Process and machine for the continuous casting of a thin metal product

A process and machine for the continuous casting of thin steel products, in which the inner casting space (13) of the mold (1) is given a substantially spindle-shaped cross-section which forms a central part (17) of large thickness (L), making it possible to introduce a jet of molten metal under normal conditions, and having on its sides (18) a small thickness (1) corresponding to that of the product to be produced (53). In a thickness-reducing stand (2) directly at the exit of the mold (1), the central part (17) of the product is progressively flattened to a thickness (1) equal to that of the sides.

FIELD OF THE INVENTION 
The invention relates to a process and a machine for the continuous casting 
of thin metal products especially steel products in the form of strips. 
BACKGROUND OF THE INVENTION 
Various processes for continuous casting of metal products are known in the 
art, but in comparison with casting of non-ferrous alloys, the continuous 
casting of steel is particularly difficult. The process used normally 
involves casting the steel in an open-ended mold with cooled walls 
limiting an inner tubular space, in which the product forms, the latter 
consisting of a liquid core surround by a skin consisting of a solidified 
surface shell. This product is discharged continuously through an orifice 
located at the exit of the mold, the latter usually having a vertical 
axis, and the product subsequently passes into a secondary cooling device, 
in which final solidification of the product takes place and which 
therefore has means of retaining and means of cooling the solidified faces 
of the product, the assembly as a whole being placed inside a frame 
forming a sort of tubular jacket surrounding the product, until 
solidification is completed. 
To ensure continuity of casting, the metal, delivered in the molten state 
in a casting ladle, is poured into an intermediate vessel which makes it 
possible to ensure continuity of casting during the replacement of the 
ladle and which is itself provided with a casting hole, the discharge rate 
of which is adjusted in the conventional way by means of a spout and via 
which the metal is poured inside the mold. To prevent oxidation of the 
metal, a submerged nozzle is generally used, comprising a casting tube 
which penetrates the mold and which dips into the bath of liquid steel. 
This process is now perfected as far as concerns the continuous casting of 
products of varied dimensions, in particular bars often called "billets" 
or sheets called "slabs". In this case, the inner tubular space of the 
mold has a rectangular shape limited by two wide walls and two narrow 
walls which correspond respectively to the long and short sides of the 
product. However, for casting under good conditions, and particularly to 
allow penetration of said nozzle into the mold, the slab must have a 
minimum width determining the thickness of the slab. To date, the minimal 
thickness of industrially cast slabs is approximately 150 mm. 
The problem of the continuous casting of a thin product, i.e., a product of 
sufficiently small thickness to pass directly into a rolling mill, has 
been studied for a long time, even since the very start of research in 
continuous casting. However, the various processes heretofore proposed are 
generally based on new techniques, for example, wheel systems or 
cooled-strip systems, and have not yet been adopted for industrial use. 
It has also been proposed to use these machines of the conventional type 
modified for the casting of thin strips. 
For example, U.S. Pat. No. 2,564,723 proposes using a mold, the lower part 
of which has a flattened rectangular cross-section corresponding to that 
to be imparted to the product and the upper part of which is flared in the 
form of a funnel, so as to provide a widened central part making it 
possible to introduce the casting tube. 
Such an arrangement solves the problem of introducing the casting tube, but 
does not make it possible to cast steel continuously on an industrial 
scale. In fact, after the filing date of the patent, it was found that, 
during casting, the mold had to be driven in vertical oscillation 
movements making it possible to prevent the solidified shell from adhering 
to the walls of the mold. The arrangement described in U.S. Pat. No. 
2,564,723 does not allow such oscillating movements. 
To solve this problem of oscillations, French Patent No. 1,505,630 also 
envisages a mold in the form of a funnel, but in which the wide walls 
converging downwards are composed of sectors of a cylindrical casing, 
allowing a circular oscillating movement. Such an arrangement is difficult 
to put into practice and, like the preceding one, assumes that the product 
is completely formed at the exit of the mold. Consequently, there is a 
risk of blockage as a result of a wedge effect, with pulling forces 
exerted in a particularly sensitive zone of the installation. 
