JP2749628B2 - Alloyed hot-dip galvanized steel sheet with excellent formability and sharpness after painting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an alloyed hot-dip galvanized steel sheet excellent in formability and sharpness after painting.

<Conventional technology> Cold rolled steel sheets are often used for thin steel sheets that require a finished appearance after painting, such as automotive body outer panels, exterior electric appliances, and exterior panels such as sheet metal furniture. The surface roughness is adjusted by temper rolling. However, the use of surface-treated steel sheets has been rapidly increasing, particularly from the viewpoint of rust prevention of automotive steel sheets, and it has been a challenge to achieve both post-painting sharpness and press formability of the surface-treated steel sheets. In the case of a relatively thin surface-treated steel sheet such as electroplating, the surface roughness of the cold-rolled steel sheet, which is the original sheet, is maintained even after the surface treatment. It is almost possible with technology.

However, if further measures to prevent rust are required, a thicker surface treatment is required, such as hot dip galvanized steel sheet subjected to alloying, and the surface roughness in that case is the same as the surface roughness of the original sheet. The problem is that they are completely different.
That is, the surface roughness of the alloyed hot-dip galvanized steel sheet greatly changes from the surface roughness of the original sheet by both the hot-dip galvanizing step and the alloying step. It is known that the surface roughness of the final alloyed hot-dip galvanized steel sheet is roughened by peculiar fine irregularities, and adversely affects both sharpness after coating and press formability.

2. Description of the Related Art Today, the finish quality of a painted surface of an automobile has recently attracted attention as an important quality control item because it can directly appeal to customers for the luxury and overall quality of the automobile. The sharpness is one of the indicators of the finish quality of the coating, and the coating technique has been mainly improved to improve the quality. On the other hand, the surface roughness of a thin steel sheet has conventionally been generally roughened by dulling because of press formability. However, with the improvement of the coating technology, the relationship between the surface roughness of the thin steel sheet that becomes the base of the coated surface and the surface roughness after painting becomes clear, and the post-painting sharpness is improved by managing the surface roughness of the steel sheet. It is becoming increasingly clear that things are possible.

Conventionally, the surface roughness of a cold-rolled steel sheet has been controlled by temper rolling using a skin pass roll subjected to shot dulling. The main purpose of this is to improve press formability. In order to improve the sharpness after painting, it is necessary to reduce the surface roughness of cold-rolled steel sheets, and this finding was introduced in, for example, the NILAN SAE (SAE Tech, Paper Ser, No. 800208) paper. ing.

However, even if this result is applied as it is, a problem remains from the viewpoint of moldability. Compatibility between moldability and clarity cannot be achieved by controlling the average roughness as in conventional shot-dal processing. JP-A-62-168602 and JP-A-62-2244
No. 05 discloses a surface roughness control technology for achieving both post-painting sharpness and formability in cold-rolled steel sheets. But,
This applicable steel type is limited to a thin-rolled type in which the surface roughness of the original sheet can be received as it is even after the surface treatment, among the cold rolled steel sheets or the surface-treated steel sheets.

That is, in the case of a thick surface treatment such as a hot-dip galvanized steel sheet or a case where the surface is roughened by further alloying treatment, conventionally, surface roughness management for sharpness, or forming. Roughness control for compatibility with gender was considered impossible, and studies for this were hardly respected.

<Problems to be Solved by the Invention> In the above-mentioned prior patents, the target steel types were all limited to cold-rolled steel sheets and thinned surface-treated steel sheets. that is,
The reason is that the surface roughness is a steel type determined in principle by temper rolling, and the desired roughness management is relatively easy in this step. On the other hand, the alloyed hot-dip galvanized steel sheet has fine irregularities on its surface, and it is said that the presence of these irregularities cannot expect the effect of controlling the roughness as in the case of a cold-rolled steel sheet.

The present invention discloses a surface roughness management technology for improving both the sharpness after painting and the formability of an alloyed hot-dip galvanized steel sheet at the same level as a cold-rolled steel sheet, and discloses the formability and sharpness after painting. It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet having both excellent properties.

<Means for Solving the Problems> That is, in the present invention, the maximum roughness Rmax is 8 μm or more,
Alloying with excellent formability and sharpness after painting characterized by a surface area ratio of 40% or more and 90% or less from the highest convex part of the steel sheet surface to a position 20% lower than Rmax A hot-dip galvanized steel sheet is provided.

