Title of Invention

FEED DIP PIPE FOR THE CONTINUOUS CASTING OF THIN SLABS

Abstract

The present invention relates to a dip pipe for feeding by gravity, with a molten metal or alloy from a ladle having a nearly constant head, a slab being formed from a bath with surface height in a thin mould with cooling walls, comprised of four walls extending in a substantially vertical direction and having a horizontal cross-section formed of two sides with a length much greater than the other two, the dip pipe comprising a length of vertical upper tube in communication with the upper ladle and, downwards, with a distributing flattened portion or diffuser with discharge holes at the bottom, relating to two distinct passages formed by a partition baffle and opening under the surface of the slab being formed at a given distance from the mould walls, characterized in that the diffuser in its highest cross-section has a surface area which is smaller than that of the upper tube; that said diffuser has inner side walls, facing the narrow sides of the mould which are symmetrically diverging from a vertical axis with respect to which they depart from above downwards to form each an angle a < 7.50 with said axis; and that the partition baffle in its lower portion narrows to the narrow sides of the thin mould, whereby between its sides and the vertical two angles ~ < 7.50 are formed.

Full Text

The present invention relates to a feed dip pipe for the continuous casting of thin slabs and more in particular a submerged nozzle for guiding in the best way as possible a molten metal or alloy from a ladle having a nearly constant head for feeding the same, without turbulence or swirling. to a level underneath the head or meniscus of a slab being formed within a cooling mouid in which the slab itself takes a shape by solidification. Thin slabs are known which are formed of four walls extending in vertical direction with horizontal cross-section having two sides of prevailing length with respect to the other two. It is also known that for introducing molten metal, especially steel, fed from a vessel above, into the inside of the mould, a connection conduit is used being called "submerged", as its lower mouth is dipped in the molten bath within the mould and is adapted as much as possible to the thin size of the same mould in order to keep a sufficient distar\ce from tiie cooling walls. Therefore dip pipes for thin slabs are usually employed in the technique as having in the lower portion horizontal cross-section of rectangular, polygonal or elliptical shape, with outlet boards directed the narrow sides and/or downwards. However these prior art dip pipes do not solve the various problems which are typical of this technology, as are widely described in the literature in this field and due to various reasons, in particular the fluid stream flowing out from the dip pipe has the tendency to circulate within the liquid mass in the core of the forming slab, solidified only externally, while having the attitude to re-emerge to the surface, thus generating stationary waves at the bath surface, especially in the proximity of the narrow faces of the thin mould. Thereby the lubricating slag will generally gather in the lower portions of the wave-shaped meniscus, while leaving the picks uncovered, with consequent lack of lubrication or poor distribution thereof, which gives rise to mould wear as well as a poor surface quality of the slab and incorrect thermal exchange of the forming slab with the mould, that is a cause of possible cracks. Furthermore, the zones where the fluid swirls come back again into the liquid bath show a marked bent of the meniscus, in which the particles of powder and lubricating slag are easily entrapped in the forming slab, thus providing an additional cause of cracks and other surface defects. The turbulence at the level of meniscus in the mould is also an important cause of wear for the nozzle the life of which is then reduced. Possible turbulence and whirlpools in Ihe fluid stream at the outlet of the nozzle have a negative influence on the solidification process occurring within the slab, which should be progressive and as homogeneous as possible in the direction parallel to the narrow taces of the mould. On the contrary steadiness of feed and a distribution as symmetrical as possible of the flow with respect to the longitudinal axis of the slab, with the maximum of homogeneity at the horizontal cross-sections would be desirable. Mention is made of tiie additional inconvenience due to the fact that oxides are present in the molten metals or alloys and have the tendency to deposit on the inner surfaces of the nozzle thus modifying its geometry and hence negatively effecting the passage cross-sections of flow. Except for the last mentioned inconvenience, which becomes worse in case of slow flow rates in the various passage cross-sections, all the other inconveniences previously mentioned worsen as the flow rate of molten metal or alloy increases, namely m correspondence with higher speeds at which ttie slab being formed in the mould is withdrawn and/or larger cross-section, areas of the slab, thereby higher flow rates in the various passage cross-sections, in particular at the discharge holes. Anyhow all these mentioned inconveniences are present in whichever known shape of dip pipe or nozzle thus negatively affecting in various ways the correct trend of the casting and of the cooling of the slab under connation with a consequence of having a final product of poor quality. The dip pipe or submerged nozzle according to DE 4142447 has the angle a comprised in the range from Lo to 22°, varying with the mould width. Furthermore, such an angle is inversely proportional to the flowing rate of the molten steel. Owing to these higher values of angle a, a detachment of tithe steel stream from the mould walls is experienced. Therefore it is an object of the present invention to provide a feed dip pipe of nozzle that can overcome the mentioned drawbacks by reducing as much as