Method for predicting erosion depth of coated steel sheet

Abstract

To provide a prediction method of corrosion depth of a painting steel plate which can grasp corrosion depth of the painting steel plate at an early stage.SOLUTION: A prediction method of corrosion depth of a painting steel plate including a steel plate and a painted film arranged on the steel plate, comprises the steps of: starting corrosion test of the painted steel plate; detecting rust generation of the steel plate included in the painting steel plate after starting the corrosion test; and predicting the corrosion depth after the rust generation of the steel plate included in the painting steel plate based on a measured value or prediction value of corrosion depth of the same kind of steel plate when performing a corrosion test under the same conditions as the corrosion test, on the same kind of steel plate as the steel plated included in the painting steel plate.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for predicting the erosion depth of a painted steel sheet including a steel sheet and a coating film provided on the surface of the steel sheet.

Background Art

[0002] Conventionally, in the development of painted steel sheets with coatings such as electrodeposition coatings, corrosion tests such as composite cycle corrosion tests, salt spray tests, and exposure tests have been conducted to evaluate the corrosion resistance of painted steel sheets, assuming the actual use of painted steel sheets as automotive outer panels.

[0003] As methods for evaluating the corrosion resistance of painted steel sheets, various methods are used for the purpose of enhancing the correlation with the corrosion of actual automotive outer panels. For example, Patent Document 1 describes a method for evaluating the corrosion resistance of a surface-treated steel sheet with an electrodeposition coating used for an automotive hood panel, in which one or more types of processing such as flanging, draw bead, and flat sliding are applied to the surface-treated steel sheet, different or the same surface-treated steel sheets after the processing is applied are overlapped to form a steel sheet joint portion, and then the steel sheet with the steel sheet joint portion formed is used as a test piece and subjected to a corrosion environment to evaluate the corrosion resistance.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] On the one hand, the erosion of the steel sheet in the painted steel sheet is affected by multiple factors such as the blocking of corrosion factors such as moisture by the coating film such as the electrodeposition coating film and the adhesion between the steel sheet and the coating film, so it has been considered difficult to predict. For this reason, in the conventional corrosion resistance evaluation method of painted steel sheets, after conducting a corrosion test such as a composite cycle corrosion test on the painted steel sheet for a long time, the corrosion depth of the painted steel sheet is actually measured to evaluate the corrosion resistance of the painted steel sheet. On the other hand, in the development process of painted steel sheets, it has been required to grasp the erosion depth of the painted steel sheet at an earlier stage.

[0006] The present invention has been made in view of such points, and an object thereof is to provide a method for predicting the erosion depth of a painted steel sheet that can grasp the erosion depth of the painted steel sheet at an early stage.

Means for Solving the Problems

[0007] In order to solve the above problems, the inventors of the present invention have intensively studied, focusing on the fact that the corrosion process of the painted steel sheet when conducting a corrosion test on the painted steel sheet is divided into a process in which the anticorrosion action of the steel sheet due to the blocking of corrosion factors by the coating film and the adhesion of the coating film is maintained, and a process of erosion progress after the generation of red rust on the steel sheet included in the painted steel sheet. As a result, it has been found that the erosion after the generation of rust on the steel sheet when conducting a corrosion test on the painted steel sheet progresses at a certain rate later than the erosion of the steel sheet when conducting a corrosion test under the same conditions on a steel sheet of the same type as the steel sheet included in the painted steel sheet. And based on such findings, the inventors of the present invention have completed the method for predicting the erosion depth of the painted steel sheet of the present invention.

[0008] The method for predicting the erosion depth of a painted steel sheet according to the present invention is a method for predicting the erosion depth of a painted steel sheet including a steel sheet and a coating film provided on the surface of the steel sheet, the method comprising: a step of starting a corrosion test of the painted steel sheet; a step of detecting the occurrence of rust on the steel sheet included in the painted steel sheet after the start of the corrosion test; and a step of predicting the erosion depth of the steel sheet included in the painted steel sheet after the occurrence of the rust using the measured value or predicted value of the erosion depth of the same type of steel sheet when performing a corrosion test under the same conditions as the corrosion test on the same type of steel sheet as the steel sheet included in the painted steel sheet.

Advantages of the Invention

[0009] According to the present invention, the erosion depth of a painted steel sheet can be grasped at an early stage.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

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Figure 6

Figure 7

Figure 8

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Figure 10

[0011] Hereinafter, embodiments of the method for predicting the erosion depth of the coated steel sheet of the present invention will be described. First, the outline of the method for predicting the erosion depth of the coated steel sheet according to the embodiments will be described by exemplifying the first and second embodiments.

