淬火_碳分配_回火Nb_Cu复合中碳钢的组织与性能研究-硕

WuhanUniversityofScienceandTechnologyIAbstractTheindustryofironandsteelistheeconomicfoundationofanation,andatthesametimeasaneconomicdevelopmentengine,thesteelindustryisalsooneofthemainCO2producers.Till2010,thesteelyieldallovertheworldhasreachuptoabout1.3billiontons.Inordertoconserveenergyandreduceenvironmentalpollution,itisurgenttodevelophigh-strengthsteeltoachievetheaimofdevicelightweight.Quenching-Partitioning-Tempering(Q-P-T)ProcessproposedbyXuZuyao(T.Y.Hsu)isanovelprocesstoobtainhighstrengthsteel.Inthispaper,inordertoevaluatetheeffectofchemicalcompositiononmicrostructuresandhardnessofquenching-partitioning-temperingsteel,Nb/Cu-free,Nb-bearingandNb/Cu-bearingmediumcarbonsteelstreatedbyquenching-partitioning-temperingprocesswereinvestigated.Inordertoevaluatetheeffectofquenchingtemperatureonmicrostructuresandhardness,differentquenchingtemperatureforNb/Cu-bearingsteelwereinvestigated.Themicrostructuralobservationsandhardnesswereanalyzedbyopticalmicroscope,scanningelectronmicroscopy,transmissionelectronmicroscope,X-raydiffractionandhardnesstest.Theresultsareasfollowing:(1)Theultrafine-grainedmicrostructuresofthethreekindsofsteelbyquenching-partitioning-temperingprocessaremainlymartensiteandretainedaustenite.Whenthepartitioning-temperingtimeisenough,wecanfindveryfinecarbidesprecipitationinthemartensitematrix.Andwiththeincreasingpartitioning-temperingtime,themartensitelathwillbecomecoarse,thevolumefractionandcarboncontentofretainedaustenitewillchange.TheadditionofNbandCucancontributetothevolumefractionofretainedaustenite,andthesteelwithadditionofNbandCuhasmorevolumefractionofretainedaustenitethanthatoftheNb/Cu-freesteel.(2)TheadditionofNbandCuisresponsiblefornotonlythemorevolumefractionofretainedaustenitebutalsotheprecipitationstrengthening.WiththeadditionofcarbideformerelementofNbandCu,theprecipitationstrengtheningeffectisobviousforthequenching-partitioning-temperingtreatedsteel.ThefineprecipitateswereidentifiedtobeNb(C,N)andCuparticles,respectively.Whenthetemperingtemperatureis450°C,thepeakvalueofhardness(450HV1)occurredatthepartitioning-temperingtimeof1800sforNb-bearingsteel.TheadditionofCucancontributetotheprecipitationofNb,andtherebyitplaysanimportantroleintheprecipitationstrengthening.AndforNb/Cu-bearingsteel,therearetwopeakvaluehardnessoccurredatthepartitioning-temperingtimeof120sand1800s.(3)Withtheincreasingquenchingtemperature,thevolumefractionofretainedaustenitewillincreaseandthehardnesswilldecrease.WuhanUniversityofScienceandTechnologyIIKeywords:Microstructure,Quenching-Partitioning-TemperingProcess,Nb-Bearing,Cu-Bearing,CarbidePrecipitationWuhanUniversityofScienceandTechnologyIIIContentAbstract............................................................................................................................................IContent..........................................................................................................................................IIIChapter1Preface............................................................................................................................11.1Introduction.......................................................................................................................11.2Thedevelopmentofhigh-strengthsteels..........................................................................21.3Retainedausteniteinsteels...............................................................................................31.4Quenching-partitioning-tempering(Q-P-T)process.........................................................41.4.1IntroductionofQ-P-Tprocess................................................................................41.4.2Heattreatmentandmicrostructure.........................................................................51.4.3Thermodynamicsandkinetics................................................................................61.5Aimofthepresentwork....................................................................................................8Chapter2ThePrinciplesofDesign..............................................................................................102.1Thedesignofmicrostructure...........................................................................................102.2Thedesignofcomposition..............................................................................................102.2.1Thedesignofcarbon(C)......................................................................................112.2.2Thedesignofsilicon(Si).....................................................................................112.2.3Thedesignofmanganese(Mn)............................................................................112.2.4Thedesignofotheralloyingelements.................................................................112.3Thedesignofheattreatment...........................................................................................12Chapter3ExperimentalProcedures................................