Recent developments of electromagnetic actuators for continuous casting of long and flat products

Transcription

1 Recent developments of electromagnetic actuators for continuous casting of long and flat products Electromagnetic actuators for continuous casting machines can be divided into two categories: electromagnetic stirrers (EMS) and electromagnetic brakes (EMB). Although they may be described as mature technologies, recent developments of Multi-Mode EMS and Multi-Mode EMB control intelligently the liquid steel in the mould to drastically reduce steelmaking defect occurrence on as-cast and finished products. These developments cover the full range of slab thickness and casting speeds. Author: Siebo Kunstreich Danieli Rotelec Electromagnetic actuators for continuous casting machines can be divided into two categories: electromagnetic stirrers (EMS) and electromagnetic brakes (EMB). The abbreviations are used both for the processes, ie, stirring and braking, as well as for the devices, ie, stirrers and brakes. (The designation, EMBr, refers specifically to the EMB systems commercialised by ABB.) The common purpose of both EMS and EMB is to control the liquid steel flow during casting in order to improve productivity and product quality. The difference is that EMS uses dynamic magnetic fields which are rotating or travelling, ie, two or three phase inductors with AC currents like the stators of AC induction motors, whereas EMB uses static magnetic fields, ie, single phase DC currents like the electromagnet of eddy current brakes. EMS is used for long and flat products; EMB only for flat products. EMS and EMB were developed in the 1970s and 1980s, respectively, and are considered mature technologies. It might therefore be surprising that there are still recent developments worth describing. Moreover, in many meetings with caster suppliers and steelmaking companies, we have noticed a general lack of knowledge or even misunderstanding about the various EMS and EMB applications. The purpose of this paper is to review the different applications and to present recent developments. EMS FOR BILLETS/BLOOMS State-of-the-art For long products, EMS is used in the mould (M-EMS), in the secondary cooling zone or the strand area (S-EMS), and in the final solidification zone (F-EMS) [1]. All three applications use rotating magnetic fields with so-called rotative stirrers. Travelling magnetic fields for S-EMS and F-EMS have been abandoned because of poor efficiency. Historically, EMS started with S-EMS to improve the internal solidification structure in terms of equiaxed zone, centre segregation and centre porosity of billets and blooms. Today, however, approximately 90% of the applications are M-EMS. Its great success is due to the fact that it improves not only surface and subsurface quality of billets and blooms, but also its internal solidification structure (and even better than S-EMS). M-EMS always uses low frequency current below 10Hz. If 50Hz or 60Hz were used (which would be easier for the electrical power supply as there would be no need for a frequency converter) this would have practically no effect on the liquid steel because the major part of the magnetic field would be consumed for induction heating of the mould copper tube. S-EMS and F-EMS use either network frequency or low frequency current, depending on the section size. S-EMS on its own has been replaced almost completely by M-EMS, but is sometimes still used in combination with M-EMS to eliminate remaining centre porosities. The more frequently used combination is M+F- EMS to further reduce centre segregation in high carbon and high alloyed steel grades. New jumbo stirrers The recent tendency to cast ever larger sections led us to build larger stirrers that go beyond the common experience so cannot be designed by simple extrapolation from the smaller sizes. For mould stirrers, one problem is that mould height does not increase for these large diameters. As a result of the limited available space (height), the stirrers become flatter a 57

2 r Fig 1 Magnetic field maps, (top) simulation, (bottom) measurement r Fig 2 The largest jumbo mould stirrer ever built r Fig 3 Top shrinkage cavity of 700mm square bloom, without (left), and with (right) M-EMS [4] (the aspect ratio stirrer-diameter over stirrer-height becomes two or more) which is extremely unfavourable for magnetic performance and requires great care in checking the design criteria. Danieli Rotelec has adapted the commercially available software tools that now show excellent agreement between simulation and measurements after the stirrer has been built (see Figure 1), and electromagnetic simulations followed by CFD simulations have been carried out to revise the design criteria. For fi nal stirrers, we do not have the problem of height limitation but, because of the longer and larger diameter stirrers in use, heat radiation from the bloom becomes another issue so that here also, we need new design criteria to avoid deformation of the stirrer casing. The largest mould stirrer we ever