JP2749627B2 - 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 automobile body outer panels and exterior panels such as household electrical appliances and sheet metal furniture. The surface roughness is adjusted by temper rolling from the both sides.
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 a damping basis for 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 the cold-rolled steel sheet.
It is also introduced in SAE (SAE Tech, Paper Ser, No. 800208) paper by NILAN et al.

However, as a result, a problem remains from the viewpoint of moldability even when applied as it is. Compatibility between formability and image clarity cannot be achieved by controlling the average roughness as in conventional shot-dal processing. JP-A-62-168602 and JP-A-62-224405 disclose surface roughness management techniques for achieving a balance between sharpness after painting and formability of a cold-rolled steel sheet. However, the applicable steel type is limited to a thin-rolled type in which the surface roughness of the original sheet can be directly received 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 by the temper rolling in principle, and the desired roughness management can be relatively easily performed in this step. In contrast, galvannealed steel sheets have fine irregularities on the surface,
It has been said that due to the presence of the irregularities, the effect of controlling the roughness as in the case of a cold-rolled steel sheet cannot be expected.

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, the present invention is characterized in that a flat portion having an average roughness Ra of 0.6 μm or less occupies 30% or more of the steel sheet surface, and Rmax is 8 μm or more and 16 μm or less. An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which is excellent in formability and sharpness after painting.

Further, it is preferable that the average waviness (Wca) is 0.45 μm or less.

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

As described above, the surface prevalence of the alloyed hot-dip galvanized steel sheet is, as described above, due to the fine irregularities formed at the stage of alloying after plating, and the surface is entirely rough as shown in FIG. Become 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 conventional management of average roughness or PPI (the number of peaks per inch) is not sufficient, and that a finer surface roughness structure is required.

Therefore, in the present invention, by controlling the surface roughness using another parameter, both the formability of the galvannealed steel sheet and the sharpness after painting are achieved.

In order to properly adjust the surface of the alloyed hot-dip galvanized steel sheet as shown in FIG. 6 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 is also conceivable, in which case it is preferable to use a roll that has been 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. The dulling roll is pressed against the plated steel sheet at a rolling reduction such that a desired transfer rate is obtained. As a result, a hot-dip galvanized steel sheet having a transfer rate in a desired range, that is, excellent moldability and sharpness can be obtained. However, in the present invention, it is not necessary to limit the manufacturing method, and the same effect can be obtained by achieving the roughness range disclosed in the present invention other than laser dulling.

That is, in the present invention, the average roughness Ra of the galvannealed steel sheet is reduced by 30%
As described above, it is disclosed that setting Rmax in the range of 8 to 16 μm is effective for both moldability and sharpness.

To explain this, a schematic diagram of a two-dimensional roughness profile of the net of the present invention is shown in FIG. ₁ , where l ₁ , l ₂ , and l ₃ are flat portions included within a predetermined length L, and Rmax Is the maximum roughness, Wca
Is the average swell.

The flat portion means a portion where Ra is 0.6 μm or less, which is 3 μm.
0% or more means (l ₁ + l ₂ + l ₃ ) /L≧0.3. The flatness area ratio can be determined by analysis of a two-dimensional roughness profile or image processing of the steel sheet surface using three-dimensional roughness data.

 Rmax is set to 8 to 16 μm.

If Rmax is less than 8 μm, there is a risk that the sliding surface at the time of pressing may cause seizure. It is because it becomes high.

More preferably, Wca (average ridge) is set to 0.45 μm or less. If Wca exceeds 0.45 μm, the sharpness after coating is impaired, which is not preferable.

Untreated and hot-dip galvanized steel sheet according to the invention (G
FIG. 2 shows the relationship between the area ratio of Ra ≦ 0.6 μm and Rmax for A). As you can see, the conventional GA
In many cases, Rmax is 10 μm or more, and since the area ratio of Ra ≦ 0.6 μm is about 10% or less, both sharpness and moldability have problems. 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 has been almost impossible until now, has been achieved. Good moldability can be achieved.

