JP3476495B2 - Sheet steel for press working - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a thin steel plate for press working,
More specifically, the present invention relates to a thin steel sheet for press processing which is excellent in image clarity after press coating and press formability, and which is less likely to generate stars.

[0002]

2. Description of the Related Art Conventionally, the quality of the coating surface has been considered as an issue of the coating material and the coating method, and improvements have been made from that point, but with the improvement of the coating technology, the thin plate which is the basis of the coating It has also been found that the effect of surface roughness cannot be ignored. Previous studies have revealed that the flatter the surface of the thin plate, the better the image clarity after painting (TG. Nilan et al SAE Tech Paper Ser. No80.
0208, 1980). However, if the roughness is too low, like a bright thin plate, in an automated press line, the blank plate is difficult to slide and handle, and scratches are noticeable, and the mold tends to be kinked during press molding. It becomes a very difficult to handle thin plate. As described above, achieving both press moldability and post-painting clarity is a contradictory and difficult task in surface roughness management.

An excellent method for solving this problem is
It is proposed in Japanese Patent Publication No. 3-38924. In this publication, the surface roughness of a thin plate is considered as a long-wavelength roughness component (waviness) having a long cycle of unevenness and a roughness component having a short wavelength, and the waviness component of the thin plate surface remains after coating. However, focusing on the fact that the roughness component with a short wavelength is hidden by the coating film, we have proposed a high-visibility thin plate that is controlled so as to eliminate the remaining swell component as much as possible.

However, when actually using the high-definition image-forming steel plate subjected to such control, it is necessary to sufficiently take measures against star-shaped defects due to the inclusion of foreign matter. Had a new problem that the yield decreased or the maintenance work increased due to the occurrence of star-shaped defects.

The high image clarity thin plate proposed in the above publication controls the surface roughness wavelength as described above, and is particularly excellent in surface appearance, so that surface defects such as stars are easily noticeable. . Furthermore, in today's press manufacturing line, automation and unmanned operation are progressing considerably, and it is difficult to inspect star eyes and the like, and it is feared that a large number of star eyes will occur, The appearance of thin plates that are hard to see is desired.

Here, the star is a so-called star that causes minute indentations on the surface of the material after pressing due to foreign matter particles adhered to the punch surface or the material surface on the side in contact with the punch, deposits of plating coating, and the like. Defects called pimples, pressure marks, etc.

Various measures such as polishing of the press die, cleaning of the press die and materials, dust protection in the factory, cleaning of press oil have been implemented as measures against star eyes, but there are various factors related to the mechanism of star eye generation. Therefore, it is extremely difficult to reduce this defect. When the number of star eyes is large, the press die and the pressed product must be frequently maintained, which requires a lot of time and labor. Also, if there are many star eyes on the panel surface, it will be scrapped as a defective product,
Reduces product yield. Furthermore, even if the number of occurrences is small, it is necessary to polish and smooth the surface of the thin plate with a grindstone, resulting in a decrease in efficiency in the manufacturing process. As described above, the star eyes have become a serious problem in the pressing process of automobile bodies, especially outer panels.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a thin steel sheet for press working, which does not impair the press formability, has excellent image clarity after coating, and is less prone to star formation.

[0009]

The above object of the present invention is to ensure that the surface condition satisfies the following conditions (1) to (3), or to secure its effect (4) and / or Alternatively, it is achieved by a thin steel sheet for press working, which satisfies the condition (5). (1) The average waviness (Wca) of the unevenness on both surfaces of the steel sheet is 0.
6 μm or less. (2) The 10-point average roughness (Rz) of the unevenness on the surface of the steel sheet in contact with the punch is 10 μm or more and 20 μm or less . (3) Average roughness of the unevenness of the steel plate surface in contact with the die (R
a) is 0.4 μm or more and 2 μm or less and the flat area ratio is 4
0% or more and 80% or less . (4) At an arbitrary position on the surface of the steel plate in contact with the punch,
200 μm x recesses with a depth of 10 μm or more from the center plane
One or more in a 200 μm square. (5) The fluctuation range of the height of the convex portions on the surface of the steel plate in contact with the punch is 20% or less of the average height of the convex portions.

