FR2460333A1 - METHOD AND APPARATUS FOR COOLING STEEL STRIPS DURING CONTINUOUS ANNEALING TREATMENT - Google Patents

Abstract

The present invention relates to a method and apparatus for cooling steel strips during a continuous annealing process. According to this process, a steel strip heated to a temperature above its crystallization temperature passes through a cooling unit 12 comprising a certain number of rollers 32 and 32a cooled by circulating water. This steel strip is caused to pass alternately around the upper surface and the lower surface of the cooling rollers 32 and 32a. By raising and lowering some rollers, the contact area, that is, the contact time between the web and the rollers is changed, which changes the cooling speed. When the cooling rollers are moved away from the web, the unit is filled with water to cool the web directly. Cold gas can be projected against the strip before it enters the cooling unit. When the strip is cooled indirectly, it passes around a series of rollers cooled by internal circulation of water, which allows the strip to be cooled without the formation of oxide films. The cooling mode can be chosen according to the nature of the steel strip. Applications to the steel industry and, in particular, to steel strip production facilities.

Description

The present invention relates to a method and an apparatus for

  to cool steel strips during an annealing treatment

run continuously.

  The methods of cooling the steel strips and plates to perform a continuous annealing treatment thereof can be divided into two categories, one being cooling by gas jets and the other by cooling. water. Each of these two cooling modes has

  its own advantages, which are accompanied by certain disadvantages.

  Specifically, in the gas jet process, a cooling gas is sprayed at high speed against a steel strip heated to a temperature corresponding approximately to its Al transition point to cool to about 400 ° C. , after what

  the strip is subjected to a maturation treatment for 3 to 5 minutes.

  The efficiency of a cooling by a gas is lower than that of a

  liquid cooling, because of its relative heat capacity

  weak. Although liquids have a greater thermal capacity

  that the gases, the film of vapor that forms on the surface of the band decreases their cooling efficiency. This is why the cooling equipment used in modern high-speed processing lines is bulky, expensive and expensive.

  lead to relatively high operating costs.

  However, the procedure of cooling by gas jets has the advantage that its cooling speed can be adjusted at will to be adapted to the treatment of mild steel strips and this

  type of thermal cycle is economical because, unlike cooling

  it is not necessary to previously cool the coolant to room temperature, then heat it to

maturation temperature.

  Among the water cooling processes, there is a distinction between

  spray cooling method in which a cooling agent

  liquid drainage is sprayed onto a uniformly heated steel strip and a process in which a heated steel strip is immersed in such

  coolant. However, to project the liquid, it is necessary to

  in addition to the fact that the configuration of the liquid

  projected cooling varies, which makes it impossible to obtain a cooling

  uniform drainage and, therefore, a homogeneous product.

  The thermal cycle of each of its cooling systems is fast. In particular, with regard to the immersion process, the cooling rate being extremely high, of the order of 1000 to 2000 ° C / second, which is to be compared with 10 to 30 ° C / second of the method of cooling by gas jets, it is easily understood that the immersion process is suitable for the manufacture of high tensile strength steel strips, which have a mixed structure of ferrite and martensite and which do not contain other special elements. Since, as explained above, this

  process ensures a high cooling rate, the equipment requires

  to this end is extremely compact. Moreover, since it is sufficient to pass the steel strip in the cooling water, the costs

  operating costs are significantly reduced.

  The setbacks of the benefits described above are, given that the

  cooling rate is too high even when the cooling water

  the boiling point, the inability to transfer to the ripening temperature during cooling and since the steel strip is subjected to cooling at about 1000 ° C or normal temperature, the obligation, to perform the ripening treatment that follows, to reheat the band that has been

  cooled to such a low temperature that it involves

  and additional equipment resulting in

poor thermal.

  Accordingly, it is an object of the present invention to provide a

  and improved apparatus for cooling a steel strip with a continuous translational movement which makes it easy to adjust

  the cooling rate of it.

  Another object of the invention is to provide a method and an apparatus for cooling at a given speed a steel strip which has been heated to its recrystallization temperature by avoiding the formation of oxide films.

