JPH02117702A - Cold rolled steel sheet having good transferability and vertical planting capability - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet having good transferability and vertical painting capability by working the sheet so that both average center line surface roughnesses of a specific direction (x) and the direction (y) orthogonal to the direction (x) meet specific conditions. CONSTITUTION:The transfer direction and the direction orthogonal to the transfer direction, that is, a vertical painting direction are denoted by (x) and (y), respectively. Respective center line average roughnesses Ra<x> and Ra<y> are made to meet conditions expressed by formulas. In the case of dull working a roll, pitches of beam irradiation are made to be different in the roll axial and peripheral directions from each other to obtain a crater distribution meeting conditions. Roughness in the transfer direction is increased to meet the relation shown by the formula I to prevent slips in transfer. As for the formula II, if roughness over the total area of the sheet is smaller than a value shown by the condition, press formability is made to be worse and if the roughness is larger than the value, image clarity after painting is made to be worse. The condition expressed by the formula III indicates a desirable range of transferability and image clarity. The formula IV shows a condition by which vertical painting capability is further increased.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention relates to a cold-rolled steel sheet used as an exterior material for automobiles, home appliances, etc. This invention relates to a cold-rolled steel sheet that has excellent paintability when painted.

(Prior technology) Cold-rolled steel sheets are widely used as exterior materials for automobiles and home appliances, but the most important properties required for such applications are press formability and paintability (beauty after painting). ), these properties are greatly influenced by the surface condition (surface roughness) of the steel sheet, but in terms of -a, a dull surface is better for press formability, and a prite surface is better for paintability. .

Recently, there has been a strong demand for sharpness after painting, especially for cold-rolled steel sheets for automobile exteriors, and various methods of processing the surface of steel sheets have been proposed to meet this requirement. A typical example is a technique in which the surface of a roll is processed with a high-energy density beam such as a laser beam, and this roll is used to roll and dull a cold-rolled steel plate (for example, The surface of a roll machined with a laser beam has regularly arranged crater-like minute irregularities, and when this roll is used, for example, as a work roll for temper rolling, the minute irregularities on the roll are When transferred onto a steel plate, a steel plate with regular minute irregularities (called a laser dull steel plate) is obtained.

Compared to dull m plates rolled with conventional shot blasting rolls (referred to as shot dull steel plates for convenience), laser dull steel plates can control the shape and distribution of unevenness to a certain extent, and have a relatively smaller flat area. Since a high ratio of dull surfaces can be obtained, the steel sheet maintains press formability and has excellent image clarity after painting. However, this laser dull steel plate has only recently begun to be used on a trial basis, and there are some unresolved problems regarding its practical use.

(Problem to be solved by the invention) On the user side of cold-rolled steel sheets, for example, in highly automated automobile factories, raw steel sheets are often transported by suction with vacuum pads during pressing and other processes. However, since the surface of Laser Dull W4@ has a relatively high area ratio of flat parts, it is more slippery than Schott Dal steel plate, and there is a high risk of falling on lines such as vacuum pad conveyance.
This is one of the practical problems of laser dull steel sheets.

Another problem is vertical paintability.

Various types of coatings are used to protect metal products and give them a beautiful appearance, and spray coating is particularly used for automobiles because it can be applied with high efficiency.

Paints generally contain a solvent, and the viscosity of the paint, the volatilization rate of the solvent, and the thickness of the coating greatly affect the leveling performance during film formation. In addition, from the viewpoint that the microscopic unevenness of the base material affects the sharpness of the painted surface, cold-rolled steel sheets that are dull-finished especially for the purpose of press forming are used.
As mentioned above, laser dull steel sheets are attracting attention.

However, conventional paintability (sharpness after painting) is often evaluated based on the beauty of painting when the steel plate is held horizontally (horizontal paintability); The beauty of the coating (vertical paintability) has not been studied much, and the excellent clarity of Laserdal steel plate is simply a reflection of its horizontal paintability.

For horizontal painted surfaces, it is possible to level the painted surface smoothly before baking by adjusting the amount of solvent in the paint.

