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
Microalloyed Steels: Production, Processing, Applications, IOM3, November 2007, London, UK
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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.
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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
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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.
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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
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Procedure• Classical modeling approach:
– Not enough to predict heterogeneities
• New model:– Particular characteristics of TSDR Technology
• Initial As-cast Structure• Specific Thermomechanical Deformation Route
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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
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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
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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)
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Final Gauge Thickness (mm)
Total Strain
Rol
ling
Entr
y Te
mpe
ratu
re (º
C)
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0.035%NbDc
Optimum Processing Zone
10
Residual unrefinedas-castgrains
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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
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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)
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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
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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
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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
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0.6 0.6
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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
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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
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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
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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
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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
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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
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Model applications
• Validation steps:– Laboratory– Plant trials
• Industrial Schedule optimization focusedon:– Thick final gauges– High microalloying levels
• Powerful tool for new grade design
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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
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Final Gauge Thickness (mm)
Total StrainR
ollin
gEn
try
Tem
pera
ture
(ºC
)
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Residual unrefinedas-cast grains
Optimum Processing Zone
0.05% Nb(a) Dc
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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
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0.3
0.4
0.5
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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
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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.
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Final Gauge Thickness (mm)
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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?
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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
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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”,...).
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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.
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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
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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.
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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)
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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
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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
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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.
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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
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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.
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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
Microalloyed Steels: Production, Processing, Applications, IOM3, November 2007, London, UK