COOLING APPARATUS FOR THICK STEEL PLATE

Abstract: A cooling apparatus for uniformly cooling a thick steel plate in order to obtain a very flat thick steel plate wherein a cooling water supply means provided with more than two systems of the header (3,4) to which a desired water volume crown is given in the width direction of the thick steel plate is provided.

Description: The present invention relates to a cooling apparatus for thick elongated steel plates according to the opening portion of claim 1. Such an apparatus is known from GB-A-1 568 483 disclosing an apparatus of cooling a strip by sprayers mounted between carrying rolls while the strip is being carried. The sprayers are supplied with air by way of first ducts and with water by way of second ducts, and the mixture of air and water is sprayed over the strip. It is also understood that the spray fully covers the width of the strip. Furthermore, a roll is cooled by the mixed air-water. Considerable research has recently been devoted to the process for production of thick steel plate with the aim of developing a process which, through the combination of low temperature rolling and forced cooling, will make it possible to reduce the amount of alloying elements required in thick steel steet production, to carry out various processes with smaller energy consumption and to develop new types of steel products. This research has already produced substantial results. The cooling apparatuses developed for the process encompassing such a combination of steps have been aimed at making it possible to carry out the cooling operation in a simple manner, thereby overcoming the problem of insufficient cooling capacity posed by the conventional cooling bed, and at realizing an improvement in steel quality, more particularly, in realizing a remarkable enhancement of steel strength and toughness. It is a requirement of the cooling operation that the cooling be uniform in order to respond to the demand for higher steel quality. Also, it is a requirement of the steel plate that, following the completion of the cooling step, it should have a degree of flatness sufficient for it to be used immediately as a commercial product. This requirement derives from the need to save energy and reduce the number of production processes. The inventors of the present invention have devoted themselves to the development of a cooling apparatus capable of providing the uniform cooling required to satisfy the aforesaid conditions. Through their study they found that no matter how perfectly a cooling apparatus is able to carry out uniform distribution of water, when a steel plate of large area is subjected to transient type cooling, a number of factors, including some intrinsic to the steel plate itself, act to obstruct uniform cooling. The main ones of these factors are listed below: (1) Plate crown It is the nature of a rolling mill to produce a steel plate which is thicker at the center (crown) than at the edges. The tendency for this to occur increases with decreasing plate thickness and increasing plate width. For instance, in a steel plate that is 12 mm thick and 4000 mm wide, the height of the crown is about 0.3 mm while, in contrast, in a steel plate that is 40 mm thick and 2000 mm wide, the height of the crown is nearly 0 mm. Therefore, even if perfectly uniform cooling could be carried out on both (top and bottom) sides of the plate, the cooling rate at the center portion would be slower proportionally to the difference in plate thickness (which in turn depends on the size of the plate) so that the plate will suffer from fluctuation in steel quality and defects in shape. (2) Temperature of the plate immediately before cooling In an industrial process for heating and rolling a steel plate, the plate will inevitably undergo a drop in temperature due to the cooling effect of the air that constitutes the environment for the operation. This effect tends to be stronger at thinner portions of the plate so that it sometimes happens that the temperature at the edge portions of the plate immediately before cooling is as much as 20°C lower than the temperature at the center. When a steel plate having such temperature differences is then subjected to cooling, these differences are greatly magnified, to such an extent that the shape of the plate is impaired, to say nothing of the fluctuation in steel quality caused by the temperature differences at the start of cooling. (3) Influence of running water on the plate When a steel plate is placed horizontally and subjected to cooling by transient running water, the cooling water applied from the top side of the plate flows outward and then downward over the edge portions. Therefore, the largest amount of water flows over the edge portions on the top surface of the steel plate so that the cooling rate is rapid in this region. Regardless of plate thickness, this action of the running water occurs only on the top surface of the plate. Moreover, the effect of this action increases with increasing plate width. In order to deal with the above-mentioned factors which tend to cause variation in the cooling rate in the width direction of the steel plate, it has generally been the practice to apply to only the top surface of the plate a fixed-type "crown of water" (wherein the amount of water applied is greater at the center portion of the plate than at the edges). The term "water volume crown" as used in this specification refers to the distribution profile of running cooling water applied to the surface of a steel plate. The water volume crown applied should desirably differ from plate to plate depending on the differences in plate thickness and width referred to in items (1)-(3) above. Moreover, these