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Kun Xu - Continuous Casting Consortium - University of Illinois at ...

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ANNUAL REPORT 2009<br />

UIUC, August 5, 2009<br />

Modeling he<strong>at</strong> transfer, precipit<strong>at</strong>e<br />

form<strong>at</strong>ion, and grain growth during<br />

secondary spray cooling<br />

<strong>Kun</strong> <strong>Xu</strong><br />

(Ph.D. Student)<br />

Department <strong>of</strong> Mechanical Science and Engineering<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 1<br />

Objectives<br />

Cracks form during cooling due to:<br />

- tensile stress<br />

- low ductility<br />

To design temper<strong>at</strong>ure histories to<br />

avoid crack form<strong>at</strong>ion, need accur<strong>at</strong>e<br />

predictive tools<br />

Models can now accur<strong>at</strong>ely predict<br />

temper<strong>at</strong>ure and stress histories<br />

Need tools to predict metallurgical<br />

behavior to estim<strong>at</strong>e hot ductility, such<br />

as grain size, precipit<strong>at</strong>e form<strong>at</strong>ion<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 2


Specific applic<strong>at</strong>ion:<br />

transverse surface cracks<br />

casting direction<br />

transverse cracks<br />

Widespread crazing and fine<br />

transverse cracks <strong>at</strong> oscill<strong>at</strong>ion<br />

marks on the as-cast surface <strong>of</strong><br />

a line pipe steel slab (top side)<br />

E. S. Szekeres,Proceedings <strong>of</strong> the 6th Intern<strong>at</strong>ional Conference on Clean Steel, Hungary, 2002.<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 3<br />

mold<br />

gap<br />

Effect <strong>of</strong> depressions on grain size and cracks<br />

Steel shell<br />

Molten steel pool<br />

Larger grains and cracks bene<strong>at</strong>h depressions &<br />

oscill<strong>at</strong>ion marks<br />

Due to:<br />

More he<strong>at</strong> flow resistance across gap<br />

Higher shell temper<strong>at</strong>ure<br />

Faster grain growth<br />

Causes:<br />

Tensile strain concentr<strong>at</strong>ion area<br />

Make hot grains actually align (Secondary<br />

recrystalliz<strong>at</strong>ion) to form “blown grains”<br />

Embrittled with the large numbers <strong>of</strong> fine<br />

precipit<strong>at</strong>es <strong>at</strong> weak grain boundaries —<br />

transverse cracks open up along boundaries<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 4


Mechanism <strong>of</strong> surface crack form<strong>at</strong>ion<br />

with precipit<strong>at</strong>e embrittlement<br />

STAGE I - Normal solidific<strong>at</strong>ion on mold<br />

wall. Surface grains are small but highly<br />

oriented.<br />

γ<br />

STAGE III - Nitride precipit<strong>at</strong>es<br />

begin to form along the blown<br />

grain boundaries. Microcracks<br />

initi<strong>at</strong>e <strong>at</strong> weak boundaries.<br />

γ<br />

STAGE IV - Ferrite transform<strong>at</strong>ion<br />

begins and new precipit<strong>at</strong>es form<br />

<strong>at</strong> boundaries. Existing microcracks<br />

grow & new ones form.<br />

STAGE II - Surface grains “blow” locally due to high<br />

temper<strong>at</strong>ure (>1350 o C) and strain, especially <strong>at</strong> the<br />

base <strong>of</strong> deeper oscill<strong>at</strong>ion marks.<br />

γ γ<br />

γ γ<br />

E. S. Szekeres,Proceedings <strong>of</strong> the 6th Intern<strong>at</strong>ional Conference on Clean Steel, Hungary, 2002.<br />

STAGE V - At the straightener,<br />

microcracks propag<strong>at</strong>e and<br />

become larger cracks, primarily<br />

on top surface <strong>of</strong> the strand.<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 5<br />

Cracks open up <strong>at</strong> weak grain boundaries<br />

<strong>Casting</strong> direction<br />

~1mm<br />

E. S. Szekeres,Proceedings <strong>of</strong> the 6th Intern<strong>at</strong>ional Conference on Clean Steel, Hungary, 2002.<br />

Crazing around a transverse<br />

crack <strong>at</strong> base <strong>of</strong> an oscill<strong>at</strong>ion<br />

mark on the as-cast top surface<br />

<strong>of</strong> a 0.2%C steel slab<br />

Note larger grain size (typical<br />

size ~1mm) <strong>at</strong> the base <strong>of</strong><br />

oscill<strong>at</strong>ion mark<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 6


Hot ductility varies with temper<strong>at</strong>ure and<br />

grade due to precipit<strong>at</strong>e form<strong>at</strong>ion<br />

B.G. Thomas et al, ISS Transactions, 1986, 7-20.<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 7<br />

