JP2519809B2 - Coating steel sheet and its evaluation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a coating steel sheet excellent in coating film clarity (image clarity on the surface of a steel sheet after coating) and coating film of various coating steel sheets. The present invention relates to a method for evaluating a coating steel plate for evaluating the image clarity, and particularly, excellent coating film clarity even for various coating steel plates with different dulling methods (for example, laser dull steel plate, shot dull steel plate, discharge dull steel plate). TECHNICAL FIELD The present invention relates to a steel sheet for coating which has an excellent property and is capable of accurately evaluating the sharpness of a coating film, and an evaluation method thereof.

<Prior Art> Conventionally, a cold-rolled steel sheet, which is a typical example of a steel sheet for painting used for automobile bodies and household electric appliances, is manufactured by degreasing and cleaning after cold rolling, and then temper-rolling after annealing. It is usually done. Here, one of the purposes of temper rolling is to impart moderate surface roughness to the steel sheet surface by performing light rolling using a work roll whose surface is dull finished, and to prevent seizure resistance of the steel sheet during press forming. Is to improve.

By the way, as a method for dull-finishing the surface of the work roll, conventionally, a method of shot blasting in which a shot is projected on the roll surface to perform dull processing, and a method of discharging in a working liquid to melt and scatter the roll surface A method by electric discharge machining for obtaining a predetermined dull surface has been put into practical use. In the case of dull finishing of work rolls by these methods, since the irregular roughness profile peculiar to each dulling method is formed on the roll surface, the rolled steel plate surface is composed of irregular peaks and valleys. If such a steel plate is pressed, the lubricating oil is stored in the valleys to reduce the frictional force between the press die and the steel sheet, facilitating the press work and at the same time peeling due to the frictional force with the die. The press formability can be improved by, for example, preventing the seizure by trapping the formed metal powder in the valley portion.

In recent years, not only passenger cars, but also light vehicles, wagons, and trucks can directly appeal to customers the overall high quality after painting, so the quality of the painted surface is extremely important. It is a quality control item. By the way, there are various evaluation items for the coated surface.

Among them, it is particularly important that the diffused reflection of the painted surface is small and the gloss is excellent, and that the distortion of the image is small, that is, the image clarity is excellent, and these glossiness and image clarity are generally combined. It is called "coating clarity".

Therefore, the film clarity of a steel sheet rolled by a work roll processed by shot blasting (hereinafter referred to as a shot dull steel sheet) and a steel sheet rolled by an electric discharge machined work roll (hereinafter referred to as a discharge dull steel sheet) In order to improve, in recent years dull processing by high-density beam energy such as laser, i.e., irradiating a rotating work roll with an intermittent laser beam of a short pitch close to the vertical while moving parallel to the rotation axis of the work roll, BACKGROUND ART A coating steel sheet is rolled by a work roll having a roll surface on which a regular roughness profile is formed by a laser dulling method in which the surface of a work roll is melted and dulled. The laser dull steel sheet obtained in this manner has less irregularity components on the surface of the steel plate that deteriorates the coating image clarity, as compared with conventional discharge dull steel plates and shot dull steel plates, while the press formability is at a good level. Has become. Generally, long wavelengths (approx. 0.8
The larger the amount of the unevenness component of ~ 8.0 mm), the lower the image clarity of the coating film, and the more the presence of the unevenness component of short wavelength (generally 0 to 0.8 mm), the better the press workability. Become.

As a method for evaluating the coating image clarity of the coated surface, there are conventional methods, for example, JP-A-63-117206 and JP-A-61-217708.
JP-A-62-103544 and JP-A-62-233712 have been proposed.

However, although the type of coating material and the coating method also affect the clarity of the coating film on the coated surface, the roughness of the surface of the steel sheet as the coating base is also strongly influenced. That is, if the flat portion of the steel plate surface occupies a small proportion and the number of irregularity components with large intervals increases, the irregularity also increases on the painted surface, resulting in diffuse reflection of light, impairing glossiness, and distortion of the image. This causes the deterioration of the above-mentioned image clarity of the coating film.

Therefore, it is necessary to evaluate the sharpness of the coating film from the microscopic morphology of the steel sheet surface before coating. As a conventional example for performing such an evaluation, the one described in JIS B-0601 (1982) Exists. In this conventional example, among the microscopic morphology of the steel plate surface, as a factor affecting the evaluation of the coating film sharpness,
The irregularities formed on the surface of the steel sheet (wavelength is about 0.8 to 8.0 mm "waviness component") are mentioned. This swell component is
As mentioned above, even if the steel sheet is painted, it cannot be filled and hinders the coating film clarity. Therefore, the coating film clarity of the coating steel sheet should be evaluated according to the degree of this waviness component on the steel sheet surface. Is going.

