JP2005144491A - Method and apparatus for cooling high-temperature steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cooling method and apparatus therefor by which cooling capacity is continuously adjusted without changing the quantity of water mass and the conveying speed of the steel sheet when cooling the steel sheet in a nucleate boiling area. <P>SOLUTION: In a cooling method by which a high-temperature steel sheet which is conveyed on table rollers after hot-rolling is cooled by using a cooling system which is divided into a plurality of cooling blocks in the conveying direction of the steel sheet with dewatering devices and provided with a nozzle for jetting cooling water toward the surface of the steel sheet in a cooling block, the cooling rate of the steel sheet is controlled by continuously adjusting effective cooling area in each cooling block by performing the cooling of the steel sheet in each cooling block in the nucleate boiling state and also shifting the jetting position of the nozzle in the conveying direction of the steel sheet. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for cooling a high-temperature steel sheet after hot rolling, and in particular, proposes a cooling method and apparatus for a high-temperature steel sheet capable of continuously changing the cooling capacity without changing the amount of cooling water and the conveying speed. To do.

  The TMCP method (Thermo-Mechanical Control Process) is one of the methods for producing thick steel plates that combines controlled rolling in hot rolling and accelerated cooling after rolling. In addition to obtaining steel plates with high strength and toughness, It is a widely used technique because it can reduce elements and save heat. However, as the quality requirements for thick steel plates increase, the accuracy requirements for cooling control technology have become more severe.

  In general, it is known that when a high-temperature steel sheet is water-cooled, three cooling modes (boiling phenomenon) occur depending on the surface temperature of the steel sheet being cooled (see, for example, Patent Document 1). FIG. 1 shows the relationship between the steel sheet surface temperature and the heat flow rate when the cooling conditions are constant. When a hot steel plate is cooled, first, a film boiling state in which a vapor film exists between the steel plate surface and the cooling water is brought about. In this state, since the temperature of the steel sheet surface is very high, the cooling water cannot evaporate before reaching the surface of the steel sheet and directly contact the steel sheet, and the heat flow rate is small and the cooling ability is low.

  When the surface temperature of the steel sheet decreases, the film boiling shifts to transition boiling. In this transition boiling region, since the vapor film covering the steel sheet surface cannot exist stably, the film boiling state collapses locally, and a part where the cooling water and the steel sheet surface are in direct contact is generated. The heat flow rate of the part increases rapidly. At this time, if the surface in direct contact does not exist uniformly, cooling is further promoted because the low-temperature portion has a large heat flow rate, while cooling is further promoted because the high-temperature portion has a smaller heat flow rate than the low-temperature portion. Become slow. As a result, the temperature difference between the two parts further increases, causing uneven cooling.

  Further, when the surface temperature of the steel plate is lowered, the vapor film is not present on the steel plate surface, and the state shifts to a state where almost the entire surface of the steel plate can be brought into contact with cooling water, that is, a nucleate boiling state. In this region, temperature unevenness is unlikely to occur. That is, the heat flow rate when transitioning from transition boiling to nucleate boiling becomes a maximum point, but the temperature of the steel plate decreases and the heat flow rate decreases, so even if there is uneven temperature in the steel plate before cooling Contrary to the transition boiling region, the temperature unevenness of the steel sheet decreases.

  As described above, the transition boiling state is a state in which the film boiling state and the nucleate boiling state coexist, so that the uniform cooling of the steel sheet is hindered and the accuracy of controlling the cooling stop temperature is deteriorated. On the other hand, both the film boiling state and the nucleate boiling state are suitable for uniform cooling because the heat flow rate of cooling has a positive gradient with respect to the steel plate temperature. That is, in order to perform uniform cooling, it is preferable to cool in the film boiling region and / or the nucleate boiling region, and particularly when the target cooling stop temperature is low, cooling in the nucleate boiling region is important. .

