Effect of Mn and Si addition on the dynamic transformation of austenite during strip rolling
John J. Jonas
Birks Professor of Metallurgy Emeritus McGill University
Acknowledgements: Chiradeep Ghosh, Vladimir V. Basabe & Clodualdo M. Aranas, Jr.
Hatchett Seminar London, July 16, 2014
• What is Dynamic Transformation?
• What are the microstructural changes that take place during DT?
• What can thermodynamics and kinetics tell us about these changes?
• What is the effect of Mn and Si addition on these phenomena?
• What is the effect of Nb addition?
Questions
3
Research Methodology
4
Strip Rolling Simulations
Strip rolling simulations carried out on a 0.06C-0.3Mn-0.01Si steel with a cooling rate of 7-8 C˚/s, interpass times of 1s, and
pass strains of 0.4 applied at a strain rate of 1s-1.
5
Why does the MFS during simulated strip rolling not increase with decreasing temperature (1s interpass times)?
Strip Rolling MFS Plots
Change in flow curve shape
6
Effect of Interpass Time in Strip Rolling
The shorter the interpass time, the greater the amount of softening due to ferrite formation. The longer the time, the more re-transformation to austenite.
.06C-.3Mn-.01Si
7
Effect of Interpass Time on Re-Transformation
The shorter the interpass time, the greater the amount of softening due to ferrite formation. The longer the time, the more re-transformation to austenite.
8
MFS vs. 1/T Diagram Courtesy of T. Schambron
9
MFS vs. 1/T Diagrams Courtesy: E. Poliak, Arcelor-Mittal, USA
ArcelorMittal, United States
0
0
0
0
0
0
What is going on?
Formation of
Widmanstätten
(displacive) ferrite
V.V. Basabe et al., ISIJ Int., 2010-13 C. Ghosh et al., ISIJ Int., 2013
Widmanstatten Microstructures (0.09%C Steel)
Widmanstatten Microstructures (0.06%C Steel)
Note that the Widmanstätten plates are only 200 nm thick and therefore cannot be seen using optical microscopy. The plates also coalesce into polygonal grains during and after rolling.
Conversion to Polygonal Ferrite
Shear Stress in Torsion
Geometry associated with formation of a pair of self-accommodating Widmanstätten ferrite plates and the corresponding shear stresses.
Time Elapsed During Rolling
ε = 0.5 (35% reduction)
ε = 100 s-1
Time in the deformation
zone = 5 ms
Incremental transformation
time = 100μs (for 1% strain)
.
16
Dynamic Transformation
Austenite
Diffusionless transformation
Allotriomorphic
ferrite
Pearlite
Widmanstätten
ferrite < Ae3
Bainite
Martensite
Dynamic
transformation
Widmanstätten
ferrite > Ae3
Diffusional
transformation
17
Carbon Diffusion Distance during Formation of Widmanstätten Ferrite
0
10
20
30
40
50
60
70
80
90
100
10080604020
Time, s
Dif
fus
ion
dis
tan
ce
, n
m
Temperature range: 700-900°C @ 20°C intervals
Increasing temperature
Increasing temperature
Diffusivity
in fe
rrite
Diffusivity in austenite
0
Plate thickness: 200nm
Distance: 100nm Time: 80μs
18 1. D. A. Porter and K. E. Easterling: Phase Transformations in Metals and Alloys, (1988), Published by Van Nostrand Reinhold (International) Co. Ltd, Molly Millars Lane, Workingham, Berkshire, England. 2. J. Kucera and K. Stransky, Czech. J. Phys. B, 30 (1980), 1315. 3. J. K. Stanley, Trans. AIME, 185 (1949), 752.