Micrographs <strong>of</strong> precipit<strong>at</strong>es in steels<br />

Complex precipit<strong>at</strong>es with TiN core<br />

TiNb(C) precipit<strong>at</strong>es <strong>at</strong> grain boundaries<br />

V. Ludlow et al, Precipit<strong>at</strong>ion <strong>of</strong> nitrides and carbides during solidific<strong>at</strong>ion and cooling, unpublished work.<br />

M. Charleux et al, Metallurgical and M<strong>at</strong>erials Transactions A, 2001, vol.32A, 1635-1647.<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 8


Project Overview<br />

He<strong>at</strong> Transfer<br />

Program: CON1D<br />

Ductility Prediction<br />

Phase fraction &<br />

temper<strong>at</strong>ure history<br />

Grain size<br />

Equilibrium<br />

Precipit<strong>at</strong>ion Model<br />

Equilibrium<br />

concentr<strong>at</strong>ion<br />

Kinetic Model<br />

(precipit<strong>at</strong>es)<br />

Grain Growth<br />

Model<br />

Volume fraction &<br />

size distribution<br />

Final goal Predict ductility and combine with other models to<br />

design casting practices to prevent cracks<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 9<br />

Equilibrium precipit<strong>at</strong>ion model<br />

Reaction xM + yX <br />

M M: metal, X: C, N, S or O<br />

x X y<br />

x y<br />

Solubility product: KM[ ] [ ] /<br />

x X = a<br />

y M ⋅a<br />

X a<br />

α (MxXy) : activity <strong>of</strong> precipit<strong>at</strong>es<br />

( MxXy) [%i]: dissolved concentr<strong>at</strong>ion <strong>of</strong><br />

i j<br />

ai = fi[% i]<br />

where log fi = ei[% i] +∑ei<br />

[% j]<br />

element i<br />

Mutual Solubility: If two precipit<strong>at</strong>es have the same crystal structures and similar<br />

l<strong>at</strong>tice parameters (


Groups <strong>of</strong> precipit<strong>at</strong>es<br />

The current model has 18 precipit<strong>at</strong>es, 13 alloying elements and 4 groups <strong>of</strong> mutually<br />

soluble precipit<strong>at</strong>es<br />

• TiN, TiC, NbN, NbC 0.87 , VN, V 4 C 3 (f.c.c structure) l<strong>at</strong>tice mism<strong>at</strong>ch ~7.92%<br />

– Valid<strong>at</strong>ed by many experimental observ<strong>at</strong>ions<br />

• Al 2 O 3 , Ti 2 O 3 (hexagonal structure) l<strong>at</strong>tice mism<strong>at</strong>ch ~8.06%<br />

• MgO, MnO (f.c.c structure) l<strong>at</strong>tice mism<strong>at</strong>ch ~5.54%<br />

• MnS, MgS (f.c.c structure) l<strong>at</strong>tice mism<strong>at</strong>ch ~0.38%<br />

• SiO 2 (trigonal structure)<br />

• TiS (trigonal structure)<br />

• Ti 4 C 2 S 2 (hexagonal structure)<br />

• AlN (hexagonal structure)<br />

• BN (hexagonal structure)<br />

• Cr 2 N (hexagonal structure)<br />

• A complete system <strong>of</strong> 35 nonlinear equ<strong>at</strong>ions is solved with Newton-Raphson method<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 11<br />

Valid<strong>at</strong>ion for mutually exclusive precipit<strong>at</strong>es<br />

NbN after adding Al<br />

NbN after adding B<br />

AlN<br />

NbN<br />

BN<br />

NbN NbN before before adding adding Al B<br />

Three hypothetical steels<br />

1. 0.02%Nb and N<br />

2. 0.02%Nb and N, 0.02% Al<br />

3. 0.02%Nb and N, 0.01% B<br />

Second order equ<strong>at</strong>ion and<br />

analytical solution exists<br />

Existing precipit<strong>at</strong>e AlN or<br />

BN delays the precipit<strong>at</strong>e<br />

NbN to form <strong>at</strong> the lower<br />

temper<strong>at</strong>ure and decreases the<br />

equilibrium amount <strong>of</strong> NbN <strong>at</strong><br />

the same temper<strong>at</strong>ure<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 12


Pseudo-ternary diagram for Al-Nb-N system<br />

Fix the sum <strong>of</strong> weight percent concentr<strong>at</strong>ion (Al+Nb+N) as 0.05%<br />