SUMMARY OF THE INVENTION 
The subject of the present invention is a new process and machine making it 
possible to produce thin products while at the same time preserving the 
advantages of conventional machines, i.e., in general terms, forming the 
solidified shell in a mold with cylindrical walls having a generatrix 
parallel to the casting axis and maintaining a core of molten metal within 
the product at the exit of the mold, complete solidification taking place 
in a secondary cooling device comprising means of cooling and retaining at 
least the large faces of the product until complete solidification. 
To make it possible to introduce the casting tube into the mold, the 
latter, as in the arrangement described above, has a widened central part 
narrowing towards the sides to a thickness equal to that of the thin 
product to be cast. This cross-section is preserved up to the exit of the 
mold, the latter having cylindrical wide walls with generatrices parallel 
to the casting axis and to the axis of oscillation. 
According to the invention, progressive flattening of the central part of 
the product is carried out to a thickness equal to that of the edges in a 
thickness-reducing stand located immediately downstream of the exit 
orifice of the mold and having two respective guide and straightening 
surfaces for the two large faces of the product, the reduction profile of 
which is determined as a function of the casting speed and the cooling 
conditions, so as to exert a pressure distributed uniformly over the 
entire surface, at the same time causing and controlling, on each of the 
big faces, the symmetrical movements of two straightening zones from the 
edge towards the axis, at a speed the respectively axial and radial 
components of which are substantially constant and coordinated with the 
casting speed, in order to realize a straightening of the large faces of 
the product up to complete solidification. 
Preferably, the mold is given an inner profile such that the large faces of 
the product are connected to the small faces at the edge by means of end 
parts arranged in planes parallel to the casting line and intersecting the 
small faces of the product perpendicularly, and in the thickness-reducing 
stand straightening of the central part is carried out with progressive 
widening of the plane end parts, the latter being maintained perpendicular 
to the small faces. 
According to another advantageous characteristic, the profile of the 
surfaces retaining the longitudinal faces of the product is determined as 
a function of the casting speed and the cooling conditions, so that each 
straightening zone moves towards the axis at a radial speed substantially 
equal to the radial speed of widening of the already solidified end part 
of the product and that the complete solidification of the product and the 
straightening of the large faces are realized substantially at the same 
moment at the exit of the thickness-reducing stand. 
The invention also comprises a machine for continuous casting of thin 
products, comprising a mold with a widened central part narrowing towards 
the two sides to a thickness of the order of that of the strip to be cast, 
and a secondary cooling device which, according to the invention, 
comprises a thickness-reducing stand located directly below the mold and 
equipped with retaining means defining two substantially continuous guide 
surfaces for the large faces of the product and delimiting an inner 
passage space for the product, the said thickness-reducing stand having on 
its sides a constant thickness equal to that of the sides of the mold and 
in its central part a thickness decreasing progressively in the casting 
direction between an inlet orifice of a cross-section identical to that of 
the exit orifice of the mold and an exit orifice of rectangular 
cross-section corresponding to that of the thin product of be cast. 
Preferably, in the thickness-reducing stand, the strip-shaped product is 
left free to widen towards the sides as a function of the flattening of 
the central part, in such a way that the perimeter cross-section relative 
to the axis remains preserved. 
In an especially advantageous embodiment, the two wide walls of the mold 
and the succeeding guide surfaces of the thickness-reducing stand, have an 
inwardly concave curved central part extended laterally by means of two 
convex parts connected tangentially, on one side, to the concave central 
part and, on the other side, to a plane end part parallel to the casting 
axis and intersecting perpendicularly the narrow walls of the mold and the 
small faces of the product, the said concave central and convex connecting 
parts being straightened progressively in the thickness-reducing stand, so 
as to come into alignment with the plane end parts at the exit of the said 
thickness-reducing stand. 
According to another preferred characteristic, the means of retaining the 
product in the thickness-reducing stand provide continuous frictionless 
support for the large faces of the product by forming, along the said 
faces, a film of supporting fluid at a pressure at least equal to the 
ferrostatic pressure in the product. 
In a particularly advantageous embodiment, the thickness-reducing stand is 
limited by two wide walls, each consisting of a plurality of connected 
caissons for the injection of fluids under pressure, which are distributed 
in such a way that their edges are aligned in two directions perpendicular 
to one another and form, on each wall, a grid retaining the product. 
