(EPD)Effect of Different Parameters on Breakouts in

EPD Congress 2014
Edited by: James Yurko, Lifeng Zhang, Antoine Allanore, Cong Wang, Jeffrey S. Spangenberger,
Randolph E. Kirchain, Jerome P. Downey, and Lawrence D. May
TMS (The Minerals, Metals & Materials Society), 2014
EFFECT OF DIFFERENT PARAMETERS ON BREAKOUTS IN
BILLET CASTER
R J Singh, Devilal, S K Jha, Sheshank Shekhar*, E Z Chacko*, Rina Sahu
National Institute of Technology, Jamshedpur.
*Tata Steel, Jamshedpur.
Keywords: Breakouts; Billets
Abstract
Breakouts are one of the biggest problems encountered during continuous casting of steel
billets. Billets get cracked locally or break completely in two parts during solidification causing
the liquid part of the partially solidified billet to empty inside the cooling chamber. Proper
control of important parameters is necessary to avoid break outs during continuous casting. In
the present work, the nature of the break outs at a billet caster at Tata Steel was studied. Data
analysis was also done for certain casting parameters and the Sulphur tracer test was conducted
to understand solidification in the mould. It was found that to reduce the occurrence of break
outs, it was necessary to tune the operations based on the grade being cast and the design of the
mould tube being used.
Introduction
A break out during the continuous casting process is a
rupture of the partially solidified shell inside the secondary cooling chamber, causing the
contained liquid steel to spill out. In the mid 1970s while continuous casting technology was still
in early stage of development, causes of breakouts was not well understood. Today, with
technological development much has been understood about breakouts but, most of the
understanding has been in the slab casting process wherein due to the higher level of
instrumentation, a lot of information has been gained. In billet casting however, due to the
geometry of the mould housing with the tube mould, multiplied by the complexity of the
multiple strands, instrumentation has not been used and obtaining data for analysis has been
difficult.
Breakouts at billet casters are still the main hindrances to production. A break out
results in a loss in production since the strand which has had a break out is now not available to
produce and thus a six strand caster would be reduced to a five strand caster, increasing the time
for casting the same tonnage. Additionally extra set up time is required as more often than not a
mould change is required in order to have the strand casting again.
Methodology Adopted:
1.
Collection of data of breakout from CC1
2.
Visit to mould room to analyze how the breakouts occur.
3.
Inspection of mould jacket.
4.
Inspection of breakout shell.
5.
Correlating it with data and to reach a conclusion about the probable cause and
mechanism of breakout.
Factor causing breakouts in billets:
There are different parameters which cause breakouts in billet caster. Some of them are:
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a) Oil flow rate,
b) Liquid temperature,
c) Mould level,
d) Mould oscillation,
e) Steel composition,
f) Casting speed,
g) Secondary cooling in different zones.
However, by controlling some of these parameters such as casting speed, mould oscillation, oil
flow rate and super heat breakouts can be avoided to a large extent.
Identifying the phenomenon
In order to understand the phenomenon, data of the types of break outs taking place was
collected accompanied by physical inspection of the break out pieces. This was done for a five
month period. It was found that there were two modes by which break outs were taking place i.e.
Shell Breakout and Bleed Break out. 73% of the break outs were shell break outs and the rest
27% were bleed break outs. The two types of break outs are defined below.
1. Shell Break out: These break outs are typically characterized by transverse rupture of the
billet at the exit of the mould. This type of break out occurs due to sticking of the billet inside
the mould (probably at the upper portions of the mould), resulting in the development of a
local thin shell inside the mould. This local thin shell travels down the mould and when it
reaches the exit of the mould the billet ruptures in two. One part of the billet will be left in the
mould (length equal to the effective mould length) and the other part will get withdrawn by
the withdrawal rolls. The broken portion of the billet in the mould will be in the form of a
hollow shell and hence this type of break out has been called a Shell Break out. Figure 1
shows a Shell Break out shell, which was taken out of the mould tube at the mould
maintenance room after there was a shell break out.