0.75<br />

0.50<br />

0.25<br />

Nb<br />

0.00<br />

1.00<br />

none<br />

0.75<br />

0.50<br />

0.25<br />

Nb<br />

0.00<br />

1.00<br />

1.00<br />

1.00<br />

AlN<br />

0.00<br />

0.00<br />

Al 0.00 0.25 0.50 0.75 1.00 N<br />

Al 0.00 0.25 0.50 0.75 1.00 N<br />

0.75<br />

0.50<br />

0.25<br />

Nb<br />

0.00<br />

1.00<br />

0.75<br />

none 0.50<br />

NbN<br />

AlN<br />

0.25<br />

1.00<br />

0.00<br />

Al 0.00 0.25 0.50 0.75 1.00 N<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 13<br />

Ti 0.26 Nb 0.24 N 0.50<br />

T=1350 o C<br />

T=1150 o C<br />

0.75<br />

0.75<br />

0.50<br />

0.50<br />

0.25<br />

none<br />

0.75<br />

T=1300 o C<br />

0.25<br />

Nb<br />

0.00<br />

1.00<br />

none<br />

AlN<br />

0.75<br />

NbN<br />

NbN+AlN<br />

T=1125 o C<br />

0.50<br />

0.50<br />

0.25<br />

0.25<br />

1.00<br />

0.00<br />

Al 0.00 0.25 0.50 0.75 1.00 N<br />

Test problem for mutually solubility<br />

(Ti-Nb-N system)<br />

Ti 0.38 Nb 0.12 N 0.50<br />

Ti 0.48 Nb 0.02 N 0.50<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 14<br />

none<br />

Three hypothetical steels<br />

1. 0.01%Nb and N<br />

2. 0.01%Nb and N, 0.005% Ti<br />

At least fourth order equ<strong>at</strong>ion<br />

and no analytical solution<br />

Mutual solubility decreases<br />

the activity <strong>of</strong> NbN, which<br />

allows more NbN to form<br />

Without consider<strong>at</strong>ion <strong>of</strong><br />

mutual solubility, NbN<br />

precipit<strong>at</strong>es would decrease<br />

with Ti additions


Molar fraction <strong>of</strong> precipit<strong>at</strong>es<br />

1E-3<br />

1E-4<br />

1E-5<br />

Valid<strong>at</strong>ion with stability <strong>of</strong> TiS and Ti 4 C 2 S 2<br />

Ti 2 O 3<br />

Ti 4 C 2 S 2<br />

TiC<br />

TiN<br />

Al 2 O 3<br />

TiS<br />

1E-6<br />

1000 1050 1100 1150 1200 1250 1300 1350<br />

Temper<strong>at</strong>ure (centigrade)<br />

1E-6<br />

1000 1050 1100 1150 1200 1250 1300 1350<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 15<br />

Steel<br />

Nb1<br />

Nb8<br />

Steel<br />

B<br />

C 0.0033 0.0040 0.081 0.0115 0.037 0.050 0.0022 0.0036<br />

Recorded types <strong>of</strong> observed precipit<strong>at</strong>es in experiments (m<strong>at</strong>ches prediction)<br />

Measurements: X. Yang et al, ISIJ Intern<strong>at</strong>ional, 1996, vol.36, 1286-1294.<br />

Steel<br />

B<br />

C<br />

C<br />

0.085<br />

0.081<br />

C<br />

0.0036<br />

1300 o C<br />

TiN<br />

TiN, TiS<br />

Si<br />

0.0050<br />

Mn<br />

0.081<br />

1250 o C<br />

TiN, TiS *<br />

TiN, TiS<br />

S<br />

0.0028<br />

1200 o C<br />

TiN, TiS<br />

TiN, TiS<br />

Molar fraction <strong>of</strong> precipit<strong>at</strong>es<br />

1E-3<br />

1E-4<br />

1E-5<br />

Al<br />

0.045<br />

1150 o C<br />

TiN, TiS<br />

Ti 4 C 2 S 2<br />

0.095<br />

TiC<br />

Ti<br />

TiN, Ti 4C 2S 2<br />

TiN<br />

Al 2 O 3<br />

TiS<br />

Temper<strong>at</strong>ure (centigrade)<br />

Steel B Steel C<br />

N<br />

0.0019<br />

1100 o C<br />

TiN, Ti 4C 2S 2<br />

TiN, TiS * , Ti 4C 2S 2<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 16<br />

O<br />

0.0028<br />

Valid<strong>at</strong>ion with equilibrium Nb(C,N) precipit<strong>at</strong>ion<br />

experiments (microalloy steel)<br />

Si<br />

0.28<br />

0.31<br />

Mn<br />

1.46<br />

1.44<br />

P<br />

0.008<br />

0.01<br />

0.002<br />

0.002<br />

Measurements: S. Zajac and B. Jansson, Metallurgical and M<strong>at</strong>erials Transactions B, 1998, vol.29B, 163-176.<br />

