Control device for an accelerated cooling system and method for controlling an accelerated cooling system

Abstract

To provide a control device for an acceleration cooling device and a control method for the acceleration cooling device, which can suppress material dispersion caused by dispersion of an acceleration cooling start temperature and can improve material stability during mass production of steel sheets.SOLUTION: A control device 19 of an acceleration cooling device 11 lowers the temperature of a steel sheet 7 to a target acceleration cooling stop temperature by performing acceleration cooling to the steel sheet 7 being conveyed. A target accelerated cooling stop temperature at a measurement point of the temperature of the steel sheet 7 and an accelerated cooling condition at an accelerated cooling device 11 at the measurement point for realizing the target accelerated cooling stop temperature are determined on the basis of a relationship between the accelerated cooling start temperature and the accelerated cooling stop temperature and the temperatures of the steel sheet 7 at least two or more points in the conveying direction T of the steel sheet 7 at a time point before entering the accelerated cooling device 11.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a control device for an accelerated cooling device and a control method for an accelerated cooling device.

Background Art

[0002] A method for manufacturing a high-strength thick steel plate that performs accelerated cooling after hot rolling has advantages such as being able to reduce alloy elements in the thick steel plate compared to conventional manufacturing methods, further being able to omit heat treatment, or reducing the number of heat treatment implementations, and is widely applied. As an accelerated cooling method, an accelerated cooling method called a passing type is generally used, in which the thick steel plate after hot rolling is continuously introduced into an accelerated cooling device from the front end to the tail end, and water is sprayed onto the upper and lower surfaces of the thick steel plate for cooling. The passing type of accelerated cooling method can reduce the size of the accelerated cooling device compared to an accelerated cooling method called a simultaneous injection type that starts accelerated water cooling after the entire thick steel plate has completely entered the accelerated cooling device. However, because the accelerated cooling start temperature of the thick steel plate in the longitudinal direction of the thick steel plate is not constant, there is a possibility of material variation in the longitudinal direction of the thick steel plate in the passing type of accelerated cooling method. Generally, from the viewpoints of production efficiency and suppression of strength reduction of the thick steel plate, the thick steel plate after completion of hot rolling is conveyed to the accelerated cooling device by the shortest distance, and then accelerated cooling is immediately started on the thick steel plate.

[0003] In order to solve such problems, various studies have been conducted on a method for manufacturing a thick steel plate that performs accelerated cooling after hot rolling.

[0004] For example, Patent Document 1 describes a method for manufacturing controlled-cooled steel sheets. In this method, between hot rolling and controlled cooling (corresponding to the accelerated cooling described above; hereinafter referred to as accelerated cooling), the entire length of the thick steel sheet before accelerated cooling is slowly cooled by a shower cooling system. Patent Document 2 describes a method for manufacturing high-strength steel sheets, in which tempering treatment is performed by induction heating after accelerated cooling. Patent Documents 3 and 4 describe methods for controlling the cooling of steel sheets. In these methods, the speed at which the steel sheets pass through the cooling device is adjusted based on the temperature of the steel sheets measured by a thermometer installed inside a pass-through type cooling device that performs accelerated cooling of the steel sheets. Patent Document 5 describes a method for controlling the cooling of steel sheets. In this method, in a pass-through type cooling device that performs accelerated cooling of steel sheets, thermometers are placed before and after the cooling device in the direction of steel sheet passage, and the speed at which the steel sheets pass through the cooling device is adjusted based on the temperature of the steel sheets measured by these thermometers. Patent Document 6 describes a method for controlling the cooling of a pass-through type cooling device that performs accelerated cooling of steel sheets. In this method, the accelerated cooling start temperature is measured for each region divided along the longitudinal direction of the steel sheet, and the transport speed in the through-type accelerated cooling device is adjusted based on the measured accelerated cooling start temperature so that the accelerated cooling stop temperature for each region becomes the target accelerated cooling stop temperature. Patent document 7 describes a method for reducing material variation in hot-rolled steel. In this method, when manufacturing hot-rolled steel sheets, data including the steel sheet composition, operating conditions, and actual material values ​​for each hot-rolled steel sheet manufactured in the past is accumulated. Then, based on the accumulated data and the required specifications including the steel sheet composition and operating conditions of the hot-rolled steel sheet to be manufactured at the present time, the material of the hot-rolled steel sheet to be manufactured at the present time is estimated. Furthermore, operating conditions that satisfy the required specifications are set based on the estimated material value and estimation error. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-57330 [Patent Document 2] Japanese Patent Publication No. 2005-120409 [Patent Document 3] Japanese Patent Publication No. 2006-272395 [Patent Document 4] Japanese Patent Publication No. 2006-281300 [Patent Document 5] Japanese Patent Publication No. 2010-167503 Public Relations [Patent Document 6] Japanese Patent Publication No. 2010-247234 [Patent Document 7] Japanese Patent Publication No. 2006-239696 [Overview of the project] [Problems that the invention aims to solve]

[0006] The method described in Patent Document 1 not only requires shower cooling equipment, but the slow cooling performed before accelerated cooling may reduce the strength of the thick steel plate, potentially preventing the achievement of high strength. Furthermore, the method in Patent Document 1 cannot suppress variations in the accelerated cooling start temperature, and therefore may not be able to suppress material variations caused by these variations. Patent Document 2 also has similar problems to those described in Patent Document 1.

