Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite
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This study examines metadynamic recrystallization in coarse-grained Nb microalloyed austenite steel. There is a transition between static and metadynamic recrystallization between the critical strain εc and transition strain εT = 2.2εc. Below εc only static recrystallization occurs. Between εc and εT both static and metadynamic recrystallization contribute to softening. Above εT only metadynamic recrystallization occurs. A new model was developed that considers the contributions of both static and metadynamic recrystallization between εc and εT. The model predictions matched experimental validations
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Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite
- 1. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite STUDY OF METADYNAMIC RECRYSTALLIZATION PHENOMENA IN COARSE GRAINED Nb MICROALLOYED AUSTENITE P. Uranga, A.I. Fernández, B. López and J.M. Rodriguez-Ibabe CEIT and Univ. of Navarra, Pº M. Lardizábal 15, 20018 - San Sebastian, Basque Country (Spain)
- 2. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Introduction Thin Slab Casting Technology – Coarse Grained As-cast Austenite Structure – Microalloying Elements in Solution – High Deformation in First Stands Possibility of Dynamic Recrystallization? Study of Post-Dynamic Softening Microstructural Modeling
- 3. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Dynamic Recrystallization in First Stand? Critical Strain 0.035% Nb 1120 0.6 1100 Strain rate: 5 s-1 0.7 1080 0.8 T(ºC) 1060 0.9 1040 1.0 1020 1.1 1000 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 D0 (μm) • Onset of Dynamic Recrystallization for Reductions > 55%, ε > 0.8 • Softening Mechanisms for Strains Higher than Critical Strain
- 4. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Usually when: ε > εc Dynamic Metadynamic Recrystallization Recrystallization Recently, Bai et al. (2000): Metadynamic ε > εT With εT = 1.5· εp Recrystallization Transition between different regions
- 5. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Material and Experimental Procedure C Mn Si S P Nb N 0.1 1.42 0.31 0.008 0.018 0.035 0.0053 T 1400 ºC Strains between εc and ε = 4 ε1 tip ε2 ε = 1 s −1 → t ip = 1 s & 1100ºC ε = 5 s −1 → t ip = 0.3 s & t
- 6. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Influence of Strain on Softening 1 1 T = 1100ºC Strain rate = 1 s -1 T = 1100ºC Strain rate = 5 s-1 t=1s t = 0.3 s Fractional Softening Fractional Softening 0.8 0.8 εT = 1.3 0.6 0.6 εT = 1.7 0.4 0.4 0.2 0.2 εC = 0.6 εC = 0.75 0 0 0 1 2 3 4 5 0 1 2 3 4 5 Strain Strain • Strain dependent softening for strains higher than εc εT = 2.2·εc • Strain independent behaviour for strains higher than εT
- 7. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Softening Kinetics 200 T = 1100ºC -1 Strain rate=1 s εc εT 150 Stress (MPa) 100 50 I II III 0 0 1 2 3 4 5 Strain
- 8. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Softening Kinetics 200 T = 1100ºC •Range I: (ε < εc) -1 Strain rate=1 s εc εT Static Recrystallization 150 Stress (MPa) •Range III: (εT < ε) 100 Metadynamic Recrystallization 50 I II III •Range II: (εc <ε < εT) 0 Static and Metadynamic 0 1 2 3 4 5 Recrystallization Strain
- 9. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite εT : Critical Value to achieve the minimum dynamic recrystallized fraction for a complete post-dynamic softening by metadynamic recrystallization Coarse Grained Austenite: εc High Industrial Conditions: – εT much higher than applied strain – Deformation and Softening Mechanisms: Ranges I / II
- 10. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Different Softening Mechanisms Range I (ε < εc): Static Recrystallization – Initial Grain Size t0.5 SRX = f (D0 ,ε ,ε ,[Nb ],T ) & – Strain – Strain Rate – Temperature Range III (εΤ < ε): Metadynamic Recrystallization – Strain Rate t0.5 MDRX = f (ε ,T ) & – Temperature Grain Refinement
