MXPA06006161A - Method and device for cooling a steel strip - Google Patents

Abstract

The invention relates to a cooling device for implementing a tempering operation of a steel strip, comprising a weir (4) in which a number of tubes (1) are completely immersed, which are, in essence, vertically stacked and arranged symmetrically on both sides of the strip (2) along this strip, and which discharge, in the form of essentially horizontal turbulent jets, a cooling fluid across a slot or a number of holes. Any two successive tubes (1), which are situated on the same size of the strip (2), are separated by the same gap (B) for all the tubes (1) for evacuating the cooling fluid. The gap (B) is set to a given specific flow rate value of the fluid in order to minimize the loss of load in the evacuation channels corresponding to said gap (B).

Description

PROCEDURE AND COOLING DEVICE OF A STEEL BAND FIELD OF THE INVENTION The present invention relates to a device for the start-up of the cooling of a steel strip, in the context of a continuous annealing process. In particular, this cooling is carried out by means of immersion water jets. This cooling operation can be carried out consecutively in a first cooling operation in a boiling water bath.

BACKGROUND OF THE INVENTION Continuous annealing is a thermochemical treatment that is applied to steel bands after cold rolling. The "band" of steel is the steel product that, cut, will provide the plates used particularly for the manufacture of car bodies, des frameworks of household appliances, and so on. The continuous annealing process consists of parading the steel strip in an oven where it is exposed to controlled heating and cooling. In the continuous annealing furnace, the steel band circulates vertically, in accordance with a series of successive, ascending and descending belt shots, and thus sequentially passes through the various stages of the treatment. The treatment of the band in the furnace generally comprises the following successive thermal stages: - preheating and heating: the band reaches a temperature of 700 ° to 850 ° C in 2 to 3 minutes; -maintenance at the maximum temperature for about 1 minute; - slow cooling, for example in boiling water; - rapid cooling (referred to as "tempering"), for example with water in liquid form projected on the strip at a temperature which can be at most up to its boiling temperature; - over-aging; - final cooling. These different stages are necessary for the start-up of the metallurgical treatment considered, in particular the recrystallization, the precipitation of carbides, the obtaining of final structures or even the obtaining of a steel that does not age, and so on. In particular, in recent years, we have seen the emergence of a growing demand, arising mainly from the automotive industry, for steel sheets that simultaneously exhibit improved strength and formability properties. In this framework, the cooling phase plays a particularly crucial role because it allows, in certain cases, to reduce the concentration in expensive alloying elements necessary for the realization of particular microscopic structures, such as, for example, "dual phase", multiphase, High Elastic Limit ("ULE", for its acronym in English), and so on. Therefore, the cooling process corresponds to a non-negligible metallurgical and economic activity. The main cooling technologies applied industrially are: - the cooling by gas jets; - immersion in. a water bath, possibly "agitated"; - cooling by passage through cooled rollers; - cooling by water jets; - the cooling by a mist of water created by spraying with a supersonic gas, this technology being called "fog jet". In the past, the Applicant developed a cooling process which consists of immersing the steel strip in a water bath close to its boiling temperature. Although this process is characterized by an exceptional cooling homogeneity and a constant thermal transfer coefficient, regardless of what the line conditions are, it also has certain limitations. On the one hand, the cooling speeds that can be achieved are relatively small, in particular of approximately 50 ° C / s for a steel strip of 1 mm thickness. This limitation stems from the fact that, when a steel strip is submerged at high temperature in a boiling water bath, in the vicinity of its surface a stable vapor film is formed, in a regime called "heating", which considerably limits the thermal exchanges. We understand by heating the presence of a vapor film, generated by important boiling, between a hot wall and a fluid, whether it is a liquid, a diphasic mixture of liquid and vapor, this presence having as a consequence a bad heat transfer between the wall and the fluid. On the other hand, the temperature of the steel strip at the outlet of the boiling water bath should remain above about 300 ° C. When the temperature of the band becomes lower than this temperature, the vapor film becomes unstable and becomes unstable. goes to the boiling regime called nucleate. In this last regime, neighboring regions of the band are subjected to different heat fluxes, which creates important temperature differences. These temperature gradients induce in the steel mechanical pressures, which have the risk of creating plastic deformations that are permanent, as well as leading to defects of the flat characteristic. Solutions have been proposed in order to reduce these faults. For example, the steel strip can be immersed in a static cold water bath. However, this solution generally leads to the occurrence of flaws in the flat feature. Other solutions have been proposed, which consist of cooling the steel strip by means of immersion jets, in order to prevent the local formation of boiling zones in the vicinity thereof. These cooling systems may or may not be preceded by slower cooling, such as "gas jet cooling" or immersion in a static water bath.

