New slab caster at Salzgitter Flachstahl GmbH The new single strand vertical bending type caster was designed both to increase total cast slab output at lower cost and to increase the range and quality of steel grades produced, particularly for the automotive and linepipe sectors. The caster incorporates a number of new and innovative features. AUTHORS: Ulrich Grethe, Thomas Müller, Peter Müller, Christian Geerkens, Wolfgang Moßner and Axel Weyer Salzgitter Flachstahl GmbH and SMS Demag AG With a crude steel production capacity of more than 8Mt, Salzgitter AG ranks among the leading steel producers in Europe. One of its subsidiaries, Salzgitter Flachstahl GmbH, produces hot strip, thin sheets and surface finished products as well as input stock for heavy plates and sections. Two bow-type continuous casters have a product range of approximately 400 steel grades including high quality grades which meet the most stringent tolerances in terms of dimensions and mechanical properties. In February 2003, SMS Demag AG was awarded a contract for the supply of a single-strand slab caster and all ancillary equipment. The objectives of constructing a third slab caster were to increase the annual production Technical data Annual production rate [t/yr] 600,000 Ladle capacity [t] 210 Caster data Caster type Vertical bending Casting radius [m] 9.0 Number of strands 1 Number of segments 12 Metallurgical length [mm] 30,949 Casting speed [m/min] 1.35 Tundish capacity [t] 34 Dummy bar system Inserted from the top Secondary cooling Air-mist type Water max [l/kg] 1.1 Slab dimensions Cast dimensions, width [mm] 850 2,100 Basic thickness [mm] 250 Slab length range [mm] 5,200 12,400 r Table 1 Technical features of caster No. 3 r Fig.1 Ladle turret rate of all three casters to a total of 4.7Mt, enhance the grade range, in particular phosphorus-alloyed and high tensile steel grades (including hydrogen induced cracking (HIC) resistant as well as interstitial-free (IF) and C steel), cut production costs, increase flexibility, improve product quality and to safeguard slab production during the standstill and scheduled maintenance of one of the existing casters. Also, this caster had to be designed in such a way so as to absorb production of one of the two existing casters in the longer term. The key technical features are shown in Table 1. The caster is designed as vertical bending machine of segment-only design for casting slabs of 250mm thickness and of widths between 850 and 2,100mm. It has a metallurgical length of 30.9m and a vertical section of approximately 2.6m. Figure 1 shows the ladle turret. The prevailing demanding product quality requirements in combination with the planned annual production capacity increase from 0.6 to 1.3Mt/yr will be satisfied by using the most up-to-date plant engineering and technologies, which will be presented in more detail. a 131
r Fig.2 Mould oscillation operating window SPRING-GUIDED RESONANCE OSCILLATION SYSTEM WITH HYDRAULIC DRIVE Precise mould oscillation allows achievement of improved surface quality. Therefore, the mould must, at regular intervals, move downward faster than the strand under compressive stress and thus prevent the newly formed strand skin sticking to the mould wall. This requires highly dynamic control as far as scanning time (Ta = 1ms) and control algorithms are concerned. The spring-guided resonance mould is a new very space-saving design. The hydraulic drive allows any oscillation parameter to be changed with casting in progress to obtain a flawless slab surface. Roller wear is also dramatically reduced. The mould oscillation system was assembled and dynamically tested in the shop prior to shipment. Figure 2 shows the extent of the investigations carried out in the operating window of frequency and oscillation stroke of the hydraulic oscillation system. The movement accuracy of the mould was checked using triaxial acceleration sensors with particular attention being paid to movement at the liquid steel level in the mould. In addition, the phase difference between the two lifting brackets was determined and the time sequence of the accelerations was analysed. During commissioning, these measurements were repeated for selected operating points, both in the idle mode and during casting where the good results of the shop test were confirmed. The oscillation parameters may be set online from the control pulpit. r Fig.3 Principle of detection of the end of liquid core r Fig.4 Measurement during operation for one strength grade, 1,150mm SEGMENTS OF CYBERLINK DESIGN CONCEPT The CyberLink segments, which are the highlights of the installation, allow the production of sophisticated steel grades and provide the preconditions for further developments in the field of HIC-resistant steel grades. The segments used in the horizontal section of the continuous caster downstream of the straightening segment excel by special mechanical and process engineering features. The top frame is guided by CyberLinks arranged at an angle, while the side frame, commonly used up to now, is now no longer required. This top frame guiding assembly ensures self centring of the top frame on the strand. A lifting beam is also no longer required and the driven roller is integrated into the top frame in such a way that no more than four hydraulic cylinders are necessary for each segment. To allow safe transmission of the drive torque to the strand even in the case of a fully solidified strand, the segments located downstream of the end of liquid core are actuated via a special combination of position and force control. A new type of load-sharing control ensures that the tightening torques are distributed on the various drives in an optimum manner. In this control system, the measured force of the segment 132
