Three main topics are covered in this paper regarding Nb-Mo interactions in microalloyed steels. First, the synergetic behavior of Nb and Mo enhancing solute drag effects and modifying recrystallization and precipitation kinetics in austenite under hot working conditions are analyzed. Then, the effect of different microalloying additions on the final phase transformations will be exposed. In addition to composition and austenite conditioning effect on the phases formed and corresponding CCT diagrams, a quantitative study using EBSD technique has been performed in order to measure unit size distributions and homogeneity of complex microstructures. Finally, the contribution of different strengthening mechanisms to yield strength has been evaluated for different coiling temperatures and compositions.

1. Effects of combining Nb ó
and Mo in
HSLA Steels: From austenite
conditioning to final microstructure
N. Isasti, B. Pereda, B. López, J.M. Rodriguez-Ibabe and Pello Uranga
puranga@ceit.es
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastian, Basque Country, Spain

2. Summary
MULTIPLE MICROALLOYING
Nb-Mo MICROALLOYED STEELS
• High strength
• Low temperature toughness
Nb-Mo microalloyed steels
Niobium
Molybdenum
Strain
Accumulation
Microstructural
refinement
Increase of
hardenability

3. Combined effect of Nb-Mo on
• Austenite Conditioning
– Softening kinetics
– Non-recrystallization temperature
• Phase Transformation
– CCT Diagrams
– Unit size and microstructural homogeneity
• Mechanical Properties
– Tensile tests
– Strengthening contributions

4. CHEMICAL COMPOSITIONS

5. Materials
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

6. 1. AUSTENITE CONDITIONING

7. Nb-Mo Steels during hot-working
• The use of Nb is well known because of its effect
retarding recrystallization.
• The addition of Mo to Nb microalloyed steels may
introduce significant changes in the microstructural
evolution during hot working.
• For example, it has been reported that Mo in solid
solution produces a strong retardation effect on
dynamic and static recrystallization.
• Therefore, the combination of both elements
enhances strain accumulation prior to final cooling
strategy.

8. Static Recrystallization Kinetics
t 0.5  9.92 x10 11  D0   5.6 D0
 0.15
 275000

 180000 


  0.53  exp 
 185   Nbeff 
  exp 
 RT 

 T

Nbeff  Nb
Nbeff
for 0.03% Nb-Mo microalloyed steels
for 0.06% Nb-Mo microalloyed steels
Nbeff
for Nb microalloyed steels
 1.19Nb  0.09Mo
 1.19Nb  0.032Mo
50
3Nb
3Nb-Mo
6Nb
6Nb-Mo16
6Nb-Mo31
DSRX  1.8D

0.56
0
1
t0.5 (cal.)
40
30
20
10
0
0
10
20
30
t0.5 (exp.)
40
50

9. Dependence of Tnr as a function of the
interpass time ( = 0.4)
1100
1075
Tnr (ºC)
1050
6Nb-Mo31
6Nb
1025
3Nb-Mo31
1000
975
3Nb
950
0
10
20
Interpass time (s)
30
40

10. Low Nb (0.03%Nb)
Fractional Softening (%)
tip = 10 s,
 = 0.4
100
Tnr = 985ºC
80
60
Precipitation
3Nb
T nr =1026 ºC
40
20
solute drag
3Nb-Mo31
0
7
7.5
8
10000/T (1/K)
8.5
9

11. High Nb (0.06% Nb)
Fractional Softening (%)
tip = 30 s,
 = 0.4
100
Precipitation
T nr =1030ºC
80
6Nb
60
T nr= 1045ºC
40
6Nb-Mo31
20
0
7
7.5
8
10000/T (1/K)
8.5
9

12. 2. PHASE TRANSFORMATIONS

13. Thermomechanical schedule
CONTINUOUS COOLING TRANSFORMATION STUDY
Precipitate
dissolution
Austenite
conditioning
Continuous
cooling
Cooling rates: 0.1-200ºC/s
Cycle A→ Undeformed austenite
Cycle B→ Deformed austenite (Strain = 0.4)
Cycle C → Deformed austenite (Strain = 0.8)

