GB1592554A

GB1592554A - Pouring of molten metal from a teeming vessel

Info

Publication number: GB1592554A
Application number: GB34593/77A
Authority: GB
Original assignee: USS Engineers and Consultants Inc
Current assignee: USS Engineers and Consultants Inc
Priority date: 1976-08-18
Filing date: 1977-08-17
Publication date: 1981-07-08
Also published as: NL7708942A; HU177768B; TR21159A; PH14710A; GR71863B; CH624863A5; FR2361963A1; NO152677B; JPS5829181B2; FR2361963B1; AU505612B2; AT358752B; CA1082421A; DE2737123C2; BR7705415A; YU42156B; MX146487A; NZ184879A; NL183440C; NO152677C

Abstract

A description is given of a method in which, by means of a special structural configuration of the restriction orifice in the flow duct of a casting vessel, it is possible to avoid an accumulation of oxidic impurities. For this purpose, the restriction orifice is given an axial length which is shorter than the constriction of the casting stream which it causes, known as the "vena contracta". Provided above this restriction orifice is a chamber which slows down the flow of the molten metal and calms turbulence. A description is furthermore given of an apparatus for carrying out this method, in which use is made of a flat disc (40) which is provided with the restriction orifice (42) and is arranged in such a way in the through hole for the passage of the molten metal that a chamber (38) is formed above the flat disc (40). <IMAGE>

Description

(54) IMPROVEMENTS IN THE POURING OF MOLTEN METAL FROM A TEEMING VESSEL (71) We, USS ENGINEERS AND CON- SULTANTS, INC. a corporation organised and existing under the laws of the State of Delaware, United States of America of 600 Grant Street, Pittsburgh, State of Pennsylvania, 15230, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: In the production of steel product, such as billets, slabs and blooms, by the continuous casting method, molten steel is poured into a mold from the bottom of a teeming vessel such as a ladle or a tundish that is operably positioned above the mold. In order to control product quality it is necessary that the level of the molten metal in the mold be maintained substantially constant. For this reason, it has been the practice to interpose a metering nozzle in the pour passage between the teeming vessel and the mold. Such metering nozzles are normally replaceably disposed in the bottom of the vessel and consist of a bore including an upper portion formed with either a conically tapered or spherically formed wall that terminates at its bottom in a flow restricting orifice formed of a diameter to produce the desired rate of molten metal flow into the mold.

In the continuous casting of steels deoxidized by aluminum, magnesium, titanium or rare earth compounds, collectively referred to herein as "aluminum-killed" steels, there is a tendency for alumina or other refractory oxide inclusions of microscopic proportions to accumulate in the pour passage through the metering nozzle. These inclusions accumulate in the flow restricting orifice causing the flow opening to constrict thereby adversely affecting casting operations by initially requiring the speed of casting to be reduced in order to maintain the required metal level in the mold, and finally, requiring replacement of the orifice when the constriction becomes excessive. If the nozzle is fixed in the teeming vessel, the casting operation must be terminated in order to replace the nozzle. If the nozzle forms part of a sliding gate valve, several nozzle changes may be required during the pouring of a single cast.

The problem is more acutely manifest in low production facilities, i.e., those in which casting operations compel the use of metering nozzles containing orifice openings of less than one inch diameter. In these facilities the production of aluminum-killed steel product in a continuous casting operation cannot be accomplished on a practical commercial basis due to the rapid obstruction of the small diameter orifice openings and the attendant frequency of nozzle replacements that result.

It has been contemplated to alleviate the described problem by cementing a thin disc containing the flow control orifice in the bottom of the pour opening of the teeming vessel. Such a device, known as a "wafer nozzle", is shown and described in Japanese Patent Application Serial No. 24208/72 which was laid open on November 11, 1973, under Serial No. 92226/73. Applicants have found, however, that "wafer nozzles" are not totally dispositive of the problem for several reasons. First, such devices which are normally formed of a refractory material of greater density than the surrounding refractory of the vessel lining are prone to rapidly deteriorate due to the imposition thereon of high thermal stresses. Secondly, the flow of metal through these nozzles is often disturbed, principally as a result of turbulent fluctuations in the molten metal bath within the teeming vessel, thereby producing instability of the flow stream through the orifice opening and flaring in the stream emerging therefrom. An unstable flow stream gives rise to some, albeit reduced as compared with other devices of the prior art, inclusion accumulation on the nozzle. Moreover, both of these characteristics present the danger of causing metal oxidation due to incrcased exposure of the metal to air.