 Hereinafter, the present invention will be described in more detail.

As described above, the surface of the alloyed hot-dip galvanized steel sheet is entirely roughened as shown in FIG. 6 due to the fine irregularities formed at the stage of alloying after plating. This has conventionally made it difficult to control the surface roughness of this steel type. However, in the present invention, on the premise of this state, the sharpness and the formability are improved by adjusting the surface roughness within a range achievable in the subsequent steps. For this purpose, it has been found that the conventional management of the average roughness or PPI (the number of peaks per inch) is not sufficient, and it is necessary to further restrict the surface roughness structure.

Therefore, in the present invention, by controlling the surface roughness using a special parameter, it is possible to achieve both the formability of the galvannealed steel sheet and the sharpness after painting.

In order to properly adjust the rough surface as shown in FIG. 6 of the alloyed hot-dip galvanized steel sheet as in the present application, it is necessary to control the surface roughness in each manufacturing process. A method of finally adjusting the surface roughness by a skin pass is also conceivable, in which case it is preferable to use a roll subjected to laser dulling. The bright roll is dulled with a laser to form a concavo-convex pattern to be applied to the galvanized steel sheet. Using this dulling roll, if the roughness level within the range of the present invention can be controlled by temper rolling so that a desired transfer rate is obtained, a hot-dip galvanized steel sheet excellent in formability and sharpness can be obtained. Can be

 However, the present invention does not limit the manufacturing method.

In the present invention, the maximum roughness Rmax of the galvannealed steel sheet is 8 μm or more, and the surface area ratio between the highest convex portion of the steel sheet surface and the position 20% lower than Rmax is 40% or more and 90% or more. The range is as follows.

Referring to FIG. 1, which is a schematic diagram for explaining this, l ₁ , l ₂ , l ₃ , and l ₄ are flat portions included within a predetermined length L, and RmaX is the maximum roughness.

Rmax is set to 8 μm or more. If Rmax is less than 8 μm, there is a risk that the sliding surface at the time of pressing may cause seizure. Further, the present invention provides a position between the highest convex portion of the steel sheet surface and a position 20% lower than the maximum roughness Rmax. The surface area ratio is between 40% and 90%. If the surface area ratio is less than 40%, moldability and paint sharpness are poor,
Further, if it exceeds 90%, the moldability is inferior, so that it is not preferable.

Untreated and hot-dip galvanized steel sheet according to the invention (G
About A), the maximum roughness R from the highest convex part of the steel sheet surface
FIG. 2 shows the relationship between the surface area ratio up to a position 20% lower than max and Rmax. As can be seen, the conventional GA material is particularly clear because the Rmax is 10 μm or more, and the surface area ratio from the highest convex part of the steel sheet surface to the position 20% lower than the maximum roughness Rmax is about 40% or less. Not excellent. On the other hand, the GA material of the present invention uses a roughness parameter that was not controlled by the conventional GA material, and by limiting the surface roughness, the high sharpness after painting, which was almost impossible in the past, was achieved. And good moldability can be achieved.

4 and 5 show the surface profile of the hot-dip galvanized steel sheet according to the present invention, and FIG. 6 shows the surface profile of an untreated conventional hot-dip galvanized steel sheet. 6, the surface is randomly roughened by crystal growth by an alloying process, whereas in the case of the present invention shown in FIG. 4 and FIG. It can be seen that they are formed at the desired ratio. The flat portion and the concave portion are shown in FIGS.
It is better to arrange them regularly as shown in the figure. In FIG. 4, SRa (average roughness obtained by a three-dimensional roughness measuring device) is 1.0 μm, SRmax (maximum roughness obtained by a three-dimensional roughness measuring device) is 11.3 μm, and in FIG. SRa 0.9μm, SR
max is 9 μm, that of FIG. 6 is SRa 1.3 μm, SRmax 14
μm.

<Examples> Next, the present invention will be specifically described based on examples.

(Example 1) A cold-rolled steel sheet having a thickness of 0.7 mm was used as an original sheet, and hot-dip galvanized with a basis weight of 45/45 g / m ² on both sides under a single condition, and 540 ° C x 3 sec.
When the alloying treatment was performed, an alloyed galvanized steel sheet as illustrated in FIG. 6 was obtained.

