Optimization of Rolling Conditions in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling Route: Processing Maps

Microalloying for New Steel Processes and Applications, September 7-9, 2005, Donostia-San Sebastián

Optimization of Rolling Conditions in Nb Microalloyed Steels Processed by Thin Slab

Casting and Direct Rolling Route: Processing Maps

P. Uranga, A.I. Fernández, B. López and J.M. Rodriguez-Ibabe

CEIT and tecnun (University of Navarra)Donostia-San SebastiánBasque Country, Spain

Objective• Definition of Optimal Processing Conditions

for different grades under a wide variety of Industrial Rolling Conditions

• 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• Processing Maps:

– Suitable for visual understanding

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


......

Rex Unrex...



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


......, , ,


[drex] i

Freq

uenc

y

D

D

3-D

Freq

uenc

y

[d0] i

[fv] i


......

Rex Unrex...



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


......, , ,


[drex] i

Freq

uenc

y

D

D

3-D

Freq

uenc

y

[d0] i

[fv] i


......

Rex Unrex...



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


......, , ,


[drex] i

Freq

uenc

y

D

Rolling Simulations

• 0.035% 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: 1040 to 1100ºC

• Interpass times: function of strain-rates

• Final cooling rate: 20ºC/s to 800ºC

RollingSchedules

• Large reductions during first passes

e (mm)Pass no. Strain Rate tip Strain Rate tip

1 0.7 5 6 0.5 5 6 352 0.7 10 4 0.5 10 4 303 0.55 15 3 0.45 15 5 304 0.45 30 2.1 ⎯ ⎯ ⎯ 305 0.35 50 1.8 0.3 20 2.7 306 0.25 70 0.25 25 ⎯

4 10 ΔT

SimulationResults

• Final Austenite Microstructure

0.0

0.1

0.2

0.3

0.4

0.5

10 30 50 70 90 MoreAustenite Grain Size (μm)

Are

a Fr

actio

n

Ti = 1100ºC

e = 4 mm

0.0

0.1

0.2

0.3

0.4

0.5


Are

a Fr

actio

n

Ti = 1040ºCTi = 1100ºC

e = 4 mm

0.0

0.1

0.2

0.3

0.4

0.5


Are

a Fr

actio

n

Ti = 1100ºC

e = 10 mm

0.0

0.1

0.2

0.3

0.4

0.5


Are

a Fr

actio

n

Ti = 1040°CTi = 1100ºC

e = 10 mmDmean= 9 μm

Dmean= 15 μmDmean= 16 μm

Dmean= 21 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

• e = 4 mm: Homogeneous Structure. ZD < 8

• e = 10 mm: Microstructural Heterogeneities. ZD > 8

Entry Rolling Temperature: 1040ºC

0

200

400

600

800

1000

1200

1400

1600

1 2 3 4 5 6Interstand

Gra

in S

ize

( μm

)

0

2

4

6

8

10

12

14

ZD P

aram

eter

DmeanDcDmaxZD

e = 4 mmTi = 1040ºC

0

200

400

600

800

1000

1200

1400

1600

1 2 3 4 5 6Interstand

Gra

in S

ize

( μm

)0

2

4

6

8

10

12

14

ZD P

aram

eter

DmeanDcDmaxZD

e = 10 mmTi = 1040ºC

Effect of thefinal gauge thickness

• Dmean

– Strain ↑: Dmean↓– Ti ↑: Dmean ↑

• Heterogeneities unrevealed

Final thickness (mm)

12.65 7 6 4 3 1.5210

0

5

10

15

20

25

30

1 2 3 4 5Total Strain

Mea

n A

uste

nite

Gra

in S

ize

( μm

)

Ti = 1100°CTi = 1040°C

a) Dmean


• Dc

– Strain ↑: Dc↓– Ti ↑: Dc ↑


12.65 7 6 4 3 1.5210

0

20

40

60

80

1 2 3 4 5Total Strain

Dc

Aus

teni

te G

rain

Siz

e ( μ

m)

Ti = 1100°CTi = 1040°C

b) Dc

• 1040ºC: sharp increase for low strains and low Ti


• ZD– Ti ↑: ZD constant– Strain ↓ and Ti ↓ :

ZD ↑• Heterogeneities

revealed


12.65 7 6 4 3 1.5210

0

5

10

15

20

25

1 2 3 4 5

Total Strain

ZD P

aram

eter

Ti = 1100°CTi = 1040°C

c) ZD

Processing Maps

• Dc isoclines combined with Processing Regions

2 2.5 3 3.5 41040

1050

1060

1070

1080

1090

1100

202525

3030

3035

35

35 35

4040

40

40

50

50

50

60

60

60

70

Final Gauge Thickness (mm)

Total Strain

Rol

ling

Entr

yTe

mpe

ratu

re(º

C)

12.65 7 6 4 3 1.52

Dc

Optimum Processing Zone

10

Residual unrefined

as-castgrains

Processing Maps

• Retained strain isoclines combined with Processing Regions

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

0.4

0.50.5

0.50.5

0.6

0.6 0.6 0.60.7 0.70.8

RetainedStrain


Total Strain

Rol

ling

Entr

yTe

mpe

ratu

re(º

C)

12.65 7 6 4 3 1.52


10

Residual unrefinedas-castgrains

Processing Maps

• e ≤ 6-7 mm: Dc ↔ Ti relationship.Homogeneous structure.

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

0.40.4

0.50.5

0.50.5

0.6

0.6 0.6 0.60.7 0.70.8

RetainedStrain


Total Strain

Rol

ling

Entr

yTe

mpe

ratu

re(º

C)

12.65 7 6 4 3 1.52


10


2 2.5 3 3.5 41040

1050

1060

1070

1080

1090

1100

2025

25

30

30

30

35

35

35 35

4040

40

40

50

50

50

60

60

60

70


Total Strain

Rol

ling

Entr

yTe

mpe

ratu

re(º

C)

12.65 7 6 4 3 1.52

Dc


10


• e > 6-7 mm: Dc independent from Ti. Minimum Ti required to avoid heterogeneities.

Conclusions

• The microstructural evolution during hot rolling of coarse grain sized austenite has been modeled considering all the microstructural reactions that can take place during an industrial TSDR production of a Nbmicroalloyed steel.

• The model is able to predict possible heterogeneities present in the final austenite microstructure before transformation.

• Based on the results obtained with the model, processing maps have been drawn for a 0.035% Nb microalloyed steel, considering the following parameters: initial rolling temperature, total reduction and rolling schedule.

• The influence of the previous parameters on the microstructural refinement and homogeneity has been evaluated. The processing maps have revealed as a very useful tool to define optimum processing conditions in order to exploit all the benefices of the Nb microalloying in thin slab direct rolling

Acknowledgments

• Basque Government• CICYT (MAT2002-01174 project)

• Materials Department - Thermomechanical Treatments Group (CEIT)

• CEIT and tecnun• Carnegie Mellon University and Professor

A.W. Cramb

Microalloying for New Steel Processes and Applications, September 7-9, 2005, Donostia-San Sebastián

Optimization of Rolling Conditions in Nb Microalloyed Steels Processed by Thin Slab

Casting and Direct Rolling Route: Processing Maps

P. Uranga, A.I. Fernández, B. López and J.M. Rodriguez-Ibabe

CEIT and tecnun (University of Navarra)Donostia-San SebastiánBasque Country, Spain

Grain Size Distribution in an Industrial Thin Slab

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000

Grain Size (μm)

Freq

uenc

y (%

)

CenterNear Surface Mixed

Columnar-Equiaxed Structure

Dγ > 2000 μm → Volume Fraction > 20%