JP2005211945A - Method for cooling high temperature steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for cooling a high temperature steel plate, which method can be applied to a longitudinally profiled steel plate having various cross sections without changing the flow rate of water to be poured on the steel plate during the pass of the plate. <P>SOLUTION: In the method for cooling the high temperature steel plate, dividing points for the steel plate in the longitudinal direction are preliminarily determined before cooling at the leading end, the trailing end, and the inflection point of the thickness gradient of the plate in the longitudinal direction. Reference positions are provided between the inlet side and the outlet side of a cooling apparatus, and the speed of the pass is changed at a constant accelerating rate or a constant decelerating rate within the period from the arrival of the dividing point at the reference point to the arrival of the next dividing point at the same reference point, and an initial speed of the pass, the accelerating rate, and the decelerating rate are determined such that the dividing point is cooled within the allowable cooling temperature range while passing the cooling apparatus. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a cooling method in a case where high-temperature LP steel plates having various plate thickness shapes are supplied in a longitudinal direction and supplied with a constant amount of cooling water to be cooled by passage.

  In recent years, steel plates used in the shipbuilding and construction industries have mechanical properties such as high strength and high toughness, as well as 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 controlled cooling, has been put into practical use.

  On the other hand, LP steel plates (Longitudinally Profiled Steel Plates), which have a linear shape with an arbitrary shape in the longitudinal direction, are attracting attention in order to significantly reduce the weight of structures and the cost of field welding work. ing. By adopting such a steel plate, it is possible to make a rational cross-sectional configuration and equal thickness of the joint according to the required cross-section when used in a region where the cross-sectional force of the steel structure changes suddenly, and steel weight reduction or Cost reduction can be expected by reducing the number of processing.

  A typical example of the shape of the LP steel sheet is shown in FIG. Here, the right side of the figure will be described below with the steel plate tip and the left side as the steel plate tail end. FIG. 1 (a) shows an LP steel plate having a uniform thickness gradient in the longitudinal direction, with the plate thickness increasing sequentially from the front end to the rear end of the steel plate. FIG. 1B shows an LP steel plate having parallel portions having different plate thicknesses at the front end portion and the rear end portion, and having a uniform thickness gradient in a taper shape in the longitudinal direction therebetween. FIG. 1 (c) has a parallel portion at the front end portion, the rear end portion, and the central portion of the steel plate, and the central parallel portion is thicker than the front end portion and the rear end portion. It is an LP steel plate having a uniform thickness gradient in a tapered shape in the direction.

  FIG. 1 (d) has a parallel part at the central part in the longitudinal direction of the steel sheet, the central parallel part is thicker than the front end part and the rear end part, and the front and rear end parts and the parallel part of the central part are LP steel plate having a uniform thickness gradient in a taper shape in the longitudinal direction. FIG. 1 (e) has a parallel portion at the front and rear ends of the steel plate, the plate thickness is thicker at the center of the steel plate than at the front and rear ends of the steel plate, and the length between the front and rear ends and the central portion is longer. It is an LP steel plate having a uniform thickness gradient in a tapered shape in the direction. FIG. 1 (f) shows that the plate thickness is thicker at the center of the steel plate than at the front and rear ends of the steel plate, and has a uniform thickness gradient that is tapered in the longitudinal direction between the front and rear ends. LP steel plate.

  FIG. 1 (g) has parallel portions at the front and rear end portions and the central portion of the steel plate, the plate thickness of the parallel portions is increased in the order of the longitudinal direction, and the space between the parallel portions is evenly tapered in the longitudinal direction. This is an LP steel sheet with a large thickness gradient. FIG. 1 (h) has a parallel part in the central part in the longitudinal direction of the steel sheet, the front end of the steel sheet is thinner than the central parallel part, and the rear end of the steel sheet is thicker than the central parallel part. The LP steel sheet has a uniform thickness gradient in a taper shape in the longitudinal direction between the rear end portion and the parallel portion of the central portion. FIG. 1 (i) has parallel portions at the front and rear end portions and the central portion of the steel plate, the plate thickness of the parallel portion at the center of the steel plate is thinner than the parallel portion at the front and rear ends of the steel plate, and is parallel. It is an LP steel plate having a uniform thickness gradient in the longitudinal direction between the portions.