The constitution of the present invention will be described in detail below, but the preferred embodiments of the present invention and advantages based on them will be apparent.

Condition (1) First, the average waviness (Wca) of the unevenness of the thin plate for press working of the present invention is set to 0.6 μm or less on both surfaces. Reducing the waviness component of the thin plate surface is important for improving the image clarity after painting. FIG. 1 shows the relationship between the average waviness of the surface roughness of the steel sheet and the image clarity after coating. The surface waviness on the die side has unevenness corresponding to the waviness even after coating, and the image clarity after coating is impaired. The surface waviness on the punch surface side (the surface that is the inside of the automobile body) also affects the die-side surface (the surface that is the outside of the automobile body) by press molding, so it is also 0.6 μm or less. FIG. 5 shows the effect of the waviness of the punch-side steel sheet surface on the post-painting clarity of the opposite surface (die surface) after forming.

As the lower limit of the swell is smaller,
Excellent image clarity after painting, but satisfy the other conditions of the present invention,
And Wca is 0. It is industrially very difficult to control to less than 1 μm. The preferred range of Wca is 0.1
It is 0.4 μm.

Here, the average Wca of the long wavelength roughness component (waviness) having a long period relating to the unevenness is as follows in JIS B0
It is defined and measured based on 610. That is, Wca is the center line waviness of the filtered wave, and the high-frequency cutoff value of 0.
It is a value obtained by the equation (1) from the waviness curve (W (x)) obtained by removing the surface roughness component having a short wavelength by applying an 8 mm filter.

[0014]

[Equation 1] (This definition is for a two-dimensional roughness meter, but for a three-dimensional roughness meter,

[Equation 2] Becomes)

Condition (2) The thin plate surface on the side in contact with the punch has a 10-point average roughness (R
z) is 10 μm or more and 20 μm or less . Stars are generated when foreign matter is mixed between the punch surface and the surface of the thin plate, but by creating a gap between the punch surface and the punch surface due to surface irregularities on the thin plate side, the generation of surface protrusions due to the mixing of foreign matter is mitigated. In addition, it is effective to carry out the foreign matter mixed in the mold together with the thin plate to the outside of the mold to prevent continuous star defects. Such an effect is shown in FIG.
As shown in, Rz is expressed when it is 10 μm or more.
However, it is currently difficult to manufacture Rz having a thickness of more than 20 μm.

Under the condition (2), 10-point average roughness (R
z) is defined in JIS B0601 as follows and is measured accordingly. That is, in the part where the reference length (L) is extracted from the cross-section curve, the average value of the altitudes of the highest to fifth peaks measured from the straight line that does not cross the cross-section curve and in the direction of the secondary magnification The value of the difference from the average value of the altitude of the valley bottom from the deepest to the fifth is expressed in micrometers (μm). Also in this case, the definition in the case of the two-dimensional roughness meter can be expanded to three dimensions as it is, and the measurement can be performed by the three-dimensional roughness meter.

The fluctuation range of the height of the convex portion means the height hi (i = 1) obtained by taking a plurality of convex portions from the center plane obtained by measuring the surface roughness with a three-dimensional roughness meter. ~ N) and the maximum absolute value of the difference between the average heights H thereof are divided by H (%). That is, it is as in the following formula.

[0018]

[Equation 3]

Incidentally, FIG. 4 shows the 10-point average roughness (Rz).
And the resistance to die galling, or FIG. 6 shows the relationship between the fluctuation range of the height of the convex portion and the number of generated star eyes.