same factors will determine whether a water volume crown should be applied from only one side or from both sides. Table 1 shows the water volume crown required on both sides of a steel plate in quantitative terms. In the case of a slit nozzle, it is technically feasible to remotely control the header mechanism so as to control the water volume crown applied to each plate in accordance with its size. However, the mechanism required for this would be so complicated as to raise many problems from the viewpoint of equipment cost and maintenance. Accordingly, as a practical matter, it has been possible to supply only specific types of water volume crown to the top surface of a plate by selection of specific slit widths or types of nozzles. Summary of the invention It is the principal object of the present invention to provide a cooling apparatus for uniformly cooling a thick steel plate of various specifications, kinds and properties. It is another object of the invention to provide a cooling apparatus for cooling a thick steel plate in compliance with the specifications, kind and properties of the plate in order to obtain uniform steel properties. This object is solved according to the invention by a cooling apparatus for thick elongated steel plate being transported in its lengthwise direction, with a base provided to extend in the direction of passage of the plate, a plurality of rod-headers provided on the base and extending in the width direction of the plate and having nozzles connected to water supply means, and which is characterized in that the headers are divided into at least two systems, each system is connected to a water supply means via a flow valve, and the headers of the systems are arranged alternately side by side, the nozzles are so constructed as to produce a different water volume crown (transverse profile and rate of supply of water) for each system, in that headers conferred with a large water volume crown and headers conferred with a small water volume crown may be arranged alternately side by side, and in that each nozzle consists of an inner tube and an outer tube, a flow rectification tube may be provided at an opening of said inner tube, said opening communicates via the outer tube with a slit nozzle formed by a nozzle plate provided with a bolt which controls the slit width. The cooling apparatus according to the invention may be successively provided with a plurality of zones in the longitudinal direction of said thick steel plate, wherein a first zone consists of two systems of first headers. a second zone consists of two systems of second headers. a third zone consists of two systems of third headers, and in each zone said headers of one system produce a first water crown, and headers of the other produce a different water crown, and said headers of the one system are alternately arranged with said headers of the other system. Brief description of the drawings Other and further objects of the present invention will be better understood from the following detailed description with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a cooling apparatus in accordance with an embodiment of the invention; Fig. 2 is a diagrammatic view showing models of the water distribution in the width direction provided by system A and system B headers respectively: Fig. 3 is a diagrammatic view showing models of water distribution obtained by controlling the amount of flow through the systems A and B; Figs. 4-6 are diagrammatic views showing models of water distribution for corrective purposes; Fig. 7 is an explanatory view of a slit lamina type nozzle; Fig. 8 is an explanatory view of the end face of a nozzle; Fig. 9 is a graph showing the relationship between slit width and the amount of water; Fig. 10 is an explanatory view showing a nozzle in accordance with another embodiment; Fig. 11 is a diagram showing the relationship between the nozzle pitch and the water volume; Fig. 12 is a diagram showing the relationship between the nozzle bore and the water volume; Fig. 13 is an explanatory view of a cooling apparatus in accordance with an embodiment of the invention; and Fig. 14 is a diagrammatic view showing the results obtained by the application of the apparatus according to this invention. Detailed description of the invention The present invention relates to a cooling apparatus for cooling a thick steel plate in which the water volume crown can be varied from plate to plate by a simple mechanism in a manner which assures the optimum crown for each plate. Fig. 1 shows a group of constituent elements provided at the cooling zone over the top surface of a steel plate in accordance with an embodiment of the present invention. The nozzle group for the top surface of the plate consists of two systems, A and B, which are provided with water control valves 1 and 2, respectively, and with numerous headers 3a₁-3a n and 4b₁-4b n . These systems use headers which supply the water volume crowns shown in Fig. 2. More specifically, in the present invention a base (not shown) is arranged in the passage for an elongated thick steel plate and a plurality of rod-like headers 3, 4 are provided on the base in the width direction of the plate so as to adequately cover the entire plate width and to extend crosswise to the running direction of the plate. The headers are provided with a plurality of flow control valves 1-a, 2-b,... capable of finely controlling the amount of water supplied to each of the headers. The headers are divided into at least two systems (system A and system B in the drawing) and each system is connected to a water supply means (not shown) via a flow valve (1 or 2). There is thus constituted a first nozzle group a₁-a n