However, in vertical painting, if the amount of solvent is adjusted so that the painted surface is sufficiently leveled, the paint will sag before baking, so the amount of solvent must be adjusted, resulting in insufficient leveling and Long-period undulations called yuzu skin tend to remain, resulting in poor image clarity after the paint is baked.

In order to improve transportability, it is effective to roughen the surface of the steel plate from the viewpoint of preventing slippage, but if the roughness becomes excessive, the clarity of the coating will deteriorate.

In addition, in order to improve vertical paintability, it is better to increase the fluidity of the paint in the vertical direction and make the steel plate surface as flat (bright) as possible to ensure sufficient leveling, but in order to ensure press formability, It is necessary to provide a certain degree of roughness.

Even with laser dull steel sheets that are currently used as sharpness steel sheets, the sharpness during horizontal painting can only be achieved by reducing the waviness of the painted surface caused by the waviness of the steel sheet. Therefore, the sharpness of vertical members such as automobile doors, which is important, is not improved much.In addition, these sharpness steel plates have a roughness (for example, center line average roughness R).
In order to obtain sharpness by reducing the roughness (roughness represented by a), it is relatively smoother than the conventional Schottdal steel plate, and is therefore inferior in transportability.

An object of the present invention is to further improve the transportability and vertical coating properties of a cold-rolled steel sheet that has both press formability and sharpness after coating.

(Means for Solving the Problems) The present inventors aim to simultaneously improve transportability and vertical paintability in a copper plate that has excellent press formability and sharpness after painting, such as the laser dull steel sheet described above. It was confirmed that imparting roughness directionality to the surface of the fs plate is an effective means for this purpose. In other words, it is possible to simultaneously satisfy the above-mentioned contradictory requirements only by giving directionality to the roughness and giving anisotropy to the many properties that depend on the surface shape.

Here, the gist of the present invention is the following cold-rolled steel sheet:
Each center line average roughness Ra'', Ra' measured in a specific direction X and a direction y perpendicular to it on one surface
A cold-rolled steel sheet with excellent transportability and vertical coating properties, characterized in that the following formulas [1] and [2] satisfy the conditions.

Ra'>Ra'・・・・・・・・・・・・■0.8≦Ra
true+Ra'≦3.2 (unit: UVa) - ■ jIn order to improve the overall properties of cold rolled steel sheets, taking into consideration press formability and sharpness after painting, the above equations [1] and [2]
] In addition to the conditions of formula [1] and [2] below, it is desirable to satisfy the conditions of formula [1] and [2].

0.9≦Ra1′≦2,2 (unit μ11)...■0.
3≦Ray≦0.8 (unit: μm 1)”・■ The cold-rolled steel sheet of the present invention has a surface roughness that differs depending on the measurement direction (the X direction and the X direction perpendicular to this).

FIG. 1 is a conceptual diagram of the surface of the steel plate of the present invention. The steel plate surface consists of a flat base surface 1 and minute depressions 2 distributed on this base surface.As shown in the figure, the horizontal direction of this surface is
Assuming that the vertical direction is the X direction, the distribution density of the minute dents 2 differs depending on these directions. Therefore, the roughness Ra'' and Ra' measured in each direction are different, and Ra1
l>Ra'.

Such a cold-rolled steel sheet can be manufactured by a method in which a roll is subjected to dull processing using the above-mentioned high energy density beam, and then temper rolling is performed using the roll. In other words, when processing a roll, if the pitch of beam irradiation, such as a laser beam, is changed appropriately in the axial and circumferential directions of the roll, the distribution density of craters formed on the roll surface can be changed in the axial and circumferential directions. The surface roughness of the steel plate rolled by this roll has a directionality as shown in Fig. 1.

(Function) Figure 2 shows the above equations 0 to 0, with RaX on the horizontal axis and
It is displayed with Rav as the vertical axis.

Hereinafter, the cold rolled steel sheet of the present invention will be explained with reference to this figure.