Solution tre<strong>at</strong>ed (1300 o C) 1 hour<br />

Followed by w<strong>at</strong>er quenching<br />

Then aged <strong>at</strong> different isothermal<br />

temper<strong>at</strong>ure for 24-48 hours<br />

Cool to ambient quickly<br />

Measure Nb amount in Nb(C,N) by<br />

the inductively coupled plasma<br />

(ICP) emission method<br />

Nb precipit<strong>at</strong>ed as Nb(C,N) (wt%)<br />

0.04<br />

0.03<br />

0.02<br />

0.01<br />

S<br />

Nb<br />

0.013<br />

0.033<br />

Al<br />

0.013<br />

0.017<br />

N<br />

0.005<br />

0.004<br />

V<br />

0.014<br />

0.011<br />

Prediction <strong>of</strong> Nb1<br />

Measurement <strong>of</strong> Nb1<br />

Prediction <strong>of</strong> Nb8<br />

Measurement <strong>of</strong> Nb8<br />

Ti<br />

0.003<br />

0.003<br />

0.00<br />

850 900 950 1000 1050 1100 1150 1200 1250<br />

Temper<strong>at</strong>ure (centigrade)


Temper<strong>at</strong>ure and steel phase fraction model<br />

CON1D Program: Solve the transient he<strong>at</strong> conduction in the mold and spray regions <strong>of</strong><br />

continuous steel slab casters using finite difference method<br />

The program is calibr<strong>at</strong>ed to predict the temper<strong>at</strong>ure <strong>of</strong> the real casters.<br />

Example: thin slab casting <strong>of</strong> low-carbon 1006N2 steel<br />

Steel<br />

1006N2<br />

Phase fraction<br />

1.0<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

C<br />

0.0472<br />

N<br />

0.0083<br />

S<br />

0.0013<br />

Liquid<br />

Delta ferrite<br />

Austenite<br />

Alpha ferrite<br />

Mn<br />

0.9737<br />

0.0<br />

650 800 950 1100 1250 1400 1550<br />

Temper<strong>at</strong>ure ( o C)<br />

Si<br />

0.2006<br />

0.0084<br />

0.0123<br />

0.0027<br />

0.0223<br />

0.0001<br />

0.0354<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 17<br />

Ti<br />

Nb<br />

V<br />

Slab size: 90mm thick, 1396mm wide<br />

Mold length: 950mm<br />

Pouring Temper<strong>at</strong>ure: 1553 o C<br />

<strong>Casting</strong> speeds: 2.8m/min and 4.6m/min<br />

Al<br />

Liquidus temper<strong>at</strong>ure: 1524.4 o C<br />

Solidus temper<strong>at</strong>ure: 1500.7 o C<br />

Equilibrium Precipit<strong>at</strong>ion in continuous casting <strong>of</strong><br />

1006N2 Steel<br />

Mold length 0.95m, start <strong>of</strong> first spray zone 0.85m, end <strong>of</strong> last spray zone 11.25m<br />

Only precipit<strong>at</strong>es predicted to form are: (Ti,Nb,V)(C,N), MnS and AlN<br />

7.5m: Ti 0.326 Nb 0.173 V 0.006 N 0.424 C 0.071<br />

0.85m: Ti 0.333 Nb 0.166 V 0.006 N 0.429 C 0.066<br />

V=2.8m/min<br />

7.5m: Ti 0.361 Nb 0.138 V 0.005 N 0.446 C 0.050<br />

15m: Ti 0.338 Nb 0.161 V 0.006 N 0.433 C 0.062 0.85m: Ti0.419 Nb 0.081 V 0.002 N 0.463 C 0.035<br />

V=4.6m/min<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 18<br />

B<br />

15m: Ti 0.428 Nb 0.072 V 0.002 N 0.466 C 0.032<br />

Higher casting speed causes higher temper<strong>at</strong>ure and less precipit<strong>at</strong>es (same spray flows)<br />

TiN is most stable <strong>at</strong> high temper<strong>at</strong>ure because <strong>of</strong> its low solubility limit. For the higher<br />

casting speed, AlN does not form in the mold.<br />

Cr


Particle Collision<br />

Particle Diffusion<br />

Kinetic model for precipit<strong>at</strong>e growth<br />

dni<br />

1<br />

= (1 + ) Φ nn − n (1 + ) Φ n ( i≥2)<br />

dt 2<br />

dn<br />

dt<br />

i<br />

i−1<br />

∑ δki , −kki , −kki−k nM<br />

i∑ δik<br />

, ik , k<br />

k= 1 k=<br />

1<br />

Gener<strong>at</strong>ion <strong>of</strong> size i particle by<br />

collision <strong>of</strong> smaller particles k and i-k,<br />

only count once for reacting particles<br />

Loss <strong>of</strong> size i particle by<br />

collision with any particle j,<br />

double loss if both particle i<br />

=− βinn 1 i + βi−1nn 1 i− 1 − αiAn i i + αi+<br />

1Ai+ 1ni+ 1 ( i≥2)<br />

Particle<br />

Diffusion<br />

(i→i+1)<br />

Particle<br />

Diffusion<br />

(i-1→i)<br />

Particle<br />

Dissolution<br />

(i→i-1)<br />

Particle<br />

Dissolution<br />

(i+1→i)<br />

Collision happens only in liquid, and diffusion happens in both liquid and solid<br />

