Patent Grant
8388775
The present invention relates to an in-line process for the thermal treatment of rolled rails for improving the mechanical properties in at least one superficial layer of the rail head, and to a device for the thermal treatment of rails, specifically to a device for the in-line thermal treatment of rails exiting from a rolling system.
Different solutions for devices and processes for the thermal treatment of rolled rails are included in the known art, the devices and processes specifically being directed to harden the head by means of the quenching operation.
Many of these devices are not arranged in-line with the rolling stands. This implies the stocking of the rolled rails and a subsequent heating thereof before proceeding to the quenching thermal treatment, with significant energy consumption and low efficiency.
In other systems the devices are instead arranged along the rolling line: the rolled rail is unloaded on a roller table, which is secured to the ground; it is then withdrawn by manipulators, including elaborate leverage systems, which control the handling of the rail during the thermal treatment to which the latter is subjected; and it is finally ejected on the cooling bed or plate by means of appropriate ejection mechanisms.
The rails which are heated or directly coming from the rolling mill are subjected to a fast cooling either by the use of spray nozzles, which inject a cooling fluid (water, air, or water mixed with air) on the rail head, or by immersion of the same in a tank containing a cooling fluid.
When spray nozzles are used, the drawback occurs of the rail warping in the direction of the length due to a temperature inhomogeneity in some segments of the rail and due to the subsequent different thermal dilatations.
When the immersion tank is used instead, a greater cooling uniformity in the direction of the length is achieved, although in any case the temperature difference between the base of the hot rail and the cooled head results in a bending of the rail; the drawback is that the manipulators employed are not sufficiently rigid and resistant to counteract and contain said bending. Another drawback of such manipulators is that, during the treatment, they are always in contact with the rail in the same fulcra thus generating undesirable “cold” areas on the rail itself.
Furthermore, with all of the known devices, the throughput of the entire line is extremely low. The throughput does not exceed 12-15 rails/hour for rails which are about 100 m long. Such devices are also not structurally simple and require a considerable maintenance, both elements determining an increase in production and management costs for the device.
The need is therefore felt to provide an innovative device for the thermal treatment of rails exiting from a rolling system allowing to overcome the above said drawbacks.
As far as the thermal treatment process is concerned, the immersion processes provide to make a continuous cooling of the rail head, which however results in a metallurgic structure which is not uniform through the entire thickness of the treated layer.
Other processes instead include the introduction of alloy elements, such as silicon and aluminium, in the steel to be treated in order to obtain the desired final features; the addition of alloying elements has the disadvantage of considerably increasing production costs.
The need is therefore felt to provide an innovative process for the thermal treatment of the head of the rails allowing to increase the mechanical properties through the achievement of an improved metallurgic structure without the addition of alloying elements in steel.
It is the primary object of the present invention to obtain a new process of in-line thermal treatment of rolled rails which ensures to obtain a fine pearlitic structure which is uniform through a whole predetermined superficial thickness of the rail head, specifically suitable for the use of the rails in very cold environments in virtue of the improved toughness.
Another object of the invention is to obtain a device for the thermal treatment of rails, placed in-line with a rolling system, which is structurally simple, has a high sturdiness and requires less maintenance as compared to the existing devices.
Therefore, the present invention aims to achieve the above disclosed objects by providing a process for in-line thermal treatment of a rail exiting from a rolling system which, according to claim 1, includes the following steps:
Another aspect of the present invention provides to make a device for the in-line thermal treatment of rails exiting from a rolling system, which, according to claim 8, includes at least one mobile trolley in turn including
Advantageously, the device of the invention includes at least one roller table allowing to guide the rail perfectly along the rolling line thus maintaining the same position with which it exits from the last rolling stand, i.e. with the symmetry axis of the rail in a substantially horizontal position. The same roller table also provides for the rigid support of the rail, the handling thereof during the thermal treatment and the unloading thereof on the cooling plate. The roller table of the device according to the invention therefore performs all of these functions in a different manner as compared to the traditional roller table which instead only serves to forward the rolled rail and therefore needs to be combined to dedicated manipulator devices.
A further advantage of the device of the invention is represented in that it provides two wheeled roller tables which, alternately and axially aligned to the rolling line, allow the almost concurrent thermal treatment of two rails improving the throughput of the system. In this manner, twice the throughput is achieved as compared to that achieved with the known devices, with a rolling rate in the range between 8 and 10 m/s.
