JPH0790243B2 - Method for producing cold-rolled steel sheet with excellent paint clarity - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a method for producing a cold-rolled thin steel sheet, which is used after being subjected to forming processing such as press working and coating, such as an outer panel of an automobile body or an exterior panel of household electric appliances. It is desired to improve the coating clarity of the product steel sheet advantageously.

In recent years, not only passenger cars, but also light cars, wagon cars, and even trucks, the quality of the painted finish of the body after painting makes it possible for the customer to directly see the overall high quality of the car. This is an extremely important quality control item because it will be sued.

By the way, there are various methods for evaluating the coating surface, and among them, it is important that the coating surface has less diffuse reflection and excellent glossiness, and that there is less image distortion and superior image clarity. , Is generally called "vividness" because of its gloss and image clarity.

Various methods have been developed to evaluate the image clarity, but most commonly, the measured value by a DORIGON meter manufactured by Hunter Associates Laboratory in the United States, that is, DOI (Distinctne
ss of Image) value is used for this DOI value when the light is incident on the sample at an incident angle of 30 °, and its specular reflection light intensity Rs and scattered light intensity R _0.3 at ± 0.3 ° to the specular reflection angle are It is expressed by the following equation using the value.

DOI value = 100 × (Rs−R _0.3 ) / Rs It can be said that the higher the DOI value is, the better the clarity of the coating film surface is. The DOI of the coating surface is
It is desired that the value is 94% or more.

(Prior Art) By the way, cold-rolled steel sheets generally used for forming processing such as outer panels of automobile bodies and exterior panels of household electric appliances are cold-rolled hot-rolled steel sheets to a required thickness by a tandem cold rolling mill. After rolling, degreasing cleaning is performed, annealing is further performed, and temper rolling is usually performed to manufacture.

One of the purposes of temper rolling in such a process is to give moderate surface roughness to the steel sheet surface by performing light rolling using a work roll whose surface is dull-finished. Is to improve the seizure resistance.

As a method for dull-finishing the surface of work rolls used for temper rolling, conventionally, a method by shot blasting (hereinafter abbreviated as SD method) and a method by electrical discharge machining (hereinafter
Abbreviated as ED method), has been put to practical use.

However, the work rolls dull-finished by the SD method or the ED method have a fairly irregular roughness profile formed on their surfaces, and therefore, when temper rolling is performed on a steel sheet using the work rolls, the steel sheet surface also has an uneven surface. It presents a rough surface composed of regular peaks and valleys with very few horizontal surfaces. When coating is applied to a steel sheet having irregular peaks and valleys in this way, the coating film is formed along the slope between the peaks and valleys, so the proportion of the horizontal coating film surface becomes extremely small, It deteriorates the clarity of the coating surface.

In the conventional temper rolling using a work roll that has been dull-finished by the SD method or the ED method, the above problems cannot be avoided, and thus it is difficult to obtain sufficiently excellent sharpness of the coating film surface.

(Problems to be Solved by the Invention) In view of the above-mentioned circumstances, the inventors previously described in Japanese Patent Application No. 61-7769 (published in JP-A No. 62-168602) microscopically By improving the surface roughness profile to reduce irregularities on the coating surface after painting, and increasing the proportion of the horizontal surface area, the specular reflectance of light is improved and the distortion of the image is reduced. Proposed a method for efficiently producing a steel sheet capable of remarkably improving the sharpness of the coated surface after coating and a steel sheet having such an excellent surface roughness profile in the temper rolling process. There is.

In addition, Japanese Patent Application No. 61-67433 (JP-A-62-22440)
No. 5 gazette), the surface of the steel sheet is not provided with undulations having a wavelength of about 300 to 1000 μm, which causes defects called yuzu skin on the surface of the coating film. We propose a steel sheet that can improve the image clarity and an efficient manufacturing method.

That is, in the invention of Japanese Patent Application No. 61-67433, in the cold rolled steel sheet manufacturing method, first, as a work roll of the final stand of the tandem cold rolling mill, a bright roll and a dull weight by laser processing (hereinafter referred to as LD method). ) Rolls (laser dull rolls), rolls dulled by the ED method (discharge dull rolls), and center line average roughness Ra
By using one of the rolls (shot blast dull rolls) dulled by the SD method at 1.0 μm or less, it is possible to prevent the occurrence of waviness on the steel plate surface after cold rolling and As the quality rolling, the same method as the method proposed in the specification of Japanese Patent Application No. 61-7769 described above is applied.

That is, the invention according to Japanese Patent Application No. 61-67433 is temper rolling a steel sheet in a temper rolling step using a roll having a surface roughened by a high density energy source including a laser. Is an essential condition.

