Precipitation of Nb in ferrite after
austenite conditioning
I. Gutierrez, A. Iza-Mendia, A. Altuna, B. Pereda
CEIT and Tecnun (University of Navarra),
Manuel de Lardizábal 15, 20018
Donostia-San Sebastián Spain
the other co-authors
Amaia Iza-Mendia Alazne Altuna Beatriz Pereda
Donostia-San Sebastian
CEIT is a non profit Research Centre working
in partnership with the University of Navarra
www.ceit.es
www.tecnun.es
Reheating Hot rolling Run-out Coiling
table
INTRODUCTION
Strip hot rolling mill
The hot rolling mill transforms as-cast steel into finished or
semifinished products
it can be considered as a tool for getting tailored
combinations of mechanical properties
Ferrite grain
refinement
Reheating Hot rolling Run-out Coiling
table
INTRODUCTION
Strip hot rolling mill
• Austenite conditioning
Nb:
- Solute drag
- Precipitation
The main role of Nb microalloying is the control of the
austenite microstructure in the hot rolling mill
Reheating Hot rolling Run-out Coiling
table
INTRODUCTION
Strip hot rolling mill
Over the last 30 years most of the research on Nb
microalloyed steels has concentrated on the hot rolling mill
Higher requirements in terms of mechanical property at
minimum cost
INTRODUCTION
Need for process and additions
optimisation
S Vervynckt et al. International Materials Review, 2012
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Nb solubility in
austenite
INTRODUCTION Tem
pera
ture
Reheating
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Nb in solution
INTRODUCTION Tem
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ture
Reheating Hot rolling
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Nb in solution
INTRODUCTION Tem
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Reheating Hot rolling
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Nb in solution
INTRODUCTION Tem
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ture
Reheating Hot rolling
Strain induced
precipitation
Run-out
table
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Nb in solution
INTRODUCTION
Tem
pera
ture
Reheating Hot rolling Run-out
table
Coiling
Reheating Hot rolling Run-out Coiling
table
INTRODUCTION
Strip hot rolling mill
Focus: extra strengthening associated with
Nb above that expected from grain
refinement
• Interphase precipitation
• Homogeneous precipitation
• Cluster formation
• Hardenability (transformation dislocations)
Nb in solution after austenite conditioning
Complex interactions, depending
on Nbfree and cooling strategies
INTRODUCTION
INTRODUCTION
“There is still a certain disagreement with regard to the
morphology of the precipitates”
“There is no definite answer to the question whether
precipitation of Nb carbides significantly contribute to
the strength or not”
Steel Reseach, 2004
OBJECTIVES
• Study the potential precipitation of Nb in ferrite during coiling.
• Estimate the precipitation strengthening
EXPERIMENTAL
C Si Mn Al Nb N
0.06 0.35 1.00 0.047 0.056 0.006
Plane strain compression + simulated coiling
Modelling assisted design of
the thermomechanical
sequences
EXPERIMENTAL
C Si Mn Al Nb N
0.06 0.35 1.00 0.047 0.056 0.006
Mechanical testing &
microstructural characterisation
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0 50 100 150 200 250 300
Tem
pera
ture
Coiling
1250ºC, 15 min
EXPERIMENTAL
1100ºC, 1s-1, =0.3; + 20s holding
Recrystallized
&
Nb in solution
Sequence S1:
750ºC
300ºC
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800
1000
1200
1400
0 50 100 150 200 250 300
Tem
pera
ture
Coiling
1250ºC, 15 min
EXPERIMENTAL
1000ºC, 1s-1, =0.3
Strained
&
Some strain induced
precipitation of Nb
Sequence S2:
