Caus of Transver Corner Cracks in Micro-Alloyed Steel in

2024年3月1日发(作者：会议室设计)

Causoftransvercornerc*1,1,1,1,1,2Transvercornercrackshavebeenfrequentlyobrvedandareextremelydifficulttopreventinsomemicroallocksareusuallyfoundonthefixed(outer)sidecorneroftheslabsthrentstudy,theslabsurfacemicrostructurewasinvestigated,andtheresultsshowthatthecausofthecrackformationarechain-likeprecipitationsandfilm-likeproelly,whenthetemperatureoftheslabsdroppedaftersolidification,theNb,VorTicarbidesand/ornitridesprecipitatedinchain-likeway,ocesshind,duetothestressmismatchbetweenthematrixandthefineprecipitatesduringbendingoperations,thechain-likeprecipitatedcarbidesand/ornitridesiile,thefilm-likeproeutectoidferriteprecipitatedalongtheaustenitegrainboundariesduringtheaustenite–ofthefactthatthestrengthofproeutectoidferritefilmislowerthanthatoftheaustenitegrains,whentheslabsweresubjectedtobendingstress,ds:Verticalbendingtypecontinuouscaster,Transvercornercracks,Film-likeproeutectoidferrite,CrackingsusceptibilityIntroductionexperimentsonthetransvercornercracks,someForthepast30years,strongnitrideandcarbiderearchersmaintainthatthecracksarecaudbytheformationelementssuchasNb,ggestaddedtomicroalloyedhighstrengthsteeltoenhanceavoidingthisbrittlenesstemperaturebykeepingthestrengththroughfinegrainedstrengtheningaswellastemperaturehigher/r,transvercornerperature3–5(700–900uC)duringbendingand/othodonlyworkswiththebowtypetheoscillationmarks,erticalbendingtypecontin-boundariesassociatedwithchain-likeprecipitatesanduouscaster,itisimpossibletomaketheslabcornerfiarefoundonthetemperatureavoidthetroughoftheembrittlementfixedsidecorneroftheslabsduringbendinginaverticaltemperatureduringbending,becauitis700–900uCbendingtypecontinuouscasterorontheloo(inner)ecorneroftheslabsduringstraighteninginthebowesmersituationhasbeenalloyedsteelinaverticalbendingcontinuousslabcastersconsideredadifficultsubjectandhasbeenagreatdidnot,forsomereason,attracttheattentionofconcernandsignificantissueformetallurgists,dieshavebeenconductedonthearemanycausfortheformationoftransver1andHarada2arguethatthecracksaretionoftransvercornercracks,especiallystudiesofthegeneratedinthemouldduetothegregationofPinthesurfacemiercornercracksoccurfrequently,whichriouslyreducesthehotdeliveryofslabsandhinders1CollegeofMaterialsScienceandEngineering,ChongqingUniversity,ightofthefeaturesChongqing2400044,ChinaoftransvercornercracksintheslabscontainingNb,VBaoshanIronandSteelCo.,Ltd,Shanghai201900,ChinaandTi,theprentstudyexaminesthesurfacemicro-*Correspondingauthor,emailmafj@ctureofslabsandthedistributionofprecipitatesofß2010InstituteofMaterials,MineralsandMiningPublishedbyManeyonbehalfoftheInstituteReceived16May2009;accepted28June2009DOI10.1179/030192309X125IronmakingandSteelmaking2010VOL37NO173

