Submerged entry nozzle.
Description.
[0001] The present invention relates to a submerged entry nozzle for the continuous casting of metal, in particular for the continuous casting of flat product such as thin slabs or thin strips. Such submerged entry nozzle comprises generally an inlet section having a generally axial symmetry, a transition section in fluid communication with the inlet section to change the nozzle inner symmetry from having generally axial symmetry to generally planar symmetry and an outlet section having a broad dimension and a narrow dimension in fluid communication with the transition section. Typically, the broad dimension will be about five to ten times the narrow dimension. [0002] The inlet section of the submerged entry nozzle is connected to the discharge orifice of an upstream vessel, generally a tundish and the outlet section deepens into a rectangular mold having also a broad dimension and a narrow dimension substantially smaller than the broad dimension. Typically, the slabs or strips cast in such molds have thicknesses of 50 to 60 mm and widths of 975 to 1625/nm. [0003] Fig. 1 shows a submerged entry nozzle known from the USP 5,785,880 which is largely used for the continuous casting of thin strips of steel. This submerged entry nozzle 1 comprises an inlet section 2 having a generally axial symmetry, a transition section 3 in fluid communication with the inlet section to change the nozzle inner symmetry from having generally axial symmetry to generally planar symmetry and an elongated outlet section 4 having a broad dimension and a narrow dimension in fluid communication with the transition section 3. Such an elongated outlet section provides the required flow area to reduce the flow velocity of the steel discharged into the mold. The reduction of the flow velocity in turn limits the turbulence caused in the mold. [0004] The top of the cylindrical inlet section is generally broader than the remaining of the inlet section to permit a strong fixation of the submerged entry nozzle in the bottom wall of the tundish. Beside that, the wall thicknesses are relatively constant across the different sections to prevent any thermal schock problems that, unavoidably, would occur with significantly varying thicknesses.
[0005] DE 41 16 723 relates to immersion nozzles designed to control the internal pressure inside the immersion nozzle and limit air suction. During normal casting operation, the pouring channel of the nozzle is filled with melt and a stopper can be omitted; the flow regulation being assured by the nozzle itself. DE 41 16 723 also discloses that the outlet opening of the immersion nozzle is enlarged in the form of a cone. The reason for this enlargement is not given. [0006] With such prior art submerged entry nozzles for the casting of flat products, there is a maximum flow area that can be reached for a determined mold and steel throughput. Indeed, the outer dimensions of the nozzle must be smaller than the inner dimensions of the mold to permit its introduction and withdrawal without interfering with the mold walls. Once the dimensions of the nozzle are close to the dimensions of the mold, it is no longer possible to further increase the
flow area. There is therefore a need to modify the design of the submerged entry nozzle of the prior art to permit a further increase of the flow area without having to modify the overall outer dimensions of the nozzle.
[0007] Alternatively, it could be economically interesting to further increase the flow area of a submerged entry nozzle without having to increase the size of the nozzle.
[0008] Generally, a submerged entry nozzle is made from conventional refractory material such as alumina graphite refractory composition. In order to extend the life of the nozzle and to allow it to resist to corrosion by the extremely aggressive slag floating on the surface of the molten metal in the mold (at the meniscus level), the nozzle generally comprises a slag protection sleeve, for example made from zirconia graphite or zirconia alumina graphite which can be copressed with the body of the nozzle. In that case, there is a compromise between the thickness of the slag protection sleeve (to extend the life) and the inner dimension of the outlet section (to increase the flow area), the overall dimension of the nozzle being limited by the mold internal dimensions. [0009] One object of the invention is therefore to provide a submerged entry nozzle according to the preamble of claim Shaving an increased flow area.
