WO2011023483A1

WO2011023483A1 - Mould for continuous casting of long or flat products, cooling jacket designed to cooperate with such a mould and assembly comprising such a mould and such a cooling jacket

Info

Publication number: WO2011023483A1
Application number: PCT/EP2010/060916
Authority: WO
Inventors: Ugo Zanelli
Original assignee: Siemens Vai Metals Technologies S.R.L.
Priority date: 2009-08-04
Filing date: 2010-07-27
Publication date: 2011-03-03
Also published as: CN102470425A; EP2292350A1; EP2461924A1; RU2012108072A

Abstract

The invention concerns a mould for continuous casting of long or flat products, said mould extending along an axis (z) and comprising an internal and an external surface delimiting a mould thicknesses (e, e', e3, e4), the internal surface defining a mould cavity, characterized in that a thickness (e3, e4) of at least a longitudinal element (16A, 16C) of the mould contained in a longitudinal cross-section of the mould (10) increases, at least on a portion of the mould (10), from a point P of minimum thickness in both directions defined by the longitudinal axis (z), the increase depending on a measured or simulated temperature gradients of a test mould in use. The invention also concerns a method of making such a mould, a cooling jacket design to cooperate with such a mould and an assembly comprising such a mould and such a cooling jacket.

Description

Mould for continuous casting of long or flat products, cooling jacket designed to cooperate with such a mould and assembly comprising such a mould and such a cooling jacket The invention relates to a mould for the continuous casting of long or flat products such as billets or blooms.

Moulds for the continuous casting of long products define a continuous sizing passage for the cast metal which enters it in the molten state via the top of the mould and leaves it via the bottom of the mould in the form of a solid shell deriving from the peripheral solidification of the cast metal on contact with the cold wall of the mould body and which contains a still-liquid core. Solidification then continues to its conclusion in the lower part of the casting machine by means of spray units. These kinds of moulds are also called "mould tube" and can have a square, rectangular, circular, or polygonal cross-section. The tubes can be straight or curved along a so called "casting radius", and the internal cavity is characterized by a progressive decrease in dimension in order to follow the natural shrinkage of steel during solidification process. Moulds for continuous casting of long products, particularly billets cast in open stream casting, must withstand high temperatures load during casting. In particular, the top part of the mould is subject to a heat transfer profile which could be characterized by isotherm at different temperatures. Such conditions, which are also enhanced by higher casting speed and oil lubrication, induce high thermal gradients in longitudinal and also in transversal direction of the mould, with additional high gradients, and therefore high internal temperature, across the thickness of mould. Such temperature differences between different points of the mould, and the high temperature reached inside the mould, induce thermal distortion, recrystalization, cracking and detachment of internal chromium plating. Additionally, especially when oil is used for lubrication, the high temperature of internal side of the mould causes partial evaporation of oil, with defective lubrication and higher oil consumption rate. Moreover, such uneven mould wall temperature and corresponding heat extraction are generating negative effect on product quality. In the past several trials have been made, reducing the mould thickness generally in a uniform way transversally and longitudinally. In other applications the mould thickness has been reduced by grooves applied vertically in the top part of the mould, or eventually along all the mould length, with the aim to increase heat transfer in that area and reduce mould temperature. Other applications were applying a differentiate cooling based on separate circuits for different parts of the mould tube itself.

However, if in one hand the decrease of mould thickness improves the temperature gradient between internal and external side of the mould, on the other hand, this decrease reduces mechanical characteristics of the mould.

The object of the invention is to solve the above mentioned problems and more specifically to increase heat transfer, decrease temperature gradients in longitudinal and transversal directions of a mould, as well as decrease the internal temperature of a mould for continuous casting of long products and at the same time keeping the mechanical characteristics of said mould when used.

For this purpose, the subject of the invention according to a first aspect is a mould for continuous casting of long or flat products, said mould extending along an axis and comprising an internal and an external surface delimiting a mould thicknesses, the internal surface defining a mould cavity, characterized in that a thickness (e3, e4) of at least a longitudinal element of the mould contained in a longitudinal cross-section of the mould increases, at least on a portion of the mould, from a point of minimum thickness in both directions defined by the longitudinal axis, the increase depending on a measured or simulated temperature gradients of a test mould in use.

The invention increases heat transfers, decreases temperature gradients in longitudinal and transversal direction, and decreases the internal temperature of the mould, creating the preconditions for a more homogeneous shell growth.

