KR900006694B1 - Method and apparatus for cooling steel strips - Google Patents

Abstract

Process includes forcefully moving a liquid coolant on the opposite face of a continuously moving strip in the form of a nonturbulance flow and in the counter direction to the advancement of the strip when it passes through the final cooling zone of a multiple-zone cooler. Cooling is speeded-up.

Description

Cooling method and cooling device of steel strip

1 is a graph showing the relationship between the temperature difference and the heat transfer coefficient between the steel strip and the cooling water in the conventional immersion cooling method

2 is a schematic diagram showing the principle of the cooling method according to the present invention.

3 is a graph showing the relationship between the relative velocity Vr of the coolant to the steel strip and the heat transfer coefficient α.

4 is a sectional view of a cooling apparatus according to the present invention.

5 is a perspective view of the cooling device of FIG.

6, 7 and 8 are schematic diagrams showing another embodiment of the present invention.

9 is a schematic diagram showing yet another embodiment of the present invention.

FIG. 10 is a sectional view of an essential part of the cooling device shown in FIG.

FIG. 11 is a graph showing the relationship between the cooling water temperature at the inlet of the water cooling jacket and the steel strip temperature at the outlet for a certain amount of steel strips. FIG.

12 is a graph showing the relationship between the amount of steel strip and the steel strip temperature at the outlet with the cooling water temperature at the inlet of the water cooling jacket being constant.

13 is a schematic view of a cooling apparatus with a control system for maintaining the temperature of a cooled steel strip in accordance with the present invention.

14 is a schematic diagram of the control system of FIG.

FIG. 15 is a graph showing the relationship between the amount of steel strip and the steel strip temperature at the water cooling jacket outlet when the temperature of the cooled steel strip is controlled in accordance with the present invention.

16 is a graph showing the relationship between the amount of steel strip and the steel strip temperature at the water cooling jacket outlet when the cooled steel strip is controlled in the actual operation according to the present invention.

* Explanation of symbols for main parts of the drawings

1a, 1b: Flow control plate 2a, 2b, 3a, 3b: Sealing roll

5a, 5b: cooling water inlet 6a, 6b: cooling water outlet

11: water cooling jacket 13: cooling water supply pipe

16: circulation pipe 17: thermometer

18: flow control valve 25: computing device

26,27: flow regulator 28: mixer

36: nozzle

The present invention relates to a method for cooling a steel strip in a continuous annealing line and an apparatus for carrying out the method, in particular, regardless of the change in the amount of steel strip and the temperature change of the supply coolant due to seasonal factors, A method and apparatus for effectively controlling cooling a steel strip to a target temperature.

Steel strips used for surface treatment or deep-drawing are generally cold-rolled, followed by continuous heat treatment such as heating, soaking and cooling, so-called "continuous, in order to impart the desired mechanical properties to the steel strip. Annealing "process.

In recent years, as the demand for very thin steel strips of 0.3 mm or less increases, a method of passing the steel strips more rapidly in the continuous annealing line has been widely used in order to greatly increase the heat treatment capacity. Therefore, the furnace equipment for continuous annealing is inevitably extended and expanded, resulting in equipment cost, equipment space, and operational problems described below.

That is, as the furnace lengthens and expands, the so-called "thermal inertia" of the furnace increases, which takes a lot of time to change the heat treatment conditions, such as meandering and heat-buckle of steel strips. Various problems arise.

The present invention is to efficiently solve the problems of the high speed operation of the furnace, especially in the final cooling stage.

As a cooling method used for continuous annealing treatment, a gas injection cooling method for injecting a cooling gas into a steel strip, a roll cooling method for winding a steel strip around a roll through which a cooling medium passes, and a immersing method for immersing the steel strip in a cooling tank cooling). The cooling rate is increased in the order of gas injection cooling, roll cooling, and immersion cooling.

In the case of cooling the steel strip from the recrystallization temperature in the cooling zone of the continuous annealing line to a temperature that does not oxidize in the air, it is generally considered that the most efficient cooling method is gas injection cooling and / or roll cooling on the high temperature side and immersion cooling method on the low temperature side. do.

