JP7647638B2 - Method for calculating ferrite transformation temperature of steel plate, cooling control method, and manufacturing method - Google Patents

Description

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.

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 rough rolling mill to produce a rough bar. Next, the rough bar is rolled to a predetermined thickness in a finishing rolling 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 the 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 caused by changes in the surface temperature of the steel sheet and changes in heat quantity caused by transformation of the steel sheet.

In recent steel sheet cooling control processes, the cooling target temperature range has become stricter, and there is an increase in steel sheets, such as high-tensile steel sheets, for which cooling control is difficult due to transformation heat generation. For this reason, there is an increasing demand for more accurate steel sheet temperature prediction model equations used in cooling control, and the coiling temperature is controlled by giving the cooling capacity using some kind of function or a correction coefficient so that the coiling temperature falls within the target temperature range depending on the cooling conditions of the steel sheet. For example, Patent Document 1 describes a technology for controlling the coiling temperature by creating an equation that represents the cooling capacity as a function of the cooling water temperature, the steel sheet temperature, and the cooling water flow rate density, and further multiplying it by a correction coefficient called learning control.

Japanese Patent Application Publication No. 6-246320

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 deterioration of the equipment over time. 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 has been 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.

The present invention has been made to solve the above problems, and its object is to provide a method for calculating the ferrite transformation temperature of a steel plate, which can accurately calculate the ferrite transformation temperature 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 can accurately control the temperature of the steel plate at the completion of cooling to a target cooling temperature. Another object of the present invention is to provide a method for manufacturing a steel plate, which can produce a steel plate having desired properties with a high yield.

The method for calculating the ferrite transformation temperature of a steel plate according to the present invention is characterized by including an estimation step of estimating the temperature change of the steel plate during cooling by injecting 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 the cooling treatment of the steel plate by injecting cooling water; a back-calculation step of back-calculating, using the temperature prediction model equation, the correction coefficient by which the end-of-cooling temperature of the steel plate estimated in the estimation step coincides with the actual value of the end-of-cooling temperature; and a correction step of correcting the ferrite transformation temperature so that the 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.

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

The cooling control method for steel plate according to the present invention is characterized by including a step of estimating the temperature change of the steel plate during cooling by spraying cooling water by calculating the temperature prediction model formula using the reference ferrite transformation temperature obtained by the calculation method for the ferrite transformation temperature of steel plate according to the present invention, and controlling the amount of spraying 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.

The method for manufacturing a steel plate according to the present invention is characterized by including a step of manufacturing the steel plate by cooling the steel plate using the cooling control method for the steel plate according to the present invention.

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 the desired characteristics can be manufactured with a high yield.

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. FIG. 2 is a flowchart showing a flow of a ferrite transformation temperature calculation process according to an embodiment of the present invention. FIG. 3 is a diagram showing the temperature change history of the steel sheet. FIG. 4 is a diagram showing the temperature change history of the steel sheet. FIG. 5 is a diagram showing changes in the temperature change history of the steel sheet with changes in the correction coefficient. FIG. 6 is a diagram showing the change in specific heat associated with the change in the correction coefficient. FIG. 7 is a diagram showing the relationship between the ferrite transformation temperature and RMSE in the examples.

Below, we will explain the method for calculating the ferrite transformation temperature of a steel plate, the cooling control method, and the manufacturing method, which are one embodiment of the present invention, with reference to the drawings.

[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.

Figure 1 is a schematic diagram showing the configuration of the cooling equipment in a hot rolling line according to one embodiment of the present invention. As shown in Figure 1, the cooling equipment 1 in the hot rolling line according to one embodiment of the present invention is provided with 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 equipment 1, at a position approximately 1/3 of the length of the run-out table 2 from the entry position, at a position approximately 2/3 of the length of the run-out table 2 from the entry position, and at the exit position. In addition, the cooling equipment 1 is provided with a computer 4 for controlling the cooling of the steel sheet S. The computer 4 is composed of an information processing device such as a computer, and controls the flow rate of the refrigerant 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 the cooling water discharged from each cooling zone, the time the steel sheet S passed through each cooling zone, etc.). The reference numeral 6 in the figure indicates a winder.