Thus, in the patent application JP-A-58 039210, the band is first cooled in a water bath whose temperature is higher than 60 ° C, up to a temperature comprised between 200 ° and 500 ° C, temperature range in which produces the transition between boiling in film and nucleated boiling. It is then proposed to cool the band just before or just after the transition by immersion water jets, until the band reaches the bath temperature. A similar solution (JP-A-60 009834) uses a set of cooling ramps, arranged on both sides of the steel strip and submerged in a water tub whose temperature is between 60 and 75% of the boiling temperature. For a given configuration of sprinkler ramps, a laminar circulation is generated, which allows to avoid the formation of a vapor film in the vicinity of the steel strip. Another solution is even to circulate water between two flat plates in parallel and countercurrent in relation to the direction of the band's run (EP-A-210847, JP-A-63 145722, JP-A-62 238334). Another document proposes to use the pressure of impact of the jets, in order to suppress the deformations of the band when the hardening is carried out (see JP-A-11 193418). The depositor intends to apply, on both sides of the steel strip, a pressure of at least 500 N / cm2.

Finally, it is also possible to control the cooling by means of additives in the tempering bath, so as to avoid boiling and thus limiting the level of internal pressures in the steel when the tempering is carried out (JP-A-57 085923) . Although numerous solutions have been proposed, the simultaneous obtaining of high thermal performances and a good flat characteristic after the rapid quenching by liquid route, continues to be an important challenge today.

OBJECTIVES OF THE INVENTION The present invention aims to perform a so-called tempering operation, typically at a speed in excess of 1000 C / s, applicable to flat metallurgical products, preferably steel, in the form of cold-rolled strips. This tempering operation must be carried out by means of jets of cold water whose temperature is comprised, preferably, between 0 ° C and 50 ° C, said jets being immersed. The invention aims to ensure cooling conditions with high powers as homogeneous as possible throughout the width of the steel strip, by controlling the circulation within the device.

Thus, the temperature of the band at the entrance of the device should be between 750 ° C and 350 ° C, while the temperature at the outlet should be comprised, preferably, between 0 ° C and 150 ° C.

BRIEF DESCRIPTION OF THE INVENTION A first purpose of the present invention relates to a base cooling device for performing a tempering operation when the continuous annealing treatment of a flat product is carried out in the form of a metallurgical strip, preferably a steel strip, said device being located in a basically vertical ascending or descending strap strip, comprising a ditch in which a plurality of tubes stacked substantially vertically and symmetrically on one and the other part of the strip is completely submerged along it, and that eject, each of them, in the form of turbulent jets basically horizontally, a cooling fluid towards the band through a slit or a plurality of holes. In addition, the device is provided in its lower part with sealing means. According to the invention, two successive tubes, arranged on the same side of the strip, are separated by an identical interval for all the tubes in order to evacuate the cooling fluid. Said interval is then chosen, with a given value of the specific flow rate of the cooling fluid, expressed in cubic meters per hour and per square meter of a face of the band, to minimize the loss of load in the evacuation channels corresponding to said interval (the loss of load for each interval and the total load loss are identical). According to a preferred embodiment of the invention, the wall of the gutter, located at the rear of the tubes, has a width at least equal to that of the tubes and the horizontal distance of this wall in relation to the rear face of the tubes is chosen in such a way that the loss of load caused by the presence of the ditch is less than 5% of the loss of load caused by the • intervals between two successive tubes, which is considered negligible. Therefore, the spill is two-dimensional. Advantageously, the invention makes it possible to avoid the phenomena of local boiling by choosing a specific flow rate of the cooling fluid on one side of the strip comprised between 250 and 1000 m3 per hour and per m2. In an example of a device tested by the Applicant, the maximum specific flow rate per face was approximately 580 m3 per hour per m2. Preferably, the pressure loss caused by the intervals is less than 150 mm of water column.

