JP4042667B2 - Steel plate cooling method - Google Patents

Description

  According to the present invention, a steel plate whose thickness is successively decreased or increased in the longitudinal direction, i.e., a high-temperature steel plate having a form of a tapered steel plate whose thickness changes in a taper shape, is supplied by a cooling device while being fed in the longitudinal direction. The present invention relates to a method of cooling the steel plate to a uniform temperature in the longitudinal direction by controlling the plate passing speed while supplying the cooling water.

  In recent years, thick steel plates employed in the shipbuilding and construction industries have mechanical properties such as high strength and high toughness, and are required to have excellent weldability in order to facilitate field welding operations. In order to obtain such characteristics, after performing controlled rolling in which rolling is performed at a predetermined temperature using a material of a predetermined component such as reducing the carbon equivalent and adding an alloy element, cooling is performed at a predetermined temperature condition. TMCP (Thermo Mechanical Control Process) technology, such as implementing controlled cooling, has been put into practical use. On the other hand, tapered steel sheets whose thickness changes in the longitudinal direction of the steel sheets are in the limelight in order to greatly reduce the weight required for the structure and the welding work on site.

  As described above, tapered steel sheets are often subjected to controlled cooling because of needs such as higher strength, weldability, and weight reduction. At this time, since the mechanical properties of the material depend on the start temperature, end temperature, and cooling rate of cooling, it is important to control these temperatures with high accuracy.

  However, since the thickness of the tapered steel plate changes sequentially when viewed in the longitudinal direction, when a constant amount of water is injected by a certain length of cooling device and a constant speed is passed, However, since the cooling rate and the cooling end temperature are different, there is a problem that the material varies.

  In order to avoid such a problem, the following is proposed about cooling of a taper steel plate.

Japanese Patent Laid-Open No. 8-230216 JP 7-68309 A JP-A-62-130222

Technique disclosed in Patent Document 1, by the addition of alloying elements was hot rolled at a finishing temperature of more than ₃ points Ar, gradually to room temperature and quenched from Ar ₃ point or more temperature to martensite transformation start temperature just above In this method, the material is transformed into a bainite structure by cooling and tempered to a temperature of Ac ₁ point or less to keep the material of the portions having different plate thicknesses constant. However, this method has a problem in that the production cost increases because of the addition of expensive elements such as Nb and Ni and a heat treatment step. In addition, since the manufacturing process increases, it is difficult to manufacture with a short delivery time.

  In the method disclosed in Patent Document 2, when the plate thickness at the tip of the steel plate is thinner than the plate thickness at the tail end, after the steel plate is once transported into the cooling device, the longitudinal direction of the steel plate is simultaneously spread over the entire length at the same time. Cooling is started, and cooling is terminated sequentially from the leading edge of the steel sheet to the tail edge while passing through the cooling device, thereby changing the cooling time for the thick part and the thin part, so that the thick part and the thin part are predetermined. It is the method of setting to the cooling end temperature. However, although the longitudinal temperature distribution after cooling can be controlled, since the cooling of the entire steel sheet is started at the same time when the steel sheet is stopped first, the jet nozzle collision point and the non-collision point of the cooling nozzle on the steel sheet surface Since the cooling capacities are greatly different from each other, there is a problem that a large temperature unevenness occurs in the initial stage, and this increases with the progress of cooling. In addition, there is a problem that it cannot be applied when the plate length is longer than the length of the cooling device.

  Patent Document 3 discloses that the hot steel sheet is divided into a plurality of sections in the length direction, and the adjacent sections are cooled while being transported with different amounts of cooling water, so that the cooling rate of the thick part and the thin part is increased. This is a method of changing to a predetermined cooling end temperature. However, since the heat capacity of the thick part is larger than that of the thin part, a very large cooling capacity is required to control the thick part so that the cooling rate of the thick part and the thin part is the same. In general, since the cooling capacity increases in proportion to the input water amount to the power of 0.5 to 0.8, the cooling capacity does not increase so much even if the amount of water is increased. Therefore, when the plate thickness ratio between the thick part and the thin part is large, the necessary variable range of the flow rate becomes large within the same plate. As a result, since it is necessary to adjust the flow rate within a very wide range within one steel plate, there are problems with the accuracy and response speed of the flow rate adjustment valve, and actual application is difficult.

  Therefore, with respect to the taper steel plate, as in the operation of a general steel plate without taper, the longitudinal temperature distribution after cooling is accurately controlled to a predetermined temperature for the cooling end temperature that is easy to manage. The method of homogenizing the material is adopted. Further, in order to solve the above-described problems, a method of cooling while passing through a steel sheet in a state where the cooling water is stabilized by injecting a predetermined amount of water in the cooling device in advance and the following has been proposed.

