role of microalloying elements during thin slab direct rolling
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Role of Microalloying Elements during Thin Slab Direct Rolling
P. Uranga, B. López and J.M. Rodriguez-Ibabe
CEIT and Tecnun (Univ. of Navarra)Donostia, Basque Country, Spain
puranga@ceit.es
Microalloyed Steels: Production, Processing, Applications, IOM3, November 2007, London, UK
Thin Slab Casting and Direct Rolling
• One of the most promising processing routes to maintain steel as a prominent technological material.
• Several metallurgical changes compared to traditional routes:
• Smaller segregation during solidification• Higher N and residual element amount (scrap based EAF
routes)• Very coarse austenite grain size prior to hot rolling • Lower total reduction during rolling
These peculiarities will have a significant effect on the behavior of microalloying elements.
Nb Microalloyed Steels
• Specific empirical equations fitted to Thin Slab Direct Rolling technology.
• Softening mechanisms:– Post-Dynamic Softening
• Static Recrystallization.• Metadynamic Recrystallization.
– Precipitation – Softening Interaction• Grain Size Evolution
Modeling• Definition of Optimal Conditions for
Microalloyed Grades using innovative Microstructural Models.
• Special attention to:
– Avoidance of microstructural heterogeneities in thick plates and high levels of microalloying additions.
– Conditioning of austenite structure prior to transformation.
600 μm
0
5
10
15
20
25
30
0 500 1000 1500 2000 2500 3000
Grain Size (μm)Fr
eque
ncy
(%)
CenterNear Surface
As-Cast Microstructure
• Mean Grain Size: ~800-1000 μm• High fraction of grains bigger than 2 mm
Procedure• Classical modeling approach:
– Not enough to predict heterogeneities
• New model:– Particular characteristics of TSDR Technology
• Initial As-cast Structure• Specific Thermomechanical Deformation Route
D
3-D
Freq
uenc
y
[d0] i
[fv] i
kpth interval np1 … …
......
Rex Unrex...
Final MicrostructureHistograms
Recrystallized Fraction Unrecrystallized Fraction
Grain Size
Are
a Fr
actio
n
Grain Size
Are
a Fr
actio
n
[ ]ird [ ]iud [ ]iX
pth rollingpass
[ ]iX
1− [ ]ir ε
Rex Unrex
1st rollingpass
i1th interval n11 … …
......, , ,
Log-normal Distribution
[drex] i
Freq
uenc
y
D
Austenite Model
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150
20
20
25
25
30
3030
35
35
35
40
40
40
4045
45
45
45
50
50
5050
50
6060
60
7070
7080
8090
100110120
Final Gauge thickness (mm)
Total Strain
Rol
ling
Entr
yTe
mpe
ratu
re(º
C)
12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
0.05%NbDc
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150
20
20
25
25
30
3030
35
35
35
40
40
40
4045
45
45
45
50
50
5050
50
6060
60
7070
7080
8090
100110120
Final Gauge thickness (mm)
Total Strain
Rol
ling
Entr
yTe
mpe
ratu
re(º
C)
12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
0.05%NbDc
Industrial Processing Simulations• Optimization of rolling schedules ⇒ Processing Maps
Austenite processing maps for the Dc isoclines: (a) 0.035%Nb; (b) 0.05%Nb
(a) (b)