possible and gradually the flow rate passing through the cross-sections in correspondence of gradually decreasing distances trom the discharge holes, thus obtaining a stabilized stream, symmetrical with respect to the vertical axis, with a kinetic energy which can be more easily dissipated within the liquid core of the slab being formed, and reducing to the minimum the presence of whirls and turbulence in the meniscus. Within the dip pipe the flow is accelerated until a point of cross-section reduction and then it is evenly slowed down while maintaining the lower portion of the diffuser filled with liquid. This object is obtained by means of a dip pipe or nozzle of the present invention. Accordingly the present invention provides a dip pipe for feeding by gravity, with a molten metal or alloy from a ladle having a nearly constant head, a slab being formed from a bath with surface height in a thin mould with cooling wails, comprised of four walls extending in a substantially vertical direction and having a horizontal cross-section formed of two sides with a length much greater than the other two, the dip pipe comprising a length of vertical upper tube in communication with the upper ladle and, downwards, with a distributing flattened portion or diffuser with discharge holes at the bottom, relating to two distinct passages formed by a partition baffle and opening under Ie scoriae of the slab being formed at a given distance from the mould walls, characterized in that the diffuser in its highest cross-section has a surface area which is smaller than that of the upper be; that said diffuser has inner side walls, facing the narrow sides of the mould which are symmetrically diverging from a vertical axis with respect to which they depart from above downwards to form each an angle a These and additional objects, advantages and features of the dip pipe or nozzle according to the invention will appear more clearly to tiiose skilled in the art from the following description of a non-limiting preferred embodiment of the invention itself, with reference to the drawings in which: FIGURE 1 shows a longitudinal, sectional view of the nozzle according to the invention being immersed in a thin mould, taken in a median plane, parallel to the large faces of the mould itself; FIGURE 2 shows a longimdinal sectional view of the nozzle immersed in the mould, taken along a plane II-II parallel to the narrow faces of the mould; and FIGURE 3 shows a sectional view along the line E-III of Figure 2. With reference to Figure 1, a dip pipe 1 feeds by gravity with a molten metal or alloy 2, contained in an upper ladle 3 having a nearly constant head, a slab 4 being formed at the inside of a thin mould 5, with cooling walls and formed of four walls extending in a vertical direction with a horizontal cross-section wherein two sides are of prevailing length with respect to the other two. Although shown in Figure 3 as having a perfectly rectangular cross-section, the mould can have slightly convex or polygonal walls or even witii a longitudinal trend slightly different from the perfectly vertical one represented in Figure 2, without departing from the features of the dip pipe according to the invention. The dip pipe comprises a length of vertical pipe of a circular cross-section, being connected to the upper ladle 3 in a known way. The dip pipe can be provided, at its upper portion, with a flow control surface 7, while downwards extends itself. Through a fitting zone 18, with a flattened distributing portion, in the following called diffuser 8, having lower discharge 9, 9". The diffuser 8 provides for feeding the material under the head 17, from which the term "dip" or "submerged", at the inside of the slab 4 being formed in the thin mould 5 while keeping a given distance from the walls of the mould itself. The slab 4 being formed as being represented with solid walls of increasing thickness from the top to the bottom, while the inner core must be still considered liquid or however not yet completely solidified. In the diffuser zone 8 a central baffle 14 is also provided, integral to both the larger walls of the diffuser, suitable to divide the flow in two distinct conduits 16, 16" ending with the two holes 9, 9" for discharging downwards. The flow passage cross-section 10, at the highest level of the diffuser height, at the end of the fitting portion 18 with the pipe 6, has been preferably represented coincident with the upper end of the baffle 14 although this is not an essential feature of the invention. 1 According to the present invention, the area of such a cross-section 10 is less than that corresponding to the cross-section area of the upper pipe 6, which has been indicated with reference numeral 11. This condition is better shown In Figure 2. It will be noted that, in spite of the fact that the side walls of fitting 18 appear to diverge downwards in Figure 1, i.e. at the cross-section parallel to the large faces of the mould, in all the other sectional planes their are convergent, thus giving rise to a reduction of cross-section in the downward direction. Furthermore the inner side walls 12, 12" of the diffuser 8 towards the narrow sides of the thin mould 5 are diverging downwards and form each with a vertical axis 13 from which they depart an angle a that is less than or equal to 7.5°. Still according to the invention, the flow partition baffle 14 is narrowing in its lower portion 15, 15" along the sides facing the narrow sides of the thin mould 5, by forming with the vertical axis 13 to angles p The two passage conduits 16, 16" which consequently are formed from opposite sides of the partition baffle 14, have a cross-section at right angles with the flow that is increasing in a downwards direction, but without making easier a flow detachment from the walls. Owing to the restriction imposed to angles a and p, a flow separation is avoided and the flow rate along the two conduits 16, 16" results to be the maximum technically obtainable in relation to the desired speed of outflow