Effect of Alloying Additions in the Final Microstructure of Nb-Mo Steels Processed by Thin

Slab Direct Rolling Technologies

P. Uranga, J. Ganzarain, D. Jorge-Badiola and J.M. Rodriguez-Ibabepuranga@ceit.es

MS&T’08 ConferenceOctober 6-9, 2008, Pittsburgh, PA

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

MS&T’08 Conference, Pittsburgh, PA

Introduction

• Multiple alloying combinations for high strength properties

• Thin slab direct rolling metallurgical peculiarities– As-cast coarse grains– Alloying elements in solid solution

• Study of combined effect in softening and precipitation kinetics

• Modeling and microstructural evolution validation for schedule optimization

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Material

• Two Nb-Mo microalloyed steels

• 0.05%C - 0.03%Nb– 0.16% Mo– 0.31% Mo

Steel C Si Mn S Al Mo Nb N

3Nb-Mo16 0.05 0.04 1.58 0.002 0.027 0.16 0.03 0.005

3Nb-Mo31 0.05 0.05 1.57 0.002 0.028 0.31 0.028 0.006

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Deformation SchedulesSoaking:

1350ºC, 15 min

600ºC, slow cooling

Quenching

Tem

pera

ture

Time

He flow

10ºC/s

700ºC, 1hour

First deformation temperatures

Tini = 1100ºC Tini = 1050ºC

ΔT = 50ºC

Quenching temperatures

Tq = 900ºC Tq = 850ºCε1 = ε2 = ε3 = 0.4ε4 = 0.5

1ºC/s

Austenite Microstructureand Modeling

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Austenite Microstructures Prior to Transformation

• Non-recrystallized deformed austenite grains– Tini = 1100ºC: Homogeneous Structures

3Nb-Mo16 3Nb-Mo31

Tin

i = 1

100º

C

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Microstructural Heterogeneities in Austenite. Tini = 1050ºC

• Non-recrystallized deformed Austenite Grains– Tini = 1050ºC: Microstructural Heterogeneities

in Homogeneous Matrix 3Nb-Mo16 3Nb-Mo31

Tin

i = 1

050º

C

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Grain Distribution Modeling• Input:

– Thermomechanical Sequence– Composition– Initial Grain Size Distribution

• Model:– Equations developed for Nb-Mo steels and wide

range of austenite grains:

• Output:– Recrystallized and unrecrystallized grain size

distributions– Mechanism history: drag, precipitation, dynamic rex

[ ]( ){ } 155,0147,00

3

78,1035,0201108,2 ZDNb

c++

⋅= −ε [ ] [ ]( )⎥⎦

⎤⎢⎣

⎡+⎟

⎠⎞

⎜⎝⎛ −⋅⎟

⎠⎞

⎜⎝⎛⋅= −−−

−

MoNbTRT

Dt DoX 09,0185275000exp180000exp1092,9 53,06,511

5,0

15,00 εε &

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Model Predictions

• Austenite grain size distribution prior to transformation– Tini = 1100ºC: Similar distributions for both

steels 3Nb-Mo16 3Nb-Mo31

Tin

i = 1

100º

C

0

0.1

0.2

0.3

0.4

0.5

20 40 60 80 100 120 140 160 180 200

Grain Size (μm)

Aus

teni

te A

rea

Frac

tion Model

Experimental

3Nb-Mo16Tini = 1100ºC

0

0.1

0.2

0.3

0.4

0.5

20 40 60 80 100 120 140 160 180 200

Grain Size (μm)

Aus

teni

te A

rea

Frac

tion

ModelExperimental

3Nb-Mo31Tini = 1100ºC

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Model Predictions

• Austenite grain size distribution prior to transformation– Tini = 1050ºC: Heterogeneity increases as Mo

content increases 3Nb-Mo16 3Nb-Mo31

Tin

i = 1

050º

C

0

0.1

0.2

0.3

0.4

0.5

20 100 180 260 340 420 500 580

Grain Size (μm)

Aus

teni

te A

rea

Frac

tion

ModelExperimental

3Nb-Mo16Tini = 1050ºC

0

0.1

0.2

0.3

0.4

0.5

20 100 180 260 340 420 500 580

Grain Size (μm)

Aus

teni

te A

rea

Frac

tion Model

Experimental

3Nb-Mo31Tini = 1050ºC

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Heterogeneous Structures

• As-cast austenite grain prior to transformation:– Lack of Recrystallization through deformation

passes and interstands– Higher fraction for 3Nb-Mo31

• Bigger maximum grain size– Strain induced Nb(C,N) precipitation interacts

with softening mechanisms mainly in highly strained grains

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Heterogeneity: As-cast Fraction

