line the optimised caster practice to achieve the best product quality, irrespective of steel grade. In addition to a uniform solidification front, knowledge of solidification parameters, such as solidification temperature and heat conductivity for peritectic reactions, is necessary. For this reason several reference steel grades are pre-calculated with the new DynaPhase model, which ensures an exact prediction of those parameters for a wide range of different steel chemistries. Based on this knowledge, with Dynacs 3D, the accurate calculation of the final solidification point and the area of the mushy zone is ensured. As a result of these highly accurate calculation models, it is possible to reach the highest surface quality and internal quality levels such as centre segregation, which can be seen in Figure 6 on a macro etched sample of 0.75% C ultra high carbon steel. Due to the wide range of steel chemistries for which CC7 is used, the behaviour during the whole solidification and cooling down range has to be known. This is why the new DynaPhase model includes an extended database which predicts the peritectic range, especially for new grades with high Al, Si and Mn content. r Fig 6 Macro etch of centre segregation in 0.75%C steel position: middle LIQUIROB The SIMETAL LiquiRob is an impressive example of modern applied mechatronics in the harsh environment of the steel industry. This highly flexible robotic system is capable of performing a wide variety of dangerous and systematic tasks. Two LiquiRob systems are installed with the aim of setting a higher safety standard for operating personnel and to increase process stability and reproducibility [5]. LiquiRob in the ladle area: ` connects the ladle shroud handling (LSH) device, which is used for shroud clamping and also acts as a slide gate cylinder. A ladle shroud holder is no longer necessary for operators working below the ladle with liquid steel ` connects multi-coupling system for services like argon and electricity ` unlocks the ladle bolt Also, for the first time in a steel plant, an optical measurement system was installed along with the 68

4 STEELMAKING AND CASTING LiquiRob. The optical measurement system serves for the identification of targets which are the basis for connection and removal of required equipment at the ladle. LiquiRob in the tundish area is used for: ` Temperature/oxygen/hydrogen measurement ` Taking steel samples (see Figure 7) ` Shroud handling ` Tundish powder dosing ` Ladle oxygen lancing The LiquiRobs are fully integrated into the plant automation system and so fully automatic operation of the LiquiRobs is performed. Additionally, operating personnel have the freedom to interact with the system from the safety of the control room and can feed socalled working queues, which LiquiRob executes sequentially. During the first months of operation, LiquiRob successfully took about 1000 steel samples/ temperature measurements from the tundish and proved its stability and high availability. With the installation of these two LiquiRob systems Siemens VAI has demonstrated its leadership in robotic applications in steel plants and has shown the future direction of plant operation with an emphasis on personal and operational safety. r Fig 7 Sample handling with LiquiRob SAFETY Another feature of the SVAI safety system comprises the design of the mould cooling system to fulfil performance level PL e/category 4. Instrumentation, sensors and the design of hydraulic controls were done to the required performance level. The control system for safety related parts fulfils the requirements of EN ISO , safety PLCs for critical functions are installed and a selection of safety-related parts of the control systems suit the safety functions and level, eg, sensors, logic units, power control elements, as well as drives and actuators. Collision a r Fig 8 Strategy for risk assessment and risk reduction Another important issue in plant design is conformity with regulations and state-of-the-art safety features. This is now mandatory for new installations in European steel plants. The European standard EN (safety requirements for machinery and equipment for continuous casting of steel) for continuous casting machines have been applied at CC7. Extensive analysis was done and several safety documents produced which were the basis for safety equipment installed in the mechanics and automation systems. These include: ` Risk analysis, which dictates technical and organisational safety measures ` Safety identification plan, showing dangerous areas, escape routes, safety signs and safety information ` Operational safety instructions 69

5 STEELMAKING AND CASTING protection functions against interference for specific equipment (eg, ladle turret/tundish car) are forseen. All these measures are based on legal and normative basics like machinery directive 2006/42/EG, standards such as EN ISO 12100:2010, including safety of machinery comprising general principles for design risk assessment and risk reduction as well as the EN 14753:2007 comprising safety of machinery safety requirements for machinery and equipment for continuous casting of steel. A summary of the above is illustrated in Figure 8. CONCLUSIONS The success of the recent thick slab caster project at voestalpine Stahl Linz is the logical result of continuous developments at Siemens VAI, together with excellent co-operation with voestalpine Stahl engineers. The installation of the caster has set a new benchmark in casting of high quality steels with increased operator safety. MS Andreas Eichinger, Ewald Reisenberger, Josef Watzinger and Jürgen Meisel, are with Siemens VAI Metals Technologies GmbH, Linz, Austria. CONTACT: eichinger@siemens.comjohannes.staudinger@ siemens.com References [1] C Bernhard, R Pierer, A Tubikanec and C M Chimani, Experimental characterization of crack sensitivity under continuous casting conditions, CCR, Paper No. 6.3, 2004 [2] R Pierer and C Bernhard, The nature of internal defects in continuously cast steel and their impact on final product quality, AIST Proceedings 2010, pp [3] F Ramstorfer, K Dittenberger, K Hauser and S Hahn, Dynacs 3D the new dimension in secondary cooling for slab casters, ECCC 2011 [4] S Ilie, R Fuchs, K Etzelsdorfer, C Chimani and K Mörwald: Slab quality improvement by soft reduction technology, ECCC 2008 [5] M Hirschmanner et al, LiquiRob Improved safety and systematic procedures on the casting floor using advanced robotics, ECCC 2011