According to the present invention, there is provided a refractory nozzle providing a passage for pouring molten metal from a bottom pour teeming vessel, the nozzle having a substantially cylindrical axial through bore and an internal annular partition extending transversely of the bore intermediate its ends to divide the bore into upper and lower sections, the partition having an upper surface which is substantially perpendicular to the axis of the bore, and the partition defining a centrally disposed flow control orifice having an axial length no more than twice its diameter and wherein the diameter and axial depth of the upper bore portion are so proportioned relative to each other and to the orifice as to define a region of relative flow stagnation which is capable of stabilizing flow through the orifice.

More specifically the invention provides a refractory nozzle providing a passage for pouring molten metal from a bottom pour teeming vessel, the nozzle having a substantially axial through bore and an internal annular partition extending transversely of the bore intermediate its ends to divide the bore into an upper flow entry section and a lower flow discharge section, the partition having an upper surface substantially perpendicular to the axis of the bore, and the partition defining a central control orifice having a diameter not less than one half the axial length of the orifice, the entry section having a diameter not less than 1.4 times the orifice diameter and a depth-to-diameter ratio not less than 1:3 and the construction being such that in use a region of relative flow stagnation develops in the entry section which is capable of stabilizing flow through the orifice.

The invention is further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partial vertical sectional view of a bottom-pour teeming vessel equipped with a pour nozzle constructed according to the present invention; Figure 2 is an enlarged elevational view of a nozzle insert constructed according to the present invention; Figure 3 is a plan view of the nozzle insert of Figure 2 and Figure 4 is a partial vertical sectional view of a bottom-pour tundish vessel having a slide gate from control valve equipped with a nozzle insert constructed according to the present invention.

For purpose of describing the invention there is shown, in Figure 1 of the drawings, the lower side portion of a teeming vessel 10 of generally well known construction that is adapted to hold molten metal. The vessel:l0 comprises a metal shell 12 having a refractory lining 14, provided with aligned openings. 16 and 18 respectively. to accommodate means for bottom-pouring molten metal from the vessel. A generally cylindrical well block 20 is disposed in the lining opening 18 and has a through-bore 22 which terminates at its upper end in an upwardly facing conical recess 24. The bore 22 communicates with the vessel interior and forms part of the molten metal pour passage from the vessel.

In the illustrated embodiment and in applications in which it is desired to intermittently open or close the pour passage, a stopper rod 26, of usual construction, can be operably arranged for seating engagement at its lower end in the well block for opening and closing the pour passage.

A refractory nozzle insert 28 constructed according to the present invention is provided with means for mounting in the well block bore 22. As shown best in Figures 2 and 3, the nozzle insert 28 comprises a body of generally hollow cylindrical configuration.

The external wall of the body is formed with a lower portion 30 that is of somewhat reduced diameter as compared with the upper portion 32 to define an annular stepped shoulder 34 intermediate the ends of the insert for seating on a complementary shoulder in the wall block 20. It should be understood, however, that means other than the illustrated stepped shoulder can alternatively be provided for mounting the insert 28 within the vessel. Fixture of the nozzle insert 28 in the well block 20 is preferably accomplished by cementing or the like (not shown).

The body of the insert 28 is provided with an axial bore having a lower portion 36 and an upper portion 38 which cooperate to define the metal pour passage from the teeming vessel. An annular, relatively thinwalled disc-like partition 40 is intergrally formed in the pour passage of the insert, projecting radially from the passage wall and transversely of the axis thereof. As shown, the member 40 is disposed intermediate the ends of the bore, forming the divider between the lower and upper bore portions, 36 and 38 respectively. A through-opening 42 penetrates the member 40 forming a flow restricting orifice therethrough.

The member 40 is formed as thin as practicable but in any event has a thickness no greater than twice the diameter of the orifice opening 42 therein. The upper surface 44 of the member 40 is formed such that the inlet edge 48 of the orifice opening 42 is a sharp edge. The surface 44 is substantially perpendicular to the bore axis of the opening 42 and at right angles to the wall thereof. The wall of the orifice opening 42 may, if desired, be slightly downwardly divergent, so that its effective diameter is at the upper end thereof.

However, if such a configuration is adopted the inlet edge 48 will be more prone to rapid erosion and, for this reason, is not deemed desirable for most applications.