[0007] The methods described in Patent Documents 3 and 4 improve the accuracy of the accelerated cooling stop temperature of thick steel plates by measuring the surface temperature of the steel plate during accelerated cooling, but may not be able to suppress material variations caused by variations in the accelerated cooling start temperature. The method in Patent Document 5 adjusts the accelerated cooling conditions after the cooling of the leading edge of the steel plate is completed, so it may only be applicable when the total length of the steel plate is long or the total length of the accelerated cooling device is short. Furthermore, the method in Patent Document 5 improves the accuracy of the accelerated cooling stop temperature of the steel plate, but may not be able to suppress material variations caused by variations in the accelerated cooling start temperature. The method in Patent Document 6 improves the accuracy of the accelerated cooling stop temperature of thick steel plates, but may not be able to suppress material variations caused by variations in the accelerated cooling start temperature. The method in Patent Document 7 may not be able to suppress material variations caused by variations in the accelerated cooling start temperature and accelerated cooling stop temperature of thick steel plates.

[0008] The present invention was made to solve the above problems, and aims to provide a control device for an accelerated cooling device and a control method for an accelerated cooling device that can suppress material variations caused by variations in the accelerated cooling start temperature, and can also improve material stability during mass production of steel sheets. [Means for solving the problem]

[0009] To achieve the above objectives, the present invention (1) A control device for an accelerated cooling device that performs accelerated cooling on a steel plate in transit to lower the temperature of the steel plate to a target accelerated cooling stop temperature, the control device for an accelerated cooling device that determines the target accelerated cooling stop temperature at a temperature measurement point of the steel plate and the accelerated cooling conditions in the accelerated cooling device at the measurement point to achieve the target accelerated cooling stop temperature, based on the relationship between the accelerated cooling start temperature and the accelerated cooling stop temperature and the temperature of the steel plate at at least two locations in the transport direction of the steel plate before it enters the accelerated cooling device. (2) The time before entering the accelerated cooling device is the time when entering the accelerated cooling device, and the temperature of the steel plate is the surface temperature of the steel plate when entering the accelerated cooling device. The control device for the accelerated cooling device as described in (1). (3) The control device for the accelerated cooling device according to (1), wherein the temperature of the steel plate is determined to be at least one of the measured value of the surface temperature of the steel plate at the exit side of the hot rolling mill upstream of the accelerated cooling device in the transport direction, and an estimated value calculated based on the measured value and the distance between the accelerated cooling device and the hot rolling mill in the transport direction. (4) The control device for the accelerated cooling device according to (2), wherein the temperature of the steel plate is determined to be at least one of the measured value of the surface temperature of the steel plate at the exit side of the hot rolling mill upstream of the accelerated cooling device in the transport direction, and an estimated value calculated based on the measured value and the distance between the accelerated cooling device and the hot rolling mill in the transport direction. (5) The control device for the accelerated cooling device according to any one of (1) to (4), wherein the temperature of the steel plate at least two locations in the conveying direction of the steel plate is at least two locations among the front, middle, and tail ends of the steel plate in the conveying direction of the steel plate. (6) The control device for an accelerated cooling device according to any one of (1) to (4), wherein the accelerated cooling device comprises a plurality of cooling units that supply a cooling fluid to the steel plate, and the accelerated cooling conditions include the transport speed of the steel plate in the accelerated cooling device and the number of cooling units that supply the cooling fluid to the steel plate in the accelerated cooling device. (7) A control method for an accelerated cooling device that performs accelerated cooling on a steel plate in transit to lower the temperature of the steel plate to a target accelerated cooling stop temperature, comprising: a temperature measurement step of determining the temperature of the steel plate at at least two locations in the transport direction of the steel plate before it enters the accelerated cooling device; and an accelerated cooling condition determination step of determining the target accelerated cooling stop temperature at the temperature measurement location on the steel plate and the accelerated cooling conditions in the accelerated cooling device at the temperature measurement location to achieve the target accelerated cooling stop temperature, based on the relationship between the accelerated cooling start temperature and the accelerated cooling stop temperature and the temperature of the steel plate. (8) A control method for the accelerated cooling apparatus according to (7), in which the steel plate is tempered immediately after the temperature of the steel plate has been reduced to the accelerated cooling stop temperature, or after the temperature of the steel plate has been reduced to the target accelerated cooling stop temperature and then air-cooled. [Effects of the Invention]

[0010] According to the present invention, it is possible to improve the material uniformity and material stability during mass production of steel plates (referred to as high-strength steel plates) to which accelerated cooling is applied, which are used in ships, bridges, buildings, offshore structures, line pipes, etc. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows an example of a thick steel plate manufacturing line equipped with an accelerated cooling device according to the present invention. [Figure 2]It is a diagram showing an example of the configuration of an accelerated cooling device. [Figure 3] It is a flowchart for explaining an example of control executed by a control device of an accelerated cooling device according to the present invention.

Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described. FIG. 1 is a diagram showing an example of a thick steel plate manufacturing line provided with an accelerated cooling device according to the present invention. The thick steel plate manufacturing line 1 shown in FIG. 1 conveys the slab 3 heated in the heating furnace 2 in order to the first rolling mill 5 and the second rolling mill 6 by the conveying device 4, and hot-rolls the slab 3 step by step with each rolling mill 5, 6 to manufacture a thick steel plate (hereinafter simply referred to as a steel plate) 7. In the example shown in FIG. 1, the conveying device 4 is composed of a plurality of conveying rolls. The first rolling mill 5 is a rough rolling mill and is configured to rough-roll the slab 3 to a predetermined thickness. The second rolling mill 6 is a finishing rolling mill and is configured to roll the slab 3 rough-rolled by the first rolling mill 5 to the product thickness to manufacture the steel plate 7. Each of the above rolling mills 5, 6 corresponds to a hot rolling device in the embodiment of the present invention. The steel plate 7 extends along the conveying direction T in the conveying device 4. Therefore, the conveying direction T of the steel plate 7 and the length direction of the steel plate 7 are substantially parallel.

[0013] A first thermometer 8 is installed on the inlet side of the second rolling mill 6 in the conveying direction T of the steel plate 7, and a second thermometer 9 is installed on the outlet side of the second rolling mill 6. These thermometers 8, 9 may be any conventionally known ones as long as they can measure the surface temperature of the steel plate 7, but are preferably non-contact thermometers such as radiation thermometers.

[0014] A first leveler 10 is provided on the downstream side of the second rolling mill 6 in the conveying direction T of the steel plate 7. The first leveler 10 corrects the shape of the steel plate 7 rolled by each of the rolling mills 5, 6 and may be a conventionally known leveler.

[0015] On the outlet side of the first leveller 10, an accelerated cooling device 11 for cooling the steel plate 7 to a predetermined accelerated cooling stop temperature (hereinafter simply referred to as the stop temperature) by controlling the cooling rate in the transformation temperature range of steel is provided close to the first leveller 10. There is a speed difference between the conveyance speed of the steel plate 7 in the accelerated cooling device 11 and the conveyance speed of the steel plate 7 that exits the second rolling mill 6 and is conveyed to the accelerated cooling device 11. Therefore, a third thermometer 12 is provided on the inlet side of the first leveller 10 to measure the surface temperature of the steel plate 7 immediately before entering the accelerated cooling device 11. The surface temperature of the steel plate 7 immediately before entering the accelerated cooling device 11 is the accelerated cooling start temperature (hereinafter simply referred to as the start temperature). In the embodiment of the present invention, the surface temperature of the steel plate 7 immediately before entering the accelerated cooling device 11 is measured at a plurality of locations in the conveyance direction T of the steel plate 7 or in the length direction of the steel plate 7 by the third thermometer 12. The third thermometer 12 is preferably a non-contact thermometer, similar to the first thermometer 8 and the second thermometer 9.

[0016] FIG. 2 is a diagram showing an example of the configuration of the accelerated cooling device 11. The accelerated cooling device 11 shown in FIG. 2 is an accelerated cooling device called a through type, and the steel plate 7 is continuously made to enter from the tip to the tail end in the length direction of the steel plate 7 inside it. Then, a cooling fluid is sprayed onto each of the upper and lower surfaces of the steel plate 7 to cool the steel plate 7 to the stop temperature set as described later. The above cooling fluid may be liquid-phase cooling water.

[0017] The accelerated cooling device 11 includes a plurality of upper header units 13, 14 for spraying cooling water toward the upper surface of the steel plate 7, and a plurality of lower header units 15, 16 for spraying cooling water toward the lower surface of the steel plate 7. In FIG. 2, for simplicity of the drawing, a pair of upper header units 13, 14 and two lower header units 15, 16 are shown. Each of the above header units corresponds to the cooling unit in the embodiment of the present invention.

