汽車零部件用微合金非調(diào)質(zhì)鋼的開發(fā)及應(yīng)用

1、,,Microalloyed Forging Steels for Automotive Applications,David K. MatlockAdvanced Steel Processing and Products Research CenterDepartment of Metallurgical and Materials EngineeringColorado School of MinesGolden, Col

2、orado USA,,Microalloy Steel Applications,Ultra Light Steel Auto Suspension, AISI (2001), www.autosteel.org,Etc. 7%,AutomotiveMaterials,,,,Example: Trends in Engine Design,,C. Lemaître, AIST Bar Conf., Winter P

3、ark, Colorado (2006),New environmental legislation – continued reductions in allowable emissionsDiesel: Increased injection pressuresGasoline: Increased use of turbo and/or compressors,European Data,Lighter weight po

4、werplants for improved fuel economyRequire higher strength materialsMicroalloyed bar steels for crankshaftsIncreased stiffness (compared to cast iron)Better fatigue resistanceReduced component weightAllow smaller a

5、nd lighter flywheels and clutch systems,,AISI Report – 2004 www.autosteel.org,Example: Trends in Engine Design,Historical: Use of Microalloyed Forging Steels for Automotive Components,Initial applicat

6、ions in Europe – 1980’sBased on direct-cooled ferrite-pearlite steelsGrowth in use in the US – 1990’sFerrite-pearlite steelsNon-traditional bainitic steels with retained austeniteWith improved processing control a

7、nd understanding of microalloying fundamentals, use continues to expand.,Requirements for New Microalloyed Bar and Forging Steels Alloying, Processing and Product Developments,Higher Strength … with …Excellent Mechanic

8、al PropertiesFormability, Toughness, Fatigue Resistance, Machinability …….. Product BenefitsMore efficient vehicle designsHigher torque-capacity componentsHigher operating stressesSafer vehiclesImproved crashworth

9、iness,,Presentation Overview,?Opportunities for use of Microalloyed Steels in AutomobilesReduce manufacturing costsBalance properties and performanceTailor materials for specific applicationElements of interest Ti,

10、Nb, V, N, C, … Al … and their T-dependent solubilities Presentation based on Selected ExamplesAutomotive SpringsForgings for GearsThermomechanical ProcessingTransmission Components,,,Microstruc

11、tural Classes of Microalloyed Steels,Ferrite-Pearlite Steels – Direct CooledStrengthened by:Pearlite volume fractionFerrite grain size and substructurePrecipitation strengthening of ferrite(e.g. V and Nb)Heat Tr

12、eated Steels Through hardened (martensitic or bainitic)Induction hardened surfacesSurface modified and heat treatedCarburized,,Direct-Cooled Forging Steels,Kaspar et al., Microalloying Forging Steels, TMS, 1996,Bene

13、fit of Direct-Cooling: Reduced processing time and costRequirements: Equipment capable of controlled cooling,,,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,?Solubilit

14、y ConsiderationsMicrostructural Control and Grain Growth during ProcessingStrengthening and Toughening Mechanisms,,Microalloy Precipitate Solubility,?Microalloy elements in metals:As solid solution alloy additions

15、In precipitatesAt grain boundariesWithin grainsPrecipitates form between solid solution elements and interstitials of C or NMicroalloy Interstitials Typical ElementsPrecipitatesNbCNbCNbN

16、VNVCVNTiTiCTiNAlAlNSpecific precipitates that form depend on composition and temperature,,Solubility Overview,?Apply equilibrium thermodynamics to predict state of microalloy elements in alloy –

17、driving force for precipitate formation controlled by solubilityReactions of Interest:M + X = MXExample:Nb + C = NbCyM + zX = MyXz4V +3C = V4C3Reaction Rate: ks = Equilibrium constantks is referred to

18、as the solubility product and is equal to the ratio of activities:,,,,,,,,Solubility Overview,?Temperature dependence of solubility product, ks, described by an Arrhenius equation; i.e. By convention ks given by:

19、Example solubility diagram for NbC,,,,,,,,,,,,,,Example: NbC Solubility Diagram,Assume:T = 1100 oC[Nb] = 0.1 wt pct[C] = 0.2 wt pct,,,,,,,,,,,,,,Solubility data from: Narita, Trans ISIJ (1975),,Effects of Compo

20、sition and Temperature,,,,,,,,,,,,,,,,Example: NbC Solubility Diagram,Solubility data from: Narita, Trans ISIJ (1975),,,,,,,,,,,,,Carbide/NitrideSolubility,Lower solubility product=Increased precipitate stabilityEx

21、amples:At 1200 oC TiN is more stable than NbC, VNAt all temperatures VC has lowest stability, e.g. VC easily dissolves at higher temperatures,,,,From: Matlock, Krauss, and Speer, Microalloying ‘05,Importance of Unde

22、rstanding Solubility,Temperature ranges in which carbides, nitrides, and carbonitrides form and dissolve determine suitability for a given microalloying design. For example:TiN, stable at temperatures in excess of 120

23、0 oC, used for austenite grain size control at high forging temperatures and during high-temperature carburizingVN (and carbonitrides) dissolve at low austenitizing temperaturesV is available for fine-scale precipitat