2 2.5 3 3.5 41040
1050
1060
1070
1080
1090
1100
2025
25
30
3030
35
35
35 35
40
40
4040
50
5050
6060
60
70
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
y Te
mpe
ratu
re (º
C)
12.65 7 6 4 3 1.52
0.035%NbDc
Optimum Processing Zone
10
Residual unrefinedas-castgrains
2 2.5 3 3.5 41040
1050
1060
1070
1080
1090
1100
2025
25
30
3030
35
35
35 35
40
40
4040
50
5050
6060
60
70
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
y Te
mpe
ratu
re (º
C)
12.65 7 6 4 3 1.52
0.035%NbDc
Optimum Processing Zone
10
Residual unrefinedas-castgrains
Industrial Processing Simulations• Optimization of rolling schedules ⇒ Processing Maps
Austenite processing maps for the retained strain isoclines: (a) 0.035%Nb; (b) 0.05%Nb
(a) (b)
2 2.5 3 3.5 41040
1050
1060
1070
1080
1090
11000.2
0.2
0.2
0.3
0.30.3
0.30.4
0.4
0.4
0.50.5
0.50.5
0.6
0.6 0.6 0.60.7 0.7 0.8
0.0350.035%Nb%NbRetainedRetained strainstrain
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
yTe
mpe
ratu
re(º
C)
12.65 7 6 4 3 1.52
Optimum ProcessingZone
10
Residual Residual unrefinedunrefined
asas--castcastgrainsgrains
2 2.5 3 3.5 41040
1050
1060
1070
1080
1090
11000.2
0.2
0.2
0.3
0.30.3
0.30.4
0.4
0.4
0.50.5
0.50.5
0.6
0.6 0.6 0.60.7 0.7 0.8
0.0350.035%Nb%NbRetainedRetained strainstrain
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
yTe
mpe
ratu
re(º
C)
12.65 7 6 4 3 1.52
Optimum ProcessingZone
10
Residual Residual unrefinedunrefined
asas--castcastgrainsgrains
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
11400.2 0.2
0.20.4
0.4
0.40.4
0.60.6
0.6 0.6
0.8
0.8
0.8 0.8
1
1 1 11.2 1.2
1.21.4 1.4
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
y Te
mpe
ratu
re (º
C)
12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
0.050.05%Nb%NbRetainedRetained strainstrain
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
11400.2 0.2
0.20.4
0.4
0.40.4
0.60.6
0.6 0.6
0.8
0.8
0.8 0.8
1
1 1 11.2 1.2
1.21.4 1.4
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
y Te
mpe
ratu
re (º
C)
12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
11400.2 0.2
0.20.4
0.4
0.40.4
0.60.6
0.6 0.6
0.8
0.8
0.8 0.8
1
1 1 11.2 1.2
1.21.4 1.4
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
y Te
mpe
ratu
re (º
C)
12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
0.050.05%Nb%NbRetainedRetained strainstrain
Phase transformation modeldescription
Frecuencia
d(dγ)0, (fV)0
Frequency
dγ(dγ)0, (fV)0
Austenite grainsize distribution
n intervals
Austenite to ferrite transformation:(Bengochea, López, Gutiérrez)
[ ] ( )( )[ ]γα ε DTD acc 015.0exp14.1335.4.5.01 5.047.0 −−++−= &
Frequency
d
Log-normal distribution
( )[ ] ⎟⎟⎠
⎞⎜⎜⎝
⎛−−= 2
2μdln
2σ1exp
dσ2π1P
( )2σDlnμ
2−= α
dα
Area Fraction Ferrite grain sizedistribution
Phase transformation modeldescription
Model parameters:
• σ : standard deviation (no significant effect) ⇒ σ = 2
• X: maximun/mean grain size ratio (each log-normal distribution offerrite grains is cut at the value of X.(Dα))
⇒ X increases with increasing the austenite grain size thickness, D* = f(Dγ , εacc)
Dγ
a)
D*
b)
εγ
32