[0010] It has now been found that this result could be achieved just by enlarging at least a part of the narrow dimension in the inner downstream portion of the outlet section. While the enlargement may seem small, the increase in flow area (and therefore the reduction in flow velocity) is much more significant than that would be achieved by increasing the broad dimension in the inner downstream portion of the outlet section. If the broad dimension is ten times higher than the narrow dimension, any increase in the narrow dimension will impact ten times more on the flow area than the same increase in the broad dimension. [0011] The enlargement of the subentry nozzle according to the invention has an inlet section and an outlet section and is defined by the formula : 0.02 <(b-a)/2L < 0.25 with L ≥ 3 x (b-a)/2 wherein a is the width of the inlet section b is the width of the outlet section taken in the same cross section than a and L the distance between the inlet section and the outlet section [0012] Such an enlargement can be constant in the enlarged part, can increase progressively or can be a combination of both. For example, the inner downstream portion of the outlet section can comprise a step enlarging constantly thereby the narrow dimension. [0013] Preferably, the inner downstream portion of the outlet section is flared out in the narrow dimension. With such an arrangement of the enlargement, the generation of turbulence by a sudden change in flow area is avoided.
[0014] . In a variant, the enlargement can comprise a flared out portion followed by a straight (enlarged) section which will guide the molten metal stream.
[0015] In a variant, the enlargement can comprise two or more flares or two or more steps or can consist of a combination of a flare and a step. The enlargement can also be curved or in the
form of a paraboloid.
[0016] In another variant, the flares or steps can be followed by a straight section which will guide the molten metal stream.
[0017] Preferably the enlargement is defined by the formula 0.04 <(b-a)/2L < 0.20 and most preferably as 0.06 <(b-a)/2L < 0.14.
[0018] The width (b-a)/2 of the enlargement and the height or length (L) of the enlargement are chosen so that there is no separation of the flow. The flow of melt within the nozzle deviates in the enlargement portion, the flow area increases and the flow velocity reduces. [0019] When (b-a)/2L is higher than 0.25 with L < to 3x(b-a)/2, the enlargement is useless for what concerns the object of the invention. The flow separates from the internal walls of the nozzle and keeps its initial trajectory. There is no significant deviation of the flow nor reduction of velocity.
[0020] (b-a)/2L corresponds to the mean angle of the deviation α. When the mean angle is small or the (b-a)/2L values are on the lower side of the range, the height or length (L) of the enlargement is preferably higher or equal to 5 times (b-a)/2, more preferably 7 times (b-a)/2 and even more preferably 10 times (b-a)/2. When the mean angle is small, the height of the enlargement is increased to obtain the targeted flow velocity reduction. Although the height of the enlargement depends on the mean angle chosen and the dimension of the nozzle depending itself from the mold dimension and steel throughput, the height (L) will generally be higher than 20 mm.
[0021] As explained above, the narrow dimension of the nozzle is limited by the narrow dimension of the mold and can not be further increased. The zirconia sleeve of the nozzle is determining the life of the nozzle. One way to extend the life of the nozzle is to increase the thickness of the sleeve. It could be interesting to increase the life of the submerged entry nozzle by increasing the thickness of the sleeve without changing the flow velocity profile.
[0022] This can also be achieved by a nozzle according to the invention. The thickness of the sleeve is increased internally increasing the life of the nozzle but reducing the flow area and downward, the enlargement in the outlet section increases the flow area to compensate the flow area reduction due to the increase of the sleeve thickness. [0023] The enlargement is preferably located downward with respect to the middle of the sleeve, the middle of the sleeve corresponding in average to the position of the slag line in the casting mold. It is advantageous, while not mandatory, that the enlargement takes place without having to reduce the thickness of the slag protection sleeve at the meniscus level. [0024] A nozzle according to the invention has at least two objectives, one being to reduce the flow velocity at the outlet of the nozzle without increasing the overall dimension of the subentry nozzle, the other one being to extend the life of the nozzle while keeping the same flow velocity at the outlet of the nozzle.
[0025] The enlargement of the inner downstream portion of the outlet section can be machined in the finished (fired) nozzle. It can also be generated during the manufacture of the nozzle.
