JPH02274856A - Alloyed hot-dip galvanized steel sheet having excellent formability and brightness after painting - Google Patents

Abstract

PURPOSE:To impart a high brightness after painting and excellent formability to the alloyed hot dip-galvanized steel sheet by forming the surface of the steel plate to the roughness specified in the area rate of flat parts, the nearest spacing between recesses and the angle formed by the flat parts and recesses. CONSTITUTION:The surface of the alloyed hot dip-galvanized steel sheet is worked by using dull rolls, etc., while adjusting the draft. The steel sheet is so worked at this time that the flat parts having <=0.6mum average roughness (Ra) occupy >=30% of the steel sheet surface. In addition, the steel sheet is worked to the surface roughness in which the recesses lower by >=2mum than the flat parts are distributed to attain 50 to 300mum nearest spacing and which has the shape to attain 5 to 30 deg. angle formed by the flat parts and the recesses. Further, the recesses are preferably distributed in such a manner that the recesses are not continued by the recesses of >=0.5mum depth to prevent the parting of the individual recesses by >=20% of the average value of the nearest spacing. The steel sheet having the excellent formability and the brightness after painting the excellent formability and the rightness after painting is obtd. by such control of the surface roughness.

Description

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

<Conventional technology> Cold-rolled steel sheets have traditionally been widely used for thin steel sheets that require a finished appearance after painting, such as the exterior panels of automobile bodies, home appliances, and sheet metal furniture. To achieve both, the surface roughness is adjusted by temper rolling.
However, the use of surface-treated steel sheets is rapidly increasing, especially from the standpoint of rust prevention for automobile steel sheets, and it has become an issue to achieve both post-painting sharpness and press formability in surface-treated steel sheets. In the case of surface-treated steel sheets with relatively thin coatings, such as electroplating, the surface roughness of the cold-rolled steel sheet, which is the original sheet, is maintained even after surface treatment, and surface roughness management is an extension of conventional cold-rolled steel sheets. This is almost possible with technology.

However, if further rust prevention measures are required, thicker surface treatment is required, such as with hot-dip galvanized steel sheets that have been subjected to alloying treatment, and in that case, the surface roughness is equal to that of the original sheet. The problem is that it is completely different.
In other words, the surface roughness of the alloyed hot-dip galvanized steel sheet changes greatly from the surface roughness of the original sheet through both the hot-dip galvanizing process and the alloying process. It is known that the surface roughness is caused by specific fine irregularities, which adversely affects both the image clarity after painting and the press formability.

Nowadays, the quality of the paint surface finish of automobiles has recently attracted attention as an important quality control item because it can directly appeal to customers the luxurious feel and high overall quality of the automobile. Image clarity is one of the indicators of paint finish quality, and improvements in painting technology have traditionally been made to improve image clarity. On the other hand, the surface roughness of thin steel sheets has conventionally been generally roughened to an appropriate degree by using dull area weight for press formability. However, with the improvement of painting technology,
The relationship between the surface roughness of the thin steel sheet that forms the basis of the painted surface and the surface roughness after painting has been clarified, and it has become possible to improve the image sharpness after painting by controlling the surface roughness of the steel sheet. has been revealed.

The surface roughness of cold-rolled steel sheets has conventionally been controlled by temper rolling using shot-dulled skin bath rolls, but 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, and this knowledge is based on, for example, the SAE (SAE Tech.

Paper Ser, No. 800208) is also introduced in the paper.

However, even if this result is applied as is, problems remain in terms of formability. It is impossible to achieve both formability and image clarity by controlling the average roughness as in conventional shot dull processing. JP-A-62-168602 and JP-A-6
No. 2-224405 discloses a surface roughness control technique for achieving both post-coating sharpness and formability in cold-rolled steel sheets. However, the applicable steel types have been limited to cold-rolled steel sheets or surface-treated steel sheets with a light weight that retains the surface roughness of the original sheet even after surface treatment.

In other words, in the case of thick surface treatments such as hot-dip galvanized steel sheets, or cases where the surface becomes rough due to further alloying treatment, conventional methods have been used to control surface roughness for image clarity or to form It was considered impossible to control the roughness so as to be compatible with quality, and research for this purpose was hardly considered.