It will be appreciated that, in forming the pour passage through which molten metal, especially aluminum-killed steels, flow from a teeming vessel as described herein certain advantages are derived. First, because the restricting orifice, i.e., orifice opening 42, is disposed in a thin, disc-like partition in the manner described, the danger of obstructing the control flow passage by oxide inclusions is, for practical purposes, eliminated. This advantage is derived from the fact that, in a thin disc, i.e., one having an L/D ratio no greater than 2, the imposition of a sharp edge defining the inlet to the orifice opening causes the stream of flowing metal to constrict, as indicated by the flow lines in Figure 1, from a maximum diameter, at the upper edge of the orifice to a "vena contracta", as defined in Chambers Dictionary of Science and Technology 1971. Constriction of the flow lines is enhanced by the fact that the surface 4 is disposed normal to the axis of the orifice opening 42 whereby the fluid flow approaching the opening is imparted with a significant component of velocity radially inwardly toward the axis of the opening.

Since the wall of the orifice opening is shorter in length than the distance within which the constricted flow stream will reduce from its maximum diameter to its vena contracta there will be no contact made by the flowing metal on the wall and, concomitantly, no boundary layer will be formed adjacent the wall within which inclusions can accumulate.

In order to insure the creation of the convergent stream of molten meal the outer periphery of the upper surface 44 of the partition 40 which is normal to the bore axis should be formed of a diameter not less than about 1.4 times that of the opening 42, preferably not less than about twice the diameter thereof.

Additionally, due to the thinness of the partition 40 that contains the restricting orifice 42, the material about the orifice opening is rapidly brought up to the temperature of the molten metal whereupon excessive heat losses which otherwise might produce material "freezing" to the opening is avoided.

A further advantage derived from the fact that the upper bore portion 38 is formed as a right circular cylinder superposed above the member 40 is that there is established a welllike plenum positioned immediately upstream of the orifice opening 42. This plenum defines a region in which the flow of molten material is relatively stagnant, which serves a two-fold purpose. First, it operates to dampen the effects of any turbulent fluctuations that may occur in the bath of molten metal within the teeming vessel as a result, for example, of the pouring of additional metal into the teeming vessel from a supply ladle, or the like. The stagnant region within the plenum can produce such dampening and thus eliminate the disruptive effect which turbulence within the bath has upon the maintenance of the convergent stream of molten metal flowing through the orifice opening as long as the diameter of the bore portion 38 does not exceed three times its depth. Preferably this depth-to-diameter ratio should be about two.

Secondly, because of the relatively stagnant flow of molten metal away from the centre of the plenum, such deposition of oxide inclusions as takes place within the nozzle is mainly confined to that portion of the plenum near the intersection of the wall of bore portion 38 with the surface 44. By sizing the plenum as taught herein the space so provided is of ample volume to receive the deposited inclusions without their entering or otherwise interfering with the opening 42 during the practical life of the device.

As shown, the lower bore portion 36 in the described arrangement is formed of a diameter that is somewhat reduced as compared with that of the upper portion 38. By sizing this portion of the bore in the manner shown adequate material is provided beneath the member 40 with which to provide bottom support thereto. The wall of this bore portion should not, however, be located so close to the lower edge 49 of the orifice opening 42 as to provide a surface which will be in contact with the stream of molten metal exiting the orifice opening.

Figure 4 of the drawing illustrates a preferred application of the nozzle insert 28 of the present invention applied in a slide gate valve installation on a tundish vessel 50 utilized in conjunction with a continuous casting installation. The vessel 50 whose bottom portion is shown in the figure comprises a shell 52 having a refractory lining 54.

The metal pour passage from the vessel is formed by the bore 56 in well block 58. A slide gate valve apparatus, indicated generally as 60, is attached to the vessel bottom by means of a mounting plate 62. A slide gate apparatus of the type contemplated is more fully described in U. S. Patent No. 3,779,424 to E. P. Shapland, Jr. and assigned to the assignee of this application. To the extent necessary for an understanding of this invention the apparatus may be described as having a stationary top plate 64 provided with an opening 66 aligned with the well block bore 56 and a pair of oppositely spaced parallel rails 68 mounted beneath the top plate for slidably guiding metal encased refractory panels or gates 70 or 70a. The gates 70 and 70a are adapted to be indexed in sequence from a position beneath the top plate opening 66 by means of a hydraulic operator 72 consisting of fluid motor 74, ram 76 and actuating rod 77. Some of the gates, indicated as 70, are blank, having no opening from the passage of molten metal and serve when in position to prevent the flow of metal from the tundish 50. Others, indicated at 70a, are provided with an axial bore 78 forming the pour passage of metal from the vessel. As shown in Figure 4, the slide gate 70a has its bore 78 formed of stepped diameters defining a shoulder for mounting a nozzle insert 28 therein of similar configuration as that described in connection with Figures 1, 2 and 3 above.

