3.4-粗轧机工作辊的使用

1©2009InnseCilindri–AllRightsReservedFLATROLLINGROUGHINGSTANDSWORKROLLAPPLICATIONS热轧板带轧机粗轧工作辊的使用Nov.200909年11月2©2009InnseCilindri–AllRightsReservedMAIMAIMS主要目标:incrementofmillproductivitythrough:通过以下方法增加产量-incrementoftripduration-增加轧制周期-reductionofrollchanges-减少换辊次数MAINPROBLEMS:主要问题：-wearresistance-耐磨性-barsurfacequality–辊面质量-slippagetendency-打滑趋势-firecrackresistance-抗热裂纹性EVOLUTIONOFROLLQUALITY:轧辊质量的评估CarbonSteelrolling:碳钢轧制：HiCrSteel高铬钢semiHSS(HSS)半高速钢（高速钢）StainlessSteelrolling:不锈钢轧制HiCrSteel高铬钢HSS高速钢REMARKS:备注Thechangeofrollgradeintowelldevelopedgradepermitstogetbetterresultsasfarwearresistance,butitisnecessarytopayattentioninusing,toavoidrisksofspallingsconnectedwithhighervaluesofresidualcompressivestressesoftheshellusingmorealloyedgrades轧辊材质的改进提高了轧辊的耐磨性，但我们在使用中必须注意避免剥落的风险,这是因为工作层中添加更多的合金材料使得残余压应力变大。ROUGHINGMILLWORKROLLS粗轧工作辊3©2009InnseCilindri–AllRightsReservedFig.5newback-upandworkrollsFig.5Fig.6wornworkrollsandnewback-uprollsFig.5wornback-upandworkrollsINCREMENTOFTRIPDURATIONisthemainaimforworkrollsofroughingstands.Inthelastyearstherewasacontinuousincrementrollingcampaigns.Presentlythetargetistoreachandtoexceed50.000tonsinCarbonSteelrollingwith5/7passesinreversingstands.Ofcourse,theaimsofincrementofrollingcampaignsmustbepursuedpayingstrongattentiontoavoidrollingaccidents(inparticularrisksofspallingofbarreledges).Infact,excessivevaluesofworkrollwear,togetherwiththeback-uprollwear,couldcausetoohighvaluesofstressesinthezonesclosedtothebarreledges,asshownhereunder.TheformationofdogboneprofileoftheslabbytheEdgercanincreasethesevaluesofstressincorrespondenceofthestripedgesBeingthebondingareasubjectedtohighfatigue,itisopportunetomake,mainlyinthelastpartoftherolllife(after50÷70%ofshellconsumption),aftereachtrip,UTtestsofthebondingarea,takingcareinparticularoftheareaofrollingedge.4©2009InnseCilindri–AllRightsReservedFig.5newback-upandworkrollsFig.5Fig.6wornworkrollsandnewback-uprollsFig.5wornback-upandworkrolls提高轧制周期是粗轧机架工作辊的主要目标。在过去几年,轧制周期不断提高。近年来，在可逆粗轧机5-7道次的碳钢轧制中，轧制周期已经达到和超过50000吨。当然，对于增加轧制周期的追求需建立在注意避免轧制事故的基础上（特别是注意避免辊身边部剥落的风险）。事实上，工作辊和支撑辊的过度磨损会造成辊身边部区域应力过大，如右图所示。立辊轧制形成的板坯狗骨头形状会增加板带边部轧制时的应力。结合层区域因此处于高疲劳，在轧辊寿命的最后阶段（工作层消耗了50-70%后），每个换辊周期之后,都要用超声波检测结合层，特别是检查位于轧制区域边部的结合层。