GB2329141A

GB2329141A - Continuous casting

Info

Publication number: GB2329141A
Application number: GB9819351A
Authority: GB
Inventors: Eddie Winder; Timothy Reynolds
Original assignee: Kvaerner Metals Continuous Casting Ltd
Current assignee: Kvaerner Metals Continuous Casting Ltd
Priority date: 1997-09-12
Filing date: 1998-09-07
Publication date: 1999-03-17
Also published as: GB9819351D0; GB9719318D0

Abstract

The invention aims to accommodate a variety of methods for allowing deviations relative to the theoretical passline with minimal strain rates and stresses, thereby providing a high quality product. A casting apparatus and a method of casting is thus provided in which any deviations of the cast strand away from or towards the theoretical passline or any deviations to the edges of the cast strand are defined, at least in part, by continuous curves. One example of such a suitable continuous curve is a clothoid which applies a uniform rate of curvature removal or application along its length, therefore, leading to the lowest possible strain rates being applied to the solidifying shell over this length. Increased equipment life, reduced withdraw resistance and wear and improved product quality result as benefits of this techniques application.

Description

IMPROVEMENTS IN AND RELATING TO CASTING This invention concerns improvements in and relating to casting, particularly, but not exclusively, with respect to accommodating changes in direction of material movement during casting and subsequent processing.

Casting machines, such as continuous casting machines, introduce liquid metal to a mould which defines the required cross section of the strand to be produced. The molten metal initially solidifies to form a shell around the periphery of the mould, but subsequent cooling causes the shell to increase in thickness and eventually solidify completely. Further externally applied cooling is normally involved in this process, once the strand has left the confines of the mould.

Once in its desired product configuration, the strand can be cut into desired lengths for further handling.

A variety of mould profiles and subsequent operations exist and the configuration of these relative to one another can vary significantly. For instance, the mould may have a straight or curved configuration and the mould may be provided vertically, horizontally or inclined between the two.

Generally vertical orientations are preferred for the casting stage whereas the subsequent processing steps, such as rolling, are normally performed in a horizontal alignment. As a consequence a variety of inclines or curves are normally employed to redirect the material from the casting direction configuration towards the subsequent processing configuration.

The back face of the mould is normally referred to as the passline and historical continuous casting techniques have had as their general aim the conveyance of the strand and the material forming it in a direction parallel to the theoretical strand passline for the mould and subsequent operations More recently, continuous casting processes have been developed in which deviation of the mould profile from the theoretical passline or other deviations lie on the edges which are generally perpendicular to the passline, are deliberately introduced to counteract one or more problems with continuous casting. Such deviations include bulges beyond and within the passline. The transitions to and from these deviations relative to the theoretical passline in the eventual product have, to date, been provided in a relatively abrupt manner which gives rise to strains, high strain rates and stresses within the strand. Such strains and stresses may give rise to cracking or other impairments of product quality.

The present invention aims to accommodate a variety of methods for allowing deviations relative to the theoretical passline with minimal strain rates and stresses thereby providing a high quality product.

According to a first aspect of the invention we provide a method of casting in which any deviations of the cast strand away from or towards the theoretical passline or any deviations to the edges of the cast strand are defined, at least in part, by continuous curves.

One example of a suitable continuous curve is a clothoid which applies a uniform rate of curvature removal or application along its length, therefore, leading to the lowest possible strain rates being applied to the solidifying shell over this length. Increased equipment life, reduced withdraw resistance and wear and improved product quality result as benefits of this techniques applicatIon.

The first aspect of the invention may include the options, possibilities and features set out elsewhere in this application.

According to a second aspect of the invention we provide a method of casting in which a strand of cast product is formed, the strand having a direction of travel, the profile of the strand being determined by strand profiling means, the thickness of the strand, perpendicular to the axis defining the direction of travel, being greater at a first location than at a second location and wherein the profile determining means and/or strand between the first and second locations are, at least in part, defined by one or more continuous curves.

By providing continuous curves the stresses and strain rates imposed on the material during any movement it may undergo between the second and first location is minimised.

