JPH0237902A - Steel sheet having partially different surface roughness - Google Patents

Abstract

PURPOSE:To improve galling resistance and brightness by forming numerous rugged parts consisting of approximately circular plateaus and valleys enclosing the same on a steel sheet surface and further forming surfaces of various roughness values with specific average roughness as a boundary on the steel sheet surface. CONSTITUTION:The steel sheet is subjected to temper rolling by dull finish rolls using a high-density energy beam to form the approximately circular plateaus 1a as well as the annular recesses 2a and annular projecting parts 2 enclosing the plateaus respectively on the steel sheet surface. The non-worked parts are flat land parts 3a. Further, the reference roughness on the surface prior to the temper rolling and the surface roughness of the flat part of the dull finish rolls are changed to dividedly form the average roughness Ra on the steel sheet surface after the temper rolling to >=0.7mum and <0.7mum. The mold galling resistance is improved in the parts having >=0.7mum roughness and the paint brightness of reflectivity is improved in the parts having <0.7mum roughness in this way.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to steel plates that are used after forming and being painted, such as outer panels for various vehicles such as automobiles, trucks, and trains, or outer panels for household electrical appliances. In particular, the present invention relates to a steel plate that is less likely to cause mold galling during molding and has improved paint clarity.

[Prior art] In steel plates used for exterior panels as described above, mold galling (peeling-like damage accompanied by seizure of the steel plate to the mold) that occurs during forming processes such as press working is prevented. For this purpose, it is customary to perform temper rolling with dull finishing rolls to adjust the surface roughness.

Traditionally, shot blasting or electric discharge machining has been used for this dull finishing, and the surface of a steel plate rolled using rolls finished by these methods is, for example,
It has a profile consisting of irregular peaks and valleys as shown in the figure. During the molding process, the valleys in the profile act as oil reservoirs for lubricating oil and trap the generated metal powder, preventing seizure and mold galling. Therefore, from the viewpoint of preventing mold galling, the larger the surface roughness, the better.

On the other hand, in the case of exterior steel plates such as those mentioned above, the quality of the paint finish is an important evaluation item in order to enhance the aesthetic impression, and among these, the clarity of the image when an object is captured on the surface of the paint film is particularly important. This characteristic is generally referred to as image sharpness.

By the way, the sharpness of the painted surface is greatly influenced by the surface roughness of the steel plate itself, as well as by the type of paint and coating method. In other words, fine irregularities on the surface of the steel plate are filled with paint and are smoothed or averaged, so there is almost no negative effect, but when the irregularities become large enough, a coating film is formed along the irregularities, causing reflected light to disappear. The particles are scattered, the gloss decreases, and the sharpness of the image also deteriorates. For example, it is said that the smaller the surface roughness Ra (center line surface roughness) of a steel plate temper-rolled with a shot blasting roll, the better the image clarity will be in terms of dimensions. .

That is, if the surface roughness Ra is increased in order to improve the anti-mold anti-stick property, the paint sharpness tends to deteriorate, and when the surface roughness Ra is decreased to improve the paint clearness, the anti-mold anti-seize property tends to decrease. Therefore, in the past, the surface roughness Ra was adjusted to an appropriate range in order to achieve both anti-moulding properties and image sharpness, but this is just a compromise measure and it is difficult to fully satisfy both performance requirements. I can not say. Moreover, when dull finishing is done by shot blasting or electrical discharge machining, the second
As shown in the figure, the surface of each rolling roll is composed of irregular peaks and valleys, and it has been impossible to control the roughness of only a specific portion of the roll by increasing or decreasing it.

On the other hand, a technique such as that disclosed in, for example, Japanese Patent Application Laid-Open No. 168602/1983 has also 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 in a unique shape on the surface of a steel sheet, and the outline is as follows.

That is, when a laser pulse is irradiated onto the surface of the roll while rotating the roll, as shown in Fig. 3(A), (
As shown in B), the metal in the laser irradiated area melts to form a crater 1, and the molten metal rises around the crater 1 to form an annular convex part 2. The formation pitch of the craters 1 and convex portions 2 in the roll circumferential direction can be arbitrarily adjusted by changing the roll rotation speed and the laser pulse irradiation period during dull finishing, and the formation pitch in the roll axial direction can be adjusted as desired. It can be freely adjusted by the laser irradiation device or the relative movement distance of the rolls each time the rolls rotate. Further, the diameter and depth of the crater 1 and the width and height of the convex portion 2 can be changed by changing the energy and irradiation time of the laser pulse. When a steel plate is temper-rolled using a dull finishing roll with countless craters 1 and protrusions 2 formed on its surface in this manner, the protrusions 2 of roll R are formed as shown in Fig. 4(A) and (B). An annular recess 2a is formed by digging into the surface of the steel plate P, and the metal in this part flows upward in the direction of the crater 1, forming a substantially circular plateau 1a, and the unprocessed area outside the protrusion 2 is formed. part(
In other words, the surface of the steel plate P pressed by the flat surface 3 (the part to which no laser energy was applied) becomes a flat plain portion 3a. The surface of the steel plate thus obtained includes a substantially circular plateau 1a, an annular recess 2a surrounding it, and a plateau 1a.
It has a slightly lower flat area 3a.

