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Effect of Alloying Additions in the Final Microstructure of Nb-Mo steels Processed by Thin Slab Direct Rolling Technologies
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Effect of Alloying Additions in the Final Microstructure of Nb-Mo steels Processed by Thin Slab Direct Rolling Technologies

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Presentation made at Materials Science and Technology 2008 Conference held in Pittsburgh

Presentation made at Materials Science and Technology 2008 Conference held in Pittsburgh

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  • 1. Effect of Alloying Additions in the FinalMicrostructure of Nb-Mo Steels Processed by Thin Slab Direct Rolling Technologies P. Uranga, J. Ganzarain, D. Jorge-Badiola and J.M. Rodriguez-Ibabe puranga@ceit.es CEIT and TECNUN (University of Navarra) Donostia-San Sebastián Basque Country, Spain MS&T’08 Conference October 6-9, 2008, Pittsburgh, PA
  • 2. Introduction• Multiple alloying combinations for high strength properties• Thin slab direct rolling metallurgical peculiarities – As-cast coarse grains – Alloying elements in solid solution• Study of combined effect in softening and precipitation kinetics• Modeling and microstructural evolution validation for schedule optimization MS&T’08 Conference, Pittsburgh, PA
  • 3. Material• Two Nb-Mo microalloyed steels Steel C Si Mn S Al Mo Nb N 3Nb-Mo16 0.05 0.04 1.58 0.002 0.027 0.16 0.03 0.005 3Nb-Mo31 0.05 0.05 1.57 0.002 0.028 0.31 0.028 0.006• 0.05%C - 0.03%Nb – 0.16% Mo – 0.31% Mo MS&T’08 Conference, Pittsburgh, PA
  • 4. Deformation Schedules Soaking: First deformation temperatures 1350ºC, 15 min Tini = 1100ºC Tini = 1050ºC ΔT = 50ºC ε1 = ε2 = ε3 = 0.4 Quenching temperatures 1ºC/s ε4 = 0.5 Tq = 900ºC Tq = 850ºCTemperature 700ºC, 1hour He flow 10ºC/s 600ºC, slow cooling Quenching Time MS&T’08 Conference, Pittsburgh, PA
  • 5. Austenite Microstructure and Modeling
  • 6. Austenite Microstructures Prior to Transformation• Non-recrystallized deformed austenite grains – Tini = 1100ºC: Homogeneous Structures 3Nb-Mo16 3Nb-Mo31 Tini = 1100ºC MS&T’08 Conference, Pittsburgh, PA
  • 7. Microstructural Heterogeneities in Austenite. Tini = 1050ºC• Non-recrystallized deformed Austenite Grains – Tini = 1050ºC: Microstructural Heterogeneities in Homogeneous Matrix 3Nb-Mo16 3Nb-Mo31 Tini = 1050ºC MS&T’08 Conference, Pittsburgh, PA
  • 8. Grain Distribution Modeling• Input: – Thermomechanical Sequence – Composition – Initial Grain Size Distribution• Model: – Equations developed for Nb-Mo steels and wide range of austenite grains: ε c = 2,8 ⋅10 −3 {1 + 20([Nb] + 0,035)} D 0,147 Z 0,155 t 0,5 X = 9,92 ⋅10 −11 Do ε −5 , 6 D − 0 ,15 ⎛ 180000 ⎞ ε −0,53 exp⎜ ⎡⎛ 275000 ⎞ ⎤ − 185 ⎟([Nb ] + 0,09[Mo ])⎥ 0 0 & ⎟ ⋅ exp ⎢⎜ 1,78 ⎝ RT ⎠ ⎣⎝ T ⎠ ⎦• Output: – Recrystallized and unrecrystallized grain size distributions – Mechanism history: drag, precipitation, dynamic rex MS&T’08 Conference, Pittsburgh, PA
  • 9. Model Predictions• Austenite grain size distribution prior to transformation – Tini = 1100ºC: Similar distributions for both steels 3Nb-Mo16 3Nb-Mo31 0.5 0.5 3Nb-Mo16 3Nb-Mo31 Tini = 1100ºC Tini = 1100ºC Austenite Area Fraction Austenite Area Fraction 0.4 Model 0.4 Model Experimental Experimental Tini = 1100ºC 0.3 0.3 0.2 0.2 0.1 0.1 0 0 20 40 60 80 100 120 140 160 180 200 20 40 60 80 100 120 140 160 180 200 Grain Size (μm) Grain Size (μm) MS&T’08 Conference, Pittsburgh, PA
  • 10. Model Predictions• Austenite grain size distribution prior to transformation – Tini = 1050ºC: Heterogeneity increases as Mo content increases 3Nb-Mo16 3Nb-Mo31 0.5 0.5 3Nb-Mo16 3Nb-Mo31 Tini = 1050ºC Tini = 1050ºC Austenite Area Fraction Austenite Area Fraction 0.4 0.4 Model Model Experimental Experimental Tini = 1050ºC 0.3 0.3 0.2 0.2 0.1 0.1 0 0 20 100 180 260 340 420 500 580 20 100 180 260 340 420 500 580 Grain Size (μm) Grain Size (μm) MS&T’08 Conference, Pittsburgh, PA