eDD−∗ =
Recrystallized Unrecrystallized
(* Plane strain deformation)
X = 1.5 for all intervals with D* < 25 μm; X = 2 for 25 μm < D* < 50 μm;
X = 2.5 for 50 μm < D* < 75 μm; X = 3 for D*>75 μm
Phase transformation modelvalidation
• Recrystallized austenite
0
0.05
0.1
0.15
5 15 25 35 45 55 65 75
Austenite grain size (μm)
Are
a Fr
actio
n
Schedule A: Dγ = 28 μm
Schedule C: Dγ = 42 μm
0
0.05
0.1
0.15
10 30 50 70 90 110 130 150
Austenite grain size (μ m)
Are
a Fr
actio
n
0
0.1
0.2
0.3
4 12 20 28 36 44 52Ferrite grain size (μ m)
Are
a Fr
actio
n
modelexperimental
⇒ Dα = 10 μm
0
0.1
0.2
0.3
4 12 20 28 36 44 52 60 68Ferrite grain size (μ m)
Are
a Fr
actio
n
modelexperimental
⇒ Dα = 14.6 μm
Phase transformation modelvalidation
• Unrecrystallized austenite
Schedule B: Dγ = 28 μm, εacc = 1 ⇒ Dα = 5.3 μm
Schedule D: Dγ = 40 μm, εacc = 1 ⇒ Dα = 7.5 μm
0
0.1
0.2
0.3
2 6 10 14 18 22 26
Ferrite grain size (μ m)
Are
a Fr
actio
n
modelexperimental
0
0.1
0.2
0.3
4 12 20 28 36Ferrite grain size (μ m)
Are
a Fr
actio
n
modelexperimental
Model applications
• Validation steps:– Laboratory– Plant trials
• Industrial Schedule optimization focusedon:– Thick final gauges– High microalloying levels
• Powerful tool for new grade design
Schedule Redesign• Initial Thickness: 55 mm• Final Thickness: 10 mm
Seq 10A Seq 10B
Pass ε ε& (s-1)
tip(s) ε ε&
(s-1)tip(s)
ΔT (ºC)
1 1 5 10 1 5 6 35 2 ⎯ ⎯ ⎯ 0.45 10 9 30 3 0.45 15 5 ⎯ ⎯ ⎯ 30 4 ⎯ ⎯ ⎯ ⎯ ⎯ ⎯ 30 5 0.3 20 2.7 0.3 20 2.7 30 6 0.25 25 0.25 25 (*)
Seq 10
Pass ε ε& (s-1)
tip (s)
ΔT (ºC)
1 0.5 5 6 35 2 0.5 10 4 30 3 0.45 15 5 30 4 ⎯ ⎯ ⎯ 30 5 0.3 20 2.7 30 6 0.25 25 (*)
From 5 to 4 stand rolling schedules
Different combinations for dummy passes
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150
20
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40
40
45
45
45
45
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50
50
50
50
60
6060
70
7070
80
8090
100110120
Final Gauge Thickness (mm)
Total StrainR
ollin
gEn
try
Tem
pera
ture
(ºC
)
12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
0.05% Nb(a) Dc
Effect of theSchedule
• Reduction in Final Austenite As-Cast Fraction– Seq10 → Seq 10A → Seq 10B
• Microstructural Homogeneity Optimum for Sec 10B: Min Ti : 1090 to 1070ºC
0
0.1
0.2
0.3
0.4
0.5
1040 1060 1080 1100
Rolling Entry Temperature (ºC)
Fina
l Aus
teni
te A
s-ca
st F
ract
ion
Seq 10Seq 10ASeq 10B
0
5
10
15
20
1040 1060 1080 1100 1120
Rolling Entry Temperature (ºC)ZD
Par
amet
er
Seq 10Seq 10ASeq 10B
Effect of Initial Slab Thickness
• Initial Thickness: 55 mm → 70 mm• Initial/Final Thickness ≥ 7 [*] → Toughness Requirements
[*] Klinkenberg C and Hensger KE, Materials Science Forum, 2005. 500-501: 253~260.
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150
2020
25
2530
30
30
35
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40
40
40
45
45
45
45
50
50
50
50
50
60
6060
70
7070
80
8090
100110120
Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
yTe
mpe
ratu
re(ºC
)12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
0.05% Nb(a) Dc
Prob
lem
s?