  By the way, as a method of controlling the cooling rate of the steel sheet after hot rolling, a method of adjusting the amount of cooling water as disclosed in Patent Document 2 and Patent Document 3, and Patent Documents 4 to 6 are disclosed. There are two methods for adjusting the conveying speed of the steel plate to be cooled. However, in the cooling in the nucleate boiling region, the latter method is general for the following reason.

Factors affecting the heat transfer characteristics (boiling curve) include cooling water temperature, water density, and cooling method. However, as shown in FIG. 1, in the film boiling region, when the temperature of the cooling water is lowered and the cooling density is increased, the temperature at which film boiling shifts to transition boiling shifts to the high temperature side, and the heat flow rate increases. To do. That is, in the film boiling region, the cooling capacity can be adjusted by increasing or decreasing the water density of the cooling water. On the other hand, in the nucleate boiling region, there is no significant change in the relationship between temperature and heat flow rate even if the cooling water temperature and water density are changed, and the temperature at which transition from nucleate boiling to transition boiling shifts to the high temperature side, and the nucleate boiling region Only spreads to the high temperature side. That is, in the nucleate boiling state, the cooling capacity does not change even if the water density of the cooling water is increased or decreased. Therefore, when cooling in a nucleate boiling state, it is common to adjust the cooling time by changing the transfer speed.If the transfer speed is not adjusted (cannot), the number of cooling blocks to be used (i.e. cooling) The cooling time is changed stepwise by increasing or decreasing the (length) to change the cooling capacity (see, for example, Patent Document 7).
Japanese Patent Laid-Open No. 10-211515 Japanese Patent Publication No. 07-061493 Japanese Patent Application Laid-Open No. 09-010823 JP-A-62-199723 Japanese Patent Laid-Open No. 01-205811 JP-A-11-169941 JP-A-10-216821

  As is clear from the above, when the steel sheet is cooled in the nucleate boiling region, the steel sheet temperature can be controlled to the target temperature because the cooling capacity can be continuously adjusted if the conveyance speed of the steel sheet can be adjusted. It's easy to do. However, in this method, when it is necessary to control the rolling speed, there is a problem in that it cannot be compatible with the conveyance speed necessary for ensuring the cooling speed. On the other hand, the method of adjusting the cooling rate by changing the number of cooling blocks to be used while keeping the conveying speed constant has a problem that the cooling capacity changes discontinuously and fine adjustment cannot be performed.

  An object of the present invention is to provide a cooling method and apparatus capable of continuously adjusting the cooling capacity without changing the water density or the conveying speed of the steel plate in cooling the steel plate in the nucleate boiling region. is there.

  The inventors diligently studied in order to solve the above-described problems of the prior art in which the cooling capacity is adjusted by changing the number of cooling blocks. As a result, the change in the number of cooling blocks in the prior art corresponds to the adjustment of the cooling area of the steel sheet. Therefore, if the area of the steel sheet to be cooled can be continuously changed in the cooling block, the cooling capacity is continuously increased. I thought that I could adjust it.

  That is, the present invention is a hot-rolled steel plate that is hot-rolled and transported on a table roll, and is partitioned into a plurality of cooling blocks in the steel-plate transport direction by a draining device, and the cooling block is cooled toward the surface of the steel plate. In the method of cooling using a cooling device provided with a nozzle for jetting water, the steel sheet in each cooling block is cooled in a nucleate boiling state, and the effective cooling area in each cooling block is continuously adjusted. By doing this, the cooling rate of the said steel plate is controlled, It is a cooling method of the high temperature steel plate characterized by the above-mentioned.

  In the cooling method of the present invention, it is preferable to continuously adjust the effective cooling area in each cooling block by moving the nozzle spraying position in the conveying direction of the steel plate.

  In addition, the present invention is a cooling device that is partitioned into a plurality of cooling blocks in the conveying direction of the steel sheet by a draining roll that contacts the surface of the hot steel sheet that is hot rolled and conveyed on the table roll. Each cooling block is provided with a nozzle that injects cooling water toward the surface of the steel plate, and the cooling water injection nozzle is provided in the plate width direction of the steel plate and cools in the conveyance direction of the steel plate. There is provided a cooling device for a high-temperature steel sheet, wherein water is injected and an effective cooling area in each cooling block can be continuously changed.

