JPH0254719A - Cold rolled steel plate excellent in image clearity after coating - Google Patents

Abstract

PURPOSE:To manufacture the title steel plate by specifying the average roughness on the surface of a steel sheet and regulating the value of the maximum roughness and average waviness by the total coating thickness and the number of times in coating. CONSTITUTION:In the roughness on the surface of the steel sheet, average roughness is regulated to >=0.3mu, and, in the value of the maximum roughness (R max) found for the components having <=400mu wave length and the average waviness (W ca) found for the components having >400mu wave length, according to the total coating thickness (T) and the number of times in coating (L), each formula of maximum roughness (R max)<=AXTXL<1/2> and average waviness (W ca)<=BXTXL<1/2> [A=0.05, B=0.0026, L(mu)] is individually satisfied. By this method, image clearity after coating in the cold rolled steel sheet is made excellent.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a cold-rolled steel sheet with excellent image clarity after coating.
Paint clarity has recently become important as a characteristic that allows users to visually communicate the quality of products using steel sheets, such as automobiles. Conventionally, by painting technology,
Improvements in image clarity after painting had been attempted, but with today's improvements in coating technology, it has become clear that further improvement in image clarity can be synergistically achieved by retroactively controlling the surface roughness of the steel plate before painting. Futa.

The present invention relates to a cold-rolled steel sheet with excellent image clarity after painting using a new technique for effectively setting the surface roughness of the steel sheet in accordance with various existing painting methods.

<Conventional technology> When considering control of the surface roughness of a steel plate as a method of improving image clarity after painting, it is necessary to consider the effect of the coating film formed on the surface of the steel plate by painting. . It has been known that the coating film acts as a LOWPASS filter for the profile of the surface roughness of the steel plate, and changes the surface roughness after coating. However, this filter effect changes depending on the coating method, and as a result, even with the same steel plate surface roughness profile, the sharpness after coating changes. Therefore, it is important to select and change the surface roughness of the steel sheet in order to obtain good image clarity after painting, not by a single roughness pattern, but by the painting method.

In recent years, the sharpness of cold-rolled steel sheets after painting has attracted attention as a characteristic that has a significant effect on the finished appearance, especially when considering their use as exterior panels for automobiles. Furthermore, when considering the use of steel plates in electrical appliances, the appearance finish after painting is an element that directly appeals to users of quality, and improvements are desired.

Improving such image clarity is primarily a challenge for painting technology.
Improvements in painting methods and paints are being actively studied. As a result, progress in coating technology has definitely improved the quality after coating, but on the other hand, the influence of the steel plate surface roughness, which was previously thought to have no effect, gradually appears on the surface after high-quality coating. It has been found that it affects

However, conventionally, the main method for controlling the surface roughness of a steel plate has been shot dulling, which imparts a random roughness pattern: control of the surface roughness of a steel plate has been limited to managing the average roughness.

Therefore, research on the relationship between post-painting sharpness and steel plate surface roughness has been published, at best, in a random manner, examining the relationship between a certain steel plate surface roughness and the average roughness.

<Problem to be solved by the invention> NlLAN et al. have reported to SAE the relationship between the average roughness of steel sheets and the sharpness after painting, but their results also show that in order to improve the sharpness, it is necessary to increase the average roughness. It simply states that it needs to be as small as possible. However, if this result is applied as is, deterioration in moldability cannot be avoided.

In other words, if only the sharpness after painting is considered, bright steel sheet has the best sharpness, but this is contradictory to the conventional method of dulling for formability and handling. It is impossible to use bright steel plates as press-formed steel plates.

The present inventors previously discovered that it is possible to achieve both formability and sharpness in a cold-rolled steel sheet by controlling the surface roughness of the steel sheet. A technique for controlling the profile of the surface roughness of a steel plate using a dulling technique was disclosed in JP-A-63-132702.