The processes disclosed in the USP 6395396 or USP 6616782 can help in the manufacture of such nozzle.
[0026] Some embodiments of the invention will now be described by way of examples with reference to the accompanying drawings in which - Fig. 1 shows a submerged entry nozzle of the prior art;
- Figs. 2 and 3 are respectively front and side views of a submerged entry nozzle according to an embodiment of the invention;
- Figs. 4 to 10 are views of different downstream portions of the outlet section of different embodiments of the invention. - Figs. 11 to 13 are views of different downstream portions of the outlet section of different embodiments of the invention illustrating the mean angle α of the enlargement. [0027] In these figures, reference 1 depicts a submerged entry nozzle. The submerged -entry nozzle 1 is divided into three sections: an inlet section having a generally axial symmetry, a transition section 3 in fluid communication with the inlet section to change the nozzle inner symmetry from having generally axial symmetry to generally planar symmetry and an elongated outlet section 4 having a b/oad dimension 6 and a narrow dimension 5 in fluid communication with the transition section 3. It is to be noted that the outlet section may (or may not) comprise one or more flow dividers or baffles of any shape, for example as depicted by reference 12. [0028] As is visible on Fig. 3, according to the invention, at least a part of the narrow dimension 6 is enlarged in the inner downstream portion 7 of the outlet section 4.
[0029] Various kind of enlargements of the inner downstream portion 7 of the outlet section 4 are depicted on Figs. 4 to 10 wherein a is the width of the inlet section b is the width of the outlet section taken in the same vertical cross section than a and L the distance between the inlet section and the outlet section
- the inner downstream portion 7 of the outlet section 4 can be flared out of an angle α in the narrow dimension 6 (Fig. 4). In this particular case, the mean angle α corresponds to the arctangent ((b-a)/2L);
- the inner downstream portion 7 of the outlet section 4 can be flared out in the narrow dimension 6 for a certain length and then, can comprise a straight portion 9 (Fig. 5). In this case, the inlet section of the enlargement corresponds to the beginning of the flare and the outlet section of the enlargement to the end of the straight portion;
- the inner downstream portion 7 of the outlet section 4 can comprise a step 8, enlarging thereby the narrow dimension 6, followed by a straight portion 9 (Fig. 6); - in a variant, the inner downstream portion 7 of the outlet section 4 can comprise a step 8, enlarging thereby the narrow dimension 6, followed by a tapered portion (Fig. 7) which itself can be followed by a further straight portion 9 (Fig. 8) or a tapered portion, followed by a step 8 and a straight portion 9 (Fig. 9);
- according to another embodiment quite similar to the embodiment shown on Fig. 5, the slag
protection sleeve 10 is further extended downwardly. It can be noted that the thickness of the sleeve is reduced downwardly to avoid interference with the enlargement of the inner downstream portion 7 of the outlet section 4 (Fig. 10);
- any combination of the depicted options would also be within the scope of the present invention.
[0030] Figs 11 and 12 illustrate what is intended by the mean angle α. [0031] Fig. 13 illustrates what is intended by mean angle α in another embodiment of the invention. The enlargement is followed by a further enlargement portion which does not correspond to the definition of the present invention. (b-a)/2L of the further enlargement is much bigger than 0.25 and there is no straight portion over a sufficient length L downstream of the flare.
The flow of metal can only separate in the further enlargement portion and this enlargement is thus useless in terms of flow velocity reduction. Such enlargement with an angle bigger than 15° is not taken into account unless followed by a straight portion as for instance illustrated in Fig 6. [0032] As to the slag protection sleeve, it can represent just a part of the wall at the meniscus level as shown on figures 2 to 8 and 10 or the whole thickness of the wall as shown on figure 9. [0033] The skilled person will easily select the most appropriate design of the inner downstream portion of the outlet section in function of the dimension of the mold and extraction speed.