Claims:WE CLAIM:

1. An improved tundish adapted to reduce the skull loss at the end of casting sequence comprising:

tundish housing having an open top and a pouring box providing a raised inlet with respect to the tundish bottom for feeding metal from ladle for controlled outflow of molten metal therefrom for casting in mould;

at least one strand spaced from said pouring box for passage therethrough of molten metal into mould;

said pouring box and its inlet cooperatively disposed with respect to a raised platform surrounding the same comprising a raised pouring box surroundings such as to block the free skull forming region surrounding said pouring box and the supporting tundish bottom thereby reducing the unwanted skull formation surrounding said pouring box.

2. The improved tundish as claimed in claim 1 comprising a wedge with embedded pouring box adapted to minimise the tundish loss in pouring box area.

3. The improved tundish as claimed in anyone of claims 1 or 2 which is a T shaped multi strand tundish wherein said pouring box and its inlet cooperatively disposed with respect to a raised platform surrounding the same constitutes. a feed area of molten metal from ladle substantially free of any skull forming space and cooperates with plurality of strands for multiple mould castings which are longitudinally disposed with strands close to the said pouring box constituting the central strands.

4. The improved tundish as claimed in claim 3 wherein said pouring box and its inlet cooperatively disposed with respect to a raised platform surrounding the same such as to block the free region surrounding said pouring box cooperates with inverted V shaped dams at the central strands for improved inclusion floatation.

5. The improved tundish as claimed in anyone of claims 1 to 4 wherein said raised pouring box surroundings are adapted for allowing liquid metal flow from pouring box to outlets through modified pouring box area (wedge) such as to minimize the residual metal in middle part of T shaped multi-strand tundish.

6. The improved tundish as claimed in anyone of claims 1 to 5 comprising of wedge in pouring box area of tundish provided with gentle slope in central strand area.

7. The improved tundish as claimed in anyone of claims1 to 6, where the overall skull weight in modified tundish is more than 40% less than the existing tundish.

8. The improved tundish as claimed in anyone of claims 1 to 7 wherein vertical height of wedge and inverted V dam is same in size.

9. The improved tundish as claimed in anyone of claims 1 to 8 comprising inverted V dams adopted to improve inclusion floatation in central strands by increasing the residence time.

10. The improved tundish as claimed in anyone of claims 1 to 9 wherein said raised pouring box surroundings is adapted to favour easy de-skulling of solidified skull from the tundish after casting sequence due to more symmetric shape.

Dated this the 28th day of June, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION

The present invention relates to an improved tundish bottom design in continuous casting of steel in multi-strand billet caster. More particularly, the present invention is directed to a tundish bottom design in T-shaped multi strand tundish to avoid problem of high skull loss and improve steel cleanliness, by incorporating an improved tundish bottom design with wedge in pouring box area to reduce the skull loss and incorporation of inverted V dams in central strand area to provide better inclusion flotation, thus improving casting quality and productivity.

BACKGROUND OF THE INVENTION
Continuous casting process of steel is one of the most accepted technologies to convert the molten steel to solidified semi-finished flat and long product by an open ended water cooled copper mould through which molten steel is passed. A thin shell formed quickly around the molten steel during passing the water cooled mould due to heat transfer. Shell strength is enough to hold the molten steel inside it when it is coming-out from mould. The rest molten steel is solidified outside of the mould by water cooling system. In such a way casting of steel through open ended water cooled copper mould is continuous process where liquid steel poured into the mould and partially solidified semi-finished product of steel is coming out.