Advantageously, immersing the rail in the tank for its whole length ensures the homogeneity of the treatment, the thermal distortions of the rail being virtually avoided or reduced to the minimum in virtue of the rigidity of the device, and also ensures a greater flexibility in the handling of the final cooling step, which is the most important to obtain the final desired structure. The result of a fine pearlitic grain depends on the cooling rate in this last step as well as on the deformation of the material obtained in the rolling stands. Therefore, high cooling rates are preferred, which do not lead in any case to the formation of undesired bainitic and/or bainitic-sorbitic structures.
The process according to the present invention advantageously provides four cooling steps, two by air and two by water with additives or by another appropriate cooling liquid. The head of the rails obtained by this process displays the following properties:
The dependent claims disclose preferred embodiments of the invention.
Further features and advantages of the invention will become more apparent in light of the detailed description of a preferred though not exclusive embodiments of a device for the thermal treatment of rails, which is shown by way of non-limitative example with the aid of the accompanying drawings in which:
a and 1b depict examples of layouts for rail production systems provided with the device according to the invention;
a depicts an example of the layout of a part of the rail production system including the device for the thermal treatment of the invention. This exemplary layout includes:
The straightening machine 6 may be placed on the right and/or on the left of the cooling plate 5.
b depicts a variant of the layout in which the device for the thermal treatment of the invention, which includes the mobile trolleys 3 and 4, is always arranged along the rolling axis X although in this case the cooling plate 5 is arranged between the last rolling stand and the device of the invention. This layout offers the possibility to treat in-line only some of the rolled rails. The rolled rails, for which the quenching thermal treatment is not required, may be unloaded onto the plate 5 which, by translating them, unloads them directly on the straightening machine 6.
In the device of the invention the trolleys 3, 4 are arranged parallel to one another and to the rolling axis X and, advantageously, are adapted to be positioned alternately along said rolling axis. Each trolley has indeed the possibility to laterally translate with respect to the rolling axis or line in virtue of the presence of handling means, for instance a rack system provided in the floor or another appropriate system.
Each trolley 3, 4, shown in
The roller table, placed immediately at the exit of the rolling-mill train, serves to withdraw, guide and grip a rolled rail to perform the thermal treatment. At the end of the treatment, the rail is unloaded onto the cooling plate 5 by means of the same roller table.
For each pair of motorized rollers 10 there is provided also a idle roller 12, having vertical axis when the trolley is in the position (b), which comes into contact with the base of the rail foot to better guide the rail to the same position as that by which it exits from the last rolling stand.
In a preferred variant, the pairs of motorized rollers 10, 10′ may be opened and, during the receiving step of a rolled rail to be treated, the lower rollers, fixed with the axis in a horizontal position, receive the rail while the upper rollers, which are mobile, are lifted from the working position. For instance, the upper rollers may be lifted by rotating about a pin to ease the insertion of the rail into the device of the invention. Once the rail is totally inserted, the upper rollers and the idle rollers 12 are respectively adhered to the web and to the foot of the rail to ensure the gripping.
Advantageously, the whole longitudinal roller table is appropriately pivoted so as to rotate about a longitudinal axis parallel to the rolling axis X to orient the rail head downwards.
Each trolley 3, 4 further includes a longitudinal tank 11, 11′ containing a cooling liquid, preferably but not necessarily water containing a synthetic additive, such as for instance glycol, in which the rail head is immersed. The tank 11, 11′ has a longitudinal extension at least equal to that of the rail and is placed on the base of the trolley.
Appropriate actuating means for the tank 11, 11′ are provided to lift it from the base of the trolley up to a predetermined height so as to perform the immersion in the cooling liquid of the rail head. Such actuating means may include, for instance, hydraulic jacks or a leverage system. Advantageously, the thickness of the rollers 10, 10′ is reduced, for instance in the range between 60 and 80 mm, so as to avoid interferences with the edge of the underlying cooling tank when the latter is lifted.
The level of the cooling liquid in the tank 11, 11′ may be close to the edges or the liquid may overflow thus spilling laterally each time the rail is immersed. In this latter case, side collection tanks 13, 13′ may be provided and, advantageously, recirculation means for the liquid may also be provided with the reintroduction in the tank of the collected liquid. Stirring means for stirring the cooling liquid in the tank, such as for instance oscillation generators, may also be provided.
Advantageously, spray nozzles 14 are provided on the roller tables 15, 16, which are intended to carry out the cooling of the rail foot in order to avoid thermal distortions as a consequence of the temperature difference which is generated between the head and the foot of the rail. A further advantage consists in that in this manner less residual stresses are obtained in the treated rail. The spray nozzles 14 preferably spray the same cooling liquid contained in the tank, possibly even mixed with air.