Preferably the thicknesses at the first and second locations are measured in a direction parallel to one another.

One or both of the first and second locations may be within the confines of the mould and/or outside the confines of a mould.

The continuous curve may join a first linear portion with a second linear portion, which may or may not be parallel with the first. Two or more continuous curves may be used to join the first and second linear portions.

The continuous curve may join a first curve, such as a radius, with a second curve, which may or may not be a radius. The first and second curves mayocurve in the same or different directions. Two or more continuous curves may be used to join the first and second curves.

The continuous curve may join a linear portion with a curved portion, which may or may not be a radius. Two or more continuous curves may be used to join the linear and curved portions.

The first and second location may be within the same cross sectional profile, the cross sectional profile being defined perpendicular to an axis, defined by the overall direction of travel, at that location.

Preferably the thickness at the first location is greater than at the second location. Preferably the first location is closer to the centre line of the strand than the second location.

The cross-sectional profile may be a cross-section through a mould. The profile determining means may be the walls of the mould The cross-sectional profile may be a cross-section through the strand after leaving the mould. The profile determining means may be rollers, grids or the like.

The profile of the cross-section may include a portion defined by a pair of linear parallel wall portions and a portion defining a bulge inward and/or outward relative to the linear portion. The bulge may be defined in two opposing walls of the strand. The bulges in the two walls may mirror one another.

Preferably the first location is defined in a bulge portion and the second portion is defined in a linear portion.

The cross-sectional profile may be defined by a series of linked continuous curves, with or without linear portions and/or curved portions. A series of bulges may thus be provided.

The bulge may be defined in part, or completely, by a continuous curve.

The bulge may be formed by a first continuous curve connected to an intervening portion, the intervening portion being connected to a further continuous curve.

The bulge may be formed by a first continuous curve connected to a second continuous curve of opposing curvature, the second continuous curve being connected to a third continuous curve, the third continuous curve curving in the same direction as the first.

The continuous curves may link to linear portions of the strand.

A continuous curve may be used to link one linear portion of a wall with another linear portion of a wall. The linear portions of the wall may be at a significant angle to one another, for instance greater than 45". In this way continuous curves may be used for the corners of strands.

The bulge, incorporating one or more continuous curves may taper in the overall direction of travel of the strand. One or more continuous curves may be present throughout the tapering of the bulge.

The first and second locations may be axially separated from one another, the axis being defining by the overall direction of travel. The first and second locations may be on the same axis or spaced relative to it.

Preferably the thickness at the first location is greater than at the second location. The second location may be further along the axis defining the direction of travel than the first location.

The first and/or second location may be defined with a mould.

The profile determining means may be the walls of the mould.

The first location is preferably defined nearer the inlet to the mould than the second location.

The first and/or second locations may be defined outside a mould. The profile determining means may be rollers, grids or the like. The first location may be defined between a first set of opposing rollers, with the second location being defined between a second set of opposing rollers. The separation of the rollers forming pairs in the second set of rollers may be less than the separation of the rollers forming pairs in the first set of rollers. The separation of the rollers between the first and second set may vary according to one or more continuous curves. Preferably a first continuous curve tending towards the reduced separation is provided in proximity to the first set of rollers. Preferably the first curve is tangential joined to another of opposing curvature to join to the second set of rollers.

The first location may be defined within a mould, the second location being defined outside the mould. Thus one or more continuous curves may be employed to link mould wall profile defining means with roller profile defining means.

The first location may be provided on a linear wall portion of the strand and the second location may also be provided on a further linear portion of the strand. The first and second linear portions may be inclined relative to one another. The first linear portion may be inclined relative to the axis defined by the overall direction of travel of the strand.

Preferably the second linear portion is parallel to the overall direction of travel. Preferably the first and second linear portions are joined by a first continuous curve tending in a first direction and a second continuous curve tending in the opposing direction to the first.

The first location may be provide on a curved wall portion of the strand and the second location may also be provided on a further curved portion of the strand. The first and second curves may be of the same or different curvature and/or may curve in the same or different direction.

