Process for starting a continuous casting mold

Abstract

A method of initiating a continuous casting mold includes steps of positioning a plug inside a continuous casting mold, sealing the plug with respect to inner walls of the mold, positioning a prefabricated chill member on a top surface of the plug; and introducing molten metal into the mold. By prefabricating the chill member, valuable time is saved during the mold startup phase. The prefabricated chill member may have a compressible metallic sealing strip secured thereto to enhance sealing against the mold wall without abrading or scratching the mold wall. Another aspect of the invention involves sealing the gaps that exist between the mold surfaces and the mold plug by use of a an elongated body that defines in cross-section a broadened upper portion and a tapered lower portion that narrows into a leading edge. This construction permits the sealing element to be quickly inserted into a gap without being caught on the mold plug.

Description

[0001] This is a continuation of application Ser. No. 09/321,245, filed May 27, 1999, the entire disclosure of which is hereby incorporated as if set forth fully herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to the field of continuous casting of metals, and more specifically processes and equipment that are used to initiate casting in a continuous casting mold.

[0004] 2. Description of the Related Technology

[0005] In the conventional continuous casting process, molten metal is poured into an open top of a mold that also has an open bottom. As it cools, the metal solidifies against water-chilled interior walls of the mold to form a solidified skin surrounding a molten core. The solidifying strand emerges from the lower end of the mold and is guided away from the mold. After the strand leaves the mold, cooling continues by the use of water sprays so that the core ultimately solidifies. The conventional process involves withdrawing the partially solidified strand downwardly, and gradually curving the strand along an arc until it continues in a horizontal direction for cutting. The movement of the strand along the arc involves guiding the strand with appropriate guide rollers. Then, the strand is cut to desired lengths for further processing.

[0006] Before the casting operation can be initiated, the lower end of the mold must be temporarily plugged to prevent the molten metal from simply running through it. In other words, the molten metal must be retained within the mold for a sufficient period of time for solidification to begin. Conventionally, the plug or “dummy bar head” is held in place by a what is commonly termed a “dummy bar,” or “starter bar,” which is an elongated strip or chain having an end fastened to the mold plug.

[0007] After solidification occurs near the bottom of the mold, the incipient strand will itself plug the bottom of the mold, so the starter plug is no longer necessary. An endwise pulling force on the dummy bar withdraws the plug from the mold and draws the plug and attached strand along the arcuate path that is defined by the guide rollers. The strand is eventually separated from the dummy bar so that the dummy bar can be reused.

[0008] When the plug is placed in the mold, clearance is provided for so that the plug may be safely inserted without damaging the mold's inner surfaces, thus a gap exists around the perimeter of the plug. However, it is essential that the plug form a tight seal with the walls of the mold. Molten steel has a viscosity approximating that of water, and can quickly leak from the mold if the plug is not well seated. In the past, asbestos cord was used as packing about the plug to ensure a tight seal. For environmental reasons, however, asbestos is no longer used and has been replaced with alternative materials such as board that is fabricated from heat-resistant ceramic material such as vitreous aluminosilicate fibers.

[0009] FIG. 1 is a diagrammatic depiction of a continuous casting machine 10, which, as is conventional, has mold walls 12 with inner mold wall surfaces 14. A dummy bar 16 is shown positioned in the mold 10 and a mold plug 18 is secured to the dummy bar 16. Currently, after the plug has been placed in the mold, strips 20 of the fibrous material are pushed downwardly from the top of the mold into the gaps between the plug and the mold walls. The fibrous material, as is shown in cross-section in FIG. 2, is tapered, but has a flat leading surface 21 that can become jammed against the plug 18 during insertion.

[0010] As is shown in FIG. 2, in order to accelerate solidification of the molten metal against the plug 18 during the startup phase, it is common to position pieces of metal scrap or metal shavings on the top surface of the plug 18, particularly near the mold side walls 14. This so-called “chill scrap” effectively absorbs heat energy from the molten metal, hastening the liquid to solid phase change of the molten metal. Typically, fine chill material 21 such as nail whiskers is first poured on top of the plug, particularly near the mold walls and on top of the fibrous sealing material, which would otherwise be quickly burned through by the molten metal were it allowed to come into direct contact with the molten metal. After this has been done, larger pieces 22 of chill scrap material are added to enhance the cooling effect.

[0011] In any continuous casting machine, it is advantageous to be able to perform the startup phase as quickly and as efficiently as possible. This is especially so in the case of a dual or twin strand machine, which includes two separate molds that are serviced by a common tundish. In such machines, it is deemed by some steel producers to be too risky for reasons of safety to perform the startup phase for one mold while the other mold is in operation and the tundish remains positioned overhead. Consequently, either the active mold must be shut down, which would be prohibitively expensive, or the startup phase for the inactive mold must be performed in the limited period of time that exists during a tundish change, which can be as little as two minutes. It is very difficult to position the dummy bar, install the starter plug, position the chill scrap and seal the gaps between the plug and the mold walls in this short period of time.

[0012] A need exists for an improved process of plugging and sealing a continuous casting mold during the startup phase that is faster and more efficient than the processes that are in conventional use.