[0018] The first upper header unit 13 is located above the steel plate 7 in the height direction H of the accelerated cooling device 11, and is positioned upstream of the second upper header unit 14 in the transport direction T at a predetermined distance. The first upper header unit 13 has a plurality of first upper nozzles 13A, which are made of, for example, cylindrical pipes. As shown in Figure 2, these first upper nozzles 13A are installed at a predetermined angle and inclination with respect to the upper surface of the steel plate 7 so as to spray cooling water at an angle from the upstream side to the downstream side in the transport direction T with respect to the upper surface of the steel plate 7.

[0019] The second upper header unit 14 is located above the steel plate 7 in the height direction H of the accelerated cooling device 11, and has a plurality of second upper nozzles 14A, for example, made of cylindrical pipes. As shown in Figure 2, these second upper nozzles 14A are installed at a predetermined angle and inclination with respect to the upper surface of the steel plate 7 so as to spray cooling water at an angle from the downstream side to the upstream side in the transport direction T with respect to the upper surface of the steel plate 7. In this way, each upper header unit 13, 14 is designed to spray cooling water towards predetermined locations inside the accelerated cooling device 11.

[0020] A first lower header unit 15 is provided on the opposite side of the collision point between the steel plate 7 and the cooling water sprayed from the first upper header unit 13. The first lower header unit 15 has a plurality of first lower nozzles 15A, which are made of, for example, cylindrical pipes, and these first lower nozzles 15A extend along the height direction H of the accelerated cooling device 11. The spacing between adjacent conveying rolls on the opposite side of the collision point is set so that the first lower header unit 15 can be positioned on the opposite side of the collision point.

[0021] A second lower header unit 16 is provided on the opposite side of the collision point between the steel plate 7 and the cooling water sprayed from the second upper header unit 14. The second lower header unit 16 has a plurality of second lower nozzles 16A, which are made of, for example, cylindrical pipes, and these second lower nozzles 16A extend along the height direction H of the accelerated cooling device 11. The spacing between adjacent conveying rolls on the opposite side of the collision point is set so that the second lower header unit 16 can be positioned on the opposite side of the collision point.

[0022] Returning to the explanation of Figure 1, a fourth thermometer 17 is provided on the outlet side of the accelerated cooling device 11. The fourth thermometer 17 measures the surface temperature of the steel plate 7 that has been accelerated cooling by the accelerated cooling device 11. It is preferable that the fourth thermometer 17 is a non-contact thermometer, similar to the thermometers 8, 9, and 12 described above.

[0023] A second leveler 18 is provided downstream of the fourth thermometer 17 in the conveying direction T of the steel plate 7, and the second leveler 18 corrects the shape deformation of the steel plate 7 caused by accelerated cooling. The second leveler 18 may be a conventionally known leveler, similar to the first leveler 10. A tempering device (not shown) is provided downstream of the second leveler 18, and the hardness of the steel plate 7 is adjusted by the tempering device. The tempering device may be a conventionally known induction heating type tempering device.

[0024] Furthermore, an electronic control unit (hereinafter referred to as ECU) 19 is provided to control the amount of cooling of the steel plate 7 in the accelerated cooling device 11, that is, the difference between the start temperature and stop temperature of the steel plate 7. The ECU 19 is mainly composed of a microcomputer and performs calculations based on input data and pre-stored data and calculation formulas. The calculation results are then output as control command signals to the accelerated cooling device 11 and the conveying device 4. Examples of data input to the ECU 19 include the surface temperature of the steel plate 7 measured by each thermometer 8, 9, and 12, the conveying speed of the steel plate 7 in each rolling mill 5 and 6, and the conveying speed of the steel plate 7 in the accelerated cooling device 11. Examples of control command signals output from the ECU 17 include signals to control the number of each upper header unit 13 and 14 that spray cooling water onto the steel plate 7 in the accelerated cooling device 11, signals to control the number of each lower header unit 15 and 16, and signals to control the conveying speed of the steel plate 7 in the accelerated cooling device 11.

[0025] Here, we will explain the speed difference between the conveying speed of the steel plate 7 in the accelerated cooling device 11 and the conveying speed of the steel plate 7 as it leaves the second rolling mill 6 and is conveyed to the accelerated cooling device 11. The conveying speed of the steel plate 7 in the accelerated cooling device 11 is slower than the conveying speed of the steel plate 7 as it leaves the second rolling mill 6 and is conveyed to the accelerated cooling device 11. Therefore, the leading edge of the steel plate 7 in the longitudinal direction can leave the second rolling mill 6 and enter the accelerated cooling device 11 while maintaining its temperature. However, the middle and trailing edges of the steel plate 7 in the longitudinal direction are exposed to the outside air and air-cooled on the conveying device 4 between the second rolling mill 6 and the accelerated cooling device 11 while the leading edge is being cooled in the accelerated cooling device 11. Therefore, the middle and trailing edges of the steel plate 7 enter the accelerated cooling device 11 at a lower temperature than the leading edge. Therefore, accelerated cooling begins at the center and tail end of the steel plate 7 when their respective starting temperatures are lower than those at the tip, which can result in material variation along the length of the steel plate 7. In this embodiment of the present invention, the control described below is implemented to suppress material variation caused by variations in the starting temperature along the length of the steel plate 7.