[0012] [First Embodiment] FIG. 1 is a diagram showing a flowchart of the procedure of the method for predicting the erosion depth of a painted steel sheet according to the first embodiment (left side), and a flowchart of the procedure of the method for obtaining the prediction formula for the maximum erosion depth of the steel sheet used in the prediction method according to the first embodiment (right side). FIG. 2(a) is a schematic cross-sectional view showing a main part of the painted steel sheet whose erosion depth is predicted by the prediction method according to the first embodiment, and FIG. 2(b) is a schematic cross-sectional view showing a main part of a steel sheet of the same type as the steel sheet included in the painted steel sheet shown in FIG. 2(a). FIG. 3 is a diagram for explaining the method for obtaining the prediction formula for the erosion depth of the painted steel sheet used in the prediction method according to the first embodiment. FIG. 4 is a diagram for explaining the method for predicting the erosion depth of the painted steel sheet according to the first embodiment. FIG. 5 is a diagram for explaining the step of detecting the occurrence of red rust on the steel sheet in the method for predicting the erosion depth of the painted steel sheet according to the first embodiment. FIG. 6 is a diagram schematically showing the configuration of a system for executing the method for predicting the erosion depth and the method for obtaining the prediction formula of the painted steel sheet according to the first embodiment. FIG. 7 is a diagram for explaining the composite cycle corrosion test in the conventional method for evaluating the corrosion resistance of a painted steel sheet.

[0013] As shown in FIG. 2(a), the painted steel sheet 1 whose erosion depth is predicted in the first embodiment includes a galvanized steel sheet 2 in which a zinc protective film 2B is provided on the surface 2As of the steel sheet 2A, and an electrodeposition coating film 4 provided on the surface 2s of the galvanized steel sheet 2 (the surface 2Bs of the zinc protective film 2B). The initial thickness of the steel sheet 2A is 0.8 mm.

[0014] In the method for predicting the erosion depth of the painted steel sheet according to the first embodiment, the maximum erosion depth of the steel sheet 2A included in the painted steel sheet 1 when a predetermined number of cycles are performed in the composite cycle corrosion test (CCT) of the painted steel sheet 1 is predicted. The composite cycle corrosion test is a corrosion test compliant with JASO M609, and is a test for evaluating the corrosion of the evaluation object by repeatedly performing a cycle of a process for accelerating corrosion on the evaluation object. Each cycle of the composite cycle corrosion test consists of three processes: a salt spray process, a drying process, and a wetting process.

[0015] In the conventional method for evaluating the corrosion resistance of coated steel sheets, when performing a composite cycle corrosion test on coated steel sheet 1, as shown in FIG. 7, for example, cycles of a specified number such as 120 times or 240 times were carried out, and the maximum erosion depth of steel sheet 2A after the implementation was actually measured. Then, whether the corrosion resistance of the coated steel sheet passed or failed was determined according to whether the actually measured maximum erosion depth was less than or equal to a certain ratio (for example, 80% or the like) of the initial thickness of steel sheet 2A. For this reason, it took time to find out whether the corrosion resistance of the coated steel sheet passed or failed, which affected the subsequent schedule.

