Advanced Vehicle Concepts

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  ULSAB-AVC Consortium26May2001 Technical Transfer Dispatch #6 ULSAB-AVC Body Structure MaterialsMay 2001 FOREWORD1.0Introduction2.0   Materials Selected for the ULSAB-AVC Body Structure3.0    Advanced High Strength Steel Microstructures, Behavior, and Alloy Design4.0Materials Selection Process for ULSAB-AVC5.0Forming Assessment Appendices:IULSAB-AVC Body Structure Parts ListIIULSAB-AVC Steel Grades PortfolioIII Considerations in the Selection of Advanced High Strength Steels for ULSAB-AVCIVExamples of ULSAB-AVC Forming Simulations Appendix III Rev’d 6 June 2001  ULSAB-AVC Consortium26May2001 FOREWORDProgram Background ULSAB-AVC(Advanced Vehicle Concepts) is the most recent addition to the global steel industry’s series of initiatives offering steel solutions to the challenges facing automakers around the world to increase the fuelefficiency of automobiles, while improving safety and performance and maintaining affordability. This programfollowsthe UltraLight Steel Auto Body (ULSAB) program(results announced worldwide in 1998). As with the ULSABProgram, the ULSAB-AVCConsortiumcommissioned P ORSCHE E NGINEERING S ERVICES , I NC ., Troy, Mich. USA, to providedesign and engineering management for ULSAB-AVC.In the ULSAB-AVCprogram, P ORSCHE E NGINEERING S ERVICES , I NC . takes a holistic approach to the development of a newvehicle architecture that offers cost-efficient steel solutions to mass reduction challenges. ULSAB-AVCwill presentadvanced vehicle concepts to help automakers use steel more efficiently and provide a steel-based structuralplatformfor achieving: ã   Anticipated crash safety requirements for 2004, ã   Significantly improved fuel efficiency, ã   Optimized environmental performance regarding emissions, source reduction and recycling, ã   High volume manufacturability at affordable cost. Technical Transfer Dispatches (TTD)  To encourage valuable dialogue, the ULSAB-AVCProgramprovides periodic communications in the formof TTDs tokey contacts within the automotive industry to keep key expert automotive staff informed about Programprogress. TTD#6 provides critical information about the application of advanced high strength steels (AHSS) to vehicledesign, offering important design considerations in using these advanced steels. Also included in this TTDareexamples of the effective collaboration process between steel suppliers and design engineers to achieve the fullyoptimized use of AHSS and documentation of properties for the steel grades used in the ULSAB-AVCbodystructures.It is important to note that the information reported in this dispatch related to ULSAB-AVC’s design is work inprogress, subject to change as the engineering process is completed. The final programresults, to be delivered tothe global automotive community in early 2002, could be different than what is included here. However, fromourexperience with previous dispatches, we believe that allowing our customers to reviewthe work in progress not onlyprovides an avenue for exchange and feedback but also contributes helpful input to our customers’ own researchefforts.For more information or to provide feedback, please contact your local ULSAB-AVCMember Company or ULSAB-AVCprogrammanagement as follows: Ed Opbroek, ProgramDirectorULSAB-AVC Tel. (513) 422-1844Fax. (513)  ULSAB-AVC Consortium26May2001 1 1.0 Introduction Engineered steels provide automotive designers and manufacturers with the unique option of combining lightweighting with the traditional steel advantages of low cost and eco-efficiency.This was clearly demonstrated by the ULSAB Program and was achieved, in part, through theextensive use of both high strength steels (HSS) and ultra high strength steels (UHSS).The HSS grades used in ULSAB utilized mostly conventional microalloy approaches. The goalsfor ULSAB-AVC are more aggressive than for ULSAB because of the need to reduce the addedmass required to satisfy future safety mandates. For ULSAB-AVC, it is therefore appropriate toalso consider the application of newer types of high strength steels, the so-called advanced highstrength steels (AHSS), to assist in achieving the overall aims of the program through the designof an efficient lightweight body structure.In contrast to ULSAB, where a key focus was to demonstrate the manufacturing     feasibility of theaggressive use of readily available HSS and modern manufacturing processes (e.g. tailored blanks, hydroforming, assembly laser welding), ULSAB-AVC is a concept  program. This provides an opportunity to expand the list of candidate steels by considering those steels that arecurrently available and those that will become available by 2004. To coordinate this, it was firstnecessary to adopt a consistent nomenclature of the various grades of steels. 