Advantageously when the mould has a plurality of longitudinal elements extending along the axis, said elements forming a transverse polygonal cross-section of the mould, a thickness of at least one mould element contained in a mould transversal cross-section continuously varies between a first and a second value depending on measured or simulated transversal temperature gradients of a mould in use. Advantageously the thickness varies continuously from the middle of a segment of said mould element in the direction of one of the internal corners of the mould.

According to the invention, the thickness of each mould element varies from the middle of a segment of each mould element in the direction of an internal corner of the mould.

According to another feature of the invention, in at least one transversal cross section, at least one portion of a the mould element is symmetrical with respect to a plan passing by the central segment of said transversal mould element and being perpendicular to said transversal mould element.

Advantageously the variation of the thickness extends only on a portion of the height of the mould. Furthermore, the variation is an increase. In one embodiment of the invention, the maximal thicknesses are equal.

Advantageously the variation of the thickness is function of a longitudinal temperature gradient measured or simulated on a mould in use.

Advantageously, the thickness contained in a longitudinal cross-section of each longitudinal element increases, at least on a portion of the mould, from a point P of minimum thickness in both directions defined by the longitudinal axis. Advantageously, the point P of minimum thickness is located on the mould depending on the location of the point P_x of maximum longitudinal temperature previously measured on said test mould in use or previously determined by simulation.

According to one embodiment, the point P of minimum thickness is located on the mould approximately between 50 to 100 mm from a point P_y determined thanks to the point P_x of maximum longitudinal temperature previously measured on said test mould in use or previously determined by simulation.

According to a second aspect, the subject of the invention is a method of making a mould for continuous casting of long or flat products according to the above mentioned definitions, the method comprises the steps of:

- measuring or simulating temperature gradients of a test mould in use,

- machining the mould until the thickness of at least one part of the mould varies depending on measured or simulated temperature gradients of the test mould.

According to the invention the method may comprises the steps:

- prior to the machining of the mould, a first point P_x of maximum longitudinal temperature is determined by measurement on said test mould in use or by simulation of said test mould in use,

- prior to the machining of the mould, determining a second point P_y located on the mould at a location depending on the location of the first point P_x on the test mould,

- machining the test mould in such a way that the point of minimum thickness P be located approximately between 50 to 100mm from the second point P_y.

In one embodiment, the mould is machined such as the point P_y be located on the mould at the same location than point P_x is located on the test mould.

According to a third aspect, the invention concerns a cooling jacket designed to cooperate with a mould as above defined, the cooling jacket comprising a body defining a plurality of cooling ducts for guiding a cooling agent along external surfaces of the mould, the cooling jacket being designed to receive and at least partially longitudinally surround said mould, a thickness of the cooling jacket varies depending on measured or simulated temperatures gradients of a mould in use.

Advantageously, the thickness of the cooling jacket at a predefined height of the cooling jacket is inversely proportional to the thickness of the mould at the same height. According to another aspect the subject of the invention is a method of making a cooling jacket as above defined, the cooling jacket being designed to cooperate with a mould as above defined, the method comprises a step wherein the cooling jacket is machined such that a thickness of the cooling jacket varies depending on measured or simulated temperatures gradients of a test mould in use.

In one embodiment, the cooling jacket is machined such that a thickness of the cooling jacket at a predefined height of the cooling jacket is inversely proportional to the thickness of the mould at the same height. In one embodiment, the cooling jacket is machined in such a way that at given height of the cooling jacket, the more the thicknesses of the mould are high the less the thicknesses of the cooling jacket are important, at least on longitudinal portion of the cooling jacket. According to another aspect, the subject of the invention is an assembly comprising a mould as above defined and a cooling jacket as above defined.

Due to this particular design of the external part of the mould and of the water cooling channels, higher stability and improved centring of the mould itself is achieved, preventing uneven cooling effects due to possible mould misalignment inside the cooling jacket. The invention will be clearly understood and other aspects and advantages will be more clearly apparent in light of the following description given by way of embodiments with reference to the appended drawings, in which:

figure 1 shows measured or simulated longitudinal temperature isotherms of a mould in use,

figure 2 is a view of internal and external temperatures profiles of a mould in use according figure 1 conditions,

figures 3A, 3B and 3C show schematic longitudinal and transversal cross-sections of a mould according to the invention.