- 11. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Softening Kinetics Region II (εc < ε < εT): Static and Metadynamic Recrystallization ⎡ ⎛ ⎛ t ⎞ ⎞⎤ X MDRX = MDRX Xf ⎢1 − exp⎜ − 0.693⎜ ⎜ ⎟ ⎟⎥ ⎟⎟ ⎜ ⎢ ⎣ ⎝ ⎝ t 0.5MDRX ⎠ ⎠⎥ ⎦ ⎡ ⎛ ⎛ t ⎞ ⎞⎤ X = X MDRX + X SRX X SRX = X f ⎢1 − exp SRX ⎜ − 0.693⎜ ⎟ ⎟⎥ ⎜ ⎜ ⎟⎟ ⎢ ⎣ ⎝ ⎝ t 0.5SRX ⎠ ⎠⎥ ⎦ ε − εc MDRX Xf = X SRX = 1 − X f MDRX εT − εc f
- 12. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Softening Kinetics 1 1 XSRX 0.8 0.8 Fractional Softening Fractional Softening εT XMDRX XSRX εT XMDRX 0.6 0.6 εc 0.4 0.4 X εc -1 0.2 Strain rate = 1 s 0.2 X t=1s Strain rate = 5 s-1 t = 0.3 s 0 0 0 1 2 3 4 5 0 1 2 3 4 5 Strain Strain
- 13. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Post-Dynamic Softening with Time 1 Strain = 0.9 Strain rate = 1 s-1 • Applied Strain in Range II Fractional Softening 0.8 0.6 • Sum of Individual Components: Overall 0.4 XMDRX Softening 0.2 XSRX 0 0.01 0.1 1 10 100 Time (s)
- 14. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Dependence of t0.5 with Applied Strain 100 • Below εc: Strong dependence 10 on strain εc εp εT • ε > εT: Strain independent t0.5 (s) 1 T = 1100ºC • εc< ε < εT: Correct Prediction Strain rate = 1 s-1 D0 = 806 μm 0.1 0.1 1 10 Strain
- 15. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Microstructural Evolution Thin Slab Casting and Direct Rolling Technology – Heterogeneous Initial Grain Size – Solid Solution Recrystallization delayed Need of a Model taking into account these singularities
- 16. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Modeling Microstructural Evolution Initial variables: – Processing variables: Strain, strain-rate, temperature… – Initial Grain Size Distribution Static and Metadynamic Softening Interaction with Precipitation Grain Size Evolution Outputs: – Recrystallized and Unrecrystallized Grain Size Distributions at the entry of any rolling pass
- 17. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Model Validation Torsion Test Schedule Initial Grain Size Distribution T 30 1400 ºC 25 Frequency (%) ε = 0.93 20 strain rate = 5 s-1 T = 1080ºC 15 1100 ºC (20 min) 10 T = 1000ºC 5 0 0 500 1000 1500 2000 2500 3000 Grain Size (μm) t Mean Grain Size: 806 μm
- 18. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Model Validation Results 0.4 Model Experimental 0.3 Area Fraction 0.2 0.1 0 20 100 180 260 340 420 500 Grain Size (μ m) XSRX = 0.86 XMDRX = 0.12
- 19. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Comparison between models Classical New Model Approach 0.4 0.4 Model a) Model b) Experimental Experimental 0.3 0.3 Area Fraction Area Fraction 0.2 0.2 0.1 0.1 0 0 20 100 180 260 340 420 500 20 100 180 260 340 420 500 Grain Size (μm) Grain Size (μm)
- 20. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Conclusions Results show that once the applied strain is higher than the critical value εc, there is a transition regime between εc and εT where both classical static and metadynamic recrystallization softenings intervene. In the analyzed 0.035%Nb microalloyed steel, for the case of coarse initial grain sizes, a value of the transition strain equal to εT = 2.2·εc has been obtained. Taking into account the typical reductions of the first rolling passes, real situations will correspond to fully classical static recrystallization (ε < εc) or to mixed conditions (ε > εc).
- 21. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite Conclusions (II) The model considers a mean recrystallized microstructure combining both classical static and metadynamic grains through the law of mixtures. The model predictions have been validated with specific tests. The model leads to good predictions when both static and metadynamic softening are included for applied strains ε > εc. Some important divergences are observed if only metadynamic softening is considered.
- 22. Study of Metadynamic Recrystallization Phenomena in Coarse Grained Nb Microalloyed Austenite