[0001] The invention relates to a continuous cast method, wherein a melt is supplied into
a frame-shaped mould through a top opening of the mould by a nozzle having at least
two opposing ports, until a surface level of the melt reaches a steady state position
above the ports, and wherein in a steady state condition, an at least superficially
solidified casting is drawn through a bottom opening of the mould opposite to the
top opening, at a velocity corresponding to the flow rate of the melt, thus basically
keeping the surface level in the steady state position.
[0002] The continuous strip cast process is well-known since the mid of the 19
th century and is currently employed with the major part of steel production world-wide,
including plain carbon, alloy and stainless steel grades in different shapes and sizes.
These include large rectangular slabs (having cross sections from 0.5 cm x 50 cm up
to 25 cm x 220 cm) and basically square blooms (up to 40 cm x 60 cm), small square
or circular billets □ (10 cm up to 20 cm of diameter) as well as other, e.g. dog -bone
shapes.
[0003] The melt flows from a ladle to a tundish and further through a ceramic nozzle that
is attached to the tundish into the mould. Once in the mould, the melt freezes against
the water-cooled walls of the bottomless copper mould to form a thin solidified shell
of the casting. After exiting the mould, the shell forms a container supporting the
remaining liquid melt in the casting. While the tundish provides a continuous stream
of melt to the mould, even during exchange of the ladle, the process runs in steady
state.
[0004] In modem continuous slab cast assemblies, the mould is assembled from four separate
copper walls: Two broad walls are fixed to the assembly and two narrow walls are movable
towards an axis of symmetry of the mould, thus allowing adjustment of the slab width
even during the cast process. After machining and assembly of the mould, the gaps
between the broad and the narrow walls are limited to about 0.3 mm. In operation,
thermal deformation as well as wearing of the copper walls allow these gaps to increase
up to 1.5 mm without impacting the cast process. In steady state condition of the
continuous cast process, the melt surface as well as the surface of the thin shell
of the casting is covered by liquid slag, both thermally insulating the copper walls
from the steel melt and lubricating the gap between the mould and the casting.
[0005] The nozzle has lateral ports, forcing the melt stream to protrude at basically right
angles to the narrow walls of the mould, thus inducing a forced flow in the melt.
Opposing pairs of drive rolls underneath the bottom opening of the mould continuously
both withdraw the casting from the mould at a velocity (or casting speed) corresponding
the flow rate of t he melt into the mould and bend the casting from the initially
vertical to a horizontal direction. Further solidification of the casting is driven
by cooling water or air mist sprays between the rolls.
[0006] For starting the cast process, the bottom opening is plugged with a dummy bar□ and
the empty mould is filled with melt like in a conventional mould cast. While the surface
level of the melt rises to a previously defined steady state position above the ports
of the nozzle (also SEN, Submerged Entry Nozzle)□ , the shell begins to solidify both
on top of the dummy bar and on the walls of the mould. After the surface level reaches
the steady state position, the dummy bar is withdrawn from the bottom opening carrying
along the casting by means of the drive rolls.
[0007] The most critical phase of the starting procedure of the known process is the initial
filling of the empty mould through up to 1.5 m of plain air: The melt splatters into
the mould and spills onto the cooled copper walls, primarily to the narrow walls of
the mould. Striking the cold walls, small spillings of melt abruptly become solid,
forming sharp-edged shapes that above all firmly adhere to the walls. A similar effect
regularly occurs in flying tundish change during the cast process as provided by modem
cast processes in order to significantly reduce downtime of the assembly: While the
melt surface inside the mould falls below the ports level, the newly starting melt
stream again falls up to 1.2 m through plain air.
[0008] Because these solidified spillings are mainly embedded to the insulating slag instead
of the streaming melt in the continuous cast process, they mostly do not melt, but
instead not only obstruct the relative movement between the mould and the casting,
but they are scratching the thin solidifying shell. Failing to close, the resulting
notches extend over the length of the casting. Apart from the visible quality defect,
these notches are severe weakness points in the thin shell of the casting: In particular
when bending the casting from vertical to horizontal direction, the thin shell may
burst open in these notches, spilling the liquid metal from inside both over the drive
rolls and over nearby parts of the casting assembly. This dreaded accident (breakout
accident)□ regularly involves not only interruption of the cast process, but damages
the assembly, requires repair and tidying and overall causes serious loss of productivity.
[0009] Furthermore, the above mentioned solidified spilling mostly occur around the narrow
walls and in particular near the mechanically delicate gaps between the narrow and
the broad walls of the mould. Spillings to the gaps not only boost wearing of the
mould when adjusting the slab width but are in particular initial nuclei for formation
of skull inside the mould.
Problem To Be Solved
[0010] The invention intends to reduce spillings and related drawbacks during the starting
procedure.
Solution
[0011] Based on the previously known continuous cast method, the invention suggests that
a splash shield is mounted to the nozzle and prevents spillings of the melt protruding
from the ports from hitting the mould, and that the splash shield is molten by the
surrounding melt at least in the steady state condition. The lost splash shield of
the invention is effective only during the most critical phase of the starting procedure,
namely while the melt is initially filling the mould through plain air. In this period,
it shields the walls of the mould from melt spillings protruding from the nozzle ports.