Various dip cooling methods have been proposed so far. For example, the cooling methods disclosed in Japanese Patent Laid-Open Nos. 11,931 / 82 and 11,933 / 82 use a plurality of cooling water tanks to control the supply of cooling water, spray cooling, or mist. By combining cooling with immersion cooling method, the steel strip is effectively cooled and the cooling water temperature after cooling is made as high as possible to use it as hot water.

The immersion cooling method is usually used for steel strip cooling from the temperature of about 250-300 degreeC in which saturated solid-solution carbon hardly changes, to the temperature where temper color does not appear in air. If the cooling rate is too fast, as in the immersion cooling method, there is a problem of ageing by solid carbon. In recent years, a material in which solid solution carbon such as ultra-low carbon steel to which Nb is added is fixed in advance by a third element has been used as an unaging material. Therefore, it is desired to improve the cooling rate in the final cooling to achieve high speed operation, high productivity and high efficiency.

However, the dip cooling method disclosed in Japanese Patent Application Publication Nos. 11,931 / 82 and 11,933 / 82 has the following problems.

(1) Cooling water must be supplied to the cooling water tank to prevent the temperature rise of the cooling water. However, in this case, the cooling water flows only in the upper layer of the cooling water tank and hardly flows in the lower layer. As a result, when the hot steel strip is immersed in the cooling water, a steam film is generated on the surface of the strip by the relatively low temperature cooling water in the upper layer. Since it is difficult to remove or destroy the intermediate film, the cooling efficiency is inevitably limited. Therefore, a large immersion cooling apparatus is required to increase the cooling rate and its efficiency, which inevitably increases the cost of equipment and the space of the equipment. However, retrofitting existing devices to speed up cooling is nearly impossible.

(2) As described above, since the cooling water does not flow uniformly over all parts of the cooling water tank, the temperature distribution is not uniform and adversely affects the cooled steel strip.

(3) In the case of using the cooling water discharged from the immersion cooling bath as hot water, at least two immersion baths are required, which not only increases the size of the apparatus but also requires complicated control such as global control.

1 shows the change of the heat transfer coefficient when cooling the steel strip by a known method. The vertical axis represents the heat transfer coefficient, and the horizontal axis represents the temperature difference between the steel strip and the cooling water. The black origin and the black triangle point represent data when the steel strip is supplied in the horizontal and vertical directions with respect to the surface of the coolant, respectively.

In the range where the temperature difference between the strip and the coolant is small, the heat transfer coefficient increases as the temperature difference increases. This is because almost no vapor film is generated on the surface of the strip. If the temperature difference further rises, the steam film is violently generated, and the cooling water and the steel strip are separated by the thick steam film, thereby adversely affecting the cooling effect. This tendency is severe in areas with large temperature differences. As a result, the heat transfer coefficient drops as shown in the right side of FIG.

As can be seen in FIG. 1, the heat transfer coefficient in the case of vertical feeding of the strip is somewhat higher than in the case of horizontal feeding. This is considered to be because steam escapes upward more easily in the former case than in the latter case.

In the above prior art cooling method, the upper limit of the heat transfer coefficient is 5000 KcaI / mz.h.t at most.

It is an object of the present invention to provide a steel strip cooling method and apparatus capable of eliminating all the disadvantages of the prior art and cooling the steel strip with high efficiency.

In order to achieve the above object, in the present invention, the coolant is forcedly flowed in a rectified state along both surfaces of the steel strip in a direction opposite to the moving direction of the steel strip.

According to the present invention, a steel strip cooling apparatus using a coolant is disposed opposite to both surfaces of the steel strip to form a flow path between the steel strips and to move between flow adjustments for forcibly flowing the coolant through the flow path. At least one coolant supply port downstream of the steel strip, and at least one coolant outlet upstream of the moving steel strip.

Another object of the invention is to control the temperature of the cooling steel strip in the mouthpiece to cool the steel strip to a certain range at all times stably without being affected by the usual expected factors such as the amount of steel strip to be cooled, the coolant temperature, etc. To provide a steel strip cooling method and apparatus.

In order to achieve the above object, in the present invention, the temperature of the cooled steel strip is controlled by mixing a portion of the cooling liquid used for cooling the steel strip with a new cooling liquid and controlling the mixing ratio of the cooling liquid to control the temperature of the cooling liquid used for cooling. It is kept within a predetermined target temperature range.