and a second nozzle group b₁-b n , each of which is capable of supplying a different water volume crown. As shown in the drawing, the headers a₁, a₂,... of system A and the headers b₁, b₂,... of the system B are arranged alternately side by side in series. Therefore, the crown conferred on the headers of system A differs from that conferred on the headers of system B. In the above explanation, the cooling water supply means is provided with two systems, A and B, but it is understood that the system will not be limited thereby. In addition, with reference to the arrangement system of headers, it is seen that the following arrangement may be adopted: a combined group of header a₁ and header a₂ is newly made, and another combined group of header b₁ and header b₂ is also made, and the combined group of headers a₁ and a₂ is alternately arranged side by side with the combined group of headers b₁ and b₂ (e.g., a₁, a₂, b₁, b₂,...). The water volume crown shown in Fig. 2(a), which is supplied by the headers of system A, is the crown suitable for treating those plates among all plates to be treated which require the smallest water crown (namely, plates corresponding to the thick and narrow plate mentioned in Table 1). The water volume crown shown in Fig. 2(b), which is supplied by the headers of system B, is the crown suitable for treating those plates among all plates to be treated which require the largest water volume crown (namely, plates corresponding to the thin and wide plate mentioned in Table 1). In the cooling apparatus of the invention, a plate of such size that it requires the largest water volume crown is cooled by water distributed by system B alone and a plate of such size that it requires the smallest water volume crown is cooled by system A alone. When a plate of an intermediate size is treated, a crown of intermediate size will do. Therefore, in such case, the amount of water supplied through the systems A and B is controlled to obtain the water volume crown appropriate for the plate concerned. Fig. 3 shows how the water volume crown changes as the proportion of the water supplied through each of systems A and B is changed. Fig. 3(a) shows the crown obtained when the water supply is 1/2A+1/2B, Fig. 3(b) shows that for 1/3A+2/3B, and Fig. 3(c) that for 2/3A+1/3B. In accordance with the present invention, it is possible to change not only the side of the water volume crown but also the pattern (profile) thereof. In general, the required pattern of the water volume crown varies somewhat from case to case. Fig. 4 shows a crown pattern used for correcting variation in cooling rate caused by the plate crown. Fig. 5 shows a crown pattern for dealing with fluctuations in temperature in the width direction of the plate. Fig. 6 shows a crown pattern used for preventing overcooling of the edge portions of a plate caused by water flowing over the plate in the width direction. As is clearly shown in Figs. 4-6, crown patterns differ according to the purpose they are intended to attain. Therefore, accurate cooling can be carried out by combining headers capable of supplying water crowns of the patterns shown in Figs. 4-6 with each other or wihh headers for supplying a flat water distribution, so as to obtain an optimum water crown for the steel plate to be cooled. Moreover, the supply of the cooling water can also be carried out as required using three or more header systems each capable of providing a different crown pattern. Next the method for causing the respective header systems to supply the desired water crowns will be explained. Fig. 7 is a detailed view of a slit lamina type nozzle comprising a nozzle plate 5, a bolt 6 for adjusting the slit width, a reinforcing flange 7, an inner tube 8, an outer tube 9, and a short tube 10 for rectifying the water flow. As shown in Fig. 8, the slit width d of the nozzle varies in the longitudinal direction of the slit (i.e. in the width direction of the steel plate) between the center and the opposite ends. This variation can be either continuous or stepwise. The view of the slit shown in Fig. 8 is somewhat exaggerated; in actual fact the difference between d₁ and d₂ in a slit 4 m long is as small as 2 mm. Fig. 9 shows the relationship between the amount of water supplied per unit width in the longitudinal direction (plate width direction) of the nozzle and the slit width in the above-mentioned header of the slit lamina type. The relation is almost linear. Thus it will be understood that by varying the slit width, it is possible to obtain a corresponding change in the amount of water supplied, by which means the required water crown can be attained. Figs. 10-12 show the method of attaining the desired crown in the case where the nozzles are of the pipe lamina type or where headers equipped with nozzles are used, and the results obtained with the crown obtained. As shown in Fig. 10, a desired water volume crown is obtained by providing nozzles (or drilled holes) of the same bore more densely toward the center of the header and more sparsely toward the ends. Fig. 11 shows the results of a test concluded using the system shown in Fig. 10. It will be noted that a desired distribution of the water in the width direction of the steel plate can be obtained. In this system it is also possible to use a fixed nozzle pitch and instead to vary the nozzle type, i.e. the nozzle bore in the lengthwise direction of the header. A combination of these two methods can also be used. Fig. 12 shows an example of the water volume crown obtained in a test of a header wherein the nozzle bore is varied so that the amount of water supplied by the end nozzles (b) is 5% less than that supplied by a center nozzle (a). In addition there