While the melt rises over the ports, the splash shield is molten and itself becomes
unrecognizable part of the melt. For the following continuous process, the ports of
the nozzle are unshielded, thus inducing the necessary flow into the melt in the mould.
[0012] In a preferred embodiment of the invention, the splash shield deflects a melt stream
basically protruding from one of the ports at right angles to a wall of the mould,
to a direction basically parallel to the wall. The stream of melt thus is guided towards
the dummy bar at the bottom of the mould, where spillings forming notches do not impact
the thin shell of the casting.
[0013] Preferably the splash shield further deflects the melt stream to an axis of symmetry
of the mould. Streams of melt protruding from different ports of the nozzle thus are
guided towards each other, and are mutually reducing flow rate and impetus. The resulting
steadied stream of melt produces less spillings and rather flows than splatters into
the mould.
[0014] In a further preferred embodiment of the invention, the nozzle is attached to a bottom
of a tundish, that is filled with the melt from a ladle. Using a tundish instead of
filling the mould directly from the ladle allows for providing a continuous stream
of melt to the mould, even during exchange of the ladle.
[0015] In another preferred embodiment of the invention, the casting is drawn vertically
from the mould and bent to a horizontal direction through paired support rolls. In
this inventive cast method the initial casting direction equals to the direction of
gravitational force, providing uniformity of the forced flow of melt inside the mould
and the of the final casting.
[0016] The invention further suggests a splash shield for use with one of the above described
methods, comprising a bar for penetrating the opposing ports of the nozzle and thus
mounting the splash shield to the nozzle. After inserting the bar through the nozzle,
the splash shield is firmly attached to the nozzle in a very simple manner.
[0017] In a preferred embodiment of the invention, the bar has a tubular shape with a centred
inlet opening for the melt and outlet openings for the melt at both ends. The melt
thus flows through the bar. Where the bar only forms the splash shield, handling,
in particular mounting of the shield to the nozzle is significantly simplified.
[0018] In a further preferred embodiment of the invention, the splash shield has an annular
ring for surrounding the nozzle above the ports, and further has deflectors being
attached to the annular ring, wherein in a mounted position of the splash shield the
deflectors are assigned to the ports for deflecting melt streams from the ports to
an axis of symmetry of the mould. This splash shield provides for significantly steadying
the stream of melt as described above.
[0019] The invention not only provides for better surface quality of the casting product
and enhances the productivity of the process by reducing the risk of shell bursts,
but also significantly eases the starting of a continuous strip cast process.
[0020] Apart from the starting of the continuous strip cast process, the splash shield of
the invention may also be effectively applied with flying tundish change, wherein
the surface level of the melt is allowed to fall below the top edge of the ports of
the nozzle.
Exemplary Embodiments
[0021] In the following, the invention will be illustrated in exemplary embodiments. As
shown in
- Fig. 1a
- a sketch view of a continuous cast assembly,
- Fig. 1b
- a sketch detail of the cast assembly, showing the nozzle inside the mould and
- Fig. 1c
- a perspective detail of the nozzle inside the mould,
- Fig. 2a
- a first inventive splash shield,
- Fig. 2b
- the first splash shield mounted to the nozzle and
- Fig. 2c
- a flow view of the melt through the first splash shield,
- Fig. 3a
- a second inventive splash shield,
- Fig. 3b
- the second splash shield mounted to the nozzle and
- Fig. 3c
- a flow view of the melt through the second splash shield.
[0022] The continuous cast assembly shown in figure 1a and in detail in figures 1b and 1c
includes a ladle 2 and a tundish 3 underneath the ladle 2. The ladle 2 provides steel
melt 4 to the tundish 3. A ceramic nozzle 5 is mounted to the tundish 3 and protruding
through a top opening 6 of a frame-shaped mould 7, ending between the copper, water-cooled
walls 8 of the mould 7.
[0023] From the tundish 3, the melt 4 is provided into the mould 7 through two opposing
ports 9 of the nozzle 5. In steady state condition of the cast process (as shown in
figures 1a and 1b), the surface level 10 of the melt 4 is basically kept in a defined
steady state position above the ports 9. Inside the mould 7, at the cold surface 11
of the walls 8, the melt 4 solidifies to form a thin shell 12 of the casting 13.
[0024] Underneath a bottom opening 14 of the mould 7, the assembly 1 includes a sequence
of paired drive rolls 15 for withdrawing the casting 13 from the mould 7, and for
bending it from vertical to horizontal direction. Along with the drive rolls 15, the
casting 13 is cooled by means of water sprays (not shown in the drawing).
[0025] The first embodiment of a splash shield 16 as shown in figure 2a is welded from steel
sheets of 3 mm thickness to form a bar 17 with tubular shape. The shield 16 has a
length 18 of 40 cm and a squared cross-section of 64 mm of height 19 and 54 mm of
width 20. The splash shield 16 has a brick-shaped stopper 21 welded to its upper surface
22, an inlet opening 23 in the upper surface 22 and two outlet openings 24 in the
lower surface 25 at both ends 26.