Preferably, the retaining grid consists of longitudinal and transverse 
bars arranged in directions respectively parallel and perpendicular to the 
casting axis, the longitudinal bars opposite one another forming, two by 
two, in the central part, a V open at the top, which progressively closes 
towards the sides, the end longitudinal bars being parallel and at a 
distance from one another substantially equal to the thickness of the 
product to be produced, and the transverse bars of one and the same grid 
forming, at the inlet of the stand, an obtuse angle which opens in the 
casting direction until it becomes a plane angle at the exit of the stand. 
In another embodiment, the means of retaining the product in the 
thickness-reducing stand consist, on each face, of a plurality of rolls, 
the distance between which and the diameter of which are determined as a 
function of the thickness of the solidified skin at this location and of 
the deformation force exerted on the product. The said rolls are limited 
by an outer surface of revolution generated by a line which, in the plane 
passing through the axis of the roll and perpendicular to the casting 
axis, envelopes the cross-section to be given to the inner space of the 
stand.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrated diagrammatically the upper part of a continuous-casting 
installation, shown in section through the mid-plane P2 perpendicular to 
the longitudinal walls and passing through the casting axis 10. The 
installation comprises an open-ended mold 1 opening into a secondary 
cooling device which, in a conventional way, comprises a first stand 2 
arranged directly at the exit off the mold 1 and a cooling and guide 
jacket 3, in which solidification is completed. 
The mold 1 consists, in the usual way, of two wide walls 11 and two narrow 
or side walls 12 forming respectively the large and small sides of an 
inner tubular space 13 and which is symmetrical relative to the two 
mid-planes, namely, a longitudinal plane P1 and a transverse plane P2, and 
are mounted inside a frame 14, itself fastened to a table 15 which is 
connected to a casting floor 20 by guide and oscillation means (not 
shown), which make it possible to drive the mold in oscillating movements 
along the casting axis 10. 
As can be seen from the cross-section, for example in FIG. 2 and 4, the two 
wide walls 11 have a cylindrical profile with generatices parallel to the 
axis, the profile being curved in cross-section so as to delimit a 
spindle-spaced inner tubular space 13 symmetrical relative to two planes, 
namely, a longitudinal plane P1 and a transverse plane P2, passing through 
the axis 10, and having a vaulted central part 17 narrowing progressively, 
in the direction transverse to the axis, towards two small sides 18 
delimitted by the two narrow walls 12 which are inserted in the 
conventional way between the two wide walls 11 and which have a small 
width (1) substantially equal to the thickness of the product to be 
formed. Preferably, as will be seen in more detail later, the longitudinal 
walls 11 of the mold comprise an inwardly concave central part 17 
connected by means of two middle parts 19 to two plane lateral parts 18 
parallel to the longitudinal plane of symmetry P1. 
The product leaving the mold via the lower orifice 16 therefore has a 
central part 55 thicker than the edges 57. 
At the outlet of the mold, the product 5 passes immediately to the first 
part 2 of the secondary cooling device, the latter consisting, in the 
conventional way, of a stand equipped with means of retaining the large 
faces of the product the constitution of which is designed for the small 
thickness of the skin solidified at this point. 
In the embodiment illustrated in FIG. 1, the means of retaining the two 
large faces of the product consists of two grid-shaped walls 4 comprising 
longitudinal bars 41 arranged in planes parallel to the mid-plane and 
transverse bars 42 at right angles to the axis 10. 
The longitudinal bars 41 arranged opposite one another in one and the same 
vertical plane form, two by two, in a central part, a V open at the top at 
an angle A which progressively closes towards the sides in such a way that 
the end longitudinal bars 410 are parallel and at a distance 1 from one 
another corresponding to the thickness of the strip leaving the mold. 
Correspondingly, the transverse bars 42, the shape of which is matched to 
that of the product, form, at the inlet 43 of the retaining stand 2, a 
curve which opens at an obtuse angle B and the radius of curvature of 
which increases progressively downwards in the casting direction, the 
lower bars 420 arranged at the exit of the stand being practically 
straight. 
