Continous casting mould

Abstract

An open-ended mold for casting metal strands with a thin slab cross section, the mold having broad-side walls which form a central widening region which extends from the mold inlet edge to the mold outlet edge and at the outlet edge opens out in a rectangular outlet cross section. To prevent to a great extent the strand from coming away from the broad-side walls, the contour of the mold cavity in the widening region is formed, as seen in sectional planes taken at right angles to the longitudinal axis of the mold, by curves which at least in partial regions form clothoids or clothoidal curves, within which sudden changes in curvature (&Dgr;k) are minimized or reduced to zero. The curvature reduces to flat at the mold outlet edge. Other cavity surface features are disclosed.

Description

[0001] The invention relates to an open-ended mold for a continuous casting installation, in particular an open-ended steel casting mold, for casting a metal strand with a thin slab cross section with a maximum thickness of 150 mm, with a mold cavity formed by cooled broad-side walls and narrow-side walls, the mutually opposing broad-side walls forming a central widening region, which extends from the mold inlet edge to the mold outlet edge and ends there in a rectangular outlet cross section of the open-ended mold and which goes over in the direction of the mutually opposing narrow-side walls into narrow-side adjusting regions.

[0002] Open-ended molds of this type are referred to in practice as “funnel molds” and are described with various configurations of the funnel-shaped widening region in the literature. The many embodiments that have been derived from the original Rossi mold (DE-C 887 990) and the first trial mold with a funnel-shaped widening region (EP-B 149 734) reflect the problems in the forming of the strand shell and the shaping of this strand shell in the spatially curved widening region.

[0003] On the inlet side, the known open-ended molds have in the region of the meniscus a spindle-shaped cavity cross section, which is constricted continuously in the direction of the narrow-side walls to the size of the strand to be cast and is made to revert to the size of the desired strand thickness in the strand drawing-out direction or casting direction, either still within the length of the mold or directly in the outlet cross section or in the downstream backup and guiding roll stand of the continuous casting installation.

[0004] By this configuration it is possible to produce in such a mold steel strands which, with customary slab widths, have strand thicknesses of less than 150 mm, preferably however 30 to 100 mm, and at the same time to introduce melt into the open-ended mold with submerged casting nozzles which are thicker than the target thickness for the cast strand.

[0005] EP-A 1 002 599, DE-A 39 07 351 and EP-A 552 501 already disclose open-ended molds in which the funnel-shaped widening region is formed in sectional planes taken at right angles to the longitudinal axis of the mold by arc portions which replicate the concave-convex-curved spindle form. In particular at the points of inflection at which arcs of a circle with radii deviating from one another merge in line with one another, there are great sudden changes in radius and curvature. At these points of discontinuity, the strand shell comes away from the mold wall, whereby inhomogeneous temperature conditions occur in the strand shell and, as a result, there is uneven strand shell development. As a further consequence, this leads to thermal stresses between neighboring strand zone and increased loads, and also under some circumstances to damage of the strand shell.

[0006] EP-A 552 501, EP-A 909 597 and DE-A 39 07 351 disclose for example open-ended molds in which the contour of the funnel-shaped widening region change in the longitudinal direction, i.e. along sectional planes parallel to the longitudinal axis of the mold, abruptly or with a great change in curvature. This usually applies to molds in which the funnel-shaped widening region already ends at a relatively great distance from the mold outlet edge. Apart from the disadvantages already described above of sudden or great changes in radius and curvature of the curves describing the widening region, additional loads on the strand shell are produced by the oscillation of the mold. The locally intensified knocking of the mold on the funnel-shaped taper of the strand brings about additional impairment of the strand shell in this region.

[0007] An open-ended mold of the type described at the beginning, with a widening region which extends from the mold inlet edge over the entire length of the mold and is only made to revert to the rectangular cross section of the strand to be cast in the mold outlet cross section is already known from DE-C 35 01 422. It is also the case with this mold that the contour of the widening region is formed in sectional planes taken at right angles to the longitudinal axis of the mold by arcs of a circle adjoining one another in line and likewise has the disadvantages described above. The shortening of the circumferential length of the strand inevitably occurring as a result of the reversion of the widening region is counteracted by a corresponding adjustment of the narrow-side walls and a distortion of the strand shell is already avoided in this way.

