JP2514692B2 - Steel sheet with excellent paint clarity and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a steel sheet used after being coated after forming, such as outer panels for various vehicles such as automobiles, trucks and trains, or outer panels for household electric appliances. The present invention also relates to a steel sheet which is less likely to cause galling during molding and has improved paint clarity, and a method for producing the same.

[Prior Art] In the steel plate used for the outer plate as described above,
In order to prevent mold galling (damaging like peeling caused by seizure of steel plate die) that occurs during forming such as pressing, it is customary to temper-roll with dull finishing rolls to adjust the surface roughness. Is. The shot blast method or the electric discharge machining method has been conventionally used for this dull finish.
The surface of the dull-finished steel sheet rolled using the rolls finished by these methods has a profile composed of irregular peaks and valleys as shown in FIG. 2, for example. Then, in the molding process, the valley portion in the profile acts as an oil reservoir for the lubricating oil, and also captures the generated metal powder to prevent seizure and suppress die galling. Therefore, from the viewpoint of preventing mold galling, the larger the surface roughness, the more preferable.

On the other hand, in the case of steel plates for outer panels such as those mentioned above, the quality of the coating finish is an important evaluation item in order to enhance the aesthetic decorative feeling, and in particular, the sharpness of the image when an object is copied on the surface of the coating film is important. This property is commonly referred to as "vividness".

By the way, the sharpness of the coated surface is largely influenced by the surface roughness of the steel sheet itself, as well as the influence of the type of coating and the coating method. That is, since the fine irregularities on the surface of the steel sheet are filled with the paint and leveled, almost no adverse effect appears, but when the irregularities become large to some extent, a coating film is formed along the irregularities, and reflected light is scattered, As the gloss decreases, the image clarity also deteriorates. For example, FIG. 3 is a graph exemplifying the relationship between the surface roughness Ra (center line surface roughness) of a steel sheet temper-rolled by a shot blasting roll and the sharpness of the coating, which has a small surface roughness Ra. The sharpness is improved almost linearly.

In contrast to the dull finish steel plate, a bright finish steel plate is also known. Since this bright finished steel sheet is temper-rolled by using a roll having a smooth surface ground by a grindstone, the surface of the obtained steel sheet is mirror-reflected with metallic luster. Therefore, if only the above-mentioned paint clarity is considered,
It can be said that bright finished steel sheets are superior to dull finished steel sheets. However, as is clear from the above-mentioned point, since this bright finished steel sheet has almost no valley portion which acts as an oil reservoir for lubricating oil, it has a problem in workability such as press forming.

That is, if the surface roughness Ra is increased in order to improve the mold galling resistance, the coating sharpness deteriorates, and if the surface roughness is reduced (or bright finish) to improve the coating sharpness, the mold galling resistance decreases. Therefore, in the past, the surface roughness was adjusted to an appropriate range to achieve both mold scuffing resistance and sharpness, but it is an eclectic measure, so both required performances are sufficient. It cannot be said to satisfy.

As a measure for coping with such a situation, for example, a technique disclosed in JP-A-62-230402 has been proposed. This method uses a temper rolling roll that has been dull-finished with a high-density energy beam such as a laser beam to form irregularities of a unique shape on the surface of a steel sheet, and the outline is as follows.

That is, when a laser pulse is irradiated toward the surface of the roll while rotating the roll, as shown in FIG.
As shown in (B), the metal of the laser irradiation portion is melted to form the crater 1, and the molten metal rises around the crater 1 to form the annular convex portion 2. The formation pitch of the craters 1 and the convex portions 2 in the roll circumferential direction can be arbitrarily adjusted by changing the rotation speed of the roll and the irradiation period of the laser pulse at the time of dull finishing, and the formation pitch in the roll axial direction can be adjusted. It can be freely adjusted by the moving distance of the laser irradiation device for each rotation of the roll. Further, the diameter and depth of the crater 1 and the width and height of the convex portion 2 can be changed by the energy of the laser pulse and the irradiation time.
Then, when the steel sheet is temper-rolled by the dull finishing roll having the crater 1 and the convex portion 2 formed innumerably on the surface by such a method, the convex portion 2 of the roll R is formed as shown in FIGS. An annular recess 2a is formed by biting into the surface of the steel plate Q, and the metal in this part flows so as to rise toward the crater 1 to form a substantially circular plateau 1a, which is unprocessed outside the protrusion 2. The surface of the steel sheet Q pressed by the flat portion 3 (that is, the portion to which the laser energy is not applied) becomes a flat plain portion 3a. The surface of the steel sheet thus obtained has a substantially circular plateau 1a and an annular recess 2a surrounding the plateau 1a,
And the flat land portion 3a slightly lower than the plateau portion 1a.