Condition (3) The average roughness (Ra) of the surface on the die side is 0.4 μm or more and 2 μm or less , preferably for the purpose of maintaining handleability and die galling resistance during press molding. It is set to 0.6 to 2 μm. It is difficult at the current technical level in manufacturing to satisfy the following conditions that the area ratio of the flat portion is 40% or more and that the thickness is more than 2 μm.

Since the surface of the thin plate on the die side is a surface where the image clarity after coating is important, in addition to the condition (1) relating to the waviness described above, a certain amount of flat area ratio is secured. The securing of the flat portion is effective for further improving the image clarity after suppressing the waviness to 0.6 μm or less. That is, by setting the flat portion to 40% or more, excellent post-painting image clarity is improved. The larger the flat area ratio, the better the image clarity, but if it exceeds 80%, the average roughness (R
It becomes difficult to satisfy the above condition of a). FIG. 3 shows the relationship between the flat area ratio and the sharpness.

The average roughness (Ra) is defined by JIS
The center line average roughness defined by B0601, which is the portion of the measured length L in the direction of the center line from the roughness curve obtained by filtering the cross-section curve with a cutoff value of 0.8 mm. Is calculated by the equation below.

[Equation 4]

The flat area ratio is the ratio of the number of contact points n _i at which the angle of the tangent to the center line is 3 ° or less when the tangent is drawn to the cross-section curve at a constant pitch (20 μm) with respect to the total number n of measured points. Is expressed as a percentage and is expressed by the following formula.

[0024]

[Equation 5]

Condition (4) Condition (2) relaxes the influence of foreign matter that has penetrated into the mold by providing a gap between the punch surface and the surface of the thin steel sheet.
Furthermore, the effect of carrying it out of the mold together with the steel plate was realized, and the effect is clearly shown in the above-mentioned graph. However, in order to further secure such an effect, it is important to surely secure voids of a certain size or more at a required distribution density or more. From such an idea, it is desirable to add the condition (4) to the condition (2). That is, at any position on the surface of the steel plate in contact with the punch, the effect of trapping foreign matter can be expected, and the distribution density of recesses with a depth of 10 μm or more from the center plane is added at least one in 200 μm square. Is. The effect of adding this condition is shown in FIG. The data shown in FIG. 2 are also shown in the figure, but it is possible to further reduce the number of star eyes even if the steel plate has the same Rz by satisfying the condition (4). Recognize.

Condition (5) The effect of preventing star-shaped defects based on this condition (2) is more effective than the fact that the height of the surface irregularities is uniform. That is, it is desirable to make the height of the uniform unevenness uniform so that the fluctuation range of the height of the convex portions is 20% or less of the average height of the convex portions. The effect of homogenization is shown in FIG.

In the star eye inspection, if the die side surface of the pressed thin plate is lightly rubbed with a grindstone, the star eye part is slightly swelled, so only that part will be strongly ground,
It will be in a white glowing state. The number of these white points is counted as the number of stars.

The above conditions (1) to (3), and further,
In order to manufacture a thin plate satisfying the conditions of (4) the number of recesses having a depth of 10 μm or more, and (5) the condition of the fluctuation width of the height of the protrusions,
The procedure is as follows. The surface roughness control during the production of the thin plate is performed by transferring the roll surface roughness used in the rolling process (cold rolling, temper rolling) to the thin plate surface.
Therefore, the roughness of the present invention can be achieved by appropriately finishing the surface roughness of the rolling roll (dull finish). Discharge dull, laser dull, etching dull, etc. are mentioned as the method.

The thin plate for press working of the present invention is a thin plate made of a non-ferrous material such as a cold rolled steel plate, a surface-treated steel plate, and an Al alloy thin plate, and is not particularly limited. By pressing the thin plate for press working of the present invention, an outer plate for an automobile body having excellent coating image clarity can be obtained with high efficiency. Further, not only automobiles, but also panels for products such as home electric appliances that require painting finish are preferably manufactured.