1.1 ULSAB-AVC Steel Nomenclature Methods used to classify steels vary considerably. To provide a consistent nomenclature, theULSAB-AVC Consortium adopted a standard practice that defines both yield strength (YS) andultimate tensile strength (UTS). In this classification system, steels are identified as:XX aaa/bbbWhereXX= Type of steelaaa= Minimum YS in MPa, and bbb= Minimum UTS in MPa.The steel type designator uses the following classification:Conventional Types_____________Advanced High Strength (AHSS) Types *_ Mild = Mild steelDP = Dual phaseIF= Interstitial-freeCP= Complex phaseIS= IsotropicTRIP= Transformation-induced plasticityBH= Bake hardenableMart= MartensiticCMn= Carbon-manganeseHSLA= High strength, low alloy * refer to Section 3.0 for further description As an example, a classification of DP 500/800 refers to dual phase steel with 500 MPa minimumyield strength and 800 MPa minimum ultimate tensile strength.  ULSAB-AVC Consortium26May2001 2 1.2 The Rationale for Advanced High Strength Steels Consistent with the terminology adopted for ULSAB, High Strength Steels (HSS) are defined asthose steels with yield strengths from 210–550 MPa; Ultra-High Strength Steels (UHSS) aredefined as steels with yield strengths greater than 550 MPa. The yield strengths of AdvancedHigh Strength Steels (AHSS) overlap the range of strengths between HSS and UHSS, as shownin Figure 1. The principal differences between conventional HSS and AHSS aredue to their microstructures. AHSS aremulti-phase steels, which containmartensite, bainite, and/or retained austenitein quantities sufficient to produce uniquemechanical properties. Compared toconventional micro-alloyed steels, AHSSexhibit a superior combination of highstrength with good formability. Thiscombination arises primarily from their highstrain hardening capacity as a result of their lower yield strength (YS) to ultimate tensilestrength (UTS) ratio.For conventional steels, reduced formability is one of the consequences when selecting steelswith higher strength levels. To overcome this, recent steel developments, which can facilitatefurther lightweighting of automotive structures, have targeted this phenomenon. The family of steels based on multi-phase microstructures typify the development of improved materialconcepts to enhance formability.The multi-phase AHSS family includes dual phase (DP), transformation induced plasticity(TRIP) and complex phase (CP), products. Figure 1 data show the relative strengths andformability (measured by total elongation) of conventional strength steels, such as mild (Mild)and interstitial free (IF) steels; conventional HSS such as carbon-manganese (CMn), bakehardenable (BH), isotropic (IS), high strength IF (IF), high strength, low alloy (HSLA). Figure 1also shows advanced high strength steels (AHSS) such as dual phase (DP), transformationinduced plasticity (TRIP), complex phase (CP), and martensite (Mart) steels.Although not displayed in Figure 1, another category of steels, known as press hardened or hot-formed steels are also of interest, especially for those components with a complicated shape butrequiring ultra high strength levels. These grades are, essentially, martensitic grades.    E   l  o  n  g  a   t   i  o  n   (   %   ) Lower Yield Strength (MPa)01020304050607004006001000 IF Mild IF H  S L A D P  , C P  T  R I  P   BH C  M  n  Low StrengthSteels (<210MPa)Ultra High StrengthSteels (>550MPa) 200800 High StrengthSteels   AHSS  IS Conventional HSS  MART 1200 Figure 1. Strength-Formability relationshipsfor mild, conventional HSS, and AdvancedHSS steels.
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