[0008] The object of the present invention is therefore to avoid the described disadvantages of the known prior art and to propose an open-ended mold with which the contour of the mold cavity is optimized in such a way that coming away of the strand from the mold wall is to a great extent prevented from the meniscus to the mold outlet edge, and consequently homogeneous forming of the strand shell in the mold is ensured.

[0009] This object is achieved according to the invention by the contour of the mold cavity in the widening region being formed in sectional planes taken at right angles to the longitudinal axis of the mold by curves which at least in partial regions form clothoids or clothoidal curves, within which the sudden changes in curvature are minimized or reduced to zero. Extensive avoidance of sudden changes in curvature creates the best conditions for optimum forming of the strand shell.

[0010] The described advantage of avoiding the strand shell coming away is equally obtained if curve portions formed approximately or exactly by arcs of a circle or straight lines are joined by curve portions formed by clothoids or clothoidal curves and sudden changes in curvature of these curve portions at their transitional points are minimized or reduced to zero.

[0011] A preferred configuration in the central region of the widening region is obtained if the contour of the mold cavity in the widening region is formed in sectional planes taken at right angles to the longitudinal axis of the mold by curves running symmetrically in relation to the narrow-side walls with a convex-concave profile, which, starting from the center of the mold cavity contour with only a small or no curvature, go over into a region with constantly increasing curvature. This effectively counteracts tendencies of the strand shell to come away in the central region.

[0012] Favorable conditions are obtained thereby if the curvature (k) in the center of the mold cavity contour meets the condition

k≦0.3*(4H)/(H2+L2),

[0013] where k=curvature (mm)

[0014] H=half the maximum mold cavity widening (mm)

[0015] L=half the width of the widening region B1 (mm)

[0016] An improvement in the forming of the strand shell and guiding of the strand shell along the broad-side walls is obtained if the sudden changes in curvature at the transition from the central widening region to the narrow-side adjusting regions and at the location of the inflectional tangents in the curve profile are minimized, preferably tend toward zero. This produces favorable conditions if the sudden changes in curvature (&Dgr;k) at the transition from the central widening region (11) to the narrow-side adjusting regions (12, 13) meet the condition &Dgr;k≦0.5*(4H)/(H2+L2), or if the sudden changes in curvature (&Dgr;k) at the location of the inflectional tangents (25) meet the condition &Dgr;k≦0.4*(8H)/(H2+L2).

[0017] The coming away of the strand shell from the broad-side wall of the mold is to a great extent avoided if the contour of the mold cavity in the widening region is formed in sectional planes taken parallel to the longitudinal axis of the mold and the narrow-side walls by straight lines between the mold inlet edge and the mold outlet edge. This linear formation of the central widening region in the longitudinal direction of the mold has the effect that a change in curvature conducive to the strand shell coming away only occurs at the mold outlet. There, however, no adverse effect caused by an inhomogeneous temperature distribution is to be expected, since in this region the effect of the direct spray cooling on the cast strand commences.

[0018] Critical regions with regard to the coming away of the strand shell from the broad-side wall of the mold are formed by the maximum widening, the transition from the central widening region to the adjoining narrow-side adjusting regions and the regions of the mold cavity that have points of inflection. Here, minimizing of the coming away of the strand shell is achieved on the one hand by the transition from the central widening region and the narrow-side adjusting region laterally adjoining the latter being formed by a straight line which, in a sectional plane taken parallel to the longitudinal axis of the mold and the narrow-side walls, joins the mold inlet edge and the mold outlet edge, on the other hand by the locations of the inflectional tangents in the contour of the mold cavity with sectional planes taken at right angles to the longitudinal axis of the mold forming straight lines between the mold inlet edge and the mold outlet edge which lie in a sectional plane taken parallel to the longitudinal axis of the mold.