14. Transformation Products
PF
DP
POLYGONAL FERRITE
DEGENERATED PEARLITE
GF
M
QF
BF
BAINITIC FERRITE
MARTENSITE
QUASIPOLIGONAL FERRITE
GRANULAR FERRITE

15. Continuous Cooling Transformation diagrams (CCT)
EFFECT OF CHEMICAL COMPOSITION
Effect of the addition of Mo
1000
Temperature (ºC)
800
PF
DP
GF+QF
600
BF
400
ºC/s
200
Cycle A
HV
6NbMo0
6NbMo31
200 100 50
20
10 5
2
1
0.5
221 220 240 196 178 170 167 158 154
285 253 229 210 207 199 197 196 176
0.1
139
136
0
0.1
1
Mo↑
10
100
Time (s)
Transformation start
temperature ↓
1000
10000

16. Continuous Cooling Transformation diagrams (CCT)
EFFECT OF CHEMICAL COMPOSITION
Effect of the addition of Nb
1000
Temperature (ºC)
LOW Nb
PF+P
Nb↑
Transformation start
temperature ↓
HIGH Nb
800
Nb↑
Transformation start
temperature ≈
DP
GF+QF
600
BF
400
Steel
200
0
Cycle A
CMn
3NbMo0
6NbMo0
0.1
HV
1
200 100 50
20
10 5
2
1
0.5
192 180 155 145 133 131 136 121 131
256 239 202 201 181 194 180 155 156
277 292 256 223 220 204 200 176 162
10
100
Time (s)
1000
Cycle A
Cycle B
CMn
ºC/s
D0 (μm)
19
105.3
121.6
3NbMo0
17
117.6
135.9
6NbMo0
14
142.9
165
3NbMo31
12
166.7
192.5
6NbMo31
14
142.9
165
0.1
107
150
130
10000
Sv (mm-1 )

17. Continuous Cooling Transformation diagrams (CCT)
EFFECT OF THE THERMOMECHANICAL SCHEDULE
Effect of the amount of deformation in austenite
1000
Temperature (ºC)
800
PF
DP
QF+GF
600
Ms
BF
400
200
0
ºC/s 200 100 50
6NbMo31
ε=0 (Cycle A) HV 285 253 229
ε=0.4 (Cycle B)
246 246 232
ε=0.8 (Cycle C)
246 261 247
0.1
1
20
10 5
1
0.5
210 207 199 197 196 176
220 215 204 192 183 181
229 217 205 199 186 175
10
100
Time (s)
ACCUMULATED
DEFORMATION ↑
2
1000
0.1
136
155
153
10000
Transformation start
temperature ↑

18. EBSD Quantification
6NbMo0 (1ºC/s)
RECRISTALLYZED γ
DEFORMED γ

19. EBSD Quantification
Mean crystallographic unit sizes
16
16
15º
4º
6NbMo0_Cycle A
6NbMo0_Cycle A
6NbMo0_Cycle B
12
Mean Grain Size (µm)
Mean Grain Size (µm)
6NbMo0_Cycle B
6NbMo31_Cycle A
6NbMo31_Cycle B
8
4
12
6NbMo31_Cycle A
6NbMo31_Cycle B
8
4
(b)
(a)
0
0.01
0.1
1
10
Cooling Rate (K/s)
Accumulation of
deformation in γ
100
1000
Cycle B
0
0.01
0.1
1
10
Cooling Rate (K/s)
Microstructural
refinement
100
1000

20. EBSD Quantification
Microstructural heterogeneity → Dc20%/Dmean
Dc20%
Cut off grain size at 80% area fraction
in a grain size distribution histogram
12
6NbMo0_Cycle B
10
6NbMo31_Cycle B
Dc20% / D mean (15º)
6NbMo0_Cycle C
8
6NbMo31_Cycle C
FORMATION OF BAINITIC
FERRITE
6
4
2
0
0.01
0.1
1
10
Cooling Rate (K/s)
100
1000
FERRITIC MICROSTRUCTURES

21. 3. MECHANICAL PROPERTIES

22. Thermomechanical schedules
COILING SIMULATIONS – PLANE STRAIN COMPRESSION
Precipitate
dissolution
Austenite
conditioning
Isothermal
maintenance
Coiling temperatures:
650ºC, 550ºC, 450ºC

