EP0705152B1 - Improved mould for steel continuous casting, particularly for the continuous casting of thin slabs - Google Patents

Abstract

An improved mould, particularly suitable for the continuous casting of thin slabs, shows the wide walls having a profile in a vertical plane being formed of a number of curved lines which are radiused to each other, in particular arcs of circumference having a bending radius of a size decreasing downward from a value approaching infinity on the inlet side where the tangent line to the profile is vertical, until reaching with a continuous line the known lower band at the outlet from the mould. The latter is an arc of circumference with the center in the oscillating center of mould, being positioned at a height comprised between the levels of inlet and outlet sides of the mould itself. Preferably the profile pattern is elliptical starting from the inlet side with vertical tangent down to said lower arc of circumference, so that the bending variation is caused to be tendentially continuous.

Description

• The present invention relates to an improved mould for steel continuous casting, particularly suitable for continuous casting of thin slabs, preferably having a thickness of less than 70 mm according to the preamble of claim 1.
• It is known that, even if a reduced homogeneity in the skin formation, during casting of conventional slabs with a thickness ≥ 240 mm, is normally tolerable, it becomes on the contrary a source of problems in case of casting of thin slabs, as defined above, owing to the velocity of casting (3-6 m/min against a maximum value for coventional slabs of 1,5 m/min), mainly when these thin slabs are foreseen to be sent directly to the rolling steps. The consequence of a dishomogeneous cooling is a product of lower quality, with formation of longitudinal, variously coloured stripes, just because of the different temperature, showing a lack of homogeneity in the slab thickness. Another, even more serious inconvenience which occurs, mainly at a higher casting speed, is a possible "break out" or rupture of skin during the solidification with unavoidable dangerous consequences for the whole casting machine.
• From DE-A-41 31 829, a mould for thin slabs is known having a central bombation or concavity of the two opposite, inner and outer plates, which have a first length, starting from the inlet region of the mould, substantially vertical, until about the 50% of the plate height, thereafter assuming a curved profile at the final portion of the mould outlet, with a bending radius that for the inner plate (intrados) is the same as the outer one (extrados) less the thickness of thin slab.
• These features are reflected in the preamble of claim 1.
• It has been observed that a mould whose plates have been manufactured according to the features of DE-A-41 31 829, although having undeniable advantages with respect to the conventional moulds, especially as far as a better cooling homogeneity is concerned, which has a great importance when the cast product has to be immediately forwarded to the rolling step without intermediate processes, does not solve the problem of a possible detachment of the cast product from the walls at the regions of sudden bending variation. This gives rise to a longitudinal discontinuity that not only results in a dishomogeneous cooling, but also can cause local mechanical stresses (compressive and tensile stresses, respectively at the intrados and extrados) with possibility of cracking in the most stressed zones until generating breakouts.
• Therefore the present invention intends to provide for a better operation of the mould by improving the profile itself of the plates thereof in order to cause the casting to be as closed as possible to the mould walls, so that the heat removed therefrom will be maximum and as uniform as possible with the bending that varies as continuously as possible from inlet having a vertical tangent down to the arc-shaped final portion. Hence the mould profile has been modified as far as the basic surfaces of both intrados and extrados plates are concerned.
• These objects are obtained with a mould showing the features of claims 1.
• It should be appreciated that the mould profile pattern, formed in a vertical plane by a number of curved lines being radiused to each other, according to the most general features of claim 1, or the elliptical pattern of the profile itself according to claim 2, may be provided for moulds having a central concavity in the upper zone, as already mentioned before with reference to DE-A-40 31 691, or even for moulds without such a concavity, with wider sides or opposite inner and outer plates (intrados and extrados) substantially parallel also at the upper inlet edge.
• These and additional objects, advantages and features of the mould according to the present invention will become clearer from the following detailed descripttion of a preferred embodiment thereof, given by way of a non-limiting example with reference to the annexed drawings in which:
