IMPACT PAD FOR A TUNDISH

Abstract: A tundish impact pad formed from refractory material, the impact pad (1) comprising a base (3) having an impact surface (5) which in use faces upwardly to receive molten metal poured onto the impact pad, and a wall (7) extending upwardly from the base around at least part of the periphery of the impact surface, the wall including an overhang (9) projecting over a peripheral region (11) of the impact surface (5), the overhang (9) including a plurality of protrusions (13) which project further over the impact surface (5) than the remainder of the overhang (5).

Description: Impact Pad for a Tundish This invention relates to the handling of molten metal, especially steel. In particular, the invention relates to a tundish impact pad, i.e. an article formed from refractory material to be placed on the floor of a tundish to receive an incoming stream of molten metal poured into the tundish from a ladle. The invention has particular utility in the continuous casting of steel. International Patent Application No. WO 96/14951 (FOSECO) discloses a tundish impact pad comprising a body of refractory material capable of withstanding contact with molten steel in a tundish. The impact pad body comprises a base having an impact surface, an outer sidewall extending upwardly from the impact surface, and a top surface connected to the sidewall and defining an opening therein. The top surface has an inner annular portion substantially parallel to the impact surface, and there is a substantially right angled corner between the sidewall and the impact surface, and also between the sidewall and the inner annular portion of the top surface. This impact pad provides increased residence time of the molten steel in the tundish, which is important to ensure that unwanted inclusions have sufficient time to float to the surface of the steel and to be removed. International Patent Application No. WO 97/37799 (FOSECO) discloses a tundish impact pad comprising a body of refractory material capable of withstanding contact with molten steel in a tundish. The impact pad body comprises a base having an impact surface for molten steel, an outer sidewall extending upwardly from the impact surface and extending around the base to completely enclose it. An annular body portion connected to the sidewall provides a top surface substantially parallel to the impact surface and defines an opening into which molten steel can be poured, the lower surface of the annular body portion and the inner face of the sidewall defining a recess having an undercut portion extending continuously around and above the impact surface. A portion of the top surface is at a lower level than the remainder of the top surface and the recess beneath the portion of the top surface is of smaller cross-section than the remainder of the recess. This impact pad is particularly useful for improving the flow characteristics in elongate tundishes in which the molten steel is poured into the tundish at an end of the tundish and the or each outlet for the steel is at the opposite end of the tundish. The impact pad is oriented such that the lower level portion of the end wall is adjacent to the end wall next to the incoming stream of molten steel, so that the steel rebounding from the pad preferentially flows towards this end wall. This has the effect of markedly reducing the surface turbulence created in the tundish, and generally improving the flow paths, resulting in the production of cleaner steel. It will be appreciated that the process of designing a new tundish impact pad which meets particular pre-determined criteria is extremely complex, since changing one aspect of the design of an impact pad generally has unforeseen ramifications on the flow dynamics of the entire tundish system. The present inventors have identified a need for a new tundish impact pad which causes the molten metal rebounding from the impact surface to be directed generally symmetrically about a longitudinal axis of the tundish. This is particularly important, for example, for tundishes in which the or each outlet is located on the longitudinal axis of the tundish, or where two or more outlets are arranged symmetrically about this axis. Furthermore, it is common for the incoming stream of molten metal (the "ladle stream") to be "off-center", i.e. not located on the longitudinal axis of the tundish. This commonly occurs because the ladle shroud (the pipe through which in-coming metal flows from the ladle into the tundish) normally moves together with a sliding gate valve which controls the flow of the metal from the ladle, and since the valve is frequently only partially open in order to limit the flow rate, the ladle shroud is frequently off-centre with respect to the tundish. According to a first aspect of the present invention, a tundish impact pad formed from refractory material is provided, the impact pad comprising a base having an impact surface which in use faces upwardly to receive molten metal poured onto the impact pad, and a wall extending upwardly from the base around at least part of the periphery of the impact surface, the wall including an overhang projecting over a peripheral region of the impact surface, the overhang including a plurality of protrusions which project further over the impact surface than the remainder of the overhang. The wall preferably extends around the entire periphery of the impact surface, fully enclosing the impact surface. According to a second aspect, the invention provides a tundish containing an impact pad according to the first aspect of the invention. The tundish is preferably elongate in shape (i.e. in plan view), it preferably has outlets near each longitudinal end, and in use the incoming stream of molten metal, and thus the impact pad, are preferably located