RIDGED TUNDISH STOPPER

Description

FIELD OF THE INVENTION

The present invention relates to a stopper for controlling the flow of molten metal through an exit hole of a tundish towards a mould or a casting tool.

DESCRIPTION OF PRIOR ART

A stopper is typically used to control the flow of liquid metal flowing out of a tundish towards a mould or a casting tool. The stopper comprises an elongated body having a lower end designed to close an exit hole at the bottom of the tundish. The stopper moves up and down, to close or open the exit hole.

The lower end of the stopper is immerged in the liquid metal. The upper end of the stopper is in the air. An intermediate part of the stopper is in contact with a slag layer floating on the liquid metal at the interface with air. The slag, flowing down in vortices along the stopper, erodes chemically and mechanically the stopper, reducing locally its diameter and decreasing its lifetime. Moreover, when the liquid metal level is low, the slag may be driven down by vortices into the exit hole.

In order to inhibit vortices, document CN110788314 A discloses a stopper having large blocking table and rib plates.

In order to increase the lifetime of the stopper, document KR20140140429A discloses a stopper made of an elongated body and a removable protection piece at the level of the slag. The slag is expected to erode the removable protection piece, which can then be removed and replaced by a new one.

CN106735153A describes a stopper with rings aiming at facilitating floating of inclusions in order to improve the cleanliness of the molten steel.

A problem of these known stoppers is that they comprise a thick relief strongly increasing locally the diameter of the stopper, and thus its resistance to motion in the liquid metal. A fine control of the stopper up and down motion is thus impossible.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a stopper reducing the vortices along its lateral wall.

The invention relates to a stopper for controlling the flow of liquid metal out of a tundish, the stopper extending in an axial direction between an upper end and a lower end and being radially limited by a lateral wall comprising a plurality of ridges, wherein the ridges extend at least partially along a circumferential direction on the lateral wall, wherein each ridge has a radial extension that is less than half a radius of the stopper taken at said ridge and is at least 0.01 times the radius of the stopper taken at said ridge, the ridges have an angular extension lower than 180°, and the ridges are disposed in quincunx along the lateral wall.

The inventors have shown, through simulations, that the ridges break up large vortices which create slag entrainment along the lateral wall and thus strong erosion. Moreover, they have shown that the average wall shear stress on the stopper is decreased with respect to a stopper without ridges.

The stopper according to the invention does not comprise any thick relief since all ridges are radially limited in size by half of the radius of the stopper at their level. This absence of thick relief provides a low resistance to motion and thus a good response to control. This upper limit in the radial dimension of every ridge of the claimed stopper is not present in the stopper of KR20140140429A where the removable protection piece, reference 200, provides an extremely large ridge. In this prior art document, the “thread”, reference 110, is not intended to be used without the removable protection piece or to be in contact with the slag, and forms a single ridge and not a plurality of ridges.

When the level in the tundish is low, the slag may be driven, by the vortices, into the exit hole of the tundish, towards the mould or casting tool in the downstream process. This impairs the quality of the moulded pieces and creates operational and safety issues, i.e. breakouts. The ridges in contact with the slag when the level in the tundish is low decrease the driven effect of the vortices on the slag. They thus decrease the problem of slag flowing into the exit hole.

Since the ridges are disposed in quincunx along the lateral wall, the ridges that are vertically adjacent are at least partially shifted circumferentially. The inventors have shown that this shift between the ridges has a strong effect on the entrainment of the slag along the stopper because the slag has to move left and right when moving down along the stopper.

Ridges that are adjacent along the axial direction are not completely aligned axially. The stopper is preferably one piece. The stopper is preferably moulded in one piece. The ridges are preferably integral with the stopper, and especially with the upper end and the lower end. At least some of the ridges are closer to the lower end than to the upper end.

The stopper does not comprise any protrusion having a radial extension that is more than half a radius of the stopper taken at said protrusion. The stopper does not comprise any removable protection piece around the lateral wall. The ridges are preferably not helicoidal.