• FIGURE 1 is a longitudinal cross-section view of both the plates forming the wider sides of the mould;
• FIGURES 2 and 3 show a diagrammatic representation of an example of variation of the bending radiuses R-R₄ of the profile, for the basic surfaces of extrados and intrados plate, respectively; and
• FIGURES 4 and 4a show a schematic representation of an ellipse passing through a point B and tangent at that point to a given circumference of radius R, having in B a bending radius equal to R, and respectively a detail thereof in enlarged scale for the sake of clarity.
• With reference to Figure 1, the continuous casting mould according to the present invention is formed, in its wider sides, of two copper plates 1, 1a respectively of extrados and of intrados, i.e. provided with a slight bending with respect to the vertical line, thus leading without discontinuity to the underlying arc-shaped path (not shown) of a continuous casting plant, particularly suitable for thin slabs.
• As previously stated, such a bending was provided in the conventional plates at the lower zone only, correspoding to about 1/2 height, the remaining portion being substantially vertical.
• The two opposite plates 1, 1a are cooled with a fluid medium fed from respective water boxes 2, 2a of whichever known type. As already mentioned before, a central concavity could be provided in the upper region of the mould in correspondence of which the two plates 1, 1a are parallel to each other, in a transverse cross-section only at the end portions, near the smaller sides of the mould. In fact they are spaced apart of a greater distance at the middle in order to allow more room to the immersion pouring nozzle, thus ensuring the presence of a sufficiently wide surface of moulten steel, which is thermally stable, being positioned between plate and nozzle, such as to favour a good melting of the covering powder for the purpose of an optimum lubrication. In any case the features that will be described later on are directed to the bending of the vertical profile of both plates irrespective of the possible central concavity thereof, whereby the profile referred to could be either the real one of the inner surface of of the plate at whichever vertical cross-section, or that of an ideal, so-called "basic" surface, with respect to which at the central zone the upper surface keeps away to form the above-mentioned concavity. Still with reference to said "basic surface" this could be the above-defined ideal surface, in case of a mould provided with concavity, or the inner, actual surface of the plates.
• To show more clearly the geometrical features of the basic surface of each plate, respectively of extrados and intrados, reference is made to Figs. 2, 3 in which there are shown examples of profiles obtained by intersecting with a vertical plane the inner surface of an embodiment of mould plate according to Fig. 1.In both drawings O designates the oscillation center of the mould, on the horizontal line at about half height of the profile, coincident with the bending center of the final portion of the profile itself at the outlet zone of the mould. According to the prior art this final portion of the profile, having the bending center coincident with the oscillating center of the mould, was extending along about one half the plate height and forming the only curved zone of the profile pertaining to the basic surface, which otherwise was substantially rectilinear.
• According to the present invention, the basic surface profile in the vertical plane is entirely curved with bent portions of different radiuses being continuously radiused to each other, having in other words common tangents at the points of connection between adjacent curvilinear lengths, without intermediate apices, until reaching the last portion, at the mould outlet, formed of an arc of circumference with center in the oscillating point of mould, thereby with prefixed radius, e.g. comprised between 5000 and 5500 mm for the two plates (in Fig. 2 5200 mm for the outer plate or extrados), with a value that for the inner (intrados) plate is less by the thickness of the thin slab with respect to the other plate.
• According to a first embodiment of the present invention, the profile is subdivided in a plurality of arcs of circumference (in number of five in Figs. 2, 3), corresponding to the same angle to the center α, while having the respective bending centers placed, as shown in the drawings, on the radial line traced from the end point of the upper adjacent arc, from which point the arc concerned starts. As a matter of fact, point O^IV is on the orizontal line drawn from the inlet edge of the mould at a distance R₄ from its mouth. The first arc portion, starting from the top between points F, E will have thereby R₄ as bending radius extending downward along an angle α until point E which, when connected with center O^IV, forms a rectilinear segment on which the center O^III of the second arc E-D will be positioned and so on. This way at point E the tangent to arc E-F is coincident with that of arc E-D and the line is perfectly radiused without apices. The same happens for all the points between adjacent arcs, until point B between arc C-B and the last arc A-B, with center in O which is also the oscillating center of the mould.