built has an external diameter of 2m yet is only 610mm high (see Figure 2). It is used on a semi-continuous vertical caster for mm square blooms. At the end of the cast, the bloom is stopped below the mould to complete solidifi cation, the stirrer is then moved below the mould and continues to stir the top of the bloom during its ongoing solidifi cation. The equiaxed zone width obtained with this process is almost 100% and the top shrinkage cavity is reduced to 300mm from 800mm, typically (see Figure 3). STRAND EMS FOR SLABS State-of-the-art of S-EMS As with long products, strand EMS for slabs is used to improve the internal solidification structure in terms of increased equiaxed zone and decreased centre porosity and center segregation [2]. Unlike long products, however, slabs require linear inductors with horizontally travelling magnetic fields that generate a thrust pushing the liquid steel horizontally across the slab width, thus generating the so-called butterfly stirring pattern. Such an effect can be produced either with box-type EMS positioned on the inside radius of the machine behind the mechanical guiding rolls, or with in-roll EMS, where two stirring rolls are positioned either side on the inside radius or face-to-face on the inside and outside radii. In-roll EMS has far better efficiency because of the reduced distance between stirrer and slab its electrical power consumption is approximately three times less than that of the box-type EMS. It is particularly recommended, therefore, for doublestage EMS, ie, one pair of rolls in an upper and one in a lower position. Double-stage EMS with box-type design would lead to excessive investment and running cost. Double-stage EMS does, however, generate a triple zero stirring pattern (see Figure 4) that moves the liquid steel over a longer distance in the strand and therefore achieves better metallurgical results, particularly when casting at higher superheat. New in-roll EMS for wide slabs Historically, in-roll EMS was limited to conventional slab casters with slab 58

3 widths up to 1,650mm, because the stirring rolls had no intermediate bearings. For larger widths there would be excessive roll defl ection because of increased bearing distance, but a mechanical arrangement with back-up rolls to limit the defl ection of the stirring rolls is not a satisfactory solution because of maintenance problems for the back-up rolls and short life time for the roll sleeve of the stirring rolls. To take advantage of stirring rolls for wide slabs, a new version has been developed that permits installation of stirring rolls with intermediate bearings. With this new design, in-roll EMS can be used on large and very large slab casters with roll defl ection less than 0.5mm. A 2,400mm wide slab caster has been equipped with this new design and has been operating successfully for more than two years (see Figure 5). MOULD EMS AND EMB FOR SLABS State-of-the-art EMS or EMB in slab moulds is used to decrease or eliminate the defects generated directly or indirectly by the liquid steel fl ow in the mould. The defects are related to mould powder entrapment, gas bubbles, non-metallic inclusions, insuffi cient or inhomogeneous meniscus temperature or insuffi cient meniscus lubrication, and accumulate mainly in the surface and subsurface region of the slabs. They translate into casting slowdowns due to sticker and breakout alarms and poor slab and cold rolled coil quality. These defects are generated by inappropriate steel fl ow in the mould which cannot be controlled by the caster operator, because the fl ow itself is determined for a given cast by the SEN geometry and slab thickness and during the cast by the actual conditions of casting speed, slab width, SEN immersion depth and argon fl ow rate that change all the time [3]. Therefore the purpose of EMS and EMB in the mould is to correct the naturally existing steel fl ow and to force it into an optimised fl ow pattern by using electromagnetic forces. Two technologies are available: ` ABB s EMB, which has evolved since the 1980s from EMBr and EMBr-Ruler into different FC-Mould generations and used on thin and thick slabs. It is not the purpose of this paper to go into details here, as many articles can be found in the literature. ` Danieli Rotelec s Multi-Mode EMS, which is used for medium thick slabs. It provides three functions for slowing down, accelerating and rotating the liquid steel in the mould (electromagnetic level slow down EMLS, level acceleration EMLA and rotative stirring EMRS). Details of this technology and its results are described in reference [3]. So, what is new in this fi eld? r Fig 4 Double-stage EMS r Fig 5 In-roll EMS for wide slabs NEW CONCEPT FOR ALL SLAB SECTIONS INCLUDING EMS AND EMB Controlling slab quality means controlling steel fl ow in the mould. This general idea is largely accepted today, however, the question of which technology should or should not be used EMS or EMB and for which application, still raises opposing views and confusion that might persist due to commercial reasons or to ignorance of physics and metallurgy. The following facts should be well understood: The common belief that liquid steel fl ow can be stopped with EMB is wrong. Unlike the solid disc of the eddy current brake in trucks and buses, the liquid fl ow does not go through the obstacle of the magnetic fi eld that generates the braking force, but goes around it. If the braking force is strong enough, ie, magnetic fi eld must be a 59

