EP0355940A2

EP0355940A2 - Continuous casting mold with removable insert

Info

Publication number: EP0355940A2
Application number: EP89300988.6A
Authority: EP
Inventors: Libor F. Rostik; Robert J. Simcoe
Original assignee: Chaparral Steel Co
Current assignee: Chaparral Steel Co
Priority date: 1988-06-27
Filing date: 1989-02-01
Publication date: 1990-02-28

Abstract

Apparatus comprising a continuous casting mold (18) for horizontal steel casting having a removable insert (32) constituting the entry end of the mold. The insert (32) may be of a material that is more wear resistant and has better thermal conductivity than the remainder of the mold (18). The insert (32) may be of a conductive ceramic material and may have an integral break ring (34, 36, 38) formed thereon, with the break ring (34, 36, 38) being of a non-conductive ceramic material.

Description

This invention relates to apparatus for the continuous horizontal casting of molten material, particularly metals and metal alloys.
In the continuous casting of molten material, particularly metals and alloys such as steel, the apparatus for this purpose includes a tundish for receiving the molten material to be cast. The tundish has a refractory, horizontally disposed nozzle through which the molten metal leaves the tundish for casting. The nozzle at the end opposite that connected to the tundish is connected to a flow-through continuous casting mold. The continuous casting mold is made of a heat-conducting material, usually copper or copper alloys, and provision is made for circulation of liquid, usually water, to cool the mold. As the molten metal enters the mold and contacts the cooled, interior mold surfaces, it is solidified to form a solidified skin of the molten metal with the interior portion remaining in the molten condition. The thickness of the solidified skin increases progressively along the length of the mold. As this partially solidified or embryo casting leaves the mold, the skin is sufficiently thick to prevent the breakout of molten metal. Thereafter, the embryo, casting is progressively cooled and eventually complete solidification of the casting is achieved.
The refractory material of the nozzle which is in contact with the molten metal leaving the tundish is at extremely high temperatures. In contrast, the mold abutting the refractory nozzle is at significantly lower temperatures. Consequently, it is conventional practice to place on the interior of the mold a break ring to serve as a transition material between the refractory of the nozzle and the heat-conducting material of the cooled mold. The break ring functions to define the point at which the shell of the casting begins to form when the molten metal initially enters the mold. It prevents solidification and hang-up of the metal at the end of the nozzle at the interface of the nozzle and mold. More specifically in this regard, as is well known, relative oscillation or vibration of the mold and casting is provided longitudinally to facilitate withdrawal of the partially solidified casting from the mold. If metal enters and solidifies at the interface or connection between the nozzle and mold a solidified metal projection, commonly termed a "fin", is formed. This, during withdrawal of the casting from the mold, results in surface irregularities on the casting skin, which can cause cracking with resulting molten metal break-out. In addition, the break ring prevents the molten metal from freezing within the pores of the refractory material adjacent the continuous casting mold. In summary, the break ring prevents damage to the newly formed solidified casting skin at the entry end of the mold. For this purpose, and particularly in the continuous casting of molten alloys, such as steel, the break ring must have chemical resistance to the steel, high resistance to thermal shock, low thermal conductivity, high resistance to wear and erosion and accurately conform to the surface on which it is mounted. To meet these requirements, the break ring is conventionally constructed from refractory oxides or nitrides, such as boron nitride, silicon nitride and zirconia, and is machined to the proper contour to achieve the required accurate mounting.
The continuous casting mold in typical steel casting operations is of a generally round and_/or rectangular configuration to permit the casting of slabs or billets which are subsequently reduced to flat-rolled sheet and strip or long structured products, including bars and beams, respectively. The configuration of the mold may also conform generally with the shape of other products to be cast. Consequently, the interior cross-section of the continuous casting mold must conform to this desired configuration. Each mold, however, due to inaccuracies in. construction will vary somewhat in dimension. Therefore, the break ring requires either machining to very close tolerances to mate with the interior of each continuous casting mold or may be formed integrally with the mold. If the break ring is not accurately dimensioned with respect to the mold interior onto which it is mounted, this will result in the molten metal propagating between the connection of the break ring and the mold interface surface. As discussed herein, this may prevent withdrawal of the casting.
Because the temperature of the molten metal issuing from the nozzle through the continuous casting mold is highest at the entry end of the mold, this end of the mold is subjected to more rapid wear than the remainder of the mold body. Consequently, it is the wear at the mold inlet end that first causes deterioration of the mold sufficient to require replacement thereof. In this instance, the remainder of the mold body is generally not worn to the extent requiring replacement. Also, thermal shock resulting at the interface at the entry end of the mold may cause a crack in the newly formed skin of the casting which remains during progressive solidification. This may result in a site for crack propagation during subsequent rolling.
It is accordingly an object of the present invention to provide apparatus for the continuous horizontal casting of molten material which enables the problem of wear at the entry end of the mold to be dealt with in a cost effective manner to provide a more economical casting practice.
Another object of the invention is to provide a continuous casting mold which can be provided with an integral break ring of non-conductive material more readily than has hitherto been the case.
The present invention provides apparatus for the horizontal continuous casting of molten material, including a tundish for receiving a quantity of molten material, such as molten metal and alloys, to be cast, a refractory nozzle providing a molten material outlet from said tundish, and a flow-through mold of heat-conducting material having an inlet end contacting said nozzle for receiving said molten material therefrom and an outlet end for discharging an embryo casting resulting from cooling of said molten material during passage through said mold, characterised in said apparatus including a removable insert comprising said mold inlet end and with one end of said insert contacting said nozzle and an opposite end thereof connected to a body portion of said mold.
The insert may be constructed from a material having higher wear resistance and/or thermal conductivity or controlled thermal conductivity with respect to the body portion of the mold. By controlled thermal conductivity, the conductivity of the insert may be varied along its length and specifically may increase progressively from the end contacting the nozzle to the opposite end. The body portion of the mold may be constructed from copper or a copper alloy in the conventional manner, with the insert constructed from a material that may be a second copper alloy having higher wear resistance and different or higher thermal conductivity than the copper or copper alloy of the body portion of the mold. For this purpose, the insert material may for example be composite material, ceramic or alloys, such as copper alloy including at least one alloying element providing increased wear resistance and/or thermal conductivity, which alloying elements may include zirconium and silver. The composite material may be for example composites of copper ceramic alloys. The insert may be constructed from a conductive ceramic having higher wear resistance than the body portion of the mold. In this embodiment, the insert may be less conductive than the body portion of the mold. The insert may include a break ring and specifically a non-conductive, ceramic break ring, formed integrally on the insert at the inlet end thereof adjacent the nozzle, without any intermediate bonding layer being provided between the break ring and the insert.
The invention will be more particularly described with reference to the accompanying drawings, in which:

Figure 1 is a somewhat schematic view in vertical cross-section of a portion of a continuous casting apparatus according to an embodiment of the present invention; and
Figures 2, 3 and 4 are similar cross-sections showing alternate embodiments of the present invention.

With respect to Figure 1 of the drawings, there is shown an assembly of a portion of a continuous, horizontal steel casting apparatus, designated generally as 10. The apparatus includes a tundish 12 containing therein a quantity of molten steel 14. A refractory nozzle 16 is connected at an outlet end to the interior of the tundish 12 and at the opposite end to the inlet end of a horizontally disposed, continuous casting mold 18. The mold 18 has a generally round and/or rectangular configuration and constitutes a copper body portion 20 having an interior water circulation chamber 22 communicating with water inlet 24 and water outlet 26. This structure provides for the circulation of water through the chamber to cool the mold. As the molten metal 14 enters the mold 18 and contacts the copper water-cooled mold body 20, it begins to solidify to form a solidified skin 28 with the interior constituting molten metal 14. A break ring 30 is provided at the entry end of the mold.
In accordance with the invention, there is provided at the entry end of the mold a removable insert 32. The insert 32, in accordance with the embodiment shown in Figure 1, constitutes an alloy containing copper and having higher wear resistance than the copper or copper alloy constituting the body portion 20 of the continuous casting mold. Since the inlet end of the mold is subjected to greater wear than the body portion of the mold because of the higher temperature of the molten metal as it enters the mold and comes into contact with the inlet end thereof, the insert 32 is subject to greater wear and degradation from the molten metal. Since the insert is removable, it may be replaced when it becomes worn, without replacing the body portion of the mold which is subjected to less wear than the entry end of the mold. This results in a more economical casting practice, since the entire mold does not have to be replaced because of wear at the entry end thereof.
Figure 2, illustrates an alternate embodiment of the invention. In Figure 2 an insert 32 of a conductive ceramic is provided. Integral with the insert 32 is a break ring 34 constructed of a non-conductive ceramic. The integral break ring 34 may be formed by conventional thermal spraying, such as plasma spraying, of the refractory compound onto the refractory of the insert 32. Plasma spraying, as is well known, includes providing a powder charge of the refractory compound to be used in the manufacture of the integral break ring. The powder charge is introduced to a high-temperature gas plasma which melts the particles and sprays them onto the surface to be coated with the refractory compound. Spraying of the refractory compound to deposit the same is achieved by accelerating the molten refractory compound by the plasma arc. Although various techniques are known for this purpose and the invention embodies any equivalent practice, various conventional thermal spraying practices suitable for use in the practice of the invention are disclosed in the article "Tomorrow's Surface Coatings," Iron Age, June 21, 1985, pages 35-49. This could also be achieved by various other techniques, included powder metallurgy and processes based thereon.
Although the break ring 34 is shown having a cross-section of generally rectangular configuration, other cross-sectional shapes may be used and are - well known in the art. For example, the surface of the break ring in contact with the molten metal may be rounded or slanted rather than being at right angles as shown in the embodiment of Figure 2. Also, as is well known in the art, the break ring may be positioned other than as shown in this Figure. For example, it may be on the outer end surface of the mold adjacent the nozzle.
