JPH082445B2 - Steel plate with excellent paint clarity - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a steel sheet used after being coated after forming, such as outer panels for various vehicles such as automobiles, trucks and trains, or outer panels for household electric appliances. In particular, the present invention relates to a steel sheet which is less likely to cause galling during forming and has improved paint clarity.

[Prior Art] In the steel plate used for the outer plate as described above,
In order to prevent mold galling (damaging like peeling caused by seizure of steel plate die) that occurs during forming such as pressing, it is customary to temper-roll with dull finishing rolls to adjust the surface roughness. Is. The shot blast method or the electric discharge machining method has been conventionally used for this dull finish.
The surface of the steel sheet rolled using the rolls finished by these methods has a profile composed of irregular peaks and valleys as shown in FIG. 2, for example. Then, in the molding process, the valley portion in the profile acts as an oil reservoir for the lubricating oil, and also captures the generated metal powder to prevent seizure and suppress die galling. Therefore, from the viewpoint of preventing mold galling, the larger the surface roughness, the more preferable.

On the other hand, in the case of steel plates for outer panels such as those mentioned above, the quality of the coating finish is an important evaluation item in order to enhance the aesthetic decorative feeling, and in particular, the sharpness of the image when an object is copied on the surface of the coating film is important. This property is commonly referred to as "vividness". By the way, the sharpness of the coated surface is largely influenced by the surface roughness of the steel sheet itself, as well as the influence of the type of coating and the coating method. That is, since the fine irregularities on the surface of the steel sheet are filled with the paint and leveled, there is almost no adverse effect. However, when the irregularities are large to some extent, the coating film is formed along the irregularities, and the reflected light is Scattering reduces the gloss and deteriorates the image clarity.

In the dull finishing of the roll by the conventionally used shot blasting method or electric discharge machining method, the roughness of the roll surface cannot be controlled accurately, so that fine roughness is provided and waviness is formed at the same time. Therefore, fine roughness and waviness are also given to the surface of the steel sheet rolled by this roll, and the vividness of the coating deteriorates due to this waviness. For example, FIG. 3 shows the surface roughness Ra (center line surface roughness) of a steel sheet temper-rolled by a shot blasting roll.
Is a graph exemplifying the relationship between the coating sharpness and the coating sharpness, and the smaller the surface roughness is, the more the sharpness is improved as a linear function.

That is, when the surface roughness Ra is increased in order to enhance the mold galling resistance, the coating sharpness tends to deteriorate, and when the surface roughness Ra is reduced to improve the coating sharpness, the mold galling resistance tends to decrease. For that reason, conventionally, the surface roughness Ra is adjusted to an appropriate range to achieve both the anti-galling property and the sharpness, but since it is an eclectic measure, it is not enough to satisfy both required performances. I can not say.

As a measure for coping with such a situation, there has been proposed a technique of forming irregularities of a unique shape on the surface of a steel sheet using a temper rolling roll dull-finished with a high-density energy beam such as a laser beam, The outline is as follows.

That is, when a laser pulse is irradiated toward the surface of the roll while rotating the roll, as shown in FIG.
As shown in (B), the metal of the laser irradiation portion is melted to form the crater 1, and the molten metal rises around the crater 1 to form the annular convex portion 2. The formation pitch of the craters 1 and the convex portions 2 in the roll circumferential direction can be arbitrarily adjusted by changing the rotation speed of the roll and the irradiation period of the laser pulse at the time of dull finishing, and the formation pitch in the roll axial direction can be adjusted. It can be freely adjusted by the moving distance of the laser irradiation device for each rotation of the roll. Further, the diameter and depth of the crater 1 and the width and height of the convex portion 2 can be changed by the energy of the laser pulse and the irradiation time.
Then, when the steel sheet is temper-rolled by a dull finishing roll (hereinafter, also referred to as a laser dull finishing roll) having an infinite number of craters 1 and convex portions 2 formed on the surface by such a method, FIG. 5 (A) , (B), the convex portion 2 of the roll R bites into the surface of the steel plate P to form an annular concave portion 2a, and the metal in this portion flows so as to rise toward the crater 1 to form a substantially circular plateau. The portion 1a is formed, and the surface of the steel sheet P pressed by the unprocessed (that is, the portion to which the laser energy is not applied) flat portion 3 outside the convex portion 2 becomes a flat plain portion 3a. The surface of the steel sheet P thus obtained has a substantially circular plateau 1a, an annular recess 2a surrounding the plateau 1a, and a flat plate 3a slightly lower than the plateau 1a.

And, in the steel sheet having such a surface shape, the surface roughness can be imparted independently of the undulation that adversely affects the image clarity, and the annular recess 2a has a lubricating oil sump and a chipping capturing portion at the time of forming. As a result, it has excellent image clarity and mold galling resistance.