As specified in JIS B-0601, as shown in the schematic view of FIG. 2, a stylus type surface roughness measuring device is moved at a speed V on the unevenness of wavelength λ formed on the sample surface. From the cross-sectional curve of the measured surface obtained by, remove the component of surface roughness with a wavelength of less than about 0.8 mm with a low-pass filter having a predetermined high-frequency cut-off value, and further straightness etc. The average height with respect to the center line of the unevenness, which is obtained by removing the component of the large wavelength (approximately 8.0 mm ~) corresponding to the long component of the wavelength corresponding to the shape deviation with the high pass filter having the predetermined low pass cutoff value. The value expressed in μm is referred to as “filtered centerline undulation (SW _CA )”. The smaller the value of the filtered center line waviness, the better the coating film sharpness.

<Problems to be Solved by the Invention> However, according to the above-described conventional evaluation method, the cutoff value in the filter when obtaining the value of the filtered centerline waviness is such that the attenuation rate is −12 dB as shown in FIG. It is the wavelength corresponding to the frequency at which the gain becomes 75% using the electric filter of / oct. Therefore, the value of the filtered center line waviness obtained by the above-mentioned conventional evaluation method is not evaluated by cutting off the wavelengths on the high band side and the low band side by 100%, and therefore, as shown in FIG. There are challenges.

(1) Measurement conditions "cut-off value (mm), measurement range (mm x
mm) ”, the value of the“ filtering center line waviness ”fluctuates significantly, that is, the influence of the error component corresponding to the shape deviation on the long wavelength side and the influence of the component on the short wavelength side are It is included in the value of “filtered center line swell”.

(2) 100% low wavelength component (improves press moldability)
Since this low wavelength component is not cut off and enters as the value of “filtering center line waviness”, the value of “filtering center line waviness” of the laser dull steel plate becomes higher than that of the shot dull steel plate and discharge dull steel plate. Since evaluation results may occur, it is not possible to accurately evaluate the surface properties of various steel sheets produced by different manufacturing methods.

As shown in Fig. 2, the low cutoff value is 0.4 to 0.8 mm.
In the low conventional example, since the laser dull steel sheet has a plurality of sharp peaks on the short wavelength side that improves press formability, the short wavelength component that improves press formability enters the value of the filter center line waviness, and the actual coating film The laser dull steel plate with good image clarity has a larger filtered center line waviness value than the shot dull steel plate and discharge dull steel plate, and the true waviness component cannot be evaluated.

As described above, the conventional evaluation method has a problem in that it is not possible to accurately compare and evaluate the coating film clarity of the dull-worked cold-rolled thin steel sheet having good press formability.

Further, as described above, the conventional coating film sharpness evaluation method is not always complete because it cannot evaluate 100% of the true waviness component in a predetermined wavelength range. Even if the image clarity is evaluated as good, there is a problem that the actual image clarity of the coating film is not always sufficient.

Since an accurate evaluation method for the coating film clarity of coating steel sheets has not been established, this evaluation method is fed back to the field of coating steel sheet manufacturing to improve the coating clarity of coating steel sheets. However, there is a problem in that the improvement of the coating film clarity of the conventional steel sheet for painting cannot be said to be sufficient.

SUMMARY OF THE INVENTION It is a first object of the present invention to provide a steel sheet for coating, which is sufficiently excellent in coating film sharpness, in order to solve the above-mentioned conventional unsolved problems.

Further, in order to solve the above-mentioned conventional problems, the present invention provides a surface evaluation method for a coating steel sheet capable of accurately evaluating the coating film clarity of various coating steel sheets. To aim.

<Means for Solving the Problems> The inventors of the present invention conducted various studies on a method for evaluating the coating film sharpness of a coating steel sheet in order to solve the above-mentioned object. In addition to obtaining a method that can be evaluated accurately, and using this evaluation method to study the coating image clarity of a steel sheet for painting, it was found that certain conditions were satisfied irrespective of the steel sheet for painting with different manufacturing methods. The inventors have found that a steel sheet for coating having a good coating film clarity can be obtained by doing so, and have reached the present invention.

That is, the first aspect of the present invention is a coating steel sheet having on its surface unevenness in a wavelength range of a film clarity hindrance that is not corrected even by press molding and reduces the film clarity after coating. In, in the range of wavelength 1.0 ~ 6.0 mm in the frequency analysis curve obtained by Fourier transforming the cross-sectional curve of the surface of the steel sheet, the coating image clarity inhibition wavelength region is set, and the power spectrum sum of the wavelength region is set. To 0.5 μm ² or less.