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, taking into account external factors and the temperature of the steel sheet S during cooling. Below, the operation of the calculator 4 when executing the ferrite transformation temperature calculation process, which is 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.

[Ferrite transformation temperature calculation process]
2 is a flowchart showing the flow of the ferrite transformation temperature calculation process according to an embodiment of the present invention. The flowchart shown in FIG. 2 starts when the cooling performance 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 performance data of the steel sheet S includes information on the performance values of the cooling start temperature, the cooling midway temperature, and the cooling end temperature 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 performance values of the cooling midway temperature for one steel sheet S (for example, the measured value of the thermometer 3b and the measured value of the thermometer 3c, etc.).

In the process of step S1, the calculator 4 sets an initial value of the ferrite transformation temperature _Tf (for example, 400° C.) This completes the process of step S1, and the ferrite transformation temperature calculation process proceeds to the process of step S2.

In the process of step S2, the calculator 4 calculates the temperature change history of the steel sheet S by the cooling equipment 1 using the ferrite transformation temperature _Tf set in the process of step S1 or step S7 and at least the actual value of the cooling start temperature of the steel sheet S , the actual value of the flow rate of the cooling water discharged from each cooling zone when the steel sheet passes through each cooling zone, and the time during which the steel sheet passes through each cooling zone, which are included in the cooling record data of the steel sheet S. 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) using a numerical solution method, which does not take into account the temperature distribution in the thickness direction of the steel sheet S. 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 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 temperature during cooling, and L is a curve showing the calculated values of the temperature change history of the steel sheet S. As shown in Fig. 3, at the processing stage of step S2, curve L showing the calculated values of the temperature change history of the steel sheet S coincides only at the actual value P1 of the cooling start temperature.

Here, in formula (1), ρ is the density of the steel sheet S, c is the specific heat of the steel sheet S, d is the plate thickness of the steel sheet S, T is the temperature of the steel sheet S, t is time, h is the water-cooling heat transfer coefficient , _Tw is the temperature of the coolant, and α is a correction coefficient.

The specific heat c of the steel sheet S includes transformation latent heat and can be derived from the ferrite transformation temperature T _f using the method proposed by the inventors of the present invention (Japanese Patent Application No. 2021-144784). When the ferrite transformation temperature T _f that is dominant in the transformation heat generation is determined, the pearlite specific heat and the magnetic transformation specific heat can also be uniquely determined. That is, the specific heat C (T) of the steel sheet is expressed by the following formula (2). However, the present invention is not limited to this specific heat model, and can also be applied to other specific heat calculation models that include the ferrite transformation temperature as a parameter.

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 ferrite transformation, and C _p (T) is the specific heat due to pearlite transformation.

The austenite specific heat C _o (T) is expressed by the following formula (3).

The specific heat _Cm (T) due to magnetic transformation is expressed by the following formulas (4) and (5): In formulas (4) and (5), _Tm indicates the magnetic transformation temperature, and is set to 770°C when the ferrite transformation temperature _Tf is 770°C or higher, and is set to the same as the ferrite transformation temperature _Tf when the ferrite transformation temperature _Tf is less than 770°C.

The specific heat C _f (T) due to ferrite transformation is expressed by the following formula (6): 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.

The specific heat _Cp (T) due to pearlite transformation is expressed by the following formula (8) using the pearlite transformation temperature _Tp . It has been experimentally found that pearlite transformation occurs at a constant temperature in low carbon _steel with a high ferrite transformation temperature _Tf , and that in steel with a high carbon content, the pearlite transformation temperature _Tp also decreases as the ferrite transformation temperature Tf decreases. For this reason, the pearlite transformation temperature _Tp shows the same characteristics as the magnetic transformation temperature, and is determined by the following formula (9).

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

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 matches 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 deterioration of the equipment over time 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.

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, if the cooling midway temperature is to be matched, the cooling end temperature also changes, so that the cooling midway temperature cannot be matched while keeping the cooling end temperature consistent. As shown in FIG. 6, the cooling midway temperature can be changed so that the cooling end temperature approaches the target value by changing the position of the peak of the specific heat due to the transformation heat calculated by the transformation temperature together with the correction coefficient α so that the cooling end temperature is matched. Therefore, in this embodiment, an optimization problem of two variables, the correction coefficient α and the ferrite transformation temperature _Tf, is solved so that the error between the actual value and the calculated value of the cooling midway temperature and the cooling end temperature is minimized. This completes the process of step S3, and the ferrite transformation temperature calculation process proceeds to the process of step S4.