Mean diffusion distances of carbon and Mn in ferrite
19
Carbon Mn
0 20 40 60 80 1000
20
40
60
80
100
120
140
160
Mea
n D
iffu
sio
n D
ista
nc
e,
nm
Time, s
743°C
753°C
763°C
773°C
783°C
793°C
803°C
823°C
0 20 40 60 80 1000.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Mean
Dif
fusio
n D
ista
nce, n
m
Time, s
743°C
753°C
763°C
773°C
783°C
793°C
803°C
823°C
O. Thuillier, F. Danoix, M. Goune and D. Blavette; Scripta Materialia, 2006
No substitutional diffusion during displacive transformation
21
Velocity of sound in steel = 6000 m s-1 Therefore time taken to travel 100 nm in steel
~ 16 pico sec
FCC BCC C diffusion Substitutional
diffusion
Displacive ~16 pico sec
Paraequilibrium ~80 micro sec
Orthoequilibrium ~10 sec
Times Required for Three Types of Transformation
Thermodynamic
Considerations
C. Ghosh et al., Acta Mater., 2013 J.J. Jonas & C. Ghosh, Acta Mater. 2013
The Free Energy Obstacles to Dynamic Transformation
Effect of Mn & Si Addition On the Obstacles to Transformation
LL: .01Mn/.24Si HL: 1.4Mn/.24Si LH: .01Mn/.95Si HH:1.4Mn/.95Si
+Si
+Mn
+ Mn +Si Wray, P.J.:
Metall. Trans. A 15A (1984), 2041
Effect of Mn & Si Addition On the Obstacles to Transformation
Note maximum because of approach to delta
ferrite phase field.
Softening as the Driving Force
Data of Peter Wray replotted: Metall. Trans. A: 15A, 1984, 2041.
Strain rate: 2.3 x 10-2s-1
Softening as the Driving Force
Effect of Mn & Si Addition on the Driving Force
These quantities can only be determined experimentally
Comparison of the Driving Forces and Obstacles to the Transformation
30
Summary of Driving and Opposing Forces
Effect of Nb Addition on Dynamic Transformation
● There is no partitioning of Nb during DT. Thus the W. ferrite is supersaturated in Nb.
● There is insufficient time for precipitation of NbCN and therefore no particle hardening.
● As a result of these considerations, deter-mining the driving force for DT (i.e. the net
softening) is more difficult than in plain C steels.
● Conversely, the thermodynamic obstacles to the transformation can be readily evaluated.
Conclusions – Strip Mills
1. MFS vs. 1/T plots exhibit low slopes due to DT.
2. Strip mill (1s) simulations indicate that DT is initiated at the beginning of each pass.
3. The amount of DT ferrite formed & retained increases with decreasing interpass time.
4. Because of the formation of DT ferrite, the volume flow rate increases during rolling.
5. There is C but not substitutional diffusion during the displacive formation of W. ferrite.
Conclusions – Thermodynamics
1. Stressing and straining raises the effective Ae3 and Ae1 temperatures.
2. Widmanstätten ferrite forms when the γ/α flow stress difference is large enough to overcome the free energy obstacles opposing transformation.
3. There is insufficient time for substitutional diffusion, so only displacive and paraequilibrium mechanisms can operate during DT.
4. C diffusion and partitioning (into austenite) during rolling can lead to undesirable ductility issues.
5. Mn addition somewhat opposes and Si addition significantly promotes the formation of Widman-stätten ferrite.
Softening as the Driving Force
Further Work
1. Effect of stress during transformation on variant selection?
2. Dependence of ferrite volume fraction on experimental conditions.
3. Modeling DT and DRX in combination.
4. Application to control of microstructure? Runout table modeling?
5. Application to gauge control. Other?
Mean Flow Stress (MFS)
36
MFS = f(C,T,ε,ε) where: C = composition T = temperature of deformation ε = strain ε = strain rate
MFS can be calculated by:
Jonas J. J., The Hot Strip Mill as an Experimental Tool, ISIJ, 2000; 40(8): 731-738.
Plate Mill Simulation: Dependence of mean flow stress (MFS) on 1000/T for 0.10%C-0.04%Nb-0.30%Mo microalloyed steel. Here, interpass intervals of 30s was employed
Ti-V steel
Interpass time: 0.5s.
Cooling rate: 10°C/s. εp = 0.1.
Pussegoda et al., Metall. Trans. A (1990)
Effect of Interpass Time
38
Strip rolling simulations showing the effect of increasing the interpass time. The MFS displays detectable load drops at short interpass times.
Ti = 1000°C 7 deformations ε = 0.4 per pass ε = 1 s-1
MFS Plots – Plate Rolling
39
Plate rolling simulations showing MFS curves at longer interpass times. The MFS curves converge to the expected MFS for the plate mills.
