Method for calculating ferrite transformation temperature of steel plate, cooling control method, and manufacturing method

Abstract

To provide a method for calculating a ferrite transformation temperature of a steel sheet capable of accurately calculating the ferrite transformation temperature considering external factors and the temperature of the steel sheet during cooling.SOLUTION: A method for calculating the ferrite transformation temperature of a steel sheet includes: an estimation step for estimating the temperature change of the steel sheet at the time of cooling by injecting cooling water by calculating a temperature prediction model equation including a correction coefficient applied to the heat transfer coefficient of the surface of the steel sheet and a specific heat calculated from the ferrite transformation temperature as variables using actual data on the cooling treatment of the steel sheet by the injection of cooling water; a back-calculation step for calculating a correction coefficient by which the cooling end temperature of the steel sheet estimated in the estimation step coincides with the actual value of the cooling end temperature using the temperature prediction model equation; and a correction step for correcting the ferrite transformation temperature so that the difference between the cooling midway temperature of one or more steel sheets estimated in the estimation step and the actual value of the cooling midway temperature becomes small.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for calculating the ferrite transformation temperature of a steel plate, a cooling control method, and a manufacturing method. [Background technology]

[0002] In the manufacturing process of hot-rolled steel sheets, first, a slab is heated to a predetermined temperature in a heating furnace, and the heated slab is rolled in a roughing mill to produce a rough bar. Next, the rough bar is rolled to a predetermined thickness in a finishing mill consisting of multiple rolling stands to produce a hot-rolled steel sheet (hereinafter, simply referred to as "steel sheet"). Finally, the steel sheet is cooled by spraying cooling water on the upper and lower surfaces of the steel sheet using a cooling device installed on a run-out table, and then the steel sheet is wound up by a winder. The temperature of the steel sheet before being wound up by the winder is called the winding temperature. This winding temperature has a significant effect on the material properties of the steel sheet, and is therefore an important index for quality control of the steel sheet. The winding temperature can be controlled by controlling the amount of cooling water sprayed according to the conveying speed of the steel sheet, but it becomes very difficult to control the winding temperature for certain materials due to changes in cooling capacity accompanying changes in the surface temperature of the steel sheet and changes in heat quantity accompanying transformation of the steel sheet.

[0003] In recent steel sheet cooling control processes, the cooling target temperature range has become stricter, and the number of steel sheets, such as high-tensile steel sheets, for which cooling control is difficult due to transformation heat generation is increasing. For this reason, there is an increasing demand for higher accuracy in the steel sheet temperature prediction model equation used for cooling control, and the coiling temperature is controlled by giving a cooling capacity by some function or a correction coefficient so that the coiling temperature falls within the target temperature range according to the cooling conditions of the steel sheet. For example, Patent Document 1 describes a technology for controlling the coiling temperature by creating an equation expressing the cooling capacity as a function of the temperature of the cooling water, the temperature of the steel sheet, and the water flow rate density of the cooling water, and further multiplying the equation by a correction coefficient called learning control. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-246320 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technology described in Patent Document 1 does not reflect the change in specific heat of the steel sheet due to transformation heat in the temperature prediction model formula. Therefore, the inventors of the present invention have proposed a method (Patent Application No. 2021-144784) in which the specific heat change amount due to pearlite transformation and magnetic transformation is calculated from the ferrite transformation temperature, and the calculated specific heat change amount is added to the austenite specific heat, thereby reflecting the change in specific heat of the steel sheet due to transformation heat in the temperature prediction model formula. However, this method does not take into account external factors such as aging of the equipment. In addition, although a thermometer that measures the temperature of the steel sheet during cooling is often installed on the runout table, the specific heat is not calculated taking into account the temperature of the steel sheet during cooling. For this reason, there was a demand for a technology that can accurately calculate the ferrite transformation temperature, which is the dominant parameter of the transformation heat amount of the steel sheet, taking into account external factors and the temperature of the steel sheet during cooling.

[0006] The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for calculating the ferrite transformation temperature of a steel plate, which is capable of calculating the ferrite transformation temperature with high accuracy by taking into account external factors and the temperature of the steel plate during cooling. Another object of the present invention is to provide a method for controlling the cooling of a steel plate, which is capable of precisely controlling the temperature of the steel plate at the completion of cooling to a cooling target temperature. Another object of the present invention is to provide a method for manufacturing a steel plate, which is capable of manufacturing a steel plate having desired properties with a high yield. [Means for solving the problem]

[0007] The method for calculating the ferrite transformation temperature of a steel plate according to the present invention is characterized in that it includes an estimation step of estimating a temperature change of the steel plate during cooling by spraying cooling water by calculating a temperature prediction model equation including, as variables, a correction coefficient multiplied by the heat transfer coefficient of the surface of the steel plate and a specific heat calculated from the ferrite transformation temperature, using actual data on cooling treatment of the steel plate by spraying cooling water; a back-calculation step of back-calculating, using the temperature prediction model equation, the correction coefficient such that the end-of-cooling temperature of the steel plate estimated in the estimation step coincides with an actual value of the end-of-cooling temperature; and a correction step of correcting the ferrite transformation temperature so that a difference between one or more mid-cooling temperatures of the steel plate estimated in the estimation step and the actual value of the mid-cooling temperature is reduced.

[0008] The method for calculating the ferrite transformation temperature of a steel plate according to the present invention is characterized in that, in the above-mentioned invention, it includes a step of determining an average value of the ferrite transformation temperatures obtained from a plurality of steel plates of the same steel type as a reference ferrite transformation temperature for each steel type of the steel plate.

[0009] The cooling control method for a steel plate according to the present invention is characterized in that it includes a step of estimating a temperature change of the steel plate during cooling by spraying cooling water by calculating the temperature prediction model equation using the reference ferrite transformation temperature obtained by the calculation method for the ferrite transformation temperature of a steel plate according to the present invention, and controlling the spray amount of the cooling water based on the estimated temperature change of the steel plate so that the temperature of the steel plate after cooling becomes a predetermined temperature.

[0010] The method for manufacturing a steel plate according to the present invention is characterized by including a step of manufacturing a steel plate by cooling the steel plate using the method for controlling cooling of the steel plate according to the present invention. Effect of the Invention

[0011] According to the method for calculating the ferrite transformation temperature of a steel plate of the present invention, the ferrite transformation temperature can be calculated with high accuracy by taking into account external factors and the temperature of the steel plate during cooling. Therefore, according to the method for controlling cooling of a steel plate of the present invention, the temperature of the steel plate at the completion of cooling can be controlled with high accuracy to the cooling target temperature. As a result, according to the method for manufacturing a steel plate of the present invention, a steel plate having desired properties can be manufactured with high yield. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a cooling facility in a hot rolling line according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a flowchart showing a flow of a ferrite transformation temperature calculation process according to an embodiment of the present invention. [Diagram 3] FIG. 3 is a diagram showing the temperature change history of the steel sheet. [Figure 4] FIG. 4 is a diagram showing the temperature change history of the steel sheet. [Diagram 5] FIG. 5 is a diagram showing changes in the temperature change history of a steel sheet with changes in the correction coefficient. [Figure 6] FIG. 6 is a diagram showing the change in specific heat with the change in correction coefficient. [Figure 7] FIG. 7 is a diagram showing the relationship between the ferrite transformation temperature and RMSE in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, a method for calculating the ferrite transformation temperature of a steel sheet, a cooling control method, and a manufacturing method according to an embodiment of the present invention will be described with reference to the drawings.

[0014] [Configuration of cooling equipment] First, with reference to FIG. 1, a configuration of a cooling facility in a hot rolling line according to an embodiment of the present invention will be described.

[0015] FIG. 1 is a schematic diagram showing a configuration of a cooling facility in a hot rolling line according to an embodiment of the present invention. As shown in FIG. 1, the cooling facility 1 in the hot rolling line according to an embodiment of the present invention includes a run-out table 2. In this embodiment, the length of the run-out table 2 is 130 m, and 13 cooling zones 2a, each 6 m long, are arranged. Each cooling zone controls the temperature of the steel sheet S to a cooling target temperature by spraying cooling water onto the upper and lower surfaces of the steel sheet S rolled by the finishing rolling mill. Thermometers 3a to 3d for measuring the temperature of the steel sheet S are arranged at the entry position of the cooling facility 1, at a position about 1 / 3 of the length of the run-out table 2 from the entry position, at a position about 2 / 3 of the length of the run-out table 2 from the entry position, and at the exit position. In addition, the cooling facility 1 includes a computer 4 for controlling the cooling of the steel sheet S. The computer 4 is configured by an information processing device such as a computer, and controls the flow rate of the coolant discharged from each cooling zone via a control panel 5. In addition, the calculator 4 can collect actual values ​​related to the cooling control of the steel sheet S (measurements of the thermometers 3a to 3d, actual values ​​of the flow rate of cooling water discharged from each cooling zone, the time the steel sheet S passed through each cooling zone, etc.). Reference numeral 6 in the figure denotes a winder.

[0016] In the cooling equipment 1 having such a configuration, the calculator 4 executes the ferrite transformation temperature calculation process described below, thereby accurately calculating the ferrite transformation temperature of the steel sheet S in consideration of external factors and the temperature of the steel sheet S during cooling. Hereinafter, the operation of the calculator 4 when executing the ferrite transformation temperature calculation process according to one embodiment of the present invention will be described with reference to Fig. 2. The operation of the calculator 4 described below is realized by the calculator 4 executing a computer program.

[0017] [Ferrite transformation temperature calculation process] Fig. 2 is a flowchart showing the flow of the ferrite transformation temperature calculation process according to one embodiment of the present invention. The flowchart shown in Fig. 2 starts when the cooling record data of the steel sheet S and an execution command for the ferrite transformation temperature calculation process are input to the calculator 4, and the ferrite transformation temperature calculation process proceeds to the processing of step S1. The cooling record data of the steel sheet S includes information on the cooling start temperature, mid-cooling temperature, and cooling end temperature record values ​​for each steel type of the steel sheet S stratified by the components and manufacturing conditions of the steel sheet S. There may be multiple mid-cooling temperature record values ​​for one steel sheet S (for example, the measured values ​​of the thermometer 3b and the thermometer 3c).

[0018] In the process of step S1, the calculator 4 calculates the ferrite transformation temperature T f Then, the process of step S1 is completed, and the ferrite transformation temperature calculation process proceeds to the process of step S2.

[0019] In the process of step S2, the calculator 4 calculates the ferrite transformation temperature T f and included in the cooling performance data for steel plate S ,at least Actual cooling start temperature of steel plate S The actual flow rate of cooling water discharged from each cooling zone when the steel plate passes through each cooling zone, and the time when the steel plate passes through each cooling zone The temperature change history of the steel sheet S by the cooling equipment 1 is calculated using the above formula. In this embodiment, the calculator 4 calculates the temperature change history of the steel sheet S, for example, as shown in FIG. 3, by solving the temperature prediction model formula shown in the following formula (1) that does not consider the temperature distribution in the thickness direction of the steel sheet S by a numerical solution method. In general, an online calculator (process computer) solves formula (1) at multiple positions (bank length pitch, etc.) in the longitudinal direction of the steel sheet S, but the temperature drop amount of the steel sheet S can be derived by numerically integrating formula (1). In addition, in FIG. 3, point P1 is the actual value of the cooling start temperature, point P2 is the actual value of the cooling end temperature, point P3 is the actual value of the cooling midway temperature, and L is a curve showing the calculated value of the temperature change history of the steel sheet S. As shown in FIG. 3, at the processing stage of this step S2, the curve L showing the calculated value of the temperature change history of the steel sheet S coincides only at the actual value P1 of the cooling start temperature.

[0020]

number

[0021] In formula (1), ρ is the density of steel plate S, c is the specific heat of steel plate S, d is the plate thickness of steel plate S, T is the temperature of steel plate S, t is time, and h is the water-cooling heat transfer coefficient. 、 T w is the temperature of the refrigerant, and α is a correction coefficient.

[0022] The specific heat c of the steel sheet S includes the transformation latent heat, and is calculated by the method proposed by the inventors of the present invention (Patent Application No. 2021-144784) to obtain the ferrite transformation temperature T f The ferrite transformation temperature T f Once the specific heat of pearlite and the specific heat of magnetic transformation are determined, the specific heat of pearlite and the specific heat of magnetic transformation can be determined uniquely. That is, the specific heat of steel sheet C(T) is expressed by the following formula (2). However, the present invention is not limited to this specific heat model, and can be applied to other specific heat calculation models that include the ferrite transformation temperature as a parameter.

[0023]

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[0024] Here, in formula (2), C o (T) is the austenite specific heat, C m (T) is the specific heat due to magnetic transformation, C f (T) is the specific heat due to the ferrite transformation, C p (T) indicates the specific heat due to pearlite transformation.

[0025] In addition, the austenite specific heat C o (T) is expressed by the following formula (3).

[0026]

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[0027] In addition, the specific heat C due to magnetic transformation m(T) is expressed by the following formulas (4) and (5). In formulas (4) and (5), T m denotes the magnetic transformation temperature, and the ferrite transformation temperature T f When the temperature is 770℃ or higher, the ferrite transformation temperature T f When is less than 770℃, the ferrite transformation temperature T f is considered to be the same as

[0028]

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[0029]

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[0030] In addition, the specific heat C due to the ferrite transformation f (T) is expressed by the following formula (6): In addition, in formula (6), f indicates the ferrite fraction, and is expressed by the following formula (7) using the carbon concentration C (%) of the steel sheet S.

[0031]

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[0032]

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[0033] In addition, the specific heat C due to pearlite transformation p (T) is the pearlite transformation temperature T p The pearlite transformation occurs at the ferrite transformation temperature T f In low-carbon steels with high carbon content, the transformation occurs at a certain temperature, while in steels with high carbon content, the transformation occurs at the ferrite transformation temperature T f As the temperature decreases, the pearlite transformation temperature T p It has been experimentally proven that the pearlite transformation temperature T pSince it shows the same characteristics as the magnetic transformation temperature, it is defined as the following formula (9).

[0034]

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[0035]

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[0036] This completes the process of step S2, and the ferrite transformation temperature calculation process proceeds to the process of step S3.

[0037] In the process of step S3, the calculator 4 back-calculates the correction coefficient α in the temperature prediction model equation so that the cooling end temperature calculated in the process of step S3 coincides with the actual value P2 of the cooling end temperature, as shown in Fig. 4. Here, the correction coefficient α is a coefficient multiplied by the water-cooling heat transfer coefficient h, and is a parameter for correcting errors when modeling physical phenomena and external factors such as aging of the equipment and clogged valves. By changing the correction coefficient α, it is possible to change the temperature change history of the steel sheet S starting from the cooling start temperature, as shown in Figs. 5(a) and (b). This means that the heat removal from the surface of the steel sheet S by the cooling water can be changed to any value.

[0038] However, although the cooling end temperature can be arbitrarily matched with the cooling start temperature as the starting point by changing the correction coefficient α, matching the cooling end temperature during cooling also changes the cooling end temperature, so it is not possible to match the cooling end temperature while keeping it consistent. As shown in FIG. 6, the cooling end temperature can be changed so that it approaches the target value while matching the cooling end temperature by changing the position of the peak of the specific heat due to transformation heat calculated from the transformation temperature together with the correction coefficient α. Therefore, in this embodiment, the correction coefficient α and the ferrite transformation temperature T are set so as to minimize the error between the actual and calculated values ​​of the cooling end temperature and the cooling end temperature. fThis completes the process of step S3, and the ferrite transformation temperature calculation process proceeds to step S4.

[0039] In the process of step S4, Calculator 4 calculates the error (temperature error) between the mid-cooling temperature calculated in the process of step S3 and the actual value P3 of the mid-cooling temperature. If there are multiple actual values ​​P3 of the mid-cooling temperature, Calculator 4 calculates the sum of the temperature errors calculated from each actual value. This completes the process of step S4, and the ferrite transformation temperature calculation process proceeds to the process of step S5.

[0040] In the process of step S5, the calculator 4 converts information about the temperature error calculated in the process of step S4 into the ferrite transformation temperature T f Then, the process of step S5 is completed, and the ferrite transformation temperature calculation process proceeds to step S6.

[0041] In the process of step S6, the calculator 4 calculates the ferrite transformation temperature T f is the maximum value T fmax (For example, 1000°C). As a result of the determination, the ferrite transformation temperature T f is the maximum value T fmax If so (Step S6: Yes), Calculator 4 advances the ferrite transformation temperature calculation process to the process of Step S8. f is the maximum value T fmax If not (Step S6: No), Calculator 4 advances the ferrite transformation temperature calculation process to the process of Step S7.

[0042] In the process of step S7, the calculator 4 calculates the ferrite transformation temperature T f The temperature range in which ferrite transformation occurs is limited to the range from 400°C to 1000°C, and the temperature range for cooling the steel sheet S after rolling is also within this range. In addition, the ferrite transformation temperature T fThe calculation accuracy of ferrite transformation temperature T f is varied from 400°C to 1000°C in 1°C increments to search for the minimum value of the temperature error. Although the brute force method is used in this embodiment, the minimum value of the temperature error may be searched for by other methods, such as a method in which initial values ​​are randomly selected and the temperature is found by the steepest descent method in order to reduce the amount of calculation and increase the increment width of the ferrite transformation temperature (improve calculation accuracy), so long as care is taken not to get caught in a local minimum value that is not the minimum value. This completes the process of step S7, and the ferrite transformation temperature calculation process returns to the process of step S2.

[0043] In the process of step S8, the calculator 4 calculates the ferrite transformation temperature T f The information is read out, and the ferrite transformation temperature T f This completes the process of step S8, and the ferrite transformation temperature calculation process proceeds to step S9.

[0044] In the process of step S9, Calculator 4 determines whether or not the process has been completed using all cooling history data related to the steel plate S of the same steel type. If the result of the determination is that the process has been completed using all cooling history data (step S9: Yes), Calculator 4 advances the ferrite transformation temperature calculation process to the process of step S10. On the other hand, if the process has not been completed using all cooling history data (step S9: No), Calculator 4 returns the ferrite transformation temperature calculation process to the process of step S1.

[0045] In the process of step S10, the calculator 4 calculates the ferrite transformation temperature T f The average value of these was calculated, and the calculated ferrite transformation temperature T f The average value of the ferrite transformation temperature T f This completes the process of step S10, and the series of ferrite transformation temperature calculation processes ends.

[0046] As is clear from the above description, in the ferrite transformation temperature calculation process according to one embodiment of the present invention, the calculator 4 calculates the correction coefficient α and the ferrite transformation temperature T f The temperature change of the steel plate during cooling by the injection of cooling water is estimated by calculating a temperature prediction model formula including the transformation heat amount calculated from the above as a variable, and the temperature prediction model formula is used to back-calculate a correction coefficient α that makes the estimated cooling end temperature of the steel plate coincide with the actual value of the cooling end temperature, and the ferrite transformation temperature T f This makes it possible to accurately calculate the ferrite transformation temperature by taking into account external factors and the temperature of the steel plate during cooling.

[0047] Furthermore, by calculating a temperature prediction model equation using the reference ferrite transformation temperature obtained by the ferrite transformation temperature calculation process according to one embodiment of the present invention, a change in temperature of the steel plate during cooling by injection of cooling water can be estimated, and by controlling the amount of injection of cooling water so that the temperature of the steel plate after cooling reaches a predetermined temperature based on the estimated temperature change of the steel plate, the temperature of the steel plate at the completion of cooling can be accurately controlled to a cooling target temperature. Furthermore, by manufacturing a steel plate by cooling the steel plate using this steel plate cooling control method, a steel plate having desired properties can be manufactured with a good yield.

[0048] [Modifications] In the above embodiment, the ferrite transformation temperature T f Therefore, when a steel sheet S of a steel type that has not been rolled in the past is rolled online, cooling control using an appropriate specific heat is not possible. Here, the inventors of the present invention applied the method of the above embodiment to all steel types for which cooling performance data could be collected, and calculated the ferrite transformation temperature T f The carbon equivalent of the steel and the ferrite transformation temperature T f It was found that there is a correlation between the carbon equivalent C eqwas calculated using the following formula (10): The variables of each element symbol in formula (10) indicate the mixing ratio (%) of the corresponding element contained in the steel grade.

[0049]

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[0050] From this, an approximation curve T showing the relationship between the average carbon equivalent and the average ferrite transformation temperature was obtained from a graph in which the horizontal axis is the average carbon equivalent for each steel type and the vertical axis is the average ferrite transformation temperature for each steel type. f =f(C eq ) can be obtained. Therefore, the calculation formula of the approximation curve is programmed in the control device, and when a steel type not included in the ferrite transformation temperature database is encountered, the carbon equivalent is calculated from the components of the steel type, and the ferrite transformation temperature is calculated from the calculation formula of the approximation curve and used for cooling control. Then, if sufficient cooling performance data can be collected to derive the ferrite transformation temperature for each steel type, the method proceeds to storing the ferrite transformation temperature for each steel type in a database based on the above embodiment and controlling the same. EXAMPLES

[0051] In this example, the effect of the present invention was verified for specific steel types of high-tensile steel classified by alloy components, rolling conditions, etc. The specific steel types are steel types that have a large amount of transformation heat generation and are considered to be difficult to control for cooling. Data on the cooling performance of the target steel types (number of data N=675) was collected, and the ferrite transformation temperature T f The ferrite transformation temperature T when the RMSE (root mean square error) of the error between the actual and calculated values ​​of the two cooling temperatures is minimized by changing the f The ferrite transformation temperature T f The calculation results are shown in Fig. 7. In the example shown in Fig. 7, the ferrite transformation temperature T f The average temperature was calculated to be 615°C, which was set as the standard ferrite transformation temperature for the target steel type.

[0052] The position of the steel plate where the ferrite transformation temperature was calculated was 180 m from the tip. This position is where tension is applied to the steel plate by the finishing mill and coils, where the effects of meandering of the steel plate and residual water on the top of the steel plate are minimal, and where feedback control does not function sufficiently, so the magnitude of the error in the temperature calculation is likely to have an effect on cooling control. The ferrite transformation temperature T f Using the specific heat calculated from the above, a simulation was performed for the actual cooling process of the target steel type. The simulation results are shown in Table 1 below, and it was confirmed that the calculation accuracy was improved compared to the conventional example. Specifically, for the mid-cooling temperature (intermediate temperature 1), the RMSE improved from 25.9°C to 20.8°C (19.6% improvement). Also, for the mid-cooling temperature (intermediate temperature 2), the RMSE improved from 36.7°C to 21.5°C (41.4% improvement). Also, for the cooling outlet temperature, the RMSE improved from 26.5°C to 17.5°C (33.9% improvement).

[0053] [Table 1]

[0054] Although the embodiments of the present invention have been described above, the present invention is not limited by the descriptions and drawings that form part of the disclosure of the present invention according to the present embodiments. In other words, other embodiments, examples, and operation techniques, etc., made by those skilled in the art based on the present embodiments are all included in the scope of the present invention. [Explanation of symbols]

[0055] 1 Cooling equipment 2 Runout Table 2a Cooling Zone 3a~3d Thermometer 4 calculator 5. Control Panel 6 Winder S steel plate

Claims

1. an estimation step of estimating a temperature change of the steel plate during cooling by spraying cooling water by calculating a temperature prediction model formula including, as variables, a correction coefficient multiplied by the heat transfer coefficient of the surface of the steel plate and a specific heat calculated from the ferrite transformation temperature, using at least a result value of the cooling start temperature of the steel plate, a result value of the flow rate of cooling water discharged from each cooling zone when the steel plate passes through each cooling zone, and a time taken for the steel plate to pass through each cooling zone, which are included in result data related to the cooling treatment of the steel plate by spraying cooling water; a back-calculation step of back-calculating the correction coefficient by using the temperature prediction model formula so that the cooling end temperature of the steel plate estimated in the estimation step coincides with an actual value of the cooling end temperature; a correction step of correcting the ferrite transformation temperature so that a difference between one or more mid-cooling temperatures of the steel plate estimated in the estimation step and an actual value of the mid-cooling temperature becomes small; A method for calculating the ferrite transformation temperature of a steel plate, comprising:

2. 2. The method for calculating the ferrite transformation temperature of a steel plate according to claim 1, further comprising a step of determining an average value of ferrite transformation temperatures obtained from a plurality of steel plates of the same steel type as a reference ferrite transformation temperature for each steel type of the steel plate.

3. 3. A cooling control method for a steel plate, comprising a step of estimating a temperature change of the steel plate during cooling by injection of cooling water by calculating the temperature prediction model formula using a reference ferrite transformation temperature obtained by the calculation method for the ferrite transformation temperature of a steel plate according to claim 2, and controlling an injection amount of the cooling water based on the estimated temperature change of the steel plate so that the temperature of the steel plate after cooling becomes a predetermined temperature.

4. A method for manufacturing a steel plate, comprising the step of manufacturing a steel plate by cooling the steel plate using the method for controlling cooling of the steel plate according to claim 3.

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