EP2185747B1 - Method and device for secondary descaling steel strip with low pressure water jets - Google Patents

Description

The present invention relates to the descaling operation of a metal strip, in particular steel, scrolling during hot rolling before entering the milling or finishing cages of the rolling mill

It is recalled that this operation is more commonly called "secondary descaling", as opposed to the "primary" descaling which intervenes on the steel slabs at the exit of the reheating furnace before rolling.

It is also recalled that the secondary descaling of the steel strips is intended to rid the surface of the band of the so-called secondary scale, which is formed by rapid reoxidation of the hot metal during the stay of the strip in the open air after its primary descaling on leaving the oven. It thus intervenes doubly during the rolling, first before the entry of the strip in the roughing, and before entering the finisher rolling mill. For the sake of simplicity, reference will be made hereinafter only to the case of secondary descaling at the finisher inlet, it being understood that what is said in this connection applies essentially also to secondary descaling at the entrance. of roughing.

The secondary scale is generally in the form of an adherent layer of metal oxides, typically between 50 and 100 microns thick, rather irregular in appearance. The secondary descaling is successful when it provides at the entrance of the finisher a steel strip having on the surface a uniform layer of residual scale of 20 to 30 microns thick just beyond, to avoid encrustations of oxides on rolling rolls.

To do this, the descaling operation consists schematically in impacting the surface of the moving strip by powerful water jets delivered by spray bars placed at a short distance and provided with injection nozzles fed under high pressure, typically above 130-150 bars, or even more than 200 bars in some cases. It is thus intended to combine a thermal effect (the surface temperature of the strip, around 1100 ° C at the inlet of the descaling, drops almost instantly to near 600 ° C) with a mechanical effect due to the high amount of movement of water jets at impact to crack the scale and eliminate the surface by the effect of hunting. This operation is conventionally carried out in a descaling box, about 1 to 2 m long, placed some 5 m upstream of the finishing cages, crossed by the steel strip in rapid rectilinear scrolling and sheltering the upper irrigation booms and lower ones with nozzles inclined counter-current of about ten degrees.

Although essential link to any chain of development of steel incorporating a hot phase (except to want to place the rolling mill in its non-oxidizing atmosphere - which is obviously not feasible), secondary descaling remains an expensive operation, less by the large amounts of water it involves (the water used is recycled) that because of the hydraulic high-pressure equipment that serves it, and for which it is necessary to question the possibilities of reducing the cost, particularly in terms of maintenance of pumps and circuits, and of electricity consumption:

The object of the invention is to provide an immediately operational response to the question of reducing the costs of the secondary descaling operation, that is to say a response compatible with a simple reorganization of existing equipment, so without necessarily involve the relocation of a new complete secondary descaling equipment.

To this end, the subject of the invention is a method for the secondary descaling of metal strips, in particular steel, in scrolling during their hot rolling by spraying water at their surface by means of watering ramps. nozzles fed with pressurized water, characterized in that the nozzles are fed under low hydraulic pressure, not exceeding 30 bars (and preferably below 10 bars, but not less than about 3 bars), and that, and that the said nozzles are adjusted so that the density of the heat flux extracted from the band (HF) resulting from the cooling of its surface by the projected water is between 6.5 and 20 MW / m ² for a strip temperature between 900 and 1100 ° C.

Advantageously, the heat flux density is between 10 and 11 MW / m for a strip temperature between 900 and 1100 ° C. In addition, in order to ensure a thermal effect of the water sprayed on the surface to be descaled quantitatively analogous to the thermal effect obtained with the usual known high-pressure secondary descaling method (ie a cooling of the strip which lowers the temperature of its oxidized surface at 600 ° C approximately), said nozzles are dimensioned so that they deliver a surface water flow rate on the strip similar to the surface water flow delivered by said high pressure method.

Preferably, the surface water flow rate is greater than 2500 l / min / m ² and preferably 7500 l / min / m ² .

• on alimente les buses avec une pression hydraulique inférieure à 10 bars, sans toutefois descendre en deca de 3 bars,
• le procédé de l'invention est mené en amont des cages finisseuses d'un train de laminage à chaud de bandes d'acier,
• le procédé de l'invention est mené en amont des cages dégrossisseuses d'un train de laminage à chaud de bandes d'acier.

