EP3099828B1

EP3099828B1 - Effective cooling tank for treating pearlitic and bainitic rails

Info

Publication number: EP3099828B1
Application number: EP15710881.2A
Authority: EP
Inventors: Daniele Andreatta
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2014-01-29
Filing date: 2015-01-29
Publication date: 2019-01-02
Anticipated expiration: 2035-01-29
Also published as: EP3099828A1; WO2015114550A1

Description

Field of the invention

The present invention relates to a cooling tank for use in an installation for the thermal treatment of rail heads to obtain fine pearlitic or bainitic structures.

State of the art

Several solutions are considered in the known art for the thermal treatment of rolled rails, in particular aimed at obtaining the hardening of the rail head.
Starting from a temperature higher than 600°C, rail heads are subjected to a quick cooling either by the use of spray nozzles, which inject a cooling liquid (water, air, or water mixed with air) onto the rail head, or by immersing the head into a cooling tank containing a cooling liquid, for example water added with additive.
With respect to the solution with spray nozzles, the use of the tank allows greater cooling uniformity in the direction of the length of the rail and a larger cooling rate range to be obtained.
To further increase the heat exchange inside the tank and thus speed up the treatment, certain solutions of the known art provide jets of cooling liquid originating from holes located on the bottom of the tank and impinging the rail head immersed in the liquid: such jets increase the heat exchange and accordingly the cooling capability.
A solution of this type is described in document GB619699 , which illustrates a tank that provides three rows of holes on the bottom, wherefrom the cooling liquid comes out, the holes being adapted to create liquid jets directed towards the immersed rail head. However, the document mentioned provides that the rail head to be treated rests on a support, which is an obstacle for the liquid coming out of the holes and causes it to divert onto the two sides of the rail head. This results in the middle area of the rail head being impacted in an irregular manner by such jets and therefore being subjected to non-uniform cooling. Furthermore, the liquid flow that forms in the tank does not effectively lap the part of the head closest to the core of the rail.
Other solutions of cooling tank are described in documents WO2013/068998A1 ; WO 2010/133666A1 ; US5183519A ; US2009/200713A1 .
Other solutions have been proposed to improve such a cooling process - as disclosed for example in document JP63203724 - which provide supporting the rail by the flange in order not to have obstacles between the jets and the immersed rail head and to be able to treat it as uniformly as possible.
In particular, document JP63203724 provides three separate jets inside the bath, which are directed on the three faces of the rail head. This solution provides the complete immersion of the rail and therefore is not suitable for differentiated thermal treatments on different areas of the head.
However, in all solutions of the known art, the jets of cooling liquid form a flow at the outlet of the nozzles, which rises vertically towards the rail and, while moving towards it, tends naturally to widen, thus accordingly losing speed, and to flit about, that is to move alternatively to the right or to the left with respect to the hypothetical point of impact desired, thus involving a non-symmetrical and non-uniform heat transfer.
On the other hand, the rail head can not be approached excessively to the holes located on the bottom of the tank to preserve treatment uniformity over the entire length of the rail and to avoid the so-called resulting "punctiform effect" caused due to the presence of the steps between the holes: a rail that is excessively close to the holes is therefore not treated uniformly along the longitudinal axis, because the areas of the rail head located perpendicular above the holes are subjected to increased cooling with respect to the stretches located at the step between two consecutive holes. Furthermore, reasons of a practical/operating nature prevent the diameter of the aforesaid holes from having too small values (e.g. approximately < 6 mm) because the holes would thereby be easily occluded by the scale that detaches from the rail and by other particles in the bath.
In addition, it is very difficult to make the procedure for cooling rail heads that meet recent standards. In relation to the quality the rails must meet, the requirements of European Standard EN 13674-1 are difficult to obtain using cooling devices known to date due to the reduced speed of impact of the cooling liquid at particular points indicated by numeral P4 in figure 1, unless the binder content (e.g. chromium) is increased, with negative consequences on the toughness and on the cost of production.
Thus, the need is felt to make a cooling tank which allows overcoming the aforesaid drawbacks.