JP-A-62-166013 Japanese Patent Laid-Open No. 10-71416 JP 2000-15321 A

  Patent Document 4 discloses that a hot steel sheet is divided into a plurality of sections in the length direction, the steel sheet temperature before cooling is measured, and the optimum cooling condition at each dividing point is calculated based on the actually measured value. In this method, the cooling start temperature and the end temperature are set to predetermined temperatures by correcting the plate passing speed during cooling according to the plate thickness of the steel plate to be passed (referred to as “Prior Art 1”).

  Patent Document 5 discloses that the steel plate conveyance speed is constant acceleration from the time when the front end of the steel plate reaches the inlet of the cooling device to the time when the rear end of the steel plate reaches the outlet of the cooling device. Alternatively, a method of adding a deceleration and controlling the conveying speed of the steel plate so that the cooling end temperatures of the front end and the rear end coincide with a predetermined temperature (referred to as “prior art 2”).

  Patent Document 6 discloses that the tip of the steel plate is conveyed at the first speed until the tip of the steel plate passes through the position in the cooling device determined by a predetermined calculation from the inlet of the cooling device. After passing through the position in the cooling device until the rear end of the tapered steel plate enters the cooling device, the first acceleration accelerates to the second speed, and after the rear end enters the cooling device. The cooling method is characterized in that the cooling is performed according to a conveyance pattern that accelerates to the third speed by the second acceleration (referred to as “prior art 3”).

  In the prior arts disclosed in Patent Documents 4 to 6, the following problems remain, and it is difficult to manufacture a tapered steel plate having a stable material.

  Patent Document 4 is a method of changing the steel plate conveyance speed in accordance with the plate thickness. In principle, although high-precision control is possible, the steel plate full length temperature before cooling is taken in as shown in the examples. Furthermore, in order to obtain the set speed V (X) when the position of X enters the cooling device from the front end of the steel sheet using the temperature, it is necessary to solve a very complicated simultaneous equation, which requires a lot of calculation time. Also, since the conveyance speed pattern is determined so that the cooling time at each point matches the value obtained from the plate thickness and cooling conditions based on the measured temperature, for example, a local temperature deviation is erroneously applied during rolling. It may be impossible to find a solution because it is affected by the measurement temperature error. Further, since there is no concept of acceleration and the steel plate speed is controlled as a function of the distance traveled by the steel plate tip, the rotational speed control of a table roll or the like that is normally used becomes very complicated, and actual application is difficult.

  Patent Document 5 adds a constant acceleration or deceleration to the steel sheet conveyance speed, and controls the steel sheet conveyance speed so that the cooling end temperatures of the front end and the rear end coincide with a predetermined temperature. However, although the actual application is easy with a very simple principle, the front end and the rear end are at a predetermined temperature, but the center of the plate is not guaranteed.

  In Patent Document 6, by performing the step of adding acceleration or deceleration to the steel plate conveyance speed during the sheet passing, the temperature at the end of cooling at the leading end portion and the trailing end portion matches the predetermined temperature. In this method, the steel plate conveyance speed is controlled, but after the steel plate tip passes through the plate at a constant speed to a certain position in the cooling device, the first acceleration is performed, and further, the steel plate tail end is moved to the cooling device. When entering, the second acceleration is started. Therefore, when the steel plate length is sufficiently longer than the cooling equipment length, most of the length from the front end of the steel plate is passed through at the first acceleration, so the temperature control cannot be guaranteed because the control is the same as in Patent Document 5. Since the region where the region is generated and the steel plate longitudinal direction position at which the second acceleration is performed is “steel plate length−cooling device length”, the acceleration / deceleration start position near the central portion in the steel plate longitudinal direction depends on the steel plate length or the cooling device length. Since it changes, there exists a fault which the temperature control precision of the steel plate longitudinal direction center part changes with steel plate length or cooling device length.

  In view of the above, the object of the present invention is to change the cooling length and the amount of cooling water supplied in the through plate in order to stabilize the temperature in the longitudinal direction without adding a special alloy element in cooling the tapered steel plate. Without passing through, as in Patent Documents 4 to 6, when the cooling is carried out in a state in which a predetermined amount of cooling water is injected in advance and the cooling is carried out by changing the plate passing speed during cooling, the problems of the conventional method are solved. Another object of the present invention is to provide a method that enables a highly accurate temperature distribution at the end of cooling by a relatively simple method.

  According to the first aspect of the present invention, a taper steel sheet whose thickness is successively decreased or increased in the longitudinal direction is passed through a cooling device, and after inserting the steel plate into the cooling device, a cooling length and a supply amount of cooling water In which the steel sheet is divided into at least two zones in the longitudinal direction, the dividing points and the tip and tail ends of the steel sheet are set as acceleration / deceleration points, A calculation means for calculating a moving speed is provided, an acceleration / deceleration point is provided at a predetermined position between the inlet side and the outlet side of the cooling device, and the steel plate temperature at the acceleration / deceleration point when the cooling device reaches the outlet side Constant acceleration or deceleration from the time when the acceleration / deceleration point reaches the acceleration / deceleration point to the time when the next acceleration / deceleration point reaches the acceleration / deceleration point, so that is within the allowable cooling end temperature range. Speed of the steel plate Those having features to be controlled.

