Dynamic Recrystallization of a Nb bearing Al-Si TRIP steel

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


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


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


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


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=


Stress-strain behavior

• Sellars and Tegart /1972/:

• n = 4.5 , α = 0.012

( )[ ]npdef sinhA

RTQexpZ ασ=⎟

⎠⎞

⎜⎝⎛ε= &

Qdef = 420 kJ/mol


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= &


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


• 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


• 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


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


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%


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%


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


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


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.


Acknowledgments

• Authors acknowledge Financial support from the Spanish Department of Industry, Tourism and Commerce (Programa de Proyectos Consorciados, FIT 170300-2007-1).


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

Dynamic Recrystallization of a Nb bearing Al-Si TRIP steel

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