Summary of the invention

It is the main object of the present invention to provide a cooling tank for the thermal treatment of rail heads, which allows reaching a particularly high cooling efficiency by means of conveniently directing cooling liquid jets, flow rate and other conditions being equal.
It is another object of the invention to provide a cooling tank which allows increased heat exchange uniformity to be obtained along the entire length of the rail, efficiency being equal.
Not less important is the objective of obtaining an increased and uniform cooling of the lateral faces of the rail head, close to the corners where the head ends and narrows in the core, that is, in the points indicated by P4 in figure 1, so as to meet the hardness requirements required by European standard EN 13674-1.
Thus, the present invention proposes to achieve the objects mentioned above by providing a cooling tank for the thermal treatment of a rail head, which, according to claim 1, comprises a container defining a longitudinal symmetry plane and adapted to be filled with a cooling liquid in which the head to be thermally treated can be at least partly immersed parallel to the symmetry plane, said container having a bottom with a single row of holes, arranged at said symmetry plane, for generating a cooling liquid flow in said container,
wherein the container is equipped with a pair of first longitudinal bulkheads arranged symmetrically with respect to the symmetry plane,
wherein said first longitudinal bulkheads have a respective upper concave-shaped stretch with respect to the symmetry plane,
and wherein the upper stretches of the first longitudinal bulkheads are separated from each other by a distance greater than the width of the rail head, so as to define an area of the container adapted to house the rail head at least partly,
whereby, when the rail head is at least partially immersed in said area of the container, the cooling liquid flow generated by the single row of holes is directed vertically towards a middle face of the rail head and is divided by said middle face into two flows which are transversal with respect to the symmetry plane and are opposite to each other, and the concave shape of the upper stretches of the first longitudinal bulkheads then receives said two transversal flows and inverts the direction thereof by directing said two transversal flows towards respective lateral faces of the rail head.
A further aspect of the invention provides a method for cooling a rail head, performed in the aforesaid cooling tank, which, according to claim 8 comprises the following steps:

at least partly immersing the rail head into the area of the container defined between the upper stretches of the first longitudinal bulkheads;
generating a cooling liquid flow, by means of the single row of holes, which is parallel to the symmetry plane and is directed towards the middle face of the rail head;
dividing said parallel flow, by means of the impact with said middle face, into two flows which are transversal with respect to the symmetry plane and are opposite to each other;
receiving said two transversal flows by the concave shape of the upper stretches and reversing the direction thereof in order to direct said two transversal flows towards respective lateral faces of the head of the rail.