However, since a roll surface roughening machine using such high-density energy is very expensive, it has not been widely used yet and maintenance work is difficult because it requires a high degree of technical skill, so it is always stable. However, it is difficult to realize the surface roughness profile on an industrial scale.

By overcoming such problems, that is, by temper rolling using rolls that are dulled by the commonly used roll surface roughening method SD method or ED method, cold rolled steel sheet with excellent image clarity is obtained. That is the object of the present invention.

(Means for Solving Problems) In the cold-rolled steel sheet manufacturing method according to the present invention, a bright roll having a center line average roughness Ra of 1.0 μm or less is first used as a work roll of a final stand of a tandem cold rolling mill. Laser dull roll By using a roll selected from discharge dull roll and shot blast dull roll, it is possible to prevent waviness from occurring on the surface of the steel sheet after cold rolling.

Next, as a work roll for temper rolling, by using a roll whose center line average roughness Ra is 1.0 μm or less by the SD method or the ED method, a dull pattern on the surface of the work roll is applied to a steel plate. Transfer to the surface.

Here, it is possible to suppress the waviness of the steel plate surface after cold rolling by using the roll of the above or as the work roll of the final stand of the tandem cold rolling mill, in Japanese Patent Application No. 61-67433. It is the same as described above.

Further, the temper rolling described above is applied again as a secondary process to a steel sheet that has been subjected to re-rolling or general temper rolling, and further, the temper rolling reduction rate is set to 0.3 to 5.0%. Is also suitable.

By applying the above means, the present invention is disclosed in Japanese Patent Application No. 61-6.
It has been found that it is possible to obtain the same effects as the invention of No. 7433.

The inclination angle θ of each portion of the roughness curve on the surface of the steel sheet after temper rolling can be measured as shown in FIG. 1, and the distribution of tan θ = α can be obtained as shown in FIG.

The probability of falling within ± 2α when α is fixed to a certain value is given by the area W _{2 α} , and the larger W _{2 α} is, the higher the area occupancy rate (flat area rate) is.

As shown in Fig. 3, the results of actual measurement of W _{2 α} for various steel plates with α = 0.03 are shown in Fig. 3. In both cases, the flat surface area ratio decreases as the surface roughness represented by the center line average roughness Ra (μm) of the steel plate decreases. Is increasing.

Steel plate by shot blast dull roll (hereinafter SD
Described as wood. ), In the case of a steel plate (referred to as ED material) by discharge dull roll, when Ra is 1.0 μm or less, it is about the same as when the steel plate by laser dull roll (hereinafter referred to as LD material) falls below Ra 1.4 μm. It is considered that the flat area ratio becomes.

Next, the work roll of the final stand of the cold rolling tandem mill (abbreviated as WR below) and the work roll of the temper rolling (hereinafter SR
To the cold-rolled steel sheet obtained by applying the roll conditions of
Steel plate with the center line average roughness Ra (μm) according to the identification mark shown in Table 1 based on the measurement results of the image clarity (DOI value) on the coating surface of the product coated with 3 coats of thickness 85 μm FIG. 4 shows the surface roughness along the horizontal axis.

As shown in Fig. 5, the DOI value is measured by injecting light into the sample S at an incident angle of 30 ° and measuring the specular reflection intensity Rs and the scattered light intensity R0.3 at ± 0.3 ° with respect to the specular reflection angle. It can be calculated by the above.

According to FIG. 4, the sharper the steel sheet surface roughness Ra after temper rolling, the worse the image clarity becomes, and the roll condition using a shot blast roll with Ra> 1.0 μm as WR (Case Nos. 3, 7, and 9). Marks ○, □ and △), SD material mark ○, ED material mark □ are LD material mark △ if the steel plate surface roughness is the same.
Based on the general tendency that the image clarity is inferior to that of the above, even when the steel plate surface roughness is Ra ≈ 0.1 (case No. 10) using a bright roll for SR even under the above roll conditions, If a laser dull roll is used for SR (case No. 9), a DOI value of 95% or more can be obtained even when the steel plate surface roughness Ra is lower than about 1.6, but here In order to obtain a DOI value equivalent to that obtained with the above bright roll using a discharge dull roll or shot blast dull roll as SR, the Ra> 1.0 μm as work roll WR of the cold rolling final stand is indicated by arrow 1. From shot blast dull roll to bright roll, again as indicated by arrow 2 Ra
Change to discharge dull roll of ≦ 1.0μm or arrow 3
It is clear that it is necessary to change to a bright roll as well as to a shot blast dull roll with Ra ≦ 1.0 μm as indicated by arrow 4. In short, as the work roll WR of the cold rolling final stand, the center line average roughness Ra is
Bright rolls, laser dull rolls of 1.0 μm or less,
After performing cold rolling using a discharge dull roll or a shot blast dull roll, especially using a discharge dull roll or a shot blast dull roll having a center line average roughness Ra of 1.0 μm or less in the temper rolling process as a work roll SR, It is equivalent to using a laser dull roll for temper rolling, which is expensive for dull finishing and requires control of roughness.Furthermore, a bright roll is used for temper rolling to crush the roughness peaks on the surface of the steel sheet and flatten the surface. It is possible to achieve an improvement comparable to the improvement of the sharpness by increasing the number.