  The invention according to claim 2 converges the initial plate passing speed and the acceleration or deceleration so that the steel plate temperature at the acceleration / deceleration point when reaching the cooling device outlet side is within the range of the allowable cooling end temperature. It is characterized by being obtained by calculation.

  According to a third aspect of the present invention, the cooling device is constituted by a plurality of cooling banks, and the steel plate is supplied to the cooling device by ON-OFF control for switching between water flow and non-water flow for each cooling bank. It is characterized by changing the cooling length before insertion.

  The invention according to claim 4 is characterized in that each cooling bank of the cooling device is provided with a draining roll for preventing the cooling water on the steel plate from flowing into another cooling bank. .

  In the invention according to claim 5, the longitudinal direction position of the steel plate in the longitudinal direction of the acceleration / deceleration point excluding the tip and tail ends of the steel plate is set so that the longitudinal temperature deviation in the longitudinal direction after cooling falls within an allowable range. It has a special feature.

  According to the present invention, the tapered steel plate is divided into at least two or more areas in the longitudinal direction, and the division point is set as an acceleration / deceleration point together with the tip and tail ends of the steel plate, and thus the acceleration / deceleration point including the division point Is used as a change point of acceleration or deceleration applied to the steel sheet, and fine speed control is possible not only at the front and rear ends of the steel sheet but also at the central part in the longitudinal direction of the steel sheet. Thus, a highly accurate cooling control can be performed, and a useful effect of stabilizing the material is brought about.

  Next, embodiments of the present invention will be described with reference to the drawings.

  1 and 2 are drawings according to an embodiment of the cooling method of the present invention, FIG. 1 is a schematic side view showing a hot rolling facility, and FIG. 2 is a schematic side view showing a cooling device.

  As shown in FIG. 1, the high-temperature tapered steel plate 2 rolled by the rolling mill 1 is moved to the cooling device 4 by the transfer table 3 and passes through the cooling device 4, whereby the upper cooling header 5 and the lower cooling header. 6 is cooled by jetting the cooling water from 6.

  As shown in FIG. 2, a draining roll 9 is disposed before and after the steel plate moving direction of the upper cooling header 5 of the cooling device 4 so that the steel plate 2 can be restrained. The cooling device 4 is composed of a plurality of cooling banks, and each cooling bank is surrounded by adjacent draining rolls 9 and 9 and conveyance rolls 31 and 31 of the conveyance table 3, and the cooling headers 5 and 6 are respectively arranged. I have. The draining roll 9 prevents the cooling water staying at the upper part of the steel plate from flowing into another cooling bank.

  In each cooling bank, the cooling headers 5 and 6 are linked in the vertical direction so that ON / OFF control for switching between water flow and non-water flow is possible. The cooling length can be controlled by this ON-OFF control, and a free cooling length can be selected.

  In the cooling device 4, it is preferable to inject cooling water from the cooling headers 5 and 6 in advance before passing the steel plate. Overcooling can be prevented and stable cooling can be performed.

  The longitudinal temperature of the steel plate 2 is measured by an entrance thermometer 7 and an exit thermometer 8 attached before and after the cooling device 4.

  As for the taper of the steel plate 2, there are cases where the plate thickness gradually increases from the tip toward the tail end, and conversely, the plate thickness decreases gradually from the tip to the tail end. In the present embodiment, cooling of a tapered steel plate in which the plate thickness gradually increases from the tip toward the tail end will be described.

  In the present embodiment, as described above, the tapered steel plate is thickened toward the tail end of the steel plate, and the rear end portion having the tail end is thicker than the tip portion having the tip. Therefore, in order to control the same cooling end temperature at each position in the longitudinal direction of the steel sheet, it is necessary to lengthen the cooling time at the rear end of the steel sheet, and the rear end compared to the front end. It is necessary to implement a control that slows the plate passing speed.

Here, the implementation conditions in the present embodiment are: the cooling start temperature is 770 ° C., the cooling end temperature is 500 ° C., the total length of the cooling device 4 is 5 m, and the cooling water volume density (input water volume) is 1000 L (liter) / Set to min · m ² . Further, the thickness of the tip of the steel plate 2 is 10 mm, the thickness of the tail end is 20 mm, and the total length of the steel plate 2 is 10 m.

