The document summarizes a study on the effect of composition and thermomechanical processing on the austenite transformation in Nb-Mo microalloyed steels. Seven steel compositions containing 0.05% C and varying amounts of Nb and Mo were subjected to two different thermomechanical cycles, involving deformation at different temperatures and cooling rates. Microstructural characterization showed the transformed phases depended on composition and processing. Dilatometry curves and continuous cooling transformation diagrams were produced to analyze the austenite transformation kinetics and phase stability regions.

1. Titulo de la
Effect of composition and thermomechanical
presentación
process on the austenite transformation in
Nb-Mo microalloyed steels
October 21, 2010 – Houston, Texas
N. Isasti, B. López and P. Uranga
puranga@ceit.es
(CEIT and TECNUN, Univ. Navarra)
A. Leff, E. Toby, M. Hartshorne, C. Wrinkler and M. Taheri (Drexel U.)
M. Grimes (Lehigh U.)

2. Introduction
• Multiple microalloying for high performance
grades:
– High strength and low-T toughness
• Combined effect – synergies Nb-Mo
• Effect of thermomechanical route:
– Recrystallized austenite
– Deformed austenite
• Reducing grain size (unit size) in transformed
phases:
– strength and toughness improved

3. Steel compositions and Techniques
EXPERIMENTAL

4. Chemical compositions of steels (wt%)
Steel C Mn Si Nb Mo Al N
CMn 0.05 1.58 0.05 - 0.01 0.03 0.005
3NbMo0 0.05 1.6 0.06 0.029 0.01 0.028 0.005
3NbMo16 0.05 1.58 0.04 0.03 0.16 0.027 0.005
3NbMo31 0.05 1.57 0.05 0.028 0.31 0.028 0.005
6NbMo0 0.05 1.56 0.05 0.06 0.01 0.028 0.004
6NbMo16 0.05 1.6 0.05 0.061 0.16 0.03 0.005
6NbMo31 0.05 1.57 0.05 0.059 0.31 0.031 0.005

5. Chemical compositions of steels (wt%)
Steel C Mn Si Nb Mo Al N
6NbMo0 0.05 1.56 0.05 0.06 0.01 0.028 0.004
6NbMo16 0.05 1.6 0.05 0.061 0.16 0.03 0.005
6NbMo31 0.05 1.57 0.05 0.059 0.31 0.031 0.005

6. Experimental Procedure
• Bähr DIL805D deformation dilatometer
• Optical Microscopy
• Philips XL30cp Scanning Electron Microscope (SEM), using
TSL (TexSEM laboratories) MSC 2002 equipment.
• Field Emission Scanning Electron Microscope (FEG-SEM)
Jeol JSM-7000F, using HKL Channel5 EBSD
• Vickers hardness (HV 1 kg)

7. Schematics of thermomechanical schedules
Temperature (ºC)
1250ºC, 5min
20ºC/s→ 12s
ε=0.3, ε&& = 1s −−11
1150ºC, ε = 0.3, ε =1s-1
20ºC/s→
5s
ε = 0.4εε = 1s −−11
900ºC, ε =0.4, ,&& =1s-1
Cycle A Cycle B
Time (s)

8. Microstructural Characterization and Dilatometry Curves
RESULTS

9. Optical Micrographs
6NbMo0
Cycle A Cycle B
0.1 ºC/s PF+GF(+P)
0.5 ºC/s PF+QF+GF

10. Optical Micrographs
6NbMo0
Cycle A Cycle B
10 ºC/s QF+GF
QF+GF+
100 ºC/s
BF+M

11. Dilatometry curves
6NbMo31
Cycle B
0.004 100
0.1ºC/s 0.1ºC/s
0.5ºC/s 0.5ºC/s
Transformed fraction (%)
10ºC/s 80 10ºC/s
0 100ºC/s 100ºC/s
60
ΔL/Lo
-0.004
40
-0.008
20
-0.012 0
0 200 400 600 800 0 200 400 600 800
Temperature (ºC) Temperature (ºC)
ΔL/L0 vs Temperature (ºC) Transformed Fraction vs
Temperature (ºC)

12. Dilatometry curves – Transfomation Rates
6NbMo31 Cycle B
0.6 60
0.1ºC/s 10ºC/s
0.5ºC/s 100ºC/s
Transformation Rate
Transformation Rate
0.4 40
0.2 20
0 0
0 200 400 600 800 0 200 400 600 800
Temperature (ºC) Temperature (ºC)

13. Phase Stability Regions
CCT DIAGRAMS

14. CCT Diagrams
6NbMo0
Cycle A Cycle B
1000 1000
900 900
800 800 A
A PF
700 PF 700
Temperature (ºC)
Temperature (ºC)
45% 65%
88%
15% 92%
GF 25% GF P
600 GF+QF 600 25%
GF+QF
500 500 BF
BF
Ms
400 Ms M 400 M
300 300
ºC/s 200 100 50 20 10 5 2 1 0.5 0.1
200 ºC/s 200 100 50 20 10 5 2 1 0.5 0.1 200
HV 268 265 257 227 215 210 209 197 185 160
100 6NbMo0 HV 279 275 272 240 220 207 204 202 197 191
100 6NbMo0
Cycle A Cycle B
0 0
0.1 1 10 100 1000 10000 0.1 1 10 100 1000 10000
Time (s) Time (s)