[0026] Figure 3 is a flowchart illustrating an example of control performed by the control device of the accelerated cooling device 11 according to the present invention. The control example shown in Figure 3 is repeatedly executed by the ECU 19 at predetermined short intervals during the operation of the thick steel sheet manufacturing line 1. First, the design conditions for the steel sheet to be shipped as a product (hereinafter referred to as "product steel sheet") are input to the ECU 19, and the relationship between the start temperature and stop temperature in the accelerated cooling device 11 for obtaining the product steel sheet is calculated based on the design conditions (step S1). Examples of the design conditions mentioned above include the target chemical composition of the product steel sheet, the target sheet thickness, the target surface temperature of the steel sheet 7 immediately after it leaves the second rolling mill 6, the tempering method, and the target tempering temperature. These values ​​are predetermined as design values ​​for the product steel sheet.

[0027] The relationship between the start and stop temperatures described above can be determined by, for example, first obtaining multiple manufacturing results for steel plates that aim for the same chemical composition, plate thickness, heating temperature, and rolling end temperature. From these manufacturing results, for example, a manufacturing result with nearly constant tensile test results is selected, and the start and stop temperatures for the selected manufacturing result are determined. Then, the relationship between the start and stop temperatures, i.e., the calibration curve, is calculated. Alternatively, the relationship between the start and stop temperatures can be determined using a machine learning model such as deep learning.

[0028] Following or almost simultaneously with step S1, the temperatures at at least two locations along the length of the steel plate 7 as it enters the accelerated cooling device 11 are acquired as starting temperatures (step S2), and the starting temperatures acquired in step S2 are input to the ECU 19. Step S2 described above corresponds to the temperature measurement process in this embodiment.

[0029] The phrase "when entering the accelerated cooling device 11" refers to the moment when the steel plate 7 enters the accelerated cooling device 11, or immediately before that. Furthermore, "at least two locations in the longitudinal direction of the steel plate 7" refers to at least two locations in the longitudinal direction of the steel plate 7 that are different from each other. More specifically, it refers to at least two locations in the longitudinal direction of the steel plate 7, namely the front, middle, and rear. In the embodiment of the present invention, it is preferable to measure the temperatures at the above three locations using the third thermometer 12, and to use at least two of these measured values ​​as the starting temperatures at the temperature measurement locations. The above-mentioned starting temperatures may be actual measured values ​​from the third thermometer 12. Alternatively, the starting temperatures may be estimated values ​​from the surface temperature of the steel plate 7 measured by the second thermometer 9 immediately after leaving the second rolling mill 6. These estimated values ​​can be calculated based on, for example, the ambient temperature, the distance between the second rolling mill 6 and the accelerated cooling device 11, and the time from leaving the second rolling mill 6 to entering the accelerated cooling device 11.

[0030] Next, the stop temperatures corresponding to the measured or estimated values ​​of each start temperature obtained in step S2 are calculated (step S3). Specifically, step S3 in Figure 3 shows a calibration curve illustrating the relationship between the start temperature and the stop temperature calculated in step S1, and the stop temperature corresponding to the start temperature obtained in step S2 is calculated using this calibration curve. The stop temperature thus calculated is the target value of the stop temperature at the location where the start temperature was measured or estimated (hereinafter referred to as the target stop temperature), and this is set in the accelerated cooling device 11. In the embodiment of the present invention, as shown in step S3 in Figure 3, the start temperatures of the steel plate 7 decrease in the order of the front, middle, and tail ends, and the stop temperatures decrease in the order of the front, middle, and tail ends. The target stop temperature may be determined for the entire steel plate, but from the viewpoint of reducing the computational load, it is preferable to determine it for the location where the temperature of the steel plate 7 was measured or estimated, as described above. The above target stop temperature corresponds to the target accelerated cooling stop temperature in the embodiment of the present invention.

[0031] Following step S3, the accelerated cooling conditions (hereinafter simply referred to as cooling conditions) for cooling the tip of the steel plate 7 from the starting temperature to the target stopping temperature are determined (step S4). Specifically, in step S4, the number of upper header units and lower header units used for accelerated cooling of the tip are determined. The amount of cooling water sprayed onto the steel plate 7 from each header unit is approximately constant. Therefore, for example, the amount of cooling water that can be sprayed to lower the temperature of the tip of the steel plate 7 from the starting temperature to the target stopping temperature, i.e., the number of each header unit, is determined. For example, a map or table defining the relationship between the temperature difference between the starting temperature and the stopping temperature and the number of upper header units and lower header units can be prepared in advance, and the number of each header unit can be determined using that map or table. Note that the number of upper header units and lower header units provided in the accelerated cooling device 11 is determined by design. Therefore, the number of upper header units and lower header units used for accelerated cooling of the tip is determined with the number of upper header units and lower header units provided in the accelerated cooling device 11 as the upper limit.

