& Development
CHINA FOUNDRY Research
Vol.11 No.1 January 2014
Formation mechanism of shrinkage and large
inclusions of a 70 t 12Cr2Mo1 heavy steel ingot
Liu Hongwei, *Fu Paixian, Kang Xiuhong, and Ma Xiaoping
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Abstract: Shrinkage cavities and large inclusions are serious internal defects of heavy steel ingot and
influence the quality of subsequent forgings. In order to remove these two types of defects, a 70 t 12Cr2Mo1
heavy ingot fabricated by vacuum carbon de-oxidation process was sectioned and investigated by means of
structure observation and EDS analysis. To further study the forming mechanism of shrinkage and inclusion
defects and find possible solutions, simulation on pouring and solidification processes was also carried out
using Fluent and ProCAST software, respectively. Results show that the shrinkage defects do not appear in the
middle-upper part of the ingot. The critical value of shrinkage cavity criterion is ascertained as 0.013 on the basis
of sectioning investigation and simulation results, which can be used in computer simulation to predict and avoid
shrinkage defects in production of 12Cr2Mo1 ingots with different weights. However, large inclusions are found
at the bottom of the ingot body. The bad thermal conditions of the ingot surface and large amount of entrained
slag are the main origin of the large inclusions. The simulation result of the pouring process shows that large
inclusions may be eliminated by combined measures of improving the top thermal condition and controlling the
height of rudimental molten steel in the ladle to above 300 mm.
Key words: 12Cr2Mo1 heavy ingot; segregation; shrinkage defect; large inclusions
CLC numbers: TG142
Document code: A
T
he internal quality of heavy ingots will influence
the quality of the final forging products. In
practical production, some forgings are frequently
rejected because of serious defects of the ingots [1-2].
With the development of large equipment used under
high temperature and pressure, in order to ensure
the reliability of the forging production in operation,
the requirement for fine steel ingots is increased. For
a steel ingot, the main internal defects include central
shrinkage cavity and porosity, macrosegregation and
large inclusions at the bottom of the ingot [3-4]. Generally,
shrinkage defects occur in the central part of the ingots
and seriously influence the quality of the products if they
can not be eliminated by forging [5]. As a means, mold
optimization through computer simulation was applied to
eliminate the shrinkage defects, but the shrinkage defect
criterion for a certain material was extremely crucial
for obtaining accurate simulation result, which can only
be obtained by combining sectioning investigation with
solidification process simulation [6]. Macrosegregation
*Fu Paixian
Male, born in 1979, Ph.D, Associate Professor. His research interests mainly
focuse on the control of the internal qualities of steel ingots and castings.
E-mail: [email protected]
Received: 2013-01-15 Accepted: 2013-08-11
46
Article ID: 1672-6421(2014)01-046-06
in a mold ingot mainly includes positive segregation,
negative segregation, A-segregation and V-segregation [7].
To reduce macrosegregation, advanced vacuum carbon
deoxidation (VCD) refining technology was applied
during steelmaking but only in some special steels due
to operation difficulty [8]. More accumulated inclusions
were observed in A-segregation or V-segregation than
in any other place. Especially, there are many MnS
inclusions. These inclusions may become the origins of
cracks and endanger the safe operation of the product.
Accumulated large inclusions in the zone of the deposit
cone are serious metallurgical problem in the bottompouring ingot, which results from the “falling of crystal
rain” from the top at the early stage of solidification [9].
The same defect was also found in top-pouring ingots,
and few studies on this aspect were found in open
literatures. Owing to its close relationship with the final
product quality, it is necessary to clarify the formation
mechanism of large inclusions at the bottom of toppouring ingots.
In this study, a detailed sectioning investigation was
performed on a 70 t 12Cr2Mo1 ingot manufactured by
VCD refining technology to study the central shrinkage
defects and inclusions of the ingot. To further study the
forming mechanism and conditions of shrinkage and
inclusion defect and find a practical possible solution
Research & Development CHINA FOUNDRY
Vol.11 No.1 January 2014
to the problem, the simulation on pouring process by Fluent
software and solidification process by ProCAST software was
carried out.