The first location may be provided on a curved wall portion of the strand and the second location may be provided on a linear portion of the strand. Preferably the linear portion is parallel to the overall direction of travel.

The profile defining means may incorporate continuous curves linking first and second locations in more than one orientation. Thus links in the direction of travel and perpendicular to may be provided in this way.

The casting method is preferably a continuous casting technique.

The cast product may be a billet, bloom, slab, beam blank or strip.

The direction of travel of the strand may be linear or curved or combinations thereof. Within the mould, linear or curved, preferably following a radius, directions of travel may be provided.

According to a third aspect of the invention we provide casting apparatus comprising strand profile determining means, the strand profile determining means defining a casting direction, the separation of the strand profile determining means at a first location being greater than at a second location, the separation being measured perpendicular to the axis of the casting direction, the profile determining means extending between the first location and the second location, at least in part, being defined according to a continuous curve.

The third aspect of the invention may include any of the options, features and possibilities set out elsewhere in this document, including the first and second aspects of the invention.

Preferably the strand profile determining means is a mould and/or one or more pairs of opposing rollers.

According to a fourth aspect of the invention we provide cast products produced by the method of the first and / or second aspects of the invention and / or using the apparatus of the third aspect of the invention.

Various applications and embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 illustrates the configuration of a strand and passline in a typical system; Figure 2a illustrates the initial cross sectional mould profile for a mould, Figure 2b illustrates the end mould cross sectional profile for a mould of Figure 2a; Figure 2c illustrates a side view of the mould profile of Figures 2a and 2b; Figure 3 illustrates an alternative side profile for the mould of Figure 2a and 2b; Figure 4a to 4e illustrate applications of the present invention to casting, Figure 5 illustrates a side view of a mould profile incorporating the present invention; Figure 6 illustrates a plan view of a mould cross sectional profile incorporating the present invention; Figure 7a illustrates a prior art strand reduction technique; Figure 7b illustrates a strand reduction technique embodying the present invention; and Figure 8 illustrates a form of continuous curve.

Figure 1 illustrates a strand produced by introducing molten metal to a mould section 1, the strand 2 passing on to a subsequent processing stage, rolling stage 3. The mould stage 1 is preferably conducted in a substantially vertical alignment to facilitate pouring of the molten material into the mould.

The subsequent rolling stage 3, however, is preferably carried out in the horizontal configuration to provide easier handling and greater room for processing. As a consequence, it is necessary to provide a transition stage 5 between mould stage 1 and subsequent rolling stage 3 during which the strand is conveyed from the substantially vertical alignment to a substantially horizontal alignment.

Bending and unbending of the cast strand in this way does not involved movement of the shell away from or towards the theoretical pass line, discussed in more detail below, and as a consequence is significantly different from the variations in profile accommodated by the present invention.

As continuous casting machines may produce sections of a variety of sizes, the axially line taken to define the path of the strand, the passline, is most frequently taken to be the lower or back face of the strand (rather than a centre line for the strand) as this is not usually moved in changing from one section size to another. Thus, in the illustrated example of Figure 1, the passline 7 is initially linear and substantially vertically aligned and passes through a transitional curve section to a substantially linear section once more, this time in a horizontal alignment.

In the general history of continuous casting the general aim has been to guide and support the solidifying strand shell in such a manner that the shell of the strand travels in a direction which is parallel to the theoretical strand passline at the corresponding position along the passline.

More recently there has been a growing trend to deliberately flex the strand shell into an offset from its nominal path parallel to the pass line or to provide it with an offset to begin with, and then flex the strand shell back into the nominal path parallel to the pass line at a later point.

An example of such deviation is provided in the slab casting mould of Figure 2a and 2b. The aim of the mould is to produce a substantially rectilinear slab conforming in cross section to the outlined profile 16 illustrated in Figure 2b. To cast thin slabs, however, it has been necessary to create a funnel shaped portion 18 in the upper part of the mould, Figure 2a, so as to provide greater space for the pouring tube through which the liquid metal is introduced. Thus in the upper profile the mould has bulges 20a, 20b on opposing sides which deviate from the nominal passline, which passes straight into the page on the plane illustrated by the dotted line 22 and linear portions of the mould profile 24a, 24b.

