65.INDUSTRIAL SKI-END MASTERING AT ARCELOR DUNKIRK

INDUSTRIAL SKI-END MASTERING AT ARCELOR DUNKIRKROUGHING MILLS. Dauphin, Ph. Cordier, P. Boutoille, C.Colin, J. Coupu 1, B. Petit 21. INTRODUCTION AND INDUSTRIAL CONTEXTDuring the rolling of the steel strips, the product may have a variety of geometrical defects including the ski-end defect. This phenomenon, widely known by all the continuous roughing mills, generates productivity and quality losses that are considerable but also degrades the smooth operation of the tool by generating very high solicitation (impacts) and the premature degradation of the equipment.The phenomenon of the ski-end is described by the straightening of the product head part that can reach a length of 800 mm to more than 1m over a sheet metal length for some rolling conditions. The associated curvature can achieve levels of around 7.10-4 to 9.10-4 mm-1 (fig.1). Essentially, this results in the differential elongation of the fibers between the upper and lower faces of the product.Figure 1:Photo of R4 raised headWidth and altimetry measurementSki-end detectionThe economic consequences at TCC Dunkirk have increased over the years until reaching, in the years of crisis, productivity losses of up to 360k€/year and a quality cost that was almost as high of approximately 340k€/year. The transitory nature of the phenomena has worsened these losses through additional costs that are difficult to quantify and that refer to the premature degradation of the equipment (impact shocks and over-solicitation of the kinematic drive chain).The production objectives for the future years has led us to identify this fault as being a major working objective for our hot rolling mill.Initially, we endeavored to define actions to protect the installation and reduce the costs by adjusting the process so as to be able to roll most of the products assigned to ski defects.In parallel, we began an approach to understand the physical mechanisms causing the defects. During this second phase, thorough bibliographical investigation enabled us to draw up a tree of the influencing factor causes that we completed by finite element modeling and achieve the hierarchy of the causes.Finally, each of the parameters were standardized and put through statistic process control (SPC) industrial monitoring.1Arcelor-Atlantique, Dunkerque, France 2Arcelor-ARSA, Maizières-les-Metz, France2. LIMITING RISKS AND INDUSTRIAL COSTSThe high amount of the industrial costs mentioned in the introduction defines a real priority in terms of the challenges, meaning the almost immediate establishing of conservative measures to: -Decrease the installation operational stoppage times.-Drastically reduce the number of by-products rejected during rolling.-Protect the equipment exposed to risks of damage (such as the tearing away of the roll cooling pipes at stand R4 exit, damaging of the entry equipment of stand R5, descaling holders, edger guide…)The provisions investigated as soon as the project was launched in 2003 then set up gradually by focusing as a priority on roughing stand R4, made it possible to cut the industrial costs by between 30 and 40%. They are described chronologically in the following:1. Adaptation of the in-bite conditions in the downstream roughing stand consisting of an upstream edger and a horizontal stand R5, operating as soon as a defect is detected on leaving R4.This adaptation resulted in the implementing of an automatic over-stroke on edger R5 (400mm) and a low speed guided introduction into the downstream stand for small amplitude defects.2. In the case of high amplitudes, design and implementation of a protocol for the "flattening" of the product head (mechanical reduction of the defect by weight handle on crane), to allow the product to be engaged according to precise rules described to the operator in the booth and according to the end forms of the bar.3. Improved ski-defect detection reliability by appropriate actions. The automation of the adjustments mentioned in point 1, requiring the rugged reliability improvement of the device, prompted the industrial plant to define a shock detector suspended from a gantry as displayed in figure 1.This device was adjusted specifically with respect to the geometry of the downstream stand (upper position of edger guide). In this way, any over- or under-detections of defects initiated by the initial roto-sensor were eliminated entirely.4. Finally, as an ultimate countermeasure, following the benchmarking within some other hot rolling mills, we adapted a "head folding" device on the exit of stand R4 by fitting our industrial configurations and carrying out risk calculations (optimal angle of shield, height with respect to roller table, distance from R4 exit…) The chosen device is of the “Mechanical head folder” type and is schematized in figure 2. This “folder” has been equipped with accelerometers to analyze the strength of the impacts when they occur, as well as their sequencing.Figure 2:Principe of the “head folding” device3. FINITE ELEMENT MODELLINGA