1 Experimental procedure
The measured chemical compositions of the 12Cr2Mo1 steel
are listed in Table 1. The melting and pouring process was EAF
(Electric Arc Furnace) → LF (Ladle Furnace) → VCD (Vacuum
Carbon Deoxidation) → MSD (Mold Stream Degassing), and
top-pouring process was used. The pouring temperature was
1,570 ℃. Figure 1 shows a sketch of the ingot with the main
parameters of H/D = 1.65 (H = 2,850 mm, height of ingot; D
= 1,727.5 mm, mean diameter of ingot) and TP = 7% (taper
of ingot body). The selection of these parameters were mainly
based on the consideration of gaining a sound ingot body and a
high utilization rate of ingot according to practical experiences.
It is known that the higher the H/D value, the higher the
utilization rate of ingot, but the more the shrinkage porosities
and cavities at the same time.
Table 1: Chemical compositions of 12Cr2Mo1 steel (wt.%)
C
0.051
Mn
0.33
Ni
Cr
0.090 2.240
1000
0.890
P
S
0.010 0.005
Ingot body
H
A
Mo
Hot top
200
0.147
Si
Fig. 2: Test positions for chemical composition and
inclusions in sample B
Model, which can predict the shrinkage defects. In simulation,
the initial conditions and thermal boundary condition were as
follows: the pouring temperature was 1,570 ℃, the liquidus
temperature was 1,518 ℃, and the solidus temperature
was 1,470 ℃. Heat transfer coefficients between the iron
mold and the outside and the iron mold and steel were
10 W∙m-2∙K-1 and 1,500 W∙m-2∙K-1, respectively. The heat transfer
coefficients between the insulation board and insulation brick
and the board and iron mold are both 20 W∙m-2∙K-1 [11].
The pouring process of molten steel was simulated using
VOF (volume of fluid) model of Fluent software, which can
simulate two or more immiscible fluids by solving a single
set of momentum equations and tracking the volume fraction
of fluids. Based on practical conditions, the height of molten
steel and slag was set as 2,295 mm and 150 mm respectively,
and other space above the slag, in the ladle, was set as air. The
initial conditions were that every phase keeps stationary state
and there was no heat transfer. In the simulation, the residual
of the solution was defined as below 10-6, which assures a good
numerical convergence.
3 Results and analysis
410
3.1 Sectioning investigation on middle-upper
part of ingot body
B
Tail cone of ingot
Fig. 1: Sizes of ingot (mm) and positions of samples
Two large samples, as shown in Fig. 1, were cut separately
using high temperature flame from the ingot. Sample A was
located at middle-upper part of the ingot about 200 mm
from the hot top, where shrinkage cavities and porosities
often occur. Sample B was located at the bottom of ingot,
where large inclusions often occur. The longitudinal sections
of samples from the center of the ingot were prepared for
investigation. To observe the segregation and shrinkage defects
of the ingot, macro examinations (sulfur print and cold acid
etching) were performed; the SEM morphology observation
and EDS analysis of inclusions were carried out. On the
eleven sampling positions of sample B, as shown in Fig. 2, the
chemical composition was measured, and detailed inclusions
analysis was conducted.
Figure 3 shows the macro-structure of sample A located at
middle-upper part of the ingot body after cold acid etching. In
the macro-structure, neither A-segregation nor V-segregation
is observed. Besides, no shrinkage cavity or porosity is
observed. A few uniformly distributed inclusions are
observed in sample A. The SEM and EDS analysis results
in Fig. 4 show that the inclusions are mainly complex
oxysulfides composed of Al2O3 and MnS, and the size of the
inclusions is commonly below 5 µm.
