161-paper-influence of casting speed variation during contin

1、InfluenceofCastingSpeedVariationduringContinuousCastingonNonMetallicInclusionsinIFSteelSlabsQiaoyingZhang1XinhuaWang1JiongmingZhang1LiZhang2HaiboTang2GuodongXu21UniversityofScienceTechnologyBeijingChina2BaoshanIronSteelC

2、pationLtd.ChinaABSTRACTTheinfluenceofcastingspeedvariationonnonmetallicinclusionsinsurfacelayersofIFsteelslabsduringcontinuouscastingwasinvestigatedwithOPA(iginalPositionStatisticDistributionAnalysis)method.Itwasfoundtha

3、twhenthecastingspeedwasevenlydecreasedfrom1.4mminto0.6mminincreasesofthenonmetallicinclusionsowingtotheincreaseofthemoldpowderentrapmentwereobservedmainlyonthoseslabswhichwerecastatthestartofcastingspeedchange.Whileinexp

4、erimentofincreasingcastingspeedevenlyfrom0.6mminto1.4mminincreasesofnonmetallicinclusionswereobservedmainlyonslabswhicharecastatthetimewhenthecastingspeedwasstoppedtoincreaseafterithadbeenincreasedto1.4mmin.Fslabswhichwe

5、recastduringthecastingspeedevenlyincreasingdecreasingperiodatthetimewhenincreasingdecreasingthecastingspeedatlowcastingspeedlevel(0.6mmin)theinfluenceofcastingspeedchangewasverysmall.Inadditionitwasfoundthatathighercasti

6、ngspeedlevel(1.4mmin)evenslightchangeofcastingspeedcouldresultinremarkableincreaseofthenonmetallicinclusions.Inthestudymathematicsimulationwasappliedwhichalsoindicatedthattheflowoftheliquidsteelismuchmelargelyinfluenceda

7、tthetimewhenthecastingspeedisstartedtodecreasecastingspeedincreasingisstoppedathighercastingspeedlevel.KeyWds:IFsteelcontinuouscastingcastingspeedmoldmoldpowdernonmetallicinclusions1INTRODUCTIONTheinterstitialfreesteel(I

8、Fsteel)iswidelyusedfautomotives.AsthesheetsespeciallythosefexposedpanelsrequiregoodsurfacequalitystrictcontrolofnonmetallicinclusionsespeciallylargesizedinclusionsinIFsteelsisrequested.Inrecentyearssurfacedefectsoftheste

9、elsheetscausedduetoproblemsofsteelmakingsecondaryrefiningetc.hasbeensignificantlydecreasedowingtotheimprovementofthesteelmakingrefiningtechnologiesequipments.Howevertheratioofthesurfacedefectsowingtomoldpowderentrapmentd

10、uringcontinuouscastinghasincreased.[1~4]Moldpowderentrapmentmainlytakesplaceinunsteadycastingperiodse.g.thetimeofcastingstartcastingendladleexchangemoldwidthchangeSENchangeetc.Duringtheunsteadycastingperiodsusuallycastin

11、gspeedVcislargelyvariedwhichdisturbstheflowoftheliquidsteelinthemoldresultsinseverelevelfluctuation.Hencemoldpowdersareeasytobecarriedintotheliquidsteelsomeareentrappedbythesolidifiedshellasthenonmetallicinclusionsinslab

12、s.Castingspeedvariationshouldbeavoidedduringcasting.Howeverinpracticalcontinuouscastingproductionchangeofcastingspeedisstillinevitable.Inmoststeelmakingplantsmethodsof“downgrade”“defectslab”areadoptedi.e.theslabswhichare

13、castduringcastingspeedchangeperiodareusedfproducinglowergradeproductseventakenasscrapedslabs.Whenthismethodissimplyusedsomegoodslabsmaybe“downgrade”scrapedsomedefectslabsmaybeescapedtakenasgoodslabs.Indertoimprovethequal

14、ityIFsteelproductsinBaoSteelinvestigationwascarriedoutontheinfluenceofcastingspeedvariationonnonmetallicinclusionsinsurfacelayersofIFsteelslabs.2EXPERIMENTALTheexperimentswerecarriedoutattheNo.2BOFshopofBaoSteel.Afterdes