The inventors studied a configuration with different roughness in the two directions so as to satisfy all of the steel plate properties required as an exterior material. As a result, by imparting large roughness in the conveying direction and suppressing the roughness in the direction of maximum inclination of vertical coating, conveyance and vertical coating properties are improved.
The two-way roughness Ra satisfies all requirements by optimizing the average roughness in two directions for press formability and suppressing the average roughness in two directions for horizontal painting clarity. ”, revealed the realm of Rav.

First, the formula (■) will be explained.

Straight line A in Figure 2 is the line of Ra''=Ra'.The area below this line is the area that satisfies equation 0, and the roughness in the X direction is
This area is larger than the directional roughness.

In this case, the X direction is the transport direction, and the X direction is the vertical coating direction. That is, the formula 0 (Rax>Ray) means that the roughness of the surface of the steel plate has directionality, and the roughness measured in the lateral direction is greater.

As mentioned above, on production lines at automobile factories and the like, steel plates and parts formed from them are picked up by vacuum rubber pads and transported. In the conveying direction of the steel plate, if there are scratches with small roughness, or if the inertial force during acceleration or deceleration during conveyance exceeds the static friction between the steel plate surface and the pad, the pad may slip, leading to accidents such as the steel plate falling. Therefore, in the present invention, the roughness in the conveyance direction (in this case, the X direction) is increased,
This prevents the above-mentioned slippage.

■Equation, 0.8μm≦Rax + Ra'≦3.2p
In Fig. 2, m is the straight line o (Ra''+Ra'=0
．． 8 ull) and straight line A (Ra"+Ra'=3.2
ttwa ).

When Ra” and RaY are smaller than 0.8 μl, the entire steel plate (
The roughness (combined length and width) is too small, resulting in poor press formability. On the other hand, when Ra''+Ra' is larger than 3.2 μm, the sharpness of the entire board after painting becomes poor.

After all, the area of the book surrounded by the straight lines A, 8, and 8 in Figure 2 is an area where the surface roughness has directionality, has excellent conveyance in the X direction, and excellent vertical painting performance in the X direction. This is a region that has both press formability and sharpness of the temporary whole.

The equation (2) means the area between straight lines 2 and E in FIG.
If a" is 0.9 μ- or more, the pad will be more securely prevented from slipping. However, if the surface is too rough, roughness in the X direction will appear on the surface even after painting, so Rax It is preferable to suppress the upper limit of 2.2 μm or less.

The equation (①) indicates the area between the straight line and G in Figure 2, and indicates that it is desirable to keep Raγ below 0.8μ in order to further improve the vertical paintability. for,
The smaller Ra' is, the better; however, if Ra' is extremely small, press formability will be poor and press flaws will be more noticeable.

Therefore, Ra' is preferably set to 0.3 μm or more.

After all, in FIG. 2, the area surrounded by straight lines 2, H, H, and G is the desirable area of Ra1I and Ra'.

Note that the surfaces satisfying the conditions of the above formulas (1) and (2) or the formulas (1) to (2) may be on both sides of the cold-rolled steel sheet, but may be on only one side depending on the case. That is, there are cases where only one side of the cold-rolled steel sheet is important for paintability and transportability, and in such a case, the other side may be finished with a normal bright finish or a dull finish.

The directionality of roughness on the surface of a cold-rolled steel sheet that satisfies the conditions of the present invention can be visually determined. Therefore, during transportation and vertical painting, it is easy to handle the steel plate in a desired direction.

(Example) Samples of cold-rolled steel sheets with different Ra'' and Ra' were prepared under the following conditions, and the sliding force in the X direction, vertical paintability in the X direction, press formability, and horizontal paintability were investigated. In addition,
Ra was measured based on JIS-BO601,
With laser dull steel plates, the values vary depending on whether the measurement line happens to pass through many micro-concave areas or through many flat areas, so
Twenty pieces were measured at a pitch of 0.05 mm, and the average was taken.

i. Test material: JIS 3310 5PCDS cold rolled steel plate (thickness 0.8
m-)11, Sample preparation method: Preparation of rolling roll...JIS 5UJ2tliil
The power density 3 XIO'ga/
cmz laser beam 1.2X1 by chopper
The irradiation was modulated into an intermittent beam of 0' Hz. Ra'' and Ra' were adjusted by changing the roll rotation speed and feed speed.