As can be seen in the side view of the mould profile of Figure 2c, the mould thus consists of a section with inclined walls 26 and linear walls 28, which in the lower part of the mould are linear only 30. These meet at transition point 32. Due to the rapid change of direction at this transition point 32, however, strains, high strain rates and stresses are introduced into the solidifying shell and these give rise to cracking and product Impairment as a result.

Similar problems occur with the alternative mould profile side view illustrated in Figure 3.

Deviations from the nominal passline have also been employed in a variety of other applications such as ** the utilisation of billet moulds which change in cross section along the length of the mould in order to achieve more even shell solidification thicknesses and to allow for faster casting; ** reduction of the section thickness or area near to final centreline solidification in order to provide inwards compaction which counteracts the volume shrinkage which occurs during solidification and so as to give improved centreline quality of the finally solidified product; ** creating deliberately bulged strand cross sections and then rolling these back to a plain cross section near to final centreline solidification in order to provide inwards compaction which counteracts the volume shrinkage which occurs during solidification and therefore gives improved centreline quality of the finally solidified product; ** casting a round cross sectional profile which is then reduced to a square section by rolling within the casting machine or subsequent roller processing; ** reducing the thickness or area of the strand whilst the core of the strand is still liquid in order to provide a cast section which is closer to the final require size.

The prior art techniques, however, are consistent in using either linear to linear transitions or linear to radius transitions which give rise to a sudden change in the direction of the shell path travel. This inevitably leads to areas of concentrated strain and high strain rates. As well as impairing product quality, this in turn leads to high loads on the mechanical support equipment giving reduce working life and increased strand withdrawal resistance and wear.

The present invention, however, ensures that any change in material movement direction, whether it be a change away from the nominal passline or towards it, is achieved in a continuous manner without abrupt changes in travel direction.

Figures 4a to 4e schematically illustrate the application of such a continuous curve to a variety of initial and subsequent movement directions.

Thus in Figure 4a a transition from a linear section A, via a continuous curve B to radius subsequent section C is achieved.

Figure 4b provides transition from a first radius direction D through continuous curve E to opposing radius direction F.

Figure 4c commences with radius G through transition continuous curve H to subsequent radius I in an equivalent direction to the first.

Figure 4d illustrates an initial linear section J which translates through first continuous curve K and then opposing direction continuous curve L to subsequent linear section M.

Figure 4e commences with initial linear section N and then translates through first continuous curve 0 and opposing direction continuous curve P to linear section Q angled relative to initial linear section N.

Continuous curves can be applied in a variety of different applications and configurations where deviation from the nominal passline exists.

Thus, as illustrated in Figure 5, the continuous curve is suitable for providing a gradual transition between the funnel portion 100 of a thin slab casting mould and the linear portion 102 of such a mould, in the direction of travel of the metal material from inlet 104 to outlet 106.

In more detail, the mould, in the initial portions of the funnel 100, has a linear taper 108 towards the desired strand product separation. The desired strand thickness is defined by the two parallel, linear sections 110 of the mould profile towards its outlet. To avoid rapid transitions in direction between the two portions 100 and 102, a continuous bend 112 is employed. The continuous curve 112 thus starts parallel to the tapering linear portion 100 of the mould and gradually curves away from the projection of this line, at a regularly increasing rate towards a maximum necessary to achieve the desired change in direction in the distance provided. From the maximum curvature, the clothoid 112 decreases in curvature once more at a constant rate. Eventually, the continuous curve 112 decreases to a direction parallel and conjoined with the linear portion 110 defining the outlet 106 for the mould.

By providing the transition in this way the metal shell during its movement in from the funnel towards the final strand thickness is subjected to the minimum strain. Uniform distribution of tensile strain over the continuous curve presents significant benefits as a result.

