This document studies the dynamic recrystallization behavior of a TRIP steel microalloyed with niobium and aluminum when subjected to hot compression tests. It was found that aluminum addition decreases grain boundary energy and increases the driving force for boundary migration, accelerating dynamic recrystallization. Peak strain was related to the Zener-Hollomon parameter and initial grain size. Dynamically recrystallized grain size decreased with increasing strain rate. The kinetics of dynamic recrystallization were characterized for the TRIP steel and compared to other microalloyed steels.

1. Dynamic Recrystallization of a
Nb bearing Al-Si TRIP steel
R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe
puranga@ceit.es
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastián
Basque Country, Spain
MS&T’09 Conference
October 25-29, 2009, Pittsburgh, PA

2. Introduction
• TRIP steels
– Higher Strength: Nb microalloyed CMnAlSiP
– Extensive research: Phase transformations,
Mechanical Properties and Finishing
Operations
• Hot Working of TRIP steels
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3. Material and Experimental
• Typical levels for an Al-Si TRIP steel
microalloyed with 0.03%Nb
C Si Mn P Al Nb Cr
TRIP1 0.18 0.48 1.87 0.018 1.18 0.03 0.05
TRIP2 0.23 0.71 1.45 0.007 0.96 0.024 0.10
• Compression tests: Bähr 805 Dilatometer
• Tdef: 950 to 1150ºC
• Strain rates: 0.1, 1 and 5 s-1
• Initial grain sizes: 20, 78 and 106 µm
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4. Flow curves
250
D0=78 μm ε '=5 s-1
200 Effect of Temperature
σ (MPa)
150
100
1150ºC
50 1100ºC
1000ºC
950ºC
0 250
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.1 s-1
ε D0=106 μm T=1100ºC 1 s-1
200
5 s-1
σ (MPa)
150
100
50
Effect of Strain Rate
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
ε
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5. Definition of Critical Strain-Peak
Strain
• εc : change in slope in the dσ/dε – σ curve
300
-1
D0=78 μm T=1000ºC ε '=1 s
200
εc
100
dσ/dε
0
125 130 135 140 145 150 155 160
εp
-100
-200
σ (MPa)
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6. Peak Strain – Critical Strain
• Constant relationship: ε c = 0.79ε p
0.6
0.5
0.4 εc = 0.79 εp
0.3
εc
0.2
20 μm
0.1 78 μm
106 μm
0
0 0.1 0.2 0.3 0.4 0.5
εp
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7. Stress-strain behavior
• Sellars and Tegart /1972/:
Z=ε
⎛ Q def ⎞
& exp⎜ ⎟ = A sinh [
(ασ p ) n ]
⎝ RT ⎠
• n = 4.5 , α = 0.012
Qdef = 420 kJ/mol
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8. Stress-strain behavior
⎛ 420000 ⎞
Z = ε exp⎜
& ⎟ = 1.153 × 10 sinh( 0.012σ p )
15
[ ]
4.5
⎝ RT ⎠
45
Ln Z (Z= ε exp (Q/RT))
40
.
35
20 μm
78 μm
106 μm
30
-1 0 1 2
Ln (sinh (ασp))
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9. Activation energy, Qdef
• Effect of Al and Nb
450 450
TRIP1
400 Present work 400
with Nb with Nb
Q, kJ/mol
Q, kJ/mol
TRIP2
350 350
no Nb
300 no Nb
300
Poliak et al. Poliak et al.
Ref. [8]
250
250
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 0.8 1.3 1.8
Al, % Al, %
E.I. Poliak, and F. Siciliano, Hot Deformation Behavior of Mn-Al and Mn-Al-Nb Steels,
MS&T 2004 Conf. Procs., 2004, p 39-45
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10. Peak Strain Dependence
• Two regions: Zlim ~ 1-2·1016 s-1
1
εp
20 μm
78 μm
106 μm
0,1
1E+14 1E+15 1E+16 1E+17 1E+18 1E+19 1E+20
Z (s-1)
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11. Peak Strain Dependence
• Two regions: Zlim ~ 1-2·1016 s-1
εp 1
20 μm
Do 78 μm
106 μm
Poliak (1.3%Al)
Poliak (1%Al)
0.1
1E+14 1E+15 1E+16 1E+17 1E+18 1E+19 1E+20
Z (s-1)
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12. Peak Strain Prediction
• Equation valid for Z < 1·1016 s-1
ε p = 1.05 ×10 −3 D o.11 Z 0.145
0
1
0.8
εp (calculated)
0.6
0.4
78 μm
0.2 20 μm
106 μm
0
0 0.2 0.4 0.6 0.8 1
εp (experimental)
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13. Dynamic Recrystallization
Evolution for low Z
Dγ = 78 μm Z = 2.52 1014 s-1
100
80 εp
Stress (MPa)
60
Xrex = 30% Xrex = 100%
40
20
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Strain
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14. Dynamic Recrystallization
Evolution for high Z
Dγ = 78 μm Z = 4.10 1018 s-1
250
εp
200
Stress (MPa)
150
Xrex = 1% Xrex = 4% Xrex = 13%
100
50
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Strain
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15. Dynamically Recrystallized
Grain Size
Z = 2.52 1014 s-1 Z = 9.15 1014 s-1 Z = 1.26 1016 s-1
Drex = 20 μm Drex = 17.4 μm Drex = 13.5 μm
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16. Dynamically Recrystallized
Grain Size
• Grain size dependence on Z
100
Drex =1381 Z-0.13
Drex (μm)
10
1
1E+14 1E+15 1E+16 1E+17
-1
Z (s )
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17. Conclusions
• The kinetics of dynamic recrystallization (DRX) have been investigated in a
TRIP steel containing Nb and Al. A relationship of εc = 0.79 εp fitted well the
experimental results.
• Two separate regions are observed in peak strain / Zener-Hollomon plot:
– Low Z values: increasing εp for increasing Z. Fitting equation for peak strain.
– Z values higher than ~1016 s-1: saturation on the peak strain. No trend with Z or D0.
• An acceleration of dynamic recrystallization is observed in the studied TRIP
steel comparing to plain C-Mn and Nb microalloyed steels.
– Aluminum addition decreases the SFE of austenite leading to an increase of the net driving
force for boundary migration.
• The dynamically recrystallized microstructure has been analyzed in the
TRIP steel; Ddyn increases as Z diminishes.
• A decrease in the dynamically recrystallized grain size is observed when
comparing to Nb microalloyed steels deformed under the same conditions.
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18. Acknowledgments
• Authors acknowledge Financial support
from the Spanish Department of Industry,
Tourism and Commerce (Programa de
Proyectos Consorciados, FIT 170300-
2007-1).
MS&T’09 Conference, Pittsburgh, PA

19. Dynamic Recrystallization of a
Nb bearing Al-Si TRIP steel
R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe
puranga@ceit.es
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastián
Basque Country, Spain
MS&T’09 Conference
October 25-29, 2009, Pittsburgh, PA
MS&T’09 Conference, Pittsburgh, PA