JPH0339761B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention relates to thin steel sheets, such as cold-rolled steel sheets, which are subjected to forming processing such as press working before or after the painting process, such as automobile body outer panels and home appliance outer panels. The present invention relates to steel sheets for painting, such as hot-dip galvanized or various electroplated steel sheets, and methods of manufacturing the same. Cold-rolled thin steel sheets, which are a typical example of this type of steel sheet for painting, are normally manufactured by degreasing and cleaning after cold rolling, further annealing, and then skin-pass rolling. One of the purposes is to give the steel sheet surface an appropriate surface roughness by performing light rolling using work rolls with a dull surface finish, thereby improving seizure resistance during press forming. By the way, as a method for dull finishing the surface of a work roll used in such temper rolling, conventionally a method using shot blasting and a method using electrical discharge machining have been put into practical use. In the case of dull finishing of work rolls for skin pass rolling by these methods, an irregular roughness profile is formed on the roll surface, so the steel plate surface after skin pass rolling is composed of irregular peaks and valleys. It exhibits a rough surface. If a steel plate with such a rough surface is pressed, lubricating oil will be stored in the valleys, reducing the frictional force between the press die and the steel plate, making the press work easier, and at the same time making it easier to bond with the die. Seizing can be prevented by trapping the flaked metal powder in the valleys due to the frictional force. In recent years, the quality of the paint finish on the body of not only passenger cars but also light cars, wagons, and even trucks after painting has become a direct visual indicator of the overall high quality of the car to customers. This is an extremely important quality control item because it can be sued. By the way, there are various evaluation items for painted surfaces, but among them, it is especially important that the painted surface has low diffused reflection and has excellent gloss, and that there are few defects in mapping, that is, excellent image clarity. The combination of glossiness and image clarity is generally referred to as image clarity. The sharpness of the painted surface is affected by the type of paint and the painting method, but it is also strongly influenced by the rough surface of the steel plate used as the base for painting. In other words, if the flat portion of the steel sheet surface has a small proportion and the rough surface is highly uneven, the painted surface will also have large unevenness, resulting in diffuse reflection of light and loss of gloss.
This results in poor mapping, resulting in a decrease in image clarity, which worsens the above-mentioned image clarity. In general, the roughness of a steel plate surface is often expressed by the centerline average roughness Ra, but the larger the centerline surface roughness Ra, the larger the amplitude of the peaks and valleys, and the more uneven the painted surface becomes. This deteriorates the sharpness of the image. Various methods have been developed to evaluate sharpness, but the most commonly used method is the method used by Hunter Associates Laboratory in the United States.
DOI, measured using a DORIGON meter manufactured by Associates Laboratory
(Distinctness of Image) value is used.
This DOI value is calculated by the following formula using the specularly reflected light intensity Rs and the scattered light intensity R _0.3 at ±0.3° with respect to the specular reflection angle when light is incident on the sample surface at an incident angle of 30°. expressed. DOI value = 100 x (Rs - R _0.3 ) / Rs By the way, as mentioned above, when a steel plate is subjected to temper rolling using a work roll that has been dull finished by the conventional shot blasting method or electric discharge machining, As shown, the steel plate surface has a rough surface consisting of irregular peaks and valleys, and there are very few planes parallel to the plate surface. When a steel plate surface with irregular peaks and valleys is painted in this way, a coating film is formed along the slopes between the peaks and valleys, so for example, as will be explained later, a coating film is formed along the slopes parallel to the plate surface. The proportion of the coating surface decreases, resulting in poor image clarity. Conventional shot blasting and electric discharge machining methods cannot avoid such problems, and therefore it has been difficult to obtain sufficiently excellent image clarity of the coating surface. (Prior art) In order to improve the sharpness of the coating surface, attempts have been made to use laser-dulled skin-pass rolling work rolls (no literature found), but conventional laser-based dulling technology has not been used. Since the area ratio η of the flat part transferred onto the plate surface was below 80%, it was difficult to achieve a DOI of 98, and the highest quality of image clarity for automobiles could not be achieved. Nakatsuta. (Problems to be solved by the invention) This invention was made against the background of the above-mentioned circumstances, and by improving the roughness profile of the surface of the steel plate, the unevenness of the coating film surface after painting can be reduced and the surface can be flattened. By increasing the proportion occupied by the steel sheet, we can improve the specular reflectance of light and reduce the problem of image mapping, thereby providing a steel plate with excellent image clarity after painting. It is an object of the present invention to provide a method for efficiently manufacturing a steel plate having a surface roughness profile. In other words, the present invention provides a steel plate for painting and a method for manufacturing the same, which can significantly improve image clarity compared to the past without making any changes to the paints and painting methods used in the past. be. (Means for solving the problem) The above purpose is advantageously achieved by the following matters. The centerline average roughness Ra of the plate surface is within the range of 0.3 to 2.0 μm, and the microscopic form constituting the surface roughness is a trapezoidal peak having a flat peak surface;
A groove-shaped valley formed so as to surround all or part of the periphery and a crest, and located outside the trough, higher than the bottom of the trough and lower than the top surface of the crest. or an intermediate flat part formed at the same height, and the average distance between the centers of adjacent peaks is Sm, the average diameter of the outer edge of the valley is D, and the flat peak of the peak. The average diameter of the surface is d ₀ , and the ratio of the sum of the areas of the flat top surface of the mountain portion and the flat surface of the intermediate flat portion to the total area of the plate surface is η
(%), and is characterized by being configured to satisfy 1.7<Sm/D3.0 Sm−D450(μm) 30(μm)d ₀ 500(μm) 80(%)η95%. A steel plate for painting (first invention). The surface of the work roll for temper rolling is made up of a collection of minute crater-shaped recesses and a ring-shaped raised part on the front side at the outer edge of the recess, and the average center distance between adjacent recesses.
