Reoxidation of Steels (PR 060 231)

Prévia do material em texto

Reoxidation Behavior in Al killed Steel 
during Casting 
 
 
Yukari Tago, Yoshihiko Higuchi, Shin Fukagawa, 
Tatsuo Kanai, Akifumi Mutoh 
 
Corporate Research & Development Laboratories 
Sumitomo Metal Industries Ltd., 
 
16, Sunayama, Hasaki-machi, kashima-gun, Ibaraki-pref., 
314-0255,Japan 
 
+81-479-46-5113(Tel) 
+81-479-46-5142(Fax) 
 
 
Key Words: secondary steelmaking; carbon steel; 
reoxidation; kinetics; mass transfer 
 
INTRODUCTION 
 
 To reduce defects in Al-killed steel, it is 
necessary to improve steel cleanliness. The origin of 
the defects is mainly Al2O3 inclusions in steel. They 
are considered to be formed by a reoxidation 
reaction between aluminum in metal and ladle slag, 
tundish slag, and air flowing into the tundish during 
casting. 
 In our previous work1, the kinetic behaviors of 
reoxidation by ladle slag and tundish slag were 
investigated in laboratory experiments, and it was 
confirmed that reoxidation behavior could be 
simulated by a Coupled Reaction Model2345. 
 In this study, reoxidation behavior during casting 
was investigated in plant experiments and the time 
variation of aluminum content in the tundish was 
then simulated with a mathematical model that 
considers all the factors of reoxidation. 
 The contribution of each factor to reoxidation 
during casting was evaluated quantitatively, and the 
most effective measure to suppress the formation of 
Al2O3 inclusions was clarified. 
EXPERIMENTAL 
 
 After 250t RH treatment, Al-killed steel was cast 
at 2 strand C.C. machine shown in Figure 1. 
 At the steady stage during casting, metal samples 
were taken at 2 positions in the tundish: (a) near by 
the stream from the ladle and (b) the outlet of 
tundish. However, just before and after ladle 
exchange (non-steady stage) sampling was done 
only at position (a) for safety reasons. 
 To examine the influence of (FeO+MnO), the 
sum of the concentrations of (FeO) and (MnO) in 
ladle slag, on reoxidation behavior, it was altered 
from 0.5 to 10wt%. 
 In addition, to estimate the quantity of air 
flowing into the tundish, gas samples were taken 
both inside and outside the pouring tube. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.1 Schematic drawing of sampling position 
during casting. 
 
EXPERIMENTAL RESULTS 
 
Change in Metal Composition at the Tundish 
 At the steady stage, soluble aluminum content, 
[sol.Al] in the tundish outlet was 0.0008% less than 
value in the inlet, whereas [Si] was 0.001% 
higher where the average values of 10ch data 
were used because each data was close to the 
analysis limit. 
Ladle
Ladle slag
Metal
Tundish
Metal
TD slag
Mold
Metal sampling position
(a) (b)
2000 STEELMAKING CONFERENCE PROCEEDINGS 231
 Figure 2 shows the change in [sol.Al]
 RH-TD: the 
change in [sol.Al] from RH to the tundish. 
[sol.Al]
 RH-TD increased for 500s after ladle opening 
and after that remained approximately constant for 
all runs. In addition, the absolute value of 
[sol.Al]RH-TD increased with increase of 
(FeO+MnO), all through casting. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.2 Change in [sol.Al] in the tundish outlet. 
 
Change in Nitrogen Partial Pressure in the 
Tundish Atmosphere 
 PN2 inside the pouring tube was as high as about 
0.8˜105 Pa at the ladle opening because air flows 
into the tube at ladle exchange, however, after that 
PN2 decreased rapidly. On the other hand PN2 
outside the tube was less than 3.0˜102 Pa all 
through casting. 
 Then it is assumed to be enough to consider the 
effect of air on reoxidation only during ladle 
exchange. 
 