- figures 4A, 4B, 4C, 4D show schematic longitudinal and transversal cross sections of a cooling jacket according to the invention.

figures 5A 5B 5C show schematic longitudinal and transversal cross sections of the assembly of a mould and of a cooling jacket according to the invention.

It has to be noted that on the drawings the view are not at the same scale.

When metal in liquid form is poured into a mould, the top part of this mould is subject to a heat transfer profile which could be characterized by isotherm at different temperatures. Figure 1 is a graphic giving the isotherms measured or simulated along the longitudinal direction and the longitudinal thickness of a test mould for continuous casting of long products in use, the liquid metal entering the mould being at a temperature comprises between approximately 1450°C and 1600°C, depending on the chemical composition of the cast product and on the casting mode, section and speed. The test mould represented in the figures is 900mm long, has a thickness of 13mm and has a squared cross section.

Figure 2 is a view of internal and external temperatures profiles of a test mould in use according to figure 1 conditions. In other words, figure 2 gives temperatures, measured or simulated, along the internal and the external faces of a test mould in use in figure 1 conditions. The temperatures T°out are the temperatures of the external face of the test mould and the temperatures T°in are the temperatures of the internal face of the test mould. These two curves give the difference of temperature across the thickness of the test mould for a given altitude of the known mould. For example, the difference of temperature between an internal and an external point of the known test mould situated at 200mm from the top of the known test mould is about 100⁰C. As above mentioned, this difference causes cracking of the mould and other problems.

The aims of the invention are to minimize the differences of temperature between two points of the mould. In other words, to have more uniform temperature distribution of the mould and at the same time increase heat extraction and decrease internal mould wall temperature.

In this purpose an object of the invention is a mould for continuous casting of long products comprising an internal and an external surface delimiting a thickness of the mould, the internal surfaces defining a mould cavity. The main feature of the invention is that the thickness of at least one portion of the mould varies depending on measured or simulated temperature gradients of another mould in use. The other mould may be a known mould having known features.

Figure 3A, is a longitudinal cross-sectional view of one embodiment of a mould 10 according to the invention and figures 3B and 3C are transversal cross-sectional views of figure 3A at two different heights or altitudes of said mould 10. For a better understanding figures 3B and 3C are provided with an orthogonal two dimensional coordinate system defined by two orthogonal half straight line x and y. The mould 10 comprises an external longitudinal surface 12 and an internal longitudinal surface 14 and can be made of copper or of an alloy including cooper. This kind of mould is also called a mould tube. Furthermore, the mould 10 comprises a plurality of longitudinal elements 16 to 16C extending along an axis z. The elements 16 to 16C may have a plate shape. In this particular embodiment and as can be seen on figure 3B and 3C, the mould 10 has a transverse polygonal cross-section and comprises a plurality of internal corners A, B, C, D and a plurality of external corners A'.E, F, B',B",C", C, C", D"\ D', D" and A'". In the transversal cross-section of the mould 10 of figure 3B, the mould element 16 is delimited by two consecutive internal corners A and B and by four consecutive external corners A', B', E, F. The vertical thickness e of said transversal mould element 16, which is the projection of the thickness e on the vertical axis y, continuously varies from one point of said element 16 of the mould 10 between a first vertical thickness e1 and a second thickness e2. For this particular mould element 16 the first thickness e1 of the element 16 is the vertical distance between point A and point E and the second thickness e2 of the element 16 is the vertical distance between point A and point A'. Of course the thickness of each of the tree other mould elements 16A to 16C respectively delimited by the corners BB'B"C"C'C, CC'C'D'" D¹¹D¹D and DD¹D¹¹A¹¹A¹A are also varied as above mentioned for the thickness e but in the corresponding horizontal or vertical direction.

The figure 3C is a cross-sectional view of figure 3A at another height or altitude of mould 10. In this view, the transversal cross-section of the mould 10 has a different shape.

In the transversal cross-section of the mould 10 of figure 3C, a closed transversal element 18 of the mould 10 is delimited by two consecutive internal corners G and J and by four consecutive external corners G', K, L, J'. The horizontal thickness e' of said transversal mould element 18, which is the projection of the thickness e¹ on the axis horizontal x, continuously varies from one point of said mould element 18 between a first horizontal thickness e1 ' and a second thickness horizontal e2'. For this particular mould element 18 the first thickness e1 ' is the horizontal distance between point L and point J and the second thickness is the horizontal distance between point J and point J'. Of course the thickness of each the tree other closed transversal mould element 18A to 18C in this cross-section of the mould 10 respectively delimited by the corners GG¹G¹¹H¹" H¹H, HH¹H¹TI¹¹I and MT¹J¹¹J¹J is also varied as above mentioned for the thickness e' but in the corresponding horizontal or vertical direction.

