Effects of combining Nb ó
and Mo in
HSLA Steels: From austenite
conditioning to final microstructure

N. Isasti, B. Pereda...
Summary
MULTIPLE MICROALLOYING

Nb-Mo MICROALLOYED STEELS

• High strength
• Low temperature toughness
Nb-Mo microalloyed ...
Combined effect of Nb-Mo on
• Austenite Conditioning
– Softening kinetics
– Non-recrystallization temperature

• Phase Tra...
CHEMICAL COMPOSITIONS
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...
1. AUSTENITE CONDITIONING
Nb-Mo Steels during hot-working
• The use of Nb is well known because of its effect
retarding recrystallization.
• The add...
Static Recrystallization Kinetics
t 0.5  9.92 x10 11  D0   5.6 D0

 0.15

 275000

 180000 


  0.53  ex...
Dependence of Tnr as a function of the
interpass time ( = 0.4)
1100
1075

Tnr (ºC)

1050

6Nb-Mo31
6Nb

1025

3Nb-Mo31
10...
Low Nb (0.03%Nb)

Fractional Softening (%)

tip = 10 s,

 = 0.4

100

Tnr = 985ºC

80
60

Precipitation

3Nb

T nr =1026 ...
High Nb (0.06% Nb)

Fractional Softening (%)

tip = 30 s,

 = 0.4

100

Precipitation

T nr =1030ºC

80

6Nb

60

T nr= 1...
2. PHASE TRANSFORMATIONS
Thermomechanical schedule
CONTINUOUS COOLING TRANSFORMATION STUDY

Precipitate
dissolution

Austenite
conditioning

Contin...
Transformation Products
PF
DP

POLYGONAL FERRITE
DEGENERATED PEARLITE

GF

M

QF
BF

BAINITIC FERRITE
MARTENSITE

QUASIPOL...
Continuous Cooling Transformation diagrams (CCT)
EFFECT OF CHEMICAL COMPOSITION
Effect of the addition of Mo
1000

Tempera...
Continuous Cooling Transformation diagrams (CCT)
EFFECT OF CHEMICAL COMPOSITION
Effect of the addition of Nb

1000

Temper...
Continuous Cooling Transformation diagrams (CCT)
EFFECT OF THE THERMOMECHANICAL SCHEDULE
Effect of the amount of deformati...
EBSD Quantification
6NbMo0 (1ºC/s)
RECRISTALLYZED γ

DEFORMED γ
EBSD Quantification
Mean crystallographic unit sizes
16

16

15º

4º

6NbMo0_Cycle A

6NbMo0_Cycle A

6NbMo0_Cycle B

12

...
EBSD Quantification
Microstructural heterogeneity → Dc20%/Dmean
Dc20%

Cut off grain size at 80% area fraction
in a grain ...
3. MECHANICAL PROPERTIES
Thermomechanical schedules
COILING SIMULATIONS – PLANE STRAIN COMPRESSION

Precipitate
dissolution

Austenite
conditioning...
Final Microstructures
6NbMo0

550ºC

QUASIPOLIGONAL FERRITE
GRANULAR FERRITE

650ºC

POLYGONAL FERRITE
PERLITE
3. MECHANICAL PROPERTIES
Strength
Mechanical properties
YIELD STRENGTH - TENSILE STRENGTH

Yield Strength/Tensile
strength (MPa)

700

600
TS

500

YS

400
...
Mechanical properties
CORRELATION BETWEEN MICROSTRUCTURE-MECHANICAL
PROPERTIES

σ y  f (σ0 ,  ss ,   ,  gs ,  ppt )
...
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



Lo...
Mechanical properties
Grain Size
500

Yield Strength (MPa)

Nb-Mo
450

Nb
400
3NbMo0
3NbMo31
6NbMo0
6NbMo31

350
3

4
5
Me...
Mechanical properties
Dislocation Density

σ ρ  αMμb ρ

2
ρ
ub

Kernel Average
misorientation for θ<2º
Mechanical properties
6NbMo31

Precipitation
Hardening

100 nm

100 nm

550ºC

650ºC

9
3NbMo0

0.5

σ ppt

f
x
 10.8 v l...
Contributions to Strength
3NbMo31

3NbMo0
MOD
EXP

423
387

426
401

407
414

456
460

56%

57%

7%

4%

22%

26%

18%

43...
CONCLUSIONS
Conclusions
• Nb and Mo show synergetic mechanisms
ideal for:
– Strain accumulation during austenite
conditioning
– Transf...
ACKNOWLEDGEMENTS
Acknowledgements
• IMOA and CBMM
• Prof. Hardy Mohrbacher
• Spanish Government MINECO (MAT200909250 and MAT2012-31056)
• B...
Effects of combining Nb ó
and Mo in
HSLA Steels: From austenite
conditioning to final microstructure

N. Isasti, B. Pereda...
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Effects of combining Nb and Mo in HSLA Steels: From austenite conditioning to final microstructure

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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.

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Effects of combining Nb and Mo in HSLA Steels: From austenite conditioning to final microstructure

  1. 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. 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. 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. 4. CHEMICAL COMPOSITIONS
  5. 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. 6. 1. AUSTENITE CONDITIONING
  7. 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. 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. 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. 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. 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. 12. 2. PHASE TRANSFORMATIONS
  13. 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. 14. Transformation Products PF DP POLYGONAL FERRITE DEGENERATED PEARLITE GF M QF BF BAINITIC FERRITE MARTENSITE QUASIPOLIGONAL FERRITE GRANULAR FERRITE
  15. 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. 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. 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. 18. EBSD Quantification 6NbMo0 (1ºC/s) RECRISTALLYZED γ DEFORMED γ
  19. 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. 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. 21. 3. MECHANICAL PROPERTIES
  22. 22. Thermomechanical schedules COILING SIMULATIONS – PLANE STRAIN COMPRESSION Precipitate dissolution Austenite conditioning Isothermal maintenance Coiling temperatures: 650ºC, 550ºC, 450ºC
  23. 23. Final Microstructures 6NbMo0 550ºC QUASIPOLIGONAL FERRITE GRANULAR FERRITE 650ºC POLYGONAL FERRITE PERLITE
  24. 24. 3. MECHANICAL PROPERTIES Strength
  25. 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. 26. Mechanical properties CORRELATION BETWEEN MICROSTRUCTURE-MECHANICAL PROPERTIES σ y  f (σ0 ,  ss ,   ,  gs ,  ppt ) Composition Dislocation Density Precipitation Grain/Unit Size
  27. 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. 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. 29. Mechanical properties Dislocation Density σ ρ  αMμb ρ 2 ρ ub Kernel Average misorientation for θ<2º
  30. 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. 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. 32. CONCLUSIONS
  33. 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. 34. ACKNOWLEDGEMENTS
  35. 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. 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

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