Objasnění příčin vzniku vady při válcování speciálního profilu pomocí MKP

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1 Objasnění příčin vzniku vady při válcování speciálního profilu pomocí MKP FEM-Aided Clarification of the Cause of Defect Formed during Hot Rolling of a Special Section Ing. Richard Fabík, Ph.D. VŠB-TU Ostrava, FMMI, katedra tváření materiálu, 17. listopadu 15, Ostrava - Poruba, ČR, Ing. Richard Baron, VÚHŽ, a. s., Válcovna speciálních profilů, Dobrá 240, ČR, Cílem této práce bylo analyzovat příčinu vzniku vady na boku válcovaného speciálního profilu. Na základě metalografických snímků byla vada definována jako přeložka. Na základě polohy vady a s ohledem na to, že je její výskyt nepravidelný, byla přijata hypotéza, že za vznikem vady stojí zvýšené opotřebení válců v prvních kalibrech. K ověření této hypotézy byla provedena počítačová simulace válcování daného profilu, jak na nových tak na opotřebovaných válcích. Míra opotřebení válců byla úmyslně zvolena výrazně vyšší než je v reálné praxi možné, aby byla zvýšena pravděpodobnost vzniku vady při matematickém modelování. K simulaci byl použit 3D simulační program Forge2005. Výsledky simulace ukázaly, že vlivem opotřebení válců došlo k výraznému zvětšení šířky profilu, před 7. průchodem. Jelikož se 7. průchod provádí v zcela uzavřeném kalibrem č.2 a tedy s omezeným šířením, byl tento již předem vytipován jako okamžik možného vzniku výronku. Tento výronek pak v následném průchodu může vést ke vzniku přeložky. Tento předpoklad byl pomocí matematického modelování potvrzen. Matematický model určil pravděpodobnost vzniku přeložky v 8. průchodu na 70 %. Nejpravděpodobnější místa výskytu přeložky stanovené modelem se shodují s místy výskytu skutečných vad. Provedená matematická analýza válcování speciálního profilu potvrdila, že za vznikem vady může být větší šíření při válcování v kalibru 1, které je důsledkem jeho většího opotřebení. Úplnou jistotu o příčině vzniku vady je možno získat poloprovozním experimentem a důsledným monitorování klíčových parametrů, které mohou způsobit zvýšení šíření při válcování. Kromě již zmíněného opotřebení kalibru to může být variabilita vstupních rozměrů, teploty (teplotního profilu), rychlosti válcování a chemického složení oceli. Vady podobného typu co do tvaru i co do vlivu na okolní mikrostrukturu se vyskytují i na jiných profilech v jiných válcovnách (např. kolejnice) a vždy k jejich výskytu dochází v místech, kde může jít o zaválcování výronku. Matematické modelování je tak ideální metodou pro stanovení příčiny jejich výskytu. The purpose of this study was to identify the cause of a defect formed on the side of a rolled special section. The defect was classified as a lap on the basis of micrographs. With respect to the position of the defect and to its irregular occurrence, a hypothesis was framed that the defect was caused by increased wear of first roll pass. In order to verify this hypothesis, computer simulation of rolling of the section was performed. It included alternatives using new and worn rolls. The simulation showed that due to the wear of rolls, the width of the section increased markedly prior to 7th reduction. As the forming process in 7th reduction takes place in completely closed set of roll pass no. 2, i.e. with restricted expansion, it was identified as the possible instant of flash formation. This flash then may form the lap in the following reduction. The mathematical model showed that the probability of formation of the lap in 8th reduction was 70%. Introduction Development of computer technology offers ever more elaborated and some novel mathematical modelling methods for description of materials response to rolling. Research in this field has been undertaken by a number of experts who proposed numerous particular models [1-5]. The study deals with the use of mathematical modelling using software FORGE 2005 for identification of a cause of defects in shaped sections [6]. This is another field, apart from finding the distribution of thermal and mechanical quantities on the cross-section of the rolled product, where mathematical modelling can be applied to real-world operation problems [7]. Application of FEM-based mathematical modelling for roll pass sequence design and verification is still disputable.

