[0001] The present invention relates to a take-up cooling control device controlling a take-up cooling device included in a hot rolling line and to a take-up cooling control method.
BACKGROUND ART [0002] In recent years, the quality of steel plates is being heightened as in the case of dual phase (DP) steel and transformation induced plasticity (TRIP) steel. Generally speaking, as is known in the art, in the rolling of DP steel and TRIP steel, the holding time at an intermediate temperature halfway through the cooling between the cooling start and the cooling completion (hereinafter referred to as the intermediate air cooling time) greatly influences the volume fraction of the ferrite phase. Thus, it is necessary for the intermediate air cooling time to be controlled to a certain time range. The quality of the steel plate deteriorates if the intermediate air cooling time is shorter or longer than the time range. In view of this, in the cooling control of the steel plate, not only the take-up temperature but also the intermediate temperature is matched with the target temperature. Further, control is performed to provide the intermediate air cooling time during

which the steel plate temperature is held in the vicinity of the intermediate temperature for a fixed period of time.
[0003] Patent Document 1 discloses an example of a cooling device enabling cooling control of such a steel plate. According to the control method, at least the temperature of the rolled material, the water cooling rate, and the air cooling time are used as the control amounts. With respect to each of these control amounts, the priority order and the permissible value are determined, and target value correcting calculation is performed in accordance with the priority order and so as to satisfy the permissible value.
[0004] Patent Document 2 discloses an example of a take-up cooling control device equipped with water cooling prohibition header calculation means which calculates the respective header patterns of upstream cooling equipment and downstream cooling equipment with an intermediate thermometer therebetween and which specifies the header in the vicinity of the intermediate thermometer suppressing the opening operation. In this take-up cooling control device, the opening operation of the header in the vicinity of the intermediate thermometer is suppressed, whereby control is performed such that the intermediate air cooling time is within the target range.
PRIOR ART DOCUMENTS Patent Documents
[0005] Patent Document 1: Japanese Patent Application

Publication No. 2007-268540
Patent Document 2: Japanese Patent Application Publication No.
2015-54322
SUMMARY OF THE INVENTION
[0006] Although the above related-art techniques allow control taking into account the cooling temperature, pattern, and intermediate temperature of the steel plate, it is to be assumed they have problems as follows, from the point of view of an improvement in terms of accuracy in temperature control and observance of the intermediate air cooling time. [0007] While Patent Document 1 discloses the performance of the target value correcting calculation in accordance with a given priority order and so as to satisfy the permissible value, it discloses no method of determining the priority order and the permissible value between the control amounts including the temperature of the rolled material, water cooling rate, and air cooling time. To determine the priority order of the control amounts and the permissible value, it is necessary to examine the quality of the steel plate when one or a plurality of the control amounts are varied within or out of the permissible value with respect to an enormous number of combinations of the control amounts.
[0008] While Patent Document 2 discloses a method of performing pre-set control such that the intermediate air cooling time is within the target range, the relationship among

the priority order of the cooling headers, the priority order of the water cooling prohibition flags, and the volume fraction of the ferrite phase are not clarified. Thus, in the case, for example, where the steel plate rate is changed, there remains a problem to be solved regarding the cooling control method for obtaining a desired ferrite phase volume fraction.
[0009] Generally speaking, the rolling of a steel plate is started at low rate, and, thereafter, the rolling is continued at a fixed maximum rate. The steel plate being rolled is reduced in rate toward the completion of the rolling when the rear end portion is approached before being driven out of the mill at low rate. In this way, the rolling rate of the steel plate is changed, so that the time between the moment when the steel plate is driven out of the mill and the moment when the position of the intermediate thermometer is reached varies depending upon the longitudinal portion of the steel plate. Thus, even if the cooling rate is controlled to a fixed level, there is the possibility of the volume fraction of the ferrite phase generated through ferrite transformation being not fixed.
[0010] It is an object of the present invention to provide a take-up cooling control device and a take-up cooling control method realizing a target take-up temperature for a metal plate such as a steel plate manufactured by a hot rolling line and allowing equalization of the volume fraction of at least one transformed phase of the metal plate between the longitudinal

plate portions.
[0011] To achieve the above object of the invention, there is provided, in accordance with the present invention, a take-up cooling control device controlling a take-up cooling device equipped with a plurality of cooling headers emitting cooling water to a rolled material rolled by a hot rolling mill and taken-up by a down coiler, including: a target temperature history calculation unit which, with respect to each section of the rolled material obtained through division of the rolled material by a predetermined length in the longitudinal direction, calculates a target temperature history in a case where the each section undergoes a change between a moment when the each section is delivered from the hot rolling mill and a moment when each section moves to the down coiler such that the volume fraction of at least one transformed phase of the rolled material is substantially fixed between the sections; a cooling command calculation unit calculating for the each section a cooling command for the each cooling header, the cooling command causing a temperature at which the each section is cooled by the take-up cooling device to coincide with the calculated target temperature history; and a header pattern output unit that calculates an opening/closing pattern of the each cooling header for each predetermined period of time based on the cooling command for the each cooling header calculated for the each section and outputs the calculated opening/closing pattern to

the take-up cooling device.
[0012] According to the present invention, it is possible to realize a target take-up temperature for a metal plate such as a steel plate manufactured by a hot rolling line and to equalize the volume fraction of at least one transformed phase of the metal plate between the longitudinal plate portions. Thus, according to the present invention, it is possible to improve the production quality of a metal plate such as a steel plate manufactured by a hot rolling line.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Fig. 1 is a diagram illustrating an example of the
construction of a take-up cooling control device according to
an embodiment of the present invention and of an object of
control;
Fig. 2 is a diagram showing an example of the chemical
composition of a steel plate constituting the object of rolling;
Fig. 3 is a diagram schematically illustrating an example of
a phase transformation start condition;
Fig. 4 is a diagram showing an example of an isothermal
transformation rate coefficient table with respect to phase
transformation start carbon concentration CFT of ferrite
transformation;
Fig. 5 is a graph showing the time tF, target when the ferrite phase
volume fraction XF attains the target ferrite volume fraction
XF, target at each temperature obtained by dividing the section