  In the cooling device of the present invention, it is preferable that the effective cooling area in each cooling block is continuously changed by moving the cooling water injection nozzle of each cooling block toward the conveying direction of the steel plate.

  According to the present invention, when the steel plate after hot rolling is cooled in the nucleate boiling region, the cooling area can be continuously changed, so that the cooling rate of the steel plate can be continuously adjusted. As a result, precise control of the cooling rate is possible, which greatly contributes to the improvement of steel plate quality.

  The cooling method of the present invention is a technique for cooling a steel sheet in a nucleate boiling region. This is because, in the film boiling region, the cooling capacity can be changed by changing the water density, so that fine adjustment of the cooling capacity is relatively easy, while in the transition boiling region, the cooling unevenness occurs, which is preferable. This is because the hot rolling end temperature of the steel sheet is usually about 1000 to 850 ° C., and therefore cooling after rolling is mainly cooling in the nucleate boiling region.

In order to cool the steel plate in a nucleate boiling state, it is necessary to secure a water amount per unit area of the steel plate, that is, a water amount density of a predetermined amount or more. The reason for this is that when the water density is low, there is a transition boiling state in which the portion where the steam film is present and the portion where the cooling water is in direct contact with the steel plate are mixed, and the temperature unevenness due to the difference in the heat flow rate between the two portions. Expands. However, as the water density is increased, the area where the cooling water and the steel plate are in direct contact with each other increases, and the cooling capacity increases. This is because the cooling capacity becomes almost constant after reaching the boiling region. Therefore, in order to secure the nucleate boiling state and prevent the occurrence of cooling unevenness, it is necessary to ensure a water density of a predetermined amount or more (for example, 2000 l / m ² min). However, as described above, even if the water density is increased further, the cooling capacity hardly increases.

  The water density is also affected by the steel plate temperature and the cooling water temperature. In addition, the temperature of the maximum heat flow point that shifts from the nucleate boiling region to the transition boiling region also varies depending on the temperature of the cooling water and the water density. Therefore, in the present invention, the minimum amount of water density at which nucleate boiling is obtained is obtained in advance by an off-line test, and the amount of cooling water is always equal to or higher than the minimum amount of water density of steam at the surface to be cooled. Need to be set. Therefore, in order to realize cooling in the nucleate boiling state, it is preferable to change the amount of water sprayed from all the nozzles depending on the cooling area. However, in the nucleate boiling region, since the change in the cooling capacity due to the water density is small, precise control as in the prior art (Patent Documents 4 to 6) in which the cooling capacity is changed by adjusting the cooling water volume is not necessary. What is necessary is just to be able to change the total amount of jetted water in 2 steps-about 3 steps according to an area.

Next, a cooling device for carrying out the present invention will be described.
As shown in FIG. 2, the cooling apparatus to which the present invention can be applied has a conveying direction of the steel plate 1 by two conveying rolls 4 that are in contact with the surface of the hot steel plate 1 conveyed on a table roll. A nozzle 3 for injecting cooling water 2 toward the surface of the steel plate 1 to be transported is disposed between the transport rolls 4 partitioned into a plurality of cooling blocks (in the rolling direction) and partitioning each cooling block. It has a structure. Although the said conveyance roll 4 conveys the steel plate 1, it also serves as a draining roll, and when the cooling water 2 sprayed from the nozzle 3 reaches the steel plate 1, it flows along the surface of the steel plate and drains. Since it is blocked by the roll 4 and flows down from the steel plate edge, the cooling water does not enter the adjacent cooling block. The cooling nozzle 3 is arranged in the plate width direction of the steel plate 1 and injects the cooling water 2 with an injection angle α toward the conveying direction of the steel plate 1. The length (area) at which the cooling water 2 sprayed from the nozzle 3 hits the steel plate 1 is an effective area for cooling.