However, coating methods vary depending on the parts used, and in order to obtain the effect of improving image clarity corresponding to these coating methods, it is necessary to control the roughness of the same pattern, but not by controlling the roughness of the same pattern. It was found that the determination and control of the roughness pattern is important.

The inventors of the present invention have repeatedly conducted research experiments and intensively considered how to select an appropriate roughness depending on the coating method.

The surface roughness of the steel plate was varied using the maximum roughness and average waviness as parameters, and the subsequent coating method was also varied in the number of coatings and coating thickness, and the sharpness of the painted surface was evaluated. The relationship between this result and the steel plate surface roughness was investigated.

The image sharpness was evaluated by both visual comparison evaluation and DOI value using a Dorigon meter, and since a correlation was observed between the two, the DOI value was used to examine the quantitative relationship with roughness.

As a result, it has become clear that there is an important correlation between the DOI value and the roughness parameter, and it has become possible to express this relationship mathematically and to perform actual roughness control using the formula.

An object of the present invention is to provide a cold-rolled steel sheet that has a steel sheet surface roughness suitable for the applied painting method and has excellent image clarity after painting.

<Means for Solving the Problems> In order to achieve the above object, according to the present invention, the steel plate surface roughness is 0.3 μs or more in average roughness and the wavelength is 400 μs or more.
The maximum roughness determined for the components below the threshold and the wavelength of 4
A cooling film with excellent post-painting sharpness, characterized in that the average waviness value calculated for components exceeding 00 μs satisfies the following formulas depending on the total coating thickness (T) and the number of coatings (L). A rolled steel plate is provided.

Maximum roughness (Rmax) ≦A X T x JT Average waviness (Wca) ≦ B X T
0.05, B = 0.0028, T = I - total coating thickness (door), L = number of coatings.

The present invention will be explained in more detail below.

The surface roughness of a steel sheet is an important element for press forming, and a certain degree of surface roughness is necessary to control the occurrence of mold galling by controlling the contact state with the press mold.

Generally, cold-rolled steel sheets are dulled for this purpose.

On the other hand, in terms of image clarity after painting, it is said that it is better for the surface roughness of the steel plate to be as bright as possible, like a mirror surface. Therefore, in order to achieve both press formability and paint clarity, it has become necessary to impart contradictory surface roughness to the steel sheet, and it has been considered difficult to achieve this.

However, as a result of analyzing the change in steel plate surface roughness due to the coating film, it was found that it is possible to achieve both characteristics by actively utilizing this change and managing the roughness. That is,
It has become clear that not all the unevenness that makes up the surface roughness of a steel plate impairs image clarity after painting, but that some long-wavelength components remain in the surface roughness after painting and impair image clarity. Ta. Therefore, it has been found that if roughness control can be performed to eliminate only the roughness components that impair image clarity after painting, image clarity can be effectively improved without making the image brighter.

However, it has been found that the appropriate roughness pattern for this purpose varies depending on the applied coating method, and that it is necessary to find an appropriate roughness pattern.

It is known that the coating film formed on the surface of a steel plate has a filter effect on the surface roughness of the steel plate. That is,
Fine irregularities disappear from the surface irregularities after painting because they are hidden by the paint film. As a result, only roughness components with relatively long wavelengths remain on the surface after painting. It is known that the image sharpness, which affects the quality of the finished product after painting, is affected by this remaining long wavelength component, and that the image sharpness is significantly impaired.

Based on this knowledge, JP-A No. 63-132702 proposes to reduce the longer wavelength component than the surface roughness of the steel plate, but the premise is that fine irregularities must be completely eliminated by the coating film. is necessary. However, the roughness filtering effect of the coating film naturally differs depending on the coating method. In particular, when the coating is thin, a sufficient filter effect may not be obtained and fine irregularities may remain even after coating.

In this case, the remaining unevenness may have an adverse effect on image clarity. Furthermore, regarding the roughness of long wavelength components with wavelengths exceeding 400 μm, the degree of smoothing varies depending on the coating method and coating thickness, and the effect on image clarity after coating also differs.

Steel plates to which the present invention can be applied are those that have not undergone various surface treatments on the surface of the steel plate, but also include cases where one side has a cold-rolled surface, such as one-sided plating, and that cold-rolled surface is a painted surface. It will be done.

Assuming constants A and B, it is essential that the maximum roughness and average waviness be values defined by the following formula.

Maximum roughness (Rmax) ≦A
t) Average waviness (Wca)≦B
・(2) Here, A = 0.05, B = 0.
It is 0026.

The maximum roughness and average waviness values defined by the above formula are determined by the degree of roughness and unevenness that can be hidden by the paint film.If the roughness is applied beyond this value, the unevenness will remain even after painting, and It has a negative effect on the sharpness of the image.

In order to avoid lowering the roughness unnecessarily for the sake of sharpness, it is necessary to set the maximum roughness and average waviness values within the ranges of the above formula.

On the other hand, in order to ensure formability, the average roughness is 0.13 μm.
The above is necessary. When the average roughness is less than 0.3 μm,
Necessary formability cannot be obtained.

In reality, it is difficult to satisfy the above formula by simply managing the average roughness of a vehicle as a method of controlling the maximum roughness or average waviness, and a technology to finely control the surface roughness of steel sheets is also required. It is. As a roughness control method that can be implemented, there is a dulling technique using a laser beam or a technique using photoetching. This dulling technology is superior in that it is possible to control the surface roughness profile of the steel plate, and the maximum roughness and average waviness can be directly controlled.

Next, the values of constants A and B in the equations defining the maximum roughness and average waviness are set to 0.05 and 0.05, respectively.
The basis for setting it as 0028 will be explained.

Steel plate samples with varying maximum roughness and average waviness were prepared and painted under various coating conditions, and the subsequent image clarity was measured.

The image clarity after painting was measured using a Dorigon meter manufactured by Hunter and quantified as a DOI value. The DOI value is the amount of specularly reflected light when irradiated from a direction of 30 degrees with respect to the normal line of the sample, and Ro is the amount of light reflected at an angle of ±0.3 degrees from the specular reflection. Then, it is given as DOI=(R5-Ro,3)/R5X100. This evaluation method has good correlation with conventional evaluation methods such as human visual judgment and the PGD method in which a test pattern is projected onto a sample to see how fine a pattern can be identified.

In addition, the surface roughness of the steel plate was measured with Talysurf, and some samples were measured with 5E-3FK manufactured by Koita Seisakusho.
Measure the dimensional roughness and calculate the maximum roughness (Rmax) from this result.
And the average waviness (Wca) was determined.

The product of the coating film thickness and the number of coatings to the 1/2 power is the parameter P.
The relationship between this value and the DOI value after painting is summarized in a graph as shown in FIG. 1. As the value of P increases, the DOI value increases. However, the transition curve of this relationship changes with the value of Rmax, and as the value of Rmax decreases, the transition curve shifts toward the low P side.

The relationship between Rmax and P value for which the DOI is 95 is shown in FIG. When the coating becomes thicker and the value of P increases, the DOI
There is a tendency for the value of Rmax, which is 95, to increase.

From this relationship, if the coating method is given as a P value, the DOI value is 9.
An allowable Rmax value of 5 or more can be determined. Also, from this linear relationship, Rmax = 0.05x
P is obtained, and A = 0.05.

Figure 3 also shows the relationship between the P value and the DOI value after painting.
This is a graph using ca as a parameter.
As in the case of Rmax, the DOI value increases as the P value increases, and this change curve shifts with the Wca value. That is, the smaller Wca is, the closer the DOI value change curve is to the lower P side.

From this result, the value of Wca that gives a DOI value of 95 is P
FIG. 4 shows the relationship between values. This knowledge is effective in determining how much the Wca value should be limited by the coating method in order to obtain high image clarity.