Casting direction
Figure 1: Shell Break out shell (portion left in the mould after a shell break out)
Shell thickness: A large number of shells left over after shell break outs were cut open down
the middle (Figure 2). It was found that shell formation was unusual. At the meniscus or in the
upper regions of the mould the shell thickness was very high and at the exit of the mould the
thickness was very low. Blazek and Saucedo [6] have discussed this type of break out
describing it as a sticker break out based on laboratory experiments. They found that if there
was sticking behavior at the meniscus, the portion just below the meniscus would tear and
move down, while the shell at the meniscus would be at a standstill and thus getting thicker.
The torn portion would heal as fresh liquid would fill the gap, but would then break again,
being the weakest portion, as the withdrawal pulled the billet down (schematic in Figure 3 ).
If one were to measure the temperature of the mould surface, the loc
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ation of the torn portion would show as a hot spot, as liquid steel would come directly into
contact with the copper mould at this point. They found that the hot spot travelled down the
mould and at the point of it leaving the mould it would result in a breakout because of the
excessively thin shell.
Casting direction
Figure 2: Profile of shell left in mould (mm)
after
Figure 3: Mechanism of formation and
propagation of sticker type break outs.
After BLAZEK and SAUCEDO[1]
Deformed Oscillation marks: There were other clues as well reinforcing the thought that
sticking was the cause of shell break outs. On observing the surface of the shell left over in
the mould, it was always found that the oscillation marks, which should be horizontal and
parallel to each other, were slanted and sometimes deformed (Figure 4 below). It was inferred
that this was because of sticking at some mid-face or corner region, causing the billet to move
differentially within the mould thus causing the oscillation marks to get deformed.
Abnormal
oscillation
marks.
Figure 4: Deformed oscillation marks on the billet
shell left in the mould after shell break out
2. Bleed break out: Local rupture of the billet limited to any one face/corner of the billet (Figure
5). This type of break out usually takes place due to a longitudinal rupture on the billet surface,
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most often at the corner/off corner location. It can take place anywhere from just at the mould
exit to the withdrawal/straightening rolls.
Bleed Breakout.
Casting direction
Figure 5: Bleed break out which took place 1070mm below the meniscus (200mm below the mould exit)
Investigation of a bleed break out by sectioning: A bleed in a high carbon cast was investigated
by sectioning the hollow billet above the location of the bleed at continuous 1inch sections.
Thirty sections were cut as shown in figure 6. The sections were then machined and
photographed, figures 7 to 9. Sections 1 and 2 cracked in the longitudinal direction during the
cutting operation (figure 7). The cracks were in line with the break out location. By examining
the cracked sections it seemed that the crack was pre-existing in most of the thickness (large
dull area on the crack face) indicating that some internal longitudinal cracks were present in the
billet in the off corner region. It is possible that these cracks opened up to cause the bleed
(sections 11 to 17, figures 8 and 9). Additionally it was found that the billet was suffering from
differential solidification inside the mould, resulting in low shell thickness at the top left corner
which bled (sections 3 and 6, figure 8). This is usually caused by a mismatch between the
shrinkage of the billet and the taper of the mould.
1
30
Casting direction
Casting direction
Casting direction
Figure 6: High carbon billet with bleed break out
Location
of break
out
Figure 7: Sections 1 and 2 of
the break out piece
Figure 8: Sections 3, 6 and 11 of the break out piece
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Figure 9: Sections 13, 14, 17 and 30 of the break out piece
Investigation into shell growth during casting
Shell thickness determination using S tracer: Understanding of solid shell formation inside the
mould is one of the key issues in understanding the cause of break outs. For getting solid shell
thickness inside the mould sulphur print technique was used by adding sulphur as a tracer in the
form of FeS, in the mould during casting. FeS powder was poured into the falling stream in the
moulds of strands 4 (low speed 2.8 m/min) & 6 (high speed 3.6 m/min) in the last billets to be
cast. The strands were closed after a few seconds. 1.2 meter length pieces were then cut from
the tail ends of the cast billets which were then cut into 12 pieces of approx 100 mm each.
