BAOSTEEL, BAC2006 May 24-26, 2006, Shanghai, China
AVOIDANCE OF MICROSTRUCTURAL HETEROGENEITIES BY HOT ROLLING
DESIGN IN THIN SLAB DIRECT ROLLED NIOBIUM MICROALLOYED
STEELS
P. Uranga, B. López and J.M. Rodriguez-Ibabe
CEIT and TECNUN (University of Navarra)Donostia-San SebastiánBasque Country, Spain
Objective• 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
Procedure
• Classical approach:– Not enough to predict heterogeneities
• New model:– Particular characteristics of TSDR Technology
• Initial As-cast Structure• Specific Thermomechanical Deformation
Route
0
5
10
15
20
25
30
0 500 1000 1500 2000 2500 3000
Grain Size (μm)
Freq
uenc
y (%
)
CenterNear Surface
ModelDescription
• Main difference:– Expansion of Classical Microstructural
Modeling to Grain Size Distributions• Input:
– Grain Size Distribution measured in a real Thin Slab
• Output:– Recrystallized and Unrecrystallized Grain
Size histograms and Retained Strain
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
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
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
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
Rolling Simulations
• 0.05% Nb Microalloyed Steel (0.06%C, 0.008%N, 1.1%Mn)
• Initial thickness: 55 mm
• Final thicknesses: 1.5 to 12.65 mm
• Rolling entry temperatures: 1060 to 1150ºC
• Interpass times: function of strain-rates
• Final cooling rate: 20ºC/s to 800ºC
RollingSchedules
• Large reductions during first passes
e = 1.5 e = 2 e = 3 e = 4 e = 6 e = 7 e = 10 e = 12.65
Pass ε ε& (s-1)
tip (s) ε ε&
(s-1)tip(s) ε ε&
(s-1)tip(s) ε ε&
(s-1)tip(s) ε ε&
(s-1) tip(s) ε ε&
(s-1)tip(s) ε ε&
(s-1)tip(s) ε ε&
(s-1)tip(s)
ΔT (ºC)
1 1 5 6 0.9 5 6 0.75 5 6 0.7 5 6 0.55 5 6 0.5 5 6 0.5 5 6 0.4 5 6 352 1 15 3 0.9 15 3 0.75 10 4 0.7 10 4 0.55 10 4 0.5 10 4 0.5 10 4 0.4 10 4 303 0.85 50 1.8 0.8 40 2 0.7 15 3 0.55 15 3 0.55 15 3 0.45 15 3 0.45 15 5 0.4 15 5 304 0.55 90 1 0.5 70 1.6 0.5 40 2 0.45 30 2.1 0.4 25 2.5 0.4 25 2.5 ⎯ ⎯ ⎯ ⎯ ⎯ ⎯ 305 0.4 150 0.7 0.4 90 1 0.4 60 1.7 0.35 50 1.8 0.35 40 2 0.3 30 2.1 0.3 20 2.7 0.3 20 2.7 306 0.3 200 0.3 120 0.3 80 0.25 70 0.25 60 0.25 50 0.25 25 0.2 25 (*)
SimulationResults
• Final Austenite Microstructure
0.0
0.1
0.2
0.3
0.4
0.5
0 – 10 20 – 30 40 – 50 60 – 70 80 – 90 More
Austenite Grain Size (μm)
Are
a Fr
actio
n
Ti = 1100ºC
e = 10 mm • Ti = 1100ºC: Homogeneous Structure
• Mean Size : 25 μm
0.0
0.1
0.2
0.3
0.4
0.5
0 – 10 20 – 30 40 – 50 60 – 70 80 – 90 More
Austenite Grain Size (μm)
Are
a Fr
actio
n
Ti = 1100ºCTi = 1060ºC
e = 10 mm
• Ti = 1060ºC: Heterogeneous Structure
• Mean Size : 22 μm
• 5% bigger than 100 μm
ParameterDefinition
• Histograms not very useful when analyzing wide spread of conditions and/or materials
• General parameters defined:– Dmean
– Dmax
– Dc (10% of the volume fraction of grains have a bigger size than Dc)
– ZD (=Dmax/Dmean)
Parameter Evolution
• Ti = 1100ºC: Homogeneous Structure → ZD < 8
• Ti = 1060ºC: Microstructural Heterogeneities for e > 3 mm → ZD > 8
0
5
10
15
20
25
1 2 3 4 5
Total Strain
ZD P
aram
eter
Ti = 1100°CTi = 1060°C
Final thickness (mm)
12.65 7 6 4 3 1.5210
Processing Maps
• Microstructural Heterogeneities: Low T and Low strain
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150
20
20
25
25
30
30
30
35
35
35
40
40
40
40
45
45
45
45
50
50
50
50
5060
6060
70
7070
80
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% Nb(a) Dc
• Grain Growth: High T
• Optimum Strain-Temperature Conditions
Schedule Redesign
• Initial Thickness: 55 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 6 352 ⎯ ⎯ ⎯ 0.45 10 9 303 0.45 15 5 ⎯ ⎯ ⎯ 304 ⎯ ⎯ ⎯ ⎯ ⎯ ⎯ 305 0.3 20 2.7 0.3 20 2.7 306 0.25 25 0.25 25 (*)
Seq 10
Pass ε ε& (s-1)
tip(s)
ΔT (ºC)
1 0.5 5 6 352 0.5 10 4 303 0.45 15 5 304 ⎯ ⎯ ⎯ 305 0.3 20 2.7 306 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
20
25
25
30
30
30
35
35
35
40
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(º
C)
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 theInitial Slab Thickness
• Initial Thickness: 55 mm → 70 mm
• Initial/Final Thickness ≥ 7 [*] → Toughness Requirements
[*] Klinkenberg C and Hensger KE, Processing Niobium Microalloyed API Grade Steel on a Thin Slab Plant. Materials Science Forum, 2005. 500-501: 253~260.
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
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150
2020
2525
3030
3035
35
35
40
40
40
40
45
4545
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
εr = +0.2
Conclusions
• The production of Nb 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
• With the help of a mathematical model, the effect of composition and processing parameters can be analyzed in order to define the optimum conditions for slab rolling.
• Rolling schedules have been modified in order to optimize final homogeneous microstructures and retained strain in austenite prior to transformation to ferrite.
Acknowledgments
• Basque Government• CICYT (MAT2002-01174 project)
• Materials Department - Thermomechanical Treatments Group (CEIT)
BAOSTEEL, BAC2006 May 24-26, 2006, Shanghai, China
AVOIDANCE OF MICROSTRUCTURAL HETEROGENEITIES BY HOT ROLLING
DESIGN IN THIN SLAB DIRECT ROLLED NIOBIUM MICROALLOYED
STEELS
P. Uranga, B. López and J.M. Rodriguez-Ibabe
CEIT and TECNUN (University of Navarra)Donostia-San SebastiánBasque Country, Spain
Processing Maps
• Dc isoclines combined with Processing Regions
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150
20
20
25
25
30
30
30
35
35
35
40
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(º
C)
12.65 7 6 4 3 1.5210
Residual unrefinedas-cast grains
Optimum Processing Zone
0.05% Nb(a) Dc
2 2.5 3 3.5 41060
1070
1080
1090
1100
1110
1120
1130
1140
1150 0.20.2
0.20.4
0.4
0.4
0.4
0.60.6
0.6 0.6
0.8
0.8
0.8 0.8
1
1 11
1.2 1.21.21.4 1.4
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% Nb(b) Retained
Strain
• Retained strain isoclines combined with Processing Regions
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