[0026] Before starting the cast process, the splash shield 16 is inserted through the ports
9 into the nozzle 5 until the stopper 21 hits the surface 27 of the nozzle 5 as shown
in figure 2b. After assembly, a slide gate (not shown) underneath the tundish 3 is
opened, the melt 4 flows through the inlet opening 23 into the shield 16 and flows
out of the shield 16 through the outlet openings 24 as shown in figure 2c.
[0027] The alternative second embodiment of a splash shield 28 as shown in figure 3a is
evenly welded from steel sheets of 3 mm thickness. The second splash shield 28 has
an annular ring 29 having a diameter 30 of about 14 cm and carrying two deflectors
31. The deflectors 31 are box-shaped having a height 32 of 16 cm and width 33 of 15
cm. The outward surfaces 34 of the deflectors 31 are arranged to have a distance of
about 33 cm. The second splash shield 28 has a separate bar 35 from a bevelled steel
sheet of 3 mm thickness and 5 cm width 36. The deflectors 31 have slits 37 for inserting
the bar 35 to the splash shield 28.
[0028] Before starting the cast process, the nozzle 5 is positioned into the annular ring
29 of the alternative splash shield 28 and the bar 35 is inserted through the slits
37 and through the ports 9 into the nozzle 5 until a stopper 38 of the bar 35 hits
the outward surface of the deflector 31 as shown in figure 3b. After assembly, the
melt 4 first is guided by the bar 35 to flow into the deflectors 31 and afterwards
to flow out of the splash shield 28 as shown in figure 3c.
[0029] Both the first splash shield 16 and the alternative second splash shield 28 are molten
at least after they are below surface level 10 of the melt 4, thus becoming part of
the melt 4 themselves.
[0030] In the figures there are
- 1
- Assembly
- 2
- Ladle
- 3
- Tundish
- 4
- Melt
- 5
- Nozzle
- 6
- Top opening
- 7
- Mould
- 8
- Wall
- 9
- Port
- 10
- Surface level
- 11
- Surface
- 12
- Thin shell
- 13
- Casting
- 14
- Bottom opening
- 15
- Drive roll
- 16
- Splash shield
- 17
- Bar
- 18
- Length
- 19
- Height
- 20
- Width
- 21
- Stopper
- 22
- Upper surface
- 23
- Inlet opening
- 24
- Outlet opening
- 25
- Lower surface
- 26
- End
- 27
- Surface
- 28
- Splash shield
- 29
- Annular ring
- 30
- Diameter
- 31
- Deflector
- 32
- Height
- 33
- Width
- 34
- Outward surface
- 35
- Bar
- 36
- Width
- 37
- Slit
- 38
- Stopper
1. Continuous cast method, wherein a melt (4) is supplied into a frame-shaped mould (7)
through a top opening (6) of the mould (7) by a nozzle (5) having at least two opposing
ports (9), until a surface level (10) of the melt (4) reaches a steady state position
above the ports (9), and wherein in a steady state condition, an at least superficially
solidified casting (13) is drawn through a bottom opening (14) of the mould (7) opposite
to the top opening (6), at a velocity corresponding to the flow rate of the melt (4),
thus basically keeping the surface level (10) in the steady state position, characterized in that a splash shield (16, 28) is mounted to the nozzle (5) and prevents spillings of the
melt (4) protruding from the ports (9) from hitting the mould (7), and that the splash
shield (16, 28) is molten by the surrounding melt (4) at least in the steady state
condition.
2. Continuous cast method as claimed in the preceding claim, characterized in that the splash shield (16, 28) deflects a melt (4) stream basically protruding from one
of the ports (9) at right angles to a wall (8) of the mould (7), to a direction basically
parallel to the wall (8).
3. Continuous cast method as claimed in the preceding claim, characterized in that the splash shield (28) further deflects the melt (4) stream to an axis of symmetry
of the mould (7).
4. Continuous cast method as claimed in one of the preceding claims, characterized in that the nozzle (5) is attached to a bottom of a tundish (3), that is filled with the
melt (4) from a ladle (2).
5. Continuous cast method as claimed in one of the preceding claims, characterized in that the casting (13) is drawn vertically from the mould (7) and bent to a horizontal
direction by means of paired support rolls (15).
6. Splash shield (16, 28) for use with one of the above claimed methods, comprising a
bar (17, 35) for penetrating the opposing ports (9) of the nozzle (5) and thus mounting
the splash shield (16, 28) to the nozzle (5).
7. Splash shield (16) as claimed in the preceding claim, characterized in that the bar (17) has a tubular shape with a centred inlet opening (23) for the melt (4)
and outlet openings (24) for the melt (4) at both ends (26).
8. Splash shield (28) as claimed in claim 6 or 7, characterized by an annular ring (29) for surrounding the nozzle (5) above the ports (9), and further
characterized by deflectors (31) being attached to the annular ring (29), wherein in a mounted position
of the splash shield (28) the deflectors (31) are assigned to the ports (9) for deflecting
melt (4) streams from the ports (9) to an axis of symmetry of the mould (7).