The apparatus of the present invention for carrying out the method includes a thermometer for the coolant discharged from the flow path, a flow regulating valve and a flow meter: a thermometer for the newly supplied coolant in the flow path, a flow regulating valve met flow meter: a steel strip leaving the flow path. Thermometer for detecting the temperature of the valve: Computation unit for calculating the amount of coolant to be mixed with the discharged coolant: Flow regulator for controlling the flow control valve in accordance with the signal of the operation unit: and discharged coolant through the flow control valve And a mixer for mixing the new coolant.

Referring to the accompanying drawings, the purpose, configuration and effects of the present invention in more detail.

2 schematically illustrates the cooling method of the present invention in which the steel strip S is rising. Cooling water W adjusted in the rectified state is supplied to one side of the steel strip S. Cooling water W pivots downward from the position in contact with the steel strip and flows in a direction opposite to the direction of movement of the strip.

As described above, the cooling water as a whole is forcedly flowed in the opposite direction to the movement of the steel strip S, so that any stagnation of the cooling water caused in the prior art can be eliminated and the vapor film generated on the surface of the steel strip can be quickly removed or crushed. Thus, high efficiency cooling of the steel strip can be achieved.

Assuming that the moving speed of the steel strip S is V _s (m / s), the flow rate of the cooling water is V _m (m / s), and the downward is a positive value, the relative speed Vr of the cooling water W to the steel strip S is m / S) is represented by the following equation.

V _r = V _w -V _s ... … … … … … … … … … … … … … … … … … … … … … … … … … (One)

In the present invention, the relationship between the relative velocity Vr and the heat transfer coefficient α (KCa1 / m ² .h. ° C.) was investigated. The results of the investigation are shown in FIG. As can be seen in FIG. 3, the heat transfer coefficient α rises in proportion to the relative speed Vr. The relationship can be approximated by the following equation (2).

α = 5600 V _r ^0.68 . … … … … … … … … … … … … … … … … … … … … … … … … … (2)

In this case, the relative velocity Vr is, for example, 10 m / s each, and the heat transfer coefficient α is 26,800 Kcal / m 2 h.

In FIG. 3, the heat transfer coefficient in the conventional cooling method is at most 5000 Kca1 / mz.h.. C, which is only one minute or less of water compared to the cooling method of the present invention.

4 is a cross-sectional view of a preferred embodiment of the present invention, the main part of which is schematically shown in FIG. The flow regulating rods 1a and 1b are arranged parallel to both ends of the steel strip S to form a water cooling jacket for supplying cooling water rectified along both surfaces of the steel strip. The sealing rolls 2a, 2b, 3a, and 3b serve to prevent the cooling water from leaking only at the lines and ends of the flow regulators. Sealing plates 4b are provided on both sides of the flow adjusting plate. These sealing plates are joined to and sealed on the side of the adjusting plate, thereby preventing cooling water from leaking from the side of the adjusting plate, and at the same time serving to position the adjusting plate to the assembled counterpart.

The coolant introduced into the jacket formed by the adjusting plate through the cooling water inlets 5a and 5b flows in the gap or flow path formed by the steel strip S and the flow adjusting plate 1a and 1b in a direction opposite to the moving direction of the steel strip S. Exit through the coolant outlets 6a, 6b provided at the other end of the jacket. Effective cooling is achieved by forcing the cooling water through a relatively narrow gap between the steel strip S and the flow control plates 1a, 1b.

6 to 8 show a modification of the cooling device according to the invention. The apparatus of FIG. 6 consists of three sets of flow control plates along the moving line of the steel strip S. FIG. The apparatus of FIG. 7 and FIG. 8 consists of a flow control plate arrange | positioned in U shape and W shape, respectively. The steel strip was finally cooled using the cooling apparatus of FIG. 4 under the following conditions.

Size of steel strip: thickness: 0.3mm, width: 900mm

Steel Strip Throughput: 60T / h

Steel strip temperature at the start of cooling: 150 ℃

Supply Coolant Temperature: 30 ℃

Steel strip temperature after cooling: 60 ℃

Thickness of coolant flow path: about 6mm

The cooling water used for the treatment was about 40 T / h. The temperature of the cooling water at the outlet was about 70 ° C. The steel strip was uniformly cooled in the width direction by the cooling treatment. The non-uniform cooling and problems caused by the vapor film were completely excluded.