The ratio Sm/D of Sm and the diameter D of the outer edge of the ring-shaped raised part is within the range of 1.7 to 3.0, and Sm-D is 450μm
A patterning process that forms the following surface pattern is applied using a high-density energy source, and the work roll with the surface pattern is used on one or both sides of the steel plate to be temper-rolled. A method for manufacturing a steel plate for coating (second invention), characterized in that a pattern on the surface of a work roll is transferred to the surface of the steel plate by temper rolling with a rolling elongation rate λ of 0.3% or more. The inventors investigated a non-traditional laser processing method for dull finishing work rolls for temper rolling, and after conducting various experiments and research, found that dull finishing using a high-density energy source such as a laser was used. Steel plates that have been temper-rolled using finished rolls have flat peaks that make up the surface roughness, and the fact that there are many flat valleys between the peaks means that the coating film is the best during painting. This means that it is advantageous for flattening the outer layer. In other words, in this case, there is less diffused reflection of light than on irregular rough surfaces such as shot blasted materials or electrical discharge machined materials.
It is thought that the sharpness of the image will be improved. As a result of further experiments, we have published Japanese Patent Application No. 61-7769 regarding a roughness profile of the surface of a steel sheet that can improve the clarity of the coating film after painting, and a method for manufacturing a steel sheet having that roughness profile. It was disclosed in the book. However, with this, it was still difficult to achieve a sharpness DOI of 98 or higher. Therefore, the processing rate in press processing of automobile outer panels is at most 10
As a result of further experiments focusing on the fact that it is small (less than %), we found that we have devised a laser-based dull pattern at such a low processing rate and have come up with an invention to improve it to be more advantageous in improving image clarity. It has been reached. As the high-density energy source, a laser is most suitable, but plasma, electron beam, etc. are also applicable. Cold-rolled steel sheets are commonly used as thin steel sheets to which the dull finish described above is applied by temper rolling, but as already mentioned, so-called surface-treated steel sheets that have been previously subjected to hot-dip galvanizing or various types of electroplating, etc. Depending on the case, it may be a hot-rolled thin plate. (Function) [1] Dulling of a work roll for temper rolling using a laser: First, a high-density energy source, such as a laser, is used to rotate the work roll for temper rolling and sequentially project laser pulses onto the surface of the roll. , the roll surface is regularly melted by laser energy to form regular crater-shaped recesses. The state is shown in FIG. In FIG. 1, reference numeral 1 indicates a crater-shaped recess (hereinafter simply referred to as a crater) formed on the surface of the roll 3, and around the crater 1, molten roll base metal is deposited on the roll surface 3A.
A flange-like raised portion (hereinafter simply referred to as flange) 2 is formed in a ring shape. Note that the inner wall layer of the crater 1 including this flange 2 forms a heat affected zone 5 with respect to the roll base material structure 4. In the laser dulling process as described above, the depth and diameter of the crater 1 on the roll surface formed by the laser pulse are determined by the magnitude of the incident laser energy and the projection time.