DISCUSSION 
 
Reoxidation Model during Casting 
 To clarify the contribution of each factor to 
reoxidation during casting, it is necessary to 
accurately simulate the change in [sol.Al] by 
reoxidation reaction during casting. However, it is 
difficult to simulate it because a couple of reactions 
occur simultaneously during casting as shown in 
Figure 3, and metal fluidity in the tundish must be 
considered. 
 Considering these points, reoxidation model was 
developed. 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3 Factors in reoxidation during casting 
 
Simulating Metal Flow in the Tundish 
 To investigate metal flow in the tundish, 30kg 
of nickel alloy was added at the inlet of the tundish 
as a metal tracer and samples were taken at the 
outlet consecutively. 
 Figure 4 shows the change in nickel content at 
the tundish outlet after Ni alloy addition. The two 
symbols( , ) indicate the observed data at two 
outlets of the tundish shown in Figure 1. There is 
very little difference between these values. The 
solid line indicates the calculated results with 
Mixing Tanks Model, where metal flow in the 
tundish is approximated by complete mixing tanks 
in series6 as shown in Figure 5 and expressed as 
Eq.(1), 
 
[Ni]n = {p/(p-1)!} (p T/T0) exp(-p T/T0) (1) 
 
where, p is total number of tanks, [Ni]n is the 
normalized increase of nickel content at the tundish 
outlet, T is time after nickel addition, and T0 is 
metal holding time in the tundish. 
0 500 1000 1500-200
-100
0
Time after ladle opening (s)
[so
l.A
l] R
H
-
TD
 
(p
pm
)
 0.5
 6
 10
(FeO+MnO)LD(%)
85˜103kg TD
1. Ladle slag
2. Air
3. Tundish slag
2
3
2
1
2000 STEELMAKING CONFERENCE PROCEEDINGS232
 As in Figure 4, the calculated results with p=3 
shows good agreement with observed ones. Thus it 
was confirmed that metal flow in the tundish can 
be simulated well with 3-tanks model. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.4 Change in nickel content in the tundish outlet 
after nickel alloy addition. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.5 Mixing Tanks Model for simulation of metal 
flow in the tundish. 
 
Change in [sol.Al] at Ladle during Casting 
 [sol.Al] in metal flowing into the tundish 
changes because of reoxidation by ladle slag. The 
reaction rate is assumed to increase while metal is 
poured into the tundish because the relative volume 
of slag to metal increases as shown in Figure 6. 
Considering that, the change in [sol.Al] at the ladle 
was calculated with the Coupled Reaction Model, 
which was developed by Robertson et al2 as a 
reaction model for desulfurization. It was confirmed 
to be applicable for reoxidation by the ladle slag 
and the tundish slag in our previous work. For 
details of this model, the original paper2 and our 
previous report should be referred. 
 
 
 
 
 
 
 
 
 
 
 
Fig.6 Increase in the relative volume of slag to 
metal in ladle during casting. 
 
 (FeO+MnO) was altered from 1% to 15%. The 
results are shown in Figure 7. [sol.Al] decrease 
gradually until the middle of pouring, and it 
decreases drastically near the end. In addition, the 
higher the (FeO+MnO) in slag is, the more rapidly 
[sol.Al] decreases. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.7 Change in [sol.Al] at the ladle calculated with 
the Coupled Reaction Model. 
0 1 2 30
0.5
1
 No.1 strand
 No.2 strand
T/T0 ( - )
[N
i] n
 
( - 
)
85˜103kg TD
3 tank model
dCj/dt=(Cj-1-Cj)/ j Cj
V1=V2= =Vtotal/p
1= 2= = total/p
where, p: total number of tanks,
 C: concentration,
 : metal holding time,
 V: volume
1
Ci
V1
2
C1
V2
3
C2
V3
j
Cj-1
Vj
Cj
p
Vp
Cp
Slag
Metal
[%
s
o
l.A
l]/[
%
so
l.A
l] i
ni
 0 500 1000 1500 2000
 Time after ladle opening(s) 
1 
0.5 
(FeO+MnO)=1% 
5% 
10% 
15% 
250t ladle 
time
W
Sl
a
g 
/ W
M
et
al
2000 STEELMAKING CONFERENCE PROCEEDINGS 233
Evaluation of Mass Transfer Coefficient of 
Reoxidation Reaction at the Tundish 
 