ni : Number density <strong>of</strong> size i particle (m-3 )<br />

Фij : Collision coefficient for collision between size i and size j particle (m3 /s)<br />

βi : Diffusion r<strong>at</strong>e constant <strong>of</strong> size i particle (m3 /s)<br />

αi : Dissolution r<strong>at</strong>e per unit area <strong>of</strong> size i particle (m-2s-1 i 4 1 i<br />

)<br />

Dr β π =<br />

α = βn exp( 2 σV<br />

/ RTr) / A<br />

i i 1, eq m i i<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 19<br />

Introduction <strong>of</strong> Particle Size Grouping (PSG) model<br />

The model always simul<strong>at</strong>es from nuclei (~ 0.1nm) up to large coarsened particles<br />

(~100μm): particles could contain 1~10 18 molecules<br />

Serious comput<strong>at</strong>ion and memory storage issues arise with such a large size range<br />

Solve with PSG method: Use N G groups (


Size group and particle radius for constant R V<br />

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Valid<strong>at</strong>ion <strong>of</strong> test problem for collision<br />

1/ 2 3<br />

Turbulent collision Φ = 1.3( ε / ν)<br />

( r + r ) with only single molecules initially<br />

Dimensionless form<br />

n 0 : Initial number density <strong>of</strong> single molecules; ε, ν, r 1 are input parameters<br />

Initial condition (t*=0 ): n 1 * =1 and ni *=0 for i≥2 (same for N i in PSG model))<br />

Boundary condition: n M *=0, or N G *=0 <strong>at</strong> all t *<br />

Exact solution: n M =12000; PSG model: N G =16 (R V =2) or N G =11 (R V =3)<br />

Dimensionless numder density<br />

1<br />

0.1<br />

0.01<br />

1E-3<br />

1E-4<br />

1E-5<br />

N 1<br />

N 2<br />

N 3<br />

ij i j<br />

n = n / n<br />

*<br />

i i<br />

1E-6<br />

1E-3 0.01 0.1 1<br />

N 4<br />

N 5<br />

Dimensionless time<br />

0<br />

t = 1.3( ε / ν)<br />

r n t<br />

R V =2.0<br />

N6 N7 N8 N9 N10 * 1/2 3<br />

1 0<br />

N11 N12 Dimensionless numder density<br />

1E-6<br />

1E-3 0.01 0.1 1<br />

Dimensionless time<br />

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1<br />

0.1<br />

0.01<br />

1E-3<br />

1E-4<br />

1E-5<br />

N 1 RV =3.0<br />

N 2<br />

N 3<br />

N 4<br />

N 5<br />

N 6<br />

N7 N8 N 9


L<br />

n j<br />

Develop new PSG method for diffusion<br />

dN V V<br />

ceil( V ) floor( V )<br />

dt V V V V<br />

U<br />

n j<br />

j<br />

= 1 U<br />

βjN1( Nj −nj ) − 1<br />

L<br />

αjAj( Nj − nj) +<br />

j− 1, j U U<br />

βj−1Nn 1 j− 1+ j, j+<br />

1 L L L<br />

αj+<br />

1Aj+ 1nj+ 1<br />

j j j j<br />

Diffusion<br />

inside group j<br />

Dissolution<br />

inside group j<br />

Diffusion<br />

group j-1→j<br />

floor( V ) ceil( V )<br />

− Nn − An ( j≥2)<br />

V V<br />

j, j+ 1 U<br />

βj 1<br />

U<br />

j<br />

j−1, j L<br />

αj<br />

L<br />

j<br />

L<br />

j<br />

j j<br />

Diffusion<br />

group j→j+1<br />

Dissolution<br />

group j→j-1<br />

Dissolution<br />

group j+1→j<br />

: Number density <strong>of</strong> particles in group j, which jumps into group j-1 by dissolution after<br />

losing only one single molecule<br />

: Number density <strong>of</strong> particles in group j, which jumps into group j+1 by diffusion after<br />

getting only one single molecule<br />

Estim<strong>at</strong>ed from geometric progression <strong>of</strong> the number densities for two neighboring size groups<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 23<br />