On the other hand, the waviness of the steel sheet surface generated in tandem cold rolling does not completely disappear in temper rolling, and from the viewpoint that this deteriorates the paint clarity, the tandem cold rolling is a pretreatment for temper rolling with a laser dull finishing roll. Japanese Patent Application Laid-Open No. 62-224405 proposes a technique for controlling the roughness of the work roll of the final stand of the machine. The outline of this technology is as follows.

That is, as the work roll of the final stand of tandem cold rolling, bright roll, laser dull finishing roll, discharge dull finishing roll or center line average roughness Ra of 1.0 μm
After cold rolling using any one type of roll selected from shot blast rolls of m or less, temper rolling is performed using a laser dull finishing roll as a work roll for temper rolling.

[Problems to be Solved by the Invention] As a result of further study by the inventors of the technique disclosed in the above-mentioned publication, the following was found. For example, the laser dull finishing roll is used as the work roll of the final stand of the tandem cold rolling mill to perform cold rolling, and then the laser dull finishing roll is used as the work roll for temper rolling. In the rolled steel plate, the dull pattern train transferred at the first time and the dull pattern train transferred at the second time interfere in the plate width direction, and interference fringes appear. The interference fringes not only impair the appearance of the steel sheet base material, but also the interference fringes stand out depending on the coating film thickness, which also impairs the appearance of the coating.

The present invention has been made to solve such a technical problem, and an object of the present invention is to make the most of the features of the disclosed invention as described above and to eliminate the inconvenience of interference fringes.

[Means for Solving the Problems] The structure of the steel sheet of the present invention that has been able to achieve the above object is a steel sheet obtained by rolling twice with a dull finishing roll using a high-density energy beam. Pitch P ₁ (μm) in the plate width direction of the dull pattern row transferred to the surface of the steel sheet during rolling, and pitch P ₁ (μm) in the plate width direction of the dull pattern row transferred to the surface of the steel sheet during the second rolling. ₂ (μm) is the following [I] formula or [I
The point is that any one of the formulas I] is satisfied.

P ₁ = P ₂ ... [I] [Operation] In the laser dull finishing roll, the dull pattern composed of the crater 1 and the annular convex portion surrounding the crater 1 forms a regular row in the roll rotation direction and is irregularly formed in the roll axial direction. Therefore, on the surface of the steel plate rolled by the laser dull finishing roll, the dull pattern composed of the plateau 1a and the annular recess 2a surrounding the plateau 1a
As shown in FIG. 3A, regular rows are formed in the rolling direction (this is the above-mentioned dull pattern row), and they are arranged irregularly in the strip width direction.

On the other hand, in the case of a steel plate that is rolled twice with a laser dull finishing roll, the pattern transferred in the first rolling does not completely disappear in the second rolling under rolling pressure like temper rolling. The surface shapes of the second rolling and the second rolling are synthesized.

Here, the pitch P ₁ (μm) in the plate width direction of the laser dull pattern row transferred to the steel plate surface at the first time, and the pitch P in the plate width direction of the laser dull pattern row transferred to the steel plate surface at the second time. _{When 2} (μm) are equal, that is, [I] below
When the expression is satisfied, no interference fringe appears [For P ₁ and P ₂ , see FIG. 1 (B) below].

P ₁ = P ₂ [I] On the other hand, when the pitches P ₁ and P ₂ are different, interference fringes may appear in the plate width direction.

 The mechanism by which this interference fringe is generated is as follows.

For a steel plate that has been rolled twice with a laser dull finishing roll, the pitch P of the laser dull pattern row in the plate width direction
_{When 1} and P ₂ are different, as shown in FIG.
The mutual laser dull pattern rows A and B transferred at the second time and the second time are periodically overlapped in the plate width direction. In other words, the ratio of the flat land portion 3a still remaining after two rollings (flat land portion area ratio) repeatedly increases and decreases in the strip width direction. However, since the array of the dull pattern composed of the plateau 1a and the annular recess 2a is irregular in the strip width direction, no periodic change in the flat area ratio occurs in the rolling direction.