In order to connect batch wise steel process and continuous casting process, tundish is used between steel carrying ladle and continuous casting mould. In modern day steel making, continuous casting tundish system has very important role. During continuous casting process, tundish has many important roles such as metallurgical vessel to provide molten steel at required flow rate, low inclusion, uniform chemistry, constant temperature to continuous casting mould etc. It acts as buffer zone between steelmaking batch wise process and continuous casting, it receives liquid steel from the ladle which is present at top and delivers the same into the mould which present at bottom of tundish after attaining a particular bath height in the presence of a slag layer constantly and continuously. In tundish, molten steel is covered with steel slag. Slag layer acts as sink for non-metallic inclusions and provide protective layer for molten steel from heat loss and air. At the end of casting sequence, the leftover steel in tundish is drained as much as possible to be converted into solidified product. During the drainage of tundish at the end of casting sequence, there is possibility of slag carry over into casting mould it may result poorer cleanliness of cast product. Since the mechanical and thermal properties of these slag is different from steel, they severely weaken the solidified steelshell which can no longer hold the pressure of liquid steel. This leads to a breakoutwhich can lead to significant downtime of the casting machine. Even without the slaglayer on top, most vortices may also entrain air with it, leading to a similar result.To avoid slag carry over to mould, considerable amount of metal is left in tundish as the solidified skull at the end of casting sequence. This leads to considerable amount of precious metal loss that is customarily referred to in the industry as ‘‘tundish skull’’ and responsible for decrease in steel yield. Complete drainage of metal from tundish is prohibited due to formation of vortex above the outlet nozzle and start of slag entrapment in molten steel. This affects product quality and may also result in break out. In the caster shop, the operator therefore routinely terminates casting before any significant slag entrainment results into continuous casting mold. Therefore, some residual steel is left behind in a tundish at the end of sequence casting. The skull loss also depends on slag vortexing phenomena and determined by the height of liquid column above the tundish nozzle at which the upper slag phase starts to get entrained into mould. Lesser is the height of liquid column, lower is the tundish loss for tundish without any furniture. It is always desired by the steelmakers to study and modify the tundish to minimize the skull loss. Similar studies have been done on slab caster tundish with single strand, where flow is unidirectional, but is rarely attempted in a multi strand billet caster tundish.

In multi-strand billet caster tundish flow of metal is different in all strands and does not have uniform flow profile. Moreover, all strand may not operate simultaneously making flow more complex. At the end of the sequence, the last strands are closed first and the central stands in the last. Additionally, in multi-strand tundish the time taken for the metal to flow from the inlet to the central nozzle (closest to the pouring box) is lowest and does not have proper residence time for inclusion flotation. Whereas the time taken for the metal to flow from the inlet to the end nozzle (farthest to the pouring box) is highest resulting is better inclusion flotation.

Inevitably some amount of steel is lost in tundish in the form of solidified skull at the end of sequence casting. This tundish loss often called as skull loss is always reported higher in multi-strand billet casters compared to single strand casters. Metal flow behaviour in multi-strand tundish is different from the single strand tundish. So is the draining out the metal at the end of the cast, which is far more complex in multi-strand tundish, specially the billet casters. To reduced tundish skull for any given caster is often not possible since geometry, dimensions and capacity of tundish systems, nozzle opening and nozzle location, etc. are fixed and therefore, cannot be independently varied without bringing in simultaneous changes in upstream and downstream processes. It can only be achieved through modification of existing tundish bottom design.

To reduce tundish skull at the end of the cast and to improve the steel cleanliness at central strand, it is necessary to modify the existing tundish system. For any given continuous casting tundish system, modification in process parameters is generally not possible since tundish capacity, geometry, dimension, nozzle location, diameter of nozzle etc are fixed thus cannot change independently without simultaneously changing the upper line or down line of process. So, solution should be evolved from entirely new idea without changing any existing process parameters. It can be done by modifying tundish bottom design on the basis of process fundamentals.

In the present work water modeling studies have been carried out to design a new tundish bottom for reducing the skull loss in a multi-strand billet caster. This invention relates to new modified design for the inside bottom surface of T shaped multi strand billet caster tundish for continuous casting molten steel to reduce the tundish skull loss during draining of tundish specially at end of casting sequence and to improve the steel cleanliness specially during casting process. New design of tundish bottom has wedge in pouring box area and inverted dams at the central nozzles. Skull loss at end of casting sequence has reduced due to presence of wedge in pouring box area and the dams helped in inclusion flotation in the central strands.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to an improved tundish bottom design for T shaped multi-strand billet caster with raised bottom at pouring box area for reducing skull loss at the end of casting sequence which leads to higher productivity of caster.
A still further object of the present invention is directed to develop an improved tundish bottom design for T shaped multi-strand billet caster which ensures more metal drainage from tundish at the end of casting without affecting the existing vortex height.
Another object of the present invention is directed to a tundish bottom design for T shaped multi-strand billet caster wherein inverted V dams are provided at central strands to improve the steel cleanliness at central strands.
Yet another object of the present invention is directed to a tundish bottom design for T shaped multi-strand billet caster to improve the minimum residence time for steel at central strands which further leads to improvement in inclusion floatation at central strand.
A still further object of the present invention is directed to develop said tundish bottom design for T shaped multi-strand billet caster which ensure the easy de-skulling of solidified skull from the tundish after casting sequence due to more symmetric shape.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to an improved tundish adapted to reduce the skull loss at the end of casting comprising:

tundish housing having an open top and a pouring box providing a raised inlet with respect to the tundish bottom for feeding metal from ladle for controlled outflow of molten metal therefrom for casting in mould;

at least one strand spaced from said pouring box for passage therethrough of molten metal into mould;

said pouring box and its inlet cooperatively disposed with respect to a raised platform surrounding the same comprising a raised pouring box surroundings such as to block the free skull forming region surrounding said pouring box and the supporting tundish bottom thereby reducing the unwanted skull formation surrounding said pouring box .

A further aspect of the present invention is directed to the improved tundish comprising a wedge with embedded pouring box adapted to minimise the tundish loss in pouring box area.

A still further aspect of the present invention is directed to said improved tundish which is a T shaped multi strand tundish wherein said pouring box and its inlet cooperatively disposed with respect to a raised platform surrounding the same constitutes. a feed area of molten metal from ladle substantially free of any skull forming space and cooperates with plurality of strands for multiple mould castings which are longitudinally disposed with strands close to the said pouring box constituting the central strands.

A still further aspect of the present invention is directed to said improved tundish wherein said pouring box and its inlet cooperatively disposed with respect to a raised platform surrounding the same such as to block the free region surrounding said pouring box cooperates with inverted V shaped dams at the central strands for improved inclusion floatation.

A still further aspect of the present invention is directed to said improved tundish wherein said raised pouring box surroundings are adapted for allowing liquid metal flow from pouring box to outlets through modified pouring box area (wedge) such as to minimize the residual metal in middle part of T shaped multi-strand tundish.

Another aspect of the present invention is directed to said improved tundish comprising of wedge in pouring box area of tundish provided with gentle slope in central strand area.

Yet another aspect of the present invention is directed to said improved tundish, where the overall skull weight in modified tundish is more than 40% less than the existing tundish.

A further aspect of the present invention is directed to said improved tundish wherein vertical height of wedge and inverted V dam is same in size.

A still further aspect of the present invention is directed to said improved tundish comprising inverted V dams adopted to improve inclusion floatation in central strands by increasing the residence time.

A still further aspect of the present invention is directed to said improved tundish wherein said raised pouring box surroundings is adapted to favour easy de-skulling of solidified skull from the tundish after casting sequence due to more symmetric shape.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: is an illustration of front and top view of an existing T shaped multi-strand tundish (8 strands in the present case).
Figure 2: is an illustration of the side view of an existing T shaped multi-strand tundish.
Figure 3: is an illustration of the isometric view of an existing T shaped multi-strand tundish.
Figure 4: is an illustration of shape of a solidified tundish skull of existing tundish at the end of casting sequence.
Figure 5: is an illustration of actual solidified tundish skull of existing tundish at the end of casting sequence.
Figure 6: is an illustration of front and top view of a modified T shaped multi-strand tundish (8 strands in the present case).
Figure 7: is an illustration of the side view of modified T shaped multi-strand tundish.
Figure 8: is an illustration of the isometric view of modified T shaped multi-strand tundish
Figure 9: is an illustration of shape of a solidified tundish skull of modified tundish at the end of casting sequence.
Figure 10: is an illustration of actual solidified tundish skull of modified tundish at the end of casting sequence.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention relates to an improved tundish with a novel tundish bottom design for T shaped multi-strand billet caster adapted to reduce the metal left in tundish (tundish skull) at the end of casting sequence. The proposed design also has newly designed and incorporated inverted V shaped dams at central strands in front of the pouring box area to improve the steel cleanliness at central strands.

