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Solidification under continuous casting
conditions, thermodynamic and experimental
studies
B. Santillana , L. Zhuang , H. Li, L. Han
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CALPHAD & steelmaking
Carbon equivalent formulae
Cp = [%C] + XMn[%Mn] + XSi[%Si] +…
2
Black & white 16 colours 256 colours
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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
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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
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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
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Origin of ferrite streaks
‘Zorro’ lines/
ferrite streaks
(and centre-
line
segregation) in
a HR strip
‘”stretched” as-cast slab
DSP FV85
dendritic
strcuture
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Ferrite streaks correlated to Mn depletion of primary dendrite formation
Mn
Ferrite
Pearlite
(hardness
indents)
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Relationship cast structure and banding
8
≈200 µm
≈ 200 µm
8
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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
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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)
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The question is not if we can cast a steel, but
how can we cast it…
Example 1: precipitation during continuous casting
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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
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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
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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
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University of Science and Technology Beijing
Microstructure of directional solidification samples
Withdrawal rate: 125 μm/s
Withdrawal rate: 125 μm/s
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Microstructure of directional solidification samples
Withdrawal rate: 25 μm/s
Withdrawal rate: 25 μm/s
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Microstructure of directional solidification samples
6 mm
liquid
Withdrawal rate: 5 μm/s
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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.
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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
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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
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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
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Microstructures of the studied surface on each
sample.
University of Science and Technology Beijing
Selected areas for EPMA
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Representative binary images of the microstructures.
University of Science and Technology Beijing
Liquid & solid determination
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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
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• 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….
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Conclusions
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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???
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