4 r Fig 6 Control concept According to this concept, the following guidelines can be expressed: ` Thick slabs ~ mm, casting speeds ~1-3m/min This is the most frequently used range, and we recommend the most fl exible tool, ie, Multi-Mode EMS with all three accelerating, braking and rotating functions: EMLA, EMLS, EMRS. The natural steel fl ow is either double-roll (high casting speed), unstable (high argon fl ow) or singleroll (low casting speed and high argon fl ow). The optimised fl ow will be double-roll fl ow with meniscus velocity in the range m/sec and stable rightstrong and liquid velocity must be high, the EMB can act like a wall that changes the fl ow direction, but neither the fl ow is stopped nor the direction is under control. EMB is a DC electromagnet with static magnetic fi eld that only generates braking forces in opposite direction and proportional to existing steel velocity, whereas EMS is an AC inductor with travelling magnetic fi eld that generates braking or accelerating forces parallel to the fi eld velocity and proportional to the differential velocity of steel and fi eld, It follows that Multi-Mode EMS can generate new fl ow in the selected direction, hence transform arbitrary, unstable or bias fl ow into stable and right-left symmetrical fl ow whereas EMB cannot. Danieli Rotelec s collaboration with NKK, which started in the 1990s, has confi rmed that medium thick slabs need the meniscus fl ow to be optimised into a rightleft symmetrical double-roll fl ow, which means one has to accelerate or to slow down (brake) the fl ow exiting from the SEN with electromagnetic forces that push steel symmetrically outwards (from SEN to narrow faces) or inwards (from narrow faces to SEN). This can only be achieved by travelling magnetic fi elds of EMS with the functions EMLA (acceleration) respectively EMLS (slow down). The conventional EMB, whatever the generation is, can only brake; it cannot impose right-left symmetrical forces and cannot accelerate. Based on this experience, we have applied the most recent knowledge on steel defect analysis, fl uid fl ow mechanics and magneto-hydrodynamics calculations to develop impressive technologies called Multi-Mode EMS and Multi-Mode EMB. These technologies control intelligently the liquid steel in the mould to drastically reduce steelmaking defect occurrence on as-cast and fi nished products. Moreover, we propose a concept that evidences which type of fl ow control must be applied according to slab thickness and casting speed (see Figure 6). This concept covers the full range of slab thickness and casting speeds and is as follows: ` Only braking for very high casting speeds (typically above 5m/min) and thin slab casters ` Braking/accelerating/rotating for high to medium to low casting speeds (ranging from ~1-3 m/min) and medium to thick slabs (in the range mm) ` Only rotating for very low casting speeds (<1m/min) and very thick slabs (>250mm) This concept is based on the following considerations: Steel fl ow in the mould is, of course, a three-dimensional phenomenon. However, fl ow control operating in all three dimensions would be extremely diffi cult, space consuming and expensive. Therefore, we reduce the three dimensional reality into a two dimension view, which means we focus either on a plane parallel to the broad slab face or on a plane parallel to the meniscus, depending in which plane the predominant fl ow is found. Slab thickness and casting speed are the two parameters that favour one or the other plane: ` For thin slabs, the predominant fl ow is in the plane parallel to the broad slab faces and for very high casting speeds the fl ow in the mould is always too fast. Hence, we need only braking forces acting in that plane. ` The thicker the slab and the slower the casting speed (which are related), the more fl ow goes out of this plane into the direction of slab thickness. Hence, for very thick slabs and very low casting speeds, the plane parallel to the meniscus becomes relevant, hence the fl ow control uses electromagnetic forces in that plane which means it generates rotative fl ow. ` Between these two extremes, we have the most frequently used medium to thick slabs and medium to high casting speeds. This is the domain of Multi-Mode EMS that uses the accelerating/braking/rotating functions EMLA/EMLS/EMRS. These functions are used alone or in combination, with the correct mode being automatically selected relative to steel grade, section size and casting speed. 60