By introducing a first pair of longitudinal bulkheads into the tank, which are conveniently shaped in cross section, or by suitably shaping the longitudinal lateral walls of the cooling liquid container of the tank in the direction of the cross section, the fluid jets can be advantageously directed onto the parts of the rail head to be cooled with increased intensity such as to lap them accurately and to significantly limit the flicker of the flow of liquid.
Advantageously, a further pair of bulkheads can be provided, arranged inside the aforesaid first pair of bulkheads, and kept separate and raised from the bottom of the container of cooling liquid, thus leaving a passage and mixing space for the fluid. The flow rate of the jets coming out of the holes being equal, the space left between the inner longitudinal bulkheads and the bottom therefore allows involving an increased volume of cooling liquid in the formation of the flows, maintaining increased rates, and therefore increased efficiency of even 50%, flow rate and other conditions being equal. Increased cooling rates can be thereby achieved, also without acting on the chemical composition of the cooling liquid. Very high cooling rates are essential to obtain bainitic rail structures, while average high cooling rates allow fine pearlitic structures to be obtained by possibly varying the chemical composition of the cooling liquid, when required.
The inner longitudinal bulkheads, which are arranged at the middle of the tank, are vertical, while the most outer bulkheads, whether or not they coincide with the sides of the container of the tank, have an initially vertical shape which inclines in the high part towards the lateral faces of the rail.
The most inner bulkheads have a height which is equal to or less than the vertical stretch of the most outer bulkheads arranged close to the lateral walls of the cooling liquid container of the tank.
The most inner bulkheads are advantageously characterized in the lower part thereof by a jointed inlet for accompanying the jet coming out of the holes.
The presence of the longitudinal bulkheads, which border and canalize the vertical flow that forms at the outlet of the holes where the cooling liquid is ejected, further allows to decrease the punctiform effect of the jets over the length of the rail because these bulkheads allow the rail head to be immersed, thus leaving it at a greater distance from the outlet holes while successfully maintaining efficiency parity of the cooling treatment.
It is indeed possible with the bulkheads to increase the distance to be placed between the rail head and the holes on the bottom of the container, with better homogenizing of the temperature of the liquid in the flow up to completely eliminating the punctiform effect, thus obtaining the maximum longitudinal uniformity, while in any event ensuring adequate cooling efficiency.
This embodiment ensures the required continuous exchange of the liquid inside the tank which, by overflowing from the top of the walls or lateral bulkheads delimiting the cooling liquid container of the tank, is collected in two lateral channels.
The effectiveness and flexibility of the thermal treatment process is increased due to the present invention because the tank also allows increased flow rates of cooling liquid to be used in functional manner. Indeed, without such bulkheads, if the flow rate of the liquid is simply increased, the jets formed at the outlet of the holes take on - already at a short distance from the outlet of the holes - a particularly turbulent and disorderly movement, whereby they move away from the "working" areas thus making the removal of heat from the rail head less uniform and therefore creating areas with different hardness. By arranging the bulkheads as described in greater detail below, the flow of the jets remains directional and can remain directed exactly towards the areas wherefrom heat must be removed, also when the flow rate is increased. The selection of the arrangement of the rail with the head downward and the base upward integrates well with this distribution of the cooling liquid, which laps the various areas of the head section with the due intensity without touching the core of the rail and the base thereof, unless required for the cooling process.
The width of the passage between the two inner bulkheads affects the efficiency of the thermal treatment because if the distance is increased between the two inner bulkheads, the speed of the jets in vertical and transversal direction is decreased, accordingly causing the lowering of the cooling rate; if instead said distance decreases, the cooling rate increases.
An advantageous variant of the cooling tank of the invention provides a system for adjusting the position of the inner bulkheads, for adjusting said distance between the two inner bulkheads and/or for adjusting the gap from the bottom of the tank, in the cases in which it is provided, so as to vary the cooling rate without modifying the flow rate of the cooling liquid, thus performing only simple operations.
Another advantageous variant of the cooling tank of the invention provides a system for adjusting the vertical position of the outer bulkheads, so as to adapt the tank to the type of rail (shape, height, geometry) to be treated, thus making it possible to vary also the distance of the rail head from the row of holes, while keeping the same depth of immersion. Regulations in all countries indeed provide different measures, shapes, mechanical features, etc. for rails. The opportunity to adjust the outer bulkheads vertically makes it possible to model the tank (in particular the overflow level) according to the distance to leave between the holes and the rail head and according to the depth of immersion of the rail.
The dependent claims describe preferred embodiments of the invention.

Brief description of the figures

Further features and advantages of the invention shall be more apparent in light of the detailed description of preferred, but not exclusive, embodiments of a cooling tank, disclosed by way of a non-limiting example, with the aid of enclosed drawings, in which:

Fig. 1 depicts a cross section of a rail head with an indication of specific points in which the hardness is to be increased to meet the European Standard (relative to pearlitic rails);
Fig. 2 depicts a perspective view of a component of a portion of a first embodiment of the tank according to the invention;
Fig. 3 depicts a cross section of the longitudinal extension of the tank in fig. 2;
Fig. 4 depicts a cross section of the longitudinal extension axis of the upper part of the tank in fig. 2, while indicating the trend followed by the cooling liquid jets;
Fig. 5 depicts a cross section of the longitudinal extension axis of a second embodiment of the tank of the invention.

