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

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35

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40

4045

45

45

45

50

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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

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6060

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7070

7080

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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%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

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1070

1080

1090

1100

2025

25

30

3030

35

35

35 35

40

40

4040

50

5050

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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

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35

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35 35

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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

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

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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

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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

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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

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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

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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