24、ion strengthening on cooling after forgingNbC (and TiC) dissolve and precipitate at temperatures intermediate to TiN and VPrecipitates prevent austenite recrystallization during finish hot rolling – results in fine fe

25、rrite grain sizes.,,,,,,,,,,,,,Carbide/NitrideSolubility,CompareAustenite and Ferrite,,,,Alloying and Process Control,Solubility controls driving force for precipitation, volume fraction, etc.Approach to use solubili

26、ty productsSpecify alloy contentEvaluate solubilities of constituentsDetermine which precipitates form/dissolve at temperature/composition of interestPredict austenite composition and precipitate volume fractionsDe

27、sign process methodologyApproach will be illustrated with Nb-modified gear steels for high temperature carburizing.,,,,,,,,,,,,,,,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar

28、Steels,?Solubility ConsiderationsMicrostructural Control and Grain Growth during ProcessingStrengthening and Toughening Mechanisms,,?Consider grain growth controlDuring hot workingDuring heat treatingGrain

29、 growth limited by second phase particlesR = stable grain sizer = particle radiusf = volume fraction particlesFiner grain size = decrease r or increase fSize and volume fraction controlled by alloying and proc

30、essingIf too small – particles redissolveIf too large – particles are ineffective Types of grain growthNormal grain growthAbnormal grain growth,,Particle Effects on Grain Growth,Steel: Controlled Rolled with 0.02

31、NbTime: 60 minutesHeating Rate: 145°C/min,NGG = NormalAGG = AbnormalIAGG = Initial Abnormal,Grain Growth Terminology,K. AlOgab, PhD, ASPPRC, CSM (2004),,Particle Effects on Grain Growth,[1]Moving Grain Boundary

32、 Approaches Particle,[3] Particle RetardsBoundary Movement,,,Particle Effects on Grain Growth,From: Krauss, Steels, ASM, (2005),Plain Carbon SteelsNormal grain growth(without particles),,Steel with ParticlesGrain gr

33、owth slowed by particles,,,Particle Effects on Grain Growth,From: Krauss, Steels, ASM, (2005),Plain Carbon SteelsNormal grain growth(without particles),,Steel with ParticlesGrain growth slowed by particles,,Particle

34、dissolution at higher temperatures results in abnormal grain growth,,,Particle Effects on Grain Growth,From: AlOgab, PhD, ASPPRC, Colorado School of Mines (2004),Example Light Optical Micrographs,Abnormal Grain Growth,

35、Normal Grain Growth,,Grain Growth Control – Ti, V, Al, Nb,From: Krauss, Steels, ASM, (2005),,From: Krauss, Steels, ASM, (2005),Grain Growth Control – Importance of Nb,,Grain Growth Control – Importance of Nb,From: Kra

36、uss, Steels, ASM, (2005) andK. AlOgab, PhD, ASPPRC, CSM (2004),,From: Krauss, Steels, ASM, (2005),Summary: Grain Growth Control,Microalloying elements produce particles that suppress grain growthParticle dissolution

37、leads to abnormal grain growth and steels with large average grain sizesParticle size, volume fraction, and distribution controlled by microalloy additions and temperatureTi and Nb effective in creating particles tha

38、t suppress grain growth,,,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,?Solubility ConsiderationsMicrostructural Control and Grain Growth during ProcessingStrengthen

39、ing and Toughening Mechanisms,,Strengthening Mechanisms,?Grain Size RefinementIncrease strengthIncrease fatigue resistanceIncrease toughnessRefines transformed microstructuresPrecipitation StrengtheningT

40、ransformation Strengthening,,,,Hall- Petch EquationGrain boundary blocks slip bandStress concentrated at head of blocked slip band,Strengthening: Grain Size Effects,Barrett et al. (1973) and Gladman (1997),,Grain Si

41、ze Refinement,From: Krauss, Steels, ASM, (2005),,Grain Size Refinement,Austenite refinement also modifies martensite packet size in quenched and tempered steels.,From: Krauss, Steels, ASM, (2005),Martensite packet size

42、 v. austenite grain size,Yield strength depends on packet size,,Fatigue Endurance Limits: Carburized Gear Steels,Cornelissen et al., ASM (2000),Modified 4320 and 8620 Gear Steels,,Finer Austenite Grain Size,,,Fatigue End

43、urance Limits: Carburized Gear Steels,Cornelissen et al., ASM (2000),Modified 4320 and 8620 Gear Steels,,Finer Austenite Grain Size,,,Refined Prior Austenite Grain Size Leads Directly to Improved Fatigue Resistance,Grain

44、 Size Effectson Toughness,Charpy V-Notch DataFine grain size lowers transition temperatureFine grains resistcleavage fracture,From: Hertzberg (1989),Precipitation Strengthening,,Strength increases with smaller pa

45、rticle sizes or greater volume fractions,,Pinned Dislocation0.1 C - 0.04 Nb (wt pct) Steel(From Gladman, 1985),,D. Ponge, www.materialsknowledge.org (2005),Combined Effects:Grain Refinement and Precipitation,,Dire