lot of studies have been done on ski-ends defects (or strip curvature, or strip warping defects), influence of the shape factor defined by the ratio [contact length in bite / average thickness], influence of the initial thickness of the bar, effect of product grade (particularly strain rate or deformation rate sensitivity of the metal deformation resistance), effect of the mean friction level between bar and work-rolls [1] [2] [3].Studies of asymmetric rolling have been conducted through the use of experimental, analytical and numerical techniques. Considering the very fast progress of Finite Element modeling to deal with that field, most approaches use now this type of powerful simulation which is able to give very precise results and tendencies very close to the reality.The main typical sources of ski-end defects can be reminded: pass-line or rolling level or bite entry angle unsuited, rolls speed or rolls diameter mismatch, product thermal gradient, asymmetrical friction between upper and lower roll/strip interfaces, asymmetrical rolls roughness, asymmetrical clearances in the kinematic chain; in fact, these primary factors can be really difficult to rank for a given industrial plant, because of lack of sufficient information and/or on-line measurements. As a consequence, Arcelor Research and Arcelor Dunkirk have highlighted the usefulness of direct driving spindle torques, spindles rotational speeds and model-based interpretation.In order to analyze complementarities between ski-end defect and differential torque, 2D FEM [4] has been used with the following process configuration and typical industrial rolling schedule for the lastAn example of FEM result is given in previous figure 3, with a global bar length of 10 m for the simulation (2-dimensional approach), with gravity influence neglected, and rear end of the bar with imposed zero vertical speed during the transfer in the roll bite. The mechanical behaviour of the product metal corresponds to an elasto-visco-plastic law for a typical steel product (0.05%C – 0.3%Mn), and the friction law is a Coulomb type, with low slip speed regularization (standard friction coefficient m = 0.2). For each simulation from R3 to R4, the temperature of the bar is simulated with a parabolic approximation (maximum value for R3, R4 and R5 respectively: 1520 K, 1503 K and 1415 K, and thermal gradient added for asymmetry study.The first result extracted from the Finite Element Modeling is the sensitivity of different asymmetry sources on ski-end defect amplitude and relative differential rolling torque, as shown figure 5. The amplitude of the ski-end defect, expressed in curvature (mm-1), is calculated with a post-treatment which searches automatically the optimum circle radius fitting the strip deformation considering about 1200 mm of curvilinear abscissa of rolled product; a positive curvature value means upwards ski-enddefect. The relative differential rolling torque is calculated by the ration [lower torque – upper torque]/[ lower torque + upper torque] expressed in %, on the basis of the mean values obtained for the last thousand numerical increments.Asymmetric friction of 10% means that the upper strip/work-roll friction coefficient is +10% and the lower one -10% comparing to the basic symmetrical case; differential temperature of 10° means that a thermal gradient of 10° between lower and upper strip surfaces is simulated (upper surface colder than lower one); differential speed of 1% means that the lower work-roll speed is +1% and the upper work-roll speed -1% comparing to the basic symmetrical case (the lower WR is the faster).The interesting following conclusions can be drawn :- The curvature and differential rolling torque sensitivities are quite linear for friction, temperature and speed parameters, apart from differential rolling torque sensitivity withspeed for [(speed upper – speed lower)/speed mean] more than 6%, with an asymptotic curvedtendency.-The curvature and differential rolling torque sensitivities are quite different between frictionand speed parameters on the one hand, and temperature parameter on the other hand;considering that product thermal gradient has negligible impact on differential rolling torque,this result will be put to very good use for the industrial plant in order to rank the real ski-enddefect sources and countermeasures as precised afterwards.Thanks to a global FEM simulation plan carried out for R3, R4 and R5 roughing stands, the following table summarizes precise tendencies and coupling effects with the shape factor (tested with constant entry thickness), depending on the roughing stand industrial working point.Table 1: coupling effect between ski-end defect and shape factor for industrial working pointsThese theoretical results show clearly :-The critical shape factor Ld/hm crit, which corresponds to an inversion of the ski-end defectbecause of speed mismatch, is not constant but sensibly increasing with the entry thicknessof the bar.