Heats of steel of 200-ton size containing .03 to .05 weight-percent aluminum have been cast in a continuous caster from a vessel equipped with a slide gate valve having a nozzle insert as described herein whose orifice opening is of one and three-eighths inch diameter. The casting operation has been characterized by a substantially constant casting speed through-out the entire casting period, typically of sixty minutes or more duration. ln contrast, use of a conventional flow nozzle in an identical installation exhibited rapid decrease in casting speed (a drop of 12 to 15 inches per minute in casting speed during a casting period of only ten minutes duration). The improved performance in the former installation is attributed to the fact that the accumulation of oxide inclusions is minimized by use of the nozzle insert.

It will be understood that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended

Claims

claims.

WHAT WE CLAIM IS: I. A refractory nozzle providing a passage for pouring molten metal from a bottom pour teeming vessel, the nozzle having a substantially cylindrical axial through bore and an internal annular partition extending transversely of the bore intermediate its ends to divide the bore into upper and lower sections, the partition having an upper surface which is substantially perpendicular to the axis of the bore, and the partition defining a centrally disposed flow control orifice having an axial length no more than twice its diameter and wherein the diameter and axial depth of the upper bore portion are so proportioned relative to each other and to the orifice as to define a region of relative flow stagnation which is capable of stabilising flow through the orifice.
2. A refractory nozzle providing a passage for pouring molten metal from a bottom pour teeming vessel, the nozzle having a substantially axial through bore and an internal annular partition extending transversely of the bore intermediate its ends to divide the bore into an upper flow entry section and a lower flow discharge section the partition having an upper surface substantially perpendicular to the axis of the bore; and the partition defining a central control orifice having a diameter not less than one half the axial length of the orifice, the entry section having a diameter not less than 1.4 times the orifice diameter and a depth-todiameter ratio not less than 1:3 and the construction being such that in use a region of relative flow stagnation develops in the entry section which is capable of stabilizing flow through the orifice.
3. A nozzle as claimed in claim 1 or 2, when formed as a replaceable nozzle insert for mounting in the vessel.
4. A nozzle as claimed in claim 1 or 2 when formed as a refractory panel of a slide gate valve for controlling flow of the molten metal from the vessel.
5. A nozzle as claimed in claim 1 or 2, when formed as a nozzle insert replacebly mounted in a refractory panel of a slide gate valve for controlling flow of the molten metal from the vessel.
6. A nozzle as claimed in claim 3 or claim 5 in which the external surface of the nozzle is of generally cylindrical configuration and has means thereon for mounting said body.
7. A nozzle as claimed in claim 6, in which said means for mounting comprises a shoulder between upper and lower portions of said external surface.
8. A nozzle as claimed in claim 1 or 2, and which forms a refractory lined wall of the teeming vessel.
9. A nozzle as claimed in any preceding claim in which said partition is formed integrally with said body.
10. A nozzle as claimed in any preceding claim in which the portion of said bore upstream of said partition is of a diameter greater than that of the portion of said bore downwstream thereof.
I I. A nozzle as claimed in claim 10 in which the walls of said upstream and downstream bore portions are parallel to the axis of said bore.
12. A nozzle as claimed in claim 10 or 11 in which said upstream bore portion is of;a diameter no greater than five times the diameter of said through-opening.
13. A nozzle as claimed in claim 12, in which said upstream bore portion is ofa diameter about twice that of said throughopening.
14. A method of preventing the accumulation of oxide inclusions on the flow surface of an orifice opening in a flow nozzle disposed in the pour passage of a teeming vessel for pouring aluminium killed steels or the like, comprising the steps of forming said orifice opening of an axial length shorter than distance over which the diameter of the molten metal stream flowing therethrough reduces from its maximum diameter to its vena contracta and providing a plenum about said orifice opening on the upstream side thereof defining a region in which the flow of molten metal is relatively stagnant.
15. A refractory nozzle for providing a passage for the pouring of molten metal from a teeming vessel, substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.
16. A refractory nozzle for providing a passage for the pouring of molten metal from a teeming vessel, when incorporated in a slide gate flow control valve, substantially as hereinbefore described with reference to and as illustrated in Figure 4 of the accompanying drawings.