外文翻译宝钢碱性氧气转炉炼钢生产及洁净钢控制.docx

外文翻译宝钢碱性氧气转炉炼钢生产及洁净钢控制.docx

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外文翻译宝钢碱性氧气转炉炼钢生产及洁净钢控制

Basicoxygenfurnacebasedsteel-makingprocessesandcleanlinesscontrolatBaosteel

L.Zhang*1,J.Zhi2,F.Mei2,L.Zhu2,X.Jiang2,J.Shen2,J.Cui2,K.Cai3andB.G.Thomas4

Opticalmicroscopy,totaloxygenmeasurementsandslimetestshavebeenconductedtoquantifythesizedistributionandamountofinclusionsatvariousprocessingstepsduringbasicoxygenfurnace（BOF）basedsteelproductionatBaosteel.Theeffectsonsteelcleanlinessofspecificoperationalimprovementsduringsteelrefiningandcontinuouscastinghavebeeninvestigated.SuchimprovementstotheseprocessesandtheresultinglevelofsteelcleanlinessatBaosteelaresummarisedinthepresentpaper.LadleslagreductionlowersFeO+MnOintheslagtobelow5%,decreasingsteelreoxidationbytheslag.CalciumtreatmentbyCaSiwireinjectionduringladlefurnace（LF）refiningisusedtomodifyinclusions.Slagdetectionisemployedattheladlebottomduringcontinuouscasting.Flowcontroldevices,CaOcontainingfiltersandhighCaObasedbasicpowderwithCaO/Si02>4areusedinthetundishtoremovemoreinclusions.WiththisBOFbasedsteelmakingprocess,impuritylevelscanbecontrolledtoachieve-totaloxygen（TO）<16ppm,[S]<5ppm,[P]<35ppm,[N]<29ppm,[H]<1ppminlinepipesteels,and[C]<16ppm,TO<19ppm,[N]<15ppmininterstitialfree（IF）steels.

Keywords:

Cleansteel,Inclusions,Impurityelements,Interstitialfreesteel,Linepipesteel

Introduction

Theimportanceofcleansteelinterrasofproductqualityisincreasinglybeingrecognised.Cleansteelrequirescontrolofthesizedistribution,morphologyandcompositionofnon-metallicoxideinclusionsinadditiontotheamount.Furthermore,sulphur,phosphorus,hydrogen,nitrogenandevencarbon[1,2]shouldalsobecontrolledtoimprovethesteelproperties.Forexample,,formability,ductilityandfatiguestrengthworsenwithincreasingsulphideandoxideinclusioncontents.Loweringthecarbonandnitrogenenhancesstrainagingandincreasesductilityandtoughness.Hardenabilityandresistancetotemperembrittlementcanbeenhancedbyreducingphosphorus.Thedefinitionof'cleansteel'varieswiththesteelgradeanditsenduse.Forexample,interstitialfree（IF）steelrequiresbothcarbonandnitrogentobe<30ppm;linepipesteelrequiressulphur,phosphorusandtotaloxygen（TO）alltobe<30ppm，lowhydrogen,lownitrogenandsuitableCa/Sandbearingsteelrequiresthetotaloxygentobelessthan10ppm.[3]Inaddition,manyapplicationsrestrictthemaximumsizeofinclusions[3,4],sothesizedistributionofinclusionsisalsoimportant.

BaoshanIron&SteelCo.,Ltd（Baosteel）iscurrentlythelargeststeelcompanyinChina.Itsannualsteelproductionwas115milliontonnesin2003,119milliontonnesin2004and14.0milliontonnesin2005.Withregardtothebasicoxygenfurnace（BOF）basedsteelmakingroute,therearethree300tandtwo250tBOFs;severalsteelrefiningunits,includingoneCAS-OBunit（controlledargonstirring-oxygenblow）,twoRH（Ruhrstahl-Heraeus）degassersandoneladlefurnace（LF）.Since1990,effortstoimprovesteelcleanlinesshavefocusedondevelopingsteelmakingpracticestolowerTO,N,S,P,HandClevelstoachievelowcarbonaluminiumkilled（LCAK）steel.ForLCAKsteelandIFsteel,theproductionprocessisBOF→RH→continuouscasting（CC）,andforlinepipesteel,theprocessisBOF→RH→LF→CC.

