紧固件被誉为“工业之米”，是应用量最广泛的机械基础件，其性能直接关系到装备的可靠性与安全性。紧固件钢作为用量最大的特殊钢品种，在高端装备制造业中承担着关键角色。在“双碳”目标与高端装备高质量发展的新形势下，紧固件钢的研发正朝着高强化、绿色化和耐热化三大方向演进。本文聚焦这一趋势，探讨高强、绿色及耐热紧固件钢的材料设计、工艺创新与工程应用。
1高强化紧固件钢
高强化是紧固件材料发展的主要趋势。8.8-12.9级紧固件通常利用马氏体相变（如碳素钢、低合金钢、合金钢）、固溶析出（如高温合金）或冷作强化（微合金钢）获得相应的强度和塑性匹配。装备轻量化要求紧固件等级达到13.9级及以上强度，需要解决伴随强度提高带来的疲劳和延迟断裂问题，惠卫军、董瀚等人在42CrMo钢的基础上，通过Nb、V微合金化导入氢陷阱，研制出具有抗疲劳和延迟断裂特性的14.9级紧固件用ADF1超高强钢。德国卡迈锡公司通过等温热处理工艺控制获得贝氏体组织，研制出延迟断裂性能优异的14.8U-17.8U级紧固件。
8.8-12.9级紧固件材料性能的稳定性是共性技术要求。上海大学及上海大学（浙江）高端装备基础件材料研究院高性能钢铁材料团队提出了纯净度、均匀度及组织度控制的理念。纯净度控制包括杂质含量及夹杂物等级；均匀度控制包括带状组织等级、元素偏析以及关键化学元素的过程控制能力；组织度包括晶粒尺寸、组织类型、析出相和微观组织精细结构等。通过冶炼工艺优化，控制SCM435钢的S、P、O、N、H等杂质含量不大于150ppm，夹杂物等级Σ（A+B+C+D）≤3；C、Cr、Mo波动±0.01%，过程能力控制（Cpk）大于1.67，带状组织不大于3级。通过以上方法，制备的紧固件硬度散差小于2HRC，晶粒度等级≥8，在107循环载荷条件下的疲劳极限σA50达到94MPa，满足汽车发动机关键紧固件材料服役要求。
此外，借助M2C与MC碳化物的二次硬化效应，并引入额外氢陷阱，团队还研发出抗氢延迟断裂性能优异的14.8-19.8级紧固件钢。关键的组织度控制单元为M2C和MC碳化物的形态、尺寸和数量，研究了C、Cr、Mo、V合金体系碳化物的合理配比，优化了强度、塑性和韧性的匹配，通过导入氢陷阱显著提高抗延迟断裂性能，如图1所示。
图18.8-19.8系列紧固件耐延迟断裂和典型力学性能
2绿色化紧固件用非调质钢
绿色化紧固件用非调质钢可替代传统调质钢，省略球化退火、调质等工序，减少30%以上能耗。为此，团队开发了8.8-10.9级紧固件用冷作强化非调质钢。主要通过冷作强化提高材料的强度，组织类型和晶粒尺寸是关键的组织度控制单元。非调质钢抗拉强度（Rm）的来源主要为：
（1）式中，为热轧盘条的强度；为冷变形产生的强度增量；为时效处理后的强度增量。
针对8.8-10.9级紧固件性能，利用Nb、V微合金化和控轧控冷工艺控制获得平均晶粒尺寸不大于10μm的铁素体和珠光体组织（如图2），结合相匹配的冷变形和时效处理工艺达到目标性能。
图2紧固件用非调质钢微观组织及强度随不同减面率的变化
3耐热紧固件钢
耐热紧固件钢一般应用于高温工作服役区域，团队开发了耐300-750℃的耐热紧固件钢。以耐高温紧固件用A286合金为例，要求满足650℃持久性能，微观组织是关键控制因素。利用Cr、Ni、V、Ti、Al、Mo、B多形合金化的组织度控制技术开发了A286合金线材，实现固溶强化、沉淀强化[Ni3（Al，Ti）]和晶界碳化物强化效果。按照ASTME292-18进行650℃持久性能测试，结果表明，开发的A286合金线材满足性能要求，如图3所示。
图3A286固溶时效处理后的微观组织及650℃条件下的持久性能
4未来发展方向
紧固件钢的未来发展方向，是要满足现代工业对于更高强度、更长使用寿命、更可靠安全、更轻量化以及更环保的要求。与此同时，还需兼顾航空、深空、深海、深地、极地开发等极端条件下，紧固件用钢在耐蚀、耐高低温等方面的综合性能需求。
AdvancedFastenerSteels:CurrentandComing
Knownasthe"grainofindustry,"fastenersarethemostwidelyusedmechanicalfoundationalcomponents,whoseperformancedirectlydeterminesthereliabilityandsafetyofequipment.Asthemostconsumedspecialsteelvariety,fastenersteelplaysacriticalroleinhigh-endequipmentmanufacturing.Underthenewcontextofdualcarbongoalsandhigh-qualitydevelopmentofadvancedequipment,theresearchanddevelopmentoffastenersteelisadvancingtowardthreekeydirections:high-strengthening,greenmanufacturing,andheatresistance.Thisarticlefocusesonthesetrends,exploringthematerialdesign,processinnovation,andengineeringapplicationsofhigh-strength,green,andheat-resistantfastenersteels.