After subjecting the hot-dip galvanized steel sheet thus obtained to a tempering pretreatment to smooth the steel sheet surface, using a dull roll that has been subjected to laser dulling, the rolling reduction is varied to obtain various types as shown in Table 1. Dulled steel sheet was obtained. These are shown in Table 1 together with the surface characteristics and the test results described below. The test results are shown in FIG.

Comparative steel 1 has a small surface area ratio of 14% from the highest convex portion of the steel plate surface, which is a requirement of the present invention, to a position 20% lower than Rmax, and therefore has a large friction coefficient and high formability. not good. Also, the sharpness (DOI value) after painting is poor.

Since Comparative Steel 2 has a small surface area ratio of 27%, similarly to Comparative Steel 1, both the formability and the sharpness after painting are poor.

Comparative steel 3 has a small Rmax of 6 μm, and therefore has a large coefficient of friction and poor formability.

Since the comparative steel 4 has a large proportion of the upper 20%, the coefficient of friction increases, and galling occurs.

Further, since the comparative steel 5 has a small Rmax in addition to the comparative steel 4, the coefficient of friction further increases, and galling occurs.

On the other hand, it is understood that the steel of the present invention is excellent in both formability and sharpness.

In addition, the measurement and test of each characteristic were performed as follows.

(1) Rmax The height difference between the highest point and the lowest point in the three-dimensional roughness profile is described. (The maximum roughness of the two-dimensional roughness parameter is three-dimensional.) (2) The surface area ratio between the highest convex part of the steel sheet surface and the position 20% lower than Rmax is measured by three-dimensional roughness. Determined by analyzing the results.

(3) Formability The press formability is closely related to the coefficient of friction between the sample and the mold. For this reason, the friction coefficient was determined from the provision of a pull-out when the sample was pulled out with both sides of the sample sandwiched by a mold (SKD11, 2 cm width), and a load of 100 kg was applied under a holding load of 100 kg.

(4) Sharpness after painting 3 coats on the sample (Electron made by Kansai Paint
After coating 9400 with 20 µm, middle coat with TP-26 sealer and top coat with Amirac TM-13202 (black) 50 µm), the DOI value was measured.

The DOI value is measured with a Hunter DORIGON meter, and the amount of specular reflection when light is irradiated from the direction of 30 ° of the sample normal is R.
s, when the amount of light reflected at an angle shifted by ± 0.3 ° from the regular reflection is R _0.3 , DOI = (Rs−R _0.3 ) / Rs × 100. This evaluation method is based on human visual judgment,
It shows good correlation with conventional evaluation methods such as the PGD method to check whether a test pattern can be identified on a sample.

<Effect of the Invention> The hot-dip galvanized steel sheet of the present invention has a maximum roughness Rmax of 8
μm or more, the surface area ratio between the highest convex part of the steel sheet surface and the position 20% lower than Rmax is 40% or more 90
%, So that both moldability and sharpness after painting are excellent.

It is also apparent that the same effect can be obtained in the case of a two-layer plated steel sheet further plated on a galvanized steel sheet.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the characteristics of the hot-dip galvanized steel sheet of the present invention. FIG. 2 is a diagram for comparison between the present invention and a conventional hot-dip galvanized steel sheet. FIG. 3 is a graph showing the results of Example 1. 4 and 5 are profile diagrams of the present invention, and FIG. 6 is a profile diagram of a conventional hot-dip galvanized steel sheet. Magnification: 100 times each in vertical and horizontal directions (X and Y axes), 500 in roughness (vertical Z axis) direction
It is twice.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Hideo Abe 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corp. Technical Research Division (56) References JP-A-2-185959 (JP, A) JP-A Sho 63-33591 (JP, A) JP-A-59-197553 (JP, A) JP-A-56-72160 (JP, A)

Claims (1)

(57) [Claims] 1. A forming method wherein the maximum surface roughness Rmax is 8 μm or more, and the surface area ratio between the highest convex portion of the steel sheet surface and a position 20% lower than the Rmax occupies 40% or more and 90% or less. Alloyed hot-dip galvanized steel sheet with excellent properties and sharpness after painting.