Effect of Initial Slab Thickness
• Initial Thickness: 55 mm → 70 mm
• Initial/Final Thickness Ratio ≥ 7 [*] → Toughness Requirements
Seq 10C
Pass ε ε& (s-1)
tip(s)
ΔT (ºC)
1 1 5 6 352 0.45 10 7 303 ⎯ ⎯ ⎯ 304 0.35 20 2.7 305 0.25 30 2.1 306 0.2 40 (*)
Similar homogeneity
Higher Retained Strain
Smaller ferrite grain size
Improvement in strength and toughness
εr = +0.2
Multiple Alloyed Steels
• Microalloying application in TSDR routes has been increasing continuously.
• For specific grades, there is no unique option:– For structural grades up to 500 MPa one element can be
selected (Nb, V).– When higher strengths are required, a combination of two
microalloying elements would be a good choice (or one element combined with Mo) (API grades).
– The selection of one element can be determined by other factors (scrap based steel, metallurgical “know how”,...).
Mo-Nb Steels
• Mo addition is a common practice to increase strength and toughness in low C steels (low temperature transformation products after hot rolling).
• On the other hand, the use of Nb is well known because of its availability to retard recrystallization.
• The addition of Mo to Nb microalloyed steels may introduce significant changes in the microstructuralevolution during hot working.
• For example, it has been reported that Mo in solid solution produces a strong retardation effect on dynamic and static recrystallization.
Solute retardation parameter (SRP) for dynamicand static recrystallization
Akben, Bacroix and Jonas, Acta Metall, 31, 1983, pp. 161-174
0
50
100
150
200
250
V Mo Ti NbElement
SRP
dynamicstatic
Drag effect of Mo on Tnr
Effect of Mo addition on the non-recrystallization temperature (Tnr) of Nb microalloyed steels processed using thin slab casting technologies:
• Tnr: interaction among deformation, recrystallizationand precipitation.
• Competition between Nb(C,N) precipitation and Nb-Mo drag mechanisms.
950
975
1000
1025
1050
1075
1100
0 10 20 30 40
Interpass time (s)
T nr(º
C)
3Nb
3Nb-Mo31
6Nb-Mo31
6Nb
Dependence of Tnr as a function of the interpass time (ε = 0.4)
Low Nb
0
20
40
60
80
100
7 7.5 8 8.5 9
10000/T (1/K)
Frac
tiona
lSof
teni
ng(%
)
Tnr =1026ºC
Tnr = 985ºC
tip = 10 s, ε = 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)
Frac
tiona
lSof
teni
ng(%
)
T nr =1030ºC
T nr= 1045ºC
tip = 30 s, ε = 0.4
6Nb
6Nb-Mo31
High Nb
Precipitation
ConclusionsMedium Nb contents (0.03%Nb):
The additional solute drag effect produced by Mo allowed the Tnr values to be higher in the Nb-Mo than in the Nb steels. Strain induced precipitation occurs at lower temperatures than Tnr in Nb-Mo grades.
Higher Nb contents (0.06%Nb):The acceleration of strain induced precipitation makes the contribution of Mo, as solute drag, less relevant.
Mo-V Steels
• New combinations of Mo and V microalloyed steelsopen new fields to research.
• Combination of strain accumulation (Mo drag) and V precipitation:– Ferrite refinement is achieved by:
• Austenite pancaking.• Ferrite nucleation enhancement on MnS+V(C,N).
– Increase in Yield Strength: ~ +200 MPa (dispersionstrengthening)[*].
[*] P.S. Mitchell, Maters. Sci. Forum, Vols. 500-501, 2005, pp. 269-278
Conclusions
• New steel grades produced by Thin Slab Direct Rolling technology are required for high-end applications.
• The production of microalloyed steels by Thin Slab Direct Rolling technology needs to adapt the chemical compositions and processing parameters to achieve the required mechanical properties for each steel grade.
• New modeling tools are a suitable way to perform optimization operations.
Role of Microalloying Elements during Thin Slab Direct Rolling
P. Uranga, B. López and J.M. Rodriguez-Ibabe
CEIT and Tecnun (Univ. of Navarra)Donostia, Basque Country, Spain
puranga@ceit.es
Microalloyed Steels: Production, Processing, Applications, IOM3, November 2007, London, UK
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