 Assuming that aluminum in the steel reacts with 
silica in the tundish slag according to Eq.(2), 
 
4[Al]+3(SiO2)=3[Si]+2(Al2O3) (2) 
 
increase of [Si] from the inlet to the outlet at the 
steady stage could be calculated from the decrease 
of [sol.Al] stoichiometrically. The calculated 
increase of [Si] is approximately equal to the 
observed one. This suggests that oxygen in the 
atmosphere inside the tundish hardly affects 
reoxidation at the steady stage. 
 Then granting that aluminum is oxidized only by 
the tundish slag at the steady stage, the mass 
transfer coefficient is evaluated in the following 
way. 
 Metal fluidity in the tundish is assumed to be 
expressed by the 3-tanks cascade model which was 
confirmed to be applicable in the previous section. 
 Combining this model with the Coupled 
Reaction Model for reoxidation reaction, the 
expression : 
 
dCi/dt=(Ci-1-Ci)/(T0/p)+Ri (3) 
 
is derived,where Ci is the concentration in the metal 
in tank i (wt%), C0 is the concentration in metal 
flowing into the tundish (wt%), Ri is the change in 
concentration in tank i during unit time, calculated 
by the Coupled Reaction Model. 
 Then the mass transfer coefficient in metal, km at 
the tundish was determined from the change in 
[sol.Al] and [Si] from the inlet to the outlet of the 
tundish. As a result, the mass transfer coefficient in 
metal, km, is determined to be 6.0˜10-5 m/s. In our 
previous laboratory experiment4 it was proved that 
the mass transfer coefficient is closely related to the 
metal velocity. The relationship between them for 
the tundish is indicated in Figure 8 compared with 
the laboratory experiment results. The plot for the 
tundish fits nearly on the line obtained by using the 
laboratory results. 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.8 Relation between metal velocity and mass 
transfer coefficient. 
 
Reoxidation by Air Flowing into the Tundish 
 Reoxidation by air inside the pouring tube at 
ladle exchange was estimated as follows: Gas 
volume entrained by metal flowing from ladle into 
the tundish is calculated with a mathematical model 
developed by Cho at el. 7 Assuming that all of 
oxygen in the entrained gas reacts with aluminum in 
the metal, the change in [sol.Al] was calculated. 
 
Reoxidation Model Considering All the Factors 
during Casting 
 Combining the models in the previous section, 
the change in [sol.Al] at the tundish with time 
during casting can be evaluated by the following 
procedure. 
 First, giving zero to [sol.Al] in the tundish as the 
initial value, the change in [sol.Al] in the tundish 
during casting is calculated from Eq.(3). Then, the 
calculated [sol.Al] is substituted as the initial value, 
and the change in [sol.Al] is calculated again. This 
calculation is iterated until the values converge. In 
addition, the change in [sol.Al] according to 
reoxidation by air just after ladle opening is 
incorporated into calculation in the first tank in the 
3-tanks model. 
10-3 10-2 10-1 100
10-6
10-4
10-2
U (m/s)
k m
 
(m
/s
)
km=3.5x10-2U1.5
 10kg Cruicible
 85x103kg TD
2000 STEELMAKING CONFERENCE PROCEEDINGS234
 The calculated values are compared with 
observed ones in Figure 9. The calculated results 
shows good agreement with the observed ones at 
both the steady and the non-steady stage. The drop 
of [sol.Al] at the non-steady stage in the calculation 
reflects the drastic decrease of [sol.Al] in the ladle 
in the preceding heat. Therefore, it should be correct 
to assume that the decreasing rate of [sol.Al] at 
ladle increases as the relative volume of slag to 
metal increases as shown in Figure 7. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.9 Comparison of change in [sol.Al]RH-TD 
calculated with the reoxidation model and 
observed ones. 
 