from the discharge holes 9, 9". Under the hydrodynamic aspect, the dip pipe or nozzle according to the invention is substantially like it would present to the flow of molten material a compression chamber in correspondence with the cross-section 11, more precisely between the latter and the reduced cross-section 10. Subsequently the flow has its maximum acceleration, then slowing down downstream, starting from cross-section 10, gradually along the two conduits 16, 16", but still preserving the continuity of contact with the walls. However it is convenient that the flow rate is stil! accelerated along the upper portion, with diverging faces of the baffle 14 in order to keep clear both conduits 16, 16" of any deposit of oxides, such a deposit already occurring in this zone at the presence of an excessive or too early slowing down of the flow. For this purpose it is preferable that the cross-section area of both conduits 16,16" is still decreasing between the highest cross-section 10 of the diffuser and that of the maximum width of the baffle. It would be possible to obtain such a condition e.g. by imposing for the above-mentioned upper zone of the baffle 14, assuming that said edges 19, 19" are provided as shown In Figure 1, that these are inclined by an angle > a. In this way the two upper zones of conduit 16, 16", where start to form by departing about the upper edges 19, 19" of baffle 14 wfl"ll be slightly convergent before starting of the divergent zone in the actual diffuser 8. Possible additions and/or modifications can be made by those skilled in the art vAih respect to the above described and illustrated embodiment of the dip pipe according to the present invention without exceeding the scope of the invention itself. In particular the dip pipe 1, instead of being provided vw"th a flow control surface 7, as indicated in Figures 1 and 2, could be directly flanged In a way per se known to the bottom of ladle 3, \A4iile the flow control surface could be provided on a different member, placed within the ladle itself. In an alternative solution the pipe 1 could also be flanged, again in a way per se know, under a "drawer" of flow control placed on the bottom of ladle 3, acting in a known way by choking at the passage port between two holed and facing plates feeding one above the other. WE CLAIM: 1. A dip pipe for feeding by gravity, with a molten metal or alloy (2) from a ladle (3) having a nearly constant head, a slab (4) being formed from a bath with surface height (17) in a thin mould (5) with cooling walls comprised of four walls extending in a substantially vertical direction and having a horizontal cross-section formed of two sides with a length much greater titian the other two, die dip pipe (1) comprising a length of vertical upper tube (6) in communication with the upper ladle (3) and, downwards, with a distributing flattened portion or diffuser (8) with discharge holes (9, 9") at the bottom, relating to two distinct passages (16, 16") formed by a partition baffle (14) and opening under the surface (17) of the slab (4) being formed at a given distance atom the mould (5) walls, characterized in that the diffuser (8) in its highest cross-section (10) has a surface area which is smaller than that of the upper tube (6); that said diffuser (8) has inner side walls (12, 12") facing the narrow sides of the mould (5) which are symmetrically diverging from a vertical axis (13) with respect to which they depart from above downwards to form each an angle a 2. The dip pipe accord to claim 1, wherein said partition baffle (14) extends from the bottom of said diffuser (8), at the same level of said discharge holes (9, 9"), up to said highest and narrower cross-section (10) of the diffuser (8) to form two passage conduits (16, 16") having an increasing cross-section area from above downwards in a direction perpendicular to the flow of molten metal or alloy, at least from a zone where said baffle has Ie greatest width, from which said sides (15, 15") stet approaching to the vertical. 3. The dip pipe according to claim 2, wherein the upper end of said baffle (14), substantially at the same level as said highest and narrower cross-section of said doilies (8), is connected to the upper tube (6) through a tapered fitting zone (18) and that between said upper end and a largest zone of the baffle (14) from which the narrowing sides (15, 15") start there are diverging upper sides (19, 19") of the partition baffle itself. 4. The dip pipe according to claim 3, wherein the diverging upper sides (19, 19") of said baffle (14) form an angle > a with the vertical where a is withm the range of 0.5"^ to 7.5", whereby an initial portion of said conduits (16, 16") has a constant or decreasing cross-section with the flow increasing its velocity until the zone of greatest width of the baffle (14). 5. The dip pipe according to one or more of the preceding claims, wherein said tube (6) is provided, at its upper portion, with a flow control surface (7). 6. The dip pipe according to one or more of claims 1-4, wherein said tube (6) is directly flanged to the bottom of ladle (3), there being provided at the inside of the latter a flow confrol surface. 7. The dip pipe according to one or more of claims 1-4, wherein said tube (6) is flanged in a way per se known to a "drawer" device of flow control on the bottom of ladle (3). 8. A dip pipe for feeding by gravity substantially as herein described with reference to the accompanying drawing.

Documents:

1135-mas-1997 abstract duplicate.pdf

1135-mas-1997 abstract.pdf

1135-mas-1997 claims duplicate.pdf

1135-mas-1997 claims.pdf

1135-mas-1997 correspondence others.pdf

1135-mas-1997 correspondence po.pdf

1135-mas-1997 description (complete) duplicate.pdf

1135-mas-1997 description (complete).pdf

1135-mas-1997 drawing duplicate.pdf

1135-mas-1997 drawing.pdf

1135-mas-1997 form-19.pdf

1135-mas-1997 form-2.pdf

1135-mas-1997 form-26.pdf

1135-mas-1997 form-4.pdf

1135-mas-1997 form-6.pdf

1135-mas-1997 others.pdf

1135-mas-1997 petition.pdf