The lateral wall is an external surface. The ridges are preferably designed to be in contact with the slag at the top surface of the liquid metal.

In an embodiment, the ridges extend partially along the circumferential direction and partially along the axial direction on the lateral wall. In another embodiment, the ridges are only along the circumferential direction.

The invention also relates to a system for continuous casting comprising:

• • a tundish comprising an exit hole;
  • liquid metal in the tundish, the liquid metal having a top surface;
  • a stopper as described in the present document, configured for closing the exit hole and disposed in such a way that the top surface of the liquid metal is in contact with at least some of the ridges; and
  • a support holding the stopper.

Preferably, the system further comprises:

• • a mould or casting tool below the tundish configured to receive liquid metal flowing through the exit hole;
  • a sensor for sensing the liquid metal in the mould or casting tool; and
  • a data processing unit connected to the sensor and to the support and configured for controlling the support as function of information received from the sensor.

With such a system, the vertical motion of the stopper, and consequently the metal flow in the exit hole, is controlled based on measurement in the mould or casting tool.

The invention also relates to a method for controlling a stopper as described in the present document in a system as described in the present document, wherein the support moves the stopper vertically, preferably with a frequency between 0.1 Hz and 100 Hz.

With the stopper according to the invention, the vertical motion of the stopper is not impaired by any thick protrusion, which is especially advantageous for quick oscillations.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a vertical cross-section of a tundish,

FIG. 2 is a vertical cross-section of a part of a stopper,

FIG. 3 is a horizontal cross-section of a part of a stopper, along the plan shown III-III at FIG. 2,

FIG. 4 is a vertical cross-section of a part of a tundish with a stopper,

FIG. 5 is a zoom on the stopper of FIG. 4,

FIG. 6 is a 3D view of a part of the stopper of FIG. 4,

FIG. 7 is a 3D view of a part of a stopper,

FIG. 8a is a vertical cross-section of a part of a tundish with a stopper according to the prior art,

FIG. 8b is a vertical cross-section of a part of a tundish with a stopper,

FIG. 9a is a vertical cross-section of a part of a tundish with a stopper according to the prior art, and

FIG. 9b is a vertical cross-section of a part of a tundish with a stopper.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto. The drawings described are only schematic and are non limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein.

Furthermore, the various embodiments, although referred to as “preferred” are to be construed as exemplary manners in which the invention may be implemented rather than as limiting the scope of the invention.

The term “comprising”, used in the claims, should not be interpreted as being restricted to the elements or steps listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising A and B” should not be limited to devices consisting only of components A and B, rather with respect to the present invention, the only enumerated components of the device are A and B, and further the claim should be interpreted as including equivalents of those components.

FIG. 1 represents a tundish 60. In the tundish 60, liquid metal 65 flows from a ladle shroud 62 to an exit hole 61. The top surface 66 of the liquid metal is a layer of slag. A stopper 1 controls the flow of metal through the exit hole 61.

The stopper 1 comprises a refractory material. The stopper 1 has an upper end 91 (which may be called first end) connected to a support 69 that moves the stopper 1 vertically, and a lower end 92 (which may be called second end) able to close the exit hole 61. The stopper 1 may comprise a sleeve 93, preferably located closer to the upper end 91 than to the lower 92, and made of a material more resistant to erosion than the rest of the stopper 1. The stopper 1 comprises a through hole 94 for gas injection. The exit hole 61 is connected to a tundish upper nozzle 71 and a submerged nozzle 70 leading the liquid metal to a mould 64 or casting tool.

The mould 64 or casting tool below the tundish 60 receives the liquid metal flowing through the exit hole 61 when it is not totally obstructed by the stopper 1. A sensor 67 detects at least one characteristic of the liquid metal 72 in the mould 64 or casting tool, for example the metal level. The sensor 67 sends information to a data processing unit 68 that controls the support 69. The information is thus used to move the stopper 1 vertically. The vertical position of the stopper controls the flow into the exit hole 61. In addition to its vertical motion regulating the flow through the exit hole 61, the stopper 1 may oscillate vertically (which may be called “dithering”). The frequency of the oscillations is preferably in the range from 0.1 Hz to 100 Hz, more preferably from 0.2 Hz to 10 Hz, and its amplitude in the range from 0.2 to 10 mm.