Ti = 1000°C 7 deformations ε = 0.4 per pass ε = 1 s-1
Experimental Quench Times & CCT Diagrams 0.79C Steel
Effect of Strain on the Phase Diagram
T
%C
Ae1
Ae3
Ae3
Ae1
Texp
Cexp increasing
strain
X
Zr? Ti?
Gleeble Tests – Prof. P. Karjalainen
Conclusions – Critical Strain
1. The double differentiation method can be used to determine the critical strains for DT (as well as for DRX).
2. The DT critical strains of 0.05-0.1 are well below those associated with DRX.
3. Free energy considerations call for three domains of transformation behavior:
i) purely displacive ferrite formation (without carbide formation) at C levels up to about 0.1%;
ii) ferrite formation followed by carbide formation at C levels up to 0.2-0.4%;
iii) ferrite formation accompanied by carbide formation at C levels above 0.2- 0.4%.
Conclusions - Microstructure
1. The displacive formation of Widmanstätten ferrite takes place during DT.
2. Appreciable C diffusion can take place during the 100 μs available (incrementally) during steel rolling; thus carbide formation generally follows ferrite formation (but see below).
3. There is insufficient time for substitutional diffusion and so only displacive and paraequilibrium mechanisms can operate during DT.
Conclusions
1. The displacive formation of Widmanstätten ferrite takes place during DT.
2. Appreciable C diffusion can take place during the 100 μs available (incrementally) during steel rolling; thus carbide formation generally follows ferrite formation (but see below).
3. There is insufficient time for substitutional diffusion and so only displacive and paraequilibrium mechanisms can operate during DT.
4. Straining raises the effective Ae3 and Ae1 temperatures and changes the nature of the phases.
Conclusions, continued
5. In the presence of Mn, DT ferrite, DT carbides and austenite (all 3) can all be present simultaneously.
6. Free energy considerations call for three domains
of transformation behavior:
i) purely displacive ferrite formation (without carbide formation) at C levels up to about 0.1%;
ii) ferrite formation followed by carbide formation at C levels up to 0.2-0.4%;
iii) ferrite formation accompanied by carbide formation at C levels above 0.2-0.4%.
Fe-C Phase Diagram
γ + cementite
≈
600
700
800
900
1000
Tem
pe
ratu
re, °
C
0.5 1.0 1.5
Weight percent, carbon
Ae3
Ae1
Acm
γ
γ + α
α + cementite
α
47
Early Strip Mill Simulations
Samuel et al., THERMEC’88 0.1C-1.27Mn-0.2Si-0.11Nb-0.08V
2s interpass; 0.5 strain
Karjalainen et al., ISIJ Int. 1995 0.08C-1.46Mn-0.04Nb
1 & 3s interpass; 0.24 strain
2013: 0.09C-1.3Mn-0.02Si-0.036Nb 1s interpass; s.r. 2s-1; cooling rate 8 °C/s
Strip Mill Simulations (0.79%C Steel)
T start = 907°C T Finish = 803°C
T start = 800°C; T Finish = 735°C
J.J. Jonas et al. BAC2013
Interpass time – 1s. Cooling rate – 8°C/s. εp = 0.25; ε = 4 s
-1.
Strip Mill Simulations (MFS)
7.5 8.0 8.5 9.0 9.5 10.040
60
80
100
120
140
M
FS
(M
Pa)
10000 / T (K-1)
+ Pussegoda et al., Metall. Trans. A (1990) * Karjalainen et al., ISIJ Int. (1995)
1060 977 903 838 780 T (°C)
727
Research methodology
52
Ae3
Testing atmosphere: Ar + H2 atmosphere Time
Tem
per
atu
re
20 min.
1150°C
1050°C
20 s 200 s
1°C / s 1 min.