hydraulic cylinders which, related to the process, may greatly differ for each segment for instance due to dynamic Soft Reduction, is taken account of in the moment distribution. Determination of the end of liquid core The detection of the end of the liquid core benefits from the fact that the mechanical or dynamic properties of the strand change with position. For this purpose the top frame of the CyberLink segment oscillates with an amplitude of up to 0.2mm. The area of hysteresis of an oscillation vibration can be computed via the phase displacement of the cylinder force and position and the location of the end of liquid core is determined from the size of the area of hysteresis. Both stationary and non-stationary methods are available for the determination of the end of liquid core. In the case of the stationary method short oscillations take place one after the other with the CyberLink segments at constant process conditions (see Figure 3). Figure 4 shows a measurement during operation for one strength grade. In the case of the non-stationary method for transient process conditions for which a change of casting speed is particularly beneficial an oscillation with one CyberLink segment is performed for a longer period of time. Operating results have shown that, due to the small amplitude, the top frame oscillation does not adversely affect the internal quality of the slabs. Figure 5 shows the characteristic evolution of the area of hysteresis computed from the cylinder force and position for a given strength grade as the end of liquid core passes segment 9. In addition to the Dynamic Solidification Calculation (DSC ) solidification model with top frame oscillation, a single calibration of the curve progressions, for instance by pneumatic gun tests, provides another effective tool to determine where the end of the liquid core is located. AIR-MIST SECONDARY COOLING SYSTEM The quality of the steel produced by a continuous caster is strongly dependant on the solidification process and hence on the cooling system. Modern concepts for continuous casters with a high casting performance make exacting demands on the nozzle system in the strand guide system. The cast strand has a thin skin as it exits the water-cooled mould, and is fully solidified by the end of the strand guide system by means of the spray cooling system and a number of other heat-transfer mechanisms. This is achieved by the implementation of the DSC cooling model which is in charge of the control loop. r Fig.5 Characteristic evolution of the area of hysteresis 1,600mm Cooling concept The entire strand guide system is split into 16 spraying zones. In addition, spraying zones 3 to 5 are each broken down into three control loops over the casting width (see Figure 6). Spraying zone 1 (spray ring) and spraying zone 2 (narrow faces) are equipped with single fluid nozzles. Flat-jet nozzles are in zone 1 and flat jet plus two additional cone nozzles are used in zone 2. Cooling in spraying zones 3 to 16 (segments 0 12) is via air-mist cooling. In the horizontal strand guide section, ie, spraying zones 13 16 (segments 8 12), the specific density of admitted water is relatively low. In spraying zones 3 12 (segments 0 7), the width of spraying is adapted to the actual slab width, whereas in spraying zones 13 16 (segments 8 12) it is not adapted. Slab width-dependent setpoint presetting for spraying zones 3 12 The adjustable total range of slab width is 850 2,100mm (cold dimensions). Spraying zone 1 always operates with a spraying width that corresponds to the maximum slab width. For smaller slab widths the spraying width of spraying zones 3 12 is adjusted to the actual slab width. In spraying zones 3 5 (segment 0 and 1) this is achieved with the so-called control concept, while in spraying zones 6 12 (segments 2 to 7) it is done via edge deactivation. Slab width-dependent control concept in spraying zones 3 5 In the area of spraying zones 3 5, cooling is implemented by the slab widthdependent control concept. Figure 6 is an example that shows the realisation of the concept. The water correction factor which depends on the slab width is a computed via the DSC. 133
4.2 MS06-36 pp131-136 4/6/06 10:27 am Page 134 r Fig.6 Control concept for spraying zones 3 to 5 r Fig.7 Edge deactivation of spraying zones 6 to 12 134 r Fig.8 Technological control and monitoring systems