The method according to the invention may also comprise different characteristics, taken alone or in combination:

• the nozzles are fed with a hydraulic pressure of less than 10 bar, without however falling below 3 bars,
• the process of the invention is carried out upstream of the finishing stands of a hot rolling mill of steel strips,
• the process of the invention is carried out upstream of the roughing cages of a hot rolling mill of steel strips.

As will have probably already been understood, the invention is based on the discovery that it is even more the thermal effect of the jets of water on the cooling of the oxide crust that acts in favor of the secondary descaling that their mechanical effect on the fragmentation of this crust of oxides on the surface of the band, or, in other words, the effect of "high-pressure cleaning" of the powerful jets on their impact, as previously thought.

To characterize this similarity of thermal effect between the process of the invention and the conventional high-pressure process, it will be referred to as the heat flux density extracted from the strip which integrates both the temperature and flow rate parameters. surface water, it being understood that this flow rate must be adjusted according to the temperature of the strip at the inlet of the descaling.

To characterize the process, it is the same basic consideration namely the use of low pressure while preserving the thermal effect generated by the use of high pressure jets.

Both upstream of the finishing cages and upstream of the roughing cages, the success of the secondary descaling is in fact directly and almost solely related to the thermal efficiency of the cooling of the oxide layer to be eliminated, independently of the supply pressure of the spray boom nozzles. In other words, with equal thermal efficiency, the quality of the secondary descaling obtained will be the same whether it is depressurized with high pressure jets or not.

It is pointed out, to avoid any confusion, that the expressions used here of "thermal effect of cooling" and "thermal efficiency" are equivalent. They express the fact that, during the brief residence time of the strip in the descaling box (of the order of one second only), it is a question of ensuring a fall of the temperature of the oxide layer up to about 600 ° C, whatever its temperature at the entrance of this box. It is known that the underlying physical quantity, and ordinarily measurable on a rolling train, is the extracted caloric flux density.

Therefore, the replacement of the usual strong jets (100 bars and beyond) by jets "low pressure" (less than 30 bars) is enough to ensure the thermal contraction of the oxide crust, a contraction that will result in detachments of this crust, completed by the energy of the jets which, although modest, suffices here amply to the task to make then easy the removal of calamine by simple action of sweeping and training with the water streaming on the surface.

These cascade effects are obtained with "low pressure" jets, in accordance with the invention, as soon as, as we have already said, they are made to ensure the same level of cooling of the oxide layer on the band that with the jets "high pressure", cooling level that will actually be achieved by simply keeping the surface flow rate of cooling water on the band.

Thus, the replacement of the usual water supply "high pressure" by a "low pressure" supply, becomes a solution immediately applicable industrially to thus enjoy a considerable economic advantage without conceding on the quality of descaling.

• la figure 1 est un tracé de courbes, dites d'ébullition, issues de l'expérience et montrant, en fonction de la température de surface de la bande, l'efficacité thermique comparée d'un décalaminage secondaire avant l'entrée au finisseur mené avec des pressions hydrauliques d'eau projeté différentes. Cette efficacité thermique est traduite quantitativement en ordonnée par la densité surfacique de flux thermique extrait (HF), donnée en MW/m2 de surface de bande métallique;
• la figure 2 montre l'efficacité de ce décalaminage secondaire, en termes d'épaisseur résiduelle de la couche de calamine en micromètres (e_c) dans une plage de températures de surface de la bande d'acier décalaminée (900-1050 °C) délibérément choisie conforme aux températures d'entrée dans les cages finisseuses.

The invention will be well understood and other aspects and advantages will appear more clearly in view of the following description given with reference to the attached single plate of figures in which:

• the figure 1 is a plot of curves, called boiling curves, resulting from the experiment and showing, as a function of the surface temperature of the strip, the comparative thermal efficiency of a secondary descaling before the entry to the paver driven with pressures different projected water hydraulics. This thermal efficiency is quantitatively translated in ordinate by the surface density of extracted heat flux (HF), given in MW / m2 of metal strip surface;
• the figure 2 shows the effectiveness of this secondary descaling, in terms of the residual thickness of the calamine layer in micrometers (e _c ) in a surface temperature range of the descaled steel strip (900-1050 ° C) deliberately chosen to comply at the inlet temperatures in the finishing cages.

On the figure 1 , the reference curve is curve A. This curve A results from a conventional secondary descaling carried out using powerful water jets from nozzles fed at 130 bar pressure. The other two curves B and C are representative of "low pressure" jets of 8 bars each, one (curve B) resulting from tests carried out with a flow rate of water equal to that of the curve A to jets "high pressure", namely 7,500 l / min / m ² , the other, curve C, resulting from tests conducted with a significantly lower surface flow rate: 1500 l / min / m ² .

