dynamic recrystallization of a nb bearing al-si trip steel
DESCRIPTION
Presentation made at Materials Science and Technology 2009 held in PittsburghTRANSCRIPT
Dynamic Recrystallization of a Nb bearing Al-Si TRIP steel
R. Zubialde, P. Uranga, B. López and J.M. [email protected]
MS&T’09 ConferenceOctober 25-29, 2009, Pittsburgh, PA
CEIT and TECNUN (University of Navarra)Donostia-San SebastiánBasque Country, Spain
MS&T’09 Conference, Pittsburgh, PA
Introduction
• TRIP steels– Higher Strength: Nb microalloyed CMnAlSiP– Extensive research: Phase transformations,
Mechanical Properties and Finishing Operations
• Hot Working of TRIP steels
MS&T’09 Conference, Pittsburgh, PA
Material and Experimental
• Typical levels for an Al-Si TRIP steel microalloyed with 0.03%Nb
• 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
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
MS&T’09 Conference, Pittsburgh, PA
Flow curves
D0=78 μm ε '=5 s-1
0
50
100
150
200
250
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
ε
σ (M
Pa)
1150ºC1100ºC1000ºC950ºC
D0=106 μm T=1100ºC
0
50
100
150
200
250
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
ε
σ (M
Pa)
0.1 s-11 s-15 s-1
Effect of Temperature
Effect of Strain Rate
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Definition of Critical Strain-PeakStrain
• εc : change in slope in the dσ/dε – σ curve
-200
-100
0
100
200
300
125 130 135 140 145 150 155 160
σ (MPa)
d σ/dε
ε c
εp
D0=78 μm T=1000ºC ε '=1 s-1
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Peak Strain – Critical Strain
• Constant relationship:
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.1 0.2 0.3 0.4 0.5εp
ε c
20 78 106
μm
μmμm
εc = 0.79 εp
pc εε 79.0=
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Stress-strain behavior
• Sellars and Tegart /1972/:
• n = 4.5 , α = 0.012
( )[ ]npdef sinhA
RTQexpZ ασ=⎟
⎠⎞
⎜⎝⎛ε= &
Qdef = 420 kJ/mol
MS&T’09 Conference, Pittsburgh, PA
Stress-strain behavior
30
35
40
45
-1 0 1 2
Ln (sinh (ασp))
Ln Z
(Z= ε
exp
(Q/R
T))
20 78 106
.
μm
μmμm
[ ] 5.4p
15 )012.0sinh(10153.1RT
420000expZ σ×=⎟⎠⎞
⎜⎝⎛ε= &
MS&T’09 Conference, Pittsburgh, PA
Activation energy, Qdef
• Effect of Al and Nb
250
300
350
400
450
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
Al, %
Q, k
J/m
ol
with Nb
no Nb
Present work
Poliak et al.Ref. [8]
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
250
300
350
400
450
0.8 1.3 1.8
Al, %
Q, k
J/m
ol
with Nb
no Nb
Poliak et al.
TRIP1
TRIP2
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• Two regions: Zlim ~ 1-2·1016 s-1
0,1
1
1E+14 1E+15 1E+16 1E+17 1E+18 1E+19 1E+20
ε p
Z (s-1)
20 78 106
μm
μmμm
Peak Strain Dependence
MS&T’09 Conference, Pittsburgh, PA
• Two regions: Zlim ~ 1-2·1016 s-1
0.1
1
1E+14 1E+15 1E+16 1E+17 1E+18 1E+19 1E+20
Z (s-1)
ε p
20 78 106 Poliak (1.3%Al)Poliak (1%Al)
μm
μmμmDo
Peak Strain Dependence
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Peak Strain Prediction
• Equation valid for Z < 1·1016 s-1
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1εp (experimental)
ε p (c
alcu
late
d)
78 20 106
μm
μmμm
145.011.0o
3p ZD1005.1 −×=ε
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Dynamic RecrystallizationEvolution for low Z
Dγ = 78 μm Z = 2.52 1014 s-1
0
20
40
60
80
100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Strain
Stre
ss (M
Pa)
εp
Xrex = 30% Xrex = 100%
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Dynamic RecrystallizationEvolution for high Z
Dγ = 78 μm Z = 4.10 1018 s-1
0
50
100
150
200
250
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Strain
Stre
ss (M
Pa)
εp
Xrex = 1% Xrex = 4% Xrex = 13%
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Dynamically RecrystallizedGrain 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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Dynamically RecrystallizedGrain Size
• Grain size dependence on Z
1
10
100
1E+14 1E+15 1E+16 1E+17Z (s-1)
Dre
x (μm
)
Drex =1381 Z-0.13
MS&T’09 Conference, Pittsburgh, PA
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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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
Dynamic Recrystallization of a Nb bearing Al-Si TRIP steel
R. Zubialde, P. Uranga, B. López and J.M. [email protected]
MS&T’09 ConferenceOctober 25-29, 2009, Pittsburgh, PA
CEIT and TECNUN (University of Navarra)Donostia-San SebastiánBasque Country, Spain