46、ct-Cooled Microalloyed Bar Steels,Sawada et al., ISS-AIME, (1994),Direct cool after forging – ferrite-pearlite or “non-traditional bainite” microstructuresHigh carbon critical for strengthMicroalloying ? precipitation

47、hardeningStrength Increase ? toughness decrease,,,Direct-Cooled Microalloyed Bar Steels,S. Thompson, ASPPRC, (2006),Pearlite + Interphase Precipitation,,Importance of Interlamellar Spacing,?Pearlite strength and tough

48、ness depend on interlamellar spacing,From: Gladman,in Microalloying Forging Steels, TMS, 1996,S = true interlamellar spacingt = pearlitic carbide thickness,,Summary: Strengthening Mechanisms,?Contributions of differe

49、nt strengthening mechanisms are additiveMany equations available in the literature to summarize properties (e.g. see Gladman’s text)Opportunities exist to design materials with specific mechanical properties,From: G

50、ladman,in Microalloying Forging Steels, TMS, 1996,,,Thermomechanical Processing of Microalloyed Bar Steels,,TMP Control of Microstructures,Example Reference:Boyd and Zhao, in New Developments in Long and Forged Products

51、, AIST, (2006)Purpose of Study: Compare forging schedules designed to control austenite recrystallization with alloy content in modified 1541 alloysVariables: Forging history (temperature), cooling rate, microalloy

52、additions,Boyd and Zhao, AIST, (2006),Boyd and Zhao, AIST, (2006),Warm forging: Designed to finish in ferrite-pearlite region,Forging Schedules Applied to Nb/V/Ti Steels,Goal: Produce refined ferrite and pearlite coloni

53、es + precipitation strengthening,Deformation in Austenite: Designed to produce non-recrystallized austenite,,Boyd and Zhao, AIST, (2006),,Warm Forged 1541 with Nb,SEM Micrographs,Result: successfully produced refined m

54、icrostructuresStrengthened also by dislocation substructure in ferrite,D. Ponge, www.materialsknowledge.org (2005),Thermomechancial Processing AlsoEnhances Precipitation,Boyd and Zhao, AIST, (2006),,Effect of TMP on Me

55、chanical Properties,Warm Forged: 1541+ Nb,Forged to produce non-recrystallized austenite,Conventional Forging,,,,1541+ Ti/V,1541+ Nb,,Boyd and Zhao, AIST, (2006),,Summary – Thermomechanical Processing,Microstructures wit

56、h refined ferrite and pearlite can be successfully produced to increase both strength and toughnessPrecipitates resulting from Nb additions successfully suppressed grain growth to yield the superior properties.Thermo

57、mechanical processing of microalloyed bar steels offers opportunities for property development in new steels.,,,ExampleAutomotive Springs,,Example: Automotive Springs,?Design Requirements:Higher strengthImproved

58、 fatigue resistanceImproved toughnessLighter weightHistorically, spring steels based on SAE 5160 and more recently, SAE 9259Recently, microalloyed steels, based on Nb + V have led to improved materials,M. Head,

59、 et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,,Spring Steel Compositions,New Microalloyed Grade: Lower C, with Si, Nb, and VUse Thermomechanical Processing to control grain size,With (max values) 0.02 P;

60、 0.021 S; 0.012 Ni (in wt pct),M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,Spring SteelProcessing,Hot RolledFormedHeat TreatedAustenitize ~ 940 oCTEM Showed Nb Precipitates for Control of

61、Austenite Grain Size,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,Front Suspension Coil Spring in North American Minivan: Improved with Microalloyed Steel,M. Head, et al., Stelco Inc., SAE (2006)

62、and GDIS, www.autosteel.org,,Summary – Spring Steel Development,Controlled thermomechanical processing is critical in order to be able to utilize benefits of microalloying to control and refine final microstructureFin

63、al properties benefit from contributions of both Nb and V in alloys.Development of improved microalloy spring steels will continue.,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,,,Example:Vacuum

64、 Carburized Gears,,Bending Fatigue In Gears,Trends for Automotive Gear Steels,Carburize at higher T: 930 oC ? 1050 oCShorter heat treat cycle = ?$$Use alternate technologiesVacuum or low-pressure plasma carburizin

65、gHigher T gas carburizingDesign alloys to respond to modified thermal histories ? use microalloy additionsImprove material propertiesFatigue resistanceBending fatigue Contact fatigue,,,Example Time Saving with “Va

66、cuum” Carburizing,,Klinkenberg and Jansto, AIST, (2006),,Grain Size and “Vacuum” Carburizing,,Advanced Carburizing: Plasma and Vacuum may operate at higher temperatures = shorter cycle times, but……..,From Davidson et a

67、l. 2001,Austenite Grain Size – Importance to Fatigue of Carburized Steels,Fine Grain Size = Increased Endurance Limit,Cornelissen et al. (2000),Trends for Automotive Gear Steels,Methods to refine austenite grain sizes

68、in carburized gear steelReheat after carburize and quench -- Refine by transformation cyclingRequires extra heat treat cycle = $$Utilize microalloy precipitates to suppress grain growthAlloy designs based on solubi