International Journal of Emerging Technology and Advanced Engineering - PDF document

International Journal of Emerging Technology and Advanced Engineering Website: www.ije tae.com ( ISSN 2250 - 2459 , Volume 2, Issue 1, Janua ry 2012) 45 Minimization o f Dross Formation During t he Continuous Galvanizing Process in t he Steel Industry 1 Pranay Sawaitul , 2 S. A. Chow riwar, 3 I.P. Lade 1,2,3 Department of Mechanical Engineering, Visvesvaraya National Institute of Technology , Nagpur 40010, India 1 pr anaay.vnit@gmail.com 2 sandeepchowriwar@gmail .com 3 kavya98.ipl@gmail.com Abstract — During the study of continuous galvanizing line, large amount of waste material is observed near the surface of bath . Dross is formed due to reaction between aluminum, iron and zinc in the zinc bath. The coupled phenomena of aluminum, strip temperature and iron dissolution were simulated in order to predict and to better understand the generation and movement of inter metallic dross particles within certain regions of a typical galvanizing bath. Solutions for the temperature and aluminum concentration can be correlated with the solubility limits of aluminum (Al) and iron (Fe) to determine the amount of precipitated aluminum in the form of Fe 2 Al 5 Zn x dross. MATLAB software is used to develop a simulation program to find the optimum percentage of the parameters in order to reduce the total dross. The developed program is flexible to calculate the optimum parameters for per coil and per shift data. The information worked out i n this paper is of major significance in the prediction of the formation of dross particles, which can cause defects on the coated product. Mathematical model is developed and parameters are optimized with the help of simulation technique. Also the dross f ormation is reduced and the process parameters are improved. Keywords — D ross minimization, galvanizing line, Al content, Zinc bath, MATLAB, steel sheets. I. I NTRODUCTION During the hot dip galvanizing, the steel strip enters into the bath at a temperature o f 465°C. At this temperature, the mobility of iron atoms within the strip enables them to escape and enter into the zinc bath. In turn, this can lead to the formation of inter metallic’s within the bath known as dross particles, which can have detrimental effects upon the finished products and bath hardware. [1] Dross can be defined as ―scum on molten metal‖ with the implication that it has no value and it an undesirable material . [ 2] Dross can be of two types, mainly top dross and bottom dross. Top dross i s composed of suspended masses of long intermetallics spikes that are usually interwoven together in clumps. This top dross is formed at the surface of molten zinc due to reaction between iron and aluminium, it forms oxides at the surface due to its greate r affinity towards iron and as it the lighter metal. This top dross is removed at regular intervals, often known as skimming. Bottom dross that is floating in the galvanizing bath, due to temperatu re inversions,adhere to surface of parts being galvanized [2]. This dross becomes encapsulated in the free zinc layers during withdrawal, causing excess coating thickness and surface irregularity. The formation of floating dross results iron is rejected from the solution. Iron has a limited solubility in molten z inc. At the temperature of 465°C, iron solubility is about 0.05 %. Above this value, iron reacts with molten zinc a nd falls to the bottom as dross [3]. In continuous galvanizing, aluminium in the bath plays an important role in product quality control and t he economics of operation. However, maintaining the bath aluminium content at an optimum level for a specified product is a challenging task, mainly due to enrichm ent of aluminium in the coating [5]. Adequate control of aluminium (Al) content in the galvaniz ing bath is critical to the coating quality, especially for galvanneal (GA) coatings. When the bath Al is too high, coating becomes more difficult to anneal resulting in silver edges, powdering, flakes and other defects. GA coatings are also more susceptib le to streaking defects at high bath Al contents. When the bath Al is too low, an excessive amount of bottom dross will form during GA production, leading to a prolonged upward product transition with a bath loaded with numerous floating dross particles, l ow bath Al also causes more severe dross build up on sink rolls and more intensive corrosion to submerged