- There is also an other particular shape factor level Ld/hm max lower than Ld/hm crit, whichcorresponds to a local maximum of the ski-end defect amplitude, and depends not only onthe entry thickness of the bar, but also on the asymmetrical source parameter considered.-The friction sensitivity for ski-end defect is for R4 stand higher than R5 stand, which is muchhigher than R3 stand; this point could explain the reason why the ski-end crisis happenmainly for R4 stand at Arcelor Dunkerque.All these considerations make it possible for process people to improve significantly the conservative countermeasures (like pass-line monitoring, anti-ski mechanical device…) and process operational control (rolls management, work-rolls temperature/oxidation conditioning, product thermal gradient control…) as explained in the next paragraphs.4. OPERATIONAL SKI DEFECT TESTING AT ARCELOR DKFollowing this end defect modelling study and the many existing bibliography references that exist, the physical variables having an influence on the industrial force encountered at Dunkirk were put into a list in order to draw up the following exhaustive causes tree.Figure 5: Influencing causes and parameters tree.4.1. The pass line:It is a known fact that the entry geometry of the product into the horizontal and/or vertical stand can have a great influence on ski-end defects because of its impact on the angle of attack and on the curve radius of the product on engagement. Systematic readjustment work of all the upstream/downstream pass lines of each roughing out stand was therefore performed to comply with: - Rectilinear transit in the rolling direction of the steel bar just before stand gap: by defining the relative position in height as a minimum between each piece of equipment from the entry to the exit ofthe stand (upstream roller table → edger side guides / central guide → horizontal tangent of the bottom work roll → output table → downstream roller table).- A minimum sheet curve obtained by the alignment of the incoming product horizontal axis with that of the stand gap within the right of way to a ½ reduction and by allowing for an estimation of the lower stack stretching of the horizontal stands. This approach resulted in revising all the standards rules under the low work support cylinders in the suspended sections.These adjustments are essential for the roughing system at Arcelor Dunkirk because they establish the configuration of the campaign for 21 days without any other means of monitoring by intermediate controls during rolling.4.2. Product temperature differential:The product temperature gradient during rolling is not known before exit from the roughing mill at Dunkirk, because of the particularity of the instrumentation technology to be implemented for measurements on the lower face of the bar in a highly unfavourable environment (dampness, fouling by dust, scale debris, high temperature…).Furthermore, the temperature measurement is still acting surface and does not give directly the thermal gradient in the thickness of the product as is desired.These reasons indicate why it is necessary to use the available thermal model to regulate the heating furnaces, in particular since they are well instrumented in the differential thermal properties for the upper and lower heating zones of the product, by means of thermocouples within the vaults and sections of the furnaces.This orientation has allowed a real operational change in the product heating regulation mode, because the control of the temperature deviation has caused of the industrial site to change over from a gas flow rate control to a heating control by temperature deviation within each sub-zone of the furnaces (preheating, heating and equalization zones).This new and original regulation mode has induced the following consequences:-Its development and implementation are crucial and there is a definite risk to industrialization!-The changeover from one mode to the other was accomplished after a statistical studylasting several years which made it possible to correlate gas flow rates and temperature.This made it completely transparent with respect to the operating point of the rolling mill.The trickiest part consisted in modifying the temperature deviations in the lower zones of thetwo furnaces, operating almost "blind" to obtain the best result on the ski-end defect at R4level, while bearing in mind that the thermal origin is one of the major components causingthe defect to occur. As a consequence, this modification made it absolutely necessary tocarry out a relevant and optimised experiment plan.-The adaptation and/or modification of the thermal settings in the furnaces still occurs withsome delay and inertia because of the volume of the enclosure and the product heatingtimes; this point must be carefully dealt with in order not to increase industrial risk.