  The invention also proposes to provide a cooling unit

  perfected in which it is possible to cool indirectly the

  tape by passing it around a series of rollers through the-

  which "a cooling water circulates, or by passing it

  right in a cooling water bath.

  According to one aspect of the present invention, a method for cooling a steel strip which has been heated to a temperature above its recrystallization temperature, during a continuous annealing treatment; characterized in that the steel strip is passed around a number of cooling rollers and in that

  varies the contact time of the belt with the cooling rollers.

  in order to vary also its cooling speed.

  The invention also relates to a method for cooling a steel strip which has been heated to its recrystallization temperature during a continuous annealing treatment, which process is characterized in that the steel strip is cooled. with a cooling gas and then with

some water.

  The invention also proposes a cooling apparatus for a

  continuous annealing treatment of a steel strip which has been heated

  above its recrystallization temperature, which is characterized by

  it includes a cooling unit with several wheels

  coolers around which the steel strip is driven and a mechanism for varying the contact area between the steel strip and the cooling rollers to vary the contact time between the strip and the rollers, an agent cooling

  circulating inside the cooling rollers.

  A gas cooling unit which projects the cooling gas against the steel strip can be provided in front of the water cooling unit. The steel strip alternately passes over the upper and lower surfaces of the cooling rolls. By moving the cooling rollers away from the belt and filling the cooling unit with water, the steel strip is subjected to

  direct cooling. A water tank is connected to the cooling unit.

  drainage through a siphon and the steel strip circulates in the cooling unit and in the water tank. In this way, if the water is removed from the cooling unit and the vessel and the upper rollers are lowered, the steel strip is forced to follow a corrugated path in contact with the upper surfaces and

  lower rollers. In this case, water is passed to the

  cooling rolls, thereby indirectly cooling the belt. It can thus be seen that, in this way, the cooling unit of the invention can be easily switched between the

  direct cooling and indirect cooling.

  Other features and advantages of the invention will emerge

  of the following description, given solely by way of example no

  limiting, with reference to the accompanying drawings, in which: - Figure I is a schematic representation of an embodiment of a continuous annealing apparatus according to the invention; FIG. 2 is a side view of the cooling rollers of the apparatus of FIG. 1; Figure 3 is a plan view of the cooling rollers of Figure 2; FIG. 4 is a schematic representation of another embodiment of a continuous annealing apparatus according to the invention; Figure 5 is an enlarged side view of the cooling unit of Figure 4 when it is used to cool the strip directly with water; - Figure 6 is a side view similar to Figure 5 but

  which shows how the strip is cooled by rollers passing through

  caused by a circulation of water; - Figure 7 is a schematic longitudinal sectional view of an example of a water cooling unit associated with a gas cooling unit; and - Figure 8 is a partial view showing a modification of

the apparatus of Figure 4.

  Referring to Figure 1, there is shown a continuous annealing apparatus according to the invention in which a steel strip (S), which unwinds from one of the reels 1a and 1b 'passes continuously in a chain of treatment, the debit strips being connected together by means of a shear 2 and a welding machine 2. Following the welding machine 2b are mounted successively, an alkali wash tank 3, a electrolytic rinsing tank 4 and a hot water spray tank 5, which together constitute a cleaning unit providing pre-treatment of the surface of the strip. After passing through this pretreatment unit, the strip is guided to an oven 10, passing through a dryer 6 and an accumulator

  7. The furnace 10 comprises a heating zone 8 in which

  A recrystallization annealing, a uniform heating zone 9 and a cooling unit 12 are carried out. The strip which has been recrystallized, annealed and cooled in this way is then reintroduced into the furnace 10.

  it passes into a reheating zone 13 to undergo a maturation process.

  The strip then passes into an outlet accumulator 16, in a water cooling unit 17, in a dryer 18 and in a refining mill 19. At its exit from the refiner mill 19, the strip (S) passes through a notch-making device 20 of variable width, a deburrer 21, a grease nipple 22, an exit shear 23 for winding up.

  finally around a receiver coil 24.