  • 11. Heterogeneous Structures• As-cast austenite grain prior to transformation: – Lack of Recrystallization through deformation passes and interstands – Higher fraction for 3Nb-Mo31 • Bigger maximum grain size – Strain induced Nb(C,N) precipitation interacts with softening mechanisms mainly in highly strained grains MS&T’08 Conference, Pittsburgh, PA
  • 12. Heterogeneity: As-cast Fraction Tini = 1100ºC Tini = 1050ºC 0.8 0.8 Tini = 1100ºC Tini = 1050ºC 3Nb-Mo31 3Nb-Mo31 0.6 3Nb-Mo16 0.6 3Nb-Mo16 As-cast fraction As-cast fraction 0.4 0.4 0.2 0.2 0 0 1 2 3 4 1 2 3 4 Interstand Interstand• Homogeneous Micro: Complete Rex prior to Precipitation• Minimum initial temperature is needed for heterogeneities to be avoided• Once homogeneity achieved: focus on austenite pancaking MS&T’08 Conference, Pittsburgh, PA
  • 13. Austenite Pancaking: Unrecrystallized Fraction• Austenite Pancaking: Drag / Precipitation Tini = 1100ºC Tini = 1050ºC 1 1 Tini = 1050ºC Unrecrystallized Austenite Fraction Tini = 1100ºC Unrecrystallized Austenite Fraction Precipitation Precipitation Solute Drag Solute Drag 0.8 0.8 3Nb-Mo16 3Nb-Mo31 3Nb-Mo16 3Nb-Mo31 0.6 0.6 0.4 0.4 0.2 0.2 0 0 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 Interstand Interstand MS&T’08 Conference, Pittsburgh, PA
  • 14. Austenite Pancaking: Nb Steel – NbMo Steels• Mo drag effect accelerates Nb(C,N) precipitation at low deformation T Tini = 1100ºC Tini = 1050ºC 11 Precipitation 11 TTini = 1050ºC ini = 1050ºC Unrecrystallized Austenite Fraction TTini==1100ºC Unrecrystallized Austenite Fraction Unrecrystallized Austenite Fraction 1100ºC Precipitation Solute Drag Unrecrystallized Austenite Fraction ini Precipitation Solute Drag Solute Drag 3Nb 3Nb-Mo16 3Nb-Mo31 0.8 0.8 0.8 0.8 3Nb-Mo16 3Nb-Mo31 3Nb 3Nb-Mo16 3Nb-Mo31 3Nb-Mo16 3Nb-Mo31 0.6 0.6 0.6 0.6 εac = 0.50 εac = 0.56 εac = 0.73 0.4 εac = 0.43 0.4 0.4 0.4 εac = 0.95 εac = 0.54 0.2 0.2 0.2 0.2 0 00 1 2 3 4 0 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 1 4 2 1 32 34 4 1 2 3 4 1 2 3 4 1 2 3 4 Interstand Interstand Interstand Interstand MS&T’08 Conference, Pittsburgh, PA
  • 15. Transformation
  • 16. Transformed Microstructures Coiling Simulation 700ºC• For Tini = 1100ºC: – Homogeneous ferrite 3Nb-Mo16 3Nb-Mo31 Tini = 1100ºC Dα = 8.6 μm Dα = 8.7 μm MS&T’08 Conference, Pittsburgh, PA
  • 17. Transformed Microstructures Coiling Simulation 700ºC• For Tini = 1050ºC: – Homogeneous ferrite with heterogeneous regions 3Nb-Mo16 3Nb-Mo31 Tini = 1050ºC Dα = 8.4 μm Dα = 8.6 μm MS&T’08 Conference, Pittsburgh, PA
  • 18. Microstructural Units EBSD• Homogenous ferrite microstructures correspond to high angle GB units Tini = 1100ºC 3Nb-Mo31 MS&T’08 Conference, Pittsburgh, PA
  • 19. Microstructural Units EBSD• Prior austenite coarse grains transform to coarse ferrite units or acicular structures: forming low angle GB areas. Tini = 1050ºC 3Nb-Mo31 MS&T’08 Conference, Pittsburgh, PA
  • 20. Conclusions• Mo addition to Nb microalloyed steels: important increase in the delay of static rex kinetics → The refinement of the initial as-cast structure is retarded.• For homogeneous microstructures: EBSD show that ferrite grains are diversely oriented with high-angle grain boundaries.• Structures transformed from non-refined as-cast grains form coarse microstructural units, bigger than those observed with the optical microscope. Toughness will be impaired.• For optimized thermomechanical schedules, Mo affects hardenability. This factor can be useful for the formation of complex microstructures with high strength and toughness levels. MS&T’08 Conference, Pittsburgh, PA
  • 21. Effect of Alloying Additions in the FinalMicrostructure of Nb-Mo Steels Processed by Thin Slab Direct Rolling Technologies P. Uranga, J. Ganzarain, D. Jorge-Badiola and J.M. Rodriguez-Ibabe puranga@ceit.es CEIT and TECNUN (University of Navarra) Donostia-San Sebastián Basque Country, Spain MS&T’08 Conference October 6-9, 2008, Pittsburgh, PA