Cooling of about 60 T / h was required to process such amounts of steel strips according to prior art methods. Thus, according to the invention a considerable amount of cooling water is saved.

As can be seen from the foregoing, according to the present invention, the steel strip is cooled at a significantly higher efficiency than the prior art. Therefore, the processing performance in the continuous annealing treatment considerably increases, and further, the present invention can provide a compact cooling apparatus.

9 illustrates another embodiment of the present invention. When the water cooling jacket is formed in a U-shape, the leg portion 31c of the U-shaped jacket into which the steel strip is introduced tends to obstruct the flow of cooling water. Therefore, the swamp needs to consider the resistance of the leg 31c to the coolant flow in order to obtain cooling performance. This resistance Δh can be calculated by the following equation according to the theory of weir.

Where h is the difference in coolant number (mm), Q is the flow rate (m3 / s), C is the resistance factor (= 0.6), B is the width of the jacket (warer width), and g is the acceleration of gravity.

Therefore, if the height difference h between the legs 31a and the surface of the coolant at the legs 31c is determined to satisfy the following equation, the coolant flows smoothly and does not interfere with the cooling effect.

In the present invention, the gap between the steel strip and the flow control plate is very small, so the pressure loss tends to be large. Therefore, an increase in the capacity of the cooling water supply pump and the power required to drive the pump is required. In order to solve this problem, it is preferable to have a nozzle 36 to promote the flow of the cooling water as shown in FIG. 9 and FIG. As shown in FIG. 10, the slit nozzle 36 is disposed in the water cooling jacket in the direction of movement of the steel strum and slightly inclined toward the steel strip for cooling water spray. This will compensate for the pressure loss caused by the water cooling jacket. The nozzle 36 is supplied with cooling water through the pipe 37 by the feed pump 38. One nozzle 36 is disposed on each side of the steel strip, but may include a plurality of nozzles on each side of the strip.

In addition, the steel strip between the deflection rolls 32 in the horizontal portion 31b of the water cooling jacket tends to cause a difference in the cooling effect between the upper and lower portions of the steel strip, resulting in catenary deformation. In this case, it is thought that the tensile force acting on the steel strip is increased to reduce the catenary deformation of the steel strip.

However, there is a limit because the tensile force is too high to break the steel strip. To prevent this, it is effective to incline the horizontal portion of the water cooling jacket as shown in FIG. In consideration of the flow direction of the cooling water, it is preferable to position the end portion of the horizontal portion 31b toward the leg portion 31a higher than the end portion of the leg portion 31c side.

According to the above embodiment, since the steel strip can be cooled at a higher efficiency than the conventional technology, the processing performance of continuous annealing can be remarkably improved. However, in the above embodiment, the temperature of the steel strip to be cooled is sometimes changed from a predetermined temperature by a change in the amount or weight of the steel strip to be treated and, for example, a temperature change of the supply cooling water according to the season.

FIG. 11 shows the relationship between the supply coolant temperature and the temperature of the cooled steel strip when the amount or weight of the steel strip passing through the cooling device is 40T / h. As can be seen from FIG. 11, when the temperature of the supply cooling water is too high or too low, the temperature of the cooled steel strip is out of the desired temperature range.

12 is a result of examining the relationship between the amount or weight of the steel strip and the temperature of the cooled steel strip in two cases where the temperature of the cooling water is 40 ° C and 30 ° C. As is evident from this, as the amount of steel strip increases, the temperature of the cooled steel strip increases and often goes out of the desired temperature range.

The following example controls the temperature of the cooled steel strip at the outlet of the cooling device to ensure that the steel strip is always at a desired range without being influenced by typically expected factors such as the amount of steel strip to be cooled, the coolant temperature, and the like. It is to provide a method for cooling stably.

13 shows a cooling device having a water cooling jacket and a control system thereof. The flow regulating plates 11a and 11b form a water cooling jacket 11 for forcibly supplying the cooling water in a rectified state along both surfaces of the steel strip. The deflection roll 12 switches the moving direction of the steel strip in the water cooling jacket 11 that receives the cooling water through the cooling water supply pipe 13. The reservoir 14 for storing the used cooling water is provided with a pump 15.