<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 light-weight surface-treated steel sheets. it is,
The reason for this is that it is a type of steel whose surface roughness is basically determined by temper rolling, and the desired roughness control can be achieved relatively easily in this process. On the other hand, alloyed hot-dip galvanized steel sheets have fine irregularities on their surfaces, and because of the presence of these irregularities, it has been thought that the effect of roughness control as in the case of cold-rolled steel sheets cannot be expected.

The present invention discloses a surface roughness control technology for improving both the post-painting sharpness and formability of alloyed hot-dip galvanized steel sheets to the same level as cold-rolled steel sheets. The purpose of the present invention is to provide an alloyed hot-dip galvanized steel sheet with excellent properties.

Means for Solving the Problem> That is, the present invention provides a method in which a flat portion with an average roughness Ra of 0.6 μm or less occupies 30% or more of the surface of a steel plate, and a concave portion lower than the flat portion by 2 μm or more is located at the nearest distance. 50-300
Alloyed hot-dip galvanizing with excellent formability and sharpness after painting, characterized by a shape with a distribution of micrometers and an angle of 5 degrees or more and 30 degrees or less between a flat part and a concave part. It provides steel plates.

Further, it is preferable that the recesses are individually distributed so as not to be connected by recesses having a depth of 0.5 μm or more.

Furthermore, it is preferable that the distance between the nearest adjacent recesses does not exceed 20% of the average value.

The present invention will be explained in more detail below.

As mentioned above, the surface of an alloyed hot-dip galvanized steel sheet is roughened overall as shown in Figure 6 due to the fine irregularities formed during the alloying stage after plating. . This has conventionally made it difficult to control the surface roughness of this type of steel. However, in the present invention, based on this condition, the surface roughness is adjusted within the range that can be realized in subsequent steps, thereby improving image clarity and formability.
For this purpose, the conventional average roughness or PPI (1
It was found that simply controlling the number of ridges per inch is not sufficient, and that it is necessary to define even finer surface roughness structures.

Therefore, in the present invention, by controlling the surface roughness using special parameters, we aim to achieve both formability and post-painting sharpness of the alloyed hot-dip galvanized steel sheet.

In order to properly adjust the 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 using a skin pass may also be considered, and in that case, it is preferable to use a roll that has been subjected to laser dulling. The bride roll is dulled using a laser to form the uneven pattern that is intended to be applied to the hot-dip galvanized steel sheet. This dull roll is pressed against the galvanized super steel sheet at a rolling reduction rate that provides a desired transfer rate. As a result, the transfer rate becomes within the desired range, and if the roughness level can be controlled within the range of the present invention, a hot-dip galvanized steel sheet with excellent formability and image clarity can be obtained. However, the present invention is not limited to its manufacturing method.

In the present invention, the area of the flat part with an average roughness Ra of 0.6 μm or less of the alloyed hot-dip galvanized steel sheet is 30% or more, and the nearest distance between the concave parts that is 2 μm or more lower than the flat part is 50 to 300 μm. It is disclosed that it is effective for both image clarity and formability to have a shape in which the angle between the flat part and the concave part is 5 degrees or more and 30 degrees or less.

To explain this, referring to FIG. 1 which is a schematic diagram of a two-dimensional roughness profile of the present invention, flr, JZ2
, Il, 3. λ4 is the flat portion included within the predetermined length L, and V, , V2 are the nearest concave spacings. Also, θ1. θ2. θ.

θ4. θ5. θ6 indicates the angle between the flat portion and the concave portion. That is, the angle here refers to the angle when a tangent is drawn at the point of contact between the flat part and the recessed part.

The flat part means a part where Ra is 0.6 μm or less, and when this is 30% or more, (It, -1-12+ms)/L≧0
．． It means 3. The flat area ratio can be determined by analyzing a two-dimensional roughness profile or by image processing of the steel plate surface using three-dimensional roughness data. If the area ratio of the flat portion is less than 30%, the sharpness of the image deteriorates, which is not preferable.