oftransvercornercracksinmicroalloyedsteel1Positionandmacromorphologyoftransvercornercracksoftransvercornercracksintheslabswereinvestigated,andatheoreticalfoundationtoeffectivelyreduce,orcompletelyeliminatethetransvercornercracks,mentalThespecimenswerelectedfromthenarrowcornersurfaceoftheslabs,ascracksarefrequentlylocatedonthefirostructuresofthetypicalslabswereanalydafterbeingetchedwith4%nitalsolutionusingthefollowinginstruments:opticalmicroscopy(OM),scan-ningelectronmicroscopy(SEM)andenergydispersivespectroscopy(EDS).tothefickswerescarcelyfoundonthenarrowsurfaceoftheloosidecounterpart(Fig.1).Crackswereldomobrvedonthenarrowsurfaceofthelooside(Fig.2a),whiledistincttransvercornercrackscouldbeeasilyidentifiedwiththenakedeyeonthefixedsidesurface(Fig.2b).Thecracks,markedbyarrowsinthefigures,romorphologyofthetransvercornercracksontheslabswasobrvedbyusingOMandSEM(Fig.3).Cracksthecracks,sofprecipitatesontransvercornercracksMicroalloyedelements(Nb,VandTi),whicharestrongnitrideandcarbideformingelementsinsteel,usuallygeneratestablehighmeltingpointcarbide,nitrideorcarbon–nitridewhentheslabsult,thegrainboundarybind,inturn,contributesile,theembrittlementtemperaturetroughbroadensandriessteelwithalargenumberofcracks,themicroalloyedelementsintheslabswerefoesof(Ti,Nb)(C,N)precipitateswereResultsanddiscussionTransvercornercrackdistributionandfeaturesInrearchingtheformationmechanismoftransvercornercracksontheslabs,alargenumberofslabswithdifferentchemicalcompositiodbythecoaroxidescaleofsteel,r,chresultsshowthatcornercracksareusuallydistributedonthenarrowcornersurfaceapproachingonthewidesurfaceintheslabfiackxtendedTable1Transvercornercrackrateandchemicalcompositionofslabs,wt-%SteelABCDEFGC0.1550.0950.14460.090.09270.15970.165Mn0.851.431.470.20.101.510.93Si0.20.180.3070.980.210.40.03Al0.0350.0310.0260.0270.0310.0460.045Ti0.010.010.0130.0150.0150.012Nb0.0150.0270.0080.0080.007V0.0020.043Transvercornercrackrate,%184Ferritefilmwidth,mm80.00174IronmakingandSteelmaking2010VOL37NO1

oftransvercornercracksinmicroalloyedsteelanarrownearbylooside;bnarrownearbyfixedside2Transvercornercracksdistributedalongoscillationmarksofslabnomorethan6,73mm,whichisconsistentwiththedatainNbgreatlyaffectstheductilityofsteelsunderhightheliterature,eantime,arch8–11ontheeffectofNbonmostprecipitatesweredisperdwithintheausteniteductilityconcludedthatNbdeepensandbroadensthegrainsintheAriessteelwithoutcracks,hSEMandEDS(Fig.5)forven0?008–0?027%ndifferentsteels,thechemicalcitatesontheausteniteboundariesareshowninNiobiumcarbon–mes,theprecipitatescanbeassociatedniteboundariesisthemaincauforlowerductilityofwithotherslaginclusion,alniobiumcompoundsandsquareastheForthisanalysis,thecarbon–nitrideofmicroalloyedtitaniumcompounds(Fig.6andTable3).Thedatasteelprecipitatedconsiderablyalongtheaustenitegraininthefollow-upinvestigationoftransvercornerboundariesattheexistingprocessparameters,whichcrackincidenceillustratedthatthecrackrateforintensifihain-likemicroalloyedsteelwashigherthanitscounterpartsprecipitatesalignedwiththeausteniteboundary,sowithoutNb,bsothersteelsinTable1showthatfewcrackscanbeweresubjectedtostress,andwhenthesurfacetempera-discoveredinsteelGthatdoesnotcontainNb,VandturewaslowerthanthelowductilitytemperaturetheTi,whereassteelscontainingNb,ingly,cracksalongthegrainboundariesafromopticalmicroscopy;bfromscanningelectronmicroscopy3MicromorphologyoftransvercornercracksincontinuouscastslabIronmakingandSteelmaking2010VOL37NO175

