L. SOCHA et al.: STUDY OF MACRO-SEGREGATIONS IN A CONTINUOUSLY CAST BILLET
237–241
STUDY OF MACRO-SEGREGATIONS IN A CONTINUOUSLY
CAST BILLET
[TUDIJ MAKROIZCEJ V KONTINUIRNO ULITI GREDICI
Ladislav Socha1, Vlastimil Vodárek2, Karel Michalek1, Hana Francová3,
Karel Gryc1, Markéta Tkadle~ková1, Ladislav Válek4
1V[B – Technical University of Ostrava, FMME, Department of Metallurgy and Foundry, 17. listopadu 15/2172,
708 33 Ostrava – Poruba, Czech Republic
2V[B – Technical University of Ostrava, FMME, Department of Material Engineering, 17. listopadu 15/2172,
708 33 Ostrava – Poruba, Czech Republic
3V[B – Technical University of Ostrava, FMME, Department of Physical Chemistry and Theory of Technological Processes,
17. listopadu 15/2172, 708 33 Ostrava – Poruba, Czech Republic
4ArcelorMittal Ostrava a.s., Research, Vratimovská 689, 707 02 Ostrava-Kun~ice, Czech Republic
ladislav.socha@vsb.cz
Prejem rokopisa – received: 2015-07-13; sprejem za objavo – accepted for publication: 2016-02-09
doi:10.17222/mit.2015.220
This paper is devoted to the study of macro-segregations on the longitudinal section of a continuously cast billet Ø 160 mm
destined for the production of seamless pipes. Its evaluation comprised a visual inspection of macro-etching, microstructure
characterisation and a quantitative X-ray microanalysis by means of an energy-dispersive X-ray microanalysis (EDX) in the
selected areas. Furthermore, segregation parameters were calculated using combustion-analysis data. The results given in this
paper present the basic information about the interior structure of a continuously cast billet. The results will be used, beside
other things, for the verification of the results of the numerical modelling of continuously cast billets.
Keywords: steel, macro-segregation, continuous casting, round billet
Ta ~lanek obravnava {tudij makro-izcej na vzdol`nem prerezu kontinuirno ulite gredice Ø 160 mm namenjene za proizvodnjo
brez{ivnih cevi. Ocena je sestavljena iz vizuelnega pregleda po makrojedkanju, iz ocene in kvantitativne rentgenske
mikroanalize s pomo~jo energijsko disperzijske rentgenske analize (EDX) izbranih podro~ij. Izra~unani so bili tudi parametri
izcejanja s pomo~jo podatkov zgorevne analize. Rezultati, prikazani v tem ~lanku, predstavljajo osnovno informacijo o notranji
strukturi v kontinuirno uliti gredici. Rezultati bodo uporabljeni, poleg drugega, tudi za preverjanje rezultatov numeri~nega
modeliranja kontinuirno ulitih gredic.
Klju~ne besede: jeklo, makro-izceja, kontinuirno ulivanje, okrogla gredica
1 INTRODUCTION
Steel crystallization occurs during the casting and
subsequent solidification of continuously cast billets.
Material solidification, the grain size, chemical hetero-
geneity and the existence of incompactness can have
negative influences on the properties of the final ma-
terial.1 A relatively big homogeneity of the distribution
of sulphur and other elements and also the absence of
thick segregate areas in the whole cross-section are due
to the fast solidification of the billet. The range and
removal of impurities are mainly connected with the
contents of phosphorus and sulphur in the steel, the steel
temperature and also the intensity of the billet cooling.2
The local chemical heterogeneity appears in the billet
axis, especially for the steels with higher contents of
carbon where the possibility of internal-hollow creation
is higher.3,4
The paper presents the results of the study of macro-
segregation on the longitudinal section of a continuously
cast billet Ø 160 mm. Samples were taken during the
casting of a high-quality billet destined for the produc-
tion of seamless pipes without the usage of the MEMS
technology. First, a visual inspection of the longitudinal
section of the billet was made for the study of macro-
segregation in a continuously cast billet. Then a micro-
structural analysis using a quantitative X-ray microanal-
ysis with the help of EDX in SEM (Quanta 450 FEG)
was carried out. The results of the microstructural
analysis were subsequently supplemented with an
analysis of the contents of carbon and sulphur with the
aim to evaluate the chosen segregation parameters for the
micro-samples taken from particular areas of macro-
etching. The obtained original results will be, beside
other things, used for verifying the results of numerical
modelling of casting continuously cast billets.