substantially in the longitudinal centre of the tundish. The tundish preferably includes at least one dam extending across its width between the impact pad and an outlet. The top surface of the dam preferably has a central portion (i.e. central with respect to the longitudinal axis of the tundish) which is lower than peripheral portions on each side of it. This generally aids in directing the flow of the molten metal towards the longitudinal axis of the tundish. The inventors of the present invention have found that the provision of a plurality of protrusions which project further over the impact surface than the remainder of the overhang of the peripheral wall generally causes the molten metal rebounding from the impact surface to be directed substantially towards the centre of the impact pad. This generally has the effect of causing substantially symmetrical flow of the molten metal out of the impact pad, even if the metal flowing into the pad onto the impact surface is "off-centre", i.e. spaced a distance from the centre of the pad. Such substantially symmetrical flow out of the impact pad generally causes symmetrical flow throughout the tundish, resulting in the reduction of so-called "dead zones" where the rate of flow of the molten metal is relatively low (causing poor replenishment of fresh metal and the possibility of solidification of the metal) and generally improving the consistency of residence time in the tundish experienced by the molten metal. The impact surface of the pad preferably has at least two corners (e.g. four corners) over which a respective protrusion projects. Advantageously, at least one (and preferably all) of the corners may be formed at the convergence of substantially straight sides of the impact surface. More preferably, the impact surface may be substantially polygonal, most preferably substantially quadrangular, especially substantially rectangular or square. A protrusion preferably projects over each corner of a polygonal impact surface. Additionally or alternatively, at least one protrusion (preferably two protrusions or more) may extend along at least most of the length of a respective side of the impact surface, and may even extend along substantially the entire length of a respective side of the impact surface. In some preferred embodiments, the distance by which the overhang extending between two said protrusions projects over the impact surface is substantially uniform along at least most of its length. The overhang extending between two protrusions may advantageously be substantially straight along at least most of its length. Additionally or alternatively, however, for one or more of the overhang(s) extending between two protrusions, the distance by which the overhang projects over the impact surface may be substantially non-uniform along at least most of its length, for example one or more of the overhang(s) extending between two protrusions may be curved in plan view, for example such that the or each overhang projects further over the impact surface at the centre of its length than it does at each end, or alternatively such that it projects further at each end than it does at its centre. It will be appreciated that the shape and size of each overhang between protrusions, and/or the shape and size of each protrusion, may be varied according to each particular circumstance, e.g. depending upon the shape and/or size of the tundish, and/or the amount by which the incoming stream of molten metal is off-centre, or according to any other particular requirements of the user. The maximum distance by which the or each protrusion of the impact pad projects over the impact surface is preferably at least 110%, more preferably at least 120%, even more preferably at least 140%, especially at least 150% of the maximum distance by which the remainder of the overhang projects over the impact surface. The or each protrusion of the impact pad projects over the impact surface by a maximum distance preferably of no more than 600%, more preferably no more than 500%, especially no more than 450% of the maximum distance by which the remainder of the overhang projects over the impact surface. Advantageously, the height of the overhang above the impact surface may be non-uniform along its length. For embodiments of the invention having at least three protrusions, the height of the overhang extending between two of the protrusions is preferably different to that of the overhang extending between one of the two protrusions and another protrusion, or between two other protrusions. Advantageously, at least one portion of the overhang which is lower than that of another portion of the overhang may project a greater distance over the impact surface than does the other, higher, portion of the overhang. This has been found to cause molten metal rebounding from the impact surface and impinging on such lower overhangs to be re-directed back towards the centre of the impact pad generally more forcefully than the metal impinging on the higher overhangs. This in turn appears to contribute to causing the metal to flow symmetrically away from the impact pad. (However, no limitation on the interpretation of the invention should be construed from the postulated explanations stated herein of the functioning of its various aspects.) In some preferred embodiments, the peripheral region of the impact surface comprises a trough extending around at least part of a central region of the impact surface. For embodiments in which the impact surface has corners, the trough is preferably wider at the corners than elsewhere. For example, the shape of the trough (in plan view) may substantially mirror that of the overhang of the wall. The invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 shows, in isometric projection, a tundish impact pad according to the