FIG. 3 shows the plate thickness and cooling rate when cooling is performed according to the above conditions (cooling start temperature: 770 ° C., cooling end temperature: 500 ° C., cooling device length: 5 m, input water amount: 1000 L / min · m ² ). Show the relationship. As can be seen from FIG. 3, the cooling rate is proportional to a value close to the first power of the plate thickness. In general, when the amount of cooling water, the cooling start temperature, and the cooling end temperature are fixed, the cooling rate can be approximated by a relation of nth power of the plate thickness. Here, n> 0. In addition, the current cooling rate is 0.95 of the plate thickness.

  Here, in order to make the cooling end temperature constant in the longitudinal direction in the tapered steel plate, the cooling time τ in the longitudinal direction of the steel plate can be obtained from the cooling rate in FIG. 3 as follows. Note that both sec and s indicate seconds.

ΔT = Cv · τ = (A / t ⁿ ) · τ (1)
τ = ΔT · t ⁿ / A (2)
However,
ΔT: Temperature drop (° C)
t: thickness A: constant Cv: cooling rate (° C./sec)
n: Constant

  On the other hand, since the thickness of the tapered steel plate changes in the longitudinal direction with a constant gradient, the relationship between the plate thickness and the distance from the steel plate tip at this time can be expressed as follows.

t = aL + b (3)
However,
a, b: constant L: distance from steel plate tip (m)

Substituting equation (3) into equation (2),
τ = ΔT · t ⁿ / A = ΔT · (aL + b) ⁿ / A (4)
In order to obtain the above formula (4) and the cooling end temperature of the tapered steel plate to be a constant temperature, the cooling time in the longitudinal direction needs to satisfy the formula (4). A detailed solution of the plate passing speed satisfying this is described in Patent Document 4.

  On the other hand, as in Patent Document 5, when the sheet is passed through the longitudinal direction at a constant deceleration, the time in the longitudinal direction of the steel sheet can be expressed as follows.

X = (1/2) ατ ² + V ₀ τ (5)
τ = {− V ₀ + √ (V ₀ ² + 2αX)} / α (6)

  Therefore, the cooling time at an arbitrary position in the longitudinal direction of the steel sheet is expressed as follows.

τ (L) = {√ (V ₀ ² + 2α (Lc + L)) − √ (V ₀ ² + 2αL)} / α (7)
However,
α: acceleration (m / sec ² )
τ: Cooling time (sec)
V ₀ : Initial speed (m / sec)
X: Travel distance (m)
L: Distance from steel plate tip (m)
L _C : cooling device length (cooling length) (m)

Here, from the relationship between the plate thickness of FIG. 3 and the cooling rate shown above, the cooling time τ of the steel plate tip portion where the plate thickness at the steel plate tip is 10 mm is 3 sec as a condition for setting the predetermined cooling end temperature. When cooling control is performed so that the cooling time of the rear end of the steel plate with a plate thickness of 20 mm at the tail end is 6 sec, the cooling time actually required in the longitudinal direction of the steel plate obtained based on the formula (4) and the conventional method The cooling time is shown in FIG. The target cooling time in FIG. 4 is a plot of τ and L in equation (4). In Patent Document 5, the initial speed, which is the speed when the tip of the steel sheet enters the cooling device, is 1.8 m / sec, and the deceleration is 0.098 m / sec ² .

  In Patent Document 5, it is possible to control the steel plate front end and rear end to a predetermined temperature by selecting the initial speed and acceleration, but the steel plate cannot be set to a predetermined cooling time in the longitudinal center. It can be seen that it is impossible to control the central portion in the longitudinal direction of the steel plate to a predetermined temperature, and the material cannot be stabilized.

  Therefore, in order to set the steel plate temperature at the central portion in the longitudinal direction of the steel plate to the predetermined cooling end temperature, the acceleration or deceleration with time is set so that the cooling time in the longitudinal direction satisfies Equation (4) as in Patent Document 4. Need to change. However, since the speed control of Patent Document 4 does not have the concept of acceleration, it is necessary to control the rotational speed of the table roll 3 at a very high level, so that it is difficult to control it by actual operation.

  Therefore, in the present invention, in order to stabilize the material within a practically acceptable range, the steel plate is divided into at least two zones (two or more zones) in the longitudinal direction so as to be a numerical value approximately approximate to Equation (4). It is characterized by dividing, setting the dividing point and each point of the tip and tail of the steel plate as acceleration / deceleration points, and changing the acceleration or deceleration applied to the steel plate at each acceleration / deceleration point.

  Next, a case where the present invention is actually applied will be described.

  FIG. 5 is a schematic side view for explaining a method of passing a tapered steel plate in which the plate thickness is gradually increased from the front end to the tail end by the cooling method of the present invention. The steel sheet is divided into two parts in the longitudinal direction, and the dividing point is set as an acceleration / deceleration point 2. Further, an acceleration / deceleration point 1 is set for the tip of the steel plate, and an acceleration / deceleration point 3 is set for the tail end. Further, as shown in FIG. 5, the cooling device 4 is constituted by five cooling banks. Between the adjacent cooling headers 5 and 5 and between the cooling headers 6 and 6 of the cooling device 4 (see FIG. 2), both are attached at a pitch of 1 m.