  FIG. 1 (j) has a parallel portion at the center, the plate thickness of the parallel portion at the center of the steel plate is thinner than the parallel portions at the front and rear ends of the steel plate, and the parallel portion is tapered in the longitudinal direction. It is an LP steel plate having a uniform thickness gradient. In FIG. 1 (k), there are parallel portions at the front and rear ends of the steel plate, and the thickness at the center of the steel plate is thinner than the parallel portions at the front and rear ends of the steel plate. This is an LP steel plate having a uniform thickness gradient in a taper shape in the longitudinal direction between the parallel portions at the ends. In FIG. 1 (l), the plate thickness at the center of the steel plate is thinner than the front and rear ends of the steel plate, and a uniform thickness gradient is formed in a taper shape in the longitudinal direction between the central portion of the steel plate and the front and rear ends. LP steel plate with

  Controlled cooling has been increasingly applied to LP steel sheets having such a wide variety of cross-sectional shapes because of needs such as high 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.

  On the other hand, LP steel plates have various cross-sectional shapes when viewed in the longitudinal direction. Therefore, the cooling rate and cooling end temperature are different between the thick and thin portions, and the constant water flow conditions and constant flow plates are different. When cooling is performed at a speed, there is a problem that material variation occurs in each part of the steel plate.

In order to avoid this, the following prior art is disclosed.
JP-8-232016 discloses disclosed in (Patent Document 1) method, with the addition of alloy elements was hot rolled at a finishing temperature of more than ₃ points Ar, martensitic transformation starting from Ar ₃ point or more temperature This is a method in which the material of the parts having different plate thicknesses is kept constant by transforming into a bainite structure by quenching to just above the temperature and gradually cooling to room temperature, and tempering to a temperature of Ac ₁ point or less.

As shown in FIG. 17, a technique disclosed in Japanese Patent Application Laid-Open No. 7-96319 (Patent Document 2) has a plurality of coolants injected to a mountain-shaped or trapezoidal tapered steel plate. In the stage (1) before being transported into the cooling device having the zone, the taper is transported at a speed V ₀ until the central portion M of the steel plate reaches the entrance of the cooling device 20. And, at the stage (2) when the central portion is transferred from the tip T of the tapered steel plate into the cooling device 20, the coolant is injected from the zone, and the cooling from the tip T to the central portion M is started simultaneously, to convey at the speed _{V 1.} Furthermore, at the stage where the tail B reaches the inlet of the cooling device 20 (3), conveyed at a speed V _2. Then, at the stage where the tip T has reached the outlet of the cooling device 20 (4), and conveyed at speed V _3, the coolant is injected from the zone at that stage the central portion M reaches the outlet of the cooling device 20 (5) the exit to cool down, then taper at a speed V ₄ - the steel sheet is extracted from the cooling device 20, which is to become such a cooling process step (6).

  In the technique disclosed in Japanese Patent Application Laid-Open No. 10-249430 (Patent Document 3), as shown in FIG. 18, a plurality of banks of cooling devices 12a to 12n and a plurality of position detection devices are provided, and each cooling bank is electromagnetically opened and closed. A cooling device with valves 13a to 13n is used. The cooling zone is a unit composed of zones S1, S2 and S3.

  Next, the cooling method of this method is demonstrated using FIG. First, cooling water is passed from the S1 and S2 zones before the mountain-shaped and trapezoidal tapered steel plates enter the cooling device. Then, with the insertion of the tapered steel plate A, after it is determined whether or not the central portion M has exceeded the first bank 12a in the inlet side cooling zone S1, the central portion M is the first one as shown in FIG. When one bank 12a is exceeded, the electromagnetic on-off valves 13a to 13c are closed sequentially each time, and the coolant injection is sequentially stopped from the first bank 12a as shown in FIGS. 19 (b) and 19 (c). As described above, when the central portion M of the tapered steel plate A is in the inlet side cooling zone S1, the electromagnetic on-off valves 13b and 13c of the second and third banks 12b and 12c are sequentially installed from the first bank 12a. It is closed and the second and third banks 13b and 13c are not cooled.

  Next, as shown in FIG. 19 (c), while the tapered steel plate A exceeds the banks 12d to 12g in the intermediate cooling zone S2, the electromagnetic on-off valves 13d to 13g are opened and cooled from the banks 12d to 12g. Since the agent is injected, the cooling from the tip T to the tail B can be performed sequentially.

  Then, after it is determined whether or not the tip T has exceeded the i-th bank 12h in the exit side cooling zone S3, as shown in FIG. The on-off valves 13h to 13j are opened, and coolant is separately injected from the i-th bank 12h to the bank 12i and the n-th bank 12j as shown in FIGS. 19 (d) and 19 (f).

  As described above, when the tip T of the tapered steel plate A is in the outlet side cooling zone S3, the electromagnetic valves 13h to 13j are sequentially opened individually as shown in FIG. The coolant is sequentially injected from ˜12j, and cooling is performed sequentially from the portion excluding the tip T portion to the tail end B. This is a cooling method according to the above procedure.