This gives a quantity that defines the roughness equivalent to the Ra roughness of a normal shot blast roll. The metal that forms the roll 3 by heating with a laser instantaneously turns into metal vapor under high irradiation energy density, and the molten metal on the roll surface is blown away by the vapor pressure generated at this time, forming the crater 1. The blown-off molten metal re-fixes around the crater 1, causing the crater 1 to become
A flange 2 surrounding the flange 2 is formed. These series of behaviors can be executed more effectively by spraying an auxiliary gas such as oxygen gas toward the irradiation point. Then, by rotating or moving the roll in the axial direction and irradiating it with regular laser pulses, the above-mentioned craters 1 are regularly formed, and the roll surface is roughened by a collection of these craters formed one after another. It will show a face. The roughness of the surface of the roll thus formed is shown in Figures 2 and 3, and as is clear from these figures, the area between adjacent craters 1 remains flat as the roll surface. The face is 6. Here, the distance between adjacent craters is
The direction in which the roll is rotated is controlled by controlling the frequency of the laser pulse in relation to the rotational speed of the roll, and the irradiation position of the laser is moved in the direction of the roll axis every time the roll rotates. It can be adjusted by controlling the pitch of movement. Although the above description has been made regarding the case where a laser is used as the high-density energy source, the same applies to the case where other high-density energy sources such as plasma or electron beams are used. [2] Transferring dull marks to the surface of a steel plate by skin-pass rolling: Light rolling is applied to a steel plate, for example, an annealed cold-rolled steel plate, in the skin-pass rolling process using a work roll that has been dulled using a laser or the like as described above. By performing rolling at a certain rate, the burrs of the rolls are transferred to the surface of the steel sheet, and a rough surface is formed on the surface of the steel sheet. If we microscopically observe the surface of the steel plate during this process, as shown in Fig. 4, the flange 2 is pressed against the surface of the steel plate 7 with strong pressure at a substantially uniform height on the surface of the roll 3. Local plastic flow of the material occurs near the surface of the thin steel plate 7, which is softer than the material of the roll 3, and the metal of the steel plate 7 flows into the crater 1 of the roll 3, forming a rough surface. At this time, the top surface 8 of the trapezoidal peak bulging inside the crater 1 becomes a flat surface as it is on the original steel plate surface, and the outside of the flange 2 between adjacent craters 1 on the roll 3 becomes flat. Tanmen 6
The intermediate flat portion 9 pressed against the intermediate flat portion 9 becomes a flat surface as it is, and the former flat top surface 8 is slightly higher than or at the same level as the latter intermediate flat portion 9. Therefore, the microscopic morphology of the rough surface of the steel plate 7 after temper rolling is as shown in FIGS. 5 and 6.
A trapezoidal peak 10 having a flat peak surface 8 and a groove-shaped valley 11 formed to surround the trapezoidal peak 10
and between the adjacent peaks 10 and the valleys 11
There is a ridge 1 higher than the bottom of the valley 11 on the outside of the
It will be formed at a level lower than or at the same level as the peak surface 8 of 0. As is clear from the above, the surface of the steel plate after temper rolling has a large proportion of the flat part consisting of the peak surface 8 of the peak part 10 and the intermediate flat part 9, and the proportion of the flat part consisting of the peak part 10 and the valley part In principle, the ratio of the sloped surfaces 13 between the slopes 11 and 11 is reduced. On the other hand, in the case of rolls that have been roughened by shot blasting or electric discharge machining, the ridges on the roll surface that form the roughness are as shown in Figure 7A.
As shown in B, the ridges have various heights close to a normal distribution, and in this case, during the temper rolling process, the ridges on the surface of the roll 3 dig into the surface of the steel plate 7, as shown in FIG. Rough surface profile of roll surface and steel plate 7
The rough surface profile of the surface of the original plate is combined with the rough surface profile of the original plate surface, so that the steel plate 7 after temper rolling has, in principle, a large proportion of sloped surfaces formed by peaks and valleys. Therefore, it can be seen that in this case, the surface structure and the formation process are completely different from a steel sheet that has been temper-rolled by a roll that has been dulled by a laser. Figure 9A shows the inclination angle distribution of the roughness of the steel plate surface after skinpass rolling when skinpass rolling is performed using rolls that have been dulled by the conventional shot blasting method, and Figure 9B shows the definition of the angle of inclination. By the way, as mentioned above, the DOI value that expresses image sharpness is expressed as the ratio of scattered light of ±0.3° to the specular reflection angle, so it is good if there is a large proportion of valleys with a flatness slope angle of 0.3° or less. However, in the case of Fig. 9A, the occupation rate of the inclination angle within ±0.3° is only 13%, and in two dimensions it is only (0.13) ² ×100 = 1.7%. On the other hand, when skin pass rolling is carried out using laser dulled rolls, a flatness ratio that is an order of magnitude higher can be obtained. [3] Definition of the dimensions of each part of the roughness profile of the steel plate surface after roll and skin pass rolling: Here, we will define the dimensions of each part of the roughness profile of the roll surface dulled by laser as described above, and the dimensions adjusted by the roll. The dimensions of each part in the roughness profile of the quality rolled steel plate are