The steel plate is decelerated when the acceleration / deceleration points 1, 2, and 3 reach the acceleration / deceleration points provided in the cooling device. The acceleration / deceleration point for changing the deceleration may be set anywhere from the inlet side to the outlet side of the cooling device as long as it is in the cooling device, but in this embodiment, the acceleration / deceleration point is the cooling device inlet side. The speed at which each of the acceleration / deceleration points 1, 2, and 3 in the longitudinal direction of the steel sheet reaches this acceleration / deceleration point (referred to as “steel entry speed”) is V ₁ , V ₂ , V ₃ , acceleration / deceleration point 1 and acceleration / deceleration. When the distance between the points 2 is L ₁ and the distance between the acceleration / deceleration point 2 and the acceleration / deceleration point 3 is L ₂ , the decelerations α ₁ and α ₂ between the acceleration / deceleration points can be obtained as follows.

α ₁ = (V ₂ ² −V ₁ ² ) / 2L ₁ (8)
α ₂ = (V ₃ ² −V ₂ ² ) / 2L ₂ (9)

Therefore, the acceleration / deceleration point 1 reaches the acceleration / deceleration point from the time when the acceleration / deceleration point 1 enters the cooling device at the approach speed V ₁ and the acceleration / deceleration point 1 reaches the acceleration / deceleration point (cooling device entry side). performing a first deceleration by the deceleration alpha ₁ to time, then deceleration point 2 deceleration alpha ₂ in a second from the time it reaches the deceleration point to the time when the deceleration point 3 reaches the deceleration point Carry out deceleration. In this case, the distances L ₁ and L _{2 in the} longitudinal direction of the plate are equal and the distance is 5 m. Here, the cooling device delivery side speed V ₄ at the acceleration / deceleration point 3 (tail end) is decelerated at the deceleration α ₂ immediately before the acceleration / deceleration point 3 reaches the cooling device entrance side as shown in FIG. (V ₃ > V ₄ ), or at a constant speed (V ₃ = V ₄ ) with the speed V ₃ when the acceleration / deceleration point 3 reaches the cooling device entrance side as shown in FIG. There is a method to pass through, but either one can be used. This time, as shown in FIG. 6, a description will be given by a method in which the plate is passed through the deceleration α ₂ immediately before the acceleration / deceleration point 3 (tail end) reaches the cooling device entrance side.

In order to make the cooling time of the front end portion described above 3 sec and the cooling time of the rear end portion 6 sec, in the present invention, the steel plate front end entry speed V ₁ is 2.18 m / s and the first deceleration is 0. FIG. 4 shows the cooling time in the longitudinal direction of the steel plate when the control is performed with the second deceleration being 0.05 m / s ² and the second deceleration being 0.05 m / s ² , it can be seen that the present invention substantially matches the target cooling time ( the first, second deceleration, given as an acceleration, -0.33m / s ^2, a 0.05M / s ^2).

  The distance between the acceleration / deceleration points in the longitudinal direction may be divided into equal distances or non-uniform distances, but the cooling method can be controlled with high precision with a small number of divisions by optimizing the division method. It is possible. However, care should be taken because the temperature deviation may be increased if the division distance is mistaken. As for the number of divisions of the steel sheet, this time, it was divided into two divisions and there was one division point. However, the temperature deviation of the entire length can be minimized as the number of divisions is increased to three or more. However, since computation time is required, it may be determined as appropriate according to the load on the computer and the allowable temperature deviation. Thus, it becomes possible to guarantee the temperature of a steel plate longitudinal direction with high control by implementing deceleration (adjustment) at a predetermined position as explained in the present invention with respect to the steel plate longitudinal direction.

  The difference between the embodiment of the present invention and Patent Document 6 is that, in Patent Document 6, the second deceleration is performed from the tail end approach, whereas in the present invention, the number of divisions in the longitudinal direction is set to 2 and the longitudinal direction of the steel sheet The distance between each acceleration / deceleration point is made equal, and the second deceleration is performed from the approach of the dividing point (acceleration / deceleration point 2) at the center in the longitudinal direction of the steel sheet (arrival to the acceleration / deceleration point). Therefore, in Patent Document 6, the effect of the second deceleration can be obtained only by the cooling length from the tail end of the steel plate. However, in the present invention, the second deceleration is performed in the vicinity of the central portion in the longitudinal direction of the steel plate. Fine speed control is possible, and cooling control is possible with high accuracy.

Here, as an example of Patent Document 6, the speed at which the tip of the steel plate reaches the cooling device is 1.72 m / s, and the first deceleration is 0.152 m / s when the tip moves 2.5 m after reaching the cooling device. in s ^2, further showing a steel plate longitudinal direction of the cooling time when the steel sheet tail was shown through plate from reaching the cooling device in the second reduction 0.002 m / s ² in FIG. It can be seen that there is a region where the cooling time largely deviates from the target cooling time.