2 Formulation of Hypothesis of Defect Formation The type of the defect is shown in the photograph in Fig. 1. They are two parallel grooves appearing irregularly on the side near the special section ridge. Microstructure analysis was performed (Fig. 2.) in order to classify the type of the defect: a crack or a lap. With regard to the shape of the defect and apparently decarburized microstructure in its vicinity, a hypothesis was formulated that the defect is a result of a flash rolled into the surface. The flash would be formed by flow of excess metal in the previous roll pass. The analysis of the schedule of reductions (table 1 only lists first 7 reductions relevant to the solution of the problem. The remaining data will not be published owing to the rules of the manufacturing secret) suggests that (with regard to the defect location) the defect forms during the eighth reduction or that there is overflow of material in the roll pass in the seventh reduction. There might be several causes of the material overflow in the roll pass 2 (Fig. 3.) in the seventh reduction (variations in the entry cross-section, temperature and rolling speed drop and resulting greater sideways expansion). Considering the temporal distribution of occurrence of the defect, the most probable cause of the material overflow in 2nd roll pass during 7th reduction is the increased wear in the box pass 1. Of the whole series, the box pass was used the most (in 5 instances in total), while the roll pass 2 was used twice and the remaining roll passes were used once each. This resulted in increased wear of the box pass, which might lead to the suggested roll pass material overflow. Obr. 2. Metalografický rozbor vady v příčném řezu Fig. 2. Microstructure analysis of defect, transverse section Tab 1 Základní parametry válcování Tab 1 Rolling parameters Obr. 3. Schéma kalibru 2 Fig. 3. Roll Pass 2 Diagram FE Analysis Obr. 1. Vady na povrchu speciálního profilu Fig. 1. Defects situated on the surface of special section The actual simulation was performed with the aid of a 3D forming simulation tool: FORGE Two alternatives were calculated. The first alternative involved roll pass 1 with no wear, while the second one concerned a worn pass 1. The roll shapes used in the model are shown in Fig. 4. As no actual worn roll was available, the amount of wear was defined by mathematical statistic techniques. The contour of top and bottom surfaces of the box pass 1 matched the Gauss curve, where highest wear, 4 mm, was in the pass centre. This is, by estimate, about twice as much as the value which might occur in actual rolling. However, it will increase the probability of identification of possible defect.

3 Obr. 4. Neopotřebovaný válec (vlevo), opotřebovaný válec (vpravo) Fig. 4. New roll (left) and a worn roll (right) As the arrangement of the problem is non-symmetric and 7 reductions were used (the length of the product during rolling and the complexity of calculation increased), coarse finite element mesh in the centre of the workpiece was used. The surface mesh was, understandably, finer to facilitate detection of defect origin. A similar procedure was used for creation of mesh in the rolls (Fig. 5.). The calculation was stabilized with additional tools simulating the function of entry guides. The correct use of such tools is described in detail in [8]. Initial and boundary conditions of the mathematical model are defined in Table 2. Chemical composition of 28Mn6 steel used is shown in Table 3. Fig. 6 shows a plot of flow stress of the steel. Obr. 5. Konečněprvková síť polotovaru a válců, situace před prvním úběrem. Fig. 5. Finite element mesh in the workpiece and rolls; prior to first reduction. Obr. 6. Přirozený deformační odpor oceli 28Mn6 Fig. 6. Flow stress of 28Mn6 steel Tab. 2 Počáteční a okrajové podmínky konečnoprvkové simulace Tab. 2 Initial and boundary conditions of the FE simulation Main proportions Boundary conditions Initial proportion h 0 = 80 mm b 0 = 80 mm Friction = 0,3 l 0 = 210 mm Final length l 8 = 820 mm Rolls velocity = 50 rpm Rolls diameter R = 243 mm Thermal exchange with rolls = W.m -2.K -1 Initial conditions Thermal exchange Air computation Rod temperature T 0 = C Symmetry NO Roll temperature T R = 20 C Plain of symmetry - Ambient temperature T a = 20 C FEM formulation Rigid-plastic Tab. 3 Chemické složení oceli 28Mn6 [hm. %] Tab. 3 Chemical composition of 28Mn6 steel [wt. %] C Si Mn P S Cr Mo Ni V Cr + Mo + Ni Note: Si, P, S, Cr, Mo, Ni a Cr + Mo + Ni: the top limits are listed