Dimensionless number density<br />

10<br />

1<br />

0.1<br />

0.01<br />

1E-3<br />

1E-4<br />

1E-5<br />

1E-6<br />

Valid<strong>at</strong>ion <strong>of</strong> test problem for diffusion<br />

Dimensionless form<br />

*<br />

ni = ni / n1,<br />

eq<br />

*<br />

t = 4π Drn 11 1, eqt<br />

Molecules produced by an isothermal first order reaction<br />

nM<br />

* * * * *<br />

n ( t ) = n ( t )/ n = i⋅ n = 9[1−exp( −0.1<br />

t )]<br />

s s 1, eq i<br />

i=<br />

1<br />

Initial condition (t*=0 ): n i *=0 for i≥1 (same for N i in PSG model)<br />

Boundary condition: n M *=0, or N G *=0 <strong>at</strong> all t*<br />

Exact solution: n M =16000; PSG model: N G =17 (R V =2) or N G =12 (R V =3)<br />

R V =2.0<br />

N 1<br />

N 2<br />

N 3<br />

N4 N5 N6 N7 N8 N9 ∑<br />

N 10<br />

N 11<br />

N 12<br />

N 13<br />

1 10 100 1000<br />

N 14<br />

0<br />

0.01 0.1 1 10 100<br />

Dimensionless time<br />

Dimensionless time<br />

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Dimensionless number density<br />

10<br />

1<br />

0.1<br />

0.01<br />

1E-3<br />

1E-4<br />

1E-5<br />

1E-6<br />

R V =3.0<br />

N 1<br />

N 2<br />

N 3<br />

N4 N5 N6 N7<br />

1 10 100 1000<br />

*<br />

N s<br />

10<br />

8<br />

6<br />

4<br />

2<br />

N 8<br />

Dimensionless time<br />

N 9


Storage<br />

Comput<strong>at</strong>ional<br />

time<br />

Compare comput<strong>at</strong>ional cost for test problems<br />

Exact<br />

n M =12000<br />

~225 hours<br />

Collision<br />

PSG (R v =2)<br />

N G =16<br />

~0.8s<br />

PSG (R v =3)<br />

N G =11<br />

~7420s<br />

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~0.4s<br />

Exact<br />

n M =16000<br />

Diffusion<br />

PSG (R v =2)<br />

N G =17<br />

~358s<br />

PSG (R v =3)<br />

For constant RV , the number <strong>of</strong> size groups in PSG method must s<strong>at</strong>isfy<br />

V<br />

NG<br />

−1<br />

= R > n → N = Ceil(log n ) + 1+ 1 (last one for boundary group)<br />

NGV M<br />

G RVM N G =12<br />

~248s<br />

* Calcul<strong>at</strong>ion is run with M<strong>at</strong>lab on Dell OPTIPLEX GX270 with P4 3.20GHz CPU and 2GB RAM<br />

The comput<strong>at</strong>ional time is proportional to n M 2 or NG 2 for collision problem (double<br />

loop), and n M or N G for diffusion problem (single loop)<br />

Comput<strong>at</strong>ion cost is dram<strong>at</strong>ically reduced, especially for the problem with a large<br />

variety <strong>of</strong> precipit<strong>at</strong>e sizes because <strong>of</strong> logarithm rel<strong>at</strong>ion<br />

Sample results for AlN particles<br />

V M =1.33×10 -5 m 3 /mol, σ=0.2~0.75J/m 2<br />

DAl =3*10-3exp(-234500/RT) m2 /s, DN =9.1*10-5exp(-168500/RT) m2 /s<br />

6770<br />

Solubility product in austenite log K<br />

1.03 , Al0 =0.011wt%, N0 =0.009wt%<br />

AlN =− +<br />

T<br />

Choose RV =2 and 30 size groups, ∆t=1×10-3s, Implicit Euler with Gauss-Seidel<br />

iter<strong>at</strong>ive method<br />

Amount <strong>of</strong> dissolved elements and precipit<strong>at</strong>e (wt%)<br />

0.0175<br />

0.0150<br />

0.0125<br />

0.0100<br />

0.0075<br />

0.0050<br />

0.0025<br />

0.0000<br />

700 750 800 850 900 950 1000 1050 1100<br />

Temper<strong>at</strong>ure ( o C)<br />

Dissolved aluminum<br />

Dissolved nitrogen<br />

AlN precipit<strong>at</strong>e<br />

1E-15<br />

700 750 800 850 900 950 1000<br />

Temper<strong>at</strong>ure ( o C)<br />

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D(m 2 /s)<br />

1E-11<br />

1E-12<br />

1E-13<br />

1E-14<br />

N<br />

Al


Choose σ=0.5J/m 2<br />

Calcul<strong>at</strong>ed size distribution<br />

Growth (10~100s) is quick,<br />

coarsening (100-500s) is slow<br />

Lower temper<strong>at</strong>ure causes higher<br />

nucle<strong>at</strong>ion r<strong>at</strong>e, thus large number <strong>of</strong><br />

fine precipit<strong>at</strong>ed particles are observed<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 27<br />