SUMMARY OF THE INVENTION

[0013] Accordingly, it is an object of the invention to provide an improved process of plugging, sealing, and inserting chill scrap into a continuous casting mold for the startup of a casting strand that is faster and more efficient than the processes that are in conventional use.

[0014] In order to achieve the above and other objects of the invention, a method of initiating a continuous casting mold includes, according to a first aspect of the invention, steps of positioning a plug inside a continuous casting mold; sealing the plug with respect to inner walls of the continuous casting mold; positioning a prefabricated chill member on a top surface of the plug; and introducing molten metal into the mold, whereby the continuous casting process is initiated quickly and efficiently.

[0015] According to a second aspect of the invention, a method of initiating a continuous casting mold includes steps of positioning a plug inside a continuous casting mold; sealing the plug with respect to inner walls of the continuous casting mold by a process comprising inserting a leading edge of a sealing element downwardly into a gap between the plug and an inner wall of the continuous casting mold; and introducing molten metal into the mold, whereby the continuous casting process is initiated quickly and efficiently.

[0016] According to a third aspect of the invention, a prefabricated chill member for use inside a continuous casting mold during a startup phase, includes a plurality of metallic scraps that are fastened together into an integral member, the member being shaped so as to approximate a shape of a portion of an inner wall of a continuous casting mold, whereby when the member is positioned within the mold on a mold plug it will provide cooling to molten metal that is inserted into the mold during a startup phase of continuous casting mold, thus accelerating solidification of the molten metal in the area of the plug.

[0017] According to a fourth aspect of the invention, a sealing element for sealing a gap between a mold plug and an inner wall of a continuous casting mold during the startup phase of a continuous casting machine, includes an elongated body that defines in cross-section a broadened upper portion, and a tapered lower portion that narrows into a leading edge, whereby the sealing element can be quickly inserted into a gap without being caught on the mold plug.

[0018] These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a diagrammatical depiction of a first step of a conventional process of preparing a continuous casting mold for operation;

[0020] FIG. 2 is a diagrammatical depiction of a second step of a conventional process of preparing a continuous casting mold for operation;

[0021] FIG. 3 is a diagrammatical depiction of a first step of a process of preparing a continuous casting mold for operation according to a preferred embodiment of the invention;

[0022] FIG. 4 is a diagrammatical depiction of a second step of a process of preparing a continuous casting mold for operation according to a preferred embodiment of the invention;

[0023] FIG. 5 is a diagrammatical depiction of a third step of a process of preparing a continuous casting mold for operation according to a preferred embodiment of the invention;

[0024] FIG. 6 is a diagrammatical depiction of a prefabricated chill member constructed according to another embodiment of the invention; and

[0025] FIGS. 7 and 8 are diagrammatical depictions of a prefabricated chill member constructed according to yet another embodiment of the invention, showing the member is uncompressed and compressed states, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0026] Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIG. 3, a continuous casting mold 10 is diagrammatically depicted which includes a plurality of mold walls 12, each having an inside surface 14. As may be seen in FIG. 3, the dummy bar 16 is positioned within the continuous casting mold 10 and includes a mold plug 18 that is shaped so as to generally conform to the space that is defined by the inner surfaces 14 of the mold walls 12. However, as is described above, gaps exist between the mold plug 18 and the inner surfaces 14 of the mold walls 12.

[0027] According to one important aspect of the invention, sealing is effected between mold plug 18 and the inner surfaces 14 by inserting at least one sealing element 24, which is shaped as an elongated body that defines in cross section a broadened upper portion and a tapered lower portion that narrows into a leading edge 25, so that the sealing elements 24 can be quickly inserted into the gaps without being caught on the mold plug 18. As is evident from the drawings and the description provided above, the insertion of the sealing elements 24 is performed in situ, or in other words the sealing elements are inserted into the gaps between the mold plug 18 and the mold wall 12 after the mold plug has already been inserted into the continuous casting mold. The sealing element is also modified or manufactured to fit perfectly in the gap between the mold walls and plug to enhance quick insertion. This insures that valuable time will not be wasted when positioning the sealing elements. Preferably, the sealing elements 24 are fabricated from heat-resistant ceramic material such as vitreous aluminosilicate fibers, which are readily commercially available. Alternatively, sealing elements may be used that are fabricated from a wire mesh rope.

[0028] Turning now to FIG. 4, another important aspect of the invention involves the use of a prefabricated chill member 26 that is prepared prior to being placed within the continuous casting mold. Preferably, the prefabricated chill member 26 is formed in a predetermined shape so as to include at least one portion that is substantially complementary to one of the inner surfaces 14 of the mold wall 12. Most preferably, as may be seen in FIG. 4, the fabricated chill member 26 is shaped so as to substantially conform to the top of mold plug 18 and to the boundaries of the space that is defined by the inner surfaces 14 of the mold walls 12. Preferably, prefabricated chill member 26 is fabricated from a plurality of metal scraps that are welded together, although alternative materials and alternative fabrication techniques could also be used within the spirit of the invention. The prefabricated chill member is also manufactured to completely cover the sealing material to reduce the chance of steel leakage.

