study of metadynamic recrystallization phenomena in coarse grained nb microalloyed austenite

Study of Metadynamic Recrystallization PhenomenaStudy of Metadynamic Recrystallization Phenomenain Coarse Grained Nb Microalloyed Austenitein Coarse Grained Nb Microalloyed Austenite

STUDY OF METADYNAMIC STUDY OF METADYNAMIC RECRYSTALLIZATION PHENOMENA IN RECRYSTALLIZATION PHENOMENA IN COARSE GRAINED Nb MICROALLOYED COARSE GRAINED Nb MICROALLOYED

AUSTENITEAUSTENITE

P. Uranga, A.I. FernP. Uranga, A.I. Fernáández, B. Lndez, B. Lóópez and pez and

J.M. RodriguezJ.M. Rodriguez--IbabeIbabe

CEIT and Univ. of Navarra, PCEIT and Univ. of Navarra, Pºº M. LardizM. Lardizáábal 15,bal 15,20018 20018 -- San Sebastian, Basque Country (Spain)San Sebastian, Basque Country (Spain)


IntroductionIntroduction

Thin Slab Casting TechnologyThin Slab Casting Technology–– Coarse Grained AsCoarse Grained As--cast Austenite Structurecast Austenite Structure–– Microalloying Elements in SolutionMicroalloying Elements in Solution–– High Deformation in First StandsHigh Deformation in First Stands

Possibility of Dynamic Recrystallization? Possibility of Dynamic Recrystallization? Study of PostStudy of Post--Dynamic Softening Dynamic Softening Microstructural ModelingMicrostructural Modeling


100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1000

1020

1040

1060

1080

1100

1120

D0 (μm)

T(ºC

)

0.60.7

0.8

0.9

1.0

1.1

Critical Strain0.035% Nb

Strain rate: 5 s-1

•• Onset of Dynamic Recrystallization for Reductions > 55%, Onset of Dynamic Recrystallization for Reductions > 55%, ε ε > 0.8> 0.8

•• Softening Mechanisms for Strains Higher than Critical StrainSoftening Mechanisms for Strains Higher than Critical Strain

Dynamic Recrystallization in First Stand?Dynamic Recrystallization in First Stand?


Usually when:Usually when:

ε > εcDynamic Dynamic RecrystallizationRecrystallization

Metadynamic Metadynamic RecrystallizationRecrystallization

Recently, Recently, BaiBai et al. (2000):et al. (2000):

Metadynamic Metadynamic RecrystallizationRecrystallization εε >> εεTT With With εεTT = 1.5= 1.5·· εεpp

Transition between different regionsTransition between different regions


C Mn Si S P Nb N0.1 1.42 0.31 0.008 0.018 0.035 0.0053

Material and Experimental ProcedureMaterial and Experimental Procedure

T

t

1400 ºC

ε1 tip1100ºC

ε2

Strains between εc and ε = 4

s 1t s 1 ip1 =→= −ε&

s 3.0t s 5 ip1 =→= −ε&


Influence of Strain on Softening

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5

StrainFr

actio

nal S

ofte

ning

εT = 1.7

εC = 0.75

Strain rate = 5 s-1

t = 0.3 sT = 1100ºC

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5

Strain

Frac

tiona

l Sof

teni

ng

εT = 1.3

εC = 0.6

Strain rate = 1 s-1

t = 1 sT = 1100ºC

εεTT == 2.22.2··εεcc

•• Strain dependent softening for strains higher than Strain dependent softening for strains higher than εεcc

•• Strain independent behaviour for strains higher thanStrain independent behaviour for strains higher than εεTT


Modeling Softening Kinetics

0

50

100

150

200

0 1 2 3 4 5Strain

Stre

ss (M

Pa)

T = 1100ºCStrain rate=1 s-1

εc εT

I II III



0

50

100

150

200

0 1 2 3 4 5Strain

Stre

ss (M

Pa)

T = 1100ºCStrain rate=1 s-1

εc εT

I II III

••Range I: (Range I: (ε ε << εεcc))

Static RecrystallizationStatic Recrystallization

••Range III: (Range III: (εεTT < < εε))

Metadynamic RecrystallizationMetadynamic Recrystallization

••Range II: (Range II: (εεcc <<ε ε << εεTT))

Static and Metadynamic Static and Metadynamic RecrystallizationRecrystallization


εεTT : Critical Value to achieve the minimum : Critical Value to achieve the minimum dynamic recrystallized fraction for a complete dynamic recrystallized fraction for a complete postpost--dynamic softening by metadynamic dynamic softening by metadynamic recrystallizationrecrystallizationCoarse Grained Austenite: Coarse Grained Austenite: εεcc HighHighIndustrial Conditions:Industrial Conditions:

–– εεTT much higher than applied strainmuch higher than applied strain–– Deformation and Softening Mechanisms: Ranges I / IIDeformation and Softening Mechanisms: Ranges I / II


Range I (Range I (ε ε << εεcc): ): Static RecrystallizationStatic Recrystallization– Initial Grain Size– Strain– Strain Rate– Temperature