In this way, the two wide walls 4 delimit an inner space 40 for the passage 
of the product, having on the sides 48 a constant thickness (1) equal to 
that of the sides of the mold, in its central part 47, a thickness which 
decreases progressively in the casting direction between the inlet orifice 
(43) of the retaining stand 2, of a cross-section identical to that of the 
exit orifice 16 of the mold, and an exit orifice 44 located in the lower 
part of the stand 2 and having a rectangular cross-section corresponding 
to that of the thin product to be formed. 
Thus, the product 5 which is formed inside the mold and which, as already 
indicated, has the shape of the latter, i.e., a thick central part 
narrowing progressively towards the side in the horizontal direction, 
enters the upper part of the retaining stand 2, the large faces of which 
determine a reduction in thickness of the central part in proportion as 
the product advances in the casting direction. This reduction in thickness 
is possible because the cast product, at the exit of the mold, consists of 
a liquid core 51 surrounded by a solidified and consequently deformable 
skin 52. 
However, to obtain this progressive flattening of the product in the 
thickness-reducing stand, at the same time avoiding defects along the wide 
walls, in particular cracks which can result in skin splitting, the 
support given to the wide walls of the product during their advance and 
straightening must not exert significant stresses on the skin during 
solidification, but must only subject the latter to uniformly distributed 
stresses having, at each point, a value compatible with its thickness at 
such point, and this thickness must both give the necessary strength and 
at the same time preserve sufficient flexibility to allow the deformation 
without a deterioration in quality. 
According to the essential characteristic of the invention, it is necessary 
first of all that the grid retaining the large faces of the product in the 
thickness-reducing stand should form a bearing surface which is 
practically continuous and, as far as possible, frictionless. For this 
purpose, it is expedient if the surfaces retaining the two longitudinal 
faces of the product in the thickness-reducing stand each consists of a 
plurality of connected caissons which can be delimited by the longitudinal 
bars 41 and transverse bears 42 or, alternatively, produced in the way 
illustrated in FIGS. 6 and 7. Each caisson is closed towards the outside 
by a bottom 61 and on the sides by edges 62 which, on the same side as the 
product, limit a wide-aperture orifice 63, into which opens a duct 64 
connected to a circuit (not shown) supplying fluid under pressure. The 
pressure of the fluid is determined as a function of the location of the 
caisson, i.e., of the level of liquid steel at this point and the shape 
and thickness of the solidified wall, so as to balance the ferrostatic 
pressure at this level, with a leakage flow along the edges 62 which, 
allowing provision of a gap of width (e) between the product and the grid 
for avoiding contacts (FIG. 4). 
The caissons are arranged in successive tiers 65 (FIG. 6), the height of 
which is determined, on the one hand, so as to avoid an excessively large 
number of tiers, and, on the other hand, so that the ferrostatic pressure 
over the height of the caisson is relatively constant. 
As shown in the left-hand part of FIG. 7, each tiers 65 can comprise a 
single caisson which extends over the entire width of the product and 
which is preferably supplied by means of several fluid injection nozzles 
66 connected to a single supply duct 64. However, it is also possible, as 
shown in the right-hand part of FIG. 7, to distribute in one and the same 
tier, over the entire width of the product, a certain number of caissons 
60 which are connected separately to a single duct 65 supplying fluid 
under pressure. 
The edges 62 of the caissons are preferably aligned in transverse and 
longitudinal directions, forming a grip supporting the product, and 
consequently it is also possible, as in the embodiment of FIG. 3, to limit 
the caissons by means of longitudinal bars 41 and transverse bars 42. 
According to another characteristic of the invention, the profile of the 
surfaces retaining the large faces of the product is determined as a 
function of the casting speed and of the cooling conditions, in order to 
ensure progressive straightening of the said large faces progressively 
with their thickening and consequently their hardening. 
FIG. 4 shows the extreme part of the product 5 and of the wide wall 4 of 
the thickness-reducing stand. It is advantageous to give the mold 1 an 
inner cross-section in the form of a spindle, such that, at the exit of 
the mold, each longitudinal face of the product comprises an outwardly 
convex central part 55 extended by means of two concave parts 56 
connecting tangentially, on one side, to the central part 55 and, on the 
other side, to a plane end part 57 parallel to the longitudinal plane of 
symmetry P1 and intersecting the small face 54 of the product 
perpendicularly and preferably at a sharp angle. Thus, in the upper part 
of the straightening stand 2, the faces 4 supporting the product 5 will 
comprise a central part 47 concave towards the axis 10, which is connected 
by means of convex middle part 49 to lateral parts 48 parallel to the 
longitudinal plane of symmetry P1. 