Test
temperature
ε = 1 s/2ε =
ε = 3 s/4ε =
Torsion test Torsion simulation
Eight Steels Investigated
Steel Composition Ortho. Ae3
1 0.019 C-0.2 Si-1.5 Mn 872
2 0.11 C-0.26 Si-1.1 Mn-0.038 Nb 848
3 0.22 C-1.56 Si-1.56 Mn-0.045 Nb 860
4 0.06 C-0.01 Si-0.30 Mn 877
5 0.09 C-0.02 Si-1.30 Mn-0.036 Nb 836
6 0.21 C-0.24 Si-1.30 Mn 822
7 0.45 C-0.24 Si-0.70 Mn 772
8 0.79 C-0.24 Si-0.65 Mn 733
Stress-strain Curves
54
J.J. Jonas et al. SRI, 2013
MFS vs. 1/T Diagrams
What is going on? What is the contribution
of DRX?
56
Critical Strain Determinations using Double Differentiation
C. Ghosh et al., SRI, 2013
J.J. Jonas et al., ISIJ Int., 2013
Two Sets of Minima
Double Minima
59
Compression Tests
Critical Strains for DT & DRX
700 750 800 850 900 950 1000 10500.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Temperature,°C
Cri
tic
al
Str
ain DRX
DT
Strip Mill Simulations (Critical Strains)
Widmanstatten Microstructures (0.09%C Steel)
Conversion to Polygonal Ferrite
Austenite
Reconstructive
transformation
Displacive transformation
Allotriomorphic
ferrite
Pearlite
Widmanstätten
ferrite
Bainite
Martensite
Dynamic
transformation
How do Non-Equilibrium Phases Form?
Displacive ferrite
Metastable carbides
G
austenite ferrite
deformation deformation
transformation re-transformation
T > Ae3
Free Energy Changes During Forward and Backward Transformations
H. Mahjoubi, M.S Thesis, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, 2010.
Phases
Free Energy-Composition Diagram
Gibbs energy-composition diagram for 0.79%C steel at 803°C (Ae3 + 70°C) illustrating the Gibbs energy changes taking place during dynamic transformation.
Dynamic Phase Diagram
Quasi-binary paraequilibrium dynamic phase diagram compared with the conventional undeformed diagram.
0.79C Steel
67
Effect of Mn on the Phase Diagram
Calculated quasi-binary paraequilibrium phase diagram for the 0.21%C steel compared with that of the 0.79%C steel.
0.21C-1.3Mn 0.79C-0.65Mn
Effect of Mn & Si Levels on the Phase Diagram
Calculated quasi-binary paraequilibrium phase diagram for the 0.06C-0.30Mn-0.50Si steel compared with that of the 0.79%C steel.
Strip Mill Simulations (1995)
Karjalainen et al., ISIJ Int., Vol. 35 (1995), pp. 1523-1531
Cooling rate: 6°C/s; εp = 0.25; ε = 2 s-1; 0.04%Nb steels
Free Energy-Composition Diagram
0.09%C Steel
Free Energy-Composition Diagram
0.06%C Steel
Free Energy-Composition Diagram
0.79%C Steel
Free Energy-Composition Diagram
0.21%C Steel
Carbide Formation
& the Role of C
Diffusion
Transmission Electron Microscopy
76
[001] Ferrite
0.79C Steel T = 763°C (Ae3 + 30°C)
Carbides within the ferrite grain
Ferrite grain
Courtesy of Dr. Ilana Timokhina and Prof. Elena Pereloma
[111] Ferrite
[660] cementite
Conversion to Polygonal Ferrite
78
Partitioning of Mn & C
~14
3.3
nm
Courtesy of Dr. Ilana Timokhina and Prof. Elena Pereloma
Concentration profiles across the boundary
~22.34 nm
O. Thuillier, F. Danoix, M. Goune and D. Blavette; Scripta Materialia, 2006
No substitutional diffusion during displacive transformation
79
Effect of DT & DRX on Rolling Load
10% drops
Strain rate = 0.4 s-1
Strip Mill Simulations (0.79%C Steel)
T start = 1070°C T Finish = 814°C
T start = 1000°C T Finish = 807°C
C. Ghosh
Interpass time – 1s. Cooling rate – 8°C/s. εp = 0.25; ε = 4 s
-1.
Effect of DT & DRX on Rolling Load
10% drops
Strain rate = 4 s-1
Mixed Grain Sizes-Strip Simulation
F12 - 798°C Cooling Rate – 6°C/s Interpass Time – 3s.