[0030]

EXAMPLES The present invention will be specifically described below with reference to examples. In the surface rolling (Zn-Ni, unit weight 20/20 g / m ² unit) thin steel plate and cold-rolled thin steel plate (0.7 mm thickness) for ordinary automobile outer plates, after temper rolling after manufacturing in the normal process Samples (steels A to K) in which various surface roughnesses were individually controlled by changing the type of dull roll were prepared. This thin steel sheet was subjected to 300 mm square rectangular tube draw forming, and the post-painting clarity of the surface after forming and the occurrence of stars were investigated. In addition, for the purpose of easy evaluation, the investigation of the generation state of the stars was carried out by molding with the foreign matter placed in the mold in advance. The experimental results are shown in Table 1. It can be seen that the steel sheet of the present invention is excellent in coating image clarity and staring resistance after forming.

The evaluation was carried out by the following method. Sharpness after painting of pressed products The panel is painted black (3 coats, 3 bake) in the coating material and painting process that are usually applied for automobiles, and then the sharpness and drigonometer on the outside of the punch surface. (DO
I) The value was evaluated.

Number of Stars of Pressed Product The punch bottom die surface of the panel was ground with a grindstone to count the number of stars generated.

Occurrence Rate of Defective Stars : For the same sample plate, 100 pieces of 300 mm square rectangular tube were continuously formed, and the number of occurrence of star eyes in 100 sheets was examined. did.

[0034]

[Table 1]

[0035]

EFFECTS OF THE INVENTION According to the present invention, there is provided a thin plate for press processing which is excellent in image clarity after coating without impairing press moldability and is less likely to generate star eyes. It is possible to obtain the car body with high efficiency.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between waviness and sharpness.

FIG. 2 is a graph showing the relationship between 10-point average roughness (Rz) and the number of generated star eyes.

[FIG. 3] Flat area ratio (%) and after coating (3 coats, 3
6 is a graph showing a relationship with bake) image clarity.

FIG. 4 is a graph showing the relationship between the average roughness Ra of the steel sheet surface and the galling resistance.

FIG. 5 is a graph showing the relationship between the waviness (Wca) on the punch-side steel plate surface and the post-painting image clarity of the molded product.

FIG. 6 is a graph showing the relationship between the maximum variation width (%) of the height of the convex portion and the number of generated star eyes.

FIG. 7: The 10-point average roughness (Rz) is further related to the number of generated star eyes in a square of 200 μm and the depth from the center plane is 1
It is a graph which plotted both relations when one or more recesses of 0 μm or more are present.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Mitsuhiro Sato Inventor, Hiroshi Sato 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. (56) Reference JP-A-62-168602 (JP, A) JP-A-SHO 63-132729 (JP, A) JP 63-255320 (JP, A) JP 2-133105 (JP, A) JP 2-274856 (JP, A) JP 5-57312 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21D 22/00-26/14 B21B 1/22 B21B 27/00

Claims (3)

(57) [Claims] 1. A thin steel sheet for press working, the surface condition of which satisfies the following conditions (1) to (3). (1) The average waviness (Wca) of the unevenness on both surfaces of the steel sheet is 0.
6 μm or less. (2) 10-point average roughness of irregularities on the surface in contact with the punch (R
z) is 10 μm or more and 20 μm or less . (3) The average roughness (Ra) of the irregularities on the surface in contact with the die is 0.4 μm or more and 2 μm or less , and the flat area ratio is 40.
% Or more and 80% or less . 2. A concave portion having a depth of 10 μm or more from the center plane is formed at an arbitrary position on the surface of the steel plate which is in contact with the punch.
The thin steel sheet for press working according to claim 1, wherein one or more pieces are present in a square of 00 μm × 200 μm. 3. The fluctuation range of the height of the convex portion of the surface contacting the punch is 20% or less of the average height of the convex portion.
The thin steel sheet for press working according to.