[0032] Following or almost simultaneously with step S4, the transport speed of the steel plate 7 when the leading edge of the steel plate 7 is accelerated cooling in the accelerated cooling device 11 is determined (step S5). Once the transport speed of the leading edge is determined in step S5, the time it takes for the leading edge to pass through the accelerated cooling device 11 is determined accordingly. This determines the cooling speed of the leading edge. The transport speed of the leading edge can be determined, for example, by preparing a map or table in advance that defines the relationship between the temperature difference between the starting temperature and the stopping temperature, the number of upper header units and the number of lower header units, and the transport speed of the steel plate 7 in the accelerated cooling device 11, and using that map or table.

[0033] Following step S5, the appropriate conveying speeds are determined for the central and tail ends of the steel plate 7 to obtain the desired metal structure and strength at those ends (step S6). This is because, as mentioned above, the number of upper and lower header units to be used in the accelerated cooling device 11 has already been determined. The conveying speeds for the central and tail ends of the steel plate 7 can be determined by, for example, preparing a map or table in advance that defines the relationship between the temperature difference between the start temperature and the stop temperature, the number of upper and lower header units, and the conveying speed of the steel plate 7 in the accelerated cooling device 11. Once the conveying speeds for the central and tail ends of the steel plate 7 are determined, the time it takes for the central and tail ends to pass through the accelerated cooling device 11 is determined accordingly. Based on this, the cooling speeds for the central and tail ends are determined. Note that the cooling speeds for the leading edge, central and tail ends of the steel plate 7 may differ from each other.

[0034] Following step S6, equipment constraints on the transport speed are acquired (step S7). Here, equipment constraints refer to the limits on the transport speed caused by the accelerating cooling device 11. Specifically, these refer to the upper and lower limits of the transport speed and acceleration determined by the design of the accelerating cooling device 11. Alternatively, if the upper and lower limits of the transport speed or acceleration are limited due to maintenance or other reasons, these limits refer to those values. Such limit values ​​are stored as data in a storage unit (not shown), and this data is input to the ECU 19.

[0035] Following step S7, it is determined whether each transport speed determined in steps S5 and S6 is less than or equal to the limit value obtained in step S7 (step S8). If each transport speed exceeds the limit value, that is, if there are equipment constraints and it is not possible to set each transport speed determined in steps S5 and S6, it is determined negatively in step S8 and the process proceeds to step S9. In step S9, the number of header units used for accelerated cooling of the steel plate 7 in the accelerated cooling device 11 is changed. After that, the process proceeds to step S5 and the previous control is executed.

[0036] If, however, each transport speed is below the limit value, a positive decision is made in step S8 and the process proceeds to step S10. In step S10, the number of header units determined in step S4 and each transport speed determined in steps S5 and S6 are determined as the cooling conditions for the steel plate 7 to achieve the target stop temperature. The cooling conditions are then set in the accelerated cooling device 11, and the accelerated cooling device 11 is activated according to the cooling conditions, and the steel plate 7 is accelerated cooling. After that, the control shown in the flowchart in Figure 3 is temporarily terminated. Steps S3 to S10 described above correspond to the accelerated cooling condition determination process in this embodiment.

[0037] As described above, the control device of the accelerated cooling device 11 according to the embodiment of the present invention measures or estimates the surface temperature (start temperature) of the steel plate 7 entering the accelerated cooling device 11 at at least two locations among the front, middle, and tail ends of the steel plate 7. Then, it sets a stop temperature according to these start temperatures. Therefore, even if variations in the start temperature are unavoidable, since a stop temperature is set according to the start temperature, substantially similar accelerated cooling can be performed at each of the above-mentioned parts. As a result, material variations in the length direction of the steel plate 7 caused by variations in the start temperature can be suppressed. This improves the material uniformity of the accelerated cooled steel plate 7 and the material stability during mass production of the steel plate 7.

[0038] In this embodiment, the temperature of the steel plate 7 was measured at three locations: the front, middle, and tail. However, instead, the temperature can be measured at at least two of these locations. For example, the temperature can be measured at the front and middle, the front and tail, or the middle and tail. Even in these cases, material variations in the length direction of the steel plate 7 caused by variations in the starting temperature can be suppressed compared to the conventional method. In other words, substantially the same effects and advantages as in the above-described embodiment can be obtained. Furthermore, the amount of cooling water injected into the steel plate 7 from each header unit of the accelerated cooling device 11 can be changed, and this amount of cooling water can be included in the cooling conditions. [Examples]

[0039] Next, an example of a test conducted to confirm the operation and effects of the control device of the accelerated cooling device 11 according to an embodiment of the present invention will be described. In this example, thick steel plates were manufactured using a thick steel plate manufacturing line similar to the thick steel plate manufacturing line shown in Figure 1. That is, slabs having the chemical composition shown in Table 1 were manufactured by a continuous casting method.