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

In the process of step S5, the calculator 4 associates the information on the temperature error calculated in the process of step S4 with the information on the ferrite transformation temperature _Tf and stores them in the performance database. This completes the process of step S5, and the ferrite transformation temperature calculation process proceeds to the process of step S6.

In the process of step S6, Calculator 4 determines whether or not the ferrite transformation temperature _Tf used in the process of step S2 is the maximum value _Tfmax (e.g., 1000°C). If the result of the determination is that the ferrite transformation temperature _Tf is the maximum value _Tfmax (step S6: Yes), Calculator 4 advances the ferrite transformation temperature calculation process to the process of step S8. On the other hand, if the ferrite transformation temperature _Tf is not the maximum value _Tfmax (step S6: No), Calculator 4 advances the ferrite transformation temperature calculation process to the process of step S7.

In the process of step S7, the calculator 4 increases the ferrite transformation temperature _Tf used in the process of step S2 by 1°C. The temperature range in which ferrite transformation occurs is limited to the range of 400°C to 1000°C, and the temperature band in which the steel sheet S after rolling is cooled is also within this range. In addition, the calculation accuracy of the ferrite transformation temperature _Tf is sufficient even in increments of 1°C. For this reason, in this embodiment, the ferrite transformation temperature _Tf is changed in increments of 1°C from 400°C to 1000°C in a brute force manner to search for the minimum value of the temperature error. Note that, 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 the initial value is randomly selected and the temperature is calculated by the steepest descent method in order to reduce the amount of calculation and improve the increment width of the ferrite transformation temperature (improve the calculation accuracy), as long as care is taken not to get caught in a local minimum value that is not the minimum value. Thereby, the process of step S7 is completed, and the ferrite transformation temperature calculation process returns to the process of step S2.

In the process of step S8, the calculator 4 reads out information on the ferrite transformation temperature _Tf with the minimum temperature error calculated in the process of step S4 from the performance database, and stores the read-out ferrite transformation temperature _Tf . This completes the process of step S8, and the ferrite transformation temperature calculation process proceeds to the process of step S9.

In the process of step S9, the calculator 4 determines whether or not the process has been completed using all the 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 the cooling history data (step S9: Yes), the 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 the cooling history data (step S9: No), the calculator 4 returns the ferrite transformation temperature calculation process to the process of step S1.

In the process of step S10, the calculator 4 calculates the average value of the ferrite transformation temperatures _Tf stored in the process of step S8, and sets the calculated average value of the ferrite transformation temperatures _Tf as a representative value (reference ferrite transformation temperature) of the ferrite transformation temperatures _Tf of the steel plate S of the steel type being processed. This completes the process of step S10, and the series of ferrite transformation temperature calculation processes ends.

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 uses the actual value of the cooling start temperature of the steel plate during cooling by spraying cooling water to calculate a temperature prediction model equation including, as variables, the transformation heat amount calculated from the correction coefficient α and the ferrite transformation temperature _Tf , thereby estimating a temperature change in the steel plate during cooling by spraying cooling water, and uses the temperature prediction model equation to back-calculate the correction coefficient α by which the estimated cooling end temperature of the steel plate coincides with the actual value of the cooling end temperature, and corrects the ferrite transformation temperature _Tf so that the difference between the estimated mid-cooling temperature of the steel plate and the actual value of the mid-cooling temperature becomes small. Therefore, the ferrite transformation temperature can be calculated with high accuracy in consideration of external factors and the temperature of the steel plate during cooling.

In addition, by calculating a temperature prediction model equation using the reference ferrite transformation temperature obtained by the ferrite transformation temperature calculation process, which is one embodiment of the present invention, the temperature change of the steel plate during cooling by spraying cooling water is estimated, and the amount of sprayed cooling water is controlled so that the temperature of the steel plate after cooling becomes a predetermined temperature based on the estimated temperature change of the steel plate, thereby enabling the temperature of the steel plate at the completion of cooling to be precisely controlled to the cooling target temperature. Furthermore, by manufacturing steel plate by cooling the steel plate using this steel plate cooling control method, steel plate having the desired characteristics can be manufactured with a good yield.