In the cooling device configured as described above, the coolant is introduced into the water cooling jacket at the feeder supply port downstream of the moving path of the steel strip S and forcedly flows through the water cooling jacket 11 in a direction opposite to the moving direction of the steel strip. During this time, the coolant cools the steel strip with high efficiency until it leaves the outlet. According to this embodiment, a part of the cooling water discharged from the cooling is circulated and reused for temperature control of the cooling water.

The cooling system includes a circulation pipe 16 for circulating the discharged cooling water, a thermometer 17 for detecting the temperature of the discharged cooling water, a flow control valve 18 for the circulation cooling water, and a flow meter for circulating cooling water 19. And a new coolant supply pipe 20 including a thermometer 21, a new coolant flow control valve 22 and a flowmeter 23, and a thermometer 24 for detecting the temperature of the steel strip leaving the cooler. It consists of.

The computing device 25 obtains the cooling water of the predetermined temperature by calculating the amounts of the circulating cooling water and the new cooling water to be mixed with each other based on the detectors detected by each detector. The calculated result is input to the flow regulators 26 and 27 and the output thereof controls the opening of the flow regulating valves 18 and 22, respectively. The circulating coolant adjusted to the required amount and the new coolant are mixed in the mixer 28 to obtain a predetermined temperature of cooling water, which is supplied to the water cooling jacket 11 through the supply pipe 13. The dewatering roll 29 functions to remove the cooling water from the steel strip leaving the water cooling jacket.

The arithmetic operation in the arithmetic unit 25 is as follows. It is assumed that the allowable range T _so of the target cooling temperature is represented by the equation (5).

T_SO

T_SO2……………………………………………………………………(5)T _SO1

T _SO

T _SO2 ... … … … … … … … … … … … … … … … … … … … … … … … … … (5)

The quantity Q of the supply cooling water is expressed as the sum of the quantity Qc of the circulating cooling water and the quantity QN of the fresh cooling water.

Q = Q _C + Q _N ... … … … … … … … … … … … … … … … … … … … … … … … … … (6)

The relation (7) between the set cooling water temperature T _WC and the new cooling temperature T _WN is as mind.

(T _W -T _WN ) Q _N = (T _WC -T _W ) Q _C ... (7)

From equations (6) and (7), the values of Q _C and Q _N are calculated.

First, it is determined whether the steel strip temperature T _SO , detected by the thermometer 24 is within the range of the formula (5). When this value is satisfied (5), the values of Qc and QN are calculated by equations (8) and (9) based on the detected temperatures T _wN and T _wc .

On the other hand, if the steel strip temperature T _SO, is higher than the upper limit, the set cooling water temperature T _w is lowered by a fixed value.

T _W = T _W -a... … … … … … … … … … … … … … … … … … … … … … … … … … 10

On the contrary, when the steel strip temperature T _SO, is lower than or equal to the lower limit, the set cooling water temperature T _w rises by a fixed value.

T _W = T _W + a... … … … … … … … … … … … … … … … … … … … … … … … … … (11)

Next, the amounts of the circulating cooling water Qc and the new cooling water Q _N are calculated based on the cooling water temperature T _w newly set by the equations (8) and (9). In this case, the coolant temperatures Twc and T _WN detected by the thermometers 17 and 21 are used.

The manipulation is summarized in the flow chart of FIG. FIG. 15 shows the relationship between the steel strip temperature at the outlet and the amount or weight of the steel strip when the cooling water temperature is controlled as described above. The steel strip was cooled under the following conditions using the chiller and control system shown in FIG.