Referring to Figure 0, it is defined as follows. D: Average outer diameter of the flange 2 on the roll surface = Average diameter of the outer edge of the trough 11 on the steel plate surface d: Average diameter of the crater 1 on the roll surface d ₀ : Average diameter of the flat peak surface 8 of the ridge 10 on the steel plate surface Average diameter H: Depth of crater 1 on roll surface h ₁ : Height of flange 2 on roll surface = Depth from middle flat part 9 to valley part 11 on steel plate surface h ₂ : Flat of peak part 10 on steel plate surface Height Sm of the flat peak surface 8 from the intermediate flat part 9: Average distance between the centers of adjacent craters 1 on the roll surface = Average distance between the centers of adjacent peaks 10 on the steel plate surface α: Flange 2 on the roll surface Width [4] Influence on the area ratio η of the flat portion of the steel plate surface after skin-pass rolling: Using the value defined above, the pattern constituting the roughness profile of the roll surface and that of the skin-pass rolling We investigated whether the conditions had the following effects on the area ratio η of the flat portion of the surface after temper rolling. Here, the area ratio η of the flat portion is the ratio of the area occupancy η ₁ of the flat top surface 8 of the mountain portion 10 and the area occupancy η ₂ of the intermediate flat portion 9, as shown in FIG. It is expressed as a sum. η=η ₁ +η ₂ ...(1). Here, the value of η ₁ changes depending on the rolling reduction in temper rolling. This is because if the rolling reduction rate changes, the degree to which the steel sheet metal flows into the inside of the crater 1 changes, and therefore the diameter d ₀ of the peak portion 8 of the peak portion 10 changes. On the other hand, the value of η ₂ is Sm/
It becomes a constant value depending on the value of the ratio of D. This Sm/D ratio is within the range of the following equation (2), as will be described later. 1.7<Sm/D≦3.0...(2) Then, η ₁ is determined by the following equation (3), and as shown in equation (4), d ₀ has a constant relationship with d, and η ₂ is
It is determined by equation (5) depending on the value of Sm/D. η ₁ = π (d ₀ /Sm) ² / 4 ...(3) d ₀ = Kd ... (4) η ₂ = 1 - π (D / Sm) ² / 4 +a {(D / Sm) ² COS ^-1 (Sm/D) -√() ² -1} ...(5) However, in equation (5), when Sm/D≧1, a=0 ...(6) When Sm/D<1, a =1...(7) From the above (2), (5), (6), and (7), η ₂ changes within the following range. 0.78<η ₂ <0.95 ...(8) Here, the roughness profile cross-sectional shape of the roll surface and steel plate surface is
Taking the axis and y axis, the cross-sectional shape of crater 1 is y =
Assuming that it is COx, setting d=π, COS d/2=0...(9) Also, from COSd ₀ /2=h ₂ , d ₀ =2COS ^-1 h ₂ ...(10) Here , the ratio h ₂ /H of the height h ₂ of the peak 10 transferred to the steel plate surface by the crater 1 and the depth H of the crater 1 can be called the roughness transfer rate. Since the depth H of 1 is 1, the roughness transfer rate can be expressed as h ₂ /1, that is, h ₂ . The roughness transfer rate is h ₂ /1, that is, the height of the peak 10
h ₂ has a relationship determined by the rolling elongation rate λ of temper rolling. That is, h ₂ =f(λ)...(11) This relationship was determined through the following experiment. The original plate is a plate with a thickness of 0.32μm and a Ra roughness of 0.38μm.
Using SPCC steel plate, the roll for temper rolling is 200mmφ Hs with Ra roughness of 3.45μm by laser.
Using a material with a hardness of 94, skin pass rolling was performed at various rolling elongation rates λ. The results are shown in FIG. From FIG. 13, the roughness transfer rate h ₂ /1 increases linearly until the temper rolling elongation rate λ is around 1.5%, but
It can be seen that when λ exceeds 1.8%, the roughness transfer rate is saturated. Furthermore, using the results in FIG. 13, the above d ₀ ,
When the values of k and k ² were determined, the results shown in Table 1 were obtained.

[Table] By the way, when dull processing is performed using a laser so that the average roughness of a cold-rolled thin steel sheet for normal press forming is Ra1.0 to 3.0μm, the width α of the flange around the crater is is approximately 0.09×D. Therefore, d can be expressed by the following formula. d=0.82D...(12) Also, if equation (12) is used in equation (4), d ₀ =0.82kD...(13) Therefore, equation (3) can be expressed as follows. η ₁ = π (0.82kD/Sm) ² /4 = 0.5281k ² (D/Sm) ² ...(14) Equations (5), (6), (7), (8), (14) and the From the results in Table 1, the area ratio η of the flat part is as shown in Table 2a, Table 2b,
The values are shown in Table 2c.