Furthermore, in the embodiment of figures 3 each transversal mould element is symmetrical with respect to a plan passing by the central segment of said transversal mould face and being perpendicular to said transversal mould face. In other words the thickness (e or e) as above defined, of a mould face has the same value for two points situated at the same distance with respect to and in both side of the central segment of said mould face.

In another embodiment not shown on the figures, the thickness e varies continuously from the middle of a segment AB, BC, CD or DA of one mould element 16 in the direction of one of the internal corners A, B, C or D of the mould. Moreover, the variation of the transversal thicknesses of a mould can be limited to the top portion of this mould, the other part of the mould having a constant transversal thickness. For example, the variation of the transversal thickness of a mould can extends longitudinally only between the first 300 and 400 mm of the mould height.

Furthermore, in the embodiment of figure 3B and 3C wherein the mould 10 has a general squared transversal cross-section, the thicknesses in the areas of the corners are equal. In other words, the distances AA', BB', CC, DD' are equal to each others and the distances GG, 'HH', II', JJ' are also equal to each others. In other words, the maximal horizontal and vertical thicknesses of the mould 10 contained in a transversal cross-section of the mould at a given height or altitude of the mould may have the same value.

Moreover, the variation above mentioned is an increase and e2 is greater than e1 whereas e2' is greater than e1 '.

Moreover, in the case of a rectangular cross-section mould it may happen that the thicknesses contained in a transversal cross-section of the mould at a given height or altitude of the mould are the same only for the two opposite elements, while the maximal thicknesses in the area of the corners, as above defined have the same value. The principle of varying the thickness of a mould also applies in the longitudinal direction of the mould 10 according to the invention. Figure 3A is a longitudinal cross- section of the mould 10. Each mould 10 element 16A and 16C comprise respectively longitudinal surfaces 20 and 22 each one having a respective thickness e3 and e4. As above mentioned the variation of the thickness e3 and e4 is function of a longitudinal temperature gradient previously measured on another mould in use or is function of a simulated longitudinal temperature gradient of a mould as presented in figures 1 and 2. More precisely, the longitudinal thicknesses e3 and e4 of each longitudinal element 16A,16C increases, at least on a portion of the mould, from a point of minimum thickness in both directions defined by a longitudinal axis z. More precisely, in the embodiment of figure 3A while moving vertically from the top point P1 of the mould 10, the thicknesses e3 and e4 progressively decrease up to the point P where the thickness is minimum. Then, the thicknesses e3 and e4 increase up to the point P2. After the point P2 the thicknesses e3 and e4 are constant. As way of example the distance between point P1 and P can be comprises between 300 and 400mm for a mould having a height of 900mm. The thickness of the element 16B and 16C in a longitudinal cross section of the mould varies also as above mentioned.

This variation of thicknesses e3 and e4 allows a more uniform temperature distribution in the mould 10 and at the same time increases heat extraction and decreases internal mould wall temperature.

The point P of minimum thickness of the mould 10 is placed approximately between 50 to 100mm from a point P_y determined on the production mould thanks to the point of maximum longitudinal temperature P_x previously measured on said test mould in use or previously determined by simulation. This point P_x and the curves of figure 1 and 2 are obtained by experimental measurement based on thermocouples or other measuring systems installed on known moulds in order to find the real temperature profile, or are calculated with proper simulation programs.

Several calculations and temperature profile measurements, as well as heat transfer calculations, show that an increased casting speed also results in an increased heat transfer and therefore increases the temperature of the mould 10 also called copper tube. Reasons for such an increase of heat transfer are:

- Due to the shorter residence time of steel in the mould, the shell is thinner and more deformable and therefore more subject to ferrostatic pressure, which brings about a more intimate contact between the shell and the mould tube, with lower gap formation;

- this brings about also higher temperatures of the shell with consequently a higher temperature gradient, which is resulting in a higher heat transfer;

- Additionally, at higher temperature, there is less contraction, which once again brings about a better shell contact and higher heat transfer conditions.

Increase in heat transfer by increasing the casting speed is particularly evident in the region immediately under the meniscus

Such values and situations are also documented by experiments made using instrumented moulds.

