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. 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
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.
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
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
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
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