Then, if the recessed part is 2 μm or more lower than the flat part, the closest distance is 5.
The recesses are arranged on the surface of hot-dip galvanized steel so that the recesses have a diameter of 0 to 300 μm. That is, 50μm≦V,,V
, ≦300 μm. Outside this range, it is not preferable because both image clarity and moldability cannot be achieved.

Further, the angle between the flat part and the recessed part is 5 degrees or more and 30 degrees or less (5°≦θ1.θ2.θ).

θ4. θ6. θ6≦30@). 5
If it is less than 30 degrees, the amount of oil retained in the recess will be reduced.
If the temperature is too high, the supply of oil present in the recesses to the sliding surface will be reduced, which is not preferable.

Furthermore, in the present invention, the recesses are connected to each other by a recess having a depth of 0.5 μm or more. It is preferable that they are individually isolated and separated by flat areas so as not to cause any damage, at a depth of 0. If a recess of 5 μm or more is clogged, oil will flow out to other recesses, reducing the supply of oil to the sliding surface, which is undesirable.

Furthermore, the nearest distance between each recess is 20% of its average value.
It is not desirable to be further apart.

Using the symbols in FIG. 1, the following conditions should be satisfied: 0.8≦vt and V2≦1.2v. Outside this range, the sharpness of the image
This is not preferable because it causes variations in sliding properties.

Untreated and hot-dip galvanized steel sheets (GA
), FIG. 2 shows the relationship between the area ratio of Ra≦0.6 μm and the nearest recess interval V (μm) or the angle θ (°) between the flat portion and the recess. As will be seen from now on, in conventional GA materials, V cannot be measured and the area ratio of Ra≦0.6 μm is on the order of several percent, so they are not particularly excellent in both image clarity and formability. On the other hand, the GA material of the present invention uses roughness parameters that were not controlled in conventional GA materials, and by limiting the surface roughness, it can achieve high post-painting heights that were previously considered almost impossible. It is possible to achieve good image clarity and moldability.

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 an untreated conventional hot-dip galvanized steel sheet.

The surface of the conventional one shown in Fig. 6 is randomly roughened due to crystal growth during alloying treatment, whereas the one of the present invention shown in Figs. 4 and 5 has flat parts and concave parts as desired. It can be seen that it is formed at a ratio of . The flat parts and recesses should be arranged regularly as shown in Figures 4 and 5. The one in Figure 4 is 5Ra (average roughness determined with a three-dimensional roughness measuring device). is 1.0μm, SRma
x (maximum roughness) is 11.38 m1 The one in Figure 5 is S
Ra is 0.9 μm and SRmax is 9 μm. In the case of FIG. 6, SRa is 13 μm and SRmax is 14 μm.

<Example> - Next, the present invention will be specifically explained based on examples.

(Example 1) A cold-rolled steel sheet with a thickness of 0.7 mm has an average roughness of 0.
86μm), area weight 45/45g/r on both sides under single conditions
r? When the steel sheet was hot-dip galvanized and alloyed at 540° C. for 3 seconds, an alloyed galvanized steel sheet as illustrated in FIG. 6 was obtained. This is Comparative Steel 1 shown in Table 1.
corresponds to

The hot-dip galvanized steel sheet obtained in this way was subjected to heat refining pretreatment to smooth the surface of the steel sheet, using a dull roll that had been subjected to laser dulling, and various dulling processes were performed as shown in Table 1 by changing the rolling reduction rate. Obtained steel plate. Table 1 shows the surface properties and the test results below. The test results are shown in Figure 3.

Comparative Steel 1 has a considerably roughened surface due to the unevenness formed during the alloying treatment, so it has a large friction coefficient and poor press formability. Further, since the area ratio of Ra≦0.6 μm is small, the image clarity after painting (Do I value) is also poor.

Comparative Steel 2 has a large distance between the concave portions and a large angle between the concave portions and the flat portion, so the coefficient of friction is large and the formability is poor.

In Comparative Steel 3, since the recesses are continuous, oil easily escapes between the recesses, resulting in a large coefficient of friction and poor formability.

