Method for heating a metal melt

The invention concerns a method for heating a metal melt, in particular molten steel covered with a casting powder, introduced via a submerged outlet into an ingot mould of a continuous casting plant. In order to ensure uniform heat dissipation over the ingot mould and constant frictional forces between the latter and the casting shell, the heat energy is introduced at given points into the surface of the melt bath and the heat energy point on the surface of the melt bath is brought to a predetermined line.

CROSS-REFERENCE TO RELATED APPLICATION 
This application is a 371 of PCT/DE95/00427 filed Mar. 30, 1995. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a method and apparatus for heating 
molten metal and more particularly, to a method and apparatus for heating 
molten metal which has been introduced into an ingot mold of a continuous 
casting installation via an immersion nozzle, especially molten steel 
covered with a casting powder. 
2. Description of the Prior Art 
It is known from Japanese Patent Abstract JP-A-61-144 243 to remove 
solidified slag adhering to the mold wall, e.g., by means of a laser beam. 
In the continuous casting of steel, adhesion forces occur between the 
strand and the ingot mold which can lead to high tensile stresses in the 
casting shell and accordingly to cracks in the surface of the billet or 
even to a tearing off of the strand. Therefore, in the continuous casting 
of steel an oscillating movement is provided between the ingot mold and 
the strand. In vertical continuous casting, this is generally produced by 
a sinusoidal up-and-down motion of the ingot mold. This mold movement 
prevents the newly formed casting shell from sticking to the wall of the 
ingot mold. Depending on the oscillating speed and casting speed, 
frictional forces occur between the ingot mold and the casting shell. 
These frictional forces depend further on the width, length, and conicity 
or amount of taper of the ingot mold, as well as on the lubrication. In 
this regard, it has been shown that a lifting platform system at a 
determined average casting speed causes lower frictional forces than at 
high or low casting speeds regardless of the dimensions of the ingot mold. 
It may be concluded from this that the mold lift and the casting 
lubrication must be optimally adjusted to the casting conditions. 
The casting powder located on the melt has an effect on the flow of heat 
carried off along the ingot mold. The differences in the heat flux caused 
by the casting aids are most pronounced in the region of the meniscus and 
decrease toward the ingot mold outlet. It may be concluded from this that 
the thickness of the casting shell is influenced by the casting aids 
substantially only in the region of the meniscus. 
It has been shown that the heat flux density in an ingot mold increases as 
the casting speed increases. The heat carried off is at its highest in the 
meniscus. This is because the liquid steel is in close contact with the 
wall of the ingot mold and has the highest temperature in this area. With 
the extensive heat extraction, the casting shell cools off and, in so 
doing, shrinks and pulls away from the wall of the ingot mold. The type of 
casting powder and its more heat is carried off from the liquid steel in 
the ingot mold when the casting powder has a low melting point than with 
higher-melting casting powder. An even greater increase in the heat 
carried off was determined when using rapeseed oil as a mold lubricant. 
Insufficient dissipation of heat is one cause of breakout in continuous 
casting. In general, a weakening of the casting shell in the ingot mold 
precedes breakout; that is, a crack occurs in the casting shell or the 
slag has prevented the heat from being carried off through the casting 
shell. Cracks in the casting shell occur, for example, because of 
suspension during or after the overflow of the ingot mold or during 
bridging between the immersion nozzle and casting shell. 
SUMMARY OF THE INVENTION 
Therefore, the object of the present invention is to provide a method and 
apparatus which ensure a uniform dissipation of heat along the ingot mold 
and additionally ensure constant frictional forces between the casting 
shell and ingot mold. 