Japanese Patent Laid-Open No. 8-230216 JP-A-7-96319 JP-A-10-249430

  However, the method described in Japanese Patent Application Laid-Open No. 8-230216 has a problem that the manufacturing 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.

  Further, in the methods described in JP-A-7-96319 and JP-A-10-249430, it is necessary to inject cooling water into the steel plate passage plate, so that the amount of cooling water is the target immediately after the injection of cooling water. Overshooting does not follow the value, or a phenomenon in which the cooling water amount gradually increases occurs, resulting in changes in temperature uniformity and cooling capacity. In addition, when the cooling water is stopped, only the upper surface is supercooled by the cooling water staying on the steel plate, which causes the shape of the steel plate to be disturbed and temperature unevenness to occur. Furthermore, these methods are applicable only to mountain-shaped and trapezoidal LP steel plates, and there is a problem that they cannot be applied to all LP steel plates having various shapes as shown in FIG.

  The present invention has been made in view of such circumstances, as shown in FIG. 1 without adding a special alloy element, and without changing the flow rate of water passing through the steel plate during the passing plate. It aims at providing the cooling method of the high temperature steel plate applicable to LP steel plate which has various shapes.

  The first means for solving the above-mentioned problem is to cool the LP steel plate when the LP steel plate whose thickness varies in the longitudinal direction is passed through a water-cooled cooling device having a plurality of cooling zones. Before starting, the cooling zone used in the cooling device and the cooling water flow rate in each cooling zone are determined. During cooling of the LP steel sheet, the cooling water flow in the cooling zone used and each cooling zone is passed. This is a method of cooling without changing the amount of water, in which the longitudinal direction of the steel sheet is set in advance at the leading edge and the rear edge and the inflection point of the thickness gradient in the longitudinal direction before cooling, while the cooling device is turned on. A reference position is provided from the side to the exit side, and a constant acceleration or a constant deceleration is performed after the division point reaches the reference position until the next division point reaches the reference position. And plate speed Acceleration / deceleration is performed, and the initial plate passing speed and the acceleration and deceleration are determined such that the dividing point is cooled to a temperature within an allowable cooling temperature range while passing through the cooling device. This is a method for cooling a high-temperature steel sheet (claim 1).

  The second means for solving the above-mentioned problem is the first means, and the initial plate passing speed, the acceleration and the deceleration are allowed to be cooled while the dividing point passes through the cooling device. It is obtained by convergence calculation so as to be cooled to a temperature within the temperature range (claim 2).

  The third means for solving the problem is the first means or the second means, wherein the reference position is a center position in the longitudinal direction of the cooling zone used by the cooling device. (Claim 3).

  The fourth means for solving the problem is any one of the first means to the third means, wherein the continuous length of the cooling zone to be used is set shorter than the length between the reference positions. (Claim 4).

  According to the present invention, it can be applied to LP steel sheets having various shapes as shown in FIG. 1 without adding a special alloying element and without changing the flow rate of water passing through the steel sheet. A method for cooling a high-temperature steel sheet can be provided.

  Hereinafter, examples of embodiments of the present invention will be described in detail with reference to the drawings. FIG. 2 is a schematic diagram showing an example of a cooling device to which the method for cooling a high-temperature steel sheet according to the embodiment of the present invention is applied.

  As shown in FIG. 2, the high-temperature steel plate 2 rolled by the rolling mill 1 is conveyed by a conveying table 3 to a cooling device 4 in which cooling water has been jetted in advance from above and below, and is cooled while being passed. The longitudinal temperature of the steel plate 2 is measured by thermometers 5 and 6 attached before and after the cooling device 4. The cooling device 4 is composed of 10 zones in the longitudinal direction, and cooling headers 7 are attached to each zone at a pitch of 1 m. Water draining rolls 8 are installed in front of and behind the cooling header 7 to restrain the steel plates and prevent the cooling water in adjacent zones from flowing into each other area.

  Thus, by controlling the water flow of each cooling header 7 on and off, the zone used for cooling can be freely selected according to the pattern required for cooling the steel plate. In the subsequent drawings, the cooling headers are painted black where the cooling water is flowing, and the non-white coatings are the cooling water passing zones.

  When cooling the steel plate 2 using such a cooling device 4, it is usual to perform control cooling to the same cooling stop temperature at each position in the longitudinal direction of the steel plate 2. In order to cool the temperature of the steel plate 2 to the target temperature, the amount of cooling water is calculated using a mathematical model of heat transfer calculation. For example, these mathematical models are introduced in documents such as the Japan Iron and Steel Institute's “forced cooling of steel materials”, and those skilled in the art can appropriately select and use these mathematical models.