Next, a method for continuously adjusting the contact area between the cooling water sprayed from the nozzle and the steel sheet, that is, the effective cooling area will be described.
In this invention, adjustment of an effective cooling area is performed by changing the position of the nozzle which injects cooling water. In the apparatus of the present invention, the position of the cooling nozzle has a structure that can be moved in the conveying direction with an electric screw jack or the like, whereby the area where the cooling water sprayed from the cooling nozzle hits the steel sheet (effective cooling area) is: It is possible to adjust continuously as the nozzle moves. FIG. 3 schematically illustrates only the upper surface side of the steel plate for the first cooling method, and FIG. 3 (a) shows a state where the effective cooling area is the smallest (that is, the state where the cooling capacity is the smallest). FIG. 3B shows a state where the effective cooling area is the widest (that is, the state where the cooling capacity is the largest). FIG. 3 (c) schematically shows the result of simulating the cooling curve in one cooling block in the case of FIG. 3 (a) and FIG. 3 (b) compared with the case of no cooling (NC). FIG. In the present invention, in this figure (c), it is possible to cool within the range indicated by the oblique lines.

  In the present invention, the injection angle α of the cooling water injected from the cooling nozzle varies depending on the draining roll interval (cooling block length) and the distance between the cooling nozzle and the steel plate, but in the maximum cooling area state, the nozzle It is necessary to set an angle at which the cooling water can cover the steel sheet between the draining rolls from directly below (or directly above).

  Further, as described above, the amount of cooling water sprayed from this nozzle needs to be equal to or higher than the minimum water amount density at which a nucleate boiling state is obtained even in the maximum effective cooling area state. In the case of the present invention, if the effective cooling area changes, the amount of cooling water for obtaining a nucleate boiling state per block also changes. However, in the nucleate boiling state, there is no difference in cooling capacity depending on the water amount density. Therefore, it is not necessary to adjust the amount of cooling water with high accuracy, and it is possible to cope with a wide range of cooling capacity changes by switching the amount of water in two or three stages.

  In addition to adjusting the effective cooling area by moving the nozzle position in the transport direction as described above, the nozzle injection direction, that is, the injection angle with respect to the steel plate or the distance between the nozzle and the steel plate is adjusted. These methods may be combined with the method of the present invention.

It is a figure explaining the factor which affects a heat transfer characteristic (boiling curve). It is a figure which shows one example of the cooling device of this invention. It is a figure which shows the cooling method and cooling curve of this invention.

Explanation of symbols

1: Steel plate 2: Cooling water 3: Nozzle 4: Conveying roll (draining roll)
5: Running water L: Effective cooling area Lmin: Minimum effective cooling area Lmax: Maximum effective cooling area

Claims (4)

A hot steel plate that is hot-rolled and transported on a table roll is partitioned into a plurality of cooling blocks in the transport direction of the steel plate by a draining device, and the cooling block is used to inject cooling water toward the surface of the steel plate In the method of cooling using a cooling device provided with a nozzle, the steel plate is cooled in the nucleate boiling state in each cooling block, and the effective cooling area in each cooling block is continuously adjusted, thereby the steel plate A method for cooling a high-temperature steel sheet, wherein the cooling rate of the steel sheet is controlled. The cooling method according to claim 1, wherein the effective cooling area in each cooling block is continuously adjusted by moving the nozzle spraying position in the conveying direction of the steel plate. A cooling device which is partitioned into a plurality of cooling blocks in the conveying direction of the steel sheet by a draining roll which is hot-rolled and conveyed on the table roll with a high temperature steel sheet sandwiched between the cooling blocks, Is provided with a nozzle for injecting cooling water toward the surface of the steel plate, the cooling water injection nozzle is provided in the plate width direction of the steel plate and injects cooling water toward the conveying direction of the steel plate, An apparatus for cooling a high-temperature steel sheet, wherein the effective cooling area in each cooling block can be continuously changed. The cooling device according to claim 3, wherein the effective cooling area in each cooling block is continuously changed by moving the cooling water injection nozzle of each cooling block toward the conveying direction of the steel plate.

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