包晶钢中裂纹的研究

武汉科技大学本科毕业设计I摘要近年来，包晶钢的应用越来越广泛。然而，表面裂纹是包晶钢的主要缺陷之一，对其表面质量影响很大，严重影响产品的合格率。因此，观察裂纹形态及组织并分析其产生原因是非常有意义的。本文的实验材料为15CrMo包晶钢连铸坯，剥皮2mm，无探伤裂纹，然后进行连轧。选取连轧10道次后的直径为100mm棒材，编号为1#；选取12道次后的直径为75mm的棒材，编号为2#。本文采用金相检验，扫描电子显微镜和化学成分分析等方法对棒材的表面裂纹进行了观察分析，了解了表面裂纹的形态及组织特征，并根据实验结果对裂纹形成原因进行了分析。结果表明，1#试样裂纹内C、O含量高，Ca、Si和Al的氧化物夹渣较多，还有少量Mg的化合物，而且裂纹周围有大量的Si和MnS等夹杂物；2#试样裂纹内C含量较高，主要为Al、Si和Ca的氧化物夹渣富集，并且有大量Mg的化合物，裂纹内还有少量的As、Cu、Ni等元素。裂纹周围有Si夹渣，大量的Si、Ca及Al的氧化物等夹杂物和MnS夹杂。结合实验结果分析可知：1#试样表面裂纹产生的主要原因是由于浇注过程中结晶器保护渣的流动性恶化，结晶器内钢水液面波动大，钢水的流动不合理，浸入式水口插入结晶器深度不够等因素使保护渣卷入到坯壳中形成Al和Si氧化物夹渣富集于钢坯表层下面。夹渣形成以后，在连轧过程中，夹杂物逐渐露出钢材表面，在应力作用下沿夹渣富集处形成裂纹。2#试样表面裂纹周围存在脱碳现象，说明表面裂纹主要是在均热炉中均热时因为温度不均匀导致局部过热而产生的，并且在随后的轧制过程中不断扩展。针对表面裂纹的产生原因，提出了以下减少表面裂纹的措施：选用合适的保护渣、采用电磁制动技术、控制钢水的过热度、选择大容量的中间包等。关键字：表面裂纹；结晶器；保护渣；夹杂物………………………………………….000000000000000000000000000000000000000000武汉科技大学本科毕业设计IIAbstrctInrecentyears,applicationofperitecticsteelhasbeenmoreextensive.However,surfacecrackisoneofthemajordefect,whichhasgreatimpactonthequalityofthesurfaceandseriouslyaffectsthepassingrateofproductsseriously.Soit’sverymeaningfultoobservethemorphologiesandthemirco-structureofthesurfacecracksandinvestigateincausesofthesurfacecracks.Thetestmaterialsaretwoperitecticsteelbars(15CrMo),whichareafterrollinginthispaper,bothofthelengthare250mm,diametersare100mmand75mm.Theformerisnumbered1#andthelatterisnumbered2#.Thepresentpaperhasanalyzedthesurfacecracksofthebarsbymircroscopictest,electiondiffractionspectrometer(EDS)andchemicalcompositionanalysisoftheinclusions,themorphologiesandthemirco-structurecharactersofthesurfacecrackshavebeenknown.Accordingtotheresultsofcracks,thecausesofthesurfacecracksareanalyzed.TheresultsshowsthattherearehighcontentofC,O;moreoxideinclusionsofCa,SiandAlandasmallamountofMgcompoundsinthecrackofthefirstsample(1#).Besides,aroundthecracktherearealargenumberofotherinclusionssuchasSiandMnS.TherearehighcontentofCinthecrackofthesecondsample(2#).Furthermore,oxideslagsofAl,SiandCaenrichinthecrackandalargenumberofMgcompoundsinthecrack,asmallnumberofAs,Cu,Niandotherelementsarealsofoundinthecrack.ThereareinclusionofSi,alargenumbermoreoxideslagsofCa,SiandinclusionsofAlandMnSaroundthecrack.Combiningtheexperimentalresults,thepapershowsthatthecausesofthesurfacecracksofthefirstsample(1#)isthatthemoldwallisinvolvedintheblankshell,formsoxideslagsofAl,SiandCaandenrichbelowthesurfacebilletbecauseofsomefactors,whicharethedeteriorationofthemobilityofmoldpowderintheprocessofcasting,thelargefluctuationsofthesurfaceofmoltensteelinmoldflux,theunreasonableflowofmoltensteel,theinappropriatedepthofsubmergednozzleetc.Aftertheformationofslag,inclusionsaregraduallyexposedinthesurfaceofsteelduringtheprocessofrolling,andformcracksalong武汉科技大学本科毕业设计IIItheslagenrichmentunderthestress.Thedecarbonizationphenomenonisfoundaroundthesurfacecrackofthesecondsample(2#).Thisshowsthatsurfacecracksaremainlyformedwhenthebarsareheateduniformlyinuniform-heatingfurnace.Forthecausesofsurfacecracks,somemeasuresareproposedasthefollowingsoastoreducethesurfacecracks:selecttheappropriatemoldflux,usethetechnologyofelectromagneticbraking,controltheoverheatingofmoltensteelandselectthelargetankmiddle-capacityetc.Keywords：Surfacecrack；Moldwall；Moldflux；Inclusions武汉科技大学本科毕业设计IV目录1文献综述..............................................................................................................................11.1本课题的主要研究意义及研究内容........................................................................11.1.1本课题的研究意义..........................................................................................11.1.2本课题的研究内容..........................................................................................11.2包晶钢的简介............................................................................................................11.2.1包晶钢的定义..................................................................................................11.2.2包晶钢的凝固特点..........................................................................................21.3棒材缺陷的研究现状................................................................................................31.3.1棒材缺陷的种类..............................................................................................31.3.2棒材缺陷形成的根源......................................................................................31.4包晶钢棒材表面裂纹的研究....................................................................................41.4.1表面裂纹的形成机理......................................................................................41.4.2表面裂纹的影响因素......................................................................................41.4.3减少铸坯表面微裂纹的措施..........................................................................82实验研究方法......................................................................................................................92.1实验材料....................................................................................................................92.2实验设备........................................................................