2 Simulation procedure
The solidification process of the ingot was simulated using
the commercial casting software ProCAST with Porosity
Fig. 3: Macro-structure of sample A with cold acid etching
47
& Development
CHINA FOUNDRY Research
Vol.11 No.1 January 2014
(a)
(b)
Fig. 4: SEM morphology (a) and EDS analysis (b) of inclusion in sample A
3.2 Sectioning investigation on bottom part
of ingot body
For sample B located at bottom part of the ingot body, the
macro-structure in Fig. 5 shows that there is a super-large
inclusion in the sample; it is located at position 2 in Fig. 2, i.e.,
in central zone of the bottom of ingot body. The size of the
inclusion is about Ф20 mm. In the position 10 mm under the
super-large inclusion, there is an area of approximately 45 mm
× 55 mm, which includes many accumulated large inclusions
(see Fig. 6).
Because the size of the above-mentioned inclusion was
so big, its typical part was selected for the analysis of
(a)
compositions in position 2, as shown in Fig. 7. It was observed
that the inclusion is composed of white, gray, and black
different zones. The compositions of each zone are listed in
Tables 2 and 3. From Table 2, it can be seen that the white
zone is Fe particle, the gray zone is spinel mainly consisted of
magnesium, chromium and aluminum oxides, and the black
zone includes Si, Ca, Na, Mn in addition to the chemical
composition of the gray zone. From Table 3, the compositions
of the white and gray zones in Fig. 7(b) are similar to the
results in Fig. 7(a), but the black zone is mainly composed
of MgO. The inclusions in other positions were also tested.
There mainly are two types of inclusions in these samples.
(b)
Fig. 5: Super-large inclusion in sample B with cold acid etching (a)
and sulphur print (b)
(a)
Fig. 6: Large conglomerated inclusions
at bottom of ingot (sample B)
(b)
Fig. 7: Partial morphology of bottom large inclusion in position 2 of sample B [(a) and (b) are photos observed
in different selected typical parts]
48
Research & Development CHINA FOUNDRY
Vol.11 No.1 January 2014
Table 2: Compositions of partial large inclusions in Fig. 7(a) (wt.%)
Elements
White zone
O
Na
Mg
Al
Si
Ca
Cr
Mn
Fe
Total
-
-
-
-
-
-
-
-
100
100
Gray zone
49.85
-
9.10
16.13
-
-
23.44
-
1.48
100
Black zone
47.75
5.74
3.04
7.68
18.02
12.28
0.50
4.71
0.28
100
Table 3: Compositions of partial large inclusions in Fig. 7(b) (wt.%)
Elements
White zone
O
Mg
Al
Ti
Cr
Mn
Fe
Total
-
-
-
-
-
-
100
100
Gray zone
49.20
15.61
27.12
0.70
7.36
-
-
100
Black zone
45.26
53.48
-
-
0.36
0.90
-
100
One type is global inclusions with above 50 μm in size, as
shown in Fig. 8(a); their chemical compositions are CaOSiO2-Al2O3 in the dark zone and Al2O3-MgO-Cr2O3 in the gray
zone, as shown in Figs. 8(b) and (c), and they mainly distribute
in positions 1, 3, and 6 in Fig. 2. The other type is irregular
morphology inclusions below 10 μm in size, as shown in Fig. 9;
their chemical compositions are mainly MnS in the dark zone
and Al2O3-MgO-Cr2O3 in the gray zone, as shown in Figs. 9(b)
and (c), and they are mainly distributed in other positions. The
(a)
amount of the complex oxide inclusions in Fig. 8 is more than
that of the oxysulfide inclusions in Fig. 9. The results above
indicate clearly the distribution, chemical composition, size of
inclusions at the bottom of the ingot, and also show that there
are large numbers of inclusions above 50 μm accumulated in
the central zone at the bottom of the ingot. Owing to these large
inclusions, the zone cannot be used for forgings, which decreases
the material's utilization ratio of the ingot.
(b)
(c)
Fig. 8: Morphology of global inclusion of above 50 μm in size (a) and its composition in dark zone (b) and gray zone (c)
(a)
(b)
(c)
Fig. 9: Irregular morphology of inclusions below 10 μm in size (a) and its composition in dark zone (b) and gray zone (c)
3.3 Analysis on shrinkage defects of
experimental ingot
Formation of shrinkage defects is closely related with
solidification process of the ingots. Based on the chosen mold
design, solidification process of the experimental ingot was
visualized by ProCAST software. The change of solidification
front at different solidification times is shown in Fig. 10. In
the early stage of the solidification, the solidification front
shows a U shape, which is favorable to the feeding of melt, as
shown in Fig. 10(a). With the development of solidification,
the solidification front gradually develops as a V shape.