Detailed description of preferred embodiments of the invention

With particular reference to figures 2 to 4 illustrating a first preferred embodiment of the invention, the cooling tank is globally indicated by numeral 1, defining a longitudinal axis parallel to the longitudinal symmetry plane X of the tank and of the rails to be treated in tank 1.
Tank 1 comprises an upper part 2 and a lower part 3 (depicted in figures 2 and 3), it nevertheless being apparent that the lower part was not depicted in the other figures for simplification, being however understood that such a lower part 3 is also present in the depictions in figures 4 and 5. A container 1' of the cooling liquid is provided inside the upper part 2, into which a row of holes or nozzles 23 opens, through which the cooling liquid coming from the lower part 3 of tank 1 flows. Said lower part 3 of tank 1 is adapted to ensure a proper and uniform distribution of the cooling liquid to the upper part 2 of the tank. The lower part 3 is a delivery conduit for delivering the cooling liquid, that is the so-called delivery volume of the cooling liquid, while container 1' of the upper part 2 is the so-called cooling volume where the thermal treatments of rail R occur.
This upper part 2 has the upper side uncovered to leave a passage through which head 20 of rail R to be thermally treated can be immersed in container 1'. Along the sides of the upper part 2 of the cooling tank 1, two lateral channels 17, 18 are provided, where the cooling liquid is collected which continuously overflows from the top of the lateral walls 11, 12 of container 1' of cooling liquid during the operation of the tank. These two lateral channels 17, 18 are equipped with discharge pipes, not illustrated in the figures, along the longitudinal extension thereof. The cooling fluid already used for the thermal treatment of rail R flows through the discharge pipes into a recirculation circuit of the cooling liquid and of any other treatment required.
Tank 1 comprises a wall or bottom 8 for separating the lower part 3 and the upper part 2, which defines the bottom of the latter. The aforesaid row of holes 23 is made in bottom 8, arranged in longitudinal direction and evenly for the entire longitudinal extension of the tank, preferably with equal distance between one hole and the next. The particular shape in section of the holes 23 causes them to operate as nozzles capable of generating very powerful jets of liquid. The row of holes 23 is arranged along a longitudinal axis coinciding with the intersection of the longitudinal symmetry plane X of the tank and of the rail with bottom 8 when the rail is inserted with head 20 in the tank. This arrangement causes the generation of a flow of cooling liquid to the jets, passing from the lower part 3 to the upper part 2 under the force of the pressure created by the liquid recirculation system, jets which are directed vertically in a first stretch until they break against the middle face 32 of head 20 of rail R, and then divert laterally in approximately perpendicular direction to said plane X, as shown in figure 4 by a series of arrows diagrammatically depicting the trend of the flow in a plane transversal to the symmetry plane.
Advantageously, a pair of longitudinal bulkheads 13, 14 is provided having a cross-section shape such as to optimize the cooling also of the lateral faces 33 of head 20 of rail R. The bulkheads 13, 14 are arranged at an equal distance from the longitudinal symmetry plane X and are symmetrical with respect to said plane X and to the row of holes 23.
The bulkheads 13, 14 have, respectively:

a vertical lower stretch 4, 5, which may coincide with or be adjacent to a lower stretch of the lateral walls 11, 12 of container 1';
an intermediate stretch 4', 5' jointed to the corresponding lower stretch 4, 5, inclined by an angle α, with respect to bottom 8, and converging towards the symmetry plane X;
an upper stretch 6, 7 which is concave in shape with respect to the symmetry plane X.