Moreover, the 2nd aspect of this invention detects the cross-section curve of the surface of the steel plate for coating, Fourier-transforms this cross-section curve, and obtains a frequency analysis curve, and also impairs the sharpness of the steel plate after painting. By setting the coating sharpness inhibition wavelength range to within the range of the wavelength 1.0 to 6.0 mm in the frequency analysis curve, by obtaining the power spectrum sum of the coating sharpness inhibition wavelength range, the coating steel sheet The present invention provides a method for evaluating a steel sheet for painting, which is characterized by evaluating the sharpness of the coating film.

<Operation> According to the coating steel sheet and the evaluation method thereof according to the present invention, an irregular random waveform of the surface shape of the coating steel sheet, that is, an input signal showing a random variation with respect to the time axis is Fourier-transformed. A frequency analysis curve (power spectrum-waveform of unevenness formed on the steel plate surface) obtained by displaying by decomposing into amplitude levels for each frequency is created. Next, the coating film clarity inhibition wavelength region that inhibits the coating film clarity of the coated steel sheet is set to a wavelength of 1.0 to 6.0 mm in the frequency analysis curve.
Set within the range of. Then, the sum of the power spectra in the wavelength range where the coating film clarity is impeded is obtained.

Here, the sum of power spectra is an integral of the power spectra in the above-mentioned coating image clarity inhibition wavelength region.

The power spectrum sum of the frequency analysis curve in the wavelength range in which the film clarity is impeded becomes smaller as the amount of the waviness component on the surface of the coating steel sheet decreases. Therefore, the smaller the power spectrum sum in the above-mentioned coating film sharpness inhibition wavelength range, the better the coating film sharpness. Therefore, by comparing the above power spectrum sums for each steel sheet, various coating steel sheets can be obtained. It is possible to accurately compare and evaluate the clearness of the coating film for each.

In the present invention, since the power spectrum sum of the frequency analysis curve is obtained, 100% of the power spectrum lying in the image clarity inhibition wavelength range is taken in, while removing all the power spectrums in other wavelengths. Therefore, the component corresponding to the shape deviation on the longer wavelength side than the above wavelength region which hinders the coating image clarity and the press moldability improving component on the short wavelength side which improves the press moldability are the above power spectra. Not added to the sum. Therefore, the coating film clarity of various coated steel sheets can be accurately evaluated for each steel sheet.

In addition, by setting the power spectrum sum in the wavelength range of the coating sharpness to a small value, it is possible to reliably provide a coating steel sheet having excellent coating clarity.

In the above-mentioned present invention, the reason why the wavelength range of the coating film image clarity inhibition is set within the above range is as follows.

When the short wavelength end side of the coating film sharpness impairing wavelength region is less than 1.0 mm, the wavelength component less than 1.0 mm that improves press formability enters the above power spectrum sum.

On the other hand, if the long wavelength end side exceeds 6 mm, since it can be corrected by press molding, the wavelength component exceeding 6 mm, which corresponds to shape deviation, does not deteriorate the coating image clarity of the coating film, The reason for this is that the sharpness of the coating film cannot be accurately evaluated among various coated steel sheets.

In the above-mentioned present invention, the coating image sharpness inhibiting wavelength region can be freely set within the range of 1.0 to 6 mm.

Next, the reason for limiting the numerical values in the first aspect of the present invention will be described.

The sum PA of the power spectra in the coating film clarity inhibition wavelength region of the steel sheet before coating is PA ≦ 0.5 μm ² . If the power spectrum sum PA is PA> 0.5 μm ² , there are many irregularities in the coating film sharpness inhibiting wavelength range on the surface of the steel sheet before painting, and these irregularities can be filled by painting even after painting. Without coating, the power spectrum sum PA must be 0.5 μm ² or less to prevent the clearness of the coating film after coating.

<Example> First, one example of an evaluation method for a coating steel sheet according to the second aspect of the present invention will be described.

FIG. 1 is a diagram showing the configuration of a coating steel plate evaluation apparatus for carrying out this evaluation method.

In FIG. 1, a coating steel plate sample 40 is placed on the steel plate sample fixing base 1. The stylus 10 comes into contact with the steel plate sample 40, and scans the dull groove formed on the surface of the sample to output a random waveform according to the degree of roughness of the unevenness on the surface of the steel plate sample 40.

This random waveform is input to the three-dimensional surface shape measuring device 2 and converted into a voltage signal. This voltage signal is fast Fourier transformed by the frequency analysis device 4 to create a frequency analysis curve.

This frequency analysis curve is decomposed and quantified by the microcomputer 5 into a power spectrum sum (integral value) between the wavelength bands according to the purpose, and the result is displayed on the display device 7.