Next, a steel sheet approach speed V ₁ , V ₂ , V ₃ is obtained. The temperature at each of the acceleration / deceleration points 1, 2, 3 after completion of cooling is determined as predetermined cooling end temperatures T ₁ , T _2. The reason for calculating by numerical simulation so that it matches within the allowable error range of T ₃ is that, unlike the analytical solution, the temperature dependence such as the thermophysical value of the steel material and the time from exiting the cooling device to reaching the thermometer This is because it is easy to consider cooling.

In the convergence calculation method, first determine an appropriate initial plate passing speed V _old and determine the deceleration rate α ₁ , α ₂ , and carry out heat transfer calculation only for the time that the steel plate is transported from the cooling device entrance side to the thermometer installation position. To do. Next, the following calculation is performed from the cooling start temperature T _s , the target cooling end temperature T _e , the initial sheet passing speed V _old , and the calculated cooling end temperature T _c to obtain a corrected sheet passing speed V _new .

V _new = V _old × (T _s −T _c ) / (T _s −T _e )

  Such an operation may be repeated until the cooling end temperature falls within an allowable error. Since there are various ways of convergence other than the method introduced in the present invention, an appropriate method may be selected according to the processing time and the stability of calculation.

  The above description has been given of the case where the taper of the steel plate increases in thickness sequentially from the tip toward the tail end, but it can also be applied to the case where the plate thickness decreases gradually from the tip toward the tail end. Of course.

  Next, the reason why the cooling device is divided into a plurality of cooling banks, and the cooling length is changed so that ON / OFF control can be performed for switching of water flow or non-water flow in each cooling bank will be described. When the above-described calculation is performed, the plate passing speed, acceleration, and deceleration may not be within the control range of the transport table roll. For example, when the steel plate thickness is large and the cooling end temperature is low, the sheet passing speed may be too low and may be outside the control range of the transport table roll. In this case, it is only necessary to change the cooling length in advance by using ON / OFF control of water flow in each cooling bank to compensate for the control of the transport table roll. That is, the cooling length is changed so that the thick part and the thin part of the steel plate are within the conveyance speed control range of the equipment. It is preferable to increase the number of cooling banks as much as possible and to shorten the length per cooling bank. This improves the controllability. The ON / OFF control of water flow is performed before the start of cooling, and is not performed after the start of cooling, that is, after the steel sheet is inserted into the cooling device.

  The draining roll prevents the cooling water staying at the upper part of the steel sheet from flowing into another cooling bank. If there is no draining roll, the staying water at the upper part of the steel sheet may flow into another cooling bank and the cooling time may not be strictly controlled. In addition, the staying water may cause local temperature unevenness, resulting in material variations.

  Next, the present invention will be described in more detail with reference to examples.

The cooling equipment shown in FIGS. 1 and 2 described in the embodiment of the present invention was used to cool the tapered steel sheet using the sheet feeding speed calculation method. The cooling device had a cooling length of 5 m, a cooling water amount of 1000 L / min · m ² , and a cooling bank number of 5. The cooling conditions were a cooling start temperature of 770 ° C. and a cooling end temperature of 500 ° C. The tapered steel plate had a tip thickness of 10 mm and a tail end thickness of 20 mm, and the thickness gradually increased from the tip toward the tail end, and the total length was 10 m. The acceleration / deceleration points of the steel plate were set as an acceleration / deceleration point 1, an acceleration / deceleration point 2, an acceleration / deceleration point 3,. The acceleration / deceleration point was on the cooling device entrance side.

Deceleration is performed when the tip of the steel plate reaches the acceleration / deceleration point (cooling device entry side), and the plate is passed through the deceleration α ₂ immediately before the tail end reaches the cooling device entry side as shown in FIG. It was. The results of Examples 1 to 4 performed by changing the number of divisions and the distance between the acceleration / deceleration points were as follows. Table 1 shows the conditions and results of the examples.

In Example 1, as shown in FIG. 5, the steel plate was divided into two sections, ie, a front end portion and a rear end portion in the longitudinal direction. An acceleration / deceleration point 1 was set at the tip of the steel plate, an acceleration / deceleration point 2 was set as a dividing point between the tip and rear ends, and an acceleration / deceleration point 3 was set at the tail end. The distance L ₁ between the acceleration / deceleration point 1 and the acceleration / deceleration point 2 was 5 m, and the distance L ₂ between the acceleration / deceleration point 2 and the acceleration / deceleration point 3 was 5 m. When this is calculated by the plate passing speed calculation method described above, the plate passing speed (speed at which the tip enters the cooling device) when the tip reaches the acceleration / deceleration point is 1.81 m / s, and the acceleration / deceleration point 1 And the acceleration / deceleration point 2 was 0.231 m / s ² , and the deceleration between the acceleration / deceleration point 2 and acceleration / deceleration point 3 was 0.036 m / s ² . When cooling was performed under these conditions, the temperature distribution in the longitudinal direction became very uniform as shown in FIG. The temperature deviation in the longitudinal direction at this time was 25 ° C. When the material was investigated later, it was satisfactory over the entire length.