• Actually for the construction of the profile according to the teaching of the present invention the starting point will be just the arc A-B, which is already defined in its geometrical characteristics. Then, on the extension of line O-B, wherein B is the point of contact with the contiguous arc B-C, point O^I is fixed, which is the bending center of the arc B-C itself, and so on. The length of arc B-C and the subsequent arcs until reaching point F, will be chosen at will, through simple calc lation of geometry and trigonometry, while preferably fixing conditions capable of rendering easier the calculation itself, such as by adopting angles at the center, all equal, upon fixation of the number of arcs in which length B-F is to be divided, or when considering points S'...S^IV of projection of points O'...O^IV on the horizontal line passing by O. These conditions of uniform division of the arcs and the respective angles may be useful to simplify calculations and the design of the plate, but are not at all determinant in view of the present invention, according to which there could be even provided arcs of continuously increasing length, as well as the width of corresponding angles.
• Still according to the shown embodiment, the center O is positioned at a level corresponding to about half height of the mould, more precisely in the middle of the length with a varying bent, ending at the top with an arc whose tangent, at the upper end F, coincident with inlet zone of the mould, is vertical, as the bending center O^IV is on the horizontal line traced at the same point F. It will be appreciated that the profile of the basic surface of both plates can be subdivided in a number of arcs which is different from that indicated in the illustrated example, preferably a number of more than five, to obtain a curve of better continuity. It it clear that when the subdivision of the profile into arcs approachs to infinity, the curve becomes a single one, with a continuously varying bending. During the oscillation of the mould, the lower region of the plates having a greater bending will be that whose profile is less different from the oscillating path, whereas the maximum difference will be present at the upper or inlet zone of the mould, where however the transverse shifts caused by the oscillation are hardly perceivable owing to the great size of the basic radius R. Therefore, even if in that zone, at the respective central portion of the inner surface of both intrados and extrados plates, the above-mentioned concavity or bombation is not formed, there will be no danger of contact between the inner walls of the plates and the pouring nozzle during oscillation.
• According to a preferred embodiment of the invention, the continuously varying curve along which the inner surfaces of both mould plates vary, without discontinuity, their position in the space from a vertical tangent orientation at the upper end to the arc of circumference trend at the lower end, is of elliptical kind. The conditions which the arc of ellipse between points F and B of Figs. 2, 3 will have to meet are: vertical tangent at the upper end; tangent coincident, at the lower end, with that of the final arc of circumference adjoining therewith (at point B of Figs. 2, 3); and size of the bending radius ρ, at that position being equal to radius R of such an arc of circumference.
• With reference to Fig. 4 and to the most meaningful portion thereof, shown by enlarged scale in Fig. 4a an example of ellipse is represented the lower right-end quarter of which may determine the arc of ellipse desired between points B and F of Figs. 2, 3. For the construction of this ellipse it is sufficient to take into consideration few and simple geometrical rules, while bearing in mind the previously indicated conditions.
• From the requirement that the tangent line on point F has to be vertical, there derives as a consequence that the center O' of the ellipse should be positioned on the horizontal line traced through point F. Thus it will be possible to draw a system of Cartesian axes with the centre in O' and the axis of abscissas x coincident with the minor axis b of the ellipse, corresponding to O'F. Therefore first unknown quantities to be determined are the length size of the minor axis b and of the major axis a along the vertical passing through O', which provides the axis of ordinates y. If called m and n respectively the coordinates of O, namely the center of oscillation of the mould with respect to the system of axes x, y having its center in O', the coordinates of point B, when considered on the ellipse, will be respectively: ${\text{x}}_{\text{B}}\text{= b cos t}$