In a steel plate with such a surface shape, the annular recess 2a functions as a lubricating oil reservoir and a cutting powder trap during forming processing, and exhibits a mold galling prevention effect. Compared to roughened steel plates, it has many flat surfaces and suppresses diffused reflection, resulting in extremely excellent image clarity.

[Problems to be Solved by the Invention] Even in a steel plate rolled using a temper rolling roll that has been dull-finished with a high-density energy beam, it is difficult to improve the sharpness of the flat surface (flat area 3a). ) is preferably large from the above-mentioned purpose. However, increasing the surface area (ratio) of the flat area 3a
This leads to a reduction in the average roughness Ra on the surface of the steel plate, and the more flat areas 3a there are, the more effective the recesses 2a are in capturing metal chips generated by the relative movement of the steel plate on the die during press forming. becomes small, and mold galling occurs in the flat area 3a. As long as a rolling roll whose surface is roughened by a high-density energy beam such as a laser beam is used, many flat areas 3a still exist, and it is almost impossible to maintain good mold galling resistance. That is, even with the above-mentioned techniques, a steel plate that fully satisfies both the required properties of image clarity and anti-mold galling properties has not been obtained.

The present invention was made to solve these technical problems, and while taking advantage of the features of the disclosed invention as described above,
The purpose of this invention is to provide a steel plate that has both the required properties of anti-mold galling and paint clarity by improving the surface properties of the steel plate from different perspectives.

[Means for Solving the Problems] The structure of the steel sheet of the present invention that has achieved the above object is that innumerable irregularities consisting of a substantially circular plateau and a valley surrounding the base of the plateau are formed on the surface of the steel plate. , and in a steel plate whose portion excluding the unevenness is a flat area, the steel plate surface is defined into two or more portions, and the average roughness in one portion is Ra≧0.
．． 7 μm, and the average roughness in the other part is R
The main point is that a<0.7 μm.

[Function] The present inventors have investigated the mold-to-die galling property and We have conducted intensive research on the influence of surface shape characteristics on both characteristics of paint clarity.

First, the present inventors determined the average surface roughness Ra (hereinafter referred to as average roughness) of the entire surface including the flat portion 3a and the uneven portions of a roughened steel sheet that has been temper-rolled with a laser dull finishing roll. We investigated the relationship between image clarity after painting. In carrying out this investigation, the
By changing the surface roughness of the flat part 3 of the roughened steel plate and the surface roughness of the flat part 3 of the dull finishing roll, or by changing the elongation rate during temper rolling, the surface roughness of the flat part of the roughened steel plate can be changed, The average roughness Ra of the roughened steel plate was varied by changing the interval of 1, and the effect of each on the image clarity after painting was investigated. The coating has a film thickness of 90 μm.
The image clarity is ATI SYS.
Evaluation was performed using a DOI meter manufactured by TEM INC. In evaluating the sharpness, the rDOI value of 93, which satisfies the requirements for a high sharpness steel plate, was set as a criterion of 3 or higher, and the sharpness was evaluated using this standard.

The results are shown in Figure 1 (A), and the paint clarity improves as the average roughness Ra of the steel plate surface becomes smaller.
It has been found that a needs to be approximately less than 0.7 μm.

Next, the present inventors investigated the relationship between the above-mentioned average roughness Ra and mold galling property for the above-mentioned roughened steel sheet. In order to evaluate the mold galling property, a U-bending test (iron bending test) is performed, and the surface pressure applied to the steel plate surface is increased by gradually narrowing the gap between the punch and the die, and the limit at which mold galling begins to occur is determined. Clearance was evaluated (see Figure 5). In addition, it is difficult to make an absolute evaluation of mold galling, as the occurrence of mold galling changes depending on the press conditions, mold conditions, production conditions, etc., so a relative evaluation is performed in comparison with the normally used Schottdal steel plate. Yes.

The results are shown in Figure 1 (CB). When the average roughness Ra is less than 0.7 μm, it exhibits galling properties equivalent to conventional Schottdal steel sheets, but when the average roughness Ra is 0.7 μm or more, it exhibits galling properties. It is understood that the steel plate produced by the laser dull finish roll has extremely good anti-mold galling properties.