1High-StrengthFastenerSteels
Highstrengthhaslongbeenadevelopmenttrendinfasteners.Forfastenersofpropertyclasses8.8-12.9,thefatigueproperty,corrosionresistance,anddurabilitydependonthecombinationofstrengthandductility,associatedwiththemartensitictransformationthatoccursinmildsteelandlow/mediumalloyedsteel.Equipmentlight-weightingentailsthatthestrengthoffastenersneedstobefurtherimproved,i.e.propertyclasses≥13.9,nevertheless,thedecreasedfatigueresistanceanddelayedfractureperformance.Grade14.9fastenerwithexcellentresistancetofatigueanddelayedfracturehasbeendevelopedbyDongHanetal,whichismadeof42CrMosteelmicroalloyedwithNbandV.Kamexie(Germany)developed14.8U-17.8Ugradefastenerswithexcellentdelayedfracture,whichisassociatedwiththeformationofbainiteduringisothermaltreatment.
Propertyandstabilityarethetypicaltechnicalrequirementsof8.8-12.9classesfastenersteels.Weputforwardtheconceptsofthecontrolofpurity,homogeneity,andmicrostructure.Specifically,(i)Impuritycontentandinclusioncontrol;(ii)Controlofbandedmicrostructuregrade,elementsegregation,andcontentofprimarychemicalelements;(iii)Adjustmentandcontrolofgrainsize,microstructuretype,precipitates,andfinemicrostructure.ThemeltingprocesswasoptimizedtoreduceimpuritiessuchasS,P,O,N,andHto≤100ppm.Restrictinginclusionlevelssumbelowgrade3.ThefluctuationofC,CrandMoiswithin±0.01%,withCpkexceedingof1.67andbandedmicrostructurecontrolledtograde3orbelow.Thefastenersexhibitedahardnessdispersionoflessthan2HRCandachievedafatiguelimitofapproximately94MPaunder10⁷cyclicloadingconditions,whichmeetstherequirementsforkeyautomotiveenginefasteners.ThecorrespondingdataarepresentedinFig.1.
Fig.1MicrostructureandS-Ncurvefor10.9fastenermaterial
Thedevelopmentoffastenersteelswithsuperiorresistancetohydrogen-induceddelayedfracture(propertyclasses14.8-19.8)entailedutilizingthesecondaryhardeningeffectofM2CandMCcarbidesandtheintroductionofadditionalhydrogentraps.Controlofcarbidemorphology,size,andquantitywascrucial.ByinvestigatingtherationalcarbidechemistrywithintheC-Cr-Mo-Valloysystemandoptimizingthestrength-ductility-toughnesssynergy,asignificantenhancementindelayedfractureresistancewasachieved,asillustratedinFig.2.
Fig.2Delayedfractureresistanceandtypicalmechanicalpropertiesof8.8-19.8seriesfasteners
2Microalloyedfastenersteels
Sincethemicrostructuretypeandgrainsizearethecriticalparametersforstructurecontrol,non-quenchedandtemperedsteelsforfastenersaremainlystrengthenedbycolddrawing.
Thesourceoftensilestrength(Rm)ofnon-quenchedandtemperedsteelisgivenbyequation1,whereisthestrengtheofhot-rolledsteelwire;andaretheincreamentofstrengthaftercolddrawingandaging,respectively.
（1）Micro-alloyingwithNbandVincombinationwithcontrolledrollingandcoolingprocessyieldsamicrostructurecomprisingferriteandpearlitewithagrainsizebelow10μm.Thepropertyisachievedfor8.8-10.9fasteneruseafterthecorrespondingtreatmentofcolddrawingandagingtreatment.
Fig.3Microstructurevolutionandtheeffectofcold-drawnreductiononstrengthfornon-quenchedandtemperedfastenersteel
3Heat-resistantsteelforfasteners
Thermal-resistancefastenersaregenerallyusedinareasoperatingathightemperatures.Wehavedevelopedheat-resistantfastenersteelsthatcanwithstandtemperaturesrangingfrom300°Cto750°C.
Asathermal-resistancefastener,A286mustmeet650°Ccreep-rupturerequirements,whichmicrostructureistheessentialfactor.
Forhigh-temperaturefastenersusedinautomobile,650˚Cdurabilityisoneofthemostimportantfactor,whichiscloselyrelatedtothemicrostructure.WedevelopedA286alloywireonthebasisofpolymorphicalloyingtheory(alloyedwithCr,Ni,V,Ti,Al,Mo,B),inwhichcombinedstrengtheningincludingsolidsolutionstrengthening,precipitation(i.e.Ni3(Al,Ti))hardening,andgrain-boundarystrengtheningisachieved.InaccordancewithASTME292-18,650℃durabilityofA286wasevaluated.TheresultsinFig.4indicatethatasoundpropertyisobtained.
Fig.4Microstructureand650˚CdurabilityofA286alloyaftersolutionandagingtreatment
4Future
Fastenersteels'futuredevelopmentwillmeetmodernindustry'sdemandsforhigherstrength,longerlife,greaterreliability,lighterweightandbetterenvironmentalfriendliness,whilealsoaddressingcomprehensivepropertiessuchascorrosionandtemperatureresistanceunderextremeconditions(aviation,deepspace,deepsea,deepearth,andpolardevelopment)