Always advantageously, the distance between the end of each tube and the band is identical for all tubes and is between 50 mm and 200 mm. Always in accordance with the invention, the ejection velocity (VJET) satisfies the following criteria, respectively: - for the holes, vjrr; _ _,! -, tí - for the slits, Where A represents the distance between the tube and the band and d represents the diameter of an orifice or the thickness of the slit. A and d are expressed in the same units of length, in meters for example. Its quotient is dimensionless. VJET is expressed in m / s. These two criteria, derived from the theory of turbulent jets, provide the attenuation of the maximum velocity of a turbulent jet with an environment of zero velocity. The criteria are calculated based on a minimum speed of 2.5 m / s. The maximum speed of the jet with A = 50 mm (position of the band in relation to the orifice of the jet) is 0.65 m / s. Therefore, the speed of 0.65 m / s is considered as the minimum speed of the jet when it reaches the band, to break the heating layer.

Preferably, the cooling fluid is liquid water maintained at a temperature lower than 50 ° C. Preferably, the device is located in a basically vertical rising belt (angular deviation in relation to the vertical less than 30 °), being preceded directly by a tub of water brought basically to the boiling temperature. The invention will advantageously be implemented in an installation in which the metallurgical product to be treated has a speed of between 0.25 m / s and 20 m / s, as well as a thickness comprised between 0.1 mm and 10 mm. An important feature of the invention resides in the fact that the cooling tubes have such dimensions that the ejection speed of the cooling fluid is homogeneous over the entire width of the band. Preferably, the tubes have dimensions such that the distribution of the speeds is such that there is a relative deviation between the maximum velocity (Vmax) and the minimum ejection velocity (Vm? N) in accordance with the width of the tube, which is less than 5% where: Preferably, the ratio between the passage section of a tube and the spray-free section of this tube, i.e. the area of the slit or the accumulated area of the orifices, is greater than 1.