From each 100mm piece a 50mm sample was prepared, from the middle. Sulphur printing was
done on both the faces of the 50mm samples. As sulphur would not dissolve in the solid shell
already formed at the time the FeS was inserted, a white band would be visible on the print
representing shell thickness at the given location in the mould (Figure 10). As one goes down
the mould, the white rim increases.
After doing the prints it was found that below 400mm from the meniscus it was difficult to
differentiate the white band around the shell. This could be due to the churning caused by the
Electro Magnetic Stirrer (EMS). The EMS had its maximum intensity at around 200mm below
the meniscus and thus it is possible that it was not allowing the sulphur to go into the strand.
Nevertheless, it was seen that there was a 1mm difference in shell thickness 300mm below the
meniscus, between the strand cast at 2.8m/min and that cast at 3.6m/min, figures 11 and 12
respectively.
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Edge of billet
Sulphur in the matrix
Absence of Sulphur in the rim
Figure 10: Sulphur print of last piece of billet impregnated with Sulphur
10
8
8.2
8
7
6
6
shell thickness in mm
shell thickness in mm
10
4.5
4
4
2
0
0
100
200
300
7
6
4
5
4
4
3.2
2
0
400
0
Distance from meniscus ( in mm )
Thickness of shell(in mm)
8
100
200
300
400
Distance from meniscus ( in mm)
Face - 0
Thickness of shell (in mm)
Face - 0
Thickness of shell (in mm ) Face - 1
Thickness of shell (in mm)
Face - 1
Figure 11: Estimation of high carbon shell
growth down the mould using Sulphur tracer
technique; Casting speed 2.8m/min
Figure 12: Estimation of high carbon shell
growth down the mould using Sulphur tracer
technique; Casting speed 3.6m/min
Analysis
Data of nine months was analyzed for the significance of various factors on breakouts. During
this period 2072 heats were analyzed. The effects of Sulphur, Carbon, Phosphorus, Mould life,
and degree of superheat were analysed. It was found that in the range of operation (0.005% to
0.035%), the sulphur content of the steel did not play a significant role in either type of break out
(figures 13 and 14). Normally one would have expected some kind of trend but it is possible that
due to the low sulphur requirements of all finished products and the higher Manganese contents
of all of the grades (Mn/S ratio was > 25 in all heats) it was not an issue.
It was found that both types of break outs were most frequent while casting
rebar grade steel which had Carbon between 0.15% and 0.25% (figures 15 and 16). This it was
226
believed was most likely due to, the faster solidification (the chemistry being beyond the
pertitectic point) leading to a larger shell thickness being formed sooner in the mould. The caster
was using a funnel mould which, though having a low overall taper (0.8%/m) , had a sharp
midface taper (9%/m) at the meniscus. The thick and hard shell would get stuck in the midfaces
causing binding and then a shell break out. As seen in the sulphur tracer test (figures 10 to 12),
there was an increase in shell thickness by around 1mm at lower speeds of 2.8m/min as
compared to 3.6 m/min. An increased shell thickness would lead to increased binding. This type
of break out was more pronounced at lower speeds. Additionally, the faster solidification would
lead to increased shrinkage in the mould. This along with the low overall taper would cause gaps
to form at corners resulting in thinning of the off corner regions, causing internal longitudinal
cracks as seen in the investigation by sectioning; figures 6 to 9. Increasing the casting speeds in
this carbon range has been beneficial in reducing the extent of both shell break outs and bleeds as
there would be thinner shells and lower gaps in the mould.
As the phosphorus content increases, there can be observed an increasing trend
in the amounts of both bleed and shell breakouts (figures 17 and 18). It is possible that the
formation of Fe 3 P compound which is brittle and gets segregated causes the breakouts. The shell
once bound in the mould would rupture more easily causing a shell break out and an internal
crack would open more easily causing a bleed break out. The effect of superheat followed
predictable lines, with the both break outs getting worse with higher superheats (figures 19 to
20). An increased superheat would result in a hotter shell, which would rupture more easily
allowing both types of break outs. With an increase in mould life the bleed break outs showed an
increasing trend (figure 22). This is possible due to a loss of taper with increasing mould life,
which would cause gaps to form between the solidifying billet and the mould and thus bleed
break outs. The trend though was reverse for shell break outs (figure 21). It is possible that at low
mould lives, the mould retains its sharp midface taper and thus caused an increase in binding.
