Computer Simulation of Microstructure Evolution in

1.IntroductionThedevelopmentofprocessmodelshasreceivedtremen-dousattentioninthesteelindustryoverthepastthreedecades.Todayawealthofsophisticatedmodelsisavail-ableformoststeelprocessingroutesthatisunparalleledcomparedtoothermaterialsproducingindustries.Inpartthisisduetothelargeproductionvolumeofsteelwhichoutnumbersthatofanyothermetalandalloybyawidemargin.Associatedwiththisisahighdegreeofautomati-zationinthesteelindustryandprocessmodelsplayacru-cialparttoproducesteelwithconsistentqualityandhighproductivity.Thismodeldevelopmenthasbeenmadepossi-blebytheenormousprogressincomputertechnologyoverthelastdecades.Increasingly,manyofthesemodelsarebasedonsoundphysicalprinciplesandincludesub-modelsthattrackthemicrostructureevolutionalongagivenprocesspath.Inparticular,thisapproachisofparamountimportancefortheproductionoflowcarbonsheetsteelsthatarerequiredforsomeofthemostdemandingapplica-tionsofsteel,e.g.intheautomotiveindustry.1)Thus,thepresentreviewpaperemphasizesthemicrostructuremodelsforthisfamilyofsteels.Inparticular,hotrollingofsteelshasbeenanemphasisofcomputermodelling.StartingwiththepioneeringworkofSellarsetal.2,3)inthelate1970’smathematicalmodelsweredevelopedinthe1980’sprimarilyforplaterolling.4–7)Theprocessingparameters(e.g.strainratesintheroll-biteandinterstandtimes)ofplaterollingcanbecomparativelyeasilyreplicatedwithlaboratorysimulationstocharacterizetheindividualmicrostructurephenomenaofgraingrowth,recrystallization,precipitationandphasetransformation.Inparallel,modelsweredevelopedforhotstripmills,i.e.themostsophisticatedprocessingfacilitiesfoundinsteelmills.Senumaetal.8)reviewedthestatusofthesemodelsin1992.Sincethisreview,theirconclusionthatcomputermodellingwouldbecomeakeytechnologyforhotstripproductionhasbecomeareality.Virtuallyallmajorsteelproducersnowemploymicrostructurehotstripmodelsroutinely,atleastonanoff-linebasis.9–19)Moreover,novelstripprocessingroutes,i.e.compactstripprocessing(CSP)/thinslabcastinganddirectrolling(TSDR),havebeenintroducedsincethe1990’s.Thisnecessitatedtheextensionofmodels,whichhadoriginallybeendevelopedforintegratedmills,tothesenewprocessingconditions.Anotherrecentaspectofsheetsteelproductionisthemoveintoadvancedhighstrengthsteels(AHSS).Thisisprimarilydrivenbytheincreaseddemandsofautomobilemanufacturerstoemploysteelswithimprovedpropertiesintermsofstrength–ductilitybalanceandcrashworthi-ness.1,20,21)ThedevelopmentofAHSSisanexcellentexam-pleoftheversatilityofsteelstobetailoredtoamultiplicityofpropertiesbymanipulatingsteelchemistryandprocess-ingroutes.Animportantadvantageofsteelsascomparedtolightweightmetalsandalloys(e.g.AlandMg)isthattheaustenite–ferritetransformationcanbeexploitedtoproducemicrostructuresthatareassociatedwithsignificantlyim-provedpropertyprofiles.Thisisindeedthekeymetallurgi-caltoolforAHSS.Processingwindowsforthisnewclassofsteelsaregen-erallytighterthanthoseforconventionalsteels.Therefore,thedesignandcontrolofrobustprocessingroutesmoti-ISIJInternational,Vol.47(2007),No.1,pp.1–15ComputerSimulationofMicrostructureEvolutioninLowCarbonSheetSteelsMatthiasMILITZERTheCentreforMetallurgicalProcessEngineering,UniversityofBritishColumbia,Vancouver,BC,CanadaV6T1Z4.(ReceivedonJuly24,2006;acceptedonSeptember14,2006)Modelsofmicrostructureevolutioninsteelsarereviewed.Theemphasisofthereviewisonlowcarbonsheetsteelsbothhot-rolledandcold-rolledandannealed.Firstthestate-of-the-artonindustrialmicrostruc-tureprocessmodelsispresented.Theindividualmodelconceptsforgraingrowth,recrystallization,precipi-tationandphasetransformationsarebrieflydiscussed.Thedevelopmentfromempirically-basedmodelstophysically-basedmodelsisidentifiedasakeyissuetohaveincreasedpredictivecapabilitiesforthesemod-elsoverawiderrangeofsteelgradesandoperationalconditions.Thechallengesinthedevelopmentofthenextgenerationofmodelsaredelineated.Inparticular,newaspectsofmicrostructureevolutionassociatedwithnovelprocessingroutesandadvancedhighstrengthsteelsareevaluated.Further,themajorityofthecurrentlyemployedmodelsareonthemacro-scalebutfuturemicrostructuremodelswillincreasinglybemeso-scalemodelsthatpredictactualmicrostructuresratherthananumberofaverageparameters(e.g.grainsize,fractiontransformed)todescribemicrostructureevolution.KEYWORDS:mathematicalmodel;microstructure;steel;hotrolling;annealing;graingrowth;recrystalliza-tion;precipitation;phasetransformation.1©2007ISIJReviewvatesthedevelopmentofadvancedmicrostructuremodelsthatprovideanincreasedinsightintotheunderlyingphysi-calmetallurgyprinciples.Further,AHSSarefrequentlyproducedascold-rolledannealedand/orcoatedsheets.Here,theintercriticalannealingstepassumesacrucialroletoobtainthedesiredcomplex,multi-phasemicrostructures.Intercriticalannealingconstitutesaparadigmshiftfroman-nealingofconventionalsteelswherenocyclingthroughthetransformationregionoccurs.TheaddedcomplexityoftheannealingroutesforAHSSrequiresthedevelopmentofnovelannealingmodelsthatalsoaddressausteniteforma-tionwhichhasreceivedcomparativelylittleattentioninprocessmodels.Currentlyavailablemicrostructuremodelsforthepro-ducti