Different Softening MechanismsDifferent Softening Mechanisms

Grain RefinementGrain Refinement

[ ]( )T,Nb,,,Dft 0SRX5.0 εε &=

( )T,ft MDRX5.0 ε&=

Range III (Range III (εεΤΤ < < εε): ): Metadynamic RecrystallizationMetadynamic Recrystallization– Strain Rate– Temperature



SRXMDRX XXX +=

cT

cMDRXfX

εεεε

−−

= MDRXf

SRXf X1X −=

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

MDRX5.0

MDRXfMDRX t

t693.0exp1XX

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

SRX5.0

SRXfSRX t

t693.0exp1XX

Region II (Region II (εεcc < < εε < < εεTT): Static and Metadynamic Recrystallization): Static and Metadynamic Recrystallization



0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5Strain

Frac

tiona

l Sof

teni

ng

Strain rate = 1 s-1

t = 1 s

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5Strain

Frac

tiona

l Sof

teni

ng

Strain rate = 5 s-1

t = 0.3 s

εc

XSRX

εc

XSRXXMDRXεT XMDRXεT

XX


0

0.2

0.4

0.6

0.8

1

0.01 0.1 1 10 100Time (s)

Frac

tiona

l Sof

teni

ng

Strain = 0.9Strain rate = 1 s-1

XSRX

XMDRX

ModelingPost-Dynamic Softening with Time

•• Applied Strain in Range IIApplied Strain in Range II

•• Sum of Individual Sum of Individual Components: Overall Components: Overall SofteningSoftening


ModelingDependence of t0.5 with Applied Strain

0.1

1

10

100

0.1 1 10Strain

t 0.5

(s)

T = 1100ºCStrain rate = 1 s-1

D0 = 806 μm

•• Below Below εεcc: Strong dependence : Strong dependence on strainon strain

ε c

•• εε > > εεTT: Strain independent: Strain independentε Tε p

•• εεcc<< εε < < εεTT: Correct Prediction: Correct Prediction


Modeling Microstructural EvolutionModeling Microstructural Evolution

Thin Slab Casting and Direct Rolling TechnologyThin Slab Casting and Direct Rolling Technology– Heterogeneous Initial Grain Size

– Solid Solution

Recrystallization delayedRecrystallization delayed

Need of a Model taking into account these Need of a Model taking into account these singularitiessingularities


Modeling Microstructural EvolutionModeling Microstructural Evolution

Initial variables:Initial variables:– Processing variables: Strain, strain-rate, temperature…– Initial Grain Size Distribution

Static and Metadynamic SofteningStatic and Metadynamic SofteningInteraction with PrecipitationInteraction with PrecipitationGrain Size EvolutionGrain Size EvolutionOutputs:Outputs:– Recrystallized and Unrecrystallized Grain Size

Distributions at the entry of any rolling pass


T

t

1100 ºC (20 min)

1400 ºC

ε = 0.93strain rate = 5 s-1

T = 1080ºC

T = 1000ºC

Model Validation

Torsion Test Schedule

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000

Grain Size (μm)Fr

eque

ncy

(%)

Initial Grain Size Distribution

Mean Grain Size: 806 μm


Model Validation ResultsModel Validation Results

0

0.1

0.2

0.3

0.4

20 100 180 260 340 420 500Grain Size (μm)

Are

a Fr

actio

n

ModelExperimental

XXSRXSRX = 0.86= 0.86

XXMDRXMDRX = 0.12= 0.12


0

0.1

0.2

0.3

0.4

20 100 180 260 340 420 500Grain Size (μm)

Are

a Fr

actio

n

ModelExperimental

a)

0

0.1

0.2

0.3

0.4

20 100 180 260 340 420 500Grain Size (μm)

Are

a Fr

actio

n

ModelExperimental

b)

New ModelClassical

Approach

Comparison betweenbetween models


ConclusionsConclusionsResults show that once the applied strain is higher than Results show that once the applied strain is higher than the critical value the critical value εεcc, there is a transition regime , there is a transition regime between between εεcc and and εεTT where both classical static and where both classical static and metadynamic recrystallization softenings intervene.metadynamic recrystallization softenings intervene.

In the analyzed 0.035%Nb microalloyed steel, for the In the analyzed 0.035%Nb microalloyed steel, for the case of coarse initial grain sizes, a value of the case of coarse initial grain sizes, a value of the transition strain equal to transition strain equal to εεTT == 2.22.2··εεcc has been obtained.has been obtained.

Taking into account the typical reductions of the first Taking into account the typical reductions of the first rolling passes, real situations will correspond to fully rolling passes, real situations will correspond to fully classical static recrystallization (classical static recrystallization (εε << εεcc) or to mixed ) or to mixed conditions (conditions (εε >> εεcc).).


Conclusions (II)Conclusions (II)The model considers a mean recrystallized The model considers a mean recrystallized microstructure combining both classical static and microstructure combining both classical static and metadynamic grains through the law of mixtures. metadynamic grains through the law of mixtures. The model predictions have been validated with The model predictions have been validated with specific tests. The model leads to good predictions when specific tests. The model leads to good predictions when both static and metadynamic softening are included for both static and metadynamic softening are included for applied strains applied strains εε >> εεcc. Some important divergences are . Some important divergences are observed if only metadynamic softening is considered.observed if only metadynamic softening is considered.