The V shape reduces the feeding capability of molten steel.
The joint angle of the V shape in the solidification front is
smaller, between 1/3 position of ingot body [see Fig. 10(b)]
49
& Development
CHINA FOUNDRY Research
Vol.11 No.1 January 2014
3.4 Formation mechanism of large inclusions
1518.00
1514.80
1511.60
1508.40
1505.20
1502.00
1498.80
1495.60
1492.40
1489.20
1486.00
1482.80
1479.60
1476.40
1473.20
1470.00
(a)
(b)
(c)
Fig. 10: Evolution of solidification front: 3.6 h (a),
5.8 h (b), 7.7 h (c)
and 4/5 position [see Fig.10(c)], than in other zones. Thus the
shrinkage defects easily occur in this zone with bad feeding
capability. But in this zone, the simulation results show
that the mush area of the solidification front is not large, so
serious shrinkage cavities do not occur. After 4/5 position,
the joint angle of the V shape increases, which means that
there is enough thermal energy transferred from the hot top
to ensure feeding capability at the late stage of solidification.
Moreover, the result shows shrinkage defects, predicted by the
porosity model in ProCAST, would not occur when the value
of criterion of shrinkage defects was above 0.013, as shown
in Fig. 11. Since the value is very small, it also indicates
the defects would not be serious. In a word, above-analyzed
results imply the chosen mold design is suitable and could
ensure a good central quality of the experimental ingot. This is
agree well with the sectioning investigation result of sample A
that there is no shrinkage defects in middle-upper part of the
ingot.
Based on the sectioning and simulation results, the critical
value of criterion of shrinkage defects was determined as 0.013.
Through application of the criterion in computer simulation,
shrinkage defects of 12Cr2Mo1 ingots with different tonnage
weights can be predicted and avoided by suitable mold
design, which is very significant for the future simulation and
production of sound 12Cr2Mo1 ingots.
Cri.
0.013200
0.013180
0.013160
0.013140
0.013120
0.013100
0.013080
0.013060
0.013040
0.013020
Based on the above results for sample B, the inclusion above
50 μm is mainly composed of CaO, SiO2, Al2O3 and MgO.
These oxides are the main composition of slag and flux from
the top of the ingot. Besides, Na 2O is found in the superlarge inclusions in Fig. 7, which demonstrates that flux is one
of the origins of the large inclusion [11]. Moreover, the high
content of MgO should come from the liner bricks suffering
serious high erosion in the ladle. The detached MgO floats to
the surface of the molten steel, and combines with other slag.
It is crucial to understand why the slag and flux appear in the
bottom of the ingot. Thermal condition of the top molten steel
is considered to have great influence. The ingot was poured
under the condition of vacuum. After the melt was poured
into the mold, the condition of vacuum was cancelled, and the
flux was added. Owing to the heat loss on the surface of the
ingot, the molten steel froze quickly. When the flux burned
on the surface, the thermal energy will be enough to re-melt
the frozen shell. At this moment, a large amount of “crystal
rain” falls from the remelted surface shell. Suzuki [12] has
demonstrated the fact by experiment that solidified crystals
fall to the bottom of the ingot from the top. If a great deal of
slag is frozen into the shell, the burned product of flux may
react with the slag; and “crystal rain” may take them together
to the bottom of the ingot. Through the above analysis, the bad
thermal condition of the ingot surface and large amounts of
entrained slag are the main reasons for large inclusions at the
bottom of the ingot. In order to avoid the flux entrapped into
the ingot, excepting for improving the thermal condition on the
surface by suitable selection of pouring temperature and flux,
the slag must be controlled not to be trapped into the ingot.