In accordance with a preferred embodiment of the invention, said tubes have a rectangular section. Preferably, the ratio of one side to an adjacent side of the rectangular section is between 0.1 and 10, while the thickness of the tubes is between 0.25 times and 10 times the diameter of the holes or the thickness of the tubes. the slit, in order to control the coherence of the jet, the relationship between the thickness of the tubes and the diameter of the orifices being, if it is still the case, preferably, equal to 2/3. In accordance with another advantageous feature of the invention, the sealing means mentioned with. Previously they comprise a watertight compartment with a double pair of rollers, which at the same time allows the passage of the band and the creation of a loss of load that limits the spillage of the ditch downwards to a minimum value. Still in accordance with the invention, these sealing means also comprise means for injecting a fluid between the rollers, whose pressure and / or temperature can be controlled. Advantageously, the upper tube is equipped with a barrier whose height is at least equal to the sum of the thickness of the water wave in the gutter and the height of the water column corresponding to the head loss between the maximum flow tubes . A second purpose of the present invention relates to a tempering process when the continuous annealing treatment of a flat product in the form of a metallurgical strip is performed., preferably a steel strip is put into operation, by starting the device described under one of the above embodiments, to achieve a specific cooling power of between 1000 kW / m2 and 10000 kW / m2 per side of the metallurgical product. According to the method of the invention, the temperature of the band at the entrance of the device is comprised between 350 ° C and 750 ° C, while the temperature at the outlet is between 50 ° C and 450 ° C, preferably between 50 ° C and 100 ° C, or between 350 ° and 450 ° C.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 schematically represents a sectional view of the cooling device according to the present invention. Figure 2 schematically represents an arrangement of holes intended for spraying water onto the steel strip in the device of the present invention. Figure 3 illustrates graphically the thermal performances of the cooling device according to the invention. Figure 4 illustrates the performances of said device in terms of the flat characteristic of the steel strip. Figures 5 and 6 illustrate the impact of the uniformity of cooling on the homogeneity of the mechanical properties of the steel strip. Figure 5 refers to a steel of the "dual phase" family, while Figure 6 refers to a steel of the multiphase steels family. Figure 7 provides schematically the different positions of the samples taken as a function of the width of the sheet, for the performance of tests relative to figures 5 and 6. Figure 8 indicates the parameters that allow calculating the index of the flat characteristic, these parameters characterizing the sinusoid in which the longitudinal profile of the band is assimilated at the beginning.

PREFERRED MODALITY OF THE INVENTION As shown in Figure 1, the cooling device is constituted by a set of tubes 1, called "ramps" or "cooling ramps", arranged symmetrically on either side of the steel strip to be cooled. These ramps are submerged and fed laterally in cooling fluid. Preferably, its section is rectangular. In the remainder of the disclosure of the invention, the terms "tubes" and "ramps" will be used interchangeably. The immersion of the ramps is carried out by means of a hermetic closing system, located in the lower part of the device, which at the same time allows the passage of the steel strip 2 and the creation of a maximum load loss, in a way that the flow of cooling fluid spill down to the bottom of the hood is limited to a minimum. In the presented application, this sealing system is constituted by a double pair of rollers 3, applied against the steel strip and located symmetrically in relation to it. A fluid is injected between the rollers whose pressure and / or temperature can be controlled. Preferably, the cooling fluid is water. The cooling ramps are located at a distance A from the line of passage of the band 2. For reasons of obstruction, on the one hand, and in order to limit the total flow in the system, to achieve equivalent performances, on the other hand The maximum distance between the band and the cooling ramps is set at 200 mm. A space B is left between two successive ramps, in order that the water injected by the ramps can be evacuated between them. This guarantees a circulation as homogeneous as possible in accordance with the width of the steel strip. The choice of distance B is derived from a compromise between a specific cooling power P maximum, the specific power which is defined as the cooling power per unit area and per side of band to be cooled and a minimum loss of load to through the evacuation channels, in order to ensure a sufficiently rapid renewal of the cooling fluid in the vicinity of the sheet and thus avoid the formation of local boiling zones in the vicinity of the band. The distance B is chosen identically between two successive ramps for all the ramps, in order to ensure identical circulation conditions on the face of all the sprinkler ramps. Therefore, this allows to obtain a vertical homogeneity of the circulation. In this way, the cooling fluid injected by a given ramp is evacuated by means of the channels directly adjacent to this ramp. This avoids creating preferential paths and minimizes the passage time of the cooling fluid in the vicinity of the band., always to avoid the local formation of boiling areas. Each cooling chute 1 is provided, on the face exposed to the band, with at least one slit or a set of holes, as shown in Figure 2, intended for the projection of the cooling fluid towards the band. . The distance between two successive orifices must be such that the circulation in the proximal vicinity of the band can be assimilated to that of a slit. The ejection speed of the fluid must be sufficient to avoid forming boiling zones in the vicinity of the band. This ejection velocity V is chosen as a function of the distance A in relation to the band and is typically between 0 and 10m / s. Downstream of the evacuation channels, the device or the cooling hood comprises a ditch 4, over the entire width of the hood and whose height corresponds to the level of the jet of the last ramp, which guarantees that, under all conditions of operation, the last ramp is submerged just like the others.