can also be used headers wherein, as disclosed in Japanese published unexamined patent application No. 153616/80, nozzle of fixed pitch and bore are provided in rows having different effective lengths. The above described systems can be applied either to a cooling system wherein the steel plate is cooled without restraint or to a cooling system wherein the plate being cooled is restrained by, for example, a roll. In the case where the plate is restrained, the spray system is more effective, but in this case too the concept of supplying a water volume crown in the width direction of the steel plate from each of the headers remains unchanged. The above explanation relates to an embodiment wherein each steel plate is cooled using one specific water volume crown throughout its entire length. In fact, however, there are cases in which it is necessary to vary the cooling pattern in the longitudinal direction of the steel plate. This is particularly true in the case of a long plate. To make it possible to satisfy this necessity, the cooling zone is divided into a desired number of sub-zones, for example, into three cooling sub-zones as shown in Fig. 13. Each of these zones if provided with two headers systems, system A and system B, and the header systems for the respective sub-zones are provided with flow control valves 1-1-1-3 and 2-1-2-3. With this arrangement, a different water volume crown can be applied at different cooling temperature regions of the plate in the longitudinal direction. In addition, when it is necessary to compensate for fabrication errors in the respective headers or to control the water volume crown with very high precision, this can be done by adjusting the flow control valve 1-a₁-1-a n and 2-b₁-2-b n shown in Fig. 1. If required, all such adjustments can be carried out automatically. Next, actual example of the application of the cooling apparatus of the present invention to the cooling of thick steel plates will be described. (1) Size of steel plate (m/m) (a) Thickness 35; Width 2500; Length 8000 (b) Thickness 25; Width 3000; Length 10000 (c) Thickness 12; Width 3500; Length 11000 (2) Cooling conditions The shapes of the plates (a)-(c) processed under the conditions shown above are shown in Fig. 14 (a)-(c), respectively. It will be noted that by overcoming the various problems mentioned earlier, it was possible to obtain steel plates with excellent flatness. If, applying the apparatus according to the present invention, the apparatus is linked with the process computer and the various sensors currently used for controlling the rolling line, it will be possible in most cases to determine the specific crown required for each steel plate. For instance; (1) the plate crown can be known from the measured value provided by the γ-ray thickness gage at the rear of the rolling mill; (2) the temperature distribution of the steel plate can be obtained in advance from a thermometer, thermovision or the like; and (3) the differences in cooling capacity resulting from differences in the volume of water flow can be worked out using computer simulation so that each and every plate can be uniformly cooled, thus making it possible to attain much greater uniformity not only in the shape of the plate but also in the quality of their steel. Furthermore, existing cooling apparatus not capable of supplying a water volume crown can be modified in accordance with this invention by incorporating therein (to the degree that this does not lead to problems regarding header pitch etc.) a separate header system capable of supplying a water volume crown.

Claims: 1. A cooling apparatus for thick elongated steel plate being transported in its lengthwise direction, with a base provided to extend in the direction of passage of the plate, a plurality of rod-headers (3, 4) provided on the base and extending in the width direction of the plate and having nozzles connected to water supply means, characterized in that the headers (3, 4) are divided into at least two systems (A, B), each system (A, B) is connected to a water supply means via a flow valve (1 and 2, respectively), and the headers of the systems (A and B, respectively) are arranged alternately side by side, the nozzles are so corstructed as to produce a different water volume crown (transverse profile and rate of supply of water) for each system (A and B), in that headers conferred with a large water volume crown and headers conferred with a small water volume crown are arranged alternately side by side, and wherein each said nozzle consists of an inner tube (8) and an outer tube (9), a flow rectification tube (10) is provided at an opening of said inner tube (8), said opening communicates via the outer tube (9) with a slit nozzle by a nozzle plate (5) provided with a bolt (6) which controls the slit width. 2. A cooling apparatus as claimed in claim 1, wherein a plurality of zones (1, 2 and 3) are successively provided in the longitudinal direction of said thick steel plate, a first zone (1) consists of two systems of first headers (a₁-a n and b₁-b n ), a second zone (2) consists of two systems of second headers (a₄-a₆ and b₄-b₆), a third zone (3) consists of two systems of third headers (a₇-a₉ and b₇-b₉), wherein in each zone said headers of one system produce a first water crown, and headers of the other produce a different water crown, and said headers of the one system are alternately arranged with said headers of the other system.

Inventors: FUKUDA, KEIJI NIPPON STEEL CORP. PROCESS TECH., ONOE, YASUMITSU NIPPON STEEL CORP. PROCESS TECH., UMENO, MASAKI NIPPON STEEL CORP. YAHATA WORKS
Applicants: NIPPON STEEL CORPORATION
Application year: 1982
Main classification: B21B45
IPC codes: B21B45, C21D9