(Function) As the final stand work roll WR of the tandem cold rolling mill, a roll selected from bright roll, laser dull roll, discharge dull roll, and shot blast dull roll having a center line average roughness (Ra) of 1.0 μm or less is selected. The reason is that when cold rolling is performed using a work roll of Ra> 1.0 μm, the waviness of the steel sheet surface becomes large, the image clarity after coating is impaired, and it does not meet the purpose of the invention. . Here, Ra is usually suitable up to about 0.3 μm.

For work roll SR of temper rolling mill, center line average roughness of both discharge dull roll and shot blast dull roll
Ra is set to 1.0 μm or less because when Ra> 1.0 μm, the unevenness on the surface of the steel sheet becomes too large and the image clarity deteriorates. Here, Ra is usually suitable up to about 0.5 μm.

When the temper rolling is performed again, the reduction rate is 0.3 to 5.0%, and if the reduction rate is less than 0.3%, it is too light reduction and the flat surface area ratio of the steel sheet surface is increased to produce a clear image. It is difficult to improve
If the rolling exceeds 5.0%, the rolling load becomes high, the power consumption of the rolling mill becomes high, and the material of the steel sheet deteriorates, which deteriorates the workability.

〔Example〕

As shown in Table 2 as the work roll WR of the final stand of the cold tandem rolling mill, the center line average diameter Ra of the roll surface is shown.
Cold rolling was carried out using bright rolls, shot blast dull rolls, discharge dull rolls, and laser dull rolls of different types.

Then, similarly under the conditions shown in Table 2, after the temper rolling, the center line average roughness Ra and the flat area ratio of the steel sheet surface were measured.

Furthermore, the result of measuring the clarity (DOI value) of the coating film after applying 3 coats of 80 μm thickness is the center line average roughness R of the steel sheet.
The results are shown in Table 2 together with the surface roughness represented by a (μm) and the flat area ratio (%).

As is clear from Table 2, symbols C and M corresponding to the center line average roughness Ra of the work roll WR of the final stand of the cold rolling mill exceeding 1.0 μm, and the center line average roughness Ra of the temper rolling work roll SR are 1.0. The center line average roughness Ra of the work rolls WR and SR of the symbols F, G, I, J and both of which exceed μm is 1.0μ.
The image clarity (DOI value) was more than m and was less than 85% in all cases, and was low along with the flat area ratio as compared with the application example of the present invention.

(Effect of the invention) As is clear from the above examples, according to the method of the present invention, the surface waviness of the steel sheet after cold rolling is reduced,
Finally, it is possible to reduce the waviness of the steel sheet surface after temper rolling, and therefore, in combination with the use of work rolls with a prescribed roughness profile in temper rolling, it is possible to obtain coatings with outstanding image clarity. It becomes possible to form a film.

[Brief description of drawings]

FIG. 1 is a drawing showing the inclination angle of the roughness curve of the steel plate surface, FIG. 2 is a drawing showing the distribution of the inclination angle (θ) of the roughness curve of the steel plate surface, and FIG. 3 is the center line average roughness (Ra). ) Is a drawing showing the relationship between the surface roughness of the temper rolled steel sheet and the inclination angle distribution (W ₂ ), and Fig. 4 is the effect of the steel sheet surface roughness after temper rolling on the sharpness (DOI value). FIG. 5 is a drawing showing a method of measuring a DOI value that expresses sharpness.

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Claims (3)

[Claims] 1. A tandem cold rolling mill is used to manufacture a cold rolled steel sheet in which a hot rolled steel sheet is cold rolled by a tandem cold rolling mill to a required sheet thickness, then annealed and then temper-rolled. After performing cold rolling using a work roll of the final stand with a center line average roughness Ra of 1.0 μm or less, selected from bright roll, laser dull roll, discharge dull roll, and shot blast dull roll, As a work roll for temper rolling, a roll with a center line average roughness of 1.0 μm or less roughened by electrical discharge machining or shot blasting is used for temper rolling. Is transferred to the surface of the steel sheet, and a method for producing a cold-rolled steel sheet with excellent coating image clarity. 2. The method for producing a cold-rolled steel sheet having excellent coating image clarity according to claim 1, wherein temper rolling is repeated. 3. The method for producing a cold-rolled steel sheet having excellent coating sharpness according to claim 1 or 2, wherein the temper rolling has a rolling reduction of 0.3 to 5.0%.