Benefits of thermodynamic and

microstructure simulations towards

reliable continuous casting of steels

M. Apel1), B. Böttger1), D.G. Eskin 3), B. Santillana2)

1) Access, Aachen, Germany

2) Tata Steel RD&T, POBox 10000, 1970CA, IJmuiden, The Netherlands

3) BCAST, Brunel University, Uxbridge UB8 3PH, U.K.

2

Outline

• Background and industrial application

• Microstructure differences

• MICRESS results

• Unexpected “peritectic solidification” and Cp

3

Breakouts!!!!

• Solidified shell bursts open

• Lost Production (24 hours)

• Machine damage

• Loss of over 200 K-euro's

• Safety

4

The iceberg…

Breakouts

A few…

Cracks

Few more…

Solidification issues

Plenty…

5

Solidification issues

Mechanical

Chemical

Thermal

?? Combined with

high heat

extraction???

ZDT, ZST,

LIT,

DCP???

Micro/macr

osegregatio

n,peritectic

solidificatio

n???

Critical

stresses &

strains???

6

Alloy compositions:

LCAK HSLA LR-HSLA

Steel grade

(in 0,001 %) min aim max

C 25 45 60

Mn 170 220 270

N (ppm) 50

Al soluble 20 35 55

Nb 5

Ti 10

V 5

Ca (ppm) 20 60

Steel grade

(in 0,001%) min aim max

C 25 45 60

Mn 725 800 875

N (ppm) 110 130 150

Al soluble 15 30 55

Nb 10 13 16

Ti 10

V 119 130 141

Ca (ppm) 20 60

Steel grade

(in 0,001%) min aim max

C 25 45 60

Mn 725 800 875

N (ppm) 80 100

Al soluble 15 30 55

Nb 10 13 16

Ti 10

V 40

Ca (ppm) 20 60

cracking! not

cracking! cracking!

Why?

Problem

7

Macrostructure (same heat transfer conditions)

HSLA LCAK

Macroetching for the two steel grades considered in this study.

8

17 mm17 mm 18 mm18 mm

HSLA LCAK

Macroetching for the two steel grades considered in this study.

9

Primary Dendrite Arm Spacing

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10 12

Distance from slab surface (mm)

Me

asu

red

PD

AS

(u

m)

HSLA

LCAK

10

Sumitomo hot tensile test

ZST ZDT

Brittle temperature range

(ΔTB)

FV85 Hot Tensile Tests

1320°C 1340°C 1360°C 1380°C 1400°C 1440°C1300°C 1460°C 1480°C1420°C

FV85 Hot Tensile Tests

1320°C 1340°C 1360°C 1380°C 1400°C 1440°C1300°C 1460°C 1480°C1420°C

ZST

ZDT

Brittle temperature range

(ΔTB)

HSLA LCAK

11

Aims of this project

• Perform Phase Field Simulations of microstructure

formation of the first solid shell under “Continuous

casting conditions”

• Quantify simulated solidification microstructure

• Find out whether the observed differences of the

cracking risk can be understood via the chemical

composition

12

Comparison LCAK and HSLA (same heat transfer conditions)

HSLA is finer and more equiaxed!

LCAK (Fe-C-Mn), r0=0.065 µm, σ=10 %*r0, N=108

HSLA (Fe-C-Mn-N-Nb-V), r0=0.065 µm, σ=10 %*r0, N=108

13

Comparison LCAK and HSLA:

Carbon composition distribution at 0.4s LCAK (Fe-C-Mn), r0=0.065 µm, σ=10 %*r0, N=107

HSLA (Fe-C-Mn-N-Nb-V), r0=0.065 µm, σ=10 %*r0, N=107

HSLA is finer, more equiaxed

and has a wider solidification range

14

LCAK vs HSLA at same heat transfer conditions

• The difference between LCAK and the HSLA steels is the as-cast

structure

• LCAK has a coarser structure and tends to form more columnar

dendrites than the HSLA

• Finer grains, more equiaxed & thinner dendrites exhibit more isotropic

and uniform mechanical properties =>less tendency for hot tearing

formation and decreased macrosegregation.

• Coarser structure also exhibits a wider BTR (also shown in the SMI hot

tensile tests)

15

Thermo-Calc

C: 0.060%

Mn: 0.875%

Nb: 0.016%

N: 0.015%

V: 0.141%

Ti: 0.010%

C: 0.060%

Mn: 0.875%

Nb: 0.016%

N: 0.015%

V: 0.141%

Ti: 0.010%

C: 0.060%

Mn: 0.875%

Nb: 0.016%

N: 0.010%

V: 0.040%

Ti: 0.010%

C: 0.060%

Mn: 0.875%

Nb: 0.016%

N: 0.010%

V: 0.040%

Ti: 0.010%

A: FV85 B: FV83

Figure 1. Scheil calculations for FV83 and FV85 considering the maximum values for the

compositions.

A (for HSLA steel grade), the phases listed are as follows:

1: LIQUID: liquid steel

2: LIQUID BCC_A2: liquid and the δ ferrite (L+ δ)

3: LIQUID BCC_A2 FCC_A1#2: liquid, δ ferrite (L+ δ) and TiN

4: LIQUID FCC_A1#1 FCC_A1#2: liquid, γ austenite (L+ γ) and TiN

Consequently, in B (for LR-HSLA steel grade), the phases listed are as follows:

1: LIQUID: liquid steel

2: LIQUID BCC_A2: liquid and the δ ferrite (L+ δ)

3: LIQUID BCC_A2 FCC_A1#2: liquid, δ ferrite (L+ δ) and TiN

4: LIQUID BCC_A2 FCC_A1#1 FCC_A1#2: liquid, δ ferrite and γ austenite (L +δ+γ) and TiN

5: LIQUID FCC_A1#1 FCC_A1#2: liquid, γ austenite (L+ γ) and TiN

16

Carbon potential-Ferrite potential

• The actual peritectic covers the range Cd to CL, but regarding

“peritectic grades” prone to defects the “peritectic” range is:

• Cδ < Cp < Cγ

• with the proximity to Cδ (left hand /low C extremity) being considered

particularly important.