oftransvercornercracksinmicroalloyedsteel4Carbidesand/ornitridesprecipitateachain-likelineupalongaustenitegrainboundariesinsteelDandbdisperwithingrainsinsteelA5Prerecipitates,sofproeutectoidferritefilmontransvercornercracksDuringcastingtheslabcornetheslabswerebent,moment,proeutectoidferriteprecipitatedfromtheausteniteboundariesandformedalayer-likeferritefilm,whichisabout20mmobrvedforDriessteelbyproperexemplificationandisconsistentwiththeliterature.12Thecontinuityoftheaustenitematrixwasdisrupted,hestrengthoftheferritewasone-quarterofthatoftheaustenitegrains,stresswouldcongregateontheproeutectoidferritefilmTable2ChemicalcompositionofprecipitatesbyEDSanalysis,wt-%SteelCABCDEFG5.638.132.665.443.737.728.51MnAlTiNbVOFe0.7039.641.864.701.653.2328.71.373.181.221.811.7846.8.1738.597.324.48623610184.681.400.5244.3718.058.591.254.1473.7590.276PrecipitatesdisperdwithinaustenitegrainbySEMandEDS76IronmakingandSteelmaking2010VOL37NO1

oftransvercornercracksinmicroalloyedsteel7Film-likeproeutectoidferriteprecieferritecouldoscillationmarksdeepen,theincidenceofcracksrisnotsupportthestrengthgeneratedbystress,dinthefilmandcracksspreadalongtheHowever,tedtotheincidenceofcracks,accordingtotheRearchresur-increadinlinewiththeincreasingthicknessofthefilm-isonofoscillationmarksonthesamplesindicatesthatlikeferrite(Table1).NbCprecipitatesalongthegrainthemarkdepthsontheloosideandthefixedsidewertion,nocrackswerefoundintoferrite,whichresultsinadecreaincarboncontentontheloosidecornerofthesamples,despitetheirnearthegrainboundaryandtheformationoffilm-likefiult,ckerthefilm-likeferrite,theextendedalongtheoscillationmarks,themarkswerelargerthestrainconcentration,andcracksformeasilyjust,NbandVcanfacilitategrainrefinementandcontributetodispersionhardeningthroughtheirpre-CausoftransvercornercrackscipitationinausteniteandthroughtheirprecipitationinAccordingtotherearchonthemicromorphologyofferriteduringoraftercRatransformationinthetransvercornercracksinslabswithOMandSEM,6,r,verylargeprecipitatescanandthecomparisonwiththeresultsofEDS,thecracksbefoundinmicroalloyedsteelsproducedbythehavethefollowingfeatures:continuouscastingprocess(Fig.4).Whenthetempera-(i)cracksappearedinthetroughoftheoscillationtureofthecornersislowerthanA3whenslabisbent,themarksontheslabsandextendedandcrackedlargeprecipitatesinthefilm-likeproeutectoidferritealongtheausteniteboundariesalongausteniteboundariesbecomethestressconcentra-(ii)noslaginclusioncanbefoundincrackstionsourcesandincreathecrackingsusceptibilityof(iii)theficksandthechain-likecarbon–nitridesAlthoughacharacteristicmicrostructurewasfoundontheloosideofslab(Fig.8),withsubstantialamountsofgranularferriteandnoferritefilm,sofoscillationmarksontransvercornercracksManyrearchers13,14holdtheviewt,theminimiarchresultsareprentedinFig.9.14Asisshown,thetransvercracksldomoccurwhenthedepthoftheoscillationmarksislessthan0?eTable3ChemicalcompositionofprecipitatesbyEDSanalysis,wt-%ElementCOAlSiSCaTiMnVFeContent3.1213.905.813.983.410.892.499.041.1556.218Microstructuremorphologywithoutfilm-likeproeutec-toidferriteIronmakingandSteelmaking2010VOL37NO177