 Altering (FeO+MnO) in ladle slag from 0 to 
15%, [sol.Al] behavior during casting is calculated. 
Figure 10 shows the relationship between 
(FeO+MnO) in the ladle slag and | [sol.Al]RH-TD| at 
steady and the non-steady state stage, which are 
represented by the value at 1100s and 60s after ladle 
exchange. The calculated values are in good 
agreement with observed ones both at the steady 
and the non-steady stage. In addition, the intercept 
of the calculated line at steady and the non-steady 
state stage indicate the reoxidation quantity by the 
tundish slag, and those for both the tundish slag 
and air, respectively. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.10 Relationship between | [sol.Al]RH-TD| at the 
steady and the non-steady stage and 
(FeO+MnO) in the ladle slag. 
 
Figure 11 shows the contribution of each 
reoxidation factor to | [sol.Al]RH-TD| at the steady 
and the non-steady stage. It can be seen that in the 
case where the (FeO+MnO) is more than 5%, 
decrease of [sol.Al] is mostly caused by reoxidation 
with ladle slag at both stages. Thus, in the operating 
conditions such as this experiment, it is wise to 
reduce (FeO+MnO) in ladle slag first of all, and 
then to take measures against reoxidation by the 
tundish slag and air. 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.11 Contribution of each reoxidation factor to 
 | [sol.Al]RH-TD| at the steady and the non-
steady stage. 
0 1000-200
-100
0
100
Time after ladle opening (s)
[so
l.A
l] R
H
-T
D 
(p
pm
)
 obs.
 cal.
85˜103kg TD
(FeO+MnO)LD=6%
 1 5 10 15 
(FeO+MnO) in ladle slag (%) 
LD slag 
Air 
TD slag 
200 
100 
0 
non-steady stage
steady stage
 
|
[%
s
o
l.A
l] R
H-
TD
| (p
pm
)
5 10 15
100
200
0
(FeO+MnO)LD (%)
[so
l.A
l] R
H
-
TD
 
(pp
m
) 85˜103kg TD
 1100s
 60s
obs. cal. Time afterLadle opening 
2000 STEELMAKING CONFERENCE PROCEEDINGS 235
CONCLUSIONS 
 
 Reoxidation behavior during casting was 
investigated in plant experiment. The change in 
[sol.Al] in the tundish during casting was simulated 
well with a calculation model considering all factors 
in reoxidaion; ladle slag, the tundish slag and air 
flowing into the tundish. And the contribution of 
each factor to the decrease of [sol.Al] at the steady 
and the non-steady stage were quantitatively 
evaluated. It became clear that reducing 
(FeO+MnO) in ladle slag is most effective to reduce 
reoxidation at both steady and the non-steady stage. 
 
REFFERENCE 
 
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Tetsu-to-Hagane,84(1998),333. 
2. D.G.C.Robertson,S.Ohguchi,B.Deo and 
A.Willis:Scaninject ,Part I,MEFOS,Sweden, 
(1983),8. 
3. S.Kitamura,H.Aoki,K.Okohira:Tetsu-to- 
Hagane,79(1993),1242. 
4. S.Mukawa,Y.Mizukami:Tetsu-to-Hagane, 
80(1994),207. 
5. S.Shinozaki,K.Mori,Y.Kawai:Tetsu-to-Hagane,67(1981),
70. 
6. S.Nagata:Mixing Principles and applications, 
Kodansha Ltd.,Tokyo,(1975),234. 
7. T.Cho,K.Iwata,M.Inoue:Tetsu-to-Hagane, 
68(1982),2461. 
2000 STEELMAKING CONFERENCE PROCEEDINGS236