[0016] In contrast, the present inventors focused on the specific content of the corrosion process in the composite cycle corrosion test of the painted steel sheet 1. Specifically, in the corrosion process of the painted steel sheet 1 in the composite cycle corrosion test, as shown in FIG. 7, the stage where erosion of the steel sheet 2A is avoided by blocking corrosion factors such as moisture in the salt water by the electrodeposition coating film 4 and the adhesion of the electrodeposition coating film 4, and the stage where erosion of the steel sheet 2A is avoided by sacrificial corrosion prevention of the zinc protective film 2B are passed through, and white rust generation due to deterioration of the electrodeposition coating film 4 and sacrificial corrosion prevention of the zinc protective film 2B occurs. As a result, when the corrosion factor reaches the steel sheet 2A, corrosion of the steel sheet 2A starts, and red rust begins to occur on the steel sheet 2A. And even after the occurrence of red rust, the erosion of the steel sheet 2A progresses, and finally holes due to erosion are generated in the steel sheet 2A. The present inventors focused on the fact that the corrosion process in such a composite cycle corrosion test of the painted steel sheet 1 is divided into a process in which the corrosion prevention action by blocking the corrosion factor by the electrodeposition coating film 4, the adhesion of the electrodeposition coating film 4, and the sacrificial corrosion prevention of the zinc protective film 2B is maintained, and a process in which the erosion progresses after the occurrence of red rust on the steel sheet 2A, and conducted intensive research. As a result, the present inventors found that when performing a composite cycle corrosion test on the painted steel sheet 1, the erosion after the occurrence of red rust on the steel sheet 2A progresses at a pace 0.2 times that of the erosion of the steel sheet 12A shown in FIG. 2(b) which exists as a single body of the same type as the steel sheet 2A provided in the painted steel sheet 1 under the same conditions of the composite cycle corrosion test. Furthermore, it was found that the relationship between the number of cycles and the maximum erosion depth in the composite cycle corrosion test of the steel sheet can be approximated by a sigmoid function and predicted. In the method for predicting the erosion depth of the painted steel sheet according to the first embodiment, based on these findings, a prediction formula for the maximum erosion depth of the steel sheet 2A provided in the painted steel sheet 1 is acquired in advance. Then, the maximum erosion depth of the steel sheet 2A provided in the painted steel sheet 1 is predicted using the prediction formula.

[0017] (Method for obtaining the prediction formula for the maximum erosion depth of the steel sheet) Here, with reference to FIGS. 1 and 3, the procedure for the method for obtaining the prediction formula for the maximum erosion depth of the steel sheet 2A provided in the painted steel sheet 1 will be described. Note that each process in the method for obtaining the prediction formula is performed using the processing device 21, the storage device 22, the input device 23, the display device 24, etc. in the system 20 shown in FIG. 6.

[0018]

[0018] In the method for obtaining the prediction formula of the maximum erosion depth of the steel plate 2A included in the painted steel plate 1, first, as shown in the flowchart on the right side of FIG. 1 and FIG. 3, after preparing the steel plate 12A that exists as a single unit of the same type as the steel plate 2A included in the painted steel plate 1, a CCT tester (not shown) is used to perform a composite cycle corrosion test on the steel plate 12A of the same type (corrosion test process of the steel plate of the same type). The steel plate 12A of the same type has an initial thickness of 0.8 mm, which is the same as that of the steel plate 2A of the painted steel plate 1.

[0019] In the corrosion test process of the steel plate of the same type, the conditions of the composite cycle corrosion test are made the same as those of the composite cycle corrosion test performed on the painted steel plate 1 in the prediction method of the erosion depth of the painted steel plate, except that the number of cycles of the cycle is adjusted. Then, in the composite cycle corrosion test of the steel plate 12A of the same type, the maximum erosion depth of the steel plate 12A is measured after each cycle. For measuring the maximum erosion depth of the steel plate 12A, for example, VR-5000 manufactured by Keyence Corporation is used. Thereby, the number of combinations of the number of cycles of the composite cycle corrosion test of the steel plate 12A and the maximum erosion depth (measured value) of the steel plate 12A at the time when the cycles of that number are performed is obtained as many as necessary for obtaining the prediction formula. Then, by using each device of the system 20, as shown in the graph of FIG. 3, a plurality of combinations of the number of cycles and the maximum erosion depth in the composite cycle corrosion test of the steel plate 12A are plotted in a two-dimensional coordinate system (x: number of cycles, y: maximum erosion depth).

[0020] Next, as shown in the flowchart on the right side of FIG. 1, based on the above findings, a prediction formula for the maximum erosion depth of the steel plate 2A included in the painted steel plate 1 is obtained from the relationship between the number of cycles and the maximum erosion depth in the composite cycle corrosion test of the steel plate 12A plotted in the two-dimensional coordinate system (prediction formula acquisition process).

[0021] In the prediction formula acquisition process, first, based on the above finding that the relationship between the number of cycles and the maximum erosion depth in the composite cycle corrosion test of the steel plate can be approximated by a sigmoid function, by using each device of the system 20, as shown in the graph of FIG. 3, the relationship between the number of cycles and the maximum erosion depth in the composite cycle corrosion test of the steel plate 12A plotted in the two-dimensional coordinate system is approximated by the following sigmoid function formula (1). Specifically, the values of coefficients a to d are appropriately set so that the relationship between the number of cycles and the maximum erosion depth in the composite cycle corrosion test of the steel plate 12A can be approximated by formula (1). Thereby, formula (1) is used as a prediction formula for predicting the maximum erosion depth of the steel plate 12A when a predetermined number of cycles are performed in the composite cycle corrosion test of the steel plate 12A of the same type as the steel plate 2A provided in the painted steel plate 1.