Effect of Polynomial Order
140 160 180 200 220 240 260 2802
4
6
8
10
12
14
16
n = 2
n = 3
n = 4
n = 5
n = 6
Stress (MPa)
-(
)
220 230 240 250 260 2701
2
3
4
5
6
7
8
9
10
11
n = 7
n = 8
n = 9
n = 10
n = 11
n = 12
n = 13
n = 14
n = 15
-(
)
Stress (MPa)
Effect of polynomial order on the second derivative/stress relationship according to the partial curve method
3 4 5 6 7 8 9 10 11 12 13 14 15 160.14
0.16
0.18
0.20
0.22
0.24
0.26
0.28
0.30
0.32
0.34
0.36
0.38
0.40
0.42
DT
DRX
Order of the polynomial
Cri
tical S
train
Order of the polynomial
Effect of Polynomial Order (Entire Curve Method)
140 160 180 200 220 240 260 280
4
6
8
10
12
14
16
18
n = 6
n = 7
n = 8
n = 9
n = 10
n = 11
Stress (MPa)
-(
)
140 160 180 200 220 240 260 280
2
4
6
8
10
12
14
16
18
n = 3
n = 4
n = 5
-(
)
Stress (MPa)
5 6 7 8 9 10 11 12 13 14 15 160.14
0.16
0.18
0.20
0.22
0.24
0.26
0.28
0.30
0.32
0.34
0.36
0.38
0.40
0.42
DT
DRX
Order of the polynomial
Cri
tical S
train
200 210 220 230 240 250 260 270 2802
4
6
8
10
12
14
n = 12
n = 13
n = 14
n = 15
Stress (MPa)
-(
)
Order of the polynomial
Strip Mill Simulations (Critical Strain)
Temperature, °C
Cri
tica
l Str
ain
T start = 907°C T Finish = 803°C
Temperature, °C
Cri
tica
l Str
ain
T start = 800°C T Finish = 735°C
DRX
DT
Ternary orthoequilibrium phase diagram at 700°C for 0.21C steel
Presence of three phases concurrently
Ternary orthoequilibrium phase diagram at 725°C for 0.79C steel
Presence of three phases concurrently
Hypo-Eutectoid steel
Ae1
Equilibrium phase diagram
T
+
%C
T
Ferrite and
Pearlite
Ferrite and
Strain
Dynamic phase diagram
Ae3
ADT
Strain
Carbides
Compression Flow Curves
0.0 0.2 0.4 0.6 0.8 1.0 1.20
20
40
60
80
100
120
140
160
950°C
1050°C
1100°C
1150°C
Str
ess, M
Pa
Strain
Nb-modified Steel=0.5 s-1 ε=0.25 s-1 ε=0.1 s-1 ε
=0.05 s-1 ε
=0.5 s-1 ε
=0.25 s-1 ε
=0.5 s-1 ε
=0.25 s-1 ε
=0.05 s-1 ε
=0.5 s-1 ε
=0.25 s-1 ε
=0.05 s-1 ε
0.0 0.2 0.4 0.6 0.8 1.00
20
40
60
80
100
120
140
160
180
900°C
950°C
1000°C
1050°C
Str
ess,
MP
a
Strain
ε =0.1 s-1
ε =0.1 s-1
ε =0.1 s-1
ε =0.1 s-1
ε =0.25 s-1
ε =0.25 s-1
ε =0.25 s-1
ε =0.25 s-1 ε =0.50 s-1
ε =1 s-1
ε =0.5 s-1
ε =0.5 s-1
ε =0.5 s-1
Low Carbon Steel
0.0 0.2 0.4 0.6 0.8 1.0 1.20
20
40
60
80
100
120
140
160
950°C
1000°C
1050°C
1100°C
1150°C
Strain
Str
ess,
MP
a
Nb-modified TRIP Steel=0.5 s-1 ε=0.25 s-1 ε
=0.05 s-1 ε
=0.5 s-1 ε
=0.05 s-1 ε
=0.5 s-1 ε
=0.05 s-1 ε
=0.05 s-1 ε
=0.5 s-1 ε
=0.05 s-1 ε
=0.25 s-1 ε
* Torsion
Torsion Flow Curves
%Carbon
Gib
bs
fre
e e
ne
rgy
X XX
α γ
T > Ae3
ΔG = ΔGchem+ΔGDeformation
X X
Effect of strain on Gibbs free energy of austenite
92
Double Differentiation Poliak Method (1996)