2 MACRO-STRUCTURAL ANALYSIS OF THE
BILLET
First, a visual inspection of the longitudinal section
of the billet was made for the study of macro-segregation
in the continuously cast billet, which is given in Fig-
ure 1a.
It is evident in Figure 1a that the surface area of
equiaxed crystals (the chill zone) penetrates into a depth
Materiali in tehnologije / Materials and technology 51 (2017) 2, 237–241 237
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:620.191.3:621.74.047 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(2)237(2017)
of approx. 18 mm. This is followed by an area of colum-
nar crystals with a thickness of approx. 35 mm and, in
the middle of the longitudinal section, there is an area of
equiaxed grains.
In the axial area of the billet, there is shrinkage poro-
sity that is a consequence of the casting conditions
during the final period of the billet solidification. The
darker contrast in the area of shrinkage porosity is
related to the processes of chemical segregation during
the solidification.
3 MICROSTRUCTURAL ANALYSIS OF THE
BILLET
After the visual inspection of macro-etching, the
sample was cut into smaller parts, evident in Figure 1b.
The samples were prepared for a microstructural analysis
aimed at evaluating the characterisation of the micro-
structure and chemical heterogeneity using the EDX
technique. A few samples – sample 2, sample 4 and
sample 5 – were chosen for the analysis. The following
elements were chosen for the X-ray microanalysis
(EDX): silicon, phosphorus, chromium, manganese, iron
and molybdenum. The results of a semi-quantitative
X-ray microanalysis were normalized to 100 %.
The microstructure of sample 2 presenting areas of
columnar crystals is given in Figure 2. Previous auste-
nite grain boundaries are decorated with fine grains of
allotriomorphic ferrite. Nodules of the pearlite compo-
nent usually grow up along the networks of allotrio-
morphic ferrite. The microstructure inside the austenitic
grains consists of a mixture of acicular ferrite and a
small fraction of pearlite, as it is evident in Figure 3.
Subsequently, a quantitative X-ray microanalysis
(EDX) was made. A spectral RTG microanalysis of the
matrix was made in the randomly chosen areas of sample
2 and the results are given in Table 1.
Table 1: Results of the EDX microanalysis – sample 2
Tabela 1: Rezultati EDX mikroanalize – vzorec 2
Analysis
Analysed elements (w/%)
Si Cr Mn Fe Mo
1 0.30 0.72 1.01 97.47 0.50
2 0.40 0.75 1.21 97.24 0.40
3 0.36 0.78 0.90 97.66 0.30
AVE 0.35 0.75 1.04 97.46 0.40
STD 0.05 0.03 0.16 0.21 0.10
Note: AVE – arithmetic mean, STD – standard deviation
The microstructure of sample 4 in the axial area of
the billet, which shows an increased fraction of the pear-
lite component in the surroundings of the shrinkage
porosity, is shown in Figure 4. Networks of allotrio-
morphic ferrite decorate the prior austenite grain boun-
daries. Austenite decomposed into a mixture of acicular
ferrite and pearlite inside the original austenitic grains.
Fine shrinkage porosity, surrounded by narrow bands
exhibiting a limited etching response, is evident in
Figure 5. Further, an increased amount of non-metallic
inclusions (manganese sulphide) and the bainite ob-
served in these areas proves that these are the last soli-
dified areas of enriched melt where the largest segrega-
tion can be expected.
L. SOCHA et al.: STUDY OF MACRO-SEGREGATIONS IN A CONTINUOUSLY CAST BILLET
238 Materiali in tehnologije / Materials and technology 51 (2017) 2, 237–241
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Figure 3: Detail of the microstructure in the area of columnar crystals
Slika 3: Detajl mikrostrukture v podro~ju stebrastih kristalnih zrn
Figure 2: Microstructure in the area of columnar crystals
Slika 2: Mikrostruktura podro~ja s stebrastimi kristalnimi zrni
Figure 1: Longitudinal section through Ø 160 mm billet: a) macro-
etching of the studied billet, b) positions of the samples taken for a
microstructure characterisation
Slika 1: Vzdol`ni prerez skozi gredico Ø 160 mm: a) makrojedkana
gredica, b) polo`aj vzorcev, vzetih za karakterizacijo mikrostrukture
Results of a semi-quantitative X-ray microanalysis of
sample 4 are given in Table 2. When comparing them
with the results given in Table 1, it is evident that the
metal matrix in the studied area with a different etching
response was enriched by all the evaluated substitution
solutes and, in addition, a significant concentration of
phosphorus was found here.
Table 2: Results of the EDX microanalysis – sample 4
Tabela 2: Rezultati EDX-mikroanalize – vzorec 4
Analysis
Analysed elements (wt. %)
Si P Cr Mn Fe Mo
1 0.40 0.33 1.22 1.46 95.84 0.75
2 0.35 0.19 1.30 1.33 95.96 0.87
3 0.42 0.21 1.26 1.42 95.98 0.71
AVE 0.39 0.24 1.26 1.40 95.93 0.78
STD 0.08 0.08 0.07 0.08 0.08 0.08
Note: AVE – arithmetic mean, STD – standard deviation
The microstructure of sample 5 (the axial part of the
billet) with an increased fraction of pearlite adjacent to
the shrinkage porosity is shown in Figure 6.
Similar to sample 4, bands with a different etching
response were found in the surroundings of the shrinkage
porosity where the microstructure was made of bainite,
and a large amount of sulphide inclusions was observed,
as seen in Figure 7.
In the case of sample 5, a semi-quantitative X-ray
microanalysis was also made and its results are given in
Table 3.
Table 3: Results of the EDX microanalysis – sample 5
Tabela 3: Rezultati EDX-mikroanalize – vzorec 5
Analysis
Analysed elements (w/%)
Si P Cr Mn Fe Mo
1 0.49 0.34 1.53 1.35 95.21 1.08
2 0.55 0.22 1.27 1.49 95.75 0.72
3 0.54 0.00 1.46 1.31 95.77 0.92
AVE 0.53 0.19 1.42 1.38 95.58 0.91
STD 0.03 0.17 0.13 0.09 0.32 0.18
Note: AVE – arithmetic mean, STD – standard deviation
4 SEGREGATION PARAMETERS
On the basis of the X-ray microanalysis results, it
was decided to carry out also a carbon and sulphur
analysis of the micro-samples taken from individual
L. SOCHA et al.: STUDY OF MACRO-SEGREGATIONS IN A CONTINUOUSLY CAST BILLET
Materiali in tehnologije / Materials and technology 51 (2017) 2, 237–241 239
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 7: Detail of the microstructure in the axial area of the billet
Slika 7: Detajl mikrostrukture na sredini gredice
Figure 5: Detail of the microstructure in the axial area of the billet
Slika 5: Detajl mikrostrukture na sredini gredice
Figure 6: Microstructure in the axial area of the billet
Slika 6: Mikrostruktura na sredini gredice
Figure 4: Microstructure in the axial area of the billet
Slika 4: Mikrostruktura na sredini gredice
areas of the macro-etching. Sample 2, sample 4 and
sample 5 were chosen for a proper analysis. The samples
were cut into the micro-samples for an evaluation of
individual areas, as seen in Figure 8.
The analysis was carried out in an ELTRA CS 2000
combustion analyser and the results obtained for indivi-
dual micro-samples are given in Table 4.
Table 4: Results of the combustion analysis of micro-samples
Tabela 4: Rezultati analize z zgorevanjem mikro vzorcev
Sample Place ofsampling
Elements (w/%) Average values(w/%)
C S Ø carbon Ø sulphur
2
Edge
0.304 0.0083
0.301 0.0081
0.297 0.0078
Centre
0.300 0.0087
0.302 0.0087
0.304 0.0086
4
Edge
0.269 0.0071
0.278 0.0074
0.287 0.0077
Centre
0.328 0.0101
0.347 0.0098
0.365 0.0095
5
Edge
0.287 0.0089
0.285 0.0090
0.283 0.0090
Centre
0.318 0.0090
0.321 0.0098
0.325 0.0105
It is evident from Table 4 that the micro-samples
taken from the centre show higher contents of carbon, by
0.045 % of mass fractions (sample 4) and by 0.021 % of
mass fractions (sample 5), than in the area of columnar
crystals (sample 2). It can be stated, in the case of
sulphur, that its contents are in the range from 0,0074 %
to 0,0098 % of mass fractions for all the micro-samples.
On the basis of the obtained results, the micro-samples
from the central area were evaluated by means of the
chosen segregation parameters5 and their relations are
given in Equations (1), (2) and (3).
Ih
SX
XS
= (1)
where Ih – is heterogeneity index, (-), SX – standard
deviation, (-), XS – mean value, (-).
Is
MAX
XS
= (2)
where Is – is segregation index, (-), MAX – maximum
value, (-), XS – mean value, (-).
k
Isef
=
1
(3)
where kef – is efficient partition coefficient, (-), Is –
segregation index. (-).
Chemical compositions of the analysed micro-sam-
ples were used for determining the segregation para-
meters, which were completed by the basic statistical
parameters, and the results for carbon are given in
Table 5, while the results for sulphur are in Table 6.
Table 5: Results of segregation parameters for carbon
Tabela 5: Rezultati parametrov izcejanja pri ogljiku
Sample Place ofsampling
Segregation parameters
Ih Is kef
2
Edge 0.019 1.024 0.977
Centre 0.020 1.020 0.980
4
Edge 0.034 1.029 0.972
Centre 0.094 1.115 0.897
5
Edge 0.010 1.012 0.988
Centre 0.048 1.044 0.958
Table 6: Results of segregation parameters for sulphur
Tabela 6: Rezultati parametrov izcejanja pri `veplu
Sample Place ofsampling
Segregation parameters
Ih Is kef
2
Edge 0.0695 1.0779 0.9277
Centre 0.0962 1.0741 0.9310
4
Edge 0.0422 1.0590 0.9443
Centre 0.1519 1.1374 0.8792
5
Edge 0.1108 1.0832 0.9232
Centre 0.1639 1.1895 0.8407
On the basis of the calculated segregation parameters
from Tables 5 and 6, the following rows were deter-
mined. Individual substitutional solutes segregate in
conformity with the decreasing value in the efficient
partition coefficient (kef):
• sample 2: Mo-Si-Mn-Cr
• sample 4: P-Mo-Cr-Si-Mn
• sample 5: P-Mo-Cr-Si-Mn
Considering the values of the efficient partition
coefficient (kef), the phosphorus in sample 5 segregates
extremely significantly, the phosphorus of sample 4
segregates significantly, while carbon, sulphur, silicon,
chromium, molybdenum and manganese segregate into
an interdendritic melt. Molybdenum segregates with the
highest intensity (kef. = 0.685) in sample 2, phosphorus
segregates with the highest intensity (kef. = 0.365) in
sample 4, phosphorus segregates with the highest inten-
sity (kef = 0.169) in sample 5, carbon segregates with the
lowest intensity (kef = 0.979) in sample 2, carbon segre-
gates with the lowest intensity (kef = 0.935) in sample 4
and carbon segregates with the lowest intensity (kef =
0.973) in sample 5.
An identical order of segregation follows from the
obtained values of the efficient partition coefficients (kef)
L. SOCHA et al.: STUDY OF MACRO-SEGREGATIONS IN A CONTINUOUSLY CAST BILLET
240 Materiali in tehnologije / Materials and technology 51 (2017) 2, 237–241
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 8: Positions of micro-samples taken for the combustion
analysis
Slika 8: Polo`aji mikrovzorcev za analizo z zgorevanjem
and from the data index of element segregation (Is)
when, in both cases, the process is finished by carbon.
5 CONCLUSIONS
A study of macro-segregations on the longitudinal
sections of continuously cast billets Ø 160 mm was made
without the usage of the MEMS technology. The
evaluation was aimed not only at a macro-structural anal-
ysis of the longitudinal section of a billet but also at an
evaluation of the microstructure, a quantitative X-ray
microanalysis (EDX) and an evaluation of the segre-
gation parameters on the chosen samples. Based on the
results of the structure characterisation, the following
conclusions can be stated:
• macro-etching revealed three typical areas (the chill
zone, columnar crystals and the central area of
equiaxed crystals) of the structure of a cast billet. In
the axial area, there was shrinkage porosity with a
diameter of approx. 2 mm. Local changes in the
etching response in that area were explained as the
consequence of the effect of the chemical segregation
during the billet solidification.
• the microstructure of the whole section of the billet
consisted of networks of allotriomorphic ferrite
grains along the previous austenite-grain boundaries,
needles of acicular ferrite and nodules of the pearlite
component. Immediately under the billet surface and
in the centre of the longitudinal section of the billet,
higher fractions of pearlite were observed. This
proved that carbonization occurred in these areas. In
the under-the-surface areas, it was probably due to
the reaction of steel with the cast powder. In the
centre of the longitudinal section of the billet, the
microstructural changes are connected with the
effects of the chemical segregation during solidifica-
tion.
• during the microstructural analysis of the axial area
of the billet, bands with different etching responses,
together with sulphide inclusions, were found in the
surroundings of the shrinkage porosity. The last por-
tions of the segregated melt were solidified in these
areas. As a result of the chemical segregation,
austenite decomposed into bainite in these areas.
• in the areas with a modified etching response, an
enrichment due to all the substitutional solutes,
including a significant increase in the phosphorus
concentration, was proved with an X-ray micro-
analysis. Apart from phosphorus, a high degree of
segregation was proved in the cases of molybdenum,
chromium and manganese.
• on the basis of a metallographic and micro-analytic
evaluation, a carbon analysis of the micro-samples
taken from individual bands of the macro-etching
was carried out by means of a combustion analyser to
perform a complex evaluation.
• it is evident from the distribution of the maximum
and minimum values of the segregation indexes (Is)
of individual elements that the highest values were
determined (for most of the analysed elements) in the
centre of the longitudinal section of the billet. On the
contrary, the lowest values of the segregation indexes
(Is) of individual elements were found (for most of
the analysed samples) in the area of columnar crys-
tals. This confirmed the precondition that the highest
tendency to segregation can be expected along the
billet axis, while the lowest tendency to segregation
can be expected in the area of columnar crystals and
in the chill zone.
• it is obvious that phosphorus is the most segregating
element because its value of the efficient partition
coefficient (kef) is lower than for the other analysed
elements. It was proved that the segregation index for
phosphorus (Is) reached the highest values.
• the obtained results made it possible to describe and
evaluate the material properties of the continuously
cast billet Ø 160 mm, which can be used for a result
verification and solidification-model development by
means of numerical modelling.
Acknowledgements
This paper was carried out with the support of the
project of Student Grant Competition, numbers
SP2015/78 and SP2015/70.
This work was created at the Faculty of Metallurgy
and Materials Engineering within Project No. LO1203
Regional Materials Science and Technology Centre –
Feasibility Program funded by the Ministry of
Education, Youth and Sports of the Czech Republic.
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L. SOCHA et al.: STUDY OF MACRO-SEGREGATIONS IN A CONTINUOUSLY CAST BILLET
Materiali in tehnologije / Materials and technology 51 (2017) 2, 237–241 241
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