processo q&p (quenching and partitioning) estudo de caso completo

1 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012

Processo Q&P (Quenching and Partitioning)

Estudo de caso completo

Fernando Rizzo

2 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012

Projeto de Cooperação Internacional

J.G. Speer, D.K. Matlock , A. Streicher

– Colorado School of Mines, USA

F. Rizzo, A. R. Aguiar

– PUC, Rio de Janeiro, Brazil

D.V. Edmonds, Kejian He

– University of Leeds, UK

NSF-CNPq (CIAM) , NSF-EPSRC

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Quenching and Partitioning:

- Background

- Fundamental Issues

- Recent Results

•J.G. Speer, D.K. Matlock, B.C. De Cooman and J.G. Schroth, Acta Mater., 51 (2003) 2611-2622. •J.G. Speer, A.M. Streicher, D.K. Matlock, F.C. Rizzo and G. Krauss, Austenite Formation and Decomposition, ed. E.B. Damm and M. Merwin, TMS/ISS, Warrendale, PA, USA, 2003, pp. 505-522. •John G. Speer, David V. Edmonds, Fernando C. Rizzo, David K. Matlock, Current Opinion in Solid-State and Materials Science, 8 (2004) 219-237

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TRIP SteelsMF

Ac3

Time

Te

mp

era

ture

MS

Ac1

B +

C

iC

f

“Conventional”Processing ofSteels with

CC and Isothermal

Transformations

Ac3

Te

mp

era

ture

C = Ci

MF

Time

MS

B +

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The “Q&P” ProcessQuenching and Partitioning

TE

MP

ER

AT

UR

E

TIME

PT,t

AT

QT

Quenchedand Partitioned

Ms

Mf

Provisional US Patent Application: September, 2003

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The “Q&P” Process

Step 1. Austenitize or Intercritically Anneal

- more austenite- lower C

- higher Ms

- less austenite- higher C

- lower Ms

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Step 2. Cool (quench?) below Ms

- Ms -TQ controls martensite formation

- intercritical annealing has more stable austeniteand higher carbon martensite

Austenitize + Quench Intercritical Anneal + Quench

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Step 3. Diffuse Carbon from Supersaturated Martensite

- Phase compositions change

- Phase boundaries stationary

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Q&P Process Schematic

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New Processing Concept (Sheet, Bar,…etc)

Use carbon partitioning intentionally…from partially transformed martensite tountransformed austenite.

Usually precluded because carbide precipitation occurs during tempering of martensite.

Result: carbon-enriched austenite

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Thermodynamics of Carbon

Partitioning

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Important Questions

How much can we enrich the austenite?

That is…what are the “equilibrium” martensite and austenite compositions?

Or…when does partitioning stop?

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“True” Metastable Equilibrium

Fe3C

% Carbon

Tem

per

atu

re

Fe3C

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“True” Metastable Equilibrium

Fe C

G

xEQ

FeFe

CC

xEQ

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“True” Metastable Equilibrium CANNOT Apply!!

Fe3C

% Carbon

Tem

per

atu

re

Fe3C

XalloyX X

- The equilibrium phase fractions are fixed by the lever rule- The actual phase fractions were fixed by cooling below Ms!

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A New Equilibrium Condition was Hypothesized

“Constrained Carbon Equilibrium” (CCE)

- Iron atoms are completely immobile (the phase boundaries are stationary).- Carbon atoms are completely mobile.- Carbon diffuses until its chemical potential (activity) is equal in ferrite and austenite.- Assume…competing reactions are precluded by

Si/Al

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Properties of “Constrained Carbon Equilibrium”

- Not a unique condition at any temperature!- Depends on initial phase fractions/compositions

Fe C

G

II

C

II

C

xI

CPE

x

II

CPE

xI

CPE

x

II

CPE

I

C

I

C

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- Austenite may be more enriched or less enriched than ortho- or para- equilibrium

Fe C

G

II

C

II

C

xI

CPE

x

II

CPE

xI

CPE

x

II

CPE

I

C

I

C

T0

A3

Properties of “Constrained Carbon Equilibrium”

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Properties of “Constrained Carbon Equilibrium”

RT

TT

CC

CCEC

CCECCEe

X)4.120105,169(8.43789,76

XX

XXX alloyCCCCECCCE CCECCE

ff

)X1()X1( iCCE CiCCCE ff

Carbon Constrained Equilibrium:

Mass balance:

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Key Characteristics of CCE

- Almost all of the carbon should partition to austenite- Enrichment levels are potentially very high

(Fe-0.5C)

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Example CCE Calculations - 1.0%C Initial Austenite

M artensite C ontent25%50%75%90%

1x10 - 4 1x10 - 2 1x10 0

Carbon in Ferrite (wt. % )

200

300

400

500

600

Tem

pera

ture

(oC

)

0 2 4 6 8 10

Carbon in Austenite (wt. % )

200

300

400

500

600

1% Carbon

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We have all the pieces to predict microstructure…

Example

Steel Composition: C=0.15 Mn=1.0 Si=1.5

Ms(oC)=539-423(%C)-30.4(%Mn)-12.1(%Cr)-17.7(%Ni)-7.5(%Mo)

Ms=445oC (Steel)

Intercritical Annealing T=810oC

f~ 22% C~ 0.68 wt. % Ms~222oC

f~ 78%

TIA=810oC

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Quench T = 150 oC

Fraction of Martensite(Koistinen and Marburger)

Final Microstructure

f~ 10%

fM~ 12%

f~ 78%

Phase Compositions After 450oC Partitioning

C~ 1.5%

CM~ .0019%

Tq=150oC

Tp=450oC

fkm 1 exp 1.1 102 Ms Tq

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Q&P Process Design Methodology