4 Discussion of Results The change in the shape of the rolled product during first 8 reductions for both alternatives of calculation is shown in Figs. 7. to 13. First two reductions took place in roll pass 1 without tilting. Apart from the difference in the product height (which is the direct result of roll wear in roll pass 1), Fig. 8. shows the expansion in the central region of the rolled product (producing a slight barrel shape). This barrel shape is even more pronounced after third reduction, which was preceded by tilting (Fig. 9.). After fourth reduction (Fig. 10.), the maximum difference in width and height of the product is 2.92 mm and 8.86 mm, respectively. Roll pass 2 was used for fifth reduction (however, the roll pass is not fully closed here, see Fig. 11.), after which not only considerable product width variation (almost 9 mm) was observed but also marked difference between smoothness of surface of products resulting from two different alternatives. The alternative involving worn rolls shows points of inflexion in locations of edges of side walls of the roll pass. The subsequent rolling in roll pass 1 (Fig. 12.) cannot reverse such tendency. In 7th reduction (Fig. 13.), in a fully closed roll pass 2, flash forms on both sides of the roll pass. The flash on the side of the nose appears to be more prone to lap formation, as it is longer and more steep. Fig. 14. shows the result of 8th reduction and isosurfaces of probability of lap formation. The side of the nose represents the probability of 0.7, while the opposite side exhibits the value of no more than 0.3. The region with highest probability of lap formation well matches the location where the actual defect was found in the rolled product. Obr. 7. Rozměry vývalku po 1. úběru; vlevo nové Fig. 7. Dimension of rolled section after 1 st reduction; left new rolls; right worn rolls Obr. 10. Rozměry vývalku po 4. úběru; vlevo nové Fig. 10. Dimension of rolled section after 4 th reduction; left new rolls; right worn rolls Obr. 8. Rozměry vývalku po 2. úběru; vlevo nové Fig. 8. Dimension of rolled section after 2 nd reduction; left new rolls; right worn rolls Obr. 9. Rozměry vývalku po 3. úběru; vlevo nové Fig. 9. Dimension of rolled section after 3rd reduction; left new rolls; right worn rolls Obr. 11. Rozměry vývalku po 5. úběru; vlevo nové Fig. 11. Dimension of rolled section after 5 th reduction; left new rolls; right - worn rolls

5 Obr. 13. Pravděpodobnost vzniku přeložky po 8. úběru, opotřebované válce Fig. 13. Probability of lap formation after 8 th reduction, worn rolls Conclusion Mathematical analysis of rolling of a special section showed that formation of the defect may be the result of greater expansion in roll pass 1 due to its greater wear. The cause of the defect can be precisely confirmed by means of a pilot experiment and by consistent monitoring of key parameters which are the suspected cause of sideways expansion. In addition to the above mentioned roll wear, these may include the variability in input dimensions, temperatures (temperature profile), rolling speed and chemical composition of steel. Defects of such type (both in terms of shape and effect on microstructure in their vicinity) can be observed in other sections in other rolling plants (e.g. railroad rails). They tend to form in locations where rolling of a flash into the surface can be expected. Mathematical modelling is the optimum technique for identification of the defect cause. Acknowledgements. The works were implemented in the framework of solution of projects MSM (MŠMT ČR). References [1] R. Inakov, Journal of Materials Processing Technology, 142, (2003), [2] J. P. Perä, R. Villanueva, Trans. of Steel Rolling, Paris, 28 (2006), [3] M. Kazeminezhad, A. Karimi Taheri, Materials Letters, 60, (2006), [4] K. Komori, Internetional Journal Mech., 40 (1998) 5, [5] M. Glowacki, et al, J. of materials processing technology, 53 (1995) 2, [6] R. Fabík, J. Kliber, S. A. Aksenov, Impact of Rolling Conditions on Propagation of a Potential Crack, Materials Science Forum, Vols (2008), Trans Tech Publications, Switzerland, pp , ISSN [7] R. Fabík, S. A. Aksenov, Optimalization of crop losses by finite element simulation of slab edging. Steel research international, čís. 79 (2008), Special Edition Metal Forming Conference 2008, Volume 1, ISBN [8] R. Baron, Využití matematického modelování při hledání příčin výskytu vad na bocích válcovaných profilů Diplomová práce, VŠB-TU Ostrava, 2008