Said angle α, which is other than zero, is less than 90°, preferably ranging between 30° and 60°.
Preferably, but not necessarily, the borderline area between the upper stretch 6, 7 and the corresponding intermediate stretch 4', 5' defines a cusp.
The concave shape of said upper stretch 6, 7, which forms a longitudinal channel with cup-shaped transversal section, may be, for example, semi-circular, semi-elliptical or more generally, curvilinear with variable radius. The concave shape must be such as to allow the cooling fluid flows, which are diverted laterally in direction approximately perpendicular to plane X from the outline of the rail head, to reverse direction to be directed towards the lateral faces 33 of the rail, thus succeeding in reaching areas which are normally difficult to lap and therefore to cool, such as the points indicated by P4 in figure 1. The flow of vertical liquid coming from the holes 23 impinges head 20 of the rail to then be divided into two transversal flows with opposite, approximately horizontal flow. The particular shape of the upper stretch 6, 7 of the bulkheads 13, 14, together with the reciprocal position with head 20 of rail R, which is at least partly immersed in the cooling liquid in an area of container 1' corresponding to the area delimited by said upper stretches 6, 7, allows said transversal flows, which are still rich with kinetic energy, to make a U-turn and to resume impinging head 20 of the rail at the lateral faces 33 thereof. At this point, most of the liquid of said transversal flows comes out of container 1' (arrows 30 in figure 4) while the portion remaining inside container 1' will keep alive two contrarotating vortexes (arrows 31 in figure 4) which promote the mixing of the liquid close to such lateral faces 33, thus improving the heat exchange.
Advantageously, to obtain excellent cooling results of the lateral faces 33 and completely meet the hardness requirements required by current European Standard EN 13674-1, the height of the third upper stretches 6, 7 is substantially equal to the height of points P4 of head 20 of rail R, the height being for example measured with respect to bottom 8.
Furthermore, the cooling tank of the invention is equipped with support means (not illustrated) for supporting rail R with head 20 facing downward, which are configured to block the rail with head 20 thereof in operating position, the head immersed in container 1', with the middle face 32 at a predetermined height H from bottom 8 and, accordingly, with the edges 34, in which head 20 ends and narrows in core 35 of the rail at a predetermined height H' from bottom 8 (figure 3) to ensure the fluid laps the surface of the head closest to points P4 during the cooling. To obtain an optimal thermal cooling of rail R, it is immersed at least partly with head 20 thereof in container 1' filled with cooling liquid, by arranging rail R with its symmetry plane in vertical direction coinciding with the symmetry plane X of container 1'. Thereby, the transversal flows of cooling liquid, which are diverted by the concave shape of the upper stretches 6, 7, are directed symmetrically onto the two lateral faces 33 of head 20 in order to have symmetry of treatment. Advantageously, height K from bottom 8 of the lower end of the upper stretches 6, 7, that is the height of the cusp, is less than height H where the middle face 32 is of the immersed rail head, so that the transversal flows, which branch off from the meeting of the vertical flow with the middle face 32 of the rail head, can be intercepted by the concave shape, or cup shape, of the upper stretches 6, 7 to cause a reversal in direction.
A further advantage lies in the fact that height K of the upper end of the upper stretches 6, 7 is designed on the basis of the points of the lateral faces 33 of the rail, which require more cooling according to production needs. When the requirements of European Standard EN 13674-1 are met, such a height K' is advantageously less than height H' of the edges 34 of the rail, so that the transversal flows, which are diverted by the concave shape of the upper stretches 6, 7, impinge the lateral faces 33 of the rail at the points P4 indicated in figure 1. In determining the operating height at which head 20 of the rail is to be supported with respect to the tank, it is apparent that the volume of fluid moved from container 1', and therefore of the flow rate of fluid introduced through the holes 23 and which overflows beyond the edges of the longitudinal walls 11, 12, is also considered.