  

  ; ${\text{y}}_{\text{B}}\text{= a sin t}$

  

  , where t is the angle comprised between axis x and the straight line connecting point O' with point H at which the circumference of radius a intersects the horizontal line from B passing also through point K on the vertical line traced at point B. When considering B as a point of the circumference with center in O and radius R, for the same coordinates thereof the following equations are found:
  ${\text{x}}_{\text{B}}\text{= R cos ξ+m}$

  

  ; ${\text{y}}_{\text{B}}\text{= R sin ξ+n}$

  

  , where angle ξ is comprised between the horizontal in O and the straight line connecting the same point O with B. By imposing the coordinates to be equal and calculating the bending radius ρ at point B as a function of the coordinates themselves, which ρ in turn must be equal to radius R (i.e. the distance OB), that is known, through a series of calculation not really of trivial nature, but however solvable by trigonometry, the following equations are obtained: $$\begin{array}{c}\begin{array}{c}\begin{array}{}\text{(I)}\text{a =}\frac{\text{R sinξ+n}}{\text{sint}}\end{array}\\ \begin{array}{}\text{(II)}\text{ab =}\frac{{\text{tg ξ (R sin ξ +n)}}^{\text{2}}}{{\text{tg t. sin}}^{\text{2}}\text{t}}\end{array}\\ \begin{array}{}\text{(III)}{\text{cos}}^{\text{4}}\text{t =}\frac{{\text{R}}^{\text{2}}}{{\text{tg ξ (R sin ξ +n)}}^{\text{2}}{\text{(tgξ +1)}}^{\text{3}}}\end{array}\end{array}\end{array}$$

  
• Although previously indicated as unknown, the ordinate n of point O may be fixed at will, as the center of oscillation is placed at a lower level with respect to the upper end of the mould. Therefore the ordinate n will be comprised between zero (in which case there is no ellipse but a single arc of circumference AF) and y_B (in which case would be again necessarily B≡F, as the arc of circumference should have a vertical tangent in B, whereby the arc of ellipse would be reduced to this point only). In the case of Fig. 2 (at the extrados) the value of n is of 400 mm. Upon fixing also angle ξ, to be determined e.g. by means of the inclination of tangent line in B (equal to 90°+ξ), and considering that R is of course known (5200 mm for the extrados plate in Fig. 2) from formula III there is obtained the value t. This value is introduced in formula I to obtain a and from formula II there is immediately found the b value. The ellipse is thus defined through the length of its axes and only its center O has to be fixed on axis x, which is found by obtaining m, i.e. the abscissa of O', which is a known point in the space, from the following relation: $\text{m=b cos t-R cos ξ}$

  

  .
• In practice, in order to determine the profile of both inner and outer plates, one has to proceed by calculating the coordinates x_M and y_M of whichever number of points M relating to an ideal ellipse, intermediate between two profiles of intrados and extrados. When indicating with Δ half a distance between the two plates (for example Δ=30 mm), the coordinates of a point P on intrados (inner plate) will be: ${\text{x}}_{\text{p}}{\text{= x}}_{\text{M}}\text{-Δ cos δ}$

  

  and ${\text{y}}_{\text{P}}{\text{= y}}_{\text{M}}\text{- Δ sin δ}$

  

  , whereas for the coordinates of a corresponding point P' on the extrados side it will be: ${\text{x}}_{\text{P'}}{\text{= x}}_{\text{M}}\text{+ Δ cos δ}$

  

  and ${\text{y}}_{\text{P'}}{\text{= Y}}_{\text{M}}\text{+ Δ sin δ}$

  

  , where δ is the angle corresponding, for each point M, to the angle ξ already defined previously for point B.
• Thus it will be possible to define point by point the profile of the two opposite surfaces with elliptical pattern, by providing for example some tables or a suitable calculation program. As stated above, when manufacturing the actual inner surfaces of the two plates, with respect with the two "basic" profiles, thus determined differences could be adopted for defining the possible central concavity or bombation whose pattern, both in horizontal cross-section and in vertical section could be chosen at will and however according to modes independent from the present invention.
• In any case it is clear that the continuous variation of bending from a value approaching infinity in correspondence with the upper end (thus allowing the best conditions for fitting of the dipping nozzle, taking into account of the transverse shifts due to oscillation) to a prefixed value, comprised between 2500 and 6000 mm in case of thin slabs, in the area of an arc-shaped lower affords the advantage of avoiding sudden deviations of the casting in this dangerous initial step wherein the product is not yet solidified but there is still present a "liquid core". In fact these sudden deviations could bring not only to a possible setting apart of the skin zone of the strand from the mould walls with negative consequences on the cooling degree and its uniformity of distribution, but the deformation stress being concentrated in a single zone could give rise to ruptures of the skin itself with flowing out of the inner liquid (break out) with remarkable damages to the plant, in terms of both material to be repaired, and of waste time for the plant standstill.
• It should be clear that the invention is not meant to be restricted to the above-suggested calculation means to obtain the result of defining the elliptical curve desired.