Also, from this linear relationship, Wca=0.0026X
P is obtained, and B = 0.0026.

It is clear from the results shown in FIGS. 1 to 4 that it is effective to keep both Rmax and Wca values as small as possible in order to obtain high image clarity. However, unnecessarily reducing this value will also lead to a decrease in moldability. Therefore, in order to ensure the formability that should be provided as a cold-rolled steel sheet for pressing to which the present invention is applied, it is necessary to control the average roughness Ra.

FIG. 5 shows the results of measuring the relationship between the average roughness Ra of a steel plate and the coefficient of friction in a completely degreased state. From this result, as a cold-rolled steel sheet for press use, the surface roughness of Ra is at least 0.
．． It can be seen that 3 μm is necessary.

That is, it can be seen that when Ra is less than 0.3 μm, mold galling is likely to occur if lubrication is poor.

Based on the above results, the steel plate surface roughness of the present invention was defined as described above.

<Example> The present invention will be specifically explained below based on Examples.

Steel plate having Ra, Rmax and Wca specified in the present invention shown in Table 1, and Ra, Rmax and W
Regarding steel plates in which any of ca is outside the specifications of the present invention,
Table 1 shows the results of a comparison of press formability and image clarity after painting.

Regarding formability, a CRM type mold galling test was conducted as an indicator for evaluating this. From the second sheet onward, the number of sheets at which mold galling occurred was compared by performing the process without lubrication.
The evaluation results are ◎ when no galling occurs even after 10 sheets are molded, O when galling is mild (two streaky galling occurs on the 10th sheet), and × when galling is large ( From the 5th sheet onwards, there will be streaky scratches, and on the 10th sheet there will be severe galling, such as peeling and digging.

The steel plate (example of the present invention) having the Ra, Rmax, and Wca values specified by the present invention has a sharpness after painting of
Excellent results with a DOI value of 95 or higher were obtained. However, when Rmax and Wca are outside the specifications of the present invention (comparative example), the image clarity after coating is not satisfied. Further, if Ra defined by the present invention does not satisfy 0.3 μm or more (comparative example), the mold galling test result will be poor. From this result, the effect of the present invention is clear.

Note that samples Nos. 21 and 22 are the results of an investigation on the cold-rolled surface side of single-sided plated steel sheets.

<Effects of the Invention> Since the present invention is configured as described above, it is possible to obtain a cold-rolled steel sheet with excellent sharpness after painting by having a steel sheet surface roughness suitable for the applied painting method. be able to.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the product P of the coating film thickness and the 1/2 power of the number of coatings and the DOI value of image clarity after coating. FIG. 2 is a diagram showing the relationship between the P value and Rmax when the DOI value is 95. FIG. 3 is a diagram showing the relationship between the product P of the coating film thickness and the 1/2 power of the number of coatings and the DOI value of image clarity after coating. FIG. 4 is a diagram showing the relationship between the P value and Wca when the DOI value is 95. FIG. 5 is a diagram showing the relationship between the average roughness of a steel plate and the mold galling property. FIG, 1 P=(TXJT: ), pm FIG, 2 Pm(Tx, /U), , pm FIG, 4 p = CTXJU ), pm FIG, 3 Pm(Txff), Pm FIG, 5 Dried up water 2pm

Claims (1)

[Claims] (1) The maximum roughness obtained for components whose steel plate surface roughness is 0.3 μm or more in terms of average roughness and whose wavelength is 400 μm or less, and the average waviness value determined for components whose wavelength exceeds 400 μm are the total coating A cold-rolled steel sheet with excellent image clarity after painting, which satisfies the following formulas depending on the thickness (T) and the number of times of painting (L). Maximum roughness (Rmax)≦A×T×√L Average waviness (Wca)≦B×T×√L Where, A=0.05, B=0.0026, T=total coating thickness (μm), L=number of coatings.