(∂/∂σ)( –(∂θ/∂σ)) = 0 Ni – (also 304 SS)
Note: Absence of phase change in Ni & SS
Double Minima Steel 1 (Compression)
50 60 70 80 90 100 110 120 1304
5
6
7
8
9
10
11
12
900°C
950°C
1000°C
1050°C
-(
)
Stress (MPa)
Strain rate = 0.1 s-1
60 70 80 90 100 110 120 130 140 1504
5
6
7
8
9
10
11
12
900°C
950°C
1000°C
1050°C
-()
Stress (MPa)
Strain rate = 0.25 s-1
70 80 90 100 110 120 130 140 150 160 170 1804
5
6
7
8
9
10
11
12
900°C
950°C
1000°C
1050°C
-()
Stress (MPa)
Strain rate = 1.0 s-1
60 70 80 90 100 110 120 130 140 150 1604
5
6
7
8
9
10
11
12
900°C
950°C
1000°C
1050°C
-()
Stress (MPa)
Strain rate = 0.5 s-1
Double Minima Steel 2 (Compression)
30 40 50 60 70 80 90 100 110 120 130 1404
5
6
7
8
9
10
Stress (MPa)
950°C
1000°C
1050°C
1075°C
1100°C
1150°C
-()
Strain rate = 0.1 s-1
30 40 50 60 70 80 90 100 1103
4
5
6
7
8
9
10
Stress (MPa)
950°C
1000°C
1050°C
1075°C
1100°C
-()
Strain rate = 0.05 s-1
60 70 80 90 100 110 120 130 1404.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
950°C
1050°C
1075°C
1100°C
1150°C
-()
Stress (MPa)
Strain rate = 0.5 s-1
40 50 60 70 80 90 100 110 120 130 1402
3
4
5
6
7
8
9
10
11
Stress (MPa)
950°C
1000°C
1050°C
1075°C
1100°C
1150°C
-()
Strain rate = 0.25 s-1
Double Minima Steel 3 (Compression)
50 60 70 80 90 100 110 120 130 140 150 1600
2
4
6
8
10
12
14
16
18
900°C
950°C
1000°C
1050°C
1100°C
1150°C
Strain rate = 0.1 s-1
Stress (MPa)
-()
40 50 60 70 80 90 100 110 120
4
6
8
10
12
14
16
18
20
Stress (MPa)
950°C
1000°C
1050°C
1100°C
1150°C
-()
Strain rate = 0.05 s-1
50 60 70 80 90 100 110 120 130 140
6
7
8
9
10
11
Stress (MPa)
950°C
1000°C
1050°C
1100°C
1150°C
-()
Strain rate = 0.25 s-1
50 60 70 80 90 100 110 120 130 140 150
5
6
7
8
9
10
11
Stress (MPa)
950°C
1000°C
1050°C
1100°C
1150°C-(
)
Strain rate = 0.5 s-1
Atom Probe Tomography
98
Courtesy of Dr. Ilana Timokhina and Prof. Elena Pereloma
~14
0 n
m
~22 nm
~37
.3 n
m
~7.83 nm
Carbon segregation to shear bands and sub-boundaries
DT & DRX Critical Strains (Torsion)
ΔT (Experimental Temperature – Ae3), °C ΔT (Experimental Temperature – Ae3), °C
ΔT (Experimental Temperature – Ae3), °C ΔT (Experimental Temperature – Ae3), °C
Double Minima (Torsion)
Conversion to Polygonal Ferrite
800 850 900 950 10000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
DRX Critical Strain
DT Critical Strain
Cri
tic
al
Str
ain
Temperature,°C
TStart
= 1000°C
TFinish
= 807°C
Strip Mill Simulations (Critical Strains)
DRX
DT
800 850 900 950 1000 10500.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
DRX Critical Strain
DT Critical Strain
Cri
tica
l S
tra
in
Temperature,°C
TStart
= 1070°C
TFinish
= 814°C
