Presentazione orale al convegno Euromat 2011, svoltosi a Montpellier, Francia, dal 12/09/2011 al 15/09/201

D’Aiuto F.

Firrao D.

Matteis P.

Scavino G.

English

MATTEIS, PAOLO

SCAVINO, Giorgio

FIRRAO, Donato

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

04 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEFatigue behavior of dual-phase and TWIP steels for lightweight automotive structures / Matteis P.; Scavino G.; D’AiutoF.; Firrao D.. - (2011).OriginalFatigue behavior of dual-phase and TWIP steels for lightweight automotive structuresPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2458380 since:Fatigue behaviorof Dual-Phase and TWIP steelsfor lightweight automotive structuresP. Matteis1, G. Scavino1, F. D’Aiuto2, D. Firrao11Politecnico di Torino, DISMIC2Centro Ricerche FIAT, Torino• To characterize and compare mass-produced (DualPhase), innovative (TWIP), and experimental highstrength steels for car weight reduction• To facilitate the industrialization of these new steels,with regards to both production processes and servicerequirements• To study these steels fatigue crack growth behavior,which may in future be considered in the car-bodydesign and verification, in pursuit of further weightreductions, as it now happens in aeronautic designOverall aimsSteel sheets for car bodies (I)Desired propertiesLower fuel consumptionLess pollution (Euro 4 – 5 …)Increased load (commercial vehicles)Lower costHigher strength→ lower weight →Plastic energy absorption → car-crash safetyFatigue endurance → ordinary car serviceDuctility, weldability → production processes• High-strength weldable steel sheets are made by:– continuous casting– hot rolling– cold rolling– continuous final heat treatment– protective coating (Zn)• Sheets are cold formed to produce car body parts• Car bodies are assembled by resistance spot weldingSteel sheets for car bodies (II)Most common overall production cycle Examined Dual Phase (DP) steel• widely used• low alloy, ferrite and martensite microstructure• made by intercritical annealing and quenching after cold rollingOES, Wt. %C 0.18Mn 2.3Cr 0.50Si 0.18Al 0.034Nb 0.025Ni 0.015Examined TWIP steel• not yet widely used• ductile, high-Mn austenite strengthened with solute C• TWinning Induced Plasticity (TWIP) effectOES, Wt. %C 0.65Mn 18.3Al 1.5Ni 0.41Si 0.05Cu 0.05V 0.04P 0.02S 0.01Resistance spot welding of TWIP steel sheets (I)process parametersSpot radius: 3 mmForce: 3.5 kNTime: 260 msCurrent: 7 kAElectrodeElectrodeForceForceFusionzonepowerResistance spot welding of TWIP steel sheets (II)microhardness (no variations)Fusion ZoneSheet 1Sheet 21001502002503003504000 2 4 6 8 10Microhardness [HV]Distance from weld spot centre [mm]FusionZoneHeatAffectedZoneUnalteredspot→radius   Resistance spot welding of TWIP steel sheets (III)X-ray diffraction (austenite only)0123430 40 50 60 70 80 90 100Counts [x1000]2θ diffraction angle [°]Weld spotSteel sheetTensile curvesS-N fatigue - proceduresDP1000 as-fabricated (with Zn coating)TWIP as-fabricated (uncoated)with one RSW spot at mid-length (joining a 20 mm sheet square)As-fabricated →with← RSW spot ↓ 20 mm wide, full thickness tensile specimens constant load amplitude fatigue tests load ratio ≈ 0 (minimum load = 50 N) 106 cycles fatigue limit (σD), staircase methodS-N fatigue - results18024030036042048054060040 1,000Max. Stress [MPa]Thousand cyclesDP1000TWIPTWIP + RSW→3→3→2→1→1→1→1→2σD = 558 MPaσD = 403 MPaσD = 210 MPaS-N fatigue – optical fractographyFatiguenucleationFatigue crack propagationFinal (slant) fractureDP1000,  σmax = 600 MPa, 888,842 cycles Paris fatigue - procedure64 mmpolished Compact Tension (CT) specimenswidth W=64 mm, thickness B=1.7 mm, B/W≈ 1/38pre-cracking & fatigue crack growth as per ASTM E647ΔK-increasing procedure: load-controlled fatigue steps≈ 0.3 mm crack growth per step≈ 8% ΔK increase between stepsLoad ratio = 0.1Optical crack-length measurements1E-91E-81E-71E-61E-5Crack growth rate, da/dN [m]Stress Intensity Factor amplitude, ΔK [MPa√m]5                       10            15      20                      40            60      80Paris fatigue - resultsDP1000da/dN = C∙ΔKmC = 8.85∙10-12m = 2.88Conclusions & Future WorkThe TWIP steel (0.65% C, 18% Mn, 1.5% Al) exhibitsremarkable UTS (≈ 950 MPa) and ductility (≈ 50%), but alower fatigue limit (403 MPa at 1 million cycles) than adual phase steel with similar strengthRSW welding spots greatly reduce the TWIP steelstrength, ductility, and fatigue resistance, even if theweld microstructure is similar to the base metalA Paris plot was successfully obtained for a dual phasesteel sheet, notwithstanding the quite reduced thicknessFurther tests are needed for a thorough characterizationand comparison of the examined steelsFatigue behaviorof Dual-Phase and TWIP steelsfor lightweight automotive structuresP. Matteis1, G. Scavino1, F. D’Aiuto2, D. Firrao11Politecnico di Torino, DISMIC2Centro Ricerche FIAT, TorinoThis work was financed by the Italian Government (contract 2008PSH7YZ). P. Olivieri, R. Picerno,and F. Rizzato contributed to part of it during their B.Sc. or M.Sc. dissertation preparation.Thank you for your attention!

Fatigue behavior of dual-phase and TWIP steels for lightweight automotive structures

https://iris.polito.it/retrieve/handle/11583/2458380/55090/Euromat-Montpellier-2011.pdf

Fatigue behavior of dual-phase and TWIP steels for lightweight automotive structures

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