[0022]

Number

[0023] Subsequently, based on the above finding that the erosion after the occurrence of red rust of the steel plate 2A in the case of performing the composite cycle corrosion test on the painted steel plate 1 proceeds at a pace 0.2 times that of the erosion of the steel plate 12A in the case of performing the composite cycle corrosion test under the same conditions on the steel plate 12A of the same type as the steel plate 2A provided in the painted steel plate 1, a prediction formula for the maximum erosion depth of the steel plate 2A provided in the painted steel plate 1 is obtained from the above prediction formula (1). Specifically, as an expression representing the relationship between the number of cycles and the maximum erosion depth when the erosion pace of the steel plate 12A changes by 0.2 times in the composite cycle corrosion test of the steel plate 12A of the same type as the steel plate 2A provided in the painted steel plate 1, the following formula (2) is obtained by adding 0.2 as a coefficient to the number of cycles x in the above prediction formula (1). And for the steel plate 2A provided in the painted steel plate 1, in order to take into account that the erosion proceeds after the occurrence of red rust, which is different from the steel plate 12A existing as a single unit of the same type, the number of cycles at the time of the occurrence of red rust of the steel plate 2A in the composite cycle corrosion test of the painted steel plate 1 is set as x 0 and, in the following formula (2), x is replaced with (x - x 0) is replaced. As a result, the following prediction formula (3) is obtained as the prediction formula for the maximum erosion depth of the steel sheet 2A included in the painted steel sheet 1. Then, the following prediction formula (3) is stored in the storage device 22.

[0024]

Number

[0025]

Number

[0026] (Method for Predicting Erosion Depth of Painted Steel Sheet) Subsequently, with reference to FIGS. 1, 4, and 5, a method for predicting the erosion depth of the painted steel sheet according to the first embodiment will be described. Each process in the method for predicting the erosion depth is performed using the processing device 21, the storage device 22, the input device 23, the display device 24, etc. in the system 20 shown in FIG. 6.

[0027] In the method for predicting the erosion depth of the painted steel sheet according to the first embodiment, first, as shown in the left flowchart of FIG. 1 and FIG. 4, a painted steel sheet 1 including a galvanized steel sheet 2 having a zinc protective film 2B provided on the surface 2As of the steel sheet 2A and an electrodeposition coating film 4 provided on the surface 2s of the galvanized steel sheet 2 is prepared, and then a composite cycle corrosion test is started on the painted steel sheet 1 using a CCT tester (not shown) (corrosion test start step). The conditions of the composite cycle corrosion test at this time are the same as those of the composite cycle corrosion test performed on the steel sheet 12A in the method for obtaining the prediction formula for the maximum erosion depth of the steel sheet as described above.

[0028] Next, as shown in the flowchart on the left side of FIG. 1, FIG. 4, and FIG. 5, the generation of red rust on the steel plate 2A included in the painted steel plate 1 is detected (rust generation detection step). At this time, after each cycle of the combined cycle corrosion test, as shown in FIG. 5, the surface 4s of the electrodeposited coating film 4 of the painted steel plate 1 is visually observed using an optical microscope to confirm the presence or absence of red rust emerging on the surface 4s of the electrodeposited coating film 4. When red rust emerging on the surface 4s of the electrodeposited coating film 4 can be visually recognized, the generation of red rust on the steel plate 2A is detected thereby, and the number of cycles (for example, 60 times) at the time when red rust can be visually recognized is input into the system 20 via the input device 23 as the number of cycles at the time of red rust generation and stored in the storage device 22. In the combined cycle corrosion test of the painted steel plate 1, the cycle is repeated until red rust can be visually recognized, and the test is terminated when red rust can be visually recognized. That is, as shown in FIG. 4, after the stage where erosion of the steel plate 2A is avoided by blocking the corrosion factor by the electrodeposited coating film 4 and the adhesion of the electrodeposited coating film 4, and the stage where erosion of the steel plate 2A is avoided by sacrificial corrosion prevention of the zinc protective film 2B, when the corrosion factor reaches the steel plate 2A, the corrosion of the steel plate 2A starts, and the test is terminated when red rust starts to occur on the steel plate 2A.