750ºC
300ºC
1100ºC, 1s-1, =0.3; + 20s holding
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400
600
800
1000
1200
1400
0 50 100 150 200 250 300
Tem
pera
ture
Coiling
(650ºC)
1250ºC, 15 min
EXPERIMENTAL
1100ºC, 1s-1, =0.3
870ºC; 1h
Rex or Strained
&
full Nb precipitation
S1 and S2 Reference tests
1100ºC, 1s-1, =0.3; + 20s holding
870ºC
Nb precipiation
model predictions
B. López et al., 2006
PRECIPITATION IN AUSTENITE: REFERENCE TEST
TEM ANALYSIS
S1-R S2-R
Reference tests
Coiling T
750ºC
600ºC
500ºC
MICROSTRUCTURE
Bainitic
microstructures
Tcoiling<600ºC
0.06%C-0.056%Nb
S1
TENSILE PROPERTIES
TENSILE PROPERTIES + FERRITE GRAIN SIZE
CONTRIBUTIONS TO YIELD STRENGTH
pptdssy
dk 2/1
0
lattice
friction
solid
solution
grain size
dislocations
precipitates
Pickering, 1993
2/14.17)(5544833254 dCNSiMn freefreeyF
CONTRIBUTIONS TO YIELD STRENGTH
yFerimentalexpyyNb
2/1
freefreeyF d4.17)CN(5544Si83Mn3254
Cfree equilibrium
Nfree was determined by modelling
EXTRA STRENGTHENING DUE TO Nb
0.06%C-0.056%Nb
EXTRA STRENGTHENING DUE TO Nb
0.06%C-0.056%Nb
Full
precipitation
in
EXTRA STRENGTHENING DUE TO Nb
0.06%C-0.056%Nb
0.13%C-0.02%Nb Reducing the Nb content
Coiling at : 750ºC 600ºC
IQ-EBSD
EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE
y Nb (MPa) 90 150
S1
Coiling at : 750ºC 600ºC
IQ-EBSD
EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE
y Nb (MPa) 90 150
S1
Coiling at : 750ºC 600ºC
IQ-EBSD
EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE
y Nb (MPa) 90 150
S1
Coiling at 750ºC
Precipitation at the transformation front
GB
TEM ANALYSIS
Thin foil
CCT CURVES
CCT + INTERPHASE PTT CURVES
T. Sakuma and R.W.K. Honeycombe: 1984
Coiling at 750ºC
CCT + INTERPHASE PTT + COILING CURVES
Coiling at 750ºC
TEM ANALYSIS
• Nb-rich precipitates:
• in few regular parallel rows (spacing
1300 nm)
• in some segments of rows
• irregularly distributed
• AlN nucleated at dislocations
Discontinuous interphase precipitation
CCT + INTERPHASE PTT + COILING CURVES
Coiling at 600ºC
Coiling at 600ºC
Homogeneous precipitation in ferrite
TEM ANALYSIS
Thin foil
0 2 4 6 8 10 12 14 16 18 20
keVFull Scale 68 cts Cursor: 19.536 (0 cts)
foto 3 grande
Cu
Cu Nb
NbAl
CuFe
Carbon extraction replica TEM ANALYSIS
Coiling at 600ºC
From the
Cu-grid
410125.6ln8.10)(
d
d
fMPa v
ppt
PRECIPITATION STRENGTHENING
ASHBY-OROWAN'S EQUATION
fv: volume fraction
d: average precipitate diameter in m
PRECIPITATION STRENGTHENING
Coiling at 650ºC
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Tem
pera
ture
1250ºC, 15 min
1100ºC, 1s-1, =0.3; + 20s holding
Sequence S1:
600ºC; 24 or 48h
Long holding at Coiling
temperature
STABILITY
S1 + holding at 600ºC
TENSILE
The tensile properties are not affected by a prolonged holding at 600ºC
S1 + 48h holding at 600ºC
TEM ANALYSIS
Baker-Nutting orientation relationship
• Coiling at 600ºC produces in this steel an homogeneous, extremely fine and stable general precipitation of NbC in ferrite, leading to maximum strengthening:
• proportional to the Nb left in solution in austenite;
• in agreement with the predictions from Ashby-Orowan’s equation.
CONCLUSIONS
• Considering homogeneous precipitation of NbC in ferrite as a possible strengthening mechanism opens the way to a better design of the composition and thermomechanical sequences for an improved use of Nb additions.
• To the Institute of Materials, Minerals and Mining (IOM3)
• To Companhia Brasileira de Metalurgia e Mineração (CBMM)
• To Beta Technology
• To Naila Croft, Ben Mico and Georgia Gomes Bemfica.
ACKNOWLEDGEMENTS
Modelling to Optimise the Processing of Niobium Steels
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