In Example 2, although not shown in the drawing, the steel plate is divided into three sections, ie, a front end portion, a middle portion and a rear end portion in the longitudinal direction. The dividing point between the front end and the intermediate portion is set as the acceleration / deceleration point 2, the dividing point between the intermediate portion and the rear end is set as the acceleration / deceleration point 3, and the tail end is set as the acceleration / deceleration point 4. The distance between acceleration / deceleration point 1 and acceleration / deceleration point 2 of the steel plate is 3.3 m, the distance between acceleration / deceleration point 2 and acceleration / deceleration point 3 is 3.3 m, acceleration / deceleration point 3 and acceleration / deceleration point 4 are The distance between them was 3.4 m. When this is calculated using the plate speed calculation method described above, the plate speed when the tip reaches the acceleration / deceleration point is 1.53 m / s, and the decrease between acceleration / deceleration point 1 and acceleration / deceleration point 2 is as follows. The speed is 0.059 m / s ² , the deceleration between the acceleration / deceleration point 2 and the acceleration / deceleration point 3 is 0.173 m / s ² , and the deceleration between the acceleration / deceleration point 3 and the acceleration / deceleration point 4 is 0. It became a 031m / s ^2. When cooling was performed under these conditions, the temperature distribution in the longitudinal direction became very uniform as shown in FIG. At this time, the in-plate temperature deviation in the longitudinal direction was 13 ° C., and when the material was examined later, it was satisfactory over the entire length.

In Example 3, as in Example 1, the steel plate was divided into two sections, ie, a front end portion and a rear end portion in the longitudinal direction. An acceleration / deceleration point 1 was set at the tip of the steel plate, an acceleration / deceleration point 2 was set as a dividing point between the tip and rear ends, and an acceleration / deceleration point 3 was set at the tail end. The distance between the acceleration / deceleration point 1 and the acceleration / deceleration point 2 was 3.3 m, and the distance between the acceleration / deceleration point 2 and the acceleration / deceleration point 3 was 6.7 m. When this is calculated by the plate speed calculation method described above, the plate speed when the tip reaches the acceleration / deceleration point is 2.19 m / s, and the decrease between acceleration / deceleration point 1 and acceleration / deceleration point 2 is as follows. The speed was 0.534 m / s ² , and the deceleration between the acceleration / deceleration point 2 and the acceleration / deceleration point 3 was 0.044 m / s ² . When cooling was performed under these conditions, the temperature distribution in the longitudinal direction was slightly higher than that of Example 1 on the tail end side as shown in FIG. The longitudinal temperature deviation in the longitudinal direction at this time was 33 ° C. When the material was investigated later, it was good over the entire length, but the variation was slightly larger than in Example 1 and Example 2. In the longitudinal division in which the distance between the acceleration / deceleration points in the longitudinal direction of the steel sheet is different as in the present embodiment, the temperature control in the longitudinal direction is slightly worse than in the first embodiment even if the number of divisions (two divisions) is the same.

In Example 4, as in Example 1, the steel sheet was divided into two sections, ie, a front end portion and a rear end portion in the longitudinal direction. An acceleration / deceleration point 1 was set at the tip of the steel plate, an acceleration / deceleration point 2 was set as a dividing point between the tip and rear ends, and an acceleration / deceleration point 3 was set at the tail end. The distance between the acceleration / deceleration point 1 and the acceleration / deceleration point 2 was 6.7 m, and the distance between the acceleration / deceleration point 2 and the acceleration / deceleration point 3 was 3.3 m. When this is calculated by the plate speed calculation method described above, the plate speed when the tip reaches the acceleration / deceleration point is 1.66 m / s, and the decrease between acceleration / deceleration point 1 and acceleration / deceleration point 2 is performed. The speed was 0.148 m / s ² , and the deceleration between the acceleration / deceleration point 2 and the acceleration / deceleration point 3 was 0.031 m / s ² . When cooling was performed under this condition, the temperature distribution in the longitudinal direction was very uniform as shown in FIG. The temperature deviation in the longitudinal direction at this time was 15 ° C., which was a temperature deviation comparable to Example 2 in which the number of divisions in the longitudinal direction was 2. When the material was investigated later, it was good over the entire length.

[Comparative Example 1]
Next, a comparative example will be described.

  Table 1 also shows the conditions and results of the comparative examples.

