sjtu-fuliming and shanaidang-high strength and high ......carbon-free or ultra-low carbon femnc...

材料科学与工程学院School of Materials Science and Engineering

1896 1935 1987 2006

1

High Strength and High Ductility UltrafineGrained Low Carbon TWIP Steels

FU Li-ming , SHAN Ai-dang付立铭单爱党

1. School of Materials Science and Engineering2. School of Environmental Science and Engineering

Shanghai Jiao Tong University2014-08-14


2

Introduction

Nb,Ti alloyed TWIP steels

UFG TWIP steels

Summary

Outline


3

TWIP steel: TWinning Induced Plasticity steel

Introduction

Hardfield steel

Grassel and Frommeyer

Fe-Mn-Al-Si alloyCarbon-free or ultra-low carbon

FeMnC alloy0.6~1.2wt.%C

Bouaziz and Allain

Product

1882

TWIP

Hard Field

1997~1998 2004~ 2005

Arcelor-TKS

TWIPFe-14Mn-1.4C

化学成分，

wt%

15~30wt.%Mn


4

2011/2012Ansteel , BaosteelTrial product/Industrial productAutomotive TWIP steel

2013TKSGI/GA/ZMGalvanized TWIP steel

2011/2012POSCOTrial production/Product supply

for Hyundai-Kia, Chrysler, General Motors, Volkswagen

Automotive TWIP steel

2009POSCOContinuous CastingFe-15Mn-2.0Al-0.6C-0.5Si

2009Salzgitter- CorusContinuous CastingFe-15Mn-0.7C-2.5Al(2.5Si)

2004VAS-CDL/ AustriaContinuous CastingFe-(15~30)Mn-1.0C-4Al-4Si

2003~2009POSCOContinuous Casting/ Continuous Casting(twin roll type)Fe-18Mn-1.5Al-0.6C(Ti, B)(4)

2005Salzgitter- CorusDirect Strip Casting /HSDFe-16Mn-2.4Al-0.08C-2.3Si

1998~2005Acelor-Thyssen Krupp SteelContinuous Casting/ L-IP, X-IPFe-22Mn-0.6C

1998MPITWIP ConceptFe-15~30Mn-2~4Al-0.03C-2~4Si

1994POSCOIngot CstingFe-25Mn-1.5Al-0.55C-0.014N(Ti,B)( 3)

1993RIST, POSCOAustenitic steel for automobile /PAK-90/Pilot Test

Fe-25Mn-1Al-0.3C; Fe-25Mn-0.3C ; Fe-25Mn-1.5Al-0.5C

-0.1N

1991POSCOPatent, TWIP steel for automobile,World fistFe-xMn-3Al-0.6Si

1882-Hadfield steelFe-14Mn-1.4C

yearCompanyManufacturingSteel


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1) High product of strength and elongation

Stress-strain curves for typical AHSS steel.

Introduction - TWIP steel

Typical characteristic


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Relationship between yield strength and elongation in various automobile steels


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2) Very high strain hardening rate


Comparison of Strain hardening rate and n-value for some typical AHHSs.


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Comparison of energy absorption for common types of the automobile

3) Very high energy absorption for anti-intrusion (crash)

From: DeCooman et.al, New Trends and Developments in Automotive System Engineering. 2011



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Mechanism of High-Mn austenitic TWIP steels

Deformation twinning

Deformation Twins

TD TD


(b)

Deformation Twins

TD TD(b)

Deformation Twins

TD TD(b)(a)

Matrix Annealing Twin

Deformation twins(c)

TDTD


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Relatively lower stacking fault energy (SFE)

by adjusting the composition : C、Mn、Al、Si… (20mJ~45mJ/m2)


From: Park et.al, Metals and Materials International. 2010, 16(1): 1-6

Plasticity mode transition of the high Mn austenitic steels with different SFE


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Limitations1) Relatively lower strength, especially for yield strength

limits implementation for anti-intrusion (crash) assemblies

Some hot rolled TWIP steel, YS even less 200MPa; Most TWIP steels: YS<500MPa

350MPa

500MPa

From: Grassel et al. Inter. J. Plast., 2000,16, 1319 From: Bouaziz et al. Curr. Opin. ST. M., 2011,15,141

C-free and ultra-low C TWIP steel Medium and high C TWIP steel


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Pre-straining

Typical methods for Improvement

From: Dini et al, Mater. Des., 2010, 31,3395

Precipitationstrengthening

Grain Refinement

From:C Scott et al.