Edge deactivation in spraying zones 6 12 In the area of spraying zones 6 12 cooling is implemented via edge deactivation. The edge nozzles of the spraying zones may be closed or opened depending on the actual slab width. The water correction factor which depends on the slab width is automatically selected as a function of the slab width. The setpoints of these spraying zones are reduced accordingly. Figures 6 and 7 illustrate these points. TECHNOLOGICAL CONTROL AND MONITORING SYSTEMS (see Figure 8) Mould level control A basic requirement for attaining a high slab surface quality is a stable mould level. Major fluctuations of the mould level make surface flaws and casting flux inclusions more likely. To exclude these impacts which affect the quality, SMS Demag has developed its own concept for mould level control including special technological control algorithms integrated in the SMS Demag control systems. With a view to improving the control accuracy and to increase operational reliability, two mould level meters are used simultaneously. r Fig.9 Cumulative production Mould width adjustment during casting To improve production flexibility, a mould adjusting mechanism has been implemented. During casting, the narrow face taper may be adjusted depending on the casting speed, steel grade and section width. During width variations, special width-change strategies are applied to keep yield losses at a low level. Thanks to an optimised section width change it is not necessary to reduce the casting speed. High adjusting speeds allow considerable width changes with short wedge lengths to be obtained. Break-out prediction system and mould heat flux visualisation The measurement of the heat flux in the mould and its 3D representation have proven very useful during commissioning. Thermocouples are fitted in several vertical levels in the mould to measure temperature distribution in the mould. The representation of the heat flux allows closer monitoring of what is happening in the mould and to detect an imminent break-out at an early stage. These measuring results also provide important information on the further development of steel grades, casting fluxes and submerged entry nozzles. r Fig.10 Heats per month Hydraulic segment adjustment and soft reduction To reduce segregation during residual solidification, soft reduction is used for specific steel grades and is achieved by using hydraulically adjusted CyberLink segments. Because the Level 2 DSC model and the technological control systems interact, soft reduction is performed at the correct position down the slab. a 135
Casters 1 and 2 High-strength grades Micro and low alloyed steel grades Reduced pearlite micro alloyed steel grades Chromium alloyed steel grades Molybdenum alloyed steel grades Nickel alloyed steel grades C steel grades >0.25% C Sheet grades IF steel grades r Fig.11 Slab exiting caster Caster 3 All of the above plus: Special grades (Si steels, IF steels, P alloyed steels, HIC-resistant grades, pipe grades) IF grades with highest purity and meeting the most stringent surface quality requirements r Table 2 Steel grade range cast at Salzgitter Flachstahl GmbH START- UP AND FIRST OPERATING RESULTS After a construction period of only 22 months the caster commenced operation on November 18, 2004, 11 days ahead of schedule. The casting speed is a maximum of 1.35m/min depending on the steel grade. Thanks to the prepared addition of three more segments, the casting speed can be increased to 1.60m/min. With the new caster, the range of grades produced has been expanded as shown in Table 2. By week 3 after completion 460 heats (~ 96,500t) were produced all of which were suitable for customer orders. By mid-march 2005, four months after completion, more than 1,000 ladles covering most of the steel grades have been cast successfully (see Figure 9). Figure 10 shows the production per month. A considerable portion of the cast slabs is dispatched to the hot rolling mill for hot charging. This continuous production can be guaranteed as long as the quality of the slabs is reliable. Figure 11 shows the cast slab exiting the machine. By implementing most advanced plant engineering and technologies the new installation satisfies the severe requirements as far as the extended product range, product quality and annual production are concerned; it is the combination of the various SMS Demag technologies used which enable Salzgitter Flachstahl GmbH to test advanced qualities and further improve them to be ready to go into production. This highly developed range of grades strengthens its position as supplier to premium car makers such as Audi, BMW, Mercedes, VW and PSA. More than 90% of the products are sold in Europe. MS Ulrich Grethe is Plant Manager Steelmaking Plant, Thomas Müller, is with Steelmaking Plant Production Management and Peter Müller is Continuous Caster Production Manager, all at Salzgitter Flachstahl GmbH, Salzgitter, Germany. Christian Geerkens is Executive Vice-President of Steelmaking/Continuous Casting Technology Division, Wolfgang Moßner is General Manager Commissioning and Process Engineering Department and Axel Weyer is General Manager Process and Systems Technology, all at SMS Demag AG, Düsseldorf, Germany. CONTACT: thilo.sagermann@sms-demag.com 136