In the comparative example 1, like patent document 5, the distance between the front-end | tip and tail end of a steel plate was 10 m. When this was calculated by the plate passing speed calculation method described above, the plate passing speed when the tip reached the cooling device was 1.53 m / s, and the deceleration between the tip and tail was 0.073 m / s. s ² . When cooling was performed under these conditions, the temperature distribution in the longitudinal direction, as shown in FIG. 8, was the target temperature at the front end (near the front end) and the rear end (near the tail end), but at the center of the steel plate The temperature became high. The temperature deviation in the longitudinal direction at this time was 65 ° C, and when the material was investigated later, the strength decreased as it became closer to the central part in the longitudinal direction of the steel sheet, and it failed because it did not fall within the intended strength specification. .

[Comparative Example 2]
In Comparative Example 2, according to the concept of Patent Document 6, the constant speed is 1.45 m / s until the tip of the steel plate reaches, and the first deceleration is performed when the steel plate tip reaches 2.5 mm after reaching the cooling device. subjected to reduction at 0.108m / s ^2, the tail end is decelerated from reaching the cooling device passing plate speed to 0.604m / s in the second deceleration 0.010 / s ^2. When cooling was performed under these conditions, the temperature distribution in the longitudinal direction was as intended at the front end (near the front end) and the rear end (near the tail end) as shown in FIG. The temperature slightly increased in the part. In this case, the longitudinal temperature deviation in the plate is 40 ° C., and when the material was investigated later, there was a region where the strength was partially reduced at the central portion in the longitudinal direction of the steel plate. Since it was cut off and shipped, the yield was low.

It is a schematic side view which shows the hot rolling equipment which concerns on embodiment of this invention. It is a schematic side view which shows the cooling device which concerns on embodiment of this invention. It is a graph which shows the relationship between plate | board thickness and a cooling rate when it cools from 770 degreeC to 500 degreeC with the cooling device which concerns on embodiment and an Example of this invention. It is the graph which compared the target time in each position of a steel plate longitudinal direction, the cooling time of patent document 5, and the cooling time of this invention. It is a schematic side view explaining the sheet passing method of the steel plate concerning an embodiment and an example of the present invention. The graph explaining the example of the speed control after the tail end enters the cooling device in the method of passing the plate with the deceleration α ₂ just before the tail end reaches the cooling device entrance side according to the embodiment of the present invention. It is. A graph tail according to the embodiment of the present invention will be described an example of the speed control from enters tail is the cooling device in the method of left sheet passing speed V ₃ when it reaches the cooling device inlet side is there. It is a graph which shows the temperature distribution of the steel plate longitudinal direction after the cooling of Comparative Examples 1 and 2 implemented by the comparative example. It is a graph which shows the temperature distribution of the steel plate longitudinal direction after the cooling of Examples 1-4 implemented in the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Rolling machine 2 Steel plate 3 Conveying table 31 Conveying roll 4 Cooling device 5 Upper cooling header 6 Lower cooling header 7 Incoming thermometer 8 Outlet thermometer 9 Draining roll

Claims (5)

  In a method in which a tapered steel plate whose thickness is successively decreased or increased in the longitudinal direction is passed through a cooling device, and after inserting the steel plate into the cooling device, cooling is performed without changing the cooling length and the amount of cooling water supplied. The calculation means for dividing the steel sheet into at least two sections in the longitudinal direction, setting the dividing point and the tip and tail ends of the steel sheet as acceleration / deceleration points, and calculating the moving speed of the acceleration / deceleration points An acceleration / deceleration point is provided at a predetermined position between the inlet side and the outlet side of the cooling device, and the steel plate temperature at the acceleration / deceleration point when reaching the outlet side of the cooling device is within the allowable cooling end temperature range. From the time when the acceleration / deceleration point reaches the acceleration / deceleration point to the time when the next acceleration / deceleration point reaches the acceleration / deceleration point, a constant acceleration or deceleration is applied to the plate speed of the steel plate Characterized by controlling Steel plate cooling method that.   The initial plate passing speed and the acceleration or deceleration so that the steel plate temperature at the acceleration / deceleration point when reaching the cooling device outlet side is within the range of the allowable cooling end temperature and the acceleration or deceleration are obtained by convergence calculation. A method for cooling steel sheets.   The cooling device is composed of a plurality of cooling banks, and the cooling length is changed before the steel plate is inserted into the cooling device by ON-OFF control for switching between water flow and non-water flow for each cooling bank. The method for cooling a steel sheet according to claim 1 or 2.   The method for cooling a steel sheet according to claim 3, wherein each cooling bank of the cooling device is provided with a draining roll for preventing the cooling water on the steel sheet from flowing into another cooling bank.   The steel plate longitudinal direction position of the said acceleration / deceleration point except the front-end | tip and tail end of the said steel plate is set so that the longitudinal temperature deviation in the longitudinal direction after cooling may be in an allowable range. The method for cooling a steel sheet according to claim 1.

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