Inter. J. Mater. Res., 2011,102,538

From: Bouaziz et al.

Curr. Opin. ST. M., 2011,15,141

Fe-22Mn-0.6C Fe-(17~22)Mn-(0.6~0.9)C Fe-31Mn-3Al-3Si


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2) Limited works focus on the study ofthe Low Carbon (≤0.2wt.%) TWIP steels



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Thermodynamic calculation by JMatPro-V8

δ-Ferrite

Austenite

Liquid Austenite

δ-FerritePeritecticReacton

From:M.N. Shiekhelsouk and O. bouaziz, et al.

Mater. Sci. forum., 2006,524-525,833

From: V. Torabinejad, et al.

Mater. Des., 2011,32,2345

Duplex

TWIP steel


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(b)

(a)

(c)

(d)

(e)

15wt.%Mn

17wt.%Mn20wt.%Mn

22wt.%Mn

25wt.%Mn

Fe-0.08C-xMn-(2~3)Si-(2.5~4)Al alloy

No δ-ferrite

From: Liming Fu, Thesis, SJTU, 2014

Our previous works


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12 papers：V precipitation；

5 papers：Nb precipitation ；

2 papers：Ti precipitation ；

3 papers：Ti+W precipitation ；

4 papers：V、Ti、Nb precipitation

3) Rather limited works paid attention to the Nb/Ti microalloyed TWIP steels


•Only about 26 papers concerned Nb,V,Ti (micro-) alloyed high Mn austenitic steel


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Recent typical research[1] F. Reyes-Calderon, et al., Mater. Sci. Eng., 2013,560,552;

Nb、V、Ti effect on the hot flow behavior and recrystallized grain refinement.[2] C. Scott, et al., Int. J. Mat. Res.(Z. Metallkd), 2011,5,538.

Comprehensive work for studying the Nb，Ti and precipitate strengthening. these works are all concerned medium and high carbon TWIP steels. (≥0.4wt.%)

[3] B. X. Huang, Y. H. Rong and L. Wang, et al., Mater. Sci. Eng., 2006, 438-440, 306.Ultralow carbon Fe-Mn-Si-Al-Nb alloy, mainly revealing the effect of solute Nb on the stacking faults energy and TWIP

From:C Scott et al.

Inter. J. Mater. Res., 2011,102,538


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Present Report

To attempted to produce the high strength and high ductility NG/UFG TWIP steel by cold-ASR process;To study the effect of the annealing temperatures on the

microstructures and properties of the TWIP steels

1. Nb,Ti (micro-) alloyed low C TWIP steels

the feasibility of the TWIP steel alloyed by Nb and/Ti;Effect of Nb and Ti additions on property, deformation behavior ;Strengthening mechanism of Nb,Ti precipitates in TWIP steels

2. Ultrafine grained TWIP steel


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Microstructure and properties of Nb,Ti (micro-) alloyed low carbon TWIP steels

Nb,Ti alloyed TWIP steels


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ExperimentalCast ingot

50kg smeltining in vacuum induction furnace.

Homogenization 2h at 1250℃ under protective argon atmosphere

TMCPhot-rolled from 120 down to 12mm at 1150 ℃

finish temperature 900 ℃water quenching

cold-rolled70~85% reduction


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Results and Disscussion

• Tensile curveshot-rolled steel


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0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40200

300

400

500

600

700

800

900

Stre

ngth

, M

pa

Nb+Ti, at.%

YS UTS

Effect Nb and Ti additions on the YS and UTS


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Contribution of the Nb and Ti additions to the YS


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Effect of Nb/ Ti additions on the grain size

10℃/s

1280℃, 600s

1200℃, 50%

600s

850~1150℃

30℃/s

Gleeble simulating process on the recrystallized grain growth


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(a) (b)

(c) (d)

(e) (f)

(g)

(h)

1150℃

1050℃

950℃

850℃

TPMn30Ti steel (a,c,e,g ) TPMn30Nb-2 steel (b,d,f,h )


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Variation of recystalizaion grain size of the Nb and Ti-alloyed TWMn30 steel with the temperatures.