15. CCT Diagrams
6NbMo16
Cycle A Cycle B
1000 1000
900 900
800 800
A PF
A PF
700 700
Temperature (ºC)
Temperature (ºC)
87% 55%
85%
15%
GF GF
600 600 GF+QF
GF+QF
500 BF 500
BF
400 Ms M 400
85%
Ms M
300 300
200 ºC/s 200 100 50 20 10 5 2 1 0.5 0.1 200 ºC/s 200 100 50 20 10 5 2 1 0.5 0.1
100 6NbMo16 HV 292 280 271 240 229 219 217 211 203 199
100 6NbMo16 HV 270 268 264 248 234 219 213 212 204 157
Cycle A Cycle B
0 0
0.1 1 10 100 1000 10000 0.1 1 10 100 1000 10000
Time (s) Time (s)

16. CCT Diagrams
6NbMo31
Cycle A Cycle B
1000 1000
900 900
A
800 800
PF
A
700 700
Temperature (ºC)
93%
Temperature (ºC)
PF 19% 41% 65%
70%
600 GF+QF GF 600 GF+QF
GF
500 500 BF
BF 75%
400 Ms M 400 Ms M
300 300
200 ºC/s 200 100 50 20 10 5 2 1 0.5 0.1 200 ºC/s 200 100 50 20 10 5 2 1 0,5 0,1
100 6NbMo31 HV 311 298 292 260 236 220 214 213 212 210
100 6NbMo31 HV 299 292 282 241 232 215 213 201 176 147
Cycle A Cycle B
0 0
0.1 1 10 100 1000 10000 0.1 1 10 100 1000 10000
Time (s) Time (s)

17. Alloying and Retained Strain Effects
CCT ANALYSIS

18. Effect of Retained Strain on CCT
6NbMo31
1000
900
800
A PF
700
Temperature (ºC)
GF
600 GF+QF
500
BF
400 Ms M
300
200 ºC/s 200 100 50 20 10 5 2 1 0.5 0.1
100
6NbMo31
0
0.1 1 10 100 1000 10000
Time (s)

19. Mo Effect on CCT
Cycle B
1000
900
800
A
PF
700
Temperature (ºC)
GF+QF GF
600
500 BF
400 Ms M
300
200 ºC/s 200 100 50 20 10 5 2 1 0.5 0.1
100
Cycle B
0
0.1 1 10 100 1000 10000
Time (s)

20. EBSD / SEM-FEGSEM
LOW COOLING RATE PHASES

21. Low CR-EBSD
Cycle B
Effect of Mo addition
6NbMo0 6NbMo16 6NbMo31
Cooling Rate= 0.1 ºC/s

22. EBSD Cooling Rate= 0.1 ºC/s
Cycle B
Effect of Mo
1.2
1
Accumulated Area Fraction
0.8
0.6
0.4
0.1ºC/s
6NbMo0
0.2
6NbMo16
6NbMo31
0
105
125
135
145
155
5
15
25
35
45
55
65
75
85
95
115
Grain Size (Diameter µm)

23. EBSD / SEM-FEGSEM
IDENTIFICATION OF
SECONDARY PHASES

24. EBSD
6NbMo16
Identification of second phases Cycle B
Degenerated
0.5 ºC/s 1 ºC/s M/A Islands
Pearlite
Fe3C-1
MAD 0.538

25. Bainite + Martensite + (GF+QF)
HIGH COOLING RATE PHASES

26. High CR products – FEGSEM EBSD
6NbMo16
Image Quality (IQ)
20 ºC/s 100 ºC/s
=50 µm; Map15; Step=0.2 µm; Grid700x700 =50 µm; Map15; Step=0.2 µm; Grid700x700

27. EBSD (Electron Backscatter
Diffraction) 6NbMo16
Misorientation Map
20 ºC/s 100 ºC/s
=50 µm; Map15; Step=0.2 µm; Grid700x700 =50 µm; Map15; Step=0.2 µm; Grid700x700
1-15º
>15º

28. EBSD (Electron Backscatter
Diffraction) 6NbMo16
Inverse Pole Figure IPF (z)
20 ºC/s 100 ºC/s
=50 µm; Map15; Step=0.2 µm; Grid700x700 =50 µm; Map15; Step=0.2 µm; Grid700x700

29. Final Remarks
• Effect of Mo and retained strain has been
evaluated
– Slight effect of Mo: increase in ferrite size
– Strong effect of strain
• EBSD unit size measurements may be useful
for toughness predictions
• The generated data will be the input for
modeling

30. Acknowledgements
• Science and Innovation Ministry of Spain
(MAT2009-09250)
• NSF and TMS. Conference Registration Fee
Funding
• N . Isasti for all the experimental results and
analysis (1st year PhD student)
• D. Jorge-Badiola: help on EBSD

31. Titulo de la
Effect of composition and thermomechanical
presentación
process on the austenite transformation in
Nb-Mo microalloyed steels
October 21, 2010 – Houston, Texas
N. Isasti, B. López and P. Uranga
puranga@ceit.es
(CEIT and TECNUN, Univ. Navarra)
A. Leff, E. Toby, M. Hartshorne, C. Wrinkler and M. Taheri (Drexel U.)
M. Grimes (Lehigh U.)