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

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5060

6060

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7070

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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

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1120

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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

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1150

2020

2525

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4545

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7070

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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

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1150

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6060

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7070

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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

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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