[0029] Next, as shown in the flowchart on the left side of FIG. 1 and FIG. 4, using the prediction formula for the maximum erosion depth of the steel plate 2A included in the pre-acquired painted steel plate 1 and the number of cycles at the time of red rust generation, the maximum erosion depth of the steel plate 2A included in the painted steel plate 1 when a predetermined number of cycles are performed in the combined cycle corrosion test of the painted steel plate 1 is predicted (erosion depth prediction step).

[0030] In the erosion depth prediction process, by using each device of the system 20, in the prediction formula (the above formula (3)) of the maximum erosion depth of the steel plate 2A included in the painted steel plate 1 stored in the storage device 22, the number of times x0 of the cycle at the time of red rust generation stored in the storage device 22 is assigned to the number of times of implementation of the cycle at the time of red rust generation (for example, 60 times). Further, in the obtained unique prediction formula, a predetermined number of times (for example, 120 times or 240 times) for which it is desired to predict the maximum erosion depth of the steel plate 2A with respect to the number of times x of the cycle is input. Thereby, as the predicted value of the maximum erosion depth of the steel plate 2A included in the painted steel plate 1 when a predetermined number of cycles are performed in the composite cycle corrosion test of the painted steel plate 1, the value of y of the above unique prediction formula is acquired. In this way, the maximum erosion depth of the steel plate 2A included in the painted steel plate 1 when a predetermined number of cycles are performed in the composite cycle corrosion test of the painted steel plate 1 is predicted.

[0031] According to the method for predicting the erosion depth of the painted steel plate according to the first embodiment, by using the prediction formula of the maximum erosion depth of the steel plate 2A included in the painted steel plate 1 acquired in advance, when a predetermined number of cycles are performed in the composite cycle corrosion test of the painted steel plate 1, the maximum erosion depth of the steel plate 2A included in the painted steel plate 1 can be predicted at the time when the occurrence of red rust is detected. Thereby, in the composite cycle corrosion test of the painted steel plate 1, it is possible to determine whether the corrosion resistance of the painted steel plate 1 is acceptable at the time when the occurrence of red rust is detected. For this reason, at an earlier stage, it is possible to take measures according to whether the composite cycle corrosion test of the painted steel plate 1 is acceptable.

[0032] [Second Embodiment] FIG. 8 is a diagram showing a flowchart of the procedure of the method for predicting the erosion depth of the painted steel plate according to the second embodiment. Each process in the method for predicting the erosion depth of the painted steel plate 1 according to the second embodiment is performed using the processing device 21, the storage device 22, the input device 23, the display device 24, etc. in the system 20 shown in FIG. 6.

[0033] In the method for predicting the erosion depth of the coated steel sheet according to the second embodiment, similar to the prediction method according to the first embodiment, when performing a predetermined number of cycles in the combined cycle corrosion test (CCT) of the coated steel sheet 1, the maximum erosion depth of the steel sheet 2A included in the coated steel sheet 1 is predicted. Then, the measured values of the maximum erosion depth of each cycle after the implementation of the combined cycle corrosion test of the steel sheet 12A existing as a single body of the same type as the steel sheet 2A included in the coated steel sheet 1 are acquired in advance and stored in the storage device 22. Note that the conditions of the combined cycle corrosion test of the steel sheet 12A of the same type are the same as those of the combined cycle corrosion test performed on the coated steel sheet 1 in the method for predicting the erosion depth of the coated steel sheet, except that the number of cycles implemented is adjusted. And in the method for predicting the erosion depth of the coated steel sheet, the maximum erosion depth of the steel sheet 2A included in the coated steel sheet 1 is predicted using the measured values of the maximum erosion depth of the steel sheet 12A acquired in advance.

[0034] In the method for predicting the erosion depth of the coated steel sheet according to the second embodiment, first, as shown in FIG. 8, similar to the method for predicting the erosion depth of the coated steel sheet according to the first embodiment, after preparing the coated steel sheet 1, the combined cycle corrosion test of the coated steel sheet 1 is started (corrosion test start step).

[0035] Next, as shown in FIG. 8, similar to the prediction method according to the first embodiment, the generation of red rust on the steel sheet 2A included in the coated steel sheet 1 is detected (rust generation detection step). At this time, similar to the prediction method according to the first embodiment, the number of cycles implemented (for example, 60 times) at the time when red rust can be visually recognized is stored in the storage device 22 as the number of cycles implemented at the time of red rust generation.