The already mentioned motorization of the roller pairs 10, 10′ determines a longitudinal alternate motion of the rail which allows the dedicated nozzles 14 to cool the foot over its entire length and therefore also the part of foot in contact with the idle rollers 12.
The working cycle related to the preferred embodiment of the device of the invention, with two trolleys 3, 4 provided with respective roller tables 15, 16 is disclosed hereinafter:
1) the first trolley 3 is initially along the rolling axis X (position b in
2) after having received the rolled rail 9, the first trolley 3 moves from the pass-line or rolling line (position b in
3) the air cooling of the rail 9 on the trolley 3 continues until reaching a temperature higher than 720° C., preferably in the range between 800 and 850° C., for a total time for instance in the range between 40 and 90 sec, preferably equal to 80 sec; at the same time, the second trolley 4 (position b) receives a second rail 9′, which is also moved forwards and backwards so as to uniform the thermal load on the gripping rollers 10′ and so as not to create cold spots on the web of the rail;
4) when the second rail 9′ is received, the second trolley 4 returns to its side position (position a) while its roller table 16 rotates by 90° counter-clockwise so that the symmetry axis of the rail 9′ is oriented on the vertical with the head facing downwards; at the same time, the first trolley 3 moves to return along the rolling line (position b) and its tank 11 is lifted by said actuating means (not shown), for instance hydraulic jacks, to perform a second cooling step by immersion of the head of the first rail 9 in the lifted tank 11. Advantageously, the above said longitudinal alternate motion of the rail 9 induces the vapour film, that tends to be formed in contact with the surface of the head during cooling, to brake when the head is immersed, thus improving the thermal exchange;
5) at the end of the time provided for the immersion step, for instance in the range between 10 and 20 sec, preferably equal to 15 sec, the tank 11 of the first trolley 3 (position b) is then lowered to perform the air equalizing step (having a duration for instance in the range between 10 and 60 seconds, preferably equal to 15 sec); at the same time, the first air cooling step of the second rail 9′ (position a) is completed and the tank 11′ of the second trolley 4 is lifted so as to perform the second cooling step by immersion of the head of the rail 9′;
6) next, the tank 11′ (position a) is then lowered to perform the air equalizing step (having a duration for instance in the range between 10 and 60 seconds, preferably equal to 15 sec) and the tank 11 (position b) is lifted again for a final cooling step by immersion of the head of the first rail 9 (having a duration for instance of about 250 sec);
7) similarly, the tank 11′ (position a) is again lifted for the last cooling step by immersion of the head of the second rail 9′;
8) when the last step of the thermal treatment for the first rail 9 is finished, the tank 11 (position b) is again lowered, the roller table 15 rotates by 90° in a direction opposite to the previous one to bring the symmetry axis of the rail back to a horizontal position, the motorized rollers 10 forward the thermally treated rail 9 by unloading it on the cooling plate 5 placed downstream and receive (step 1) a third rail to be treated when it exits from the last rolling stand;
9) when the third rail is received, the first trolley 3 repeats the operations described in steps 2) and 3) while in the second trolley 4, when the last thermal treatment step is finished for the second rail 9′, the tank 11′ (position a) is lowered again, the roller table 16 rotates by 90° in an opposite direction to the previous one to bring the symmetry axis of the rail back to a horizontal position while the same trolley 4 moves to return to the position aligned with the rolling line (position b), the motorized rollers 10′ forward the second thermally treated rail 9′ by unloading it onto the cooling plate 5 placed downstream and receive a fourth rail to be treated when it exits from the last rolling stand.
The cycle continues by repeating the above described steps 4 to 9. The rails 9, 9′ are always loaded on and unloaded from the respective trolleys along the rolling axis X.
According to a variant, instead of lifting and lowering the tank 11, 11′, it is possible to respectively provide the lowering and lifting of the roller table 15, 16, already being rotated by 90° counter-clockwise.
The process for the thermal treatment of rails for hardening the head, object of the present invention, comprising the four cooling steps above described, may also be performed by using devices different than that one described above.
In any case, the process according to the invention is performed in-line, i.e. at the exit of the rolling-mill train when the rail has reached the area of thermal treatment, so that the rolling residual temperature equal to about 900÷950° C. is exploited advantageously. In this manner, a considerable energy saving is obtained with respect to off-line processes which provide for heating the rail again before the quenching thermal treatment.
Hereinafter there is disclosed a preferred embodiment of the process according to the invention relating to a steel having a percentage of carbon in the range between 0.7 and 0.9% and a manganese content in the range between 0.75 and 1.25%.