FIGS. 2 and 3 illustrate a stopper 1 in an embodiment of the invention. The stopper 1 has an axis 100. The stopper 1 is hereby described referring to an axial direction 101 (which is vertical in use), a radial direction 102 (horizontal in use and extending away from the stopper axis 100), and a circumferential direction 103 (horizontal and perpendicular to the radial direction 102).

The stopper 1 has a lateral wall 10 shaped by ridges 12 that protrude radially. The lateral wall 10 circumferentially surrounds the stopper 1. The ridges 12 have a top side 51, a tip 52 and a bottom side 53.

Considering any of the ridges 12, its radial extension 21 can be determined and the radius 20 of the stopper 1 at the considered ridge 12 can also be determined. In the invention, the radius 20 of the stopper 1 at any considered ridge 12 is more than twice the radial extension 21 of said considered ridge 12, preferably more than three times the radial extension 21 of said considered ridge 12, more preferably more than four times the radial extension 21 of said considered ridge 12. As visible at FIG. 2, the radius 20 of the stopper 1 measured at a ridge 12 may be equal the sum of the radial extension 21 of said ridge 12 with the radius up to said ridge 12.

The stopper 1 comprises a group 16 of ridges 12 comprising at least two adjacent ridges 12 separated along the axial direction 101 by a gap 19. These two ridges 12 are not completely aligned axially. The radial extension 21 of a ridge 12 is preferably measured with respect to the deepest point of all the gaps 19 adjacent to said ridge 12. Preferably, the stopper 1 comprises up to 100 ridges. The ridges 12 are not totally along the axial direction 101: any of the ridges 12 is either only circumferential, or partially circumferential and partially axial. At least some of the ridges 12 may be in the sleeve 93.

For conciseness in the present document, the diameter 22 of the stopper 1 at the uppermost ridge 121 is identified hereby as DU. Preferably, the ridges 12 have an axial extension 41 between 0.01 DU and 0.4 DU, preferably between 0.03 DU and 0.24 DU, more preferably between 0.05 DU and 0.16 DU. The axial extension 41 is preferably measured as close as possible to the axis 100, i.e., at the beginning of the ridge 12. Preferably, the gaps 19 have an axial extension 42 between 0.02 DU and 0.3 DU, preferably between 0.04 DU and 0.3 DU, more preferably between 0.05 DU and 0.1 DU.

For conciseness in the present document, the radius 20 of the stopper 1 at a ridge 12 is identified hereby as RS. Preferably, each ridge 12 has a radial extension 21 between 0.01 RS and 0.4 RS, preferably between 0.05 RS and 0.3 RS, more preferably between 0.1 RS and 0.2 RS.

The stopper 1 may be cylindrical or have a conical vertical section. The conical vertical section may be such that an angle α between 1° and 30° exists between the lateral wall 10 and the axial direction 101. The ridges 12 have an angular extension, in a plan perpendicular to the axis 100, β lower than 180°, preferably lower than 90°, 60° or 45°.

FIG. 4 illustrates a stopper 1 with its lower end 92 closing the exit hole 61 of the tundish 60. The exit hole 61 is connected to the tundish upper nozzle 71. The lower end 92 is covered by an anti-stick coating 95.

FIG. 5 is a zoom into the stopper 1 of FIG. 4, showing the radial extension 21 of the ridge 12, and the radius 20 of the stopper 1 at this ridge 12. The radial extension 21 of the ridge 12 is measured from the line 190 which is axial and passes at the deepest point of the gap 19 adjacent to said ridge 12.

