control of centre line crack in billets

1. Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=yirs20
Download by: [FU Berlin] Date: 19 March 2017, At: 01:03
Ironmaking & Steelmaking
Processes, Products and Applications
ISSN: 0301-9233 (Print) 1743-2812 (Online) Journal homepage: http://www.tandfonline.com/loi/yirs20
Analysis and control of central cracks in the bloom
continuous casting of microalloy 49MnVS3 steel
J. Zeng, W. Q. Chen & H. G. Zheng
To cite this article: J. Zeng, W. Q. Chen & H. G. Zheng (2016): Analysis and control of central
cracks in the bloom continuous casting of microalloy 49MnVS3 steel, Ironmaking & Steelmaking,
DOI: 10.1080/03019233.2016.1228571
To link to this article: http://dx.doi.org/10.1080/03019233.2016.1228571
Published online: 09 Sep 2016.
Submit your article to this journal
Article views: 68
View related articles
View Crossmark data

2. Analysis and control of central cracks in the
bloom continuous casting of microalloy
49MnVS3 steel
J. Zeng∗1
, W. Q. Chen1
and H. G. Zheng2
In order to control central cracks in continuous casting of microalloy 49MnVS3 steel, the formation
mechanism of central cracks has been studied by analysing the element segregation, crack
morphology, hot ductility, precipitates and centre macrosegregation. It was found that the centre
macrosegregation of carbon and sulphur and the precipitation of (Mn, Fe)S in grain boundary at
the later stage of solidification could decrease the solidus temperature and enlarge the high
temperature brittle zone, meanwhile, the precipitations of MnS and Ti(C, N) in grain boundary
could make cracks easier to propagate, both of which lead to the formation of central cracks.
Based on the above analysis, the centre macrosegregation of carbon and sulphur and the
inclusions including MnS and the total in the centre of bloom were decreased by using final
electromagnetic stirring and machine soft reduction together, thus, the central cracks of bloom
were controlled successfully.
Keywords: 49MnVS3 bloom, Central crack, Macrosegregation, MnS, High temperature brittle zone
Introduction
It is well-known that the formation of internal cracks
has long been recognised as a serious problem for con-
tinuous casting of steels. It is particularly difficult to
control in the continuous casting of special steels
especially sulphur-comprising steels.1
Generally speak-
ing, internal cracks usually include the following six
types: midway cracks, triple point cracks, centreline
cracks, diagonal cracks, bending/straightening cracks
and pinch roll cracks.2
All of the internal cracks result
from high tensile strains and stresses acting on regions
of the solid shell that are in the high temperature
zone of low strength and ductility at the middle and
final stage of solidification.3
In order to prevent these
defects, much effort has been spent to investigate the
formation of internal cracks by proposing various the-
ories such as the strain theory4,5
that cracks take
place when liquid films between the dendrites are
existed as well as a localised strain caused by the ther-
mal gradient, and propose solutions to these cracks
by reducing casting speed and increasing spray cool-
ing.6,7
El-Bealy8
concluded that the effect of homogen-
eity degree of cooling pattern between the rolls is a vital
factor in the growth rate of solid shell resistance which
has a complete responsibility to affect centreline macro-
segregation and resist the mechanical stresses. Xu et al.9
pointed out that the final electromagnetic stirring
(FEMS) can effectively decrease the amount of cracks
in 1Cr13 stainless steel by reducing the gradients of
temperature and concentration during solidification of
liquid steel.