Π= N1/ N1,<br />

eq<br />

Temper<strong>at</strong>ure<br />

900 o C<br />

850 o C<br />

800 o C<br />

750 o C<br />

Gener<strong>at</strong>e PTT diagram<br />

Nt / N1,<br />

eq −Π<br />

f =<br />

N / N −1<br />

5% precipit<strong>at</strong>ed<br />

178.1s<br />

60.2s<br />

40.3s<br />

38.5s<br />

t 1, eq<br />

Precipit<strong>at</strong>ed (500s)<br />

94.4%<br />

97.0%<br />

97.3%<br />

97.6%<br />

Experimental PTT curves <strong>of</strong><br />

AlN precipit<strong>at</strong>ed in austenite <strong>of</strong><br />

0.051%Al-0.0073%N steel<br />

Nose temper<strong>at</strong>ure (~800 o C)<br />

J. P. Michel et al, Acta Metallurgica,<br />

1981, 513-526.<br />

Quicker precipit<strong>at</strong>ion and higher<br />

nose temper<strong>at</strong>ure: interfacial energy<br />

value are between 0.5-0.75J/m 2<br />

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Grain growth model<br />

Grain growth in austenite under the presence <strong>of</strong> precipit<strong>at</strong>es<br />

(1/ n−1)<br />

dD * ⎛ Qapp<br />

⎞⎡1 1 f ⎤<br />

= M 0 exp⎜−<br />

⎟⎢<br />

−<br />

dt RT ⎣Dkr⎥ ⎝ ⎠ ⎦<br />

M0 *: Kinetic constant th<strong>at</strong> represents grain boundary mobility (m2 /s)<br />

Qapp : Apparent activ<strong>at</strong>ion energy for grain growth (J/mol)<br />

n: Exponent to measure resistance to grain boundary motion<br />

f and r: the volume fraction and radius <strong>of</strong> precipit<strong>at</strong>es<br />

K: Zener coefficient rel<strong>at</strong>ed to pinning efficiency <strong>of</strong> the precipit<strong>at</strong>es.<br />

The maximum grain diameter in the presence <strong>of</strong> precipit<strong>at</strong>es is defines as D = kr/ f<br />

Calcul<strong>at</strong>ion begins from the temper<strong>at</strong>ure <strong>of</strong> totally austenite structure, the initial austenite<br />

grain size is assumed to be with the order <strong>of</strong> the primary dendrite arm spacing (PDAS)<br />

( )<br />

m n<br />

1 K CR( C0)<br />

λ = K=278.75, m=–0.20628, n=–0.3162+2.0325C 0 for 0≤C 0 ≤0.15 (wt pct)<br />

I. Andersen and Ø Grong, Acta Metallurgica M<strong>at</strong>erialia, 1995.<br />

M. E. Bealy and B. G. Thomas, Metallurgical and M<strong>at</strong>erials Transactions B, 1996.<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 29<br />

Grain growth <strong>of</strong> 1006N2 steel in continuous casting<br />

Neglect the influence <strong>of</strong> precipit<strong>at</strong>es<br />

dD * ⎛ Qapp<br />

⎞⎡1 1 f ⎤<br />

= M 0 exp⎜−<br />

⎟⎢<br />

−<br />

dt RT ⎣Dkr⎥ ⎝ ⎠ ⎦<br />

Fully austenite temper<strong>at</strong>ure 1384 o C<br />

Cooling r<strong>at</strong>e estim<strong>at</strong>ed by CON1D<br />

Without precipit<strong>at</strong>es: the grains large<br />

enough to cause cracks<br />

Rough estim<strong>at</strong>ion: At surface, volume<br />

fraction <strong>of</strong> TiN 1.45*10 -4 <strong>at</strong> mold exit<br />

by choosing ρ steel =7500kg/m 3 and<br />

ρ TiN =5420kg/m 3 , typical average TiN<br />

particle size 10~100nm and k=4/3. the<br />

maximum grain size is calcul<strong>at</strong>ed<br />

within the range <strong>of</strong> 90~900µm<br />

(1/ n−1)<br />

Grain size (mm)<br />

1.6<br />

1.2<br />

0.8<br />

0.4<br />

Lim<br />

M 0 * =4×10 -3 m 2 /s, n=0.5,<br />

Q app =167686+40562(wt%C p )<br />

wt%C p =wt%C-0.14wt%Si+0.04wt%Mn<br />

J. Reiter, C. Bernhard and H. Presslinger, MS&T 2006,<br />

Cincinn<strong>at</strong>i, USA.<br />

surface (V=2.8) surface (V=4.6)<br />

0.5mm (V=2.8) 0.5mm (V=4.6)<br />

0.0<br />

10mm (V=2.8) 10mm (V=4.6)<br />

0 3 6 9 12 15<br />

Distance from meniscus (m)<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 30<br />