[0036] Next, as shown in FIG. 8, using the measured value of the maximum erosion depth of the steel sheet 12A acquired in advance and the number of cycles implemented at the time of red rust generation, the maximum erosion depth of the steel sheet 2A included in the coated steel sheet 1 when performing a predetermined number of cycles in the combined cycle corrosion test of the coated steel sheet 1 is predicted (erosion depth prediction step).

[0037] In the erosion depth prediction step, based on the above finding that the erosion after the occurrence of red rust of the steel plate 2A in the case of performing a combined cycle corrosion test on the painted steel plate 1 progresses at a pace that is 0.2 times the erosion of the steel plate 12A in the case of performing a combined cycle corrosion test under the same conditions on the steel plate 12A of the same type as the steel plate 2A provided in the painted steel plate 1, when performing a predetermined number of cycles in the combined cycle corrosion test of the painted steel plate 1, a predicted value of the maximum erosion depth of the steel plate 2A provided in the painted steel plate 1 is obtained. In this case, an appropriate measured value is selected from the measured values of the maximum erosion depth of the steel plate 12A stored in the storage device 22 (the measured values of the maximum erosion depth after each cycle in the combined cycle corrosion test of the steel plate 12A) and obtained as the predicted value. Specifically, first, the number of times obtained by further multiplying by 0.2 the number of times obtained by subtracting the number of cycles at the time of red rust occurrence (for example, 60 times) stored in the storage device 22 from the predetermined number of cycles (for example, 120 times or 240 times) of the cycle for which the maximum erosion depth of the steel plate 2A provided in the painted steel plate 1 is to be predicted is obtained. Subsequently, from among the measured values of the maximum erosion depth of the steel plate 12A (the measured values of the maximum erosion depth after each cycle in the combined cycle corrosion test of the steel plate 12A), the measured value of the maximum erosion depth after the implementation of the cycle of the obtained number of times is selected and obtained as the predicted value. In this way, the maximum erosion depth of the steel plate 2A provided in the painted steel plate 1 when performing a predetermined number of cycles in the combined cycle corrosion test of the painted steel plate 1 is predicted.

[0038] According to the method for predicting the erosion depth of the painted steel plate according to the second embodiment, by using the measured value of the maximum erosion depth of the steel plate 12A obtained in advance, when performing a predetermined number of cycles in the combined cycle corrosion test of the painted steel plate 1, the maximum erosion depth of the steel plate 2A provided in the painted steel plate 1 can be predicted at the time when the occurrence of red rust is detected. Thereby, in the combined cycle corrosion test of the painted steel plate 1, it is possible to determine whether the corrosion resistance of the painted steel plate 1 is acceptable or not at the time when the occurrence of red rust is detected. For this reason, at an earlier stage, it is possible to take measures according to whether the combined cycle corrosion test of the painted steel plate 1 is successful or not.

[0039] (Function and Effect) Therefore, according to the method for predicting the erosion depth of the coated steel sheet according to the embodiment, as in the first and second embodiments, the erosion depth of the coated steel sheet can be grasped at an early stage.

[0040] Subsequently, the details of the configuration of each process and the like in the method for predicting the erosion depth of the coated steel sheet according to the embodiment will be described.

[0041] 1. Corrosion test start process In the corrosion test start process, the corrosion test of the above-mentioned coated steel sheet is started. Here, the "corrosion test" refers to a test for evaluating the corrosion resistance of the evaluation object under conditions that promote the corrosion of the evaluation object. Examples of the method of the corrosion test include a composite cycle corrosion test (CCT) conforming to JASO M609. In the composite cycle corrosion test conforming to JASO M609, for the evaluation object, for example, a cycle consisting of three processes: a salt spray process (2 hours, 35 ± 1°C, 5% NaCl solution), a drying process (4 hours, 60 ± 1°C), and a wetting process (2 hours, 50 ± 1°C, > 95% RH) is repeated to evaluate the corrosion of the evaluation object.

[0042] The type of the coated steel sheet is not particularly limited as long as it is a coated steel sheet including a steel sheet and a coating film provided on the surface of the steel sheet. For example, in addition to a galvanized steel sheet having a zinc protective film plating film provided on the surface of the steel sheet and a coating film provided on the surface of the galvanized steel sheet, there are also those including a steel sheet and a coating film directly provided on the surface of the steel sheet. Further, the type of the coating film is not particularly limited, and examples thereof include an electrodeposition coating film.