4 and 5 show the surface profile of a hot-dip galvanized steel sheet according to the present invention, and FIG. 6 shows the surface profile of a conventional hot-dip galvanized steel sheet. 6 has a random surface roughened by crystal growth during the alloying process, whereas the present invention shown in FIGS. 4 and 5 has a flat portion and a concave portion. It can be seen that they are formed at the ratio of The flat portions and the concave portions are regularly arranged as shown in FIGS. 4 is SRa.
(Average roughness obtained with a three-dimensional roughness measuring instrument) is 1.0 μm, S
Rmax (maximum roughness obtained with a three-dimensional roughness measuring instrument) is 11.3μ
m, FIG. 5 shows SRa of 0.9 μm and SRmax of 9 μm, and FIG. 6 shows SRa of 1.3 μm and SRmax of 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 (average roughness 0.86μ).
m), hot-dip galvanized with a basis weight of 45/45 g / m ² on both surfaces under a single condition, and subjected to alloying at 540 ° C x 3 sec. An alloyed galvanized steel sheet as illustrated in Fig. 6 I got This corresponds to Comparative Steel 1 shown in Table 1.

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. Table 1 shows the surface properties and test results of the following. The test results are shown in FIG.

Since the comparative steel 1 is considerably roughened by the unevenness formed during the alloying treatment, the coefficient of friction is large and the press formability is poor. Also, the area ratio of Ra ≦ 0.6 μm is small,
Since the undulation Wca is large, the sharpness (DOI value) after painting is also poor.

Comparative steel 2 has a large undulation similar to comparative steel 1, and Rm
Since ax is too large, moldability and sharpness are poor.

The comparative steel 3 has a large undulation similarly to the comparative steel 2, and the flatness area ratio is too small, so that the formability and the sharpness are not sufficient.

Relative Rmax of Comparative Steel 4 is too small, so that mold 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) Ra and flatness area ratio A three-dimensional roughness curve was measured (FIGS. 4 to 6), and the raw data was input to an image processing apparatus Luzex 5000.
By performing the analysis, the flatness area ratio can be measured. Ra measures the conventional definition to three dimensions. That is, Here, S = L × L, f (x, y) is a function indicating a surface curve. (2) Rmax Indicates the height difference between the highest point and the lowest point in the three-dimensional roughness profile (the two-dimensional roughness parameter Three-dimensional maximum roughness).

(3) Wca This represents a center line swell specified in JIS B0610, and evaluates a component showing a long wavelength in surface roughness.

(4) 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 pull-out resistance when the sample was pulled out with a load of 100 kg being pressed by holding the sample from both sides with a die (SKD11, 2 cm width).

(5) Sharpness after painting 3 coats on sample (Electron 94 manufactured by Kansai Paint)
00 was applied to 20 μm, the middle coat was applied to TP-26 sealer, and the top coat was applied to Amirac TM-13 # 202 (black) at 50 μm), and then 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 a good correlation with conventional evaluation methods such as the PGD method for checking 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 rough surface after the alloying treatment in which the area ratio of Ra ≦ 0.6 μm is 30% or more and Rmax is 8 to 16 μm.
By controlling the thickness of the steel sheet, a steel sheet having excellent formability and sharpness after painting can be achieved. Further, when Wca is set to 0.45 μm or less, a good result in sharpness can be obtained.

Further, it is apparent that the effect of the present invention can be similarly obtained when two-layer plating is further performed on hot-dip galvanizing.

[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. The magnification is 100 times in each of the vertical and horizontal directions (X and Y axes), and the roughness (vertical Z axis) direction is 5 times.
It is 00 times.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideo Abe 1 Kawasaki-cho, Chiba City, Chiba Pref. 63-33592 (JP, A) JP-A-59-1636 (JP, A) JP-A-2-57670 (JP, A)

Claims (2)

(57) [Claims] 1. A flat portion having an average roughness Ra of 0.6 μm or less occupies 30% or more of the steel sheet surface, and Rmax of 8 μm or more.
An alloyed hot-dip galvanized steel sheet having excellent formability and sharpness after painting, characterized by being 16 μm or less. 2. The galvannealed steel sheet according to claim 1, wherein the average waviness (Wca) is 0.45 μm or less.