oftransvercornercracksinmicroalloyedsteel9Transvercornercrackrateversusdepthofosceaturesindicatethatthetransvercornercrackswerefterisaverticalbendingmachineinwhichthefourastingstage,heatdeliveryonslabcornersistwo-dimensional,sothecoolingisintensifiedandthetemperatureoftheslabcornersfalldramaticallytotherange15ofthelowductilitytemperatureofsteel(750–900uC).Duetothelowerhotductility,whenbeingbent,thefixed(outer)sideofslabissubjectedtoalargetensilestressand,inadditiontothenotcheffectofthetroughofoscillationmarks,rmore,itisreportedthatthestressatthecorneroftheslabisfargreaterthanthatatthecentre.16Whiletheloo(inner)sideoftheslabalsosuffersstressthroughthestraightgment,thenarrowsurfacecoolinghasbeenhaltedbelowtheverticalgmentstage,creasinglatentheatofsolidificationfromthemoltensteelsolidificationintheslabcentre,thetemperaturesofslabsurfaceandthecornerbothri,whichisfavourabletobeoutofthelowductilitytemperaturerange,htheaboveanalysis,itisbelievedthatthetransvercornercracksofmicroalloyedsteelcontainingNb,VandTifromtheverticalbendingtypeslabcaster,areformedduetothefilm-likeproeutectoidferriteandthecarbon–nitrideofallotofthepreviousaccounts,whentheslabsaredrawnintotheverticalgment,thetemperatureoftheslabcornersbeginstofallfromaround1200to750uC,whichTable4DepthofoscillationmarksSteelABCDEFGDepth,mmLooside0.50..650.650.750.700.700.50Fixedside0.650750.700.800.750.850.6078IronmakingandSteelmaking2010VOL37NO1providesthetemperaturerangeforprecipitatingthecarbon–nitrideofthealloyelementsandthusalargeamos,theprecipitatesactasthenucleatingagentsthatgiveritotheproeutectoidferriteprecipitateanditsfierticalgmentofthecondcoolingzone,thetemperatureofitscornerislowerthanA3temperatureandthemicrostrhefilm-likeproeutectoidferriteobstructsthecontinuityoftheaustenitegrainmatrixandthestrengthofproeutectoidferriteisfarlowerthanthatofaustenite,thelargeprecipitatesinthefilm-likeproeu-tectoidferritealongausteniteboundariesbecomestressconcentrationsourcesduringbending,whichdramati-callyintensifition,thenotchesionsThecausoftheformationoftransvercornercracksonmabsweredrawnintotheverticalgmentofthecaster,two-dimensionalcoolingtookplaceandthetemperatureoftheslabcornerdroppedsharplytoabout750–900uC,sothesurfacecrackmergedatheslabssolidified,withdecreasingtempera-ture,thecarbidesand/ornitridesofNb,VandTiprecipitatedinachain-likewayalongtheaustenitegrainsboundariesandpinnedontotheausteniteboundaries,whichrestrainedtheslipuently,etemperatureoftheslabswaslowerthanthephatransformationtemperature(A3),thefilm-likeproeutectoidferritewiththenitrideand/orcarbideofNb,VandTirvedasanucleatingagentthatcommencedtheprecipitationalongtheaustenitegrainsboundariesandformednetworkproeutectoidferritefilm,hestrengthoftheproeu-tectoidferritefilmisfarlowerthanthatofaustenitegrains,,:‘StudyonsurfacefinecracksofshipbuildingsteelplatecontainingNbandTi’,IronSteel,2002,37,41–,,,chi:‘AformationmechanismoftransvercracksonCCslabsurface’,ISIJInt.,1990,30,310–ombe:‘Effectofoscillation-markformationonthesurfacequalityofcontinuouslycaststeelslabs’,.B,1985,9B,605–chi:‘Crackformationinthecontinuouscastingofsteel’,.B,1977,9B,489–:‘Theinfluenceofcompositiononthehotductilityofstealsandtotheproblemoftransvercracking’,ISIJInt.,1999,39,(9),833–855.

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