Presentation made at Materials Science and Technology 2008 Conference held in Pittsburgh

1. Effect of Alloying Additions in the Final
Microstructure of Nb-Mo Steels Processed by Thin
Slab Direct Rolling Technologies
P. Uranga, J. Ganzarain, D. Jorge-Badiola and J.M. Rodriguez-Ibabe
puranga@ceit.es
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastián
Basque Country, Spain
MS&T’08 Conference
October 6-9, 2008, Pittsburgh, PA

2. Introduction
• Multiple alloying combinations for high strength
properties
• Thin slab direct rolling metallurgical peculiarities
– As-cast coarse grains
– Alloying elements in solid solution
• Study of combined effect in softening and
precipitation kinetics
• Modeling and microstructural evolution
validation for schedule optimization
MS&T’08 Conference, Pittsburgh, PA

3. Material
• Two Nb-Mo microalloyed steels
Steel C Si Mn S Al Mo Nb N
3Nb-Mo16 0.05 0.04 1.58 0.002 0.027 0.16 0.03 0.005
3Nb-Mo31 0.05 0.05 1.57 0.002 0.028 0.31 0.028 0.006
• 0.05%C - 0.03%Nb
– 0.16% Mo
– 0.31% Mo
MS&T’08 Conference, Pittsburgh, PA

4. Deformation Schedules
Soaking: First deformation temperatures
1350ºC, 15 min Tini = 1100ºC Tini = 1050ºC
ΔT = 50ºC
ε1 = ε2 = ε3 = 0.4 Quenching temperatures
1ºC/s
ε4 = 0.5 Tq = 900ºC Tq = 850ºC
Temperature
700ºC, 1hour
He flow
10ºC/s 600ºC, slow cooling
Quenching
Time
MS&T’08 Conference, Pittsburgh, PA

5. Austenite Microstructure
and Modeling

6. Austenite Microstructures Prior to
Transformation
• Non-recrystallized deformed austenite
grains
– Tini = 1100ºC: Homogeneous Structures
3Nb-Mo16 3Nb-Mo31
Tini = 1100ºC
MS&T’08 Conference, Pittsburgh, PA

7. Microstructural Heterogeneities in
Austenite. Tini = 1050ºC
• Non-recrystallized deformed Austenite
Grains
– Tini = 1050ºC: Microstructural Heterogeneities
in Homogeneous Matrix
3Nb-Mo16 3Nb-Mo31
Tini = 1050ºC
MS&T’08 Conference, Pittsburgh, PA

8. Grain Distribution Modeling
• Input:
– Thermomechanical Sequence
– Composition
– Initial Grain Size Distribution
• Model:
– Equations developed for Nb-Mo steels and wide
range of austenite grains:
ε c = 2,8 ⋅10 −3
{1 + 20([Nb] + 0,035)} D 0,147 Z 0,155 t 0,5 X = 9,92 ⋅10 −11 Do ε
−5 , 6 D − 0 ,15 ⎛ 180000 ⎞
ε −0,53 exp⎜
⎡⎛ 275000 ⎞ ⎤
− 185 ⎟([Nb ] + 0,09[Mo ])⎥
0
0
& ⎟ ⋅ exp ⎢⎜
1,78 ⎝ RT ⎠ ⎣⎝ T ⎠ ⎦
• Output:
– Recrystallized and unrecrystallized grain size
distributions
– Mechanism history: drag, precipitation, dynamic rex
MS&T’08 Conference, Pittsburgh, PA

9. Model Predictions
• Austenite grain size distribution prior to
transformation
– Tini = 1100ºC: Similar distributions for both
steels
3Nb-Mo16 3Nb-Mo31
0.5 0.5
3Nb-Mo16 3Nb-Mo31
Tini = 1100ºC Tini = 1100ºC
Austenite Area Fraction
Austenite Area Fraction
0.4 Model 0.4
Model
Experimental
Experimental
Tini = 1100ºC
0.3 0.3
0.2 0.2
0.1 0.1
0 0
20 40 60 80 100 120 140 160 180 200 20 40 60 80 100 120 140 160 180 200
Grain Size (μm) Grain Size (μm)
MS&T’08 Conference, Pittsburgh, PA

10. Model Predictions
• Austenite grain size distribution prior to
transformation
– Tini = 1050ºC: Heterogeneity increases as Mo
content increases
3Nb-Mo16 3Nb-Mo31
0.5 0.5
3Nb-Mo16 3Nb-Mo31
Tini = 1050ºC Tini = 1050ºC
Austenite Area Fraction
Austenite Area Fraction
0.4 0.4 Model
Model
Experimental
Experimental
Tini = 1050ºC
0.3 0.3
0.2 0.2
0.1 0.1
0 0
20 100 180 260 340 420 500 580 20 100 180 260 340 420 500 580
Grain Size (μm) Grain Size (μm)
MS&T’08 Conference, Pittsburgh, PA

