Meijie_et_al_2011
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Meijie_et_al_2011

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Wuhan University of Science and Technology, the Hubei Province Key Laboratory of
Refractories and Ceramics, Wuhan 430081, China
(Received 20 January 2011; accepted 20 April 2011)
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Gas blowing at the bottom of tundish is an efficient metallurgy technique in clean steelmaking. In
this paper, the removal of small size inclusions in the gas bottom-blowing tundish was studied by
numerical simulation and industrial practice. The residence time distribution (RTD) of molten steel
in the tundish was calculated by mathematical modeling. The content of small size inclusions in the
slab was analyzed using a oxygen probing and metallographic images. The results show that the
molten steel characteristics obviously change when applied gas bottom-blowing, the average
residence time of molten steel in the tundish prolongs more than 100s and the dead volume fraction
decreases about 5%. Therefore, the removal efficiency of small size inclusions greatly increases
because of bubbles attachment and long moving path. Industrial experiment results show that the
average inclusions content of less than 20μm decreases more than 24%, the average overall oxygen
content decreases about 15% when controlling the reasonable blowing parameters.
Keywords: Tundish; Inclusion removal; Gas blowing; Modeling; Industrial practice.
#Corresponding author: major6886@gmail.com
Journal of
Mining and
Metallurgy
J. Min. Metall. Sect. B-Metall. 47 (2) B (2011) 137 - 147
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The tundish plays an important role in
cl e a n st e e l ma k i n g b y r e mo v i n g s o m e
unwanted inclusions from molten steel. So,
various kinds of flow control devices (FCD),
such as dam, weir, baffles and turbulence
inhibitor (TI) have been developed in recent
de c a d es [1 - 3 ] . I n a dd i t i on , so m e ne w
me t h o ds ha v e al s o b ee n pr o p os e d in
continuous casting process including plasma
heating for maintaining appropriate casting
DOI:10.2298/JMMB11012006M
st ee l te mp er a tu re [ 4,5], c en tr if u ga l f l ow
tundish [6], swirling flow tundish [7] and gas
bottom-blowing [8-10]. In these renovations,
ga s bo t to m - b lo w i n g ha s at t ra c t e d mo r e
at t e n ti o n s be c a u se o f h ig h e r re m o v al
efficiency for small size particle inclusions
an d l ow in ve st me nt . Th e i ne rt ga s w as
injected through porous brick at the bottom
of tundish. Gas bubble curtains are formed.
Wei et al [11] have demonstrated through the
cold model study that when the particles are
not wetted by the liquid, they can be captured
by gas bubbles and floated up to the free
surface. Solid inclusions such as alumina and
silica are not wetted by the liquid steel and
therefore can be removed by attachment to
gas bubbles. Wang et al [12] have developed
a m a th e ma t ic a l m o de l to d et e rm i ne t he
optimum bubble size for the removal of
inclusions from molten steel by floation. The
model suggests that the optimum bubbles
sizes for the removal from steel of alumina
inclusions smaller than 50×10-6 m in size are
in the range of 0.5 to 2×10-3 m in diameter.
On th e b a s i s o f s tu d i e s o f p ar t i c le - g a s
intercation in water based mineral processing
systems, Lifeng Zhang and S. Taniguchi [13]
di s c u ss e d th e m ec h a n is m o f i n c lu s i o n
removal by bubbles. The overall process of
inclusion floation in steel by a gas bubble
ca n be d iv i d e d i n to s ub p r o ce s s e d: ( 1)
approach of a bubble to an inclusion; (2)
formati on of a thin li quid fil m betwee n
inclusion and bubble; (3) oscillation and / or
sl i d i ng of th e in c l us i o n o n th e b u b b le
surface; (4) drainage and rupture of the film
with the formation of a dynamic three-phase
co n ta c t; ( 5) s ta b il i sa ti o n of t he b ub b le -
inclusion aggregate with respect to external
stresses; (6) flotation of the bubble-inclusion
ag g r e ga t e . T he n a s im p l e m a t h em a t i ca l
mo d e l of i n c lu s i o n r e m o va l b y bu b b le
flotation was described.
The contribution of gas bubbling curtains
in the tundish to the removal of small size
particles has been verified by physical and
ma t h e ma t i c al mo d e li n g [1 4 - 1 6] . I n t h i s
subject, the flow field, RTD curves and
inclusion removal efficiency were analyzed
by mathematical modeling at first, then the
industrial experience was experimented in
Laiwu IRON and Steel Lmtd. The average
Z. Meijie / JMM 47 (2) B (2011) 137 - 147
138
Fig. 1. Geometric dimensions of the industrial tundish
contents of overall oxygen and small size
(<20μm) inclusions were detected.
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Th e s el e c t ed t u nd i s h f o r t h e p r e s en t
analysis is a 30t tundish of Lai Wu Iron &
Steel Ltd., China. The dimensions of this
tundish is shown in Figure 1. This two-strand
tun dish was equ ippe d wi th a tur bul ence
inhibitor (TI) and a pair of baffles whose
dimensions were shown in Figure 2 and
Figure 3 respectively. Under steady state
condi tions , the liqu id st eel d epth in th e
tundish is 0.96m. The inner diameters of
la d le s h ro u d a n d su b me rg ed n o zz l e a r e
70 m m an d 5 0m m r es p e c ti v e l y. Th e
nu mer ic al si mu lat io n w as pe rfo rm ed fo r
casting slabs (175×1260mm) with casting
speed 1.4m·min-1.
In order to study the effect of gas bottom-
blowing on removal of inclusions, a pair of
porous plugs were established on the bottom
of the tundish. Dimensions of the plug were
shown in Figure 4.
  3'$, 3(" + ,.#$+
.-31.+ $04 3(.-2 A three-dimensional,
steady, turbulent bubbly flow in the tundish
Z. Meijie / JMM 47 (2) B (2011) 137 - 147 139
Fig. 2. Geometric dimensions of the TI
Fig. 4. Schematic of gas curtain bricks for industrial tundish
Fig. 3. Geometric dimensions of the baffle