The ridges 12 are disposed in a plurality of circumferential rows 25, 26 on the lateral wall 10. Each row horizontally surrounds the stopper 1. The plurality of circumferential rows 25, 26 comprises preferably only two rows 25, 26: a first row 25 and a second row 26, the first row 25 being closer to the upper end 91 than the second row 26. The ridges 12 of the lowest of all the rows 25, 26 may be configured to contact the tundish upper nozzle 71. The quincunx disposition of the ridges 12 implies a circumferential offset between ridges 12 of adjacent rows 25, 26. Each row 25, 26 may comprise between three and twelve ridges 12. The number of ridges 12 in the rows 25, 26 is preferably identical. Within each row 25, 26, the ridges 12 have preferably an identical shape. The ridges 12 of different rows 25, 26 may have identical or different shapes. For example, in the stopper 1 of FIGS. 4 and 5, the ridges 12 are identical within each row 25, 26 but are different between rows 25, 26. The ridges 12 of the second row 26 are preferably located, at least partially, around a narrowing 96 of the through hole 94 of the stopper 1.

The top side 51 of the ridges 12 make an angle θ1 with the axis 100, θ1 being measured downwards. The bottom side 53 of the ridges 12 make an angle θ3 with the axis 100, θ3 being measured upwards. The angles θ1 and θ3 are preferably such that the top side 51 and the bottom side 53 extend radially away towards each other. In other words, such top and bottom sides are getting closer as extending away from the axis 100. The angles θ1 and θ3 may be between 20° and 60°. As shown on FIGS. 4 and 5, θ1 may be lower than θ3 for the first row 25 and higher than θ3 for the second row 26. The tip 52 of the ridges 12 may be parallel to the axis 100.

The stopper 1 preferably comprises a higher part 55, a ridged part 56 comprising the ridges 12, and a bottom part 57 comprising the lower end 92. The higher part 55 may comprise the upper end 91. The ridged part 56 is directly between the higher part 55 and the bottom part 57. In a preferred embodiment of the invention, the ridged part 56 has a diameter, taken without the ridge 12 (which is the difference between the diameter 22 of the stopper 1 at the uppermost ridge 121 and twice the radial extension 21 of the uppermost ridge 121), that is higher than any of the diameter 58 of the higher part 55 or the diameter 59 of the bottom part 57, irrespective of where the diameter 58 of the higher part 55 is measured or where the diameter 59 of the bottom part 57 is measured. In other words, the ridges 12 are located in the thickest section of the stopper 1.

FIGS. 6 and 7 illustrate two stoppers 1 according to the invention, wherein the ridges 12 are closer to the lower end 92 than to the upper end 91. The angles θ1 and θ3 are below 90° in the embodiment of FIG. 6, and, in the embodiment of FIGS. 7, θ1 is higher than 90° and θ3 is lower than 90°. The tip 52 of the ridges 12 may be planar, sharp, or rounded.

FIGS. 8a to 9b compare Computional Fluid Dynamics (CFD) simulations for a stopper 2 without any ridge (called below standard stopper) and a stopper 1 with ridges 12 as illustrated at FIG. 6 (called below ridged stopper). On these figures, reference 111 is the direction of the flow, reference 112 is the slag and reference 113 is the steel.

FIG. 8a (for the standard stopper 2) and 8b (for the ridged stopper 1) show the direction of the flow 111. It appears that, under the same conditions, the steel 113/slag 112 interface bent downwards for the standard stopper 2 while this interface is globally flat for the ridged stopper 1.

FIG. 9a (for the standard stopper 2) and 9b (for the ridged stopper 1) show the slag entrainment. It appears that, under the same conditions, the slag 112 is entrained into the tundish upper nozzle 71 for the standard stopper 2 while the slag 112 is not entrained into the tundish upper nozzle 71 for the ridged stopper 1.

Although the present invention has been described above with respect to particular embodiments, it will readily be appreciated that other embodiments are also possible. Moreover, the feature(s) of any described or illustrated stopper is (are) hereby considered as combinable with feature(s) of any other stopper(s).