11. Heterogeneous Structures
• As-cast austenite grain prior to
transformation:
– Lack of Recrystallization through deformation
passes and interstands
– Higher fraction for 3Nb-Mo31
• Bigger maximum grain size
– Strain induced Nb(C,N) precipitation interacts
with softening mechanisms mainly in highly
strained grains
MS&T’08 Conference, Pittsburgh, PA

12. Heterogeneity: As-cast Fraction
Tini = 1100ºC Tini = 1050ºC
0.8 0.8
Tini = 1100ºC Tini = 1050ºC
3Nb-Mo31 3Nb-Mo31
0.6 3Nb-Mo16 0.6 3Nb-Mo16
As-cast fraction
As-cast fraction
0.4 0.4
0.2 0.2
0 0
1 2 3 4 1 2 3 4
Interstand Interstand
• Homogeneous Micro: Complete Rex prior to Precipitation
• Minimum initial temperature is needed for heterogeneities
to be avoided
• Once homogeneity achieved: focus on austenite pancaking
MS&T’08 Conference, Pittsburgh, PA

13. Austenite Pancaking:
Unrecrystallized Fraction
• Austenite Pancaking: Drag / Precipitation
Tini = 1100ºC Tini = 1050ºC
1 1
Tini = 1050ºC
Unrecrystallized Austenite Fraction
Tini = 1100ºC
Unrecrystallized Austenite Fraction
Precipitation Precipitation
Solute Drag Solute Drag
0.8 0.8
3Nb-Mo16 3Nb-Mo31
3Nb-Mo16 3Nb-Mo31
0.6 0.6
0.4 0.4
0.2 0.2
0 0
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Interstand Interstand
MS&T’08 Conference, Pittsburgh, PA

14. Austenite Pancaking:
Nb Steel – NbMo Steels
• Mo drag effect accelerates Nb(C,N)
precipitation at low deformation T
Tini = 1100ºC Tini = 1050ºC
11 Precipitation 11
TTini = 1050ºC
ini = 1050ºC
Unrecrystallized Austenite Fraction
TTini==1100ºC
Unrecrystallized Austenite Fraction
Unrecrystallized Austenite Fraction
1100ºC Precipitation Solute Drag
Unrecrystallized Austenite Fraction
ini Precipitation
Solute Drag Solute Drag
3Nb 3Nb-Mo16 3Nb-Mo31
0.8
0.8 0.8
0.8 3Nb-Mo16 3Nb-Mo31
3Nb 3Nb-Mo16 3Nb-Mo31
3Nb-Mo16 3Nb-Mo31
0.6
0.6 0.6
0.6
εac = 0.50
εac = 0.56 εac = 0.73
0.4 εac = 0.43
0.4 0.4
0.4 εac = 0.95
εac = 0.54
0.2 0.2
0.2 0.2
0
00 1 2 3 4
0 1 2 3 4 1 2 3 4
1 2 3 4 1 2 3 1
4 2 1 32 34 4 1 2 3 4 1 2 3 4 1 2 3 4
Interstand Interstand
Interstand Interstand
MS&T’08 Conference, Pittsburgh, PA

15. Transformation

16. Transformed Microstructures
Coiling Simulation 700ºC
• For Tini = 1100ºC:
– Homogeneous ferrite
3Nb-Mo16 3Nb-Mo31
Tini = 1100ºC
Dα = 8.6 μm Dα = 8.7 μm
MS&T’08 Conference, Pittsburgh, PA

17. Transformed Microstructures
Coiling Simulation 700ºC
• For Tini = 1050ºC:
– Homogeneous ferrite with heterogeneous
regions
3Nb-Mo16 3Nb-Mo31
Tini = 1050ºC
Dα = 8.4 μm Dα = 8.6 μm
MS&T’08 Conference, Pittsburgh, PA

18. Microstructural Units EBSD
• Homogenous ferrite microstructures
correspond to high angle GB units
Tini = 1100ºC
3Nb-Mo31
MS&T’08 Conference, Pittsburgh, PA

19. Microstructural Units EBSD
• Prior austenite coarse grains transform to
coarse ferrite units or acicular structures:
forming low angle GB areas.
Tini = 1050ºC
3Nb-Mo31
MS&T’08 Conference, Pittsburgh, PA

20. Conclusions
• Mo addition to Nb microalloyed steels: important
increase in the delay of static rex kinetics → The
refinement of the initial as-cast structure is retarded.
• For homogeneous microstructures: EBSD show that
ferrite grains are diversely oriented with high-angle grain
boundaries.
• Structures transformed from non-refined as-cast grains
form coarse microstructural units, bigger than those
observed with the optical microscope. Toughness will be
impaired.
• For optimized thermomechanical schedules, Mo affects
hardenability. This factor can be useful for the formation
of complex microstructures with high strength and
toughness levels.
MS&T’08 Conference, Pittsburgh, PA

21. Effect of Alloying Additions in the Final
Microstructure of Nb-Mo Steels Processed by Thin
Slab Direct Rolling Technologies
P. Uranga, J. Ganzarain, D. Jorge-Badiola and J.M. Rodriguez-Ibabe
puranga@ceit.es
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastián
Basque Country, Spain
MS&T’08 Conference
October 6-9, 2008, Pittsburgh, PA