[0043] 2. Rust generation detection process In the rust generation detection step, rust generation on the steel sheet included in the painted steel sheet is detected after the start of the corrosion test. The method for detecting rust generation on the steel sheet included in the painted steel sheet is not particularly limited, and an appropriate method is applied as necessary according to the corrosion test method. When the corrosion test method is a combined cycle corrosion test, for example, after each cycle of the combined cycle corrosion test is carried out, the surface of the electrodeposited coating film of the painted steel sheet is visually observed to detect rust generation on the steel sheet.

[0044] 3. Erosion depth prediction step In the erosion depth prediction step, the erosion depth after the rust generation of the steel sheet included in the painted steel sheet is predicted using the measured value or predicted value of the erosion depth of the same type of steel sheet as the steel sheet included in the painted steel sheet in the corrosion test under the same conditions as the corrosion test.

[0045] The method for predicting the erosion depth after rust generation of the steel sheet included in the painted steel sheet using the measured value of the erosion depth of the same type of steel sheet as the steel sheet included in the painted steel sheet is not particularly limited. When the corrosion test method is a combined cycle corrosion test, for example, the measured value of the maximum erosion depth after each cycle in the combined cycle corrosion test of the same type of steel sheet as the steel sheet included in the painted steel sheet is used to predict the erosion depth after rust generation of the steel sheet included in the painted steel sheet.

[0046] The method for predicting the erosion depth after rust generation of the steel sheet included in the painted steel sheet using the predicted value of the erosion depth of the same type of steel sheet as the steel sheet included in the painted steel sheet is not particularly limited. When the corrosion test method is a combined cycle corrosion test, for example, a prediction formula for predicting the maximum erosion depth of the steel sheet when a predetermined number of cycles are carried out in the combined cycle corrosion test of the same type of steel sheet as the steel sheet included in the painted steel sheet is used to predict the erosion depth after rust generation of the steel sheet included in the painted steel sheet.

[0047] As a prediction formula for predicting the maximum erosion depth of a steel sheet when performing a specified number of cycles in a composite cyclic corrosion test of a steel sheet of the same type as the steel sheet included in the painted steel sheet, for example, the above formula (1) can be cited. And, as a prediction formula for predicting the maximum erosion depth of the steel sheet included in the painted steel sheet when performing a specified number of cycles in the composite cyclic corrosion test of the painted steel sheet, for example, the above formula (3) may be used, but the following formula (4) obtained by generalizing the above formula (3) can be used.

[0048] [Number]

[0049] For example, when the erosion after rust generation of the steel sheet in the case of performing a corrosion test on the painted steel sheet progresses at a pace that is 0.5 times the erosion of the steel sheet in the case of performing a corrosion test under the same conditions on a steel sheet of the same type as the steel sheet included in the painted steel sheet, t becomes 0.5 in formula (4). In the case of the first embodiment, t becomes 0.2 in formula (4), and formula (4) becomes formula (3).

[0050] 4. Method for Predicting Erosion Depth of Painted Steel Sheet The method for predicting the erosion depth of a painted steel sheet is a method for predicting the erosion depth of a painted steel sheet including a steel sheet and a coating film provided on the surface of the steel sheet, and includes a corrosion test start step, a rust generation detection step, and an erosion depth prediction step. [Examples]

[0051] Hereinafter, with reference to reference examples and examples, the method for predicting the erosion depth of the painted steel sheet according to the embodiment will be further described.

[0052] [Reference Example] Composite cyclic corrosion tests under predetermined conditions were conducted on the painted steel sheets of Reference Examples 1 to 10. The painted steel sheets of Reference Examples 1 to 10 are painted steel sheets of the same specification, comprising a galvanized steel sheet with a zinc protective film provided on the surface of the steel sheet (thickness: 0.8 mm) and an electrodeposited coating film provided on the surface of the galvanized steel sheet. Further, a composite cyclic corrosion test under the same conditions as the test of the painted steel sheet was conducted on a steel sheet of the same type as the steel sheet comprised in the painted steel sheets of Reference Examples 1 to 10. The steel sheet of the same type is a single steel sheet of the same type and the same thickness as the steel sheet comprised in the painted steel sheet.