[0029] As may be seen in FIG. 5, after the prefabricated chill member 26 is placed in position, molten metal 30 is introduced into the continuous casting mold 10. In one embodiment of the invention, fine chill scrap material may be placed on top of the prefabricated chill member 26, particularly around the outer edges, prior to introducing the molten metal. The presence of the prefabricated chill member 26 will help quickly conduct heat away from the portion of the molten metal 30 that is adjacent to the plug 18, thus accelerating the phase change of the molten metal 30 from a liquid to a solid. It also acts to prevent the molten metal from sticking to the plug. During this time, the molten metal 30 is prevented from leaking out of the continuous casting mold 10 by the presence of the sealing element or elements 24. As the incipient casting 28 continues to form, the dummy bar 16 and plug 18 are pulled from the bottom of the continuous casting mold 10, thus initiating the continuous casting process.

[0030] FIG. 6 depicts another embodiment of the invention, wherein, instead of the sealing elements 24, a compressible sealing strip 32 is pre-attached to the prefabricated chill member 26 prior to the insertion of the chill member 26 into the mold. In the embodiment of FIG. 6, the compressible sealing strip 32 is generally V-shaped, and is fabricated from a resilient metallic material, such as a spring steel.

[0031] The compressibility of the sealing strip 32 is beneficial because the walls of a continuous casting mold are tapered, and it is important that the mold plug both effectively seal against the mold walls and not scratch or abrade the mold walls when it is inserted into or withdrawn from the mold. It will be apparent from FIG. 6 that when the prefabricated chill member 26 is inserted from above into the mold that the strip 32 will deflect inwardly when it contacts the mold wall, thus forming a tight seal with the mold wall without scratching it. Fine chill material such as nail whiskers 21 may be poured in from above after the mold plug and dummy bar have been positioned, thus filling the area above the sealing strip 32 and protecting the sealing strip 32 from bum-through when the molten metal is introduced.

[0032] FIGS. 7 and 8 are diagrammatical depictions of a prefabricated chill member constructed according to yet another embodiment of the invention, showing the member in uncompressed and compressed states, respectively. Specifically, FIGS. 7 and 8 depict a modification of the embodiment of FIG. 6 wherein the interior area of the V-shaped sealing strip 32 is pre-filled with a compressible foam or filler material 34, such as expandable low density polystyrene foam, which will remain flexible after introduction and maintain the flexibility of the sealing strip 32 during the mold start-up phase. The filler material 34 would also act as a barrier between the mold wall and the fine chill material 21 after the fine chill material 21 is introduced to further reduce the possibility of mold wall scratching or abrasion.

[0033] Through use of the invention as described herein, the operator of a continuous casting machine will be able to re-start a down strand quickly, such as during the short interruption of casting that occurs during a tundish change.

[0034] It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

1. A method of initiating a continuous casting mold, comprising steps of:

(a) positioning a plug inside a continuous casting mold;

(b) sealing the plug with respect to inner walls of the continuous casting mold by a process comprising inserting, in situ within the mold, a leading edge of a sealing element downwardly into a gap between the plug and an inner wall of the continuous casting mold;

(c) positioning a prefabricated chill member on a top surface of the plug; and

(d) introducing molten metal into the mold, whereby the continuous casting process is initiated quickly and efficiently.

2. A method according to claim 1, further comprising a step, performed prior to step (a), of fabricating said chill member into a predetermined shape out of metallic components.

3. A method according to claim 2, wherein said step of fabricating said chill member into a predetermined shape out of metallic components comprises fabricating said chill member out of metal scrap.

4. A method according to claim 2, wherein said predetermined shape includes portions that are shaped so a to be substantially complementary to the outer portion of a dummy bar head.

5. A method according to claim 2, wherein said predetermined shape includes portions that are shaped so a to be substantially complementary to the inner walls of the continuous casting mold.

6. A method according to claim 2, wherein said prefabricated chill member comprises at least one compressible metallic sealing strip on an edge thereof, and wherein step (c) comprises compressively engaging a mold wall with the sealing strip.

7. A method according to claim 6, wherein said sealing strip is generally V-shaped.

8. A method according to claim 7, further comprising a step of pre-filling said V-shaped sealing strip with a compressible non-metallic material.

9. A method according to claim 1, wherein step (b) comprises inserting a leading edge of a sealing element downwardly into a gap between the plug and an inner wall of the continuous casting mold.

10. A method of initiating a continuous casting mold, comprising steps of:

(a) positioning a plug inside a continuous casting mold;

(b) sealing the plug with respect to inner walls of the continuous casting mold by a process comprising inserting, in situ within the mold, a leading edge of a sealing element downwardly into a gap between the plug and an inner wall of the continuous casting mold; and

(c) introducing molten metal into the mold, whereby the continuous casting process is initiated quickly and efficiently.

11. A method according to claim 10, wherein step (b) is performed with a sealing element that is fabricated from a material comprising board that is fabricated from heat-resistant ceramic material.