At the exit from the rolling-mill train, when the rail has reached the thermal treatment area at the time t=0 (
Thus it is provided the second step of cooling of the only rail head by means of a cooling liquid, until reaching a surface temperature of the head little more above the Ar3 temperature of transformation from austenite into pearlite. Specifically, the value of this surface temperature is from 50 to 150° C. above the Ar3 temperature and thus such as to avoid the phase transformation from austenite into pearlite. This second step, having a duration for instance in the range between 10 and 20 seconds, preferably equal to 15 sec, may be performed by immersion of the rail head in a tank of water, or another appropriate liquid, eventually containing a synthetic additive, or may be performed by means of jets of water, or another appropriate liquid, eventually containing a synthetic additive, directed on the rail head and coming from dedicated nozzles provided in cooling boxes and arranged so as to cover the whole length of the rail.
Advantageously the process according to the invention provides to interrupt the cooling by means of said liquid and to carry out a third cooling step again in air, lasting for instance in the range between 10 and 60 seconds, preferably equal to 15 sec, in order to equalize the temperature of the rail head surface up to that one corresponding to a predetermined superficial layer of the rail head, having a depth preferably from 25 to 25 mm measured starting from said surface. Indeed the heat of the inner layers tempers the superficial layers up to a temperature of about 720÷840° C.
This third step may be performed by bringing the rail head out from the above mentioned tank or by closing the nozzles which generate the jets directed on the head.
Subsequently, there is provided a fourth cooling step, again by means of the same cooling liquid, until reaching a surface temperature lower than 500° C., preferably lower than 450° C. This fourth step, lasting for instance about 250 seconds, may be performed either by immersion of the rail head in said tank, or by means of the sprays of the above said nozzles of the cooling boxes.
The maximum duration of the fourth step is in the range between 180 and 350 seconds and it is such as to generate a cooling rate sufficiently high in order to obtain a fine grain pearlitic structure and avoid at the same time the formation of bainitic and bainitic-sorbitic structures, notoriously rigid but brittle. For the example of embodiment just described said cooling rate is not higher than 3-4° C./sec.
The total duration of the thermal treatment cycle, including all the four cooling steps, depends on the composition of the steel constituting the rail in terms of percentage of carbon (in the range between 0.45 and 1.2%) and of the alloy elements contained therein. The total time of the thermal treatment above disclosed is in the range, for instance, between 240 and 520 seconds, preferably equal to 360 seconds.
Further, the duration of the first three cooling steps also depends on the conditions in which the rail arrives from the exit of the rolling-mill train, such as the residual surface temperature and the condition of equalization of the temperature between the surface and the above mentioned superficial layer of the rail head. The duration of said first three steps can be also considerably reduced as much as the rail exits from the rolling mill with a relatively low temperature and with a good equalization condition of the temperature between the surface and the core of the rail head.
When the process is carried out by using the method of immersion in a tank of liquid, an advantageous embodiment according to the invention provides for:
In the process according to the invention the fact of performing a first liquid cooling in which no phase transformation occurs, allows to reduce the total time of the rail head thermal treatment cycle; further the fact of interrupting the second cooling step by liquid and of performing the third step of cooling by air (tempering) allows to equalize, from the metallurgical point of view, the temperature of the above mentioned superficial layer of the rail head with the temperature of the external surface. In this manner, for the following fourth cooling step by means of liquid, there will occur about the same starting temperature of the austenite-pearlite phase transformation both for the surface and for all said superficial layer and, accordingly, about the same cooling rate. Therefore at the end of said phase transformation, a fine and uniform pearlitic structure is advantageously obtained in a superficial layer or thickness which is about 15-25 mm thick, preferably at least 20 mm. A fine and uniform pearlitic structure is required for operational use of the rail at very low temperatures, for instance up to −60° C.
In the cooling step by immersion in the liquid it is possible to use water with the addition of an appropriate polymer at a temperature in the range between 35 and 55° C. or pure water at a temperature close to the boiling point.
Specifically, the curve 20 shows the trend of the surface temperature of the rail head, while the curve 21 shows the trend of the temperature of an inner layer, 20 mm thick, of the rail head. Both the curves 20, 21 include four segments corresponding respectively to the first, second, third and fourth cooling steps.
A high slope of the curves 20 and 21 in
Advantageously, the preferred cooling rate in the final step of the thermal treatment of the invention is in the range between 2 and 7° C./s, preferably 2÷5° C./s. In the example of a rail made of a steel having a percentage of carbon in the range between 0.7 and 0.9% and a manganese content in the range between 0.75 and 1.25% the optimal cooling rate is equal to 3÷4° C./s.
Advantageously, providing the intermediate third step of cooling in air (equalization in
From the comparison, it may be noted that with the process according to the invention (
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