At the corners of the mould, the heat transfer is lower with respect to the centre of the mould face, therefore the highest heat transfer value, and as a consequence the highest temperature in mould tube is reached respectively in the centre of the mould tube face and in the area which is under the meniscus.

The higher temperature is detrimental to the mould tube mechanical characteristics as it may be close to the copper recristallization temperature, which would produce alteration in tube hardness and stability, and additionally is giving higher possibility for cracking of the chromium plating in meniscus area.

Additionally, as the heat conduction is depending on the thickness of conductive material, a thinner copper wall is clearly giving possibility for a higher heat transfer. Based on all facts mentioned above, the position of the minimum thickness of the copper tube according to the invention is to be placed in a region which is under meniscus area, exact position being dependant on casting speed and cast grades, and can be approximately estimated at a distance between 70 and 100 mm from the meniscus itself.

This position and the corresponding copper thickness, as well as the possibility to maintain the actual copper thickness in other areas of the tube, by a continuous variation in copper thickness for a minimum to a maximum, will have the following positive effects:

- Increase of the local heat transfer where it is more effective (area where shell thickness still does not act as a barrier to heat transfer;

- More stability of the copper tube with respect to a tube with same minimum thickness equally extended in all length/perimeter;

- Decrease of the temperature of the copper tube and a more equal

temperature distribution, which brings about better mechanical characteristics, maintaining adequate hardness also in casting conditions;

- Increased mould life due to increased stability, decreased temperature and due to prevention of chromium plating cracking/detaching created by high temperature gradients;

- Decreased possibilities of mould tube deformation related to excessive

temperature increase which results in thermal expansions of tube against existing constraints (e.g. water gap duct, flanges etc.).

In order to obtain the mould according to the present invention, in one embodiment a mould having the same geometrical features than the known mould used to determine the curves of figures 1 and to 2 is machined until the shapes as above mentioned described and shown on the figures are obtained.

To obtain a mould according to the invention the following steps can be performed:

- measuring or simulating temperature gradients of a test mould in use, - machining the production mould 10 such that the thickness e, e', e3, e4 of at least one part of the production mould 10 varies depending on measured or simulated temperature gradients of test mould. Furthermore prior to the machining of the mould, a first point P_x of maximum longitudinal temperature is determined by measurement on said test mould in use or is previously determined by simulation. Prior to the machining of the production mould 10, a second point P_y located on the mould 10 is determined depending on the location of the first point P_x. For example, the point P_y may be located on an external face of the second mould, the longitudinal distance between the top of the second mould and the point P_y being a multiple of the distance between the point P_x and the top of the first mould. Subsequently the mould 10 is machined in such a way that the point of minimum thickness P be located approximately between 50 to 100mm from the second point P_y. In other words, the position of the point P_y on the production mould may be extrapolated from the position of the point P_x on the test mould.

In case the production mould 10 has a different size with respect to the test mould, the geometry of the production mould, and in particular his thickness variation, is calculated by fitting to new size the data taken from the test mould.

Furthermore, values, and in particular the location of point P_x, determined with one test mould can be used for machining a plurality of production moulds having or not the same geometrical characteristics between each others.

In case the first and the second mould have the same geometrical features, the point Py and P_x may be located at the same location respectively on the production and on the test mould. For example, the point P_y is located at the same longitudinal distance from the top of the mould 10 than the point P_x is from the top of the test mould.

Moulds for continuous casting of long products are cooled during casting process by mean of a cooling agent, flowing trough an external guided channel surrounding the mould, in order to avoid problems with mechanical distortion and to additionally improve the heat extraction, increase water turbulence and helping in centring the mould inside the water jacket. Another aspect of the invention is then a cooling jacket designed to receive and at least partially longitudinally surround mould 10. Figure 4A is a longitudinal cross-sectional view of a cooling jacket according to the invention and figures 4B to 4D are transversal cross-sectional views of figure 4A at different height or altitude of the cooling jacket. The cooling jacket 30 for a mould as previously defined comprises internal 32 and external faces 34 delimiting two longitudinal cross-sectional cooling jacket faces 36 and 38. The cooling jacket 30 defines a plurality of cooling ducts 36 for guiding a cooling agent along external surfaces of a mould according to the present invention. The cooling ducts extend at least on a portion of the height of the cooled jacket 30. The cooling jacket 30 is designed to receive and at least partially longitudinally surround said mould 10 and has a complementary form with respect to the mould. In particular, as can be seen on figure 4A, the thicknesses i and i" of the cooling jacket varies along the height of the cooling jacket 30. In the embodiment shown on the figures, the cooling jacket 30 has squared cross- sectional shape comprising a plurality of portions 3OA to 30 D. The thicknesses i, and i' of the cooling jacket 30 delimited by internal 32 and external faces 34 at a predefined height of the cooling jacket 30 are inversely proportional to the thickness e3 or e4 of a cross sectional longitudinal face 20 or 22 of the mould at the same height of the cooling jacket 30.