Experimentalmethodandexaminationofinclusionsinsteel

Experimentalmethods

Ladlesteelsamplesweretaken500-600mmbelowthetopslagintheladle,tundishsteelsamplesfrom300mmaboveitsoutletandmouldsteelsamplesfrom150mmbelowthemeniscusand300mmawayfromthesubmergedentrynozzle（SEN）outports.Thesamplerwasacylindricalsteelcupwithaconeshapedcoppercovertoprotectitfromslagentrainmentduringimmersion.Attachedtoalongbar,thesamplerwasimmerseddeepintothemoltensteel,wherethecoppermeltedandthecupwasfilled.Smallsteelsamples,80mminlengthand30mmindiameter,weremachinedinto5（dia.）x5mmcylindersforTOandnitrogenanalysis,and20（dia.）×15mmcylindersformicroscopeexamination.Thesteelpowdersresultingfrommachiningwereusedforanalysisofthecarbon,phosphorusandsulphurcontents.LargeSteelsamplesfromtheladleandtundish,200mminlengthand80mmindiameter,weremachinedinto60（dia.）×150mmcylinders;asshowninFig.1.TOandnitrogenmeasurement.Analysisincludedthechemicalcompositionofslagandsteelsamples,microscopeexaminationofmicroinclusions,slimeextractionofmacroinclusionsandSEManalysisofthemorphologyandcompositionofinclusions.

Fig.1Samplinglocationsforcontinuouslycastslab:

TOtotaloxygen

Inthepresentwork,'macroinclusions'werethosegreaterthan50umindiameter.Mostoftheseweredetectedintheresiduesextractedbyelectrolyticisolation（'slimetest'）fromthelargersteelsamples.The'microinclusions'dataderivefrommicroscopicassessmentscarriedoutonplanarsections,mostofwhichweresmallerthan~50μm

Morphologyandcompositionoftypicalinclusions

Themorphology,compositionandlikelysourcesoftypicalinclusionsfoundinLCAKsteelsamplesformtheladle,tundishandmoundareshowninFigs.2and3respectively.Themorphologiesincluded:

（a）angularaluminate（Fig.2dandfandFig.3b）;（b）aluminacluster（Fig.2bandc）;and（c）sphericalsilicate（Fig.2aandcandFig.3a）.

a.ladle;b.tundish;c,d.mound;e,f.slab

Fig.2Typicalinclusionsfromgivensamplesexaminedbymicroscope

（a）tundish（b）slab

Fig.3Typicalinclusionsfromgivensamplesextractedusingslimemethod

Thepossiblesourcesweredeoxidationproducts,reoxidationproductsorbrokenrefractoryliningbricks.Inlinepipesteel,besidesthesecommoninclusions,manynanoscaleTiNinclusionswerefoundalonggrainboundaries.ThesenanoTiNchangedfromsquaretoellipsoidifcombinedwithTi2O3,asshowninFig.4[5]