between a plurality of temperatures at predetermined intervals;
Fig. 6 is a diagram showing an example of the processing flow
to be executed by the target temperature history calculation
unit;
Fig. 7 is a diagram showing an example of the target temperature
history at each steel plate rate Vi < V2 < V3 < V4 obtained in
a comparative example (related-art technique);
Fig. 8 is a diagram showing an example of the target temperature
history at each steel plate rate Vi < V2 < V3 < V4 obtained in
an embodiment of the present invention;
Fig. 9 is a diagram showing the ferrite phase volume fraction
of a hot-rolled DP steel manufactured based on the comparative
example (related-art technique);
Fig. 10 is a diagram showing the ferrite phase volume fraction
of a hot-rolled DP steel manufactured based on the embodiment
of the present invention; and
Fig. 11 is a diagram showing the ferrite average crystal grain
size of the hot-rolled DP steel of the embodiment of the present
invention as compared with the comparative example (related-art
technique).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] In the following, an embodiment of the present
invention will be described in detail with reference to the
drawings. In the drawings, the same components are indicated
by the same reference characters, and a redundant description

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thereof will be left out.
[Outline of the Construction of a Take-Up Cooling Control Device 100]
[0015] Fig. 1 is a diagram illustrating an example of the construction of a take-up cooling control device 100 according to an embodiment of the present invention and of an object of control 150. As shown in Fig. 1, the take-up cooling control device 100 receives various signals (actual values of the steel plate rate, take-up temperature, etc.) from the object of control 150, and outputs control signals in accordance with the actual values to the object of control 150.
[0016] Here, the construction of the object of control 150 will be described first. In the case of the present embodiment, the main component of the object of control 150 is a take-up cooling device 160 in a hot rolling system. The take-up cooling device 160 is arranged between a hot rolling mill 152 and a down coiler 154, and cools a steel plate 151 at a temperature of approximately 850°C to 900°C rolled by a mill 153 of the hot rolling mill 152. The down coiler 154 takes up the steel plate 151 cooled by the take-up cooling device 160. While in the present embodiment the rolled material rolled by the hot rolling mill 152 is a steel plate the main component of which is iron such as DP steel and TRIP steel, the rolled material is not restricted to a steel plate. [0017] The take-up cooling device 160 is composed of an upper

9
cooling device 161 water-cooling the steel plate 151 from the upper side and a lower cooling device 162 water-cooling the steel plate 151 from the lower side. The upper cooling device 161 and the lower cooling device 162 have a plurality of cooling headers 163 emitting cooling water and respectively arranged at upper and lower positions with the steel plate 151 therebetween, along the longitudinal direction of the steel plate 151. Each cooling header 163 is formed by a large number of nozzles arranged along the width direction of the steel plate 151.
[0018] The plurality of cooling headers 163 arranged along the longitudinal direction of the steel plate 151 are divided into sections each having a predetermined headers. Each section of the plurality of cooling headers 163 is referred to as a bank 164. Here, the banks 164 arranged along the longitudinal direction of the steel plate 151 on the mill 153 side will be referred to as a front side bank group 165. Similarly, the banks 164 arranged along the longitudinal direction of the steel plate 151 at the middle portion will be referred to as an intermediate bank group 166, and the banks 164 arranged on the down coiler 154 side will be referred to as a rear side bank group 167.
[0019] Further, in the object of control 150, there are provided, in order to detect the temperature of the steel plate 151 under cooling control, measuring instruments such as a

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finishing delivery side thermometer 170, an intermediate thermometer 171, and a take-up thermometer 172. The finishing delivery side thermometer 170 measures the temperature of the steel plate 151 immediately after the rolling at the hot rolling mill 152. The intermediate thermometer 171 installed in the vicinity of the middle portion of the take-up cooling device 160 measures the temperature of the steel plate 151 passing the installation position thereof. The take-up thermometer 172 measures the temperature of the steel plate 151 immediately before the taking-up by the down coiler 154.
[0020] Further, the construction of the take-up cooling control device 100 will be described with reference to Fig. 1. In the present embodiment, the object of the take-up cooling control executed by the take-up cooling control device 100 is to cause the temperature of the steel plate 151 measured by the take-up thermometer 172 to coincide with the target take-up temperature, and to cause the ferrite volume fraction of the steel plate 151 to coincide with the target volume fraction. To achieve this object of the control, the take-up cooling control device 100 outputs an opening or closing operation command to each cooling headers 163 constituting the take-up cooling device 160. In the present embodiment, the opening or closing operation command is a command indicating whether or not cooling water is to be emitted from the cooling headers 163. It may be a command indicating the amount of cooling water

11
emitted.
[0021] The take-up cooling control device 100 consists of an ordinary computer equipped at least with a processing unit 110 and a storage unit 101. Here, the processing unit 110 includes function blocks such as a phase transformation start condition calculation unit 111, a holding condition calculation unit 112, a plate temperature estimation unit 113, a steel plate rate pattern correction unit 114, a target temperature history calculation unit 120, a cooling command calculation unit 130, and a header pattern output unit 140. Further, the storage unit 101 stores various items of control information such as a target finishing delivery temperature, a target take-up temperature steel plate rate pattern, an isothermal transformation rate coefficient table, a phase transformation start condition table, a target volume fraction, and steel plate chemical composition data.
[0022] The take-up cooling control device 100 constructed as described above is realized by an ordinary computer equipped with a central processing unit (CPU) and a storage device (semiconductor memory, magnetic hard disk device, etc.) and configured to execute various computation processing operations and control processing operations. In this case, the function of each function block constituting the processing unit 110 is realized by causing the CPU to execute a predetermined program stored in the storage device. Further,

12
the storage unit 101 is realized by storing predetermined data in a storage region allotted to a part of the storage device. [0023] In the present embodiment, control information designated by the user is a target finishing delivery temperature TF, target take-up temperature TC, target phase volume fraction of ferrite or the like, and steel plate rate pattern. These items of control information may be directly input by the user through an input device belonging to the take-up cooling control device 100, or may be input from a host computer 50 via a network.
[0024] In the following, each function block constituting the processing unit 110 of the take-up cooling control device 100 will be described in detail with reference also to from Fig. 2 onward.
[Phase Transformation Start Condition Calculation Unit 111] [0025] The phase transformation start condition calculation unit 111 obtains the phase transformation start carbon concentration CFT and CMT and the phase transformation start temperatures TFT and TMT with respect to ferrite and martensite for realizing the target phase volume fraction χF of the ferrite phase based on the phase transformation start condition table, the target phase volume fraction, and the steel plate chemical composition data stored in the storage unit 101. [0026] Referring to the storage unit 101, the phase transformation start condition calculation unit 111 first

13
obtains a phase transformation start condition corresponding to the chemical composition of the steel plate 151 to be rolled. Fig. 2 is a diagram showing an example of the chemical composition of the steel plate 151 to be rolled. In Fig. 2, the data of each column of each line represents the content of an element contained in the steel plate 151 identified by “Slub #.” The “Slub #” of the steel plate 151 is designated by the user before its rolling is started.
[0027] The phase transformation start condition is expressed as the relationship between carbon concentration C and temperature T for each transformation type (TRF_TYPE). In the present specification, this is expressed as {TRF_TYPE, C, T}. Fig. 3 is a diagram schematically illustrating an example of the phase transformation start condition. In Fig. 3, the phase transformation start condition is shown in a graph, in which the horizontal axis indicates carbon concentration C and in which the vertical axis indicates temperature T, as the relationship between carbon concentration C and temperature T for each of four kinds of phase transformation types TRF_TYPE. Here, characters Tf(C), Tp(C), Tb(C), and Tm(C) respectively represent the phase transformation start condition for ferrite transformation, pearlite transformation, bainite
transformation, and martensite transformation. While in Fig. 3 the phase transformation start conditions are depicted as linear lines for the sake of simplification, they are generally

14
to be expressed as curves.
[0028] The phase transformation start condition table {TRF_TYPE, C, T} can be calculated by using, for example, the well-known calculation of phase diagram (CALPHAD) method. In this case, it is possible to attain a more accurate result by adding the dislocation density energy of the austenite phase, which depends on the rolling condition. In the present embodiment, the phase transformation start condition {TRF_TYPE, C, T} is stored as a phase transformation start condition table in the storage unit 101. However, the program of the CALPHAD method may be mounted.
[0029] Subsequently, the phase transformation start condition calculation unit 111 calculates the carbon concentration at the time of phase transformation start based on the target phase volume fraction χF of the ferrite phase input by the user beforehand. For example, in the case of a DP steel consisting of the ferrite phase and the martensite phase, the carbon concentration CFT when starting the transformation from the austenite phase to the ferrite phase can be calculated by using the following equation (1):
CFT = C0 (1), where C0 is the carbon concentration of the steel plate 151. [0030] The carbon concentration CMT when starting the transformation from the austenite phase to the martensite phase can be calculated by using the following equation (2):

15
CMT = (C0 - XF × CF)/(1 - XF) (2), where CF is the carbon concentration of the ferrite phase and XF is the phase volume fraction of the ferrite phase.
[0031] Further, the phase transformation start condition calculation unit 111 obtains a temperature corresponding to the carbon concentration at the time of phase transformation start from the phase transformation start condition {TRFTYPE, C, T}. In the example of the DP steel, the temperature corresponding to the condition: TRFTYPE=FT (ferrite transformation) and CCFT is regarded as the ferrite transformation start temperature TFT, and the temperature corresponding to the condition: TRFTYPE=MT
(martensite transformation) and CCMT is regarded as the martensite transformation start temperature TMT. Here, the symbol “~” means interpolation. As the interpolation method, it is possible to use Lagrangian interpolation. Apart from this, a plurality of kinds of interpolation including linear interpolation are known.
[0032] The phase transformation start condition calculation unit 111 outputs the phase transformation start carbon concentration CFT, CMT and the phase transformation start temperature TFT, TMT for realizing the ferrite phase target phase volume fraction XF.
[Holding Condition Calculation Unit 112]
[0033] To hold the temperature of the steel plate 151 at a fixed temperature during cooling for a fixed period of time,

16
the holding condition calculation unit 112 calculates the holding temperature and time as the holding temperature TH and the holding time AH. Here, the holding temperature TH and the holding time AH correspond to the intermediate temperature and the intermediate air cooling time as generally referred to.
[0034] The holding condition calculation unit 112 first refers to the isothermal transformation rate coefficient table stored in the storage unit 101, and obtains the isothermal transformation rate coefficients (hereinafter referred to as time-temperature transformation (TTT) rate coefficients) n and k in the case of the chemical composition corresponding to the phase transformation start carbon concentration CFT, CMT . Further, the holding condition calculation unit 112 calculates the TTT rate by using the TTT rate coefficients n and k. The TTT rate can be calculated by using, for example, the following equation (3) known as Johnson-Mehl-Avrami-Kolmogorov (JMAK) model:
DxF/dt = n × k × t(n-1) × XA (3), where xA is the volume fraction of the austenite phase, and n and k are TTT rate coefficients.
[0035] Fig. 4 is a diagram showing an example of the isothermal transformation rate coefficient table with respect to the phase transformation start carbon concentration CFT of the ferrite transformation. As shown in Fig. 4, in the present embodiment, the rate coefficient table is expressed as

17
{Temperature T, n, k}, using the JMAK model.
[0036] The TTT rate coefficient can be prepared, for example, through regression of the data of the TTT chart (TTT diagram) obtained from a phase transformation experiment. Further, it can be calculated by using a phase transformation rate calculation model. An example of the phase transformation rate calculation model is on pp. 423-432 of ISIJ International Vol. 32 (1992).
[0037] Referring to Fig. 5, a method of obtaining the holding temperature TH and the holding time ∆H by utilizing the isothermal transformation rate coefficient table will be described. Fig. 5 is a graph showing the time tF, target at which the volume fraction χF of the ferrite phase attains the target ferrite volume fraction χF, target through time integration of equation (3) by a plurality of temperatures obtained, for example, through division of the interval between TFT and TMT into sections each being 5°C. The temperature tx at which the time tF, target obtained by this graph is shortest is selected, and used as a nose temperature Tnose. Then, the time tF, target at the nose temperature Tnose is obtained as tx. The holding condition calculation unit 112 outputs Tnose and tx thus obtained respectively as the holding temperature TH and the holding time ∆H.
[Plate Temperature Estimation Unit 113] [0038] The plate temperature estimation unit 113 calculates

18
the temperature change in each section of the steel plate 151 moving at a steel plate velocity V. In the following example, the temperature change during the movement of one section of the steel plate 151 from the installation position of a finishing delivery side thermometer 170 to the installation position of a take-up thermometer 172 is differentiated while advancing the time by a fixed period of time ∆. The temperature change calculated may be any of the temperature change from the delivery side of the mill 153 to the installation position of the take-up thermometer 172, the temperature change from the installation position of the finishing delivery side thermometer 170 to the down coiler 154, and the temperature change from the delivery side of the mill 153 to the down coiler 154.
[0039] Here, the position at the current time of the section of the steel plate 151 constituting the object of the temperature change calculation is expressed by the distance Ln from the installation position of the finishing delivery side thermometer 170, and the plate temperature estimation unit 113 calculates this distance Ln from the following equation (4):
Ln = Ln-1 + ∆ × V (4), where Ln is the current distance (m), Ln-1 is the distance (m) prior to the current moment by ∆, and ∆ is divisional calculation time (s) in plate temperature estimating calculation. [0040] Next, the plate temperature estimation unit 113 determines the operation of the cooling headers 163 at the

19
distance Ln from a previously set header pattern (information designating whether or not cooling water is to be emitted from the cooling headers 163), and calculates the surface heat flux of the steel plate 151.
[0041] Here, when the cooling headers 163 are performing the water cooling operation in which they emit cooling water, the surface heat flux qw thereof can be calculated, for example, by the following equation (5):
qw = 9.72 × 105 × Q0.355 × {(2.5 - 1.15 × logTw) × D/(pl × pc)}0.646 (5),
where o is a water amount density (L/m2/s), Tw is a water temperature (°C) , pl is a nozzle pitch (m) in the line direction, and pc is a nozzle pitch (m) in a direction orthogonal to the line.
[0042] On the other hand, when the cooling headers 163 are not performing the water cooling operation, the surface heat flux qr can be calculated by the following equation (6):
qr = a × e × [(273 + Tsu) 4 - (273 + Ta)4] (6), where a is Stefan-Boltzmann constant (W/m2/K4), e is an emissivity, Ta is an air temperature (°C), and Tsu is a surface temperature (°C) of the steel plate.
[0043] The plate temperature estimation unit 113 calculates the surface heat flux at the upper surface and the lower surface of the steel plate 151 with using the equation (5) or (6), and quantifies the heat transfer amount at each steel plate surface.

20
Then, based on the temperature before the elapse of the calculation divisional time A, the heat amount transferred during the time A is added or subtracted, whereby the temperature of the section concerned of the steel plate 151 is calculated. [0044] Here, in the case where the heat conduction in the thickness direction of the steel plate 151 is to be neglected, the temperature of the section concerned of the steel plate 151 can be calculated by using the following equation (7):
Tn = Tn-1 - (qt + qb) × A/(p × C × B) (7), where Tn-1 is a plate temperature (°C) before the time A elapses, qt is a heat flux (W/m2) at the upper surface of the steel plate, qb is a heat flux (W/m2) at the lower surface of the steel plate, p is a density (kg/m3) of the steel plate, C is specific heat (J/kg/K) of the steel plate, and B is a thickness (m) of the steel plate.
[0045] In the case where the heat conduction in the thickness direction of the steel plate 151 is taken into consideration, the well-known heat conduction equation is solved, whereby it is possible to calculate the temperature in the thickness direction of the steel plate 151. The heat conduction equation is given by the following equation (8). Various documents disclose the method of dividing this equation (8) in the thickness direction of the steel plate 151 and performing differentiation thereon by a computer.
dT/dt = {A/(p × C)} × (32T/dx2) (8),

21
where λ is a heat conductivity of the steel plate, T is inner temperature of the steel plate, and X is position in the thickness direction.
[Steel Plate Rate Pattern Correction Unit 114]
[0046] The steel plate rate pattern correction unit 114 corrects and outputs the maximum rate in the case of the steel plate rate pattern as designated by the user. For this purpose, the steel plate rate pattern correction unit 114 contains a steel plate rate upper limit calculation unit 1141.
[0047] Generally speaking, the rate of the steel plate 151 varies with the progress of the rolling. The forward end portion of the steel plate 151 travels within the take-up cooling device 160 in a non-tension state due to the pushing-out of the mill 153 at the rear. Thus, when its rate is high, the steel plate 151 is raised from the conveyance portion and is likely to cause a take-up error at the down coiler 154. Further, the rear end portion of the steel plate 151 also travels within the take-up cooling device 160 in a non-tension state due to the taking-up of the down coiler 154 at the front side. Thus, when its movement rate is high, the steel plate 151 vertically undulates and is likely to cause a take-up error at the down coiler 154. To prevent such an error, it is common practice to lower the steel plate rate at the forward end and the rear end of the steel plate 151. [0048] On the other hand, regarding most of the steel plate

22
151 except for the forward end portion and the rear end portion, the rate of the taking-up by the down coiler 154 and that of the pushing-out by the mill 153 are adjusted to control the tension applied to the steel plate, whereby it is possible to suppress a traveling error within the take-up cooling device 160. Thus, in order to increase the production amount of the steel plate 151 per unit time, control is performed to increase the rate of the steel plate in most of the steel plate 151. Also from the viewpoint of enhancing the temperature uniformity in the longitudinal direction of the steel plate 151, it is advantageous to increase the steel plate rate and to shorten the rolling time.
[0049] The steel plate rate upper limit calculation unit 1141 calculates the steel plate rate upper limit when the cooling rate CRFH from the finishing delivery temperature TF to the holding temperature TH and the cooling rate CRHC from the holding temperature TH the take-up temperature TC are set to the maximum possible cooling rate that is possible in the take-up cooling device 160.
[0050] The time tIMT it takes for the section of the steel plate 151 moving at the steel plate velocity V to move from the installation position of the finishing delivery side thermometer 170 to the intermediate thermometer 171 can be calculated by using the following equation (9): tIMT = LIMT/V (9),

23
where LIMT is a distance from the installation position of the finishing delivery side thermometer 170 to the installation position of the intermediate thermometer 171.
[0051] To perform dynamic control on the cooling headers 163 belonging to the front side bank group 165 by using the measurement temperature of the intermediate thermometer 171 such that the steel plate temperature measured by the intermediate thermometer 171 coincides with the holding temperature TH, the following inequality must be satisfied: AFR + AFH < tIMT < AFR + AFH + AH (10) .
[0052] Here, AFR is the time that elapses until the section having left the finishing delivery side thermometer 170 enters the take-up cooling device 160, and AFH is the requisite time for the cooling from the finishing delivery temperature TF to the holding temperature TH. They can be respectively calculated by the following equations (11) and (12):
AFR = LFR/V (11) , where LFR is a distance from the installation position of the finishing delivery side thermometer 170 to the installation position of the first cooling header 163 of the take-up cooling device 160, and
AFH = (TF - TH)/CRFH (12) .
[0053] Similarly, to perform dynamic control on the cooling header 163 belonging to the rear side bank group 167 by using the measurement temperature of the take-up thermometer 172 such

24
that the steel plate temperature measured by the take-up thermometer 172 coincide with TC, the following inequality must be satisfied:
AFH + AH + AHC + ARC < tCT (13) . [0054] Here, ARC is the time that elapses until the section having left the take-up cooling device 160 reaches the position of the take-up thermometer 172, and AHC is the requisite time for the cooling from TH to TC. They can be respectively calculated by the following equations (14) and (15):
ARC = LRC/V (14) , where LRC is a distance from the last header of the take-up cooling device 160 to the take-up thermometer 172, and
AHC = (TH - TC)/CRHC (15) . [0055] Further, tCT is the time that elapses until the section reaches the position of the take-up thermometer 172 from the position of the finishing delivery side thermometer 170. It can be calculated by the following equation (16):
tCT = LCT/V (16) , where LCT is a distance from the finishing delivery side thermometer 170 to the take-up thermometer 172. [0056] The temperature change during air cooling is smaller than that during water cooling. Thus, in the above equations (11) through (16), the temperature change other than that during water cooling is neglected. To take into consideration the temperature change during air cooling, TF of equation (12) and

25
TC of equation (15) are corrected taking into consideration the temperature change during air cooling. On the other hand, TH of equations (12) and (15) need not be corrected in particular since the latent heat generated with the progress of the ferrite transformation and the air cooling offset each other.
[0057] The two inequalities obtained as described above (10) and (13) are adjusted with respect to the steel plate velocity to obtain the following equation for determining the steel plate rate upper limit (Upper Bound velocity) VUB:
VUB = a × Min[(LIMT - LFR)/AFH, (LCT " LFR - LRC)/(AFH + AH + AHC)] (17) , where a is a safety factor (0 < a < 1).
[0058] As described above, CRFH and CRHC are regarded as the maximum cooling speed that can be realized by the take-up cooling device 160, and the steel plate rate upper limit calculation unit 1141 calculates the steel plat rate upper limit VUB using the equation (17) and outputs it.
[0059] The steel plate rate pattern correction unit 114 compares the maximum rate Vmax in the steel plate rate pattern as designated by the user with VUB calculated by the above-mentioned steel plate rate upper limit calculation unit 1141. In the case where Vmax is larger than VUB, it corrects Vmax to VUB. Further, the steel plate rate pattern correction unit 114 corrects the steel plate rate pattern so as to match it with the correction of Vmax and outputs the corrected steel plate rate

26
pattern.
[0060] The correction of the steel plate rate pattern can be performed by adjusting the acceleration/deceleration time while maintaining the former acceleration/deceleration rate before and after Vmax. Alternatively, the
acceleration/deceleration rate may be adjusted while maintaining the former acceleration/deceleration time, or both the acceleration/deceleration time and the acceleration/deceleration rate may be adjusted.
[0061] Further, while in the above example Vmax is corrected to VUB only when Vmax is larger than VUB, it may be so arranged that Vmax is always corrected to VUB. Alternatively, when Vmax is larger than VUB, a warning may be issued so that the user may set VUB.
[Target Temperature History Calculation Unit 120]
[0062] The target temperature history calculation unit 120 calculates the target temperature history of the steel plate 151 between the moment when the steel plate 151 is discharged from the mill 153 and the moment when it reaches the position of the down coiler 154 (that is, during the movement from the installation position of the finishing delivery side thermometer 170 to the installation position of the take-up thermometer 172). This target temperature history is calculated so as to satisfy all of the finishing delivery side temperature TF and the take-up temperature TC, the steel plate

27
rate pattern as output by the steel plate rate pattern correction unit 114, and the holding temperature TH and the holding time ∆H output from the holding condition calculation unit 112. Further, the target temperature history calculation unit 120 calculates the opening/closing pattern of the cooling headers 163 for realizing this target temperature history.
[0063] The steel plate 151 is divided in the longitudinal direction into sections of a predetermined length, and the target temperature history calculation unit 120 prepares the target temperature history steel plate 151 and the
opening/closing pattern of the cooling headers 163 mentioned above for each of the sections.
[0064] Fig. 6 is a diagram showing an example of the processing flow executed by the target temperature history calculation unit 120. First, in step S01, the target temperature history calculation unit 120 receives the steel plate rate V, the target finishing delivery temperature TF, the target take-up temperature TC, the holding temperature TH, the holding time ∆H, etc. of a certain section of the steel plate 151 as input information, and starts the processing. [0065] Here, it is to be assumed that the installation position XIMT of the intermediate thermometer 171, and the minimum length Lair of the air cooling section provided in the direction of the mill 153 from the intermediate thermometer 171 are constants already known. Further, in the present