-The implementation of differential furnace control by temperature deviation also made itpossible to harmonize the results with the products between various furnaces and betweenvarious teams.The impact of controlling the thermal gradient using this new control has reduced the ski defect costs by approximately 20%.One of the results obtained is illustrated in the following figure 6 which shows an example of a temperature deviation monitoring in the equalization zone of furnace No. 2, in the form of an SPC (Statistic Process Control) card made available to the operator:C8060402020032004-2005Gaz flow rate controlControl by temperature deviation Adaptation of the T° differentialFigure 6:result of the product cheating regulation mode4.3. Interface differential frictionDifference of friction and rolling torque:To integrate the theoretical results concerning the correlated effects between sheet curvature and differential rolling torque, Arcelor Dunkirk has implemented prototype measurements for driving spindle torques and rotational speeds, for upper and lower parts of the last roughing stand R5. An illustration of these on-line measurements is given figure 8, for a bar width of 1640 mm.The spindle torque measurement is supported by telemetry signal coming from a strain gage (2 couples of elementary bi-directional gages 90° oriented) placed between the WR spindle bearing support and the work-roll neck coupling; its absolute precision is estimated less than 5%. The spindle rotational speed is obtained by a eddy-current sensor which detects the passage of 96 conductive targets regularly spaced around the spindle circumference; its digital precision is estimated at 1 rpm at best.From these experimental results, considering that differential rolling torque is mainly impacted by friction asymmetry in comparison with product thermal gradient (very low effect on the differential torque) and differential speed (low mean value on the ski-end defect time window, non-twin roughing stand with synchronous drive), the importance of symmetry for upper and lower interfaces between the strip and the work-rolls is clearly highlighted for Dunkirk Roughing Mill, and must be further considered with process improvements.-0,25-0,2-0,15-0,1-0,0500,050,10,150,20,250,3T i m e (m s e c )0200400600800100012001400T i m e (m s e c )Figure 7: typical experimental results for R5 roughing standArcelor Dunkirk has also set up torque measurements on the working spindles of stand R4. At the exit of this stand, two signals are used for detecting any defects: the impact detector mentioned in paragraph 2.1 supplies binary information and a measurement of the altimetry (available per side of the bar) supplied by the stereoscopic gauge EES which is used for quantifying and qualifying the head-end geometry (figure 8.a).These two items of information are used for following the bar-to-bar evolution during a campaign (figure 8.b). Concerning this campaign the behaviour of the bar head defect clearly changes. During a period of approximately 60 products (case 2), the elevation of the ski-end defect is far greater, in which case the values are around 100 mm.geometry measurement during a rolling campaignFigure 8: measured head defect of the barAnalysis reveals that for this period, the average couple differential is highest. The previous figure 9 illustrates the signals observed for two bars, one at the beginning of the campaign with few defects and at the other during the period including major defects. For the defected bar, the couple difference is over 20%. This difference is significant at the head of the bar but also throughout the rolling of the product. However, this behaviour is not systematic, because dependant on the product grade, the working-point for the critical stand, and the different competitive sources of defect. Long term observation reveals that a differential torque, even significant, does not necessarily cause a major extremity defect. This analysis must be continued more accurately in particular using descriptors that take into consideration the dynamics of the elevation defect, the torques difference and the other time information highlighting differential behaviour (temperature, speed…).Differential friction induced by a differential roughness/wear of the work rolls:Throughout the previous developments, we have observed that the rolling process is very rarely symmetrical between the two faces of the product: from the thermal point of view and rolling torque point of view for instance. Another factor of dissymmetry is caused by the differential of the rolls characteristics. Although the roughness and the surface condition of the rolls during assembly at thebeginning of the campaign are designed to be symmetrical, there are always small hardness deviations that can result in non-symmetric rolling taking place very quickly during the campaign.We have observed radically different behaviour in the pairs of rolls depending on whether the hardness (define by [hardness of upper cylinder – hardness of lower cylinder]) is positive or negative on assembly. The figure 10 gives a clear illustration that his hardness variation is discriminating starting from a value ≥ +5 shores.Fig.10: Definition of upper/lower hardness differential standard.We formulate the