【table】

【table】

[Table] If this η is illustrated according to the value of Sm/D, it can be expressed as shown in FIG. Moreover, this relationship can be generalized by the following equation (15). η=η ₁ +η ₂ =0.5281k ² (D/Sm) ² +1−π/4(D/
Sm) ² +a{(D/Sm) ² COS ^-1 (Sm/D) -√() ² -1)} ...(15) From Figure 14, the ratio of the area ratio Sm/D of the flat part is It is clear that there has been a major change. In addition, η changes depending on the elongation rate λ for temper rolling, especially
When Sm/D is small, it is greatly affected by changes in λ. [5] Lower limit of skin pass rolling elongation rate λ: As mentioned above, skin pass rolling elongation rate λ influences η, but if λ is too small, the skin pass rolling operation itself becomes unstable and the steel plate surface is affected. It becomes difficult to transfer dull marks. According to experiments by the inventors, if the temper rolling elongation rate is 0.3% or more, dull transfer is possible, so the temper rolling elongation rate λ was set to be 0.3% or more. [6] Lower limit of flat area ratio η and Sm/D: When dulling a skin-pass rolling work roll with a laser, change Sm, D, and d, and also change the elongation rate of skin-pass rolling. By changing λ,
Steel plates with various flat part area ratios η (all
After applying black paint using three coats, the DOI of the surface was measured, and the results shown in Figure 15 were obtained. From Figure 15, as η increases,
It is clear that the DOI value increases, that is, the image clarity improves. In recent passenger car body coatings, it is desirable that the DOI value be 98 or more in order to exhibit the highest image clarity, and for that purpose it is desirable that η be 80% or more. Therefore, from FIG. 15, the lower limit of η is set to 80%. Therefore, in order to ensure η = 80% at the normal temper rolling reduction of 0.8 to 1.2%, the lower limit of Sm/D must be determined from Figure 14.
Set it to 1.7. [7] Upper limit of Sm/D, Sm-D, and upper limit of η: The dimensions of each part of the roll roughness profile such as D, Sm, and H, which were already defined in [3], are clear from the explanation so far. The number of roll rotations, laser pulse frequency, laser output, feed rate at the laser irradiation point, laser irradiation time, or O ₂
It can be changed by adjusting the spraying conditions of auxiliary gas such as gas. here,
When achieving an Ra roughness of 0.5 to 5 μm, which is suitable for general cold-rolled steel sheets for processing, by skin pass rolling using laser-dulled rolls, the flange width α on the roll surface is approximately 20 to 40 μm; Further, the flange height _h1 is approximately 5 to 30 μm. On the other hand, the roughness profile formed on the surface of the steel plate has three types of patterns shown in FIG. 16A, B, and C depending on the value of Sm/D. That is, Sm/
When D is 1, as shown in No. 15A, adjacent continuous groove-shaped troughs 11 are in contact with each other at their tops,
When Sm/D>1, the adjacent valleys 11 are separated as shown in FIG. 16B, and conversely, when Sm/D<1, the adjacent valleys 1 are separated as shown in FIG.
1 are in a state where they overlap each other. Various roughness profile patterns can be obtained by changing the value of Sm/D in this way, but the inventor manufactured skin pass rolling rolls with various values of Sm/D by laser processing. A cold-rolled steel sheet that had been annealed to an appropriate skin-pass rolling reduction ratio was subjected to skin-pass rolling to perform dulling. Pressing tests and painting tests were conducted on each steel plate, and the following findings were obtained. That is, if the value of Sm/D of the roll becomes significantly large, when the steel plate 7 is subjected to skin pass rolling with the roll 3 to give it a dull weight, as shown in FIG. The area of the intermediate flat part 9 that exists between the peak part 10 becomes too large, so if such a steel plate is press-formed as shown in FIG. The metal flaking powder 13 generated during the work is difficult to be captured by the mountain part 11, and the metal flaking powder 13 remains on the press tool 1 forever.
It remains between 4 and intermediate flat 9. Further, a significantly large Sm/D means that the space of the valley portion 11, which serves to store press lubricating oil, becomes relatively small, so that poor lubrication is likely to occur. As a result, if Sm/D is too large, seizure is likely to occur during press working. Further, here, the width of the intermediate flat 9, that is, the absolute value of (Sm/D), also needs to be regulated for the following reason. The size of the flange formed on the roll surface by laser dulling, that is, the width α and height _h1 , is determined by the fact that part of the metal in the crater-shaped recess 1 melted by the laser rises around it and re-fixes. Since it _is related to the process of In other words, when D is large, the ability to store lubricating oil during press working and the ability to trap metal flaking powder 13 is large, and this has an important meaning in preventing the occurrence of seizure, but its effectiveness is limited in the following cases. limited to. That is, metal flaking powder 13
After this occurs, it gradually accumulates as the press processing progresses, and eventually within the range of the relative sliding length between the press die and the pressurized material, until seizing occurs. This is the case when the surface of the workpiece has groove-like recesses that can trap metal powder. In order to satisfy such conditions, it is necessary to make the absolute value of the width (Sm-D) of the intermediate flat portion smaller than a certain value. According to the above-mentioned experiments conducted by the present inventors, in the case of a steel plate that is used for the outside of a passenger car body and is not processed at a very high rate, the strain rate of press working is within 10%.