The stylus 10 is configured to be movable with respect to the sample fixing base 1. After moving the stylus 10 in the X-axis direction on the sample surface, the stylus 10 is moved in the Y-axis direction at a predetermined pitch. By repeating this, the dull processed groove formed on the sample surface is scanned.

The frequency analysis device 4 performs a fast Fourier transform on the irregular waveform of the surface shape obtained by the three-dimensional surface shape measuring device 2, that is, an input signal showing random fluctuations with respect to the time axis, and performs an amplitude for each frequency. Create a frequency analysis curve obtained by disassembling and displaying the levels.

This frequency analysis curve is obtained by mathematically converting the time axis signal to the frequency axis, and the vertical axis is the power spectrum in which the amplitude is displayed as a level from the random waveform to the frequency axis. Therefore, this frequency curve can evaluate 100% of the amplitude level between the wavelength ranges of interest.

In a predetermined storage area of the storage device of the computer 5, a coating film clarity inhibiting wavelength region that inhibits coating film clarity is set. Next, the computer 5 calculates and outputs the power spectrum sum of the frequency analysis curve in this set wavelength range.

The sum of the power spectra does not include a component that can be corrected by press molding and that corresponds to shape deviation on the long wave number side, or a component that improves press moldability that improves press moldability. Therefore, it becomes possible to accurately compare and evaluate the coating image clarity of various coating steel sheets.

FIG. 4 shows an output curve of the surface roughness of a steel sheet temper-rolled by a work roll that has been shot dull. In FIG. 4, the horizontal axis represents the amount of displacement of the sample surface in the X-axis (or Y-axis) direction, and the vertical axis represents the depth direction of the sample (Z-axis direction).
Indicates. FIG. 5 shows a two-dimensional average power spectrum distribution (three-dimensional power spectrum distribution is linearly added and two-dimensionally averaged) obtained by fast Fourier transforming the output curve of FIG. 4 in the frequency analysis device 4. , Frequency analysis curve). In FIG. 5, the horizontal axis represents the wavelength (μm) of the irregularities formed on the sample surface, and the vertical axis represents the power spectrum. Further, λ _{1 to} λ ₂ represent the coating image clarity inhibition wavelength region, and the black coating portion represents the power spectrum sum (integral value) in the coating image visibility inhibition wavelength region.

The wavelength range of the coating film clarity inhibition can be set in the computer 5 in advance through an interface circuit.
The coating film clarity inhibiting wavelength range means a wavelength range of unevenness formed on the surface of the steel sheet that cannot be corrected even by press working because the unevenness on the surface of the steel sheet is not filled with the coating.

In the present invention, this wavelength range 0.8 ~ 8 mm preferably,
It is set within the range of 1.0 to 6.0 mm. When the low wavelength side of the above wavelength range is set to less than 0.8 mm, the short wavelength component that can be filled by coating is calculated as the coating image clarity inhibition wavelength, and the above-mentioned wavelength is longer than 8 mm. When the upper limit of the wavelength range is set, a component corresponding to shape irregularity that can be corrected by press molding and does not affect the coating clarity will be calculated as the coating clarity inhibition wavelength. This is because it is not possible to accurately evaluate the sharpness of the coated steel sheet coating film.

It should be noted that irregularities having a wavelength of less than 0.8 mm improve the press formability and can be filled by painting. Therefore, the press workability can be evaluated by calculating the power spectrum sum of the frequency analysis curve at the wavelength of less than 0.8 mm. As a result, when the power spectrum sum of less than 0.8 mm is large, the evaluation result is that the press workability is good.

Then, the sum of the power spectrums of the frequency analysis curves in the coating sharpness inhibition wavelength region becomes small, and thus the evaluation result that the coating sharpness is good is obtained.

For this reason, by comparing the power spectrum sums in the above-mentioned coating film sharpness inhibition wavelength regions, it becomes possible to accurately and comparatively evaluate the coating film clarity of various coating steel sheets.

On the other hand, in the conventional evaluation method shown in JIS B-0601, the low-wavelength component for improving press formability at 0 to 0.8 mm and the long-wavelength component corresponding to shape deviation exceeding 8 mm are filtered center line waviness. Enter as. In the method of the present invention, since the power spectrum of the low-wavelength component and the long-wavelength component does not fall within the power spectrum sum of the wavelength range that hinders the coating clarity, the conventional evaluation method described in JIS B-0601 described above. By comparison, it is possible to accurately evaluate the coating film sharpness after coating of the coating steel sheet.