hypothesis that the differential hardness would cause differential wearing of the rolls during the campaign. Accordingly we would be confronted by a roll surface topography evolution resulting in differential slipping and friction within the roll bite.This experimental result is of particular importance and we have included this new rule for working roll matches to the specification for R4.In the same line of thinking, when at the metallurgical differentials are reached regarding our back-up rolls (for instance regarding the chromium content), we have observed a differential change in the working bite area, in particular at the end of the campaign.These 2 points were the subject of the revision and adjustment of the new rolling standards in effect today.4.4 Tangential roll speed difference:The driving of the working rolls is accomplished by a single motor (non-twin drive roughing stand) through a kinematic line comprising fully mechanical elements (reduction units, gear box, spindles, couplings). This means that it is impossible to adjust directly the angular velocity differential of the two work rolls by the motor actuation.On the other hand, a roll diameter difference can generate a linear velocity difference between the top and bottom WR. Then this difference is limited to less +/-1 mm. In this case the steady-state velocity difference is less than 0.3% and the effect on the product curvature is then negligible (fig.4). This choice is made to minimise the risk of the premature wear of the rolls and to avoid mechanical over-solicitation on some elements in the kinematic chain.5. INDUSTRIAL RESULTS:All the measures described in this document make it possible to accumulate substantial industrial gains when ski defects appear. The actions relative to the definition of a suitable process of reducing the defect number and their consequences, has been estimated at approximately 30/40% in reduction of the induced costs.The actions attached to the mastering of the physical characteristics involved in the defect appearance mechanism, including in particular control of the thermal aspects of the product and adaptation of the roll characteristics, have resulted in an further improvement of approximately 20%.The total cost related to ski-end defects at roughing mill R4 has thus been reduced by €400 K (60%) between 2003 and the end of 2005.Fig.11: productivity cost evolutionCONCLUSIONS AND PERSPECTIVESInitially, Arcelor Dunkirk HSM established industrial actions that reduced significantly the head-end defects observed on the roughing mill. The standards on the pass-line and the new matching rules for the rolls were applied to render the process symmetrical. New mechanical tools also limited successfully the amplitude of the defect at the exit of the most critical stand R4.The further theoretical study based on calculations using FEM clarified the influencing parameters in the configuration of Dunkirk HSM. Accordingly, control of product heating made it possible to manage the thermal gradient in the thickness of the product, keeping it to a higher temperature on the upper face.The theoretical analysis developed here indicates in particular a differential friction effect at the strip/roll interfaces inducing a considerable differential behaviour in rolling torque as well as an emphatic ski-end defect. The first experimental investigations confirmed the fact during a period with major defects when it is found that the differential torque is also high: the evolution of the differential torque is then directly linked to the friction between the two strip/roll interfaces. In the case of Dunkirk HSM, this differential friction behaviour and the product thermal gradient are clearly preponderant parameters regarding the generation of the head-end defect observed, bearing in mind that this defect changes during rolling as a function of the production rate and of progress in the campaign.The next investigations will be carried out to control the evolution of the strip/roll interfaces, in particular by the management and optimization of the historical conditions under which the work rolls are cooled. The efficiency of this new operating device will be explored in 2006 following substantial investments decided by Arcelor Dunkirk, consisting in dedicated instrumentation of each WR cooling system for its separate control.REFERENCES[1] Analysis of front end bending in plate rolling by finite element method. B.H.Park, S.M.Hwang, Transaction of ASME, August 1997, vol 119, pp.314-323[2] A steady state thermo-elastoplastic finite element model of rolling with coupled thermo-elastic roll deformation. A.Hacquin, Journal of.Materials Processing Technology 60 (1996) 109-116[3] Influence of roll speed mismatch on strip curvature. C.W. Knight, S.J. Hardy, A.W. Lees, K.J. Brown, Journal of.Materials Processing Technology 168 (2005) 184-188[4] Validation of finite-element models for asymmetric rolling. M.Pietzyk, K.Wilk, I.Pillinger, P.Hartley. Modelling of metal rolling processes, December 1996, London, pp.432-441.。