  In the present invention, the cooling water amount when cooling the steel plate 2 is kept constant, and the change in the required cooling water amount accompanying the change in the thickness of the steel plate 2 is changed by changing the plate passing speed of the steel plate 2. I have to.

  For example, the front end of a steel plate (in the present specification and claims, the front end of a steel plate means the end on the side that first enters the cooling device when passing through the cooling device, and the rear end refers to cooling When it passes through the device, it means the end on the side that finally enters the cooling device.) When the thickness is thinner than the thickness of the rear end, it is necessary to increase the cooling time as the steel plate thickness increases. For this reason, it is necessary to implement a control that slows the plate passing speed at the rear end compared to the front end of the steel plate.

  In order to implement such control, division points are set in the steel plate 2 in the present embodiment. The front end portion and the rear end portion are unconditionally set as dividing points, and the other dividing points are points at which the longitudinal thickness gradient changes. This dividing point may be referred to as a contact point below. And about the division | segmentation point, the plate | board speed for cooling to target temperature is calculated | required by numerical formula model.

  As shown in FIG. 3, examples of dividing points (contact points) in the length direction of the steel plate have parallel portions at the front end portion, the rear end portion, and the central portion of the steel plate, and the plate thickness of the parallel portion goes in the rear end direction. An LP steel plate that is thick and has a uniform thickness gradient tapered in the longitudinal direction between the parallel portions will be described. In the steel sheet shown in FIG. 3, the right side of the drawing is the leading end and the left side is the trailing end.

  The steel plate as shown in FIG. 3 is divided into five as shown in the drawing, and the divided steel plates are connected by six contacts (contacts 1 to 6). Between the contact points is one section, and in the case of FIG. 3, the length of each section is shown as L1 to L5. The thickness of the section indicated by the length L1 is T1, the thickness of the section indicated by the length L3 is T2, and the thickness of the section indicated by the length L5 is T3. In the section indicated by the length L2, the thickness decreases linearly from T1 to T2, and in the section indicated by the length L4, the thickness decreases linearly from T2 to T3. Yes.

  As shown in FIG. 4, this contact point can be determined as a position where the gradient changes when a longitudinal thickness gradient with respect to the front and rear ends of the steel plate is taken. 4 shows a longitudinal thickness gradient of the plate thickness with the rear end portion (right side of FIG. 3) of the tapered steel plate of FIG. 3 as the origin. As described above, the contact point of the steel plate is defined as a position where the front end portion, the rear end portion, and the plate thickness gradient in the longitudinal direction change.

  Next, an actual plate passing state of a steel plate having a cross-sectional shape as shown in FIG. 3 will be described with reference to FIG. In the cooling device, only the first two zones out of the 10 zones are allowed to pass through water, and the steel sheet is allowed to enter the cooling device in this state. In this case, the central position of the two passing zones is selected as the acceleration / deceleration point.

  The steel plate has six contact points in the longitudinal direction, and the distances L1 to L5 between the contacts are the same 2 m, and the right side of FIG. In this case, the distance between the contacts of the LP steel plate and the cooling zone length are the same 2 m. In the longitudinal direction, the LP steel sheet has a thin steel plate tip and increases in thickness toward the rear end. Therefore, in order to keep the longitudinal cooling temperature of the steel plate constant, the sheet passing speed of the steel plate is It is necessary to make the conveyance pattern decelerate as it goes to the end.

5 If the transition from (a) to the state as of FIG. 5 (b), i.e. to the contact 6 enters into the cooling device deceleration point, passing the plate at a constant speed V _1. Then, when the transition from FIG. 5 (b) to the state of Fig. 5 (c), i.e. to the contact 5 enters into the cooling device deceleration point, performs deceleration with alpha ₁ deceleration. In this embodiment, since the plate thickness of the contact 6 and the contact 5 is the same, the deceleration alpha ₁ becomes zero, the steel sheet speed at the time of the contact 5 enters into the cooling device deceleration point becomes V _1.

Next, when transitioning from the state shown in FIG. 5C to the state shown in FIG. 5D, that is, until the contact 4 enters the acceleration / deceleration point in the cooling device, the vehicle is decelerated at the deceleration α2. Figure 5 (d) state, i.e. the contact 4 is the speed entering into the cooling device deceleration point becomes V _2. In this way, cooling is performed by sequentially changing the steel plate speed in the same manner until the final contact 1 enters the acceleration / deceleration point in the cooling device, as shown in FIG.

Also, when the transition from FIG. 5 (h) to the state of FIG. 5 (i), i.e. the contact 1 of the steel sheet rear end from entering into the cooling device deceleration point, the transport contacts 1 at a constant speed V ₃ Let As a result, the speed pattern of the steel plate becomes as shown in FIG. 6, and the steel plate having a uniform temperature distribution in the longitudinal direction can be manufactured by performing sequential deceleration when the steel plate contact enters the acceleration / deceleration point in the cooling device. It becomes.