[0019] In keeping with operational practice, it is advantageous if the straight lines are arranged inclined by the extent of strand shrinkage. The inclination corresponds in its spatial orientation and order of magnitude to the values customary for a taper setting of the narrow sides. This measure is intended to avoid the strand shell having to run through a transition from a parallel wall region into the widening region in the strand drawing-off direction.

[0020] In order to meet special solidifying and shrinking conditions of particular grades of steel, it is expedient if the contour of the mold cavity in the widening region has in sectional planes taken parallel to the longitudinal axis of the mold and the narrow-side walls a curved, preferably parabolic, profile between the mold inlet edge and the mold outlet edge, in dependence on the solidifying conditions of the metal to be cast. This also applies especially to the regions of the transition from the central widening region to the adjoining narrow-side adjusting regions and the locations of the points of inflection or inflectional tangents, if appropriate with allowance for a shrinkage-dependent inclination.

[0021] Further favorable influences on the quality of the strand to be cast are obtained if the radii of curvature of the contour of the mold cavity in the widening region in sectional planes taken at right angles to the longitudinal axis of the mold become greater between the mold inlet edge and the mold outlet edge in the direction of the mold outlet edge and are infinite at the outlet edge. With an overall mold concept in mind, it is expedient if the broad-side walls in the narrow-side adjusting regions are arranged essentially parallel to one another and the narrow-side walls are convexly formed in dependence on the shrinking behavior of the metal respectively to be cast.

[0022] Further details, features and advantages emerge from the explanations and representations which follow of an exemplary embodiment not restricting the extent of protection.

[0023] In the drawings:

[0024] FIGS. 1a, 1b, 1c show a schematic representation of the open-ended mold according to the invention in a plan view (view Z), front view and side view (section A-A) according to one possible embodiment of the invention,

[0025] FIG. 1d shows a further embodiment of the open-ended mold according to the invention in side view (analogous to section A-A in FIG. 1c),

[0026] FIG. 2 shows the open-ended mold according to the invention in a longitudinal section with submerged casting nozzle and cast strand,

[0027] FIG. 3 shows the coming-away behavior of the strand from the broad-side wall when there are great changes in curvature in a central widening region of a rectangular shape in a continuous casting mold according to the prior art,

[0028] FIG. 4 shows the coming-away behavior of the strand from the broad-side wall when there are great changes in curvature in a central widening region of a trapezoidal shape in a continuous casting mold according to the prior art,

[0029] FIG. 5 shows the coming-away behavior of the strand from the broad-side wall when there are minimized changes in curvature in a central widening region of a rectangular shape in a continuous casting mold according to the invention,

[0030] FIG. 6 shows a comparison of the contour of the central widening region and the associated curvature profiles in the case of the mold according to the invention and a mold according to the prior art.

[0031] An open-ended mold of the type according to the invention is represented in FIGS. 1a to 1c in a schematic representations. It comprises two mutually opposing broad-side walls 1, 2 and two likewise mutually opposing narrow-side walls 3, 4, which can be adjusted between two positions B, B1 in the sense of a format adjustment. Bounded by the broad-side walls 1, 2 and the narrow-side walls 3, 4, between the mold inlet edges 5 and the mold outlet edges 6, is the mold cavity 7 of the open-ended mold, in which a cast strand, comprising a strand shell 9 and a liquid core 10, is formed between a meniscus (liquid metal level) 8 and the mold outlet edge 6, as illustrated in FIG. 2.

[0032] The broad-side walls 1, 2 have a central widening region 11, which is bounded essentially by the narrowest adjusting region B1 of the narrow-side walls 3, 4 and goes over on both sides into the narrow-side adjusting regions 12, 13. These narrow-side adjusting regions are formed by mutually plane-parallel wall parts of the broad-side walls 1, 2 and extend from the mold inlet edge 5 to the mold outlet edge 6. As a departure from this embodiment, it is quite possible for the broad-side walls to form planar surface areas arranged such that they converge in the strand drawing-off direction or the direction of the narrow-side walls in the region of the narrow-side adjusting regions.