In the present invention, heat energy is introduce d from a heat energy 
source onto the surface of a metal melt or metal bath in a punctiform or 
concentrated point-like manner. As used herein, punctiform means that the 
heat energy is provided as a concentrated or point-like source of energy, 
as is characteristic of laser energy sources and laser beams. In a 
preferred embodiment, the heat energy source is a laser beam. The heat 
energy point provided by the heat energy source or laser beam is guided on 
the surface of the metal melt along a predefinable line or path. The 
distinguishing characteristics of a laser beam, e.g. high 
monochromaticity, coherence, parallelism and energy density, make it 
possible to heat or melt materials, including metals, within narrowly 
defined regions or areas. The quality of the laser beam depends, in part, 
on the adjustment, diameter, performance stability and focus of the laser 
beam source. In turn, the laser beam quality and its intensity influence 
the quantity of work that can be performed by a particular laser beam 
source. By varying the quantity of work being performed, the intensity of 
the laser beam can likewise be varied. 
When using steel as the material in a continuous casting process, the 
critical region or area of the material for heat dissipation is the 
concave or convex upper surface or meniscus of the metal melt. This area 
or region can be directly influenced by the laser beam, which can be 
located outside of the continuous casting or ingot mold. 
According to the present invention, a molten metal is introduced into an 
ingot mold of a continuous metal casting operation via an immersion 
nozzle. The ingot mold and immersion nozzle are generally rectangular in 
shape, both having a pair of relatively long side walls and a pair of 
relatively short side walls. The longitudinal side walls of the immersion 
nozzle are shorter in length than the longitudinal side walls of the ingot 
mold. The immersion nozzle is partially submerged in the molten metal 
within the ingot mold thereby defining a region or area on the surface of 
the metal which extends along and between the longitudinal side walls of 
the ingot mold and the longitudinal side walls of the immersion nozzle. 
This region or area extends only between the opposite ends of the 
longitudinal walls of the immersion nozzle and is referred to as the 
shadow region or area. The remaining region or area on the surface of the 
molten metal, i.e. that area beyond the opposite ends of the longitudinal 
side walls of the immersion nozzle, is referred to as the free region or 
area. 
The molten metal has a natural flow characteristic within the ingot mold 
which is determined by the specific composition of the metal. 
A casting powder placed on the surface of the molten metal further ensures 
heat dissipation from the ingot mold. 
In a preferred embodiment of the present invention, heat energy from a 
laser beam is introduced as a heat energy point onto the surface of the 
molten metal at a starting point defined at the center of the shadow 
region or area. The heat energy point is then moved from its starting 
point to a point located on the surface of the metal in the free region or 
area. In so moving the heat energy point, the starting point, end point, 
path and velocity of travel are controllable and selectable to maximize 
the heat dissipation from the surface of the molten metal. In a 
particularly preferred embodiment, the travel path of the heat energy 
point follows the natural flow characteristic of the molten metal. 
According to present invention, the heat energy which is introduced in a 
punctiform manner, is adjusted in a predefinable manner not only with 
respect to the level of its heat energy, but also with respect to its 
period of use. Thus, it is proposed to move the heat energy point in the 
regions between the immersion nozzle and the corresponding longitudinal 
side of the ingot mold edge. In so doing, the starting point, the end 
point, and the path and velocity of the heat energy source, i.e. laser 
beam, between these points can be freely selected. 
The equipment for generating the laser beam can be arranged at a safe 
location outside the ingot mold and immersion nozzle. The laser beam can 
be guided via a mirror to the desired region at the surface of the melt.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Referring now to the drawings, FIGS. 1a and 1b show, respectively, 
cross-sectional and diagrammatic views of a of the continuous casting 
arrangement 10 configured in accordance with the present invention. A melt 
S on which a casting powder G floats is located in an ingot mold 11. An 
immersion nozzle 12 is submerged in the melt S. 