Although various ceramic compounds may be suitable for use in the practice of the invention for constructing the insert 32 and break ring 34 examples of suitable conventional compounds for this purpose are molybdenum and copper alloys, alloys of titanium boride and the like. Likewise, suitable alloys of copper for use in constructing the insert 32 in the embodiment of Figure 1 may be copper silver bearing or zirconium bearing alloys. Specifically in this regard, Figure 3 shows the insert 32 having an integral break ring 36 with the cross-sectional area thereof decreasing progressively from the end thereof contacting the nozzle to the opposite end. In this manner, controlled thermal conductivity is achieved, whereby with a break ring of non-conductive material as the cross-section of the break ring decreases relative to the cross-section of the insert the thermal conductivity of the insert correspondingly increases. This may also be achieved with the embodiment of Figure 4 where the break ring 38 is of uniform cross-section along its entire length by varying the thermal conductivity of the break ring material from end to end thereof by using materials of different thermal conductivity in constructing the break ring. This may be achieved for example by flame spraying the different materials during construction of the break ring. Alternately, the break ring may be constructed by powder metallurgy techniques where different materials of varying thermal conductivity in powder form are consolidated to form the break ring. Consolidating techniques may include conventional hot isostatic pressing as by the use of a gas pressure vessel, commonly termed an autoclave.
It will readily be appreciated that because the break ring is formed integrally with the removable insert 32 prior to the insert being inserted into the mold the provision of the break ring is facilitated as is its replacement. Moreover the insert 32 can be replaced when worn without the need for replacing the body of the mold.

Claims

1. Apparatus for the horizontal continuous casting of molten material, including a tundish (12) for receiving a quantity of molten material (14), such as molten metal and alloys, to be cast, a refractory nozzle (16) providing a molten material outlet from said tundish, and a flow-through mold (18) of heat-conducting material having an inlet end contacting said nozzle (16) for receiving said molten material (14) therefrom and an outlet end for discharging an embryo casting resulting from cooling of said molten material (14) during passage through said mold (18), characterised in said apparatus including a removable insert (32) comprising said mold inlet end and with one end of said insert (32) contacting said nozzle (16) and an opposite end thereof connected to a body portion of said mold.

2. Apparatus according to claim 1, wherein said insert (32) is - constructed from a material more resistant to wear than said body portion of said mold (18).

3. Apparatus according to claim 1 or 2, wherein said insert material has better thermal conductivity than said body portion of said mold (18).

4. Apparatus according to claim 1, 2 or 3, wherein the thermal conductivity of said insert (32) increases progressively from the end thereof contacting the nozzle (16) to the opposite end.

5. Apparatus according to any one of the preceding claims, wherein said insert material is a copper alloy.

6. Apparatus according to claim 5, wherein said copper alloy includes at least one of the alloying elements zirconium or silver. _'

7. Apparatus according to any one of claims 1 to 4, wherein said insert material is a conductive ceramic having higher wear resistance than said body portion of said mold (18).

8. Apparatus according to any one of the preceding claims, wherein a break ring (34, 36, 38) is integrally formed on said insert (32) at the inlet end thereof without any intermediate bonding layer between said break ring (34, 36, 38) and said insert (32).

9. Apparatus according to claim 8 wherein said break ring (34, 36, 38) is of a non-conductive ceramic material.

10. Apparatus according to claim 8 or 9, wherein said break ring (36) decreases in cross-sectional area from an end thereof adjacent the nozzle (16) to the opposite end.

11. Apparatus according to claim 8, 9 or 10, wherein the material of said break ring (34, 36, 38) decreases in thermal conductivity from an end thereof adjacent the nozzle (16) to the opposite end.