5 left symmetry. This is achieved with the braking (EMLS) function slowing down the too strong double-roll fl ow, or with the accelerating function (EMLA) accelerating the too weak double-roll fl ow or transforming singleroll and unstable fl ow into double-roll fl ow, the stirrers being located at mid height of the mould. Certain steel grades develop gas bubbles that are trapped in the solidifi cation front close to the meniscus. For such cases, the rotative stirring (EMRS) is the preferred application. To obtain an effi cient rotative fl ow at the meniscus, the required stirring power is higher than for EMLS and EMLA, the slab must not be too thin, the casting speed not too high, and the stirrers must be close to the meniscus. In these conditions, an additional design called up and down mechanism, is provided that shifts the stirrers automatically up into a position close to the meniscus for EMRS operation and down to mid-mould height for EMLS/EMLA operation. ` Medium thick slabs ~ mm These sections are too fl at and do not allow generation of an effi cient rotative fl ow at the meniscus. Therefore, the EMRS function and the up and down mechanism can be ignored, which reduces the investment as well as the operating cost (because the EMRS function requires the highest electrical power). The fl ow control purpose here is only to obtain the optimised double-roll fl ow with meniscus velocity in the range m/sec and stable right-left symmetry under all casting conditions. ` Very thick slabs >300mm The thicker the slab, the lower the casting speed, and the natural fl ow in the mould does not require any braking. Here the purpose of fl ow control is washing the solidifi cation front to eliminate gas bubbles and particle inclusions in the surface and sub-surface region and to equalise the meniscus temperature. This is achieved with the EMRS function of Multi-Mode EMS located in a high position close to the meniscus. We have developed two additional stirring modes that can be generated due to the fact that we have four independent stirrers around the mould (see Figure 7). The fi rst mode, Optimised EMRS, modulates the stirring force on the same broad mould face between right and left side to achieve more regular fl ow velocity around the meniscus and avoid turbulences in the corners near the narrow faces. The second mode, Double EMRS, produces two rotating fl ow loops where the meniscus rotates clockwise on one side and counterclockwise on the other side of the SEN. Both, optimised and double EMRS are currently used on a very thick 400 mm slab caster. r Fig 7 Multi-Mode EMS ` Thin slabs <150mm The natural fl ow is either doubleroll or unstable; single-roll fl ow does not exist because no argon is used on thin slab casters. These casters aim at high productivity, hence use high casting speed. The double-roll fl ow in most cases is too strong, hence acceleration is not required. Furthermore, rotational fl ow cannot be generated with such fl at sections. Therefore, for thin slab casters, EMS can be replaced by EMB, provided, however, that one can develop an advanced braking system that provides more sophisticated functions for the steel fl ow control inside the mould than the global braking of conventional EMBr or EMBr Ruler. Danieli Rotelec, in cooperation with Danieli R&D and Danieli Davy Distington, has developed a new advanced EMB with exactly that purpose. It is described below. NEW MULTI-MODE EMB FOR THIN SLAB CASTERS In thin slab casters, the problems generated by high casting speed/high throughput are as follows (see Figure 8): A The fi rst problem is excessive meniscus fl ow velocity with too rapid double-roll fl ow, which is like that in thick slab casters, generates standing waves and turbulences close to the narrow mould faces. The consequence is a 61

6 r Fig 8 Four flow pattern phenomena in the mould of thin slab casters that generate slab defects r Fig 9 New EMB multi-pole configurations developed to address the specific problems of Figure 8 a thin powder layer, hence bad lubrication close to the narrow faces, uneven shell thickness and powder entrapment. These problems exist if the meniscus steel velocity exceeds 0.3m/sec. B The second problem, connected to the first, and also generated by too rapid double-roll flow is too strong downward flow that can generate too strong central return flow, which, in turn, creates right-left instabilities. C The third and worst problem is bias flow and asymmetrical flow with respect to the SEN axis, right-left instabilities, meniscus waves and turbulences. Unstable flow is the worst case for surface/subsurface quality. D The fourth problem, again the consequence of a too fast double-roll fl ow, is excessive vertical meniscus fl uctuation/meniscus waviness. It generates local perturbation of mould powder lubrication, uneven shell formation, and longitudinal cracks. These four problems are not independent of each other, which is a typical problem in fl uid mechanics and which makes it particularly diffi cult to fi nd a good solution to control them. A wide variety of magnetic confi gurations with AC or DC fi elds have been investigated with coupled electromagnetic and CFD simulations using free