[0033] The central widening region 11 has at the mold inlet edges 5 a cross section which narrows in the form of a spindle to the narrow-side adjusting regions 12, 13. It is provided in its central region with a width adequate for receiving a submerged casting nozzle 14 and is reduced toward the narrow-side adjusting regions 12, 13 to the thickness D of the metal strand to be cast by curves 15 with a curvature profile that is to a great extent free of any sudden changes. These curves 15 comprise for example arcs of a circle 16, 17, which are joined in line by clothoids 18, 19, 20, 21 or clothoidal curves and the curvature profile of which runs without any abrupt changes or with abrupt changes minimized.

[0034] The central widening region 11 is continuously constricted between the mold inlet edge 5 and the mold outlet edge 6 and is reduced at the mold outlet edge 6 to the size of a rectangular outlet cross section 22, which corresponds to the desired casting cross section. The contour profile of the central widening region 11 between the mold inlet edge 5 and the mold outlet edge 6 is formed, in relation to a sectional plane which is taken parallel to the longitudinal axis 23 of the mold and parallel to the narrow-side walls 3, 4, by straight lines 24a, 24b, . . . 24n. This linear configuration of the central widening region in the casting direction ensures freedom from deformation in the vertical strand drawing-off direction. This definition of the sectional plane is based on the assumption that the narrow-side walls 3, 4 are arranged plane-parallel to one another and there is therefore no taper adjustment.

[0035] Consequently, the end faces of the planar narrow-side walls are oriented parallel to the longitudinal axis of the mold. A taper adjustment of the narrow-side walls that is provided in practice therefore does nothing to change the orientation of the straight lines described. The central widening region 11 can also be constricted between the mold inlet edge 5 and the mold outlet edge 6 by a curve with a parabolic profile 26. (FIG. 1d)

[0036] FIG. 3 illustrates results of numerical investigations on a finite-element basis in a half-section of a broad-side wall between the meniscus and the mold outlet edge by an example of an open-ended mold according to the prior art with a central widening region which is of a rectangular shape in an end-on view and extends in the transverse direction between the line I, which corresponds to the longitudinal axis (23) of the mold in FIG. 1a, and the narrow-side adjusting region, and in the longitudinal direction between the meniscus and the line A. The light areas show regions at which the coming away of the strand shell from the broad-side wall is particularly pronounced. In these regions, a great change in curvature occurs in the contour of the central widening region. At these points, inhomogeneous temperature conditions and reduced strand shell development are to be expected. A distinction can be made here between essentially three regions in the transverse direction, indicated by the vertical lines I, II, III, and a region in the longitudinal direction, indicated by the horizontal line A. Particularly at risk is the region with the greatest widening in the mold (line I), followed by two regions (lines II, III) at the curve transitions toward the edge of the mold. In the longitudinal direction there follows at the end of the funnel an extended region (line A) with diminishing contact of the strand shell with respect to the broad-side wall. This may be regarded as a serious disadvantage of funnel molds in which the funnel ends inside the mold.

[0037] FIG. 4 shows analogous results with respect to a funnel mold with a V-shaped constriction of the widening region between the meniscus and the mold outlet edge, as is likewise known from the prior art. The structural composition of the representation corresponds to FIG. 3. In the case of this type of mold too, great regions with disturbed contact between the strand shell and the mold wall occur. This is attributable to the V-shaped funnel edge (line V), which lies obliquely in relation to the casting direction and which the strand/strand shell must pass. The associated changes in curvature in the longitudinal direction cause these disturbances of the contact behavior.

[0038] FIG. 5 shows the results of the numerical investigation on a finite-element basis with reference to an open-ended mold according to the invention. The structural composition of this representation corresponds in turn to that of FIG. 3. As a result of the optimization according to the invention of the contour of the mold cavity of the mold, light regions with disturbed contact between the strand shell and the broad-side wall now occur in the direct vicinity of the mold outlet edge, where no adverse effects on the forming of the strand shell occur any longer.