[0040] [Table 1]

[0041] (Examples 1-10 of the invention) After loading the aforementioned slab into the heating furnace, hot rolling was performed in the first and second rolling mills. Immediately after the completion of rolling in the second rolling mill, the surface temperature of the central part of the steel sheet in the width direction was measured using a second thermometer. The steel sheet was then immediately transported to the front of the first leveler, and the surface temperature of the steel sheet was measured multiple times in the length direction using a third thermometer. Specifically, while continuously feeding (transporting) the steel sheet into the first leveler and the accelerated cooling device, the surface temperature of at least two locations (the front, middle, and tail) of the steel sheet in the length direction was measured using the third thermometer. These measured values ​​were used as the starting temperature of the steel sheet, and the cooling conditions for the steel sheet were determined by performing the control shown in the flowchart in Figure 3. That is, similar to the embodiment of the present invention described above, the stopping temperature was changed according to the starting temperature. Then, the accelerated cooling device was activated with the above cooling conditions to perform accelerated cooling of the steel sheet.

[0042] After accelerated cooling, the steel plate was thoroughly reheated, for example, by air cooling for 50 seconds or more. Subsequently, while continuously entering (conveying) the second leveler, the surface temperature of the steel plate was measured with a fourth thermometer, and it was further air cooled.

[0043] (Examples 11-20 of the invention) The steel plates were manufactured in the same manner as in Invention Examples 1 to 10, except that the surface temperature of the central part of the steel plate in the width direction, measured by a second thermometer, was used to estimate the surface temperature of the steel plate entering the accelerated cooling device.

[0044] (Examples 21-30 of the invention) The steel plate was manufactured in the same manner as in Invention Examples 1 to 10, except that the temperature of the tip and tail end of the steel plate was measured, and after passing through the second leveler, tempering was performed using an induction heating device.

[0045] (Examples 31-40 of the invention) The steel plates were manufactured in the same manner as in Invention Examples 11-20, except that they were tempered using an induction heating device after passing through the second leveler.

[0046] (Examples 41-50 of the invention) The steel plates were manufactured in the same manner as in Invention Examples 31-40, except that tempering was performed in a heating furnace instead of an induction heating device.

[0047] (Comparative Examples 1-10) Regardless of the starting temperature, the steel plate was accelerated in an accelerated cooling device with the stopping temperature set to a predetermined temperature. Furthermore, tempering was performed using an induction heating device installed immediately after the second leveler, followed by air cooling. Aside from these steps, the steel plate was manufactured in the same manner as in Invention Examples 1 to 10.

[0048] (Comparative Examples 11-20) Except for the cooling conditions for the leading edge of the steel plate, which were determined by the control shown in the flowchart in Figure 3, and accelerated cooling was performed under those conditions, the steel plate was manufactured in the same manner as in Comparative Examples 1 to 10.

[0049] (Comparative Examples 21-30) The steel sheets were manufactured in the same manner as in Comparative Examples 1 to 10, except that they were tempered using an induction heating device after passing through the second leveler.

[0050] (Comparative Examples 31-40) The steel sheets were manufactured in the same manner as in Comparative Examples 21-30, except that tempering was performed in a heating furnace instead of an induction heating device.

[0051] (Evaluation of materials) Material properties were evaluated using tensile testing. Tensile test specimens were taken from three locations: 250 mm from the leading edge of the steel plate, in the center of the steel plate, and 250 mm from the trailing edge of the steel plate. At each location, the tensile test specimen was taken from the center of the steel plate in the width direction. The sampling and tensile testing of the tensile test specimens were carried out in accordance with ASTM A370.

[0052] For each steel plate in Invention Examples 1-50 and Comparative Examples 1-40, as described above, test specimens were taken from three locations along the length of the steel plate and tensile tests were performed to determine the tensile strength. The standard deviation of these tensile strengths was calculated, and specimens were deemed acceptable if six times the standard deviation was 60 MPa or less; otherwise, they were deemed unacceptable.

[0053] Table 2 lists the methods for determining the stop temperature for Invention Examples 1-50 and Comparative Examples 1-40, the manufacturing conditions for each steel sheet, and the tensile strength and the value of the standard deviation of the tensile strength multiplied by 6 for each steel sheet. For Invention Examples and Comparative Examples in which the stop temperature was determined by the control shown in the flowchart in Figure 3, "Logic" is written in the column for the method of determining the target accelerated cooling stop temperature in Table 2. The manufacturing conditions are the heating temperature of the slab, the final rolling temperature (i.e., the measured value of the second thermometer), the starting temperature in the accelerated cooling device, the target stop temperature, the measured value of the stop temperature (i.e., the measured value of the fourth thermometer), the tempering method, and the tempering temperature.