invention; Figure 2 shows, in isometric projection, a longitudinal cross-section of the tundish impact pad shown in Figure 1 ; Figure 3 is a schematic representation of the asymmetric flow of molten metal in an elongate tundish from an off-centre ladle-shroud; Figure 4 is a schematic representation of the symmetric flow of molten metal in an elongate tundish from an off-centre ladle-shroud using an impact pad and novel dams according to the invention; and Figure 5 shows a dam as shown schematically in Figure 4. Figure 1 shows a tundish impact pad 1 according to the invention, comprising a base 3 having an impact surface 5 which in use faces upwardly to receive molten metal poured onto the impact pad, and a wall 7 extending upwardly from the base around the periphery of the impact surface, the wall including an overhang 9 projecting over a peripheral region 11 of the impact surface, the overhang including a plurality of protrusions 13 which project further over the impact surface than the remainder of the overhang. The impact pad 1 is rectangular in plan view, both the base 3 and the impact surface 5 being rectangular in shape. Each of the protrusions 13 is located at one of the four corners of the pad. Along each of the longer sides of the rectangular pad, between the protrusions 13, the overhang 9a is relatively small, and is substantially straight and uniform. Along each of the shorter sides of the pad, between the protrusions 13, the overhang 9b is relatively large, and is curved in plan view. Furthermore, the overhangs 9b are at a lower level above the impact surface than are the overhangs 9a. The peripheral region 11 of the impact surface comprises a trough, the shape of which substantially mirrors that of the entire overhang (9a, 9b, 13). Figure 2 shows a longitudinal cross-section of the impact pad of Figure 1. The extent of the overhangs 9b is clearly visible, as is the depth of the trough 11. It is also clear that the shorter walls 7b are perpendicular to the impact surface 5, and the overhangs 9b are perpendicular to the walls 7b. The longer walls 7a are, however, inclined to the perpendicular, i.e. they slope outwardly from the impact surface to the overhangs 9a. Figure 3 is a schematic representation of the asymmetric flow of molten metal in an elongate tundish from an off-centre ladle-shroud. In each of parts (a) - (c), the tundish is shown in plan view, with the incoming ladle-stream of molten metal represented by a circle in the longitudinal centre of the tundish but off-centre in a transverse direction, i.e. closer to one longitudinal sidewall of the tundish than the other. The tundish also contains a pair of dams 15 (containing holes to permit flow-through of the molten metal) on either longitudinal side of the tundish, and outlets indicated by circles at each end of the tundish. The arrows indicate the flow paths of the molten metal; in part (a), the initial flow paths of metal entering the tundish are shown. (According to normal working practice, the tundish is already full of molten metal flowing through it - the flow paths shown merely indicate the predominant flow paths at various distances from the tundish.) Even at this early stage it can be seen that because the ladle-stream is off-centre, the flow is asymmetric with respect to the longitudinal axis of the tundish. In part (b), the molten metal has begun to flow through the dams, and the asymmetry of the flow paths is even more apparent. In part (c), the molten metal has reached the outlets. Indicated by the numeral 17 are so-called "dead-zones" where there is little or no flow of the molten metal during these steady-state conditions. The presence of dead zones in a tundish is a major problem, since the metal cools in these regions and may start to solidify; they also cause variations in the composition of the metal flowing out of the tundish. Figure 4 shows how the flow paths of the molten metal are changed by the use of an impact pad 1 according to the invention. Also present in the tundish of Figure 4 are dams 19 located closer to the outlets than the dams of Figure 3. These dams have a central portion 21 of their top surface which is lower than the peripheral portions 23 on either side of it. As in Figure 3, the in-coming ladle-stream of Figure 4 is once more off-centre, but this time the presence of an impact pad according to the invention causes the flow paths of the molten metal leaving the impact pad to be more symmetrical. By the time that the molten metal has reached the outlets the flow paths are substantially symmetrical, and no dead-zones are present. Figure 5 shows a front view and a cross-sectional view of a dam 19 (shown only schematically in Figure 4). The dam has a bottom surface 25 and side surfaces 27 shaped and sized to fit tightly in a tundish across its width. Its top surface has a central portion 21 which is lower than peripheral portions 23 on either side of it. This has the effect of channelling molten metal flowing over the dam along a generally axial flow path in the tundish. It is therefore complementary to the impact pad, in causing symmetrical and/or axial flow of molten metal in the tundish. The dam also has a plurality of holes 29 extending through it, to allow molten metal to flow through the dam in addition to the molten metal which can flow over the dam. The holes 29 are inclined upwards in the direction of flow of the molten metal, so as to impart an upwardly directed flow path to the metal flowing through the holes. The angle of inclination of the holes is preferably greater for the lower holes and preferably decreases on rising up the dam. As indicated in the drawing, the lowest holes preferably have an angle of inclination of 40°, reducing to 30° and then 20° respectively for the higher holes. The dimensions shown are in millimetres.