So, from the outset of casting a product having right-angled edges is 
produced within the mold itself, and in this way the straightening forces 
exerted on the large faces of the product are transferred to a certain 
distance from the small face 54. The direct production of a product with 
plane edges is useful per se, because it makes it possible to reduce the 
metal losses but, within the scope of the invention, this particular form 
also makes it possible to prevent, during flattening, an angular 
deformation of the lateral end which would be particularly difficult and 
could be detrimental in this zone. 
On the other hand, because the right-angle shape of desired thickness is 
obtained directly on the sides of the product, a bearing point is 
provided, and starting from this the deformation process can be carried 
out progressively by advancing towards the center in proportion as the 
product advances in the straightening stand, in order to obtain, at the 
exit of the latter, or a little downstream thereof, a completely 
solidified product having the desired thin rectangular cross-section. 
FIGS. 4 and 5 illustrate the straightening process diagrammatically. FIG. 5 
shows in perspective the development, inside the thickness-reducing stand 
(not shown), of the product 5 which comprises a core of liquid metal 51 
surrounded by a solidified skin 52, the thickness of which increases in 
the direction of advance of the product, i.e., in the direction oz, the 
figure relating to a trihedron Oxyz. 
If the development of a large face 53 of the product is considered, the 
plane end part 57, which extends from the corner A to the point B1, must 
have a width (d) such that the force straightening the face 53 does not 
exert any effect on the edge A of the solidified product. According to the 
essential characteristic of the invention, a thrust distributed 
continuously over the two curved parts 55 and 56 is exerted, in such a way 
that the width (d) of the plane part 57 increases continuously in 
proportion to the advance of the product and to the thickening of the 
solidified skin 52. Thus, considering a transverse section of the product 
going through level Z1 from the point A, to level z2, the plane part 57 
extends at this level substantially up to the zone c2 which, at the level 
z1, was located at c1. The plane P tangent to the big face 53 at the point 
c1 forms an angle U with the axis Ox and an angle V with the axis Oz, and 
these two angles close progressively from the point c1 to the point c2 
during the advance of the product and the straightening of the face 53. 
The profile of the wide walls 4 of the straightening stand is determined 
in such a way that the speed dy/dt of transverse displacement of the point 
c1 and the rotational speeds dU/dt and dV/dt of the tangent plane P in the 
horizontal direction and the vertical direction respectively are 
compatible with the cast speed dz/dt of the product and sufficiently low 
to ensure that the elongations resulting therefrom between the points c1 
and c2 do not cause any defect in the solidified skin, taking into account 
the thickness of the latter and the quality of the metal. In fact, the 
present knowledge of a technician specialized in continuous casting, which 
is based on several years of operating casting machines, makes it possible 
to define with a certain amount of accuracy the resistance to deformation 
of the solidified skin 52. Bearing in mind the small thickness of the 
product, this skin preserves sufficient flexibility until complete 
solidification to allow the progressive straightening which is carried out 
by means of a uniformly distributed thrust, with the solidified wall 
pivoting about the zone 58 of connection to the plane part 57 which forms 
a bearing point for the straightening force, since solidification takes 
place more rapidly on the lateral edges of the liquid core 51. 
The force to be generated in order to deform the shell 52 during 
solidification comprises two components: 
(a) the force necessary to overcome the ferrostatic pressure level opposite 
the element of volume to be displaced, this force depending on the height 
of metal at this point; and 
(b) the force necessary to deform the metal surfaces linking this element 
of volume to its environment. 
The means described above, allowing a fluid film to flow between the cast 
product and the wall 4 of the stand, make it possible, by virtually 
eliminating friction, to reduce these forces as much as possible and to 
prevent deterioration of the surface of the product and of the geometry of 
the cavity of the reducing stand 4. 
It will be noted that it is preferable if the fluid used is not a cooling 
medium, or at least is a cooling medium to as small an extent as possible, 
and this makes it appropriate preferably to choose air or a neutral gas 
instead of water. 
