thermal stress analysis using finite element method in area

1.ANA JOSAN, 2. IMRE KISSTHERMAL STRESS ANALYSIS USINGFINITE ELEMENT METHOD IN AREAOF THE ROLLING MILLS ROLLSAbstract:The paper presents the analysis of thermal stress field distribution that occurs in the mill rolls, usingfinite element method. The finite elements method or the analysis with finite elements is based onthe concept of building complicated object out of simpler ones, or dividing complicated objects intosimpler ones, for which known schemes of calculation can be applied. The main idea in the methodof finite element is to find the solution of a complicated problem by replacing in with a simpler one.The analysis of the thermal strains in rolling rolls, using finite elements method, has been carried outon Adamit-type rolls, cast of hypereutectoid steel and used in rolling profile I on the middle profilerolling train of Arcelor Mittal Hunedoara branch.The industrial experimental data (rolling temperature, cooling water temperature, materialcharacteristics etc.) are the basics for simulation program.Keywords:Rolling rolls, Simulation, Thermal Stress Analysis, Finite Element MethodINTRODUCTIONA particularly actual problem for the steelmaking companies is the low exploitationendurance of the rolling rolls, as they are themost stressed parts in the rolling train.By taking into consideration thermal strains wecould carry out a complete study, very close tothe real conditions of rolling roll exploitation, asthe thermal influences constitute one of thebasic causes leading, even under favourableexploitation conditions, to thermal fatiguefissures, that reduce the use of rolls in rollingsessions [1].THE EXPERIMENTAL ANALYSEThe analysis of the thermal strains in rolling rollshas been carried out on Adamit-type rolls, castof hypereutectoid steel and used in rollingprofile I on the middle profile rolling train ofArcelor Mittal Hunedoara branch.The method of the finite element implies thegeneration of a discretizing lattice on thesurfaces previously defined, taking intoconsideration the fact that all the knots andelements of the lattice are numbered [2], [3]. Wemention that, on discretizing, we obtained atotal number of 253613 elements and 49768knots.© copyright FACULTY of ENGINEERING – HUNEDOARA, ROMANIA 65

ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGThe plane discretizing of the rolling rolls and ofthe rolled profile are given in Figure 1 andFigure 2.Fig. 1. Diagram of the plane modeling of the rollingrolls and the rolled profile IFig. 2. Diagram of the plane modeling of the rollingrolls and the rolled profile I. Detail AIn order to determine the material thermalphysicalproperties, we took into account thefact that the steel that the rolling rolls are castfrom is an Adamit-type, hypereutectoid steel,grade OT- A3, having the chemical structuregiven in Table 1. The steel to be rolled is an allpurpose steel (OL 37), having the chemicalstructure given in Table 2 [4].The thermal-physical and material properties ofthe Adamit-type, hypereutectoid steel that therolling rolls are cast from are [4]:• the coefficient of linear expansion:−6 1α = 13 ⋅10;K• thermal conductivity:Wλ = 31 ;m ⋅ K• specific heat:Jc p= 620 ;kg ⋅ K• initial temperature: T initial = 20 o C.The thermal-physical and material properties ofthe steel to be rolled, i.e. (OL37), respectively ofprofile I, are [4]:• the coefficient of linear expansion:−6−1−6−1α o = 12,2 ⋅10grdC ; α o = 14,9 ⋅10grdC ;100 C700 C• thermal conductivity:WWλ = 25λ = 50 ;800 o C20 Cm ⋅Ko m ⋅K• specific heat:cp 20oCJ= 452 ; ckg ⋅ KJ753 ,3 ; ckg ⋅ Kp 800oCp 600oCJ= 933 ,3kg ⋅ K• initial (rolling) temperature: T rolling = 800 o C.=DETERMINATION OF THE STRAIN STATE IN CASEOF APPLYING THE COOLING WATER ON TWOSURFACESIn case of calculating the thermal strains at800 o C, the rolling rolls being cooled on twosurfaces, we took into consideration the fact thatthe strains in the neck areas are not relevantbecause movements were null here (“expansionblocked”), as the bearing clearance was ignored.Table 1. The chemical structure of the Adamit-type,hypereutectoid steelSteelChemical structure, %grade C Si Mn P max S maxOTA31.8…2.0 0.6…0.8 0.7…0.9 0.04 0.02Cr Ni max Mo Cu max Ti max1.0…1.2 1.6…2.0 0.3…0.5 0.2 0.06Table 2. The chemical structure of the steelto be rolledSteelChemical structure, [%]grade C max Mn max Si max P max S maxOL370.20 0.80 0.07 0.06 0.06Fig. 3. The rolling roll cooling diagram, withoutdisplaying the surfaces the cooling water is appliedon (position a) and displaying them(surfaces 68 and 179) (position b)662010/Fascicule 1/January‐March/Tome III

ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGThe diagram of the water cooled surfaces withrespect to the position of the rolled section isgiven in figure 3.In order to draw the diagrams corresponding tothe thermal strains in the knots placed on thecritical rolling areas of the roll circumference, itis necessary to place the knots on thecircumference of the groove pass. In order todraw the diagrams corresponding to the thermalstrains in the knots placed on the critical rollingareas of the roll circumference, it is necessary toplace the knots on the circumference of thegroove pass.Fig. 5. The variation of Sigma Von Mises strainsFig.4. Variation of the Sigma Von Mises strainsIn Figure 4 we gave the ensemble of the rollcross-section, as well as the magnitudes of theVon Mises strains for the following knots(schematically pointed out on the surface of theupper roll): 35215, 35218, 35221, 35224, 35227,35230, 35233, 35236, 35239 and 35242. In theplan given in fig.3, one can notice an increase inthe thermal strains towards the rolling area (theprofile). Thus, for knot 35215 Von Mises strain is121.73 N/mm 2 and for knot 35242, of 928.52N/mm 2 . The distance between the first and thelast knot under study represents 167.88 mm (inthe graph given in the figure, it is considered tobe equal to the unit).Fig. 6. The variation of Sigma2010/Fascicule 1/January‐March/Tome III 67

ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGIn Figure 5 we gave the ensemble of the rollcross section, as well as the magnitudes of theVon Mises strains for the following knots(schematically pointed out on the surface of theupper roll): 35126, 35129, 35132, 35135, 35138,35141, 35144, 35147, 35150, 35153 and 35156.The diagram shows a decrease in the thermalstrains away from the rolling area, knot 35126having a Von Mises strain of 930.77N/mm 2 andknot 35156 of about 246 N/mm 2 . In this case, thecalculated length is 185.1947mm.Fig. 7. The variation of Sigma Von Mises strainsSimilarly, in Figure 6 we gave the ensemble ofthe roll cross section, as well as the magnitudesof the Von Mises strains for the following knots(schematically pointed out on the surface of theupper roll): 4277, 4281, 4285, 4289, 4293, 4297,4301, 4305, 4309, 4313, 4317 and 4321. In theplan given in Figure 6 one can notice a decreasein the thermal strain away from the rolling area.Thus, for knot 4277 Von Mises strain is of902,51N/mm 2 and for knot 4321 of 173.26N/mm 2(in this case, the calculated length is 185.1947mm). In Figure 7 we gave the ensemble of theroll cross section, as well as the magnitudes ofthe Von Mises strains for the following knots(schematically pointed out on the surface of theupper roll): 4423, 4427, 4431, 4435, 4439, 4443,4447, 4451, 4455, 4459, 4463 and 4467. Figure 7shows that for knot 4467 Von Mises strain is of822.58 N/mm 2 and for knot 4423 of 99.203 N/mm 2(the calculated length is 185.1947 mm).CONCLUSIONAs a result of simulating the rolling process bymeans of the finite element method in view ofquantitatively and qualitatively determiningthermal strains, we came to the followingconclusions:• thermal strains are the basic cause of fissuringthe rolling surface of rolls as a result of thermalfatigue, caused by the high values in the areasneighboring the contact area with theincandescent semi-finished part; the graphsshow that the values of the thermal strains inthe area under consideration rank within121,73 and 930.77 N/mm 2 for the upper rolland 99.303 and 902.51 N/mm 2 for the lowerroll;• thermal strains have significantly higher valuesas compared to the mechanical ones and actat relatively short intervals of time (withinfractions of a second);• the rolling rolls break during the rollingprocess because of thermal fatigue;• a most precise knowledge of the character ofthe strains generated by the complex stressesthe hot rolling rolls undergo, allows thedetermination of the duration in exploitationunder safe conditions, by their comparison tocertain limit values imposed from the verybeginning;• strains in the rolling rolls have a cyclicalcharacter and the strain state is mainly theresult of the action of the fields of symmetricaland asymmetrical temperatures that causethermal fatigue on their surface and superficiallayer.Engineering is concerned with the design of asolution to a practical problem. A scientist mayask why a problem arises, and proceed toresearch the answer to the question or actuallysolve the problem in his first try, perhapscreating a mathematical model of hisobservations. By contrast, engineers want toknow how to solve a problem, and how toimplement that solution. In other words,scientists attempt to explain phenomena,682010/Fascicule 1/January‐March/Tome III

ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGwhereas engineers use any available knowledge,including that produced by science, to constructsolutions to problems.The technological manufacturing process of therolling mills rolls, as well as the quality ofmaterial used in manufacturing them, can havea different influence upon the quality and thesafety in the exploitation. The proposalapproaches the issue of quality assurance of therolling mills rolls, from the viewpoint of thequality of materials, which feature can causeduration and safety in exploitation.REFERENCES[1.] BUDAGHIANT, N. A., KARSSKI, V.E.,Cilindri de laminor turnati, E.T., Bucuresti1986[2.] HEUBNER, K.H., The Finite ElementMethod for Engineers - Tohn Wiley andSons, N.Y. - Toronto, 1975[3.] HUGHES, T.J.R., The Finite ElementMethod - Pretince-Hall, Enghvood Cliffs,N.J., 1987[4.] STURZA, S., BOZGA, L., RomanianStandards and Norm Pack (internal usageArcelor Mittal Hunedoara branch, 1992[5.] KISS, I., The quality of rolling mills rollscast by iron with nodular graphite, Mirton,Timisoara, 2005[6.] KISS, I., Researches concerning theimprovement quality of rolling mill rollscast from cast-iron with nodular graphite,doctoral thesis, Bucuresti, 2005[7.] KISS, I., Researches regarding the qualityassurance of the rolling mills cast-iron rollsthrough mathematical molding of themanufacturing process and theexperimental study of durability inexploitation, National contract No5889/2005, Bucuresti[8.] STEVENS-OOST, B.: Roll quality and rollcost development: where do we comefrom and where are we going?, atInternational Symposium “Rolls 2003”,United Kingdom, 2003[9.] KERR, E.J., WEBBER, R.: Roll performance- present overview and look to the future,at: International Symposium “Rolls 2003”,Birmingham, United Kingdom, 2003[10.] CARLESS, P.: Present and future hot stripmill finishing train work rolls, Ironmakingand Steelmaking, vol.27, No.1, 2000[11.] SCHRODER, K. H.: A basic understandingof the mechanics of rolling mill rolls,Eisenwerk Sulzau-Werfen, ESW-Handbook,2003[12.] SCHRODER, K. H.: Rolling conditions inhot strip mills and their influence on theperformance of work rolls, MetallurgicalPlant and Technology, no.4/88, 44-56,Dusseldorf[13.] SCHRODER, K. H.: Questions, answers,more questions – Twenty-five years ofexperience in discussing rolls and rollingtechnology, 42 nd Mechanical Working andSteel Processing Conference Proceedings,Toronto, 2000[14.] PELLIZZARI, M., MOLINARI, A., BIGGI, A.:Optimization of the hot roll performancesthrough microstructural tailoring, atConference of University Trento, Brescia,Italy, 2002[15.] McCANN, J.: Overview of work rolls forcold rolling, Ironmaking and Steelmaking,vol.27, No.1, 2000[16.] MOLINARI, A., TREMEA, M., PELLIZZARI,M., BIGGI, A., CORBO, G.: High speedsteels for hot rolls with improved impactand thermal fatigue properties, in:Materials Science and Technology, 18,2002[17.] TAKIGAWA, H.: Developement of HighSpeed Tool Steel Rolls and TheirApplication to Rolling Mills, in: NipponSteel Technical Report, Nr.74, 1997[18.] TISZA, M.: Modelling and simulation inmaterials science and manufacturingtechnologies, la: Conference of UniversityTrento, Brescia, Italy, 2002AUTHORS & AFFILIATION1.ANA JOSAN,2.IMRE KISS1. 2. 3DEPARTMENT OF ENGINEERING & MANAGEMENT,FACULTY OF ENGINEERING HUNEDOARA,UNIVERSITY “POLITEHNICA” TIMISOARA, ROMANIA2010/Fascicule 1/January‐March/Tome III 69

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