FIG. 6 shows the power spectrum distribution of the shotdal steel plate. And FIG. 7 shows the power spectrum distribution of the laser dull steel plate. In FIGS. 6 and 7, the larger the power spectrum sum, the more the unevenness of the wavelength exists on the steel sheet. Comparing FIG. 6 and FIG. 7, the laser dull steel plate has a wavelength range (λ ₁
It can be seen that the sum of power spectra is small at ˜λ ₂ , for example, λ ₁ = 0.8 mm, λ ₂ = 8 mm.

As a result, according to the method of evaluating the quality of the coating film clarity of the coating steel sheet by using the sum of power spectra in the wavelength range of the coating clarity sharpness as in the present invention, the laser dull steel sheet is the most This is consistent with the actual result of good image clarity.

Next, the present invention will be specifically described based on specific examples.

(Example 1) Fig. 8 shows the results of the evaluation of the film clarity of various coated steel sheets under various measurement conditions. In the examples of this evaluation, the wavelength range (λ _{1 in} FIG. 6 and FIG. 7) that changes the measurement range and reduces the coating image clarity of the steel sheet after coating is used.
To [lambda] ₂₎ at a low wavelength end side (lambda ₁₎ is 0.8mm, and the long wavelength end side (lambda ₂₎ is set to 0.8mm~8mm is 8 mm, the power spectrum sum (PS sum of this wavelength range) The results obtained are shown below.

The evaluation results shown in FIG. 8 indicate that the laser dull steel plate has a smaller power spectrum sum in the coating image sharpness impeding wavelength region than the other steel plates. This is in agreement with the actual result that the laser dull steel plate has less unevenness components in the coating image clarity inhibiting wavelength region than the shot dull steel plate and the discharge dull steel plate. Here, regarding the laser dull steel sheet, the laser dull steel sheet has many irregularities for improving press formability on the short wavelength side as compared with other steel sheets, but these irregularities hinder the coating film sharpness. Therefore, the laser dull steel sheet has better coating film clarity and press formability than other steel sheets.

(Comparative Example 1) On the other hand, according to the surface evaluation method using the conventional filter shown in FIG. 3, there is a case where the laser dull steel plate has a larger wave center line waviness than other types of steel plates. is there. This is because the irregularities of the press moldability improving component in the low wavelength region are not cut by the filter and are taken into the value of the filtered center line waviness. Therefore, in the evaluation method of FIG. 3, the sharpness of the coating film of the laser dull steel plate is worse than that of other types of steel plates, which may be the opposite of the actual result.

Further, as shown in FIG. 3, the value of the filtered centerline waviness varies greatly depending on the difference in the measuring range. However, according to the evaluation method shown in FIG. 8, compared with the evaluation method shown in FIG. It shows that the fluctuation of the power spectrum sum is small even if the measurement range changes, and that not only the manufacturing method is different but also the coating film sharpness can be correctly evaluated for various coating steel plates with different measurement ranges. ing.

(Example 2) Fig. 9 shows the results of evaluation by the DOI value of the coating film clarity of various steel sheets after coating. Then, FIG. 10 shows the result of the sensory evaluation of whether or not the sharpness of various steel sheets after coating was good.
In the examples of this evaluation, various steel plate samples of 8 mm × 8 mm (laser dull steel plate, shot dall steel plate, discharge dull steel plate)
Was prepared, and the power spectrum sum of this steel sheet was measured using the apparatus described in FIG. 1 above. Next, this various steel plates were coated with 3 coats and a film thickness of 85 μm, and after the coating, the DO that correlates with the quality of the film clarity
The I value was measured. Then, in the sensory evaluation, the quality of the coating film clarity is expressed in five stages, and the higher the value, the better the sensory evaluation of the coating clarity.

In the examples of FIGS. 9 and 10, the short wavelength side of the coating image clarity inhibition wavelength range is set to 1 mm, while the long wavelength side is set to 6 mm.
Set to.

The above-mentioned DOI (Distinctness of Image) is a general evaluation of the image clarity of a coating film, and is a measured value by a Dorigon meter manufactured by Hunter Associates Laboratory, USA.

This DOI value is ± 0.3 with respect to the regular reflection light intensity R _s and the normal semi-emission angle when light is incident on the surface of the sample at an incident angle of 30 degrees.
It is expressed by the following equation using the value of scattered light intensity R _{0.3 in} degrees.

DOI value (%) = 100 × (R _s −R _0.3 ) / R _{s The} higher the DOI value, the better the coating clarity.

As shown in FIG. 9, it can be seen that the laser dull steel plate has a smaller power spectrum sum than the other steel plates. Further, it can be seen that the laser dull steel sheet has a higher DOI value and has a better coating film clarity after coating. This is in agreement with the actual sensory evaluation, as shown in FIG. Therefore, by obtaining the sum of the power spectra in the wavelength range of the coating clarity sharpness in the frequency analysis curve, the coating clarity of various steel sheets can be evaluated in a state of being consistent with the evaluation result after coating.