Experimental High-Al TRIP Sheet Steel C Al Mn Si P N Cr S

0.19 1.96 1.46 0.022 0.01 0.0018 0.08 0.002

ASSUME:

- Complete partitioning of carbon to austenite

- No competing reactions (carbide formation)

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Calculations for Experimental Al-Steel (at QT)

0 100 200 300 400

Q uench Tem perature

0

0.1

0.2

0.3

0.4

0.5

Pha

se F

ract

ion

M in itia l quench in itia l quench

AlSiMnCCo 305.74.30423539)(M s

IA=0.5

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Martensite Formation During Final Quench

0 100 200 300 400

Q uench Tem perature

0

0.1

0.2

0.3

0.4

0.5

Pha

se F

ract

ion

M in itia l quench

M fina l quench

in itia l quench

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Calculated Final Austenite Fraction in High-Al Steel

final

0 100 200 300 400

Q uench Tem perature

0

0.1

0.2

0.3

0.4

0.5

Pha

se F

ract

ion

M in itia l quench

M fina l quench

in itia l quench

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Effect of Intercritical Annealing Step

0 100 200 300 400 500

Q uench Tem perature - oC

0

0.05

0.1

0.15

0.2

0.25

Fin

al A

uste

nite

Fra

ctio

n

IC = 0%

IC = 50%

IC = 75%

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Effect of Manganese Content

0 100 200 300 400 500

Q uench Tem perature - oC

0

0.05

0.1

0.15

0.2

0.25

Fin

al A

uste

nite

Fra

ctio

n

0.5% M n1.46% M n

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Effect of Carbon Content

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Example of DICTRA SimulationSolid-Solid Phase Transformations in Inorganic Materials 2005

Edited by J. HoweTMS (The Minerals, Metals & Materials Society), 2005

CARBON ENRICHMENT OF AUSTENITE AND CARBIDE PRECIPITATION DURING THE QUENCHING AND

PARTITIONING (Q&P) PROCESS

F.C. Rizzo 1, D.V. Edmonds2, K. He 2, J.G. Speer3, D.K. Matlock3and A. Clarke 3

1Department of Materials Science and Metallurgy; Pontifícia Universidade Católica-Rio de Janeiro; RJ 22453-900, Brazil

2School of Process, Environmental and Materials Engineering; University of Leeds; Leeds LS2 9JT, United Kingdom

3Advanced Steel Processing and Products Research Center; Colorado School of Mines; Golden, CO 80401, USA

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Simulation Conditions

Steel composition:

•0.19C-1.59Mn-1.63Si wt%

Heat treatment:

•Fully austenitized at 900oC, quenched to 293oC to produce 68% martensite and partitioned at 400oC. The thickness of the ferrite and austenite plates used in the simulation were 0.30 and 0.14 microns, respectively (obtained by TEM).

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Carbon Concentration Profiles for ferrite and austenite

0

400

800

1200

1600

2000

2400

2800

3200

AC

R(C

)

0 5 10 1510-8

DISTANCE

DICTRA (2005-07-01:11.46.47) :Ferrite C activityTIME = 1E-04,.001,.01,.1,1,10

CELL #2

2005-07-01 11:46:47.76 output by user Fernando from RIZZO1

0

5

10

15

20

25

30

10-3W

(C)

0 1 2 3 4 5 6 710-8

DISTANCE

DICTRA (2005-07-01:11.59.50) :Austenite C ProfileTIME = 1E-04,.001,.01,.1,1,10

CELL #1

2005-07-01 11:59:50.50 output by user Fernando from RIZZO1

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Carbon concentration profiles in and during partitioning under CCE at 400C, for a 0.19C-1.59Mn-1.63Si steel

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Average carbon concentration as a function of time for

(0.30m) and (0.14m) plates during partitioning at 400oC

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Variation of (a) carbon flux and (b) carbon activity at the interface during partitioning. Time plotted in a log scale

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Carbon concentration (wt%) at and interfaces as a function of time during partitioning at 400 oC

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Carbon flux and concentration in the center of (a) ferrite and (b) austenite plates as a function of time

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CONCLUSIONS

1. For the scale of microstructure investigated, carbon depletion from the ferrite during partitioning at 400C occurs quite rapidly, around 10-1 seconds, while the austenite takes much longer, around 10 seconds, to achieve a uniform concentration.

2. Due to its rapid depletion, the carbon concentration in the center of the ferrite plate starts to decrease after 10-3 seconds. After this time the driving force for carbide precipitation is gradually reduced.

3. Carbon enrichment of the austenite will promote, initially, a substantial increase in the carbon concentration at the interface and a progressive stabilization of the plate, advancing from the interface to the center. Full stabilization is achieved when the composition of the central region reaches a carbon concentration corresponding to room temperature Ms.

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Some Q&P Experimental

Results

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TEM micrographs of the Q&P microstructure produced in a 0.19%C-1.59%Mn-1.63%Si TRIP steel composition

Quenching to 260°C and partitioning at 400°C for 100 s: (a) bright-field image and (b) dark-field image using a (200) austenite reflection.

(a) (b)

42 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012

Total elongation vs. ultimate tensile strength for TRIP, Dual phase (DP), martensitic (M), and Q&P sheet steel products

400 800 1200 1600U ltim ate Tensile S tre ng th , M Pa

0

10

20

30

40T

ota

l Elo

ng

atio

n,

%TRIP (ULSAB)A

TRIP (GM )A

Conventional TRIPB

1-Step Q&PB

2-Step Q&PB

M (ULSAB)A

M (GM)A

M (Ispat-In land)C

DP (ULSAB)A

DP (GM )A

DP (Ispat-Inland)CT R IP

D P

M

Q & P

Note A: 80 mm gauge lengthNote B: 25.4 mm gauge length Note C: Gauge length unspecified