1350

1370

1390

1410

1430

1450

1470

1490

1510

1530

1550

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Composition (wt% C)

Tem

pera

ture

(ºC

)

δδ+γ

δ+L

γ γ+L

LδP

LP

L2γ2

Cδ

δ1

CLCγ

γ1

1350

1370

1390

1410

1430

1450

1470

1490

1510

1530

1550

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Composition (wt% C)

Tem

pera

ture

(ºC

)

δδ+γ

δ+L

γ γ+L

LδP

LP

L2γ2

Cδ

δ1

CLCγ

γ1

Reaction:

L

Transformations:

LL

Figure 1. Schematic Fe-C phase diagram showing the peritectic reaction and transformation

[7]

17

Carbon potential-Ferrite potential

• The changes in the Fe–C phase diagram through chemistry can be described using an effective

carbon content Cp expression of the form:

• Cp = [%C] + XMn[%Mn] + XSi[%Si] + … with Xx as a coefficient.

• This approach works on the principle whereby the equivalent concentrations of austenite forming

elements are added to the Cp value, and the equivalent concentrations of ferrite forming elements are

subtracted.

• However, from the +/- signs of various coefficients, the Cp approach may no be very straightforward

for determining the selectivity between Austenite and Ferrite.

• The TRICO formula is calculated from a statistical analysis done at TRICO steel Decatur, Alabama,

USA (now Nucor Decatur).

• The SMS formula is based on plant experience.

Reference Mn Si Al(tot) N P S Nb V Ti Cu Sn Cr Ni Mo B Ca

Wolf 1991 0.02 -0.1 -0.7 -0.04 0.04 -0.1

Trico 1999 0.01 0.009 0.05 0.5 0.008 0.17 0.04 0.009 0.007 0.007 0.0006 0.003 0.02 -0.007 1.32 -0.24

BSSTC 1998 0.043 -0.14 1.06 0.029 0.11 -0.13 -0.024 0.037 -0.083 0.1 -0.063

SMS 0.014 -0.037 -0.04 -0.222 0.003 0.023 -0.004

Howe 0.04 -0.14 0.7 -0.24 -0.04 0.1 -0.1

18

BS-STC & Howe formulae

BS STC

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500

Ladle

Cp

No BO's FV83

No BO's FV85

NO BO's F12L

No BO's FN 80

No BO's FN 81

BO's

upper limit

lower limit

BS STC

0.02

0.04

0.06

0.08

0.1

0.12

0.14

6000 6250 6500 6750 7000 7250 7500

Ladle

Cp

No BO's FV83

No BO's FV85

BO's

upper limit

lower limit

A: Cp for all the ladles from 1st January 2009

up to 31st December 2010.

B: Ladles corresponding to FV83 and FV85

Figure 1. Carbon potential calculated with BS STC formula.

Howe

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500

Ladle

Cp

No BO's FV83

No BO's FV85

NO BO's F12L

No BO's FN 80

No BO's FN 81

BO's

upper limit

lower limit

Howe

0.02

0.04

0.06

0.08

0.1

0.12

0.14

6000 6250 6500 6750 7000 7250 7500

Ladle

Cp

No BO's FV83

No BO's FV85

BO's

upper limit

lower limit

A: Cp for all the ladles from 1st January 2009

up to 31st December 2010.

B: Ladles corresponding to FV83 and FV85

Figure 1. Carbon potential calculated with A. Howe formula.

LR-HSLA

HSLA

19

Wolf formula & Blazek limits

Wolf

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500

Ladle

Cp

No BO's FV83

No BO's FV85

NO BO's F12L

No BO's FN 80

No BO's FN 81

BO's

upper limit

lower limit

Wolf

0.02

0.04

0.06

0.08

0.1

0.12

0.14

6000 6250 6500 6750 7000 7250 7500

Ladle

Cp

No BO's FV83

No BO's FV85

BO's

upper limit

lower limit

A: Cp for all the ladles from 1st January 2009

up to 31st December 2010.

B: Ladles corresponding to FV83 and FV85

Figure 1. Carbon potential calculated with Wolf formula.

LC

AK

Cδ

LC

AK

Cγ

MA

Cδ

MA

Cγ

MA

2 Cδ

MA

2 Cγ

LR

-HS

LA

Cδ

LR

-HS

LA

Cγ

HS

LA

Cδ

HS

LA

Cγ

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Peri

tecti

c r

an

ge (

Cp

)

LCAK Cδ

LCAKCγ

MA Cδ

MA Cγ

MA2 Cδ

MA2 Cγ

LR-HSLA Cδ

LR-HSLA Cγ

HSLA Cδ

HSLA Cγ

20

CONCLUSIONS

• LCAK is more sensitive to cracking because the microstructure is

coarse

• LR-HSLA may have cracking issues because is “almost” a peritectic

grade so may have late peritectic solidification…

• Next step: DSC-DTA

• Aims of the project achieved!

Perform Phase Field Simulations of microstructure formation of the first solid

shell under “Continuous casting conditions”

Quantify simulated solidification microstructure

Find out whether the observed differences of the cracking risk can be

understood via the chemical composition

• Next step: other steel grades to simulate

Thank you for your attention

Questions???

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