(Comparative Example 2) Next, the wavelength range that impairs the sharpness of the coating film is set to 0.2 mm, which is a wavelength shorter than 0.8 mm on the short and long sides defined in the present invention, and the long wavelength side is within the range of the present invention. Setting to 6 mm, the power spectrum sum of the steel plate (original plate) is calculated, and for this steel plate,
Based on the method explained in Fig. 9 above, paint the above-mentioned DO
I value was calculated. Then, for this coated steel sheet, the first
The same sensory evaluation as described with reference to FIG. FIG.
Fig. 12 shows the relationship between the power spectrum sum of the steel plate (original plate) and the DOI value, and Fig. 12 shows the relationship with the sensory evaluation of the coating film clarity of the coated steel plate.

It can be seen from FIGS. 11 and 12 that the power spectrum sum of the laser dull steel sheet is the highest. The laser dull steel plate has sharp peaks and depressions that improve press formability on the short wavelength side (<0.8 mm) as compared with other types of steel plates. Therefore, 0.8 mm on the short wavelength side of the coating image clarity inhibition wavelength range.
If it is set to be less than this, the short wavelength component is added as the coating image clarity inhibiting wavelength component, so that the power spectrum sum becomes a large value. On the other hand, according to FIG. 11 and FIG. 12, even if the power spectrum sum of the laser dull steel plate is high,
It can be seen that the DOI value and sensory evaluation are good. As a result, when evaluating the coating sharpness of the laser dull steel sheet from the power spectrum sum in the coating sharpness inhibiting wavelength range of the frequency analysis curve, the short wavelength side of the coating sharpness inhibiting wavelength is 0.8 m
It is necessary to set m or more, preferably 1 mm or more. Further, since the wavelength component of less than 0.8 mm is buried and disappears by the coating, it may be excluded from the wavelength range of the coating clearness after coating.

On the other hand, if the long-wavelength side of the coating image clarity inhibition wavelength range is set to exceed 8 mm, the shape error component that can be corrected by press working of the main body will be added to the above power spectrum sum, so The long wavelength end of the
It is necessary to set it to 8 mm or less, preferably 6 mm or less.

In the present embodiment, the three-dimensional surface shape measuring device has been described using a stylus that comes in contact with the surface of a steel plate sample. However, for example, an ultrasonic wave generator and a wave receiver are provided.
A non-contact type may be used, such as irradiating the surface of the steel sheet sample with ultrasonic waves to detect irregularities on the surface of the steel sheet.

(Example 3, Comparative Example 3) Next, an example of the coating steel sheet according to the present invention will be specifically described.

A cold rolled steel plate of low carbon aluminum killed steel was used as the test steel. This is temper-rolled at a reduction rate of 0.8% using skin pass rolls that have been dull processed by shot blasting, dull processing by electric discharge machining, and dull processing by a laser beam, and 12.0 x 6.0 mm for each steel plate.
The test pieces were prepared. Of these, the test piece ~ is a shot dull material, ~ is a discharge dull material,
Are laser dull materials.

Then, the three-dimensional surface profile curve of each test piece was obtained by the three-dimensional surface shape measuring device (SAS-2001 manufactured by Myojin Koki) of each test piece. At this time, the roughness pattern of the roll surface was changed by changing the conditions of each dulling method to change the roughness pattern of the dull processed steel sheet surface after skin pass rolling.

Fig. 13 shows 3 of the surface of the test piece prepared in this way.
A three-dimensional surface cross-section curve figure is shown.

In FIG. 13, (1) is the three-dimensional surface cross-section curve diagram of the test piece which is a shot dull material, (2) is the three-dimensional surface cross-section curve figure of the test piece which is a discharge dull material, 3) is a test piece which is a laser dull material
It is a three-dimensional surface section curve figure.

Then, with respect to each of the above-mentioned test pieces, an irregular random waveform of the surface shape obtained by the above-mentioned three-dimensional surface shape measuring apparatus, that is, an input showing a random variation with respect to the time axis, using a frequency analysis apparatus. A frequency analysis curve obtained by subjecting the signal to Fast Fourier Transform and decomposing and displaying the amplitude level for each frequency of the unevenness was prepared as shown in FIGS. 6 and 7.

Next, the power spectrum sum of these curves in the wavelength range of the film sharpness obstruction was calculated. At this time, the coating image sharpness inhibiting wavelength region λ _{1 to} λ ₂ was set to 1.0 to 6.0 mm.