  Hereinafter, a method for obtaining the above-described steel plate speed (intrusion speed, conveyance speed) will be described. This is calculated by performing a convergence calculation by numerical simulation so that the temperature after completion of cooling falls within a predetermined allowable error range at each contact point of the steel plate.

  The reason for the calculation by numerical simulation is that, unlike the analytical solution, it is easy to consider the temperature dependence of the thermophysical property value of the steel material, the cooling after leaving the cooling device, and the time until it reaches the thermometer. The simulation method is also described in the “Japan Steel Association Rolling Theory and Practice” in addition to the above-mentioned Japan Iron and Steel Institute's “Forced Cooling of Steel” and is well known. The description is omitted.

The convergence calculation method is as follows. At each contact point of the steel sheet, first determine an appropriate initial sheet passing speed _Vold , determine the acceleration or deceleration α _i between the contact points, and the steel sheet is transported from the cooling device entry side to the thermometer installation position. The heat transfer calculation is performed for the time that is Next, from the cooling start temperature Ts, the target cooling end temperature T _e , the initial plate passing speed V _old , and the calculated cooling end temperature T _c

Become calculate with, seek the modification through plate velocity _{V new.} 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 described above, an appropriate method may be selected according to the processing time and the stability of calculation.

Hereinafter, a method for determining acceleration and deceleration will be described. Assuming that the approach speeds at adjacent contacts are V _i and V _{i + 1} and the distance between the contacts is L _i , the deceleration α _i between the dividing points is obtained as follows.

Therefore, the steel sheet enters the cooling device at the steel sheet tip approach speed V ₁ , and the acceleration or deceleration between the contacts can be obtained from the speed of each contact obtained by convergence calculation from Equation (1).

  In this way, by controlling the plate feed speed, for example, FIGS. 1B, 1C, 1D, 1E, 1G, 1H, 1I, 1J, and 1K are used. In the case of a steel plate in which there is a portion where the plate thickness in the longitudinal direction becomes a parallel portion in a part of the steel plate as in), the acceleration or deceleration may be treated as zero (that is, not accelerated or decelerated) in the parallel portion. It is possible to calculate the speed of steel plates having various shapes with the same control program as when there is a longitudinal thickness gradient.

  The steel sheet may enter the cooling device from the right side in FIG. 1 or from the left side. However, when the plate thickness is successively increased in the longitudinal direction as in FIGS. 1A, 1B, 1G, 1H, etc., it is preferable to first enter the cooling device from the thin portion side. . This is because, in the present invention, when passing through the cooling device from the thick part side in order to perform the passage cooling, the approach timing of the rear end of the steel sheet is delayed compared to the front end, and therefore the intrusion temperature of the rear end of the steel sheet is intrusion of the front end temperature. This is because there is a risk that the temperature at the tail end of the steel sheet becomes lower than the temperature and the temperature at the tail end of the steel sheet becomes lower than a predetermined cooling start temperature. On the other hand, if a thick part having a slow cooling rate during air cooling enters with a delay, the temperature deviation becomes small at the front and rear ends of the steel plate, and the steel plate longitudinal temperature distribution before insertion of the cooling device can be made more uniform.

  Moreover, it is preferable to make the cooling zone length which performs water flow shorter than the minimum value of the distance between the contacts of a steel plate. For example, an LP steel plate having a cross-sectional shape as shown in FIG. 3 and a distance between contact points L1 to L5 of 2 m is passed through all 10 zones as shown in FIG. Consider the case of cooling in a shorter state. The acceleration / deceleration point is the center point of these 10 zones. The steel plate contact 6 enters the cooling device at the time of FIG. 7 (a), and exits the cooling device at the time of FIG. 7 (f) through the states of FIGS. 7 (b) to (e). During this time, since the steel plate contacts 1 to 5 have already entered the cooling device, the influence of the speed of each contact obtained by the convergence calculation is affected by the other contacts and the cooling controllability is lowered.

  As the cooling zone length is shorter than the distance between the contact points of the steel plate, the controllability is improved. However, the water flow rate is reduced by that amount, and in order to secure the cooling end temperature, it is necessary to slow down the plate passing speed. Therefore, there is a possibility that the deviation of the cooling device entering timing at the front and rear ends of the steel plate becomes large and a predetermined cooling start temperature cannot be secured at the tail end. Therefore, if the cooling zone length is set to a value about the minimum value of the distance between the contacts, the temperature controllability is good and the temperature difference between the front and rear ends of the steel sheet can be reduced.