As can be seen on the figures 4A to 4D the thicknesses i and i' increase in the top part of the water jacket 30 up to a maximum value and then diminish up to a minimum value and remain constant in the last part of the water jacket 30.

A method of making the cooling jacket 30 designed to cooperate with the mould 10 comprises a step wherein the cooling jacket is machined such that a thickness i, i' of the cooling jacket 36,38 varies depending on measured or simulated temperatures gradients of the test mould in use.

In one embodiment, the cooling jacket is machined such that a thickness i, i' of the cooling jacket 30 at a predefined height of the cooling jacket 30 is inversely proportional to the thickness e3, e4 of the mould 10 at the same height. In other words, the cooling jacket is machined in such a way that at a given height of the cooling jacket 30, the more the thicknesses e3' and e4' of the mould 10' are high the less the thicknesses i and i' of the cooling jacket are important, at least on a longitudinal portion of the cooling jacket 30.

Figure 5A is a longitudinal cross-sectional view of an assembly comprising a cooling jacket 30 and a mould 10' according to the invention and figures 5B and 5C are transversal cross-section of figure 5A at different heights or altitudes of the cooling jacket.

The cooling jacket 30 has a longitudinal profile and a transversal profile which follows the different thicknesses e3' and e4' of the external faces(s) of the mould 10' and then has also thicknesses i and i' which depend, at least on a portion of the cooling jacket 30, on a temperature gradient previously measured on another mould in use or on a previously simulated temperature gradient of a mould in use.

Moreover, in order to keep the total thickness of the assembly 40 at a constant value, at given height of the cooling jacket 30, the more the thicknesses e3' and e4' of the mould 10' are high the less the thicknesses i and i' of the cooling jacket are important.

Besides, the cooling jacket 30 comprises to parts which that can be assembled around the mould 10, the assembly being made in a factory. Moreover the casting mould above described may also be curved extending along a curved radius and may have different length and/or thickness, depending on the cast product size and chemical composition and depending on the required productivity.

The particular design of the assembly mould/cooling jacket prevents mould deformation, and grant higher casting speed, longer mould life and better product quality due to a more uniform heat extraction not only along the mould perimeter but also in longitudinal direction. The uniformity of cooling in billet perimeter allows a more uniform shell growth, preventing possible shape deformations due to differentiate cooling and resulting thermal tensions, while the extension of the mould decreased thickness in the longitudinal direction will increase the heat extraction capability of the mould itself, granting a faster shell growth with related possibility of increased casting speed and productivity.

All of these advantages give a benefit in term of reduction of machine costs, i.e. more productivity with the same number of strands or reduced number of strands with same productivity. Moreover it is also possible to install the invention on existing machines exchanging old systems with a mould and a mould cooling jacket according to the invention.

Claims

1. Mould for continuous casting of long or flat products, said mould extending along an axis (z) and comprising an internal and an external surface delimiting a mould thicknesses (e, e', e3, e4), the internal surface defining a mould cavity, characterized in that a thickness (e3, e4) of at least a longitudinal element (16A, 16C) of the mould contained in a longitudinal cross-section of the mould (10) increases, at least on a portion of the mould (10), from a point P of minimum thickness in both directions defined by the longitudinal axis (z), the increase depending on a measured or simulated temperature gradients of a test mould in use.

2. Mould according to claim 1 or 2 having a plurality of longitudinal elements (16, 16A, 16B, 16C) extending along the axis (z), said elements forming a transverse polygonal cross-section of the mould characterized in that a thickness (e, e') of at least one mould element contained in a mould transversal cross-section continuously varies between a first (e1 , e'1 ) and a second value (e2, e'2) depending on measured or simulated transversal temperature gradients of a test mould in use.