[0039] A comparison of the contours of the mold cavity in the widening region in the case of a funnel mold according to the prior art and a mold according to the invention in a sectional plane taken at right angles to the longitudinal axis of the mold is represented in FIG. 6. Represented in the upper half of the figure is the contour profile of the mold width, starting from the center of the mold (longitudinal axis of the mold), and the lower half of the figure shows the assigned curvature profile over the mold width. The contour profile according to the prior art, represented by dotted lines, is formed by two arc portions merging one into the other and ends in a straight line which identifies the narrow-side adjusting region. The associated curvature profile, represented by a dotted line, jumps at the contact point of the two arc portions (point of inflection 25′) from an until then constant negative value to a constant positive value. Occurring here is the massive point of discontinuity in the curvature profile that was described as disadvantageous and contributes significantly to the coming away of the strand shell. Conditions with the same effect occur at the transition from the arc to the straight line at the transition to the narrow-side adjusting region. Here, the curvature jumps at a point of discontinuity back to the value zero. In comparison with this, represented by the solid line is the contour profile according to an embodiment of the invention, which, starting from the center of the mold, is formed by a clothoid 18 or clothoidal curve with a constantly increasing curvature, which is adjoined by an arc portion 16 with a constant curvature. This is adjoined by a transitional portion which is formed by two clothoids 19, 20 or clothoidal curves and in the profile of which the curvature is changed continuously from a negative maximum value to a positive maximum value as it passes through a point of inflection 25. There further adjoins an arc portion 17 with a positive curvature value, which is then transformed by a clothoid 21 or clothoidal transition curve to the curvature value zero of a straight line. The avoidance of sudden changes in curvature is provided by this contour profile.

[0040] Instead of the arc portions in the contour profile that are specified in this exemplary embodiment, curve portions formed to approximate arcs of a circle may also be used. In the region of maximum widening in the widening region, a curve portion formed by a straight line or approximately by a straight line may be provided. All these modifications are within the extent of protection of the invention.

Claims

1. An open-ended casting mold for a continuous casting installation for casting a metal strand with a thin slab cross section the mold comprising: a mold cavity formed by mutually opposing, spaced apart, cooled broad-side walls and of mutually opposing, spaced apart, narrow-side walls between the broad-side walls, the mold having a mold inlet edge and a mold outlet edge; the mutually opposing broad-side walls forming a central widening region between the broad-side walls, and which extends from the mold inlet edge to the mold outlet edge and the widening region ending at the mold outlet edge in a rectangular outlet cross section of the open-ended mold; the widening region extending in the directions toward and narrowing toward the mutually opposing narrow-side walls and merging into laterally adjoining narrow-side adjusting regions where the broad-side walls are closer together than in the widening region; the contour of the mold cavity in the widening region is formed, as seen in sectional planes taken at right angles to the longitudinal axis of the mold, by curves which at least in first partial regions form clothoids or clothoidal curves such that within the clothoids or clothoidal curves, the sudden changes in curvature are minimized or reduced to zero.

2. The open-ended mold as claimed in claim 1, wherein the curves include second partial regions between successive first partial regions, and the second partial regions are either formed approximately or exactly by arcs of a circle or straight lines and are joined by the first partial regions of the curves, whereby sudden changes in curvature (&Dgr;k) of these curve portions at their transitional points are minimized or reduced to zero.

3. The open-ended mold as claimed in claim 1, wherein the contour of the mold cavity in the widening region is formed in sectional planes, taken at right angles to the longitudinal axis of the mold, and the curves run symmetrically in relation to the narrow-side walls with a convex-concave profile, the profile starting from the center of the mold cavity contour with only a small curvature or no curvature, and going over into a region with constantly increasing curvature.

4. The open-ended mold as claimed in claim 3, wherein the curvature (k) in the center of the mold cavity contour meets the condition

k≦0.3*(4H)/(H2+L2),

where k=curvature (mm)

H=half the mold cavity widening (mm)

L=half the width of the widening region B1 (mm).