mold<br />

exit


Conclusions (equilibrium precipit<strong>at</strong>ion<br />

model)<br />

1) An equilibrium precipit<strong>at</strong>ion model is established, which includes<br />

consider<strong>at</strong>ion <strong>of</strong> solubility limit with Wagner interaction effect, mutual<br />

solubility and complete mass conserv<strong>at</strong>ion <strong>of</strong> alloying elements.<br />

2) The influence <strong>of</strong> mutual solubility on precipit<strong>at</strong>ion behaviors is carefully<br />

considered. Pseudo-ternary precipit<strong>at</strong>ion diagram is produced.<br />

3) The model is valid<strong>at</strong>ed with analytical solutions <strong>of</strong> simple cases and<br />

experimental measurements <strong>of</strong> liter<strong>at</strong>ures. Good agreements are found.<br />

4) The model is applied for practical 100N2 steel and continuous casting<br />

with two casting speed. TiNbV(CN), Mns and AlN are only possible<br />

precipit<strong>at</strong>es th<strong>at</strong> could form. TiN is the mainly precipit<strong>at</strong>es <strong>at</strong> high<br />

temper<strong>at</strong>ure especially in the mold because <strong>of</strong> its lowest solubility limit.<br />

With higher speed, AlN does not form in the mold.<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 31<br />

Conclusions (kinetic model)<br />

1) Comparing with some empirical or semi-empirical kinetic precipit<strong>at</strong>ion models,<br />

the suggested precipit<strong>at</strong>e growth mechanisms by collision and diffusion are<br />

fundamentally based and all parameters are physically significant.<br />

2) The PSG method is suggested and valid<strong>at</strong>ed for particle agglomer<strong>at</strong>ion due to<br />

collision and diffusion. Within a wide range <strong>of</strong> volume r<strong>at</strong>ios, R V , mass balance<br />

is s<strong>at</strong>isfied and good m<strong>at</strong>ches are found. Because the accuracy <strong>of</strong> the PSG<br />

method increases with decreasing R V , an R V <strong>of</strong> 2.0 seems optimal.<br />

3) PSG method needs only a small number <strong>of</strong> size groups to simul<strong>at</strong>e particle<br />

agglomer<strong>at</strong>ion, and huge saving <strong>of</strong> memory storage and comput<strong>at</strong>ional time is<br />

found by comparison with solving initial problem without losing mass balance<br />

and sacrificing the desired accuracy.<br />

4) In addition to gener<strong>at</strong>e volume fraction and size distribution <strong>of</strong> precipit<strong>at</strong>ed<br />

particles, the PSG method is used to gener<strong>at</strong>e the PTT diagram.<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 32


Conclusions (grain growth)<br />

1) In continuous casting, the grain growth model predicts grains approaching 50%<br />

<strong>of</strong> their final size by mold exit. The grains under 0.5mm oscill<strong>at</strong>ion marks are <strong>at</strong><br />

least 10% larger than those on the slab surface due to local high temper<strong>at</strong>ure.<br />

Without precipit<strong>at</strong>es, the grains are large enough to cause ductility problems<br />

2) It is expected th<strong>at</strong> the larger grains and lack <strong>of</strong> fine precipit<strong>at</strong>es are likely the<br />

controlling factors to cause coarse grains and susceptibility to surface cracks<br />

bene<strong>at</strong>h the oscill<strong>at</strong>ion marks. A program for coupling all the above models is<br />

necessary and important in future work<br />

<strong>University</strong> <strong>of</strong> <strong>Illinois</strong> <strong>at</strong> Urbana-Champaign • Metals Processing Simul<strong>at</strong>ion Lab • <strong>Kun</strong> <strong>Xu</strong> • 33<br />

Acknowledgements<br />

• <strong>Continuous</strong> <strong>Casting</strong> <strong>Consortium</strong> Members<br />

(ABB, Arcelor-Mittal, Baosteel, Corus, Delavan/Goodrich,<br />

LWB Refractories, Nucor, Nippon Steel, Postech, Steel<br />

Dynamics, ANSYS-Fluent)<br />

• Pr<strong>of</strong>. B. G. Thomas<br />

• Other gradu<strong>at</strong>e students in CCC groups<br />

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