Effects of combining Nb and Mo in HSLA Steels: From austenite conditioning to final microstructure
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Transcript of Effects of combining Nb and Mo in HSLA Steels: From austenite conditioning to final microstructure
ó Effects of combining Nb and Mo in
HSLA Steels: From austenite
conditioning to final microstructure
N. Isasti, B. Pereda, B. López, J.M. Rodriguez-Ibabe and Pello Uranga
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastian, Basque Country, Spain
Summary
• High strength
• Low temperature toughness
Nb-Mo microalloyed steels
Niobium
Strain Accumulation
Microstructural refinement
Molybdenum
Increase of hardenability
Nb-Mo MICROALLOYED STEELS
MULTIPLE MICROALLOYING
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
CHEMICAL COMPOSITIONS
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
Materials
1. AUSTENITE CONDITIONING
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.
156.0
08.1 DDSRX
for Nb microalloyed steels
for 0.03% Nb-Mo microalloyed steels
for 0.06% Nb-Mo microalloyed steels
eff
DNb
TRTDxt 185
275000exp
180000exp1092.9 53.06.5
0
11
5.0
15.00
NbNb eff
MoNbNb eff 09.019.1
MoNbNb eff 032.019.1
0
10
20
30
40
50
0 10 20 30 40 50t0.5 (exp.)
t 0.5 (
ca
l.)
3Nb3Nb-Mo6Nb6Nb-Mo166Nb-Mo31
Static Recrystallization Kinetics
950
975
1000
1025
1050
1075
1100
0 10 20 30 40
Interpass time (s)
T nr
(ºC
)
3Nb
3Nb-Mo31
6Nb-Mo31
6Nb
Dependence of Tnr as a function of the
interpass time ( = 0.4)
Low Nb (0.03%Nb)
0
20
40
60
80
100
7 7.5 8 8.5 9
10000/T (1/K)
Fra
cti
on
al
So
fte
nin
g (
%)
T nr = 1026 ºC
T nr = 985ºC
tip = 10 s, = 0.4
Precipitation
solute drag
3Nb
3Nb-Mo31
0
20
40
60
80
100
7 7.5 8 8.5 9
10000/T (1/K)
Fra
cti
on
al
So
ften
ing
(%
)
T nr =1030ºC
T nr = 1045ºC
tip = 30 s, = 0.4
6Nb
6Nb-Mo31
High Nb (0.06% Nb)
Precipitation
2. PHASE TRANSFORMATIONS
Thermomechanical schedule
CONTINUOUS COOLING TRANSFORMATION STUDY
Cooling rates: 0.1-200ºC/s
Precipitate dissolution
Austenite conditioning
Continuous cooling
Cycle A→ Undeformed austenite
Cycle B→ Deformed austenite (Strain = 0.4)
Cycle C → Deformed austenite (Strain = 0.8)
Transformation Products
BAINITIC FERRITE
MARTENSITE
BF
M
QUASIPOLIGONAL FERRITE
GRANULAR FERRITE
GF
QF
POLYGONAL FERRITE
DEGENERATED PEARLITE
DP
PF
0
200
400
600
800
1000
0.1 1 10 100 1000 10000
Te
mp
era
ture
(ºC
)
Time (s)
Cycle A
6NbMo0
6NbMo31
PF
GF+QF
DP
BF
ºC/s 200 100 50 20 10 5 2 1 0.5 0.1
HV 221 220 240 196 178 170 167 158 154 139285 253 229 210 207 199 197 196 176 136
Continuous Cooling Transformation diagrams (CCT)
Effect of the addition of Mo
EFFECT OF CHEMICAL COMPOSITION
Mo↑ Transformation start
temperature ↓
0
200
400
600
800
1000
0.1 1 10 100 1000 10000
Te
mp
era
ture
(ºC
)
Time (s)
ºC/s 200 100 50 20 10 5 2 1 0.5 0.1
HV 192 180 155 145 133 131 136 121 131 107 256 239 202 201 181 194 180 155 156 150277 292 256 223 220 204 200 176 162 130
Cycle A
CMn
3NbMo06NbMo0
PF+P
GF+QF
BF
DP
HIGH Nb Nb↑ Transformation start temperature ≈
Steel D0 (μm) Sv (mm-1)
Cycle A Cycle B
CMn 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
LOW Nb Nb↑ Transformation start temperature ↓
Continuous Cooling Transformation diagrams (CCT)
EFFECT OF CHEMICAL COMPOSITION
Effect of the addition of Nb
0
200
400
600
800
1000
0.1 1 10 100 1000 10000
Te
mp
era
ture
(ºC
)
Time (s)
ºC/s 200 100 50 20 10 5 2 1 0.5 0.16NbMo31
ε=0 (Cycle A) HV 285 253 229 210 207 199 197 196 176 136ε=0.4 (Cycle B) 246 246 232 220 215 204 192 183 181 155 ε=0.8 (Cycle C) 246 261 247 229 217 205 199 186 175 153
QF+GF
BFMs
PF
DP
Continuous Cooling Transformation diagrams (CCT)
EFFECT OF THE THERMOMECHANICAL SCHEDULE