As well as being suitable for handling transitions in mould profile portions in the direction of travel of the strand through the mould such continuous curves are also suitable for handling changes of direction in other orientations. Thus, as illustrated in Figure 6, the continuous curves find application in handling the transitions between the bulges 150 of a thin slab mould and the linear lateral potions 152 of such a mould.

Moving along the side of the mould from the left hand side of the cross section, therefore, the mould is defined by an initial linear section 154 which is parallel to the opposing side 156 of the mould profile. This linear section 154 then enters a continuous curved section 158 which gradually increases in curvature before entering into an opposing handed continuous curve portion 160. The continuous curves 158, 160 define one half of the side of the bulge 150 of the mould, the subsequent half of the bulge being defined by a further continuous curve 162 which then transfers into an opposing handed continuous curve 164 and finally into the linear portion 166 of the mould profile forming the right hand side of the mould.

Once again, the minimising of the rate of change of direction ensures that metal moving from the bulge laterally into the parallel sided portions of the mould during progress of the strand down through the mould, ie into the page, is achieved without undue strain levels being imposed.

As illustrated in Figures 7a and 7b, the concept of continuous curve transition is not only applicable to direction changes within the mould itself.

Figure 7a illustrates a linear strand reduction process in which a mould 200 produces a slab 202 of thickness X at its exit. The slab 202 is then subjected to two parallel pairs of rollers 204 of equivalent spacing to the slab thickness before entering a linear reduction stage 206 designed to reduce the slab thickness down to Y, the product thickness for this stage.

In prior art arrangements, as shown in Figure 7a, the rollers 208 used to affect this reduction in slab thickness are provided with a set of rollers 208a parallel to one another and parallel to the passline 210 for the strand/slab 202 and with the opposing rollers 208b parallel to one another but inclined between the first rollers 204 defining slab thickness X and the subsequent set of rollers 212 defining slab thickness Y. As a consequence of this arrangement, whilst the strand/slab 202 is reduced in thickness evenly during the passage of the strand between the inclined rollers 20Bb and their opposing rollers 208a, the rate of change in slab thickness is far higher at the introduction 214 to this portion and at the exit 216 from this portion. As a consequence, at these points 214, 216 a high level of strain rate is applied to the strand. Equivalent problems to those discussed above occur in terms of equipment and product quality as a result.

By employing the technique of the present invention, as illustrated in Figure 7b, such locations of high strain rate can be avoided.

In the embodiment of Figure 7b, once more, a slab 202 of metal thickness X is cast in a mould 200 and then passes on to a pair of parallel rollers 204 of equivalent separation to the slab 202. In the subsequent stage 250 whilst the rollers 252a on one side of the system are maintained parallel to one another and to the passline 210, the rollers 252b on the opposing side take the form of two opposing handed continuous curves. Thus the position of rollers 252b gradually moves towards the opposing rollers 252a from spacing X towards the midpoint 258 according to a first continuous curve. From the midpoint 258 the opposing hand continuous curve is employed to gradually reduce the curvature of the rollers 252b towards the output linear section in which the pairs of opposing rollers 260 are parallel to one another and define the desired product thickness Y.

In this embodiment, as a result of the continuous curve involvement, the rate of change of curvature in the transition from the first linear spacing to the second linear spacing is kept to a minimum so reducing tensile strains and spreading these over a larger distance. Improved equipment performance and product quality result.

The continuous curve may be provided in a number of manners.

A clothoid (or Cornus's spiral) defined by a spiral with parametric cartesian equations:

may be used. The radius of curvature being 1/s.

A simplified version of the clothoid which can be employed is given by: Y = X3/6 Ro L Where: X and Y - cartesian co-ordinates of curve Ro = final (or initial radius of curve) L = horizontal length of curved section.

Such a curve is illustrated in Figure 8.

The present invention provides a useful technique for accommodating changes in slab thickness or other dimensions in any direction whether these be axially aligned with the direction of travel of the strand, perpendicular to the direction of travel of the strand, in either direction, or at an intervening angle thereto.