Therefore, the present inventors conducted further research while taking into consideration the aspects of actual press working of steel plates. When press-forming an automobile outer panel or the like, a steel plate P is press-formed by advancing a punch 7 while being held between an upper mold (die) 5 and a wrinkle holding plate 6, as shown in FIG. In this type of forming process, mold galling is most likely to occur in areas where the steel plate P moves (slides) on the die under strong surface pressure as the punch advances, such as the die shoulder. This is the radius portion 8 and the beat portion 9 that undergoes repeated deformation of bending and unbending. On the other hand, the part in the final product where the paint clarity is a problem is the part 10 related to the punch 7. That is, in the steel plate P, the portion where the anti-mold galling property is a problem and the portion where the sharpness is a problem are located at different locations, and each is local.

For this reason, the present inventors set the surface average roughness Ra of the part where mold galling is a problem as large as described above, and set the surface average roughness Ra of the part where image sharpness is a problem as described above. It has been discovered that it is of extremely high industrial value to make steel sheets finer than others, that is, to realize steel plates with partially different surface roughnesses, and the present invention has now been completed.

Hereinafter, the present invention will be explained in more detail with reference to examples, but the following examples do not limit the performance of the present invention, and
Any design changes for the purposes described below are included within the technical scope of the present invention.

[Example 7] A low carbon aluminum guild steel plate was used as a material, and after cold rolling, annealing was performed, and further temper rolling was performed. As rolls for skin pass rolling, one was prepared with a dull finish using a laser beam and another with a dull finish using shot blasting. In addition, when performing dull finishing processing using a laser beam,
A roll with a uniform roughness distribution (pattern A), a roll with different roughness on both sides in the roll axis direction and a different roughness in the center (pattern B), and a roll with a roughness of a part of the circumferential direction in addition to the width of the roll Prepare the same type of roll (pattern C) as the side,
A steel plate was subjected to a rolling work roll using each roll. Steel plate P obtained using rolls of each of these patterns
The roughness distribution on the surface of (8) and (B
) and (C), respectively. In addition, in Figure 7 (I
) and (11) indicate regions with the same average roughness Ra. In this way, various steel plates P having different patterns and average roughnesses Ra were obtained.

Each of the obtained steel plates P was pressed using a door model mold, and the mold-gagging property of the portion (II) was investigated. At this time, for the first sheet, rust preventive oil is applied to the surface of the steel plate and press working is performed, and for the second and subsequent sheets, no rust preventive oil is applied, and the number of sheets to be processed is determined by the number of sheets processed until mold galling occurs on the steel plate. The gender was evaluated. Further, the press-molded product was coated with paint three times to a film thickness of 90 μm, and the sharpness of the portion (1) was evaluated using the DO* meter described above.

The results are as shown in Table 1, and the products that meet the specified conditions of the present invention (actual travel examples) have both anti-mold galling and image sharpness characteristics compared to the comparative examples, and are suitable for various exterior panels. [Effects of the Invention] The present invention is constructed as described above, and the surface of the steel plate is divided into two or more parts, and the part where the anti-mold galling property is a problem and the part where sharpness is a problem are separated. By individually specifying the average roughness of each part where imageability is a problem, it has become possible to provide a roughened steel sheet with good anti-mould galling properties and excellent paint clarity.

[Brief explanation of the drawing]

Figure 1 (A) is a graph showing the relationship between the average roughness Ra of a steel plate and image clarity, Figure 1 (B) is a graph showing the relationship between the average roughness Ra of a steel plate and clearance, and Figure 2 is a graph showing the relationship between the average roughness Ra of a steel plate and clearance. Figure 3 is an explanatory diagram showing the surface profile of a roughened steel plate using a roll that has been dull finished using a high-density energy beam, and Figure 4 is an explanatory diagram showing the surface profile of a roughened steel plate using a roll that has been dull finished using Figure 3 is an explanatory diagram showing the surface shape of a steel plate roughened using a dull finishing roll, Figure 5 is an illustration to explain the state of the ladder bending test, and Figure 6 is a schematic diagram of press forming the steel plate. Explanatory diagram, Figure 7 (A), CB
) and (C) are plan views showing steel plates with various roughness distributions. 1... Crater la... Plateau part 2... Annular convex part 2a... Annular recessed part 3... Flat part (unprocessed part) 3a... Flat part 5... Dice 6... Wrinkle holding plate 7... Punch 8... Die shoulder radius 9... Beat part R... Roll P...
Steel plate Fig. 2 Fig. 1 (A) Average roughness of steel plate Ra () 1 m> Average roughness of steel plate Ra () tm) Fig. 5 Fig. 6 Fig. 7 CA) Fig. 7 (B) Fig. 7 (C) Hi-board width! - Handling/Tl← Deck width→

Claims (1)

[Claims] In a steel plate in which innumerable irregularities consisting of a substantially circular plateau and a valley surrounding the base of the plateau are formed on the surface of the steel plate, and the part excluding the unevenness is a flat area, the surface of the steel plate is divided into two or more parts. and the average roughness in one part is Ra≧0.
7 μm, and the average roughness of the other part is Ra.
A steel plate with partially different surface roughness, characterized in that the surface roughness is <0.7 μm.