In order to ensure identical circulation conditions on the face of each ramp: - the upper cooling ramp is topped with a barrier 5 whose height is at least equal to the sum of the thickness H of the water layer in the gutter and of the height of the water column? H corresponding to the head loss? P through the evacuation channels, for the maximum flow rate Qmax; - an evacuation channel is made below the last ramp. Thus, when the system works, there is a difference in water level between the front face or side of the belt and the rear face or side of the gutter of the ramps. This difference is equal to the height of the water column corresponding to the loss of load between two ramps, for a given flow. The cooling performances of the device, illustrated in Figure 3, were measured in industrial conditions by thermal balance based on the following magnitudes: steel strip temperatures at the input and output of the device, length of the cooling section - and scroll speed of the steel band through the device. Figure 3 shows that the specific cooling power, expressed in kW per square meter and per side of the bank, is a linear function of the specific flow, expressed in cubic meters per hour and per square meter for the two accumulated faces. Under the conditions considered here, the specific power is between 4000 and 6000 kW / m2 -and per product face.

Figure 4 illustrates the performance of the device in relation to the flat characteristic of the steel strip. They are the image of the homogeneity of cooling and, as a consequence, of the domain of circulation in the device. The characterization of the flat characteristic here refers to long edges. Each point in the figure represents a point of operation of the device - defined by the specific cooling power associated with a given time during the industrial test campaign. A flat characteristic index, expressed in units "I", is associated to each operating point. A unit "I" corresponds to a relative elongation of 1 mm per 100 m of steel strip. In the case of a "long edge" type defect, the longitudinal profile of the band at the edge can be assimilated to a sinusoid, with a wavelength L and an amplitude X. The flat feature index is calculated based on the measurements of L and X (see Figure 8) by the following relationship: Figure 4 shows two reference thresholds, 120 and 240"I" units, which correspond to the tolerances of the flat characteristic admissible by two electrogalvanization lines with zinc. The figure shows that most of the operating points are located without reaching the threshold of the most demanding line.

Figures 5 and 6 illustrate the impact of the uniformity of cooling on the homogeneity of the mechanical properties. Figure 5 refers to a steel of the "dual phase" family. Figure 6 refers to a multiphasic steel (ferrite, martensite, bainite, perlite). In both cases, the mechanical properties are characterized by a tensile test. - The samples are taken in different positions according to the width of the sheet, according to the scheme shown in figure 7: 1) Extreme edge, 2) Edge , 3) Fourth, 4) Center, 5) Center, 6) Fourth, 7) Edge, 8) Extreme edge. Figures 5 and 6 show, respectively, the breaking load, the elastic limit (only in Figure 6) and the elongation at 80% of the breaking load. It can be concluded from these observations that there is a good homogeneity of the mechanical properties according to the width of the band.

Claims (20)

NOVELTY OF THE INVENTION CLAIMS

1. - A cooling device for carrying out a tempering operation when the continuous annealing treatment of a flat product is carried out in the form of a metallurgical strip 2, preferably a steel band, said device: - being located in a belt shot basically vertical ascending or descending; - comprising a ditch 4 in which completely submerges a plurality of tubes 1 stacked basically vertically and symmetrically in one and the other part of the strip 2, along the same, and which eject each one, in the form of turbulent jets basically horizontal, a cooling fluid towards the band through a slit or a plurality of holes; - being provided in its lower part with sealing means 3; characterized in that two successive tubes 1, arranged on the same side of the band 2, are separated by an identical interval B for all the tubes, in order to perform the evacuation of the cooling fluid, said interval B being chosen, in a given value of the specific flow rate of the cooling fluid, expressed in cubic meters per hour and per square meter of a face of the band, to minimize the loss of load in the evacuation channels corresponding to said interval B.