28
embodiment, in order to express the installation position XIMT of the intermediate thermometer 171, etc., it is to be assumed that there is provided an imaginary coordinate axis (X-axis) along the advancing direction of the steel plate 151. The direction of this coordinate axis (X-axis) is the direction from the mill 153 side to the down coiler 154 side, and the origin is the installation position of the finishing delivery side thermometer 170.
[0066] The minimum length Lair of the air cooling section is a distance provided for the purpose of maintaining a fixed surface condition of the steel plate 151 at the point in time when temperature measurement is performed by the intermediate thermometer 171, and securing the requisite temperature measurement accuracy. The specific length of the minimum length Lair of the air cooling section differs depending upon the measurement system of the intermediate thermometer 171. For example, it is a length corresponding to three cooling headers 163.
[0067] Next, in step S02, the target temperature history calculation unit 120 obtains the length LH of the temperature holding section, the requisite number NF, open of open headers for cooling the steel plate temperature from TF to TH, and the requisite number NR, open of open headers for cooling the steel plate temperature from TH to TC. An open header is a cooling header 163 in an open state in which cooling water is emitted.

29
[0068] LH is obtained by the following equation (18), and NF, open and NR, open are obtained by the following equations (19-1) and (19-2):
LH = V × ∆H (18),
NF, open = (TF - TH)/∆Topen (19-1), and
NR, open = (TH – TC)/∆Topen (19-2).
Here, ∆Topen in equations (19-1) and (19-2) is the substantial temperature change amount in one open header, and can be calculated by using equation (7) or (8) of the plate temperature estimation unit 113.
[0069] Next, in step S03, the target temperature history calculation unit 120 sets initial value of the position XC3e of the open header nearest to the down coiler 154 (hereinafter referred to as the most downstream open header), and the initial value of the initial value of the opening/closing pattern Popen of the cooling headers 163 included in the take-up cooling device 160.
[0070] At this time, as the initial value of the position XC3e of the open header nearest to the down coiler 154, the position of the cooling header 163 nearest to the down coiler 154 is set. The initial value of the opening/closing pattern Popen is set by the following two steps of processing. That is, first, as the first step, all the cooling headers 163 are set to be closed. After this, as the second step, NF, open cooling headers 163 of the front side bank group 165 sequentially from the one nearest

30
to the mill 153, and NR, open cooling headers 163 of the rear side bank group 167 sequentially from the one nearest to the down coiler 154 are set to be open headers.
[0071] Next, in step S04, the target temperature history calculation unit 120 calculates from the opening/closing pattern Popen the expected take-up temperature value TC’, and adjusts Popen and NR, open such that the differential amount between the expected take-up temperature value TC’ and the target take-up temperature TC, |TC - TC’|, is minimum. Here, in the case where TC > TC’, the closed headers are changed to open headers sequentially from the one closest to the down coiler 154, and increases NR, open by an amount corresponding to the differential amount. In the case where TC < TC’, the open headers are changed to closed headers sequentially from the one farthest from the down coiler 154, and decreases NR, open by an amount corresponding to the differential amount.
[0072] Further, in step S04, the target temperature history calculation unit 120 calculates the length LC3 of the water cooling region of the rear side bank group 167 (hereinafter referred to as the third water cooling region) in accordance with the following equation (20) by using the NR, open adjusted as described above.
LC3 = NR, open × Lhead + (NBank, R, open -1) × Lgap (20), where NBank, R, open = Floor (NR, open/Hbank), Lhead is distance between the cooling headers 163, Lgap is a distance between the banks

31
164, NBank, R, open is a number of banks 164 at which all cooling headers are set to be open headers, Hbank is a number of cooling headers 163 included in 1 bank, and Floor is a reduction function to a natural number.
[0073] Subsequently, in step S05, the target temperature history calculation unit 120 calculates the temperature holding start position XHs in accordance with the following equation (21):
XHs = XC3e – LC3 - LH (21).
[0074] Next, in step S06, the target temperature history calculation unit 120 determines which is larger, XHs or XIMT + Lair. When, as the result of the determination, XHs is larger than XIMT + Lair (NO in step S06), the procedure advances to step S07. When XHs is XIMT + Lair or less (YES in step S06), the procedure advances to step S08. Thus, due to the determination processing in step S06, the temperature holding start position XHs in step S08 is always on the mill 153 side with respect to XIMT + Lair. [0075] In step S07, the target temperature history calculation unit 120 corrects the coordinate position XC3e of the open header nearest to the down coiler 154 in accordance with the following equation (22):
XC3e = XC3e + ∆XC3e (22), where ∆XC3e = Round ((XIMT + Lair - XHs)/Lhead) × Lhead, and Round is a function approximating a real number to a nearest integer. [0076] Thus, ∆XC3e is an integral multiple of the interval of

32
one header. This means, in step S07, the opening/closing pattern Popen of the third water cooling region and the holding region is shifted in the front-rear direction in accordance with the corrected XC3e. For example, when ∆XC3e = -2 × Lhead, the opening/closing pattern Popen of the third water cooling region and the holding region is shifted in the direction of the mill 153 by a distance corresponding to two headers, and the two headers nearest to the down coiler 154 is set to closed headers. Through the above processing, at the point in time when step S08 is started, the opening/closing pattern Popen with respect to all the headers on the down coiler 154 side of the temperature holding start position XHs is determined.
[0077] In step S08, the target temperature history calculation unit 120 compares the temperature holding start position XHs with the distance LF, open from the inlet of the take-up cooling device 160 to the NF, openth cooling header 163. Here, NF, open is the number of open headers needed for cooling the steel plate temperature obtained in step S02 from TF to TH. When, as the result of the comparison, XHs is larger than LF, open (YES in step S08), the target temperature history calculation unit 120 causes the procedure to advance to step S09. When XHs is LF, open or less (NO in step S08), the procedure advances to step S12. [0078] Subsequently, in step S09, the target temperature history calculation unit 120 calculates the waiting temperature Tw by using the phase transformation rate model shown in the