  The cooling unit used in the annealing apparatus is shown in FIGS. 2 and 3. As can be seen, the strip (S) passes

  around four rollers 32 and 32a. The two rollers 32 can be elevated

  With or without their support 31, they are lowered or lowered by means of a screw or a hydraulic cylinder 30. Water or another cooling medium circulates inside one or both rollers 32 and 32a. By varying the contact angle or the length of the portion of the web that is in contact with the upper surfaces of the rollers 32 and with the lower surfaces of the rollers 32a, the contact time can be varied without changing the band speed, thereby varying the cooling rate and thus the temperature of the cooled band, which allows the latter to be directed

  to the next step while keeping it at a set temperature.

  This cooling unit may be used at any point in the path of the strip, for example, at the cooling zone

  by gas 15 or the water cooling zone 17.

  The cooling unit shown in Figures 3 and 4 can also be used as a direct acting cooling unit, depending on the characteristics of the steel strip to be cooled. Groups

  steel which does not require maturation treatment can be cooled

  dies at speeds of 1000 to 2000C / sec, by removing oxide films as appropriate. In particular, it is possible to obtain in strips or plates of steel with a high tensile strength a mixed structure of ferrite and martensite with only a small proportion of special elements, thanks to such fast cooling due to direct contact. . Consequently, the cooling unit according to the invention can be constructed so that it can be used at the

  times for indirect cooling with rollers and cooling

dissenmt live with water.

  FIG. 4 shows a modified continuous annealing apparatus having a transformable cooling unit which is also shown on FIG.

  Figures 5 and 6. As can be seen, the band (S) passes between four wheels.

  32 and 32a, the upper rollers 32a being vertically movable

  securely by means of a screw mechanism or a hydraulic cylinder. More precise-

  When the rollers 32a are raised, as shown in FIG. 5, the web (S) passes directly around the guide rollers 12a and 12b without coming into contact with the cooling rollers 32 and 32a.

  while when the upper rollers 32a are lowered, as

  In FIG. 6, the strip (S) passes alternately around the rollers

32 and 32a.

  The contact air between the belt and the rollers can be adjusted according to the degree of lowering of the upper rollers 32a. Accordingly, as shown in Fig. 6, the cooling rate and the final temperature of the web can be adjusted at will by properly raising and lowering the upper rollers 32a. When the transformable cooling unit 12 shown in Fig. 4 is used to cool the strip directly with water as shown in Fig. 5, it is necessary to remove the oxide film which is deposited on the strip. during water cooling. For this purpose, means are provided between the cooling unit 12 and the reheating zone 13, as shown in FIG. 4, to remove this oxide film. Thus, the strip which has been cooled in the unit 12 passes into a pickling tank 25, into a hot water wash tank 26, into a neutralization tank 27 and into a dryer 28. Alternatively, as shown in FIG. 8, the means provided for removing the oxide film may be installed between the output accumulator 16 and the refiner mill 19, said means comprising a pickling tank 48, a hot water washing tank 49, a tank 50, a second hot water wash tank 51, a water wash tank 52 and a dryer 53. On the other hand, when the cooling unit is used to cool the strip indirectly, by means of the rollers, as shown in FIG. 6, oxide films are not formed, so that it is unnecessary to provide the

  pickling, the wash tank with hot water and the neutralization tank.

  In this case, only the second hot water wash tank 52 and the

dryer 53 are to be used.

  A unit for cooling by gas jets It may be added to the cooling unit 12 described above, as shown in FIG. 7. The gas jet cooling unit It comprises a fan 60, driven by a motor and a number of nozzles 41 which project the cooling gas sent by the fan against the band. The cooling gas circulates in the unit 11 and is cooled by water tubes 11a placed in front of the fan 60 until reaching a temperature of 50 to 150 ° C from a starting temperature of 150 to 2500C. As explained above, the operating mode of the cooling unit 12 can be transformed as that of the devices of FIGS. 5 and 6. To the right of a cooling chamber 35 is provided a circulation vessel 36 which receives cold water through a feed pipe 46, if necessary. From either side

  of the circulation tank 36 are provided valves 37 and 37a which allow

  attempt to adjust the level of water in the cooling chamber 35 and in the tank 36. An outlet passage 34 is formed on the left side of the cooling chamber 35 to guide the strip to the pickling tank 17 or directly to the heating zone 13. The water contained in the reservoir 36 is directed towards the nozzles 43 by a pipe 38 in which are interposed a filter 33 and a pump 42 which projects the water

  cooling against opposite sides of the band (S).