• Homogeneous Micro: Complete Rex prior to Precipitation• Minimum initial temperature is needed for heterogeneities

to be avoided• Once homogeneity achieved: focus on austenite pancaking

Tini = 1100ºC Tini = 1050ºC

0

0.2

0.4

0.6

0.8

1 2 3 4Interstand

As-

cast

frac

tion

3Nb-Mo313Nb-Mo16

Tini = 1100ºC

0

0.2

0.4

0.6

0.8

1 2 3 4Interstand

As-

cast

frac

tion

3Nb-Mo313Nb-Mo16

Tini = 1050ºC

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Austenite Pancaking: Unrecrystallized Fraction

Tini = 1100ºC Tini = 1050ºC

0

0.2

0.4

0.6

0.8

1

Unr

ecry

stal

lized

Aus

teni

te F

ract

ion Precipitation

Solute Drag

Interstand

3Nb-Mo16

Tini = 1100ºC

3Nb-Mo31

3 41 2 3 41 20

0.2

0.4

0.6

0.8

1

Unr

ecry

stal

lized

Aus

teni

te F

ract

ion Precipitation

Solute Drag

3 4Interstand

3Nb-Mo16

Tini = 1050ºC

3Nb-Mo31

1 2 3 41 2

• Austenite Pancaking: Drag / Precipitation

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Austenite Pancaking: Nb Steel – NbMo Steels

Tini = 1100ºC Tini = 1050ºC

0

0.2

0.4

0.6

0.8

1

Unr

ecry

stal

lized

Aus

teni

te F

ract

ion Precipitation

Solute Drag

Interstand

3Nb-Mo16

Tini = 1100ºC

3Nb-Mo31

3 41 2 3 41 20

0.2

0.4

0.6

0.8

1

Unr

ecry

stal

lized

Aus

teni

te F

ract

ion Precipitation

Solute Drag

3 4Interstand

3Nb-Mo16

Tini = 1050ºC

3Nb-Mo31

1 2 3 41 2

0

0.2

0.4

0.6

0.8

1

Unr

ecry

stal

lized

Aus

teni

te F

ract

ion

PrecipitationSolute Drag

Interstand

Tini = 1100ºC

3 41 2 3 41 2 3 41 2

3Nb-Mo16 3Nb-Mo313Nb

• Mo drag effect accelerates Nb(C,N) precipitation at low deformation T

0

0.2

0.4

0.6

0.8

1

Unr

ecry

stal

lized

Aus

teni

te F

ract

ion

3 4Interstand

3Nb-Mo16

Tini = 1050ºC

3Nb-Mo31

1 2

3Nb

3 41 2 3 41 2

εac = 0.50

εac = 0.43εac = 0.56 εac = 0.73

εac = 0.54εac = 0.95

Transformation

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• For Tini = 1100ºC:– Homogeneous ferrite

Transformed Microstructures Coiling Simulation 700ºC

3Nb-Mo16 3Nb-Mo31

Tin

i = 1

100º

C

Dα = 8.6 μm Dα = 8.7 μm

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• For Tini = 1050ºC:– Homogeneous ferrite with heterogeneous

regions

Transformed Microstructures Coiling Simulation 700ºC

3Nb-Mo16 3Nb-Mo31

Tin

i = 1

050º

C

Dα = 8.4 μm Dα = 8.6 μm

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Microstructural Units EBSD

• Homogenous ferrite microstructures correspond to high angle GB units

Tini = 1100ºC 3N

b-M

o31

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Microstructural Units EBSD• Prior austenite coarse grains transform to

coarse ferrite units or acicular structures: forming low angle GB areas.

Tini = 1050ºC

3Nb-

Mo3

1

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Conclusions• Mo addition to Nb microalloyed steels: important

increase in the delay of static rex kinetics → The refinement of the initial as-cast structure is retarded.

• For homogeneous microstructures: EBSD show that ferrite grains are diversely oriented with high-angle grain boundaries.

• Structures transformed from non-refined as-cast grains form coarse microstructural units, bigger than those observed with the optical microscope. Toughness will be impaired.

• For optimized thermomechanical schedules, Mo affects hardenability. This factor can be useful for the formation of complex microstructures with high strength and toughness levels.

Effect of Alloying Additions in the Final Microstructure of Nb-Mo Steels Processed by Thin

Slab Direct Rolling Technologies

P. Uranga, J. Ganzarain, D. Jorge-Badiola and J.M. Rodriguez-Ibabepuranga@ceit.es

MS&T’08 ConferenceOctober 6-9, 2008, Pittsburgh, PA

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