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Recent Development of Light-weighting Steels

Recent Development of Light-weighting Steels. Graduate Institute of Ferrous Technology Dong-Woo Suh. Contents. What is light-weighting steels? Brief review on related research activities Some results on prototype alloys Concluding remarks. Why light-weighting?. 0.2. 0.14.

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Recent Development of Light-weighting Steels

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  1. Recent Development of Light-weighting Steels Graduate Institute of Ferrous Technology Dong-Woo Suh

  2. Contents • What is light-weighting steels? • Brief review on related research activities • Some results on prototype alloys • Concluding remarks

  3. Why light-weighting? 0.2 0.14 CO2 emission (kg/km) 0.02 1,088 ~1,360 kg (over 75% of car weight)

  4. Evolution of automobile • Light-weighting materials • w/o loss of crashworthiness • More efficient combustion • engine Improvement of fuel efficiency • Hybrid vehicle • Fuel cell vehicle Development of alternative vehicles

  5. Needs for light-weighting in alternative vehicles battery system / motor / fuel cell / gas storage battery system / motor + 90~100kg + 250kg

  6. One way – Higher strength steels 1500

  7. Needs for another approach Double strength Half thickness? • Front structure • 500 ~ 800MPa  1500MPa • : 30% reduction in thickness considering • the structural stiffness (Toyota)

  8. lattice expansion lower atomic mass of substitutional atom ● Specific weight of Fe-Al alloy ■ Specific weight of Fe-14~28Mn-Al-C [Frommeyer] Basic concept of light-weighting steels (M. Shiga et al. J. Phy. Soc. J.) • Al : 26.98 g/mol • Fe: 55.85 g/mol

  9. Alloy developments • 3 main research activities • Steels with ferrite as major phase • Based on Fe-Al binary system with microalloying elements • TS 400~600MPa, T.EL 30~35% • Automotive application, corrosion resistance • Multiphase steels • Based on Fe-Mn-Al-C system with microalloying elements • TS 800~1200MPa, T.EL 30~60% • Automotive structural components, reinforcements • Steels with austenite as major phase • Based on Fe-Mn-Al-C system with larger amount of austenite stabilizer • TS 600~1500MPa, T.El < 60% • Automotive structural component, military armor plate

  10. Issues of Fe-Al ferritic alloys • Alloy design - Fe-Al binary phase diagram - Role of carbon and -carbide - Effect of microalloying elements • Phase transformation and properties - Process related microstructure evolutions - Formability and texture

  11. Binary Fe-Al equilibrium •  phase: - Limited domain due to Al ( stabilizer) - Disappear with addition of ~3 atomic % Al •  (A2) phase • - Solid solution of Fe and Al (disordered) • - Two transitions: k-state and magnetic • k-state • - Short range ordering • - Decreases the uniform and total elongation

  12. Workability and Al content Fe-13at.% Al Fe-17at.% Al (D.W. Suh, unpublished work)

  13. Ferritic light-weight steels

  14. Mechanical properties (low carbon) • Increase of strength • - solid solution hardening • Ductility loss • - short range ordering • - decrease of dislocation mobility • : absence of cell formation during • straining Fe-8.5%Al (G. Frommeyer et al., La Revue de Metall.)

  15. Mechanical properties (high carbon) B C D A (R.G. Baligidad et al., La Revue de Metall.) • Tensile strength ~1000MPa is achieved by hard 2nd phase • Effect of -carbide on ductility does not seems to be fully understood

  16. Development of Fe-Mn-Al-C steels • Fe-Mn-Al-C alloys ( in 60~80’s) • - replacement of Cr, Ni containing stainless steel • - Fe-34.4Mn-10.2Al-0.76C (corrosion / oxidation) • - TS ~730MPa, El ~70% • TWIP steel (from 90’s) • - Fe-18Mn-1.5Al-0.6C • Age hardenable Fe-Mn-Al-C alloys • - light-weighting steels • - TRIPLEX steel (18-28Mn, 9-12Al, 0.7-1.2C) • - austenite / -carbide / ferrite