Grain Refinement


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Microstructure(a) (b)

(c) (d)

OP image, before tension OP image, after tension

SEM, after tensionSEM, after tension

TPMn30NbTi-1 steel


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(a)

Nb

(b)

The Nb-Ti precipitates in TPMn30NbTi-2 steel


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析出相分布

0 5 10 15 200

20

40

60

TiNb NbNb

Cou

nts

Energy (KeV)

Nb

Ti

Cu

Cu

(c)

0 2 4 6 8 10 12 14 16 18 200

15

30

45

60

Nb NbNb

Cou

nts

Energy (KeV)

Nb

Cu

Cu

(c)

(a) (b)

TEM extraction replica of TPMn30Nb-2 steel showing morphology and distribution of Nb-precipitates

TEM extraction replica of TPMn30NbTi-2 steel showing the morphology of the Nb-Ti precipitates


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Heavily cold-rolled Nb-Ti microalloyed TWIP steel

The OP image of the three cold-rolled TWIP steels(a) TWMn30; (b)TPMn30NbTi-1;(c)TPMn30Nb-2


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(c) 500℃annealing, 84%；

(d) 600℃ annealing, 84%

Typical SEM image of the annealed Mn30NbTi-1 steel

(a) 50% reduction；(b) 84% reduction

(e) 700℃ annealing, 84%(f) 800℃ annealing, 84%


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TEM micrographs of the cold rolled and annealed samples annealed at (a) 500,550℃, (b) 600℃, (c) 650℃ and (d)700℃


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Tensile properties

Engineering stress-strain curves of the selected TWIP steels with different annealing temperatures.


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The effect of annealing temperature on a) the offset yield strength


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The effect of annealing temperatures on the ultimate tensile stress-total elongation balance with Nb,Ti alloyed and unalloyed steels


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Comparison of the mechanical properties in present workwith the reported results of ultra-low- carbon or carbon-freeand medium- and high-carbon TWIP steel.From: Liming Fu, Thesis,SJTU, 2014


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Discussion• Influence of Mn on Nb precipitating in high Mn TWIP steel

γγγγ ε ′′′ ++= MnMnCMnFeCFeMnC xxkkaa )(lnln 10

γγγγ ε ′′′ ++= MnMnNbMnFeNbFeMnNb xxkkaa )(lnln 10

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00

0.01

0.02

0.03

0.04

0.05

xMn=30

xMn=15

xMn=10

xMn=8

xMn=6

xMn=4xMn=2

xMn=1

xMn=2

x C ,

wt.%

xNb, wt.%

The calculated NbC solubility curves at 950℃ with different Mn content.


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Precipitation strengthening

0.1 0.2 0.3 0.4 0.5

200

300

400

500

600

700

ASR+SR-Annealed

ASR+SR-Anneal

ed

HR-Annealed

Y

ield

Str

engt

h,

MPa

dγ-1/2 , μm-1/2

σ0=153.78MPa

ky=1072.78MPa.μm1/2

TWMn30, Experimental Hall-Petch relation, Fitted

2/10Y

−+=Δ γσσ dky

0 10 20 30 40 50 60 70200

300

400

500

600

700

TWMn30Nb-2, Experimental Hall-Petch relation calculation

Yiel

d St

reng

th, M

pa

Grain Size, μm

Comparison of experimental YS with grain size with calculationfrom Hall-Petch relation for TPMn30Nb-2 steel.


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Effect of the precipitation on the deformation behavior

0.0 0.1 0.2 0.3 0.4 0.5 0.60

200

400

600

800

1: no Nb-Ti 2: 0.06Nb3: 0.04Nb+0.022Ti4: 0.5Nb5: 0.26Ti 6: 0.3Nb+0.2Ti

σ−σ 0

, M

Pa

True strain, ε−εoffset

123

4

5

6

(a)

0.0 0.1 0.2 0.3 0.4 0.5

-1000

0

1000

2000

3000

4000

Stra

in h

arde

ning

rate

, MPa

True strain, ε

no Nb-Ti 0.06Nb 0.04Nb+0.022Ti 0.26Ti 0.5Nb 0.3Nb+0.2Ti

(b)

Effect of Nb and Ti additions on the deformation behavior of hot-rolled low carbon TWIP steel. (a) the offset true stress-train curves and (b) the corresponding strain-hardening rate curves.


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UFG TWIP steelsHigh strength-ductility UFG low carbon TWIP

steels produced by heavily asymmetrically rolling (ASR)

grain size below 500nm


42

Method of severe plastic deformation (SPD) TWIP steels

ECAP HPT Rolling

Products

Relatively small dimensions

Products

Relatively small dimensions

Products

Large Sheet


43

SPD by Asymmetric rolling (ASR)

Asymmetric rolling (ASR)

Symmetric rolling (SR)


44


45

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

058.11

1ln3

2=⎟

⎠⎞

⎜⎝⎛− r

Equi

vale

nt S

train

Distance from high speed side (mm)

SR=1 (conventional rolling) SR=1.2 SR=1.5 SR=1.7 SR=2

• ASR : Different circumferential velocities of the two working rolls ( different diameters or rotation speeds); Greatly enhance the total strain or increase the equivalent strains.