As any change in the tundish configuration, will change the flow dynamics and may affect other parameters, a detailed investigation must be carried before implementation. To study the effect of flow control modifiers like wedge, dams, baffles etc on the fluid dynamics inside the tundish, water modelling studies were carried out. For water modelling studies, a 0.25 scale plexi-glass model of 45 T billet caster tundish was fabricated based on Froude number criterion.RTD measurement was carried out using the electrical conductivity measurement technique through stimulus response of injected saturated salt solution. Different type of bottom designs was formulated and studied. Each experiment was conducted at least thrice and then average values were taken for Minimum Residence Times for different configurations so as to ensure better accuracy of the results. Minimum Residence Time was measured at central and end strands under each set of configuration using a conductivitymeter. Vortex formation phenomena were also studied during drainage of tundish for both the cases. Based on the water modelling studies and considering fabrication difficulties, use of wedge at turbo stopper region was found to be most optimized design for starting the plant trials.The volume occupied by the wedge does not allow the left metal to accumulate and solidify in this region.For validation of lab scale studies, plant trials at billet caster were conducted. One tundish was selected and turbo-stopper area shape was modified. Tundish skull has been collected weighed at the end of every sequence with and without modified tundish.Plant trials confirmed that the new wedge near the pouring box and dam configuration reduced the skull loss by 40% had no adverse effect on the cleanliness of the steel.

The basic aspect of the present invention is thus directed to develop the novel type tundish bottom for T shaped multi-strand billet caster tundish by changing inner refractory lining of bottom of tundish which alone would provide considerable advantage in reducing the tundish skull at the end of casting sequence. Most importantly, there is no change in tundish external structure. The objective of modification is to generate the raised bottom at the poring box area in the shape of wedge with slope at central strand side.The volume occupied by the wedge does not allow the left metal to accumulate and solidify in this region.

A further aspect of the present invention is directed to incorporate the new designed inverted V shaped dam at central strands with raised bottom in front of pouring box area to improve the steel cleanliness at central strand. The vertical height of wedge and inverted V dam would be decided by other parameters of the continuous casting system.

The tundish bottom configuration according to an embodiment of the present invention are described where under with reference to illustrative drawings wherein accompanying Figure 1 showsfront (1) and top view (2) of an existing T shaped multi-strand tundish (8 strands in the present case). The T shaped tundish has a round pouring box (3) where the metal is poured from the ladle. The molten metal is then passed into the moulds through various strands which allow parallel casting. Due to this complex shape the flow dynamics of the metal is different at the central strand (4) and at the end strand (5).

Accompanying Figure 2 shows side view (6) of an existing T shaped multi-strand tundish along the central axis, where the proximity of pouring box (7) and central strand (8) can be seen without any flow modifier between them. This does not provide time for inclusion flotation and it gets entrapped into the solidifying metal.

Accompanying Figure 3 shows isometric view of an existing T shaped multi-strand tundish.

Accompanying Figure 4 shows the shape of the solidified metal (9) at the end of the casting from the existing tundish.

Accompanying Figure 5 shows the actual picture of the solidified metal (10) at the end of the casting from the existing tundish. It can be concluded from the pictures that thicker metal skull is getting formed at the central region mainly near the pouring box area.

Accompanying Figure 6 shows front (11) and top view (12) of the modified T shaped multi-strand tundish (8 strands in the present case). The T shaped tundish has a wedge (13) with embedded pouring box (14) where the metal is poured from the ladle. The volume occupied by the wedge does not allow the left metal to accumulate and solidify in this region. It also has two inverted V shaped dams (15) at the central strands which helps in increasing the residence time of the metal in the tundish and helping in inclusion flotation.

Accompanying Figure 7 shows side view (16) of the modified T shaped multi-strand tundish along the central axis, where the wedge (17) is made fixed in the tundish with refractory and the pouring box (18) is embedded during every new casting. An inverted V dam (19) is introduced in front of the central strand (4) which act as flow modifier between them. This helps in inclusion flotation.

Accompanying Figure 8 shows isometric view of the modified and improved T shaped multi-strand tundish.

Accompanying Figure 9 shows the shape of the solidified metal (20) at the end of the casting from the modified tundish.

Accompanying Figure 10 shows the actual picture of the solidified metal (21) at the end of the casting from the modified tundish. It can be concluded from the pictures that no thicker metal skull is getting formed at the central region due to the wedge (17) near pouring box area. The overall skull weight in modified tundish is more than 40% less than the existing tundish. The symmetric shape of the skull allows easily de-skulling/cleaning from the tundish after casting sequence.

It is thus possible by way of the present invention to provide new modified bottom design for the inside bottom surface of tundish for T shaped multi strand billet caster in continuous casting of molten steel to reduce the tundish skull loss during draining of tundish specially at end of casting sequence and to improve the steel cleanliness specially during casting process. The novel tundish bottom configuration comprises a wedge in pouring box area and inverted dams at the central nozzles. Skull loss at end of casting sequence has reduced due to presence of wedge in pouring box area and the dams helped in inclusion flotation in the central strands.