Rolling of steel plate: The above rolls were used as upper and lower work rolls for temper rolling, and a predetermined roughness was transferred to both sides of the E+ffi.

The evaluation method is as follows.

*Permeability (sliding force): With anti-rust oil applied to the steel plate, use a rubber pad with a diameter of 75φ■ to create a vacuum of 600+nmHg, apply force from the side, and measure the load when slipping occurs. and evaluated.

11. Vertical paintability After chemical conversion treatment and electrodeposition coating (film thickness 20 μs) of the steel plate,
30% commercially available automotive top coat paint with Ford Cup #4
Adjust it so that it is a second, apply vertical spray painting, and PGD
The image sharpness was measured using a meter (Japan Color Research Division).

1j, press formability evaluated by limit drawing ratio (LDR) by cylindrical drawing.

iv. Horizontal paintability After applying the same surface treatment as the vertical paintability test, Ford
Spray the paint adjusted for 35 seconds with cup #4,
Image clarity was measured using a PGD meter.

The evaluation results are summarized in Table 1, and the relationship between Ra'' and Ra' of samples N11l to 10 in this table is shown in Figure 2.
It is indicated by [phase].

As shown in Table 1 and FIG. 2, the steel plate (sample Nα5.6) in which Rax and Ra' are within the range defined by the present invention has all properties at a satisfactory level. Sample demon 7 and 9
The steel plate has problems in press formability and horizontal painting properties, but it can be used for applications where these properties are not important.

(Margin below) FIG. 3 is a diagram showing the relationship between sliding force and Ra''.

When Ra" is 0.9 μ- or more, a high sliding force of 4 kgf or more is obtained.

FIG. 4 is a diagram showing the relationship between the PGD value and Raγ due to vertical painting. PGD value 0.85 at Ra'0.8μm or less
It can be seen that the above-mentioned good image clarity can be obtained. however,
When Ra" is 2.4 .mu.m, streaks are visible on the painted surface even if Ra.gamma. is 0.8 .mu.m or less, resulting in poor image clarity.

Figure 5 shows the average roughness, Ra - (RaX+Ray)/
2 and horizontal coating properties. As shown in the figure, P C,D (io, 8
Good image clarity of 5 or more was obtained.

(Effects of the Invention) The cold-rolled steel sheet of the present invention has not only basic properties such as press formability and sharpness after painting, but also excellent transportability and vertical paintability. Therefore, there is no risk of accidents on the conveyor line (and it can be widely used for automobile doors, fenders, etc., as well as exterior materials for home appliances, building materials, etc.).

[Brief explanation of the drawing]

FIG. 1 is an enlarged plan view showing the surface condition of the steel plate of the present invention. Fig. 2 is a diagram showing the desirable ranges of centerline average roughness Ra'' and Ra' in the X direction and Y direction of the steel plate. Fig. 3 is a diagram showing the relationship between sliding force and RaX, and Fig. 4 Figure 5 shows the relationship between PGD value and Ra' due to vertical painting. Figure 5 shows the average roughness, Ra = (Ra''+Ra')/
2 is a diagram showing the relationship between 2 and horizontal paintability.

Claims (1)

[Claims] (1) Each centerline average roughness Ra^x, R measured in a specific direction x of the surface and in a direction y perpendicular thereto
A cold-rolled steel sheet with excellent transportability and vertical coating properties, characterized in that a^y satisfies the conditions of formulas [1] and [2] below. Ra＾x＞Ra＾y・・・・・・・・・・・・・［1] 0.8≦Ra＾x+Ra＾y≦3.2 (unit: μm)
[2] (2) The cold rolled steel sheet according to claim 1, which satisfies the conditions of formulas [3] and [4] below in addition to the conditions of formulas [1] and [2] above. 0.9≦Ra^x≦2.2 (unit: μm)...[3] 0.3≦Ra^y≦0.8 (unit: μm)...[4]