3. Mould according to claim 2 characterized in that the thickness (e,e') varies continuously from the middle of a segment (AB, BC₁CD, DA) of said mould element (16) in the direction of one of the internal corners (A₁B₁C₁D) of the mould.

4. Mould according to one of the claims 2 and 3 characterized in that the thickness of each mould element (16,16A,16B,16C) varies from the middle of a segment of each mould element (16,16A,16B,16C) in the direction of an internal corner (A,B,C,D,G,H,I,J) of the mould (10).

5. Mould according to anyone of the previous claims characterized in that at least one mould element (16, 16A, 16B, 16C,18,18A18B18C) is symmetrical with respect to a plan passing by the central segment of said transversal mould element (16, 16A, 16B, 16C) and being perpendicular to said transversal mould element.

6. Mould according to anyone of claims 2 to 5 characterized in that the variation of the thickness (e) extend only on a portion of the height (h) of the mould (10).

7. Mould according anyone of claims 2 to 6 characterized in that the variation is an increase.

8. Mould according to anyone of the previous claims characterized in that the maximal thicknesses (e2, e2') are equal.

9. Mould according to anyone of the previous claim characterized in that the variation of the thickness (e3, e4) is function of a longitudinal temperature gradient measured or simulated on a test mould in use.

10. Mould according anyone of the previous claims characterized in that the thickness (e3, e4) contained in a longitudinal cross-section of each longitudinal element (16, 16A, 16B, 16C, 18, 18A, 18B, 18C) increases, at least on a portion of the mould (10), from a point (P) of minimum thickness in both directions defined by the longitudinal axis (z).

11. Mould according to the previous claim characterized in that the point P of minimum thickness is located depending on the location of the point P_x of maximum longitudinal temperature previously measured on said test mould in use or previously determined by simulation.

12. Mould according to the previous claim characterized in that, the point P of minimum thickness is located on the mould approximately between 50 to l OOmm from a point P_y determined thanks to the point P_x of maximum longitudinal temperature previously measured on said test mould in use or previously determined by simulation.

13. Method of making a mould (10) for continuous casting of long or flat products according to anyone of the previous claims characterised in that the method comprises the steps of:

- measuring or simulating temperature gradients of a test mould in use,

- machining the mould (10) until the thickness (e, e', e3, e4) of at least one part of the mould (10) varies depending on measured or simulated temperature gradients of the test mould.

14. Method according to claim 13 wherein :

- prior to the machining of the mould (10), determining a second point P_y located on the mould (10) at a location depending on the location of the first point P_x on the test mould,

- machining the test mould in such a way that the point of minimum thickness be located approximately between 50 to 100mm from the second point Py.

15. Method according to claim 14 wherein the mould is machined such that point Py is located on the mould (10) at the same location than point P_x is located on the test mould.

16. Cooling jacket (30) designed to cooperate with a mould (10) according to anyone of claims 1 to 12, the cooling jacket comprising a body defining a plurality of cooling ducts (6) for guiding a cooling agent along external surfaces (12) of the mould (10) the cooling jacket (30) being designed to receive and at least partially longitudinally surround said mould characterized in that, a thickness (i, i') of the cooling jacket (36,38) varies depending on measured or simulated temperatures gradients of a mould in use.

17. Cooling jacket according to claim 16 characterized in that the thickness (i, i') of the cooling jacket (30) at a predefined height of the cooling jacket (30) is inversely proportional to the thickness (e3,e4) of the mould (10) at the same height.

18. Method of making a cooling jacket (30) designed to cooperate with a mould (10) according to anyone of claims 1 to 15, characterized in that the method comprises a step wherein the cooling jacket is machined such that a thickness

(i, i') of the cooling jacket (36,38) varies depending on measured or simulated temperatures gradients of a mould in use.

19. Method according to the previous claim wherein the cooling jacket is machined such that a thickness (i, i') of the cooling jacket (30) at a predefined height of the cooling jacket (30) is inversely proportional to the thickness (e3,e4) of the mould (10) at the same height.

20. Method according to claim 18 wherein the cooling jacket is machined in such a way that at given height of the cooling jacket (30), the more the thicknesses (e3',e4') of the mould (10') are high the less the thicknesses (i,i') of the cooling jacket are important, at least on longitudinal portion of the cooling jacket (30).

21. Assembly comprising a mould according to anyone of the claims 1 to 12 and a cooling jacket according to anyone of the claims 16 and 17.