  For this reason, the cooling device is divided into a plurality of zones, and equipment capable of turning on and off the cooling water in each cooling zone is required. In addition, when the calculations described above are performed, the plate passing speed, acceleration, and deceleration may not be within the control range of the transport table roll. In that case, the length of the cooling device is changed to The acceleration and deceleration must be within the control range of the transport table roll. From the above, the controllability is improved by increasing the number of cooling zones as much as possible and shortening the length per zone.

  Moreover, it is preferable that the acceleration / deceleration point of the cooling device is a central portion of a zone through which water is passed in the cooling device. The reason for this will be described with reference to FIGS. FIG. 8 is an example in which the acceleration / deceleration point of the cooling device is on the inlet side of the cooling device. The steel sheet has contacts 1, 2, and 3 from the tip, and enters the cooling device from the contact 1. After the contact 2 enters the acceleration / deceleration point, deceleration is performed to cool the thick contact 3. At this time, since the thickness of the steel plate does not change in the longitudinal direction and is uniform between the contact 1 and the contact 2, the hatched region in the figure is a region that should be passed through at a constant speed. Nevertheless, it is affected by deceleration. This region is the same as the zone length through which the cooling device passes. On the other hand, if the acceleration / deceleration point of the cooling device is the central portion in the longitudinal direction of the cooling zone length through which the cooling device passes water as shown in FIG. 9, the region affected by the deceleration is hatched, and the contact 2 The region includes the rear taper portion. For this reason, in the parallel portion, which is an area that should be passed through at a constant speed, the area affected by the deceleration is narrowed, so that the temperature accuracy in the longitudinal direction can be increased.

  Furthermore, if a draining roll is not installed, the cooling water at the top stays and flows into another cooling zone, and the cooling time cannot be strictly controlled. Therefore, it becomes a cause of material variation.

(Example 1)
The LP steel plate as shown in FIG. 10 was cooled. The right side in the figure was the steel plate tip and entered the cooling device. The plate thickness at the front end of this steel plate is 10 mm, the plate thickness at the tail end is 17.5 mm, and the plate thickness at the longitudinal center of the steel plate is 20 mm, which is thicker than the front and rear ends. The length from the steel plate tip to the thickest part is 4 m, and the length from the steel plate tail to the thickest part is 3 m. This LP steel sheet was cooled using the cooling device (cooling length 10 m, cooling water amount 1000 L / min.m ² ) described in the embodiment of the present invention. The cooling conditions were a cooling start temperature of 770 ° C. and a cooling end temperature of 500 ° C., and cooling was performed using the method of the present invention.

  In addition, in this example and other examples and comparative examples, since the plate passing speed is calculated by convergence calculation, the heat transfer coefficient of the cooling device is obtained in advance offline before the cooling is performed. In Example 1 and Comparative Example 1, the sheet passing speed was determined by numerical calculation using this heat transfer coefficient. In Comparative Examples 2 and 3, the cooling time during water cooling was calculated from this experiment.

  At this time, the proper cooling time when the plate thickness is 10mm at the front end of the steel plate is 2.4sec, the proper cooling time when the plate thickness is 17.5mm at the rear end of the steel plate is 4.8sec, and the proper cooling time when the plate thickness of the steel plate M is 20mm is 5.9sec It became.

  In Example 1, splitting was performed in two splits and three contacts with the steel plate front and rear ends and the position where the plate thickness was 20 mm as the steel plate contact, and the convergence calculation was performed by the method described above. In the calculation, the radiant heat transfer coefficient and the natural stagnation heat transfer coefficient are added between the time from entering the cooling device 4 from the thermometer 5 and the time from reaching the thermometer 6 after leaving the cooling device 4. In the cooling device, the heat transfer coefficient during the water cooling previously obtained was used. The plate passing speed of each contact was obtained by convergence calculation so that the temperature was 770 ° C. at the thermometer 5 position and 550 ° C. at the thermometer 6 position.

As the cooling condition at this time, since the minimum value of the distance between the contacts is 3 m, 3 zones out of 10 zones of the cooling device were used. At this time, the cooling zone length for passing water is 3 m. The acceleration / deceleration point of the cooling device was set at a position of 1.5 m from the cooling device entrance side. As a result of calculating the temperature from the convergence calculation, the plate passing speed at the front end was 80 mpm, the plate passing speed at the contact point with a plate thickness of 20 mm near the longitudinal center was 21 mpm, and the plate passing speed at the rear end of the steel plate was 39 mpm. From contact point 1 to contact point 2, the vehicle was decelerated at a deceleration of 0.208 m / s ² and from contact point 2 to contact point 3 was accelerated at an acceleration of 0.051 m / s ² . As shown in FIG. 11, the temperature distribution in the longitudinal direction of the steel sheet after cooling was substantially uniform in the longitudinal direction, and when the material was investigated later, the entire length was within the specifications.