In order to control the entrapment of the slag, the pouring
process of molten steel in ladle was reappeared by Fluent
software. Figure 12(a) shows the initial distribution of molten
steel. When the sliding nozzle is opened, the molten steel
flows out with high velocity from the outlet at the bottom of
the ladle, as shown in Fig. 12(b). When molten steel is below
300 mm in ladle, the slag above the sliding nozzle will be
entrapped into the molten steel [see Fig. 12(c)]. Because of
the strong down-flow of molten steel, the entrapped slag flows
together with the molten steel from the ladle.
Based on the simulation results obtained using Fluent,
through controlling the residual molten steel to have a height
above 300 mm in the ladle can avoid the slag being entrapped
into the mold. The combined measures of improving the top
thermal condition and controlling the height of rudimental
molten steel may be effective in solving the problem of large
inclusions at the bottom of the ingot in practical production.
0.013000
0.012980
0.012960
0.012940
0.012920
0.012900
Fig. 11: Simulated central shrinkage defects
50
4 Conclusions
The internal quality of the 70 t 12Cr2Mo1 steel ingot produced
by vacuum carbon de-oxidation (VCD) process was investigated
by detailed sectioning investigation including analysis on central
shrinkage cavity and large inclusions at the bottom of the ingot.
Research & Development CHINA FOUNDRY
Vol.11 No.1 January 2014
7.00e+03
7.11e+00
7.00e+03
6.65e+03
6.75e+00
6.65e+03
6.30e+03
6.40e+00
6.30e+03
5.95e+03
6.04e+00
5.95e+03
5.60e+03
5.68e+00
5.60e+03
5.25e+03
5.33e+00
5.25e+03
4.90e+03
4.97e+00
4.90e+03
4.55e+03
4.62e+00
4.55e+03
4.20e+03
4.26e+00
4.20e+03
3.85e+03
3.91e+00
3.85e+03
3.50e+03
3.55e+00
3.50e+03
3.15e+03
3.20e+00
3.15e+03
2.80e+03
2.84e+00
2.80e+03
2.45e+03
2.49e+00
2.45e+03
2.10e+03
2.13e+00
2.10e+03
1.75e+03
1.78e+00
1.75e+03
1.40e+03
1.42e+00
1.40e+03
1.05e+03
1.07e+00
1.05e+03
7.01e+02
7.11e-01
7.01e+02
3.51e+02
3.55e-01
3.51e+02
1.22e+00
0.00e+00
1.22e+00
(a) Contours of density (kg·m3)
(b) Contours of velocity magnitude (m·s-1)
(c) Contours of density (kg·m3)
Fig. 12: Simulation results of pouring process: initial distribution of molten steel (red), slag (green),
and air (blue) (a); starting velocity (b); slag entrapment (c)
Moreover, by numerical simulation, the forming mechanism of
shrinkage and slag entrapment in the ladle were further studied.
Some important conclusions are summarized as follows:
(1) The 12Cr2Mo1 large steel ingot can be manufactured
through the EAF → LF → VCD → MSD process. Under the
experimental conditions, no shrinkage cavity or porosity forms
in the middle-upper part of the ingot.
(2) In this experiment, the critical value of criterion of
shrinkage cavity occurrence is ascertained as 0.013 by means
of the shrinkage porosity model in ProCAST combined with
sectioning investigation results of sample in middle-upper
part of the ingot, which can be used for future simulation and
production of sound 12Cr2Mo1 ingots.
(3) The large inclusions are mainly composed of slag and flux,
which are carried together with “remelted surface shell” from
the hot top of the ingot in the early stage of solidification. The
bad thermal conditions of the ingot surface and large amounts
of entrained slag are the main reasons for large inclusions at
the bottom of the ingot. The simulated results show that the
large inclustions may be eliminated by combined measures of
improving the top thermal condition and controlling the height
of rudimental molten steel in ladle to above 300 mm.
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This research is financially supported by the Program of National Technological Cooperation and Communication
(Project 2010 DFR 70640), and Chinese National S&T Major Project (2011ZX06004-016).
51