Moreover, to avoid generating additional stress in the solidified shell 52, 
it is preferable for the perimeter of the product in cross-section to be 
preserved during its descent and flattening in the retaining stand 2. 
Consequently, as indicated in FIG. 3, the latter will only have wide walls 
4 so as to allow the product to widen slightly at the sides. Due to the 
small width of the product on its sides, there is no disadvantage inherent 
in the absence of walls retaining the small faces 54, since the solidified 
skin is sufficiently thick at the exit of the mold. 
Because the profile of the supporting faces 4 is determined in such a way 
that the liquid core 51 continues to be present practically up to the exit 
orifice 44 of the straightening stand, there is no danger that the product 
will be jammed at this point as a result of a wedging effect. 
The product is then completely solidified, and the lower part 3 of the 
secondary cooling device, located underneath the straightening stand 2, 
may consist, in an entirely conventional way, of a series of rolls 31. 
Moreover, all the conventional arrangements can be used in this part, and 
it should be noted that the small thickness of the formed product 53 makes 
it possible to direct it into a horizontal position more quickly than in 
installations for the casting of relatively thick slabs. 
The second part 3 of the secondary cooling device can therefore be 
shortened or in some cases, omitted, and even if the first stand 2 has to 
be lengthened, the total height of the installation should, in the end, be 
reduced. 
In general terms, as indicated in FIGS. 2 and 4, the width (L) of the 
central part of the inner space 13 is determined so as to make it possible 
to introduce into the mold the casting tube 55 which extends the outlet 
spout of the intermediate vessel. In this way, it is possible to carry out 
casting in an entirely conventional manner, in particular with regulation 
of the flow of liquid steel and of the level of the latter inside the 
mold. 
According to another improvement illustrated in FIG. 1, the wide walls 4 of 
the straightening stand 2 are each mounted on a supporting frame 21 
equipped, in its upper part, with suspension lugs 46 articulated about 
horizontal axles on a fixed structure 22 or directly on the casting axles 
on a fixed structure 22 or directly on the casting floor 20. Furthermore, 
two identical jacks 23 arranged on either side of the stand and supplied 
and/or controlled simultaneously are articulated, at their two ends, 
respectively on the two frames 21 supporting the walls 4, Thus, by 
supplying the double-acting jacks 23 simultaneously in either direction, 
it is possible to control the opening or closing of the supporting walls 
4. It is thus possible to open the straightening stand 2 quickly, for 
example to release the product in the event of a breakout or jamming for 
any reason. 
The two frames 21 supporting the walls 4 can be articulated on one another 
or, alternatively, as shown in the drawings be equipped, in their lower 
part, with rollers 25 which bear directly on the strip 53 at the exit of 
the straightening stand 2. 
Supporting and deformation elements forming the wide wall 4 must provide a 
practically continuous support for the product over the entire periphery 
of its large faces and, as described hereabove, it is better realizing 
wide walls 4 in shape of grids. However, in some cases, particularly if 
the properties of the metal give the solidified shell sufficient strength, 
it would also be possible as shown in FIGS. 8 and 9, to support the large 
faces and straighten them by means of tools 7 having horizontal axes or, 
at all events, at right angles to the casting axis 10. The 
center-to-center distances and the diameters of the rolls are determined 
as a function of the thickness of the solidified skin and of the height of 
liquid metal at the corresponding level, to ensure that the wall is 
retained without the risk of the tearing. 
As seen in FIG. 9, the profile of each roll must be determined so that it 
corresponds at each level to the shape of the product, and consequently, 
since the latter preferably has a spindle-shaped form, the outer surface 
of the roll will be formed by a surface of revolution generated by a line 
which, in the plane passing through the axis of the roll perpendicular to 
the casting axis 10, envelops the cross-section to be given to the 
product, so that the rolls as a whole define a retaining and straightening 
wall of the desired form. Consequently, as illustrated in FIG. 9, the 
profile of the rolls will depend on their level inside the straightening 
stand 2, and, because of the width of the product, it may be necessary to 
arrange an intermediate support in the mid-plane of the product, so that 
each roll only extends over half the width of the product. It would be 
possible to use in the thickness-reducing stand other means of retaining 
the big faces of the product, making it possible to ensure progressive 
flattening of the product with continuous supporting of its big faces on 
their all width.