(Comparative Example 1)
The same steel plate as in Example 1 was passed through the cooling device at a constant speed using the same cooling device as in Example 1. In this case, when the cooling zone length is 3 m and the plate passing speed is obtained by numerical calculation so that the cooling end temperature of 500 ° C. is obtained at the position where the plate thickness is 20 mm in the center in the longitudinal direction of the steel plate, 35 mpm is obtained. It became. Therefore, cooling was performed by passing the plate at a constant speed of 35 mpm. As shown in FIG. 12, the temperature distribution in the longitudinal direction of the steel plate after cooling reached the target cooling end temperature at the center in the longitudinal direction of the steel plate. Was 230 ° C. Later, when the material was investigated, the strength was extremely exceeded at the tip of the steel plate, and it was rejected.

(Comparative Example 2)
The same steel plate as in Example 1 was cooled by the method described in Patent Document 2 using the same cooling device as in Example 1. The cooling zone length was 10 m, and the cooling water was passed simultaneously at the same time after the steel plate tip entered, and the plate was passed at a constant speed of 130 mpm to the position of 20 mm where the plate thickness was the thickest at the longitudinal center. Next, the plate was passed at 199 mpm until the rear end of the steel sheet entered the cooling device, and was passed at 62 mpm after the front end of the steel plate reached the outlet side of the cooling device. As shown in FIG. 13, the temperature distribution in the longitudinal direction of the steel plate after cooling is low due to the occurrence of supercooling in some places where the cooling water is passed simultaneously at the tip of the steel plate, and the temperature deviation in the plate is 120 ° C. became. Later, when the material was investigated, there was a part where the strength was extremely exceeded at the tip of the steel plate, and it was rejected.

(Comparative Example 3)
The same steel plate as in Example 1 was cooled by the method described in Patent Document 3 using the same cooling device as in Example 1. When estimating the cooling zone length and the sheet passing speed, which are the conditions for appropriate cooling time at the respective positions of the front end, the rear end, and the central part of the steel plate, the zone in FIG. 19 is 2 m for the S1 zone, 2 m for the S2 zone, The S3 zone was 6 m and the plate feed speed was 100 mpm. When cooled by the procedure shown in Patent Document 3 under this condition, the temperature distribution in the longitudinal direction of the steel plate after cooling was uneven in the entire steel plate as shown in FIG. This is because, as described above, supercooling occurred at the site where the cooling water was injected into the steel plate passage and at the site where the cooling water was stopped. Later, when the material was investigated, it was rejected because there was a part that was extremely strong.

(Example 2)
The LP steel plate as shown in FIG. 15 was cooled. The right side in the figure was entered into the cooling device with the steel plate tip. There are three parallel parts, and the shape gradually increases from the tip of the steel plate. The plate thickness at the tip is 10 mm, the plate thickness at the tail end is 17.5 mm, and the plate thickness of the parallel part at the longitudinal center of the steel plate is 15 mm. The length of the parallel portion is 2.5 m at the front end of the steel plate, 2 m at the longitudinal center of the steel plate, and 4 m at the tail end of the steel plate. Moreover, between the parallel part of the steel plate front end and the parallel part of the steel plate longitudinal center and between the parallel part of the steel plate tail end and the parallel part of the steel plate longitudinal direction center is a taper shape, and the length is respectively It is 2m and 3.5m.

The LP steel sheet was cooled using the cooling device (cooling length 10 m, cooling water amount 1000 L / min.m ² ) described in the embodiment of the present invention. The cooling conditions were a cooling start temperature of 770 ° C., a cooling end temperature of 500 ° C., and a cooling zone length of 2 m. As in Example 1, the numerical calculation of the plate passing speed was performed based on the heat transfer coefficient obtained offline in advance, and the plate passing speed at each contact point was obtained.

In this embodiment, since the leading end is thin and the trailing end is thick, the control is performed to sequentially decelerate the steel plate speed as shown in FIG. The steel sheet was divided into 5 and 6 contacts as the contacts as shown in FIG. 3, and the convergence calculation was performed by the method described above. As the cooling condition at this time, since the minimum value of the distance between the contacts is 2 m, only 2 zones out of 10 zones of the cooling device were used. The cooling zone length for passing water at this time is 2 m. The acceleration / deceleration point of the cooling device was set at a position of 1 m from the cooling device entrance side. As a result of calculating the temperature from the convergence calculation, the respective contact speeds were 61 mpm, 61 mpm, 31 mpm, 31 mpm, 28 mpm, and 28 mpm from the tip. In this case, the deceleration of the contacts are each a steel plate front ^{end, 0m / s 2, 0.19m /} s 2, 0m / s 2, a ^{0.0084m / s 2, 0m / s} 2. As shown in FIG. 16, the temperature distribution in the longitudinal direction of the steel plate after cooling was substantially uniform in the longitudinal direction, and when the material was examined later, the entire length was within the specifications.