  A hot water tank 39 is installed under the tank 36 to receive the hot water leaving the left side of the valve 37 and the one that

  overflows above the valve 37a. Hot water-collected in the tank

  see 39 is evacuated by a pump 40. The reservoir 39 is connected to the tank

36 by a pipe 45 and a valve 47.

  The operation of the unit described above is as follows: the level of the water in the cooling chamber 35 is adjusted by means of the valve 37, while the strip is pre-cooled by the

  gas that is thrown against it by the nozzles 41 (which may be on the

  10 or more), before being cooled by the water of the chamber 35. The water in the chamber 35 also serves as isolation between the treatment steps. The hot water discharged by the pump 40 can be used in the hot water wash tank. When the valve 47 is open, the water contained in the cooling chamber 35, to the left of the valve 37, flows completely into the reservoir 39 and the strip is cooled, as shown in FIG. 6, by the rollers. 32 and 32a. Meanwhile, a stream of cooling water is sent into these rolls.

  So we can easily switch from the cooling mode

  Figure 5 to that of Figure 6 without draining all the water from the large tank 36, but simply the relatively small amount of water present to the left of the valve 37. Moreover, even after the water present to the left of the valve 37 has been evacuated, the cooling gases contained in the gas cooling unit II are prevented from escaping through the siphon 44 interposed between the chamber of

  cooling 35 and the reservoir 36.

  So we see that thanks to the represented embodiments

  in FIGS. 4 to 7, it is possible to cool the strip very thinly.

  with water at a low temperature close to room temperature, relatively slowly with rollers cooled by water circulation, avoiding the formation of oxide films on the web during cooling, which allows regularly perform a series of treatments, including maturation treatments, to produce different kinds of steel strips or

different applications.

  The indirect cooling process will now be described in more detail. The temperature of the steel strip entering the cooling unit 12 through the heating zone 8 for annealing and by the uniform heating zone 9 varies only slightly, generally between 500 and 800 ° C, according to its thickness and its composition and the band 1o is cooled by the rollers 32 and 32a. The water flowing through these

  cooling rollers can be at room temperature, varia-

  temperature of this water between 50C and 300C does not appreciably change the cooling effect. As a result, even when the web is cooled from LaC or heated from 60 to 700C, this does not affect the cooling action of the rollers. When the cooling rollers, in which circulates a stream of water, are brought into contact with the steel strip and a steel strip 0.6 mm thick, heated between 300 and 6000C, for example , comes into contact with these cooling rolls for about one second, the band is cooled by about 180 ° C, while when in contact for 2 seconds,

  it is cooled by about 2600C. When a steel strip having a thick

  1.2 mm is in contact for one second, it is cooled by about 90c, while it is cooled by about 1400C after

  contact of 2 seconds. When a 0.8 mm thick steel strip

  speed of 150m / min comes into contact with the circulating rollers.

  water 32 and 32a, at a contact angle of 0.81 after being raised to 5800C by the annealing treatment, it is cooled to about 505-515 ° C by the first roll 32 and about 465-480 'C by the second

  roll 32a. The band is cooled to 410 and 4200C by the third wheel.

  water 32 and finally at 3800C by the fourth roller 32a. The same result can be achieved by reducing the contact angle and decreasing the roll diameter. Even when the thickness of the tape or its speed of movement varies, a similar result can be obtained with a contact time of less than 2 seconds, by varying the others.

settings.

  The band thus cooled between 350 and 3800C is directed towards the

  next heat treatment area (13-15). As a result,

  The fuel consumption required to heat the belt can be reduced by 25 to 30% compared to the case in which the belt is cooled.

  around room temperature.

  The following example, which of course has no limiting character,

  will better understand the particularities of the invention.