After a few heats get cast, the mould yields a little, allowing the billet to be withdrawn.
Conclusions
1.
Shell break outs take place due to binding or sticking of the billet within the mould. It is
impacted by the Phosphorus content of steel and by the superheat. Given that the Carbon range
> 0.15% and low mould lives give the maximum trouble it is possible that the sharp midface
taper of the existing mould was causing the shell break outs. An increase in casting speed in
this carbon range was seen to lower the shell thickness significantly and would be helpful to
reduce shell break outs.
2.
Bleed breakouts took place due to the opening up of pre-existing longitudinal cracks in
billets. They worsened with increasing Phosphorus, superheat and mould life. Reducing the gap
between the mould and the billet would help in even solidification of the shell and the reduction
of the occurrence of bleed break outs. With the present moulds, an increase in casting speeds
would be helpful to reduce these types of break outs.
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95%
90%
85%
80%
SHELL BO
NO SHELL
BO
Frequency of B/O
Frequency of B/O
100%
100%
99%
98%
97%
96%
S, %
Figure 13: Effect of SULPHUR on shell break
out
Figure 14: Effect of SULPHUR on bleed break
out
100%
95%
SHELL BO
90%
NO
SHELL BO
85%
80%
Frequency of B/O
Frequency of B/O
100%
99%
98%
97%
96%
SHELL BO
NO SHELL
BO
Figure 16: Effect of CARBON On bleed break
out
100%
Frequency of B/O
Frequency of B/O
80%
98%
96%
94%
92%
90%
B/O
NO B/O
P, %
P, %
Figure 17: Effect of PHOSPHORUS. On shell
break out
NO B/O
C, %
Figure 15: Effect of CARBON. On shell break
out
100%
85%
B/O
95%
C, %
90%
NO B/O
95%
S, %
95%
B/O
Figure 18: Effect of PHOSPHORUS On bleed
break out
228
Frequency of B/O
100%
98%
B/O
96%
NO
B/O
94%
92%
0-30 30-4040-50 >50
Frequency of B/O
100%
98%
98%
97%
B/O
NO
B/O
o
0-30 30-40 40-50 >50 Temp, C
Figure 20: Effect of SUPERHEAT On bleed
break out
100.0%
Figure 19: Effect of SUPERHEAT. On shell
break out
99%
99%
96%
Temp,o C
SHELL
B/O
NO SHELL
B/O
97%
Frequency of B/O
Frequency of B/O
100%
Life, hrs
Figure 21: Effect of Mould life. On shell break
out
99.8%
99.6%
99.4%
99.2%
BLEED
NO
BLEED
Life, hrs
Figure 22: Effect of Mould life On bleed break
out
References
[1] M.M.Wolf, “History of Continuous casting” in steel making Conference Proceedings, Iron
and steel society ,Warrendale, 1992.
[2]H.F.Schrewe, Continuous casting of steel, Fundamental Principles and Practice, Stahlund
Eisen, Dusseldorf, Germany.
[3] W.R. Irving, Continuous casting of steel, London, UK, Inst. of Materials, 1 Carlton House
Terrace, 1993.
[4] Continuous Casting, in the making, Shaping and treating of steel ,2, A. Cramb, ed.,
Pittsburgh, PA: Assoc. Iron & Steel Engineering, 2001.
[5]Lawson, G.D., S.C. Sander, et.al. 1994, Prevention of Shell Thinning Breakouts Associated
with Widening Width Changes, Steelmaking Conference Proceeding, S. Warrendale, PA,
Iron and Steel Society, 329-336.
[6] Kenneth E. Blazek and Ismael G. Saucedo: ISIJ Internaional, Vol 30 (1990), No. 6, 435
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