Comparisons #44 and 5 are based on the angle θ between the flat part and the concave part.
Since the friction coefficient is large, the moldability is poor.

In comparison tR6, since the angle θ between the flat portion and the recessed portion is small, the coefficient of friction is large and galling occurs.

In contrast, it can be seen that the steel of the present invention, in which the surface roughness was controlled within the range of the present invention, was excellent in both image clarity and formability after painting.

Note that the measurements and tests for each characteristic were performed as follows.

(1) Measure the three-dimensional roughness curve of Ra and flatness area ratio (Figures 4 to 6), manually input this raw data to the image processing device Luzex 5000, and analyze it to determine the area ratio of the flat part. can be measured.

Ra is measured by expanding the conventional definition to three dimensions.

In other words, Ra (3D) S=LXL.

f (x, y) is a function indicating a surface curve (2) Closest spacing between recesses Three-dimensional roughness raw data was analyzed using an image processing device, and the spacing between recesses was measured.

(3) Angle consisting of a flat part and a concave part Data obtained by scanning the central part of a concave part was extracted from the three-dimensional angle data, and an angle made by a flat part and a concave part was determined.

(4) Formability Press formability is closely related to the coefficient of friction between the sample and the mold material. For this reason, the coefficient of friction was determined from the pull-out resistance when the sample was held between both sides by mold materials (SKDll, 2 cm width) and a load of 100 kg was applied to the sample.

(5) Sharpness after painting After applying 3 coats to the sample (electrodeposition is 20 μm of Kansai Paint Electron 9400, 1 intermediate coat is TP-26 sealer, and top coat is 50 μm of Amirac TH-13202 (black)) DOI The value was measured.

The DOI value is measured using a DORICON meter manufactured by Hunter, and is the amount of specularly reflected light when irradiated from a direction of 30 degrees to the normal line of the sample, Rs, which is reflected at an angle ±0.3 degrees deviated from the specular reflection. When the amount of light is Ro, it is given as DOI-(Rs-Ro, 3)/RsxlOO. This evaluation method shows good correlation with conventional evaluation methods such as human visual judgment and the PGD method, which checks whether a test pattern can be identified on a sample.

Table 1 ◆ mark: The interval between concave and convex parts is within ±20% of the average value in the table. , the rough surface after alloying treatment has an area ratio of Ra≦0.6 μm that occupies 30% or more, and the concave portions that are lower than the flat portion by 2 μm or more have the closest distance of 50 to 3
00 μm, and the angle between the flat part and the concave part is 5 degrees or more and 30 degrees or less, so it has excellent moldability and sharpness after painting.

Furthermore, it is even better if the individual recesses are not connected and the distance between their nearest neighbors is within 20% of the average value. The present invention is based on the G.A.
A similar effect can be obtained even in the case of two-layer plating, in which plating is further applied on top of the .

Ru.

4 and 5 are enlarged views of the profile of the present invention, and FIG. 6 is an enlarged view of the profile of a conventional hot-dip galvanized steel sheet. In addition,
The magnification is 100 times in the vertical and horizontal directions (X, Y axes) and 500 times in the roughness (vertical Z@) direction.

Claims (3)

[Claims] (1) A flat part with an average roughness Ra of 0.6 μm or less occupies 30% or more of the steel plate surface, and is 2 μm from the flat part
The moldability and sharpness after painting are characterized by a shape in which the concave portions are distributed so that the closest distance between the flat portions and the concave portions is 50 to 300 μm, and the angle between the flat portion and the concave portion is 5 degrees or more and 30 degrees or less. Superior alloyed hot-dip galvanized steel. (2) The alloyed melt having excellent formability and post-painting sharpness according to claim 1, wherein the recesses are individually distributed so as not to be connected by recesses having a depth of 0.5 μm or more. Galvanized steel. (3) The alloyed hot-dip galvanized steel sheet with excellent formability and post-painting sharpness according to claim 1 or 2, wherein the nearest distance between the individual concave portions is not more than 20% of the average value thereof.