2. - The device according to claim 1, further characterized in that the wall of the count 4, located at the rear of the tubes 1, has a width at least equal to that of the tubes 1 and the horizontal distance of this wall in relation to the rear face of the tubes 1 it is chosen in such a way that the loss of load caused by the presence of the gutter 4 is less than 5% of the loss of load caused by the intervals B between two successive pipes 1.

3. The device according to claim 1 or 2, further characterized in that the specific flow rate of the cooling fluid is between 250 and 1000 m3 per hour, per m2 and per side of the band.

4. The device according to claim 1, 2 or 3, further characterized in that the pressure loss caused by the intervals B is less than 150 mm of water column.

5. The device according to any of the preceding claims, further characterized in that the distance A between the end of each of the tubes 1 and the band 2 is identical for all tubes and is between 20 mm and 200 mm .

6. The device according to claim 5, further characterized in that the ejection velocity (VJET) in each tube satisfies the following criteria, respectively: - for the holes, A a - for the slits, where A represents the distance between the tube and the band and d represents the diameter of an orifice or the thickness of the slit.

7. The device according to any of the preceding claims, further characterized in that the cooling fluid is liquid water maintained at a temperature below -50 ° C.

8. The device according to any of the preceding claims, characterized also because the device is located in an ascending vertical belt shot and is directly preceded by a tub of water that is basically brought to the boiling temperature.

9. The device according to any of the preceding claims, further characterized in that the flat metallurgical product to be treated has a thickness comprised between 0.1 mm and 10 mm.

10. The device according to any of the preceding claims, further characterized in that the flat metallurgical product to be treated has a scroll speed between 0.25 m / s and 20 m / s.

11. The device according to any of the preceding claims, further characterized in that the cooling tubes 1 have such dimensions that the distribution of the ejection velocities are such that there is a relative deviation (Vmax - Vm / n / Vmax) between the maximum speed (Vmax) and the minimum ejection speed (Vmin) according to the width of the tube, that is less than 5%. The device according to claim 11, further characterized in that the ratio between the passage section of a tube and the spray-free section of said tube, i.e. the area of the slit or the accumulated area of the orifices, is greater than 1. 13. The device according to any of the preceding claims, further characterized in that said tubes 1 have a rectangular section. 14. The device according to claim 13, further characterized in that the ratio of one side to an adjacent side of the rectangular section is between 0.1 and 10. 15. The device according to claim 13 or 14 , further characterized in that the thickness of the tubes is between 0.25 times and 10 times the diameter of the holes or the thickness of the slit, the ratio between the thickness of the tubes and the diameter of the orifices being, if the case, preferably, equal to 2/3. 16. The device according to any of the preceding claims, further characterized in that said sealing means 3 comprise a sealed compartment with a double pair of rollers, while allowing the passage of the band 2 and the creation of a loss of load that limits to a minimum value the spills of gutter 4 downwards. 17. The device according to claim 16, further characterized in that said sealing means 3 further comprise means for injecting a fluid between the rollers, whose pressure and / or temperature can be controlled. 18. The device according to any of the preceding claims, further characterized in that the upper tube 1 is equipped with a barrier 5, whose height is at least equal to the sum of the thickness of the water layer (H) in the gutter and the height of the water column (? H) corresponding to the loss of load between the maximum flow tubes. 19. A hardening process when performing the continuous annealing treatment of a flat product in the form of a metallurgical strip, preferably joining a steel strip, starting the device according to any of the preceding claims, to achieve a specific power of cooling comprised between 1000 kW / m2 and 10000 kW / m2 per side of the metallurgical product. 20. The procedure according to the claim 19, further characterized in that the temperature of the band at the inlet of the device is between 350 ° C and 750 ° C, while the temperature at the outlet is between 50 ° C and 450 ° C, preferably between 50 ° C and 100 ° C or between 350 ° C and 450 ° C