It has been found that as long as the value of Sm/D does not exceed 3.0, seizures during press forming as described above do not occur frequently. (FIG. 23) It has also been found that in order to prevent frequent seizures, it is necessary to make the absolute value of the width (Sm-D) of the intermediate flat portion 9 smaller than 450 μm. (FIG. 24) Some results of the experiment are shown in Table 3. Note that the values of (Sm-D) ₁ and (Sm-D) ₂ in Table 3 are the first
As shown in Figure 9.

【table】

[Table] Furthermore, the value of Sm/D is correlated with the area ratio η of the flat portion on the surface of the steel sheet, as shown in FIG. 14 described above. According to the inventors' experiments described above, as can be seen from Table 3, when the area ratio η of the flat portion exceeds 95%, seizure occurs frequently. Therefore, in this invention, in order to obtain a steel sheet with good press formability without seizure, Sm/
The upper limit of the ratio of D is 3.0, the upper limit of the flat portion area ratio η is 95%, and the upper limit of (Sm-D) is less than 4.0 μm.

[9] Diameter of the flat peak surface of the peak on the surface of the steel plate d ₀
Upper limit of: The flat peak surface 8 of the peak portion 10 that constitutes the microscopic profile of the rough surface of the steel sheet surface is a surface that bears a press load in press forming, and corresponds to a so-called bearing area. If the diameter d ₀ of this mountain top surface 8 is large, the flat area of the mountain top surface will be large, and seizure will occur during press working as in the case where Sm/D and η are large as described in item [7] above. It shows a tendency to become easier to do. According to the inventor's experiments, it has been found that when d ₀ is 500 μm, seizure is likely to occur. In addition, in order to form a wide mountain top surface 8 with d ₀ exceeding 500 μm, it is necessary to increase the diameter of the crater 1 of the roll, and in this case, the amount of energy required for laser pulse irradiation to generate the crater is If the result is excessive, it becomes necessary to use a laser oscillator with a higher output than necessary, or to slow down the rotation speed of the roll and lengthen the irradiation time. This leads to a decrease in processing efficiency and reliability. Therefore d ₀ is 500μm
It is necessary that the following is true. On the other hand, if the diameter d ₀ of the peak surface 8 of the peak part 10 is too small, the peak part 10 will be easily destroyed by compressive stress and shear stress during press working, and therefore more metal powder will be generated at the peak part. In the case of levers, seizure is also likely to occur. According to experiments conducted by the present inventors, it has been found that seizure is likely to occur particularly when d ₀ is less than 30 μm. Furthermore, if d ₀ is made smaller, the value of D will also be reduced accordingly, so in order to make d ₀ smaller and to satisfy Sm/D≦3.0 as stated in the above item [7], for,
The value of Sm itself must also be made small. In other words, it is necessary to reduce the interval between roll craters. To achieve this, it is necessary to either extremely reduce the roll rotational speed when performing laser processing on the roll or to extremely increase the laser pulse frequency.
In either case, it will be economically disadvantageous. For these reasons, the diameter d ₀ of the peak surface 8 of the peak portion 10 needs to be 30 μm or more. In addition, here, the diameter d ₀ of the mountain top surface 8 is the average diameter of 30 ~
It is fine if it is within the range of 500 μm, but if the crater 1 is actually formed on a roll using a high-density energy source such as a laser and the peak part 10 is formed by temper rolling, the peak part 10 will have a flat peak. The planar shape of the surface 8 is not necessarily a perfect circle, but is often oval or irregular. Therefore, in that case, the average value of the long axis of each mountain peak is
500μm or less, and the average value of the minor axis of each mountain top surface is
It is desirable to adjust the thickness to 30 μm or more.
Of course, it is appropriate that the largest length of all the peak surfaces is 500 μm or less, and the smallest length of the short diameter of all the peak surfaces is 30 μm or more. [10] Centerline surface roughness Ra of steel plate: As mentioned above, in this invention, it is most important to control the microscopic profile that forms the rough surface of the steel plate. Surface roughness also needs to be regulated. In other words, even if the microscopic profile of the rough surface is regulated as described above, the centerline average roughness Ra
If Ra exceeds 2.0 μm, the sharpness of the coating film will not be sufficiently good, while if Ra is less than 0.3 μm, seizure will easily occur during press working. Therefore, Ra is
It was set within the range of 0.3 to 2.0 μm. [11] Summary: From the above, it is clear that steel sheets temper-rolled with rolls that have been dulled using a high-density energy source such as a laser have good press formability (especially seizure resistance),
In addition, in order to achieve the excellent paint film clarity required for passenger cars after painting, preferably with a DOI value of 98% or more, the microscopic roughness profile of the steel plate surface must be (i) The ratio of the sum of the areas of the flat parts (the top surface of the mountain part and the middle flat part) to the area of the entire board surface (flat area occupancy rate) μm is 80% or more and 95% or less (ii) The ratio Sm/D of the average center-to-center distance Sm of the peaks and the average diameter D of the outer edges of the valleys is in the range of 1.7 or more and 3.0 or less, and Sm-D is less than 450 μm; (iii) The average diameter d ₀ of the top surface of the mountain is 30 μm or more;
It must be within the range of 500 μm or less, and in addition, the center line average roughness must be within the range of 500 μm or less.