It is important to recall here again that the criterion for setting a successful "low pressure" secondary descaling according to the invention lies in the maintenance in the oxide layer of a thermal effect similar to that conventionally achieved with jets "high pressure" (curve A). This must ultimately result in a drop in the temperature of the blank from 20 to 100 ° C (depending on the grade of steel to be rolled) between its entry into the watering box (typically 1100 ° C approximately for a carbon steel for example) and its entry into the finishing cages of the mill (typically about 1030 ° C).

To achieve this, given the low residence time of the band under the irrigation ramps (of the order of one second), it is therefore necessary to ensure under these ramps a cooling which causes the surface of the band to fall sharply. up to about 600 ° C, so that, on the one hand, the cooling rate of the oxide crust is high enough that the differential thermal contraction oxides-metal resulting from it succeeds in detaching this crust by fragmenting it. moreover possible, and on the other hand, that the inevitable subsequent heat input of the core of the strip towards the surface makes the latter reach the desired temperature at the entrance of the finishing cages.

This thermal effect, which is thus expressed by a high rate of momentary cooling of the surface of the strip (several hundred degrees / sec) has been expressed, for the parameterization of the three curves of the graph, by a classically accessible physical quantity. from the measurement, namely the density of heat flux extracted from the product being rolled by the projected water (Heat Flux in English abbreviated, or HF), magnitude expressed in MW / m ² . However, this characteristic quantity is particularly suitable for dimensioning a plant. descaling because it is correlated to the cooling water flow rate per m ² of strip (the surface water flow), which is a parameter that can easily be obtained from the definition of the operation of the descaling: schematically, at a value of HF corresponds a surface flow rate of cooling water.

Thus, as can be seen, the HF of the "High Pressure" descaling of reference (curve A) was kept constant around 10 MW / m ² all along the watering operation (surface temperature ranging from 1100 to 600 ° C). Those of the "low pressure" descaling according to the invention were maintained respectively over the same temperature range between 10 and 18 MW / m ² in the experimental case representative of curve B and between 6 and 10 MW / m ² for the case. of curve C.

Note that the HF value is actually calculated from data specific to each descaling equipment that are, to mention only the most important, the temperature of the cooling water (here at 20 ° C for all tests) , the type of projection nozzles, the water outlet pressure of these nozzles, the distance separating the nozzle nose of the surface of the band to be descaled, as well as the opening angle of the jet at the outlet of the nozzle.

It will be observed that the general appearance is the same for the curve B and the curve C: a common rise up to a strip surface temperature of about 450 ° C, followed by a bump having a maximum between 550 and 600 ° C for both, but with differentiated intensities this time. Then, the decay takes place almost in parallel between the two curves up to 1100 ° C, which is the common input temperature of the test strips in the descaling boxes.

It will be noted that it is precisely at this level of the temperature range (1100 to 900 ° C to be wide) that the industrial interest of the process according to the invention must be especially appreciated since almost all the rolling trains hot-rolled steel belts work with belt temperatures at the entrance of finishing cages between 900 and 1100 ° C.

However, it is precisely in this temperature range that we observe an almost equivalent quality of descaling between the reference curve at high pressure A and the low pressure curve B, equivalent to correlate of course with that of the values of HF on the graph (between 10 and 11 MW / m ² ). On the other hand, with respect to them, the low-pressure curve C, which displays a significantly lower HF (slightly less than 7 MW / m ² ), reflects a correlatively lower quality of descaling.

As shown by the tests carried out on an industrial pilot and recorded on figure 2 it is in this temperature range that a fine residual calamine layer, hardly exceeding 23 μm in thickness, is obtained which is used in a 6 bar BP configuration, or HP 100 bars, reflecting a virtually identical quality of descaling for these two options.

It is specified that these tests were conducted on a strip of low carbon steel type ISF with a distance "nozzle-steel strip" of 160 mm identical in each case, the same with respect to the flow of water jet-sprayed, namely 110 l / min, also with regard to the speed of travel of the steel strip at 60 m / min and the temperature of the projected water (20 ° C). The efficiency of the descaling was evaluated (on the ordinate) from the measurement of the residual calamine thickness at the surface of the strip by observation of micrographic sections of the descaled product.