The cycle of increase / decrease in the flat area ratio in the plate width direction is as follows. Taking the position where the laser dull pattern rows A and B completely overlap as a reference, the distance between the pattern rows A and B widens due to the difference in pitch as the laser dull pattern rows move away from the reference, and the n-th pattern In columns
_They are separated by P ₁ -P ₂ | × n (μm). However, when this interval is equal to the pitch P ₁ of the pattern row A (or the pitch P ₂ of the pattern row B), that is, | P ₁ -P ₂
When | × n = P ₁ (or | P ₁ -P ₂ | × n = P ₂ ),
The nth pattern row B and the n + 1th pattern row A (or the nth pattern row A and the n + 1th pattern row B) overlap again. Therefore, the number of pattern rows B before overlapping again is n = P ₁ / | P ₁ -P ₂ | (the number of pattern rows A is n
= P ₂ / | P ₁ -P ₂ |). Here, when n is an integer, it means that the pattern strings completely overlap, and when n is not an integer, the pattern strings do not completely overlap.
It means the case where the area ratio of the flat ground becomes maximum in the vicinity.
This interval number n = P of pattern rows B to again overlap the pitch P ₂ of the pattern array _{B 1 / | P 1 -P 2} | ( or pattern sequence A pitch P ₁ to n = P ₂ / | P ₁ -P ₂ |) can be obtained by multiplying (P ₁ × P ₂ ) / | P ₁ -P ₂ |
(Μm). That is, the increase / decrease cycle of the flatland area ratio can be expressed by (P ₁ × P ₂ ) / | P ₁ -P ₂ | (μm). Then, a portion having a high flat land area ratio and a portion having a low flat land area ratio appear as interference fringes.

However, when the inventors further studied,
It was found that the interference fringes became indistinguishable to human eyes as the period of increase / decrease in flatland area ratio became smaller. Then, as a result of observing the surface of several kinds of steel plate samples which were rolled twice with the laser dull finishing rolls having different pitches P ₁ and P ₂ , when the increase / decrease cycle of the flat area ratio is 2000 μm or less, that is, the following [II] It was found that the interference fringes became indistinguishable when the equation was satisfied.

From the above, if the rolling is performed twice with the laser dull finishing roll so as to satisfy the relation of the formula [I] or the formula [II], interference fringes that impair the appearance do not appear on the surface of the steel sheet.

It should be noted that here, the two-time rolling with the laser dull finishing roll means, of course, the case in accordance with the technique of the above-mentioned publication, that is, the case where the laser dull finishing roll is cold-rolled and then another laser dull finishing roll is temper-rolled. This also includes the case where the temper rolling is performed twice with a laser dull finishing roll.

[Example] A low carbon aluminum killed steel sheet was used as a material, and the material steel sheet was rolled using a laser dull finishing roll as a work roll of a tandem cold rolling mill final stand.
Cleaning and annealing were performed, and then temper rolling was performed using a laser dull finishing roll as a work roll. At this time, the dull pattern pitches P ₁ , P ₂ in the work roll for the final stand of the tandem cold rolling mill and the work roll for temper rolling
Was changed as shown in Table 1 below to obtain cold-rolled steel sheets (sheet thickness 0.8 mm) having various surface shapes. The cold-rolled steel sheet thus obtained was subjected to electrodeposition coating with a film thickness of 20 μm.

About cold rolled steel plate (original plate) and steel plate after electrodeposition coating,
The presence or absence of interference fringes was visually inspected. The results are also shown in Table 1. As is clear from the results shown in Table 1, it is understood that those satisfying the requirements specified in the present invention (Examples) do not show interference fringes and there is no problem in the appearance of the steel sheet.

[Advantages of the Invention] The present invention is configured as described above, and in a steel sheet that is rolled twice with a dull finishing roll by a high-density energy beam, each pitch of the dull pattern row is defined by the formula [I] or [II] It was possible to prevent the appearance of interference fringes that adversely affect the appearance of the raw steel sheet or the appearance after coating by satisfying the above condition.

[Brief description of drawings]

FIG. 1 (A) is an explanatory view showing a dull pattern on the surface of a steel plate rolled by a laser dull finishing roll, FIG. 1 (B).
Is an explanatory view showing a dull pattern of a steel plate that has been rolled twice with laser dull finishing rolls having different pitches, and FIG. 2 is an enlargement exemplifying the surface profile of a roughened steel plate using a roll that has been dull finished by shot blasting. Sectional drawing, FIG. 3 is a graph showing the relationship between the surface roughness Ra of the roughened steel sheet and the sharpness, and FIG. 4 is an explanatory view showing the surface shape of the dull finishing roll using a high-density energy beam. The figure is an explanatory view showing the surface shape of the steel sheet roughened by using the dull finishing roll of FIG. 1 ... Crater, 1a ... Plateau part 2 ... Annular convex part, 2a ... Annular concave part 3 ... Flat part, 3a ... Flat part

Claims (1)

[Claims] 1. A steel sheet obtained by rolling twice with a dull finishing roll using a high-density energy beam, wherein the pitch of the dull pattern array transferred to the steel sheet surface in the first rolling in the sheet width direction. P ₁ (μm) and 2
The pitch P ₂ (μm) in the plate width direction of the dull pattern row transferred to the surface of the steel plate at the time of the second rolling is as shown in [I] below.
A steel sheet with excellent paint clarity that is characterized by satisfying either the formula or the formula [II]. P ₁ = P ₂ ... [I]