[0054] [Table 2]

[0055] Note that the accelerated cooling start temperature and accelerated cooling stop temperature at the tip, as listed in Table 2, are the maximum temperatures at a position approximately 500 mm from the tip of the steel plate entering the accelerated cooling device. The accelerated cooling start temperature and accelerated cooling stop temperature at the central part are the maximum temperatures near the center in the longitudinal direction of the steel plate. The accelerated cooling start temperature and accelerated cooling stop temperature at the tail end are the maximum temperatures at a position approximately 500 mm from the tail end in the longitudinal direction of the steel plate.

[0056] As shown in Table 2, Invention Examples 1 to 50 meet the target performance. In contrast, Comparative Examples 1 to 40 do not meet the target performance.

[0057] 1. Thick steel plate manufacturing line 2 Furnace 3 Slabs 4. Conveying device 5. First Rolling Mill 6. Second Rolling Mill 7 Steel plate 8 1st thermometer 9 Second thermometer 10. Leveler 1 11 Accelerated cooling device 12 Third thermometer 13. First Upper Header Unit 14. Second Upper Header Unit 15. First Lower Header Unit 16. Second Lower Header Unit 17 4th thermometer 18. Second Leveler 19 Electronic Control Unit (ECU)

Claims

1. A control device for an accelerated cooling system that performs accelerated cooling on a steel plate during transport to lower the temperature of the steel plate to a target accelerated cooling stop temperature, Based on the relationship between the accelerated cooling start temperature and the accelerated cooling stop temperature, and the temperature of the steel plate at at least two locations in the transport direction of the steel plate before it enters the accelerated cooling device, the target accelerated cooling stop temperature and the accelerated cooling conditions in the accelerated cooling device at each measurement location for the steel plate temperature are determined. Control device for an accelerated cooling system.

2. The point in time before entering the accelerated cooling device refers to the moment of entering the accelerated cooling device. The temperature of the steel plate is the surface temperature of the steel plate when it enters the accelerated cooling device. A control device for an accelerated cooling device according to claim 1.

3. The temperature of the steel plate is at least one of the following: the measured surface temperature of the steel plate at the exit of the hot rolling mill upstream of the accelerated cooling device in the transport direction, and an estimated value calculated based on the measured value and the distance between the accelerated cooling device and the hot rolling mill in the transport direction. A control device for an accelerated cooling device according to claim 1.

4. The temperature of the steel plate is at least one of the following: the measured surface temperature of the steel plate at the exit of the hot rolling mill upstream of the accelerated cooling device in the transport direction, and an estimated value calculated based on the measured value and the distance between the accelerated cooling device and the hot rolling mill in the transport direction. A control device for an accelerated cooling device according to claim 2.

5. The temperature of the steel plate at at least two locations in the conveying direction of the steel plate is at least two of the leading edge, middle, and trailing edge of the steel plate in the conveying direction of the steel plate. A control device for an accelerated cooling system according to any one of claims 1 to 4.

6. The accelerated cooling device comprises a plurality of cooling units that supply a cooling fluid to the steel plate, The control device for an accelerated cooling device according to any one of claims 1 to 4, wherein the accelerated cooling conditions include the transport speed of the steel plate in the accelerated cooling device and the number of cooling units that supply the cooling fluid to the steel plate in the accelerated cooling device.

7. A control method for an accelerated cooling device that performs accelerated cooling on a steel plate during transport to lower the temperature of the steel plate to a target accelerated cooling stop temperature, A temperature measurement step to determine the temperature of the steel plate at at least two locations in the transport direction of the steel plate before it enters the accelerated cooling device, The system includes an acceleration cooling condition determination step, which determines, for each temperature measurement point on the steel plate, the target acceleration cooling stop temperature and the acceleration cooling conditions in the acceleration cooling device at the temperature measurement point in order to achieve the target acceleration cooling stop temperature, based on the relationship between the acceleration cooling start temperature and the acceleration cooling stop temperature and the temperature of the steel plate. A control method for an accelerated cooling system.

8. The steel plate is tempered immediately after its temperature has been reduced to the accelerated cooling stop temperature, or after its temperature has been reduced to the target accelerated cooling stop temperature and then air-cooled. A control method for an accelerated cooling device according to claim 7.

9. A method for manufacturing a steel sheet, comprising manufacturing a steel sheet by controlling the accelerated cooling performed on the steel sheet using the accelerated cooling control method described in Claim 7 or 8, in order to lower the temperature of the steel sheet to a target accelerated cooling stop temperature.

Images