[0053] Figure 9(a) is a graph showing the relationship between the number of cycles executed in the composite cyclic corrosion test of a steel sheet of the same type as the steel sheet comprised in the painted steel sheets of Reference Examples 1 to 10 and the maximum erosion depth of the steel sheet. Figure 9(b) is a graph showing the relationship between the number of cycles executed after the occurrence of red rust in the composite cyclic corrosion test of the painted steel sheets of Reference Examples 1 to 10 and the maximum erosion depth of the steel sheet. Figure 9(c) shows the graph of Figure 9(b) and the predicted graph together. The predicted graph shown in Figure 9(c) is obtained by multiplying the number of cycles executed in the graph of the composite cyclic corrosion test of a steel sheet of the same type as the steel sheet comprised in the painted steel sheet shown in Figure 9(a) by 5.

[0054] In Figure 9(c), since the predicted graph overlaps with the graph of Figure 9(b), it can be seen that the erosion of the steel sheet after the occurrence of red rust in the composite cyclic corrosion test of the painted steel sheet proceeds at a pace of 0.2 times the erosion of the steel sheet in the composite cyclic corrosion test of a steel sheet of the same type as the steel sheet comprised in the painted steel sheet.

[0055] [Examples] A combined cycle corrosion test under predetermined conditions was conducted on the painted steel sheets of Examples 1 to 4. The painted steel sheets of Examples 1 to 4 are painted steel sheets of the same specification, and include a galvanized steel sheet with a zinc protective film provided on the surface of the steel sheet (thickness: 0.8 mm), and an electrodeposited coating film provided on the surface of the galvanized steel sheet. Further, a combined cycle corrosion test under the same conditions as the test of the painted steel sheets of Examples 1 to 4 was conducted on a steel sheet of the same type as the steel sheet included in the painted steel sheets of Examples 1 to 4. The steel sheet of the same type is a single steel sheet of the same type and the same thickness as the steel sheet included in the painted steel sheet. Then, using a plurality of combinations of the number of cycles and the maximum erosion depth (measured value) in the combined cycle corrosion test of the steel sheet of the same type, in the same manner as in the first embodiment, a prediction formula for the maximum erosion depth of the steel sheet included in the painted steel sheets of Examples 1 to 4 (the number of cycles at the time of red rust generation is assigned to x0 in the above formula (3)) was obtained.

[0056] FIG. 10(a) plots the relationship between the number of cycles and the maximum erosion depth (measured value) of the steel sheet in the combined cycle corrosion test of the painted steel sheet of Example 1, and shows a graph of the prediction formula for the maximum erosion depth of the steel sheet included in the painted steel sheet of Example 1. FIGS. 10(b) to 10(d) are similar graphs for the painted steel sheets of Examples 2 to 4.

[0057] As shown in FIGS. 10(a) to 10(d), in any of Examples 1 to 4, the plots of the maximum erosion depth (measured value) of the steel sheet in the combined cycle corrosion test of the painted steel sheet overlapped with the graph of the prediction formula.

[0058] As described above, the embodiments according to the present invention have been described in detail. However, the present invention is not limited to the above embodiments, and various design changes can be made without departing from the spirit of the present invention described in the claims.

Explanation of Reference Numerals

[0059] 1 Painted steel sheet 2 Galvanized steel sheet 2s Surface 2A Steel sheet 2As Surface 2B Zinc Protective Film 2Bs Surface 4 Electrodeposited Coating Film

Claims

【Claim 1】 A method for predicting the maximum erosion depth of a painted steel sheet comprising a steel sheet and a coating film provided on the surface of the steel sheet, the method comprising: starting a combined cycle corrosion test on the painted steel sheet; detecting the occurrence of rust on the steel sheet comprised in the painted steel sheet after the start of the combined cycle corrosion test, and setting the number of cycles executed at the time of detection of the occurrence of rust as the number of cycles executed at the time of rust occurrence (x0); predicting the maximum erosion depth after the occurrence of rust on the steel sheet comprised in the painted steel sheet when the number of cycles executed in the combined cycle corrosion test is x times, using the following prediction formula (4). A method for predicting the erosion depth of a painted steel sheet, characterized by comprising the above steps. 【Number 1】 x: Number of cycles executed y: Maximum erosion depth a: Gain (rate of change) b: Adjustment magnification c: Parallel movement in the x direction (point where the change in y with respect to x is maximum) d: Parallel movement in the y direction x0: Number of cycles executed at the time of rust occurrence t: Ratio of the erosion rate of the steel sheet comprised in the painted steel sheet to the erosion rate of a single steel sheet of the same type as the steel sheet comprised in the painted steel sheet (t < 1)

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