It is necessary that Ra is within the range of 0.3 to 2.0 μm. Further, as a condition for skin pass rolling, it is necessary that the elongation rate λ of the skin pass rolling is 0.3% or more. Among the above conditions, the relationship between the particularly important Sm-D ratio and the appropriate range of d ₀ is shown in FIG. 20 together with a summary of the reasons for limiting the range. (Example) As material steel plate, C0.04%, Mn0.2%, P0.02
%, S0.015%, N0.003%, O0.005%,
A cold-rolled steel sheet with a thickness of 0.8 mm that was cold-rolled with a cold-rolling reduction of 69.2% and further annealed in a box-type annealing furnace was used. As work rolls for temper rolling, we prepared dull rolls that were dulled by laser pulse processing, dull rolls that were dulled by conventional shot blasting, and prite rolls that were not dulled. The rolled steel sheet was subjected to temper rolling using each roll so that the temper rolling elongation λ was within the range of 0.5 to 2.5%. Here, the surface roughness Ra of bright roll is 0.15μm
, and the surface roughness of the dull roll is Ra1.1 ~
Various changes were made within the range of 5.6 μm. In particular, the surface roughness profile of the roll that was dulled by laser processing _is as follows: and a sample with 1.7≦Sm/D≦3.0 Sm−D<450 50≦d≦500 35≦H≦120 h ₁ ≒1/3H were prepared. The surface roughness of the steel plate after temper rolling as described above is Ra 0.08 μm for steel plates using bright rolls (prite finish material), and Ra 0.6 to 2.25 for steel plates using dull rolls (dull finish material). It was μm.
In particular, the surface roughness profile of a steel plate temper-rolled using a roll that has been subjected to dull processing rather than laser processing is as follows. For the 500 μm B sample, 1.7≦Sm/D≦3.0 Sm−D<450 30<d500. Next, each steel plate after temper rolling was subjected to chemical conversion treatment under the following conditions. Treatment material: Fine-grained phosphate-based chemical for dip treatmentDeep conditions: 43℃ x 120 seconds Film weight: 2.3±0.2g/cm ² Pretreatment: Degreasing, water washing, surface conditioning Post-treatment: Water washing, pure water washing, drying After the chemical conversion treatment, three coats were applied under the following conditions. Painting position: horizontal painting Undercoat: cationic ED paint 18-20 μm thick Intermediate coat: sealer 30-35 μm thick Top coat: top coat 30-35 μm thick Sanding was not performed in each step. The DOI value of the painted surface was measured using a DORIGON meter. The results are shown in FIG. 21 for the case of three coats corresponding to the surface roughness Ra of each steel plate. In addition, in Figure 21 and in the text below, LT
The material is a steel plate that has been temper-rolled using a roll that has been dulled by laser, the EDT material is a steel plate that has been temper-rolled using a roll that has been dulled by electrical discharge machining, and the SB material is temper-rolled using a roll that has been dulled by shot blasting. Represents a steel plate. As is clear from Figure 21, the LT material instep when painted with 3 coats is compared to EDT material and SB material.
The image clarity is excellent at a DOI value of 10 to 11.
LT material Otsu was even better by one point in terms of DOI value, achieving a DOI value of 98. As already mentioned, the sharpness of the coating film is determined by the DOI
It is said that a value of 98% or more is desirable, but in the above example, it is clear that a DOI value of 98 or more can be obtained with the 3-coat LT material, as shown in FIG. Furthermore, according to a separately conducted press working test for outer panel parts for passenger cars, the LT shown in Figure 21
It was confirmed that no seizure occurred in the material O sample during press working. (Effects of the Invention) According to the steel plate for painting of the present invention, a remarkable effect of improving the sharpness of the coating film than before without impairing press formability can be obtained, and the steel plate for painting of the present invention According to the steel sheet manufacturing method, a steel sheet with excellent coating film clarity can be practically manufactured as described above.