(Comparative Example 4)
Using the same steel plate as in Example 2, using the same cooling device as in Example 2, the position of the portion where the cooling zone length is 2 m and the thickness at the center in the longitudinal direction of the steel plate is 15 mm, as in Example 2. Then, when the plate passing speed was calculated by numerical calculation so that the cooling end temperature was 550 ° C., it was 32 mpm. Therefore, cooling was carried out by passing the plate at a constant speed of 32 mpm. As shown in FIG. 16, the temperature distribution in the longitudinal direction of the steel plate after cooling was almost the target cooling end temperature at the longitudinal center of the steel plate. The deviation was 170 ° C. Later, when the material was investigated, the strength was extremely exceeded at the tip of the steel plate, and it was rejected.

It is a figure explaining the example of the shape of the LP steel plate which can apply this invention. It is a figure explaining the example of the cooling device used for embodiment of this invention. It is a figure explaining the contact division | segmentation of the steel plate in embodiment of this invention. It is a figure explaining the method of determining the contact division position of the steel plate in embodiment of this invention. It is a figure explaining the speed control method of the steel plate in embodiment of this invention. It is a figure which shows the example of the speed pattern of the steel plate in embodiment of this invention. In embodiment of this invention, it is a figure explaining the reason which controllability becomes low when cooling zone length is long. In embodiment of this invention, a steel plate is a figure explaining temperature control property in case the acceleration / deceleration point in a cooling device exists in a cooling device entrance side. In embodiment of this invention, it is a figure explaining temperature control property in case the acceleration / deceleration point in a cooling device exists in the center part of the cooling zone longitudinal direction which a cooling device uses in embodiment of this invention. It is a figure which shows the shape of the LP steel plate cooled in Example 1 and Comparative Examples 1-3. It is a figure which shows the longitudinal direction temperature distribution after cooling in Example 1. FIG. It is a figure which shows the longitudinal direction temperature distribution after the cooling in the comparative example 1. It is a figure which shows the longitudinal direction temperature distribution after the cooling in the comparative example 2. It is a figure which shows the longitudinal direction temperature distribution after the cooling in the comparative example 3. It is a figure which shows the shape of the LP steel plate cooled in Example 2 and Comparative Example 4. It is a figure which shows the longitudinal direction temperature distribution after cooling in Example 2 and Comparative Example 4. It is a figure explaining the cooling control method of patent documents 2. FIG. It is a figure explaining the cooling device of patent documents 3. It is a figure explaining the cooling control method of patent documents 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Rolling mill, 2 ... Steel plate, 3 ... Conveying table, 4 ... Cooling device, 5 ... Incoming thermometer, 6 ... Outlet thermometer, 7 ... Cooling header, 8 ... Draining roll,

Claims (4)

When the LP steel sheet whose thickness changes in the longitudinal direction is passed and cooled by passing through a water-cooled cooling device having a plurality of cooling zones, before the cooling of the LP steel plate, The cooling water flow rate in each cooling zone is determined, and during the cooling of the LP steel plate, cooling is performed without changing the cooling water flow rate in the cooling zone to be used and each cooling zone, Before the cooling, set the dividing point at the inflection point of the longitudinal thickness of the steel sheet in the longitudinal direction and the thickness gradient in the longitudinal direction at the front and rear ends, while providing a reference position between the inlet side and the outlet side of the cooling device. From the time when the dividing point reaches the reference position to the time when the next dividing point reaches the reference position, the plate speed is accelerated or decelerated at a constant acceleration or constant deceleration. Initial speed and above Speed and deceleration, the dividing point, the cooling method of hot steel plate characterized by determining that such is cooled to a temperature within the allowable cooling temperatures range while passing through the cooling device. The initial passage speed, the acceleration, and the deceleration are obtained by convergence calculation so that the dividing point is cooled to a temperature within an allowable cooling temperature range while passing through the cooling device. Item 2. The method for cooling a high-temperature steel sheet according to Item 1. The method for cooling a high-temperature steel sheet according to claim 1 or 2, wherein the reference position is a center position in a longitudinal direction of a cooling zone used by the cooling device. The method for cooling a high-temperature steel sheet according to any one of claims 1 to 3, wherein a continuous length of a cooling zone to be used is set shorter than a length between the division points.

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