Claims

CLAIMS:

1. A method of casting in which any deviations of the cast strand away from or towards the theoretical passline or any deviations to the edges of the cast strand are defined, at least in part, by continuous curves.

2. A method of casting in which a strand of cast product is formed, the strand having a direction of travel, the profile of the strand being determined by strand profiling means, the thickness of the strand, perpendicular to the axis defining the direction of travel, being greater at a first location than at a second location and wherein the profile determining means and/or strand between the first and second locations are, at least in part, defined by one or more continuous curves.

3. A method according to claim 1 or claim 2 in which the continuous curve joins a first linear portion with a second linear portion and / or joins a first curve with a second curve and / or joins a linear portion with a curved portion.

4. A method according to claim 2 or claim 3 in which the first and second location are within the same cross sectional profile, the cross sectional profile being defined perpendicular to an axis, defined by the overall direction of travel, at that location.

5. A method according to claim 4 in which the thickness at the first location is greater than at the second location, the first location being closer to the centre line of the strand than the second location.

6. A method according to claim 4 or claim 5 in which the profile of the cross-section includes a portion defined by a pair of linear parallel wall portions and a portion defining a bulge inward and/or outward relative to the linear portion, the bulge being defined in two opposing walls, the bulges in the two walls mirroring one another.

7. A method according to any preceding claim in which a continuous curve is used to link one linear portion of a wall with another linear portion of a wall, the linear portions of the wall being at a significant angle to one another, for instance greater than 459.

8. A method according to claim 2 or claim 3 in which the first and second locations are axially separated from one another, the axis being defining by the overall direction of travel.

9. A method according to claim 8 in which the thickness at the first location is greater than at the second location, the second location being further along the axis defining the direction of travel than the first location.

10. A method according to any of claims 2 to 9 in which the first and/or second locations are defined with a mould.

11. A method according to any of claims 2 to 10 in which the first and/or second locations are defined outside a mould.

12. A method according to claim 11 in which the profile determining means are rollers, the first location being defined between a first set of opposing rollers, with the second location being defined between a second set of opposing rollers.

13. A method according to any of claims 2 to claim 11 in which the first location is defined within a mould, the second location being defined outside the mould.

14. A method according to any of claims 2 to 13 in which the first location is provided on a linear wall portion of the strand and the second location also provided on a further linear portion of the strand, the first and second linear portions being inclined relative to one another, the first linear portion being inclined relative to the axis defined by the overall direction of travel of the strand.

15. A method according to any of claims 2 to 14 in which the profile defining means incorporate continuous curves linking first and second locations in more than one orientation.

16. Casting apparatus comprising strand profile determining means, the strand profile determining means defining a casting direction, the separation of the strand profile determining means at a first location being greater than at a second location, the separation being measured perpendicular to the axis of the casting direction, the profile determining means extending between the first location and the second location, at least in part, being defined according to a continuous curve.

17. Apparatus according to claim 16 in which first and second location are within the same cross sectional profile, the cross sectional profile being defined perpendicular to an axis, defined by the overall direction of travel, at that location, the profile of the cross-section including a portion defined by a pair of linear parallel wall portions and a portion defining a bulge inward and/or outward relative to the linear portion, the bulge defined in two opposing walls of the strand, the bulges in the two walls mirroring one another.

18. Apparatus according to claim 15 or claim 17 in which the first and second locations are axially separated from one another, the axis being defined by the overall direction of travel, the thickness at the first location being greater than at the second location, the second location being further along the axis defining the direction of travel than the first location.

19. Apparatus according to claim 16, claim 17 or claim 18 in which the profile determining means are rollers, the first location being defined between a first set of opposing rollers, with the second location being defined between a second set of opposing rollers, the separation of the rollers forming pairs in the second set of rollers being less than the separation of the rollers forming pairs in the first set of rollers.

20. Apparatus according to any of claims 16 to 19 in which the first location is provided on a linear wall portion of the strand and the second location is provided on a further linear portion of the strand, the first and second linear portions being inclined relative to one another, the first linear portion being inclined relative to the axis defined by the overall direction of travel of the strand.