The bulkheads 13 and 14 at which upper end the upper stretches 6, 7 define cups for conveying/returning the flow of cooling liquid advantageously have a position that can be adjusted by means of an appropriate system for adjusting their vertical position, not illustrated in detail in the figures. Therefore, the bulkheads 13, 14 can take on a position of height which is lower than, equal to or greater than the lateral walls 11, 12 of container 1', without departing from the scope of the invention. This adjustment can therefore also occur manually during the steps of changing the types of rail, in absence of cooling liquid or when its temperature is so low to allow access by operators.
Such a feature allows maintaining the desired distance from the nozzles 23 with different rail types/dimensions, the immersion being equal.
In a second advantageous embodiment illustrated in figure 5, where all the elements described above for the first embodiment are present, there can be provided, inside container 1' of the upper part 2, a further pair of longitudinal bulkheads 21, 22, which are arranged substantially vertical and at equal distance from the longitudinal symmetry plane X and symmetrically with respect to the row of holes 23. The bulkheads 21, 22 are arranged inside the pair of bulkheads 13, 14 and extend for a height which is equal to or less than the lower vertical stretch 4, 5 of the most outer bulkheads 13, 14, close to the lateral walls of the cooling liquid container of the tank.
Advantageously, the pair of longitudinal bulkheads 21, 22 with the relative row of holes 23 extends for the entire longitudinal extension of tank 1, in each module in case the tank consists of modules.
Preferably, the axis of each hole 23 is arranged parallel to the bulkheads 21, 22. Advantageously, the diameter of the holes 23 is approximately 6-12 mm, preferably equal to 10 mm, while the step between the holes 23 is approximately 1,5-5 times the diameter of the holes, preferably equal to 3 times the diameter of the holes.
Preferably, distance "L" between the two bulkheads 21, 22 has a minimum value, which is not less than the diameter of the holes 23. On the other hand, it is preferable to select distance "L" equal to a maximum value of twice the diameter of the holes 23 so that the positive effect of the presence of the bulkheads 21, 22 does not disappear and therefore the speed of the jet of liquid coming out of the holes 23 does not fall excessively. Preferably, distance "L" is selected greater than diameter "d" of the holes by approximately 4-6 mm. Advantageously, thickness "s" of the bulkheads 21, 22 which are preferably, but not necessarily, made of metal material, is as small as possible while ensuring the bulkheads have a predetermined sturdiness and rigidity, for example of approximately 5 mm.
Gap 25, 26 between the lower edge of the bulkheads 21, 22 and bottom 8 must not be too large because if the flow of the cooling liquid created at the outlet of the holes 23 is not canalized, it would continue expanding with respect to the axis of the holes 23 and would break against the lower part of the bulkheads 21, 22, thus significantly losing speed and risking not being sufficiently canalized into the slit or longitudinal channel formed by the parallel bulkheads 21, 22. Advantageously, the height of gap 25, 26 ranges between 0 and 1,5 L. Said slit or longitudinal channel between the two bulkheads 21, 22 serves to better direct the cooling liquid flow which forms with the jets that come out of the row of holes 23 towards the middle face 32 of head 20 of rail R.
When there is provided a height other than zero of gap 25, 26 between the lower end of the bulkheads 21, 22 and bottom 8, the lower ends of said bulkheads 21, 22 are advantageously tapered or bevelled so as to facilitate the conveying of the cooling liquid flow into the respective longitudinal slit.
Figure 4 (where the bulkheads 21 and 22 are not illustrated) diagrammatically shows, with a cross-section view of container 1', the trend of the flows that form in both the embodiments of tank 1 of the invention, in the presence of a rail subjected to the thermal treatment. If there is the pair of inner bulkheads 21, 22, they allow the jets of cooling liquid coming out of the holes 23 towards the middle face 32 of the rail head to be further better directed, thus decreasing any flicker.
The vertical lateral walls 11, 12 can not be provided in both the afore-described embodiments. Therefore, in this case, the lateral walls of container 1' coincide completely with the longitudinal bulkheads 13, 14, and during the operation of the tank, the cooling liquid overflows from the upper ends of the bulkheads 13, 14 while passing directly in the lateral channels 17, 18.
The cooling tank 1 can advantageously consist of a plurality of longitudinal modules, connected to each other by means of flanges or other suitable means of connection, so as to form a single element.
The cooling tank according to the invention described above has the following advantages:

it is possible to direct the jets of fluid also towards the sides of the rail in areas which are difficult to lap with tanks of known type, thereby increasing the surface of the rail lapped directly by the flow and in particular allowing to obtain increased hardness in the points P4 of the rail head;
increased cooling rates are reached, flow rate being equal (efficiency increased by 50%);
the flow rate can be increased while maintaining the directionality of the jets of fluid (increased effectiveness);
significantly high fields of variation of the cooling rate can be obtained without modifying the composition of the cooling liquid, which generally can be water, oil or aqueous solutions of salts and/or polymers, thus significantly increasing the operating flexibility of the tank while allowing to have a single device for the treatment of several types of steel (pearlitic and bainitic);

longitudinal bulkheads

the flicker of the jets of cooling liquid coming out of the holes is significantly limited;
the punctiform effect of the jets is reduced, thus obtaining a uniform treatment along the longitudinal extension of the rail.

Claims

A cooling tank (1) for the thermal treatment of a head (20) of a rail, comprising therein a container (1'), adapted to house the rail in its entire length, defining a longitudinal symmetry plane (X) and adapted to be filled with a cooling liquid in which the head (20) to be thermally treated can be at least partly immersed, according to the direction of the height of the rail, parallel to the symmetry plane (X), with head downward and base upward, said container (1') having a bottom (8) with a single row of holes (23) aligned along said symmetry plane (X), which communicate with a delivery conduit (3) for delivering the cooling liquid to generate a cooling liquid flow in said container (1'), from the bottom upward, wherein the container (1') is equipped with a pair of first longitudinal bulkheads (13, 14) arranged symmetrically with respect to the symmetry plane (X),
wherein said first longitudinal bulkheads (13, 14) have
- a respective vertical lower stretch (4, 5), adjacent to a lower stretch of the lateral walls (11, 12) of the container (1');

- a respective intermediate stretch (4', 5'), inclined by an angle α, with respect to the horizontal, and converging towards the symmetry plane X; of value greater than zero and less than 90°,

- a respective upper stretch (6, 7) having a concave shape with opening facing toward the symmetry plane (X),
and wherein the upper stretches (6, 7) of the first longitudinal bulkheads are separated from each other by a distance greater than the width of the head (20) of the rail, so as to define an area of the container (1') adapted to house the head (20) of the rail at least partly,
whereby, when the head (20) of the rail (R) is at least partially immersed in said area of the container (1'), the cooling liquid flow generated by the single row of holes (23) is directed vertically towards a middle face (32) of the head (20) of the rail and is divided by said middle face (32) into two flows which are transversal with respect to the symmetry plane (X) and are opposite to each other, and the concave shape of the upper stretches (6, 7) of the first longitudinal bulkheads (13, 14) then receives said two transversal flows and inverts the direction thereof by directing said two transversal flows towards respective lateral faces (33) of the head (20) of the rail.
A tank according to claim 1, wherein the borderline area between each upper stretch (6, 7) and the corresponding intermediate stretch (4', 5') defines a cusp.
A tank according to any one of the preceding claims, wherein the concave shape of each upper stretch (6, 7) can be semi-circular or curvilinear with variable radius, such as semi-elliptical.
A tank according to any one of the preceding claims, wherein the container (1') is equipped with a pair of second longitudinal bulkheads (21, 22) arranged at the single row of holes (23) so as to define a longitudinal channel between them for better directing the cooling liquid flow towards the middle face (32) of the head (20) of the rail (R).
A tank according to claim 4, wherein said second longitudinal bulkheads (21, 22) are arranged inside said pair of first longitudinal bulkheads (13, 14) and symmetrically with respect to the symmetry plane (X) and to said single row of holes (23).
A tank according to claim 5, wherein said second longitudinal bulkheads (21, 22) form a gap (25, 26) with respect to the bottom (8), and are completely vertical, or are vertical and have, in the lower part thereof, an edge which is folded with respect to the vertical to form a tapered inlet to the longitudinal channel.
A tank according to one of the claims from 4 to 6, wherein the second bulkheads (21, 22) have a height which is equal to or less than the lower stretch (4, 5) of the first bulkheads (13, 14).
A method for cooling a head (20) of a rail in a cooling tank according to any one of the preceding claims, the method comprising the following steps:
- at least partially immersing the head (20) of the rail (R), with the head downward and the base upward, in the area of the container (1') defined between the upper stretches (6, 7) of the first longitudinal bulkheads (13, 14);

- generating a cooling liquid flow, by means of the single row of holes (23), which is parallel to the symmetry plane (X) and is directed from the bottom upward against the middle face (32) of the head (20) of the rail;

- dividing said parallel flow, by means of the impact with said middle face (32), into two transversal flows which are diverging with respect to the symmetry plane (X) and opposite to each other;

- receiving said two transversal flows by the concave shape of the upper stretches (6, 7) and reversing the direction thereof in order to direct said two transversal flows in a converging direction towards the lateral faces (33) of the head (20) of the rail.