23. Final Microstructures
6NbMo0
550ºC
QUASIPOLIGONAL FERRITE
GRANULAR FERRITE
650ºC
POLYGONAL FERRITE
PERLITE

24. 3. MECHANICAL PROPERTIES
Strength

25. Mechanical properties
YIELD STRENGTH - TENSILE STRENGTH
Yield Strength/Tensile
strength (MPa)
700
600
TS
500
YS
400
3NbMo0
3NbMo31
6NbMo0
6NbMo31
300
400
450
500
550
600
Coiling temperature (ºC)
650
700

26. Mechanical properties
CORRELATION BETWEEN MICROSTRUCTURE-MECHANICAL
PROPERTIES
σ y  f (σ0 ,  ss ,   ,  gs ,  ppt )
Composition
Dislocation
Density
Precipitation
Grain/Unit Size

27. Mechanical properties
Grain Size

 1
π
σ gs  1.05 * αMμ b   f i θ i 
 f i ·d 2º 2
10 θi 15º 
2θi 15º


Low angle
boundary fraction
High angle
boundary fraction
A. Iza-Mendia, I. Gutierrez, Materials Science and Engineering A, vol. 561, 2013, pp, 40-51

28. Mechanical properties
Grain Size
500
Yield Strength (MPa)
Nb-Mo
450
Nb
400
3NbMo0
3NbMo31
6NbMo0
6NbMo31
350
3
4
5
Mean grain size 2º (μm)
MICROSTRUCTURAL
REFINEMENT
6
• Mechanical strentgh ↑
•Toughness ↑

29. Mechanical properties
Dislocation Density
σ ρ  αMμb ρ
2
ρ
ub
Kernel Average
misorientation for θ<2º

30. Mechanical properties
6NbMo31
Precipitation
Hardening
100 nm
100 nm
550ºC
650ºC
9
3NbMo0
0.5
σ ppt
f
x
 10.8 v ln(
)
4
x
6.125  10
Mean size of precipitates (nm)
3NbMo31
6NbMo0
6NbMo31
8
7
6
400
450
500
550
600
Coiling Temperature (ºC)
650
700

31. Contributions to Strength
3NbMo31
3NbMo0
MOD
EXP
423
387
426
401
407
414
456
460
56%
57%
7%
4%
22%
26%
18%
431
397
15%
14%
80%
σy (MPa)
61%
62%
58%
60%
40%
2%
2%
20%
20%
σy (MPa)
80%
435
457
19%
100%
100%
MOD
EXP
63%
60%
40%
5%
0%
20%
20%
20%
17%
17%
17%
0%
0%
650
650
550
450
Coiling temperature (ºC)
428
406
454
410
445
408
100%
100%
80%
MOD
EXP
457
426
482
434
461
473
57%
59%
12%
5%
80%
59%
59%
σy (MPa)
σy (MPa)
60%
60%
40%
4%
20%
20%
16%
8%
10%
19%
17%
14%
63%
60%
40%
14%
0%
21%
18%
20%
16%
13%
22%
14%
0%
0%
650
550
450
Coiling temperature (ºC)
650
UNIT SIZE
PRECIPITATION
DISLOCATIONS
6NbMo31
6NbMo0
MOD
EXP
550
450
Coiling temperature (ºC)
550
450
Coiling temperature (ºC)
COMPOSITION

32. CONCLUSIONS

33. Conclusions
• Nb and Mo show synergetic mechanisms
ideal for:
– Strain accumulation during austenite
conditioning
– Transformation start temperature control
– Microstructural refinement after transformation
• The formation of low-angle boundary
substructure is the main contribution to
strength

34. ACKNOWLEDGEMENTS

35. Acknowledgements
• IMOA and CBMM
• Prof. Hardy Mohrbacher
• Spanish Government MINECO (MAT200909250 and MAT2012-31056)
• Basque Government (PI2011-17)
• Thermomechanical Treatments Group at CEIT

36. Effects of combining Nb ó
and Mo in
HSLA Steels: From austenite
conditioning to final microstructure
N. Isasti, B. Pereda, B. López, J.M. Rodriguez-Ibabe and Pello Uranga
puranga@ceit.es
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastian, Basque Country, Spain