5. The open-ended mold as claimed in claim 1, further comprising transitions between the central region and the narrow-side adjusting regions, wherein sudden changes in curvature at the transitions from the central widening region to the narrow-side adjusting regions and at locations of inflectional tangents in the curve profile are minimized to tend toward zero.

6. The open-ended mold as claimed in claim 5, wherein the sudden changes in curvature (&Dgr;k) at the transition from the central widening region to the narrow-side adjusting regions meet the condition

&Dgr;k≦0.5*(4H)/(H2+L2)

where k=curvature (mm)

H=half the mold cavity widening (mm)

L=half the width of the widening region B1 (mm).

7. The open-ended mold as claimed in claim 5, wherein the sudden changes in curvature (&Dgr;k) at the locations of the inflectional tangents meet the condition

&Dgr;k≦0.4*(8H)(H2+L2)

where k=curvature (mm)

H=half the mold cavity widening (mm)

L=half the width of the widening region B1 (mm).

8. The open-ended mold as claimed in claim 1, wherein the mold cavity in the widening region has the contour thereof, as seen formed in sectional planes taken parallel to the longitudinal axis of the mold and the contour of the narrow-side walls, by straight lines between the mold inlet edge and the mold outlet edge.

9. The open-ended mold as claimed in claim 1, further comprising transitions from the central widening region to the laterally adjoining narrow-side adjusting region regions, the transitions are formed by a straight line which, in a sectional plane, taken parallel to the longitudinal axis of the mold and the narrow-side walls, joins the mold inlet edge and the mold outlet edge.

10. The open-ended mold as claimed in claim 5, wherein in the contour of the mold cavity, in sectional planes taken at right angles to the longitudinal axis of the mold, the locations of the inflectional tangents form straight lines between the mold inlet edge and the mold outlet edge and the straight lines lie in a sectional plane taken parallel to the longitudinal axis of the mold.

11. The open-ended mold as claimed in claim 8, wherein the straight lines are arranged inclined by the extent of strand shrinkage of the strand being molded.

12. The open-ended mold as claimed in claim 1, wherein in sectional planes taken parallel to the longitudinal axis of the mold and the narrow-side walls, the contour of the mold cavity in the widening region has a curved profile between the mold inlet edge and the mold outlet edge which is dependent on the solidifying conditions of the metal to be cast.

13. The open-ended mold as claimed in claim 12, further comprising transitions between the central widening region and the laterally adjoining narrow-side adjusting region are formed by a curve with an essentially parabolic profile and, in a sectional plane parallel to the longitudinal axis of the mold and the narrow-side walls, that curve joins the mold inlet edge and the mold outlet edge.

14. The open-ended mold as claimed in claim 12, wherein in sectional planes taken at right angles to the longitudinal axis of the mold, the mold has inflectional tangents at locations in the contour of the mold cavity which form curves having an essentially parabolic profile between the mold inlet edge and the mold outlet edge, and the curves lie in a sectional plane parallel to the longitudinal axis of the mold.

15. The open-ended mold as claimed in claim 14, wherein the curves with the essentially parabolic profile are arranged inclined by the extent of strand shrinkage.

16. The open-ended mold as claimed in claim 1, wherein in the widening region, in sectional planes taken at right angles to the longitudinal axis of the mold, the radii of curvature of the contour of the mold cavity become greater between the mold inlet edge and the mold outlet edge in the direction of the mold outlet edge until they are infinite at the outlet edge.

17. The open-ended mold as claimed claim 1, wherein the broad-side walls are essentially parallel to one another in the narrow-side adjusting regions laterally of the central regions.

18. The open-ended mold as claimed in claim 1, wherein the narrow-side walls are convexly formed with respect to the cavity in dependence on the shrinking behavior of a respective metal to be cast.

19. The open-ended mold as claimed in claim 1, wherein the mold is shaped for casting a metal strand with a maximum thickness of 150 mm.

20. The open-ended mold as claimed in claim 12, wherein the contour of the mold cavity in the widening region has a curved parabolic profile.