Tata Steel Slide

Solidification under continuous casting

conditions, thermodynamic and experimental

studies

B. Santillana , L. Zhuang , H. Li, L. Han

Tata Steel Slide

CALPHAD & steelmaking

Carbon equivalent formulae

Cp = [%C] + XMn[%Mn] + XSi[%Si] +…

2

Black & white 16 colours 256 colours

Tata Steel Slide

The solidification of a steel will be a fingerprint

that marks many properties of the final

product.

Example 2: Segregation and banding

Ennis et al., Acta Mater., 115, 132-142. 10.1016/j.actamat.2016.05.046

Tata Steel Slide 4

As-cast dendritic structure

Mo

uld

Sla

g film

λ1

λ2

λ1

λ2

0

20

40

60

80

100

120

140

160

180

200

0 2 4 6 8 10 12

Distance from slab surface (mm)

Measu

red

PD

AS

(u

m)

HSLA LCAK

Tata Steel Slide

Relationship cast structure and banding

5

Elemental line profiles of

martensite/austenite bands embedded in

the ferrite matrix in CRA, showing inverse

segregation of Al and Mn

5

Tata Steel Slide 6

Origin of ferrite streaks

‘Zorro’ lines/

ferrite streaks

(and centre-

line

segregation) in

a HR strip

‘”stretched” as-cast slab

DSP FV85

dendritic

strcuture

Tata Steel Slide 7

Ferrite streaks correlated to Mn depletion of primary dendrite formation

Mn

Ferrite

Pearlite

(hardness

indents)

Tata Steel Slide

Relationship cast structure and banding

8

≈200 µm

≈ 200 µm

8

Tata Steel Slide

Homogenisation during reheating; fact vs. fiction

9

Single component diffusion of Al in Fe indicates homogenisation times from seconds to hundreds of hours at 1250°C

9

Tata Steel Slide

Homogenisation during reheating; fact vs. fiction

10

DICTRA multi-component calculations:

diffusion of solid-state elements (Al/Mn) during reheating is too slow to result in chemical homogenisation

segregation profiles present after casting remain throughout the down-stream processes

Distributions of C, Mn, Al along the diffusion path/ boundary for time steps of 100, 1000 and 100,000 s (28 h)

10

Tata Steel Slide

The question is not if we can cast a steel, but

how can we cast it…

Example 1: precipitation during continuous casting

Tata Steel Slide

AHSS

1

2End of solidification

TS 1=1297ºC

(steel with B)

End of solidification

TS 2=1474ºC

(steel without B)

Oscillation Mark Bleeder Cracking

Example of the defect formation

Tata Steel Slide

University of Science and Technology Beijing

Two methods have been adopted in the experimental work.

Method A: Directional

solidification and quenching

(DSQ)

Method B:

Solidification and

quenching (SQ)

Bridgman furnace Vertical-tubular furnace

Experimental methods

13

Tata Steel Slide

University of Science and Technology Beijing

The directional transformation of the alloy

can be carried out from liquid to room

temperature so that the consecutive liquid–

solid and solid–solid transformation fronts in a

given sample can be studied under well-

defined steady-state conditions.[3]

As fluid flow strongly influences the

microstructures, its control and absence in

directional solidification by the use of small

diameter tubes is a powerful technique,

complementary to space experiments. [3]

[1]Seo S M, Kim I S, Lee J H, et al. Eta phase and boride formation in directionally solidified Ni-base superalloy IN792+ Hf[J]. Metallurgical and Materials Transactions A, 2007, 38(4): 883-893.

Bridgman-furnace [1]

Method A

14

Tata Steel Slide

University of Science and Technology Beijing

Microstructure of directional solidification samples

Withdrawal rate: 125 μm/s

Withdrawal rate: 125 μm/s

15

Tata Steel Slide

University of Science and Technology Beijing

Microstructure of directional solidification samples

Withdrawal rate: 25 μm/s

Withdrawal rate: 25 μm/s

16

Tata Steel Slide

University of Science and Technology Beijing

Microstructure of directional solidification samples

6 mm

liquid

Withdrawal rate: 5 μm/s

17

Tata Steel Slide

Schematic illustration of mushy zone that reveals nucleation and

development of primary grains in different zones.[1]

[1] M. Bagheri, M. Alizadeh, Morphological approach to hot tearing during continuous casting of steels, International Journal of Cast Metals Research 28(5) (2015) 276-282.

University of Science and Technology Beijing

Microstructure which is similar to the cross

section can also be obtained by a method

called solidification and quenching (SQ).

For directional solidification, the structure

along with the growth direction can be

obtained at first. Then the sample can be

cut to investigate the cross section.

18

Tata Steel Slide

Schematic diagram of the solidification and quenching process ranging

from 1505 ℃℃℃℃ to 1415 ℃℃℃℃at the interval of 10 ℃℃℃℃.Schematic diagram of the experimental equipment.

University of Science and Technology Beijing

Method B; Solidification & quenching

19

Tata Steel Slide

Schematic diagram of measuring the temperature The measured temperature history of the sample

The measured position is heart of the sample

University of Science and Technology Beijing

8.2℃℃℃℃/min

Method B; Solidification & quenching

20

Tata Steel Slide

University of Science and Technology Beijing

1475℃℃℃℃ 1465℃℃℃℃ 1455℃℃℃℃

1445℃℃℃℃ 1435℃℃℃℃ 1425℃℃℃℃ 1415℃℃℃℃

Microstructures of the quenched samples which can

indicate the solidification process.

Etched samples

21

Tata Steel Slide

Microstructures of the studied surface on each

sample.

University of Science and Technology Beijing

Selected areas for EPMA

22

Tata Steel Slide

Representative binary images of the microstructures.

University of Science and Technology Beijing

Liquid & solid determination

23

Tata Steel Slide

Mass fraction of solid

T fm,s

1475 58%

1465 76%

1455 88%

1445 92%

1435 96%

1425 97%

1415 100%

Experimental results compared with calculations.

University of Science and Technology Beijing

,

, ,s,

m s

s

m s m L

s L

f

V f ff

4 3 3

s ,4.80 10 7.41 10startT T kg m

Tγ,start =1478

TL,start =1505

4 3 3

,7.5 10 7.02 10L L startT T kg m

fV,s

Liquid & solid fraction

24

Tata Steel Slide

• Calphad method is very useful for the steel industry, many simpleapplications…

• The whole microstructure of the mushy zone can be obtained with directional solidification and S&Q methods, convenient to study the dendrite structure, element distribution, etc.

• Not really enough yet, but we are working on….

25

Conclusions

Tata Steel Slide

Persistence is to the character of man what carbon is to steel (Napoleon Hill), unless other elements are added to thecomposition…*

* Preposition No. 8, PhD thesis B. Santillana

Questions???

Tata Steel Slide