meniscus surface and reproducing the geometry of the SEN and the funnel mould with the purpose of identifying the position, area and strength of the magnetic fi elds that would be useful [4]. The chosen solution is a multiple pole geometry addresses the specifi c fl ow problems shown in Figure 8 with the following distinctive functions: (i) braking for excessive horizontal meniscus fl ow and too strong downward fl ow, (ii) stabilising for the bias fl ow instabilities (and for a too strong central return fl ow) and (iii) damping for the vertical meniscus fl uctuations (see Figure 9). These different functions correspond to different magnetic confi gurations that can be activated in different operating modes, ie, by combining three different power supplies with appropriate current settings. Because of this design for different operating modes and in analogy to our technology for thick slab casters, we have named this new EMB: Multi-Mode Electro-Magnetic Brake (MM-EMB). We have validated these design fl uctuations and with a laser scanner on one side of the SEN [4]. With this instrumentation, we can measure the meniscus shape and the liquid metal fl ow on the left and right side of the SEN and compare the fl ow under condition with EMB switched on experimentally on a 1:1 scale demonstration plant using casting speeds from 4.2 to 11.7m/min, mould sections of thickness 102.5mm, width (1,800, 1,530, 1,250, 990) mm and different SEN geometries. The mould is equipped with four sensors, two on each side of the SEN, to measure horizontal meniscus velocity and vertical meniscus or 62

7 off. Testing parameters are mould width, throughput (ie, casting speed) and electrical current settings of the electromagnets. Figure 10 shows the top the meniscus shape of the natural fl ow, ie, MM-EMB switched off. The example is for a four-port SEN, mould size 1,800 x 100mm and casting speed 6.9m/min (throughput 1,100l/min). The lower fi gure shows the meniscus shape under same conditions with EMB switched on. Figure 11 shows in the upper fi gure the horizontal meniscus fl ow velocity on the left and right side of the SEN versus time: With EMB switched OFF, meniscus velocity is biased at 0.5 and 0.3m/sec at the left and right side of SEN, respectively. With EMB switched ON, the bias fl ow is stabilised and the meniscus velocity is braked to 0.19 m/sec. The lower fi gure shows the vertical meniscus fl uctuations in terms of standard deviation of the fl uctuations, versus time. With brake switched OFF, the standard deviation of the vertical fl uctuations is 1.8mm, with brake switched ON, the damping function has reduced the fl uctuations from sigma 1.8mm to 0.35mm. This example illustrates the outstanding ability of the new EMB to simultaneously stabilise the right-left asymmetry (equalise the bias fl ow), brake the horizontal meniscus fl ow velocity and damp the vertical meniscus fl uctuations. Figure 11 refers to a magnetic field configuration (operating mode) that enhances the damping function against the braking function. Different magnetic field configurations (operating modes) can be chosen for the inverse purpose to enhance the braking function against the damping function. MM-EMB is a flexible tool because the braking and damping functions can be modulated according to casting conditions or steel grades/type of defects. For example, the braking function can be increased versus the damping function, if the main problem is excessive meniscus velocity, eg, powder based inclusions, whereas the braking function can be decreased vs the damping function, if the main problem is excessive meniscus fluctuation and/or cold meniscus, eg, bad lubrication, insufficient powder melting, stickers, alumina based inclusions. This new EMB is commercialised under the name of Multi-Mode EMB and we believe it will be a breakthrough in terms of fl ow control on thin slab casters. MS Siebo Kunstreich is President of Danieli Rotelec, Bagnolet, Paris, France. CONTACT: s.kunstreich@danieli-rotelec.fr REFERENCES [1] Optimising the application of electromagnetic stirring, in Millennium Steel 1999, p173; Electromagnetic stirring, nozzle erosion, slag entrapment and internal quality of r Fig 10 Meniscus flow measured without and with EMB switched on. Section 1,800 x 100mm, casting speed 6.9m/min (1,100l/min) r Fig 11 Horizontal meniscus flow velocity (top) and vertical meniscus fluctuations (bottom) measured without and with EMB in an operating mode set to enhance the damping function billet, in Millennium Steel 2001, p223. [2] Strand electromagnetic stirring (S-EMS) for thick slab casters: Box-type or in-roll stirrers?, in Millennium Steel 2008, p122. [3] Steel fl ow, slab quality and the need for dynamic electromagnetic control in the mould, in Millennium Steel 2003, p126). [4] Kunstreich Siebo, Gautreau Thierry, Ren Jean- Yves, Codutti Andrea, Guastini Fabio, Petronio Marco, Development and Validation of Multi-Mode EMB, a New Electromagnetic Brake for Thin Slab Casters, in Proceedings of the 8th European Continuous Casting Conference, Graz June