Dimension of steel strip: thickness: 1mm, width: 2000mm

Amount of coolant: 13.2T / h (new coolant: 3.3T / h, circulating coolant: 9.9T / h)

Coolant temperature: (new coolant: 10 ℃, circulating coolant: 50 ℃)

Target temperature after cooling: 50 to 60 ° C Steel strip Initial amount: 20T / h

The amount or weight of steel strip to pass under these conditions was gradually increased. When the amount of steel strip was 40T / h, the steel strip temperature at the outlet tried to exceed the upper limit of 60 ° C of the target temperature. The ratio of fresh coolant to circulating coolant was changed from 7.1T / h to 6.1T / h to lower the coolant temperature to 30 ° C. As a result. The temperature of the cooled steel strip was lowered to 50 ° C. However, whenever the amount of steel strip was 65 T / h, the cooled strip temperature tended to exceed 60 ° C. The ratio of fresh coolant to circulating coolant was changed from 11T / h to 2.2T / h to lower the coolant temperature to 20 ° C.

As a result, the cooled steel strip temperature dropped to 50 ° C. After that. Even when the amount of Kang Strip was increased to 80 T / h, the cooled steel strip temperature did not exceed 60 ° C. Even when the amount of steel strip was changed from 20T / h to 80T / h, the cooled steel strip temperature at the outlet was maintained in the target temperature range of 50 ° C to 60 ° C.

According to the present invention, the temperature of the cooled steel strip during the cooling treatment of the continuous annealing line can be maintained in the desired target temperature range without being affected by variables such as the change in the amount of the steel strip.

What has been described so far is to describe some of the most preferred embodiments of the present invention in order to facilitate understanding of the present invention. Therefore, the present invention is not limited to the above embodiments, and various modifications and alterations can be made without departing from the spirit and scope of the present invention.

Claims (12)

A method for cooling a steel strip using a coolant in a continuous annealing line, the method comprising forcibly flowing the coolant in a counter flow rectified along both surfaces of the steel strip in a direction opposite to the direction of movement of the steel strip. Steel strip cooling method. The method of claim 1, wherein the steel strip is cooled by the coolant while being guided through the U-shaped passageway. The method of claim 1, wherein the steel strip is cooled by the coolant while being guided through the W-shaped passageway. The method of claim 1, wherein a part of the cooling liquid used for cooling the steel strip is mixed with the new cooling liquid and the mixing ratio is controlled to control the temperature of the cooling liquid to maintain the temperature of the cooled steel strip within the desired target temperature range. Steel strip cooling method characterized in that. The method of claim 1, wherein the steel strip is cooled in the final cooling step of the continuous annealing line. A chiller for cooling a steel strip using coolant, comprising: a flow control plate disposed opposite to both surfaces of the steel strip to form a flow path between the steel strip and forcing the coolant to flow through the flow strip; A steel strip chiller comprising at least one coolant supply port downstream of the steel strip and at least one coolant discharge port upstream of the migrating steel strip. 7. The steel strip cooling apparatus according to claim 6, further comprising sealing plates connected to both sides of the flow adjusting plate to prevent leakage of coolant from the side of the flow adjusting plate. 7. The steel strip cooling apparatus according to claim 6, wherein the flow path is formed of a U-shaped passage. 7. The steel strip cooling apparatus according to claim 6, wherein the flow path is formed of a W-shaped passage. 9. The steel strip chiller according to claim 8, wherein the coolant surface height at the coolant supply side is higher than the surface height at the coolant outlet side by a distance h calculated in the following equation.

Where Q is the flow rate of the coolant in the flow path (m3 / s), and B is the width of the flow path. 9. The connection portion according to claim 8, wherein the connecting portion connecting the two vertical portions of the U-shaped flow path is inclined so that the end portion of the connecting portion is more vertical than the end portion of the connecting portion on the vertical portion with the coolant discharge port. High steel strip chiller. In a chiller for cooling a casing strip using a coolant. A flow control plate disposed opposite both surfaces of the steel strip to form a flow path between the steel strips and forcing the coolant to flow through the flow strip: at least one coolant supply downstream of the moving steel strip; At least one coolant outlet in the upstream layer of the steel strip; a coolant thermometer discharged from the flow path, a flow control valve and a flow meter; a coolant thermometer newly supplied to the flow path, a flow control valve and a flow meter: a steel strip leaving the flow path. Thermometer for detecting the temperature of the controller: Computation unit for calculating the amount of the discharged coolant and the new coolant to be mixed: Flow regulator by controlling the flow control valve in accordance with the signal of the operation unit: and discharge coolant through the flow control valve and Steel strip chiller consisting of a mixer for mixing new coolant .

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