Claims: Claims 1. A tundish impact pad formed from refractory material, the impact pad comprising a base having an impact surface which in use faces upwardly to receive molten metal poured onto the impact pad, and a wall extending upwardly from the base around at least part of the periphery of the impact surface, the wall including an overhang projecting over a peripheral region of the impact surface, the overhang including a plurality of protrusions which project further over the impact surface than the remainder of the overhang. 2. An impact pad according to Claim 1 , in which the impact surface has at least two corners over which a respective protrusion projects. 3. An impact pad according to Claim 2, in which at least one of the corners is formed at the convergence of substantially straight sides of the impact surface. 4. An impact pad according to Claim 2 or Claim 3, in which the impact surface is substantially polygonal. 5. An impact pad according to Claim 4, in which the impact surface is substantially quadrangular, preferably substantially rectangular or square. 6. An impact pad according to Claim 4 or Claim 5, in which a protrusion projects over each corner of the polygonal impact surface. 7. An impact pad according to any preceding claim, in which the distance by which the overhang extending between two said protrusions projects over the impact surface is substantially uniform along at least most of its length. 8. An impact pad according to any preceding claim, in which the overhang extending between two said protrusions is substantially straight along at least most of its length. 9. An impact pad according to any one of claims 1 to 6, in which the distance by which the overhang extending between two said protrusions projects over the impact surface is substantially non-uniform along at least most of its length. 10. An impact pad according to Claim 9, in which one or more overhang(s) extending between two protrusions is curved in plan view 11. An impact pad according to any preceding claim, in which the maximum distance by which the or each protrusion projects over the impact surface is at least 110% of the maximum distance by which the remainder of the overhang projects over the impact surface. 12. An impact pad according to any preceding claim, in which the maximum distance by which the or each protrusion projects over the impact surface is no more than 600% of the maximum distance by which the remainder of the overhang projects over the impact surface. 13. An impact pad according to any preceding claim, in which the height of the overhang above the impact surface is non-uniform along its length. 14. An impact pad according to Claim 13, comprising at least three said protrusions, the height of the overhang extending between two of the protrusions being different to that of the overhang extending between one of the two protrusions and another protrusion, or between two other protrusions. 15. An impact pad according to Claim 13 or Claim 14, in which at least one portion of the overhang which is lower than that of another portion of the overhang projects a greater distance over the impact surface than does the other, higher, portion of the overhang. 16. An impact pad according to any preceding claim, in which the peripheral region of the impact surface comprises a trough extending around at least part of a central region of the impact surface. 17. A tundish containing an impact pad according to any preceding claim. 18. A tundish according to Claim 17, also containing one or more dams extending across its width, the or each dam having a top surface with a central portion which is lower in height than laterally peripheral portions on each side of it.

Inventors: LOPEZ-RAMIREZ SIMON, MORALES RODOLFO D, PALAFOX-RAMOS JORGE, LOPEZ-RAMIREZ, SIMON, MORALES, RODOLFO, D., PALAFOX-RAMOS, JORGE
Applicants: FOSECO INT, LOPEZ RAMIREZ SIMON, MORALES RODOLFO D, PALAFOX RAMOS JORGE, FOSECO INTERNATIONAL LIMITED, LOPEZ-RAMIREZ, SIMON, MORALES, RODOLFO, D., PALAFOX-RAMOS, JORGE
Application year: 2000
Main classification: B22D41
IPC codes: B22D11, B22D41