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Effect of Alloying Additions in the Final Microstructure of Nb-Mo steels Processed by Thin Slab Direct Rolling Technologies
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
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