Figure 14 shows the results of the sum of power spectra of each test steel sheet.

As shown in FIG. 14, the power spectrum sum of the shot dull steel plate and the discharge dull steel plate in the coating image visibility inhibiting wavelength region is a larger value than the power spectrum sum of the laser dull steel plate. These are for the following reasons.

In order to improve press formability (performance to prevent seizure between the steel plate and the press forming die) when dull weighting the steel plate with a roll that has been dull processed by the shot blasting method or the electric discharge machining method, the steel sheet surface It is necessary to form unevenness in the wavelength region of less than 1.0 mm.

In order to sufficiently form the unevenness for improving the press formability on the surface of the steel sheet to improve the press workability, it is necessary to form a large number of such unevenness on the roll surface. However, along with this, many irregularities in the wavelength range that hinder the coating clarity are also formed on the roll surface. Is a large value.

As shown in FIGS. 13 (1) and 13 (2), it can be seen that the surface roughness of the shot dull steel plate and the discharge dull steel is large. As shown in FIG. 6, it can be seen that the power spectrum sum of the coating image clarity inhibiting wavelength region of the shotdal steel plate is larger than that of the laser dull steel plate of FIG.

On the other hand, in the laser dull steel plate rolled by the roll that is laser dull processed, the test piece,
As described above, the sum of the power spectra in the wavelength range where the film clarity is impeded is a small value. The laser-dulled roll has a large number of microlators that improve press formability on the surface, but other parts are rich in flatness. As shown in (3), it is rich in flatness except for the uneven portion which improves press formability, and as shown in FIG.
The power spectrum distribution in the coating image clarity inhibition wavelength region is smaller than that of the shot dull steel plate and discharge dull steel plate, and as a result, the sum of the power spectrum in the coating image clarity inhibition wavelength region is shown in Fig. 14. Is a small value.

Next, the following chemical conversion treatments were performed on the above test pieces 1 to 3, and then three coat coating was applied.

Chemical conversion treatment Treatment agent: Fine-grained phosphate agent for dip treatment Dip condition: 43 ° C x 120 seconds Coating weight: 2.2 ± 0.2 g / cm ² Pretreatment: Degreasing, water washing, surface conditioning Posttreatment: Water washing, pure water washing , Dry coating Coating posture: Vertical posture Undercoat: Cationic ED paint 18-20 μm thick Middle coat: Sealer 30-35 μm Top coat: Topcoat 30-35 μm Sanding was not performed in each process.

The following DOI values were measured on the coating film surface of each steel sheet after coating. Fig. 15 shows the relationship between the power spectrum sum and the DOI value in the wavelength range where the clarity of the coating film is impaired before coating.

Here, it is shown that the higher the DOI value (90% or more), the better the coating film sharpness.

As shown in FIG. 15, when the power spectrum sum in the frequency resolution curve coating sharpness inhibition wavelength region is 0.5 μm ² or less, the above DOI becomes 90% or more, and the frequency analysis curve coating film after coating The sum of the power spectra in the image sharpness inhibition wavelength region is 0.05 μm ² or less, which indicates that the image clarity of the coating film is good. That is, it can be seen that the test pieces ,,, have good image clarity of the coating film. On the other hand, the test piece ...
Since the DOI value of is less than 90%, it can be seen that the coating image clarity is poor.

In order to keep the sum of the power spectra in the coating image clarity inhibition wavelength range of the steel sheet before coating to 0.5 μm ² or less, the work roll controlled by the shot dull processing method, the discharge dull processing method, and the laser dull processing method according to the purpose. , It is necessary to roll the steel sheet for painting with controlled rolling reduction, oil coating, etc.

Regarding the laser dull processing method, it is necessary to control the amount of unevenness in the wavelength range that hinders the coating clarity of the work roll surface by changing the laser beam irradiation time, laser beam feed pitch, laser beam intensity, etc. However, since these are easily controlled, they are most suitable for the production of the coating steel sheet of the present invention.

Further, it is necessary to control the amount of unevenness in the wavelength range that hinders the coating clarity of the work roll surface by controlling the energy during shot blasting and electric discharge machining.

Then, the power spectrum sum of the coating image clarity inhibition wavelength region before coating is 0.5 μm ² or less, and the DOI value of the coated surface is 90% or more by using an appropriate coating method. Further, it is possible to provide a steel sheet for coating which has a coating film sharpness which is 0.05 μm ² or less in the coating film sharpness inhibition wavelength region after coating and has excellent coating film clarity. In the above-mentioned embodiment, the case where the coating image clarity inhibition wavelength range is set to 1.0 to 6.0 mm has been described, but the present invention is not limited to this, and if necessary, 0.8 to 8.0 mm range, preferably 1.0 to 6.0 mm. It is possible to set the above-mentioned wavelength range of hindrance to the coating film clarity to another wavelength range of 6.0 mm.