46

Our previous works: SPD by Asymmetric rolling (ASR)

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Jiang and Shan et.al, Scripta Mater, 2009, 60 , 905

Pure Alless 1μmPure Fe

0.9 μmDing and Shan et.al, Mater. Sci. Eng. 2009, 509,76

Pure TiLess 100nm

Li and Shan et.al, Mater. Sci. Eng. 2012, 558,309


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NG/UFG TWIP steels

Matoso et.al.Scripta Materialia

2012,67:649.450HVHTPFe-24Mn-3Al-

2Si-1Ni-0.06C

Saha and Ueji et.al,Scripta Mater., 2013,68:81348849702400Cold rolling+

RecrylstallizationFe-31Mn-3Al-

3Si

Lee, et.al, Mater.Lett., 2012, 75:169; Koyama et.al, Mater. Design, 2013, 49:234.

441280840460cold rolling+Recrylstallization

Fe-17Mn-0.6C steel

Timokhina et.al, Mater. Sci. Eng.A, 2014, 593:163.2116241480260~

600ECAP+

Recovery0.61C-22.3Mn -0.19Si-0.14Ni

Referenceε%

UTSMPa

YSMPa

Grain sizenmFabricationTWIP

steels

No information shows the UFG high-Mn austenitic steel was produced by ASR method so far.


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Process and heat treatment

Rolling Laminar Cooling

1250℃、2h

Reheating

Start 1200℃End 880~900℃ Rate 20-30℃/s

400℃ air cooling

HR plate 20mm


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Materials: Fe-25Mn-2.8Al-2.4Si-0.08C (wt.%)Deformation: ASR +SR

Experimental

ASR+SR


50

(a)

(c) (d) (e)

Microstructures

Optical micrographs of the high-Mn steel.(a) hot-rolled , 950℃ annealing, (b) 98% ASRed-SRed; (c) 500℃;(d) 600℃;(e)700℃.

(b)


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TEM observation of the 98% ASRed+SRed steel (a) bright field image and (b) dark field image .

TEM Observation


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Annealed at 500℃

Annealed at 700℃Annealed at 600℃


53

ASR-SRed Recovery

Partially recrystallized

Fully recrystallized

406135836254Ave. sizenm

700600500400ASR+SRT, ℃

Grain size change with annealing temperatures


54

XRD patterns of the high-Mn steel by cold-ASR-SRedand annealing at different temperatures.


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/ YS(MPa)

UTS(MPa)

EL(%)

Hot-rolled, 900℃ 307 677 72.598% deformation 1582 1591 3.1

400 ℃ 1442 1473 7.2 500℃ 1216 1278 17.8600℃ 980 1126 32.6 700℃ 687 968 46.4

Tensile curves


56

Strain hardening rate


57

TEM observation in 600℃ annealed steelafter tensile

Deformation twins


58

Deformation micro-twins in 700℃ annealed steelsafter tensile deformation

Micro-bands

Deformation twins


59

UFG TWIP steel exhibit good balance of strength and ductility and high strain hardening.1) dislocations recovery during annealing and

partial recrystalliztion of the nanostructures. 2) lower stacking faults energy of the steel;plasticity mode: combination of dislocation gliding, deformation twinning, active stacking faults and glide or shear banding


60

SummaryIt is feasible to produce the Nb-Ti alloyed high strength-ductility high Mn TWIP steel. The reasonable amount of Nb and Ti addition should exceed 0.1wt.% level; Combination of the grain refinement strengthening and precipitate strengthening are the major strengthening mechanism of the Nb and/or Ti alloyed low carbon TWIP steels.

The nano-grained high-Mn austenitic steel with grain size of 60nm is successfully produced through heavy cold-processing by combination of the asymmetric rolling (ASR) and symmetric rolling (SR);

The UFG high-Mn austenitic steels exhibit relatively higher strain hardening and good balance of high strength and ductility. This is due to the relatively lower stacking fault energy of the steel and the partial recrystallization of the nanostructures.


61

Thank you !

sjtu-fuliming and shanaidang-high strength and high ......carbon-free or ultra-low carbon femnc...

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