[Modifications]
In the above embodiment, only the ferrite transformation temperature _Tf for the steel sheet S of a steel type that has been rolled a certain number of times in the past can be obtained, so 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 cannot be performed. Here, the inventors of the present invention calculated the ferrite transformation temperature _Tf by applying the method of the above embodiment to all steel types for which cooling performance data could be collected, and found that there is a correlation between the carbon equivalent of the steel type and the ferrite transformation temperature _Tf . The carbon equivalent _Ceq was calculated using the following formula (10). The variables of the element symbols in formula (10) indicate the blending ratio (%) of the corresponding element contained in the steel type.

In this way, an approximation curve _Tf = f(Ceq) showing the relationship between the average carbon equivalent and the average ferrite transformation temperature can be obtained from a graph with the average carbon equivalent for each steel type on the horizontal axis and the average ferrite transformation temperature for each steel type on the vertical _axis . Therefore, the calculation formula for the approximation curve is programmed into the control device, and when a steel type not 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 for 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 and controlling it based on the above embodiment.

In this embodiment, the effect of the present invention was verified for a specific steel type of high tensile steel classified by alloy components, rolling conditions, etc. The specific steel type is a steel type that is considered to have a large transformation heat generation amount and difficult to control cooling. The cooling performance data (number of data N=675) of the target steel type was collected, and the ferrite transformation temperature T _f was changed from 400°C to 1000°C for each steel plate in a round-robin manner, and the correction coefficient α was calculated by a method of back-calculating the correction coefficient α, and the ferrite transformation temperature T _f was calculated when the RMSE (root mean square error) of the error between the actual value and the calculated value of the two cooling intermediate temperatures was minimized. The calculation result of the ferrite transformation temperature T _f for a certain steel plate is shown in FIG. 7. In the example shown in FIG. 7, the average value of the obtained ferrite transformation temperatures T _f was calculated by removing low-reliability data such as data with abnormal thermometer data, and was 615°C. This was set as the reference ferrite transformation temperature for the target steel type.

The position of the steel plate where the ferrite transformation temperature was calculated was 180 m from the tip. This position is an area where tension is applied to the steel plate by the finishing rolling mill and the coil, the influence of meandering of the steel plate and residual water at the top of the steel _plate is minimal, and feedback control does not function sufficiently, so that the magnitude of the error in the temperature calculation is likely to affect the cooling control. Then, a simulation calculation of the actual cooling process of the target steel type was performed using the specific heat obtained from the ferrite transformation temperature Tf. The simulation calculation 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 intermediate cooling temperature (intermediate temperature 1), the RMSE improved from 25.9°C to 20.8°C (19.6% improvement). Also, for the intermediate 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).

The above describes an embodiment of the invention made by the inventors, but the present invention is not limited to the descriptions and drawings that form part of the disclosure of the present invention according to this embodiment. In other words, other embodiments, examples, and operational techniques made by those skilled in the art based on this embodiment are all included in the scope of the present invention.

1 Cooling equipment 2 Run-out table 2a Cooling zone 3a-3d Thermometer 4 Computer 5 Control panel 6 Winder S Steel plate

Claims (4)

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 intermediate cooling temperatures of the steel plate estimated in the estimation step and an actual value of the intermediate cooling temperature becomes small;
A method for calculating the ferrite transformation temperature of a steel plate, comprising: The method for calculating the ferrite transformation temperature of a steel plate according to claim 1, characterized in that it includes a step of determining the average value of the ferrite transformation temperatures obtained from each of a plurality of steel plates of the same steel type as the reference ferrite transformation temperature for each steel type of the steel plate. A cooling control method for a steel plate, comprising the steps of: calculating the temperature prediction model formula using the reference ferrite transformation temperature obtained by the method for calculating the ferrite transformation temperature of a steel plate described in claim 2; estimating the temperature change of the steel plate during cooling by injecting cooling water; and controlling the amount of injection 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. A method for manufacturing a steel plate, comprising a step of manufacturing the steel plate by cooling the steel plate using the steel plate cooling control method described in claim 3.

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