15、ulphurizationpretreatmentthehotmetalwasblownina250ttopbottomcombinedblowingfurnacetheliquidsteelafterBOFsteelmakingwasfurtherrefinedinaRHdegasser.TherearetwoslabcastersinNo.2BOFshopwhichmainspecificationsareshowninTable1

16、.Intheexperimentsslabsof1150mmwidth230mmthicknesswereproduced.withoutcastingspeedchange.Itwasfoundthatmostofthenonmetallicinclusionslargerthan10?mexistedinthesubsurfacelayerswhichare0.5~3.5mmfromthesurfacesveryfewnonmeta

17、llicinclusionsweredetectedonthesubsurfacespecimensmethan4.5mmdeepfromtheslabsurfaces.Photo.1showsseverallargesizednonmetallicinclusionsfoundinsubsurfacespecimensbySEMtheirchemicalcompositionsanalyzedbyEDSareshowninTable2

18、.TheselargenonmetallicinclusionsallcontainNa2Oindicatingthattheywereentrappedmoldpowdersthereactionproductsbetweentheinvolvedmoldpowdersnonmetallicinclusionsinliquidsteels.Table2.Chemicalcompositionsofthetypicallargenonm

19、etallicinclusionsanalyzedbySEMEDSNo.Al2O3(wt%)CaO(wt%)SiO2(wt%)K2O(wt%)MgO(wt%)Na2O(wt%)MnO(wt%)S(wt%)a5.4233.6326.040.761.8615.741.2615.29b75.740.386.720.414.315.237.21c19.007.0425.100.7731.4111.435.24d3.5936.2840.844.8

20、610.553.88AsshowninFig.1slabswhichwerecastatlowcastingspeeds(0.6mmin~0.8mmin)containmenonmetallicinclusionsinthesubsurfacelayers.ThisisbecausethatwhencastingspeedwaslowthequantitythevelocityofthesteelfromtheSENinthemoldw

21、eredecreased.Heatsupplytothetopsurfaceoftheliquidsteelwasreducedbecausetheupflowofthesteeltothesurfacebecameweak.Themeltingthefluidityofthemoldfluxwereinfluencedmoldpowderentrapmentbecameeasytooccur.HoweverasshowninFig.1

22、whencastingspeedwasincreasedto1.4mmincontentsofnonmetallicinclusionsinslabsubsurfacelayerstendedtoincrease.Inthisexperimentslabscastat1.0mmin1.2mmincastingspeedcontainedtheminimumamountofnonmetallicinclusionsinsubsurface

23、layers.Fig.2showsthecontentsofnonmetallicinclusionsinsurfacelayerspecimensofslabswhichwerecastintheexperimentofdecreasingcastingspeed.ThecontentofthenonmetallicinclusionsateachVcistheaverageresultofOPAmeasurementonthesub

24、surfacespecimenswhichwere0.5mm1.5mm2.5mm3.5mmfromtheslabsurface.Alsothecastingspeedchangeisillustratedinthefigure.BefedecreasingthecastingspeedVcwasstabilizedat1.4mminthecontentofthenonmetallicinclusionsoftheslabscastat1

25、.4mminwasabout0.0019%.AsshowninFig.2whendecreasingcastingspeedwasstartedthecastingspeedwasdecreasedfrom1.4mminto1.2mminthecontentofthenonmetallicinclusionsincreasedremarkablyto0.0022%.Howeverinthelatercastingspeeddecreas

26、ingperiodwhenVcwasevenlydecreasedfurtherto1.0mmin0.8mmincontentsofthenonmetallicinclusionsweredecreasedto0.00132%~0.00137%lessthanthecontentoftheslabscastat1.4mmin.MeoverwhenthecastingspeedchangingwasstoppedafterVcwasdec

27、reasedto0.6mminonlyaveryslightincreaseofthenonmetallicinclusionswasfound.TheexperimentalresultshownbyFig.2isverymeaningfulwhichindicatesthatifthecastingspeedisdecreasedfromrelativelyhigherspeedlevel(e.g.1.4mmin)theincrea

28、seofthenonmetallicinclusionsduetothemoldpowderFigure2.ContentsofnonmetallicinclusionsinsubsurfacespecimensofslabscastincastingspeeddecreasingexperimentPhoto1.Photographsoftypicallargesizednonmetallicinclusionsinsubsurfac