More generally, it has been evaluated that the descaling according to the invention can be carried out for a heat flux density extracted from the product of between 6.5 and 20 MW / m 2 and, when referring to the flow rate of surface water, for a flow rate higher than 2500 l / min / m2.

The flux densities expressed above are measured under the ramp in the impact zone of the descaling jets.

Here we find, in figures, what was already stressed before, namely the importance of working with a thermal efficiency (HF) preserved compared to what is traditionally practiced, when we go from a "high pressure" descaling at a "low pressure" descaling according to the invention.

The choice of the level of the low pressure to maintain is indeed of second order importance with respect to maintaining the HF, this, of course, as long as we do not go down too low in pressure, say around 3 -5 bars minimum. Otherwise the required surface water flows, therefore the required HF levels (of the order of 10 MW / m ² ) could no longer be achieved, except to multiply the watering ramps, but with the risk nevertheless of not being to be able to ensure the thermal contraction effect of the oxide crust necessary for its unhooking of the metal support surface.

Conversely, the economic interest of working industrially with a "low pressure" that would be more than 30 bar suddenly fades to this level of pressure since the necessary equipment are those, or close to those, that are used already today for "high pressures".

It will be understood that the invention can easily be implemented by operating with pumps supplied at low pressure, thereby saving energy and reducing maintenance costs, provided that the conformation of the nozzles is adapted as necessary. to ensure a flow of surface water equivalent to that which would have been practiced in high pressure configuration.

The nozzles used for the implementation of the method of the invention will be arranged at the same distance from the strip as the distance applied during the known descaling process at high pressure.

• la possibilité de fractionner les rampes basse pression à faible cout. Le fractionnement des rampes permettra d'arroser au plus juste, à savoir la bande à décalaminer uniquement et non pas toute la largeur du train de laminage, ce qui induit des économies d'eau, une réduction de la masse d'eau qui circule en boucle et donc une réduction corrélative du coût énergétique supplémentaire;
• la possibilité d'utiliser les rampes "basse pression" comme un actionneur de réglage de la thermique de la bande à son entrée dans le finisseur;
• l'usure moindre des buses de projection d'eau;
• la réduction globale des coûts de maintenance de l'installation (pompes, vannes, circuits...).

Other additional advantages of using low pressure ramps in place of high pressure to achieve secondary descaling will be noted, such as:

• the possibility of splitting the low pressure ramps at low cost. The splitting of the ramps will make it possible to water as accurately as possible, namely the strip to be de-scaled only and not the entire width of the rolling mill, which induces water savings, a reduction in the mass of water flowing through. loop and therefore a correlative reduction of the additional energy cost;
• the possibility of using the "low pressure" ramps as an actuator for adjusting the thermal of the strip as it enters the finisher;
• less wear of the water spray nozzles;
• the overall reduction of maintenance costs of the installation (pumps, valves, circuits ...).

It goes without saying that the invention can not be limited to the examples described above, but applies to multiple variants and equivalents. In particular, it is recalled that it relates to any form of secondary descaling, that is to say removal of calamine previously formed by hot oxidation of a metal surface in contact with the ambient air.

Claims (7)

1. A method for secondary descaling running steel strips during their hot rolling, by projecting water at their surface by means of spraying ramps with nozzles supplied with pressurized water, characterized in that it consists of supplying said nozzles under low hydraulic pressure, not exceeding 30 bars and in that said nozzles are adjusted so that the heat flow density extracted from the strip (HF) resulting from the cooling of its surface by the projected water is comprised between 6.5 and 20 MW/m² for a strip temperature comprised between 900 and 1,100°C.
2. The method according to claim 1, characterized in that the heat flow density is comprised between 10 and 11 MW/m² for a strip temperature comprised between 900 and 1,100°C.
3. The method according to claim 1 or 2, characterized in that said nozzles are adjusted so that they deliver a surface water flow rate of more than 2,500L/min/m².
4. The method according to claim 3, characterized in that the surface water flow rate is 7,500L/min/m².
5. The method according to any of claims 1 to 3, characterized in that the nozzles are supplied with a hydraulic pressure of less than 10 bars, without however going below 3 bars.
6. The method according to any of the preceding claims, characterized in that it is conducted upstream from finishing stands of a hot roll train for steel strips.
7. The method according to any of the preceding claims, characterized in that it is conducted upstream from roughing stands of a hot roll train for steel strips.

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