5©2009InnseCilindri–AllRightsReservedThesetestscanpermittotakeundercontroltheseareasasfarthepossibleformationofmisbondingsduetofatiguefortheheavyrollingconditions.Tolimittheproblemofmisbondingduringtheuse(normallymorefrequentinsecondpartofrolllife)rollsarenormallycastwithincreasedshellthicknessatscrapdiameterupto25mm.SHELLTHICKNESSTheincrementofshellthickness,mainlyfornewhighalloyedgrades(semiHSSandHSS),shouldbeanywaylimitedinordertoavoidthecreationofexcessivevaluesofresidualstressesinthecore.Forinstance,HSSrollswithdiameter1200mmshouldhaveamaximumtotalshellthicknessof145mminthepartwithlowerthickness.Infactfortheserolls(duetotheverticalcentrifugation)itispossibletohaveadifferenceinthicknessthroughthebarrellengthof15÷20mm,andconsequentlyashellthicknessof165mminthebottompartofthecasting:thisvalueisconsideredasthelimit.6©2009InnseCilindri–AllRightsReserved这些检测可控制在这些区域由于轧制周期过长产生的疲劳而可能引起的结合不良。为了避免在使用过程中产生结合不良（通常更频繁地出现在轧辊寿命的后半部分），通常轧辊浇铸时要增加工作层的厚度,其增加的直径厚度超出报废直径可达25毫米。工作层厚度为了避免芯部残余应力过大，主要是在新的高合金材质（半高速钢和高速钢）上，工作层厚度的增加一定要受到限制。比方说，直径1200mm的高速钢轧辊在厚度较薄的地方工作层厚度不能超过145mm。实际上，这些轧辊（由于立式离心铸造的缘故）辊身有可能存在15-20mm的厚度差，因而轧辊铸造底部的厚度会达到165mm：这是工作层厚度的上限。7©2009InnseCilindri–AllRightsReservedWEARPROFILEInthewearprofilesitisnotablebiggerwear(+20÷50%)ofbottomrollcomparedwithtoproll;thereforethewearcontrolshouldbemadeonthebottomroll:usuallyforthisrollwearof1,5mmondiameterisnotexceeded.Itisrecommendedanywaytodonotexceed1÷1,2mm.Furtherdressingmustremovetotallyanycrackpresentonthebarrel(crackscouldoriginatespallings).TeststhroughEddyCurrents(andeventuallythroughUTtoo)permittoverifythepresenceofcracks.Incaseofsurfacefreefromdefects,aftereliminationofwear,normallyafurtherdressingof0,5÷1mmondiameterpermittoobtainasuitablesurfaceoftherollfornextuse.Theaimofincrementoftripdurationmustbepursuedanywayforeachspecificmill,foreachspecifickindofroll,andforeachkindofrolledsteel.Remarks:onthebasisofourexperience,itispossibletoavoidthecompleteremovaloffirecrackingnetworkifitisuniformdistributedonthebarrelsurfaceandifitisverysmallinsize.8©2009InnseCilindri–AllRightsReserved磨损曲线从磨损曲线可以看出，下辊比上辊的磨损要大得多（20-50%）；因此需对下辊进行磨损控制：通常，在直径方向的磨损不应超过1.5mm。建议磨损值不要超过1-1.2mm。辊面上任何存在的裂纹都必须修磨干净（裂纹会引起剥落）。涡流检测（配合超声波探伤检测）可确定裂纹的存在。假如辊面没有缺陷，在去除磨损后，在直径方向上再多修磨0.5-1mm可达到良好的辊面效果，为下次上机使用。增加轧制周期的目标必须根据每一特定的轧机、每种特定的轧辊材质以及每种轧材情况来完成。备注：根据我们的经验，如果辊面上的网状热裂纹分布均匀且尺寸很小就不需要完全将其修磨掉。9©2009InnseCilindri–AllRightsReservedSTARTINGOFROLLING(tolimittheproblemsofslippage)Itshouldpermittotherolltoreachathermalconditionsuitableforrolling,limitingalsotherisksofslippingThermalconditionofrollshouldbereachedslowly,toavoidthermalstresses,through:-Reductionofabsorbedpowerandspeed,increasingthenumberofpassesfrom5to7(thisprocedurecanlastseveralhours).-Maintainingthenumberofpasses,butstartingrollingwithsteelgradesthatrequiredlowerrollingstrengthTheses