Effect of the amount of deformation in austenite
ACCUMULATED DEFORMATION ↑
Transformation start temperature ↑
EBSD Quantification
6NbMo0 (1ºC/s) RECRISTALLYZED γ
DEFORMED γ
EBSD Quantification
Accumulation of deformation in γ
Cycle B Microstructural
refinement
Mean crystallographic unit sizes
0
4
8
12
16
0.01 0.1 1 10 100 1000
Me
an
Gra
in S
ize
(µ
m)
Cooling Rate (K/s)
6NbMo0_Cycle A
6NbMo0_Cycle B
6NbMo31_Cycle A
6NbMo31_Cycle B
4º
(a)0
4
8
12
16
0.01 0.1 1 10 100 1000M
ea
n G
rain
Siz
e (
µm
)
Cooling Rate (K/s)
6NbMo0_Cycle A
6NbMo0_Cycle B
6NbMo31_Cycle A
6NbMo31_Cycle B
15º
(b)
0
2
4
6
8
10
12
0.01 0.1 1 10 100 1000
Dc
20
% /
Dm
ean
(15
º)
Cooling Rate (K/s)
6NbMo0_Cycle B
6NbMo31_Cycle B
6NbMo0_Cycle C
6NbMo31_Cycle C
EBSD Quantification
Dc20% Cut off grain size at 80% area fraction
in a grain size distribution histogram
Microstructural heterogeneity → Dc20%/Dmean
FORMATION OF BAINITIC
FERRITE
FERRITIC MICROSTRUCTURES
3. MECHANICAL PROPERTIES
Thermomechanical schedules
Coiling temperatures:
650ºC, 550ºC, 450ºC
Precipitate dissolution
Austenite conditioning
Isothermal maintenance
COILING SIMULATIONS – PLANE STRAIN COMPRESSION
Final Microstructures
POLYGONAL FERRITE
PERLITE
QUASIPOLIGONAL FERRITE
GRANULAR FERRITE
6NbMo0 550ºC 650ºC
3. MECHANICAL PROPERTIES
Strength
300
400
500
600
700
400 450 500 550 600 650 700
Yie
ld S
tre
ng
th/T
en
sile
str
en
gth
(M
Pa
)
Coiling temperature (ºC)
3NbMo0
3NbMo31
6NbMo0
6NbMo31
TS
YS
Mechanical properties
YIELD STRENGTH - TENSILE STRENGTH
Mechanical properties
CORRELATION BETWEEN MICROSTRUCTURE-MECHANICAL
PROPERTIES
),,,,σ(σ 0y pptgsssf
Composition
Dislocation
Density Grain/Unit Size
Precipitation
Mechanical properties
A. Iza-Mendia, I. Gutierrez, Materials Science and Engineering A, vol. 561, 2013, pp, 40-51
Grain Size
2
1
2º
θ
i
15ºθ2
iigs d·f10
πθfbαMμ*1.05σ
15ºii
Low angle
boundary fraction High angle
boundary fraction
350
400
450
500
3 4 5 6
Yie
ld S
tre
ng
th (
MP
a)
Mean grain size 2º (μm)
3NbMo0
3NbMo31
6NbMo0
6NbMo31
Mechanical properties
• Mechanical strentgh ↑
•Toughness ↑
MICROSTRUCTURAL REFINEMENT
Nb-Mo
Nb
Grain Size
Mechanical properties
Dislocation Density
ραMμbσ ρ
ub
2ρ
Kernel Average
misorientation for θ<2º
Mechanical properties
6
7
8
9
400 450 500 550 600 650 700
Me
an
siz
e o
f p
rec
ipit
ate
s (
nm
)
Coiling Temperature (ºC)
3NbMo0
3NbMo31
6NbMo0
6NbMo31
6NbMo31
)106.125
xln(
x
f10.8σ
4
0.5
vppt
550ºC 650ºC
100 nm 100 nm
Precipitation
Hardening
16% 13% 14%
21%18%
22%
0% 12% 5%
63%57% 59%
0%
20%
40%
60%
80%
100%
650 550 450
σy
(MP
a)
Coiling temperature (ºC)
6NbMo31MOD 457 482 461EXP 426 434 473
16% 14% 14%
20%19% 17%
4% 8% 10%
60% 59% 59%
0%
20%
40%
60%
80%
100%
650 550 450
σy
(MP
a)
Coiling temperature (ºC)
6NbMo0MOD 428 454 445EXP 406 410 408
18% 15% 14%
19% 22% 26%
0%7% 4%
63%56% 57%
0%
20%
40%
60%
80%
100%
650 550 450
σy
(MP
a)
Coiling temperature (ºC)
3NbMo31MOD 431 435 456EXP 397 457 460
17% 17% 17%
20% 20% 20%
2% 2%5%
61% 62% 58%
0%
20%
40%
60%
80%
100%
650 550 450
σy
(MP
a)
Coiling temperature (ºC)
3NbMo0MOD 423 426 407EXP 387 401 414
UNIT SIZE
PRECIPITATION
DISLOCATIONS
COMPOSITION
Contributions to Strength
CONCLUSIONS
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
ACKNOWLEDGEMENTS
Acknowledgements
• IMOA and CBMM
• Prof. Hardy Mohrbacher
• Spanish Government MINECO (MAT2009-
09250 and MAT2012-31056)
• Basque Government (PI2011-17)
• Thermomechanical Treatments Group at CEIT
ó Effects of combining Nb and Mo in
HSLA Steels: From austenite
conditioning to final microstructure
N. Isasti, B. Pereda, B. López, J.M. Rodriguez-Ibabe and Pello Uranga
CEIT and TECNUN (University of Navarra)
Donostia-San Sebastian, Basque Country, Spain