a.compoundinclusionswithcompositionTi2O3+MnS;b.TiNinclusion

Fig.4Nanoprecipitatesinlinepipesteel

Totaloxygenmeasurementisanindirectmethodofevaluatingoxideinclusionsinasteel.3Thetotaloxygen（TO）inthesteelisthesumofthefreeoxygen（dissolvedoxygen）andtheoxygencombinedasnon-metallicinclusions.Freeoxygen,or'active'oxygen,canbemeasuredrelativelyreadilyusingoxygensensors.Itiscontrolledmainlybyequilibriumthermodynamicswithdeoxidationelements,suchasaluminium.If[%A1]=0.03-0-06,thefreeoxygenis3-5ppmat1600°C.Becausethefreeoxygendoesnotvarymuch,thetotaloxygenisareasonableindirectmeasureofthetotalamountofoxideinclusionsinthesteel.OwingtothesmallpopulationoflargeinclusionsinasteelandthesmallsamplesizeforTOmeasurement（normally<20g）,itisraretofindalargeinclusioninasample.Evenifasamplecontainsalargeinclusion,itisprobablydiscountedbecauseoftheanomaloushighreading.Thus,theTOcontentactuallyrepresentsthelevelof<50umsmalloxideinclusionsonly.ThecurrentTOinIFandlinepipesteelslabsatBaosteelis<16ppm.TheTOintheladle,tundish,mouldandslabintwotypicalsequencesofLCAKsteelisshowninFig.5,indicatingthattheTOdecreasedfromtheladletothetundish,tothemouldandtothecontinuouslycastslab.

Fig.5Totaloxygeninsteelfromladletoslab

Ladleoperationstoremovemoreinclusions

Ladleslagreductiontreatment

ReoxidationtoformaluminaintheladleduringsteelrefiningismainlycausedbySi02intheslagandliningrefractory,andMnOandFeOintheladleslag,bythefollowingreactions:

3/2（Si02）+2[Al]=（Al203）+3/2[Si]

3（MnO）+2[Al]s=（Al203）+3[Mn]

3（FeO）+2[Al]s=（A1203）+3[Fe]

SlagreductiontreatmentiscarriedoutbyaddingaluminiumandlimeontothetopoftheladleslagtoreduceitsFeOandMnOcontent.TheeffectofladleslagreductiontreatmentontheTOcontentinthesteelisshowninFig.6.AlargerFeO+MnOcontentintheladleslagcorrespondstohighertotaloxygen.Withtheslagreductiontreatment,MnOandFeOintheladleslagwerereducedto<5%,correspondingto<20ppmTOinthetundish.

Fig.6EffectofFeOandMnOcontentinladleslagonTOinsteel

Calciumtreatment

Nozzleclogginginducesseriouscastabilityproblemsinaluminiumkilledsteels,suchasloweringthecastingspeed,inducingasymmetricalfluidflowandlevelfluctuationsinthemould,thusentrappingmoreinclusions,andsometimescausingabreakout.Removingmoreinclusionsbeforecontinuouscastingisthebestwaytopreventnozzleclogging,andistheonlyapproachforsteelswithverystrictrequirementsonformability[6].

AtBaosteel,CaSiwireisfedintothemoltensteelduringladlerefining.AluminareactswithCaO,formingcalciumaluminates.Tfthegeneratedcalciumaluminateshavealowmeltingpoint,thencloggingisimproved.ThepossiblecompoundinclusionsgeneratedbycalciumtreatmentincludeCA6,CA2,CA,Ci2A7andC3A,whereCandArepresentCaOandA1203,respectively.Thefirsttwoshouldbeavoidedowingtotheirhighmeltingpointover1700°C.

CurrentpracticeatBaosteelindicatesthat[Ca]shouldbe>25ppminordertopreventsolidaluminabasedinclusionclogs.ToomuchcalciumcanalsogenerateCaSwithahighmeltingpoint（2450°C）.ToomuchsulphurinthesteelandtoolowatemperaturealsoenablesCaSgeneration.Baosteelpracticeindicatesthat<50ppm[Ca]inthesteelcanpreventCaSgeneration,and[Ca]/[Al]>0.09favourspreventionofnozzleclogging（Fig.17）.Hence,[Ca]needstobecontrolledwithintherange25-50ppm,and[Ca]/[Al]>0-09,toavoidnozzlecloggingproblems.

Controlofnitrogen,carbon,sulphurandphosphorusi