EXAMPLE

  A strip of low carbon steel 0.8 mm thick and 1000 mm wide is passed through the heating zone 8 and the uniform heating zone 9 at a speed of 150 m. / min, in order to perform a recrystallization anneal for 1 minute, at a temperature of 7000C before directing it to the cooling unit 12, shown in FIG. 1. Each of the cooling rollers 32 and 32a at a diameter of 600 mm and is traversed by a cooling water stream at 150C having a flow rate of 250 1 / min, the strip (S) being cooled while coming into

  contact with these cooling rollers at an angle of 0 to 0.9t.

  The strip (S), whose inlet temperature in the cooling unit is approximately 600 ° C., is cooled to between 395 and 415 ° C., the variations in the temperature of the cooled strip being less than 200 ° C. The thus cooled strip then undergoes a maturation at a temperature of 400 to 3500C for 3 minutes, in the reheating zone 13 and in the zone of

  maturation 14, which makes it possible to obtain a strip of steel having

  uniform mechanical tees and which is suitable for use under

pull-ups and compressions.

Claims (16)

  Process for cooling a steel strip which has been heated to a temperature above its recrystallization temperature during a continuous annealing treatment, characterized in that it consists in passing the steel strip around a number of cooling rollers and varying the contact time of this steel strip with the cooling rollers to vary its cooling rate.   2. Method according to claim 1, characterized in that the steel strip is subjected to a maturation treatment after it has   cooled by the cooling rollers.   3. Method according to claim 1, characterized in that the angles of contact between the steel strip and the rollers are varied.   in order to change the contact time.   4. A process according to any of claims 1-3, characterized   controlled by passing a coolant inside the cooling rollers.   5. Process according to any one of claims 1 to 4, characterized   In that we pass the steel strip alternately around the   upper and lower surfaces of the cooling rollers.   6. Process according to claim 1, characterized in that   also says the steel strip with a cold gas before submitting   to the cooling action of the rollers.   7. Apparatus for carrying out the method according to claim 1, characterized in that it comprises cooling rollers around which the steel strip passes and a mechanism for varying the contact area between the strip of steel. steel and the rollers and so vary the   contact time between the steel strip and the cooling rollers.   8. Apparatus according to claim 7, characterized in that cooling water passes inside the rolls of cooling.   Apparatus according to claim 7, characterized in that the   means for varying the contact area between the steel strip and the rolls   The cooling fins are designed to pass the alternating   around the upper and lower surfaces of cooling rollers.   10. Apparatus according to claim 7 or 8, characterized in that   includes means for raising or lowering certain cooling rollers   compared to others so that, when the rollers have been raised, the band passes without coming into contact with them, whereas when the cooling rollers are lowered, the band passes alternately around the upper and lower surfaces of the rollers. cooling rollers.   He. Apparatus according to claim 10, characterized in that when the cooling rollers have been moved away from the steel strip, the cooling unit is filled with cooling water of steel band.   12. Apparatus according to claim 7 or 10, characterized in that it comprises means for cooling, the steel strip with a   cooling gas before it enters the cooling unit.   13. Apparatus according to claim 12, characterized in that the means for cooling the strip with a cooling gas comprising a number of nozzles which project a cold gas against the steel strip, a fan for circulating the gas in the nozzles and one   water cooler to cool the circulating gas.   14. Apparatus according to claim 7 or 10, characterized in that it also comprises a water tank, connected to the cooling unit.   by a siphon, a tank connected to the water tank and a circulating pump.   to introduce water into the water tank for the cooling unit. is lying.   15. Apparatus according to claim 14, characterized in that it also comprises nozzles arranged in the cooling unit, on either side of the steel strip, the nozzles being connected to the outlet of the circulation pump.   16. Apparatus according to claim 14, characterized in that it comprises a valve located at an intermediate point of the water tank and in that the reservoir is connected to the water tank at a point between   the valve and the siphon, by a valve.   17. Apparatus according to claim 16, characterized in that a water supply pipe is connected to the water tank, on the side of the   valve that is the opposite of the siphon.