Claims (9)

1. A mould for the continuous casting of thin slabs, with two opposite wider sides formed of copper plates (1, 1') being both water-cooled, each of said wider sides being provided with basic surface which is coincident with the respective inner wall surface such that a deviation from said basic surface may be provided as a concavity in a central zone of said inner wall, wherein the profile of said basic surface in a vertical plane shows an arc-shaped bent (A-B) at the lower zone, with the bending center coincident with the oscillation centre (O) of the mould and having radius (R), characterized in that said profile of the basic surface is formed of a number of curves adjoining to each other in continuity with said arc-shaped lower zone (A-B), said curves having an increasing bending radius as the distance from the latter mentioned zone (A-B) increases, without apices and discontinuity at the points of junction between adjoining curves, until approaching an almost infinite radius at an upper end (F) where the tangent to the curve is substantially vertical, said point (O) being at a height comprised substantially midway between the upper (F) and the lower (A) ends of the profile.
2. A mould according to claim 1, characterized in that said curves forming the profile in a vertical plane of the basic surface of the mould plates are all formed of arcs of circumferences having the respective bending center at the intersection of the straight lines connecting the ends of the arc itself with the bending center of the adjacent arcs, until said final arc of circumference (A-B) whose bending center is coincident with center of oscillation (O) of the mould, with all the angles to the centre (α) of the various arcs of circumference being identical to each other, with the bending centre (O^IV) relating to the first upper arc starting from said point (F) being located on the horizontal line passing by the same point (F).
3. A mould according to claim 2, characterized in that said series of arcs of circumference is approaching an infinite number, whereby the only resulting curve is of elliptical kind, having a bending radius decreasing downwards, until the continuously variable bending radius (ρ) reaches the minimum value corresponding to said prefixed radius (R) when adjoining in (B) said final arc-shaped portion (A-B).
4. A mould according to claim 3, characterized in that said elliptical curve extends between said upper end point (F) of said basic profile, corresponding to the inlet edge of the mould, where the tangent is vertical, until the point (B) where the tangent is coincident with that relating to the adjacent lower end arc (A-B) and is inclined by an angle (90°+ ξ) with respect to the horizontal, the angle (ξ ) being that, of negative value, comprised between the horizontal at point (O) and line connecting (O) and (B).
5. A mould according claim 4, characterized in that said elliptical curve (A-F) is a length of an ellipse having the center in (O') on the horizontal through (F) on the same side of point (O) which can be considered the origin of a system of Cartesian axes with axis (x) on the connecting line (O'-F) the distance of which is coincident with minor axis (b) of said ellipse.
6. A mould according to claim 5, characterized in that the ordinate (n) of said point (O) coincident with the center of oscillation of the mould has a negative value comprised between zero and the ordinate (y_B) of said point (B) at which the arc of circumference (A-B) with the centre on the same point (O) begins.
7. A mould according to claim 6, characterized in that said ordinate (n) of point (O) is equal to (y_B/2).
8. A mould according to claim 6 or 7, characterized in that, for a total height of the mould equal of about 1 m, said arc-shaped final portion (A-B) has a height of about 100 mm, considered as the difference between the ordinates of the two end points (A, B) with respect to said system of axes having its origin at point (O').
9. A mould according to any of claims 3-8, characterized in that each one of the two basic profiles relating respectively to the extrados and intrados plates can be determined by calculating the coordinates (x_M; y_M) of a number whichsoever of points (M) corresponding to an elliptical profile, intermediate between said two profiles which in turn can be defined through the coordinates of points (P) pertaining to intrados and (P') pertaining to extrados by means of the relations: $${\text{y}}_{\text{P}}{\text{=y}}_{\text{M}}{\text{-Δcosδ; y}}_{\text{P}}{\text{=x}}_{\text{M}}{\text{-Δsinδ and x}}_{\text{P'}}{\text{=x}}_{\text{M}}{\text{+Δcosδ; x}}_{\text{P'}}{\text{=y}}_{\text{M}}\text{+Δsinδ,}$$

   

   where Δ is equal to half a distance between said two basic profiles and δ is an angle, of varying value, defined for each point M of said intermediate ellipse similarly to said angle (ξ) for said point (B).

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