33
above equation (3). Here, the waiting temperature Tw is the temperature at which the ferrite phase volume fraction is less than a predetermined tolerance range δF when the holding is performed for c × XIMT/V hours at that temperature. Here, the constant c is a value ranging from 0.1 to 0.9. For example, it is 0.5. δF is a value which is substantially 1/10 of the target volume fraction.
[0079] Subsequently, in step S10, the target temperature history calculation unit 120 calculates the number NC1, open of the open headers of the first water cooling region cooling the steel plate temperature from TF to Tw and the length LC1 of the first water cooling region. Further, the target temperature history calculation unit 120 calculates the number NC2, open of the open headers of the second water cooling region cooling the steel plate temperature from Tw to TH and the length LC2 of the second water cooling region.
[0080] Subsequently, in step S11, the target temperature history calculation unit 120 calculates the length Lw of the waiting region in accordance with the following equation (23):
Lw = max (XHs – LC1 – LC2, 0) (23). [0081] In step S12, the temperature holding start position XHs is LF, open or less, so that the target temperature history calculation unit 120 sets all the cooling headers 163 on the mill 153 side of the temperature holding start position XHs to open headers.

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[0082] Through the above operation, at the point in time when the processing by the target temperature history calculation unit 120 is completed, the final opening/closing pattern Popen with respect to all the cooling headers 163 in the take-up cooling device 160 is obtained. In view of this, in step S13, the target temperature history calculation unit 120 outputs this final opening/closing pattern Popen to the cooling command calculation unit 130 to complete the processing of the target temperature history calculation unit 120 with respect to one section of the steel plate 151.
[0083] The above processing by the target temperature history calculation unit 120 is executed for each section with respect to all the sections of the steel plate 151.
[Cooling Command Calculation Unit 130, Header Pattern Output Unit 140]
[0084] The cooling command calculation unit 130 calculates a cooling command corresponding to the opening/closing pattern Popen for each section calculated by the target temperature history calculation unit 120 in accordance with the position of each section when each section of the steel plate 151 is actually cooled by the take-up cooling device 160. Header pattern output unit 140 converts the cooling command calculated by the cooling command calculation unit 130 to a header pattern in which the cooling headers 163 are opened and closed, and outputs it to the object of control 150. The processing by the

35
cooling command calculation unit 130 and the header pattern output unit 140 is executed for each predetermined divisional time from the passing of the forward end of the steel plate 151 by the finishing delivery side thermometer 170 until the rear end of the steel plates passes by the take-up thermometer 172. [0085] To clarify the features and effects of the invention according to the present embodiment, the temperature history and metal texture attained in the case of the DP steel according to the embodiment of the present invention were compared with those of a comparative example (related-art technique). The comparison results are shown in Figs. 7 through 11. [0086] Fig. 7 is a diagram illustrating an example of the target temperature history at the steel plate rates V1 < V2 < V3 < V4 in the comparative example (related-art technique). In the comparative example, the temperature holding regions are provided on the mill 153 side and the down coiler 154 side substantially in symmetry with the intermediate thermometer 171 being at the center. Thus, at V4, which is the highest, there is a shortage of the number of cooling headers 163 from the completion of the temperature holding region to the down coiler 154, and it is impossible to attain the target take-up temperature TC. Further, the temperature is lowered at a substantially uniform cooling rate from the target finishing delivery temperature TF to the holding temperature TH for each section of the steel plate 151, so that the cooling rate from

36
the target finishing delivery temperature TF to the holding temperature TH greatly varies from section to section. [0087] Fig. 8 is a diagram illustrating an example of the target temperature history at the steel plate rate V1 < V2 < V3 < V4 obtained in the embodiment of the present invention. In the present embodiment, the opening/closing pattern is set starting from the side near the down coiler 154, so that, even at the steel plate rate V4, which is the highest, it is possible to attain the target take-up temperature TC. Further, the waiting time at the waiting temperature Tw is changed in accordance with the rate change for each section of the steel plate 151, so that the fluctuation in cooling rate from Tw to TH due to the difference in steel plate rate from section to section is mitigated as compared with the comparative example of Fig. 7.
[0088] Fig. 9 is a diagram illustrating the volume fraction of the ferrite phase of the hot-rolled DP steel manufactured based on the comparative example (relate-art technique). At the steel plate rate V1, which is low, the ferrite phase is generated during cooling from the target finishing delivery temperature TF to the holding temperature TH, so that the volume fraction of the ferrite phase increases, and the strength of the steel plate 151 decreases. At the steel plate rate V4, which is high, the taking-up is performed at a temperature higher than the martensite transformation start temperature, so that a

37
bainite texture is formed, resulting in deterioration in strength and tenacity.
[0089] Fig. 10 is a diagram showing the volume fraction of the ferrite phase of the hot-rolled DP steel manufactured based on the embodiment of the present invention. As can be seen from Fig. 10, in the case of the present embodiment, it is possible to attain a substantially uniform ferrite phase volume fraction even if the steel plate rate is changed.
[0090] Fig. 11 is a diagram showing a comparison example between the embodiment of the present invention and the comparative example (related-art technique) in the average crystal grain size of the ferrite of the hot-rolled DP steel. In the comparative example (related-art technique), in the hot-rolled DP steel manufactured at the steel plate rate V1, which is low, the ferrite grain size tends to increase due to the ferrite phase generated at a relatively high temperature. In the hot-rolled DP steel manufactured based on the present embodiment, the ferrite transformation progresses at the holding temperature TH, so that the ferrite grain size is substantially uniform independently of the steel plate rate. [0091] According to the embodiment of the present embodiment, the volume fraction of the ferrite phase and the crystal grain size of the ferrite are substantially fixed even if the steel plate rate is changed, so that it is possible to achieve a uniform quality of the steel plate manufactured.