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional view showing the state of the roll cross section when the surface of the work roll is dulled using laser pulses as high secret energy in the method of the present invention, and Fig. 2 is a dulling process due to the laser pulses described above. A schematic cross-sectional view showing the rough surface profile of the surface of the processed roll; FIG. 3 is a plan view of FIG. 2; and FIG. Figure, 5th
The figure is a schematic cross-sectional view showing the profile of the rough surface of the steel plate that has been temper-rolled by the rolls mentioned above, Figure 6 is a plan view of Figure 5, and Figure 7A is a dull area formed by conventional shot blasting. A diagram showing the height distribution of the rough surface of the processed roll surface, Figure 7B
Figure 8 shows the peak height distribution of the rough surface of the roll surface that has been dulled by conventional electric discharge machining, and Figure 8 shows the roughness distribution of the rough surface of a roll that has been dulled by conventional electrical discharge machining. Figure 9A is an explanatory diagram showing the situation, and Figure 9A is a diagram showing the inclination angle distribution of roughness °C of the steel plate surface when skin pass rolling is performed using rolls that have been dulled by the conventional shotplast method. A diagram showing the definition of the inclination angle in Figure A, Figure 10 is an explanatory diagram showing the definition of the dimensions of each part of the profile that forms the rough surface of the temper rolling roll and the steel plate, and Figure 11 is a diagram showing the definition of the flat surface. area ratio η
(=η ₁ + η ₂ ), Figure 12 is an explanatory diagram for approximate calculation of the roughness profile of the steel plate surface on the roll surface, and Figure 13 is the temper rolling elongation rate λ. Figure 14 is a diagram showing the relationship between the roughness transfer rate h ₂ /1 and the relationship between the area ratio η of the flat portion of the steel plate surface and the temper rolling elongation rate λ at various Sm/
Correlation diagram shown according to the value of D, Figure 15 is a correlation diagram showing the relationship between the steel plate flat area ratio η and the DOI value of the coating film when three coats are applied, Figure 16A,
B and C are explanatory diagrams showing the changes in the planar roughness profile of the steel plate surface when Sm/D is changed;
Figure 7 is a schematic cross-sectional view showing the microscopic profiles of the roll surface and the steel plate surface when the Sm/D ratio is excessive, and Figure 18 shows the effect of press working on the steel plate shown in Figure 17. Fig. 19 is a schematic diagram to explain the relationship between (Sm/D) ₁ and (Sm/D) ₂ , and Fig. 20 is a diagram showing the value of Sm/D and the peak surface of the peak of the steel plate surface. An explanatory diagram showing the relationship between the appropriate range of the diameter d ₀ , Figure 21 is a correlation diagram showing the relationship between the center line average roughness Ra of the steel plate and the DOI value of the coating film in the case of 3-coat painting in the example, and Figure 22 The figure shows the sharpness of the image.
Explanatory diagram to show how to measure DOI value, No. 23
Figure 24 shows seizure occurrence and Sm/D, (Sm-D) ₂
It is a relationship diagram. 1...A crater-shaped recess on the roll surface, 2...A ridge-like raised flange on the roll surface, 3...
...Roll, 7... Steel plate, 8... Flat peak surface of the mountain part on the surface of the steel plate, 9... Middle flat part on the surface of the steel plate,
10... Peaks on the surface of the steel plate, 11... Valleys on the surface of the steel plate, 13... Inclined surface.

Claims (1)

[Claims] 1. The centerline average roughness Ra of the plate surface is within the range of 0.3 to 2.0 μm, and the microscopic form constituting the surface roughness is trapezoidal with a flat peak surface. between the ridge and a groove-like trough formed to surround all or part of the periphery of the ridge, and which is higher than the bottom of the trough and located outside the trough. and an intermediate flat part formed at the same height as or lower than the peak surface of the mountain, and the average distance between the centers of adjacent peaks is Sm, the average diameter of the outer edge of the valley is D, and the peak is The average diameter of the flat mountain top surface of
d ₀ , the ratio of the sum of the areas of the flat top surface of the mountain portion and the flat surface of the intermediate flat portion to the total area of the plate surface is defined as η (%), and 1.7<Sm/D3. 0 Sm-D450 (μm) 30 (μm) d ₀ 500 (μm) 80 (%) η95% A steel plate for painting, characterized in that it is configured to satisfy the following. 2 The surface of the work roll for skin pass rolling is made up of a collection of minute crater-shaped recesses and a ring-shaped raised part on the front side at the outer edge of the recess, and the average center distance between adjacent recesses. The ratio Sm/D between the distance Sm and the diameter D of the outer edge of the ring-shaped raised portion is within the range of 1.7 to 3.0, Sm-D
A patterning process that forms a surface pattern with a diameter of 450 μm or less is applied using a high-density energy source, and the work roll with the surface pattern is used on one or both sides of the steel plate to be temper rolled. ,
A method for manufacturing a steel plate for painting, characterized in that a pattern on the surface of a work roll is transferred to the surface of the steel plate by temper rolling with a temper rolling elongation rate λ of 0.3% or more. 3. The method of manufacturing a steel plate for painting according to claim 2, wherein a laser is used as the high-density energy source.