A laser energy source 21 is arranged outside the continuous casting 
arrangement 10. A laser beam is guided from the laser energy source 21 via 
a laser optical system 27 onto the surface of the melting bath S on 
opposit sides of the immersion nozzle 12 via a movable central mirror 22 
and a movable external mirror 23, respectively. The laser energy source 21 
can be positioned so that the laser beam contacts the mirrors 22, 23 
directly. In an alternative embodiment, the laser energy source 21 can be 
positioned outside of the continuous casting arrangement 10 and the laser 
beam directed onto the surface of the metal melt via positionable mirrors 
24. In this configuration, a single laser energy source 21 can be used to 
contact the surface on both sides of the immersion nozzle 12 by using two 
positionable mirrors 24. The mirrors 22, 23 move in an oscillating manner 
under the control of the control unit 32 to direct the heat energy point 
from the laser energy source 21 onto the surface of the metal melt. The 
heat energy point is directed onto the surface of the metal melt beginning 
in the center of the shadow region, following a predefinable path toward 
the free region, and returning therefrom to the shadow region. 
The mirrors 22 and 23 are swivelable about an axle 26. The axle 26 is 
connected to a control unit 32 which communicates with a computing element 
31. This computing element 31 is connected by way of measurement circuits 
with a temperature gauge 33 and by way of control circuits with a laser 
energy source 21. 
Referring now to FIG. 1b, and in particular, to the embodiment illustrated 
on the right-hand side thereof, two positionable mirrors 24 are disposed 
external to the continuous casting arrangement 10 and located on opposite 
longitudinal sides of the nozzle 12. In this configuration, the laser 
energy source 21 can be directed onto the surface of the metal melt in the 
shadow region on either side of the nozzle 12. By impinging on the 
positionable mirror 24 near or proximal to the laser energy source 21, the 
laser beam can be directed onto the surface of the metal melt located at 
the top (in the figure) of the ingot mold 11. The stationary mirror 24 
near or proximal to the laser energy source 21 can then be swiveled out of 
the path of the laser beam so that the beam impinges on t he distal 
positionable mirror 24 and is directed onto the surface of the metal melt 
located at the bottom (in the figure) of the ingot mold 12. In this 
manner, the laser energy source 21 can contact the surface of the metal 
melt S on either side of the nozzle 12. 
FIG. 2a diagrammatically illustrates the path L followed by the energy 
point from a laser energy source 21 as it moves along the surface of a 
metal melt in the region or area between the ingot mold 11 and the 
immersion nozzle 12. 
FIG. 2b graphically illustrates the relationship between time and the path 
L of an energy point from a single laser energy source 21 as the energy 
point oscillates between the free regions located at opposite ends of the 
ingot mold 11. The energy point begins at the center of shadow region, 
travels toward the free region at one end of the ingot mold 11, and passes 
through the shadow region as it travels in the opposite direction toward 
the free region at the other end of the ingot mold 11. The energy point 
oscillates from one end of the ingot mold 11 to the other as it is guided 
back and forth uniformly by the movement of the mirrors 22, 23 under the 
control of the control unit 32. In this embodiment, the energy point 
contacts the surface of the melt S only on one side of the metal melt or 
bath. 
FIG. 2c graphically illustrates the relationship between time and the path 
L of two energy points from two laser energy sources 21 as each energy 
point oscillates between the center of the shadow region and the free 
regions located at opposite ends of the ingot mold 11. The energy points 
begin at the center of the shadow region and travel in opposite directions 
as they oscillate between the center of the ingot mold 11, i.e. the shadow 
area or region, and the end of the ingot mold 11, i.e. the free region or 
area. The energy points move slowly along the surface of the melt S as 
they travel from the shadow region to the free region and then return more 
rapidly from the free region to the shadow region in a jerking manner. 
FIG. 2d graphically illustrates the relationship between time and the path 
L of an energy point from a single laser energy source 21 as the energy 
point oscillates between the shadow region and the free regions located at 
opposite ends of the ingot mold 11. The energy point begins in the center 
of the shadow region and then travels slowly along the surface of the 
metal melt outward towards the free region. From the free region, the 
energy point is jerked rapidly back toward the center of the shadow 
region. The energy point then travels slowly in the opposite direction 
outward toward the free region, and is jerked back toward the center of 
the shadow region as described above.