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[0092] The present invention is not restricted to the above-described embodiment and modification but further includes various modifications. For example, the above embodiment and modification have been described in detail in order to facilitate the understanding of the present invention, and the present invention is not restricted to a construction equipped with all the components described above. Further, a part of the construction of an embodiment and of a modification may be replaced by the construction of another embodiment and another modification. Further, to the construction of an embodiment or of a modification, the construction of another embodiment or of another modification may be added. Further, with respect to a part of the construction of each embodiment and modification, it is possible to perform addition, deletion, or replacement of a construction included in another embodiment or modification.
Description of Reference Characters [0093] 50: Host computer 100: Take-up cooling control device 101: Storage unit 110: Processing unit
111: Phase transformation start condition calculation unit 112: Holding condition calculation unit 113: Plate temperature estimation unit 114: Steel plate rate pattern correction unit

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120: Target temperature history calculation unit
130: Cooling command calculation unit
140: Header pattern output unit
150: Object of control
151: Steel plate (rolled material)
152: Hot rolling mill
153: Mill
154: Down coiler
160: Take-up cooling device
161: Upper cooling device
162: Lower cooling device
163: Cooling header
164: Bank
165: Front side bank group
166: Intermediate bank group
167: Rear side bank group
170: Finishing delivery side thermometer
171: Intermediate thermometer
172: Take-up thermometer
1141: Steel plate rate upper limit calculation unit
TF: Target finishing delivery temperature
TC: Target take-up temperature
TH: Holding temperature
Tw: Waiting temperature
∆H: Holding time

NF, open: Number of open headers needed for lowering the steel
plate temperature from TF to TH
NR, open: Number of open headers needed for lowering the steel
plate temperature from TH to Tc
LH: Length of the temperature holding section
Lair: Minimum length of the air cooling section
LC3: Length of the third water cooling region
XIMT : Position of the intermediate thermometer
XC3e: Position of the most downstream open header
XHs: Temperature holding start position
Popen: Opening/closing pattern of the cooling headers

What is claimed is:
1. A take-up cooling control device controlling a take-up cooling device equipped with a plurality of cooling headers emitting cooling water to a rolled material rolled by a hot rolling mill and taken-up by a down coiler, comprising:
a target temperature history calculation unit which, with respect to each section of the rolled material obtained through division of the rolled material by a predetermined length in the longitudinal direction, calculates a target temperature history, in a case where the each section undergoes a change between a moment when the each section is delivered from the hot rolling mill and a moment when the each section moves to the down coiler, such that the volume fraction of at least one transformed phase of the rolled material is substantially fixed between the sections;
a cooling command calculation unit calculating for the each section a cooling command for the each cooling header, the cooling command causing a temperature at which the each section is cooled by the take-up cooling device to coincide with the calculated target temperature history; and
a header pattern output unit that calculates an opening/closing pattern of the each cooling header for each predetermined period of time based on the cooling command for the each cooling header calculated for the each section and

outputs the calculated opening/closing pattern to the take-up cooling device.
2. The take-up cooling control device according to claim
1, further comprising a holding condition calculation unit
calculating a holding temperature and a holding time for
generating an isothermal transformation with respect to at least
one transformed phase of the rolled material,
wherein the target temperature history calculation unit calculates the target temperature history so as to satisfy the holding temperature and the holding time calculated by the holding condition calculation unit.
3. The take-up cooling control device according to claim
2, wherein the holding condition calculation unit calculates
the holding temperature and the holding time based on a chemical
composition of the rolled material set by a user, a target
finishing delivery temperature, a target take-up temperature,
a target volume fraction of a transformed phase.
4. The take-up cooling control device according to claim
3, wherein, assuming that a number of the cooling headers in
an open state needed for cooling the rolled material from the
target finishing delivery temperature to the holding
temperature is N, and in a case where it is determined that a
position of the Nth cooling header as counted from the inlet
of the take-up cooling device is on the hot rolling mill side
of the position where the holding of the holding temperature

is started, the target temperature history calculation unit calculates the target temperature history such that, when the temperature of the rolled material is lowered to a waiting temperature higher than the holding temperature, the waiting temperature is maintained for a fixed period of time.
5. A take-up cooling control method for use in a take-up cooling control device controlling a take-up cooling device, which is equipped with a plurality of cooling headers emitting cooling water to a rolled material rolled by a hot rolling mill and taken-up by a down coiler,
the method comprising:
a first step in which, with respect to each section of the rolled material obtained through division of the rolled material by a predetermined length in the longitudinal direction, there is calculated a target temperature history in a case where the each section undergoes a change between a moment when the each section is delivered from the hot rolling mill and a moment when the each section moves to the down coiler, such that a volume fraction of at least one transformed phase of the rolled material is substantially fixed between the sections;
a second step in which there is calculated for the each section a cooling command for each cooling header, the cooling command causing a temperature at which the each section is cooled by the take-up cooling device to coincide with the calculated target temperature history; and

a third step in which an opening/closing pattern of the each cooling header for each predetermined period of time is calculated based on the cooling command for the each cooling header calculated for the each section and is output to the take-up cooling device.
6. The take-up cooling control method according to claim
5, further comprising a fourth step in which a holding
temperature and a holding time for generating an isothermal
transformation with respect to at least one transformed phase
of the rolled material are calculated, wherein,
in the first step, the target temperature history is calculated so as to satisfy the holding temperature and the holding time calculated in the fourth step.
7. The take-up cooling control method according to claim
6, wherein, in the fourth step, the take-up cooling control
device calculates the holding temperature and the holding time
based on a chemical composition of the rolled material set by
a user, a target finishing delivery temperature, a target
take-up temperature, and a target volume fraction of a
transformed phase.
8. The take-up cooling control method according to claim
7, wherein, in the first step, assuming that a number of the
cooling headers in an open state needed for cooling the rolled
material from the target finishing delivery temperature to the
holding temperature is N, and in a case where it is determined

that a position of the Nth cooling header as counted from an inlet of the take-up cooling device is on the hot rolling mill side of the position where the holding of the holding temperature is started, the take-up cooling control device calculates the target temperature history such that, when the temperature of the rolled material is lowered to a waiting temperature higher than the holding temperature, the waiting temperature is maintained for a fixed period of time.