And in the steel plate with such a surface shape, the annular recess
2a functions as a lubricating oil sump and a cutting dust capturing part at the time of molding, exerting a mold galling prevention effect, and
Compared with the conventional roughened steel sheet as shown in the figure, since there are many flat surfaces and irregular reflection can be suppressed, sharpness is also very excellent.

Based on these findings, the above-mentioned published invention further affects the surface shape of the steel sheet that affects the image clarity after coating.
It was made by paying attention to the long wavelength component exceeding 400 μm, and specifically, the adjacent plateau parts.
Let Sa be the average center-to-center distance between 1a and 1a, D be the average diameter of the outer edge of the annular recess 2a, d be the average diameter of the inner edge, and Lap be the average diameter of the plateau at the center line of the roughness curve. The feature is that the adjustment is performed so as to satisfy the relationship of the following formula.

Sa ≦ 800 μm Lap ≦ 150 μm [Problems to be Solved by the Invention] The present invention makes use of the features of the disclosed invention as described above, and improves the microscopic shape characteristics of the steel sheet surface from different angles, thereby The purpose is to further improve the anti-galling property and the coating sharpness.

[Means for Solving the Problems] The structure of the steel sheet of the present invention capable of achieving the above object is a steel sheet obtained by temper rolling with a dull finishing roll by a high-density energy beam, 585μm ≦ λ <2730μm when the shape is analyzed by frequency
The gist is that the wavelength component intensity of the wavelength λ included in the range is 0.2 or less.

Moreover, such a steel sheet accounts for 20% of the total surface area of the flat part of the dull finishing roll by the high-density energy beam.
It is obtained by setting the above and performing temper rolling at least twice using the roll.

[Operation] The present inventors have temper-rolled a steel sheet (hereinafter referred to as a dull finished steel sheet) that has been temper-rolled by the dull finishing roll (hereinafter referred to as a laser dull finishing roll) with the high-density energy beam as described above. ) Shows good anti-galling resistance, and the effects of surface shape characteristics on the coating image clarity were investigated from various angles.

First, the present inventors conducted the following experiment from the viewpoint that only the component having a long wavelength is involved in the image clarity after coating. That is, the present inventors prepared laser dull finished steel plates of various patterns, applied 3-coat coating with a total coating film thickness of 90 μm to the steel plates, and investigated the wavelength distribution in the surface shape before and after coating.

The results are shown in FIG. The residual rate in Fig. 1 is
A value obtained by dividing the wavelength component intensity P after coating by the wavelength component intensity P before coating is shown. As is clear from FIG. 1, the residual rate shows a sharp rise when the wavelength λ becomes 585 μm or more. From this, it can be judged that particularly long wavelength components of 585 μm or more remain even after coating, which adversely affects the image clarity.

Therefore, the present inventors have found that the wavelength λ of the unpainted steel sheet is 58
When the relationship between the wavelength component intensity P in the range of 5 μm ≦ λ <2730 μm and the image clarity after coating with three coats was investigated, the results shown in FIG. 6 were obtained. The conditions for frequency analysis at this time are as follows. First, 8.192mm for one sample
We prepared 10 profiles of measurement length, set the number of data points to 8,192 points per profile, and calculated the wave height (h) distribution after filtering the profile by eliminating the slope by the least squares method and using the moving average. did. Then, the obtained 10 wave height distributions are linearly added and averaged to obtain 1 wave height distribution,
In this wave height distribution, the sum of squares of the wave height (Σh ² ) was obtained for the data points existing in the wavelength range, and this was taken as the wavelength component intensity P (μm ² ).

The range of the wavelength λ is less than 2730μm because the long-wavelength component of 2730μm or longer on the surface of the original plate did not significantly affect the image clarity after coating in the visual sensitivity evaluation of the coated surface and the evaluation by the image clarity measuring instrument. And In addition, the DOI meter manufactured by ATI SYSTEMS INC was used to evaluate the sharpness.

As is clear from the results of FIG. 6, it is clear that the wavelength component intensity P of the surface shape of the steel sheet in the above wavelength range has a good correlation with the DOI value.

Next, the present inventors measured the wavelength component intensity P at the wavelength λ of 585 μm ≦ λ <2730 μm on each steel plate surface when temper rolling the steel plate using the dull finishing rolls obtained by various methods. did. At this time, for the purpose of comparison, the same measurement was performed on the bright finished steel sheet.

The results are shown in FIG. 7. The wavelength component intensity P on the surface of the obtained steel sheet is
The sizes are smaller in the order of electrical discharge machining dull finish, laser dull finish, and bright finish. In the steel sheet temper-rolled by the dull finishing rolls obtained by the shot blasting method and the electric discharge machining method, the wavelength component intensity P varies, but this seems to be due to the roll manufacturing process itself. On the other hand, in the laser dull finished steel sheet, the wavelength component intensity P can be controlled within an appropriate range, but in the conventional product, the wavelength component intensity P (that is, P =
It was difficult to control to less than 0.2).

In this way, the present inventors have found that the steel sheet has a coating sharpness of 585.
As long as a laser-dull finished steel sheet having a wavelength distribution that depends on the wavelength component intensity P in the wavelength region of μm ≦ λ <2730 μm and has the intensity P of 0.2 or less can be manufactured, it is suitable for a bright finished steel sheet. The idea was to obtain a steel sheet with clear painting.

Therefore, the present inventors have made various studies on a method for producing a steel sheet having the above surface characteristics. As a result, temper rolling is performed at least twice using a laser dull finishing roll having an area ratio F (hereinafter referred to as flat area area ratio) of the flat portion 3 [see FIG. 5 (A)] of 20% or more. It was found that the desired steel sheet can be obtained in this way, and the present invention was completed. The flat area ratio F means the ratio of the flat part 3 shown in FIG. 5 (A) to the total surface area.

Ordinary temper rolling is performed by passing the sheet through the roll once, but no matter how the laser dull finishing method is improved, only one pass of the sheet is 585 μm ≦ λ <2730 μm.
It is difficult to set the wavelength component intensity in the wavelength region of 0.2 to 0.2 or less. Therefore, in the present invention, the wavelength component intensity P can be reduced to 0.2 or less by carrying out temper rolling at least twice with the above laser finishing roll. In the present invention, the flat land area ratio F of the laser dull finishing roll is set to 20% or more.
If it is less than 20%, even if the temper rolling is performed twice or more, the crater 1 (see FIG. 4) has a pattern in which the craters 1 overlap with each other, so that the improvement of the coating sharpness of the steel sheet cannot be expected.

[Example] Using a laser dull finishing roll having various flat area ratios F, a low carbon aluminum-killed steel plate having a plate thickness of 0.85 mm was temper-rolled. The surface of the steel sheet after temper rolling is 3
Frequency analysis was performed using a dimensional roughness analyzer to measure the wavelength component intensity P when the wavelength λ on each steel plate surface was 585 μm ≦ λ <2730 μm.

On each of the obtained steel sheets, paint was applied three times to a film thickness of 90 μm (electrodeposition 20 μm, intermediate coat 35 μm, top coat 35
The image clarity was evaluated by the DOI meter described above.

As a comparative example, the same experiment was conducted on a steel sheet temper-rolled by a shot blast dull finishing roll and a bright finishing roll.

The results are collectively shown in Table 1, and those satisfying the prescribed requirements of the present invention (Examples) have a sharpness of "DOI value of 95 or more" which is almost equal to that of the blast finished steel sheet. I understand.

[Advantages of the Invention] As described above, according to the present invention, by adopting the above-described configuration, the dull finish steel plate having the sharpness comparable to that of the bright finish steel plate was obtained. This steel sheet is optimal as a material that requires particularly excellent paint clarity such as an automobile outer panel.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between wavelength and residual ratio, and FIG. 2 is an enlarged cross-sectional view illustrating the surface profile of a roughened steel sheet using a roll that has been dull-finished by shot blasting.
FIG. 4 is a graph showing the relationship between the surface roughness of the roughened steel sheet and the image clarity, FIG. 4 is an explanatory view showing the surface shape of a dull finishing roll using a high-density energy beam, and FIG. 5 is that of FIG. Explanatory drawing showing the surface shape of the steel sheet roughened using the dull finishing roll, FIG. 6 is a graph showing the relationship between the wavelength component intensity P and the DOI value after coating, and FIG. 7 is using various finishing rolls. It is a graph which shows the wavelength component intensity | strength P in the steel plate surface tempered and tempered. 1 ... Crater, 2 ... Convex part 3 ... Flat part (non-processed part) 1a ... Plateau part, 2a ... Annular recess 3a ... Flat part

Claims (2)

(57) [Claims] 1. A steel sheet obtained by temper rolling with a dull finishing roll by a high-density energy beam, wherein 585 μm ≦ λ <2 when the shape of the surface of the steel sheet is subjected to frequency analysis.
Wavelength component intensity of wavelength λ included in the range of 730 μm (μ
A steel sheet with excellent paint clarity that is characterized in that m ² ) is 0.2 or less. 2. A high-density energy beam is used to set the flat surface of a dull finishing roll to a total surface area of 20% or more, and the roll is used to perform temper rolling at least twice. A method for producing a steel sheet having excellent paint clarity, which comprises obtaining the steel sheet according to 1).