The non-quenched and tempered 49MnVS3 steel has
the advantage of excellent mechanical properties and sav-
ing thermal energy without quenching-tempering treat-
ment, which is used widely in auto parts such as the
crankshaft and connecting rod of the engine. The
upstream produced processes of the 49MnVS3 steel
were as follow: EAF → secondary metallurgy (LF +
VD) → SEN bloom caster. The continuous casting of
the sulphur-comprising 49MnVS3 steel bloom with the
section size of 320 mm × 425 mm has serious internal
cracks especially the central cracks, which make the qua-
lified ratio of flaw inspection on final product bar as low
as 20%. Centreline cracks or central cracks always appear
in the central region of the bloom section and form in the
end of solidification. The four most important variables
that affect the high temperature mechanical properties
of steel are strain rate, grain size, precipitation and
inclusion content (their size, volume fraction and distri-
bution being important).10–12
It has been found that
with the accumulation of P and S in the interdendritic
liquid, the freezing temperature of the liquid decreases
obviously, thus the internal crack tendency is greatly
increased.13
The objective of the present study is to ana-
lyse the formation mechanism of the central cracks during
solidification and control these cracks by optimising oper-
ating variables in continuous casting. The formation
mechanism of central cracks during the solidification of
1
State Key Laboratory of Advanced Metallurgy, University of Science and
Technology Beijing, Beijing 100083, PR China
2
Technology Institute of Metallurgy, Baosteel Institute, Shanghai 201900,
PR China
∗
Corresponding author, email zengjie2014@126.com
© 2016 Institute of Materials, Minerals and Mining
Received 17 March 2016; accepted 19 August 2016
DOI 10.1080/03019233.2016.1228571 Ironmaking and Steelmaking 2016 1

3. the 49MnVS3 steel bloom was investigated by analysing
the element segregation, crack morphology, high temp-
erature mechanical properties, precipitates and centre
macrosegregation. Based on the analysis, these central
cracks were controlled finally.
Experimental
The bloom was sectioned on a transverse plane, mechani-
cally machined and etched with an acid solution (50%
water and 50% hydrochloric acid, 80°C) to reveal the
macroscopic of central cracks. Specimens including
cracks which sampled from the transverse sections were
etched with the following solution: (60 g picric acid,
15 g CuCl2, 60 cm3
liquid soap and 3000 cm3
water)14
for revealing the micro-morphologies of cracks. SEM
(Model JSM-6480LV) was performed to observe the sur-
face morphology of cracks, and the elements distribution
and constituent phase of precipitates were identified by
the EDS analysis. The size, number and composition of
inclusions or precipitates in the old and improved con-
tinuous casting processes were automatically statistical
and analysis using an Aspex explorer SEM.
The chemical composition of the 49MnVS3 steel is
listed in Table 1. The element macrosegregation was
determined by a drilling in the final solidification position
(shrinkage cavity appearing) of the transverse bloom.
Each sample was drilled out 4 mm in depth with a 5-
mm diameter drill along the central longitudinal direc-
tion. The contents of the main segregation elements
including the carbon, sulphur and manganese were
detected by the infrared carbon–sulphur determinator.
For example, the macrosegregation degree of carbon
was defined as C/C0, where C is the centre carbon content
(drilling test) and C0 is the carbon content in the liquid
steel (tundish test).15
Hot tensile test specimens with dimensions of 10 mm in
diameter and 120 mm in gauge length were machined
from the bloom along the solidification direction. Tensile
tests were performed using the computerised thermal
stress/strain simulator Gleeble 3800 and the reduction of
area (RA) was measured to evaluate the hot ductility of
this steel. The specimens were heated from room tempera-
ture to 1320°C at 10°C s−1
, held for 5 min and then cooled
to the deform temperature (D.T.: from 700 to 1320°C,
total nine points) at 3°C s−1
. Specimens were held at the
D.T. for 2 min and then strained to failure at a strain
rate of 10−3
s−1
. After rupture, the specimens were
immediately quenched by water spraying to preserve the
microstructure and precipitates at the D.T. The fracture
morphology of the specimens was examined using the
SEM (MLA 250).
Results and discussion
Formation mechanism of central cracks
In order to eliminate central cracks in the 49MnVS3 steel
bloom, the formation mechanism of these cracks has been
studied by analysing the element segregation, crack mor-
phology, hot ductility, precipitates and centre
macrosegregation.
Element segregation calculation
In order to calculate the elements segregation between the
solidus and liquidus temperature range of the 49MnVS3
steel, the thermodynamic calculation software Thermo-
Calc 4.1 was used in the research. Figure 1a shows the
variation of the solid fraction in the equilibrium and
non-equilibrium solidification process, respectively. It
can be seen from the non-equilibrium solidification curves
that the liquidus and solidus temperature are 1485 and
1344°C, respectively, while the liquidus and solidus temp-
erature in the equilibrium solidification are 1485 and
1400°C, respectively. The solidus temperature in the
non-equilibrium solidification decreases by 56°C com-
pared with the equilibrium calculation results. The non-
equilibrium calculation results are closer to the actual
solidification process in which the element distributions
in the solid and liquid phases are hardly homogenised.
Figure 1b–d demonstrates the variation of the solute seg-
regations and precipitates in the investigated steel. Con-
sidering the diffusion of interstitial carbon in the solid
phase, the non-equilibrium solidification Schiel model is
applied in the calculation.16
It can be seen from the figure
that the main elements including C, S, P, Mn and Si are
strongly segregating in the solidification stage, but the
influences of S and Mn are limited in the later solidifica-
tion stage by the precipitation of MnS in 1410°C, and
the proportion of MnS accounts for 70.6% of the total
in 1400°C and 94% of the total in 1350°C. The microalloy
elements Nb and V are also accumulate in the solidifica-
tion process, but the Ti element is decreased by the pre-
cipitation of TiN which begin to precipitate in 1403°C
and accounts for more than 70% of the total in 1350°C.
It can be concluded from the above analysis that the
MnS is precipitating almost completely and a large num-
ber of TiN precipitates are formed near the solidus temp-
erature. These calculation agree well with the results from
Luo et al.17
that the MnS inclusions began to precipitate
in the solid fraction of 0.9.
Crack morphology
Central cracks or core cracks appear in the central region
of a cast section and form toward the end of solidification.
Looking at Fig. 2, the crack can be observed in the central
of the sulphur-comprising microalloy 49MnVS3 bloom
which is perpendicular to the outer-arc side and pass
across the centre with 60–80 mm in length close to the
inner-arc. The crack showing zigzag lines in the equiaxed
zone is 260 μm in width and the final position of the crack
is almost located in the junction of the equiaxed-
columnar zone. It also can be seen from Fig. 2 that the
crack in the columnar zone is along the primary dendritic
interfaces. These interface show very narrow, smooth and
almost linear edges and the crack width is almost 75 μm
in the columnar zone, and the central cracking formation
Table 1 Chemical composition of 49MnVS3 steel (wt-%)
C Si Mn P S Nb V Ti Ni Cr N
0.46 0.35 0.71 0.0075 0.055 0.022 0.094 0.034 0.21 0.19 0.014
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
2 Ironmaking and Steelmaking 2016

4. in this case is probably to be caused by the residual liquid
film between the primary dendrites. These observations
correspond to the results from other research18
that hot
cracking formation was caused by the residual liquid
between the dendrites.
Figure 3 shows the central crack surface of the steel.
The crack surface showing smooth liquid film and exhi-
biting the smooth topography characteristic of cracks of
‘hot tears’ formed in regions of these liquid films. Evi-
dence of liquid films in the interior surface of a central
1 Thermodynamic calculation of the solid fraction, solute segregation and precipitates in 49MnVS3 steel
2 The central crack in a bloom
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
Ironmaking and Steelmaking 2016 3

5. crack displayed in Fig. 3 means that the crack formed in
the mushy zone at the later solidification stage.
Hot ductility
The high temperature mechanical properties of 49MnVS3
steel were investigated through hot tensile tests. The tran-
sition temperature from ductile to brittle fracture was
obtained by measuring the RA as a function of tensile
temperature. The RA is defined as19,20
:
RA =
Ab − Aa
Ab
× 100
where Ab is the original cross-sectional area of the speci-
men before test and Aa is the cross-sectional area of the
specimen after fracture test. Besides, the tensile strength
and stress–strain curve during and after solidification
were measured to investigate the reason why hot ductility
is different under different test temperatures.
The hot ductility and stress–strain curves for the tested
steel are demonstrated in Fig. 4. As shown, the plotted
curves RA and tensile strength are measured as a function
of the temperature. According to some research
results,21,22
the temperature range in which the RA is
less than or equal to 60% is a crack sensitive range for
continuous casting, which is called the hot brittle range.
Apparently, the RA of the tested steel is significantly
smaller at a temperature over 1300°C and this zone can
be called as the high temperature brittle zone. With the
temperature increasing from 700 to 1320°C, the tensile
strength decreases from 180 to 10 MPa continuously.
Besides, it also can be seen from Fig. 4b that the
maximum strain is only 0.07% and the corresponding
stress is just 10 MPa at the test temperature of 1320°C,
and the steel failed in a completely brittle manner in
this temperature. This indicates that the transition temp-
erature between brittle and ductile fracture occurred
from 40 to 70°C below the solidus temperature. Wein-
berg20
investigated the carbon steel in high temperature
tensile tests and found the same results.
Figure 5 illustrates the fracture surface of the examined
steels, which was tensioned at 1320°C and 1000°C,
respectively. It can be observed in Fig. 5a that there exist-
ing liquid films in the crack surface and these films are
consistent with their almost zero ductility. By compari-
son, many large dimples can be seen in Fig. 5b and
these dimples are in accord with their excellent high temp-
erature mechanical properties. The high temperature zone
of low strength and ductility is at the root of most of the
cracks found in continuous casting.2
Precipitates
The variables which have a major influence on the crack
formation in the microalloyed steel are precipitates or
inclusions. The SEM mappings of element distribution
in the interior surface of a crack are shown in Fig. 6. It
can be seen that there are C, N, P and S elements enrich-
ment and lots of MnS and Nb–V–Ti (C, N) precipitates in
the crack surface. Segregating of C, S and P elements have
a harmful influence on the solidification cracking sensi-
tivity of the studied steel at high temperatures by decreas-
ing the solidus temperatures in the interdendritic zone.
Moreover, Fig. 6 clearly indicates that the manganese
3 SEM of exposed surface of a central crack with different magnifications a 180× and b 650×
4 Hot ductility of 49MnVS3 steel a RA and tensile strength and b stress–strain curve
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
4 Ironmaking and Steelmaking 2016

6. sulphides as well as the titanium carbides contain signifi-
cant amounts of niobium. As shown in Fig. 1b, the con-
tent of Nb is decreased in 1370°C by the precipitation
of diphase precipitate contains Nb. In the continuous
casting of Nb containing steels, a large part of the diphase
precipitation contains Nb in grain boundary comes out
dynamically during the straightening operation and so
can be very detrimental to ductility.23
Figure 7 demon-
strates some typical precipitates in the crack surface.
The compositions of the corresponding precipitates or
inclusions in Fig. 7 are shown in Table 2. The main pre-
cipitates are block or long strips MnS and (Ti, Nb)C,
and also including some sphere shapes (Mn, Fe)S. The
melting point of (Mn, Fe)S decreased with the increasing
of Fe content.
It could be inferred from the above analysis that the
central crack is along the grain boundary because a
large amount of precipitated phases existed. Turkdogan24
has suggested that during the later stages of solidification,
most of the N would be expected to precipitate in a rela-
tively coarse form as TiN in the interdendritic regions
which may subsequently become the austenite grain
boundaries. It is known that the grain boundary precipi-
tations are particularly deleterious. For a given volume
fraction of precipitates and/or inclusions, the more the
particles at the boundaries, the closer they are to each
other and the easier it is for cracks to interlink under
external force.23
Centre macrosegregation
The centre macrosegregation and central cracks in a ran-
domly selected 21 blooms for a period of time are
measured and demonstrated in Fig. 8. The acceptable
level of internal cracks in a bloom is of crack length less
than 20 mm which appear small when compared with
5 SEM of fracture surface at various test temperatures a 1320°C and b 1000°C
6 Element distribution in a crack surface
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
Ironmaking and Steelmaking 2016 5

7. the width of the bloom and thus could be healed after roll-
ing process. As shown in Fig. 8, the length of central
cracks is about 50–80 mm (see red dashed box) for most
of the cracking blooms. If the centre segregation degrees
of carbon and sulphur elements are over 1.30, meanwhile,
the centre segregation degree of manganese element is
over 1.05 and the Mn/S ratio is less 12, the possibility of
crack appearance is over 60%. Instead, there is little risk
of serious central cracks in a low segregation degree of
carbon and sulphur elements bloom. That is to say, a sig-
nificant positive correlation exist between centre macrose-
gregation and central cracks, thus, improving centre
macrosegregation is probably an effective measure in
terms of preventing the arise of these central cracks. It
has been found in this investigation and other’s result25
that the macrosegregation behaviour during solidification
in the mushy zone is a significant parameter of interden-
dritic crack formation.
Some researchers have studied the influence of sulphur
content and Mn/S ratio on the crack susceptibility of bil-
let, they concluded that a low (Mn/S) ratio gives way to
the formation of low-melting interdendritic liquid FeS
phases during solidification and these low-melting phases
7 Typical precipitates in a crack surface
Table 2 Chemical compositions of typical precipitates in a
crack surface (wt-%)
C S Ti Mn Fe Nb
MnS 37.92 1.77 60.31
(Ti, Nb)C 18.11 48.80 10.49 22.60
(Mn, Fe)S 37.23 55.45 7.32
8 Relation between centre macrosegregation and length of central cracks in the bloom
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
6 Ironmaking and Steelmaking 2016

8. cause the internal billet cracking during continuous cast-
ing.26–28
Kinoshita and Kuroki29
had proposed that an
increase in the Mn/S ratio has a beneficial effect on the
strain-to-fracture after quantitatively investigated the
change of FeS and MnS in sulphide inclusions and
found that the FeS content decreased with increasing of
the manganese content.
In summary, the centre macrosegregation of carbon
and sulphur elements and the precipitation of (Mn, Fe)S
in the grain boundary during the later stages of solidifica-
tion could decrease the liquidus temperature and enlarge
the high temperature brittle zone, meanwhile, the precipi-
tation of MnS and Ti(C, N) in the grain boundary could
make cracks easier to propagate, both of which lead to the
formation of central cracks.
Control of central cracks
The formation of central cracks is related to centre macro-
segregation which is strongly influenced by machine and
operating variables such as spray water intensity, casting
speed, final electromagnetic stirring and machine soft
reduction (MSR) parameters.30–32
According to the
crack formation analysis, cracking can be reduced by
decreasing the centre macrosegregation of carbon and
sulphur elements. Therefore, improving continuous cast-
ing operating variables is of great importance to control
these cracks. The conditions of the bloom continuous cas-
ter were that the casting speed was 0.60–0.70 m min−1
, of
four secondary cooling water zones and the superheat of
the molten steel was about 35–50°C. Compared with the
old process of only final electromagnetic stirring, the
improved process is using FEMS and MSR at the same
time in an appropriate solidification position. The pos-
ition of MSR is behind the FEMS.
The comparison results of element centre macrosegre-
gation between the old and improved process are shown
in Fig. 9. As shown in the figure, encouraging results
are obtained for 49MnVS3 steel bloom by using the
FEMS and MSR together. In comparison to the conven-
tionally produced bloom, the application of the improved
process leads to a lower and more homogeneous segre-
gation level. The mean centre segregation degree of car-
bon decreases from 1.43 to 1.20, the mean centre
segregation degree of sulphur decreases from 1.50 to
1.31 and the mean centre segregation degree of manga-
nese decreases from 1.08 to 1.03.
Figure 10 demonstrates the macrographs of transverse
bloom section in the old and improved process. The left
of Fig. 10 is the macrograph of the bloom transverse sec-
tion in the old process, it is clear that the central cracks
are serious. In the right photo of Fig. 10, the macrostruc-
ture of centre bloom section become homogeneous, mean-
while, the central crack is eliminated after the improved
process. Therefore, another important consequence of
decreasing these centre macrosegregation is that the central
cracks can be eliminated in the continuous casting bloom.
The size, number and composition of inclusions (or pre-
cipitates) which including MnS, Al2O3, CaO and (Nb, Ti)
C in the old and improved casting process specimens
sampled from the central transverse bloom sections were
automatically statistical and analysis using an Aspex
explorer SEM. The MnS and total inclusion distributions
in the two specimens are listed in Table 3. As shown in the
Table, in comparison to the mean size of MnS and total
inclusion in the conventionally produced bloom with cen-
tral cracks, the mean size of these inclusions decreased by
more than 70% after application of the improved process
9 Comparison of centre macrosegregation between old and
improved process
10 Macrographs of transverse bloom section in old and improved process
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
Ironmaking and Steelmaking 2016 7

9. free from cracks or defects in the bloom. Figure 11
demonstrates that the number of large size MnS and
total inclusion per unit area decreases significantly in
the no-cracking bloom (improved process), the number
of MnS per unit area with size over 3 μm decreases by
75.3%, the number of MnS per unit area with size over
10 μm decreases by 94.2% and the number of total
inclusion per unit area with size over 10 μm even
decreases by 97.8%. In conclusion, the central cracks in
the bloom can be controlled by improving the centre
macrosegregation which results in the reducing of the
number and size of MnS and total inclusion.
Conclusions
The formation mechanism of central cracks has been
studied during continuous casting of microalloy
49MnVS3 steel bloom by analysing the element segre-
gation, crack morphology, hot ductility, precipitates and
centre macrosegregation. Based on the formation mech-
anism, the central cracks were controlled finally by
improving continuous casting process. The following are
the major conclusions:
(i) The centre macrosegregation of carbon and sulphur
elements and the precipitation of (Mn, Fe)S in the
grain boundary at the later stage of solidification
could decrease the solidus temperature and enlarge
the high temperature brittle zone, meanwhile, the
precipitations of MnS and Ti(C, N) in the grain
boundary could make cracks easier to propagate,
both of which lead to the formation of central
cracks.
(ii) The formation of central cracks in the 49MnVS3
steel bloom is related to centre macrosegregation
and inclusions (or precipitates), using final electro-
magnetic stirring and MSR together in an
appropriate solidification position could control
these cracks by decreasing the centre macrosegrega-
tion of carbon and sulphur elements and reducing
the MnS and total inclusion.
References
1. G. Alvarez de Toledo, O. Campo and E. Lainez: ‘Influence of sulfur
and Mn/S ratio on the hot ductility of steels during continuous cast-
ing’, Steel Res., 1993, 64, 292–299.
2. J. K. Brimacombe and K. Sorimachi: ‘Crack formation in the con-
tinuous casting of steel’, Metall. Trans. B, 1977, 8, 489–505.
3. M. W. Young, N. H. Heung and F. J. Ye: ‘Analysis of solidification
cracking using the specific crack’, ISIJ Int., 2000, 40, 129–136.
4. H. F. Bishop, C. G. Ackerlind and W. S. Pellini: ‘Metallurgy and
mechanics of hot tearing’, Trans. Am. Foundry Soc., 1952, 60,
818–833.
5. W. S. Pellini: ‘Strain theory of hot tearing’, Foundry, 1952, 80, 125–
133.
6. R. K. Ozeki and J. D. Duke: Proceedings to be published, Metals
Society/ IRSID Conference on Continuous Casting, Biarritz, 1976.
7. W. R. Irving and A. Perkins: Proceedings to be published, Metals
Society/ IRSID Conference on Continuous Casting, Biarritz, 1976.
8. M. O. El-Bealy: ‘Spray cooling pattern and microthermomechanical
rigidity criterion for improving inner quality in continuously cast
steel slabs’, Ironmak. Steelmak., 2013, 40, 167–188.
9. Y. Xu, R. Xu, Z. Fan, C. Li, A. Deng and E. Wang: ‘Analysis of
cracking phenomena in continuous casting of 1Cr13 stainless steel
billets with final electromagnetic stirring’, Int. J. Min. Met.
Mater., 2016, 23, 534–541.
10. W. T. Lankford: ‘Some considerations of strength and ductility in the
continuous-casting process’, Metall. Trans., 1972, 3, 1331–1357.
11. Y. Maehara, K. Yasumoto, H. Tomono, T. Nagamichi and Y.
Ohmori: ‘Surface cracking mechanism of continuously cast low car-
bon low alloy steel slabs’, Mater. Sci. Technol., 1990, 6, 793–806.
12. B. Mintz: ‘The influence of composition on the hot ductility of steels
and to the problem of transverse cracking’, ISIJ Int., 1999, 39, 833–
855.
13. Z. Han, K. Cai and B. Liu: ‘Prediction and analysis on formation of
internal cracks in continuously cast slabs by mathematical models’,
ISIJ Int., 2001, 41, 1473–1480.
14. J. M. Cabrera-Marrero, V. Carreno-Galindo, R. D. Morales and F.
Chavez-Alcala: ‘Macro-micro modelingofthedendriticmicrostructure
Table 3 MnS and total inclusion distributions in the bloom, No./mm2
Inclusion type Specimen <1 μm 1–3 μm 3–5 μm 5–10 μm >10 μm Total Mean length Mean width
MnS Crack 1.52 8.63 2.29 1.47 1.39 15.29 13.75 6.56
No-crack 2.27 7.66 0.92 0.27 0.08 11.20 2.81 1.72
Total Crack 11.49 63.66 21.04 15.79 9.92 121.89 9.99 4.8
No-crack 10.37 32.15 1.57 0.47 0.22 44.77 2.73 1.39
11 The number and size variations of a MnS and b total inclusion in old and improved process
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
8 Ironmaking and Steelmaking 2016

10. of steel billets processed by continuous casting’, ISIJ Int., 1998, 38,
812–821.
15. J. Zeng, W. Chen, G. Wang, C. Cao and Y. Gao: ‘Development and
application of an off-line soft reduction model during continuous
casting of high-carbon rectangular billet’, Metall. Res. Technol.,
2015, 112, 403–411.
16. G. E. Fuchs and B. A. Boutwell: ‘Modeling of the partitioning and
phase transformation temperatures of an as-cast third generation
single crystal Ni-base superalloy’, Mater. Sci. Eng. A, 2002, 333,
72–79.
17. Y. Luo, J. Zhang, Z. Liu, C. Xiao and S. Wu: ‘In situ observation
and thermodynamic calculation of MnS in 49MnVS3 non-quenched
and tempered steel’, Acta Metall. Sin., 2011, 24, 326–334.
18. S. K. Kim, Y. D. Lee, K. Hansson and H. Fredriksson: ‘Influence of
cooling rate on the hot cracking formation of nickel rich alloys’, ISIJ
Int., 2002, 42, 512–519.
19. B. Rogberg: ‘An investigation on the hot ductility of steels by per-
forming tensile tests on “in situ solidified” samples’,
Scand. J. Metall., 1983, 12, 51–66.
20. F. Weinberg: ‘The ductility of continuously-cast steel near the melt-
ing point – hot tearing’, Metall. Trans. B, 1979, 10, 219–227.
21. B. Mintz and R. Abushosha: ‘Effectiveness of hot tensile test in
simulating straightening in continuous casting’, Mater. Sci.
Technol., 1992, 8, 171–178.
22. E. Lopez-Chipres, I. Mejia, C. Maldonado, A. Bedolla-Jacuinde
and J. M. Cabrera: ‘Hot ductility behavior of boron microalloyed
steels’, Mater. Sci. Eng. A, 2007, 460, 464–470.
23. B. Mintz, S. Yue and J. J. Jonas: ‘Hot ductility of steels and its
relationship to the problem of transverse cracking during continu-
ous casting’, Int. Mater. Rev., 1991, 36, 187–220.
24. E. T. Turkdogan: ISS Steelmaking Conf. Proc., 1987, 70, 399–415.
25. M. O. El-Bealy: ‘On the formation of macrosegregation and inter-
dendritic cracks during dendritic solidification of continuous casting
of steel’, Metall. Trans. B, 2014, 45, 988–1017.
26. H. Nakata and H. Yasunaka: ‘Influence of sulfur and manganese on
hot ductility of steel’, Tetsu-to-Hagane, 1990, 76, 376–382.
27. F. Weinberg: ‘The strength and ductility of continuously cast steels
above 800°C’, Metall. Trans. B, 1979, 10, 513–522.
28. W. T. Lankford: ‘Some considerations of strength and ductility in the
continuous-casting process’, Metall. Trans., 1972, 3, 1331–1357.
29. T. Kinoshita and S. Kuroki: ‘Influence of sulphide inclusions on
the hot-tearing behaviour of steel castings’, Imono, 1972, 44,
1080–1089.
30. J. Zeng and W. Chen: ‘Effect of casting speed on solidification struc-
ture and central macrosegregation during continuous casting of
high-carbon rectangular billet’, Metall. Ital., 2015, 7–8, 43–50.
31. J. Zeng and W. Chen: ‘Effect of secondary cooling conditions on
solidification structure and central macrosegregation in continu-
ously cast high-carbon rectangular billet’, High Temp. Mater.
Proc., 2015, 34, 577–583.
32. K. S. Oh and Y. W. Chang: ‘Macrosegregation behavior in con-
tinuously cast high carbon steel blooms and billets at the final
stage of solidification in combination stirring’, ISIJ Int., 1995,
35, 866–875.
Zeng et al. Analysis and control of central cracks in the bloom continuous casting
Ironmaking and Steelmaking 2016 9