<Effects of the Invention> As described in detail above, according to the first aspect of the present invention, coating is performed within the wavelength range of 1.0 to 6.0 mm in the frequency analysis curve obtained by Fourier transforming the sectional curve of the surface of the coating steel sheet. By setting the wavelength range of film sharpness inhibition and setting the power spectrum sum of this wavelength range to 0.5 μm ² or less, the DOI value after coating can be increased, for example, 90% or more. It is possible to provide a coating steel sheet having excellent image clarity.

In addition, according to the evaluation method for a coating steel sheet according to the second aspect of the present invention, an irregular random waveform of the surface shape of the coating steel sheet, that is, an input signal that shows a random fluctuation with respect to the time axis is Fourier-transformed. To create a frequency analysis curve and set the coating image clarity inhibition wavelength range to the wavelength range 1.0-
Since it is set in the range of 6.0 mm and the power spectrum sum of the frequency analysis curve in this wavelength range where the coating clarity is impeded is used, the coating clarity of various coating steel sheets can be compared accurately. Can be evaluated.

Therefore, by using the evaluation method according to the present invention, it becomes possible to control the production and coating of the steel sheet for coating according to the purpose of improving the image clarity of the coating film.

[Brief description of drawings]

FIG. 1 is a block diagram of an apparatus for carrying out an example of an evaluation method according to the present invention, FIG. 2 is a schematic diagram for explaining the contents of a conventional evaluation method of a coating steel sheet, and FIG. 3 is FIG. Fig. 4 is a characteristic diagram showing the values of the surface waviness of the shotdal steel plate and the laser dull steel plate obtained by the evaluation method of Fig. 4, and Fig. 4 shows the output curve obtained by the three-dimensional surface shape measuring device which measured the surface of the laser dull steel plate. FIG. 5, FIG. 5 is a diagram showing a two-dimensional average power spectrum distribution obtained by performing a fast Fourier transform on the output curve of FIG. 4, and FIG. 6 is a diagram showing a power spectrum distribution of a shotdal steel plate. , FIG. 7 is a diagram showing a power spectrum distribution of a laser dull steel plate, FIG. 8 is a characteristic diagram showing results of coating film clarity evaluation of various coating steel plates under various measurement conditions, and FIG. Power spectrum sum of painting steel sheet and painting Characteristic diagram showing the relationship with the DOI value of the steel sheet, FIG. 10 is a characteristic chart showing the result of a sensory evaluation of the quality of the coating film clarity of various steel sheets after coating, and FIG. 11 is a coating steel sheet. Fig. 12 is a characteristic diagram showing the relationship between the sum of power spectra and the DOI value after coating. Fig. 12 is a characteristic diagram showing sensory evaluation of coating film clarity of coated steel sheets. Fig. 13 (1), (2), ( 3) is a three-dimensional surface cross-sectional curve diagram of the test piece that has been temper-rolled by rolls that have undergone various dulling processes, and FIG. 14 shows the coating film of various dulled steel sheets according to the examples of the present invention. A characteristic diagram showing the value of the power spectrum sum in the image inhibition wavelength region, FIG. 15 is
FIG. 3 is a characteristic diagram showing the relationship between the value of DOI after coating and the sum of power spectra in the wavelength range of the film clarity hindrance before coating. Explanation of symbols 2 ... 3D surface shape measuring device, 4 ... Frequency analysis device, 5 ... Microcomputer

Claims (2)

(57) [Claims] 1. A steel sheet for coating, which is not corrected even by press molding and has unevenness in a wavelength region of a coating film visibility hindering deterioration of coating film clarity after coating, wherein the surface of the steel sheet is In the range of wavelength 1.0 ~ 6.0 mm in the frequency analysis curve obtained by Fourier transforming the cross-section curve, the coating image clarity inhibition wavelength range is set, and the power spectrum sum of the wavelength range is 0.5 μm ² or less. A steel sheet for painting which is characterized by being present. 2. A coating film sharpness that inhibits the sharpness of a coated steel sheet by detecting a cross-section curve on the surface of the coating steel sheet and Fourier transforming the cross-section curve to obtain a frequency analysis curve. By setting the inhibition wavelength region within the range of wavelength 1.0 to 6.0 mm in the frequency analysis curve, and obtaining the power spectrum sum of the coating image clarity inhibition wavelength region, the coating image clarity of the coating steel sheet. A method for evaluating a steel sheet for painting, which comprises: