JP2015167976A - Winding temperature control method of hot rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a winding temperature control method of a hot rolled steel sheet which can accurately control a winding temperature of the hot rolled steel sheet by properly correcting a heat transfer coefficient model part at a water cold heat transfer coefficient of the hot rolled steel sheet.SOLUTION: When controlling a hot rolled steel sheet which is rolled by hot rolling equipment to a prescribed winding temperature by cooling the hot rolled steel sheet by cooling water on a runout table, there is used, as a model formula of a heat transfer coefficient, a formula which is obtained by multiplying a heat transfer coefficient learning value to a heat transfer coefficient model part which is expressed as a function of a water temperature of the cooling water, a temperature of the hot rolled steel sheet, and the density of a water amount of the cooling water, and by adding a correction item which enlarges the heat transfer coefficient accompanied by an increase of a transfer speed when the transfer speed of the hot rolled steel sheet is not lower than a specified threshold.

Description

  The present invention relates to a method for controlling a coiling temperature of a hot-rolled steel sheet, in which a hot-rolled steel sheet rolled in a hot rolling mill is controlled to be cooled on a run-out table to control a coiling temperature. Each hot-rolled steel sheet has a coiling temperature target for coiling with a coiler, and the coiling temperature control device uses a runout table so that the coiling temperature of the hot-rolled steel sheet falls within the target tolerance range. Determine the cooling pattern of the hot-rolled steel sheet. And a cooling device implements control which cools a hot-rolled steel plate according to the cooling pattern determined by the coiling temperature control device.

  The coiling temperature control is performed on the runout table (ROT) and the temperature record on the exit side of the finishing mill (finishing mill) for each control unit (called a cutting plate) obtained by dividing the hot-rolled steel sheet in the longitudinal direction at constant length units. Feed forward (FF) control that performs heat transfer calculation of hot-rolled steel sheet during ROT cooling multiple times based on the planned passing speed, and determines the optimal cooling pattern to achieve the coiling temperature target by convergence calculation And feedback (FB) control for correcting the cooling water amount from the winding temperature result.

  The water-cooled heat transfer coefficient of the hot-rolled steel sheet used for heat transfer calculation is generally the same as the heat transfer coefficient model part calculated using a model formula as a function of water temperature, water volume density, steel sheet surface temperature, etc. A value obtained by multiplying a learning term (heat transfer coefficient learning unit) obtained by back-calculating the actual heat transfer coefficient from the recent cooling record of the steel type is adopted.

  Regarding the heat transfer coefficient learning unit, various methods have been devised regarding how to reflect the heat transfer calculation including Patent Document 1.

  On the other hand, for the heat transfer coefficient model part during water cooling, as in Patent Document 2, a model expressed as a function of water density, water temperature, and steel plate temperature, and as in Patent Document 3, water density, material surface temperature, A model that expresses the difference between the material surface temperature and the water temperature, a model expressed as a function of the steel plate speed, a model that introduces a function that takes into account changes in the behavior of heat transfer in the transition boiling region, such as Patent Document 4 and Patent Document 5, A model that corrects the heat transfer coefficient with the plate thickness, FDT (finishing rolling exit temperature) target value, and CT (winding temperature) target value, as in Reference 6, and water volume density, thickness, Various types of model formulas have been proposed, such as a model expressed as a function of material temperature and cooling water temperature.

  In these model formulas, there is a term that expresses a physical phenomenon in boiling heat transfer, such as the surface temperature correction term in Patent Document 4, but the correction term for the water temperature and the water density is a value obtained from experiments. Is adopted. Further, other correction terms (speed, plate thickness, FDT target, CT target) are correction terms having no physical meaning that are introduced in accordance with actual results.

  In the heat transfer calculation, even if the accuracy of the heat transfer coefficient predicted by the heat transfer coefficient model unit is poor, the heat transfer coefficient learning unit can improve the accuracy to some extent. This is because the heat transfer coefficient shows a close value in the cooling of steel plates with similar operating conditions and steel types, and if the appropriate heat transfer coefficient learning value is reflected in the calculation, the model part has an inaccuracy. Can also cover it. For this reason, the current coiling temperature control is actually calculated by relying on the heat transfer coefficient learning unit, and there are few proposals for improving the accuracy of the heat transfer coefficient model.

Japanese Patent Publication No. 6-88060 JP-A-6-218414 Japanese Patent Laid-Open No. 9-10822 JP 2000-317513 A JP 2004-331992 A Japanese Patent Laid-Open No. 9-267113 JP 2000-167615 A

  As described above, in performing heat transfer calculation of hot-rolled steel sheet, the heat transfer coefficient during water cooling of the hot-rolled steel sheet cannot be expressed by an appropriate model based on physical phenomena, There is room for improvement, and calculations relying on the heat transfer coefficient learning unit are performed. However, if the operating conditions and steel types are complicated, the influence of model errors increases and cannot be covered by the heat transfer coefficient learning unit. An error occurs.

  This is because the process of learning by calculating the heat transfer coefficient from the cooling performance is eventually calculated back using the model formula of the heat transfer coefficient model part, and the error of the heat transfer coefficient model part affects the learning value. Because.

  That is, in the prior art, the phenomenon that the heat transfer coefficient changes depending on the speed of the hot-rolled steel sheet is not properly reflected in the model. For this reason, even if it is the same steel type, when the steel plate speed is different, or when the acceleration / deceleration during winding is large even within the same steel plate, the temperature prediction accuracy by the steel plate speed deteriorates.

Specifically, in Patent Document 3 described above, speed correction (v ^−0.08 ) for reducing the heat transfer coefficient with respect to an increase in the steel sheet speed is proposed, but actually, in the heat transfer coefficient model part, When the model is applied with terms of water temperature, water density, and steel plate surface temperature, excluding the steel plate speed, the learning value of the heat transfer coefficient is calculated back from the actual results, and the correlation between the learned value and the steel plate conveyance speed is examined. The phenomenon that the hot-rolled steel sheet cools well (the heat transfer coefficient is large) has been confirmed, and the speed correction of Patent Document 3 does not match the phenomenon actually occurring.

  The present invention has been made in view of such circumstances, and appropriately corrects the heat transfer coefficient model part in the water-cooled heat transfer coefficient of the hot-rolled steel sheet to control the winding temperature of the hot-rolled steel sheet with high accuracy. It is an object of the present invention to provide a method for controlling the coiling temperature of a hot-rolled steel sheet.

  As a result of studying to solve the above problems, the present inventor is an important factor for changing the heat transfer coefficient is the speed of the hot-rolled steel sheet, and appropriately reflects the conveyance speed of the hot-rolled steel sheet in the heat transfer coefficient. It was found that the coiling temperature of the hot-rolled steel sheet can be controlled with high accuracy.

  Conventionally, the heat transfer coefficient learning unit is grouped according to steel type, sheet thickness, FDT target, and CT target, and the heat transfer coefficient calculated backward from the actual results greatly varies depending on the sheet thickness even in the same group. Although obtained, the heat transfer coefficient expresses the ease of heat transfer on the surface of the steel sheet, and originally does not change with the plate thickness. A typical operation condition that varies depending on the plate thickness is a conveyance speed. As a general operation pattern, a steel plate with a large plate thickness has a low conveyance speed, and a steel plate with a small plate thickness has a high conveyance speed. This led to the idea that the factor that fluctuates the heat transfer coefficient is not the sheet thickness but the conveyance speed of the hot-rolled steel sheet.

  Then, the physical phenomenon was considered in detail, the relationship between the conveyance speed of the hot-rolled steel sheet and the heat transfer coefficient was accurately grasped, and these were correctly reflected in the model formula, thereby reaching the present invention.

  The present invention has been completed based on the above findings and provides the following (1) to (3).

(1) A hot rolled steel sheet winding temperature control method for controlling a hot rolled material rolled in a hot rolling facility to a predetermined winding temperature by cooling with a cooling water on a run-out table,
As a model formula for the heat transfer coefficient, the heat transfer coefficient model part expressed as a function of the cooling water temperature, the hot rolled steel sheet temperature, and the cooling water volume density is multiplied by the heat transfer coefficient learning value. It is characterized by controlling the coiling temperature of a hot-rolled steel sheet using a correction term that increases the heat transfer coefficient with an increase in the conveyance speed when the conveyance speed is equal to or higher than a specific threshold value. A method for controlling the coiling temperature of a hot-rolled steel sheet.
(2) The method for controlling the coiling temperature of a hot-rolled steel sheet according to (1), wherein the following formula is used as a model formula of the heat transfer coefficient.
α = KKW · f ₁ (T _W ) · f ₂ (W) · f ₃ (T _S ) · K _V
When V <Vth: K _V = 1
When V ≧ Vth: K _V = 1 + f ₄ (V)
Where α: heat transfer coefficient [W / m ² · K], KKW: heat transfer coefficient learning value [−], T _W : coolant temperature [K], T _S : steel plate temperature [K], W: cooling Water density of water [m ³ / m ² · min], V: Conveying speed of hot-rolled steel sheet [m / s], Vth: Conveying speed threshold value of hot-rolled steel sheet [m / s], f ₄ (V) : A function of the conveyance speed that increases as the conveyance speed increases.
(3) The winding temperature control method for a hot-rolled steel sheet according to (2), wherein the f ₄ (V) is expressed by the following formula.
f ₄ (V) = a × (V−Vth) ^b
However, a and b are positive integers.

  According to the present invention, as a model expression of the heat transfer coefficient, when a conveyance speed of the hot-rolled steel sheet is equal to or higher than a specific threshold, a correction term for increasing the heat transfer coefficient with an increase in the conveyance speed is added. Therefore, the change of the heat transfer coefficient according to the conveyance speed of the hot rolled steel sheet can be appropriately reflected. For this reason, it is possible to improve the variation in temperature prediction accuracy due to acceleration / deceleration during winding of a hot-rolled steel sheet, and to improve the temperature prediction accuracy of steel sheets with the same steel type and different conveyance speeds. The temperature can be controlled with high accuracy.

It is the schematic which shows an example of the rolling cooling equipment for enforcing the winding temperature control method of the hot-rolled steel plate of this invention. It is a figure which shows the change by the steel plate conveyance speed of the speed correction term of patent document 3, Formula 5. It is a figure which shows the correlation with the steel plate conveyance speed in an actual operation, and a heat-transfer coefficient learning value. It is a figure which shows the conventional heat transfer model (a) which considered only the boiling heat transfer, and the heat transfer model (b) of this invention which also considered the influence of the convective heat transfer. It is a figure which shows the result of having examined in detail about the correlation with the steel plate conveyance speed of FIG. 3, and a heat transfer coefficient learning value.

Hereinafter, the present invention will be specifically described with reference to the accompanying drawings.
FIG. 1 is a schematic view showing an example of a rolling cooling facility for carrying out the method for controlling the coiling temperature of a hot-rolled steel sheet according to the present invention. The rolling cooling equipment 1 is provided on a runout table 22 provided on the downstream side of a finishing mill 21 (only the final stand is shown). The hot-rolled steel sheet S rolled by the finish rolling mill 21 travels on the run-out table 22 and is wound on a winder (down coiler) 23.

  The rolling cooling facility 1 has a cooling unit 2 that is arranged above and below and injects cooling water into the hot-rolled steel sheet S. Cooling water is supplied to the cooling unit 2 from a cooling water supply means (not shown). Each of these cooling units 2 is divided into a plurality of cooling banks 3. And the cooling amount with respect to the hot-rolled steel sheet S is controlled by adjusting the amount of water injected to each bank 3 of such a cooling unit 2.

  The rolling cooling equipment 1 includes a cooling equipment entrance thermometer 4 that measures the temperature of the hot-rolled steel sheet S that leaves the finishing mill 21 and is sent to the cooling equipment 1, and the thickness of the hot-rolled steel sheet S after finish rolling. A thickness gauge 5 for measuring the thickness, a steel plate speed detector 6 for measuring the unwinding speed of the hot-rolled steel sheet S of the finish rolling mill 21, which is the supply speed of the hot-rolled steel sheet S to the cooling facility, and hot rolling at the time of winding A delivery-side thermometer 7 that measures the temperature of the steel sheet S, and a winding speed detector 8 that measures the winding speed of the hot-rolled steel sheet of the winder 23, which is the carry-out speed of the hot-rolled steel sheet S from the cooling facility. Have. Further, the rolling cooling facility 1 includes a cooling control unit 9 that calculates a water injection pattern to the cooling unit 2 in order to control the hot rolled steel sheet S to a desired winding temperature, and a calculation of the water injection pattern by the cooling control unit 9. Therefore, the learning value calculation unit 10 that calculates the learning value of the heat transfer coefficient and outputs it to the cooling control unit 9 and the bank opening and closing that outputs the water injection command to the cooling unit 2 based on the water injection amount output from the cooling control unit 9 And an input / output unit 11.

Next, the cooling control operation by the rolling cooling facility 1 will be described.
The cooling control unit 9 calculates a water injection pattern to each bank 3 of the cooling unit 2 in order to cool the hot rolled steel sheet S to a desired winding temperature over the entire length, thereby controlling the winding temperature of the hot rolled steel sheet S. To do. At this time, the control is performed by setting the cooling pattern on the basis of the temperature results obtained by the cooling equipment inlet side thermometer 4 on the exit side of the finish rolling mill 21 for each cut plate which is a constant length unit in the longitudinal direction of the hot rolled steel sheet S. The feed forward (FF) control to be determined and the feedback (FB) control to correct the cooling amount based on the actual winding temperature obtained by the outlet thermometer 7 are included. For example, in the FF control, the temperature drop obtained by using the actual temperature obtained by the cooling facility entry-side thermometer 4 and the heat transfer coefficient of the hot-rolled steel sheet, and further, the hot-rolled steel sheet S obtained by the thickness gauge 5 The water injection pattern is calculated on the basis of the plate thickness and the conveyance speed measured by the steel plate speed detector 6 and the winding speed detector 8.

  At this time, the heat transfer coefficient of the hot-rolled steel sheet used when the temperature calculation is performed in the FF control and the FB control of the cooling control unit 9 depends on the steel type, the plate thickness, the target temperature, etc. A value obtained by multiplying the learning value of each group of rolled material groups is adopted. The learning value of the heat transfer coefficient is calculated by the heat transfer coefficient learning value calculation unit 10 and reflected in the water injection pattern calculation of the cooling control unit 9. In the heat transfer coefficient learning value calculation unit 10, the learning value calculated and updated based on the results when the same rolling group was rolled last time is used.

  In this way, the water injection amount of each bank 3 is calculated, the calculated water injection amount of the bank 3 is output to the bank opening / closing input / output unit 11, and the water injection command is output from the bank opening / closing input / output unit 11 to each bank 3. Cooling control is performed.

In the present invention, in such cooling control, as described above, it is assumed that the heat transfer coefficient of the hot-rolled steel sheet being water-cooled on the run-out table changes depending on the conveying speed of the hot-rolled steel sheet. Although the influence of the conveyance speed of the hot-rolled steel sheet is considered in the prior art (Patent Document 3), the actual physical phenomenon cannot be correctly reflected in the model formula. That is, in Patent Document 3, the heat transfer coefficient h _Wt on the upper surface of the steel sheet and the heat transfer coefficient h _Wb on the lower surface are represented by the following equations.
(Top)
h _Wt = 1.163 × 10 ⁶ W ^0.425 (θ _S −θ _W ) ⁻¹ v ^−0.08 {1−0.02 (θ _S −40)} z
(Bottom)
h _Wb = 7.020 × 10 ⁵ W ^0.771 (θ _S −θ _W ) ⁻¹ {1-0.02 (θ _S −40)} z
W: Water density [m ³ / m ² · min], θ _S : Material surface temperature [° C.], θ _W : Water temperature [° C.], v: Steel plate speed [m / min], z: Correction coefficient

Therefore, the speed correction term (v ^−0.08 ) of Patent Document 3 is as shown in FIG. 2, and is corrected so that the heat transfer coefficient decreases as the speed increases.

  In contrast, in actual operation, the learning value of the heat transfer coefficient is calculated back from the actual results using a heat transfer coefficient model as a function of the water temperature, water density, and steel sheet surface temperature, excluding the steel plate speed, and the learned value and the steel plate transport When the correlation of speed is obtained, as shown in FIG. 3, the heat transfer coefficient is increased by increasing the steel sheet speed. In other words, the heat transfer coefficient calculated by the model of Patent Document 3 is a result opposite to the actual heat transfer coefficient, and the actual heat transfer coefficient is more than the heat transfer coefficient calculated by the model of Patent Document 3. large.

  Then, based on an actual physical phenomenon, it examined how the heat transfer coefficient in hot-rolled steel sheet water cooling changed.

  The heat transfer in water cooling of the hot-rolled steel sheet is boiling heat transfer because the hot-rolled steel sheet has a temperature of 100 ° C. (boiling point of water) or higher. On the other hand, the heat transfer of a steel plate of less than 100 ° C. is called convection heat transfer, and the state in which the fluid (cooling water) is stopped (that is, the state in which the steel plate is stopped) is natural convection, and the fluid (cooling water) is The moving state (that is, the state where the steel plate is moving) is called forced convection.

There are three forms of boiling heat transfer, nucleate boiling heat transfer, transition boiling heat transfer, and film boiling heat transfer, depending on the surface temperature of the steel sheet. Patent Document 4 focuses on the difference in heat flux in each form. A heat transfer model has been proposed. However, boiling heat transfer is based on the premise that the fluid (cooling water) is stationary as shown in FIG. 4A, and the influence of convection caused by the conveyance of the hot-rolled steel sheet is not reflected. That is, as a conventional heat transfer coefficient model considering boiling heat transfer, generally, a heat transfer coefficient model part expressed as a function of cooling water temperature, hot-rolled steel sheet temperature, and cooling water volume density is used. A product obtained by multiplying the heat transfer coefficient learning value is used. Specifically, a model formula represented by the following formula (1) is used as a model formula of the heat transfer coefficient α ′ [W / m ² · K].
α ′ = KKW · f ₁ (T _W ) · f ₁ (W) · f ₃ (T _S ) (1)
However, KKW: Heat transfer coefficient learning value [−], T _W : Cooling water temperature [K], T _S : Steel plate temperature [K], W: Cooling water quantity density [m ³ / m ² · min]

  Therefore, in the present invention, as shown in FIG. 4 (b), while assuming the heat transfer to the boiling point, the effect of convective heat transfer due to the conveyance of the hot-rolled steel sheet is taken into account, and the heat is increased as the hot-rolled steel sheet conveyance speed increases. Build a model that can represent the actual phenomenon where the transfer coefficient increases.

  In constructing such a model formula, the correlation between the learning value calculated backward from the results shown in FIG. 3 and the steel plate conveyance speed will be examined in more detail. As shown in FIG. 5, in the region where the conveyance speed is low, the conveyance speed is low. In contrast, it can be seen that the heat transfer coefficient tends to increase with respect to the transport speed in a region where the heat transfer coefficient is substantially constant and the transport speed is equal to or greater than a certain threshold value (transport speed threshold Vth).

  That is, the conveyance speed threshold Vth indicates a boundary where the speed dependency of the heat transfer coefficient changes, and the cooling water smoothly flows over the hot-rolled steel sheet at a conveyance speed lower than the conveyance speed threshold Vth. For this reason, the change in the heat transfer coefficient due to the increase in the transfer speed is small, whereas the cooling water becomes a turbulent state at the transfer speed threshold value Vth or higher, so that the change (increase) in the heat transfer coefficient is caused by the increase in the transfer speed. It appears greatly.

  Therefore, in the present invention, as a model expression of the heat transfer coefficient, a conventional model obtained by multiplying the heat transfer coefficient learning part by the heat transfer coefficient model part expressed as a function of the coolant temperature, the steel plate temperature, and the coolant water density. A formula in which a heat transfer coefficient speed correction term reflecting the above tendency is added is used.

Specifically, as a model expression of the heat transfer coefficient alpha, (1) the model expression represented by the heat transfer coefficient velocity correction term K _V below in consideration of the equation (2) into equation is used.
α = KKW · f ₁ (T _W ) · f ₂ (W) · f ₃ (T _S ) · K _V (2)

Here, when the transport speed is V [m / s] and the transport speed threshold is Vth [m / s],
When V <Vth: K _V = 1
When V ≧ Vth: K _V = 1 + f ₄ (V)
However, f ₄ (V) is a function that increases as the conveyance speed increases.
f ₄ (V) = a × (V−Vth) ^b (where a and b are positive integers). Appropriate values are set for a and b depending on the steel type and the like.

  Thus, in the present invention, it is possible to appropriately reflect the change in the heat transfer coefficient according to the conveyance speed of the hot-rolled steel sheet, thus improving the variation in temperature prediction accuracy due to acceleration / deceleration during hot-rolled steel sheet winding, And the improvement of the temperature prediction precision of the steel plate from which the conveyance speed differs by the same steel type can be implement | achieved, and it becomes possible to control the coiling temperature of a hot-rolled steel plate with high precision.

Next, the result of actually grasping the effect of the present invention will be described.
In the case of the conventional example using the above formula (1) that does not consider the steel plate conveyance speed as the heat transfer coefficient applied to the heat transfer coefficient model part, the material test is not good due to CT failure (the characteristic value obtained in the material test is not the target (step The ratio was 0.2%. On the other hand, in the case of the example using the above formula (2) reflecting the steel plate conveyance speed based on the present invention as the heat transfer coefficient, the material test failure rate due to CT failure was 0.17%. That is, since the coiling temperature can be controlled with high accuracy according to the present invention, the CT defect rate can be reduced as compared with the prior art.

DESCRIPTION OF SYMBOLS 1 Rolling cooling equipment 2 Cooling part 3 Cooling bank 4 Cooling equipment inlet side thermometer 5 Sheet thickness meter 6 Steel plate speed detector 7 Outlet side thermometer 8 Winding speed detector 9 Cooling control part 10 Heat transfer coefficient learning value calculating part 11 Bank open / close input / output unit 21 Finishing mill 22 Runout table 23 Winder S Hot rolled steel sheet

Claims (3)

A hot rolling steel sheet rolling temperature control method for cooling a hot rolled material rolled in a hot rolling facility with a cooling water on a run-out table to control to a predetermined winding temperature,
As a model formula for the heat transfer coefficient, the heat transfer coefficient model part expressed as a function of the cooling water temperature, the hot rolled steel sheet temperature, and the cooling water volume density is multiplied by the heat transfer coefficient learning value. It is characterized by controlling the coiling temperature of a hot-rolled steel sheet using a correction term that increases the heat transfer coefficient with an increase in the conveyance speed when the conveyance speed is equal to or higher than a specific threshold value. A method for controlling the coiling temperature of a hot-rolled steel sheet. The winding temperature control method for a hot-rolled steel sheet according to claim 1, wherein the following formula is used as a model formula of the heat transfer coefficient.
α = KKW · f ₁ (T _W ) · f ₂ (W) · f ₃ (T _S ) · K _V
When V <Vth: K _V = 1
When V ≧ Vth: K _V = 1 + f ₄ (V)
Where α: heat transfer coefficient [W / m ² · K], KKW: heat transfer coefficient learning value [−], T _W : coolant temperature [K], T _S : steel plate temperature [K], W: cooling Water density of water [m ³ / m ² · min], V: Conveying speed of hot-rolled steel sheet [m / s], Vth: Conveying speed threshold value of hot-rolled steel sheet [m / s], f ₄ (V) : A function of the conveyance speed that increases as the conveyance speed increases. Wherein f _{4 (V)} is the winding temperature control method of the hot-rolled steel sheet according to claim 2, characterized by being represented by the following formula.
f ₄ (V) = a × (V−Vth) ^b
However, a and b are positive integers.

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