  17. Deformation and work-hardening • Fe-(0~1)C-(10~30)Mn-(3~9)Al alloys 100mJ/m2 20mJ/m2 40mJ/m2 SFE TRIP TWIP S(M)BIP Mn Al C

  18. Effect of Mn content (G. Fromeyer et al., ISIJ Int.)

  19. Austenitic light-weighting steels • Alloys with 20-30%Mn, 8-10%Al, 0.8-1.2%C • - Above 850oC, supersaturated austenitic structure • - During isothermal aging in 350-700oC, -carbide • precipitation occurs to raise strength Aged at 823K for 180min. (Choo. et al., Acta Mater.)

  20. Light-weighting TRIPLEX steel • Fe-26Mn-11Al-1.2C • - austenite, ferrite and -carbide (G. Frommeyer et al., Steel Research)

  21. -carbide and cracking • (0.3~0.5)C-(5~6)Mn-(6.5~7.5)Al (S.Y. Shin et al., Met. & Mater. Trans.)

  22. Recent results on prototype alloys • 1st year results from project supported by government • Target properties • - 780MPa / 980MPa grade steels • - 10% density reduction • Fe-Mn-Al-C multi-phase steel • - to make the alloy with lean system • compared with reported alloys • - utilizing metastable austenite • - suppression of -carbide • in ferrite phase Underbody of battery-electric vehicle

  23. Fe-Mn-Al-C thermodynamic database 0.2C-7Mn-4Al (fM=0.73) 0.25C-7Mn-4Al (f M =0.82) large discrepancy

  24. Fe-Mn-Al-C thermodynamic database • Update thermodynamic database to improve reliability • (collaboration with B.J Lee, POSTECH)

  25. Austenite fraction control 0.12C-15Mn-7Al 0.2C-8Mn-7Al

  26. Fe-0.08C-6Mn-3Al TRIP steel 3.1Al (720oC))   3Al f=26.0% (D.W Suh et al., Met. & Mater. Trans.)

  27. Prototype alloys and processing soaking annealing for 2min. at 740oC, 780oC, 820oC hot-rolling 10oC/s 10oC/s cold-rolling (~75%)

  28. Density of the alloys

  29. Microstructure after cold-rolling 0.2C-8Mn-7Al 0.12C-15Mn-7Al

  30. Microstructure after annealing (820oC) 0.12C-15Mn-7Al 0.2C-8Mn-7Al

  31. Austenite fraction after annealing

  32. Properties of annealed sheets 0.2C-8Mn-7Al 0.12C-15Mn-7Al

  33. TRIP effect from metastable austenite?

  34. Change of austenite fraction • Austenite is too stable to expect strain-induced martensite formation • Alloy design considering partitioning of alloying elements are required

  35. Concluding remarks • Light-weighting steels • - alternatives for mass reduction • - potent material for future vehicles application • Three main research activities • - Fe-Al based ferritic alloys • - Fe-Mn-Al-C based multiphase alloys • - Fe-Mn-Al-C based austenitic alloys • Challenges • - Thermodynamic approaches • - Understandings on microstructure-property control • - Difficulties on production (casting and rolling)

  36. Properties of annealed sheets

  37. Fe-Al binary phase diagram

  38. Mechanical properties (low carbon) (G.Frommeyer et al., La Revue de Metall.) • Lower carbon(~0.03%) content because C decreases ductility owing to • carbide precipitates Fe3AlC, -carbide • Utilizing microalloying elements to avoid -carbide formation

  39. Microstructure constituents in light-weighting steel Matrix Ferrite+Aus. Austenite +Fer. (+) Ferrite Stable  Metastable  TRIP type TRIPLEX type Duplex type ELC type

  40. Precipitation of (Fe,Mn)3AlC (Fe,Mn) Al perovskite E21 metastable L12 Chemical modulation : austenite <100> (C and Al) spinodal decomposition

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