hardware. [6] In general, GA production is rather unforgiving to bath Al fluctuations. The situation becomes even more complicated if sheet steels con sists of different grades, with some containing significant amount o f P and other alloying elements. [6]. International Journal of Emerging Technology and Advanced Engineering Website: www.ije tae.com ( ISSN 2250 - 2459 , Volume 2, Issue 1, Janua ry 2012) 46 To achieve excellent product quality, the bath Al content should change to accommodate the changes in steel grades. This can be an impossible task as bath assays are taken once every few hours to analyze the assays. Product transition from galvanizing to GA by converting the galvanizing bath can be very challenging. Predicting the exact duration of this long process based on experience may be inaccurate as the duration is affected by many operational conditions such as coating weight, line speed, strip width and temperature. If the bath Al decreases faster than anticipated, some GI products will be produced at low bath Al content, resulting in formabilit y problems and coating defects due to incomplete inhibition at the coating/substrate interface. If the bath Al decreases slower than expected, the production schedule may need to be altered because the unforgiving GA products cannot be produced when the ba th Al is too high. [8] Fig 1 . Schema of Galvanizing Bath Operation [ 3] To minimize the dross, aluminium is added into the zinc bath to lower the iron dissolution. The iron solubility with molten zinc is reduced by aluminum addition and temperature variat ion of strip entering into the bath. The dross formation is highly dependent on aluminium % wt of bath, iron dissolution and strip entry temperature. The dross minimization is done using these three parameters using the theory of heterogeneous nucleation a nd the objective function is formed using this theory. And the minimization of dross is done by optimizing these parameters using simulation. II. S IMULATION O F G ALVANIZING P ROCESS P ARAMETERS The overall expression for iron dissolution is [ 1] q= D (C 1 - C 1 ) / (IL T) 2 (SW) (DiB) (CL) / (LS) Where, D=diffusion coefficient of iron atoms through inhibition layer. D=D o exp ( - Q / (R*T )) C 1 = concentration of iron within substrate. C 2 =concentration of iron in bath. ILT=inhibition layer thickness, m DiB=distance in bath, m CL=coil length, m LS=line speed, m/s The chemical formula for defining dross is given by Dross = fe 2 Al 5 Zn x Dross = Fe + Al + Zn Where, Fe = ((Fe % wt * net consumption)/100) + iron dissolution (q ) Al= ((Al % wt * net consumption)/100) As 95% of zinc i s present in dross, Zn = (0.95 * dross) Therefore, Dross = Fe + Al + (0.95 * dross) So, the objective function is, Dross = (Fe + Al) / 0.05 International Journal of Emerging Technology and Advanced Engineering Website: www.ije tae.com ( ISSN 2250 - 2459 , Volume 2, Issue 1, Janua ry 2012) 47 III. C ALCULATIONS Example considering coil parameters. t=0.36mm w=1095mm L=5930m Strip entry temp, SET=497°C Net consumption, Zn=3768/9 kg Al=0.29 % wt Fe=0.15 %wt DiB= 3m D=D o exp ( - Q / (R*T)) =1.29 * E - 19 LS=140m/min , Inhibition layer thickness, ILT=866+ 376ln ( EAL) =400.56nm Iron dissolution, q = D (C 1 - C 1 ) / (ILT) 2 (SW) (DiB) (CL) / (LS) [1] q=1 .04 E - 5 m 3 =82.153 gm Fe = ( (Fe % wt * net consumption)/ 100) + iron dissolution ( q) =0.80815kg Al = ((Al % wt * net consumption)/ 100) =1.5989kg Dross = ( Fe+Al)/0.05 =80.16kg Area=12991 m 2 Dross/m2=3.8612 g/m 2 Dross/shift=dross * no of coils =443 .1kg Area/shift=57745 m 2 Cal. Dross /shift = 7.6733 g/m 2 Act. dross / shift =12.07 g/m 2 % deviation in dross=(cal. dross – act. dross) / act. Dross = - 36.427 % IV. S I MULATED P ARAMETERS Al=0.15 %wt Fe=0.15 %wt Fe=0.628 kg Al=0.628kg Dross=25.12 kg Dross aft er considering deviation = ( simulated dross + (simulated dross * % deviation)) =34.27 kg Reduction in dross= (act . dross – simulated dross) / (act . dross) = ( 50.16 – 25.12 ) / 50.16 = 49.921 % Reduction in dross = (act dross – deviated dross) / (act dross) = ( 50.16 – 34.27) / 50.16 = 31.628 % V. SIMULATION PROGRAMMING The simulation programming is done with the help of flow chart described below. It helps in understanding the program in a simple and logical manner. International Journal of Emerging Technology and Advanced Engineering Website: www.ije tae.com ( ISSN 2250 - 2459 , Volume 2, Issue 1, Janua ry 2012) 48 Al l =Aluminium lower limit Al u =Aluminium upper limit C=Total no of coils n=No of coils t l =Te mperature lower limit t u =Temperature upper limit A= area of coil S=speed of coil q=iron dissolution The above flow chart showing the logic of simulation programming. The programming is done for calculation purpose so as to get the optimum value. This pro gramming helps to reduce manual calculations. VI. A) O PTIMIZATION O F A LUMINIUM C ONTENT During hot dip galvanizing, the small amount of Al is added into the zinc bath so as to form the protective layer on the steel strip. Al addition is done as it restricts the iron strip to dissolve into the molten zinc. Al has greater affinity towards iron and also it is lighter metal. The melting point of Al is 423°. So the temperature of zinc bath has to maintain above this temperature. The zinc bath is maintained at the tem perature of 460°C to 470°C under standard processing parameters. The iron dissolution is calculated using above equation and is compared with Al %wt. q min = D* (C 1 - C 2 ) / [(ILT)* 2* (SW)* (DiB)*[(CL) / (LS)]] Where, ILT=866 + 376 ln (EAL) D=D o exp ( - Q/RT) C 1 = concentration of iron within substrate. =100 Input: Al l , Al u , t l , t u , C, Fe%, A, S, q, n Start with Al l and t l , Al u , t u for a coil Check Al l Al Al u Check t l t t u Calculate q Increment t Al increment Calculate q (min) and identify optimum Al and t Calculate dross based on optimum parameters & original data Calculate %age reduction in dross If nC Yes No Stop Yes Yes No No International Journal of Emerging Technology and Advanced Engineering Website: www.ije tae.com ( ISSN 2250 - 2459 , Volume 2, Issue 1, Janua ry 2012) 49 C 2 =concentration of iron in bath. =0.14 ILT=inhibition layer thickness, m DiB=distance in bath, m =3m CL=coil length, m LS=line speed, m/s Fig. 2 Al content Vs iron dissolution F ig.3 Mat lab M file Environment B ) O PTIMIZATION O F S TRIP E NTRY T EMPERATURE The strip entry temperature plays an important role in galvanizing industry. If the strip is hotter than the bath when the strip immersed in the bath, the bath can overheat, which c auses increased dissolution of iron from the strip into the bath. The strip is hotter than the bath at the snout (i. e., near the point of immersion) unless the strip is sufficiently cooled following heat treatment that occurs prior to immersion in the bath . In conventional processes, the temperature of the bath is relatively high to avoid freezing of zinc bath at the bath surface. Use of a significantly cooler bath, however can cause zinc to freeze at the bath surface because of poor circulation in conventi onal bath and because the small difference between the strip immersion temperature and bath temperature. Fig. 4 Mat labs M file Environment C .T ESTING A ND V ALIDATION Testing of the program has been carried out with the help of actual data. The work invol ved taking samples of dross formed from the ISPAT STEEL INDUSTRIES line during galvanizing production. Knowing the process parameters and iron dissolution rate, it is possible to calculate the dross formed and to minimize it. An average effective aluminium content of 0.18 wt - % and strip entry temperature of 470°C is suggested. International Journal of Emerging Technology and Advanced Engineering Website: www.ije tae.com ( ISSN 2250 - 2459 , Volume 2, Issue 1, Janua ry 2012) 50 D . V ALIDATING T HE M ATHEMATICAL M ODEL For validating the mathematical model, the math model dross is equated with actual data, so as to get the approximate figures for further calcula tions, and the developed math model shows the approximate figures and the deviation in math model and actual dross is also within the same range as shown in above graph . TABLE 1 The practical dross is greater than the math model calculated dross, also graph shows the deviation between above drosses are approximately in same range, so the mathematical model developed is computed true for calculation purpose. TABLE 2 Fig.5) Actual Dross Vs Math Model Dross VII. V ALIDATION O F S IMULATED P ROGRAM For validating the present simulated program, the values of simulated program are compared with previous data at which the input parameters are taken same i.e. Al content and SET are taken into consideration. Al=0.18 %wt Fe=0.15 %wt SET=470°C As the validati on is done for same values of Al and SET, so dross/shift is compared in grams/m2. And it is observed that the simulated dross after programming and actual dross of previous data taken for comparison is approximately same and there is deviation of 4.34 % wh ich proves the validation of program and can be useful for practical application in galvanizing industry for processing steel. VIII. C ONCLUSIONS A ND S UGGESTIONS A mathematical model has been produced and the amount of iron dissolution that occurs during the hot dipping of a steel substrate has been calculated. The dross reduction takes place on an average of 20 - 30% from previous one. The main conclusions from the work are as follows. Actual dross(kg) Dross after math model dev.(kg) 443.1 339.273 428.39 356.0 94 436.35 195.895 445.67 174.321 402.19 219.609 Math Model Shift Actual Dross Calculated Dross g/m2 g/m2 1 8.41 6.5815 2 4.43 6.0364 3 10.32 10.301 4 5.69 7.6929 5 6.69 9.1479 6 3.97 6.4532 7 18.33 15.904 8 9.3 7.9524 11 8.01 6.4705 12 8.75 7.5712 / Actual Dross( g/m2) Simulation Output( g/m2) Deviation 8.048 7.68 4.34% TABLE 3 International Journal of Emerging Technology and Advanced Engineering Website: www.ije tae.com ( ISSN 2250 - 2459 , Volume 2, Issue 1, Janua ry 2012) 51 A . S TRIP E NTRY T EMPERATURE At higher strip entry temperature, the inhibition lay er forms much more rapidly upon the surface of the steel, thus leading to a decrease in the amount of iron dissolution before inhibition layer formation. However at higher temperatures the diffusion of iron through the inhibition layer increases, resulting in an increase in the amount of dissolution. So the optimum strip entry temperature for reducing iron dissolution is 470°C. B . E FFECTIVE A LUMINIUM C ONTENT O F B ATH Increasing the effective aluminium content of the bath results in a considerable decrease in the amount of iron dissolution. The biggest effect is before the inhibition layer formation, where as increase in aluminium content leads to quicker formation of the inhibition layer upon steel substrate. In addition, it can be seen that Al content increa ses the dross formation after particular range of addition. So the optimum aluminium content in the bath is 0.18 % wt of bath. S UGGESTIONS An entry temperature that is too high can also result in more enrichment of Al in the ternary alloy layer, causing a higher depletion rate of Al from the bath and overall poor Al level control. It also results in more dissolution of the steel strip – leading to more dross generation. So the difference between the strip entry temperature and bath temperature has to be kep t as minimum as possible. The higher the Al level is above 0.14%, the higher the bulk Al level in the coating. If the bulk Al content of the coating reaches levels above 0.30%, spot weld ability problems could result. Since the amount of Al extracted fro m the bath by the ternary alloy layer is independent of line speed and coating weight, it is important that operators know the rate of Al extraction at all times so that it can be replenished. So, the Al content and strip entry temperature should be mainta ined at 0.18 %wt of bath and 470° C respectively so as to reduce dross without affecting the quality of steel strip processed. R EFERENCES [1] S. O’Dell , J. Charles, M. Vlot and V. Randle, 2003 . Model ing of iron dissolution during hot dip galvanizing of stri p steel. [2] N. Y. Tang, Demystifyin g CGL bath chemistry management, Tech Cominco Ltd. Product Technology Centre, Mississauga, Ontario, Canada. [3] F.Ilinca, F. Ajersch, C. Baril and F. E. Goodwin, Numerical simulation of the galvanizing process during GA to GI tra nsition , Int. J. Meth. Fluids 2007; 53, 1629 - 1646. [4] Daniel Groteke, March 2004, pg 32 - 37, Aluminium Drosses. [5] Arturo Lazacano - Navarro, mar 28.1994, Process to reduce dross in molten aluminium , United States Patent. [6] Ramchandra S. Patil , Nov . 23, 1998 . Method for galvanizing and galvannealing employi ng a bath of zinc and aluminium, United States Patent. [7] Yoshikazu Fukuoka, Hiroshi Miwa , July 7 , 1975. Continuous hot - dip galvanizing process for steel strip , United States Patent. [8] G. K. Mandal, R. Balasubramaniam an d S. P. Mehrotra, Jan. 13, 2009. Theoretical investigation of the interfacial reactions during hot - dip galvanizing of steel. [9] Perti J. Sippola, Jan 29. 1998. Method of producing hot - dip zinc coated steel free of dross pick - up defects on coating and associat ed apparatus. [10] L. Chen, R. Fourmentin, J. R. Mc Dermid, 2008. Morphology and kinetics of interfacial layer formation during galvanizing hot - dip galvanizing